Note: Descriptions are shown in the official language in which they were submitted.
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BIO-OXIDATION PROCESS AND APPARATUS
FIELD OF THE INVENTION
This invention relates to bio-oxidation processes and reactors for the
liberation of
metals from minerals, especially sulphides, containing them.
s BACKGROUND OF THE INVENTION
Metal containing minerals, especially metal sulphides, may be oxidised
using specific types of micro-organism, especially bacteria in a
bioextraction,
particularly a bio-oxidation process. Oxidation of sulphide minerals may be
used
to put the metals into solution, from for example iron, copper, zinc, nickel
and
to cobalt sulphides, or to release precious metals, such as gold, silver and
platinum,
encapsulated in metal sulphides, particularly in refractory ores. The process
is
known as bacterial oxidation, bioextraction, bio-oxidation, bioleaching, or
bacterial
leaching. In this respect, pyrite, an iron sulphide, and arsenopyrite, an iron-
arsenic sulphide are the most common minerals occluding gold in the so-called
is refractory gold ores and treatment of such materials by microorganisms may
therefore assist in liberation of precious metals from refractory ores
containing
them.
Certain kinds of bacteria used in such processes have been documented
and include the mesophiles, Thiobacillus ferrooxidans, Thiobacillus
thiooxidans,
2o Leptospirillum ferrooxidans, moderate thermophiles, and thermophiles such
as
Sulpholobus.
The above processes are the subject of an extensive literature
encompassing papers and patents. However, conventional methods of bacterial
oxidation involve oxidation in agitated stirred tanks into which air is
introduced or
2s in heaps using rocks with a size of 6mm or greater in which fine particles
may
have been agglomerated or heaps using palletised fine material. An acidic
bacteria containing liquor is developed and typically used to irrigate the
heap and
facilitate metal extraction. The heap leaching technique has been used for
many
years and Brierley et al have patented a process (US Patent No. 5246486) for a
3o two stage agglomeration process for the recovery of gold from sulphide
minerals
in heaps where the mineral particles are of large size and not slurried.
Van Aswegen in the article "The Genmin Experience", has described
bacterial oxidation in tanks made of stainless steel or other lined steel
which
contain a slurry of concentrate and water. The tanks are typically 9 metres
high
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and 9 metres diameter and agitated with axial flow impellers which are driven
by
large motors. Compressed air is piped into the tank and sheared by the
impellers
into a fine dispersion of bubbles to maintain a desired dissolved oxygen
concentration in the slurry. A variety of impeller types other than axial flow
may
s be used such as turbines.
The present applicant has found that such methods suffer from a number
of disadvantages. Primary among these is the high power consumption and high
capital cost encountered in employing bacterial oxidation reactors agitated by
impellers.
Although proposals have been made to employ diffusion means to maintain
particles in suspension, notably in Envirotech US Patent Nos. 4728082,
4732608,
4968008, 4974816 and 5007620, it is uniformly recognised that there is a need
to
provide some mechanical means, for example in the form of a rake to ensure
that
significant quantities of solids are not deposited on the floor of the reactor
vessel,
is reducing reactor efficiency.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide an improved process and
apparatus for carrying out bacterial oxidation especially of metal sulphide
minerals.
2o With this object in view, the present invention provides, in its first
aspect, a
process for recovering metals from materials containing them by bio-oxidation
including treating, in a non-mechanically agitated reactor, a slurry
containing a
metal containing material with bacteria capable of promoting extraction of
metals
from said metal containing material; and maintaining said material in
suspension
2s and bacterial viability in the reactor by introducing an oxygen-containing
gas to
the slurry within the reactor by aeration means.
The aeration means advantageously introduces gases to the reactor in the
form of bubbles of controlled size, generally of small diameter to enhance
mass
transfer of oxygen to bacteria. It will be understood, in this respect, that
the
so bacterial oxygen demand is extremely high and oxygen diffusion
characteristics
important. Control of bubble size by shearing is not employed.
Suitable aeratioNagitation means may particularly and advantageously
include diffusers as described hereinbelow. Diffusers are devices that will
both
introduce air or other gases to the slurry by diffusion mass transfer of gas
from
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fine bubbles- advantageously of controlled size-to the solution or slurry and
maintain solids in suspension for bacterial oxidation. Dome, tubular, disc or
doughnut type diffusers may appropriately be used in accordance with the
invention.
s Aeration means may include hydraulic shear devices. In such devices,
very fine bubbles are formed by dislodgment thereof before full development
under the influence of shear forces.
Surface aerators such as paddle aerators may be used to break liquid
surface and assist in entrainment of gas such as air in the bulk reaction
slurry.
io Reactor configuration may be varied to maximise aeration.
A further alternative would be the use of liquid jets where a stream of liquid
from a reactor is pumped through a venturi which draws air into the liquid
thereby
aerating it. The aerated liquid is returned to the reactor.
