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a—3

The Waihi Company erected a Cassel plant to treat only a portion of their pulverised ore, in order that they could test the different methods of treatment against each other. Therefore, by having the ore from 40-head of stamps treated by amalgamation, and the same ore from another battery of 30-head of stamps treated in a cyanide solution, a good comparison could be made. The result showed that, while by pan-amalgamation 65 per cent, of gold and 35 per cent, of silver was obtained, the ore treated by the cyanide solution yielded 90 per cent, of its gold and about 50 per cent, of its silver. That is, if the assay value of the ore showed it to contain loz. of gold and 4oz. silver to the ton, by adopting the cyanide prosess lSdwts. of gold and 2oz. silver is recovered; while, by pan-amalgamation, only 13dwts. of gold and loz. Bdwts. of silver is recovered : thus showing a saving in favour of the cyanide process of sdwts. gold and Bdwts. of silver to the ton, less the extra expense of treatment and royalty to the Cassel Company. Many experiments have been made by this company, in regard to the treatment of the ore, with the view of ascertaining the cheapest and most economical methods to adopt. Dry crushing is now strictly adhered to. It was considered at one time that the tear and wear was far greater by dry crushing than by wet, while the quantity of pulverised ore was less. Experiments were made by having 30 heads of stamps crushing dry, and the same number of stamps crushing wet. The result of this was that the quantity of ore pulverised was slightly more by wet crushing, but the amount of slimes was also considerably more, and also the amount of bullion by pan-amalgama-tion was less than that obtained from the ore that was crushed dry. After several months' trial by the two systems the wet crushing was abandoned, and at the present the whole of the sixty heads of stamps is crushing the ore in a dry state. In a conversation with the manager, Mr. Barry, as to the advantages and disadvantages of dry crushing, he stated that, although the wear-and-tear on the bearings in a dry-stamp mill may be slightly more than when the ore is crushed wet, still the wear-and-tear of shoes, dies, screens, and mortars is considerably less by adopting dry crushing. When this mill was first erected dry crushing raised a considerable quantity of dust in the air within the building, but by means of ventilation and dust-chambers the air inside the building is now rendered comparatively pure to what it was at first. In my previous reports, attention was directed to the mode of drying the ore, and the waste of heat in the open kilns which were then used. They were made in the shape of an inverted cone, having a hole in the bottom, from which the ore was drawn out when dried, and the kiln emptied ready for another charge. The company have now constructed much larger kilns, which arc 20ft. in diameter at the top and 37ft. in depth, capable of holding 100 tons of ore. These kilns are not emptied after every charge, but every third day about 50 tons are drawn out, and a similar quantity of wet ore and firewood added on to the top of the charge. By this means the kilns are never allowed to get cold, and there is always sufficient heat below to set fire to the firewood, which is added and mixed with the new charge of ore. With these kilns the ore is dried more cheaply than previously. In reference to the process of treating the ore by the Cassel process at this company's works, the manager (Mr. Barry) states: "The ore is first dried in open kilns, excavated in tufaceous sandstone. The last new ones, 37ft. deep by 20ft. in diameter at the top, and tapering down to the bottom, where it is finished off with a brick arch, having a door and iron chute for discharging the dried ore into trucks. These kilns are first charged with wood and ore in layers, each layer of wood being about sft. apart. After the kiln is fully charged the wood is lighted, and after being all burnt up, about one-half of the charge is withdrawn—so tons—and another 50 tons of raw ore, together with wood, added on to the top; after which about 50 tons is withdrawn every third day. This method of drying the ore is found to be very economical as regards fuel, as there is not a large surface of cold material to heat up as is the case with the smaller kilns, which are emptied at each charge. The cost of firewood used in large kilns is about Is. 6d. per ton of ore dried. After the ore is taken from the kiln it is then put through the rock-breaker, from which it falls into a hopper, and thence, by automatic feeders, it is fed into the stamp-mortars, when it is pulverised until it passes through a 30-mesh and sometimes a 60-mesh screen. The company intend in the future to use a 40-mesh standard. As the pulverised dust pass through the screens it falls into a narrow trough, when it is conveyed by means of an Archimedian screw into a dust-bin at one end of the battery, and from this bin the pulverised material is lifted with a bucket-belt elevator and discharged on to an Bin. rubber belt with rope edges, and conveyed to and across the hopper 110 ft. long, running the entire length of the cyanide plant-house. This hopper has twenty doors for discharging the sand into trucks, which are then run straight out over the percolating vats on to travellers running on rails, which are fitted with hand traversing gearing, enabling a truck to be tipped at any part of the vat. This is an important point, as sand has a tendency to pack if moved about or touched in any way after being tipped into the vat. As a further preventive against packing there is a small traveller running under the main traveller, with a platform just at the height that the sand is to be filled up to. All trucks are tipped over this platform, which breaks the fall and throws the sand off in a light shower all around. When the vats are filled up to a depth of about 2ft., a strong solution of cyanide —0-4 per cent, is introduced into the bottom of the vat under the filter-cloth, and forced up through the sand until it stands about 2in. above it; the solution remaining under the filter-cloth is then drawn off, and the filtration commences; the 2in. on the surface taking about twenty-four hours to percolate through. After the whole of the strong solution has been taken out of the ore, a weak stock solution is run on the top of the ore to a depth of about 6iin. The cock connecting with the vacuum cylinder is then opened, and in about thirty hours the second solution has passed through; after which about 10in. of water is run on to the top, and when this has gone through the ore the operation is completed. The slud»e-door in the vat is opened, and the sand sluiced out by means of two 2in. hose-pipes under a head of 150 ft. The vats are all circular, 22ft. 6in. in diameter and 4ft. in depth, of which sin. is taken up by the filter bottom, which consists of a wooden grating with edges rounded off on. the upper side, having a