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The flotation processand related flotation cells are widely used for treating metallic and nonmetallic ores and in addition, it is receiving an ever widening application in other industries. A greater tonnage of ore is treated by flotation than by any other single process. Practically all the metallic minerals are being recovered by the flotation process and the range of non-metallics successfully handled is steadily being enlarged. In recent years the art of flotation has been successfully applied in other than the mining industry, such as flotation of wheat, and other industrial applications. As flotation reagents are further developed, the application of flotation will be more widespread.
The Sub-A Flotation Cellhas been applied to all types of flotation problems and these machines have continuously demonstrated their superiority. They have given very successful results through a wide range of problems, and their supremacy is fully proven by world-wide acceptance and application.The feature of the Sub-A is the design. The Sub-A incorporates all of the basic principles and requirements of the flotation process and these, coupled with the special and exclusive wear features, make it the ideal Flotation Cell.
Sub-A Flotation Cells have been developed over the intervening years since 1927 until today there are over 26,000 cells in operation. Flotation cells are standard equipment for an ever widening range of metallurgical and industrial problems. They are being used in plants of all types and sizes and they are giving excellent results at minimum cost at tonnages of a few tons up to 35,000 tons per 24 hours.
To take care of the wide range of problems confronting the flotation process, the Sub-As are built in a wide and flexible range of commercial sizes, from the No. 8 through the No. 12, No. 15, No. 18, No. 18 Special, No. 21, No. 21 Deep, No. 24 and the No. 30.
There is a particular size cell for every problem and tonnage, with each cellhaving incorporated into itsdesign features to take care of any condition. This is the basis on whichSub-A Cells have been designed. Standard cells are as follows:
The construction of the Sub-A Standard Flotation Cell is with double welded steel tank, alloy iron cell side liners, rubber bonded to steel bottom liners, impellers and diffuser wearing plates of molded rubber or alloy iron, individual cell pulp level control, rubber protected shafts and rubber sand relief bushings. To the standard cells the supercharging principle can be quickly adapted as all recent cells are furnished with automatic air seal and air bonnet to which low pressure air is easily connected. Variations from the standard cellallow the pulp to bypass through convenient ports which can be opened or closed while the unit is operating. This feature makes the Sub-A either the exclusive positive circulation unit or an open type cell.
Sub-A Flotation Cells are built with special design for work in acid and corrosive circuits. The construction of this typecellis similar to the standardSub-A except that the parts in contact with the pulp are of special materials and the tank itself is of wood construction. There is also a variation in design to take care of the special conditions usually associated with acid circuits and for convenience the acid-proof cells are built in 2-cell units.
The Sub-A Grain Peeling Flotation Cell is a special adaptation of the Sub-A for the peeling of wheat and other grains by flotation. The peeling cellis a continuous, straight-sided tank without spitzkasten overflow. This unit with its special features, loosens the outer brans. From the peeling unit feed passes to the Sub-A special grain Flotation Cell which is similar in design to the standard Sub-A, except for the special construction to meet the requirements of this type flotation. These peeling-Flotation Cells are built for a capacity of approximately 150 bushels per hour. The number of cells per unit and the number of units required for an installation is dependent upon the feed rate required and also upon the type of grain to be peeled.
The widespread success of the Sub-A Flotation Cell is attributed to the basic qualities of the design of this type Flotation Cell. Successful metallurgy results from the distinctive gravity flow feature, which assures positive circulation of all pulp fractions with reagents from cell to cell and hence results in high efficiency.
The pulp flows by gravity into each cell through the feed pipe, from which it drops directly on top of the rotating impeller below the stationary hood. As the pulp cascades over the impeller blades it is thrown outward and upward from the impeller and diffuser wearing plate by the centrifugal action of the impeller. The pulp is kept in complete circulation by the impeller action and as the flotation reaction takes place, the pulp is passed from cell to cell. Pulp overflows to each succeeding cell over an adjustable weir gate in the partition. This gate gives accurate control of pulp level as the pulp passes through the machine. To take care of coarse oversize each cell has a rubber sand relief opening in the partition weir casting which feeds oversize direct to the impeller of the next cell without short circuiting. Circulation within each cell itself and return of middlings is by means of adjustable openings in the hood above each impeller, although for normal operation these are kept closed, except for middling return.
