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Developing micro-wave technology

By Andrew Perks

It never fails to amaze me how you can open your mind to new concepts when it comes to conversations with people you meet when you travel.

Andrew Perks Andrew Perks is a subject expert in ammonia refrigeration. Since undertaking his apprenticeship in Glasgow in the 1960s he has held positions of contracts engineer, project engineer, refrigeration design engineer, company director for a refrigeration contracting company and eventually owning his own contracting company and low temperature cold store. He is now involved in adding skills to the ammonia industry, is merSETA accredited and has written a variety of unit standards for SAQA that define the levels to be achieved in training in our industry.

I was recently at Harmony Mine at Fochville (deepest gold mine in the world) in the North Western Cape training a whole bunch of Site Incident Response Training Teams where I met Jon Tweedt from Iowa in my guest house. Very interesting guy: over a cup of coffee he certainly opened my horizons. I thought the micro-wave in the corner of the kitchen was where I warmed up my supper. I never thought it comes from the invention of radar in the Second World War and what its uses are today.

It’s an ongoing process but developing micro-wave technology is making big inroads into mining productivity throughout the world, and it is good to see that we are also embracing it in South Africa. This is not one of my normal articles but read on and, according to Jon, see where they are using the principles these days.

Thermal processing of ore minerals has been understood to improve processing rates in mining operations for some time. Inducing a thermal shock causes micro-fracturing to occur within ore samples, reducing the total amount of energy required to pulverise the material into a fine enough powder for further chemical processing which removes precious metals from the ore mineral. Conventional heating techniques are costly and time-consuming, and do not yield sufficient energy reductions to justify their use. Microwave heating, however, is capable of rapidly super-heating only the target mineral while leaving the surrounding gangue material almost entirely unprocessed.

This rapid heating causes very fast thermal expansion of the target mineral, which increases the internal stresses in ore samples and can cause fracturing on both a micro- and macro-scale. Increased mineral content in certain ores contributes to this phenomenon, and it has been tested and proven by applying microwave power in the range of hundreds of kilowatts up to over a megawatt. Larger sample sizes obviously require increased power to produce comparable results.

The magnetron tube – which converts standard DC power to microwave power – is limited to a maximum of 100kW at 915MHz. This functions as a cavity resonator which was developed by British scientists and physicists during World War II for use in radar systems. This technology was later applied to industrial thermal processing and remained the standard for nearly 80 years. Research into the optimal ISM frequency band at which to apply microwaves is ongoing, with possibilities including 2450MHz, 915MHz, and 400MHz. Industrial-scale microwave generators capable of producing upwards of 100kW within the allowable frequency band of 900-930MHz have been unavailable until recently.

In the meantime, transistor technology became ubiquitous and subsequently replaced vacuum tube technology in nearly every facet of electronics before the early 2010s. It was at this point that transistors were capable of generating sufficient power necessary to compete with the existing magnetrons through the use of complex RF combining structures. Due to the imprecise frequency, phase angle, and harmonic characteristics of the magnetron tube, efficient combining of multiple generator units through waveguide had been impossible.

However, by utilising a corporate combining hierarchy, hundreds or thousands of transistors are now able to be operated in unison to produce microwave power levels ranging from as little as one kilowatt up to or exceeding one megawatt; the upper limit of this technology is yet to be determined.

In addition to the vastly increased power, ongoing research shows solid-state microwave generators are capable of shifting their frequency within the government-allocated ISM bands. These changes in frequency cause changes in the electromagnetic response of the target material, shifting hot- and cold-spots within the material and producing more evenly distributed temperature rise.

Frequency can be shifted at a time scale that is several orders of magnitude above that of thermal phenomenon, allowing the temperature increase to be effectively averaged across almost the total volume of the sample.

The harsh conditions of the mining industry preclude the use of magnetron-based microwave

Generators. Vacuum seals can crack, environmental contaminants can ruin components, and maintenance/ replacement costs can ultimately derail projects. The use of transistor-based microwave generators eliminates many of these problems and allows the use of high-power microwaves in locations that were previously impossible, including the direct application of microwaves at underground mining sites. Significant testing and experimentation is being performed to quantify the savings in cost and energy that can be expected from utilising this technology.

One example of this testing includes the design and commissioning of a large-scale microwave cavity which is designed to hold immense rock samples and is capable of receiving up to one megawatt of microwave power through multiple waveguide feeds which are strategically located on the cavity walls. Extensive simulation work has been performed to justify the construction of this system by optimising the expected energy absorption by sample materials such as granite, pyroxinite, platreef, and others. The cavity itself is highly instrumented to collect as much thermal and visual data as possible, as well as operational data from the microwave units such as absorbed and reflected power and optimal operating frequency.

Once samples have completed thermal processing in the microwave system, they will undergo extensive physical testing to quantify the performance improvements of the relevant mining operation. This could include reductions in energy expenditure and processing time due to in-situ micro-fracturing, life-extension for consumable components of mining machines such as milling heads or drill bits, and reductions in total vibration within the mining area that can cause various equipment problems and pose health hazards to mining personnel.

Additionally, the implementation of high-power microwaves may ultimately save lives by allowing the removal of humans from dangerous underground mine sites and making it possible for mining operations to be monitored and controlled remotely from the safety of external control centres.

Phew, for you scientists this has been quite an interesting article, stay safe till next time.

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