阅读说明：本技术 高纯度的硅和氧化铝的联合生产 (Integrated production of high purity silicon and alumina ) 是由 蒂尔·巴特尔 马蒂亚斯·豪尔 于 2019-10-23 设计创作，主要内容包括：本发明涉及一种生产硅和氧化铝的工艺。在促进铝热反应的条件下,使铝与氧化钙和SiO-2的熔融炉渣接触,由此形成处于分开的两相中的硅和铝酸盐炉渣。使铝酸盐炉渣转化为氧化铝和氧化钙,其被重新投入到反应中。可通过在700℃至1000℃温度下熔化铝屑或不同铝合金的组合来获得铝。将初级铝熔体中的硅含量调节为8％至14％,然后使该初级铝熔体冷却至660℃以下,由此形成沉淀,且得到高纯度的铝,该铝可被投入到反应中。(The present invention relates to a process for producing silicon and alumina. Under the condition of promoting aluminothermic reaction, making aluminium and calcium oxide and SiO 2 Thereby forming the silico-and aluminate slags in separate phases. The aluminate slag is converted to alumina and calcium oxide, which are reintroduced into the reaction. The aluminium may be obtained by melting aluminium scrap or a combination of different aluminium alloys at a temperature of 700 to 1000 ℃. Adjusting the silicon content in the primary aluminium melt to 8% to 14%, and then cooling the primary aluminium melt to below 660 ℃, whereby a precipitate is formed and a high purity aluminium is obtained, which can be put into the reaction.)

1. A process for producing silicon and alumina comprising the steps of:

i. in the thermite step, aluminum metal is contacted with:

slag consisting of alkaline earth metal oxides and silicon dioxide, or

-said alkaline earth metal oxide and said silica

In particular wherein the alkaline earth metal oxide comprises or consists essentially of calcium oxide;

thereby forming silicon and alkaline earth metal oxide aluminate slag in separate phases;

separating the silicon from the alkaline earth metal oxide aluminate slag in a separation step; and

converting the alkaline earth metal oxide aluminate slag into alumina and alkaline earth metal oxide in a conversion step, wherein the alkaline earth metal oxide is reintroduced into the aluminothermic step;

the method is characterized in that:

the aluminium metal in the aluminothermic step is obtained by:

-melting aluminium scrap or a combination of different aluminium alloys, in particular in a temperature range of 700 ℃ to 1000 ℃, more in particular in a temperature range of 800 ℃ to 900 ℃, to produce a primary aluminium melt;

-adjusting the silicon content in the primary aluminium melt to the following level:

a silicon content of 8 to 14% by weight, in particular 11.5 to 12.5% by weight, more in particular 11.8 to 12.2% by weight or 12.0% by weight

-cooling the primary aluminium melt to below 660 ℃, in particular 580 to 620 ℃, over a period of 5-50h, thereby forming a precipitate, and then separating the precipitate from the aluminium melt, thereby obtaining a secondary aluminium melt, which is then used in the aluminothermic step in the following form:

after curing, or

Directly in molten form.

2. The process of claim 1, wherein the slag comprising alkaline earth metal oxide and silica is obtained by heating alkaline earth metal oxide and silica to produce slag, and the aluminum metal is added in the following morphology:

i.a. solid, or

The secondary aluminum melt.

3. The process of claim 1, wherein the alkaline earth metal oxide and the silica are added to the secondary aluminum melt in the form of:

a mixture of the alkaline earth metal oxide and the silica; or

i.d. is added in the form of a solid slag obtained by forming the alkaline earth metal oxide and the silica into a molten slag and then cooling the molten slag.

4. The process according to any one of the preceding claims, wherein the silica is present in a content of between 40% and 88.5% by weight (for the alkaline-earth oxide being CaO), in particular between 47% and 57%, more particularly about 52%, relative to the total amount of silica and alkaline-earth oxide used in the aluminothermic step.

5. The process according to any one of the preceding claims, wherein the aluminothermic step is repeated, removing residual silica from the aluminate slag, and removing residual aluminum from the silicon, by performing one or both of the following steps:

i.e. reacting the silicon obtained in the separation step with alkaline earth metal oxide and silica or a slag comprising alkaline earth metal oxide and silica in a aluminothermic silicon treatment step under conditions promoting aluminothermic reactions, thereby forming silicon and alkaline earth metal oxide aluminate slag in separate phases; and/or

i.f. reacting the alkaline earth metal oxide aluminate slag obtained in the separation step with aluminium metal obtained by steps a, b, c of claim 1 in a aluminothermic slag treatment step under conditions promoting aluminothermic reactions;

thereby forming silicon and alkaline earth metal oxide aluminate slags with an oxidation state of 0 in separate phases in each reaction;

separating the silicon from the alkaline earth metal oxide aluminate slag.

6. The process of any one of the preceding claims, wherein the converting step comprises a Pedersen process step, wherein:

treating the alkaline earth oxide aluminate slag with an alkali carbonate to obtain an alkali aluminate-containing solution and an alkaline earth carbonate;

separating the alkaline earth carbonate from the alkali metal aluminate-containing solution;

contacting the alkali metal aluminate-containing solution with carbon dioxide, thereby forming an aluminum hydroxide precipitate and an alkali metal carbonate solution;

collecting the aluminum hydroxide precipitate; and

optionally, in step iii.a, the alkali metal carbonate solution formed in step iii.c is reused.

7. A process according to claim 6, wherein the alkali metal carbonate is sodium or potassium carbonate, especially sodium carbonate.

8. Process according to any one of claims 6 to 7, wherein the alkaline earth carbonate is calcined to obtain an alkaline earth oxide and CO₂。

9. The process according to any one of claims 6 to 8, wherein the alkaline earth carbonate is converted to an alkaline earth hydroxide, wherein the conversion comprises:

contacting the alkaline earth metal carbonate with a transition metal compound, in particular an iron compound, more in particular iron (II) carbonate, thereby forming a reaction mixture;

contacting the reaction mixture with an acid, in particular HCl (aqueous solution);

subsequently adding a base (especially ammonia or an alkali metal hydroxide, especially sodium hydroxide) to the reaction mixture to bring the pH to 3.0-9.5 (especially 7.0-9.5) but not higher than 9.5, thereby forming a first alkaline solution and a precipitate;

i. separating said precipitate from said first alkaline solution, in particular by filtration;

subsequently adding a base, in particular ammonia or an alkali metal hydroxide, in particular sodium hydroxide, to the first basic solution to increase the pH by at least 1, thereby increasing the pH to 9.5-12.5, thereby forming a second basic solution and a hydroxide precipitate of an alkaline earth metal, in particular calcium;

k. separating off the hydroxide precipitate of the alkaline earth metal, in particular calcium; and

calcining the hydroxide precipitate of the alkaline earth metal, in particular calcium, to form the oxide of the alkaline earth metal.

10. The process according to claim 8 or 9, wherein the CO generated by calcining the alkaline earth metal carbonate₂Is reintroduced into the Pedersen process step.

