Note: Descriptions are shown in the official language in which they were submitted.
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SELECTIVE RECOVERY OF COPPER AND SILVER FROM COMPLEX SULPHIDE
ORE, CONCENTRATE, TAILINGS, CRUSHED ORE OR MINE SLUDGE
BACKGROUND OF THE INVENTION
Copper, silver and gold are characterized by their unique physico-chemical
characteristics and are essential commodities for industrial applications
outside of their
monetary or decorative value. All three metals are excellent conductors of
electricity.
This invention deals with a selective leaching and recovery of copper and
silver from
composite copper and silver bearing sulphides, which are either in the form of
complex
metal containing sulphidic minerals, or in the form of sulphide concentrates,
in-situ or
ex-situ in an economic and environmentally friendly manner.
Copper is the third most common metal in use, trailing only iron and
aluminium. Copper
sulphides, in naturally occurring mineral deposits, are normally found in
association with
sulphides of iron, nickel, lead, zinc and molybdemum and often contain traces
of silver
and gold. Chalcopyrite is one of the most common ores from which it is
extracted.
Copper finds its wide-ranging applications in electrical wires, roofing and
plumbing and
industrial machinery.
Conventional extractive metallurgical process generally involves
pyrometallurgical
methods for recovering copper values from copper sulphides. Known recovery
process
mostly involves grinding the ore, froth flotation (which selectively separates
minerals
from gangue by taking advantage of differences in hydrophobicity) to get an
ore
concentrate, roasting and reduction with carbon or electrowinning. However,
such
treatment often entails expensive mining and beneficiation process steps to
concentrate
the sulphides. In addition, the production of copper employing the known
technology
from sulphidic copper ores produces large amounts of sulfur dioxide, carbon
dioxide
and cadmium vapor. Smelter slag and other residues of process also contain
significant
amounts of heavy metals. Further, strict adherence to environmental
regulations
governing mining operations may substantially increase the cost of recovering
copper
from its ores by conventional processes.
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The hydrometallurgical processes of this invention to recover copper and
silver from
their respective sulphide minerals employ either air or oxygen as oxidants to
convert
sulphidic mineral to their oxidized forms followed by the dissolution of
oxidized mineral
to recover target metals. Because of the limited solubility of oxygen or air
in aqueous
system, the time taken to oxidize the minerals for leaching without employing
air or
oxygen as oxidants becomes excessively burdensome for commercial deployment of
the process. Further, high pressure and temperature requirement to increase
oxygen
supply without employing air or oxygen as oxidants adds to the cost of the
overall
recovery process. This invention deals with the utilization of an oxidant for
rapid
oxidation of the mineral, providing an economic route for the
commercialization of this
technology. The oxidant is electrochemically regenerated for further leaching.
As a
result, the overall process runs as a closed-loop operation.
A patent search revealed alkaline leaching approach to recover metals. United
States
Patent 3,967,957 (Fonseca) describes use of aqueous ammonia oxidative leach
and
recovery of metal values. United States Patent 5,308,381 (Hau et al.)
describes
ammonia extraction of gold and silver from ores and other materials. Neither
of these
methods suggests the recovery process of the present invention.
SUMMARY OF THE INVENTION
A new hydrometallurgical method has been found for selective dissolution of
copper and
silver from complex sulphidic minerals.
The invention comprises a process for selective leaching of at least one of
copper and
silver from mixtures and ores containing at least one of copper sulphide and
silver
sulphide, comprising:
a. contacting the mixture or ore sequentially with an aqueous oxidant followed
by a leachant comprising: 1) an oxidant selected to oxidize the sulphur
present only to elemental sulphur, and 2) ammonium hydroxide in amounts
sufficient to form soluble ammine complexes of at least one of copper and
silver;
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b. extending the contact time between leachant and solids to give the desired
recovery of at least one of copper and silver in the leachate while
maintaining
operative reagent concentrations;
c. separating the desired leachate from the residual solids;
d. recovering at least one of copper and silver from the leachate; and
e. regenerating the lixiviant used in the process.
The oxidant may be selected from the group consisting of an oxygen-containing
gas, a
water-soluble peroxide, a water-soluble perchlorate and a water-soluble
hypochlorite.
Preferably the oxidant is a hypochlorite in a concentration sufficient to
oxidize all of the
sulphides present.
