| WO1995033870A1 | 1995-12-14 |
| DE2933164A1 | 1981-02-26 | |||
| US2866701A | 1958-12-30 | |||
| US4292145A | 1981-09-29 |
| 1. | Method for producing high purity silicon metal and A120„ and other valuable species formed in a cathode layer in an electrolysis bath, from alkali alkaline earth alumino silicates preferably feldspars or feldspar containing rocks, dissolved in fluoride, preferably cryolite, by a series of refining steps, having a cathode layer comprising high grade Si, Al2O3, Al, Na and Na2O dissolved in the fluoride, characterised in that said layer is removed from the cathode and treated with water, whereby graphite originating from the cathode layer with some Si float to the top and are decanted off with formation of NaOH solution, H2 gas and a precipitate containing Si, Al2O3, Al(OH)3 and fluoride (cryolite), the precipitate is milled and washed with water, whereafter Al(OH)3 is decanted off with NaOH, the residue is then treated with acid, preferably HCl and/or H2SO4 whereafter the fines, containing cryolite, are decanted off from the Si/Al2O3 solids, which then are separated in a jig, the Si fractions are melted above 1410 °C, treated with CO2 and crystallised when cooled (59), the Al2O3 enriched fraction (14) is treated with Almetal to remove residual Si (11), and can be used for production of pure aluminium, and AlSialloys of all compositions. |
| 2. | Method according to claim 1, characterised in that Si is refined by water and by the formed NaOH, which has a concentration below 0.1 M. |
| 3. | Method according to claims 12, characterised in that AI2O3 is freed from silicon by using the purified Al, and a mixture of purified cryolite and Al2O3 is electrolysed for production of pure Al. |
| 4. | Method according to claims 12, characteri sed in that a Si and graphite mixture is heated above 1410 CC to produce refined Si and/or SiC by bubbling a controlled amount of CO2 through the mixture. |
The present invention concerns a series of refining steps for producing high purity silicon (Si) metal, Al 2 O 3 , hydrogen gas and high purity aluminium (Al), AISi alloys of all compositions, and silicon carbide (SiC), after the required conditions in a electrolysis bath, preferably using feldspars ((Na,Ca,K)Al,. 2 Si 3 . 2 O g ) or feldspar containing rocks dissolved in fluorides, preferably cryolite (Na 3 AlF 6 ).
The high grade Si is produced, deposited together with Al 2 O 3 and Na 2 O dissolved in cryolite in a cathode layer in an electrolysis process (1) (Fig. 1). The species in the cathode layer are refined by water (2) with formation of a sodium hydoxide (NaOH) solution, H 2 gas, and an aluminium hydroxide (AI(OH) 3 ) precipitate (Figs. 2 and 3). The fines, containing Si and graphite (C) floats to the top, and are decanted with the NaOH solution and filtered (Fig. 3) and heated above 1410 °C to produce SiC (1-9) (Fig. 2). The residual precipitate, containing Si, Al 2 O 3 and cryolite is milled, stirred washed and separated with water, all species are treated with HCl and/or H 2 SO 4 and at different temperatures (2-4), The fines, containing cryolite, are decanted, washed, filtered and separated from the Si/AI 2 O 3 deposits, which is then separated in a jig. The enriched Si fraction is melted above 1410 °C, treated with CO 2 and crystallized when cooled (5-6). The Al 2 O 3 enriched fraction (1-4) is treated with Al-metal to remove residual Si (11) (Figs. 2 and 3). The enriched two fractions, one containing cryolite and the other containing Al 2 O 3 , are mixed and treated with Al-metal to remove the residual Si, for then to produce high grade Al-metal by coupling the graphite cathodes as anodes in an Al- electrolysis bath (10). The high grade Si alloyed with high grade Al gives high purity AlSi-alloys (ll).
