Plasma arc process and apparatus for producing fumed silica

Abstract

An apparatus for producing fumed silica from silicon dioxide is described, wherein the plasma arc reactor comprises at least one top electrode extending into the molten silicon dioxide contained in the reactor, an electrically conductive plate arranged below the molten silicon dioxide, and a bottom anode. The apparatus is adapted to generate a plasma arc, wherein the plasma arc is provided at the tip of the electrode and is adapted to be directly transferred to the molten silicon dioxide to form SiO. A quenching system is also provided, such as a hydrogen and oxygen containing gas injected within the reactor. The quenching system is adapted to re-form SiO2, but re-forms it as nano-sized amorphous particles, and a reactor outlet is provided arranged for allowing the amorphous SiO2 nano-particles in the form of fumed silica to exit the reactor.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 189,069, filed May 15, 2021, which is now pending and incorporated herein by reference. Technical Field

[0003] The subject matter of this invention relates to the production of fumed silica, and more specifically, to the production of fumed silica using plasma arc. Background Technology

[0004] Fumed silica is an inert and harmless substance, a common thickener used in various industrial applications. Its large surface area and low bulk density make it a valuable raw material for a wide range of products, including paints, food, cosmetics, and catalysts, and it is most commonly used as a thickener or drying agent. Small amounts of fumed silica (1 wt.% to 5 wt.%) can significantly affect the rheological properties of liquids, such as the viscosity of paints. Fumed silica is also used as a lightweight abrasive and free-flowing agent in bulk materials.

[0005] Fumed silica is composed of long 3D chains of nano-sized silica molecules. The complex structure of these chains results in a product with low bulk density and a very large specific surface area (+50 m²). 2 ( / g), and has a strong thickening effect.

[0006] Fumed silica is conventionally produced via flame hydrolysis, and it is the product of a complex silicon production process. Silica in quartz form is extracted from mines, pulverized to a specific size range, and then reduced to silicon or ferrosilicon in an electric arc furnace in the presence of a carbon source, and further reduced to iron in the case of ferrosilicon. This process consumes a large amount of energy, results in significant carbon dioxide emissions, and produces solid byproducts such as silica ash, another form of silica, and slag. The silicon is then transported to different equipment, typically in a remote location, where it is converted to SiCl4 using HCl and Cl2 gases. The SiCl4 is then burned in a flame hydrolysis process using hydrogen and oxygen. The resulting product is silica (SiO2), a different type of silica from the starting material, differing in physical morphology, structure, and surface chemistry. The entire process is multi-step and highly polluting, emitting both greenhouse gases (GHG) and acid gases.

[0007] Considering the entire product lifecycle, conventional methods for manufacturing fumed silica have high carbon emissions of 16.4 kg CO2 equivalent per kg of product [Reference 1]. Furthermore, the conversion rate in each step of this method is less than 100%; for example, in best industrial practice, the silicon conversion rate in silicon production is only 80%, resulting in the loss of approximately 20% of the silicon as silica ash, thus leading to material loss.

[0008] Therefore, it is desirable to directly convert silica into fumed silica in a single step, while simultaneously reducing emissions of pollutants, including GHG, and lowering costs. This can be achieved by directly evaporating SiO2 at high temperatures and decomposing it into SiO, then re-oxidizing the SiO back into SiO2. Because this method requires high temperatures (+1700°C), conventional heating methods, such as flame heating via burners, are unsuitable. Conventional electric heating methods (e.g., resistance heating elements) are also unsuitable because they cannot reach the required high temperatures, and the elements would be coated with soot, affecting their efficiency. One method to achieve the high temperatures required for this method is to use a plasma arc reactor. Plasma arcs can reach temperatures exceeding the decomposition temperature of silica, which meets the requirements of this method. Plasma arcs are not contaminated during fumed silica production, and their efficiency is not compromised. Furthermore, the plasma arc method offers high scalability.

[0009] Research conducted by several universities has led to the development of current technologies for the production of fumed silica using a transferred arc plasma torch.

[0010] Addona (Master's Thesis in Engineering, Addona, 1993, McGill University, Montreal, Canada) Production of fumed silica using a transferred DC arc water-cooled plasma torch on a laboratory-scale plasma method. This project investigated how different quenching conditions affect the properties of fumed silica. Fumed silica was successfully produced using the radiative energy of the plasma method. Large surface area powders were produced by high pre-quenching temperatures, high quenching rates, and low pre-quenching supersaturation ratios.

