How to Specify Filter Cloth for a Copper Electrolyte Duty: What Actually Sets Cloth Life

Short answer: No supplier can quote you a filter cloth life for a copper electrolyte duty, because the interval is set by the process conditions and by what the plant itself decides is acceptable, and it varies from customer to customer. What you can specify are the four things that decide it: the chemistry and temperature the fabric sits in, whether it holds the precoat and releases the cake, whether it blinds, and whether it sees flow in both directions. The trap is that almost every published cloth selection guide is written for a dewatering duty, where the cloth is the filter. In a precoat duty the filter aid does the retention, and the same comparison tables then point at the opposite construction.

What actually sets cloth life

"No number" sounds like an evasion until you look at what the cloth is actually being asked to do in electrolyte clarification:

• survive the chemistry and temperature it sits in

• hold the precoat and let the cake go

• resist blinding

• take flow in both directions

None of those is a figure a catalogue can quote. All of them can be written into an enquiry, and getting them right is the difference between a consumable you plan for and one that surprises you.

The environment the cloth lives in

Start with the conditions, because they eliminate more options than anything else.

A copper electrorefining tankhouse runs its electrolyte at 60 to 65 degrees C, a range that balances conductivity, copper solubility and additive stability, with 160 to 200 g/L of free sulphuric acid, 40 to 50 g/L of copper in solution and 20 to 50 mg/L of chloride. Those figures are Glencore Technology's, from its published guidance on electrorefining. A described operating plant reported by 911 Metallurgist sits inside them: 60 degrees C, 200 g/L free sulphuric acid, 49 g/L copper sulphate, specific gravity 1.2. Current density typically runs 200 to 350 A per square metre, with some plants pushing toward 400 and beyond.

Read that as a materials problem rather than a process one and the significance changes. The cloth is held at elevated temperature, continuously, in a strongly acidic solution, for its entire working life, apart from the discharge cycle. It is not a fabric that spends its time cool, dry or rinsed free of acid. Two stresses at once, and that combination is what separates the media. It is also exactly what a datasheet phrase like "acid resistant" fails to capture.

Why "acid resistant" is not a specification

The filtration industry publishes fibre ratings, and they are useful, but they have to be read against the actual duty rather than a one word summary of it.

Polyester is the default cloth of the filter press world, with high tensile strength, good abrasion resistance and dimensional stability through repeated cleaning, and the filtration literature describes it as acid resistant. Read further and the rating narrows: suppliers put polyester's suitable range at roughly pH 3 to 9, note that it cannot withstand strong acids such as concentrated sulphuric, and describe its hydrolysis resistance as poor, which is another way of saying it is not a good choice for moist heat.

Polypropylene is rated across a far wider chemical window, roughly pH 1 to 13, with excellent resistance to both acids and alkalis and excellent hydrolysis resistance. Its constraint is thermal rather than chemical: suppliers give it a working ceiling near 100 degrees C, with continuous service generally quoted at 90 to 100.

Now place the duty against the ratings. A tankhouse electrolyte applies strong acid and moist heat at once, continuously, and those are the two conditions polyester's published ratings single out as its limits. Polypropylene's published window covers the chemistry comfortably, and 60 to 65 degrees C sits below its thermal ceiling, though not by a margin you would ignore if your plant ran warm.

We have no field test that says polyester fails in an operating tankhouse, and we are not claiming one. The point is narrower: on the published ratings, the fibre a general purpose cloth guide is most likely to put in front of you is the one whose stated limits this duty sits closest to. If a quotation says "acid resistant cloth" and nothing else, it has not told you which of these two you are getting.

Nylon is rated weaker on acids than either. Higher specification fibres exist, PVDF for higher temperatures and PTFE for particular chemistries, but for a conventional copper electrolyte the question is usually settled before them.

The cloth is not the filter, the precoat is

This is the part the general guides cannot help with, because they assume a different machine.

In a precoat filter the cloth is a septum. A slurry of filter aid is laid down on its upstream face first, and the solution then passes through the precoat before it reaches the cloth. The precoat is the filtration medium, and it typically has a finer pore structure than the septum it sits on. That is the general precoat arrangement, described in the patent literature for precoat filters across duties rather than for copper specifically. The cloth's job is to retain the precoat and build a cake on its upstream surface, not to retain the process solids.

That reframes the specification completely. You are not selecting a cloth by micron rating against the particles in your electrolyte. You are selecting one that will hold a precoat evenly, release a cake cleanly, and survive the chemistry. How the aid is applied, as a precoat or as body feed, is a separate decision that sits on top of this one.

It also explains where the real retention limit sits, and here the copper specific patent literature is candid. Conventional precoating, it states, is limited in filtering the minute electrolytic by-products and dirt of 0.5 micron or less that come from the decomposition of additives such as glue and thiourea. The obvious response is a finer filter aid, and the same source explains why that fails: when the aid is made finer, "filtration efficiency sharply declines; in other words, permeation of electrolyte worsens."

That is the industry's own literature stating the trade-off plainly. Retention and throughput pull against each other, the tension is resolved in the filter aid grading, and no choice of cloth resolves it for you. The same patent describes a working answer as a blend, filter aid graded across roughly 3 to 40 microns, mixed from a coarser and a finer fraction rather than chosen as a single cut.

For the solids the aid is there to catch, the range is wide. Floating slimes in copper electrorefining have been characterised at around 25 microns, amorphous and irregular, arriving in two populations, small agglomerates and fine particles, with some material at colloid dimensions. The spectrum runs from tens of microns down past the point where precoat stops being effective, in the same stream. On the leach and solvent extraction side the feed characterisation question is the same one, and we have set out what the solids loading in a Copperbelt circuit means for a specification.

