FGD Gypsum Filter Cloth for Horizontal Vacuum Belt Filters

Recommended material direction

This gypsum comes out of a wet limestone scrubber, not an acid plant, and the species that defines it is chloride. Chloride is not what limits the polymer. It is what puts the metal in the joint in question, and what the wash train has to get out of the cake before it meets a wallboard specification.

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

  The reference polymer for belt duties: abrasion resistance and dimensional stability under tension. Its published weak side is hot alkaline liquor, so the question to answer here is what your cleaning cycle runs at, not what the scrubber runs at.

• 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

  Comfortable across the pH scale and indifferent to chloride. The constraint is the polymer reference temperature above, and creep under sustained load on a long tensioned belt.

What actually decides it Scrubber liquor temperature at the filter, and the cleaning chemistry. Then the cake chloride target, because it is the wash stage that has to hit it, and the wash is a cloth decision as much as a process one.

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

Wash performance is what this duty is bought on. A surface that releases the cake cleanly and washes back is worth more here than the last increment of retention.

• 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.

Smoother surfaces release and wash back better and retain more coarsely at the same yarn count; that trade is the whole of the weave decision on a wash-critical duty.

Yarn form. Monofilament is the direction where wash-back dominates. Multifilament buys finer retention and pays for it by holding solids inside the yarn bundle, which is the opposite of what a wash stage wants.

Yarn form, weave and surface finish are settled as one decision. Standard woven options are monofilament, multifilament and mono+multi combination; staple / spun yarn is evaluated under engineering review.

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.

A cloth that filters beautifully and washes badly fails the chloride specification, not the filtration specification. Ask for the permeability figure with its method and pressure, then ask what the wash does to it.

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 an assertion of completed mill testing.

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.

Chloride-bearing liquor puts the hook material of a clipper joint in question — the metal is the chloride-sensitive part of that assembly, not the fabric. A PAD or welded overlap takes the metal out of the equation and costs a longer change-out. Send a close-up of the joint you run now, both faces.

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.

Edge treatment and the condition of the machine edge seal are assessed together — a vacuum bypass at the edge shows up as a wet cake long before anyone blames the edge.

Edge constructions drawn in cross-section: how vacuum belt edges wear and what each treatment addresses.

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.

Wallboard-grade duty is a dimensional argument as much as a chemical one: a belt that arrives out of square wanders, and a belt that wanders leaks vacuum at the seal.

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.