Process for smelting magnesium by using electric furnace slag through silicothermic method

Abstract

The invention discloses a process for smelting magnesium by using electric furnace slag through a silicothermic method. The process comprises the steps of screening of magnesium-containing raw materials, preparation of a magnesium reduction sample and a magnesium reduction experiment process. The method comprises the following steps: firstly, deeply analyzing dolomite and power plant ash element components, and exploring the standardization of impurity control in raw materials through a corresponding reduction process: determining the impurity threshold value of the raw materials, and ensuring that the volatilization rate of magnesium is greater than 85%; a novel acidification calcination method and a CaO neutralization method are adopted, the S content is reduced from 0.43% to 0.006%, sintering is avoided, and the magnesium volatilization rate is increased; caO neutralizes residual acidity, meanwhile, the Ca / Mg ratio is increased to 1.055, generation of dicalcium silicate is promoted, and therefore the magnesium reduction yield of the power plant ash is greatly increased; the magnesium reduction yield of the power plant ash and dolomite mixed raw material is close to or exceeds the yield of pure dolomite, the dolomite consumption is reduced, the energy consumption per ton of magnesium is reduced, the reducing slag amount is reduced, and the solid waste amount is reduced.

Description

Technical Field

[0001] This invention relates to the field of metallurgical solid waste treatment technology, and in particular to a magnesium smelting process using electric furnace slag silicothermic process. Background Technology

[0002] Magnesium and its alloys have important applications in aerospace, automotive lightweighting, and other fields due to their low density, high specific strength, and excellent damping properties. Currently, the Pidgeon process (silicothermic reduction process) is mainly used in industry to smelt metallic magnesium, which uses calcined dolomite (calcined white, MgO·CaO) as raw material and ferrosilicon (FeSi) as reducing agent to reduce magnesium oxide under high temperature and vacuum conditions.

[0003] However, the traditional Pidgeon method has the following problems:

[0004] 1) High energy consumption: Dolomite calcination requires temperatures above 1200℃, and the magnesium reduction process requires long-term heat preservation, with energy consumption accounting for more than 40% of production costs.

[0005] 2) Raw material limitations: High-quality dolomite resources are becoming increasingly scarce, and the impact of impurities (such as Si, Al, Fe, S) on magnesium reduction efficiency is still unclear. In particular, the threshold of the impact of the content of each impurity element on magnesium volatilization rate is currently unknown. This makes it difficult to select raw materials, and the output value of different raw materials varies greatly, resulting in unpredictable economic losses.

[0006] 3) Environmental burden: The calcination process of raw material dolomite emits a large amount of CO2, and the accumulation of reduction slag (mainly calcium silicate) causes waste of land resources.

[0007] On the other hand, power plant ash, namely dust generated during the production and smelting of calcium carbide (the main components of the dust are calcium oxide, magnesium oxide, and silicon dioxide), is recycled after calcium oxide is extracted and reduced in a fluidized bed furnace, with a recovery rate of 60%. The remaining calcium oxide, magnesium oxide, and silicon dioxide become solid waste and are sold as cement raw materials.

[0008] The power plant ash recovered from the electric furnace is rich in MgO (10-30%), but due to its high sulfur, high silicon and aluminum content and other impurities, the reduction rate is unstable and cannot be guaranteed when it is used directly as a magnesium raw material. At the same time, it causes low activity and does not solve the problems of power plant ash desulfurization and synergistic effect with dolomite. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of existing technologies by proposing a magnesium smelting process using electric furnace slag silicothermic method.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A magnesium smelting process using electric furnace slag silicothermic method includes the following steps:

[0012] 1) Screening of magnesium-containing raw materials:

[0013] A mixture of dolomite and power plant ash is selected as the magnesium-containing raw material. The dolomite in the magnesium-containing raw material is pre-calcined, and the power plant ash in the magnesium-containing raw material is pre-activated or modified. The mass ratio of power plant ash to dolomite is 3:7-5:5, and the raw material content in the mixture is: Si < 2.49%, Al < 0.098%, Fe < 0.235%, S < 0.006%. The power plant ash is any one of untreated power plant ash, activated power plant ash, or acidified power plant ash.

[0014] 2) Preparation of magnesium reduction samples:

[0015] The magnesium-containing raw material and the ferrosilicon reducing agent (Zheng Neng Company) were thoroughly mixed according to the molar ratio of Mg in the magnesium-containing raw material to Si in the ferrosilicon reducing agent of 1:1.05-1.1 to obtain a mixed raw material.

