Decomposition Characteristics Of Phosphogypsum And Piecewise Calcination For Preparing Sulphoaluminate Cement

Phosphogypsum(PG) is industrial waste residue from the production of phosphoric acid by the wet process. As the emissions of PG increase year by year, solving the problem on the utilization of PG is extremely urgent. Preparing sulphoaluminate cement with PG is an important way to utilize PG. Based on the concept of PG partially decomposing for preparing sulphoaluminate cement, namely Ca O generated from PG partially decomposing partly replace limestone and undecomposed PG replace natural gypsum for forming clinker mineral. Taking the improvement of the decomposition rate of PG and promoting PG concentrated decomposing to release the SO2 in a certain temperature period as the breakthrough point, the decomposition characteristics of PG and its piecewise calcination for preparing sulphoaluminate cement are studied in this paper in order to improve the utilization ratio of PG and achieve the goal of consuming PG in large quantities. This paper has important reference value on the follow-up studies about preparing sulphoaluminate cement with PG as the main component.The thermal decomposition characteristics of PG was studied first and it was discovered that atmosphere, temperature and ratio of C/S(the ratio of amount of substance for carbon and sulfur in the system of PG and activated carbon) had certain influence on the decomposition rate of PG. When the other conditions were the same, the decomposition rate of PG in CO2 atmosphere was higher than that of N2 and air atmosphere. Unlike traditional reducing atmosphere, the incorporation of activated carbon in CO2 atmosphere could form weak reducing atmosphere and promote the decomposition of PG. The decomposition characteristics of PG were researched mainly by adopting piecewise calcination system with weak reducing atmosphere in the low temperature and oxidizing atmosphere in the high temperature. The results showed that the piecewise calcination system of CO2-950?-1h/air-1200?-1h could make the decomposition rate of PG with the ratio of C/S being two reach 87.69%. The decomposition product didn't contain Ca S and didn't reach the molten state. Meanwhile, SO2 gas could be controlled to concentratedly release in the temperature range of 900~1200? and the theoretical concentration of SO2 gas was 9.11% which met the requirement of the concentration for preparing sulfuric acid.The PG by piecewise calcination with the decomposition rate of 87.69% was used to prepare sulphoaluminate cement, which could obviously raise the utilization ratio of PG compared with the traditional ingredient of using the original PG for preparing sulphoaluminate cement. There were C2 AS that was middle transitional mineral besides C4A3S?, C2 S and C4 AF in the cement clinker, which might be related to high content of impurities such as P2O5 in the precalcined PG. The three days' compressive strength of sulphoaluminate cement prepared with the precalcined PG could reach over 40 MPa, but the hydration rate and compressive strength were lower than sulphoaluminate cement prepared with natural gypsum, PG according to the traditional ingredient. Early hydration products were mainly needle bar ettringite and aluminum hydroxide gel and ettringite was interwoven with aluminum hydroxide gel. The cement paste became more and more dense with the development of hydration age. It could be preliminary showed that separating the decomposition of PG from the sintering process of sulphoaluminate cement clinker and preparing sulphoaluminate cement with piecewise calcined PG instead of natural gypsum and partly replacing limestone were feasible at present.Besides, in order to promote the decomposition of PG in the calcination process of clinker and achieve the purpose of raising the utilization ratio of PG, the original PG was used to prepare sulphoaluminate cement under the piecewise calcination system of CO2-950?-1h/air-1270?-1h. The research found that the decomposition degree of PG would be affected by other components in the raw materials and the form of raw materials. The decomposition rate of PG could go up to 73.31% in the process of piecewise calcining clinker, which had obvious improvement compared with the traditional calcined system. When the actual decomposition rate of PG was relatively close to the expected decomposition rate, the mineral composition of clinker was basically identical with the expectations. When using partially decomposed PG to prepare sulphoaluminate cement, PG could replace natural gypsum but only partly replace limestone in previous studies. The feasibility that PG entirely replace natural gypsum and limestone for preparing sulphoaluminate cement was analysed and the theoretical analysis and calculation showed that when using PG with low content silicon to prepare sulphoaluminate cement, partially decomposed PG could entirely replace natural gypsum and limestone if the decomposition rate of PG reached over 92% by reasonably adjusting calcined condition. The content of PG in the raw material could increase to over 73%.