AU663212B2 - High purity silicon dioxide - Google Patents

Description

HIGH PURITY SILICON DIOXIDE TECHNICAL FIELD The invention relates to a process for producing purified a silicon containing substance and to an apparatus for use in such a process. The process of the invention may be used, for example, to produce very high purity silicon dioxide or very high purity silicon.

BACKGROUND ART Very high purity silicon dioxide and very high purity silicon are in demand for an increasing number of uses including electronics and solar energy, Known methods for the production of high purity silicon dioxide are based predominantly on the removal of all elements other than silicon and oxygen by treatment with acid. Such methods have a high energy cost and lead to the production of a number of by-products which are pollutants and who>e disposal is therefore difficult. There is S thus a need for a method for purifying silicon dioxide which has a relatively low energy 15 requirement and which results in the removal of impurities in a form which enables them tc be sold or disposed of without significant environmental impact.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a process for producing a purified silicon containing substance. Other objects of the invention are to provide a process for producing purified silicon dioxide and a process for the production of purified silicon.

Further objects of the invention are to provide an apparatus for the production of a purified silicon containing substance, an apparatus for the production of purified silicon dioxide and an apparatus for the production of purified silicon.

DISCLOSURE OF INVENTION S. 25 As used herein, the term "water at least fully saturated with hydrofluosilicic acid" means hydrofluosilicic acid at at least a saturated concentration in water at the particular temperature and pressure of operation and includes supersaturation. At room temperature this concentration is at least equal to the azeotrope concentration of hydrofluosilicic acid in water at standard temperature and pressure, According to a first embodiment of the present invention there is provided a process for producing a purified silicon containing substance which comprises the steps of: reacting impure silica with hydrogen fluoride to produce impure silicon tetrafluoride, and contacting the impure silicon tetrafluoride with water at least fully saturated with hydrofluosilicic acid, to remove at least some of the impurities in the silicon tetrafluoride.

Step of the process of the first embodiment produces, in addition to the purified silicon tetrafluoride, a liquid product which is relatively enriched in the impurities present in the impure silicon tetrafluoride from step Optionally, step of the process of the first embodiment may be repeated a plurality of times by contacting the purified silicon tetrafluoride produced from a previous step with water at least fully saturated with hydrofluosilicic acid, Suitably, where step is repeated a plurality of times, the water at least fully saturated with hydrofluosilicic acid which is contacted with the silicon tetrafluoride may be obtained from another step such that the water at least fully saturated with hydrofluosilicic acid used at each successive step contains a relatively lower concentration of impurities than does the water at least fully saturated with hydrofluosilicic acid at any previous step It will be appreciated that the liquid digestion product from step as well as the liquid product from step contains impurities originally present in the raw silica digested. Optionally, these liquid products may suitably be distilled in order to concentrate the impurities, thereby facilitating the removal of the impurities from the process. Such a distillation also serves to recover for reuse the volatile fluorine containing substances present in the liquid products. Distillation of the liquid products produces a vapour comprising steam, hydrogen fluoride and silicon tetrafluoride, and 20 may concentrate the impurities sufficiently to cause them to crystallise, for example as aluminium trifluoride, potassium fluoride, magnesium fluoride, sodium fluoride, calcium fluoride, etc. Thus, such a distillation step can produce a slurry of crystals of mixed fluorides. The crystals may be separated from the slurry and disposed of or sold.

Suitably, the hydrogen fluoride used in step of the process according to the first embodiment may contain silicon tetrafluoride. Thus, the hydrogen fluoride may be obtained by drying the vapour obtained from distillation of the liquid product from step and/or spent hydrogen fluoride obtained from step Thus, according to a second embodiment of the present invention, there is provided a process for producing a purified silicon containing substance which comprises distilling the liquid product from step of the process of the first embodiment and/or the spent hydrogen fluoride from step of the process of the first embodiment, thereby producing a vapour stream comprising silicon tetrafluoride, hydrogen fluoride and steam and a waste stream comprising fluorides of elements other than silicon and hydrogen; contacting the vapour stream with a fluoride based water removing material to produce a substantially dried gaseous stream comprising hydrogen fluoride and silicon tetrafluoride; and supplying the dried gaseous stream as the hydrogen fluoride source for step of the process of the first embodiment.

