Phosphogypsum vs FGD Gypsum

1. Origin — two different plants

Phosphogypsum leaves a fertilizer complex. In the wet-process route, phosphate rock is attacked with sulphuric acid; phosphoric acid is the product and calcium sulphate is what has to be filtered out of it and washed. The filtration step is part of the acid plant's production line, so filtration performance is tied directly to acid recovery, not only to solids handling.

FGD gypsum leaves a scrubber. A limestone or lime absorber takes sulphur dioxide out of a flue gas stream and the resulting calcium sulphate is dewatered before it is stacked, landfilled or sold on to a board or cement plant. The filtration step sits on an emissions-control system, downstream of combustion, and the buyer of the cake is often a different business from the plant that produced it.

That is the first practical consequence: the two cakes are judged by different people against different targets. Ask which plant the slurry comes from before anyone starts talking about cloth.

2. Chemistry — the fluoride side and the chloride side

The published distinction between the two materials is not subtle. Phosphogypsum belongs to the acid side of a phosphate plant and is described in the literature as fluoride-bearing. FGD gypsum belongs to a scrubber circuit and is described as chloride-bearing. Different aggressive species, different plant, different process water.

What follows from that for cloth is a question, not an answer: polymer choice, finishing and seam hardware all have to be set against the chemistry the cloth will actually sit in. We do not publish a polymer recommendation keyed to the word gypsum, because the word does not determine the chemistry.

At a glance

Cells marked Site-specific depend on your plant. They are deliberately empty of figures rather than filled with an industry average presented as fact.

3. Steel-sinter FGD gypsum: the third duty that blinds standard cloths

Integrated steelworks operate flue-gas desulphurisation scrubbers downstream of iron ore sintering strands. While the chemistry is ostensibly "calcium sulphate dihydrate", the process environment departs radically from utility power scrubbing:

• Abrasive iron dust & fine coke breeze: The gas stream carries sub-micron iron oxides and carbon particles that escape electrostatic precipitators, entering the absorber and embedding deeply into open mono-filament yarn structures.
• Fine, sticky cake consistency: Unlike the clean, prismatic crystals of utility FGD, sinter gypsum cakes often exhibit a dense, thixotropic mud texture that resists cake wash and clings to yarn interstices.

4. Particle size and crystal habit

This is the part of the comparison where most published material becomes vague, and we are going to be explicit about why. Crystal habit and particle size distribution are the properties that decide how a cake builds, how it washes and how quickly a cloth blinds — so they matter more than almost anything else on this page.

We do not have confirmed particle size data for either material that we are willing to put our name to, so the table above gives no figure for it. What we can say is structural: crystal habit in a phosphoric acid circuit is a function of the acid plant's operating regime, and crystal habit in a scrubber is a function of absorber conditions. Neither is a constant of the material. The distribution from your plant is the one that matters, and a current analysis from your lab beats any figure we could publish.

5. The machine these duties usually land on

Phosphogypsum dewatering on rubber-belt horizontal vacuum belt filters is a real, well-documented duty, and so is FGD gypsum dewatering on the same machine family. That shared machine family is exactly why the two duties get merged in people's heads.

Two honest qualifications. First, the vacuum belt filter is not the only route for FGD gypsum — drum filters also dewater it, and a site running drums is not doing anything unusual. Second, "vacuum belt filter" is an ambiguous phrase: some machines marketed under vacuum-belt language are indexing tray machines rather than continuous rubber-belt filters, and the cloth mechanics are not the same. If you are not certain which machine family you have, send a photo of the machine and its nameplate before anyone quotes a belt.

6. What the difference means for cloth selection

These are two duties rather than one, and cloth selection follows the duty rather than the shared label. A mill can weave for both — we do — but that is a statement about the loom, not about the duty.

In practice, selection runs from the slurry backwards: the chemistry, temperature and particle size of your stream drive polymer, weave and finish; the machine drives width, length, seam and edge; the failure you are trying to escape drives the trade-off between permeability and cake release. Candidate polymers commonly used on this duty include PET monofilament, PET monofilament–multifilament combinations, polypropylene, and alkali- and hydrolysis-resistant specials. Which of them belongs on your machine is a conclusion of the review, not an input to it.

6. What does not transfer between the two duties

• A validated spec. A cloth that performs on FGD gypsum is not evidence for a phosphogypsum belt, and the reverse is equally true.
• A service life figure. Cloth life is duty-specific and driven by the plant's operating regime. We do not publish a service-life promise for either duty.
• A cake moisture target. It belongs to the machine, the vacuum system and the slurry, not to the word gypsum.
• A wash regime. Acid recovery washing on a phosphoric acid plant and chloride washing on a scrubber circuit are different jobs with different success criteria.
• A seam and edge package. These follow the machine and the failure mode, and are confirmed per machine.

Evidence and limits of this page

The origin, host industry, chemistry and machine statements above come from third-party manufacturer literature and trade coverage collected during our market research, cross-checked against a second independent source before being used. The belt-class split is an observation about a competitor's published range, not a claim about our own products.

The limits are equally worth stating. Market and industry reports frequently lump both materials together as "gypsum filtration", which is where the confusion originates. And every process value on this page is left to your plant to confirm — no temperature, pH, particle size, solids content, cake moisture or life figure appears anywhere above, because we do not have confirmed values to publish.

Parameters we need from you

This is the list an engineer works through before quoting either duty. Items you cannot answer are fine — mark them unknown and we will work around them.

1. Machine brand and model, or a photo of the nameplate if the model is unreadable
2. Belt width and total length, measured on the old cloth rather than taken from a drawing
3. Seam type currently fitted — clipper, PAD, spiral or overlap — with a close-up photo
4. Edge treatment currently fitted, and whether the edge or the seam is what failed
5. Which gypsum this is: phosphogypsum, FGD gypsum from power, or FGD gypsum from sinter
6. Slurry temperature and pH, if your process sheet or a lab report gives them
7. Particle size distribution, if the plant has a current analysis
8. What went wrong on the last cloth: blinding, tracking, seam leak, edge fray or cake sticking