In the production of solar grade silicon by metallurgical route the distribution of B and P between slags and liquid silicon is the most important key issue. The equilibrium and thermochemistry of reactions between liquid silicon and BaO-SiO slags and up to 10% BaO-containing CaO-BaO-SiO slags is studied through experimental work and using thermodynamic calculations. It is shown that the distribution coefficient of B (L ) is higher for the CaO-BaO-SiO slags than that for B 2 BaO-SiO slags and it is not significantly affected by temperature and composition changes of the slags. In contrast, the distribution coefficient of P (L ) is higher for BaO-SiO slags than that for the CaO-BaO-SiO slags, and it is higher at P 2 2 lower temperatures. The chemical activities of the dilute solutions of Ba in liquid silicon, and the dilute solutions of B O , 2 3 P O and BaO in the slags are calculated. Moreover, the reaction mechanisms for B, P, Ba and Ca transport between liquid 2 5 silicon and the slags are explained. Keywords Silicon · Boron · Phosphorous · Barium · Calcium · Slag · Distribution coefficient 1 Introduction energy consumption [2, 3]. However, the production of SoG-Si through metallurgical refining processes is more Silicon is liberated from its natural oxide form (quartz) energy efficient and environmentally friendly than chemical route which in turn may encourage a faster growth of through a carbothermic reduction process in submerged electric arc furnace; the product being metallurgical grade the global PV market. This has been the motivation for silicon (MG-Si), which is further purified to reach solar the development of several refining processes, where MG- Si is refined through the combination of sub-processes grade silicon (SoG-Si) quality for PV applications. The purity of MG-Si isusually above 99% Si and it contains to produce SoG-Si. Almost all the present impurities in MG-Si except B and P can be effectively removed by impurities such as Fe, Al, Ti, Ca, B, and P , while SoG-Si has much higher purity of above 99.9999 %Si. The majority directional solidification, which is a final key process step in metallurgical approaches. Boron is the most difficult of SoG-Si feedstock in the market is currently produced from MG-Si through the well known Siemens process element to be removed by directional solidification due to its large distribution coefficient between solid and liquid or newly developedfluid bed reactor (FBR) technology.In phases, which is K =0.8 . In order to remove this these chemical processes, pure silicon is deposited on rods impurity, many potential processes have been studied such or silicon seeds from a gas phase, which is produced as slag refining, plasma refining, gas refining, solvent through the conversion of MG-Si to purified gaseous compounds of silicon; SiHCl or SiH . The Siemens process refining, leaching, etc. . In particular, the slag refining 3 4 technique is a part of the only commercial metallurgical in particular is an expensive process with regard to high process of ELKEM Solar for SoG-Si production, where the dissolved B in silicon is adsorbed to a silicate slag through Jafar Safarian oxidation. Hence, the potential of slags for B removal is Jafar.Safarian@ntnu.no the main important parameter to consider for the process. In this case, the thermodynamic equilibrium for B distribution Norwegian University of Science and Technology (NTNU), Alfred Getz Vei 2, No-7491, Trondheim, Norway between liquid silicon and molten slags is studied, which Silicon is defined based on the weight percentages of B in the two the fundamental thermochemical approaches, employing phases as: thermodynamic tables and software. (wt%B) slag L = (1) [wt%B] met al 2 Experimental Procedure The magnitude of L -value is depending on slag type and composition, temperature, gas phase composition and as In the present study, specific silicon and slag samples were it is a quite important parameter it has been extensively produced and further interacted at elevated temperatures studied through many experimental works for many silicate through the following described methodology. slag systems. For instance, many slags such as CaO-SiO [5–10], CaO-CaF -SiO [6, 8, 11, 12], CaO-BaO-SiO , 2.1 Materials Preparation 2 2 2 CaO-MgO-SiO [5, 6, 13], CaO-Al O –SiO , CaO- 2 2 3 2 O-SiO [14, 15], Al O -CaO-MgO-SiO , Al O - Na Two types of BaO-containing slags were prepared by 2 2 2 3 2 2 3 BaO-SiO and Al O -CaO-MgO-SiO , Na O- mixing high purity powders of BaO, CaO and SiO powders 2 2 3 2 2 2 SiO  and CaO-Na O-SiO slags  have been studied (+99%) and melting the mixtures in high purity graphite 2 2 2 experimentally. In general L is increased with increasing crucibles. The slags were BaO-SiO binary slags, and B 2 temperature as been observed through the studies using BaO-CaO-SiO slags containing up to 10%BaO, the initial CaO-SiO2 slags , CaO-CaF2-SiO2 slags [6, 12], and compositions for