Patent Application
20220073361
The present invention relates to a process for the production of silicon and alumina (aluminium oxide), particularly for the production of silicon and alumina in high purity. The process of the invention is energy and resource saving in comparison to existing processes for the production of high purity silicon and alumina, as it comprises internal recycling loops to recover and reuse key materials of the process.
Solar cells play a key role in the transition of energy generation from non-renewable resources to energy generation from emission free and renewable resources. High purity aluminium oxide and silicon are resources that are required in the production of light emitting diodes and solar cells. However, their generation is according to the state of the art cost- and energy intensive.
In order to be usable in the solar cell production, silicon needs to be purified from other elements to achieve at least 6N purity (which refers to 99.9999% silicon). This has prompted the development of complicated chemical purification processes like the Siemens process. Alternatively impurities can be removed readily either via simple metallurgical treatments such as slag treatments or segregation during directional solidification. The development of metallurgical purification of silicon has been developed in the last decades to provide cost efficient methods to produce high volumes of material.
High purity aluminium oxide is required in the electronic industry, for example in the production of LEDs or displays. Aluminium oxide is commonly produced with hydrometallurgical methods but only in 98-99% purity, which is not sufficient for use in electronic applications that necessitate purity above 99%. In particular, the use in electronics is hampered by an excessive concentration of the elements sodium, iron and other impurities carried in from the production process using natural raw material.
Currently, aluminium oxide is transformed to aluminium metal, purified and then oxidized to obtain high purity aluminium oxide. This is wasteful, expensive and inefficient.
Based on the above mentioned state of the art, the objective of the present invention is to provide means and methods of a process for the coupled production of silicon and aluminium oxide in high purity. This objective is attained by the claims of the present specification.
The following US-patents and patent applications are incorporated herein by reference: U.S. Pat. Nos. 1,618,105, 4,634,581, and 4,457,903.
Terms and Definitions
The term aluminium in the context of the present specification relates to pure aluminium as well as to alloys and mixtures containing aluminium. One particular source of aluminium of the present invention is aluminium scrap. Another particular source of aluminium of the present invention is aluminium dross.
The term aluminium dross in the context of the present specification relates to a mixture of aluminium metal and aluminium oxide which is obtained during melting of aluminium. Every open aluminium melt exhibits a surface oxidation and this surface layer is skimmed off in certain steps of aluminium processing. The skimming product contains a lot of aluminium metal (up to 70%) and therefore is subject to methods for recovering aluminium metal. One particular form of aluminium dross is the so called “black dross” which is coming from a conventional dross re-working process where part of the residual aluminium metal was recovered. Black dross has therefore a lower content of aluminium metal and normally can't be further recycled.
The term silicon in the context of the present specification relates to pure silicon as well as to silicon that contains impurities and mixtures comprising silicon.
The terms alumina, aluminium oxide and aluminium(III)oxide in the context of the present specification are used synonymously and relate to the compound of the general formula Al2O3.
The term silicon dioxide and silica are used synonymously within the present specification and relate to the compound of the formula SiO2. Main sources for silicon dioxide used in the present invention comprise quartz as well as quartz sand.
The terms aluminium hydroxide and aluminium(III)hydroxide are used synonymously within the present specification and relate to the compound of the formula Al(OH)3.
The terms aluminium chloride and aluminium(III)chloride are used synonymously within the present specification and relate to the compound of the formula AlCl3.
The terms aluminium chloride hexahydrate and aluminium(III)chloride hexahydrate are used synonymously within the present specification and relate to the compound of the formula AlCl3.6H2O.
The term alkaline earth metal oxide aluminate slag in the context of the present specification relates to a composition comprising an alkaline earth metal oxide and an aluminium oxide. In some exemplary (but not limiting) embodiments, the composition can be described by a general formula (MO)12.(Al2O3)7, whereby M refers to the alkaline earth metal, and Al and O have the common atomic symbol meanings.
The terms aluminothermic reduction and aluminothermic reaction are used synonymously in the context of the present specification and relate to the reduction of silicon dioxide to silicon of oxidation state 0 in the presence of aluminium, whereby aluminium is oxidized during this reaction.
