Calcium carbonate reduction method based on microwave and heat conduction coupling

Abstract

The invention discloses a calcium carbonate reduction method based on microwave and heat conduction coupling, and relates to the technical field of metal smelting. The method comprises the steps of raw material pretreatment, ingredient forming, sectional microwave pretreatment, microwave-inductive coupling reduction and product separation and recovery, and cooperative regulation and control of microwave internal heating and inductive heat conduction are achieved by optimizing the raw material ratio, introducing a wave-absorbing auxiliary material and combining vacuum degree and temperature parameters designed for high activity of metal calcium. The method provided by the invention overcomes the defects of difficulty in adaptation of high activity of calcium, insufficient temperature control precision, high energy consumption, low product purity and poor environmental protection property in the prior art, remarkably improves the quality and reduction efficiency of metal calcium, reduces the production energy consumption and cost, and is free of secondary pollution, high in process stability and suitable for industrial continuous production.

Description

Technical Field

[0001] This invention relates to the field of metal smelting technology, specifically to a calcium carbonate reduction method based on microwave and heat conduction coupling. Background Technology

[0002] As a highly reactive lightweight metallic material, metallic calcium plays an irreplaceable role in aerospace, electronics manufacturing, special alloy smelting, and chemical deoxidation and desulfurization. Its excellent reducing properties and physicochemical characteristics make it a key raw material in high-end industrial production. Currently, the industrial production of metallic calcium is mainly based on traditional processes such as the silicothermic and aluminothermic methods. With the rise of microwave metallurgy technology, the advantages of microwave internal heating, uniform temperature rise, and high thermal efficiency are gradually being applied to the field of metal smelting. Related technologies have been explored in the preparation of metals such as magnesium, providing a feasible direction for the innovation of metallic calcium smelting processes. However, given the special property that metallic calcium is more reactive than magnesium, there is an urgent need to develop suitable, efficient, and precise smelting technologies.

[0003] In the existing technology, most of the microwave smelting-related patents are for metals with relatively low activity, such as magnesium. For example, the "New process method for continuous electric furnace smelting of metallic magnesium" in publication number CN105420516A and the "Microwave heating smelting technology for magnesium" in publication number CN103882247A have achieved energy consumption optimization and continuous production in magnesium smelting. However, the reduction reaction of metallic calcium has higher requirements for the control precision of temperature and vacuum degree. The existing magnesium smelting process cannot be directly adapted, which leads to problems such as incomplete reduction and low product purity in the calcium smelting process. Meanwhile, while there are preliminary explorations in existing technologies that attempt to directly apply microwave heating to the reduction of calcium carbonate, such as the authorized patent CN116496007B "A process for microwave decomposition of lime," this patent discloses the application of microwave heating in the decomposition of calcium carbonate raw materials. It improves the absorption characteristics of the raw materials by forming a CaO layer through pre-decomposition and completes the calcination and decomposition of limestone using microwave heating. However, this technology is only designed for the production of calcium oxide from limestone. It does not optimize the vacuum degree and temperature control parameters for the high reactivity of metallic calcium, nor does it design a dedicated microwave absorption auxiliary material and a microwave-heat conduction coupling heating mechanism. It only achieves basic microwave absorption through the modification of the raw materials themselves, and the heating method is still single microwave heating. When it is applied to the process of reducing calcium carbonate to prepare metallic calcium, problems such as incomplete reduction reaction, low product purity, and high energy consumption will occur, which cannot meet the needs of industrial production. Traditional calcium smelting processes rely on a single heating method, which has drawbacks such as difficulty in temperature control, low thermal efficiency, and high energy consumption. Furthermore, the environmental emission indicators during production are difficult to meet the requirements of current green metallurgy policies. In addition, there is a lack of optimized design for the absorption characteristics of calcium smelting raw materials, which further limits the improvement of production efficiency and product quality. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a calcium carbonate reduction method based on microwave and thermal conduction coupling. This method solves the problems of existing metallic calcium smelting technologies (such as the aluminothermic process, electrolysis process and existing microwave smelting processes), which have difficulties in adapting to the high reactivity of calcium, the single heating method leading to low temperature control accuracy and thermal efficiency, high energy consumption and environmental emissions, insufficient product purity, and lack of optimization of raw material absorption characteristics.

