| With the aggravation of global energy crisis, the rapid development of new energy materials especially for Photovoltaic industry is provided with chance of a lifetime. The Photovoltaic industry in the world has been rising at a speed of growth rate for30%annually. In2009, the production of solar cells had reached to10.5GWp on a global scale. It was4382MW in China and was more than40%for global totals. The solar grade silicon (SoG-Si) is base of Photovoltaic industry and the crystalline silicon solar cells had a share of88%in market in2009so the supply and cost of silicon materials is crucial for solar cells. Presently, the production of SoG-Si has mainly two methods:Siemens technology and thermal decompositon of SiH4, which have restricted the development of Photovoltaic industry for their high cost and heavy environmental load. To our inspiration, the new technology for SoG-Si preparation with metallurgical method, for its low cost, low energy consumption and friendly environment, has been growing up and applying. But the critical points and puzzled problems are the removal of impurities in metallurgical grade silicon (MG-Si) especially for B and P and the requirement for6N purity for SoG-Si.In this paper, the features and removal of boron in MG-Si raw materials were systemically studied based on the MG-Si product with18ppmw for boron content. It was found that the impurities take on deep, light and white color inclusions in MG-Si. The deep inclusions confirmed as slag distribute in the whole MG-Si and the light and white ones that are thought to be metals or intermetallic compounds are lie in grain boundary of silicon. Based on binary phase diagrams between Si and impurity elements, the conformation of impurities Fe, Al, Ca, Ti, Ni, Cu etc in MG-Si raw materials was studied. The transformation among Si-B phases was analyzed and it was confirmed that SiB6is the form of Si-B phase in silicon melt. Based on Me-B, Si-Fe-B and Si-Al-B phase diagram systems, the probability of binary compounds FeB, Fe2B, AlB2, AlB12, CaB6and ternary compounds Fe4.7Si2B, Fe2Sio.4Bo.6were also analyzed. It was found that the preferential order for the combination between B and other elements is TiB2>MnB2>AlB2>Co2B>NiB>Fe2B>MgB2>SiB3. The phase equilibrium experiments indicated the existence of SiB4in Si-B binary system and also FeSi2, CaB6and SiB6in Si-Me-B systems, which was consistent with Si-Fe, Ca-B and Si-B phase diagrams. The thermodynamic process of boron removal by O2and H2O-O2gas blowing was studied andit was found that the impurity boron is volatilized in forms of gaseous boron oxide species (BxOy)and gaseous boron hydrate species (BxHzOy) and the thermodynamic relationships betweenboron removal limitation and partial pressures of gaseous species at1412?2230°C wasobtained. It showed that the partial pressure of gaseous boron hydrate species is much higherthan that of gaseous boron oxide species so it is preferable to for boron removal with H2O-O2gasblowing. At1450°C, the volatilization of BHO2and B3H3O6is most among all gaseous boronhydrate species. The partial pressures of BHO2reach to105Pa and2500Pa when the boron levelis reduced to5ppmw and O.lppmw respectively and the thermodynamic relationships displaythat on the contrary the partial pressures of gaseous boron hydrate species will reduce with theincrease of temperature. The experimental results by O2gas blowing in ladle showed that theremoval rates of impurities Fe,Cu, V and C in MG-Si is lower than20%and it is98.2%and92.9%for A1and Ca. B is reduced from35ppmw to18ppmw which is close to50%and P isreduced from184ppmw to112ppmw which is volatilized in forms of gaseous P2. Theexperimental results by Ar-H20-02gas blowing in DC arc furnace showed the removal rate ofboron increased with the prolongation of refining time and it was reduced from18ppmw to2ppmw when the time reached to1Omin. But it has great difference between the boron removalresults in experiments and the calculated value for boron removal limitation in thermodynamicfor the effects and constraint for dynamic factors.The thermodynamic relationships among boron level in silicon [%B], boron level in slag asi0and aCre obtained asthe same time, the electrochemical reactive model between Ca0-Si02slag and boron in MG-Siwas established, which displays the reactive mechanism between boron in silicon and molten slag. Theexperiments of slag refining in induction furnace showed it is not helpful to boron removal forpure Si2,which is accordant to thermodynamic results. The maximal value for partitioncoefficient of boron in silicon is1.58for Ca0-Si02slag when the composition of CaO/SiC>2is1.5. The boron level in silicon is reduced to1.8ppmw with the slag and silicon ratio2.5andrefining time3.Oh. The expeirmental results adding MgO, Li2,LiF and K2O to Ca0-Si02binary molten slag showed it is disadvantageous for boron removal with the addition of MgO.The boron level is respectively reduced to1.3ppmw and1,4ppmw when the slag and silicon ratio is4:1with the addition of20%LbO and30%K2O. The efficiency of boron removal is92.7%,The XRD spectrogram of refined slag showed that the experimental results with alloyed Si-Bsample are accordant to those of MG-Si refining and boron in MG-Si is oxidized into B2O3which combines into Li20*2B203with Li20.The method of boron removal for MG-Si with molten salt MeClx refining was brought forward.The thermodynamic relation between boron level in silicon and equilibrium partial pressure ofBCly°with molten salt CuCb,FeCb,MgCh,NaCK CaCl2and AICI3at1550C was obtained.It was found by experiments the boron level in silicon is reduced from18ppmw to3.1ppmw withadding50%~60%FeCl3to MG-Si. The efficiency of boron removal increases with the higherratio of molten salt in mixed FeCl3-Si02and MgCb-Si02systems. It is presumed that FeCb isreduced to Fe, which combines into intermetallic compound FeSi: with Si when using FeCl3refining. However, Mg is not found in slag with MgCb refining maybe the decomposition ofMgCb and volatilization of Mg. |
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