Method for obtaining cast chrome in a technical combustion mode

Abstract

The invention relates to the field of metallurgy, and more particularly to a technique for obtaining cast chrome. The present method is carried out using centrifugal SHS casting and a reaction mixture. The reaction mixture is prepared by mixing chromium (III) oxide, metallic aluminium, metallic magnesium and calcium bifluoride in the following proportions: 71.5-72.5 wt% chromium (III) oxide; 23.0- 24.0 wt% metallic aluminium; 2.3-3.2 wt% metallic magnesium; 1.2-2.3 wt% calcium bifluoride. The resulting reaction mixture is placed in a refractory mould which is disposed on a centrifuge rotor, the mixture is ignited and synthesis is carried out in combustion mode during centrifugal acceleration within a g-force range of 45-55 g, with the subsequent separation of a cast chrome ingot from a slag phase based on aluminium oxide АL2О3. The result is a non-furnace technique for obtaining cast chrome having a purity of up to 99.7% and a reduced impurity content.

Description

[0001] METHOD OF PRODUCING CAST CHROME IN THE TECHNOLOGICAL COMBUSTION MODE

[0002] DESCRIPTION

[0003] The invention relates to the field of special metallurgy, in particular, to the technology of producing cast chromium by metallothermic reduction of chromium (III) oxide in a process combustion mode under the influence of centrifugal forces created in centrifugal units.

[0004] Synthesized cast chromium has a reduced content of impurities and can be used as a consumable (precursor) in protective coating technologies. Cast chromium is also used in metallurgy to produce precision alloys for special purposes and in the electronics industry as a component of electronic devices.

[0005] A method is known (RU 2027788C1, published 27.01.1995) for producing chromium by aluminothermic reduction, by loading and melting a charge of chromium oxide, aluminum, oxidizer and flux at the first stage, and a charge of chromium oxide and aluminum in an amount of 1.1 - 1.65 of the stoichiometry at the second stage. At the first stage, the charge is melted, containing 55 - 75% of chromium oxide from its total mass for melting and aluminum in an amount of 0.77 - 0.92 of the stoichiometrically required, to reduce this chromium oxide when loading the charge at a rate of 180 - 260 kg / m2-min.

[0006] The disadvantages of this method are the low content of Cr (no more than 98.5 wt.%), the multi-stage nature and energy-intensive nature of the production process.

[0007] A method is known (RU 2260630 C1, published 20.00.2005) for the aluminothermic production of metallic chromium, which includes melting the charge and releasing the smelting products. The charge contains chromium oxide, aluminum, lime, an oxidizer - sodium nitrate in variant 1 or sodium anhydride and dichromate (potassium) in variants 2, 3, additionally in variants 2, 3 - calcium hydroxide and table salt, in variant 2 - fluorspar concentrate. Variant 2 produces chromium with a nitrogen content of up to 0.05 wt. %, in variant 3 - with a nitrogen content of up to 0.01 wt. %. The differences in the variants consist of the established ratio of lime (and calcium hydroxide in variants 2, 3) to aluminum, limiting the carbon content in the lime, introducing most of the lime with a minimum carbon content directly into the charge, and the rest of the lime with a slightly higher carbon content into the blast furnace, certain ratios of components in the charge, loading and melting the charge in one stage at high speeds of 310-450 kg / m3.2 -min, loading of fluorspar concentrate onto the liquid slag remaining after draining onto the skull in option 2.

[0008] The disadvantages of this method are the impossibility of obtaining cast Cr (with a concentration of more than 99.0 mass%) of a multi-component mixture composition, including highly toxic chemical compounds; as well as the staging of extra-furnace smelting with lower ignition of the charge, including the stage of slag and metal drainage.

