Note: Descriptions are shown in the official language in which they were submitted.
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a
METHOD FOR INITIATING HEAP BIOLEACHING OF SULFIDIC ORES
FIELD OF THE INVENTION
The present invention is directed generally to
bioleaching of sulfidic ores and specifically to heap and vat
bioleaching of sulfidic ores.
BACKGROUND OF THE INVENTION
A major source of many metals, particularly copper and
gold, is sulfidic ores. In sulfidic ores, the metals are
either present as or immobilized by stable metal sulfides,
which are frequently nonreactive or slow reacting with
lixiviants such as cyanide, ferric ion or sulfuric acid.- To
promote the dissolution of the metals in a lixiviant, the
elements compo»nded with the metal (e.g., sulfide sulfur) are
first be oxidised. In one approach, oxidation of the sulfide
sulfur is induced by organisms, such as Thiobacillus
Ferrooxidans and Thiobacillus Thiooxidans (commonly referred
to as biooxidation or bioleaching).
Although biooxidation can be performed in a continuous
stirred tank reactos, a common technique is to perform
biooxidation in a heap. Compared to biooxidation in a
continubus stirred reactor, heap biooxidation generally has
lower capital and operating costs but a longer residence time
and lower overall oxidation rate for the sulfide sulfur in the
feed material.
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In design~.ng a heap biooxidation process, there are a
number of considerations. First, it is desirable to have a
relatively high heap permeability and porosity. Fine material
can decrease heap permeability and porosity and result in
channeling. Channeling can cause a portion of the material in
the heap to have a reduced contact with the lixiviant, thereby
limiting the de-gree of biooxidation of the material. Second,
it is desirably that the residence time of the feed material
in the heap (i.e., the time required for an acceptable degree
l0 of biooxidation) be as low as possible. Existing heap
leaching processes typically have residence times of the heap
on the pad of 12 months or more for an acceptable degree of
biooxidation to occur.
SUMMARY OF THE INVENTION
These and other objectives are addressed by the process
of the present invention. The process includes the steps of:
(a) biooxidizing a first portion of a feed material
containing metal sulfides to form a biooxidized .fraction;
(b) combining the biooxidized fraction and a second
portion of the feed material to form a combined feed, material;
and
a
(c) thereafter biooxidizing the combined feed material.
The metal in the metal sulfides can be copper, gold, silver,
nickel, zinc, arsenic, antimony, and mixtures thereof." As
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will b~ appreczated, precious metals, such as gold, generally
are not compounded with sulfide sulfur but are rendered
immobile in the lixiviant by close association with metal
sulfides, especially pyrite and arsenopyrite.
Because the biooxidized fraction includes large active
cultures of organisms, such as Thiobacillus Ferrooxidans;
Thiobacillus Thiooxidans; Thiobacillus Organoparus;
Thiobacillus Acidphilus; Sulfobacillus Thermosulfidooxidans;
Sul folobus Acidocaldarius, Sulfolobus . BC; Su1 folobus
Solfataricus; Acidanus Brierley; Leptospirillum Ferrooxidans;
and the like for oxidizing the sulfide sulfur and other
elements in the feed material, the combination of -the
biooxidized fraction and the second portion of the feed
material (which typically has not been biooxidized) '.jump
starts" the biooxidation of the second portion. 'Tn other
words, the time required to substantially complete
biooxidation of the second portion is significantly reduced
relative to existing heap leaching processes, thereby reducing
heap pad area and capital and operating costs.
The biooxidatiQn in step (a) can be performed in a
continuous stirred reactor or on a heap. A continuous stirred
reactor. is preferred because of the relatively rapid rate of
biooxidation in such reactors and the high concentration of
microbes on the biooxidized residue. After inoculation of the
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slurried portion of the feed material, the continuous stirred
reactor preferably is sparged with oxygen and supplied with
suitable nutrients for the microbes to foster biooxidation.
