WO2010131199A1

WO2010131199A1 - Method and apparatus for purifying silicon

Info

Publication number: WO2010131199A1
Application number: PCT/IB2010/052078
Authority: WO
Inventors: Emanuele Centurioni; Daniele Iencinella
Original assignee: RAYSOLAR Srl
Current assignee: RAYSOLAR Srl
Priority date: 2009-05-12
Filing date: 2010-05-11
Publication date: 2010-11-18
Anticipated expiration: 2011-11-12
Also published as: IT1394029B1; CN102803139A; ITBO20090301A1

Abstract

The invention relates to a novel method for purifying silicon, in particular in order to obtain solar grade silicon, and an apparatus for realizing the method. The apparatus for purifying silicon comprises: a chamber, which is longitudinally extending and of dimensions such as to receive a vessel for containing silicon; means for heating a selected portion of said chamber; the heating means and the chamber being movable with respect to each other in a direction that is parallel to the longitudinal extension of the chamber (1) in order to realize a directional solidification of the silicon contained inside the vessel, when the vessel is placed inside the chamber. The method for purifying silicon, comprises the steps of: introducing a vessel containing silicon internally of a longitudinally-extending chamber; predisposing means for heating a selected portion of the chamber; performing a relative movement of the heating means and the chamber In a direction that is parallel to the longitudinal extension of the chamber in order to obtain a directional solidification of the silicon contained in the vessel.

Description

METHOD AND APPARATUS FOR PURIFYING SILICON

TECHNICAL FIELD

The Invention relates to the field of methods and apparatus used for purifying silicon and in particular for performing purification of 98-99% pure metallurgical-grade silicon, and obtaining a silicon having greater purity. The purified silicon can be applied in a photovoltaic or electronic field, and for the production of MEMS (Micro Electro Mechanical Systems),

BACKGROUND ART

Silicon destined for the photovoltaic Industry, which can be used to produce solar cells, typically has a purity of 99.999% - 99.9999% (5-6N). This silicon, known as solar grade, can contain only limited quantities of determined impurities, Although a standard classification does not exist for solar-grade silicon, the most recent literature reports that the silicon must have a total content of metal impurities of < 160 ppm, boron < 3 ppm and phosphorus <10 ppm.

Demand for silicon for production of photovoltaic cells was up to a short time ago satisfied by the remaining amount left over from convention production of electronic grade silicon, which is 99.9999999% pure (9N). This grade of purity is obtained only in large plants, by means of processes which require high amounts of energy in order to perform the dissolution of metallurgical silicon in hydrochloric acid, fractioπed distillation of the resulting volatile compounds, thermal decomposition and solidification by means of deposition in the steam phase (CVD)

When solar grade silicon underwent a great Increase In demand, the left-overs from electronic grade silicon were no longer sufficient. Attempts were made to construct new plants for production of solar-grade silicon which exploited the same technology as that used for the production of electronic-grade silicon. Although solar-grade silicon obtained in this way is of excellent quality, its production requires the use of large complex plants which have high energy consumption; for this reason the obtained product Is expensive.

There is therefore a need for obtaining solar-grade silicon by means of alternative processes to the ones cited herein above, with less complex plants and smaller energy demands.

It is Known that the process of direction solidification of molten silicon in a crucible enables obtaining a silicon ingot in which the part that solidifies first has a smaller content of metal impurities while the part that solidifies last contains the majority of the metallic impurities.

