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A SEMINAR ON
ELECTROLYTIC PRODUCTIONOF METALLIC POWDER
PREPARED BY:
DARSHIT FADADU
ROLL NO: 938
GUIDED BY:Dr. V.V.MATHANE SIR
DEPARTMENT OF METALLURGICAL AND MATERIAL ENGINEERING
FACULTY OF TCHNOLOGY & ENGINEERING
M.S UNIVERSITY
VADODARA
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INTRODUCTION
These methods are based on the electrolysis of molten solutions of metals
or fused salts. The metals are electrically deposited on the cathode of an
electrolytic cell as a sponge or powder or at least in a physical form in
which it can be easily disintegrated into a powder.
An electrolytic process is the use of electrolysis industrially to refine
metals or compounds at a high purity and low cost. Some examples are
the hall-heroult process used for aluminum, or the production of hydrogen
from water. Electrolysis is usually done in bulk using hundreds of sheets of
metal connected to an electric power source. In the production of copper,
these pure sheets of copper are used as starter material for the cathodes,
and are then lowered into a solution such as copper sulfate with the large
anodes that are cast from impure (97% pure) copper. The copper from the
anodes are electroplated on to the cathodes, while any impurities settle tothe bottom of the tank. This forms cathodes of 99.999% pure copper.
Brief history
The very earliest uses of metal powders have been traced to severalparts of the world. For example, gold powder was fired ontojewellery by the Incas, and the Egyptian uses of iron powder date
back to 3000 BC. During the 1800's, the use of powder metallurgy techniques began
in earnest. Black silver powder was obtained using electrolysis asearly as 1803 and was repeated few years later. Smee recognized,as early 1842, the main features of the dependence of crystallitesize on current density and concentration and he described theconditions under which black metals are formed, whose dark colorhe correctly explained as being caused by the very small size of thedeposited particles. The conditions of formation and the propertiesof electrolytic black silver powder were studied in detail already inthe nineteenth century, and the growth of the highly disperseddeposits of silver and copper were followed under the microscope.In an extended study of the electrodeposition of copper, the currentdensities above which copper is deposited in powder form weredetermined in 1886.
Besides copper and silver, the nature and the causes of formation ofzinc sponge have drawn the attention of various early investigators.One process by which iron powder (mainly for magnets) wasmanufactured around 1920 was described in details. Further olderwork on the formation of powders, flakes, etc., of various metals(lead-sponge, tungsten, platinum, etc.) is reported in a period up to
1925.
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Before 1910, many papers dealt in detail with the formation ofelectrolytic metal powders and sponges; however, this problem wasrarely considered for its own sake between 1910 and 1935. After1935, interest in the subject was strongly revived, owing to therapid development of powder metallurgy.
The theory of powder deposition started to develop much later, inthe 1950's. Electrodeposition of metal powders evolved from an artto a science by the chapter of Ibl (1962) and especially by the bookof Calusaru (1979). Calusaru reviewed all theoretical and practicalknowledge up to 1979, and provided the basis for the scientificapproach to the field. The essence of it is the conclusion that forelectrochemical production of metal powders the correspondingelectrochemical processes must be under diffusion control.
The properties of metal powders depend on the properties of thepowder particles, which, in turn, depend on the conditions of
electrodeposition. A methodology for modeling of powder particles,and hence properties of the powder, for electrodeposition at aperiodically changing rate was introduced by Popov and Pavlovic(1993). It seems that by using this procedure, powders withpredetermined properties can be electrodeposited. Thedevelopment of this idea will probably be the future ofelectrochemical powder formation.
Basic principle
The basic principle is the electrolysis process in which decomposition of a
molten salt/aqueous solution into its ions is obtained by the passage of
electric current. The metallic ions are deposited at the cathode which can
be removed with a brush and collected at the bottom.
The equipment used is an electrolytic bath made of steel, and lined from
inside with rubber. Two electrodes are inserted in the bath.
