Production of Potassium Phosphate by a Conversion Method
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Fibre Chemistry, Vol. 52, No. 3, September, 2020 (Russian Original No. 3, May-June, 2020)
PRODUCTION OF POTASSIUM PHOSPHATE BY A CONVERSION METHOD K. S. Kirish,* P. P. Vlasov,** and B. A. Dmitrevskii***
UDC 661.32-661.5
A conversion method using a closed cycle for production of potassium dihydrogen phosphate from available feedstock, i.e., hot-leached potassium chloride (98%) or its mixture with sodium chloride, purified extraction phosphoric acid, and technical sodium carbonate (bicarbonate), with separate addition of the reagents is reported. Solubility diagrams of the KCl–NaH2PO4–NaCl–KH2PO4–H2O system with joint and separate addition of the starting reagents are calculated. A product of higher quality with lower chloride content is shown to be produced in the second instance. The calculations are confirmed by laboratory experiments.
New technologies for growing agricultural crops and widespread use of enclosed ground for year-round production of expensive produce impose more rigid requirements on the variety and quality of mineral fertilizers. In particular, hothouses experience the most acute demand for chloride-free potassium-containing fertilizers, especially potassium phosphate. The need to expand the scale of chloride-free potassium phosphate production for agriculture is predetermined by the increased demand for them from traditional consumers and the expansion of their applications [1, 2]. Potassium phosphate production is based on phosphoric acid (H3PO4) produced by decomposition of natural phosphates by sulfuric acid and purified from impurities by chemical methods or treatment with organic extractants. The use of extraction H3PO4 to produce potassium dihydrogen phosphate (KH2PO4) is preferred because of its low cost. However, the presence of large amounts of impurities in it that are transferred from the phosphate feedstock into the extracted solution cause serious problems [3, 4]. H3PO4 is neutralized in the traditional process by potassium carbonate, which increases the cost of the product. Acid produced from phosphate feedstock by an electrothermal method can be used to product potassium phosphate in limited quantities. However, this process has not been widely used in industry because it is highly energy-intensive and contaminates the environment with highly toxic gaseous and solid wastes. Furthermore, H3PO4 produced by the electrothermal method contains heavy-metal salt impurities that limit the scope of its use [4]. Extraction H3PO4 directly neutralized or preliminarily purified of impurities is more suitable for production of mineral fertilizers. In the first instance, fertilizers suitable only for use on open ground in fields are obtained because of the impurity content. The second method can produce purer fertilizers suitable for enclosed ground. However, the process in this instance becomes expensive. The greatest difficulty in producing chloride-free potassium-containing fertilizers is selection of the potassium component. Expensive potassium carbonate (K2CO3, potash) or potassium hydro
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