Methods for production of silicon, silumin and aluminium by electrolysis in a continuous or a batch process from feldspar is described in WO 95/33870, which is the inventors' own application. The production step (step I) (Fig. 1) and refinement steps (1,3 and 4) (Fig. 2) of Si are approximately the same as in the application WO 95/33870. Feldspars dissolved in cryolite are electrolysed wάth a carbon anode placed at the bottom and the
cathode placed at the top (Fig. 1 , step I). The cathode, containing Si deposited in a residual electrolyte is taken out of the bath and placed in water/HCl/H 2 SO 4 mixtures and separated from the crushed electrolyte with organic heavy liquids.
The AlSi-alloys are produced by alumino thermal reduction in step II (Fig. 1) at about 1000°C in WO 95/33870 and Al metal is produced in step III (Fig. 1 ).
SiAl-alloys are produced today by adding Si to AlSi-alloys or Al at 680 to 1410°C.
SiC is produced, today, from carbothermal reduction of quartz at high temperature.
Al 2 O 3 is produced from the well known Bayer process, using bauxite as an raw material.
Al(OH) 3 is an intermediate product formed in this process. NaOH and H 2 are produced from the well known alkali/Cl 2 electrolysis processes containing saturated NaCl in the cell.
Na 2 CO 3 and NaHCO 3 are produced by the well known Solvay process from CO 2 , NH 3 and saturated NaCl.
The present invention produces high purity Si and Al 2 O 3 . Si and Al 2 O 3 are deposited with the compound Na 2 O dissolved in cryolite in a layer deposited at the cathode surface in an electrolysis bath (step I) (Fig. 1), which is the refinement step (1) (Figs. 1 and 2). (The equations are not balanced.)
2 NaAlSi 3 O 8 + 24 e → 6 Si + Na 2 O + Al 2 O 3 (+ Al + Na) (eq.l)
When the feldspar concentration is large in the silicate rock, Al 2 O 3 is deposited in the cathode layer by electrolysis because of saturation. The cathode layer is separated from the electrolyte in the bath. The higher the voltage applied and therefore the current density, the larger the deposition of Al and Na.
The species in the cathode layer are then separated from the CO, flushed and refined graphite cathode (1) by picking, milling and grinding to a fine powder so that the largest parts of the Si grains with purity above 99.7 % Si are almost free from Al metal, Na metal, Al 2 O 3 crystals and of Na 2 O dissolved in cryolite. High applied voltage or high current density favour formation of a separate layer at the cathode of Si metal in the inner part of the layer instead of deposited Al 2 O 3 . The purity of the separated Si ( >99.7 % Si)
grains deposited as solids is not influenced by the voltage applied and formation of Al and Na formed, because they do not form any alloy.
In refinement step (2), water is added to the powder, the mixture is kept at room temperature or tempered and NaOH, Al(OH) 3 and H 2 -gas starts to form. Graphite containing small Si grains floats to the top and is removed by decanting. Al(OH) 3 precipitates as a gel and is separated together with the NaOH solution from the solid precipitated cathode species (Si, cryolite and Al 2 O 3 ) by stirring, decanting and filtering operation. The impurities as Na, K and Ca alloyed with Si reacts with water and dissolve (eq.2).
Elemental P and Al metal alloyed with Si, and Al metal react with NaOH and purify the Si grains.
Na + H 2 O → NaOH + H 2(g) (eq.2)
The NaOH which is formed, has a concentration below 0.1 M.
Al(Si) + NaOH → Al(OH) 3(s) + Si + H 2 (eq.3)
Na 2 O forms partly from electrolysis, where Na metal formed, oxidate in air to form Na 2 O, and is formed by partly released alkali oxides (Na 2 O and K 2 O) from electrolysed and dissolved feldspars in cryolite (eq. l). Ca or Ca 2+ forms CaF 2 in the electrolysis bath (step I) (Fig. 1).
Na 2 CO 3 and NaHCO 3 can be produced by adding self-produced CO 2 from the anodes (steps I and III) (Fig. 1 ) to the solution of NaOH (2).
