Method for rendering inert dust residue containing silicon metal

Abstract

The invention relates to a method for rendering inert dust residue containing silicon metal, left over from trichlorosilane synthesis. The inventive method produces valuable materials containing silicon which can be used in metallurgical processes. According to said method, the process of rendering the residue inert is carried out in several steps. In a first step, 10 to 50 wt. % water, in relation to the quantity of residue, and an at least equimolar quantity of an alkaline compound, in relation to the chloride content of the residue, are added to the residue. Subsequently or simultaneously, the residue is heated to a temperature of 50 to 200° C. In a further step, at least twice the quantity of water is added to this mixture, which is extensively liberated from dissolved salts by filtration and subsequent washing with water.

Description

[0001] The invention relates to a method for rendering inert dust residue containing silicon metal originating from the trichlorosilane synthesis in a fluidised bed as well as the use of such inert residues in metallurgical processes.

[0002] Trichlorosilane is produced in the technical scope in accordance with the “Studies in Organic Chemistry 49; Catalyzed Direct Reactions of Silicon”, Elsevier, 1993, p. 445 to 457 by reacting metallurgical silicon with hydrogen chloride or by reacting silicon with tetrachlorosilane and hydrogen. As a rule, such processes are carried out in fluidised-bed reactors adding catalysts, like e.g. copper or copper compounds, as well as without adding any catalyst.

[0003] All such processes have in common that the trichlorosilane-containing gaseous product mixture that leaves the fluidised bed is contaminated by solid dust containing silicon and, if any, catalyst. The removal of such dust is achieved by separation in a cyclone or in several cyclones which are connected in series or by hot-gas filtration with ceramic or sintered metal candles. Any dust residue remaining in the trichlorosilane-containing gaseous product mixture is subsequently removed by washing with liquid chlorosilanes.

[0004] Alternatively, procedures are known, wherein the dust is removed from the trichlorosilane-containing product mixture exclusively by washing with liquid chlorosilanes or by total condensation of the product mixture and subsequent distillation without previous separation by means of cyclones or hot-gas filters. The suspensions containing chlorosilanes or dust which occur in this process are usually processed to regain chlorosilanes.

[0005] According to the methods specified in DE 36 42 285 C1, DE 37 09 577 A1, DE39 41 825 A1, DE39 41 827 A1, DE41 16 925 A1, DE41 30 880 A1, DE41 30 881 A1, DE42 43 223 C1, EP0 201 200 A1, EP 0521309 A1, U.S. Pat. No. 4,690,810 they can be processed by distillation, wherein a distillation bottom product occurs containing silicon, metal chlorides and, if any, catalyst, which needs to be disposed of after rendering inert by alkali or by a treatment with water vapour. A use of the mixtures of hydrolysed metal chlorides and hydrolysed chlorosilanes occurring in such process, e.g. in metallurgical processes, is not economic because of the high contents of chloride and the low contents of valuable metals, particularly silicon metal. In order to obtain a valuable product, it is therefore advantageous, to extensively separate the solid dust compounds of the trichlorosilane-containing product mixture originating from the trichlorosilane synthesis prior to the gas washing or the total condensation and to process them separately.

[0006] The dust occurring in the cyclones and/or the hot-gas filter can be composed differently depending on the operating conditions in the fluidised-bed reaction, the composition of the metallurgical silicon used and the application of catalyst. It consists essentially of non-reacted residues of the metallurgical silicon used, chlorides like FeCl2 and CaCl2, and catalyst and/or a catalyst compound, if any. Such dust material is highly reactive and must be rendered inert prior to its use. Inerting in this sense means that the reactivity of the dust material is reduced and its handleability is improved.

[0007] According to EP-A1-0 201 200 inerting can be achieved by mixing the dust material with water and granulation of such mixture. The product occurring is now rendered inert, but cannot be used directly as a raw material in metallurgical processes, however, because of its high chloride contents and low pH-value, combined with the easy elutability of metal ions, requiring a reprocessing of the product.

[0008] DE-C1-195 07 602 specifies another possibility of rendering inert, wherein the dust material is reacted in hydrous suspension with alkaline compounds, such as e.g. NaOH, CaO, Na2CO3, NaHCO3 or cement, and subsequently filtered. Inerting is achieved also in this procedure, but the suspensions occurring are often difficultly filterable, resulting in filter cake with a high water and chloride content. Also such products cannot be used directly as raw material in metallurgical processes and must be reprocessed in expensive aftertreatment.

[0009] EP 0 416 584 A1 discloses the use of a heatable solids mixer as well as a heatable plough blade mixer within the scope of processes of chemical engineering.