The purified silicon tetrafluoride produce I by the process of the first or second embodiments may be converted to other purified silicon containing substances by llib1|219399.doc methods generally known in the art. For example, hydrolysis of the silicon tetrafluoride produces purified silicon dioxide, and reacting the silicon tetrafluoride with hydrogen at elevated temperature produces purified silicon.

Thus, according to a third embodiment of the present invention, there is provided a process for the production of purified silicon dioxide comprising hydrolysing silicon tetrafluoride purified by the process of the first or second embodiments with water to produce hydrated silicon dioxide and a hydrolysate and separating the hydrated silicon dioxide from the hydrolysate.

Generally, the water used is high purity water, typically very high purity water, and the silicon dioxide produced is high to very high purity silicon dioxide.

Suitably, in the process of the third embodiment, the hydrated silicon dioxide may be dried to produce purified silicon dioxide.

The hydrolysate from the process of the third embodiment comprises water at least fully saturated with hydrofluosilicic acid and this hydrolysate may suitably be used as the is. water at least fully saturated with hydrofluosilicic acid in step According to a fourth embodiment of the present invention there is provided a process for the production of purified silicon comprising heating silicon tetrafluoride purified by the process of the first or second embodiments with hydrogen to produce purified silicon and hydrogen fluoride, and separating the purified silicon from the 20 hydrogen fluoride.

Generally, the hydrogen used is high purity hydrogen, typically very high purity hydrogen, and the silicon produced is high to very high purity silicon.

The present invention also relates to an apparatus for the production of a purified silicon containing substance, an apparatus for the production of purified silicon dioxide and an apparatus for the production of purified silicon.

Thus, according to a fifth embodiment of the present invention there is provided an apparatus for producing a purified silicon containing substance, comprising: a digestion vessel comprising means for admitting a digestion fluid, means for admitting silicon dioxide to be purified, means for contacting the digestion fluid with the silicon dioxide, vapour outlet and liquid outlet; and one or more purification vessels, the purification vessels being arranged in series when there is more than one of them, each purification vessel comprising means for admitting a liquid and a vapour, means for contacting the liquid and the vapour, means for separating the contacted liquid and vapour, and liquid and vapour outlet means; wherein the vapour outlet means of the digestion vessel communicates with the means for admitting a vapour to the first of the purification vessels in the series, the vapour outlet means of each purification vessel except the last in the series communicates with the means for admitting a vapour to the next purification vessel in the series, and the liquid outlet means of each purification vessel except the first in the series communicates with the means for admitting a liquid to the previous purification vessel in llibz1219309,doc the series.

According to a sixth embodiment of the present invention there is provided an apparatus for the preparation of purified silicon dioxide, comprising the apparatus of the fifth embodiment and further comprising: a hydrolyser, comprising means for admitting silicon-containing fluid to be hydrolysed, means for admitting high purity water, means for contacting the admitted high purity water and the fluid to be hydrolysed, means for separating high purity hydrated silicon dioxide from a liquid hydrolysate and outlet means for the high purity hydrated silicon dioxide and the liquid hydrolysate; and a drier comprising means for admitting high purity hydrated silicon dioxide, means for heating the high purity hydrated silicon dioxide and outlet means for dried high purity silicon dioxide; w.Arein the vapour outlet means of the last purification vessel in the series in the apparatus of the fifth embodiment communicates with the means for admitting silicon- S. 15 containing fluid to be hydrolysed of the hydrolyser and the outlet means for very high purity hydrated silicon dioxide communicates with the means for admitting very high purity hydrated silicon dioxide of the drier.

