Phosphoric acid hemihydrate process

Developments of recent years include plants designed to precipitate the calcium sulfate in the form of the hemihydrate instead of gypsum, hi special cases, hydrochloric acid is used instead of sulfuric acid for rock digestion, the phosphoric acid being recovered in quite pure form by solvent extraction. Solvent-extraction methods have also been developed for the purification of merchant-grade acid, which normally contains impurities amounting to 12 18% of the phosphoric acid content. Processes for recovering part of the fluorine in the phosphate rock are in commercial use. 

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 ... 

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. 

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. 

The fluoride portion is either removed as gaseous silicon tetrafluoride, if silica is added, or as the sparingly soluble sodium hexafluorosilicate, which remains with the calcium sulfate. 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 hemihydrate. The variants also differ in the concentration of the phosphoric acid produced, but it is never more than 43%. The basic process was patented by Lawes in England in 1842, but the presendy used variant was developed by the Dorr-Oliver company in the 1930s. 

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 hemihydrate. 

Fig. 1.5-2. Flow Sheet for Phosphoric Acid Manufacture using the Fisons Hemihydrate Process (variant 3).

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... 

Table 11.20. Process Requirements of the Hemihydrate Process Product Phosphoric Acid Process Hemihydrate...

Phosphoric Acid 55 221-261 105-127 U u (40% PjO ), sulfuric acid 3.0% (2.5% SO3). calcium sulfate (hemihydrate) slurry fluorine compounds. Liquid phase, gases containing HjO and Si i are evolved Foam distribution process... 

Since 1969, a substantial number of commercial hemihydrate process plants have been built in various modes. In the hemihydrate process, reaction conditions are higher in temperature and phosphoric acid strength so that the stable solid phase is calcium sulfate with one half molecule of water. 

The principal processes currently in use are shown in Table 10.8. Over 90 percent of the world s phosphoric acid is produced by the dihydrate method, but there is likely to be increased production by the hemihydrate method because of advantages in some situations. The dihydrate processes listed have been modified over the years so that many slightly different configurations of the same process may exist. 

Potassium sulfate and ammonium sulfate solution obtained as a byproduct from caprolactam production are used industrially. In the feba phosphoric aad process (Figure 13.8), phosphate rock is reacted with a mbcture of sulfuric and nitric acids under conditions that form calcium sulfate in the hemihydrate form, which is re moved by fittiation 115]. In this process, part of the sulfuric add may be replaced by ammonium sulfate. 

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