An entropy approach to optimizing heat recovery in high-pressure autoclave circuits

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 preheating 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 is generated using fossil fuels, the recovery of flashed steam also reduces the associated carbon footprint.

This paper introduces the concept of entropy, Clausius inequality, the second law of thermodynamics, and the use of temperature-entropy (T–s) diagrams to describe the overall thermodynamic efficiency. It explores the benefit of adding multiple stages of pressure letdown and feed preheating, akin to the use of regenerative feed water heating in thermal power plants to improve overall Rankine Cycle efficiency. This paper then shows how this concept may 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, this 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 and fuel consumption and CO2 emissions for each incremental stage.