COUPLED PRODUCTION OF HIGH PURITY SILICON AND ALUMINA

Abstract

The invention relates to a process for the production of silicon and alumina Aluminium is contacted with a molten slag of a calcium oxide and SiO.sub.2 under conditions facilitating an aluminothermic reaction, thereby forming silicon and an aluminate slag in two phases which are separated. The aluminate slag is converted to alumina and calcium oxide, which is re-fed in the reaction. The aluminium is provided by melting of aluminium scrap or a combination of different aluminium alloys at a temperature of 700 to 1000° C. The primary aluminium melt is adjusted to a content of 8 to 14% of silicon and then cooled to below 660° C., whereby precipitates are formed, and high purity aluminium is obtained to be introduced into the reaction.

Claims

1. A process for the production of silicon and alumina comprising the following steps: i. In an aluminothermic step, contacting an aluminium metal with a molten slag comprising an alkaline earth metal oxide and a silicon dioxide, or an alkaline earth metal oxide and a silicon dioxide particularly wherein the alkaline earth metal oxide comprises or essentially consists of calcium oxide, under conditions facilitating an aluminothermic reaction; thereby forming silicon and an alkaline earth metal oxide aluminate slag in separate phases; ii. separating the silicon and the alkaline earth metal oxide aluminate slag in a separation step; iii. in a conversion step, converting the alkaline earth metal oxide aluminate slag to alumina and alkaline earth metal oxide, wherein said alkaline earth metal oxide is re-fed in the aluminothermic step; characterized in that the aluminium metal of the aluminothermic step is provided by a procedure comprising the steps of melting of aluminium scrap or a combination of different aluminium alloys to yield a primary aluminium melt, particularly at a temperature of 700 to 1000° C., more particularly 800° C. to 900° C.; adjusting said primary aluminium melt to a content of 8 to 14% (w/w) of silicon, particularly 11.5% to 12.5% of silicon, more particularly 11.8% to 12.2% of silicon, or to 12.0% of silicon cooling said primary aluminium melt to below 660° C., particularly to 580 to 620° C., over 5 to 50 hrs, whereby a precipitate is formed, and said precipitate is separated from said aluminium melt, whereby a secondary aluminium melt is obtained which is used in the aluminothermic step after solidification or directly in molten form.

2. The process according to claim 1, wherein said molten slag comprising an alkaline earth metal oxide and a silicon dioxide is provided by heating said alkaline earth metal oxide and said silicon dioxide to yield said molten slag, and said aluminium metal is added i.a. as a solid or i.b. as said secondary aluminium melt.

3. The process according to claim 1, wherein said alkaline earth metal oxide and said silicon dioxide are added to said secondary aluminium melt i.c. as a mixture of said alkaline earth metal oxide and said silicon dioxide, or i.d. as a solid slag obtained by forming a molten slag from said alkaline earth metal oxide and said silicon dioxide, and cooling said molten slag to form said solid slag.

4. The process according to claim 1, wherein the content of silicon dioxide relative to the sum of silicon dioxide and alkaline earth metal oxide used in the aluminothermic step ranges from 40% to 88.5% (w/w for alkaline earth metal oxide being CaO), particularly from 47% to 57%, more particularly wherein the content of silicon dioxide relative to the sum of silicon dioxide and alkaline earth metal oxide is approximately 52% (w/w).

5. The process according to claim 1, wherein the aluminothermic step is repeated to remove residual silicon dioxide from the aluminate slag, and to remove residual aluminium from the silicon, by performing one or both of the following steps: i.e. in an aluminothermic silicon workup step, the silicon obtained in the separation step is reacted with a molten slag comprising an alkaline earth metal oxide and a silicon dioxide, or an alkaline earth metal oxide and a silicon dioxide under conditions facilitating an aluminothermic reaction; thereby forming silicon and an alkaline earth metal oxide aluminate slag in separate phases; and/or i.f. in an aluminothermic slag workup step, the alkaline earth metal oxide aluminate slag obtained in the separation step is reacted with an aluminium metal obtained by the procedure specified in steps a, b and c of claim 1 under conditions facilitating an aluminothermic reaction; thereby in each reaction forming silicon of oxidation state 0 and an alkaline earth metal oxide aluminate slag in separate phases; separating the silicon and the alkaline earth metal oxide aluminate slag.

