Note: Descriptions are shown in the official language in which they were submitted.
~ 13~0169
COMPOSITION AND METHOD FOR RECOVERY
OF GOLD AND SILVER FROM SOURCES THEREOF
Backqround of the Invention
This invention relates to the hydrometallurgical
recovery of precious metals, and more particluarly to an
improved hydrometallurgical process for extracting gold and
silver from ores and other sources by use of a
bromate/perbromide leaching solution.
Conventionally, precious metals such as gold and
silver have been recovered from ores by leaching with alkaline
cyanide solution. By reaction with cyanide ion and oxygen the
precious metal is converted to a cyanide complex (gold cyanide
anion) which is taken up in the leaching solution. The
dissolution of gold, for example, is illustrated by the
following reaction:
4Au+8CN- +02+2H20 ~4Au(CN)2 +40H- (1)
Because of the high stability of the gold cyanide complex
anion, even oxygen of the air is sufficent to oxidize gold in
the presence of cynanide ion. Recovery of gold from the
cyanide solution by precipitation may be illustrated by the
following reaction equation:
Au(CN)2+2NaCN+Zn+2H20 Zn(CN)4 2 ( )
Alternatively, gold may be recovered from cyanide solution by
adsorption of the gold cyanide complex anion onto activated
carbon, desorption with a hot alkaline solution, and recovery
by electrowinning or by raising the pH. A
*
.,
'"X
1340169
.
typical scheme for recovery via activated carbon is
illustrated by the reaction equations set forth below:
AU(cN)- activated > Au(CN) adsorbed (3)
Hot
alkaline
OH
5Au AU(cN)2 desorbed
While widely practiced on a commercial scale,
cyanide leaching suffers from well known disadvantages.
Thus, leaching rates with alkaline cyanide solutions are
slow, contact times in the range of 24-72 hours being
common in the case of gold ores. Because of the toxicity
of cyanide, care must be exercised to maintain cyanide
solutions on the alkaline side in order to prevent the
release of hydrogen cyanide gas. Severe environmental
restrictions must be observed, requiring careful monitoring
and control of all process purge streams. Spent cyanide
leaching solutions must be sub~ected to waste treatment
operations before discharge to the environment.
Gold has also been leached commercially by use of
aqua regia, a mixture of concentrated hydrochloric and
concentrated nitric acid, according to the following
reaction equation:
Au+4H +4Cl +NO3 > AuC14+NO+2H2O (4)
Gold may then be recovered by reduction with zinc metal or
raising of the leaching solution pH. However, this method
is relatively unattractive because aqua regia is expensive,
and highly corrosive and emits toxic fumes. Moreover, it
1~40169
~ .
readily dissolves base metals and dissolves gold only
relatively slowly in aqueous solution.
Thiourea has also been used as a lixiviant for
the dissolution of gold from ores according to the
following reaction equation:
Au+Fe +2CS(NH2)2 > Au[CS(NH2)2] +Fe 2+ (5)
Although thiourea is effective, it is subject to oxidative
degradation and is, thus, prone to high consumption levels
in extracting gold from its ore.
Other processes have been developed for the use
of halogens, halides or other halide-bearing compounds for
the recovery of precious metals from ores. For example,
Shaeffer U.S. patent 267,723 describes a process in which
ore is roasted in a vat, water added to the roasted ore,
and liquid bromine added to produce a mixture which is
agitated, thereby dissolving the gold in the water in the
form of a bromide. After filtration to separate the
solids, gold is precipitated from the solution by oxalic
acid or sulfate ion.
Fink et al. U.S. Patent 2,283,198 note that
chloride and bromide ions accelerate the dissolution of
gold in aqueous bromine solution. This disclosure states
that chlorine or hypochlorite may be used as intermediate
oxidizing agents, as indicated by the reactions:
2Br +C12 2 Cl- (6)
or
2Br +ClO +2H Cl +Br2+H2O (7)
They, therefore, proposed a process of extracting gold from
its ores by leaching with a solvent prepared by adding free
'' 13~016~
chlorine to a solution containing a bromide and a large
excess of chloride salt. Alternatively, a leaching agent
is prepared by adding hypochlorite and a mineral acid to a
solution containing a bromide and a large excess of
chloride salt. Fink et al. describe precipitation of gold
from the solution by any of the well known methods, such as
addition of zinc or ferrous sulfate.
Harrison U.S. patent 2,304,823 describes the
recovery of gold from ores by dissolution in a treatment
solution containing iodine, potassium iodide, water and
nitric acid. Mercury or zinc may then be added to recover
the precious metal from solution. The method is said to be
applicable to treatment of refractory material such as
refractory sulfide, telluride or the like.
Jacobs U.S. patent 3,625,674 describes oxidizing
gold with an alcoholic solution of molecular iodine, pro-
ducing AuI which may be decomposed by heating to produce
metallic gold and iodine vapors, the iodine being recovered
and recycled for treatment of additional ore. AuI obtained
from the process is also suitable for forming other gold
compounds for use in the industrial arts, such as gold
sodium thiomalate.
Wilson U.S. patent 3,709,681 describes a process
in which a finely divided source of noble metal is treated
with a solution containing a ketone solvent, dissolved
iodine, bromine or chlorine, a halide salt, and preferably
glacial acetic acid. The noble metal content of the
treatment solution is recovered by displacement onto a
non-noble metal surface such as aluminum foil.
Homick et al. U.S. patent 3,957,505 describe a
process in which gold bearing material is treated in an
aqueous solution consisting of iodine and a water soluble
iodide salt to produce a solution containing dissolved gold
iodide salts. Gold metal is precipitated from the solution
by mixing of the solution with a reducing agent such as
13~016~
hydroxylamine, hydrazine, sodium thiosulfate and the like.
