Transcript PRODUCTION OF METAL POWDERS

PRODUCTION OF METAL POWDERS
The selection of materials in powder metallurgy is
determined by two factors.
i) The alloy required in the finished part.
ii) Physical characteristics needed in the powder.
Both of these factors are influenced by the process
used for making powder.
i)
There are numerous ways for powder production
which can be categorized as follows.
1) Mechanical methods of powder production:
i) Chopping or Cutting
ii) Abrasion methods
iii) Machining methods
iv) Milling
v) Cold-stream Process.
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
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.
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.
a) Rubbing of Two Surfaces:
When we rub two surfaces against each other, hard surface removes the
material from the surface of soft material.
* Contamination
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.
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.
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.
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;
i) Ball Milling
ii) Attrition Milling.
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
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.
Figure: Tumbler mill used for milling metal powders
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.
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.
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.
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.
Figure: Effect of impact. (a) Brittle single particle.
(b) Ductile single spherical particle
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
 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.
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.
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.
Figure: Attrition ball mill
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.
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.
Figure: Raw material steam impacting a target and
shattering in Coldstream impact process.
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.
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
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
The chemical processes can be discussed under
the headings of:
(i) Decomposition of solid phases.
(ii) Precipitation of Aqueous Solutions
(iii) Precipitation from Melts
(iv) Decomposition of Gaseous Phases
Classification of Chemical Methods:
The well known techniques which are based on
chemical/thermal decomposition are;
(i) Reduction of oxides
(ii) Precipitation from solutions
(iii) Thermal decomposition
(iv) Hydride decomposition
(v) Thermite reaction
(vi) Electro-chemical method
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.
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.
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.
(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.
The important implication of these facts is that,
(a) All metal oxide are reducible by carbon from very
low to very high temperatures ------ although
practically the temperatures necessary may be too
high, but
(b) The reaction must be prevented from reversing on
cooling, and
(c) 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.
(a)
(b)
(c)
(d)
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.
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.
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
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.)
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
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.
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.
PYRON PROCESS
•
•
Mill scale
Reducing agent ---- Hydrogen gas
Raw Material
(cleaned)
Milling
Screening
Oxidation
Reduction
Milling
Screening
Storage
• 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.
• 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
2Fe+3H2O
• The reduction product is ground and mechanically
densified to make it suitable for production of
structural parts.
• Fine particle size -----small pores ------------faster
sintering.
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.
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:
(i) Hydride Formation:
In this step turnings of metals (Ti, U, Zr etc) are heated
in hydrogen resulting in the formation of hydrides.
(ii) Milling:
Hydrides are brittle in nature and thus can be easily
crushed and ground to fine powder.
(iii) 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.
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:
i) Formation of insoluble compounds/precipitates:
The salts of metals are converted/precipitated as insoluble
hydroxides, carbonates or oxalates etc.
ii) 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.
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.
**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.
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.
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.
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.