The strength of the cyanide solutions is tested by silver nitrate, using 10 c.c. of the strong and weak solutions respectively (diluted to about 100 c.c. with water) and taking 100 c.c. of wash-water undiluted. In each case a few drops of a 5 per cent, solution of potassium iodide are added. When the titration is done very rapidly and carried to a pronounced opalescence, this method in the hands of ordinary tankmen gives results fair enough for comparison in commercial millwork.
As a careful check is kept on all the ore and solution in every stage of the treatment, the number of gold and silver assays and other determinations on control samples, headings, tailings, “ finals,” flue dust, bag dust, slimes, gold bullion, lead bullion, slags, sweepings and solutions is necessarily very large.
Strong and weak solutions are assayed for gold by evaporating 1 A.T. in a small lead tray and then scorifying and cupelling the tray; but this method does not give satisfactory amounts to weigh in the case of wash-waters. If 20 or 30 A.T. are taken for evaporation, it must be begun in glass or porcelain vessels and finished in a lead tray after having been very carefully transferred. This entails much tedious work if some 15 or 20 determinations are required per day. Two rapid methods have been devised and used at Cyanide for this determination.
In one method a reagent of zinc dust coated with lead is prepared as follows:
500 grammes of zinc dust known as “ blue powder ” are mixed with water; to this is added an aqueous solution of 25 grammes of lead acetate, and the whole thoroughly stirred. When the zinc becomes coated with lead 20 A.T. of wash-water are placed in a bottle, and to this is added about 10 grams (usually measured in a wooden mustard spoon) of the lead-zinc mud. The solution is agitated in the bottle for an hour or more, the lead-zinc mud allowed to settle, and all but 50 c.c. of the solution decanted through a filter leaving the lead-zinc in the bottle. Ten c.c. of oil of vitriol are added, and, when all action has ceased, the residue is thrown into the same filter and washed with water. When drained, the wet filter and residue are placed on a large scorifier with 15 grams of sand, burnt in the front of the muffle furnace, and then assayed by the fusion method. This gives very good results if the “ blue mud ” is in excess, and if not too much zinc is left in the residue to be fluxed off with the sand.
In the second method 20 A.T. of “ wash-water ” are placed in a large beaker, and 10 to 15 grammes of the dry “blue powder ” are shaken into it from the tip of a spatula, 50 or 75 c.c of strong commercial H2SO4 are added and the whole vigorously stirred and allowed to stand for one hour, or until all decided action has ceased. The contents of the beaker are then filtered, the residue is washed, and the wet filter is burnt on a scorifier with ½ A.T. sand and fused in a small crucible with litharge, soda carbonate, borax, and fluorspar, and the assay finished in the ordinary way. This method has been found to give very fair results with the “ wash-water ” solutions, and, besides not taking very long, allows of a large number of determinations being made at the same time.
These methods are technical methods, in which extreme rapidity is aimed at rather than extreme accuracy. However, they give fairly good comparative results.
All the methods in use at this mill have stood the test of time, and some of the modifications in the practice now employed are kept secret, a necessary consequence of the close competition of other custom mills in the district.