these slags are given in Table 1. All the CaO-Na2O-SiO2 slags . The relationship between L target slag compositions are in molten state at the target and slag chemical composition is complicated an L in reaction temperatures, according to the related binary and a wide range from 0.3 for CaO-SiO2 slags to 9.3 for ternary slag systems. The slags were prepared through CaO-Al2O3-SiO2 slags  have been reported by different heating up the mixtures to 1923±30 K (1650±30 C) and researchers. Inspecting the literature data it is seen that holding for around one hour for complete melting, followed the measured L values for a given slag system are not by slow cooling to the room temperature. in agreement. For instance, the reported L values by High purity electronic grade silicon was doped by B Teixeira et al.  using CaO-SiO2 slags are in the range and P elements through mixing with two high purity Si-B of2to5.5, L is in the minimum at CaO/SiO2=0.85 and and Si-P master alloys, which were containing 500 ppm B it is increased with both decreases and increases of the and 1300 ppm P, respectively. The applied procedures for slag basicity. However, much narrower L range for the making these two master alloys were described previously same slag system has been observed in which L is not , . The silicon mixtures were melted through their significantly affected by the slag chemical composition and heating up to 1823 K (1550 C) in a high purity graphite it is increaseed minimally from 2.2 to 2.5 with increasing crucible in an induction furnace, holding at this temperature the basicity from 0.6 to 1.3 . It has to be noticed that for 30 minutes and casting the melt in a water-cooled copper the analysis of B in low concentrations in both Si and mold to attain a homogeneous silicon regarding the B and P slag phases is a challenge and this issue may be a reason concentrations. As a result, a silicon containing 30±1 ppmw for observing different results in similar experiments, in B and 25±0.5 ppmw P was produced as analyzed by a high addition to other sources of errors. The recent studies on resolution Inductively Coupled Plasma-Mass Spectrometer using Na2O-SiO2  and CaO-Na2O-SiO2 slags (ICP-MS) on four samples. It is worth mentioning that the have shown that when Na2O-containog slags are contacted contamination of silicon by carbon is not significant as the with silicon the dissolved B is also gasified in the form of solubility of carbon in silicon is low, i.e. 150 ppm at 1550 sodium metaborate (Na2B2O4) due to its relative high vapor C. This small amount of carbon have no significant effect pressure in the system as its mechanism has been explained on the chemical properties of B and P components, which [17, 18]. is more acceptable when the concentrations are very low as In the present study the distribution of B and also this study. P between silicon and BaO-SiO2 and CaO-BaO-SiO2 slags is studied. In addition to B distribution, the P 2.2 Silicon and Slag Interaction distribution between slags and silicon is studied as it is more concentrated in the silicon phase and it may affect The B- and P-doped silicon sample was added into the slag- further silicon purification processes, i. e. acid leaching. containing crucibles, while the slag/silicon mass ratio was The involved reactions for the mass transport of Ba, Ca, fixed equal to 2. Then the crucibles were heated to 1773 K ◦ ◦ B and P between the silicon and slag phases are studied. (1500 C) and 1873 K (1600 C) in an induction furnace The thermodynamic activities of the solute elements in the under high purity Ar (99.999%Ar) flow. The samples were silicon and slag phases are determined through applying hold at the target temperatures for 2 hours, followed by slow Silicon Table 1 The experimental conditions for interacting silicon and slag, and slag compositions before and after reaction Experiment no. Temperature ( C) Initial slag compositions (wt%) Final slag compositions (wt%) BaO CaO SiO BaO CaO SiO 2 2 1 1500 45.01 0 54.95 42.6 0 57.4 2 1500 49.6 0 50.2 45 0 55 3 1500 55.03 0 44.93 49.5 0 50.5 4 1500 60.04 0 39.95 56.8 0 43.2 5 1600 45.05 0 54.92 41.4 0 58.6 6 1600 50.03 0 49.95 44.8 0 55.2 7 1600 55.1 0 44.9 49.0 0 51.0 8 1600 60.01 0 39.94 53.4 0 46.6 9 1500 0 40.05 59.9 0 40.0 60.0 10 1500 2.5 39.0 58.5 2.5 38.9 58.6 11 1500 4.95 38.05 57.0 4.9 37.95 57.15 12 1600 0 40.02 59.93 0 39.9 60.05 13 1600 5.01 37.95 57.03 4.6 35.3 60.1 14 1600 10.02 36.05 53.91 8.6 33.9 57.5 cooling to the room temperature. As the electromagnetic in low concentrations has a high affinity into the CaO-SiO forces extensively stir the molten silicon, it causes high slags (with CaO/SiO =0.67) and it is not reduced at mixing of the system and equilibrium is reached in the 1500 C when up to 2.5 %BaO exists in the slag. The mea- experiments duration. The solidified samples were then sured concentrations of Ba in silicon interacted with the crushed and silicon and slag