[0009] A method is known (RU 2103401 C1, published 27.01.1998) for the aluminothermic production of metallic chromium, which includes the staged loading and melting of a charge containing chromium oxide, an oxidizer (sodium nitrate or chromium anhydride), lime, aluminum and the release of smelting products. At the first stage, the charge is melted, consisting of components from the total mass per melt: chromium oxide 53-65%, oxidizer 60-80%, lime 30-40 and aluminum, in an amount of 0.8-0.94 of the stoichiometrically required for the reduction of chromium oxide at a charge loading rate of 180-280 kg / m 2 »min. At the second stage, the melting of the charge components from the total mass for melting is carried out: chromium oxide 36-47%, oxidizer 20-40%, lime 60-70% and aluminum, in the amount of 1.02-1.02 of the stoichiometrically required for the reduction of chromium oxide with a charge loading rate of 170-275 kg / m3 2»min, and when producing metallic chromium with a low nitrogen content (0.05%) in the batch at both stages, chromic anhydride and potassium or sodium dichromate are used as an oxidizer with the addition of calcium hydroxide, table salt and fluorite concentrate to the batch in a ratio to the weight of the chromium oxide batch / 0.1-0.2 / : / 0.02-0.04 / : / 0.03-0.08 / : / 0.001-0.02 / : / 0.001-0.02 / . The disadvantages of this method are the impossibility of producing cast Cr (with a concentration of more than 99.0 wt.%), the multi-stage melting process, including the stage of draining slag and metal; as well as increased heat losses.

[0010] A method is known (RU2352662 published: 20.04.2007) for producing a cast heat-resistant alloy based on cobalt (Co-Cr-Nb-W-Mo-Al-Re-C), where the content of elements in the target product is mass. %: Cr - 19, Nb - 15, W - 2.7, Mo - 1.9, Al - 0.9, Re - 1.3, C - 1.95, Co - the rest.

[0011] The method for producing a cast cobalt-based alloy in combustion mode (including preparing a reaction mixture of starting components containing chromium III oxide, niobium oxide, tungsten oxide, molybdenum oxide, aluminum, graphite and cobalt oxide, placing the reaction mixture in a refractory mold, placing a functional layer of aluminum oxide between the mixture and the mold wall, placing the mold on a centrifuge, igniting the mixture and carrying out synthesis in combustion mode under centrifugal acceleration) is characterized in that rhenium and / or ruthenium are additionally introduced into the reaction mixture in the following ratio of starting components, mass %: chromium III oxide 15.0-25.0 niobium oxide 13.0-22.0 tungsten oxide 0.7-4.8 molybdenum oxide 1.0-1.7 aluminum 20.0-26.0 graphite 2.2-4. 5 rhenium and / or ruthenium 0.5-2.0 cobalt oxide the rest and the synthesis is carried out at centrifugal acceleration of 50-100 g.

[0012] The method includes: preliminary preparation of the mixture by mixing the initial components (oxides of target metals and the reducing agent metal); pouring the mixture into a refractory graphite mold, where a thin-walled cylinder made of non-ferrous metal or thick paper is installed with a gap from its wall to the inner surface of the graphite mold of 5 mm. The finished mixture is poured into the cylinder, a functional layer of aluminum oxide is poured into the gap between the cylinder and the mold. The cylinder is removed, and the loaded mold from the reaction mixture and the functional layer 5 mm thick is placed in a centrifugal unit. The centrifuge rotor is set in rotation and an overload of 50 g is created, after which the reaction mixture is ignited with an electric coil. After the combustion process is complete, the synthesis product is cooled and removed from the reaction mold.The synthesis product consists of two layers: the lower one is the target product in the form of a cast heat-resistant hard alloy based on cobalt (Co-Cr-Nb-W-Mo-Al-Re-C), and the upper one is a cast oxide material ACO3 (corundum). The layers are easily separated from each other.

[0013] The disadvantages of this method are the limitations of obtaining alloys with a high Cr content (above 50 wt.%) and the impossibility of obtaining cast Cr (with a concentration of more than 99.0 wt.%).