Circulation in Sub-A Flotation cells is highly efficient due to the distinctive gravity flow feature. This method of pulp circulation assures the positive circulation of all pulp fractions with resultant maximum treatment of each and every particle. It is an established fact that the mechanical method of circulating material is the most positive and economical, particularly where the impelleris below the pulp. A Flotation Cell must not only be able to circulate coarse material (encountered in practically every mill circuit) but also must re-circulate and retreat the difficult middling products.
An alternate pulp flow is obtained from cell to cell by the side ports in the partitions. The ports are adjustable so that a portion of the pulp can pass from cell to cell through these ports and consequently bypass the impeller and weir overflow.
It is not essential to have each individual cell with separate weir gate control; however, for most installations this is recommended. An alternate arrangement is with gate control every two to four cells for pulp level control, and free pulp passage from cell to cell, by means of the ports, as well as cell to cell overflow. The arrangement is actually a grouping without sacrificing the positive circulation feature.
The passage of pulp through the cell and the action created in the impeller zone draws air down the stationary standpipe and from the partition along the feed pipe. This positive suction of air gives the ideal condition for average flotation and the action in the impeller zone thoroughly mixes the air with the pulp and reagents. As this action proceeds, a thoroughly aerated live pulp is produced and furthermore, as this mixture is ground together by the impeller action, the pulp is intimately diffused with exceedingly small air bubbles which support the largest number of mineral particles.
For particular problems the aeration in the Sub-A can be augmented by the application of Supercharging, whereby fully controlled air under low pressure is diffused into the pulp. This feature is accomplished by the introduction of air from a blower or turbo-compressor through the standpipe connection into the aerating zone where it is premixed with the pulp by the impeller action. This supercharging of the pulp creates a highly aerated condition which is maintained by the automatic seal in the cell partition. Supercharging is of particular advantage for low ratio of concentration and slow-floating ores.
Throttling of air in the Sub-A Flotation Cell is of benefit when suppressed flotation is required. This is accomplished by cutting off or decreasing the size of air inlet on the standpipe. Suppressed flotation finds its chief use in grain flotation, certain non-metallics and occasionally in cleaner or recleaner operations.
The aeration and mixing of the pulp with reagents all takes place in the lower zone of the cell. This thorough mixing, which is below the stationary hood, is to a considerable degree responsible for the metallurgical efficiency of the cell.
Supercharging Sub-A Flotation Cells by increasing the sub-aeration is obtained with a small volume of air at low pressure. The air bonnet and automatic air seal are integral parts of all standard Sub-A Cells; hence, to increase the aeration all that is required is a connection from the air bonnet to a source of air supply.
The aerated pulp, after leaving the mixing zone, passes upward by displacement to the central section of the cell. This is a zone of quiet and is free from cross currents and agitation. In this zone, the mineral-laden air bubbles separate from the worthless gangue and pass upward to the froth column without dropping their load, due to the quiescent condition. The gangue material follows the pulp flow and is rejected at the discharge end of the cell.
It is in the separation zone that effective aeration is essential and this is assured in the Sub-A as the air is broken up into minute bubbles. These finely diffused bubbles are essential for carrying a maximum load of mineral.
The mineral-laden bubbles move from the separation zone to the pulp level and are carried forward to the overflow lip by the crowding action of succeeding bubbles. To facilitate the quick removal of mineral-laden froth, Flotation Cellsare equipped with froth paddles. Froth removal can be further facilitated by the use of crowding panels which create a positive movement of froth to the overflow.
Concentrates produced by Sub-As are noted for their distinctive high grade and selectiveness. The spitzkasten built into Sub-A cells is partially responsible as it allows a quiescent zone just before froth removal in order that middling fractions may fall back into the pulp flow. Cells are built with single overflow as standard, but double overflow can be supplied.
These are several of the distinctive advantages obtained with the use of Sub-A Flotation Cells which are found in no other Flotation Cell. The combination of these several advantages is necessary to obtain successful flotation results.
