DE10143685B4 - Structured silicon carbide particles, process for their preparation and use - Google Patents

Abstract

Structured Silicon carbide particles in the form of primary and secondary particles manufactured by a method according to at least one of claims 10 to 24, the secondary particles essentially composed of silicon carbide primary particles, where the secondary particles morphologically and structurally whole, essentially whole or in part the morphology and structure of the particles of the starting material kieselguhr / diatomaceous earth correspond.

Description

The The invention relates to the field of ceramics and relates to structured Silicon carbide particles used, for example, in technical products, like porous Filter materials, as a filter aid, catalyst support, auxiliaries for analytical Method, as a storage medium for liquid or gaseous Substances, as a constituent of surface coatings or as Abrasives may be used to process theirs Production and its use.

silicon carbide (SiC) has long been for technical Ceramics used. For Many technical products will use silicon carbide powder as starting material used. Silicon carbide has a very high chemical and thermal Resistance.

Usually, SiC is synthesized via the so-called Acheson process, in which petroleum coke (C) and sand (SiO ₂ ) react in a high-temperature bed. The powder is then prepared by grinding processes from the bulk present reacted SiC. The morphology of such powders is usually splintery with smooth surfaces. The powder particles themselves have no internal porosity. The specific surface area of these powders typically reaches 10 to 15 m ² / g for very small particles.

Alternative methods for producing silicon carbide powders are very numerous. For example, chemical reactions of Si and C containing gases (eg, SiCl ₄ and CH ₄ , respectively), wherein the reaction to SiC is initiated by plasma or laser, or hot areas (eg, CVD) are exploited. The resulting products consist of particles (secondary particles), which in turn are composed of very fine SiC primary particles. As a result, the secondary particles have a fissured surface and z. T. also internal porosity. The primary particles themselves have smooth surfaces and a morphology resulting from the crystal forms of the SiC polytypes. The specific surface area of such powders is thereby higher than that of the ground Acheson powder.

For some Applications, eg. B. as a filter aid, catalyst support, etc., be chemically and thermally stable powder with a high specific surface area and application-specific morphology needed.

The use of silicon carbide powder as a filter aid for beverage production is in DD 215577 A1 proposed. The advantage of using silicon carbide is seen in chemical resistance and cost. These are silicon carbide powders from heating conductor production which correspond to the grain size spectrum of kieselguhr / diatomaceous earths.

Generally is known, in the filtration of liquids in addition to the actual filter, filter aid to use. At all known methods for precoat are the usual Filter aids such as cellulose, kieselguhr / diatomaceous earth, perlite, Charcoal, wood flour, silica gels after more or less prolonged life discarded. This causes waste quantities, which are not insignificant lead to financial expenses. It is likely that this financial expense in the next few years with the introduction of the Kreislaufwirtschaftsgesetz will increase considerably.

in the In essence, the recycling concepts can be centralized and decentralized Divide procedure. In the central process, the ones to be regenerated Filter aid collected from multiple systems and in a central Plant prepared.

In the process of Tremonis ( DE 3935953 C2 ), the kieselguhr / diatomaceous earth used are collected centrally and heated thermally at about 600-700 ° C. Here, organic contaminants are removed from the kieselguhr / diatomaceous earth. A central processing is essential because of the expensive processing plant.

The Procedure has the significant disadvantage that it is due to the central processing of kieselguhr / diatomaceous earth for mixing different diatomaceous earth / Diatomeenerdefraktionen from different Enterprises come, which leads to an inferior kieselguhr / Diatomeenerdemischung, the does not have the same filtration property as Neugur.

In DE 195 07 930 A1 brewery kieselguhr / diatomaceous earth sludge is regenerated at elevated temperature and pressure using a wet oxidation, pressure hydrolysis or chemolysis plant. Even with this method, the expensive system allows only a central processing concept.

The Mixing different kieselguhr / diatomaceous earth fractions from different companies here also to an inferior kieselguhr / Diatomeenerdemischung which does not have the same filtration property as Neugur.

In DE 3623484 A1 the treatment of used kieselguhr / diatomaceous earth is described by means of sodium hydroxide in a low concentration and subsequent rinsing with hydrochloric or sulfuric acid in a decentralized process.

In this process, the chemi regeneration of the filtration-active structure so strong that the filtration properties are adversely affected. This is also reflected in the low permissible concentrations of the detergents or alkalis and acids used.

The reduced stability of kieselguhr / diatomaceous earth in relation to a liquor / acid regeneration takes into account a newer process ( EP 0611249 A1 ), which uses an enzymatic treatment of kieselguhr / diatomaceous earth for regeneration. Here, the kieselguhr / diatomaceous earth is not chemically attacked.