When the starting solids also contain lead sulphide, the process is carried
out
sequentially by recovering the lead employing alkali metal hydroxide followed
by
treatment of the washed residue with ammonium hydroxide to dissolve at least
one of
copper and silver as ammine complexes. The process for selective leaching of
lead
and at least one of copper and silver from mixtures and ores containing lead
sulphide
and at least one of copper sulphide and silver sulphide comprises:
a. contacting the mixture or ore sequentially with an aqueous oxidant followed
by a leachant comprising: 1) an oxidant selected to oxidize the sulphur
present only to elemental sulphur, 2) alkali metal hydroxide in amounts
sufficient to form soluble plumbate ion, and 3) ammonium hydroxide in
amounts sufficient to form soluble ammine complexes of at least one of
copper and silver;
b. extending the contact time between leachant and solids to give the desired
recovery of at least one of copper and silver in the leachate while
maintaining
operative reagent concentrations;
c. separating the desired leachate from the residual solids;
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d. recovering at least one of copper and silver from the leachate; and
e. regenerating the lixiviant used in the process.
The desired oxidation potential of the leachant for steps a) and b) is
maintained by
reagent addition. The desired hydroxide content of the leachant is maintained
throughout the leaching steps a) and b). Majority of the oxidation of the
mineral occurs
within 15-30 minutes. The contact time in steps a) and b) can be extended to
attain
desired recovery.
The invention includes an aqueous leachant composition selected to solubilize
at least
one of copper and silver selectively from mixtures and ores containing at
least one of
copper sulphide and silver sulphide, comprising:
1) an oxidant selected to oxidize the sulphur from the sulphides only to the
elemental sulphur stage, and
2) ammonium hydroxide selected to form soluble ammine complexes of at least
one of copper and silver from at least one of copper sulphide and silver
sulphide oxidation products.
The invention also includes an aqueous leachant composition selected to
solubilize lead
and at least one of copper and silver selectively from mixtures and ores
containing lead
sulphide and at least one of copper sulphide and silver sulphide, comprising:
1) an oxidant selected to oxidize the sulphur from the sulphides only to the
elemental sulphur stage;
2) an alkali metal hydroxide selected to form soluble alkali metal plumbate
from
lead sulphide oxidation product; and
3) ammonium hydroxide selected to form soluble ammine complexes of at least
one of copper and silver from at least one of copper sulphide and silver
sulphide oxidation products.
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In a preferred aspect the composite sulphides are treated with sodium
hypochlorite at
ambient temperature and pressure. Sodium hypochlorite is used as an oxidant to
oxidize sulphide in the composite mineral to elemental sulphur. If lead
sulphide is
present in the original mixture, lead oxide formed by treatment with sodium
hypochlorite
is reacted with sodium hydroxide to form soluble sodium plumbate which is
subsequently treated to recover lead as high purity lead carbonate. Lead
carbonate can
be easily converted to other lead products based on end-user requirements. The
lead
depleted residue is washed with water and treated with ammonium hydroxide to
dissolve at least one of copper and silver as ammine complexes and recovered
as at
least one of pure copper compound and silver metal. Copper compound thus
formed
can be easily converted into any other copper products based on end-user
needs.
In another embodiment of the invention, unconsolidated minerals containing
lead
sulphide and at least one of copper sulphide and silver sulphide, including
discrete
blocks of rocks and agglomerated ore particles and concentrate, agglomerated
and
unagglomerated sulphide bearing mill tailings of mineral beneficiation and
similar
sulphide containing by-products and waste products of recycling processes, are
leached
ex-situ, at ambient temperature and pressure, with sodium hypochlorite and the
selected hydroxide sequentially. The pregnant leach solution is subsequently
removed
and is treated for desired metal recovery. The lixiviant can be either used as
a mixture
of oxidant and the selected hydroxide or oxidant followed by the lixiviant.
The leaching
process can also be used if lead sulphide is not present in the unconsolidated
minerals.
DESCRIPTION OF PREFERRED EMBODIMENTS
In one aspect of the present process for solubilizing at least one of copper
and silver
from composite sulphidic minerals in the ore body, crushed ore or tailings, an
aqueous
solution of ammonium hydroxide is used to dissolve at least one of copper and
silver as
ammine complexes after oxidation of the sulphidic minerals by sodium
hypochlorite in
the previous step. In one of the preferred embodiments of the present
invention the
sulphide bearing minerals in the ore are brought into contact with ammonium
hydroxide
at high pH. The leach solution reacts with the sulphidic minerals to attain
the highest
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metal ion concentration to render the leaching process economical as
determined by
the kinetics of the process. The pregnant solution containing the dissolved
value metals,
in particular at least one of solubilized copper and silver, are recovered
from the leach
solution by precipitation. Ammonium hydroxide (most common laboratory reagent)
combined with sodium hypochlorite (commonly referred to as bleach) ensures
that the
reagents utilized in the leaching process are not likely to damage the
environment. The
leaching process is conducted at ambient temperature and pressure.
