METHOD AND APPARATUS FOR PRODUCTION OF AMORPHOUS SILICA FROM SILICON AND SILICON-CONTAINING MATERIALS

Abstract: The present invention relates to a method for production of amorphous silica from particulate materials containing at least 50 % elemental silicon, where the silicon-containing material is supplied to a combustion reactor which is heated by an oil- or gas burner and where amorphous silica is recovered after the outlet from the combustion reactor. At least a part of the particulate silicon containing materials are supplied to the combustion reactor in the form of an aqueous slurry. The invention further relates to an apparatus for production of amorphous silica.

Description: Title of Invention Method and apparatus for production of amorphous silica from silicon and silicon-containing materials. Field of Invention The present invention relates to a method for producing amorphous silica from silicon and silicon-containing materials by flame combustion of silicon- containing materials with air, oxygen-enriched air or oxygen. The invention further relates to apparatus for production of amorphous silicon. Background Art Amorphous silica having a small particle size and a high surface area is today mainly produced by reacting volatile silicon compounds, particularly silicontetrachlor.de with an oxygen-containing gas in a flame. This product which is called "fumed silica" has a number of applications, particularly as a filler in plastic and rubber, it is further known that similar amorphous silica products can be produced in plasma furnaces and electric furnaces where the reactants are quarts and carbon or silica and quarts. These products are, however, costly and have not found much use. From EP-B-151490 and US patent No. 4,755,368 it is known methods for flame combustion of silicon where particulate silicon fluidized in a gas such as air or oxygen, is supplied to a reactor where the silicon is combusted in a flame to produce SiO 2 particles. From Norwegian patent No. 304366 it is further known a method for production of amorphous silica from particulate silicon-containing materials where particulate silicon is supplied in fluidized form to a combustion reactor using an oxygen-containing fluidizing gas and where additional oxygen- containing gas is supplied to the combustion reactor whereby the particulate silicon is burned to SiO 2 without the addition of further heat energy. It is a disadvantage of the known method for combustion of silicon powder to amorphous SiO 2 that it is difficult to obtain an even and consistent process. It is believed that a main reason for this is that it is difficult to obtain an even and constant supply of Si-powder to the reactor when the silicon powder is supplied to the reactor fluidized in a gas. Further it is, as can be seen from Norwegian patent No. 304366, only obtained a specific surface area of about 20 m 2 /g for the produced SiO 2 while it for many applications is needed amorphous silica having a much higher surface area of 80 m 2 /g or higher. Disclosure of Invention It is an object of the present invention to provide a method and an apparatus for combustion of silicon-containing materials to amorphous SiO 2 , whereby silicon is supplied to the combustion reactor in a very even and stable way and whereby it is obtained amorphous silica having a surprisingly high specific surface area. The present invention thus relates to a method for production of amorphous silica from particulate materials containing at least 50 % elemental silicon, where the silicon-containing material is supplied to a combustion reactor which is heated by an oil- or gas burner and where amorphous silica is recovered after the outlet from the combustion reactor, which method is characterized in that at least a part of the particulate silicon containing materials are supplied to the combustion reactor in the form of an aqueous slurry. According to a preferred embodiment the silicon is supplied to the combustion reactor in the form of an aqueous slurry containing between 20 and 80 % by weight silicon-containing material, and preferably between 40 and 60 % by weight of silicon-containing material. According to another preferred embodiment a viscosity regulating agent such as a mineral acid or organic dispersing agents is added to the aqueous slurry containing silicon-containing material. In order to modify the surface of the produced amorphous silica it can be added surface modifying agents to the aqueous slurry, such as for instance salts of aluminium, preferably aluminium sulphate. According to yet another embodiment the amorphous silica is quenched by addition of water in the area of the outlet from the combustion reactor. It has surprisingly been found that by supplying silicon in the form of an aqueous slurry it is obtained a steady state operation in the reactor at the same time as the produced amorphous silica has a surprisingly high specific surface of more than 100 m 2 /g. As silicon-containing material it can be used any particulate material having a content of elemental silicon of at least 50 %. Examples of such materials are metallurgical grade silicon in particulate form, high purity silicon in particulate form, atomized silicon, kerf from the sawing of silicon wafers for electronics and for solar cells, filter dust obtained from crushing of silicon and residues from reactors for the production of organohalosilanes and inorganic silanes. The present invention further relates to an apparatus for the production of amorphous silica from particulate materials containing at least 50 % by weight of elemental silicon, which apparatus comprises an elongated reaction chamber having an oil- or gasburner in one end, at least one opening arranged along the sidewall of the combustion reactor for supply of silicon containing material in the form of an aqueous slurry, an outlet opening for produced amorphous silica and means for collecting amorphous silica. The combustion reactor has preferably an inlet opening for supply of air, oxygen enriched air or oxygen. Short description of the Drawing Figure 1 shows a sectional view of the apparatus according to the invention. In Figure 1 there is shown a cylindrical shaped combustion reactor 1. The reactor 1 comprises an outer steel cylinder 2 having an inner refractory lining 3. In one end the reactor 1 has an oil- or gas burner 4. In its other end the reactor 1 has an outlet opening. A cooling chamber 5 for circulating cooling air is arranged on the outside of the steel cylinder 2. The cooling chamber 5 has an inlet opening 6. The reactor 1 is further equipped with at least one inlet opening 7 for supply of silicon containing material in the form of an aqueous slurry. The cooling air supplied through the inlet opening 6 to the cooling chamber 5, leaves the cooling chamber 5 and into a chamber 8 which via an outlet 9 is connected to a filter or the like for collecting the produced amorphous silica. It is further arranged an inlet opening 10 for supply of air, oxygen enriched air or oxygen to the reactor 1. When starting the process, the reactor 1 is first heated by the oil- or gas burner 4. Thereafter the supply of silicon is started by supply of an aqueous slurry containing silicon through the inlet opening 7 and supply of air, oxygen enriched air or oxygen through the opening 10. The silicon particles which enter the reactor will ignite and be burned to amorphous silica as they pass through the reactor 1. EXAMPLE 1 Silicon dust collected from a plant for crushing metallurgical grade silicon was mixed with water to a slurry containing 60 % by weight of silicon. The slurry was supplied to a reactor as shown in figure 1 after the reactor had been heated to a temperature of about 1550°C at the inlet end of the reactor. The slurry was pumped into the inlet opening 7 of the reactor in an amount of 0.2 kg per. minute together with 500 liters of oxygen per. minute. The produced amorphous silica was collected in a filter unit connected to the outlet opening 9 in the chamber 8. The specific surface area of the amorphous silica was measured by DIN 66132 (BET one point measurement). The produced amorphous silica had a specific surface area of 114.2 m 2 /g. The amorphous silica was examined in order to measure content of crystalline silica using a Philip PW1710 x-ray diffraction apparatus having a copper anode in the x-ray tube. No crystalline silica was found. EXAMPLE 2 The test in Example 1 was repeated except that 0.4 kg slurry per. minute was supplied to the reactor. The produced amorphous silica had a specific surface area of 213.6 m 2 /g. EXAMPLE 3 The test in Example 1 was repeated, except that it was used a slurry containing 50 % by weight silicon. The produced amorphous silica had a specific surface area of 117.9 m 2 /g. EXAMPLE 4 The test in example 1 was repeated except that the silicon containing material used was kerf from diamond cutting of silicon wafer for solar cells which had been subjected to milling in 15 minutes in a ball mill. The produced amorphous silica has a specific surface area of 103.2 m 2 /g. Examples 1 - 4 show that by the method according to the present invention it is obtained amorphous silica having a very high specific surface area compared to the amorphous silica produced according to Norwegian patent No. 304366.