particles were separated. It is binary BaO-SiO slags (Fig. 1) show that the solubility of worth mentioning that the solidified silicon and slag phases Ba in silicon is increased with increasing the slag basicity showed similar configuration in all crucibles (silicon melt (BaO/SiO ratio), indicating there is larger driving force for is surrounded by slag phase) as observed previously in the mass transport of Ba from the barium-silicate slags into using Na O-SiO slag  indicating complete melting and the liquid metal for higher basicities. Moreover, the amount 2 2 proper contact of the two phase. The samples were then of Ba in silicon is depending on the process temperature. analyzed for measuring the main components by ICP-MS. Figure 1 shows that more Ba is transferred from the slags In this case, three parallels of each sample were analyzed into silicon at the higher temperature, while there is slightly and then the average compositions were determined. different trends at the two temperatures for Ba concentration dependence on basicity changes. The measured concentrations for experiments 9 to 14 3 Results show that the mass transport of both Ca and Ba from the slag into the silicon occurs; Ca for CaO-SiO slags and the The results of the experiments are described as follows. both elements in CaO-BaO-SiO slags. Figure 2 shows that Ba transfer into liquid silicon is increased with increasing 3.1 Mass Transport between the Phases and Ba the concentration of BaO in the initial slag for a given and Ca Distribution temperature and initial CaO/SiO ratios. Regarding the concentrations in Table 1, there is significant reduction in basicity ((wt%CaO+wt%BaO)/wt%SiO ) when BaO exists The measured chemical compositions of the slags after interac- tion with silicon are given in Table 1. The chemical com- in the slag, and considerable reductions of BaO and CaO occurs, which is accompanied with Si transfer as SiO into position changes in this table show that there is significant mass transport between the two phases so that the concentra- the slag. According to Table 1 and Figs. 1 and 2, the transfer tions of Ba, Ca and Si in the slags are significantly different of Ba and Ca into liquid silicon for a given initial chemical compare to the initial slag concentrations. However, no sig- composition is significantly higher at 1873 K (1600 C) nificant changes in the main slag components is observed than 1773 K (1500 C). Figure 3 shows the relationship for the experiments 8, 9 and 12. This may indicate that the between X /X molar ratio and X /X molar Ba Ca BaO CaO slags 8 and 9 are in almost equilibrium with molten sili- ratio in metal and slag phases, respectively. A direct con. However, experiment 10 result may indicate that BaO relationship between these concentration ratios in the two Silicon Fig. 1 Relationship between Ba concentration in silicon and the basicity of BaO-SiO slags phases is observed and larger X /X ratio for higher P concentrations in the two phases using the following Ba Ca BaO concentrations in the initial ternary slags. However, expression: it is observed that for a given BaO/CaO ratio there is higher Ba/Ca ratio in the metal phase at the lower (wt%P) slag temperature, evident of CaO reduction at 1873 K (1600 C) L = (2) [wt%P ] met al is significantly more than that at 1773 K (1500 C). It is worth noting that Ca and Ba transferred into the silicon are easily removed by directional solidification in the integrated Figure 4 shows the relationship between L and basicity solar silicon process. for the binary BaO-SiO slags in contact with silicon. Obviously, the L value is depending on temperature and 3.2 The Distribution of Phosphorus it is larger for lower temperatures. Moreover, L is not significantly depending on the basicity, and it is increased The distribution coefficient of P between the slag and silicon or decreased minimally with basicity changes, however, phases (L ) can be calculated based on the measured different trends for the two temperatures are observed. The Fig. 2 Relationship between Ba concentration in silicon and the basicity of CaO-(BaO)-SiO slags Silicon Fig. 3 Relationship between X /X ratio in silicon and the Ba Ca X /X in BaO SiO2 CaO-(BaO)-SiO slags at equilibrium, X denotes molar fraction of componenti distribution of P between CaO-(BaO)-SiO slags and silicon to 1.3 was studied by Johnston and Barati and the in Fig. 5 indicates that there is again larger L values for the measured L between 0.1 to 0.2 at 1773 K (1500 C), which P P lower temperature for a given slag basicity. Moreover, the is lower than the L in this study. L value is increased at 1773 K (1500 C) with increasing the basicity, or in another word the introduction of BaO- 3.3 The Distribution of Boron containing slags possess high phosphate capacity. However, the L change with basicity change is not significant at The measured concentrations of B between the slag 1873 K (1600 C). This