11. The process of claims 6 to 10, further comprising:

-a first HCl step, in which the aluminium (III) hydroxide precipitate is treated with concentrated aqueous hydrochloric acid, obtaining a concentrated aluminium chloride solution and an aluminium trichloride hexahydrate precipitate;

-a dilution step, in which water is added to dissolve the aluminium trichloride hexahydrate, thus obtaining a diluted aluminium chloride solution, leaving the impurities as solid residues;

-a first separation step, in which said impurities are removed from said aluminium chloride solution;

-a second HCl step, in which the diluted aluminium chloride solution is treated with gaseous hydrochloric acid to a final concentration of HCl of 25% to 30%, thereby producing a precipitate of aluminium trichloride hexahydrate;

-a second separation step, in which the precipitate of aluminium trichloride hexahydrate is separated and collected; and

-calcining the precipitate to produce alumina and hydrochloric acid (gas).

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

13. The process according to any one of claims 1 to 5, wherein the alkaline earth metal oxide aluminate slag is directly converted to alumina in the slag conversion process comprising:

-a slag dissolution step, wherein the alkaline earth metal oxide aluminate slag is contacted with an aqueous HCl solution, in particular concentrated aqueous HCl solution, thereby forming a solution comprising aluminium (III) chloride and alkaline earth metal chloride;

-a precipitation step of aluminium (III) chloride hexahydrate comprising contacting the solution formed in the slag dissolution step with concentrated HCl, in particular gaseous HCl, thereby forming and precipitating aluminium (III) chloride hexahydrate, and leaving the alkaline earth metal chloride in solution; and

-separating and calcining the aluminum (III) chloride hexahydrate precipitate to obtain alumina and gaseous HCl.

14. The process according to claim 13, wherein the alkaline earth chloride is converted to an alkaline earth hydroxide under basic conditions, and the alkaline earth hydroxide is separated and calcined to give an alkaline earth oxide, and which can be reused in the aluminothermic step.

15. Process according to any one of the preceding claims, wherein the primary aluminium melt is an aluminium alloy containing the elements Mn, Mg, Cu, Si and Zn or combinations thereof (e.g. aluminium alloy "AlSi"), or a combination of parts made of these alloys₁₀MnMg ") and the part also contains a so-called grain refiner, especially Al-Ti-B.

Background

The following U.S. patents and patent applications are incorporated herein by reference: US1618105, US4634581 and US4457903.

Terms and definitions

In the context of the present specification, the term "aluminium" relates to pure aluminium as well as to alloys and mixtures containing aluminium. One specific source of aluminum in the present invention is aluminum scrap and another specific source is aluminum dross.

In the context of the present specification, the term "aluminium dross" relates to a mixture of aluminium metal and aluminium oxide obtained during the melting of aluminium. Surface oxidation occurs in the exposed aluminum melt and the resulting surface layer is skimmed off during certain steps of the aluminum process. The skimming contains a significant amount of aluminum metal (up to 70%) and it is therefore necessary to recover the aluminum metal by means of a process for recovering it. One particular form of aluminium dross is the so-called "black dross", which results from a conventional dross reprocessing process in which part of the remaining aluminium metal is recovered. Therefore, the content of aluminum metal in the black slag is low and cannot be further recovered.

In the context of the present specification, the term "silicon" relates to pure silicon as well as to silicon and silicon-containing mixtures containing impurities.

In the context of the present specification, the terms "alumina", "aluminium oxide" and "aluminium (III) oxide" are synonymous and correspond to the general formula Al₂O₃The compound of (1).

The terms "silica" and "silica" are used synonymously in this specification and are identical to the chemical formula SiO₂The compound of (1). The main sources of silica used in the present invention include quartz and quartz sand.

The terms "aluminum hydroxide" and "aluminum (III) hydroxide" are used synonymously in the present description and correspond to the chemical formula Al (OH)₃The compound of (1).

The terms "aluminum chloride" and "aluminum (III) chloride" are used as synonyms in the present description and have the chemical formula AlCl₃The compound of (1).

The terms "aluminium chloride hexahydrate" and "aluminium (III) chloride hexahydrate" are used as synonyms in the present description and are related to the chemical formula AlCl₃·6H₂And O is related to the compound.

In the context of the present specification, the term "alkaline earth metal oxide aluminate slag" relates to a composition comprising an alkaline earth metal oxide and alumina. In some exemplary, but non-limiting, embodiments, the composition may be represented by the general formula (MO)₁₂·(Al₂O₃)₇Wherein M denotes an alkaline earth metal and Al and O have the usual atomic symbol meanings.

The terms "aluminothermic reduction" and "aluminothermic reaction" are used as synonyms in the context of the present specification and relate to the reduction of silica to silicon in the oxidation state 0 in the presence of aluminum, whereby aluminum is oxidized during the reaction.

In the context of the present specification, the term "Pedersen process" relates to the conversion of alkaline earth metal oxide aluminate slag to aluminium (III) hydroxide and alkaline earth metal carbonate.

In the context of the present specification, the term "alkaline earth metal" or "group 2 metal" relates to the alkaline earth metals beryllium, magnesium, calcium, strontium and barium, especially to magnesium, calcium, strontium and barium, even especially to calcium.

In the context of the present specification, the term "transition metal" relates in particular to transition metals of groups 4, 5, 6, 7, 8, 9, 10, 11 of the periodic system of the elements.

In the context of the present specification, the term "alkali metal" especially relates to the alkali metals sodium and potassium.

Drawings

FIG. 1 is a phase diagram of Al and Si. The mass percent of Si in the Al/Si composition is shown on the x-axis and the temperature (in:. degree. C.) is shown on the y-axis. For a composition with 12% by weight of Si, the eutectic point of the Al/Si composition is seen to be 577 ℃.

FIG. 2 is a schematic view of the overall integrated process for producing high purity silicon and alumina.

FIG. 3 is a schematic representation of the steps for conversion from alkaline earth metal oxide aluminate slag to alumina.

Fig. 4 shows the Pedersen process and hydrometallurgical steps in detail.

Figure 5 shows a thermite reduction treatment step in which the silicon and alkaline earth metal oxide aluminate slag in separate phases are treated again under thermite reduction conditions to react unreacted aluminium with silica, respectively.

FIG. 6 shows CaO and SiO₂Phase diagram of (a). SiO 2₂In CaO/SiO₂The mass percent in the composition is expressed on the lower x-axis, SiO₂The mole fraction of (c) is shown on the upper x-axis and the temperature (unit:. degree. c.) is shown on the y-axis.

Fig. 7 shows silicon produced in the aluminothermic process step floating on calcium aluminate slag that can be used as a precursor to HPA.

FIG. 2 illustrates one embodiment of an integrated process for producing silicon and alumina.

In certain embodiments, the process begins with the purification (1) of aluminum metal, and thus the aluminum metal may be provided in the form of pure aluminum, aluminum scrap, aluminum alloys, and mixtures thereof. In one embodiment of the invention, the content of aluminum is adjusted to include further additives, in particular silicon and alloying elements present in the aluminum scrap in different combinations. The aluminum containing the additive is melted in a high temperature furnace, thereby forming a primary aluminum melt. The primary aluminum melt was allowed to cool slowly. During this cooling process, impurities precipitate out and sink to the bottom or grow on the crucible wall, whereby the secondary aluminum melt formed becomes one phase, while impurities remain in the second phase. Thus, the secondary aluminum melt may be separated from impurities by methods including decantation, solidification and cutting, filtration, promotion of liquid salt flux, and removal of flux from the melt. These methods are known to those skilled in the art.