Then the brittle cryolite is crushed and milled and its suspension is removed by decantation after stirring. The Si and Al 2 O 3 grains are settled in one fraction at the bottom after cryolite is removed (Fig. 3). Si and Al 2 O 3 are then partly separated in a jig (Fig. 3). The milled C from the cathode can be mixed with the Si-rich portion and melted above 1410 °C to produce SiC (7) after various steps (1-4), and (5) (Fig. 2). It is possible in this invention to refine SiC further after steps (3-5), for a short and controlled time, by flushing with CO 2 (8 and 9), to make it more pure (eq.4) (Figs. 2 and 3).
SiC(Al) + CO 2 → SiC + Al 2 O 3 (eq.4)
Either the whole fraction containing Si, Al 2 O 3 and cryolite could be added HCl (3) or just any ofthe enriched fractions with HCl (3) in various concentrations (Figs. 2 and 3). Either the the whole fraction containing Si, Al 2 O 3 and cryolite could be added H 2 SO 4 (4) or just any of the enriched fractions with H 2 SO 4 (4) in various concentrations. Especially high concentrated H 2 SO 4 reacts at high temperature with residual cryolite to produce fluoric acid (HF) (eq.5), which is very active to remove foreign elements in Si. H 2 SO 4 should be added before (4) and after the melting/freezing process of Si and then be washed with water to remove the salt from the Si metal. Either the the whole fraction could be added a mixture of HCl and H 2 SO 4 (3-4) or just any of the enriched fractions with H 2 SO 4 (3-4) with various concentrations and temperature.
Na 3 AlF 6 + H 2 SO 4 → HF + sulfates (eq.5)
After washing the settled fraction of Si or/and the decanted graphite/silicon fraction they are dried and melted and crystallised above 1410 °C in an Ar atmosphere (5) or in a CO 2 atmosphere (6) produced from the anodes (steps I and I1I)( Fig. 1). To get the molten Si in better contact with CO 2 , CO 2 is bubbled through the molten Si before crystallising it (6). The high grade Si produced in (5 and 6) are obtained because of the manner of cryolite (fluoride) to complex foreign elements (5) in the molten state in addition to that the CO 2 has a purifying effect on Si.
By adding the pure Al metal (10) to the Si containing cryolite/Al 2 O 3 rich fractions by stirring and at the temperature about 1000 °C , the cryolite/Al 2 O 3 rich fractions are free from impurities as Fe and Si, which are removed . The purified cryolite/ A1 2 0 3 mixture can be used for production of super pure Al metal by electrolysis (10).
Super pure AlSi-alloys or SiAl-alloys are produced by mixing super pure Si with super pure Al at 580 °C to 1410°C and then solidify the alloys (11) (Fig. 2).
Example 1 :
The Si crystals deposited in the cathode layer is milled and treated with water at different temperature (2). A high grade Si with a purity around 99.99 % Si is obtained.
Example 2:
The Si crystals mixed with a portion of cryolite are melted above 1410 °C (5). A high grade Si crystallised with purity in the range 99.999 - 99.99999 % Si is obtained.
Example 3: The Si crystals mixed with cathode carbons in the fines are treated with water and in addition acids at different temperatures (2) and heated above 1410 °C (7). High grade SiC of purity in the range 99.99-99.999 % SiC is obtained.
Example 4: The milled and crushed cathode layer is added water. After a time NaOH, Al(OH) 3 and H 2 are formed (12). NaOH is dissolved in the water. Al(OH) 3 forms a gelatinous precipitate, which is decanted from the cathode layer compounds, containing Si, Al 2 O 3 and cryolite.
Example 5: High grade Al metal is produced by electrolysis (10) in a cryolite and Al 2 O 3 purified fraction, where iron is removed from the mixture by treating it with HCl and/or H 2 SO 4 . Silicon is removed after treating the cryolite/Al 2 O 3 mixture with Al at about 1000 °C .
Example 6: A mixture of high grade Si and of Al gives a high purity AlSi-alloys and SiAl-alloys (11) with low Fe content, when heated at about 580 to 1410°C and crystallised.
Example 7:
The dissolved NaOH is added CO 2 , produced at the anodes, to form Na 2 CO 3 and NaHCO 3 by addition of equivalent amounts of CO 2 .
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