DREDGING FOR METALS 221 



machinery is also similar to those employed in the ladder dredges 

 of equal capacity. 



The appliances used for the treatment of the dredged materials 

 can be divided into two classes, namely, the screen and elevator 

 and the sluice-box. The former is provided with a screen through 

 which the material raised is washed onto tables and discharged astern 

 of the dredge in a semicircular chute. The coarser material passes 

 out of the screen into a tail chute also to be deposited astern, 

 either by passing over a sluice run or being conveyed in the trays 

 of a tailings elevator. The sluice-box dredge is the one in which 

 the gold is caught in a long sluice run, fitted with ripples, into which 

 the buckets tip their burden. 



In the screen type of dredge the buckets discharge the material 

 into a hopper, through which it passes into either a revolving or 

 shaking screen of sufficient size and length to enable it thoroughly 

 to screen and wash the most difficult material. The revolving screen 

 is usually made 16 or 17 ft. long and 54 in. in diameter and built 

 with a frame of steel rings and perforated steel plates so arranged 

 that they can be easily renewed when worn out without removing 

 the entire screen. The screen is mounted on adjustable steel rollers 

 on a pitch of 1 in. to the foot and is driven by special shafts and gears 

 connected with the main engine. Streams of water are introduced 

 into the upper end of the screen to wash the material as soon as 

 it enters; water is supplied by a centrifugal pump operated by a 

 special engine. In the screen all the fine materials being washed 

 will fall through the holes while the stones and the coarse materials 

 will pass out of the lower end and be deposited on the banks by 

 means of an elevator or other device. The gold, together with the 

 materials and water falling beneath the screen, are collected in a 

 specially constructed box which will equally distribute the materials 

 upon the gold-saving tables. These can be made of different designs, 

 but as a rule are constructed of steel plates and angles. They can 

 be built 14 ft. long and 12 ft. wide, made in four widths of 3 ft. each 

 stepped one above another with division plates between them. 

 The tables have an inclination of 1 in 10 and are covered with some 

 fibrous substance held down by strips of expanded metal acting as 

 ripples. Besides the water from the screen, which is conveyed onto 

 the gold -saving tables, together with the materials, to facilitate 

 the washing operations additional water is supplied by a special 

 pipe whose flow on each division is regulated by a sliding door. 



