Filter Cloth in High-Alkali (Caustic) Duty: Causes and Cloth-Side Fixes
The cloth has not worn out — it has lost its strength. In strongly alkaline liquor, polyester is chemically attacked rather than abraded, and the belt tears one shift while still looking serviceable the shift before.
Alkaline cloth failure links three things that have to be looked at together: a polymer with a hydrolysable backbone — ordinary polyester (PET) — a liquor carrying free hydroxide at temperature, and a machine that keeps the cloth in contact with that liquor for its entire running life. Bayer-process alumina liquor is the classic case, and some zeolite and chemical circuits behave the same way. The failure does not present as wear. It presents as strength loss and embrittlement in a fabric that still looks like a filter cloth.
Does this match what you are seeing?
- The cloth tears or splits under normal running tension, with no impact damage and no obvious wear track.
- Yarns snap instead of bending when a sample is flexed by hand; monofilaments crack rather than deform.
- The fabric has gone stiff, chalky or papery, and edges crumble when rubbed between finger and thumb.
- The seam pulls out of sound-looking fabric, or the sewing thread has gone before the cloth around it.
- Life collapsed after a change in liquor strength, liquor temperature, or the cleaning chemistry.
- Damage is spread over the whole cloth, including areas that never see cake or slurry.
If that is not quite it
The cloth is thinning where the slurry first lands, with a directional wear pattern Abrasive Slurry Wear
Belts are short-lived and no single cause stands out Short Cloth Life
The joint is the only thing failing and the fabric is sound Joint / Seam Failure
Alkaline hydrolysis, in plain terms
This section is industry-level knowledge, not data from our own failure records. It is here because the mechanism decides everything downstream: what you should look for, what a replacement cloth has to be, and which questions actually determine whether a cloth survives your circuit.
- 01
Polyester has a hydrolysable backbone
PET is a polyester: its chain is held together by ester linkages. Hydroxide ions cleave those linkages, so strongly alkaline liquor does not merely swell or stain the fibre — it cuts the polymer chains that carry the load.
- 02
The attack works from the fibre surface inward
Alkaline hydrolysis of PET is described in the literature as a surface-erosion process: the fibre is eaten back from the outside and the filament diameter falls. Tensile strength follows the load-bearing cross-section down with it.
- 03
Rate rises with temperature and with concentration
Both drive the same reaction. A duty that is survivable warm and dilute becomes aggressive hot and strong, which is why two circuits that a process sheet would both call "alkaline" are not one duty.
- 04
Polypropylene has no such linkage
PP is a polyolefin — a hydrocarbon backbone with nothing for hydroxide to cleave — which is why caustic duties push selection toward PP or toward hydrolysis-resistant treatments on polyester. PP carries its own limits in exchange, principally a lower working-temperature ceiling than PET and more creep under sustained tension.
Where this duty is found
Bayer-process alumina liquor
The classic caustic filtration duty. Bauxite is digested in hot concentrated sodium hydroxide; the alumina dissolves as sodium aluminate and the undissolved residue — red mud — is separated and washed before the liquor goes forward. Residue separation, washing and liquor security filtration all handle strongly alkaline liquor at temperature.
Alumina / red mud filtration →4A zeolite and detergent-builder circuits
Zeolite A is crystallised from sodium aluminate and sodium silicate solution, so the mother liquor the cloth sits in is alkaline. Published equipment literature puts this duty on both horizontal vacuum belt and leaf filters, so the machine is not a given either.
4A zeolite filtration →Chemical circuits and caustic cleaning regimes
Caustic recovery, causticisation and alkaline precipitation duties behave the same way. So does a cleaning regime: a cloth can spend its process life in a mild liquor and take its real chemical dose during clean-in-place, when hot caustic is run deliberately through the fabric.
→Red mud separation and washing are commonly run on drum, disc, leaf or pressure filters rather than on horizontal vacuum belt filters. If your alkaline duty is on one of those machines, tell us the machine before anything else — we will say honestly whether the cloth form we supply fits it at all.
The opposite end of the chemistry scale, for contrast: phosphogypsum filtration is an acidic, fluoride-bearing duty, and nothing about polymer selection carries across from one to the other.
Cloth, machine, process
Three columns, kept separate on purpose. Two of them are not ours to fix, and on a caustic duty the machine and process columns are where the dose is usually being set.
- The polymer was chosen on filtration and release behaviour and the chemical duty was treated as secondary — that is how a polyester belt ends up in caustic service.
- No hydrolysis-resistant treatment was specified, because nobody asked what the liquor actually was.
- The sewing thread, clipper or fastener is a different material from the fabric, so the joint reaches the end of its chemical life first and the belt is scrapped around a sound cloth.
- The construction is fine-filament where the duty is chemically aggressive: surface attack removes the same depth from every filament, so it takes a larger proportion of a thin one.
- Cloth wash on the return run is weak, blocked or misaimed, so the fabric completes every circuit still wet with liquor instead of rinsed. Contact time, not just contact, sets the dose.
- Steam, condensate or a heat-exchanger fault puts the cloth at a higher temperature than the process sheet says it sees.
