separator spiral mineral

gravity spiral concentrator working principle

gravity spiral concentrator working principle

The gravity spiral circuit is designed to extract and concentrate coarse gold from the recirculating load in the mill grinding circuit and hence prevent a build up within that circuit and the eventual escape of some of that gold into the C.I.L. tanks and thereon into the final tails. (See fig. 4)

For the spirals to work efficiently the feed supply must have consistent characteristics and be of a constant rate. Variations in the flow rate, the feed size distribution and percentage solids will have adverse effects upon separation. Generally the solids tonnage should give adequate loading of the concentrate and middlings areas and the pulp density should be low enough to ensure mobility of particles in these areas. BUY SPIRALS

Feed to the spirals may be adjusted by the moving of two splitter arms on either side of the cyclone underflow discharge box, this altering the volume of the feed passing over the splitter screen. (See fig. 5)

The feed may also be adjusted by varying the speed of the gravity feed pump. This is necessary when the mill feed has been dropped and it is impossible to get sufficient feed for the gravity pump by adjustment of the splitter arms. At such times the speed will need to be dropped and the water additionadjusted to provide optimum feed density.

The pulp density may be altered by the addition of water, before the splitter screen, in the gravity feed pump hopper or to the concentrate launder beneath the primary spirals. The latter option adjusts the density of the feed to the cleaner spiral only.

The static distributor (See fig. 6) at the head of the primary spirals ensures an accurate division of the pulp stream to the spirals. For maximum efficiency a constant head should be maintained in the head pot. The head can be adjusted by either altering the flow rate from the splitter screen and/or altering the annular gap between the head pot and the distributor body, by moving the head pot up or down as required.

Feed from the splitter screen passes down into the gravity feed pump hopper and from there it is pumped to the static distributor above six triplex type primary spirals. As the pulp passes down these spirals; separaration of particles occurs according to specific gravity and the heavier minerals progress to the inner profile while lighter minerals are forced towards the outer profile, along with most of the water and slimes. At the bottom of each spiral layer there are splitters which can be adjusted to ensure the optimum recovery of coarse gold. (See fig. 7)

The middlings and tailings from the primary spirals are directed to both the mill feed and the mill discharge pump. The proportion going to either may be adjusted so as to help achieve optimum grinding conditions.

The concentrate from the inner outlet of the cleaner spiral is fed directly on to the Wilfley table and the middlings and tailings report to the gravity sump pump which feeds into the mill discharge pump feed hopper.

The Humphreys Spiral Concentrator, which was invented by I. B. Humphreys and first used in 1943 for concentrating chromite in Oregon beach sands, consists of five or six spiral turns of a modified semicircular launder which is about the size of a conventional automobile tire. Feed enters the top spiral and the tailing discharges from the bottom one, while concentrate and middlings are cut off by outlet ports regularly spaced at each turn of the spiral, and the products passed through rubber hoses to common launders which run the full length of a bank of spirals. Wash water is supplied from a small wash-water channel paralleling the main channel.

Operating entirely by gravity flow and involving no mechanical parts, the separation of the heavy constituents of the feed is effected by the same centrifugal forces and flow gradients encountered in ordinary river or stream concentration.

A capacity of 38 tons per spiral was obtained in the 1000-ton per 24 hr. Oregon plant operating on about a minus 40-mesh feed and in the 5000-ton plant recently installed near Jacksonville to concentrate ilmenite 174 roughing, and 12 finishing spirals have replaced an installation of tables and flotation cells.

The Humphreys Spiral has been successfully applied to recovery of chromite from chrome sands, rutile, ilmenite, and zircon from sand deposits, tantalum minerals and lepidolite from their ores, gravity concentration of base metal ores and in the cleaning of fine coal.

How it works: Pulp is introduced at the top of the spiral and flows downward. As the pulp follows the spiral channel the light particles in the pulp stream move outward and upward into the fast flowing portion of the stream while the heavy particles move to the inner slow moving portion of the stream, where they are drawn off through concentrate ports.

Adjustable splitters allow any portion to be removed through the ports. Tailing is discharged at the bottom of the spiral. Spirals are usually installed in double units, two spirals to a frame, in rows of two to twelve. Feed is split evenly to all spirals. At one plant 21 rows of 12 spirals each are fed by one pump.

The Humphreys Spiral Concentrator is a simple, efficient gravity concentrator which effects a separation between minerals of the proper size range that have sufficient difference in their specific gravity.