In another embodiment of the invention, the content of the additive in the aluminium is controlled by the control unit, whereby the control unit is configured to quantify the chemical element. In another embodiment of the present invention, the temperature of the primary and secondary aluminum melts is controlled by thermocouples during the purification step (1).

In a second step, silica and alkaline earth metal oxide (especially calcium oxide) are melted in a vessel in a furnace, thereby forming a molten alkaline earth metal oxide-silica slag. The secondary aluminium melt of process (1) is added in molten or solidified form to the molten or solidified slag, whereby aluminothermic reduction (2) is initiated. During aluminothermic reduction (2), the silica is reduced to silicon while the aluminum is oxidized, thereby forming an alkaline earth metal oxide aluminate slag (4). The reaction is exothermic and therefore generates heat. In the present invention, the reaction is carried out continuously and the heat required for the reaction is generated by the reaction itself, thereby reducing the overall energy consumption of the process.

In this reaction, the alkaline earth metal oxide aluminate slag (4) and the silicon form two distinct phases and can be separated mechanically, in particular by decanting, solidifying and cutting or discharging. In a further course of the process of the invention, the alkaline earth metal oxide aluminate slag (4) and the silicon formed during the aluminothermic reduction (2) are treated separately.

Further purifying the silicon (3), thereby obtaining high purity silicon. However, this purification step is optional, as less pure silicon is also produced in the process.

A possible silicon purification method (3) comprises the following steps:

-etching of the silicon surface;

-remelting of silicon;

-directional solidification of molten silicon;

-top cutting of solidified silicon; and

-reusing the top incision.

The alkaline earth metal oxide aluminate slag (4) is solidified and crushed (5). Possible methods of comminution (5) are grinding and sieving.

The alkaline earth metal oxide aluminate slag (4) is further reacted in a Pedersen process (6) and by a hydrometallurgical treatment (7) to produce alumina and an alkaline earth metal carbonate. The alkaline earth carbonate is purified (8) and calcined to the alkaline earth oxide, which is then fed back into the process.

Purification of alkaline earth metal carbonates (8) involves conversion of alkaline earth metal carbonates to alkaline earth metal hydroxides. The alkaline earth metal carbonate is contacted with HCl in solution to produce an alkaline earth metal chloride and carbon dioxide. The carbon dioxide may be reintroduced into the Pedersen process. Furthermore, a transition metal carbonate or a transition metal (especially iron carbonate or iron) is added to the solution of alkaline earth metal chloride, thereby forming an iron-containing solution of alkaline earth metal chloride. The pH of this iron-containing alkaline earth metal chloride solution is raised, thereby precipitating iron-containing impurities. These precipitated precipitates were removed from the solution. When the pH is further increased, the very pure alkaline earth hydroxide precipitates and is converted into alkaline earth oxide by calcination (9).

Figure 3 shows the conversion of the alkaline earth metal oxide aluminate slag (4) to alumina in further detail. The solidified slag is crushed, which crushing comprises grinding and/or sieving. The slag (4) is solidified and crushed (5) and then further processed in the Pedersen process (6) to yield aluminium hydroxide (III) and alkaline earth metal carbonate. Aluminum hydroxide (III) is further reacted in acid treatment (7) to produce aluminum oxide.

Fig. 4 shows the Pedersen process (6) and acid treatment (7) in detail. In the Pedersen process, crushed and solidified alkaline earth metal oxide slag (5) is treated in an aqueous alkaline solution (6 a). The basic conditions (6a) are obtained by treating the slag with an aqueous solution of an alkali metal carbonate, in particular sodium or potassium carbonate, in which the aluminate is dissolved in the form of an alkali metal aluminate. Furthermore, alkaline earth metal carbonates are formed, which precipitate from the alkaline aqueous solution. Thus, the alkali metal aluminate remains in solution and can be separated from the alkaline earth carbonate by methods known to those skilled in the art. The remaining solution containing the alkali metal aluminate is further treated with carbon dioxide, whereby aluminum (III) hydroxide precipitates (6b) and alkali metal carbonate remains in solution.

The precipitated aluminum (III) hydroxide may be recovered from the solution by methods known in the art. Specific methods include filtration and decantation. The alkali metal carbonate is recovered from the solution by methods known in the art and is reintroduced into the process.

In the hydrometallurgical step (7), the aluminium (III) hydroxide separated off in the Pedersen process (6) is treated (7a) with an acid, in particular HCl, whereby aluminium (III) chloride and aluminium (III) chloride hexahydrate are formed, whereby aluminium (III) chloride hexahydrate is precipitated. The precipitated aluminium (III) chloride hexahydrate is redissolved by the addition of water, whereby a solution and possibly a residual precipitate are formed, whereby the residual precipitate is removed from the solution. Treating (7b) the solution with a further acid, in particular HCl (gas), to salt out aluminum (III) chloride hexahydrate, whereby aluminum (III) chloride hexahydrate is precipitated out. The precipitated aluminum (III) chloride hexahydrate is separated from the solution by methods known in the art. The separated aluminum (III) chloride hexahydrate is calcined (7c) in a two-step procedure to alumina, wherein aluminum (III) hydroxide is formed at a temperature of 300 ℃ to 400 ℃ (in particular 300 ℃ to 350 ℃), thereby recovering HCl (gas). The aluminum (III) hydroxide is converted to aluminum oxide at temperatures of 900 ℃ to 100 ℃, in particular 1000 ℃.

FIG. 5 shows an embodiment of the invention in which aluminum and SiO are treated in aluminothermic reduction (2) after purification of aluminum (1)₂And alkaline earth metal oxides. Two-phase separation after aluminothermic reduction to produce a mixture containing residual SiO₂The alkaline earth metal oxide aluminate slag (4a) and silicon (4b) containing residual aluminum. With new aluminium (in the case of 4a) and new SiO, respectively₂Alkaline earth metal oxide or novel SiO₂Alkaline earth metal oxide slag (10) (in case of 4b) a second aluminothermic reduction (2a) of the two phases is carried out in order to increase the yield of alumina and silicon at the end of the process.

Alternatives to the various features are presented herein as "embodiments. It should be understood that these alternatives can be freely combined to form separate embodiments of the invention disclosed herein.

The invention is further illustrated by the following examples and figures, from which further embodiments and advantages of the invention can be derived.

