Research On Removal Of Impurity Elements Of Boron And Metals In Silicon

Depletion of energy and environment problem make people focus on the clean energy.Among the solar cells,polycrystalline silicon is the most important material for Si solar cells production and makes up 50%.The Simens method is of high cost and pollutes the environment.So amount of researchers have started to study the metallurgical method which is low cost and environment friendly.However,boron element becomes the main impediment for its industrialization.The segragation coefficient of boron in metallurgical silicon is 0.8 and can't be reduced effectively by using directional solidificaiton.And its saturated vapor pressure is too low to be removed effectively through vacuum treatment.Plasma melting is an effective method to remove impurity B,but the cost is high and silicon loss is serious.Thus this paper attempts to adopt two ways to remove the boron impurity in the metallurgical grade silicon.Firstly boron is oxidized and spread to achieve effectively remove through slagging agent method,two kinds of quaternary slag systems of CaO-SiO2-BaO-CaF2 and CaO-SiO2-Al2O3-MgO are utilized to remove boron impurties in medium frequency induction melting furnace.Secondly,on the basis of the principle that the lower the temperature is,the less the boron content is in silicon,boron impurity is removed by separating the precipitation,accumulation layers of primary silicon solidified from Al-Si-(Sn)low melting alloy system under action of electromagnetic fileds.The conclusions are shown as follows:When the ratio of slag to silicon,CaO/SiO2 and Al2O3 content were 1:4,1.0 and 20%,respectively among CaO-SiO2-Al2O3-MgO slag system,a maximum distribution coeffcient LB of 4.7 could be obtained.With fixed slag composition and its ratio to silicon,LB increased with increasing melting time.When the electromagnetic induction melting time was 120 minutes,a maximum LB of 5.5 could be obtained.Considering factors such as to save energy and time,the melting time of 60-80 minutes were suggested.The best component of slag was found to be 35.5%CaO-35.5%SiO2-20%Al2O3-9%MgO.When the ratio of slag to silicon and refining time is 1:4 and 60 minutes,respectively,B content was reduced from 29 ppmw to 5.2 ppmw and total remove fraction of B was 82.1%after twice slag treating.Remove fraction of metallic impurities such as Al,Ti,Mn,Cu and P was 89.9%,49.9%,50%,70.8%and 33.3%,respectively.When the quaternary slag systems of CaO-SiO2-BaO-CaF2 was used to refine,LB increased with increasing the melting time,and most of the impurities could be reduced during 60 minutes.Considering production efficiency,60-80 minutes was best refining time.When 15%BaO and 20%CaF2 were added into CaO-SiO2 slag,LB could be increased significantly.When CaO/SiO2,BaO content and CaF2 content were 1.1,15%and 20%,respectively,a maximum LB of 6.94 could be obtained.The best slag component was found to be 34.1%CaO+30.9%SiO2+15%BaO+20%CaF2.When the best slag composition was adopted and the ratio of slag to silicon and refining time werel:4 and 60 minutes,B content could be reduced from 35ppmw to 3.7ppmw and the removal fraction of B was 89.4%after twice slag treating.When the Al-30%Si,Al-35%Si and Al-40%Si were solidified respectively at 845?(above liquidus temperature 30?),887?(liquidus temperature),930?(liquidus temperature)under action of 25 mT rotating magnetic field,primary silicon could be segregated and the biggest area of silicon rich layer could be obtained.With the increase of silicon content in Al-Si alloy,the area of silicon rich layer and the size of primary silicon increased,which was helpful to raise the purification efficiency of impurities,but the removal fraction of impurities decreased slightly.The best purification effect could be obtained by using Al-30%Si alloy.Except Al matrix,Fe could be reduced from 2339 to 4.3ppmw,Ca could be reduced from 75 to 0.3 ppmw,Ti could be reduced from 512 to 1.8 ppmw,B could be reduced from 65 to 7.2 ppmw,P could be reduced from 65 to 5.9 ppmw.When Al-30%Si-10%Sn was solifified under 10 mT travelling magnetic field,the primary silicon began to accumulte in the bottom and periphery of the sample.When magnetic flux density was greater than or equal to 25 mT,the segregation of the primary silicon became better.After the 50 mT magnetic flux density was adopted,B could be therefore effectively removed,and the B impurity could be reduced from 65 ppmw to 8 ppmw.