Silicon sheets, silicon ingots, and silicon carbide (SIC) from silica-rich indisutrial wastes

Abstract

A method for obtaining silicon sheets, silicon ingots, and silicon carbide (SiC) from silica-rich industrial wastes such as steel slag, steel sludge, refractory bricks, wherein the silica-rich industrial wastes are first dissolved in a strong acid to separate the salts and minerals other than silica, and then the precipitate is powdered and dissolved in a very strong acid to form a silicon-rich solution, wherein the silicon-rich solution is electrochemically reduced in an electrocatalytic reactor to elemental Si, wherein Si is deposited on a carbon-based cathode surface, and the Si sheets are peeled off from the carbon-based cathode surface Allowing sufficient deposition of the Si on the carbon-based cathode surface creates thicker Si sheets, from which Si ingots are obtained. The Si deposited carbon-based cathode when subjected to thermal treatment at high temperature, solid phase diffusion of C and Si results in the formation of SiC.

Description

[0001] TITLE SILICON SHEETS, SILICON INGOTS, AND SILICON CARBIDE (SiC) FROM SILICA-RICH INDISUTRIAL WASTES CROSS REFERENCE TO RELATED APPLICATIONS The present application is based upon and claims priority to India complete patent application number 202441024270 filed on May 21, 2024, which claims priority to India provisional patent application number 202441024270 filed on March 27, 2024, the entire contents of which are herein incorporated by reference. FIELD The present disclosure relates to obtaining silicon sheets, silicon ingots, and silicon carbide (SiC) from silica-rich industrial wastes. BACKGROUND The global demand for silicon ingots is primarily driven by the growth of the semiconductor industry for the manufacture of microchips for electronic devices, as well as for the solar energy market for manufacturing of solar cells. Silicon ingots are preferred for their high-purity levels and uniform crystalline structure, which result in better electrical performance. In recent days, the demand for Si ingots as raw material to fabricate Si wafers for semiconductor applications becomes high. To cater to this need, the demand vs supply chain flow needs to be addressed, at the same time providing an environmental advantage. One way to address this gap between demand and supply is an environmentally effective way of large-scale recovery of silicon from steel-slag or other silica-rich industrial wastes. A method that is highly significant for reducing the loss of silicon resources, as well as paves the way for clean and sustainable recovery of silicon and manufacturing of the Si ingot in an effective and efficient manner from otherwise waste and polluting materials such as steel-slag and other industrial wastes rich in silicon, so that the semiconductor fabrication industry is self-sustained in supply chain. IN202341000490 discloses a method of recovery of value-added products (VAP) such as native metals, metal alloys, and compounds from combustion wastes or refuse derived fuels (RDF). These elements or compounds when extracted by chemical or physical process can result in value addition in diverse fields of applications such as wastewater treatment to semiconductor devices. To emerge as a global leader and hub in semiconductor designing and manufacturing industry, India Semiconductor Mission (ISM) led by global experts is set up to serve as a focal point for the inclusive, comprehensible, efficient, and seamless deployment of the development and exhibit Semiconductor ecosystem in association with the Government ministries / departments / agencies, industry, and academia. In the year 2021, Government of India had approved the comprehensive Semicon India programme with a financial outlay of INR 76,000 crore for the development of a sustainable semiconductor ecosystem. This includes attractive incentive support to companies / consortia that are engaged in Silicon Semiconductor Fabs, Display Fabs, Compound Semiconductors / Silicon Photonics / Sensors (including MEMS) Fabs, Semiconductor Packaging (ATMP / OSAT) and Semiconductor Design. The program will give an impetus to semiconductor and display manufacturing by facilitating capital support and technological collaborations (Ref: https: / / ism.gov.in / about-semiconindia- programme.html#:~:text=Semicon%20India%20Program %20aims%20to,%2F%20OSAT)%20and%20Semiconductor%20Design). Several global industries from several countries such as from The United States, Czech Republic, China, Kyrgyzstan, Taiwan, Korea, Japan, Switzerland, Italy, France, United Kingdom, and Qatar, Russia supply Si ingots in both polycrystalline and monocrystalline forms. The cost associated with the Si ingot import amounts to $ 1000 to 1500 per ton (Ref. https: / / trade-metal.com / silicon-ingots-c945.html). This creates lot of supply vs demand related issues. This can be mitigated and supply of ingots at all thickness can be achieved by the extraction of elemental Si from the slag. Slag is a waste material sold to cement and pavement industries at a very low cost. This when diverted and utilized as source for Si ingot production by cost-effective electrochemical process, this will pave way for the early achievement of semiconductor fabrication in India before the targeted deadline of Govt of India and also place the country in global map as leaders in semiconductor fabrication as well as support Make-In-India initiatives. Silica (SiO2) rich industrial wastes such as Steel slag or sludge is a by-product obtained in the steel production plants, and presents huge challenge in the solid waste disposal. This slag or sludge although considered as solid waste, also possess several valuable elements such as titanium, nickel, Zinc, iron, aluminium, silica so on and so forth. These elements or compounds when extracted by chemical or physical process results in value addition in diverse fields of applications such as wastewater treatment to semiconductor devices. Knowledge from various literature indicates that leaching is the technique that has been most widely used in