Extraction of valuable metals from tailings dumps (or residues or low grade stockpiles) traditionally regarded as waste, has long been researched by metallurgists. A number of developments in mineral processing technology have combined to make the retreatment of numerous tailings dumps a profitable proposition. This is particularly the case for tailings dumps created from the processing of platinum group elements (PGEs) from the UG2 reef in the Bushveld Complex (BC) of South Africa, which are now being retreated to recover chrome. Similarly, tailings dumps arising from the processing of chrome ore in the Lower Group reefs of the BC are also being retreated to recover PGEs. Several commercially profitable enterprises have also been set up with the sole intention of retreating tailings dumps arising from South Africa’s Witwatersrand gold processing operations.
According to Munyar Chirisa, a chemical engineer and senior manager of Deloitte, any tailings dump can be considered a potential mineral asset as long as it contains economic quantities of the mineral or metal of interest.
It is important for businesses that intend to engage in the reprocessing of tailings dumps to determine the economic viability of their mineral assets. In this regard, Venmyn Deloitte has often been requested to conduct scoping through feasibility studies intended to investigate optimal ways of reprocessing tailings dumps.
Mineral Resources are the cornerstone to any mineral asset and thus form the basis of any economic study.
In order to estimate Mineral Resources for a tailings dump a comprehensive technical study must be undertaken to assess the quantity and quality of the tailings material and demonstrate reasonable prospects for eventual economic extraction as required by the 2016 edition of the South African Code for the Reporting of Exploration Results, Mineral Resources and Mineral Reserves (the SAMREC Code).
To determine the quality of the tailings material, the grade variability and mineralogy must be assessed. To determine the quantity, the tonnage of the tailings dump must be established through the measurement of volume and bulk density.
In Venmyn Deloitte’s experience, any tailings dump must first be surveyed by an accredited surveyor to determine the volume and to also create a digital terrain model (DTM) which is required for 3 dimensional (3D) modelling. This is then followed by the drilling of a sufficient number of evenly spaced exploration drillholes (i.e. auger drillholes) on a regular grid to obtain representative samples for use in determining the bulk density, moisture content, grade, mineralogy and metallurgical characteristics of the tailings material.
The correct measurement of dry bulk density to account for moisture content is often overlooked in tailings dump Mineral Resource estimation. Often, an overly simplistic method is used to estimate the bulk density and this can cause significant errors in tonnage estimation and, consequently, an over estimate of metal content. Venmyn Deloitte relies on a method that is based on the principle of collecting representative samples both laterally and vertically. The method also requires that the both the volume and length of the auger barrel used to collect the samples are known with a high level of confidence.
Each sample is dried and weighed to obtain its dry weight. The dry weight is then divided by the volume of the auger barrel in order to obtain the dry bulk density per sample. Venmyn Deloitte recommends that a variance analysis be conducted on the individual density measurements in order to establish the variance around the mean. The variance to be determined is required to be within 15% from the mean at 95% confidence limit in order for the average value to be considered applicable to the entire tailings dump (i.e. to be considered as the bulk density).
Venmyn Deloitte uses Datamine Studio RM software to create a 3D block model of tailings dumps based on the DTM supplied by surveyors as well as a drillhole database containing collar, density and assay data. The model is created to demonstrate the distribution of grade throughout a tailings dump and to also verify the bulk volume using the survey data produced by surveyors. Knowing the deposition history of a tailings dump and, possibly, past process plant efficiencies, gives an idea of what to expect in terms of mineralogy and metals of interest.
Since tailings dumps are mostly mined in bulk and not selectively, the mean value of the grade must be determined with a high level of confidence. This is critical for financial modelling and feasibility studies.
Prior to Mineral Resource classification, Venmyn Deloitte analyses the variance of the moisture content, grade and bulk density results using in-house proprietary Variance Towers. Venmyn Deloitte has developed the Variance Towers based upon traditional statistics to utilise historical and on-going information in order to quantify variance of geological and chemical parameters and the boundaries and logic for quantitative Mineral Resource classification. In conjunction with the results of the Variance Towers, the Mineral Resource classification for a particular tailings dump is assigned using the criteria detailed in a “Logic Table” created in-house following the guidelines set out in the SAMREC Code, with a justification applied for each of the critical parameters used in determining the Mineral Resource classification alongside the judgement of a Competent Person.