Filter Cloth Blinding: 7 Causes and How to Fix Them

Last updated: 2026-10-31

When fill time climbs past roughly 1.4 times its commissioning value — 22 to 31 minutes with feed solids and pressure unchanged — filter cloth blinding is the first hypothesis to test, not the last. Measure air permeability drop and dry cloth mass gain before touching pump curves or plate alignment: both move within two cycles, while mechanical faults drift slowly.

Key Takeaways

• Trend fill time every cycle; a 10–20% drift is the earliest reliable signal of cloth plugging.
• Weigh a dried cloth sample: more than 8% mass gain per square meter indicates depth loading.
• Surface deposits wash off; precipitates grown inside the yarn do not, at any backwash pressure.
• Select weave on P80 and liquor chemistry, never on “tightest available” — tighter cloth blinds faster.
• Retire cloth on measured air permeability, not on visible holes or a fixed calendar.

What Actually Happens Inside a Blinded Filter Cloth?

A filter cloth is a permeable fabric that separates solids from liquor across a pressure difference and supports the cake that performs most of the real filtration. On a healthy press the cloth does not filter; it seeds a bridge in the first 60–120 seconds (typical) and steps aside. How hard that seeding is depends on what the circuit upstream sends, which is why solid liquid separation is a train problem before it is a cloth problem.

Solid-Liquid Separation, step by step

Three loading modes explain nearly every case. Surface loading is a deposit layer on the upstream face that closes pores from above and usually washes off. Depth loading is a penetration of fines into the yarn bundle that leaves particles trapped between individual filaments, behind the weave crossover and often under the knuckle. Pore plugging is a mineral growth inside the opening that connects the two faces, and it closes the passage from within.

Where solids lodge follows the geometry. In a monofilament cloth the openings are interstices between adjacent warp and weft yarns, typically 20–60 µm across (indicative, to be confirmed against the weave data sheet). A multifilament cloth adds a far larger internal surface: capillaries between the hundreds of filaments inside each yarn, commonly 2–15 µm, which capture everything below about 10 µm and never release it. The weave knuckles, where warp crosses weft, are the tightest spots and blind first. Behind the face, a backing scrim adds strength but provides a tortuous drainage path that collects migrated fines.

The irreversibility is chemical, not hydraulic. Gypsum, iron hydroxide, basic zinc sulfate, and silica nucleate on the fiber from a liquor supersaturated at the cloth, then grow until the crystal bridges the pore. A 40 µm pore closes after 10–20 µm of growth on each side, which in a supersaturated neutralization liquor takes days (indicative). Once keyed into the weave, backwashing pushes water past the deposit, not through it. The window for washing is therefore before the precipitate grows; the window for prevention is in the chemistry.

Blinding is also the only one of four failures that starts silent. Cloth wear thins the fabric and opens holes at plate edges, showing as rising filtrate turbidity rather than fill time. Cloth stretching sags the fabric into the chamber, so cake thickness becomes uneven while permeability is barely affected. Mechanical damage is a tear or puncture from misalignment or a wash lance, and shows instantly as cloudy filtrate.

How Do You Recognize Filter Cloth Blinding Before Throughput Collapses?

You recognize it by trending, because a blinded cloth gives no alarm until the press is short of capacity. Fill time is the primary indicator, and the comparison that matters is the unit’s own commissioning baseline at the same feed solids and pressure. Fill time also falls when feed solids rise, so log density with it: a thickener upset from 45% to 55% underflow masks a blinding cloth for a week. A drift of 10–20% over two to three weeks justifies inspection; waiting for 50% means weeks of lost throughput. The filter press for mining guide lists the baseline data worth recording.

Thickener vs Filter Press in practice

Filter Press Area Calculator from Leaf Test Data: selection rules

Cake moisture is the signal operations notices first, because wet cake sticks and plugs chutes. A rise of 3 percentage points (wet basis) at constant squeeze pressure means the drainage path is partly closed. Filtrate clarity runs the opposite way: a blinding cloth often produces clearer filtrate, because the blinded weave retains fines it previously passed. Clear filtrate plus slow fill is a blinding signature, not a good sign. The filter cake moisture guide covers the measurement basis.

Two direct measurements settle the question. Cut a 100 × 100 mm coupon from a marked location, dry it at 105 °C, and express it as g/m² against the new-cloth figure, typically 450–600 g/m² (indicative). Above 8% gain is depth loading; above 15% is usually precipitate. Then run an air permeability test on the dried coupon per ISO 9237: a drop below 60% of the new value means the weave, not the cake, controls the cycle.

