Gravity Concentration

How to Evaluate Washery Performance

Many attempts have been made during the last forty years to evaluate the performance of gravity separation equipment, that is, the effectiveness with which light and heavy particles are separated. The most comprehensive treatment of the subject was made at the 1st International Conference on Coal Preparation held in Paris (1950) by Cerchar.. The float and sink analysis of the product is presented on a Gaussian distribution curve, resulting in an easier visualization of the inherent difficulties of separation. The gives of the distribution curve are then plotted, giving a quantitative measure of the deviation from perfect separation as an error distance instead of an error area.

Heavy Media Separation (HMS) Performance Criteria

In the ideal HMS process, all material lower in density than the specific gravity of separation (S.G.S.) would be recovered as floats and all material of higher density would appear as sinks.

In order to evaluate the misplaced material, the washery products are tested at the density at which the washing unit is operated. The original type of plot is shown and was developed primarily for coal cleaning units. The curve for raw coal represents the cumulative percentages of sink material. The refuse curve is also plotted as

Separation Efficiency

The technical excellence of separation achieved in a mineral concentration process, or any other process where two constituents of any kind are physically separated from each other, is expressed uniquely and quantitatively by the Separation Efficiency:

Es = (R – Rg)

where R is the percentage of the valuable constituent and Rg the percentage of the waste appearing in the concentrate. The Separation Efficiency meets all the usual requirements of a true efficiency and is independent of the nature of the process used. It can be visualized as the percentage of the feed material that, in effect, actually undergoes complete separation while the rest of the feed is distributed randomly and unchanged into the two separation products.

Mineral Concentration Processes and their Evaluation

The overall concentration process may be subdivided into one of liberation, which severs the bonds between unlike mineral species, and another of separation, which sorts the resulting liberated particles into two products, a concentrate, rich in mineral “A”, and a tailing, lean in mineral “A”.

A perfect mineral concentration process is represented by the idealized scheme shown in Figure 1 in which liberation and separation are not differentiated. The feed mixture consists of two types of minerals which

Rheological Properties of Heavy Media Suspensions Stabilized by Polymers

Heavy media separation (HMS) is one of the most widely used gravity methods in the mineral industry. The feed is separated, according to the specific gravities, into two products; float (light) and sink (heavy) particles. The universal acceptance of the process is due to simplicity, efficiency and robustness for ease of control. However, if optimum separation is desired during the operation of heavy media plants, a delicate balance of a number of variables is essential. Among the variables, the two which are generally the most frequently checked and controlled are specific gravity and consistency of the medium.

Rheology of Heavy Media

The finely ground solids dispersed in water make up the heavy media suspension. The particles must be such a size, generally not greater than 65 mesh, so that they can be readily maintained in homogeneous distribution throughout the fluid. Heavy media suspensions of this type, volumetric concentrations greater than 25%, do not follow the same rules of fluid mechanics as simple fluids (like water or oil), therefore to understand their behavior viscosity will be defined.

When a body is moving in a fluid, it experiences a viscous drag or retarding force. The amount of viscous drag is a function of

Batac Jig for Cleaning Fine and Coarse Coal Sizes

The cleaning of coal by the use of jigs is one of the oldest methods of separating coal from its impurities. Although in many situations jigs have been replaced by other methods, by far the largest percentage of coal today is still being cleaned by some form of jigs.

Description of the Batac Jig

In the Batac, the pneumatically pulsated principles of the Baum jig are still used; however, the method of air distribution has been changed, the pulsations of the air causing the jigging action have been greatly improved by new compressed air valves electronically controlled, and the bed stratification level operated by the reject ejectors is more precisely controlled by inductive-hydraulic apparatus.

In the Baum jig, air under pressure is forced into a large air chamber on one side of the jig vessel causing pulsations and suction to the jig water, which in turn cause pulsations and suction through the screen plates upon which the raw coal is fed. The distribution of this force, being on one side of the jig, tends to cause unequal force along the width of the jig screen area and, therefore, tends to cause unequal stratification and some loss in the efficiency of separation

Copper Recovery from Reverberatory Slag

White Pine reverberatory furnace slags contained economically recoverable amounts of copper for a variety of reasons. Among them:

  1. White Pine concentrates yield extremely high matte grades (65+% Cu) and viscous slags. The viscous slags hinder matte settling causing matte “prill” entrapment in the slag.
  2. White Pine’s total refined copper output is accomplished through fire refining requiring special techniques for impurity removal, especially small amounts of nickel introduced from purchased pyrite and chalcopyrite fluxes. These special techniques result in high-copper slags, refractory to smelting, being reintroduced into the reverberatory furnace.
  3. Furnace bottom build-ups resulting in mechanical matte entrapment through the formation of “dams” hindering matte flow.

The plant flowsheet utilized a dual-density heavy media drum and jigging to recover the chunk and “BB” sized prill matte following wet screen sizing for media preparation. Coarse slag, -1″ x 4 mesh, is treated in a 3-product Wemco heavy media system yielding a coarse final concentrate, a copper-enriched middling and a reject tailing. Screen undersize, -4 mesh x 0, is cycloned at approximately 100 mesh with the +100 mesh distributed to three 24″ x 36″ Denver Duplex mineral jigs. Jig concentrates are dewatered and join the heavy media concentrates.

copper-recovery-metallurgical-balance

Process Plant Design

The radial system simplifies and improves control and supervision to a very considerable extent and can lead to a form of continuous plant operation with the separate sectors shutting down in succession for necessary maintenance while maintaining the overall plant operation.