This concentrator is a spiral conduit of modified semi-circular cross-section, with outlets for removal of concentrate and middling. Pulp is introduced at the top of the spiral. As the pulp follows the spiral channel, lighter particles in the pulp stream move outward and upward into the fast moving part of the pulp stream. The heavy particles move to the inner, slow moving portion of the stream, where they are drawn off through concentrate or middling outlets. Adjustable splitters allow any portion of the concentrate or middling to be diverted through the outlets. Tailing discharges from lower end of spiral. A full- size spiral is used for laboratory testing. Two arrangements are suggested for test work.

In the closed circuit test unit illustrated, although a full-size spiral is used, as little as 20 pounds of material will indicate the possibility of spiral concentration in a batch test. By removing measured quantities of products, and adding like amounts of feed in repeated steps, substantial samples may be taken for analysis and estimates of capacity. Results from this procedure, using 100 to 300 pounds of material, are close to pilot test results.

Another arrangement, also using a full-size spiral, is a small pilot plant, and is suitable for test work where a larger quantity of material can be handled. The storage tanks may be built on the job from drawings supplied. This unit allows continuous feeding of material and permits accumulation of concentrate and tailing in separate tanks, which may then be re-run as feed for second stage concentration or scavenging of tailing.

Spiral concentrators are modem, high capacity, low costunits developed for the concentration of low grade ores. Spirals consist of a single or double helical sluice wrapped around a central support with a wash water channel and a series of concentrate take-off ports placed at regular intervals along the spiral (Figure 17). To increase the amount of material that can be processed by one unit, two or more starts are constructed around one central support. New spirals have been developed that do not use wash water. These new units have modified cross sections and only one concentrate-take-off port, which is

rapid wear of the rubber lining and irregular wash water distribution resulted in major production problems. Although still in use, the Humphreys cast iron spirals have been largely superseded by a variety of other types, notably the fiberglass Reichert spirals and new, lightweight Humphreys spirals.

The processes involved in mineral concentration by spirals are similar for all models. As feed containing 25-35% solids by volume is fed into the channel, minerals immediately begin to settle and classify. Particles with the greatest specific gravity rapidly settle to the bottom of the spiral and form a slow-moving fluid film. Thus the flow divides vertically: one level is a slow-moving fluid film composed of heavy and coarse minerals; the other level, the remainder of the stream, is composed of lighter material and comprises the bulk of the wash water. The slow-moving fluid film, its velocity reduced by friction and drag, flows towards the lowest part of the spiral cross-section (nearest the central support) where removal ports are located. The stream containing the lighter minerals and the wash water develops a high velocity, and is thrust against the outside of the channel (Figure 18). Separation is enhanced by the differences in centrifugal forces between the two: the lighter, faster flowing material is forced outward towards the surface, and the heavier, slower material remains inward towards the bottom.

Spiral concentrators are capable of sustained recoveries of heavy minerals in the size range of 3 mm down to 75 microns (6 to 200 mesh). They are suitable for use as roughers, cleaners, or scavengers. Feed rates may vary from 0.5 to 4 tons per hour per start, depending on the size, shape, and density of the valuable material. Some factors that affect recovery are the diameter and pitch of the spiral, the density of the feed, the location of splitters and take-off points, and the volume and pressure of thewash water. Individual spirals are easily monitored and controlled, but a large bank of spirals requires nearly constant attention.

Advantages of spiral concentrators include low cost, long equipment life, low space requirements, and good recovery of fine material. They can also be checked visually to determine if the material is separating properly. For maximum operating efficiency, feed density should remain constant, the particle-size distribution of the feed should be uniform, and fluctuations in feed volume should be minimized. Spiral concentrators will tolerate minor feed variations without requiring adjustment. Spiral concentrators, like cone concentrators, are efficient, low-maintenance units that should be considered for any large- scale gravity separation system.

The newer Humphreys spirals are capable of recovering particles as small as 270 mesh (53 microns). In a test at CSMRI, a new Mark VII Reichert spiral recovered 91.3% of the free gold contained in the feed in a concentrate representing only 5.4% of the feed weight. The unit showed little decrease in gold recovery efficiency with material down to 325 mesh (45 microns) (Spiller, 1983).

gravity separation

gravity separation

Our Australian based head office houses the world's largest spiral manufacturing facility and produces over 20,000 starts annually. In 2010/11, we manufactured HC33 and WW6 spirals for ArcelorMittal's Mont Wright mining operations in Canada to deliver the largest single spiral order in our history.

spiral separator

spiral separator

We have 3-turn, 5-turn and 8-turn spirals with high-, medium- and low-gradient profiles. Our high-capacity spiral separator (the HX series) provides a per-start tonnage rating of 5 to 9 tph, depending on the mineral type, and is especially suited for the treatment of low-grade ore, where large tonnages are processed.

