WO2008145236A1

WO2008145236A1 - Economical process for the production of si by reduction of sicl4 with liquid zn

Info

Publication number: WO2008145236A1
Application number: PCT/EP2008/003276
Authority: WO
Inventors: Eric Robert; Tjakko Zijlema
Original assignee: Umicore
Priority date: 2007-05-25
Filing date: 2008-04-24
Publication date: 2008-12-04
Also published as: TW200909349A

Abstract

The invention relates to an economical process for manufacturing high purity silicon as a base material for the production of e.g. crystalline silicon solar cells. SiCl4 is converted to Si metal by contacting gaseous SiCl4 with liquid Zn, in a process comprising the steps of: - providing an initial quantity of molten Zn bath in a reactor; - blowing gaseous SiCl4 into said molten bath 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 at a temperature above the boiling point of Zn, thereby vaporising Zn and obtaining Si metal; wherein the contacting and the separation steps are performed in a single reactor, and characterised in that the contacting step is performed by injecting SiCl4 at a molar flow rate between 0.1 and 0.8 mol%/mol.min, and preferably between 0.4 and 0.8 mol%/mol.min of the initial Zn quantity, and with a maximum areal supply rate of 50 kg/min per m of bath surface. This process does not require complicated technologies and preserves the high purity of the SiCl4 towards the end product, as the only reactant is Zn, which can be obtained in very high purity grades and continuously recycled.

Description

Economical process for the production of Si by reduction of SiCU with liquid Zn

The invention relates to the manufacture of solar grade silicon (Si) as a feedstock material for the manufacture of crystalline silicon solar cells. The Si metal is obtained by direct reduction of SiCl₄, 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 SiHCl₃ 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 SiCl₄ 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 SiCl₄ 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 SiCl₄ 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 ZnCl₂ by-product. There is however no explanation why the entrainment of fine powder Si with ZnCl₂ 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.

Generally speaking, the production of the silicon as a fine powder is not particularly desirable. Indeed, such a powder can very easily oxidize, particularly at the surface of the grain. This oxidation makes it very difficult to subsequently re-melt the silicon to produce poly- or monocrystalline ingots. A pelletization/compactation step would probably be needed, which complicates the process.

In WO2006/100114 Al a process of direct reduction of SiCl₄ with liquid Zn was disclosed, wherein the contacting step of SiCl₄ with Zn, and the separation of the obtained Si and ZnCl₂ were performed in the same reactor. However, the process proved to be uneconomical, since the flow rate, which is equivalent to the molar supply rate of the SiCl₄ was much too low. Moreover, it was stated that by increasing the flow rate above the figures dislosed in this document (e.g. 0.023 mol%/mol.min in Example 1) the loss of Si by entrainment would become unacceptable.

In WO2007/013644 Al chlorosilane is fed into a metal melt, preferably aluminum, at a flow rate of less than 1.0 mol%/mol.min of the metal. The process is not further explored for the reaction between SiCl₄ and liquid Zn, and hence no optimisation is proposed. In the Examples according to the invention, the yield of the reaction with Al, calculated by dividing the Si (metallic) recovered by the Si input (as SiC14), is 17.1% or lower. This means that (at least) 82.9% of the Si fed into the reactor is lost by entrainment, or has not reacted. It is clear that a system using Al as reductant is highly uneconomical. Furthermore, aluminum is certainly not desirable as an impurity in HP silicon, which fact puts the whole process at risk.

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 in an economical way by a process for converting SiCl₄ into Si metal, comprising the steps - providing an initial quantity of molten Zn bath in a reactor;

- blowing gaseous SiCl₄ into said molten bath 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 at a temperature above the boiling point of Zn, thereby vaporising Zn and obtaining Si metal; wherein the contacting and the separation steps are performed in a single reactor, and characterised in that the contacting step is performed by injecting SiCl₄ at a molar flow rate between 0.1 and 0.8 mol%/mol.min, and preferably between 0.4 and 0.8 mol%/mol.min of the initial Zn quantity, and with a maximum areal supply rate of 50 kg/min per m² of bath surface.

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.

