KSSSS
CHAPTER XVII
THE RECOVERY OF CYANOGEN
The presence of hydrocyanic acid amongst the produots of the distillation of coal has been recognized for a number of years, and many and varied attempts have been made to cffcct its recovery on profitable lines. The amount of hydrocyanic acid occurring under normal conditions is, however, comparatively small, so tliat m many cases the working expenses of the process adopted have reu der ed recovery unprofitable, except in the case of the largest concerns. From a commercial pomt of view, cyanogen derives its valne from the faet that the principal metliod of treat-ing rocks and quartz for the recovery of gold is by the cyanide process, witli or with-out amalgamation on copper plates. The rationale of the process is the crushing of the gold-bearing rock in a suitable machine, the admixture of the fines with water, so as to form a sludge, and, finally, the passing of this sludge over copper plates amalgamated with mercury. The fine gold then commixes with the mercury, whilst the residue of crushed ore flows away. The waste is known as ‘‘ tailings. ’ ’ Penodic-ally that is after every three or four weeks, the copper plates are scraped, and the amalgam obtained is distilled in a retort, whence the mercury is evolved and the gold left befeind. The mercury treatment is still employed in conjunction with cyanide, although in many cases the ore is treated direct and the amalgamation process is dispensed with. To-day, the greater portion of the gold output of the world is obtained by means of the cyanide metliod. Briefly, the process consists of two essential parts :—
(a) Dissolving the gold from the ore in a cyanide solution.
(b) Precipitation of the gold from the solution.
The mineral as won from the mines is primarily subjected to several sortmgs, and is then treated in large vats with a solution of sodium, or occasionally potas-sium, cyanide. An extremely weak solution is usually employed, owing to the tact that’the greater the percentage strength of the cyanide the more easily are metals other than. gold dissolved. The period of treatment generally vanes from twelve to twenty-foür hours, and in cases where the mineral may require extended contact the cyanide solution should be renewed every three or four days. Precipitation oi the gold from the cyanide solution is usually carried out by means of zmc, the latter displacing the gold from the cyanide as follows :
2 NaAu(CN)2 + Zn = Na2Zn(CN)4 + 2 Au.