PROCESS FOR TREATING SILICA-BEARING MATERIAL

Abstract: A process (10) for treating silica-bearing material comprises maintaining a hot reducing atmosphere in a reaction zone (11), and passing a particulate solid silica-bearing material through the reaction zone (11) in an unsupported manner. The material is allowed to heat up and to react as it passes through the zone (11), thereby to produce a silicon-rich fraction (line 16) and a silicon-poor fraction (line 18).

Description: PROCESS FOR TREATING SILICA-BEARING MATERIAL THIS INVENTION relates to a process for treating silica-bearing material. According to the invention, there is provided a process for treating silica-bearing material, which comprises maintaining a hot reducing atmosphere in a reaction zone; passing a particulate solid silica-bearing material through the reaction zone in an unsupported manner; allowing the material to heat up and to react as it passes through the zone, thereby to produce a silicon-rich fraction and a silicon-poor fraction. The silica-bearing material can, at least in principle, be any silica-bearing material such as a silica containing composite mineral eg low grade aluminium silicate, nickel silicate, manganese silicate, or the like. The metal component will then form part of the silicon-poor fraction. However, the silica-bearing compound may, in particular, be zircon (ZrSi0 4 ) , and the invention will further be discussed with reference to zircon only; however, as set out hereinbefore, the invention is not necessarily limited to zircon being the only silica-bearing compound which can be used. The zircon may be heated, as it passes through the hot reducing atmosphere, to such a degree that the zircon starting material dissociates to form a dissociated product (Zr0 2 -Si0 2 ) which is converted further in the reaction zone into the silicon-rich fraction and the silicon-poor fraction. The silicon-poor fraction is thus desilicated zircon or zirconia (Zr0 2 ) . In other words, the zircon starting material is heated to a suitable dissociation temperature. Maintaining the hot reducing atmosphere in the reaction zone may be effected by feeding a reducing agent into the reaction zone, while heating the reaction zone. While the reducing agent or reductant can be in solid form, it is preferably in gaseous form. Thus, it may be a volatile hydrocarbon such as methane, a volatile inorganic substance such as hydrogen sulphide (H 2 S) , or hydrogen (H 2 ) . The heating of the reaction zone may be effected by means of plasma generating means. In other words, the heating may be effected by means of a plasma flame. However, the zircon starting material may also be heated in other ways to the dissociation temperature, such as in a suitable thermal oven or furnace providing the reaction zone, by means of a flame reactor or furnace providing a suitable flame in the reaction zone, or the like. The use of a plasma flame is generally preferred, since a thermal plasma of intense localised high enthalpy is then provided in the reaction or high temperature plasma zone, in which zone the required dissociation temperatures are achieved. The zircon starting material may be introduced, using suitable techniques, into the thermal plasma in the reaction zone, where dissociation of the zircon will take place to yield a plasma dissociated zircon ('PDZ') . Any suitable plasma generating means may be used, such as a high voltage DC plasma burner, or radio frequency or microwave generated plasmas; however, at least one non-transfer-arc plasma gun or burner is preferred. In accordance with known technology, an inert plasma gas, such as argon (Ar) or nitrogen (N 2 ) , may be used with the gun or burner to create the thermal plasma. The plasma may thus be generated by a plurality of the non-transfer-arc plasma guns. In particular, three such guns, arranged in star fashion with their operative ends being downwardly inwardly directed, may be provided. The dissociation of the zircon may be achieved at reaction zone or plasma temperatures above about 1700°C, preferably above about 2200°C, and up to about 5000°C. The higher the temperature, the quicker will be the dissociation of zircon. The passing of the zircon through the reaction zone in said unsupported fashion may be effected by feeding the particulate zircon into a feed zone above the high temperature plasma or reaction zone provided by the plasma flame, and allowing the zircon to free-fall through the plasma or reaction zone while being subjected to the reducing atmosphere, thereby to be dissociated into the plasma dissociated zircon ('PDZ') and thereafter converted into the silicon-rich and zirconia fractions, ie the dissociation and conversion are effected while the zircon particles are in flight. The process may include feeding the zircon and reductant continuously into the reaction zone, and withdrawing the product fractions from