ELECTROCHEMICAL PROCESS

1. 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.

2. 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 whileanode is made of the same metal whose powder is being produced.

3. 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.

4. Figure: Electrolytic Cell Operation for Deposition of Powder --- Schematic.

5. 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.

6. * 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

7. * 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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

16. 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

17. 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.

18. 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

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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).

28. 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.

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