METHOD AND APPARATUS FOR PRODUCING SILICON

- EPRO DEVELOPMENT LIMITED

An apparatus for producing pure silicon from an electrolyte including a first crucible for receiving the electrolyte, a heat source for heating the electrolyte in the first crucible to form a molten electrolyte, an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte wherein electrolysis is able to be applied to the molten electrolyte when a potential difference is provided between the anode and the cathode. A stirring device is adapted for stirring the molten electrolyte when electrolysis is being applied whereby pure silicon is produced which is soluble with the anode to form an alloy.

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Description
TECHNICAL FIELD

The present invention relates to a method and apparatus for producing silicon, and in particular, silicon of suitable purity for use in solar-cell applications and the like.

BACKGROUND OF THE INVENTION

Solar-cell technologies are perceived to be an environmentally-friendly alternative to traditional forms of energy production such as those utilising fossil fuels. Accordingly, solar-cell technologies represents a significant commercial market.

Currently, technology developed by Siemens (the “Siemens Process”) is widely considered to be the leading process for the production of silicon which is of suitable purity for use in solar-cell applications. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150° C. such that the trichlorosilane gas decomposes and deposits additional silicon onto the rods to enlarge them. The Siemens process is however considered to be both relatively expensive and not environmentally-friendly. In energy terms, it is estimated that the Siemens process expends approximately 200 MW·Hr of electricity in order to produce 1 ton of solar-grade silicon.

Carbothermic reduction processes have also been developed as an alternative to the Siemens process. However, these processes do not produce silicon which is of solar-grade quality since impurities such as boron and phosphorous, which are inherently contained in carbon, cannot be removed to suitably low levels (i.e. to levels in the parts-per-million or parts-per-billion).

Accordingly, there is a perceived need for an alternative solution in seeking to address the above-described problems associated with production of solar-grade silicon.

SUMMARY OF THE INVENTION

The present invention seeks to alleviate at least one of the above-described problems.

The present invention may involve several broad forms. Embodiments of the present invention may include one or any combination of the different broad forms herein described.

In a first broad form, the present invention provides a method of producing pure silicon from an electrolyte wherein the method includes the steps of:

(i) heating the electrolyte in a first crucible to form a molten electrolyte; and

(iii) applying electrolysis to the molten electrolyte by providing a potential difference between an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte;

wherein the molten electrolyte is stirred as electrolysis is being applied whereby pure silicon is produced which is soluble with the anode to form an alloy.

Advantageously, the stirring of the molten electrolyte during electrolysis may assist in both generating ion-flow within the molten electrolyte, and, increasing the contact between the electrolyte and the molten alloy anode. The pure silicon may thereafter be readily segregated from the alloy using a segregation technique as discussed further below.

Preferably, the electrolyte may include cryolite, calcium oxide particles and quartz particles. More preferably, the electrolyte may comprise approximately between 82-94% cryolite particles by weight of the electrolyte. Also typically, the electrolyte may comprise approximately between 3-15% calcium oxide particles by weight of the electrolyte. Also preferably, the electrolyte may comprise approximately 3% quartz particles by weight of the electrolyte. More preferably, the electrolyte may comprise approximately 87% cryolite particles, 10% calcium oxide particles and 3% quartz particles by weight of the electrolyte. Also preferably, the present invention may include a step of controllably adding quartz particles to the molten electrolyte during electrolysis to substantially maintain approximately 3% quartz particles in the electrolyte by weight of the electrolyte.

Typically the first crucible may include at least one of a carbon, silicon nitrate and silicon carbide material. Preferably, the first crucible material may be adapted to generate heat if placed within an induction furnace, or, to conduct heat in a relatively efficient manner if placed in a direct heating furnace. Preferably, the first crucible may include a recess defined by an inner peripheral wall and a base for receiving the electrolyte. Typically, the recess may include a cylindrical shape.

Preferably, a first crucible lining may be arranged inside the first crucible recess between the first crucible and the electrolyte. More preferably, the first crucible lining may be contoured to substantially complement the inner peripheral wall of the first crucible. Preferably, the first crucible lining may include a quartz material. Alternatively, the first crucible lining may include at least one of a calcium oxide, magnesium fluoride, sodium fluoride and silicon material. Preferably, the material used as the first crucible lining may be temperature resistant above at least approximately 1000° C. Preferably, the material used as the first crucible lining may also be corrosion-resistant when exposed to the electrolyte during the electrolysis process. Advantageously, the first crucible lining may assist in preventing electrolyte from being absorbed into the first crucible wall which tends to decrease efficiency in the electrolysis process into the alloy anode. Additionally, because the first crucible lining assists in blocking absorption of the electrolyte into the first crucible wall, this assists in preventing a short circuit where the efficiency of the electrolysis of silicon into the alloy anode will be affected tremendously.

Alternatively, the first crucible lining may be formed from the molten electrolyte itself by generating a temperature gradient within the molten electrolyte wherein a portion of the molten electrolyte may be solidified adjacent the inner peripheral wall of the first crucible. Typically, electric arc heating may be used to create the temperature gradient between the cathode and molten electrolyte adjacent the inner peripheral wall of the first crucible.

Typically, the electrolyte may be heated using at least one of an arc furnace and an induction furnace.

Typically, the molten electrolyte may be stirred using at least one of an induction furnace to magnetically stir the molten electrolyte, and, a mechanical stirrer.

Typically, the anode may include at least one of a copper, gold, silver, zinc and a magnesium material. Preferably, the anode includes a copper material. More preferably, the anode may comprise an alloy of copper and pure silicon. Typically, the pure silicon may comprise approximately 3% by weight of the alloy.

Preferably, prior to step (i) of the first broad form, the alloy may be positioned in the first crucible adjacent the base of the first crucible and melted in the first crucible whereby said electrolyte may be thereafter deposited into the first crucible on top of the melted alloy in readiness for step (i) of the first broad form to be performed. Typically, the alloy may be melted at a temperature of between approximately 950-980° C.

Typically, step (i) of the first broad form may include heating the electrolyte in the first crucible to a temperature of at least approximately 900° C. in order to form the molten electrolyte.

Preferably, during step (ii) of the first broad form, the molten electrolyte may be maintained at a temperature whereby the molten electrolyte does not solidify. Typically, the temperature may be maintained between approximately 900-1000° C. to alleviate solidification of the molten electrolyte. Preferably, the temperature may be maintained at approximately 980° C. to prevent solidification of the molten electrolyte.

Typically, the molten electrolyte may be exposed to a current density of between approximately 0.1 A/cm2 to 2.0 A/cm2 in the first crucible when electrolysis is applied.

Typically, the cathode may include at least one of a carbon, copper and a platinum material. Preferably, the cathode may be formed by pressing purified carbon powder into a solid rod. More preferably, the carbon powder may be purified by exposing the carbon powder to an acid wash and chlorine flushing to remove impurities.

Preferably, the cathode may be positioned in the first crucible after the electrolyte has been heated to form the molten electrolyte wherein the cathode is in electrical/ionic communication with the molten electrolyte.

Preferably, the present invention may include a step of segregating the pure silicon from the alloy after step (ii). Typically, the step of segregating the pure silicon from the alloy may be performed when pure silicon produced as a result of electrolysis may no longer be soluble with the alloy. Also typically, if the temperature of the alloy is between approximately 950-980° C., the pure silicon may no longer be soluble with the alloy when the pure silicon in the alloy is approximately 25% per weight of the alloy.

Typically, the step of segregating the pure silicon from the alloy may include adjusting the temperature of the alloy to between approximately 800-850° C. whereby pure silicon is able to naturally segregate from the alloy. Also preferably, before adjusting the temperature of the alloy to between approximately 800-850° C., the alloy may be transferred into a second crucible. Preferably, the second crucible may include a material which is inert with respect to the alloy and/or molten salts. Typically, molten salts may be used to cover the segregated pure silicon to alleviate re-oxidisation of the pure silicon. Typically, the alloy in the second crucible may be reintroduced back into the first crucible when a predetermined percentage of pure silicon particles by weight of the alloy has segregated from the alloy. More typically, the predetermined percentage of pure silicon particles by weight of the alloy may include approximately 11% of pure silicon particles by weight of the alloy when the alloy in the second crucible is reintroduced into the first crucible.

