212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



be absorbed by water, the resulting acid from the first tower having 

 a strength of 30 per cent, this being known as weak nitric acid. 



The reaction which thus has taken place in the three first towers 

 is as follows: 3 N0 2 4-HX>=2 HN0 3 +NO. 



The liberated nitric oxide, in the presence of oxygen and water, is 

 again converted into nitrogen peroxide and nitric acid. 



The remaining 20 per cent of the gas leaving the third tower is now 

 so weak that another solution than water must be provided for its 

 absorption. An alkali solution such as soda ash is used for this pur- 

 pose in towers 4 and 5, the resulting product being a mixture of 

 sodium nitrate and sodium nitrite, which is evaporated and may be 

 used directly as a fertilizer. If only sodium nitrite is desired the gas 

 is passed into a solution of caustic soda. 



The weak acid from the first tower is, however, the main product ; 

 but as such a weak acid can not be economically transported, it must 

 be converted into some neutral salt or changed into concentrated acid 

 of 95 per cent strength, which can be shipped in aluminum tank cars. 

 The concentration is accomplished by means of sulphuric acid, which 

 has a greater affinity for the water in the weak nitric acid, thus ab- 

 sorbing it, leaving the nitric acid in concentrated form. 



The neutral salt, generally produced from the weak acid, is cal- 

 cium nitrate, also known as nitrate of lime or Norway saltpeter, 

 because it is the main product of the large nitrate plants in Nor- 

 wajr. The calcium nitrate Ca(N0 3 ) 2 , is thus produced by treating 

 ordinary limestone with the weak acid, the reaction being as fol- 

 lows: CaC0 3 +2 HN0 3 ==Ca(N0 3 ) 2 +H 2 0+C0 2 . 



The resulting solution is evaporated by the waste heat from the 

 steam boilers before mentioned, and the product is then ready for the 

 market. 



This calcium nitrate contains 13 per cent of fixed nitrogen, and 

 is, like Chile saltpeter, an excellent fertilizer, although somewhat 

 hygroscopic. It is, as stated, the main product of the large Rjukan 

 nitrate plants in Norway, which have an annual productive capacity 

 of about 200,000 tons of nitrate, equivalent to about 26,000 tons of 

 fixed nitrogen. Over 300,000 electrical horsepower are used for 

 this, generated in magnificent high-head water-power plants. The 

 power requirements with this process are thus about 12 horsepower 

 years per ton nitrogen fixed, and from this high rate of consump- 

 tion it follows that the price of the power must be very low in order 

 to make the process an economic success. 



The majority of the furnaces used at Rjukan are of the Birke- 

 land-Eyde type, the latest designs having a capacity of 4,000 to 

 5,000 horsepower each. There are also some Schoenherr furnaces of 

 1,000 horsepower capacity each. Numerous other types of furnaces 