Yarn and weave, where the standard guidance points the wrong way

Filter cloths are built from three yarn constructions, and the published comparison is consistent across suppliers. Micronics sets it out:

• Monofilament, single continuous extrusions, has high tensile strength, excellent cake release properties and resistance to blinding, but low particle collection efficiency.

• Multifilament, strands twisted together, has high tensile strength, average particle collection efficiency and average cake release. Increased twisting reduces blinding.

• Staple, or spun, made from chopped filaments combed and twisted, has excellent particle retention but low tensile strength, and cake release is typically poor.

Read that table with a dewatering duty in mind, where the cloth itself is the filter, and it points at spun, because retention is the whole job.

A precoat electrolyte duty reads the same table and should reach the opposite conclusion. Retention is not the cloth's job, it is the filter aid's. What the cloth has to do is hold a precoat, let a cake go and resist blinding, which is the monofilament column, the construction with the worst retention figure on the table. The property that makes spun the obvious choice for a filter press is the property a precoat septum does not need.

Weave follows the same logic. Plain weave gives a balanced combination of strength and permeability, twill gives higher mechanical strength, and satin gives a smooth surface with excellent particle release. Permeability, usually expressed in CFM per square foot, is controlled by the fibre, how tightly the fabric is woven and how thick it is, with thicker fabrics generally less permeable.

The specification instinct is to tighten the weave, because tighter feels safer. In a precoat duty a tighter cloth mostly costs throughput and makes the precoat harder to release, without buying the retention you are imagining, because the precoat was doing that work already.

What actually blinds a cloth in this duty

Blinding is the failure mode that ends a cloth's life, and it is worth naming the mechanisms rather than treating it as general wear. The filtration literature lists particles accumulating and bridging across the pores, chemical precipitation forming hard or gelatinous deposits that adhere to the fabric, and oils or greases clinging to the fibres. The symptom is the same in each case, falling flow or rising pressure drop across the medium.

Two of those are live in a copper electrolyte circuit. Bridging is fed by the fine end of the slimes population described above. The gelatinous precipitate mechanism is the one worth watching, because this duty deliberately doses organic additives, glue and thiourea, and their decomposition products are precisely the extremely fine material the patent literature identifies as the hard fraction. Connecting those two is our reading rather than a published finding, but it points at something practical: a cloth that resists blinding is worth more here than a cloth that retains well.

Cake release belongs in the same discussion, since a cake that will not let go tends to be handled by force, and scraping or hand finishing works the fabric mechanically on top of everything the chemistry is already doing to it. How the cake comes off is a machine question as much as a media one, and we have set out separately what a discharge cycle actually costs.

Why nobody can quote you a change interval

Everything above explains the answer at the top. Cloth life here is set by the acid concentration and temperature the fabric sits in, the solids loading and the character of the solids, the filter aid and how it is dosed, how the cake is discharged, and the plant's own judgement about when clarity or throughput has drifted far enough to justify a change. Those inputs differ between refineries, and between circuits inside the same refinery.

So the honest answer is that the interval depends on the process conditions and on the customer's own requirement, and it differs from customer to customer. Anyone who quotes you a single figure without asking about any of the above is quoting a figure from a different plant.

The useful conclusion is not to chase the number. It is to specify the conditions properly, so the cloth you are given is matched to them, and the interval you eventually observe is a property of your circuit rather than an accident.

What to put in your enquiry

If you are specifying replacement cloth, or a new filter, these are the items that change the answer, and sending them is the difference between a matched cloth and a generic one:

• Free sulphuric acid concentration in g/L and operating temperature in degrees C. Not "acidic" and not "warm". These two together decide the fibre.

• Chloride content, and any other halide. It bears on materials selection for the wetted parts as well as the fabric.

• Solids loading and, if you have it, particle size distribution. The feed characterisation drives the filter aid grading, which is where retention is actually set.

• Which additives are dosed, and at what point in the circuit the filter sits relative to them.

• Filter aid type and dosing. For a copper duty this is normally a flux calcined diatomite. As a rule of thumb for precoat dosing, allow around 1 kg of filter aid per square metre of filtration area, recognising that this is a general figure rather than a consumption rate.

• Filtration area and configuration, and whether the machine backwashes. In the reversible installation described by 911 Metallurgist a fresh cake forms on the opposite side of the septum during the backwash, so where a unit works that way the fabric sees flow in both directions and has to be specified for it.

• How the cake is discharged, and whether it comes off dry and friable or wet and sticky.

• What you are optimising for. Clarity, throughput, or interventions per year. These pull in different directions, and the media choice should follow the priority rather than the reverse.

Where this fits

Sharpenn builds pressure filters for electrolyte clarification in non-ferrous refining. Around 1,000 units are installed worldwide across more than 460 installations, 25 of them in copper refining, at capacities from 5 to 350 cubic metres per hour, and the installed base has produced repeat orders after 20 to 25 years.

The cloth is a consumable in this duty, replaced on a cycle rather than washed and reused, and we can supply the replacement cloths with the equipment and on reorder. We can also supply filter bags for any make of filters, so an existing installation does not have to be ours for us to specify and supply the consumable. Where change frequency rather than cloth cost is the real driver, reusable filter elements are something we have built for other industries and can develop for an electrolyte duty on request.

Send us your acid concentration, temperature and solids loading, and we will tell you what the cloth should be.