Understanding filtration › 4. Diagnosing a filter cloth

CHAPTER 4.4

Filter cloth blinding and washing

A filter cloth does not always end up torn: often it slowly clogs until it no longer filters fast enough. Understanding this blinding makes it possible to wash at the right time, with the right method, and to postpone replacement.

Nicolas Ract · 8 min read · October 2026

What is blinding?

The finest particles in the product, in the micrometre range, lodge in the fabric. The pores narrow, resistance to flow increases and cycles get longer, until the cloth has to be replaced. Together with mechanical failure, it is one of the two main reasons for replacing filter cloths [1].

These particles are sometimes joined by chemical deposits: salts that precipitate in the fabric and form a crust that water alone does not remove [2].

The signs

• filtration cycles getting longer, for the same product;
• thinner or wetter filter cakes;
• pressure rising faster at the start of filling;
• a cloth that is darker, stiffer and heavier once dry (see chapter 4.1).

Field experience: track cycle time in a simple table, cloth by cloth or pack by pack. A slow drift indicates blinding; a sudden jump more likely indicates a change of product or an incident.

What encourages blinding

• Filling too fast at the start: at a high initial flow rate, fines penetrate deep into the still-clean cloth; regulating the pump flow rate can extend cloth life [2].
• A cloth chosen for abrasion resistance only: it must also be chosen for its resistance to blinding [2].
• Process chemistry: hard water, sparingly soluble salts, a change of raw material.
• Field experience: flocculant overdosing or the presence of oils and greases also quickly foul the cloths.

The three levels of washing

1. Rinsing at every cycle

On fast presses, a high-flow, low-pressure rinse can accompany every cycle. It flushes pieces of filter cake off the sealing surfaces (which prevents leaks, see chapter 4.3) but does little against blinding [2].

2. High-pressure washing

Plate by plate, with a spray bar or a cloth washer: according to operator feedback, around one to two minutes per plate, at most every three cycles, often once a day or every other day. On tower filters, high-pressure washing at every cycle is common [2]. Watch the distance and angle of the nozzles: a jet that is too close wears the cloth (see chapter 4.2, cause 5).

3. Chemical washing

When water is no longer enough because the blinding is chemical, an acid or alkaline wash may be needed. Examples reported by operators include 2 to 5 % oxalic acid against copper and iron deposits, or 1 % nitric acid applied by a contractor [2]. The right product depends on the nature of the deposit and on the cloth fibre.

A study published in 2021 on filter press cloths from the mining industry showed that cleaning with acids, bases or ultrasound could, depending on the fibre and the solvent, clean the cloths almost completely without degrading their mechanical strength [1].

Wash or replace?

Field experience: compare the cost of a wash (downtime, labour, product, effluents) with the cycle-time gain it brings and with the remaining service life of the cloth. When cycle time no longer returns close to its original value after washing, or washing has to be repeated more and more often, replacement becomes the better calculation. This is the subject of chapter 6.3, Cost per tonne.