Waihi Dredging Plant

by Robert M. Aitkin. 1911

Australasian Institute of Mining Engineers.

30th JUNE, 1911. [No. 2.]

the following paper is a description of the plant and operations of the above company in their recovery and treatment of the quartz tailings deposited as final residues by the various mining companies at Waihi, Karangahake, and Waitekauri into the Ohinemuri River.

During the past few years the silting of the Ohinemuri River has increased to such an extent as to cause considerable interest and discussion, and has lately been the subject of a most extensive inquiry by a Royal Commission, to consider the best means of dealing with what is generally spoken of as a public nuisance.

Ever since the commencement of mining in this district, which was opened as a goldfield in 1875, the various mines operating have been discharging their tailings into this river, it having been proclaimed a sludge channel in 1895.

Up to the year 1900 the quantity of quartz crushed was not large, probably not exceeding 500,000 tons; but since that date the annual tonnages discharged, have increased enormously, and at the present time would total from 550,000 to 600,000 tons per year. Records show that up to the end of 1910 at least 5,000,000 tons have been crushed, for an actual yield of bullion considerably exceeding £10,000,000 in value, and all the valuable tailings from these crushings were deposited into this river. That these tailings are valuable is beyond question, for, prior to the introduction of the cyanide process, so small a percentage of the bullion contents was extracted that the bulk of the ores were unpayable by ordinary methods, and ever since its successful introduction highly payable values have been, and are still being, discharged in the tailings.

Although tube mills have been installed at some of the mills for about seven years, it is only recently that metallurgists have realized that exceedingly fine grinding or "all sliming" with cyanide solutions is the only satisfactory method of obtaining a good extraction and low residues from these ores, and even to-day they are slow to adopt the most up-to-date systems of treatment.

The bulk of the ore is by no means complex, and presents little or no chemical difficulties with cyanide treatment, but the bullion (gold or silver) is contained, partly in a free state and partly combined, in an exceedingly fine state of division, so that "all sliming" to the finest degree is necessary to liberate the bullion contents.

It being purely a question of grinding, the value of the tailings deposited into the river—whether cyanide is in use or not— depends almost entirely on the coarseness or fineness to which the sand is ground.

The Ohinemuri is a small river with a watershed of about 90,000 acres, having a fall of about 20 ft. per mile from Waihi, where tailings are first deposited, to Mackaytown. The Talisman and Crown mills are situated just above Mackaytown, and some distance below this township the river debouches into flat lands, on which Paeroa stands. The tailings from the various mills do not deposit to any great extent above Mackaytown, but immediately below is a very large accumulation of coarser sands. It is, however, in the river where it meanders through the flat country, a distance of over 6½ miles to its junction with the main Waihou River, that a natural settling basin for the coarser sands is formed. Practically all this portion of the river is tidal, and, with its willow growths and obstructions, forms an ideal settler, the flow of the water being just sufficient in velocity to allow the coarser particles containing the bullion values to settle, while the fine slimes, from which a good extraction has been obtained, are carried on to sea. These sands have not only considerably contracted the banks of the river, but have filled the bed of this large settling area to within two miles of the Waihou Junction, and this portion of the river, once navigable by fairly large steamers, can now only be traversed by small oil launches at high tide. It is well known that where 20 ft. of water existed it will not show as many inches today, and probably these deeper sections of the river, which were not sampled by boring, were the first to be filled, and therefore contain the very valuable tailings from the earlier crushings.

Without extensive cross-sections an accurate estimate of the quantity could not be arrived at; but it is generally admitted that the approximate estimate given of over 2,000,000 tons is on the safe side, and this quantity is being daily supplemented from the mills now operating above. These large deposits on the banks and in the bed of the river have all been carefully sampled and tested with a system of boring proved to be accurate, and show that similar values exist in these tailings to those actually treated at Waihi.

The average assay value of sands taken from the banks of the river was 8s. 4d. per ton (gold 6s. 1d. and silver 2s. 3d.), while the coarser sands in the bed (composing by far the greater proportion) vary in different sections of the river from 11s. 2d. (gold 8s. 4d., silver 2s. 10d.) to 13s. 4d. (gold 10s., silver 3s. 4d.) per ton of tailings.

There are parts where much higher values can be obtained owing to concentration, but these were discarded, as the available quantity of these sands is not large.

