Coal handling equipment has to cope with a more than movement of the black fuel, as Alex Hughes finds out
Anomalous objects have long been an unwanted feature of coal consignments. Scrap metal, rocks and even railway sleepers are some of the more interesting objects fished out of coal shipments at various specialist terminals around the world. From time to time, stories also emerge of radioactive contamination
However, with extraneous objects being so common, dedicated handling equipment needs to be ready for any eventuality. Those terminals deploying grabs have an easier time of it, but the real problem occurs when terminals rely either on continuous loaders or on continuous unloaders, which feature a sophisticated screw mechanism.
Juha Huovilainen, sales director at Siwertell, nowadays part of the Cargotec group of companies, says unwanted objects do not represent major practical problems for the company's unloaders. “It's true, there are unscreened consignments of coal, originating in places such as China or Russia, which contain large stones, tramp metal and lumps of wood,” he says. “However, stones either pass right through, or never enter into the screw in the first place. Wood, for its part, cannot stop the screw conveyor; it either passes right through or is chopped in smaller pieces so it fits into the screw.”
Steel tramp, which can be anything from a small nut to large, steel beams, can pose problems. Bigger pieces of metal are able to stop the vertical conveyor. They do this by releasing the hydraulic safety system that the screw conveyor is equipped with. If released, this system can be easily reset, allowing the vertical screw to be reversed with exact same force as it had applied when operating normally, allowing it to be freed of tramp material.
Lars-Eric Lundgren, the company's regional sales manager (Europe), says: “These different tramp materials can be found in any type of cargo. However, irrespective of the type of machinery deployed - be it unloaders, conveyors, crushers, or hoppers - having any form of tramp material there is unwelcome.”
On some of Siwertell's high capacity unloaders – capable of handling flows of between 2,000 and 2,500 tons of coal per hour - a very large magnet system is installed at the back of the gantry conveyor just to pick out steel tramp, thereby helping the customer avoid having to subsequently remove this material from the handling chain.
“All this collected steel tramp can go through our Siwertell 70 – 100m conveyor system before entering the magnet,” says Mr Huovilainen, adding that the company's unloaders are designed to handle any kind of dry bulk material, not just coal.
He points out that, when Siwertell designs dry bulk handling equipment, it does have to do so with the knowledge that extraneous material will be present in consignments and that this will have to be dealt with.
“Stones of up to 30cm-40 cm in diameter will easily go inside a St 790 vertical screw easily and, just as easily, pass all the way through, as we mostly have continuous flights on the vertical screw,” he says.
Mr Lundgren stresses that, because of this ability, operators don't have to ask Siwertell to adapt its machinery specifically to avoid damage from anomalous objects, if these are quite common in the type of consignments it normally handles.
“Furthermore, extraneous material has no impact on the life cycle cost of a Siwertell unloader, either,” he says.
Both men concedes that tramp material in coal does, from time to time, cause damage to handling equipment – this is inevitable – although this tends not to be reported back to manufacturers, who have nevertheless done all they can to ensure that machinery copes as best it can.
“You have to remember,” says Mr Huovilainen, “that in an environment where it is inevitable that there will be some form of metal tramp in consignments, it is best to design equipment to allow smaller objects to simply pass through the unloader; they won't cause any major damage. Larger objects that do get into the vertical screw – and we have to assume they will be there – will simply cause the screw to stop, after which rewinding allows the anomalous object to be removed. As for really large objects, they are too big anyway, so never get into the vertical screw and hence are not a functional problem.”
Both men remain adamant that, safety features being as good as they are, coal terminals can quite happily use continuous unloaders and don't need to switch to grabs if consignments are known to particularly badly contaminated.
“I would stress again,” says Mr Lundgren, “ that the presence or not of extraneous material in consignments need have no additional operational or maintenance costs for unloaders.”
He says that Siwertell has, more or less, done what is needed to deal with tramp material and that no plans exist to invest major amounts of resources or money to improve that functionality of its unloaders.
“However, if any of our customers were to experience problems with extraneous material, we would probably undertake a joint development project aimed at detecting the material earlier or handling it in a different way,” he says, while pointing out that most cargo being shipped today tends to be screened before being loaded into vessels.
Finally, asked whether Siwertell had encountered problems with radioactive materials, particularly with terminals handling Russian coal, Mr Huovilainen said that this has not been reported back to the company.
“From a theoretical view point, radioactive material could be detective inside our unloaders, since the technology exists, but no customers has so far asked us to equip one of our machines with it,” he says.
Open cast mining remains the culprit
JSC Baltic Coal Terminal (BCT), located at the Latvian port of Ventspils, began operations in 2008. The facility, which has a six million ton capacity, handled 2.4m tons of export Russian coal in 2014, up 5% on 2013.
The problem of anomalous objects in coal consignments is not that big, says BCT board member Ilya Sokolov, because the terminal handles high calorific value coals, which means that the coal is cleaned at washing plants before being loaded into wagons and despatched to the port.
“However, for those terminals in the Baltic handling coal from open-cast mines, there might well be a bigger problem,” says Mr Sokolov.
Nevertheless, BCT is not complacent. All inbound coal consignments are subjected to an initial magnetic cleaning before being added to the stockpile for temporary storage or during the direct unloading of rail wagons into the vessel. Where coal is reclaimed, it also undergoes a second coal magnetic separation.
According to Mr Sokolov, various objects do make their way into coal shipments, including plastic bottles, pieces of metal, wood, paper and rubber, some of which can accidentally get into wagons on the inbound transit.
“Of all the various anomalous objects that turn up in coal consignments, a large amount of subsurface rocks can cause excessive wear and tear to crushing equipment, while lumps of metal can damage handling machinery, such as conveyor belts and crushing machines. However, all our equipment was designed specifically on the understanding that it would, from time to time, have to deal with anomalous objects in coal shipments, which is why we introduced three stages of automated magnetic separation, while other unwanted objects are removed manually,” says Mr Sokolov.
As to whether clients are prepared to pay for coal consignments to be cleaned prior to shipment, he explains that many of them deliberately choose BCT specifically because it can comprehensively remove metal tramp from coal and that this forms part of the terminal's standard package for users.
“Magnetic separation is an integral part of the overall handling process,” he says. “If clients want coal cleaned and other anomalous objects removed, we can do that, too, and are happy to discuss this with clients,” says Mr Sokolov. In respect of suggestions that some Russian coal consignments do, from time to time, suffer from low level radioactive contamination, BCT has had no experience of this.
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