PHOSPHORUS FERTILIZERS 141 



1*6. A certain amount of heat is evolved, chiefly owing to 

 the action of sulphuric acid on calcium carbonate, but the 

 temperature can be regulated to some extent by the degree 

 to which the acid has been diluted. The temperature to which 

 the mass rises will vary, but may occasionally reach 200 C. 

 (400 F.). The results of the heat produced, are that much 

 steam is given off, and the superphosphate is partially dried. 

 After having been mixed for two or three minutes, the sludge 

 is discharged from the mixer into a channel, down which 

 it flows into the dens. It is convenient that this channel 

 should be set at an angle, so that it may work by gravity, but 

 where this is not practicable a screw conveyor must be 

 arranged to help the sludge along. 



The dens are brick chambers of any convenient size up 

 to 1000 cub. ft. capacity, to suit the particular factory, 

 arid are best built of bricks which resist the action of acids. 

 "Unless they are fairly resistant, the bricks gradually crumble, 

 and the den ultimately falls down ; this is especially impor- 

 tant if any weight is placed upon the bricks at the top, as is 

 very frequent with vertical mixers. With the ordinary type 

 of den, which is a square brick building, the front is com- 

 posed of loose wooden boards, which are fastened to the front 

 without any special device excepting that of propping them 

 up with other planks. Special doors on hinges give endless 

 trouble, as the material of which the hinges are made becomes 

 corroded and worn. The planks become much damaged by 

 the action of the acids, and it is therefore desirable that they 

 themselves should be protected in some way. Planks made 

 of ordinary deal, or any soft wood, like pine or larch, can be 

 made very highly resistant by dipping into melted naphthalene. 

 Naphthalene at 100 C. (212 F.), penetrates the wood fibres 

 with great ease, and on cooling crystallizes in the spaces 

 between the wood fibres. Painting the planks with sodium 

 silicate solution also helps to preserve them. 



During the manufacture of superphosphate, there are 

 given off poisonous gases, consisting of carbon dioxide, 

 hydrofluoric acid, hydrochloric acid, sulphur dioxide and 

 volatile organic compounds. To protect the workmen from 



