Solid heat carrier heating method for magnesite calcination

Abstract

This invention discloses a method for calcining magnesite using a solid heat carrier. The method employs magnesia as a specific solid heat carrier, and the process includes: magnesia pretreatment and heating, magnesite pretreatment, drying and preheating, calcination, separation of the solid heat carrier from the product, and waste heat recovery. The pretreated and heated magnesia is brought into full contact with the pretreated and preheated magnesite in the calcination reactor for heating. After calcination, the magnesia is separated from the product and recycled for further heating. Simultaneously, the waste heat from the recovered carbon dioxide and magnesium oxide is used to preheat the drying of the magnesite. This application proposes using a solid heat carrier for magnesite calcination, utilizing the homogeneity of magnesia and magnesium oxide to prevent the introduction of impurities, overcoming the limitations of conventional gaseous heat carriers with stringent airflow requirements, adapting to various types of reactors, and achieving a clean and efficient process through waste heat recovery.

Description

Technical Field

[0001] This invention relates to a method for calcining magnesite, specifically a method for calcining magnesite using a solid heat carrier. Background Technology

[0002] Magnesite resources are a national strategic resource, and its resource utilization industry guarantees the national demand for magnesium materials and products containing 8 million tons / year of magnesium. Calcination of magnesite is the core process for producing magnesium oxide, and it is divided into two methods according to the calcination temperature: light calcination (700-1000℃) and heavy calcination (1400-1800℃).

[0003] Currently, the commonly used industrial methods for calcining magnesite employ a gaseous heat carrier heating mode. This involves using high-temperature flue gas generated from fuel combustion as a gaseous heat carrier, forming a gas-solid two-phase system with the magnesite raw material, and achieving calcination through gas-solid heat exchange. Patents related to using flue gas as a heat carrier include Chinese Patent Application Nos. 201810734458.8, 201110049511.9, 201611101736.3, and 202411047363.0. Some patents also disclose using carbon dioxide generated during the magnesite decomposition process as a heat carrier for the calcination process, such as Chinese Patent Application Nos. 200810079443.9, 201910825735.0, and 202010841608.2. However, carbon dioxide is also a gaseous heat carrier.

[0004] According to heat balance calculations, the gaseous heat carrier needs a mass much larger than the magnesite raw material to meet the heat requirements of calcination. This results in the gas velocity not being too low and a relatively high gas velocity must be maintained to ensure sufficient heat transfer. This makes such processes generally difficult to control airflow and have high equipment loads. At the same time, it is difficult to adapt to the high-efficiency utilization requirements of flotation fine tailings. Summary of the Invention

[0005] The purpose of this invention is to provide a magnesite calcination method using a solid heat carrier. This method uses magnesia as a dedicated solid heat carrier, avoids the introduction of impurities from the source, is compatible with various types of reactors, solves many weaknesses of existing gas heat carrier calcination processes, and achieves a clean and efficient process through waste heat recovery and utilization, thus contributing to the upgrading of magnesite calcination technology.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for calcining magnesite using a solid heat carrier, wherein the solid heat carrier is used to provide heat during the calcination of magnesite to prepare magnesium oxide, the method utilizes the homogeneity between magnesia and magnesium oxide to prevent the introduction of impurities, and selects magnesia as a dedicated solid heat carrier. The specific steps of this method include: 1) Magnesia pretreatment and heating: Magnesia is pretreated to the particle size required by the heat carrier; the main component of the magnesia is magnesium oxide, which is homogeneous with the calcined magnesium oxide product; the pretreated magnesia solid heat carrier is sent into the heating device and heated to the target temperature. 2) Magnesite pretreatment: The magnesite raw material is crushed and screened to a particle size that meets the requirements for calcination; 3) Magnesite drying and preheating: The pretreated magnesite is sent to the drying and preheating unit, where the sensible heat of the subsequently recovered carbon dioxide and magnesium oxide products is used to dry and preheat the magnesite. 4) Calcination: The dried and preheated magnesite is fed into the calcination reactor; at the same time, the high-temperature magnesia solid heat carrier heated in step 1 is fed into the calcination reactor. The high-temperature magnesia solid heat carrier is in full contact with the magnesite raw material to exchange heat to the calcination temperature, providing heat for the calcination of magnesite. 5) Separation of solid heat carrier and product: After calcination, the solid heat carrier of magnesia and magnesium oxide are separated by means of sieving or gas purging, taking advantage of the difference in particle size. The solid heat carrier of magnesia is fed back into the heating device in step 1 and recycled after heating. The high-temperature carbon dioxide and magnesium oxide sensible heat are used for subsequent waste heat recovery. 6) Waste heat recovery and utilization: The separated carbon dioxide and magnesium oxide are exchanged with magnesite to recover waste heat for the drying and preheating process of magnesite.