6. The process according to claim 1, wherein the conversion step comprises a Pedersen process step, by which iii.a. the alkaline earth metal oxide aluminate slag is treated with alkali carbonate to yield an alkali aluminate containing solution and an alkaline earth metal carbonate, and iii.b. the alkaline earth metal carbonate is separated from the alkali aluminate containing solution; iii.c. the alkali aluminate containing solution is contacted with carbon dioxide, whereby an aluminium hydroxide precipitate and an alkali carbonate solution are formed; iii.d. the aluminium hydroxide precipitate is collected, iii.e. optionally, the alkali carbonate solution formed in step iii.c. is re-used in step iii.a.

7. The process according to claim 6, wherein the alkali carbonate is sodium carbonate or potassium carbonate, particularly sodium carbonate.

8. The process according to claim 6, wherein the alkaline earth metal carbonate is calcined to yield alkaline earth metal oxide and CO.sub.2.

9. The process according to claim 6, wherein the alkaline earth metal carbonate is converted to an alkaline earth metal hydroxide, wherein this conversion comprises iii.f. contacting the alkaline earth metal carbonate with a transition metal compound, particularly with an iron compound, more particularly with iron(II)carbonate, wherein a reaction mixture is formed, iii.g. contacting of said reaction mixture with an acid, particularly with HCl(aq), iii.h. subsequently adding a base, particularly ammonia or an alkali hydroxide, particularly sodium hydroxide, to said reaction mixture, to attain a pH of 3.0-9.5 but not higher than 9.5, particularly to pH of 7.0-9.5, whereby a first basic solution and a precipitate are formed, iii.i. separating said first basic solution from said precipitate; particularly by filtration; iii.j. subsequently adding a base, particularly ammonia or an alkali hydroxide (particularly sodium hydroxide), to said first basic solution, to attain a pH increase of at least 1, to pH of 9.5-12.5, whereby a second basic solution and an alkaline earth metal hydroxide, (particularly: calcium) hydroxide precipitate, are formed iii.k. isolation of said alkaline earth metal hydroxide (particularly: calcium) hydroxide precipitate, iii.l. calcination of said alkaline earth metal hydroxide, (particularly: calcium) hydroxide precipitate to form said alkaline earth metal oxide.

10. The process according to claim 8, wherein CO.sub.2 generated by calcination of the alkaline earth metal carbonate is re-fed into the Pedersen process step.

11. The process according to claim 6, further comprising a first HCl step, wherein the aluminium(III)hydroxide precipitate is treated with concentrated aqueous hydrochloric acid, which yields a concentrated aluminium chloride solution and a precipitate of AlCl.sub.3 hexahydrate, a dilution step, in which water is added to dissolve the AlCl.sub.3 hexahydrate which yields a diluted aluminium chloride solution, leaving impurities as a solid residue; a first separation step, wherein said impurities are removed from said aluminium chloride solution; a second HCl step, wherein the diluted aluminium chloride solution is treated with gaseous hydrochloric acid to a final concentration of 25-30% HCl, which yields precipitate of AlCl.sub.3 hexahydrate; a second separation step, wherein said precipitate of AlCl.sub.3 hexahydrate is removed and collected; and calcination of the precipitate to yield alumina and hydrochloric acid (gas).

12. The process according to claim 11, whereby the hydrochloric acid (gas) generated in the calcination of the precipitate is re-used in the first HCl step and/or the second HCl step.

13. The process according to claim 1, wherein the alkaline earth metal oxide aluminate slag is directly converted into alumina in a slag conversion process comprising a slag dissolving step, wherein the alkaline earth metal oxide aluminate slag is contacted with aqueous HCl, particularly concentrated aqueous HCl, whereby a solution comprising aluminium(III)chloride and alkaline earth metal chloride is formed, an aluminium(III)chloride hexahydrate precipitation step, comprising contacting the solution formed in the slag dissolving step with concentrated HCl, particularly with gaseous HCl, whereby aluminium(III)chloride hexahydrate is formed and precipitated and whereby the alkaline earth metal chloride remains in solution, separating and calcinating the aluminium(III)chloride hexahydrate precipitate, yielding alumina and gaseous HCl.

