DOCTORAL THESIS IN
STOCKHOLM, SWEDEN 1998
Weathering of mining
waste rock containing
alum shale and
limestone
ERIK PUURA
Academic Dissertation which, with due permission of the KTH Royal
Institute of Technology, is submitted for public defence for the on
Tuesday the 26th May 1998, at 12:00 a.m. in
KTH ROYAL INSTITUTE OF TECHNOLOGY
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 twostage 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 magnesiumcontaining 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.
ISBN: 99-2600537-3 | Trita-KET 84 | ISSN 1104-3466
http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-2644