Oxidation of cobalt powder

The rates of oxidation of cobalt powders were measured in the thin film region at —78, —22, 0, and 26°. During the first oxidation at each temperature, multiple oxide layers formed to some limiting thickness. 

Calciothermic reduction of samarium oxide, in the presence of cobalt powder, yields samarium-cobalt alloys in the powder form. The process is popularly known as reduction diffusion. Samarium oxide, mixed with cobalt powder and calcium hydride powder or calcium particles, is heated at 1200 °C under 1 atm hydrogen pressure to produce the alloys. Cobalt oxide sometimes partly replaces the cobalt metal in the charge for alloy preparation. This presents no difficulty because calcium can easily reduce cobalt oxide. A pelletized mixture of oxides of samarium and cobalt, cobalt and calcium, with the components taken in stoichiometric quantities, is heated at 1100-1200 °C in vacuum for 2 to 3 h. This process is called coreduction. In reduction diffusion as well as in coreduction, the metals samarium and/or cobalt form by reduction rather quickly but they need time to form the alloy by diffusion, which warrants holding the charge at the reaction temperature for 4 to 5 h. The yield of alloy in these processes ranges from 97 to 99%. Reduction diffusion is the method by which most of the 500 to 600 t of the magnetic samarium-cobalt alloy (SmCOs) are produced every year. 

A powder which burns with a green flame is obtained by the addition of nitrate of baryta to chlorate of potash, nitrate of potash, acetate of copper. A white flame is made by the addition of sulfide of antimony, sulfide of arsenic, camphor. Red by the mixture of lampblack, coal, bone ash, mineral oxide of iron, nitrate of strontia, pumice stone, mica, oxide of cobalt. Blue with ivory, bismuth, alum, zinc, copper sulfate purified of its sea water [sic]. Yellow by amber, carbonate of soda, sulfate of soda, cinnabar. It is necessary in order to make the colors come out well to animate the combustion by adding chlorate of potash.15... 

Cobalt Orthosilicate, Co2Si04, is obtained by heating to bright redness a mixture of amorphous silica, and the chloride and oxide of cobalt. Excess of silica is removed by treatment with concentrated soda solution, the residue consisting of the orthosilicate in the form of a violet crystalline powder. The crystals are isomorphous with peridote density 4-63. They are decomposed by hydrochloric acid.3... 

Heaton (1928) records that cohalt oxide forms an intense and beautiful black , noting that it is used to some extent in fresco painting . Mayer (1991) describes this as a rather coarse black powder . This colour (as black oxide of cobalt ) was found in the paintbox belonging to Joseph Southall (1861-1944). Dunkerton (1980) states that this is a potter s colour which was made by Sir William Burton in collaboration with. .. John Batten and available through the society of tempera painters and Lechertier Barbe , thereby being also known as Burton s cobalt. See cobalt oxides and hydroxides group. 

Heaton (1928) records that cobalt oxide forms an intense and beautiful black , noting that it is used to some extent in fresco painting . Mayer (1991) on the other hand describes friis as a rather coarse black powder . It is also known as black oxide of cobalt and cobalt black. 

A critical issue is the stabiUty of the hydride electrode in the cell environment. A number of hydride formulations have been developed. Table 5 shows hydride materials that are now the focus of attention. Most of these are Misch metal hydrides containing additions of cobalt, aluminum, or manganese. The hydrides are prepared by making melts of the formulations and then grinding to fine powers. The electrodes are prepared by pasting and or pressing the powders into metal screens or felt. The additives are reported to retard the formation of passive oxide films on the hydrides. 

Cobalt compounds can be classified as relatively nontoxic (33). There have been few health problems associated with workplace exposure to cobalt. The primary workplace problems from cobalt exposure are fibrosis, also known as hard metal disease (34,35), asthma, and dermatitis (36). Finely powdered cobalt can cause siUcosis. There is Htfle evidence to suggest that cobalt is a carcinogen in animals and no epidemiological evidence of carcinogenesis in humans. The LD q (rat) for cobalt powder is 1500 mg/kg. The oral LD q (rat) for cobalt(II) acetate, chloride, nitrate, oxide, and sulfate are 194, 133, 198, 1700, 5000, and 279 mg/kg, respectively the intraperitoneal LD q (rat) for cobalt(III) oxide is 5000 mg/kg (37). 

The reduction diffusion process has also been used for the production of powders of the magnetic neodymium-iron-boron alloy (Nd15Fe77B8). The reaction involves use of a powder mix of neodymium oxide, iron, ferroboron and calcium. The reaction is conducted by heating the powder charge mixture at 1200 °C for 4 h under vacuum. Neodymium-iron-boron alloys are much more prone to oxidation than samarium-cobalt alloys and a proprietary leaching procedure is used for the separation of the alloy and calcium oxide. 

Cobalt represents an interesting contrast to the many activated metal powders generated by reduction of metal salts. As will be seen, the cobalt powders are highly reactive with regard to several different types of reactions. However, in contrast to the vast majority of metals studied to date, it shows limited reactivity toward oxidative addition with carbon halogen bonds. 

Rhabdomyosarcomas developed in rats injected intramuscularly with the powder of either pure cobalt metal or cobalt oxide. In other studies implantation of cobalt caused local fibrosarcomas in rabbits, but inhalation studies in hamsters did not reveal any increase in tumors from cobalt oxide. Lifetime exposure to cobalt sulfate by inhalation resulted in increased incidence of alveolar/bronchiolar neoplasms and a spectrum of inflammatory, flbrotic, and proliferative lesions in the respiratory tract of male and female rats and mice. ... 

Tetrammino - cobaltous Chloride, [Co(NH3)4]C12, is produced if ammonia gas is passed over anhydrous cobaltous chloride. Ammonia is rapidly absorbed, increase in volume takes place, and a reddish-white powder is formed. This decomposes on heating with evolution of ammonia, leaving a residue of cobalt oxide. 

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