Production of Si by reduction of SiCU with liquid Zn, and purification process

The invention relates to the manufacture of solar grade silicon as a feedstock material for the manufacture of crystalline silicon solar cells. The Si metal is obtained by direct reduction of SiCl ₄ , a precursor that is commonly available in high purity grades.

Silicon suitable for application in solar cells is commonly manufactured by the thermal decomposition of SiHCl ₃ according to the Siemens process or its variants. The process delivers very pure silicon, but it is slow, highly energy consuming, and requires large investments.

An alternative route towards the formation of Si for solar cells is the reduction of SiCl ₄ with metals such as Zn. This process has the potential for significant cost reduction because of lower investment costs and reduced energy consumption.

The direct reduction of SiCl ₄ by Zn in the vapour phase is described in US 2,773,745, US 2,804,377, US 2,909,411 or US 3,041,145. When Zn vapour is used, a granular silicon product is formed in a fluidised bed type of reactor, enabling easier Si separation. However, an industrial process based on this principle is technologically complex.

The direct reduction of SiCl ₄ with liquid Zn is described in JP 11-092130 and JP 11- 011925. Si is formed as a fine powder and separated from the liquid Zn by entraining it with the gaseous ZnCl ₂ by-product. There is however no explanation why the entrainment of fine powder Si with ZnCl ₂ can take place. It proved impossible to repeat the process as described in these patents. The essential technical features enabling to discharge substantial amounts of the generated polycrystalline silicon powder together with the vapour of the zinc chloride are missing.

It is an object of the present invention to provide a solution for the problems in the prior art.

To this end, according to this invention, high purity Si metal is obtained by a process for converting SiCl ₄ into Si metal, comprising the steps of:

- contacting gaseous SiCl ₄ with a liquid metal phase containing Zn, thereby obtaining a Si-bearing metal phase and Zn-chloride;

- separating the Zn-chloride from the Si-bearing metal phase; and

- purifying the Si-bearing metal phase by heating to a temperature above the melting point of Si and injecting Cl ₂ and/or a gaseous Si chloride compound into said metal phase, thereby vaporising Zn, eliminating impurities and obtaining Si metal. The Si chloride is preferably SiCl ₄ , i.e. the same compound as used in the contacting step.

The contacting and the separating steps are performed in a single reactor. This is rendered possible by the fact that a major part (more than 50% by weight) of the formed Si is retained in the liquid metal phase.

It is useful to combine the contacting and the separating steps, by operating the contacting step at a temperature above the boiling point of Zn-chloride, which evaporates. The Zn-chloride can be permitted to escape so as to be collected for further processing.

The Si-bearing metal phase as obtained in the contacting step can advantageously contain, besides Si as solute, also at least some Si in the solid state, e.g. as suspended particles. Formation of particular Si may indeed occur during the contacting step, when the Zn metal gets saturated in Si. Solid state Si can also be obtained by cooling the Si- bearing metal phase as obtained in the contacting step, preferably to a temperature of between 420 and 600 ⁰ C. The solid state Si can preferably be separated from the bulk of the molten phase, e.g. after settling. This Si metal phase is however still impregnated with Zn and has to be further processed in the purification step.

It is advantageous to perform the contacting step by injecting SiCl ₄ into a bath comprising molten Zn at a flow rate adapted to limit the loss of Si by entrainment with

evaporating Zn-chloride, to less than 15% (weight). Flow rates of SiCl ₄ up to 50 kg/min per m ² of bath surface are compatible with the abovementioned low Si losses. Preferably the gaseous SiCl ₄ is adequately dispersed in the bath, e.g. by using multiple submerged nozzles, a submerged nozzle equipped with a porous plug, a rotating gas injector, or any other suitable mean or combination of means. The SiCl ₄ can be injected along with a carrier gas such as N ₂ . A flow rate over 10, and preferably 12 or more kg/min per m ² of bath surface is advised to perform the process in a more economical way.

It is useful to add a vacuum evaporation step after the purification step. The purification can advantageously be performed in again the same reactor as the first two process steps.

It is also advantageous to recycle one or more of the different streams which are not considered as end-products:

- the obtained Zn-chloride can be subjected to molten salt electrolysis, thereby recovering Zn, which can be recycled to the SiCl ₄ reduction step, and chlorine, which can be recycled to a Si chlorination process for the production of SiCl ₄ ;

- Zn that is vaporised in the purification step can be condensed and recycled to the SiCl ₄ contacting step; and/or- the fraction of SiCl ₄ that exits the contacting and the purification steps un-reacted can be recycled to the SiCl ₄ contacting step, e.g. after condensation.

