GOLD FROM ORE

Manawatu Standard, Volume LIII, Issue 51, 27 January 1933, Page 12

GOLD FROM ORE

NEW ZEALAND’S LEADERSHIP

HISTORY OF ENTERPRISE.

New Zealand was' the first country in the world to adopt the cyanide process in the treatment of gold, and the first working plant was established at Karangahake in 1889. Paying tribute to the pioneering spirit of scientific enterprise which marked the development of the mining industry of this country in the early days, Mr W. Donovan, M.Sc., F.1.C., Dominion Analyst, revealed this fact at Palmerston North last evening when, as chairman, lie delivered a most interesting address at the annual meeting of the New Zealand section of the Institute of Chemistry of Great Britain and lieland. He dealt with the results which had attended the application of chemistry to the industry, of technical mistakes made in the past, of near success, and then success. RECOVERY OF GOLD.

Gold in sand and gravel may be separated by simple washing, as in alluvial mining, and little scientific aid is required from chemistry,” stated Mr Donovan. “Gold disseminated in quartz reefs or masses of rock, frequently in association with other metallic minerals, requires scientific methods of extraction, and presents' problems that have always appealed to the chemical worker. The property which gold possesses of amalgamating with mercury enabled the recovery of finely divided gold that would be lost in any process of washing. Amalgamation is still largely used in the recovery of gold. The crashed ore passes in a stream of water over copper plates surfaced with mercury, the gold being caught and retained by the mercury as gold amalgam. Another method is the use of rotating pans in which the crushed ore is still more finely ground in the presence of mercury and water, the fine gold being freed from entangling mineral or rock and amalgamating with the mercury. By ‘retorting,’ i.e., distilling off the mercury and melting the residue, the gold with some silver, if present, is rendered fit for use or sale. In many cases amlgamation failed to recover gold in an ore; for example, when it was finely divided, or when lead, antimony, and other minerals were present which also amalgamated with the mercury and spoiled it for gold recovery. A search for other methods of extraction was made. These were essentially chemical processes.

SMELTING PROCESSES.

“Some attention was given to smelting. A ‘La Monte’ furnace was erected at Thames in 1885 for the treatment of ore not amenable to amalgamation processes. It was a shaft furnace, water cooled, in which ore was smelted with coke as fuel and iron oxide and limestone as fluxes. Metallic lead or lead ore was also mined with the charge to alloy with the gold and silver present. The lead was tapped off at the bottom and cupelled on a bone ash hearth to recover the precious metals. It will be seen that the process required efficient chemical control. It was effective, but the costs were excessive. “The Parkes process developed in London was tried at Karangahake. Ore was roughly crashed, mixed with fluxes of lime, iron ore, salt cake, fluorspar and coal, and smelted for three to four hours until molten. The slag on top was skimmed off, and tire regulus, consisting of fused sulphides of the baser metals, and containing all the gold and silver in the original ore, was tapped and cooled. _ It was then partly calcined, mixed with oxide of lead, and smelted. The metallic lead formed collected the gold and silver, and yielded it again on enpellation. This also required strict chemical supervision. The process failed for a curious reason. The furnace was operated for not quite a month. The slag was small in amount and contained hard lumps. Working was discontinued. Two years later the furnace was dismantled when it was found that the bricks used for the hearth were highly porous and had absorbed all the regulus. Had operations been continued until the limit of absorption had been reached, the furnace would have operated successfully. The cost of operation, however, was very high.

ADOPTION OF CHLORINE. “Chomical solvents for gold were also investigated. The first successfully tried on a commercial scale was chlorine. Gold is readily attacked by chlorine forming gold chlorides, which are soluble in water. The ore was roasted, crushed in the dry state, and filled into vats about seven feet in diameter, provided with a false bottom of inert material, usually ne.bbles and sand. It was moistened with water, and chlorine gas, generated in airtight lead vessels, introduced through the false bottom until it had permeated the whole mass. After 24 to 48 hours’ contact to dissoive the gold water led in at the top washed the gold chloride out at the bottom. Gold was precipitated usually by fer- ■ rous sulphate or sulphur dioxide. A chlorination plant was operated for jmany years at the Progress Mine, ißeefton.

