Innovative materials handling technology is being used by Onslow Salt Pty Ltd at its new $100m salt project at Onslow, 1,400km north of Perth in the Pilbara region.
The project will service the rapidly growing chlor-alkali and other salt consuming industries across the Asia-Pacific region, and represents a further step in the development of Australia's $300m a year solar salt export industry.
Onslow Salt is 92% owned by the Dutch pharmaceuticals, coatings and chemicals group, Akzo Nobel, which has been developing the project since 1997 as a state-of-the-art solar salt facility. Gulf Holdings Pty Ltd, an Australian company that initiated the project, holds the balance of an 8% interest.
The project, which dispatched its first shipload of 45,000t of salt in July, is designed eventually to reach a full production of 2.5mtpa.
The Onslow salt field occupies 90 sq km of salt flats, with seawater pumped by three pumps at a maximum rate of 4,000L/s each for approximately 18 hours per day depending on tides.
Salt is naturally evaporated, with a system of evaporation ponds for precipitating impurities and growing salt crystals. The salt is harvested by a custom designed and manufactured “harvester” supplied by Hydraulic Contracting and Supply Co of Perth. This cuts the salts off the pan floor like a continuous miner at a rate of 750tph, and elevates the salt for loading into trucks for transport via a haul road to a washing and screening facility.
The salt is then transported, first, by a stainless steel mesh conveyor to help drainage and then by conventional rubber conveyor to a stockpile stacker at speeds of 3.7m/s.
The dual boom stacker is raised 25m above the ground and stacks either side - east and west - of the conveyor up to 24m overall height to a stack width of 50m and length of 500m. The stacks hold a total of some 850,000t of salt.
The salt stacker is normally operated remotely from the wash plant operator's control room and travels out and back along a set of rails which extend the length of the stockpile area.
The motor control centre is located on the upper platform of the stacker with an 11kv trailing cable in a cable tray connected to the conveyor's frame structure extending the length of the stockpile. A cable reeler on the stacker picks up or feeds out cable as required.
When a ship is waiting at the jetty, a purpose-built reclaimer - the first of its type in Western Australia, says general manager, operations, Ron van Velzen - is used instead of conventional dozers which can cause contamination and noise problems.
The reclaimer was built by Austman Pty Ltd of Perth, with specialist design work in Sydney by HMIDE. The 100t reclaimer has a capacity of 2,200tph and delivers stockpiled salt to a hopper car and then to the stockpile conveyor for transport to the jetty conveyor.
The hopper car shares the same set of rails as the stacker and is positioned and locked onto the rails prior to operating the reclaimer. It remains there as long as the rake-type reclaimer can operate in that position.
The scissor action of the two mobile conveyors which transport the salt from the reclaimer to the hopper car allows the reclaimer to move back and forwards across the face of the stockpile and advance into it.
A lattice-rake attached to the reclaimer is positioned parallel to the stockpile cut face so that its cutting edge slices layers of salt from the stockpile as the reclaimer moves the rake across the face of the stockpile.
The sliced salt tumbles down the face of the stockpile, where an elevator reclaimer picks it up and delivers it, via the head chute, to the first of the two mobile conveyors which transports the salt to the hopper car and onto the stockpile conveyor.
At the head-end of the stockpile conveyor, a transfer chute delivers the salt onto the jetty conveyor for transport to the ship loading facility.
When the reclaimer is unable to advance any further into the stockpile because of the fully extended position of the two mobile conveyors, reclaiming operations are stopped. The hopper car is moved back along the stockpile conveyor until the scissor action of the two mobile conveyors is in the fully compressed position. The hopper car is locked into this new position and reclaiming operations begin again.
Because the stacker and reclaimer's hopper car share the same set of rails, both facilities cannot operate at the same time. Instead the plant will be either stacking salt and not reclaiming, or vice versa.
Onslow salt is able to include a second conveyor running parallel to the stockpile conveyor so that stacking and reclaiming can occur simultaneously in the future.
The shiploader is located on a platform positioned at the end of the jetty which extends some 1.5km out from the shore. It is controlled normally via a remote handset from the deck of the ship or the shiploader platform, slewing and luffing to deliver salt evenly in the ship holds at 2,200tph.
Both the salt stacker and the shiploader were designed by Tasmanian company, SEMF Projects. Fabrication was in modules by a Tasmanian joint venture called EMTC, between SEMF Holdings and The Engineering Company, which specialises in ship loading and unloading systems and mobile structures.
Both machines make maximum use of closed members such as square or circular hollow sections to minimise corrosion opportunities.
Site-wind standards apply to both. Operation ceases once wind speeds exceed 80kph, and tie-down locations are proved for cyclone speeds.
The modules were assembled on site by the erection contractor, Barclay Mowlem, which was also responsible for the 3.5km of fixed conveyors.
Because of the corrosion danger, all steelwork is painted with International paint from an Akzo Nobel group company. Constant touching up is needed where there is mechanical damage.