Alternatively and/or additionally, instead of reintroducing the alloy in the second crucible back into the first crucible when the predetermined percentage of pure silicon particles has segregated from the alloy in the second crucible, the following steps may be applied:

(i) forming the alloy in the second crucible into at least one of a tape and a powder; and
(ii) applying a secondary electrolysis to the tape or powder;

whereby pure silicon particles may be segregated from the alloy in the form of nano-sized pure silicon particles.

Typically, the tape may be formed by at least one of casting and extruding the alloy. Also typically, the powder may be formed by mechanically grinding or milling the alloy. Preferably, the powder may be formed by grinding the alloy in a controlled environment. Typically, the powder may include micron to nano sized alloy particles.

Preferably, the step of applying the secondary electrolysis may include submerging the tape or powder in an electrolyte solution. Typically the electrolyte solution may include at least one of hydrochloric acid (HCl), dehydrated acetic acid, iodine solutions (i.e. potassium iodine solution), hydrazoic acid (HN3), acetone, concentrated alcohol and a combination thereof. Preferably, during the secondary electrolysis, the acidity of the electrolyte solution may be replenished.

Typically, a current of less than 1 A may be applied during the secondary electrolysis.

Preferably, after the secondary electrolysis has been performed, the electrolyte solution containing the nano-sized pure silicon particles may be further processed to separate the nano-sized pure silicon particles from the electrolyte solution. Typically, the nano-sized pure silicon particles may be centrifugally separated from the electrolyte solution. Also preferably, the separated nano-sized pure silicon particles may thereafter be stored in a substance suitable for alleviating reaction with free oxygen. Typically the substance may include concentrated alcohol or dehydrated alcohol

Yet alternatively, it would be understood by a person skilled in the art that any one of the above steps of segregating pure silicon from the alloy in the second crucible may be performed directly upon the alloy in the first crucible without being removed into a second crucible.

Yet alternatively, after step (ii) of the first broad form of the present invention, the step of segregating pure silicon from the alloy produced in the first crucible may include performing a secondary electrolysis upon the alloy whereby the polarities of the anode and cathode are reversed. Preferably, by reversing the polarities, the anode may provide a positive node and the cathode may provide a negative node during the secondary electrolysis. Typically, the secondary electrolysis may result in a composite containing silicon and electrolyte being deposited on the anode. Typically the composite formed on the anode may include silicon of around 300 mesh particle size.

Preferably, the secondary electrolysis may be performed upon the alloy in a separate crucible from the first crucible which may typically include a SiC material.

Typically, the secondary electrolysis may be performed using at least one of the following electrolyte compositions (by approximate percentage weight):

(i) 10% K2SiF6, 25% AlF3, 25% NaF, 35% BaF2, 5% CaF2; (ii) 40-70% Na3AlFe, 5-20% K2SiF6, 5-15% CaF2, 5-10% CaO; and

(iii) 95-99% Na3AlF6, 1-5% SiO2.

Preferably, the composite that is deposited on the anode may be melted at a temperature of at least approximately 1450° C. or above whereby silicon conglomerates into pellets or ingots within the electrolyte upon cooling. Typically the composite may be melted by use of an induction furnace or the like.

Typically, upon cooling, the conglomerated silicon pellets or ingots may be filtered from the electrolyte by use of a filter such as a mesh or the like. Alternatively, the composite may be melted back into the alloy and cooled at a controlled rate so that silicon floats to the top for collection. Typically, the composite may be melted back into the alloy at a temperature in accordance with the prescribed melting temperature depicted in the phase diagram of FIG. 19.

In a second broad form, the present invention provides an apparatus for producing pure silicon from an electrolyte including:

a first crucible for receiving the electrolyte;

a heat source for heating the electrolyte in the first crucible to form a molten electrolyte;

an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte wherein electrolysis is able to be applied to the molten electrolyte when a potential difference is provide between the anode and the cathode; and

a stirring device for stirring the molten electrolyte when electrolysis is being applied whereby pure silicon is produced which is soluble with the anode to form an alloy.

Preferably the electrolyte may include cryolite, calcium oxide particles and quartz particles. More preferably, the electrolyte may comprise approximately between 82-94% cryolite particles by weight of the electrolyte. Also typically, the electrolyte may comprise approximately between 3-15% calcium oxide particles by weight of the electrolyte. Also preferably, the electrolyte may comprises approximately 3% quartz particles by weight of the electrolyte. More preferably, the electrolyte may comprise approximately 87% cryolite particles, 10% calcium oxide particles and 3% quartz particles by weight of the electrolyte.

Preferably, the present invention may include a dispenser for dispensing quartz particles in to the electrolyte during electrolysis to substantially maintain approximately 3% quartz particles in the electrolyte by weight of the electrolyte.

Typically, the first crucible may include at least one of a carbon, silicon nitrate and silicon carbide material. Preferably, the first crucible may include a recess defined by an inner peripheral wall and a base for receiving the electrolyte. Typically, the recess may include a cylindrical or rectangular shape.

Preferably, the present invention may include a first crucible lining arranged inside of the first crucible recess between the first crucible and the electrolyte. Also preferably, the first crucible lining may be contoured to substantially complement the inner peripheral wall of the first crucible. Typically, the first crucible lining may include at least one of a quartz, calcium oxide, magnesium fluoride, sodium fluoride and silicon material.

Alternatively, the first crucible may include a first crucible lining formed from solidification of a portion of the molten electrolyte as a result of a temperature gradient being generated within the molten electrolyte. Typically, the present invention may include an electric arc heating device for generating the temperature gradient between the cathode and molten electrolyte adjacent the inner peripheral wall of the first crucible.

Typically, the heat source may include at least one of an arc furnace and an induction furnace. Preferably, the arc furnace may be a primary source of heat.

Typically, the molten electrolyte may be stirred using at least one of an induction furnace to magnetically stir the molten electrolyte, and, a mechanical stirrer. Advantageously, an induction furnace may alleviate production costs by providing a dual function as a heat source and as a stirring device.

Typically, the anode may include at least one of a copper, silver, gold, zinc and a magnesium material. Preferably, the anode may include a copper material. More preferably, the anode may include an alloy of copper and pure silicon. Typically the pure silicon may comprise approximately 3-7% by weight of the alloy.

Preferably, the alloy may be melted in the first crucible adjacent the base of the first crucible and the electrolyte may be positioned on top of the alloy. Typically, the heat source may be adapted to melt the alloy at a temperature of between approximately 950-980° C.

Typically, the heat source may be adapted to heat the electrolyte in the first crucible to a temperature of at least approximately 900° C. in order to form the molten electrolyte.

Preferably, during electrolysis of the molten electrolyte, the heat source may be adapted to maintain the molten electrolyte at a temperature whereby the molten electrolyte does not solidify. Typically, the heat source may be adapted to maintain the temperature of the molten electrolyte at between approximately 900-1000° C. to prevent solidification of the molten electrolyte. Preferably, the heat source may be adapted to maintain the temperature of the molten electrolyte at approximately 980° C. to prevent solidification of the molten electrolyte.

Typically, the molten electrolyte may be exposed to a current density of between approximately 0.1 A/cm2 to 2.0 A/cm2 in the first crucible when electrolysis is applied.

Typically, the cathode may include at least one of a carbon, copper and a platinum material. Preferably, the cathode may include a solid rod formed from pressed purified carbon powder. More preferably, the carbon powder may be purified by exposing the carbon powder to an acid wash and chlorine flushing to remove impurities. Also preferably, the cathode may be adapted for positioning in to the first crucible after the electrolyte has been heated to form the molten electrolyte whereby the cathode is in electrical/ionic communication with the molten electrolyte.

