Waihi Dredging Plant

In 1897 the Waihi Dredging Company was formed by E.H. Barber with capital of £5000. Buildings were constructed on the banks of the Ohinemuri and Waitete streams, using a simple dragline to recover tailings from the Ohinemuri River. In 1903 the plant was sold to Messrs Brown and Thompson under the name of the Ohinemuri River Syndicate (Townsend 2003).

As a new venture, much experimentation occurred and the first plant soon proved to be ineffectual. It was realised a better way to grind the tailings was needed. Very fine grinding was found to be necessary to allow the cyanide solution to effectively dissolve the bullion still in the tailings and so, in 1904, a small tube or flint mill was erected. This could finely grind up to 100 tons of tailings per week, but this led to further difficulties, as the fine material required perfect agitation in the cyanide tanks to effect a high extraction of bullion (Townsend 2003).

Various air agitation tanks were trialed before Brown and Thompson invented their own system, which they patented worldwide (Papers and Reports Relating to Minerals and Mining 1905). These tanks are the now famous B & M agitation tanks, used all round New Zealand and the world, to treat slimes (very finely ground ore). In 1903 the syndicate employed 10 men and made a profit of £884. However by 1905 it was determined to be commercially unsuccessful despite the large amounts of capital invested (Townsend 2003).

The syndicate was sold in October 1908 to the Waihi Paeroa Gold Extraction Company, which expanded the plant and installed new machinery, over a period of nine months.

Robert Mark Aitken, in 1911, described the reconstructed plant as: "a steam-driven plant of 4 tube mills, 9 conical air agitators, vacuum filter slimes plant, pumps, compressors, and all other accessories necessary for an up-to-date "all slime" cyanide plant, were erected, in all costing about £16,000. The plant was designed to work wholly on cyanide solutions in which all grinding, separating, and agitating was done—the company being the first to adopt that system, which was afterwards installed by the Grand Junction Co., and must sooner or later be adopted on these ores by every up-to-date plant in the district." ("Waihi-Paeroa Gold Extraction Co. Ltd." in: Australasian Institute of Mining Engineers. 30th JUNE, 1911. [No. 2.])

The company was confident of success as crushing the tailings was no longer a problem and the new cyanide process was under control.

After eighteen months of experimentation with grinding and treatment processes, the plant finally proved it could be economically run. Figures for 1909 show that 23,950 tons of tailings were reprocessed and 30 men were employed. On 4 March 1910 a public company was formed, "The Waihi Paeroa Gold Extraction Company Limited", as the management wished to start erecting their new plant at Paeroa. This new plant, with four times the capacity of the Waihi plant, would be modelled exactly on the perfected processes of the old plant. The Waihi plant closed that same year after processing 35,034 tons of tailings for a return of 36,770oz. bullion, valued at £14,615 (Downey 1935:251 cited by Ritchie n.d.:l).

Thus this plant played a vital role in improving appliances for lifting, grinding, and cyaniding tailings. In April 1910 tenders were advertised for dismantling the Waihi plant and removing it to the new Waihi Paeroa Gold Extraction Plant situated off Mill Road, Paeroa (Townsend 2003).

Robert Mark Aitken, manager of the plant for the Waihi Paeroa Gold Extraction Company, describes the operation of the plant thus:

"First of all, the tailings are dredged from the river by air-suction dredges, and conveyed down the river in barges. Floated into hoppers, cleaned in vanners, and screened and ground in cyanide-solution in tube mills. The whole will pass a 200-mesh screen—40,000 holes to the square inch. That is the fineness of the coarse particles. From 60 to 70 per cent. would probably pass a 300-mesh screen and 30 per cent. would probably pass a 400-mesh screen, or possibly finer.

"After passing the tube mills it goes into the separation-boxes. These separation-boxes are from 5 ft. to 7ft. deep. conical shape, and the finely ground slime has to rise in these boxes to overflow; rises through the cyanide-solution in the boxes, any coarse material separating out. Any material that is not actually slimes is returned to the mills from the bottom of the separator-boxes by gravitation. This underground—an insufficiently ground—portion is constantly returned to the mills for regrinding until slimed sufficiently to overflow the separator-boxes. Every particle that will not pass the 200-mesh contains enough gold to make it worth while to regrind. No force is use to make slimes overflow. They rise in the still water and overflow.

"The overflow that is going into the boxes—the sludgy matter—after passing through the mills, is diluted with a small amount of cyanide-solution to cause the necessary overflow. The cyanide-solution is very weak--2 lb of cyanide dissolved in 1 ton of water. When sufficiently ground, lime, or lime-water, is added to coagulate the slimes and facilitate the settling in the settlers. The thickened slime from the bottom of the settlers is pumped into tall tanks for agitation.

"When the bullion is dissolved the sludge is drawn off for the vacuum filter, which extracts the cyanide-solution, washes the slimes, and the slime is then discharged. It is washed in cyanide-solution or water before being discharged. We just dump it down on the ground, and a small trickle of water over it washes it into the main river. It goes into the main river just above the Waihi Company's dam.

"Our dredges are working in the main river. We dump the slimes just below where we take the coarse tailings out of the river—not more than 20 yards from the elevator. We dredge from the elevator for two miles up the river. We have treated about 33,000 tons since we started operations—about sixteen or eighteen months."

(Waihou and Ohinemuri Rivers. Report of Commission Appointed to Enquire into Silting of: 1910. Page 354).

[This historical summary written collaboratively by Rachel Darmody and Eric Lens.]