Abstract: Iron whiskers are produced by thermal decomposition of iron pentacarbonyl vapor in an indirectly heated empty-space decomposer in which the cross-sectional area for entry of the iron pentacarbonyl into the empty-space decomposer is from 10 to 40% of the cross-sectional area of the empty-space decomposer, the mass flow density of the iron pentacarbonyl vapor, based on the cross-sectional area of the decomposer, is from 0.01 to 0.07 kg per square meter per second, and the temperature in the empty-space decomposer should at no point be below 360.degree. C.

Abstract: Iron powder is prepared by thermal decomposition of iron pentacarbonyl in a cavity decomposer, with an average amount of from 4 to 8 kg per m.sup.2 per hour of iron carbonyl striking the heating surface, by a process in which the velocity of the inflowing stream is brought to about 0.2-4 kg per m.sup.2 per second of carbonyl vapor or vapor/inert gas mixture by changing the inlet cross-section.

Abstract: Finely divided metal oxides produced by combustion of metal carbonyls are separated from the stream of gas by filtration. Before filtration, the stream of gas is subjected to a permanent change of direction in a tubular zone. This change of direction must be sufficient to cause a centrifugal acceleration which is from at least 1,000 to 10,000 times the acceleration due to gravity.