Cathode is made of lead while anode is made of the same metal whose
powder is being produced.
The electrolytic tanks have conical bottoms with a valve. Suction pipes are
connected to these bottoms and powder is removed from the tank.
The efficiency of the tank/process depends on the deposition rate.
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Figure: Electrolytic Cell Operation for Deposition of
Powder
Faradays law of electrolysis:
Faraday's 1st Law of Electrolysis - The mass of a substance altered at an
electrode during electrolysis is directly proportional to the quantity of electricity
transferred at that electrode. Quantity of electricity refers to the quantity of
electrical charge, typically measured in coulomb.
Faraday's 2nd Law of Electrolysis - For a given quantity of electricity (electric
charge), the mass of an elemental material altered at an electrode is directly
proportional to the element's equivalent weight. The equivalent weight of a
substance is its molar mass divided by an integer that depends on the reaction
undergone by the material
http://en.wikipedia.org/wiki/Electrolysishttp://en.wikipedia.org/wiki/Quantity_of_electricityhttp://en.wikipedia.org/wiki/Electrical_chargehttp://en.wikipedia.org/wiki/Equivalent_weighthttp://en.wikipedia.org/wiki/Molar_masshttp://en.wikipedia.org/wiki/Quantity_of_electricityhttp://en.wikipedia.org/wiki/Electrical_chargehttp://en.wikipedia.org/wiki/Equivalent_weighthttp://en.wikipedia.org/wiki/Molar_masshttp://en.wikipedia.org/wiki/Electrolysis8/3/2019 Darshit Content
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Mathematical form:
Faraday's laws can be summarized by
where:
m is the mass of the substance liberated at an electrode in grams Q is the total electric charge passed through the substance F= 96,485 C mol1 is the Faraday constant M is the molar mass of the substance zis the valency number of ions of the substance (electronstransferred per ion).
Note that M / zis the same as the equivalent weight of the substance altered.
For Faraday's first law, M,F, andzare constants, so that the larger the value ofQ the
larger m will be.
For Faraday's second law, Q,F, andzare constants, so that the larger the value ofM / z
(equivalent weight) the larger m will be.
In the simple case of constant-current electrolysis, Q =Itleading to
and then to
where:
n is the amount of substance ("number of moles") liberated:n = m / M
tis the total time the constant current was applied.
In the more-complicated case of a variable electrical current, the total charge Q is
the electric currentI() integrated over time :
Here tis the totalelectrolysis time. Please note that tau is used as the currentI is a function of tau.
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Powder production at cathode is favored by:
high current density;
weak metal concentration;
addition of acids;
low temperature;
avoidance of agitation, and;
Suppression of convection.
* Very fine powder can be obtained when the current flowing is so
strong in relation to the strength of the solution that hydrogen is
strongly evolved from the cathode.
Hydrogen evolution is encouraged by:
Increasing cell voltage;
Diminishing the size of the cathode;
Bringing the anode and cathode closer together;
Increasing the temperature;
Weakening the strength of the metallic solution
Adding acid
* When metal is deposited without evolution of hydrogen, thedeposit may be ductile and compact if the current is just not great
enough to cause hydrogen formation, or very hard with large
crystals using strong solutions and large quantities of electricity, or
sandy and brittle with little cohesion using very small current.
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DESIGN CONSIDERATIONS:
An outstanding characteristics of electrolytic powder process is the
large number of variables which either have to be selected and
fixed before plant is erected, or which have to be controlled during
operation. The most important are;
Electrolytes
Electrodes
Current
Flow of electrolyte
Structural considerations
After treatment
Electrolytes:
The choice of the type of electrolyte will depend largely upon the
cost of the chemicals involved.
Electrolyte should not corrode the apparatus i.e., it should be of
non-corrosive nature.
Concentration of the electrolyte should remain same with the
passage of time.