Mineral spiral separator components include the spiral trough; sliding or auxiliary splitters; launders; distributor and piping; feed box; product box and stainless steel product splitters; and the discharge chute at the bottom of the modular housing frame. Our spiral separators have a diameter of 1 000 mm.

Oursingle-anddouble-stagespiral separators are optimised for coal particles in the size range of 1 to 0.1 mm, providing enhanced coal washing for slurries. Their compact, modular design provides flexibility when building or upgrading your plant.

Double-stage spiral separators reduce the requirements of height and floor space in a plant, and reduce capital and operating costs. TheMX7spiral separator is ideal for difficult-to-wash coals, improving the overall efficiency of tougher separation applications.

Your local Multotec branch will help you with end-to-end solutions for your gravity concentration plant, from testing equipment through to field service, ongoing optimisation and any maintenance. We keep a wide range of spares and accessories for your spiral concentrator, reducing lead times to ensure maximum processing uptime.

Your local Multotec branch will help you with end-to-end solutions for your gravity concentration plant, from testing equipment through to field service, ongoing optimisation and any maintenance. We keep a wide range of spares and accessories for your spiral concentrator, reducing lead times to ensure maximum processing uptime.

Spiral concentrators are simple low energy-consuming devices that separate minerals mainly on the basis of density or by shape. Spirals are widely used in mineral processing as a method for pre-concentration and have proven to be metallurgically efficient and cost-effective.

Separation on a spiral is achieved through a combination of forces that act on particles as they move down the trough of the spiral. These forces include gravitational forces, centrifugal force, hydrodynamic drag, and lift and friction forces. Apart from the forces acting on a spiral, the properties of the slurry flowing on a spiral including, solids concentration, feed rate and wash water also plays an important role in the separation on the spiral.

During separation, the heavy particles migrate toward the inner region of the trough, with lighter-density particles migrating to the outer edge of the trough. In a dry spiral separation, materials are sorted by shape. Rounder particles travel faster, are forced to the perimeter of the spiral, from where they can be collected separately from non-round material.

Our process engineers will work with you to optimise your spiral concentrator plant according to feed rate, the solids concentration, the wash water addition and the position of the cutters or splitters used to separate the tailings, middling and concentrate streams.

spiral (concentrators) - an overview | sciencedirect topics

spiral (concentrators) - an overview | sciencedirect topics

The spiral concentrator is a modern high-capacity and low-cost device. It is developed for concentration of low-grade ores and industrial minerals in slurry form. It works on a combination of solid particle density and its hydrodynamic dragging properties. The spirals consist of a single or double helical conduit or sluice wrapped around a central collection column. The device has a wash water channel and a series of concentrate removal ports placed at regular intervals. Separation is achieved by stratification of material caused by a complex combined effect of centrifugal force, differential settling, and heavy particle migration through the bed to the inner part of the conduit (Fig.13.31). Extensive application is the treatment of heavy mineral beach sand consisting of monazite, ilmenite, rutile, zircon, garnet, and upgrade chromite concentrate. Two or more spirals are constructed around one central column to increase the amount of material that can be processed by a single integrated unit.

The spiral concentrator first appeared as a production unit in 1943 in the form of the Humphrey Spiral, for the separation of chrome-bearing sands in Oregon. By the 1950s, spirals were the standard primary wet gravity separation unit in the Australian mineral sands industry.

In the spiral concentrator the length of the sluicing surface required to bring about segregation of light from heavy minerals is compressed into a smaller floor space by taking a curved trough and forming into a spiral about a vertical axis. The slurry is fed into the trough at the top of the spiral and allowed to flow down under gravity. The spiralling flow of pulp down the unit introduces a mild centrifugal force to the particles and fluid. This creates a flow of pulp from the centre of the spiral outwards to the edge. The heaviest and coarsest particles remain near the centre on the flattest part of the cross-section, while the lightest and finest material is washed outwards and up the sides of the launder (Fig. 15.15). This separation may be assisted by the introduction of additional water flowing out from the centre of the spiral either continuously or at various locations down the length of the spiral. This wash water may be distributed through tubes or by deflection from a water channel that runs down the centre of the spiral. Some present designs have overcome the need for this wash water. Once the particle stream has separated into the various fractions, the heavy fraction can be separated by means of splitters at appropriate positions down the spiral. A concentrate, middlings mid tailing fraction can be recovered.