may show that the introduction and metal phases were used to calculate the distribution of small amount of BaO into CaO-SiO slag is beneficial coefficient of B between the two phases by Eq. 1 and the for P removal. However, the both diagrams in Figs. 6 and results for the BaO-SiO slags and CaO-BaO-SiO slags are 2 2 7 indicate that the effect of temperature on P distribution shown in Figs. 6 and 7, respectively. These figures show is the main parameter. It is worth mentioning that L for that L is significantly lower for BaO-SiO slags than that P B 2 20%Al O -BaO-SiO slags for BaO/SiO between 0.45 for low BaO-containing CaO-SiO slags. L is in the range 2 3 2 2 2 B Fig. 4 L -value changes with basicity of BaO-SiO slags at different temperatures Silicon Fig. 5 L -value changes with basicity of CaO-BaO-SiO slags at different temperatures of 0.85 to 1.35 for BaO-SiO slags, while for the other given slag composition, meaning that more B is possible to slag is in the range of 2.0 to 2.4. On the other hand, the be removed from silicon at higher temperatures, this may relationship between the L and basicity in these slags be the reason of obtaining higher L -values in this study B B is different. It is worth mentioning that Suzuki et al.  than Suzuki et al. . The changes of L with basicity for measured lower L -values for using CaO-10%BaO-SiO the CaO-(BaO)-SiO slags in Fig. 7 show that L is not B 2 2 B slags at 1723K ( 1450 C) and the obtained L between 1.45 significantly affected by the introduction of small amount to 1.9 for (CaO+BaO)/SiO ratios between 0.8 to1.3, which of BaO into the CaO-SiO slag. Moreover, it is difficult 2 2 are higher basicity range than the present study. As seen in to see a clear effect of temperature and L -value in a ◦ ◦ Fig. 6, when the binary BaO-SiO slag is contacted with short range of 2.1±0.1 at 1600 C, and 2.2±0.2 at 1500 liquid silicon, the L value is showing smaller value when C. This insignificant temperature dependence of L is in B B the basicity is close to unity. However, for other studied agreement with literature for CaO-SiO slag systems . basicities, up to around 30% higher L -value is observed. In contrast toL parameter, L is more dependent on the B P B Moreover, L -value is higher for higher temperature for a slag composition and less on process temperature. Fig. 6 L -value changes with basicity of BaO-SiO slags at different temperatures Silicon Fig. 7 L -value changes with basicity of CaO-BaO-SiO slags at different temperatures 4 Discussion activity coefficient for the dilute solutions of Ca in silicon as 0.0032  at the silicon melting point at 1687 K (1414 The obtained results presented in the previous section C) supports this explanation. In order to calculate the ther- are discussed and they are used to determine some modynamic activity of Ba in liquid silicon the results of thermodynamics parameters in the studied slag-metal experiments 1 to 8 can be used. Considering the changes in systems. Gibbs energy of reaction (3) at equilibrium, G ,wemay express the chemical activity of the dissolved Ba in Si as: 4.1 Mass Transport of Ba into Silicon and its 2 2 Chemical Activity a a −G Si BaO 3 a =[ exp( )] (5) Ba a RT SiO2 When the binary BaO-SiO slag reacts with the high purity In this study a denotes the activity of component i in liquid silicon, a portion of Ba is transferred into the molten solution, R is the universal gas constant and T is absolute silicon through the following reaction: temperature. Regarding the low measured equilibrium −5 Si+2(BaO) = 2Ba+(SiO )K (at 1773 K) = 1.05×10 2 3 concentrations of Ba in silicon, we have a dilute solution of Ba in Si and it is a fair approximation to assume (3) ideal behavior for silicon solvent; a equal to unity. The Si In addition, when a BaO-CaO-SiO ternary slag is chemical activities in BaO-SiO solutions has been studied contacted with silicon, chemical reaction (4) takes place in literature through experimental and theoretical works simultaneously in the system, yielding some dissolved Ca in [22–25]. Based on the measured activities for BaO and SiO the silicon melt: by Tyurnina et al. [23, 24] at 1910 K (1637 C), the chemical −9 activities of these slag components were calculated by a Si + 2(CaO) = 2Ca + (SiO )K (at 1773 K) = 2.4 × 10 2 4 regular solution approximation at 1773 K (1500 C) and (4) 1873 K (1600 C). The calculateda and a curves and BaO SiO2 The measured slag concentrationsat two different tempera- their comparison with the reported data by Tyurnina et al. tures show that reaction (3) proceeds more at higher tem- are illustrated in Fig. 8. Negative deviation from the ideal peratures as observed in Figs. 1 and 2 for the two types of solution is clearly observed for the both slag components slags. The standard Gibbs energy of formation for reactions in a large composition range, in particular for the chemical (3)and (4) are both positive, i.e. 168.7 kJ/mol and 289.5 composition