Patent ID: 7172742

Claim:
A process for preparing phosphoric acid, which comprises: decomposing phosphate rock, separating the decomposed solution, converting the dihydrate gypsum and filtrating, is characterized in that, the process comprises the steps following: (1-1) in a first reactor ( 1 ), feeding phosphate rock slurry, sulfuric acid, and recycled low-concentration phosphoric acid ( 9 ) from a filter to conduct decomposition; and obtaining a first reaction solution; wherein the weight ratio of phosphate rock slurry to sulfuric acid to recycled low-concentration phosphoric acid is 1: (0.6â€“0.8): (1.0â€“2.5); (1-2) in a second reactor ( 2 ), feeding the first reaction solution obtained in step 1-1, and other phosphate rock slurry and sulfuric acid thereby making the reaction complete; (1-3) dividing the reaction solution obtained from the second reactor into two parts: one part ( 8 ) comprising 10â€“30% of the reaction solution flows into conversion cell ( 4 ), and the other ( 7 ) flows into a separator ( 3 ), wherein the product phosphoric acid is extracted; (1-4) in a conversion cell ( 4 ), feeding the dihydrate gypsum ( 10 ) separated out from the separator, some additional sulfuric acid and the one part ( 8 ) of the second reaction slurry from the second reactor to conduct the conversion reaction; and obtaining a conversion solution; and (1-5) in a filter ( 6 ), after filtering the conversion solution, feeding the filtrate ( 9 ) into the first reactor; wherein the filter cake is the crude low-concentration phosphoric acid hemihydrate gypsum useful for producing sulfuric acid and cement.