Assessment of blended waste rock material at Zinifex Century Mine: consequences for acid drainage generation

There is little doubt that acid rock drainage (ARD) is the largest and most testing long-term environmental issue facing the global minerals industry (Lawrence and Day, 1997, Munchenberg, 1998, Olson et al. 2006). The aim of this study was to assess leachate quality from different blends of waste rock at Zinifex Century Mine and determine consequences for acid drainage generation. This thesis describes the results of research undertaken to predict the risk of ARD associated with a possible change to waste rock disposal practices at the Zinifex Century Mine. Zinifex Century Mine Limited (ZCML) comprises a zinc, lead and silver mining and milling operation at Lawn Hill in northwest Queensland, concentrate dewatering and shipping facilities at Karumba in the Gulf of Carpentaria, and a 304km slurry pipeline connecting the two operations. Similar to other mines where sulphide minerals are present, ARD may be generated at the ZCML mine site from exposed pit surfaces, ore stockpiles, removed waste rock and deposited tailings (Bates et al. 2000). To minimise the generation of ARD in the waste rock dumps, waste rock is classified into the following three classes based on competence and acid forming / consuming capabilities: • Class 1: Competent rock, non-acid forming or acid-consuming material. • Class 2: Non-competent, non-acid forming or acid-consuming material • Class 3: Acid-forming material. The generalised waste rock dump design for current operations comprises an outer zone of class 1 material and inner zone containing class 2 and 3 materials. Cambrian Limestone (CLS), a class 1 material, is used for its structural and acid neutralising capabilities in waste rock dump construction. Current mine waste rock placement procedures specify that class 1 rock with greater than 5% contamination of class 3 material is to be placed within the inner zone of the waste rock dumps due to uncertainty surrounding the long-term acid producing capabilities of this rock. This procedure, as well as additional restrictions on waste rock dump design, has the potential to cause significant scheduling problems due to the limited reserve of class 1 rock available for waste rock dump rehabilitation. This is particularly the case in the latter stages of the mine life following movement of most of the waste rock to access the ore. Due to finite reserves of CLS material and the need to develop strategies to maximise the beneficial use of available limestone reserves, as well as the belief by mine personnel that the abovementioned contamination percentage may be conservative, this project was commissioned to enable the leachate from different blends of waste rock to be assessed. The objectives of the study were to: • Quantify blending levels of acid consuming (Cambrian Limestone) and acid forming (Hanging Wall Siltstone (HWD)) rock that will produce pH neutral leachate low in metal and salt concentrations; • Determine the validity of current waste rock placement procedures; • Investigate the influence of particle size on leachate quality; and • Attempt to establish links between results from various ARD prediction tests. Various static (acid base accounting, net acid generation tests, acid buffering characteristic curves and cyclic voltammetry) and kinetic (column leach tests (CLTs) and heap leach pads (HLPs)) ARD prediction tests were conducted on blended waste rock material from ZCML. Twenty CLTs, representing five lithology blends and three particle size distributions were assembled in a laboratory environment. Five replicate CLTs were established. Three HLPs representing three lithology blends were constructed from run of mine material at ZCML. The CLTs and HLPs were periodically watered to simulate rainfall events. In addition, the HLPs were exposed to wet season rainfall events. Results from the static and kinetic tests were compared to address the issue of scale-up. Findings from the study were: • Static tests conducted on blended CLS and HWD samples classified each sample as non acid forming (NAF). Results from kinetic testwork confirmed this finding, indicating that limestone blending may be effective in controlling the pH of leachate generated from waste rock, however elevated sulphate concentrations in leachate would ensue. • Results obtained from the column leach tests were repeatable. • Sulfate production rates were equivalent to neutralising potential depletion rates in most CLTs and HLPs. This confirmed neutralisation of the sulfuric acid produced by pyrite oxidation by calcium and magnesium carbonates. • The calculated time to NP depletion exceeded the time to sulfide depletion in all CLTs and HLPs, however these times were misleading for columns containing armoured CLS material. • Despite first flush events, total metal concentrations in CLT and HLP leachate generally complied with maximum limits specified in Environmental Authority No. MIM800020402 (Ecoaccess, 2004). • Dissolved zinc concentrations were significantly greater in CLT samples with low pH values, however were not as high as thermodynamically predicted using the geochemical modelling program MINTEQA2. Dissolved zinc concentrations in HLP leachate were also not as high as thermodynamically predicted at pH values less than 7.8. This was due to the control of dissolved zinc concentrations by the rate of sphalerite oxidation at low pH values and the solubility of Zn(OH)2 at high pH values. • Dissolved lead concentrations in CLT and HLP samples were extremely small or undetectable (i.e. less than 4.8x10-4μmol L-1) and less than those thermodynamically predicted using MINTEQA2 due to low oxidation rates of galena resulting from surface passivation at lower pH values, and Pb(OH)2 solubility controlling Pb2+ concentrations at neutral-alkaline pH values. • Median dissolved zinc and copper production rates were comparable between the CLTs and HLPs, however rates were greater in the HLPs during large wet season flushes. • Geochemical modelling confirmed the absence of gypsum precipitation in the CLT comprising the most reactive waste rock blend and smallest particle size distribution and confirmed gypsum precipitation in HLP2 and HLP3. Sulfate production, neutralising potential depletion and oxygen consumption rates were therefore underestimated in these HLPs. The main conclusions of the study were: (1) Results from CLTs comprising smaller particle size distributions were most comparable with results from the HLPs. Anomalous behaviour was observed in CLTs comprising larger particle size distributions. (2) There was no apparent benefit in leaving the HLPs unwatered for periods of time greater than one week. (3) Oxygen consumption rates for CLT samples were significantly slower and consequently not directly comparable to those for the HLPs, despite greater flushing of the CLTs. (4) Blending CLS and HWD material was effective in maintaining neutral pH values and regulatorycompliant total metal concentrations in drainage from CLTs and HLPs comprising blends up to 75%CLS / 25%HWD. However, sulfate concentrations in drainage from all blended samples exceeded current regulatory discharge limits. (5) Visual inspection of material comprising the HLPs after excavation highlighted the presence of armouring layers on HWD material. (6) The effectiveness of CLS in neutralising sulfuric acid increased with decreasing particle size. Corresponding recommendations were: (1) Construct CLTs from particle size distributions passing 10mm as a maximum to best emulate the particle size distributions and residence times in waste rock dumps. (2) For future HLP testwork at ZCML, or sites of similar climatic conditions, accelerate the rate of pyrite oxidation in the HLPs by watering at weekly intervals on a continual basis in the dry season (i.e. no prolonged drought period). (3) For future CLT testwork for sites with distinct seasonal variations, locate the CLTs in field conditions on the site under investigation, to ensure samples are exposed to similar ambient temperatures and humidity. Alternatively, where this is not possible, site temperature and humidity conditions should be simulated in a laboratory environment. (4) To prevent a potential increase in sulfate concentrations in WRD discharge, the current 5% contamination limit of class 3 in class 1 material should not be reduced. (5) Where there is insufficient CLS material available to cover both the dump surface and batters, priority should be given to the WRD surface, with other non-acid forming competent material used on the batters. (6) Investigate the reduction in particle size of class 1 material placed within / on the WRDs