Quick answer: Kaolin, ball clay, bentonite and other clay slurries are among the hardest materials to dewater because the particles are flat, sub-micron and hold water between their layers. A membrane filter press is the standard tool for kaolin producers: it takes a 25–35% solids slurry from the thickener to a 65–75% solids cake (roughly 25–35% moisture) in a 60–120 minute cycle, which a belt press or centrifuge cannot reach. The keys are a tight monofilament cloth, a gentle feed pump, a membrane squeeze at 12–16 bar, and accepting a long cycle rather than forcing it.
Why clay behaves differently from other minerals
Most mineral concentrates dewater easily: the particles are hard, roughly round and a few tens of microns across, so a cake forms an open structure through which water drains. Clay minerals are the opposite. Kaolinite particles are hexagonal platelets, typically 0.2–2 μm across and a fraction of that in thickness. They stack face to face like a deck of cards, the stacks trap water between the plates, and the cake that forms is dense and nearly impermeable. Smectite clays such as bentonite are worse still, because water enters the crystal lattice itself and the particle swells.
Two practical consequences follow. First, filtration is slow: the specific cake resistance of a fine kaolin can be a thousand times that of a sand or a metal concentrate. Second, a mechanical squeeze after the cake has formed removes far more water than simply pumping at higher pressure, because it compresses the cake rather than trying to push water through an ever-thickening barrier. That is why the membrane filter press dominates the industry.
Where the filter press sits in a kaolin plant
Kaolin processing is a wet process from the pit onward. The crude clay is slurried with water and dispersant, degritted through screens and hydrocyclones, classified by particle size in centrifuges or hydroseparators, and often bleached, magnetically separated or flotated to raise brightness. All of this happens at 20–40% solids. At the end the product must be delivered either as a 70% solids slurry for paper coating, or as a dry lump, noodle or spray-dried powder for ceramics, paint, rubber and fibreglass. Both routes pass through dewatering.
The classic route is thickener → filter press → either re-dispersion (for slurry products) or dryer (for dry products). The filter press does the heavy lifting: it removes the bulk of the water mechanically at a fraction of the energy that evaporating it in a dryer would cost. Every percentage point of moisture removed in the press is water the dryer does not have to boil off, and thermal drying is the largest energy cost in the plant. The same logic applies in ceramic tile plants, where clay-rich body slip and glaze wastewater are dewatered before the sludge is recycled or dried.
Which press type for clay
| Equipment | Cake solids on kaolin (typical) | Cycle | Comments |
|---|---|---|---|
| Recessed chamber filter press (no membrane) | 55–65% | 2–4 h | Works but slow; cake often wet in the centre; long feed at high pressure wears pumps and plates |
| Membrane filter press | 65–75% | 1–2 h | Industry standard; squeeze compresses the cake and equalizes moisture; shortest cycle for a given moisture |
| Belt filter press | 35–50% | Continuous | Too wet for lump or dry products; used only for waste clay tailings |
| Decanter centrifuge | 40–55% | Continuous | Fine platelets pass through with the centrate; high polymer demand |
| Tube press (very high pressure) | 75–82% | Short cycles, small batches | Highest solids, but small throughput per unit and higher capital and maintenance cost |
For the great majority of kaolin, ball clay, china clay and bentonite plants the membrane filter press is the right answer. The chamber press is a reasonable choice for waste clay from sand washing or aggregate plants where a slightly wetter cake is acceptable and capital is tight; our sand washing guide covers that case. See also our comparison of filter press vs centrifuge vs belt press.
Designing the press for clay: what changes
Feed solids and conditioning
Feed the press from a thickener at 25–35% solids. Feeding a dilute slurry wastes cycle time filling the chambers with water and produces a soft, uneven cake. Kaolin is usually dispersed for processing (with sodium silicate or polyacrylate), and a dispersed slurry filters much more slowly than a flocculated one; many plants add a small dose of acid or a cationic flocculant just before the press to collapse the dispersion. Trials on your own slurry settle this quickly: a 10–20% cut in cycle time from conditioning is common.
Feed pump
Clay is not abrasive, so the concern is control rather than wear. The filtration pressure must rise slowly from 1–2 bar to the final 6–8 bar as the cake builds; a pump that hits full pressure in the first minutes drives fines into the cloth and blinds it. A variable-speed centrifugal pump, a progressive cavity pump or a large air-operated diaphragm pump with a pressure ramp all work. Long feed times at low pressure are normal for clay; do not treat them as a fault.
