Caliche, leaching

Guggenheim A process for extracting sodium nitrate from caliche, a native sodium nitrate found in Chile. The ore is leached at 40°C with water containing controlled concentrations of magnesium and calcium sulfates. Operated on a large scale in Chile. See also Shanks. 

Shanks An obsolete process for extracting sodium nitrate from caliche, a Chilean mineral. The ore was leached with sodium chloride solution at 70°C and the sodium chloride and nitrate were separated by fractional crystallization. See also Guggenheim. 

Potassium nitrate, essential in the manufacture of black gun powder, was produced by the Chinese, who had developed gun powder by the tenth century AD. The process involved the leaching of soil in which nitrogen from urine had combined with mineral potassium. By the early 1800s, potassium nitrate had become a strategic military chemical and was still produced, primarily in India, by using the ancient Chinese method. The caliche deposits in Chile are the only natural source of potassium nitrate (2). These deposits are not a rich source of potassium nitrate, purifying only to about 14% as K O. 

Fig. 2. SQM s heap leaching of caliche tailings for iodine recovery from diluted iodate solutions.

Diluted iodate solution is obtained by hydrothermal vat leaching of caliche ore during nitrate recovery. Concentrated iodide solutions are obtained by heap leaching of old waste dumps (tailings) and low grade nitrate caliche, such as blasted overburden, left over by former nitrate producers. 

The third method (that is, the use of suitable selective solvents) is almost universally used in effecting selective dissolution. It may be used in connection with the temperature effect as in leaching caliche (nitre ores) or alone as in organic work. 

Caliche may have about 20 per cent sodium nitrate and 25 per cent sodium chloride. Now if we look at Fig. 5 we see that one method of treatment would be to leach the ore at a temperature of 80°C. At this point we would dissolve 118 parts of sodium nitrate and 13 parts of sodium chloride per 100 parts of water. Now by cooling this... 

The preceding description of soil formation is extremely generalized and does not include many soil types. For example, peat soils are predominantly composed of partially decomposed plant remains. In arid areas, upward water flow and evaporation at the soil surface can produce a hard caliche soil. In very warm and humid areas, leaching and organic matter decomposition occur quite rapidly and soil minerals can become highly weathered, resulting in saprolite soils. In ecosystems in these areas, biomass, instead of soil particles, may become the major repository of plant nutrients. 

In the subsoils of arid and semiarid soils, Ca commonly precipitates as cakite (CaCC>3) rather than being leached away. It is found as indurated layers (caliche and other local names) in many arid soils and as more diffuse CaC03 in Aridisols and Mollisols. Precipitation of CaCCTj in soils is affected by the rates of soil water movement, CO2 production by roots and microbes, CO2 diffusion to the atmosphere, and water loss by soil evaporation and plant transpiration. CaCC>3 layers are also derived from upward movement and evaporation of Ca-rich waters. Calcium carbonate accumulations can amount to as much as 90% of the mass of affected soil horizons. Gypsum precipitates in some arid soils, despite being about 10 x as water soluble as Ca carbonate. 

Chile, production is based on the Guggenheim method, which permits ores containing under 10% nitrate to be profitably worked. Caliche is mined by open-pit operations, using draglines and power shovels, and is crushed to about 80% between 3/4 and 3/8 inches (1.9-0.95 cm). This product is leached at about 40°C wth water in a series of vats each vat has a capacity of some 10,000 tonnes of crushed ore. The underflow from each vat is heated before passing to the next since sodium nitrate has a negative heat of solution. After extraction and washing are complete, the residue is removed from the vats and transported to a waste dump. 

Such leaching tanks and arrangements are used extensively in the metallurgical industries, for recovery of tannins from tree barks and woods, for leaching sodium nitrate from Chilean nitrate-bearing rock caliche), and in many other processes. 

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