[0014] The closest analogue to the claimed method is the method (RU2495945 published 20.10.2013) of aluminothermic production of metallic chromium using dichromate-anhydride technology, which includes the preparation, loading and melting of a charge containing chromium oxide, aluminum, an oxidizer in the form of chromium anhydride and sodium or potassium dichromate, calcium hydroxide, table salt, fluorspar concentrate, lime with a carbon content of no more than 0.2 wt.%, introduced directly into the charge, loading lime with a carbon content of no more than 0.5 wt.% into the furnace top 2-4 minutes before the end of the charge melting at a ratio of lime with a carbon content of no more than 0.2 wt.%, lime with a carbon content of no more than 0.5 wt.%, equal to (50-60) : (40-50), respectively, the release of smelting products by draining slag and metal, while loading and melting of the charge is carried out in one stage at a rate of 360-460 kg / m 2min, and after draining part of the slag into the mold, fluorite concentrate is loaded onto the remaining liquid slag on the skull, after dissolving which the slag and metal are drained, along with this, in the claimed method, before loading the charge components into the mixing drum, calcium and sodium hydroxide or potassium dichromate are placed and mixed for 4-5 minutes, and the charge components are taken at the following content, wt.%: chromium oxide - 56.5-57.3, aluminum - 24.2-25.4, sodium or potassium dichromate - 8.4-8.6, chromium anhydride - 2.8-4.3, calcium hydroxide - 2.55-2.65, table salt - 0.4-0.45, fluorite concentrate 0.9-1.1 from the weight of chromium oxide, lime with a carbon content of not more than 0.2 wt.% - 1.4-1.7, lime with a carbon content of no more than 0.5 wt.% - 1.15-1.45 with a ratio of sodium or potassium dichromate and chromic anhydride equal to 1:(0.4-0.5), as well as lime in the batch with a carbon content of no more than 0.2 wt.% and calcium hydroxide equal to 1:(1.7-1.9).To minimize the impurity content of carbon in chromium, calcium hydroxide containing carbon no more than 0.2 wt.% is used.

[0015] The known methods have common disadvantages - a relatively low (27.0 - 40.4%) yield of metallic chromium with a purity of at least 99.7 wt.% (X99N3 - X99N1), the need for strict balancing of the mixture composition (charge exothermicity) as a factor determining the stability and safety of the technological process of chromium production (high exothermicity leads to a "hot" smelting process and causes a tendency to emissions of burning charge and molten smelting products, with low exothermicity, the smelting process becomes "cold" and the residual content of CrO3 in the slag increases and the formation of remelted (dirty) metal on the hearth of the furnace). A common feature of the above methods is also the multi-stage nature, energy consumption of the production process, a high level of gas evolution during the smelting process due to the use of functional additives that intensify gas evolution.

[0016] The objective of the invention is to eliminate the disadvantages inherent in known solutions.

[0017] The technical result of the claimed invention is the development of a new out-of-furnace technology for producing cast chromium with a reduced content of the main impurity components (Al, O, N, C) by the centrifugal SHS casting method (in combustion mode), reducing energy costs, increasing productivity and the yield of the target product to 96 mass. % of the calculated values, reducing the content of non-metallic inclusions due to the positive effect of the centrifugal force on the melt of combustion products (mixing the melt in the combustion zone and subsequent gravitational separation of the synthesis products). The method makes it possible to increase the purity of the synthesized alloy to 99.7%, reduce the formation of technogenic waste based on aluminum oxide formed during the synthesis process, due to its subsequent technical application in refractory production technologies.

[0018] In addition, the technical result is a reduction in volatile man-made emissions at the combustion (synthesis) stage of the initial compositions for obtaining cast chrome, due to the absence of additional energy additives in the initial mixture.

[0019] The said results are achieved due to the fact that a method is claimed for producing cast chromium in a process combustion mode, including preparing a reaction mixture from the initial components (chromium (III) oxide, metallic aluminum, metallic magnesium), characterized in that the production of cast chromium is carried out by centrifugal SHS casting using a reaction mixture prepared by mixing the initial components, additionally containing calcium difluoride (CaE2), wherein the reaction mixture is placed in a refractory mold, which is placed on a centrifuge rotor, where the ignition of the mixture and the synthesis in the combustion mode are carried out at a centrifugal acceleration in the range of overload values ​​of 45-55 g, followed by separating the cast chromium ingot from the slag phase based on aluminum oxide (AlO3), wherein the initial mixture is prepared at the following ratio of components, mass.%: chromium III oxide - 71.5-72.5; metallic aluminum - 23.0-24.0; metallic magnesium - 2.3-3.2; calcium difluoride (CaFr) - 1.2-2.3.