ranges of the slags in Table 1. ◦ ◦ kJ/mol at 1500 C and 1600 C, respectively. Therefore, we The chemical activity of the dissolved Ba in silicon in may conclude that the reason for the mass transport of Ba contact with the BaO-SiO slags can be calculated using and Ca through chemical reactions (3)and (4)isthe large the determined activities for BaO and SiO components in negative standard deviation of silicon rich Si-Ba and Si-Ca Eq. 5 as the results are illustrated in Fig. 9. Considering melts from the ideal solution, or in other word a driving the low concentrations of Ba in liquid silicon, the activity force existence for the reactions. The previously determined coefficient of Ba in dilute solutions can be calculated as Silicon Fig. 8 Chemical activities of BaO-SiO slags components at different temperatures ◦ ◦ −5 −5 γ = 8.8 ×10 and γ = 12.5 ×10 at 1773 K (1500 which causes small Ba transfer into Si (Fig. 2). Obviously, Ba Ba C9 and 1873 K (1600 C), respectively. These in turn yield a equilibrium is established by small BaO concentration in the temperature relationship between the activities coefficients slag, i.e. 2.5wt%BaO, while it occurs with more Ba transfer of Ba in Si-Ba dilute solutions as: at higher concentrations and temperatures. ◦ 1.229 −4 γ = 7.81 × 10 − (6) Ba 4.2 Chemical Activity of BaO in Low BaO-Containing CaO-SiO Slags As observed above, there is a large negative deviation from ideal solution for the silicon-rich Si-Ba solutions and due The mass transport of Ba through chemical reactions (3) to the low chemical activity of Ba, the chemical reaction occurs rapidly through the contact of slag and liquid silicon. (3) proceeds when the BaO-SiO slags are contacted with Considering this reaction at equilibrium, for the BaO- silicon, which causes significant Ba transport into the containing ternary slag, the chemical activity of BaO in the liquid silicon as shown in Fig. 1. The more Ba transport CaO-BaO-SiO slag can be expressed as: from the slag into silicon at higher temperatures for a given slag composition is mainly attributed to the higher a a SiO2 activity of BaO in the slags at higher temperatures, while Ba a =[ ] (7) BaO ◦ −G the other chemical activities in chemical reaction (3)are a exp( ) Si RT less temperature dependent. In other word, there is a larger driving force for the chemical reaction (3) at higher The concentration of Ba in liquid silicon is low and dilute solu- temperatures. However, not significant BaO transport to tions of Ba in silicon are in contact with the slags containing silicon when the concentration of BaO is low in CaO-BaO- low BaO concentrations. Assuming no significant inter- SiO is due to the low chemical activity of BaO in the slag, action between dissolved Ba and Ca in silicon, we may 2 Silicon Fig. 9 Chemical activities of dilute solutions of Ba in silicon calculate the chemical activities of Ba in silicon using the in the slag, very low concentrations of Ba and BaO in the above calculated γ for Ba in liquid silicon. On the other silicon and slag phases were measured in ppmw level, and Ba hand, as the concentration of BaO in the slags is low and Ba source is the trace impurity in the slag phase and this X <0.034 according to Table 1, it is a fair approximation gave points in Fig. 10 for very low BaO concentrations. BaO to consider the thermodynamics data for binary CaO-SiO The large difference between the activities of BaO at the slags to calculate the chemical activity of SiO in the slags. two studied temperatures may indicate that there is interac- Based on the activity data for SiO in CaO-SiO system by tions between the BaO, CaO and SiO components in the 2 2 2 Rein and Chipman  which are reliable as compared with slag phase, which are significantly dependent on tempera- literature previously , a can be estimated around 0.8 ture. Further precise work on this ternary slag system may SiO2 ◦ ◦ and 0.85 at 1773 K (1500 C) and 1873 K (1600 C), respec- provide more information to gain a better understanding of tively. Assuming the activity of silicon solvent as unity, we BaO thermochemical behavior in these slags. can calculate the chemical activity of BaO in the slag phases asshowninFig. 10 for the two temperatures. The calculated 4.3 Mass Transport of Ca ◦ ◦ −4 −4 results give γ = 1.1 × 10 and γ = 17.8 × 10 BaO BaO for the dilute solutions of BaO in CaO-SiO slags at 1773 As mentioned above, when silicon is contacted with ◦ ◦ K (1500 C) and 1873 K (1600 C), respectively. It is worth the CaO-BaO-SiO ternary slag, the partial silicothermic mentioning that for the experiments 9 and 12 with no BaO reduction of CaO from slag occurs through reaction (4) Fig. 10 Chemical activities of dilute solutions of BaO in CaO-BaO-SiO slags 2 Silicon simultaneously with BaO reduction. As BaO is more readily [22, 25] Employing the HSC Chemistry thermodynamic reduced due to considerably larger K value compared to software for calculating