Abstract
The laterite nickel (Ni) ore smelting operation in Niquelandia (Goias state, Brazil) produced large amounts of smelting wastes, stockpiled on dumps (slags) and in settling ponds (fly ash). In this study we present data on the chemistry, mineralogy and pH-dependent leaching behaviours of these two waste materials.
Bulk chemical analyses indicated that both wastes contained significant amounts of potentially toxic elements (PTEs), with substantially higher concentrations in the case of the fly ash (up to 2.51 wt% Ni, 1870 mg/kg Cr and 488 mg/kg Co). The mineralogical investigations carried out using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron microprobe (EPMA) indicated that the slag was mainly composed of silicate glass, olivine and pyroxene.
In contrast, the fly ash was composed of Ni-bearing serpentine-like phases (originating from the furnace feed), Ni-bearing glass, olivine, pyroxene and spinel. The pH-dependent leaching behaviour was performed according the EU standard experimental protocol (CEN/TS 14997) in the pH range of 3-12.
The leaching was highly pH-dependent for both materials, and the highest releases of PTEs occurred at pH 3. The slag generally exhibited an U-shaped leaching behaviour of the PTEs as a function of pH, and was found to release up to 48.0 mg/kg Ni, 25.6 mg/kg Cr, and 1.42 mg/kg Co.
The fly ash was significantly more reactive, and exhibited its highest leaching level of PTEs between pH 3 and 7. The maximum observed release corresponded to 5750 mg/kg Ni, 4.35 mg/kg Cr, and 112 mg/kg Co.
The leached Ni concentrations after 24 h of leaching in deionized water exceeded the limit for hazardous waste by more than 100x according to the EU legislation (40 mg/kg Ni).