Method for preparing magnesium oxide by calcining magnesite

Abstract

The invention provides a method for preparing magnesium oxide by calcining magnesite, which comprises the following steps: introducing fuel into a decomposer or into the decomposer and an activity regulator, igniting, and discharging generated high-temperature flue gas after flowing through a multi-stage preheating unit, a drying unit and a tail gas separation unit, feeding a water-containing magnesite raw material into a dryer to be in contact with hot flue gas from a preheater so as to remove moisture; the dried material passes through a multi-stage preheater and is subjected to countercurrent flow heat exchange with high-temperature flue gas from a decomposer, so that the temperature of the material is increased; the preheated material is heated to the decomposition temperature in a decomposer, and magnesium carbonate is decomposed to generate magnesium oxide; separating the decomposed gas-solid mixture to obtain a high-temperature magnesium oxide solid; and performing countercurrent flow heat exchange on the high-temperature magnesium oxide solid and combustion-supporting air, and outputting cooled magnesium oxide. Through multi-stage countercurrent flow heat exchange and product sensible heat recovery, an efficient heat energy circulation system is constructed, and the fuel energy consumption in the calcination process is remarkably reduced; the method realizes flexible regulation and control of magnesium oxide product activity, and is suitable for continuous clean production of fine particle raw materials.

Description

Technical Field

[0001] This invention relates to the field of magnesium oxide preparation technology, and more particularly to a method for preparing magnesium oxide from magnesite. Background Technology

[0002] For decades, the production of light-calcined magnesite oxide from magnesite using high-temperature calcination has relied on the traditional process of using a water-gas generator combined with a reverberatory furnace or a vertical kiln. This outdated process has resulted in low production efficiency, poor product quality, and significant energy waste. Due to incomplete combustion, the vertical kiln's thermal efficiency is only 26.47%, and the flue gas temperature exceeds 230°C. A large amount of heat is lost with the dense smoke. The temperature of the light-calcined magnesite powder after calcination reaches 800°C. This high-temperature product releases energy into the environment through natural cooling, resulting in substantial energy waste annually due to the inability to recover sensible heat. When a reverberatory furnace is used for light-calcining magnesite, the heat consumption reaches 6.63 GJ / t-MgO, and the quality of the light-calcined MgO powder is unstable, with dust concentrations in the exhaust gas exceeding 500 mg / Nm³. 3 Even as high as 1000 mg / Nm 3 The above describes a system with a labor productivity of only 1100 tons per person per year, low automation, and is a very outdated light-calcination system and method. In summary, the currently used traditional calcination technology for magnesite is outdated, energy-intensive, uses only large-particle raw materials, has a slow calcination reaction rate, makes product quality difficult to control, and employs an intermittent operation mode, resulting in significant dust pollution. Therefore, how to reduce energy consumption during the roasting process and how to control product quality during roasting are urgent problems that need to be solved by those skilled in the art.

[0003] A patent published on June 28, 2019, with publication number CN109942210A, discloses a method for preparing highly active magnesium oxide from magnesite, including the following steps: crushing, grinding, two-stage roasting, and magnetic separation. This invention employs a neutral-reduction two-stage roasting process, with both stages conducted at relatively low temperatures. This ensures that magnesite decomposes into amorphous MgO under low-temperature roasting conditions. Magnesite is first roasted under neutral conditions to achieve partial decomposition, and MgCO3 undergoes incomplete decomposition to produce CO. This CO is used in the subsequent roasting stage as a reducing gas to reduce weakly magnetic iron minerals in the magnesite to strongly magnetic iron minerals, which are then removed by magnetic separation. During the reduction roasting process, undecomposed magnesite continues to decompose. After two stages of low-temperature roasting, the magnesite is finally completely decomposed, avoiding the rapid formation of MgO crystals under high-temperature conditions. However, this method generates high-temperature flue gas that is directly emitted or naturally cooled, resulting in significant energy waste and high energy consumption. Furthermore, this method, through a specific low-temperature two-stage process, can only produce products with fixed ultra-high activity (13-15 seconds), which is difficult to meet the diverse market demand for magnesium oxide activity. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method for preparing magnesium oxide by calcining magnesite, which solves the problems of high energy consumption and the inability to obtain only fixed ultra-high activity products in the existing magnesium oxide preparation process.