[0008] In step 1, the particle size range of the magnesia heat carrier is 1-10 mm, preferably 1-3 mm; the heating method in step 1 includes, but is not limited to, fuel combustion heating or electric heating. In step 2, the particle size range of the magnesite raw material is 0-5 mm, preferably 0-1 mm.

[0009] Step 4 involves calcining magnesite, which includes light calcination and heavy calcination. The light calcination process corresponds to a calcination temperature of 700-1000℃, preferably 800-900℃, and a calcination time of, but not limited to, 1-120 minutes. The heavy calcination process corresponds to a calcination temperature of 1300-1800℃, preferably 1400-1700℃, and a calcination time of, but not limited to, 1-240 minutes. The magnesite calcination reactor in step 4 can be one of the following: a descending bed, a fluidized bed, or a conveying bed. The calcination reactor does not require strict control of the airflow parameters.

[0010] In step 5, the sieving and separation is achieved by a double-layer vibrating screen. The upper screen retains the large-diameter magnesia solid heat carrier, while the lower screen allows the small-diameter magnesia product to pass through. The gas purging separation includes, but is not limited to, blowing away magnesia and retaining the magnesia solid heat carrier by introducing one of air, carbon dioxide, or nitrogen.

[0011] In step 6, waste heat recovery and utilization are achieved using heat exchange tubes or heat exchange chambers. The carbon dioxide after heat exchange can be recovered, purified, and reused separately, while the magnesium oxide after indirect heat exchange is stored in the product storage tank.

[0012] The significant features and positive effects of this invention are: 1. This invention is the first to apply magnesia sand as a dedicated solid heat carrier in magnesite calcination, eliminating the introduction of impurities: This invention breaks with the current practice of using gaseous heat carriers in magnesite calcination, and for the first time applies solid heat carrier technology to magnesite calcination. Magnesia sand is selected as the dedicated solid heat carrier. Magnesia sand is homogeneous with magnesium oxide products, and even if it undergoes slight wear and pulverization due to long-term cyclic use, it will not introduce foreign impurities, thus eliminating impurity contamination at the source. This effectively ensures the purity and quality of magnesium oxide products, meets the production needs of high-purity magnesium oxide, and solves the industry pain point of flue gas impurities contaminating products in existing gaseous heat carrier calcination processes.

[0013] 2. This invention avoids the gas velocity problem caused by the gas-solid two-phase structure of the gas heat carrier, and has strong adaptability and simple operation: This invention uses magnesia solid heat carrier for heating, which does not require the formation of a gas-solid two-phase system with the raw materials, and has no strict requirements on the airflow, greatly reducing the difficulty of operation and the load on equipment. This solid heat carrier can be adapted to various types of reactors such as descending bed, fluidized bed, and conveying bed, enhancing process flexibility and adapting to production scenarios of different scales and product requirements.

[0014] 3. This invention utilizes waste heat recovery to achieve a clean, efficient, and energy-saving process: This invention recovers the sensible heat of carbon dioxide and magnesium oxide products for use in the drying and preheating process of magnesite, making full use of waste heat resources and further reducing process energy consumption, which meets the current development needs of the magnesite calcination industry for low-carbon, energy-saving, clean, and efficient development.

[0015] 4. The present invention features a closed-loop circulation of magnesia heat carrier, which reduces costs: The solid magnesia heat carrier can be used in a closed-loop cycle. During the circulation process, the solid heat carrier can be reused through simple separation. The operation is convenient and suitable for large-scale industrial applications.

[0016] 5. The process of this invention is simple and reliable: the pretreatment, heating, separation, and circulation of magnesia solid heat carrier, as well as the waste heat recovery and gas-solid separation processes, all adopt mature industrial technologies. The operation is convenient and stable, and it can be quickly promoted for industrialization, which will help upgrade the calcination technology of magnesite. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process flow of Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the process flow of Embodiment 2 of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0019] Example 1 A method for calcining magnesite using a solid heat carrier, such as... Figure 1 As shown, the specific steps for preparing active magnesium oxide using the light calcination process of magnesite are as follows: 1. Magnesia pretreatment and heating: Industrial grade magnesia is selected as the dedicated solid heat carrier. It is crushed and screened to a particle size of 1-2 mm to remove impurity particles and set aside. The pretreated magnesia solid heat carrier is sent into a heating furnace and heated by natural gas combustion. The heating temperature is controlled at 850-900℃. After the magnesia heat carrier is heated to the target temperature, it is kept at the temperature for later use.

