3 minute read
No flash in the pan
SOME NUMBERS AND DIMENSIONS ARE EASIER TO UNDERSTAND THAN OTHERS, like the height of a person, the distance from Cape Town to Durban, even a country’s GDP. But those involved with fly ash and zeolites are a little mindboggling at both ends of the spectrum.
Consider this. Eskom generates 90% of its electricity through coal-fired power stations, with the balance produced by hydroelectric and nuclear facilities. Together, they consume about 90 million tons of coal and 320 Ml litres of water (1,5% of South Africa’s total water consumption) per annum and spew 42% of the country’s greenhouse gases into the air.
Because most of the coal used by Eskom is low quality with a high ash content of 40%, burning it produces 36 million tons of fly ash waste annually, the equivalent, as UWC’s Professor Leslie Petrik likes to imagine it, of at least six dumps the size of the 146 metre-high Great Pyramid of Giza.
Fly ash is the main component of the coal ash produced by incineration in the power station boilers and it’s pretty horrible stuff, consisting of very fine particles of unburnt carbon, boron, selenium, cobalt, chromium, molybdenum and vanadium; oxides of silicon, aluminium, iron, calcium, magnesium, potassium, titanium and sulphur; toxic elements such as mercury, arsenic and thallium; as well as soluble salts. Whether the dumps are irrigated or dry, rain and chemical weathering leach these pollutants into the groundwater and the water catchment areas, rivers, lakes and dams.
Until recently, fly ash only had limited use as an asphalt filler or additive to cement and concrete. It sounds like a contradiction, but Prof Petrik, a material scientist who runs the Environmental and Nano Sciences (ENS) research group in UWC’s Department of Chemistry, realised that fly ash could actually be applied to acid mine drainage (AMD) to remove the toxic impurities left behind in mine water after gold, copper and coal mining. Once the mine water was purified, the fly ash solids could be used to backfill and seal disused mine shafts, preventing AMD from recurring and simultaneously reducing our mountains of fly ash.
The logical next step was for the ENS to develop synthetic nano-fi lters called zeolites from the fl y ash itself.
The name zeolite means ‘boiling stone’ which hints at zeolites’ very interesting properties. They release any adsorbed water when heated. They are highly stable at high temperatures and pressures, insoluble in water and organic solvents, and do not oxidize in air.
In their natural or synthetic form, zeolites are crystalline aluminosilicate minerals with a highly porous structure and tiny pore sizes measured in billionths of a metre that can ‘trap’ elements or molecules of a particular size, e.g. metals or cations, and allow others to pass through them. About 40 zeolites occur in nature, but many more have been synthetically manufactured with predictable uniform pores, creating molecular sieves. Zeolite A, for example, is widely used as a component in laundry detergent to remove water hardness ions like calcium and magnesium and soften the water.
Much of the ENS group’s work to make zeolites from fl y ash involved the development of completely new workable processes and purpose-built technology suited to South African conditions, for which the ENS team holds three patents.
The ENS’s work in other environmental problem areas also regularly excites public interest. In 2020, for example, the team measured alarmingly high levels of toxic contaminants in several species of fi sh commonly caught and consumed by Capetonians and proved that the contamination was a direct consequence of the City’s poor management of sewage effl uent.
Along with a plethora of conference presentations (approaching 500), reports and more than 180 peer-reviewed journal articles, Prof Petrik’s contributions to her fi eld includes supervising 29 PhD and 66 MSc/MEng students to completion (and currently supervising a further 17 postgrads through the ENS).
