ELECTROCHEMICAL PROCESS

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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 as a sponge or
  powder or at least in a physical form in which it
  can be easily disintegrated into a powder.
• 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.

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• 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.
• Basic principle of the process and equipment
  used
• 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.

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• 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 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 --- Schematic.

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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.

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• 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
• (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

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• 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 is erected, or which have to be controlled
  during operation. The most important are
• (i) Electrolytes
• (ii) Electrodes
• (iii) Current
• (iv) Flow of electrolyte
• (v) Structural considerations
• (vi) 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.

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• Cost 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.

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• In the case of sulphate electroytes, 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.
• Iron powder ----- sulphate or chloride baths.
• Having selected the type of bath, the exact
  composition must then be chosen and thereafter
  maintained with considerable care.

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• 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.

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• 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.

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• 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.
• 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

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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 sheets

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• 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.

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• 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

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• 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.
• 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.

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• 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.

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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 the compressibility.

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• 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.
• 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.

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• 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.

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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 that stearic acid
  dissolved in ammonia was suitable for a
  commercial process.

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• 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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• Copper Powder Production by Electrolysis
• Similar to electro-refining of copper
• However, in this process electrolytically refined
  copper anodes are used instead of impure cast
  anodes as in refining process.
• Powdery or spongy deposit instead of strongly
  adherent product.
• Copper refining ---- electrolyte with 150 g/l of
  CuSO4.5H2O and current density of 100 200 A/m2.
• Copper powder production ----- ---- electrolyte
  with 50 g/l of CuSO4.5H2O and current density
  increased to 500 A/m2.

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• The powder deposited on the cathode is removed
  and washed, filtered and then given an annealing
  and reducing treatment at temperature between 500
  1000 oC.
• Various operating conditions can affect the
  formation and size of particles.
• High current density favors the formation of the
  powder (i.e. the efficiency is increased).

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• Raising the operating temperature of the cell,
  current efficiency is increased and it reduces
  the cell voltage.
• However, temperature higher than 60 oC, produces
  coarser powder than the powder produced at lower
  temperature.
• 40 oC 15 oC
• The brush-down intervals help in control of the
  particle size of the deposit. The powder becomes
  coarser as the interval is increased.
• Frequent brush-down also limits variations in
  cathode current density.

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• Table Various set values of conditions.
• CONDITIONS SET VALUE
• Copper (solution) 5 15 g/l
• Sulfuric acid 150 175 g/l
• Temperature 30 55 oC
• Cathode current density 700 1100 A/m2
• Anode current density 430 550 A/m2
• Cell potential 1.5 V