flotation machine pumped

flotation machines

flotation machines

As pneumatic and froth separation devices are not commonly used in industry today, no further discussion about them will be given in this module. The mechanical machine is dearly the most common type of flotation machine currently used in industry, followed by the column machine which has recently experienced a rapid growth.

A mechanical machine consists of a mechanically driven impeller that disperses air into the agitated pulp. In normal practice this machine appears as a long tank-like vessel having a number of impellers in series. Mechanical machines can have open flow of pulp between the impellers or can be of cell-to-cell design with weirs between them. Below is a typical bank of flotation cells used in industrial practice.

The procedure by which air is introduced into a mechanical machine falls into two broad categories: self-aerating, where the machine uses the depression created by the impeller to induce air, and supercharged, where air is generated from an external blower. The incoming feed to the mechanical flotation machine is usually introduced in the lower portion of the machine. At the very below is shown a typical flotation cell of each air delivery type (Agitair & Denver)

The most rapidly growing class of flotation machine is the column machine, which is, as its name implies, a vessel having a large height-to-diameter ratio (from 5 to 20) in contrast tomechanical cells. This type of machine provides a counter-current flow of air bubbles and slurry with a long contact time and plenty of wash water. As might be expected, the major advantage of such a machine is the high separation grade that can be achieved, so that column cells are often used as a final concentrate cleaning step. Special care has to be exercised in the generation of fine air bubbles and the control of the feed rate to the column cell for such cells to be effective. Column cell use is often of limited value in the recovery of relatively coarse valuable particles; because of the long lifting distances involved, the bubbles can not carry large particles all the way to the top of the cell.

Probably the most significant area of change in mechanical flotation cell design has been the dramatic increase in machine cell volume with a single impeller. The idea behind this approach is that as machine size increases (assuming no loss of recovery performance with the larger machines), both plant capital and operating cost per unit of throughput decrease. In certain industrial applications today, cells of even a thousand cubic meters in volume (a large swimming pool) are being used effectively.

The throughput capabilities of various cell designs will vary with the flotation machines residence time and pulp density The number of cells required for a given operation is determined from standard engineering, mass balance calculations. In the design of a new plant, the characterization of each cells volume and flotation efficiency is generally calculated from data gathered on a laboratory scale flotation using the same type of equipment for the same material mixture in question. This procedure is then followed by the application of semi-empirically derived scale-up factors. Research work is currently under way to improve the understanding and performance of commercial flotation cells.

Currently, flotation cell design is primarily a proprietary material of the various cell manufacturers. Flotation plants are built in multiple cell configurations (called banks), and the flow through the various banks is adjusted in order to optimize plant recovery of the valuable as well as the grade of the total recovered mass from flotation. Up above is a typical flotation bank scheme. The total layout of a given flotation plant (including all of the various banks) operating on a given feed is called a flotation circuit.

The application of the air-lift to flotation is not new, but the first attempts to make use of the principle were not successful because the degree of agitation in the machine was insufficient to enable the heavy oils then in use as collecting reagents to function effectively. The advent of chemical promoters, however, made agitation of secondary and aeration of primary importance, with the result that the application of the air-lift principle became practicable and led to the introduction of the Forrester and the Hunt matless machines. South western Engineering Corporation are the owners in most countries of the rights to license and manufacture these and other types operating on the air-lift principle, and they have developed a machine based chiefly on the Welsh and Hunt patents which may be considered as representative of the type that is now most commonly used.

The Southwestern Air-Lift Machine, as it is called, consists of a V-shaped wood or steel trough of any length but of the standard cross-section shown in Fig. 40, the area of which is 9.85 sq. ft. and the interior depth 36 in. Low- pressure air is delivered from a blower through a main supply pipe to an air-pipe or header which runs longitudinally over the top of the machine. The air enters the trough itself through a seriesof vertical down-pipes , which are screwed into sockets welded tothe underside of the header at 4-in. intervals along its length and are open at their lower ends. They are from to 1 in. in diameter for roughing machines and from to in. for cleaners, and they reach to within 6 in. of the bottom. The air-lift chamber is formed by two vertical partitions, one on each side of the line of down-pipes, both of which extend from one end of the trough to the other, forming a compartment 6 in. wide. The lower edges of the partitions are an inch or two above the ends of the down-pipes and their upper edges are about level with the froth overflow lips at each side of the machine. A few inches above the top of the air-lift chamber is a deflector cap which serves to direct the rising pulp outwards and downwards against two vertical baffles. These extend the length of the trough parallel to and outside the partitions, their loweredges being several inches below the normal pulp level. The spacebetween the baffles and the sides of the machine forms two spitzkasten- shaped zones of quiet settlement where the froth collects.

The feed enters near the bottom of one end of machine and the tailing is discharged over an adjustable weir at the other end. The air, issuing in a continuous stream from the open ends of the down-pipes, carries the pulp up the central chamber on the principle of an air-lift pump. The air is subdivided into minute bubbles and more completely mixed with the pulp as the rising mass hits the cap at the top and is deflected and cascaded on to the baffles at each side, which direct it downwards, distributing the bubbles evenly throughout the pulp in the body of the machine and giving them ample opportunity to collect a coating of mineral. Rising under their own buoyancy, the bubbles enter the spitzkasten zones, up which they travel without interference, dropping most of the gangue particles mechanically entangled between them as they ascend. They collect on the surface of the pulp at the top as a mineralized froth, which is voluminous enough to pass over the lip into the concentrate launders without the need of scrapers. The pulp, on the other hand, continues its downward passage and enters the air-lift chamber again. In this way a continuous circulation of the pulp is maintained, its course through the machine being more or less in the form of a double spiral.

The aeration is generally controlled by a single valve in the header of each machine, but for selective flotation the machine is sometimes divided by transverse partitions into sections 4 ft. long, the header over each section being provided with a separate air-valve. The depth of the froth is regulated by means of the adjustable gate of the tailing weir. If difficulty is likely to be experienced in making a clean tailing with the normal amount of aeration, it is preferable to use two machines. The second one is run as a scavenger with an excess of air as compared with normal requirements, the low-grade froth so produced being pumped back to the head of the primary or roughing machine, in which the aeration is more normal in order that a comparatively clean concentrate may be produced. It is often possible to take a concentrate off the first few feet of the rougher rich enough to be sent to the filters as a finished product, the froth from the rest of the machine being pumped back to the head. When this method of flotation is adopted, it is an advantage to have the header divided into sections, each with its own valve, so that the aeration can be varied along the length of the machine. By increasing the volume of air at the discharge end the froth can be given a slight flow towards the head of the machine, with the result that the minerals are concentrated there to the exclusion of partially floatable gangue which might otherwise enter any bubbles not fully loaded with mineral.

If the froth from the feed end of the rougher is not of high enough grade, it must be re-treated in a separate cleaning machine, the length of which usually varies from one-quarter to one-half of the total length ofthe roughing and scavenging machines according to the amount of concentrate to be handled. Should still further cleaning be necessary, it is performed in a recleaner, which is generally of the same length as the cleaner. The tailings from these operations are often, but not necessarily, returned to the head of the rougher.

It is usual to prepare the pulp for flotation by adding the reagents to the grinding circuit or in a conditioning tank ahead of the flotation section, but soluble frothers such as pine oil and quick-acting promoters such as the xanthates can be added at the head of the machine if desired, since the air-lift provides enough agitation to emulsify and distribute them throughout the pulp. It is not as a rule advisable to introduce reagents into the air-lift chamber itself ; should it be necessary to do so to obtain a satisfactory recovery of the minerals, it is best to employ separate roughing and scavenging machines and to make the extra additions at the head of the scavenger.

Southwestern Air-Lift Machines are made of standard cross-section, as already stated, and in a series of lengths ranging, for ordinary purposes, from 4 to 48 ft. There is no limit to the possible length, however, and 100-ft. machines are in actual use. The tonnage capacities under different conditions will be found in Table 26. The pressure of air needed at the machine is from 1.6 to 1.7 lb. per square inch, which under normal conditions requires a pressure of about 2 lb. per square inch at the blower. It is usual to allow 75 to 100 cu. ft. of free air per minute at this pressure per foot of rougher and 45 to 70 cu. ft. per minute per foot of cleaner and recleaner. From these figures the approximate volume of air required for a machine or machines of any given length can be calculated. The power necessary to supply the air can then be found from Table 30.

The Callow Cell consists of a shallow horizontal trough, the bottom of which is covered with a porous medium, usually termed a blanket, consisting of a few layers of canvas or of a sheet of perforated rubber. Air is introduced at low pressure under the blanket, and, in passing through it, is split up into minute bubbles, which rise through the pulp in the cell, collecting a coating of mineral in the process.

Fig. 41 shows a section of the type of cell commonly employed. Its width is usually from 24 to 36 in., and its interior depth from 18 to 22 in. measured from the overflow lip ; the length varies according to requirements and is generally a multiple of the width. On the bottom are placed, side by side, the square open-topped cast-iron blanket frames or pans . The blanket covering the top of each pan is securely held in place by flat iron strips bolted round the edges, while one or two pipe grid-bars across the top prevent it from bulging. This arrangement allows a blanket to be changed in a few minutes should it becomedamaged. The air inlet to each pan projects through the bottom of the cell and is connected by a pipe and regulating valve to a header, which is provided with a main control valve.

The pulp enters one end of the cell through a feed opening and is discharged over an adjustable weir at the other end. There is no agitation, but the continuously rising stream of air bubbles keeps the particles of ore in suspension and induces a certain amount of circulation as the pulp passes along the cell. In this way the minerals are given many chances of becoming attached to the bubbles and thus of being carried over into the concentrate launder. The froth that forms on the surfaceof the pulp, usually to a depth of 8 to 10 inches, is voluminous enough to overflow the lips on each side of the cell without the use of mechanical scrapers.

