Patent Application
20180201513
This invention is concerned with the reduction of metal and/or semi-metal oxides. More particularly the invention relates to a method and apparatus adapted to produce silicon by reduction of silicon oxides.
The inventor has determined that the reaction between a strong oxidiser and a reducer can provide sufficient energy for metallothermic reduction of silicon oxides to silicon to be completed at relatively low temperatures, such as less than 580 deg C., and that the reduction can be effected with no or minimal dwell time even at such a maximum temperature. The method can be simple, quick, and efficient without producing greenhouse gases. This method can also be used for reduction of other metal or semi-metal oxides such as for example only Ta2O5, Nb2O5WO3 and MoO2; and also used in the co-reduction of two or more metal or semi-metal oxides to produce alloys and composites of them.
Silicon is the eighth most abundant element in the universe, and the second most abundant in the earth's crust after oxygen. Silicon dioxide (silica) which is commercially used as a resource of silicon is very widely available. Elemental Silicon has a vast array of applications including deoxidising or alloying element for steel, in cast iron and aluminium alloys, raw material in the semiconductor industry (such as in electronic devices, photovoltaic cells, and biosensors), photonics and as a promising anode candidate within rechargeable lithium ion batteries.
The world production of silicon was about 7.7 million tonnes in 2014, from which about 80% was in the form of ferrosilicon with an average 77% silicon content. China was the largest producer of both ferrosilicon (about 6 million tonnes) and elemental silicon (about 1.3 million tonnes)[1].
Silicon is produced industrially in the form of either ferrosilicon or metallurgical-grade silicon. The latter is the precursor for preparation of polycrystalline or solar grade silicon used in the semiconductor and battery industry and also the precursor for preparation of silicon halides useful in the production of silicones.
Traditionally, elemental silicon is produced in an industrial scale by carbothermal reduction of silica in submerged-arc electric furnaces at temperatures of about 2000 deg C. [2]. At this temperature, molten silicon dioxide is reduced to molten silicon, but this process also generates CO2 emissions (reaction 1).
SiO2+C=Si(l)+CO2(g) ΔG°=+103 kJ (at about 2000 deg C.) (reaction 1)
Moreover, carbothermic deoxidization of silicon oxide requires substantial energy (11 kWh/kg of Si), and adversely affects the environment through emission of carbon dioxide. The product of reaction 1 is bulk silicon with a purity of about 95-98% known as metallurgical-grade silicon; and is mainly produced in China, Russia, Brazil, Norway, South Africa, and USA. Metallurgical-grade silicon is typically ground into a powder form for further processing.
Considering the increasing world demand for silicon, and the need to comply with international agreements to reduce carbon emissions, the development of sustainable, green and simpler processes to obtain this element is highly desirable.
The main applications of silicon powder are as follows:
Polysiloxanes (or Silicones) are versatile polymers of silicon and oxygen with carbon and hydrogen, and can be synthesized to exhibit a wide variety of properties as fluids, elastomers or resins, for use in a wide variety of silicone compositions. Silicones can be used in diverse applications such as in implants, skin care products, artificial tears, burn treatments and other wound care, leather finishing, lubricating oils, adhesives, sealants, protective coatings for construction as well as in electrical and electronic products.
In industry, Silicones are produced by reacting pulverized metallurgical-grade silicon with methyl chloride in a fluidised bed to form chlorosilanes at 250 to 350 deg C. and at pressures of 1 to 5 bars; followed by polymerisation and polycondensation.
Global demand for silicones increased from 1.7 to 2.4 million tonnes between 2012 and 2018, representing average annual growth rate of nearly 6 percent. China is the largest producer of silicones and owns nearly 40 percent of global silicone-production capacity. Companies such as Dow Corning and Wacker Chemie are global leaders in silicone production.
Photovoltaics (PV) is a fast growing market with an annual growth rate of PV installations of 44% between 2000 to 2014. Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon used as raw material in the solar photovoltaic and electronics/semiconductor industry. Polycrystalline solar grade silicon is obtained by dissolving metallurgical grade silicon powder in hydrogen chloride generating a silane gas such as trichlorosilane; this is followed by the Siemens Process in which polycrystalline silicon is grown at very high temperatures.
