50 



The dried rock is loaded into cars, which is then hoisted up 

 an inclined plane into the mill, and dumped by the crushers. 

 These are three in number, made of iron, by Baugh & Sons, of 

 Philadelphia, and are mounted on heavy frames, independent 

 of the mill building ; they are driven by belts from the main 

 shaft, and run at a speed of 450 revolutions per minute ; a man 

 feeds these crushers with the dried rock, which passing though 

 crushed, is picked up by elevators and delivered into the hop- 

 pers of the mill-stones. 



Of these mill-stones there are six pair; they are of the best 

 French buhr stone, and are driven by the crank shaft of one 

 engine; they are four feet in diameter and are make 170 revo- 

 lutions per minute. The amount ground depends entirely upon 

 the degree of fineness to which it is ground; in this mill the 

 rock is ground so that all will pass through a screen of 80 wires 

 to the inch, and the product is about 3 tons per pair of stones 

 per day of 10 hours. After passing through the stones the 

 powdered rock is received into elevators which deliver it into 

 a box through which it is screwed from the mill house into the 

 mixing house at an elevation of about 30 feet, and there is de- 

 livered, not having been touched by hand since it was fed as 

 crude rock to the crusher. 



The mixing is done in a tub of cast iron 8 feet in diameter, 

 which revolves 20 times per minute, and in which are small 

 ploughs, which revolve 160 times per minute. Into this tub a 

 weighed quantity of the powdered rock is thrown by simply 

 overturning a large scoop, which hangs from a steelyard, the 

 scoop and steelyard being suspended from a frame which 

 runs on a trainway from the pile of ground rock to the 

 mixing tub; a known weight of acid is now run in and the 

 revolving ploughs thoroughly incorporate the phosphate and 

 the acid. When a certain time has passed, an iron plug, 

 which stops up a hole in the centre of the tub, is raised and 

 the mixed mass, either in a semi-fluid or dry condition, 

 depending on the amount of acid added, falls through into a 

 space below. The amount of acid which is mixed with the 

 phosphate depends upon the grade of solubility desired, the 

 higher the percentage of Soluble Phosphoric Acid wanted the 

 larger the amount of acid to be added, and here is the chief diffi- 

 culty in the manufacture, for the higher the grade the more 

 pasty is the mass, and, therefore, the more difficult is the after 

 manipulation ; up to 5 per cent, of Soluble Phosphoric Acid the 

 mass comes from the mixing tub dry, and can be screened at 

 once and packed in sacks; but when enough acid is added to 

 render 11, 12, and 13 per cent, soluble, the mass comes from 

 the mixing tub a semi-fluid and will flow like mud 30 or 40 feet, 

 and must be left for a time varying, from two weeks to two 

 months, to harden before it can be handled ; the intermediate 

 grades also of 6, 7, and 8 per cent., when left, harden into a rock 



