In the heart of modern industry, a silent chemical reaction takes place every day in huge electrolytic
cells. This is copper electrolytic refining - a key process that electrochemically transforms crude
copper into high-purity copper. At the heart of the process are the seemingly simple but meaningful
chemical equations that, like precise instructions, direct the mysterious journey of copper atoms
from the anode to the cathode.
Dissolution at the anode: Cu - 2e- → Cu²⁺
Inside the electrolysis bath, when direct current is passed through the system, an oxidation reaction begins to take
place in the crude copper plate that serves as the anode. The central equation here is: Cu - 2e- → Cu²⁺. Behind this
simple equation is a complex microscopic metamorphosis.
The copper atoms in the crude copper anode lose two electrons to the current and are transformed into copper
ions into the electrolyte. This process does not occur in isolation, but in competition with impurity elements in the
anode. Metallic elements that are more active than copper, such as iron and zinc, also undergo oxidation reactions
and enter the solution as ions: Zn - 2e- → Zn²⁺, Fe - 2e- → Fe²⁺. And those precious metal elements which are less
active than copper, such as gold and silver, are difficult to oxidize and fall off from the anode directly, sinking to the
bottom of the tank to form precious “anode mud”.
Precipitation at the cathode: Cu²⁺ + 2e- → Cu
At the other end of the electrolyzer, the opposite reduction reaction is taking place on the pure cathode plate: Cu²⁺ +
2e- → Cu. Copper ions migrating from the anode gain electrons at the cathode and are transformed back into metallic
copper, which is deposited on the surface of the cathode in an extremely organized manner.
This reaction process requires precise control. The current density has to be just right - too high results in too rapid a
deposition of copper and the formation of a loose, porous structure; too low compromises productivity. The concentration
of copper ions in the electrolyte needs to be balanced - too thick will produce rough crystals, while too thin may lead to
competing discharges of hydrogen ions, generating hydrogen gas and reducing current efficiency.
The art of balance in the electrolyte
Connecting the cathode and anode reactions is the copper sulfate-rich electrolyte, which is not only the medium for ion
transfer, but also the equalizer of the entire reaction. The main ion migration and diffusion process takes place in the
electrolyte: CuSO₄ ⇌ Cu²⁺ + SO₄²-.
This reversible reaction maintains the dynamic balance of ions in the electrolyte. As the anode continues to dissolve to produce
new copper ions and the cathode continues to consume copper ions, the electrolyte composition is constantly changing. This
requires the maintenance of optimum process conditions through the circulation system and temperature control, which typically
maintains the copper ion concentration in the electrolyte in the range of 40-50 g/L and the sulfuric acid concentration in the
range of 180-200 g/L.
Destination of impurity elements
During the electrolysis process, impurity elements go to different destinations according to their chemical properties. Zinc,
iron, nickel and other elements with potentials negative to copper enter the electrolyte in the form of ions, and need to be
purified periodically after gradual enrichment. Gold, silver, platinum and other precious metals whose potential is positive to
copper do not dissolve and sink into the anode mud, which becomes an important raw material for the recovery of these valuable
elements. The elements such as arsenic, antimony and bismuth whose potential is close to that of copper are most difficult to deal
with. They are partly dissolved and partly precipitated, which can easily affect the quality of copper cathode, and it is necessary to
add specific reagents to inhibit their unfavorable effects.
Chemical wisdom in process control
In actual production, it is not enough to understand the basic reaction equations. Workers improve the quality of copper cathode
crystals by adding gummy substances such as gelatin and thiourea. These additives promote the uniform deposition of copper ions
by adsorbing them onto the active spots on the cathode surface, resulting in a dense, flat copper cathode.
Temperature control is equally critical and is usually maintained between 50 and 60 degrees Celsius. Too high a temperature
accelerates the decomposition of additives and the generation of acid mist; too low a temperature increases the electrolyte resistance
and energy consumption. The electrolyte circulation rate also needs to be carefully regulated to ensure uniform distribution of
concentration and temperature.
Copper electrolytic refining in the chemical reaction equation, seemingly simple but contains a wealth of industrial practice wisdom.
From Cu - 2e- → Cu²⁺ to Cu²⁺ + 2e- → Cu, behind these two simple half-reactions is a set of complex and sophisticated industrial
systems. It is this perfect combination of chemistry and engineering that allows us to refine electrolytic copper from crude copper
with a purity of up to 99.99%, providing a vital base material for the modern electrical society.