Grade A Copper Cathode Impurities: Which Ones Electrolyte Filtration Controls, and Which It Cannot

The short answer

Grade A copper cathode, under the London Metal Exchange's contract rules, allows at most 25 ppm silver, 5 ppm lead, 5 ppm arsenic, 4 ppm antimony, 2 ppm bismuth, 2 ppm each of selenium and tellurium, and 15 ppm sulphur, with 65 ppm for all the listed impurities together (BS EN 1978:2022 figures). An electrolyte filter can act only on the part of that load that reaches the cathode as particles, and only on particles that leave the cell and pass through it.

A 2014 paper by Moats and co-authors names four ways an impurity gets into the deposit: electrolytic co-deposition, slime occlusion, electrolyte occlusion and precipitation. A filter removes suspended particles from the circulating electrolyte. Slime released at an anode can reach the neighbouring cathode in the same cell without passing a filter, and dissolved or trapped impurities are beyond any filter.

The literature is clearer on some elements than others. Under normal conditions, a 2012 review puts lead, antimony and bismuth on the particle side, though one refinery technology supplier treats antimony and bismuth as soluble and liable to co-deposit. Arsenic's route is not settled. Silver has an ionic route and a particle route, and part of the sulphur can come from an additive. So when a cathode fails on a named element, the first question is which route that element took, and only then whether the filter is the right lever.

What Grade A actually limits

The LME's special contract rules for Copper Grade A set out three qualities, each in its own table: BS EN 1978:2022 (cathode grade Cu-CATH-1), China's GB/T 467-2010 Cu-CATH-1, and ASTM B115 Grade 1. We quote BS EN 1978 below, converted from percentages to ppm (0.0001 percent is 1 ppm):

• Silver: 25 ppm, the largest single allowance on the list.

• Lead: 5 ppm.

• Arsenic: 5 ppm, inside a group limit of 15 ppm for arsenic, cadmium, chromium, manganese, phosphorus and antimony.

• Antimony: 4 ppm, in the same group.

• Bismuth, selenium, tellurium: 2 ppm each, with a group limit of 3 ppm for the three together.

• Iron: 10 ppm, inside a group limit of 20 ppm for cobalt, iron, nickel, silicon, tin and zinc.

• Sulphur: 15 ppm, which the rules say "shall be determined on a cast sample".

• Total: 65 ppm for every listed element other than copper.

The Chinese table carries the same numbers. ASTM B115 Grade 1 is built differently: it sets bismuth at 1 ppm rather than 2, gives nickel (10 ppm) and tin (5 ppm) individual limits, has no group limit except the one for selenium, tellurium and bismuth, and applies its 65 ppm total to a shorter list of elements. If a cathode result is in dispute, check which standard the contract names before anything else.

None of these figures is a filtration number. They describe the cathode, and the electrolyte is one of several things that put impurities into it.

Four routes into the cathode

Moats, Aslin, Pranowo and Alvear, in a 2014 Conference of Metallurgists (CIM) paper hosted on Glencore Technology's website, put it in one sentence: "The mechanisms for cathode contamination are electrolytic co-deposition, slime occlusion, electrolyte occlusion or precipitation."

They map to a filter like this:

• Co-deposition. An impurity ion is reduced at the cathode with the copper. The impurity is dissolved, so a filter has nothing to catch.

• Slime occlusion. A slime particle reaches the cathode face and is overgrown. A filter helps only with particles that are carried out of the cells into the circulating stream. A 2026 Glencore Technology article on electrorefining notes that if slimes detach from the anode surface and float into the electrolyte, "they can deposit onto the cathode". In a tankhouse that can happen within one cell, before the electrolyte ever reaches a filter.

• Electrolyte occlusion. A pocket of electrolyte is trapped in the growing deposit, carrying whatever is dissolved in it. A filter cannot change what is dissolved.

• Precipitation. A compound comes out of solution on or near the cathode. Whether the filter helps depends on where and when the precipitate forms, which is the subject of a later section.

One complication sits over all four. The same Glencore article describes nodules forming where local current density exceeds the bulk level, and states that, left uncontrolled, "nodules trap electrolyte and entrain slime particles, driving up impurity content in the deposit." The 2014 paper says the same of the rough, nodular surface left by a precipitation event, and a 2011 SAIMM journal paper on electrolyte additives (Muhlare and Groot, citing earlier work) says rough surfaces cause anode slimes and suspended solids to adhere and become entrapped. A rough deposit amplifies the particle route and the trapped-electrolyte route together, so a nodulation problem can look like a filtration problem and an electrolyte problem at once. We covered the additive side of that coupling in our article on suspended anode slimes.