Description of the numbering

1 purification of aluminium

2 aluminothermic reduction

2a aluminothermic reduction II

3 purification of silicon

4 alkaline earth metal oxide aluminate slag

4a contains residual SiO₂Of alkaline earth metal oxide aluminate slag

4b silicon containing residual aluminum

5 grinding/sieving

6 Pedersen process

6a alkaline leach

6b precipitation

Hydrometallurgical treatment of 7 alumina precursors

7a acid treatment

7b precipitation

7c calcination

Purification of 8 alkaline earth metal oxide precursors

9 recovery of alkaline earth metal precursors

10 New alkaline earth metal oxide-SiO may optionally be added₂Slag of furnace

Examples

Example 1 general Process overview

Additional purification of solar silicon and high purity alumina would be greatly simplified by a cost-effective method to produce raw silicon precursors with lower levels of boron, phosphorus and other impurities and aluminate precursors with lower levels of alkali and other metals. This can be achieved by SiO₂Aluminothermic reduction of the sand.

Obtaining pure components:

o low-pollution SiO in the market₂Sand

o Using our method to remove impurities from Al-Si or Al-Si

Then reduction of SiO by aluminothermic reduction using the Al or Al-Si₂To reduce the impurity content in the silicon; the aluminium will also reduce other metals present in the melt which will remain in the silicon, thus removing residual aluminate from the metal;

preparing high purity alumina by using aluminate slag (low in metal content) generated during aluminothermic reduction as a synthesis precursor;

producing high purity alumina from the obtained slag using a hydrometallurgical process, whereby the slag components are recovered; and

the joint production of pure silicon and high-purity alumina is realized by an internal circulation loop and low energy consumption.

EXAMPLE 2 aluminum silicon (Al-Si) purification

Melting aluminum, aluminum scrap or a combination of different aluminum alloys (with a weight percentage content of Si of 8% to 14%) at a temperature of 800 ℃ to 900 ℃;

slowly cooling the obtained melt to 580-620 ℃ within 8-48h to separate out impurities; and

separating the solid from the pure liquid by: decantation, solidification and cutting, filtration, promotion of liquid salt flux and removal of flux from the melt.

Example 3 aluminothermic reduction

Melting pure CaO-SiO with lime and silica₂Slag (ratio of about 1:1, variable);

MgO-SiO may also be used₂Slag, SrO-SiO₂Slag, BaO-SiO₂Slag;

adding pure Al-Si for aluminothermic reduction;

separating the formed silicon/metal melt from the aluminate slag;

possibly repeatedly, thus producing a silicon/metal melt and a new slag;

the slag from the above step can be used a second time; and

establishing a counter-current principle: in the operation process of inputting raw materials of waste slag and new Al-Si, the new slag and the silicon-rich melt react.

Example 4 further treatment of silicon separated from the aluminothermic reduction

Surface etching/leaching of the silicon block by aluminothermic reduction with HCl and CaCO with spent HCl₃Conversion to Ca (OH)₂；

Remelting the silicon;

directional curing;

top cutting the ingot and slicing the material; and

creating a recycle and re-processing cycle in this processing step.

EXAMPLE 5 treatment of calcium oxide aluminate slag

Grinding and milling;

recovery of CaO (as CaCO) using the Pedersen process₃Form (d) and generate Al (OH)₃(ii) a And

mixing Al (OH)₃Dissolved in HCl and worked up according to the HCl route described in EP0157503B 1.

Example 6-Pedersen Process (see left side of FIG. 4)

1) Crushing and grinding the alkaline earth metal oxide aluminate slag;

2) 130g of Na were dissolved in 1L of deionized water₂CO₃；

3) 300g of aluminate slag (4) prepared by aluminothermic reduction is stirred in the solution for 1.5h at the temperature of 90 ℃;

4) forming strong alkaline sodium aluminate solution and solid precipitate; the precipitate may contain undissolved impurities and CaCO₃；

5) Removing the precipitate by filtration;

6) addition of CO to the clarified solution obtained by the previous step₂(g) So that Al (OH)₃Separating out and separating the separated precipitate from the solution by filtration;

7) repeating steps 3) to 6) with the remaining alkaline solution in step 6) and the precipitate in step 5); in this step, no fresh aluminate slag will be used, but only the precipitate separated off from step 5); in this case, further Al (OH) should be separated from the slag₃；

8) Washing and drying the combined separated Al (OH)₃(ii) a And

9) reacting the resulting Al (OH)₃The reaction was further carried out as described below.

Example 7 hydrometallurgical treatment of alumina precursor (see right side of FIG. 4)

1) Stirring was continued at 60 ℃ to add 156.6g of Al (OH)₃Dissolved in 610mL of HCl (aqueous solution) (36% by weight);

2) there is a possibility of forming an aluminium (III) chloride solution which reacts slightly with acids and solid residues including AlCl₃·6H₂O and consisting essentially of; al (OH)₃With AlCl in the presence of HCl₃·6H₂O reaction, but since the amount of liquid is small, Al (OH)₃Can not be completely dissolved;

3) 92mL of H were added₂O to dissolve the remaining AlCl₃·6H₂O, then cooling the solution to room temperature;

4) removing the solids remaining in the solution by filtration;

5) adding HCl (g) to the solution to salt out AlCl₃·6H₂O; however, due to impure AlCl₃·6H₂O is precipitated by too high a concentration of HCl in the solution, so the HCl concentration in the solution should not exceed 30% by weight, and most preferably 25% to 28.5%; the precipitated AlCl is then separated off by filtration₃·6H₂O；

6) Washing AlCl with concentrated hydrochloric acid₃·6H₂O, then separating and drying; AlCl₃·6H₂O melts at a temperature of 100 ℃ and starts to decompose at this temperature, forming HCl; if further heated, Al (OH) is formed₃Which releases water at above 400 ℃ and reacts with Al₂O₃Carrying out reaction;

7) the AlCl in step 6 is placed in a fume hood suitable for HCl on a hot plate at 300 ℃ to 350 ℃₃·6H₂Conversion of O to Al (OH)₃(ii) a And

8) reacting the resulting Al (OH)₃Reacting for 1h in a furnace at 1000 ℃ to generate Al₂O₃。

₃Example 8-after the Pedersen Process onFurther treatment of CaCO

Addition of FeCO₃Or Fe metal;

dissolving in HCl and recovering CO₂For step 4 (Pedersen);

separating solids and liquids;

by NH₄Or NaOH or optional additives to raise the pH to precipitate impurities (iron hydroxides, phosphides, borides, etc.);

separating solids and liquids;

sufficiently raising the pH to precipitate Ca (OH)₂(ii) a And

separate solids and liquids.

Example 9

Preparation of 1380g CaO and 1620g SiO₂Is used in the process and is added to a crucible made of isostatically pressed graphite. 1000g of Al (P1020) can be provided in the form of flakes, each flake weighing 75 g. The thermocouple was placed in a crucible protected by a closed-end graphite tube. The crucible was closed with a graphite lid provided with ports for placing thermocouples and subsequent addition of material.

The covered crucible was placed in an induction furnace and heated to 1650 ℃ to melt the mixture of oxides. After the melting was completed, aluminum flake (T ═ 1650 ℃) was added to cause a reaction, thereby raising the temperature to T ═ 1850 ℃. The system was kept at this temperature for 40min to complete the reaction and the silicon and slag formed were separated. The power was turned off and the furnace and crucible were then cooled to 700 ℃ in preparation for removal of the crucible.

