A study of the uptake of cu2+ by calcium silicate by batch and continuous reactors for potential commercialisatión

Abstract

This study presents a significant advancement in the understanding of the uptake of Cu2+ by nanostructured calcium silicate (NCaSil) and to develop a strategy of using it in a continuousmanner using packed columns. The NCaSil structure consists of micro-sized agglomeration ofnanometre-sized platelets of calcium silicate. This arrangement grants the material a largesurface area of 400 to 600 m2 g-1.The kinetics and thermodynamics ofthe adsorption ofCu2+ onto NCaSil in batch were studiedat temperatures ranging from 277 to 333 K The reaction between Cu2+ and NCaSil occurredrapidly, being endothermic and exhibiting an increase in the entropy meaning that theadsorption process became more spontaneous when the temperature was increased.Furthermore, the uptake resulted in the formation of copper sulfate hydroxide minerals in theform of Cu4(OH)6SO4·nH2O, where n is equal to 2 for wroewolfeite, 1 for posnjakite and 0for brochantite. Using powder X-ray diffraction and scanning electron microscopy it wasproven that at temperatures between 293 and 313 K wroewolfeite and posnjakite wereintermediates in the formation of brochantite. Specifically at high temperatures of 333 K andCu2+ concentrations higher than 15.7 mmol L-1 the reaction proceeded directly to the formation of the thermodynamically stable compound brochantite. A kinetic study of thecrystal growth was carried out using powder-XRD which showed that the rate determiningstep towards the formation ofbrochantite is the nucleation of So/·. Additionally, a value forthe activation energy of 42 kJ mor1 using powder-XRD data was obtained for the formationofthe crystallographic plane 420 in the brochantite crystal.A sample of a real mining waste was collected and analysed. Based on this sample anemulated waste was generated. The NCaSil was tested for the uptake of Cu2+ ions from thisemulated mining waste, showing that the use ofNCaSil is feasible at pH values greater than 3.The production and use of NCaSil may be coupled to existing mining waste treatmentprocesses in order to remove dissolved copper from solution and produce a copper rich solidas the by-product.NCaSil was packed inside a conventional axial flow column anda radial flow colurnn, whichwas developed as part of this project. The former proved to be impractical due to a largepressure drop through the colurnn, while the latter was impractical due to short operationaltimes before breakthrough. Nonetheless, the radial flow column was operated by immersionin a tank exhibiting similar kinetics of copper ions uptake to those observed in batchprocesses. Therefore, the scale-up of this process was proposed including the necessaryequations keeping the ratio ofthe tested radial flow column.