The application of coal discards for acid mine drainage neutralization Table II AMD solution and coal sample assays Coal sample (%) Constituent AMD1 (mg/L) A B C 3.32 Al 424 9.88 3.89 As <2 NA NA NA Ca 492 0.33 1.00 1.21 Cd <2 NA NA NA Co 2.1 <0.05 <0.05 <0.05 Cr <2 <0.05 <0.05 <0.05 Cu Fe 10.9 6325 <0.05 <0.05 <0.05 K 10.6 0.50 0.02 0.35 Mg 494 103 0.17 <0.05 0.31 <0.05 0.18 Mo <2 NA NA Li <2 Mn Na 79.9 Pb <2 Ni 1.78 NA 0.08 0.98 NA 0.001 0.83 NA <0.05 NA 0.07 <2 <0.05 <0.05 STOT 6455 NA NA Si 90.6 V <2 <0.05 <0.05 <0.05 NA 0.48 2.32 2.33 S2– Ti <0.05 <0.05 NA 1.27 1.13 <2 0.47 0.25 21.6 NA 0.56 9.52 0.20 12.5 SiO23 NA 24.2 3.40 13.0 Al2Si2O5(OH)4 <0.05 <0.05 Zn CaCO32 <0.05 5.64 <0.05 <0.05 NA 47.3 18.6 15.9 Ash NA 70.0 25.3 NA Moisture NA 1.47 3.20 NA Volatiles NA 14.5 23.5 NA 3 Fixed carbon NA 14.4 48.0 NA Notes 1. Initial conditions of the AMD solution: pH 2.5 to pH 2.8 and 392 mV at 15°C 2. Alkalinity expressed as CaCO3 equivalent from acid-base accounting (ABA) method 3. Calculated from Al and Si contents on the assumption that Al occurs only in kaolinite (Al2Si2O5(OH)4) and Si in both kaolinite and quartz (SiO2). NA: not assayed 40% (Lloyd, 2000; Coal Resources, 2001; North, Engelbrecht, and Oboirien, 2015). Sample A can therefore be classified as waste coal. Samples B and C had ash contents of less than 30% and can therefore be classified as power station coal (Cornish, 2016). Samples B and C had a higher degree of alkalinity expressed as CaCO3 (2.3%) compared with sample A (0.48%), which suggests that these two samples should show greater neutralizing capacities. The AMD solution contained elevated concentrations of Fe (6.3 g/L) and (sulphate) sulphur (6.4 g/L), which is in agreement with AMD characteristics (Johnson and Hallberg, 2005). Column neutralization test work The accumulated drainage pH profiles of the 1 m columns are presented in Figure 3. Sample A at the coarser crush sizes (–40 mm and –12.5 mm) did not achieve a drainage pH above 7, whereas the –6.3 mm crush size achieved a drainage pH above 7 for 11 days. Sample B at the coarse (–40 mm) crush size never achieved a drainage pH above 7, whereas the finer crush sizes achieved a drainage pH above 7 for 18 days and 51 days for the –12.5 mm and –6.3 mm crush sizes, respectively. Sample C (–6.3 mm) maintained a pH above 7 for 23 days. The accumulated drainage pH profiles of the 6 m columns are presented in Figure 4. The columns containing sample A maintained an accumulated drainage pH above 7 for 36 days (–40 mm), 39 days (–12.5 mm), and 54 days (–6.3 mm), respectively. The test on sample B –40 mm crush size showed an accumulated drainage pH above 7 for 121 days, whereas the –12.5 mm crush size test continued to maintain an accumulated drainage pH above 7 for 204 days. The neutralizing capacities are summarized in Table III. Sample A neutralized more than 150 L AMD per ton coal at the coarser crush sizes (–40 mm and –12.5 mm) in the 6 m columns, and 258 L AMD per ton coal (6 m columns) to 282 L AMD per ton coal (1 m columns) at the finer crush size (–6.3 mm). For sample B, the capacity increased from 804 L AMD per ton coal at the –40 mm crush size (6 m columns) to 1410 L AMD per ton coal at the –12.5 mm crush size (6 m columns), and as high as 2112 L AMD per ton coal at the –6.3 mm crush size (1 m columns). Lime equivalents ranged from 2.28 kg Ca(OH)2 to 31.7 kg Ca(OH)2 per ton coal. Figure 3—Accumulated drainage pH profiles, 1 m columns ▶ 534 SEPTEMBER 2020 VOLUME 120 The Journal of the Southern African Institute of Mining and Metallurgy