It is almost impossible to take a sample of the clean tailings from the banks of this river, even below the company's claims, that will not show an assay value of 5s. to 6s. per ton, and these tailings are usually called slimes in comparison to the deposits of coarser sands above.

It is for the treatment of these deposits (probably the greatest of their kind in the world) that the Waihi-Paeroa Gold Extraction Company is erecting their tube milling and cyanide plant on a site about 1½ miles above the Junction, or about 2 miles by river below Paeroa township. The company was formed about September, 1908, by Mr. P. N. Kingswell, now managing director, who, after viewing and testing the properties, quickly realized that, by treating large quantities by the most up-to-date methods, the proposition was a highly payable one, and he arranged to take over the river claims and the old plant at Waihi (which had been closed down after repeated failures), while the writer was engaged to manage the concern.

Owing to unworkable conditions on the Paeroa titles at that time (since removed), it was decided to reconstruct the Waihi plant, where a large accumulation had been formed by the Waihi Company's dam, and 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. The system effects a considerable saving in cyanide, shorter treatment is necessary, and fewer agitating vats are required; while, in conjunction with vanners (as practised by the Grand Junction Co.), for returning to the tube mills the mineralized portion of the ore, the bullion contents are more easily extracted, and consequently the silver residues are reduced in a much shorter time.

The first supposed difficulty in the river proposition was the recovery of the sands, but this was easily overcome by a novel air-lift dredger, designed and perfected by Mr. Kingswell and the writer, which lifted the tailings into barges at a rate of 40 tons per hour, the loaded barges being towed by oil launch to the plant and raised into storage hoppers by belt bucket-elevators a height of about 60 ft.— the total cost, inclusive of all plant renewals, being about 5d. per ton.

The treatment of the sands was simple, everything being ground to pass (at least) a 200-mesh screen; but the extractions varied greatly, independent of the fineness of the grinding, and at times exceptionally high residues would result. A careful investigation showed that the charcoal deposited in the river from the various steam plants was the cause of the trouble, and thus was re-precipitating the bullion from the solutions; consequently, the extractions depended on the quantity of charcoal present, which at times was sufficient to re-precipitate the whole of the bullion dissolved. The importance of completely cleaning the sands was at once recognized, and, after experimenting, Union vanners were installed, which not only removed this deleterious matter, but at the same time separated a large proportion of the valueless river sands and increased the value of the heads.

It may be interesting to note the apparent selective action for gold in the precipitation by charcoal—the quantities of gold and silver precipitated being about equal by weight, and not in proportion to the relative values of the gold and silver in solution. Charcoal taken from the river showed an assay value of 10s. 1d. per ton; but it is evident that this material loses its precipitating effect on exposure to air, and the properties are again renewed by re-grinding. It is also important to note that the ordinary kiln-burnt lime used in all batteries for the settling of their slimes in cyanide treatment contains more or less charcoal, and samples of this substance, taken from the company's settlers at the Waihi plant, assayed £7 10s. 3d. per ton (gold £7 6s.-8.d., silver 3s. 7d.) This indicates distinctly that lime used for this purpose must be free from charcoal, otherwise bullion losses will occur.

The slime product from the river tailings is practically pure, finely-ground quartz, free from what may be called "natural slime" ; consequently, it is more difficult to agitate and more easily settled than that produced in ordinary battery practice. This natural slime, produced from mullock and the softer portions of the ore, greatly assists the working of the slime, and composes from 20 % to 35 % of the ore crushed. At the same time, a high percentage reduces the rate of vacuum filtration, owing to its-close nature. It is, however, easily understood that tube mills working on the original ore should have a far greater grinding capacity (in tonnage) than the Waihi-Paeroa Co.'s mills, which only receive the harder particles of the quartz.

The company's plant at Waihi worked about 18 months, and a large amount of experimental work was done, besides experimental runs to arrive at the economic limit of the grinding. During that period over 32,000 tons of sand were treated for, approximately, £14,000 worth of bullion, equal to an actual extraction of nearly 8s. 7d. per ton. The average residue, on account of charcoal troubles and experimental work, was high—over 4s. per ton ; but this past experience will enable the company to reduce the residues at Paeroa plant to 2s. or 2s. 3d. per ton, probably lower, if power costs can be reduced. Even at the extraction of 8s. 7d. per ton a substantial profit was made over working expenses. The actual working costs, plus plant renewals, were reduced to a minimum, being for one period 4s. 9d. per ton. The average working cost at this plant, exclusive of bank and bullion charges and management, was about 5s. 2d. per ton, the principal item being labour; and, when treating 500 tons per day at Paeroa as against about 80 tons per day at Waihi, this item, especially, must show a very considerable reduction. Fuel, oils, packings, grease, sundries, and plant renewals would also be much less per ton on the larger tonnage, especially when machinery more economical and better suited for the class of work will be used. The total cost of cyanide, flints, lime, and zinc at the Waihi plant was under 1s. 3d. per ton, and this can be reduced to under 1s. per ton at the Paeroa plant. Basing the estimates on the actual costs of treating over 32,000 tons at Waihi, the management considers that 3s. 6d. per ton will pay the total costs of treatment at Paeroa, allowing for a tonnage of 500 tons per day.

