MANUFACTURE OF SUPERPHOSPHATE. 105 



In other words, they wrought hke masons making mortar. But for 

 thirty years this work has been done exchisively by mechanical 

 means, which enables the work to be done more rapidly and in 

 larger quantities at a time. 



Consequently the mixing pit has been enlarged so much that in 

 its new form it constitutes the chamber, or more familiarly the " den " 

 or " house " in which the phosphate is rendered soluble. This 

 chamber is closed, and care has to be taken to eliminate and render 

 inoffensive the toxic gases which are disengaged from the material 

 during its decomposition. 



To mix the acid with the phosphate a "mixer" or mixing 

 machine is used, constructed and installed thus : The mixer con- 

 sists of an egg-shaped pan 1-6 metre wide at top (say 64 in.), and 

 1-20 metre (say 48 in.) wide at bottom, fitted with two discharge 

 doors, with lever and counterpoise, which enables the mixing to be 

 run into an enclosed space, called the decomposition chamber (" den " 

 or " house "), which is built on the ground floor or sunk in the ground. 

 In the pan a vertical shaft turns, driven by cog-wheel gearing, 

 and carrying blades of a special form arranged in a helicoid 

 manner ; . these lift, throw down, and triturate the mass, after the 

 style of a plough as it works the ground, and prevent it at the 

 same time from being deposited and attached to the sides. It 

 suffices to pull the bent levers to open the discharge doors, and 

 thus let the hquid a jniree fall into the decomposition chamber 

 ("den" or "house"). 



The work is easy and rapid. The pan is made of cast-iron, with 

 2 per cent of a special alloy which renders it very resistant to acid. 

 The arms of the agitator and the blades as well as the valves are of 

 cast-steel. The mixing shaft makes sixty turns a minute ; the mix- 

 ing is triturated (churned) until the pulverized phosphate is intimately 

 mixed with the acid. When the ground phosphate is too coarse to 

 pass through a 70 mesh sieve the mixture remains longer in the 

 liquid state, and then the length of time occupied in mixing must 

 be prolonged. The acid, contained in a lead-lined tank, is drawn into 

 a measuring tank by turning a valve ; it flows through a If inch 

 lead pipe into the mixer in the form of a shower of rain. At the 

 same time the crushed phosphate — previously weighed and laid on 

 sacks on two inclined planes to right and to left of the mixer — is run 

 into the mixer. In certain factories the phosphate is brought to the 

 mixer by an elevator, and received in buckets by means of which it 

 is run into the mixer. The bags retain about 1 per cent of phos- 

 phate in the fabric. The mixer can take a charge of 225 to 250 

 kg. (495 to 550 lb.). When the phosphate is rich in carbonate of 

 lime the mixture froths and threatens to prime. Such a mishap 

 is obviated by diminishing the amount of x^hosphate. The acid and 

 phosphate ought to be run in simultaneously and never after each 



