| The high yield and insufficient utilization of phosphogypsum(PG)in China limit the sustainable development of phosphorus chemical industry.Promoting the resource utilization of PG is of great significance for improving resource utilization efficiency,green transformation of economic and social development,and achieving carbon peaking and carbon neutrality.However,soluble impurities,high pretreatment cost,and unstable reductive decomposition limit the resource utilization of PG.The development of PG utilization technology with high utilization rate,stable product performance,and low carbon emission is the key to large-scale and low-carbon utilization of PG.In this paper,an innovative technical route for the copreparation of calcium sulfoaluminate(CSA)cementitious material and sulfuric acid using nonreductive decomposition of PG was proposed.Thenon-reductive decomposition of PG provides all CaO and CaSO4 required for the formation of CSA clinker,and SO2 is collected for production of sulfuric acid.Aiming at the non-reductive decomposition of PG to prepare CSA cementitious materials,the feasibility of non-reductive decomposition of PG to prepare CSA cementitious materials and the formation mechanism of clinker minerals were firstly studied,and the influence mechanism of Fe2O3,phosphorus and fluorine impurities on the formation of clinker minerals and the hydration characteristics of cementitious material was revealed.Then,on this basis,the pilot system design and experimental research were carried out,and the technological process of the co-production CSA cementitious materials and sulfuric acid using non-reductive decomposition of PG was established.Finally,a complete system of process integration,environmental assessment and economic analysis of the co-production process was formed.The main work of this paper is as follows:PG,aluminum ash,fly ash,and red mud were used as raw materials to prepare CSA cementitious material.The non-reductive decomposition characteristics of PG and CSA raw meal were explored.The effects of calcination regime and raw material ratio on the decomposition of CaSO4,formation of clinker minerals,and properties of cementitious material were studied.The concentration of SO2 in flue gas during the preparation of CSA clinker using non-reductive decomposition of PG was analyzed.Compared with PG,CSA raw meal has lower initial non-reductive decomposition temperature and higher decomposition efficiency.When the calcination temperature is higher than 1270℃ and the holding time is longer than 60 min,the decomposition rate of CaSO4 exceeds 65.78%,the formation of CSA clinker minerals is fine,and the compressive strength of CSA cementitious material meets the requirements of grade 525 CSA cement in the national standard.PG accounts for more than 70%of the raw materials,and its utilization rate is greatly improved.The alkalinity coefficient and aluminum silicon ratio are controlled to be 0.95-1.10 and 2.0-3.0 respectively.Based on the simulation analysis of Aspen Plus,when the excess air coefficient is lower than 2.34,the SO2 concentration in the flue gas is higher than 7.0%,which meets the requirement of sulfuric acid production.Therefore,it is feasible to co-produce CSA cementitious material and sulfuric acid using nonreductive decomposition of PG.The reaction process of non-reductive decomposition of CaSO4 and mineral formation process of C4A3S,C2S,C4AF,C4A3S-C2S,C4A3S-C4AF,and C4A3S-C2S-C4AF were studied using chemical reagents as raw materials to prepare target minerals.The reaction kinetic mechanisms of non-reductive decomposition of CaSO4 and its preparation of single mineral and composite mineral were studied by universal integral method.The reaction mechanism of preparing C4A3S-C2S using non-reductive decomposition of CaSO4 is power function law,while the reaction mechanism of non-reductive decomposition of CaSO4 and preparation of other minerals is phase boundary reaction.The reaction activation energy of non-reductive decomposition reaction of CaSO4 is 364.94 kJ/mol.In contrast,the reaction activation energies of target mineral formation decrease in varying degrees,and the coupling reaction rate of nonreductive decomposition of CaSO4 and mineral formation is higher.The effects of Fe2O3 on the formation of clinker minerals and hydration characteristics of cementitious materials were explored using chemical reagents and solid waste as raw materials,respectively.Furthermore,the solid solution characteristics and mechanism of Fe in minerals were clarified,which lays a theoretical foundation for reducing Al2O3 content in clinker minerals.With the increase of Fe2O3 content,the decomposition of CaSO4 and the formation of clinker minerals are first slightly promoted and then hindered.When the design content of C4AF is 10 wt.