EP2331459A2

EP2331459A2 - Method for the pyrolysis of carbohydrates

Info

Publication number: EP2331459A2
Application number: EP09783459A
Authority: EP
Inventors: Jürgen Erwin LANG; Alfons Karl; Hartwig Rauleder; Ekkehard MÜH; Guido Stochniol
Original assignee: Evonik Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2008-09-30
Filing date: 2009-09-28
Publication date: 2011-06-15
Also published as: JP2012504089A; BRPI0919109A2; US20110182796A1; EA201100570A1; AU2009299911A1; NZ591239A; DE102008042498A1; WO2010037699A2; CA2739049A1; KR20110063498A; TW201026635A; WO2010037699A3; CN102171140A; ZA201102323B

Abstract

The present invention relates to methods for the technical pyrolysis of a carbohydrate or carbohydrate mixture at an elevated temperature while adding silicon oxide, to a pyrolysis product obtainable in this way, and to the use thereof as a reducing agent for the production of solar silicon from silicic acid and carbon at a high temperature.

Description

Process for the pyrolysis of carbohydrates

The present invention relates to a technical process for pyrolysis of carbohydrates, in particular of sugar, the pyrolysis product thus obtainable and its use as a reducing agent in the production of solar silica from silica and carbon at high temperature.

It is known to pyrolyze carbohydrates, for example mono-, oligo- and polysaccharides, in gas chromatographs.

US 5,882,726 discloses a process for producing a carbon-carbon composition wherein pyrolysis of a low-melting sugar is performed.

GB 733 376 discloses a process for purifying a sugar solution and pyrolysis at 300 to 400 ^° C.

It is also known to pyrolyze sugar at high temperature to produce an electron-conductive substance (WO 2005/051840).

The large-scale pyrolysis of carbohydrates can lead to problems due to caramelization and foaming, which can significantly disturb the process control and the process sequence.

It is also known u. a. Use sugar as a reducing agent with a low level of impurities (US 4,294,811, WO 2007/106860) or as a binder (US 4,247,528) in the production of pure silicon.

It is an object of the present invention to provide a process for the pyrolysis of carbohydrates, in particular of sugars, in which foaming is avoided. The object is achieved according to the information in the claims.

Thus, it has surprisingly been found that by adding silicon oxide, preferably SiO 2, in particular precipitated silica and / or fumed silica, the foaming effect can be suppressed. So you can operate industrial processes for the pyrolysis of carbohydrates in a simple and economical way now without disturbing foaming. In addition, caramel formation was no longer observed when carrying out the method according to the invention.

In addition, in a preferred embodiment, it was possible to lower the pyrolysis temperature of, for example, 1 600 ^° C. to about 700 ^° C., particularly in an energy-saving manner (low-temperature operation). Thus, the inventive method is advantageously operated above a temperature of 400 ⁰ C, more preferably at 400 to 700 ⁰ C and most preferably at 400 to 600 ⁰ C. This method is extremely energy efficient and also has the advantage that the caramel formation is reduced and the handling of the gaseous reaction products is facilitated. Also preferably, the reaction between 800 and 1 600 ⁰ C, more preferably between 900 and 1 500 ⁰ C, in particular carried out at 1 000 to 1 400 ⁰ C, wherein advantageously obtains a graphite-containing pyrolysis product. If one prefers a graphite-containing pyrolysis product, one should aim for a pyrolysis temperature of from 1 300 to 1 500 ^° C. The present process is advantageously carried out under protective gas and / or reduced pressure (vacuum). Thus, the process according to the invention is advantageously carried out at a pressure of from 1 mbar to 1 bar (ambient pressure), in particular from 1 to 10 mbar. Conveniently, the pyrolysis apparatus used is dried before the beginning of the pyrolysis and rinsed by purging with an inert gas, such as nitrogen or Ar or He, virtually free of oxygen. The pyrolysis time in the process according to the invention is generally between 1 minute and 48 hours, preferably between 1/4 hour and 18 hours, in particular between ^Λ A hour and 12 hours at said pyrolysis, while the heating time can be up to reach the desired pyrolysis additionally of the same order of magnitude, in particular between 1/4 hour and 8 hours , The present process is usually carried out batchwise; but you can also do it continuously.

A C-based pyrolysis product obtained according to the invention contains carbon, in particular with graphite constituents and silica, and optionally fractions of other carbon forms, such as coke, and is particularly poor in impurities, such. B. B, P, As and AI compounds. Thus, the pyrolysis product according to the invention can be advantageously used as a reducing agent in the production of silicic acid silicic acid at high temperature. In particular, the graphite-containing pyrolysis product according to the invention can be used on account of its conductivity properties in an arc reactor.

The subject of the present invention is therefore a process for technical, d. H. industrial pyrolysis of a carbohydrate or carbohydrate mixture at elevated temperature with the addition of silica.

