In September 2001 Kanowna Belle installed three Eimco Agidisc filters at its paste backfill plant to prepare tailings for use as backfill in the site’s mined-out stopes. Twelve on, Kanowna Belle paste fill coordinator Matthew Doherty says they are very happy with the results.
At Kanowna Belle, tailings are taken from the mill through the thickener and then on to the Agidisc filters. The slurry enters the filter at 60% solids and is dewatered and expelled as cake at 80% solids. Each filter can process up to 130tph (wet).
Initial design requirements for the plant specified that the three filters process up to 255tph (wet) in total, which is being achieved with just two of the Eimco filters.
“We actually purchased three filters because we thought they would all be needed to provide the throughput we required. However, we have found that only two filters are required to give us the throughput and we are able to keep the third on stand-by for when maintenance is performed on either of the other two,” Doherty says.
After being expelled, the dewatered cake goes into a high intensity twin shaft mixer that adds cement to the mixture. This mixture is then gravity fed to the stopes, usually around 750m underground, where it sets to form a strong, safe block.
Kanowna Belle engineering specialist Alan Welsh says that although backfilling with tailings is a long used method, their way of doing it with a paste made from the tailings and cement provides a much safer fill than other backfill methods. “Once we have filled a stope and the fill has set, we are able to mine the next stope right up to the fill as it will support itself underground,” Welsh says.
“The process is actually very difficult to perfect. The paste must be exactly the right consistency to achieve a certain yield stress to enable gravity feeding and for it to set at the required strength.
“The Agidiscs’ place in this process is extremely important. The filters’ excellent dewatering capabilities reduce the amount of cement required in the mix and this greatly reduces our overall costs. We are filling stopes around 30,000m³, which require around $0.4m worth of cement each. If the tailings had a higher water content, more cement would be required and our costs would escalate. Basically, the filters give us control over the product's water content and keep process costs to a minimum.”
Each of Kanowna Belle’s Agidisc filters has a filtration area of 245m² and was supplied with an external pipe barrel filtrate piping for longer life and lower replacement costs. The filters also have a snap blow cake release system for efficient cleaning; lightweight disc sectors for easy replacement; a floating stuffing box on the bath agitator for superior life over stationary seal arrangements; and a twin form/dry zone receiver system for improved vacuum control.
“These features were all considered when selecting the filters for our plant, but basically the decision came down to fundamental factors such as cost, the availability of spares, having local support available through Eimco’s Perth office and the flexibility of the filters and their suitability to our application,” Welsh says.
“Special adaptions were also made to the filters to suit our site conditions. The corrosive nature of saline process water on site required that all wetted steel parts on the filters either be rubber lined or fabricated from 316 stainless steel. As well as dual vacuum receivers, a moisture trap was also supplied for each filter to protect the vacuum pumps from corrosion.
“During commissioning Eimco representatives maintained a strong presence on site and have retained an excellent level of service ever since,” Welsh says.
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