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The cyaniding of concentrate proceeds in the main upon similar lines to those of any ordinary ore and the method is worked out by a series of comparative tests in the ordinary way. In general it may be said that concentrates if they are to be treated raw need exceptionally fine grinding in order to yield a high recovery by cyanide; a long period of contact with the solution and extra cyanide strength are other factors that usually play an important part.

A word should be said here about cyanide consumption. This naturally has a tendency to be high considered in terms per ton of material because the concentrate contains most of the base mineral of the ore and also a high metal content that is soluble in cyanide. Part of this loss may sometimes be avoided and part recovered by regeneration.

One fertile source of cyanide consumption is the formation of ferrocyanide, and this may be prevented to a large extent by protecting the product from oxidation. Concentrates that are to be cyanided should be taken straight from the concentrators with the least possible removal of moisture and exposure to the air. When concentrate is exposed to the air in a porous moist condition an oxidation more or less rapid takes place with formation of ferrous and ferric sulphate and insoluble basic ferric sulphate. The first two can be removed by water washing but the latter cannot, and a plan was proposed in the early days of the Rand and has since been actually used on a commercial scale at the Goldfield Consolidated, Nevada, of giving a preliminary treatment with sulphuric acid, water washing, neutralizing and cyaniding. Removal of the iron sulphates in one or both of these ways reduces the formation of ferrocyanide to inconsiderable proportions.

Another plan which is sometimes very effective is to agitate with lime water and compressed air until all the ferrous compounds are oxidized to the ferric state. In one instance in the writer's experience a concentrate when treated direct, showed a cyanide consumption of 100 lb. per ton, 50 lb. of which was found to be combined in the form of ferrocyanide. By giving a

preliminary treatment with lime water and compressed air agitation for 48 hours, the subsequent cyanide loss was reduced to 54 lb. per ton, the resulting solution showing only traces of ferrocyanide.

The remaining chief source of cyanide loss is due to metals soluble in the solution, among which may be named copper, zinc, and the precious metals. If the copper is soluble in weak acid, it might be profitable in some cases to give a preliminary acid treatment, but if the cyanide loss cannot be reduced by this means and is prohibitive in amount, it is probable that the cyanide process is not suitable for that material, though it might be worth while to experiment on the regenerative effect or electrolytic precipitation.

When zinc is present in the concentrate as a factor in cyanide consumption it can be removed from the solution with regeneration of the cyanide by means of sodium sulphide. This would be done, of course, after the precipitation of the silver as so to produce a zinc sulphide free from precious metal.

The amount of cyanide combining with silver in a rich silver concentrate will account for a large proportion of the apparent consumption. For instance, a concentrate assaying 400 oz. of silver per ton would need 32 lb. of cyanide (100% KCN) to form the compound KAg(CN)2. In some cases the value of such an amount of cyanide would constitute the difference between profit and loss on the treatment. This can all be recovered and rendered available by using as precipitant either sodium sulphide or aluminium dust. The former has a limited sphere of usefulness since it does not precipitate gold, so that it would need to be supplemented by some additional process when gold is present. Moreover, if the silver were to be first thrown down with sodium sulphide and the gold afterward precipitated by zinc, the stock solution would become zinky with the result that when next the silver was precipitated the zinc would also be thrown down with it, yielding a base product for the clean-up.

Aluminium dust seems to offer an ideal method in such cases as it precipitates both gold and silver, regenerates all the com

bined cyanide, and yields a high grade metallic product easily put into the form of bullion.

Cyanidation of Flotation Concentrate.-At the time of writing very little work has been done along this line. There seems to be a widespread anticipation of difficulties arising due to the presence of the oil, but in the cases thus far investigated by the writer, the samples of flotation concentrate gave no evidence of being much different from concentrates recovered by mechanical methods, as regards their amenability to direct cyanidation. Some writers, however, have found considerable difficulty in cyaniding such material, and suggestions have been made as to the preliminary treatment of it with solvents for the oil such as caustic soda. There are indications that the kind of oil used in the flotation has an effect on the subsequent amenability of the concentrate to cyanidation, and there is a considerable field for investigation of the subject.

CHAPTER X

PRECIPITATION

Whatever method of precipitation be used it is of great importance that the solution should be absolutely clear and free from suspended matter, and in practice it is almost impossible to obtain solution from slime treatment, either by decantation or by means of the various types of slime filter, that is sufficiently clean to give the best results. It is therefore almost always advisable to clarify it by filtration before precipitation.

There are special presses on the market for this purpose, or a cheap and efficient filter may be made by using submerged filter leaves of the Butters type, operated either by vacuum in an open tank or by pressure in a closed receptacle. Perhaps the simplest and cheapest device is a sand filter, or better a pair of such filters for alternative use. They may be circular tanks 20 to 30 feet in diameter and 3 or 4 feet deep with a filter bottom such as is used for leaching tanks, covered with six inches of clean river sand of from 20 to 40 mesh. The suspended matter will collect in a thin film on top of the sand, and when percolation becomes too slow it is only necessary to drain the tank dry, skim off the surface with shovels, and sprinkle a little more clean sand over it, the whole operation taking only a few hours.

To ensure the maximum efficiency and minimum of trouble with these filters it is advisable to run the solution first to a settling tank of about the same size fitted with a baffle board extending right across it, and from the top downward about 18 or 20 inches; this baffle gives a tranquil surface and good settling conditions. The solution is introduced on one side of the baffle and flows out at the opposite side into the sand filter, the tank remaining all the time full of solution. Most of the suspended matter will

settle down to the bottom of this tank which is periodically decanted and the collected slime hosed out either to the agitators or slime filter. By this device a minimum of work is thrown on the clarifying filter and under ordinary circumstances it should need skimming only about once a month and may even run two months without attention.

Precipitation by Means of Zinc.-The recovery of the precious metals from the cyanide solution is almost universally accomplished by precipitation with zinc, either in the form of fine threads or of dust (the condensed fume recovered in the process of retorting the metal).

The phenomena of precipitation are essentially electrical and may be traced to the action set up by two metals of different potentials forming a galvanic couple, with the cyanide solution as electrolyte.

The chemical changes theoretically accompanying this action are expressed by Park1 in the equation.

2KAg(CN)2 + Zn =K2Zn (CN) + 2Ag

and Julian and Smart2 apparently assume the same reaction though they do not give the equation. According to this 0.166 parts by weight of zinc should be needed to replace 1 part of gold, and 0.303 parts of zinc to replace 1 part of silver. Clennell, however, prefers the reaction

3

[blocks in formation]

in accordance with which it would require 0.332 parts of zinc to 1 part of gold and 0.606 parts of zinc to 1 of silver. In favor of the former equation it may be noted that the normal practice in the Pachuca district of Mexico when using zinc dust shows a consumption of zinc of not over 1 pound per pound of bullion and often as low as 0.8 lb. of zinc, while with the Crowe Vacuum Process even lower zinc consumptions have been ob

1 The Cyanide Process, page 180 (Fifth Edition).

2 Cyaniding Gold and Silver Ores, page 139 (Second Edition).

3 The Cyanide Handbook, page 123 (Second Edition).

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