Comparison of 12 Cobalt Powder Preparation Processes

Cobalt is an important raw material for the production of high-strength, high-temperature and corrosion resistant alloys. It is widely used in cemented carbide, ceramic chemical industry, catalysis and battery industries. In recent years, with the rapid development of battery, cemented carbide, ceramics and other industries, the amount of ultra-fine cobalt powder has also risen rapidly. At present, there are many methods to prepare cobalt powder at home and abroad. Such as oxide, oxalate hydrogen reduction method, oxalate thermal decomposition method, high-pressure water spray method, electrolysis method, polyol method γ X-ray irradiation method, microemulsion method, high-pressure hydrogen reduction method, gas phase hydrogen reduction method, hydrazine liquid phase reduction method, etc. Some of these methods have been industrialized, while others are still in the stage of laboratory exploration and research and development. Next, this paper analyzes the different production processes of cobalt powder for your reference.

Hydrogen Reduction of Oxides and Oxalates

In the oxide reduction method, oxalate or carbonate precipitation is first decomposed into oxides by heating, and then reduced to metal cobalt powder by H2. This method can only produce coarse cobalt powder>38um. Oxalate reduction is to directly convert oxalate into metal cobalt powder in H2 atmosphere. Essentially, it is a mixed process of oxide reduction and direct thermal decomposition of oxalate. The product purity is 99.5%, the average particle size is 2~4 um, and the shape is dendritic and the distribution is uneven.

Oxalate Thermal Decomposition Method

This is a relatively new production process of cobalt powder. This method is the same as the old process in obtaining high-purity cobalt oxalate, that is, high-purity electrolytic cobalt flakes or cobalt particles are dissolved into CoCI2 solution with pure hydrochloric acid, and CoC2O4-2H2O is precipitated with oxalic acid or ammonium oxalate, washed and dried. Drying is the key process of this process. The temperature should be accurately controlled to ensure that all crystal water is removed without causing the material layer surface to become oxidized into cobalt oxide. The production cost of this process is significantly reduced because hydrogen is not used. The purity of cobalt powder produced is 99.9%, so it is suitable for the production of cemented carbide. However, because the drying temperature is not easy to control, and the need for protective gas, large-scale production cannot be carried out. Generally, the average particle size of cobalt powder obtained is 1-1.5 um.

High Pressure Water Spray Method

This method is a physical method, that is, the cobalt block weighed according to the formula requirements is placed in the medium frequency induction smelting furnace for heating and melting, then poured into the leaky ladle furnace, and the high-pressure water atomization pulverizing device is started before. The metal liquid in the leaky ladle furnace flows out through the bottom leaky nozzle, penetrates the sealing sheet and enters the atomizing device. Under the impact of high-pressure water flow from the annular nozzle, it is broken into numerous small liquid beads and quickly condensed, forming a mixture of water and powder that falls into the current stabilizer below the atomizing cylinder. The water and powder rotate in a vortex shape on one side and are absorbed by the ejector below the atomizing cylinder on the other side and sent to the hydrocyclone to form a fast cyclone. Under the effect of centrifugal force, the metal powder is thrown to the periphery and discharged from the powder outlet into the powder container. The prepared wet powder is dehydrated and dried to obtain cobalt powder. Compared with the reduction method, the production process of cobalt powder by water mist method is simple, safe, pollution-free, and the labor intensity of workers is low.

Electrolytic Method

Electrolytic method uses electrolytic cobalt as anode, stainless steel water jacket as cathode, electrolytic cobalt sulfate and other solutions containing cobalt ions, and colloidal additives are added to the solution to prevent particles from agglomeration. When cobalt ion moves to cathode, it is realized by diffusion, convection and migration. These three processes are related to temperature, electrolyte composition, solution viscosity and ion concentration. Compared with other chemical liquid phase methods, the electrolytic method does not introduce impurities, the reaction process is easy to control, and it can produce cobalt powder with higher purity. In addition, the powder prepared by electrolysis is dendritic or acicular, which is not suitable for preparing spherical cobalt powder.

Polyol Method

The solid compound of cobalt is suspended in a liquid polyol, and then the suspension is heated to a certain temperature. In most cases, the boiling point of the liquid phase can be reached, so these initial compounds are reduced metal cobalt powder is produced. The role of polyols is to make the initial compounds in suspension as a liquid phase; Secondly, it is also a solvent and reducing agent. The cobalt powder produced by this process is mainly characterized by spherical particles with uniform particle size uniform and subtle. Another feature of this method is that the production process is simple and feasible, the raw materials are various, and the product particle size can be adjusted and controlled.

