Note: Descriptions are shown in the official language in which they were submitted.
2~323~7
PRESSURE OXIDATION METHOD~ FOR
GOLD EXTRACTION AND TOXIC WASTE TREATMENT
Technical Field of the Invention
This invention relates to the extraction of
gold from feed mixtures of auriferous sulphidic ores,
concentrates of such ores or reclaimed sulphide
concentrates and previously generated toxic wastes,
such as roaster calcine and roaster arsenic sludge.
Backaround Of the Invention
Metallic gold is extracted from gold-
containing ores or concentrates formed from such ores.
Higher grades of auriferous ores have a higher gold
content. Since sources of high-grade ores are rapidly
being depleted, the attention of the gold mining
industry is becoming more and more focused upon methods
of extracting gold from lower-grade ores. The lower-
grade ores contain higher amounts of impurities, such
as sulphidic minerals. Typical sulphidic minerals
present in gold-containing ores are arsenopyrite and/or
pyrite, and may also include appreciable amounts of
pyrrhotite. Lesser amounts of base metals in sulphide
form, such as copper, zinc, and lead may also be
present.
Extraction of metallic gold from ores or ore
concentrates is conventionally accomplished by
cyaniding (i.e., dissolution in cyanide solutions).
The cyaniding extraction of gold from refractory
auriferous sulphidic material has been shown to be
improved if the material is first subjected to a
pressure oxidation treatment. See, for example, U.S.
--1--
2~323~7
Patent No. 2,777,764. In the pressure oxidation
treatment, maximum gold recovery is obtained through
complete oxidation of the sulphide sulphur atoms to
sulphate form.
A problem with this pre-cyaniding pressure
oxidation treatment is the formation of elemental
sulphur as an intermediate or as a primary oxidation
product. As a result of the elevated temperature at
which the pressure oxidation treatment is conducted,
the elemental sulphur so produced will be in molten
form. Molten sulphur tends to wet and/or coat many of
the sulphides present in the slurry being oxidized.
Such coated sulphides tend to agglomerate. Formation
of agglomerates containing molten sulphur and unreacted
sulphides limits sulphide oxidation, ultimately
resulting in decreased gold recovery. Additional
problems arise in continuous operations, because
agglomerates may build up to the point where they
accumulate in the reaction vessel. Moreover,
elemental sulphur detrimentally affects subsequent
cyaniding processing, even further decreasing gold
recovery.
One method of obviating the molten elemental
sulphur problem is described in U.S. Patent
No. 4,605,439. That patent discusses the addition of
"relatively inert solids" to the feed of iron-
containing, refractory auriferous sulphidic material to
provide a feed slurry of high pulp density. As a
result, the patented pressure oxidation process could
be carried out at a lower temperature with less
agglomeration. The inventors of this patented
technique theorized that the added inert solids
promoted the dispersion o~ the elemental sulphur formed
in the pressure oxidation reaction (reduc1ng the amount
of agglomeration) and suspen~ion of any agglomerates
formed (permitting the agglomerates to oxidize more
-2-
2 t~3~3 ~ 7
completely). The only example recited in the patent of
a relatively inert solid suitable for admixture with
the pressure oxidation autoclave refractory feed stream
is a recycle stream (i.e., a portion of the process
stream constituting material previously passed through
the autoclave, either before or after liquid-solid
separation and prior to cyaniding).
Other alternatives for overcoming the
problems created by the formation of molten elemental
sulphur have been suggested. A survey of some of these
alternatives and the deficiencies thereof with respect
to practical, continuous, industrial-scale gold
recovery operations are discussed in U.S. Patent No.
4,605,439.
An additional problem facing the mining
industry is an increased emphasis on the environmental
impact of both present and past mining and metal
recovery processes. Consequently, heightened interest
has been shown in processes for removing inorganic
contaminants from wastewater generated during gold
milling and other industrial processes as well as for
removing toxic substances ~rom previously generated
waste from such processing.
In the past, refractory auriferous materials
were sometimes treated by roasting, a technique that
resulted in the production of toxic by-products, such
a~ arsenic-containing calcines and sludges. As a
result of greater environmental awareness, roasting is
being replaced by pressure oxidation in the treatment
of auriferous refractory material. Pressure oxidation
techniques, utilizing correct feed stoichiometry and
followed by appropriate neutralization processing,
typically generate waste effluent process streams
containing non-directly dischargeable levels of toxic
contaminants. The toxic wastes previously produced by
the roasting process remain; however, and pressure is
~2~7
being brought to bear upon the mining industry to
institute clean up procedures.
Specifically, wastewater from a large
percentage of previously conducted roasting processes
has been retained in tailings ponds. In fact,
retention of wastewater in tailings ponds is the oldest
and still a common method for treatment of gold mill
wastewater effluent. Natural degradation,
photodegradation, precipitation, and volatilization of
contaminants are the primary mechanisms of tailings
pond wastewater treatment. Many metals are soluble in
wastewater as a result of cyanide complexing, however.
