PRODUCTION OF METAL POWDERS

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PRODUCTION OF METAL POWDERS

• The selection of materials in powder metallurgy
  is determined by two factors.
• The alloy required in the finished part.
• Physical characteristics needed in the powder.
• Both of these factors are influenced by the
  process used for making powder.

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• There are numerous ways for powder production
  which can be categorized as follows.
• Mechanical methods of powder production
• i) Chopping or Cutting
• ii) Abrasion methods
• iii) Machining methods
• iv) Milling
• v) Cold-stream Process.

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• 2. Chemical methods of powder production
• i) Reduction of oxides
• ii) Precipitation from solutions
• iii) Thermal decomposition of compounds
• iv) Hydride decomposition
• v) Thermit reaction
• vi) Electro- chemical methods

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• 3. Physical methods of powder production
• i) Water atomization
• ii) Gas atomization
• iii) Special atomization methods
• The choice of a specific technique for powder
  production depends on particle size, shape,
  microstructure and chemistry of powder and also
  on the cost of the process.

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• 1. Chopping or Cutting
• In this process, strands of hard steel wire, in
  diameter as small as 0.0313 inches are cut up
  into small pieces by means of a milling cutter.
• This technique is actually employed in the
  manufacturing of cut wire shots which are used
  for peening or shot cleaning.
• Limitations
• It would, however, be difficult and costly to
  make powders by this method and for this reason
  it is not profitable to discuss the technique in
  detail.
• 2. Rubbing or Abrasion Methods
• These are all sorts of ways in which a mass of
  metal might be attacked by some form of
  abrasion.
• Rubbing of Two Surfaces
• When we rub two surfaces against each other, hard
  surface removes the material from the surface of
  soft material.
• Contamination

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• b) Filing
• Filing as a production method has been frequently
  employed, especially to alloy powders, when
  supplies from conventional sources have been
  unobtainable.
• Such methods are also used for manufacture of
  coarse powders of dental alloys.
• Filing can also be used to produce finer powder
  if its teeth are smaller.
• commercially not feasible.
• c) Scratching
• If a hard pin is rubbed on some soft metal the
  powder flakes are produced.
• Scratching is a technique actually used on a
  large scale for the preparation of coarse
  magnesium powders.
• scratching a slab of magnesium with hardened
  steel pins.
• a revolving metal drum to the surface of which
  is fixed a scratching belt.

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• The drum, which is about 8 inches in diameter,
  rotates at a peripheral speed of approximately
  2500 ft./min. The slab of magnesium metal, 14 in.
  wide by 1.75 in. thick enters through a gland in
  the drum casing and presses against the steel
  pins.
• d) Machining
• A machining process, using for example a lathe or
  a milling cutter in which something more than
  just scratching is involved, since the attacking
  tool actually digs under the surface of the metal
  and tears it off.
• On lathe machine by applying small force we get
  fine chips.
• A large amount of machining scrap is produced in
  machining operations. This scrap in the form of
  chips and turnings can be further reduced in size
  by grinding.
• small scale production.

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• Disadvantages
• Lack of control on powder characteristics,
  including chemical contamination such as
  oxidation, oil and other metal impurities.
• The shape of the powder is irregular and coarse.
• Advantages
• For consuming scrap from another process,
  machining is a useful process.
• Presently the machined powder is used with high
  carbon steel and some dental amalgam powders.

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COMMERCIAL METHODS

• These are the methods used for high production
  rate. Best examples of mechanical production
  methods are the Milling Process and Cold Stream
  Process.
• Milling
• The basic principal of milling process is the
  application of impact and shear forces between
  two materials, a hard and a soft, causing soft
  material to be ground into fine particles.
• Milling techniques are suitable for brittle
  materials.
• Two types of milling are
• Ball Milling
• Attrition Milling.

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• Objectives of milling include
• Particle size reduction (comminution or grinding)
• Shape change (flaking
• Solid-state alloying (mechanical alloying)
• Solid-state blending (incomplete alloying)
• Modifying, changing, or altering properties of a
  material (density, flowability, or work
  hardening)
• Mixing or blending of two or more materials or
  mixed phases

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• Ball Milling
• Ball milling is an old and relatively simple
  method for grinding large lumps of materials into
  smaller pieces and powder form.
• Principle of the process
• The principle is simple and is based on the
  impact and shear forces.
• Hard balls are used for mechanical comminution of
  brittle materials and producing powders.
• Milling Unit
• The basic apparatus consists of the following
• A ball mill or jar mill which mainly consists of
  a rotating drum lined from inside with a hard
  material.
• Hard balls, as a grinding medium, which continue
  to impact the material inside the drum as it
  rotates/rolls.

