Powder metallurgy: Atomization and chemical reduction

1.  Manufacture of Metal Powders
Atomization is the process used commercially to produce the largest tonnage of metal powders.  In water and gas atomization (Figures 2-1 and 2-2, respectively) the raw material is melted then the liquid metal is broken into individual particles.  To accomplish this, the melt stock, in the form of elemental, multi-element metallic alloys, and/or high quality scrap, is melted in an induction, arc, or other type of furnace.  After the bath is molten and homogenous, it is transferred to a tundish which is a reservoir used to supply a constant, controlled flow of metal into the atomizing chamber.  As the metal stream exits the tundish, it is struck by a high velocity stream of the atomizing medium (water, air, or an inert gas).  The molten metal stream is disintegrated into fine droplets which solidify during their fall through the atomizing tank.  Particles are collected at the bottom of the tank.  Alternatively, centrifugal force can be used to break up the liquid as it is removed from the periphery of a rotating electrode or spinning disk/cup (Figure 2-3).

Figure 2-3         Figure 2-3

Here is a video link for better understanding:

Additional alloying can be performed in the liquid metal bath after the original charge has become molten.  Also, the bath can be protected from oxidation by maintaining an inert gas atmosphere as a cover over the liquid metal.  Alternatively, the top of the furnace can be enclosed in a vacuum chamber.  The furnace type and degree of protection are determined by the chemical composition of the bath and the tendency of the metal to oxidize.
These manufacturing techniques result in powders with different characteristics and appearance, for use in specific applications (Figure 5).  Water atomization usually produces irregularly shaped particles free of internal porosity, whereas the shape of gas atomized particles is spherical, also without internal porosity.  Metal powders produced by oxide reduction are irregular in shape, have a large surface area, and usually contain a substantial amount of internal porosity.  Particles fabricated by milling or other mechanical methods exhibit a spectrum of shapes, depending on the relative ductility or brittleness of the feed material.  Ductile powders are generally flat with a high aspect ratio whereas brittle particles can be angular and regularly shaped.  The milling of agglomerated particles can cause the agglomerates to break up, sometimes with little effect on the shape of the individual particles.  Powder particles produced chemically can have shapes ranging from spherical to angular.  Electrolytic powders are of high purity with a dendrite morphology.

ATOMIZER

Figure 5: Representative Metal Powders: (a) Chemical; Sponge Iron-Reduced Ore; (b) Electolytic: Copper; (c) Mechanical: Milled Aluminum Powder Containing Disperoids (17); (d) Water Atomization : Iron; (e) Gas Atomization: Nickel-Base

Figure 5: Representative Metal Powders: (a) Chemical; Sponge Iron-Reduced Ore; (b) Electrolytic: Copper; (c) Mechanical: Milled Aluminum Powder Containing Disperoids (17); (d) Water Atomization : Iron; (e) Gas Atomization: Nickel-Base

Chemical reduction.

  • It involves chemical reaction generally a halide or oxide. This may be carried out
  • SOLID STATE: Reduction of iron oxide with carbon and tungsten oxide with hydrogen.
  • THE FORMATION OF SPONGE IRON
    By Hoganas process:
  • The batch does not move in the process .
  • Quite pure iron ore (fe304) used with carbon and limestone.
  • Contain in cylindrical ceramic container(silicon carbide).
  • Total reduction time is 24 hours and 1200 *C .
  • Limestone used to uncontaminated the sulphur from iron ore. so that pure iron can be obtain.

TUNGESTON POWDER

  • The ore of tungsten is wolfermite(FeWO4) and scheelite(CaWO4)
  • The ore is mixed in Hcl to form tungestic acid.
  • Tungestic acid is mixed in ammonia to form APT (ammonium paratungastate)
  • APT calcined to give blue oxide.
  • Blue oxide and caustic soda are mixed to give tungsten ppt as Wo3.
  • At 850*c reduction take place and the hydrogen used should be 99.4% pure to give high quality powder.


chemical reduction chamber

chemical reduction chamber

Picture4

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