These metals therefore do not precipitate out of
solution and contribute substantially to the high
levels of inorganic contaminants found in tailings
ponds. Also, severe climatic conditions limit the
efficacy of tailings ponds, since the aforementioned
mechanisms of tailings pond wastewater treatment do not
occur when that pond is frozen.
Implementation of more strict environmental
regulation would appear to limit the use of tailings
ponds. Speci~ically, wastewater retention in tailings
ponds is ef~ective to reduce wastewater contamination;
however, ~uch wastewater purification is inadequate to
permit direct discharge. Oxidized residues of, for
example, high iron content constitute toxic byproducts
o~ the roasting process. These contaminants are
discharged into and remain in tailings ponds in soluble
(i.e., toxic) form.
Also, ores containing significant amounts of,
for example, arsenopyrite (generally in excess of 1%
arsenic) have been previou~ly subjected to roasting.
Such roasting released oxidized forms of arsenic from
the ore, thereby producing effluent having arsenic
contaminants. Effluent containing high levels of
soluble arsenic was typically stored in underground
-4-
2~23~7
vaults dedicated to that purpose. Such storage
techniques are susceptible to breach, thereby releasing
the toxic residue into the environment. For example,
a fissure may develop in the vault structure.
Alternatively, ground water drainage may occur. As a
result, methods of treating these previously produced
toxic wastes are being sought.
Summary of the Invention
The present invention provides methods
whereby currently mined refractory auriferous
materials, preferably auriferous sulphidic materials,
are mixed with koxic waste from old mill tailings to
extract gold from both the new ore production and the
old production wastes. Gold may also be recovered from
reclaimed sulphidic concentrates by the methods of the
present invention. These methods also produce an
environmentally acceptable, gold extraction waste
product.
The methods of the present invention include
the steps of: mixing currently mined refractory
auriferous material and/or reclaimed sulphidic
concentrates with at least one low sulphur content
toxic waate generated in p.evious gold mill processing,
such as roaster calcine or sludge to produce a low
sulphur content mixture; pressure oxidizing the
mixture; and cyaniding the oxidized mixture.
Preferably, the mixing step of the methods of the
present invention includes the addition of a waste
having low sulphur, high iron, and low arsenic content,
such as calcine, and a waste having low sulphur, low
iron, and high arsenic content, such as arsenic sludge.
In a preferred method, both a reduction in the amount
of sulphur present in the pressure oxidation feed
mixture and an increase in the formation of insoluble
ferric arsenate during pressure oxidation are achieved.
2 ~ 3 2 3 ~ 7
Reduction in feed sulphur content results in a decrease
in elemental sulphur and concomitant agglomeration
formation in the pressure oxidation step. In addition,
the insoluble molecules constitute a more manageable
waste product.
If the currently mined refractory auriferous
material or a reclaimed sulphidic concentrate contains
carbonate compounds, an acidification step can be
utilized prior to the pressure oxidation treatment. In
this manner, a high level of carbonate conversion to
carbon dioxide during the pressure oxidation process
can be avoided. Acid liquor generated in the pressure
oxidation process is preferably recycled for use in
this acidification step.
Brief Description of the Drawing
The Figure shows a flow diagram of a gold
extraction/toxic waste treatment method of the present
invention.
Description o~ the Preferred Embodiments
The present invention is directed to methods
o~ extracting gold from a re~ractory auriferous
materlal, preferably an auri~erous sulphide material,
including mixing, pressure oxidation and cyaniding
steps. The present invention is also directed to
methods of simultaneously extracting gold and treating
(detoxifying) toxic waste.
For the purpose of this description, the
values stated as percentages are percentages by weight,
unless otherwise indicated. With the exception of
usage ln connectlon with gold, the term "content" also
re~ers to weight percent, albeit less de~initively than
a numerical percentage value. Gold content is
expressed in ounces per ton (oz/ton).
-6-
2~32~7
For the purpose of this description, the term
"refractory auriferous material" encompasses any
refractory material containing gold. Exemplary of such
materials are auriferous sulphide ores, concentrates
5derived from such ores, concentrates reclaimed from
gold mill tailings, or the like. A combination of one
or more of these materials might also be used as a
refractory auriferous sulphidic material in accordance
with the present invention. A "currently mined
10refractory auriferous sulphidic material" encompasses
auriferous ores, concentrates thereof or combinations
of the two that have not previously been treated to
extract gold therefrom.
As mentioned above, typical sulphide minerals
15present in auriferous ores are arsenopyrite, pyrite,
pyrrhotite, base metal sulphides, such as copper, zinc,
and lead sulphides, and the like. An exemplary ore
that might be processed in accordance with the present
invention contains from about 0.05 to about 0.8 oz/ton
20gold (Au); from about 0.2 to about 3% arsenic (As);
from about 2 to about 5% iron (Fe); and from about l to
about 5% sulphur (S). An exemplary concentrate derived
from ~uch an ore contains from about 1 to about
10 oz/ton Au; from about 8 to about 20% As; from about
2520 to about 40% Fe; and from about 15 to about 35% S.
