Filtration Duties in a Copper SX-EW Plant: Which Filter Goes Where

Short answer: A copper SX-EW plant has at least six distinct filtration duties, they sit at different points in the circuit, and they are not one class of equipment. Pressure and precoat filters own the duties where the target is suspended solids and a solution has to reach a clarity target before the next stage will behave. They do not own the duties where the target is entrained organic, which belongs to dual media and carbon equipment. Splitting the circuit on that line, what is being removed rather than where the filter sits, stops a filter being bought for the wrong job.

An SX-EW plant is not a tankhouse

Almost everything written about electrolyte filtration in copper assumes an electrolytic refinery: cast anodes, anode slimes falling into the cells, a clarification filter keeping the electrolyte clean enough for high purity cathode. That describes a refinery well, and we have set out those duties in our complete guide to electrolyte filtration for copper refineries. It does not describe a solvent extraction and electrowinning plant.

In an SX-EW circuit the copper never passes through a cast anode. It is leached from ore, collected as pregnant leach solution, upgraded and purified by solvent extraction, then plated from the resulting electrolyte using inert lead anodes. There are no anode slimes in the refinery sense. Instead there is a set of problems a refinery does not have: colloidal silica and suspended solids in the leach solution, crud at the solvent extraction interface, entrained organic carried into the electrolyte, and lead dioxide sludge from anode corrosion. Different contaminants, different failure modes, different equipment. An engineer specifying from tankhouse literature will get the wrong filter in at least two places.

The duty map, stage by stage

Here is where each duty sits, what is actually being removed, and which class of equipment owns it.

Leach and pregnant solution collection

Pregnant leach solution arrives from the heap or vat carrying whatever the ore and the irrigation gave it. The first separation is bulk: ponds, clarifiers and thickeners, with a polymer flocculant, usually a polyacrylamide, to accelerate settling.

This is not a pressure filter duty. The volumes are large, the solids loading is high and variable, and the objective is settling rather than a clarity specification. Specify clarifiers and thickeners.

PLS clarification ahead of solvent extraction

Once the bulk solids are down, the solution still carries fine suspended solids and colloidal silica, and this is where the circuit starts paying for them. High total suspended solids reporting to the PLS greatly increases crud formation and the associated losses of extractant and diluent. Published work on crud mechanism finds the major mineral of crud samples is silica, and identifies silica and colloidal particles in the PLS as a main obstacle to running solvent extraction well. Plant practice reflects this: solution from the pregnant pond is pumped to a clarifier where remaining slimes are removed before extraction.

This is a pressure or precoat filter duty, and it has the signature of one: moderate to low solids loading after the bulk stage, a clarity target rather than a recovery target, a closed circuit, and a downstream stage that is expensive to upset. It is also the cheapest place in the circuit to attack crud, because every kilogram of solids stopped here is a kilogram that cannot stabilise an emulsion later.

Solvent extraction crud

Crud is amorphous third phase material that accumulates at the aqueous and organic interface in the settlers: a voluminous, low density mixture of aqueous phase, organic phase, solids from various sources, and air.

This is not a pressure filter duty, and it is worth being precise about why. Once crud has formed it is a three phase separation problem, not a solid liquid separation. Plant practice is to vacuum the crud layer off, store it, and pump it to a three phase decanter centrifuge that splits it so the organic returns to the circuit and the aqueous goes to neutralisation. That is centrifuge territory. What a filter contributes to crud is preventive, and it happens one stage upstream at PLS clarification. Anyone offering a single box that both prevents and treats crud is selling rather than specifying.

The recovered organic is then commonly regenerated by clay treatment, typically bentonite, before it returns to the circuit. That is not a pressure filter duty either. Adsorbing degradation products onto clay belongs to clay and carbon columns. A filter may sit downstream to catch carryover of the clay itself, which is a solids duty, but the treatment is not ours.

Advance electrolyte filtration ahead of electrowinning

This is the duty most often labelled "the electrolyte filter" in an SX-EW plant, and it is the one where the honest answer takes a paragraph rather than a sentence.

