April 2017

Field testing methods to improve sampling for acid rock drainage assessments

  • By James Tuff and Brent Usher, Klohn Crippen Berger Ltd, Brisbane, Australia

In-field geochemical testing has the power to significantly reduce the time taken for laboratory testing and can result in cost-savings to mining operations

Introduction: A new approach to field sampling

The environmental impacts of mining activities are increasingly coming under the scrutiny of regulatory bodies. Mining operations are progressively developing waste characterisation programs to improve the understanding of potential impacts and the methods that may be employed to provide mitigation.

Crucial to impact moderation is a strong focus on the geochemical behaviour of waste material, which may cause adverse effects, such as acid rock drainage (ARD) and metalliferous leaching (ML). Mining operations attempt to mitigate these potential issues by working within regulatory frameworks (eg freshwater discharge guidelines; ANZECC 2000), developing internal waste placement and discharge criteria and continually reviewing the frameworks with external audits by known experts in the field.

Underpinning all waste handling plans will be a geochemical characterisation program, which itself is dependent on extensive laboratory testing, a laborious, time-consuming and often high-cost process. Geochemical characterisation programs need to provide a detailed understanding of the geochemistry and to provide regulators and community stakeholders with results that comply with industry-accepted practice. Laboratory testing is, nevertheless, essential and not something that can be neglected if an operation is serious about its environmental obligations.

To balance the due diligence that must be performed for waste characterisation with the need to conduct cost-effective laboratory programs, most geochemical investigations involve in-field observations prior to – and during – sampling. However, many in-field programs are primarily observational, which places the onus of appropriate sample selection with the laboratory and may significantly increase the time taken to conduct the investigation.

Geochemical analysis using readily available equipment may be conducted in the field during sampling to improve the screening process prior to laboratory testing. Here we show that employing in-field geochemical testing techniques provides at least a semi-quantitative framework in which to choose samples for laboratory testing. We show that the in-field techniques may be employed across a large number of mine sites, ranging from metalliferous deposits to coal fields and mineral sands. In the hands of experts and with well-defined methods, in-field geochemical testing has the power to significantly reduce the time taken for laboratory testing, reduces the potential for analysing superfluous samples and results in potentially significant cost savings to mining operations.

In-field geochemical testing techniques

The main aim of field geochemical testing is to improve the decision-making process to select appropriate mine-derived material for detailed laboratory testing, which will then provide fully-quantitative information to inform mine planners. To achieve this, several field geochemical tests are employed:

Field pH measurements

Field pH (pHf) measurements are used to give an indication of what will happen to the pH of water (such as rain water) that may come into contact with mine-derived waste. The pHf analysis typically involves measuring the pH of an approximate 1:5 soil to deionised water suspension. The pH is measured using portable pH meters (Stone and Hopkins, 1998; Watling et al, 2004).

Hydrogen peroxide oxidised pH measurements

Hydrogen peroxide (H2O2) oxidised pH (pHfox) measurements are used to provide an indication of the impact that mine-derived waste may have on water (such as rain water) that comes into contact with it. These measurements are intended to induce rapid oxidation of species such as sulfides. For pHfox analysis, the sample is typically treated with hydrogen peroxide to oxidise the iron sulfides (such as pyrite) that may be present. During mixing, the sample may be seen to react with the peroxide; this and any other observations (such as smell, any colour change etc) are recorded and the pH is measured (using the same meter) once the reaction has finished. Generally a pHfox value less than four is considered to be potentially acid forming (Stone and Hopkins, 1998; Watling et al, 2004). Figures 1 and 2 show pH readings in progress in the field.

Figure 1. Example of field sampling and testing.
Figure 2. Example of field sampling and testing.

Handheld X-ray fluorescence testing

Handheld X-ray fluorescence (HHXRF) testing provides a semi-quantitative indication of the composition of mine-derived waste. A typical HHXRF device is shown in Figure 3 and many variations are available. A wide range of elements may be analysed depending on the specific site requirements. For ARD assessments, this has been successfully used at several sites for rapid in-field indication of the mine waste’s chemical composition. This can be used as a screening tool to identify layers with elevated metal content and often elevated sulfur. The XRF can be correlated to site assay data, and together with available site geochemical, geological and assay data, can be used to identify samples or lithologies that may result in acidic or metal-laden drainage.

The three techniques can be utilised in conjunction to identify areas of concern, such as high sulfur concentrations, pH decrease as a result of rapid oxidation and high metal concentrations. In this way, field data can assist in selecting appropriate samples for detailed laboratory analysis and, in some cases, in providing a rapid indication of potential geochemical risks associated with mining waste.

Figures 1 and 2 show a typical set up for field geochemical testing. Although unconsolidated samples are being tested in this example, all types of material, from all lithologies, may be screened for further analysis. The techniques particularly lend themselves to drill core samples, but with a GPS tracker (increasingly included as a feature in the HHXRF) all aspects of the site may be analysed, including in situ waste rock dumps and run-of-mine pads, etc.