Iodine from the spent aqueous solution is recovered by
acidification of the solution and addition of an oxidizing
agent such as hydrogen peroxide, potassium permanganate,
sodium chromate, chlorine or bromine to precipitate
elemental iodine.
McGrew et al. U.S. patent 4,557,759 describe a
process for the hydrometallurgical recovery of gold from
materials containing gold by leaching the materials with a
lixiviant containing iodine. The lixiviant is prepared by
saturating an aqueous solution of iodide with iodine. When
a sulfide is added to this reagent, iodine reacts with the
sulfide and is converted to iodide. Additional elemental
iodine is then added to this iodide bearing solution until
the desired concentration of total iodine and ratio of
iodine to iodide are achieved for maximum leaching
efficiency. The lixiviant is then circulated through the
ore zone until all the gold is dissolved. Gold is
subsequently recovered on activated charcoal. The excess
iodide formed during the process is reoxidized to iodine
electrochemically in a special diaphragm cell to regenerate
the lixiviant. The desired concentration of iodine in the
lixiviant is between 1 and about 20 grams per liter.
In addition to ores, there is a substantial
number of additional sources of gold and silver which offer
the opportunity for economical recovery. In fact, many of
these secondary sources are substantially richer than ores
with respect of the content of the metal to be recovered.
Gold is available from numerous scrap sources, including
industrial wastes, gold plated electronic circuit boards,
and as an alloy with copper, zinc, silver or tin in the
karat gold used in jewelry. Silver is available from
photographic and x-ray film emulsions, from scrap sterling,
and from numerous industrial sources.
13~101fi9
-
Bazilevsky U.S. patent 3,495,976 describes a
process for recovering gold from a plated substrate by
dissolving the gold in an aqueous solution of potassium
iodide and free iodine. Gold is recovered from the
solution by addition of concentrated sulfuric acid and
heating of the resultant mixture near the boiling point,
which distills off molecular iodine and effects
precipitation of the gold.
Bahl et al. U.S. patent 4,190,489 describe a
composition and method for etching gold, particularly gold
layers on ceramic substrates. The composition is prepared,
for example, by mixing potassium bromide (75g), elemental
bromine (25g), and water (100 ml.) This solution is
effective for recovery of gold from ceramic substrates at
essentially room temperature.
Bahl et al. 4,375,984 describe a process in which
an alkaline metal bromide/bromine solution is used to etch
gold from a substrate. In this disclosure, the etching
solution may be prepared, for example, by mixing potassium
bromide (2g), bromine (one gram), and water (25 ml.) Gold
dissolved in the etching solution is recovered as metallic
gold, either by precipitation using an alkali metal
hydroxide or by decomposition in which the etching solution
is driven off. The alkali metal bromide/bromine etchant
solution may be regenerated by the addition of an acid
thereto.
Kalocsai Bl 4,684,404, based on re-examination of
U.S. patent 4,684,404, describes dissolution of metallic
gold in a reagent comprising a protic solvent such as water
or alcohol, a nonreducing cation source such as sodium,
potassium, ferrous or ammonium, and a source of free
bromine such as molecular bromine, bromine water, or an
inorganic or organic bromine containing compound from which
bromine can be liberated in the reagent. Optionally, the
solution further contains a strong oxidizing agent such as
~ 13 10169
hydrogen peroxide, sodium peroxide, potassium peroxide,
sodium permanganate, potassium permanganate, potassium
chromate or ferric sulfate. Among the exemplary reagents
disclosed by Kalocsai is a composition (Example 28)
containing 1.0% v/v Br2, 1% w/v NaBr, and 0.6% w/v NaOH,
and having a pH of 7.35.
Sergent et al. U.S. patent 4,637,865 describe a
process for extracting a precious metal from a source
material by contacting the source material with an aqueous
leaching solution containing a leaching agent comprised of
an N-halohydantoin compound. Leaching solutions are
described containing 1,3-dibromo-5,5-dimethylhydantoin,
l-bromo-3-chloro-5,5 dimethylhydantoin and 1,3-dichloro-
5,5-dimethylhydantoin. Precious metal may be removed from
the leaching solution by precipitation of the less noble
metal, ion exchange, treatment with activated carbon,
solvent extraction or electrowinning.
Simpson U.S. patent 4,439,235 describes a process
for removing precious metal values from comminuted
carbonaceous ores in which the comminuted ore is contacted
with an aqueous solution of hypochlorite, iron ion, and
acid.
Falanga et al. U.S. patent 4,319,923 teaches a
process in which gold and palladium are etched with a
potassium iodide/iodine etching solution and the metal is
recovered from the solution by addition of alkaline
compound, preferably KOH, to increase the pH to at least
12.5 and precipitate metallic gold from the solution. A
borohydride is used to precipitate palladium. A similar
process is described in MacDonald U.S. patent 4,319,922.
Jolles, "Bromine and its Compounds", Academic
Press, New York, 1966, page 173, states that a mixture of
sodium bromate and sodium bromide has been used under the
name of "mining salts" in the extraction of gold ore.
Jolles states that the proportion of the respective
~ 13401~
components vary but are usually 43% sodium bromate and 57%
sodium bromide, i.e., two moles sodium bromide to one mole
sodium bromate.
Belohlav et al. U.S. patent 3,222,276 describe a
process for producing an aqueous bromine solution from an
aqueous solution of bromide/bromate salts and mineral
acid. In accordance with the process, a concentrated
aqueous solution of bromide and bromate salts at a 5:1
molar ratio is pumped to a mixing zone where it is mixed
with a mineral acid to convert the bromide/bromate solution
into a concentrated aqueous bromine solution. This
concentrated aqueous bromine mixture is pumped to a second
mixing zone where it is diluted with a larger volume of
water to produce a diluted stream of aqueous bromine, and
the latter stream is further diluted with a large body of
water to produce a highly dilute aqueous bromine solution
in substantially quantitative yield from the mineral acid
and the bromide/bromate salt solution. The process as
described is useful in the bromination of swimming pool
water. The reference contains no mention of the use of
bromine or bromides in the recovery of precious metals from
sources thereof.
