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Preparation process and performance comparison of high-purity magnesia

Hebei Messi Biology Co., Ltd. stated that the preparation of high-purity magnesia uses magnesium-containing compounds as raw materials. Some are made into light-burned magnesium oxide and then electrofused or re-fired to obtain high-purity magnesia. Some are directly calcined or pyrolyzed to obtain high-purity magnesia.

Hydromagnesite resources in Naqu, Tibet

Preparation of high-purity magnesite from magnesite

(1) Magnesite direct calcination method

This method can be divided into two-step calcination and one-step calcination. The second-step calcination uses high-quality magnesite. First, the first step is calcined in a calcining furnace. The temperature is controlled at about 1273K to generate light-burned magnesium oxide. It is then mechanically crushed and ball-milled. During the ball-milling process, “pseudomorphs” can be eliminated. phenomenon, and at the same time reduce the particle size of magnesium oxide powder as much as possible. The magnesium oxide powder is pressed and then sintered at a controlled temperature to obtain the final product. This method has wide sources of raw materials, simple process and low production cost. However, since the impurities contained in magnesite are difficult to separate, the purity of magnesia is generally difficult to reach above 99%. However, the purity of the product can be improved by selecting high-purity raw materials and improving the impurity removal methods of raw materials. Magnesite and graphite powder are mixed and roasted into light-burned magnesia powder. The light-burned powder is then mixed with rare earth magnesium alloy and evenly pressed into balls, and then sintered at high temperature to form high-purity magnesia. This process can produce high-purity magnesia with a purity greater than 99.8% and a volume density greater than 3.50g/cm3.

Compared with the two-step calcination, the one-step calcination cancels the light burning activation step and directly sinters high-purity magnesia. A production process that utilizes magnesite concentrate to roast high-purity magnesia in one step, including a mixed ball press unit and a high-temperature firing unit. After the magnesite concentrate and binder are fully mixed in the mixer, they enter the briquetting machine and are pressed into high-density pellets. The pellets are then roasted in a high-temperature shaft kiln in one step to obtain magnesium oxide with a mass fraction of ≥97.5%. Magnesia with density ≥3.20g/cm3.

(ii) Magnesite ammonium leaching method

Lightly burned magnesium oxide obtained by calcining low-grade magnesite is reacted with ammonium chloride solution. The ammonia produced in the reaction is absorbed by pure water. After the reaction, impurities will remain in the slag. After the solution and waste residue are separated, the leachate reacts directly with the recovered ammonia water to form magnesium hydroxide. The magnesium hydroxide is calcined in two steps to obtain magnesia. In this process, the ammonium chloride mother liquor can be recycled. Magnesia with a MgO content of 99.97% and a density of 3.41g/cm3 can be produced through the ammonium leaching method. The main reactions of this method are reaction formulas (1) to (4). The magnesia obtained by this method has high purity and good performance, but the process flow is long and the operation is difficult in actual production.

(iii) Magnesite carbonization method

Solid minerals such as magnesite are calcined into powder and then digested, and then CO₂ is introduced for carbonization. After filtration, it is pyrolyzed to precipitate basic magnesium carbonate, which is then dehydrated and dried to produce light magnesium carbonate, which is then lightly burned to obtain light magnesium oxide. High-purity magnesia is obtained after the compact is burned. Zhang Koning et al. used the carbonization method to obtain high-purity magnesia with a MgO mass fraction of 99.21% and a volume density of 3.38g/cm³. The carbonization method has the advantages of strong selectivity, non-corrosiveness, high recovery rate, wide source of raw materials, and easy recycling. However, there are still shortcomings such as large equipment investment and long production process. The main reaction processes are shown in formulas (5) to (8).

(4) Magnesite hydration method

Magnesite is calcined and decomposed into magnesium oxide, and then the magnesium oxide is completely hydrated into magnesium hydroxide. After fine grinding-calcination-fine grinding-forming-sintering, sintered magnesia with a volume density of 3.47g/cm³ is obtained. This method is simple and easy to implement, but requires high quality of raw materials.

