Semi-calcined dolomite magnesium smelting sintering material ball and preparation method thereof

Abstract

The application discloses a kind of semi-calcined dolomite magnesium smelting sintered material balls and preparation method thereof, belong to metal magnesium smelting technical field, including steps A semi-calcined dolomite is obtained by semi-calcining dolomite raw materials;Step B according to mass ratio, semi-calcined dolomite: silicon iron: fluorite = 100: (10-15): (0-3) preparation raw materials;Step C the obtained raw material is placed in ball mill jar and is ball-milled mixed treatment, and is pressed into material ball;Step D material ball is placed in high temperature furnace and heated sintering, in the sintering process, low vacuum condition is kept in high temperature furnace, or inert gas is introduced into high temperature furnace, so that CO2 produced in material ball is promptly discharged, and molten state silicon calcium compound is generated in material ball interior;The semi-calcined dolomite of the application reduces energy consumption, so that the stability of semi-calcined dolomite is obviously enhanced, not easy to " absorb moisture ", reduce the loss of reducing agent, improve the reduction efficiency of subsequent sintered material ball, reduce production cost, and be applicable to common crystal structure and grain size dolomite.

Description

Technical Field

[0001] This invention belongs to the field of magnesium smelting technology, and specifically relates to a semi-calcined dolomite magnesium sintering pellet and its preparation method. Background Technology

[0002] Magnesium and its alloys possess numerous excellent physical and chemical properties, making them widely used in aerospace, automotive, electronics, and biomedical fields. As the world's largest producer of primary magnesium, my country supplies the global demand. Currently, the silicothermic process, as a representative of thermal reduction methods, produces over 90% of the world's primary magnesium. The silicothermic process uses calcined dolomite (calcined white) as raw material. The calcined white dolomite and ferrosilicon are powdered and then mixed evenly with fluorite powder (containing ≥95% CaF2) to prepare magnesium smelting pellets. These pellets are then placed in a heat-resistant stainless steel reduction vessel and placed in a reduction furnace. Under a vacuum of approximately 10 Pa, crude magnesium is produced. The crude magnesium is then refined into magnesium ingots using flux, and the ingots undergo surface treatment to become finished magnesium ingots.

[0003] In the preparation of magnesia pellets, the main equipment used for calcining dolomite includes vertical kilns, double-hearth kilns, sleeve kilns, and rotary kilns, among which rotary kilns are the most numerous and widely used. The temperature of dolomite calcined in a rotary kiln is approximately 1200–1250℃. The calcined dolomite from the rotary kiln is cooled to <200℃ in a vertical cooler and then conveyed to the dolomite silo. Calcining 1 ton of dolomite consumes approximately 0.32 tce (tons of standard coal equivalent). This process not only involves huge energy consumption but also generates a large amount of CO2. Based on the chemical reaction formula of dolomite decomposition, it can be estimated that for every 1 ton of MgO raw material produced, 2.2 t of CO2 will be directly generated and emitted, causing serious environmental damage. Under the global context of "dual carbon" (carbon peaking and carbon neutrality) goals and high-quality development, the high energy consumption and high emissions of the traditional siliceous thermal method are dramatically amplified, becoming the biggest bottleneck restricting the sustainable development of the industry. If CO2 emissions during the preparation of magnesium smelting pellets can be reduced and recovered, resource reuse can be effectively achieved, generating significant economic and environmental benefits. Furthermore, due to the high chemical reactivity of calcined white precipitate, it readily reacts with moisture in the air. Once calcined white precipitate absorbs moisture, the magnesium smelting pellets will not only suffer from reduced agent loss during subsequent vacuum reduction processes, but the high-temperature water vapor and carbon dioxide will also re-oxidize the reduced magnesium vapor back to MgO, leading to a significant decrease in magnesium yield. This high reactivity of calcined white precipitate necessitates strict management of its storage and transportation, and its immediate deployment, increasing production costs. Summary of the Invention