Although prior art processes which use molecular
bromine have been effective for recovery of gold from
source materials, pure bromine is a toxic, fuming liquid
which generates a suffocating vapor and must be subjected
to special handling. Bromine may be dissolved to a certain
extent in water, or incorporated in an alkali metal
perbromide solution, but these aqueous materials exhibit a
substantial bromine vapor pressure, so that their use also
commonly entails special handling. Alternatively,
molecular bromine can be generated from the acidification
of alkali metal bromates, but by themselves bromates
provide only a limited source of molecular bromine.
Additionally, bromate salts have a high crystallization
,~ 1310169
temperature which makes them inconvenient to use as leaching
agents for precious metals. Mlxtures of bromides and
bromates, such as the "mining salts" described by Jolles,
have found their place ln the technology of precious metal
recovery, but have not provlded a sufficient source of
molecular bromine to be as effective as those known solutions
whose bromine vapor pressure is relatively high.
SUMMARY OF THE INVENTION
Briefly, therefore, the present invention is
directed to an aqueous composition havlng a pH of between
about 6.5 and about 7.5 comprising bromide ion, perbromide
ion, molecular bromlne, at least about 2% by welght bromate
ion and a metal ion selected from among alkali metal and
alkaline earth metals. The equivalent molecular bromine
content of the composition is between about 10% and about 40%
by weight and the ratio of the molar concentration of bromate
ion to the sum of the molar concentrations of molecular
bromine and perbromide ion in the composition is between
about 0.05 and about 0.8.
The invention is further directed to a leachlng
solution having a pH of between about 6.5 and about 7.5
adapted for leachlng of a preclous metal comprising gold or
silver from a source thereof. The solution contains between
about 0.01% and about 1% by weight equivalent molecular
bromine, between about 0.01% and about 1% by weight bromide
ion, and between about 0.005% and about 15% by weight total
halide lon. The equlvalent molecular bromine concentration
in moles per liter is equal to the sum of the actual molar
g
B
~ 1340169
concentration of molecular bromlne, the molar concentratlon
of perbromlde lon, three tlmes the molar concentration of
bromate ion, and the molar concentratlon of hypobromlte ion
and hypobromous acld ln the solutlon.
The inventlon is further directed to a method for
leaching of a precious metal comprlsing gold or silver from
an ore contalnlng such metal. The process comprlses
contacting the ore wlth an aqueous bromlne leachlng solutlon,
the leaching solutlon having a pH of between about 6.5 and
about 7.5 and containing between about 0.01~ and about 20% by
weight equivalent molecular bromine, between about 0.005% and
about 20% by weight bromide ion, and between about 0.005% and
about 30% by weight total hallde lon. The equivalent
molecular bromine concentration in moles per liter ls equal
to the sum of the actual molar concentratlon of molecular
bromlne, the molar concentratlon of perbromlde lon, three
tlmes the molar concentration of bromate lon, and the molar
concentratlon of hypobromite lon and hypobromous acld ln the
solutlon. An aqueous leachate contalnlng the preclous metal
ls thereby produced.
Other obiects and features wlll be ln part apparent
and ln part polnted out herelnafter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance wlth the present invention, lt has
been dlscovered that gold and sllver may be efflclently
recovered from source materials by use of a leaching solution
derived from a unlque precursor composltlon that contalns a
high concentration of avallable bromine in the form of a
-- 10 --
1340169
combinatlon of an alkali metal or alkaline earth metal
perbromide and an alkali metal or alkallne earth bromate. In
partlcular, lt has been found that the
- lOa -
n
13901~9
precursor composition, which is prepared by mixing a
perbromide salt component solution and a bromate salt
component solution, may contain a substantial concentration
of equivalent molecular bromine, yet have a very low
bromine vapor pressure which facilitates handling and
minimizes the hazards of using molecular bromine for
leaching of gold and other precious metals. Moreover, the
leaching solution of the invention may be readily prepared
by dilution of the precursor composition and used for the
leaching of precious metals without any serious problems of
containment of bromine vapor.
Use and handling of the precursor composition is
not hampered by bromate salts crystallizing or otherwise
precipitating from the solution. The leaching solution
prepared from this composition has been demonstrated to be
highly effective for the leaching of gold from refractory
ores, without the need for any preparatory processing other
than conventional roasting. If preferred, however, a clean
ore concentrate can be prepared by conventional processing,
which may include pressure oxidation.
In accordance with a particularly preferred
embodiment of the invention, a leaching solution precursor
concentrate containing perbromide and bromate salts is
initially produced. In the preparation of the leaching
solution of the invention, this concentrate is diluted to
provide the leaching solution. If desired, the pH may be
adjusted either before or after dilution by addition of an
acid such as HBr, HCl, H2SO4, or C12, or a base, such as
NaOH, KOH or Ca(OH)2.
In the preparation of the precursor composition
of the invention, a component solution of an alkali metal
or alkaline earth metal perbromide is mixed with a
1340169
~ .
component solution of alkali metal or alkaline earth metal
bromate. The perbromide solution is prepared by addition
of bromine to an aqueous solution of a bromide ion. For
example, sodium perbromide and calcium perbromide are
prepared by saturating the Br- content of the respective
aqueous NaBr or CaBr2 solution with molecular bromine:
NaBr + Br2 > NaBr3 (8)
CaBr2 + 2Br2 ~ ~ Ca(Br3)2 (9)
When prepared in the course of providing a concentrate, the
metal bromide solution initially has a concentration of at
least about 25% by weight, preferably essentially saturated
to its solubility limit, i.e., 45-50% by weight in the case
of NaBr, or 55-60% by weight in the case of CaBr2.