In a particularly preferred embodiment, a gas appropriate for maintenance
is of bacterial viability include oxygen containing gases, such as air, oxygen
enriched air or oxygen; optionally, with addition of carbon dioxide, such as
air
enriched with carbon dioxide, may be introduced to the aqueous solution by.
aeration means, particularly diffusers.
Where a bio-oxidation process is involved, various chemolithotrophic bacteria
2o may be employed. Various categories of bacteria may be employed in these
processes. These categories are as follows:
1 ) mesophiles which are suitable for oxidation in the range 10°C to
45°c
2) moderate thermophiles, which oxidise the sulphide minerals in the
2s range 40°C to 60°C; and
3) thermophiles which oxidise from above 50°C to 90°C.
The process may be carried out employing any one or more such
microorganisms, especially bacterial species, often referred to as a single or
mixed culture respectively, which will oxidise sulphides or other minerals or
metal
3o containing materials in the required temperature range. Preferred
mesophiles for
use in bacterial oxidation are Thiobacillus ferrooxidans, Thiobacillus
thiooxidans
and Leptospirillum ferrooxidans. Most of the moderate thermophiles do not have
specific names, but some have been referred to as Sulphobacillus
thermooxidans. Preferably, the thetmophilic bacteria will be of the type
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Sulpholobus brierleyi, Sulpholobus BC, and Sulpholobus acidocaldarius.
Notwithstanding naming of species herein, similar iron and sulphur oxidising
bacteria within the overall temperature range of 0°C to 90°C
identified but not yet
named are included within the scope of the present invention. The process is
s most advantageously conducted generally in the temperature ranges for which
the bacteria are tolerant as described above. Reaction may be undertaken in
the
mesophilic, moderately thermophilic or extreme thermophilic temperature range.
Though the process may be employed exclusively for metal liberation,
pre- or post- treatment by other metallurgical operations, such as CIP or CIL
to processes for recovery of precious metals for example, may be employed
where
desirable. Solid/liquid separation may typically follow bio-oxidation, liquor
being
further treated for metal recovery.
The process may be conducted in any suitable reactor, inciuding those
types of reactor already generally known to the art, operating advantageously
on
is a continuous basis. Aeration may be achieved by including, in such
reactors,
suitable aeration means even in large capacity systems.
However, it is intended that no mechanical means be employed within the
reactor, that the introduction of gas be employed as the sole agency by which
particles are maintained in suspension without significant build-up of mineral
2o containing particles constituting the metal containing material on the
floor of the
reactor. Such a reactor may be said to be rakeless.
In a particularly preferred embodiment, the metal containing materiai is, for
example, a non-ferrous base metal sulphide ore, such as a copper, nickel,
zinc,
lead or cobalt containing ore, including mixed or polymetallic ores, or a
refractory
2s gold ore incorporating occluding sulphides amenable to dissolution by
bacterial
action. Other materials in this class may include flotation concentrates,
gravity
concentrates, tailings, precipitates, mattes and sulphidic fume. However, the
above process is also suitable for carrying out bacterial oxidation and
bioleaching
of non-sulphide ores and other generally inorganic materials containing metals
in
3o economic concentrations, where suitable bacteria are available to carry out
the
process. For example, without intending to limit the invention in any way,
bioleaching of rare earth ores, oxidic manganese ores and phosphate rock is
possible in accordance with process of the invention.
In a further embodiment of the invention, there is provided a reactor
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system for bio-oxidation treatment of metal containing materials including at
least
one rakeless reactor having a reaction volume provided with aeration means for
introducing an oxygen containing gas for maintaining said metal containing
material in suspension and bacterial viability.
s The reactor may take various forms, though incorporates no mechanical
means of agitating the reactor volume, and may for example be in the form of
tanks or vats. In this case, the reactor may be fabricated from suitable
materials,
steel, metal alloys or concrete optionally lined with an acid resistant
medium.
Additional materials of construction include wood, plastic, fibre-glass or any
to suitable aeration means may be as known in the art.
Alternatively, a reactor may take a reservoir configuration being formed by
excavation above or below ground. In such a case, the reservoir may be lined
with a liquid impermeable barrier, such as clay or plastic membrane, to
prevent
solution entering the surrounding ground or rock. The reactor may be built
from
is rock or other suitable material above the surrounding ground surface.
In a reservoir configuration, though the following may be applicable to
other types of suitable reactor, the reactor is preferably rectangular in plan
section. The reactor may be tapered towards either end. When viewed from an
end the reactor may also be rectangular in section though other shapes may be
2o employed. The walls of the reactor may be substantially vertical or sloped.
The
depth of the reactor may typically be between 4 and 8 metres, and the width
between 5 and 20 metres though both dimensions could be larger or smaller.
Very large reactors, possibly having length greater than 100 metres, may be
built
and a number of reactors may be employed. Such reactor systems may be
2s combined in series or operated in parallel.
