Among harmful reducing agents may be mentioned organic matter, sulphuretted hydrogen, sulphurous acid, ferrous sulphide (all of which may be present in old accumulated slime dams), ferrous sulphate (which in alkaline solutions becomes ferrous hydroxide), soluble sulphides (rarely found in normal working solutions), and thiosulphates.

Effect of Temperature. The rate of dissolution of metals in cyanide solution increases with rise of temperature. Julian and Smart1 state that in a special series of experiments made to elucidate this point the solubility of gold increased to a maximum at 85 deg. Cent. (185 deg. F.) and then slightly decreased to the boiling point. This principle has in some instances been applied in practice, especially in the treatment of silver ores, notably in the Tonopah, Nevada, district. In most cases, however, the increased consumption of cyanide due to heating the solutions in the presence of ore more than offsets the increased extraction. From 38 deg. C. (100 deg. F.) upward the decomposition of cyanide is usually very marked, but in cold climates it is often found advantageous to maintain the temperature of the working solutions at between 15.5 and 21 deg. C. (60 to 70 deg. F.) by artificial heating.

Dissolving Effect of Zinc-potassium Cyanide. Many writers, including Feldtmann, and Julian and Smart, maintain that gold will dissolve in pure K2Zn(CN)4 in the absence of free cyanide. If this be true of gold it would probably apply also to metallic silver, but silver compounds such as argentite do not appear to be acted on in this way. A series of experiments carried out by the writer with pure silver sulphide precipitated dried and pulverized in the laboratory showed that zinc-potassium cyanide solution equivalent to 0.3% KCN had almost no dissolving effect upon it during a 24 hour agitation in a bottle open to the air, while in solutions of the double cyanide to which varied amounts of free cyanide were added the dissolution of silver was in every case closely proportional to the quantity of free cyanide present, and corresponded with the weights of silver dissolved by solutions 1 Cyaniding Gold and Silver Ores, page 92 (Second Edition).

containing similar strengths of free cyanide, but with the double cyanide absent.

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Cyanide Regeneration.-Arising out of the subject of the dissolving effect of the double cyanide is the question whether any part of this combined cyanide becomes available for dissolution purposes in the course of its circulation through the plant. Although the fact is doubted by some writers the indications certainly point to such a regeneration of free cyanide. This may take place from two causes, first, a precipitation of zinc by soluble sulphides formed during contact of the solution with the ore, K2S + K2Zn(CN), = ZnS + 4KCN, and second, a reaction of the free alkali on the double cyanide with the formation of an alkaline zincate and free cyanide, K2Zn(CN), + 4KOH K2ZnO2 + 4KCN + 2H2O. This reaction is no doubt reversible so that the dissociated cyanide would tend to re-combine with zinc if any change occurred in the solution to disturb the equilibrium. Where the alkali employed is lime instead of caustic soda or potash there may be a formation of insoluble calcium zincate which renders the reaction nonreversible. In the operation of cyanide plants many metallurgists have observed that a saving in cyanide is effected by raising the lime content from a merely "protective" amount up to 0.1 % or 0.13% CaO. When the solution from the barren sump is thrown back to the mill and comes in contact with ore and fresh lime the free cyanide indicated by titration will often be found to rise in strength 40 or 50%, and its extractive efficiency will be increased in proportion to the increase of free cyanide as shown by the free cyanide determination (method No. 2, page 22). In a laboratory experiment made by the writer a solution of zincpotassium cyanide was made up showing no free cyanide when tested by method No. 2, and having practically no dissolving effect on prepared Ag2S. An excess of lime was added to a portion of the solution, and after a few hours' agitation it was filtered clear. A titration now indicated that about half of the combined cyanide originally present had been decomposed, and was recorded as free cyanide. This solution, when tested on prepared silver sul

phide, had a dissolving efficiency equal to a freshly prepared cyanide solution of the same titration strength.

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Crosse's Process.Some years ago a process of cyanide regeneration was patented by Andrew F. Crosse based on the principle of precipitating the zinc as sulphide by addition of sodium sulphide to the solution, K2Zn(CN)4 + Na2S = ZnS + 2KCN + 2NaCN. For this purpose the solution was to be heated to about 65 degrees C. (149 deg. F.), and the reagent added until a faint permanent trace of soluble sulphide was indicated in the solution on addition of a few drops of lead acetate. This, however, did not complete the process, because a difficulty arose due to the almost invariable presence in the stock solution of sodium zincate, Na2ZnO2: this zincate was acted upon by the sodium sulphide to form caustic soda; Na2ZnO2 + Na2S + 2H2O ZnS + 4NaOH. The caustic soda, gradually building up in the stock solutions, attacked the zinc in the precipitation boxes, involving an additional zinc consumption and also causing such an evolution of hydrogen as to carry precipitate out of the boxes and send up the assay of the tails to a prohibitive extent. Crosse proposed to meet this condition as follows: when the caustic soda had accumulated to a dangerous extent in the stock solutions the use of Na2S was to be temporarily suspended and the reaction was to be effected by charging the tankful of solution with hydrogen sulphide generated from the zinc sulphide already filtered out and accumulated during the first stage of the process, causing the reaction, H2S + 2NaOH = Na2S+ 2H2O. This part of the procedure, however, has so far proved impracticable on a working scale, and the process as a whole has not found any useful application.