The conventional advance electrolyte filter is a dual media granular filter, anthracite over garnet. The anthracite layer on top removes organic, and the finer garnet below removes solids down to the micron scale. Design duty is better than 90 percent efficiency for both, with an outlet target below about 2 ppm organic against a solids loading typically around 20 ppm, at a specific flow rate of 12 cubic metres per hour per square metre and a minimum of three units. Anthracite media life is roughly one year of continuous service. Real installations run banks of them: one published expansion describes seven parallel anthracite and garnet filters installed to reduce organic entrainment and solids.

Sharpenn does not build the dual media granular filter. It is configured that way because its primary target is entrained and dissolved organic, and removing organic from an aqueous stream is not a solid liquid separation. If your specification for this position reads "below 2 ppm organic", that is a dual media duty and you should buy one.

A pressure or precoat filter belongs in this part of the circuit wherever the target is the solids rather than the organic: a solids clarity requirement tighter than the granular bed is designed to deliver, a polishing stage protecting the cells, or a plant whose organic entrainment is already controlled and whose remaining problem is particulate. Precoat reaches the fine end, because a filter aid cake laid on the septum traps particles a cloth alone would pass, which is why it is chosen in general process practice where suspended solids must come down towards the single figure parts per million range.

Electrowinning electrolyte recirculation and anode sludge

Lead anodes corrode slowly in service, mainly as lead dioxide, and the sludge accumulates in the cells. Operating practice is to clean each cell around three times a year, and excessive build up eventually compromises cathode quality. Circulation rate interacts with this directly: typical tankhouse flow is about 0.12 cubic metres per hour per square metre of plating area with a maximum around 0.14, and above that lead dioxide particles tend to stay in suspension instead of settling.

This is a pressure or precoat filter duty, and it is the one that most resembles a refinery duty. The contaminant is a suspended solid, the objective is cathode quality, the circuit is closed and recirculating, and it is the duty most often left to periodic cell cleaning alone.

Raffinate bleed and impurity precipitation

Most raffinate returns to leach, but a portion is bled to keep impurities in check. Rising iron in the electrowinning solution reduces current efficiency, increases energy consumption and changes cathode surface morphology, and the options are bleed, chemical precipitation or ion exchange. Manganese has its own control problem, driven by high valent species generated in the electrowinning environment.

This one splits. Where precipitation is used, the precipitate is bulk solids and belongs to thickeners and presses. Where a clarified solution has to go back to the circuit afterwards, that is a clarification duty and a pressure filter suits it. Which half you are buying depends on whether the solids or the solution is your product, which is the question to settle before specifying anything.

Where a pressure or precoat filter earns its place

Across all of the above, four conditions mark a duty as ours:

• The target is suspended solids, not organic. The single most useful test in an SX-EW circuit. A specification written in ppm of organic is a dual media or carbon duty. One written in ppm of suspended solids or in turbidity is a pressure filter duty.

• The objective is clarity, not recovery. If you are measuring cake moisture and metal recovery, you want a press or a thickener. If you are measuring what is left in the filtrate, you want a pressure filter.

• The duty is closed and pressurised, because the solution is acidic, hot, or valuable enough that an open circuit is unattractive.

• The clarity target is tight enough to justify precoat. Body feed and precoat modes exist for exactly this decision, and we covered that trade off in our guide to precoat and body feed filtration.

Sharpenn builds pressure filters for these duties in horizontal, vertical and rotary horizontal precoat configurations, in leaf and precoat modes, at capacities from 5 to 350 cubic metres per hour, and in filtration areas up to the 200 and 500 square metre range where the footprint demands it. The installed record is approximately 1,000 units worldwide across 460 or more installations across all applications, of which 25 units are in copper refining, with repeat orders after 20 to 25 years and the original filters still in service. In copper SX-EW specifically there is an installation in the DRC rated at 350 cubic metres per hour, at the top of the major plant range.

