Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE TREATMENT OF MOLYBDENUM CONCENTRATE
FIELD OF THE INVENTION
This invention relates to the treatment of a
molybdenum concentrate. It also relates to the
treatment of a copper-molybdenum concentrate for the
recovery of metal values therefrom.
BACKGROUND OF THE INVENTION
Molybdenum occurs chiefly as molybdenite (MOS2)
and is often present in small amounts in ores also
containing copper.
Typically, such an ore contains about 0.5% to 1%
Cu (as sulphide) and about 0.01 to 0.03% Mo, by
weight, although these concentrations vary widely.
The ore is usually treated in a concentrator to
produce a bulk concentrate containing about 25%-40%
Cu and about 0.3%-2% Mo. The bulk concentrate is
then treated in a molybdenum separation plant to
produce a Cu concentrate and a Mo concentrate
containing about 50% Mo and about 0.1 - 10% Cu as
copper sulphide. The % Cu in the Mo concentrate
varies widely as the separation of Cu from Mo is
sometimes difficult and expensive. If the % Cu in
the Mo concentrate is above about 0.25% Cu, it may
incur a penalty in the market, or be difficult to
market at all when the market is over-supplied. Mo
concentrates containing high Cu, above 0.75%, are
sometimes referred to as dirty Mo concentrates.
The dirty Mo concentrate is sometimes subjected
to a leach process with a concentrated or strong
ferric chloride solution, typically about 50 to 100
g/L chloride to produce a solution containing Cu and
a low copper Mo concentrate containing less than
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about 0.25% Cu, as required by the market, in order
to avoid the payment of a penalty. The amount of the
penalty increases as the amount of Cu in the Mo
concentrate increases.
The ferric chloride leach, however, is a capital
intensive process with high operating costs. Copper
recovery from the leach solution is difficult because
of the high chloride content so that the solution is
often discarded or blended with a heap leaching
operation, which however results in chloride
contamination and reagent losses. As a result of the
high cost, some mining operations do not use the
leaching process. They simply endeavour to produce
the best quality Mo concentrate possible under the
circumstances, which concentrate is then marketed
taking the penalty for having a Cu concentration
which is above the required maximum value.
There is, however, a trade-off between the
amount of Cu in the high copper Mo concentrate
produced by the process and the amount of Mo recovery
to the Mo concentrate. The smaller the amount of Cu
present in the Mo concentrate, the poorer the Mo
recovery from the concentrate. Thus, it is
beneficial to have a copper presence in the high
copper Mo concentrate but, as indicated above, the
presence of copper is problematical when using
conventional processes.
It is accordingly an object of the present
invention to provide an alternative process for the
recovery of Mo from the so-called dirty Mo
concentrate referred to above.
It is also an object of the invention to remove
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impurities, such as Mo and As, from a copper
containing solution during a copper recovery process.
SUMMARY OF THE INVENTION
According to the invention there is provided a
method for the treatment of a molybdenum concentrate
also containing copper and/or zinc comprising the
step of subjecting the concentrate to a pressure
oxidation process which leaches the copper and/or
zinc into solution leaving the bulk of the molybdenum
in the residue, which residue is substantially copper
and/or zinc free.
According to a further aspect of the invention
there is provided a method of purification of a
molybdenum concentrate contaminated with copper
and/or zinc, comprising the step of subjecting the
molybdenum concentrate to pressure oxidation in the
presence of oxygen and a feed solution containing
halide to produce a pressure oxidation solution
containing copper and/or zinc and a solid residue
containing molybdenum.
According to another aspect of the invention,
there is provided a process for the extraction of
molybdenum and copper, comprising the steps of
subjecting a molybdenum-copper concentrate to a
first pressure oxidation in the presence of oxygen
and a first feed solution containing halide and
copper (e.g. CuC12 and CuSO4) to produce a first
copper containing solution and a solid residue
containing molybdenum; subjecting a copper
concentrate to a second pressure oxidation in the
presence of oxygen and a second feed solution
containing halide and copper (e.g. CuC12 and CuSO4)
to produce a second copper containing solution and a
solid residue; subjecting the second copper
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containing solution to solvent extraction to produce
a pregnant copper solution and a raffinate;
recycling a first portion of said raffinate as said
second feed solution to the second pressure
oxidation along with said first copper containing
solution; and recycling a second portion of said
raffinate as said first feed solution to the first
pressure oxidation.