Regarding the bullion recovered from these tailings, it may be interesting to point out that this is of a very low value per oz., notwithstanding an average fineness of 950 in bullion. At Waihi the average value was only 8s. per oz., the proportion by weight being 1 of gold to 11.6 of silver, and by value 3.4 of gold to 1 of silver. At Paeroa the total bullion will increase with the better extraction, but the increase of gold will be much greater proportionately than the silver, owing to the absence of the injurious effects of charcoal, consequently the value of the bullion will be greater per oz.

The low value of the bullion goes to show that the fine grinding and long-continued agitation not only liberates the gold (which dissolves immediately this is done), but gives a much better extraction of the silver contents than that obtained by the ordinary treatment. The bulk of this silver is contained in the mineralized portion of the ore, and for its extraction continued grinding or agitation by air is necessary.

Paeroa plant.

The Paeroa plant, now being erected, is to be capable of treating 500 tons of coarse sands per day, and is practically an "improved edition" of the Waihi plant on a very much larger scale. It has been designed on similar lines to work the " all slime " cyanide process, all alterations and improvements added being the outcome of practical experience gained whilst working sands of a similar nature at the Waihi plant.

Care has been taken to save labour in every department without undue expense, and at the same time maintain its full efficiency. Parts subject to breakage or wear are made easy of renewal, and duplicates are stocked where necessary to enable the plant to be worked continuously. A wharf, with a jib crane capable of lifting 10 tons, is erected on the river, and all plant and material can be landed by. steamer at the site. Smithy, store buildings, &c., are erected, and a complete system of tram lines connects from wharf to main road, where unloading gear is also provided. From the main tram line branch lines run in and about the works, so that everything can be easily and cheaply handled. A fitting shop, with lathe, radial drill, punching and shearing machine, screwing machines, &c., is being erected, to be worked by an oil engine during erection of plant, and, later, from the main power plant, for repairs and renewals.


The power plant will consist of 4 Babcock and Wilcox water-tube boilers arranged in two batteries, fitted with chain-grate mechanical stokers. Green's economizers, feed-water heater, feed pumps, and other necessary accessories, to work at a pressure of 160 lb. per sq. in. Slack coal will be used as fuel, which can be delivered on the site at about 9s. per ton, and storage hoppers will be provided with bucket elevators to fill coal bunkers placed over the stokers. Provision is also made for easy discharge of ashes from pits.

The boilers supply steam to two compound condensing engines, the " No. 1 " being a cross compound of 630 b.h.p. by Hick, Hargreaves and Co., and " No. 2 " a 150 b.h.p. tandem by Yates and Thorn. The " No. 1 " will be capable of driving the whole plant with a small overload, and the " No. 2," besides acting as a spare, will be able (when the tube mills are not working) to operate the balance of the machinery on Sundays, or at any time the mills are idle. The shafting and clutches are so arranged that either engine will take the load exclusive of mills, or both engines may be worked together. In the latter case the smaller, or "No. 2 " "engine, will drive the generator (for power and lighting) air compressors, and fitting-shop gear.

The power is transmitted by cotton ropes to two main lines of shafting, and also to the intermediate shafting. From each main line of shafting 10 tube mills are driven—two by belting, with fast and loose pulleys on the pinion shaft, and the other eight are arranged in pairs, one on each side of the shaft, each pair being driven from the one steel pinion, with a friction clutch on the main shaft for stopping and starting. This latter method of driving obviates the use of belting, and balances the weight on the shafting, besides saving considerable room; the only reason for driving four by the first method being that they were already constructed for this drive.

There is no disadvantage in driving a pair of mills with the one clutch, as sole-plates of bearings are designed to allow either mill being disengaged from the pinion.

