Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia. 1 field study and modelling

A field survey and modelling of the oxidation and carbonate buffering react ions inside the alum-shale-containing waste rock dumps located in Maardu, E stonia, wa accomplished. In the slope areas, the shale has been altered at high temperatures due to the spontaneous combustion and the pyritic acidity has been eliminated through migration of SOx gases out from the dump. In t he central parts of the waste rock plateaus, low temperature pyrite oxidati on fronts develop towards the dump depth and towards the centres of individ ual shale lumps. The main secondary phases precipitating in the weathering profile are gypsum, ferric oxyhydroxide, I-jarosite and smectite. The respe ctive field data made it possible to calibrate the two-stage oxygen diffusi on model and the characteristic pyrite oxidation rate 0.06-0.08 mol of pyri te reacted per kg of available water (pyrox/H2O value) was estimated to des cribe the first tens of years of dump performance. The model is capable to compare different shale disposal strategies that are illustrated with two c ase scenarios. The buffering of sulphuric acid my Mg-calcite appears to be an incongruent reaction with gypsum precipitating that lads to the build-up of the high Mg/Ca ratio in the leachate. Application of the Mg/Ca method e stimates the pyrox/H2O value in the range of 0.05-0.14 mol/kg.