Filtration Duties in a Nickel Cobalt Hydrometallurgy Plant: Which Filter Goes Where

Short answer: A nickel cobalt hydrometallurgy plant does not have one filtration duty. It has six or seven, they sit at different points in the flowsheet, and they belong to different classes of equipment. Pressure and precoat filters own the clarification and polishing duties, where a solution has to reach a clarity target before the next stage will behave. They do not own the bulk solid liquid separation duties, which belong to thickeners and presses. Getting that split right at the specification stage is what keeps a filter from being bought for the wrong job.

Most nickel cobalt plants do not have a tankhouse

Almost everything written about filtration in non-ferrous refining assumes an electrolytic tankhouse: an electrolyte loop, anode slimes, a clarification filter keeping the cells clean. That describes a copper refinery well. It does not describe most nickel cobalt plants.

The dominant routes for laterite feed end in chemical reduction or in an intermediate product, not in electrowinning. Glencore's Murrin Murrin operation in Western Australia is the clearest published example: high pressure acid leach on laterite ore, solvent extraction, and then a hydrogen reduction circuit of six parallel autoclaves in which hydrogen precipitates nickel metal from solution. The powder is briquetted and sintered. There is no cell house anywhere in that description. The company reports roughly 47,000 tonnes per year of nickel and 3,000 tonnes per year of cobalt briquettes from 4.5 million tonnes per year of ore.

The ammonia leach route reaches the same place by different chemistry, again through reduction rather than electrolysis. Other operations stop earlier still and ship a mixed hydroxide intermediate rather than producing metal at all.

This matters for equipment selection because it changes what the filter is protecting. In a copper tankhouse, the filter protects cathode quality. In a nickel cobalt hydromet plant, it usually protects an organic phase, a precipitation reaction, or an autoclave. The clarity targets and the failure modes are different, and so is the correct equipment.

The duty map, stage by stage

Here is where each filtration duty sits, what it is really asking of the equipment, and which class of equipment owns it.

Leach discharge and counter current decantation

Autoclave discharge is a hot slurry at roughly 20 to 25 percent solids, leaching at temperatures around 240 to 270 degrees C and pressures in the range of 3800 to 5400 kPa, according to published process descriptions of the HPAL route. Separating that is a bulk washing duty, not a clarification duty. Industry practice is counter current decantation, typically six to seven thickener stages, with the underflow washed by recycled process water.

This is not a pressure filter duty. The solids loading is enormous and the objective is washing efficiency and metal recovery, not clarity. Specify thickeners.

Neutralisation and iron aluminium removal

Primary neutralisation is generally carried out ahead of the CCD circuit to drop out the bulk of the solid impurities. Downstream, partial neutralisation with limestone removes iron, aluminium and chromium from the pregnant leach solution before it reaches solvent extraction.

These stages generate freshly precipitated, often gelatinous solids in quantity. Mostly not a pressure filter duty. Filter presses and similar high solids equipment own it. The exception is the final trim, which shades into the next duty.

Pregnant leach solution clarification ahead of solvent extraction

Once the bulk impurities are down, the solution still carries fine suspended solids, and those solids go straight into the organic phase if nothing stops them. This is a pressure filter duty. It has the signature of one: moderate to low solids loading, a real clarity target, a closed pressurised circuit, and a downstream stage that is expensive to upset.

Solvent extraction crud and entrained organic

Crud is a genuine and well documented failure mode here, and it is not simply a solids problem. Work published in Industrial and Engineering Chemistry Research found that crud and gel formation was driven by dissolved silica and its speciation, and that in one case nickel could only be separated after silica gel filtration because crud formed at pH 7.

Be careful with this one. Part of it is a filtration duty and part of it is not. Removing solids upstream so that crud has less to build on is a clarification duty. Removing entrained organic from an aqueous stream is a different separation problem with its own equipment, and in copper solvent extraction circuits it is typically handled by dual media granular filters rather than by leaf or precoat filters. Anyone who tells you a single box solves both is selling, not specifying.

Mixed hydroxide precipitation and thickening

Mixed hydroxide precipitation using magnesia is increasingly the chosen way to make the nickel cobalt intermediate from laterite feed. The precipitate is the product. Recovering it is a high solids dewatering duty, measured in moisture content and recovery.

This is not a pressure filter duty. Thickeners and dewatering equipment own it.

Solution purification and polishing

Purification ahead of the recovery step removes residual copper, iron and zinc, commonly by staged counter current precipitation with lime and recycled magnesium hydroxide. Whatever the chemistry, it leaves fine solids in solution, and the stage that follows will not tolerate them.