[0016] Take no less than 3.5g of mixed raw materials, place them in a mold with a size of φ20, and place them in a press for pressing. Set the pressure to 50KN and press for 2min to obtain a magnesium reduction sample.

[0017] 3) Magnesium reduction experiment process

[0018] The magnesium reduction sample was placed in a burning boat and then loaded into a high-purity quartz tube. The sealed quartz tube was then inserted into a horizontal resistance furnace for heating, and a reduction experiment was carried out in conjunction with a vacuum pump.

[0019] The reduction temperature was set to 1200℃. Under a vacuum of 10Pa, the sample was heated to 1200℃ in the furnace and then held for 1-3 hours (i.e., reduction time). The magnesium volatilization rate (magnesium reduction yield against standard / %) was then tested.

[0020] Magnesium vapor enters the condenser connected to the reduction furnace. Argon gas is pre-circulated into the condenser to protect it from oxidation. A condensation gas circulation pipe at 450-500℃ is installed on the condenser wall. The magnesium vapor condenses into solid magnesium crystals on the inner wall of the condenser. After condensation is completed, the magnesium ingots are taken out after natural cooling to room temperature.

[0021] After the heat preservation is completed, the sample is cooled to 250°C with the furnace, the quartz tube is pulled out, and the temperature is further cooled until the temperature of the quartz tube is below 60°C. The vacuum pump is turned off, and the slag is taken out to test the MgO residue and obtain the magnesium content (%) of the slag.

[0022] Preferably, the calcination process of the dolomite in 1) is as follows:

[0023] Dolomite particles with a diameter range of 2-3 cm were sieved out, and 250-300 g were weighed and placed in a corundum crucible of a lifting pit furnace. The corundum crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the dolomite in the corundum crucible was measured in real time. The furnace was heated, and the weight change curve of the dolomite sample was recorded in real time. The heating rate was 10℃ / min, and the calcination was set to 1200℃. When the temperature reached 1200℃, it was held until the sample no longer lost weight. The furnace mouth was covered with a heat insulation board to reduce heat loss from the furnace mouth, and calcined dolomite was obtained.

[0024] Preferably, the activation treatment process for the ash from the power plant is as follows:

[0025] 250g of power plant ash was placed in an alumina crucible of a lifting pit furnace. The alumina crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the power plant ash in the alumina crucible was measured in real time. The furnace was heated, and the weight change curve of the power plant ash sample was recorded in real time. The heating rate was 10℃ / min, and the activation roasting temperature was set to 600℃. The temperature was maintained at 600℃ until the sample no longer lost weight. The furnace mouth was covered with a heat insulation board to reduce heat loss from the furnace mouth, thus obtaining activated power plant ash. The activation treatment improved the activity of the magnesium reduction reaction.

[0026] In some preferred solutions, the modification treatment of power plant ash in 1) includes the following steps:

[0027] Take a 1-5 mol / L HNO3 aqueous solution, and adjust the mass ratio of power plant ash to HNO3 aqueous solution to 10:1-2. Calcine the resulting sludge at 150℃ for 0.5-1 h. This step mainly oxidizes SO42-. 2- This causes it to oxidize into SO2 or H2S, thereby reducing the S content and simultaneously producing NO. x The acid mist waste gas generated needs to be treated with alkaline solution, and the dry residue obtained by calcination is acidified power plant ash.

[0028] In some more preferred solutions, the modification process of power plant ash 1) also includes the following steps:

[0029] Acidified power plant ash and CaO powder are mixed at a weight ratio of 9:1 and calcined at 150℃ for 0.5-1h to neutralize the acidity and prevent subsequent protons from affecting the magnesium reduction process, thus obtaining neutralized power plant ash.

[0030] Preferably, before mixing, the magnesium-containing raw material and the ferrosilicon reducing agent in 2) are pulverized in a pulverizer until the particle size is less than 74 μm (i.e., passing through a 200-mesh sieve). The magnesium reducing activity of the small-particle-size raw material is higher.

[0031] Preferably, the reduction reaction in step 3) also requires testing and calculation:

[0032] First, based on the different element contents in the magnesium-containing raw materials and the ratio of magnesium-containing raw materials to ferrosilicon reducing agent, the element contents include the contents of S, Si, Al, Fe, Ca and Mg, different magnesium reduction samples are prepared.

