Development Of Compound Gluing Material For Phosphate Gypsum Low Temperature Decomposition And Calcination

Phosphogypsum(PG) is a kind of industrial by-products of phosphoric acid produced with wet process, whose main composition is CaSO4·2H2O, and contains P2O5, fluoride and organic matters, heavy metals and other impurities. Currently, about 15% of the PG has been recycled, mainly used in the agricultural soil improvement, construction materials and cement retarder, etc. Otherwise, 85% of PG is disposed directly as solid waste with serious potential environment impact. In recent years, in allusion to the chemical and physical properties of PG, to explore diverse pathways of recycling is the key point of PG resource utilization research. Using PG as building raw materials has been researched and developed in diverse aspects, mainly focus on gypsum board production, or PG additive in producing H2SO4-cement process. PG-H2SO4-cement process can produce acid and cement, solve the difficulty of PG stockpiling, and reduce the exploitation of cement raw materials, which provides great social and environmental benefit. But in terms of technical and economic benefits, there are many problems to be solved by using this technology. From the engineering examples at home and abroad, the main problems of PG- H2SO4-cement process to be solved is given as follows in two aspects: one is the high heat consumption and low production capacity, materials is easy to circle, binding, and even jams in calcining kiln, resulting in the significantly worse kiln ventilation, incomplete fuel combustion, and production fell sharply; the other is the unstable quality of the clinker.In order to reduce the PG cement energy consumption, improving the product quality, to achieve the objective of commercial utilization of PG, this study taking PG as main raw materials, mixed with carbon, modified components (fly ash, FA) to produce a new kind of PG-based composite cementitious material(PBCCM) under the low temperature calcination conditions. PBCCM worked as a kind of additive, mixed with Portland cement clinker to form composite Portland cement. In the production of PBCCM, some part of PG is intented to be decomposited which can reduce the energy consumption. On the other hand, some residual CaSO4 can be used as cement retarder, reduce or with no need of the additive gypsum, which not only can save resources but also improve the cement strength.The paper uses FactSage thermodynamic software to carry on the thermodynamic calculation of the PG decomposition and formation of clinker mineral process under the condition of carbon addition, involved in the preparation of PBCCM. The study identifies the main way of CaSO4 decomposition. The main products of CaO-SiO2, CaO-Al2O3 binary system and CaO-Al2O3-SiO2 ternary system are identified by software at the temperature range of 1100-1250 ℃. The most possible minerals synthesized at low temperature in PBCCM are determined from the point of view of thermodynamics, laying the foundation for subsequent experimental study.On the basis of thermodynamics, two orthogonal experiments are carried out to support the preparation methods study of PBCCM. The orthogonal experiment I took the calcination temperature, retention time, dosage of activated carbon and fly ash as four factors of the orthogonal research. The research shows that when the calcination temperature is 1200℃, retention time is 30 min, dosage of activated carbon is 10%, dosage of fly ash is 5%, new cementitious material can obtain a high compressive strength, the strength nearly equals to the pure Portland cement clinker’s strength under the same maintenance condition. As for the change of the chemical composition in raw materials will lead to the change of raw material dosage, to make the preparation methods of PBCCM more universality, the orthogonal experiment II takes HM instead of the dosage of fly ash as one of the impact factors of the research, and adjusts the value range of calcination temperature, retention time and dosage of activated carbon, in order to optimize the reaction conditions. Experiment shows the optimal reaction conditions of the PBCCM preparation is that the calcination temperature is 1200℃, retention time is 30 min, activated carbon dosage is 10%, and HM is 1.3. The 3d, 7d and 28d compressive strength of PBCCM is 67.59MPa, 83.01MPa and 99.11MPa respectively. In actual process, HM can be considered as the primary productivity modulus, n for the auxiliary modulus. The control range of HM and n is suggested to be 1.3 - 1.5 and 3.29 - 3.64 respectivly. Due to the different product characteristics, PBCCM obtains a lower calcination temperature, a shorter retention time and a more stable product quality compared with PG-H2SO4-cement process.In this paper, XRD is undertaken to analyze the typical clinker samples of orthogonal experiment I and II, to determine the composition of clinker minerals. The study finds that most samples with better compressive strength contains C2S and C2AS, which are considered to perform hydraulicity. For the local inhomogeneities of calcination, some other minerals will be formed during the calcination, such as C3S, C3A and CA2. Among the clinker minerals, C2S has high activity and fast hydration performance, making significant contribution to the development of the strength; C2AS is regarded as weak hydraulicity minerals, hydrate slowly, which can accelerate the long-term strength. Low temperature calcined PBCCM can be considered as a kind of low bailey aluminate cementitious material. Paper analyzes the hydration products of the new cementitious material by XRD, SEM and other testing means, and finds that the hardened cement paste is mainly composed of C-S-H gel, calcium hydroxide, 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.The research gives the best preparation method of PBCCM and determined the clinker mineral composition and hydration products. On the next step, reaction kinetic of PBCCM is studied in order to promote the new process into industry realization. The paper carries on thermal decomposition reaction kinetic study of CaSO4, PG and PG-FA system with different dosage of activated carbon by isothermal method, under similar reaction conditions of industrial calcination furnace. The three systems’ kinetic research all take SO3 decomposition ratio as an evaluation standard of the degree of the reaction, to calculate the apparent activation energy and pre-exponential factor of the system, which determines the most probable mechanism functions of each system. Results show that with the increase of the dosage of activated carbon in the system, apparent activation energy and pre-exponential factor of the three systems are declining, and kinetic compensation effect exists in the process of calcination. The paper matches the dynamic compensation function of the three systems. The most probable mechanism function of CaSO4 and PG system is the same, which is Z-L-T equation. On the other hand, the most probable mechanism function of the PG-FA system is Avrami - Erofeev equation (n=2), which reflects the random nucleation and whereafter growth mechanism. The kinetic research accumulates basic data for the large scale calcination furnace development, so as to realize the industrial application of PBCCM.In addition, the industrial process of PBCCM has been designed, the whole system is composed of clinker calcination, sulfuric acid preparation and waste heat power generation. The traditional PG-H2SO4-cement process was optimized. In the new process, the raw materials takes one-step of decomposition and calcination in the kiln after preheat. Through the heat balance calcination, the formation heat and burning heat of PBCCM clinker are referred to be 1728.92kJ/kg and 3063.14kJ/kg respectively. Compared with PG-H2SO4-cement process, the system energy consumption has been greatly reduced. The SO2 concentration of the flue gas exhausted from the process was calculated to be 15.32%, which increased greatly compared with the traditional process. Under the scale of the output of PBCCM clinker is 300,000 tons per year, the raw material consumption and the production cost have been accounted. The calculation results show that the annual consumption of PG and FA are 650,000 tons and 30,000 tons respectively. Besides producing clinker, PBCCM production system also can output 230,000 tons sulfuric acid per year. Thus it can be seen the new process eqiupps with excellent technical and economical benefits, realizing the resource utilization of PG.In allusion to the serious situation of comprehensive utilization of PG, the thesis develops a low cost, low energy consumption, strong operability of PG-based new high quality building materials under the condition of low temperature calcination. The PBCCM process makes up the disadvantages in the traditional PG-H2SO4-cement process, such as high-energy consumption, poor product quality and so on. The theory of strength sources, rotary kiln development, process design, system heat balance, production cost have been discussed from the point of view of thermodynamics, kinetics and engineering, improved the theoretical basis of PBCCM process, to provide technical support and lay the foundation to the next practical engineering application.