Processing Theory And Application Technology Research Of Phosphogypsum Based Gypsum Piaster

Phosphogypsum (PG) is solid waste of calcium sulfate discharged from wet process phosphoric acid production.Generally,4.5-5.5tons PG are generated when per ton of P2O5are produced and most of the calcium sulfate is in the form of dihydrate (DH, CaSO4·2H2O). Currently the mainly way of disposal PG is stockpilling for the phosphorus, fluorine, organic compounds and other ingredients containing in PG affect its performance and increae its using cost compared to natural gypsum. In2011, with the growing demand of phosphoric fertilizer the production of PG was up to70million tons in China and less than20.0%is used at present.It is estimated that the accumulative total of PG is over300million tons throughout China, which not only occupies plenty of lands, waste of resources, and harmful constituents also cause serious pollution to the surrounding soil, vegetation, water and air. In2006, PG was classified as "hazardous solid waste"by SEPA (state environmental protection administration),the problem of PG disposal has been a worldwide problem that seriously hampered the sustainable development of the phosphorus chemical industry and environmental protection.The main problems of cementitious materials preparation from PG instead of natural gypsum at home and abroad are:(1) PG based cementitious materials have higher preparation costs for the harmful impurities influencing the performace despite the high Ca2SO4·2H2O content, while PG pretreament has no superiority compared to natural gypsum not only in the price but in products performance;(2)widely useage of a-hemihydrate sulphsate calcium (a-HH) with high performance and high value-added prepared under autoclave atmosphere is restricted by the complex process, high energy consumption, inconvenience to the control, unstable performance and high price;(3) preparation of a-HH in hydrothermal electrolytic solutions with characteristic of mild reaction conditions and easy to conrol under atmospheric pressure is still at the laboratory or theoretical stage, meanwhile the raw materls are limited to natural gypsum, desulfurized gypsum or gypsum with little impurities.Funded by Chinese National Technology Support Project "Low-cost and Energy-saving Technology Research and Demonstration of Energy-efficient Building" (Project No.2011BAJ03B03), Major Science and Technology Plan of Hubei Province" Utilization Efficiently and Industrialization of Phosphorus Chemical Byproducts"(Project No. DZS0005), and Research and Plan Project of Hubei Province"Comprehensive Utilization of Phosphogypsum"(Project No.2009BCB030), based on theoretical and technical problems of high-strength gypsum cementitious materials preparation, design and selection of Phosphogypsum Based Gypsum Plaster (PBGP) preparation system are presented in the paper. Influences of raw material pretreatment to the formation and crystal morphology control of PBGP is studied, then effects of impurities on the performance of PBGP hydration and hardening are discussed, next optimization of PBGP preparation process and industrialization technology are considered, meanwhile the relation of PBGP composition-structure-performance is diccovered, and the critical theory and technology of PBGP preparation process, crystal morphology control, performance optimization and industrialization are established. The main research work and compliments are listed are as follows:1. Design and selection of PBGP preparation system.At hydrothermal temperature of85℃～100℃, NaCl, CaCl2and its mixtures are selected as activity agent, respectively.Influences of activity agent type and concentration, solid-liquid ratio and the reaction temperature and time to the products are researched. The experiment results show that phase transition of PG from DH to HH can conduct in strong NaCl solution, but NaCl is not favourable for preparation of a-HH from PG phase transition, for the phase transition product of PG is a mixture of a-HH and omongwaite (Na2Ca5(SO4)6·3H2O)-a structural isomer of HH; α-HH can be obtained as the product of PG phase transiton in strong CaCl2solution; the reaction time of PG transiton to a-HH shortens from240min to50min with increasing NaCl content from0to4%in24%Ca-Na-Cl solution, but it is accompanied with increasing of Na2O content and irregular uncompleted crystal shapes in the reaction product; NaCl increases the solubility of PG and promote the nucleation and growth of HH crystal, while CaCl2reduces the solubility of PG for the common ion effect, a-HH crystal growth can be greatly accerlated by surface diffusion and adsorption of relatively high activity of Ca2+2. Influences of raw material pretreatment to the formation and crystal morphology of PBGPInfluences of the acidic impurities, soluble salts, insoluble impurities contained in PG to phase transiton process, product shapes and compressive strength are studied, crystal modifier commonly used are choosed to modify PGGP crystal morphology. It turns out that soluble phosphate and fluoride impurities elongate a-HH crystal aspect ration, decrease a-HH crystal diameter while provide the necessary acidic conditions for PG phase transition; homemade crystal modifier NS and EN have obvious effects on a-HH cryatal habit, short columnar PBGP crystal of aspect ratio nearly1with compressive strength beyond35MPa can be obtained when the dosages of NS and EN are0.15%and0.4%respectively;solube acidic impurities increase appropriate dosage of crystal modifier and decrease the product compressive strength;soluble salts and organic matters fine a-HH crystal partles, and reduce the product strength as well; insoluble impurities shorten a-HH crystal to rode-like shape, uneven length and distribution,improve product strength;unpretreated PG is not appropriate for PBGP preparation,water-washing of raw material is beneficial for subsequent crystal modifier usage, crystal morphology control and strength of the product; linear correlation of the product dry compressive strength and crystal volume is showed when PBGP has a uniform crystal shapes of diameter≥8μm and1-3aspect ratio.3. Effect of impurities on PBGP hydration and hardening performanceSeting time, strength variation and corresponding mechanism of high-strength gypsum in dilute solution of H2SO4、H3PO4and HC1of different pH values and in different contents of CaCl2are studied, the factors influencing PBGP performance are analyzed.The results reveal that acid solutions accelerate initial and final setting of high-strength gypsum at roughly the same rule, high-strength gypsum show different setting performance in the same acidic solutions of the different pH values or in the different acidic solutions of the same pH values;2h and dry strength values of gypsum plaster are lowered markly by H2S04,H3PO4and HC1solutions of pH values less than3.0, and the strength values vary with the kind and content of acid radical ions when the pH value is between3to7; high-strength gypsum setting may also be accerlerated by activity agent CaCl2, its strength especially dry strength loss increasing with the addition of CaCl2content; acceleration periods are all prolonged and phase change heats are all enhanced by these impurities; product strength is effected by DH phase content, while impurities ions, such as E2PO4,H+,Cl" play important impacts through changing the nature of contact points; hydration reaction of high-strength gypsum is inhibited by CaCl2addition, the higher the CaCl2content, the higher the proportion of high-strength gypsum unhydration, more obviously the dry strength decreased.4. Optimization of PBGP preparation process and applicationPBGP preparation-scal is expanded in the laboratory, setting and strength properties of PBGP are tested, and the PBGP preparation process is optimized, the results indicate that waste water from raw material pretreatment can be recycled after disposal; the product compressive strength is still more than40MPa, though crystal boundary become unsharp after PBGP slurry was washed and dried; filtrate from PBGP slurry treated by precipitator can be recycled, the compressive strength of the product is greater than35MPa and25MPa when the diltrate cycle times are4and5respectively; silicon, aluminum, iron and other insoluble impurities containing in PG can leach out during hydrothermal reaction process, thereby the purity of PBGP is improved; the optimized preparation process of PBGP can be devied into four stages,that is, raw material pretreatment, hydrothermal reaction under atmospheric pressure,slurry preparation, PBGP power and gypsum products preparation.