One greater Disadvantage of the method, however, is the very specific Effectiveness of enzymes and especially in brewing technology discussed residue problem of minor traces of foreign enzymes in diatomaceous earth / diatomaceous earth.

Another possibility is the synthetic production of filter aids such as dry-milled or shortened plastic and cellulose fibers, as well as fibrillated plastic and cellulose fibers ( DE 41 10 152 C1 ). Here, filter media with a large inner surface and good filtration properties are to be produced synthetically. The plastics used are polyethylene (HDPE), polypropylene (PE), halogenated polyethylenes, polyoxymethylenes and polyamides. The celluloses used are freed of all soluble constituents by a chemical treatment and are largely resistant to leaching.

One greater Disadvantage of these synthesized filter aids are their comparison to diatomaceous earth / diatomaceous earth worse filtration properties. at Diatomaceous earth / diatomaceous earth is the exoskeleton of seaweed, which have a very fine microstructure with a large inner surface, the main responsible for the excellent filtration properties of kieselguhr / diatomaceous earth are.

The reaction of kieselguhr / diatomaceous earth into silicon carbide or silicon nitride is described by Huang et al. TL Huang, et al., Ts'ai Liao K'o Hsueh (1985), 17A (1), 51-62 and Y. Mizuhara, et al., J. Ceram. Soc. Jpn. (1994), 102 (July), 640-645. From them a rapid conversion of kieselguhr / diatomaceous earth is observed. This good feasibility in silicon carbide or silicon nitride is attributed to the large specific surface area of the diatomaceous earth / diatomaceous earth powder. As target products of the described method silicon carbide powder or Siliziumnitridwhisker be called. In the methods used, the kieselguhr / diatomaceous earth used decomposes as a result of the formation of gaseous SiO according to the following reaction: SiO ₂ + C = SiO + CO

The resulting gaseous silicon monoxide (SiO) serves as an intermediate for the further formation of silicon carbide or silicon nitride according to: SiO + 2C = SiC + CO respectively. 3SiO + 3C + 2N ₂ = Si ₃ N ₄ + 3CO

Due to the above reactions, which usually occur in the carbothermic reduction of SiO ₂ , it is not possible to maintain the morphological structure of diatomaceous earth / diatomaceous earth.

The synthesis of silicon carbide materials while maintaining given structures is being investigated in several works. For example, Greil, P. et al., J. Europ. Ceram. Soc. 18 (1998), Biomorphic Cellular Silicon Carbide of Wood: I. Processing and Microstructure, 1961-1973 the preservation of wood structures by pyrolysis of the wood and subsequent reaction of the remaining carbon with vaporous elemental silicon. Likewise, there are proposals gaseous silicon compounds such. B. SiCl ₄ with carbon bodies to react. With the aid of this method, the production of carbon fiber composites with silicon carbide matrix as described by Fitzer, E. et al, Am. Ceram. Soc. Bull. 65 [2] (1986), Fiber-reinforced silicon carbide, 326-335, and W. Krenkel, Possibilities and limitations of fiber-reinforced ceramics, DKG. Seminar: Material Appropriate Design for Ceramic Components, Stuttgart 1990 has been shown. Characteristic of the examples mentioned is the presence of carbon structures as starting material and structural units to be preserved with minimal dimensions in the range> 10 μm.

The preparation of nano-sized powders of silicon carbide (<0.1 μm) has been reported in the literature (Kamlag, Y. et al, J. of Aerosol Science 31 (2000), Sept., 630-631 and Zhang, B. et al al, Chinese Journal of Lasers 26 (1999) Jan., 93-96). To produce such small particles serve reactions in the gas phase, the implementation of precursors and the implementation of finely dispersed SiO ₂ raw materials such as silica, silica sol or gels.

The resulting structures are obtained in these methods from the thermodynamically induced growth directions of the crystals, so that either spherical-like particles or formed according to the crystal habit particles. The formation of certain shapes can in some cases be controlled via the process conditions, in that selected growth directions in the synthesis are thermodynamically advantageous, which leads to Training whiskers, platelets or balls can lead. The shaping possibilities are always limited to simple geometries.

A continuous synthesis of silicon carbide powder is described in US Pat WO 81/02292 described. In this case, a system with continuous feed of the starting materials, electrical heating and a gas inlet and outlet for generating a fluidized bed is called, which is suitable for the production of various SiC powder types. The advantages presented are mainly in a continuous operation, the use of very different starting materials (solid, liquid, gaseous) and the variability in the generated SiC grain size spectrum. As with the already mentioned methods, it is intended here to produce compact SiC particles without a microstructural design with respect to a different geometry from the formed crystal form.

