Weathering of mining waste rock containing alum shale and limestone

Abstract: Geochemical processes occurring in the waste rock dumps ofan old phosphate mine, located in Maardu, northem Estonia, havebeen studied and modelled. The waste rock contains pyritic,metalliferous and organic-rich shale, as well as limestone.Production of acidity during pyrite oxidation is a potentialmechanism for heavy metal release. Under certain conditions,such as close proximity to steep slopes, active oxidation ofpyritc and organic matter leads to the spontaneous combustionof the waste rock.The study combines the results of field observations andsample analyses with physico-chemical modelling of thepredominating processes. The main approach has been to specifythe solid phase transformations and use reaction frontmovements for indicating and quantifying the development ofthese processes. The model of low temperature shale pyriteoxidation calculates two-stage diffusional transport of oxygen:(1) in the gaseous phase, through large dump pores, and (2)dissolved in the water, through shale pores. The concept oflotal hydrochemical equilibrium is used to describe theinteraction between acidic water and calcite. From themodelling results for different scenarios it appests, that inthe long-tenn, the areas where the shale was dumped selectivelybelow the rest of tbe overburden, are most likely to generateacid leachate.The other important phenomena are the dissolution of illitewith the formation of smectite and K-jaros&and thedissolution of magnesium-containing limestone followed bygypsum formation and a build-up of a high Mg/Ca ratio in theleachate. Inclusion of these factors into the model explainsthe leachate formation as a sequence of chemical reactions. Themain uncertainty remaining is the decrease in availablecarbonate buffering capacity due to the encapsulation of thelimestone lumps by secondary precipitates. The phenomena ofspontaneous combustion removes pyritic acidity from the dump ina gaseous phase. A variety of different altered shale types andnew minerals appear in the heated areas.The basic message forenvironmentally aware shale handling in association with theestablishment of new phosphate open pits in Estonia is toundertake storage under anoxic conditions.Keywords:Estonia, modelling, waste rock, alum shale,heavy metals, pyrite, oxidation, leachate, diffusion,buffering, pH, PHREEQC, transport, equilibrium, convection,spontaneous combustion, jarosite, ferrit oxyhydroxides, gypsum,illite, smectite.