"Composition of an anode alloy and method for using said composition"

Introduction

The present invention comprises a composition of an anode alloy for use in a three layer electrorefining of silicon and a method for producing silicon with a purity in the range of 99,99-99,999% by weight in a three layer electrochemical refining process using an anode alloy and an electrolyte.

Background

Super purity aluminium is today produced commercially by the so called three layer refining process. This is an electrochemical process comprising three molten layers where impure metal is alloyed with a heavy, noble metal (Cu) and placed in the bottom as anode of an electrochemical cell. Over this, a liquid layer of electrolyte with intermediate density is positioned. Liquid super purity aluminium with density of 2.3 g/cm ³ is deposited cathodically on top of this by sending electric current through the system. This principle may also be used for purifying other less-noble metals as Si and Mg, and may yield a product with ultra high purity.

99.9999+% (6N) is achieved for Al.

Yoshikawa et al. [1] describe removal of B by metallurgical solidification refining of Si using an Al-Si alloy in which precipitation of stabile substances in a liquid phase is followed by solidification refining of the contaminates between two phases in which one of the phases (Al) has a higher affinity for the undesirable contaminate (B) than the other phase (Si).

In three layer refining processes there has been a problem concerned with the production of super purified Si due to a narrow chemical window which implies use of low current densities which subsequently leads to low yield of Si. A high yield of Si in a three layer refining process can be obtained according to prior art, but the product Si will not satisfy the necessary requirements as to purity for use in solar cells due to contaminants of boron (B).

Summary of the invention

The present invention is conceived to solve or at least alleviate the problems identified above.

The objective of the present invention is to provide a composition of an anode alloy and method for producing silicon with a purity in the range of 99,99-99,999% by weight for use in a three layer electrorefining process involving an anode alloy and an electrolyte.

Specially, an object of the invention is to obtain an anode alloy and a method involving said anode alloy for producing super purified silicon with a high yield and reduce the problems with contaminants. In the present invention it was surprisingly found that addition of a metal to the anode alloy in a three layer electrorefining cell resulted in the possibility of widening of the electrochemical window and allowing the use of higher current densities and subsequently higher yield, along with lower content of contaminants and a purity of the refined Si in the range of 99,99- 99,999% by weight.

The invention provides in an aspect an anode alloy composition for use in an anode that constitutes a lower layer in a three layer arrangement for electrorefining of silicon, said composition comprising the following components: 60-90 weight% Si, 0-40 weight% Cu, 0-10 weight% B, 0-10 weight% AI, wherein the composition comprises at least one of the following components: 0-40weight% of a metal from group 8 of the transition metals, 0-10 weight% of a metal from group 4 of the transition metals, 0-10 weight% of a metal from group 5 of the transition metals, and possibly at least one precipitated intermetallic compound. The density of said anode alloy is in the range of from 2,7-6 g/cm ³ , more preferably in the range of from 2,7-4,4 g/cm ³ . The intermetallic compounds are at least one compound comprising metal boride.

In a further embodiment the present invention comprises a method for producing silicon with a purity in the range of 99,99-99,999% by weight in an electrorefining process using a three layer arrangement comprising an anode, an electrolyte, and a cathode, said electrolyte comprising at least one alkaline earth metal fluoride, said anode being a metal alloy anode, the alloy comprising the following components: 60-90 weight% Si, 0-40 weight% Cu, 0-10 weight% B, 0-10 weight% Al, wherein at least one of the following components is added to said anode alloy: 0-40 weight% of a metal from group 8 of the transition metals, 0-10 weight% of a metal from group 4 of the transition metals, 0-10 weight% of a metal from group 5 of the transition metals.

At least one of the components added to said anode alloy is chosen from the transitions metals: Ti, Fe and V.

A further aspect of the present invention is to keep the hydrostatic pressure high at the bottom of the cell in order to avoid contact between the intermetallic particles and the electrolyte. This can be obtained by keeping the height of the column of the anode alloy in the range of from 20-100cm, preferably 20-80cm and most preferably 20-50cm. Further, the added metal component is formed into at least one compound having a densitiy p < Panodeaiioy , and at least one compound with a densitiy p < P _a n _odea ii _o y is removed from the upper part of the forwell.

In another aspect, the added metal component is formed into at least one compound having a densitiy p > Panodeaiioy - and at least one compound having a density p > P _anodea ii _o y is precipitated. Said at least one compound is precipitated at the bottom of a forwell.