Publication WO2008/031229 describes a method for obtained low-purity solar- grade silicon in which the removal of the metal impurities is obtained by directional solidification. The silicon to b© purified is melted in a rotating kiln In which it is subjected to a purification from other impurities and is then poured into a recipient that is external of the kiln. The directional solidification is achieved by electromagnetically shaking the molten silicon. In a particular embodiment described in WO2008/031229, the molten silicon is poured into a superiorly-open recipient having thermally-insulated walls, According to WO2008/031229, the directional solidification has to be interrupted when the molten mass is partially solidified (40-80%), producing an ingot which has an external shell comprising solid poly-crystalline silicon, having a greater purity than the desired purity, and a central part which comprises silicon in the liquid state enriched with impurities with respect to the starting material. Apart from requiring electromagnetic shaking, this method implicates use of a further electromagnetic apparatus and a consequent further high energy consumption, and includes a following stage of breaking the solid shell to cause the liquid silicon to exit. Overall this method requires two mass transfers in the molten state at a temperature of beyond 141O⁰C These transfers necessarily lead to implications relates to personnel safety and/or automated plants.

In WO2008/031229, the removal of the phosphorus is don© by directional solidification, while the removal of the boron Is done by oxidation. Hydrogen/combustible torches are situated internally of the rotating kiln, which provide an oxidizing atmosphere. To complete the removal of the boron it is however necessary to add slagging agents to the molten silicon and consequently to remove the slag from the purified silicon.

USS972107 describes a method and an apparatus with which it is possible to perform direction solidification of silicon previously purified from boron and phosphorus internally of the same reaction chamber in which the silicon purification reactions take place. The chamber is immersed in a cooling liquid which in turn is located internally of a bath provided with an inlet valve and a plurality of outlet valves arranged vertically in order to cause the cooling liquid to flow into the bath. Hydrogen/oxygen immersion torches are located internally of the chamber, which are mobile vertically and shake the molten mass to be purified. The directional solidification is performed both acting on the vertical valves in order to control the coolant fluid level in the bath and vertically moving the torches, distancing them from the molten mass. Although this method enables a single apparatus to be used both for the directional solidification and for purification from other impurities (for example B and P), it exhibits considerable disadvantages. It is not, for example, energetically efficient inasmuch as particularly complex and energy-absorbing means are used both as a source of heat and for shaking, these means being the immersion torches, and furthermore, in order to perform the directional solidification it is necessary to cool the molten mass, It follows that in order to perform the direction solidification heat has to be removed from the reaction chamber by means of the coolant liquid and the chamber must be newly heated in order to perform the following purification operation. Additionally, an important consideration is that in order to perform the directional solidification, the reaction chamber has to be internally clad with a special ceramic cladding. Following formation of the ingot this cladding breaks and has to be restored before proceeding with a further purification. This leads to necessary machine down-time with a consequent negative effect on costs and production times.

In US5972107 the immersion torches not only generate th© heat necessary for keeping the silicon and slagging agents molten and creating turbulence In the reaction environment, but they can also provide oxygen, hydrogen, water, inert gas and the slagging agents too, Thus the boron and phosphorus can be oxidized by trapping them in the slag present in the reaction environment. Further, following the reaction with water steam and hydrogen, boron and phosphorus can be transformed into volatile compounds.

According to WO2008/031229 and USS972107, in order to obtain solar grade silicon it is essential to use slagging materials; for fusion of these materials a great deal of energy is required, and a large quantity of byproduct is consequently produced.

Other methods are also known for purifying silicon which, similarly to WO2008/031229 and US5972107. include the addition of slagging agents in the molten silicon and consequent removal of the slag, in particular to remove the boron

For example, in "Purification of metallurgical grade silicon up to solar grade", N. Yuge, M. Abe, K. Hanazawa, H. Baba, N. Nakamura, Y. Kato, Y. Sakaguchi, S. Hiwasa, F. Aratani, Prog. Photovoltaics: Res. Appl. 2001; 9:203-209, a two-stage silicon purification system is illustrated. In the first stage the metallurgical grade silicon is melted with an electronic beam gun, left to drip in the liquid state in a vacuum in order to facilitate evaporation of the phosphorus and then directionally solidified in a copper recipient In order to segregate the metal Impurities. The silicon Ingot thus obtained Is reduced to pieces and in the second stage, also described in US 5192091 , is newly melted by electromagnetic induction and treated with a plasma torch in the presence of silicon and slagging components to remove boron and carbon by means of oxidation. The plasma torch also functions as a means for shaking the molten mass. A second directional solidification In a graphite recipient completes the removal of the metal Impurities. This process, though leading to good results in terms of purity, is particularly complex due to the various types of machinery required for fusion and purification of the silicon and for the numerous steps required, among which transfer of materials in the molten state with a consequent high total energy cost.