14. The process according to claim 13, wherein the alkaline earth metal chloride is converted into alkaline earth metal hydroxide under basic conditions, wherein the alkaline earth metal hydroxide is isolated and calcined to yield alkaline earth metal oxide, which is reused in the aluminothermic step.

15. The process according to claim 1, wherein said primary aluminium melt is a combination of aluminium alloys containing elements selected from Mn, Mg, Cu, Si and Zn or a combination thereof (e.g. “AlSi10MnMg”) or parts made from these alloys further containing so called grain refiners, particularly Al—Ti—B.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0194] FIG. 1 shows a phase diagram of Al and Si. The mass % of Si in an Al/Si composition is represented on the x-axis, whereas the temperature in ° C. is represented on the y-axis. The eutectic point of an Al/Si composition is found at 577° C. for a composition of 12% (w/w) Si.

[0195] FIG. 2 shows a schematic representation of the entire coupled process of the production of high purity silicon and aluminium oxide.

[0196] FIG. 3 shows a schematic representation of the conversion step starting from the alkaline earth metal oxide aluminate slag to aluminium oxide.

[0197] FIG. 4 shows a detailed representation of the Pedersen process and the hydrometallurgical step.

[0198] FIG. 5 shows an aluminothermic reduction work up step, where the separated phases of silicon and alkaline earth metal oxide aluminate slag are each treated again under aluminothermic reduction conditions to bring unreacted aluminium and silicon dioxide to reaction.

[0199] FIG. 6 shows a phase diagram of CaO and SiO.sub.2. The mass % of SiO.sub.2 in the CaO/SiO.sub.2 composition is represented on the lower x-axis, the mol fraction of SiO.sub.2 is represented on the upper x-axis, whereas the temperature in ° C. is represented on the y-axis.

[0200] FIG. 7 shows Silicon made in an aluminothermic process step floating on a Calciumaluminate slag which can be used as a HPA precursor.

[0201] FIG. 2 shows an embodiment of the coupled process of producing silicon and aluminium oxide.

[0202] In certain embodiments, the process starts with the purification of an aluminium metal (1), whereby the aluminium metal may be provided in the form of pure aluminium, in the form of aluminium scrap and in the form of aluminium alloys and mixtures. In one embodiment of the present invention, the aluminium is adjusted to contain further additives, in particular silicon and the alloying elements found in different combinations in aluminium scrap. The aluminium comprising said additives is molten in a high temperature furnace, whereby a primary aluminium melt is formed. This primary aluminium melt is slowly cooled down. During this cooling process, the impurities precipitate and sink to the ground or grow at the crucible walls, whereby a secondary aluminium melt is formed as one phase and the impurities remain in a second phase. The secondary aluminium melt may thus be separated from the impurities by methods comprising decanting, solidification and cutting, filtration, impelling of liquid salt flux and removing flux from melt. These methods are known to a person skilled in the art.

[0203] In a further embodiment of the present invention, the content of the additives in the aluminium is controlled by a control unit, whereby the control unit is configured to quantify chemical elements. In a further embodiment of the present invention, the temperature of the primary and secondary aluminium melt is controlled by a thermocouple during the purification step (1).

[0204] In a second step, silica and an alkaline earth metal oxide, in particular calcium oxide, are molten in a vessel in a furnace, whereby a molten (alkaline earth metal oxide)-silica slag is formed. The secondary aluminium melt of process (1) is added to this molten or solidified slag in molten or solidified form, whereby an aluminothermic reduction (2) starts. During the aluminothermic reduction (2), silicon dioxide is reduced to silicon at the same time as aluminium is oxidized, whereby an alkaline earth metal oxide aluminate slag (4) forms. This reaction is exothermic and thereby heat is generated. In one aspect of the present invention, this reaction is run continuously and the heat required for the reaction is produced by the reaction itself, thereby reducing the total energy consumption of the process.

[0205] One aspect of this reaction is that the alkaline earth metal oxide aluminate slag (4) and silicon form two distinct phases and may be separated mechanically, in particular by decanting, solidification and cutting or draining. The alkaline earth metal oxide aluminate slag (4) and silicon formed during the aluminothermic reduction (2) are treated separately in the further course of the process of the invention.

[0206] Silicon is further purified (3), whereby high purity silicon is obtained. This purification step is however optional, as also silicon of lower purity may be produced within this process.

