Carbonyl Iron Powders

Revised by Sotiri Papoulias, International Specialty Products

Very fine iron powders, like nickel powders, can be produced by carbonyl decomposition (vapor metallurgy) techniques. The first patents covering the carbonyl process were registered to Ludwig Mond in 1890. A brief review of early metal carbonyl research, with emphasis on his work, is presented in the article "Production of Nickel-Base Powders" in this Volume. In addition, some of the different types of metal carbonyls that can be formed and the formation and decomposition reactions of carbonyls used for the production of primary metal and powder products are described in that article in the discussion of nickel carbonyl powders. This section, however, deals only with the production of iron powders produced by the decomposition of iron pentacarbonyl, Fe(CO)5.

Process Conditions. Iron pentacarbonyl, the raw material from which carbonyl iron powder is produced, is a liquid with a boiling point of 102.8 °C (217 °F). It is formed by passing carbon monoxide over a reduced sponge iron at relatively high pressures and at temperatures ranging from 170 to 200 °C (340 to 390 °F). The physical properties of iron pentacarbonyl are:

Formula

Fe(CO)s

Color and state (room temperature)

Viscous yellow liquid

Molecular weight

195.9

Iron, %

28.51%

Melting point, °C (°F)

-21 (-5.8)

Boiling point, °C (°F)

102.8 (217)

Specific gravity (room temperature)

1.457

Heat of formation, kJ/g • mol (kcal/g • mol)

-964.0 (-230.2)

Typically, the feed stocks used are high-surface-area oxidized iron powders or iron turnings that are reduced in hydrogen or another suitable atmosphere prior to carbonylation. The presence of oxygen or oxides on the surface of the iron hinders the reaction, whereas the presence of catalysts improves the rate of formation.

The reaction is exothermic. To achieve commercial production rates, carbonylation is conducted at 130 to 180 atm (1900 to 2600 psi) and temperatures of 170 to 175 °C (340 to 350 °F). As shown in Fig. 8, iron pentacarbonyl formation increases with increasing carbon monoxide pressure. Increasing the temperature increases the rate of reaction. Increasing pressure prevents excessive decomposition of the carbonyl. At temperatures above 200 °C (390 °F), carbonyl yield is reduced rapidly by the increased conversion of carbon monoxide to carbon and carbon dioxide by the disproportionation reaction. Iron pentacarbonyl is then condensed and purified by distillation, whereupon it reverts to di-iron nonacarbonyl, Fe2(CO)9.

Fig. 8 Effect of system pressure of 100 to 300 atm (1470 to 4400 psi) and temperature on the formation of iron pentacarbonyl. Source: J. Chem. Soc., Vol 121, 1922, p 29-32

The rate of decomposition for di-iron nonacarbonyl is temperature dependent, with maximum rates achieved at about 200 to 250 °C (390 to 480 °F). At higher temperatures, the iron produced oxidizes in the carbon monoxide atmosphere, and high-carbon powder is formed.

Present Commercial Processes. In the United States, International Specialty Products (formerly GAF Corporation) currently produces high-purity iron powder using carbonyl decomposition technology. In Europe, BASF produces carbonyl iron powders using the same decomposition product, but employing scrap iron as source material. The GAF (ISP) process starts with high-purity iron. Iron powder, or sponge, which is initially treated under hydrogen to reduce surface oxides to metallic iron. This feedstock is then reacted with carbon monoxide under pressure at elevated temperature to form liquid iron pentacarbonyl. Subsequently, the carbonyl is vaporized and thermally decomposed to form "crude" carbonyl iron powder. The crude powder is refined chemical processing and mechanically separated to yield various size grades of powder. The carbon content of the powder may reach a maximum of 0.8%, which is reduced to as low as 0.075% to satisfy specific requirements.

Powder Properties. High-purity iron powders produced using carbonyl technology are typically spherical and are available in several grades that range from 2 to 9 /'m Fisher subsieve size, with apparent densities of 1.2 to 3.2 g/cnr\ The powder can be produced at or near atmospheric pressure. Particles characteristically exhibit an onionskin structure due to minute carbon deposits in alternate layers. The high purity of these powders makes them an excellent starting material for the production of magnetic cores and electronic components. This powder oxidizes readily in air and is packaged under an inert gas to facilitate storage.

These powders are essentially free of nonferrous metals and contain 0.3% O, 0.075 to 0.8% C (depending on the grade), and 0.05 to 0.9% N. Alloy powders also are available that nominally contain 92.5% Fe (minimum), 0.8% C, 0.3% O, and 6.0% N. Primary applications of these powders include:

• High-frequency cores for radio transmitters, telephones, televisions, direction finders, VHF and UHF circuitry, and radar equipment

• Magnetic-fluid clutch and brake systems

• Carbide and diamond cutting tools

• Pharmaceuticals (iron supplements, multivitamins)

• P/M materials and alloys

• Magnetic coatings and tapes

Radar-absorbing materials Food enrichment and animal feed

More information on applications is given in the article "Specialty Applications of Metal Powders" in this Volume. Production of Iron Powder*

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