The base of the reactor may be sloped so that there is a gradient from the
feed end at which ore or other metal containing material is fed into the
reactor to
the discharge end. This slope may be a descending slope. The gradient of the
reactor may be variable. This may assist in transfer of particles from feed to
3o discharge. Volume of the reactor is calculated in dependence upon the rate
at
which the metal containing material or mineral is being treated.
Aerating means, or diffusers, may be located at even spacings along the
base of the reactor. A concentration of diffusers may be determined as a
function
of oxygen demand in the reactor.
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Feed may be introduced at one end of the reactor through multiple points
located along the width of one end. Distribution of slurry to these points may
be
via a ring main or splitter of conventional type.
However, it is not intended to restrict possible reactor designs to the above
s configurations and other configurations could be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
Description of preferred embodiments of the invention now follows. The
description is made with reference to the accompanying drawings in which:
Figure 1 is a side sectional view of an aeration means to be used in
io accordance with a first embodiment of the process of the present invention;
Figure 2 is a plan view of the aeration means shown in side section in
Figure 1;
Figure 3 is a side sectional view of an aeration means to be used in
accordance with a second embodiment of the process of the present invention;
is Figure 4 is a plan view of the aeration means shown in side section in
Figure 3;
Figure 5 is a schematic diagram showing flow circulation in a reactor,
including a tubular aeration means, as operated in accordance with a third
embodiment of the process of the present invention;
2o Figure 6 is a 'schematic diagram showing flow circulation in a reactor
including disc aeration means, as operated in accordance with a fourth
embodiment of the process of the present invention.
Figure 7 is a side schematic diagram showing another embodiment of a
reactor according to the present invention; and
2s Figure 8 is a plan schematic diagram for the reactor of Figure 7.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring specifically to the Figures 1 to 4, the gas is delivered through
diffusers, i.e. a means for diffusing a gas into the liquid portion of a
slurry or
solution. Typically, compressed air - though other gases as described above
may
3o be suitable - is blown through gas supply line 10 to the diffuser unit 1
passing
through a perforated wall 11 thereof forming air bubbles which serve to
transfer
oxygen and carbon dioxide into a slurry 2 in a reactor (not shown) for
bacterial
respiration and for oxidation of minerals. Gas supply line 10 may service a
number of reactors. A slug flow introduction of air or gas to the reactor,
that is,
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without control over bubble size at introduction to the reactor, is not
desirable and
is, most advantageously, to be avoided.
Diameter or size of the perforations 13 in wall 11 dictates bubble size, as
desired, in a controlled manner. The smaller the bubble size the greater the
s surface area per unit of volume providing efficiency of gas-liquid contact
allowing
the high values of oxygen transfer that are necessary for efficient bacterial
oxidation. Bubble size is to be maintained at 7.5mm average diameter or less,
preferably 5mm average diameter or less.
Disc, doughnut or tubular designs may be employed, the first two types
to being shown in Figures 1 to 4.
A disc diffuser, as seen in side section in Figure 1 and plan in Figure 2, is
manufactured from a flexible membrane 15 that closes when no air is being
delivered. This ensures that substantially no slurry enters gas supply line 10
during processing. The membrane 15 may be retained in position by a retaining
is device such as a clamping ring 16 which fixes the membrane in position 15
about
an outlet 20 of the gas supply line 10. As the membrane 15 expands with air
flow, perforations in the form of pores in the membrane open allowing air flow
therethrough. The pores 13 may advantageously be microscopic, of the order of
1 to 5N in diameter. As a result, impeller action is not required to shear air
slugs
2o to an appropriate bubble size distribution to optimum transfer of oxygen to
the
slurry. Further the mass transfer occurs in a readily controllable fashion and
the
bubble size may be controlled by selecting a membrane or other material (see
below) with desired pore or hole size.
A doughnut shaped diffuser is shown in side section in Figure 3 and plan in
2s Figure 4. Again, a membrane 15 is employed which has the same
characteristics
as discussed above. However, here the gas supply line 10 serves further supply
lines 23 feeding the doughnut shaped tube 24, the walls of which are formed
from
membrane 15 thereby providing greater contact area of membrane 15 with
solution or slurry.
3o Characteristics of membrane 15 may inciude flexibility, relatively low
cost,
durability and easy replacement. The membrane 15 may be formed from acid
resistant rubber or other polymers with a pore size distribution as desired
for use
in the process. The membrane may be manufactured in a known manner in the
field of membrane technology.
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A further type of diffuser is of typically dome shape and manufactured from
plastics or ceramics. The material is perforated with holes of similar
diameter to
that above that are fixed and open all the time allowing air to flow
therethrough to
form a bubble swarm in the slurry within the reactor.
s A particularly effective type of diffuser is tubular with the tubular body
perforated with holes for gas exit preferably having a U-shaped configuration.
The tubular body may be connected to the gas supply line and may be formed
from a membrane or suitable plastic or ceramic material.