In the present method for producing silicon by three layer electrorefining using an anode alloy and an electrolyte, contaminants such as at least one metal

component is added to the anode alloy in the forewell. The anode alloy primarily consists of SiCu, SiCuFe or SiFe. The contaminants have a residence time previous to the formation of different borides which is precipitated and settled at the bottom and is further removed from the forewell. Possibly lighter precipitated compounds will rise to the top of the forewell where it is removed. A definite anode alloy and further contaminants such as preferably at least one of the following components Ti, Fe and V is added to the anode alloy in order to remove active boron of the anode alloy in the main cell which again lead to widening of the electrochemical window. The effect of the widening of the electrochemical window is that the current density is increased simultaneously as achieving high purity of the product Si at the cathode. The addition of contaminants in a three layer electrorefining process results in the widening of the electrochemical window, the increase in the current density and the production of super purity Si which is a surprising and unexpected feature. The precipitated particles of intermetallic compounds will settle at the bottom of the cell. In order to avoid contact between the electrolyte which tend to form a film around the anode alloy, the hydrostatic pressure at the bottom of the cell must be kept high enough in order to prevent the electrolyte from forming a film below the anode alloy. This is achieved having a layer of the anode alloy in the range of from 20 - 100cm. Further, it is required to collect the precipitated particles and segregation crystals/compounds in the forewell. This can be achieved by constructing the forewell with a depressed bottom compared to the main area of the cell. The particles and contaminants will be collected in the forewell which makes removal of these more easily.

Brief description of drawings

Embodiments of the invention will be described with reference to the following drawings, where:

Figure 1 : The principle of three-layer electrorefining depicted schematically. Figure 2: X-ray diffractogram of particles collected from the top of the anode alloy. Densities: alloy, 4.0 g/cm ³ , electrolyte 3.0 g/cm ³ . AI ₂ Cu has been oxidized to AI ₂ CuO ₄ due to air exposure.

Figure 3: Schematic drawing of a cell for electrorefining of Si. lntermetallic particles of Ti, Fe, Al, B etc. form in the anode alloy. Particles lighter than the alloy will float up in the forewell while heavy particles will accumulate on the bottom. Figure 4: The relationship between the content of B and the content of Ti in a Si+20wt%Cu molten alloy.

Figure 5: The relationship between the content of B and the content of Fe in a Si+20wt%Cu molten alloy.

Detailed description The principle of three layer refining process for Si is shown schematically in Figure 1. The principle relies on the electrochemical series which lists the energy needed for anodically dissolving metals in an electrolyte according to Eq. 1.

^F _{2 +} M = MF _n [1]

This is again funded on fundamental thermodynamic properties of the elements. The electrolyte may be based on different anions such as F ^" , Cl ^" , SO ₄ ^2" or O ^2" , to mention some. For the refining of Al, fluoride based electrolytes is utilized. For the refining of Si, oxide based electrolytes has been investigated, but without success due to a number of reasons, most notably the high viscosity of these melts. Silicon may, however, be refined in a three layer process above its melting point by incorporating a Si-Cu alloy as anode under a fluoride based electrolyte based on CaF ₂ with additions of BaF ₂ (density modifyer) and SiF ₄ as Si-carrying agent. The electrochemical series in the fluoride system is listed in Table 1. The elements with E°<E°(Si) will thermodynamically be more stable than Si and so not go anodically into solution during polarization of the Si-Cu alloy. The elements with E°>E°(Si) will be less stable than Si and enter the electrolyte in the form of fluorides together with Si during polarization. The difference in E ⁰ between Si and its neighbours is termed the electrochemical window in the refining of Si. In a real electrochemical process, the E°-values is modified and the actual potential where the process starts is termed E ^rev . This effect is described by Eq. 2 and arises due to chemical activities deviating from unity.

E ^> - ^{" δG} and E ^rev = E° - —\n{-^ [2] nF nF a _B - a,

Chemical activites less than unity will tend to push the potential for the reduction or oxidation reactions away from the theoretical ε°-value. From Table 1 , it can be seen that the values for E ⁰ for the anodic dissolution of B is very similar to the value of E ⁰ for Si (δE=13mV) on the cathodic side, while on the anodic side, the electrochemical window to Al is substantial (δE=147mV).

Table 1: Gibbs free energy and related electrochemical potential for the dissolution of metals in fluoride media at 1700K.

This implies that Al is not easily reduced into the purified Si at the cathodic side of the cell, while B may more easily be codissolved anodically with Si if the activity of

B approaches unity in the anode alloy. B is not tolerated in quantities >~2ppm in

Si for the use in solar cells, while it occurs in definite quantities (~25-40 ppm) in the raw-material metallurgical Si feedstock. While an electrochemical window of

13mV may be tolerated in a high-temperature three-layer electrorefining process by keeping the anodic current density low, it is desirable to keep the activity of B in the anode alloy low in order to push the E _rev -value for the dissolution of B away from that of Si-dissolution. This in order to increase the anodic current density

employed and thereby the yield from the reactor. This may be accomplished by using feedstock without or very low in B. This is not easily accomplished since B is inherently difficult to remove from Si due to its high segregation coefficient (0.8- 1.0). If low-B feedstock may be found, boron will never the less accumulate in the anode alloy until it reaches activities approaching unity in the long term.

