Gold cyanidation effluent often contains high levels of cyanide and metals. While removal in synthetic water is well demonstrated, real process water introduces complexities that need further study. Batch tests with process waters from three Peruvian sites evaluated an electrochemical approach for contaminant removal. Cyanide destruction was two orders of magnitude slower in real water, and copper removal varied with water composition. The results underscore the need to understand process water chemistry to create a reliable treatment design framework. Additional research is essential for optimizing cyanide and metal removal in practical applications.

Mining activities are ancient in Portugal, dating back to the Pre-Roman and Roman periods. They played an important role in the Portuguese economy of the 19^th and 20^th centuries associated mainly with polymetallic sulphides in the south, and uranium minerals in the north. In the second half of the twentieth century, almost all the mining activities were suspended and consequently many mine sites were left untreated. The fact that much of Portugal mineralization is associated with sulphide minerals provided great potential for acid mine drainage (AMD) production and an estimate of about 14 % of the Portuguese mines were found to generate acidic waters. São Domingos mine, a deposit of polymetallic sulphides explored mainly for copper, located in Southeast Portugal in the Iberian Pyrite Belt, is an example of a serious problem resulting from the existence of AMD. The inactive open-pit from the mining operations resulted in the creation of a large pit-lake with acid water with pH close to 2 containing high concentrations of sulphate and heavy metals (mainly aluminium, iron, zinc and copper). The contamination is not only confined to the pit-lake since the downstream area is also affected and its impact extends to several water bodies located nearby.

The problems associated to AMD in Portugal and in São Domingos in particular were the driving force that stimulated the studies of our research group for the development of alternative treatment processes based on the use of Sulphate-Reducing Bacteria (SRB), since traditional neutralization do not address the reduction of sulfate, are expensive and result in the formation of high volumes of sludge. Moreover, the use of biologic techniques offers several opportunities, such as the possibility to recover the metals, namely as metal sulphide nanoparticles and nanocomposites with functional applications.

Since the first studies several carbon sources have been tested, mainly waste locally available, such as wine waste that have been very efficient. In addition to the characterization of the consortia and their dynamics, other optimizations were introduced to the bioremediation systems aiming to improve the efficiency of the process and the quality of the effluents for irrigation purposes.

As a result, a pilot plant has been installed on-situ at São Domingos mine. Presently, the characterization and potential use of acidophilic SRB consortia are under investigation and new projects aiming at their utilization and the eventual co-treatment with other waste are envisaged, expecting to bring new challenges and opportunities.

The mine water levels in large parts of the Ruhr region are currently rising to an optimized level, among other things with the aim of reducing the number of dewatering locations. This offers the opportunity to convert the sites to modern submersible pumps wells. These developments also place new demands on numerical modelling. A method was therefore developed to be able to depict the pump management typical for well pumping stations using numerical modelling with the established box model.

Submersible pumps wells are characterized by only a few pumps, each with a comparatively high flow rate. This means that, depending on the water inflow and the void volume available as storage, the water levels rise and fall depending on the number of active pumps. Stationary water levels, such as those maintained by underground pumping systems, will hardly exist in the future. For the planning of pump levels, pump design and water level variations to be applied for, predictions of the expected rise and lowering behaviour under realistic operating conditions are required.

To date, only a few pumping scenarios have been taken into account in the box model. A distinction was made between pumps that keep the water level at a certain level and pumps with a predetermined constant pumping rate. However, in order to model well water management, it is necessary to be able to switch between different pumping rates at short notice and to comply with upper and lower water level limits.

In addition to these internal mine factors, external conditions also influence future pumping management. The most important parameter in planning is the mixed water concentrations that arise in the receiving waters. For this, the pre-loading of the mine water and the discharge quantities must be forecast as accurately as possible. Many mine drainage systems also have seasonally fluctuating inflow rates, which show a climatic connection. The receiving waters also have these seasonal variations, albeit at a different time than the mine water, which increases the risk of limit values in the receiving waters being exceeded, especially in the summer months. By integrating pumping strategies adapted to these conditions in the box model, well-founded forecasts for discharge rates and loads can be made that also take seasonal requirements into account.

Sedimentation pond facilities are integrally associated with the treatment of mine water characterized by Suspended Solid (SS) content. Design criteria for sedimentation ponds are typically determined utilizing Stokes' law with an estimated particle settling velocity or by assuming particle-specific gravity. However, under actual conditions, particle characteristics may vary depending on the location. Finer sediments, such as colloidal clay (<2 µm), frequently deviate from Stokes' law due to inter-particle interactions and agglomerative behaviour. Consequently, additional data, including particle size and settling velocity, are necessary to establish design criteria that accurately represent the specific conditions of each site.

This investigation focuses on settling tests of mine water with existing water quality characteristics. Six water samples from diverse locations were analyzed for physical parameters including pH, Oxidation Reduction Potential (ORP), Total Suspended Solid (TSS), and Iron and Manganese Metals (both total and dissolved). Discrete particle sedimentation tests, jar tests utilizing lime or aluminum sulfate, and floc particle sedimentation tests were conducted for each water sample. Column settling tests with a water level of 100 cm and a sampling port at a depth of 80 cm were employed to assess the settling rate of discrete particles from various sites.

This study has identified that natural gravity settling (without the addition of chemicals) can occur for water with acidic characteristics, with pH values ranging from 3.16 to 4.05. In acidic water, up to 57.4% of SS removal can be achieved within 50 minutes as the zeta potential approaches zero. Conversely, neutral pH water required chemical coagulation, with optimum doses ranging from 20 to 200 mg/L, contingent upon the initial water characteristics. Furthermore, the settling test was conducted subsequent to jar testing, with sampling ports at 20 cm depth intervals and measurements taken at 15-minute intervals over 60 minutes. Iso-removal curves were utilized to determine parameters such as detention time and overflow rate. Additionally, iron and manganese concentrations post-chemical treatment were evaluated.

This research is anticipated to serve as a reference for sedimentation pond design strategies, addressing both dimensional considerations and the selection of chemicals using a non-Stokes' law approach.