Question

Define Double Refractory Ores as associated with gold deposits and how they

affect the mineral recovery in extractive metallurgy.

ii. Briefly explain Pregrobbing and propose three ways of reducing pregrobbing in

gold metal recovery.

b.

i. State and explain briefly five (5) types of mills and describe their grindings

3.

ii. Describe the motion of a charge in a tumbling mill

Answer 1

Define Double Refractory Ores as associated with gold deposits and how they affect the mineral recovery in extractive metallurgy.

Ans:A double refractory gold ore usually contains both sulfide minerals and carbonaceous matter and results in very low gold extraction. The micro-sized gold particles are enclosed inside the mineral lattice and cannot be extracted even with very fine grinding. The carbonaceous matter preg-robs gold leading to loss of gold values to the residue. Thus, pre-treatment is required to decompose the sulfide minerals and carbonaceous matter prior to cyanidation. The conventional process involves roasting the ores at 450°C - 700°C or to pressure oxidize the ores in an autoclave at 180°C - 200°C. To avoid the elevated temperature operation, the microbial pre-treatment process was used in this investigation. Three kinds of micro species were used in this study. The chemolithotrophic bacteria (Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans) were used to decompose the sulfide minerals and expose the gold particles to the lixiviants. The actinomycete, Streptomyses setonii, decomposed the carbonaceous matter and eliminated the preg-robbing problem. The white rot fungus, Trametes versicolor, indirectly decomposed the sulfide minerals and passivated the carbonaceous matter. The pre-treatment could be conducted in two stages with the combination of the micro species or with one micro species. Three different ore samples were used for the test works and the results demonstrated that over 85% - 94% of the gold could be extracted by cyanidation after microbial pre-treatment.

ii. Briefly explain Pregrobbing and propose three ways of reducing pregrobbing in gold metal recovery.

Ans:

Preg-robbing is the phenomenon whereby the gold cyanide complex, Au(CN)2−, is removed from solution by the constituents of the ore. The preg-robbing components may be the carbonaceous matter present in the ore, such as wood chips, organic carbon, or other impurities, such as elemental carbon.

The various techniques are described below

1. Flotation and Depression

This method has been employed successfully where small amounts of gold are associated with the carbonaceous matter in the ore. In such circumstances, the carbonaceous matter can be floated off and discarded. The remaining ore is then processed using conventional cyanidation techniques. This technique, however, does not work for ores in which considerable quantities of gold are associated with the carbonaceous component. J. Orlich, J. Fuestenau, & D. Horne, Column Flotation of Carbon at the Royal Mt. King Mine, SME Annual Meeting, Phoenix, Ariz., February 1992.

2.Blanking

Blanking agents are used to passivate the surfaces of activated carbon in carbonaceous ores. The blanking agents work by selectively adsorbing on the surface of the activated carbon preferentially to the gold-lixiviant complexes in solution. Kerosene, fuel oil, and RV-2 (para nitro benzol azo salicylic acid) have been used as blanking agents. This method is not applicable where considerable quantities of gold are associated with the carbonaceous matter. And as explained in U.S. Pat. No. 3,574,600, blanking is also not applicable to ores that contain significant quantities of organic acids as carbonaceous matter. One of the objects of the present invention is to permit the processing of carbonaceous ores regardless of native carbon content and regardless of the amount of gold originally associated with the carbonaceous matter.

3. Activated Carbon or Resin In Leach or Pulp

Activated carbon or resin can be added to leach solutions to preferentially adsorb aurocyanide. This process rests on the principle of using a stronger aurocyanide adsorbent than the carbonaceous matter in the ore. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991. However, this process is not effective when the ore contains large amounts of carbonaceous matter, because native carbonaceous matter has the ability to adsorb gold cyanide complex four times faster than activated carbon. B. J. Scheiner, Relation of Mineralogy to Treatment Methods for Carbonaceous Gold Ores, Society of Mining Engineers, 87-96, pp 1-6, 1987. Furthermore, CIL processes use relatively large carbon particles, whereas the ore is fine ground, so that the added carbon and its adsorbed gold values may readily be separated from the ore after cyanidation by size.

Q.i. State and explain briefly five (5) types of mills and describe their grindings

Ans:

Following are types of mills:

1.Roller mills are mills that use cylindrical rollers, either in opposing pairs or against flat plates, to crush or grind various materials, such as grain, ore, gravel, plastic, and others. Roller grain mills are an alternative to traditional millstone arrangements in gristmills. Roller mills for rock complement other types of mills, such as ball mills and hammermills, in such industries as the mining and processing of ore and construction aggregate; cement milling; and recycling.

2. horse mill is a mill, sometimes used in conjunction with a watermill or windmill, that uses a horse engine as the power source. Any milling process can be powered in this way, but the most frequent use of animal power in horse mills was for grinding grain and pumping water. Other animal engines for powering mills are powered by dogs, donkeys, oxen or camels. Treadwheels are engines powered by humans.

3. treadwheel, or treadmill, is a form of engine typically powered by humans. It may resemble a water wheel in appearance, and can be worked either by a human treading paddles set into its circumference (treadmill), or by a human or animal standing inside it (treadwheel). These devices are no longer used for power or punishment, and the term "treadmill" has come to mean an exercise machine for running or walking in place.

4.arrastra (or arastra) is a primitive mill for grinding and pulverizing (typically) gold or silver ore. Its simplest form is two or more flat-bottomed drag stones placed in a circular pit paved with flat stones, and connected to a center post by a long arm. With a horse, mule or human providing power at the other end of the arm, the stones were dragged slowly around in a circle, crushing the ore. Some arrastras were powered by a water wheel; a few were powered by steam or gasoline engines, and even electricity.

For gold ore, the gold was typically recovered by amalgamation with quicksilver. The miner would add clean mercury to the ground ore, continue grinding, rinse out the fines, then add more ore and repeat the process. At cleanup, the gold amalgam was carefully recovered from the low places and crevices in the arrastra floor. The amalgam was then heated in a distillation retort to recover the gold, and the mercury was saved for reuse.

5.stamp mill consists of a set of heavy steel (iron-shod wood in some cases) stamps, loosely held vertically in a frame, in which the stamps can slide up and down. They are lifted by cams on a horizontal rotating shaft. As the cam moves from under the stamp, the stamp falls onto the ore below, crushing the rock, and the lifting process is repeated at the next pass of the cam.

Each one frame and stamp set is sometimes called a "battery" or, confusingly, a "stamp" and mills are sometimes categorised by how many stamps they have, i.e. a "10 stamp mill" has 10 sets. They usually are arranged linearly, but when a mill is enlarged, a new line of them may be constructed rather than extending the line. Abandoned mill sites (as documented by industrial archaeologists) will usually have linear rows of foundation sets as their most prominent visible feature as the overall apparatus can exceed 20 feet in height, requiring large foundations. Stamps are usually arranged in sets of five.

Some ore processing applications used large quantities of water so some stamp mills are located near natural or artificial bodies of water. For example, the Redridge Steel Dam was built to supply stamp mills with process water.

3

ii. Describe the motion of a charge in a tumbling mill.

Ans:

A tumbling mill is a machine for reducing the particle size of a solid, with a drum which turns to lift and drop the feed mixed with a grinding medium.

The extra centrifugal effect enhances the normal grinding effects of a tumbling mill.

The small balls which are moving under the influence of gravitational forces in a tumbling mill can no longer provide sufficient impact force for particle fracture.

A tumbling mill is a machine for reducing the particle size of a solid, with a drum which turns to lift and drop the feed mixed with a grinding medium.