Diffusers suitable for the application will typically be sized to allow a
sufficient
io volume of gas to be diffused into the slurry to achieve between 0.5 and 15
mg/I
dissolved oxygen concentration in the slurry depending upon the oxygen
requirement of the bacteria used in the bio-oxidation process. There may be
provided a number of diffusers as necessary to maintain solids in suspension
and
supply sufficient gas for oxidation.
~s Reactor configuration may be selected to achieve optimal aeration.
Diffusers) 24 may be . placed adjacent, that is at, or just off, the base of
the
reactor, if applicable, spaced at intervals - possibly with different rates of
gas
introduction - to give a suitable bubble distribution for the material being
treated.
The diffusers are fixed in position. Under different circumstances, the number
of
2o diffusers required to maintain the solids in suspension may be greater than
that
required for oxidation, at other times the number required may be lower. A
uniform arrangement throughout the reactor base may be preferred.
Conveniently, the number of aeration means or diffusers per unit area within
the
reactor may vary such that there is a greater proportion at the feed end of
the
2s reactor where higher rates of bacterial oxidation would be expected than at
the
discharge end.
Figures 5 and 6 show, without intending to limit the invention, likely flow
circulation patterns for a number of tubular and disc diffusers located in a
reactor
100. It may be understood that each arrangement allows efficient mixing and
3o bacteriaUsolid contacting though a disc diffuser arrangement may be
preferred.
Figures 7 and 8 show a reactor 200 of rectangular plan having a sloped
base 204. The gradient of base 204 descends from feed end 208 (to which input
slurry is introduced) to discharge end 212 (at which the output leached
particles
are removed). An overflow liquor stream 218 is removed to a metal recovery
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stage (not shown}.
A number of diffusers 224 of tubular type are located evenly spaced along
the base of the reactor 200.
Diffusers 224 are supplied with air by line 232 air is induced to flow through
s line 232 by induction fan 230. Carbon dioxide supplement is introduced by
line
226 to line 232. Suitable diffusers 224 are of tubular fine bubble membrane
type
available in Australia from MRE, Adelaide, the distributor of Enviroquip O
tubular
diffusers. Such diffusers are particularly suitable for attachment to PVC pipe
which may be conveniently used for line 232.
to The tubular diffusers 224 are supplied with oxygen containing gas by a
main pipe or manifold 225 of which the tubular diffusers 224 form lateral
extensions covering the base of the reactor 200 as shown in Figure 8. As the
core of each diffuser 224 fills with water they will stay on the base of the
reactor
200.
is Diffusers may also be provided in pipelines communicating with a reactor,
for example those connecting the reactor with others in the reactor system.
Alternatively, gas may be introduced at these points by other suitable means.
This may be especially applicable in the case of a heap or dump leaching
process
in which ore particles are not placed into suspension in the leach liquor.
2o Materials treated by the process and reactor systems of the invention may
include ores, concentrates obtained from ores, tailings, wastes and other
materials having sufficient metal values present to economically remove the
metal
or to remove metals detrimental to the environment or other processes. It is
to be
understood that the present invention is not limited to the treatment of
sulphides.
2s The metal value containing material may require to be pre-treated, for
example crushed, to a size sufficiently small to enable it to be ground in
conventional comminution equipment such as ball mills.
Conveniently, the solids requiring oxidation are ground to a particle size
sufficiently fine for the gas to be effectively used in maintaining a
suspension
3o thereof and without allowing significant build-up on the floor of the
reactor.
Typically, grinding to a suitable particle size distribution is necessary to
meet this
criterion. The size of the product after grinding is preferably 90% passing 50
microns or less. It is advantageous for the ore or other metal containing
material
to be ground in an ultra fine grinding machine to particle size between 80%
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passing 2 microns and 80% passing 30 microns in accordance with a further
embodiment of the invention. The bottom particle size is as small a size as is
practically attainable by grinding. The size of the particles should be chosen
to
be the optimum for maintaining the solids in suspension and carrying out a
s bacterial oxidation process.
Conveniently, after crushing and/or grinding the material concentration
may occur by gravity, flotation or other beneficiation processes to increase
the
proportion of the sulphides or other desired minerals in the product. However,
the
process is equally applicable to the treatment of concentrates of residue
io produced by others and comminution pre-treatment is not necessarily
required in
each case.
The ground metal containing ore or concentrate is preferably slurried with
an aqueous solution, especially water. Slurries from grinding may be diluted
within additional water and pumped into the reaction zone in which the bio-
~s oxidation process is to take place. The pulp density of the slurry
introduced to the
reactor of the invention may be of great importance to the attainment of the
object
of the invention, accordingly a range of 5 to 15% or upward may be selected.
At
lower pulp density, as stated, efficient bacterial activity may be achieved as
toxic
effects due to high ionic concentration; and/or mechanical grinding of
bacteria are
zo avoided. Leaching is enhanced by the very low shear, low ionic
concentration
environment. Bacteria are ideally previously added to the reactor in the form
of a
culture with a mineral and grown in sufficient numbers so that there are
preferably
between 105 and 109 bacteria per millilitre of slurry. The bacteria may be
maintained in the reactor according to accepted practice or by a method of
2s immobilisation.
