Calcium oxide material has excellent thermodynamic stability, desulfurization and dephosphorization performance, carbon adsorption performance, etc., and is an important alkaline material in industrial production and application. However, due to its poor hydration resistance, it seriously damages the service performance of the material and limits the application scope of calcium oxide materials. In this paper, the application progress of calcium oxide materials is expounded, and the main measures to optimize the performance of this material system, such as thermodynamic stability, desulfurization and dephosphorization, carbon adsorption and hydration resistance, are summarized.
Calcium oxide has excellent thermodynamic stability, desulfurization and dephosphorization performance and carbon adsorption performance, and is widely used in tundish filters, ladle, glass kiln regenerator, cement kiln firing zone, induction furnace, LD converter and electric furnace and other fields . The crystal structure of calcium oxide is sodium chloride type, the ion coordination number is 6, and calcium ions are located in the octahedral voids of oxygen ions. g·cm-3), so the lattice structure is loose and prone to hydration. When calcium oxide undergoes a hydration reaction, since the volume of the reaction product calcium hydroxide accounts for a large proportion, the material is easy to expand, resulting in increased cracks or even cracking. The degree of hydration of calcium oxide clinker increases with the increase of reaction temperature. At present, the hydration resistance of calcium oxide material is the main factor restricting its service performance and application range.
In order to promote the sustainable development of industrial production and application, improve the service performance of related products, and expand the application scope of calcium oxide materials, it is of great significance to prepare calcium oxide materials with excellent performance. This paper mainly expounds the research progress of calcium oxide thermodynamic stability, desulfurization and dephosphorization performance, carbon adsorption performance and hydration resistance, and focuses on the optimization of hydration resistance. and development direction.
1. Thermodynamic stability
Calcium oxide has high thermodynamic stability, and its melting point is second only to MgO, ZrO2 and ThO2, etc., and the operating temperature can reach 2 000 °C. Because it is difficult to react with molten metal, it is often used in the preparation of superalloy crucibles. Compared with magnesium oxide crucibles, calcium oxide crucibles are more thermally stable and add less oxygen to the molten metal. This is mainly due to the fact that the saturated vapor pressure of Ca is about 20 times smaller than that of Mg when the smelting temperature is the same.
Calcium oxide crucible replaces magnesium oxide and other material crucibles with higher thermodynamic stability and lower oxygen increase, which meets the melting of ultra-pure nickel-based superalloys. Nickel-based superalloys are obtained by smelting using alumina crucibles and calcium oxide crucibles, and superalloys using calcium oxide crucibles have higher oxidation resistance and better creep resistance. After thermodynamic analysis of calcium oxide and magnesium oxide, titanium and titanium-aluminum alloys, the results show that calcium oxide crucibles can be used for smelting titanium alloys, but magnesium oxide is not suitable. Using calcium oxide crucible and magnesium oxide crucible to smelt Cr12 steel under vacuum at 1 600 °C, it is found that when the pressure in the furnace is 5-10 Pa, calcium oxide does not decompose and does not supply oxygen to molten steel, and calcium oxide crucible is used. The average particle size, number and area ratio of inclusions in Cr12 steel are smaller.
2. Desulfurization and dephosphorization performance
Aerospace, nuclear energy, transportation, and petrochemicals have continuously improved performance requirements for clean steel and superalloys. It is necessary to improve the purity and reduce the content of non-metallic inclusions and harmful elements such as O, S, P, H, and N in the alloy. The impurity control technology not only depends on the alloy smelting technology, but also requires the smelting materials to have the function of cleaning the metal melt. If clean steel requires high cleanliness, the content of sulfur and phosphorus in molten steel should be less than 0.01 wt%. Studies have shown that, compared with other materials, calcium oxide materials have stronger adsorption capacity for impurities such as sulfur, phosphorus, Al2O3, and SiO2 in molten steel, so they have great application potential in the field of clean steel preparation. The basic principle of desulfurization of calcium oxide material is to react with S in molten steel to generate CaS; and the basic principle of dephosphorization is to oxidize phosphorus in molten steel to P2O5 gas. Due to the poor stability of P2O5 gas, it reacts with calcium oxide to form phosphate and dissolves into the steel slag, and the phosphorus content in the molten steel decreases.
In the study of the conversion of calcium oxide to phosphorus during sludge pyrolysis, the results showed that calcium oxide not only promoted the conversion of orthophosphoric acid diester to orthophosphoric acid monoester, but also transformed inorganic phosphorus into phosphorus mineral hydroxyapatite. And with the increase of calcium oxide addition, the generation of hydroxyapatite increases. The effect of calcium oxide on the adsorption of phosphate under different conditions. As a phosphorus removal agent, calcium oxide can effectively remove phosphorus in wastewater.
