المحاضرة الرابعة

2 METAL POWDER PRODUCTION
The lecture of metal powder production is a very extensive one. In this lecture the description of most important principles and processes is given, which are of technological significance. The significant manufacturing methods may be classed as follows:
1. Chemical methods
2. Physical methods
3. Mechanical methods
Virtually any material can be made into a powder by one or more of these methods. The exact method chosen must be related to the type of application and the desired properties and structure of the final product.
2.1. Chemical Methods
For detailed description of basic thermodynamics and kinetics aspects readers are referred to follow standard text1 as an introductory guide. These methods can be further classified as chemical reduction and decomposition.
2.1.1 Chemical Reduction
Chemical reduction involves chemical compound most frequently an oxide, but sometimes a halide or other salt of the metal. This may be carried out:
(a) from the solid state – as in the reduction of iron oxide with carbon or of tungsten oxide with hydrogen.
Sponge iron powder produced by the H?gan?s process is a typical example of this production method. The H?gan?s process is based in the use of quite pure magnetite (Fe3O4) ores found in northern Sweden.
The iron ore is reduced with a carbonaceous material. Figure 2. Shows the steps involved in producing such powder. The ore is ground to a particle size distribution determined by each of the desired iron powder. The ore powder is placed in the center of cylindrical ceramic containers (‘saggers’made of silicon carbide) surrounded on the outside by a concentric layer of a mixture of coke and limestone. The saggers are placed in layers upon cars which are pushed through a fuel fired tunnel kiln. The carbon monoxide produced from the coke reduces the ore to iron. Total reduction time is of the order of 24 hours at a reduction temperature of 1200oC. The limestone serves to bind any sulphur in the coke and prevents its contaminating the iron. The sponge iron is mechanically removed from the saggers, ground and the resulting powder magnetically separated from impurities. In a final reduction step the powder is carried through a continuous furnace in hydrogen atmosphere on a belt made of stainless steel.
Another iron powder with trade name ‘Pyron iron powder’ is produced from mill scale by reduction with hydrogen. The mill scale is ground, magnetically separated and first roasted in air to convert the Fe3O4 to Fe2O3 because the rate of reduction of Fe2O3 with hydrogen is faster than that of Fe3O4. The oxide is then reduced in a belt furnace at temperature near 980oC. The reduction product is ground. It is noticed that the pores in Pyron iron powder particles are considerably finer than those in sponge iron powder particles, because of the coarsening of pores at relatively higher reduction temperature in the latter case.