Below is a list of things/factor which can cause cyanide loses in the leach process:
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 + O + H20 = 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 the latter out of account for the present, the quantity of cyanide tied up in combination with silver amounts to about 0.08 lb. KCN or 0.06 lb. NaCN per troy oz., and 1.2 grm. KCN or 0.9 grm. NaCN per gram of silver, so that the apparent loss of cyanide from this cause may be considerable.
Loss in Zinc Precipitation
In precipitating the silver from the silver-potassium-cyanide combination by means of zinc, not only is the combined cyanide not liberated, but there is a further consumption involved. The reaction in the zinc boxes is rather obscure, but tests for free cyanide made on entering and leaving almost invariably show a marked loss of strength, amounting sometimes to as much as 0.03% or 0.6 lb. per ton of solution. A part of this combined cyanide is rendered available later on when the solution comes in contact with fresh ore and lime in the mill, but part of it, varying in amount according to circumstances, may be classed as a permanent loss.
For reactions in zinc precipitation see Chapter X.
Omitting losses of solution by leakage and runovers, there is always a loss of cyanide in the residue, both in the percolation and agitation processes, because it is not possible to give sufficient water wash at the finish to displace all the solution. The amount of such mechanical loss varies within wide limits and will depend upon the strength of the treatment solution, the degree of moisture discharged in the residues, and the method used for the final dewatering of the slime. For instance, in the case of slime a filter provides a nearer approach to displacement of the solution by water than dilution of the pulp followed by settlement and decantation, and therefore employs the water wash to better advantage.
The loss of cyanide in sand residue is usually inconsiderable, but in the treatment of slime it may often be from ¼ to ½ lb. per ton of ore, and has even been known to run as high as 1½ lb.
In the absence of protective alkali, acids present in the ore decompose the cyanide with formation of hydrocyanic acid. Heating of solutions tends to the formation of formates, HCO2R and possibly acetates, CH3CO2R. Oxidation by atmospheric air forms cyanate, KCNO, but Clennell states that this reaction is slow in the case of solutions and that air or oxygen may be injected into a cyanide solution for a long time without any appreciable oxidation of cyanide taking place. On the other hand the writer in making some comparative tests between mechanical and Pachuca agitation found that the air agitation increased the consumption of cyanide by ½ to 1 lb. per ton of ore when working with solutions of 0.2% KCN. This, however, may have been due rather to the oxidizing action of the air on the finely divided particles of an ore rather sensitive to oxidation. When insufficient free alkali is present carbonic acid gas (CO2) reacts with the cyanide thus:
KCN + CO2 + H2O = HCN + KHCO3
The bicarbonates, KHCO3 are no protection against CO2 and it is for this reason that in the titration for protective alkali phenolphthalein is preferred to methyl orange since the bicarbonates are alkaline to the latter but acid to phenolphthalein.
There is also, especially in weak solution, a tendency to a decomposition known as hydrolysis:
KCN + H2O = KOH + HCN
This is, however, a reversible reaction and in the presence of a sufficient excess of caustic alkali the tendency is minimized to a large extent.
Recently some interesting experiments have been made, and the loss of cyanide in the form of HCN demonstrated by covering the tops of the tanks, drawing the gas off by suction, and passing it into a solution of caustic soda, thus recovering a large proportion of the cyanide lost in treatment. It is to be noted, however, that in these instances the protective alkalinity was low, that is, in the neighborhood of 0.01 % CaO whereas when tried at a mill where the alkalinity was higher, that is, 0.025% CaO and was able to detect only traces of HCN gas being given off. H. A. White in criticizing the above mentioned article is inclined to attribute the loss of HCN rather to the action of the carbonic acid in the atmosphere and sees further losses by hydrolytic decompositions resulting in the formation of ammonium compounds such as ammonium formate and carbonate. Mr. White in an estimate of the sources of cyanide loss on the Rand, where the total consumption is about 0.4 lb KCN per ton of ore gives the following tabulation
Loss by dilution (i.e., mechanical loss in residue)……………30 per cent.
