Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the treatment of tailings
produced during the processing of hard rock ore. Such tail-
ings may for example be tailings from a flotation operation
or tailings from a hydrometallurgical operation.
Hard rock ores generally consist of metal-bearing
igneous and metamorphic rock which is composed of a mixture
of quartz, other silicate minerals and metallic sulphides.
Metals produced through the processing of hard rock ores
include gold, silver, nickel, copper, lead and zinc.
lo Processing of hard rock ores generally includes
crushing the ore to a fine size and then separating the
valuable metals from the remainder of the rock using a
variety of methods such as flotation, flocculation, hydro-
metallurgical processes, etc. The tailings from such
processing have to be disposed of. The volume of the tail-
ings is frequently greater than the volume of the original
rock ore so that it is not possible to return all the tail-
ings to the excavated area produced in mining the ore.
Other mining operations also provide other practical limita-
tions on replacing the tailings underground.
The general practice of above-ground tailings
disposal consists of dyking off an area and pumping the tail-
ings into the area, since the tailings from most operations
will be in the form of a slurry. Problems related to this
disposal method include concern as to the long term stability
of the dykes and a dust problem if the tailings dry out. The
practice of adding cement to tailings so that they can be used
as mine backfill is widespread. However, the relatively high
cost of using cement to solidify tailings has generally
precluded its use in treating tailings disposed of above-
ground.
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An object of the present invention is to provide an
economic process which can be utilized in the treatment of
tailings resulting from the processing of hard rock ores.
According to the invention, a process for the
solidification of tailings produced by the processing of
hard rock ore comprises conditioning the tailings with
Sulphuric acid, and subsequently mixing with the conditioned
tailings an additive compound comprising an alkaline earth
metal oxide or hydroxide in an amount sufficlent to raise
the pH of the tailings to a value in the range of from about
10 to about 14 and produce a solid product.
The amount of sulphuric acid added is preferably
i`n the range of from about 0. ol % to about 5% of the dry
weight of the tailings.
The alkali earth metal oxide or hydroxide may be
selected from the group consisting of calcium oxide, calcium
hydroxide, magnesium oxide and magnesium hydroxide. The
amount of alkaline earth metal hydroxide is preferably in
the range of from about 0.1% -to about 5.0% of the dry
weight of the tailings.
Waste slag may be added to the tailings either
prior to or after the acid addition stage, the amount of
slag being in the range of from about 0.1% to about 70%
of the dry weight of the tailings.
According to a preferred embodiment, the tailings to
be treated by the process of the present invention comprise
a slurry of crushed rock and water, with the solids making
up between about 5 and about 85% of the total weight of the
slurry.
In the first stage of the process of the invention,
sulphuric acid is mixed with the tailings to condition -the
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solids for solidification. The acid added should be rela-
tively concentrated in order to ultimately produce solidified
material of superior strength. The amount of acid to be
added is a function of the weight of the solids being treated,
not of the pH of the mixture. The amount of sulphuric acid
added is preferably in the range of from about 0.01 to about
5~ of the dry weight of the tailings.
After the acid addition, an alkaline earth metal
oxide or hydroxide, preferably lime, is added to and mixed
with the mixture to raise the pH of the mixture to a value
in the range of from about 10 to about 14. Following this
addition, the treated tailings may be placed in the tail-
ings disposal area or in some cases used as mine backfill.In a preferred embodiment, the tailings slurry is mechani-
cally thickened sufficiently such that it can be immediately
deposited in such a manner as to form a hill. After a
period of time, the mixture gels and sets to a single solid
mass which does not break down when wet.
The sulphuric acid used in the conditioning step
may be waste acid produced in the scrubbing of sulphur oxide
em;ssions from smelting operations. Also, finely ground
slag may be incorporated into the mixture to add to the
physical strength of the waste product and to partially sub-
stitute for some of the alkaline earth metal oxide or
hydroxide required for the process.
Examples of the invention will now be described.
ElXAMPLE 1
Unclassified mine tailings were used in this
example and were primarily composed of pyrrhotite, feldspar,
quartz, mica, talc and chlorite.
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3 grams of concentrated sulphuric acid (93% H2S04)
were mixed with 1250 grams of tailings (76.3% solids wt/wt).
After 15 minutes of mixing, 25 grams of calcium hydroxide
were rnixed with the tailings. The mixture was then poured into
sample cups and allowed to air-dry to the solid state over
a period of several days. A sample of untreated tailings was
also poured into a sample cup and allowed to dry.
After three days, the tailings solidified in accord- ¦
ance with the invention had an unconfirmed compressive strength
in excess of 7000 psi. When samples of both the solidified
and untreated tailings were immersed in water, the untreated
tailings collapsed within 10 minutes while the solidified
tailings had not broken down after 2 days.
EXAMPLE 2
Other mine tailings were found to have the following
chemical composition: I
Al 7.67% ',
Ca 4-33%
Fe 4-49%
K 2.32%
Na 1.29%
sio2 56.9%
10 grams of concentrated sulphuric acid were mixed
with 2 kilograms of tailings (71.0% solids). After the acid
and the tailings were well mixed, 80 grams of CaO were mixed
with the tailings and the mixture poured into sample cups and
allowed to air-dry to the solid state.
After 5 days of drying, the solidified product did
not crumble when immersed in water. The unconfined compres-
sive strength of a sample, as determined using a concretepenetrometer, exceeded 700 psi.
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Other examples and embodiments of the invention
will be apparent to a person skilled in the art, the scope of
the invention being defined in the appended claims.
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