According to this invention, SiCl ₄ is reduced with liquid Zn. The technology for this process is therefore much more straightforward than that required for the gaseous reduction process. A Si-bearing alloy containing both dissolved and solid Si can be obtained, while the chlorinated Zn either forms a separate liquid phase, containing most of the solid Si, or is formed as a vapour. Zn can be retrieved from its chloride, e.g. by molten salt electrolysis, and reused for SiCl ₄ reduction. The Si-bearing alloy is purified at high temperatures, above the melting point of Si, which is well above the boiling points of both Zn and Zn-chloride. The evaporated Zn can be retrieved and

reused for SiCl ₄ reduction. Any other volatile element is also removed in this step. It is thus possible to close the loop on Zn, thereby avoiding the introduction of impurities into the system through fresh additions.

In a preferred embodiment according to the invention, gaseous SiCl ₄ is contacted with liquid Zn at atmospheric pressure, at a temperature above the boiling point OfZnCl ₂ (732 °C) and below the boiling point of Zn (907 ⁰ C). The preferred operating temperature is 750 to 880 ⁰ C, a range ensuring sufficiently high reaction kinetics, while the evaporation of metallic Zn remains limited.

In a typical embodiment, the molten Zn is placed in a reactor, preferably made of quartz or of another high purity material such as graphite. The SiCl ₄ , which is liquid at room temperature, is injected in the zinc via a submerged tube. The injection is performed in the lower part of the Zn-containing vessel. The SiCl ₄ , which is heated in the tube, is actually injected as a gas. It can also be vaporized in a separate evaporator, and the obtained vapours are then injected in the melt. The end of the injection tube can be provided with a dispersion device such as a porous plug or fritted glass. It is indeed important to have a good contact between the SiCl ₄ and the Zn to get a high reduction yield. If this is not the case, partial reduction to SiCl ₂ could occur, or SiCl ₄ could leave the zinc un-reacted. With an adequate SiCl ₄ - Zn contact, close to 100% conversion is observed. Finely dispersing the SiCl ₄ is an important factor in limiting the entrainment of finely dispersed Si with the gaseous flow.

The reduction process produces ZnCl ₂ . It has a boiling point of 732 ⁰ C, and is gaseous at the preferred operating temperature. It leaves the Zn-containing vessel via the top. The vapours are condensed and collected in a separate crucible.

The process also produces reduced Si. The Si dissolves in the molten Zn up to its solubility limit. The Si solubility in the Zn increases with temperature and is limited to about 4% at 907 °C, the atmospheric boiling point of pure Zn.

In a first advantageous embodiment of the invention, the amount of SiCl ₄ injected is such that the solubility limit of Si in Zn is exceeded. Solid, particulate Si is produced, which may remain in suspension in the molten Zn bath and/or aggregate so as to form dross. This results in a Zn metal phase with a total (dissolved, suspended and drossed) mean Si concentration of preferably more than 10%, i.e. considerably higher than the solubility limit, and thus in a more efficient and economic Si purification step. The particulate Si can be subject to losses by entrainment with the ZnCl ₂ gaseous stream, however, in practice the Si loss by entrainment is less than 15% of the total Si input, and this is to be considered as acceptable.

In a second advantageous embodiment according to the invention, the Si-bearing alloy is allowed to cool down to a temperature somewhat above the melting point of the Zn, e.g. 600 °C. A major part of the initially dissolved Si crystallizes upon cooling, and accumulates together with any solid Si that was already present in the bath, in an upper solid fraction. The lower liquid fraction of the metal phase is Si-depleted, and can be separated by any suitable means, e.g. by pouring. This metal can be directly re-used for further SiCl ₄ reduction. The upper Si-rich fraction is then subjected to the purification as mentioned above, with the advantage that the amount of Zn to be evaporated is considerably reduced.

Both of the above first and second advantageous embodiments can of course be combined.

When the purification step is performed above the melting point of Si, the molten silicon can be solidified in a single step, chosen from the methods of crystal pulling such as the Czochralski method, directional solidification and ribbon growth. The ribbon growth method includes its variants, such as ribbon-growth-on-substrate (RGS), which directly yields RGS Si wafers.

Alternatively, the molten silicon can be granulated, the granules being fed to a melting furnace, preferably in a continuous way, whereupon the molten silicon can be

solidified in a single step, chosen from the methods of crystal pulling, directional solidification and ribbon growth.

The solid material obtained can then be further processed to solar cells, directly or after wafering, according to the solidification method used.

The Zn, together with typical trace impurities such as Tl, Cd and Pb, can be separated from the Si-bearing alloy by vaporisation at a temperature above the boiling point of Zn (907 °C). Si with a purity of 5N to 6N is then obtained. A special high temperature sparging or bubbling step with Cl ₂ and/or a gaseous Si chloride typically leads to Si with even superior purity. For this operation, the temperature is further increased above the melting point (1414 °C) but below the boiling point (2355 ⁰ C) of Si. Some of the elements that can be eliminated efficiently by this process step are Cr, Cu, Mn, Al, Ca, B and P.