the changes in the reaction constant, K value, i.e. 4000 larger at 1773 K (1500 C), there will K , the activity of P O can be calculated for the both 4 10 2 5 be larger extent of Ba transfer than Ca transfer from the slag types of slags as shown against the P O molar fraction in 2 5 into the melt, as seen in Fig. 3. In addition to reactions (3) Fig. 11. and (4), the other reaction that can show equilibrium in the The calculated activities for P O in the slags can be 2 5 system regarding these components is: used to determine the activity coefficient of dilute solutions −32 of P O in the slags. This yields γ = 2 × 10 2 5 P 2O5 Ca + (BaO) = Ba + (CaO)K = (at 1773 K) = 65.74 (8) −30 and γ = 1 × 10 for illustrated system in Fig. 11 P 2O5 ◦ ◦ for BaO-SiO slags at 1500 C and 1600 C, respectively. This reaction at equilibrium condition may yield: Similarly for low BaO containing CaO-SiO slags at 1773 −33 ◦ ◦ K (1500 C) and 1873 K (1600 C), γ = 4 × 10 X γ γ X Ba Ca BaO BaO P 2O5 = (K ) (9) 8 −31 and γ = 3×10 are obtained, respectively. Although X γ γ X Ca CaO Ba CaO P 2O5 these very small activity coefficients for P O in the slags 2 5 The term in the parenthesis is a constant value for a given are obtained, not significant phosphorous is removed from temperature and depending on the interaction between the silicon into the slag, which is due to the very small reaction slag components (CaO and BaO), Ba and Ca are distributed. constant for chemical reaction (10). The slope of the lines in Fig. 3 may show the magnitude of The other type of chemical reaction for P transport from the term in parenthesis in Eq. 9 and as we see more extent silicon to the adjacent slag is the formation of barium of Ca transfer into the melt (relative to Ba transfer) occurs phosphide according to the following reaction: at higher temperatures. Although the reaction constant K decreases with increasing temperatures, the most important 2P + 3Ba = Ba P K (at 1773 K) = 3.7 × 10 (12) 3 2 12 parameter for Ca and Ba distribution will be the structure The chemical activity of Ba P in the slags can be calculated of the slag and the corresponding interactions between CaO 3 2 using the above calculated chemical activities for P and Ba and BaO in it, appeared in their activity coefficient terms in and this yields larger activities for this compound compared Eq. 9. to the calculated P O activities. This may indicate that the 2 5 formation of P O is a more stable form than Ba P in the slag. 4.4 Phosphorous Distribution Thermochemistry 2 5 3 2 However, in order to study the thermodynamics of P removal from silicon in BaO-SiO system through these mechanisms, Although a couple of chemical reactions may occur for 2 the following overall chemical reaction can be considered: the oxidation of the dissolved P in silicon or the reduction of its oxide from the slag, the equilibrium can be studied considering the following reaction for the both BaO-SiO 2 −21 Ba P +4(SiO ) = 3(BaO)+(P O )+4Si K (at 1773 K) = 1.6×10 3 2 2 2 5 13 and CaO-BaO-SiO slags: (13) −46 4P +5(SiO ) = 2(P O )+5Si K (at 1773 K) = 4.15×10 (10) 2 2 5 10 Considering the equilibrium constant for this reaction, K , we may write: Considering the equilibrium for reaction (10), we obtain the K a following expression for the chemical activity of P O in a 13 P 2O5 2 5 SiO2 ( ) = (14) 3 4 the slag: Ba3P 2 a a eq BaO Si 5 4 2 Based on the above calculated chemical activities for SiO a a SiO2 P a =[ K ] (11) P 2O5 10 and BaO and considering Raoultian behavior for silicon, the Si right part of Eq. 14 can be calculated as illustrated in Fig. 12 for two temperatures. It is found observed that for all the The chemical activity of P in liquid silicon, a , can be a a P 2O5 P 2O5 experiments ( )<( ) , indicating that P O is 2 5 calculated for different measured chemical compositions a a Ba3P 2 Ba3P 2 eq considering the phosphorous activity coefficients as γ = the more stable P-containing phase in the slag. Therefore, ◦ ◦ 0.47 and γ = 0.49 at 1500 C and 1600 C, respectively. we may conclude that the mechanism of P adsorption into  Activities of SiO in the slags for the given chemical the slag is through the chemical reaction (10). compositions at equilibrium in Table 1 can be determined The distribution of P, and in another word, the extent of by the outlined approach above for BaO-SiO slags and P transfer between the two phases is depending on the type the literature data for low BaO-containing CaO-SiO slags. of slag as observed in Figs. 4 and 5. Considering chemical 2 Silicon Fig. 11 Calculated chemical activities of dilute solutions of P O in BaO-SiO and 2 5 2 CaO-BaO-SiO slags at given representative conditions, symbols: calculated using experimental data reaction (10), the formation of P O is in relation with 3BaO·P O . The main chemical reactions for the formation 2 5 2 5 the source of oxygen in the system (SiO ), and therefore of these components can be written as: there