Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for the recovery of valuable metals
The present application relates to a multistage
process for the recovery of valuable metals from aqueous,
acidic leachate solutions.
Many valuable metals such as copper, silver, gold or
palladium occur in nature not alone but in association with
other ores. To recover the valuable metals from these ores,
hydrometallurgical processes by means of which the valuable
metal ions are separated from the other constituents of the
ores by liquid/ liquid extraction using suitable complexing
agents have become established. For this purpose, the ores or
the ore concentrates obtained by flotation are first broken
down chemically in a leaching process, forming aqueous
solutions of the metal salts. The valuable metal ions are
subsequently separated out by means of liquid/ liquid
extraction. Metal recovery occurs in various ways depending on
the type of metal, for example by electrowinning of copper.
This is followed by a refining process dependent on the type
of metal, for example the electrolytic refining of copper. The
details of the leaching process are dependent on the type of
valuable metal, the way in which it is bound in the ore and
the type of materials accompanying the ore (gangue type). The
ore is frequently leached using aqueous acids, for example
sulphuric, hydrochloric or nitric acid, or alkalis such as
ammonia/ammonium carbonate or ammonium sulphate. However,
apart from the known chemical processes, biological leaching
by means of suitable strains of bacteria has become
established in recent years. Thus, US 4,571,387 describes a
process for leaching sulphidic copper ores, for example
chalcopyrite (CuFeS2), in which the copper ores are brought
into contact with sulphide-oxidizing strains of the bacterium
Thiobacillus ferrooxidans in acidic aqueous solution and Cu2+
ions as well as sulphur and sulphate or sulphuric acid are
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formed by oxidation of the ore. The Cu2+ ions can then be
processed further by liquid/liquid extraction. A similar
process is described in US 4,729,788 which discloses the use
of thermophilic bacteria of the Sulfolohus type for leaching
sulphidic gold or silver ores.
Although microbiological leaching has advantages over
the classical methods (in particular, it requires less
complicated and expensive plants and the amounts of inorganic
acids or alkalis required for leaching can be reduced),
microbiological leaching does cause problems in the subsequent
processing stage of solvent extraction (SX). This is because
the resulting leachate solutions contain not only the desired
high proportions of valuable metals but also, as an intrinsic
aspect of the leaching process, relatively large amounts of
accompanying elements such as iron. In particular, the
leachate solutions obtained have relatively high proportions
of valuable metal ions. Furthermore, for example,
microbiological leaching of sulphidic Cu ores (or ore
concentrates) can give aqueous leachate solutions containing
Cu ions in amounts of up to 50 g/1. Such high valuable metal
concentrations are, on the one hand, desirable for efficient
leaching, but on the other hand these high valuable metal
concentrations cause problems in the SX process. This is
because when such concentrated leachate solutions are treated
with complexing agents, a significant drop in the pH of the
aqueous phase occurs during extraction, since the complexing
agents generally exchange the valuable metal ions for protons.
However, as the pH drops the acid-dependent equilibrium
between free complexing agents and loaded complexing agents is
quickly shifted to the side of the free complexing agents, so
that the proportion of valuable metal which can still be
extracted from the aqueous leachate solution also falls. For
example, the extraction of copper can virtually no longer be
carried out economically at pH values in the region of 1Ø
It is therefore an object of the present invention to
provide an improved extraction process by means of which even
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leachate solutions containing high proportions of valuable
metal ions can be subjected to liquid/liquid extraction.
A further problem is posed by the accompanying metals
in the leachate solution. Thus, most commercially exploited
copper ores also contain iron which is likewise brought into
solution by the leaching process. In the extraction process,
this iron can lead to selectivity problems which reduce the
yield of valuable metal. It is therefore a further object of
the present invention to provide a process in which valuable
metal ions can be extracted selectively and in economically
viable yields from leachate solutions in which they are
present in association with other metal ions.
It has been found that the above objects are achieved
by combining the liquid/liquid extraction process known per se
with particular pH control.