γ Ray Irradiation

This method is to prepare a certain concentration of cobalt solution, add an appropriate amount of isopropanol and polyvinyl alcohol as free radical scavenger and dispersant respectively, adjust the pH value with acetic acid and NaOH, then conduct ultrasonic degassing treatment on the prepared solution and add nitrogen to reduce the oxygen content in the solution, and then γ Irradiation by γ - ray, use a cobalt source with a dose rate of 70Gy/min, and collect it by conventional methods after irradiation. The preparation conditions of this method are easy to control, the obtained ultrafine cobalt powder has narrow particle size distribution, strong oxidation resistance and high productivity, but it requires high equipment and high cost, so it is not easy to realize industrial production.

Microemulsion Method

First, the anionic surfactant is dissolved in isooctane at a certain concentration, and then CoCl is dissolved. And NaBH4 were dissolved in this solution at a certain concentration to obtain two kinds of micro lotion. Mix the two micro lotion, and the solution turns from light pink to black. Acetone and water are used as flocculants to coagulate the colloid. After filtering and washing with water to remove excess surfactant, the colloid is sent to dry to obtain ultra-fine cobalt powder. The basic principle is that CoCI2 and NaBH4 respectively exist in the water core of their respective micro lotion. When the two micro lotion are mixed, due to the collision between micellar particles, mutual exchange or material transfer of substances in the water core occurs, causing chemical reactions in the core. Since the radius of the water core is fixed, the particle size generated in it is also controlled.

The particle size of cobalt powder produced by this method is generally less than 100 nm, the shape is spherical, the distribution is uniform, but the fluidity is poor. Due to the high cost of raw materials and the complex process control, large-scale production is limited.

Redox Method

Cobalt powder or small cobalt flakes are calcined in an air atmosphere heating furnace at 850 ～ 950 ℃ for 48 hours. The metal cobalt particles or small cobalt flakes are oxidized to loosen the structure, and then they are milled with cemented carbide balls. After sieving, they are reduced with hydrogen at 500 ℃ to obtain fine metal cobalt powder of the required purity. The average particle size of the metal powder obtained by this process is 2.5 um. Although the process is relatively simple, it must be reduced with hydrogen finally.

High Pressure Hydrogen Reduction

The cobalt solution is converted into cobalt sulfate ammonia solution in an autoclave, or C02X is precipitated into cobalt hydroxide with ammonia, and then reduced to ultrafine cobalt powder with hydrogen under high pressure. The reduced cobalt powder is generally spherical, depending on the preparation conditions, with a particle size of 0.1~0.5um. Except hydrogen partial pressure, other factors have certain effects on the particle size. Relatively speaking, the particle size of cobalt powder obtained at higher temperature, catalyst concentration and lower cobalt concentration is smaller.

Gas Phase Hydrogen Reduction Method

Preheat the steam and H2 of cobalt halides (such as CoCl2) to a certain temperature, mix them evenly and then enter the reaction chamber, and then heat them to the reaction temperature. The mixed gas is injected into the reaction chamber at high speed for reduction reaction to generate ultrafine cobalt powder. Since the reduction reaction is exothermic, once the reaction starts, it can proceed rapidly. However, the by-product HCI of the reaction is easy to be adsorbed on the surface of the particles, and the unreacted chloride is easy to mix with the product. In addition, the particle size of powder can be controlled by adjusting the reaction temperature and the ratio of mixed gas Hz. This process is characterized by no need for high temperature conditions, low raw material cost and low requirements for equipment.

Hydrazine Liquid Phase Reduction Method

The liquid phase reduction method of hydrazine has been the focus of research on the synthesis of metal nano drill powder because of its advantages such as fast, high efficiency, easy control of the particle size, morphology and properties of the product, simple process, low cost, and suitability for large-scale industrial production. The molecules in hydrazine are associated by strong hydrogen bonds and are widely used as reducing agents. As a reducing agent, it has the greatest advantage of strong reduction ability under alkaline conditions, its oxidation product is N2, and will not introduce impurities metal ions into the product. This method is characterized by cheap raw materials, easy availability, simple equipment, convenient process, and the reaction can be completed in one step without heating.

Ultrasonic Atomization Thermal Decomposition Method

Ultrasonic atomization thermal decomposition method (USP for short) is an advanced method for preparing nano powder, which has been used to prepare nano metal powder, metal alloy powder and ceramic materials. The method can also be used to prepare nano cobalt powder.