Method for preparing chromium metal, chromium alloy and fused alumina through two-stage reduction

Abstract

The invention discloses a method for preparing metal chromium, chromium alloy and fused alumina through two-stage reduction, relates to the technical field of metal smelting, and comprises the steps of material preparation and primary reduction, primary separation and metal chromium extraction, secondary reduction and fused alumina preparation, and secondary separation and product collection. The high-temperature liquid slag produced in the first stage is directly and thermally loaded into the direct-current electric arc furnace for the second-stage reaction, so that huge energy consumption caused by remelting of the solid slag is thoroughly avoided. Meanwhile, the waste aluminum-chromium slag is used for replacing aluminum oxide to serve as the fused alumina raw material, the raw material cost is greatly reduced, and harmless treatment and resource utilization of hexavalent chromium in the slag are achieved. The metal chromium is produced in the first stage and is widely applied to the fields of aerospace, navigation, high-end alloy and the like; and in the second stage, chromium alloy is efficiently recycled through a direct-current electric arc furnace and applied to the fields of stainless steel, welding materials and the like, meanwhile, fused corundum is synchronously generated, and a complete and efficient industrial chain is formed.

Description

Technical Field

[0001] This invention relates to the technical field of metal smelting, specifically to a two-stage reduction method for preparing metallic chromium, chromium alloys, and fused alumina. Background Technology

[0002] Metallic chromium, a key additive in high-performance alloy steels, stainless steels, and special alloys, is primarily produced via the aluminothermic reduction method. This method uses Cr₂O₃ and aluminum powder as raw materials, generating metallic chromium through a high-temperature reduction reaction. However, this traditional process has significant drawbacks: The aluminum-chromium slag produced after the aluminothermic reaction still contains 8%-12.5% ​​unreduced chromium (in the form of Cr₂O₃ and Cr(VI)) and 80%-86% alumina (Al₂O₃). This type of slag is classified as hazardous waste due to the presence of toxic hexavalent chromium, requiring additional costs for harmless treatment, resulting in a serious waste of chromium and aluminum resources. The stockpiling of aluminum-chromium slag not only occupies land, but the leached Cr(VI) can easily pollute the water and soil environment. Existing recycling technologies generally suffer from high energy consumption, complex processes, and low metal recovery rates, making it difficult to achieve economically feasible large-scale application.

[0003] High-quality fused alumina typically requires alumina as raw material, which is smelted at temperatures above 2400℃ in an electric arc furnace. This process is costly in terms of raw materials and energy consumption. In recent years, although some studies have attempted to recover valuable components from aluminum-chromium slag, these studies have mostly focused on single products, such as extracting residual chromium or preparing low-value-added building materials. They have failed to simultaneously achieve the co-production of metallic chromium, chromium alloys, and fused alumina.

[0004] Therefore, there is a need to develop a simplified processing method that reduces energy consumption and increases metal recovery rates, thereby enabling economically feasible and environmentally friendly large-scale applications. The aim is to address the problems existing in current recycling technologies, promote resource recycling, and reduce environmental pollution. Summary of the Invention

