High-purity magnesium oxide generally refers to magnesium oxide products with a purity greater than 99%. Because high-purity magnesium oxide has excellent physical properties such as ultra-high melting point, good magnetic permeability and excellent insulation properties, as well as specific chemical properties, it is widely used in ceramics, metallurgy, medicine, electronics, national defense and other fields [1 ].
The development and industrialization of high-purity magnesium oxide will greatly promote the development of industries such as electronics, national defense, aerospace, and advanced ceramics . China has a large output of ordinary grade magnesium oxide products, which are exported in large quantities and the market is weak; while some fine magnesium oxides such as high-purity magnesium oxide, activated magnesium oxide, and silicon steel magnesium oxide have a large market demand, and the products are imported in large quantities .
1 Preparation method of ordinary magnesium oxide
The main sources of raw materials for the production of magnesium oxide include liquid ore and solid ore. The former is mainly seawater, underground brine, and salt lake brine; the solid minerals mainly include magnesite, brucite, dolomite, serpentine, and asbestos tailings. The main methods for preparing magnesium oxide from brine or hydrochlorite include milk of lime method, ammonium carbonate method, ammonia method, soda ash method, and hydrochlorite direct pyrolysis method; the main methods for preparing magnesium oxide from solid ore include calcined magnesite Mining method, carbonization method, acid hydrolysis method, ammonium sulfate method and ammonium sulfate double salt method, etc. [4,5].
2 Preparation method of high-purity magnesium oxide
The production of high-purity magnesium oxide is generally achieved by using the preparation method of ordinary magnesium oxide and carrying out certain purification processes.
2.1 Brine precipitation method
2.1.1 Brine spray thermal decomposition method
After the brine is concentrated to a certain concentration, it is directly sprayed into the thermal decomposition reactor and thermally decomposed at 800°C to 1000°C to produce crude MgO. The crude MgO is washed with water to remove soluble chlorides, and the crude MgO is completely converted into Mg(OH)2, and then calcined at 1600°C to 2000°C to generate high-purity MgO. Its process principle is:
This method uses a secondary calcination process, and the calcination temperature is higher, resulting in higher production costs. At the same time, the by-product HCl gas during the primary calcination process will cause serious corrosion to the production equipment.
2.1.2 Dolomite/lime method
The dolomite/lime method is a mature and widely used high-purity magnesium oxide production technology. At present, except for a few manufacturers such as Israel’s Dead Sea Periclase (DSP) and Tateho Dead Sea Fused Magnesia (TDF), nearly 20 major high-purity magnesium oxide manufacturers in the world use the dolomite/lime method to produce high-purity magnesium oxide. product.
The dolomite/lime method is also called the lime milk precipitation method. It adds lime milk to brine containing a certain concentration of MgCl2 to react to generate Mg(OH)2, which is washed, dried, and calcined to obtain MgO . This method requires high activity of Ca(OH)2, and the generated precipitate is Mg(OH)2. The particles are small and easy to absorb impurities, so the product purity is low; at the same time, the Mg(OH)2 precipitates into a gel, making it difficult to filter. , the filter cake has high moisture content, high energy consumption in the drying process, and the generated CaCl2 has low added value, causing serious environmental pollution .
2.1.3 Ammonia precipitation method
Similar to the dolomite/lime method, the ammonia precipitation method mainly produces magnesium oxide by adding an alkaline precipitant to the brine to prepare a magnesium hydroxide intermediate. The alkaline precipitant used in the ammonia method is liquid ammonia or ammonia gas [10 ]. The process of producing high-purity magnesium oxide by ammonia method is roughly as follows: first, the old brine solution is decolorized, impurities removed and refined, and the selected treatment technology is similar to the dolomite/lime method; then liquid ammonia or ammonia gas is introduced into the refined old brine solution The precipitant reacts to generate magnesium hydroxide intermediate; finally, the magnesium hydroxide intermediate is washed, filtered, and calcined to produce high-purity magnesium oxide products, and the filtrate can be used to produce ammonium chloride by-product.
2.1.4 Soda ash method
The soda ash method adds a soda ash solution precipitant to the brine  to first generate heavy magnesium carbonate (MgCO3·3H2O) precipitate. The heavy magnesium carbonate undergoes water washing, pyrolysis, etc. to obtain basic magnesium carbonate. Finally, it is calcined. Lightweight and high-purity magnesium oxide products can be obtained [12,13]. The production of high-purity magnesium oxide by the soda ash method is a traditional production method with simple process, low equipment requirements and high purity of magnesium oxide products. It has a production history of more than 60 years in my country. At present, most small and medium-sized high-purity magnesium oxide production enterprises in my country still use the soda ash method for production .
2.1.5 Ammonium bicarbonate method
NH4HCO3 is used as a precipitant to react with MgCl2 in brine to generate 4MgCO3·Mg(OH)2·4H2O, which is washed, dried and calcined to obtain MgO. Using this method, the utilization rate of CO2 in NH4HCO3 is low, the production consumes a large amount of NH4HCO3, and the cost is high .
2.2 Solid mineral calcination and carbonization method
2.2.1 Magnesite calcination carbonization method
In this method, magnesite is calcined to produce light-burned magnesia. The light-burned magnesia is digested and carbonized to obtain a magnesium bicarbonate solution. Activated carbon is used as the adsorbent to remove iron ions in the magnesium bicarbonate solution. After adsorption and impurity removal, The magnesium bicarbonate solution is then pyrolyzed, and the basic magnesium carbonate obtained by pyrolysis is filtered, washed, dried, and then calcined to obtain a high-purity magnesium oxide product.
2.2.2 Dolomite calcination carbonization method
The steps for preparing high-purity magnesium oxide by dolomite calcination and carbonization are basically the same as the magnesite calcination and carbonization method, which mainly include five steps: calcination, digestion, carbonization, pyrolysis and calcination. The specific process is as follows:
2.3 Other preparation methods
In addition to some of the traditional preparation methods introduced above, some other preparation methods have appeared in recent years, such as microwave radiation method, metal alkoxide hydrolysis method, direct precipitation method, uniform precipitation method, gas phase method and sol-gel method, etc. [14,17].
Recently, the “primary cell method ultra-high purity magnesium oxide” technology independently developed by the Translational Research Center of Beijing Institute of Technology (Tangshan) has achieved a breakthrough, solving the technical and industrialization problems that our country has been tackling since the 1970s, and breaking the long-standing tradition of this technology. The field is “stuck” by foreign technology. It is understood that during the production process of using magnesium as a fuel cell using the original cell method, it was discovered that ultra-high purity magnesium oxide can be generated. The original cell method uses magnesium as the anode metal material of the fuel cell, which increases the safety of fuel cell production, transportation, storage, filling and other aspects, and converts magnesium into magnesium oxide in a short time. At the same time, no industrial waste emissions can be achieved in the process of obtaining ultra-high purity magnesium oxide. Compared with the magnesite calcination method and the seawater/brine precipitation method, the primary cell method can obtain ultra-high purity magnesium oxide with a purity of up to 99.95%, with lower cost, shorter process route, and higher product yield. And the production process is accompanied by the generation of a large amount of high-quality direct current.
With the development of industry, the application scale of high-purity magnesium oxide in traditional fields is expanding day by day, new application fields are also constantly explored and developed, and the market capacity continues to expand. From the perspective of development trends, the proportion of high-purity magnesium oxide consumption is increasing year by year. However, our country has always lagged behind foreign countries in this field, and has been “stuck” by foreign countries in industrialized production technology. Therefore, it is urgent to increase the research and development and industrialization of high-purity magnesium oxide preparation technology.