Relatively pure salts of copper which are cheap and freely available
are uncommon, and therefore most copper powder production has
been derived from sulphate-sulphuric acid baths.
Some scientists are in favor of copper chloride bath because of
better cathode efficiency, lower cell voltage and less power
consumption. It is claimed that the chloride bath produces a more
dendritic powder with better pressing properties
In the case of sulphate electroyte, the presence of a small amount
of chloride improves the anode current efficiency. Such additions
may, however, cause corrosion problems in the cells and
deterioration of the keeping qualities of the powder.
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Having selected the type of bath, the exact composition must then
be chosen and thereafter maintained with considerable care
With copper sulphate/sulphuric acid electrolytes, it has been found
that cathode current efficiency improved as the copper content
increased, reaching a maximum of 96.4 % at 35 gm./liter, and
decreased with increasing acid, being 91.9 % at 25 gm./liter. The
apparent density of powder produced increased to a maximum of
0.663 gm./ml. at 8.6 amp./dm.2 and thereafter decreased, and
similarly attained a maximum of 0.42 gm./ml. at 100 125 gm./liter
of H2SO4. The copper concentration had a large effect on the apparent
density which varied from 0.42 gm./ml. at 5 gm./liter to 2.44 gm./ml. at 45
gm./liter. Increasing the copper content substantially increased particle
size of the powder
The electrolyte composition does not necessarily stay constant
during electrolysis. Variations are usually caused mainly by
differences in anodic and cathodic current efficiencies.
In the case of copper, the concentration of metal in the bath
generally rises. Subsidiary effects are caused by evaporation, by
drag-out when the powder is removed, and by the chemical solution
of the electrodes when the current is interrupted. Replace the
electrolyte with fresh solution.
Control of temperature is also important. It was found that as the
temperature increases from 15 to 60 C, the current efficiency
increased from 66.8 to 91.4 % and the apparent density from 0.451
gm./ml. to 0.746 gm./ml.
Electrodes:
The size, shape and disposition of electrodes may vary widely.
The anode may be soluble or insoluble and may be placed directly in
the electrolyte or within a porous pot, or be separated by a
diaphragm.
The anode may be of pure or impure metal, or in the form of scrap
supported in a basket. Unless, however, special precautions are
taken, impure anodes may cause operating difficulties or at least
contamination of the powder by the formation of slimes.
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It is not unusual for the area of the anode to be larger or smaller
than that of the cathodes, for the purpose of balancing the electrode
efficiency. For similar reasons, in order to improve the distribution of
powder deposit on the cathodes, it is recommended to use anodes
with rows of holes bored in them
In the case of cathodes, the choice may depend upon whether the
deposit is going to be stripped off or allowed to fall off in the form of
a sponge or powder, or weather it is intended to make a coherent
brittle deposit. In the former case, the choice is mainly a matter of
minimizing corrosion, especially at the liquid level, and facilitating
clean stripping.
For copper ----- copper rod, Al sheets, Pb sheet.
For iron -------- Nb, Mo, Ta, W or Pb sheets
When the deposit is of a brittle nature, it may be removed either by
knocking it off or flexing the sheet cathode.
Sponge deposits may be removed using brushes.
Layers of graphite paint or oils may be employed to facilitate the
separation. Castor oil oxidized with 1-3 % perchloric acid applied by
pre-immersion has been used.
It is not unusual to make the deposition upon a cathode starting
sheet which is substantially crushed along with the deposit. For
example, iron gauze has been recommended and used. This
becomes embrittled during the electrolysis and is readily crushed.
It has even been proposed to employ cold-pressed and un-sintered
or sintered cathode which easily disintegrate.
Rotating electrodes
Current:
The choice of a specific operating current density will depend mainly
upon whether a coherent brittle or powdery spongy deposit is to be
made. In the former case the current density will be low, in the
latter it will be high.
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In each case there may be an optimum density which gives the
highest current efficiency, but this may not necessarily be the same
density which produces the most suitable grade of powder.