According to a seventh embodiment of the present invention there is provided an apparatus for the production of purified silicon, comprising the apparatus of the fifth 20 embodiment and further comprising a hydrogenator, comprising means for admitting silicon-containing fluid to be hydrogenated, means for admitting high purity hydrogen, means for heating the admitted high purity hydrogen and the fluid to be hydrogenated, thereby forming high purity silicon and hydrogen fluoride, means for separating high purity silicon from the hydrogen fluoride and outlet means for the high purity silicon and the hydrogen fluoride; wherein the vapour outlet means of the last purification vessel in the series in the apparatus of the fifth embodiment communicates with the means for admitting siliconcontaining fluid to the hydrogenator.

In the digestion step of the process of any of the first to fourth embodiments, silicon dioxide is digested by hydrogen fluoride to produce silicon tetrafluoride and water according to the following reaction 4HF SiO 2 SiF 4 21120.

The digestion step is therefore supplied with at least 4 moles of hydrogen fluoride for each mole of silicon dioxide supplied. The use of excess hydrogen fluoride results in the formation of hydrofluosilicic acid according to the following reaction 2HF SiF 4 H2SiF 6 The liquid digestion product comprises hydrofluosilicic acid and water together with dissolved impurities digested from the raw silica. The gaseous digestion product comprises silicon tetrafluoride, water vapour, and other gaseous impurities derived from the digestion of the raw silica Where the digestion fluid contains silicon tetrafluoride in 1libld210399.doc addition to hydrogen fluoride, the silicon tetrafluoride participates in the digestion reaction to produce hydrofiuosilicic acid. Thus, the relative amounts of the gaseous digestion product and the liquid digestion product from the digestion step may be controlled by the quantity and composition of the digestion fluid supplied to the digestion s step per unit weight of the raw silica supplied. Further control is obtained by the temperature at which the digestion step is carried out. Usually, the fluid used in the digestion step will be water at least fully saturated with hydrofluosilicic acid and comprising hydrogen fluoride in an amount of from about 25% to about 50% of the weight of the water. More usually, the amount of hydrogen fluoride will be from about 40% to about 50% of the weight of the water. The amount of hydrogen fluoride is desirably controlled so that the vapour pressure of the hydrogen fluoride in the digester remains relatively low. It has been found that this objective may be achieved if the amount of hydrogen fluoride in the digestion fluid is not more than 50% by weight of the weight of the water.

is The digestion step may suitably be carried out at a temperature of from about to about 80°C. Generally, the digestion step is carried out at a temperature in the range of from 50 0 C to 75°C. Most usually, the digestion step is carried out at a temperature of about 700C.

Tho digester vessel is constructed of material substantially inert to hydrogen 20 fluoride, silicon tetrafluoride and hydrofluosilicic acid. Suitably, it may be constructed from polytetrafluoroethylene, natural rubber or a material lined with polytetrafluoroethylene or natural rubber, or it may be of graphite.

The purity of the purified silicon containing substance may be determined by number of times step of the process of the invention is repeated. Generally, step (b) will be repeated from 1 to 100, typically 1 to 10, or more times. The numbef of repeats will be selected depending on the desired degree of purity of the purified silicon containing suostance which is to be produced.

Generally, in each repeat of step intimate contact is provided between the input impure silicon tetrafluoride and water at least fully saturated with hydrofluosilicic acid, whereby the impurities in the impure silicon tetrafluoride are distributed between the vapour and liquid phases. This distribution favours the liquid phase, As a result, the liquid stream leaving step is relatively enriched in the impurities compared to the liquid stream entering that step, and the silicon tetrafluoride exiting step at each repeat is relatively impoverished in the impurities compared to the vapour stream entering that step.

The purification vessels may be gas liquid absorption towers of generally known design. Usually, the purification vessels will be packed towers, plate towers or spray towers. Suitably, the purification vessels may be constructed of carbon. Usually, the purification vessels will be constructed of graphite.

