In Situ Evaluation of Mechanical Properties of Phosphate Tailings for Exploring Reuse Potential: Case Study of a Phosphate Mine, South Africa

Abstract

Mining and subsequent processing of phosphate rock for phosphate-bearing mineral recovery result in the large production of waste material, also known as tailings material. At the studied phosphate mine, the waste tailings are processed into slurry at a tailings plant, which is eventually pumped out to its final location, the tailings dam. At this point, the material poses environmental challenges, including consumption of surface land, dust hazard, potential for surface and subsurface water pollutant. The main objective of this study was to evaluate in situ mechanical properties of phosphate tailings, with the scope of exploring their reuse potential, through determination of material’s bearing capacity, penetration indices, as well as overall mechanical strength capabilities. The field dynamic cone penetrometer (DCP) tests were conducted to obtain material’s in situ California bearing ratio (CBR), bearing pressures and penetration indices. As a complementary measure, material’s gradation tests were performed at the laboratory to give further insight into material’s mechanical strength. The resultant data collected through these tests revealed that the material exhibits low bearing strength and low penetration resistance, which were accompanied by uniform-graded particle distribution. Observed penetration indices were well above the 10 mm/blow mark, at some points going as high as 90 mm/blow. CBR values were found to be very low, with some as low as 02–20. This corresponded with the low values of bearing capacities, with most values falling below 100 kPa and some as low as 20–40 kPa. Sieve analysis revealed that material consists mostly of sand-sized particles, with a coefficient of uniformity of 3. From the assessment and understanding of this data, it was concluded that the tailings are of poor mechanical strength, and for this reason, they could not be reused readily for engineering purposes. Hence, it is advisable that before any attempt to reuse the material for such purposes, innovative measures must be explored to enhance tailings mechanical strength. The work presented in this paper forms part of the mini-dissertation submitted by the first author to the Department of Mining and Environmental Geology, School of Environmental sciences, University of Venda, Thohoyandou, South Africa.