US5788945 describes a method in which, starting from a silicon which contains 40 ppm of boron, It Is possible to obtain silicon containing 1 ppm of boron by continuous addition of slagging agents to the molten silicon and continuous removal of the unactivated slag from the reaction environment. In this method a double quantity of slag is used with respect to the quantity of silicon at the outset, and therefore a considerable quantity of heat has to be created to melt both the materials, and a double quantity of sub-products (unactivated slag) has to be managed with respect to the quantity of silicon produced having a low boron component,

The methods described in US5788945, WO2008/031229 and US182091 involve at least a transfer of silicon in the molten state, Also they are not suitable for effecting purification of the low-purity silicon either from metals or from boron and/or carbon, from phosphorus and/or their compounds with a single apparatus; in fact more apparatus is required, which leads, especially, to a higher energy consumption,

In spite of the effort expended in developing methods for purifying low-purity silicon or metallurgical silicon, a simple and economically efficient method is still required, which uses a single non-complex apparatus that enables removal of the metal impurities and other relevant impurities present in the low-purity silicon and cited herein above.

As th© directional solidification requires melting the whole mass of silicon to be purified and maintained in the molten state for a relatively long period, and since usually a single direction solidification is not sufficient to obtain metal impurity value of less than 150 ppm, a silicon purification method is required which has a high level of efficiency in removing the metal impurities. - Q -

SUMMARY OF INVENTION

The main aim of the present invention is to provide a method and an apparatus which enables silicon to be purified while avoiding expensive stages of heat subtraction from the purification chamber and/or the molten mass, renewal of the chamber cladding and in which transfers of molten material are not required, which lead to considerable implications relating to the safety of operators and/or automated plants.

A further aim of the present invention further consists in providing a silicon purification method in which an electromagnetic shaking device of the molten mass is not indispensable.

An additional aim of the present invention is to provide a simple and economically efficient method for purifying low-purity silicon or metallurgical silicon and obtaining solar grade silicon suitable for applications in photovoltaic or electronic fields.

The above-indicated aims and objectives are obtained with the silicon purification method and with the proposed purification apparatus which are free of the drawbacks of known methods and apparatus.

The apparatus of the invention, with which silicon can be purified, comprises:

a chamber having a longitudinal development of such dimensions as to receive a vessel for containing silicon;

means for heating a selected portion of the chamber;

the heating means and the chamber being movable with respect to each other according to a direction parallel to the longitudinal extension of the chamber in order to realize the directional solidification of the silicon contained inside the vessel, when the vessel is placed inside the chamber,

The apparatus enables a lower energy consumption with respect to the prior art, as no part thereof requires cooling and the energy is used almost exclusively for melting the mass of the silicon.

It is obvious that the relative movement between the heating means and the chamber mean that heat extraction from the chamber can be avoided, as the chamber cools following reciprocal distancing of the heating means. The heating means, then, can always be maintained at a constant temperature. This advantageously enables extracting the vessel from the relatively*coo⁾ chamber in which the silicon is solidified, inserting a second vessel containing the silicon to be purified and realizing a relative movement between the heating means and the chamber in a parallel direction to the longitudinal development of the chamber in order to melt the silicon contained in the second recipient. In this way it Is not necessary to cool, leave to cool or lower the temperature of the heating means which are immediately operative for a further melting operation with a consequently greater energy efficiency with respect to the prior art.