At the hydrolysis step in the process of the third embodiment, hydrolysis of the |libzl21lB3o.do 6 silicon tetrafluoride occurs according to the following reaction 3SiF 4 H20 SiO 2 2H 2 SiF 6 The silicon dioxide is precipitated in the form of a hydrate.

In the hydrolysis, control of the quantity of water used, and the reaction temperature, is desirable to prevent the hydrated silicon dioxide forming as a gel.

Suitably, the quantity of water used may be in the range of from 5 mole to 10 mole in excess of the stoichiometric requirement for the hydrolysis. Any excess water reacts with silicon tetrafluoride in the last of the purification stages. Suitable temperatures for the hydrolysis reaction are in the range of from 35 0 C to 70 0 C. The input water temperature is suitably cor.rolled at a temperature of from 35 0 C to 50 0 C. Usually, the input water temperature used will be in the range of from 35 0 C to 40 0

C.

The hydrolyser vessel may suitably be constructed of very high purity silicon dioxide or of ultra pure graphite.

.:Drying of the purified silicon dioxide hydrate is suitably by means of electrical S is heating, although other forms of heating, for example steam, may be used. Usually, the heating will be under vacuum, although the drying conditions used depend on the level of purity required in the high purity silicon dioxide product. Drying may be at temperatures of from 90 C to 110 0 C and at pressures ranging from ultra-high vacuum to ambient. Usually, drying will be at temperatures of about 105°C. The drier vessel may 20 be constructed of pure silica, ultra high purity graphite or an inert metal, for example platinum. The actual material of construction may be selected depending on the level of purity, or the nature of the residual impurities, which can be tolerated in the final product.

Suitably, a proportion of the dried purified silicon dioxide obtained from the i:s drying step may be returned to the hydrolyser. When this is done, the crystal size of the dried very high purity silicon dioxide is usually larger than it is when no dried silicon dioxide is returned to the hydrolyser. The crystal size of the purified silicon dioxide product may be further controlled by adjusting the temperature of the hydrolysis reaction.

no In the process according to the second embodiment, the distillation of the liquid streams comprising hydrofluosilicic acid, water and impurities may be achieved at a pressure in the range of about 1 to about 3 atmospheres and at a temperature in the range of 100 0 C to 120°C. Usually, the pressure and temperature will be about 100 kPag and 105 0 C. The distillation vessel may be constructed of any suitable material substantially 3s resistant to hydrogen flucride, silicon tetrafluoride and hydrofluosilicic acid. Such materials include graphite, teflon, acid resistant stainless steel, or natural rubber. In the latter case, the natural rubber may be provided as a lining on a suitable structural material. Usually, the distillation vessel will be constructed of graphite.

Drying of the vapour from the distillation stage may be achieved by contacting the steam-containing vapours with any water absorbing fluoride based substance which is ltibI121039O.doc 7 substantially inert to hydrogen fluor il and silicon tetrafluoride and which is soluble in hydrofluosilicic acid. Generally the difference in the temperature of absorption and desorption of water by the water absorbing substance is a small one. Suitably, the substance absorbs and desorbs water at a temperature less than about 100°C. Generally, the water absorbing substance is potassium fluoride and/or aluminium fluoride.

The steam-containing vapours may be passed through a tower packed with the water-absorbing substance. Suitably, such a tower may be made of polypropylene, polytetrafluoroethylene, natural rubber or graphite. Polypropylene is relatively inexpensive but has a limited life. Graphite is the preferred material.

When its water-absorbing capacity is exhausted, a tower may be taken off-line and heated, which causes the water absorbing substances to be dried, with the formation of a water vapour containing stream which may be vented to air or condensed and run to waste.

