RU2601484C1 - Method of producing fine carbides of transition metals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to chemical engineering of inorganic substances, particularly, to production of highly-dispersible refractory carbides of transition metals in granular form, including mixed composites based thereon. Described is a method for preparing fine carbides of transition metals, including production of carbon-containing gel-forming solution of metal salts, further hydrolysis using a sol-gel technology, drying, thermal treatment and carbothermal reduction, in which gel-forming solution is produced as follows: mixing aqueous solution of nitrates of transition metals with urea, after dissolving urea as carbon-containing substance is introduced lamp soot, which in solution is subjected to dispersion, then urotropin is added and dissolved, in obtained gel-forming is added a metal hydroxide, subsequent hydrolysis is carried out by drop dispersion of solution in heated oil, formed granules are washed from oil and then treated with aqueous ammonia solution.

EFFECT: method of producing highly-dispersible refractory carbides of metals in form of granules, which makes it possible to reduce duration of process and replace metal-organic compounds and carbon-containing polymers or their solutions.

8 cl, 3 ex

Description

The invention relates to the field of chemical technology of inorganic substances, specifically to the production of highly refractory refractory carbides of transition metals in granular form, including mixed composites based on them.

The known method is that carbides of transition metals of groups 4-6 of the periodic system of elements are obtained by reduction of metal oxides with carbon (see "Refractory carbides", pp. 16-19, Kiev 1970, "Naukova Dumka"). As the transition from metal oxides of the 4th group to metal oxides of the 5th and 6th groups, the temperature of reduction and formation of carbides decreases from 2100 ° C - for the 4th group to 1500 ÷ 1700 ° C - for the 6th group.

Also known is a method for the production of carbides of refractory metals, including the preparation of a reactive mixture of powders of the starting components, loading it into the reaction volume and synthesizing carbide by melting with an electric arc created between the two electrodes, with the sequential supply of an additional amount of the mixture to the melting zone. As charge materials, a powder of refractory metal, in particular tungsten, and soot taken in the ratio respectively (96: 4) are used (RF patent No. 2060934, IPC СВВ 31/30, publ. 05.27.1996). The disadvantage of this and the above described method is that as a result, sintered and fused carbide of low dispersion is obtained, which requires an additional and laborious grinding operation to obtain a finely divided powder.

A known method of producing porous carbides of refractory metals by mixing a metal oxide powder with a carbon-containing component, molding and heat treatment (RF patent No. 2181913, IPC G21C 3/64, publ. 04/27/2002). As a carbon-containing component, coal tar pitch is used in the form of its solution in trichlorethylene. Carbidization and sintering are carried out at 1900 ÷ 2200 ° C. The disadvantage of this method is that the resulting sintered and fused carbide, as in the previous method, requires an additional and time-consuming grinding operation. In addition, the use of a solution of coal tar pitch is associated with the need for a long process of its carbonization to produce carbon. The duration of only this stage of the process for producing carbides of refractory metals is 40 ÷ 50 hours.

The closest in technical essence to the claimed invention is a method for producing highly dispersed refractory metal carbides, which consists in mixing organic solutions of metal compounds with carbon-containing polymers or their solutions, subsequent hydrolysis of the mixture by sol-gel technology, drying at 20 ÷ 250 ° C, pyrolysis-carbonization at 350 ÷ 600 ° C and high-temperature processing at 600 ÷ 1200 ° C at a pressure of 10 ^-1 ÷ 10 ^-4 Pa (RF patent No. 2333888, IPC СВВ 31/30, publ. 09/20/2008). The main disadvantages of the known technical solution include: the need to use scarce and expensive components - organometallic compounds, as well as the duration of the process due to the use of carbon-containing polymers or their solutions, requiring a long (up to 30 hours) pyrolysis operation. Although the metal carbide formed by this method is a highly dispersed spherical particle (less than 100 nm), however, as in the above known technical solutions, this method does not allow to obtain carbide in the form of granules with a diameter of 0.1 ÷ 3.0 mm and requires an additional operation of converting the powder material into a granular granular product.

