There are three main issues when unloading hazardous goods such as sulphur: a high explosion risk, extreme corrosion, and stringent regulations for environmental protection. Accommodating these issues requires state-of-the-art environmentally-friendly equipment, which was the trigger for Cargotec looking at different solutions and developing a range of unloaders that can offer economical, clean and safe sulphur-handling.

The risk of explosions and fires are amongst the largest hazards when handling sulphur and they also involve the most serious consequences. To ensure safe handling of this hazardous material, Cargotec has developed a system, called ‘4S’ (Siwertell Sulphur Safety System), which is designed to minimise the risk of explosions and detect fires.

To prevent explosions and fires, the unit is equipped with nozzles that spray water at the inlet feeder and in the conveyor transfer points. An automatic lubrication system is installed to lubricate and cool down end and intermediate bearings.

Even with preventive technology in place, there are rare occasions on which an explosion occurs anyway or a fire starts, so a system that reacts immediately upon detection and extinguishes any fire has also been designed. There are fire detectors along the conveying line, and when activated, these automatically start the fire extinguishing system spraying water and stop the conveyors. To manage any explosion, the conveyor’s steel casings are reinforced with extra-thick steel, and explosion-venting valves are fitted along the conveyors and dust collectors to relieve pressure.

Corrosion is another issue when handling sulphur. Its specially-designed conveying line with stainless steel components ensures that a unit is protected from corrosion and it is also practically maintenance-free. There are also few moving parts in a screw conveyor and wear is considered minimal.

Its negative effect on the environment means that there are often restrictions where sulphur can be unloaded. Handling sulphur, therefore, calls for operators to eliminate spillage and minimise dust emissions.

For unloading operations, cargo pick-up is carried out below the cargo surface in the ship’s holds. With a unique inlet feeder mechanically guiding the cargo into the screw conveyor, the Siwertell unloaders are designed for a layer-by-layer unloading method, which minimises dust creation from cargo ‘avalanches’ inside the hold. They are also equipped with loading chutes that reduce dust creation.

The unloader can be configured to discharge sulphur either to a jetty conveyor, for further transportation to storage areas, or directly into trucks or rail cars. The most common and efficient way, especially for travelling rail-gantry systems, is to integrate the unloader in a complete conveying and storage system. In such cases, the transfer point from the unloader to the jetty conveyor is rendered dust-proof through a specially-designed transfer trolley with integrated dust suppression system.

Direct discharge from the unit to trucks – which is the most common application for the road-mobile unloader range – or rail cars is carried out through specially-designed loading bellows also fitted with an integrated dust suppression system.

The conveying lines of each unit are based on totally-enclosed screw conveyors configured in such a way that negative pressure is created in the conveying line and all connection and transfer points are totally enclosed and sealed.

As a result operators can continue to unload any hazardous bulk good even in areas where the strictest environmental regulations apply, and there are many examples of unloaders operating in environmentally-sensitive or close to populated areas.

One such example is in the Port of Lüderitz, in Namibia. In 2006, Cargotec was approached by Anglo Base Metal, on behalf of its Namzinc subsidiary, to investigate how to solve the problem of unloading sulphur in a port that was predominately used by the fishing industry.

It was not possible to install a fixed unloader in the port as the quay needed to be cleared after each sulphur discharge operation. Based on the demands for a high unloading capacity, the size of ships in question, and the requirement for dust-free unloading, the client selected a Siwertell 15 000 S. Since its delivery in 2008, the road-mobile unit has been continuously used for unloading prilled sulphur.

Using a dual loading bellows system for truck loading, and a 4S-design to deal with sulphur’s explosive and corrosive qualities, the unloader has a rated capacity of 350 t/h and can discharge a ship or barge up to 17000 dwt, and a total weight of 72 tonnes, including semi-trailer.

Operating a unit on a dedicated sulphur mission is very cost efficient. The unloaders are designed to consume the minimal amount of energy possible, for example, only 0.2 litres of fuel per tonne is required with a Siwertell 5 000S unloading prilled sulphur – which is granular and therefore free flowing – or about 0.7 kWh/t with a larger 1600 t/h unloader.

The unloaders can be powered from a diesel-driven engine or directly from the grid and to comply with the latest exhaust and noise emission regulations, the diesel engines are continuously upgraded to fulfil the latest emissions regulations.

Road mobile units – which can be used to discharge barges and smaller ships, up to about 15000 dwt – are normally powered from a diesel-driven engine onboard the trailer, while the larger gantry rail-based units are powered via a mains high-voltage cable on a reel attached to the gantry.

For larger ships, unloaders are normally installed on travelling rail gantries. This type of installation is the most efficient set-up from an operational perspective and also provides the best opportunity to adopt a truly environmentally-friendly sulphur-handling system.

In ports where there are no rails, or where the unloader needs to be moved away from the jetty, installation on a rubber-tyred gantry is possible. Also, as an alternative solution, the unloaders can be mounted in fixed positions. Overall system efficiencies can be further noted at the clean-up stage, because the Siwertell unloader has a flexible and strong unloading arm. This means that a high through-ship capacity with minimal unloading time can be achieved, which substantially reduces shipping and crew costs.

*The author is expert at Cargotec, Sweden.