the resource recovery domain followed by other techniques like fusion, hydrothermal treatment. Steel sludge possesses > 70% of SiO2. Refractory bricks are another Silica (SiO2) rich industrial waste from steel plants are generally 70-98% silicon dioxide. As SiO2is the major constituent of Silica (SiO2) rich industrial wastes such as steel slag, sludge, and refractory bricks, the present inventors envisage utilizing the silica content in the slag to electrochemically reduce the same to form Si or SiC wafers. The Si is selectively deposited electrochemically on electrode surface and lead to the formation of wafers or ingots of Si that can be utilized in the semiconductor fabrication as source material. The inventors also envisage that the key advantage can be realised as the electrochemical extraction of Silicon from these silicon-rich industrial wastes can be at concentrations as low as parts per million (ppm) or parts per billion (ppb). SUMMARY The present disclosure provides a method for obtaining silicon sheets, silicon ingots, and silicon carbide (SiC) from silica-rich industrial wastes, wherein the extraction of silicon from silica-rich industrial wastes is done by electrochemical method, wherein Si is deposited on a carbon-based cathode surface, and separation of Si sheets from the carbon-based cathode surface is done by peeling off the deposited Si sheets. The silica-rich industrial wastes such as steel slag, steel sludge, refractory bricks are first dissolved in a strong acid to separate the salts and minerals other than silica, and then the precipitate is powdered and dissolved in a very strong acid to form a silicon-rich solution, and then the silicon-rich solution is electrochemically reduced in an electrocatalytic reactor to elemental Si, wherein Si is deposited on a carbon-based cathode surface. In an aspect, the carbon-based cathode on which Si is deposited is selected from a graphite electrode, a carbon black electrode, a Glassy carbon electrode, a carbon nano tube (CNT) electrode. The strong inorganic acid can be selected from HF, HCl, H2SO4, HNO3, and H3PO4, wherein in order to dissolve / digest the minerals other than SiO2 in the silica-rich industrial waste, the strong acid is selected from HCl, H2SO4, HNO3, and H3PO4; and in order to dissolve / digest SiO2 in the silica-rich industrial waste, very strong acid is required, wherein the very strong acid used is HF. Permitting the deposition of the Si on the carbon-based cathode surface for sufficiently longer time, while providing sufficient amount of silicon-rich solution creates thicker Si sheets, wherein the thickness of the Si sheets may range from several millimetres to several centimetres. Si ingots can be obtained from such thicker Si sheets. The Si deposited carbon-based cathode is subjected to thermal treatment in Muffle furnace at high temperature (950oC) to allow solid phase diffusion of C and Si resulting in the formation of SiC. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 illustrates Linear Sweep Voltammetry (LSV) study of the silica-rich industrial wastes steel slag, steel sludge, and refractory bricks. Fig.2 illustrates SEM image of deposited Si from refractory bricks-1 and its EDS image showing 98.40% Si presence. Fig. 3 illustrates p-XRD of the deposited Si on graphite electrode surface from refractory bricks-1. Fig. 4 illustrates p-XRD of the deposited SiC on graphite electrode surface from refractory bricks-1. Fig. 5A illustrates atomic force microscopy (AFM) images of Si coatings on graphite electrode from Slag-1. Fig.5B illustrates AFM images of Si coatings on graphite electrode from Sludge. Fig. 5C illustrates AFM images of Si coatings on graphite electrode from Refractory bricks-1. Fig.6A illustrates AFM images of SiC coatings on graphite electrode from Slag- 1. Fig. 6B illustrates AFM images of SiC coatings on graphite electrode from Sludge. Fig. 6C illustrates AFM images of SiC coatings on graphite electrode from Refractory bricks-1. Fig. 6D illustrates AFM images of SiC coatings on graphite electrode from Refractory brick-2. Fig.7 illustrates thermogravimetric analysis of the coated Si and SiC on graphite electrode. Fig.8 illustrates thermal oxidation of SiC coated and uncoated graphite electrode at 13270C in air inside muffle furnace. Fig.9 illustrates Thermal oxidation of SiC coated and uncoated graphite electrode at 15270C in air inside muffle furnace. Fig. 10 illustrates flow chart of the method of Si separation from SiO2 or other industrial wastes. DETAILED DESCRIPTION The subject matter of the present disclosure is described in detail with reference to the accompanying drawings. Unless otherwise specified, all the technical and scientific terms used herein have the same meaning as is generally understood by a person skilled in the art pertaining to the present disclosure. Headings are used solely for organizational purposes, and are not intended to limit the disclosure in any way. The use of the singular includes the plural unless specifically stated otherwise. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well. The use of “or” means “and / or” unless stated otherwise. Unless otherwise indicated, all numbers used herein to express quantities, dimensions, and so forth used should be understood as being modified in all instances by the term "about." It is to be understood that wherein a numerical range is recited, it includes all values within that range, and all narrower ranges within that range, whether specifically recited or not. As used herein, "including," "containing" and like terms are understood to be synonymous with "comprising" and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases, or method steps. In addition, it should be appreciated that any figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings of them are not necessarily drawn to scale. Any method / process steps and / or operations and / or instructions used in this disclosure, are for illustrative purposes in a particular order and / or grouping. Other orders and / or grouping of the process steps or its portions and / or operations