Oxygen Consumption Tests

Download our Rapid Kinetic Geochemical Testing for AMD Prediction – OxCon and OPT Capability Statement.

Oxygen consumption tests can be used to directly measure the sulfide oxidation rate of a material, overcoming the limitations of column leach and humidity cell tests. The key benefits of this form of kinetic test work are:

  • Rapid determination of oxidation rates (usually 1–6 weeks) compared with column leach and humidity cell tests, which usually take many months to years.
  • Lower cost relative to other techniques due to significantly lower analytical costs and shorter test durations.
  • Vastly improved accuracy of sulfide oxidation rates by direct measurement of oxygen concentration decrease as an indication of sulfide oxidation.
  • Testing under field or specific test conditions in terms of moisture content, temperature, solar radiation, particle size, cover materials, temperature, and more.
  • Small sample size (typically 2–5 kg) and suitability for scaling to larger or smaller samples with relative ease.
  • The oxidation rate is reported as a reactive sulfide ‘half-life’ and annualised Pyrite Oxidation Rate (POR), as well as various other conventional units, which allows the result to be applied directly to bulk average materials — a result with direct management applications.

Earth Systems has developed an advanced, rapid oxygen consumption test technology called OxCon for the accurate estimation of sulfide oxidation rates and acidity generation rates for sulfidic geological materials including waste rock, tailings and wall rock. Now routinely conducted by ESAnanlytical this technology is used to conduct two kinetic tests:

OxCon Rapid Oxygen Consumption Cell Test

This testwork is ideal for accurate and rapid assessment of sulfide oxidation rates of waste rock and tailings materials. Most sample sizes can be accommodated.

  • Determination of intrinsic sulfide oxidation rate, reactive lifetime of materials and the rate of ANC consumption under simulated site conditions.

OxCon Oxygen Penetration Test (OPT)

It is clear from most tailings deposits that sulfide oxidation is limited to the upper portion of the deposit only. To fully understand the pollution generation from tailings, we therefore need their oxidation rate as well as determining the full depth of oxidation (ie. the oxygen penetration depth). Beyond this depth, tailings can remain unsaturated but completely inert due to the lack of any oxygen (ie. nitrogen pore gas only).

  • Determination of intrinsic sulfide oxidation rate, reactive lifetime of materials and the rate of ANC consumption,
    PLUS
  • Depth of oxidation penetration and the effectiveness of cover materials at full scale under simulated site conditions.

OxCon Wallrock Oxygen Consumption

Direct measurement of the sulfide oxidation rate of coherent slabs of different lithologies (of measured surface area) that occur as wallrock materials in open pits. Sulfide oxidation rates are reported as a function of surface area and normalised to 1 wt% pyrite to permit accurate pollution assessment from pit walls.

  • Determination of intrinsic sulfide oxidation rate, reactive lifetime of materials and the rate of ANC consumption,
    PLUS
  • Sulfide oxidation rate as a function of surface area (m2) under simulated site conditions.

All of these tests both determine the intrinsic rate of oxidation for sulfidic materials by direct measurement of oxygen consumption by the material over time. In combination with the detailed mineralogy and geochemistry of the sample, these tests can provide:

  • Annual rate of acidity generation (per kg, per m2 or wt% FeS2 – depending on material),
  • The reactive lifetime of the material,
  • Timing and magnitude of peak acidity,
  • Rate of ANC consumption
  • ‘Lag time’ or onset to acid conditions,
  • The thickness of ‘reactive zones’,
  • Cover system performance, and
  • Geochemical performance of closure strategies.

All within a short 1 to 6 week time frame.

OxCon testwork can be conducted over a broad range of oxygen and moisture concentrations, making it possible to define the sulfide oxidation rates over a range of conditions.  This is of particular importance in some sulfidic waste rock and tailings facilities where sub-atmospheric oxygen concentrations and variable moisture contents are routinely recorded.

Using the OPT tests it is now possible for the first time to directly test the geochemical performance of proposed closure strategies such as covers for waste rock dumps and tailings storage facilities, replacing or augmenting conventional unsaturated zone modelling. As better covers result in thinner ‘reactive zones’, the relative performance of different strategies can be compared directly under carefully controlled conditions, and the thickness, compaction and target moisture content of each cover component can be optimised to minimise cost and maximise long-term geochemical stability — with significant opportunities for cost savings in construction.

