Descripción del área de estudio y datos MERRA2

A global, standard practice for the characterisation of ARD generation potential, known as the “wheel approach” (Morin and Hutt, 1998), caters for laboratory, field based and whole rock geochemical assessments. The common laboratory test used as first indicator of ARD

Didi X Makaula Chapter 7

approach uses extreme oxidants, providing worst case data, but also is carried out in batch, allowing overall neutralisation of acid formed based on neutralising capacity present in the rock in a manner independent of their relative rates. The latter may hide acid forming capacity in the long term (Lapakko, 2002; Hesketh et al., 2010). By providing ‘snapshot’ data rather than a time course, no relative rates of acid consumption and production are indicated. The kinetic leach test provides an advantage when compared to static tests since these account for relative rates and are more representative of the actual field conditions (Bradham and Caruccio, 1990). Commonly used kinetic tests include laboratory-based column tests, humidity cells and field-based test pads (Sapsford et al., 2009). These kinetic tests require longer periods to generate meaningful data (several months to years), making them costly to run, often ~US$ 700-1000s (Broadhurst et al., 2013; Parbhakar-Fox and Lottermoser, 2015) with a delay in data availability to inform appropriate disposal approaches. Furthermore, these tests do not consider the microbial-mineral interaction between the sulfide-bearing waste material and the indigenous iron- and sulfur-oxidising microorganisms that catalyse ARD formation.

To address these shortcomings and complement the existing static and kinetic characterisation tests, a batch biokinetic test was developed by researchers at the University of Cape Town (Hesketh et al., 2010; Broadhurst et al., 2013). The biokinetic test holds several advantages over conventional kinetic tests for measuring the ARD potential, including the delivery of meaningful data regarding the long term ARD generating potential and its kinetics in a relatively short space of time (± 3 months) and being relatively inexpensive to operate (Broadhurst et al., 2013). The configuration of the batch biokinetic test does not, however, represent the typical fluid contacting mechanism in the waste rock dump nor does it consider the washout of neutralising capacity in a flow-through system, such as a waste rock dump, where the kinetics of acid neutralisation and acid generation may differ.

Refinement of the batch biokinetic test to develop a flow-through lab-scale ARD characterisation test is desired to remove the limitations encountered in batch conditions and allow assessment of the mineral-microbe interactions within the waste rock. Microbial activity and the association and colonisation of the mineral surface by the microorganisms plays a key role in the generation of ARD from waste rock dumps.

7.3 Research approach

The mixed mesophilic culture (Section 3.2) was used in this study. Two pyrite bearing waste rock samples were used in this study: PEL-HS and PEL-LS (Section 3.3.2), coated onto glass beads (Section 3.3). The flow-through column reactor system was set-up and operated as

described in Section 3.5.2 and 3.5.3 Solution chemistry analysis was conducted, including measurement of pH, redox potential and Fe (Section 3.6). Microbial coverage analysis via the detachment method was conducted (Section 3.7), microbial distribution and growth on the mineral surface was analysed using SEM visualization (Section 3.8) and microbial activity measurement was conducted using IMC (Section 3.9).

7.3.1 Static tests

The static tests were conducted on both pyrite-bearing waste rocks using acid-base accounting (ABA) and net acid generation (NAG) tests. The ABA test, conducted according to Smart et al. (2002), measures the net acid producing potential (NAPP), which represents the balance between maximum potential acidity (MPA) and the acid neutralising capacity (ANC). The measured values were expressed as kg H2SO4 tonne-1 of solid waste rock. A negative

NAPP indicates that the sample had sufficient ANC to counterbalance acid production. Similarly, when the MPA is greater than the ANC value and the NAPP was positive, this means that the sample is acid generating. The NAG test was determined according to Stewart et al. (2006) and allows both acid forming and neutralising reactions to occur simultaneously. These tests were performed in triplicate.

