Research On Phosphorus Removal From MG-Si Using CaO-Based Slag Under Air Atmosphere

With the rapid development of the photovoltaic industry,route to solar grade Silicon(SoG-Si)refined by low cost metallurgical process has attracted much attention.Generally,metal impurities in silicon(Si)are removed by acid leaching and directional solidification,boron(B)by slag treatment and phosphorus(P)by vacuum treatments.However,due to the rigorous vacuum conditions required by P removal,it is of great theoretical value and practical significance to systematically explore a new process of effectively removing P from Si.In this paper,the lowcost slag treatment is adopted to study the P removal process and mechanism of CaO-SiO2-CaF2 slag under air atmosphere of the actual industrial production,and the results obtained are as follows.(1)A comparative experiment was conducted to study the B removal and P removal with CaO-SiO2-CaF2 by use of the orthogonal analysis method.It is shown that the slag/Si ratio and slag refining time are the key factors for B removal and the best B removal fraction is 94%,and the efficiency of B removal is higher within half an hour.However,the most important factors affecting P removal by slag refining are basicity and slag/Si ratio,which is different from B removal.The secondary phase CaSi2 will be formed in the cooling process of liquid Si,and the content of CaSi2 increases with the increase of basicity of slag.Moreover,P is precipitated in phase CaSi2 which is removed by acid leaching,and the maximal P removal fraction is 99%?It is also found that the slag after slag treatment with basicity higher than 1.39,will become powder at a slow cooling rate due to the ?-? phase transition of Ca2SiO4,which is in favor of separation of Si from slag.(2)The formation mechanism of secondary phase CaSi2 was studied.The results showed that part of P in Si was oxidized into the slag during the slag refining of Si using CaO-based slag,and the other P in Si is reduced,and distribution ratio of P between slag and Si,between CaSi2 and Si both reach maximums of 0.82 and 33.63,respectively,at the beginning refining period of around 10 min.It's shown that Si reacts with CaO to form Ca which dissolves into Si in the short period of slag refining.And the reaction of Ca,Si,and P happens during the cooling process after casting,to generate CaxSiyPz and CaSi2 which is in favor of removal of metal impurities,such as Fe,Al and Ca.Si and Ca will be oxidized after 15 minutes'slag treatment,which reduces the slag basicity and results in P in slag entering Si.Therefore,the low basicity slag is not good for P removal.(3)The feasibility of dephosphorization of Si by employing Si-Ca alloy was studied.It is found that P in Si can be removed by Si-Ca alloy,and the removal effect increases with the increasing of the amount of Si-Ca alloy.Distribution ratio of P between CaSi2 and Si comes up to 74.39 when the ratio of Si-Ca alloy to Si is 2,while P removal fraction reaches 95.45%.