[0033] Secondly, a discrete coordinate graph showing the effect of element content on magnesium volatilization rate was plotted. After fitting, an element content / magnesium volatilization rate curve was obtained, and the content threshold of each element on the magnesium reduction yield was obtained. This allows for more accurate preparation of raw materials and provides precise theoretical support for the feasibility of using power plant ash as a magnesium reduction raw material.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention first conducts an in-depth analysis of the elemental composition of dolomite and power plant ash, and explores the standardization of impurity control in the raw materials through corresponding reduction processes: clarifying the impurity thresholds of the raw materials (Si < 2.49%, Al < 0.098%, Fe < 0.235%, S < 0.006%), and ensuring that the magnesium volatilization rate is > 85%;

[0036] 2. This invention employs a novel acidification and calcination method (HNO3 oxidation) + CaO neutralization method to reduce the sulfur content from 0.43% to 0.006%, avoiding sintering and increasing the magnesium volatilization rate; that is, using HNO3 to neutralize SO4 2- The oxidation process results in the release of SO2 / H2S, while CaO neutralizes the residual acidity and increases the Ca / Mg ratio to 1.055, promoting the formation of dicalcium silicate. This significantly increases the magnesium reduction yield of power plant ash.

[0037] 3. This invention also combines calcined dolomite and power plant ash, resulting in a magnesium yield exceeding the average of single raw materials. The free SiO2 in the power plant ash reacts with CaO in the calcined dolomite to generate low-melting-point silicates, improving material mass transfer. CaO can also fix sulfur to form CaSO4, reducing its inhibition of MgO reduction. By blending dolomite with power plant ash, the yield of power plant ash can be increased. Blending experiments have also shown that the product after blending is higher than the sum of the yields of the individual raw materials, exhibiting a typical synergistic effect. Furthermore, acidified and neutralized power plant ash, with fewer additives (both acid and alkali dosage not exceeding 20%), can achieve magnesium reduction yields close to and exceeding those of pure dolomite blends, while simultaneously reducing dolomite usage, lowering energy consumption per ton of magnesium, reducing reduction slag, and decreasing solid waste. Attached Figure Description

[0038] Figure 1 This is a schematic diagram illustrating the effect of Ca content in magnesium-containing raw materials on magnesium volatilization rate studied in this invention.

[0039] Figure 2 This is a schematic diagram illustrating the effect of Si impurities in magnesium-containing raw materials on the magnesium volatilization rate studied in this invention.

[0040] Figure 3 This is a schematic diagram illustrating the effect of Al impurities in magnesium-containing raw materials on the magnesium volatilization rate studied in this invention.

[0041] Figure 4 This is a schematic diagram illustrating the effect of Fe impurities in magnesium-containing raw materials on the magnesium volatilization rate studied in this invention.

[0042] Figure 5 This is a schematic diagram illustrating the effect of sulfur impurities in magnesium-containing raw materials on the magnesium volatilization rate studied in this invention.

[0043] Figure 6 This is a schematic diagram illustrating the effect of Mg content in magnesium-containing raw materials on magnesium volatilization rate studied in this invention.

[0044] Figure 7 This is a schematic diagram illustrating the effect of Ca / Mg (molar ratio) on magnesium volatilization rate in magnesium-containing raw materials studied in this invention.

[0045] Figure 8 This is a schematic diagram illustrating the effect of heat preservation time on the magnesium volatilization rate of magnesium-containing raw materials reduced in this invention (tested under single magnesium-silicon ratio conditions).

[0046] Figure 9 This diagram illustrates the effect of the magnesium-to-silicon ratio (Mg / Si, referring to the molar ratio of Mg in the magnesium-containing raw material to Si in the ferrosilicon reducing agent) on the magnesium volatilization rate after reduction of the magnesium-containing raw material, as studied in this invention. Figure 1 (Tested under conditions of single heat preservation time);

[0047] Figure 10 This is a schematic diagram illustrating the effect of S content in magnesium-containing raw materials on the magnesium volatilization rate after reduction of magnesium-containing raw materials, as studied in this invention.

[0048] Figure 11 This is a schematic diagram illustrating the effect of Si content in magnesium-containing raw materials on the magnesium volatilization rate after reduction of magnesium-containing raw materials, as studied in this invention.

[0049] Figure 12 This is a schematic diagram illustrating the effect of Fe content in magnesium-containing raw materials on the magnesium volatilization rate after reduction of magnesium-containing raw materials, as studied in this invention.

[0050] Figure 13 This is a schematic diagram illustrating the effect of Al content in magnesium-containing raw materials on the magnesium volatilization rate after reduction of magnesium-containing raw materials, as studied in this invention.

[0051] Figure 14 This is a schematic diagram illustrating the effect of the Ca / Mg (molar ratio) in the magnesium-containing raw material on the magnesium volatilization rate after reduction of the magnesium-containing raw material, as studied in this invention.

