RU2731094C1 - Method of producing powder containing silicon carbide and aluminum nitride from natural coal ash - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: disclosed is a method of producing powder containing silicon carbide and aluminum nitride from natural coal ash, comprising mixing and grinding ash in a ball mill, heat treatment in gaseous medium for 30 minutes, where powder mixture of ash and graphite in weight ratio of 80:20 is ground in a ball mill for 1 hour and piled with a layer of not more than 1 mm on the bottom of the cathode in the form of a vertically arranged graphite shell, generation of DC arc discharge in air medium by contact of anode in form of solid graphite rod with said powder mixture at current intensity from 165 to 200 A for 10 to 20 seconds, then interruption of discharge burning, removing anode, and after cathode cooling to room temperature, powder mixture is extracted from cathode cavity, is placed in corundum dishes and heated in an atmospheric furnace to 900 °C, held at this temperature for 30 minutes, obtained powder is removed from the furnace and cooled in natural conditions.

EFFECT: shorter time for obtaining powder containing silicon carbide and aluminum nitride from ash of natural coal.

1 cl, 2 dwg, 1 tbl, 1 ex

Description

The invention relates to inorganic chemistry, namely, to obtain a powder containing silicon carbide and aluminum nitride from natural coal ash, which can be used for the production of superhard ceramics.

A known method of producing a powder containing silicon carbide and aluminum nitride from ash formed during the combustion of natural coal at the Baqiao station in the city of Xian [Wang Jongjie, Wang Yonglan, Jin Zhihao SiC powders prepared from fly ash Journal of Materials Processing Technology 117 (2001) 52 -55]. The ash contained 57.07 wt. % SiO ₂ ; 31.30 wt. % Al ₂ O ₃ ; 3.30 wt. % Fe ₂ O ₃ ; 1.93 wt. % CaO; 5.04 wt. % MgOc and 1.36 wt. % of other components with an average particle size of about 8 microns. Soot is added to the ash, with an average particle size of about 8 μm in an amount of 10 wt. % and mixed in a ball mill in ethanol for 24 hours. The resulting mixture is placed in a vacuum oven, into which a stream of gaseous nitrogen is supplied and kept in the oven for 30 - 180 minutes at a temperature of 1200 - 1500 ⁰ C. Then the power supply to the oven is turned off and the resulting powder, cooled naturally, is removed.

The resulting powder contained Al ₂ O ₃ , C, SiO ₂ , AlN, SiC (cubic) and SiC (hexagonal).

Disadvantages of the known method: the need to grind a mixture of ash and soot in ethanol, the long duration of the ash grinding process in a ball mill, the process in vacuum with a nitrogen flow fed into the vacuum furnace chamber, the long duration of the ash heat treatment process.

The technical result of the proposed method is to reduce the time for obtaining a powder containing silicon carbide and aluminum nitride from natural coal ash.

A method for producing a powder containing silicon carbide and aluminum nitride from natural coal ash, as in the prototype, includes mixing and grinding the ash in a ball mill, heat treatment in a gas atmosphere for 30 minutes.

According to the invention, a powder mixture of ash and graphite with a particle size of not more than 50 microns in a mass ratio of 80:20 is ground in a ball mill for 1 hour and poured with a layer of not more than 1 mm on the bottom of the cathode in the form of a vertically arranged graphite cup. DC arc discharge is generated in air by contact of the anode in the form of a solid graphite rod with the specified powder mixture at a current from 165 to 200 A for 10 to 20 seconds, then the discharge combustion is interrupted by removing the anode. After the cathode has cooled down to room temperature, the powder mixture is removed from the cathode cavity, placed in a corundum dish and heated in an atmospheric oven to 900 ° C, kept at this temperature for 30 minutes. The resulting powder is removed from the oven and cooled naturally.

When a DC arc discharge occurs, the temperature in the zone of its formation rises to several thousand degrees, and after disconnecting the DC source, the temperature drops to room temperature, as a result of which conditions arise for the synthesis of silicon carbide and aluminum nitride from the silicon oxides and oxides contained in the ash of natural coal aluminum. When an arc discharge burns in the cathode cavity, oxygen is consumed to form gaseous carbon monoxide CO, which escapes into the atmosphere and, as a result, the discharge combustion zone does not come into contact with atmospheric oxygen and the oxidation of the resulting powder is prevented by atmospheric oxygen. Since the erosion of the graphite anode occurs during the combustion of the arc discharge, a significant amount of graphite is also contained in the resulting powder containing silicon carbide and aluminum nitride. Taking into account the resistance of silicon carbide and aluminum nitride to high temperatures in air, unbound carbon in the form of graphite is removed from the resulting powder by heating in an atmospheric furnace to a temperature higher than the combustion temperature of graphite with the formation of gaseous carbon dioxide CO ₂ .

The powder obtained by the proposed method contains up to 60.3% silicon carbide and 9.7 - 11.9% aluminum nitride.