the reactor continuously or on an intermittent basis. According to one aspect of the invention, the silicon-rich fraction may comprise mainly silica (Si0 2 ) , with it and the zirconia fraction collecting at the bottom of the reaction zone in admixed solid particulate form. Thus, the particles of the silicon-rich fraction are physically distinct or separate from those of the zirconia fraction. The average particle size of the mixture will then be less than that of the zircon. Thus, when the average particle size of the zircon is 3-150 μm, eg 5-100 μm, the majority of particles of the admixture may be of submicron size. Thus, the average particulate size of the admixture may be <1 μm. This admixture is suitable for use as an opacifier without further comminution thereof being required. In fact, the particle size thereof can be regulated as desired by controlling the dissociation temperature and/or the intensity of heating of the zircon starting material and/or the proportion of reducing agent fed into the reaction zone. The zirconia fraction or component, which can readily be separated from the finer and pure silica component using known physical or other separating techniques, can also be used as a substitute for baddeleyite. In addition, separation of the mixture of submicron zirconia component and submicron silica component from residual unreacted zircon and other larger products, can readily be effected using such known separating techniques. The average particle size of the silica component can be as small as 10 nm. The average particle size of the zirconia fraction can also be as small as 10 nm, but the predominant average particle size thereof is about 100 nm. Depending on the reducing agent used, the mixture may contain impurities. Thus, if a CH 4 /neutral gas, eg nitrogen, atmosphere is used, carbon and/or nitrogen contamination may be experienced. The process may then include injecting a suitable agent, eg air, into the tail end of the plasma, ie at the lower end or below the reaction zone, to reduce the contamination. According to another aspect of the invention, the zircon and/or the fractions produced are heated sufficiently, eg subjected to a sufficiently high temperature in a preferred reducing atmosphere, so that the silicon-rich fraction is in the form of a vapourized reduced silicon oxide, with this fraction being withdrawn from the reaction zone in vapourized form leaving a substantially desilicated zirconia product as residue in the reaction zone. This can naturally be recovered. The reduction of the silica preferably takes place in situ as it is formed. The reduced silicon-oxide may be silicon monoxide. The withdrawn vapourized fraction may then be treated to convert the silicon monoxide to a substantially purified reconverted silica (Si0 2 ) product. This can then be effected by means of hydrolysis. The process according to this embodiment of the invention thus entails suitably heating zircon in the heating or reaction zone to form dissociated zircon (Zr0 2 -Si0 2 ), reacting the dissociated zircon with the reductant, to reduce the silica (Si0 2 ) component, preferably in situ, to the silicon monoxide (SiO) product at a temperature sufficient to cause simultaneous vaporisation of the silicon monoxide, continuously removing the vaporised silicon monoxide product to yield a separated substantially desilicated zirconia (Zr0 2 ) product, and subsequently converting the silicon monoxide to a substantially purified silica (Si0 2 ) product. An important feature of this aspect of the invention comprises the reduction of the silica component of the dissociated zircon, and the removal of the reduced silicon monoxide product, while the zircon is in the dissociated state, and before the back reaction usually associated with dissociated zircon production, whereby the silica component and the zirconia component associate again to form zircon, takes place. The importance of this feature relates to the need to provide a cost-effective and technically feasible mechanism for separating the silica component of a zircon starting material from the zirconia component once these components are dissociated such as by plasma dissociation. In the plasma dissociation of zircon, a distinct glassy phase (Si0 2 ) and a well dispersed submicron polygonal crystallite phase (Zr0 2 ) are formed. Although it is possible for the glassy phase to be leached from the crystallite phase under relatively mild conditions, such as with alkali solutions at about 150°C, such chemical processing adds to the overall production costs and is thus unattractive from a commercial point of view. However, with plasma dissociation of zircon a commercially attractive zirconia product may be obtained, having inherent attributes of fineness of the crystallite particles and high reactivity towards acid dissolution. The invention provides a method whereby the glassy silica phase