[0052] Figure 15This diagram illustrates the effect of the magnesium-to-silicon ratio (Mg / Si, referring to the molar ratio of Mg in the magnesium-containing raw material to Si in the ferrosilicon reducing agent) on the magnesium volatilization rate after reduction of the magnesium-containing raw material, as studied in this invention. Figure 2 (Under different heat preservation times). Detailed Implementation

[0053] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0054] 1. Experimental Procedure Formulation

[0055] 1.1 Inventive Concept

[0056] Dust is generated during the production and smelting of calcium carbide (the main components of the dust are calcium oxide, magnesium oxide, and silicon dioxide). The generated dust is recycled by extracting calcium oxide in a fluidized bed furnace, with a recovery rate of 60%. The remaining calcium oxide, magnesium oxide, and silicon dioxide become solid waste and are sold as cement raw materials.

[0057] Tests revealed that the dust recovered from the electric furnace contained high levels of magnesium oxide and calcium oxide, resulting in low activity. This activity could be improved before it could be used as a raw material for magnesium smelting. Therefore, a silicothermic process for magnesium smelting using electric furnace slag is proposed.

[0058] 1.2 Preliminary Plan

[0059] The dust generated during calcium carbide production is heated to 550-600℃ using a gas-fired drying drum. This drying process simultaneously classifies the sulfur content in the electric furnace slag (sulfur content greater than 0.4% will sinter during smelting and cause agglomeration during reduction). After further drying and desulfurization, the mixture is blended with ferrosilicon in a ball mill, ground, and then pressed into granulated pellets to prepare date-shaped pellets, which are then sent to the reduction system for smelting.

[0060] Using the dust generated during the calcium carbide production process to smelt magnesium can gradually reduce the amount of dolomite used, thereby reducing the energy consumption of magnesium smelting and the amount of reducing slag and greenhouse gas generated, thus protecting the environment and reducing raw material costs.

[0061] 1.3 Preparation of calcined white powder and activation of power plant ash

[0062] In the lifting pit furnace, the various types of dolomite selected in the previous stage are calcined and the power plant ash is activated.

[0063] Dolomite particles with a diameter range of 2–3 cm were sieved out, and 250–300 g were weighed and placed in an alumina crucible of a lifting pit furnace. The alumina crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the dolomite in the crucible was measured in real time. The furnace was heated, and the weight change curve of the dolomite sample was recorded in real time. The heating rate was 10 °C / min, and the calcination temperature was set to 1200 °C. Once 1200 °C was reached, the temperature was held until the sample no longer lost weight. The furnace opening was covered with an insulation board to reduce heat loss from the furnace opening.

[0064] 250g of power plant ash was placed in an alumina crucible within a lifting pit furnace. The crucible was connected to a precision balance via a nickel-chromium alloy wire, and the weight loss of the power plant ash in the crucible was measured in real time. The furnace was heated, and the weight change curve of the power plant ash sample was recorded in real time. The heating rate was 10℃ / min, and the activation roasting temperature was set to 600℃. The temperature was maintained at 600℃ until the sample no longer experienced weight loss. An insulation board was placed over the furnace opening to reduce heat loss.

[0065] 1.4 Preparation of magnesium reduction samples

[0066] The ferrosilicon raw materials provided by Zheng Neng Company, the calcined white raw materials produced during the calcination process, and the activated power plant ash raw materials were pulverized in a pulverizer so that the particle size of all the raw materials for the reduction experiment was less than 200 mesh.

[0067] According to two different reducing agent ratios of Mg:FeSi of 1:1.05 and 1:1.1, calcined silicide and ferrosilicon, and activated power plant ash and ferrosilicon were prepared separately and thoroughly mixed. At least 3.5g of the mixed raw material was placed in a φ20 mold and pressed in a press at a pressure of 50KN for 2 minutes. Sample A (1:1.05) and sample B (1:1.1) were prepared for calcined silicide and ferrosilicon, respectively. Sample information is shown in Table 1.

[0068] 1.5 Magnesium Reduction Experiment Procedure

[0069] For each MgO-containing raw material, two sets of samples, A and B, were prepared according to different amounts of reducing agent, and then subjected to silicothermic reduction experiments. The samples were placed in a heated boat and then loaded into a high-purity quartz tube. The sealed quartz tube was inserted into a horizontal resistance furnace for heating, and a vacuum pump was used for the reduction experiment. The reduction temperature was set at 1200℃, and experiments were conducted under a vacuum of 10 Pa with different reduction times and amounts of reducing agent. For the same raw material, both samples A and B were loaded simultaneously in each experiment to analyze the effect of reducing agent dosage on the reduction results under otherwise constant conditions. During the experiment, the samples were heated to 1200℃ with the furnace, and then held at that temperature for 1 h, 2 h, and 3 h, respectively, to analyze the effect of reduction time on the reduction results. After the holding period, the samples were cooled to 250℃ with the furnace, the quartz tube was removed, and the temperature was further reduced until the quartz tube temperature was below 60℃. The vacuum pump was then turned off, and samples A and B were removed and weighed. After the experiment, the residual MgO in samples A and B was measured using chemical analysis, and the degree of reduction was calculated.

