Phosphoric acid dihydrate processes

The impurity content can be affected by the way in which the phosphoric acid manufacturing process is carried out single stage processes, or newer processes, which lead to the hemihydrate, have a higher impurity level, so that further purification steps are necessary. Two step processes, which either lead via the dihydrate to the hemihydrate (Central-Prayon process) or via the memihydrate to the dihydrate (Nissan process), provide products, which can be utilized without further purification steps. Should further purification of the phosphogypsum be necessary, two types of process are possible ... 

A broad comparison of the main types of processes, the strength and quaUty of phosphoric acid, and the form and quaUty of by-product calcium sulfate are summarized in Table 7. Because the dihydrate process is the most widely used, the quaUty of its acid and calcium sulfate and its P2O3 recovery are taken as reference for performance comparisons. Illustrative flow diagrams of the principal variations in process types have been pubUshed (39). Numerous other variations in process details ar also used (40—42). The majority of plants use a dihydrate process and some of these have production capacity up to 2100 of P2O3 per day. 

There are several variants of the Dorr process which differ according to the treatment of the calcium sulfate. Some variants produce the dihydrate, gypsum, others produce the hemi-hydrate. The variants also differ in the concentration of the phosphoric acid produced, but it is never more than 43 percent. The basic process was patented by Lawes in England in 1842 but the presently used variant was developed by the Dorr-Oliver company in the 1930s. Stevens, H. M., Phosphorus and Its Compounds, Interscience, New York, 1961. 

Guillini A process for making gypsum from the waste product from the Wet Process for making phosphoric acid. The waste is heated with water in an autoclave this removes impurities and converts the calcium sulfate dihydrate to the hemi-hydrate. 

HDH (1) [Hemi dihydrate] A Wet Process for making phosphoric acid. The calcium sulfate is first produced as the hemihydrate and then in another stage this is converted to the dihydrate. Developed by Fisons, UK, and operated in Yugoslavia and the United Kingdom. 

Prayon One of the Wet processes for making phosphoric acid by reacting phosphate rock with sulfuric acid. The byproduct is gypsum, calcium sulfate dihydrate. It uses a compartmentalized, multi-section, lined, concrete reactor, with finishing tanks in which the gypsum crystals mature. In 1990 one third of the wet process phosphoric acid made in the Western World was made in this way. The process was developed in 1977 by the Societe de Prayon, Belgium. Variations are known as PH2, PHI 1, and PH12. One variation uses solvent extraction with isopropyl ether and tri-n-butyl phosphate. 

In the wet process, phosphate rock is reacted in a slurry of phosphoric acid and calcium sulfate crystals containing a controlled quantity of sulfuric acid. The simplified reactions for the dihydrate process is as follows ... 

The principal dihydrate processes in use as of 2000 are shown in Table 23.6. During the decade 1990-2000, many of the smaller plants and even some larger than 450 tons per day P205 plants have been shut down due to environmental or market conditions. This has resulted in more production in Morocco, Jordan, and India, and less in Europe. Most of the world s phosphoric acid is produced by the dihydrate method, but there is likely to be... 

TABLE 23.6 Phosphoric Acid Plants, Worldwide Dihydrate Process, 2000... 

Hemihydrate processes are available for new facilities and also for the retrofitting of existing dihydrate plants. Several conversions to higher-strength acid have been made, where the steam saved in evaporation can replace fuel. Plants that make super-phosphoric acid, or where sulfuric acid plant steam is not available, are likely targets for conversion. 

Leyshon, D. W The Origin of the Modern Dihydrate Phosphoric Acid Process—Cominco 1931, A.I.Ch.E., Orlando, FL, 1990. 

The phosphoric acid obtained in the dihydrate process has a concentration of 28 to 32% (as P2O5). The yield, based on the phosphorus content of the apatite, is ca. 95%. Modern plants have reactors with capacities up to over 1000 t P2O5 per day. Fig. 1.5-1 shows a (simplified) flow sheet of a dihydrate plant. 

Fig. 1.5-1. Flow Sheet for Phosphoric Acid Manufacture using the Dihydrate Process.

The reaction involved in producing phosphoric add from fluorapatite and sulfuric acid by the dihydrate process may be r resented by the follounng equation ... 

Hemihydiate processes have the significant advantage of producing phosphoric acid with a relatively high con- centratioh without usiri my concentration step. There is also some interest in two-stage processes that involve crystallization in the hemihydrate form followed by re-crystallization in the dihydrate form (or vice verseO, with... 

BIPROKWAS Engineering, Ltd., Gliwice, Poland, oF fers a single-reactor dihydrate process with capacities of 30,000- to 200,000-tpy per train. The prc uct is 54% P2O5 acid containing about 1.5% F. BIPROKWAS has built eight phosphoric add trains and revamped two, the latest a 160,000-tpy unit. 

Most phosphate rocks of the fluoroapatite - type contain a significant quantity of fluorine, usually 3%-4% F by weight. In some cases up to 60% of this can be evolved during the manufacture of wet-process phosphoric acid. The remainder of the fluorine is retained in the gypsum (depending mainly on the rock composition), and most of remainder is in the filter acid 130], The fluorine is evolved from various stages of phosphoric acid processes from the reactor slurry surface, in the flash cooler, and in the concentration plant. In a dihydrate process the proportions are ------ --------------... 

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