McAra: The Cassel plant, which was completed in February 1894 to treat the tailings lying in the gully below the Waihi Gold Mining Company's battery (tailings which the Cassel Company had purchased), was of great significance in demonstrating beyond question the fact that the Martha ore was suitable for treatment by this method which gave excellent results with low-grade ore. It was the first successful use of cyanide at Waihi.

The plant, which was very systematically arranged, was situated in a hollow below the tailings dam, to allow the tailings to be run on a suitable down-grade into the leaching vats and to be discharged from them after treatment by sluicing. The sludge eventually found its way into the Ohinemuri River a short distance away. Utilising gravity to the fullest possible extent rendered the operation a very simple one.

The building that housed the plant had a frontage of 116 feet and a breadth of 77 feet. It included a laboratory and offices in a lean-to at one end, as well as a manager's office, and also provided space for experimental work. The bullion room was situated apart from the assay office to avoid possible contamination of samples. There were eight circular leaching vats in the plant (20 ft in diameter and 4 ft deep), arranged in two rows and built of special grade three-by-three-inch kauri timber. The bottoms sloped two inches, with a discharge launder running the length of the. plant to enable the residue to be sluiced directly into it by opening the twelve-by-eighteen-inch cast-iron discharge door. Two vats of the same size as the leaching vats were used as solution sumps and were kept covered.

The reservoir (13 ft 9 ins in diameter and 5 ft deep) and vacuum chamber (13 ft 9 ins by 3 ft 9 ins deep) were situated in the same line as the sumps, and beneath them was a rectangular tank (12 ft by 8 ft by 18 ins deep), capable of holding the contents of the vacuum chamber. The reservoir was at a high enough level to allow the solution to gravitate into the leaching vats. There were three precipitation boxes (12 ft 8 ins by 19 ins), with side discharge for slimes and a settler for cleaning up. The dissolver (an iron pan about 3 feet 6 inches diameter by 2 feet 6 inches high) was capable of dissolving four boxes (1000 lbs.) of cyanide a day; it was so arranged that the required amount of strong solution could be added to the reservoir by simply turning a tap.

A four-inch centrifugal pump was used for returning solutions to the reservoir and there was also an eight-inch vacuum pump which could produce a vacuum reading of twenty-six inches of mercury; the power was supplied by a four-foot-diameter Pelton wheel working under a sixty-foot head of water from the Company's race. The plant was said to run very smoothly, with ample power and water for sluicing purposes. Water connections were placed over each vat and one man could empty its forty tons in two hours. The tailings pits were connected by tramway to the works, with two sets of lines passing over the top of each vat to allow side-tipping trucks to spread the material. Open launders were used to distribute the solutions to the vats, in preference to pipes, as they not only gave good visual control but also enabled the solutions to be sampled easily.

The procedure was to tip sixty-five truckloads — about thirty-three tons dry weight — into the vat and add about six tons of "strong" solution (0.7%), allowing this to percolate through to the bottom below the filter bed for twenty-four hours, then to apply "weak" solution (0.25%) which was also allowed to gravitate through the ore, after which ten tons of water in two charges was drawn through quickly by suction, removing all the cyanide and dissolved bullion. After passing through the extractor boxes the solutions were returned to the reservoir and used again with "make up" solution added. The zinc slimes from the precipitation boxes were cleaned up and melted fortnightly.

The Cyanide Process, James Park, 1896: Some 25,000 tons of these tailings were successfully treated by the Cassel Gold Extracting Company, whose works have recently been acquired by the Waihi Gold and Silver Mining Company, who are now treating the remainder of the tailings on their own account. The plant consists of eight leaching vats, each 22½ft. in diameter and 4ft. deep, together with all the necessary appliances.

From the Annual Report from the Directors, for the year 1897 (HP Barry’s Report): During the latter part of the year the tailings plant has been dismantled and the vats, eight in number, vacuum cylinder and building have been re-erected at the Mill, an arrangement which will prove and in fact has already proved to be an advantageous one.