Fly ash recycled into high performance products

Paul Boughton
Philip Michael describes the process that recycles the waste from coal-fired power stations into valuable industrial minerals.

Using a proven mining technique, the RockTron Beneficiation Process separates the waste fly ash components into 100 per cent recycled high performance products.

RockTron has developed advanced mineral processing technologies that enable greater sustainable sourcing of minerals from waste fly ash that, up until now, has been discarded and land-filled. This technology could enable billions of tonnes of stored fly ash to be recovered, beneficiated and sold to industries worldwide, thus reducing the mining and energy intensive processing of billions of tonnes of virgin minerals. As a result, it can make an incremental and effective contribution towards reducing the carbon intensity of cement production and carbon emissions, while conserving natural resources through recycling waste fly ash material currently consigned to long term landfill.

Fly ash is the waste product generated from burning hard coal at coal-fired power stations. It comprises the heat-treated constituents of coal itself, the major ones being alumino-silicates, iron oxides in the form of maghemite and carbon in the form of char. All fly ashes vary in their percentage of these three components but together these three account for 100 per cent of fly ash residues. Approximately 1-2 per cent of the alumino-silicates are hollow glass spheres known as cenospheres.

The utilisation of fly ash by the construction industry is regulated by technical standards, such as the EN450 standards in Europe, the ASTM C-618 standards in the USA and their equivalents in Asia. Various methods have been attempted to improve the quality of fly ash in an effort to make it more suitable for industrial applications. The most simple and commonly applied process is to grade the fly ash by particle size, which categorises it for a range of cementitious applications. This is referred to as Classified Ash. Additional improvements are made by the removal of some carbon in an effort to bring the overall Loss on Ignition (LOI) content below the 7 per cent demanded by BSEN450 Category A & B for use as a CEM I replacement in ready-mixed concrete.

Most technologies to date have employed dry fly ash beneficiation techniques, fractioning the finer particles of fly ash captured in either electrostatic precipitators or bag filters, while other systems aim to remove some of the deleterious carbon fraction from the fly ash. However, none of these approaches provide a holistic solution that deals with 100 per cent of all fresh fly ash produced by power stations each year. Moreover, no dry ash beneficiation system can even attempt to recycle, recover and beneficiate the billions of tonnes of fly ash in long term storage around the world.

The RockTron Beneficiation process is the first of its kind to provide a total processing solution for both fresh and stockpiled fly ash. RockTron plants do not require a working coal-fired power station to operate effectively, as they can work on legacy ash deposits as well as fresh ash from power stations' electrostatic precipitators. Despite the obvious commercial and environmental opportunities of this technology, it has taken almost 20 years for the founders to secure funding for the first large scale beneficiation plant. This was due in part to the major corporate reorganisation of the UK Electricity Generating Sector in the early 1990s and the bursting of the dot com bubble in 2002. RockTron's beneficiation technology has now been proven with the construction in 2008 of the world's first 800000 tonne per annum capacity commercial plant in the UK, licensed to a major energy utility company. Further plants are already under construction around the world.

RockTron's innovation is that its technology enables consistent production of quality controlled cementitious minerals, despite the often poor initial quality of the original fly ash produced by the power station. RockTron's process uses 'Froth Flotation', a proven wet mineral processing technique, and other specialist processes to treat the total fly ash feedstock.

The result is a very green system that produces neither solid nor liquid waste streams and enables the highly efficient separation of the valuable minerals that make up fly ash.

The first stage involves the removal of cenospheres, hollow glass spheres for use in applications such as lightweight fillers and coatings. This improves the density of the remaining alumino-silicate particles.

The second stage involves the removal of carbon. First, this carbon can be returned to the power station to be re-burnt as fuel, thereby reducing the overall energy impact of the production plant. Second, carbon provides a negative contribution to strength development in concrete and has a deleterious effect on air entraining agents and additives, which it preferentially adsorbs. This often requires the more expensive CEM I to be added per cubic metre of concrete produced. The RockTron process is very efficient at removing the carbon from fly ash, improving the water to cement ratio, increasing concrete strength development at a lower cost base and producing a consistent lighter coloured concrete. RockTron Alpha cement substitute has a typical LOI (carbon content) of <5 per cent in the UK, making it the lowest carbon/finest particle size cement substitute available on the market.

The process then recovers the much higher specific gravity spherical maghemite particles, which further helps lighten the colour of the final alpha product and thus produces a whiter concrete. This spherical maghemite product can be used in a variety of shielding and sound absorption applications including specialist high density concretes, often required in hospitals and the wider nuclear industry.

The remaining beneficiated material comprises clean, beneficiated solid alumino-silicate glass spheres, which are classified into cementitious products graded into RockTron Alpha and Delta.

Philip Michael is Co-Founder of RockTron, Keynsham, Bristol, UK.

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