Table of Contents
The cyanide is usually dissolved in a little water before being added to the stock solution, as the amount of KCy present is more easily determined in a strong solution than in any other form. A special dissolving vat of small size is often provided and is placed at a higher level than the large vats used for the storage of the stock solution. High grade cyanide is now largely used containing 98 to 99 per cent, of KCy. Sodium cyanide is also used, and this, calculated as potassium cyanide, contains 125 to 130 per cent, of available KCy. The contents of these boxes are broken into lumps and placed in wire-gauze trays, which are immersed in water contained in the stock vat. The soluble salts are soon dissolved, potassium cyanide in particular being very rapidly removed, and the tray is then lifted out, still containing the insoluble material, which is thrown away. The strength of the concentrated solution, which is kept to raise the stock solution to its normal strength, may be from 10 to 25 per cent. It should not be prepared until it is wanted, as it undergoes somewhat rapid decomposition.
Strength of the Leach Solution
The strong solution usually contains from 0.25 to 0.35 per cent, of cyanide and the weak solution from 0.05 to 0.15. A strength of 0.5 per cent, is, however, necessary in the treatment of some ores, and, on the other hand, it is sometimes better to use no solutions stronger than 0.1 per cent, or even more dilute.
Much stronger solutions than these were used in the early days of the process, but were found to be unnecessary. A series of experiments on a working scale to test the dissolving powers of solutions of different strengths. Five similar vats were charged with 34.03 tons each of fresh slightly-pyritic tailings of uniform composition from the Jumpers battery, containing 100 grains of gold per ton. The treatment that each vat received was exactly the same except that the strength of solution employed was different in every case. To each charge, 15.31 tons of solution were added, and circulated for three days, after which the solution was drained off and displaced by 8.4 tons of wash-water. When the charges had drained dry, they were sampled, the result being as follows:
The stronger solutions may have dissolved the gold much more rapidly than the weaker ones did although the final results were the same. The conditions under which the experiments were carried out prevented this point from being determined. Since, however, it takes two or three days in practice to wash out soluble salts from Transvaal tailings, there is little or no real advantage to be gained by dissolving the gold quickly. Crosse also found that almost equally good results were obtained by treating rich Bonanza tailings with solutions containing from 0.1 to 0.5 per cent. KCy, the time of treatment being twenty-four hours.
Testing Cyanide Solutions Strenght
The ordinary method of estimating the amount of potassium cyanide present in a liquid is by titration with a standard solution of silver nitrate. Silver cyanide is formed, and re-dissolves in the excess of potassium cyanide until one-half of the latter has been decomposed. The equations are as follows:
AgNO3 + KCy = AgCy + KNO3
AgCy + KCy = KAgCy2
When one-half of the KCy present has been converted to AgCy, an additional drop of AgNO3 solution causes the formation of a permanent white precipitate of AgCy. The amount of silver solution added is then read off, and the percentage of cyanide calculated. The equation of the end reaction is:
KAgCy2 + AgNO3 = 2 AgCy + KNO3
A few drops of a solution of potassium iodide are often added, in accordance with a suggestion made by J. S. MacArthur, to make the end reaction sharper, and to prevent inaccuracy through the presence of ammonia or other substances in which silver chloride is soluble.
The presence of soluble sulphides prevents the test from giving any result, owing to the formation of brown sulphide of silver on the addition of the first drop of silver nitrate. In this case, the sulphides are precipitated by shaking with freshly precipitated carbonate of lead, before titrating.