Fly-ash a valuable resource

It is time Australia stopped treating fly-ash from coal-fired power generation as a waste — and started to regard it as a valuable resource with many end uses, according to Professor Colin Ward of the CRC for Coal in Sustainable Development and University of NSW.

“Australia’s 40 power stations produce 12-13 million t of fly-ash a year, only a sixth of which currently goes to economic products like cement and concrete. Most of it is currently used as landfill and is both a cost and a lost opportunity. There is a lot more we can do with fly-ash,” Ward said.

Ward claims that fly-ash produced from the burning of Australian coals is low in toxic elements by world standards, and most of these are locked up in any case and unlikely to leach out. In such cases the ash is suitable for a range of valuable applications including improving acid or sandy soils, production of synthetic zeolites, as a solution to the intractable problem of acid mine drainage and as backfill to stabilise former underground and open-cut mines.

“Fly-ash usually acts as a form of cement and you can also produce concrete products and different types of aggregate from it for a host of end uses. Using fly-ash for these purposes also means we do not need to mine and burn as much limestone for cement production, reducing limestone use as a source of greenhouse gas emissions,” Ward said.

“The content of potentially toxic trace elements in Australian coal ash is low by world standards and, of these elements, only a small proportion — about 20% or less — is actually mobilisable. Many of our coal ashes are likely to be environmentally safe — and indeed desirable — for other uses.

“Where there is a tendency for trace elements to leach out of the ash, we have found that if it is put back into a coal mine, the surrounding rocks will generally lock them up to a great extent and stop them getting into the groundwater.”

At the same time up to half of Australia’s fly-ashes are alkaline, which offers scope for their use in correcting problems such the growing acidity of the nation’s agricultural soils, or acidic water draining from metal sulphide mines. The ability of fly-ash to soak up water also makes it promising for helping to ‘drought-proof’ sandy soils by making them more absorbent, according to Ward.