must be a higher rate of P oxidation from silicon CaO + P O = CaO · P O K (at 1773 K) = 2.03 ×10 2 5 2 5 15 for lower basicity. For a given slag composition, observing (15) lower L value at higher temperature is attributed to the increase of P O activity in the slag, while the activity of P 2 5 in liquid silicon is not significantly changed by temperature 19 3CaO+P O = 3CaO·P O K (at 1773 K) = 2.07×10 2 5 2 5 16 change. On the other hand, the value of L is dependent (16) on the basicity and the structure of slag and as P O is 2 5 an acidic agent in the slag, L is expected to be increased 3BaO+P O = 3BaO·P O K (at 1773 K) = 5.53×10 with increasing basicity. This is observed for the BaO-SiO 2 2 5 2 5 17 slag, while it is less dependent on basicity for CaO-BaO- (17) SiO at 1873 K (1600 C). However, it is hard to explain the observed not significant L dependent on basicity for the The magnitude of the reaction constants K ,K , and 15 16 K indicates that for the slags in experiments 9 to 14, other conditions in this study. The L values in Figs. 4 and 5 P 17 for calcium-silicate slags are obviously larger than barium- where CaO is the main basic component, we may have a silicate slags. This may be explained considering the affinity better affinity of P O in the slag through the formation of 2 5 of the acidic agent P O in the slags with the main basic 3CaO•P O compared to barium silicate slags, which is 2 5 2 5 components as it can be in the form of calcium phosphates observed in significantly higher L -values for CaO-(BaO)- of CaO·P O and 3CaO·P O , or barium phosphate of SiO slags compared to BaO-SiO slags (Figs. 4 and 5). 2 5 2 5 2 2 Silicon Fig. 12 Relationship between P 2O5 ( ) and composition of Ba3P 2 eq BaO-SiO slags in equilibrium with silicon containing low P concentrations, symbols: calculated using experimental data 4.5 Chemical Activity of Boron Oxide For the above expressions L =1 was considered, which is a fair approximation (Fig. 6) and the calculations for It is generally accepted in literature that boron exists in the experimental points are well correlated in Fig. 13. the silicate slags in the form of oxide B O .Asthereis Similarly, the following expressions are obtained for low 2 3 always significant amount of silicon oxide in the slag in BaO-containing CaO-BaO-SiO : contact with liquid silicon, the equilibrium can be studied −9 2 lnγ (at 1773 K) =7×10 +0.23X +0.246X B2O3 B2O3 considering the following reaction for the both studied B2O3 binary and ternary silicate slags: (22) −2 2B+3/2(SiO ) = (B O )+3/2Si K (at 1773 K) = 9.3×10 (18) 2 2 3 18 −8 2 lnγ (at 1873 K) =1×10 +0.51X +0.543X B2O3 B2O3 B2O3 and the chemical activity of B O at equilibrium can be (23) 2 3 expressed as: It is worth noting that the activity coefficient of B O is 2 3 3/2 not significantly affected by L changes and close γ a a B B2O3 SiO2 B a = K (19) B2O3 18 values are obtained for different L -values by Eqs. 20–23. 3/2 B Si These equations show higher activity coefficients for boron The chemical activity of B in molten silicon can be oxide in the BaO-SiO slags than CaO-BaO-SiO slags, 2 2 calculated using the data literature,  which yields γ = which may indicate the higher affinity of boron into CaO- 3.87 and γ = 3.65 at 1773 K (1500 C) and 1873 K SiO slags which causes the significantly higher L value 2 B (1600 C), respectively. It is a fair approximation to consider for these slags as observed through comparing Figs. 6 and 7. Raoutian behavior for silicon solvent, and we can consider a equal to its molar faction, which is close to unity. 4.6 Boron Removal Mechanism Si Employing the HSC Chemistry thermodynamic software for calculating the changes in the reaction constant, K ,the When a boron-containing silicon melt is contacted with a activity of B O can be calculated for the both types of slags BaO-SiO slag, the oxidation of the dissolved boron occurs, 2 3 2 as presented against the B O concentration in Fig. 13 for assuming smaller initial B O than equilibrium concentra- 2 3 2 3 typical conditions. The calculations here yield the following tion in the slag. The oxidation can occur via the chemical expressions for the activity coefficient of B O in specific reaction (18) or via the following reaction: 2 3 BaO-SiO slags at different temperatures: −9 2B+3(BaO) = (B O )+3Ba K (at 1773 K) = 3.18×10 2 3 24 −10 2 lnγ (at 1773 K) = 7×10 +0.554X +0.48X (20) B2O3 B2O3 (24) B2O3 Calculating the changes in the standard Gibbs energy for ◦ ◦ −9 2 lnγ (at 1873 K) = 2×10 +1.39X +0.62X (21) B2O3 B2O3 chemical reactions (18)and (24), G and G ,wefind B2O3 18 24 Silicon Fig. 13 Calculated chemical activities of dilute solutions of B O in BaO-SiO and 2 3 2 CaO-BaO-SiO slags at representative equilibrium conditions, , symbols: calculated using experimental data that pure oxides of Si and Ba are stable in contact with pure phases, we can obtain the following equations by rearrang- B. However, when BaO-SiO slag