[0005] (II) Technical Solution To achieve the above objectives, the present invention provides a calcium carbonate reduction method based on microwave and thermal conduction coupling, comprising the following steps: S1. Raw material pretreatment: Calcium carbonate with a purity of ≥95% is calcined at 850-950℃ for 2-3 hours and ball-milled to 80-120 mesh to obtain calcined calcium powder; the reducing agent is crushed and ground to 100-150 mesh, the mineralizing agent is ground to 120 mesh, and the microwave absorbing auxiliary material is treated to a particle size of <40 mesh for later use. S2. Ingredients and Molding: Calcined calcium powder: reducing agent: mineralizing agent: microwave absorbing auxiliary material: impurity removal agent = (60-75):(15-25):(2-5):(5-10):(1-3) by mass ratio, mixed, high-energy ball milled to a particle size ≤50μm, and then extruded into pellets with a diameter ≤20mm and a porosity of 15-20% under a pressure of 20-30MPa. S3. Segmented pretreatment: The pellets are fed into a closed pretreatment furnace, evacuated to 10-20 Pa, microwave preheated to 900-1100℃ and kept at that temperature for 1-2 hours, and the CO2 generated during preheating is recovered. S4. Microwave-Induction Heat Conduction Coupled Reduction: The pretreated pellets are transferred into the coupled reduction furnace, the vacuum is drawn to 1-5 Pa, and the temperature is first heated to 1400-1500℃ by microwave. Then, the induction heat conduction heating is started, the temperature is adjusted to 1600-1800℃, and the temperature is held for 3-5 hours to carry out the reduction reaction. S5. Product separation and recovery: Calcium vapor is guided by argon to a condenser at 850-950℃ for crystallization and collection. By-product waste residue is recovered after cooling, and argon is purified and recycled.

[0006] Preferably, the reducing agent in S1 is 75% ferrosilicon alloy or aluminum powder with a purity of ≥99%, and the ferrosilicon alloy needs to have its surface oxide layer removed beforehand.

[0007] Preferably, the mineralizing agent in S1 is fluorite with CaF2 ≥ 98%, the microwave absorbing auxiliary material is silicon carbide or graphite powder, and the impurity removal agent is sodium carbonate with a purity ≥ 99%.

[0008] Preferably, the microwave frequency in S4 is 2450MHz, the microwave power in the preheating stage is 25-35kW, and the microwave power in the reduction stage is 40-60kW.

[0009] Preferably, the coupling reduction furnace described in S4 adopts a double-layer insulation structure, with a built-in microwave generator and induction heating coil. Interference is avoided through frequency isolation layout, thereby achieving coordinated control of microwave heating and induction heat conduction.

[0010] Preferably, the byproduct in S5 is calcium silicate or calcium aluminate, wherein the calcium silicate contains ≥45% CaO and can be used as a raw material for cement clinker or building materials.

[0011] Preferably, the vacuum level described in S3 and S4 is controlled by a combination system of jet pump and mechanical pump, and the vacuum level fluctuation range is ≤ ±0.5 Pa.

[0012] Preferably, the condenser in S5 adopts an inert gas sealed structure, with argon purity ≥99.99% and recycling rate ≥90%.

[0013] (III) Beneficial Effects This invention provides a calcium carbonate reduction method based on microwave and thermal conduction coupling. It has the following beneficial effects: 1. This invention addresses the highly reactive nature of metallic calcium by employing an innovative design that couples microwave and thermal conduction heating. Combined with microwave-absorbing materials to enhance microwave utilization efficiency, it achieves precise temperature control and efficient reduction, effectively solving the problems of incomplete calcium reduction and insufficient product purity in traditional processes, thus significantly improving product quality. Simultaneously, the process drastically reduces production energy consumption, optimizes industrial economics through scientific cost accounting, and produces no secondary pollution, aligning with the trend of green metallurgy. Its process exhibits strong stability and adaptability, meeting the needs of continuous industrial production and possessing broad application prospects. Detailed Implementation