[0004] The purpose of this invention is to address the problems that rotary kiln calcination processes are only applicable to dolomite with network-like or fine-grained structures, and that the calcination process generates significant energy consumption and large amounts of CO2. Furthermore, due to the high chemical reactivity of calcined dolomite, it readily reacts with moisture in the air. Once calcined dolomite absorbs moisture, the magnesium sintering pellets will not only suffer from reduced agent loss in the subsequent vacuum reduction process, but the high-temperature steam and carbon dioxide will also re-oxidize the reduced magnesium vapor back to MgO, leading to a significant decrease in magnesium yield. Therefore, this invention proposes a semi-calcined dolomite magnesium sintering pellet and its preparation method. The technical solution adopted to achieve the above objectives is as follows: A method for preparing semi-calcined dolomite magnesia sintering pellets includes the following steps: Step A: Semi-calcining the dolomite raw material yields semi-calcined dolomite; Step B: Prepare raw materials according to the mass ratio of semi-calcined dolomite: ferrosilicon: fluorite = 100: (10-15): (0-3); Step C: Place the obtained raw materials into a ball mill jar for ball milling and mixing to obtain a uniformly mixed raw material powder of 80-200 mesh, and press it into pellets; Step D: Place the pellets from step C into a high-temperature furnace for heating and sintering. During the sintering process, maintain a low vacuum condition inside the high-temperature furnace, or introduce inert gas into the high-temperature furnace so that the CO2 generated by the decomposition of the pellets can be discharged in time, and molten silicon-calcium compounds can be generated inside the pellets.

[0005] Preferably, in step A, the dolomite raw material is semi-calcined at 500-1000℃ to obtain semi-calcined dolomite with a composition of CaCO3:MgO≈1:1 molar ratio.

[0006] Preferably, in step A, the dolomite raw material includes dolomite with a hexagonal rhombic structure, a network structure, and coarse or fine grains.

[0007] Preferably, in step D, the sintering temperature T is 1000℃ < T < 1200℃. When a low vacuum condition is adopted to ensure that the CO2 generated by the decomposition of the pellets is discharged in time and does not affect the formation of silicon-calcium compounds, the absolute pressure P satisfies -5.594×10-6T2+0.0187T-10.757 < lgP < 4.0, where lgP is the commonly used logarithmic value of the absolute pressure P, the unit of the absolute pressure P is Pa, and the unit of the sintering temperature T is ℃.

[0008] Preferably, in step D, the sintering temperature T is 1000℃ < T < 1200℃. In order to ensure that the CO2 generated by the decomposition of the pellets is discharged in time and does not affect the formation of silicon-calcium compounds, an inert gas is introduced. The high-temperature furnace is in a dynamically flowing inert gas atmosphere. When the inert gas flows out of the high-temperature furnace, it carries the generated CO2 and discharges it outside the high-temperature furnace.

[0009] A type of magnesium sintering pellet is obtained by the method described above for preparing magnesium sintering pellets from semi-calcined dolomite.

[0010] A magnesium smelting method using the above-mentioned magnesium sintering pellets as raw materials.

[0011] The beneficial effects of this invention are as follows: (1) Compared with the calcined dolomite (CaO·MgO) obtained by traditional calcination of dolomite (>1200℃), the semi-calcined dolomite (CaCO3·MgO) is obtained by semi-calcination of dolomite at a lower temperature (500-1000℃), in which CaCO3 is retained, thus reducing energy consumption compared with the traditional calcination method; on the other hand, the CaCO3 retained in the semi-calcination process significantly enhances the stability of the semi-calcined dolomite, making it less prone to "moisture absorption", reducing the storage and transportation conditions of raw materials, reducing the loss of reducing agent at high temperature, and improving the reduction efficiency.

[0012] (2) Semi-calcined dolomite has a lower calcination temperature, which increases the activity of MgO, improves the reduction efficiency of subsequent sintering pellets, and reduces production costs.