In practice spirals are arranged in stacks or modules of roughers, scavengers and cleaners, where the initial concentrate is retreated to upgrade the fraction to its final grade. Spiral length is usually five or more turns for roughing duty and three turns in some cleaning unite. For coal concentration, 6 turns providing a gentler slope with longer residence time for the more difficult separation.

The performance of spirals is dependent on a number of operating parameters, summarised in Table 15.9. Spirals generally a chieve an upgrade ratio of 3:1 (heavy fraction:feed grade) and hence multiply treatments are required [13]. The presence of slimes adversely affects the spiral performance. More than 5% of 45m slimes will affect the separation efficiency.

With the steep pitch of a spiral, two or three spirals can be wound around the same common column and these types of spirals have been used in Australia for more than 20years. The multistart spirals conserve floor space and launder requirements. These triple-start spirals are built into a twelve spiral module and for these modules, the design of the distributor is critical to ensure that each spiral has a uniform feed.

The splitter blades on these spirals are all adjustable to direct the heavy fraction into pipes or a collecting launder. The current range of spirals available consist of a number of different profiles which have individual separation characteristics. The dimensions of some of the available spirals range from 270 406mm pitch, 590 700mm diameter and 2.1 2.4m high.

The advantages that modem spirals offer are simple construction requiring little maintenance, low capital cost and low operating cost - no reagents required, no dense media losses occur, low operating personnel required.

This is another variation of gravity separation, using density differences and centrifugal force; Figure 3.13. Originally known as Humphreys spiral (after the inventor) a wide range of devices are now available. A spiral concentrator consists of a helical conduit of semi-circular cross-section. Feed pulp of between 15 and 45 percent solids in the size range 3 mm to 75 m is introduced at the top of the spiral. As it flows downwards, the particles stratify due to the combined action of centrifugal force, the differential settling rates of the particles, and the effect of interstitial trickling through the flowing particle bed. The higher specific gravity particles are removed through the port located at the lowest point in the cross-section. Wash water added at the inner edge of the stream, flows outwardly across the concentrate band. Adjustable splitters control the width of the concentrate band removed at the ports. The grade of concentrate drawn from descending ports decreases progressively, with tailings discharged from the lower end of the spiral conduit.

Gravity concentration is a proven process for mineral beneficiation. The gravity concentration techniques are often considered where flotation practice is less efficient and operational costs are high due to extremely complicated physical, chemical and mechanical considerations. The gravity separations are simple and separate mineral particles of different specific gravity. This is carried out by their relative movements in response to gravity along with one or more forces adding resistance to motion offered by viscous media such as air or water. Particle motion in a fluid depends on specific gravity, size and shape of the moving material. The efficiency increases with coarser size to move sufficiently but becomes sensitive in presence of slimes. There are many types of gravity separators suitable for different situations. There are many devices for gravity concentration. The common methods are manual pans, jigs, pinched sluice and cones, spiral concentrator and shaking table to name a few.

Panning as a mineral or metal recovery technique was known to ancients since centuries past. Gold panning was popular and extensively practiced in California, Argentina, Australia, Brazil, Canada, South Africa and India during the nineteenth century. Panning is done by manual shaking of tray containing riverbed sand and gravels, alluvial deposits containing precious metals like gold, silver, tin, tungsten etc. The shaking of the tray separates sand, stones and fine-grained metals into different layers by differential gravity concentration (Fig. 12.28). The undesired materials are removed. This is primitive practice at low cost and generally in practice at small scale by the local tribal people.

Jigs are continuous pulsating gravity concentration devices. Jigging for concentrating minerals is based exclusively on differences in the density of the particles. The elementary jig (Fig. 12.29) is an open tank filled with water. A thick bed of coarse heavy particles (ragging) is placed on a perforated horizontal jig screen. The feed material is poured from the top. Water is pulsated up and down (the jigging action) by pneumatic or mechanical plunger. The feed moves across the jig bed. The heavier particles penetrate through the ragging and screen to settle down faster as concentrate. The concentrate is removed from the bottom of the device. Jigging action causes the lighter particles to be carried away by the cross flow supplemented by a large amount of water continuously supplied to the concentrate chamber. Jig efficiency improves with relatively coarse feed material having wide variation in specific gravity. Jigs are widely used as efficient and economic coal cleaning device.