[0005] The purpose of this invention is to provide a two-stage reduction method for preparing metallic chromium, chromium alloys and fused alumina, in order to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing metallic chromium, chromium alloys and fused alumina by two-stage reduction, comprising at least the following steps: S1, material preparation and initial reduction: Cr2O3, aluminum granules, sodium chlorate, caustic soda flakes, etc., in quantitative proportions are added to a mixer to prepare a mixture. Part of the mixture is spread at the bottom of the smelting furnace as a bottom material. The furnace is ignited to initiate the reaction, and the furnace temperature is maintained at 1900-2000℃. After the bottom material undergoes an aluminothermic reduction reaction, the remaining mixture is added in batches until the reaction is complete. S2. Initial separation and extraction of metallic chromium: After the reaction is completed, the metal chromium and liquid aluminum-chromium slag are allowed to stand until they are fully separated into layers. Then, the upper layer of liquid aluminum-chromium slag is introduced into a slag bag and transported to the next process through the slag discharge port located on the side wall of the smelting furnace. The lower layer of liquid metal chromium remains in the smelting furnace and is cooled and shaped to obtain the first stage of reduced chromium. S3. Second-stage reduction and preparation of fused alumina: The liquid aluminum-chromium slag remaining in the slag bag after the first reaction is poured into a DC electric arc furnace, and petroleum coke and coke are added as reducing agents. The electromagnetic stirrer at the bottom of the DC electric arc furnace is turned on, and the furnace temperature is raised to 2200℃ to 2400℃ and kept constant to carry out the reduction reaction again. S4. Secondary Separation and Product Collection: After the reaction is completed, the slag outlet of the DC electric arc furnace is opened to discharge the slag. After static cooling, the lower layer of second-stage reduced chromium and the upper layer of fused alumina are fully separated and solidified. Then, mechanical separation is performed to obtain the second-stage reduced chromium and fused alumina products respectively.

[0007] Preferably, the smelting furnace is a split structure that opens to the left and right, consisting of two halves of the furnace body. The joint surface of the two halves of the furnace body is filled and sealed with refractory sealing material. A sealing plug is installed at the slag discharge port on the side wall of the furnace body. Each half of the furnace body is provided with multiple sets of ear plates corresponding to the position, and each set of ear plates is fixedly connected by pins.

[0008] Preferably, in step S1, the material ratio by mass percentage is: Cr2O3 65%-70%, aluminum granules 25%-30%, sodium chlorate 5%-9%, caustic soda flakes 1%-2%, and the aluminum granule particle size is controlled at 0.5-3mm.

[0009] Preferably, in step S3, based on the total mass of the liquid aluminum chromium slag, the amount of petroleum coke added is 3.7%-4.0%, the amount of coke added is 0.1%-0.3%, the fixed carbon content of the petroleum coke is not less than 98.5%, and the particle size range is 3 to 7 mm; the fixed carbon content of the coke is not less than 84%, and the particle size is less than 10 mm.

[0010] Preferably, in step S1, the isothermal reaction time is 25-40 minutes.

[0011] Preferably, the refractory sealing material is a mixture of slag and refractory mud.

[0012] The present invention proposes a two-stage reduction method for preparing metallic chromium, chromium alloys, and fused corundum, which has the following advantages: 1. By directly charging the high-temperature liquid slag produced in the first stage into a DC electric arc furnace for the second stage reaction, the huge energy consumption of remelting solid slag is completely avoided. At the same time, using waste aluminum-chromium slag instead of alumina as the raw material for fused corundum not only significantly reduces the cost of raw materials, but also achieves the harmless treatment and resource utilization of hexavalent chromium in the slag, truly turning the waste slag of the traditional aluminothermic process into a valuable resource.

[0013] 2. The first stage produces metallic chromium that meets the GB / T 321-2023 standard, widely used in aerospace, marine, and high-end alloy fields. The second stage utilizes liquid slag hot charging to efficiently recover chromium alloys in a DC electric arc furnace for use in stainless steel and welding materials. Simultaneously, fused alumina is generated to meet the needs of refractory materials and precision abrasives. This achieves the synergistic production of high-value products across multiple fields, including metallic chromium, chromium alloys, and refractory materials, forming a complete and efficient industrial chain.

[0014] 3. For the first time, a DC electric arc furnace combined with electromagnetic stirring technology was applied to the production of fused alumina. This combination of technologies effectively reduced electrode consumption and significantly improved energy utilization, thereby further reducing overall production costs. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the smelting furnace structure of the present invention.

