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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/Electrolysis


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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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