Recovery and repurposing of low-grade thermal resources in the mining and mineral processing industry

Abstract

A substantial quantity of energy is currently lost as waste heat to the atmosphere in the mining and mineral processing industry. The recovery and application of this waste heat to on-site processes is an opportunity to reduce primary fuel usage, operational cost and environmental impacts. To investigate this, research has been conducted within a nickel smelter’s sulphuric acid plant, where operational data was collected on a process water cooling loop that incorporates cooling tower systems. The cooling towers discharge in excess of 50 MW of heat without any recovery. Collected data was then used to develop a model to quantify the potential for low-grade heat recovery and repurposing within the sulphuric acid manufacturing process. Heat pumps were examined as a method to repurpose this waste heat, and use it to replace electric heaters currently used in the mist precipitators and weak acid stripper. The model allows for an economic and environmental impact comparison between various recovery and application scenarios. Results obtained from the model indicated that the implementation of a heat pump system would provide a reduction in annual operating costs that allows a payback period of 3 years. Furthermore, there would be from less primary energy consumption a reduction in CO2 emissions of about 42% from heat pump operation compared to electric heaters for the system. To further quantify environmental benefits from implementing the proposed recovery strategy, a comparative life-cycle assessment (LCA) model was also constructed and applied to the sulphuric acid plant. The LCA showed a 20% reduction in emissions in the cooling tower and heating system would be achieved from the impact categories of global warming, acidification, eutrophication, and human toxicity potentials. This includes emissions from cooling tower fans and water pumps, as well as the mist precipitator air heating. The concepts and models can be applied to a wide range of energy intensive industrial sectors, to help identify and quantify reductions in consumption and improvements in long-term sustainability performance.