Petrographic And Physico-Chemical Characterisation Of Tailings Dump And Soils Around A Nickel-Copper Mining And Smelting Environment

Abstract

Recent concerns of mining and smelting of nickel-copper (Ni-Cu) in Selebi Phikwe, Botswana possibly affecting the environmental physico-chemistry motivated this study. Physico-chemical analyses which included particle size distribution (PSD), pH, electrical conductivity (EC), cation exchange capacity (CEC), color and descriptive petrography on samples of tailings dump and soils were performed in order to understand the significance of these properties on the surrounding physical environment. The PSD of soil samples revealed the average wt % of the < 2 μ m fraction (clay fraction) was between 3 and 9 wt %, the > 2 μ m to < 50 μ m fraction (silt fraction) was between 34 and 44 wt %, and the > 50 μ m to < 250 μ m fraction (sand fraction) was between 47 and 63 wt %. Soil pH ranged from 3.5 to 6.0. Electrical conductivity values were significantly low, and the range was from 50 μS/cm to 250 μS/cm. Soils with low pH correspondingly had low EC and were close to the mine. The CEC values occurred between 2 meq/100 g and 20 meq/100 g. Hand specimens of tailings dump viewed with a microscope and physical tests performed on the samples for hardness, cleavage, fracture, colour, streak, lustre, and crystal appearance depict albite, cristobalite, chalcopyrite, pyrrhotite, tremolite, and pentlandite to be contained in them. Soil colour varied from pale yellow, reddish yellow to dark reddish brown. Grains were poorly sorted, with subangular grains located further from the plant, which is indicative of windblown particles transported at a short distance. Reddish brown soils were closer to the tailings dump, mining areas and the smelter/concentrator plant. The observation is indicative of both Fe and Cu containing minerals enriching the soils of the study area. The findings of this study are indicative of increase in soil acidity due to mining and smelting of Ni-Cu at Selebi Phikwe area. High acidity favoured the leaching of heavy ions from tailings dump and related mining waste to the soils. Fewer exchange sites have resulted in a low CEC for ions adsorbed on the sediment surfaces, consequently they remain in solution and are bioavailabe for plant uptake.

KEY WORDS: tailings dump; soils; physico-chemistry; particle size distribution; petrography; environment

Global Journal of Environmental Sciences Vol.4(1) 2005: 31-39