Twenty Tonnes a Day: Dust Control at the Hopper, Not the Room

Twenty tonnes of powder a day works out at 830 kilograms an hour, or about fourteen kilograms a minute going into a hopper. At that rate the air around the bag-split point carries a visible plume for most of the shift, and the respirable fraction of it — the part under 10 micrometres that reaches the deep lung — is the part nobody can see from three metres away.

Personal sampling at one site running this volume put the operator at 3.2 milligrams per cubic metre over an eight-hour shift, with peaks above 20 during bag changes. The site’s response was to add a general extraction fan in the roof. The number went up.

Why Room Extraction Made It Worse

A roof fan pulls air from everywhere at once. It dilutes the plume and moves it, which improves visibility and does nothing for the man standing in the middle of the source. Worse, the induced draught pulled dust off the hopper lip and spread it across a wider area of the shed before it could be captured.

Dust control is a capture problem, not a ventilation problem. You have to get the air moving at the point where the dust is generated, and you have to do it before it has a chance to mix with the room.

The Hopper Is the Only Place That Matters

Three interventions, in order of effect:

• Enclose the bag-split point. A hood with a hinged lid over the hopper throat, an apron of flexible curtain down to the bag, and a 900 cubic metre per hour extraction connection at the back. Capture velocity at the bag mouth needs to be around 0.5 to 1.0 metres per second; anything less and the plume escapes at the first gust.
• Cut the bag low and slow. A knife slit down one side, bag tipped into the hood, empty bag folded and bagged immediately. Ripping a bag open at arm’s length is the single biggest peak exposure on the shift.
• Filter the extracted air and return it, or discharge outside away from intakes. A cartridge filter with a 0.5 micrometre rating, pulse cleaned on a timer, keeps the shed under 1 milligram per cubic metre if the hood is doing its job.

With local capture in place, the same site measured 0.4 milligrams per cubic metre on the operator and 0.15 in the general shed air. Nothing else changed.

What Not to Do

Do not use compressed air to blow down the hopper area. It converts settled dust back into airborne dust with no capture at all, and it is the fastest way to turn a 0.4 into a 4.0. Vacuum instead, with a rated unit and a grounded hose.

Do not use a standard workshop vacuum with a plastic hose. Polymer dust generates static, and a plastic hose on a dry day is a spark risk in a cloud of fine organic powder. Grounded, anti-static hose and a bonded and earthed unit.

Do not wet the floor to keep dust down unless you have somewhere for the water to go. Wet polymer is slippery enough to be a fall hazard on a concrete slab, and the runoff is a leachate stream with a very high organic load that nobody has accounted for.

The Dust Is Also Product

There is a cost argument buried in the exposure argument. Powder that leaves the hopper as dust does not gel leachate. At 2,200 dollars a tonne, 1 percent loss is 22 dollars a tonne. The site that installed proper capture also found its consumption dropped by about 1.5 percent against the batch sheets, which paid for the hood in a season.

Respiratory Protection as the Last Line

If the hood is working, a half-mask with a P2 or P3 filter is enough for bag handling and it should be available at the hopper, not locked in a store. Fit testing is not optional — a badly fitted mask gives the wearer a false sense of protection that is worse than no mask, because it removes the incentive to keep the hood in order.

Beard growth, a strap over a hood, and a mask worn under the chin between bags are the three things you will see on any site that has stopped paying attention.

Dust and the Wider Batch

Dust also affects the batch itself. Powder that settles on the tank rim and the floor is powder that is not in the gel, and at 20 tonnes a day a 1 percent drift is 200 kilograms a day of unaccounted consumption. Reconcile bags issued against batches made at the end of each week, and treat any gap above 2 percent as a handling problem rather than a measurement problem.

The particle size and handling data for each grade is worth checking before designing a hopper: finer grades fluidise more readily and need a slightly higher capture velocity, and the difference between grades is enough to change the fan specification.

If the site is heading toward 20 tonnes a day, design the dust control before the volume arrives. Retrofitting a hood into a shed built for 3 tonnes a day is a week of disruption and a lot of ducting that does not quite fit.