[0070] 2. Experimental Results and Analysis

[0071] 2.1 Raw material formulation screening

[0072] Table 1. Main Raw Material Formula of the Invention

[0073]

[0074] It should be noted that Ca / Mg in Table 1 refers to the molar ratio of Ca and Mg, and % in Table 1 refers to the percentage of each component in the total mass of the raw materials.

[0075] The preparation process of Examples 1-5 is as follows:

[0076] Dolomite calcination: Dolomite particles with a diameter range of 2-3 cm were sieved out, and 250-300 g were weighed and placed in a corundum crucible of a lifting pit furnace. The corundum crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the dolomite in the corundum crucible was measured in real time. The furnace was heated, and the weight change curve of the dolomite sample was recorded in real time during the heating process. The heating rate was 10℃ / min, and the calcination was set to 1200℃. When the temperature reached 1200℃, it was held until the sample no longer lost weight. The furnace mouth was covered with a heat insulation board to reduce heat loss from the furnace mouth, and calcined dolomite was obtained.

[0077] The preparation process of Preparation Example 6 is as follows:

[0078] Activation and calcination: 250g of power plant ash was placed in an alumina crucible of a lifting pit furnace. The alumina crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the power plant ash in the crucible was measured in real time. The furnace temperature was increased, and the weight change curve of the power plant ash sample was recorded in real time during the heating process. The heating rate was 10℃ / min, and the activation and calcination temperature was set to 600℃. The temperature was maintained at 600℃ until the sample no longer lost weight. The furnace opening was covered with a heat insulation board to reduce heat loss from the furnace opening, thus obtaining activated power plant ash.

[0079] The preparation process of Preparation Example 7 is as follows:

[0080] Acidification reaction: Take a 3 mol / L HNO3 aqueous solution, and use power plant ash to HNO3 aqueous solution at a mass ratio of 10:1.5. Calcine the resulting sludge at 150℃ for 0.8 h. This step mainly oxidizes SO42-. 2- This causes it to oxidize into SO2 or H2S, thereby reducing the S content and simultaneously producing NO. x The acid mist waste gas generated needs to be treated with alkaline solution, and the dry residue obtained by calcination is acidified power plant ash.

[0081] The preparation process of Example 8 is as follows:

[0082] Neutralization reaction: The acidified power plant ash obtained in Preparation Example 7 was mixed with CaO powder at a weight ratio of 9:1 and calcined at 150°C for 0.8 h to neutralize the acidity and prevent subsequent protons from affecting the magnesium reduction process, thus obtaining neutralized power plant ash.

[0083] 2.2 Reduction Test Results

[0084] 2.2.1 Reduction Test of Raw Materials from Single Mg Source

[0085] The calcined dolomite or power plant ash from Preparation Examples 1-6 were prepared separately according to two different reducing agent ratios of Mg:FeSi of 1:1.05 and 1:1.1, and then thoroughly mixed to obtain a mixed raw material.

[0086] Take at least 3.5g of mixed raw materials, place them in a mold with a size of φ20, and place them in a press for pressing. Set the pressure to 50KN and press for 2 minutes to obtain the sample.

[0087] The sample was placed in a flammable boat and then loaded into a high-purity quartz tube. The sealed quartz tube was then inserted into a horizontal resistance furnace for heating, and a reduction experiment was conducted using a vacuum pump. The reduction temperature was set at 1200℃, and experiments were carried out under a vacuum of 10Pa with different reduction times and different amounts of reducing agent. The specific experimental results are shown in Table 2 below.

