An Introduction into Plasma for Waste Recovery, by Dr Tim Johnson
Entry by Dr Tim Johnson, Technical Director at Tetronics.
It is one of those perennial pub-quiz questions: what is the collective noun for a group of somethings – skylarks for example? And of course, you can never remember the correct answer at the time. Well, in the unlikely event you ever get asked at your local drinking establishment what a collection of ionised gas particles is called, then there is no need to be embarrassed: the answer is ‘a plasma’. You might even wish to amaze your friends further by telling them that this simple change from gas to plasma brings with it a series of highly useful properties that have made plasma-based processes some of the most widespread in the world today.
Plasmas come in a wide variety of types, but fluorescent lighting and arc welding are probably the most common. These ubiquitous technologies typify the low-pressure and high-pressure (or ‘thermal’) forms of plasma respectively and it is the latter that I have spent over twenty years developing, researching and writing about.
Like arc welding, the heart of most thermal plasma processes is an electric arc struck between an electrode and a workpiece in the presence of a flowing gas that acts to stabilise, shield and direct the arc towards the target. What has helped arc welding become such a feature of industry today is its winning combination of intense heat, operating at normal atmospheric pressures in a highly controllable and extremely flexible manner – features which are shared by thermal plasma processes in general.
One key feature of plasma is it encourages the various components of raw materials to separate out (or ‘partition’ in technical-speak) between the three main output streams: valuable metals tend to be collected at the base of the plasma furnace in a bath of metal, heavy metals tend to be captured very effectively in the liquid slag that floats on top of the metal and materials with a low boiling point (such as nasty organic compounds) tend to go into the exhaust gas cleaning system.
This attractive mix of properties means that thermal plasmas have been used for a huge range of high-temperature industrial processes for a long time. In more recent decades, plasma-based processes have been used in many applications in the environmental sector, where the ability of the intense heat of the plasma arc to melt or vaporise the various components of waste materials has enormous benefits.
However, although Tetronics thermal plasmas are being used to address some of our more intractable hazardous waste problems, wastes are not simply made less hazardous before being landfilled. Instead, the ability of plasma to recover so many products of value from wastes means that the residue at the end of the process that has to be disposed of as a ‘waste’ is often only a tiny fraction of the original material. In this sense, these facilities should be regarded not as waste treatment plants, but rather as ‘recovery from waste’ plants. Not only does this maximise the ‘green’ credentials of any proposed waste facility based on Tetronics plasma technology (which makes it easier for new plants to obtain the critical planning permissions and other permits that they need), but the valuable by-products help to improve the economics of plasma waste treatment still further. And since such plants still have to operate in a hard commercial world, such things should be very important to all of us if we are to change the way our society deals with the balance of recycled waste versus natural resources.