Understanding filtration › 5. Processes by sector

CHAPTER 5.4

Chemicals and pigments

In inorganic chemistry, a solid is often produced by precipitating it from a solution. It then has to be recovered, washed free of its mother liquor, and the effluents treated. The cloth works in an acidic or alkaline medium, often hot, and must deliver a pure product.

Nicolas Ract · 8 min read · October 2026

An example process: titanium dioxide by the sulphate route

Titanium dioxide is the white pigment in paints, plastics and paper. Its production by the sulphate process is a good illustration of the role solid-liquid separation plays in a chemical process. According to a study published by the German Federal Environment Agency [1]:

• 1. Digestion: the ground ore (ilmenite or titanium slag) is digested with sulphuric acid; the resulting cake is redissolved in water or in recycled dilute acid.
• 2. Clarification: the insoluble solids are separated from the solution, as far as possible, by a thickener, then by a vacuum filter or a filter press. This is the first filtration, where the goal is a clear filtrate.
• 3. Crystallisation of iron sulphate, by vacuum cooling, when the ore is ilmenite.
• 4. Hydrolysis: titanium oxide hydrate precipitates selectively, at around 95 to 110 °C.
• 5. Filtration and washing of the hydrate: the hydrate is separated from the waste acid by filtration, washed, treated, then filtered again. This is the second filtration, where the goal is the purity of the precipitate.
• 6. Calcination of the hydrate, between 800 and 1,100 °C, which yields the pigment.
• 7. Treatment of the waste acid, containing around 20 to 22 % sulphuric acid: it is concentrated and recycled, or neutralised with milk of lime, which produces gypsum [1].

This neutralisation gypsum is the third filtration. A Spanish study describes a "red gypsum" produced by neutralising the weak acid effluents of a titanium dioxide plant with lime: it consists mainly of calcium sulphate dihydrate and iron hydroxides; the authors propose using it as a setting regulator in cement, whereas it had until then been sent to controlled landfill in the country studied [2].

General guidance: this pattern of clarification, precipitation, washing and neutralisation recurs in many inorganic chemistry processes: metal salts, oxides and hydroxides, phosphates, inorganic pigments, catalysts.

The equipment

• Recessed chamber and membrane filter presses, for precipitates to be washed and for gypsum (see chapters 2.1 and 2.2).
• Vacuum filters, for clarification and some continuous washing [1] (see chapter 2.3).
• Tower filter presses: manufacturers list titanium dioxide among their applications [4] (see chapter 2.5).
• Candle filters: general guidance, they are used for the fine clarification of solutions.

Filter cake washing, the key to purity

A filtered precipitate retains salt-laden mother liquor in its pores. Washing it means replacing that mother liquor with a purer liquid. A technical article distinguishes three regimes: direct displacement of the mother liquor, diffusion of impurities out of the pores, and an intermediate regime. Efficiency is tracked through the wash ratio, i.e. the number of wash volumes passed, by plotting the fraction of impurities remaining in the cake against that ratio [3].

According to the same author, the chamber-and-membrane filter press is among the most effective at removing impurities with little water, because the consolidated cake limits shrinkage and short-circuiting of the wash liquid. He also notes that the distribution of the wash near the cloth depends on the cloth support specific to each plate design [3].

Field experience: a partially blinded cloth sends the wash liquid through the areas that are still open. The cake is then well washed in some places and not in others, and product purity drifts without any visible defect on the cloth.

What wears or blinds the cloths

• Chemical attack on the fibre: strong acids, bases, oxidisers. Each fibre has its limits (see chapter 3.1).
• Temperature: around 80 to 90 °C continuous for polypropylene, 100 to 110 °C for polyamide, 130 to 135 °C for polyester according to a fibre resistance table [5]. Hot solutions and steam washes must stay within these limits.
• Precipitation within the fabric: a salt that crystallises on cooling, or gypsum forming in the pores, creates a crust (see chapter 4.4).
• The very fine particles of precipitates, of the order of a micrometre, which penetrate the fabric.
• Field experience: gypsum is known to cause blinding, and its crystallisation depends heavily on the neutralisation conditions (pH, temperature, residence time). A poorly crystallised gypsum filters badly, whatever the cloth.

What matters to the operator

• Product purity: residual salts, traces of iron or other metals, colour. For a pigment, whiteness is a commercial criterion.
• Wash liquid consumption: every volume of wash water has to be pumped, then treated or evaporated.
• Filtrate clarity at the clarification stage: solids that pass through end up in the product.
• Gypsum dry solids content and its disposal route: reuse, if there is an outlet, or paid storage.
• Availability: the process is continuous; a stopped filtration blocks everything upstream.

Indicators to track

The common baseline in chapter 4.5 applies, with indicators specific to each stage:

Also record each chemical wash of the cloths and its effect on cycle time: this is what shows when blinding is no longer reversible.

Choosing the medium: points to watch

• List the actual conditions: pH, type of acids or bases, oxidisers, maximum temperature of the product and of the washes, cleaning chemicals. The fibre is chosen for the most severe case (see chapter 3.1).
• One cloth per stage: clarification calls for retention, the precipitate to be washed for uniform permeability, gypsum for good cake discharge and resistance to blinding (see chapters 3.2 and 3.3).
• Edge sealing: an acid leak at the plate edge is as much a safety issue as a yield issue (see chapters 4.3 and 3.4).
• Test before rolling out: an immersion test of a sample in the actual medium, then a trial on a few plates (see chapter 6.2).