Gravity Concentration

Gravity concentration Introduction Gravity techniques to recover residual valuable heavy minerals in flotation tailings are being Gravity methods of separation are used to treat a increasingly used. Apart from current production, great variety of materials, ranging from heavy metal there are many large tailings dumps which could be sulphides such as galena (sp. gr. 7.5) to coal (sp. gr. excavated cheaply and processed to give high value 1.3), at particle sizes in some cases below 50 txm. concentrates using recently developed technology. These methods declined in importance in the first half of the twentieth century due to the develop- ment of the froth-flotation process, which allows Principles of gravity concentration the selective treatment of low-grade complex ores. Gravity concentration methods separate minerals of They remained, however, the main concentrating different specific gravity by their relative move- methods for iron and tungsten ores and are used ment in response to gravity and one or more extensively for treating tin ores, coal and many other forces, the latter often being the resistance to industrial minerals. motion offered by a viscous fluid, such as water In recent years, many companies have or air. re-evaluated gravity systems due to increasing It is essential for effective separation that a costs of flotation reagents, the relative simplicity marked density difference exists between the of gravity processes, and the fact that they mineral and the gangue. Some idea of the type of produce comparatively little environmental pollu- separation possible can be gained from the concen- tion. Modern gravity techniques have proved effi- tration criterion cient for concentration of minerals having particle D h - Df sizes in the 50txm range and, when coupled with (10.1) improved pumping technology and instrumenta- D 1 - Df tion, have been incorporated in high-capacity plants where D h is the specific gravity of the heavy (Holland-Batt, 1998). In many cases a high propor- mineral, D I is the specific gravity of the light tion of the mineral in an orebody can at least be mineral, and Df is the specific gravity of the fluid pre-concentrated effectively by cheap and ecolog- medium. ically acceptable gravity systems; the amount of In very general terms, when the quotient is reagents and fuel used can be cut significantly greater than 2.5, whether positive or negative, then when the more expensive methods are restricted gravity separation is relatively easy, the efficiency to the processing of gravity concentrate. Gravity of separation decreasing as the value of the quotient separation of minerals at coarser sizes as soon decreases. as liberation is achieved can also have signifi- The motion of a particle in a fluid is depen- cant advantages for later treatment stages due to dent not only on its specific gravity, but also on decreased surface area, more efficient dewatering, its size (Chapter 9); large particles will be affected and the absence of adhering chemicals which could more than smaller ones. The efficiency of gravity interfere with further processing. processes therefore increases with particle size, and 226 Wills' Mineral Processing Technology the particles should be sufficiently coarse to move are used to prepare the feed it may be preferable to in accordance with Newton's law (Equation 9.6). de-slime at this stage, since the high shear forces Particles which are so small that their movement is produced in hydrocyclones tend to cause degrada- dominated mainly by surface friction respond rela- tion of friable minerals. tively poorly to commercial high-capacity gravity The feed to jigs, cones, and spirals should, methods. In practice, close size control of feeds if possible, be screened before separation takes to gravity processes is required in order to reduce place, each fraction being treated separately. In the size effect and make the relative motion of the most cases, however, removal of the oversize particles specific gravity-dependent. by screening, in conjunction with de-sliming, is adequate. Processes which utilise flowing-film Gravity separators separation, such as shaking tables and tilting frames, should always be preceded by good Many different machines have been designed and hydraulic classification in multi-spigot hydrosizers. built in the past to effect separation of minerals by Although most slurry transportation is achieved gravity, and they are comprehensively reviewed by by centrifugal pumps and pipelines, as much Burt (1985). Many gravity devices have become as possible should be made of natural gravity obsolete, and only equipment that is used in modern flow; many old gravity concentrators were built mills will be described in this chapter. Contact on hillsides to achieve this. Reduction of slurry details for manufacturers of gravity concentrators pumping to a minimum not only reduces energy can be found in the annual Buyer's Guide of prod- consumption, but also reduces slimes production ucts and services published by Mining Magazine in the circuit. To minimise degradation