Typically, the second portion of the feed material has
not been biooxidized. In one embodiment, the second portion
is coarsely sized while the biooxidized traction (i.e., the
first portion) is finely sized. The biooxidized fraction
typically has a Peosize ranging from about 5 to about 200 mesh
microns and more preferably from about 10 to about 200 microns
while the second fraction has a Peo size in excess of that of
the biooxidized fraction.
The combining step can be performed in a number of ways.
For example, the biooxidized fraction can be agglomerated with
the second portion of the feed material. "Agglomeration"
refers to the formation of particles into a ball (i.e., an
agglomerate), with or without the use of a binder.
Alternatively, the biooxidized fraction can be placed on the
conveyor belts along with the second portion of the feed
material and be carried by the belts to the heap.
In another embodiment, the process includes the step of
floating a portion of the feed material to form a tailing
fraction and a concentrate fraction. The first portion of the
feed material includes the concentrate fraction. A
substantial portion of the fine material is discarded in the
tailing fraction so that the porosity and permeability of the
heap remains unaffected by the fine size of the relatively
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- ~ small quantity of concentrate fraction (which is incorporated
into the heap after partial or complete biooxidation of the
concentrate fraction). Commonly, the first portion of the
feed material constitutes no more than about 15 wt~ of the
feed material while the concentrate fraction constitutes no
more than about 30 wt~ of the first portion (i.e., no more
than about 4.S wt~ of the feed material). Accordingly, the
tailing fraction constitutes at least about 70 wt~ of the
first portion.
BRIEF DESCRIPTION OF THE DRA~dINGS
Fig. 1 depicts an embodiment of the present invention_for
the recovery of base metals: and
Figs. 2A and B depict an embodiment of the present
invention for the recovery of base and/or precious~metals.
DETAILED DESCRIPTION
Referring to Figure 1, ~an embodiment of the present
invention is depicted for recovering base metals from
sulfidic ores. The recoverable base. metals include copper,
,.
iron, nickel, zinc, antimony, arsenic, and mixtures thereof.
The metal generally occurs in the ore as a metal sulfide,
such as chalcopyrite (CuFeSz), bornite (Cu;FeS4), chalcocite
(Cu2S) , digenite (Cu,S;) , covellite (CuS) , and the like .
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A feed material 10 containing the metal sulfide is
comminuted 14 to produce a comminuted material 18. The Pea
size of the comminuted material preferably ranges from about 5
to about 20 mm.
The comminuted material 18 is subjected to primary size
separation 22 to form an undersized fraction 26 and an
oversized fraction 30. Primary size separation 22 can be
performed by any suitable technique, with screening being most
preferred. The preferred screen size ranges from about 1 to
about 3mm. Typically, the undersized fraction 26 represents no
more than about 30 wt% of the comminuted feed material 18
while the oversized fraction 30 represents at least about.70
wt% of the comminuted feed material 18.
The undersized fraction 26 is subjected to secondary size
separation 34 to produce a sand 38 and a fine portion 42 of
the feed material. Secondary size separation 34 can be
performed by any suitable techniques such as by cycloning or
screening. The secondary size separation 34 is performed such
that the fine portion 42 represents no more than about 20 wt%
of the undersized fraction 26. The secondary size separation
34 is typically performed such that the fine portion 42 has a
Peosize ranging from about 5 to about 200 microns.
The sand 38 is combined with the oversized fraction 30 to
form a coarse portion 46 of the feed material. Preferably, the
coarse portion 46 represents at least about 90 wt% of the
comminuted feed material 18.
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The fine portion 42 is subjected to flotation 50 to
produce a tail~.ng fraction 54 and a concentrate fraction 58.
