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8/3/2019 Darshit Ppt

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A SEMINAR ONELECTROLYTIC PRODUCTION

OF METALLIC POWDER

PREPARED BY:

DARSHIT FADADU

ROLL NO: 938

GUIDED BY:

Dr. V.V.MATHANE SIR


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ELECTROCHEMICAL PROCESS

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 asa sponge or powder or at least in a physical form in which it can be easilydisintegrated into a powder.

Principle:

The basic principle is the electrolysis process in which decomposition of amolten salt/aqueous solution into its ions is obtained by the passage of electriccurrent. The metallic ions are deposited at the cathode which can be removedwith a brush and collected at the bottom.

The electrolytic tanks have conical bottoms with a valve. Suction pipes areconnected 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.


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Faradays law of electrolysis:

Faraday's 1st Law of Electrolysis - The mass of a substancealtered 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 molarmass divided by an integer that depends on the reaction undergone

by the material.


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Powder production at cathode is favored by:(1) High current density;

(2) Weak metal concentration;

(3) Addition of acids;

(4) Low temperature;

(5) Avoidance of agitation, and;

(6) Suppression of convection.

(7) Hydrogen evolution.

* 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 evolvedfrom the cathode.


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Hydrogen evolution is encouraged by:(i) increasing cell voltage.

(ii) diminishing the size of the cathode.

(iii) bringing the anode and cathode closer together.

(iv) increasing the temperature.

(v) weakening the strength of the metallic solution.

(vi) adding acid.

* When metal is deposited without evolution of hydrogen, the deposit 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 iserected, or which have to be controlled during operation. The most important

are;

Electrolytes

Electrodes

Current

Flow of electrolyte

Structural considerations

After treatment


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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.

Cost: Relatively pure salts of copper which are cheap and freelyavailable 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.


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In the case of sulphate electrolytes, 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.

Having selected the type of bath, the exact composition must then be chosen

and thereafter maintained with considerable care.

The electrolyte composition does not necessarily stay constant during

electrolysis. Variations are usually caused mainly by differences in anodic andcathodic 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.


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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 contaminationof the powder by the formation of slimes.

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 onthe cathodes, it is recommended to use anodes with rows of holes bored in

them


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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 sheetsWhen 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 hasbeen used.


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Current:

The choice of a specific operating current density will depend mainlyupon 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.

In each case there may be an optimum density which gives the highest

current efficiency, but this may not necessarily be the same densitywhich 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.


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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 thecompressibility.


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Structural:

Owing to the substantial changes in behavior of an electrolyticpowder cell when its size is increased, it is advisable that, whensuch a process is advised in the laboratory, it should be operatedas a unit cell with full-sized electrodes before an attempt is madeto design the final plant.

Structural design factors involve taking decision upon the sizeand nature of the electrodes, whether they should be stationaryor rotary, or be sheets, tubes or rods, etc., whether the cathodesshould be lifted out of the cell for scrapping or not, whether thescrapping should be manual or mechanical.

Other problems concern with the corrosive nature of theelectrolyte: such as tank construction and linings, contacts,electrolyte handling, cooling or heating, used anode treatment,etc.


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After-treatment: An electrolyte powder is generally in a reactive condition, and is

also wet with reactive electrolyte, there are considerableproblems 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.


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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 thatstearic 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 onstorage. It is customary, at least with copper powder, to add corrosion

inhibitors to the powder.


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Advantages:

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.

The cost of operation may be high in some cases.


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References:

ELECTRO-CHEMISTRY AND CORROSION SCIENCE. BY,

NESTOR PEREZ.ELECTRO-CHEMISTRY ENCYLOPEDIA.

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