The recovery of metals from their sulphides by hydrometallurgical methods
usually
necessitates the oxidation of the sulphide ion in the metal sulphide to render
the metal
soluble and hence recoverable from the solution. It has been found that for
best results
the sulphide in the sulphidic minerals is oxidized only to elemental sulphur,
hence the
oxidation potential of the oxidant in the leach solution is adjusted such that
it is
insufficient to oxidize the sulphide to the hexavalent state. The oxidation
potential of a
reagent is understood to mean the power of the reagent to remove electrons and
it may
be expressed quantitatively in millivolts. In the present process for leaching
at least one
of copper and silver from at least one of copper sulphidic and silver
sulphidic minerals
by sodium hypochlorite followed by ammonium hydroxide, the oxidant (sodium
hypochlorite) could be potentially replaced by oxygen or air or even electro-
oxidation.
Other alkali metals e.g. potassium could replace sodium.
The chemistry involved in the sequential leaching of lead followed by at least
one of
copper and silver in alkaline leaching process is as follows:
Lead Recovery
1. Chlorine and sodium hydroxide are produced by electrolysis of aqueous
sodium
chloride solution.
2NaCI + 2H20 ¨> Cl2 + H2 + 2NaOH
2. Sodium hypochlorite is produced by mixing chlorine with sodium
hydroxide.
4 Cl2(g) + 8 NaOH ¨*4 NaCIO + 4 NaCI +4 H20
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3. Sodium hypochlorite reacts with lead sulphide in presence of sodium
hydroxide
to produce soluble sodium plumbate, sodium chloride and elemental sulphur.
NaCIO + PbS(s) + NaOH ---> NaPbOOH + NaCI + S
4. Soluble sodium plumbate produced in step 3 is treated with carbon
dioxide gas to
precipitate insoluble lead carbonate.
NaPbOOH + NaOH + 2 CO2(g) --> PbCO3(s) + Na2CO3 + H20
5. Sodium hydroxide is regenerated by treating sodium carbonate produced in
step
4 with quick lime.
CaO + H20 + Na2CO3 ¨> CaCO3(s) +2 NaOH
carbon dioxide gas, which are recycled.
CaCO3 ¨> CaO + CO2
A bleed solution is intermittently treated to remove the impurities built up
during the
leaching process.
Silver and Copper Recovery
The lead depleted residue containing at least one of oxidized copper and
silver is
washed with water and treated with ammonium hydroxide solution to dissolve at
least
one of copper and silver as at least one of copper and silver ammine
complexes.
Cu2+(aq) + 4 NH3(aq) --> Cu(NH3)42+(aq)
Ag+(aq) + 2 NH3(aq) --> Ag(NH3)2+(aq)
Silver is precipitated as metallic silver by employing copper compounds.
Ammonia is
recovered by distillation and recycled for further use. Copper is precipitated
as either
copper hydroxide or carbonate.
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The present invention has the additional advantage that it does not entail
preconcentration of the minerals, which may require costly mining expenditures
and
equipment. The process does not create acid drainage problems and uses
relatively
environmentally benign reagents. Since iron does not dissolve under alkaline
conditions,
the process can recover metals from the iron containing complex minerals as
high purity
products. Other complex sulphides containing gold, nickel, cobalt and
molybdenum in
addition to copper, silver, lead, zinc and iron can also be treated by the
process
proposed herein.
EXAMPLE
Column test was conducted to simulate leaching. Approximately 120 g crushed
ore,
containing composite lead, silver and copper sulphidic minerals was lightly
ground with
a mortar/pestle and packed in a 1.27 cm-ID (internal diameter) X 51cm-L clear
vinyl
tube. Small plugs of glass wool were placed on the ends of the tubing, acting
as
particulate filters as the liquid goes through the column. Tapping the sides
of the column
ensured uniform packing. Prior to leaching, N2 sparged deionized water was
pumped
through the column to remove any entrapped air. The deionized water was left
in the
sealed column overnight.
The oxidant (0.48M Na0C1) was pumped upward through the column, at relatively
constant flow rate using a peristaltic pump for an hour. The column was washed
with
water. The lixiviant (1.35 M NR4OH) was then pumped upward through the column,
at
relatively constant flow rate using a peristaltic pump. The effluent was
collected in a
separatory funnel. 10-15 ml aqueous samples were collected at the exit of the
column at
pre-set time intervals and quantitatively analyzed for copper and silver
concentration.
The target flow rate was 1 ml/min, translating into approx 20 minutes
residence time in
the column. The actual average flow rate throughout the 22.5 hours testing
period was
1.05 ml/min.
Although the present invention has been described with reference to the
preferred
embodiments, it is to be understood that modifications and variations may be
resorted
to without departing from the spirit and scope of the invention, as those
skilled in the art
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readily understand. Such modifications and variations are considered to be
within the
purview and scope of the invention and the appended claims.
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