[0010] EP 0 428 338 A2 discloses a method of rendering inert metallic silicon containing residues that originate during the generation of organic halogenide silane compounds. Water and an alkali compound are added to these residues. In the process, the residues are added to an aqueous basic solution, which has been heated to 50 to 100° C., and then filtered.

[0011] EP 0 433 600 A2 describes a method of processing silicon containing residues by reaction with calcium bases or calcium carbonate.

[0012] The object of the invention is therefore to provide a method for rendering inert dust residue containing silicon metal originating from the trichlorosilane synthesis, providing an easy way of obtaining valuable materials containing silicon metal, which can be used in metallurgical processes.

[0013] This object is achieved by a method for rendering inert dust residue containing silicon metal originating from the trichlorosilane synthesis, characterised in that the process of rendering the residue inert is carried out in several steps. In a first step, 10 to 50 weight percent water, in relation to the quantity of residue, and an at least equimolar quantity of an alkaline compound, in relation to the chloride content of the residue, are added to the residue. Subsequently or simultaneously, the residue is heated to a temperature of 50 to 200° C. In a further step, at least twice the quantity of water is added to this mixture, which is extensively liberated from dissolved salts by filtration and subsequent washing with water.

[0014] It is preferred to use the alkaline compound at a molar excess ratio of 2 to 10%, in relation to the chloride content of the residues. Such alkaline compounds can be, for example, NaOH, Na2CO3, NaHCO3, CaO, Ca(OH)2 or cement. Preferred are CaO, Na2CO3 and cement or any mixtures thereof.

[0015] In a particularly preferred modification of the inventive method the mixture is heated to a temperature in the range from 100 to 150° C., preferably 120 to 140° C.

[0016] In a modification of the inventive method the mixture having been heated to 50 to 200° C. is maintained at this temperature for a period of 10 to 60 minutes.

[0017] When the material according to the invention is applied the products obtained have as a rule a chloride content of <1 weight percent in relation to the dry substance and a water content of <40 weight percent. Such products can be used in metallurgical processes, such as e.g. the manufacture of iron alloys or the manufacture of copper, without any further aftertreatment.

[0018] The preparation of the mixture in the first step can be carried out in a usual heatable solids mixer. Preferably mixers with in-built mixing tools are used, e.g. plough blade mixers.

[0019] Filtration and washing is preferably carried out in a filter press. It showed that washing the filter cake requires only small amounts of water in order to remove the dissolved salts extensively. The preferred amount of water is in a range of a 1.5 to double the amount in relation to the mass of the filter cake.

[0020] The method according to the invention will be explained in the following by means of some examples.

[0021] The composition and the silicon corn spectrum of the samples used in the experiments correspond to typical dust residue containing silicon metal originating from the reaction of metallurgical silicon with silicon tetrachloride and hydrogen to trichlorosilane.

[0022] The samples used had the following composition: 1 Sample A Sample B Si 57.2 wt. percent Si 48.4 wt. percent Fe  4.8 wt. percent Fe 18.7 wt. percent Cu 26.0 wt. percent Cl 29.0 wt. percent Cl 10.2 wt. percent residue  3.9 wt. percent residue  1.8 wt. percent (Ca etc.) (Ca etc.)

[0023] The composition of Sample A corresponds to a residue originating from a trichlorosilane synthesis which was catalysed with CuCl and wherein metallurgical silicon with an Fe contents of 0.4 weight percent was used. The composition of Sample B corresponds to a residue originating from a trichlorosilane synthesis wherein metallurgical silicon with an Fe contents of 1.9 weight percent was used.

EXAMPLE 1 (COMPARATIVE EXAMPLE)

[0024] 200 g of Sample A were mixed with 20 g cement and 1000 ml water and heated to 75° C. Subsequently the pH-value was adjusted to 8 in the suspension by 10 molar soda lye. After 60 min the suspension was filtered over a vacuum laboratory filter and washed on the filter with 600 ml of water. The filter cake sucked off had a water content of 44.5 weight percent.

EXAMPLE 2 (COMPARATIVE EXAMPLE)

[0025] 200 g of Sample B were mixed with 20 g cement and 1000 ml water and heated to 75° C. Subsequently the pH-value was adjusted to 8 in the suspension by 10 molar soda lye. After 60 min the suspension was filtered over a vacuum laboratory filter and washed on the filter with 600 ml of water. The filter cake sucked off had a water content of 70 weight percent.

