There are many methods for preparing iron oxide, which can be roughly divided into two types, dry method and wet method according to the state of reaction materials. Dry method is divided into gas phase method and solid phase method. The gas phase method mainly includes roasting method, thermal decomposition method, Ruthner method and so on. Wet method mainly include sol-gel method, air oxidation method, hydrolysis method, precipitation method, etc… In addition, there are some improvement methods, such as, hydrothermal method, catalytic method and nucleation method.
The vapor phase method often uses base iron or ferrocene as raw material, and is prepared by flame thermal decomposition, vapor deposition, low temperature plasma chemical deposition or laser thermal decomposition. The solid-phase law is to mix metal salts or metal oxides according to the formula, and the calcine them after grinding. After the solid-phase reaction, nanoparticles can be obtained directly or by grinding.
The traditional roasting method mainly refers to the green inkstone roasting method, which is to obtain iron oxide red by high-temperature calcination of ferrous sulfate. This method is only suitable for small-scale production due to the serious pollution of the environment caused by SO2 and SO3 produced. In addition, there are iron yellow and iron black calcining methods. The process of preparing iron oxide by roasting method is simple and easy to operate, but the method has high energy consumption, the product purity is low, high temperature sintering easily causes crystal agglomeration, and the obtained powder has poor dispersion.
Thermal decomposition methods often use base iron or ferrocene as raw materials, and are prepared by gas phase decomposition, flame thermal decomposition or laser decomposition. This method has the characteristics of simple process, few influencing factors, good operating environment. The products produced are characterized by high quality, ultra-fine particles and food dispersion. But the technical difficulty is high, the structure and material requirements of the equipment are high, the one-time investment is also large.
Ruthner method, also known as spray roasting method is a major method that early large and medium-sized steel companies use the FeC1 waste liquid produced during the processing of pickling steel and recover hydrochloric acid to prepare iron oxide. It uses iron chloride as raw material, add iron scraps to consume acid, and the solution is purified and then spray roasted at high temperature. The ferrous chloride solution is hydrolyzed and oxidized under high temperature conditions to generate hydrogen chloride and iron oxide. Hydrogen chloride is recovered with hydrochloric acid and returned to the pickling steel workshop for reuse. The produced iron oxide can be used to produce soft ferrite. This method has the characteristics of simple process, short cycle, large output and low cost. However, due to the production of hydrochloric acid in the production process, the corrosion to the equipment is large and it has high requirements for production equipment.
Wet method, also known as liquid phase method, is currently the main method for preparing powder materials widely used in laboratories and industrial fields. The main advantages of this method are easy control of the components, simple equipment and low production cost. The disadvantage is that there are many impurities and it is difficult to obtain high-performance particle powder. The generated particles are easy to form aggregate pseudo particles and difficult to disperse.
The main process of preparing iron oxide powder by sol-gel method is to add a certain amount of surfactant and alkali to the solution containing Fe3+ to make Fe(OH)3 solution, then filter it after heating up and wash it with deionized water several times. Finally, α-Fe2O3 particles are prepared by drying. This method has the advantages of easy availability of raw materials, low price, simple process, etc… It is also beneficial to promote the balance of chlor-alkali. This method can prepare ultrafine, uniform and spherical ideal iron oxide powder. The disadvantages is that the organic solvents used are flammable and toxic, the product cost is high.
Air oxidation method can be divided into acid method and alkali method. The acid method is to precipitate ferrous ions into Fe(OH) with alkali, which is less than the theoretical amount of alkali, pass oxidation to prepare FeOOH seed crystals. Then add ferrous salt into seed crystals to continue the aeration oxidation, so that Fe2+ is directly oxidized to FeOOH, and deposited on the seed crystal to make the seed crystal grow. Finally, it need to be filtered, washed, dried and calcined. This is an important method for preparing iron oxide. The process is simple. However, because the air oxidation method is a three-phase reaction of gas, solid and liquid, its reaction mechanism and process conditions are complicated and the particle shape is difficult to control.
The hydrolysis method is to hydrolyze a certain concentration of metal salt water by controlling the temperature and PH value to generate hydroxide and oxide precipitation. During the preparation process, some crystallization aids are usually added to reduce the rate of hydrolysis precipitation and crystal growth, as well as ensure complete and uniform particle growth. Then the precipitate is dried to prepare the corresponding nanoparticles. The hydrosis method includes forced hydrolysis and homogeneous hydrolysis.
This is the earliest method to synthesize metal oxide nanoparticles by liquid-phase chemical reaction. Usually, substances with different chemical compositions are mixed in a solution state and then an appropriate precipitating agent is added to make a precursor precipitate. Then the precipitate is dried or calcined to obtain the corresponding nano-sized particles.
Hydrothermal method refers to the reaction and crystallization of raw materials in a sealed pressure vessel, using fluids such as aqueous solutions or water vapor as solvents under high temperature and high pressure conditions. Since the reaction is carried out in an aqueous solution under high temperature and high pressure, a certain form of precursor will exhibit different properties from normal temperature, such as, enhanced ion cleanliness, increased solubility, compound crystal structure transformation and easy dehydration of hydroxides, etc…