[0011] Addona (PhD dissertation, Addona, 1998, McGill University, Montreal, Canada) researched a new technique for producing fumed silica that significantly improves the energy efficiency of the process by transferring a plasma arc to molten silica using a transferred DC arc water-cooled plasma torch. The successful arc transfer resulted in a patent: a method for forming oxide ceramic electrodes in a transferred plasma arc reactor (Canadian Patent No. 2,212,471, published April 1, 2003, and U.S. Patent No. 6,060,680, published May 9, 2000). In this study, the produced fumed silica had a competitive surface area but lacked thickening ability.

[0012] Pristavita (Master's thesis in Engineering, Pristavita, 2006, McGill University, Montreal, Canada) investigated the effect of coalescence on the rheological properties of fumed silica. The conclusion was that coalescence did not improve rheological properties; the lack of thickening was due to the absence of free hydroxyl groups on the product surface. Quenching conditions were tested, and the resulting product exhibited high overall quality and a competitive surface area, with measured values ​​up to 260 μm. 2 / g.

[0013] As described in the above references, the production of fumed silica using a transferred arc plasma torch has several disadvantages: high operating costs due to the relatively poor thermal efficiency of the torch, as most of the energy is dissipated and lost in the torch water cooling loop; poor scalability; and the risk of water leakage into the reactor, which could lead to a catastrophic steam explosion due to the reaction of water with the molten silica in the bath.

[0014] Therefore, it is desirable to provide a new method and apparatus for producing high-quality fumed silica in a single step. Summary of the Invention

[0015] Therefore, it is desirable to provide a new method and equipment for producing high-quality fumed silica.

[0016] The embodiments described herein provide, in one aspect, a plasma method for the continuous production of fumed silica with lower energy requirements and carbon emissions than conventional methods.

[0017] Furthermore, the embodiments described herein provide, in another aspect, an apparatus for melting, evaporating, and decomposing silica in one step, and subsequently quenching the vapor phase to form and functionalize fumed silica.

[0018] Furthermore, the embodiments described herein also provide a plasma arc method for directly converting silicon dioxide into fumed silicon dioxide.

[0019] Furthermore, the embodiments described herein provide, in another aspect, a plasma arc method for preparing fumed silica that is substantially waste-free and does not generate any hazardous waste.

[0020] Furthermore, the embodiments described herein also provide an apparatus for thermally decomposing silicon dioxide into silicon monoxide without any reducing agent.

[0021] Furthermore, the embodiments described herein also provide a plasma arc method for producing fumed silica, the plasma arc method comprising the following steps:

[0022] Silica, such as crushed quartz, is supplied to the plasma arc reactor;

[0023] A plasma arc is generated at the tip of at least one top electrode inside the reactor.

[0024] The plasma arc is directly transferred to molten silica contained in the reactor, thereby forming SiO;

[0025] SiO is quenched to reform SiO2, but reformed into nano-amorphous particles; and

[0026] SiO2 nanoparticles in the form of fumed silica were removed from the reactor.

[0027] Furthermore, the embodiments described herein also provide an apparatus for producing fumed silica, the apparatus comprising: a reactor adapted to generate a plasma arc; at least one top electrode extending into molten silica contained within the reactor; a conductive plate disposed below the molten silica; a bottom anode, wherein the plasma arc provided at the tip of the electrode is adapted to be directly transferred to the molten silica for forming SiO; a quenching system, such as a hydrogen- and oxygen-containing gas injected into the reactor, the quenching system being adapted to reform SiO2 into nanoscale amorphous particles; and an outlet for allowing the fumed silica to exit the reactor.

[0028] Furthermore, the embodiments described herein provide, in another aspect, that the current path flowing through the reactor begins at the electrode, forms a plasma arc between the electrode and the molten silica, flows through the conductive molten silica to the conductive plate, and then flows through the bottom anode.

[0029] Furthermore, the embodiments described herein provide, in another aspect, that the bottom anode is provided with cooling fins and a blower for cooling the cooling fins.

[0030] Furthermore, the embodiments described herein provide, in another aspect, that the quenching system includes at least one gas injection port.

