Application — High-Temperature Filtration
Phosphoric Acid & Phosphogypsum Filtration
Hot acid, hard abrasive crystals and scale that builds inside the weave — three loads acting at once, on one of the largest continuous filtration duties in mineral processing.
Acid, Abrasion and Scale Together
In wet-process phosphoric acid production, phosphate rock is digested with sulphuric acid and the resulting gypsum has to be separated from the product acid — usually on a horizontal belt, pan, table or tilting pan filter, with several counter-current washing stages. The filtration step is not an auxiliary operation here; it directly determines acid recovery and therefore plant economics.
What makes the fabric selection hard is that three loads arrive together. Gypsum crystals are hard and abrasive, giving continuous mechanical wear. The hot acid attacks chemically, and at temperature the polymer is at its weakest precisely when abrasion is highest. And gypsum scale grows inside the weave, so permeability can collapse long before the fabric is worn through.
Selecting on any single one of those three produces a fabric that fails on the other two. The construction, the surface finish and the wash procedure have to be designed together with the polymer.
Typical Duties
What We Optimise For
On a continuous acid filter, small differences in the fabric translate directly into acid recovery and belt life. These are the objectives we work against.
Acid Stability at Temperature
Polypropylene across the full pH range to roughly 90 °C; PVDF, PPS, PEEK or PTFE above that. Polyamide is excluded here despite excellent abrasion resistance, because strong mineral acids attack it.
Abrasion Resistance Assessed Hot
Gypsum crystals wear a fabric continuously, and the polymer is weakest at temperature. Abrasion has to be judged under hot conditions, not at ambient.
Scale-Resistant Surface
A smooth monofilament or calendered surface gives crystals fewer anchoring points, so cake and scale release more readily and the wash actually restores permeability.
Washing Efficiency
Acid recovery depends on an even, crack-free cake through several wash stages. Uniform permeability across the whole belt width matters more than a low nominal pore size.
Dimensional Stability & Tracking
A belt that stretches unevenly at temperature runs off track, chafes at the edges and fails mechanically. Thermal fixing and construction decide whether it tracks straight.
Seams and Joints
The belt joint is the limiting component. Welded, spiral or clip joints each behave differently in hot acid, and the choice follows the machine and the load.
Not the Same as FGD or Water-Treatment Gypsum
Gypsum from flue gas desulphurisation or water treatment is filtered from a broadly neutral to mildly alkaline slurry at moderate temperature. That duty is dominated by dewatering rate and cake release, and a well-chosen polypropylene fabric usually handles it comfortably.
Phosphogypsum comes out of a hot, strongly acidic process liquor. The chemical and thermal load changes the polymer decision entirely, and a fabric specification carried across from an FGD plant will not survive. If your duty is the neutral case, see our gypsum and lime sludge page instead.
What We Need to Quote
Related Topics
High-Temperature Filtration
Polymer limits, dry vs. wet heat, confection and cost logic.
Learn MoreGypsum & Lime Sludge
The neutral FGD and water-treatment case, covered separately.
Learn MoreFilter Belts
Woven, spiral and drainage belts for continuous vacuum filters.
Learn MoreMining & Minerals
Abrasive mineral slurries and hydrometallurgical circuits.
Learn MoreShort Filter Cloth Lifespan
Systematic diagnosis when belts and cloths fail early.
Learn MoreSpecialty Fabrics — PVDF, PPS, PTFE
The polymers for the hot, acidic end of the process.
Learn MoreFrequently Asked Questions
Which filter fabric is used for phosphoric acid filtration?
Wet-process phosphoric acid filtration combines hot acid with abrasive gypsum crystals, so the fabric needs chemical stability, temperature capability and abrasion resistance simultaneously. Polypropylene is the workhorse: hydrolysis-stable, tolerant across the full pH range as an orientation figure, and economical — with a temperature limit around 90 °C continuous. Where the acid runs hotter, PVDF, PPS, PEEK or PTFE come into consideration. Polyamide is excluded despite its excellent abrasion resistance, because strong mineral acids attack it.
Why do phosphoric acid filter belts and cloths wear out so quickly?
Three loads act together. Gypsum crystals are hard and abrasive, so there is continuous mechanical wear. The hot acid attacks chemically, and at temperature the polymer is at its weakest just when the abrasion is highest. And gypsum scale builds inside the weave, so permeability falls even where the fabric is not yet worn through. Any one of these is manageable; the combination is what makes this one of the harder duties in mineral processing.
How do you deal with gypsum scaling in the fabric?
Scaling is addressed through surface design and through the wash. A smooth monofilament or calendered surface gives crystals fewer anchoring points, so the cake and the scale come away more readily. Beyond that, the acid or hot water wash procedure is part of the specification rather than an operational detail — the wash frequency, temperature and chemistry determine whether permeability recovers between cycles or declines cumulatively.
Is this the same as the gypsum filtration in flue gas desulphurisation?
No, and treating them the same is a common mistake. FGD and water treatment gypsum is filtered from a broadly neutral to mildly alkaline slurry at moderate temperature, so the duty is dominated by dewatering and cake release. Phosphogypsum comes out of a hot, strongly acidic process liquor, which adds a chemical and thermal load that changes the polymer decision entirely. We cover the neutral case separately on our gypsum and lime sludge page.
What do you need in order to quote?
Acid concentration and composition, continuous and peak temperature, machine type — horizontal belt, pan, table or tilting pan filter — the belt or cloth dimensions and fixing method, solids content and crystal size, the required filtrate clarity and cake washing efficiency, the wash and descaling procedure, and the current fabric with its service life and observed failure pattern.
Belt life or acid recovery below target on your acid filter?
Send us the acid concentration, the temperature, the machine and belt dimensions and your current service life — we will propose a construction built for all three loads.
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