[0088] Table 2. Sample conditions and experimental results for reduction experiments

[0089]

[0090] 2.2.2 Raw material reduction test of power plant ash mixed with dolomite

[0091] Continuing with the experimental conditions of Example 12, the source of Mg was changed to calcined dolomite from Preparation Example 2 (with a maximum Ca content of 41.242%) and power plant ash from Preparation Examples 6, 7, or 8, and the resulting experimental data are shown in Table 3 below:

[0092] Table 3. Results of reduction test after mixing power plant ash and dolomite

[0093] Example Sources of power plant ash Dolomite: Power Plant Ash Ca / Mg Magnesium content in slag / % Magnesium volatility / % Example 31 Preparation Example 6 No white dolomite 0.699 9.90 57.04 Example 32 Preparation Example 6 2:8 0.762 6.58 67.51 Example 33 Preparation Example 6 5:5 0.857 4.56 76.97 Example 34 Preparation Example 6 8:2 0.951 3.16 86.62 Example 12 Preparation Example 6 No power plant ash 1.014 2.63 91.98 Example 35 Preparation Example 7 5:5 0.857 4.07 80.23 Example 36 Preparation Example 8 5:5 0.914 3.27 86.24

[0094] As shown in Table 3, compared with Examples 12 and 31, the Ca / Mg ratio of Examples 32-34 increased with the increase of dolomite content. Simultaneously, due to the decrease in sulfur content, the Mg yield of Examples 32-34 was higher than the mean yield of Examples 12 and 31, but not the mean yield of Examples 12 and 31. Furthermore, the ferrosilicon and aluminum content in the power plant ash was controlled, demonstrating that the blending of power plant ash and dolomite has a synergistic effect. It also proves that using dust generated during the calcium carbide production process to smelt magnesium can gradually reduce the amount of dolomite used, thereby reducing energy consumption in magnesium smelting and decreasing the generation of reducing slag and greenhouse gases, thus protecting the environment while reducing raw material costs.

[0095] Compared with Example 33, Examples 35-36 reduced the S content, and Example 36 also increased the Ca / Mg ratio, thus obtaining magnesium reduction yields comparable to or even higher than those obtained from some pure dolomite in Examples 1-30. This fully demonstrates that the acid addition for desulfurization and the subsequent CaO neutralization are effective operations. From a market analysis, the calcined dolomite prepared in Examples 1-5 does not differ significantly in price. Therefore, dolomite with lower silicon, iron, aluminum, and sulfur content can be selected based on the power plant ash.

[0096] 2.3 Results Analysis

[0097] 2.3.1 Effects of impurities and magnesium in dolomite on magnesium volatilization rate

[0098] Based on the impurities routinely tested during the production process, the experiment mainly tested Ca, Si, Fe, Al, and S in dolomite. The effects of these five impurities on the magnesium volatilization rate during magnesium reduction are shown in [reference needed]. Figure 1-7 ,Depend on Figure 1-7It can be seen that increasing the Ca content in calcined white magnesium increases the magnesium volatilization rate; conversely, increasing the Si, Al, Fe, and S content in calcined white magnesium decreases the magnesium volatilization rate. A higher Mg content in calcined white magnesium corresponds to a higher magnesium volatilization rate. Based on the Pidgeon process for magnesium refining, where calcium reacts with silicon dioxide to form dicalcium silicate, the Ca / Mg ratio (calcium-magnesium ratio) is chosen as the indicator for evaluating magnesium volatilization rate. A higher Ca / Mg ratio results in a higher magnesium volatilization rate; when Ca / Mg is greater than 1, the magnesium volatilization rate is generally higher.

[0099] 2.3.2 Effect of reduction control conditions on magnesium volatilization rate

[0100] The reduction temperature was 1200℃. The holding time was varied. The amount of ferrosilicon added during the batching process was expressed as the magnesium-to-silicon ratio (Mg / Si). The effect of control conditions on the magnesium reduction rate is shown in [reference needed]. Figure 8 and Figure 9 ,in Figure 8 The magnesium-to-silicon ratio (Mg / Si) of each sample was controlled at 1:1.1. Figure 9 The heat preservation time for each sample was controlled at 3 hours.

[0101] Depend on Figure 8 and Figure 9 It can be seen that the longer the holding time, the higher the magnesium volatilization rate. A longer holding time helps the material to be heated sufficiently, thus resulting in a higher magnesium volatilization rate. Under the experimentally studied mixing ratios, the magnesium volatilization rates were almost identical with both ferrosilicon mixing ratios. Therefore, it can be considered that a ferrosilicon mixing ratio of 1.05 times is sufficient to ensure the complete reduction of magnesium in the calcined white metal. The amount of ferrosilicon added during the experiment was calculated based on the assumption that all magnesium in the calcined white metal was in the Mg form. Furthermore, the amount of ferrosilicon added exceeded the theoretically required amount by 1.05 times and 1.1 times. Since the calcined white metal contains other impurities, these impurities may cause the magnesium oxide in the calcined white metal to transform into other, more difficult-to-reduce forms. Therefore, the amount of ferrosilicon added can be appropriately reduced.

