Understanding filtration › 5. Processes by sector

CHAPTER 5.3

Kaolin, talc and carbonates

Industrial minerals processed by the wet route go through water to be washed, classified and purified. That water then has to be removed. Filtration sits at the point where water is most expensive: just before thermal drying.

Nicolas Ract · 7 min read · October 2026

Where separation sits in the process

Kaolin

Kaolin can be processed by the dry route or the wet route. The dry route is simpler but gives a lower-quality product; the wet route is widely used for paper-grade kaolin [1]. It involves, in sequence:

• slurrying the ore in water (blunging);
• classification into coarse and fine fractions, using centrifuges, hydrocyclones or hydroseparators;
• refining, by physical, magnetic or chemical methods, for example sulphuric acid leaching followed by the addition of a reducing agent;
• filtration and dewatering, by filter press, centrifuge, rotary vacuum filter or tube filter;
• drying, optional, in an apron dryer, rotary dryer or spray dryer, and sometimes calcination [1].

Losses in washing and classification can reach 30 to 40 % of the ore [1]. These secondary fractions are stored in ponds or, according to one published example, dewatered on recessed chamber filter presses, granulated and then dried on a belt dryer to be put to use, with a moisture content of around 12 % for filler kaolin granules [3].

Talc

Talc is mainly processed dry: crushing, grinding, air classification. The fines can, however, go through a shaking table and then flotation to remove unwanted minerals; the resulting talc slurry is then dewatered and filtered before flash drying [2].

Calcium carbonates

General guidance: natural calcium carbonate for paper, paints or plastics is often finely wet-ground and delivered as a concentrated slurry; part of the production is filtered and then dried to be delivered as powder. Tower filter manufacturers indeed list calcium carbonate and industrial minerals among their applications [4].

The equipment

• Recessed chamber filter presses and membrane filter presses: the most common for obtaining a compact filter cake (see chapters 2.1 and 2.2). Membrane squeezing improves dry solids content at the end of the cycle.
• Tower filters: general guidance, stacked horizontal plates with a continuous cloth running through them, and membrane squeezing; they operate automatically at a high cycle rate, and high-pressure washing of the cloth at every cycle is common (see chapter 4.4). One manufacturer's datasheet states an operating pressure of up to 1.6 MPa [4] (see chapter 2.5).
• Rotary vacuum filters [1], which operate continuously; general guidance, they are suitable when the target dry solids content is less demanding (see also disc filters in chapter 2.4).
• Tube filters, listed among kaolin dewatering equipment [1]; general guidance, they press the slurry at high pressure against a tubular medium.

What wears or blinds the cloths

• Abrasion: mineral particles, especially when quartz or other hard impurities remain, wear the cloth in high-flow areas (feed neck, edges) and where it contacts the plates (see chapter 4.2). General guidance: carbonates and talc are soft minerals; it is mainly slurry velocity and impurities that cause wear.
• Very fine particles: the highest-value products are also the finest, in the micrometre range. They penetrate the fabric and blind it (see chapter 4.4).
• Process additives: field experience, dispersants added to keep the slurry fluid, or flotation reagents, can alter filterability and encourage fouling.
• Frequent washing: on tower filters, the cloth is washed at every cycle; poorly adjusted nozzles wear the fabric.
• Repeated flexing: on a tower filter, the continuous cloth advances and bends at every cycle; its mechanical durability matters as much as its permeability.

What matters to the operator

• Filter cake dry solids content: whatever water remains in the filter cake has to be evaporated in the dryer. Mechanical filtration removes water at a far lower cost than thermal drying (see chapter 1.2).
• Product purity and brightness: a cloth that sheds fibres, or a filtrate that returns loaded to the process, can degrade quality.
• Throughput: filtration is often the bottleneck between the wet plant and the dryer.
• Filtrate clarity: it is generally recycled into the process; a loaded filtrate sends fines back to the head of the circuit.
• Cake discharge: a sticky filter cake lengthens cycles and requires manual intervention.

Indicators to track

The "Minerals or industrial sludge" typical case in chapter 4.5 provides the framework. In practice:

• Service-life unit: dry tonnes filtered, or cycles on a tower filter.
• Per shift or per day: filter cake dry solids content as measured by the site, cycle time, filtrate turbidity.
• At the dryer: energy consumption per tonne of dry product. This is what translates a difference in dry solids content into euros.
• Per cloth: the position and nature of the defect (localised wear or general blinding), to separate mechanical causes from process causes.

Choosing the medium: points to watch

• Retention suited to the particle size distribution: a cloth that is too open lets fines through into the filtrate; too tight, it blinds (see chapter 3.3).
• Smooth surface for cake discharge: monofilament, satin weave or calendering depending on the case (see chapter 3.2).
• Resistance to abrasion and flexing, particularly on a tower filter with a continuous cloth.
• Fibre: process chemistry is generally not very aggressive, but an acid leaching step or a chemical wash makes it necessary to check compatibility (see chapter 3.1).
• Judge on cost per tonne: a more expensive cloth that gains one point of dry solids content can pay for itself at the dryer (see chapter 6.3), provided this is verified by a trial (see chapter 6.2).