[0005] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0006] A method for preparing magnesium oxide by calcining magnesite includes the following steps: S1. System preheating: Fuel is introduced into the decomposer and ignited, and the generated high-temperature flue gas flows sequentially through a multi-stage preheating unit, a drying unit, and a tail gas separation unit before being discharged; S2. Raw material drying: Hydrated magnesite raw material is fed into a dryer and comes into contact with the hot flue gas from the preheater to remove moisture; S3. Multi-stage preheating: The dried material is sequentially passed through a multi-stage preheater and exchanged with the high-temperature flue gas from the decomposer in a countercurrent manner to raise the material temperature; S4. Decomposition reaction: The preheated material is heated to the decomposition temperature in the decomposer to decompose magnesium carbonate into magnesium oxide; S5. Product separation: The decomposed gas-solid mixture is separated to obtain high-temperature magnesium oxide solid; S6. Product cooling and waste heat recovery: The obtained high-temperature magnesium oxide solid is exchanged with combustion air in a countercurrent manner, and the cooled magnesium oxide is output as a product, while the heated air is used as combustion air.

[0007] Furthermore, in step S6, the combustion air is preheated to 400-550°C.

[0008] Furthermore, the citric acid color development time of the final magnesium oxide product is 50-90 seconds.

[0009] Furthermore, in step S1, fuel is introduced into the activity regulating unit and ignited; in step S3, the dried material undergoes multi-stage preheating and countercurrent heat exchange with high-temperature flue gas from the activity regulator; after step S5 and before step S6, an activity regulating step is also included: the separated high-temperature magnesium oxide solid is heated to 1100-1200℃ in the activity regulator and held for 5-30 seconds.

[0010] Further, in step S1, air and natural gas are introduced into the decomposer and the activity regulator, respectively; the magnesite raw material has a particle size of 74-106 μm and a moisture content of 8-12%.

[0011] Furthermore, in step S2, the temperature of the material after moisture removal is 80-100℃; in step S3, the temperature of the material reaches 600-700℃ after multi-stage preheating.

[0012] Furthermore, in step S4, the decomposition temperature is 800-900℃.

[0013] Furthermore, in step S5, the temperature of the separated high-temperature magnesium oxide solid is 800-850℃; in step S6, the magnesium oxide product is cooled to below 80℃, and the combustion air is preheated to above 400℃.

[0014] Furthermore, the citric acid color development time of the final magnesium oxide product is 120-150 seconds.

[0015] Furthermore, it also includes step S7: removing and purifying the dust-laden flue gas separated in step S2 and / or step S5, and returning the collected magnesium-containing dust to the preheating or decomposition process.

[0016] The beneficial effects of this invention are:

[0017] 1. This invention utilizes a two-stage thermal energy comprehensive utilization network of multi-stage countercurrent preheating of raw materials and waste heat recovery from product cooling to maximize the use of sensible heat from high-temperature flue gas and high-temperature products for preheating raw materials and combustion air. The system exhaust temperature is reduced to 88°C, the product discharge temperature is below 60°C, and energy consumption is significantly reduced.

[0018] 2. This invention, through an optional active adjustment unit, can precisely control the calcination temperature and residence time of the product, and can stably produce magnesium oxide products of different activity levels with flexibly adjustable citric acid color development time within the range of 50-150 seconds in the same system, meeting diversified market demands and overcoming the shortcomings of existing technologies such as single product activity and limited application range.

[0019] 3. This invention, by operating the system in a fully enclosed and continuous state, combined with efficient dust removal measures, can significantly reduce dust emissions during the production process;

[0020] 4. This invention uses conventional fuels such as natural gas for open flame combustion. Compared with existing technologies that rely on "incomplete decomposition to produce flammable and explosive CO gas and then recycling it", the process of this invention is safe, stable, and easy to control, and is more suitable for large-scale continuous industrial production.

[0021] 5. The method of the present invention is an automated process of continuous feeding and continuous production, which has high labor productivity. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: A method for preparing magnesium oxide by calcining magnesite, used to prepare highly active magnesium oxide with a citric acid color development time of 120-150 seconds.

