The unsung heroes of copper electro-refinery

Copper is a soft, malleable, and ductile metal with very high thermal and electrical conductivity and because of these properties, it is the undisputed material of choice for conducting electricity. The quality of the copper should be very good for electrical applications. Bad quality dramatically changes the electrical conductivity and workability. The following qualities are important for copper;

• Chemical property: Purity of > 99.98 %
• Physical property: smooth and shiny
• Mechanical property: Not brittle, sufficiently ductile

Figure 1: Copper electrorefining tankhouse (https://copperalliance.org.uk/)

Electro-refinery is a process which uses electrolysis to increase the purity and quality of copper and many other metals. In the copper electro-refining cell, there are two electrodes immersed in an aqueous solution (figure 2). One is the impure copper anode and other is cathode blank usually made of steel where the copper will deposit. The aqueous solution contains copper sulphate and sulphuric acid. The electrolyte temperature and current density are highly important for good quality cathode.

During the process, direct electric current is supplied to the electrolytic cell and this results in dissolution of metals from the impure anode and the metals of interest diffuse through the electrolyte and subsequently deposit on the cathode. During the copper electro-refining, the metallic copper is oxidized to cupric ions at the anode, which is then reduced to metallic copper at the cathode. The impurities from the anode either remain dissolved in the electrolyte or settle at the bottom of the cell as slimes. The redox reactions happening during copper electrolysis are shown in equation (1) and (2).

Anode:  Cu(s) → Cu²⁺(aq) + 2 e^–            E°_ox(Cu) = – 0.34 V                          (1)

Cathode:  Cu²⁺(aq) + 2 e^–  → Cu(s)        E°_red(Cu²⁺) = + 0.34 V                      (2)

Figure 2: Simplified diagram of copper electro-refining cell (Image obtained from www.funscience.in )

But, these factors are not enough to produce a high purity and high-quality copper cathode. There are other additives introduced in the solution for smooth copper deposition and minimum co-deposition of impurities on the cathode. The additives that used generally in all copper electro-refining process are the following:

Figure 3: Additives

Figure 4: Chemical structure of thiourea

Grain refiners control the nucleation process and orientate the grain structure. The chemical family of grain refiners usually are organic sulfides, disulfides, thioethers and thiocarbamates. Thiourea is commonly used grain refiner during copper electro-refining.  Thiourea was found to react with copper to form a CuS which adsorbs on the cathode surface and thus passivate the nucleation process. The CuS adsorptions inhibit the grains from further growth and maintains the optimum grain size.¹ Grain refiner is a very important additive for giving the cathode required amount of elasticity. Cathodes cannot be too elastic like the rubbers but it must not be brittle as well. If there are no or too low grain refiners, nucleation process goes on resulting a large grain size and this makes the cathode highly elastic. If there is a too high amount of grain refiner, the nucleation processes are stopped too early, making the grain size too small and this makes the cathodes highly brittle.

Figure 5: Effect of thiourea on the grain size of Cu on the surface of cathode

Levelers are leveling or smoothing agent for the cathodes. Gelatin is an example of leveler used in copper electrolysis. They tend to be adsorbed to the positions of highest current density/electro-potential which are on protrusions, edges and nodules of the cathode surface and temporarily packs the area while forcing copper to deposits on the positions of low current density which are on the depressions of the cathode surface.² As the copper deposit levels, the local point of high potential disappears and the levellers drift away and move on to other areas of high potential.  A too low gelatin concentration result in nodulated surfaces, since not all surface bulges can be covered with gelatin to inhibit further growth. A too high concentration leads to dendritic formation of copper deposits, since almost all surface of the cathodes is covered by gelatin, forcing the copper to deposit longitudinally through the few gaps. However, the levellers are known to decomposed rapidly by acid hydrolysis at high temperatures and therefore, it is required to continuously add levellers to the electrolysis bath.

Figure 6: Levelling effect of gelatin on the surface of the Cu cathode

Figure 7: Chemical structure of polyacrylamide flocculent

Flocculent: Flocculation is a process wherein tiny particles suspended in a medium form a floc or flake, either spontaneously or due to the addition of a flocculent. Flocculents are usually highly charged or large molecules that help suspended solids particles bind together to form a heavier particle, which are then big enough to settle to the bottom of the container to be drained off. Polyacrylamide is a commonly used flocculent. Small particles are trapped in the long polymer chain and subsequently settles down to the bottom of the container.

Figure 8: effect of flocculent in Cu electro-refining

It must be noted that these additive should be added in correct dosing and ratio, otherwise it will cause lot of problems during the production. These additives are used in all copper electro-refineries and they are crucial to produce a good quality copper cathodes. However, most of the people do not know it because these are not mentioned in the electrolysis courses in the high school or at university. Nevertheless, it is highly interesting to know the important roles they play in the electrorefining process.

References

1. Moo Seong. Kang, Soo-Kil Kim, Keeho Kim, Jae Jeong Kim (2008).“The influence of thiourea on copper electrodeposition: adsorbate identification and effect on electrochemical nucleation,” Thin Solid Films,12 3761-3766.
2. Stelter M., Bombach H., (2007). Fundamentals and effects of additives in copper electrorefining. Proceedings of the 6^th international copper-cobre conference (Copper 2007), Toronto (Canada), 25-30 August 2007. Eds G.E. Houlachi, J.D. Edwards, T.G. Robinson. 6^th international copper-cobre conference (Copper 2007), Toronto (Canada), 25-30 August 2007 (pp. 575-586)

About The Author

Thupten Palden is the ESR 1 of SOCRATES project. He obtained his BSc in chemistry from Jacobs University in Bremen, Germany. Later, he continued his MSc in nanomaterial science at Utrecht University in the Netherlands. After completing the masters, he worked at Dutch chemical company called DSM in the Netherlands for two year where he investigated different materials including catalyst, polymers, glasses etc using electron and optical microscopy techniques. On january 2017 he started his Phd at KU Leuven under the supervision of Prof. Koen Binnemans. His topic is Biocompatible Solvometallurgical leaching methods for low-grade industrial process residues.

If you are interested in his research and wish to learn more about it, you can reach him through thupten.palden@kuleuven.be