Prevention of Acid Mine Drainage by Passivation of the Surface of Highly Reactive Sulphide Containing Wastes
Filippova, Inna V; Filippov, Lev O.
Université de Lorraine, CNRS, GeoRessources Laboratory, Nancy, France
The work aims to select efficient inhibitors of the meteoric and bacterial dissolution of sulphides and gangue minerals to prevent the acid mine drainage (AMD) during conservation of the mining wastes. The pyrite containing tailings samples from an old French waste dump revealed the capacity of the sand and clayey wastes to produce acid waters (pH 1.8-2.4) and to release heavy metals. Comparative tests on the passivation of the surface of sulphide minerals using various inorganic (CaO, NaOH, phosphates) and organic (alkyl dithiocarbamates) chemicals allowed to identify optimal treatment conditions and efficient inhibiting reagents.
The batch tests revealed the capacity of the processing tailings to generate acid waters (pH 1.8-2.4) and to release heavy metals. Mineralogical analysis revealed not only the profound sulphide oxidation, but also the complex transformations of aluminosilicates. Continuous leaching tests were carried out in percolation columns during 68 weeks. A single treatment with carbamate inhibits the oxidation of the pyrite and the other sulphides, and totally prevents the formation of acid waters (the stabilised pH was 5.3-5.8). As studied by IR spectroscopy and XPS, passivation of sulphides with carbamate is the result of formation of superficial organometallic compounds, which are stable in acid medium and lead to a considerable reduction in hydration rate of the surface avoiding corrosion of pyrite.
The two efficient processes have been highlighted: in situ treatment by a chemical and chemical treatment of waste followed by preparation of cement matrix, if the quantity of quartz in the waste is important. The stabilization of waste becomes problematic, if the clay minerals form aggregates with fine particles (<10 µm) of pyrite. Confining trials show inefficacy of direct stabilisation of clayey wastes in cement concrete. The preliminary treatment samples with phosphates and carbamate increases concrete matrix resistance. The highest neutralisation capacity during sulphate attack is observed for concrete with addition of 0.96 % by mass of phosphate.
A 2 years pilot campaign was performed on the highly reactive fine-grained pyrite containing samples using a preliminary treatment followed by encapsulation in the cement matrix. The long-time leaching tests were designed and performed to model the actual conditions of on-site storage of the wastes. The samples were washed continuously with the leach solution alternately to the atmosphere exposure. The results allowed to conclude about an efficient AMD inhibition and the stable solution pH near to the neutral pH.