Electrolyte Filtration in a Copper Recycling Plant: What Scrap Anodes Change

The short answer

A copper recycling plant that ends in an electrolytic tankhouse filters its electrolyte for the same reason a primary refinery does: suspended anode slime contaminates the cathode. What scrap feed changes is the anode, and through the anode, the solids. Anodes cast from secondary materials are described in the research literature as "generally rich in nickel, lead, antimony, and tin", and one research group characterises them by a nickel content above 0.3 wt%. The same group refined deliberately extreme laboratory anodes, at about 5 and 10% nickel, and measured slime at 3.55 to 6.47% of the dissolved anode mass, against 0.2 to 0.8% for commercial electrorefining on the same basis. That shows direction, not a figure for a real secondary plant.

Nickel may also change how the slime behaves. A 2024 laboratory study from Aalto University, run at 25 °C under conditions chosen to make the anode passivate, found that raising anode nickel from about 0.1% to about 0.3% gave significantly more upward-moving slime particles, and called the effect "a major detrimental effect on the settling of anode slimes." At 60 °C the same higher-nickel anode shed no slime during five days. So this is a laboratory signal under forced conditions, not a plant result.

The filter itself does not change class. It is the same clarification duty, sized by the same method. What changes is the solids basis, and on a scrap-fed tankhouse that basis should come from the plant's own anode assay and electrolyte nickel, not from a primary refinery's figures.

Why scrap feed reaches the electrolyte

Electrorefining is where most of the world's high-purity copper is made. The Aalto authors cite the International Copper Study Group for a figure of over 80%. It is also the step that has to absorb whatever the furnace upstream did not remove. The same paper puts the trend plainly: "as ore grades are becoming poorer and copper scrap is being used increasingly, refineries are faced with more impurities and challenges in production. Increased impurity levels also result in more anode slimes being generated."

The impurities that scrap brings are specific. The Aalto paper notes that printed circuit boards can contain up to 5.35 wt% nickel and that cupronickel alloys typically carry about 10 to 30% nickel. A 2025 study by Morales-Aragón, Sánchez-Rodas, Ríos and Moats doped commercial anodes with 2,500 to 6,500 g/t nickel (0.25 to 0.65%), 300 to 900 g/t tin and 450 to 950 g/t antimony to simulate a higher share of recycled copper, and its abstract names nickel, tin and antimony as the elements from recycled copper that "may significantly impact the electrorefining process."

For a sense of the ceiling conventional practice works to, a 1982 Amax patent on refining nickel-, antimony- and tin-bearing copper states that enough of these impurities should be blown into the converter slag that anodes cast from the blister copper "contain less than about 1% nickel and about 0.4% or less antimony, and less than about 0.2% tin."

One trend sits behind all of this. Citing a survey of operating refineries by Moats and co-authors, the Aalto paper reports that average nickel in refinery electrolyte rose from 9.7 g/dm³ in 1999 to 15.3 g/dm³ in 2022, an increase of over 50%. It calls the cause complex and attributes it "probably partly" to falling ore grades and "partly due to the recycling of e-waste and nickel-rich copper scrap." Its authors add that copper capacity has risen "but the electrolyte bleed capacity, which is used to control the Ni2+ concentration of the electrolyte, may not have increased at the same rate."

A recycling plant does not automatically send dirty anodes to the tankhouse

It would be easy to read the above as "secondary means dirty". The evidence does not support that as a rule. Aurubis describes the anodes at its Lünen recycling site as plates "weighing about 400 kg with a copper content of up to 99.5 % that are recovered by melting down recycling raw materials in multiple steps." A paper from Bor gives the commercial anode range as 98.0 to 99.5 wt% copper.

Copper content alone does not settle the question, though. A 99.5% anode still has room for 0.3% nickel, so the element-by-element assay is what counts, not the copper figure and not the label on the plant. A plant running mixed scrap through a short refining route can be expected to see something different from one with several pyrometallurgical steps, even if both call themselves copper recycling plants.

What nickel in copper electrorefining does to the slimes

Most of the nickel in an anode dissolves, because nickel is less noble than copper; the Aalto paper notes that some reports to the slime as sulphates and oxides. The dissolved nickel accumulates in the electrolyte and has to be bled off. That is the familiar half of the problem. The Aalto 2024 study looked at the less familiar half: what nickel does to the slime that leaves the anode.