(5) Magnesite hydrochloric acid acidolysis method

Mix magnesite and magnesium sulfate aqueous solution, then use a ball mill to wet-grind to remove calcium. After acid leaching with hydrochloric acid, magnesite slurry is obtained. After the pH reaches 6~7, it is filtered. The filtrate is concentrated in the pre-concentrator and then sent to the hydrolysis and calcining furnace for hydrolysis and calcining at a temperature of 700~800°C. The hydrolyzed and calcined material is pelletized and sent to a rotary calcining kiln for calcining. After calcining, high-purity magnesia is obtained. This method is mainly used to process low-grade magnesite, and can prepare high-grade magnesite with a magnesium oxide content greater than 99% and a bulk density greater than 3.40g/m³. This process has a wide range of raw materials and can improve resource utilization, but it has high energy consumption, large equipment investment, is prone to corrosion, and has a long production process.

Brine precipitation method

Preparing high-purity magnesia Using brine or seawater to prepare high-purity magnesia, you must first obtain light magnesium oxide. A precipitant can be added to seawater or brine, and impurity ions can be removed through washing and chemical refining methods to ensure the purity of the obtained basic magnesium carbonate or magnesium hydroxide. The purity of the high-purity magnesia ultimately obtained by this method can reach 99.9 %above. Depending on the precipitant, these methods include brine soda ash method, brine ammonium carbonate method, brine lime method and brine ammonia method, etc.

Hard Burned Magnesium Oxide

(1) Brine soda ash method

This method is the earliest method for producing light magnesium oxide in my country. First, bittern or other magnesium chloride solution and sodium carbonate are reacted to form basic magnesium carbonate precipitate. After separation and light burning, the final product is obtained through high-temperature calcination. The main reactions are as shown in formula (9) and formula (10).

Brine is pretreated with hydrogen peroxide or sodium hypochlorite to remove iron, manganese and other impurities, which can improve product quality. This method is easy to operate and the raw materials are simple to purify. The generated precipitate is easier to filter than magnesium hydroxide. The prepared magnesium oxide has high purity. However, the consumption of soda ash is large and the price is high. The by-product sodium chloride has a small added value and a high cost.

Using the brine soda ash method and using coconut shell activated carbon as the second phase additive, sintered magnesia with a bulk density higher than 3.4g/cm³ and a purity higher than 98.5% was prepared. The study found that adding a trace amount of nano-magnesia reagent during the sample preparation process can increase the volume density of magnesia and reduce the porosity.

(ii) Brine ammonium carbonate method

After pretreatment, the brine undergoes metathesis reaction with ammonium carbonate, ammonium bicarbonate or carbonized ammonia water, and under suitable conditions generates basic magnesium carbonate precipitate with larger particles and easy to be filtered and washed. After calcination to obtain light magnesium oxide, it is ball milled and pressed into tablets, and finally sintered in a high-temperature sintering furnace to obtain the product. There is a small amount of sodium chloride and a large amount of ammonium chloride in the mother liquor of filtered magnesium carbonate. After filtration, the chloride can be separated. Sodium, ammonium chloride can be crystallized and recovered after cooling. The reaction process is shown in formula (11) to formula (15).

The innovative point of this method is that it can introduce CO₃²- to form a precipitate without introducing other impurity ions. However, the concentration of free ammonium in the entire process reaction system is high, leading to serious environmental pollution and other problems. Although ammonium bicarbonate is cheaper than soda ash, its cost is still high.

(iii) Brine lime method

This method uses brine or seawater as the raw material, and uses lime or dolomitic lime as the precipitant. The generated Mg(OH)₂ precipitate is lightly burned at a lower temperature to obtain magnesium oxide powder, which is then pelletized and sintered under a certain pressure to produce magnesia. The main reactions are shown in formulas (16) to (19).

The raw materials of the lime method are widely available, cheap and low in cost. If the by-product CaCl₂ can be fully utilized, the total cost will be lower. However, in production, the lime method has shortcomings such as poor magnesium hydroxide filtration performance, high energy consumption, high requirements for lime activity, and low product purity. It is usually not suitable for preparing high-purity MgO. In addition, this method produces a large amount of CaCl₂ by-product. If it cannot be effectively utilized, new industrial waste will be generated. This method is suitable for treating low-concentration brine (such as seawater). It is a common technical route used in the world to produce magnesium oxide using seawater as raw material. Japan, the United States, the United Kingdom, the Netherlands, Mexico and other countries have all established production plants using this method.