Test-work Protocols

Prior to testing, representative samples of sulfidic material are analysed for numerous relevant parameters including sulfur speciation, carbon speciation, acid-base accounting, the NAG (net acid generation) suite, major and trace element chemistry by XRF, modal mineralogy by XRD, acid buffering characteristic curve and moisture content using standardised tests at NATA accredited laboratories.

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Waste Rock sample

Tailings sample

Wallrock sample

OxCon Rapid Oxygen Consumption Cell Test

The OxCon apparatus (shown) operates by permitting a known mass of well characterised sulfide bearing material to react with a known volume of oxygen inside a sealed vessel.  Once testwork is instigated, oxygen consumption proceeds via sulfide oxidation as indicated in Reaction 1 below.  The decrease in oxygen concentration is measured, along with other key gases, such as carbon dioxide, which is often released during the decomposition of carbonate (eg. calcite, dolomite siderite) minerals.

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OxCon sample vessels.

FeS2  +  3.75 O2  +  3.5 H2O  =  Fe(OH)3  +  2 H2SO4                                                                                 Reaction 1

The measured oxygen consumption rate is assumed to be proportional to the mass of pyrite in the sample and is converted into a sulfide oxidation rate using the stoichiometry in the reaction above.
The effects of CO2 gas release during OxCon testwork is quantitatively accounted for in the calculation of FeS2 oxidation rates as it can have the effect of diluting the measured oxygen concentration.

OxCon Oxygen Penetration Test (OPT)

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OPT test columns

Vertical hermetically sealed vessels of 1-3 metres or more of sulfidic materials, with or without full-scale cover systems can be tested in the laboratory under simulated field conditions. Gas sensors directly measure gas consumption / production at intervals along the vertical section providing direct measurements of level of oxygen penetration and the gas composition with depth.  From this the thickness of the reactive zone can be determined along with the other outputs from the conventional OxCon Rapid Oxygen Consumption Cell Test (eg. oxygen consumption rate, pyrite oxidation rate, acidity generation rate).

OPT

OxCon Wallrock Oxygen Consumption

Coherent slabs of wallrock from open pits with known surface area are sealed in an OxCon apparatus. The decrease in oxygen concentration is measured, along with other key gases, such as carbon dioxide. The measured oxygen consumption rate is assumed to be proportional to the mass of pyrite in the sample and is converted into a sulfide oxidation rate using the stoichiometry in Reaction 1 above. Sulfide oxidation rates are then reported as a function of surface area and normalised to 1 wt% pyrite to permit pollution assessment from pit walls.

Leachate Analysis

Once OxCon testwork is completed, the sample is bottle-rolled with deionized water and the leachate analysed for the key mobile elements at NATA accredited laboratories.

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Bottle-roll leachate tests.

Data Outputs and Reporting

Data collected from the tests is pulled together in a standardised graphical output that puts key management metrics at your fingertips, clearly showing:

  • Sample mineralogy,
  • Acid-base accounting tests,
  • Net acid generation,
  • Sulfur speciation,
  • AMD / ARD – Neutral Mine Drainage (NMD)/ Metal Leaching (ML) – Salinity  risks,
  • Acid buffering Characteristic Curves (ABCC),
  • Oxygen consumption,
  • Pyrite oxidation,
  • Net acid generation rates,
  • Estimated acid-free ‘lag’ period before the onset of acid conditions,
  • Estimated peak net acid generation rate,
  • Estimated half-life of reactive sulfides,
  • Estimated longevity of acid generation, and
  • Detailed leachate chemistry.

Earth Systems reports sulfide oxidation rate results in units of weight percent of pyrite (exposed to atmospheric oxygen) that is converted to sulfuric acid per year (wt.% FeS2 converted / year).  This is a convenient unit that facilitates the rapid determination of acidity generation data for individual heap leach piles or entire mine sites including sulfidic wastes.  Alternatively, OxCon data can be reported as conventional kg O2 / tonne / sec or kg S / tonne / week.

Reporting is designed to provide data in a usable, ready to use format.

Download an example report:

Contact Earth Systems for more information and to find out how OxCon Kinetic test work can assist with the ongoing management of sulfidic materials a development of effective closure strategies.