110 CHEMICAL MANUEES. 



installations and use for this purpose indestructible fans with washer 

 purifiers which purify the gas completely. The fans should be rather 

 powerful, so that the amount of air drawn into the "den " during 

 •discharge is sufficient to allow the labourers to empty the " den " 

 under good conditions. Fig. 23 represents the tow^er constructed 

 by F. Benker and E. Hartmann. It has several compartments and 

 no packing. The gas penetrates into the first compartment, and 

 ^ascending it meets a jet of w^ater in the form of rain, produced by 

 ^ebonite pulverisers (Kestner Lille). From the first compartment 

 they pass into the second through the top, from the second they 

 pass into the third through the bottom, and so on up to the seventh 

 ■compartment, when they are exhausted. The silicic fluoride is de- 

 'Composed by w^ater into silica and hydrofluoric acid ; the first can 

 be separated by the filter press and the second can be concentrated 

 to 12 to 15 per cent by volume and by mixing w^ith alkaline chlorides, 

 •or even sulphates into the corresponding fluosilicates, which find 

 a use in opaque glass manufacture and in flux enamelling.^ 



1 The translator thinks it advisable to offer here a few criticisms on the 

 method of making superphosphate as described by the author. First of all, 

 the measuring tank has to discharge itself before it can be refilled, so the re- 

 ifilling cannot start before the phosphate is all in the mixer. However, it is just 

 possible that it can be refilled in the two minutes taken up in continuing the 

 mixing process, after all the acid and phosphate are run into the mixer — that 

 is to say if no hitch occurs. But if the same principle were adopted, with 2 to 

 2J ton mixers, as used in Great Britain, and the agitation prolonged, in accord- 

 ance with the weight of the mixing, then a 5 cwt. mixing taking two minutes' 

 -agitation, a 50 cwt. mixing ought to take twenty minutes, whereas it is shot into 

 the " den " shortly after the last bag is up the cups and the last drop of acid run 

 in. When the mixer is hand-fed and the lid removed, the fumes escape into the 

 air of the building, and that is bad for the men at work. Again, a cast-iron 

 mixer is a dead-weight on the boundary wall between the two " dens," and these 

 eaten away by acid are none too strong. Again, in Fig. 22 the phosphate should 

 all be weighed before it is shot at the foot of the cups, and the elevator should 

 then discharge right into the mixer, and the spray of acid and the spray of 

 phosphate should meet at the same spot. Then with a horizontal instead of 

 a vertical mixer a wooden tank fined with 5 lb. lead can be used. The only cost 

 is the mixing shaft and blades and the gun-metal discharge sluice valves. With 

 such a mixer, to take 2J tons of superphosphate, the mixing shaft is best made 

 in two pieces, and the middle extremities ending in projecting studs turned in a 

 lathe are screwed up with bolts and nuts. Then if one-half of the shaft break, 

 the other part can remain i7i situ, and the whole shaft need not be recast. But 

 care must be taken that the two halves revolve in perfect symmetry. Again, 

 it is a very awkward thing to empty a vertical mixer when it sets. All that has 

 to be done with a horizontal lead-lined mixer is to remove the (loose) boards by 

 which it is covered and set a man at work to dig it out, and if the mixer is near 

 the eaves all that he has to do is to remove a tile or two to secure ventilation. 

 But in a vertical mixer, were it not for the tiny mixings described by the author, 

 it must be a more than ordinary trying task. Coming now to hot acid versus 

 cold acid: some phosphate can be mixed very well with acid, e.g. Carolina 

 TDhosphate. But Somme phosphate requires hot acid, and the translator found 

 it advantageous to keep a 10 ton tank at 140° F. This he diluted with water 

 as mixed as the author suggests. Somme is not in any way the heau ideal of 

 a raw material. — Tr.. 