(IV) Brine ammonia method

Ammonia is used to react with refined brine to form magnesium hydroxide precipitate. The reaction product is filtered to obtain magnesium hydroxide precipitate, and the magnesium hydroxide is washed, dried, and calcined to obtain a product. The advantages of this method are that the particle size of the obtained magnesium hydroxide can be controlled, the precipitation speed is fast, and it is easy to filter and wash. The disadvantage is that the yield is low and the ammonia recovery rate is low. Because after adding ammonia water, the system quickly forms a buffer system of NH³-NH4Cl, which keeps the pH of the system at around 9.0 for a long time. Mg²⁺ is not easy to precipitate completely, and the consumption of ammonia water is large. If the ammonia recovery rate can be increased, the production cost can be greatly reduced.

Using chlorite from Qalhan Salt Lake in Qinghai as raw material, high-purity magnesia with a MgO mass fraction >99.5% and an apparent density of 3.55g/cm³ was prepared based on the ammonia method. A reverse feeding and slurry part was developed. The Mg(OH)₂ prepared by the new process has high precipitation purity, good filterability and low moisture content of the filter cake. It is an excellent precursor for preparing high-purity magnesia.

Preparation of high-purity magnesia by direct pyrolysis of brine

The main principle of the pyrolysis method is as follows. Magnesium chloride exists in the form of magnesium chloride hexahydrate at room temperature. When the temperature gradually increases, magnesium chloride begins to lose crystal water and is subsequently hydrolyzed, finally generating MgO and HCl gas.

㈠Aman method

The Aman method is a typical process for the direct pyrolysis of magnesium chloride hydrate to produce high-purity MgO. The process flow of this method is to concentrate the brine after potassium extraction to a certain concentration and directly spray it into the Aman reactor for pyrolysis. The pyrolysis product is crude magnesium oxide; a multi-stage water washing method can be used to remove incomplete components contained in the crude magnesium oxide. Impurities such as sodium chloride, calcium chloride and potassium chloride are decomposed, and the crude magnesium oxide is completely hydrated to form magnesium hydroxide; the filtered magnesium hydroxide filter cake is calcined, pressed into balls, and then reprocessed in a shaft kiln. By calcining, high-purity magnesia with a purity greater than 99% and a density greater than 3.40g/cm³ can be sintered. After the calcined exhaust gas is absorbed, about 20% hydrochloric acid is produced as a by-product. The main reaction formulas of this method are as formula (22) to formula (24).

This process has the advantages of simple process operation, short process flow, short decomposition time, good powder sintering performance, continuous operation of the equipment, and no need for acid pretreatment of water used in washing and other processes. However, there are also some disadvantages and shortcomings, such as: the process energy consumption is high; the hydrogen chloride gas generated in the spray pyrolysis seriously corrodes the equipment, which makes the manufacturing level and corrosion resistance of the main equipment higher; the process produces Dust capture and the absorption and concentration of HC1 are difficult, and environmental pollution is serious.

(ii) Direct pyrolysis process proposed in China

Many domestic researchers have done a lot of research on the technology of direct pyrolysis of brine to produce magnesium oxide. Some work does not involve magnesia, but given that magnesia is usually produced by high-temperature calcination of light-burned magnesium oxide prepared from raw materials such as magnesite, seawater, and salt lake chlorite, it is still necessary to review this part here.

A short process route for preparing high-purity magnesia by “light burning-ball milling-forming-sintering”. In the experiment, brine without any pretreatment, industrial controlled crystallized chlorite and analytically pure magnesium chloride were used as raw materials to obtain magnesia with a mass fraction of more than 99% and a maximum volume density of 3.33g/cm³. Several tests were conducted. The influence of various impurities and additives on the preparation process of magnesia. The results show that TiO₂ is the most suitable sintering aid for magnesia. Adding different amounts of TiO₂ to salt lake brine, industrial controlled crystalline hydrochlorite and analytically pure magnesium chloride can produce bulk densities of 3.15g/cm³ and 3.43g respectively. /cm³, 3.49g/cm³ products, the MgO mass fraction is higher than 98% magnesia. The process has short process flow, low calcination temperature, energy saving and consumption reduction. However, the water content of the pyrolysis tail gas in this method is too high, and only dilute hydrochloric acid can be obtained, which seriously corrodes the equipment.