- Drip, spray or liquor carry-over wets parts of the belt that the process was never supposed to reach, which is why chemical damage shows up outside the cake track.
A new cloth will not fix these.
- Liquor strength or temperature was raised to lift throughput, and cloth life was not re-examined against it.
- The cleaning chemistry is harsher than the process chemistry, and no one specified the cloth against the cleaning step.
- Upsets and excursions: the cloth is specified against the normal case and killed by the abnormal one.
- A weakened fabric is then finished off mechanically — abrasion, tracking, tensioning — so the post-mortem records a mechanical failure and the chemical cause is never found.
Correct these first, or the new belt inherits the fault.
Chemical attack is one of the candidates behind a belt that simply does not last: short cloth life sorts the others. If the joint is failing on its own, start at joint / seam failure instead.
Work through it in order
Every step says what to look at, what to measure and what to photograph. Do it in this order and you finish holding everything an engineering quote needs — and, more to the point, holding proof of whether the cause was chemical at all.
- 00
Stop, isolate, and treat the cloth as caustic
- Look at
- Machine stopped, isolated and tagged out. A cloth taken off an alkaline duty is still wet with liquor — handle it under your site's procedure for that chemical, not as a piece of fabric.
- 01
Separate chemical attack from mechanical wear
- Look at
- Find a part of the cloth that never touches slurry or cake — under the edge seal, or the outer edge of the return run. If that area is brittle too, the cause is chemical. If only the cake track is thinned, you are looking at abrasion instead.
- Measure
- How far the damage extends beyond the cake track, across the width.
- Photograph
- One shot of the damaged area and one of an area that never sees slurry, in the same frame if you can.
- 02
Flex test a sample
- Look at
- Whether yarns bend or snap, and whether the surface powders when rubbed. A chemically degraded monofilament breaks short and clean instead of yielding.
- Measure
- Nothing to gauge — but do it at several points across the width and along the length, and note where it is worst.
- Photograph
- The broken ends, close up. If you kept an offcut of the cloth when it was new, photograph both together.
- 03
Check the seam and the thread separately
- Look at
- Whether the sewing thread, clipper or fastener failed while the fabric around it is still sound. That points at the joint material, not at the cloth.
- Measure
- How much of the joint has gone, across the width.
- Photograph
- Close-up of the joint, both faces. This is the same photo the engineering RFQ asks for.
- 04
Write down what the cloth is actually in
- Look at
- The liquor as the plant measures it — free caustic or alkalinity in your own units, and what else is dissolved in it.
- Measure
- Temperature at the cloth, not at the reactor or the digester. They are frequently not the same number.
- Photograph
- The relevant line of the process sheet or the log, if that is easier than transcribing it.
- 05
Include the cleaning step
- Look at
- What is run through the cloth during clean-in-place or descaling, at what strength, at what temperature, and how often. This step is easy to leave out of a post-mortem, and it is sometimes the whole answer.
- Measure
- Minutes of cleaning contact per cycle, and cycles per week.
- 06
Check the cloth wash
- Look at
- Whether the return-run wash is actually rinsing liquor out of the fabric, or just wetting the face. Blocked nozzles and a misaimed header both leave the cloth loaded.
- Photograph
- The wash header running, from the side, so the spray pattern is visible.
- 07
Build the life history
- Look at
- How long this cloth ran, how long the one before it ran, and what changed between them — liquor, temperature, throughput, cleaning regime, supplier.
- Measure
- Days or weeks in service, per belt, as far back as your records go.
- Photograph
- Nothing to photograph — write it down. It goes straight into the failure description on the RFQ.
- 08
Copy the nameplate
- Look at
- Machine brand, model and serial number.
- Photograph
- The nameplate, lit from an angle so the flash does not wash it out.
Not sure how to measure the belt itself? See the measuring guide.
What can be changed in the cloth
Polymer selected against the liquor, not against the cake
For caustic duty the industry answer is to move away from untreated polyester — polypropylene, or polyester with a hydrolysis-resistant treatment. Which of those suits your circuit depends on your temperature as much as on your alkalinity, and it is fixed against your figures during the engineering review.
Joint materials specified with the fabric, not after it
Sewing thread and mechanical fasteners are separate materials with their own chemical behaviour. Specifying them in the same breath as the cloth stops the belt being scrapped for a joint that died early.
Construction chosen with surface attack in mind
Because alkaline attack removes material from the fibre surface, the same depth of loss costs a coarse filament a smaller share of its cross-section than a fine one. That trades against filtration fineness, so it is a decision to be made on your duty rather than a rule.
Built to your measured dimensions
Width and length taken from your old belt and your machine, with the seam type you actually run. A chemically correct cloth that is the wrong size is still a scrapped cloth.
These cases are not solved by new cloth
If the cloth wash is not rinsing liquor out, if a temperature fault is putting the fabric above what the process sheet claims, or if the cleaning regime is the real chemical dose, a replacement belt takes the same punishment as the last one and goes the same way. Those are machine and process problems. Fix them first, then change the cloth.