The term Pedersen process in the context of the present specification relates to a conversion of an alkaline earth metal oxide aluminate slag to aluminium(III)hydroxide and alkaline earth metal carbonate.
The term alkaline earth metal or group 2 metal in the context of the present specification relates to the alkaline earth metals beryllium, magnesium, calcium, strontium and barium, particularly to magnesium, calcium, strontium and barium, even more particularly to calcium.
The term transition metal in the context of the present specification relates in particular to transition metals from groups 4, 5, 6, 7, 8, 9, 10 and 11 of the periodic system of elements.
The term alkali metal in the context of the present specification relates in particular to the alkali metals sodium and potassium.
The invention relates to a process for the production of silicon and alumina from aluminium and/or secondary aluminium containing sources and silicon dioxide, in particular in the form of quartz sand comprising the following steps:
In an aluminothermic step, contacting an aluminium metal with
characterized in that
In certain embodiments, the aluminium metal of the aluminothermic step is provided by aluminium scrap, aluminium dross and a combination of different aluminium alloys or a combination of the previous.
In certain embodiments, the aluminothermic step is performed with aluminium metal, wherein the aluminium metal is provided by melting of aluminium scrap or a combination of different aluminium alloys:
In an aluminothermic step, contacting an aluminium metal with
characterized in that
Advantageously, the content of silicon in the primary aluminium melt is determined initially, and silicon is added to about 12% Si (w/w) with respect to the Al content. At this point, the Al—Si system has the lowest melting point (eutectic point at 577° C., see
Aluminium scrap comes in different alloys depending on the former use of the material. These alloys contain distinct alloying elements to provide ideal mechanical properties of the aluminium products. For instance there are very common alloys containing Mn, Mg, Cu, Si and Zn (e.g. “AlSi10MnMg”) and parts made from these alloys further containing so called grain refiners, like Al—Ti—B and others.
In certain embodiments, specific combinations of mentioned alloys are co-melted and then cooled afterwards which results in the precipitation of intermetallic phases like Al-Ti-Si-compounds or borides like TiB2. Other elements like Mg are selectively oxidized and transferred into the dross on top of the melt in form of MgO. Therefore the combination, co-melting and cooling of the mentioned alloys results in a purification effect, because compounds formed from impurities have a low residual solubility at low temperature.
The separation of precipitates and cooled melt is an important part of the process of purifying the aluminium. If the precipitates remain in the melt, they would re-dissolve during the next temperature elevation (e.g., during aluminothermic reduction) and release the precipitated impurities.
The sequence of steps a and b (providing the primary melt, combining the scrap alloys and adjusting of the Si content) is not important; the steps can also be performed in parallel or interchanged, as long as the target of joint melting of Al and a quantity of Si and other alloying elements is reached at high temperature, particularly at 800-900° C. All elements must be dissolved at high temperature in the Al melt in order to be precipitated subsequently upon cooling together with the impurities in step c. Solids floating only in the melt, would be ineffective.
Alternatively to the aluminium or aluminium scrap which is processed in the above mentioned steps a to c an aluminium dross or “black dross” can be used directly as an aluminium source for the above mentioned aluminothermic step without any pre-processing of the aluminium dross.
In certain embodiments, 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 in step i
Alternatively, if both components (slag and aluminium) are present as a melt, the aluminium may be provided and the alkaline earth metal oxide slag may be added thereto.
In certain embodiments, said alkaline earth metal oxide and said silicon dioxide are added to said secondary aluminium melt
In certain embodiments, 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, the content of silicon dioxide relative to the sum of silicon dioxide and alkaline earth metal oxide is approximately 52% (w/w).
In certain embodiments, the aluminothermic step comprises a workup step for one of the two products silicon and slag, or for both. In this workup, the products of a primary aluminothermic step are reacted with fresh reactant to remove impurities. Specifically, 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:
In each workup reaction, silicon of oxidation state 0 and an alkaline earth metal oxide aluminate slag are formed in separate phases. The silicon and the alkaline earth metal oxide aluminate slag are then separated. In certain embodiments, the silicon derived of the aluminothermic silicon workup step is further processed by methods known in the art to yield ultra-high purity silicon.
