3 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. 2. 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. 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. A 3. Mechanical Losses.-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 de-watering 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 14 to 1⁄2 lb. per ton of ore, and has even been known to run as high as 11⁄2 lb. 4. Decomposition Loss.-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. Oxida tion by atmospheric air forms cyanate, KCNO, but Clennell states1 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 12 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: 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. 2 Recently some interesting experiments have been made on the Rand by H. M. Leslie, 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 Mr. Leslie was able to detect only 1 The Cyanide Handbook, page 109 (Second Edition). 2 Journal Chem. Met. and Min. Soc. of S. A., Vol. 16, page 36. 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.. Loss as HCN gas.. Loss as NH, gas and N compounds.. 25 per cent. 25 per cent. 20 per cent. 5. 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 Smart1 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,2 Cu2+ 8KCN + H2O + 0 = 6KCN. Cu2(CN)2 + 2KOH, with a consumption of four parts of cyanide to one of copper. For copper carbonate the same writer gives, 2CuCO, +7KCN + 2KOH = Cu2(CN)2. 4KCN + KCNO + 2K2CO2 + 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. 3 6. 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. 1 Cyaniding Gold and Silver Ores, page 112 (Second Edition). 2 The Cyanide Handbook, page 80 (Second Edition). 3 Ibid., page 112. Pyrite and marcasite also dissolve to some extent, with formation = 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.) 7. 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 KS + KCN+0 + H2O = 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 Caldecott1 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. The Use of Lead Salts. In the treatment of silver ores the addition of a lead compound is often a material aid to extraction. This is commonly explained by the reaction K2S + PbO + H2O = PbS+ 2KOH but since K2S is rarely or never found in working solutions Clennell2 suggests that the lead is rather to be considered as an aid in the attack on the silver, Ag2S+PbO + 4KCN = 2KAg (CN)2 + PbS+ K2O. Results obtained by various metallurgists in actual practice go to show that the efficacy of any specific lead compound is usually proportional to the lead content, and that therefore litharge is more efficient than lead acetate, weight for weight. In regard to the amount of the lead compound used a critical point is usually found where the benefit is at a maximum and beyond which it gradually declines to zero and may even become a minus quantity. Premature Precipitation.-It sometimes happens that gold and silver which have passed into solution are precipitated while still in contact with the ore and pass out of the plant with the residue. This may result in a serious lowering of recovery and 1 Mining and Scientific Press, May 2, 1908. 2 The Cyanide Handbook, page 118 (Second Edition). |