Phosphate rock processes, examples

Large amounts of calcium sulfate are also produced as the by-product of other reactions or by synthetic processes. The production of phosphoric acid (H3P04) from sulfuric acid (H2S04) and phosphate rock, for example, results in the formation of calcium sulfate, which can be recovered and purified. Calcium sulfate can also be produced in the reaction between other calcium compounds, such as calcium hydroxide (Ca(0H)2) and sulfuric acid. 

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. 

This input to design refers to the long-term stability of the raw material sources for the plant. It is only of importance where the raw materials can or do contain impurities which can have profound effects on the corrosivity of the process. Just as the design should cater not only for the norm of operation but for the extremes, so it is pertinent to question the assumptions made about raw material purity. Crude oil (where HjS, mercaptan sulphur and napthenic acid contents determine the corrosivity of the distillation process) and phosphate rock (chloride, silica and fluoride determine the corrosivity of phosphoric acid) are very pertinent examples. Thus, crude-oil units intended to process low-sulphur crudes , and therefore designed on a basis of carbon-steel equipment, experience serious corrosion problems when only higher sulphur crudes are economically available and must be processed. 

All phosphorus fertilizers come from wet process phosphoric acid or directly from phosphate rock. Normal superphosphate, triple or concentrated superphosphate, and ammonium phosphate are the three common types used. Normal or ordinary superphosphate (NSP or OSP) is mostly monocalcium phosphate and calcium sulfate. It is made from phosphate rock and sulfuric acid and is equated to a 20% P2O5 content. It led the market until 1964. The production of normal superphosphate is similar to that for the manufacture of wet process phosphoric acid (Chapter 2, Section 3) except that there is only partial neutralization. Normal superphosphate is no longer used to any great extent. The following reaction is one example of an equation that represents this process. 

Partially Acidulated Phosphate Rock. Not all phosphate rock is suitable for direct application and may require the addition of a more soluble form of phosphate. An alternative is partial acidulation to render its P205 more available. The partially acidulated phosphate rock (PAPR) process depends on treating ground phosphate rock with only a portion of the stoichiometric value of acid, for example 50 percent PAPR.19 The amount of water-soluble phosphate in PAPR varies according to the degree of acidulation. The crop response to the product is often quite similar to fully acidulated products such as SSP or TSR... 

Dicalcium phosphate is used as a supplement to animal food. Food-grade phosphoric acid is used as an antioxidant and acidulant, for example, giving a sharp taste to soft drinks. U.S. 3,988,420 (to Israel Chemicals Ltd.) describes a process for making both products from phosphate rock and hydrochloric or nitric acid. Determine which acid leads to the highest net present value for a plant that produces 5000 metric tons per year of food grade dicalcium phosphate. 

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... 

A wide variety of techniques arxl many types of equipment are used to mine arxl process phosphate Tockr-Phosphate rock is mined by both surface (open cast or strip mining and urxlerground methods. In the following sections, several examples of mining operations illustrate the diversity of mining methods and equipment that may be employed. 

Uranium occurs in most phosphate rocks, but its concentration varies from deposit to deposit. Some sedimentary rocks show notably higher concentrations than most igneous rocks. The most uranium-rich rocks found to date are those of Florida (up to 300 ppmw U), Morocco (up to 230 ppmw U), and Jordan (up to 240 ppmw U) [32,33[. In comparison, conventional uranium deposits such as pitchblende typically have a uranium content of 1,000-3,000 ppmw. When the phosphate rock is acidulated, up to 80% or 90% of the ur ium passes into solution in the phosphoric add. The exact amount of PR acidulated depends on its characteristics and the parameters of the phosphoric acid process employed. Examples of the uranium contents of different phosphate rocks and of phosphoric acids produced from them are indicated in Table 11.29. 

The partially acidulated phosphate rock (PAPR) process depends on treating grourxi phosphate rock with only a portion of the stoichiometric value of add, for example 50% PAPR. The amount of water-soluble phos- phate in"PAPR"varies according to the degree of acidulation. 

This process requires that 12 of the 18 moles, or 67%, of phosphoric acid produced be recycled to treat more phosphate rock. There are other metals in the phosphate rock and upon acidification they become salts. The insoluble salts remain with the calcium sulfate precipitate but the soluble salts such as magnesium sulfate or iron sulfate remain in solution with the phosphoric acid. Generally, high purity is not required for fertilizer grade phosphoric acid. The crude phosphoric acid reacts, for example, with ammonia to make ammonium dihydrogen phosphate. 

---