Microwaving minerals
Microwaves can exfoliate vermiculite using less energy and producing far less dust than gas or oil-fired furnaces, claim engineers from the University of Nottingham, UK.
The group at the National Centre for Industrial Microwave Processing at Nottingham has built an industrial microwave prototype that exfoliates raw vermiculite, which is used for fire protection or water retention insulation, in a matter of seconds. The device has received an Environmental Impact award from UK research professional organisation UNICO.
‘We have studied the material’s structure, and have coupled that with our understanding about the mechanisms by which microwaves – or electromagnetic waves – heat different materials of varying properties,’ explains Professor Sam Kingman, who led the research. ‘We designed a box, made out of aluminium or stainless steel, that supports a certain electric field pattern. By constricting the waves in a certain way, we can set up a pattern that is very controlled, so the electric field doesn’t vary across the width of the cavity.’
Exfoliating vermiculite involves heating the water within, causing the material to expand and take on a ‘popcorn’-like appearance. The group studied selective heating of the mineral to maximise the energy density in the water phase, allowing it to expand rapidly. This heating ‘takes less than a second’.
‘One of the major challenges in vermiculite exfoliation is the huge volume of hot gas that is present in conventionally heated furnaces,’ says Kingman. ‘As you’re moving a significantly large amount of hot air around, you have massive issues with dust generation. The [resulting] energy consumption of the dust and fume separation system is almost as high as the furnace itself.
‘But because we don’t have a gas line in our system, we don’t have significant problems with dust.’
The throughput of the microwave system is the same as a conventional process, says Kingman, but its energy consumption is between 100-150kWh/tonne, compared to up to 800MWh/tonne for a furnace.
Another benefit is the temperature of the final exfoliated mineral. ‘In a conventional furnace, the material will come out at around 700-800ºC, and because it’s a good insulator, it takes ages to come down,’ he says. ‘Our material comes out at 80-90ºC, maybe a little higher, so it can be bagged straight away.
‘We can change the properties of the exfoliation as well, since we can control the electric field strength. This means we can control the way the vermiculite grows. So we have the ability to create a greater yield from the same material.’
The group has created a prototype device that can handle 300kg an hour, but Kingman says it can be scaled up by adjusting the microwave frequency. He also believes that the final product will be cheaper to operate than a furnace. His team has been working with an industrial firm to make equipment suitable for a commercial environment, and aims to make the system available within the next six months.Materials World Magazine, 01 Oct 2009
The group at the National Centre for Industrial Microwave Processing at Nottingham has built an industrial microwave prototype that exfoliates raw vermiculite, which is used for fire protection or water retention insulation, in a matter of seconds. The device has received an Environmental Impact award from UK research professional organisation UNICO.
‘We have studied the material’s structure, and have coupled that with our understanding about the mechanisms by which microwaves – or electromagnetic waves – heat different materials of varying properties,’ explains Professor Sam Kingman, who led the research. ‘We designed a box, made out of aluminium or stainless steel, that supports a certain electric field pattern. By constricting the waves in a certain way, we can set up a pattern that is very controlled, so the electric field doesn’t vary across the width of the cavity.’
Exfoliating vermiculite involves heating the water within, causing the material to expand and take on a ‘popcorn’-like appearance. The group studied selective heating of the mineral to maximise the energy density in the water phase, allowing it to expand rapidly. This heating ‘takes less than a second’.
‘One of the major challenges in vermiculite exfoliation is the huge volume of hot gas that is present in conventionally heated furnaces,’ says Kingman. ‘As you’re moving a significantly large amount of hot air around, you have massive issues with dust generation. The [resulting] energy consumption of the dust and fume separation system is almost as high as the furnace itself.
‘But because we don’t have a gas line in our system, we don’t have significant problems with dust.’
The throughput of the microwave system is the same as a conventional process, says Kingman, but its energy consumption is between 100-150kWh/tonne, compared to up to 800MWh/tonne for a furnace.
Another benefit is the temperature of the final exfoliated mineral. ‘In a conventional furnace, the material will come out at around 700-800ºC, and because it’s a good insulator, it takes ages to come down,’ he says. ‘Our material comes out at 80-90ºC, maybe a little higher, so it can be bagged straight away.
‘We can change the properties of the exfoliation as well, since we can control the electric field strength. This means we can control the way the vermiculite grows. So we have the ability to create a greater yield from the same material.’
The group has created a prototype device that can handle 300kg an hour, but Kingman says it can be scaled up by adjusting the microwave frequency. He also believes that the final product will be cheaper to operate than a furnace. His team has been working with an industrial firm to make equipment suitable for a commercial environment, and aims to make the system available within the next six months.Materials World Magazine, 01 Oct 2009
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