Pinched sluice and cones is an inclined trough made of wood, aluminum, steel and fiberglass, 60-90cm long. The channel tapers from about 25cm in width at the feed end to 3cm at the discharge end. Feed consisting of 50-65% solids enters the sluice and stratifies as the particles flow through the sluice. The materials squeeze into the narrow discharge area. The piling causes the bed to dilate and allows heavy minerals to migrate and move along the bottom. The lighter particles are forced to the top. The resulting mineral strata are separated by a splitter at the discharge end (Fig. 12.30). Pinched sluices are simple and inexpensive device. It is mainly used for separation of heavy mineral sands. A large number of basic units and recirculation pumps are required for an industrial application. The system is improved by development and adoption of the Reichert cone. The complete device is comprised of several cones stacked vertically in circular frames and integrated.

Spiral concentrator is a modern high-capacity and a low-cost device. It is developed for concentration of LGOs and industrial minerals in slurry form. It works on a combination of the solid particle density and its hydrodynamic dragging properties. Spirals consist of a single or double helical conduit or sluice wrapped around a central collection column. It has a wash water channel and a series of concentrate removal ports placed at regular intervals along the spiral. Separation is achieved by stratification of material caused by a complex combined effect of centrifugal force, differential settling and heavy particle migration through the bed to the inner part of the conduit (Fig. 12.31). The most extensive application is treatment of heavy mineral beach sand consisting of monazite, ilmenite, rutile, zircon, garnet etc. It is widely used to upgrade chromite concentrate. Two or more spirals are constructed around one central column to increase the amount of material that can be processed by a single integrated unit.

Shaking table consists of a sloping deck with a rifled surface. A motor drives a small arm that shakes the table along its length, parallel to the rifle pattern. This longitudinal shaking motion drives at a slow forward stroke followed by rapid return strike. The rifles are arranged in such a manner that heavy material is trapped and conveyed parallel to the direction of the oscillation (Fig. 12.32). Water is added to the top of the table and perpendicular to the table motion. The heaviest and coarsest particles move to one end of the table. The lightest and finest particles tend to wash over the rifles and to the bottom edge. Intermediate points between these extremes provide recovery of the middling (intermediate size and density) particles.

Shaking tables find extensive use in concentrating gold. It is also used in the recovery of tin and tungsten minerals. These devices are often used downstream of other gravity concentration equipments such as spirals, Reicherts cone, jigs and centrifugal gravity concentrators for final cleaning prior to refining or sale of product.

Multi-gravity separator (MGS) is a new development in flowing film concentration expertise which utilizes combined effect of centrifugal force and shaking (Fig. 12.33). Centrifugal force enhances the gravitational force and obtains better metallurgical performance by recovering particles down to 1m in diameter. It would otherwise escape into tailing stream if other conventional wet gravity separators like jigs, spiral, table etc. are used. The principle of the system consists essentially in wrapping the horizontal concentrating surface of a conventional shaking table into a cylindrical drum and then rotates. A force, many times greater than the normal gravitational pull, is exerted by this means on particles in the film flowing across the surface. This enhances the separation process to a great extent. MGS in close circuit with lead rougher cells of graphite schist-hosted sulfide ore improves the lead concentrate metallurgy from 20 to +40% Pb. Graphitic carbon content reduces simultaneously from >10 to less than 3%. Presence of graphitic carbon interferes with the flotation of sulfide ore resulting in low metal recovery and unclean concentrate. MGS improves the metallurgical recovery and quality of concentrate for graphite carbon-bearing sulfide ore and high alumina-bearing fine iron ore. MGS technique is working successfully at Rajpura-Dariba zinc-lead plant and all iron ore plant in India by decreasing graphitic carbon and alumina respectively. MGS improves 42.9% Cr2O3 with 73.5% recovery from the magnetic tailings of Guleman-Sori beneficiation plant in Turkey.

The spiral concentrator applies differential density separation between particles to separate the valuable minerals from the gangue minerals. They have been widely utilized in coal washing plants worldwide to treat material in the particle size range 1mm150 m (other reports show that spirals are capable of treating material down to 45um). Material of such size range is too coarse to be treated using froth flotation and too fine to be treated in large diameter heavy medium cyclones (Atasoy and Spottiswood, 1995; Holland-Batt, 1995; Honaker etal., 2007; Mohanty etal., 2014; Shi etal., 2018). The major factors that makes this concentrator attractable for its application is low capital and operating cost, higher recoveries and no reagents used.

The drawback of spirals is that they are density separators that are unable to obtain a D50 lower than 1.65g/cm3 but tend to have a D50 between 1.7 and 2.1g/cm3 and misplaces a significant amount of ash fines into the clean coal. Even the application of wider diameter spirals couldnt solve this problem. The D50 was further reduced by either reducing the feed rate, but that is uneconomic or by utilizing two stage spirals in a series platform, but such a setup is still not sufficient to obtain a fine coal product of 10% ash value (Barry etal., 2015; de Korte, 2016; Shi etal., 2018; Ye etal., 2018). With the development of LC3 spiral model, lower separation cut point densities (1.41.55) were achieved (de Korte, 2016; Palmer, 2016). Palmer (2016) investigated the ash level in both the middling stream and clean coal product using LC3 spiral model and the results are shown in Fig.10. The results show that LC3 spiral model has high separation efficiency at both low and high D50.

