IMPURITIES OF PHOSPHATE ROCK. 83 



ings. The fine materials of the matrix pass through while the 

 coarse materials, including phosphate, flint, limestone boulders, and 

 clay balls, are lodged on the grating. The phosphate boulders are 

 then thrown by hand into a rock crusher near by, while the flint 

 and limestone boulders and clay balls are discarded. That part 

 of the matrix which passes the grating together with the rock 

 from the crusher is dropped into a log washer beneath. The prac- 

 tice in the land pebble mines is somewhat different from that fol- 

 lowed in the hard rock section, the matrix, as pumped from the 

 pit, being thrown as a rule onto a large revolving tube, known 

 as a separator, punched "hit and miss" with holes one or two inches 

 in diameter. As the separator revolves, the phosphate pebbles, as 

 well as the finer materials of the matrix, fall through the openings 

 and lodge on a screen beneath, while the coarser materials, in- 

 cluding sand, rock and clay balls, remain in the separator from 

 which they are carried to the waste dump. From the screen be- 

 neath the separator the phosphate rock passes into the log washer. 

 While this is the usual arrangement in the land pebble phosphate 

 mines, yet in some of the newer plants it has been found practi- 

 cable to omit the separator altogether, the rock from the dump 

 being allowed to enter the log washer after passing over a screen 

 of about 1-16 inch mesh. When the separator is omitted, prac- 

 tically all the matrix from the pit passes through the log washers, 

 and it has usually been found necessary in these plants to install 

 a crusher, which is then placed between the two logs. The larger 

 pieces of bone and phosphate rock, as well as the clay balls, if not 

 disintegrated by the washer, are broken up in the crusher, and 

 the phosphate which they contain is saved. 



The log washer, through which the phosphate rock is passed, 

 consists of two cylinders or logs placed side by side in a box or 

 trough. A series of blades arranged in a spiral is fastened to each 

 cylinder. The trough is inclined, the phosphate being run in at 

 the lower end, and as the logs are made to revolve in opposite 

 directions, the phosphate rock is pushed forward by the blades, 

 meeting as it goes a constant stream of water. By this means the 

 rock is fairly well washed, the water, carrying all the finer mate- 

 rials of the matrix, escaping at the lower end or in the newer 

 washers through an opening at the side of the trough. Frequently 

 the phosphate rock is passed through a second log of the same 

 type as the first, and in all cases receives a final rinsing while pass- 

 ing over screens. 