Preferably the retention time in the reactor is 2 to 8 days. The retention
time may be longer to allow treatment of less finely ground material, which
requires a longer time to be processed, or to achieve higher levels of mineral
oxidation and dissolution of the metal. The amount of gas introduced to the
slurry
3o may be changed in accordance with retention time but this is not essential.
Conveniently, the bacterial oxidation process is carried out in an acidic
solution which is suitable for growth of the specific bacteria, for example
thiobacilli, used. This is expected to be in the range of 0.5 to 3Ø
Preferably, the
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pH will be in the range 0.8 to 2.5. An initial acid addition may be required
at the
start of the process to neutralise acid consuming minerals and maintain the pH
in
the required range. The process of oxidising sulphide minerals produces acidic
by-products and it is likely that the pH will decrease during processing. If
s necessary, the pH may be maintained in the required range by controlled
addition
of a base or basic agent such as lime, limestone or any other suitable alkali.
Where the material to be treated is not a sulphide, reaction may be carried
out under alkaline conditions. In these situations the pH is to be kept at a
level
suitable to the specific leaching conditions.
io Support of the bacteria is likely to acquire the addition of nutrients to
maintain growth. Typical nutrients to be introduced to the reaction zone are
nitrogen, sulphur and phosphorus containing materials such as ammonium
sulphate, potassium dithiophosphate and magnesium sulphate. In addition, other
nutrients may be required for specific ores and concentrates as is known in
the
is art.
Further, as oxidation of sulphide minerals is an exothermic process, heat
will be released during bacterial oxidation. This may be a problem where ores
are treated but may be especially true where an enriched sulphide concentrate
is
treated. In this case, the release of heat may increase the temperature of the
2o slurry in the reactor above that tolerated by the bacteria, especially
thiobacilli.
Sulpholobus bacteria are more temperature tolerant. Therefore, the reactors
are
typically to be provided with cooling systems. Water may be a suitable coolant
and the cooling system may be direct, with coolant water introduced at the
base
of the reactor. Alternatively, indirect cooling may be employed using
conventional
2s heat exchanger technology. For example, a cooling tower could produce cold
water which feeds cooling coils, tube bundles or like means suitably located
in the
reactor. Tube bundles and coils may be preferred in the absence of strong
currents generated by stirring or agitation by turbine blades. Evaporative
cooling
may be promoted by reducing depth of a reservoir and increasing the surface
3o area. The coolant may be introduced at sufficient velocity to assist in
maintaining
solids in suspension and admitted water may counteract evaporation losses.
More generally, a temperature control system with heating/cooling
functions may be employed. The location and nature of temperature control
system, internal or external, direct or indirect, may be varied to achieve the
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required temperature control of the reactor. For example, when thermophilic
bacteria are used, heating to above 60°C may be required.
Other uses of the process may be exemplified, without limitation, as
follows:
s FERRIC ION GENERATION
The process of the invention is suitable for converting ferrous ions in
solution to ferric ions which may attack minerals, for example uranium
minerals,
to liberate metal values of economic interest.
TREATMENT O>= MANGANESE CONTAINING ORES
io Manganese ores may be treated by mixing with a metal sulphide with the
parameters for bacterial oxidation being as above described.
Alternatively, manganese ores may be bioleached with the organism
Enterobacter at pH controlled between 4 and 8.
SULPHUR REMOVAL FROM COAL
is . The mineral pyrite occurs with coal and bacterial oxidation using
chemolithotrophic bacteria has been used to remove pyrite and reduce sulphur
content. The process of the invention may be employed for sulphur removal with
the parameters for bacterial oxidation being as above described.
In accordance with the process and reactor systems of the invention the
2o grade of ore, concentrate or other metal containing material may be lower
than
that for agitated tanks and like reactor systems because of expected lower
capital
and operating costs.
The invention will be more fully understood from the description of the
following examples.
Example A Inert Material
A trial was conducted on an inert mineral used to duplicate sulphide ores
and concentrates.
3o A 10% weight percent slurry with a sizing of 90% passing 16 micrometers
was maintained in uniform suspension in a tank with fine air bubbles injected
from
a close packed array of membrane disc diffusers of EPOM synthetic rubber
commercially available from Nopol's under the trade mark PIK300. The diameter
of the disc is 304 millimetres at the base of a 1.4 metre diameter by 4 metre
high
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tank. The height of tank is similar to that used in commercial practice.
Air flows of at least 2.4 Nm3/h/diffuser was required to prevent settling of
solids. The air flow rates required were within the manufacturer's recommended
operating range.
s Solids that settled because of a reduction or disruption in air flow were re-
suspended when the air flow was re-established at 2.4 Nm3/h/diffuser or
higher.
These results showed that a typical ground sulphide concentrate with a
sizing Qf 90% passing 12 micrometres would stay in suspension under the same
operating conditions.
to The operating conditions with respect to air flow were set to duplicate an
oxygen demand of 1.5 kilograms per cubic metre hour which is equivalent to
that
required for bacterial oxidation of sulphide minerals.
