THE ROLE OF ULTRASOUND IN THE ELECTROWINNING AND ELECTRO-REFINING O F METALS
ROBERT WALKER
Department of Metallurgy and Materials Technology, University of Surrey, Guildford (U.K.)
(Received June 26th, 1978; accepted in revised form February l l t h , 1979)
Electrowinning is a process by which metals can be recovered from solution by electrolysis. For many metals an aqueous electrolyte is used, e.g. copper, zinc, cobalt, bu t a fused salt is used for certain more reactive metals e.g. aluminium and magnesium. The metal is deposi ted from the solution at the cathode and the anode is an inert electrode. The concentrat ion of the solution is maintained by adding metal to the solution in the form of an ore, concentrate or metallurgical by-product , from which the desired ions can be leached. Impurities also enter the solution with the ore etc., so regular purifi- cation of the solution is necessary.
Electro-refining is an electrolytic process in which the impurities are removed from metals. In this case the anode is the impure metal and dissolu- t ion occurs of both metal and certain impurities, however, some impurities are insoluble. Ideally only the metal is deposited at the cathode and the impurities either remain in the solution or form an insoluble sludge. Many metals can be purified by this process.
The rate of any electrochemical process is measured by the current or current density flowing in a cell. It is generally economically desirable to have as high a current density as possible in a process. Hence for a given voltage across a cell the current can be increased by raising the conduct ivi ty in the cell and this can be achieved by:
(1) increasing the concentrat ion of metal ions in solution; (2) adding salts, acids or alkalies to the solution; (3) heating the bath; (4) adding depolarizing agents to the solution; (5) using electrodes o f a large area placed parallel and close together.
In practice all these factors can be utilized but there are certain restraints such as the solubilities o f metal ions and salts, the high cost o f heating, etc.
Agitation has been found to increase the maximum current still further. Several forms of agitation have been used in electrodeposit ion including:
(1) solution circulation across the ca thode by pumping, which is expensive; (2) mechanical agitation by propellors (fairly ineffective); (3) electrode rotat ion (used on a large scale for dilute solutions but not
for commercial plating as yet; e.g. the "Ecocel l") ;
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(4) continuously moving cathodes (suitable for plating on wire or strip); (5) air agitation which is good but may introduce oxygen from the air into
the solution; (6) ultrasonic agitation. If is also possible to use ultrasonic agitation in conjunction with other
techniques to increase mass transfer. Hence moving steel wire is plated com- mercially at a high rate with brass from an ultrasonically agitated bath.
The effect of agitation on the maximum current density in a copper sul- phate/sulphuric acid bath has been studied. Luk 'yanov and Pavlov [1] ob- tained values of:
15 A/dm 2 in a stationary electrolyte, 50 A/dm 2 with mechanical mixing of the solution, 75 A/dm 2 with solution vibrating at a frequency of 100 Hz,
125 A/dm 2 with ultrasonic agitation. Thus, under the conditions used with this particular bath, ultrasonic agita- t ion was very much better than either mechanical mixing or solution vibra- tion.
The role of agitation is beneficial because it increases convection of metal- lic ions in the bulk electrolyte and reduces the effect of the electrolytic diffusion layer. This layer, in which the concentrat ion of ions and molecular species is different from the bulk concentration, is close to the electrode sur- face. Hence diffusion of ions across this diffusion layer is necessary before deposition can occur at the cathode. Agitation is beneficial because it has been found to reduce the effective thickness of this barrier. Lyons [ 2] gives the following values:
thickness about 0.2 mm with no agitation, thickness about 0.1 mm with streaming velocity of 25 cm/sec, thickness about 0.046 mm with rotating disc 120 rpm, thickness about 0.0145 mm with rotating disc 1200 rpm.
The use of a high frequency (up to 10 kHz) current superimposed on direct current has also been found to decrease the thickness of the diffusion layer [3]. Ultrasonic agitation is also useful.