or its portions and / or instructions or its portions are possible and, one or more of the process steps and / or operations and / or instructions can be combined and / or deleted. The present disclosure provides a method for obtaining silicon sheets, silicon ingots, and silicon carbide (SiC) from silica-rich industrial wastes, wherein the extraction of silicon from silica-rich industrial wastes is done by electrochemical method, wherein Si is deposited on a carbon-based cathode surface, and separation of Si sheets from the carbon-based cathode surface is done by peeling off the deposited Si sheets. The method of obtaining a silicon sheet from a silica-rich industrial waste such as steel slag, steel sludge, and refractory bricks includes the steps of: − grinding the silica-rich industrial waste into a silica-rich powder; − digesting the silica-rich powder in a strong acid to form a silica-rich concentrate; − subjecting the silica-rich concentrate to centrifugation and filtration to separate the precipitate, and a first filtrate, which includes minerals or salts other than silica; − diluting the first filtrate with water, and storing the first diluted filtrate; − drying the precipitate, and grinding the dried precipitate to form a fine precipitate powder; − digesting the fine precipitate powder with a very strong acid, and subjecting to centrifugation and filtration to separate a second filtrate, and subsequently washing with water to form a second diluted filtrate, which is a silicon rich solution; − subjecting the silicon-rich solution to electrochemical reduction in an electrocatalytic reactor, where Si is deposited on a carbon-based cathode surface as a Si sheet; and − peeling off the Si sheet from the carbon-based cathode surface. While several metals / metal alloys / semiconductors can be used as cathode in the electrocatalytic reduction of the present disclosure, the inventors have chosen a carbon-based cathode due to the fact that metal cathodes would not provide good adhesion to the Si to be deposited on it, whereas carbon-based cathodes provide good adhesion to Si over any other metal cathode. For anode, a carbon-based anode is used in the electrocatalytic reactor of the present disclosure, wherein the carbon-based anode is selected from a graphite anode, graphene anode, a carbon black anode, a Glassy carbon anode, a carbon nano tube (CNT) anode. In order to obtain Si ingots, it is necessary to have thicker Si sheets. For this purpose, the deposition of the Si on the carbon-based cathode surface is allowed for sufficiently longer time, which creates thicker Si sheets, wherein the thickness of the Si sheets formed on the carbon-based cathode surface may range from several microns, to several millimetres to several centimetres. Si ingots can be obtained from such thicker Si sheets. As is evident to a person skilled in the art, based on the desired thickness of the Si ingot, the thickness of the deposition of Si on the carbon-based cathode surface can be modulated by providing sufficient time for deposition, and also providing sufficient amount of silicon-rich solution for electrochemical reduction. In this aspect, when required, when the silicon content in the silicon-rich solution is exhausted, the silicon-rich solution may be replenished in the electrocatalytic reactor. The sufficiency or exhaustion of the silicon content in the silicon-rich solution can be estimated based on the rate of Si deposition i.e. the steadiness of the rate of deposition, or the rate of decrease or increase of thickness of the Si sheet on the carbon-based cathode surface. The thickness of the deposition of Si on the carbon-based cathode surface also depends on the concentration of the Si content in the silicon-rich solution. In an embodiment, the elemental Si is allowed to grow on the carbon-based cathode surface and peeled off as Si sheet of 5 mm to 60 mm thickness. In an aspect, where Silicon carbide (SiC) is the requirement, the Si formed on the carbon-based cathode surface is not peeled off. Rather the Si coated carbon-based cathode is sintered in a furnace at high temperature to form SiC. In an aspect, the carbon-based cathode on which Si is deposited is selected from a graphite cathode, a carbon black cathode, a graphene cathode, a Glassy carbon cathode, a carbon nano tube (CNT) cathode, preferably a graphite cathode. The strong acid can be selected from HF, HCl, H2SO4, HNO3, and H3PO4. In an aspect, in order to dissolve / digest the minerals other than SiO2in the silica- rich industrial waste, the strong acid is selected from HCl, H2SO4, HNO3, and H3PO4. In an aspect, in order to dissolve / digest SiO2 in the silica-rich industrial waste, very strong acid is required, wherein the very strong acid used is HF. In order to understand the mineral composition in the silica-rich industrial wastes collected from a steel plant, energy dispersive X-ray Fluorescence (ED-XRF) studies were carried out. Five samples of silica-rich industrial wastes were collected from a steel plant. Sample 1: Steel sludge Sample 2: Refractory bricks-1 Sample 3: Refractory bricks-2 Sample 4: Steel slag-1 Sample 5: Steel slag-2 Table 1 represents the chemical composition of Sample 1 (Steel sludge) identified through the ED-XRF study. Table 1 Minerals % Remarks analysed composition Al2O30.834 Large variety of products like cans, foils, kitchen utensils, window forums, beer kegs, and aeroplane parts. Recovery by proposed method is 100% CaO 3.71 Binder agents, cement, aggregates, Recovery by proposed method is 100% Fe2O3 0.953 The manufacturing of body parts of ubiquitous machine tools, automobile components, ships hulls, and buildings. Can also be put back in steel production. Recovery by proposed method is > 95% SiO2 93.70 Semiconductor in computers and microelectronics industries, used as fertilizer, and in the production of glass and other ceramics. Recovery by proposed method is > 75% SrO 0.006 Pharmaceutical industry, recovery is 80% TiO20.689 Photocatalyst, pigmentation in drugs, catalyst, paint pigment etc, recovery is 80% Zr2O3 0.057 Semiconductors, optoelectronics, drug