Example B Leaching of Refractory Gold Sulphide Concentrate
is
A sample of gold concentrate containing arsenopyrite and pyrite was
ground to a size such that 80% of the particles were less than 15 micrometers
in
diameter.
The bio-oxidation reactor, constructed from conventional acid resistant
material,
2o had a volume of 200 litres and was provided, at its base, with a membrane
diffuser as described in Example A. Air was supplied to the diffuser at a rate
which maintained the ground solids in suspension. No other agitation was used.
The ground concentrate was mixed with water to produce a slurry at 10%
solids on a weight to volume basis. Sulphuric acid was added to the slurry to
2s obtain an acidity level of pH 1.2. Approximately five litres of an inoculum
slurry
containing a mixture of mineral concentrate and moderately thermophilic
bacteria
was added to the slurry in the reactor. Thereafter a mixture of nutrients
comprising hydrated magnesium sulphate, potassium orthophosphate, and
ammonium sulphate was added to the slurry.
3o The reactor was then heated such that the temperature of the slurry was
maintained at 48°C, in the moderate thermophile range.
Air was added to the reactor from a standard compressor, through a
flowmeter with a flow valve and thence into a pipe connected to the diffuser.
The air fiowrate was controlled to allow a steady stream of bubbles to be
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emitted from the diffuser. The air was used to maintain the particles in
suspension and also to supply the oxygen requirement of the bacterial culture.
No additional agitation of the reactor volume was employed. Samples of the
slurry were taken on a daily basis to determine the extent of oxidation of the
s arsenopyrite/pyrite concentrate.
Time Oxidation Level (%)
(days) Arsenic Iron
6 100 na
l0 32 100 94
The final residue in the tank was analysed after the test had proceeded for 32
days.
is Assay (wt%)
As Fe S2
Initial Concentrate3.05 21.0 23.0
Final Residue 0.03 0.4 0.1
Oxidation (%) 99 96 99
S~ is the amount of sulphide sulphur present and determines the amount
of total oxidation that has occurred. Based on the analysis of the residue,
the
oxidation extent was determined.
Gold was then extracted from the residue using conventional cyanide
2s leaching. The residue assayed 67 g/t gold of which 99% was extracted by
cyanidation.
Example C Leaching of Copper Concentrate
3o A sample of copper concentrate containing chalcopyrite (and designated "CP1
")
was ground to a size such that 80% of the particles were less than 10
micrometers in diameter.
Again, the bio-oxidation reactor had a maximum volume of 200 litres and a
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membrane diffuser, as described in Example A, was installed at the base. Air
was supplied at a rate to maintain the ground solids in suspension and
maintain
oxygen requirement of the bacteria. No other agitation was employed.
The ground concentrate was mixed with water to produce a slurry at 5%
s solids on a weight to volume basis. Sulphuric acid was added to the slurry
to
obtain an acidity level of pH 1.2. Approximately 5 litres of an inoculum
slurry
containing a mixture of mineral concentrate and moderately thermophilic
bacteria
was added to the slurry in the reactor. Thereafter, a mixture of nutrients
comprising hydrated magnesium sulphate, potassium orthophosphate, and
io ammonium sulphate was added to the slurry.
The reactor was then placed in a heated room such that the temperature of
the slurry was maintained at 48°C.
Air was introduced to the reactor from a standard compressor, through a
flowmeter with a flow valve and thence into a pipe connected to the diffuser.
is The air flowrate was controlled to allow a steady stream of bubbles to be
emitted from the diffuser. The air was used to maintain the particles in
suspension and provide the oxygen requirement of the bacteria. No additional
agitation of the reactor was employed. Samples of the slurry were taken on a
daily basis to determine the extent of oxidation of the arsenopyrite/pyrite
2o concentrate.
Conditions Cpl
Copper Content (%) 1 g
2s Copper Extraction (%) Acid Leach only 4
Copper Extraction (%) Bacterial oxidation leach
using moderate thermophilic bacteria 84
Example D Polymetallic Concentrate
D.1 LEACHING WITH MESOPHILIC BACTERIA
so In this example, the reactor used was 125mm diameter and 1.7m high. A
diffuser, as described in Example A, was installed in the base of the reactor.
A sample of mixed polymetallic concentrate (designated "M1 ") containing a
mixture of mineral sulphides was bio-oxidised to extract zinc, nickel, copper
and
cobalt. The concentrate contained the nickel mineral pentlandite, as well as
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nickel contained within the structure of the mineral pyrrhotite. Copper was
predominately present in the form of chalcopyrite though chalcocite and bomite
were also present. Zinc was present as sphalerite. The metal cobalt was
associated with the nickel minerals and as the mineral cobalt. The concentrate
s was ground to a size such that 80% of the particles were less than 15
micrometers in diameter.
