A New Process Of Producing Sulfuric Acid From Phosphogypsum

Phosphogypsum is still a by-product of wet-process phosphoric acid (WPA). Ingeneral, the amount of phosphogypsum is ranged from 4.8 to 5.5 tons whenproduced one ton WPA. By now, the whole amounts of phosphogypsum producedexceeded 0.28 billion tons per year all around the world, and among of thosephosphogypsum, more than 22 million tons were made per year in china. However,the utilizing ratio of the phosphogypsum was less than ten percent in our country.The deposit of the phosphogypsum is not only occupying much cultivable soil andseverely polluting environment, but also bringing prodigious economic burden torelated production enterprises. So, the comprehensive utilization of phosphogypsumhas become one of the key factors in the continuable development of the phosphatefertilizer industry.On the comprehensive utilization of phosphogypsum, the research wasconcentrated on the three aspects: in agriculture, the phosphogypsum was mainlyused to be fertilizer in the form of sulphur or calcium and for the ameliorable reagentin the salt or alkali soil; in the field of construction materials, the phosphogypsumwas mainly used for making gypsum powder, the gypsum plank, gypsum brick,cement and cement products; in the industry, the phosphogypsum was mainlytransformed into other sulfur forms, such as making ammonium sulfate, potassiumsulfate and sulfuric acid etc.Based on the amount of impurities in phosphogypsum,this study was focused on the effect of different impurities including phosphor and fluorine on the gypsum products,the pretreatment process,such as washing process,neutralizing process with lime and heat treating process, etc. and the comprehensiveutilization process of phosphgypsum. The process of producing sulfuric acid andcement from phosphogypsum was thought to be one of the most valuable methodsby comparing the various processes of the domestic and foreign phosphogypsumutilization and the production of sulfuric acid with the different raw materials. Also,as a recycle economic engineering encouraged by the nation, the process could carryout the recycling of sulphur resources and resolve the pollution problem of thephosphogypsum effectively. This investigation put forward a novel method byreferring to a great deal of literatures and summarizing the advantages anddisadvantages on the traditional production processes. The spirit of the process wassubstituting a new reducing agent-sulphur for coke. Through the thermodynamiccalculation, the process was feasible theoretically because anhydrous calciumsulphate could be decomposed at 721 degree under the reducing atmosphere ofsulphur.This article studied the gasification speed of the sulphur under the differenttemperature firstly, and the optimal speed of sulphur gasification was 11.0g/h. Underthe optimal gasification speed, the reaction condition, such as reaction temperature,reaction time, additives etc. could influence the phosphogypsum conversion ratio,With help of the comparison research of single factor experiment and orthogonaldesign experiment, The single factor experiment result showed that two additives canimprove the phosphogypsum conversion ratio significantly, they are B₂O₃ and V₂O₅.A mathematics equation Y=16.76+0.66×e^x/37.17 about the conversion ratio ofphosphogypsum and the Tyler standard sieve was obtained under 850 degreecentigrade and 1.5hr. The article also discussed the influence factors on theconversion ratio by orthogonal experimental design, and the regression equation wasobtained by multiply linear regression.T=-2.40128+0.00273A+0.15657B+0.04812C-0.01795D+0.30416E+0.32753F-0.17997G.The results showed that the significancyof effective factors was A＞B＞F＞E＞C＞G＞D in turn and the optimum combination was A₆B₃F₃E₃C₃G₁D2 (above T,A,B,C,D,E,F,G mean the conversion ratio, reactiontemperature, reaction time, SiO₂,AlPO₄,Fe₂O₃,Na₂SiF₆ and K₂SiF₆ in turn; n.conermark mean levels of every factor) by the range analysis method. Further, under theas-received optimum experiment conditions, the actual conversion ratio ofphosphogypsum reached to 99.12%. Finally, a schematic diagram that producingsulfuric acid and cement from phosphogypsum reduced by sulphur was put forward.The novel process was thought to be a sulphur-resources recycling green process,which can effectively reduce the pollution from phosphogypsum, and significantlyincrease the concentration of SO₂. Higher conversion ratio and lower investmentmade the process have a good future in the application of phosphogympsum.