Materials for acidic copper SX-EW solutions

SS316 is the baseline wetted material for our leaves and internal manifolds, and it covers a great deal of acidic copper service. Two variables move that answer, and both belong in an enquiry rather than in an assumption. The first is chloride: chloride assisted leaching, chloride in the ore, and saline or seawater make up water all change the corrosion picture and turn stainless selection from a default into an engineering decision. The second is temperature, which is mild in a heap leach circuit and less so in parts of the electrolyte loop.

Where either pushes past SS316, we have built in SS316L, Hastelloy and titanium. Which one is right follows from your solution chemistry rather than from a catalogue, so send the chemistry.

The vessel itself is a pressure vessel and is designed to a pressure vessel code, and which code applies depends on the jurisdiction. For Australian projects that means AS 1210, and we have written separately on what an Australian tankhouse specification actually requires.

What to put in your duty sheet

If you are preparing an enquiry for any of the clarification or polishing duties above, these items let a supplier answer properly instead of guessing:

1. Which stage in the circuit the filter sits at, named as a duty rather than as "filtration".

2. Flow rate, and whether it is continuous or batch.

3. Suspended solids in the feed and the required clarity in the filtrate, in the same units.

4. Entrained organic, stated separately from solids. If both numbers are in the specification, you are probably looking at two pieces of equipment, and it is better to find that out now.

5. Full solution chemistry, including chloride and silica.

6. Operating temperature and pressure, and pH.

7. Whether the solids or the solution is the product.

8. Available footprint and headroom.

9. The pressure vessel code that applies in your jurisdiction.

Item 4 saves the most time. A duty sheet that separates the solids target from the organic target gets you correctly scoped equipment. One that merges them into "electrolyte filtration" gets you three quotations for three different machines.

Frequently asked questions

Do you supply the advance electrolyte filter for an SX-EW plant? Not the conventional one. That filter is a dual media granular filter, anthracite over garnet, configured that way because its primary target is entrained organic at an outlet specification of a few ppm. Removing organic is not a solid liquid separation and we do not build it. Where the duty in that part of the circuit is defined by suspended solids instead, that is a pressure or precoat filter duty and it is ours.

Which SX-EW duties suit a pressure filter? Clarification of pregnant leach solution ahead of solvent extraction, solids polishing of electrolyte where the target is particulate, the electrowinning recirculation duty where lead dioxide sludge is the contaminant, and clarification of a bled stream after impurity precipitation.

Which SX-EW duties do not suit a pressure filter? Bulk pregnant solution settling, crud treatment once crud has formed, and clay treatment of organic. Those belong to clarifiers and thickeners, three phase centrifuges, and adsorption columns.

Will filtering the PLS reduce crud? It is the most effective single thing you can do about crud, because crud needs solids to stabilise it. Published work on crud mechanism points at silica and colloidal particles in the PLS. Filtering before crud forms works; filtering after it has formed does not.

Is precoat necessary, or is a cloth enough? It depends on the clarity target. A cloth septum alone handles moderate targets. Precoat lays a filter aid cake on the septum and reaches a much finer cut, which is why it is chosen where a downstream stage needs clarity approaching the single figure parts per million range.

How often does the filter cloth get changed? There is no single number and we will not invent one. The interval is set by the process conditions and by what the customer requires of the filter, and it varies from plant to plant. Treat the cloth as a planned consumable rather than a fixed service interval. We can supply filter bags, including for filters we did not build.

Getting the duty right

The recurring error in SX-EW filtration specification is treating "electrolyte filtration" as one line item. In this circuit it is at least six duties, and more of them belong to other equipment classes than to ours. That is worth saying plainly, because a duty map is only as useful as it is honest about where it stops. The same discipline applied to a different circuit gives a different map, which is why we wrote a separate one for filtration duties in a nickel cobalt hydrometallurgy plant.

Send us the duty, with the flow, the solids loading, the solution chemistry and the temperature, and we will tell you whether it is a pressure filter duty and size it if it is.