Nature of Cobalt and Its Smelting Methods

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Cobalt is a silvery white ferromagnetic metal which belongs to VIII in the periodic table with the element symbol Co, atomic number 27, atomic weight 58.9332, and valence +2 or +3. Light blue lustrous color can be noticed on the surface after polishing. There are about 100 kinds of minerals containing cobalt known so far, mainly including linnaeite, fiber pillar, cobaltite, smaltite, etc., but the cobalt content is relatively low. Seabed manganese nodules are prominent prospective resources of cobalt, in addition, the recycling of cobalt is also gaining people’s attention.

Cobalt is very stable in air and water at room temperature. When the temperature is above 300 degrees Celsius, cobalt starts oxidizing in the air and the hot cobalt can split water to release hydrogen. Cobalt powder prepared by hydrogen reduction can ignite spontaneously in the air and generate cobalt oxide. Cobalt superalloys have high strength and creep resistance ability in 900 ~ 1000 degrees Celsius, and usually used in jet engine’s high-temperature resisting parts. Cobalt also can improve the magnetic saturation and Curie point of iron-based, aluminum-nickel and rare earth alloys, and endow them with high coercive force, making them excellent magnetic material in the electrical industry. The mechanical property of those metal parts can be improved significantly after coating with cobalt alloy. Cobalt oxide can be the bleaching agent and pigment of ceramic products as the glazes containing cobalt allow the enamel bonded more closely with steel. Cobalt compounds also can work as driers in the paint. Besides, cobalt is also frequently applied in the hydration, desulfurization, oxidation and reduction of hydrocarbons during the chemical production. Physical, chemical, biological research and medical sectors often use cobalt as gamma ray source.

 

Occurrence of cobalt minerals are rather complex, and usually in low grade ores, so there are various extraction methods, but complex in process and low in recovery. Usually first use fire to gather or convert cobalt into a soluble state, then further enrich and purify cobalt with wet process, and finally get cobalt compound or cobalt.

 

Steps for smelting cobalt from smaltite are as follows: get the cobalt concentrates containing 10 to 20 percent of cobalt and 20 to 50 percent of arsenic by dressing. Two principal options processing smaltite are as follows: one is to use fire first, then go through the wet process to extract cobalt; another is to get cobalt solution by pressure leaching and then extract cobalt. Most industries in China prefer the former method, put the cobalt together with coke and flux in the reverberatory furnace or electric furnace to get part of arsenic volatilized as arsenic trioxide. If raw materials contain too much sulfur, it might also generate some cobalt matte. Calcine the mixture of arsenic ice cobalt and cobalt matte after grinding for the further removal of arsenic and sulfur; then leach with dilute sulfuric acid, using sodium hypochlorite oxidation to get rid of iron; finally use soda to adjust the solution’s pH to 3 to 3.5, making iron precipitate as iron oxide and ferric arsenate. Then use iron to remove copper in the filtrate, add alkali to generate high cobalt hydroxide precipitation and separate with nickel after oxidizing cobalt with sodium hypochlorite. Calcine cobalt hydroxide under 1000 to 1200 degrees Celsius in the reverberatory furnace to get cobalt oxide and remove sulfur by sulfate decomposition. Afterwards put that with charcoal in the rotary kiln to get reverted cobalt powder under the temperature around 1000 degrees Celsius, or smelt cobalt hydroxide into raw cobalt and get the pure one after electrolysis. The above-mentioned method can be applied to the leaching liquid to extract cobalt as well.