CHEMICAL FERTILIZERS 



nitric acid, with the subsequent production of ammonium 

 nitrate. For this purpose ammonia and air are passed 

 together over heated platinum wire gauze. It is very 

 essential that the mixture should be very well incorporated 

 and move with a steady velocity, for which purpose the 

 most satisfactory type of apparatus consists of an expanding 

 cone followed by two or three perforated diaphragms, then a 

 platinum wire gauze and lastly a reducing cone leading to 

 the exit pipe. For the purpose of heating the platinum wire 

 gauze an electric current is used. The oxidation of ammonia 

 to nitric oxide gives rise to a considerable amount of heat, 

 and this may suffice to maintain the platinum at the correct 

 temperature. 



In ordinary practice a pure, strong ammonia liquor, 

 containing about 25 % of ammonia, is allowed to flow down 

 a coke tower, up which air and a small amount of steam are 

 allowed to pass. The mixture of air and ammonia needs to 

 be filtered, to remove any particles of dust. Oxide of iron 

 must be especially carefully removed, since it has a very 

 harmful action upon the platinum catalyst. The platinum 

 wire gauze is usually made from wire one four-hundredth 

 part of an inch in diameter (0*06 mm.), woven into gauze 

 with 80 meshes to the inch (32 per cm.), and mounted on 

 an aluminium frame. Greater efficiency is obtained by 

 using two or three gauzes close together. An output of 

 about ij tons of pure nitric acid per square foot of catalyst 

 in 24 hours, with an efficiency of 95 %, has been regularly 

 obtained. Without external sources of heat the efficiency 

 drops to about 85 %. The hot gas which leaves the 

 converter at about 40o-6oo C. (75o-noo F.) ; passes 

 through a cooler, where its temperature is reduced to 

 30 C. (88 F.). The subsequent absorption of the oxides 

 of nitrogen presents the same problem as does the arc 

 method described above, but as the concentration of nitric 

 oxide in the gas is very much greater, the size of the plant 

 is correspondingly reduced. The gases obtained may 

 easily contain about 10 % by volume of oxides of nitrogen. 

 The dilute acid obtained may be neutralized by adding 