Cloth
Fine clay platelets penetrate and blind an open cloth within a few cycles. Use a tight monofilament or a monofilament/multifilament combination cloth in polypropylene, with a calendered surface, and an air permeability at the low end of the range. Cake release is usually good on a calendered cloth because the compressed clay cake is coherent. Our cloth selection guide explains the parameters, and our article on cloth blinding describes the warning signs.
Membrane squeeze
Squeeze at 12–16 bar with water or air for 15–30 minutes. The last few minutes of squeeze remove very little water; the plant should set the squeeze time from a filtrate flow meter, not from a fixed timer, to avoid wasting cycle time. Kaolin cakes squeeze to a very uniform moisture, which is valuable if the cake goes straight to a dryer or extruder that needs a consistent feed.
Chamber depth and cake thickness
Because filtration resistance rises sharply with cake thickness, clay presses use thinner cakes than mineral presses: 25–32 mm chambers rather than 40–50 mm. More plates of a thinner chamber give a shorter cycle for the same daily tonnage than fewer deep chambers.
Sizing example
A plant producing 100 dry tonnes per day of filter cake at 70% solids from a 30% solids thickener underflow must remove roughly 190 m³ of water per day. At a 90 minute cycle and 20 hours of operation the press completes about 13 cycles per day, so each cycle must produce 7.7 dry tonnes, or about 11 tonnes of wet cake. At a cake density around 1.75 t/m³ that is 6.3 m³ of cake per cycle, which a 1500 mm × 1500 mm membrane press with about 100 chambers of 30 mm can hold. Real sizing uses your own filtration test data; the example only shows the scale. Our price by plate size guide gives the range of press sizes involved.
Bentonite and other swelling clays
Bentonite does not filter the way kaolin does. Its swelling makes even a dilute slurry viscous, and the cake it forms is almost impermeable. Bentonite plants therefore rarely dewater the product in a filter press; they dry it from the pit moisture. Where a filter press meets bentonite is in waste streams: tunnel boring machine slurry, drilling mud and diaphragm wall slurry, where the bentonite is a minor component of a mostly sand and silt slurry. That is a different duty with different rules, covered in our drilling and TBM slurry article.
Related industries
Everything above applies with small changes to ball clay for sanitaryware, china clay for porcelain, feldspar and quartz fines from the same deposits, titanium dioxide pigment slurries, calcium carbonate slurries for paper filler, and precipitated silica. All are fine, low-abrasion, slow-filtering mineral products where a membrane squeeze pays for itself in dryer energy. Our mining and mineral processing hub links the related guides.
Frequently asked questions
What cake moisture can a filter press reach on kaolin?
A membrane filter press typically reaches 65–75% solids (25–35% moisture) on kaolin and ball clay. A plain chamber press reaches 55–65% solids with a longer cycle. Very high pressure tube presses can reach about 80% solids in small batches.
Why is kaolin so slow to filter?
Kaolinite particles are flat platelets under 2 microns across that stack into a dense, nearly impermeable cake and hold water between the layers. The specific cake resistance can be a thousand times that of a sand or metal concentrate, so the cake must be compressed by a membrane rather than merely pumped against.
Do I need a membrane filter press for clay?
For a saleable kaolin, ball clay or china clay product, yes: the squeeze step is what gets the cake below 35% moisture in a reasonable cycle and saves dryer energy. For waste clay from sand washing or aggregate plants a chamber press with a wetter cake is often acceptable.
What filter cloth is used for kaolin?
A tight, calendered polypropylene monofilament or mono/multifilament combination cloth with low air permeability. Open cloths blind within a few cycles because the platelets penetrate the weave.
Can a centrifuge or belt press replace a filter press for kaolin?
Not for a product cake. Belt presses reach 35–50% solids and decanters 40–55%, and fine platelets escape with the centrate. Both are used only on waste clay streams where a wet, low-value cake is acceptable.
How is bentonite different?
Bentonite swells with water and forms an almost impermeable cake, so bentonite products are dried rather than filtered. Filter presses handle bentonite only as a minor component of drilling, tunnelling and diaphragm wall slurries.
Send us a sample or a filtration test report and the tonnage, and we will size the press, chamber depth and cloth for it: request a clay dewatering proposal.
References: USGS Mineral Commodity Summaries 2024 — Clays (production, uses and processing of kaolin, ball clay and bentonite), US EPA — Mineral Mining and Processing Effluent Guidelines, 40 CFR Part 436 (includes the kaolin and ball clay subcategories).
This article is part of our guide to industrial filter press applications.