An exemplary concentrate reclaimed from gold mill
tailings contains from about 0.2 to about 2 oz/ton Au;
from about 5 to about 15% As; from about 10 to about
35% Fe; and from about 8 to about 30% S.
30The methods of the present invention feature
a mixing step, where the refractory auriferous material
is blended with at lea~t one toxic residue (i.e.,
waste) in order to substantially decrease the sulphur
content in the pressure oxidation feed stream. Such a
35decrease in sulphur content improves the performance of
the pressure oxidation treatment by preventing the
--7--
2~3'~7
generation of excessive heat and decreasing the amount
of elemental sulphur formed during that treatment. The
mixing step may be accomplished in any conventional
manner. Exemplary mixing techniques are vibrational
agitation, stirring, and the like. This mixing
procedure may be conducted using commercially available
equipment designed for that purpose.
Preferable toxic wastes useful in the present
invention are those that were generated by gold mill
processes using techniques such as roasting. One
preferred toxic residue for admixture with the
refractory auriferous material is a residue exhibiting
a low sulphur content, a high iron content, and a low
arsenic content. For the purpose of describing such a
toxic residue, a low sulphur content indicates a weight
percentage of sulphur below about 5%: a high iron
content ind~cates a weight percentage of iron in excess
of about 30%; and a low arsenic content indicates a
weight percentage of arsenic below about 50%.
Exemplary of such a toxic residue is calcine derived
~rom wastee produced in roasting processes. Exemplary
¢al¢lne ¢ompo~itions use~ul in practicing the present
invention include from about 0.25 to about l oz/ton Au;
from about 2 to about 12% As~ from about 25 to about
50% Fe; and from about 2 to about 15% S.
A second pre~erred toxic residue for
admixture wlth the refractory auriferous material is a
residue exhibiting a low sulphur content, a low iron
content, and a high arsenic content. For the purposes
of describing such a toxic residue, a low sulphur
content indicates a weight percentage of sulphur below
about 3%; a low iron content indicates a weight
percentage of iron below about 10%; and a high ar~enic
content indicates a weight percentage of ar~enic in
excess of about 50%. Exemplary of such a toxic residue
i~ ar~enic-containing sludge derived ~rom wastes
~? ~ 3 ~r~
produced in previous roasting processes. Exemplary
arsenic-containing sludge compositions useful in
practicing the present invention include from about
0.25 to about l oz/ton Au: from about 45 to about 65%
As; from about 5 to about 10% Fe; and from about l to
about 5~ S.
Arsenic sludge is typically derived from
precipitators or from dedicated pits, dedicated ponds
or underground vaults in which effluent from roaster
processing is stored. High iron content calcine is
typically derived from the oxidized solid residues
resulting from the roasting process.
In order to disperse the sulphur present in
the refractory auriferous sulphide material and
decrease the sulphur content of the pressure oxidation
feed, the mixing step must be conducted with
appropriate components in appropriate amounts.
Specifically, an amount of toxic residue(s) sufficient
to reduce the sulphur content of the pressure oxidation
feed ~i.e., refractory auriferous sulphide material and
toxic residue(s)) below about 12% by weight, preferably
below about 10% by weight, must be used. Such
stoichiometric ad~ustment o~ the pressure oxidation
feed to achieve a speci~ied sulphur content is within
the purview of a practitioner in the art.
The methods oP the present invention may also
feature addition of toxic residues having soluble toxic
contaminants capable of reacting with one another or
with other components of the pressure oxidatian feed
stream under oxidizing conditions to form insoluble
molecules having decreased toxicity. When arsenic
sludge is used in admixture with a refractory sulphidic
material containing iron, some of that iron reacts with
the ~oluble arsenic present in the sludge to form
ferric arsenate as described more fully below. In
addition or in the alternative, iron may be added to
_g_
2~323~
the pressure oxidation feed to react with the soluble
arsenic in the sludge as is more fully described below.
Since ferric arsenate is insoluble in water, the amount
of soluble arsenic is decreased, and the toxicity of
the pressure oxidation effluent is therefore reduced.
In a preferred method, calcine and arsenic
sludge are mixed with the refractory auriferous
sulphidic material. The soluble iron present in
calcine is capable of reacting with the soluble arsenic
present in arsenic-containing sludge under oxidizing
conditions to form insoluble ferric arsenate and acid.