may be removed in situ, as it is formed, to obviate the need for and cost of subsequent chemical separation. According to this aspect of the invention, the reduction of the silica component thus preferably takes place at a temperature at which the zircon is still in a dissociated state and the formed silicon monoxide product is vaporised while the zirconia remains in a finely divided crystalline phase, so that the vaporised silicon monoxide can be removed with a gaseous stream including also excess reductant and/or gaseous or vaporous reaction products, such as steam, and the inert plasma gas which is used in the generation of the thermal plasma. It will be appreciated that after such removal of a vaporised silicon compound, a substantially desilicated zirconia product will remain, the product being in a fine crystallite form if a suitable cooling procedure is followed. An important and beneficial consequence of this feature of the invention, is that controlled quenching of the dissociated zircon, to avoid re-association of the two components, is no longer critical. Cooling of the zirconia product remains important, and should preferably take place rapidly, to ensure that a fine particulate product is formed. Furthermore, chemical processing of the plasma dissociated zircon product, after controlled quenching, to remove the silica component for the production of a desilicated zirconia product, is eliminated. A factor which is of particular relevance to ensure that reduction and vaporisation of the silica component, and removal of the vaporised product, will take place effectively, is the operating temperature at which the reduction and vaporisation and the removal of the vaporised product take place. At operating temperatures above 1700°C, and preferably above 2200°C, reduction of the silica component and simultaneous evaporation of the silicon monoxide formed, will take place. Removal of the vaporised product should take place before condensation of the silicon monoxide occurs, at a temperature of about 1400°C and below. This aspect of the invention thus envisages the continuous removal of a gaseous stream including the vaporised silicon monoxide, excess reductant, inert plasma gas, and gaseous or vaporous reaction products, such as steam, at operating temperatures not below, and preferably above 1400°C. Sufficient reductant should be present to ensure substantially complete reduction of the silica to silicon monoxide. For example, a thermal Ar/H 2 plasma containing at least about 40% by volume of hydrogen may accordingly be provided, to bring about substantially complete reduction of the silica component (Si0 2 ) to silicon monoxide (SiO) . The reductant, such as CH 4 or H 2 , may conveniently be fed into the plasma burners with the inert plasma gas (Ar) in accordance with known technology. The removed gaseous stream may be fed to a converter stage, where the silicon monoxide is converted to silica. The temperature in the converter stage may be allowed to fall to about 1400°C and below, when condensation of the SiO will take place, resulting in disproportionation to form highly reactive fine silicon metal (Si) particles, and silica (Si0 2 ) . According to another embodiment of this aspect of the invention, the silicon metal particles may be reacted with steam or a condensed water phase, to form silica (Si0 2 ) with the liberation of hydrogen (H 2 ) . It will be appreciated that the removed gaseous stream including the vaporised silicon monoxide, will also contain steam as a reaction product if the reductant is hydrogen, so that no or little additional steam or water will need to be added to permit the conversion reaction of silicon metal particles into silica to proceed. Excess steam/water may in fact have to be removed from the Ar/H 2 recycle stream prior to entering into the initial process feed line. To this end, a dehumidification stage may be provided after the SiO conversion stage, and where moisture may be removed. The silica (Si0 2 ) formed in the condensation stage and by the reaction of silicon particles with steam/water, may be recovered as a substantially pure, fine particulate product of pyrogenic quality which is suitable to replace purposely manufactured pyrogenic silica in all known applications thereof. The zirconia product thus obtained is suitable for use in the ceramic industry as well for use as a raw material in the zirconium industry. The invention and the manner in which it may be carried out will now be explained by way of example, with reference to the accompanying diagrammatic drawings. In the drawings, FIGURE 1 shows a schematic representation, in the form of a flow diagram, of a process according to one aspect of the invention, for treating silica-bearing material; and FIGURE 2 shows a similar schematic representation of a process according to a second aspect of the invention, for treating silica-bearing