Loss by cyanicides……………………………………………………..25 per cent.
Loss as HCN gas………………………………………………………..25 per cent.
Loss as NH3 gas and N compounds………………………………20 per cent.
Losses Due to Base Metals
All base metals in the ore that are soluble in cyanide solution of course involve consumption of cyanide, but the most common instance is probably copper. For the dissolution of metallic copper Julian and Smart give the following equation: 2Cu + 4KCN + 2H2O = K2Cu2(CN)4 + 2KOH + H2, showing two parts by weight of cyanide consumed by one part of copper.
Clennell, however, gives an equation in which oxygen has a part and free H is not evolved,
Cu2 + 8KCN + H2O + O = 6KCN. Cu2(CN)2 + 2KOH,
with a consumption of four parts of cyanide to one of copper. For copper carbonate the same writer gives, 2CuCO3 + 7KCN + 2KOH = CU2(CN)2 . 4KCN + KCNO + 2K2CO3 + H2O, showing a consumption of three and a half parts of cyanide to one of copper. The sulphides of copper are less readily acted on but these also will gradually go into solution, with the formation, probably, of cupric thiocyanate.
Loss in the Form of Ferrocyanide
Finely divided metallic iron, such as may result from crushing and grinding operations, is slowly acted on by cyanide, as expressed in the equation
Fe + 6KCN + 2H2O = K4Fe(CN)6 + 2KOH + H2.
Pyrite and marcasite also dissolve to some extent, with formation of ferrocyanide. Ferrous hydroxide is acted on rapidly,
Fe(OH)2 + 2KCN = Fe(CN)2 + 2KOH, and Fe(CN)2 + 4KCN = K4Fe(CN)6.
Insoluble basic ferric sulphate, 2Fe2(SO4)3.Fe2O3, commonly present in partly oxidized pyrite, is another source of formation of ferrocyanide. When ferrocyanides are found in any considerable amount in the solution after treatment of an ore it is often possible to effect a large saving in cyanide by attacking the trouble at its source. In ordinary milling practice metallic iron is not an important consumer of cyanide, as witnessed by the fact that in many mills only traces of ferrocyanide may be found in the cyanide solution, but should any considerable loss of cyanide be suspected from this cause, the remedy would lie in reducing as much as possible the iron surfaces used for grinding, more particularly by the use of flint pebbles in place of iron balls for regrinding and silex or El Oro liners instead of iron or steel. The effect of iron may often be seen to a very marked degree in experimental work. When grinding samples to a slime or to pass a fine mesh sieve on a small scale, the ore is often put into a small tube mill and ground until the whole of it will pass the desired mesh screen with the result that the sample is seriously overground, not only resulting in a product much finer, on the whole, than that aimed at, but containing an amount of infinitesimally fine iron far in excess of that which would be obtained in practice. If such a sample is weighed up wet and at once transferred to cyanide treatment, the cyanide consumption due to formation of ferrocyanide may run into several pounds per ton of ore. Precautions to minimize the formation of ferrocyanide from other sources are dealt with under the heading of “Cyaniding of Concentrate. ” (Chap. IX.)
Loss in the Form of Sulphocyanate
In the treatment of silver ores this compound is almost invariably present, as seen from the equations,
Ag2S + 4KCN = 2KAg(CN)2 + K2S
K2S + KCN + O + H2O = KCNS + 2KOH
where, although five molecules of KCN are present, only four are used to combine with the silver, the fifth being consumed by combination with sulphur. It might be thought that this fifth molecule of cyanide could be kept out of combination by the addition of a lead salt, and consequent removal of the sulphur as lead sulphide, but Caldecott has found that PbS is acted on by cyanide with formation of KCNS, which probably accounts for the fact that the amount of KCNS usually present in a working solution does not seem to vary greatly whether lead be in use or not. Thus, when a lead compound is not used, one-fifth of the cyanide consumed usefully in dissolving silver sulphide is lost in this form. When lead is added during treatment there is a tendency for this loss to be less than one-fifth. Sulphocyanate is also formed by decomposition products of the base sulphides in the ore.