A further advantage of the invention is that the Si can be recovered in the molten state at the end of the purification process. Indeed, in the state-of the art Siemens process and its variants, the Si is produced as a solid that has to be re-melted to be fashioned into wafers by any of the commonly used technologies (crystal pulling or directional solidification). Directly obtaining the Si in the molten state allows for a better integration of the feedstock production with the steps towards wafer production, providing an additional reduction in the total energy consumption of the process as well as in the cost of the wafer manufacturing. The liquid Si can indeed be fed directly to an ingot caster or a crystal puller. Processing the Si in a ribbon growth apparatus is also possible.

If one does not wish to produce ready-to-wafer material, but only intermediate solid feedstock, it appears advantageous to granulate the purified Si. The obtained granules are easier to handle and to dose than the chunks obtained in e.g. the Siemens-based processes. This is particularly important in the case of ribbon growth technologies. The

production of free flowing granules enables the continuous feeding of a CZ furnace or a ribbon growth apparatus.

Example 1 The following example illustrates the invention. 8500 g of metallic Zn is heated to 850 ⁰ C in a graphite reactor. The height of the bath is about 19 cm and its diameter is 9 cm. A Minipuls™ peristaltic pump is used to introduce SiCl ₄ in the reactor via a quartz tube. The immersed extremity of the tube is fitted with a porous plug made of quartz. The SiCl ₄ , which has a boiling point of 58 °C, vaporises in the immersed section of the tube and is dispersed as a gas in the liquid Zn. The SiCl ₄ flow rate is ca. 250 g/h, and the total amount added is 5200 g. The flow rate corresponds to 0.66 kg/min per m ² of bath surface. The ZnCl ₂ , which evaporates during the reaction, is condensed in a graphite tube connected to the reactor and is collected in a separate vessel. Any un- reacted SiCl ₄ is collected in a wet scrubber connected to the ZnCl ₂ vessel. A Zn-Si alloy, saturated in Si at the prevalent reactor temperature and containing additional solid particles of Si, is obtained. About 5.3 kg of Zn-Si alloy is obtained. The total Si content of this alloy is 14.6%. The Si reaction yield is thus about 91%. The Si losses can be attributed to the entrainment of particles of Si with the escaping ZnCl ₂ vapours, and to the incomplete reduction of SiCl ₄ into Si metal. Of the remaining Si, 62 g is found in the ZnCl ₂ and about 10 g in the scrubber.

The Zn-Si alloy is separated in two fractions of about equal weight. Each fraction is heated to 1550 °C for 1 h in a quartz crucible, in order to evaporate the Zn and melt the Si. A quartz tube is dipped into the molten bath corresponding to a first alloy fraction, and SiCl ₄ is bubbled at a flow rate of 200 g/h. The second alloy fraction is not sparged. Both fractions are allowed to cool down to room temperature, under inert atmosphere. Two samples are taken from each fraction, and analysed by Glow Discharge Mass Spectrometry (GDMS). The average impurity levels are reported in the Table below. The concentration of most impurities is significantly reduced by the SiCl ₄ sparging step.

Table: Impurities in Si after thermal treatment with and without SiC14 bubbling

Example 2

170 kg of metallic Zn are heated to 850 °C in a graphite reactor placed in an induction furnace. The height of the bath is about 50 cm and its diameter is 26 cm. A membrane pump is used to transport SiCl ₄ (bp 58°C) into an evaporator . The gaseous SiCl ₄ is then bubbled through the zinc bath via a quartz tube. The SiCl ₄ flow is ca. 12 kg/hour, and the total amount added is 110 kg. The flow rate corresponds to 3.8 kg/min per m ² of bath surface. The ZnCl ₂ , which is formed during the reaction, evaporates and is condensed in a graphite tube connected to the reactor and is collected in a separate vessel. Any un-reacted SiCl ₄ is collected in a wet scrubber connected to the ZnCl ₂ vessel. About 99 kg of Zn-Si phase is obtained. The total Si content of this phase is about 16.5%. The Si reaction yield is thus about 90%. The Si losses can be attributed to the entrainment of particles of Si with the escaping ZnCl ₂ vapours, and to the

incomplete reduction of SiCl ₄ into Si metal. Of the remaining Si, about 1.4 kg are found in the ZnCl ₂ arid about 0.4 kg in the scrubber.

The Zn-Si phase is separated in two fractions of about equal weight. Each fraction is heated to 1550 °C for 2 h in a silica crucible, in order to evaporate the Zn and melt the Si. A quartz tube is dipped into the molten bath corresponding to a first fraction, and SiCl ₄ is bubbled at a flow rate of 1000 g/h during 1 hour. The second alloy fraction is not sparged. Both fractions are allowed to cool down to room temperature, under inert atmosphere. As in the previous example, the concentration of most impurities is significantly reduced by the SiCl ₄ sparging step.