[0020] 2. Magnesite pretreatment: Crush and screen the magnesite raw material to a particle size of 100-150μm, remove impurities and set aside for use.

[0021] 3. Magnesite drying and preheating: The pretreated magnesite is sent into a drying and preheating chamber. The sensible heat of the subsequently recovered carbon dioxide and magnesium oxide products is used to dry and preheat the magnesite, removing the moisture content to ≤1% and raising the temperature to 300-350℃, thereby reducing the heat consumption during the calcination process.

[0022] 4. Calcination: The dried and preheated magnesite is fed into a fluidized bed calcination reactor; simultaneously, the heated high-temperature magnesia solid heat carrier is fed into the fluidized bed reactor, and an appropriate amount of air is introduced to fluidize and suspend the magnesite fine powder. Due to its larger particle size, the magnesia solid heat carrier maintains an aggregated state, allowing for full contact and heat exchange with the fluidized magnesite fine powder. Compared to gas heat carriers, which require maintaining a high gas velocity, in this embodiment, the magnesia solid heat carrier does not require controlling the gas velocity or strictly controlling the airflow intensity. The temperature inside the reactor is controlled at 750-800℃, and the calcination time is 15-30 minutes. Through the heat transfer of the magnesia heat carrier, the magnesite is decomposed by calcination to generate active magnesium oxide.

[0023] 5. Separation of solid heat carrier and product: After calcination, the fluidizing gas and the generated carbon dioxide form a mixed gas flow, which blows away the generated magnesium oxide fine powder. The powder enters the separation equipment, such as a cyclone separator, through the top outlet of the reactor via a sealed pipeline. The carbon dioxide and magnesium oxide fine powder are separated for subsequent waste heat recovery. Due to its larger particle size, the magnesia solid heat carrier remains in the fluidized bed, achieving efficient separation of magnesia and product. The magnesia solid heat carrier is then fed back into the heating furnace, heated to 850-900℃, and then recycled.

[0024] 6. Heat exchange: The separated carbon dioxide and magnesium oxide powder are transported to the drying and preheating chamber and exchanged with the pretreated magnesite to recover waste heat and provide heat for drying and preheating of the magnesite. The carbon dioxide after heat exchange is collected and used separately, and the magnesium oxide powder is cooled and sealed for storage to obtain the active magnesium oxide product with a purity of ≥98%.

[0025] Example 2 A method for calcining magnesite using a solid heat carrier, such as... Figure 2 As shown, the specific steps for preparing active magnesium oxide using the light calcination process of magnesite are as follows: 1. Magnesia pretreatment and heating: Industrial grade magnesia is selected as the exclusive solid heat carrier. It is crushed and screened to a particle size of 2-3 mm to remove impurity particles and set aside. The pretreated magnesia solid heat carrier is sent into a heating furnace and electrically heated. The heating temperature is controlled at 850-900℃. After the magnesia heat carrier is heated to the target temperature, it is kept at the temperature for later use.

[0026] 2. Magnesite pretreatment: Crush and screen the magnesite raw material to a particle size of 0.4-0.8 mm, remove impurities and set aside for use.

[0027] 3. Magnesite drying and preheating: The pretreated magnesite is sent into a drying and preheating chamber. The sensible heat of the subsequently recovered carbon dioxide and magnesium oxide products is used to dry and preheat the magnesite, removing the moisture content to ≤1% and raising the temperature to 300-350℃, thereby reducing the heat consumption during the calcination process.

[0028] 4. Calcination: The dried and preheated magnesite is fed into a descending bed light calcination reactor; simultaneously, the heated high-temperature magnesia solid heat carrier is fed into the descending bed reactor. The magnesia heat carrier and the magnesite raw material move synchronously from top to bottom, ensuring full contact and heat exchange. Compared to gas heat carriers, which require maintaining a high gas velocity, in this embodiment, the magnesia solid heat carrier does not require controlling the gas velocity or strictly controlling the airflow intensity. The temperature inside the reactor is controlled at 750-800℃, and the calcination time is 20-35 minutes. Through the heat transfer of the magnesia heat carrier, the light calcination decomposition of magnesite is completed, generating active magnesium oxide.

