Purification of Phosphoric Acid

For most fertilizer production processes, purification of wet-process phosphoric acid is not necessary. However, there are two common fertilizer uses that may call for partial purification  

Although ammonium polyphosphate sequesters most of the common impurities, excessive amounts of some impurities (especially magnesium and organic matter) cause precipitate formation. Superphosphoric acids usually do not form sludge, but magnesium and titanium have been known to cause sludgeTorming precipitates. 

A major fraction of sludge in most merchant-sludge acids is the compound (Fe,AI)3KHi4(P04)a 4H2O. It precipitates slowly over a period of several weeks therefore, long storage periods are required to ensure reasonable completion of the precipitation reaction. 

In the production of sodium tripolyphosphate and other condensed phosphates for use as builders in detergents, the whiteness of the product and all metal ions (without sodium and potassium) are the principal con- 

Even if wet-process phosphoric acid is concentrated to the superphosphoric acid stage, it still contains too much fluorine for production of feed phosphates though the other impurity levels may be acceptable for animal feed. For animal feed preparation, it is desirable that the phosphorus fluorine ratio should be at least 100 P IF, but normal wet-process phosphoric acid has a P F ratio of between 15 1 and 54 1, depending on its concentration, the composition of the rock from which it is made, and its production conditions. 

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FIGURE 2 Solvent extraction process for purification of phosphoric acid. 

Chemistry ndProperties. The chemistry of phosphoric acid manufacture and purification is highly complex, largely because of the presence of impurities in the rock. The main chemical reaction in the acidulation of phosphate rock using sulfuric acid to produce phosphoric acid is... 

Wet Process. Over 90% of the phosphoric acid produced, both in the United States and worldwide, is wet-process phosphoric acid used almost exclusively for agricultural appHcation as both fertilizers and animal feed supplements. Although constituting a small proportion of the total wet-acid production, a significant amount of phosphoric acid for food and technical appHcations is made by purification of wet-process acid. 

Modem commercial wet-acid purification processes (see Fig. 4) are based on solvents such as C to Cg alcohols, ethers, ketones, amines, and phosphate esters (10—12). Organic-phase extraction of phosphoric acid is accompHshed in one or more extraction columns or, less frequently, in a series of countercurrent mixer—settlers. Generally, 60—75% of the feed acid P2 s content is extracted into the organic phase as H PO. The residual phosphoric acid phase (raffinate), containing 25—40% of the original P2O5 value, is typically used for fertilizer manufacture such as triple superphosphate. For this reason, wet-acid purification units are almost always located within or next to fertilizer complexes. 

The estimated world production of wet-process phosphoric acid was 24,001,000 metric tons of P20 in 1993. Capacity was 34,710,000 metric tons. Over 90% of phosphoric acid production is wet-process (agricultural-grade) acid the remainder is industrial-grades (technical, food, pharmaceutical, etc) made by the thermal route or by the purification of wet-process acid. Table 11 fists U.S. production of wet-process and industrial-grade acids. 

Methods of decreasing the fluoride concentrations of phosphoric acid have been surveyed [46]. Apart from endorsement of the addition of a reactive form of silica prior (Eq. 10.22) to stripping, acid dilution prior to reevaporation has been recommended. The additional water vapor removal assists purification by entraining hydrogen fluoride and silicon tetrafluoride as they vaporize. 

The reactor effiuent passes into a feed-product heat exchanger, where it is partially condensed. After washing with dilute caustic soda to neutralize traces of phosphoric acid, it passes into a second exchanger and on to a high-pressure separator tO give a liquid and a vapor stream. The condensate goes to purification and the vapor to recycle. The vapor is cooled by the recyde-gas cooler and scrubbed with water to remove alcohol. The build-up of impurities like methane and ethane is controlled at this point by venting a small stream of the recycle gas. 

TLC is still regarded as a cheap and effective technique for ochratoxin A estimation, particularly because of its low cost and adaptability. New methods include extraction with a mixture of phosphoric acid and dichloromethane and purification by liquid-liquid partitioning into sodium hydrogen carbonate, before separation by normal-phase TLC and detection by fluorescence as usual (Pittet Royer, 2002) and extraction with a mixture of methanol and aqueous sodium bicarbonate solution, followed by partitioning into toluene before TLC (Ventura et al.,2005). 

A typical downstream process includes (1) removal of production microbes (biomass) and solids (e.g., gypsum) from the broth, (2) recovery of crude lactic acid, and (3) purification of lactic acid. The biomass and solid waste can be separated from the liquid streams by various means, such as filtration, centrifugation, and decantation. If calcium alkali is used to control the fermentation pH, it produces calcium lactate precipitates which must be dissolved by acids such as phosphoric or sulfuric acid to extract lactic acid back into solution. After sulfuric acid has been added, calcium sulfate (CaS0 -2H20, known as gypsum) is formed and must be removed from the liquid stream as a major solid waste. 

Acid hydrolysis of nylon 6 wastes [21, 22] in the presence of superheated steam has been used to produce aminocaproic acid, which under acid conditions is converted to e-caprolactam, and several patents have been obtained by BASF [23, 24]. Acids used for the depolymerization of nylon 6 include inorganic or organic acids such as nitric acid, formic acid, benzoic acid, and hydrochloric acid [23, 25]. Orthophosphoric acid [24] and boric acid are typically used as catalysts at temperatures of 250-350°C. In a typical process, superheated steam is passed through the molten nylon 6 waste at 250-300°C in the presence of phosphoric acid. The resulting solution underwent a multistage chemical purification before concentration to 70% liquor, which was fractionally distilled in the presence of base to recover pure e-caprolactam. Boric acid (1%) may be used to depolymerize nylon 6 at 400°C under ambient pressure. A recovery of 93-95% e-caprolactam was obtained by passing superheated steam through molten nylon 6 at 250-350°C [23]. 

Many phosphate rock deposits contain quantities of radioactive elements such as uranium and thorium. Selective leaching of uranium from raw phosphate ores is difficult because the U(V1) ion is incorporated into the crystal structure of apatite (Ca5(P04)3(0H,F,Cl)), rather than adsorbtively associated with it. Uranium is, therefore, typically recovered from phosphate rocks by recovering it from phosphoric acid produced by sulfuric acid leaching of phosphate ores. The radioactive species are also leached and must be removed during purification of the acid. Uranium in... 

Simplicity of production, high analysis, and excellent agronomic quaUty are reasons for the sustained high production and consumption of TSP. A contributing factor is that manufacture of the triple superphosphate has been an outlet for so-called sludge acid, the highly impure phosphoric acid obtained as a by-product of normal acid purification. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

Fig. 4. Schematic diagram of the solvent extraction purification of wet-process phosphoric acid.

Trickle bed reaction of diol (12) using amine solvents (41) has been found effective for producing PDCHA, and heavy hydrocarbon codistiUation may be used to enhance diamine purification from contaminant monoamines (42). Continuous flow amination of the cycloaUphatic diol in a Hquid ammonia mixed feed gives >90% yields of cycloaUphatic diamine over reduced Co /Ni/Cu catalyst on phosphoric acid-treated alumina at 220°C with to yield a system pressure of 30 MPa (4350 psi) (43). 

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