[0102] 2.3.3 Effects of various factors during the reduction process on the magnesium volatilization rate

[0103] The effects of various factors on magnesium volatilization rate are shown in the figure. Figure 10-15 ,Depend on Figure 10 It can be seen that when the S content is low, sulfur has little effect on the volatilization rate of magnesium. Within the range of S content studied in the experiment, when the sulfur content is 0.006%, it can promote the volatilization of magnesium. The main reason is that sulfur in dolomite is mainly sulfate, and a small amount of sulfate can act as a mineralizing agent, promoting the formation of dicalcium silicate during the reduction process. When the sulfur content is high (0.43% power plant ash), the material is prone to sintering during calcination and reduction. During the reduction process, the sintering of the material causes magnesium oxide to be converted into more stable silicates that cannot be reduced. This sintering during the reduction process reduces the permeability of the material. Figure 11It can be seen that when the silicon content is low, the effect on the magnesium volatilization rate is small. However, when the silicon content exceeds 2.49% (in the experimental raw material), the magnesium reduction rate decreases significantly, mainly because magnesium oxide forms silicates with silicon, which are more difficult to reduce. Figure 12 It can be concluded that within the iron content range studied in the experiment, when the iron content is less than 0.235%, the effect of iron on the volatilization rate of magnesium is small, while when the iron content exceeds 0.235%, the volatilization rate of magnesium decreases; from Figure 13 It can be concluded that within the studied range, aluminum content below 0.098% has virtually no impact on magnesium volatilization, while content above 0.098% will decrease magnesium volatilization. Figure 14 It can be seen that when the calcium-magnesium ratio is high, between 0.98% and 1.055%, the magnesium volatilization rate is relatively high; when the calcium-magnesium ratio is low (0.699%), the magnesium volatilization rate is relatively low. Therefore, when preparing magnesium reduction feedstock, the calcium-magnesium ratio in the feedstock should be considered to be around 1. Figure 15 It can be seen that when the amount of ferrosilicon added exceeds the theoretically required amount of ferrosilicon, the effect on the reduction rate of magnesium is small when the excess coefficient is small or large.

[0104] 3. Conclusion

[0105] Preliminary factor exploration experiments were conducted on the four types of dolomite and various modified power plant ash selected in the previous stage, and the following conclusions were obtained:

[0106] 1) When dolomite is heated to 1200℃ with a silicon excess coefficient between 1.05 and 1.1 and held for 3 hours, the magnesium content in the reduction slag decreases to below 5%, indicating that the selected dolomite is suitable for magnesium smelting.

[0107] 2) After the power plant ash is reduced, the magnesium content in the reduction slag is reduced to about 10%. Since the silicon, aluminum and sulfur content are relatively high, preliminary experiments show that it is expected to be used as a raw material for magnesium smelting.

[0108] 3) The influence of impurities in dolomite on the magnesium volatilization rate and the limiting range of impurity content were explored, and specific element content thresholds were obtained, such as the tendency for material sintering when the S content is >0.006%. Figure 10 Al > 0.098% or Fe > 0.235% will inhibit magnesium reduction. Figure 12 , Figure 13 );

[0109] 4) A novel calcination oxidation method is used to acidify and desulfurize power plant ash, which can significantly reduce the sulfur content of raw materials and improve the magnesium reduction yield.

[0110] 5) By adding CaO to acidified power plant ash for neutralization, the Ca / Mg ratio can be further increased, thereby further increasing the magnesium reduction yield;

[0111] 6) By blending dolomite with power plant ash, the yield of power plant ash can be increased. Blending tests have also shown that the product after blending the two is higher than the sum of the yields of the individual raw materials, exhibiting a typical synergistic effect. In addition, acidified power plant ash and neutralized power plant ash can achieve magnesium reduction yields close to and exceeding those of pure dolomite with fewer additives (both acid and alkali dosages not exceeding 20%).