[0031] Furthermore, the embodiments described herein provide, in another aspect, the provision of a cyclone separator for collecting larger-sized fumed silica agglomerates as the hot gas stream and fumed silica particles exit the reactor through the outlet.

[0032] Furthermore, the embodiments described herein also provide, in another aspect, a gas / liquid cooler for cooling the hot gas stream is provided downstream of the cyclone separator.

[0033] Furthermore, the embodiments described herein provide, in another aspect, that a bag filter is provided downstream of the gas / liquid cooler, the bag filter being used to separate most of the finer fumed silica from the gas stream.

[0034] Furthermore, the embodiments described herein provide, in another aspect, that a fine particulate filter is provided downstream of the bag filter for further filtering of the gas and removing trace amounts of fumed silica.

[0035] Furthermore, the embodiments described herein provide, in another aspect, that an induced draft fan is provided downstream of the fine particulate filter, the induced draft fan being used to draw gas out of the reactor and provide a pressure below atmospheric pressure.

[0036] Furthermore, the embodiments described herein also provide a plasma arc method for producing fumed silica, the plasma arc method comprising the following steps:

[0037] Silica, such as crushed quartz, is supplied to the plasma arc reactor;

[0038] Additives are added to the supplied silica to improve the conductivity of the silica melt, and / or reduce the melting temperature of the silica melt, and / or increase the production rate and / or quality of fumed silica.

[0039] A plasma arc is generated at the tip of at least one top electrode inside the reactor.

[0040] Injecting gas through the top electrode to increase the yield of fumed silica by:

[0041] Reduce the evaporation energy of silica.

[0042] Increasing the arc power can improve the evaporation rate of silica.

[0043] By introducing reactive substances such as H, O, and OH through plasma arc heating of injected gases such as vapor, the surface chemistry and properties of amorphous nano-sized silica particles in the form of gas-phase silica are enhanced.

[0044] The plasma arc is directly transferred to the molten silica contained in the reactor, thereby evaporating the silica and forming SiO;

[0045] SiO was quenched to reform SiO2, but reformed into amorphous nanoparticles; and

[0046] Amorphous SiO2 nanoparticles in the form of fumed silica were removed from the reactor. Attached Figure Description

[0047] To better understand the embodiments described herein and to more clearly illustrate how the embodiments described herein can be implemented, reference will now be made only to the accompanying drawings, which illustrate at least one exemplary embodiment, and in the drawings:

[0048] Figure 1 is an exemplary schematic vertical cross-sectional view of a furnace for producing fumed silica according to an exemplary embodiment;

[0049] Figure 2 is an exemplary schematic diagram of a method for producing fumed silica according to an exemplary embodiment. Detailed Implementation

[0050] The aforementioned disadvantages can be overcome by the subject matter of the present invention, which uses an electroplasma arc reactor in which a plasma arc is generated at the tip of the top electrode and directly transferred to molten silica without any water cooling, thereby improving the energy efficiency of the process, eliminating the chance of water leakage, and improving the stability of the process.

[0051] Referring to Figure 1, a schematic diagram of a plasma reactor R (plasma vapor phase silica reactor) is shown, wherein a stream of silica, such as crushed quartz, preferably crushed quartz in the size range of less than 2 cm, is continuously or intermittently supplied to the furnace through a supply port 1. The reactor R is constructed of a steel shell with a refractory lining 8, which is designed to maintain the internal temperature of the reactor R at a temperature above the melting point of the silica source, preferably +1700 degrees Celsius.

[0052] The reactor R is heated using two or more electrodes 2 (graphite electrodes with gas injection sections), which are preferably made of graphite to ensure that the electrode erosion material vaporizes and does not contaminate the final product of the fumed silica process. The electrodes 2 are sealed using a high-temperature sealant (sealant) 3 to prevent excessive air ingress into the reactor R and to allow the method to operate under a slight vacuum. At the start of the method, a plasma arc 6 is initially generated between the electrodes 2 and the lower conductive plate 9, creating a pool of molten silica 7 (molten silica bath). This molten silica bath serves as the conductive medium between the plasma arc 6 and the conductive plate 9 and is consumed by the plasma arc 6 due to the evaporation process.

