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TAH-HO HUANG Taiwan Fertilizer Co., Ltd., Taipei, Taiwan (Formosa), China
Making Strong Phosphoric Acid b y .
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Modified Wet-Process Using Solvent Extraction Extraction with 1 -butanol yields an unusually pure acid which contains up to 56% phosphorus pentoxide
MANY (4-6, 8,
ATTEMPTS HAVE been made 71) to produce stronger wet phosphoric acid for use in fertilizers, but the strongest acid made has not exceeded 51% phosphorus pentoxide. Most acids marketed contain only 28 to 327,. I n the work reported here, the phosphoric acid produced contains up to 567, phosphorus pentoxide. Other wet-process modifications for making stronger phosphoric acid result in high viscosity solutions which are difficult to filter. However, solvents such as acetone, ether, ethyl alcohol, and 1butanol ( 2 ) have been used in fertilizer analysis or research to extract free phosphoric acid. These solvents not only can lower viscosity of the strong acid, but also may improve its purity. Tests with a mixture that simulated the reaction product of sulfuric acid and phosphate rock showed that acetone and ethyl alcohol gave high acid recovery but solvent losses during handling \cere excessive. O n the other hand I-butanol, added successively in several equal aliquots a t temperatures above 50' C., yielded extraction well above 907,. After these preliminary investigations, the conventional digesting procedure for wet processes was standardized with pure monocalcium phosphate. This standardized digesting procedure \vas then applied to triple superphosphate and sulfuric acid mixtures. The strong phosphoric acid produced was finally extracted with 1-butanol according to a modified procedure which yielded 98 to 100% recoverv.
Experimental
Three types of phosphate rock (Table I) and technical grade 857c orthophosphoric acid were used. I t was desired to dilute the acid to 50.7'% phosphorus pentoxide. which gives maximum conversions in triple superphosphate manufacture (7). The actual concentration
based on specific gravity a t 20". 25', and 30' C. ( 9 ) was 49.27,. C.P. sulfuric acid was diluted to 757, acid. To prepare under- or over-acidulated triple superphosphates, 25-gram portiois of phosphaie rock were added to calculated quantities of 49.27, phosphorus pentoxide acid a t 29' C. in 250ml. borosilicate glass beakers. The mixtures were well agitated until the plastic stage was reached and then allowed to cure in the laboratory for 3 or 4 weeks, or heat-cured a t 120' C. for 4 hours. The cured mixtures were mixed with calculated quantities of 757c sulfuric acid and digested a t 130' C. in an oil bath for 5 hours. These conditions ensured that the resulting sulfate would be in the form of anhydrite (5, 6). The digested mixtures were extracted with three. four. or five successive portions of I-butanol. each measuring 1 ml. per gram of raw mixture, a t about 70' C. for 30 minutes or longer, and filtered with suction. The residues were dried a t about 125' C. until free from I-butanol, weighed. and analyzed for total phosphorus pentoxide and free phosphoric acid. For free phosphoric acid determinations. a modified procedure using 955% ethvl alcohol was used ( 3 ) . Conversion and extraction efficiencies were computed : Rock P?O, to acid PzO, conversion =
Acid P,Oi extraction efficiency =
\\-here LVT is grams of phosphate rock; per cent phosphorus pentoxide in the rock; re;, grams of filter cake; Pc,. per cent of total phosphorus pentpr.
oxide in the filter cake; and P
