Waihi Dredging Plant

Waihou and Ohinemuri Rivers: Report of Commission Appointed to Enquire into Silting of: 1910


Robert Mark Aitken Page 354

Robert Mark Aitken, sworn: I am a mining engineer, and manager for the Waihi-Paeroa Gold-extraction Company, the applicants, and have been since its inception in charge of the works at Waihi. I hold first-class certificates from the Thames School of Mines in surveying, mining, metallurgy, and all branches of mining. I hold first-class battery-superintendent's certificate. I was for eight years and a half Lecturer to the School of Mines on the West Coast of the South Island, and metallurgist to a number of companies for about eight years—principally the Keep-it-Dark Company. I have been connected with all forms of battery treatment, and also with bucket dredging in the rivers. Pretty well the whole of the eight years on the West Coast was spent in treating slimes and tailings: also the two years I hare been in this district. For two years—a little over—I have been in charge of Waihi-Paeroa reduction-works at Waihi. 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 you get past the 200-mesh screen you cannot get any finer mesh, and we judge the fineness of the slimes by the rate at which it will settle in water. It is only an estimate when I speak of a fineness beyond the 200-mesh screen. 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. At times the water overflows the dam, but only in flood-time, because the Waihi Company practically take all the water in the river excepting in wet weather. We have dumped thousands of tons of tailings into the river, and there has been no overflow from the dam at all. and those slimes have remained in the dam for months. They will not go away unless there is an overflow from the dam. As soon as an overflow occurs we can see the thickened sludge water going over the dam, carrying away these slimes. The Waihi Company has 90-stamp mills discharging into the river. The Junction Company and the Union Mill (40 stamps) both also discharge into the river. [Draws sketch-plan of locality, which is put in.] The depositing of all that sludge has never blocked up the river except in dry weather: it remains in the bed of the river. When rain comes it all goes away: it does not interfere with our dredges or pontoon. All that remains in the river from other batteries is the coarse sand. The slimes are all gone. They travel on down the river. The slimes have never, so far as I know, interfered with the Waihi Company's water-race. A fairly large portion of them must go down the water-race. After rain there is no sludge left at the back of the dam. When it has been lying there four or five months it has gone clean away as soon as there is water enough to carry it over the dam. The Waihi Company's race is very flat, and I have not known them clean it out since I have been there. I have been all the way down the river. Not been there all the time; but examined it here and there all the way down. I have been all over these nine claims now applied for. We went over them, and bored them at regular intervals, and rook samples of sand for testing. I had two assistants. Took fortnight or three weeks: made thorough examination. I cannot find any slimes in the river at all; very, very small percentage of slimes: all the deposit is sand (coarse tailings), with small percentage of river-sand. Our operations have cleared the river considerably. Taken large quantity of coarse sand out of the river, and we can now go up and down without any trouble. When we first started we could not work our barges over the coarse sand : now we have no trouble even in dry weather. We are going to put our works three-quarters of a mile or a mile above the junction of the rivers. We intend to install much the same plant, but we intend to grind finer. The finer the grinding, the better extraction. Pays us to grind finer.


151. The Chairman.} What proportion do the two plants bear to one another?—The Waihi plant deals with 100 tons a day, and the proposed plant at Paeroa will deal with 500 tons. .

153. So it is an addition of a 400-ton plant to the existing 100-ton plant?—Yes. It will take about twenty to twenty-five more men than on the smaller plant. We employ fifteen men on the small plant

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28. Mr. Hanna.] Will you describe the method you adopt of collecting the tailings and treating them, and the method of your operations?—The method of dredging is a combination of the Pohle air lift with the application of water-jets to assist in raising the sand from the river. The ordinary Pohle air lift will raise water, but it would not raise sand without these water-jets. It can suck or lift from any depth, providing the depth below water is about twice the height to be lifted above water. We raised the sand about 5 ft. above water-level in the barges. We lift about 1 of water to 1 of sand. Any excess of water overflows from the barge. Then we take it to our works, and elevate it by bucket elevator.

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41. The Chairman.] Do you propose to have any process for discriminating between river-sand and tailings—any mechanical process?—Yes, we have a separate process. We take out as much as we can of the coarser river-sands, and any fine slimes or material which it does not pay us to treat. There may be about from 1 to 2 per cent of fine slime-material that it will not pay to treat.

42. That you propose to put back again direct into the river?—Yes.

43. Mr Meuller.] At your Waihi works you deposited the tailings in the Waititi Creek!—Yes.

44. And from there they ran into the dam?—Yes.

45. Did you notice, after the last flood, a large deposit of slimes in the Waititi Creek from your works?—There was absolutely no tailings or battery slime in the Waititi Creek. We used the water for our boilers.

4C. I was referring to the flat where the railway crosses the Waititi : were not your deposits thrown up the Waititi Creek as well as in the Ohinemuri River?—No; that deposit on the grass is a yellowish mud that came down with the flood of the Waititi.

47. That deposit has nothing at all to do with the material that comes from your works?—No.