What Each Term Actually Means
Electrolytic copper describes copper that has been refined by electrolysis and is used in trade as a general term for that product route. Cathode copper is the standardised product name for the cathode plate that the refinery harvests from the electrolytic cell, and it is the term that carries weight in contracts and standards. ASTM B115 covers electrolytic copper cathode, GB/T 467-2010 defines two grades, high-purity cathode copper with copper plus silver of 99.99 % minimum and standard cathode copper with 99.95 % minimum, and the EN 1978 registration used for exchange delivery requires the higher chemistry. Because cathode is an intermediate, wrought products made from it are specified separately, for example electrolytic tough pitch copper C11000 as sheet, strip and plate under ASTM B152/B152M or as busbar under ASTM B187/B187M, and copper wire rod under ASTM B49 or GB/T 3952.
How Electrorefining Produces Cathode
Crude copper of roughly 99 % purity is cast into thick anode plates, and thin starter sheets of pure copper serve as cathodes. The plates are suspended in an electrolyte of sulphuric acid at about 150-200 g/L together with copper sulphate giving roughly 40-50 g/L of copper. A cell voltage of only 0.2-0.4 V is applied at a current density of about 200-350 A/m2, with the electrolyte held near 55-65 C, and the cathode is harvested after a cycle of about seven to fourteen days. Metals less noble than copper, such as iron, nickel and zinc, dissolve into the electrolyte and are controlled by bleeding and purification; metals more noble, such as gold, silver and the platinum-group metals, do not dissolve and settle as anode slime, which is recovered for its precious metal content. The harvested plate is then washed, sampled and bundled, and the refinery records the lot chemistry that will follow the material through the supply chain.
Composition and Quality Tolerances
| Element | High-purity cathode copper | Standard cathode copper |
|---|---|---|
| Copper plus silver | 99.99 % min | 99.95 % min |
| Tin | 0.0007 % max | 0.0007 % max |
| Arsenic | 0.0006 % max | 0.0006 % max |
| Nickel | 0.0005 % max | 0.0005 % max |
| Antimony | 0.0005 % max | 0.0005 % max |
| Zinc | 0.002 % max | 0.002 % max |
| Iron | 0.0006 % max | 0.0006 % max |
| Sulphur | 0.0012 % max | 0.0012 % max |
| Lead | 0.0010 % max | 0.0010 % max |
| Phosphorus | 0.0001 % max | 0.0001 % max |
The values above are the maximum impurity levels applied to cathode in the GB/T 467-2010 classification and are consistent with ASTM B115 requirements for electrolytic copper cathode. Silver is counted together with copper because it does not harm conductivity, and the same tolerance structure is used as the basis for the copper plus silver figure that a certificate of analysis reports.
Why the Distinction Matters to Buyers
Cathode is traded as a raw material by weight, priced against an exchange quotation plus a premium, and delivered in strapped bundles that normally weigh between two and four tonnes. What the buyer receives is therefore a documented lot of metal rather than a finished product: the refinery brand, the lot number, the bundle weight and the certificate of analysis carry more commercial weight than dimensional attributes. By contrast, an order for electrolytic copper rod, strip, billet or busbar is a specification order, where diameter tolerance, temper, surface condition, elongation and conductivity are all separately guaranteed. Confusing the two leads to orders that cannot be accepted cleanly, such as asking a cathode supplier for a guaranteed elongation figure, or asking a rod mill to certify cathode-grade impurities that are outside its process control.
Common Confusions and Verification Practice
Two confusions appear regularly. The first is the loose use of electrolytic copper for the wrought product C11000, which is made from cathode but is a different item with its own standard and its own tolerance framework. The second is the assumption that oxygen-free copper is a refined-grade name, when in fact oxygen-free is a property defined by oxygen content limit, as in ASTM B170 for oxygen-free copper and ASTM F68 for oxygen-free electronic copper, regardless of the refining route. Verification of a cathode delivery should include checking the bundle marking and refinery brand against the contract, sampling plates across the lot rather than from the top layer, confirming copper plus silver and each controlled impurity by spectrometric analysis, and inspecting for entrapped electrolyte, adherent slime and lead contamination from anode hangers, all of which can pass an average chemical check while damaging the melt.
Frequently Asked Questions
Q: Are electrolytic copper and cathode copper the same thing?
A: They describe the same production route from different angles: electrolytic copper refers to copper refined by electrolysis, while cathode copper is the standardised product name for the cathode plate that the refinery produces and sells.
Q: What purity does cathode copper reach?
A: GB/T 467-2010 sets 99.99 % minimum copper plus silver for high-purity cathode copper and 99.95 % minimum for standard cathode copper, with individual impurity elements limited to a few parts per million.
Q: Why is silver counted together with copper?
A: Silver is more conductive than copper, so it is included in the copper plus silver figure rather than treated as a harmful impurity; the analytical convention is defined that way in the cathode standards.
Q: How long does a cathode cycle take?
A: The plates are harvested roughly seven to fourteen days after loading, depending on current density and refinery practice, with cell voltage held at about 0.2-0.4 V and the electrolyte at 55-65 C.
Q: Does cathode copper need a mill certificate?
A: Cathode is supplied with a certificate of analysis per lot covering copper plus silver and the controlled impurities, rather than the mechanical properties that apply to wrought products.