delivery systems Recovery by proposed method is> 90% Table 2 represents the chemical composition of Sample 2 (Refractory bricks-1) identified through the ED-XRF study. Table 2 Minerals % Remarks analysed composition CaO 1.003 Binder agents, cement, aggregates, Recovery by proposed method is 100% CuO 0.007 Electrical wires, current collectors, electrodes, utensils etc. Recovery by proposed method is 100% Fe2O3 0.461 The manufacturing of body parts of ubiquitous machine tools, automobile components, ships hulls, and buildings. Can also be put back in steel production. Recovery by proposed method is > 95% SiO2 98.40 Semiconductor in computers and microelectronics industries, used as fertilizer, and in the production of glass and other ceramics. Recovery by proposed method is > 75% TiO2 0.09 Aerospace, aircraft, and engines. Pharma grade as pigment. Drug delivery systems etc. Recovery by proposed method is > 90% ZnO 0.001 Semiconductors, optoelectronics, drug delivery systems Recovery by proposed method is > 90% Zr2O30.008 Catalyst, water purifying support etc. recovery is 90% Table 3 represents the chemical composition of Sample 3 (Refractory bricks-2) identified through the ED-XRF study. Table 3 Minerals % Remarks analysed composition CaO 1.03 Binder agents, cement, aggregates, Recovery by proposed method is 100% CuO 0.007 Electrical wires, current collectors, electrodes, utensils etc. Recovery by proposed method is 100% Fe2O3 0.461 The manufacturing of body parts of ubiquitous machine tools, automobile components, ships hulls, and buildings. Can also be put back in steel production. Recovery by proposed method is > 95% SiO2 98.40 Semiconductor in computers and microelectronics industries, used as fertilizer, and in the production of glass and other ceramics. Recovery by proposed method is > 75% ZnO 0.001 Pharmaceutical industry, recovery is 80% TiO20.090 Photocatalyst, pigmentation in drugs, catalyst, paint pigment etc, recovery is 80% Zr2O3 0.008 Semiconductors, optoelectronics, drug delivery systems Recovery by proposed method is> 90% Table 4 represents the chemical composition of Sample 4 (Steel slag-1) identified through the ED-XRF study. Table 4 Minerals % Remarks analysed composition Al2O319.60 Large variety of products like cans, foils, kitchen utensils, window forums, beer kegs, and aeroplane parts. Recovery by proposed method is 100% CaO 4.28 Binder agents, cement, aggregates, Recovery by proposed method is 100% CuO 0.012 Electrical wires, current collectors, electrodes, utensils etc. Recovery by proposed method is 100% P2O52.31 Pesticides, baking powder, incendiary bombs, and light-emitting diodes. Recovery by proposed method > 85% Fe2O38.30 The manufacturing of body parts of ubiquitous machine tools, automobile components, ships hulls, and buildings. Can also be put back in steel production. Recovery by proposed method is > 95% Mn2O30.095 Produce a variety of different alloys, dry cell batteries, and black-brown pigment in paint. Recovery by proposed method is > 90% NiO 0.012 Pesticides, baking powder, incendiary bombs, and light-emitting diodes. Recovery by proposed method > 75% K2O 3.19 fertilizer. Recovery by proposed method is > 70% SiO251.90 Semiconductor in computers and microelectronics industries, used as fertilizer, and in the production of glass and other ceramics. Recovery by proposed method is > 75% TiO23.38 Aerospace, aircraft, and engines. Pharma grade as pigment. Drug delivery systems etc. Recovery by proposed method is > 90% Table 5 represents the chemical composition of Sample 5 (Steel slag-2) identified through the ED-XRF study. Table 5 Minerals % Remarks analysed composition Al2O3 15.40 Large variety of products like cans, foils, kitchen utensils, window forums, beer kegs, and aeroplane parts. Recovery by proposed method is 100% CaO 41.80 Binder agents, cement, aggregates, Recovery by proposed method is 100% MgO 7.50 Semiconductor, drug delivery systems. Recovery by proposed method is 100% Fe2O3 2.10 The manufacturing of body parts of ubiquitous machine tools, automobile components, ships hulls, and buildings. Can also be put back in steel production. Recovery by proposed method is > 95% Mn2O3 0.091 Produce a variety of different alloys, dry cell batteries, and black-brown pigment in paint. Recovery by proposed method is > 90% ZnO 0.034 Semiconductors, optoelectronics, drug delivery systems Recovery by proposed method is> 90% SiO231.20 Semiconductor in computers and microelectronics industries, used as fertilizer, and in the production of glass and other ceramics. Recovery by proposed method is > 75% S2O61.71 Anodes for Li-S batteries, gypsum. Recovery by proposed method is > 90% When the silicon-rich solution is subjected to electrochemical reduction, Si and the other elements such as Mg, Mn, Cr, Al, Ti, Ni, Ca, Zn are isolated based on their unique fingerprint in the electrochemical series with respect to their redox chemistry and the reference electrode employed. The elements Mg, Mn, Cr, Al, Ti, Ni, Ca, Zn may be separated in their metallic form or as alloys. According to the electrochemical series, the following are the redox potentials of the elements generally present in the silica-rich industrial wastes such as steel slag, steel sludge, and refractory bricks. Silica – being a source material for any semiconductor industry, electrochemically SiO2 can be reduced to Si and deposited on electrode surface at – 0.909 V. SiO2(s) + 4H++ 4e−⇌ Si(s) + 2H2O(l) - 0.909V The acid used is a strong acid as HF, where the reaction path followed is as follows. SiO2 + 4HF ^ SiF4 + 2H2O SiF4 reacts in water to produce silicon rich solution H2SiO3 and HF as follows. SiF4+ 3H2O ^ H2SiO3+ 4HF Electrochemical reduction of Metasilicic acid is represented as: SiO32-+2H++ 2e ^ Si + H2O + O2 E = -0.909 V The elemental Si is allowed to grow on the carbon-based cathode surface and can be removed as Si sheets of mm to cm thickness, from which ingots can be obtained. Alternately, the Si deposited carbon-based cathode is subjected to thermal treatment (sintering) in Muffle furnace at high temperature (950oC) to allow solid phase diffusion of C and Si resulting in the formation of SiC. The obtained SiC would be of semiconductor grade and possess high thermal conductivity and hence can be utilized in semiconductor as well as thermal insulation applications. Magnesium – all of its oxidation states from +2 to native metal lies in the potential of -2.37 V and hence can be deposited on the electrode surface MgO ⇌ Mg2++ 2e−- 2.37 V Manganese MnO2(s) + 4H++ 2e−⇌ Mn2++ 2H2O(l) 1.23 V Mn2++ 2e ⇌ Mn - 1.17 V Overall reaction: MnO2(s) + 4H++ 4e−⇌ Mn + 2H2O(l) 0.06 V (SHE) Hence Mn can be deposited on electrode surface at near zero potential of just 60 mV Phosphorous – P2O5 can be converted to phosphoric acid in acidic medium as follows P2O5+6H++6e−⇌2PH3(g) + 5 / 2 O20.06V 2PH3(g) + 4H2O(l) ⇌ 2H3PO2+ 8H++ 8e−-0.5 V 2H3PO2 + 2H2O (l) ⇌ 2H3PO3+ 4H++ 4e−-0.5 V 2H3PO3 + 2H2O (l) ⇌ 2H3PO4+ 4H++ 4e−-0.28 V Overall reaction: P2O5 + 3H2O(l) ⇌ 2H3PO4 -1.34 V Hence phosphorous is removed electrochemically at -1.34 V as phosphoric acid Chromium – Chromium Oxide exists in +3 oxidation state and would involve the following electrochemical reaction Cr2O3 + e−⇌ 2CrO + ½ O2 - 0.424 V 2CrO + 4e−⇌ 2Cr + O2 - 0.9 V Overall reaction: Cr2O3+ 5e−⇌ 2Cr + 1½ O2- 1.324 V Thus, at – 1.324 V Cr can be deposited on the electrode surface in metallic state Aluminium – Electrochemically Al can be recovered from steel slag as below Al(OH)4−+ 3e−⇌⇌ Al(s) + 4OH−- 2.31 V Specifically at -2.31V, Al can be deposited on the electrode surface from the slag. Titanium TiO2 + e−⇌ Ti3++ O2 - 0.7 V Ti3++ e−⇌ Ti2+- 0.37 V Ti2++ 2e−⇌ Ti - 0.163 V Overall reaction TiO2 + 4e−⇌ Ti + O2 -1.233 V Thus, at -1.233 V, Ti can be extracted electrochemically from the slag to metallic state and deposited on the electrode surface. Nickel – Ni exists mostly as hydrated oxides or hydroxides and can be easily reduced to metallic Ni electrochemically at – 0.72V Ni(OH)2+ 2e−⇌ Ni(s) + 2OH−- 0.72 V Calcium – Calcium Oxide is extracted as Ca on electrode surface at – 2.84V CaO + 2e−⇌⇌ Ca(s) + ½ O2 - 2.84 V Carbon – If the steel slag is rich in carbon as well, it can be electrochemically oxidized or reduced to CO2⇌CO cycle at specific potential as shown below C + 2H2O + 2e−⇌ CO2 + 2H2 - 1.15 V CO2(g) + 2H++ 2e−⇌ CO(g) + H2O(l) - 0.106 V Overall reaction C + H2O(l) + 2H++ 4e−⇌ CO + 2H2 - 1.256 V Sulphur – sulphur trioxide is electrochemically reduced or converted to sulphites as follows: S2O62−+ 4H++ 2e−⇌ 2H2SO30.569 V Potassium – Potassium exists as oxide and can be converted to elemental K at - 2.93 V and deposited on electrode surface. This is a good source of K metal anode in K-ion batteries. K++ e- ⇌ K - 2.93 V Copper – Copper existing as oxide, can be recovered electrochemically as metallic copper and deposited on electrode surface at Cu2++ e−⇌ Cu+0.159 V Cu++ e−⇌ Cu(s) 0.52 V Overall reaction: Cu2++ 2e- ⇌ Cu 0.3419 V Zinc – In slightly acidic solution, ZnO forms metallic Zn as follows: ZnO + 2H++ 2e−⇌ Zn(s) + 2H2O 1.473 V Thus, after electrochemically removing all the other minerals of the steel slag and / or sludge either simultaneously or sequentially and then the slag would contain iron oxide as the only constituent. The material or elements recovered as indicated in the above points for the individual elements can be upscaled to appropriate industry as they will be in their purest form. Fig. 1 illustrates Linear Sweep Voltammetry (LSV) study of the silica-rich industrial wastes steel slag, steel sludge, and refractory bricks. Examples: The present disclosure will now be explained in further detail by the following examples. These examples are illustrative of certain embodiments of the disclosure without limiting the scope of the present disclosure. Example 1: Refractory bricks-1 − 500 grams of refractory bricks (Sample name: Refractory bricks-1) was ground and ball milled, and dissolved in 500 ml of Conc. HCl in an acid digestion chamber coated with polymer to avoid the SS or glass walls of the chamber dissolving in Conc. HCl. The acid digested concentrate was washed with water, and was subjected to centrifugation and filtration to separate the precipitate, and a first diluted filtrate. This first diluted filtrate primarily contains the salts and minerals other than SiO2. − The precipitate is dried, and ground to form a fine precipitate powder, which was then dissolved in 500 ml of HF in the acid digestion chamber, washed with water and subjected to centrifugation and filtration to separate a second diluted filtrate which is a silicon-rich solution. Mechanism: The silica dissolves in HF to form silicon tetra fluoride (SiF4). SiO2 + 4HF ^ SiF4 + 2H2O The SiF4 is reacted with water to produce silicon rich solution metasilicic acid (H2SiO3) and HF. SiF4 + 3H2O ^ H2SiO3 + 4HF The metasilicic acid (silicon rich solution) is electrochemically reduced, which is represented as: SiO32-+2H++ 2e ^ Si + H2O + O2E = -0.909 V An electrocatalytic reactor with graphite cathode, and graphite anode was taken, and the silicon rich solution containing H2SiO3 was added to the electrocatalytic reactor, where the H2SiO3was electrochemically reduced to Si at -0.909 V, and 1.0 A current. The elemental Si is allowed to grow on the graphite cathode surface and peeled off as Si sheet of 53 mm thickness. SiC formation In another experiment, instead of peeling of the Si sheet obtained in Example 1, the Si deposited graphite cathode was removed from the reactor, and was sintered at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC. Example 2: Refractory bricks-2 − 500 grams of refractory bricks (Sample name: Refractory bricks-2) was ground and ball milled, and dissolved in 500 ml of Conc. HCl in an acid digestion chamber coated with polymer to avoid the SS or glass walls of the chamber dissolving in Conc. HCl. The acid digested concentrate was washed with water, and was subjected to centrifugation and filtration to separate the precipitate, and a first diluted filtrate. This first diluted filtrate primarily contains the salts and minerals other than SiO2. − The precipitate is dried, and ground to form a fine precipitate