[0016] In the diagram: 1. Furnace body, 2. Sealing plug, 3. Ear plate, 4. Pin. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Please see Figure 1This invention provides a technical solution: a two-stage reduction method for preparing metallic chromium, chromium alloys, and fused alumina, characterized by at least the following steps: S1, material preparation and initial reduction: the material ratio by mass percentage is: Cr2O3 65%-70%, aluminum granules 25%-30%, sodium chlorate 5%-9%, caustic soda flakes 1%-2%, and the aluminum granule particle size is controlled at 0.5-3mm. The quantitatively proportioned Cr2O3, aluminum granules, sodium chlorate, and caustic soda flakes are added to a mixer to prepare a mixture. Part of the mixture is spread at the bottom of the smelting furnace as a base material. The furnace is ignited to initiate the reaction, and the furnace temperature is maintained at 1900-2000℃. After the aluminothermic reduction reaction occurs in the base material, the remaining mixture is added in batches until the reaction is complete. The specific reaction time is 25-40 minutes to ensure sufficient reduction of chromium and optimize product quality. This step is based on the principle of aluminothermic reduction, which uses aluminum particles to reduce Cr2O3. Sodium chlorate decomposes upon heating to generate sodium chloride, oxygen, and heat. Soda ash (NaOH) melts at high temperature and forms a low-melting-point eutectic with the alumina generated in the reaction. This can effectively reduce the viscosity of the system and promote the separation of metallic chromium from the slag.

[0019] S2. Initial Separation and Acquisition of Metallic Chromium: After the reaction is complete, the metallic chromium and liquid aluminum-chromium slag are allowed to settle and separate into layers. Due to the higher density of metallic chromium, it settles in the lower layer, while the lighter liquid aluminum-chromium slag floats in the upper layer. The specific component contents of the liquid aluminum-chromium slag are detailed in the table below.

[0020] Table 1 Composition of liquid aluminum-chromium slag The smelting furnace is a split structure that opens from left to right, consisting of two halves of the furnace body 1. The joint surface of the two halves of the furnace body 1 is filled and sealed with refractory sealing material, which is a mixture of slag and refractory mud. A sealing plug 2 is installed at the slag discharge port on the side wall of the furnace body 1. Each half of the furnace body 1 is provided with multiple sets of ear plates 3 with corresponding positions, and each set of ear plates 3 is fixedly connected by pins 4.

[0021] During operation, operators first calculate the material ratio, density, and product quantity based on theoretical calculations. They then control the slag discharge port height to be below the surface of the molten aluminum-chromium slag, but not lower than the theoretical slag / metal interface. Using a high-temperature specialized opening machine, they align the sealing plug 2 and apply mechanical force to open / pinch it open, allowing the upper layer of liquid aluminum-chromium slag to be introduced into the slag bag and transported to the next process. The lower layer of liquid metallic chromium remains in the furnace to cool and solidify completely. After the metallic chromium ingot has completely solidified and cooled to a safe operating temperature, the pins 4 securing each set of ear plates 3 are removed sequentially. Then, using hoisting equipment, the two halves of the furnace body 1 are simultaneously and smoothly moved and separated, fully exposing the solidified metallic chromium ingot. Next, the chromium ingot is removed from one half of the furnace body 1 using hoisting equipment. After removal, the residual aluminum-chromium slag at the original boundary with the aluminum-chromium slag on the upper surface of the chromium ingot is polished to remove it, ensuring a clean metallic chromium surface. Finally, the waste residue generated from polishing is cleaned, completing the removal process of the metallic chromium ingot and obtaining the first stage of metallic chromium.

[0022] S3. Second-stage reduction and preparation of fused corundum: The liquid aluminum-chromium slag remaining in the slag bag after the first reaction is poured into a DC electric arc furnace. Petroleum coke and coke are added as reducing agents. Based on the total mass of the liquid aluminum-chromium slag, the amount of petroleum coke added is 3.7%-4.0%, and the amount of coke added is 0.1%-0.3%. The fixed carbon content of the petroleum coke is not less than 98.5%, and the particle size range is 3 to 7 mm; the fixed carbon content of the coke is not less than 84%, and the particle size is less than 10 mm. The electromagnetic stirrer outside the furnace bottom is turned on and stirred at a medium intensity. This ensures that the liquid phase inside the furnace is uniform, accelerates the contact between the reducing agent and the slag liquid, promotes the reaction, and helps the generated metal settle. At the same time, the DC furnace is heated to between 2200℃ and 2400℃ and maintained at a constant temperature for the reduction reaction.