material. Referring to Figure 1, a process 10 according to one aspect of the invention for treating a silica-bearing material, has a thermal reaction zone 11 in which a thermal plasma of high enthalpy and temperature is provided. The temperature of the plasma is at least 1700°C, preferably at least about 2200°C, and may even be as high as about 5000°C. The thermal plasma may be generated by any suitable plasma generation means, such as one or more high voltage DC plasma burners. The plasma burners (not shown) may be arranged in a suitable configuration, to provide a zone of intense localised heat, into which a particulate material to be treated may be effectively introduced. A solids feed line 12 is provided, for feeding a particulate zircon starting material, having an average particle size less than 100 μm, into the thermal zone 11. If desired, the feed line 12 may be heated, to pre-heat the starting material, and also to remove impurities. It should be understood that the zircon starting material may be obtained from beach sands as a source material so that organic and other impurities may be present. A gas feed line 14 is provided, to feed an inert plasma gas, which is preferably argon, and a gaseous reductant, which is preferably methane or hydrogen, into the thermal zone 11, In the thermal zone, the zircon is dissociated into a zirconia component and a silica component, according to the following formula: plasma ZrSi0 4 → Zr0 2 -Si0 2 dissociation Simultaneously, the reductant reduces the formed silica to form silicon monoxide, with the formation of steam. This is effected according to the following formula when the reductant is hydrogen: Si0 2 + H 2 → SiO + H 2 0 At the prevalent temperature in the thermal zone 11, the SiO is vaporised, so that a first gaseous stream is continuously removed from the thermal zone 11 via gas output line 16, the gaseous stream including vaporised SiO, steam, excess H 2 and argon. It will be appreciated that such continuous removal of vaporised SiO leaves behind the zirconia component, which is recovered through solids output line 18 as a substantially desilicated fine crystallite product. The gas output line 16 leads to a converter stage 20, where SiO is converted to Si0 2 . The temperature in the converter stage 20 is permitted to fall below 1400°C, so that the vaporised SiO condenses, resulting in a disproportionation to form Si0 2 and highly reactive silicon metal particles, according to the formula 2SiO → Si0 2 + Si The metal Si particles will react with the steam present in the first gaseous stream, or with a water phase if condensation of steam has taken place, to form Si0 2 with the liberation of hydrogen, according to the formula Si + 2H 2 0 → Si0 2 + 2H 2 It should be noted that the temperature in the gas output line 16 should not be allowed to fall to the condensation temperature of SiO, ie about 1400°C, to avoid solidification of SiO in the feed line. The gaseous phase in the converter stage 20, including liberated hydrogen, excess steam and argon, is withdrawn from the converter stage 20 as a second gaseous stream, via line 22, while the formed Si0 2 is recovered through the solids output line 24 in the form of a substantially pure fine particulate product. For the separation of the solid Zr0 2 and Si0 2 products from the gaseous phases, any suitable separation procedure may be used, such as for example suitable cyclone or filter bag filtration devices. The feed line 14 may lead directly into the thermal zone 11, as shown at 26, or into the solids feed line 12, as shown at 28. Referring to Figure 2, reference numeral 100 generally indicates a process according to a second aspect of the invention, for treating a silica-based material. Parts of the process 100 which are the same or similar to those of the process 10 are indicated with the same reference numerals. In the process 100, the silicon-rich fraction is not withdrawn from the zone 11 as a vapourized stream. Instead, the reaction temperature is controlled such that the silicon-rich fraction accumulates, as a silica component, below the reaction zone together with the zirconia fraction, to form an intimate admixture of submicron average particle size. The mixture, after being withdrawn along the flow line, is suitable for use as an opacifier. In the admixture, the zirconia particles are physically distinct from the silica particles. The process 100 was simulated on laboratory scale in the following examples: EXAMPLE 1 Zircon (3-15 μm) was allowed to free-fall, in a reaction zone, through a 12kW methane/nitrogen plasma, having a temperature of at least 2000°C, at a rate of 0,5 kg/h. The methane rate into the plasma torch was 0,32 to 0,37 m 3 /h. The zircon was dissociated and desilicated, and the solid product collected. The product was in the form of a fine black powder consisting of submicron Zr0 2 crystallites and Si0 2 amorphous