powder, which was then dissolved in 500 ml of HF in the acid digestion chamber, washed with water and subjected to centrifugation and filtration to separate a second diluted filtrate which is a silicon-rich solution. Mechanism: The silica dissolves in HF to form silicon tetra fluoride (SiF4). SiO2+ 4HF ^ SiF4+ 2H2O The SiF4is reacted with water to produce silicon rich solution metasilicic acid (H2SiO3) and HF. SiF4+ 3H2O ^ H2SiO3+ 4HF The metasilicic acid (silicon rich solution) is electrochemically reduced, which is represented as: SiO32-+2H++ 2e ^ Si + H2O + O2E = -0.909 V An electrocatalytic reactor with graphite cathode, and graphite anode was taken, and the silicon rich solution containing H2SiO3was added to the electrocatalytic reactor, where the H2SiO3 was electrochemically reduced to Si at -0.909 V, and 1.0 A current. The elemental Si is allowed to grow on the graphite cathode surface and peeled off as Si sheet of 38 mm thickness. SiC formation In another experiment, instead of peeling of the Si sheet obtained in Example 2, the Si deposited graphite cathode was removed from the reactor, and was sintered at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC. Example 3: Steel slag-1 − 500 grams of Steel slag (Sample name: Steel slag-1) was ground and ball milled, and dissolved in 500 ml of Conc. HCl in an acid digestion chamber coated with polymer to avoid the SS or glass walls of the chamber dissolving in Conc. HCl. The acid digested concentrate was washed with water, and was subjected to centrifugation and filtration to separate the precipitate, and a first diluted filtrate. This first diluted filtrate primarily contains the salts and minerals other than SiO2. − The precipitate is dried, and ground to form a fine precipitate powder, which was then dissolved in 500 ml of HF in the acid digestion chamber, washed with water and subjected to centrifugation and filtration to separate a second diluted filtrate which is a silicon-rich solution. Mechanism: The silica dissolves in HF to form silicon tetra fluoride (SiF4). SiO2+ 4HF ^ SiF4+ 2H2O The SiF4is reacted with water to produce silicon rich solution metasilicic acid (H2SiO3) and HF. SiF4+ 3H2O ^ H2SiO3+ 4HF The metasilicic acid (silicon rich solution) is electrochemically reduced, which is represented as: SiO32-+2H++ 2e ^ Si + H2O + O2 E = -0.909 V An electrocatalytic reactor with graphite cathode, and graphite anode was taken, and the silicon rich solution containing H2SiO3was added to the electrocatalytic reactor, where the H2SiO3 was electrochemically reduced to Si at -0.909 V, and 1.0 A current. The elemental Si is allowed to grow on the graphite cathode surface and peeled off as Si sheet of 14 mm thickness. SiC formation In another experiment, instead of peeling of the Si sheet obtained in Example 3, the Si deposited graphite cathode was removed from the reactor, and was sintered at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC. Example 4: Steel sludge − 500 grams of Steel sludge (Sample name: Steel sludge) was ground and ball milled, and dissolved in 500 ml of Conc. HCl in an acid digestion chamber coated with polymer to avoid the SS or glass walls of the chamber dissolving in Conc. HCl. The acid digested concentrate was washed with water, and was subjected to centrifugation and filtration to separate the precipitate, and a first diluted filtrate. This first diluted filtrate primarily contains the salts and minerals other than SiO2. − The precipitate is dried, and ground to form a fine precipitate powder, which was then dissolved in 500 ml of HF in the acid digestion chamber, washed with water and subjected to centrifugation and filtration to separate a second diluted filtrate which is a silicon-rich solution. Mechanism: The silica dissolves in HF to form silicon tetra fluoride (SiF4). SiO2 + 4HF ^ SiF4 + 2H2O The SiF4 is reacted with water to produce silicon rich solution metasilicic acid (H2SiO3) and HF. SiF4 + 3H2O ^ H2SiO3 + 4HF The metasilicic acid (silicon rich solution) is electrochemically reduced, which is represented as: SiO32-+2H++ 2e ^ Si + H2O + O2E = -0.909 V An electrocatalytic reactor with graphite cathode, and graphite anode was taken, and the silicon rich solution containing H2SiO3 was added to the electrocatalytic reactor, where the H2SiO3was electrochemically reduced to Si at -0.909 V, and 1.0 A current. The elemental Si is allowed to grow on the graphite cathode surface and peeled off as Si sheet of 7 mm thickness. SiC formation In another experiment, instead of peeling of the Si sheet obtained in Example 4, the Si deposited on graphite cathode was removed from the reactor, and was sintered at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC. Evaluation: Example 1: SEM, EDS, p-XRD of Si and SiC The surface of Si coated graphite is studied for its surface morphology by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to understand the composition of the Si deposited. Fig.2 illustrates SEM image of deposited Si from Example 1 (refractory bricks-1) and its EDS image showing 98.40% Si presence. The presence of Si in amorphous or crystalline phase is determined by XRD studies. Fig.3 illustrates p-XRD of the deposited Si on graphite electrode surface from Example 1 (refractory bricks-1). As in Fig. 3, it can be seen from the diffraction patterns that the structure and phase of Si coatings on the graphite substrate. The phases present in the Si deposits are: i. Si cubic structure (Si5) with reflections of (111), (220), (311) and space group of F-43m (216), ii. Si cubic structure with reflections of (111), (200), (220) and space group Fm-3m (225), iii. graphite hexagonal structure with reflections of (002), (100), (004), (110) and space group P63 / mmc (194). The Si deposited graphite electrode is subjected to thermal treatment in Muffle furnace at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC. Fig.4 illustrates p-XRD of the deposited SiC on graphite electrode surface from Example 1 (refractory bricks-1). As in Fig.4: i. the cubic structure of SiC with the reflections of (111), (220), (311) and space group of F-43m (216)

