Sekundäre Prozesse in Bergbauhalden und Aufbereitungsabgängen - Steuerung von Stoffmobilität und Krustenbildung

Tailings from mining of sulfidic ores cause a number of environmental hazards, such as Acid Mine Drainage and dust emissions. Supporting hardpan formation represents an effective method of controlling such pollution. A hardpan is a method of passive in-situ treatment. Hardpans are able to reduce the discharge of heavy metals. Of great importance is the water economy within a dump. If the permeability of the tailings is high together with high amounts of rain water, the water flow inside a dump is downwards-directed. Consequently, contaminants flow out together with the seepage. In the case of evaporation-oriented conditions porewater solutions rise due to the presence of capillary forces and evaporate at the dump surface, leading to the precipitation of secondary phases. The secondary processes are subject to the influence of climatic conditions. In this project the natural conditions prevailing in dumps were simulated on a laboratory scale. To gain insight into the process of hardpan formation two dumps in the Freiberg mining district (Erzgebirge, FRG) having an age of 40 years were examined by drilling. The concentrations of several metals are enriched in the surface zones of the dumps. A surface roughness factor, calculated from the specific surface areas of the components, served as an indicator for the extent of these secondary processes. Bacteria, influence the metal mobility. It was found that Thiobacilli, such as T. ferrooxidans and T. thiooxidans, increase the mobility of zinc and copper, but reduce the mobility of iron due to the increased rate of oxidation. These bacteria therefore promote the hardpan formation, although the pH which they induce becomes lower. The encapsulation of components in secondarily formed precipitates reduce the accessibility of the components and immobilizes metals. The mainly sulfidic tailings used were investigated with standard leaching tests. Nevertheless, newly developed column tests are able to extensively characterize tailings. The evaporation on the open column surface generated an upward-directed capillary water flow which best simulated natural conditions. The column tests were performed to describe and quantify the controlling parameters for hardpan formation. Experiments were performed to elucidate their reproducibility, the time dependence of the processes taking place, the mobility of sulfidic ore concentrates, and the hardpan formation potential of original tailings. Of special interest was the climatic dependence of the hardpan formation. Because of grain size differences within and differences in the exposure to air movement, the hardpan thickness depends on its position on the dump. On increasing the air movement, an increase in hardpan formation could be observed. This growth was limited, because the capillary transport in the columns was interrupted if the evaporation rate, increased by the air movement, became too high. Increased temperature also led to an increase in the evaporation rate. This increased the upwards-directed capillary flow until reaching a certain point, at which the column surface dried out because the water requirement exceeded the transport capacity of the capillaries. Besides the total amount of rain water, its temporal distribution is also a controlling factor for element mobility and hardpan formation. In the case of continuous precipitation from rain, no superficial hardpan will form, because the waterflow is downwards-directed. Contaminants are discharged along with seepage. If rain precipitation events alternate with periods of dryness, changes between downwards-directed water flow and upwards-directed capillary flow occur. In periods of dryness evaporation predominates and a hardpan can form. Since the accessibility of the components is reduced, the freshly formed hardpan will not be dissolved entirely by new rain precipitation events. The depth at which a hardpan is located is influenced, in addition to neutralization and redox reactions, by the evaporation rate as well. If the rate of evaporation is too high, the capillary transport will be interrupted and mobile metals will not reach the surface of the dump. Instead, secondary phases will precipitate at a deeper level of hardpan formation which is still supplied by the capillaries. Besides evaporites, several partially metastable sulfates have formed in the hardpans of the columns. Mobile metals are not only transported as real solutions, but also as colloids which can not be thermodynamically entirely described. Moreover, the investigations have shown that not only oxidized, but also reduced or not entirely oxidized heavy metals were transported in the porewater. Mine dumps affected by hardpan formation, which leads to the enrichment of metals, may act as deposits again.