Figure 3. A typical handheld XRF instrument used for rapid field analysis; this example is from a mineral sands site.

 

Results: field checks, comparison with laboratory results

The pH methods generally follow the outlines above and are mainly dependent on maintaining the appropriate calibration of the pH meters. Handheld XRF analysis does require a degree of care to set up and a more rigorous calibration regime. Figure 4 shows a typical calibration performed at a coalfield comparing the laboratory-derived and in-field derived concentrations of National Institute of Standards and Technology Hard Rock Mine Waste Standard 2780 (NIST 2780). The calibration is performed prior to, during and after each daily analysis campaign to check internal consistency. As Figure 4 shows, with appropriate care, the laboratory and HHXRF concentrations are within 20 per cent of each other for a wide range of elements.

The usefulness of field geochemical testing is illustrated in Figure 5, which compares the results for HHXRF (pXRF in the figure) with laboratory ICP-AES analysis for over 1000 samples from a metalliferous deposit. The strong agreement between the two independent techniques highlights the reproducibility of the HHXRF results in the field. As a bonus, the comparison can then be used to ‘fine tune’ the HHXRF calibration to develop increasingly accurate in-field analysis.

Advantages and disadvantages of field geochemical testing

The most significant advantages of developing geochemical field testing programs are that the methods allow for a rapid ‘first approximation’ of the potential issues at a given mine site and they may be used to provide a reasonably accurate, semi-quantitative analysis of material on-site that may speed up and enhance the decision of which samples to refer for detailed analysis. Taking equipment procurement into account, our experience is that these field techniques cost approximately $10 per sample. Laboratory analysis is typically ten times the cost per sample (depending on the desired tests), so each $10 spent on analysis in the field has the potential to cost more than $100 per sample in the laboratory. While this may not seem significant, in the site characterisation of many larger mine sites, several hundreds of samples (or more) are often required to provide sufficient confidence in the understanding of the longer-term water quality. Obtaining representative samples is a vital component of any geochemical characterisation, since interpretation of results requires adequate and representative samples. As an example, guidance from the Department of Industry Tourism and Resources (DITR, 2006) indicates that the requirement would typically involve several hundred detailed laboratory tests, supported by data that can be used to infer potential acid generation (sulphide or pyrite content) and acid neutralisation capacity (often inferred from the availability of calcium and/or magnesium containing carbonates).

However, field techniques do have some disadvantages, primarily that the results cannot be considered as fully-quantitative. Field geochemistry analysis is no substitute for detailed laboratory testing but where good comparison to laboratory data can be shown, these can substantially increase the number of samples assessed and the confidence in the geochemical assessment. The pHf and pHfox techniques described above should only be used as indicators of which material may cause issues and warrants further investigation. HHXRF testing is limited to elements heavier than sodium in the periodic table and elements up to potassium require careful set-up and calibration to provide consistent analysis. However, with these caveats in mind  we have been able to produce detailed and internally consistent results that for many mine sites have been shown to be a cost-effective compliment to the standard approach to geochemical characterisation of mine sites.

Conclusion

Field analysis techniques, which include field pH (pHf), hydrogen peroxide oxidation pH (pHfox) and handheld XRF analysis of concentrations, are effective screening and validation tools for deleterious mine material and are gaining increased use in the industry. Handheld XRF analyses may be used to provide a compositional screening tool for laboratory testing and may be used in conjunction with field pH tests to develop internal consistency at a particular site. As results are developed, the laboratory testing may be used in turn to ‘fine tune’ the field analysis to allow for greater confidence and consistency in in-field measurements. A number of example sites have demonstrated that field techniques have a wide application across a variety of mines. Geochemical field testing therefore provides a potentially powerful sampling method that may decrease the time spent in laboratory testing, increase confidence in sample selection and identification of units that may pose environmental risks and can reduce geochemical investigation costs for mine operators.

References

Australian and New Zealand Environment and Conservation Council (ANZECC), 2000. Australian and New Zealand guidelines for fresh and marine water quality: Volume 1 – The Guidelines.

Department of Industry, Tourism and Resources, (DITR) 2006. Mine Rehabilitation, Leading Practice Sustainable Development Program for the Mining Industry.

Stone Y and Hopkins G, 1998. Acid Sulfate Soils Planning Guidelines. Published by the Acid Sulfate Soil Management Advisory Committee, Wollongbar, NSW, Australia.

Watling KM, Ahern CR and Hey KM, 2004. Acid Sulfate Soils Field pH tests. In Acid Sulfate Laboratory Methods Guidelines (eds: C R Ahern, A E McElnea, L A Sullivan). Department of Natural Resources, Mines and Energy, Indooroopilly, Queensland, Australia.

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