For estimating purposes the average capacity of a Callow Cell may be taken as 2.5 tons of feed per square foot of blanket area per 24 hours and the air consumption as 9 cu. ft. of free air per minute per square foot of blanket at a pressure of 4 lb. per sq. in. A greater pressure is likely to be required if the blankets become blinded .

The Callow Cell has proved satisfactory for many types of ores, but it has the disadvantage that coarse or heavy sand settles on the blankets, and can only be kept in motion by flogging the latter with short rubber-buffered poles. Moreover, if lime is employed in the circuit, the blankets become impregnated and clogged with calcium carbonate, which necessitates periodical acid treatment for its removal. The use of perforated rubber sheets in place of canvas in the Callow Cell mitigates without entirely curing these difficulties, which at one time were thought to be inherent in the use of a porous medium. They have been overcome, however, by the development of the Callow-Maclntosh Machine.

The Callow-Maclntosh, or the Macintosh Machine, consists of a shallow trough or cell at the bottom of which is a hollow revolving rotor covered with a porous medium. Fig. 42 shows its construction. The pulp enters through a feed opening at one end, and is discharged at the other in much the same way as in a Callow Cell. The rotor, made of seamless steel tubing with a cast-steel ring welded in each end, is perforated with -in. holes at 7-in. centres; it is about 8 in. shorter than the length of the cell and is usually 9 in. in diameter. Its weight is taken by two hollow shafts, each fitted with a flange, which are bolted to the ends of the rotor by means of four studs. This method of attachment enables the rotor to be changed and a new one inserted with little loss of time, usually not more than 15 minutes. The shafts project through the ends of this cell and are supported on self-aligning ball and socket bearings outside, so placed that the rotor itself is a few inches clear of the bottom of the trough. A rubber gasket, shown in Fig. 43, seals the opening at each end by simple pressure on a cone-faced disc mounted on the shaft. The joint is not completely watertight and a slight leakage takes place through it at the rate of about one quart per minute. At the discharge end this escaping pulp gravitates to the tailing launder, while at the feed end it is usually led to one of the pumps returning a middling product to the roughing circuit. The gasket is preferable to a stuffing-box, as it contains no grease and requires no gland water.

The rotor covering consists of a canvas sock or of a single sheet of perforated rubber. The latter is now far more commonly employed, since it lasts five times as long as the other, its life generally exceeding 18 months ; moreover it seldom becomes blinded withcalcium carbonate, and requires an air pressure of only 2 lb. per square inch instead of the 3-lb. pressure needed for canvas. The rubber sheets are made of pure gum about 5/64 in. thick with 225 holes per sq. in., the holes being made so as to allow the air to pass through while preventing the percolation of the pulp into therotor in the event of a temporary shut-down. Two scraper bars of angle iron, 1 by 1 in., are bolted to opposite sides of the rotor on the top of the covering. They project 2 in. beyond the ends of the rotor, and their purpose is to keep in circulation any sand that settles on the bottom of the cell, at the same timeprotecting the porous medium from undue wear by contact withsuch material. Air is introduced into the rotor through one or bothof the hollow shafts, which are connected by special inlet joints with themain supply. When both ends are employed for the admission of air,the rotor is usually divided into two sections by a central partitionto enable each half to be controlled separately. The rotor is driven ata speed of about 15 r.p.m. by an individual motor connected with theshaft at one end of the cell; either a worm drive directly coupled to themotor or a chain drive coupled to the motor through a speed reducercan be employed.

The principle on which theCallow-Maclntosh Machine worksis very similar to that of a CallowCell. The air bubbles actuallyissue from the top of the rotor,where the hydraulic pressure islowest, and spread out as theyrise, their distribution throughthe pulp being quite as even andeffective as when a flat blanket isused. The cell never needs flogging since the movementof the rotor prevents sand fromsettling on it, and the scraperbars keep in circulation theheavy particles that would otherwise settle on the bottom. Themachine can, if necessary, handle ore as coarse as 20 mesh at a W/Sratio of 1/1 without choking.

The control of a pneumatic cell is different from that of a machine of the mechanically agitated type, of which each cell is capable of performing the function of a high-speed conditioner. Little conditioning takes place once the pulp has entered a pneumatic cell, and provision must therefore be made for its proper preparation when employing heavy oils or chemical reagents which need a long contact period. The froth is usually maintained at a depth of 8 to 10 in., giving an effective pulp depth of 18 to 20 in. The very large volume of air bubbles released enables flotation to be effected more rapidly than in any other type of machine, the actual time required depending mostly on the degree to which the minerals have been rendered floatable. The upward stream of bubbles is so voluminous that, under ordinary conditions, the froth overflows the lips on both sides of the cell without the need of scrapers. For the same reason a considerable quantity of gangue is often carried over into the concentrate launder by mechanical entanglement with the bubbles, and one, sometimes two, subsequent cleaning operations are generally necessary in consequence. This, however, is by no means therule ; a concentrate of high enough grade to be sent to the filtering section as a finished product can sometimes be made in a single rougher- cleaner cell. When the Callow-Macintosh Machine is run in this way (counter-current operation) a partitioned rotor is employed, since, by increasing the volume of air at the tailing-discharge end, the froth can be made to flow towards the head of the cell with the result that the minerals are concentrated there to the exclusion of gangue particles. The same effect can be obtained in a Callow Cell by regulating the admission of air to the individual pans in a similar way. If is often the practice, especially in counter-current operation, for the rougher to be followed by a scavenging cell, which is run with an excess of air as compared with the former, the froth being returned to the head of the first cell.

Callow-Macintosh Machines are made in lengths of 10, 15, and 20 ft. and in widths of 24, 30, and 36 in. with a rotor 9 in. in diameter. The vertical distance from the centre-line of the rotor to the overflow lip is about 24 in. The design of the machine, however, lends itself to the construction of larger sizes for big scale operationsi.e., up to a 30-ft. cell 48 in. wide with one or two 9-in. rotors. The 30- and 36-in. cells are sometimes fitted with rotors up to 15 in. in diameter to meet special requirements.

The capacity of the standard machine varies considerably according to the grade and character of the ore. The average capacity of a rougher or rougher-cleaner cell is from 8 to 12 tons of dry feed per foot of rotor length per 24 hours. When cleaning is practised, the tonnage per foot of total rotor length (roughers, scavengers, and cleaners) may vary from 4 tons for a slow-floating ore needing double cleaning to 10 tons for an easily-floated ore with single cleaning, the average being about 6 tons per foot of total rotor length. The cleaning section usually amounts to between one-quarter and one-half of the combined length of the roughing and scavenging cells. The width of cell employed depends on the character of the ore, the time of treatment, and the tonnage.

The quantity of air necessary varies from 5 to 7 cu. ft. per minute per square foot of aerating surface at 2- to 2-lb. pressurethat is, from 12 to 16.5 cu. ft. per minute per linear foot of rotor. With a Roots type blower the power consumption in respect of the air supply is about 12 h.p. per 1,000 cu. ft. of free air per minute at a pressure of 2 lb. per square inch. The power needed to turn the rotor averages 0.5 h.p.

metso microcel flotation columns - metso outotec

metso microcel flotation columns - metso outotec

Metso Microcel sparger consists of in-line static mixers and a centrifugal pump. Tailings slurry is pumped from the base of the columnthrough the static mixers, where air and slurry are mixed under high-shear conditions to create the bubble dispersion.

Fine uniform bubble dispersions, with bubble size typically ranging from 400 to 1200 microns, are generated by the sparger system. The bubble suspension is introduced near the column base, and bubbles rise throgh the column collection zone.

jjf type flotation machine

jjf type flotation machine

Improvement: Shallow groove, the stator lower than the impeller, large slurry circulation volume, low energy consumption; the stator is a cylinder with an elliptical hole which is conducive to the dispersion and mixing of pulp and air. Umbrella shaped dispersion cover with hole keeps the pulp surface stable.

JJF flotation machine(floatation cell) is a new type of flotation equipment advanced in China. It can be widely used in the selection of non-ferrous metals, ferrous metals and non-metallic minerals. It is suitable for rough selection and sweeping of large and medium-sized flotation plants.

Large clearance between impeller and stator, the stator is a cylinder with elliptic hole, and it is good for mixing and dispersing the gas and pulp. The height of stator is lower than the impeller, pulp circulation volume is large, and it can be reached at 2.5 times of others.

When the impeller rotates, eddy current is generated in the vertical cylinder and the draft tube. The eddy current forms a negative pressure, and the air is sucked from the intake pipe and sucked in the impeller and stator regions and through the draft tube. Mix the pulp. The slurry gas mixing flow is moved by the impeller in a tangential direction, and then converted into a radial motion by the action of the stator, and uniformly distributed in the flotation tank. The mineralized bubbles rise to the foam layer, and the unilateral or bilateral scraping is the foam product.

flotation cell - an overview | sciencedirect topics

flotation cell - an overview | sciencedirect topics

The MAC flotation cell was developed by Kadant-Lamort Inc. It can save energy comparedto conventional flotation systems. The MAC flotation cell is mainly used in the flotation section of waste paper deinking pulping, for removal of hydrophobic impurities such as filler, ash,ink particles, etc. It can increase pulp whiteness and meet the requirements of final paper appearance quality. Table11.11 shows the features of MAC flotation cell. Kadants MAC flotation cell deinking system uses air bubbles to float ink particles to the cell surface for removal from the recycled material. The latest generation of the MAC cell deinking system incorporates a patented bubble-washing process to reduce power consumption and also fiber loss. It combines small, new, auto-clean, low-pressure injectors with a flotation cell. The function of injectors is to aerate the stock before it is pumped and sent tangentially to the top of the cell. The air bubbles collect ink particles in the cell and rise up to the top to create a thick foam mat that is evacuated because of the slight pressurization of the cell. The partially deinked stock then goes to a deaeration chamber and is pumped to the next stage. Here, the operation is exactly the same as for the first stage. This stage also has the same number of injectors and same flow (Kadant,2011). This operation is repeated up to five times for a high ink removal rate. Remixing of the air coming from downstream stages of the process helps the upstream stages and improves the overall cell efficiency. Adjustable and selective losses of fiberdepend on the application and technical requirements inks, or inks and fillers. The use of low-pressure injectors in the MAC flotation cell could save about 2530% of the energy used in conventional flotation systems (ECOTARGET,2009). The benefits of the MAC flotation cell are summarized in Table11.12.

