Assessment of life cycle based environmental footprints from nickel processing

2017-04-09T23:51:49Z (GMT) by Janelle Zhiyun Khoo
This thesis presents an integrated study of a life cycle assessment, process modelling and exergy analysis on three nickel laterite processing routes, including a new state-of-the-art processing technology using atmospheric leaching with nitric acid, the Direct Nickel Process. <br><br>    The stainless steel and nickel industry is an example of an energy intensive process. According to several life cycle studies, nickel production is one of the most carbon and energy intensive process in comparison to metals such as copper, lead, and zinc. The environmental impacts and energy demand of nickel processing will continue to increase with the increasing demand for stainless steel and other nickel-related alloys. Furthermore, the nickel sulphide reserves currently extracted for nickel production are depleting and future nickel production is expected to come from nickel laterites. Consequently, there is a need to improve current nickel laterite processing technologies or to find new alternatives.<br> <br>    Firstly, a comparative life cycle assessment (LCA) was conducted in this study with a functional unit of one tonne of stainless steel to allow a fair environmental evaluation of three nickel processing routes, which are (1) ferronickel production, (2) high-pressure acid leaching route (HPAL), and (3) the newly developed Direct Nickel (DNi) Process. A cradle to gate approach was adopted and covers the scope from laterite ore mining to final metal production. It was determined from the LCA that the DNi Process has a lower global warming (7t CO<sub>2</sub>-eq/ t stainless steel) and non-renewable energy scores (78 GJ/ t stainless steel). Ferronickel had impact scores of 13t CO<sub>2</sub>-eq/ t stainless steel and 181 GJ/ t stainless steel for global warming and non-renewable energy, respectively. The impact scores for Ferronickel is contributed by the significant amount of electricity required, especially for smelting carried out in an electric arc furnace. The global warming and non-renewable energy impact scores of the DNi Process in comparison to HPAL are quite similar but the water depletion scores for HPAL (187 m<sup>3</sup>/ t stainless steel) is five times higher compared to the DNi Process (38 m<sup>3</sup>/t stainless steel). This is due to the larger amount of sulphuric acid required in HPAL which is three times the amount of nitric acid required in the DNi Process. It can be deduced from this life cycle study that the DNi Process has a better environmental performance with the capability of processing different types of laterite ores (from limonitic to saprolitic ores) which is an important feature in future nickel production.<br> <br>    An exergy analysis was conducted following the LCA to determine the overall efficiency as well as efficiency for individual sub-processes of the three nickel laterite processing routes regarding exergy. To determine the inputs/outputs of a sub-process in the overall processing route, three complete process simulations were generated using Aspen Plus process modelling software. A sensitivity analysis was also conducted to study the effect of mineral composition on the amount of reagent required and output from the process and to validate the convergence of the process models. From the exergy analysis, it was determined that HPAL has the highest exergy efficiency of 78% compared to Ferronickel and the DNi Process, both of which have an exergy efficiency of 15%. In contrast, the mixed hydroxide product has a higher specific exergy (60.2 MJ/kg nickel hydroxide) compared to the nickel produced from HPAL (9.0 MJ/kg nickel) and ferronickel (5.6 MJ/kg ferronickel). This suggests that the exergy efficiency of HPAL is mostly contributed by other outputs such as tailings and offgas. Therefore, the DNi Process is more exergy efficient in terms of the main product output. In addition to the exergy analysis, a Pareto analysis was conducted to prioritize sub-processes in terms of evaluating improvements that can be made to improve the overall nickel processing route. A suggestion for improving the efficiency of the power generator for the DNi Process includes changing the type of fuel used. However, the type of fuel used is dependent on the location of the plant and the DNi Process is assumed to be based in Western Australia, where black coal is used for power generation.<br> <br>    The life cycle study along with the exergy analysis and process modelling is a new combination for identifying improvements and energy reduction of the overall processing route. This methodology can be used as a tool to study the effect of different parameters (i.e. types of reagent used, operating temperature and pressure) on the environmental impacts for any process. A proposed framework was prepared based on the integrated study of LCA and exergy analysis done above. The integration of simulating a process, then performing an LCA and an exergy analysis can help in both improving the process efficiency and at the same time reducing its environmental impacts by identifying the inefficient sub-processes.<br> <br>    To complement the above studies, a thermodynamic equilibrium study using FactSage was carried out to predict the products formed during smelting of iron and nickel ores. Two ore samples were smelted in a high temperature muffle furnace up to a temperature of 1600°C in a reducing environment. The XRD results of the ore samples before and after heat treatment have revealed that the product compositions are similar to the predictions from the thermodynamic equilibrium study. This shows that FactSage can be used to generate some of the unavailable data for LCA and process simulation, at least for the pyrometallurgical route. The thermodynamic study was also incorporated into the proposed framework if the nickel processing route being evaluated involves pyrometallurgical process.