powder, as well as some unreacted zircon. The product had the following characteristics: particle sizes ranging from 0,01 to 15 μm chemically bound carbon and nitrogen contaminant levels of up to 1% each were obtained desilication efficiency of approximately 80% was obtained. EXAMPLE 2 Zircon (3-15 μm) was allowed to free-fall, in a reaction zone, through a 12kW methane plasma, having a temperature of at least 2000°C, at a rate of 0,5 kg/h. The methane flow rate into the torch was 0,32 to 0,37 m 3 /h. Dissociation and desilication of the zircon took place in the reaction zone, and the product was subjected to secondary air contact at the plasma tail to oxidize all the Si-species. The product was collected as a fine white powder. The product had the following characteristics: - specific surface area of approximately 27 m 2 /g average particle size of Zr0 2 of 0,1 μm average particle size of Si0 2 of 0,01 μm desilication efficiency of approximately 80% was obtained A product consisting mainly of very fine amorphous Si0 2 was collected on filters through which the gaseous product from the reaction zone was passed. EXAMPLE 3 In a pilot scale apparatus zircon (average particle size 100 μm) was allowed to free-fall through a 100 kW multiple nitrogen plasma burner system, at a feed rate of approximately 30 kg/h, under the mildly reducing atmosphere in the plasma or reaction zone. Quenching was achieved by a countercurrent air stream. This air stream was cleaned of dust, firstly, in a cyclone and, secondly, in a system of filters. In the cyclone, predominantly PDZ (plasma dissociated zircon) was collected, while the material on the downstream filter consisted to a degree of more than 90% of very fine amorphous silica. In a similar laboratory experiment, the collected material was again essentially pure very fine amorphous silica. Experiments showed that the more aggressive the reducing atmosphere, the more efficient the desilication. For example, better desilication is achieved with methane than with hydrogen, under prevailing plasma conditions. SUBSTITUTESH&T(RULE.26)

Claims: CLAIMS : 1. A process for treating silica-bearing material, which comprises maintaining a hot reducing atmosphere in a reaction zone; passing a particulate solid silica-bearing material through the reaction zone in an unsupported manner; allowing the material to heat up and to react as it passes through the zone, thereby to produce a silicon-rich fraction and a silicon-poor fraction. 2. A process according to Claim 1, wherein the silica-bearing material is zircon (ZrSi0 4 ) with the zircon being heated to such a degree that the zircon dissociates to form a dissociated product (Zr0 2 -Si0 2 ) which is converted further into the silicon-rich fraction and the silicon-poor fraction, with the silicon-poor fraction thus being desilicated zircon (Zr0 2 ) . 3. A process according to Claim 2, wherein the maintaining of the hot reducing atmosphere in the reaction zone is effected by feeding a reducing gas into the reaction zone while heating it by means of a plasma flame generated by at least one non-transfer-arc plasma gun. 4. A process according to Claim 3, wherein the passing of the zircon through the reaction zone in said unsupported fashion is effected by feeding the particulate zircon into a feed zone above a high temperature plasma zone provided by the plasma flame, and allowing the zircon to free-fall through the plasma zone while being subjected to the reducing atmosphere, thereby to be dissociated into plasma dissociated zircon (Zr0 2 -Si0 2 ) and thereafter converted into the silicon-rich and zirconia fractions. 5. A process according to Claim 4, wherein the silicon-rich fraction comprises mainly silica (Si0 2 ) , with it and the zirconia fraction collecting at the bottom of the reaction zone in intimately admixed solid particulate form, and with the average particle size of the mixture being substantially less than that of the zircon. 6. . A process according to Claim 5, wherein the average particle size of the zircon is 3-150 μm, with the majority of particles of the admixture being of submicron size. 7. A process according to Claim 4, wherein the zircon and/or the fractions produced are heated sufficiently so that the silicon-rich fraction is in the form of a vapourized reduced silicon oxide, with this fraction being withdrawn from the reaction zone in vapourized form. 8. A process according to Claim 7, wherein the reduced silicon-oxide is silicon monoxide, with the withdrawn vapourized fraction being treated to convert it to a silica (Si0 2 ) product.

Inventors: DE BEER JACOB ADRIAAN, DE WET WILLEM JOHANNES, DE BEER, JACOB, ADRIAAN, DE WET, WILLEM, JOHANNES
Applicants: BEER JACOB ADRIAAN DE, PELTEK INC, WET WILLEM JOHANNES DE, DE BEER, JACOB, ADRIAAN, DE WET, WILLEM, JOHANNES, PELTEK, INC.
Application year: 1995
Main classification: C01G25
IPC codes: C01B33, C01G25