[0014] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0015] Example 1: This invention provides a method for reducing calcium carbonate based on microwave and thermal conduction coupling, comprising the following steps: S1. Raw material pretreatment: Calcium carbonate with a purity of ≥95% is calcined at 850-950℃ for 2-3 hours and ball-milled to 80-120 mesh to obtain calcined calcium powder; the reducing agent is crushed and ground to 100-150 mesh, the mineralizing agent is ground to 120 mesh, and the microwave absorbing auxiliary material is treated to a particle size of <40 mesh for later use. S2. Ingredients and Molding: Calcined calcium powder: reducing agent: mineralizing agent: microwave absorbing auxiliary material: impurity removal agent = (60-75):(15-25):(2-5):(5-10):(1-3) by mass ratio, mixed, high-energy ball milled to a particle size ≤50μm, and then extruded into pellets with a diameter ≤20mm and a porosity of 15-20% under a pressure of 20-30MPa. S3. Segmented pretreatment: The pellets are fed into a closed pretreatment furnace, evacuated to 10-20 Pa, microwave preheated to 900-1100℃ and kept at that temperature for 1-2 hours, and the CO2 generated during preheating is recovered. S4. Microwave-Induction Heat Conduction Coupled Reduction: The pretreated pellets are transferred into the coupled reduction furnace, the vacuum is drawn to 1-5 Pa, and the temperature is first heated to 1400-1500℃ by microwave. Then, the induction heat conduction heating is started, the temperature is adjusted to 1600-1800℃, and the temperature is held for 3-5 hours to carry out the reduction reaction. S5. Product separation and recovery: Calcium vapor is guided by argon to a condenser at 850-950℃ for crystallization and collection. By-product waste residue is recovered after cooling, and argon is purified and recycled.

[0016] The reducing agent in S1 is 75% ferrosilicon alloy or aluminum powder with a purity of ≥99%. The ferrosilicon alloy needs to have its surface oxide layer removed beforehand.

[0017] In S1, the mineralizing agent is fluorite with CaF2 ≥ 98%, the microwave absorbing auxiliary material is silicon carbide or graphite powder, and the impurity removal agent is sodium carbonate with a purity ≥ 99%.

[0018] The microwave frequency in S4 is 2450MHz, the microwave power during the preheating stage is 25-35kW, and the microwave power during the reduction stage is 40-60kW.

[0019] The coupling reduction furnace described in S4 adopts a double-layer insulation structure, with a built-in microwave generator (fixed frequency 2450MHz) and an induction heating coil (using low-frequency induction design). Through frequency isolation layout, interference rejection is avoided, and the coordinated control of microwave internal heating and induction heat conduction is realized.

[0020] The byproducts of S5 are calcium silicate or calcium aluminate, of which calcium silicate has a CaO content of ≥45% and can be used as raw material for cement clinker or building materials.

[0021] In S3 and S4, the vacuum level is controlled by a combination system of jet pump and mechanical pump, and the vacuum level fluctuation range is ≤ ±0.5Pa.

[0022] The S5 condenser adopts an inert gas sealed structure with argon purity ≥99.99% and a recycling rate ≥90%.

[0023] Example 2: Using 99.2% metallic aluminum powder as a reducing agent, the proportion of ingredients was adjusted to “calcined calcium powder: metallic aluminum powder: fluorite: silicon carbide: sodium carbonate = 70:18:4:6:2”; the remaining pretreatment parameters (calcination temperature, ball milling mesh, pellet size, etc.) were the same as in Example 1. The reduction stage was carried out under 2Pa vacuum and coupled heating at 1650℃ for 3.5h.

[0024] Comparative Example 1: Only microwave heating (60kW power during the reduction stage) was used, with no inductive heat conduction coupling; the other ingredient ratios, pretreatment, vacuum, temperature, and holding time parameters were completely consistent with those in Example 1.

