Influence Factors Of Recycling Utilization Of Phosphorus Gypsum By Bipolar Membrane Electrodialysis

Phosphorus gypsum is a by-product in the production of wet-process phosphoric acid.As of 2011,the cumulative production of phosphorus gypsum in China is more than 300 million tons,and stacking quantity of more than 250 million tons.Long-term stacking of phosphorus gypsum not only takes up a lot of land resources,pollutes the environment,but also brings great economic burden to the enterprises for land acquisition and construction yard.But current technical level of phosphorus gypsum's exploitation and utilization is backward,the cost of comprehensive utilization is high,and the product has a low market demand,so it's difficult to extend.Therefore,this paper puts forward a new process that recycling phosphorus gypsum by bipolar membrane electrodialysis(BMED).Firstly,decomposing phosphorus gypsum into Soluble sulfate solution,and produce calcium carbonate or calcium hydroxide as by-product.Then,decomposing soluble sulfate solution into sulfuric acid and alkali by BMED.Sulfuric acid can be recycled for the decomposition of phosphate rock,alkali can be recycled for the decomposition of phosphorus gypsum.The main research contents and results are as follows:1.We found the technology that decomposing phosphorus gypsum into sodium sulfate and calcium hydroxide by sodium hydroxide has more advantages according to the comparison between the effects of decomposing phosphorus gypsum by sodium hydroxide and sodium carbonate.And the sodium hydroxide produced in alkali room can be directly reused in the decomposition of phosphorus gypsum.So we chose sodium hydroxide as the decomposition agent.The optimum technological conditions for the preparation of sodium sulfate were determined after the effect of many factors on the conversion rate of sulfate in phosphate gypsum was investigated by the method of single factor experiments:dosage ratio of sodium hydroxide(the mass ratio of the actual quantity and the theoretical quantity)1.2,liquid to solid ratio 3.5,reaction temperature 20?,reaction time 3h.Under these conditions,the recovery rate of the sulfate in the phosphate gypsum can reach 87.57%.2.We did pre experiment to investigate the feasibility of using pure sulfate solution to produce acid and alkali by BMED before using decomposition solution of phosphorus gypsum.The result shows that sulfate solution can be successfully decomposed into acid and alkali by BMED.And the process has the advantages of high selectivity,stable process,short process and simple operation.3.After determined the feasibility of using sulfate solution to produce acid and alkali by BMED,we investigated and compared the electrodialysis process of pure ammonium sulfate and decomposition solution of phosphorus gypsum.We studied several major process parameters such as the starting concentration in the salt chamber,the current density and the electrifying time to the effect of the decomposition rate,energy consumption and current efficiency.The result shows that for ammonium sulfate solution,when the concentration of the salt chamber is 0.758mol/L and the current density is constantly 15.8m A/cm2,the decomposition rate can reach 95.78%,current efficiency reach 93.4%,and the energy consumption of 2.0643 k Wh/kg is relatively low.For the decomposition of phosphorus gypsum,in the case of electrifying to voltage increases significantly,with the increase of the current meter,the decomposition rate of sodium sulfate barely changes,but the energy consumption increases,and the current efficiency decreases.With the electrifying time becomes longer,the decomposition firstly rises and then remains at a certain level,and the energy consumption rises,and the current efficiency decreases.The result shows that enterprises should make the current electric as small as possible according to the daily production of phosphorus gypsum and daily processing capacity of electrodialysis equipment in the production of phosphoric acid to decrease production cost.The process performance of the decomposition solution of phosphorus gypsum is obviously poor in the comparison of two solutions,it shows that impurities and other ions in decomposition solution of phosphorus gypsum have a negative impact on the electrodialysis process that we should remove them before electrodialysis.4.We analyzed the recycling approach and feasibility of solutions in alkali chamber and acid chamber after electrodialysis.Solutions in alkali chamber can be reused in the decomposition of phosphorus gypsum,and that in acid chamber can be reused in the production of wet process phosphoric acid.However,the concentrations of acid and alkali are lower than the requirements of the production that need to increase their concentration before being reused.It is concluded that the method does not have economic benefit by economic evaluation,but with the increase of the country's demand for environmental protection,the method has good prospects for development.5.We analyzed the mass transfer process of electrodialysis and transfer process of ions in bipolar membrane,and found that the main factors affecting the mass transfer process of bipolar membrane electrodialysis is the concentration difference and potential difference.The mass transfer process is simplified and the mass transfer rate equation is obtained.The calculation formula of production of acid and alkali and the time varying formula of alkali concentration are deduced according to the mechanism of water dissociation of bipolar membrane and the mass transfer process in the membrane.The equation shows that the mass transfer rate increases with the increase of the working current,and decreases with the increase of the concentration of alkali chamber solution?The process of recycling phosphorus gypsum by BMED has the advantages of energy saving and emission reduction,green environmental protection.It effectively decomposes phosphorus gypsum and turns sodium sulfate into sodium hydroxide and sulfuric acid,and cyclic utilization in the production of wet process phosphoric acid and the decomposition of phosphorus gypsum.It has a good development prospect that solved the problem of cost and product outlet of the comprehensive utilization of phosphorus gypsum.