Whatever the initial concentration of the metal bromide
solution, liquid or vapor Br2 is added to the solution to
the extent of saturating the bromide ion therein, i.e. in
full stoichiometric equivalence with the Br- content.
Where the Br2 is added to a NaBr solution that is initially
at its solubility limit, the amount of bromine introduced,
as may be determined by iodometric titration, is equivalent
to a weight concentration in the resulting perbromide
solution of about 40-50% Br2. Because of the reversibility
of the reactions of equations (8) and (9), a portion of the
bromine is present as Br2, but most is present as Br3-. In
a solution saturated with respect to both initial NaBr
solubility and bromination of Br- ion, the equilibrium is
such that the solution contains about 63-64% by weight
NaBr3, 4 to 4 1/2% Br2 and 2 1/2 to 3% NaBr.
The alkali metal or alkaline earth metal bromate
component solution is prepared by addition of liquid
134016~
bromine or bromine vapor to an aqueous solution of metal
hydroxide, most preferably an alkali metal hydroxide.
Hydroxyl ions and molecular bromine react in accordance
with the following reaction equation to produce both
S bromate and bromide ions:
3Br2 + 60H- - > 5Br- + BrO3- + 3H2O (10)
Under alkaline conditions, this reaction proceeds
essentially quantitatively to the right. Preferably, the
strength of the initial caustic (or other alkaline)
solution and the amount of molecular bromine added thereto
are controlled so that, when the bromate solution is mixed
with the solution of alkali metal or alkaline earth metal
perbromide in predetermined relative proportions, the
resulting mixture has a pH of between about 6.5 and about
7.5. Where the bromate solution is used in the preparation
of a concentrate, the strength of the initial caustic
solution and the degree of bromination are selected so that
the bromate solution contains at least about 15% by weight
equivalent molecular bromine, i.e., at least about 4% by
weight bromate ion. Preferably, the bromate solution
component of the concentrate contains between about 5% and
about 8% by weight bromate ion, roughly equivalent to
between about 20% and about 30% by weight molecular
bromine. To provide a bromate component solution having
such concentration of equivalent molecular bromine and
satisfying the stoichiometric requirement set forth by
equation (10) the initial concentration of the caustic
solution is preferably in the range of 10-20% by weight in
the case of sodium hydroxide. Equivalent molar proportions
may be computed for other alkalis.
13
-~ 1340169
Alternatively, the bromate component solution may
be prepared by dissolving an alkali metal bromate or
alkaline earth metal bromate salt in water. This in fact
is the preferred method for preparing a component solution
comprising an alkaline earth metal bromate, since
difficulty may be encountered in the preparation of such
solution by addition of molecular bromine to a lime or
magnesia solution or slurry. In this alternative method of
preparing the component solution, an alkali metal or
alkaline earth metal bromide is also incorporated so as to
produce an overall composition essentially equivalent to
that obtained by dissolving Br2 in a caustic solution.
In the preparation of the precursor composition
of the invention, the perbromide solution and bromate
solution are mixed in proportions of between about 4 parts
by weight perbromide solution per part by weight bromate
solution and about 4 parts by weight bromate solution per
part by weight perbromide solution. Preferably,
approximately equal portions of the two component solutions
are mixed. Whatever relative proportions are used, the pH
of the resultant composition should be between about 6.5
and about 7.5, and the ratio of the molar concentration of
bromate ion to the sum of the molar concentrations of
molecular bromine and perbromide ion in the composition is
between about 0.05 and about 0.8. Where the bromide ion
has been fully saturated with bromine in the preparation of
the perbromide component solution, the molar concentration
of bromide ion in the precursor composition of the
invention is equal to the sum of the molar concentration of
molecular bromine and five times the molar concentration of
bromate ion.
14
1340169
In the concentrate of the invention, the bromate
ion concentration is at least about 2%, typically ranging
from about 2% to about 6% by weight, the equivalent
perbromide content is preferably at least about 10%,
ranging from about 55% to about 10% by weight, and the
concentration of bromide ion (as computed on the basis of
no dissociation of perbromide ion) generally ranges from
about 3% to about 19%, the preferred compositions typically
containing bromide ion weight concentrations in the range
of about 6% to about 17%.
With respect to the potency of leaching
compositions that may be prepared from it, the most
significant parameter of the concentrate is its equivalent
molecular bromine content. This is the proportion of
molecular bromine that is made actually available upon the
acidification of the precursor concentrate in the
preparation of the leaching agent. The equivalent
molecular bromine concentration of the concentrate is
defined in molar terms as the sum of the actual molar
concentration of molecular bromine, the molar concentration
of perbromide ion, and three times the molar concentration
of bromate ion. The equivalent bromine concentration
further includes the molar concentration of hypobromite ion
and hypobromous acid, as reflected by the equilibrium:
Br2 + H2O > H+ + HOBr + Br- (11)
HOBr ~ ~ H + OBr (12)
The equivalent molecular bromine content of the concentrate
is between about 10% and about 40%, preferably between
about 20% and about 40%, by weight. More preferably the
~ 1340169
equivalent Br2 content is at least about 25% by weight. By
using the highly concentrated component solutions as
described above, a concentrate can be prepared containing
34% by weight or more equivalent molecular bromine.
At the desired pH of between about 6.5 and about
7.5, the molecular bromine content of the concentrate is
generally not converted to bromate and bromide, i.e.,
equation (10) does not proceed to the left. As a
consequence, there is a stable equilibrium between
perbromide ion and Br2, and the composition of the
concentrate is stable within the ranges discussed above.
