Oxidation of pyritic waste rock and the subsequent generation of acid mine
drainage (AMD) is controlled to a large extent by the availability and transport of
oxygen to the reaction sites. An understanding of the interaction of the gas transfer
mechanisms within a waste rock pile is key to developing cost effective
management strategies to control AMD.
The purpose of this document is to provide the rationale and description of the
various techniques and procedures proven to be effective for measuring the bulk
physical parameters of:
- thermal conductivity;
- gas diffusion; and
- gas permeability
which are recognized as being the principal parameters required to define gas
transfer. Measurement of thermal conductivities provides a means for determining
oxidation rates from measured temperature distributions since oxidation, being
exothermic, leads to a temperature rise, which in turn relates to the thermal
conductivity of the bulk material. To reach oxidation sites within the waste rock
piles, oxygen must flow through available pore spaces. There are two mechanisms
that lead to this flow through gas filled pore space. The first is diffusion where the
flux results from the oxygen gradient consequent on oxygen consumed in the
oxidation process. The second, addressed through the measurement of gas
permeability, is advection where the flux results from a pressure gradient set up in
the pile.
In addition, techniques and procedures are provided in the report for monitoring
temperature and oxygen concentrations within the pile. Since the pyritic oxidation
reaction consumes oxygen and generates heat measurement of oxygen
concentrations and temperature profiles within a waste rock pile can provide
insight into the oxidation process and the pollution potential from waste rock
material.
This document provides details of field installations and procedural guidelines for
the measurement and monitoring of these principal gas transfer mechanisms
associated with waste rock oxidation. Section l provides a general introduction.
Section 2 provides the scientific background to the techniques used for in situ
measurements of the key bulk physical parameters (thermal conductivity, gas
diffusion, and gas permeability). Section 3 outlines the field installation techniques
and instrumentation required while Section 4 outlines how the measurements are
carried out and gives general guidance to data interpretation. The details of the
measurement techniques, instrumentation and data reduction to obtain the bulk
parameters are contained in the appendices. These appendices are sufficiently
detailed to be used in the field as selfstanding guides.
Computer models such as FIDHELM (developed by Australian Nuclear Science
and Technology
Organization, ANSTO) can be used to predict the impact of pyritic waste rock
oxidation processes as well as evaluate various options for controlling acid
drainage contamination. The measured field parameters described in this report are
required as inputs to the model