As a carbohydrate component is preferably used in the process according to the invention monosaccharides, d. H. Aldoses or ketoses, such as trioses, tetroses, pentoses, hexoses, heptoses, especially glucose and fructose, but also corresponding oligomeric and polysaccharides based on said monomers, such as lactose, maltose, sucrose, raffinose, to name just a few or derivatives thereof - including starch, including amylose and amylopectin, glycogen, glycosans and fructosans, to name but a few polysaccharides.

Optionally, one may additionally purify the aforementioned carbohydrates by a treatment using an ion exchanger, wherein the Carbohydrate in a suitable solvent, preferably water, dissolves, via a column filled with an ion exchange resin, preferably an anionic or cationic resin, the resulting solution is concentrated, for example by removing solvent fractions by heating - in particular under reduced pressure - and the thus purified carbohydrate advantageously crystalline win, for example by cooling the solution and subsequent separation of the crystalline fractions, including by filtration or centrifugation.

However, it is also possible to use a mixture of at least two of the abovementioned carbohydrates as carbohydrate or carbohydrate component in the process according to the invention. In the process according to the invention, it is particularly preferred to use a crystalline sugar which is available in economic quantities, a sugar which can be obtained, for example, by crystallization of a solution or a juice from sugarcane or beets in a manner known per se, d. H. commercial crystalline sugar, such as refined sugar, preferably a crystalline sugar with the substance-specific melting point / softening range and an average particle size of 1 .mu.m to 10 cm, particularly preferably from 10 .mu.m to 1 cm, in particular from 100 .mu.m to 0.5 cm. The particle size can be determined, for example, but not exclusively, by sieve analysis, TEM, SEM or light microscopy. However, it is also possible to use a carbohydrate in dissolved form, for example-but not exclusively-in aqueous solution, the solvent, of course, evaporating more or less rapidly before the actual pyrolysis temperature is reached.

SiO _x with x = 0.5 to 1.5, SiO, SiO 2, silicon oxide (hydrate), aqueous or water-containing SiO 2, in the form of fumed or precipitated silica, moist, dry or calcined, for example, Aerosil ^® or Sipernat ^®, or a silica sol or gel, the above-mentioned components porous or dense silica glass, silica sand, quartz glass, for example optical fibers, quartz glass beads, or mixtures of at least two. In the process according to the invention, preference is given to using silicic acid having an internal surface area of from 0.1 to 600 m ² / g, particularly preferably from 10 to 500 m ² / g, in particular from 100 to 200 nrvVg. The determination of the inner or special surface can be carried out, for example, by the BET method (DIN ISO 9277).

Preference is given to silicic acid having an average particle size of 10 nm to 1 mm, in particular from 1 to 500 microns, a. Again, the determination of the particle size u. a. by TEM (Transelectron Microscopy), SEM (Scanning Electron Microscopy) or Light Microscopy.

The silica used in the process according to the invention advantageously has a high (99%) to highest (99.9999%) purity, the content of impurities, such as B, P, As and Al compounds, in total advantageously <10 Ppm by weight, in particular <1 ppm by weight should be. The determination of impurities can be carried out, for example, but not exclusively, by means of ICP-MS / OES (induction coupling spectrometry - mass spectrometry / optical electron spectrometry) and AAS (atomic absorption spectroscopy).

Thus, in the process according to the invention, carbohydrate can be made into defoamer, ie. H. Silica component, calculated as SiO 2, in a weight ratio of 1 000: 0.1 to 0.1: 1000 use. The weight ratio of carbohydrate component to silicon oxide component may preferably be adjusted to 800: 0.4 to 1: 1, more preferably 500: 1 to 100: 13, most preferably 250: 1 to 100: 7.

As an apparatus for carrying out the process according to the invention, it is possible, for example, to use an induction-heated vacuum reactor, wherein the reactor can be made of stainless steel and coated or lined with a suitable inert material, for example high-purity SiC, Si ₃ N ₃ , high-purity quartz or silicic acid glass, high-purity carbon or graphite, Ceramics. But you can also use other suitable reaction vessels, such as an induction furnace with a vacuum chamber for receiving appropriate reaction crucible or trough.

In general, the process according to the invention is carried out as follows:

The interior of the reactor and the reaction vessel are suitably dried and purged with an inert gas, which may be heated, for example to a temperature between room temperature and 300 ⁰ C. Subsequently, the carbohydrate or carbohydrate mixture to be pyrolyzed together with the silicon oxide as defoamer component is filled into the reaction space or the reaction vessel of the pyrolysis apparatus. Beforehand, the starting materials can be thoroughly mixed, degassed under reduced pressure and transferred under protective gas into the prepared reactor. In this case, the reactor may already be slightly preheated. Subsequently, the temperature can be advanced continuously or stepwise to the desired pyrolysis temperature and the pressure reduced in order to be able to remove the gaseous decomposition products escaping from the reaction mixture as quickly as possible. It is particularly by the addition of silica advantageous to avoid foaming of the reaction mixture as far as possible. Upon completion of the pyrolysis reaction can be thermally aftertreated the pyrolysis some time, preferably at a temperature in the range 1000 to 1500 ⁰ C.