Agitated flotation cells are widely used in the mineral processing industry for separating, recovering, and concentrating valuable particulate material from undesired gangue. Their performance is lowered, however, when part of the particulate system consists of fines, with particle diameters typically in the range from 30 to 100m. For example, it was observed difficult to float fine particles because of the reduction of middle particles (of wolframite) as carriers and the poor collision and attachment between fine particles and air bubbles; a new kinetic model was proposed [34].

As an alternative to agitated cells, bubble columnsused in chemical engineering practice as chemical reactorswere proposed for the treatment of fine particle systems. Flotation columns, as they came to be known, were invented back in the 1960s in Canada [35]. The main feature that differentiates the column from the mechanical flotation cell (of Denver type) is wash water, added at the top of the froth. It was thought to be beneficial to overall column performance since it helps clean the froth from any entrained gangue, while at the same time preventing water from the pulp flowing into the concentrate. In this way, it was hoped that certain cleaning flotation stages could be gained.

Let us note that the perhaps insistence here on mineral processing is only due to the fact that most of the available literature on flotation is from this area, where the process was originated and being widely practiced. The effect of particle size on flotation recovery is significant; it was shown that there exists a certain size range in which optimum results may be obtained in mineral processing. This range varies with the mineral properties such as density, liberation, and so on, but was said to be of the order of 10100m [36].

Regulating the oxidation state of pyrite (FeS2) and arsenopyrite (FeAsS), by the addition of an oxidation or reduction chemical agent and due to the application of a short-chain xanthate as collector (such as potassium ethyl xanthate, KEX), was the key to selective separation of the two sulfide minerals, pyrite and arsenopyrite [37]. Strong oxidizing agents can depress previously floated arsenopyrite. Various reagents were examined separately as modifiers and among them were sodium metabisulfite, hydrazinium sulfate, and magnesia mixture. The laboratory experiments were carried out in a modified Hallimond tube, assisted by zeta-potential measurements and, in certain cases, by contact angle measurements.

This conventional bench-scale flotation cell provides a fast, convenient, and low-cost method, based on small samples (around 2g), usually of pure minerals and also artificial mixtures, for determining the general conditions under which minerals may be rendered floatableoften in the absence of a frother (to collect the concentrate in the side tube) [38]. This idea was later further modified in the lab replacing the diaphragm, in order to conduct dissolved air or electroflotation testssee Section 3.

Pyrite concentrates sometimes contain considerable amounts of arsenic. Since they are usually used for the production of sulfuric acid, this is undesirable from the environmental point of view. However, gold is often associated with arsenopyrite, often exhibiting a direct relationship between Au content and As grade. There is, therefore, some scope for concentrating arsenopyrite since the ore itself is otherwise of little value (see Fig.2.2). Note that previous work on pyrites usually concentrated on the problem of floating pyrite [40].

In the aforementioned figure (shown as example), the following conditions were applied: (1) collector [2-coco 2-methyl ammonium chloride] 42mg/L, frother (EtOH) 0.15% (v/v), superficial liquid velocity uL=1.02cm/s, superficial gas velocity uG=0.65cm/s, superficial wash water velocity uw=0.53cm/s; (2) hexadecylamine, 45mg/L; pine oil, 50mg/L; EtOH, 0.025%; uL=0.84cm/s; uG=0.72cm/s; uw=0.66cm/s; (3) Armoflot 43, 50mg/L; pine oil, 50mg/L; EtOH, 0.025%; uL=0.84cm/s; uG=0.71cm/s; uw=0.66cm/s [39]. The pyrite (with a relatively important Au content of 21g/ton) was a xanthate-floated concentrate. The presence of xanthates, however, might cause problems in the subsequent cyanidation of pyrites when recovering their Au value, which perhaps justified the need to find alternative collectors. In general, the amines exhibited a behavior similar to that of the xanthates (O-alkyl dithiocarbonates). The benefit of the amine was in its lower consumption, as compared with the xanthate systems.

The arsenic content of the pyrite was approximately 9% (from an initial 3.5% of the mixed sulfide ore). The material was sieved and the75m fraction was used for the laboratory-scale cylindrical column experiments. The effect on metallurgical characteristics of the flotation concentrate of varying the amount of ferric sulfate added to the pulp was studied; three collectors were used and their performance was compared (in Fig.2.2). Both hexadecylamine and Armoflot 43 (manufactured by Akzo) exhibited an increased recovery but a very low enrichment, whereas 2-coco 2-methyl ammonium chloride (Arquad-2C) showed a considerable enrichment; a compromise had to be made, therefore, between a high-grade and a low recovery.

Electroflotation (electrolytic flotation) is an unconventional separation process owing its name to the bubbles generation method it uses, i.e., electrolysis of the aqueous medium. In the bottom of the microcell, the two horizontal electrodes were made from stainless steel, the upper one being perforated. The current density applied was 300 Am2. It was observed that with lime used to control pH, different behavior was observed (see Fig.2.3). Pyrite, with permanganate (a known depressant) also as modifier, remained activated from pH 5.0 to 8.0at 80% recovery, while it was depressed at the pH range from 9.0 to 12.0. A conditioning of 30min was applied in the presence of modifier alone and further 15min after the addition of xanthate. The pure mineral sample, previously hand collected, crushed, and pulverized in the laboratory, was separated by wet sieving to the45 to+25m particle size range.

Pyrite due to its very heterogeneous surface, consisting of a mosaic of anodic and cathodic areas, presents a strong electrocatalytic activity in the anodic oxidation of xanthate to dixanthogen. It is also possible that the presence of the electric field, during electroflotation, affected the reactions taking place. In order to explain this difference in flotation behavior thermodynamic calculations for the system Fe-EX-H2O have been done [41]. It was concluded that electroflotation was capable of removing fine pyrite particles from a dilute dispersion, under controlled conditions. Nevertheless, dispersed air and electroflotation presented apparent differences for the same application.

The size of the gas bubbles produced was of the order of 50m, in diameter [21]. Similar measurements were later carried out at Newcastle, Australia [42]; where it was also noted that a feature of electroflotation is the ability to create very fine bubbles, which are known to improve flotation performance of fine particles.

In fact, the two electrodes of a horizontal electrodes set, usually applied in electroflotation, could be separated by a cation exchange membrane, as only one of the produced gases is often necessary [43]. In the lower part/separated electrode, an electrolyte was circulated to remove the created gas, and in the meantime, increase the conductivity; hence having power savings (as the electric field is built up between the electrodes through the use of the suspension conductivity). Attention should be paid in this case to anode corrosion, mainly by the chloride ion (i.e., seawater).

Microorganisms have a tremendous influence on their environment through the transfer of energy, charge, and materials across a complex biotic mineralsolution interface; the biomodification of mineral surfaces involves the complex action of microorganism on the mineral surface [44]. Mixed cationic/anionic surfactants are also generating increasing attention as effective collectors during the flotation of valuable minerals (i.e., muscovite, feldspar, and spodumene ores); the depression mechanisms on gangue minerals, such as quartz, were focused [45].

Another design of a flotation cell which applies ultrasound during the flotation process has been developed by Vargas-Hernndez et al. (2002). The design consists of a Denver cell (Koh and Schwarz, 2006) equipped with ultrasonic capabilities of performing ultrasound-assisted flotation experiments. This cell is universally accepted as a standard cell for laboratory flotation experiments. In Figure 35.25, a schematic of the Denver cell equipped with two power transducers is shown operating at 20kHz. The ultrasonic transducers are in acoustic contact with the body of the flotation cell but are not immersed in the same cell. Instead, they are submerged in distilled water and in a thin membrane that separates the radiant head of the transducer from the chamber body. The floatation chamber has a capacity of 2.7l and is also equipped with conventional systems to introduce air and mechanical agitation able to maintain the suspension of metallurgical pulp. In the upper part of the cell there is an area in which the foam is recovered for analysis by a process called skimming. The block diagram of Figure 35.25 further shows that the experimental system was developed to do ultrasonic-assisted flotation experiments. The transducers operate at 20kHz and can handle power up to 400W. In the Denver cell an acoustic probe, calibrated through a nonlinear system and capable of measuring high-intensity acoustic fields, is placed (Gaete-Garretn et al., 1993, 1998). This is done in order to determine the different acoustic field intensities with a spatial scanner during the experimentation. Figure 35.26 shows the distribution of ultrasonic field intensity obtained by a spatial scanner in the central area of the flotation chamber. The Denver cell with ultrasonic capabilities, as described, is shown in Figure 35.27. The obtained results were fairly positive. For example, for fine particle recovery it worked with metallurgical pulp under 325mesh, indicating floating particles of less than 45m, and the recovery curves are almost identical to those of an appropriate size mineral for flotation. This is shown in Figure 35.28, where a comparison between typical copper recovery curves for fine and normal particles is presented. The most interesting part of the flotation curves is the increase in recovery of molybdenum with ultrasonic power, as shown in Figure 35.29. The increase in recovery of iron is not good news for copper mines because the more iron floating the lower grade of recovery. This may be because the iron becomes more hydrophobic with ultrasonic action. According to the experts, this situation could be remedied by looking for specific additives to avoid this effect. Flotation kinetics shown in Figure 35.30 with 5 and 10W of acoustic power applied also show an excellent performance. It should be noted that the acoustic powers used to vary the flotation kinetics have been quite low and could clearly be expanded.