Limited work has been done on the structural parameter of spirals and too much attention is given to the particle size distribution, feed rate, solid concentration and splitter position(Gulsoy and Kademli, 2006). Trough profile of the spirals is important factor to consider on separation performance of fine coal. Spirals come in three forms of trough profiles (ellipse, cubic parabola and synthetic curves) as illustrated in Fig.11 (Atasoy and Spottiswood, 1995; Kapur and Meloy, 1998; Kwon etal., 2017; Ye etal., 2018). It was observed that coal tends to move to the peripheral end of the spiral as the feed rate and solid concentration increases. The D50 also increases with feed rate and concentration of solids. Spiral concentrator with the elliptical profile is preferable to collect the light particles in outer place, while spiral concentrator with the trough profile of cubic parabola is effective for the accumulation of heavy particles in the inner region of the trough. The spiral concentrator with the trough profile of cubic parabola in inner place, together with the elliptical profile in outer place, is more desirable to separate the coal fines and ultrafines.

Run-of-mine (ROM) chromite (mined ore, prior to beneficiation) is usually beneficiated with relatively simple processes. The most commonly applied processes include primary and secondary crushing, screening, milling, dense media separation and gravity separation methods (Murthy etal., 2011). More sophisticated processes such as flotation can also be used (Wesseldijk etal., 1999), but are usually not economically feasible.

In order to generate beneficiated lumpy, chip and/or pebble chromite ore (the coarser fractions, typically 6150mm) crushing, screening and dense media separation would be applied. The finer fraction (typically<6mm) of ROM chromite would normally be milled to approximately<1mm and then upgraded with a series of hydrocyclones and spiral concentrators to generate metallurgical and/or chemical grade chromite concentrate (Murthy etal., 2011). Fig.5 presents a process flow diagram for the beneficiation of chromite concentrate (<1mm) adapted from Murthy etal. (2011), who reviewed chromite beneficiation. The shaking tables (slime and scavenger tables) in this diagram would probably not be used in large-scale operations and the single spiral concentrators would probably consist of numerous banks of spiral concentrators operating in parallel.

Milling is the only process step applied during chromite beneficiation that has been implicated in the possible generation of Cr(VI). However, only dry milling of chromite has been proven to generate Cr(VI) (Beukes and Guest, 2001; Glastonbury etal., 2010). Extreme grinding (i.e. pulverization), which is not a typical comminution technique, was applied in both the afore-mentioned referenced studies and it could therefore be argued that Cr(VI) is less likely to be formed by industrial dry milling. However, Beukes and Guest (2001) also report relatively high levels of Cr(VI) in samples gathered from a dry ball mill circuit at a FeCr producer. In contrast, wet milling does not seem to generate Cr(VI) (Beukes and Guest, 2001). Wet milling would also be the obvious choice during chromite concentrate beneficiation, since hydrocyclones and spiral concentrators are wet processes. Also, during chromite concentrate beneficiation the milling step would be aimed only at liberating the chromite crystals from the gangue minerals. This is in contrast to the dry milling tests conducted by Beukes and Guest (2001) and Glastonbury etal. (2010), during which Cr(VI) was generated, where the intent was to obtain particle sizes fine enough for pelletization (which will be discussed in Section 3.2.2 and 3.2.3).

Rare earth minerals are good candidates for gravity separation as they have relatively large specific gravities (47) and are typically associated with gangue material (primarily silicates) that is significantly less dense (Ferron et al., 1991). The most commonly utilized application of gravity separation is in monazite beneficiation from heavy mineral sands. Beach sand material is typically initially concentrated using a cone concentrator to produce a heavy mineral pre-concentrate (2030% heavy minerals) before a more selective gravity separation step, often employing a spiral concentrator, is used to achieve concentrations of 8090% heavy minerals (Gupta and Krishnamurthy, 1992). At this point, a series of magnetic, electrostatic and further gravity separation operations must be applied, according to each individual deposits mineralogy (Ferron et al., 1991).