[0053] Electrode 2 can be a hollow cylindrical component, allowing the injection of the following gases: inert plasma-forming gases, such as argon, to obtain plasmas at very high temperatures; and / or reactive plasma-forming gases, such as vapor and / or a mixture of O2 as an oxygen source, to re-oxidize the decomposition products of silica, primarily SiO; and H2 as a hydrogen source for hydrogen bonding of fumed silica particles. Other gases, such as ammonia, can be injected through the hollow electrode 2 to lower the evaporation / decomposition temperature of silica and / or increase the production rate of fumed silica and / or for the same reasons as H2 injection.

[0054] Silica simultaneously evaporates and decomposes at the junction of the plasma arc 6 and the molten silica bath 7. The intense heat of the plasma arc 6 causes silica, i.e., SiO2 (in quartz form), to melt, evaporate, and decompose to form SiO. SiO is rapidly quenched using hydrogen- and oxygen-containing gases, such as steam or a mixture of steam and air, via gas injection port 5 (quenching gas injection port) to oxidize SiO to SiO2 and introduce hydroxyl groups (OH-) onto the surface of nanoscale amorphous silica particles. Other reactants can be introduced into reactor R via quenching port 5 to enhance the surface properties of fumed silica, such as making the surface of fumed silica hydrophobic or hydrophilic. Various quenching configurations can be used to produce different product characteristics. SiO reacts with oxygen to reform SiO2, but in nanoscale amorphous particles. The nanoparticles then aggregate to form a three-dimensional chain structure as they exit reactor R along with the gas flow through reactor outlet 4 (fumed silica reactor outlet).

[0055] The electrons in reactor R travel from the graphite electrode 2, forming a plasma arc 6 between electrode 2 and the molten silica bath 7, and flow through the conductive molten silica to a conductive plate 9, preferably made of a carbon-based material, such as graphite. The current then flows through a copper rod serving as the anode 10, which is equipped with cooling fins and cooled using forced air cooling. The furnace design also allows the arc 6 to be ignited, or reignited if it extinguishes during operation, using only the top electrode in the anode-cathode configuration to generate a plasma arc between the electrodes, first remelting the solidified silica, and then transferring it to the molten silica by switching to the bottom anode configuration. Helium gas can be passed through the electrode injection section to aid in arc ignition.

[0056] Turning to Figure 2, silica in the form of crushed quartz 11 is introduced into the plasma reactor R via an automated supply system. Additives can be premixed with the quartz feed and co-fed or intermittently supplied with the quartz to improve the conductivity of the molten silica, and / or to improve the plasma arc process by lowering the melting temperature of the silica, and to provide a higher operating temperature range to minimize the chance of solidification of the melt in the reactor during operation. The additives are, for example, preferably miscible metals or metal oxides in the molten silica, meaning that only one phase, i.e., a single slag phase, exists at any operating temperature. The additives preferably have a higher vapor pressure than the silica vapor pressure under reactor operating conditions (e.g., temperature and pressure), so that the additives do not co-evaporate / co-decompose with the silica and contaminate the gas-phase silica product, or co-evaporate / co-degrade in the same form as the quartz feed or in powder form at a rate significantly lower than that of the silica. For example, according to the SiO2-Al2O3 phase diagram [see reference 5], adding only 0.043 mol% Al2O3 to the silica melt can reduce the melting temperature of silica from 1723 °C to 1597 °C, and at the same time increase the conductivity of the silica melt by 10 to 20 times [see reference 6].

[0057] The supply system includes a feed hopper and mixer 13 and a screw conveyor 14. Quartz 11 is introduced into reactor R intermittently or continuously, with or without additives. Electrode 2 generates a plasma arc 6 (Fig. 1) within reactor R using an AC / DC power supply 15, with a switch set at 15'. This plasma arc 6 melts and decomposes the quartz 11. Quenching gas, such as steam, is generated at 16 (steam generator) and injected into reactor R. As the gaseous silica rapidly cools and solidifies, it forms chains of amorphous SiO2 nanoparticles in the form of fumed silica, which escape from reactor R along with the hot gas stream. A blower 17 (cooling fin blower) is used to cool the bottom anode 10 and its electrical connections. The hot gas stream and fumed silica particles exit reactor R, and larger fumed silica agglomerates are collected by a cyclone separator 18. The hot gas stream is cooled using an indirect gas / liquid cooler 19. A bag filter 20 (bag-type fumed silica collector) then separates most of the finer fumed silica particles from the gas stream. The gas is then filtered again using a fine particulate filter 21 to ensure that no silica is released into the atmosphere. An induced draft fan 22 is used to draw the gas out of the furnace and maintain the system at a pressure slightly below atmospheric pressure.

