Reduction of the As content in a colemanite concentrates

This research study aims to identify the best way of processing colemanite concentrates, which is sometimes characterised by a high content of As sulphides, finely disseminated in the matrix of the boron mineral. Scanning Electron Microscopy analyses have been carried out to identify all the mineralogical constituents and to evaluate their distribution, frequency and mutual relationships. Colemanite concentrates are mainly used by chemical plants producing boric acid and gypsum. A high As content is unacceptable for further use of tailings coming from the leaching process and is very dangerous if the tailings are released into the environment. Grinding tests were conducted under dry and wet conditions in order to assess how As sulphide minerals are released from colemanite minerals. Some of the difficulties involved in obtaining selective liberation of As sulphides from colemanite minerals were overcome, although a certain decrease in As grade in the higher size classes was registered. All comminution products have been analysed by SEM, monitoring the degree of liberation of the mineralogical constituents. Flotation tests have been carried out, following different comminution flow sheets and adopting different modulating conditions in order to evaluate the influence of the following parameters: ! collector concentration versus contaminants concentration (As minerals) in the froths; ! kind and quantity of the frothing agent to maximise mineral recovery in the froths; ! kind and quantity of the modulating agents to maximise the selectivity of the process; ! conditioning modalities in order to obtain a suitable dispersion of fines and avoid segregation of Asbearing particles together with colemanite ones; ! solid/liquid ratio of the pulp to minimise the water flow into the plant. Information obtained from SEM analysis has been adopted to evaluate segregation phenomena in pulp and in flotation products. The influence of the chemical characteristics of industrial waters flowing in the flotation process has been investigated. On the basis of the results of all the experiments, performed under optimum operating conditions, it has been possible to model different process flow sheets with the objective of maximising of boron recovery and minimisation of both As recovery and As grade in the colemanite product. Results demonstrate how to obtain a level of As in the colemanite product suitable to yield, after the chemical process, a gypsum characterised by an As content acceptable for further industrial uses.