[0207] Possible purification methods (3) of the silicon comprise the steps of [0208] surface etching of the silicon using an acid [0209] re-melting of the silicon [0210] directional solidification of the molten silicon [0211] top cutting of the solidified silicon [0212] and reusing the top cut.

[0213] The alkaline earth metal oxide aluminate slag (4) is solidified and crushed (5). Possible methods of crushing (5) are milling and sifting.

[0214] The alkaline earth metal oxide aluminate slag (4) is further reacted in a Pedersen process (6) and by hydrometallurgical work up (7) to yield aluminium oxide and alkaline earth metal carbonate. The alkaline earth metal carbonate is purified (8) prior to calcination to alkaline earth metal oxide, which is then re-fed into the process.

[0215] The purification (8) of the alkaline earth metal carbonate involves the conversion of the alkaline earth metal carbonate to an alkaline earth metal hydroxide. The alkaline earth metal carbonate is contacted with HCl in solution, to yield alkaline earth metal chloride and carbon dioxide. The carbon dioxide may be re-fed into the Pedersen process. Further, a transition metal carbonate or a transition metal, in particular iron carbonate or iron, is added to the solution of the alkaline earth metal chloride, whereby an iron containing solution of alkaline earth metal chloride is formed. The pH of this iron containing alkaline earth metal chloride solution is increased, whereby iron containing impurities precipitate. These precipitates are removed from the solution. When the pH is increased further, very pure alkaline earth metal hydroxide precipitates which is converted to alkaline earth metal oxide by calcination (9).

[0216] In FIG. 3, the conversion of the alkaline earth metal oxide aluminate slag (4) to aluminium oxide is further detailed. The solidified slag is crushed, whereby the crushing comprises milling and/or sifting. The slag (4) is solidified and crushed (5) and further treated in a Pedersen process (6), yielding aluminium(III)hydroxide and alkaline earth metal carbonate. Aluminium(III)hydroxide is further reacted in an acid treatment (7) to yield aluminium oxide.

[0217] In FIG. 4, the details of the Pedersen process (6) and the acid treatment (7) are illustrated. In the Pedersen process, the crushed and solidified alkaline earth metal oxide slag (5) is treated in aqueous basic conditions (6a). The basic conditions (6a) are a result of a treatment of the slag with aqueous alkali carbonate, in particular sodium or potassium carbonate, whereby aluminate is dissolved as alkali aluminate. Further, an alkaline earth metal carbonate is formed, which precipitates from the aqueous basic solution. Thus, the alkali aluminate remains in the solution and may be separated from the alkaline earth metal carbonate by methods known to the person skilled in the art. The remaining solution containing the alkali aluminate is treated further with carbon dioxide, whereby aluminium(III)hydroxide precipitates (6b) and alkali carbonate remains in the solution.

[0218] The precipitated aluminium(III)hydroxide may be recovered from the solution by methods known in the art. Particular methods comprise filtration and decanting. Alkali carbonate is recovered from the solution by methods known in the art and re-fed into the process.

[0219] In the hydrometallurgical step (7), the aluminium(III)hydroxide isolated in the Pedersen process (6) is treated with an acid, in particular with HCl (7a), whereby alumium(III)chloride and aluminium(III)chloride hexahydrate form, whereby the aluminium(III)chloride hexahydrate precipitates. The precipitated aluminium(III)chloride hexahydrate is re-dissolved by addition of water, whereby a solution and possibly remaining precipitates are formed, whereby the remaining precipitates are removed from the solution. The solution is treated with further acid (7b), in particular with HCl (gas), to salt out aluminium(III)chloride hexahydrate, whereby aluminium(III)chloride hexahydrate precipitates. The precipitated aluminium(III)chloride hexahydrate is isolated from the solution by methods known in the art. The isolated aluminium(III)chloride hexahydrate is calcined (7c) to aluminium oxide in a two-step procedure, wherein Aluminium(III)hydroxide is formed at temperatures between 300-400° C., particularly at temperatures between 300-350° C., whereby HCl (gas) is recovered. Aluminium(III)hydroxide is converted to aluminium oxide at temperatures in the range 900-100° C., in particular at 1000° C.

