A Study of the Uptake of Cu²⁺ by Calcium Silicate by Batch and Continuous Reactors for Potential Commercialisation

Abstract

<p>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 continuous manner using packed columns. The NCaSil structure consists of micro-sized agglomeration of nanometre-sized platelets of calcium silicate. This arrangement grants the material a large surface area of 400 to 600 m2 g-1. The kinetics and thermodynamics of the adsorption of Cu2+ onto NCaSil in batch were studied at temperatures ranging from 277 to 333 K. The reaction between Cu2+ and NCaSil occurred rapidly, being endothermic and exhibiting an increase in the entropy meaning that the adsorption process became more spontaneous when the temperature was increased. Furthermore, the uptake resulted in the formation of copper sulfate hydroxide minerals in the form of Cu4(OH)6SO4·nH2O, where n is equal to 2 for wroewolfeite, 1 for posnjakite and 0 for brochantite. Using powder X-ray diffraction and scanning electron microscopy it was proven that at temperatures between 293 and 313 K wroewolfeite and posnjakite were intermediates in the formation of brochantite. Specifically at high temperatures of 333 K and Cu2+ 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 the crystal growth was carried out using powder-XRD which showed that the rate determining step towards the formation of brochantite is the nucleation of SO4 2-. Additionally, a value for the activation energy of 42 kJ mol-1 using powder-XRD data was obtained for the formation of the crystallographic plane 420 in the brochantite crystal. A sample of a real mining waste was collected and analysed. Based on this sample an emulated waste was generated. The NCaSil was tested for the uptake of Cu2+ ions from this emulated mining waste, showing that the use of NCaSil is feasible at pH values greater than 3. The production and use of NCaSil may be coupled to existing mining waste treatment processes in order to remove dissolved copper from solution and produce a copper rich solid as the by-product. NCaSil was packed inside a conventional axial flow column and a radial flow column, which was developed as part of this project. The former proved to be impractical due to a large pressure drop through the column, while the latter was impractical due to short operational times before breakthrough. Nonetheless, the radial flow column was operated by immersion in a tank exhibiting similar kinetics of copper ions uptake to those observed in batch processes. Therefore, the scale-up of this process was proposed including the necessary equations keeping the ratio of the tested radial flow column.</p>