After DE 27 22 866 C3 Further, there is known a method for producing silicon carbide mainly composed of β crystal. As starting materials, silicon dioxide and carbon are mixed and then introduced continuously or intermittently into a vertical reaction vessel from above, then lowered by gravity through the preheating in the heating zone, heated in the heating zone by indirect electrical heating to 1600-2100 ° C, then in the Cooling zone cooled in a non-oxidizing atmosphere and then removed from the reaction vessel. In this case, the product has a composition by weight of SiC, SiO ₂ and free C in the marked region of the diagram according to FIG 1 this is achieved by keeping the molar ratio C to SiO _{2 of} the starting material in the range of 3.2 to 5.0.

Of the Disadvantage of this solution consists in the use of relatively coarse particles and low heating rates.

task The present invention is structured silicon carbide particles indicate a high specificity with high chemical and / or thermal stability surface have, an easily reproducible method for their preparation and their use.

The The object is achieved by the invention specified in the claims. Trainings are Subject of the dependent claims.

The silicon carbide particles according to the invention, produced by the process according to the invention have a high degree of agreement with the morphology and structure of the kieselguhr / diatomaceous earth used as starting materials. The terms kieselguhr and diatomaceous earth describe the same same substance.

Diatomaceous earth / diatomaceous earth is SiO ₂ as a naturally occurring raw material from sedimented diatoms, which is mining-mined.

Celite / diatomaceous earth is morphologically shaped through the structure of the diatoms.

Diatom types and their structures have been extensively described (Toma, C. R. (ed.) Identifying marine diatoms and dinoflagellates, Academic Press Inc. San Diego, 1996; Drebes, G. Marine's phytoplankton, One Selection of Heligoland plankton algae (Diatoms, Peridineen), Georg Thieme Verlag Stuttgart, 1974).

The Appearance and size of diatoms differs very strongly. There are about 200 different diatom species known.

in the In general, diatoms and their mineralized fossils are prominent through a micro to nanoporous symmetrical structure. The size of the diatoms can ever Algae type in the range of 2 to 300 microns. The outer shape the diatoms varies. You can as a tile, Discs, cylinders, spheres, cuboids and geometrical hybrids occurrence.

By the reaction with carbon by the process according to the invention silicon carbide particles are obtained, which have an internal porosity with a structuring on submicron scale and thereby have a high specific surface ≥ 20 m ² / g, advantageously ≥ 50 m ² / g. The absolute value of the specific surface area depends on the specific surface area of the kieselguhr / diatomaceous earth used. In the reaction, the primary particles of the starting material SiO ₂ are essentially converted to silicon carbide primary particles. Advantageously, at least 30% of the primary SiO ₂ particles should be converted into silicon carbide primary particles.

The present invention enables the production of complex shaped particles by transformation of course organically grown and silicified structures of silica in Silicon carbide particles. Here is the retention of morphology the silicified structures in the silicon carbide particles of particular Meaning and according to the known prior art not yet possible.

According to the invention, the silicon carbide particles according to the invention are produced by adding kieselguhr / diatomaceous earth together with carbon or first carbon and then kieselguhr / diatoms earth are introduced into a reaction space.

advantageously, can the diatomaceous earth / diatomaceous earth with a suspension of a soluble or coated dispersible carbon precursor and then in the Reaction space are introduced or the conversion is through Introducing gaseous Carbon precursors in the reaction space atmosphere allows.

It is possible and advantageous that the reaction space is already preheated.

When Reaction space is advantageously a tilted or vertical stovepipe. The stovepipe may be rotatably mounted to support the transport of the particles through the reaction space can. The transport of the particles can be done via a Gas flow, by utilizing gravity or rotary motion of the reaction space can be achieved.

It is also possible to preheat the starting materials used, wherein the temperatures must not be so high that the SiO ₂ softens.

The synthesis of the starting materials kieselguhr / diatomaceous earth to SiC proceeds essentially via solid reactions. A homogeneous distribution of the starting materials with each other and a fast heating rate are required. As a result, the formation of gaseous SiC, which occurs in the previously known SiC synthesis processes with SiO ₂ starting materials as an intermediate, suppressed and prevents the disintegration of the favorable morphology of diatomaceous earth / diatomaceous earth.

The Synthesis conditions can by a good homogenization of the starting materials before introduction into the reaction space or the use of gaseous carbon precursors be achieved in the reaction space. The very fast heating of the Powder particles, with one possible rapid heating rate is advantageous, by introducing preheated Starting materials are achieved in a preheated reaction space.