[0013] (3) In the traditional rotary kiln calcination process of dolomite, dolomite with hexagonal rhombic structure or coarse grains is easily broken during rotary kiln calcination, resulting in poor calcination effect (over-calcination or under-calcination), which in turn reduces the efficiency of the subsequent reduction stage. However, the semi-calcined dolomite used in this application to prepare magnesium sintering pellets has sufficient strength and stability and will not break during the semi-calcination process. Therefore, dolomite with hexagonal rhombic structure, network structure, and dolomite with coarse and fine grain sizes can all be used in the process of this application without adverse effects on the subsequent reduction, thus solving the problems existing in the traditional rotary kiln calcination of the above-mentioned types of dolomite raw materials.

[0014] (4) The main components of semi-calcined dolomite are CaCO3 and MgO with a molar ratio close to 1:1. CaCO3 is retained during the semi-calcination process, thus reducing carbon dioxide emissions by half compared with traditional dolomite calcination. The high-purity CO2 produced by the subsequent decomposition of CaCO3 can be collected as a by-product.

[0015] (5) The existing traditional silicothermic magnesium smelting process does not have a sintering process. The material balls have low mechanical strength and high water absorption. They are easily broken and pulverized during transportation and feeding. A large amount of powder enters the reduction tank, causing "sticking to the tank and glazing", which shortens the life of the reduction tank and reduces the magnesium yield. However, the magnesium smelting sintering material balls of this application do not have this problem. Attached Figure Description

[0016] Figure 1 This is a comparison diagram showing the moisture absorption of semi-calcined dolomite and traditionally calcined dolomite after being placed in the air for the same period of time. Figure 2 Line graph showing the weight gain due to moisture absorption of semi-calcined dolomite and conventionally calcined dolomite of equal mass after being exposed to air for 10 days. Detailed Implementation

[0017] The present application will now be further described with reference to the accompanying drawings.

[0018] Example 1 A semi-calcined dolomite magnesia sintering pellet and its preparation method include the following steps: Step A: Semi-calcining the dolomite raw material yields semi-calcined dolomite; Step B: Prepare raw materials according to the mass ratio of semi-calcined dolomite: ferrosilicon: fluorite = 100: (10-15): (0-3); Step C: Place the obtained raw materials into a ball mill jar for ball milling and mixing to obtain a uniformly mixed raw material powder of 80-200 mesh, and press it into pellets; Step D: Place the pellets from step C into a high-temperature furnace for heating and sintering. During the sintering process, maintain a low vacuum condition inside the high-temperature furnace, or introduce inert gas into the high-temperature furnace so that the CO2 generated by the decomposition of the pellets can be discharged in time, and molten silicon-calcium compounds can be generated inside the pellets.

[0019] Dolomite is semi-calcined at a lower temperature (500-1000℃) to obtain semi-calcined dolomite (CaCO3·MgO), in which CaCO3 is retained, thus reducing energy consumption compared with calcined dolomite (CaO·MgO) obtained by traditional calcination. On the other hand, the CaCO3 retained in the semi-calcination process significantly enhances the stability of semi-calcined dolomite, making it less prone to moisture absorption. This reduces the storage and transportation requirements of the raw materials, decreases the loss of reducing agent in the subsequent magnesium reduction stage, and improves the reduction efficiency.

[0020] Semi-calcined dolomite, due to its lower calcination temperature, increases the activity of MgO, thereby improving the reduction efficiency of subsequent sintering pellets and reducing production costs.

[0021] The existing traditional silicothermic magnesium smelting process does not have a sintering process. The sintered spheres have low mechanical strength and high water absorption. They are easily broken and pulverized during transportation and feeding. A large amount of powder enters the reduction tank, causing "glaze sticking to the tank", which shortens the life of the reduction tank and reduces the magnesium yield. However, the magnesium smelting sintered spheres of this application do not have this problem.

[0022] Example 2 Based on Example 1, in step A, the dolomite raw material is semi-calcined at 500-1000℃, for example, at temperatures of 500℃, 600℃, 700℃, 800℃, 900℃, 1000℃, etc., to obtain semi-calcined dolomite with a composition of CaCO3:MgO≈1:1 molar ratio.

[0023] The main components of semi-calcined dolomite are CaCO3 and MgO in a molar ratio close to 1:1. During the semi-calcination process, CaCO3 is retained, thus reducing carbon dioxide emissions by half compared to traditional dolomite calcination. The high-purity CO2 produced by the subsequent decomposition of CaCO3 can be collected as a byproduct.