The method for purifying silicon of the invention, adtuable with the apparatus of the invention, comprises following stages:

introducing a vessel containing silicon inside a longitudinally extending chamber;

arranging means for heating a selected portion of the chamber;

performing a relative movement with respect to the heating means and the chamber in a direction which is parallel to the longitudinal extension of the chamber in order to obtain the directional solidification of the silicon contained in the vessel.

A directional solidification velocity of less than 1 cm/min can advantageously be used, preferably comprised between 0.01 and 0.5 cm/min.

According to the velocity of directional solidification used, the direction solidification can preferably be repeated several times, after removal of the part of the silicon ingot enriched with impurities. In the method of the invention the stage of melting of the silicon is performed by means of the heating moans.

The material to bø purified with the proposed method can be constituted by low-purity silicon, for example metallurgical silicon which has a minimum purity of 98% in weight/weight, working slag of the electronic grade silicon, working slag from other silicon-based metallurgical industries,

The stage of melting the silicon can be performed using apparatus of the invention in which the heating means have not been activated to provide heat to the selected portion, positioning the recipient in the selected portion of the chamber and then activating the heating means to supply heat.

In an advantageous alternative, the melting can be achieved by using the apparatus, in which the means have already been activated to supply heat, by realizing a relative movement between the heating means and the chamber in a parallel direction to the longitudinal direction of the chamber in order to melt the silicon contained in the vessel.

This can be realized in a special embodiment by inserting the recipient In a different position to the selected portion and realizing a relative movement between the heating means and the chamber such that the selected portion and the position of the vessel can be coincide by performing a relative movement between the heating means and the chamber such that the selected portion and position of the vessel diverge.

In the proposed method, the energy efficiency of the method is maximized as it is not necessary either to subtract heat from the heating means before inserting a new quantity of silicon to be melted and purified in the chamber.

The apparatus and the method of thθ invention further enable preventing the stage of restoring the cladding in the chamber as the solidification is performed in a recipient that is extractable from the chamber and no transfer stages are necessary for the molten mass as the silicon to be purified is introduced into the chamber in the solid state, molten and directionally solidified internally of the vessel.

The heating means are advantageously external of the chamber. With this arrangement the heating means are not subjected to the action of volatile oxides of impurities present in the silicon or impurities present In the silicon and having vapour tensions which are greater than that of silicon. This means a reduction in the cleaning operations and/or the restoring operations of the heating means, with a consequently smaller number of apparatus down-times, shorter working times and lower production costs,

In a preferred embodiment of the apparatus, the heating means partially surround the longitudinal external wall or walls of the chamber, and provide the further advantage of minimizing the energy required for maintaining the temperature in the chamber.

The preferred apparatus of the invention comprise a furnace which incorporates the heating means and slidingly engages with the chamber. The embodiment is advantageous as the furnace, which further comprises the heat insulating means, enables coupling the heat insulating means while enabling heat loss to be minimized.

In a particularly preferred embodiment of the apparatus the furnace is tubular. Therefore it can surround at least a longitudinal portion of the chamber; furthermore, if the furnace and the chamber have a circular transversal section the quantity of heat insulation is minimized, and the heating efficiency further optimized.

When the heating means are an electrical resistance, the apparatus consumes less energy, When the heating means are a gas burner an apparatus having a lower operating cost is obtained. Gas burners are advantageously of conventional type as it is not necessary to use combustible/oxygen burners. This means that methods in which the heating means predisposed are a gas burner or an electric resistance are preferred,

Also advantageous are the methods of the invention In which the silicon contained in the vessel is shaken during the directional solidification in which the shaking facilitates the migration process of the metal impurities.

Consequently particularly preferred are the proposed apparatus comprising means for moving an end of the chamber in order to obtain the mixing of the molten silicon contained in the vessel when the vessel is placed internally of the chamber, especially when the moving means are suitable for raising and lowering the end of the chamber, as the means are simple to realize. This is realizable for example by providing a raising organ, which can be a piston, on the base of the chamber.