Element by element: what the literature says

Silver

Silver has the largest allowance and two documented routes. A 2023 study in the Transactions of Nonferrous Metals Society of China (Feng and co-authors) concluded from thermodynamic calculation that "the discharge reduction of free Ag+ is inevitable", which is the ionic route. The same study found that "the entrainment of floating anode slime formed by As, Sb and Bi is the key factor for the inclusions of AgCl and Ag colloidal particles in cathode copper", which is a particle route, and one that travels with the antimony and bismuth slimes discussed below. Its remedy was chemical: purifying the electrolyte to reduce floating slime brought cathode silver below 7.5 mg/kg in their laboratory electrolysis.

Glencore's article agrees that silver reports predominantly to the slime and that detached slime can deposit on the cathode. So filtration bears on one of silver's two routes, and the study that identified that route controlled it through electrolyte chemistry.

Lead

A 2012 TMS review by Moats, Wang and Kim found that "lead essentially enters the slimes", and that "under normal electrorefining conditions, lead, antimony and bismuth are thought to enter the cathode through the encapsulation of slime particles." Glencore places lead with gold, silver and the platinum group metals as reporting predominantly to the slime. Of the elements on the Grade A list, lead has the clearest case for being a particle problem. Whether the filter is the lever then depends on whether those particles reach the cathode in their own cell or after circulating. Our article on scrap-fed tankhouses covers how scrap feed can bring more lead and tin.

Antimony and bismuth

Here the sources frame the route differently, and it is worth saying so rather than choosing one.

The 2012 review, as quoted above, puts antimony and bismuth with lead under normal conditions. The 2014 paper explains where those particles come from: bismuth and antimony precipitate as arsenates, and, citing Braun, Rawling and Richards (1976), "floating slimes form by precipitation away from the anode surface. In the electrolyte, they can float to the cathode where they can be entrapped." The Glencore article lists antimony and bismuth among the soluble impurities and says they "will co-deposit at the cathode if not adequately suppressed through arsenic balance and electrolyte bleed."

The 2014 paper and Glencore both point to arsenic balance as the control, and Glencore adds bleed. That is electrolyte chemistry. Filtration can remove floating slime that survives into circulation, which our slimes article treats as part of the precoat filter's duty.

Arsenic

The route for arsenic is not settled. The 2012 review states that "the literature indicates that arsenic enters the cathode predominately by electrolyte entrainment." The 2014 paper is more cautious. It reports that Hoffmann (JOM, 2004) stated arsenic contamination is "likely caused by electrolyte entrapment", that Braun and co-authors (1976) concluded slime or electrolyte occlusion "could not be the sole source", and that recent industrial data showed no correlation between electrolyte arsenic and cathode arsenic. It judges co-deposition unlikely "except in rare incidents where flow to a cell is disrupted", and concludes that arsenic could enter "by any of the other methods". The same paper links elevated cathode arsenic at one refinery to arsenate precipitation events, covered below. So particles cannot be ruled out for arsenic, and neither can trapped electrolyte.

Selenium and tellurium

A US patent on tellurium recovery (US 5,939,042) describes copper and lead electrorefining slimes as "a major source of tellurium", and its worked examples treat slimes that already carry selenium. That tells us selenium and tellurium concentrate in the slime. It does not tell us how they reach the cathode. Our reading is that slime particles are the likely route, as for lead. We did not find a paper that measures it, so treat that as an inference.

Nickel, iron and the other dissolved metals

Glencore describes nickel as dissolving readily, and its effect on the cathode as "generally a concentration management issue rather than a direct deposit contamination risk." A nickel sulphate patent (US 4,314,976) gives the textbook simplification: impurities less noble than copper dissolve into the electrolyte, while the more noble ones settle as slime. It has known exceptions, lead and tin among them, as the sections above and our recycling article set out. For nickel, iron, zinc and cobalt, our reading is that the route left is electrolyte occlusion, controlled by bleed and by deposit quality.

Sulphur

Sulphur has a route that does not start with suspended solids. The 2011 SAIMM paper states that drawbacks of thiourea "include contamination of up to 10 ppm sulphur in the cathodes", and, citing earlier work, that an excess of thiourea causes striations and "entrapment of sulphur with slimes", so the additive route and the slime route interact. The 10 ppm figure appears in that paper without its own citation, but it is worth setting against a 15 ppm limit. On our reading, sulphate in trapped electrolyte is a further source.