Alternatively, the water-containing desiccant may be continuously removed from is the water-absorbing tower and continuously transferred to a heated vessel where the absorbed water is driven off from the desiccant, the dried desiccant then being i. continuously returned to the water absorbing tower. The continuous removal of moist desiccant from the water absorbing tower, and the continuous return of dried desiccant thereto, may be achieved by known methods. A suitable device is the "Floveyor", 20 comprising an endless moving cable equipped with a plurality of discs projecting from it and housed in a tube, supplied by G.P.M. Australia Pty. Ltd.

It will be appreciated that when the removal, regeneration and return of desiccant to the water absorbing tower is continuous, the system is capable of absorbing a greater amount of water and the overall purification process is thereby enhanced.

In the process of the fourth embodiment, the gaseous product stream of purified silicon tetrafluoride is reacted with high purity hydrogen in a hydrogenator. The hydrogenator is suitably an ultra high purity graphite vessel. Suitably, the reaction is conducted at a temperature of at least the melting point of silicon, That is, the hydrogenation reaction may be conducted at any temperature from about 1410 0

C

upwards. Generally, the reaction will be conducted at a temperature of from about 1410 °C to about 1800 0

C.

In the hydrogenation reaction, silicon is produced according to the reaction: SiF 4 2H 2 4HF Si.

The hydrogen fluoride is separated from the silicon, and may advantageously be used in the digestion fluid of step Generally, the hydrogenation reaction will be conducted at or near ambient pressure so as to reduce the likelihood of hydrogen embrittlement of the si,: n produced.

The process of the invention has a number of advantages over known methods for the purification of silicon containing substances. The process of the invention has a relatively low energy requirement and can produce silicon dioxide, for example, of up to lilbz)219399.do 99.999999999% purity. Further, the process of the invention can be adapted to produce a solid or aqueous slurry waste of fluorides of elements other than silicon after removal of such elements from the impure silicon dioxide used as raw material for the process.

The waste product thereby obtained is in a suitable form for disposal, or may itself have commercial value.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a preferred form of the process of the third embodiment.

Figure 2 is a schematic representation of an alternative preferred form of the process of the third embodiment, incorporating the process of the second embodiment.

Figure 3 is a schematic representation of a preferred form of the process of the fourth embodiment.

BEST MODE AND OTHER MODES OF CARRYING OUT THE INVENTION Referring to figure 1, raw silicon dioxide is provided through li e 103 to a.

is digester, and a mixture of hydrogen fluoride and silicon tetrafluoride is provided as digestion fluid to the digester through line 101. Digestion of the raw silicon dioxide by the hydrogen fluoride and silicon tetrafluoride occurs in the digester and the vapour and liquid products are physically separated. Liquid product is removed from the digester by draining through line 103. Vapour from the digester, containing silicon tetrafluoride, S: 20 water vapour and other gases, is led via line 104 to the first of a series of purification vessels. In the process represented in figure 1, three purification stages are depicted, S'.comprising purification vessels 1, 2 and 3 and associated input and outlet lines. Vapours leaving the digester via line 104 are supplied to the first purification vessel. Liquid from the second purification vessel, containing water at least fully saturated with hydrofluo- 25 silicic acid, is supplied to the first purification vessel via line 112. The liquid stream exiting the first purification vessel is led off via line 113, and vapour containing relatively purified silicon tetrafluoride leaving the first purification vessel is led off into the second purification vessel via line 111.

Similarly, the second purification vessel is provided with vapour inlet and outlet lines 111 and 121 respectively and liquid inlet and outlet lines 122 and 112 respectively.

The third purification vessel is provided with vapour inlet and outlet lines 121 and 131 respectively and liquid inlet and outlet lines 132 and 122 respectively. Where more than three purification vessels are used, each (except the first and last in the series) is provided with connections as shown for the second purification vessel.

Vapour exiting the third purification vessel is led via line 131 to a hydrolyser.

High purity water to very high purity water is provided to the hydrolyser via line 142.