The reaction proceeds as follows:
2FeS2+ 72 + 2HzO --> 2FeSO4 + 2H2SO4
4FeAsS + 1102 + 2H20 --> 4HAs02 + 4FeSO4
4FeS04 + 2H2S04 + 2 --> 2Fe2(SO4)3 + 2H20
2HAsO~ + 2 + 2H2 --> 2H3As04
Fe2(SO4)3 + 2H3As04 --> 2FeAsO4 + 3H2S04
Preferably, the Fe:As ratio in the pressure
oxidation feed is about 1.2:1 or greater. If
necessary, iron may be added to the pressure oxidation
feed to provide the appropriate component ratio. This
iron may be added at any time prior to pressure
oxidation, i.e., before, during or after the mixing
step. The iron BO added may be in any chemical form
sultable ~or this purpose, such as shredded form. The
iron i8 pre~erably dissolved in acid prior to admixture
with the pressure oxidation feed. Recycled pressure
oxidation-produced acid or any other acid capable of
dissolving iron may be used for this purpose.
When a decrease in the toxicity of toxic
residue(s) contained in the pressure oxidation feed is
also desired, the stoichiometry and other parameters of
the reaction converting the soluble toxic
contaminant(s) into insoluble non-toxic or less toxic
molecules must also be taken into account to determine
the composition of the pressure oxidation feed stream.
--10--
2~323~
For example, a pressure oxidation feed stream composed
of 20% sulphide concentrates, which can comprise
arsenopyrite, pyrite, pyrrhotite, or a combination
thereof and up to about 25% gangue minerals, 40%
calcine (<10% S, 40% Fe, 2% As), and 40% arsenic sludge
(2-3% S, 8% Fe, 55% As) is characterized by a total
sulphur content of less than 10%. Pressure oxidation
of such a feed stream will result in the formation of
ferric arsenate. A practitioner in the art would be
able to determine an appropriate pressure oxidation
feed stream composition to achieve both gold extraction
and toxicity reduction.
The pressure oxidation step of the present
invention may be conducted in accordance with known
techniques. See, for example, U.S. Patent No.
2,777,764. Moreover, toxic wastes generated in
alternative gold recovery techniques, such as roasting,
contain gold. For example, roaster calcine wastes may
contain from about 0.25 to about 1 oz/ton Au.
Similarly, arsenic sludge wastes may contain from about
0.25 to about 1 oz/ton Au. By reprocessing these
wastes, some of the gold contained therein may be
recovered, thereby increaeing the total amount of gold
extracted.
The pressure oxidation feed stream used in
the practice of the present invention is a slurry.
Aqueous slurries are preferred. Slurries useful in the
methods o~ the present invention are characterized by
pulp densities of from about 15 to about 60% solids by
weight, with from about 20 to about 40% solids by
weight being preferred.
The pressure oxidation step of the present
invention may be conducted in commercially available
equipment, such as a multi-compartment autoclave, under
conventionally accepted sulphide-oxidizing conditions.
For example, a 50 L capacity, 6-stage, equal volume per
--11--
3~
stage autoclave may be used in practicing the present
invention. The pressure oxidation treatment may be
conducted using other reactor types, such as industrial
autoclaves of, for example, 3 m diameter and 20 m
length. Exemplary equipment useful in continuous
operations are tubular reactors, a plurality of
reactors arranged in series, and the like. In
addition, a batch reaction vessel may be used when the
methods of the present invention are employed in a non-
continuous fashion.
Oxidation conditions include a temperaturefrom about 150 to about 250C and a total pressure
ranging from about 400 to 6000 kPa (50-700 psi). The
oxidation will be allowed to proceed for a time
sufficient to achieve adequate oxidation of feed
sulphides to sulphates. An adequate oxidation may be
characterized by the conversion of at least about 98%
sulphides by weight to sulphate form. When toxic waste
treatment is also involved, the time required for
formation of insoluble molecules from one or more
soluble toxic contaminants must also be considered,
sincs this reaction will generally require more time.
The time required to sufficiently complete these
reactions will vary with feed composition and total
feed volume. A practitioner in the art would be able
to determine an appropriate pressure oxidation
parameters.
The cyaniding step of the present invention
may be conducted in accordance with well known
conventional techniques. For example, the cyaniding
feed stream is mixed with an excess of an alkaline
cyanlde solution, such as a NaCN solution, to
effectuate gold extraction. Specifically, the
cyaniding feed is treated with the alkaline cyanide
solution for a time sufficient to dissolve the metal to
be extracted (i.~., gold) therein. After filtration
-12-
~3'~7
and filter washing, the gold present in the filtrate
may be recovered by any conventional method therefor.
Carbon in pulp procPssing or the like may alternatively
be used to recover gold.
An exemplary cyaniding process useful in the
practice of the present invention is described below.
The pressure oxidation effluent is subjected to a
solid/liquid separation step. Solids separated therein
are repulped to a pulp density of about 45~ solids.
Lime is then added as a slurry (e.a., about 20% w/w) to
the pulp, and the mixture is agitated until the pH
remains constant at from about 10.5 to about 11Ø An
amount of NaCN sufficient to dissolve gold from the
repulped solids, as is calculable by a practitioner in
the art, is added to the neutralized mixtures.