The Si-bearing metal phase as obtained in the contacting step will contain, besides Si as solute, also at least some Si in the solid state. Indeed, when the Zn metal gets saturated in Si, solid Si is also formed as suspended particles, but also a Si-Zn dross is obtained, floating on top of the remaining Zn bath. By increasing the molar supply rate Of SiCl₄ above 0.1 mol%/mol.min of the initial Zn content the Si-Zn dross layer is formed quickly, and the losses of particulate Si by entrainment of evaporating Zn- chloride are adequately limited. For economical reasons, the molar supply rate is preferably more than or equal to 0.4 mol%/mol.min. The upper limit of 0.8 mol%/mol.min guarantees that the dross layer is not disturbed to the extent that solid Si is entrained too much by the rising gasses. The presence of a Si-Zn dross on top of the bath helps to reach higher flows, as it also prevents the liquid bath from splashing too high and reduces the entrainment of small droplets with the evolving gases.

The maximum areal supply rate of the Zn bath should be limited to 50 kg/min per m² of bath surface, enabling to blow large quantities Of SiCl₄ without causing excessive splashing. By performing the process within these limits an optimum in process economy can be obtained, whereby the loss of Si by entrainment with evaporating Zn- chloride is limited to less than 15% (weight). An areal supply rate over 10, and preferably 12 or more kg/min per m² of bath surface is advised to perform the process in an economical way. Indeed, a high conversion yield of SiCl₄ into Si can be attained even at high injection speed. In that case, the productivity of the installation is directly linked to the injection flow rate. Thus for a given production of Si, the higher the flow rate, the lower the investment is. Also, it is clear that a bath surface of at least 300 cm², aanndd pprreeffeerraabbllyy aatt llee∑ast 500 cm is advisable in combination with the above mentioned working conditions.

Preferably the gaseous SiCl₄ 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 SiCl₄ can be injected along with a carrier gas such as N₂.

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.

It is useful to increase the temperature during the purification step to a value above the melting point of Si, and, in particular, operate the process at reduced pressure or under vacuum. 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 SiCl₄ reduction step, and chlorine, which can be recycled to a Si chlorination process for the production of SiCl₄; - Zn that is vaporised in the purification step can be condensed and recycled to the SiCl₄ converting process; and/or

- the fraction of SiCl₄ that exits the contacting step un-reacted can be recycled to the SiCl₄ converting process, e.g. after condensation.

According to this invention, SiCl₄ 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 is preferably formed as a vapour. Zn can be retrieved from its chloride, e.g. by molten salt electrolysis, and reused for SiCl₄ reduction. The Si-bearing alloy can be purified at high temperatures, above the boiling 5 points of both Zn and Zn-chloride, but below the boiling point of Si itself (2355 °C). The evaporated Zn can be retrieved and reused for SiCl₄ 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.

10 In a preferred embodiment according to the invention, gaseous SiCl₄ is contacted with liquid Zn at atmospheric pressure, at a temperature above the boiling point of ZnCl₂ (732 °C) and below the boiling point of Zn (907 °C). The preferred operating temperature is 750 to 880 °C, a range ensuring sufficiently high reaction kinetics, while the evaporation of metallic Zn remains limited.

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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 SiCl₄, 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 SiCl₄, which is heated in

20 the tube, is actually injected as a gas. Alternatively, the SiCl₄ can be vaporized in a separate device and the vapours fed to the injection tube. 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 SiCl₄ and the Zn to get a high reduction yield. If this is not the case, partial reduction to SiCl₂ could occur, or SiCl₄

25 could leave the zinc un-reacted. With an adequate SiCl₄ - Zn contact, close to 100% conversion is observed.

The reduction process produces ZnCl₂. It has a boiling point of 732 °C, and is gaseous at the preferred operating temperature. It leaves the Zn-containing vessel via the top. 30 The vapours are condensed and collected in a separate crucible. The process also produces 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 another 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 the 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 SiCl₄ 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.

When all the residual zinc has been evaporated and the silicon is in the molten state, 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. Si with a purity of 5N to 6N is then obtained. A special high temperature sparging or bubbling step with Cl₂ 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 °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. Hence a preferred embodiment consists of purifying the Si-bearing metal phase by heating to a temperature above the melting point of Si and injecting Cl₂ and/or a gaseous Si chloride compound into said metal phase, thereby vaporising Zn, eliminating impurities and obtaining Si metal, whereby the contacting and the separation steps are performed in a single reactor.

