The grab, or “ clamshell ” type of dredge, consists of a self-filling and discharging scoop or bucket, with the necessary engine power for working this from a crane. Well-known makers of this form of dredge are Messrs. Rose, Downs, & Thompson, Limited (see illustration), and Messrs. Priestman Bros. Both firms recommend, for this purpose, the double-chain grab. A one-ton capacity Kingston dredge, with 7-in. diameter cylinder, 10½-in. stroke engine, 50 lbs. working pressure, and surplus boiler powder to drive a centrifugal pump, trommel, and winches would cost about £600. The weight of such a grab, with engine and boiler, would be about 17 tons; platform dimensions, 5 ft. by 6 ft. The vertical lift from deck board, for stability, should not be over 20 ft., or, deducting the length of the grab, say, 14 ft. effective. As the working of the grab causes considerable oscillation of the barge, it is well to place the gold-saving tables, sluices, &c., on a separate barge. It is, therefore, probable that a complete dredging plant, capable of dealing with 40—50 tons per hour at a moderate depth would cost about £2,250, pontoons included, but built locally ….Read more
The Siberian Sluice and the apparatus at Voltchanka, which may be taken as a type, consists of a head sluice and three secondary sluices, which are placed at right angles to the head sluice, and which leave it at different points and converge to a common centre, where the tailings are discharged. The head sluice begins at a height of 13 feet from the ground ; it is about 90 feet long by 2 feet wide, and has a fall of about 1 in 14. The sands are dumped from the waggons on to a wooden platform situated above the sluice-head, and shovelled into the latter, a stream of water being turned in at the same time. After passing through a grizzly, the gravel runs over a series of cast-iron cross-bar riffles, which form a number of rectangular depressions (or pigeon-holes) in the bed of the sluice, by which the disintegration is favoured. The stream then flows over an iron screen, through which a part of it falls into the first secondary sluice, while the remainder continues its course over more pigeon-hole riffles. This arrangement resembles the Californian undercurrent. A second and a third screen open on to the other ….Read more
To understand How Does a Gold Dredge Work, we may want to start by looking at California or Alaska. Gold dredging in now practised on the rivers of California, has is now superseded river mining. In river mining, an entire river was frequently deflected from its course so as to lay bare a section of its bed. This was usually done by building two dams from bank to bank, with their foundations on bedrock, the water being carried off in a wooden flume, starting above the head-dam and terminating below the foot-dam.
Sometimes, tunnels were made to drain permanently large reaches and deliver the water at a lower point. Sometimes wing-dams were built out from the bank above and below the part of the river it was desired to work, and a third dam connecting their mid-stream ends was constructed parallel to the direction of the current. The space cut off was then pumped dry.
The river bed exposed by such methods was prospected and the pay-dirt when found taken out and washed, particular attention being paid to the surface of the bed-rock. The operations were usually terminated by the autumn floods.
River ….Read more