The term ultrasonics relates to sound waves of such pitch as to be beyond the threshold of human audibility and it applies to frequencies between about 13 kHz and 100 kHz. Ultrasonic vibrations are produced by the use of a generator and transducer. The generator converts mains electricity to electrical energy at an ultrasonic frequency. The transducer, which may be piezoelectric or magnetostrictive, is fed by the electrical output of the generator and converts it into ultrasonic vibratory energy of the same fre- quency . These transducers may be fit ted directly to existing equipment and, in practice, can be used externally or internally on the sides or the bo t tom of plating tanks to agitate the solution. The recent developments of newer and more efficient generators of a higher intensity, with a greater reliability, should lead to the greater application of ultrasonic energy to large scale industrial processes.
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The role of ultrasonic vibrations in aqueous media is rather complex. When applied to a liquid many effects are produced of which cavitation, microstreaming and degassing are very important. In most liquids there is a very large number of finely dispersed minute bubbles and when these are subjected to ultrasonic agitation some grow and rise to the surface, i.e. the liquid is degassed while others grow and violently collapse giving cavitation. Ultrasonic vibrations also produce a vibratory movement of particles close to the surface of the transducer to give a net f low or streaming mot ion away from the transducer.
The net effect o f these different actions is to give an intense mixing effect at surfaces immersed in ultrasonically agitated solutions. The liquid becomes degassed and this increases the influence of the cavitation which produces shock waves at the surface and these may work-harden the surface as well as agitating the solution. The effect of ultrasound on electrodeposit ion processes has been discussed and the change in some properties of the plated metals has been reviewed [4] . Thus the increased mixing effect of the electrolyte assists in diffusion in the bulk electrolyte and decreases the con- centration polarization at bo th the cathode and the anode so permits the use of higher cathodic and anodic current densities. The removal of any gaseous phase near the electrodes is also beneficial. The shock waves produced by cavitation at the surface of the cathode can harden the electrodeposit [5] as well as reducing any whisker formation or growth perpendicular to the growing surface so giving smoother and brighter coatings [6] . The specific effects, however, depend upon the particular plating solution as will the geo- metry of the tank and the intensity and frequency of the ultrasound. Thus using the same geometry and ultrasound it is possible to improve the appearance of nickel deposited from some solutions but not from others [7].
According to Yeager and Hovorka [8] ultrasonic agitation of an electro- lyte is so violent that the diffusion layer is completely eliminated at bo th electrodes so that the current/potential relationship obeys Ohm's Law. In a degassed copper sulphate/sulphuric acid bath the presence of ultrasound has been found to reduce the effect o f the diffusion layer until it is almost insig- nificant [9] . Hence ultrasonic agitation can reduce the polarization and so increases the maximum current densi ty in many electroplating baths. The actual improvement depends upon the particular bath and plating condi- tions. The maximum current density has been increased in the copper sulphate/sulphuric acid bath by factors of 3 [10] and 8 [1] and with the cyanide bath by 4 [11] , 8 [10] and 20--30 [12] fold. With ultrasound [13] of frequency 30 kHz and intensity 0.25 W/cm 2 the current efficiency of the cyanide bath was increased significantly from 53 to 86%, while another benefit was the reduction in the voltage from 2.93 to 1.83 V for a current density of 4 A/dm 2 . With the copper pyrophosphate bath [ 14] an ultrasonic frequency of 23 kHz and intensity of 2.5 W/cm 2 raised the maximum current density from 3--5 A/dm 2 to 12--20 A/dm 2 , lowered the anodic
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polarization and decreased the cathodic polarization by 100--200 mV at low current densities and by 400--500 mV at high current densities (greater than 3 A/dm 2 ). Similar results have been obtained for silver (3 to 10--20 A/din 2 [15], and by 15 fold [16]), chromium (by 15 [17] or 3 - 5 [12] fold), nickel (cyanide bath X 15 [12], Watts bath X 7.5 [18] and high sulphate bath X 25 [19], cadmium (from cyanide X 3 [10, 12]) and zinc (× 20--30 [20] ). The intensity of energy required to produce cavitation increases rapid- ly at frequencies above about 50 kHz so is not often used whereas low fre- quency (20 kHz) was beneficial and for cadmium, chromium, copper and silver it eliminated anode polarization, accelerated plating, minimized or elim- inated edge build-up and produced uniform, well-bonded deposits [21]. Hence there is considerable scope for the application of ultrasound to increase the rate of electrowinning and electrorefining processes.