[0023] Process Principle: This step utilizes the high-temperature environment of a DC electric arc furnace to reduce chromium oxide and hexavalent chromium in liquid aluminum-chromium slag to form a chromium alloy. At high temperatures, low-melting-point oxides such as potassium and sodium volatilize, and the alumina in the chromium slag undergoes high-temperature remelting and corundization to form fused alumina. The electromagnetic stirrer operates based on electromagnetic induction. When a coil outside the furnace bottom is energized, it generates an alternating magnetic field that penetrates the conductive molten slag inside the furnace. According to Faraday's law of electromagnetic induction, eddy currents are induced in the slag. These eddy currents interact with the external magnetic field, generating Lorentz forces, which drive the slag to circulate. This stirring action effectively promotes thorough mixing of the reducing agents (petroleum coke and coke) and the slag, increases the contact area of ​​the reactants, and accelerates the kinetics of the carbothermic reduction reaction. Simultaneously, it helps to uniformly distribute the temperature within the furnace and promotes the rapid aggregation and settling of newly formed chromium alloy droplets to the furnace bottom, achieving better separation from the upper layer of fused alumina.

[0024] Starting from liquid slag eliminates the enormous energy consumption of heating from solid to molten state, thus improving overall energy efficiency. The DC electric arc furnace can stably provide and maintain such high temperatures. This process is highly adaptable to raw materials, has high energy utilization, and relatively low electrode consumption. Electromagnetic stirring accelerates the contact between the reducing agent and the molten slag, increasing the reaction rate.

[0025] S4. Secondary Separation and Product Collection: After the reaction is completed, the liquid outlet of the DC electric arc furnace is opened to discharge the liquid. After static cooling, the chromium alloy reduced in the second stage in the lower layer and the fused alumina in the upper layer are fully separated and solidified. Then, mechanical separation is performed to obtain chromium alloy and fused alumina products respectively.

[0026] One of the most significant advantages of this application is its ability to simultaneously and efficiently produce metallic chromium, chromium alloys, and fused alumina. This is not simply a matter of combining two independent production lines, but rather a process of two-stage reduction and separation that enables the synergistic production of different types and specifications of products within the same process flow.

[0027] In the production of metallic chromium, liquid metallic chromium is generated in the furnace, which is the first stage of chromium reduction (meeting the national standard GB / T 321-2023 for metallic chromium). Simultaneously, the liquid aluminum-chromium slag produced in the first stage reaction is not waste, but a byproduct rich in chromium. Because the aluminothermic reduction is not complete, to control the cost and efficiency of the initial reduction, the slag will be enriched with incompletely reduced chromium oxides, alumina, and other impurities. In the second stage, the liquid aluminum-chromium slag produced in the first stage is directly introduced into a DC electric arc furnace, where petroleum coke and coke are added as reducing agents for secondary reduction.

[0028] In the second stage, under a high-reducing atmosphere, the alumina in the slag melts and crystallizes at extremely high temperatures to form dense α-alumina, i.e., fused alumina. This alumina contains trace amounts of chromium, forming alumina with specific properties, distinct from traditional alumina, showing significant improvements in hardness, compressive strength, thermal shock resistance, and chemical purity. After reduction, the residual chromium oxides in the slag have their lattice positions filled by alumina, and the chromium separates from the alumina in a metallic state, forming a chromium alloy. In traditional metallic chromium production, the aluminothermic process produces aluminochromium slag, which is often considered waste, occupying storage space and requiring treatment costs. The hexavalent chromium in the slag not only causes environmental pollution, but the residual chromium in the slag is also not effectively utilized.