[0034] ii. the cubic structure of SiC with reflections of (111), (200), (220) and space group Fm-3m (225)

[0035] iii. graphite hexagonal structure with reflections of (002), (100), (004), (110) and space group P63 / mmc (194) are noticed. It may be due to the high intensity reflection of graphite and SiC phases. Although a slight X-offset of the graphite phase is observed, it is consistent with Si and SiC formation. Evaluation: Example 1, 2, 3 and 4: Surface Roughness (through AFM) of Si and SiC coatings on graphite electrode surface Surface Roughness analysis of the Si and SiC coating on graphite electrode was conducted employing Atomic Force Microscopy (AFM). Surface roughness of the coating has tremendous influence on its functionality. Roughness affects not only surface properties–such as hydrophobicity, optical and plasmonic behaviour, adhesion, friction and Casimir forces, but also “bulk” properties such as fracture toughness and fatigue resistance. Surface roughness of electrodeposited Si and SiC coatings on graphite is calculated. Tapping mode is used to image the coatings by Atomic Force Microscopy (AFM) to avoid adhesion and shear forces between the tip and the deposited layer. Gwyddion software has been used for evaluation of parameters from AFM images. In the following, two types of surface roughness values are calculated through the software: i.e., average surface roughness Ra and root mean square (rms) surface roughness Rq. The average surface roughness can be defined as, (1) where Ra is average surface roughness, Y is total area of scan, and L represents total number of point which can be taken for the calculation of the surface roughness. Similarly, the rms surface roughness can be defined as, (2) Fig. 5A illustrates atomic force microscopy (AFM) images of Si coatings on graphite electrode from Slag-1. Fig.5B illustrates AFM images of Si coatings on graphite electrode from Sludge. Fig. 5C illustrates AFM images of Si coatings on graphite electrode from Refractory bricks-1. Fig.6A illustrates AFM images of SiC coatings on graphite electrode from Slag- 1. Fig. 6B illustrates AFM images of SiC coatings on graphite electrode from Sludge. Fig. 6C illustrates AFM images of SiC coatings on graphite electrode from Refractory bricks-1. Fig. 6D illustrates AFM images of SiC coatings on graphite electrode from Refractory brick-2. The calculated surface roughness values using Gwyiddon software is shown in Table 6 and Table 7. The surface roughness values of Si coatings is higher compared to SiC coatings on graphite. Low surface roughness on SiC on graphite is mainly due to the dense thick coating on the substrate and thermal treatment. Table 6 Example Si coatings on Average surface Root mean surface graphite electrode roughness Ra(nm) roughness Ra(nm) 1 Refractory bricks-1 95 117 2 Refractory bricks-2 166 198 3 From Steel slag-1 138 175 4 From Steel sludge 164 195 Table 7 Example SiC formed on Average surface Root mean surface graphite electrode roughness Ra(nm) roughness Ra(nm) 1 Refractory bricks-1 244 282 2 Refractory bricks-2 244 282 3 From Steel slag-1 158 196 4 From Steel sludge 259 286 Evaluation: Example 1, 2, 3 and 4: Thermogravimetric analysis of Si and SiC coatings on graphite electrode surface in N2 atmosphere Fig.7 illustrates thermogravimetric analysis of the coated Si and SiC on graphite electrode. From the thermogravimetric analysis, it is observed that Si coated graphite undergoes oxidation at a rapid rate in N2atmosphere with 85% weight loss within 250oC. In case of SiC coated graphite, Refractory brick precursor possessed least weight loss of 2.5% and stable upto 700oC, sludge precursor exhibited weight loss of 16% whereas slag precursor contributed a weight loss of 30%. This variation in the weight loss of SiC from different wastes is attributed to the variation in the composition of SiO2 (30 to 50% in slag and sludge, 70 to 98% in refractory bricks) in the source material. Evaluation: Example 1, 3 and 4: Thermal Oxidation of the samples in air Fig.8 illustrates thermal oxidation of SiC coated and uncoated graphite electrode at 13270C in air inside muffle furnace. As shown in Fig. 8, it is obvious that SiC coated graphite electrode from Steel slag (Sample: Steel slag-1) precursor exhibit a totally different anti-oxidation behavior. Throughout the oxidation process, the weight loss of SiC coated graphite increases linearly with oxidation time. After 15 hours of oxidation, the weight loss of the coated graphite is up to (i) 0.45% (Steel slag-1), (ii) 2.4% (Steel sludge), (iii) 6.4% (Refractory Bricks-1) and (iv) 12.9% (uncoated graphite electrode), which suggests that in the uncoated graphite, the oxygen can directly attack the graphite substrate. In contrast, the SiC coated graphite exhibits excellent oxidation resistance in air at 13270C. After 15 hours of oxidation, the weight loss of the coated sample is 0.45%, indicating a better oxidation protection of the coating. Fig.9 illustrates Thermal oxidation of SiC coated and uncoated graphite electrode at 15270C in air inside muffle furnace. In addition, high temperature oxidation test is also carried out in air at 15270C. As shown in Fig.9, for the SiC coated graphite oxidized at 15270C, the weight loss as a function of time is still linear and after 15 hours of oxidation, the weight loss is around 1.20%, which further confirms the highly efficient protection ability of the coating obtained from refractory brick precursor. With respect to the SiC coated graphite, the oxidation process is fast and enormous weight loss is observed (14.25%) indicating that the as-prepared coating from Steel slag (Sample: Steel slag-1) and Steel sludge precursor has very less oxidation resistance at high temperatures. Thermal oxidation of Si coated on graphite is not studied due to the fact that at such high temperature of 1300 to 1500oC, Si gets oxidised easily and leads to SiO2 within 500oC, and beyond that there would not be any change in the degradation profile. Fig. 10 illustrates flow chart of the method of Si separation from silica-rich industrial wastes. Advantages: The method of obtaining Si sheets, Si ingots, SiC from silica-rich industrial wastes of the present disclosure has the following non-limiting advantages. − Conversion of silica-rich industrial wastes into Si sheets, Si ingots, SiC help in restoration of the bio-diversity, agricultural soil rejuvenation − Eliminates entering of wastes in surface and sub-surface water table, and alleviates the chances of soil contamination, public health, and provides environmental safety. Applications: The method of obtaining Si sheets, Si ingots, SiC from silica-rich industrial wastes of the present disclosure has the following non-limiting industrial applications. − Si-sheets, Si-ingots, and SiC are useful for fabrication of semiconductors. Although the present disclosure is described in terms one or more embodiments, it is to be understood that they have been presented by way of example, and are not limiting. Thus, the present disclosure should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS 1. A method of obtaining a silicon sheet from a silica-rich industrial waste includes the steps of: − grinding the silica-rich industrial waste into a silica-rich powder; − digesting the silica-rich powder in a strong acid to form a silica-rich concentrate; − subjecting the silica-rich concentrate to centrifugation and filtration to separate the precipitate, and a first filtrate, which includes minerals or salts other than silica; − diluting the first filtrate with water, and storing the first diluted filtrate; − drying the precipitate, and grinding the dried precipitate to form a fine precipitate powder; − digesting the fine precipitate powder with a very strong acid, and subjecting to centrifugation and filtration to separate a second filtrate, and subsequently washing with water to form a second diluted filtrate, which is a silicon rich solution; − subjecting the silicon-rich solution to electrochemical reduction in an electrocatalytic reactor, where Si is deposited on a carbon-based cathode surface as a Si sheet; and − peeling off the Si sheet from the carbon-based cathode surface.