Optionally, cyanide and/or lime consumption may be
monitored to ascertain whether additional material need
be added. Also, additional stages of cyanide leaching
may be conducted to enhance gold recovery.
Monitoring the level of free cyanide and/or
lime may be conducted in accordance with known
techniques. An exemplary procedure for making the free
cyanide and lime level determinations is set ~orth
below. 10 ml of a cyanide-containing solution is
transferred by pipette into a 100 ml beaker. 5-6 drops
of a cyanide indicator, such as potassium iodide (e.g.,
5 g dissolved in 100 ml distilled water), 5 - (p-
dimethylamino benzylidene) rhodamine (e.g., 0.02 g
dissolved in 100 ml acetone), or the like, are added.
Titration for free cyanide is then conducted to the
endpoint (e.a., a cloudy yellow color for potassium
iodide) with a silver nitrate solution (e.a., 5.2 g
dis~olved in 3 1 distilled water). 2-3 excess drops of
the silver nitrate solution are added prior to the
addition of 3-4 drops of a lime indicator, such as
phenolphthalein (~g~, O.05 g dissolved in 50 ml ethyl
-13-
2~23~7
alcohol and followed by the addition of 50 ml distilled
water) or the like. Titration for lime is then
conducted with oxalic acid (e.a., 6.75 g of oxalic acid
dissolved in 3 l distilled water) until the red color
disappears. The respective levels may be calculated as
follows:
mls silver nitrate . 10 = g/l free
cyanide
mls oxalic acid . 10 = g/l lime
The refractory auriferous material useful in
the practice of the present invention may contain
additional components, such as carbonates, that
exhibit undesirable properties upon oxidation.
Carbonates generate carbon dioxide under pressure
oxidation conditions. Excessive venting which results
in oxygen loss and decreases the efficiency of the
pressure oxidation treatment, is therefore required.
Carbon dioxide formation under oxidizing conditions
can be avoided by neutralizing the carbonates prior to
oxidation. Addition of an amount of an acid, such as
H2S04, sufficient to neutralize substantially all of
the carbonates in the refractory auriferous sulphide
material will accomplish this objective.
Alternatively, acid liquor produced as a byproduct of
the pre~sure oxidation treatment can be recycled to
serve this purpose. If an insuf~icient amount o~ acid
liquor is produced ~ , substantially all of the
carbonate~ in the refractory aurtferous sulphidic
material will not be neutralized by admixture with the
recycle acid liquor), an amount of fresh acid, such as
H2SO4, may be added.
The invention will now be described by way of
example with reference to the Figure, which shows a
~low diagram o~ a gold extraction/toxic waste
treatment process. Ground currently mined re~ractory
auriferous sulphidic ore or concentrate is slurried by
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2~323~7
admixture with water in a slurry formation step 10.
If necessitated by the presence of carbonates or like
components in the refractory auriferous sulphide
material, the slurry may be formed in step 10 with the
addition of recycle acid liquor to the slurry
components to neutralize those carbonates. In this
embodiment of the present invention, slurry formation
step 10 must extend for a time period sufficient to
permit neutralization of substantially all of the
carbonates. This option is indicated by the dashed
line in the Figure. Alternatively, the acid may be
added in a separate step, either prior or subsequent
to slurry formation step lO.
When the slurry is formed, ground calcine and
arsenic-containing sludge are blended with the slurry
in a mixing step 12 to form the pressure oxidation
feed stream. The feed is subjected to a pressure-
oxidation step 14. The oxidized slurry resulting from
pressure oxidation step 14 proceeds to a liquid/solid
separation step 16, including a plurality of
thickeners in series. The industry practice of
¢ounter current decantation (CCD) may be utilized to
achieve liquid/solid separation. A practitioner in
the art would be able to ascertain and employ an
appropriate CCD protocol ~or this purpose.
~ portion o~ the liquid (i.e., acid liquor)
may be recycled to neutralize carbonates, if
necessary. Excess acid liquor is neutralized with
alkaline mill ~lotation tailings or with other
appropriate alkaline compounds in an acid treatment
step 18, and the treated acid is discharged as process
ef~luent. After pH ad~ustment with a neutralizing
agent in a neutralizing step 20, the solids are
~ub~ected to a cyaniding step 22. One output stream
~rom cyaniding step 22 is the gold product stream.
The waste effluent from cyaniding step 22 may be
-15-
2~ ~2~7
directly discharged as a result of the decrease insoluble toxic residues contained therein.
Neutralizing step 20 involves the pH
adjustment of the solid effluent from liquid/solid
separation step 16. In neutralizing step 20, a
neutralizing agent, such as lime, is added to the
solids-containing slurry in an amount sufficient to
elevate the pH of the slurry to a level conducive to
cyaniding (i.e., a pH from about 8 to about 11).
