U.S. patent application number 10/557410 was filed with the patent office on 2007-02-08 for treatment of base metal concentrate by a two-step bioleaching process.
Invention is credited to Clint Bowker, Paul Harvey.
Application Number | 20070028721 10/557410 |
Document ID | / |
Family ID | 33453094 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070028721 |
Kind Code |
A1 |
Bowker; Clint ; et
al. |
February 8, 2007 |
Treatment of base metal concentrate by a two-step bioleaching
process
Abstract
A method of recovering at least one base metal from a
concentrate wherein a residue of a primary bioleach of the
concentrate, under mesophilic and moderate themophilic conditions,
is processed to recover at least one metal, and the base metal is
recovered from a solution, produced by a secondary bioleach under
thermophilic conditions, of a residue of the metal recovery
process.
Inventors: |
Bowker; Clint;
(Johannesburg, ZA) ; Harvey; Paul; (Johannesburg,
ZA) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
33453094 |
Appl. No.: |
10/557410 |
Filed: |
May 15, 2004 |
PCT Filed: |
May 15, 2004 |
PCT NO: |
PCT/ZA04/00052 |
371 Date: |
October 13, 2006 |
Current U.S.
Class: |
75/743 |
Current CPC
Class: |
C22B 3/18 20130101; C22B
15/0089 20130101; Y02P 10/20 20151101; C22B 11/04 20130101; Y02P
10/236 20151101; Y02P 10/234 20151101; C22B 15/0063 20130101 |
Class at
Publication: |
075/743 |
International
Class: |
C22B 3/18 20070101
C22B003/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 2003 |
ZA |
2003/3621 |
Claims
1.-21. (canceled)
22. A method of treating a concentrate containing at least one base
metal which includes the steps of subjecting the concentrate to a
primary mesophilic and moderate thermophilic bioleaching process to
leach sulphides in the concentrate, processing a residue of the
primary bioleach process to recover at least one metal from the
primary bioleach residue, subjecting a residue from the metal
recovery process to a thermophilic secondary bioleaching process to
release the at least one base metal from the metal recovery residue
into solution, and recovering the at least one base metal at least
from the solution produced by the thermophilic secondary
bioleaching process, and which is characterized in that it includes
the step of preleaching the concentrate, before the primary
bioleaching process, using leach solution from at least one of the
bioleaching processes which increases ferric in the preleaching
step thereby to increase the leaching of easily leachable base
metal before the primary bioleaching process.
23. The method according to claim 22 wherein the primary
bioleaching process is carried out at a temperature of from
35.degree. C. to 50.degree. C.
24. The method according to claim 22 wherein the at least one base
metal is also recovered from a solution produced by the primary
bioleach bioleaching process.
25. The method according to claim 22 wherein the leach solution is
derived from the primary bioleaching process and the thermophilic
secondary bioleaching process.
26. The method according to claim 22 wherein base metal which is
leached during the preleaching step is removed from the concentrate
before the primary bioleaching process.
27. The method according to claim 22 wherein elemental sulphur
which accumulates during the preleach step due to rapid leaching of
easily leachable sulphides is removed during the bioleaching
stages.
28. The method according to claim 22 which includes the step of
adjusting the pH of a solution produced in the preleaching step to
maximize recovery of the at least one base metal using solvent
extraction techniques.
29. The method according to claim 28 which includes the step of
causing arsenic in the solution to precipitate as ferric arsenate
by increasing the pH of the solution to at least 2.
30. The method according to claim 28 wherein the pH adjustment is
carried out at a temperature of from 60.degree. C. to 80.degree.
C.
31. The method according to claim 22 wherein the primary
bioleaching process is carried out in a series of continuously
stirred tank reactors which are operated at a temperature of from
35.degree. C. to 50.degree. C. in the presence of an active mixed
culture of mesophilic and moderate thermophilic microorganisms.
32. The method according to claim 22 wherein a mixed culture of
mesophile and moderate thermophile microorganisms are used to
maximize sulphide bioleaching and sulphur biooxidation during the
primary bioleaching process.
33. The method according to claim 32 wherein the mixed culture
contains. Leptospirillum ferrooxidans and Acidithiobacillus
caldus.
34. The method according to claim 32 wherein the mixed culture of
microorganims also contains thermophilic microorganims, which are
not effectively active at the temperature range of 35.degree. C. to
50.degree. C.
35. The method according to claim 22 wherein the pH of the
concentrate during the primary bioleaching processes is controlled
at a value of from 1.2 to 1.7.
36. The method according to claim 22 which includes the step of
supplying air with an oxygen content of from 95% to 98% to the
concentrate during at least part of the bioleaching processes.