Despite the very high proportions of equivalent
molecular bromine, including significant fractions of Br2
and Br-3, it has been discovered that the vapor pressure of
the concentrate of the invention is quite low. For
example, a concentrate containing about 34% by weight
equivalent bromine exhibits a total vapor pressure of only
23mm Hg at 0~C, and a total vapor pressure of only 112.5mm
Hg at 35~C. By comparison, the vapor pressures of liquid
bromine are 75mm Hg at 0~C and 357.5mm Hg at 35~C and
sodium perbromide are 44mm Hg at 0~C and 214mm Hg at 35~C.
Thus, the concentrate of the invention can be used in the
preparation of a precious metal leaching solution with
greater convenience, assurance, and safety than can either
liquid bromine or sodium perbromide.
Effective aqueous bromine leaching solutions for
recovery of precious metals may be prepared by dilution of
the concentrate of the invention. Prior to or after
dilution, the pH may be adjusted by addition of either a
mineral acid such as HBr, HCl, or C12, or a base, such as
NaOH or KOH. Where the concentrate is acidified, it is
preferred that HBr be added rather than HCl. The leaching
16
1340169
.
solution is effective over a wide range of pH, but
operation is preferably carried out within a range of about
2 to about 10, more preferably in a range of between about
5 and about 7.5. A somewhat acid pH is generally preferred
to promote the conversion of bromate ion to molecular
bromine. Acidification is preferably carried out prior to
dilution, thus producing a more acidic concentrate having a
pH of between about 0.25 and about 2.5 and an equivalent
molecular bromine concentration in the range of between
about 28% and about 40% by weight.
In conjunction with dilution, a portion of NaBr
or other halide salt is advantageously incorporated into
the solution. Eh/pH diagrams constructed from
thermodynamic data show progressively larger solubility
field at lower Eh values for the formation of the AuBr4~
complexion (see equations 12 and 13 infra) as Br~ ion
concentration increases from 10-5 to l.OM. Water, and
optionally the halide salt, are added in such relative
amounts that the equivalent molecular bromine content of
the leaching solution is between about 0.01% and about 20%
by weight, the bromide ion concentration is between about
0.005% and about 20%, and the total halide ion
concentration is between about 0.005% and about 30% by
weight. In most applications, particularly in the recovery
of precious metals from ores, the leaching solution should
contain between about 0.01% and about 1% by weight,
preferably about 0.02% to about 0.5% by weight, equivalent
molecular bromine, between about 0.005% and about 10%,
preferably about 0.01% to about 1%, by weight bromide ion,
and between about 0.005% and about 15%, preferably about
0.01% to about 1.5%, by weight total halide ion. However,
in certain applications such as, for example, recovery of
17
1340169
metallic gold from an electronic circuit board or jewelry
scrap, a more concentrated leaching solution may be used.
Such may be prepared from the above described concentrates
by modest dilution with water. In some instances, the
concentrate may even be used directly for dissolution of
metallic gold or silver.
The rate of dissolution of precious metal in the
leaching solution is in some instances accelerated if it
contains a concentration of halide ions that is higher than
that provided by a bromine saturated concentrate. For this
reason, preparation of the leaching solution preferably
involves incorporation of chloride salt or bromide salt
from a source other than the concentrate. It may be noted
that both the actual molecular bromine and the ultimate
bromide ion content are also affected by the shifts in
equilibria which accompany the acidification and dilution
process. Thus equations (8) and (9), supra, are driven to
the left, converting perbromide ion to bromide and
molecular bromine; and equation (10) is also driven to the
left, converting bromate ion and bromide ion to molecular
bromine. Dilution tends to drive equation (11) to the
right, resulting in conversion of molecular bromine to
bromide ion and hypobromous acid. As a net result, the
hypobromous acid concentration is a significant component
of the equivalent molecular bromine content of the leaching
solution.
Where the precursor concentrate or leaching
solution is acidified by addition of C12, not only the
bromate but the bromide ion content thereof are converted
to molecular bromine. This may further enhance the
oxidizing and complexing power of the leaching solution for
leaching of gold or silver from a source material.
18
- 1340169
Gold and silver are recovered from a source
thereof, such as comminuted gold ore, by contacting the
source material with the aqueous bromine leaching
solution. In the case of gold, oxidation and complexing of
the gold is believed to proceed in accordance with the
equations:
2Au +3Br2 + 2Br- > 2AuBr4 (12a)
or
H+
2Au + 3HOBr + 5Br- > 2AuBr4 + 3H2O (13)
Depending on the nature of the ore, the relative
proportions of ore (or other source material) and leaching
agent may be such that the leaching slurry contains between
about 1 and about 100 lbs. active agent per ton of source.
Active agent in this instance is defined as the sum of the
amounts of metal bromide, metal bromate, metal perbromide,
metal hypobromite, hypobromous acid, and molecular bromille
in the leaching solution.
If the source material is a refractory ore, it
may be necessary to pretreat it for removal of sulfur and
carbonaceous material. Such may be accomplished by methods
known to the art such as roasting or pressure oxidation.
Roasting may be sufficient pretreatment if carried out at a
temperature of at least about 500~C. The leaching
composition and method of the invention also may be used
advantageously for recovery of gold from high grade
non-refractory ores, low grade refractory and clean ores,
electronic component scraps, jewelry scrap and similar low
grade refractory and clean ores. The composition and
method may be used for recovery of silver from various
sources, including photographic film.
19
,~
.
~~e
1340169
The slurry of ore in leaching solution is
preferably agitated to promote transfer of precious metal
from the ore particles to the aqueous phase. A leachate is
thus produced containing gold or silver complexed with
bromide ions. Leaching may be carried out at ambient
temperature. Preferably, contact is maintained for a
period of 2 to 6 hours to achieve the maximum transfer of
gold or silver from the ore.
After treatment of the precious metal source with
the leaching solution is completed, the leachate is
separated from the leached ore, as by filtration. The
filtrate (leachate) is washed with an aqueous washing
medium, the spent wash solution is combined with the
filtrate, and the combined filtrate and wash solution is
treated for recovery of the precious metal therefrom.