Thus the methods of the invention comprising a further movement stage of an end of the chamber in order to obtain mixing of the molten silicon contained in the vessel are advantageous, in particular when the stage of moving consists in rhythmically raising and lowering the end of the chamber.

The method and apparatus of the invention in which the chamber is conformed such that its larger dimension Is horizontal and the smaller dimension is vertical are preferred. In this way, apart from minimizing the heat loss, a melting vessel can be used which has a horizontal maximum dimension, in which the contact surface of the molten silicon with the atmosphere within the chamber is particularly large. This advantageously enables performing further purification of the silicon from other elements, the effectiveness of which depends on the contact surface.

These further purifications consist in removing the impurities having greater vapour tension than that of the silicon, and impurities the oxides of which are volatile.

Therefore the methods of the invention which comprise the following and final stage are advantageous:

maintaining the melting of silicon by means of said heating means and feeding an oxidizing gaseous mixture inside said chamber, in order to remove impurities, whose oxides are volatile. The impurities can comprise boron end carbon.

The oxidizing gaseous mixture preferably combines hydrogen and oxygen or water vapour, Water vapour is preferred as it enables operating with greater safety than with oxygen.

The oxidizing gaseous mixture Is advantageously obtained by mixing argon and steam-saturated hydrogen at a ratio of 0:1- 10:1 , preferably 0:1- 1:1 , the saturation being performed at a temperature comprised between 0 and 6O⁰C.

It is preferable that the stage of supply lasts for a period of between 15 and 600 minutes, with a supply of between 0.01 and 10 !/min.

In a preferred embodiment this stage is performed by regulating the atmospheric pressure contained in the chamber at between 100-10⁵ to facilitate evaporation.

The stage preferably has a duration of less than 60 minutes, and the supply is comprised between 0.01 and 10 l/min, and still more preferably between 0.1-1 l/min.

Also advantageous are methods comprising the following stage:

maintaining the silicon in the molten state by means of said heating means and adjusting the atmospheric pressure contained inside said chamber to a value lower than 1000 Pa to remove the impurities having vapour tensions that ara greater than that of the silicon (12),

The pressure value is preferably comprised between 1 and 100 Pa and still more preferably is comprised between 1 and 10Pa.

The impurities can comprise P, Cu, Ca, Al, Mn, Mg and compounds thereof.

The pressure value is preferably regulated between 1 and 10 Pa, as in this way volatilization of the impurities having a vapour tension greater than silicon's is facilitated, and the silicon is prevented from being volatilized. The additional stage preferably has a duration comprised between 15 and 600 minutes. During this stage the pressure is advantageously comprised between 1 and 10 Pa for at least 15 minutes.

It is further evident that the method does not comprise either transfers of molten material or use of slag, which enables removing both the metal impurities and the impurities in which the oxides are volatile (in particular boron), and the impurities having vapour tensions higher than silicon's (in particular phosphorus) without the aid of slagging agents, internally of a single chamber, using a single low-energy consumption apparatus and without any need for complex means for administrating the reagents and shaking such as immersion or plasma torches, or restoring the cladding in the melting chamber. This is because the reactions involved in the boron and phosphorus purification advantageously take place along the whole contact surface between the silicon in the molten state and the atmosphere present internally of the chamber and are not only limited to a minimum portion of molten silicon- It follows that this method is more efficient and effective than known silicon purification methods as it enable a smaller energy consumption and a smaller formation of production slag.

The effectiveness is further improved when the mass is shaken in accordance with the embodiments already described as the continuous mixing continually changes the molten mass in contact with the reaction atmosphere.

It follows that the apparatus of the invention comprising means for supplying a gas or an oxidizing gaseous mixture communicating with the chamber and activatable to perform stages of removal of the impurities having volatile oxides are advantageous, as are apparatus which comprise depression means communicating with the chamber and activatable to perform stages of removal of impurities having vapour tensions that are greater than silicon's.

