Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a process for removing
dissolved selenium values from an acidic aqueous copper
sulphate solution.
Many processes for the recovery of copper from its
ores include an electrowinning step in which an acidic aqueous
copper sulphate solution is electrolyzed to deposit elemental
copper on the cathode of an electrolytic cell. Copper ores
also frequently contain selenium and, as a result of the
treatment of such ores, culminating in the production of
the acidic aqueous copper sulphate solution to be electro-
lyzed, dissolved selenium values will probably be present
in the copper sulphate solution. The dissolved selenium
values are usually present both as tetravalent selenium (IV)
and as hexavalent selenium (VI).
Since selenium tends to co-deposit with copper
in the electrolytic cell, thereby contaminating the copper -~
product, it is frequently necessary to reduce the concentration
of dissolved selenium to an adequate low value. Various
selenium removal processes have been proposed in which the
copper sulphate solution is treated with sulphur dioxide
or a sulphite solution to produce a selenium-containing
precipitate which is subsequently separated from the solu-
tion. To achieve adequate selenium (VI~ removal, it is
necessary to carry out the treatment with sulphur dioxide
or sulphite solution at a temperature of at least about 140C.
Although selenium (IV) and (VI) can be adequately removed at
such temperatures, subsequéntly cooling of the solution to
below 140~C is likely to result in the precipitation of
metallic copper and consequent deposition of the precipi- -
tated copper on the walls of the equipment concerned. 5uch
deposition is extremely disadvantageous, especially in
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continuous flow processes where the copper may be deposited
in heat exchangers, valves or narrow passages to cause
blockages.
It is therefore an object of the invention to
provide a selenium removal process of the kind referred
to about but in wh.ich the likelihood of copper being
deposited when the solution is cooled to a temperature
below 140C is substantially reduced.
According to the invention, an aqueous acidic.
copper sulphate solution containing dissolved selenium
values and with substantially all the dissolved copper in
cupric form is treated at a temperature of at least about
140C with a stoichiometric excess relative to the dis-
solved selenium of sulphur dioxide or a sulphite solution
to produce a selenium-containing precipitate and some
dissolved cuprous copper and, while maintaining the solu-
tion at a temperature of at least about 140C, an oxygen
containing gas is passed into the solution under a pres-
sure of at least about 350 kPa to oxidize substantially
all the dissolved cuprous copper to dissolved cupric
copper.
The invention is based upon the realization that
the stoichiometric excess of sulphur: dioxide or sulphite
solution required for an adequately fast reaction time in
the selenium-precipitation.step also causes the reduction
of at least some of the copper in the copper sulphate solu-
tion from the cupric form to the cuprous form. Th,e dis-
solved cuprous copper is unstable and readily dispropor-
tionates to cupric copper and elemental copper when the
temperature of the solution falls below about 140C.
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Preferably, the solution is treated with a stoichio-
metric excess of from about 5 to about 200 moles of selenium
reducing compound per mole of dissolved selenium to produce
the selenium-containing precipitate.
The selenium-containing precipitate may be
separated from the solution prior to the oxidation step.
The initial acidic aqueous copper sulphate solu-
tion may typically contain from about 10 to about 100 g/L
dissolved copper, from about 5 to about 500 mg/L dissolved
selenium, and from about 10 to about 100 g/L sulphuric
acid. The solution may also contain from about 1 to about
120 g/L dissolved nickel.
The oxidation step is preferably carried out at
a temperature in the range of from about 1~0 to about 175C,
and at a pressure in the range of from about 800 to about
1500 kPa. The selenium-precipitation step may advantageously
be carried out at a similar pressure to the oxidation step,
that is to say at a pressure of at least about 350 kPa, and
preferably at a pressure in the range of from about 800 to
about 1500 kPa.
One embodiment of the invention will now be des-
cribed, by way of example, with reference to the accompany- -
ing drawing whlch shows a flow diagram of a selenium re-
moval process.