A method of calcining hydrochlorite in stages, recovering HCl in stages, and preparing basic magnesium chloride and magnesium oxide by pyrolysis. After removing part of the crystal water from the refined chlorite, it is calcined in the first stage at 250-300°C. The product is basic magnesium chloride. The tail gas is absorbed by water to obtain hydrochloric acid; the second stage is calcined at 450-500°C and washed with alcohol. MgO is obtained in the latter step, and the HCl produced is recovered by cooling to obtain hydrochloric acid. This method can be directly calcined to obtain MgO with a purity greater than 99% and hydrochloric acid with a mass fraction of 28% to 32%. At the same time, the calcination temperature is low and the energy consumption is small. However, the decomposition rate of chlorite in this method is low, which increases the burden of post-processing, the equipment is seriously corroded, and the hydrochloric acid obtained still needs to be concentrated. In addition, according to research, the reason for the low decomposition rate of this method may be that MgCl₂ is generated in the first stage of calcination.

A one-step technical route for producing magnesium oxide through spray pyrolysis of hydrochlorite. The atomized saturated magnesium chloride solution droplets are rapidly dehydrated and pyrolyzed in the reactor to produce a product with higher purity. Based on the former, Du et al. studied and calculated the flow field in the spray pyrolysis furnace, designed a new spray pyrolysis furnace structure, established a spray pyrolysis pilot test device, and prepared a product with a purity of 98.87% and an activity value as low as 45s ( CAA value/s) magnesium oxide particles. This technical route has a short process, fast pyrolysis speed, and low calcination temperature. It uses corrosion-resistant materials to prepare the furnace body and post-processing device, which overcomes the corrosion problem of the equipment. The main problem of this method is that the water content of the pyrolysis tail gas is too high, and only dilute hydrochloric acid can be obtained; the corrosion resistance and sealing requirements of the equipment are high, and the equipment cost is high.

Based on the Aman method, a spray dehydration-dynamic calcination method is proposed to produce high-purity magnesium oxide. The brine raw material undergoes vacuum evaporation, crystallization and other processes to remove impurities, and then undergoes spray dehydration to generate magnesium chloride dihydrate. After dynamic pyrolysis, High-purity magnesium oxide is generated after washing, drying and dynamic calcination. This method has been industrialized and has successfully produced low-calcium and low-boron MgO products with a purity of 99.0%. This technology adds a dehydration step before the traditional Aman method, which can by-produce hydrochloric acid with a higher concentration. This method has low production cost, continuous production process, automatic operation, and is suitable for large-scale industrial production; the existing problems are that there are many steps in the purification process and the process flow is long. In addition, it does not solve the equipment problem of magnesium chloride pyrolysis.

Using old brine as raw material, magnesium chloride dihydrate particles are obtained after purification, crystallization and dehydration; after pyrolysis and separation in a fluidized bed pyrolysis furnace, crude magnesium oxide and pyrolysis tail gas are obtained. The pyrolysis tail gas is used to preheat solid materials. The crude magnesium oxide preheats the air; the crude magnesium oxide is post-processed to obtain high-purity magnesium oxide with a purity greater than 99%; the pyrolysis tail gas is used to prepare industrial concentrated hydrochloric acid, with a mass fraction greater than 31%. The inner layer of the pyrolysis furnace used in this process is cast from acid-resistant and high-temperature resistant inorganic refractory materials, and the exhaust gas absorption equipment is mainly made of graphite. The advantages of this method are low pyrolysis energy consumption, less “three wastes” produced, high process thermal efficiency, high product purity, high concentration of hydrochloric acid obtained, high resource utilization rate, almost 100%, and is suitable for large-scale production. The main problem with this method is that the equipment cost is high and the process flow is long.

The technology of direct pyrolysis of brine has been studied, but the technology of direct pyrolysis of brine to produce high-purity magnesia is not yet mature in my country. The current more effective measure to reduce pyrolysis energy consumption is to dehydrate the raw materials to magnesium chloride dihydrate before pyrolysis. Experiments have shown that magnesium chloride can only remove approximately 4 molecules of water in the air without causing serious side reactions. Continued dehydration will cause complex side reactions. In addition, pre-dehydration can also reduce the water vapor content in the pyrolysis tail gas, obtain higher concentration of hydrochloric acid, and reduce the burden on the concentration section. However, at present, no low-cost, low-energy consumption, corrosion-resistant, large-scale, continuous pyrolysis device suitable for the brine pyrolysis process has been developed in China; nor has a short-process, low-cost, corrosion-resistant equipment, Pollution-free production process.

