Phosphoric Acid Filter Cloth for Horizontal Vacuum Belt Filters

Recommended material direction

Hot wet-process acid carrying fluoride and fluorosilicate species is the most chemically demanding of the belt duties on this site, and here the temperature is part of the chemistry rather than a separate axis. The hemihydrate routes run hotter and stronger than the dihydrate ones, which is why the route has to be settled before the polymer is.

This narrows the field; it does not pick. Which polymer goes on your machine is settled against your measured numbers and not on a web page — the same position the selection section above takes. What follows is the reasoning behind that position, not a departure from it.

• Polyester (PET) PET

  Continuous reference up to ~150 °C — a published property of the polymer, not a value of ours

  • Acid Good
  • Alkali Limited
  • Hydrolysis Limited
  • Abrasion Excellent

  Acid is not polyester's weak side — the hydrolysis problem it is known for is a hot alkaline problem, not an acid one — and its reference continuous temperature gives headroom that polypropylene does not have.

• Polypropylene PP

  Continuous reference up to ~90 °C — a published property of the polymer, not a value of ours

  • Acid Excellent
  • Alkali Excellent
  • Hydrolysis Excellent
  • Abrasion Good

  The better acid resistance of the two common polymers, and its constraint is entirely temperature. On a hemihydrate route the ceiling above is the question, not the chemistry.

• Polyvinylidene fluoride (PVDF) PVDF

  Continuous reference up to ~140 °C — a published property of the polymer, not a value of ours

  • Acid Excellent
  • Alkali Moderate
  • Hydrolysis Excellent
  • Abrasion Good

  Halogen-bearing and strongly oxidising liquors are where a fluoropolymer earns its cost step. Note its weak side is strong alkali — the opposite of the usual assumption that a fluoropolymer resists everything.

What actually decides it Which route the plant runs — dihydrate, hemihydrate, or a hemi-dihydrate variant — and the liquor temperature and fluoride level measured at the filter rather than at the reactor.

Everything in this block is reasoning from published polymer behaviour applied to the chemistry described further up this page. It is a direction, not a selection — and the selection is made against your measured numbers during the engineering review, then written into the quotation. The full polymer envelope is on the materials page.

Weave options

The wash train is the product here: what the belt recovers is P2O5 that would otherwise leave with the cake. Wash efficiency is a structural property of the cloth before it is a process setting.

• Double layer / multi-layer Two or more interlaced fabric layers woven as one. A fine filtering face carried on a coarse structural back — high strength with fine retention, and drainage channels inside the fabric. Watch: Thicker and heavier; more expensive to weave. The usual answer on heavy belt duties.
• Twill The crossing point steps sideways each pick, giving a diagonal rib. More flexible and more permeable than plain at the same yarn count, with better cake release. Watch: Slightly less dimensionally stable than plain.
• Satin Long floats, few crossing points. The smoothest surface of the three, so the best cake release and the easiest to wash back. Watch: Floats snag and abrade; retention is coarser at a given yarn count.

The structure has to displace liquor evenly rather than let the wash channel through the path of least resistance, and it has to keep doing it while the cake profile changes down the machine.

Yarn form. Monofilament for release and wash-back; the finer retention of a multifilament is paid for with solids held inside the yarn bundle, which on a wash duty is a cost rather than a benefit.

Yarn form, weave and surface finish are settled as one decision. The four yarn forms available are monofilament, multifilament, mono + multi combination and staple / spun.

Permeability considerations

Air permeability is the figure everyone compares and the easiest one to quote meaninglessly. It is measured per ISO 9237 at a stated pressure drop — often 200 Pa — or per ASTM D737 (Frazier). A value with no stated pressure and no stated unit is not comparable to anything, including ours. The same applies to a pore or retention rating: on a woven cloth that is a nominal figure and not an absolute cut-off, because woven media do not have a single pore size and the cake quickly becomes the real filter.

On a multi-stage wash train the permeability that matters is the one the cloth still has at the last stage, not the one it had at the feed end. Ask for the method and the test pressure with any figure, ours included.

How each property is measured, and what to ask any supplier for alongside the number, is set out on the testing and inspection page. Naming the method is a fact about a public standard, not a claim about our laboratory.

Joint options

• Clipper (metal hook) Interlocking metal hooks crimped onto both cloth ends, joined by a pintle wire. Fitted and replaced on the machine without dismantling it. The fastest change-out. Watch: The joint is a discontinuity: it leaks vacuum as it passes the box, and it is where most belt failures start. Hook material has to suit the liquor.
• PAD / welded overlap The two ends are overlapped and thermally fused. A smoother, quieter joint that seals better than a clipper and carries load more evenly. Watch: Made off the machine or with a portable press; a change-out takes longer.
• Spiral Spiral coils on each end interlaced and pinned. Flexible, drains through the joint, distributes load along the full width. Watch: Adds an open line across the belt — not appropriate where the joint must seal.

Every seam crosses every zone divider once per revolution on this duty, so the joint is a process variable and not a shop detail. Hook material in a hot fluoride-bearing acid is a chemical decision; a welded overlap removes the metal from the equation and costs a longer change-out.

The four constructions drawn in cross-section: how joints fail and what each one costs you.

Edge treatment

• Cut and sealed The cut edge is thermally sealed against fraying. The baseline. Adequate where the machine edge seal is in good condition.
• Folded / hemmed The selvedge is folded back and secured. Adds thickness and stiffness at the edge where the seal rubs.
• Reinforced edge band A separate band is bonded along the selvedge. For machines that chew edges — the usual answer when a belt keeps failing at the edge while its body is sound.
• Coated / sealed band A polymer band is applied to the edge. Seals the edge against vacuum bypass as well as protecting it mechanically.
• Guide strip / tracking profile A profile bonded to the underside to run in the machine guide. Only where the machine is built for it. Fitting one to a machine that is not is a common and expensive mistake.

Seam type, squareness and edge treatment are process variables on this machine. A bypass at the edge does not just wet the cake, it puts wash water where the zone dividers were supposed to keep it out.

Relevant equipment

A filter belt is decided at the loom before it is decided anywhere else: what a weaving platform can hold — pick density, tension, width — sets what the finished cloth can be asked to do. Our weaving runs on DORNIER (Germany) and Sulzer (Switzerland) platforms, machines built for technical textiles rather than for apparel.

A wash-train belt is long as well as wide, and its developed length has to stay inside the take-up range once it is hot. That is settled by construction and heat setting, not by tensioning it harder on site.

Model designations, working widths and installed quantities are being confirmed against the machine nameplates and are not published until they are. A weaving platform is easy to name and hard to fake, and we would rather you were able to check.

The full platform description, the eleven process stations from yarn to crate, and the nine laboratory checks: manufacturing.

Factory capability for this duty

The rows below are the part of our capability envelope that this duty touches. The last group is the one that makes the rest of it worth reading — a supplier who can do everything has told you nothing.

The full envelope, including the machine families and services we decline: what we build to order and what we will not take on. Fifteen pages on how a specification like this is built and read: the filter cloth specification guide.