Fig. 7 shows some of the resulting materials, in which the silicon formed during the process floats on top of the calcium aluminate slag, which is a precursor of HPA.

Table 1 shows the X-ray fluorescence analysis (XRF) results of the calcium aluminate slag and records the excellent purity obtained by performing the process using pure components. This shows that it is very suitable for further processing of the material to high-purity alumina (HPA).

TABLE 1 XRF results of the resulting calcium aluminate slags

Molecular formula	Concentration of	Statistical error (%)
			Al2O3	60.3％	0.0880
SiO2	4.9％	0.0176
			P2O5	0.0％	0.000
SO3	0.0％	0.000
			CI	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

The crude Silicon may be further refined as described in "a.ciftja, t.a.enggh and m.tangstad (2008) Refining and recycling of Silicon: a review, Norwegian University of Science and Technology, Faculty of Natural Science and Technology, Department of Materials Science and Engineering, Trondheim 2008: section partial section 3.2 "Refining" on pages 11and 12 ".

Crude silicon in liquid form is tapped from a large ladle (containing up to 10MT silicon) and, while still in liquid form, is treated with oxidizing gases and slag-forming additives, mainly silica Sand (SiO)₂) And lime/limestone (CaO/CaCO)₃) Is treated to form CaO-SiO₂Slag. Elements such as aluminum, calcium and magnesium, which are more noble than silicon, are oxidized and the degree of refining is determined by the distribution equilibrium, where the components dissolved in the slag phase are shown in brackets, and the elements dissolved in the liquid silicon are underlined:

4A1+3(SiO₂)＝3Si(1)+2(Al₂O₃)

2Ca+SiO₂＝Si(1)+2(CaO)

2Mg+SiO₂＝Si(1)+2(MgO)

Si(1)+O₂＝(SiO₂)

theoretically, the aluminum and calcium content can be reduced to very low levels; but in practice this is difficult to achieve due to the large heat losses that occur during this operation. The temperature is reduced from 1700 c to 1500 c and to avoid freezing of the melt, some of the silica required for slag formation can be provided by direct oxidation of Si (1) to heat the silicon to keep it liquid. One disadvantage of this operation is the loss of silicon (Ciftja, Engh and Tangstad 2008).

CaO-SiO as described above₂Slags are ladle refining by-products of metallurgical grade silicon (MG-Si) and are produced in high quantities (720 million tons/year, including ferrosilicon).

Because these slags still contain a large amount of SiO after MG-Si refining₂And thus can be used in a aluminothermic reduction process to produce a calcium aluminate slag, and the calcium aluminate slag can be further processed to alumina according to the processes described herein.

Detailed Description

The invention relates to a process for producing silicon and aluminum oxide from aluminum and/or aluminum-containing secondary sources and silicon dioxide, in particular silicon dioxide in the form of quartz sand, comprising the following steps:

in the thermite step, aluminum metal is contacted with:

slag consisting of alkaline earth metal oxides and silicon dioxide, or

Alkaline earth metal oxides and silica

Thereby forming silicon and alkaline earth metal oxide aluminate slags in the oxidation state 0 in separate phases;

separating the silicon from the alkaline earth metal oxide aluminate slag in a separation step;

in a conversion step, converting the alkaline earth metal oxide aluminate slag into alumina and alkaline earth metal oxide, wherein the alkaline earth metal oxide is reintroduced into the process of the aluminothermic step;

the method is characterized in that:

the aluminium metal in the thermite step is obtained by:

a. melting aluminum scrap, aluminum dross or a combination of different aluminum alloys, especially in a temperature range of 700 ℃ to 1000 ℃, more especially in a temperature range of 800 ℃ to 900 ℃, to produce a primary aluminum melt;

b. adjusting the silicon content in the primary aluminum melt to the following levels:

a silicon content of 8 to 14% by weight, in particular 11.5 to 12.5% by weight, more in particular 11.8 to 12.2% by weight or 12.0% by weight

c. Cooling the primary aluminium melt to below 660 ℃, in particular 580 ℃ to 620 ℃, over a period of 5-50h, thereby forming a precipitate, then separating the precipitate from the cooled aluminium melt, thereby obtaining a secondary aluminium melt, which is then used in the aluminothermic step in the following form:

after curing, or

Directly in molten form.

In certain embodiments, the aluminum metal in the aluminothermic step is provided by aluminum scrap, aluminum dross, a combination of different aluminum alloys, or a combination of these aluminum sources.

In certain embodiments, the aluminothermic step is performed with aluminum metal, wherein the aluminum metal is obtained by melting aluminum scrap or a combination of different aluminum alloys:

in the thermite step, aluminum metal is contacted with:

slag consisting of alkaline earth metal oxides and silicon dioxide, or

Alkaline earth metal oxides and silica

Thereby forming silicon and alkaline earth metal oxide aluminate slags in the oxidation state 0 in separate phases;

separating the silicon from the alkaline earth metal oxide aluminate slag in a separation step;

in a conversion step, converting the alkaline earth metal oxide aluminate slag into alumina and alkaline earth metal oxide, wherein the alkaline earth metal oxide is reintroduced into the process of the aluminothermic step;

the method is characterized in that:

the aluminium metal in the thermite step is obtained by:

a. melting aluminum scrap or a combination of different aluminum alloys, especially in a temperature range of 700 ℃ to 1000 ℃, more especially in a temperature range of 800 ℃ to 900 ℃, to produce a primary aluminum melt;

b. adjusting the silicon content in the primary aluminum melt to the following levels:

a silicon content of 8 to 14% by weight, in particular 11.5 to 12.5% by weight, more in particular 11.8 to 12.2% by weight or 12.0% by weight

c. Cooling the primary aluminium melt to below 660 ℃, in particular 580 ℃ to 620 ℃, over a period of 5-50h, thereby forming a precipitate, then separating the precipitate from the cooled aluminium melt, thereby obtaining a secondary aluminium melt, which is then used in the aluminothermic step in the following form:

after curing, or

Directly in molten form.

Advantageously, the silicon content of the primary aluminum melt is first determined and Si is added to about 12% by weight thereof relative to the Al content. At this time, the melting point of Al-Si system was the lowest (eutectic point was 577 ℃ C., see FIG. 1). It is desirable to cool the melt as much as possible in order to precipitate as much as possible of the compounds formed by the impurities. These compounds formed from impurities have low residue solubility at low temperatures. Subsequently, the precipitate and the melt are separated from each other, whereby an extremely pure aluminum melt is obtained, while impurities are removed as precipitate.

The aluminum scrap contains different alloys depending on the previous use of the material. These alloys contain unique alloying elements to impart desirable mechanical properties to the aluminum article. For example, there are very common alloys containing Mn, Mg, Cu, Si, Zn (e.g. "AlSi₁₀MnMg ") and parts made from these alloys also contain so-called grain refiners, such as Al-Ti-B and the like.