Radial Layout for Increased Treatment-Plant Productivity

A great deal of time and trouble is spent by highly qualified and experienced metallurgists in developing the right treatment process for a given ore. This process is then often expressed in a treatment flowsheet. This flowsheet, when completed, is commonly handed over to the mechanical engineers who then design the layout of the equipment indicated on the flowsheet and produce the working drawings upon which the detailed construction of the treatment plant will be based. It is an unfortunate fact that the tedious and time-consuming juggling of equipment and inter-plant transportation is often left to hard working draughtsmen who, however capable in their field, lack practical experience in the day-to-day operation and maintenance of the treatment plants they design. As a result many plants have been produced which, while they satisfy the requirements of the flowsheet with minimum capital requirements, cost far more than should be necessary to operate and maintain.

Gravity Concentration

Gravity concentration was developed in the dark ages – long indeed before that. Agricola cites examples of gravity concentration, crude examples admittedly, being practiced over 3000 years ago. That same author devoted a whole book of his masterly treatise to Gravity Concentration. Incidentally to those authors knowledge, that was the last full book ever published solely on that subject – a fact which they are presently rectifying.

gravity-concentration-mineral

Size Range

The size range to which gravity separation can be applied is large, some separations are possible as coarse as 500 microns with the practical cut-off regarded as fine as 5 microns.

The top particle size treatable by those units in the broad classification of coarse concentrators is rarely limited by the unit, rather by the top liberation size of the ore. Gravity concentration is the only process available to the mineral processing engineer over such a vast range of particle sizes.

gravity-concentration operating range

In addition to the particle size constraints, of course, there are constraints on specific gravity differentials and liberation, both of which will be dealt with later.

Gravity Concentration – Unit Processes

Heavy medium separation

Air Pulsated Jigs

An air-pulsated jig has teen widely adopted as a main coal washer. It may be mainly because of its many technical and economic features such as large capacity, low running cost, long life, etc. However, the recent increasing demand of fuel coal requires a higher productivity and efficiency of the air-pulsated jig. To fill this requirement theoretical and experimental investigations have been conducted on model and practical jigs in Japan.

Reduced to its simplest terms, the air-pulsated jig essentially consists of two separate mechanisms: one of jigging which occurs in the jig bed; and the other of pulsating which causes the water pulsation.

However, it should be noted that the relations between these major factors are complicated. For example, the air chamber pressure acting as an exciting force is inversely affected by the water pulsation. Hence, in order to fully understand the phenomena taking place in the air-pulsated jig it is necessary to take into account of such a feed back effect.

It is well known that the separation and stratification of particles in a jig are directly related to the bed expansion (mobility), which is governed by the water pulsation. In order to understand the basic relations between these factors theoretical and experimental

Jigs & Coal Preparation

Traditionally in coal processing jigs have been used to clean the coarse coal fraction, the +3/8″ or +¼” material. Today with the advent of modern fine coal jigs in the United States, its place was moved from the coarse to the intermediate coal circuit, the 3/8″ x 100 mesh.

jig-coal-preparation-processing-cost

 

 

jig-coal-preparation-processing-cost-of-table

Recent theoretical performance and economic evaluations indicate the Batac jig may be the most economical unit for washing intermediate size coal in a metallurgical coal cleaning plant. Consol has traditionally used heavy media cyclone hydrocyclone circuits, for washing the intermediate size coal at low gravities. However, recent theoretical calculations using Batac jig performance data indicate a Batac jig circuit that incorporates a heavy media cyclone rewash of the jig’s middlings product can produce the same yield at a given quality specification. The jig also has a processing advantage over the heavy media cyclone circuit. Heavy media cyclones, like tables, are refuse sensitive and have a tendency with increases in refuse load to kick some of this refuse out with the clean coal.

jig-coal-preparation-heavy-media-cyclones

If jigs are the intermediate size coal

Reprocessing Gold Tailings by Gravity Methods

Tailings originating from the gravity concentration of a gold ore containing about 14 g/t Au have been treated by various beneficiation methods. These tailings have been screened on the 147 and 74 micron sieves and the two coarsest fractions have been separated on shaking tables with unsatisfactory results, but the concentrates have been examined in polished sections for mineragraphic analysis.

Five elements only have been assayed, with the following results:

SiO2 81,56 % Ti 1,45 % PM 14 g/t Fe 4,3 % S 0,28 %

Gold is sometimes free, in small grains of 5-6 microns, and sometimes associated with quartz pyrite, hematite and goethite, in tiny spots of about 1 micron size. Other associations include pyrite embodied in quartz, and martitisation patterns where magnetite is replaced by hematite.

A preliminary test has been carried out on a superpanner, on the tailings as such, and it has shown that the concentrates consist mainly of coarse particles containing mixed grains of gold and quartz, iron oxides and some tiny grains of gold and pyrite; however, very small grains of gold are entrained with the slimes into the residue.

The shaking table used for the two tests is a WEDAG table, that has no riffles, but grooves along the

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