The researchers filmed slime detaching from laboratory anodes and tracked the particles. They describe four ways slime leaves the anode surface: cloud formation, individual particles, clusters and avalanche. Cluster detachment was more common with high-impurity anodes, and the authors note that "high impurity anodes generate more slime." Two of their observations bear on filtration:

• Anode nickel and settling. "Anode nickel has a major detrimental effect on the settling of anode slimes, as significantly more upward flow is observed when the nickel concentration increases from ~0.1% to ~0.3%." The authors could not tell whether the upward-moving particles contained nickel, and suggest the effect could be related to nickel oxide formation, which occurs at about 0.3 wt% nickel.

• Electrolyte nickel and the slime layer. "Increasing the amount of Ni2+ and H2SO4 can cause a more fragile anode slime layer, thus increasing the risk of cathode contamination due to suspended and slowly settling anode slime particles."

Three limits apply, and they matter. The settling tests were run at 25 °C because major slime detachment could not be seen without passivation; at 60 °C, with electrolyte nickel below 30 g/dm³, no slime detached in five days, though the authors describe the slime layer as very fragile at every composition. The study compared anodes at about 0.1% and about 0.3% nickel, with nothing in between. And the tracked particles were 0.1 mm and larger, far coarser than the fine suspended population a clarification filter removes, so the study does not show how much upward-moving slime reaches the filter.

The 2021 Metals paper by Marković and co-authors places secondary anodes at the same nickel level: "Anodes obtained from the copper-based secondary materials are characterized by the nickel content of more than 0.3% wt.%", in a passage about nickel oxide. Neither paper links settling to secondary anodes. The overlap is our reading, and both 0.3% figures sit in discussions of nickel oxide, so they may share a root and should not be read as independent confirmation. What the overlap does say is that the anode nickel at which the laboratory effect appeared is not exotic: the Aalto study's own higher-nickel samples were cut from industrial anodes, and it is where scrap-derived anodes begin on one group's description, and inside the 0.25 to 0.65% range the 2025 study chose to simulate recycled feed. The authors' own conclusion is that anode nickel "significantly increases the upward flow of anode slimes, increasing the probability of cathode contamination." Our article on suspended anode slimes covers the fine population and why the cathode is effectively part of the filtration circuit.

There is a second nickel effect at the cathode. A 2025 laboratory study by the same Aalto group found that raising nickel in the electrolyte from 0 to 40 g/L increased cathode surface roughness (Rz) from 469 µm to 945 µm, with pores of up to 500 µm in some rougher cathodes that "may entrap electrolyte and cause cathode contamination." Higher electrolyte nickel also raises viscosity, which slows flow through the filter cake; we cover the measured effect in our article on electrolyte viscosity and filter sizing.

What tin and lead do

Tin is one of the three impurities the 2025 doped-anode study names as arriving with recycled copper, and the Amax ceiling above sets the conventional target for blister anodes below about 0.2% tin. Its behaviour in the cell is unusual because much of it does not simply stay in the anode slime.

A 1973 Boliden patent on floating slime puts it in one sentence: "The tin present in the system primarily passes into solution as Sn [ions], but is then precipitated as tin(IV)hydroxide in gel form and also follows the anode slime." The 2021 Metals paper agrees on the outcome, "Sn reacts with electrolyte and precipitates in anode slime", and lists Sn(OH)2SO4 among the insoluble salts that "pass from electrolyte into anode slime." Not all tin takes that route: the 2025 study found tin and antimony tended to form oxidised inclusions in the anode, including SnO2, and its authors conclude that nickel, antimony and tin "may impact the deportment of these impurities to slimes or electrolyte."

For the filter, the point is that part of the tin solid forms in the solution, and forms as a gel. Our filter cloth article lists the mechanisms that blind a cloth in electrolyte duty, and one of them is chemical precipitation forming hard or gelatinous deposits on the fabric. If part of that tin gel is carried to the filter with the circulating electrolyte, it is that mechanism. Whether it is carried, and how much, is not measured in anything we read. It is still a reason to ask about tin before sizing, and to protect the septum with a properly laid precoat.

Lead behaves more simply. According to the 2021 paper, lead "dissolves from anodes along with Cu and immediately precipitates as the PbSO4 insoluble salt." In a high-lead anode that means more lead sulphate in the slime. The 1982 Amax patent adds that excess antimony or tin "form relatively refractory complexes with copper and nickel in the anodes; these complexes collect in the slimes phase." Antimony's other route, into floating slime with arsenic and bismuth, is covered in our suspended slimes article and is not repeated here.