The present application accordingly provides a
multistage process for the recovery of valuable metals from an
acidic, aqueous leachate solution in which valuable metal ions
are present in concentrations of from 10 to 40 g/1, which
process comprises
(I) liquid/liguid extraction of the leachate solution,
where the leachate solution is brought into contact
at least once with an organic, water-insoluble
extractant,
(II) and subsequently bringing the extractant loaded with
valuable metal ions obtained from (I) into contact
at least once with an acidic aqueous phase, with the
major part of the valuable metal ions being
transferred into the acidic aqueous phase,
(III)and subjecting the acidic aqueous phase obtained
from (II) to an electrolysis or crystallization
process for recovering the valuable metal,
wherein the pH of the aqueous leachate solution is set to at
least 1.2 and not more than 2.5 before and/or during the
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liquid/liquid extraction process (I).
Valuable metals in the context of the present
invention are metals from the group consisting of copper,
silver, gold and palladium which can be brought in ionic form
into the aqueous phase by leaching of their ores or ore
concentrates obtained by flotation. It is immaterial whether
the valuable metal is present in the ore in elemental or ionic
form. The ores typically contain the valuable metals in the
form of oxides, sulphides, selenides, tellurides, carbonates,
nitrates, silicates or halides. In the process of the
invention, particular preference is given to using leachate
solutions containing copper ions as valuable metal. The
leachate solutions are therefore preferably obtained by
leaching sulphidic ores, for example chalcopyrite or bornite.
Apart from the valuable metal ions, further metal ions
can be present in the leachate solutions. These are preferably
iron ions which are present in amounts of from 0.001 to 25
g/1. The iron ions are preferably in the form of Fe3+ ions with
lesser amounts of Fe2+ ions . From 60 to 99 mol% of Fe3+ and from
40 to 1 mol% of Fez+ ions are preferably present. In the
process of the invention, particular preference is given to
iron-containing leachate solutions containing virtually
exclusively Fe3+ ions.
It is also preferred that the aqueous leachate
solutions are obtained by biological leaching of ores
containing valuable metals or their flotation concentrates
using thermophilic microbes of the Sulfolobus type. Here, the
ores or concentrates are leached with the aid of the
thermophilic bacteria in the temperature range from 40°C to
90°C.
The process as set forth in steps (I) to (III) is
known per se and can be carried out in the plants or
apparatuses described in the prior art.. It is an essential
aspect of the invention that the aqueous leachate solution is
subjected to pH control at particular points within the
process .
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In the interests of providing an overview, Figure 1
shows an embodiment of the invention on which the following
description is based but which is purely by way of example and
has no limiting character. In the figure, the solid lines
relate to the transport of organic phases while the broken
lines show the transport of aqueous phases.
The aqueous, acidic leachate solution (feed) is
introduced into a mixer/settler E1 and brought into contact
with a solution of an organic complexing agent. The organic
phase is separated off and the pH of the remaining aqueous
phase is adjusted by bringing the aqueous phase into contact
with a suitable inorganic or organic base which may be present
in solid form or as a solution or dispersion. The aqueous
phase which has thus been set to a pH in the range from 1.2 to
2.5 is again brought into contact with the organic complexing
agents in further mixer/settlers E2 and E3. The aqueous phase
resulting from the last extraction stage ("raffinate",
solution depleted in copper) is discharged from the system.
The organic phase loaded with metal ions is then conveyed to
step (II) and stripped in S1 and S2 using a strongly acidic
aqueous solution (H2S04). This is preferably carried out in
mixer/settler systems, preferably using two mixer/ settlers
connected in series. As a result of contact with the strongly
acidic aqueous solution, the metal ions go back into the
aqueous phase and can now be passed to a further recovery step
(III), preferably electrolysis (electrowinning).
