Dusting off the handling questions

“Cement, fly ash, ground slag and other materials with fine particles of 40 microns and less have a high propensity to be entrained in the air,” says Bernard Zavatone of FL Smidth, so dealing with them can be “difficult”.

“Grabs, for example, are terrible from an environmental point of view, as fine particles just cascade from them, getting dust everywhere. But cement poses the obvious problem as you can’t use a water spray to contain the dust – it will just hydrate the cement and turn it into rock.”

So, you are really down to the choice of either using a pneumatic or mechanical screw unloader he says. The pneumatic vacuum units have a clear environmental benefit as they draw air and material inside the suction pipes and are a completely enclosed system, but there are constraints on capacity, mostly to do with the power consumption related to conveying high tonnage.

Power draw
“Firstly,” he says, “any unloader over 1,000 tonnes per hour likely will be a screw unloader; because the sheer power needed to move that volume of material makes a vacuum system uneconomical.”

“Secondly, the distance from the ship to the silo or storage area makes a difference, as if it’s a long distance – greater than 1,000 metres – the power needs, again, makes the pneumatic process very power consuming. All in all, you can be looking at potentially 50% higher power cost over mechanical.”

But, given these issues, pneumatic unloaders have a number of good points that are hard to ignore. For a start, minimising the dusting issue.

Although some dust can escape when material cascades inside the ship’s hold, as soon as it is in the unloader system, it’s almost completely contained. While it’s true that some dust can potentially escape the filters, much of this has been overcome by new filter technology: it’s certainly less dusty than the mechanical screw unloaders which often employ an open belt conveyor system running alongside the ship, says Mr Zavatone.

Further, the pairing with a pressure delivery system keeps everything contained until the load gets right into the silo – whereas mechanical belts, although enclosed in a gallery, are generally a lot more open. However, there may be very good reasons to mix the two, such as a vacuum head with a mechanical transfer in cases where you have a long distance to cover before the bulk reaches the silo.

Quay space
Lastly, a lot of operators don’t have exclusive rights over the port’s quay space and can’t leave the unloader in place at the edge of the wharf. Screw unloaders usually require a belt conveyor running along the ship’s side on the quay, which could cause problems.

Vacuum solutions get round this. “A cement operator that had to share the wharf area with vegetable and wood products went for a mobile pneumatic unloader with a 700 tonne per hour throughput, and we simply buried the pipe that linked to the storage area, leaving the quay completely flat,” says Mr Zavatone.

However, the environmental aspect does actually include the holds’ clean up phase especially as it will be used for other bulk goods. Mr Zavatone explains: “An owner doesn’t care about maximum rated capacity, he wants to know how long it will take to unload the entire ship, including clean up. Three quarters of the way down a hold, screw unloaders usually become much, much less efficient – and they are a lot more rigid – than their pneumatic equivalent, which can literally just suck the floor clean.”