The task and the technical result achieved by using the invention is the development of a method for producing highly dispersed refractory metal carbides in the form of granules, which allows to reduce the duration of the process and replace organometallic compounds and carbon-containing polymers or their solutions.

The technical result is achieved by the fact that in the method for producing finely divided transition metal carbides, including obtaining a carbon-containing gel-forming solution of metal salts, subsequent hydrolysis by sol-gel technology, drying, heat treatment and carbothermic reduction, according to the invention, the gel-forming solution is obtained as follows: a solution of inorganic metal salts is mixed with urea, after dissolution of urea, carbon black is introduced into the solution as a carbon-containing substance, which w is subjected to dispersion in a solution, and then introduced into the solution and dissolved methenamine, in the thus obtained solution is added a gelling metal hydroxide, the subsequent hydrolysis is carried out by dispersing the solution in a drip heated oil, the resulting pellets are washed free of oil and is then treated with aqueous ammonia.

A significant difference of this method is the preparation of a gelling solution based on inorganic metal salts and soot as a carbon-containing substance, as well as the addition of metal hydroxide to it, which avoids lengthy pyrolysis and carbonization processes and, thereby, shortens the process of obtaining highly dispersed metal carbides. The addition of metal hydroxide to the gel-forming solution prevents the rapid deposition of soot in the solution and, thus, increases its stability. The hydrolysis of the gel-forming solution by dispersing it dropwise into heated oil makes it possible, in contrast to known methods, to obtain highly dispersed metal carbides in the form of granules. In addition, the proposed method can significantly reduce the cost of the process by abandoning the use of scarce and expensive components - organometallic compounds and carbon-containing polymers or their solutions.

In this case, the quantitative composition of the gelling solution can be as follows:

inorganic metal salt solution 80 ÷ 150 g / liter (for metal) urotropin 0.45 ÷ 0.75 moles per mole of metal urea 0.7 ÷ 1.0 moles per mole of metal lampblack 2.5 ÷ 3.0 mol per mol of metal

The upper and lower limits on the concentration of metal in the solution are determined by the maximum possible solubility of nitrate salts in water. So, for example, for two-water zirconyl nitrate ZrO (NO ₃ ) ₂ 2H ₂ O it is 80 ÷ 100 g / l, and for six-water yttrium nitrate Y (NO ₃ ) ₃ 6H ₂ O - 150 g / l.

The content of urea and urotropine in the solution was selected taking into account the concentration of the metal in the solution and is also due to the need for gelation of the solution by dropping it into hot oil.

Carbon black is taken in the amount necessary for carbothermic synthesis of metal carbide. For example, 3 moles are taken for zirconium oxide (1), and 2.5 moles of carbon per mole of metal for yttrium oxide (2):

In a solution of inorganic metal salts and urea, metal hydroxide is added in an amount of not more than 0.5 of the volume of the gelling solution. With a larger amount of hydroxide (more than 0.5 of the solution volume), gelation (curing) of the drops of the prepared solution in hot oil becomes difficult due to the deficiency of inorganic metal salts in the solution. In addition, the high viscosity of such a solution makes it difficult to drip dispersion through a vibrating nozzle. With a smaller amount of metal hydroxide (less than 0.2 of the volume of the solution), its stabilizing effect on the stability of the solution to soot deposition becomes less noticeable.

The dispersion of soot in solution is carried out using ultrasonic vibrations.

Urotropin is dissolved at 3 ÷ 10 ° C.

The droplet dispersion of the gel-forming solution is carried out through a vibrating nozzle into oil heated to 85 ÷ 95 ° C. The oil temperature is determined by the requirements for the droplets to be fully warmed up to a given temperature and complete gelation, i.e. before turning them into solid granules.

The granules are washed from the oil with an organic solvent.