[0024] The specific steps are as follows:

[0025] S1. System Preheating and Start-up: First, air is introduced into the decomposer and activation regulator via a blower. The air flow rate in the decomposer is 31056 Nm³ / h, and the air flow rate in the activation regulator is 7764 Nm³ / h. Subsequently, natural gas is introduced into the decomposer and activation regulator at flow rates of 3000 Nm³ / h and 478 Nm³ / h, respectively. Ignition ensures stable combustion of the fuel, producing high-temperature flue gas. The system induced draft fan is started, causing the high-temperature flue gas to flow sequentially through a four-stage preheater, dryer, and exhaust gas separator before being discharged, completing the overall system preheating.

[0026] S2. Raw Material Drying and Preliminary Separation: Magnesite raw material with a moisture content of 10% and a particle size of 74-106μm is continuously fed into the Venturi dryer via a feeder at a flow rate of 61674 kg / h and a temperature of 30℃. Inside the dryer, the raw material directly contacts and efficiently exchanges heat with hot flue gas from the primary preheater at approximately 277℃, removing most of the free moisture. The dried material, after moisture removal, is discharged from the bottom of the dryer at a rate of 55507 kg / h and a temperature of 88℃. The flue gas carrying moisture and a small amount of fine material powder enters the tail gas separator for gas-solid separation. In this embodiment, 88°C gas carrying 6167 kg / h of solids is discharged from the top of the tail gas separator and sent to the gas filter by the induced draft fan. The filtered gas enters the flue gas exhaust fan for discharge. The 70°C gas collected in the gas filter is sent into the pipeline by the ash return device and heated to 660°C by the 811°C flue gas (containing 2948 kg / h of MgO solid material) from the product separator and sent into the four-stage preheating separator. The solid material enters the decomposer after passing through the four-stage preheating separator to undergo a decomposition reaction, completing the decomposition of magnesium carbonate into magnesium oxide and carbon dioxide.

[0027] S3. Multi-stage countercurrent preheating: The dry material flowing out from the bottom of the tail gas separator in step S2 is magnesite raw material after moisture removal at 88℃. This dry material sequentially enters the first, second, third, and fourth stage preheaters, where it undergoes countercurrent heat exchange with the high-temperature flue gas from the decomposer and activation regulator. The material temperature increases step by step, and is finally heated to approximately 647℃ before entering the decomposer.

[0028] S4. Decomposition reaction: The material, preheated to about 647°C, enters the decomposer and undergoes a magnesium carbonate decomposition reaction at 850°C: MgCO3→MgO+CO2, producing highly active magnesium oxide and carbon dioxide.

[0029] S5. Product Separation and Activity Regulation: The decomposed gas-solid mixture enters the cyclone separator, i.e., the product separator, where gas-solid separation occurs. The separated product is high-temperature magnesium oxide solid at approximately 811℃. A flow rate of approximately 26536 kg / h is discharged from the bottom of the separator and immediately sent to the activity regulator. In the activity regulator, the magnesium oxide is further heated to 1100℃ and held at this temperature for 5-10 seconds, completing the enhancement and stabilization of the product's activity.

[0030] S6. Product Cooling and Waste Heat Recovery: The high-temperature magnesium oxide product, after activity regulation, sequentially enters the primary and secondary coolers, where it undergoes counter-current heat exchange with ambient-temperature combustion air drawn in by the system's blower. The magnesium oxide product is gradually cooled, and the final discharge temperature drops to approximately 60°C before being sent to the finished product silo for storage. Simultaneously, the combustion air is heated to approximately 536°C and then, at a mass flow ratio of approximately 1:4, is delivered to the activity regulator and decomposer, respectively, to be mixed with natural gas for combustion and used as combustion air.

