NITROGEN FIXATION LOF 211 



The ammonium sulphate production is, however, closely related 

 to the steel-mill business, and the recent slack in this industry was 

 clearly reflected in the coke production, and naturally also the am- 

 monium sulphate output, as shown in the foregoing tabulation. 



THE ARC PROCESS 



The principle underlying this process is the possibility of chemic- 

 ally uniting part of the free nitrogen and oxygen in the air at such 

 high temperatures as only the electric arc is capable of producing, 

 this being around 5,000° to 6,000° F. 



Air, which is only a mechanical mixture of nitrogen and oxygen, 

 is thus passed through an electric arr, furnace, when a small part 

 (about 2 per cent) of these elements combine chemically and form 

 nitric oxide, NO. This gas, when leaving the furnace, has a tem- 

 perature of around 1,500° to 1,800° F., and in order to prevent the 

 NO from dissociating or breaking up, it must be rapidly cooled to 

 a temperature around 100° F., which is done in two steps. The hot 

 gas is first passed through ordinary steam boilers, which thus serve as 

 coolers, with recovery of heat in the form of steam for use in other 

 parts of the process. The temperature of the gas is lowered to about 

 300° F. by these boilers, but below this point it becomes necessary to 

 carry out the further cooling in aluminum coolers through which 

 water circulates, without, of course, coming in direct contact with 

 the gas. The reason for this is the fact that nitric oxide at these low 

 temperatures, in the presence of moisture in the gas from water leak- 

 ing through the tubes, begins to oxidize to nitric acid, which cor- 

 rodes iron but not aluminum. 



From the aluminum coolers the gas mixture is now conveyed to an 

 oxidation chamber, a big sheet-steel tank lined with fire brick. The 

 purpose of this oxidation tank is to give the gas sufficient time to 

 oxidize the nitric oxide NO to nitrogen peroxide, N0 2 or N 2 4 , this 

 being desirable for the absorption of the gas which is to follow. 



From the oxidation tank the gas is carried through an absorption 

 system consisting of several groups of absorption towers, usually 

 five towers in series per group. These towers are of enormous size, 

 the inside being filled with lumps of quartz or other materials which 

 the acid will not attack. Water is admitted to the top of the third 

 tower, and when it trickles clown over the quartz filling it meets the 

 ascending gas and is converted into a weak nitric acid. This acid is 

 then pumped to the top of the second tower, where it is used as the 

 absorption liquid, and similarly the acid from the second tower is 

 used for absorption in the first tower, thus gradually increasing in 

 strength. After having thus passed the third tower, about 80 per 

 cent of the N0 2 gas has been absorbed, and this is about all that can 