Figure 35.28. Compared recovering percent versus applied power in an ultrasonic-assisted flotation process in a Denver cell: (a) fine and ultrafine particles recovering and (b) normal particles recovering.

These experiments confirm the potential of power ultrasound in flotation. Research on assisted flotation with power ultrasound has been also carried out by Ozkan (2002), who has conducted experiments by pretreating pulp with ultrasound during flotation. Ozkhans objective was to recover magnesite from magnesite silts with particles smaller than 38m. Their results show that under ultrasonic fields the flotation foam bubbles are smaller, improving magnesite recovery rates. When Ozkhan treated magnesite mineral with a conventional treatment the beneficial effect of ultrasound was only manifested for mineral pretreatment. The flotation performed under ultrasonic field did not show improvement. This was because power ultrasound improves the buoyancy of clay iron and this has the effect of lowering the recovery of magnesite.

Kyllnen et al. (2004) employed a cell similar to Jordan to float heavy metals from contaminated soils in a continuous process. In their experiments they obtained a high recovery of heavy metals, improving the soil treatment process. Alp et al. (2004) have employed ultrasonic waves in the flotation of tincal minerals (borax Na O710 B4 H2O), finding the same effects as described above, i.e., that power ultrasound helps in the depression of clay. However, the beneficial effect of ultrasound is weakened when working with pulps with high mineral concentration (high density), probably due to an increase in the attenuation of the ultrasonic field. Safak and Halit (2006) investigated the action mechanisms of ultrasound under different flotation conditions. A cleaning effect on the floating particles was attributed to the ultrasonic energy, making the particles more reactive to the additives put in the metallurgical pulp. Furthermore due to the fact that the solid liquid interface is weaker than the cohesive forces of the metallurgic pulp liquids, it results in a medium favorable to creation of cavitation bubbles. The unstable conditions of a cavitation environment can produce changes in the collectors and even form emulsions when entering the surfactant additives. In general, many good properties are attributed to the application of ultrasound in flotation. For example, there is a more uniform distribution of the additives (reagents) and an increase in their activity. In fact in the case of carbon flotation it has been found that the floating times are shortened by the action of ultrasound, the bubble sizes are more stable, and the consumption of the reagents is drastically lowered.

Abrego Lpez (2006) studied a water recovery process of sludge from industrial plants. For this purpose he employed a flotation cell assisted by power ultrasound. In the first stage he made a flotation to recover heavy metals in the metallurgical pulp, obtaining a high level of recovery. In the second stage he added eucalyptus wood cones to the metallurgical pulp to act as an accumulator of copper, lead, nickel, iron, and aluminum. The author patented the method, claiming that it obtained an excellent recovery of all elements needing to be extracted. zkan and Kuyumcu (2007) showed some design principles for experimental flotation cells, proposing to equip a Denver flotation cell with four power transducers. Tests performed with this equipment consisted of evaluating the possible effects that high-intensity ultrasonic fields generated in the cell may have on the flotation. The author provides three-dimensional curves of ultrasonic cavitation fields in a Denver cell filled with water at frequencies between 25 and 40kHz. A warming effect was found, as expected. However, he states that this effect does not disturb the carbon recovery processes because carbon flotation rarely exceeds 5min. They also found that the pH of tap water increases with the power and time of application of ultrasound, while the pH of the carbonwaterreagentsludge mixture decreases. The conductivity of the metallurgical pulp grows with the power and time of application of ultrasound, but this does not affect flotation. The carbon quality obtained does not fall due to the application of ultrasound and the consumption of lowered reagents. They did not find an influence from the ultrasound frequency used in the process, between 25 and 40kHz. They affirmed that ultrasound is beneficial at all stages of concentration.

Kang et al. (2009) studied the effects of preconditioning of carbon mineral pulp in nature by ultrasound with a lot of sulfur content. They found that the nascent oxygen caused by cavitation produces pyrite over oxidation, lowering its hydrophobicity, with the same effect on the change of pH induced by ultrasonic treatment. Additionally, ultrasound decreases the liquid gas interfacial tension by increasing the number of bubbles. Similar effects occur in carbon particles. The perfect flotation index increases 25% with ultrasonic treatment. Kang et al. (2008) continued their efforts to understand the mechanism that causes effects in ultrasonic flotation, analyzing the floating particles under an ultrasonic field by different techniques like X-ray diffraction, electron microscopy, and scanning electron microscopy techniques. In carbon flotation it is estimated that ultrasonic preconditioning may contribute to desulfurization and ash removal (deashing) in carbon minerals. Zhou et al. (2009) have investigated the role of cavitation bubbles created by hydrodynamic cavitation in a flotation process, finding similar results to those reported for ultrasonic cavitation flotation. Finally, Ozkan (2012) has conducted flotation experiments with the presence of hard carbon sludge cavitation (slimes), encountering many of the effects that have been reported for the case of metallurgical pulp with ultrasound pretreatment. This includes improved flotation, drastic reduction in reagent consumption, and the possible prevention of oxidation of the surface of carbon sludge. A decrease in the ash content in floating carbon was not detected. However, tailings do not seem to contain carbon particles. All these effects can be attributed to acoustic cavitation. However, according to the author, there is a need to examine the contribution of ultrasound to the probability of particlebubble collision and the likelihood of getting the bubbles to connect to the particles. The latter effects have been proposed as causes for improvements in flotation processes in many of the publications reviewed, but there is no systematic study of this aspect.

In summary, power ultrasound assistance with flotation processes shows promising results in all versions of this technique, including conditioning metallurgical pulp before floating it, assisting the continuous flotation process, and improving the yields in conventional flotation cells. The results of ultrasonic floating invariably show a better selectivity and an increase, sometimes considerable, in the recovery of fine particles. Paradoxically, in many experiments an increase has been recorded in recovering particles suitable for normal flotation. These facts show the need for further research in the flotation process in almost all cases, with the exception perhaps of carbon flotation. For this last case, in light of the existing data the research should be directed toward scale-up of the technology.

The concentrate obtained from a batch flotation cell changes in character with time as the particles floating change in size, grade and quantity. In the same way, the concentrate from the last few cells in a continuous bank is different from that removed from the earlier cells. Particles of the same mineral float at different rates due to different particle characteristics and cell conditions.

The recovery of any particular mineral rises to an asymptotic value R which is generally less than 100%. The rate of recovery at time t is given by the slope of the tangent to the curve at t, and the rate of recovery at time t1 is clearly greater than the rate at time t2. There is a direct relationship between the rate of flotation and the amount of floatable material remaining in the cell, that is:

The process is carried out in a flotation cell or tank, of which there are two basic types, mechanical and pneumatic. Within each of these categories, there are two subtypes, those that operate as a single cell, and those that are operated as a series or bank of cells. A bank of cells (Fig. 8) is preferred because this makes the overall residence times more uniform (i.e., more like plug flow), rather than the highly diverse residence times that occur in a single (perfectly mixed) tank.

FIGURE 8. Flotation section of a 80,000t/d concentrating plant, showing the arrangement of the flotation cells into banks. A small part of the grinding section can be seen through the gap in the wall. [Courtesy Joy Manufacturing Co.]

The purpose of the flotation cell is to attach hydrophobic particles to air bubbles, so that they can float to the surface, form a froth, and can be removed. To do this, a flotation machine must maintain the particles in suspension, generate and disperse air bubbles, promote bubbleparticle collision, minimize bypass and dead spaces, minimize mechanical passage of particles to the froth, and have sufficient froth depth to allow nonhydrophobic (hydrophilic) particles to return to the suspension.

Pneumatic cells have no mechanical components in the cell. Agitation is generally by the inflow of air and/or slurry, and air bubbles are usually introduced by an injector. Until comparatively recently, their use was very restricted. However, the development of column flotation has seen a resurgence of this type of cell in a wider, but still restricted, range of applications. While the total volume of cell is still of the same order as that of a conventional mechanical cell, the floor space and energy requirements are substantially reduced. But the main advantage is that the cell provides superior countercurrent flow to that obtained in a traditional circuit (see Fig. 11), and so they are now often used as cleaning units.

Mechanical cells usually consist of long troughs with a series of mechanisms. Although the design details of the mechanisms vary from manufacturer to manufacturer, all consist of an impeller that rotates within baffles. Air is drawn or pumped down a central shaft and is dispersed by the impeller. Cells also vary in profile, degree of baffling, the extent of walling between mechanisms, and the discharge of froth from the top of the cell.

Selection of equipment is based on performance (represented by grade and recovery), capacity (metric tons per hour per cubic meter); costs (including capital, power, maintenance), and subjective factors.

Among all processing industries, only in the ore and mining industries is the accent more on wear resistance than corrosion. In mining industries, the process concerns material handling more than any physical or chemical conversions that take place during the refining operations. For example, in the excavation process of iron ore, conventional conveyer systems and sophisticated fluidized systems are both used [16,17]. In all these industries, cost and safety are the governing factors. In a fluidized system, the particles are transported as slurry using screw pumps through large pipes. These pipes and connected fittings are subjected to constant wear by the slurry containing hard minerals. Sometimes, depending on the accessibility of the mineral source, elaborate piping systems will be laid. As a high-output industry any disruption in the work will result in heavy budgetary deficiency. Antiabrasive rubber linings greatly enhance the life of equipment and reduce the maintenance cost. The scope for antiabrasive rubber lining is tremendous and the demand is ever increasing in these industries.