The ground concentrate was mixed with water to produce a slurry at 10%
solids on a weight to volume basis. Sulphuric acid was added to the slurry to
obtain an acidity level of pH 1.2. Approximately half a litre of an inoculum
slurry
io containing a mixture of mineral concentrate and mesophilic bacteria
(Thiobacillus
ferrooxidans and Thiobacillus thiooxidans) was added to the slurry in the
reactor.
Thereafter, a mixture of nutrients comprising hydrated magnesium sulphate,
potassium orthophosphate, and ammonium sulphate was added to the slurry.
The reactor was then placed in a heated room such that the slurry
is temperature was maintained at 35°C.
Air was introduced to the reactor from a standard compressor, through a
flowmeter with a flow valve and thence into a pipe connected to the diffuser.
The air flowrate was controlled to allow a steady stream of bubbles to be
emitted from the diffuser. The air was used to maintain the particles in
zo suspension and to supply the oxygen requirement of the bacteria. No
additional
agitation of the reactor was employed. The concentrate was oxidised with
metals
being released into solution. The level of extraction of the metals was
determined
by analysing a portion of the liquid fraction of the slurry. When the reaction
was
complete, the solid and the liquid phases were separated and the solution and
the
2s solids analysed.
Conditions M1
Copper Content (%) 5.7
Nickel Content (%) 0.9
3o Cobalt Content (%) O.Ofi
Zinc Content (%) 0.21
Sulphur Oxidation (%) 98
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7
Copper Extraction (%) _
Bacterial oxidation leach 92
Nickel Extraction (%) using thermotolerant bacteria 97
Cobalt Extraction (%) g7
Zinc Extraction (%) 95
s
D.2 LEACHING WITH MODERATE THERMOPHILES
In this example, the reactor used was 125mm diameter and 1.7m high. A
diffuser, as described in Example A, was installed in the base of the reactor.
A sample of mixed polymetallic concentrate (designated "M2") containing a
to mixture of mineral sulphides was bio-oxidised to extract zinc, nickel,
copper and
cobalt. The concentrate contained the nickel mineral pentlandite, as well as
nickel contained within the mineral pyrrhotite. Copper was predominately
present
in the form of chalcopyrite though chalcocite and bomite were also present.
Zinc
was present as sphalerite. The metal cobalt was associated with the nickel
is minerals and as the mineral cobalt. The concentrate was ground to a size
such
that 80% of the particles were less than 15 micrometers in diameter.
The ground concentrate was mixed with water to produce a slurry at 10%
solids on a weight to volume basis. Sulphuric acid was added to the slurry to
obtain an acidity level of pH 1.2. Approximately half a litre of an inoculum
slurry
2o containing a mixture of mineral concentrate and moderately thermophilic
bacteria
(Thiobacillus ferrooxidans and Thiobacillus thiooxidans) was added to the
slurry
in the reactor. Thereafter, a mixture of nutrients comprising hydrated
magnesium
sulphate, potassium orthophosphate, and ammonium sulphate was added to the
slurry.
2s The reactor was then placed in a heated room such that the slurry
temperature was maintained at 47°C.
Air was introduced to the reactor from a standard compressor, through a
flowmeter with a flow valve and thence into a pipe connected to the diffuser.
The air flowrate was controlled to allow a steady stream of bubbles to be
3o emitted from the diffuser. The air was used to maintain the particles in
suspension and to supply the oxygen requirement of the bacteria. No additional
agitation of the reactor was employed. The concentrate was oxidised with
metals
being released into solution. The level of extraction of the metals was
determined
by analysing a portion of the liquid fraction of the slurry. When the reaction
was
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complete, the solid and the liquid phases were separated and the solution and
the
solids analysed.
Conditions M2
s Copper Content (%) 7,3
Nickel Content (%) O,g
Cobalt Content (%) 0.05
Zinc Content (%) 0.24
io Sulphur Oxidation (%) 77
Copper Extraction (%) Acid leach only 12
Nickel Extraction (%) 10
Cobalt Extraction (%) 13
is Zinc Extraction (%) 24
Copper Extraction (%) Bacteriai oxidation leach 93
Nickel Extraction (%) using thermotolerant bacteria 98
Cobalt Extraction (%) g7
2o Zinc Extraction (%) 96
D.3 LEACHING WITH THERMOPHILES
In this example, the bio-oxidation reactor was of 40mm diameter and 1.Om
height.
A commercially available sintered polymer diffuser was installed in the base
of the
2s reactor.
A sample of mixed polymetallic concentrate (designated "M3") containing a
mixture of mineral sulphides was bio-oxidised to extract zinc, nickel, copper
and
cobalt. The concentrate contained the nickel mineral pentlandite, as well as
nickel contained within the mineral pyrrhotite. Copper was predominately
present
so in the form of chalcopyrite though chalcocite and bomite were also present.
Zinc was present as sphalerite. The metal cobalt was associated with the
nickel
minerals and as the mineral cobalt. The concentrate was ground to a size such
that 80% of the particles were less than 15 micrometers in diameter.