Preferably the present invention may include an apparatus for segregating the pure silicon from the alloy. Preferably, the apparatus for segregating the pure silicon from the alloy may be adapted to segregate the pure silicon from the alloy when the pure silicon produced as a result of electrolysis is no longer soluble with the alloy. Also typically, if the alloy is at a temperature between approximately 950-980° C., the pure silicon may no longer be soluble with the alloy when the pure silicon in the alloy is approximately 25% per weight of the alloy.

Typically, the apparatus for segregating the pure silicon from the alloy may be adapted to adjust the temperature of the alloy to between approximately 800-850° C. whereby pure silicon may be able to segregate from the alloy. Also preferably, the apparatus for segregating the pure silicon from the alloy may be adapted to transfer the alloy into a second crucible before adjusting the temperature of the alloy to between approximately 800-850° C. Also preferably, the second crucible may include a material which may be inert with respect to the alloy and/or molten salts. Preferably, molten salts may be used to cover segregated pure silicon to alleviate re-oxidisation of the pure silicon. Typically, the present invention may be adapted for reintroducing the alloy in the second crucible back into the first crucible when a predetermined percentage of pure silicon particles by weight of the alloy has segregated from the alloy. More typically, the predetermined percentage of pure silicon particles by weight of the alloy may include approximately 11% of pure silicon particles by weight of the alloy.

Alternatively and/or additionally the apparatus for segregating the pure silicon from the alloy in the second crucible may include:

(i) an apparatus for forming the alloy in the second crucible into at least one of a tape and a powder; and
(ii) an apparatus for applying a secondary electrolysis to the tape or powder;

whereby pure silicon particles may be segregated from the alloy in the form of nano-sized pure silicon particles.

Typically, the apparatus for forming the alloy into a tape may be adapted to cast or extrude the alloy into the tape. Also typically, the apparatus for forming the powder may be adapted for mechanically grinding or milling the alloy into the powder in a controlled environment. Preferably, the powder may be formed by grinding the alloy. Typically, the powder may include micron to nano-sized alloy particles.

Preferably, the apparatus for applying the secondary electrolysis may be adapted for submerging the tape or powder in an electrolyte solution. Typically the electrolyte solution may include at least one of hydrochloric acid (HCl), dehydrated acetic acid, iodine solutions (i.e. potassium iodine solution), hydrazoic acid (HN3), acetone, concentrated alcohol and a combination thereof. Preferably, during the secondary electrolysis, the acidity of the electrolyte solution may be replenished.

Typically, the apparatus for applying the secondary electrolysis may be adapted for applying a current of less than 1 A during the secondary electrolysis.

Preferably the present invention may include an apparatus for separating nano-sized pure silicon particles in the electrolyte solution from the electrolyte solution after the secondary electrolysis has been applied. Typically this may include a centrifuge device for centrifugally separating the nano-sized pure silicon particles from the electrolyte solution. Also preferably, the present invention may include a storage apparatus for storing the separated nano-sized pure silicon particles in a substance suitable for alleviating reaction with free oxygen. Typically the substance may include concentrated alcohol.

Yet alternatively, it would be understood by a person skilled in the art that the apparatus for segregating the pure silicon from the alloy in the second crucible may be adapted to similarly separate pure silicon directly from the alloy in the first crucible without being removed into a second crucible.

Yet alternatively, the apparatus for segregating the pure silicon from the alloy produced in the first crucible may be adapted for performing a secondary electrolysis upon the alloy whereby the polarities of the anode and cathode are reversed during the secondary electrolysis. Preferably, by reversing the polarities, the anode may provide a positive node and the cathode may provide a negative node. Typically, the secondary electrolysis may result in a solid composite containing silicon and electrolyte being deposited on the anode. Typically the silicon in the composite formed on the anode may be of around 300 mesh particle size.

Preferably, the apparatus for segregating the pure silicon from the alloy may include a separate crucible from that of the first crucible in which the secondary electrolysis is able to be performed upon the alloy with the anode and cathode polarities reversed. Typically, the separate crucible may include an SiC material.

Typically, the apparatus for segregating the pure silicon may be configured for performing the secondary electrolysis using at least one of the following electrolytes (by approximate percentage weight):

(i) 10% K2SiF6, 25% AlF3, 25% NaF, 35% BaF2, 5% CaF2; (ii) 40-70% Na3AlFe, 5-20% K2SiF6, 5-15% CaF2, 5-10% CaO; and

(iii) 95-99% Na3AlF6, 1-5% SiO2.

Preferably, the apparatus may include a heating mechanism adapted for melting the composite deposited on the anode at a temperature of at least approximately 1450° C. or above whereby silicon conglomerates into pellets or ingots in the electrolyte upon cooling. Typically the heating mechanism for melting the composite may include an induction furnace or the like.

Preferably, the apparatus may include a filter such as a mesh filter adapted for filtering the conglomerated silicon pellets or ingots from the electrolyte upon cooling.

Alternatively, the apparatus may include a mechanism for melting the composite back into the alloy and cooling the composite at a controlled rate so that silicon floats to the top for collection. Typically, the composite may be melted back into the alloy at a temperature in accordance with the prescribed melting temperature depicted in the phase diagram of FIG. 19.

In a third broad form, the present invention provides a method of producing pure silicon from an electrolyte wherein the method includes the steps of:

(i) heating the electrolyte in a first crucible to form a molten electrolyte; and

(iii) applying electrolysis to the molten electrolyte by providing a potential difference between an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte;

wherein a first crucible lining is arranged inside the first crucible to substantially separate the molten electrolyte from an inner peripheral wall of the first crucible as electrolysis is being applied whereby pure silicon is produced which is soluble with the anode to form an alloy.

In a fourth broad form the present invention provides an apparatus for producing pure silicon from an electrolyte including:

a first crucible for receiving the electrolyte;

a heat source for heating the electrolyte in the first crucible to form a molten electrolyte;

an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte wherein electrolysis is able to be applied to the molten electrolyte when a potential difference is provide between the anode and the cathode; and

a first crucible lining arranged inside the first crucible to substantially separate the molten electrolyte from an inner peripheral wall of the first crucible as electrolysis is being applied to the molten electrolyte;

wherein pure silicon is produced which is soluble with the anode to form an alloy.

In a fifth broad form, the present invention provides a method of segregating pure silicon from an alloy containing at least one of a copper, gold, silver, zinc and a magnesium material alloyed with pure silicon particles, the method including the steps of:

(i) forming the alloy into at least one of a tape and a powder; and
(ii) applying electrolysis to the tape or powder;

whereby pure silicon particles may be segregated from the alloy in the form of nano-sized pure silicon particles.

Typically, the tape may be formed by at least one of casting and extruding the alloy. Also typically, the powder may be formed by mechanically grinding or milling the alloy in a controlled environment. Preferably, the powder is formed by grinding the alloy. Typically, the powder may include micron to nano sized alloy particles.

Preferably, the step of performing the electrolysis may include submerging the tape or powder in an electrolyte solution. Typically the electrolyte solution may include at least one of hydrochloric acid (HCl), dehydrated acetic acid, iodine solutions (i.e. potassium iodine solution), hydrazoic acid (HN3), acetone, concentrated alcohol and a combination thereof. Preferably, during the electrolysis, the acidity of the electrolyte solution may be replenished.

Typically, a current of less than 1 A may be applied during the electrolysis.

Preferably, after the electrolysis has been performed, the electrolyte solution containing the nano-sized pure silicon particles may be further processed to separate the nano-sized pure silicon particles from the electrolyte solution. Typically, the nano-sized pure silicon particles may be centrifugally separated from the electrolyte solution. Also preferably, the separated nano-sized pure silicon particles may thereafter be stored in a substance suitable for alleviating reaction with free oxygen. Typically the substance may include concentrated alcohol.

In a sixth broad form, the present invention provides an apparatus for segregating pure silicon from an alloy containing at least one of a copper, gold, silver, zinc and a magnesium material alloyed with pure silicon particles, the apparatus including:

(i) an apparatus for forming the alloy into at least one of a tape and a powder; and
(ii) an apparatus for applying electrolysis to the tape or powder;

whereby pure silicon particles may be segregated from the alloy in the form of nano-sized pure silicon particles.