The results of various tests conducted in
connection with the present invention are described by
way of example only, and the present invention is
therefore not limited thereby.
EXAMPLE 1
Conventional cyaniding tests were done on
individual materials that are useful as components of
pressure oxidation feed streams contemplated for use
in methods of the present invention. Batch cyaniding
tests may be conducted in accordance with the
~ollowing procedure. If necessary, the test example
i~ ground or pulverized. The test sample is then
transferred into a suitable container (e.g., a glass
bottle) suitable for placement on a set of rolls to
achieve agitation thereof. The pulp density o~ the
test sample is ad~usted to a preferred value through
the addition of water. A "neutralizing amount" of
lime and an "extracting amount" of cyanide (i.e.,
amounts of these reagents necessary to carry out the
respective reactions) are added to the test sample.
Preferably, lime is added and admixed with the test
sample prior to cyanide addition. The sample
container i8 then secured on the rolls and agitated.
A sample of currently mined re~ractory
sulphidic concentrate containing 3.9 oz/ton gold (Au),
13.7% arsenic (As), 32.1% iron (Fe), and 29.5% sulphur
-16-
2~32~7
(S) was subjected to a 48 hr. cyaniding. The gold
extraction was 62.3%, with a residue of 1.47 oz/ton.
A 48 hr. cyaniding of the refractory ore itself gave a
gold extraction of 48.1%. A sample of refractory
5 sulphidic concentrate obtained from old tailings
contained 0.63 oz/ton AU, 3.12% As, 35.1% Fe, and
38.9% S. The gold extraction after a 48 hr. cyaniding
was 57.4%, with a residue of 0.27 oz/ton. Ground
calcine, containing 0.40 oz/ton Au, 2.26% As, 43.9%
Fe, and 22.5% S, gave a gold extraction of 26.5%, with
a residue of 0.36 oz/ton following a 72 hr. cyaniding.
Conventional cyaniding tests on samples of arsenic
sludge containing 0.52 oz/ton Au, 56.2% As, 7.8% Fe,
and 2.7% S were unsuccessful, as a result of the
toxicity and acidity of the sludge. A total of
~everal hundreds of pounds/ton of lime was added for
alkalinity. This treatment was followed by a
consumption of at least 100 lb/ton of NaCN. Less than
10% of the gold was extracted after subjecting the
treated sludge to 72 hr. of cyaniding.
EXAMPLE 2
The materials described in Example 1 were
subjected to individual batch pressure oxidation tests
at a pulp density of 9%, a total pressure of 2200 kPa,
and a temperature of 190C. Samples were taken at
pre-determined time intervals and the amount of
~ulphide sulphur oxidation to sulphate form was
measured as well as gold extraction obtained in
subsequent cyaniding. The results of these tests are
summarized in Table 1.
203i2357
TAB~E 1
%S Oxidation Oxidation Time in Minutes
to Sulphate 20 40 60 90 120
Refractory Conct. 16.4 46.9 91.6 99.9 99.9
Refractory Tails Conct. 58.1 84.3 99.2 99.9 99.9
Calcine 48.7 88.1 90.8 95.4 98.1
Sludge 94.7 96.1 97.1 97.4 98.1
% Gold Extraction
Refractory Conct. 75.3 92.1 94.5 95.3 95.5
Refractory Tails Conct. 74.3 85.8 86.6 90.5 94.2
Calcine 68.9 72.4 72.8 72.8 72.9
Sludge 89.3 90.8 92.3 92.8 95.3
In general, the results, when compared with
those obtained in Example 1, confirm that pressure
oxidation enhances gold recovery. The results also
show increased gold extraction upon increased sulphur
oxidation. With respect to calcine, the increase in
gold extraction after 20 minutes of oxidation is
slight as a result of encapsulation of gold in the
iron oxlde matrix o~ calcine.
EXAMPLE 3
Batch pressure oxidation tests were conducted
on ~eed compo~ites o~ di~erent compositions under the
conditions employed in Example 2 (i.e., 9% solids by
weight at a temperature 190'C and at a pressure of
2200 kPa ~or 2 hours). The results of these tests are
shown in Table 2.
TABLE 2
Feed Composite & Total Weight
Composite Gold
CQnct. Calcine Sludge % S Extraction %
37.0 38.0 25.0 18.2 90.4
30.0 30.0 40.0 12.8 91.2
37.0 38.0 25.0 18.2 89.6
- 50.0 50.0 11.9 86.6
45 50.0 30.0 20.0 17.7 90.5
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These results show that 86.6% of the gold contained in
the toxic wastes may be recovered in the practice of
the present invention. In addition, decreased
percentages of sulphur in the pressure oxidation feed
composite appear to result in increased gold
extraction. Such results indicate that the process
works metallurgically.