Positive circulation of all pulp fractions from cell to cell is assured by the distinctive gravity flow principle of the Sub-A. No short circuiting can occur through the cell; hence, every particle is subject to positive treatment. In instances where successful metallurgy demands the handling of a dense pulp containing an unusually large percentage of coarse material, the sand relief opening aids in the celloperation. This opening removes from the lower part of the cell the coarse fractions and passes them through the feed pipe to the impeller of each succeeding cell. The sand relief openings assure the passage of slow floating coarse mineral to each impeller and therefore it is subject to the intensive mixing, aeration and optimum flotation condition of each successive cell. The finer pulp fraction passes over the weir or through the intermediate ports. The passage of the coarse fractions through each impeller eliminates short circuiting and thus, both fine and coarse mineral are subject to positive flotation.
A Sub-A cell will not choke up, even when material as coarse as one quarter inch is circulated. Choking cannot occur as the feed to each cell is to the top of the impeller. After a shut-down, it is not necessary to drain the Flotation Cell as the stationary hood with diffuser wearing plate protects the impeller and feed pipe from sanding-up. Even though the flotation feed is finely ground, coarse material occasionally gets into the circuit and if the Flotation Cell does not have the gravity flow feature, sanding and choke-ups will occur. This gravity flow principle of pulp circulation has made possible the widespread phenomenal success of a flotation cell between the ball mill and classifier. The recovery of mineral, as coarse and as soon as possible, in a high grade concentrate is now considered a requisite to a maximum metallurgical efficiency and hence Sub-A operators value its 24-hour per day service and freedom from shut-downs.
Middling products from Sub-As can be returned by gravity from any cell to any other cell in the average flotation circuit. The flexibility is possible without the aid of pumps or elevators. The middling pulp flows to the required cell and by means of a return feed pipe, falls directly on top of the impeller, assuring positive treatment and areation of the middling product without impairing the action of the cell. This feature, exclusive with Sub-A, is of particular advantage in circuits where several cleaning steps are required to bring middling products to final grade. The initial feed can also enter into the front or back of any cell through the return feed pipe.
Sub-A cells are under full control with normal operating conditions. The pulp level is maintained at the desired place with adjustable weirs. Aeration is controlled with flexible methods of air addition which allow variable aeration for different conditions. Any overloads or surges of coarse material from the grinding circuit are effectively taken care of with the sand relief openings, ports or quickly adjustable weirs. With these control features the operator has every opportunity to maintain his circuit in balance. Pulp fluctuations can be minimized and absorbed due to the control features.
Sub-A Flotation Equipment is metallurgically unsurpassed for the production of concentrates most suitable for subsequent thickening, filtering and smelting.The selectivity of Sub-A through all mesh sizes is one of the outstanding features of this Flotation Cell. Selectivity in Cells is not by chance, but results from the basic principle in design. The distinct gravity flow feature, coupled with the individual cell construction, controlled individually or in groups, and positive circulation through the cell, is to a large degree responsible for the recovery of coarse products by flotation. The advantage of positive circulation becomes obviously important with coarser grinds. A homogeneous and thoroughly mixed pulp is circulated at all times in each cell and there is no tendency towards. classification and segregation. Thorough mixing and aeration of all pulp fractions by positive circulation is the only means of obtaining selective flotation and metallurgical efficiency through all mesh sizes. The absence of pulp stratification prevents slime recovery from surface pulp or drift of heavy granular fractions through the cell. Selective flotation with Sub-A results in several major features, such as:
Recovery in flotation is of prime importance. In studying recoveries it is essential also to investigate thoroughly the intermediate products produced. It is a simple matter to make a high recovery or a low tailing if no thought is given to the nature of the concentrate produced or circulating load. Sub-A Flotation Cells will produce a high recovery, coupled with a high grade concentrate, low volume of middling, and a final concentrate most acceptable for subsequent treatment. The overall efficiency of this Flotation Cell will assure an equitable balance between recovery and nature of products produced.
Sub-A Flotation Cells have demonstrated that they alone produce products most acceptable for economic efficiency. In competitive tests where all phases of the operation are studied in thorough detail, it has been proven time and again that Sub-As show metallurgical advantages which contribute to the highest overall efficiency of an entire mining operation. Sub-A cells are:
(1) More selective through all mesh sizes. (2) Produce a coarser concentrate. (3) Produce a concentrate more acceptable to subsequent treatment. (4) Produce equal or higher recovery in conjunction with a higher grade concentrate and higher ratio of concentration.