Different rubber compounds are used in the manufacture of flotation cell rubber components for various corrosion and abrasion duty conditions. Flotation as applied to mineral processing is a process of concentration of finely divided ores in which the valuable and worthless minerals are completely separated from each other. Concentration takes place from the adhesion of some species of solids to air bubbles and wetting of the other series of solids by water. The solids adhering to air bubbles float on the surface of the pulp because of a decrease in effective density caused by such adhesion, whereas those solids that are wetted by water in the pulp remain separated in the pulp. This method is probably the more widely used separation technique in the processing of ores. It is extensively used in the copper, zinc, nickel, cobalt, and molybdenum sections of the mineral treatment industry and is used to a lesser extent in gold and iron production. The various rubber compounds used in the lining of flotation cells and in the manufacture of their components for corrosive and abrasive duties are:

Operating above the maximum capacity can cause the performance of flotation cells to be poor even when adequate slurry residence time is available (Lynch et al., 1981). For example, Fig. 11.21 shows the impact of increasing volumetric feed flow rate on cell performance (Luttrell et al., 1999). The test data obtained at 2% solids correlates well with the theoretical performance curve predicted using a mixed reactor model (Levenspiel, 1972). Under this loading, coal recovery steadily decreased as feed rate increased due to a reduction in residence time. However, as the solids content was increased to 10% solids, the recovery dropped sharply and deviated substantially from the theoretical curve due to froth overloading. This problem can be particularly severe in coal flotation due to the high concentration of fast floating solids in the flotation feed and the presence of large particles in the flotation froth. Flotation columns are particularly sensitive to froth loading due to the small specific surface area (ratio of cross-sectional area to volume) for these units.

Theoretical studies indicate that loading capacity (i.e., carrying capacity) of the froth, which is normally reported in terms of the rate of dry solids floated per unit cross-sectional area, is strongly dependent on the size of particles in the froth (Sastri, 1996). Studies and extensive test work conducted by Eriez personnel also support this finding. As seen in Fig. 11.22, a direct correlation exists between capacity and both the mean size (d50) and ultrafines content of the flotation feedstock. The true loading capacity may be estimated from laboratory and pilot-scale flotation tests by conducting experiments as a function of feed solids content (Finch and Dobby, 1990). Field surveys indicate that conventional flotation machines can be operated with loading capacities of up to 1.52.0t/h/m2 for finer (0.150mm) feeds and 56t/h/m2 or more for coarser (0.600mm) feeds. Most of the full-scale columns in the coal industry operate at froth loading capacities less than 1.5t/h/m2 for material finer than 0.150mm and as high as 3.0t/h/m2 for flotation feed having a top size of 0.300mm feeds.

Froth handling is a major problem in coal flotation. Concentrates containing large amounts of ultrafine (<0.045mm) coal generally become excessively stable, creating serious problems related to backup in launders and downstream handling. Bethell and Luttrell (2005) demonstrated that coarser deslime froths readily collapsed, but finer froths had the tendency to remain stable for an indefinite period of time. Attempts made to overcome this problem by selecting weaker frothers or reducing frother dosage have not been successful and have generally led to lower circuit recoveries. Therefore, several circuit modifications have been adopted by the coal industry to deal with the froth stability problem. For example, froth launders need to be considerably oversized with steep slopes to reduce backup. Adequate vertical head must also be provided between the launder and downstream dewatering operations. In addition, piping and chute work must be designed such that the air can escape as the froth travels from the flotation circuit to the next unit operation.

Figure 11.23 shows how small changes in piping arrangements can result in better process performance. Shown in Fig. 11.23 is a column whose performance suffered due to the inability to move the froth product from the column launder although a large discharge nozzle (11m) had been provided. In this example, the froth built up in the launder and overflowed when the operators increased air rates. To prevent this problem, the air rates were lowered, which resulted in less than optimum coal recovery. It was determined that the downstream discharge piping was air-locking and preventing the launders from properly draining. The piping was replaced with larger chute work that allowed the froth to flow freely and the air to escape. As a result, higher aeration rates were possible and recoveries were significantly improved.

Some installations have resorted to using defoaming agents or high-pressure launder sprays to deal with froth stability. However, newer column installations eliminate this problem by including large de-aeration tanks to allow time for the froth to collapse (Fig. 11.24a). Special provisions may also be required to ensure that downstream dewatering units can accept the large froth volumes. For example, standard screen-bowl centrifuges equipped with 100mm inlets may need to be retrofitted with 200mm or larger inlets to minimize flow restrictions. In addition, while the use of screen-bowl centrifuges provides low product moistures, there are typically fine coal losses, as a large portion of the float product finer than 0.045mm is lost as main effluent. This material is highly hydrophobic and will typically accumulate on top of the thickener as a very stable froth layer, which increases the probability that the process water quality will become contaminated (i.e., black water).

This phenomenon is more prevalent in by-zero circuits, especially when the screen-bowl screen effluent is recycled back through the flotation circuit, either directly or through convoluted plant circuitry. Reintroducing material that has already been floated to the flotation circuit can result in a circulating load of very fine and highly floatable material. As a result, the capacity of the flotation equipment can be significantly reduced, which results in losses of valuable coal. Most installations will combat this by ensuring that the screen-bowl screen effluent is routed directly back to the screen bowl so that it does not return to the flotation circuit. The accumulation of froth on the thickener, which tends to be especially problematic in by-zero circuitry, is also reduced by utilizing reverse-weirs and taller center wells, as this approach helps to limit the amount of froth that can enter into the process water supply. Froth that does form on top of the clarifier can be eliminated by employing a floating boom that is placed directly in the thickener (Fig. 11.24b) and used in conjunction with water sprays. The floating boom can be constructed out of inexpensive PVC piping, and is typically attached to the rotating rakes. The boom floats on the water interface and drags any froth around to the walkway that extends over the thickener, where it is eliminated by the sprays.

Column cells have been developed over the past 30 years as an alternative to mechanically agitated flotation cells. The major operating difference between column and mechanical cells is the lack of agitation in column cells that reduces energy and maintenance costs. Also, it has been reported that the cost of installing a column flotation circuit is approximately 2540% less than an equivalent mechanical flotation circuit (Murdock et al., 1991). Improved metallurgical performance of column cells in iron ore flotation is reported and attributed to froth washing, which reduces the loss of fine iron minerals entrained into the froth phase (Dobby, 2002).

The Brazilian iron ore industry has embraced the use of column flotation cells for reducing the silica content of iron concentrates. Several companies, including Samarco Minerao S.A., Companhia Vale do Rio Doce (CRVD), Companhia Siderrgica Nacional (CSN), and Mineraes Brasileiras (MBR), are using column cells at present (Peres et al., 2007). Samarco Minerao, the first Brazilian producer to use column cells, installed column cells as part of a plant expansion program in the early 1990s (Viana et al., 1991). Pilot plant tests showed that utilization of a column recleaner circuit led to a 4% increase in iron recovery in the direct reduction concentrate and an increase in primary mill capacity when compared to a conventional mechanical circuit.

There are also some negative reports of the use of column cells in the literature. According to Dobby (2002), there were several failures in the application of column cells in the iron ore industry primarily due to issues related to scale-up. At CVRD's Samitri concentrator, after three column flotation stages, namely, rougher, cleaner, and recleaner, a secondary circuit of mechanical cells was still required to produce the final concentrate.

Imhof et al. (2005) detailed the use of pneumatic flotation cells to treat a magnetic separation stream of a magnetite ore by reverse flotation to reduce the silica content of the concentrate to below 1.5%. From laboratory testing, they claimed that the pneumatic cells performed better than either conventional mechanical cells or column cells. The pneumatic cells have successfully been implemented at the Compaia Minera Huasco's iron ore pellet plant.

This chapter presents a novel approach to establish the relationship between collector properties and the flotation behavior of goal in various flotation cells. Coal flotation selectivity can be improved if collector selection is primarily based on information obtained from prior contact angle and zeta potential measurements. In a study described in the chapter, this approach was applied to develop specific collectors for particular coals. A good correlation was obtained between laboratory batches and large-scale conventional flotation cells. This is not the case when these results are correlated with pneumatic cell trial data. The study described in the chapter was aimed at identifying reasons for the noncorrelation. Two collectors having different chemical compositions were selected for this investigation. A considerable reduction in coal recovery occurred at lower rotor speeds when comparing results of oxidized and virgin coal. The degree to which a collector enhances flocculation in both medium- and low-shear applications and also the stronger bubble-coal particle adherence required for high-shear cells must, therefore, all be taken into consideration when formulating a collector for coal flotation.

flotation machine - an overview | sciencedirect topics

flotation machine - an overview | sciencedirect topics

Industrial flotation machines can be divided into four classes: (1) mechanical, (2) pneumatic, (3) froth separation, and (4) column. The mechanical machine is clearly the most common type of flotation machine in industrial use today, followed by the rapid growth of the column machine. Mechanical machines consist of a mechanically driven impeller, which disperses air into the agitated pulp. In normal practice, this machine appears as a vessel having a number of impellers in series. Mechanical machines can have open flow of pulp between each impeller or are of cell-to-cell designs which have weirs between each impeller. The procedure by which air is introduced into a mechanical machine falls into two broad categories: self-aerating, where the machine uses the depression created by the impeller to induce air, and supercharged, where air is generated from an external blower. The incoming slurry feed to the mechanical flotation machine is introduced usually in the lower portion of the machine.Figure 7 shows a typical industrial flotation cell of each air delivery type.

