Gold!

Manawatu Times, Volume LVI, Issue 7068, 30 January 1933, Page 10

Gold!

ADDRESS TO N.Z. SECTION OF INSTITUTE OF CHEMISTRY. Delivering a most interesting address at the annual meeting of the Institute of Chemistry of Great Britain and Ireland, Mr. W. Donovan, M.Sc., F.1.G., Dominion analyst, 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. New Zealand, he said, 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 Karangahalce in 1889. Pays ing tribute to the pioneering spirit of scientific enterprise which marked thd development of the mining industry of this country in the early days, he went) on to state that gold in sand and gravel might be separated by simple washing, as in alluvial mining, and little scientific aid was required from chemistry. “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. Tho property w r hich gold possesses of amalgamating with mercury enabled tho recovery of finely divided gold that would be lost in any process of abashing. Amalgamation is still largely used in the recovery of gold. The crushed ore passes in a stream of water over copper plates surfaced with mercury, tho gold being caught and retained by the mercury as gold amalgam. Another method is the use of rotating pans in which tho crushed ore is still more finely ground in the presence of mercury and water, tho line gold being freed from entangling mineral or rock and amalgamating with the mercury.! By ‘retorting/ i.e., distilling off tlnsj mercury and melting the residue, tho gold with sonic silver, it present, is rendered fit for use or sale. In many cases amalgamation 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 18S5 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 Emestone 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 Karaugahake. Ore was roughly crushed, mixed with fluxes of lime, oron ore, salt calcc, fluorspar and coal and smelted for three to four hours until molten. Tho slag on top was skimmed off, and the regulus, consisting of fused sulphides of tho 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. Tho metallic lead formed collected the gold and silver, and yielded it again on cupellation. 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 Jumps. Working was discontinued. Two years later the furnace was dismantled when it was found that the bricks used for tho hearth were highly porous and had absorbed all the regulus. Had operations been continued until tho limit of absorption had been reached, tho furnace would have operated successfully. Tho cost of operation, however, was very high. Adoption of Chlorine.

“Chemical 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 pebbles 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 dissolve the gold water led in at the top washed the gold chloride out at the bottom. Gold was precipitated usually by ferrous sulphate or sulphur dioxide. A chlorination plant was operated for many years at the progress Mine,- Reefton. “A variation of the process was the

uso of a solution of chlorine in -water instead of gaseous chlorine. Professor J. fcj. Black, of tlic Utago University, patented the use of a solution of 121ba common salt, 141b,s sulphuric acid, and 6 to 7 oz permanganate of potash peif 100 gallons of water. The solution; smelled only slightly of chlorine and through 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 line gold only. The process, in the opinion of many, would have had extended use but for tho fact that the use of another solvent, cyanide of potassium, was being developed at tho time. Chlorination had the following disadvantages :—(1) 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 ISS6, J. .S. Macarthur, in attempting to improvo tho chlorination process, was engaged in investigating a method of generating tho chlorine eiectrolyptically 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 from 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 worldwide adoption, and probably at the present time at least three-fourths ot the gold won each year in the world is extracted from ore by cyanide. Mechanical improvements in grinding appliances, agitation tanks, iiitration systems followed rapidly, and on the chemical side New Zealand played an important part in development. Tho first working plant in the world was established at Karangahake in ISS9. In 1892 Dr. Maclaurin investigated the chemistry of tho solution of gold in potassium cyanide. His results, published in 1893, conlirmed 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 soiution of gold in potassium cyanide according to Pilsner's equation. This -work was 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 tho 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 beneiicial effect on the extraction. (3) When agitation was employed to increase the 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 o fthe zinc and increased tho efficiency o fprecipitation. Purification of Bullion. “Mr. William .Skey, analyst to the Mines Department, conducted many experiments with cyanide solution. He showed in 1895 that tho 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. Tho metallurgists at Waihi and Karangahake, whore tho largest cyanide plants were in operation, affected many modifications and minor improvements of which thcro arc no records available. Ono outstanding achievement of Mr. E. G. Banks, metallurgist to tho Waihi Company, not claimed as original, was tho 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. Tho gold was then melted and any remaining trace of lead which made the gold brittlo when alloyed with copper, was removed by passing a stream of chlorine through the molten metal. The gold then assayed 996 to 998 fine. “In addition to conducting special investigations assayers and chemists attached to each plant controlled the operations by daily assays of oro 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.”