Typically, the apparatus for forming the alloy into a tape may be adapted to cast or extrude the alloy into the tape. Also typically, the apparatus for forming the powder may be adapted for mechanically grinding or milling the alloy into the powder. Preferably, the powder may be formed by grinding the alloy in a controlled environment. Typically, the powder may include micron to nano sized alloy particles.

Preferably, the apparatus for performing the electrolysis may be adapted for submerging the tape or powder in an electrolyte solution. Typically the electrolyte solution may include at least one of hydrochloric acid (HCl), dehydrated acetic acid, iodine solutions (i.e. potassium iodine solution), hydrazoic acid (HN3), acetone, concentrated alcohol and a combination thereof. Preferably, during the electrolysis, the acidity of the solution may be replenished.

Typically, the apparatus for performing the electrolysis may be adapted for applying a current of less than 1 A during the electrolysis.

Preferably the present invention may include an apparatus for separating nano-sized pure silicon particles in the electrolyte solution from the electrolyte solution. Typically this may include a centrifuge device for centrifugally separating the nano-sized pure silicon particles from the electrolyte solution. Also preferably, the present invention may include a storage apparatus for storing the separated nano-sized pure silicon particles in a substance suitable for alleviating reaction with free oxygen. Typically the substance may include concentrated alcohol.

In any one of the preceding broad forms of the present invention, a second crucible lining may be arranged in the first crucible between the alloy and the first crucible lining. Typically, the second crucible lining may include at least one of an SiN or an SiC material to extend the life of the first lining since SiC and SiN are abrasion resistant. Preferably, the second crucible lining may include a substantially non-porous surface.

Typically, the second crucible lining may be substantially submerged below a top level of the alloy (when melted) in the first crucible and maintained substantially submerged below the top level of the alloy during the electrolysis in the first crucible so as to alleviate absorption of the electrolyte into the second crucible lining.

In a seventh broad form, the present invention provides a method of producing pure silicon including the steps of:

(i) performing an electrolysis upon a quartz-containing electrolyte in a crucible wherein a solid composite is formed containing silicon and the electrolyte;
(ii) thereafter, melting the composite wherein the silicon particles in the composite conglomerates in the electrolyte upon cooling;
(iii) thereafter, when the temperature of the melted composite falls below approximately 1414° C., filtering the conglomerated silicon from the electrolyte.

Typically, in step (i) the electrolyte may include a combination of quartz pellets/blocks and powdered quartz. Preferably, the step (i) of performing the electrolysis includes periodically adding powdered quartz to the crucible to maintain a supply a silicon particles during the electrolysis.

Typically, the composite that is formed may include approximately 20% silicon particles by percentage weight and approximately 80% electrolyte particles by percentage weight.

Preferably, the electrolysis may be performed using 2 carbon nodes having a suitable amount of raw material available to undergo the electrolytic process.

In an eighth broad form, the present invention provides an apparatus for producing pure silicon including:

(i) a crucible adapted for receiving a quartz-containing electrolyte therein;
(ii) an electrolysis system adapted for performing an electrolysis upon the electrolyte within the crucible wherein a solid composite of silicon and electrolyte is deposited therein;
(iii) a heating mechanism adapted for melting the composite wherein the silicon particles in the composite conglomerate in the electrolyte upon cooling; and
(iii) a filter adapted for filtering the conglomerated silicon from the electrolyte when the temperature of the melted composite falls below approximately 1414° C.

Typically, the electrolyte may include a combination of quartz pellets/blocks and powdered quartz. Preferably the present invention includes a mechanism for periodically delivering powdered quartz into the crucible during the electrolysis to maintain a supply of silicon particles during the electrolysis.

Typically, the composite formed by the electrolysis of the electrolyte may include approximately 20% silicon particles by percentage weight and approximately 80% electrolyte particles by percentage weight.

Preferably, the electrolysis system may include 2 carbon nodes having a suitable amount of raw material available to undergo the electrolytic process.

In a ninth broad form the present invention provides a pure silicon produced in accordance with any one of the broad forms of the present invention described herein.

In an tenth broad form the present invention provides a solar cell including pure silicon produced in accordance with any one of the broad forms of the present invention described herein.

In an eleventh broad form the present invention provides a battery including pure silicon produced in accordance with any one of the broad forms of the present invention described herein. Typically, an anode of the battery is formed from the pure silicon.

In a twelfth broad form the present invention provides a crucible lining adapted for use in accordance with any one of the broad forms of the present invention described herein. In a thirteenth broad form, the present invention provides an electrical-grade silicon material suitable for use in forming an integrated circuit element, said electrical-grade silicon material being produced in accordance with any one of the broad forms of the present invention described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the following detailed description of a preferred but non-limiting embodiment thereof, described in connection with the accompanying drawings, wherein:

FIG. 1 shows a flowchart of a method for producing pure silicon from an electrolyte in accordance with an embodiment of the present invention;

FIG. 2 shows a flowchart of an embodiment method for segregating pure silicon from the alloy formed in accordance with the method steps depicted in FIG. 1,

FIG. 3 shows a side cut-away view of an apparatus for producing pure silicon in accordance with an embodiment of the present invention;

FIGS. 4(a) and 4(b) show an EDX chart and corresponding SEM image of a first sample taken of an alloy containing pure silicon after the first electrolysis in the first crucible;

FIGS. 5(a) and 5(b) show an EDX chart and corresponding SEM image of a second sample taken of an alloy containing pure silicon after the first electrolysis in the first crucible;

FIGS. 6(a) and 6(b) show an EDX chart and corresponding SEM image of a third sample taken of an alloy containing pure silicon after the first electrolysis in the first crucible;

FIGS. 7(a) and 7(b) show an EDX chart and corresponding SEM image of a fourth sample taken of an alloy containing pure silicon after the first electrolysis in the first crucible;

FIG. 8 shows EDX data and a corresponding SEM image of a sample of pure silicon which has naturally segregated from the alloy transferred from the first crucible into the second crucible in accordance with embodiments of the present invention;

FIG. 9 shows EDX data and a corresponding SEM image of a sample of nano-sized pure silicon which has segregated from the alloy in the second crucible during a secondary electrolysis involving use of a 10% hydrochloric acid (HCl) electrolyte solution in accordance with embodiments of the present invention;

FIG. 10 shows EDX data and a corresponding SEM image of a sample of nano-sized pure silicon which has segregated from the alloy in the second crucible during a secondary electrolysis involving use of a 20% hydrochloric acid (HCl) electrolyte solution in accordance with embodiments of the present invention;

FIG. 11 shows a further EDX data and a corresponding SEM image of a further sample of nano-sized pure silicon which has segregated from the alloy in the second crucible during a secondary electrolysis in accordance with embodiments of the present invention;

FIG. 12 shows an amorphous tape casted from an alloy remaining in the second crucible after natural segregation of some of the pure silicon particles in the alloy in accordance with embodiments of the present invention;

FIG. 13 shows testing parameters and test result data observed in producing an alloy containing pure silicon in the first crucible in accordance with embodiments of the present invention described herein when a direct heat furnace and induction furnace are variably used in heating the electrolyte during the first electrolysis;

FIGS. 14 and 15 shows a side cut-away view of an alternative embodiment of an apparatus for segregating silicon from the alloy;

FIG. 16 shows a side cut-away view of an embodiment of the present invention in which a second crucible lining is disposed in the first crucible between the first crucible lining and the alloy anode; and

FIG. 17 shows a side cut-away view of a further embodiment of the present invention; and

FIG. 18 shows experimental data indicative of testing on samples of silicon produced in accordance with embodiments of the present invention.

FIG. 19 shows a phase diagram of copper silicon alloy.

 PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be described below with reference to the accompanying drawings.