7.3.1.1 Acid neutralising capacity (ANC) test

ANC was determined by adding a known volume of hydrochloric acid (HCl) to a 250 ml shake flask containing 2 g of -75 µm pulverised sample. Each sample was allowed to react at 90 °C for a maximum of 2 hours. The mixture was cooled and back-titrated with a standardised solution of sodium hydroxide (NaOH) to determine the quantity of HCl reacted, according to Smart et al. (2002). A detailed procedure is found in Appendix G.

7.3.1.2 Maximum potential acidity (MPA) test

MPA, given by (Equation 7.1), quantifies the total potential for acid generation from the total sulfur content of the sample (in wt %), with the assumption that all sulfur exists as pyrite and that sulfides completely oxidize to form sulfuric acid, hence requiring a conversion factor of 30.6 (Weber et al., 2005).

𝑀𝑃𝐴 𝑘𝑔 𝐻2𝑆𝑂4 𝑡−1= (total S %) × 30.6 Equation 7.1

7.3.1.3 Net acid generation (NAG) test

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simultaneous acid generation and neutralisation, with outcomes being the net acid generated by the sample (Stewart et al., 2006).

A 2.5 g aliquot of each milled waste rock sample was weighed and transferred into a 500 ml shake flask. To this, 250 ml 15 % H2O2 solution was added. This solution was corrected to a

pH between 4.5 and 5.0 using a 2 M NaOH solution. The mass of the flask was recorded and allowed to react overnight. After 24 hours, the flasks were boiled for two hours where minimal effervescence was observed. Thereafter, dH2O was used to correct the mass of the flask to

its original weight, accounting for evaporation. The sample was cooled and allowed to settle for one hour. Prior to filtering, the mass of the filter paper was recorded. Both the filtrate and the filter cake were recovered. The filtrate was titrated against a 0.1 M NaOH solution to a pH of, firstly, 4.5 and, secondly, 7.0, with the volumes of NaOH used being recorded. The filter cake was recovered and once again reacted with a 15 % H2O2 solution, with the mass of

sample lost, i.e. as unrecoverable from the filter paper, being recorded. This was repeated until the filtrate pH was pH 4.5 or more. The NAG values for each titration were calculated.

𝑁𝐴𝐺 =

W Equation 7.2

where V is the total volume of 0.1 M NaOH used in each titration step (ml), M is the molarity of the NaOH solution used and W is the mass (g) of ore samples used in each reaction step. 7.3.2 Biokinetic tests

7.3.2.1 Batch biokinetic tests

Biokinetic experiments were conducted in shake flasks containing 0K media (pH 1.6) supplemented with 0.5 g L-1 _Fe2+_{, as FeSO}

4 7H2O, and incubated at 30°C. The biotic

conditions were inoculated with 108 _{cells ml}-1_{. Two approaches were used for the flask}

biokinetic experiments: (i) mineral slurry and (ii) mineral coated glass beads. For the slurry, 7.5 g milled mineral was added to each flask containing a total working volume of 150 ml (0.05 g ml-1_{). For the mineral coated beads, 100 beads of 6 mm diameter were coated with the}

respective mineral waste rock (3.6 g; Section 3.3) and added to each flask with a total working volume of 100 mL (~0.036 g ml-1_{). Prior to inoculation, both slurry and waste rock coated beads}

were washed and conditioned with 0K media (pH 1.6) for 24 hours to initiate neutralising reactions. After 24 hours, the pH in the respective shake flasks was re-adjusted to pH 1.6 using concentrated H2SO4.

7.3.2.2 Flow-through biokinetic test

The columns were inoculated under saturation conditions using an upward flow of 100 ml 0 K media supplemented with 1010 _{mixed mesophilic microbial cells kg}-1 _{of ore and 0.5 g L} -1 _Fe2+

as FeSO4 7H2O in a closed circuit. The inoculum suspension was recycled for 18 h to allow

microbe-mineral contacting. Thereafter, the columns were drained, and the liquid fraction collected. A continuous downward flow of sterile fresh 0 K media (pH 1.6) supplemented with 0.5 g L -1 _Fe2+ _(FeSO

4 7H2O) was introduced and operated until the end of the experimental

run.