The Seven Causes, Ranked by How Often They Are the Answer

Most filter cloth blinding traces to seven mechanisms, ranked here by frequency, not severity; they come from troubleshooting visits on presses handling leach residues, neutralization sludges, concentrates, and tailings.

1. Fine particle population above what the weave can hold. Symptom: fill time rises from the first week, filtrate stays clear, and the face looks clean after washing. Mechanism: particles below about 10 µm migrate through the cake into the yarn capillaries, where the cake no longer protects them; the sub-10 µm fraction, not the P80, sets the rate of loss. Fix: measure that fraction, then cut fines upstream with cyclones or a tighter flocculant regime, or move to a monofilament weave with a defined pore and a pre-coat.
2. Precipitate formed inside the cloth. Symptom: mass gain above 15%, hard scale in the weave, and washing that restores permeability for only a few cycles. Mechanism: the filtrate cools or degasses at the cloth, or pH shifts a fraction of a unit across the cake, so gypsum, iron hydroxide, basic zinc sulfate, or silica becomes supersaturated where flow is slowest. Fix: analyze the scale and control the chemistry — pH adjustment ahead of the press, temperature maintenance, or a sequestrant. No wash program beats a liquor still supersaturated at the medium.
3. Residual flocculant or polymer gel. Symptom: a slimy cloth, poor cake release, and permeability that partly returns after caustic wash but not acid. Mechanism: high-molecular-weight polyacrylamide carried over from the thickener adsorbs on the fiber and forms a gel layer that traps fines; overdosing by a factor of two is common during thickener upsets. Fix: cut dose to the minimum that holds the bed, move the dosing point upstream, and run a 2–4% caustic wash with a wetting agent.
4. Organic phase and crud. Symptom: blotchy hydrophobic patches, uneven cake, and filtrate carrying fine droplets, typically after a solvent-extraction upset. Mechanism: organic droplets wet the fiber preferentially and block pores with a film water cannot displace, so open area falls while the cloth still looks clean. Fix: reduce entrained organic ahead of the press, add a detergent wash at 40–50 °C, and try a more oleophobic fiber.
5. Inadequate or one-sided cloth washing. Symptom: one face dark and loaded while the other is clean, with partial and short-lived recovery. Mechanism: the wash header reaches the cake side but not the drainage side, so deposits that migrated through the weave sit against the backing and are never contacted by the jet. Fix: wash both faces at 40–60 bar with the cloth tensioned, verify nozzle coverage with a spray test, and log each wash.
6. Over-pressure and over-squeeze. Symptom: normal fill time, rising moisture, a stiff compressed cloth, and flattened drainage channels. Mechanism: feed pressure above the cloth rating, or a squeeze held too long, compresses the weave and backing and reduces the void volume that carries filtrate; part of the effect is permanent. Fix: cap terminal pressure at the cloth rating, shorten the squeeze to where moisture gain flattens, and confirm closing force against the plate-pack limit, as in filter press cycle optimization.
7. Wrong weave or fiber for the duty. Symptom: a new cloth blinds within days while the previous one lasted months. Mechanism: multifilament or plain-weave fabric with high internal surface holds fines by design, and a fiber that hydrolyzes or swells changes its pore structure. Fix: specify weave, pore rating, air permeability, and polymer against the measured duty, and prove it on a laboratory pressure leaf filter before committing a full set.

How Do You Select a Cloth That Resists Blinding?

Selection starts with particle size and liquor chemistry, then the catalog. The controlling number is the fines fraction: if 20% of solids are below 10 µm, no weave holds them indefinitely, so the question becomes which cloth tolerates them longest and cleans best. A monofilament with a defined pore and a calendered face is the better answer for fine, precipitating duties.

Weave choice follows. Plain weave gives the tightest, most stable fabric per unit of air permeability and suits coarse, free-filtering solids. Twill and satin float longer yarn runs over the surface, improving cake release and reducing knuckle blind spots at a slightly more open structure. For sticky precipitates and gypsum, calendered satin monofilament is the usual selection: cake lifts cleanly and fewer sites exist where a crystal can anchor.

Air permeability is the practical specification handle, quoted in L/dm²/min at 200 Pa. New cloths for hydrometallurgical duty sit in the 20–200 band (typical), with 80–150 covering most fine-solids presses and 20–60 for duties where clarity outweighs rate. Every 30–40% reduction in permeability roughly halves initial fill rate. Read thread count with pore rating: a high count over coarse yarn is a loose weave. Calendering closes surface pores without changing internal structure; it improves clarity and release but blinds easily under slimy cake, so pair it with monofilament.