Referring to the drawing, aq~eous acidic copper
sulphate solution is supplied to an agitated feed tank 1
from a leaching process in which nickel-copper matte is -~
leached in sulphuric acid solution in at least two stages,
including a preliminary stage in which a substantial pro-
portion of the nickel in the matte is dissolved and re-
moved in a separate leach solution. The copper sulphate
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leach solution supplied to feed tank 1 contains about 60 g/L
copper, 50 g/L nickel, 10 g/L sulphuric acid, 5 mg/L selenium
(IV) and 30 mg/L selenium (VI). Spent electrolyte from a
subsequent copper electrowinning step is fed into the feed
tank 1 at a rate sufficient to maintain the acidity of the
solution in the feed tank 1 at about 20 g/L sulphuric acid.
The copper sulphate solution is pumped at a temperature of
about 50C from the feed tank 1 by a pump 2 at a pressure
of about 1200 kPa through two spiral heat exchangers
3, 4. In the heat exchanger 3, the copper sulphate solution
is pre-heated by solution at a temperature of about 160C
passing from the selenium removal process to a copper
electrowinning step. In the heat exchanger 4, the pre-
heated copper sulphate solution is heated by steam to a
temperature of 175C, the heat exchanger 4 being automatic-
ally controlled such that the desired solution temperature
is maintained.
The heated copper sulphate solution then passes
to a static mixer 5 where the solution is mixed with liquid
sulphur dioxide pumped by a diaphragm metering pump 6
from a sulphur dioxide storage system 7 at a rate of 2 to
3 g of sulphur dioxide per litre of feed solution, thereby
providing a stoichiometric excess of about 70 to about
100 moles of sulphur dioxide per mole of dlssolved selenium.
The mixed solution then passes to a ~eduction step 8 com-
prising four vertical reduction autoclaves ln series under
a pressure of about 1,000 kPa. Each autoclave 8 has a
safety bursting disc set at 1400 kPa at the top, with the
outlet therefrom being connected through a flash tank 9
to the feed tank 1. Each autoclav~ 8 also has a manually
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controlled heating coil to enable the temperature in each
autoclave 8 to be maintained at about 175C.
The mixed solution passes upwardly through each
autoclave 8 with no agitation and, during passage there-
through, dissolved selenium is precipitated as cuprous
selenide, it having been found that substantially no
elemental copper is prec;pitated by the sulphur dioxide
at temperatures above about 150C. However, the sulphur
dioxide does reduce some of the dissolved cupric copper
to the cuprous state. The sulphur dioxide treatment in the
autoclaves 8 reduce5the selenium content of the solution
to about Ool ~g~L dissolved selenium ~IV) and about 1 mg/L
dissolved selenium (VI).
After leaving the fourth autoclave 8, the copper
sulphate solution with precipitated cuprous selenide is
passed to a filter assembly 10 where the cuprous selenide
precipitate is filtered from the copper sulphate solution.
Since the solution is still at a pressure of about 1,000
kPa, the fllter assembly 10 actually comprises two filter
units in parallel, wlth only one filter unit being in
operation at a time. The operating filter unit is operated
until the differential pressure across the filter reaches
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about 200 kPa, and the solution~flow is then switched to
the other filter unit. Solution in the first filter unit
is transferred to the second filter unit by high pressure
air, and the first filter unit is then opened to remove
the cuprous selenide precipitate.