A comparison of product assays does not give true and complete information with respect to the performance of a Flotation Cell. Product assays for two flotation machines operating in parallel could quite conceivably be identical, yet the physical characteristics of the products recovered and discarded would be entirely dissimilar. Wide differences which would be obvious in detailed investigation might not be indicated by a cursory examination. A detailed study of flotation concentrates shows that Sub-As recover the coarser more granular sulphides which parallel cells lose in the tailing. The higher recovery of coarse concentrate has been the story in every instance where Sub-A cells have been on a comparative basis. The use of Sub-A cells is responsible for the trend in concentration by flotation of coarse granular concentrates with minimum slimes. Higher recoveries have been possible in many instances by changes in grinding and removal of coarse primary concentrates. Recovery at a coarser grind means a decreased amount of slime mineral in the pulp. Absence of slime in concentrates is reflected in the analysis of the insoluble fraction. Sub-A cells always show a lower percentage of slime in concentrate due to selectivity and this means minimum refractories in subsequent treatment.
Screen analysis of products recovered and rejected clearly demonstrate the absence of sanding and segregation in Sub-A cells and the patented positive circulation principle assures balanced products.
The capacity of a flotation cell, treating any ore, depends upon facts and conditions which can best be determined by experience and test work. The pulp density and flotation contact period required materially affect the capacity of a Flotation Cell. With these factors known from previous work or test results, the size machine can be determined. Three conditions are factors in determining the proper size celland number of cells.
Flotation contact time required for the ore is one of the determining factors in calculating capacity. If an ore is slow floating and requires twelve minute treatment time, and another ore is fast floating and requires but six minute treatment, it is evident that a cellof only half the capacity is necessary in the last instance. Pulp density and specific gravity of dry solids control the cubic feet of pulp handled by the Flotation Cell, so are determining factors in calculating the flotation contact period. The Sub-A capacity recommendations are conservative figures which are based on years of actual field operation, treating many kinds of material.
The volume of the flotation cell must be known, as the volume in the Floatation Cell determines the time available for flotation of the values to take place. Therefore, the capacity of any Flotation Cell is dependent on the volume. All flotation cells having the same volume will have approximately the same capacity, with allowance made for horsepower, the efficiency of the impeller and aeration. As the flotation contact period is very important in any Flotation Cell, the actual cubical content of any machine should be carefully checked as well as accurate determinations on average pulp specifications.
Metallurgical results required from the floatation machine will have considerable bearing on the installed capacity. Several stages of cleaning may be required to give a high grade concentrate and this can be accomplished by the Sub-A, usually in one machine without resort to pumps for middling return. Results with cells of equal volume will not necessarily be equal because they may not be equally efficient. It may be easy enough to pass pulp through a Floatation Cell but to have a machine give a high-grade concentrate, to retreat middlings, and to give a low tailing, is an advantage obtained by use of Sub-As.
Under the table, problems are given to illustrate the methods of calculating the number of cells required. In order to secure the maximum positive treatment of the mineral, and to produce a high grade concentrate, it is best to have the necessary total volume divided into at least four cells and preferably six cells, each a separate cell, so that they may be used for roughing, cleaning, or recleaning purposes.
To determine the number of Sub-A cells requiredmultiply the proposed tonnage per day (24 hours) by the time (number of minutes necessary to float the mineral) then divide this product by the proper figure given in the table. This figure is secured by taking the size cellunder consideration (find the horizontal line giving the dilution of mill pulp and the vertical line giving the specific gravity of your ore); the figure will be at the point of intersection.For conservative estimates on a gold, silver, copper, or lead ore, use the tonnage capacities in the following table.
PROBLEM 2How many No. 18 Sp. (3232) Sub-A Cells are required to treat 125 tons of lead-zinc ore per day, with treatment time 14 minutes for the lead, dilution 3 to 1, and with treatment time 16 minutes for the zinc, dilution 3 to 1, and sp. gr. 3.4?
Continuous 24-hour per day service depends upon the mechanical design and construction of a Flotation Cell. There is no unit so rugged, nor so well built to meet the demands of the process, as the Flotation Cell. The ruggedness of each cell is necessary to give long life and to meet the requirements of the process. Numerous competitive tests all over the world have conclusively proved the real worth of these cells to many mining operators who demand Maximum results at the lowest cost.