The duration of treatment with an aqueous solution of ammonia granules is determined by the formula:

t = 0.5 (2d-C / 100), where:

t is the duration of washing the granules, hours

d is the diameter of the granules, mm

C is the concentration of ammonia in an aqueous washing solution, vol. %

The essence of the proposed method for producing finely divided granular carbides of refractory metals is illustrated by the following examples.

Example 1

A gel-forming solution was prepared to obtain highly dispersed granular zirconium carbide. We took 600 ml of a zirconyl nitrate solution with a zirconium concentration of 100 g / l, 0.88 mol of urea per mole of zirconium was added and dissolved in it, 3.0 mol of carbon black per mole of zirconium, which was subjected to ultrasonic dispersion in a solution using ultrasonic generator (22 KHz) UZDN-2T for 20 minutes to obtain a homogeneous pulp. The resulting pulp was cooled to 3 ° C and 0.67 mol of urotropin per mole of zirconium was dissolved in it with stirring. Zirconium hydroxide (26.5 wt.% ZrO ₂ ) was added to the solution thus prepared in an amount of 0.2 (140 ml) of the volume of the gelling solution. The solution thus obtained was subjected to drop dispersion through a vibrating (270 Hz) nozzle with a diameter of 1.0 mm into silicone oil of the PMS-40 type at 85 ° C. Moreover, due to the decomposition of urotropine and the formation of ammonia, hydrolysis occurred in heated droplets, leading to their curing and the formation of granules. The obtained spherical granules (d = 2.1-2.2 mm) of hydrated zirconium oxide with carbon black were washed from PMS-40 oil with carbon tetrachloride, and then in aqueous ammonia (25% by weight) for 90 minutes. Then the granules were dried in air at 95 ° C. The heat treatment of the dried granules (d = 0.8-0.9 mm) was carried out in an inert atmosphere at 350 ° C, and the carbothermic reduction was carried out in vacuum at 1800 ° C. Spherical granules (d = 0.5-0.6 mm) of highly dispersed zirconium carbide were obtained. X-ray phase analysis of the obtained finely dispersed carbide showed that, as a result of synthesis, a cubic phase of zirconium monocarbide is formed.

Example 2

To obtain highly dispersed granular zirconium and yttrium carbides, a gel-forming solution was prepared. We took 300 ml of a solution of zirconyl nitrate and yttrium nitrate with a concentration of zirconium and yttrium of 100 g / l and 3.0 g / l, respectively, and mixed with 1.0 mol of urea per mole of zirconium. After dissolution of the urea, 2.9 mol of carbon black per mole of zirconium was added, which was subjected to ultrasonic dispersion in a solution using an ultrasonic generator UZDN-2T for 20 min until a homogeneous pulp was obtained. The resulting pulp was cooled to 3 ° C and 0.75 mol of urotropine was dissolved in it with stirring. 150 ml of zirconium hydroxide (26.5 wt.% ZrO ₂ ) were added to the gel-forming solution thus prepared. The resulting solution was subjected to drop dispersion through a vibrating (280 Hz) nozzle with a diameter of 1.0 mm into silicone oil of the PMS-40 type at 95 ° C. Granules of hydrated metal oxides with carbon black were washed from PMS-40 oil with carbon tetrachloride and then washed in an aqueous 25% ammonia solution. After that, air drying was performed at 95 ° C, heat treatment at 400 ° C, and carbothermic reduction in vacuum at 1600 ° C. As a result of carbothermic reduction, spherical granules (d = 0.5-0.6 mm) of highly dispersed zirconium and yttrium carbides were obtained.