[0029] 5. Separation of solid heat carrier and product: After calcination, carbon dioxide in the reactor is directly collected for subsequent waste heat recovery. The remaining solid material (magnesia solid heat carrier + magnesium oxide) is discharged from the bottom of the descending bed by gravity and sent to the screening device. Utilizing the particle size difference between magnesia (particle size 2-3 mm) and magnesium oxide (particle size 0.4-0.8 mm), the two are efficiently separated by screening. The magnesia solid heat carrier is sent back to the heating furnace, heated to 850-900℃ and then recycled. The magnesium oxide is used for subsequent waste heat recovery.

[0030] 6. Heat exchange: The separated carbon dioxide and magnesium oxide are transported to the drying and preheating chamber and exchanged with the pretreated magnesite to recover waste heat and provide heat for drying and preheating the magnesite. The carbon dioxide after heat exchange is collected and used separately. The magnesium oxide powder is cooled and sealed for storage to obtain the active magnesium oxide product with a purity of ≥98%.

[0031] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calcining magnesite using a solid heat carrier, characterized in that: The method uses magnesite calcination as the solid heat carrier to heat the process of preparing magnesium oxide. It utilizes the homogeneity between magnesia and magnesium oxide to prevent the introduction of impurities, and selects magnesia as the exclusive solid heat carrier. The specific preparation steps are as follows: 1) Magnesia pretreatment and heating: Magnesia is pretreated to the particle size required by the heat carrier; Magnesia is composed of magnesium oxide, which is homogeneous with the calcined magnesium oxide product; The pretreated magnesia solid heat carrier is sent to the heating device and heated to the target temperature. 2) Magnesite pretreatment: The magnesite raw material is crushed and screened to a particle size that meets the requirements for calcination; 3) Magnesite drying and preheating: The pretreated magnesite is sent to the drying and preheating unit, where the sensible heat of the subsequently recovered carbon dioxide and magnesium oxide products is used to dry and preheat the magnesite. 4) Calcination: The dried and preheated magnesite is fed into the calcination reactor; at the same time, the high-temperature magnesia solid heat carrier heated in step 1 is fed into the calcination reactor. The high-temperature magnesia solid heat carrier is in full contact with the magnesite raw material to exchange heat to the calcination temperature, providing heat for the calcination of magnesite. 5) Separation of solid heat carrier and product: After calcination, the solid heat carrier of magnesia is separated from the product by means of sieving or gas purging, taking advantage of the difference in particle size between the solid heat carrier of magnesia and magnesium oxide. The solid heat carrier of magnesia is fed back into the heating device in step 1) and recycled after heating. The high-temperature carbon dioxide and magnesium oxide sensible heat are used for subsequent waste heat recovery. 6) Waste heat recovery and utilization: The separated carbon dioxide and magnesium oxide are exchanged with magnesite to recover waste heat for the drying and preheating process of magnesite.

2. The method for calcining magnesite using a solid heat carrier according to claim 1, characterized in that, The particle size range of the magnesia heat carrier in step 1) is 1-10 mm, preferably 1-3 mm; the heating method in step 1) includes fuel combustion heating or electric heating.

3. The method for calcining magnesite using a solid heat carrier according to claim 1, characterized in that, In step 2), the particle size range of the magnesite raw material is 0-5 mm, preferably 0-1 mm.

4. The method for calcining magnesite using a solid heat carrier according to claim 1, characterized in that, In step 4), the calcination of magnesite includes light calcination and heavy calcination. The calcination temperature for the light calcination process is 700-1000℃, preferably 800-900℃, and the calcination time is 1-120 minutes. The calcination temperature for the heavy calcination process is 1300-1800℃, preferably 1400-1700℃, and the calcination time is 1-240 minutes. In step 4), the magnesite calcination reactor can be one of a descending bed, a fluidized bed, or a conveying bed.

5. The method for calcining magnesite using a solid heat carrier according to claim 1, characterized in that, In step 5), the sieving separation is achieved by a double-layer vibrating screen. The upper screen retains the large-diameter magnesium oxide solid heat carrier, while the lower screen allows the small-diameter magnesium oxide product to pass through. The gas purging separation is achieved by introducing air, or one of carbon dioxide or nitrogen, to blow away the magnesium oxide and retain the magnesium oxide solid heat carrier.

6. The method for calcining magnesite using a solid heat carrier according to claim 1, characterized in that, In step 6), waste heat recovery and utilization are achieved by heat exchange tubes or heat exchange chambers. The carbon dioxide after heat exchange is recovered, purified and reused separately, and the magnesium oxide after indirect heat exchange is stored in the product storage tank.

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