In certain embodiments, a particular combination of the alloys are eutectic and then cooled, thereby resulting in an intermetallic phase (e.g., Al-Ti-Si compounds) or boride (e.g., TiB)₂) And (4) precipitating. Other elements (e.g., Mg) are selectively oxidized and transferred as MgO into the dross at the top of the melt. Thus, the combination of the above alloys, eutectic and cooling can produce a purification effect due to the low residue solubility of compounds formed from impurities at low temperatures.

The separation of the precipitate from the cooled melt is an important component of the aluminium purification process. If the precipitates remain in the melt, they will re-dissolve during the next temperature increase (e.g., during aluminothermic reduction) and release precipitated impurities.

The order of steps a and b (obtaining the primary melt, combining the scrap alloy and adjusting the Si content) is not critical; these steps may also be carried out in parallel or interchangeably, as long as Al and a certain amount of Si and other alloying elements can be co-melted at high temperatures, in particular at temperatures between 800 ℃ and 900 ℃. All elements must be able to dissolve in the aluminium melt at high temperatures in order to precipitate later on when cooling with impurities in step c. Only solids floating in the melt are ineffective.

In addition to the aluminum or aluminum scrap treated in the above steps a to c, the aluminum dross or "black dross" can also be used directly as the aluminum source in the above aluminothermic step without any pretreatment of the aluminum dross.

In certain embodiments, alkaline earth metal oxide and silica are heated to produce a slag, thereby obtaining the slag comprising the alkaline earth metal oxide and the silica, and the aluminum metal is added in step i in the following form:

a solid consisting of a secondary aluminium melt, or

i.b. said secondary aluminium melt.

Alternatively, if both components (slag and aluminum) are present in the form of a melt, aluminum may be provided and an alkaline earth metal oxide slag may be added thereto.

In certain embodiments, the alkaline earth metal oxide and the silica are added to the secondary aluminum melt in the following form:

i.c. in the form of a mixture of the alkaline earth metal oxide and the silica, in other words, providing and adding the two components separately, after which the two components form a mixture in the melt, or, advantageously, mixing the two components in the form of granules at ambient temperature and adding them together; or

i.d is added in the form of a solid slag obtained by forming the alkaline earth metal oxide and the silica into a molten slag, and then cooling and grinding the molten slag into lumps or particles.

In certain embodiments, the silica is present in an amount of 40 to 88.5% by weight (for the alkaline earth oxide being CaO), particularly 47 to 57%, and more particularly about 52%, relative to the total amount of silica and alkaline earth oxide used in the thermite step.

In certain embodiments, the aluminothermic step comprises a treatment step for one or both of the two products (silicon and slag). During this treatment, the product of the initial thermite step is reacted with fresh reactants to remove impurities. Specifically, the aluminothermic step is repeated to remove residual silica from the aluminate slag and residual aluminum from the silicon by performing one or both of the following steps:

i.e. reacting the silicon obtained in the separation step with alkaline earth metal oxide and silica or with a slag comprising alkaline earth metal oxide and silica in a aluminothermic silicon treatment step under conditions promoting aluminothermic reactions, thereby forming silicon and alkaline earth metal oxide aluminate slag in separate phases; and/or

i.f reacting the alkaline earth metal oxide aluminate slag obtained in the separation step with aluminium metal obtained by steps a, b, c of claim 1 in a aluminothermic slag treatment step under conditions promoting aluminothermic reactions.

In each treatment reaction, the silicon and alkaline earth metal oxide aluminate slag in the oxidation state 0 form in separate phases. The silicon is then separated from the alkaline earth oxide aluminate slag. In certain embodiments, the silicon resulting from the aluminothermic silicon treatment step is further treated using methods known in the art to obtain ultra-high purity silicon.

In certain embodiments, the alkaline earth metal oxide aluminate slag from the aluminothermic slag treatment step is further processed in a conversion step to obtain high purity alumina.

When the thermite step is repeated using the product of the previous thermite step, the yield of alumina and silicon at the end of the process can be increased because unreacted materials are converted to the corresponding products.

In certain embodiments, the converting step comprises a Pedersen process step, wherein:

iii.a treating the alkaline earth metal oxide aluminate slag with an aqueous solution of an alkali metal carbonate to obtain an alkali metal aluminate-containing solution and an alkaline earth metal carbonate remaining as a precipitate;

iii.b separating the alkaline earth carbonate from the alkali metal aluminate-containing solution;

iii.c contacting the alkali metal aluminate-containing solution with carbon dioxide, thereby forming an aluminum hydroxide precipitate and an alkali metal carbonate solution;

iii.d collecting the aluminum hydroxide precipitate; and

iii.e optionally, in step iii.a, the alkali metal carbonate solution formed in step iii.c is reused.

In certain embodiments, the alkali metal carbonate is sodium carbonate.

In certain embodiments, the alkali metal carbonate is potassium carbonate.

In certain embodiments, the alkaline earth metal carbonate is calcined to obtain an alkaline earth metal oxide and CO₂。

In certain embodiments, the alkaline earth carbonate is converted to an alkaline earth hydroxide in an aqueous solution.

In certain embodiments, the alkaline earth carbonate is converted to an alkaline earth hydroxide, wherein the conversion comprises:

contacting an alkaline earth metal carbonate in an aqueous solution with a transition metal compound, especially an iron compound, more especially iron (II) carbonate, thereby forming a reaction mixture;

iii.g. contacting the reaction mixture with an acid, especially HCl (aq), whereby alkaline earth metal chloride, especially CaCl, is formed₂) A solution in which a transition metal chloride (in particular FeCl) is dissolved in a concentration of 0.1 to 6%₂) Thereby forming CO₂；

iii.h subsequently adding a base (especially ammonia or an alkali metal hydroxide, especially sodium hydroxide) to the reaction mixture to bring the pH to 3.0-9.5 (especially 7.0-9.5) but not higher than 9.5, thereby forming a first alkaline solution and a precipitate of iron-containing compound;

i separating said precipitate from said first alkaline solution, in particular by filtration;

j subsequently adding a base, in particular ammonia or an alkali metal hydroxide, in particular sodium hydroxide, to the first alkaline solution to increase the pH by at least 1, thereby increasing the pH to 9.5-12.5, thereby forming a second alkaline solution and an alkaline earth metal hydroxide precipitate, in particular a calcium hydroxide precipitate;

k separating off the alkaline earth metal hydroxide precipitate (especially the calcium hydroxide precipitate); and

calcining the alkaline earth metal hydroxide precipitate, in particular the calcium hydroxide precipitate, to obtain an alkaline earth metal oxide, in particular calcium oxide.

In general, this step of the process promotes group 2 metal carbonates (particularly CaCO)₃) Solubilization in acids, especially aqueous HCl. CO thus produced₂Can be reintroduced into the process during the Pedersen process step to reduce CO in the process₂And (4) discharging the amount. The solution thus produced is a group 2 metal salt solution, in particular a group 2 metal chloride salt solution; if the alkaline earth metal oxide used is calcium oxide, the resulting solution is CaCl₂And (3) solution.

Addition of a transition metal compound, especially iron (II) carbonate, to FeCl₂Is 2% to 4%, wherein 2% to 4% is relative to saturated FeCl₂In terms of the volume of the solution (which is added to the aqueous solution of the alkaline earth metal carbonate). Iron may also be added to the alkaline earth metal carbonate in the form of metallic iron, which is subsequently converted to an iron salt upon contact with an acid.

