May 2018 Project Martha – Historical & Archaeological Background 17 Extraction Advances and Development, 1893-1901 Despite improvements in amalgamation processes throughout the late 19th century, recovery percentages remained low. From 1889 trials of gold extraction using potassium cyanide were undertaken in Karangahake by the Cassel Company, and the results significantly altered methods of extraction across the Ohinemuri goldfield.73 According to P. Rainer: ‘The MacArthur-Forrest process had been patented in Glasgow during 1887, and the patent purchased by the Cassel Gold Extracting Company. It dealt with the precipitation of the gold from the crushed quartz, which up to then had been done by pan amalgamation. The new process was very simple. Ore was crushed by the stampers into an exceedingly fine state, before being passed on as a powder to circulate through tanks containing a weak solution of potassium cyanide – which had an affinity for gold. The cyanide brought the gold into solution without affecting the rock particles (or more accurately rock pulp), which could be easily drained off. The solution containing the dissolved gold was then passed over zinc shavings: the zinc replaced the gold in solution, resulting in a black sludge called ‘black slimes’ which were then refined.’74 Tests with cyanide were made by the Waihi Goldmining Company as early as 1891 using the Bohm process, although all proved unsuccessful.75 The Silverton Company were the first in Waihi to fully utilise the Cassel extraction method from April 1893 and the Waihi Company soon followed, with a cyanide plant completed at the Waihi Battery by May 1894.76 The plant proved highly suitable for processing Waihi ores, where the gold naturally occurred in a very fine state, and rendered pan amalgamation obsolete.77 The new methods saw extraction for gold rise to 90% of the assay value, compared with 66% from pan amalgamation, and about 50% for silver, up from 35%.78 In subsequent years further refinements increased this to as much as 96% for gold.79 73 AJHR 1890 C-03, p.30; Thames Star, 2 May 1889, p.2; Thames Advertiser, 28 September 1889, p.2. 74 P. Rainer, Company Town: An industrial history of the Waihi Gold Mining Company Limited, 1887-1912, np, 1976, np; Clough 2004, p.65. 75 AJHR 1892 C-03A, p.6; AJHR 1893 C-03, p.69. 76 Lens 2017, p.55; Clough 2004, p.66. Note: around early 1893 the Cassel Company also erected a tailings plant at the Waihi Battery to rework the tailings accumulated between 1888 and 1892 from the pan amalgamation process. The undertaking was mentioned in the annual warden’s report, for the year ending March 1894, on the Waihi Goldfield: ‘Cassel's Company's Works — This company has erected suitable plant for treatment of the tailings and slimes which have accumulated at the Waihi Company's works. These tailings, estimated at from 25,000 to 30,000 tons, were purchased from the Waihi Company for £5,000 early in last year. Advantage was taken of the fine weather to get the materials on the ground and erect the buildings and necessary vats and machinery. water-race was constructed to bring in water from the creek receiving water from the swamp, and also that flowing from the adit-level of the Waihi Mine. The plant is situated near the river, and below the place where the tailings are deposited. The cost of the plant, Mr. James, the company's manager, informed me, was about £2,500. The first month's run was very satisfactory 1,425 tons of tailings were run through for 1,1260z. of bullion, worth £1,500. The number of men employed is two laboratory men, two cyanide plant men, seven contractors to deliver tailings, one carpenter, one labourer, and the manager: total, fourteen men.’ AJHR 1894 C-03A, p.13. 77 Clough 2004, p.66. 78 McAra 1988, p.89. 79 Clough 2004, p.66.
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