EXAMPLE 4
Batch pressure oxidation tests were conducted
a~ in Example 2 on a feed composite consisting of 27%
refractory sulphidic concentrate, 23% reclaim tailings
refractory sulphidic concentrate, 25% calcine, and 25%
sludge by weight. The tested feed composites
exhibited pulp densities of either 20 or 25% solids.
Oxidation time and temperature were varied. The
results of these tests are shown in Table 3.
TABLE 3
Pulp Time % S Gold
DensitY % min Temp C Oxidation Extraction %
20 120 190 97.6 85.8
20 60 210 98.4 94.9
25 60 210 97.3 90.9
Thess results appear to indicate that 20~
pulp density and Z10C constitute the preferred
pressure oxidation parameters in terms of sulphur
oxidation and gold recovery. Such results indicate
that the process works metallurgically on different
feed composites.
EXAMPLE 5
The feed composite tested in Example 4 was
used in four continuous runs in a 50 1. capacity, 6-
compartment autoclave (Titanium Ltd., Quebec, Canada).
All compartments were of equal volume and eguipped
--19--
2032~7
with individual agitators. In each run, the oxygen
overpressure was 700 kPa.
The first continuous run was conducted at 20%
solids by weight at 190C following acidification of
the concentrates with 100 kg/ton of H2SO4. The gold
extraction was 84.6~. Cyaniding consumed 9.4 lb/ton
of lime and 1.79 lb/ton of NaCN. Over 97% sulphide
oxidation to sulphate was achieved. Chemical analyses
showed that all of the arsenic dissolved and combined
with iron to form ferric arsenate.
The second continuous test was conducted at
20~ solids by weight at 210C following acidification
treatment as described above. The gold extraction was
87.9%. Cyaniding consumed 8.96 lb/ton of lime and
2.02 lb/ton of NaCN. Over 98% sulphide oxidation was
achieved. Ferric arsenate was formed. An increase in
pressure oxidation temperature resulted in increased
gold recovery.
The third continuous test was conducted at
25% solids by weight at 210C following acidification
treatment as described above. The gold extraction was
88.1%. Cyanidlng consumed 8.96 lb/ton of lime and
2.91 lb/ton o~ NaCN. Over 98% sulphide oxidation was
agaln achieved. Chemical analyses showed an exaess o~
~errlc lron ln solution with less than 1 g/L o~ As.
The ~ourth continuous run was conducted at
25% solids by weight at 220C following acidification
treatment as described above. The gold extraction was
88.4%. Cyaniding consumed 8.29 lb/ton of lime and
2.24 lb/ton of NaCN. 98% sulphide oxidation was
achieved, while ~erric arsenate was formed. Again, an
increase in pressure oxidation temperature resulted in
lncreased gold recovery. Cyanidlng of composites
havlng pulp densities o~ 20% and 25% solids resulted
in substantially e~uivalent gold extraction.
-20-
2~3~3~7
. .
The process ran smoothly in all four
continuous tests. No evidence of agglomeration of
solids or elemental sulphur formation inside the
autoclave was observed in any of the continuous tests.
Consequently, the tests show that addition of toxic
wastes to the pressure oxidation feed composite
improves pressure oxidation performance over non-
diluted, refractory auriferous sulphidic material,
prior art pressure oxidation treatments.
Environmental (EPA-type) testing, i.e.,
acetic acid extraction was done on selected
neutralized products from the autoclave. The
environmental testing procedure was as follows:
1. A solid sample was prepared for
extraction by crushing, cutting or grinding, until the
sample was able to pass through a 9.5 mm mesh sieve.
2. The moisture content of the de-watered
sample was determined by drying a suitable aliquot to
constant weight at 50~C in an oven.
3. The equivalent of 50 g dry weight of the
de-watered, undried material was placed into a 1250 ml
wide mouth cylindrical bottle.
4. 800 ml tless the moisture content of the
sample in ml) of reagent water was added to the
bottle.
5. The bottle was capped and agitated in a
commercially available rotary extractor for 15
minutes.
6. The pH of the solution was measured with
a commercially available pH meter, and calibration was
undertaken with buffers at pH 7.00 and pH 4.00. The
solution was stirred during pH measurement.
7. If the pH was less than 5.2, step lOa
was next conducted.
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2~32~7
8. If the pH was greater than 5.2, a
sufficient volume of 0.5 N acetic acid was added to
bring the pH to 5.0 + 0.2.
9. The bottle was capped and placed in a
commercially available tumbling apparatus and rotated
at 10 rpm for 24 hours at room temperature end for end.
10. The pH was monitored and manually
adjusted during the course of the extraction in
accordance with the following procedures.
a. The pH of the solution was measured
when 1 hour, 3 hours and 6 hours had elapsed from the
extraction starting time. If the pH was above 5.2, it
was reduced to 5.0 + 0.2, with no adjustments being
made.
b. If the pH was below 5.0 + 0.2 after
6 hoùrs, the volume of the solution was adjusted to
1000 ml with reagent water.
c. The pH was measured and reduced to
5.0 + 0.2, if required, after 22 hours, and the
extraction was continued for an additional 2 hours.