Under the numerical simulation to study the sulfation characteristics of porous calcium oxide particles during the desulfurization process, the desulfurization efficiency of calcium oxide reaches the maximum value in the temperature range of 850-900 °C. The initial reaction rate of calcium oxide particles with large particle size will be slightly higher than that of particles with small particle size, but as the external CaSO4 blocks the pores, the internal reaction rate of large particles decreases. When the reaction is finally stopped, there will be more calcium oxide that does not participate in the reaction inside. , the overall conversion rate is lower; while for the particles with smaller particle size, due to the smaller concentration difference within the particles, the overall reaction rate is relatively consistent, so a larger desulfurization rate can be finally achieved. Under the same temperature conditions, by adding rare earth compounds and other metal compounds as modifiers of calcium oxide, the desulfurization rate of calcium oxide can also be effectively improved. Because of the simple process, the method has good industrial application prospects.
3. Carbon adsorption performance
With the development of human industrial activities, the concentration of carbon dioxide has gradually increased, and the temperature has increased year by year. According to statistics from the National Oceanic and Atmospheric Administration, the concentration of carbon dioxide in the Earth’s atmosphere in May 2021 was 419 ppm. It is predicted that, if unchecked, the concentration of carbon dioxide in the earth’s atmosphere will increase to 600-700 ppm by the end of this century, while the surface temperature will rise by 4.5-5.0°C. The intensification of the greenhouse effect not only leads to the rise of global sea levels, but also threatens the future sustainable development of mankind. Therefore, controlling carbon dioxide emissions and curbing the intensification of the greenhouse effect is an urgent problem that the world needs to solve.
Calcium oxide-based materials have the advantages of high theoretical adsorption capacity (adsorbent), wide source of raw materials (limestone, various shells, etc.) and low cost, and have become the research object of carbon dioxide solid adsorbents in recent years. The adsorption process of calcium oxide can be divided into two steps: chemical reaction and internal diffusion. First, carbon dioxide reacts with the surface of calcium oxide, and then gradually diffuses into the interior of calcium oxide along the pores. With the progress of the reaction, the newly formed calcium carbonate on the surface of the sample increased, which eventually blocked the pores on the surface of the calcium oxide and prevented the further diffusion of carbon dioxide. Therefore, the adsorption capacity of calcium oxide materials for carbon dioxide is often lower than its theoretical adsorption capacity. In addition, during the carbonation/calcination cycle of the calcium oxide-based adsorbent to capture carbon dioxide, the porosity and specific surface area of the adsorbent decrease due to the agglomeration and growth of calcium oxide particles and the collapse of the pore structure, resulting in the sintering and deactivation of the adsorbent, and the adsorption capacity is sharp. decline.
The anti-sintering modification treatment of calcium oxide can maintain the good pore structure and specific surface area of the adsorbent, thereby improving the carbon dioxide adsorption capacity of the calcium oxide-based adsorbent. At present, the research on anti-sintering modification mainly includes hydration modification, acid solution modification and doping modification. Hydration modification can make the calcium oxide adsorbent collapse and crack in the carbonation reaction stage to obtain a larger specific surface area, reduce the reaction temperature and residence time in the calcination stage, and slow down the sintering; acid solution modified adsorbent will produce More gases and small molecular substances increase the porosity; doping modification can promote the adsorption and diffusion of carbon dioxide by calcium oxide, and can also act as a framework to separate calcium oxide particles to prevent their migration and diffusion.
In addition, the pore size and specific surface area of the calcium oxide-based adsorbent have a great influence on its carbon dioxide adsorption performance. When the specific surface area of calcium oxide is the same, increasing the average pore size of the sample, especially the median pore size at 47-96 nm, is beneficial to promote the adsorption rate and adsorption capacity of the calcium oxide adsorbent. When the average pore size of calcium oxide is similar, increasing the specific surface area of the sample can improve the adsorption rate and adsorption capacity of calcium oxide for CO2.
The reaction between calcium oxide and carbon dioxide is an exothermic reaction, and the product is limestone, namely calcium carbonate. As a raw material for capturing carbon dioxide, lime can be recycled and used to eliminate carbon dioxide emissions from cement and steel plants. Therefore, the use of lime to capture carbon dioxide is not only environmentally friendly but also has great economic prospects.