Advantageously, particularly in the case of silver, the
filter cake is washed with a strong acid solution, such as
HBr but preferably HCl. Washing the filter cake in such
fashion may be effective to leach further quantities of
metal from the cake. A solution of 6N HCl is especially
Gold may be recovered from the combined filtrate
and wash solution by conventional means such as zinc or
aluminum precipitation, ion exchange, carbon adsorption, or
electrowinning. In similar fashion, the leaching solution
of the invention may be used for recovery of silver.
The following examples illustrate the invention.
Example 1
A solution was prepared by dissolving sodium
bromide (27.7 grams) in water (29.3 grams). A sodium
perbromide solution was prepared by adding liquid bromine
1340169
.~
in an amount (43.0 grams) sufficient to saturate the
bromide ion, i.e., stoichiometrically equivalent to the
initial bromide ion content, in the solution. The
resulting sodium perbromide component solution contained
5 43% equivalent molecular bromine.
A sodium hydroxide solution was prepared
containing 16.7% by weight sodium hydroxide. Liquid
bromine (25.0 grams) was added to this solution (75.0
grams) producing a composition which contained 6.7% by
10 weight bromate ion (7.9% by weight as sodium bromate; 25%
by weight equivalent molecular bromine). A concentrate was
prepared by mixing equal parts by weight of the perbromide
and bromate component solutions. The concentrate so
prepared contained 31.82% by weight sodium perbromide,
15 2.14% by weight bromine, 14.80% by weight sodium bromide,
3.94% by weight sodium bromate and 47.30% by weight water.
It had an equivalent molecular bromine concentration of 34%
by weight.
The bromine concentrations of both the precursor
20 concentrate and the sodium perbromide component solution
were confirmed by adding to the respective solutions an
excess of potassium iodide and then titrating the iodine
released with sodium thiosulfate using starch as an
indicator. Titration of the total equivalent molecular
25 bromine content of the concentrate was effected by the
addition of a strong mineral acid to convert the bromate
content to Br2. The concentrate was also titrated without
addition of acid in order to determine the actual bromine
concentration in terms of molecular bromine and perbromide
30 ion. This titration showed a 21.5% bromine in the
concentrate.
21
134~169
Using the Isoteniscope method, the total vapor
pressure was measured as a function of temperature for
liquid Br2, the sodium perbromide component solution of
this example, and the precursor concentrate of this
example. From the data obtained, the corresponding
enthalpies of vaporization were calculated. The results of
these measurements and calculations are set forth in Table
1.
Table 1
Vapor Pressure Data
Vapor Pressure/mm Hg
Temperature/~C Br2 a NaBr3 b ConcentrateC
0 75.0 44.0 23.0
95.5 56.0 30.5
15 10 120.5 68.0 38.0
151.0 86.0 48.0
189.0 108.5 60.0
234.0 138.0 69.0
289.0 173.0 86.0
20 35 357.5 214.0 112.5
a. ~ Hv = 7.29 Kcal mole -1
b. ~ Hv = 7.65 Kcal mole -1
c. ~ Hv = 7.36 Kcal mole -1
Example 2
Sodium perbromide and sodium bromate component
solutions were prepared in the manner described in Example
1. A series of concentrates was prepared using varying
proportions of the two component solutions. The
composition of the concentrates obtained are set forth in
Table 2.
22
1340169
Example 3
In order to compare the vapor pressure of
solutions prepared according to the invention with
previously known aqueous bromine-based solutions, a
solution was prepared by a formulation method comparable to
Bahl, et al. U.S. patent No. 4,190,489, and a composition
was prepared according to the invention, each containing
34% by weight equivalent bromine. For the Bahl et al.
formulation, 26 g KBr was dissolved in 40 g water and then
34 g Br2 was added to the resulting solution. For the
composition prepared according to the invention, 14.26 g
NaBr, 45.49 g H2O, 6.25 g NaOH and 34 g Br2 were mixed.
Bromate content and vapor pressure were calculated as
follows: Titration with Thiosulfate-KI using a weak acid
determines actual Br2 content (Br2 + Br3~) while titration
with Thiosulfate-KI using a strong acid converts bromate to
bromine and determines the sum of bromate and bromine
concentration. Therefore, the bromate content of the two
solutions was determined by Thiosulfate-KI titration first
with acetic acid to determine the actual bromine
concentration, and then by Thiosulfate-KI titration with
H2SO4 to determine the total equivalent molecular bromine
(Br2 + Br3- + BrO3-) and subtracting the difference.
Solution vapor pressure at 25~C was obtained by using the
Isoteniscope method.
1~40169
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1340169
Table 3
pH, Br2 Concentration, and Vapor Pressure Measurements:
Composition of Invention * vs. Bahl Formulation **
~c~ o
~~
~r~ ~mcndcd
5 Parameter Claim 1Bahl Formulation
pH 6.6 3.0
Wt. % Br2 (with HAC) 24.2 33.1
Wt. % Br2 (with H2SO4)32.5 33.4
Wt. % Br2 (present as BrO3-) 8.3 0.3
10 Vapor Pressure at 25~C 70.5 106
mm-Hg
* 14.26 g NaBr, 45.49 g H20, 6.25 g NaoH, 34 g Br2.
** 26 g KBr, 40 g H2O, 34 g Br2.
Example 4
A leaching solution was prepared from the
concentrate of Example 1 and used in leaching tests for
recovery of gold and silver from a refractory ore
concentrate initially containing 12.5% by weight carbon and
15.5% by weight sulfur. A fire assay of this ore performed
by Chemex of Canada showed 7.07 oz. gold per ton and 6.39
oz. silver per ton. A similar fire assay provided by Hazen
of the U.S. showed 6.61 oz. gold per ton and 5.83 oz.
silver per ton. Because of the high concentration of
carbon and sulfur in this ore, it was necessary to pretreat
the ore prior to leaching. Pressure oxidation and roasting
are among the commonly used methods for oxidizing carbon
and sulfur in carbonaceous and refractory ores before the
recovery of precious metals therefrom by leaching. In this
instance, the ore was pretreated by roasting.