11. Sufficient reagent water was added at
the end of the extraction period so that the total
volume of the liquid was 1000 ml.
12. The material was separated into its
component liquid and solld pha~es by filtering through
a 0.45 ~ filter.
13. The ~olution from Step 12 wa6 analyzed
for the contaminants listed in Schedule 4 that are
likely to be present.
14. Concentrations in the combined leachate
and the free liquid solution at pH 5.0 i 0.2 were
obtained.
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~B32~7
SCHEDULE 4
REG 309 LEACHATE OUALITY CRITERIA AND EXEMPLARY RESULTS
Criteria (ma/L) Results (mq/L)
Arsenic 0.05 <0.05
Barium 1.0 0.06
Boron 5.0 not tested
Cadmium 0.005 0.02
Chromium 0.05 <0.05
Cyanide (free~ 0.2 not tested
Lead 0.05 ~0.05
Mercury 0.001 not tested
Selenium 0.01 <0.01
Silver 0.05 not tested
Uranium 0.02 not tested
TABLE 4 - Semi-Ouantitative ICP Scan
Detection Detection
Limit Results Limit Results
Element (ma/L) (ma/L) Element (ma/L) (mq/L)
Al 0.2<0.50 Na 0.05230
As 0.1<0.05 Ni 0.050.11
Ba 0.050.06 P 0.2<0.02
Bo 0 01<0.001 Pb 0.1<0.05
Ca 0 2500 S 2.02400
Cd 0.050.02 Sb 0.1<0.10
Co 0.050.08 Sc [?~ 0.5<0.05
Cr 0.05<0.02 Se 0.5~0.20
Cu 0.050.81 Si 0.10.45
Fe 0.05<0.02 Sn 0.2<0.10
Mg 0.051700 Te 0.1<0.05
Mo 0.1<0.05 Zn 0.050.84
TABLE 5 - Ion Chromatoaraph Scan
Results
IonDetection Limit (mg/L)(ma/L)
Br 0.02
F 0.02
NO3 0.02
S04 0.02
Cl 0.02
NO2 0.02
PO4 0.1
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2~3~3~7
TABLE 6 - Leachate Ouality Criteria
Element Acceptable Registerable Hazardous Results
(mg/L) (ma/L~ (mq/L) (ma/L)
Arsenic <0.5 >0.5 <5 ~5 <0.05
Barium <10 >10 <100 >100 <0.10
Boron <50 >50 <500 >500 <5.0
Cadmium <0.05 >0.05 <0.5 >0.5 0.38
Chromium <0.5 >0.5 <5 >5 <0.02
Lead <0.5 >0.5 <5 >5 ~0.05
Mercury <0.01 >0.01 <0.1 >0.1 <0.001
Selenium <0.1 >0.1 <l >l <0.01
Silver <0.5 >0.5 <5 >5 <0.03
Uranium <0.2 >0.2 <2 >2 <0.01
NOTE: For each series of tests a "blank" is run for
quality control purposes, using the same reagents and
procedure.
In all cases, soluble arsenic was present at
less than 0.10 ppm, well below the maximum allowable
environmental discharge of 1 ppm. The addition of
toxic wastes to the pressure oxidation feed composite
therefore also resulted in a reduction in the level of
toxic contaminants present in those wastes to
acceptable discharge levels. These results confirm
that a 9 hour continuous run duplicated the batch
results.
EXAMPLE 6
A fifth continuous test was conducted on a
different feed composite (15% re~ractory sulphide
concentrates, and 42.5% each by weight calcine and
sludge). The test was run at 20% solids by weight at
210'C following acidification as described above, with
the exception of the use of 100 kg/ton recycle acid
liquor instead of H2S04. The gold extraction was 83.6%.
Cyaniding consumed 3.14 lb/ton of lime and 1.57 lb/ton
of NaCN. 98% sulphide oxidation was achieved. 95% of
the arsenic was converted to ferric arsenate, thereby
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20323~7
indicating that additional iron was required in the
pressure oxidation feed.
This test also ran very smoothly. No
formation of agglomerates or elemental sulphur was
observed, indicating that no excessive heat was
generated in the autoclave. Consequently, this test
also shows that the addition of toxic wastes to the
pressure oxidation feed composite improves pressure
oxidation performance over non-diluted, refractory
auriferous sulphidic material, prior art pressure
oxidation treatments. The gold extraction obtained for
this feed composite was lower (when compared with the
second continuous run of Example 5); however, the
amount of lime and NaCN consumed was considerably
reduced.
While in the foregoing specification this
invention has been described in relation to certain
preferred embodiments thereof, and many details have
been set forth for the purpose of illustration, it will
be apparent to those skilled in the art that the
lnvention is susceptible to additional embodiments and
that certain details described herein may be varied
con~iderably without departing from the basic
principles of the invention.
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