[0020] Preferably, to obtain cast chromium with a low Al content, metallic magnesium is additionally introduced into the initial composition of the mixture.

[0021] Preferably, to obtain cast chromium with a low content of non-metallic inclusions based on aluminum oxide (AlO3), the synthesis is carried out with a steady rotation of the rotor, ensuring the effect on the sample of centrifugal acceleration (overload) in the range of values ​​of 45-55 g, while the total mass of the combusted mixture should be from 2.5 kg to 6.0 kg.

[0022] Preferably, at least one selected from the series: alundum, perclase or copper water-cooled with an internal lining of a protective thermal barrier layer formed on the basis of inorganic materials selected from the series: aluminum oxide (fused corundum), yttrium oxide, aluminum nitride, boron nitride is used as a refractory form placed on the rotor of the centrifugal unit.

[0023] The invention is illustrated by drawings.

[0024] Fig. 1 shows the external appearance of the centrifugal SHS installation used for synthesis.

[0025] Fig. 2 shows the appearance of the obtained samples of cast chrome (after removal from the mold.

[0026] Fig. 3 shows the appearance of cast chromium ingots obtained using centrifugal SHS metallurgy methods with an increase in the mass of the fill, where (a) is the mass of the mixture 2.5 kg, (b) is the mass of the mixture 3.4 kg, (c) is the mass of the mixture 4.5 kg.

[0027] Fig. 4 shows micrographs of the cleavage structure (SEM) of the obtained sample of cast chromium in the process combustion mode for different sections (A - bottom, B - middle, C - top).

[0028] Fig. 5 shows the results of X-ray phase analysis of the synthesized alloys, which showed the presence of peaks related only to chromium.

[0029] In the drawings: 1 - base of the centrifugal SHS unit, 2 - rotor, 3 - suspended reactors for burning SHS compositions.

[0030] Implementation of the invention

[0031] The claimed method allows to increase the purity of the synthesized cast chromium ingots to 99.7%; reduce the loss of combustion products (as a result of scattering) and increase the efficiency of the synthesis process; reduce the concentration of volatile technogenic emissions at the combustion (synthesis) stage of the initial compositions, due to the absence of additional energy additives in the composition of the initial mixture; reduce the formation of technogenic waste based on aluminum oxide (secondary products) formed in the synthesis process, due to its subsequent technical application in refractory production technologies.

[0032] The synthesis product is an ingot consisting of two layers: the lower one is a cast chromium ingot and the upper one is a cast oxide material based on AlO3 (corundum), the layers are separated from each other and used for their intended purpose. The synthesized cast chromium has a reduced content of impurities and is used as a starting material for alloying special-purpose precision alloys in metallurgy, in protective coating application technologies, in the electronics industry as a component of electronic devices, and the cast oxide material based on AlO3 (corundum) is used to make a suspension and subsequently produce high-temperature casting molds.

[0033] The resulting cast chromium ingot (see Fig. 2) with a fine-grained structure and low impurity content has a chromium concentration of at least 99.7 wt.%, while the content of the main impurity components can be: 0.003 - 0.01 wt.%, carbon 0.01 - 0.04 wt.%, oxygen 0.03 - 0.1 wt.%, nitrogen 0.002 - 0.04 wt.%.

[0034] The technical result of the claimed invention is achieved in that the method for producing cast chromium by the centrifugal SHS casting method includes preparing a powder reaction mixture of the initial components, containing, by weight: chromium (III) oxide; metallic aluminum; metallic magnesium; calcium difluoride (CaF2); loading the mixture into a refractory mold, at least one selected from the series: alundum, perclase, copper, water-cooled with an internal lining of the surface, a functional protective layer of a refractory inorganic compound, which is at least one selected from the series: aluminum oxide, yttrium oxide, aluminum nitride, boron nitride; placing the mold on a centrifuge, igniting the mixture and carrying out synthesis in combustion mode under centrifugal acceleration in the range of overload values ​​of 45-55 g, followed by separating the target cast alloy from the synthesis products.

