Geochemistry, mineralogy and environmental impact of precipitated efflorescent salts at the Kabwe Cu-Co chemical leaching plant in Zambia

Research highlights: → Efflorescent salts of Cu and Co precipitate at Kabwe site in Zambia during long-term dry period. → They comprise an assemblage of Cu and Co minerals, some of them are very exotic in nature. → Evaporation experiment in laboratory gave more hydrated forms of minerals found in the field. → During rainy period, efflorescent salts dissolve with resulting high metals and acid pulse. → During on-going climatic change, environmental impact may become more serious. - Abstract: Precipitated efflorescent salts in Cu-Co chemical leaching plant wastes in Kabwe, Zambia, have been studied using XRF analysis, powder X-ray diffraction, scanning electron microscopy (SEM), Raman spectroscopy, and evaporation experiments combined with geochemical modeling. Field samples of efflorescent salts contained up to 14.32 wt% Cu and 1.42 wt% Co. In the field, the principal minerals in the salts were gypsum (CaSO4.2H2O), moorhouseite (Co0.6Ni0.3Mn0.12+(SO4).6H2O), bloedite (Na2Mg(SO4)2.4H2O), starkeyite (MgSO4.4H2O), chalcanthite (CuSO4.5H2O) and kroehnkite (Na2Cu(SO4)2.2H2O). In the evaporation experiment, gypsum precipitated in the first and second evaporation steps, chalcanthite and bieberite (CoSO4.7H2O), started to precipitate in Step 3, after evaporation of 60% of water, and maximum amounts of Cu and Co sulfate salts accumulated in Step 4 after precipitation of 80% of water. Epsomite (MgSO4.7H2O), and hexahydrite (MgSO4.6H2O), precipitated only after evaporation of 97.5% of water in Step 5. Presence of chalcanthite, bloedite, and kroehnkite in precipitates of Step 4 was also confirmed by Raman spectroscopy. Magnesium and Na content in Cu and Co-sulfate phases gradually increased with evaporation progress, e.g. bieberite was replaced by Co-bloedite and chalcanthite was replaced by kroehnkite in later stages of evaporation. The precipitation order was consistent with results of geochemical modeling. The principal difference between field data and data obtained in the evaporation experiment was the presence of less hydrated sulfate phases in the field, which can be explained by different time scales available for evaporation and consecutive dehydration. Dissolution experiments using efflorescent salts collected in the field indicated fast dissolution with an instantaneous drop in pH to about 4.0 and a very fast increase of dissolved species concentrations. Such behavior may have a serious environmental impact at the beginning of the rainy period in November and December, when seepage through the impoundment dam was recorded. The wastes at the Kabwe site represent a long-term source of contamination, which may be especially significant as a consequence of on-going climatic change with increasing intensity of precipitation.