containing is contacted ing the above equations: with the liquid silicon containing small amounts of B, the 3/2 a ◦ B2O3 mass transfer of B from silicon into the slag occurs due Si F = G − RT ln( ) = G + RT ln( ) 18 18 2 3/2 to the very small activity coefficient of B O in the slag. 2 3 a SiO2 We can determine the reaction mechanism for B oxidation (27) through calculating the changes in Gibbs energy of reactions (18)and(24): a ◦ B2O3 Ba 3/2 F = G − RT ln( ) = G + RT ln( ) 24 24 2 3 ◦ a a B2O3 Si a a B BaO G = G + RT ln( ) (25) 3/2 a a (28) B SiO2 Obviously, the magnitude of F and F for given process 18 24 conditions will show that which chemical reaction occurs 3 for B oxidation from thermodynamics point of view; the ◦ a a B2O3 Ba G = G + RT ln( ) (26) 24 reaction with lower F-value. Figure 14 shows the calculated 2 3 a a B BaO F and F values for different given Ba concentrations 18 24 in silicon and for a wide composition range of BaO-SiO ◦ ◦ In order to find out the reaction with lower Gibbs energy slags at 1773 K (1500 C) and 1873 K (1600 C) using independent of the concentrations of B and B O in the two the above determined activities for the involved species. 2 3 Silicon Fig. 14 Calculated changes of F and F functions with 18 24 BaO-SiO slags compositions for different temperatures and Ba concentrations in silicon As seen, for a large slag composition range up to 60% SiO The mass transfer of Ba from slag into liquid silicon occurs through silicothermic reduction, which causes and up to 0.04 wt% Ba in silicon the chemical reaction (24) is the dominant reaction for B removal. However, for dilute solutions of Ba in silicon, and the temperature dependence of the activity coefficient of Ba in silicon the high SiO concentrations and when high concentrations can be presented by Eq. 6. of Ba in silicon is maintained, the chemical reaction (18) The mass transport of P from silicon into the silicate slags is may be the mechanism for B removal. For the experiments through oxidation by SiO and the formation of P O is 1 to 8 the initial slags are in the range of X =0.63 to 2 2 5 SiO2 dominant compared to formation of Ba P . Although 0.76 and after the reaction the slags contain X =0.66 3 2 SiO2 very small chemical activity for P O in slags is calcu- to 0.78. Therefore, we may conclude that at initial the B 2 5 lated, P is not significantly removed from silicon. removal occurs by chemical reaction (24) and after reaching Boron is removed from silicon through oxidation by a specific level of Ba in silicon, the further B removal occurs BaO from the slag at high BaO concentrations and until by chemical reaction (18). However, for some experiments the Ba concentration in silicon reaches an specific concen- with small Ba transfer to silicon, only the chemical reaction tration. Further B removal occurs through oxidation by (24) is the one involved in B oxidation. SiO in the slag. Acknowledgements The present research has been supported by 5 Conclusions Research Domain 3-Recycling and Refining and Society in SFI Metal Production (a Norwegian Center for Research-driven Innovation in The interactions of B- and P-doped silicon with BaO-SiO metal production) through project number 237738. slags and low BaO containing CaO-BaO-SiO at 1773 K ◦ ◦ Open Access This article is distributed under the terms of the (1500 C) and 1873 K (1600 C) were studied, and the main Creative Commons Attribution 4.0 International License (http:// conclusions for our temperature and compositions can be creativecommons.org/licenses/by/4.0/), which permits unrestricted summarized as: use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a The distribution coefficient of B for BaO-SiO slags link to the Creative Commons license, and indicate if changes were is L =2.2±0.2, which is higher than that for low B made. BaO-containing CaO-SiO slags (L =1.1±0.2), and 2 B L is not significantly affected by temperature and composition changes of the slags. References The distribution coefficient of P, L , is in the ranges of 0.25-0.7, which is significantly smaller than L . 1. Schei A, Tuset J, Tveit H (1998) Production of high silicon alloys. However, L is higher for BaO-SiO slags than that for Trondheim, Tapir Forlag P 2 2. Braga AFB, Zampieri PR, Bacchin JM, Mei PR (2008) Review: low BaO-containing CaO-SiO slags, and L is higher 2 P new processes for the production of solar-grade polycrystalline at lower temperatures. silicon. Solar Energy Mater Solar Cells 92:418–424 Silicon 3. Safarian J, Tranell G, Tangstad M (2012) Processes for upgrading 17. Safarian J, Tranell G, Tangstad M (2013) Thermodynamic and metallurgical grade silicon to solar grade silicon. Energy Procedia kinetic behaviour of B and Na through the contact of B-doped 20:88–97 silicon with Na O-SiO slags. 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Silicon – Springer Journals
Published: May 31, 2018
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