The most rapidly growing class of flotation machine is the column machine, which is, as its name implies, a vessel having a large height-to-diameter ratio (from 5 to 20) in contrast to mechanical cells. The mechanism behind this machine to is provide a countercurrent flow of air bubbles and slurry with a long contact time and plenty of wash water. As might be expected, the major advantage of such a machine is the high separation grade that can be achieved, so that column cells are often used as a final concentrate cleaning step. Special care has to be exercised in the generation of fine air bubbles and controlling the feed rate to column cells.

Good mixing of pulp. To be effective, a flotation machine should maintain all particles uniformly in suspension within the pulp, including those of relatively high density and/or size. Good mixing of pulp is required for maximizing bubble-particle collision frequency.

Appropriate aeration and dispersion of fine air bubbles. An important requirement of any flotation machine is the ability to provide uniform aeration throughout as large a volume of the machine as is possible. In addition, the size distribution of the air bubbles generated by the machine is also important, but experience has shown that the proper choice of frother type and dosage generally dominates the bubble size distributions being produced.

Sufficient control of pulp agitation in the froth zone. As mentioned earlier, good mixing in the machine is important; however, equally important is that near and in the actual froth bed at the top of the machine, sufficiently smooth or quiescent pulp conditions must be maintained to ensure suspension of hydrophobed (collector coated) particles.

Efficient mass flow-mechanisms. It is also necessary in any flotation machine that appropriate provisions be made for feeding pulp into the machine and also for the efficient transport of froth concentrate and tailing slurry out of the machine.

Probably the most significant area of change in mechanical flotation machine design has been the dramatic increase in machine size. This is typified by the data ofFig. 8, which shows the increase in machine (cell) volume size that has occurred with a commonly used cell manufactured by Wemco. The idea behind this approach is that as machine size increases, both plant capital and operating costs per unit of throughput decrease.

The throughput capabilities of various cell designs will vary with flotation residence time and pulp density. The number of cells required for a given operation is determined from standard engineering mass balance calculations. In the design of a new plant, the characterization of each cell's volume and flotation efficiency is generally calculated from performing a laboratory-scale flotation on the same type of equipment on the ore in question, followed by the application of empirically derived design (scale-up) factors. Research work is currently under way to improve the understanding and performance of commercial flotation cells. Currently, flotation-cell design is primarily a proprietary function of the various cell manufacturers.

Flotation plants are built in multiple cell configurations (called banks), and the flow through various banks is adjusted in order to optimize plant recovery of the valuable as well as the valuable grade of the total recovered mass from flotation. This recovery vs grade trade-off is economically important in flotation, as increased recovery of the valuable is associated with decreased grade. For example, a 95% recovery of copper in the feed ore might give a concentrate grade of 18% Cu in the total recovered mass, while 80% Cu recovery might give a grade of 25% in the concentrate. Obviously, the higher the valuable recovery is, the higher the potential income, but if this higher recovery requires a great deal more grinding and/or expensive downstream processing (including further flotation) in order to upgrade the concentrate for metal refining such as smelting, the increase in potential recovery income may actually cause a net loss of total income. This grade-recovery optimization is generally worked out by individual flotation operators in each plant (and each mineral) and sets the operating philosophy of that plant.Figure 9 shows a typical industrial recovery vs grade trade-off curve for a copper sulfide ore containing pyrite. The higher the copper recovery is, the greater the amount of undesired pyrite contained in the concentrate.

The various banks of flotation cells in an industrial plant are given special names to denote the particular purpose of the banks. The rougher bank is the first group of cells that the pulp sees after size reduction. The goal of the roughers is to produce a concentrate with as high a recovery of valuable as possible with generally low grade of the valuable. The rejected gangue material from any bank of cells is commonly denoted as the tails or tailings. Usually, rougher tails are discarded so that valuable mineral not recovered in the rougher bank is lost. The concentrate of the rougher bank can be further concentrated, sometimes after additional grinding, in banks of cells called cleaners or recleaners. The tailings from the cleaners or recleaners can be recirculated to a bank of cells known as scavengers in order not to lose any valuable material in the upgrading process. Various banks of cells are also sometimes known by the particle size of the particular pulps being floated. Coarse particles, fine or slime particles, and middle-sized particles, denoted as middlings, can all be treated in separate banks.

As to overall capacities of flotation plants, the range is quite variable, depending on the type and value of the mineral being processed, the amount of valuable mineral in the feed ore to flotation, the degree and cost of size reduction involved, and the relative response of the valuable(s) to the flotation process. Smaller plants ranging in size from 500 to 5000 metric tons of feed per day are common, with feed materials having high amounts of valuable per ton of feed ore (>40%), such as coal, phosphate, and oxide ores. On the other hand, the sulfide minerals that are typically a small percentage of the ore (<10% and often less than 1%) require much greater capacity in order to achieve a reasonable economic return on investment. Thus, typical copper sulfide plants have capacities in the range of 20,000 to more than 60,000 metric tons of feed ore per day.

Conventional flotation machines house two functions in a single vessel: an intense mixing region where bubbleparticle collision and attachment occurs, and a quiescent region where the bubbleparticle aggregates separate from the slurry. The reactor/separator machines decouple these functions into two separate (or sometimes more) compartments. The cells are typically considered high-intensity machines due to the turbulent mixing in the reactor (see Section 12.9.5). The role of the separator is to allow sufficient time for mineralized bubbles to separate from the tailing stream which generally requires relatively short residence time (when compared to mechanical cells or columns).

Some of the earliest machine designs were of the reactor/separator-type. Figure 12.80 shows a design from a patent by Hebbard (1913). Feed slurry was mixed with entrained air in an agitation box (reactor) and flowed into the separation vessel where froth was collected as overflow. The design would be the basis for the Minerals Separation Corporation standard machine and early flotation cells used in the United States (Lynch et al., 2010).

The Davcra cell (Figure 12.81) was developed in the 1960s and is considered to be the first high-intensity machine. The cell could be thought of as a column or reactor/separator device. Air and feed slurry are contacted and injected into the tank through a cyclone-type dispersion nozzle, the energy of the jet of pulp being dissipated against a vertical baffle. Dispersion of air and collection of particles by bubbles occurs in the highly agitated region of the tank, confined by the baffle. The pulp flows over the baffle into a quiescent region designed for bubblepulp disengagement. Although not widely used, Davcra cells replaced some mechanical cleaner machines at Chambishi copper mine in Zambia, with reported lower operating costs, reduced floor area, and improved metallurgical performance.

Several attempts have been made to develop more compact column-type devices, the Jameson cell (Jameson, 1990; Kennedy, 1990; Cowburn et al., 2005) being a successful example (Figure 12.82). The Jameson cell was developed in the 1980s jointly by Mount Isa Mines Ltd and the University of Newcastle, Australia. The cell was first installed for cleaning duties in base metal operations (Clayton et al., 1991; Harbort et al., 1994), but it has also found use in coal plants and in roughing and preconcentrating duties. The original patent refers to the Jameson cell as a column method, but it can also be considered a reactor/separator machine: contact between the feed and the air stream is made using a plunging slurry jet in a vertical downcomer (the reactor), and the airslurry mixture flows downwards to discharge and disengage into a shallow pool of pulp in the bottom of a short cylindrical tank (the separator). The disengaged bubbles rise to the top of the tank to overflow into a concentrate launder, while the tails are discharged from the bottom of the vessel. Air is self-aspirated (entrained) by the action of the plunging jet. The air rate is influenced by jet velocity and slurry density and level in the separator chamber.

The Jameson cell has been widely used in the coal industry in Australia since the 1990s. Figure 12.83 shows a typical cell layout where fine coal slurry feeds a central distributor which splits the stream to the downcomers. Clean coal is seen overflowing as concentrate from the separation vessel. The major advantage of the cell in this application is the ability to produce clean concentrates in one stage of operation by reducing entrainment, especially when wash water is used. It also has a novel application in copper solvent extraction/electrowinning circuits, where it is used to recover entrained organic droplets from electrolyte (Miller and Readett, 1992).

The Contact cell (Figure 12.84) was developed in the 1990s in Canada. The feed slurry is placed in direct contact with pressurized air in an external contactor which comprises a draft tube and an orifice plate. The slurryair mixture is fed from the contactor to the column-type separation vessel, where mineralized bubbles rise to form froth. Contact cells employ froth washing similar to conventional flotation columns and Jameson cells. Contact cells have been implemented in operations in North America, Africa, and Europe.

The IMHOFLOT V-Cell (Figure 12.85(a)) was developed in the 19801990s and evolved from earlier designs developed in Germany in the 19601970s (Imhof et al., 2005; Lynch et al., 2010). Conditioned feed pulp is mixed with air in an external self-aeration unit above the flotation cell. The airslurry mixture descends a downcomer pipe and is introduced to the separation vessel via a distributor box and ring pipe with nozzles that redirect the flow upward in the cell. The separation vessel is fitted with an adjustable froth crowding cone which can be used to control mass pull. The concentrate overflows to an external froth launder, while the tailings stream exits at the base of the separation vessel. The V-Cell has been used to float sulfide and oxide ores with the largest operation being an iron ore application (Imhof et al., 2005).

The IMHOFLOT G-Cell (Figure 12.85(b)) was introduced in 2001 and employs the same external self-aerating unit as the V-Cell. The airslurry mixture which exits the aeration unit is fed to an external distributor box (located above the separation vessel) where pulp is split and fed to the separation vessel tangentially via feed pipes. The cell is unusual as an internal launder located at the center of the vessel collects froth. The centrifugal motion of the slurry enhances froth separation with residence times being ca. 30s.