Some workers have found that rising temperature increases the
current efficiency.
Apparent density of the product is unaffected by current density.
The frequency at which the current is interrupted has a most
important influence upon the particle size of the powder, and the
longer the intervals the larger the particle.
The greater the interval between current interruptions, the higher is
the apparent density. If this effect is important in practice, it can be
counteracted by suitable increases in current density.
Flow of Electrolyte:
In practice, convection and development of gas bubbles cause a
considerable flow of electrolyte over the cathodes, and an important
practical difficulty is to maintain this reasonably constant. It would
appear that a certain minimum forced circulation would be helpful in
attaining this.
In an experiment it was found that stirring the electrolyte coarsened
the powder and increased the apparent density.
As stirring is advantageous from the point of view of evening out
bath variables, but to some extent disadvantageous in increasing
the density and therefore reducing the compressibility.
Structural:
Owing to the substantial changes in behavior of an electrolytic
powder cell when its size is increased, it is advisable that, when
such a process is advised in the laboratory, it should be operated as
a unit cell with full-sized electrodes before an attempt is made to
design the final plant.
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Structural design factors involve taking decision upon the size and
nature of the electrodes, whether they should be stationary or
rotary, or be sheets, tubes or rods, etc., whether the cathodes
should be lifted out of the cell for scrapping or not, whether the
scrapping should be manual or mechanical.
Other problems concern with the corrosive nature of the electrolyte:
such as tank construction and linings, contacts, electrolyte handling,
cooling or heating, used anode treatment, etc.
After-treatment:
An electrolyte powder is generally in a reactive condition, and is also
wet with reactive electrolyte, there are considerable problems in
washing and drying it and bringing it to a dry powder which is not
only low in oxide but reasonably stable on storage.
For example, with electrolytic iron powder, it was found necessary
to wash the cathode deposit with water, 2 % H2SO4, water, dilute
citric acid, water, dilute ammonia, and finally with distilled water
before filtering, and then moistening with acetone before drying.
Even then it is recommended that the powder should be annealed in
hydrogen to reduce the oxide content.
Tyrrell, with copper powder, recommends annealing in a reducing
atmosphere. He found, however, that treating the powder in a
cracked ammonia atmosphere often led to rapid subsequent
deterioration on storage. He recommended treating the powder with
suitable water-repellent chemicals and indicated that stearic acid
dissolved in ammonia was suitable for a commercial process.
Many manufacturers avoid washing and drying difficulties by
annealing the powder in a reducing atmosphere.
When a brittle electro-deposit is the first product, annealing may be
absolutely necessary in order to produce a powder having
reasonable pressing qualities, and is customary among iron powder
producers.
Owing to the reactive nature of many electrolytic metal powders,
difficulties are frequently observed in preventing them from
oxidizing or corroding on storage. It is customary, at least with
copper powder, to add corrosion inhibitors to the powder.
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CONDITION SET-VALUE
Copper (solution) 5-15 g/l
Sulfuric acid 150-175 g/l
Temperature 30-55C
Cathode current density 700-1100 A/m
Anode current density 430-550 A/m
Cell potential 1.5 V
Table: Various set values of conditions in copper powder
production.
Advantages of the process:
The technique has a number of advantages, e.g.
The product is usually of a high commercial purity.
A considerable range of powder qualities can be obtained by varying
bath compositions.
Frequently the product has excellent pressing and sintering
properties.
The cost of the operation may in some cases be low.
Limitations:
Alloy powders cannot be produced.
The product of process is frequently in active condition (presence of
chemicals on powder particles) which may cause difficulties in
washing and drying it (contamination/oxidation with atmospheric
oxygen may occur).
The cost of operation may be high in some cases.
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References :
o ELECTRO-CHEMISTRY AND CORROSION SCIENCE. BY,
NESTOR
PEREZ.
o ELECTRO-CHEMISTRY ENCYLOPEDIA.
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