Lithium-ion batteries are widely used as a power source in portable electrical and electronic products. Graphite as a traditional anode material in lithium ion batteries (with a theoretical capacity of 372 mAh g−1) cannot fulfil the requirements of automotive applications needing a high energy density; hence a new generation of high power batteries must be developed using advanced lithium storage materials as electrodes. According to the reactions between silicon and lithium, silicon can be electrochemically alloyed with lithium up to 4.4 atoms of lithium per one silicon atom to form Li22Si5 intermetallic phase. Therefore silicon is considered as the most promising anode material due to its high theoretical specific capacity of 4200 mAh g−1. However, silicon shows severe volumetric changes up to 323% upon lithium insertion and extraction cycling, leading to microcracks or pulverization and therefore poor cyclability. Silicon containing nanocomposites are commonly used to overcome this problem.
Silicon nitride (Si3N4) is a ceramic with an excellent combination of properties including low density, very high fracture toughness, good flexural strength, and very good thermal shock resistance and operating temperature in an oxidizing atmosphere up to about 1300 deg C. These properties make silicon nitride ceramics appropriate candidates for applications as balls and rolling elements for light and extremely precise bearings, heavy-duty ceramic forming tools and automotive components subject to high stress. Si3N4 is prepared by heating powdered silicon between 1300 deg C. and 1400 deg C. in an atmosphere of nitrogen.
Magnesium silicide (Mg2Si) is used as an additive for some important aluminium alloys such as the 6xxx series. Mg2Si is also a lightweight indirect gap narrow band semiconductor that can be used in a range of applications such as thermoelectric applications. The other applications of Mg2Si include reinforcement for composites, anticorrosive coatings, interconnections in silicon planar technology, infrared optical devices, photovoltaic applications, as an alternative for anode materials in rechargeable lithium batteries, and in hydrogen storage.
Direct Production of Si Powder from SiO2
The smelting carbothermic reduction of SiO2 leads to the production of molten silicon. Silicon powder might be produced directly from SiO2 by solid state metallothermic reduction methods. Below is a summary of methods developed for solid state reduction of SiO2.
The reaction of SiO2 with molten aluminium can lead to the formation of silicon and alumina, according to the following reaction [3]:
4Al+3SiO2→2Al2O3+3Si (reaction 2)
However, preparation of Si by this reaction is hindered by its dissolution in the molten aluminium and by formation of an alumina layer which suppresses progression of the reduction process. This process has been adapted for the preparation of alumina reinforced Al—Si composites [4].
Electrochemical deoxidation of SiO2 in molten salt electrolytes was discussed by Nohira et al. [5] and Jin et al. [6]. However, bulk electrodeoxidation of SiO2 is difficult, considering the fact that SiO2 is an insulator.
Magnesiothermic reduction of SiO2 is the most promising alternative method for producing Si (reaction 3) from SiO2 [7-15]. Silica can also be used to prepare Mg2Si—MgO (reaction 4), which can be subsequently used in the production of some composites.
2Mg(g)+SiO2=Si+2MgO ΔG°=−333 kJ (at 700 deg C.) (reaction 3)
4Mg(g)+SiO2=Mg2Si+2MgO ΔG°=−477 kJ (at 700 deg C.) (reaction 4)
The intermetallic compound Mg2Si is an interesting material with a wide range of possible applications such as use as the strengthening phase in metal matrix composites [16], use as a hydrogen storage medium for renewable energy systems [17], use as anode materials for lithium ion batteries [18] and within certain thermoelectric applications [19].
The magnesiothermic reduction of SiO2, however, takes place at relatively high temperatures above 650 deg C. requiring a dwell time at maximum temperature of 2-5 h [7-15]. In fact, this process in which magnesium vapour acts as the reducing agent suffers from poor scalability. Even a very low concentration of oxygen in the reaction atmosphere is sufficient to oxidise magnesium vapour over long process times, therefore special equipment must be used. Hence, simultaneously lowering the temperature of the reaction between SiO2 and Mg whilst reducing the reaction time would significantly increase scalability of the reduction process, being desirable conditions not yet achieved together.
There have been reported attempts towards lowering the reaction temperature of the metallothermic reduction of SiO2. Xing et al. [19] reported that solid silicon monoxide, SiO, which is less stable than SiO2, can be converted into Si when heated with Mg at 500 deg C. for 1-12 h. At such a modest temperature, the reaction could be conducted with a solid mixture of SiO and Mg powders in a flowing H2—Ar mixture within a tube furnace without appreciable magnesium vapour loss [19].
Ning Lin et al. [20] produced Si by reacting SiO2 with AlCl3 and either aluminium or magnesium at temperatures of 200-250 deg C., according to the following reactions:
4Al+3SiO2+2AlCl3→3Si+6AlOCl (reaction 5)
2Mg+SiO2+6AlCl3→2MgAl2Cl8+2AlOCl+Si (reaction 6)
This process, however, required a stainless steel autoclave packed in a N2 glove box, as AlCl3 is very sensitive to moisture. Moreover, the process was performed in a 20 mL autoclave using only about 1 g SiO2 but for which 8 g AlCl3 was required. Although the reaction temperature was found to be 250 deg C., the reaction was prolonged and required 10 h to reach only 75 percent completion.
