Fines Below 150 Microns: The Hidden Tonnage in Every Bulk Bag You Unload

A hygiene convertor in Southeast Asia opened a 1,000 kg bulk bag and found 38 kg of it would pass a 150 micron screen. Nobody had asked for a fines limit in the purchase order, because nobody had thought fines were a specification. Six months later the converting line was running 4 percent slower, the dust collector was overloaded, and the technical manager was blaming the polymer supplier for a problem that started in the supplier’s own classification step.

Fines are the least discussed and most expensive form of off-spec in bulk SAP. This article covers where they come from, why they change dose and line speed rather than just dust, a 12-minute sieve test any QC bench can run, and the contract clause that stops the argument.

Where the 4 Percent Went

A granule does not arrive as a single size. It is ground and classified, and every classification step has an efficiency limit: a 30/60 mesh cut typically ships with 2 to 5 percent below 150 microns unless the supplier runs an extra screening pass. Add pneumatic conveying, which generates another 1 to 2 percent per 100 m, and a bag that left the plant at 2 percent can arrive at 5 percent.

Handling adds more. Every drop into a hopper, every screw transfer and every rotary valve shears edges off. On a line with six transfer points between truck and machine, the fines fraction can double between the supplier’s Certificate of Analysis and the machine inlet. The certificate is not lying; it describes a sample taken at the bagging line.

Why Fines Change Dose, Not Just Dust

Fines absorb faster than coarse granules because of surface area, so the first effect looks like an improvement: the gel forms in 8 seconds instead of 20. Then the problems start. Fine particles gel on contact with the first trace of moisture and form a sticky layer on the mixer wall, the feeder screw and the classifier screen, and that layer grows until it sheds as lumps.

In a diaper core, fines below 150 microns are largely lost to the process: they are entrained in the air stream, they block the forming screen, and they contribute to gel blocking because they swell and seal the voids between the larger particles. A core that should hold 320 g/g can drop to 275 g/g with 6 percent fines, a 14 percent loss of performance from a fraction that is only 6 percent of the mass.

A Sieve Stack That Takes 12 Minutes

Use 20 cm diameter sieves at 300, 150 and 75 microns, plus a pan. Weigh 100 g of granules, sieve for 5 minutes at an amplitude that keeps the material moving without throwing it, and weigh each fraction to 0.1 g. The result is a three-number distribution you can track batch to batch and compare with the certificate.

Two practical notes. Sieve the sample as it arrives, before any handling, or you are measuring your own plant. And sieve a retained sample from every bag you open for a month, because the variation between bags in one delivery is often larger than the variation between deliveries, which points at the bagging line rather than the reactor.

Contract Language: A Fines Ceiling With a Price Adjustment

A specification that says fines under 3 percent is a start, but it has no teeth without a method and a consequence. Write three things into the purchase order: the test method by name, the sampling point at the supplier’s bagging line, and a price adjustment of 0.5 percent of invoice value for every percentage point above the ceiling, capped at 5 percent with a right of rejection.

Suppliers who can meet the number will accept it, because the clause costs them nothing if the process is under control. Suppliers who cannot will negotiate, and their counter-offer tells you what you need to know about their screening capacity. On a 200 tonne a year contract, a 3 percent fines ceiling is worth roughly 6 tonnes of usable product and a measurable amount of line speed.

If fines are already a problem, a coarser cut is the fastest fix, and the classified SAP granule grades can be specified with a guaranteed fines ceiling.