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

700 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 50 cm. A peristaltic pump is used to transport SiCl₄ (bp 58°C) into an evaporator (jacketed heated vessel). The gaseous SiCl₄ is then bubbled through the zinc bath via a quartz tube. The SiCl₄ flow is ca. 150 kg/hour, and the total amount added is 450 kg. The flow rate corresponds to 12.7 kg/min per m² of bath surface. Expressed differently, the quantity Of SiCl₄ supplied is 0.137 mol%/mol.min of the initial number of moles of Zn. It was observed that, once the Zn bath was saturated with Si, a Si-Zn dross layer was rapidly formed on top of the bath. The ZnCl₂, which is formed during the reaction, evaporates and is condensed in a silicon carbide tube connected to the reactor and is collected in a separate vessel. Any un-reacted SiCl₄ is collected in a wet scrubber connected to the ZnCl₂ vessel. A Zn-Si phase is obtained, with a total Si content of about 16.7 %. This phase is heated up progressively, to evaporate the Zn, which is condensed in a separate vessel. The temperature is increased up to 145O⁰C and maintained at this level for 1 hour. The molten silicon is then cast into a quartz vessel and allowed to solidify to room temperature. 66 kg of metallic silicon are collected. The Si reaction yield is thus about 88 %. The Si losses can be attributed to the entrainment of particles of Si with the escaping ZnCl₂ vapours, and to the incomplete reduction Of SiCl₄ into Si metal. Of the remaining Si, about 6 kg are found in the ZnCl₂ and about 3 kg in the scrubber. The obtained silicon contains less than 5 ppb of B, 0.05 ppm of P, 0.2 ppm of Al and 0.3 ppm of total metallic impurities. Zn is below 50 ppb.

Claims

1. Process for converting SiCl₄ into Si metal, comprising the steps of:

- providing an initial quantity of molten Zn bath in a reactor; - blowing gaseous SiCl₄ into said molten bath 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 at a temperature above the boiling point of Zn, thereby vaporising Zn and obtaining Si metal; wherein the contacting and the separation steps are performed in a single reactor, and characterised in that the contacting step is performed by injecting SiCl₄ at a molar flow rate between 0.1 and 0.8 mol%/mol.min, and preferably between 0.4 and 0.8 mol%/mol.min of the initial Zn quantity, whereby a Si-Zn dross layer is formed on top of the Zn bath, and with a maximum areal supply rate of 50 kg/min per m² of bath surface.

2. Process according to claim 1, characterized in that the areal supply rate is at least 10, and preferably 12 or more kg/min per m² of bath surface.

3. Process according to claims 1 or 2, wherein the contacting and the separating steps are performed simultaneously, by operating them at a temperature above the boiling point of Zn-chloride, which evaporates.

4. Process according to any one of claims 1 to 3, whereby during the purification step, the temperature is raised above the melting point of Si, thereby forming purified liquid Si.

5. Process according to claim 4, whereby the purification step is performed at reduced pressure or under vacuum.

6. Process according to any one of claims 1 to 5, further comprising the steps of:

- subjecting the separated Zn-chloride to molten salt electrolysis, thereby recovering Zn and chlorine;

- recycling the Zn to the SiCl₄ reduction step; and - recycling the chlorine to a Si chlorination process for the production of SiCl₄.

7. Process according to any one of claims 1 to 6, wherein the Zn that is vaporised in the purification step, is condensed and recycled to the SiCl₄ converting process.

8. Process according to any one of claims 1 to 7, wherein the fraction of SiCl₄ that exits the contacting step un-reacted, is recycled to the SiCl₄ converting process.

9. Process according to claims 4 or 5, comprising a single solidification step of the purified liquid Si, using a method chosen from the group of crystal pulling, directional solidification and ribbon growth.

10. Process according to claims 4 or 5, comprising the granulation of the purified liquid Si.

11. Process according to claim 10, comprising the steps of:

- feeding the granules to a melting furnace; and

- applying a single solidification step, using a method chosen from the group of crystal pulling, directional solidification, and ribbon growth.

12. Process according to claims 9 or 11, whereby the solid material is wafered and further processed to solar cells.