An example of a flowsheet designed to concentrate monazite from Egyptian beach sands containing approximately 30wt.% valuable heavy minerals can be seen in Fig. 3 (Moustafa and Abdelfattah, 2010). In this flowsheet, low specific gravity gangue is discarded via wet gravity concentration (the authors employed a Wifley shaking table for this purpose), then low intensity magnetic separation is used to discard any ferromagnetic minerals without removing paramagnetic monazite (Moustafa and Abdelfattah, 2010). The non-magnetic stream that remains contains most of the valuable monazite, zircon and rutile as well as a portion of the gangue minerals which were not removed in the first two steps. A series of gravity, magnetic and electrostatic separations are then applied to exploit the different properties of the monazite, zircon and rutile minerals and produce the final concentrate streams. Rutile is removed as it reports to the conductor fraction after electrostatic separation (monazite and zircon are non-conductive) and then diamagnetic zircon may be removed from the paramagnetic monazite via further magnetic separation (Moustafa and Abdelfattah, 2010).

Fig. 3. Flowsheet for concentrating monazite from Egyptian beach sand. For each stream, the first and second percentages represent total weight recovery and monazite grade respectively. Reproduced with permission from (Moustafa and Abdelfattah, 2010).

In addition to the processing of beach sands, gravity separation, (shaking tables, spiral concentrators, and conical separators) is used in combination with froth flotation at many rare earth mineral processing operations throughout China (Chi et al., 2001). An example of this is at Bayan Obo, where gravity separation has been employed between the rougher and cleaner flotation circuits to efficiently separate monazite and bastnsite from the iron-bearing and silicate gangue material (Chi et al., 2001; Jiake and Xiangyong, 1984). Some challenges associated with gravity separation of the Bayan Obo ore are that gangue minerals (e.g. barite) have similar specific gravities to the desired rare earth minerals and report to the concentrate stream. In addition, gravity separation is ineffective at separating very fine particles resulting in large losses of rare earths (Ming, 1993). Some separation of very fine particles can be achieved for minerals with very large differences in specific gravity, such as gold from silicate gangue, by employing centrifugal gravity separators such as the Knelson, Falcon and Mozley Multi-Gravity Separators (Falconer, 2003; Gee et al., 2005). Most of these fine particle separators are designed for semi-continuous operation where the valuable dense material is present in low concentrations (<0.1wt.%) which may limit their suitability to REE mineral separation (Fullam and Grewal, 2001). The ongoing development of centrifugal separators capable of continuous operation (e.g. Knelson CVD) may address this issue as the manufacturers claim to be able to process feed materials with valuable heavy mineral contents of up to 50wt.% (Fullam and Grewal, 2001).

Outside of China, lab-scale gravity separations have been successfully completed on Turkish and Australian deposits with very fine-grained mineralizations (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). In both of these cases, one of the key findings was that the rare-earth minerals were concentrated into the very fine (<5m) particle size range (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). This was dealt with by either modifying the grinding steps to prevent excess fine generation or by employing a Multi-Gravity Separator, specifically designed to recover ultrafine particles via gravity separation (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). The modified grinding procedure employed an attrition scrubbing step prior to further grinding to produce a product that was 100% 300m (the size which was identified as the maximum limit for downstream flotation), while reducing the slime losses to the 5m size fraction by an average of 7.8% (Guy et al., 2000). The results from Guy et al. (2000) can be seen in Fig. 4. The importance of adequately liberating rare earth minerals without excessive fine production has also been shown by Fangji and Xinglan (2003) who employed screening and secondary grinding steps after gravity and magnetic separations at a mine in Maoniuping, China to produce a bastnsite flotation concentrate with a grade of 62% REO and a recovery of 8085%.

A final interesting application of gravity separation to rare earth mineral concentration is the use of roasting operations prior to gravity separation as outlined in a 1956 patent (Kasey, 1956). The idea presented involved roasting a rare earth carbonate ore at temperatures in excess of 1000C to convert the carbonates into oxides, thereby increasing the mineral density and susceptibility to gravity separation (Kasey, 1956). The process proposed by Kasey (1956) included an industrial application involving quenching the roasted ore particles from high temperatures; a process that would likely significantly decrease the energy required for crushing and grinding operations as detailed by Fitzgibbon (1990) in their research into thermally assisted liberation. To the best of authors knowledge, this process was never successfully applied on an industrial or pilot scale.

spiral separators - mineral processing

spiral separators - mineral processing

FRP spiral chute combines the advantages of spiral concentrator, spiral chute, shaker, and centrifugal concentrator, and is the best equipment for mining and beneficiation, especially the sand mining in the seashore, riverside, sand beach and stream. Spiral chute is used to select fine-grained iron, tin, tungsten, tantalum, niobium, gold ore, coal mine, monazite, rutile, zircon and other metal and non-metallic minerals with sufficient specific gravity difference. The sorting process is stable and easy to control, the allowable range of feed concentration is wide, the enrichment ratio is high and the recovery rate is high.