[0025] Comparative Example 2: The silicon carbide absorbing material was removed from the ingredients, and the adjusted ratio was "calcined calcium powder: ferrosilicon alloy: fluorite: sodium carbonate = 73:21:3:3"; the microwave + microwave-induction heat conduction coupling reduction method was retained, and the other parameters remained the same as in Example 1.

[0026] Experimental example: I. Sample Preparation All test samples were prepared according to the corresponding process, following the principle of a single variable to provide a unified basis for comparative experiments: Sample 1 (Process of Example 1): Using 75% ferrosilicon alloy as a reducing agent, the raw materials were prepared according to the ratio of calcined calcium powder: ferrosilicon alloy: fluorite: silicon carbide: sodium carbonate = 68:20:3:7:2; after calcination at 900℃ and ball milling at 100 mesh, the pellets were made into 18mm pellets under a pressure of 25MPa; the pellets were preheated in a microwave at 1000℃ under a vacuum of 15Pa for 1.5h, and then heated in a microwave-induction coupling at 1700℃ under a vacuum of 3Pa for 4h to obtain a metallic calcium sample.

[0027] Sample 2 (Process of Example 2): Using 99.2% metallic aluminum powder as a reducing agent, the proportion of ingredients was adjusted to "calcined calcium powder: metallic aluminum powder: fluorite: silicon carbide: sodium carbonate = 70:18:4:6:2"; the other preparation parameters were the same as those of Sample 1, except that the reduction stage was carried out under 2Pa vacuum and coupled heating at 1650℃ for 3.5h to obtain a metallic calcium sample.

[0028] Sample 3 (Comparative Example 1 process): The preparation process is the same as that of Sample 1, except that the induction heat conduction is eliminated in the reduction stage, and a single microwave heating (power 60kW) is used. All other parameters remain unchanged to obtain a metallic calcium sample.

[0029] Sample 4 (Comparative Example 2 process): The preparation process is the same as that of Sample 1, except that the silicon carbide absorbing material is removed from the ingredients and the ratio is adjusted to "calcined calcium powder: ferrosilicon alloy: fluorite: sodium carbonate = 73:21:3:3", with the other parameters remaining unchanged, to obtain a metallic calcium sample.

[0030] II. Comparative Experimental Testing Methods: To objectively verify the advantages and disadvantages of different preparation processes, a unified testing method conforming to the metal smelting industry standards was used on the four groups of samples: Calcium purity testing: The calcium content and impurity percentage are detected by inductively coupled plasma optical emission spectrometry (ICP-OES), and the purity is calculated. Calcium recovery rate calculation: The actual output is calculated by weighing, and the recovery rate is calculated by combining the theoretical calcium content of the raw materials (actual output / theoretical output × 100%). Energy consumption statistics: The energy consumption monitor records the total power consumption throughout the process and converts it into power consumption per ton of calcium (kW·h / ton). Cost accounting: Unify the accounting of raw material, equipment loss, energy consumption and other costs to obtain the production cost per ton of calcium (yuan / ton). Environmental indicator testing: Gas analyzer detects CO2 emissions during the production process and calculates the emission value per ton of calcium (kg / ton). The specific test results are shown in Table 1 below: Table 1

[0031] In summary: Superior product quality: The purity of metallic calcium prepared by this patented process (samples 1 and 2) reaches 98.2%-98.5%, which is 2.1-2.4 percentage points higher than that of sample 3 (96.1%) without coupled heating, and 2.5-2.8 percentage points higher than that of sample 4 (95.7%) without microwave absorbing material; the calcium recovery rate reaches 94.8%-95.3%, which is 6.3-6.8 percentage points higher than that of sample 3 (88.5%), and 4.6-5.1 percentage points higher than that of sample 4 (90.2%), thus solving the problems of incomplete calcium reduction and insufficient purity.

[0032] Energy consumption costs are significantly reduced: the electricity consumption per ton of calcium is only 20,500-21,200 kWh, which is 26.39%-28.82% more energy-efficient than sample 3 (28,800 kWh) which uses microwave heating alone, and 20.00%-22.64% more energy-efficient than sample 4 (26,500 kWh) which does not have microwave absorbing materials; the cost per ton of calcium is reduced to 16,800-17,500 yuan, which is 24.57%-27.59% less than sample 3 (23,200 yuan) and 18.60%-21.86% less than sample 4 (21,500 yuan), demonstrating outstanding economic efficiency for industrialization.