The power for driving vanners, elevators, pumps, mixers, &c., is transmitted by belting from the two main lines of shafting. The intermediate shaft, which can be driven separately by the " No. 2 " engine, transmits power to the fitting shop, and operates two large air compressors for slime agitation and air-lifts, besides driving a 50 kw. electrical generator.

This generator supplies lighting for the whole plant, and will operate 3 motors—"No. 1" for working the river elevators, "No. 2" the electric crane for vacuum filter slimes plant, and "No. 3" for pumping a fresh water supply for condensing and other purposes from the Upper Waihou River, a distance of about one mile from the plant.

Recovery of Sands from the River.—

The river plant will consist of one air-lift dredger, capable of raising 80 tons of sand per hour into the barges, four 65-ton sand barges, and two oil launches for towing barges between dredger and river elevators.

On the dredger pontoon is a 30 b.h.p. Crossley oil engine, worked on kerosene, which supplies power to operate an air compressor, centrifugal pump, and belt bucket-elevators. The air-lift is a telescopic 10-in. pipe, which can be raised or lowered at pleasure, with delivery chute at the top. At the bottom of this pipe, which is open to the full size, the air from the compressor is allowed to escape, the pressure being just sufficient to over-come the hydraulic head. Around the bottom of the air-lift is a series of water-jets supplied by the centrifugal, which thins the sand to the desired consistency before being raised by the air-lift. By this means the thickness or proportion of sand and water can be regulated at will, and at Waihi the company was able to raise a continuous stream of equal parts of sand and water by volume to a height of over 4 ft. above water level. This shows the great advantage of the system over centrifugal pump suction-dredging, which wastes its power by throwing large volumes of water with the sand, and would flood the sand barges. The air-lift is only required to raise the sand a few feet above water-level, when it is raised by bucket elevator for distribution into-the sand barges. The power supplied is in excess of that required to work the dredger, to allow for increased capacity, and other work, if necessary. There are very few wearing parts—-practically none on the air-lift—so plant renewals will be very small; and the actual cost of kerosene, oil, and sundries will be under 1d. per ton.

When filled, the loaded barges are towed by oil launches to the site, and there unloaded by means of double-bucket belt-elevators capable of lifting 100 tons per hour, and designed to be raised or lowered to allow for variable heights of river or barges. The tailings here pass through a revolving screen to remove debris and coarse material before passing into ferro-concrete storage-hoppers (partly built into the ground) of 2750 tons capacity. This large storage makes provision against floods or breakdowns to dredging machinery, holding a five days' supply of sand.

The sand is raised from the storage hopper with air-lifts, and conveyed to the vanner room in the main building, where ten 6-ft. Union vanners will be used to remove any deleterious matter (carbonaceous or otherwise), together with a large proportion of the valueless river sands. The vanners deliver the clean sand at the head of the machines as a thick and fairly dry cake, which, is washed cleanly off the belt by jets of cyanide solution, and raised to a steel storage tank of 100 tons capacity, the solution being returned to the vanner jets for re-use.

An air-lift maintains a constant supply of fresh sand from the storage tank to the distributor pan feeding the tube mills, this being mixed with the returns from the separator boxes, and carried in even proportions to each of the 20 tube mills.

The mills are the same size and design as used at the Waihi plant—16 ft. long and 4 ft. inside diameter, with cast-iron ribbed-liners, and filled to within 4 in. of the centre line with flints, driven at 28 r.p.m., and using from 22 to 25 h.p. per mill. Each mill will grind not less than 25 tons per day to a fineness only ascertainable, by very careful water separation, everything easily passing a 200-mesh screen. Much could be written on tube milling, but the conditions which govern the rate of grinding are so varied that there is room for considerable experimenting in this direction.

Wheel elevators, capable of raising 12,000 tons of pulp per day of 24 hours, will lift the tube-milled product to the separator boxes, where the sufficiently-ground slime overflows to settlers, and the balance is returned to the mills for re-grinding. To effect this separation the pulp must be thinned considerably with solution, and a quiet overflow maintained from the boxes.