As will be appreciated, the concentrate fraction 58 contains
most of the metal sulfide and preferably at least about 80~ of
the metal sulfide in the fine portion 42. The collectors and
frothers and conditions used during flotation 50 depend, of
course, on the particular metal sulfide being recovered. They
may include, but are not limited to, xanthates and
dithiophosphates. Typically, the concentrate fraction 58
constitutes no more than about 20 wt~ of the fine portion 42.
The concentrate fraction 58 is slurried and biooxidized
62 in a series of continuous stirred tan~C reactors, to produce
a biooxidized slurry 66. Biooxidation 62 is preferably
conducted at a slurry temperature ranging from about 20 to
about 60°C; a slurry pH ranging from about pH 1.2 to~.about pH
2.5; and a sulfuric acid content in the slurry ranging from
about 1 to about 20 g/1. During biooxidation 62, air is
sparged through the slurry to provide molecular oxygen for
biooxidation. The slurry further includes microbes and
suitable energy source and nutrients for the microbes, namely
from about 0.1 to about 10 g/1 of Fe2'; from about 0.1 to about
10 g/1 of ammonium sulfate (NH4)Z SO3, from about 0.'05 to about
5 g/1 of a phosphate.
The microbes that can be used for biooxidation are
discussed in U.S. Patent 5,246,480 entitled "Biooxidation
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Process for Recovery of Gold from Heaps of Low-Grade Sulfidic-
and Carbonaceous Sulfidic Ore Materials". The microbes include
Thiobacillus Ferrooxidans; Thiobacillus Thiocxidans;
Thiobacillus Organoparus; Thiobacillus Acidphilus;
Sulfobacillus Thermosulfidooxidans; Sulfolobus Acidocaldarius,
Sulfolobus BC; Sulfolobus Sulfataricus; Acidanus Brierley;
Leptospirillum Ferrooxidans; and the like. The microbes can
be classified as either, (a) facultative thermophile, i.e.,
the microbe is capable of growth at mid-range temperatures
l0 (e. g., about 30°C) and high (thermophilic) temperatures (e. g.,
above about 50°C to about 55°C) or (b) obligate thermophile
which are micro-organisms which can only grow at high
(themophilic) temperatures (e. g., greater than about 50°C).
The biooxidized slurry 66 is subjected to liquid/solid
separation 70 to form a pregnant leach solution 74 and a
biooxidized residue 78. The pregnant leach solution 74 is
subjected to metal recovery 82 to produce a metal product 86.
Metal recovery 82 can be performed by any suitable technique
including solvent extraction/electrowinning.
The biooxidized residue 78, which contains active
cultures of microbes, is combined with the coarse portion 46
of the feed material to form a combined feed material. The
combined material can be agglomerated 90 with or without a
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suitable binder to form agglomerates 94. The combined feed
material can be contacted with additional microbes prior to
agglomeration. In some cases, it may be desirable to
introduce different cultures of microbes that flourish at
temperatures different from the cultures of microbes present
on the biooxidized residue 78. As will be appreciated, a
temperature profile will generally exist in the heap.
As shown in Fig. 1, the biooxidized material can
alternatively be placed directly on a conveyor belt to the
heap along with the second portion of the feed material or on
top of the heap formed from the second portion of the feed
material. -
The agglomerates 98 are formed into a heap 102. The heap
102 is formed on a lixiviant-impervious liner, and an
irrigation system for the lixiviant is erected om the heap.
A cooling and/or heating system can be installed on the
process solution flowstream for temperature control. Air may
be introduced to the body of the heap through a pipe network
under positive pressure to promote ingress of molecular oxygen
through the heap.
.The heap 10'2 is biooxidized 106 to produce a solid waste
material 11.0 and a primary pregnant leach solution 114
containing most of the metal values in the comminuted material
18. Biooxidation is performed by applying a lixiviant,
preferably sulfuric acid and containing an innoculate capable
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of biooxidizing sul~ide sulfur and other elements compounded
with the metal and/or nutrients for the microbes, to the top
of the heap; percolating the lixiviant and nutrients through
the heap; and removing the primary pregnant leach solution 114
from the base of the heap 102.