Another problem that can occur in electrodeposit ion at high current densities is the formation of whiskers or single crystals that protrude from the surface. Diffusion o f ions can occur most easily to areas that project from an electrode surface so that the current density is greatest at these local areas and rapid growth, of ten dendritic in nature, can occur. This may cause shorting, or flash-overs, be tween the electrodes which are very close together in electrowinning and electrorefining processes. It is possible to produce a more level deposi t by the use of periodic reversal of the current. Plating occurs normally for a certain period and then the current is reversed so that the cathode becomes the anode and dissolution of the projecting areas occurs. The advantages of periodic reversal in the electrolytic refining of copper have been discussed elsewhere [22] . The combinat ion of ultrasonic agitation and periodic current reversal has been studied for an alkaline copper cyanide bath [23] : a combinat ion of 6 minutes at 15 A/din 2 with ultrasonics, 1 minute reversed current at 5 A/din 2 followed by 2 minutes at 7 A/dm 2 with ultrasonics gave a deposit 3.3 pm thick which would have taken 12 minutes to plate under conventional methods.
Bulgarian workers have employed ultrasonic vibrations during the electro- refining of copper. Enchev et al. [24] used ultrasound of frequency 22 kHz and intensity 1.5 W/cm 2 with a solution containing 45 g/1 copper, 150 g/1 sulphuric acid and 0.5 g/1 glue at 30--40 ° C. The ultrasonics had a strong de- polarizing effect and reduced the voltage drop between the electrodes from 6.6 V to 2.0 V at a current density of 20 A/din 2 : the consumpt ion of electri- cal power was reduced from over 14,000 MJ/ ton to less than 6,000 MJ/ ton of copper produced. The use of ultrasound of a higher frequency (400--800 kHz) gave less satisfactory results.
In another paper, also on the electrolytic refining of anodic copper, Enchev et al. [25] found that ultrasonic vibrations sharply increased the pos- sible current density and gave a more intensified process. The op t imum ultra- sonic condit ions ensuring a good yield of high purity copper together with a uniform dissolution of the anodes were a low frequency 22 kHz at 0.5-1.5 V/cm 2 . The cathodic current efficiency was 99.94--99.98% in the current
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density range of 5--20 A/dm 2 and the deposit had a compact, dendrite-free structure, the anodes dissolved uniformly and the slurry was dispersed throughout the whole volume of the electrolyte.
In a comprehensive review [26] on the electrowinning of copper the various methods used to achieve a high current density are discussed. The techniques include ultrasonic agitation as well as air sparging, periodic current reversal, improved cell design, and rotating electrodes, and the consequences of operating at high current densities are covered.
Skarbo and Harvey [27] investigated the conditions required to produce copper in the form of coherent, high-purity electrodeposits directly from dilute leach solutions. Although with ultrasonic agitation a reasonably high current density was achieved, they concluded that the use of ultrasonic energy to obtain efficient mass transport over a large area would require exorbitant amounts of electrical power. This statement, however, only applies to the specific conditions that they used and disagrees with the results obtained elsewhere [28].
Eggett et al. [28] have studied the effect of ultrasonic agitation to reduce the thickness of the boundary layer and permit the use of significantly higher current densities in copper electrowinning. Although intimate contact between the probe and cathode is desirable for maximum efficiency, this is not practical because no suitable connecting device is available at a low cost. Hence the technique Eggett et al. [28] used was to locate the probes at the bottom of the cell. Using this system the use of 200 W of ultrasonic power (97 W/dm 2 of electrolyte plan area) reduced the operating cell voltage by about 50 mV throughout the considered range of current density, a further increase to 600 W producing a reduction in voltage of only 70 mV.