[0024] In step D, specifically, the sintering temperature T is 1000℃ < T < 1200℃, for example, sintering is carried out at temperatures such as 1050℃, 1100℃, and 1150℃. When a low vacuum condition is used to ensure that the CO2 generated by the decomposition of the pellets is discharged in a timely manner and does not affect the formation of silicon-calcium compounds, the absolute pressure P satisfies -5.594 × 10⁻⁶. -6 T 2 +0.0187T-10.757<lgP<4.0, where lgP is the commonly used logarithmic value of absolute pressure P, the unit of absolute pressure P is Pa, and the unit of sintering temperature T is °C.

[0025] Example 3 Based on Example 1, in step D, the sintering temperature T is 1000℃ < T < 1200℃, for example, sintering is carried out at temperatures such as 1050℃, 1100℃, and 1150℃. In order to ensure that the CO2 generated by the decomposition of the material balls is discharged in time and does not affect the formation of silicon-calcium compounds, an inert gas is introduced. The high-temperature furnace is in a dynamically flowing inert gas atmosphere. When the inert gas flows out of the high-temperature furnace, it carries the generated CO2 and discharges it outside the high-temperature furnace.

[0026] In step D of Examples 2 and 3, the sintering process effectively collects the CO2 byproduct generated by the decomposition of CaCO3, reducing carbon emissions and improving the utilization rate of magnesium smelting products. On the other hand, the generation of molten silicon-calcium compounds inside the pellets gives the sintered pellets sufficient strength to meet the strength requirements of the pellets in the reduction stage.

[0027] Example 4: This application also discloses a magnesium sintering pellet obtained by the above preparation method. After the magnesium sintering pellet is semi-calcined and decomposed into micropores in step A, it forms a sintering pellet with micropores, which is conducive to the migration of magnesium vapor. Moreover, calcination at 750°C ensures that MgO has high activity, so it can improve the reduction rate of magnesium.

[0028] Example 5: Further experiments were conducted on the magnesium smelting pellets obtained by the method of this application as follows: like Figure 1 As shown in the figure, the moisture absorption of the semi-calcined dolomite of this application and the conventional calcined dolomite after being placed in the air for the same period of time are compared. Figure a shows the conventional calcined dolomite after being calcined at 1200℃ for 2 hours, and Figure b shows the semi-calcined dolomite after being calcined at 750℃ for 2 hours. As can be seen from Figure a, the conventional calcined dolomite is extremely easy to absorb moisture and turn into powder, while the surface of the semi-calcined dolomite in Figure b has not changed. Therefore, it can be seen that the semi-calcined dolomite of this application has extremely strong moisture-proof properties.

[0029] like Figure 2 As shown in the figure, the graph illustrates the weight gain due to moisture absorption of equal masses of semi-calcined dolomite (as described in this application) and conventionally calcined dolomite after being exposed to air for 10 days. Figure 2 It can be seen that the moisture absorption rate of traditionally calcined dolomite is highest in the first three days, then decreases and then increases again. Semi-calcined dolomite only absorbs a small amount of moisture in the first three days, and then absorbs almost no moisture from the air. By the ninth day after being placed in the air, the moisture absorption of traditionally calcined dolomite has reached 16.8 times that of semi-calcined dolomite, indicating that semi-calcined dolomite has excellent moisture-proof properties.

[0030] Furthermore, reduction experiments were conducted on the prepared pellets. The results showed that the average reduction rates of magnesium smelting using conventional calcined dolomite magnesium smelting pellets and the semi-calcined dolomite magnesium sintering pellets of this application were 88.75% and 90.13%, respectively. The reduction rate of the sintering pellets of this invention is higher than that of conventional pellets.