High purity silicon tetrafluoride entering the hydrolyser via line 131 is contacted with the water to provide hydrated silic'n dioxide of high to very high purity and a mixture of lib|2s1930.doc 9 hydrofluosilicic acid and water. The hydrated silicon dioxide is separated from the hydrofluosilicic acid and water in the hydrolyser by filtration. The hydrated silicon dioxide is transferred from the hydrolyser via line 141 to a drier, and the mixture of hydrofluosilicic acid and water is transferred via line 132 from the hydrolyser to the third purification vessel. Water vapour produced during the process of drying the hydrated silicon dioxide is removed from the drier via line 153, and dried very high purity silicon dioxide may be taken from the drier via line 152 to a packaging station.

Means is provided via line 151 to recycle a proportion of dried silicon dioxide from the drier to the hydrolyser.

The digester is a stirred vessel. Line 101 for supply of hydrogen fluoride is constructed of teflon. Line 103 is also of teflon, although other suitable plastic, for example polypropylene, may be used. Vapour outlet line 104 is constructed of graphite.

Natural rubber or silica are also suitable to be used. Lines 111 to 113, 121, 122 and 132 are also made of graphite.

"O 15 The final purification vessel in the series is larger than the others and includes a de- Sentrainment space downstream of the liquid/vapour contact space so that any graphite or other particles carried by the flowing vapour stream may be substantially separated before the vapour stream is passed via line 131 to the hydrolyser.

Line 131 and the hydrolyser are constructed of a very high purity silicon dioxide, 20 although ultra pure graphite is also suitable.

The high to very high purity water supplied to the hydrolyser will be of a purity in accordance with the purity requirement for the silicon dioxide end product. The water may be obtained by known purification means. For example, the high to very high purity water may be obtained by reverse osmosis. Addition of high purity water to the hydrolyser is controlled by means of level control on the hydrolyser and by means of conductivity measurement on the stream leaving the hydrolyser via line 132. By controlling the level in the hydrolyser and the quantity of high purity water added via line 142, the conductivity of the solution leaving the hydrolyser via line 132 is maintained at that corresponding to a solution of about 35 to 36 percent by weight of hydrofluosilicic acid in water.

Lines 141, 151 and 152 for transport of the very high silicon dioxide product are constructed of very high purity silica.

In operation, depending on the purity level desired in the very high purity silicon dioxide produced, it may be necessary initially and optionally at intervals thereafter to circulate high purity hydrogen fluoride (anhydrous or 70% w/w aqueous) through the purification vessels to remove surface impurities in the graphite. In order to facilitate this operation, the purification vessels may be each adapted to be bypassed by the inlet and outlet streams and provided with inlet and outlet valves for hydrogen fluoride.

Figure 2 provides a schematic representation of a process according to the third embodiment, incorporating the process of the second embodiment. The process ibil213300.doc comprises a digester, purification vessels, a hydrolyser, a drier and packaging station, together with associated input and output lines which perform the same functions as the correspondingly named vessels described with reference to Figure 1.

The process represented in Figure 2 further comprises a distillation vessel. Liquid phase from the digester, containing hydrofluosilicic acid, water and impurities, is supplied to the distillation vessel via line 203. Additionally, liquid draining from the first purification vessel also containing hydrofluosilicic acid, water and impurities, is supplied to the distillation vessel via line 213. Water vapour and other exhaust from the drier is also transferred to the distillation vessel via line 253 (connection not shown).

The distillation vessel is supplied with a make-up of hydrogen fluoride or hydrofluosilicic acid via line 273. Distillation of hydrofluosilicic acid in the distillation vessel results in thermal decomposition of the hydrofluosilicic acid to produce silicon tetrafluoride, hydrogen fluoride and steam which leave the distillation vessel as vapour via line 271, and are led into a water removal system. The vapour stream exiting the distillation vessel may further comprise gaseous impurities such as chlorine, hydrogen, and volatile compounds of sulphur. The distillation vessel is adapted for removal of the residue remaining from the distillation of the hydrofluosilicic acid, water, and impurities. The distillation residue consists of non volatile fluorides of elements such as aluminium, titanium, potassium, magnesium, etc. The residue is removed as a solid 20 cake, or as a slurry in water, depending on the nature of the impurities and the preferred method of their disposal.