[0220] In FIG. 5, an embodiment of the present invention is represented, wherein aluminium and SiO.sub.2 and alkaline earth metal oxide are treated in an aluminothermic reduction (2) after purification of aluminium (1). Separation of the two phases after the aluminothermic reduction yields an alkaline earth metal oxide aluminate slag with residual SiO.sub.2 (4a) and silicon with residual amounts of aluminium (4b). Both phases are separately subjected a second time to an aluminothermic reduction (2a) with either fresh aluminium (in the case of 4a) and fresh SiO.sub.2/alkaline earth metal oxide or fresh SiO.sub.2/alkaline earth metal oxide slag (10) (in the case of 4b) in order to increase the yield of alumina and silicon at the end of the process.

[0221] Wherever alternatives for single features are laid out herein as “embodiments”, it is to be understood that such alternatives may be combined freely to form discrete embodiments of the invention disclosed herein.

[0222] The invention is further illustrated by the following examples and figures, from which further embodiments and advantages can be drawn.

REFERENCE CHARACTERS

[0223] 1 Purification of aluminium [0224] 2 Aluminothermic reduction [0225] 2a Aluminothermic reduction II [0226] 3 Purification of silicon [0227] 4 Alkaline earth metal oxide aluminate slag [0228] 4a Alkaline earth metal oxide aluminate slag with residual SiO.sub.2 [0229] 4b Silicon with residual aluminium [0230] 5 Milling/Stifting [0231] 6 Pedersen process [0232] 6a Alkaline leaching [0233] 6b Precipitation [0234] 7 Hydrometallurgical treatment of aluminium oxide precursor [0235] 7a Acid treatment [0236] 7b Precipitation [0237] 7c Calcination [0238] 8 Purification of alkaline earth metal oxide precursor [0239] 9 Recovery of alkaline earth metal precursor [0240] 10 fresh alkaline earth metal oxide-SiO.sub.2 slag which may be added optionally

EXAMPLES

Example 1

General Process Overview

[0241] A cost efficient way to produce raw silicon precursors with low B, P and other impurities as well as an aluminate precursor with low alkali and other metal content, would greatly simplify additional purification to solar silicon and high purity aluminium oxide. An aluminothermic reduction of SiO.sub.2 sands realizes this, if it is carried out like proposed in the following. [0242] Obtain clean ingredients: [0243] SiO.sub.2 sands low in contaminants are available at the market [0244] Scrap—aluminum or Al—Si can be cleaned from impurities using our method [0245] Then use this Al or Al—Si to reduce SiO.sub.2 to make silicon low in impurities via an aluminothermic reduction. The aluminium will also reduce other metals present in the melt. These metals will stay in the silicon therefore purifying the residual Aluminate from metals. [0246] Use the aluminate-slag (low in metals) created during the aluminothermic reduction as a synthetic precursor to make high purity alumina [0247] Use hydrometallurgical processing to make high purity aluminium oxide from obtained slag and thereby recycle/recover the slag ingredients [0248] Create a coupled production of pure silicon and high purity aluminium oxide with internal recycling loops and low energy consumption.

Example 2

Al—Si Purification

[0249] Melt Al, Al-scrap or a combination of different aluminium alloys with 8-14% (w/w) Si at 800-900° C. [0250] Slowly cool the obtained melt to 580 to 620° C. in 8 to 48h to precipitate out impurities [0251] Separate solids from the clean liquid by the following methods: decanting, solidification and cutting, filtration, impelling of liquid salt flux and removing flux from melt

Example 3

Aluminothermic Reduction

[0252] Melt a pure CaO—SiO.sub.2 slag from lime and silica (approx. 1:1 ratio, variations) [0253] Also possible are MgO—SiO.sub.2 slag, SrO—SiO.sub.2 slag, BaO—SiO.sub.2 slag [0254] add purified Al—Si for aluminothermic reduction [0255] separate the formed Si/metal melt and aluminate slag [0256] might be repeated with resulting Si/metal melt and fresh input of slag [0257] slag of this step can be used a 2.sup.nd time [0258] establish counter-flow principle: fresh slag is reacted with Si-rich melt from a run which was carried out with spent slag and fresh Al—Si as an input

Example 4

Further Processing of Si Isolated from Aluminothermic Reduction

[0259] surface etch/leach Si— chunks from the aluminothermic reduction using HCl, use spent HCl in conversion of CaCO.sub.3 to Ca(OH).sub.2 [0260] remelting of this Si [0261] directional solidification [0262] top-cut the ingot and chunk the material [0263] creation of recycling and re-processing loops within this processing step

