2 CIRCULAR 436, U. S. DEPARTMENT OF AGRICULTURE 



are not continuous but are located over an area almost 450 miles long 

 and from 5 to 40 miles wide. The origin of these nitrate deposits has 

 been attributed to the decomposition by nitrifying bacteria of ancient 

 accumulations of seaweeds or guano, of the manure of vicunas and 

 llamas, or of the organic matter in the soil between the Andes and the 

 locations of the deposits, as well as to the oxidation of atmospheric 

 nitrogen to nitric acid during electrical storms in the Andes, to nitrog- 

 enous fumes from volcanoes, and to the fixation of atmospheric 

 nitrogen by alkali carbonates in the presence of oxidizable matter. 

 None of the theories proposed, however, account for all the conditions 

 associated with the deposits. The salts are usually intermingled with 

 sand, gravel, and clay, which they cement together, and the hard 

 cemented material usually lies beneath a loose aggregate consisting of 

 clay, gypsum, and other materials. The cemented material and the 

 overburden may vary from a few inches to several feet in thickness. 

 The sodium nitrate content of the cemented material also varies 

 greatly. The richer material is called caliche. The cementing salts 

 consist of nitrates, chlorides, borates, perchlorates, chlorates, sulphates, 

 and iodates of sodium, potassium, calcium, and magnesium. 



All the ore was formerly mined by hand, the operations consisting 

 of blasting the cemented material into large blocks by means of explo- 

 sives, breaking these into smaller pieces with sledges or crowbars, and 

 sorting out those pieces that were considered of sufficient richness 

 in nitrate to warrant further treatment at the refinery. The older 

 method of refining required an ore with at least a 15-percent nitrate 

 content for economical operation. In recent years, however, drag lines 

 to remove the overburden and electric shovels for excavation of the 

 ore have partially displaced manpower, and the introduction, in 1927, 

 of an improved refining process allows the working of ore of much lower 

 grade at certain plants. 



The older of the refining procedures, or Shanks process, involves 

 leaching the ore with boiling water, crystallizing the sodium nitrate 

 from the hot liquor by cooling, and draining the mother liquor from 

 the crystals. The material, as shipped, contains about 2 percent of 

 water. In the newer, or Guggenheim, process, the ore is extracted at 

 temperatures not exceeding 40° C, and the sodium nitrate is crystal- 

 lized from the solution by refrigeration to about 5° C. The crystals 

 obtained are centrifuged to remove mother liquor, melted in a furnace, 

 and the molten nitrate finally pumped through spray nozzles, the spray 

 solidifying in the form of pellets as it cools. 



SYNTHETIC SODIUM NITRATE 



Sodium nitrate is now produced synthetically at several nitrogen- 

 fixation plants. In the arc process for fixing atmospheric nitrogen, the 

 oxides of nitrogen formed by passage of air through an electric arc are 

 led upward through towers down which water flows. The oxides of 

 nitrogen are absorbed in the water and form nitric acid. The residual 

 oxides of nitrogen that escape absorption in these acid towers are 

 passed to alkali towers where they are absorbed in a solution of sodium 

 carbonate (washing soda). The liquor that flows from these alkali 

 towers is a solution of sodium nitrite, together with smaller quantities 

 of sodium nitrate, sodium carbonate, and sodium bicarbonate (baking 

 soda). Upon treatment of this liquor with the nitric acid obtained 

 from the acid towers, a solution of sodium nitrate is formed which is 



