Process for manufacturing silicon carbide

Abstract

A process for manufacturing SiC wherein the emissions of polluting gases are minimized, by reduction of silicon oxide by an excess of carbon, the process including electrically heating a resistor at the heart of a mixture of raw materials consisting of a carbon-based source chosen from petroleum cokes and a source of silicon, especially a silica having a purity of greater than 95% of SiO.sub.2, in order to give rise, at a temperature above 1500° C., to the simplified reaction: SiO.sub.2+3C=SiC+2CO (1), wherein the carbon-based source first undergoes a treatment for removing the contained hydrogen, so that its elemental hydrogen content (EHWC) is less than 2% by weight.

Claims

1. A process for manufacturing SiC, wherein the emissions of polluting gases are minimized, comprising reducing silicon oxide by carbon according to an Acheson process, by electrically heating a resistor in a mixture of raw materials consisting of a carbon-based source selected from cokes and a silica having a purity of greater than 95% of SiO.sub.2, in order to give rise, at a temperature above 1500° C., to the simplified reaction:
SiO.sub.2+3C=SiC+2CO  (1), wherein said carbon-based source first undergoes a treatment for removing the contained hydrogen, so that its elemental hydrogen content (EHWC) is between 0.5% and 0.01% by weight, wherein the hydrogen removal treatment is a controlled heat treatment of the coke in an electric furnace, under an inert atmosphere or is a heat treatment in a fired furnace between 1000° C. and 1350° C.

2. The process as claimed in claim 1, wherein the residual elemental hydrogen content of the coke is between 0.1% and 0.01%.

3. The process as claimed in claim 2, wherein the residual elemental hydrogen content of the coke is between 0.05% and 0.01%.

4. The process as claimed in claim 1, wherein the hydrogen removal treatment is carried out in an atmosphere so that less than 5% by weight of the fixed carbon on the dry product is removed during the hydrogen removal treatment.

5. The process as claimed in claim 4, wherein less than 1% by weight of the fixed carbon on the dry product is removed during the hydrogen removal treatment.

6. The process as claimed claim 1, wherein the hydrogen removal treatment is a heat treatment in a fired furnace between 1250° C. and 1350° C.

7. The process as claimed in claim 1, wherein the oxygen partial pressure of the calcination atmosphere is less than 5% of the total pressure of the gases.

8. The process as claimed in claim 7, wherein the oxygen partial pressure of the calcination atmosphere is less than 1% of the total pressure of the gases.

9. The process as claimed in claim 1, wherein the gases resulting from the hydrogen removal step are used at least partly as fuel for said fired furnace.

10. The process as claimed in claim 1, wherein, on top of the reaction mixture positioned around the electrical resistor of the Acheson furnace, additional coverage means are put in place, coupled to means for collecting the gaseous products emitted during the synthesis of the SiC.

11. The process as claimed in claim 10, wherein the gases collected are treated by burning off.

12. The process as claimed in claim 1, wherein the carbon-based source is a petroleum coke.

Description

EXAMPLES

Example 1 (Comparative):

(1) Firstly, silicon carbide was initially manufactured in a pilot furnace, from a mixture of petroleum coke from the company Petrobras described previously and silica, under the conventional and well-known conditions of the Acheson process. The silica has a particle size distribution such that 50% by weight of the grains is less than 600 microns. The particle size of the petroleum coke is less than 10 mm. The mixture is placed in contact with an electrical resistor. The electric resistor is sized such that the power delivered is of the order of 300 kW. The mixture is brought to a temperature above 1500° C. in the reaction zone, in order to obtain the reaction (1) for manufacturing SiC. Throughout the SiC manufacturing process, that is to say over the time during which the electric current passes through the resistor, the gases resulting from the reaction are collected and analyzed in line, in order to determine the composition and content thereof in the various pollutants. The results obtained are assembled in Table 1 which follows.

Example 2 (According to the Invention):

(2) The experimental protocol from example 1 is repeated in an identical manner but a prior step of treatment (calcination) of the petroleum coke is carried out in a fired rotary furnace, consisting of a step of heating at a temperature of 1350° C., followed by a drop to ambient temperature, the oxygen partial pressure in the calcination atmosphere being less than 1% of the total pressure of the gases, so that the EHWC is close to 0.03%. After milling the dehydrogenated coke, the mixture is placed in contact with a resistor, the cross section of which is reduced in order to compensate for the reduction in the electrical resistivity of the dehydrogenated coke (and therefore of the mixture), the power delivered by said resistor being maintained at around 300 kW, as for the preceding example.

(3) In the same way as before, during the SiC manufacturing process, the gases resulting from the reaction are collected and analyzed in line, in order to determine the content thereof in the various pollutants. The results obtained are also assembled in Table 1 which follows.

(4) TABLE-US-00001 TABLE 1 Example 1 Example 2 CO* 62 95 (vol %) H.sub.2* 30 1 (vol %) CH.sub.4* 4 0.3 (vol %) H.sub.2S* 2000 1000 (ppm vol.) SO.sub.2* 1000 500 (ppm vol.) NH.sub.3* 5000 <500 (ppm vol.) PAH** 2000 <100 (ng/m.sup.3) *average value over the whole of the SiC generation cycle, at the point of emission into the furnace (before dilution in air) **average value over the whole of the SiC generation cycle, at a height of 1 meter above the furnace

(5) On reading Table 1,it is seen that the amount of pollutants emitted is substantially reduced when the process according to the invention (example 2) is used. A marked reduction in organic and sulfur-containing odors is thus observed compared to the reference situation of the Acheson furnace, in which the coke is used without prior dehydrogenation.

(6) Such a phenomenon is even more surprising since the sulfur content of the coke after dehydrogenation was measured as being close or even identical to that of the crude coke (before dehydrogenation).

(7) Such a composition of gases emitted by the Acheson furnace, consisting essentially of CO and in which the content of pollutants, especially sulfur-containing or PAH-type condensable pollutants is substantially reduced, may be easily collected without danger of explosion or of clogging of the recovery circuit, especially in order to be treated, especially by burning.

(8) Moreover, the results of measuring, by screening, the particle size of the hydrogenated and non-hydrogenated cokes, listed in table 2 below, indicate that the coke fines are largely removed by the supplementary calcination treatment according to the invention, either by combustion or by entrainment in the calcination furnace (which is not desirable and is in the minority in our test) or else by agglomeration. Such a particle size distribution characteristic results in a substantially reduced dust accumulation in the vicinity of the Acheson furnace, especially during the assembly and disassembly thereof.

(9) TABLE-US-00002 TABLE 2 Example 1 Example 2 d.sub.10* 100 μm 250 μm *median grain diameter for which 10%, by weight, of the grains of the coke powder having an equal or smaller size.

(10) Finally, as a function of the source of coke used and of its residual hydrogen value, after the step of removing the contained hydrogen according to the invention, it was measured by the applicant company that the measured energy efficiency of an Acheson type furnace modified in order to treat a dehydrogenated coke according to the invention may be between around 10% and around 25% higher than a conventional Acheson furnace, as used for the treatment of a non-dehydrogenated coke. The expression “energy efficiency” of the Acheson furnace is understood to mean the efficiency calculated on the basis of the kWh expended for the final formation of one kilo of SiC.