Stduy On The Formation Mechanism Of C₂S-C₃P In Multi-Phase Dephosrization Slag

The crude steel production of China in 2014 was about 7 hundred million tons,the slag for dephosphorization was about 70 million tons,an average of about 100kg/t steel,which was far from the target of"slag is less than 50?70kg/t steel in 2011?2020 by iron and steel industry development science and technology guide”.There are a lot of unreacted free CaO in dephosphorization slag which is difficult to recycle,resulting in a tremendous waste of resources,the main reason is that the researchers almost take the assumption that steel slag is uniform liquid phase while the system of slag with solid phase has not been attentioned.Workers in Japan in recent years propose to improve the dephosphorization ability of slag by solid phase,to achieve efficient dephosphorization of low basicity,who use solid-liquid coexistent phase for refining,while the formation mechanism of 2CaO-SiO₂-3CaO-P₂O₅ solid solution is not clear.Now firstly,add calcium oxide and dicalcium silicate particles of different sizes to the CaO-SiO₂-FeO-P₂O₅slag to study the diffusion mechanism of 2CaO-SiO₂-3CaO-P₂O₅solid solution;secondly,do isothermal crystallization process to research the precipitation mechanism of 2CaO-SiO₂-3CaO-P₂O₅ solid solution.(1)After adding lime particles of different sizes to CaO-SiO₂-FeO-P₂O₅ system,find that:1)There is C₂S-C₃P solid solution layer at the interface instead of the C₂S layer,the diffusion mechanism and precipitation mechanism coexist,and the solid solution by the precipitation mechanism contains higher phosphorus,the P₂O₅ content in solid solution by precipitation mechanism may be closely related to the FeO content in the slag;2)When lime dissolutes in CaO-SiO₂-FeO-P₂O₅,forming 6 regions along the CaO:lime layer;phase between calcium ferrite layer and lime;calcium ferrite layer;discontinuous solid solution layer;dense solid solution layer;slag layer.The dissolution process is:the diffusion of FeO from the surface to the inside of lime particles,producing calcium ferrite,with lime dissolving into liquid slag dicalcium silicate supersaturated slag is formed in the layer,and solid solution produces,which makes FeO and SiO₂ content in the slag increase and P₂O₅ content decrease.The viscosity of slag decrease,and permeability increases,the shell around lime is destroyed,the C₂S-C₃P solid solution layer continuously dissolves,forming solid solution particles which gradually grow up around lime particles;3)C₂S-C₃P solid solution layer appears in 5 s,P₂O₅content in solid solution gets the maximum at about 120 s,then with the increase of time,solid particle size increases,while the content of P₂O₅ of the solution reduces;4)the higher the basicity of slag,the closer the slag composition of the slag with the tip of CaO-SiO₂-FeO phase diagram,the faster the dissolution of lime is,the faster the formation rate of the solution is,and the higher the content of phosphorus in solid solution is.With the increase of lime particle size,the formation of solid solution around lime particles slowed down,and the phosphorus content decreased.(2)After adding dicalcium silicate particles of different sizes to CaO-SiO₂-FeO-P₂O₅ system,find that:1)after adding calcium silicate to the slag system,dicalcium silicate dissolved,solid solutions with diffusion mechanism and precipitation mechanism coexist,the phosphorus content in solid solution of precipitation mechanism is higher;2)dicalcium silicate dissolves in CaO-SiO₂-FeO-P₂O₅ system,there are 2 areas:calcium silicate particles with liquid slag whose FeO is high;liquid phase and solid solution region.The dissolution process is:(1)due to the presence of P₂O₅ in the CaO-SiO₂-FeO-P₂O₅ system,the slag preferential-ly appears the C₂S-C₃P solid solution of precipitation,which makes C₂S in the slag unsaturated,when calcium silicate particles were added to slag,calcium silicate particles dissolve to the liquid slag,FeO spreads to particles along the gap gradually,making calcium silicate crack and drop into the liquid slag;(2)P₂O₅and CaO in the slag diffuse to the shedding of calcium silicate to form solid solution,which further increases the FeO content in slag;(3)with calcium silicate dissolving to slag,which CaO content is increased,the slag becomes C₂S-C₃P supersaturated,making C₂S-C₃P directly precipitate from the liquid slag;(4)the solid solution is very stable in the slag,calcium silicate can continuously dissolved and transformed to solid solution,after 1000 s calcium silicate particles completely dissolve.(3)By comparing putting solid particles into the slag phase and isothermal experiment,find that:1)For the slag whose basicity is 1.82,solid solution appears after 45min,which is slower than the slag whose basicity is 3;2)P₂O₅ content for isothermal experiment is lower than the putting idea,compound slag is easier to form solid solution phase,and could accumulate more P₂O₅.