In certain embodiments, the alkaline earth metal oxide aluminate slag derived of the aluminothermic slag workup step is further processed in the conversion step to yield high purity alumina.
Repetition of the aluminothermic step with the products from a previous aluminothermic step increases the yield of alumina and silicon at the end of the process, as unreacted species are converted to the respective product.
In certain embodiments, the conversion step comprises a Pedersen process step, by which
In certain embodiments, the alkali carbonate is sodium carbonate.
In certain embodiments, the alkali carbonate is potassium carbonate.
In certain embodiments, the alkaline earth metal carbonate is calcined to yield alkaline earth metal oxide and CO2.
In certain embodiments, the conversion of alkaline earth metal carbonate to an alkaline earth metal hydroxide proceeds in aqueous solution.
In certain embodiments, the alkaline earth metal carbonate is converted to an alkaline earth metal hydroxide, wherein this conversion comprises
calcination of said calcium hydroxide precipitate), yields the alkaline earth metal oxide (in particular calcium oxide).
In principle, this step of the process facilitates solubilisation of the group 2 metal carbonate, particularly CaCO3, in acid, particularly in aqueous HCl. The CO2 generated thereby can be re-fed into the process in the Pedersen process step in order to cut CO2 emissions of the process. The solution generated thus is a group 2 metal salt solution, particularly a group 2 metal chloride salt solution; if the alkaline earth metal oxide employed is calcium oxide, the solution generated is a CaCl2 solution.
The transition metal compound, particularly iron (II) carbonate, is added to a final content of 2-4% FeCl2, wherein the 2-4% is in relation to the volume of a saturated FeCl2 solution being added to the alkaline earth metal carbonate in aqueous solution. The iron can also be added as metallic iron to the alkaline earth metal carbonate, and will then convert to an iron salt upon exposure to the acid.
Upon raising the pH of the solution of the alkaline earth metal salt, particularly calcium chloride, in step iii.h, primarily impurities present in the mixture are precipitated as insoluble iron compounds, or co-precipitate with iron hydroxide as a flocculating agent in a process that can be seen as a fractionated precipitation, while the alkaline earth metal component remains in solution at below pH 9.5, which allows removing of the impurities. The pH of the alkaline earth metal, in particular calcium containing solution, is then further raised and the alkaline earth metal, in particular calcium, is precipitated as hydroxide, which as a result of the process is very clean and can be re-employed in the aluminothermic step after drying and calcination.
In certain embodiments, CO2 is re-fed into the Pedersen process.
In certain embodiments, the process based on the Pedersen process further comprises a hydrometallurgical step, comprising
In certain embodiments, the hydrochloric acid (gas) is re-fed into the process.
In certain embodiments, the alkaline earth metal oxide aluminate slag obtained in the aluminothermic step is directly converted into aluminium(III)chloride hexahydrate in a slag conversion process. This process alternative, which can be employed instead of the Pedersen step, comprises the following steps:
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 to alumina. The solution formed in the slag dissolving step may still be slightly acid after all slag has been dissolved. The alkaline earth metal chloride remains in solution in the aluminium(III)chloride hexahydrate precipitation step, as alkaline earth metal chlorides, particularly calcium chloride, have a higher solubility than aluminium(III)chloride hexahydrate at elevated concentrations of HCl.
In one embodiment, the aluminium(III)chloride precipitation step is repeated by re-dissolving the aluminium(III)chloride hexahydrate isolated in the precipitation step in water and contacting the obtained solution was gaseous HCl. The precipitated aluminium(III)chloride hexahydrate obtained by this repeat process is particularly pure.
In another embodiment, the alkaline earth metal chloride is converted into alkaline earth metal oxide by conversion of the dissolved alkaline earth metal chloride after the precipitation step to alkaline earth metal hydroxide under basic conditions. The alkaline earth metal hydroxide may be precipitated at elevated pH, isolated and calcined, yielding alkaline earth metal oxide, which may be re-fed into the aluminothermic step.