For convenience, a concentrate containing more
copper than molybdenum is referred to as a "copper-
molybdenum concentrate" and a concentrate containing
more molybdenum than copper is referred to as a
"molybdenum-copper concentrate".
The pressure oxidations may be carried out at a
temperature of from about 115 C to about 175 C,
preferably 130 C to 155 C.
The halide may be chloride and the chloride in
the first and second feed solutions may be maintained
at a value of about 8 to 15 g/L, preferably 12 g/L.
The second feed solution may contain from 0 to
50 g/L free acid as H2SO4.
The second feed solution may contain about 10 to
20 g/L, preferably 15 g/L, copper.
Further objects and advantages of the invention
will become apparent from the description of
preferred embodiments of the invention below.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is now described, by way of
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examples, with reference to the accompanying
drawings, in which:
Figure 1 is a flow diagram of a molybdenum and copper
5 recovery process;
Figure 2 is a flow diagram of another molybdenum and
copper recovery process;
Figure 3 is a flow diagram showing a first pressure
oxidation stage of the processes of Figures 1 and 2
in greater detail; and
Figure 4 is a flow diagram showing a second pressure
oxidation stage of the processes of Figures 1 and 2
in greater detail.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
According to one aspect of the process, the
dirty Mo concentrate referred to above, containing
about 50% Mo and about 0.5 - 10% Cu as copper
sulphide, is subjected to a pressure oxidation
process which leaches the copper into solution
leaving the molybdenum in the residue, which residue
is substantially copper free.
An integrated process 3 for the treatment of the
high copper Mo concentrate (dirty concentrate) is
shown in Figure 1. Figure 1 also shows how the high
copper concentrate is obtained from a copper-
molybdenum ore. The ore from mine 4 used in the
present example contains about 0.5% Cu and about
0.01% Mo. The ore is processed in a concentrator 6,
incorporating crushing, grinding and flotation, to
produce a bulk concentrate containing about 28% Cu
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and 0.5% Mo. The ore is then subjected to a re-grind
if necessary to reduce it to a particle size suitable
to allow the next step (selective flotation) to
proceed efficiently. The concentrate then proceeds to
a Mo-separation plant 8. The plant 8 uses flotation
under conditions specifically designed to suppress
flotation of copper sulphide minerals while allowing
MoS2 to float. Special reagents (notably sodium
sulphide or its derivatives) and specific conditions,
such as high pH and low oxidative conditions, are
necessary to achieve this separation.
The above Mo separation process is well-known
and is therefore not described here in any further
detail. As applied in practice, this process has its
limitations, i.e. complete rejection of the copper
minerals to the tailing (Cu concentrate) can only be
achieved at the expense of some rejection of Mo
minerals at the same time. Thus, the Cu concentrate
contains significant amounts of Mo which represents a
loss of Mo since the Mo in the Cu concentrate is not
recoverable by normal accepted methods, e.g.
smelting. This loss of Mo is minimized in the
present process by allowing some copper minerals to
float, thereby maximizing Mo recovery to the float
concentrate. This results in the Mo concentrate
having significant copper content. The Mo
concentrate may also include zinc as an impurity
along with the copper.
The float from the separation plant 8,
therefore, comprises the high copper Mo concentrate
containing about 45-50% Mo and 0.5-10% Cu. The tail
from the separation plant comprises a copper
concentrate containing about 28% Cu and a minimal
amount of Mo.
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The high copper Mo concentrate is subjected to
pressure oxidation 11 in a stirred autoclave at a
temperature of about 150 C in the presence of oxygen
and a feed solution containing copper (as CuSO4) and
chloride (as CuC12) for a retention time of about 1
hour. The copper content in the feed solution is
maintained at about 15 g/L and the chloride'content
at about 12 g/L. The solids content in the pressure
oxidation 11 is up to about 1000 g/L solids, but
preferably about 500 g/L solids.
The slurry from the pressure oxidation 11 is
flashed down to atmospheric pressure in a flash tank
19 (Figure 3) and cooled. It is then filtered, as
shown at 13, to produce a liquid 15 and a solid 17.
In the pressure oxidation 11, about 95% of the
copper in the Mo concentrate is leached into the
solution 15, thus reducing the amount of copper in
the Mo concentrate to about 0.15% Cu, while only
about 1-2% of the molybdenum present in the
concentrate is leached into the solution 15, thereby
resulting in a significantly improved copper removal.
