RU2539593C1 - Electrochemical method of obtaining of powder of calcium hexaboride - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: salt melt is used which contains calcium chloride with the additives of calcium oxide and boric anhydride at aggregate concentration of additives 6÷7 wt %, during electrolysis a mass ratio of boric anhydride to calcium oxide is maintained within 3.72±0.01 wt %, electrolysis is conducted in the temperature interval 800÷850°C, with initial impulse of density of cathode current 0.50 A/cm² within 1.5 s, and then at constant current 4.5 A and current density 0.10 A/cm² within 1.5÷3 hours.

EFFECT: method allows to obtain a target product by direct electrochemical deposition of calcium hexaboride on the cathode, it has an increased current efficiency at the simplified method of obtaining of the target product.

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Description

The invention relates to the production of powder borides, in particular to the production of powdered calcium hexaboride (CaB ₆ ) with a particle size of 3 ÷ 45 μm, which can be used in high-current electronics for the manufacture of cathodes - high-power thermoemitters, as well as a matrix of composite anode material in chemical current sources.

A known method of producing powdered calcium hexaboride in which a powder mixture of boron and calcium carbonate with a particle size of 50 ÷ 60 μm is fed into a stream of argon-nitrogen-hydrogen plasma with a temperature of 5600 ÷ 6000 ° C, where the thermosynthesis of calcium boride SU takes place within a very short time 1810298, publ. 04/23/1993 [1]. The synthesized product is collected in a cooled condenser, the average particle size of the resulting particles 95 ÷ 110 nm. According to this method, the synthesis of boride occurs in the gas phase. The method has high performance, but requires high temperatures, sophisticated equipment, and also powder elemental boron, the cost of which is today from 8 to 12 thousand rubles / kg, depending on its purity. Based on this, the cost of producing a unit mass of the target product will be very high.

A known method of producing powdered calcium hexaboride by thermal reduction of calcium carbonate (CaCO ₃ ) with elemental boron at a temperature of 1400 ÷ 1500 ° C, followed by mechanical grinding of the obtained sintered product and water washing of boride powder (High-temperature borides. Serebryakova TI, Neronov VA , Peshev PD - M: Metallurgy. Chelyabinsk branch. 1991) [2]. The method requires high temperatures and mechanical grinding of the sintered product, where the product is contaminated with impurities from crushing bodies and the walls of the mill.

An electrochemical method for producing powdered calcium hexaboride (CN 102242371, published on November 16, 2011) [3], which can be adopted as a prototype of the claimed method, is aimed at lowering the synthesis temperature, increasing the yield of the target product, and using less expensive reagents. This method is carried out in two stages. At the first stage, powders of calcium and boron-containing raw materials in a given atomic ratio of calcium to boron of 1: 6 are thoroughly mixed, the mixture is loaded into water, the resulting suspension is heated with stirring at a temperature of 80 ÷ 100 ° C for 2 hours. During this time, hydrothermal synthesis of calcium borate of an approximate total composition of CaC ₆ O ₁₀ occurs with its precipitation. The precipitate is separated from the water by filtration, dried to a granular state and pressed into tablets with a diameter of ≈5 and a thickness of ≈3 mm. The obtained tablets are sintered in a muffle furnace at a temperature of 650 ° C for 3 hours, their residual porosity is in the range of 20 ÷ 30%. Thus prepared tablets of porous calcium borate are placed in a wire basket suspended on a molybdenum rod-current lead.

At the second stage, an eutectic mixture of CaCl ₂ -NaCl salts is deposited in an electrochemical reactor, a basket with calcium borate tablets is placed in a current lead (cathode) into this melt, an anode of sintered tin dioxide (SnO ₂ ) is immersed at a shallow depth and a direct current of electrolysis is turned on. The composition of the molten electrolyte allows you to conduct the electrolysis process in the temperature range 520 ÷ 700 ° C. The solubility of calcium borate in the CaCl ₂ -NaCl melt at the indicated temperatures is negligible, therefore, B ³⁺ ions are practically absent in the melt, which impedes the implementation of the electrochemical method of producing calcium hexaborate as the target product for the following reasons. Since solid calcium borate (in tablets) does not conduct an electric current, i.e. is an insulator, it cannot be restored electrochemically at the cathode, therefore, it is chemically reduced, that is, calcium released on the metal wire of the basket-cathode during electrolysis. Thus, electrolysis in this case is needed only for continuous reproduction of the reducing agent — calcium, dissolved in the salt melt.

The mechanism of reduction of non-conductive oxides during the electrolysis of calcium chloride melts (Cambridge process) is known (NI Shurov, Yu.P. Zaikov, AP Khramov, VA Kovrov, AV Suzdaltsev. Physical chemistry and electrochemistry of molten and solid electrolytes: Reports of the XVI Russian Conference, Yekaterinburg, Publishing House of the Ural Federal University. 2013. S.399-402) [4]. The disadvantage of such processes of secondary reduction of oxides is their very low speed and low yield of the target product by the amount of consumed electricity, rarely exceeding 25-30%. This is due to the fact that the active reducing agent, calcium, dissolved in the salt melt, is uselessly consumed for oxygen oxidation at the air-salt melt interface and other side reactions, for example, with a graphite container for salt melt.

