Lakefield Research Limited was retained by Falconbridge Limited in August, 1994, to
conduct a testing program. The purpose of the program was to evaluate the effects of
the subaqueous deposition of pyrrhotite and fine tailings from the Strathcona Mill on
the water quality in Upper Strathcona tailings treatment system. The results of the test
program were to be used to determine if subaqueous deposition would be a viable
alternative method to control acid generation from the Strathcona Mill tailings. The test
program included tailings characterization and subaqueous column test work. Two
types of tailings were used in the subaqueous column test program: pyrrhotite tailings
and blended Strathcona Mill tailings (50:30 pyrrhotite : slimes).
The characterization of the water in Upper Strathcona tailings treatment system water
showed that it was acidic (pH 2.9), high in conductivity (2270 μmhos/cm), and high in
concentrations of sulphate (941 mg/L), iron (6.9 mg/L) and nickel (2.1 mg/L). Water
from the lower end of the Strathcona tailings treatment system has been treated and
used as process water for the Strathcona Mill. The process water was alkaline (pH
7.4), low in conductivity (1630 μmhos/cm), and low in concentrations of sulphate (542
mg/L), iron (
The pyrrhotite tailings used during the program consisted predominantly of pyrrhotite
(92-95%), with lesser amounts of magnetite and non-opaque minerals. The slime
tailings consisted predominantly of non-opaque minerals such as quartz, albite,
actinolite and white mica.
A multi-element scan indicated the presence of similar metals in the pyrrhotite tailings
to those in the blended Strathcona Mill tailings (SM tailings). The pyrrhotite tailings
contained 30.2% sulphur and 56% iron, whereas the SM tailings contained 19.0%
sulphur and 37% iron.
The liquid portion of the pyrrhotite tailings had a pH value of 7.9, while the liquid portion
of the SM tailings had a slightly less alkaline pH value of 6.8. The liquid portions of
both tailings types were characterized by high hardness (3430-3790 mg/L), high
conductivity (5354-5550 μmhos/cm), high concentrations of sulphur (2810-3180 mg/L),
sulphate (991-1129 mg/L), thiosulphates (1882-2761 mg/L expressed as sulphate) and
calcium (1290-1480 mg/L). For each of these parameters, the higher values were
associated with the liquid portion of the pyrrhotite tailings and the lower values with the
liquid portion of the SM tailings. Concentrations of aluminum, cobalt, copper, iron, lead
and zinc in the liquids of both tailings types were not detected above the method
detection limits.
Column leach tests were conducted on the pyrrhotite tailings and the SM tailings using
water from Upper Strathcona tailings treatment system as influent. The results
indicated that copper, cobalt, zinc, nickel, lead and aluminum were removed from the
influent and retained by the tailings, whereas the iron, calcium, magnesium,
manganese, silicon, potassium, sodium, and sulphate were released from the tailings.
The effluent pH remained above 5.
Five pilot columns half filled with tailings and covered with an overlying water column
were monitored over a 13 month period to evaluate the effects of subaqueous tailings
deposition on a stagnant layer of acidic surface water. The five columns tested
contained: the Upper Strahcona tailings treatment system water over 1) pyrrhotite, 2)
thickened SM tailings, 3) pyrrhotite with a 10 cm cover of organic substrate collected
from the treatment system, 4) thickened pyrrhotite and 5) SM tailings. The following
findings are based on the data collected from the five columns over a 13 month testing
period:
- A stagnant 1000 mm water cover is capable of maintaining the underlying tailings
in a reducing condition. - A conductivity gradient was observed to exist between the tailings layer and the
water cover, which caused a diffusion of salts upwards into the overlying water
column. - A pH increase from 3.0 to almost 6.0 was only noted in the water cover of
thickened pyrrhotite tailings column and the pyrrhotite tailings with substrate
column. The pH of the water cover in the remaining three columns remained at
about 3.0. - The iron concentrations in the water cover in all the columns decreased with
time. This decrease was interpreted to be due to the precipitation of hydrous
ferric oxides. - The nickel concentrations in the water cover remained similar to the initial nickel
concentration observed in the Upper Strathcona tailings treatment system water
(2.08 mg/L) in all the columns except in the SM tailings column. The
concentrations of nickel in the water covering the SM tailings increased to 3.2
mg/L. The nickel concentrations of tailings pore water in all columns were below
0.2 mg/L. - The sulphate concentrations in the water covering the thickened pyrrhotite
tailings column remained at approximately 500 mg/L. The sulphate concentration
in the water cover of the remaining four columns remained at about 1000 mg/L.
There was a sulphate concentration gradient between the tailings and the water
cover in all columns, which may lead to the diffusion of sulphates from the
tailings into the water cover. - The increase in concentrations of iron and sulphate in the tailings pore water and
the nickel in the water cover of the SM tailings column may be attributable to
changes in redox potential (from reducing to oxidizing) at the interface. - The formation of precipitates and a biological growth layer at the interface and
the consolidation of the tailings may have contributed to the observed decrease
in the diffusion of salts from the tailings into the water cover at the later stage of
the test program.
Based on the data collected to date, it was concluded that subaqueous disposal of
pyrrhotite tailings, with a shallow cover layer of sediment substrate from the Strathcona
tailings treatment system, would be the most effective of the deposition scenario
studied for reducing acid generation. Methods of promoting the rapid formation of the
organic rich substrate should be investigated.
Because the subaqueous column test was designed to simulate the deposition of fresh
tailings above the chemocline and thermocline in the shallow acidic waters, the results
of the column tests have provided a better understanding of the worst case impacts
that can be expected on the Strathcona tailings treatment system. Investigations
carried out in a parallel study by Rescan on the Strathcona tailings treatment system
suggest that sulphidic mine tailings should be highly stable at the bottom of the
Strathcona tailings treatment system. This stability would be due to high concentrations
of organic matter in the sediments, ample sulphate in the water column for reduction,
bottom water anoxia, sulphate reduction and the generation of alkalinity.
The results of tests conducted to date indicate that the main processes acting in the
water-tailings columns are molecular diffusion and precipitation under static conditions.
It will, therefore, take considerable time to reach equilibrium in the columns. It has
been suggested that continued monitoring of the columns would be useful to the
long-term prediction of expected impacts.
Geochemical computer modeling using existing data is recommended to assist with the
interpretation of the observed data and to help explain the precipitation of hydrous
ferric oxides and the changes of metal concentrations with pH. Microbiological
examinations are also recommended to identify the microorganisms present and
determine their roles in metal release and mobilization.
