The High Density Sludge ( HDS ) Process and Sulphate Control

The high-density sludge (HDS) process has been the standard in the mining industry for treating metal-containing acid rock drainage (ARD), providing low metal levels in the final effluent, and reducing waste sludge volumes compared to other lime based processes. It has been applied for 40 years at mine sites all over the world to meet discharge limits for metals and pH. Recently, various jurisdictions have introduced low discharge limits for sulphate concentrations that are difficult to meet with conventional treatment systems. Also, when treated water is recycled, high sulphate concentrations can cause some undesired downstream impacts. To assist in reducing effluent sulphate concentrations, the recent trend shows that HDS plants have been constructed with considerably higher retentions times in the neutralisation reactors. The reason for this increase in retention time is due primarily to the slow gypsum-precipitation reactions that are the key to minimising sulphate concentrations. These reactions and their effects are explained in detail in this paper. Also discussed are the potential synergies between the HDS process and other processes that treat sulphate more directly. This includes the combination with membrane filtration (reverse osmosis or nanofiltration), with ettringite precipitation, with ion exchange, and even barium precipitation.