Note: Descriptions are shown in the official language in which they were submitted.
CA 02728519 2010-12-17
WO 2009/153409 PCT/F12009/050540
1
METHOD FOR LEACHING NICKEL MATTE IN THE PRESENCE OF ADDED COPPER
Field of the invention
The present invention relates to a method for leaching pyrometallurgically pro-
duced nickel mattes in a multistep hydrometallurgical process.
Prior art
A large portion of the world's nickel is produced hydrometallurgically from
pyrome-
tallurgically produced nickel mattes. Generally, such a production process
involves
the leaching of nickel contained in the matte by a multistep process, followed
by
purifying the nickel-bearing aqueous solution and reclaiming the nickel. The
leach-
ing process is determined according to the composition and properties of em-
ployed nickel matte. These, on the other hand, are influenced by the grade of
con-
centrate used as a raw nickel material and by its metallurgical smelting
process.
There is a prior known method described in US patent publication 6,039,790 for
recovering nickel in one and the same process from two pyrometallurgically pro-
duced nickel mattes, one of which contains a remarkable percentage of iron. In
the
method, leaching of the nickel matte that contains iron is carried out in one
step by
conducting solution from the leaching cycle of the less iron containing matte
into
the leaching of the iron containing matte at a stage where the iron of the
less iron
containing matte is in soluble form. The iron contained in the mattes is
advanta-
geously precipitated as jarosite and the solution created in the leaching of
the iron
containing matte is conducted back into the leaching cycle of the less iron
contain-
ing matte.
A similar method has been described in US patent publication 6,206,951,
wherein
copper-rich sulfidic and metallic nickel mattes are leached in a multi-stage
process. Nickel is leached from sulfidic nickel matte by means of copper
sulfate in
a pressure leach process. The required copper sulfate is obtained from the
leach-
ing of metallic nickel matte, as well as by leaching the copper-rich material
in a
separate copper leaching process.
GB patent publication 2,108,480 describes a process developed for sulfidic
nickel
mattes rich in copper, wherein nickel is first treated by a pressure leach
under oxi-
dizing conditions with an anolyte obtained from copper electrolysis. Once at
least
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70% of nickel values in the matte are extracted, the leaching is continued
under non-oxidizing conditions. The solution from this stage is conducted to
an
atmospheric purification leach, in which the solution is stripped of copper by
means of finely powdered matte. Precipitate from the purification leach is
conducted to a nickel pressure leach, and the precipitate remaining after that
is conducted to an oxidizing pressure-leaching process of copper. The residual
precipitate mainly comprises iron previously contained in the matte.
US patent publication 5,628,817 discloses still another multi-stage leaching
process developed for nickel matte, wherein the leaching of nickel takes place
in at least two stages in conditions where free sulfuric acid is essentially
absent. The method is of the same type as the one used in US publication
6,039,790 in the leaching of matte with a low iron content. The finely ground
matte is first subjected to a two-step oxidizing atmospheric leach and
thereafter to a two-step pressure leach, wherein the first step is carried out
in
non-oxidizing or mildly oxidizing conditions and the second step in oxidizing
conditions. As opposed to the previously described method, the entire copper
content is leached in the latter atmospheric leaching and the removal of iron
takes places by precipitation from the solution downstream of the first
pressure
leaching step.
The most severe limitation of prior known leaching processes, such as those
described above, is the applicability thereof to certain types of nickel matte
only. For example, the method disclosed in GB patent publication 2,108,480 is
only applicable to sulfidic nickel mattes with a remarkably high copper
content
(the weight ratio of nickel/copper less than 2,5, i.e. the weight ratio of
copper/nickel more than 0,4). On the other hand, the processes described in
US patent publications 6,039,790 and 6,206,951 are not suitable for metallic
nickel mattes containing precious metals as the precious metals therein end
up in the iron precipitate.
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2a
Accordingly, there is still demand for a method with a versatility of adapting
to
a variety of nickel mattes, whose contents of copper, nickel, iron, sulfur, as
well as precious metals may fluctuate without a major effect on the recovery
of
various metals and on the quality of products recovered from the leaching
process.