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Figure Tumbler mill used for milling metal
powders
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• Important Parameters
• 1. The most important parameter to consider is
  the speed of rotation of the drum. An
  optimum/critical speed is adjusted for maximum
  impact velocity.
• Critical speed is the speed above which the
  ball will centrifuge.
• Very slow speed of rotation will not carry the
  balls to the top, these will roll back down the
  drum sides.
• Very fast speed (higher than critical speed) will
  not let the balls drop down as they will be
  carried around due to centrifugal forces. Thus,
  an optimum speed is required. This speed of
  rotation varies with the inverse square root of
  the drum diameter.

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• 2. The material of grinding media and its size
  and density.
• The size and density of the milling medium is
  selected according to the deformation and
  fracture resistance for metals.
• For hard and brittle materials large and dense
  media is used. Whereas, small balls are used for
  finer grinding.
• As a general rule, the balls should be small and
  their surface should be a little rough. The
  material of the balls and lining of the drum
  should be same as that of the material being
  ground.

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• 3. The rate of milling of a powder is a function
  of quantity in the total space between the balls.
• 4. Lubricants and surface active agents are used
  to nullify the welding forces which causes
  agglomeration.
• Grinding Mechanism
• During milling the following forces cause
  fracture of material into powder.
• Impact Forces These are caused by instantaneous
  striking of one object on the other. (Impact is
  the instantaneous striking of one object by
  another. Both objects may be moving or one may be
  stationary).
• Shear Forces These are caused as one material
  slides/rubs against the other.

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The impact process is shown in Fig. 1. This model
represents the moment of collision, at which
particles are trapped between two colliding balls
within a space occupied by a dense cloud,
dispersion, or mass of powder particles. This
phenomenon is typical in dry and wet milling
operations that use colliding milling mediums
such as tumbler, vibratory, and attrition ball
mills.
Figure Model of impact event at a time of
maximum impacting force showing the formation of
a micro-compact.
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Figure Effect of impact. (a) Brittle single
particle. (b) Ductile single spherical particle
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Figure Process of trapping an incremental volume
of powder between two balls in a randomly
agitated charge of balls and powder. (a) through
(c) Trapping and compaction of particles. (d)
Agglomeration. (e) Release of agglomerate by
elastic energy
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• Corrosion of metal in grinding fluid also
  facilitates comminution.
• Ball milling is used for brittle materials.
• This method is not suitable for most of the
  metals due to their ductility and cold welding.
• Limitations
• Rubbing action causes contamination of powder
  since balls may also get rubbed.
• Working hardening of metal powder is caused
  during milling.
• There is a possibility of excessive oxidation of
  final powder.
• Quality of powder is poor.
• Particle welding and agglomeration may take place.

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ATTRITION MILLING

• Attrition is the term which means to wear or rub
  away. It is a process of grinding down by
  friction.
• Milling Unit
• In attrition milling a very high efficiency ball
  mill is agitated by a vertical rotating shaft
  with horizontal arms.
• In these mills the rotational speeds are nearly 6
  80 rpm while the size of medium (balls) used is
  3 6 mm.
• Power is used to rotate the agitator and not the
  vessel as in case of ball mills. The central
  rotating shaft of attrition mill is equipped with
  several horizontal arms. When rotated, it exerts
  the stirring action to tumble the grinding medium
  randomly throughout the entire chamber.

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• Mechanism of milling
• The milling action is done by impact and shear
  forces. The charge is impacted by balls traveling
  in various trajectories that collide within the
  area.
• Impaction is caused by constant impinging of
  grinding medium due to irregular movements.
• Shearing action is produced by random movement of
  balls in different rotational directions which
  exert shearing force on adjacent slurry.
• Continuous attrition mills
• Powders of very hard materials such as ceramics,
  carbides and hard metals are being produced by
  this technique.
• The particle size becomes finer with increasing
  milling time and the shape of particle is
  angular.
• To avoid possible contamination, the balls,
  stirring rods and the tank may be made from same
  material as the powder.

