Coal fly ash electrostatic beneficiation

Paul Boughton

Fly ash produced from coal combustion can be a valuable in concrete production. Scott D Ziegler and Dr James D Bittner report.

The use of fly ash improves many of the properties of concrete in both the hardened and plastic states. However, with increased regulation, pollution controls at power plants, and fluctuating fuel prices, fly ash meeting concrete product grade quality continues to decline.

In order to avoid loss of this valuable material in concrete production, financially and environmentally viable fly ash beneficiation processes are required.

Separation Technologies, LLC (ST) operates fly ash beneficiation facilities that produce high quality pozzolan, ProAsh(R), from poor quality fly ash. Its patented electrostatic separation process reduces the carbon content of fly ash to produce a low loss on ignition (LOI) product used as a partial portland cement replacement in concrete products.

Additionally, a high carbon content product stream, Ecotherm(R), is produced which is returned to the utility boiler to recover fuel value or to cement kilns as a fuel and raw material.

ST has commissioned 12 ash beneficiation facilities in North America, the United Kingdom and Poland. ST's newest fly ash benefication installation in South Korea will be commissioned in late 2013.

Material is fed to the separator between two parallel planar electrodes. The particles are tribo-electrically charged by inter-particle contact. The positively charged carbon and the negatively charged mineral are attracted to opposite electrodes. A continuous belt moves the particles toward opposite ends of the separator. The high belt speed enables very high throughputs, up to 40tph on a single separator.

By controlling various process parameters, such as belt speed, feed point, and feed rate, the process produces low LOI fly ash at carbon contents of less than 2 to 4 per cent from feed fly ashes ranging in LOI from 4 per cent to over 25 per cent. The process is dry, requiring no additional materials other than the fly ash and produces no wastewater or air emissions.

The high carbon produced in the separator is utilised at coal burning power plants as a fuel substitute and at cement kilns as both a fuel and as a source of mineral for adjusting clinker chemistry. Utilising the high carbon ash either at a power plant or a cement kiln maximizes the energy recovery from delivered coal, reducing the need to mine and transport additional coal.

Some fly ash is contaminated with ammonia due to the injection of ammonia into the power plant's flue gas handling system. While ammoniated ash is not detrimental to concrete performance, fly ash containing greater than 100mg/kg of ammonia is considered unacceptable for production of concrete due to worker safety.

To address this problem ST has developed a process for removing ammonia from fly ash that can be used after carbon separation. The ammonia removed from the ash is catalytically converted into nitrogen.

Scott D. Ziegler is Manager, Business Development and Dr Jmes D. Bittner is Vice President of Technology, Separation Technologies, LLC, a Titan America business Roanoke, VA, USA.

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