Microwaving vermiculite

29 May 2009

Vermiculite, a mineral with a sheet-like structure, is mined and then processed into different grades for applications in fireproofing and horticultural markets.

In the process of turning the raw material into a commercially useful material, vermiculite is heated, whereupon it expands. The expansion process, called exfoliation, takes place in commercial furnaces that turn the vermiculite into a low-weight, high-volume product.

Traditionally, however, the gas or oil-fired furnaces have been highly inneficient. And while microwave processing has shown significant promise in reducing energy consumption, a commercial process has never been developed.

Until recently that is. Prof Sam Kingman and his team in the National Centre for Industrial Microwave Processing at Nottingham University have successfully developed a microwave process technology that consumes significantly less energy than traditional techniques.

‘One of vermiculite’s key properties is that it is a poor heat conductor and this leads to extremely high energy consumptions (up to 1Mhh/t) in conventional gas-fired furnaces during the exfoliation process,’  Prof Kingman told The Engineer Online.

Many people have tried to use microwave oven devices to exfoliate vermiculite, the concept being to selectively heat the water in between the microwave transparent aluminosilicate sheets.

‘While it has been possible to use microwaves to process large-grade vermiculite in small batches, until very recently, it has not been possible to process all grades - large, medium, fine, superfine and micron -  in a continuous process at industrially relevant scales,’ added Kingman.

Kingman’s new microwave technique came about after a thorough investigation of how the microwave energy interacted with the layered mineral structures, and the subsequent development of microwave cavities that allow the distribution of the electric field, which is responsible for heating the interlayer water in the vermiculite to be precisely controlled.

‘So far we have developed scaled machines that have the capability of processing multiple cubic metres of the material per hour to a specification that is better than conventional processing furnaces,’ said Kingman.

Dave Wilson