It seems likely that the first stage of Crosse's process might attain practical importance when aluminium is used as the precipitant, in the case of certain ores in the cyaniding of which zinc is detrimental to extraction of the silver, and yet which contain some zinc soluble in the cyanide solutions.

Several other methods of regenerating cyanide solutions have been proposed, chiefly based on the principle of precipitating 1 Journal Chem. Metall. and Min. Soc. of S. A., Mar., 1903.

the base metal contents with acids, but none of them has attained to any commercial importance.

Fouling of Solutions.-This expression is indefinite, and is usually used to convey the idea of gradual deterioration in the dissolving power of a mill solution during repeated contact with the ore, due to a progressive accumulation in the solution of deleterious substances contained in the pulp. One class of such substances, already alluded to, consists of reducing agents, organic or otherwise, that have the property of denuding the solutions of their dissolved oxygen, and the remedy is obviously oxidation in some form. In regard to the accumulation of base metals, there is a tendency for the activity of the solution to be retarded by the presence of any unnecessary substance, due to an increase in viscosity and the consequent resistance to the migration of the ions. It seems doubtful, however, whether there is any ulterior and more serious harm done by the presence of these metals. Copper is often said to diminish the dissolving activity of a cyanide solution to a very serious extent, but the writer has not seen conclusive evidence of this in solutions assaying up to 2 or 3 lb. of copper per ton. Julian and Smart1 state that "the solubility of silver is usually small when copper is present, whereas the solubility of gold is not generally affected to any thing like the same extent." The zinc derived from precipitation seems not to affect extraction to any appreciable extent in the majority of instances, but there are cases especially when treating ores containing antimony and arsenic, where the presence of zinc lowers the extraction of the silver as much as 10% and in a less degree that of the gold. When zinc is found to have this effect, lead in solution will often act similarly, and the addition of lead salts may have to be avoided. The writer has, however, recently come across several instances where the presence of zinc from zinc precipitation was distinctly detrimental to silver extraction though the addition of litharge was beneficial.

Strength of the Solution in Cyanide. When the cyanide process was first introduced a great point was made of the selective action 1 Cyaniding Gold and Silver Ores, page 113 (Second Edition).

of weak solutions, it being asserted that a weak solution had a tendency to select the gold and silver for attack in preference to the base metals. Julian and Smart's experiments,1 on the contrary, went to show that the ratio of gold to pyrite dissolved in a weak solution did not differ from the ratio of that in a strong solution. However that may be, it is a fact well known in practice that the use of a solution stronger than is necessary involves an increased chemical loss of cyanide without any corresponding gain in extraction of the precious metals. The mechanical loss in residues will obviously be greater with strong solutions than with weak, so that from every point of view it is important not to use a stronger solution than is necessary to obtain the maximum extraction that will be commercially profitable. The most suitable strength for any given ore can only be found by experiment, but, in general, silver ores require much stronger solutions than gold ores. In the treatment of slime the most usual cyanide strength for all-gold ores ranges from 0.005% to 0.05%, while for silver and silver-gold ores it may be 0.1% to 0.15 %, in terms of KCN, and in Pachuca strengths up to 0.4% KCN are frequently used. In the leaching of sand it is usual to employ a much stronger solution than in the agitation of slime, the idea being to obtain dissolution with a small volume of strong liquor in a short period of time so as to allow as long a time as possible for the subsequent washing out of the dissolved metals. For sand leaching, whether of gold or silver ores, the strength of cyanide commonly ranges from 0.2% to 0.3% KCN.

Sources of Cyanide Loss. (1) The Precious Metals.-The amount of cyanide that combines with the gold in an average ore is negligible, but with silver it is different owing to the much larger quantity of metal that has to be dealt with to produce commercial results. If the final result be summarized by the equation, Ag2S + 5KCN + 0 + H2O = 2KAg(CN)2 + KCNS + 2KOH it appears that 4 molecules of cyanide are appropriated by 2 atoms of silver, while a fifth molecule goes to form sulphocyanate, a substance which will be alluded to later. Leaving 1 Julian & Smart, p. 86 (2d ed.).