The Staged Flotation Reactor (SFR) (Figure 12.86) is a recent development in the minerals industry. By sequencing the three processesparticle collection, bubble/slurry disengagement, and froth recoveryand assigning each to a purpose-built chamber, the SFR aims to optimize each of the three processes independently.

The SFR incorporates an agitator in the first (collection) chamber designed to provide high energy intensity (kWm3) and induce multiple particle passes through the high shear impeller zone, hence giving high collection efficiency. Slurry flows by gravity through the reactor stages, that is, there is no need to apply agitation to suspend solids, only for particle collection. As such, impeller speed can be adjusted online in correlation with desired recovery without sanding. The second tank is designed to deaerate the slurry (bubble disengagement) and rapidly recover froth to the launder without dropback. The froth recovery unit is tailored for use of wash water and for high solids flux. Efficient particle collection and high froth recovery translate into fewer, smaller cells, resulting in a smaller footprint and building height, with lower power consumption, and the potential for good selectivity in both roughing and cleaning applications.

Induced air flotation machines have gained a degree of popularity within certain sections of the minerals processing industry because of their ability to produce small bubbles at relatively high energy efficiency. The most common of such machines is the Jameson Cell. A downcomer protrudes out of the bubbly liquid in which is housed a plunging jet. Because this jet is at high velocity the pressure within the downcomer is low due to the Bernoulli equation, and air is induced into the downcomer creating a plume of bubbles within the liquid, which rise to form a foam. There are major problems with operating Jameson Cells because their high demand for surfactant causes downstream residual frother issues. (It is noted, as an aside, that frother strippers are being developed to remove residual frother in flotation circuits, and these are identical to foam fractionation units.) Notwithstanding that Jameson Cell technology has failed to live up to its promise, it has been successfully used as a pilot-scale foam reactor to effect the autothermal thermophilic aerobic digestion (ATAD) of high strength wastewater sludge produced at a chicken processing factory. The advantage that induced gas systems have over alternative pneumatic foam systems is their very high gasliquid surface area per unit volume of foam due to their small bubbles. This feature of the foams would also be an advantage in foam fractionation because it creates high flux of gasliquid surface. However, to the authors knowledge, no attempt has ever been made to use induced gas systems as foam fractionators.

The Denver DR flotation machine, which is an example of a typical froth flotation unit used in the mining industry, is illustrated in Figure 1.47. The pulp is introduced through a feed box and is distributed over the entire width of the first cell. Circulation of the pulp through each cell is such that, as the pulp comes into contact with the impeller, it is subjected to intense agitation and aeration. Low pressure air for this purpose is introduced down the standpipe surrounding the shaft and is thoroughly disseminated throughout the pulp in the form of minute bubbles when it leaves the impeller/diffuser zone, thus assuring maximum contact with the solids, as shown in Figure 1.47. Each unit is suspended in an essentially open trough and generates a ring doughnut circulation pattern, with the liquid being discharged radially from the impeller, through the diffuser, across the base of the tank, and then rising vertically as it returns to the eye of the impeller through the recirculation well. This design gives strong vertical flows in the base zone of the tank in order to suspend coarse solids and, by recirculation through the well, isolates the upper zone which remains relatively quiescent.

Froth baffles are placed between each unit mechanism to prevent migration of froth as the liquid flows along the tank. The liquor level is controlled at the end of each bank section by a combination of weir overflows and dart valves which can be automated. These units operate with a fully flooded impeller, and a low pressure air supply is required to deliver air into the eye of the impeller where it is mixed with the recirculating liquor at the tip of the air bell. Butterfly valves are used to adjust and control the quantity of air delivered into each unit.

Each cell is provided with an individually controlled air valve. Air pressure is between 108 and 124 kN/m2 (7 and 23 kN/m2 gauge) depending on the depth and size of the machine and the pulp density. Typical energy requirements for this machine range from 3.1 kW/m3 of cell volume for a 2.8 m3 unit to 1.2 kW/m3 for a 42 m3 unit.

In the froth flotation cell used for coal washing, illustrated in Figure 1.48, the suspension contains about 10 per cent of solids, together with the necessary reagents. The liquid flows along the cell bank and passes over a weir, and directly enters the unit via a feed pipe and feed hood. Liquor is discharged radially from the impeller, through the diffuser, and is directed along the cell base and recirculated through ports in the feed hood. The zone of maximum turbulence is confined to the base of the tank; a quiescent zone exists in the upper part of the cell. These units induce sufficient air to ensure effective flotation without the need for an external air blower.

Most of the industrial flotation machines used in the coal industry are mechanical, conventional cells. These machines consist of a series of agitated tanks (usually 48 cells) through which fine coal slurry is passed. The agitators are used to ensure that larger particles are kept in suspension and to disperse air that enters down through the rotating shaft assembly (Fig. 11.13). Air is either injected into the cell using a blower or drawn into the cell by the negative pressure created by the rotating impeller. For coal flotation, trough designs that permit open flow between cells along the bank are more common than cell-to-cell designs that are separated by individual weirs.

Some of the major manufacturers of flotation equipment include Wemco (FLSmidth), Metso, Svedala, and Outokumpu. The commercial units are very similar in basic design and function, although some slight variations exist in terms of cell geometry and impeller configuration. Machines with large specific surface areas are generally preferred for coal flotation, due to the fast flotation kinetics of coal and the large froth solids loadings. Flotation machines with individual cell volumes of up to 28m3 are commonly used due to advantages in terms of capital, operating and maintenance costs. Some manufacturers also offer tank machines, which consist of relatively short cylindrical tanks equipped with conventional impellers. The simplified structural design, which allows these machines to be much larger, can offer significant savings in terms of capital and power costs for some installations. Tank cells with volumes as large as 100m3 are already in operation at coal plants in Australia.

Unlike conventional, mechanically agitated flotation machines, which tend to use relatively shallow rectangular tanks, column cells used in the coal industry are usually tall vessels with heights typically ranging from 7 to 16m depending on the application. Unlike conventional flotation machines, columns do not use mechanical agitation and are typically characterized by an external sparging system, which injects air into the bottom of the column cell. The absence of intense agitation promotes higher degrees of selectivity and can aid in the recovery of coarse particles.

In general, feed slurry enters the column at one or more feed points located in the upper third of the column body and descends against a rising swarm of fine bubbles generated by the air sparging system (Fig. 11.14). Hydrophobic particles that collide with, and attach to, the bubbles rise to the top of the column, eventually reaching the interface between the pulp (collection zone) and the froth (cleaning zone). The location of this interface, which can be adjusted by the operator, is held constant by means of an automatic control loop that regulates a valve on the column tailings line. Varying the location of the interface will increase or decrease the height of the froth zone. The froth is transported from the froth zone into the product launder via mass action.

Methods of sparging in columns are numerous and include air lances, porous tubes, eductors, static mixers, and Cavitation-TubesTM. The air rate used in a column is selected according to the feed rate and concentrate-production requirements. This parameter typically has the largest effect on the operating point of the column with respect to the ash/yield curve. The bubbles generated by the air sparging system are sized to provide the maximum amount of bubble surface area given a constant energy input. In other words, the designs of the various sparging devices are engineered to provide the smallest size and largest number of bubbles possible.

For an equivalent volumetric capacity, the cross-sectional surface area of a column cell is much smaller than that of a conventional cell. This reduced area is beneficial for promoting froth stability and allowing deep froth beds to be formed. This is an important aspect of column flotation, as a deep froth bed facilitates froth washing for the removal of unwanted impurities from the float product. Wash water, added at the top of the column, percolates through the froth zone displacing dirty process water and non-selectively entrained particles trapped between the bubbles. In addition, froth wash water serves to stabilize and add mobility to the froth. Sufficient water must be added to ensure that all of the feed water that would otherwise normally report to the froth product is replaced with fresh or clarified water. It has been reported that less than 1% of the feed pulp and associated clays will report to the froth in a well-operated column (Luttrell et al., 1999). The ability to maintain and wash a deep froth layer is the main reason cited for the improved product grades when comparing column cells to conventional cells.

In contrast, conventional mechanical cells do not operate with deep froths. Therefore, these devices allow some portion of the ultrafine mineral slimes to be recovered with the water that reports to the froth. Consequently product quality is reduced by this non-selective hydraulic conveyance (i.e., entrainment) of gangue into the product launder. In fact, fine particles (<0.045mm) have a tendency to report to the froth concentrate in direct proportion to the amount of product water recovered. As such, the flotation operator is often forced to make the decision to either pull hard on the cells to maintain yield (e.g., wet froth), or run the cells less aggressively to maintain grade (e.g., dry froth).

The primary advantage of utilizing wash water is the ability to provide a superior product grade when compared to conventional flotation processes. This capability is illustrated by the test data summarized in Fig. 11.15, which compares column flotation technology with an existing bank of conventional cells. As shown, the separation data for the column cells utilizing wash water are far superior to those obtained from the conventional flotation bank. In fact, the data for the column cells tend to fall just below the separation curve predicted by release analysis (Dell et al., 1972). A release analysis is an indication of the ultimate flotation performance and is often regarded as wash-ability for flotation. This figure suggests that columns provide a level of performance that would be difficult to achieve even after multiple stages of cleaning by conventional machines.