For these reasons, the prototype method cannot have high performance, the maximum yield of the target product in terms of the amount of electricity consumed should not exceed 30%.

It should be noted in addition that according to the method according to CN 102242371, ultrafine (submicron) CaB ₆ powder is obtained for use in chemical current sources. The ultrafine powder CaB _{6 is} very inconvenient in operation due to its loss in the form of dust during technological operations of weighing, packaging, pressing, etc. Submicron powders are many times more difficult to clean from impurities than powders with micron-sized crystals. Aerosols of such powders are harmful to the human body.

The objective of the present invention is to increase the productivity of the process of obtaining pure micro-sized powder of calcium hexaboride while reducing financial costs and energy resources for its production, as well as expanding the scope of the powder.

To solve this problem, an electrochemical method for producing calcium hexaboride powder is proposed, including electrolysis of a salt melt containing calcium and boron components, using a salt melt containing calcium chloride with additives of calcium oxide and boric anhydride at a total concentration of additives of 6 ÷ 7 wt.% , during the electrolysis, the mass ratio of boric anhydride to calcium oxide is maintained at 3.72 ± 0.01, the electrolysis is carried out in the temperature range 800 ÷ 850 ° C with an initial density pulse of cathodic current of 0.50 A / cm ² for 1.5 seconds, and further at a constant current of 4.5 A and a current density of 0.10 A / cm ² for 1,5 ÷ 3 hours.

Unlike the prototype, in the inventive method, at the beginning of the process, the electrolysis bath is loaded not with a eutectic mixture of CaCl ₂ -NaCl salts, as in the prototype, but with an individual dehydrated salt of calcium chloride CaCl ₂ . This is because in the individual calcium chloride, the solubility of the B ₂ O ₃ additive in the presence of another CaO additive increases many times, reaching their total concentration of 6% at 800 ° C and 7% at 850 ° C.

The use of electrolysis temperatures in the range of 800 ÷ 850 ° C is caused by the need for a sufficiently rapid dissolution of the indicated reagents in the molten electrolyte with simultaneous dissociation into ions. In this case, the lower temperature limit is limited by the melting point of calcium chloride (772 ° C), and the melt overheating above the melting temperature of the salt must be at least 20 ° for successful electrolysis. The upper temperature limit is due to the fact that at higher than 850 ° C the volatility of the melt rapidly increases with the loss of CaCl ₂ in the form of vapors, as well as a valuable boron-containing component in the form of BOCl vapor. The supply to the cathode of a pulse of increased current density (0.50 A / cm ² ) for 1.5 ÷ 2 seconds at the beginning of the process is needed for the instant formation of a large number of nanosized CaB ₆ nuclei - growth centers of future crystals. Together with the subsequent electrolysis mode in the form of a constant current value of 4.5 A and current density of 0.10 A / cm ² for 1.5 ÷ 3 hours in the temperature range 800 ÷ 850 ° C, this technological technique allows to obtain calcium hexaboride powder with particle size from 3 to 45 microns.

Thus, the electrolysis in the claimed method is used not for the reproduction of the reducing agent — calcium dissolved in the salt melt, as follows from the description of the process in the prototype, but for its main purpose, that is, for direct electrochemical deposition of calcium hexaboride on the cathode. In this case, the yield of the consumed amount of electricity is about 80%. Since the method allows to obtain calcium hexaboride by direct electrochemical deposition at the cathode, the chemical reduction of calcium borate described in the prototype becomes unnecessary. This allows you to get a powder of calcium hexaborate in one step, without preparing calcium and boron-containing raw materials, including the manufacture of pressed and calcined tablets of calcium borate, as is done in the prototype. The crystal size of the powder obtained by the claimed method is larger than that obtained in the prototype, however, this is not a disadvantage of the method. As applied to the manufacture of cathodes for high-current electronics, micro-sized calcium hexaboride powder is preferred.

The technical result of the claimed invention consists in direct electrochemical deposition of calcium hexaboride at the cathode, increasing current efficiency while simplifying the technology for obtaining the target product.

The method was carried out as follows. As calcium and boron components used:

- Calcium chloride (CaCl ₂ ) technical, calcined, granular, GOST 450 - 77, rev. 1-3;

- Calcium oxide (CaO) brand "h";

- Boric anhydride (B ₂ O ₃ ) - fused, grade "h", TU 6-09-17-249-88.