Example 3

A gel-forming solution was prepared to obtain highly dispersed granular zirconium carbide. We took 600 ml of a zirconyl nitrate solution with a zirconium concentration of 100 g / l, 0.88 mol of urea per mole of zirconium was added and dissolved in it, 3.0 mol of carbon black per mole of zirconium, which was subjected to ultrasonic dispersion in a solution using ultrasonic generator (22 KHz) UZDN-2T for 20 minutes to obtain a homogeneous pulp. The resulting pulp was cooled to 3 ° C and 0.67 mol of urotropin per mole of zirconium was dissolved in it with stirring. Zirconium hydroxide (26.5 wt.% ZrO ₂ ) was added to the solution thus prepared in an amount of 0.5 (150 ml) of the volume of the gelling solution. The solution was dispersed through a vibrating (270 Hz) nozzle 1.0 mm in diameter into PMS-40 type silicone oil at 95 ° C. Granules of hydrated metal oxides with carbon black were washed from PMS-40 oil with carbon tetrachloride and then washed in an aqueous 25% ammonia solution. After that, air drying was performed at 95 ° C, heat treatment at 400 ° C, and carbothermic reduction in vacuum at 1800 ° C. Spherical granules (d = 0.5-0.6 mm) of highly dispersed zirconium carbide were obtained.

The proposed method allows to obtain highly dispersed granular carbides of transition metals, abandoning the use of scarce and expensive components - organometallic compounds and carbon-containing polymers or their solutions and the need for a long process of pyrolysis and carbonization. The use of inorganic metal salts and their hydroxides, as well as lamp black as a carbon-containing substance, can reduce the process time from several tens to several (8 ÷ 10) hours. The resulting spherical granules (d = 0.2 ÷ 3.0 mm) of finely dispersed (less than 500 nm) metal carbides, depending on the sintering temperature, can have a strength of several tens of grams per particle to 1-2 kg. Low-strength granules can be used for forming compact carbide products by powder metallurgy - pressing and subsequent sintering. High-strength (1-2 kg per particle) spherical carbide granules can be used as simulators of microspherical fuel elements for testing the technology of coating them. X-ray phase analysis of the obtained fine carbide showed that the synthesis results in the cubic phase of zirconium monocarbide (examples No. 1 and No. 3) and the phase of the solid solution of zirconium and yttrium carbides (example No. 2). The dispersion of the formed carbides in granules d = 0.4 ÷ 0.6 mm depends on the dispersion of the used soot and can be 200 ÷ 500 nm.

Claims (8)

1. A method of obtaining highly dispersed carbides of transition metals, including obtaining a carbon-containing gel-forming solution of metal salts, subsequent hydrolysis by sol-gel technology, drying, heat treatment and carbothermic reduction, characterized in that the gel-forming solution is prepared as follows: an aqueous solution of transition metal nitrates is mixed with urea , after dissolution of urea, carbon black is introduced as a carbon-containing substance, which is subjected to dispersion in a solution, then methenamine lead and dissolved in the thus obtained solution is added a gelling metal hydroxide, the subsequent hydrolysis is carried out by dispersing the solution in a drip heated oil, the resulting pellets are washed free of oil and is then treated with aqueous ammonia. 2. The method according to p. 1, characterized in that the gel-forming solution contains:
transition metal nitrate solution 80 ÷ 150 g / liter (for metal) urotropin 0.45 ÷ 0.75 moles per mole of metal urea 0.7 ÷ 1.0 moles per mole of metal lampblack 2.5 ÷ 3.0 mol per mol of metal 3. The method according to p. 1, characterized in that the metal hydroxide in the gelling solution is added in an amount of 0.2 ÷ 0.5 of the volume of the gelling solution. 4. The method according to p. 1, characterized in that urotropin is dissolved at 3 ÷ 10 ° C. 5. The method according to p. 1, characterized in that the dispersion of soot in solution is carried out using ultrasonic vibrations. 6. The method according to p. 1, characterized in that the droplet dispersion of the gelling solution is carried out through a vibrating nozzle into oil, heated to 85 ÷ 95 ° C. 7. The method according to p. 1, characterized in that the granules formed are washed from the oil with an organic solvent. 8. The method according to p. 1, characterized in that the duration of treatment with an aqueous solution of ammonia granules of metal oxides is determined by the formula
t = 0.5 (2d-4C / 100), where:
t is the duration of washing the granules, hours;
d is the diameter of the granules, mm;
C is the concentration of ammonia in solution, wt. share%