The quality of the copper deposits produced with ultrasonic agitation is good [28]. The deposits have good adhesion and are less nodular, more com- pact and less porous than those produced under normal conditions. As the deposits are more compact the cathodes can be spaced closer together because of the reduced possibility of nodular growth causing shorting between plates. This, in turn, means that more electrodes can be used in the cell and this increases the efficiency. Unfortunately, the copper deposited at high current densities (108 A/dm 2 ) was porous and contained high levels of impurity. There are, however, possible technical problems with using ultra- sonic agitation and higher current densities. These include noise, acid mist levels and heavier sections for the cathode suspension bars and intercell bar systems. These problems and economic evaluation are discussed in detail by Eggett et al. [28] who conclude that a significant decrease in the total capital cost of a 100 t.p.d, tankhouse can be made. It is difficult, in general, to assess the economics of applying ultrasonic agitation to a system. The cost depends upon the volume of solution to be agitated, the geometry of the sys- tem and the type, number and size of the transducers or probes.
These findings on the quality of the copper deposits are in good agreement with my work. Thus, copper plated from ultrasonically agitated
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solutions of copper sulphate/sulphuric acid is brighter and smoother than from still or stirred baths [6] , the puri ty of the deposit is not affected by the agitation [29] and the hardness is increased [5] . Similarly the hardness of deposited nickel is increased and the porosi ty decreased by ultrasound [7] . These results are considered due to the collapse of cavities at the cathode surface which produces shock waves [30] : these waves also reduce the concentrat ion polarization at the surface so permitting a higher current density. Another advantage of ultrasound is that it can produce smooth and hard deposits wi thout the use of addition agents: these are of ten used bu t are co-deposited so are present as impurities in the electrodeposited metal.
In another application Wragg [31] has applied ultrasonics to copper plating in a fluidised bed electrochemical cell for metal extraction processes. The preliminary results have been extremely promising and the two-fold increase in the mass transfer rate has been achieved. The possible advantages of ultrasonics may include:
(1) operat ion at sub-fluidisation velocities to achieve high residence times and consequent high extraction whilst maintaining a pseudo-fluidised condition,
(2) avoidance of agglomeration and blocking which of ten occurs in metal deposit ion from fixed beds,
(3} bet ter metal phase conduction, (4) enhanced mass transfer coefficients.
It may also be possible for cont inuous particle feed, classification and extrac- t ion to enable cont inuous operat ion with respect to the metal electrode material. In another paper on the application of ultrasound to a fluidised bed, Alkatsev [32] found that the rate of copper deposit ion on to nickel powder was increased. Ultrasound also reduced the consumption of nickel and it was found to be more useful when applied for short intervals of t ime than continuously.
In a completely different application Khavskii et al. [33] have developed a method to increase the puri ty of cathodic copper. The metal is flushed with water subjected to ultrasound of frequency 18--22 kHz and intensity 2--4 W/cm 2 for 30 seconds. This process removes electrolytic and slag inclusions and the concentrat ion of sulphur and nickel was reduced 6- to 8- fold. This t reatment also removed the necessity for preliminary acidification of the wash water and the use of steam.
Hence it may be concluded that ultrasonic agitation can be beneficially applied to commerical electrowinning and electrorefining of metals. It can be applied directly to existing cells in conjunct ion with other techniques to increase the maximum current density and accelerate the rate of deposition, reduce the surface roughness permitting closer spacing of the electrodes and increase the hardness and densi ty of the deposited metal.
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REFERENCES
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Series, No. 42 on Hydrometallurgy, 1975, Paper No. 27, pp. 1--17. 29 C.T. Walker PhD Thesis, Surrey University, 1974. 30 C.T. Walker and R. Walker, J. Electrochem. Soc. 124 (5) (1977) 661--669. 31 A.A. Wragg, Chem. Ind., (1975) 333--335. 32 M.I. Alkatsev, Izv. Vyssh. Uchebn. Zaved. Insvetn. Metall., ( 1 ) (1976 )49 - -53 . 33 N.N. Khavskii, Yu.R. Smirnova and F.M. Zainatdinov, Khim. Ind. (Sofia) 48 (3)