The fusion of the bath was carried out as follows. Salt CaCl _{2 was} melted in a crucible at 850 ° C, calcium oxide CaO, which was dissolved in the melt, was poured in small portions on top of the melt, then granules B ₂ O ₃ were poured in portions, which were dissolved in the CaCl ₂ melt as it was heated. After complete dissolution of B ₂ O _{3, the} molten salt usually becomes transparent. During the deposition process and during the entire electrolysis, the ratio of boric anhydride to calcium oxide was maintained at 3.72 ± 0.01 wt.%.

When conducting electrolysis, round graphite rods were used as electrodes. The working part of the anode and cathode was lowered into the melt; above the melt level, they were protected from air oxidation by alundum tubes.

The phase composition of CaB ₆ powders was determined using a Rigaku D / Max - 2000 X-ray diffractometer; the particle size distribution was studied on a Malvern Mastersizer 2000 laser diffraction analyzer.

Example 1. In a corundum crucible at a temperature of 850 ° C, 800 g of a calcium chloride melt was deposited, onto which calcium oxide was deposited on top, and then boron oxide, based on their total concentration of 5 wt.%, The rest was calcium chloride. After 10 min exposure, the graphite anode and cathode were lowered into the melt. Initially, the melt was cleaned for 30 minutes to remove dissolved water. Further, the electrolysis to produce CaB _{6 was} carried out at a temperature of 850 ° C with an initial pulse of cathode current density i _k = 0.50 A / cm ² for 1.5 s, and then at a constant current value of 4.5 A and current density i _k = 0.10 A / cm ² for 1.5 hours.

After electrolysis was completed, the cathode with the precipitate was removed from the melt and, after cooling to room temperature, it was placed in a glass cup with boiling distilled water, in which the precipitate was washed from the salt residues. Next, the precipitate was washed into a funnel on a paper filter and washed additionally with dilute hydrochloric acid, then with distilled water and alcohol. After that, the washed precipitate, together with the filter, was dried in an oven at a temperature of 50 ° C. After removal from the filter, fine dark cinnamon powder weighing 1.1 g was obtained. Figure 1 shows an X-ray diffraction pattern of the obtained cathode deposit. X-ray phase analysis showed that the powder is a pure calcium hexaboride CaB ₆ . Elemental chemical analysis showed the presence of aluminum and iron impurities, the total content of which is 2.5 wt.%. Since the CaB ₆ crystal lattice is not distorted (see Fig. 1), it can be assumed that iron and aluminum impurities were on the surface of CaB ₆ crystals most likely in the form of hydroxide films of nanosized thickness; after additional washing of the powder with dilute hydrochloric acid and water, a CaB ₆ powder of 99.8% purity was obtained. Figure 2 shows the particle size distribution of the obtained CaB ₆ powder. Granulometric analysis showed that the average size of its crystals across is in the range of 3–15 μm. The theoretical mass of CaB ₆ , calculated according to the Faraday law, should be 1.323 g, but in fact 1.100 g was obtained. Thus, the yield of the product according to the consumed amount of electricity 6.75 A * hour is 83.1%. The productivity of the process is 0.73 g / hour.

Example 2. The experiment was carried out in the same technological procedure as in example 1, but the temperature of the experiment was 800 ° C. The average size of the obtained crystals is in the range from 8 to 45 μm with the same process performance as in example 1.

The description of the method according to CN 102242371 does not provide such important basic characteristics of the process as the cathodic current density (A / cm ² ), which characterizes the intensity of the process, the yield of the target product by current (in%), which characterizes the energy efficiency of the process, productivity of the process (g of product / hour electrolysis) is also not indicated. For this reason, a direct comparison of the results of the known and claimed methods is impossible. However, indirectly, a lower productivity of the prototype process is indicated by its longer duration (from 10 to 20 hours) instead of 1.5 ÷ 3.0 hours for the claimed method. From published data by other authors it is known that the current efficiency in such processes is not more than 25–28%. In the claimed method, it is three times higher. From these data it follows that the inventive method has significantly lower specific energy consumption for the production of a unit mass of the target product - calcium hexaboride. In addition, the lack of technological operations for the preparation of calcium and boron-containing raw materials in the form of tablets of calcium borate exclude additional energy and labor costs.

Thus, the claimed electrochemical method for producing a pure powder of calcium hexaboride has a higher productivity while reducing financial costs and energy resources, and the powder obtained by the claimed method has an expanded scope.

Claims (1)

An electrochemical method for producing calcium hexaboride powder, comprising electrolysis of a molten salt containing calcium and boron components, characterized in that a molten salt containing calcium chloride with additives of calcium oxide and boric anhydride is used at a total concentration of additives of 6 ÷ 7 wt.%, In the process the electrolysis support the mass ratio of boric anhydride to calcium oxide of 3.72 ± 0.01 wt.%, the electrolysis is carried out in the temperature range 800 ÷ 850 ° C, with an initial pulse of cathode current density of 0.50 A / cm ² for 1.5 seconds, and then at a constant current of 4.5 A and current density of 0.10 A / cm ² for 1.5 ÷ 3 hours.

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