An immediate benefit of the invention is the ability to maintain diatomaceous earth morphology in conjunction with the particle to silicon carbide conversion. The silicon carbide is chemically much more resistant to acids and alkalis and is additionally thermally more stable with respect to softening and thermal shock than SiO ₂ . Such a silicon carbide "kieselguhr / diatomaceous earth" powder can be used as a resistant and chemically regenerable filter aid in the beverage and pharmaceutical industries. Likewise, other applications using the structure of the particles are possible.

at According to the present invention, kieselguhr / diatomaceous earth is prepared according to the inventive method converted into highly resistant silicon carbide particles, the for the outstanding filtration properties responsible diatomaceous earth / diatomaceous earth structure and morphology wholly, substantially or partially preserved remains and in the regeneration of the new filter aid "silicon carbide", due to the increased chemical resistance, significantly more effective detergents, or more concentrated Alkalis and acids can be used. Due to the better regeneration possibilities and the wear resistance the silicon carbide particle may be the regenerated filter aid frequently regenerated and used for filtration, resulting in a significant Increasing the efficiency of the process leads. by virtue of reduced wear is a resharpening the kieselguhr / diatomaceous earth particles are not necessary or diminished.

One Powder of the silicon carbide particles according to the invention can thus be used as a chemically resistant filter aid in the food industry, brewing industry, chemical as well as pharmaceutical industry. It according to the current state of the art is not a process for precoat filtration, Mass filtration and regeneration of filter aids known, that silicon carbide particles match the structure and morphology of Kieselguhr / diatomaceous earth.

The silicon carbide is chemically much more resistant to acids and alkalis and it is also thermally more stable in terms of softening and thermal shock than SiO ₂ . Such a powder of the silicon carbide particles according to the invention can be used as a resistant and chemically regenerable filter aid in the precoat filtration with and without prewarming and in the mass filtration of beverages, chemical, pharmaceutical or similar liquids.

at the precoat filtration becomes the silicon carbide as a running dosage introduced into the product to be filtered (unfiltered) and forms together with the turbid substances in the precoat a filter cake, which the insoluble turbidity the unfiltrate holds back. Frequently becomes applied before the current dosage one or two Voranschwemmungen. The filtration is after reaching the allowable differential pressure or complete Filling the to disposal standing turbid space completed. Subsequently the filter cake in the precoat filter system or in a separate Apparatus prepared using detergents, acid and alkali be that again a filter suitable for filtration filter aid arises.

at the mass filtration flows the unfiltered through a filter cake of filter aid, wherein the turbidity be deposited in the filter aid bed. The filtration will on reaching the permissible Print canceled. The filter aid bed can then under Use of detergents, acid and lye are processed.

Generally can account for 0.5-70 % By weight of the silicon carbide particles according to the invention in the form of a powder, a bed or a depth filter layer.

in the Furthermore, the invention will be explained in more detail in several embodiments.

example 1

A diatomaceous earth / diatomaceous earth (Celite 350, Lehmann & Voss & Co.) with an SiO ₂ content of 92.8% by weight is mixed with a phenolic resin (VPW 1744, Vianova Resins) in an aqueous suspension. In this case, 1 kg of kieselguhr / diatomaceous earth are mixed with 0.9 kg of phenolic resin in 2 l of water with a stirrer.

The Suspension is added in a spray dryer dried granules.

The dried granules are heated in an oven under Ar atmosphere to 800 ° C and the phenolic resin decomposes, leaving only the carbon residue in the granules remain.

The Granules produced in a perpendicular to a Tube furnace connected storage tank filled. In the reservoir is the granules in Ar atmosphere at 1000 ° C heated. The connected furnace is heated to 1500 ° C in the reaction zone. If the temperatures in the reservoir and reached in the reaction zone of the furnace, trickles part of the granules over one Feeder of the reservoir in the tube furnace. About one opposing Argon flow in the furnace tube is the residence time of the powder particles in the reaction zone controlled. The powder particles remain in about 30-60 s the reaction zone. After the reaction time, the gas flow reduced, so that the resulting SiC particles in a collecting vessel at the bottom Fall of the tube furnace. The process described is repeated until the granules in the reservoir is used up.

The resulting powder is characterized by a density of 3.12 g / cm ³ , a specific surface area of 51 m ² / g and a particle morphology substantially corresponding to the starting materials.