In the roasting operation, an ore concentrate
(451 g) was charged into a 100 mm diameter quartz batch
kiln. The kiln was placed in an electrically heated
13~0169
clamshell furnace sealed with rotary fittings at the ends,
and rotated at about 5 rpm. Oxygen was passed through the
kiln while the contents thereof were heated to a
temperature ranging from 600-707~C, averaging approximately
650~C. Temperature was controlled by application of
electric power to heat and opening of the furnace to the
surroundings for cooling. After the ore was heated in the
presence of a stream of oxygen for 120 min., the kiln was
cooled and the calcine products sampled and analyzed. A
22% weight loss occurred during roasting. The calcine
contained 3% total sulfur and 8.5% sulfate, indicating that
the residual sulfide level was 0.17%.
A series of leaching tests was carried out in
which gold and silver were recovered from the calcine using
an aqueous bromine leaching agent. To prepare the leaching
agent, a portion of the concentrate of Example 1 (1.4
grams) and 48% hydrobromic acid (0.8 grams) were introduced
into a small capped bottle. The contents were mixed well
to assure conversion of sodium bromate to bromine. The
mixture was then transferred into a 100 ml. flask
containing sodium bromide (1 g), and water was added up to
the mark, i.e., to produce a total solution volume of 100
ml.
A specimen of calcine (22.75 grams; one assay ton
equivalent of dried unroasted concentrate) and the aqueous
bromine leaching solution from the volumetric flask (100
ml.) were placed in a capped 250 ml. Erlenmeyer flask. The
resultant slurry was mixed using an automatic mixer for a
predetermined period of time at room temperature.
Individual runs were made in which mixing was terminated
after 4, 8, 12 and 24 hours, respectively. After
termination of the mixing cycle, the slurry was filtered
26
13~0169
and the residue washed with 4M hydrochloric acid. Head
filtrate and wash solution were combined and analyzed for
gold and silver. The results are presented in Table 4.
The data for percent extraction in this table are based on
the concentrations of silver and gold in the solution and
the average assay values of Hazen and Chemex.
Table 4
LEACHING OF REFRACTORY CONCENTRATE
Sample Size: 22.75 g Calcine or 29.16 g unroasted
10 Fire Assay: 6.84 oz/t Au; 6.11 oz/t Ag
(unroasted ore)
Feed Preparation: 325 mesh, roasted at 650~C
B Conditions: 22~ C, pH = 5.0 - 5.5, 18.5~ ~olids~ O 40\;~
Solution
Leach Time Au Ag % Extraction
Test No. Hour oz/t oz/t Au Ag
1 4 7.10 3.98 100 65
2 8 7.13 4.13 100 68
3 12 6.86 4.54 100 74
4 24 7.00 4.33 100 70
27
1340169
Example 5
To determine the effect of pH on leaching gold
from refractory concentrate, a series of leaching tests
was conducted in the manner described in Example 4 but at
varying pH. After the calcine was mixed with the aqueous
bromine leaching reagent, the pH of the slurry was
adjusted to a specified value in the range of pH 2 to pH
by the addition of hydrobromic acid or sodium
hydroxide. After pH adjustment, the slurry was mixed for
four hours and filtered. The filter cake was washed and
the gold value in the combined filtrate and wash solution
was determined. The test results of this example are set
forth in Table 5. These data demonstrate that the
precursor concentrate of the invention may be diluted to
produce leaching solutions which are effective lixiviants
at varying pH levels for leaching gold.
Table 5
LEACHING OF REFRACTORY CONCENTRATE
Sample size: 22.75 g Calcine or 29.16 g unroasted
20 Fire Assay: 6.84 oz/t Au; 6.11 oz/t Ag
(unroasted ore)
Feed Preparation: 325 mesh, roasted at 650~C
B Conditions: 22~C; pH = 2 - 10; 18.5~ aolid~A ~ ~ID 5O\;~
Leach Time pH pH Au Extraction
25 Test No. Hour Initial Final oz/t %
1 4 2.1 3.2 6.58 96
2 4 5.6 6.3 6.86 100
3 4 8.3 7.5 6.83 100
4 4 9.8 9.1 6.90 100
28
13 10169
Example 6
In order to optimize the concentration of active
agents needed to leach gold from a refractory concentrate,
a series of leaching tests was carried out under conditions
comparable to those of Example 4 but at varying dilutions
of the concentrate. No pH adjustment was made in the tests
of this example. The slurry of calcine and aqueous bromine
reagent was mixed for 4 hours and filtered. The filter
cake was washed and the gold value of the combined filtrate
and wash solution was measured. The results of the
experimental runs of this example are set forth in Table 6,
each test result reported in this table being based on the
average of 3 runs. Note that 44 pounds of the concentrate
or 15 pounds of bromine equivalent was found necessary to
leach about 7 oz. of gold from 1 ton of refractory
concentrate ore.
29
Table 6 1340169
LEACHING OF REFRACTORY CONCENTRATE
Sample Size: 22.75 g calcine or 29.16 g unroasted
Fire assay: 6.84 oz/t Au; 6.11 oz/t Ag
(unroasted ore)
Feed Preparation: 325 mesh; roasted at 650~C
~a
Conditions: 22~C; pH = 5.0 - 6.5; 18.5~ Eolids ~ O
4 hrs. mixing
Usage of Conc.* Au Extraction
10 Test No. lb/t ore oz/t %
1 123 6.86 100
2 88 6.94 100
3 44 6.85 100
*Concentration of reagent of Example 1 per ton of ore in
leaching slurry.