Claims: CLAIMS 1. A method for production of amorphous silica from particulate materials containing at least 50 % elemental silicon, where the silicon-containing material is supplied to a combustion reactor which is heated by an oil- or gas burner and where amorphous silica is recovered after the outlet from the combustion reactor, characterized i n that at least a part of the particulate silicon containing materials are supplied to the combustion reactor in the form of an aqueous slurry. 2. Method according to claim 1, ch a ra cte rized i n that the silicon is supplied to the combustion reactor in the form of an aqueous slurry containing between 20 and 80 % by weight of silicon-containing material. 3. Method according to claim 2, cha ra cte rized i n that the silicon is supplied to the combustion reactor in the form of an aqueous slurry containing between 40 and 60 % by weight of silicon-containing material. 4. Method according to claim 1, c h a ra cte ri ze d i n that a viscosity regulating agent is added to the aqueous slurry containing silicon- containing material. 5. Method according to claim 4, c h a ra cte r i ze d i n that a mineral acid is added to the aqueous slurry. 6. Method according to claim 4, characterized in that organic dispersion agents are added to the aqueous slurry. 7. Method according to claim 1 , characterized in that surface modifying agents is added to the aqueous slurry. 8. Method according to claim 7, characterized in that salts of aluminium is added as a surface modifying agent. 9. Method according to claim 8, c h a ra ct e r i z e d i n that aluminium sulphate is added as a surface modifying agent. 10. Method according to claim 1, ch a racte rized i n that the amorphous silica is quenched by addition of water in the area of the outlet from the combustion reactor. 11. Apparatus for the production of amorphous silica from particulate material containing at least 50 % by weight of elemental silicon, characterized in that the apparatus comprises an elongated reaction chamber (1) having an oil- or gasbumer (4) in one end, at least one opening (7) arranged along the sidewall of the combustion reactor (1 ), for supply of silicon containing material in the form of an aqueous slurry, an outlet opening (9) for produced amorphous silica and means for collecting amorphous silica. 12. Apparatus according to claim 11 , characterized in that the combustion reactor (1) has an inlet opening (10) for supply of air, oxygen enriched air or oxygen.

Inventors: HENRIKSEN KNUT, VEGGE OLAF TRYGVE, HENRIKSEN, KNUT, VEGGE, OLAF, TRYGVE
Applicants: ELKEM MATERIALS, HENRIKSEN KNUT, VEGGE OLAF TRYGVE, ELKEM ASA, HENRIKSEN, KNUT, VEGGE, OLAF, TRYGVE
Application year: 2000
Main classification: C01B33
IPC codes: B01J19, C01B33