Dewatering sludge in mining with Flowmax

This is based on an actual case study to dewater sludge in order to recover fine sand that would normally go to a pond after washing and would therefore be lost.

Sea water and sand from a harbour dredging operation were pumped to a Flowmax 2200 at Whangamata on the Coromandel Peninsula in New Zealand. An average flow rate of 130m3/h and 1-2.5% feed solids concentration was processed by the Flowmax, producing 28-40% dry solids.

With the granular material in the feed a high flow rate of up to 540m3/h was achieved, although optimal separation was achieved at around 350m3/h, with a clear filtrate and a dry solids concentration of around 70%. The pile of solids you see in the following video is actually as dry as it looks!

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Dewatering sludge tailings to higher degrees than paste produces a filtered wet (saturated) and dry (unsaturated) cake that can no longer be transported by pipeline due to its low moisture content. These filtered tailings are normally transported by conveyor or truck, deposited, spread and compacted to form an unsaturated tailings deposit. This type of tailings storage produces a stable deposit requiring no retention bunding and is referred to as ‘dry stack’.

A typical moisture content of less than 20% is achieved by using a combination of belt, drum, horizontal and vertical stacked pressure plates and vacuum filtration systems. The term ‘dry cake’ or ‘dry stack’ is not entirely correct as the tailings have a moisture content several percent below saturation.

As with thickened and paste tailings, the mechanical sludge dewatering process compared to conventional slurry deposition increases costs. Producing wet and dry cake further increases this cost particularly if high throughputs are required.

Some of the many advantages to using dry stacking of tailings are:

· Can be used in areas where water conservation is critical and any water losses can jeopardise plant performance.

· Dry stacking is suited to areas of high seismic activity as the construction of retention embankments is prevented.

· For cold climates, dry stacking prevents pipe freezes and frosting problems with conventional impoundments.

· Groundwater contamination through seepage is virtually eliminated.

· Filtered and dewatered sludge tailings allow better recovery of dissolved metals and process chemicals (e.g. gold and cyanide).

Dry stack facilities are also easier to close and rehabilitate, require a smaller footprint compared to other surface tailings storage options (i.e. higher density), can be utilised in aggressive environments (e.g. undulating and steep terrain) and generate better regulator and public perceptions of tailings storage.