Tailings dewatering: filter press, thickener or both?

The question "thickener or filter press?" is badly framed. The two do not do the same job: the thickener recovers the water that separates easily, the filter press recovers what is left. The real question is: how far does your site need to go in dewatering — and at what price? The answer depends less on the equipment than on what you will do with the tailings afterwards.

This article describes the three levels of dewatering, what each delivers and costs, and the data that lets you pick the right level.

Three levels, three ways of managing tailings

Level 1 — Thickened tailings. A thickener (conventional, high-rate or high-density) receives the plant slurry, adds a flocculant, and separates by gravity a clear overflow and a thickened underflow. The underflow is still pumpable; it goes to a tailings storage facility, where it keeps settling and releasing water. This is the simplest level, continuous, with the lowest operating cost.

Level 2 — Paste tailings. A high-density or paste thickener pushes the separation further: the underflow becomes a homogeneous paste that no longer segregates and spreads at a shallow slope instead of forming a pond. It is deposited on surface with a reduced footprint, or used as underground backfill with a binder. Paste requires pumps and pipelines suited to its rheology.

Level 3 — Filtered tailings. A filter press (or a belt or disc filter, depending on the ore) turns thickened slurry into a cake that can be handled by loader and truck. The cake is dry-stacked and compacted; water recovery is maximal. This is the level that costs the most in capital and operating expense, and the one that changes site management the most.

The point that is often misunderstood: the filter press is not an alternative to the thickener, it is its complement. A filter fed with dilute slurry runs long cycles for little cake; fed with thickened underflow, it becomes efficient. In most filtered-tailings circuits, the answer to "one or the other?" is "both."

What each level delivers and costs

The data that decides

What regulation and the site impose. Topography, seismicity, proximity to watercourses, land available for a storage facility, closure requirements: that is the frame. On a constrained site, or where a retaining dam is not acceptable, the dewatering level is no longer an economic choice but an operating condition.

Water. On a site where water is scarce or expensive, or where discharge is limited, every cubic metre recovered at the plant has a value — and it funds part of the equipment. Where water is abundant, that item weighs less.

Tonnage. Filtering large tailings tonnages takes many filters, with space and handling to match; it is the parameter that drives the cost of level 3 up the most. Conversely, a moderate tonnage makes filtered tailings far more accessible.

Mineralogy. Clayey or very fine tailings flocculate and filter with difficulty: long cycles, wet cakes, blinding cloths. Sandy or coarse tailings dewater easily at every level. No decision should be made without tests on the actual tailings.

Climate. In Northern Quebec, Northern Ontario or the territories, a filtered cake is handled and stacked in freezing weather under particular rules, and the filtration building is heated; paste or slurry raise other questions (pipeline freezing, deposition under ice). Climate does not rule out any level, but it changes the cost of each.

Mine life. A heavy investment in filtration is amortized over a long period; a short project may prefer a simpler level, if the site allows it.

The tests that decide

The decision is made in the laboratory before it is made in a meeting. Three families of tests, on representative samples — several if the ore varies:

1. Settling and flocculation: flocculant choice, dosage, settling rate, achievable underflow density — this sizes the thickener and says whether paste is reachable.
2. Rheology: yield stress and viscosity of the underflow or paste — this sizes the pumps, pipelines and deposition slope.
3. Pressure filtration: cake moisture versus time, with and without membrane squeeze, cake strength, cloth behaviour — this sizes the filters and says how many are needed. Our comparison membrane or chamber filter press details what these tests reveal.

With those results, each level is costed — capital, operating, footprint, water — and compared with the others on identical grounds. That is the kind of trade-off our feasibility studies service covers upstream; supply of the filter presses and their ancillary equipment, then their commissioning, come next. See the Filter presses & hydrocyclones product page for the configurations offered, or tell us about your tailings and your site.

Frequently asked questions

Can an operating mine move from a conventional storage facility to filtered tailings? Yes, it is a common case, often driven by a facility expansion that has become impossible or by a regulatory requirement. The project adds a thickener if the circuit has none, a filtration unit and a handling chain, and is planned with production running.

Can a filter press treat plant slurry directly? Technically yes, but with long cycles and low productivity. Upstream thickening is almost always justified: it reduces the volume to filter and stabilizes the filter feed.

Are filtered tailings always dry-stackable? No: it depends on the moisture achieved, the particle size and the climate. The filtration test and geotechnical tests on the cake say whether stacking is possible and under what conditions.