187

C.—3

In a series of experiments that I undertook I found that a solution of 0.5 per cent, gave the following results upon the same areas of gold-leaf of uniform thickness when paired, as follows : — Time required for the Gold to dissolve. Iron-pyrites, paired with gold-leaf in the cyanide ... ... & minutes. Chalcopyrites ... ... ... ... ... ... 5 „ Galena ... ... ... ... ... ... ... 20 Copper-glance ... ... ... ... ... ... 30 „ Graphite ... ... ... ... ... ... ... 36 „ Anthracite ... ... ... ... ... ... 50 „ Zinc-blende The cyanide unassisted ... ... ... ... ... 100 „ The non-conducting sulphides, such as the yellow kind of zinc-blende and stibnite, displayed no such effect when placed on the gold-leaf, the dark kind of zinc-blende, however, manifested a very feeble accelerating effect. This piece of blende I afterwards found to show conducting-power for electricity if connected by a large surface to the platinum used for making connection with the galvanometer. This blende contained sulphide of iron in small quantity. Taking next the question as to the correctness of the statement that cyanide solutions of but -03 per cent, have no effect on copper compounds generally, I made the following experiments :— 1. Ghalcopyrites, crushed, then thoroughly well washed, was afterwards kept in a cyanide solution of this strength (-03 per cent.) for one hour. The filtered solution contained a very perceptible quantity of copper as cyanide, also traces of sulphur and oxidized compounds of sulphur. 2. Copper-glance, crushed and washed, then subjected to the cyanide solution of o'o3 per cent. for one hour. The filtered solution gave the same reaction as in the case of chalcopyrites. 3. Govelline (sub-sulphide of copper). As I have already shown, this sulphide is very easily attacked by weak solutions of cyanide, sulpho-cyanide of copper resulting.* The 0-03 per cent, solution dissolves the mineral as a sulpho-cyanide and cyanide of copper. The carbonates and silicates of copper, as they naturally occur even—that is, in the dense form— are also decomposed by cyanide of this strength. These results take in all the compounds of copper that are of general occurrence at the goldmines, and they show that potassic cyanide, however weak, will decompose copper ores, generally, when in contact with them. In this connection I would like to state my results upon certain other ores that are frequently associated with gold in the reef: — Stibnite (sulphide of antimony). —This ore is generally supposed to be unaffected by potassic cyanide, but I found it to be very easily and largely affected by this salt. The sulphur of this mineral, like the sulphur in certain copper sulphides, shows great tendency to combine with potassium to form that most objectionable salt—sulphide of potassium, the rest of the sulphur combines to form sulpho-cyanogen. Galena (sulphide of lead). —This mineral is slowly attacked by cyanides, but all its sulphur combines with cyanogen to form the harmless compound, sulpho-cyanogen, and lead comes into solution combined with that radical. Zinc-blende (sulphide of zinc). —Hardly affected by cyanide of any strength, it is commonly held to be perfectly neutral thereto, but a strip of silver, buried in the crushed ore, was blackened in one hour, showing that a slight decomposition had taken place. Iron-pyrites (bi-sulphide of iron). —Almost, if not quite, unaffected by cyanide solutions of any strength. Sulphur (free). —The Government Geologist informs me that he has met with sulphur in a free state in tailings at Boatman's Creek, in the Eeefton district, as a product of the decomposition of stibnite. It is asserted by Wiggersf to be insoluble in potassic-cyanide solutions, but it is very easily soluble therein if only the air, with which it is generally charged, be driven off, say, by boiling water. I This I have shown. As in the cases of the solution of copper, this means a loss of solvent power for gold and silver. I used cold solutions throughout these experiments. The formation of alkaline sulphides during the cyanide process, as applied to ores containing stibnite and copper-sulphides, is, undoubtedly, the chief cause of the loss of gold that so often occurs in that process. With oxidized ores, as malachite or dioptase, a loss of available cyanide will necessarily occur, but with these sulphides, in addition to loss of cyanide, there will be a loss of gold, and a still greater loss of silver, in proportion to the quantities present. This loss is brought about by the sulphur —that is, the alkaline sulphide —sulphurising these metals to form sulphides with them,§ and the sulphide film so formed upon the metal prevents, or greatly retards, the proper action of the cyanide solution. That gold does combine, and very readily, with the sulphur of both the alkaline and hydrogen sulphide I have already shown.* It is to precipitate the sulphur that gets into the cyanide in the cyanide process that Mr. McArthur has proposed to use, or does use (as per patent) a soluble lead salt dissolved in the cyanide. The problem for the chemist at the cyanide works is to find a practical method whereby all the sulphur of antimonial and cupreous sulphides can be made to combine with the cyanogen rather than with the potassium of the cyanide.

♦ Vol.xxi., Trans. N.Z. Inst., 1888, "On the Preparation of Artificial Chromes." f Ann. Ch. Pharm., Vol xxxix., page 319. { Vol. iv., Trans. N.Z. Inst., page 330. § Vol. iii., Trans. N.Z. Inst., page 216.