How much more slime

No published figure we found gives slime generation for an operating secondary tankhouse. What exists is a laboratory comparison on one basis and a commercial benchmark on another.

• Same paper, same basis. The 2021 Metals study refined two deliberately non-standard anodes, one at 4.67 wt% nickel and one at 10.04 wt% nickel, each with about 0.4% each of lead, tin and antimony. Slime came to 3.55 to 3.58% of the dissolved anode mass for the first and 5.01 to 6.47% for the second. The authors put slime in commercial electrorefining at 0.2 to 0.8 wt% of dissolved anode. Part of the second anode's slime was copper that fell from a poorly formed cathode deposit, so its figure overstates true anode slime.

• Commercial benchmark, different basis. A 2024 review in Processes states that copper anode electrorefining generates 5 to 10 kg of anode slime per ton of copper cathode.

The laboratory anodes were chosen to sit far outside the commercial range, so these figures show direction, not a ratio to apply to a real plant. The defensible statement is the Aalto one: high impurity anodes generate more slime. How much more, for your feed, comes from your own anode assay and a test.

What this changes in the filter specification

The clarification filter for a scrap-fed tankhouse is the same machine as for a primary refinery. The specification needs more inputs, because the solids are less predictable. Send these with an enquiry:

1. The anode assay, by element. Nickel, lead, tin, antimony and oxygen at minimum, and the range across your feed blend, not one good batch. On the evidence above, anode nickel at or above about 0.3% and any meaningful tin are the two lines to watch.

2. Electrolyte nickel and the bleed practice. The design Ni²⁺ concentration and how high it is allowed to run before the bleed catches up. In laboratory work, electrolyte nickel moved slime layer behaviour and cathode roughness, and it raises viscosity.

3. Slime generation and how it is measured. If you have a figure per tonne of cathode, give it. If the plant is new, say so, and expect the solids basis to be set from the anode assay and confirmed by a test.

4. Flow rate, filter inlet temperature and clarity target. The standard inputs, unchanged.

5. Whether the feed is expected to shift. A recycling plant's anode chemistry follows its scrap purchasing. If the blend will move toward e-scrap or nickel-bearing alloys, the filter should be sized for where the feed is going.

Our operating response to a heavier or more variable solids load is to tune the body-feed rate to the slime load and keep the precoat right-sized, so the septum is protected without wasting filter aid.

India: a policy reason to get this right now

The Ministry of Mines' Copper Vision Document states that "refining low-grade scrap in India is currently limited, with direct melting predominantly used for secondary production", that India's scrap refining capability is "currently negligible", and that India should aim to refine 15 to 20% of its available scrap in the long term. It expects the Quality Control Order to expand secondary refining capacity, and it cites S&P Global for a rise in the secondary share of world refined copper production from about 17% in 2022 to about 22% by 2035.

A secondary tankhouse specified in India now will have few domestic secondary plants to compare against, so the anode assay is the line we would check first.

What we could not establish

• Slime generation in an operating secondary tankhouse. We found laboratory figures and commercial benchmarks only.

• Whether the Aalto settling effect holds at plant temperature. It was measured at 25 °C with a passivating anode; at 60 °C the higher-nickel anode shed no slime in five days.

• Whether upward-moving slime reaches the filter. The tracked particles were 0.1 mm and larger.

• The full 2025 doped-anode study. We read its abstract; the publisher's site refused automated access, so its deportment figures are not used.

• How much tin stays in solution, or reaches the filter, before it precipitates. None of the sources we read gives a figure.

Where Sharpenn fits

Sharpenn builds pressure filters for electrolyte clarification, running in leaf (body-feed) or precoat mode on a disposable filter cloth, with a flux-calcined diatomite filter aid on copper duty. Our record: ~1,000 units installed worldwide across 460+ installations (all applications); 25 units in copper refining; capacities 5 to 350 m³/hr; repeat orders after 20 to 25 years. For Indian buyers, see our page for Indian copper refineries.

If you are specifying electrolyte filtration for a copper recycling plant, send us the anode assay, the design electrolyte nickel and bleed practice, the flow rate and the filter inlet temperature, and we will tell you what we would size and why.