This is the strongest pressure and precoat filter duty in the flowsheet, and it is the one where published industry practice is most explicit. Filter cloth suppliers to nickel refineries describe a security or polishing stage immediately before solution enters the recovery step, carried out on chamber presses or horizontal pressure filters with high retention fabrics, precisely to guarantee solution clarity. Where the target is finer still, precoat is the mode that gets you there: a filter aid cake laid on the septum traps particles the cloth alone would pass, and precoat filtration is applied in general process practice where suspended solids have to come down to around the 1 ppm range for a downstream process to work.

Where a leaf or precoat filter earns its place

Across all of the above, four conditions mark a duty as belonging to a pressure filter:

• The solids loading is moderate to low, and 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 turbidity or suspended solids in the filtrate, you want a pressure filter.

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

• The clarity target is tight enough to need a precoat. Body feed and precoat modes exist for exactly this, and choosing between them is its own decision. We covered that trade off in detail in our guide to precoat and body feed filtration.

• The chemistry and temperature are compatible with the wetted materials. See below, because this is where nickel cobalt duties differ most from copper.

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 m³/hr. 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 nickel cobalt specifically, machinery has been delivered to a nickel cobalt hydromet plant in India, on the hydrometallurgical duties described above rather than on a tankhouse. For the copper side of the same equipment, our complete guide to electrolyte filtration for copper refineries covers that duty end to end.

Materials and design notes for acidic nickel cobalt solutions

SS316 covers a great deal of non-ferrous electrolyte and process solution service, and it is the standard wetted material for our leaves and internal manifolds. Nickel cobalt duties push harder on it than copper electrorefining does, in two directions worth naming in your enquiry.

The first is temperature. Hydromet streams run hot, and a material that is comfortable in a copper tankhouse is not automatically comfortable at leach or purification temperatures. The second is chloride, which changes the corrosion picture completely and turns stainless selection from a default into an engineering decision. Neither is a reason to avoid a pressure filter. Both are reasons to state the temperature and the full solution chemistry in the enquiry, not the flow rate alone.

The vessel itself is a pressure vessel and is designed to a pressure vessel code. Which code depends on where the plant is. 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 nine items are what let a supplier answer properly instead of guessing:

1. Which stage in the flowsheet the filter sits at, named as a duty and not just 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.

4. Full solution chemistry, including chloride and any silica.

5. Operating temperature and pressure.

6. pH.

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

8. Available footprint and headroom, and whether the filter is indoors.

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

The single most useful line is the first one. A duty stated as "polishing ahead of the reduction autoclaves" gets you a sized filter. A duty stated as "a filter for a nickel plant" gets you a question.

Frequently asked questions

Do nickel cobalt plants use electrolyte filtration? Some do, but most do not, because most do not electrowin. Laterite routes based on high pressure acid leach or ammonia leach typically end in hydrogen reduction or in a mixed hydroxide intermediate. Those plants still have several filtration duties, but they sit in the hydrometallurgical train rather than around a cell house.

Which nickel cobalt duties suit a pressure filter? Clarification of pregnant leach solution ahead of solvent extraction, and solution purification and polishing ahead of the recovery step. Both are low to moderate solids duties with a clarity target in a closed pressurised circuit, which is what a pressure filter is for.

Which duties do not suit a pressure filter? Counter current decantation after the autoclave, the bulk neutralisation stages, and mixed hydroxide precipitation and dewatering. Those are high solids washing, separation and dewatering duties owned by thickeners, presses and dewatering equipment.

Is precoat needed, or is a filter 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 takes you to a much finer cut, which is why it is the mode chosen where a downstream stage needs solution clarity in the low parts per million range.

What material should the filter be? SS316 is the standard for our leaves and internal manifolds and covers most acidic non-ferrous process solutions. Temperature and chloride content are the two variables that can change that answer, so both should be stated in the enquiry rather than assumed.

Can one filter cover several duties in the flowsheet? Rarely, and it is worth resisting. The duties differ in solids loading, clarity target and chemistry, and a filter sized for a polishing duty will not behave on a bulk separation duty. Specify per duty.

Getting the duty right

The recurring error in nickel cobalt filtration specification is treating filtration as one line item. It is not one line item. It is a set of separate duties, and roughly half of them belong to equipment we do not build. That is worth saying plainly, because the value of a duty map is only as good as its honesty about where it ends.

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.