Example 5

Processing of Calcium Oxide Aluminate-Slag

[0264] crushing and milling [0265] use Pedersen process to recover CaO (as CaCO.sub.3) and produce Al(OH).sub.3 [0266] dissolve the Al(OH).sub.3 in HCl and follow the HCl route as described in EP0157503B1

Example 6

Pedersen Process (See FIG. 4, Left)

[0267] 1) Crushing and pulverizing of the alkaline earth metal oxide aluminate slag. [0268] 2) Dissolving of 130 g Na.sub.2CO.sub.3 in 1 L of deionized water. [0269] 3) Stirring of 300 g of aluminate slag (4) in the solution prepared in the aluminothermic reduction for 1.5 h at 90° C. [0270] 4) A strongly basic sodium aluminate solution is formed as well as a solid precipitate. The precipitate is expected to contain undissolved impurities as well as CaCO.sub.3. [0271] 5) Removing the precipitate by filtration. [0272] 6) Introducing CO.sub.2(g) into the obtained clear solution of the previous step, causing Al(OH).sub.3 to precipitate. Isolation of the precipitate from the solution by filtration. [0273] 7) Repeating steps 3) to 6) with the remaining basic solution of step 6) and with the precipitate of step 5). No new aluminate slag shall be used in this step, only the precipitates isolated from step 5). Here, further Al(OH).sub.3 shall be isolated from the slag. [0274] 8) Washing and drying of combined isolated Al(OH).sub.3. [0275] 9) The as-obtained Al(OH).sub.3 will be reacted further as described below.

Example 7

Hydrometallurgical Treatment of Aluminium Oxide Precursor (See FIG. 4 Right)

[0276] 1) 156.6 g of Al(OH).sub.3 is dissolved in 610 mL of HCl (aq) (36% (w/w)) under constant stirring at 60° C. [0277] 2) A solution of aluminium(III)chloride is expected to form that reacts slightly acid as well as a solid residue, comprising and essentially consisting of AlCl.sub.3.6H.sub.2O. Al(OH).sub.3 reacts to AlCl.sub.3.6H.sub.2O in the presence of HCl, however, it is not completely soluble given the small amounts of liquid. [0278] 3) 92 mL of H.sub.2O are added in order to dissolve the remaining AlCl.sub.3.6H.sub.2O. The solution is cooled down to room temperature. [0279] 4) Remove remaining solids in the solution by filtration. [0280] 5) HCl (g) is introduced into the solution in order to salt out AlCl.sub.3.6H.sub.2O. However, the concentration of HCl in the solution shall not exceed 30% (w/w), as impure AlCl.sub.3.6H.sub.2O would precipitate consequently. The optimum conditions comprise 25-28.5% (w/w) of HCl in the solution. Precipitated AlCl.sub.3.6H.sub.2O is isolated by filtration. [0281] 6) AlCl.sub.3.6H.sub.2O is washed with concentrated HCl, separated and dried. AlCl.sub.3.6H.sub.2O melts at 100° C. and starts to decompose at this temperature, forming HCl. If further heated, Al(OH).sub.3 is formed, which releases water above 400° C. and reacts to Al.sub.2O.sub.3. [0282] 7) Converting the AlCl.sub.3.6H.sub.2O from step 6 to Al(OH).sub.3 on a hotplate at 300-350° C. under a HCl suitable ventilation hood. [0283] 8) The as-obtained Al(OH).sub.3 is reacted in a furnace at 1000° C. for 1 h to yield Al.sub.2O.sub.3.