Summary of the invention
According to one embodiment, a method for leaching pyrometallurgically
produced, copper-containing sulfidic nickel matte, wherein a sulfidic nickel
matte is fed to an atmospheric leach step from which is obtained a nickel-
bearing solution and a copper- and nickel-bearing leach residue, whereby a
copper-bearing solid matter is added into the nickel matte before the leaching
or to the copper- and nickel-bearing leach residue after the leaching in such
an
amount that the weight ratio of copper to nickel in the copper- and nickel-
bearing leach residue is more than about 1, wherein the solid matter has its
source in a latter stage of the method or the solid matter consists of
synthetically produced copper sulfide, and the copper- and nickel-bearing
leach residue is conducted to pressure leaching.
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A feature of some embodiments is that the copper-nickel weight ratio of the de-
livered solid matter may be raised, to higher than about 1, by the addition of
a
copper-bearing solid substance. The copper-bearing solid substance may be
added downstream of the atmospheric leaching step into leach residue.
Solid matter may be obtained by circulating the leach residue from a latter
stage
of the method. Another possibility is, for example, the use of synthetically
pro-
duced copper sulfide, precipitated especially with hydrogen sulfide.
Some embodiments of the method comprise processing nickel mattes in such a
way that the nickel and iron contained in the mattes are converted into
soluble
form and the copper and precious metals end up in leach residue.
Sulfidic nickel matte may be leached first in an atmospheric step by means of
sulfuric acid and air, such that soluble copper precipitates but iron remains
in the
solution. Following a solid-liquid separation, the solid matter is conducted
to a
pressure leach. The pressure leach may be a two-step process and of the type
in
which the first step comprises leaching mostly copper and the second step
leaches nickel and iron as copper is precipitating. After the pressure leach,
the
leach residue may consist of copper sulfide precipitate quite pure in terms of
iron
and contains the precious metals (such as Au, Pt and Pd) of nickel mattes.
Metallic nickel matte may be processed in a separate atmospheric step by
means of sulfuric acid and oxygen for leaching metallic components of the
matte,
i.e. most of the nickel and iron. The solution, as well as sulfidic components
and
precious metals remaining in the matte, may be conducted to the atmospheric
leaching step of sulfidic nickel matte.
The method according to some embodiments may be applicable for a wide vane-
ty of nickel mattes, which may contain for example varying amounts of copper,
iron, and precious metals, as well as may be sulfidic and/or metallic in terms
of
their composition. According to some embodiments of the method the leach resi-
dues of mattes may end up in the same precipitate, which contains copper and
precious metals coming along with nickel mattes. In addition, the discussed
cop-
per sulfide precipitate may be relatively pure and contain just a little iron.
The res-
idence time for the first pressure leach step may be kept remarkably short,
less
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than 45 minutes, thus providing a high leaching capacity considering the size
of
process equipment.
Some embodiments of the invention will now be described in more detail with
reference to the flowchart depicted as a figure.
Description of the drawing
Fig. 1 shows in a flowchart the leaching of pyrometallurgically produced
nickel
mattes by a multistep hydrometallurgical process. In the figure, reference
numer-
als designate unit processes as follows:
1 Atmospheric removal of copper In
2 Atmospheric leach A Sulfidic Ni-matte
3 I pressure leach B Metallic Ni-matte
4 ll pressure leach C Air (oxygen)
5 Removal of iron Out
6 Solution purification D Fe-precipitate
7 Nickel electrolysis E Cu-precipitate
F Ni-cathode
Detailed description of a few embodiments of the invention
As shown in the flowchart, finely ground sulfidic nickel matte is fed to an
atmos-
pheric copper removal step 1. The term sulfidic nickel matte refers to
pyrometal-
lurgically produced smelter matte, in which most of the metal content is
present
in the form of sulfidic components, such as for example nickel sulfide Ni3S2,
cop-
per sulfide Cu2S, and nickel-iron sulfide (Ni,Fe)9S8. The sulfur concentration
of
sulfidic nickel matte is typically 15-25%, nor is it strongly magnetic. The
concen-
trations of various metal and impurities may fluctuate even quite extensively,
but
the total weight percentage of nickel, copper and iron is nevertheless
typically
more than 60%.