FIG. 1 shows a diagram of a device for producing a powder containing silicon carbide and aluminum nitride from natural coal ash.

FIG. 2 shows the X-ray diffraction pattern of the obtained powder containing silicon carbide and aluminum nitride (example 1).

Table 1 shows the conditions for obtaining powders containing silicon carbide and aluminum nitride and the results of their analysis by X-ray diffractometry.

Used ashes formed during the combustion of natural coal at Tomsk CHP 3, containing 59.53 wt. % SiO ₂ ; 21.83 wt. % Al ₂ O ₃ ; 4.08 wt. % Fe ₂ O ₃ ; 2.51 wt. % CaO; 1.15 wt. % MgO and 10.5 wt. % of other components. The ash was added 20 wt. % of graphite powder with a purity of at least 90% with an average particle size of not more than 50 microns.

The resulting mixture was ground in a ball mill for 1 hour.

To implement the method, a device was used, which contains a cylindrical graphite cathode 1 (Fig. 1) in the form of a vertically located glass with an outer diameter of 30 mm, a height of 30 mm, to the wall of which a dielectric holder is attached 2. A screw 3 is inserted into the threaded hole of the dielectric holder 2, connected to one end of the cylindrical graphite anode 4 in the form of a solid rod with a diameter of 8 mm. The free end of the anode 4 is located coaxially to the cathode 1 with the possibility of longitudinal movement in its cavity. Anode 4 and cathode 1 are connected to a direct current source 5 (IPT).

0.5 g (m _out ) of the resulting mixture of ash and graphite powders was placed on the bottom of cathode 1 with a layer thickness of 6 not more than 1 mm. When a direct current source 5 (DCS) was turned on, a potential difference appeared between the mixture of ash and graphite powders at the bottom of the graphite cathode 1 and the graphite anode 4. By rotating the screw 3, the anode 4 was moved inside the cavity of the cathode 1 coaxially to it until it came into contact with the powder mixture of ash and graphite. The arc discharge was ignited by short-term contact of the anode 4 with the powder mixture at a current of I = 200 A. Then, using the screw 3, the anode 4 was pulled vertically upwards coaxially to the cathode 1, forming a discharge gap L = 0.5 mm. In the process of burning an arc discharge, a mixture of ash and graphite powders, as well as anode 4 and cathode 1, are heated. After burning of the arc discharge for t = 20 seconds, the direct current source 5 (IPT) was turned off. After cooling the anode 4 and cathode 1, the resulting powder settled on the surface of the cavity of the cathode 1 was collected and placed in an atmospheric oven in a corundum dish with a layer thickness of 1 mm. In an atmospheric furnace, the resulting powder was heated to 900 ° C ( _heating t) at a heating rate of 30 ° C / min and held for 30 minutes ( _holding t) at 900 ° C. Then the oven was turned off. After the furnace cooled down, the resulting dark gray powder was collected in a plastic flask.

The analysis of the obtained powder was carried out on a Shimadzu XRD 7000s X-ray diffractometer (CuKα radiation). Comparison of the obtained X-ray diffraction pattern, namely, the positions of the diffraction maxima with the standards of various materials according to the structural data base showed (Fig. 2) the presence of many diffraction maxima, of which five correspond to the cubic phase of silicon carbide (SiC cube), six maxima correspond to the hexagonal phase of silicon carbide (SiC hex), eight maxima correspond to the hexagonal phase of aluminum nitride (AlN). Also in the X-ray diffraction pattern, nine low-intensity diffraction maxima belonging to other materials are identified.

Quantitative X-ray phase analysis was performed according to the intensity of diffraction maxima. As a result, it was found that the resulting powder contains 60.3% SiC cube and 17.5% SiC hex; 11.9% AlN; 5.1% SiO ₂ ; 3.4% Al _x Si _y O _z and 1.8% Al ₂ O ₃ .

Conditions for the preparation of other examples of powders containing silicon carbide and aluminum nitride and the results of their analysis by X-ray diffractometry are shown in Table 1.

Claims (2)

1. A method of obtaining a powder containing silicon carbide and aluminum nitride from natural coal ash, including mixing and grinding the ash in a ball mill, heat treatment in a gas atmosphere for 30 minutes, characterized in that the powder mixture of ash and graphite in a mass ratio of 80 : 20 are ground in a ball mill for 1 hour and poured with a layer of no more than 1 mm on the bottom of the cathode in the form of a vertically arranged graphite cup, generate a DC arc discharge in air by contacting the anode in the form of a solid graphite rod with the specified powder mixture at a current strength of 165 to 200 A for 10 to 20 seconds, then the discharge combustion is interrupted by removing the anode, and after the cathode has cooled to room temperature, the powder mixture is removed from the cathode cavity, placed in a corundum dish and heated in an atmospheric oven to 900 ° C, keeping at this temperature for 30 minutes, the resulting powder is removed from the oven and cooled in natural conditions. 2. The method according to claim 1, characterized in that graphite is used with a particle size of not more than 50 microns.

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