The question as to whether concentration shall be included in the treatment of a given ore will often depend on the possibility or otherwise of recovering the precious metals from the concentrate at the mine. It may be suggested that if the concentrate can be cyanided after being separated from the gangue, why can it not be equally well cyanided if allowed to remain in the pulp going to the cyanide plant? The answer is that a small bulk of refractory high grade material can be treated by special methods which would not pay if applied to the whole of the ore. For instance, a few tons a day may be profitably roasted or slimed or treated with extra strong solution or for extra long periods, all of which might be prohibitive if applied to the entire ore.
There are two ways of dealing with a concentrate in a cyanide mill; one is to regrind to the degree of fineness found to be necessary by experiment and then to discharge the resulting pulp into the stream of mill pulp flowing to the cyanide plant, where the mixture goes through the usual routine of the process; and the other is to keep it apart and treat it in a small plant specially designed for the purpose. The latter has the advantages of greater flexibility and of affording an opportunity to ascertain exactly what results are being obtained.
Roasting prior to cyanidation is often the only way to get a satisfactory extraction from refractory gold concentrates, and the resulting material may be either leached, or reground and treated by agitation. It is seldom found profitable to treat raw concentrates by leaching though this was sometimes done before the development of processes for cyaniding slimed products.
It is to be noted that desulphurizing-roasting of concentrate containing silver has the effect of rendering a large proportion of that metal insoluble in cyanide, so that for silver or silver-gold concentrates some kind of raw treatment must be devised, unless there are reasons for preferring a preliminary chloridizing roast.
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.
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 combined 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.