The common mistake is a tighter cloth to fix clarity: it moves capture to the surface, raises the pressure needed to hold rate, and cleans poorly, so filtrate improves for weeks while fill time degrades permanently.

1. Characterize the duty. Size distribution including the sub-10 µm fraction, density, pH and temperature range, and an analysis of any scale on the old cloth.
2. Confirm the mechanism. Coupon mass gain, air permeability, and scale analysis separate fines from precipitate from organic; the fix differs.
3. Write the specification. Polymer, monofilament versus multifilament, weave, pore rating, permeability target, thickness, and edge construction — a spec, not a brand.
4. Test the actual slurry. A bench leaf test or pilot-scale plate and frame filter press run for 10–20 cycles gives fill rate, moisture, and release before a full set is ordered.
5. Verify the first article. Measure permeability and mass per unit area on delivery and after 50 cycles, and keep the delivery coupon as the baseline.
6. Review at replacement. Compare measured life with the spec and change one variable at a time; three changes in a month teach nothing.

What Cleaning Program Actually Restores Permeability?

Washing works when the foulant sits on the surface or in the upper weave, and fails when it has grown inside the yarn. Build the program around early, frequent, mild washes rather than late, aggressive ones: a weekly wash costs hours of press time per month, while cloth past recovery costs weeks of throughput.

Start every cycle with high-pressure water on both faces at 40–60 bar, cloth tensioned, lance held 100–150 mm away so the jet penetrates rather than deflects. Wash the drainage side as deliberately as the cake side; the deposits that matter most are the ones that migrated through. Water at 40–50 °C removes organics and flocculant gel markedly better, and a small wetting agent dose helps water wet a hydrophobic surface.

Chemical washes are targeted. For carbonate and gypsum scale, 3–5% hydrochloric acid at 40–50 °C with a 30–60 minute soak, or 5–10% citric acid at 50–60 °C for a slower attack where chloride is unwelcome. For flocculant, gel, and organic films, 2–4% caustic at 50–60 °C for 60–120 minutes, then a detergent rinse. Never follow acid with caustic without an intermediate rinse: the heat generated can precipitate the salts you just removed.

Soak time matters more than concentration once the deposit is thin. Circulating liquor through the cloth in a wash station gives 30–120 minutes of contact; spraying alone gives seconds. The rinse endpoint should be a conductivity within about 50 µS/cm of incoming wash water (indicative), not a fixed time, because residual acid or caustic attacks the fiber and precipitates salts on the next cycle. Dry the cloth before restart where possible.

Retire the cloth when a washed, dried coupon holds less than 50–60% of new air permeability, or when mass gain stays above 10% after two consecutive chemical washes. Washing beyond that point consumes acid and labor while the press runs below capacity.

What Does Blinding Cost?

The cost of filter cloth blinding spreads across four lines that are budgeted separately, which is why it is chronically underestimated. Cloth is the visible one; labor, lost throughput, and moisture penalty decide whether a wash program pays.

Take a 200 m² press, 22 cycles per day, 1.4 t of dry cake per cycle, 330 operating days, cloth at USD 65/m² supply only (indicative planning figures, to be confirmed by quotation). A set costs about USD 13,000: three sets a year at a four-month blinded life is USD 39,000, against USD 13,000 at twelve months. The delta is USD 26,000.

Wash labor is 1.5 hours per week at USD 40/h loaded, about USD 3,100 per year. Throughput dominates. A cycle stretched from 95 to 118 minutes by slow fill cuts capacity by roughly 19%, which on 31 t/d of dry solids is about 6 t/d; at USD 45/t contribution that is USD 89,000 per year of margin at risk.

The moisture line is smaller and easier to defend internally because it appears on a haulage invoice: from 20% to 24% moisture (wet basis) adds about 2 t of water per day to a 31 t/d cake, roughly USD 4,600 per year at USD 7 per wet tonne.

The decision rule: if a wash program costs less than the cloth delta it avoids, run it. Where the press itself is the constraint, run pressure filter vs filter press before buying area.

Field Data: Fill Time From 22 to 48 Minutes and Back

A zinc neutralization circuit, single recessed-plate press, 630 m² effective area in 80 chambers of 1,500 mm plates. Feed was gypsum-bearing neutralized liquor at 46% solids by weight, P80 of 48 µm, with roughly 22% of solids below 10 µm. Terminal feed pressure 10 bar, squeeze 14 bar, target cake moisture 22–24% wet basis.

Commissioning fill time was 22 minutes and cycle time 96 minutes. Over eleven weeks fill time reached 48 minutes and cycle time 127 minutes with no change in feed density or pressure setpoint. Cake moisture rose from 24% to 30% and discharge became difficult. Filtrate clarity improved, which delayed diagnosis: operations read it as good performance.

