84 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



In the hard rock phosphate mines of Florida the coarse phos- 

 phate after leaving the rinser is made to pass over a picker belt, 

 which is usually made in the form of a large revolving table. The 

 phosphate rock remains on the picker belt during one complete 

 revolution of the table, being carefully inspected by men and boys 

 stationed around the table. The inferior rock, clay balls, flint and 

 limestone fragments, so far as recognized, are picked out and dis- 

 carded at this time, thus bringing up the grade of the shipment. 

 In the land pebble mines the phosphate rock from the last washer 

 falls on jig screens, the finer of which are 3-64 or 1-32 inch mesh. 

 From these screens the rock is elevated by endless cup chains to 

 the loading bin. 



DRYING. 



After being taken from the pit the phosphate rock must be 

 dried before being marketed. Two methods of drying are in use. 

 The first of these, which is adapted to drying coarse rock, consists 

 in piling the phosphate rock on. ricks of wood. The wood is then 

 burned, thus drying the rock. This method assists somewhat in 

 cleaning, since clay and sand, adhering to the rock, tend after dry- 

 ing to loosen and fall away in subsequent handling. 



The second method of drying, which is now largely employed, 

 is by the use of heated rotary cylinders through which the rock 

 is passed. The rock is introduced usually at the- cool end of the 

 cylinder, and by means of various devices is made to pass through, 

 escaping at the furnace or heated end. Although well adapted to 

 drying small pebble rock, the coarser rock when dried by this meth- 

 od must first be crushed. 



IMPROVEMENTS IN MINING METHODS. 



An important factor in the cost of producing phosphate rock 

 is the necessity of discarding the low grade rock, as well as that 

 which cannot be properly cleaned or separated from the minerals 

 with which it is associated. It is encouraging, however, to find 

 that through improved methods of mining the amount of phosphate 

 thus discarded is being gradually reduced, while the grade of rock 

 produced is maintained or advanced. In the modem phosphate 

 mining plants of Florida, practically no phosphate rock reaches the 

 dump except that which will paSs a 1-16, 3-64 or 1-32 inch mesh 

 screen, or is carried out with the overflow from the side opening 