In step iii.h, when the pH of the alkaline earth metal salt (especially calcium chloride) solution is raised, mainly the impurities in the mixture precipitate as insoluble iron compounds or co-precipitate with ferric hydroxide (as a flocculant) during what can be considered as fractional precipitation, while the alkaline earth metal components remain in solution at pH values below 9.5, so that the impurities can be removed. The pH of the alkaline earth metal solution, in particular the calcium-containing solution, is then further increased to precipitate the alkaline earth metal, in particular calcium, as a hydroxide which is very pure as a product of the process and can thus be reused in the thermite step after drying and calcination. In certain embodiments, the CO is₂Reintroduced into the Pedersen process.

In certain embodiments, the process based on the Pedersen process further comprises a hydrometallurgical step, comprising:

-a first HCl step, in which the separated aluminium (III) hydroxide precipitate is treated with a concentrated aqueous hydrochloric acid solution, obtaining a concentrated aluminium chloride solution and an aluminium trichloride hexahydrate precipitate;

-a dilution step, in which water is added to dissolve the aluminium trichloride hexahydrate, thus obtaining a diluted aluminium chloride solution, leaving the impurities as a solid residue;

-a first separation step, in which said impurities are removed from said aluminium chloride solution;

-a second HCl step, in which the diluted aluminium chloride solution is treated with gaseous hydrochloric acid to a final concentration of HCl of 25% to 30%, thereby producing a precipitate of aluminium trichloride hexahydrate;

-a second separation step, in which the precipitate of aluminium trichloride hexahydrate is separated and collected; and

-calcining the precipitate to produce alumina and hydrochloric acid (gas).

In certain embodiments, hydrochloric acid (gas) is reintroduced into the process.

In certain embodiments, the alkaline earth metal oxide aluminate slag obtained in the aluminothermic step is directly converted to aluminum (III) chloride hexahydrate in a slag conversion process. The process that can replace the Pedersen step includes the following steps:

-contacting the alkaline earth metal oxide aluminate slag with an aqueous HCl solution, in particular concentrated aqueous HCl solution, thereby forming a solution comprising aluminium (III) chloride and alkaline earth metal chloride, in a slag dissolving step;

-in the precipitation step of aluminium (III) chloride hexahydrate, increasing the HCl concentration in the solution formed in the slag dissolution step by contacting it with concentrated HCl, in particular gaseous HCl, thereby forming and precipitating aluminium (III) chloride hexahydrate, while alkaline earth metal chlorides remain in solution; and

-finally, the aluminium (III) chloride hexahydrate precipitate is separated and calcined, obtaining alumina and gaseous HCl, and this HCl can be reused in the precipitation step of aluminium (III) chloride hexahydrate or in the previous slag dissolution step. In this way, the alkaline earth metal oxide aluminate slag obtained in the aluminothermic step is not treated in the Pedersen process step, but is directly converted into aluminium (III) chloride hexahydrate and subsequently calcined into alumina. After all the slag is dissolved, the solution formed in the slag dissolving step may still be weakly acidic. Since alkaline earth metal chlorides (especially calcium chloride) have a higher solubility in high-concentration HCl than aluminum (III) chloride hexahydrate, the alkaline earth metal chlorides remain in solution during the precipitation step of aluminum (III) chloride hexahydrate.

In one embodiment, the precipitation step of aluminum (III) chloride is repeated by redissolving in water the aluminum (III) chloride hexahydrate separated off in the precipitation step and contacting the solution obtained with gaseous HCl. The aluminum (III) chloride hexahydrate precipitate obtained from this repeated process is particularly pure.

In another embodiment, the alkaline earth chloride dissolved after the precipitation step is converted to an alkaline earth hydroxide under alkaline conditions, thereby converting the alkaline earth chloride to an alkaline earth oxide. When the pH rises, alkaline earth metal hydroxides can precipitate, which can be separated and calcined to give alkaline earth metal oxides, and the alkaline earth metal oxides can be fed back to the aluminothermic step.

In this way, long purification steps can be eliminated, making the process more resource and energy efficient.

In certain embodiments, the process further comprises purifying the separated silicon, the purifying comprising the steps of:

-etching of the silicon surface;

-remelting of silicon;

-directional solidification of molten silicon;

-top cutting of solidified silicon; and

-reusing the top cut in the thermite step.

In certain embodiments:

the purity of the silicon obtained by the process is greater than 96%, in particular greater than 99%, in particular greater than 99.99%, even in particular greater than 99.9999%; and

the purity of the alumina obtained by the process is greater than 96%, in particular greater than 99%, even in particular greater than 99.99%.

In certain embodiments, the alkaline earth metal oxide comprises or consists essentially of calcium oxide.

100% CaO is most practical in view of cost and availability. Oxides of other group 2 elements, such as SrO, BaO, or mixtures thereof, may also be used. MgO produces slag having a relatively high melting point and the use of MgO is limited to few, if any, applications.

The process of the invention is particularly energy efficient. The aluminothermic reduction step requires high temperatures; however, since the process can be carried out continuously, the thermal energy generated during the exothermic thermite reaction can be reused in the process, i.e. for melting the raw materials. Thus, part of the energy required for the process is generated during the process without the use of an external energy source.

In addition, the invention also saves resources. The key components of the process, including alkaline earth metal oxides, carbon dioxide, alkali metal carbonates and hydrochloric acid, can be regenerated during the process and can be reintroduced into the reaction cycle. Thus, there is little waste and non-recoverable resources in the production process.

This combined production of silicon and alumina therefore differs from the prior art in that:

the impurities in the aluminum-silicon alloy can be reduced through one-time treatment;

using such purified aluminium-silicon alloys, silicon is obtained by pure feed materials, for example, in a thermite reduction process;

-obtaining high purity alumina by a combination of the Pederson process and hydrometallurgical steps using slag produced during the aluminothermic reduction;

-optionally recovering, purifying and recycling other slag components;

-the joint production of pure silicon and high purity alumina is achieved by means of an internal circulation loop and low energy consumption; and

possible calcination of Ca (OH)₂To produce pure CaO.

A second aspect of the invention relates to a process for producing silicon and alumina from aluminium dross and silica, in particular in the form of silica sand, the process comprising the steps of:

a process for producing silicon and alumina comprising the steps of:

i. in the thermite step, aluminum metal is contacted with:

slag consisting of alkaline earth metal oxides and silicon dioxide, or

Alkaline earth metal oxides and silica

Especially wherein the alkaline earth metal oxide comprises or consists essentially of calcium oxide;

thereby forming silicon and alkaline earth metal oxide aluminate slag in separate phases;

separating the silicon from the alkaline earth metal oxide aluminate slag in a separation step; and

converting the alkaline earth metal oxide aluminate slag into alumina and alkaline earth metal oxide in a conversion step, wherein the alkaline earth metal oxide is reintroduced into the aluminothermic step;

the method is characterized in that:

the aluminum metal is provided from aluminum dross.