Coupon tests were decisive. Cloth mass gain was 17% above the new figure and air permeability had fallen from 95 to 28 L/dm²/min at 200 Pa. Scale analysis showed gypsum with iron hydroxide, and the wash log showed two chemical washes in eleven weeks, both from the cake side only.

The original cloth was a multifilament twill at 480 g/m². It was replaced with a calendered monofilament satin at 520 g/m², air permeability 110 L/dm²/min at 200 Pa, on the same plate pattern. Washing became a weekly 8% citric soak at 55 °C for 60 minutes, both faces, with a conductivity endpoint.

Eight months later the cloth was still in service, mass gain 5%, fill time 26 minutes. It cost 30% more per square meter than the multifilament it replaced and had outlived two of them.

Frequently Asked Questions About Filter Cloth Blinding

What is filter cloth blinding?
Filter cloth blinding is the progressive loss of permeability caused by solids, precipitates, or organic films occupying the weave. It differs from a hole because the cloth still retains particles, often better than before, while passing less liquor. The signature is fill time rising cycle after cycle at constant feed conditions.

How often should filter cloth be washed?
High-pressure washing on both faces every cycle, chemical washing every two to four weeks on precipitating duties and every four to eight weeks on clean, coarse feeds. If washing is needed more often than every two weeks, the cause is upstream chemistry or the wrong cloth.

Can a blinded filter cloth be restored?
Partly, and only early. Surface deposits and fresh gel layers respond to high-pressure water plus a targeted acid or caustic wash. Once gypsum or iron hydroxide has grown inside the yarn, washing recovers perhaps 10–30% of lost permeability (indicative) and the cloth declines again within weeks.

Does a tighter weave prevent blinding?
No, it usually accelerates it. A tighter weave captures fines at the surface, raises the pressure needed to hold rate, compresses the cake, and cleans poorly because deposits sit in the knuckles. Choose weave on the sub-10 µm fraction, and fix clarity with feed conditioning instead.

Why does fill time increase while filtrate stays clear?
Because a partly blinded weave retains fines it previously passed. Clear filtrate is consistent with advanced blinding, and treating it as evidence of good performance is a common diagnostic error. Compare fill time against the commissioning baseline at matched feed solids before drawing conclusions on cloth condition.

How do I know when to replace cloth rather than clean it?
Replace when a washed, dried coupon holds less than 50–60% of its new air permeability, when mass gain stays above 10% after two consecutive chemical washes, or when measured life has fallen below half the expected interval. Holes and tears are a separate, more urgent trigger.

Does blinding affect cake moisture?
Yes, often before it affects throughput visibly. A restricted drainage path means the squeeze pushes water into a blocked channel rather than out of the cake, so moisture rises by several points at unchanged squeeze pressure. That moisture costs haulage and can put a contract limit at risk.

Conclusion

Filter cloth blinding is a chemistry and geometry problem that presents as a hydraulic one, and the plants that manage it do three things: they trend fill time against their own baseline, they test coupons instead of judging by appearance, and they wash early and often rather than hard and late. The verdict: if fill time has drifted more than 25% at constant feed, stop adjusting pumps and start measuring the cloth. The action is to characterize the sub-10 µm fraction and the scale on the current cloth before ordering a replacement set, so the next cloth is specified against the duty rather than the last invoice. Send the duty data and coupon results through our quotation request and we will return a cloth specification with the test work behind it; more guides sit in the resources library, and a pilot drum unit is listed at lab-scale rotary drum vacuum filter.

References

• SME Mineral Processing and Extractive Metallurgy Handbook, Society for Mining, Metallurgy & Exploration.
• Wakeman, R. J. and Tarleton, E. S., Filtration: Equipment Selection, Modelling and Process Simulation, Elsevier.
• Purchas, D. B. and Sutherland, K., Industrial Filtration of Liquids, Elsevier.
• Rushton, A., Ward, A. S. and Holdich, R. G., Solid-Liquid Filtration and Separation Technology, Wiley-VCH.
• Tarleton, E. S. and Wakeman, R. J., Solid/Liquid Separation: Scale-up of Industrial Equipment, Elsevier.
• ISO 9237, Textiles — Determination of the permeability of fabrics to air.
• USGS, Mineral Commodity Summaries, U.S. Geological Survey, annual series.
• CSIRO Mineral Resources, technical publications on tailings dewatering and solid-liquid separation.

Changelog

• 2026-10-31: First publication.