MAXIMISING THE VALUE DERIVED FROM LABORATORY TESTWORK TOWARDS HEAP LEACHING DESIGN

Heap leaching can be considered for a wide range of ore and mineral types, and the design and operation of heap leach plants have continued to be adapted and refined towards optimization and towards meeting the requirements of increasingly challenging applications. For heap leach testwork, coarsely crushed aggregate is required, which dictates that underground resources must be sampled by obtaining relatively expensive diamond drill cores that can be crushed to a range of sizes typically between 6 and 50 mm. Every attempt should therefore be made to extract the maximum information from these expensive samples. It is common laboratory practice to perform chemical, mineralogical and physical characterization of the ore, followed by bottle and column leach tests. But several possibilities exist for extracting much more than this basic information from the ore samples, before reverting to pilot or demonstration plant studies. For example, geochemical modeling coupled to the mineralogical information can be used to extrapolate better quality estimates of the acid consumption, the formation of inheap precipitates and (from integrated flowsheet simulations) the composition of the feed to metals recovery on the ultimate commercial scale plant. Furthermore, in the case of sulfidic ores, oxygen uptake measurements over the laboratory leach columns can be used to infer the rates of chemical reaction and heat generation. These rates allow the development of the vertical temperature profile with time to be calculated, which can be applied to the heating jackets of the columns to better simulate commercial-scale hightemperature heap leaching. Further insight into the factors limiting the process kinetics at any given temporal or spatial point could be gained by studying the bacterial populations obtained from such columns, which may suggest further means of process optimization. Comment is made on the effect of inoculation during heap bioleaching applications, and on the combination of inoculation and agglomeration of ore destined for heap bioleaching. During column leach experiments on uranium ores, a bacterially inoculated column ought to be included to verify the effect on operating cost of bacterial acid generation and bacterial ferrous iron oxidation throughout a heap. A case study is provided of how the results from batch column tests, coupled with flowsheet calculations, were used to optimize the acid balance of a counter-current laterite heap leach and downstream recovery process. The Southern African Institute of Mining and Metallurgy Base Metals Conference 2009 P J van Staden, S W Robertson, M Gericke, J W Neale, A Seyedbagheri. ________________________________________________________________________ Page 186 Introduction The remaining minerals resources of the world are gradually becoming more challenging to exploit by the conventional processing technologies. As a result, ongoing innovation is called for to provide economically feasible technologies, while also placing greater emphasis on minimizing the impact on the immediate and the global environment over the life of a project and after decommissioning. Incremental innovation is continuously taking place in heap leaching (as it is no doubt occurring in other technologies), but may not be very visible since most of it does not involve entirely new process routes or novel reagents. Rather, it mostly involves such aspects as more detailed attention to the mineralogical, chemical and physical characterisation of the feed material, more complex mine management and ore preparation routes (although it still draws on existing technologies), more intensive instrumentation and automation and/or more restrictive boundaries to the operating parameters. This could require a larger number of processing parameters to be defined during the testwork program, and for a particularly challenging ore type the window of feasible parameters that the testwork program is attempting to identify may be quite narrow. However the number of column leach tests that can be performed to define these parameters are limited since, for heap leaching testwork, relatively large amounts of diamond drill core sample are required, which are more expensive to obtain than reversecirculation drilling samples. It is therefore essential that the maximum scope of information is derived from the sample provided. Certain aspects of heap leaching design have been quite extensively implemented commercially with little fundamental understanding. One such area that still requires investigation is the management of bacteria in sulfide ore heap leaching, where one reason for the scant attention being paid to it has been a lack of techniques for analyzing bacteria on solid surfaces. Efforts are being made to obtain more quantifiable information on bacterial performance during metallurgical test programs, and to be able to provide guidelines for its active management together with the other heap leach control actions. This paper deals with a number of aspects that should be included in a metallurgical testwork program aimed at the heap leaching of an ore, and particularly of the more challenging ore types, to maximize the value derived from metallurgical testwork.