In certain embodiments of the second aspect of the present invention, the aluminium metal in the aluminothermic step is obtained by a process comprising the steps of:

-melting the aluminium dross to obtain a primary aluminium melt, in particular at a temperature of 700 ℃ to 1000 ℃ (more in particular at 800 ℃ to 900 ℃);

-adjusting the silicon content in the primary aluminium melt to the following level:

a silicon content of 8 to 14% by weight, in particular 11.5 to 12.5% by weight, more in particular 11.8 to 12.2% by weight or 12.0% by weight

-cooling the primary aluminium melt to below 660 ℃, in particular 580 ℃ to 620 ℃, over a period of 5-50h, thereby forming a precipitate, and then separating the precipitate from the aluminium melt, thereby obtaining a secondary aluminium melt, which is then used in the aluminothermic step in the following form:

after curing, or

Directly in molten form.

In certain embodiments of the second aspect of the present invention, the alkaline earth metal oxide and silica are heated to produce a slag, thereby obtaining the slag comprising the alkaline earth metal oxide and the silica, and the aluminum metal is added in the following form:

i.a. solid, or

The secondary aluminum melt.

In certain embodiments of the second aspect of the present invention, the alkaline earth metal oxide and the silica are added to the secondary aluminum melt in the following form:

a mixture of the alkaline earth metal oxide and the silica; or

i.d. is added in the form of a solid slag obtained by forming the alkaline earth metal oxide and the silica into a molten slag and then cooling the molten slag.

In certain embodiments of the second aspect of the present invention, the silica is present in an amount of 40 to 88.5% by weight (for the alkaline earth oxide being CaO), especially 47 to 57%, more especially about 52%, relative to the total amount of silica and alkaline earth oxide used in the thermite step.

In certain embodiments of the second aspect of the present invention, the aluminothermic step is repeated to remove residual silica from the aluminate slag and residual aluminum from the silicon by performing one or both of the following steps:

i.e. reacting the silicon obtained in the separation step with alkaline earth metal oxide and silica or with a slag comprising alkaline earth metal oxide and silica in a aluminothermic silicon treatment step under conditions promoting aluminothermic reactions, thereby forming silicon and alkaline earth metal oxide aluminate slag in separate phases; and/or

i.f. reacting the alkaline earth metal oxide aluminate slag obtained in the separating step with the aluminium metal obtained by steps a, b, c as described in the embodiments of the second aspect of the invention in a aluminothermic slag treatment step under conditions promoting aluminothermic reactions;

thereby forming silicon and alkaline earth metal oxide aluminate slags with an oxidation state of 0 in separate phases in each reaction;

the silicon is separated from the alkaline earth metal oxide aluminate slag.

In certain embodiments of the second aspect of the present invention, the converting step comprises a Pedersen process step by which:

treating alkaline earth metal oxide aluminate slag with an alkali metal carbonate to obtain an alkali metal aluminate-containing solution and an alkaline earth metal carbonate;

separating the alkaline earth carbonate from the alkali metal aluminate-containing solution;

contacting the alkali metal aluminate-containing solution with carbon dioxide, thereby forming an aluminum hydroxide precipitate and an alkali metal carbonate solution;

collecting aluminum hydroxide precipitate; and

optionally, in step iii.a, the alkali metal carbonate solution formed in step iii.c is reused.

In certain embodiments of the second aspect of the present invention, the alkali metal carbonate in the converting step is sodium or potassium carbonate, especially sodium carbonate.

In certain embodiments of the second aspect of the present invention, the alkaline earth metal carbonate is calcined in the conversion step to produce an alkaline earth metal oxide and CO₂。

In certain embodiments of the second aspect of the present invention, the alkaline earth carbonate is converted to an alkaline earth hydroxide in the converting step, wherein the converting comprises:

contacting an alkaline earth metal carbonate with a transition metal compound, in particular an iron compound, more in particular iron (II) carbonate, thereby forming a reaction mixture;

contacting the reaction mixture with an acid, in particular HCl (aqueous solution);

subsequently adding a base (especially ammonia or an alkali metal hydroxide, especially sodium hydroxide) to the reaction mixture to bring the pH to 3.0-9.5 (especially 7.0-9.5) but not higher than 9.5, thereby forming a first alkaline solution and a precipitate;

i. separating said precipitate from said first alkaline solution, in particular by filtration;

subsequently adding a base, in particular ammonia or an alkali metal hydroxide, in particular sodium hydroxide, to the first basic solution to increase the pH by at least 1, thereby increasing the pH to 9.5-12.5, thereby forming a second basic solution and a hydroxide precipitate of an alkaline earth metal, in particular calcium;

k. separating off the hydroxide precipitate of the alkaline earth metal, in particular calcium; and

calcining the hydroxide precipitate of the alkaline earth metal, in particular calcium, to form the oxide of the alkaline earth metal.

In certain embodiments of the second aspect of the present invention, the CO in the conversion step is generated by calcining an alkaline earth metal carbonate₂And reintroduced into the Pedersen process step.

In certain embodiments of the second aspect of the present invention, the converting step further comprises:

-a first HCl step, in which the aluminium (III) hydroxide precipitate is treated with a concentrated aqueous hydrochloric acid solution, obtaining a concentrated aluminium chloride solution and an aluminium trichloride hexahydrate precipitate;

-a dilution step, in which water is added to dissolve the aluminium trichloride hexahydrate, thus obtaining a diluted aluminium chloride solution, leaving the impurities as a solid residue;

-a first separation step, in which said impurities are removed from said aluminium chloride solution;

-a second HCl step, in which the diluted aluminium chloride solution is treated with gaseous hydrochloric acid to a final concentration of HCl of 25% to 30%, thereby producing a precipitate of aluminium trichloride hexahydrate;

-a second separation step, in which the precipitate of aluminium trichloride hexahydrate is separated and collected; and

-calcining the precipitate to produce alumina and hydrochloric acid (gas).

In certain embodiments of the second aspect of the present invention, hydrochloric acid (gas) generated during calcination of the precipitate is reused in the first and/or second HCl steps.

In certain embodiments of the second aspect of the present invention, the alkaline earth metal oxide aluminate slag is directly converted to alumina in a slag conversion process comprising:

-a slag dissolution step, in which the alkaline earth metal oxide aluminate slag is contacted with an aqueous HCl solution, in particular concentrated aqueous HCl solution, thereby forming a solution comprising aluminium (III) chloride and alkaline earth metal chloride;

-a precipitation step of aluminium (III) chloride hexahydrate comprising contacting the solution formed in the slag dissolution step with concentrated HCl, in particular gaseous HCl, thereby forming and precipitating aluminium (III) chloride hexahydrate, and leaving the alkaline earth metal chloride in solution; and

-separating and calcining the aluminum (III) chloride hexahydrate precipitate to obtain alumina and gaseous HCl.

In certain embodiments of the second aspect of the present invention, the alkaline earth metal chloride is converted to an alkaline earth metal hydroxide under basic conditions, and the alkaline earth metal hydroxide is separated and calcined to give an alkaline earth metal oxide, and which can be reused in the aluminothermic step.