Abstract
In many hydrometallurgical pressure oxidation (POX) processes and almost all high-pressure acid leaching (HPAL) processes, the recovery and efficient reuse of energy from flashed steam in the pressure letdown circuit is an essential element of sustainable process design and economic processing. In whole-ore POX circuits, the use of recovered process steam for pre-heating feed slurry enables autogenous conditions to be maintained at sulphide grades as low as 1.5 - 2% w/w S2-. In HPAL circuits, increasing the recovery of flashed steam minimizes the requirement for direct injection of boiler steam to the autoclave, the associated dilution of downstream liquor tenors, and where process steam (or power) are generated using fossil fuels, the associated carbon emissions.
This paper introduces the concept of entropy, Clausius inequality, the 2nd Law of Thermodynamics, and the use of temperature-entropy (T - s) diagrams to describe the overall thermodynamic efficiency. It explores the efficiency of adding multiple stages of pressure letdown and feed pre-heating, akin to the use of regenerative feed-water heating in thermal power plants to improve overall Rankine Cycle efficiency. The paper then shows how this concept can be applied to more complex metallurgical models for POX and HPAL processes, and the impact of non-ideal equipment constraints such as approach temperatures. Finally, the paper examines the benefits of increasing the number of heat recovery stages from 2 to 3, 4, and 5 stages, with the expected reduction in steam consumption, CO2 emissions and economic payback for each incremental investment.