FIGS. 1 and 2 depict flowcharts of method steps in accordance with embodiments of the for producing pure silicon. In embodiments herein described, the term “pure silicon” refers to solar-grade silicon. An apparatus (1) which is used in performing the method steps depicted in FIGS. 1 and 2 is shown in FIG. 3 and includes a first crucible (2) for receiving an electrolyte, a first crucible lining (3) arranged within the first crucible (2) which separates the electrolyte from an inner peripheral wall (2a) of the first crucible (2), a heat source for heating the first crucible (2), an electrolysis device for applying electrolysis to the electrolyte (4) after the electrolyte (4) has been melted by the heat source, and a device for stirring the molten electrolyte (4) during electrolysis. Pure silicon produced by electrolysis forms an alloy with the anode (5). The pure silicon can thereafter be extracted from the alloy through use of a segregation apparatus and processes as will be described in greater detail below.

The electrolyte used in embodiments described herein comprise approximately 87% cryolite particles, 10% calcium oxide particles and 3% quartz particles by weight of the electrolyte (4). Before being deposited into the first crucible (2), the electrolyte (4) is manufactured by melting together the cryolite, calcium oxide and quartz particles at a temperature of 1200° C. and then allowing the melted substance to solidify. The resulting density of this electrolyte (4) is approximately 3 g/cm3.

The first crucible (2) is formed from a carbon material and has an internal recess (2c) for receiving the electrolyte (4). The recess (2c) is cylindrically-shaped and is defined by an inner peripheral wall (2a) and a base (2b). The first crucible (2) and internal recess (2c) need not be cylindrically-shaped in alternative embodiments. The first crucible (2) can alternatively be formed from materials such as silicon nitrate or silicon carbide material. Whichever material is used, it is desirable that the first crucible (2) material be able to generate heat if placed within an induction furnace, or, to conduct heat in a relatively efficient manner if placed in a direct heating furnace.

The electrolysis device includes an anode (5) and a cathode (7) which are connected to positive and negative terminals of a power supply (not shown). When the cathode (7) and anode (5) are placed in contact with the molten electrolyte (4) and a potential difference is generated between the anode (5) and cathode (7), the molten electrolyte (4) becomes subject to electrolysis thereby resulting in production of pure silicon.

The anode (5) includes an alloy of copper and approximately 3% pure silicon particles by weight of the alloy. The alloy is initially formed by melting the copper and combining the approximately 3% pure silicon particles at a temperature of approximately 1200° C. The inclusion of the 3% pure silicon particles in the alloy anode assists in setting the melting temperature of the alloy at a temperature which is applied during electrolysis. Although it is conceivable that more than 3% per weight of pure silicon could be initially combined with the copper to form the alloy anode, it is considered that this amount is a suitable minimum level in setting a suitable melting point.

The cathode (7) includes a carbon rod which is formed by pressed purified carbon powder. The carbon powder is purified before pressing by applying an acid wash and chlorine flushing to remove any impurities in the carbon powder. The carbon rod is controllably positionable and only lowered into the crucible recess into contact with the electrolyte (4) after the electrolyte (4) has been heated to form the molten electrolyte (4). In large-scale production, positioning of the carbon cathode (7) would be actuated by any automated mechanical positioning system known to persons skilled in the art. In the course of testing embodiments described herein, the carbon rod was lowered manually into contact with the molten electrolyte.

The first crucible lining (3) is a cylindrical tube-shaped configuration which fits snugly within the first crucible recess. The first crucible lining (3) complements and sits flush against the inner peripheral wall (2a) of the first crucible recess (2c). Accordingly, in use, the first crucible lining (3) provides a barrier for separating the electrolyte (4) in the recess (2c) from the inner peripheral wall (2a) of the first crucible recess (2c). Notably, the presence of the first crucible lining (3) assists in preventing electrolyte (4) from being absorbed into the first crucible wall (2a) which would otherwise decrease efficiency in the electrolysis process. Additionally, because the first crucible lining (3) assists in blocking absorption of the electrolyte (4) into the first crucible wall (2a), this results in more electrolyte (4) being directed towards the alloy anode to further improve efficiency of the electrolysis process.

In this embodiment the first crucible lining (3) is formed from quartz. As the quartz lining (3) is gradually corroded by the electrolyte (4) during electrolysis, this creates a time limitation upon the duration at which electrolysis can occur, and hence, the total amount of time which may be utilised in production of pure silicon via electrolysis. In seeking to prolong the duration of electrolysis, other less corrosive materials such as calcium oxide, magnesium fluoride, and sodium fluoride could be used in place of quartz. If a quartz material is used as the first crucible lining (3), it can conveniently provide a suitable amount of additional quartz particles necessary to maintain the proportion of quartz particles in the molten electrolyte (4) at a predetermined level throughout the duration of electrolysis.

Yet alternatively pure silicon could be used to form the first crucible lining (3) as this material does not tend to melt at temperatures applied in the electrolysis process, and, it does not tend to corrode due to interaction with the electrolyte (4). As a pure silicon first crucible lining (3) does note tend to corrode, the selected thickness of the pure silicon first crucible lining (3) is a less critical consideration than when a corrodible first crucible lining (3) such as a quartz first crucible lining (3) is used. Accordingly, in large scale production of pure silicon it is envisaged that the use of a pure silicon first crucible lining (3) would be particularly advantageous because it can be reused to save ongoing costs of production. Also, in practical terms, the use of a pure silicon first crucible lining (3) would assist in alleviating the practical complexities associated with use of a corrodible first crucible lining (3) in which case a new first crucible lining (3) would need to be regularly re-inserted into the first crucible (2). During testing of embodiments of the present invention, a quartz lining of approximately 9.5 cm diameter was used.

Alternatively and/or additionally it is also possible to create a natural first crucible lining (3) within the first crucible (2) by suitably regulating the temperature at regions within the first crucible (2). For instance this could be implemented by generating a suitable temperature gradient whereby the temperature at the cathode (7) could be set relatively higher than the electrolyte situated adjacent the inner peripheral wall (2a) of the first crucible (2). Electric arc heating would be a particularly well-suited to generating the suitable temperature gradient due to relatively high temperatures arising at the cathode (7) relative to the electrolyte adjacent the inner peripheral wall of the first crucible (2). If the temperature of the cathode (7) were for example set at 980° C. and the temperature of the electrolyte (4) adjacent the inner peripheral wall (2a) of the first crucible set at 900° C. or lower, this would allow solidification of a natural first crucible lining (3) adjacent the inner peripheral wall (2a) of the first crucible (2) whilst the remaining electrolyte (4) inwardly of the first crucible (2) would remain in molten form by virtue of the cathode (7) temperature. The creation of a natural first crucible lining (3) is advantageous in that it obviates the need to provide a stand-alone first crucible lining (3) thereby alleviating production costs and process implementation complexities.

In testing of embodiments described herein, a direct heat furnace was used as the primary heat source for heating the first crucible (2), the electrolyte (4), and, the alloy within the first crucible (2). However, it is contemplated that electric arc heating could be used as an alternative heat source in alternative embodiments whereby electrical resistance in the electrolyte medium between the anode (5) and cathode (7) gives rise to heat being generated. It would be readily understood that the further apart the anode (5) and cathode (7) are positioned, the higher the resistance that will be generated and hence the greater the amount of heat that will be transferred to the electrolyte (4) in the first crucible (2). Alternatively and/or additionally, an induction furnace could be used to provide induction heating.

The stirring device is used to constantly stir the molten electrolyte (4) during electrolysis. Stirring of the molten electrolyte (4) assists in both generating ion-flow in the electrolyte (4), and, increasing the contact between the electrolyte (4) and the molten alloy anode (5). Conveniently, the use of an induction furnace would inherently provide a stirring effect within the molten electrolyte during electrolysis. Accordingly, an induction furnace could be utilised both as a secondary heat source for regulating temperature, but also as the mechanism for stirring the molten electrolyte (4) during electrolysis. This would assist in alleviating costs incurred in acquiring a separate specialised stirring equipment. Furthermore, as the stirring provided by the induction furnace is due to electromagnetic force rather than direct mechanical interaction with the molten electrolyte (4), the time and cost required to repair a mechanical stirring devices may be alleviated. The induction furnace could be configured to apply current pulsing to stir the molten electrolyte (4). Of course, it is possible to utilise a mechanical stirrer in other embodiments of the present invention if required.

