Polymer fillers meet minerals processing

Materials World magazine
1 Nov 2009
Solid alumino-silicate spheres beneficiated from waste fly ash

Mining technology is transforming waste fly ash from coal-fired power stations into cheaper and resource efficient polymer fillers, claims RockTron, based in Keynsham, UK.

The firm says it has achieved a ‘world’s first’ in fly ash beneficiation by developing a means of using pneumatic flotation to produce inorganic fillers from pulverised fuel ash (PFA). The 100% recycling process delivers carbon for potential use as fuel in power stations, reducing agent or activated carbon, and magnetite for magnetic shielding.

Professor Peter Hornsby at Queen’s University Belfast (QUB), UK, completed independent research to compare the properties of the resulting filler material with conventional glass microspheres. He notes, ‘Fly ash has too many impurities [and] it had been dismissed [for this application], but the way this is beneficiated creates something that’s nothing like fly ash. The particle size is controlled by the process – it is this control that’s crucial’.

RockTron’s Technical Director Philip Michael, who has a mining background, has applied his knowledge to help the company manufacture hollow (CenTron) and solid (MinTron) alumino-silicate microspheres of up to 300µm and solid paramagnetic microspheres.  

He describes, ‘In the late 80s, I began designing plants to clean up power station waste – separating carbon from alumina silicates for use in the cementitious industries. I thought this is similar to separating coal from ash and ultra-fine particles, and the normal means of doing that is by flotation.

‘I was involved in developing a new flotation vessel with Dr Rainer Imhof of Maelgwyn Mineral Services targetting ultra-fine particle froth flotation to below 10µm in size – in base metal flotation you don’t come down much below 25µm. Pneumatic flotation is the process used in the Imhof float cell. We have worldwide exclusive rights to use it in this application’.

The Fiddler’s Ferry site in Warrington, UK, is now operating at a commercial scale and the company plans to produce 800,000t of ‘eco-minerals’ each year. RockTron says the materials can replace conventional inorganic fillers and extenders such as talc, ground calcium carbonate, glass fibres, glass microspheres, barytes, dolomite and clay.

Research examining the MinTron 7 product, by Artis (Avon rubber), QUB and Bespoke Mould Solutions, has yielded promising results. At QUB the filler was dispersed in polypropylene, polyethylene, polybutylene terephthalate and polyamide at different loadings per polymer weight.

Hornsby says, ‘It was comparable to the glass spheres’ in all key areas. ‘They dispersed very easily. Agglomeration has been a problem in the past, for example, with untreated fly ash. The rheology was [also] not affected, and, in some cases, it gave better flow properties. If you are going to process any filled plastic by conventional processing, you want to see the impact the filler has in terms of viscosity.’

He adds, ‘The second thing is the mechanical properties – the modulus, strength, etc, at different filler loadings. We also looked at the heat deflection resistance. There will be applications where the plastic is used at high distortion temperature, and fillers are often used to reduce that’.

In a silica filled passenger tyre compound, RockTron has demonstrated that partial replacement with MinTron 7 increased tan d at 0ºC (for improved wet tyre grip) and reduced tan d at 70ºC (for reduced tyre rolling resistance). However, key properties such as hardness, modulus (M300) and abrasion loss require improvement.

Godfrey Short, Director of Advanced Products at the company says the products can also provide improved physical properties, including sound absorption, electrical conductivity, electro magnetic interference and radio frequency interference shielding, as well as improved wear resistance. Their surface chemistry can also be modified.

Michael notes, however, possible future challenges in the supply of waste PFA to the business. Co-firing coal with biomass at power stations affects ash composition, as does the use of low NOx burners and lignite coal.