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Figure Attrition ball mill
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COLD STREAM PROCESS

• This process is based on impact phenomenon caused
  by impingement of high velocity particles against
  a cemented carbide plate.
• The unit consists of
• A feed container
• A compressor capable of producing a high velocity
  stream of air (56 m3/min.) operating at 7 MPa
  (1000 psi)
• A target plate, made of cemented tungsten
  carbide, for producing impact
• A classifying chamber lined with WC while the
  supersonic nozzle and target generally are made
  of cemented tungsten carbide.

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• Mechanism of the Process
• The material to be powdered is fed in the chamber
  and from there falls in front of high velocity
  stream of air.
• This air causes the impingement of material
  against target plate, where material due to
  impaction is shattered into powder form. This
  powder is sucked and is classified in the
  classifying chamber. Oversize is recycled and
  fine powder is removed from discharge area.
• Rapidly expanding gases leaving the nozzle
  create a strong cooling effect through adiabatic
  expansion. This effect is greater than the heat
  produced by pulverization.

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Figure Raw material steam impacting a target and
shattering in Coldstream impact process.
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CHEMICAL METHODS

• Almost all metallic elements can be produced in
  the form of powders by suitable chemical
  reactions or decomposition.
• For example all chemical compounds can be
  decomposed into their elements if heated to
  sufficient high temperatures.
• If the non-metallic radical could be removed, for
  example by continuous evacuation or by
  entrainment in an inert gas, then practical
  methods of making metal powders might be feasible.

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• Theory of the process
• Mostly chemical methods are based on the
  decomposition of a compound into the elemental
  form with heating or with the help of some
  catalyst.
• In most cases such processes involve at least
  two reactants.
• (i) a compound of the metal
• (ii) a reducing agent

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• Either of the two may be in the state of a
  solid, liquid (melt), solution or gas and it
  would seem therefore that from this point of view
  at least sixteen types of such reactions could be
  possible.
• Solid
• Liquid
• Solid
• Solution
• Gas

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• The chemical processes can be discussed under
  the headings of
• Decomposition of solid phases.
• Precipitation of Aqueous Solutions
• Precipitation from Melts
• Decomposition of Gaseous Phases

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• Classification of Chemical Methods
• The well known techniques which are based on
  chemical/thermal decomposition are
• Reduction of oxides
• Precipitation from solutions
• Thermal decomposition
• Hydride decomposition
• Thermite reaction
• Electro-chemical method

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• REDUCTION OF METAL OXIDES
• Manufacturing of metal powder by reduction of
  oxides is extensively employed, particularly for
  Fe, Cu, W and Mo. As a manufacturing technique,
  oxide reduction may exhibit certain advantages
  and disadvantages. These are listed below
• Advantages
• A variety of reducing agents can be used and
  process can be economical when carbon is used.
• Close control over particle size --- because
  oxides are generally friable, easily pulverized
  and easily graded by sieving.
• Porous powders can be produced which have good
  compressive properties.
• Adoptability either to very small or large
  manufacturing units and either batch or
  continuous processes.

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• Limitations
• Process may be costly if reducing agents are
  gases.
• Large volumes of reducing gas may be required,
  and circumstances where this is economically
  available may be limited in some cases, however,
  costs may be reduced by recirculation of the gas.
• The purity of the finished product usually
  depends entirely upon the purity of the raw
  material, and economic or technical
  considerations may set a limitation to that which
  can be attained.
• Alloy powders cannot be produced.

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• Mechanism of Reaction
• Most metal powders manufactured by reduction of
  oxides are produced using solid carbon or
  hydrogen, cracked ammonia, carbon monoxide, or
  mixture of such gases. As a reducing agent for
  metal oxides, carbon holds an important and
  peculiar position because of its general
  cheapness and availability, and peculiar for the
  following reasons.
• According to circumstances and temperature, three
  carbon/oxygen reactions can occur
• (i) C O2 CO2
• In this reaction, the number of gaseous molecules
  remain constant and the entropy change is very
  small. The free energy change of the reaction is
  almost constant from room temperature to 2000 oC.

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• (ii) 2CO O2 2CO2
• The reaction is accompanied by a decrease in the
  number of gas molecules and in entropy with a
  considerable free energy change.
• (iii) 2C O2 2CO
• This reaction involves an increase in the number
  of gaseous molecules and a considerable increase
  in entropy and a considerable free energy change.
  This implies that within temperatures normally
  used metallurgically, carbon monoxide becomes
  increasingly stable the higher the temperature.
• Consequently, the free energy change temperature
  curves for these reactions intersect ------- at
  about 700 oC.