[0031] S7. Flue Gas Treatment and Fine Powder Recirculation: The high-temperature flue gas, approximately 811°C, exiting from the top of the product separator, flows sequentially through a four-stage, three-stage, two-stage, and one-stage preheater, a dryer, and a tail gas separator. After counter-current heat exchange with the raw materials, the temperature drops to approximately 88°C. This low-temperature flue gas then enters a bag filter for deep purification, achieving ultra-low dust emissions (concentration far below 500 mg / Nm³). The magnesium-containing fine powder collected by the bag filter is quantitatively returned to the system by a return ash fan. After being heated by the high-temperature flue gas from the product separator, it is sent to the preheating system to participate in subsequent reactions, achieving a near 100% comprehensive utilization rate of raw materials.

[0032] Results: The magnesium oxide product produced in this embodiment has an MgO purity of ≥97%. Detected using the citric acid colorimetric method, its color development time is 120-150 seconds, classifying it as a highly reactive product. Calculations show that producing 1 ton of this type of magnesium oxide product consumes approximately 131 standard cubic meters of natural gas, equivalent to about 160 kilograms of standard coal.

[0033] Example 2: A method for preparing magnesium oxide by calcining magnesite, used to prepare magnesium oxide with a citric acid color development time of 50-90 seconds. The specific steps are as follows:

[0034] S1. System Preheating and Start-up: Only air and natural gas are introduced into the decomposer for system preheating and operation. The air intake is 37981 Nm³ / h, and the natural gas flow rate is 3402 Nm³ / h. Ignition and combustion are initiated, and the induced draft system is started, allowing the high-temperature flue gas to flow sequentially through each stage of equipment to complete preheating.

[0035] S2. Raw material drying and S3. Multi-stage countercurrent preheating: The raw material drying and multi-stage preheating steps are the same as in Example 1. Magnesite raw material with a water content of 10% is dried and preheated in four stages of countercurrent flow, and then enters the decomposer at a temperature of about 647°C.

[0036] S4. Decomposition reaction: The material is heated to 850°C in the decomposer to complete the decomposition reaction of magnesium carbonate.

[0037] S5. Product Separation: The decomposed gas-solid mixture is separated by a cyclone separator to directly obtain high-temperature magnesium oxide solid (approximately 811°C, flow rate approximately 26536 kg / h). This embodiment does not use an activity regulator.

[0038] S6. Product Cooling and Waste Heat Recovery: The separated high-temperature magnesium oxide solid directly enters the primary and secondary coolers for countercurrent heat exchange with the combustion air. The product is stored after being cooled to approximately 60°C. The combustion air is heated to approximately 404°C and then entirely sent to the decomposer for use as combustion air.

[0039] S7. Flue gas treatment and fine powder circulation: This step is the same as S7 in Example 1, to achieve flue gas purification and fine powder circulation.

[0040] Results: The magnesium oxide product produced in this embodiment has an MgO purity of ≥97% and a citric acid color development time of 50-90 seconds. Calculations show that producing 1 ton of this type of magnesium oxide product consumes approximately 128 standard cubic meters of natural gas, equivalent to approximately 156 kilograms of standard coal.

[0041] Comparative Example

[0042] The conventional vertical kiln calcination process, well-known in the art and mentioned in the background of this invention, is used as a comparative example. This process uses lumpy magnesite as raw material and performs intermittent calcination in a vertical kiln at approximately 800-1000°C. The temperature of the calcined product reaches approximately 800°C, and a large amount of sensible heat is lost to the environment through natural cooling; simultaneously, the flue gas emitted from the kiln tail also reaches a temperature of over 230°C and is directly discharged into the atmosphere, resulting in significant heat waste. This process has low thermal efficiency (approximately 26.47%), and the energy consumption per ton of magnesium oxide produced is far higher than that of this invention, and the product activity is unstable, causing severe dust pollution.

[0043] The main technical and economic indicators of Examples 1 and 2 are compared with those of the comparative example as follows:

[0044] Comparison Projects Example 1 Example 2 Comparative Example Product activity (citric acid color development time) 120-150 seconds (high activity) 50-90 seconds Unstable, usually long Fuel consumption (standard coal / ton of product) ~160kg ~156kg Far exceeding the industry standard of 175kg for similar products Smoke temperature ~88℃ ~88℃ >230℃ Product discharge temperature ~60℃ ~60℃ ~800℃ Raw material adaptability Fine particles (74-106μm) Same as Example 1 Large-particle ore only Dust control Baghouse dust collector, ultra-low emissions Same as Example 1 Serious unorganized emissions Production continuity Continuous automated production Same as Example 1 Intermittent operation

