Accelerated Carbonation and Granulation for Treatment of Mine Tailings Using MgO/GGBS Binder

About 4.92 million tons of abandoned mine tailings exist in Korea. Accelerated carbonation and granulation treatment were simultaneously applied to mine tailings using MgO (magnesium oxide) and GGBS (grounded granule blast furnace slag) mixed binder, in order to remediate the mine tailings and reuse them as materials for construction and mine back filling. The MgO/GGBS binder has shown high compressive strength under the accelerated carbonation condition and great stabilization capacity for heavy metals when it was applied to contaminated mine tailings in the previous study. To obtain the optimal granule size and the high granule strength, operational factors (rotational speed and time) and binder dosages were tested as the following: water/solid ratio (0.16-0.21), binder dosage (10-30% to the mine tailings) and MgO and GGBS weight ratio (1:1, 1:4, 3:7). The 50% MgO and 50% GGBS (MgO_0.5GGBS_0.5) at 30% binder dosage was the best conditions, which yielded the greatest granule strength. The MgO_0.5GGBS_0.5 granule was cured under 0%, 20%, 50% and 100% CO₂ environments for 60 days to evaluate its physical and chemical characteristics. The granule strength was greater in accelerated CO₂ than in 0% CO₂ and the strength dramatically increased as the curing time passed until 28^th day. The increase of the granule strength was faster in the higher CO₂ conditions on the early stage of curing days. However, the strength decreased on 60^th day except the granule cured in 20% CO₂. The highest granule strength was obtained at 20% CO₂ with a value of at 0.47 MPa on 28^th days of curing. The maximum amount of CO₂ uptake was 0.025 kg CO₂/ kg binder for the 100% CO₂ cured granule on 28^th day. The porosity of 20% CO₂ cured MgO_0.5GGBS_0.5 granule was lower (0.19) than the 100% CO₂ cured (0.26), showing the moderate formation of hydromagnesite in the structures. The MgO_0.5GGBS_0.5 treated granule also showed a greater acid-buffering capacity at around pH 9 than the granule treated with ordinary Portland cement (OPC) treated. This implies the heavy metal leaching around pH 9 is possibly lower for MgO_0.5GGBS_0.5 granules than for OPC granules. The XRD results showed the decreases in peaks for pyrite and arsenopyrite in MgO_0.5GGBS_0.5 treated granules at 20% CO₂. This implies that those minerals could be transformed into the different minerals binding with MgO_0.5GGBS_0.5.