Synthesis and characterisation of nickel-bearing hydrous silicates
2014-07-30T15:12:40Z (GMT) by
This study of nickel-bearing hydrous silicates has been funded by European Nickel Plc. and Loughborough University, with support from the Natural History Museum, London. European Nickel Plc., the industrial sponsors of the project, is a nickel mining company based in the UK with mine sites around the globe including Turkey, Albania and the Philippines. European Nickel developed a process of nickel extraction for lateritic ore deposits known as atmospheric heap-leach (AHL). This new technique is not only economically favourable to more traditional nickel extraction methods, with relatively low plant and process costs, but it is also more environmentally responsible with lower CO2 emissions. These factors make AHL an attractive choice for new nickel extraction ventures. However, speed, efficiency and percentage recovery of nickel have proven to be variable across mined ore bodies using this technique. This is largely due to the fact that lateritic ores, particularly nickel-bearing hydrous silicate ores, are poorly understood. To develop the extraction process and understand the variability inherent to the AHL process thus requires a greater understanding of the ore types, more specifically of their chemistry. This project has been undertaken to develop the understanding of these materials and has done so by focusing on two main themes. The first focus was on the characterisation of a suite of natural mineral samples, to broaden the understanding of the types of phases present in lateritic material. Samples were sourced from nickel-laterite mine sites across the globe and historical collections at the Natural History Museum, London. Characterisation of the materials was initiated with powder x-ray diffraction (PXRD) studies, using both standard laboratory and synchrotron-based instruments. PXRD studies allowed a database of mineral phases to be compiled and common groups and themes to be identified. This found that serpentine (Mg3Si2O5(OH)4), talc (Mg3Si4O10(OH)2) and quartz (SiO2) were the most significant phases in these types of material. Following the PXRD studies, a selection of specimens samples were then investigated further using electron probe micro-analysis (EPMA). EPMA data allowed compositional information to be incorporated in context. This complementary information further emphasised the importance of talc and serpentine phases to the study, with nickel substitution most common and at the highest levels in these phases. Nickel-bearing talc was observed with up to 39.89wt.% NiO (implying over 70% octahedral cation substitution) and nickel-bearing serpentine was observed with up to 48.67wt.% NiO (implying over 85% octahedral cation substitution). As well as this, EPMA elemental mapping also found evidence of Mg-Ni substitution within mineral phases on a fine scale, observed as an, in-situ, solid solution. The second focus of this project was a parallel program of synthesis of mineral phases, which was used to allow relevant phases to be studied in isolation, away from the complex matrices inherent within the natural materials. Synthesis of the relevant mineral phases also allowed elemental compositions to be controlled, and allowed systematic studies of these materials in greater detail. This also allowed the synthetic materials to be used as standards for comparison to their natural analogues. The program of synthesis used various synthetic methods to produce the materials of interest. These included high temperature methods, low temperature hydrothermal methods and high temperature and pressure methods using specialist equipment. Early synthesis of hydrous materials was performed using Teflon-lined autoclave type apparatus but the achievable temperatures and pressures were too low to synthesise products that were crystalline enough for detailed characterisation. Synthesis using these low temperature and pressure methods found poor crystallinity of the products to be a problem for detailed characterisation. This led to the use of specialist high-temperature and pressure, Tuttle-type, cold-seal apparatus, which was able to achieve harsher reaction conditions, enabling the synthesis of single phase ordered hydrous mineral phases which permitted more detailed characterisation. Synthetic methods were gathered from the literature and adapted and optimised for the synthesis of materials of interest to the project. The most significant development in the synthetic materials project was the synthesis of a full Mg-Ni talc series ((Mg,Ni)3Si4O10(OH)2). The full series was then characterised using PXRD, infrared spectroscopy, Raman spectroscopy and thermal analysis. PXRD allowed confirmation of the synthetic products as well as structural refinements to be performed. The refined unit cell parameters match closely with reported unit cell parameters and indicate there are no miscibility gaps in the series. The similarities in the cation sizes of Ni2+ and Mg2+ produce only subtle changes in the lattice parameters between the end members and no systematic trends are observed. Infrared spectroscopy has not only showed agreement with previously published literature on the series, but also offered a more systematic approach to the compositional series and increased the amount of data on the materials. Raman spectroscopy, which has not been previously reported on these materials, has also been used to show the same shifts in the OH-stretching region as observed in the infrared studies. Thermal analysis, shows increasing thermal stability of Ni-substituted talc, increasing by c. 60°C from the Mg to Ni end-member. Carrying out these two programs of work in parallel meant characterisation of the natural materials helped to focus the direction of the synthesis program, whilst information from the synthetic studies was fed back to improve the understanding of the natural materials. This synergistic approach allowed greater benefits to be gained from each side as the project progressed and, combining the two focuses of the project, it was then possible to compare aspects of the synthetic materials with the natural materials and increase the understanding of the links between them. This has been exhibited here specifically by comparison of infrared spectra of the natural nickel-bearing talc specimens to the spectra of the synthetic talc series, which has allowed nickel substitution in the natural materials to be identified by this technique.