EXAMPLE 3 (COMPARATIVE EXAMPLE)

[0026] 100 g of Sample B were mixed with 45.5 g Na2CO3 (equivalent to 1.05 mol pro mol chloride), 5 g cement and 289 ml water. This adjusted a temperature of 35° C. After 45 minutes the suspension was filtered over a vacuum laboratory filter and washed on the filter with 250 ml of water. The filter cake sucked off had a water content of 45.1 weight percent and a chloride content of 0.24 weight percent based on dry substance.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

[0027] 100 g of Sample A were mixed with 16 g Na2CO3 (equivalent to 1.05 mol pro mol chloride), 5 g cement and 14 ml water. This adjusted a temperature of 52° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 26 weight percent and a chloride content of 1.2 weight percent based on dry substance.

EXAMPLE 5 (COMPARATIVE EXAMPLE)

[0028] 100 g of Sample B were mixed with 45.5 g Na2CO3 (equivalent to 1.05 mol pro mol chloride), 5 g cement and 39 ml water. This adjusted a temperature of 53° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 40.3 weight percent and a chloride content of 1.4 weight percent based on dry substance.

EXAMPLE 6 (ACCORDING TO THE INVENTION)

[0029] 100 g of Sample A were mixed with 16 g Na2CO3 (equivalent to 1.05 mol pro mol chloride) and 14 ml water and heated to 130° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 23.4 weight percent and a chloride content of 0.27 weight percent based on dry substance.

EXAMPLE 7 (ACCORDING TO THE INVENTION)

[0030] 100 g of Sample A were mixed with 16 g Na2CO3 (equivalent to 1.65 mol pro mol chloride), 5 g cement and 14 ml water and heated to 130° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 25.5 weight percent and a chloride content of 0.7 weight percent based on dry substance.

EXAMPLE 8 (ACCORDING TO THE INVENTION)

[0031] 100 g of Sample B were mixed with 45.5 g Na2CO3 (equivalent to 1.05 mol pro mol chloride) and 39 ml water and heated to 130° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 32 weight percent and a chloride content of 0.24 weight percent based on dry substance.

EXAMPLE 9 (ACCORDING TO THE INVENTION)

[0032] 100 g of Sample B were mixed with 45.5 g Na2CO3 (equivalent to 1.05 mol pro mol chloride), 5 g cement and 39 ml water and heated to 130° C. After 45 minutes the mixture was mixed with 250 ml water and filtered over a laboratory pressure filter at a filtration pressure of 2.5 bar and subsequently washed on the pressure filter with 250 ml water. The filter cake had a water content of 37 weight percent and a chloride content of 0.38 weight percent based on dry substance.

[0033] As can be seen from the examples, the method according to the invention provides products with water contents below 40 weight percent and chloride contents below 1 weight percent (in relation to the dry substance). Therefore these products are particularly suitable for a utilisation in metallurgical processes.

Claims

1. A method for rendering inert dust residue containing silicon metal originating from the trichlorosilane synthesis, characterised by the following steps:

the residues are mixed with 10 to 50 weight percent water, in relation to the quantity of residue, and an at least equimolar quantity of an alkaline compound, in relation to the chloride content of the residue;

the mixture is heated to a temperature ranging from 50 to 200° C.;

the heated mixture is mixed with at least the double amount of water; and

subsequently the mixture is extensively liberated from dissolved salts by filtration and subsequent washing with water.

2. A method according to claim 1, characterised in that the alkaline compound is provided at a molar excess of 2 to 10%, in relation to the chloride content of the residues.

3. A method according to claim 1 or 2, characterised in that the alkaline compounds used are NaOH, Na2CO3, NaHCO3, CaO, Ca(OH)2 or cement.

4. A method according to claim 1 or 2, characterised in that the alkaline compounds used are CaO, Na2CO3 and cement or mixtures thereof.

5. A method according to any one of claims 1 to 4, characterised in that in the first step the mixture is heated to a temperature ranging from 100 to 150° C., preferably 120 to 140° C.

6. A method according to any one of claims 1 to 5, characterised in that after heating the mixture is maintained at this temperature for a period of 10 to 60 minutes.

7. A method for the execution of a metallurgical process, in particular for the production of an iron alloy or copper, comprising the following steps:

a) rendering inert dust residue containing silicon metal originating from the trichlorosilane synthesis, comprising the following steps of rendering inert:

the residues are mixed with 10 to 50 weight percent water, in relation to the quantity of residue, and an at least equimolar quantity of an alkaline compound, in relation to the chloride content of the residue;

the mixture is heated to a temperature ranging from 50 to 200° C.;

the heated mixture is mixed with at least the double amount of water; and

subsequently the mixture is extensively liberated from dissolved salts by filtration and subsequent washing with water;

b) utilisation of these inert residues in the subsequent metallurgical process.