![]() A typical New Zealand dredge is shown in Figs. Ladder-bucket dredges are made in great numbers both in New Zealand and in Western America. This is, in the majority of cases, the most satisfactory and economical type, and the one in general use. Suction dredges, however, are not much used. The material handled ranged from fine sand to boulders 8 inches in diameter. A pit, 20 feet deep, was first excavated, and the sides were then sluiced down by jets of water, and sucked up through a hose pipe until a hole 200 feet square had been made, when the dredging apparatus was moved and a second pit excavated, the tailings being discharged into the first pit. Nevertheless, dredges of this type are in successful operation on the Snake River, Idaho, where a suction pump was working in 1899 on the material of the bank. The suction pipe is soon worn out, especially by coarse gravel, and the power required per ton of gravel is considerably greater than in the case of ladder-bucket dredges. It is difficult to regulate the relative amounts of gravel and water raised by suction pumps, the latter tending to be in great excess. weight were lifted by this pump, but larger ones occasionally blocked up the suction pipe. The water for washing was supplied from a reservoir by means of an 18-inch pipe. The gold was very finely divided, and was caught on plush mats which were washed every eight hours. The large stones were caught and separated from the fine stuff by a riddled hopper-plate. The pump was 3 feet 6 inches in diameter, and the suction pipe, which was 13 inches in diameter, could be applied at any point in an area within a radius of 40 feet. As long ago as 1891, a Welman suction dredge was in successful operation at Waipapa Creek, New Zealand. In this system, a centrifugal pump draws material through a large suction hose reaching to the bottom of the river. a crane and bucket or shovel are used to raise the gravel.Dredges may be divided into three classes, according as: In dredging, gravel is raised from the bottom of the river and delivered into a barge, and the material is there washed, the gold extracted, and the tailings sluiced back into the river. Grothe states that the first dredge was operated on the Clutha River in New Zealand in 1864, but even in 1891 the method had made but little progress, and was generally looked on with disfavor everywhere except in New Zealand. ![]() This method of recovering gold from the gravel of river beds has of late years made remarkable progress, and its extension to the working of all flat placers has completely changed the aspect of shallow alluvial mining. ![]() ![]() The gravel won by either of these three methods is washed in the usual way by sluicing.
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