Mineral Waste Beneficiation

Group Focus

Our research relates to the characterization and valorisation of industrially obtained mineral wastes and by-products such as:

  • coal fly ash
  • phosphogypsum
  • mine tailings and slimes

Depending on the raw material studied and the intended application, this may include studies on:

  • solid-state thermochemical reactions
  • solid-liquid conversions
  • wet and dry surface modification techniques etc.

Current Projects

  • investigations on finding environmentally friendly, cost-effective methods for extraction of major elements from mineral wastes
  • studies which probe the application of coal fly ash as inorganic filler in polymers
  • production of hybrid-alkali activated aluminosilicate cement systems containing up to 70% coal fly ash
  • thermogravimetric investigations of solid-state reactions

Examples of projects

Coal fly ash as inorganic filler

The most significant and large-scale application of coal fly ash is in its partial replacement for Portland cement in the cement and concrete industry. The use of fly ash as an engineering material primarily originates from its pozzolanic nature, spherical shape, and relative uniformity. Inorganic fillers are generally used in polymers to reduce production costs and improve certain physical characteristics. Surface treatment is usually performed on mineral fillers to enhance workability, improve compatibility and facilitate interaction between the polymer and filler.

Extraction of aluminium from coal fly ash

The potential for extracting major elements from coal fly ash and various types of mine tailings are pursued. In most of these studies, we make use of a preceding thermochemical treatment step to increase the reactivity and dissolution behaviour of the elements of interest. Subsequent aqueous leaching of the thermochemically activated products instead of the conventionally acid-leaching procedures used in most hydrometallurgical processes is preferred. An example of a process flow diagram for the multi-stage extraction process studies, and FESEM images of thermochemically activated coal fly ash is shown below:

Utilisation of mine waste products such as tailings and slimes as secondary metal and mineral resources

South Africa's energy and mining sectors represent an important part of the country’s economy, but they are also major contributors to solid mine residues and other waste streams such as acid mine drainage. Mine residues are becoming increasingly problematic due to the large volumes generated; the ongoing financial burden associated with their long-term management, and in some instances their acid-generating potential. There is therefore a constant need to create innovative and sustainable solutions, which can include the development of valorisation technologies for metal or mineral recovery.

These projects focus on the reactivity and dissolution behaviour of minerals contained in mine tailings and slimes for elemental extraction. Elemental extraction is achieved via a thermochemical solid-solid treatment using ammonium sulphate ((NH4)2SO4), a widely available, low-cost, recyclable extracting agent to recover and synthesise added-value products from mine tailings and slimes. This extraction process offers the significant advantage of recovering major elements or individual mineral phases from tailings in a readily available soluble form, which could be subsequently converted to value-added products, which may assist in minimizing their disposal and in making treatment processes more viable economically. 

Left: Process flow diagram for the multi-stage extraction process using ammonium sulphate,
Right: FESEM images illustrating morphological changes due to thermochemical treatment 

Analytical Techniques

The analytical techniques that are mostly applied include Thermogravimetric analysis (TGA and TGA-FTIR), Differential Scanning Calorimetry (DSC), Surface area determinations (BET), X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Raman and Fourier-transform infrared (FTIR) spectroscopies, Particle size distribution (PSD) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).


Team Members

Prof Liezel van der Merwe

Room 3-39, Nat Science Building 1

[email protected]

Tel: 012 420 5379


Mrs Barbara Castleman

Room 3-46, Nat Science Building 1

[email protected]

Tel: 012 420 2043



Postgraduate students

Ms Sameera Mohamed

PhD Student

Topic: Chemical reactivity of Bushveld Igneous Complex mine residues for mineral beneficiation

Mrs Barbara Castleman

PhD Student

Topic: Beneficiation of diamond mine residues for the synthesis of Mg-containing compounds

Mr Dennis Moyo

PhD Student

Topic: Insecticide filled polyethylene films for malaria vector control

Mr Sydney Ngetu

MSc Student

Topic: Calcined kaolinitic clay in highly substituted pozzolanic cements

Ms Rachel McCondochie

MSc Student

Topic: Extraction of Al and Si from coal fly ash for the production of catalyst supports


List of graduates

  1. Dr Elias Aphane, PhD(Chemistry), Silica nanoparticles from South African coal fly ash derived sodium silicate solutions, 2020.
  2. Mr Nqgondi Nxokwana, MSc(Geology), Physicochemical properties of South African shales in the context of geological CO2 storage, 2020.
  3. Dr Grizelda du Toit, PhD(Chemistry), Chemical and mechanical activation of hybrid fly ash cement, 2018.
  4. Mr Dennis Moyo, MSc(Chemistry), Application of coal fly ash as filler in natural rubber. University of Pretoria, 2018.
  5. Mr Benni Vilakazi, MSc(Chemistry), A thermogravimetric investigation into the synthesis of cobalt fluoride, 2018.
  6. Ms Sameera Mohamed, MSc(Geology), Extraction of major elements from PGE tailings in view of nanoparticle synthesis for environmental technological applications, 2015.
  7. Mr Lillian Ondo-Ndjimbi, MSc(Chemistry), Indirect mineral carbonation of phosphogypsum wastes, 2014.
  8. Ms Lethabo Mathebula, MSc(Chemistry), Surface modification of coal fly ash by sodium lauryl sulphate, 2013.
  9. Mr Thembane Mlambo, MSc(Geology), Improving Geological Saline Reservoir Integrity through Applied Mineral Carbonation Engineering, 2011.
- Author Liezel van der Merwe

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