[0045] As shown in the table above, the method provided by this invention constructs a highly efficient thermal energy circulation network through multi-stage countercurrent preheating to recover sensible heat from flue gas and product cooling to recover sensible heat and preheat combustion air. This results in significantly lower fuel consumption than the industry benchmark (175 kg standard coal / ton) for both high-activity and ordinary-activity products, solving the core problem of extremely high energy consumption in traditional processes. Furthermore, this invention achieves flexible and controllable product activity, efficient utilization of fine particulate raw materials, and continuous and clean production, representing comprehensive technological advancements.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modifications to the technical solutions described in the foregoing embodiments, or equivalent substitutions of some or all of the technical features thereof, within the spirit and principles of the present invention, do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for preparing magnesium oxide by calcining magnesite, characterized in that, Includes the following steps: S1. System preheating: Fuel is introduced into the decomposer and ignited. The resulting high-temperature flue gas flows through a multi-stage preheating unit, a drying unit, and a tail gas separation unit before being discharged. S2. Raw Material Drying: The hydrated magnesite raw material is fed into a dryer and comes into contact with hot flue gas from the preheater to remove moisture. S3. Multi-Stage Preheating: The dried material is passed sequentially through a multi-stage preheater, where it exchanges heat counter-currently with high-temperature flue gas from the decomposer, raising the material temperature. S4. Decomposition Reaction: The preheated material is heated to its decomposition temperature in the decomposer, causing magnesium carbonate to decompose into magnesium oxide. S5. Product Separation: The decomposed gas-solid mixture is separated to obtain high-temperature magnesium oxide solid. S6. Product Cooling and Waste Heat Recovery: The obtained high-temperature magnesium oxide solid undergoes counter-current heat exchange with combustion air. The cooled magnesium oxide is output as a product, while the heated air is used as combustion air.

2. The method for preparing magnesium oxide by calcining magnesite according to claim 1, characterized in that, In step S1, fuel is introduced into the activity regulating unit and ignited; in step S3, the dried material is preheated through multiple stages and also exchanged heat countercurrently with the high-temperature flue gas from the activity regulator; after step S5 and before step S6, an activity regulating step is also included: the separated high-temperature magnesium oxide solid is heated to 1100-1200℃ in the activity regulator and held for 5-30 seconds.

3. The method for preparing magnesium oxide by calcining magnesite according to claim 2, characterized in that, In step S1, air and natural gas are introduced into the decomposer and the activity regulator, respectively; the magnesite raw material has a particle size of 74-106 μm and a moisture content of 8-12%.

4. The method for preparing magnesium oxide by calcining magnesite according to claim 3, characterized in that, In step S2, the temperature of the material after moisture removal is 80-100℃; in step S3, the temperature of the material reaches 600-700℃ after multi-stage preheating.

5. The method for preparing magnesium oxide by calcining magnesite according to claim 4, characterized in that, In step S4, the decomposition temperature is 800-900℃.

6. The method for preparing magnesium oxide by calcining magnesite according to claim 5, characterized in that, In step S5, the temperature of the separated high-temperature magnesium oxide solid is 800-850℃; in step S6, the magnesium oxide product is cooled to below 80℃, and the combustion air is preheated to above 400℃.

7. The method for preparing magnesium oxide by calcining magnesite according to any one of claims 2-6, characterized in that, The final magnesium oxide product has a citric acid color development time of 120-150 seconds.

8. The method for preparing magnesium oxide by calcining magnesite according to claim 1, characterized in that, In step S6, the combustion air is preheated to 400-550°C.

9. The method for preparing magnesium oxide by calcining magnesite according to claim 1 or 8, characterized in that, The citric acid color development time of the final magnesium oxide product is 50-90 seconds.

10. The method for preparing magnesium oxide by calcining magnesite according to any one of claims 1-6 and 8, characterized in that, It also includes step S7: removing and purifying the dust-laden flue gas separated in step S2 and / or step S5, and returning the collected magnesium-containing dust to the preheating or decomposition process.