The results of particular tests illustrating this are
given in Examples 1 to 3 below. The liquid 15, in
the present example, contains about 30 g/L Cu, 3-7
g/L Mo, 0-30 g/L free acid and some iron. The
composition of the liquid will of course vary
depending on the type of ore being treated.
If the Mo concentrate also contains zinc, the
solution 15 will also contain zinc which can be
removed by precipitation or a bleed circuit. If
economical amounts of zinc are present then the zinc
may be removed by solvent extraction and
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electrowinning.
The Cu concentrate from the tail of the
separation plant 8 is also subjected to pressure
oxidation 12 in an autoclave under the same
conditions as the pressure oxidation 11, except that
the solids content is lower, e.g. about 120-300 g/L
solids.
After the pressure oxidation 12, the resulting
pressure oxidation slurry is flashed down to
atmospheric pressure in a flash tank 29 (Figure 4)
and cooled. It is then filtered as indicated at 14
(Figure 1), to produce a liquid 16 and a solid
residue 18.
The solid residue 18 can be further treated for
copper recovery by subjecting it to washing followed
by leaching at atmosphere pressure with H2SO4, as
shown in the process of Figure 2, to be described
below.
The pressure oxidations 11 and 12 can be carried
out batchwise or in continuous fashion. The latter
is to be preferred since the autoclave is
continuously in use and no external heating is
required to initiate the exothermic pressure
oxidation reactions.
Both pressure oxidations 11 and 12 are carried
out under an oxygen pressure of about 350 to 1800
kPa, preferably 1000 kPa. This is partial oxygen
pressure over and above the steam pressure in the
autoclave.
When the pressure oxidations 11, 12 are carried
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out in continuous fashion, oxygen, which is about 95%
to 98% pure (the remainder being inert gases such as
nitrogen and argon) is fed to the autoclave. As the
oxygen reacts during pressure oxidation, the oxygen
content vis a vis the inert components is reduced. A
gas bleed is, therefore, taken from the autoclave in
order to maintain a steady amount of about 80-85%
oxygen content in the autoclave. A minimum oxygen
content of at least about 60% in the autoclave is
considered necessary for the pressure oxidation to
proceed satisfactorily.
A surfactant, such as LignosolTm may be added to
the pressure oxidation 12 to counteract wetting of
unreacted sulphides by liquid elemental sulphur
inside the autoclave, which would otherwise prevent
such sulphide particles from reacting completely.
The surfactant has the effect of changing the
physical properties of the liquid sulphur.
The pressure oxidation liquid or solution 16,
which in the present example contains about 20 g/L
free acid and about 40 g/L Cu and a negligible amount
of Mo is subjected to Cu solvent extraction 20 with a
suitable extractant (typically an organic) to produce
a pregnant copper solution and a raffinate. The
pregnant solution (loaded organic) is subjected to
stripping, as shown at 22, and is then subjected to
electrowinning 24 to recover copper. The copper
stripping 22 is effected by means of spent
electrolyte which is recycled from the electrowinning
24, as indicated by arrow 21.
The raffinate resulting from the solvent
extraction 20, which now contains a reduced amount of
copper (about 13 g/L) as well as chloride and about
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60 g/L free acid, is split into two portions 25 and
27, as indicated at 26. The portion 27 is recycled
to the pressure oxidation 12, as shown.
5 The portion 25 is subjected to neutralization 28
with limestone so that it contains essentially no
free acid and is subjected to a liquid/solid
separation 30 to produce a liquid 32 and a solid
residue 35. The residue 35 is essentially gypsum and
10 may be discarded after washing.
The liquid 32 is split, as indicated at 40, in a
certain ratio. The ratio is determined by factors
such as the relative amounts of Mo concentrate and Cu
concentrate being treated in the pressure oxidations
11 and 12 (the Mo concentrate usually being a much
smaller amount) and the relative solids densities at
which the pressure oxidations 11, 12 are carried out,
i.e. 500 and 140 g/L, respectively, in the present
example. The split may vary from 5:1 to 500:1 to the
pressure oxidation 12 and 11, respectively, for these
reasons.
The pressure oxidation 11 is carried out at a
relatively high solid density in order to maintain
operating temperature. This is in view of the fact
that it is the copper in the Mo concentrate which
takes part in the exothermic reaction during pressure
oxidation and the amount of copper in the concentrate
is small.