There are a significant number of full-scale column installations currently in commercial service around the world. The most popular brands of columns include the CPT CoalPro (Eriez), Jameson, and Microcel columns. Although the Jameson cell does not have the traditional column geometry, it is included since it typically uses wash water to improve ash rejection. Details related to the specific design features of the various column technologies are available in the literature (McKay et al., 1988; Finch and Dobby, 1990; Yoon et al., 1992; Manlapig et al., 1993; Davis et al., 1995; Rubinstein, 1995; Wyslouzil, 1997). The primary difference between the various columns used in the coal industry is the type of air sparging system employed. These include porous bubblers, static mixers, and dynamic air injectors. Details related to the features and operation of these systems have been discussed extensively in the literature (Dobby and Finch, 1986a; Xu and Finch, 1989; Huls et al., 1991; Groppo and Parekh, 1992; Yoon et al., 1992; Finch, 1995). Ideally, the spargers should produce small, uniformly sized bubbles at a desired aeration rate. Other factors, such as equipment costs, mechanical reliability, wear resistance, and serviceability also need to be carefully considered prior to selecting an industrial sparging system.

Due to economy of scale, recent trends in the coal industry have shifted away from the installation of large numbers of smaller units toward fewer, large units with diameters up to 5m or more. Although most column installations involve the treatment of particles finer than 0.150mm, several recent column operations have been installed to treat coarser particles, such as minus 1mm feeds or deslimed 0.1500.045mm feeds. Additionally, a move to more economical cells in terms of energy efficiency has been realized as manufacturers focus on the generation of the required air bubble dispersions while using significantly less power than traditional approaches. One such device is the Eriez StackCell, which utilizes both pre-aeration methods in conjunction with traditional froth washing (Davis et al., 2011) to maximize efficiency with regard to both installation and operating cost.

The two most important requirements of laboratory flotation machines are reproducibility and performance similar to commercial operations. These two criteria are not always satisfied. The basic laboratory machines are scaled down replicas of commercial machines such as Denver, Wemco and Agitair. In the scale down, there are inevitable compromises between simplification of manufacture and attempts to simulate full scale performance. There are scaling errors, for example, in the number of impeller and stator blades and various geometric ratios. Reproducibility in semi-batch testing requires close control of impeller speed, air flow rate, pulp level and concentrate removal.

Until now, deaeration tanks always had to be placed underneath the flotation machine and also frequently in the cellar of a facility in order to ensure a sufficient height difference for the conveyance of foam. In addition, the tanks are open on top and can overflow with excess foam. That is now a thing of the past with the Deaeration Foam Pump (DFP) 4000. The new pump can be linked directly to the deinking machine and forms a clean and closed disposal system. Because it can be placed at the same level as the flotation cells, the entire flotation system saves more space than previous systems. A cellar or an additional floor height for the flotation is no longer required. The deaeration results are very impressive with the DFP 4000 from Voith Paper. The air content of the foam mass is reduced when passing through the pump from 80% to an average of 8%. Conventional deaeration systems offer approximately 12%. In addition, by using the DFP 4000, upstream foam destroyers, downstream long piping as well as pumps with high head pressures to overcome the floor height can be dispensed. With the DFP 4000, it is possible to deaerate and convey the foam, which is loaded with inks and other impurities, within a single machine. As a compact unit, it fully replaces the foam destroyer, foam tank stirring unit, and pump of previous deaeration systems. This means a clear reduction in investment costs for the tank, stirring unit, pipes, pumps, and floor space.

The DFP 4000, developed by Voith, is a compact unit that integrates several elements of the flotation deinking system. This combines the pump and deaeration machine into one unit. The deaeration foam pump replaces the foam destroyer, foam tank, stirring unit, and pump and costs less than the current suite of equipment. The DFP 4000 achieves better deaeration of the foam than conventional systems.

The DFP 4000 has two parts. In the upper part, foam is predeaerated by a mechanical foam destroyer. In the lower part, centrifugal force produced by a quick rotational movement further deaerates the foam. The resulting low-air-content suspension is brought to the required pressure so that it can be conveyed out of the machine to the next process stage. The air released during deaeration is conveyed out of the machine through a special air chamber on the side so that the airflow does not prevent the foam entering from above (Dreyer,2010).

The new pump can be linked directly to the deinking machine, forming a clean and closed disposal system. Because the deaeration pump can be placed at the same level as the flotation cells, the entire system requires less space than previous systems, so a cellar (or additional floor height) is no longer required to accommodate the system. When the foam mass passes through the DFP 4000, the foams air content is reduced from 80% to an average of 8% (Voith,2011a). Conventional deaeration systems reduce the air content to approximately 12%. The first DFP 4000 operating in a paper mill has been in service since September 2009 (Dreyer,2010). The benefits of the DFP 4000 are summarized in Table11.9 (Dreyer,2010; Voith,2011a).

Batch testing has been carried out using a specially designed 21 tumbler for mixing, and a standard Denver flotation machine for separation. A typical charge of the soil sample ranged from 200 to 600g, and the amount of coal varied depending on the contaminant concentration.

Figure 1 shows the block diagram of the 6T/day continuous unit. A slurry of contaminated soil and coal is fed at optimal solids concentration to a specially designed tumbler. In the front section of the tumbler, as a result of rotary motion, the solids are mixed and dispersed. In another section of the tumbler, layering, compaction and abrasion take place. After being discharged from the tumbler, the contents are screened into two streams. The 1mm particle size stream is directed to a high shear mixer where the oil-wetted coal particles are conditioned. The slurry is then transfered to flotation cells, where the coal microagglomerates, in the form of froth, are separated from clean soil. To facilitate dewatering and improve handleability of the combustible product, the froth can be subsequently fed into the low shear mixer for further agglomeration.

Flotation has progressed and developed over the years; recent trends to achieve better liberation by fine grinding have intensified the search for more advanced means of improving selectivity. This involves not only more selective flotation agents but also better flotation equipment. Since the froth product in conventional flotation machines contains entrained fine gangue, which is carried into the froth with feed water, the use of froth spraying was suggested in the late 1950s to eliminate this type of froth contamination. The flotation column patented in Canada in the early 1960s and marketed by the Column Flotation Company of Canada, Ltd., combines these ideas in the form of wash water supplied to the froth. The countercurrent wash water introduced at the top of a long column prevents the feed water and the slimes that it carries from entering an upper layer of the froth, thus enhancing selectivity.

The microbubble flotation column (Microcel) developed at Virginia Tech is based on the basic premise that the rate (k) at which fine particles collide with bubbles increases as the inverse cube of the bubble size (Db), i.e., k1/Db3. In the Microcel, small bubbles in the range of 100500m are generated by pumping a slurry through an in-line mixer while introducing air into the slurry at the front end of the mixer. The microbubbles generated as such are injected into the bottom of the column slightly above the section from which the slurry is with drawn for bubble generation. The microbubbles rise along the height of the column, pick up the coal particles along the way, and form a layer of froth at the top section of the column. Like most other columns, it utilizes wash water added to the froth phase to remove the entrained ash-forming minerals. Advantages of the Microcel are that the bubble generators are external to the column, allowing for easy maintenance, and that the bubble generators are nonplugging. An 8-ft diameter column uses four 4-in. in-line mixers to produce 56 tons of clean coal from a cyclone overflow containing 50% finer than 500 mesh.

Another interesting and quite different column was developed at Michigan Tech. It is referred to as a static tube flotation machine, and it incorporates a packed-bed column filled with a stack of corrugated plates. The packing elements arranged in blocks positioned at right angles to each other break bubbles into small sizes and obviate the need for a sparger. Wash water descends through the same flow passages as air (but countercurrently) and removes entrained particles from the froth product. It was shown in both the laboratory and the process demonstration unit that this device handles extremely well fine below 500-mesh material.

Another novel concept is the Air-Sparged Hydrocyclone developed at the University of Utah. In this device, the slurry fed tangentially through the cyclone header into the porous cylinder to develop a swirl flow pattern intersects with air sparged through the jacketed porous cylinder. The froth product is discharged through the overflow stream.

The process is carried out in a flotation cell or tank, of which there are two basic types, mechanical and pneumatic. Within each of these categories, there are two subtypes, those that operate as a single cell, and those that are operated as a series or bank of cells. A bank of cells (Fig. 8) is preferred because this makes the overall residence times more uniform (i.e., more like plug flow), rather than the highly diverse residence times that occur in a single (perfectly mixed) tank.

FIGURE 8. Flotation section of a 80,000t/d concentrating plant, showing the arrangement of the flotation cells into banks. A small part of the grinding section can be seen through the gap in the wall. [Courtesy Joy Manufacturing Co.]

The purpose of the flotation cell is to attach hydrophobic particles to air bubbles, so that they can float to the surface, form a froth, and can be removed. To do this, a flotation machine must maintain the particles in suspension, generate and disperse air bubbles, promote bubbleparticle collision, minimize bypass and dead spaces, minimize mechanical passage of particles to the froth, and have sufficient froth depth to allow nonhydrophobic (hydrophilic) particles to return to the suspension.

Pneumatic cells have no mechanical components in the cell. Agitation is generally by the inflow of air and/or slurry, and air bubbles are usually introduced by an injector. Until comparatively recently, their use was very restricted. However, the development of column flotation has seen a resurgence of this type of cell in a wider, but still restricted, range of applications. While the total volume of cell is still of the same order as that of a conventional mechanical cell, the floor space and energy requirements are substantially reduced. But the main advantage is that the cell provides superior countercurrent flow to that obtained in a traditional circuit (see Fig. 11), and so they are now often used as cleaning units.

Mechanical cells usually consist of long troughs with a series of mechanisms. Although the design details of the mechanisms vary from manufacturer to manufacturer, all consist of an impeller that rotates within baffles. Air is drawn or pumped down a central shaft and is dispersed by the impeller. Cells also vary in profile, degree of baffling, the extent of walling between mechanisms, and the discharge of froth from the top of the cell.

Selection of equipment is based on performance (represented by grade and recovery), capacity (metric tons per hour per cubic meter); costs (including capital, power, maintenance), and subjective factors.

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