Description of invention

The present method is carried out in a three layer electrorefining cell in which the density of the bottom layer, the anode alloy, is in the range of from 2,7 g/cm ³ to 6 g/cm ³ , the middle layer, the electrolyte, is in the range of from 2,6 g/cm ³ to 4 g/cm ³ , and the top layer Si, the product, has a density of 2,57 g/cm ³ .

In three layer electrorefining of Si, a range of intermetallic compounds may form in the anode alloy at the bottom of the cell. Boron and aluminium are known to form very stable borides and aluminates with Ti, Fe and Cu. The anode alloy comprised of mainly Si and Cu will, invariably, contain substantial amounts of other metallic impurities more electropositive than Si during long term operation of an industrial cell. Among these, Fe and Ti will be the most abundant as these are the main impurity elements in metallurgical Si used as feedstock. As in the Al-Si melt, Ti and B will have a high affinity for each other and the reaction described by Eq. 4 will proceed in the molten alloy. For titanium, the boride-forming mechanism can be described by Eq. 5.

xM + yB = M _x B _y [4]

Ti + 2B = TiB ₂ δG°= -264kJ/mol [5]

Similar mechanisms exist for Fe-B, Cu-B and a number of other transition metal borides. The compounds are very stable with Gibbs free energies of formation (δG°) in the range from 60kJ/mol - 270kJ/mol. As in the Al-Si alloy, the reaction will be characterized by a solubility constant K _sp , although it will have another value in Si-Cu than in Al-Si melt. The higher affinity for B in the Al-Si melt than in the segregated Si crystallites described in [1] indicates that the value of K _sp (Ti, B)

in molten Si-Cu alloy will be lower than in Al-Si alloy as the B seems to have a higher affinity for Al than for Si.

A number of experiments have been conducted to study the combined effects of precipitation and electrorefining in fluoride media. By keeping the density of the anodically polarized alloy high compared to the density of the intermetallic particles being formed, the particles will accumulate on the surface of the electrolyte and can easily be collected after cool-down for further investigation by XRD to determine which phases are present. A diffractogram of particles collected from the surface of the anode in such an experiment is shown in Figure 2. A number of intermetallic phases can positively be identified. The density of the particles will determine whether they accumulate on the surface of the alloy or under the alloy. XRD has also been performed on particles detected beneath the anode alloy. A total list of particles detected is found in Table 2. In an industrial cell for electrochemical refining of silicon, keeping the density of the electrolyte as well as the anode alloy as low as possible while maintaining the three-layer system will be of paramount importance for the purity of the product. Particles lighter than the anode alloy must be allowed to form in the forewell compartment where they can be skimmed off and not entering the volume covered by the electrolyte as indicated in Figure 3.

Table 2: Intermetallic phases detected in particles collected from the top and bottom of the anode alloy layer and their respective densities.

Manipulation of the relative contents of impurities

As described above, the solid intermetallic phases being formed in the anode alloy will exhibit a fundamental, characteristic solubility product K _sp both in the alloy and in the electrolyte. Hence, it is extremely important to avoid contact between the boron-containing intermetallic particles and the electrolyte. This can be accomplished by keeping the hydrostatic pressure high at the bottom of the cell by having a high column of anode metal present (~50cm) and so inhibit the formation of electrolyte film below the anode alloy.

Experiments have been performed to determine the K _sp -values in Si+20%Cu for TiB ₂ and FeB respectively. Chemical analysis data for the content of Ti-B and Fe- B and their interdependence are shown in Figure 4 and Figure 5 respectively. The K _s p -values found were 2.8-10 ¹⁴ (TiB ₂ )and 9.5-10 ⁹ (FeB). These are low values reflecting the high stability of the boride compounds. By purposely keeping the content of Ti and Fe high in the anode alloy compared to that of the feedstock, it is possible to manipulate the solubility of B to a very low value. This is described by Equation 6.

For the values found in the present invention, a content of 0.5% of iron and 3% of Ti in the anode alloy should each push the equilibrium value of B to 1ppma. These numbers will, however, not necessarily be accurate in a real system since non-ideal behaviour may be encountered at such extremely low activities of B. The order of magnitude should on the other hand be correct, and this implies that steady state concentrations on the order of % of Fe and Ti is capable of reducing the content of B in the molten anode alloy to values which can be tolerated in silicon for solar cells after subsequent electrorefining. Such additions will also make it possible to increase the current densities in the industrial cells as the electrochemical window will be widened due to Eq. 2. Typical values of the parametes (PBF ³⁼ 10 ^"4 - 10 ^"2 , 8B=IO ^"6 ) shows the electrochemical window to increase from 13mV to up to 58mV. This is more than enough to achieve a high grade of electrorefining at industrial relevant current densities (2-300mA/cm ² ).

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.