For optimal results, the conditions in the heap 102 are
carefully controlled. The lixiviant preferably has a pH less
than about pH 2.5 a~d more preferably ranging from about pH
1.3 to about pH 2Ø The lixiviant can include from about 1
to about 10 g/1 0. ferric ion sulfate to aid in the
dissolution of metals. The lixiviant can also contain an
energy source and nutrients for the microbes, such iron
sulfate,ammonium sul-ate and phosphate.
If the combined feed material contains significant
amounts of arsenic, the arsenic can be removed by
coprecipitation with iron under suitable conditions.
Typically, pentavalent arsenic and trivalent iron will
coprecipitate when the solution ratio of Fe:As exceeds 4:1 and
the solution pH exceeds 3.
The primary pregnant leach solution 114 can be subjected
to metal recovery 82 to produce the metal product 86. When
biooxidation is complete, the fully biooxidized material in
the heap becomes waste material 110.
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Figures ~A and B depict a second embodiment of the
present invention for recovering precious and base metals from
a sulfidic feed material. The feed material 100 is comminuted
I4 to form a comminuted material 118. The comminuted material
118 is subjected to primary size separation 22 to form an
oversized fraction 130 and an undersized fraction 126. The
undersized fraction 126 is subjected to secondary size
separation 34 to produce sand 138 and a fine portion 142 of
the feed material. The fine portion 142. is subjected to
l0 flotation 50 to form a tailing fraction 154 and a concentrate
fraction 158. The concentrate fraction 158 is biooxidized 62
to form a biooxidized slurry 166, which is subjected to
liquid/solid separation to form a secondary base metal
pregnant leach solution 174 and a residue 178. The residue
178 contains most of the precious metal content of 'the fine
portion 142 of the feed material.
The oversized fraction I30 and sand 138 are combined to
form a coarse portion 146 of the feed material, and the coarse
portion 146 is combined with the residue 178 and the combined
material agglomerated 90 to form agglomerates 194. The
agglomerates I94 are formed 98 into a heap 202. The heap is
biooxidszed lfl6 to form biooxidized agglomerates 206 and
primary base metal pregnant leach solution 214. The primary
and secondary base metal pregnant leach solution 174 is
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subjected to base metal recovery 82 to form a base metal
product 186 where applicable.
The biooxidized agglomerates 206, which contain most of
the precious metal content of the comminuted material 118, are
repeatedly and thoroughly washed 218, preferably with an
aqueous solution, to remove the lixiviant from the
agglomerated particles and form washed biooxidized material
222. During washing, the agglomerates will commonly brea:<
apart, thereby facilitating lixiviant removal.
to The washed biooxidized material 222 is neutralized 226 by
contact with a base material to form neutralized material 230
and agglomerated 234 to form agglomerates 230. The base
material, which is preferably lime, limestone, Portland
cement, caustic soda, cement dust, or mixtures of these
materials, can be utilized as a binder during agglomeration.
As will be appreciated, neutralization is important as the
washed biooxidized material 222 is fairly acidic and can cause
uneconomically high cyanide consumption during cyanidation
246.
The agglomerates 234 are formed into a reconstituted heap
,.
242 which is subjected to cyani.dation 246 (using a cyanide
lixivi~nt) to dissolve the precious metal in the agglomerates
234 in a precious metal pregnant leach solution 254. The
precious metal pregnant leach solution 254 can be subjected to
precious metal recovery 258 by known techniques to produce a
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precious metal product 262. After cyanidation 246 is
completed, the agglomerates 234 can be discarded as waste
material 250.
While various embodiments of the present invention have
been described in detail, it is apparent that modifications
and adaptations of those embodiments will occur to those
skilled in the art. However, it is to be expressly understood
that such modifications and adaptations are within the scope
of the present invention, as set forth in the following
to claims.
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