A crushing plant has one crusher, a few screens — and dozens of transfer points: every time one belt discharges onto another, a screen feeds a conveyor, a conveyor builds a stockpile. That is where dust forms most often, where idlers and belts wear fastest, and where suppression systems disappoint most — because they are installed before anyone has understood where the dust comes from.
A transfer point is a system: the head of the upstream conveyor, the chute, the loading zone of the downstream conveyor with its skirtboards and belt support, and the belt return. Dust there has five distinct origins. Each has its own answer, and treating them in the wrong order is expensive.
Source 1 — Induced air
When material leaves the head pulley and falls into the chute, it drags air with it. That air, compressed in the loading zone at the moment of impact, looks for a way out — and finds it through every opening: the chute entry, the skirtboards, the end of the hood. It comes out loaded with the fines it stripped from the stream. This is the main dust source of a transfer, and it grows with tonnage, drop height and chute opening.
What fixes it. First, reduce the air entrained: a shaped chute that guides the stream (a hood at the head, a "spoon" at the landing) rather than a box the material explodes in; the smallest possible drop height; a chute entry closed as tightly as possible around the stream. Then give the remaining air time to drop its fines: a settling zone — a hood extended over the loading zone, with internal curtains that slow the air. Finally, if the air volume is still too large to contain, capture it: an extraction take-off on the hood to a dust collector, sized on the induced-air flow rather than on a rule of thumb.
Source 2 — Impact and stream break-up
Where the material hits the receiving belt — or a chute wall — the stream breaks up: lumps bounce, fines separate, and part of them is thrown above the material level. The higher the drop and the sharper the impact angle, the more violent the break-up. This is also the source of belt wear (impact) and product degradation.
What fixes it. A chute that carries the material down to the receiving belt in the direction of travel and at a speed close to belt speed — which is what a shaped chute does. Continuous belt support under the loading zone (impact beds rather than spaced idlers), so the belt does not sag between rollers and the skirt seals stay in contact. Material centred on the belt: an off-centre stream works one side of the skirting and leaves the other open.
Source 3 — Skirtboards and sealing
The skirtboards and their seals close the loading zone on the sides. Worn, badly adjusted, or resting on a belt that sags between two idlers, they leave a gap through which induced air and fines escape — and through which material wedges between the seal and the belt, wearing both. This is the most visible source, the one you see walking along the conveyor, and often the simplest.
What fixes it. An adjustable-seal skirtboard system, mounted over a flat, continuous belt support, with a hood long enough to hold the settling zone. Adjustment and inspection at fixed intervals: a skirt seal is a wear part, not a structural part. And a hood closed at its end, otherwise the air simply leaves that way.
Source 4 — Belt return and carryback
Some material stays stuck to the belt after discharge — wet fines above all. On the return run, over the return idlers, it dries, comes loose and falls along the conveyor: this is carryback. It creates dust all along the return, fouls the idlers, seizes them, and forces manual clean-up under the conveyor — one of the most exposed jobs on a site.
What fixes it. Primary and secondary belt cleaners at the head pulley, properly tensioned and maintained; a belt in good condition (a worn or damaged belt holds more); and, for very sticky materials, a return-side cleaner or belt washing. No dust suppression system acts on this source: it belongs to belt cleanliness.
Source 5 — Wind and draughts
An outdoor transfer, an open hood, a ventilated gallery: the slightest draught crosses the loading zone, strips the fines and carries off the fog being sprayed there. This is the source that defeats otherwise well-designed dry fog systems — the fog is outside before it has had time to agglomerate the dust.
What fixes it. Close it: full hoods, curtains at entry and exit, cladding on exposed outdoor transfers. No dry fog in a transfer open to the wind; at most misting at the discharge, or a cannon if the space is truly open.
The order of operations
The classic mistake is to start at the end — install nozzles — when sources 1 to 3 are matters of transfer geometry and sealing. The order that gets results:
| Step | Action | Sources treated |
|---|---|---|
| 1 | Reduce the drop, shape the chute, centre the stream | 1, 2 |
| 2 | Continuous belt support, adjustable skirtboards, closed hood with settling zone | 1, 2, 3 |
| 3 | Head-pulley belt cleaners, belt maintenance | 4 |
| 4 | Close the transfer to draughts | 5 |
| 5 | Suppress what remains: dry fog inside the hood | 1 (residual) |
| 6 | Capture if the induced-air volume exceeds what the hood can hold, or if water is not allowed | 1 (residual) |
On a well-contained transfer, steps 1 to 4 remove most of the visible dust before a drop of water has been sprayed. Dry fog then treats the fines suspended in a calm air volume — the conditions it works in. The choice between dry fog and water spray at that step is detailed in dry fog vs water spray, and the other technologies in the comparison by emission point.
What changes in winter
An outdoor transfer in Quebec or Ontario adds three constraints: frozen material that sticks to chutes and belts (more carryback, chute plugging), misting water that freezes in the lines and on the skirtboards, and dry-fog compressed air that freezes in the lines if it is not dried. A system designed for winter includes heat tracing, heated nozzle enclosures, automatic drain-down on stop and an air dryer — or gives up on water and moves to capture on that transfer.
Going further
Surveying a plant's transfers, choosing between containment, dry fog and capture for each, and designing for deep cold are part of our Dust suppression systems page. Installing hoods, skirtboards and networks is planned inside a shutdown window — see our installation service. For a specific transfer, tell us the tonnage, the drop height and the material.
Frequently asked questions
Why do my dry fog nozzles make no difference? Almost always because the transfer is not contained: induced air or wind carries the fog off before it has agglomerated the dust. Close the hood and adjust the skirtboards before touching the nozzles.
Does every transfer need a dust collector? No. A well-shaped, well-contained transfer holds its own induced air; capture is justified when the air flow exceeds what the hood can contain, when the process forbids water, or inside an enclosed building.
Is carryback really a dust source? Yes, and a diffuse one: it dries and comes loose all along the belt return, far from any suppression system. Head-pulley belt cleaners are the only answer.
Hicham Marouazi, ing., PMP
Engineer and project manager (PMP) at Induscoat Solutions. Over 16 years of experience on mining, energy and petrochemical sites in Canada and internationally — selecting, installing and commissioning industrial equipment.