Other major problems of the available metallothermic reduction methods used to obtain Si relate to the incompleteness of the process. Formation of a layer of silicon on the surface of SiO2 hinders progress of the reduction process, and hence, the core of silica particles may not be converted to Si.
Above 500 deg C., Mg2Si reacts with O2 in air to yield MgO and Si, according to the following reaction [21].
Mg2Si+O2→Si+2MgO (reaction 7)
However, this process is time consuming, and also the resulting Si product itself can be oxidized. For example, Si was produced by air-oxidation of Mg2Si at 600 deg C. for 10 hours [22].
Reaction of Mg2Si with Acids
It is known that Mg2Si dissolves in dilute acids to evolve monosilane [23]:
Mg2Si+2H2SO4=2MgSO4+SiH4(g) ΔG° (at 25 deg C.)=−783 kJ (reaction 8)
The current invention has two aspects:
In a first aspect, the present invention concerns a process for the production of Si and/or Mg2Si from SiO2 in which the reaction temperature is below 580 deg C. and there is virtually no need for a dwell time. For example the reaction temperature can be from 350 deg C. to less than 580 deg C., preferably 360 deg C. to 570 deg C., even more preferably 370 deg C. to 530 deg C.
According to this first aspect of the invention there is provided a method of reducing one or more single or mixed oxides of metal and/or semi-metal other than titanium, which involves use of an initial reaction at a temperature of less than 580 deg C. between a strong oxidising agent or metal halide with a reducing agent to effect reduction of said oxide(s).
This aspect of invention embraces very effective methods for the preparation of Si (for example but without limitation in reaction 9) and Mg2Si (for example but without limitation in reaction 10) from SiO2—containing raw materials, which can take place at a relatively low temperature of 350-580 deg C., the actual reaction temperature being dependent on the SiO2 particle size, with virtually no dwell time, whereby the reaction can be completed immediately or within seconds, at the reaction temperature.
(4+4/x)Mg+2SiO2+1/xKClO4=(4+4/x)MgO+1/xKCl+2Si ΔG°=−[2353/x+542] kJ (at 350 deg C.) (reaction 9)
(4+4/x)Mg+1/xKClO4+SiO2=(2+4/x)MgO+1/xKCl+Mg2Si ΔG°=−[2353/x+344] kJ (at 350 deg C.) (reaction 10)
The temperature of the reduction process can be controlled by the particle size of the metal/semi-metal oxide.
These processing conditions (low temperature of 350-580 deg C. and no, or practically no, dwell time) provide an opportunity for large-scale production of Si using SiO2 and Mg. No vacuum condition, autoclave, or prolonged treatment is required, although in some embodiments a vacuum can be deployed.
Conventionally, when SiO2 reacts with Mg, a layer of Si forms on the SiO2 particles, and therefore in conventional such methods, the core of SiO2 particles cannot be reduced to Si. The reduction process here described and claimed however can be carried through to completion in a remarkably short timescale which means that unlike conventional reactions, the core of SiO2 particles can also be reduced to Si and/or Mg2Si as well as their surface.
Other reducing agents, such as, for example only, Ca and Na and other oxidising agents can potentially be employed.
Preferred oxidising agents include metal perchlorate salts such as potassium perchlorate (KClO4), magnesium perchlorate (Mg(ClO4)2), sodium perchlorate (NaClO4), calcium perchlorate (Ca(ClO4)2) and iron perchlorate (Fe(ClO4)2). The oxidising agent may alternatively be a metal chromate such as barium chromate (BaCrO4) and lead chromate (PbCrO4). The oxidising agent may be a metal oxalate such as magnesium oxalate (MgC2O4), iron oxalate (FeC2O4), copper oxalate (CuC2O4). Oxidising agent may be a metal chlorate such as potassium chlorate (KClO3), sodium chlorate (NaClO3) and magnesium chlorate, Mg(ClO3)2. Oxidising agent may be ammonium dinitramide, ammonium perchlorate or chlorite.
Oxidising agent also may be a metal oxide which is energetically less stable than the oxide form of the reducing agent. Reaction of these oxidiser/reducing agents provides energy to promote the metallothermic reduction of SiO2 at usefully much lower temperature and dwelling time. Therefore oxidiser agent may be a metal oxide such as Fe2O3, Pb2O3, SnO2, AgO, Cu2O and NiO. Reaction of such metals oxides with strong reducing agents such as Mg and Ca may provide sufficient energy, more than the activation energy needed for the reduction of SiO2.
