ioo CHEMICAL FERTILIZERS 



at a temperature of 8oo-iooo C. (i5oo-i8oo F.) are by 

 all systems cooled as rapidly as possible by passing through 

 boiler tubes to prevent decomposition due to reversion 

 of the reaction. The temperature of the furnace gases is 

 generally lowered to about 20O-25o C. (36o-48o F.). 

 The second part of the reaction, where nitric oxide combines 

 with oxygen, according to the equation NO + O = NO 2 , 

 does not take place at temperatures above 500 C. 

 (980 F.), and only reaches a workable rate at tempera- 

 tures below 200 C. (390 F.). To accelerate the oxidation 

 of nitric oxide, the gases are conducted through a special 

 cooling apparatus to lower the temperature. Part of this 

 cooling may be carried out by passage through evaporators, 

 where calcium nitrate is evaporated by this waste heat. 

 In the end the waste gases pass through aluminium con- 

 densers, where the temperature drops to about 50 C. 

 (122 F.). Owing to the slow rate of oxidation, the gas 

 is then passed through some vertical iron cylinders with 

 acid-proof lining, until the composition of the gases is about 

 98 % common air, and 2 % of a mixture of nitrogen peroxide 

 and a little nitric oxide. The gases then pass away to the 

 absorption towers, which usually consist of three series of 

 stone toweis about 60 ft. high and 18 ft. wide, down 

 which water flows and absorbs the nitric acid. The re- 

 maining gases contain some nitric oxides, and pass through 

 two wooden towers, filled with lumps of quartz, down 

 which a solution of sodium carbonate is allowed to flow, 

 producing sodium nitrite. The absorption follows the 

 usual counter current-system, water flowing in an opposite 

 direction to the flow of gas, so that the first tower contains 

 the strongest acid and the third tower the weakest. The 

 strength in the weakest tower is usually about 5 % of nitric 

 acid, and that in the middle tower about 20% of nitric 

 acid, while the strength in the strongest tower is about 

 40% of nitric acid. In the acid absorption towers the fiist 

 action of nitrogen peroxide is to act upon water, with the 

 production of nitrous and nitric acid, according to the 

 equation : NO 2 + H 2 O = HNO 3 + HNO 2 . The nitrous acid 