In alternative embodiments, a specialised magnetic stirring device could be used to magnetically stir the alloy in the first crucible (2).

A method of using the above-described apparatus to produce pure silicon will now be described in accordance with embodiments of the present invention.

Firstly, the quartz first crucible lining (3) is fitted inside of the crucible recess by sliding the lining into the recess. This step is represented by block (100) in FIG. 1. The anode alloy (5) is thereafter deposited inside the crucible recess (2c) such that it sits in contact with the base of the first crucible (2). The first crucible (2) is then heated by the direct heat furnace at a temperature of between approximately 950-980° C. so as to melt the alloy. This step is represented by block (110) in FIG. 1.

As soon as the alloy (5) has melted, the solid electrolyte (4) is deposited into the first crucible recess (2c) on top of the melted alloy (5) and the first crucible (2) is further heated at a temperature of at least 900° C. to melt the electrolyte into a molten electrolyte (4). These steps are represented by blocks (120) and (130) in FIG. 1.

Thereafter, electrolysis of the molten electrolyte (4) is commenced by lowering the carbon rod into contact with the molten electrolyte (4). This step is represented by block (140) in FIG. 1. As the cathode (7) is electrically connected directly to a negative terminal of the power supply, and, the alloy anode is electrically connected to a positive terminal of the power supply via the first crucible (2) (which is itself a conductive material) the potential difference between anode and cathode (7) facilitate electrolysis.

During testing of embodiments herein described, voltages in a range of 6-8V were applied across the anode (5) and cathode (7) which resulted in currents of between approximately 40-60 A flowing between the anode (5) and cathode (7) via the molten electrolyte medium. The molten electrolyte (4) being contained in a quartz crucible of approximately 9.5 cm diameter and exposed to a current density of approximately 1 A/cm2 across an approximate electrolyte surface area of 70.8800938 cm2.

It should be noted that if the current density is set too high, this may result in accelerated damage to the first crucible lining (3) and/or the first crucible (2). It would be understood by a person skilled in the art that variation in the dimensions of the first crucible (2), the magnitude of the current density applied and the surface of the electrolyte will affect the size of the yield of pure silicon produced in accordance with embodiments described herein. The yield can be generally approximated in accordance with the following formula:


YIELD=Current(A)×[Electrolysis Constant(0.262g/A h)]×[Hours of Electrolysis(h)]u

The molten electrolyte (4) is maintained at a temperature between approximately 900-1000° C. to prevent solidification of the molten electrolyte (4) during electrolysis. Generally, when the molten electrolyte (4) is maintained at a relatively higher temperature during electrolysis, the efficiency of pure silicon production is improved. However certain limitations regarding the selected temperature should be noted. Firstly, it has been found that during testing of embodiments herein described, maintaining the temperature of the molten electrolyte (4) at a temperature of approximately 980° C. during electrolysis, produces particularly desirable results. Temperatures above 980° C. tend to cause an increase in evaporation and viscosity of the molten electrolyte (4) which impedes efficiency of pure silicon production and which may ultimately result in halted of electrolysis. Temperatures at or below 980° C. will not tend to cause increases in evaporation and viscosity of the molten electrolyte (4) however the temperature should not drop below 900° C. as the molten electrolyte (4) will solidify below this temperature.

As electrolysis takes place, the molten electrolyte (4) is stirred to increase flux of pure silicon particles produced during the electrolysis, into contact with the alloy anode (5). During testing, automated mechanical stirring was employed. However, an induction furnace could be conveniently used to magnetically stir the molten electrolyte (4) by applying current pulsing to the molten electrolyte (4). This step is represented by block (150) in FIG. 1.

During electrolysis, the proportion of quartz particles in the electrolyte (4) should be maintained at approximately 3% by weight of the electrolyte (4). To achieve this requirement, a sensor is used to periodically measure the quartz particles in the electrolyte (4) and a dispensing means is used to controllably dispense additional amounts of quartz particles into the molten electrolyte (4) to compensate for any depletion as required. As mentioned above, if a quartz material is used as the first crucible lining (3) it may be feasible to utilise the quartz particles in the lining to compensate for depletion in quartz particles within the molten electrolyte (4) during electrolysis thereby maintaining the proportion of required quartz particles. This step is represented by block (160) in FIG. 1.

The pure silicon (6) which forms an alloy with the anode (5) by use of the above embodiment is shown in FIGS. 5 and 6. The pure silicon (6) is able to be segregated from the alloy (5) by use of segregation techniques.

Electrolysis in the first crucible (2) will cease, after a limited timeframe, when the solubility limit of pure silicon in the alloy is reached. This step is represented by block (170) in FIG. 1. It has been found that during testing of embodiments of the present invention, when the alloy (5) is heated at a temperature between approximately 950-980° C., the pure silicon (6) is no longer soluble with the alloy (5) when the pure silicon (6) in the alloy (5) reaches approximately 25% by weight of the alloy (5). The time taken for the solubility limit to be reached will depend upon several factors including the surface area of the molten electrolyte (4) undergoing electrolysis. However, in testing of embodiment described herein, this solubility limit was found to be reached at around 8 hours. In terms of the yield and degree of purity of pure silicon (6) produced in accordance with the embodiments herein described, this would be understood to represent a considerable improvement in efficiency over existing production methods.

In this embodiment, with reference to FIG. 2, segregation of pure silicon from the alloy is thereafter commenced by first transferring the molten alloy (5) into a second crucible (not shown) by use of a suction device or any other suitable mechanical extraction means. The second crucible can be made from any material suitable for exposure to temperatures between approximately 800-1000° C. The material used to form the second crucible should however be inert with respect to the alloy and/or molten salts—that is, it should not chemically react with the alloy or any other molten salts. This step is represented by block (200) in FIG. 2.

The molten alloy is maintained at a temperature of between approximately 800-850° C. in the second crucible at a cooling rate of approximately 2-3° C./min whereby some of the pure silicon (6) within the molten alloy (5) will tend to naturally segregate from the alloy (5) as a result of thermodynamics. The pure silicon (6), which is of lower density than copper, naturally floats to the top of the molten alloy (5). This step is represented by block (210) in FIG. 2. During testing of embodiments of the present invention it has been found that approximately 11% pure silicon (6) particles by weight of the alloy (5) will float to the top of the melt as solid pure silicon (6). The now floating pure silicon (6) is able to be re-melted into ingots.

This form of naturally segregated pure silicon (6) has been found during testing to be suitable for solar-cell grade applications and is expected to provide at least around 18% efficiency which is consistent with the performance of solar-cell grade poly-silicon conventionally produced by Siemens. Molten salts are used to cover the molten alloy (5) in the second crucible to alleviate the now floating solid pure silicon (6) from re-oxidising. In certain embodiments the alloy remaining in the second crucible may be reintroduced back into the first crucible when approximately 11% pure silicon (6) by weight of the molten alloy has naturally segregated from the alloy in the second crucible. The reintroduced molten alloy will sink to the bottom of the first crucible (2) and contribute further to the electrolysis process.

Alternatively, instead of reintroducing the remaining alloy in the second crucible back into the first crucible when the predetermined percentage of pure silicon particles has segregated from the alloy in the second crucible, the alloy remaining in the second crucible could be cast or extruded into an amorphous tape, or, mechanically grinded or milled into a powder of micron to nano sized alloy particles. It would be appreciated by a person skilled in the art that mechanical milling of the alloy has a greater tendency to introduce significant amount of impurities. Testing of embodiments of the present invention to date have involved mechanical grinding of alloys in the second crucible instead of tape casting. This step is represented by block (220) in FIG. 2.

The process and apparatus used in the casting of the amorphous tape can be configured to control the microstructure of the nano-sized pure silicon particles in the tape itself depending upon the rate of cooling during the tape casting. The nano-sized pure silicon particles which are thereafter able to be segregated from the tape in accordance with embodiments described herein could be in a range approximately between 10 nm-60 nm. FIG. 12 shows an amorphous tape which has been cast from the alloy.