[0029] This application directly uses the "waste residue" from the first stage, namely liquid aluminum-chromium slag, as a high-value raw material for the second stage production of fused alumina and further reduction of chromium. This fundamentally changes the definition of waste, realizes the deep utilization of chromium and aluminum resources across the entire industrial chain, significantly improves the overall resource utilization rate, and reduces waste treatment costs.

[0030] The slag from the first stage is in a liquid, high-temperature state and is directly introduced into the electric arc furnace in the second stage, avoiding the slag cooling and reheating process. This significantly saves a large amount of remelting energy and reduces overall energy costs. Compared to the completely independent production of fused alumina, which requires the purchase of alumina raw materials, this method utilizes the slag, a byproduct of chromium production, which is equivalent to producing high-value fused alumina with low-cost secondary raw materials, thus reducing raw material costs.

[0031] The first stage produces metallic chromium that meets the GB / T 321-2023 standard, widely used in aerospace, marine, and high-end alloy fields. The second stage utilizes hot charging of liquid slag in a DC electric arc furnace to efficiently recover chromium alloys for use in stainless steel and welding materials. Simultaneously, fused alumina is generated, which can be used in various industries such as abrasives, refractories, ceramics, and precision casting. This achieves the synergistic production of high-value products across multiple fields, including metallic chromium, chromium alloys, and refractories, forming a complete and efficient industrial chain.

[0032] Regarding the equipment used, the smelting furnace of this application employs an innovative left-right split-type smelting furnace structure. The furnace body consists of two halves 1, with the joint filled and sealed with refractory sealing material to ensure a stable high-temperature environment and effective material reaction within the furnace. This split design facilitates charging, slag removal, and maintenance. Particularly noteworthy is the sealing plug 2 installed at the slag discharge port on the side wall of the furnace body, which effectively controls slag discharge during the reaction process, precisely achieving the separation of metallic chromium from liquid aluminum-chromium slag. Furthermore, to facilitate the fixing and opening of the furnace body, each half 1 is equipped with multiple sets of corresponding ear plates 3, which are fixedly connected by pins 4. This structural design effectively ensures the safety and efficiency of the smelting process.

[0033] Compared to traditional single-unit smelting furnaces, this split structure with left and right openings not only improves the flexibility and convenience of operation, but more importantly, it can meet the requirement of accurately separating the upper liquid aluminum-chromium slag after the first stage of aluminothermic reduction reaction is completed.

[0034] In the second stage, to achieve the synergistic preparation of chromium alloy and fused alumina, this application employs a DC electric arc furnace equipped with a bottom electromagnetic stirrer. The DC electric arc furnace can provide stable high temperatures up to 2200℃ to 2400℃, ensuring efficient secondary reduction of liquid aluminum-chromium slag after the addition of petroleum coke and coke as reducing agents. The bottom electromagnetic stirrer effectively promotes uniform mixing and heat and mass transfer of materials within the furnace, accelerating the reduction reaction and facilitating effective stratification of subsequent products. Compared to AC electric arc furnaces or other smelting equipment, the DC electric arc furnace offers a more stable arc, more concentrated energy, lower electrode consumption, and superior operability and economy in the ultra-high temperature range. Its precise power control system better achieves the process requirement of "maintaining a constant temperature."

[0035] This DC electric arc furnace also features a slag outlet design, allowing for precise control of the slag discharge operation after the reaction is complete, enabling effective stratification and solidification of the chromium alloy reduced in the second stage below and the fused alumina above. Subsequent mechanical separation equipment further ensures the independent acquisition of these two high-value-added products.