The filtered copper sulphate solution proceeds
from the filter assembly 10 through a static mixer 12
where high pressure air is injected to provide a stoichio-
metric excess of oxygen relative to the cuprous ion
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concentration. The aerated solution is then passed to an
oxidation step 13 comprising two vertical autoclaves 13 in
series with the solution still being under a pressure of
about 1,000 kPa. Each autoclave 13 has a manually controlled
heating coil to enable the solution temperature therein to
be maintained at about 160-170C, and also has a safety
bursting disc set at 1400 kPa at the top with an outlet
connected to the flash tank 9. In the autoclaves 13, cuprous
ions in solution are oxidized to the cupric state. Typi-
cally, the solution entering the first autoclave 13 maycontain about 1 to 5 g/L cuprous ion~, about 99% of which
are oxidized to the cupric state by the time the solution
leaves the first autoclave 13. The residualcuprous ions
are then almost completely oxidized in the second autoclave
13. Spent gas leaves the top of each autoclave 13 and
passes through a level control tank 14 where the gas is
demisted and i9 then released to atmosphere through a valve
15 controlled by a pressure sensor in the tank 14 to auto-
matically maintain a pressure of about 1,000 kPa in the
system. The oxygen concentration in the off-gas emitted
from valve 15 may be measured to enable appropriate adjust-
ment to be made to the supply of high pressure air to the
mixer 12. Advantageously, the supply of air is such that
there is a stoichiometric excess of oxygen of about 3 times
that required for oxidation of the cuprous ions to ensure
at lea&t 99% oxidation in eac~ auto~lave 13.
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The deselenized copper sulphate solution, withdissolved copper now substantially only in cupric form,
leaves the second autoclave 13 through an overflow pipe
somewhat below the top of the autoclave and enters the
level control tank 14, which is equipped with level probes
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maintaining a desired solution level in the oxidation
autoclaves 13. The solution passes from the level control
tank 14 through a filter assembly 16 for clarification
purposes. the filter assembly 16 comprising two filter
units operated in a similar manner to those of the filter
assembly 10. After leaving the filter assembly 16, the
clarified solution passes through the heat exchanger 3 to
pre-heat the selenium-containing copper sulphate solution,
as previously mentioned, and then proceeds to a copper
electrowinning step through a release valve 17 controlled
by level probes in the level contrcl tank 14.
Thus, the copper sulphate solution is deselenized
in a continuous process under a pressure of about 1,000 kPa
from the pump 2 to the release valve 17 and at a temperature
of at least 160C from the heat exchanger 4 to the level
control tank 14, with cuprous ions formed in the selenium
precipitation autoclaves 8 being oxidized to the cupric
stage in the oxidation autoclaves 13. A solution containing
cuprous ions is therefore always at a temperature of at
least 160C, thereby substantially eliminating the possibility
of elemental copper belny formed by disproportionation of
cuprous ions and deposition in valuable parts of the system.
In the described embodiment, the cuprous selenide
precipitate was removed prior to the oxidation step, with
such a procedure preventing any selenium redissolution in
the oxidation step. If desired, the cuprous selenide pre-
cipitate may be present during the oxidation step without
any substantial selenium redissolution, particularly when
the temperature of the oxidation step is maintained below
about 160C. However, lt has been found that, if an
intermediate filtration step between the reduction and
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the oxidation autoclaves is provided, copper selenides with
a copper to selenium molar ratio close to 1:1 can be obtained,
probably due to the leaching of copper from cuprous selenide
by the cupric ion. In this case, less copper is removed
from the solution as selenide.
The removal of dissolved selenium (IV) values
reduces selenium contamination in the copper product
produced in copper electrowinning. The removal of dissolved
selenium (VI) values is particularly advantageous where
the solution also contains dissolved nickel which is
subsequently recovered in a nickel electrowinning step, since
generally speaking both dissolved selenium (IV) and dissolved
selenium (VI) contaminate electrowon nickel.
The cuprous oxidation step of the present inven-
tion is of course applicable to selenium precipitation
steps other than that described in the preferred embodiment,
and is in fact applicable to selenium precipitation steps
in which an acidic aqueous copper sulphate solution is
treated at a temperature of at least about 140C with sul- -
phur dioxide or a sulphite solution. For example, the
selenium precipitation step may be similar to that des-
cribed in Canadian patent application No. 358,594 filed
August 19, 1980 or in Canadian application No. 389,144
filed October 30, 1981.
The selenium reducing compound may be a sulphite
solution, and this term is intended to include bisulphite
and pyrosulphite solutions.
Other embodiments will be readily apparent to a
person skilled in the art, the scope of the invention
being defined in the appended claims.
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