140 CHEMICAL FERTILIZERS 



with an equal volume of water. The cement should be used 

 within an hour from mixing, and the finished work heated 

 to about 100 C. (212 F.), for several days, or at a 

 higher temperature for a shorter time. Another recipe 

 consists of 10-100 parts of ground brick or fine sand, 

 mixed with i part barium sulphate, to which is added 

 20-50 parts of sodium silicate of i -3 specific gravity, that is 

 sufficient to be able to mix the material to a convenient 

 consistency for trowel work. These cements stand the 

 action of sulphuric acid fairly well. The barium sulphate 

 cement sets more slowly than the lime kinds, but it ulti- 

 mately becomes very hard and resistant to acids. A thinner 

 mixture of barium sulphate, clay and sodium silicate makes a 

 very good wash to rub over the surface of existing and some- 

 what damaged brickwork, as it penetrates into the cracks, 

 crevices and pores, and fills them up with a meal which 

 resists the action of acid very well. 



The metal part of the screw which works inside the mixer 

 can be made from what is commonly known as chemical 

 cast iron, some kinds of which are sufficiently resistant for 

 the purpose. There are now several alloys, such as tantiron, 

 duriron, ironac, etc., which contain about 15 % of silicon, 

 and 85 % of iron, which are very suitable for this purpose. 

 Sometimes vertical mixers of an egg shape are used placed 

 over the dens, in which case the phosphate and acid are 

 fed into the mixer, and blended by beaters inside. These 

 vertical mixers are nearly always made of an acid-resisting 

 iron, and are rather more expensive and more difficult to 

 repair than the older forms. When the phosphate has been 

 ground up fairly fine, and the acid is of reasonable strength, 

 two or three minutes in the mixer is enough. The amount 

 of phosphate used at any time in the mixer depends upon 

 whether the phosphate is rich in carbonate or not. The 

 acid and the phosphate are run into any type of mixer simul- 

 taneously, which can either work on the intermittent or the 

 continuous system, according to the method of working. 

 The temperature of the acid used should generally be about 

 2 5 C. (77 F.), and its specific gravity a little under 



