Fertilizers from mineral phosphates, processing

Rock phosphate is used as fertilizer only for acid and humus-rich soils. Generally, phosphates are fertilized in the form of water- or acid-soluble compounds, derived from rock phosphate (processed mineral phosphate fertilizers). An example is superphosphate, a mixture of monobasic calcium phosphate and calcium sulfate (7-9% P). In double or triple superphosphate the P-con-tent rises to about 20%. Manures contain... 

Phosphate rock, mined widely throughout the world for its fertilizer value (see Fertilizers), in certain regions contains a few percent of lanthanides. For example, the apatite deposits in the Kola peninsula on the Russian/Finnish border. The Ln content is recoverable from the various processing residues, and because other Ln-containing minerals, such as loparite [12173-83-0], are also found there, the location suppHes a significant part of the demand in Eastern Europe. 

Major routes of mineral phosphate (phosphate rock) into finished fertilizers are outlined in Fig. 11.5 and are discussed below. It is obvious from the figure that although phosphate rock is used directly in several major fertilizer production processes (ordinary superphosphate, nitric phosphates), most important processes require that the rock phosphate first be converted to phosphoric acid (H3PO4). Phosphoric acid production, then, is a very significant component of the phosphate fertilizer industry. The processes used for the production of fertilizer-grade phosphoric acid are known collectively as wet processes, and involve, initially, dissolution of the rock in sulfuric acid. (The highly specialized technology of wet-process acid production was discussed in Chapter 10.)... 

EPA (1984) estimated that about 0.2 Ci of thorium-230 is annually emitted into the air from uranium mill facilities, coal-fired utilities and industrial boilers, phosphate rock processing and wet- process fertilizer production facilities, and other mineral extraction and processing facilities. About 0.084 Ci of thorium-234 from uranium fuel cycle facilities and 0.0003 Ci of thorium-232 from underground uranium mines are emitted into the atmosphere annually (EPA 1984). 

In the United States, the situation was in many ways different. With its large sulfur, natural gas, phosphate, and even potash resources, America s fertilizer industry rested on a sound base. It was an exporter of minerals and fertilizers, and did not have to worry to the same extent as Europe s industry about competing imports from Socialist countries. But reserves of sulfur extracted by the Frasch process have been depleted in Louisiana and Texas, and President Ronald Reagan s payment in kind (PIK) farm-acreage cuts reduced the fertilizer requirement of American farmers. These farmers are also much in debt and are having trouble selling their products on saturated markets. 

Essentially all fertilizer phosphorus now is derived from mined ores. (The occurrence, mineral characteristics, mining, and benefici-ation of major phosphate ores were described in some detail in Chapter 23.) Worldwide, about 85 percent of the mined phosphate eventually finds its way into fertilizer.3 As mentioned earlier, the most conservative estimates indicate a sufficiency for hundreds of years at expected consumption levels. Supply problems of the immediate future will relate chiefly to exhaustion of the better ores, with the result that ores of lower grades and higher impurity contents will have to be processed. 

Naturally occurring F associated with hydrous minerals has low mobility because it is occluded in structures. Airborne fluoride pollutants (from smelters, rock phosphate fertilizer factories, etc.) are, in contrast, easily dissolved on contact with the soil. These forms of fluoride can be bioaccumulated by plants before leaching, sorption, or precipitation processes have a chance to lower solubility. 

Sodium fluoride occurs naturally as the mineral villiau-mite, although the compound is not produced commercially from that source. Some sodium fluoride is obtained as a byproduct of the manufacture of phosphate fertilizers. In that process, apatite (a form of calcium phosphate that also contains fluorides and/or chlorides) is crushed and treated with sulfuric acid (H2S04). The products of that reaction include phosphoric acid (H3P04), calcium sulfate (CaS04),... 

If the objective is to prepare the safest mineral fiber that it is theoretically possible to manufacture, no trace of a toxic substance can be tolerated There was never any doubt in the author s mind that only food grade raw material could be used in this process, although the question of using purified wet phosphoric acid, derived from fertilizers, was raised from time to time. In my judgement, the best commercially available purified wet phosphoric acid is inferior to furnace acid and, although it probably could be used, it would be done with some loss of a safety margin that has been paramount throughout the history of this project. Safety is the hallmark of phosphate fibers. When safety is not an issue, some other fibers have superior properties. 

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