The organic complexing agents used in the
liquid/liquid extraction (I) in the process of the invention
are essentially hydrophobic compounds which undergo specific
or nonspecific interactions and/or coordination with the
valuable metal ions so as to balance and/or shield the charge
of the valuable metal ion so that the polarity is reduced
sufficiently for the solubility of the ions in the aqueous
phase to be decreased while the solubility of the complex in
the organic phase is increased. Such complexing agents include
particular phosphorus compounds, organic carboxylic acids,
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phenols, ketoximes, aldoximes and further chelating agents
known in this context to those skilled in the art. Examples of
specific classes of substances are organic phosphoric,
phosphonic or phosphinic acids, in particular di(2-
ethylhexyl)phosphoric acid or trioctylphosphine oxide (TOPO),
tributyl phosphate (TBP) or dibutyl carbinol (DBBP), also
acetylacetone, oxalic acid, citric acid, 2,2'-bipyridyl,
salicylaldehyde or ethylenediaminetetraacetic acid.
According to the invention, particular preference is
given to the use of aldoximes and/or ketoximes as complexing
agents. Such compounds are commercially available under the
trade name LIX~ from Henkel Corp.
In a preferred embodiment, step (I) is carried out
using organic extractants containing from 1 to 35% by weight,
preferably from 2 to 25% by weight and in particular from 2 to
20% by weight, of organic complexing agents. The complexing
agents are dissolved in suitable organic solvents, for example
aliphatic or cycloaliphatic or else aromatic hydrocarbons or
mixtures thereof, chlorinated hydrocarbons, ketones or ethers
having high flash points (from at least 70°C to about 110°C)
and also mixtures of these compounds.
The flow rates of the organic (O) and aqueous (A)
phases can be or have to be matched as a function of the Cu
concentration in the leachate solution ("advanced flow rates"
of O/A). At a Cu content of from about 12 to 15 g/1 in the
leachate solution, it has been found to be advantageous to set
an O/A ratio of from 1.6 to 2Ø This ratio can also be
increased in principle, but the extraction performance then
deteriorates.
The stripping of the loaded organic phase is
preferably carried out using an aqueous sulphuric acid
solution. The aqueous solutions for stripping typically
contain from 100 to 220 g/1 and in particular from 160 to 180
g/1 of HZS04. The latter concentration is particularly
preferred for subsequent Cu electrowinning, with the stripping
solution (B.E.) containing about 30-35 g/1 of Cu. During the
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stripping stage (II), the acid-labile formation equilibrium of
the valuable metal ion complexes is shifted back toward the
starting compounds as a result of the high proton
concentration. As a result, the valuable metal ions are set
free and go into the aqueous phase. This acidic aqueous phase
loaded with valuable metal ions is then subjected in (III) to
known work-up procedures such as crystallization or,
preferably, electrolysis in order to recover the valuable
metal.
The way in which process step (I), namely the
liquid/liquid extraction, is carried out is of particular
importance, since the pH control according to the invention is
carried out before and/or during this process step.
In the simplest case, the pH control step can be
carried out prior to introduction of the aqueous leachate
solution into the extraction stage (I). This pH control prior
to the extraction is preferably achieved by diluting the
untreated leachate solution with water. However, other methods
of pH control described further below can, in principle, also
be employed in this process step. The dilution method is
particularly advantageous when the leachate solution contains
large amounts of iron ions which would precipitate in the form
of solid iron hydroxides on addition of bases. However, this
method does produce comparatively large volume flows which
require the entire plant to be made appropriately large. In
this method, the leachate solution is preferably diluted with
two or three times its volume of water.
For this reason, the pH control of the aqueous
leachate solution is preferably carried out only after the
first extraction step. For the purposes of the present
application, the term "aqueous leachate solution" is also used
to refer to such a solution which has already been subjected
to one or more extraction steps. Here, pH control can take the
form, for example, of a titration using a suitable base.