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• The important implication of these facts is
  that,
• All metal oxide are reducible by carbon from very
  low to very high temperatures ------ although
  practically the temperatures necessary may be too
  high, but
• The reaction must be prevented from reversing on
  cooling, and
• The product of the reduction will be mainly CO2
  below 700 oC and mainly CO above this
  temperature. At high temperatures, any carbon
  dioxide is reduced by any excess carbon, forming
  more stable CO.

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• When using a reducing gas, continued contact
  between the oxide and the reducing gas must take
  place by
• Diffusion of gas through the metal to the oxide,
• Diffusion of oxygen, or oxide, through the metal
  to the gas,
• Both (a) and (b), or
• Movement of one kind or another through pores.

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• Production of Iron Powder
• by Reduction of Iron Oxide
• (Direct Reduction Process)
• Iron powders are commercially used for a large
  number of applications such as fabrication of
  structural parts, welding rods, flame cutting,
  food enrichment and electronic and magnetic
  applications.
• The classical technique for production of iron
  powder is the reduction of iron oxide.

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• Theory of the process
• It is the oldest process of production of iron
  powder by using carbon as the reducing agent.
• In this process pure magnetite (Fe3O4) is used.
  Coke breeze is the carbon source used to reduce
  iron oxide. Some limestone is also used to react
  with the sulphur present in the coke. The mixture
  of coke and limestone (85 15) is dried in a
  rotary kiln and crushed to uniform size.
• Hoganas Process

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• The ore and coke-limestone mixture is charged
  into ceramic tubes (Silicon Carbide) with care so
  that ore and reduction mixture are in contact
  with each other but not intermixed. It can be
  achieved by using concentric charging tubes with
  in the ceramic tube.
• (A pair of concentric steel charging tubes is
  lowered to the bottom of the ceramic tubes. The
  ore is fed between the steel tubes. The
  coke-limestone mixture is fed within the inner of
  the two concentric charging tubes and between the
  outer charging tube and the inner wall of the
  ceramic tube, leaving the ore and the reduction
  mixture in contact with one another, but not
  intermixed.)

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• Charged ceramic tubes are loaded on the Kiln cars
  (thirty six tubes on each) and cars are pushed
  into 170 meter long tunnel kiln where the
  reduction occurs.
• The total time a car is present in the kiln is 68
  hrs. Gas burners heat the 150 meter tunnel at a
  temperature of 1200-1260 oC and remaining length
  is cooled by air circulation.
• Within the hot zone, several chemical reactions
  occur and metallic iron is formed in the form of
  sponge cake.
• The main reaction is
• MO R M RO

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• If magnetite ore is used, then the following
  reactions will take place
• Fe3O4 3CO FeO 3CO2
• FeO CO Fe CO2
• C ½ O2 CO
• Decomposition of the limestone generates carbon
  dioxide, which oxidizes the carbon in the coke to
  form carbon monoxide. The ferrous iron oxide is
  further reduced by the carbon monoxide to
  metallic iron.
• Desulphurization occurs in parallel with
  reduction by reaction between gas and sulphides
  present in the ore resulting in gaseous sulphide
  compounds which in turn react with lime to form
  calcium sulphide.

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• The sponge cake is removed from ceramic tubes and
  dropped into a tooth crusher where this is broken
  into pieces.
• After these pieces are ground to desired particle
  size. During grinding the powder particles are
  considerably work hardened. The powder is
  annealed at 800 - 870 oC in the atmosphere of
  dissociated ammonia.
• The powder is loosely sintered, but requires only
  light grinding and screening to produce a
  finished product.

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PYRON PROCESS

• Mill scale
• Reducing agent ---- Hydrogen gas

Raw Material (cleaned)
Milling
Screening
Oxidation
Reduction
Milling
Screening
Storage
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• Mill scale is basically obtained from steel mills
  which produce sheets, rods, wires, plates and
  pipes.
• The mill scale mainly consists of Fe3 O4, and
  also contains oxides of tramp elements normally
  associated with steel, especially Si, Mn and Cr
  in the form of very finely dispersed oxides -----
  difficult to reduce.
• The mill scale is dried and ground up to the
  desired particle size in a continuous ball mill.
  (- 100 mesh)
• Oxidation of the mill scale at 870 to 980 oC
  converts Fe O and Fe3 O4 to ferric oxide (Fe2
  O3). This process is essential to ensure uniform
  properties of Pyron-iron Powder.

