There is probably no potassium cyanide now made for commercial purposes. With the development of the cyanide process the custom grew up, for economic reasons, of manufacturing what was called a “double salt” composed of potassium and sodium cyanides in varying proportions. This material was compounded so as to have a cyanogen content of 98% to 99% in terms of KCN. On account of the fact, however, that sodium cyanide contains so much more cyanogen per unit of weight than the potassium salt, the mixture could contain a considerable percentage of impurities and still show the standard strength when titrated in terms of KCN, in fact it is probable that the strength was adjusted to the desired point by “diluting” the product with some presumably inert substance such as sodium carbonate or sodium chloride. It is this product that is here referred to as commercial potassium cyanide. Sodium cyanide on the other hand does not lend itself to adulteration in this way and a material that gives a high titration in cyanogen content may be taken at its face value as regards composition. The sodium cyanide now usually put on the market will test up to about 97% to 98% in NaCN (51.4% to 52% in cyanogen content out of a possible maximum of 53%) and is therefore a very high grade product.
Which cyanide types is best at gold leaching – dissolution
The question then arises, is the relative efficiency of the two kinds of cyanide strictly proportional to their cyanogen content per unit of weight? This is usually assumed to be the case in practice and no doubt it is so in many instances. Clennell states that he has always found that “solutions containing equal amounts of cyanogen give identical results, whatever the nature of the metal with which it is combined.” This, however, is not the point. The real question is, do the two kinds of dry cyanide salts when added to a mixture of water and ground ore undergoing extraction treatment produce a solution with a cyanogen content proportional to that shown by titration to be contained in the dry salts respectively? And it would seem that in some cases at least this question must be answered in the negative.
At the mill of the Creston Colorada Company we made a comparison between the use of the double salt (98 to 99% in terms of KCN) and sodium cyanide (130% in terms of KCN). Each kind of cyanide was used exclusively every alternate month, the test extending over eight or nine months. The result showed that the weight of cyanide charged out of the store per ton of ore was the same, pound for pound, regardless of which kind was used, the cyanide strength of the stock solutions being maintained uniformly at 0.1% free cyanide in terms of KCN during the whole test. This meant that a pound of sodium cyanide, paid for at the rate of 130% KCN, did no more work than a pound of the double salt paid for at the 99% rate.
Again, at the Butters Divisadero Company’s mill we made a careful survey of the figures for the previous seven years. During all this time the treatment conditions had been uniform and also the character and grade of ore, so far as could be observed. Sodium cyanide (130% KCN) was in use. “Cyanide” (99% KCN) was in use again. The solution strength maintained throughout was 0.1% free cyanide in terms of KCN by the usual titration methods. The number of pounds of each kind of cyanide charged out of the store was then worked out per ton of ore over the several periods and also the combined extraction in gold and silver. The resulting figures were,
The result of this comparison is even more remarkable than that at the Creston Colorada since not only was the number of pounds of 99% “cyanide” consumed per ton less than that of sodium cyanide (130%) but the extraction obtained during the use of the 99% cyanide was over 3% higher.
The writer has no explanation of this phenomenon to offer, and it seems all the more difficult to account for since there is little doubt that the “ Cyanide” (99%) in question was the double salt and contained a considerable proportion of sodium cyanide as well as some presumably inert substance such as sodium chloride (though as a matter of fact no analyses are available). Whatever be the reason the conclusion would appear to be that at least in the case of some ores the “Cyanide” (double salt, 99% KCN) is more economical in use than sodium cyanide (130% KCN) and that it would be worthwhile investigating the question in each individual case.
In connection with the apparent increase in extraction at Divisadero due to the use of the double salt the question suggests itself whether the presence of a certain proportion of the potassium base may have the effect of making the solution more active for dissolving the precious metals. R. C. Kline states that he has noticed a tendency to an increased extraction when potassium salts are present in the solution.