In the pressure oxidation 12 on the other hand,
the amount of copper in the ore is higher so that
much more heat is generated by the exothermic
reaction, so that the solids concentration must be
lower for temperature control.
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The smaller portion containing essentially no
free acid is recycled to the pressure oxidation 11,
to serve as the feed solution to the autoclave for
the pressure oxidation 11. The major portion is
combined with the copper bearing solution 15 from the
pressure oxidation 11, as shown. The combined stream
23 which in the present example, contains about 10-50
g/L free acid (H2SO4) , chloride (e.g. as CuC12) and
copper (e.g. as CuSO4) is recycled to serve as feed
solution to the autoclave for the pressure oxidation
12. The chloride concentration in the feed solution
is maintained at a value of about 12 g/L and the
copper concentration at about 10-20 g/L, preferably
15 g/L. Although in the present example, the feed
solution is shown as the recycled raffinate stream
23, it will be appreciated that the required amounts
of acid, copper and chloride may be provided from any
other suitable source, e.g. an external source.
It has been found that effectively all Mo in the
solution 15 is precipitated in the pressure
oxidiation 12 and forms part of the solid residue 18.
In this way Mo is removed from the copper bearing
solution 15. If the solution 15 contains arsenic,
this impurity is removed in a similar fashion. The
results of particular tests illustrating this are
given in Examples 4 an 5 below.
In the present example the split at 26 is about
50-50, i.e. about half of the raffinate recycled to
the pressure oxidation 12 is neutralized. However,
this ratio may vary depending on the amount of acid
generated in the autoclave during the pressure
oxidation 12. Thus the feed solution may contain up
to 50 g/L free acid in cases where little acid is
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generated in the pressure oxidation 12 and down to
essentially no free acid in cases where large amounts
of acid are generated.
The solid residue 17 is a clean Mo concentrate,
the copper content of which has been reduced to
market acceptable values, e.g. <0.2% Cu. The
concentrate 17 can be further treated for Mo recovery
or, alternatively, the concentrate can be marketed.
Figure 2 shows a process 50 in which treatment
of the dirty Mo concentrate forms part of a larger
copper recovery process. The copper recovery process,
as well as the recovery of other metals present in
the pressure oxidation solution 16 is more fully
described in U.S. Patents 5,645,708 and 5,874,055, as
well as 5,855,858.
The process 50 is similar to the process 3,
except that, in addition to copper recovery from the
Cu concentrate tail of the Mo separation plant 8,
copper from a separate Cu concentrate 51 is also
extracted. Parts of the process 50 corresponding
with the process 3 are indicated by the same
reference numerals.
The process 50 is an expanded version of the
process 3 in that copper recovery from the residue 18
is effected by subjecting the residue 18 to an
atmospheric leach 52 an then to liquid/solid
separation (counter current decantation 53) to
produce a copper solution which is subjected to
solvent extraction 54 to recover copper therefrom.
As shown, the solvent extractions 54 and 20 are
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effected with the same extractant (indicated by the
stream 55) which is recycled after the stripping 22.
As shown, the loaded extractant from the solvent
extractions 54 and 20 is washed with water at 57
prior to the stripping 22.
As shown, each solvent extraction 20 and 54 is
preceded by a kerosene pre-treatment step 60 and 62,
respectively. In the solvent extractions 20 and 54,
kerosene together with a suitable copper extractant,
typically an hydroxy-oxime is used. In the pre-
treatment steps 60, 62, kerosene only is used to
mimic to some extent, or to anticipate the physical
problems that may occur in the solvent extractions 20
and 54. These problems may be due to:
(a) suspended solids in the aqueous feed
solution, which can result in "crud" in the
solvent extraction circuit; or
(b) surfactants in the aqueous feed solution,
which can result in contamination of the
solvent used in solvent extraction, such that
the solvent is no longer as immiscible as
before, resulting in turn in poor separation
of aqueous and organic phases in the settlers
after solvent extraction (often referred to
as high aqueous entrainment in the organic
phase, and/or high organic entrainment in the
aqueous phase produced form the settlers).
It is to minimize the above problems that the
pre-treatment steps 60, 62 are introduced. In this
way the problems occur in the pre-treatment steps
where the phenomenon can be managed economically,
because of the absence of the extractant which is
much more costly than the kerosene.
