Evaluating mining projects generally start with an initial scoping-level study, and if needed, more detailed prefeasibility and feasibility evaluations continue.
A scoping-level study is generally conducted at the roughly 50% level of accuracy, with flow sheet development and processing assumptions based on limited test work. A prefeasibility-level study is normally conducted to +/-25% level of accuracy, and metallurgical test work is sufficient for preliminary flow sheet development and equipment selection. A feasibility-level study is usually conducted to +/-15% level of accuracy and metallurgical testing is sufficient for definitive flow sheet development, process design and equipment selection.
Scoping-level metallurgical programs are conducted to establish how the resource material will respond to standard metallurgical processes, such as flotation, gravity concentration and leaching. They are conducted on coarse assay reject material using standard test conditions. The objective is to determine how the resource material reacts to commonly accepted recovery processes and gain a preliminary estimate of metal recoveries and, in the case of flotation, concentrate grades that are likely to be achieved.
Prefeasibility-level metallurgical programs are significantly more comprehensive than scoping studies and are typically conducted on drill core samples combined together, or composite, to represent the various major ore types that have been identified. Test work will include chemical and mineralogical analyses of each test composite followed by metallurgical testing to evaluate the recovery process, whether it is flotation, agitated cyanidation for gold or silver recovery, heap leaching for gold or copper recovery, etc. Testwork is directed at developing a preliminary flowsheet and material balance and establishing preliminary process design criteria, such as ore hardness, crush and grind size requirements, flotation or leach retention times, reagent consumptions, metal recoveries and concentrate grades.
Feasbility-level metallurgical programs are often an extension of earlier prefeasibility studies, but are conducted to a level needed to set up detailed flow sheets, material balances, process design criteria, equipment sizing and specification. Test work should be conducted on drill core samples that have been composited to represent the various ore types that are anticipated. In many cases “metallurgical” holes are drilled specifically to obtain sufficient quantities of material for testing. In addition, variability composites are developed to assess the range of metallurgical performance that might be expected throughout the ore body. Variability composites are developed to assess ore variations of specific concern. They could include ore grade, hardness, contaminant levels, lithology and spatial location within the deposit. Flotation test work at the feasibility-level will include locked-cycle testing to evaluate the impact of recirculating intermediate products within the process and may include large bulk tests, or even pilot plant testing to generate sufficient quantities of intermediate product for definitive process evaluation. Test work for heap leach evaluation will include full lift-height test columns at the crush sizes that have been determined appropriate for the ore. For processes that use cyanidation for gold and silver recovery, it is important to run cyanide detoxification studies on the leach residues to demonstrate the residues can be detoxified to required limits before being discharged to the tailing storage facility.