of friable in December each year. minerals, slurry pumping velocities should be as A classification of the more commonly used low as possible, consistent with maintaining the gravity separators on the basis of feed size range is solids in suspension. shown in Figure 1.8. One of the most important aspects of gravity The dense medium separation (DMS) process circuit operations is correct water balance within is widely used to preconcentrate crushed material the plant. Almost all gravity concentrators have prior to grinding and will be considered separately an optimum feed pulp-density, and relatively little in the next chapter. deviation from this density causes a rapid decline in It is essential for the efficient operation of efficiency. Accurate pulp-density control is there- all gravity separators that the feed is care- fully prepared. Grinding is particularly important fore essential, and this is most important on the in adequate liberation; successive regrinding of raw feed. Automatic density control should be used middlings is required in most operations. Primary where possible, and the best way of achieving this is grinding should be performed where possible in by the use of nucleonic density gauges (Chapter 3) open-circuit rod mills, but if fine grinding is controlling the water addition to the new feed. required, closed-circuit ball milling should be used, Although such instrumentation is expensive, it is preferably with screens closing the circuits rather usually economic in the long term. Control of pulp than hydrocyclones in order to reduce selective density within the circuit can be made by the use of overgrinding of heavy friable valuable minerals. settling cones preceding the gravity device. These Gravity separators are extremely sensitive to thicken the pulp, but the overflow often contains the presence of slimes (ultra-fine particles), which solids, and should be directed to a central large increase the viscosity of the slurry and hence reduce sump or thickener. For substantial increase in pulp the sharpness of separation, and obscure visual density, hydrocyclones or thickeners may be used. cut-points. It is common practice in most gravity The latter are the more expensive, but produce less concentrators to remove particles less than about particle degradation and also provide substantial 10 txm from the feed, and divert this fraction to the surge capacity. It is usually necessary to recycle tailings, and this can account for considerable loss water in most plants, so adequate thickener or of values. De-sliming is often achieved by the use cyclone capacity should be provided, and slimes of hydrocyclones, although if hydraulic classifiers build-up in the recycled water must be minimised. Gravity concentration 227 If the ore contains an appreciable amount of dilate the bed of material being treated and to sulphide minerals then, if the primary grind is finer control the dilation so that the heavier, smaller than about 300p~m, these should be removed by particles penetrate the interstices of the bed and froth flotation prior to gravity concentration, as the larger high specific gravity particles fall under they reduce the performance of spirals, tables, etc. a condition probably similar to hindered settling If the primary grind is too coarse for effective (Lyman, 1992). sulphide flotation, then the gravity concentrate must On the pulsion stroke the bed is normally lifted be reground prior to removal of the sulphides. The as a mass, then as the velocity decreases it tends sulphide flotation tailing is then usually cleaned by to dilate, the bottom particles falling first until further gravity concentration. the whole bed is loosened. On the suction stroke The final gravity concentrate often needs it then closes slowly again and this is repeated cleaning by magnetic separation, leaching, or some at every stroke, the frequency usually varying other method, in order to remove other mineral between 55 and 330cmin -1. Fine particles tend to contaminants. For instance, at the South Crofty pass through the interstices after the large ones have tin mine in Cornwall, the gravity concentrate become immobile. The motion can be obtained was subjected to cleaning by magnetic separa- either by using a fixed sieve jig, and pulsating the tors, which removed wolframite from the cassiterite water, or by employing a moving sieve, as in the product. simple hand-jig (Figure 10.1). The design and optimisation of gravity circuits is discussed by Wells (1991). Jigs Jigging is one of the oldest methods of gravity concentration, yet the basic principles are only now beginning to be understood. A mathematical model developed by Jonkers et al. (1998) shows considerable promise in predicting jig performance on a size by density basis. The jig is normally used to concentrate relatively Figure 10.1 Hand jig coarse material and, if the feed is fairly closed sized (e.g. 3-10mm), it is not difficult to achieve good separation of a fairly narrow specific gravity range in minerals in the feed (e.g.

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