To prevent external contamination, primarily it is advisable to perform the following stage after the stage of Introduction of the recipient;

regulating the atmosphere of the chamber at a pressure that is lower than atmospheric pressure and supplying an inert gas or an inert gaseous mixture in order to obtain an inert atmosphere internally of the chamber.

During the stages of melting or maintaining the molten state the chamber advantageously is at a temperature of 1410 - 1600⁰C.

BRIEF DESCRIPTION Of THE DRAWINGS

The characteristics of the invention are described with reference to the accompanying tables of drawings, in which some preferred embodiments are illustrated, and in which;

figure 1 is a longitudinal section view of an embodiment of the apparatus of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1 the components are designated with the same numbers as in the following description of the various embodiments of the invention, and will have similar characteristics unless expressly indicated.

Figure 1 illustrates a design of an advantageous embodiment of the apparatus of the invention in which the chamber 1 has a horizontal development and a transversal section, not illustrated, which is circular. The furnace 17 advantageously has a tubular conformation, surrounds the chamber 1 and comprises the heating means 2. The heating means 2 face a portion of the external wall of the chamber 1 and are surrounded in the remaining part thereof by heat insulating means 3 with the aim of minimizing heat loss.

The tubular geometry of the furnace 17 and the chamber 1 and the heating means 2, insulated from the reaction atmosphere, internal of the chamber 1 , make possible a simple and effective apparatus for confining the molten silicon 12. The energy required for maintaining the temperature Is minimized thanks to the tubular geometry, which enables heat insulation over a solid angle, almost a complete one, Further, the h©at exchange surface between the silicon and the atmosphere is intrinsically large, given the tubular geometry of the furnace and the consequent elongate boat-shaped form of the melting vessel 11.

In the embodiment of figure 1 , the apparatus comprises a cylindrical tubular element made of refractory material and two closing elements 4, 5 of the ends of the tubular elements, one or which is an openable hatch. In this embodiment the tubular element and the closing elements 4, 5 internally define the chamber 1 ,

The walls of the tubular element are preferably made of a ceramic material able to work at temperatures of above 1SOO⁰C; the material is preferably alumina, mullite or zircon.

As shown in figure 1, the apparatus advantageously comprises a frame 20 bearing the chamber 1 and the furnace 17. This enables the raising and lowering means to act directly on the frame 20,

The raising and lowering means are preferably composed of at least a piston 32 positioned below a longitudinal end of the frame 20. To keep the chamber horizontal when the raising and lowering means are not activated, a support 13 of the same height as the piston 32 is provided below the opposite ends of the frame 20.

The shaking of the molten silicon 12 is obtained by cyclically varying the inclination of the chamber 1 with respect to the horizontal plane, repeatedly obtaining a flow of the molten silicon 12 from right to left and from left to right. This type of shaking is maximized by the boat shape of the vessel 11 , the longitudinal development of which is parallel to the development of the chamber.

In the embodiment of figure 1, the depression means 7 and the supply means 9 can advantageously comprise a depression pump and at least a butterfly valve, not illustrated, and the supply means 9 are preferably a gas flow system, not illustrated.

The chamber 1 is connected to the depression means 7 and supply mean^β 9 which enable operation under depression or at controlled pressures which are lower than or equal to atmospheric pressure, while at the same time causing the process gas or the gaseous mixtures to flow internally of the chamber 1.

Considering that the chamber 1 is connected to the depression means 7 and the supply means 9, the relative movement Is preferably achieved by moving the heating means 2 with respect to the chamber 1 rather than moving the chamber 1 with respect to the heating means 2.

In the embodiment of figure 1 , the furnace 17 is mobile with respect to the chamber 1. The furnace 17 rests on at least a rotating support element 16, and is mobile horizontally with respect to the chamber 1 along a rail 30 which is present on the frame 20, within a trajectory delimited by a first and a second support element 21, 21' of the chamber 1 comprised in the frame 20.