The location of the feed pipe and the stationary hood over the rotating impeller account for the simplicity of the Sub-A cell construction. These parts eliminate swirling around the shaft and top of the impeller, reduce power load, and improve metallurgical results.
Improvements in construction of Sub- A cells during the last ten years have been gradually made as a result of plant scale testing and through suggestions from the mining fraternity. Today the Sub-A is mechanically unexcelled with rugged construction, pressure cured wearing parts, heavy duty, dependable drives. The abrasive cell zone is protected with rubber bottom liners and hard iron or Decolloy side liners. The heavy duty shafts are also rubber protected so the entire abrasive zone is sheathed for protection against wear.
The Sub-A, with its distinctive advantages, is moderately priced, due to standardization and quantity production. There is a definite mechanical or metallurgical reason behind the construction of every part of the Sub-A as explained in the following specifications.
The tank for the Flotation Cell is made of heavy steel joints are electric welded both inside and out. Partition plates are furnished with gaskets and arranged for bolting to partition channels so that if necessary all of the plates can be changed at any time in the field to provide either a right or left hand machine. Right hand machine is standard and will be furnished unless otherwise noted.
Flotation Cells are also available in wood tank construction especially suitable for corrosive circuits. These machines can be supplied with modifications so that they are ideal for use in special applications.
All cells are placed at a common floor level and due to the gravity flow principle of Flotation Cells almost any number of cells can be used in any circuit at one elevation without the necessity of pumps or elevators to handle the flow from one cellto the next. Operation and supervision is thus simplified.
For export shipments all of the items for the Flotation Cell are packed, braced, and blocked inside of the steel tank so that minimum volume is required. Safe delivery of parts without damage is thus assured.
The shaft and bearings of the Sub-A are supported in an enclosed ball bearing housing designed to properly carry and maintain the rotating impeller. Both the upper and lower heavy duty, oversized, anti-friction bearings are seated in this housing, insuring perfect alignment and protection against dirt.
Bearings have grease seals to prevent grease or oil getting into the cells; lubrication is only needed about once in six months. Many thousands of these standard bearings are in daily use on Sub- A cells, giving continuous service and low horsepower.
The hood, which is located near the bottom of the cell, is an important part of the assembly as it serves a number of purposes. The vanes on this hood prevent swirling of the pulp in the cell, producing a quiet action in the central or separation zone. The hood also supports the stationary standpipe and the hood wearing plate. Aeration of the pulp takes place in the impeller zone just below the stationary hood. The wearing plate is bolted to the bottom of the hood and prevents the impeller from being buried by pulp when the Flotation Cell is shut down.
Data from large operations have shown that the life of rubber parts is from six to fifteen times longer than the life of hard iron wearing parts. The slightly greater cost of these parts is therefore more than offset by the longer life. The advantages gained not only in lower maintenance but also in reduction in horsepower (because of the lower coefficient of friction when using molded rubber impellers) make them most economical. Both receded disk and conical disk wearing parts are also available in special hard alloy iron.
The receded disk impellers and diffuser wearing plates have been proved in commercial installations for many years and are one of the important developments made in the Flotation Cell. The receded disk impellers and diffuser wearing plates are furnished with all machines unless otherwise specified. The advantages of these parts are as follows:
Agitation is intense in the agitation zone but elsewhere it is held at a minimum and at the same time the air is finely dispersed throughout the pulp so that the cell surface presents the appearance of a smooth and quiet blanket of froth, conducive to good flotation. Molded rubber parts are recommended due to their lighter weight, perfect balance, and longer life.
In keeping with a long established policy, it is possible to use these parts on any Flotation Cell irrespective of age, without the necessity of making any major changes, thus adhering to the standard policy of No yearly models but continually improving.
The conical disk impellers and wearing plates, as illustrated, are obtainable for all sizes of machines. The conical disk impellers and wearing plates have been used in Flotation Cells for many years but are rapidly being replaced by the receded disk impellers and diffusers for general purposes. Conical disk impellers are recommended for Unit Flotation Cells and applications such as treatment of dense pulp and coarse material. Diameters of all impellers have a definite relationship to cell sizes, thus insuring uniform circulation of the pulp.
