Technical Field The present invention relates to a method for removing impurities from residual silicon powder obtained in the production of organochlorosilanes and chlorosilanes.

Background Art The commercial method for manufacturing organohalosilanes is well known and is described in US patent No. 2,380, 995. This patent discloses the direct reaction of an organohalide such as methylchloride with silicon particles to produce organochlorosilane. A copper catalyst is mixed with the silicon particles to form a reaction mass, also called contact mass. The reaction is normally carried out in a fluidized bed type reactor. A part of the silicon particles are carried out of the reactor with the organochlorosilane gases produced and is recovered in cyclones or filters. The residue recovered from the cyclone or the filter has a high content of unreacted, elemental silicon contaminated by compounds of copper, iron, chloride and others.

Further from time to time the reactor has to be stopped, the used reaction mass removed, whereafter fresh reaction mass is added. The used reaction mass still contains an appreciable amount of elemental silicon, but is contaminated with compounds of a number of elements, particularly copper, carbon, calcium, iron, aluminum and chlorine, as well as oxide and carbide particles from slag. These contaminents accumulate in the reactor during the process and after a certain period of time the used reaction mass has to be removed from the reactor as a residue. This used reaction mass or residue has conventionally been deposited or has been treated and upgraded for the use in other processes.

The commercial process for producing trichlorosilane (TCS) is also well known and is normally carried out in a fluidized bed reactor or stirred bed reactor by reaction of silicon particles with HCI gas. This process is generally

carried out at a temperature between 250°C and 550°C. Also in this process it is obtained a residue containing an appreciable amount of elemental silicon, but which is contaminated by iron-aluminum-and calcium compounds as well as oxide and carbide particles from slag. This residue therefore cannot be recycled to the reactor. Further, in the TCS process some of the boron in the silicon particles accumulates in the residue, and since the main use of TCS is to produce electronic grade silicon requiring a very low content of boron, recycling of the residue would give a TCS having a too high boron content.

TCS can also be produced by reacting silicon particles with silicon tetrachloride and hydrogen at about 500°C in a fluidized bed reactor. Also in this process silicon-containing residues are produced.

Silicon tetrachloride together with TCS is produced in a so-called solid bed reactor at about 1000°C where silicon lumps are reacted with HCI gas.

Residues having a similar chemical composition but larger particle size is produced in this process.

From US patent No. 4,307, 242 it is known a process for removing impurities from contact mass from the direct reaction for production of organohalosilane.

According to the process of US patent No. 4,307, 242 the particle size distribution of the used contact mass is analyzed, whereafter the analyzed contact mass is classified into a relative pure fraction and a relative impure fraction. The relative pure fraction is the coarse fraction and the relative impure fraction is the fine fraction. The coarse fraction is recycled to the organohalosilane reactor. Due to the very small particle size of the used contact mass, from about 5 um to about 500 um, the classification process is difficult and additional equipment such as filters are needed.

It is an object of the present invention to provide a simple, low cost process for removing impurities from residues from the process for producing organochlorosilanes and residues from the process of producing chlorosilanes where the residues are separated into a relative pure fraction and a relation

impure fraction and where the relative pure fraction can be recycled to the organochlorosilane reactor or the chlorosilane reactor.

The present invention thus relates to a method for removing impurities from elemental silicon-containing residues from the processes of producing organochlorosilane and chlorosilane, which method is characterized in that the residues are subjected to magnetic separation to provide a relative pure non-magnetic fraction having an increased silicon content and a relatively impure magnetic fraction having a lower silicon content than the non-magnetic fraction.

The magnetic separation is preferably carried out using a high intensity, high gradient magnetic separation apparatus. The magnetic field strength needed to obtain the necessary separation varies with the source and the particle size of the residue. Good results have been obtained by using a magnetic field strength of about 10000 Gauss and excellent results have been obtained by using a magnetic field strength of 17000 Gauss. It may, however, be obtained satisfactory results using a magnetic field strength below 10000 Gauss. Thus the necessary magnetic field strength for a certain residue must be determined for each particular residue.

Best results are obtained by using a short belt conveyor which has a magnet as its head pulley. The particulate residue is the feed onto the moving conveyor belt via a feed hopper and a vibration feeder. As the material is conveyed over the magnet, ferromagnetic and paramagnetic particles adhere to the conveyor belt whilst non-magnetic particles fall freely off the end of the conveyor.

The non-magnetic fraction having a high silicon content is preferably recycled to the organochlorosilane reactor or to the chlorosilane reactor. Since the residues are very hygroscopic, it is preferred to carry out the magnetic separation in an atmosphere which avoid moisture and oxidation of the residue and of the produced non-magnetic fraction. This is preferably done by carrying out the magnetic separation under an inert atmosphere.

It has surprisingly been found that even if the silicon-containing residues were believed to be virtually non-magnetic, it is possible to use magnetic separation to remove impurities from the silicon particles in the residue. Thus it has been found that for a used contact mass for production of TCS which contained 17.8 % by weight of elemental silicon it was obtained a non-magnetic fraction containing 40.9 % by weight elemental silicon, while the magnetic fraction contained only 8.6 % by weight of elemental silicon.

Short description of the drawing Figure 1 shows a magnetic separator which can be used by the method of the present invention.

Detailed description of the invention The examples set out below were carried out using a magnetic separation apparatus shown in figure 1.