The vessel 11 is located internally of the chamber where the molten material 12 to be purified is contained. The vessel 11 too must be mad© of a material able to work at temperatures of above 1500⁰C, preferably alumina, pourable alumina, mullite, zircon, boron nitride, silicon nitride.

The length of the recipient 11 can be the same as the length of the chamber 1 , but is preferably the same as or less than the length of the furnace 17 as shown in figure 1.

In the example that follows the apparatus used in figure 1 is again used, in which the chamber 1 has a length of 150 cc, an internal diameter of 75 mm, the tubular furnace 17 has a length of 60 cm, and an internal diameter of 86 mm and a vessel 11 has been used which has an elongate form (boat- shaped) having a length of 20 cm and a width of 6 cm, and a depth of 3 cm.

In the apparatus the raising and lowering means enable the horizontal axis of thθ chamber 1 to b© inclined by 0.5 degrees, with a 1 -minute cycle.

EXAMPLE

100 g of metallurgical grade silicon was positioned on the vessel 11. the quantity of preselected such as to reach a filling level which is not greater than 90% of the internal volume of the vessel 11.

Then the vessel 11 was positioned in zone B of the chamber 1, not surrounded by the furnace 17. The openable hatch was closed and an inert atmosphere was obtained internally of the chamber 1 , by causing a flow of Ar at 0,7 l/min via the supply means.

A relative movement was realized between the heating means 2 and the chamber 1 in a parallel direction to the longitudinal direction of the chamber 1 in order to melt the silicon 12 contained in the vessel 11, displacing the furnace 17 from position A to position B, maintaining the flow of argon.

Melting temperature was 1500⁰C.

On obtaining melting, the raising and lowering means were activated to incline the horizontal axis of the operating tube by 0,5 degrees, at a rate of one cycle per minute.

REMOVAL OF THE IMPURITIES BY OXIDATION ANP VOLATILISATION (B.jC and thPlr compounds!.

The flow of Ar was stopped and a hydrogen and water vapour mixture was made to flow into the chamber 1. The mixture was obtained by saturating hydrogen with water vapour at a temperature of 15'C,

After 60 minutes the water vapour-hydrogen mixture flow was stopped and a flow of Ar at 0.7 l/min was resumed,

REMOVAL OF VOLATILE IMPURITIES (P. Cu. Ca. Al. Mn. Kg and their compounds! The Ar flow was stopped and the pressure inside the chamber was regulated to a pressure of 5 Pa.

After 60 minutes a flow of Ar at 07 l/min was resumed, up to reaching atmospheric pressure.

DIRECTIONAL SOLIDIFICATION (removal of: Fe, Al. Ca. Mn. Mq. Cu. Nl. V. Na, Zr. Cr. Mo. Co. Nb. Ba. K. W. Ta and P and their compounds)

A relative movement was realized between the heating means 2 and the chamber 1 in a parallel direction to the longitudinal development of the chamber 1 in order to obtain directional solidification of the silicon 12 contained in the recipient 11 ,

During this stage the velocity of solidification was controlled by acting on the velocity of displacement of the furnace 17 from position B to position A,

The velocity of solidification used is 0.25 cm/min along the longitudinal axis of the chamber 1, which coincides with the longitudinal axis of the recipient 11 used,

The part of the ingot which solidified last was removed by mechanical action and the remaining silicon, corresponding to 70% of the initial weight, was analyzed.

Table 1 reports the results of the chemical analysis of the silicon before and after purification according to the method of the present invention. From the reported data it can be observed that the purification method Is particularly efficient for removal of metal impurities, boron and phosphorus.

a e ; hem ca ana ys s o the silicon before and after purificat on.

Table 1 reports, as indices of the total contents of the metal impurities in the silicon, the data relating to the contents Fe, Al₁ Mn and Ti. The other reduction factors of the contents of the metals (833 for Fe, 167 for Al, 233 for Mn and 75 for Ti) sufficiently demonstrate that the method of the invention enables obtaining impurity contents of less than 150 ppm with a single directional solidification.

