Mechanism Research On Deep Reductive Dephosphorization Of High Phosphorus Manganese-Silicon Alloy

Manganese-silicon(Mn-Si)alloy is one of the most produced and widely used alloys in ferroalloy industry.In recent years,with the rapid development of iron and steel industry in China,manganese alloy production is not only excess seriously but also brings double pressure to the sales and application.In fact,the manganese ore grade in China is generally low,which leads to the phosphorus impurity content in Mn-Si alloy production seriously exceed the standard.Thus,it is urgent to find effective ways to remove phosphorus impurities deeply at low cost and meet the requirements of the national standard.In this paper the problem of dephosphorization of high phosphorus Mn-Si alloy of ferroalloy enterprises in Chongqing Chengkou County were investigated.Based on the basic principle of thermodynamics,the thermodynamic behavior and mechanism of removal of phosphorus in the melt from two aspects,namely,Mn-Si melt and slag.At the same time,building a kind of fluorine free and harmless dephosphorization slag(such as CaO-SiO2-Al2O3 slag system)phosphide capacity is an effective way to be friendly to the environment.The experiment of reductive dephosphorization was carried out in MoSi2 furnace.The effect silicon and dephosphorizing alloys content,basicity of slags with little Na2 O,fluorine free of CaO-Al2O3-SiO2 on phosphorus distribution ratio and thermodynamic properties of Mn-Si melt was investigated.By using the analysis method such as ICP,SEM,XRD and XRF,conclusions are listed as follows:(1)With the increase of Si content in Mn-Si alloy,the dephosphorization rate had been significantly improved.Si and CaO reacted in the slag/metal interface and generated free Ca.Free Ca spreaded from the slag to Mn-Si melt and combined with P followed by Ca3P2 formation.Then,Ca3P2 floated up to the slag,thereby improving the dephosphorization rate.With the increasing Si content,the activity coefficient of phosphorus increased and the oxygen activity of Mn-Si melt decreased.In addition,the increasing Si content helped to decrease C content by the formation of SiC,which would float up to dephosphorization slag.(2)Ca,Ce and Al elements have theoretically good affinity capacity with P.However,Ca is easy to volatile and lost.The dephosphorization products formed by Ce and P attached to the graphite ball and could not expelled from the Mn-Si melt.The results of comprehensive comparison showed that the Al element had a better dephosphorization effect.The interaction coefficient of Al and P at the temperature of 1673 K was calculated as follows: 7)= 14.97.(3)With the increasing Na2 O content,the phosphorus distribution ratio and phosphorus capacity of the dephosphorization slag gradually increase.w(Na2O)in the initial slag should be controlled at 2%.With the increase of the amount of CaO substitution by Na2 O,phosphorus distribution ratio increases gradually.When the substitution is over 3.19%,2-in the slag was saturated and basicity of dephosphorization slag was maintained,as well as the phosphorus distribution ratio.(4)When the basicity of CaO-SiO2-Al2O3 slag was 1.0,the degree of polymerization of polymer gradually decreased with the decreasing w(Al2O3).The viscosity of slag gradually decreased and the fluidity of slag was improved,which is conducive to the rapid conversion between melt and slag.Thus,phosphorus distribution ratio increased.(5)The basicity of CaO-SiO2-Al2O3 slag is an important factor that affects the dephosphorization of Mn-Si alloy in the slag/metal interface.In the low-melting-point area of CaO-SiO2-Al2O3 phase diagram,oxygen ion activity in the slag increased with the increasing basicity of slag.Meantime,with the increasing basicity of slag,the viscosity decreased significantly and the liquidity of slag was improved remarkably,as well as phosphorus distribution ratio.(6)In the low-melting-point area of CaO-SiO2-Al2O3 phase diagram,due to the high concentration of Al2O3,there were numbers of large molecular polymer alumina complex ion formation.Thus,the viscosity of slag increased.At the same time,the free calcium produced by the reaction between Si and CaO could not quickly diffuse into the melt,which weakened the dephosphorization effect of the slag.When w(SiO2)was maintained,with the increasing CaO/Al2O3,macromolecular complex ions gradually disintegrated and the fluidity of slag turned better.Phosphorus distribution ratio increased slowly.When CaO/Al2O3 is over 1.3,it increased significantly.