In addition to sulfidic nickel matte, the atmospheric copper removal step 1 is
also
supplied with a copper- and iron-bearing solution from an atmospheric leach
step
2 and from a second pressure leach step 4. In addition to these, the discussed
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step is supplied with air. The removal of copper is performed in atmospheric
con-
ditions and at a temperature of 80-100 C. The elemental nickel and nickel
sulfide
present in the matte precipitate copper existing in the solution as copper
sulfate
upon being themselves oxidized into nickel sulfate according to the following
re-
5 action equations:
Ni + CuSO4 N1SO4 + Cu (1)
Ni3S2 + 2 CuSO4 ¨0 Cu2S + NiS +2 NiSO4 (2)
Ni3S2 + 2 CuSO4 +1/2 02 2 MS + NiSO4 + Cu20 (3)
In a continuously running operation, the reactors present a rising pH-profile
in
which the precipitation of copper occurs, in response to the above reactions,
in
acidic conditions at the start of the process step as the solution has a pH of
less
than 4. The leaching of metals consumes oxygen and, as pH rises, copper pre-
cipitates also as alkaline copper sulfate CuS0412Cu(OH)2. From the standpoint
of the effective extraction of copper and iron, however, this zone may be kept
as
brief as possible in terms of its residence time in order to minimize the
concurrent
precipitation of iron. The precipitate, i.e. the leach residue from this step,
is deliv-
ered after the liquid-solid separation to a first pressure leach step 3 and
the solu-
tion is conducted to an iron removal 5.
The finely ground metallic nickel matte is conducted to the atmospheric leach
step 2. The term metallic nickel matte refers to pyrometallurgically produced
smelter matte, in which most of the metal content is present in the form of
metal-
lic components, such as for example elemental nickel Ni , copper Cu , and iron
Fe and/or in compositions (alloy) of these metals. The sulfur concentration
of
metallic nickel matte is typically less than 15% and it is highly magnetic.
The
concentrations of various metals and impurities may fluctuate even quite exten-
sively, but the total weight percentage of nickel, copper and iron is
nevertheless
typically more than 80%.
The leaching of metallic nickel matte is performed in atmospheric conditions
and
at a temperature of 80-100 C. In addition to nickel matte, the atmospheric
solu-
tion is supplied with oxygen and sulfuric acid. Some of the sulfuric acid can
be
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replaced by an acid-containing anolyte obtained from a nickel electrolysis 7.
The
principal leaching reactions are:
Ni + H2SO4 +1/202 N1SO4 + H20 (4)
Cu + H2SO4 + % 02 CuSO4 + H20 (5)
Fe + H2SO4 + 1/2 02 FeSO4 + H20 (6)
The formation of hydrogen during leaching may be prevented by an abundant in-
troduction of oxygen and the precipitation of iron by maintaining the solution
at a
low pH, lower than 2,0. The leach residue and the solution from this step are
conducted to the atmospheric copper removal 1.
The first pressure leach step 3 is supplied with oxygen as well as with an
acid-
containing anolyte from the nickel electrolysis 7. The copper concentration of
a
solid matter delivered to the pressure leach step is increased by circulating
some
copper-bearing solid substance from a liquid-solid separation downstream of
the
second pressure leach step 4 into the precipitate coming from the atmospheric
copper removal 1. The input solid substance has a preferred copper/nickel
weight ratio of 1-2,5. The temperature in the first pressure leach step is
higher
than 100 C, and may be within the range of 110-120 C and the partial oxygen
pressure may be higher than 200 kPa. The principal leaching reactions of the
first
pressure leach step are:
5 Cu1.8S +4 H2SO4 + 12 02 ¨> 9 CuSO4 +4 H20 (7)
Ni3S2 + H2SO4 + 02 NiSO4 +2 NiS + H20 (8)
The step is mainly intended for leaching a sufficient amount of copper for the
leaching of nickel effected in the second pressure leach step. This is enabled
by
recycling, if necessary, a copper-bearing solid matter into an autoclave,
which
solid matter in this case consists of precipitate containing copper-sulfide
from a
liquid-solid separation downstream of the second pressure leach step 4. By
virtue
of recycling the precipitate, the method according to some embodyments may
enable leaching also nickel mattes of low copper contents. Besides, by virtue
of a
high copper/nickel weight ratio of the solid matter, the first pressure leach
step
can be managed in a remarkably brief residence time, less than about 1 hour,
even less than about 45 minutes, thus preventing nickel sulfide from reacting
too
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far according to a reaction (9) presented below. This is of importance as
regards
the proper functioning of the second pressure leach step.