[0035] To obtain cast chromium with a low impurity content, two reducing agents, Al and Mg, are simultaneously introduced into the initial mixture.

[0036] Characteristics of methods for synthesizing cast chromium.

[0037] Synthesis is carried out with a steady rotation of the rotor, ensuring the effect of a given value of centrifugal acceleration (overload) on the sample, while the mass of the mixture placed in the mold can vary from 2.5 kg to 6.0 kg, but should not be less than 2.5 kg. Examples of the appearance of cast chromium ingots obtained by centrifugal SHS metallurgy methods with an increase in the mass of the filling are shown in Fig. 3.

[0038] The essence of the method consists in using thermal energy released during the course of exothermic reactions in the combustion wave after the initiation of the mixture of initial reagents. As a result of the implementation of high temperatures directly in the combustion wave, the synthesis process is short-term and takes several tens of seconds. Synthesis is carried out in an air atmosphere in a centrifugal unit.

[0039] The external appearance of the centrifugal SHS installation, with the help of which the method can be implemented, is shown in Fig. 1.

[0040] The initial exothermic SHS composition is loaded into a refractory mold, at least one selected from the series: alundum, perclase or copper water-cooled with an internal lining of the surface, a functional protective layer of a refractory inorganic compound, which is at least one selected from the series: aluminum oxide, yttrium oxide, aluminum nitride, boron nitride; then the reactor is placed in a basket located on the beam of the centrifugal SHS unit, the beam is provided with baskets with suspended reaction blocks (3), which are symmetrically installed on the beam, the center of which is located on the axis of the rotor (2). The rotor (2) itself is fixed on the base (1) of the SHS unit. The reaction blocks (3) are placed in a water-cooled shell of the basket or a segmented cooling structure.The overload level is set by the number of rotor revolutions (2) per unit of time or by accelerometer readings; the electronic control system automatically reaches the set overload level or number of revolutions within a few minutes.

[0041] The SHS process is initiated by applying a short-term laser pulse to the surface of the sample, and then combustion (from several seconds to tens of seconds) of the composition occurs under the conditions of a given overload effect.

[0042] Rotation of the rotor (2) continues until crystallization of the synthesis products (3-5 minutes, depending on the mold material and the mass of the mixture being burned). Then the rotor (2) stops, and after cooling, the refractory mold is removed from the reaction block (3). A metallic chromium ingot (see example Fig. 2) and slag are removed from the cooled refractory mold. In the case of using a water-cooled mold with a heat-barrier coating, the lining is restored to prepare it for reuse.

[0043] Laser ignition is achieved by two lasers mounted near the rotation axis (not shown); the lasers are powered through the centrifuge shaft.

[0044] The effect of overload from a centrifugal SHS unit during combustion leads to intensive mixing of the melt and to a homogeneous distribution of components in the alloy (absence of liquation), a decrease in the content of impurities (non-metallic inclusions) and an increase in the yield of the target alloy to 99.7% of the calculated values ​​due to the positive effect of centrifugal force on the phase separation of combustion products into a metallic and slag phase.

[0045] Thus, the synthesis using the claimed installation is highly safe, does not require large energy costs, is environmentally friendly due to the absence of gaseous products in the synthesis products, and the resulting target alloy has a high level of uniform distribution of components by volume with a low content of impurities. Since the main technological stage (SHS) is single-stage and its duration does not exceed 1-2 minutes, and the low overload level allows the use of centrifuge designs with high productivity, the use of a centrifugal SHS - installation for producing cast chromium gives good efficiency.

[0046] Fig. 4 shows the fine-grained structure of the dendritic alloy, which is typical for cast materials. It can be seen from Fig. 4 that the alloy has a uniform structure.