Thereafter, a secondary electrolysis is applied to the tape or the powder after submerging the tape or powder in an electrolyte solution. In the embodiments described herein, electrolyte solutions having 10% hydrochloric acid (HCl) and 20% hydrochloric acid (HCl) were each used on different occasions with the results of these variations being indicated in FIGS. 9 and 10 respectively. The acidity of the electrolyte solution is regularly replenished during the secondary electrolysis by pumping HCl gas into the electrolyte solution to maintain suitable pH and Cl− levels. It would be appreciated by a person skilled in the art that in alternative embodiments, the alloy could be submerged in other electrolyte solutions including dehydrated acetic acid, iodine solutions (i.e. potassium iodine solution), hydrazoic acid (HN3), acetone, concentrated alcohol or combinations thereof. This step is represented by block (230) in FIG. 2.

A current of less than approximately 1 A is applied during the secondary electrolysis to suitably remove the copper from the alloy. After the secondary electrolysis is applied, the electrolyte solution contains Cu/Cu+/Cu++ and nano-sized pure silicon particles. A centrifuge device is then used to separate the nano-sized pure silicon particles from the electrolyte solution by way of centrifugal motion. This step is represented by block (240) in FIG. 2. It is possible in alternative embodiments to apply a current of greater than 1 A during the secondary electrolysis however this may result in oxidation of the silicon. If that is the case, then an acid such as HF (hydrofluoric acid) could be used to corrode the oxidized layer of silicon (quartz) to obtain nano-silicon.

As the nano-sized pure silicon particles have a greater tendency to react with oxygen, the nano-sized pure silicon particles are stored in a substance such as concentrated alcohol to alleviate reaction with free oxygen. This step is represented by block (250) in FIG. 2.

Copper which is extracted from the casted alloy tape or powder after the segregation process can be reintroduced back in to the first crucible (2) to assist in the electrolysis process within the first crucible (2). As would be appreciated by a person skilled in the art, this may be considered the more efficient manner in saving energy since the already segregated molten alloy is at 800-850° C. and would merely require 100-150° C. worth of energy to continue electrolysis in the first crucible (2).

In other embodiments, copper could be removed from the alloy in the second crucible by flushing the tape or powder with HCl gas or chlorine gas in an enclosed area that is oxygen free to allow the tape or powder to react with the HCl gas or chlorine gas. CuCl2 will then have to be removed via mechanical methods or rinsing with suitable solutions in which CuCl2 is soluble in. Nano-sized pure silicon particles can then be removed. CuCl2 is paramagnetic as a person skilled in the art would readily appreciate. Accordingly, a magnetic field could be applied to the powder that contains CuCl2 to remove CuCl2 from the powder whilst alleviating introduction of contaminants if it were to be rinsed by oxygen containing solutions. Advantageously, CuCl2 which is formed as a result of flushing the tape or powder with HCl gas or chlorine gas would readily dissolve into any concentrated alcohol which is used to alleviate re-oxidation of the nano-sized pure silicon particles.

It would be further appreciated by a person skilled in the art that in yet alternative embodiments, the above process and apparatus for segregating pure silicon from the alloy in the second crucible could be similarly performed directly upon the alloy in the first crucible without first removing the alloy in the first crucible (2) into a second crucible and without first allowing natural segregation of pure silicon from the alloy. Hence, the alloy in the first crucible (2) containing 25% pure silicon particle by weight of the alloy could be exposed to the above-described segregation apparatus and process by forming the alloy into a tape or powder and applying electrolysis to the tape or powder before separating formed nano-sized pure silicon particles from the electrolyte solution.

It would be further understood by a person skilled in the art that the above-described apparatus and method for segregating pure silicon from an alloy need not necessarily be applied to a pure silicon containing alloy formed in accordance with embodiments described herein and could be applied to alloys produced in accordance with alternative apparatuses and methods.

FIGS. 4(a)-4(b), 5(a)-5(b), 6(a)-6(b) and 7(a)-7(b) show EDX data and corresponding SEM images for 4 different sample regions of an alloy produced in the first crucible after the first electrolysis in accordance with embodiments of the present invention described herein. It would be understood by a person skilled in the art that the variation in proportion of pure silicon to copper in the alloy for each analysed sample varies as a result of the location upon the alloy at which the sample reading has been taken. For instance, FIGS. 5(a) and 5(b) which indicate a relatively high component of pure silicon (i.e. 100%) compared to copper (negligible) has been taken on a region of the alloy which is mostly pure silicon. In contrast FIGS. 4(a) and 4(b) indicate a relatively lower component of pure silicon in the alloy (i.e. 68.83%) due to the sample reading being taken at or near an interface between silicon and copper in the alloy.

FIG. 8 shows EDX data and a corresponding SEM image of pure silicon which has naturally segregated from the alloy in the second crucible due to different densities between the pure silicon and the copper in the alloy. The pure silicon shown in FIG. 8 is suitable for melting into ingots.

FIGS. 9, 10 and 11 show EDX data and a corresponding SEM images of nano-sized pure silicon particles which have been produced and segregated from alloys following the secondary electrolysis and processing described herein. The nano-sized pure silicon particles shown in FIG. 9 involved the use of an electrolyte solution having 10% hydrochloric acid (HCl) whilst the nano-sized particles shown in FIG. 10 involved the use of an electrolyte solution having 20% hydrochloric acid (HCl) during the secondary electrolysis. The presence of aluminium indicated in the EDX data is introduced during the mechanical grinding process of the alloy and can be substantially eliminated by use of higher precision grinding of the alloy in future experimentation. The oxygen component is also able to be substantially eliminated in further experimentation by storing the segregated nano-sized pure silicon particles in concentrated alcohol to alleviate re-oxidation in to quartz. Accordingly, it is considered that purity substantially above 99% silicon is achievable in view of the above.

FIG. 13 shows data produced during testing of embodiments of the present invention in producing pure silicon. An average efficiency of approximately 71% and 75% is achieved using direct heat furnacing and induction furnacing during the first electrolysis in the first crucible respectively. The fourth columns (“g (before)”) in FIG. 13 indicates the amount of pure silicon in the alloy anode before the first electrolysis commences whilst the fifth columns (“g (after)”) indicates the pure silicon in the alloy after the first electrolysis in the first crucible (2) has taken place. The efficiency is able to be determined by the net gain in pure silicon against energy expended in the process.

In alternative embodiments, after the electrolysis is performed in the first crucible (2), segregation of silicon (6) from the alloy (5) can be performed by placing the alloy (5) in a second crucible (7) made of SiC and performing a secondary electrolysis upon the alloy (5) in the second crucible (7). The secondary electrolysis is conducted with the polarities of the anode (5) and cathode (10) being reversed. The second crucible (7) should not be made from carbon since silicon would react with the carbon material to form SiC. An exemplary segregation apparatus is shown in FIG. 14

The secondary electrolysis is performed using a current in the range of 600-800 A and a voltage of approximately 6-7V. The current density used is approximately between 0.1 A/cm2 to 2.0 A/cm2.

The secondary electrolysis is performed using at least one of the following electrolyte compositions (8) (by approximate percentage weight) in the second crucible (7):

(i) 10% K2SiF6, 25% AlF3, 25% NaF, 35% BaF2, 5% CaF2; (ii) 40-70% Na3AlFe, 5-20% K2SiF6, 5-15% CaF2, 5-10% CaO; and

(iii) 95-99% Na3AlF6, 1-5% SiO2.

The secondary electrolysis results in a solid composite (11) containing silicon and electrolyte being deposited on the alloy anode (5) of around 300 mesh. The ratio of silicon to electrolyte in the composite (11) will increase the closer the composite is to the anode (5). However, on average, the composite (11) will contain approximately 20-30% silicon by percentage weight and approximately 70-80% electrolyte by percentage weight.

Thereafter, an induction furnace (12) or other suitable heating element is used to melt the composite (11) at a temperature of at least approximately 1450° C. or above whereby the silicon conglomerates into pellets or ingots within the electrolyte upon cooling. Upon cooling, the conglomerated silicon pellets or ingots can then be filtered from the electrolyte by use of a mesh filter (13) as shown in FIG. 15. A sample of silicon produced in accordance with this embodiment of the present invention was tested with the results being indicated by (B) in FIG. 18.