Claims

1. Apparatus for purifying silicon, characterized In that it comprises;

a chamber (1), which is longitudinally extended and of such dimensions as to receive a vessel (11) for containing silicon (12);

means (2) for heating a selected portion of said chamber;

the heating means (2) and the chamber (1) being movable with respect to each other in a direction that is parallel to the longitudinal extension of the chamber (1) in order to realize a directional solidification of the silicon (12) contained inside the vessel (11), when the vessel (11) is placed inside the chamber (1).

2. Apparatus according to claim 1, comprising a furnace (17) which incorporates the heating means and engages the chamber (1) slidingly.

3. Apparatus according to claim 2, wherein the furnace (17) is tubular.

4. Apparatus according to any one of claims 1 to 3, wherein the heating means (2) are a gas burner or an electrical resistance.

5. Apparatus according to any one of the preceding claims, comprising means for moving an end of the chamber (1), such that when the vessel (11) is placed inside the chamber (1), the molten silicon (12) contained in the vessel (11) undergoes mixing.

6. Apparatus according to claim 5, wherein said moving means are suitable for raising and lowering the end of the chamber (1),

7. Apparatus according to claim 6, comprising a frame (20) that supports the chamber (1) and the furnace (17), and wherein the raising and lowering means act directly on the frame (20).

8. Apparatus according to claim 6 or 7, wherein the raising and lowering means are at least a piston (32).

9. Apparatus according to any one of the preceding claims, comprising; a cylindrical tubular element made of refractory material and two closing means (4, 5) of the ends of the tubular element, one of the closing means being an openable hatch, the tubular element and the closing means (4, 5) defining an internal configuration of the chamber (1).

10. Apparatus according to any one of the preceding claims, comprising depression means (7) and supply means (9) for a gas or an oxidizing gaseous mixture, which means communicate with said chamber (1) and are activatable in order to perform stages for removing Impurities having vapour tensions that are greater than the vapour tension of the silicon (12) and impurities, oxides of which are volatile.

11. Method for purifying silicon, comprising steps of;

- introducing a vessel (11) containing silicon (12) internally of a longitudinally- extending chamber (1);

- predisposing means (2) for heating a selected portion of the chamber;

» performing a relative movement of the heating means (2) and the chamber (1) in a direction that is parallel to the longitudinal extension of the chamber (1) in order to obtain a directional solidification of the silicon (12) contained in the vessel (11).

12- Method according to the preceding claim, comprising a further stage of moving an end of the chamber (1) so that mixing of the molten silicon (12) contained in the vessel (11) is achieved.

13. Method according to the preceding claim, wherein said moving stage consists of lifting and lowering said end of the chamber (1) rhythmically.

14. Method according to the any one of the preceding claims, comprising a following and further stage of:

- performing a relative movement of the heating means (2) and the chamber (1) in a direction that is parallel to the longitudinal extension of the chamber (1) in order to melt the silicon (12) contained In the vessel (11).

15. Method according to any one of the preceding claims, comprising a following and further stage of:

- maintaining the silicon (12) in a molten state by means of said heating means (2) and supplying an oxidizing gaseous mixture internally of the chamber (1), in order to remove impurities, oxides of which are volatile.

16. Method according to the preceding claim, wherein said oxidizing gaseous mixture Is obtained by mixing argon and saturated hydrogen from water vapour, with a ratio of 0 : 1 - 10 ; 1 , the saturation being performed at a temperature of between 0 and 60".

17. Method according to any one of the preceding claims, comprising the following and further stage of:

- maintaining the silicon (12) in a molten state by means of said heating means (2) and adjusting the atmospheric pressure contained internally of the chamber (1) to a value of lower than 1000 Pa in order to remove Impurities having vapour tensions that are larger than a vapour tension of the silicon (12).

18. Method according to the preceding claim, wherein the pressure Is maintained at a value of between 1 and 10 Pa for at least 15 minutes.