Study On The Treatment Of Phosphogypsum To Produce Sulfuric Acid And Co-production Of Light Calcium Carbonate By Bipolar Membrane Electrodialysis Technology

Phosphogypsum is a solid by-product from the production of wet-process phosphoric acid.The production of per ton of phosphoric acid?100%in terms of??P₂O₅??will produce about 4.5-5.5 tons of phosphogypsum residue.Due to the characteristics that large discharge of phosphogypsum and low utilization rate and difficulty in comprehensive utilization result in mostly handled by stockpiling.So far,the cumulative stock of phosphogypsum in China has exceeded 300 million tons,which not only occupies a large amount of land,but also causes environmental pollution and development of enterprises will also cause great waste of resources and restricts phosphorus chemical industry.Phosphogypsum has a long history in China,over the years,through the unremitting efforts of researchers,the development of phosphogypsum resource utilization technology has made great progress,but the existing production technology still has complicated process,high utilization cost and drawbacks of serious secondary pollution lead to not conducive to promotion.Bipolar membrane electrodialysis technology is to split water in the bipolar membrane interface layer into H⁺and OH^-under the action of external DC electric field,H⁺and OH^-pass through the positive and negative membranes in the bipolar membrane respectively,Which convert the salt solution to corresponding acid and base.The technique without the introduction of new chemical components,and has the function of cleanly preparing acid and alkali without causing secondary pollution.At present,how to improve the effective utilization of phosphogypsum needs to be solved.In view of the advantages of advanced technology,simple operation,environmental protection,low energy consumption and low investment cost,the bipolar membrane electrodialysis technology is proposed to use phosphogypsum as raw material and sodium bicarbonate,and sodium hydroxide as a precipitant,select suitable additives,using the double decomposition reaction to prepare directly light calcium carbonate products,and using bipolar membrane electrodialysis technology to decompose the phosphogypsum decomposition liquid?the double decomposition reaction solution Na₂SO₄?to prepare sulfuric acid.Effective use of phosphogypsum resources to achieve the recycling of sulfur in the process of wet-process phosphoric acid production and to make full use of the calcium resources in the phosphogypsum to lay a solid foundation.The paper is divided into three parts:1)determining the optimal process conditions for Ca²⁺conversion in phosphogypsum in the double decomposition reaction;2)screening and determining suitable additives to prepare light calcium carbonate products;3)using bipolar membrane electrodialysis technically dispose phosphogypsum decomposition solution?Na₂SO₄ solution?to prepare sulfuric acid.The main research contents and results of this paper are as follows:1)The influence factors of Ca²⁺conversion rate in phosphogypsum during double decomposition reaction were discussed by the reaction time,reaction temperature,molar ratio of phosphogypsum to sodium bicarbonate,molar ratio of phosphogypsum to sodium hydroxide and mass concentration of phosphogypsum.The optimal optimization condition was established.The reaction time is 120 min,the reaction temperature is 40?,the molar ratio of phosphogypsum to sodium bicarbonate was 1:1.3,the molar ratio of phosphogypsum to sodium hydroxide is1:1.2,and the mass concentration of phosphogypsum is 50 g/L,the Ca²⁺conversion rate is 98.13%under the optimum conditions.2)Under the optimal conditions of Ca²⁺conversion in phosphogypsum,the average particle size of the prepared light calcium carbonate sample particles is used as a screening index to screen and determine sodium dodecyl sulfate,trisodium citrate citric acid and sucrose as crystal control agents.The results show that the addition of the above additives can reduce the particle size of the prepared light calcium carbonate samples,and also have a certain influence on the morphology.The product particle size distribution is normal distribution,belonging to hexagonal crystal,calcite crystal calcium carbonate,particle agglomeration is more serious;when adding 1.5%trisodium citrate and 1.5%sodium dodecyl sulfate to prepare light calcium carbonate has the finest particle size the average particle diameters are 10.45?m and 12.86?m respectively,and the prepared calcium carbonate particles are relatively uniformly distributed.3)The operating voltage,feed concentration,electrodialysis time,additive performance index of bipolar membrane electrodialysis process were studied and were optimized respectively.The bipolar membrane electrodialysis technology is used to treat the phosphogypsum decomposition liquid to prepare sulfuric acid.The optimum process conditions are:operating voltage is 20 V,feed concentration is 75g/L,1.5%trisodium citrate,and electrodialysis time is 220 min.Under this condition,the sulfuric acid concentration can be prepared to be 0.41 mol/L,the recovery rate of SO₄^2-in phosphogypsum is 97.87%,the current efficiency is 71.42%,and the energy consumption is 0.3003 kwhmol^-1.4)Through the economic feasibility evaluation of the whole experimental process,it shows that the research method of this paper is economically feasible,which can not only realize the recycling of sulfur element in phosphogypsum in the production of wet-process phosphoric acid,but also make full use of calcium resources in phosphogypsum.This method finds a new way for the effective utilization of phosphorus gypsum comprehensive utilization,which is in line with China's sustainable development strategic and energy-saving emission reduction development strategy goals.