Lime is added to the finished pulp before reaching the settlers, which are deep conical tanks, the thickened sludge (containing about 50 % dry slime) being pumped from these cones to the tall conical agitators, while the clear overflow solution is sent to the zinc-extractor boxes, and the bullion recovered. These tall conical agitators are to be built on a new style, the cones being of ferro-concrete set in the ground, and the cylinders of steel. They are of sufficient capacity to allow of an agitation by compressed air for at least 72 hours, which time is necessary for a good extraction of the silver contents. The cost of air-agitation is small, and when vanners are not used to return the mineralized portion to the mills the silver contents take longer to dissolve. The gold values dissolve almost immediately they are liberated by grinding; so, were it not for the silver, the finished slime product could be run direct to the vacuum filters. From the agitators the slime is drawn off to the vacuum filters, where solutions are extracted and slime washed. These filters are of the Moore submerged filter type; with considerable improvements in the style of working. There will be one pulp tank and two wash tanks, all of steel and circular in shape, with 6-ft. vertical sides and steep conical bottoms. The pulp tank must be continuously air-agitated, even when cakes are forming, to prevent settlement, which shows how this slime differs from ordinary battery slime. The cakes form quickly on the canvas-covered, corrugated iron frames which compose the baskets, the solution being drawn off by vacuum pumps in the usual way to the gold-solution tanks.

The frames are parallel, of three different lengths to suit the-round tanks, and of sufficient area to allow of about 9 tons of dry slime per lift. The baskets are raised and lowered vertically by an electrically-driven overhead crane, and traversed horizontally by the same power. When the cakes are sufficiently washed the vacuum is disconnected, and, assisted by a little compressed air, the load is discharged into the wash tank. At the bottom of the cone in each wash-vat is a mechanically-driven disintegrator, and water is admitted to thin the slime, which is discharged as a slurry through a valve at this point. This method of working considerably increases the capacity of the plant, obviates the necessity of raising the load a second time for discharging, and saves wear and tear by scraping on the canvas-covered frames. Before commencing operations at the Waihi plant, many doubts were expressed as to whether a slime could be produced from the river sands which would adhere to the frames by vacuum, but this was easily overcome; and, moreover, they were able to form and wash the cakes in one-third of the ordinary time required. This rapid formation of the cakes is not solely due to-the nature of the material, as usually supposed, but depends largely on the system of treatment.

The bullion-bearing solutions from the slimes plant and overflow of the settlers are sent to the head tanks of the precipitating boxes, and are sand-filtered before passing through the extractors. The usual finely-turned zinc will be used for precipitating. This costs 4½d. per lb., and the consumption is under one-third of a pound per ton. After extraction of the bullion the solutions pass to the sumps for re-use.

The extractor boxes are dressed and cleaned at regular intervals, and the slime dried by vacuum before roasting to oxidize the zinc. The oxidized slime, with fluxes, will be smelted in a kerosene blast tilting furnace, which has been found much more economical than coke, whilst the bullion produced is seldom less than 950 fine. As the bullion produced is of low value per oz., the question of electrolytic refining is under consideration, but it is questionable if this will pay on a 500-ton a day plant, as the costs of disposal, as raw bullion, have been reduced to a minimum.

The erection of the Paeroa plant is being pushed on as fast as possible, and all contracts let are so arranged that everything will be completed, and a trial run of the machinery obtained, before the end of the year (1911).

The plant, when completed, will be one of the most up-to-date tube milling cyanide plants in the world, and every section of the work has been carefully considered and discussed before finally decided upon. The experience gained in working the same system of treatment on similar sands at the smaller plant at Waihi has been of exceptional value in the designing, arranging, and constructing of all the plant and machinery, and, besides overcoming the difficulties of treatment and improving the extraction, the actual results—from over 32,000 tons treated— have supplied practical figures on which to base estimates for capacity, extraction of bullion, and working costs per ton.

The whole proposition, prior to actual results from the Waihi plant, was considered by outsiders a very speculative one, and the company was referred to as "an adventurous syndicate" for attempting to treat tailings from modern mills. However, the past bullion returns and substantial nature of the Paeroa plant will show that it has passed the speculative stage, and is now a thoroughly sound investment, which, with economical working and careful management, must return substantial profits.

The life of the concern, independent of the time required to work the present large accumulations of sand, will depend upon when the mills above cease to deposit tailings of a payable nature into the river, which acts as a huge concentrator, washing away the worthless slime and depositing the coarser sands on the company's claims for re-treatment.

This deposition of payable sands will apparently continue as long as mining companies can obtain better profits from their ore by ordinary methods than by a more complete treatment of their sands. At the present time better profits can be made by increasing the tonnage crushed than by improving the extraction.