Metal halides include fluoride, chloride, bromide, and iodide. Instead of oxidising agents, we may use a halide agent. A halogeniser agent is a metal halide. In this case, the stability of the halide should be much less than that of the halogen form of the reducing agent used (Mg, Ca, Al etc), so that their reaction can provide sufficient energy to initiate the metallothermic reduction of SiO2. Therefore the halogeniser can be, for example only, FeCl3. In this case the general reaction is:
SiO2/Mg/FeCl3→Si(Fe)/MgCl2/MgO (Reaction 10a)
Si(Fe) represents an alloy of Si and Fe.
The invention also provides in a second aspect a process for the conversion of Mg2Si to Si by acid leaching of Mg2Si for instance: For example only and without limitation:
Leaching of this silicide compound in sulphuric or nitric acids according to the following reations;
Mg2Si+2H2SO4=2MgSO4+2H2(g)+Si ΔG° (at 25 deg C.)=−840 kJ (reaction 11)
Mg2Si+4HNO3=2Mg(NO3)2+2H2(g)+Si ΔG° (at 25 deg C.)=−780 kJ (reaction 12)
It should be noted that in relation to this acid leaching in general and reactions 11 and 12 above, the acid dissolves magnesium oxide/hydroxide MgO (Mg(OH)2) formed in the reaction as per reactions 9 and 10 above.
In order that the invention may be illustrated, more easily appreciated and readily carried into effect by those skilled in the art, embodiments of the invention will now be described purely by way of non-limiting example with reference to the accompanying drawings, graphs and photomicrographs, wherein:
Referring to the drawings, graphs and photomicrographs, the reactor used for the reduction of SiO2 is shown in
The presence of a reaction dampener, such as an inert salt, for example NaCl powder above the reaction mixture and between the crucible and steel container is desirable to damp the shock generated by the reactions in the alumina crucible. It also further protects the reactive mixture and the products from the environment. It is easy to remove, (e.g. by simple aqueous washing) after the reaction has completed without deleterious effect upon the recovered silicon or silicide.
The steel container is placed in a retort furnace equipped with gas inlet and outlet. An argon flow is passed through the steel retort as the retort was heated in a resistance pot furnace, and the temperature was recorded by a thermocouple.
1.1 Low Temperature Conversion of SiO2 to Mg2Si
37 g SiO2 nanoparticles (Sigma Aldrich 637238, 10-20 nm), 51 g Mg chips (Sigma Aldrich 254118, 4-30 mesh), and 4.5 g KClO4 powder (Sigma Aldrich 241830) was mixed and loaded into the reactor shown in
After completion of the reaction, the furnace was turned off and the reactor was left to cool to room temperature. Then, the alumina crucible was removed from the retort and its content was washed with distilled water to remove NaCl and then vacuum filtered. The material obtained (which is called the as-synthesised product) was subjected to x-ray diffraction analysis (XRD).
It should be noted that heating of the as-synthesised product to 630 deg C. in air leads to the formation of Mg2Si and MgO composite powder which is useful in its own right.
Ten gram of the as-synthesised material was gradually added to 100 mL HNO3 (70%) at 50 deg C. while the solution was stirred by a magnet, which led to the release of gas. After 1 h stirring, the solution was diluted by distilled water and vacuum filtered, and then further washed with distilled water. The filtrate (3.8 g) was dried at 50 deg C. overnight. The final product which was 1.8 g light yellowish powder was subjected to XRD analysis, and the result is presented in
The Raman spectrum of the silicon product taken using 633 nm laser excitation wavelength is shown in
The surface properties of the silicon product was studied through the nitrogen adsorption-desorption technique.
SEM and bright field TEM micrograph of the Si powder produced is shown in
12.82 g SiO2 (Sigma Aldrich, 0.5-10 μm, 80% 1-5 μm), 16.44 g Mg chips (Sigma Aldrich 254118, 4-30 mesh) and 3.01 g KClO4 powder (Sigma Aldrich 241830) was mixed and the mixture was placed in an alumina crucible.
The mixture was heated to 530 deg C., and then the reactor was allowed to cool down. Then, the material inside the crucible was aqueously leached with distilled water, to remove NaCl which might be mixed with the product, and filtered. The XRD result of the material obtained is shown in
The filtrate was dried at 30 deg C., and washed with H2SO4 (95%) and HNO3 (70%). For acid washing, first 250 ml of H2SO4 acid was transferred in a 1 L beaker and that beaker was placed in a 2 L beaker. Then, the empty space between the two beakers was filled with ice. The acid was stirred by an application of a magnet stirring system and the as-synthesised product containing Mg2Si and MgO was gradually added to the acid. This addition of the Mg2Si—MgO mixture to the acid solution causes small sparks. The application of an ice bath leads to the control of the temperature and thus minimises any oxidation of silicon produced.