Vapours entering the water removal system via line 271 are contacted in the water removal system with potassium fluoride. The dried vapours containing hydrogen flu )ride, silicon tetrafluoride and gaseous impurities leave the water removal system via line 201 and are supplied thereby to the digester. Water removed in the water removal system is led to exhaust or waste, as liquid or vapour via line 282.

HHydrofluosilicic acid supply lines 203 and 213 are constructed of teflon. Other suitably resistant plastic material, for example, polypropylene can also be used. Vapour outlet line 271 from the distillation vessel is also constructed of teflon. Heating of the distillation vessel may be by steam, electricity or gas. The most economical heating means will be selected depending on the size of the distillation vessel.

Figure 3 provides a schematic representation of the process of the fourth embodiment. In Figure 3, the distillation vessel, water removal system, digester and purification vessels 1-3, together with their associated lines are as in Figure 2, except that the supply of water saturated with hydrofluosilicic acid to the last purification stage is from a separate make-up supply (not shown) via line 333. The purified silicon tetrafluoride from the last purification vessel is supplied via line 231 to a hydrogenator.

The hydrogenator is also equipped with hydrogen inlet via line 345. Silicon produced by the reaction of hydrogen, generally very high purity hydrogen, with silicon tetrafluoride is discharged via line 348 to the packaging station, and gaseous hydrogen fluoride is Ilibil210390.doc separated in the hydrogenator and led via line 302 to the digester (connection not shown).

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Claims (16)

1. A process for producing a purified silicon containing substance which comprises the steps of: reacting impure silica with hydrogen fluoride to produce impure silicon tetrafluoride, contacting said impure silicon tetrafluoride with water at least fully saturated with hydrofluosilicic acid, as hereinbefore defined, to remove at least some of the impurities in said silicon tetrafluoride, and optionally converting purified silicon tetrafluoride from step to another purified silicon containing substance.

2. A process according to claim 1 wherein step is repeated a plurality of times by contacting the purified silicon tetrafluoride produced from a previous step with water at least fully saturated with hydrofluosilicic acid. 3 A process according to claim 2, wherein said water at least fully saturated with hydrofluosilicic acid which is contacted with said silicon tetrafluoride is obtained from another step such that the water at least fully saturated with hydrofluosilicic acid used at each successive step contains a lower concentration of impurities than does the water at least fully saturated with hydrofluosilicic acid at any previous step

4. A process according to claim 1, further comprising the step of distilling a liquid product from step and/or spent hydrogen fluoride from step thereby producing a vapour stream and a waste stream comprising fluorides of elements other than silicon and hydrogen; contacting said vapour stream with a fluoride based water removing material to produce a substantially dried gaseous stream comprising hydrogen fluoride and silicon tetrafluoride; and *supplying said dried gaseous stream as the hydrogen fluoride source for step A process according to claim 4, wherein said fluoride based water removing material comprises potassium fluoride and/or aluminium fluoride.

6. A process for the production of purified silicon dioxide comprising hydrolysing silicon tetrafluoride purified by a process according to any one of claims 1 to 5 with water to produce hydrated silicon dioxide and a hydrolysate and separating said hydrated silicon dioxide from said hydrolysate.

7. A process according to claim 6, wherein said water has been purified by reverse osmosis.

8. A process according to claim 6 or claim 7, further comprising drying said hydrated silicon dioxide.

9. A process according to any one of claims 6 to 8, wherein said hydrolysate is used as said water at least fully saturated with hydrofluosilicic acid in at least one step A process for the production of purified silicon comprising heating silicon o tetrafluoride purified by a process according to any one of claims 1 to 5 with hydrogen lib12t19399.doC to produce purified silicon and hydrogen fluoride, and separating said purified silicon from said hydrogen fluoride.