The ground concentrate was mixed with water to produce a slurry at 3%
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solids on a weight to volume basis. Sulphuric acid was added to the slurry to
obtain an acidity level of pH 1.2. Approximately 100m1 of an inoculum slurry
containing a mixture of mineral concentrate and thermophilic bacteria
(Sulpholobus) was added to the slurry in the reactor. Thereafter, a mixture of
s nutrients comprising hydrated magnesium sulphate, potassium orthophosphate,
and ammonium sulphate was added to the slurry.
The reactor was then heated such that the slurry temperature was
maintained at 70°C.
Air was introduced to the reactor from a standard compressor, through a
Fo flowmeter with a flow valve and thence into a pipe connected to the
diffuser.
The air flowrate was controlled to allow a steady stream of bubbles to be
emitted from the diffuser. The air was used to maintain the particles in
suspension and to supply the oxygen requirement of the bacteria. No additional
agitation of the reactor was employed. The concentrate was oxidised with
metals
is being released into solution. The level of extraction of the metals was
determined
by analysing a portion of the liquid fraction of the slurry. When the reaction
was
complete, the solid and the liquid phases were separated and the solution and
the
solids analysed.
Conditions M2
Copper Content (%) 5.0
Nickel Content (%) 0.78
Cobalt Content (%) 0.07
2s Zinc Content (%) 0.67
Sulphur Oxidation (%) 84
Copper Extraction (%) Bacterial oxidation leach 99
3o Nickel Extraction (%) using thermotolerant bacteria 100
Cobalt Extraction (%) 98
Zinc Extraction (%) 97
EXAMPLE E ZINC CONCENTRATE
In this example, the bio-oxidation reactor was of 40mm diameter and 1.Om
height.
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A sintered polymer diffuser, as used in Example D.3, was installed in the base
of
the reactor.
A sample of zinc concentrate (designated NZ1 ") containing a mixture of
mineral sulphides was treated to extract zinc. The concentrate contained the
zinc
s as the mineral sphalerite. The sample also contained lead sulphide as
galena.
The concentrate was ground to a size such that 80% of the particles were less
than 15 micrometers in diameter.
The ground concentrate was mixed with water to produce a slurry at 5%
solids on a weight to volume basis. Sulphuric acid was added to the slurry to
io obtain an acidity level of pH 1.2. Approximately 100m1 of an inoculum
slurry
containing a mixture of mineral concentrate and moderately thermophilic
bacteria
(Thiobacillus ferrooxidans and Thiobacillus thiooxidans) was added to the
slurry
in the reactor. Thereafter, a mixture of nutrients comprising hydrated
magnesium
sulphate, potassium orthophosphate, and ammonium sulphate was added to the
>s slurry.
The reactor was then heated such that the slurry temperature was
maintained at 48°C.
Air was introduced to the reactor from a standard compressor, through a
flowmeter with a flow valve and thence into a pipe connected to the diffuser.
2o The air flowrate was controlled to allow a steady stream of bubbles to be
emitted from the diffuser. The air was used to maintain the particles in
suspension and to supply the oxygen requirement of the bacteria. No additional
agitation of the reactor was employed. The concentrate was oxidised with zinc
being released into solution. The level of extraction of the zinc was
determined
2s by analysing a portion of the liquid fraction of the slung.
Conditions Z1
Zinc Content (%) 25
3o Sulphur Oxidation (%) 60
Zlnc EXtraCtlon (%) Bacterial oxidation leach
using moderate thermophilic bacteria 97
EXAMPLE F FERROUS TD FERRIC CONVERSION
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A further test was carried out in a reactor which was 25mm diameter and
i .Om high. A sintered polymer diffuser, as used in Examples D.3 and E, was
installed in the base of the reactor.
A sample of ferrous sulphate was added to the reactor and diluted with
s water to obtain a strength of 9 g/I iron. Sulphuric acid was added to the
slurry in
the reactor to obtain an acidity level of pH 1.2. Extremely thermophilic
bacteria
(Sulpholobus), extracted onto a filter paper to separate them from other
residual
solution, were added to the slurry in the reactor. Thereafter, a mixture of
nutrients
comprising hydrated magnesium sulphate, potassium orthophosphate, and
to ammonium sulphate was added to the slurry.
The reactor was then heated such that the solution temperature was
maintained at 70°C.
Air was introduced to the reactor from a standard compressor,
through a flowmeter with a flow valve and thence into a pipe connected to the
is diffuser.
The air flowrate was controlled to allow a steady stream of bubbles to be
emitted from the diffuser. The air was used to maintain the particles in
suspension and to supply the oxygen requirement of the bacteria. No additional
agitation of the reactor was employed.
2o The ferric solution was oxidised so that the ferrous ion was converted into
ferric ion. The level of conversion of the ferrous ion was determined by
titration
with potassium dichromate.
Modifications and variations may be made to the process and reactor
system of the present disclosure without departing from the scope of the
Zs invention.
