Hydrogen Reduction Preparation Of Ultrafine Cobalt Powder

Hydrogen reduction is an effective method to prepare spherical ultrafine cobalt powder which is small, uniform, and has better mobility and can mix well. So hydrogen reduction method is a commonly used way to prepare cobalt powder in China. Hydrogen reduction methods include high-temperature solid-phase hydrogen reduction method and high-pressure hydrogen reduction method. High-temperature solid-phase hydrogen reduction method is to obtain cobalt powder by reducing cobalt oxide or cobalt salt precipitations with hydrogen at high temperature. This method is an ordinary cobalt powder preparation method in industry, but its products agglomerate seriously, and its particles are coarse and contain higher quantities of oxygen. So it is very difficult to prepare ultrafine cobalt powder by using this method. High-pressure hydrogen reduction method is to prepare ultrafine cobalt powder with the cobalt solution or slurry in the conditions of high pressure. However, this method requires a higher hydrogen pressure and the purity can only reach about 95%. It is difficult to achieve requirements of ultrafine cobalt powder products by using this way.Taking into consideration the advantages and disadvantages of high-pressure hydrogen reduction and high-temperature solid-phase hydrogen reduction, high purity spherical ultrafine cobalt powder can be prepared by high-pressure hydrogen reduction and high-temperature solid-phase hydrogen reduction from the Co(OH)2 slurry which is obtained by mixing the solutions of water-soluble cobalt salt with excess alkali. The research investigated the influence of the amount of alkali, reaction temperature, the amount of catalyst, hydrogen partial pressure, cobalt ion concentration, and high-temperature solid-phase hydrogen reduction on the ultrafine cobalt powder performance. The sample structure, morphology, particle size and size distribution were characterized by X-ray diffraction, scanning electron microscopy, BET surface area analysis, laser particle size analysis and thermal analysis.The research results show that the amount of alkali is a key factor which influences the reaction process. When the reaction proceeded in alkaline medium, the longer the reaction time is, the larger the particle size of ultrafine cobalt powder is, along with the increasing of the amount of alkali. The proper terminal PH value should be controlled at around 7 by adding the appropriate amount of alkali. The cobalt crystal grains can generate easily at low temperatures, but is not favorable for growing up, and the cobalt crystal grains are generally small at low temperatures. On the contrary, when the temperature increased, the crystal grains grow up and agglomerate. So, the proper reaction temperature is 180℃. PdCl2 catalyst is favorable for speeding up the hydrogen reduction reaction, but when the concentration of PdCl2 catalyst is less than 10 mg/L, the cobalt powders adhere on reactor walls and stirring paddle easily, so the concentration of PdCl2 catalyst must be greater than 10 mg/L. If hydrogen partial pressure is too high, it is not conducive in chemical industry, then the proper hydrogen partial pressure should be maintained at 1MPa.The Co(OH)2 slurry becomes viscous and the reaction is very slow with the increasing of initial concentration of cobalt ions, it is appropriate that the initial concentration of cobalt ions should be less than 50g/L. The purity of ultrafine cobalt powder is higher if it is treated by high-temperature solid-phase hydrogen reduction. With the increasing of temperature, the crystalline of ultrafine cobalt powder change from hexagonal close-packed(hcp) into face-centered cube(fcc) at 500℃, and the hexagonal close-packed(hcp) change into face-centered cube(fcc) completely at 700℃. The ultrafine cobalt powder prepared by high-pressure hydrogen method is reduced for one hour of time at 400℃, the particle grow up to about 200nm. When the temperature is 700℃, the particles only grow up to 250 nm.