[0047] Microphotographs (Fig. 4) of the obtained cast chromium ingots (from a cleavage) for different sections (top, bottom, middle) did not reveal any noticeable differences in the composition and structure of the alloy, which is primarily explained by the features of SHS metallurgy and the effect of centrifugal forces, which contribute to more complete mixing of the melt components before its crystallization.

[0048] The results of X-ray phase analysis of the synthesized alloys (see Fig. 5) showed the presence of peaks related only to chromium.

[0049] The essence of the method is explained by examples.

[0050] Example 1.

[0051] A reaction mixture of the starting components is prepared in the following ratio, mass%: chromium (III) oxide - 70.5; metallic aluminum - 23.0; metallic magnesium - 3.2; calcium difluoride (CaF2) - 2.3.

[0052] Then the finished mixture is poured into a refractory form and placed in a centrifugal unit. The centrifuge rotor is set in rotation by an electric drive and at the specified rotor values, providing an overload effect of 45 g, the reaction mixture is ignited by applying a short-term laser pulse to the surface of the sample and then combustion (from several seconds to tens of seconds) of the composition occurs under the conditions of the specified overload effect.

[0053] After the combustion process is complete, the synthesis product is cooled and removed from the reaction mold. The synthesis product consists of two layers with a clear boundary: the lower one is the target product, cast chromium, and the upper one is cast oxide material based on AlO3 (corundum). The layers are easily separated from each other. The chromium content in the target product is at least 99.7 wt.%. The impurity content in the alloy is no more than 0.3%.

[0054] Examples of implementing the method by analogy with example 1, but in a different composition of the initial mixture, as well as example 1 itself, are presented in Table 1.

[0055] Table 1. Composition of the reaction mixture of components, mass%

[0056] As can be seen from the presented data, the proposed method allows to obtain cast chromium in one technological stage, while the chromium content in the target product is not less than 99.7 mass %. The content of impurities in the alloy is not more than 0.3%.

[0057] The effect of overload during combustion leads to intensive mixing of the high-temperature melt in the chemical interaction zone and, as a consequence, to an increase in the completeness of the chemical reduction of chromium (III) oxide, a decrease in the content of impurities (non-metallic inclusions) due to the intensification of the process of gravitational separation of oxide and metallic synthesis products, and an increase in the yield of the target alloy to 96% of the calculated values.

[0058] The production method is highly safe, does not require large energy costs, is environmentally friendly due to the extremely low concentration of gaseous products formed in the combustion zone, and the resulting target alloy has a high level of purity, at least 99.7 mass. %, with a low content of impurities. The proposed method is technological, since the main technological stage (SHS) is single-stage and its duration does not exceed 1-2 minutes, and a relatively low overload level allows the use of centrifuge designs with high productivity.

Claims

CLAUSE OF THE INVENTION 1. A method for producing cast chromium in a process combustion mode, including preparing a reaction mixture by mixing the starting components and centrifugal SHS casting thereof, characterized in that the reaction mixture is prepared by mixing chromium (III) oxide, aluminum metal, magnesium metal, and calcium difluoride (CaFr), in the following ratio of components, by weight %: chromium III oxide - 71.5-72.5; aluminum metal - 23.0-24.0; magnesium metal - 2.3-3.2; calcium difluoride (CaF2) - 1, 2-2,3, the resulting reaction mixture is placed in a refractory mold, which is placed on the rotor of a centrifuge, and the mixture is ignited and synthesis is carried out in combustion mode under centrifugal acceleration in the range of overload values ​​of 45-55 g, followed by separation of the cast chromium ingot from the slag phase based on aluminum oxide Al2O3.

2. The method according to item 1, characterized in that metallic magnesium is additionally introduced into the reaction mixture.

3. The method according to item 1, characterized in that the total mass of the mixture being burned is from 2.5 kg to 6.0 kg.

4. The method according to item 1, characterized in that the refractory mold used is a mold selected from the series: alundum, perclase or copper water-cooled with an internal lining, a protective thermal barrier layer formed on the basis of inorganic materials selected from the series: aluminum oxide in the form of fused corundum, yttrium oxide, aluminum nitride, boron nitride.

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