Preparation of high-purity magnesia by electrofusion method

The process route for preparing high-purity magnesia by electrofusion method is short and simple, but the equipment investment and resource consumption are large. Light magnesium oxide or light-burned magnesium powder is pressed into a ball and put into an electric furnace, inserted into the electrode, and electromelted under high current and low voltage conditions. The purity of the resulting high-purity magnesia can reach 99.9%, and the density can reach more than 3.50g/cm³ . Product quality mainly depends on the size of the current and voltage and the length of the electrofusion time. The process route of the electrofusion method is generally shown in Figure 1.

At present, my country’s fused magnesite manufacturers still mostly use electric arc furnaces to fuse magnesite ore to prepare products. The technology and equipment are backward, the product grade is not high, dust pollution is serious, and the energy utilization rate is low. Foreign countries are relatively advanced. Modern electric arc furnaces can automatically control temperature and adjust electrode lifting. The raw material is high-purity magnesium oxide extracted from seawater and salt lake brine. The resulting fused magnesia product has high purity, high density, and strong high-temperature stability. Foreign countries use unsupervised learning technology and digital image processing technology to select magnesia products of different grades. At present, most domestic fused magnesia companies still rely on manual selection, resulting in varying grades of magnesia and waste of labor. In view of the current situation, existing processes and equipment should be improved and updated to improve production efficiency.

The current research on fused magnesite is on the one hand the exploration of the process, including the optimization of the smelting process, the development of additives and the exploration of energy-saving ways. On the other hand, there is research on electric arc furnaces, including the application of intelligent control technology in electric arc furnaces and the flow of melt inside the electric arc furnace.

In terms of process exploration, high-purity magnesia was prepared by using salt lake brine pyrolyzed light-burned magnesium oxide as raw material through an electric arc furnace. Under optimized process conditions, high-purity magnesia with a purity greater than 99.8% and a density greater than 3.5g/cm³ was prepared. In addition, it was found that adding TiO₂ can improve product purity. An additive for the production of large crystalline fused magnesite was developed. The main ingredients are high-purity graphite powder, rare earth oxides and zirconium oxide. Combined with appropriate technological processes and targeted addition of corresponding impurity removal substances according to the impurity types and contents of the raw materials, large pieces of white transparent high-purity fused magnesia can be obtained. The amount of additives is small but the effect is significant. The optimization of additives reduces the smelting power and time, improves the utilization rate of raw materials, and achieves energy saving and emission reduction effects;

The additive is easy to obtain, the process is simple, the required equipment is not high, the cost is low, and it is suitable for large-scale industrial production and application. A new energy-saving process for the fused magnesia system has been developed, including 4 processes including raw material pretreatment process, fused magnesia production process, segmented and graded recovery and cascade comprehensive utilization process of fused magnesia lump and flue gas, and CO₂ recovery process. Sub-process. The new process makes full use of the material energy of the entire process system, achieves “zero emissions”, avoids environmental pollution, and obtains high-grade fused magnesite with the lowest energy consumption.

In terms of research on electric arc furnaces, in order to improve the automation level and control performance, different researchers have conducted research from different angles. A heating furnace model based on the three-dimensional finite element method was proposed to investigate the convection conditions of the melt in the fused magnesium furnace. In order to solve the problems of overshoot, electrode frequency increase and arc current instability caused by two-point control algorithm and three-phase electrode control respectively in the production of fused magnesia, a fused magnesium based on improved BP neural network was designed. Furnace three-phase integrated control model. This control model solves the overshoot problem in traditional manufacturing methods, reduces production energy consumption, and improves product quality.

In order to better control the smelting process in the fused magnesia furnace, a real-time embedded control system for process control of the fused magnesia furnace based on intelligent control strategy and model-based design technology is proposed. Experimental results show that the embedded control system works well in both laboratory and industrial environments. In view of the fact that magnesite is used as the raw material, the traditional process of directly smelting fused magnesite at high temperature through an electric arc furnace has shortcomings such as low product quality, high production energy consumption, serious environmental pollution, and high work intensity. In the future, the mainstream direction for the preparation of high-purity fused magnesia will be to use high-purity light magnesium oxide obtained from brine and seawater as raw materials to produce high-purity fused magnesia; while the focus of research on the preparation process of fused magnesia will be automated arc Research on furnace equipment.

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