In figure 1 there is shown a magnetic separator comprising a conveyor belt 1 running over two pulleys 2 and 3. The pulley 3 is a permanent magnet while the pully 2 is an ordinary conveyor pulley. Below the conveyor belt there is arranged a splitter blade 4 to split the material into a magnetic fraction and a non-magnetic fraction. The two fractions are collected in hoppers 5,6. The material to be treated is placed in a hopper 7 above the conveyor belt 1 and a vibration feeder 8 or the like is arranged to feed material from the hopper 7 to the conveyor belt 1.

The specific magnetic separator used in the examples below was a PERMROLLO Laboratory Separator delivered by Ore Sorters (North America) Inc., Colorado, USA. The thickness of the conveyor belt was 0.25 mm which gave a magnetic field strength of about 17000 Gauss.

EXAMPLE 1 297 grams of a reactor residue from a TCS reactor having the chemical analysis set out in Table 1 was treated in the magnetic separator apparatus described above in connection with Figure 1.

TABLE 1 Reactor residue from TCS Element % by weight Si total 66. 8 Si elemental 17.8 Fe 2. 45 Al 3. 99 Ca 2. 48 Ti 0. 11 Mn 0. 069 Cu 0. 048 K 0. 11 Mg 0. 16 P 0. 144 Ba 0. 13 Sr 0. 064 Zr 0. 018 Cl 1. 54 B 130 ppm It was obtained a non-magnetic fraction of 158 grams and a magnetic fraction of 139 grams. The chemical composition of the non-magnetic fraction and of the magnetic fraction are set out in Table 2.

TABLE 2 Non-magnetic Magnetic Element % by weight % by weight Si total 72. 6 43. 2 Si elemental 40. 9 8. 6 Fe 0. 68 7. 09 Al 3. 46 6. 10 Ca 3. 09 2. 33 Ti 0. 02 0. 11 Mn 0. 02 0. 08 Cu 0. 01 0. 02 K 0. 09 0. 07 Mg 0. 20 0. 41 0. 058 0. 117 Ba 0. 06 0. 13 Sr 0. 03 0. 02 Zr >0. 005 0. 01 CI 0. 86 7. 47 B 49 ppm 193 ppm

As can be seen by comparing the analysis in Table 1 with the analysis of the two fractions in Table 2, the amount of elemental silicon in the non-magnetic fraction is increased substantially compared to the untreated reactor residue.

It can also be seen that the amount of elemental silicon in the magnetic fraction is low. Further it can be seen that the iron content in the non-magnetic fraction is very low and that most of the iron in the untreated reactor residue is separated into the magnetic fraction. It can also be seen that there is a reduction in the amount of aluminum and a number of the trace elements. The reduction in the content of chlorine in the non-magnetic fraction compared to the chlorine content in the untreated reactor residue is due to the fact that iron, aluminum, calcium and most of the trace elements are present in the reactor residue as chlorides.

Finally it can be seen that the non-magnetic fraction is low in boron and phosphorous as most of the boron and phosphorous contained in the reactor residue are found in the magnetic fraction.

The non-magnetic fraction obtained thus has such a composition that it can be recycled to the TCS reactor, thus increasing the yield of the silicon in the reactor.

EXAMPLE 2 844 grams of a reactor residue from a reactor for production organochlorosilane by the direct reaction having the chemical analysis set out in Table 3 was treated in the magnetic separator described above in connection with Figure 1. It can be seen from Table 3 that the reactor residue was little reacted as the content of elemental silicon is very high.

TABLE 3 Element % Si total 99. 2 % Si elemental 88.9 % Al 0.2 % Ca 0. 03 % Fe 0. 3 ppmw Mg <10 -ppmw-Zr--43-~ ppmw Sr <10 ppmw Na <10 ppmw Pb 16 ppmw Mg <10 ppmw As <10 ppmw Zn 2475 % Cu 5. 8 ppmw Ni 27 ppmw Mn 27 ppmw Cr 42 ppmw V <10 ppmw Ba 32 ppmw Ti 225 ppmw Sb <10 ppmw Sn 327

It was obtained a non-magnetic fraction of 772 grams and a magnetic fraction of 72.2 grams. The chemical composition of the non-magnetic fraction and of the magnetic fraction is shown in Table 4.

TABLE 4 Element Magnetic Non-magnetic % Si total 98. 1 99. 2 % Si elemental 70. 9 90. 0 % Al 0. 4 0. 2 % Ca 0.08 0.02 % Fe 1.0 0.2 ppmw Mg <10 <10 ppmw Zr 90 39 ppmw Sr <10 <10 ppmw Na <10 <10 ppmw Pb 27 15 ppmw Bi <10 <10 ppmw As <10 <10 ppmw Zn 4600 2139 % Cu 13. 4 5. 3 ppmw Ni 77 22 ppmw Mn 132 23 ppmw Cr 197 38 ppmw V 65 <10 ppmw Ba 46 16 ppmw Ti 908 197 ppmw Sb <10 <10 ppmw Sn 686 218

By comparing the analysis of the reactor residue set out in Table 3 with the chemical analysis of the magnetic and the non-magnetic fractions set out in Table 4, it can be seen that most of the iron and a major part of the aluminum in the reactor residue have been transferred to the magnetic fraction. Both the iron and the aluminum content in the non-magnetic fraction are at the same level as what would be expected in the original silicon particles used in the organochlorosilane reactor. Also the content of most of the trace elements are much lower in the non-magnetic fraction than in the magnetic fraction. The non-magnetic fraction thus has a composition which makes it a very suitable silicon source for recycling to the organochlorosilane reactor.