| Phosphogypsum(PG) is a kind of industrial by-products of phosphoric acid producted with wet process, whose main composition is CaSO4·2H2O, and contains P2O5, F-, organics, heavy metals and other impurities. At present, about 15% of the PG has been recycled, mainly used in the agricultural soil improvement, construction materials and cement retarder, etc, but there are still 85% of the PG is stacked directly as solid waste, not only occupying the land resource, but also causing serious pollution to the environment. After years of research, scholars all over the world mostly used PG in preparing gypsum building materials and PG producing H2SO4-cement process. PG producing H2SO4-cement process can turn waste into treasure by using PG to produce clinker and sulfuric acid. not only making use of large quantities of PG to reduce its accumulation and pollution, but also reducing the exploitation of limestones and sulfide ores, which provides great environmental and social benefits. However, in terms of technical and economic problems, there are still many obvious shortcomings to be solved by using this process. From the engineering examples of PG producing H2SO4-cement process at home and abroad, the main problems exisiting in the following three aspects:first is there are a lot of impurities in PG, the composition of the raw materials changes a lot and it’s difficult to control in the industry production; second is the high heat consumption and large energy consumption of the system, materials is easy to circle, binding, and even jams in calcining kiln, affecting the normal operation of the process; third is the unstable quality of the clinker.In order to reduce the energy consumption of PG cement process, solving the decomposition of PG and calcinations of clinker problems, this study takes PG as main raw materials, mixed with activated carbon and fly ash, taking calcinations process, and then compounding with Portland cement clinker to produce high-performance calcined high-silicon phosphogypsum-based cement under the condition of low temperature calcination (as opposed to the calcination temperature of clinker). In the process of calcination, the PG decomposition product CaO with SiO2, Al2O3 and Fe2O3 containing in fly ash produce a variety of silicate minerals and aluminosilicate minerals which is helpful to improve the strength of hardened paste The activated carbon contributes to the formation of a reducing atmosphere in the calcination process to promote the decomposition of PG, and some residual CaSO4 can be used as cement retarder, reduce or with no need of the additive gypsum, which can not only improve the strength of cement, but also save resources and bring economic benefits.This paper uses orthogonal experiment to explore the process parameters of preparing high-performance calcined high-silicon phosphogypsum-based cement. Due to the mineral composition of phosphogypsum-based hydraulic binder which is calcine(?) at low temperature is different from the mineral composition of ordinary porti and cement clinker, so we use hydraulic modulus(HM) instead of lime saturation factor(KH) to be the production target in this orthogonal experiment. We have done research on the reaction conditions of preparing high-performance calcined high-silicon phosphogypsum-based cement by using calcination temperature, holding time, dosage of activated carbon and HM as four factors in the orthogonal experiment. The results showed that high-performance calcined high-silicon phosphogypsum-based cement can get high early strength and late strength when the calcination temperature is 1200℃, holding time is 30min, dosage of activated carbon is 10% and HM is 1.3. In the actual industrial production, HM and SM can respectively be used as the primary productivity modulus and the auxiliary modulus.This paper analyzes the representative samples’clinker of the orthogonal experiment by XRD. The results showed that most samples preserving better compressive strength contains C3S, C2S, C3A, C2AS, CA2 and other minerals. Among them, C3S and C3A hydrate faster, which has a significant contribution to the early strength of hardened paste; C2S calcined at low temperature has higher activity and faster hydration rate, which have a greater role in promoting early and late strength; C2AS and CA2 were regared as weak hydraulicity minerals, hydrate slowly, which has no significant contribution to the early strength but can accelerate the long-term strength. This paper also analyzes the hydration products of high-performance calcined high-silicon phosphogypsum-based cement by XRD, SEM and EDS. The results showed that the hardened cement paste is mainly composed of calcium hydroxide, C-S-H gel, AFt. C2ASH8 and hydration alumina gel, etc. The cement stone treats AFt as skeleton, with C-S-H and hydration alumina gel packing in, forms a dense network structure and performs a high compressive strength macroscopically.This paper carries on thermal decomposition reaction kinetics study of PG-AC-FA system under reaction conditions of actual industrial production. The system takes SO3 decomposition ratio as an evaluation standard of the degree of decomposition, to calculate the apparent activation energy and pre-exponential factor of the system, which determines the most probable mechanism functions of the system. The results showed that the apparent activation energy of PG-AC-FA system is 313.78～338.42KJ·mol-1, the pre-exponential factor of the system is 22.22～24.10s-1, and the most probable mechanism functions of the system is Avrami-Erofeev equation (n=2). which reflects the random nucleation and whereafter growth mechanism.This paper calculates the flue gas volume and the content of SO2 produced during the calcination of preparing high-performance calcined high-silicon phosphogypsum-based cement and makes a reasonable recovery plan. The results showed that the flue gas volume by producing, every lkg clinker is 1.11Nm3/kg, in which the concentration of SO2 is 14.95%; among the means of controlling SO2 emmission, wet organic amine FGD technologies has better technical feasibility and economic rationality, and can be a effective method to recycle the SO2 produced during the preparation of high-performance calcined high-silicon phosphogypsum-based cement.In allusion to the serious situation of comprehensive utilization of PG, this thesis develops a low energy consumption, low cost, strong operability of high-performance calcined high-silicon phosphogypsum-based cement under the condition of low temperature calcination. The high-performance calcined high-silicon phosphogypsum-based cement process overcomes the disadvantages in the traditional PG producing H2SO4-cement process. The paper has made an elaboration on the source of high strength properities of the cementitious binder, improved the theoretical basis of the high-performance calcined high-silicon phosphogypsum-based cement, to provide technical support and lay the foundation to the next practical engineering application. |
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