4 NiS + H2SO4 + % 02 Ni3S4 + NiSO4. + H20 (9)
Leaching is manageable also in highly acidic conditions, but it may be carried
out
in such a way that some of the leached copper precipitates in the form of
alkaline
copper sulfate, the solution having a pH of not lower than about 3 at the end
of
the step.
Alternatively, the copper/nickel weight ratio of a solid matter to be fed into
the first
pressure leach step 3 can be increased by supplementing the precipitate coming
from the atmospheric copper removal 1 for example with synthetic copper
sulfide
precipitated with hydrogen sulfide or with some other copper-containing solid
of
the type that dissolves rapidly in the conditions of the first pressure leach
step 3.
From the first pressure leach step, the slurry is passed in its existing state
as the
only feed to the second pressure leach step 4. The passage of slurry can be im-
plemented by pushing the flurry first to a normal air pressure and by pumping
therefrom, by direct pumping, or without pumping by maintaining the first pres-
sure leach step at a pressure higher than the second pressure leach step. Tem-
perature in the second pressure leach step may be higher than 140 C, and may
be within the range of 140-160 , and nickel sulfides dissolve as nickel
sulfate
functions as the oxidizer:
8 Ni3S2 + 27 CuSO4 + 4 H20 24 NiSO4 + 15 Cui.BS + 4 H2SO4 (10)
6 NiS + 9 CuSO4 + 4 H20 6 NiSO4 + 5 Cul BS + 4 H2SO4 (11)
As the acid concentration rises in non-oxidizing conditions, the slurry
leaches al-
so iron and arsenic during the second pressure leach step. The liquid-solid
sepa-
ration downstream of the step is a source of relatively pure copper sulfide
precipi-
tate, having just a low iron concentration and containing the precious metals
(such as Au, Pt and Pd) received along with nickel mattes and practically non-
leached in the above-defined conditions. A desired portion of the precipitate
is re-
turned the feed of the first pressure leach step 3. The copper sulfide
precipitate
separated from the process can be fed for example to a copper smelting
facility
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or subjected to metallurgical further processing by some prior known method.
The solution from liquid-solid separation is conducted to the atmospheric
copper
removal 1.
A leaching process product solution (PLS), subsequent to the atmospheric cop-
per removal 1, is conducted, downstream of the liquid-solid separation, to the
iron removal 5. The iron removal is effected with some prior known method,
such
as by precipitating the iron with oxygen 02 and by neutralizing the acid
evolved in
the precipitation process for example with lye NaOH. Following the iron
removal,
the precipitate is separated and the solution is conducted to a solution
purifica-
tion 6, wherein the solution is stripped of cobalt and other impurities
detrimental
to nickel production by some prior known method, for example by liquid-liquid
ex-
traction. The pure nickel sulfate solution is used for producing nickel
products by
known methods, such as for example cathode by means of the nickel electrolysis
7. Nickel electrolysis provides a source of an acid-containing nickel solution
or
anolyte, the sulfuric acid contained therein being useful in the presently de-
scribed leaching process.
Some embodiments ofthe invention will be described further by way of the ac-
companying non-limiting examples:
Example 1 Atmospheric copper removal step
Ground sulfidic nickel matte with a composition of 67% Ni, 3,0% Cu, 2,0% Fe
and 23% S was leached in laboratory into a copper-containing nickel sulfate
solu-
tion. Leaching was conducted in a heated cover-equipped steel reactor provided
with agitation and aeration. The solution volume was two liters, air feed 50
L/h,
temperature 85 C and leaching time one hour. Other conditions and results are
shown in table 1. The results reveal that there is achieved a practically
complete
precipitation of the parent solution copper while leaching nickel and iron.
