APC residues are delivered in purpose designed, sealed powder tankers and are conveyed into the silos using compressed air, in a completely sealed system.
Clean water and liquid wastes (lightly contaminated water) are stored in two tanks within a bund wall. Most of the contaminated water is pumped to the tanks from around the landfill sites. Some of the liquid waste is delivered to the tanks from external sources in liquid waste tankers. The water is pumped to the plant through a delivery system which allows the proportions of clean and dirty water to be varied as required.
Each silo can hold 75 tonnes of residues. At the base of each silo is a vibration system to ensure the residues flow freely. The flow of residues is regulated by a slide valve which can be adjusted by the control computer. The silos are inspected internally every two years and the associated fittings are regularly inspected as specified by the manufacturers.
When the silos are being filled, the displaced air is exhausted through double bag filters which remove 99.97% of dust entrained in the air. The filters are monitored automatically and warnings are provided when the filter efficiency is impaired.
The residues are lifted from the base of each silo by large screw conveyors and delivered to a 'tun dish'. This diverts the residues from each silo to one of two small hoppers located above the two mixing vessels. The tun dish allows the contents of any silo to be delivered to any mixing vessel and also enables different proportions of residues to be blended prior to treatment.
The mixing vessels are long enclosed troughs with two contra-rotating paddles running the length of each vessel. The flow of residues into each vessel is regulated by a surge hopper which ensures there is always sufficient residue available for the mixing process. In the roof of each trough are spray bars which spray large quantities of water onto the residues as the mix is agitated by the paddles. The final mix typically contains 40% water by weight. The treated residues are dropped into a dump truck below the mixing vessel and are then taken to the landfill.
The entire system is monitored and controlled by a computer. Among the parameters being constantly monitored are level of residues in each silo, slide valve opening, screw conveyor rotation speed, level of residues in tun dish and surge hopper, rotation of mixing paddles and quantity of pumped water.