The Sub-A was the first Flotation Cellto use the Multi-V-Belt Horizontal Drive, which has proved so successful. Sub-A Flotation Cells have been carefully designed to be driven either by a motor and V-belts or by V-belts to a main drive shaft. In the motor driven type the impeller shafts are driven by V-belts, sheaves, and vertical ball bearing motor. This type of motor drive is much more economical and desirable than a direct motor driven unit because it makes any speed range available and does not require a special motor shaft assembly.
The standard drive on all flotation machines of an even number of cells is one motor driving two cells through V-belt drives. If an odd number of cells is ordered, a drive which will prove most economical in first cost and provide the greatest operating efficiency will be furnished. Adjustment of belt tension is provided for in the motor mounting.
The paddleshaftdrive is taken generally from the last impeller shaft by means of V-belt drive to a speed reducer, which in turn drives the paddleshaft at slow speed. The No. 30 paddleshaft drive is from a gear motor. The quick removal of the mineral froth, in the form of a concentrate, increases the recovery; quick removal of this mineral froth is very important and when a high grade concentrate is desired, the rotating paddles can be regulated as desired.
Every Sub-A Cell is actually an individual flotation machine with its own pulp level, controlled by its weir overflow. Correct overflow normally requires this positive pulp level control in each cell even though this adjustment when once made is infrequently changed. There are three methods of regulating pulp level:
Weir Blocks, as illustrated, slide easily into place at the weir, and consist of wood slats held down by means of a steel wearing bar. On the smaller machines, especially, adjustment by this means is easy as the weir is readily accessible. Actual plant practice shows in the normal circuit that it is not necessary to change the pulp level frequently.
Handwheel operated weir gates can be provided, as illustrated, so that changes in pulp level in each individual cell can be accomplished by turning the handwheel which is located far above the froth level. Changes in level can be made quickly and easily with minimum effort.
Gear driven handwheel gates, as illustrated, can be provided and are especially useful on large size Flotation Cells. This arrangement brings the control of the pulp level out to the front of the machine making it unnecessary to reach over the froth lip. The use of a gear box with handwheel control reduces the effort required for raising or lowering the gate, and provides a method of quick and easy adjustment.
Cell liners fit easily into the cell and consist of four cast iron liners and a rubber bottom liner. This bottom liner consists of a rubber compound similar to that used on the molded rubber parts, firmly bonded to a steel backing so that it does not rip or blister. This liner is held in place at the edges by the side liners.
Cell drainage is through an easily accessible port at the back of each cell.A small recirculation gate is provided near the top of each cell so that if desired, a portion of the pulp can be removed from the middling zone and returned to the impeller for retreatment. This recirculation feature influences the production of high grade concentrates in some cases. A gate is provided for this recirculation opening so that an adjustment of the zone and amount of recirculation can be varied.
Flotation Cells are provided with openings in the partition plates for by-passing the pulp from cell to cell without the pulp circulating through each hood feed pipe. In normal operation these partition gates are left closed; however, this arrangement is advantageous when large tonnages are fed to the Flotation Cell. This arrangement also allows the machine to be operated in groups of cells with the same positive control and circulation applied to each group.
The impeller assembly, consisting of steel shaft, totally-enclosed spindle bearings, standpipe, air- bonnet, hood, wearing plate, and impeller, fits easily into the cell as a unit. Diffuser and receded disk impellers, as illustrated, are furnished but conical disk impellers can be used if desired. The hood rests on corners of the cell side liners and is provided with keystone plug plates in front and back with recirculation openings. These recirculation opening in the plug plates can be opened, closed or bushed to small sizes as desired but in normal operation are closed. The keystone plug plate can be removed to provide an opening to the impeller for the return of middlings or feed, by means of return feed pipe which is easily placed to fit between the hood and front or back plate of the cell. Openings are provided in front of each cell for the return of middlings into any cell by gravity or these openings can be used to introduce feed into the cell if desired.
Adjustment of the impeller is easily and quickly accomplished from the rear of the Flotation Cell by means of the threaded rod holding the end of the spindle bearing housing. Proper adjustment of the clearance between impeller and wearing plate is important, and is easily done by loosening the bolts holding the spindle bearing housing and raising or lowering the entire housing by means of the adjustment provided by this threaded rod. After the proper clearance is secured the housing is tightened in place. Guides on each bearing housing keep rotation of impeller in perfect alignment and make vertical adjustment easy.