And there is a duty we may not be able to serve at all. Moving away from untreated polyester solves the hydrolysis mechanism; it does not raise the temperature ceiling. A duty that is both strongly caustic and hot can sit outside the ordinary alternatives. If yours is one of those, the honest answer is that a cloth change is not the fix, and we will tell you that rather than take the order.
Tell us your machine, your liquor and your cleaning regime, and we will say honestly whether we can help.
Which of these we can actually supply
Our own confirmed range for caustic duty is not published yet. These are exactly the rows a buyer on an alkaline circuit wants — and exactly the rows we are not willing to fill with a plausible number. Send your case and we will come back with what the factory has confirmed in writing, stated against your temperature rather than in the abstract.
| Cloth-side option | What it addresses | Confirmed range |
|---|---|---|
| Polymers available for caustic duty | Whether the chain can be attacked by hydroxide at all | UNKNOWN — pending factory confirmation |
| Hydrolysis-resistant treatments on polyester | Extending polyester into duties that would otherwise take PP | UNKNOWN — pending factory confirmation |
| Continuous temperature limit, per polymer and finish | The other half of the rate equation, and PP's main trade-off | UNKNOWN — pending factory confirmation |
| Alkalinity window, stated with the temperature it applies at | A chemical limit quoted without a temperature is not a limit | UNKNOWN — pending factory confirmation |
| Sewing thread and fastener materials | Whether the joint outlives the fabric | UNKNOWN — pending factory confirmation |
| Maximum fabricated width and length | Whether your machine is inside our range at all | UNKNOWN — pending factory confirmation |
| Chemical-resistance testing available on a sample | Whether a duty can be checked before a full belt is committed | UNKNOWN — pending factory confirmation |
Where the cloth family and its seam and edge options are set out: horizontal vacuum belt filter cloth. Process and quality control: factory and quality control.
Replace it, or fix the duty first?
- Yarns snap rather than bend anywhere on the cloth — strength loss is not recoverable, and a cloth that has lost it will not get it back.
- Brittleness reaches areas that never see slurry: the whole fabric has taken the dose, not one track.
- The fabric tears repeatedly at new places under normal tension.
- The seam has pulled out of fabric that is itself degraded — re-jointing it buys days.
- The fabric is sound and only the joint material failed — the joint specification is the thing to change.
- Damage is confined to the cake track with a directional wear pattern — that is abrasion, and it is a different problem.
- Life collapsed immediately after a cleaning-regime change, and the cleaning regime has not been reviewed yet.
- The cloth wash or a temperature fault is identified and not yet corrected — fix it, or the replacement takes the same dose.
Chemical degradation is one-way: a fabric that has lost tensile strength does not recover, so on this problem the replace column is usually correct once the mechanism is confirmed. What is not correct is replacing like for like. We do not publish expected service life in months for any duty, and on an alkaline circuit a life figure without the liquor temperature beside it means nothing at all.
Decided it needs replacing? Next step is measuring the old belt →
The same facts, whether you ask us to diagnose or to quote
Tick these off as you collect them. Nothing here is stored or sent from this page — it is a scratch list for your own use.
Where alkaline duties come up
Alumina / Red Mud Filtration
The Bayer circuit in full: what the liquor is, where the cloth sits, and why the machine there is often not a belt at all.
4A Zeolite Filtration
Detergent-builder zeolite from alkaline mother liquor, on both belt and leaf machines.
Short Cloth Life
When belts go early and no single cause stands out — the page that sorts the candidates, chemical attack among them.
Before you enquire
Does a high pH reading on its own tell you the cloth will fail?
No. Hydrolysis rate depends on hydroxide concentration and on temperature together, and on how long the cloth is in contact — including the return run and the cleaning step. Two circuits at the same pH, one warm and one hot, are two different duties for the fabric. Send alkalinity and temperature as a pair, plus the cleaning chemistry, and the picture is usable.
Why did the same cloth last far longer on one alkaline duty than on another?
Because the variables that set the rate moved. Higher temperature, stronger liquor, longer contact per revolution because the cloth wash is weak, or a cleaning step that nobody counted as exposure. Any one of those compresses life sharply while the process sheet still reads "alkaline" on both plants.
Is polypropylene simply the answer for caustic duty?
It removes the hydrolysis mechanism, which is the main thing killing polyester there. It does not remove the temperature question — PP has a lower working-temperature ceiling than PET and creeps more under sustained tension, so a hot caustic duty can be outside both. If yours is one of those, we would rather say so than sell you a belt that will not last.
What chemical limits can you publish for your cloth?
None yet, and we are not going to invent them. Our factory specification package is not confirmed in writing, so every capability row on this page reads UNKNOWN. Anything we tell you about a chemical window will come with the temperature it applies at and will be stated in the quotation, not taken off a web page.
These answers are engineering judgement applied to public process knowledge, not measured results, and no FAQ structured data is published from this page until they have been verified by our technical staff.
Measure your old belt and send it to us
Width, length, seam type, nameplate, the photos from the checks above — and, for this duty, the liquor alkalinity with the temperature it is measured at, plus the cleaning chemistry. That is enough for an engineering answer instead of a guess. No account, no registration.
Replacement cloth fabricated to your machine dimensions. Not an OEM spare part.