[0058] The table below summarizes the environmental benefits of the plasma method and apparatus (reactor) of the present invention for the production of fumed silica compared to conventional methods:

[0059]

[0060] Therefore, compared with existing industrial fumed silica manufacturing methods, the innovative plasma arc method and apparatus of the present invention for manufacturing fumed silica can provide a reduction of GHG emissions by about 85% and a reduction of energy consumption by about 89%.

[0061] While the foregoing description provides examples of embodiments, it will be understood that some features and / or functions of the embodiments are readily modifiable without departing from the spirit and operating principles thereof. Therefore, the foregoing is intended to be illustrative and not restrictive, and those skilled in the art will understand that other variations and modifications may be made without departing from the scope of the embodiments as defined in the appended claims.

[0062] References:

[0063] [1] Source: PCI calculation, data used from: Brandt, B et al., “Silicon-Chemistry Carbon Balance – An assessment of Greenhouse Gas Emissions and Reductions”, Executive Summary, Global Silicones Council et al., 2012.

[0064] [2]Assuming a Canadian average for electricity carbon intensity (0.15tCO2eq / MWh).

[0065] [3] Everest, D.A., Sayce, I.G. and Selton, B., “Preparation of Ultrafine Silica powders by Evaporation Using a Thermal Plasma”, Symposium on Electrochemical Engineering, Institution of Chemical Engineers, I p.2.108-2.121 (1971).

[0066] [4] IEAPVPS Task 12, Subtask 2.0, LCAReport IEA-PVPS12-04:2015-January2015ISBN 978-3-906042-28-2.

[0067] [5] Strelov, KK, Kashcheev, ID. Phase diagram of the system Al2O3-SiO2. Refractories 36, 244–246 (1995).

[0068] [6] Thibodeau, E., Jung, IH. Structural Electrical Conductivity Model for Oxide Melts, Metallurgical and Materials Transactions B, Vol. 47, No. 1, 355–383 (2016). https: / / doi.org / 10.1007 / s11663-015-0458-z.

Claims

1. An apparatus for producing fumed silica, the apparatus comprising: - A reactor adapted to generate a plasma arc; - At least one top electrode, said at least one top electrode extending into molten silica contained in the reactor; - A conductive plate disposed below the molten silica; - Bottom anode, wherein the plasma arc provided at the tip of the electrode is directly transferred to the molten silicon dioxide for the formation of SiO; - A quenching system comprising hydrogen- and oxygen-containing gases injected into the reactor, and the quenching system being adapted to reform SiO2 into nano-sized amorphous particles; and - Outlet, which allows fumed silica to leave the reactor.

2. The apparatus of claim 1, wherein, The current path flowing through the reactor begins at the electrode, forms the plasma arc between the electrode and the molten silica, flows through the conductive molten silica to the conductive plate, and then flows through the bottom anode.

3. The apparatus of claim 1, wherein, The bottom anode is provided with cooling fins, and a blower for cooling the cooling fins is provided therein.

4. The device according to claim 1, wherein, The quenching system includes at least one gas injection port.

5. The device according to claim 1, wherein, A cyclone separator is provided to collect larger-sized gaseous silica agglomerates as the hot gas stream and gaseous silica particles exit the reactor through the outlet.

6. The device according to claim 5, wherein, A gas / liquid cooler for cooling the hot gas stream is provided downstream of the cyclone separator.

7. The device according to claim 6, wherein, A bag filter is provided downstream of the gas / liquid cooler to separate most of the finer fumed silica particles from the gas stream.

8. The device according to claim 7, wherein, A fine particulate filter is disposed downstream of the bag filter, the fine particulate filter being used to further filter the gas and remove trace amounts of fumed silica.

9. The device according to claim 8, wherein, A blower is provided downstream of the fine particulate filter to draw the gas out of the reactor and provide a pressure below atmospheric pressure.

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