PEBBLE PHOSPHATE ORE DRESSING 41 



used to move the material to the sump. After 

 the face of the rock is mined so far away from the 

 sump that the material does not flow properly 

 through the ditch, a new sump is blasted 50 to 75 

 ft. nearer the working face, and the pump is 

 moved to it. The pressure required for hydraulic 

 mining of this kind is not high, but the work 

 demands a large volume of water in order to trans- 

 fer the material through the ditch to the sump and 

 not choke the pump by an excess of solids. 



The outfit for transferring the ore to the washer 

 consists of a centrifugal pump of the volute box 

 type, with chilled cast-iron facing, directly con- 

 nected to the slip-ring motor. The tail-piece of 

 the pump, which must be flexible, is raised from 

 the sump by means of a set of triple blocks sus- 

 pended from a tripod. This arrangement is nec- 

 essary, because whenever the mouth of the suc- 

 tion pipe becomes clogged with grass, roots, etc., 

 it must be raised out of the water for cleaning. 

 In case the distance is more than 800 to 900 ft. 

 between the mine pump and washer, a relay pump 

 is usually installed. In such cases the mine pump 

 discharges into a sump from which the relay pump 

 draws its material and forces it to the washer. 

 This method is used owing to the difficulty arising 

 from back flow and water hammer when operating 

 two centrifugal pumps in series. Where the 

 centrifugal pump discharges without much head, 

 the end of the pipe line is elevated for a pipe 

 length at an angle of 45 in order to furnish suf- 