[0031] In traditional rotary kiln calcination processes for dolomite, dolomite with a hexagonal rhombic structure or coarse grains is prone to breakage during calcination, leading to poor calcination results (over- or under-calcination) and consequently reducing the efficiency of the subsequent magnesium reduction stage. The above embodiments demonstrate that the semi-calcined dolomite used in this application for preparing magnesium sintering pellets possesses sufficient strength and stability, and will not crack during the semi-calcination process. Therefore, dolomite with hexagonal rhombic structure, network structure, and both coarse and fine grain sizes can be used in this process without adversely affecting subsequent reduction, thus solving the problems existing in traditional rotary kiln calcination of the above-mentioned types of dolomite raw materials.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. However, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention. For example, in the step of semi-calcining dolomite at a relatively low temperature (500-1000℃) to obtain semi-calcined dolomite (CaCO3·MgO) in which CaCO3 is retained, in actual work, the semi-calcination temperature (500-1000℃) can be appropriately adjusted according to the actual situation of the dolomite material. The temperature value within and near this range is sufficient as long as semi-calcined dolomite (CaCO3·MgO) is obtained. This temperature range is not a substantial limiting feature.

[0033] Similarly, in step D, the sintering temperature T is 1000℃ < T < 1200℃. In actual operation, the sintering temperature T can be appropriately adjusted within or near this range according to the actual situation of the pellets. As long as CaCO3 in the pellets decomposes to produce CO2 during sintering and molten silicon-calcium compounds are generated inside the pellets, this temperature range is not a substantial limiting feature.

Claims

1. A method for preparing semi-calcined dolomite magnesia sintering pellets, characterized in that, Includes the following steps: Step A: Semi-calcining the dolomite raw material yields semi-calcined dolomite; Step B: Prepare raw materials according to the mass ratio of semi-calcined dolomite: ferrosilicon: fluorite = 100: (10-15): (0-3); Step C: Place the obtained raw materials into a ball mill jar for ball milling and mixing to obtain a uniformly mixed raw material powder of 80-200 mesh, and press it into pellets; Step D: Place the pellets from step C into a high-temperature furnace for heating and sintering. During the sintering process, maintain a low vacuum condition inside the high-temperature furnace, or introduce inert gas into the high-temperature furnace so that the CO2 generated by the decomposition of the pellets can be discharged in time, and molten silicon-calcium compounds can be generated inside the pellets.

2. The method for preparing semi-calcined dolomite magnesia sintering pellets according to claim 1, characterized in that, In step A, the dolomite raw material is semi-calcined at 500-1000℃ to obtain semi-calcined dolomite with a composition of CaCO3:MgO≈1:1 molar ratio.

3. The method for preparing semi-calcined dolomite magnesia sintering pellets according to claim 1, characterized in that, In step A, the dolomite raw material includes dolomite with hexagonal rhombic structure, network structure, and coarse or fine grains.

4. The method for preparing semi-calcined dolomite magnesia sintering pellets according to any one of claims 1 to 3, characterized in that, In step D, the sintering temperature T is 1000℃ < T < 1200℃. To ensure the timely removal of CO2 generated from the decomposition of the sintering pellets and to prevent it from affecting the formation of silicon-calcium compounds, a low vacuum condition is used. The absolute pressure P must satisfy -5.594 × 10⁻⁶. -6 T 2 +0.0187T-10.757<lgP<4.0, where lgP is the commonly used logarithmic value of absolute pressure P, the unit of absolute pressure P is Pa, and the unit of sintering temperature T is °C.

5. The method for preparing semi-calcined dolomite magnesia sintering pellets according to any one of claims 1 to 3, characterized in that, In step D, the sintering temperature T is 1000℃ < T < 1200℃. In order to ensure that the CO2 generated by the decomposition of the pellets is discharged in time and does not affect the formation of silicon-calcium compounds, an inert gas is introduced. The high-temperature furnace is in a dynamically flowing inert gas atmosphere. When the inert gas flows out of the high-temperature furnace, it carries the generated CO2 with it and discharges it outside the high-temperature furnace.

6. A type of magnesium sintering pellet, characterized in that, Obtained by any one of the preparation methods according to claims 1 to 5.

7. A magnesium smelting method using the magnesium sintering pellets as described in claim 6 as raw materials.

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