2. The method as claimed in claim 1, wherein the silica-rich industrial wastes is selected from steel slag, steel sludge, and refractory bricks.

3. The method as claimed in claim 1, wherein the carbon-based cathode on which Si is deposited is selected from a graphite cathode, graphene cathode, a carbon black cathode, a Glassy carbon cathode, a carbon nano tube (CNT) cathode.

4. The method as claimed in claim 1, wherein the carbon-based cathode on which Si is deposited is a graphite cathode.

5. The method as claimed in claim 1, wherein the anode is a carbon-based anode, wherein the carbon-based anode is selected from a graphite anode, graphene anode, a carbon black anode, a Glassy carbon anode, a carbon nano tube (CNT) anode.

6. The method as claimed in claim 1, wherein the strong acid used for forming the first filtrate is selected from HCl, H2SO4, HNO3, and H3PO4.

7. The method as claimed in claim 1, wherein the very strong acid used for forming the second filtrate is HF.

8. The method as claimed in claim 1, wherein the silicon-rich solution includes metasilicic acid (H2SiO3).

9. The method as claimed in claim 1, wherein the minerals or salts other than silica include are salts and minerals of Mg, Mn, P, Cr, Al, Ti, Ni, Ca, C, S, K, Cu, Zn, Zr, Sr, and Fe.

10. The method as claimed in claim 1, wherein the Si deposited carbon-based cathode is subjected to thermal treatment at 950oC to allow solid phase diffusion of C and Si resulting in the formation of SiC.

11. The method as claimed in claim 1, wherein the deposition of the Si on the carbon-based cathode surface is continued for longer time to create thicker Si sheets, wherein the thickness of the Si sheets may range from several millimetres to several centimetres, wherein Si ingots are obtained from the thicker Si sheets.

12. The method as claimed in claim 1, wherein larger amount of silicon-rich solution was subjected to electrochemical reduction for longer time to create thicker Si sheets, wherein the thickness of the Si sheets may range from several millimetres to several centimetres, wherein Si ingots are obtained from the thicker Si sheets.

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