SGS MINERALS SERVICES
Humidity cell tests are designed to
model the geological processes of
weathering at the laboratory scale. The
purpose of the test is to determine the
rate of acid generation and variation
over time in leachate water quality.
They are often performed to confirm
or reduce the uncertainty in the results
of static prediction tests and to provide
a preliminary assessment of acid rock
drainage (ARD) control options.
TESTING PROTOCOL
The standard test is conducted at
the bench scale on a sample size of
approximately 1 kilogram. Typically
the samples particle size is less than 6
mm. The material is then subjected to
alternating cycles of dry and moist air.
The cell is soaked for a specific period
with deionized water, and the leachate
is analyzed for a number of parameters
including pH, sulphate, acidity, alkalinity,
conductivity and metals (including Ca and
Mg). The test design can be customized
at your request to better account for
sample matrix effects which may include
increasing or reducing particle size, or
possibly altering the shape of the cell to
vary the flow rate.
LARGE SCALE PROJECTS
Larger scale cells, including columns and
rectangular cells are to test the larger
particle sizes expected in waste rock
materials and to test cover design options
and the effectiveness of cover systems
in reducing ARD. Other options in
humidity cell design include the addition
of bacteria, flooding of the sample,
temperature cycling, variation of the
humid-dry air cycle, and variable duration
of the air/leach cycle.
DATA INTERVENTION
The test results are reviewed on a
monthly basis, to check on their progress
and determine the need for extended
tests. In the early stages of humidity cell
tests, results can be erratic if the sample
contains previously generated oxidation
products. Tests typically run from 16 to
24 weeks, while longer duration tests
are typical for the evaluation of control
options as these are designed to dampen
the effects of ARD production. Because
of the complexity of ARD reactions, SGS
can help you with the interpretation and
presentation of the results of humidity cell tests.