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The raffinate from the solvent extraction 54 is
combined with wash water from the organic wash 57 and
is then a split as indicated at 69. The one portion
of the split 69 recycled to the atmospheric leach 52
and the other portion is subjected to neutralization
70 and subsequent liquid/solid separation 71 to
produce a liquid which is recycled as wash water to
counter current decantation (CCD) 72 and a solid
which is combined with the gypsum solid 35.
In the present example, the split 69 is about
50-50. However, this ratio may vary in the same way
as with the split 26 described above.
The process 50 also provides for a bleed stream
80 to be taken at the split 40. The stream 80 is
subjected to solvent extraction 82 for copper
recovery and subjected to neutralization 84 to
precipitate impurities such as Ni, Co and Zn, (if
present) , as shown at 86, after passing through a
thickener 88. The solids separated in the thickener
88 are sent to the neutralization 28.
Example 1
A molybdenite concentrate, containing 45.2% Mo,
3.6% Cu, 3.6% Fe and 34.2% total sulphur, was
subjected to pressure oxidation at 500 g/L solids
density, using a feed solution containing 15 g/L Cu,
12 g/L C1 and 20 g/L free acid. The pressure
oxidation was carried out at 150 C at 200 psig (1480
Kpa) total pressure for a retention time of one hour.
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After the pressure oxidation, the product slurry
was filtered and the residue was washed with water.
The pH of the filtrate was 0.91. The filtrate
contained 31.34 g/L Cu, 3.64 g/L Mo and 5.87 g/L
5 iron. Copper and molybdenum extraction was 96.2% and
1.6%, respectively (The copper extraction is solid
based; the molybdenum extraction is solution based).
Example 2
In another test the same Mo concentrate was
subjected to pressure oxidation under the same
conditions as above, except that the feed solution
contained no free acid. The pH of the filtrate from
the pressure oxidation was 1.06. The filtrate
contained 33.04 g/L Cu, 6.64 g/L Mo and 7.24 g/L
iron. Copper and molybdenum extraction was 95.2% and
2.7%, respectively.
Example 3
In another test the same Mo concentrate was
subjected to pressure oxidation under the same
conditions as above, except that the feed solution
contained 30 g/L free acid. The pH of the filtrate
from the pressure oxidation was 1.03. The filtrate
contained 30.60 g/L Cu, 3.04 g/L Mo and 6.16 g/L
iron. Copper and molybdenum extraction was 94.1% and
1.3%, respectively.
As can be seen from the above three examples,
high copper extraction is obtained while molybdenum
extraction (representing Mo loss from the
concentrate) is low.
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Exam~ple 4
A high copper concentrate containing about 32%
Cu and 23% Fe by weight was subjected to pressure
oxidation at 240 g/L solids density, using a feed
solution containing 15 g/L, Cu, 12 g/L Cl and 31 g/L
free acid. The feed solution also contained 0.298
g/L Mo and 6.65 g/L As.
The pressure oxidation was carried out at 150 C
at 200 psig (1480 Kpa).total pressure for a retention
time of one hour.
After the pressure oxidation, the product slurry
was filtered and the residue washed with water. The
pH of the filtrate was 2.14. The filtrate contained
54.72 g/L Cu, 0.151 g/L Fe, 0.002 g/L Mo and no
detectable arsenic.
Example 5
In another test a medium copper concentrate
containing about 21.4% Cu and 25% iron by weight was
subjected to pressure oxidation at 200 g/L solids
density using a feed solution containing 15 g/L Cu,
12 g/L Cl and 13 g/L free acid. The feed solution
also contained 0.201 g/L Mo and 5.75 g/L As.
The pressure oxidation was carried out at 150 C at
200 psig (1480 Kpa) total pressure for a retention
time of one hour.
After the pressure oxidation, the product slurry
was filtered and the residue washed with water. The
pH of the filtrate was 2.53. The filtrate contained
51 g/L Cu, 0.064 g/L Fe, 0.001 g/L Mo and no
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detectable arsenic.
As can be seen from the results of Examples 4
and 5 the molybdenum and arsenic contained in the
feed solution are effectively precipitated during the
pressure oxidation.
Although certain preferred embodiments of the
present invention have been shown and described in
detail, it should be understood that various changes
and modifications may be made therein without
departing from the scope of the appended claims.