[0112] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A magnesium smelting process using electric furnace slag silicothermic method, characterized in that, Includes the following steps: 1) Screening of magnesium-containing raw materials: A mixture of dolomite and power plant ash is selected as the magnesium-containing raw material. The dolomite in the magnesium-containing raw material is pre-calcined, and the power plant ash in the magnesium-containing raw material is pre-activated or modified. The mass ratio of power plant ash to dolomite is 3:7-5:5, and the raw material content in the mixture is: Si < 2.49%, Al < 0.098%, Fe < 0.235%, S < 0.006%. The power plant ash is any one of untreated power plant ash, activated power plant ash, or acidified power plant ash. 2) Preparation of magnesium reduction samples: The magnesium-containing raw material and the ferrosilicon reducing agent are thoroughly mixed according to the molar ratio of Mg in the magnesium-containing raw material to Si in the ferrosilicon reducing agent of 1:1.05-1.1 to obtain a mixed raw material. Take no less than 3.5g of mixed raw materials, place them in a mold with a size of φ20, and place them in a press for pressing. Set the pressure to 50KN and press for 2min to obtain a magnesium reduction sample. 3) Magnesium reduction experiment process The magnesium reduction sample was placed in a burning boat and then loaded into a high-purity quartz tube. The sealed quartz tube was then inserted into a horizontal resistance furnace for heating, and a reduction experiment was carried out in conjunction with a vacuum pump. The reduction temperature was set to 1200℃. Under a vacuum of 10Pa, the sample was heated to 1200℃ in the furnace and then held for 1-3 hours. The magnesium volatilization rate was then tested. Magnesium vapor enters the condenser connected to the reduction furnace. Argon gas is pre-circulated into the condenser to protect it from oxidation. A condensation gas circulation pipe at 450-500℃ is installed on the condenser wall. The magnesium vapor condenses into solid magnesium crystals on the inner wall of the condenser. After condensation is completed, the magnesium ingots are taken out after natural cooling to room temperature. After the heat preservation is completed, the sample is cooled to 250°C with the furnace, the quartz tube is pulled out, and the temperature of the quartz tube is further cooled until it is below 60°C. The vacuum pump is turned off, and the slag is taken out to test the MgO residue and obtain the magnesium content of the slag. The reduction reaction mentioned in 3) still requires testing and calculation: First, based on the different element contents in the magnesium-containing raw materials and the ratio of magnesium-containing raw materials to ferrosilicon reducing agent, the element contents include the contents of S, Si, Al, Fe, Ca and Mg, different magnesium reduction samples are prepared. Next, a discrete coordinate graph showing the effect of element content on magnesium volatilization rate was plotted. After fitting, an element content / magnesium volatilization rate curve was obtained, and the content threshold of each element on magnesium reduction yield was obtained.

2. The magnesium smelting process using electric furnace slag silicothermic method according to claim 1, characterized in that, The calcination process of the dolomite in 1) is as follows: Dolomite particles with a diameter range of 2-3 cm were sieved out, and 250-300 g were weighed and placed in a corundum crucible of a lifting pit furnace. The corundum crucible was connected to a precision balance by a nickel-chromium alloy wire, and the weight loss of the dolomite in the corundum crucible was measured in real time. The furnace was heated, and the weight change curve of the dolomite sample was recorded in real time. The heating rate was 10℃ / min, and the calcination was set to 1200℃. When the temperature reached 1200℃, it was held until the sample no longer lost weight. The furnace mouth was covered with a heat insulation board to reduce heat loss from the furnace mouth, and calcined dolomite was obtained.

3. The magnesium smelting process using electric furnace slag silicothermic method according to claim 1, characterized in that, The activation treatment process for the power plant ash in 1) is as follows: Take 250g of power plant ash and place it in the corundum crucible of the lifting pit furnace. The corundum crucible is connected to a precision balance with a nickel-chromium alloy wire, and the weight loss of the power plant ash in the corundum crucible is measured in real time. The weight change curve of the power plant ash sample was recorded in real time during the heating process, with a heating rate of 10℃ / min and an activation roasting temperature set at 600℃. The sample was kept at 600℃ until it stopped losing weight. The furnace opening was covered with a heat insulation board to reduce heat loss from the furnace opening, thus obtaining activated power plant ash. The activation treatment improved the activity of the magnesium reduction reaction.

4. The magnesium smelting process using electric furnace slag silicothermic method according to claim 1, characterized in that, The modification treatment of power plant ash in step 1) includes the following steps: Take a 1-5 mol / L HNO3 aqueous solution, and take the mass ratio of power plant ash to HNO3 aqueous solution as 10:1-2. Calcine the resulting sludge at 150℃ for 0.5-1 h. The dry sludge obtained from calcination is the acidified power plant ash.

5. The magnesium smelting process using electric furnace slag silicothermic method according to claim 4, characterized in that, The modification process of power plant ash in step 1) further includes the following steps: Acidified power plant ash and CaO powder are mixed at a weight ratio of 9:1 and calcined at 150℃ for 0.5-1h to obtain neutralized power plant ash.

6. The magnesium smelting process using electric furnace slag silicothermic method according to claim 1, characterized in that, Before mixing, the magnesium-containing raw material and the ferrosilicon reducing agent in step 2) are pulverized in a pulverizer until the particle size is less than 74 μm.

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