! “A variation of the process was |the use of a. solution of chlorine in 'water instead of gaseous chlorine. Professor J. S. Black, of the Otago University, patented the use of a ' solution of 12ihs common salt. 141bs I sulphuric acid, and 6 t-o 7oz permanigauate of potash per 100 gallons of . water. The solution smelled only shghtjly of chlorine and though dissolving gold at less than one-tenth the rate of a saturated solution of chlorine, was much less unpleasant in use and was effective with ores containing fine

gold only. The process, in the opinion of many, would have had extended use but for the fact that the use of another solvent, cyanide of potassium, was being developed at the time. Chlorination had the following disadvantages:—(l) It was necessary to wash prior to treatment. (2) Soluble sulphates required to be leached out. (3) Chlorine was a very unpleasant substance to work with. A VALUABLE DISCOVERY.

“In 1886, J. S. Macarthur, in attempting to improve the chlorination process, was engaged in investigating a method of generating the chlorine eleetrolytically in a solution in contact with the ore. It was not satisfactory, so he turned his attention to other solvents and discovered that cyanide of potassium, which had been known for many years as a solvent of gold,' would in dilute solution extract gold Iroin many of its ores. The process eventually worked out consisted of the following stages: (1) Contact of ore and solution for several days. (,2) Separation of the solution by decantation or filtering. (3) Precipitation of the gold, usually on zinc filaments, but occasionally by electrolysis. (4) ■ Smelting the precipitated gold. “The process soon secured world-wide adoption, and probably at the present time at least three-fourths of the gold won each year in the world is extracted from ore by cyanide. Mechanical improvements' in grinding appliances, agitation tanks, filtration systems followed rapidly, and on the chemical side New Zealand played an important part in development. The first working plant in the world was established at Karangahake in 1889. In 1892 Dr Maclaurin investigated the chemistry of the solution of gold in potassium cyanide. His results, published in 1893, confirmed the fact that the rate of solubility is greatest in a solution of 0:25 per cent, strength. They also proved that the solubility of oxygen is also a maximum in a solution of this strength, and that oxygen is necessary for the solution of gold in potassium cyanide according to Ellsner’s equation. This work wtss the basis of several improvements in the process: (1) The effect of oxidising agents was tried and one of these, bromine, proved particularly successful in the treatment of telluride ores. (2) In leaching crushed ore in vats, withdrawal of the solution and exposure of the ore to air by transferring to a second vat for further treatment had a beneficial effect on the extraction. (3) When agitation was employed to increase tlie rate of solution a stream of compressed air was used in preference to agitation by mechanical means. (4) In precipitation of the gold on zinc, de-aeration of the solution lessened the consumption of the zinc and increased the efficiency of precipitation. PURIFICATION OF BULLION.

“Mr William Skey, analyst to tlie Mines Department, conducted _ many experiments with cyanide solution. He showed in 1895 that the so called selective action of potassium cyanide for gold was not as great as had been supposed, and that potassium cyanide readily attacked native sulphides of copper and antimony. The metallurgists at Waihi and Karangahake, where the largest cyanide plants were in operation, effected many modifications and minor improvements of which there are no records available. One outstanding achievement of Mr E. G. Banks. metallurgist to the Waihi Company, not claimed as original, was the purification of the bullion. The presence of small amounts of selenium and lead made it brittle and prejudiced its sale. _ This .was overcome by electrolysing in nitric acid solution with the gold-silver bullion as the anode, and a thin silver sheet as kathode. Practically pure silver over 990 fine, was deposited on the cathode. The gold sludge, unaffected by nitric acid, was boiled with strong sulphuric acid from _ 2 to 3 hours in an iron kettle. This removed selenium and most of the lead. The gold was then melted and any remaining trace of lead which made the gold brittle when alloved with copper, was removed by passing a stream of chlorine through the molten metal. The gold then assaved 996 to 998 fine.

“In addition to conducting special investigations assayers and chemists attached to_ each plant controlled tho operation by daily assays of ore before and after treatment, regular tests of cyanide solutions for strength and alkalinity, also for gold. content before and after precipitation, and corrected occasional minor troubles such as acidity of ore, undue consumption of cyanide, and falling off in efficiency of the zinc precipitation boxes.”