Study On The Preparation Of Type II Anhydrous Gypsum By Calcination Of Phosphogypsum And Liquid Phase Metho

Phosphogypsum（PG）is a solid waste discharged from wet phosphoric acid process.Its application is restricted by the existence of various impurities,organic matter and residual phosphoric acid and sulfuric acid.Type II anhydrite is a good building material because of its potential gelling property.At the same time,it can also be used as filler in polymer materials,which has broad application prospects.Preparation of type II anhydrite from PG is a way to expand the utilization of PG,and the impurities in PG are removed or converted into inert substances.Compared with semi-hydrous gypsum,type II anhydrite has the advantages of low water requirement and high hardening strength,and is the main raw material for preparing gypsum self-leveling mortar.However,type II anhydrite has dense microstructure,low activity,low hydration rate and slow coagulation.The key to its effective utilization is to select appropriate activators to stimulate activity.Therefore,in this paper,PG is used to prepare type II anhydrite.Two kinds of PG from different origin are used as raw materials to prepare type II anhydrite by calcination method and liquid phase method,and its hydration hardening is excited by activator.First,anhydrite II was prepared by calcination method.The calcination temperature was determined according to TG-DSC results of PG,and the effects of calcination temperature and residence time on the phase composition of the two PG were compared.The results show that PG-1 can be completely transformed into type II anhydrite when calcined above 500℃for 30 min,while PG-2 can be completely transformed into type II anhydrite when calcined above 700℃for 30 min.Moreover,Type II anhydrite with good crystallization can be obtained by calcination of PG at800℃for more than 30 min.Secondly,micron-grade type II anhydrite was successfully prepared from PG using phosphoric acid and sulfuric acid as reaction media by in situ two-step solution-recrystallization process.The effects of different experimental factors on the phase conversion of PG were discussed.The optimum conversion conditions are as follows:phosphoric acid concentration of 32%P₂O₅,H₂SO₄/H₃PO₄（mass ratio）of20%,reaction temperature of 90°C,H₃PO₄/PG（mass ratio）of 5,gypsum phase conversion of PG is more than 99%,and the micromorphology of type II anhydrite is laminate with an average particle size of 13.45μm.Purity 98.45%,whiteness 81.8%.The transformation process of PG type II anhydrite synthesized by dehydration in PG mixed acid follows the dispersion kinetic model.Impurities(Fe³⁺,Al³⁺,Mg²⁺,Si F₆^2-)reduce the kinetic parametersαandβof PG transformation,increase the activation enthalpy and decrease the activation entropy,and increase the nucleation potential barrier of type II anhydrite crystal.Thus,the process of synthesis of PG into type II anhydrite by dehydration is inhibited.With the increase of impurity concentration in phosphate-sulfur mixed acid,the chemical shift of calcium,sulfur and oxygen elements of type II anhydrite shifts to the left,resulting in weakened binding energy between Ca²⁺and SO₄^2-ions,loose crystal structure,and increased energy required to complete DH-AH dehydration transformation.The proposed scheme of PG synthesis type II anhydrite has a good application prospect.Finally,the hydration of type II anhydrite is stimulated by sulfate.Sulfate has no significant effect on the early hydration of type II anhydrite,but no significant effect on the later hydration.When 2%sodium sulfate,potassium sulfate or alum is used to stimulate the hydration of type II anhydrite,the order of action is alum>potassium sulfate>sodium sulfate.Sulfate activator can not only accelerate the hydration process of type II anhydrite,but also improve the microstructure of type II anhydrite,so as to show different mechanical properties.Under the same sulfate excitation,the hydration properties and mechanical strength of type II anhydrite prepared by liquid phase method are superior to those prepared by calcination method.By comparing the hydration and mechanical properties of C-AH-1,C-AH-2,L-AH-1 and L-AH-2,it can be concluded that L-AH-2 shows the best gelling performance.