Before carrying out the process of the invention, the
leachate solutions have pH values of less than 1.2, with
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typical values being in the range from 0.8 to 1.1. The copper
contents are from 10 to 40 g/1. The iron contents can be in
the range from 0.0001 to 25 g/1. In addition, depending on the
type of ore which has been leached, other metal ions such as
aluminium, lead, cadmium, calcium, chromium, cobalt,
manganese, magnesium, sodium, nickel and zinc ions may be
present in amounts of from 0.001 to 1.0 g/1.
As bases for setting a pH in the range from 1.2 to 2.5
as specified according to the invention, it is possible to use
either inorganic or organic bases. Suitable inorganic bases
are the oxides or hydroxides of alkali metals and/or alkaline
earth metals, for example sodium hydroxide or calcium
hydroxide (for example as "slaked lime" or "quicklime").
Preference is given to using calcium hydroxide or sodium
hydroxide in the form of aqueous solutions or slurries. These
solutions or slurries typically contain the inorganic base in
amounts of from 10 to 30% by weight, preferably from 20 to 25%
by weight. The neutralization can also be carried out using
solid bases. During the pH control of the aqueous leachate
solution, for example, the bases are added until the desired
pH range has been set. The aqueous leachate solution which has
been adjusted in this way is then subjected to the further
process steps. Typical amounts of base solution, as 25%
strength by weight NaOH, are in the range from 5 to 30 g of
base solution/litre of leachate solution.
In principle, organic bases such as tertiary amines
can also be used for controlling the pH. These are marketed by
the Applicant under the trade name ALAMINE~. Quaternary
ammonium compounds, for example of the ALIQUAT~ type (Henkel),
are also suitable. However, in the case of these organic
bases, the pH control step has to be carried out in the form
of a separate SX stage, i.e. a separate extraction circuit. In
such a procedure, the amines in the form of 30 - 40% strength
by weight solutions in suitable solvents, e.g. kerosene, are
brought into contact with the aqueous leachate solution.
It is also preferred that the pH control of the
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aqueous leachate solution be carried out during a three-stage
liquid/liquid extraction as shown in Figure 2, where the
leachate solution is first brought into contact with the
organic extractant in two mixer/ settlers, pH control is
subsequently carried out and this is followed once more by a
mixer/settler system for extraction using the organic
extractant.
In all cases, the process of the invention can be
carried out continuously or batchwise. However, it is
generally carried out continuously.
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Examples
The process of the invention was carried out in a
plant as shown in Figure 1. Here, E1 to E3 are each
mixer/settlers in which the aqueous leachate solution is
sequentially brought into contact with the organic extractant.
S1 and S2 are stripping stages in which the loaded organic
phase is brought into contact with the aqueous acidic phase.
B.E. (barren electrolyte) is the Cu-containing sulphuric acid
stripping solution which is used for stripping the Cu-laden
organic phase. P.E. (pregnant electrolyte) is the Cu-enriched
sulphuric acid solution obtained after stripping; this is
passed to Cu electrowinning (Cu-EW). Buffer vessels serve for
intermediate storage of the stripped organic phase and the Cu
laden organic phase and make it possible for the process to be
carried out continuously.
A leachate solution (feed) having a Cu concentration
of 13.9 g/1 and a pH of 3.8 was used. The iron content was 8
mg/1. In addition, according to the invention, after the first
mixer/settler E1 in the liquid/liquid extraction there was
installed a further mixer/settler for adjusting the pH ("pH
adjustment") in which the organic phase loaded with valuable
metal was brought into contact with an aqueous 25% strength by
weight NaOH solution. The extraction was carried out using a
30% v/v solution of LIX 973N~ in Shellsol~ D70 as organic phase
in each case.
Four trials with and without pH adjustment were
carried out. The results are shown in Tables la to 1d:
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Table la: Process without pH adjustment:
Time g/1 0/A g/1 of pH in g/1 of g/1 of g/1 NaOH/1
of Cu Cu Cu of
in Cu in E1 controlin E2 in E3 Cu of
h in 0/A O/A O/A in feed
feed stage H.E.