In this way, the process is even more resource and energy saving, as long purification steps may be omitted.
In certain embodiments, the process further comprises a purification of the separated silicon comprising the steps of
In certain embodiments,
In certain embodiments, the alkaline earth metal oxide comprises or essentially consists of calcium oxide.
100% CaO is the most practical example due to cost and availability. Other group 2 element oxides might be employed, such as SrO, BaO or mixtures thereof. Employment of MgO will be limited to few if any applications, as the melting point of the resulting slag is high.
The process of the present invention is particularly energy efficient. The aluminothermic reduction step requires high temperatures; however, as this process may be run continuously, the heat energy generated during the exothermic aluminothermic reaction may be re-used to run the process, i.e. to melt the starting materials. Hence, part of the energy required for the process is generated during the process and does not need to be provided by external sources.
Further, the present invention is also particularly resource saving. Key components of the process, comprising the alkaline earth metal oxide, carbon dioxide, alkali carbonate and hydrochloric acid may be re-generated during the processes and may be re-fed into the reaction cycles. Therefore, the production process is practically free of waste and non-recycled resources.
The coupled production of silicon and alumina thus differs from existing processes by
A second aspect of the invention relates to a process for the production of silicon and alumina from aluminium dross and silicon dioxide, in particular in the form of quartz sand comprising the following steps:
In certain embodiments of the second aspect of the invention, the aluminium metal of the aluminothermic step is provided by a procedure comprising the steps of
In certain embodiments of the second aspect of the invention, 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
In certain embodiments of the second aspect of the invention, said alkaline earth metal oxide and said silicon dioxide are added to said secondary aluminium melt
In certain embodiments of the second aspect of the invention, 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).
In certain embodiments of the second aspect of the invention, 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:
In certain embodiments of the second aspect of the invention, the conversion step comprises a Pedersen process step, by which
In certain embodiments of the second aspect of the invention, the alkali carbonate in the conversion step is sodium carbonate or potassium carbonate, particularly sodium carbonate.
In certain embodiments of the second aspect of the invention, the alkaline earth metal carbonate in the conversion step is calcined to yield alkaline earth metal oxide and CO2.
In certain embodiments of the second aspect of the invention, the alkaline earth metal carbonate in the conversion step is converted to an alkaline earth metal hydroxide, wherein this conversion comprises
In certain embodiments of the second aspect of the invention, CO2 in the conversion step is generated by calcination of the alkaline earth metal carbonate is re-fed into the Pedersen process step.
In certain embodiments of the second aspect of the invention, the conversion step further comprises
In certain embodiments of the second aspect of the invention, 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.
In certain embodiments of the second aspect of the invention, the alkaline earth metal oxide aluminate slag is directly converted into alumina in a slag conversion process comprising
In certain embodiments of the second aspect of the invention, 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.
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.
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).
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.
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.
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.
Possible purification methods (3) of the silicon comprise the steps of
The alkaline earth metal oxide aluminate slag (4) is solidified and crushed (5). Possible methods of crushing (5) are milling and sifting.
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.
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).
In
In
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.
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.
In
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.
The invention is further illustrated by the following examples and figures, from which further embodiments and advantages can be drawn.
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 SiO2 sands realizes this, if it is carried out like proposed in the following.
A mix of 1380 g CaO and 1620 g SiO2 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.
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).
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
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.
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 (SiO2) and lime/limestone (CaO/CaCO3) to form CaO—SiO2-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(SiO2)=3Si(I)+2(Al2O3)
2Ca+SiO2═Si(I)+2(CaO)
2Mg+SiO2═Si(I)+2(MgO)
Si(I)+O2═(SiO2)
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).
The above mentioned CaO—SiO2-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).
Since these slags still contain a lot of SiO2 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18202168.3 | Oct 2018 | EP | regional |
This is the U.S. National Stage of International Patent Application No. PCT/EP2019/078946 filed on Oct. 23, 2019, which in turn claims the benefit of European Patent Application No. 18202168.3 filed on Oct. 23, 2018.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/078946 | 10/23/2019 | WO | 00 |
