Applicazione delle tecniche elettrosmotiche alla consolidazione dei fanghi di risulta dei processi mineralurgici

Mining activity carried out in Sardinia between 1850 and 1990 has left its mark on the environment in the form of considerable and numerous sources of pollution. One of the most important problems is linked to the tailing basins which represent around 40% of the mining waste in the territory. The risk these pose is related to the huge volume of the deposited material, to its chemical and mineralogical characteristics (high concentrations of heavy metals) and to its high water content. Most of the tailing dams were built before 1980 and they are not really dams in the conventional water-retaining sense. They are more akin to a thick facing or veneer at the front of a waste dump so have no impermeable barrier preventing leakage of contaminated water. These, therefore, represent sources of continual contamination for the surrounding soil and groundwater. Remediation interventions aimed to remove the contamination sources or to reduce the concentration of contaminants in the wastes to below the acceptable limit (CLA) are extremely costly and unsustainable due to the high volume of material. A more suitable solution could be, instead, the isolation of the contamination sources. To this regard, waterproofing basins (both the bottom and the sides) is an aspect of primary importance. Among the different waterproofing techniques used in other fields, the most common are those based on clogging the soil pores by injecting them with sealing materials (jet-grouting and chemical or cement injections). The electroosmotic technique offers an alternative approach by which it is possible to produce the quick consolidation of the slurry contained in the basins and the reduction of its hydraulic conductivity. The research into the electroosmosis phenomenon started from the hypothesis that its application could reduce the consolidation time of the slurry material typically deposited in the tiling dams. The technique can be adopted to realize barriers of consolidated and low permeability material along the perimeters and bottom of disused basin while in active basins the quick consolidation of the slurry could make it possible to accumulate a greater mass of tails and to increase the daily amount brought to the basin. The effectiveness of the technique has been confirmed by experimenting on a tail material deriving from the floatation of lead and zing mineral. An experimental apparatus was built consisting of a cylindrical cell with a variable volume, able to reproduce the consolidation process, both under a static load (static consolidation) and a static load plus an electric field (combined consolidation). The research showed that the application of the electric gradient of 1V/cm reduces the consolidation time by about 80%. At the end of consolidation, the deformation of the sample is about 33% in the case of combined consolidation, and 22% in that of only the static load. The electroosmotic process, therefore, on the one hand nearly halves the consolidation time and on the other reduces the volume of the slurry by about 33%. The measurement of the permeability carried out at the end of each test demonstrated that the combined consolidation reduces the slurry’s permeability coefficient by about two orders of magnitude (from 1.02•10ˉ6 m sˉ¹ to 1•10ˉ8 m sˉ¹). The method is then suitable for the realization of impermeable barriers on the inside of disused tailing dams which need to be permanently secured and for accelerating the consolidation of the tail in active basin where, as a consequence, a greater amount of tail can be accumulated and a greater flow rate can be adopted.