In general, one obtains such a pyrolysis product or a composition containing high purity carbon. It is particularly advantageous to use the process product according to the invention as a reducing agent for the production of solar silicon from one or high purity silica. These pyrolysis product according to the invention with the addition of other components, such as pure or highly pure SiO 2, activators, such as SiC, binders, such as organosilanes, organosiloxanes, carbohydrates, silica gel, natural or synthetic resins, and high-purity processing aids, such as molding, tableting or extrusion aids, such as Graphite, in a defined form, for example, by granulation, pelleting, tableting, extrusion - to name just a few examples.

The present invention thus relates to a composition or the pyrolysis product, as obtained by the process according to the invention.

Therefore, the present invention also provides a pyrolysis product having a carbon to silica content (calculated as silica) of from 400 to 0.1 to 0.4 to 1000, preferably from 400: 0.4 to 4: 10; more preferably from 400: 2 to 4: 1, 3; in particular from 400: 4 to 40: 7.

In particular, the direct process product of the method according to the invention is characterized by its high purity and usability for the production of polycrystalline silicon, in particular of solar silicon for photovoltaic systems, but also for medical applications.

Such a composition of the invention (also called short pyrolysate or pyrolysis) can be especially beneficial as a feedstock in the production of solar-grade silicon by reduction of SiO ₂ at a higher temperature, in particular in an electric arc furnace may be used. Thus, the direct process product according to the invention can be used in a simple and economical manner as a C-containing reducing agent in a process, as can be seen for example from US 4,247,528, US 4,460,556, US 4,294,811 and WO 2007/106860.

The subject matter of the present invention is also the use of a composition according to the invention (pyrolysis product) as starting material in the production of solar silicon by reduction of SiO ₂ at a higher temperature, in particular in an electric arc furnace. The present invention is further illustrated and illustrated by the following example and the comparative example without limiting the scope of the invention.

Examples:

Comparative Example 1

Commercially available refined sugar was melted in a quartz glass under protective gas and then heated to about 1 600 ⁰ C. During this time, the reaction mixture foamed strongly, partially precipitated - also caramel formation was observed, and the pyrolysis remained adhering to the wall of the reaction vessel, see. FIG. 1a).

example 1

Commercially available refined sugar was mixed with SiO2 (Sipernat ^® 100) in the weight ratio 20: (: SiO 2 Sugar) were mixed, melted and heated to 800 ⁰ C. 1 No caramel formation was observed and no foaming occurred. A graphite-containing, particulate pyrolysis product was obtained, which was advantageously not substantially adhered to the wall of the reaction vessel, cf. Figure 1 b) and Figure 2 (electron micrograph of the pyrolysis product of Example 1).

Claims

claims:

1. A process for the technical pyrolysis of a carbohydrate or carbohydrate mixture at elevated temperature with the addition of silica.

2. The method according to claim 1, characterized in that at least one silicon dioxide form, in particular pyrogenic or precipitated silica of high to highest purity, is used as the silica.

3. The method according to claim 1 or 2, characterized in that one uses as the carbohydrate component at least one crystalline sugar.

4. The method according to any one of claims 1 to 3, characterized in that one uses carbohydrate and silica (in each case calculated in total) in a weight ratio of 1 000 to 0.1 to 0.1 to 1 000.

5. The method according to any one of claims 1 to 4, characterized in that one carries out the pyrolysis in a reactor with the exclusion of oxygen.

6. The method according to any one of claims 1 to 5, characterized in that one carries out the pyrolysis at a temperature between 400 and 700 ⁰ C.

7. The method according to any one of claims 1 to 5, characterized that one carries out the pyrolysis or above 700 ⁰ C at a pressure between 1 mbar and 1 bar in an inert gas atmosphere.

8. Composition (pyrolysis product), obtained according to one of claims 1 to 7.

9. Pyrolysis product having a carbon to silica content (calculated as silica) of 400 to 0.1 to 0.4 to 1000.

10. Use of a composition (pyrolysis product) according to claim 8 or 9 or prepared or obtained according to one of claims 1 to 7 as a feedstock in the production of solar silicon by reducing SiO ₂ at a higher temperature, in particular in an electric arc furnace.

11. A process for the preparation of silicon, preferably solar silicon, which comprises reacting a mixture of a carbohydrate, preferably a carbohydrate purified by means of ion exchange columns, and a silicon oxide, preferably a high-purity silicon oxide containing B, P, As - And AI compounds in total of <10 ppm by weight, at temperatures of 400 to

700 ⁰ C pyrolyzed and that pyrolysis product then used to produce high purity silicon.