Trattamento di stabilizzazione-solidificazione mediante leganti idraulici e additivi applicato agli sterili mineralurgici: il caso di Masua

Environmental issues related to abandoned mining sites are an emergency that finds many examples around the world, with serious adverse effects, both on the people living in those territories, either as impact on the surrounding environment, with serious damage to natural resources. Sardinia is among the Italian regions where mining work had a key role in economic development, and was one of the most important site in Western Europe for high productivity. Nowadays, the legacy left by the mining activity is a valuable historical and cultural heritage that needs to be valorized through the recovery and redevelopment of disused mining areas. However, it is known that mining activity produce a serious environmental impact, sometimes altering permanently the ecosystem, the morphology of the territory, the hydrological and hydrogeological structures of large areas. Furthermore, among the long period effects, there are problems related to instability of the cultivation sites and of the tailing dams(e.g. landfills, settling ponds).The biggest impact is certainly represented by the dispersion of highly toxic contaminants, such as heavy metals, that are concentrated especially in mine tailings. These constitute an important source of environmental pollution, which can have serious impact on soil and water resources of vast land areas, so as to constitute, in some cases, a risk to human health. In the context mentioned above , fits this research work, that while studying a particular case, constitutes a representative model of specific problems associated with abandoned mining areas. The object is the mining site of Masua, located in South-West of Sardinia (Sulcis-Iglesiente): This area has an elevate natural value and a great tourist potential. In the past, the site was the most important in Europe for the production of lead and zinc. Inside are two big settling ponds of tailings, arising from the processes of flotation of minerals mined in the surrounding areas. The volume of wastes amounts to about 2.5 Mm3 and the occupied surface is 12 hectares. The aim of research is the treatment of the tailings of Masua, by the application of stabilization-solidification techniques, using hydraulic binders and different additives, which, mixed with the waste, allow to minimize the mobility of contaminants. The material is characterized by high concentration of cadmium, lead and zinc. The leachate reports high content of lead, above the limits of Italian Law concerning inert wastes. Among the possible reuses of the treated material there is the backfill of mining voids. These are characterized by stability problems, processes of superficial subsidence and collapse of the cultivation halls. Stabilization/solidification techniques using hydraulic binders are an ideal operational strategy to achieve the twin objectives: make the waste less hazardous and reuse of the treated material in operations of mine backfilling. Binders, like cement, lime and fly ash, give a solid structure and the physical properties required for the operations of backfilling. Moreover, the solidification process reduces the mobility of metal contaminants through different processes, such as precipitation, adsorption, physical encapsulation, that take place between the pollutants and constituents of the solid matrix. An important aspect is that the reclamation should be economically sustainable. The hydraulic binder, generally, constitutes about 75% of the cost of the entire operation of backfilling. Therefore, it is essential minimize the proportion of this component in the constitution of the mixtures, without, however, reducing physical-mechanical properties, nor the efficiencies of inertization. Low contents of Portland cement, between 3% and 5% by weight, such as those tested in the present study, allow the development of a sufficient cohesive strength and give to the filling mass the self-supporting capacity. However, low percentages of binders, may not be enough to ensure the appropriate efficiencies of stabilization of the waste. To solve this problem, It was used a small percentage of certain additives, which, according to their properties, reduce the mobility of contaminants. In particular, have been employed two types of clays (bentonite and a montmorillonite clay). It 'well known that such minerals have high adsorption capacity towards heavy metals. Moreover, in strongly alcaline conditions, such as those that are established during the hydration of Portland cement, they release silicate and alumina compounds, improving the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH). These are the main components of the solid matrix, and compounds involved in the fundamental mechanisms of immobilization of heavy metals. Some chemicals additives were used as Bauxsol, potassium dihydrogen phosphate and ferric chloride hexahydrate. The first is a product resulting from the treatment of residues of the bauxite processing, with high adsorption capacity towards the metal contaminants. The combined use of potassium phosphate and ferric chloride, in small quantities, can convert lead in highly stable forms with low solubility, such as the pyromorphite. The research analyzes the interactions between the treated material and the various components of the tested mixtures. Some key factors that characterize the process of stabilization and solidification have been identified, and some possible solutions to the problems that arise with the use of the adopted techniques have been highlighted . The study focuses solely on the investigation of the chemical characteristics of the inert waste, without, however, analyze the physical and mechanical properties of the same. Although these aspects are important for the possible reuse of the material, they are subordinate to the legislative constraints, which, as a first , provide for the preservation and protection of environmental from contaminants that may be released from the treated wastes, when they are deposited in the storage site.