[0036] Through Lorentz force-driven non-contact physical stirring, the electromagnetic stirrer at the bottom of the DC electric arc furnace forces the high-viscosity aluminum-chromium slag melt to form a high-speed rotating flow field in a high-temperature environment. This innovation effectively solves the following problems: First, it ensures sufficient contact between the petroleum coke / coke reducing agent and the residual chromium oxide in the slag through strong convection, thereby increasing the reduction reaction rate and ensuring chromium recovery. Second, it achieves ultra-homogenization of the melt, eliminating temperature gradients and component segregation, and ensuring the uniform growth of fused corundum crystals. After this operation, the purity of Al2O3 can be effectively increased from 80%-86% to 94.5%-98%. Finally, it promotes efficient separation of the metallic phase, accelerates the collision aggregation and sedimentation of chromium metal droplets, and simultaneously reduces the chromium content in the slag and improves the purity of the chromium alloy. Compared with traditional mechanical stirring, this technology has the advantages of "low pollution, no wear and tear on moving parts, and high energy conversion efficiency".

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for the production of metallic chromium, chromium alloys and electrofused corundum in two stages of reduction, characterized in that: At least comprising the following steps: S1, material preparation and primary reduction: quantitative proportioning of Cr2O3, aluminum particles, sodium chlorate, flake caustic, etc. is added to a mixer to prepare a mixture, part of the mixture is laid on the bottom of a smelting furnace as a bottom material, ignition is initiated, the furnace temperature is maintained at 1900-2000°C, after the aluminum thermal reduction reaction of the bottom material, the remaining mixture is added in batches until the reaction is completed; S2, primary separation and extraction of metallic chromium: after the reaction is completed, the liquid aluminum-chromium slag is guided into a slag ladle through a slag discharge port on the side wall of the smelting furnace and is transported to the next process after the liquid aluminum-chromium slag is fully separated from the lower layer of liquid metallic chromium, the lower layer of liquid metallic chromium is left in the smelting furnace and is cooled to form a first-stage reduced chromium; S3, second-stage reduction and preparation of electrically fused corundum: the liquid aluminum-chromium slag in the slag ladle after the first reaction is added to a direct current arc furnace, petroleum coke and coke are added as reducing agents, an electromagnetic stirrer at the bottom of the direct current arc furnace is turned on, the furnace temperature is raised to 2200-2400°C, and the temperature is maintained, and a reduction reaction is performed again; S4, secondary separation and product collection: after the reaction is completed, the slag discharge port of the direct current arc furnace is opened to discharge, the second-stage reduced chromium alloy in the lower layer is fully separated from the electrically fused corundum in the upper layer after cooling, and mechanical separation is performed to obtain chromium alloy and electrically fused corundum products, respectively.

2. The process for preparing metallic chromium, chromium alloys and electrofused corundum in two stages according to claim 1, characterized in that: The smelting furnace is a left-right split structure composed of two half furnace bodies (1), the joint surfaces of the two half furnace bodies (1) are filled and sealed with refractory sealing materials, a plugging plug (2) is installed at the slag discharge port of the side wall of the furnace body (1), and each group of ear plates (3) is fixedly connected by a pin (4).

3. The process for preparing metallic chromium, chromium alloys and electrofused corundum in two stages according to claim 1, characterized in that: In step S1, the material proportions are as follows: Cr2O3 65%-70%, aluminum particles 25%-30%, sodium chlorate 5%-9%, and flake caustic 1%-2% by mass percentage, and the particle size of the aluminum particles is controlled to be 0.5-3 mm.

4. The process for preparing metallic chromium, chromium alloys and electrofused corundum in two stages according to claim 1, characterized in that: In step S3, based on the total mass of the liquid aluminum-chromium slag, the petroleum coke is added in an amount of 3.7%-4.0%, the coke is added in an amount of 0.1%-0.3%, the fixed carbon content of the petroleum coke is not less than 98.5%, and the particle size range is 3-7 mm; the fixed carbon content of the coke is not less than 84%, and the particle size is less than 10 mm.

5. The process for preparing metallic chromium, chromium alloys and electrofused corundum in two stages according to claim 1, characterized in that: In step S1, the constant temperature reaction time is specifically 25-40 minutes.

6. The process for preparing metallic chromium, chromium alloys and electrofused corundum in two stages according to claim 1, characterized in that: The refractory sealing material is a mixture of slag and refractory mud.

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