7 14.26 1.64 12.5/4.561.56 6.94/0.674.80/0.3131.2
18 14.26 1.62 13.3/5.26~ 1.61 7.92/0.87~ 5.42/0.36~ -
~ ~ ~ ~ 28.3
Table 1b: Process with pH adjustment between E1 and E2 (pH -
1.8)
Time g/1 0/A g/1 of pH g/1 of g/1 of g/1 NaOH/1
of Cu in Cu Cu
in Cu in E1 controlin E2 in E3 of of
h in 0/A O/A O/A Cu feed
feed stage in
B.E.
3 13.6 1.60 -/5.84 1.8 -/0.87 -/0.25 29.1 12
6 14.16 1.64 14.06/5.861.8 8.90/0.805.7/0.2529.4 10
~ ~ ~ ~ ~ ~
Table lc: Process with pH adjustment between E1 and E2 (pH -
2.0)
Time g/1 O/A g/1 of
of Cu pH g/1 of g/1 of g/1 NaOH/1
in Cu Cu
in Cu in E1 controlin E2 in E3 of of
h in 0/A O/A O/A Cu feed
feed stage in
B.E.
2 14.3 1.8 -/- 2 -/- -/0.16 - 20
6 19.3 1.8 13.4/5.372 8.44/0.375.64/0.1424.4 18
~ ~ ~ ~
Table 1d: Process with pH adjustment between E1 and E2 (pH -
2.2)
Time g/1 0/A g/1 of pH g/1 of 1 of
of Cu in Cu Cu g/1 NaOH/1
g/
in Cu in E1 controlin E2 in E3 of of
h in 0/A O/A O/A Cu feed
feed stage in
B.E.
2 14.3 1.8 -/5.26 2.2 -/0.28 -/0.10 - 32
6 14.0 1.8 13.2/5.272.2 8.32/0.295.44/0.0933.7 30
~ ~ ~ ~
Two further trials in which the pH control step was
located downstream of the second mixer/settler E2 in the
liquid/liquid extraction as shown in Figure 2 were carried
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out.
The results are shown in Table 2a and 2b:
Table 2a: Process with pH adjustment between E2 and E3 (pH -
1.4):
Timeg/1 O/A g/1 of g/1 of pH g/1 of g/1 NaOH/1
of Cu Cu in Cu
in Cu in in E1 in E2 controlin E3 of of
h O/A O/A O/A Cu feed
feed stage in
B.E.
3 13.8 1.7 13.42/6.07-/1.75 1.4 -/0.05 33.0 45
6 13.8 1.7 13.40/6.008.72/1.111.4 5.86/0.0532.5 41
~ ~ ~ ~
Table 2b: Process with pH adjustment between E2 and E3 (pH -
1.5)
Timeg/1 O/A g/1 of
of Cu g/1 of pH g/1 of g%1 NaOH/1
Cu in Cu
in Cu in E1 in E2 controlin E3 of of
h in O/A O/A O/A Cu feed
feed stage in
H.E.
3 13.92 1.8 12.64/5.157.80/0.821.5 5.48/0.0234.0 45
6 14.06 1.8 12.50/5.257.80/0.821.5 5.40/0.0233.8 43
Comparison of the concentration in the raffinate (the
aqueous phase in E3) shows that the copper concentration can
in each case be reduced to a value of about 10 mg/1 of Cu or
less by means of the process of the invention, while values of
greater than 0.3 g/1 of Cu are obtained without pH control.
The process of the invention thus leads to considerably
improved efficiency of the extraction step compared with
conventional processes.
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Key to Figures for Finisher (numbers refer to copies marked up
in red)
1. Figure 1
2. Raffinate
3. pH adjustment
4. Buffer vessels
P.E. and B.E. and Feed remain unchanged (also in Figure 2).
5. Figure 2
6. Raffinate
7. pH adjustment
8. Buffer vessels
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