The spiral chute mainly uses the inertial centrifugal force generated by the minerals of different densities in the spiral rotation to achieve the separation of light and heavy minerals. Because of its simple equipment structure, low power consumption, and large processing capacity, it is widely used in the gravity separation process. Chute beneficiation belongs to bevel flow separation process. The slurry is given to a certain inclined chute. Under the impetus of water flow, the ore particles are loose and layered. The upper layer of light minerals is quickly discharged from the tank, and the lower layer of heavy minerals is retained in the tank or discharged from the lower part at a low speed. After that, concentrate and tailings are obtained.

The spiral chute is composed of six parts: ore feed divider, ore feed groove, spiral groove, product intercepting groove, ore receiving hopper and groove bracket.The spiral groove composed of spiral pieces is the main component. The spiral pieces are made of glass steel (glass fiber reinforced plastic) and are connected together by bolts. The glass fiber reinforced plastic spiral sheet is light and strong, and has a wear-resistant layer on the surface, which is durable.

The first end of the trough is equipped with a multi-tube ore-feeding splitter, which is evenly distributed and easy to control. The evenly divided pulp is slowly fed to the spiral groove surface by feeding minerals.The end of the spiral groove is equipped with a valve block type product intercepting groove, which divides the sorted ore flow into several products according to grade. Use the position of the adjustment valve block to change the interception width of each product.The receiving hopper is a concentric ring cylinder, which collects and exports multiple intercepted streams according to products.

The spiral chute is based on the density difference between the minerals to achieve the purpose of sorting, and at the same time, it can recover the iron minerals that have been separated by monomers at a relatively coarse particle size level (generally not affected by the type and magnetic properties of iron minerals, surface) chemical properties etc.).Therefore, it can be widely used in various combined processes of processing magnetite (including roasted magnetite), hematite and other minerals such as weak magnetic-strong magnetic-flotation. It can supplement, assist, improve and improve processes such as magnetic separation, flotation, centrifuge reselection, and fine sieve.

The main advantages of the chute are the simple structure of the equipment, the low investment and production costs, and the coarse and medium-grain chute also have a high processing capacity. The disadvantage is that the separation accuracy is low, so it is suitable for use as a rough separation equipment. It is widely used in the treatment of tungsten, tin, gold, platinum, iron and some rare metal ores, especially in the treatment of low-grade sand ores. It has no moving parts and consumes no power; The equipment covers a small area and has a large processing capacity; Because the quality score of the ore is high and no washing water is added, water is saved; The conditions required for operation (such as ore feeding granularity, quality score, etc.) are not harsh, and the selection index is relatively stable. Light weight, moisture-proof, rust-proof, corrosion-resistant and noise-free.

gold centrifugal concentrator - mineral processing

gold centrifugal concentrator - mineral processing

The centrifugal concentrator rotates at a high speed, generating a high G force, which can separate small gravity recovery gold (less than 50 microns), which was of previously unrecoverable with traditional mineral jigs, spiral separator, and gravity tables.In recent years, the application of centrifugal force has proved to be an effective technology for recovering fine and heavy minerals. The centrifugal force acting on the mineral particles can reach 50 times its own gravity, thus significantly increasing the sedimentation speed of the particles. As the intensity of the centrifugal force increases, the size of the particles that can be captured becomes finer.

The centrifugal concentrator can be used to recover the tailings of chromite, gold, scheelite and other heavy minerals, so as to make better use of mineral concentrates. Their operating and equipment costs are relatively low. Thanks to its less environmental impact and good recovery of fine-grained minerals, the application of centrifugal concentration has become increasingly important worldwide.

The gold centrifugal concentrator is a gravity-enhancing device. The feed slurry is introduced into a rotating drum, and the impeller rotates to form a high-gradient centrifugal field. The ore particles flow on the inner wall of the rotor and are continuously layered and deposited on the smooth inside the centrifuge wall.The lighter particles outside the bed are removed from the rotor assembly due to their lower specific gravity or smaller size. Heavy particles remain in the concentration zone, where the concentrate is cleaned with fluidized water.A rotor spins the bowl, which throws the feed coming into the center against the walls of the bowl. Light and fine particles are carried out of the bowl with the tailings while heavy and coarse particles are collected and removed from the bowl. Industrially they can operate as batch or continuous units, with pores opening intermittently to collect concentrate.

Related Equipments