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• Reduction of ferric oxide by hydrogen is done in
  an electric furnace (30 40 meter long) at 980
  oC . (continuous belt furnace).
• Hydrogen is supplied by NH3 cracking plant and
  reduction is done at 980 oC.
• Fe2O3 3H2 2Fe3H2O
• The reduction product is ground and mechanically
  densified to make it suitable for production of
  structural parts.
• Fine particle size -----small pores
  ------------faster sintering.

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• Powder Characteristics
• The Pyron Powder is a porous and finer.
• It has sponge like microstructure.
• It sinters faster as compared to powder formed by
  other commercials processes.
• Advantages
• There is no relative movement of particles of the
  charge to each other or to the belt, therefore
  sticking and welding is avoided.
• Low carbon contents in the final product because
  of use of hydrogen.
• Low labor cost.
• Thin beds and continuous flow of reducing gases
  lead to a comparatively short time of reduction.
• The purity of the iron powder product is
  entirely a function of the raw mill scale.

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HYDRIDE DECOMPOSITION

• This method of powder production is used for
  precious metals. Hydrides are binary compounds of
  metals and hydrogen.
• The main steps are as follows
• Hydride Formation
• In this step turnings of metals (Ti, U, Zr etc)
  are heated in hydrogen resulting in the formation
  of hydrides.
• Milling
• Hydrides are brittle in nature and thus can be
  easily crushed and ground to fine powder.
• Dehydridation
• The fine powder of hydrides is heated under
  vacuum at elevated temperature to eliminate
  hydrogen from metal, and consequently a fine
  metal powder is obtained.

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PRECIPITATION FROM SOLUTIONS

• This method is used for precious metals.
• Leaching an ore or ore concentrate, followed by
  precipitating the metal from leach solution.
• Steps Involved
• Formation of insoluble compounds/precipitates
• The salts of metals are converted/precipitated
  as insoluble hydroxides, carbonates or oxalates
  etc.
• Decomposition
• On heating, these compounds/ppts. decompose into
  metal or metal oxides and gaseous products.
• The examples of this technique are the
  production of uranium dioxide, platinum,
  selenium, silver, nickel and cadmium oxides.

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• Powder characteristics
• The chemically precipitated powders can have high
  purity and have fine particle size and tendency
  towards agglomeration.
• The particle shape is irregular or cubic or
  sometime it is sponge like.
• The flow properties of these powders are poor and
  the packing densities are low.

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• In some cases, powder is produced by gaseous
  reactions, i.e. metal chlorides, fluorides or
  oxides of vanadium, niobium, tungsten, uranium,
  titanium, and zirconium are reduced with sodium,
  magnesium or hydrogen. The reaction product is
  leached with dilute hydrochloric acid to remove
  sodium and magnesium chlorides. The resulting
  powder is spongy like with irregular shape.

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THE CARBONYL PROCESS

• The only method for the manufacture of metal
  powder by the pyrolysis of a gaseous compound
  which has been used industrially on a substantial
  scale is the carbonyl iron or nickel process.
• When iron and nickel ores react under high
  pressure (70 300 atm.) with carbon monoxide,
  iron pentacarbonyl Fe(CO)5 or nickel
  tetracarbonyl Ni(CO)4 is formed, respectively.
• Both compounds are liquids at room temperature.
• Fe(CO)5 evaporates at 103 oC and Ni(CO)4 at 43
  oC.

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• Precipitate Formation
• This step of the process is carried out according
  to the following scheme
• The liquid carbonyles are stored under pressure
  in tanks submerged in water.
• The distilled and filtered liquids are conveyed
  to steam heating cylinders, where they are
  vaporized.
• The vapors of liquid are sent to decomposers. The
  decomposers are jacketed and heated, giving an
  internal temperature of 200 250 oC. These
  cylinders are 9 10 feet high with an internal
  dia of 3 feet, with conical bottoms.
• The incoming stream of vapors meets a tangential
  stream of ammonia gas. CO is removed here and
  precipitates of metals are formed which are then
  sieved, dried and may be milled to break up the
  agglomerates.
• The CO gas arising from the decomposition is
  recovered and re-used.

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• Carbonyl iron powder is used for the production
  of magnetic powder cores for radio or television
  applications.
• In P/M it is used for the manufacture of soft
  magnetic materials and permanent magnets.
• Because of its high price and poor die filling
  properties, it is not suitable for the
  manufacture of sintered structural components.
• The carbonyl process is also well suited for the
  extraction of both metals from lean ores. The
  process can be controlled so as to yield a
  spherical metal powder.