Where the filter sits, and where it does not

The filter is outside the cell. It sees only what the electrolyte carries out of the cells into circulation, and it returns clarified electrolyte to them. Contamination happens inside the cell. Sorted by what the literature says about each route:

• Where the literature points to particles under normal conditions: lead; antimony and bismuth on the 2012 review's account (Glencore differs); silver's particle route, which travels with the floating slime. On our reading, selenium and tellurium belong here too.

• Where the route is open: arsenic.

• Dissolved, so no filter reaches it: silver's ionic route, nickel, and on our reading iron, zinc and cobalt.

• Additive-linked: sulphur from thiourea, which interacts with slime entrapment when thiourea is overdosed.

Even the particle group has a limit. As we read it, slime that detaches at an anode can deposit on the cathode beside it. The 2014 paper, citing Braun (1976), places floating slime formation away from the anode surface, and it reports that when electrolyte becomes supersaturated in arsenates it precipitates suddenly, with cathode data from one operating refinery showing elevated arsenic, antimony and bismuth during those events. It also states that arsenate and antimonate precipitation forms scale "in pipelines, cells, filters and heat exchangers", and that precipitation "will inevitably occur whenever electrolyte is exposed to cooler surfaces during and immediately after harvesting activities."

Our reading of those statements, and it is a reading rather than a published finding: particles that detach or precipitate within a cell can reach a cathode before that electrolyte next passes the filter. Filtration reduces how much of that load the circulating stream returns, which still matters, and the chemistry that prevents the precipitate is the control for the rest.

None of this makes filtration optional. Particles in the circulating electrolyte can reach the cathode, and the circulating stream is where a pressure filter in precoat mode does its work on the fine fraction (see precoat versus body feed). The point is narrower: the filter controls part of one route, and Grade A is decided across four.

What to put in the specification, and in the investigation

Do not write "Grade A cathode" as the filter's performance requirement. The filter cannot deliver that result on its own, and a supplier who accepts the line is accepting responsibility for arsenic balance, additive dosing and current density. Write a filtrate clarity target with the test method behind it, and state which elements the target is meant to protect.

Name the standard. BS EN 1978 and ASTM B115 differ on bismuth (2 ppm against 1 ppm) and in how they group the other elements.

When a cathode fails, ask which route before which equipment. A useful sequence:

1. Which element failed, and against which standard?

2. What does the literature say about that element's route: particles under normal conditions (lead), open (arsenic), two routes (silver), dissolved (nickel) or additive-linked (sulphur)?

3. Does the excursion track the electrolyte's suspended solids record, or its dissolved arsenic, antimony and bismuth, its additive dosing, or a run of nodulated cathodes?

4. Did it follow a harvest or a disruption of flow to a cell? The 2014 paper says arsenate and antimonate precipitation inevitably occurs on cooler surfaces at harvest, and judges arsenic co-deposition unlikely except in rare incidents where flow to a cell is disrupted. That harvest-time precipitates then reach the cathode is our reading.

5. Only then: is the filter performing to its clarity target, measured the way the specification says?

Ask for the right data at enquiry. For a new or replacement filter, the useful inputs are the anode assay, the electrolyte assay including arsenic, antimony and bismuth, the circulation and bleed rates, the electrolyte temperature at the filter inlet, and the suspended solids record with its sampling point. The cathode assay history helps too, because it shows which elements are causing the trouble.

What we could not establish

• How the four routes divide by mass for any element in an operating tankhouse. The sources give qualitative routes, not percentages.

• How much contaminating slime reaches cathodes within its own cell, against how much circulates first.

• A published route for selenium and tellurium into the cathode. Our particle-route reading rests on their concentrating in the slime.

• The source behind the SAIMM paper's "up to 10 ppm" sulphur figure, which is stated there without its own citation.

• Hoffmann's 2004 JOM paper directly. The publisher's site refused the request, and we report it only as the 2014 paper cites it.

Where Sharpenn fits

Sharpenn builds pressure filters for electrolyte clarification, running in leaf (body-feed) or precoat mode. That is the circulating-particle part of the picture above, and we size against your suspended solids, your flow and your electrolyte, not against a cathode grade a filter does not control. Our record: ~1,000 units installed worldwide across 460+ installations (all applications); 32 units in copper refining; capacities 5 to 350 m³/hr; repeat orders after 20 to 25 years. For the wider picture of the duty, see the guide to electrolyte filtration in copper refineries.

If a cathode element is drifting and you want a second view on whether the filter is the right lever, send us the failing element, the standard, the electrolyte assay and the suspended solids record.