11. A process according to claim 10 wherein said hydrogen is high purity hydrogen.

12. A process according to claim 1.1 wherein said hydrogen is very high purity hydrogen.

13. An apparatus for producing a purified silicon containing substance, comprising: a digestion vessel comprising means for admitting a digestion fluid, means for admitting silicon dioxide to be purified, means for contacting the digestion fluid with the silicon dioxide, vapour outlet and liquid outlet; and one or more purification vessels, the purification vessels being arranged in series when there is more than one of them, each purification vessel comprising means for admitting a liquid and a vapour, means for contacting the liquid and the vapour, means for separating the contacted liquid and vapour, and liquid and vapour outlet mea.; wherein the vapour outlet means of the digestion vessel communicates with the means for admitting a vapour to the first of the purification vessels in the series, the vapour outlet means of each purification vessel except the last in the series communicates with the means for admitting a vapour to the next purification vessel in the series, and the liquid outlet means of each purification vessel except the first in the series communicates with the means for admitting a liquid to the previous purification vessel in ,0 20 the series.

14. An apparatus according to claim 13, further comprising: a hydrolyser, comprising means for admitting silicon-containing fluid to be hydrolysed, means for admitting water, means for contacting the admitted water and the fluid to t'e hydrolysed, means for separating hydrated silicon dioxide from a liquid hydrolysate and outlet means for the hydrated silicon dioxide and the liquid hydrolysate; and a drier comprising means for admitting hydrated silicon dioxide, means for heating the hydrated silicon dioxide and outlet means for dried silicon dioxide; wherein the vapour outlet means of the last purification vessel in said series 30 communicates with the means for admitting silicon-containing fluid to be hydrolysed of the hydrolyser and the outlet means for hydrated silicon dioxide communicates with the means for admitting hydrated silicon dioxide of the drier. An apparatus according to claim 13, further comprising a hydrogenator, comprising means for admitting silicon-containing fluid to be hydrogenated, means for admitting hydrogen, means for heating the admitted hydrogen and the fluid to be hydrogenated, thereby forming silicon and hydrogen fluoride, means for separating silicon from the hydrogen fluoride and. outlet means for the silicon and the hydrogen fluoride; T qn wherein the vapour outlet means of the last purification vessel in said series .4 communicates with the means for admitting silicon-containing fluid to the hydrogenator. Mlibz1219399.doc

16. A silicon containing substance whenever purified by a process according to any one of claims 1 to 12.

17. Purified silicon dioxide whenever produced by the process of any one of claims 6 to 9.

18. Purified silicon whenever produced by the process of any one of claims 10 to 12.

19. A process for purifying a silicon containing substance, which process is substantially as hereinbefore described with reference to any one of the drawings. A process for the production of purified silicon dioxide, which process is substantially as hereinbefore described with reference to Figure 1 or Figure 2,

21. A process for the production of purified silicon, which process is substantially as hereinbefore described with reference to Figure 3. Dated 25 July, 1995 Larkden Pty. Ltd. "Patent Attorneys for the Applicant/Nominated Person SPRUSON&FERGUSON *fee o *S 0 g llibz1219399,doc ABSTRACT HIGH PURITY SILICON DIOXIDE The invention relates to a process for producing a purified, silicon containing substance which comprises the steps of: reacting impure silica with hydrogen fluoride to produce impure silicon tetrafluoride, and contacting the impure silicon tetrafluoride with water at least fully saturated with hydrofluoililicic acid to remove at least some of the impurities in the silicon tetrafluoride. The process of the invention may be used, for example, to produce very high purity silicon dioxide or very high purity silicon. .I. 14* (Fig. 1)