Example 8

Further Processing of CaCO.SUB.3 .After Pedersen Process

[0284] adding some FeCO.sub.3or Fe-metal [0285] dissolving in HCl and recover CO.sub.2 for using in step 4 (Pedersen) [0286] separating solids and liquid [0287] increasing pH by NH.sub.4 or NaOH or optional addition to precipitate impurities (Fe-hydroxide, Phosphides, Borides etc.) [0288] separating solids and liquid [0289] fully increase pH to precipitate Ca(OH).sub.2 [0290] separating solids and liquid

Example 9

[0291] A mix of 1380 g CaO and 1620 g SiO.sub.2 was prepared for the process and filled into the crucible made of iso-graphite. 1000 g Al (P1020) was available in 75 g pieces. A Thermocouple was placed into crucible protected by a closed end graphite tube. The crucible was closed with a lid made of graphite containing ports for the Thermocouple and later material addition. Crucible with lid were placed into an induction furnace and heated to 1650° C. to melt down the oxide mix described before. After melting was complete Al-pieces were added (T=1650° C.) causing a reaction which increased temperature to T=1850° C. The system was held at this temperature for 40 minutes to finish the reaction and separate silicon and slag formed. After shutting down power the furnace and crucible cooled down to 700° C. and was ready to remove the crucible.

[0292] FIG. 7 shows some resulting material with silicon formed during the process floating on top of a Calciumalminate slag which is a precursor for HPA.

[0293] An X-ray fluorescence analysis (XRF) of the Calciumaluminate slag is shown in table 1 and documents the superior purity which was obtained by using pure ingredients for the process. It suggests the material is well suited to be further processed into high purity alumina (HPA).

TABLE-US-00001 TABLE 1 XRF-results of the obtained Calciumaluminate slag. Formula Concentration Stat Error(%) Al2O3 60.3% 0.0880 SiO2  4.9% 0.0176 P2O5  0.0% 0.000 SO3  0.0% 0.000 Cl  0.0% 0.000 K2O  0.0% 0.000 CaO 34.2% 0.0241 Sc2O3  0.0% 0.000 TiO2  0.0% 0.000 V2O5  0.0% 0.000 Cr2O3  0.0% 0.000 MnO  0.0% 0.000 Fe2O3  0.4% 0.00219 CoO  0.0% 0.000 NiO  0.0% 0.000 CuO  0.0% 0.000 ZnO  0.0% 0.00019 Ga2O3  0.0% 0.000 GeO2  0.0% 0.000 As2O3  0.0% 0.000 SeO2  0.0% 0.000 Br  0.0% 0.000 Rb2O  0.0% 0.000 SrO  0.0% 0.00006 Y2O3  0.0% 0.00001 ZrO2  0.1% 0.00026 PbO  0.0% 0.00003 MoO3  0.0% 0.00000

Example 10

[0294] Crude silicon may be further refined as described in A. Ciftja, T. A. Engh and M. Tangstad (2008) Refining and Recycling of Silicon: A Review. Norwegian University of Science and

[0295] Technology, Faculty of Natural Science and Technology, Department of Materials Science and Engineering, Trondheim 2008: see particularly section 3.2 “Refining” on pages 11 and 12.

[0296] The crude silicon is tapped as liquid in large ladles (containing up to 10 MT of silicon) and treated when still liquid with oxidative gas and slag-forming additives, mainly silica sand (SiO.sub.2) and lime/limestone (CaO/CaCO.sub.3) to form CaO—SiO.sub.2-slags. Elements less noble than silicon such as Al, Ca and Mg are oxidized and the degree of refining is determined by distribution equilibriums, where the (parentheses) refer to components dissolved in a slag phase and the underscored symbols refer to dissolved elements in liquid silicon:

4Al+3(SiO.sub.2)=3Si(I)+2(Al.sub.2O.sub.3)

2Ca+SiO.sub.2═Si(I)+2(CaO)

2Mg+SiO.sub.2═Si(I)+2(MgO)

Si(I)+O.sub.2═(SiO.sub.2)

[0297] Theoretically it is possible to remove Al and Ca to very low levels, but in practice this is prevented by the large heat losses occurring during this operation. Temperature drops from 1700 to 1500° C., and to avoid freezing of the melt, some of the silica needed for slag formation is provided by direct oxidation of Si(I) in order to heat silicon to keep it liquid. A disadvantage of this operation is the loss of silicon (Ciftja, Engh and Tangstad 2008).

[0298] The above mentioned CaO—SiO.sub.2-slags are an off product from ladle refinements of metallurgical grade silicon (MG-Si) which is produced in high volumes (7.2 million tons per year including ferrosilicon).

[0299] Since these slags still contain a lot of SiO.sub.2 after the MG-Si refinement they can be used in an aluminothermic reduction process to create Calciumaluminate slags being further processed into alumina according the process described herein.