This policy of continual improvement is the aim for advancement of the Sub-A. Experiences gained in field studies have shown the factor of safety to use on shafting, bearings, and other operating parts.
Miningpioneered and developed the method of producing molded rubber wearing parts in 1932. Experience in handling practically all types of abrasive pulps, and in circuitswith various types of flotation reagents, and oils, have facilitated the development of suitable rubber compounds to meet all conditions. The use of Sub-A Flotation with molded rubber wearing parts is extremely valuable to the operator, with the assurance of the lowest possible maintenance cost. Molded rubber wearing parts are still the leaders in the field, giving trouble-free service for much longer periods than any other make impeller. Molded rubber impellers have handled some very large tonnages and records of 4 and 5 years continuous operation are common.
These features, combined with the sturdy construction of the cells, oversize bearings, heavy duty shafting and rugged cell liners, are showing average maintenance costs including labor for installation of less than $0.001 per ton in many cases. Even under the most adverse conditions Sub-A Cells rarely show total repair cost in excess of $0.003 per ton.
Each Sub-A Cell is provided with an air bonnet on the shaft assembly so that low pressure air may be connected if desired. To assure complete diffusion of air in the pulp an automatic seal is built in each weir casting.
Feed may enter any cell of a Flotation Cell, through the front or back. The hand of the Sub-A may be easily changed in the field by reversing the position of the weir casting with plate and partition plate. The hood assembly is turned through 180 degrees and the feed liner is changed with the liner in the opposite segment.
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 .
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 . 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 .
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 . 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) . 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 .
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 . 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 . 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 . Similar measurements were later carried out at Newcastle, Australia ; 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 . 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 . 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 .
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, in mineral processing, method used to separate and concentrate ores by altering their surfaces to a hydrophobic or hydrophilic conditionthat is, the surfaces are either repelled or attracted by water. The flotation process was developed on a commercial scale early in the 20th century to remove very fine mineral particles that formerly had gone to waste in gravity concentration plants. Flotation has now become the most widely used process for extracting many minerals from their ores.
Most kinds of minerals require coating with a water repellent to make them float. By coating the minerals with small amounts of chemicals or oils, finely ground particles of the minerals remain unwetted and will thus adhere to air bubbles. The mineral particles are coated by agitating a pulp of ore, water, and suitable chemicals; the latter bind to the surface of the mineral particles and make them hydrophobic. The unwetted particles adhere to air bubbles and are carried to the upper surface of the pulp, where they enter the froth; the froth containing these particles can then be removed. Unwanted minerals that naturally resist wetting may be treated so that their surfaces will be wetted and they will sink.
This ability to modify the floatability of minerals has made possible many otherwise difficult separations that are now common practice in modern mills. Flotation is widely used to concentrate copper, lead, and zinc minerals, which commonly accompany one another in their ores. Many complex ore mixtures formerly of little value have become major sources of certain metals by means of the flotation process.
We describe a sequential multi-scale modelling method for processing operations.We detail the application of this method to the mineral flotation process.Multi-phase CFD models of flotation cells serve as the macro-scale component.We describe micro-scale models of bubbleparticle collision and attachment.Sequential multi-scale modelling of other processing operations is summarised.
Processing of minerals and metals involves phenomena functioning over a wide range of length scales. Equipment is generally large in order to process the required large flow rates, yet achieving the functional requirements of the operation requires successful execution of micro-scale processes. Multi-scale modelling approaches to process simulation are firstly reviewed, and similarities and differences compared with the more common multi-scale approaches to materials modelling are given.
The sequential multi-scale method is then illustrated with reference to the mineral flotation process. In this case, multi-phase CFD (computational fluid dynamics) models of large-scale cells has been complemented by micro-scale CFD simulations of bubbleparticle collision, and experimental and modelling studies of the bubbleparticle attachment process itself.
Finally, other examples of sequential multi-scale modelling are summarised, highlighting progress on unit operations including aluminium reduction cells, leaching heaps, copper solvent-extraction settlers, and fluidised beds.