Example 2

A commercially available kieselguhr / diatomaceous earth powder (Celite 281, Lehmann & Voss & Co.) with an SiO ₂ content of 89.6 m% is charged into the storage tank of the furnace according to Example 1 and preheated. The further process is essentially analogous to Example 1, wherein the kieselguhr / diatomaceous earth particles are held in the reaction zone by a counterflow consisting of a gas mixture of 80% by volume of Ar and 20% by volume of methane (CH ₄ ). The reaction takes place with the carbon of the gas atmosphere.

The resulting powder is characterized by a density of 3.09 g / cm ³ , a specific surface area of 47 m ² / g and a particle morphology completely corresponding to the starting materials.

Claims (29)

Structured silicon carbide particles in the form from primary and secondary particles manufactured by a method according to at least one of claims 10 to 24, the secondary particles essentially composed of silicon carbide primary particles at the secondary particles morphologically and structurally whole, essentially all or part of the Morphology and structure of the particles of the starting material kieselguhr / diatomaceous earth correspond. Silicon carbide particles according to claim 1, wherein said morphological and structural agreement to the starting material kieselguhr / diatomaceous earth is between 50 and 100%. Silicon carbide particles according to claim 1, which have a specific surface area of ≥ 20 m ² / g. Silicon carbide particles according to claim 3, which have a specific surface area of ≥ 50 m ² / g. Silicon carbide particles according to claim 1, wherein said Diatomaceous earth / diatomaceous earth morphology and thus the silicon carbide particles is substantially flat, elongated oval to circular and they have partially symmetrically arranged pores, wherein the Morphology and structure of kieselguhr / diatomaceous earth particles the cell structure of each silicified algae decreases. Silicon carbide particles according to claim 5, wherein said Pores predominantly have a circular shape with diameters of 20-200 nm or a elongated slit-like shape with a width of 50-300 nm and lengths of up to 5 μm exhibit. Silicon carbide particles according to claim 5, wherein said Pores are substantially open and run without curvature through the particles. Silicon carbide particles according to claim 1, wherein said primary particle a particle size ≤ 1 μm and the secondary particle have a particle size of 5-300 microns. Silicon carbide particles according to claim 1, wherein said secondary particle to ≥ 30% made of silicon carbide primary particles is constructed. Process for the preparation of silicon carbide particles according to one of Claims 1 to 9, in which kieselguhr / diatomaceous earth having a particle size of the secondary particles of 5-300 μm and carbon simultaneously or first the carbon and then the kieselguhr / diatomaceous earth are introduced into a reaction space and there at heating rates of ≥ 10 Ks ^-1 . Process according to claim 10, wherein the starting materials outside preheated the reaction space become. Process according to claim 11, wherein the starting materials at temperatures up to 1100 ° C preheated become. The method of claim 10, wherein carbon, its precursors and / or compounds in solid or gaseous form is introduced into the reaction space. The method of claim 10, wherein the reaction space preheated to temperatures of 1400 ° C to 2000 ° C before introducing the starting materials. The method of claim 10, wherein the kieselguhr / diatomaceous earth with dispersible or soluble Carbon precursors coated is introduced. A method according to claim 10, wherein a diatomaceous earth / diatomaceous earth suspension with or without carbon, its precursors and / or compounds, these by means of spraying or freeze granulation dried and as granules in the reaction space is introduced. A method according to claim 10, wherein heating rates between 50 and 600 Ks ^{-1 are} realized. Process according to claim 10, wherein the starting materials at maximum temperature above a period of 2 to 1000 s. The method of claim 10, wherein diatomaceous earth / diatomaceous earth used in pure form or with impurities of up to 15%. The method of claim 10, wherein carbon used in the form of its precursors and / or its compounds becomes. The method of claim 20, wherein carbon in the form of methane or other gaseous hydrocarbons is used The method of claim 10, wherein the carbon used in solid form with a particle size of 0.01 to 10 microns becomes. The method of claim 10, wherein a mixing the starting materials within the reaction space or before introduction carried out in the reaction space becomes. The method of claim 10, wherein the heating the starting materials by means of electrical resistance heating, inductive Warming, Microwave energy, infrared radiation or laser radiation is performed. Use of the silicon carbide particles produced according to claim 10 according to claim 1 as a filter means for the filtration of fluids. Use according to claim 25 as filter means for Depth filtration or precoat filtration of fluids. Use according to claim 25 as filter means for Filtering drinks, chemical and pharmaceutical products and liquids. Use according to claim 25 as filter medium in Form of a powder, a bed or a depth filter layer. Use according to claim 25 as a filter aid in a proportion of 0.5-70 Ma .-% in the filter medium.