%,the performance of CSA cementitious material is the best.In C4A3S,the highest solid-soluted content of Fe is 6.76 wt.%and the substitution of Al for Fe reaches 12.96 mol.%.The highest solid-soluted content of Fe in C2S is 1.00 wt.%.The formation of iron phase is based on C2F,and Al is continuously added to form C6AF2,C4AF,and C6A2F The first principle calculation results show that Fe tend to replace Al in C4A3S and Si in C2S.With Ca3(PO4)2,CaHPO4·2H2O,and Ca(H2PO4)2·H2O as phosphorus impurities and NaF,CaF2,and Na2SiF6 as fluorine impurities,the effects of phosphorus and fluorine impurities on the mineral formation of CSA clinker and hydration characteristics of cementitious materials were studied.Furthermore,the occurrence state and distribution regularity of P and F were revealed.With the increase of P2O5 content,Ca3(PO4)2 and Ca(H2PO4)2 promoted the decomposition of CaSO4 and formation of clinker mineral,whereas CaHPO4 shows a promoting effect only when the P2O5 content is higher than 1.5 wt.%.The presence of phosphorus impurities reduces the 1-day strength of CSA cementitious material but does not affect its 3-day and 28-day strength development.With the increase of fluorine impurities,the decomposition of CaSO4 is first inhibited and then promoted.Among them,NaF promotes the formation of clinker mineral,while CaF2 and Na2SiF6 shows a promoting effect only when their content is more than 1.0 wt.%.The presence of fluorine impurities reduces the 1-day strength of CSA cementitious material but promotes its 3-day and 28-day strength development.P is dissolved in C4A3S,C2S,and C4AF,but mainly in C2S.F mainly exists in the form of CaF2,which is uniformly distributed in clinker.Based on the experimental data,a pilot system of producing CSA cementitious material using the non-reductive decomposition of PG was designed and the pilot experiment was carried out.The technical feasibility of co-producing CSA cementitious material and sulfuric acid was verified.CSA cementitious material was used to make non-steamed and static-pressure brick for research on its industrial application.The pilot system includes three links:raw meal preparation,clinker calcination,and cementitious material preparation.Based on the pilot system,the prepared CSA cementitious material has good mineral formation,and its compressive and flexural strength meet the requirements of grade 525 CSA cement in the national standard,but the setting time is relatively fast.In addition,the SO2 concentration of the flue gas exceeds 7.0%,meeting the requirement of sulfuric acid production.The leaching concentration of harmful elements in CSA cementitious materials meets the requirement of class III groundwater and it is friendly to the environment.The preparation experiment and pilot production of non-steamed and static-pressure brick are carried out using CSA cementitious material as binder and garbage ash and stone powder as aggregates.The obtained non-steamed and static-pressure brick has excellent performance.Technological process of co-producing CSA and sulfuric acid using non-reductive decomposition of PG was established.Based on the life cycle assessment theory,the environmental impact and carbon emission reduction effect of the co-production process were evaluated.The co-production process has a lower environmental impact than conventional process.The key links of the co-production process affecting environment are clinker calcination and sulfuric acid preparation,and the key substances are raw coal and electricity.The reduction of raw coal and electricity consumption in clinker calcination and the reduction of pollutant emissions in sulfuric acid preparation will help to reduce the environmental impact of the co-production process.Due to the low-carbon emission of sulfuric acid preparation and raw material mining,the carbon emission of the co-production process was 18.36%lower than that of conventional process.A plant with an annual output of 150 kilotons of CSA cementitious material and 65 kilotons of sulfuric acid(98%)consumes 194.3 kilotons of PG,66 kilotons of aluminum ash,2.1 kilotons of fly ash,and 10.7 kilotons of red mud,and reduce the emission of 28 kilotons of CO2 eq.The co-production process has excellent economy and realizes largescale and low-carbon utilization of PG. |
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