Example 7
To study the effect of NaBr concentration on the
leaching of Au and Ag from a refractory concentrate, a
series of leaching tests was carried out. The tests were
similar to those of Example 4, but the concentration of
NaBr was varied. The aqueous bromine leaching solution
contained 1 wt.% concentrate of Example 1 and varying
amounts of NaBr. The residue was washed with water instead
of 4M HCl. The results are presented in Table 7.
Considering the fire assay of head ore (Hazen) as the
1340169
.
basis, the gold recovery ranged from 96% to 100% and silver
recovery range was 2-11%. It is interesting to note that
the addition of NaBr does not have any effect on the Au
recovery, whereas the recovery of Ag is affected by the
concentration of NaBr. Comparing the Ag recovery in these
leaching tests with those of Example 4, it may be concluded
that washing the residue with 4M HCl definitely improves
the Ag recovery without having any effect on the Au
recovery. The residues of the leaching tests (Table 7)
were fire assayed by Hazen. Set forth in Table 8 are the
metallurgical balance and the calculated percentage of gold
solubilized on the basis of the calculated head.
Table 7
LEACHING OF REFRACTORY CONCENTRATE
15 Sample Size: 50.00 g Calcine
Fire assay: 8.52 oz/t Au; 6.60 oz/t Ag
(Calcine)
Feed Preparation: 325 mesh; roasted at 650~C
B Conditions: 22~C; pH = 5.0 - 6.5; 20.0~ Eolids~ 50\'~
4 hrs. mixing
Solution
NaBr Au Ag % Extraction
Test No.Wt. % oz/t oz/tAu Ag
1 0 8.77 0.12100 1.8
2 2.5 8.44 0.1799 2.6
3 5.0 8.27 0.6097 9.1
4 10.0 8.18 0.8596 12.9
5 20.0 8.97 0.73100 11.1
31
1340169
Table 8
LEACHING OF REFRACTORY CONCENTRATE
METALLURGICAL BALANCE
Sample Size: 50.00 g Calcine
Fire assay: 8.52 oz/t Au (14.62 mg Au)
Feed Preparation: 325 mesh; roasted at 650~C
Conditions: 22~C, pH - 5.0 - 6.5; 20.0~ solidEA ~ .~ 5
~LJ 4 hrs. mixing
Run No. 1 2 3 4 5
Filtrate
Volume, ml 370 360 294 302 486
Au Conc., mg/L 40.9 40.8 47.0 46.3 31.8
Au Conc., mg 15.13 14.69 13.82 13.98 15.46
Residue
15 Au Conc., oz/t 0.372 0.322 0.186 0.276 0.342
Au Conc., mg 0.64 0.55 0.32 0.47 0.58
Au Solubilized, % 96.0 96.4 97.8 96.7 96.4
Calculated Head 9.19 8.88 8.24 8.42 9.35
oz/t
20 Overall Balance, % 108 104 97 99 109
1340169
Example 8
A series of leaching tests was carried out in
which gold and silver were recovered from a low grade clean
ore using an aqueous bromine leaching agent. The procedure
of Example 4 was followed. The leaching solution contained
0.5 wt.% of the concentrate of Example 1 and 1 wt.% NaBr.
The residue was washed with 4 M HCl. The head filtrate and
wash solution were analyzed for Au and Ag to obtain the
solubilized metals. The results are presented in Table 9.
Considering the fire assay of head ore (Hazen) as the
basis, the gold recovery was 100%. The silver recovery
ranged between 50-100%.
Table 9
LEACHING OF LOW GRADE CLEAN ORE
15 Sample Size: 29.16 g ore
Fire Assay: 0.148 oz/t Au; 1.99 oz/t Ag
Feed Preparation: 200 mesh
~'~
Conditions: 22~C; pH = 5.0 - 6.5; 23 Eoli~S ~t.~o ~0\~5
1340169
Solution
Leach Time Au Ag % Extraction
Test No.Hour oz/t oz/t Au Ag
1 4 0.148 2.35 100 100
2 4 0.150 2.30 100 100
3 4 0.166 1.00 100 50
4 4 0.146 1.10 99 55
4 0.167 1.02 100 51
6 4 0.177 2.30 100 100
7 4 0.195 2.30 100 100
Example 9
In a further series of leaching tests using the
concentrate of Example 1, the concentration of concentrate
in the leaching solution was varied from 2.0 to 6.0 g/L.
These tests indicated that gold recovery was maximized at
about 4.0 g/L concentrate.
Further tests were conducted at leaching times of
2, 4.6, 12, 18 and 24 hours. The results of these tests
indicated that over 98% of all leachable gold was
solubilized after 2 hours. Based on the results of the
latter tests a leaching time of 6 hours was chosen for
further tests.
Triplicate confirmatory tests were conducted on
two separate ore calcines that had been obtained by
roasting samples of Canadian flotation concentrate at 650~C
- 750~C. The confirmatory tests were conducted using what
were considered generally optimum conditions: 4g/L
concentrate, pH 5.0-6.0, and leaching time 6 hours. In the
tests on the first calcine, gold in the residue ranged from
0.592 to 0.650 oz/t, Au recovery ranged from 94.2% to 94.5%
and Au Head was calculated as ranging from 9.51 to 9.96
oz/t. In the tests on the second calcine, the
34
1340169
,,
corresponding figures were 0.714 to 0.768 oz/t Au in
residue, 96.0 to 96.3% Au extraction, and 17.29 to 17.73
oz/t Au Head.
In view of the above, it will be seen that the
several objects of the invention are achieved and other
advantageous results attained.
As various changes could be made in the above
methods and products without departing from the scope of
the invention, it is intended that all matter contained in
the above description shall be interpreted as illustrative
and not in a limiting sense.