This in-
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dicates a separation of copper and iron.
Table 1
Solution Solid
V (L) pH Ni (pIL) Cu (a(L) Fe m (a) Ni % Cu %
Fe %
Feed 2.0 1.2 109 2.1 2.6 164 67.3 3.0
2.0
End 2.0 4.3 119 0.004 3.2 156 60.8
5.5 1.8
Example 2 Two-step pressure leach
The leach residue of nickel matte, treated as in example 1 and with a composi-
tion of 63% Ni, 6,2% Cu, 2,2% Fe and 23% S, was leached with a laboratory au-
toclave in a two-step fashion. Prior to leaching, the solid matter had its cop-
per/nickel weight ratio changed by having copper sulfide precipitate
containing
66% Cu and 28% S admixed within the leach residue. In the series of tests, the
solution volume was 1 liter and the pressure in autoclave was 8 bar. During
the
first pressure leach step, the temperature was 112 C and the autoclave was
supplied with oxygen under the agitator. Leaching time was 45 minutes. After
this, the supply of oxygen was shut off and the autoclave was heated for the
sec-
ond pressure leach step to the temperature of 147 C. Leaching time in the sec-
ond pressure leach step was two hours and during this period the autoclave was
not supplied with oxygen. Other conditions and the results of pressure leach
tests
are shown in table 2.
The results reveal that the leaching of nickel is also managed in highly
acidic
conditions, but may be carried out in such a way that the solution pH at the
end
of the first pressure leach step is not lower than 3. Between tests 1 and 2 is
also
visible the effect of a copper/nickel weight ratio in the solid matter. At a
higher
copper/nickel weight ratio, substantially more copper becomes leached during
the first pressure leach step and thereby an improved recovery of nickel is
achieved in the second pressure leach step. The results further reveal that
the
leaching of iron takes place at the latest during the second pressure leach
step,
such that the ob-
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9
tamed leach residue comprises relatively pure copper sulfide precipitate which
can
be recycled into the feed of a two-step pressure leaching process.
Table 2
Solution Solid
nzs 04 (g/L pH Ni (WO Cu WO Fe (g/t m Ni %
Cu % Fe % C tieNi ratio
Test
Feed 52.0 73 1.0 1.0 140 26.0 40.5 1.0 1,5
End of step I 1.1 83 20.2 1.6 10.3 45.4 0.2
End of step II 0.0 112 0.3 1.0 85.6 2.4 67.1 0.06
Test 2
Feed 62.0 73 1.0 1.0 180 20.0 40.2 0.7 22
End of step I 1.2 GO 20.3 1.6 10.7 54.8 0.1
End of step II 0.6 111 7.8 1.7 103 0.8 70.4 0.04
Test 3
Feed 17.0 73 1.0 1.0 170 22.1 45.0 0.8 2D
End of step I 3.0 83 24.3 0.06 10.1 40.8 1.6
End of step II 0.0 105 8.1 1.6 83.5 0.05 74.8 0.07
Example 3 Atmospheric leaching
Ground metallic nickel matte with a composition of 48% Ni, 7,4% Cu, 30% Fe and
5,7% S was leached in laboratory into an acidified nickel sulfate solution.
Leaching
was conducted in a heated cover-equipped steel reactor provided with agitation
and aeration. The solution volume was two liters, oxygen feed 50 L/h,
temperature
85 C and leaching time three hours. Other conditions and the results are shown
in
table 3. The results indicate that metallic nickel matte can be leached in
atmos-
pheric conditions in such a way that practically all that remains in the leach
residue
are sulfidic components. The test also demonstrates that iron remains in a dis-
solved state as long as the solution is maintained at a low pH.
Table 3
Solution Solid
H2SO4. (gI-) pH Ni (W Cu (04 Fe WO m (g) Ni % Cu % Fe % S %
Feed 72.0 80 0.1 124 48.1 7.4 30.3
5.7
End 1.8 85 0.002 16 27.5 27.1 29.8
9.8 26.8