A sample of sand was collected from the beach of Winterton-On-Sea (a village in the English county of Norfolk).
37 g sand of the same sample was dried at 100° C. and mixed with 51 g Mg chips (Sigma Aldrich 254118, 4-30 mesh) and 4.0 g KClO4 powder (Sigma Aldrich 241830). The mixture was placed in an alumina crucible and the powder mixture was further pounded by means of a mallet. The extra space left in the alumina crucible above the reaction mixture was filled with NaCl salt. The crucible was then sealed by means of a ceramic bung, and placed in a second alumina crucible and the gap between the two alumina crucibles until the bung level was filled with additional NaCl. Then a cylindrical copper weight was placed on the ceramic bung.
The alumina crucible was placed in a retort furnace equipped with gas inlet and outlet. An argon flow was passed through the steel retort as the retort was heated in a resistance pot furnace to 570 deg C. The retort was then left to cool down to room temperature, the alumina crucible was removed from the retort and its content was washed with distilled water to remove NaCl and then vacuum filtered. The material obtained was dried under vacuum at room temperature for 1 h. The dried material (which is called the as-synthesised product) was subjected to XRD analysis, and the result can be seen in
5 g of the material obtained was washed with 100 mL H2SO4 with a concentration of 91% in an ice cooled container for 1 h. Then, the acid was diluted by adding distilled water to 20% H2SO4 causing an increase in temperature to 80 deg C. The solid material (with a yellow-dark brown colour) was subsequently washed with HNO3 with concentration of 67% at 50 deg C. Then, the solid material was filtered and the filtrate was dried under vacuum. The XRD result of the product is shown in the
A sample of sand from the same origin as Example 3 was ball milled for 72 h by a low energy rotating ball milling device using a plastic container and alumina balls with the ball:sand ratio of 10:1. The SEM morphology of the milled powder is shown in
37 g ball milled sample was dried at 100° C. and mixed with 51 g Mg chips (Sigma Aldrich 254118, 4-30 mesh) and 4.1 g KClO4 powder (Sigma Aldrich 241830). The mixture was placed in an alumina crucible and the powder mixture was further pounded by means of a mallet. The extra space left in the alumina crucible above the reaction mixture was filled with NaCl salt. The crucible was then capped by means of a ceramic bung. The crucible was placed in a second alumina crucible and the gap between the two alumina crucibles up to the bung level was filled with additional NaCl. Then a cylindrical copper weight (about 1 kg) was placed on the ceramic bung.
The alumina crucible was placed in a steel retort equipped with gas inlet and outlet, and an argon gas flow was maintained through the retort, whilst it was heated in a resistance pot furnace with a heating rate of about 6° C. min−1. The temperature was continuously recorded by the thermocouple attached to the ceramic bung. The temperature-time profile of the run is shown in
12.00 g tantalum pentoxide (Ta2O5, particle sizes 5-10 μm), 2.25 g Mg chips (Sigma Aldrich 254118, 4-30 mesh) and 0.71 g KClO4 powder (Sigma Aldrich 241830) were mixed and the mixture was loaded into an alumina crucible with a diameter of about 3 cm and height of about 8 cm. The crucible was filled with NaCl. Then the alumina crucible was covered by an alumina lid and loaded into a larger alumina crucible of 6 cm in diameter and 11 cm in height. The empty space inside the crucible was filled with NaCl, and the alumina crucible was closed by an alumina lid. The crucible was then loaded into an electric furnace, and heated to 620° C. with a heating rate of 6° C. min−1. The furnace was immediately turned off at the maximum with no dwell time. At the room temperature, the content of the alumina crucible was washed in distilled water and vacuum filtered. The filtrate was dried at 50° C. under a vacuum of 10−4 mbar. The x-ray diffraction pattern of the product is seen in
The processing conditions described in Examples 1-5 (a low processing temperature of 350-580 deg C. and no, or practically no, dwell time) provide an opportunity for large-scale production of metals using metal oxides. No vacuum condition, autoclave, or prolonged treatment is required, although in some embodiments a vacuum can be preferred.
Some preferred embodiments of the present invention:
| Number | Date | Country | Kind |
|---|---|---|---|
| GB1604975.1 | Mar 2016 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2017/076254 | 3/10/2017 | WO | 00 |