[0033] Superior environmental performance: CO2 emissions are only 310-320 kg / ton, which is 33.33%-35.42% lower than sample 3 (480 kg / ton) and 23.81%-26.19% lower than sample 4 (420 kg / ton), and there is no secondary pollution throughout the process, which meets the requirements of green metallurgy policy.

[0034] Scientific process design: Vacuum and temperature parameters are optimized to address the high reactivity of metallic calcium. Precise temperature control is achieved through microwave-thermal conduction coupling. Microwave-absorbing materials are used to improve microwave utilization. The process has strong stability and adaptability, meeting the needs of continuous industrial production.

[0035] 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 reducing calcium carbonate based on microwave and thermal conduction coupling, characterized in that: Includes the following steps: S1. Raw material pretreatment: Calcium carbonate with a purity of ≥95% is calcined at 850-950℃ for 2-3 hours and ball-milled to 80-120 mesh to obtain calcined calcium powder; the reducing agent is crushed and ground to 100-150 mesh, the mineralizing agent is ground to 120 mesh, and the microwave absorbing auxiliary material is treated to a particle size of <40 mesh for later use. S2. Ingredients and Molding: Calcined calcium powder: reducing agent: mineralizing agent: microwave absorbing auxiliary material: impurity removal agent = (60-75):(15-25):(2-5):(5-10):(1-3) by mass ratio, mixed, high-energy ball milled to a particle size ≤50μm, and then extruded into pellets with a diameter ≤20mm and a porosity of 15-20% under a pressure of 20-30MPa. S3. Segmented pretreatment: The pellets are fed into a closed pretreatment furnace, evacuated to 10-20 Pa, microwave preheated to 900-1100℃ and kept at that temperature for 1-2 hours, and the CO2 generated during preheating is recovered. S4. Microwave-Induction Heat Conduction Coupled Reduction: The pretreated pellets are transferred into the coupled reduction furnace, the vacuum is drawn to 1-5 Pa, and the temperature is first heated to 1400-1500℃ by microwave. Then, the induction heat conduction heating is started, the temperature is adjusted to 1600-1800℃, and the temperature is held for 3-5 hours to carry out the reduction reaction. S5. Product separation and recovery: Calcium vapor is guided by argon to a condenser at 850-950℃ for crystallization and collection. By-product waste residue is recovered after cooling, and argon is purified and recycled.

2. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The reducing agent mentioned in S1 is 75% ferrosilicon alloy or aluminum powder with a purity of ≥99%. The ferrosilicon alloy needs to have its surface oxide layer removed beforehand.

3. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The mineralizing agent mentioned in S1 is fluorite with CaF2 ≥ 98%, the microwave absorbing auxiliary material is silicon carbide or graphite powder, and the impurity removal agent is sodium carbonate with a purity ≥ 99%.

4. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The microwave frequency described in S4 is 2450MHz, the microwave power during the preheating stage is 25-35kW, and the microwave power during the reduction stage is 40-60kW.

5. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The coupling reduction furnace described in S4 adopts a double-layer insulation structure, with a built-in microwave generator and induction heating coil. Through frequency isolation layout, interference rejection is avoided, and the coordinated control of microwave internal heating and induction heat conduction is realized.

6. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The byproducts mentioned in S5 are calcium silicate or calcium aluminate, wherein the calcium silicate contains ≥45% CaO and can be used as raw materials for cement clinker or building materials.

7. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The vacuum level described in S3 and S4 is controlled by a combination system of jet pump and mechanical pump, and the vacuum level fluctuation range is ≤ ±0.5 Pa.

8. The calcium carbonate reduction method based on microwave and thermal conduction coupling according to claim 1, characterized in that: The condenser described in S5 adopts an inert gas sealed structure with argon purity ≥99.99% and recycling rate ≥90%.

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