The composite (11) can also be melted back into the copper silicon alloy at approximately 1000° C. to a minimum of 50% weight silicon. It would be understood that the melting temperature of the alloy will vary depending upon the specific percentage weight of silicon in the alloy. Thereafter, a controlled cooling process can be performed so that silicon floats to the top due to solubility limits and density differences. The cooling rate could be at or around 2-3° C./min from a predetermined percentage weight of silicon within the alloy melt to approximately 800° C. At or around 800° C., the alloy is still in liquid phase, silicon has already segregated itself out of the alloy melt, and can be poured out to obtain the silicon. After segregation of pure silicon, the remaining alloy will be approximately 13-15% weight silicon. A minimum of approximately 50% weight silicon particles in the alloy is necessary in order for the silicon to nucleate and grow and hence segregate. Nucleation equations, crystal growth equations, free energy of the said silicon particles are just some of the factors which will affect how silicon can segregate from the alloy based on the specific and controlled cooling rate used.

In any of the preceding embodiments described herein, a second crucible lining (9) made from a non-porous SiN or an SiC material can be arranged in the first crucible (2) between the alloy (5) and the first crucible lining (3). The presence of the second crucible lining (9) between the alloy (5) and the first crucible lining (3) assists in prolonging the electrolysis process in the first crucible (2).

The second crucible lining (9) is maintained substantially submerged below the top level of the melted alloy (5) during the electrolysis in the first crucible (2). The submersion of the second crucible lining (9) below the top level of the melted alloy (5) alleviates absorption of the electrolyte (4) into the second crucible lining (9).

A further embodiment for producing pure silicon is shown in FIG. 17. The embodiment involves an apparatus for performing electrolysis upon a quartz-containing electrolyte (15) in a crucible (14) without the use of any crucible linings. The electrolysis is conducted using two carbon rods (17a,17b) as electrodes in the electrolyte where the two carbon nodes have a suitable amount of raw material available to undergo the electrolytic process. Materials such as carbon or Al2O3 should not be used in this embodiment as they will tend to react with the electrolyte material and form contaminants.

Blocks or pellets of quartz are initially placed inside the crucible (14) and powdered quartz is periodically added during the electrolysis to ensure supply of silicon particles during the electrolysis. The electrolysis results in a packed solid composite (16) being deposited in the crucible (14) of around 20% weight silicon of the composite and 80% weight electrolyte of the composite (16). The silicon in the composite (16) is of approximately 300 mesh particle size.

An induction furnace (18) or other suitable heating mechanism is used to melt the composite (16) whereby the silicon conglomerates into small ingots in the electrolyte upon cooling. A mesh filter (13) such as is shown in FIG. 15 can then be used to filter out the silicon ingots from the electrolyte when the temperature of the composite falls below around 1414° C. (i.e. the solidification point of silicon). Alternatively, it has been found that ethanol of 40-100% concentration can be used to segregate silicon from the composite (16) whereby the composite (16) is first broken down and rinsed with the ethanol. The solid electrolytes will float and the silicon will stay submerged.

Boron and phosphorous are found to form compounds with the electrolyte and evaporate during electrolysis phases. A sample of silicon produced in accordance with this embodiment of the present invention was tested with the results being indicated by (A) in FIG. 18.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described without departing from the scope of the invention. All such variations and modification which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope of the invention as broadly hereinbefore described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps and features, referred or indicated in the specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge.

Claims

1. A method of producing pure silicon from an electrolyte wherein the method includes the steps of:

(i) heating the electrolyte in a first crucible to form a molten electrolyte; and
(ii) applying electrolysis to the molten electrolyte by providing a potential difference between an anode and a cathode which are adapted for electrical/ionic communication with the molten electrolyte;
wherein the molten electrolyte is stirred as electrolysis is being applied and pure silicon produced as a result of the electrolysis is soluble with the anode to form an alloy.

2. A method as claimed in claim 1 wherein the electrolyte comprises cryolite, calcium oxide particles and quartz particles.

3. A method as claimed in claim 2 wherein the electrolyte comprises approximately between 82-94% cryolite particles by weight of the electrolyte.

4. A method as claimed in claim 2, wherein the electrolyte comprises approximately between 3-15% calcium oxide particles by weight of the electrolyte.

5. A method as claimed in claim 2, wherein the electrolyte comprises approximately 3% quartz particles by weight of the electrolyte.

6. A method as claimed in claim 2, wherein the electrolyte comprises approximately 87% cryolite particles, 10% calcium oxide particles and 3% quartz particles by weight of the electrolyte.

7. A method as claimed in claim 2, including a step of controllably adding quartz particles to the electrolyte during electrolysis to substantially maintain approximately 3% quartz particles in the electrolyte by weight of the electrolyte.

8. A method as claimed in claim 1, wherein the first crucible includes at least one of a carbon, silicon nitrate and silicon carbide material.

9. A method as claimed in claim 1, wherein the first crucible includes a recess defined by an inner peripheral wall and a base for receiving the electrolyte.

10. A method as claimed in claim 9 including the step of arranging a first crucible lining inside the first crucible recess between the first crucible and the electrolyte.

11. A method as claimed in claim 1 including the step of arranging a second crucible lining inside the first crucible recess between the alloy anode and the first crucible lining.

12. A method as claimed in claim 11, wherein the second crucible lining includes at least one of an SiC and an SiN material.

13. A method as claimed in claim 11, wherein the second crucible lining is submerged beneath the anode during electrolysis in the first crucible.

14. A method as claimed in claim 1, wherein silicon is segregated from the alloy after step (ii) by performing a secondary electrolysis upon the alloy whereby a solid composite of silicon and electrolyte is formed.

15. A method as claimed in claim 14, wherein the polarities of the anode and cathode are reversed.

16. A method as claimed in claim 14, wherein the secondary electrolysis is performed in a crucible separate to the first crucible.

17. A method as claimed in claim 14, wherein the separate crucible includes an SiC material.

18. A method as claimed in claim 14, wherein the secondary electrolysis is performed using at least one of the following electrolyte compositions: (i) 10% K2SiF6, 25% AlF3, 25% NaF, 35% BaF2, 5% CaF2; (ii) 40-70% Na3AlFe, 5-20% K2SiF6, 5-15% CaF2, 5-10% CaO; and (iii) 95-99% Na3AlF6, 1-5% SiO2.

19. A method as claimed in claim 14, wherein the composite that is deposited on the anode is melted at a temperature of at least approximately 1450° C. whereby silicon conglomerates into pellets or ingots within the electrolyte upon cooling.

20. A method as claimed in claim 19, wherein the conglomerated silicon pellets or ingots are filtered from the electrolyte using a filter.

21. A method of producing pure silicon including the steps of:

(i) performing an electrolysis upon a quartz-containing electrolyte in a crucible wherein a solid composite is formed containing silicon and the electrolyte;
(ii) thereafter, melting the composite wherein the silicon particles in the composite conglomerates in the electrolyte upon cooling;
(iii) thereafter, when the temperature of the melted composite falls below approximately 1414° C., filtering the conglomerated silicon from the electrolyte.

22. A method as claimed in claim 21 including the step of periodically adding powdered quartz to the crucible to maintain a supply a silicon particles during the electrolysis.

23. A method as claimed in claim 21, wherein two carbon nodes are used in the electrolysis.

Patent History
Publication number: 20130277227
Type: Application
Filed: Dec 20, 2011
Publication Date: Oct 24, 2013
Applicant: EPRO DEVELOPMENT LIMITED (Kwai Chung, New Territories, Hong Kong)
Inventors: Albert Pui Sang Lau (Kowloon), Lee Cheung Lau (Kowloon)
Application Number: 13/996,403
Classifications
Current U.S. Class: Silicon, Boron, Or Phosphorus Containing (205/358)
International Classification: C25B 1/00 (20060101);