37. The method according to claim 22 which includes the step of
removing toxic silver from the primary bioleaching residue.
38. The method according to claim 22 wherein the thermophilic
secondary bioleaching process is carried out at a temperature of
from 65.degree. C. to 80.degree. C. in the presence of active
quantities of extreme thermophilic microorganims.
39. The method according to claim 22 which includes the step of
controlling the pH of the concentrate during the secondary
bioleaching process at a value of from 1.0 to 1.7.
40. The method according to claim 22 which includes the step of
supplying air with a carbon dioxide content of from 1% to 5% to the
concentrate during at least part of the bioleaching processes.
41. The method according to claim 22 wherein the at least one base
metal is copper which is recovered by concentrating the copper and
stripping, followed by cathode production by electrowinning.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to the treatment of a base metal
containing concentrate.
[0002] The recovery of a base metal such as copper from a
concentrate, which contains arsenic, using a bioleaching process,
can be problematic for it is necessary to avoid producing a
bioleach residue which is contaminated with arsenic.
[0003] U.S. Pat. No. 6,461,577 addresses the problem of arsenic
toxicity of extremely thermophilic bacterial cultures by means of a
two-stage leaching process. In a first mesophilic stage a major
part of the arsenic contained in the material being treated is
leached from the material and then oxidised from As(III) to As(V).
The remaining leachable metal content of the material being treated
is leached out in a second thermophilic stage. The concentration of
pentavalent arsenic falls quickly and the toxic effect thereof on
the thermophilic bacteria thus falls at the same rate.
[0004] It is desirable though to remove the soluble arsenic as
ferric arsenate which passes EPA limits and is safe for land
disposal and which, to the extent possible, is not in a bioleach
residue.
SUMMARY OF INVENTION
[0005] The invention provides a method of treating a concentrate
containing at least one base metal which includes the steps of
subjecting the concentrate to a primary mesophilic and moderate
thermophilic bioleaching process to leach sulphides in the
concentrate, processing a residue of the primary bioleach process
to recover at least one metal from the primary bioleach residue,
subjecting a residue from the metal recovery process to a
thermophilic secondary bioleaching process to release the at least
one base metal from the metal recovery residue into solution, and
recovering the at least one base metal at least from the solution
produced by the thermophilic secondary bioleaching process.
[0006] The primary bioleaching process may be carried out at a
temperature of from 35.degree. C. to 50.degree. C.
[0007] Preferably the at least one base metal is also recovered
from a solution produced by the primary bioleach bioleaching
process.
[0008] The method preferably includes the step of preleaching the
concentrate, before the primary bioleach bioleaching process, using
leach solution from at least one of the bioleaching processes.
Preferably the leach solution is derived from the primary
bioleaching process and the thermophilic secondary bioleaching
process.
[0009] The preleaching step is used to remove easily leachable base
metal from the concentrate before the primary bioleaching process.
Elemental sulphur which may accumulate during the preleach step due
to rapid leaching of easily leachable sulphides, may be removed
during the bioleaching stages, especially during the thermophilic
secondary bioleaching process at elevated temperatures.
[0010] The primary bioleaching process may be carried out in a
series of continuously stirred tank reactors which are operated at
a temperature of from 35.degree. C. to 50.degree. C. in the
presence of an active mixed culture of mesophilic and moderate
thermophilic microorganisms.
[0011] A mixed culture of mesophile (20.degree. C. to 40.degree.
C.) and moderate thermophile (40.degree. C. to 55.degree. C.)
microorganisms is preferably used to maximise sulphide bioleaching
and sulphur biooxidation during the treatment process. The mixed
culture may contain microorganisms like Leptospirillum ferrooxidans
and Acidithiobacillus caldus, a good iron oxidiser and a good
sulphur oxidiser respectively.
[0012] The primary bioleach process may also contain thermophilic
microorganisms, which are not effectively active at the temperature
range of 35.degree. C. to 50.degree. C. Such microorganisms will,
however, still be living but will be dormant or slowly
metabolising. When the temperature increases during the
thermophilic secondary bioleaching process these microorganisms
will reactivate their activity. This may be very useful for base
metal concentrates, as the thermophilic secondary bioleaching stage
would be continuously re-inoculated.
[0013] The pH of the concentrate or pulp in reactors in which the
primary bioleaching is carried out may be controlled at a value of
from 1,2 to 1,7. This may be achieved by the addition of limestone
or raffinate produced in the base metal recovery step, to the
reactors.
[0014] Oxygen may be supplied to the concentrate in the reactors in
the form of enriched air which may contain from 95% to 98% oxygen,
during at least part of the bioleaching processes.
[0015] An objective of operating the primary bioleaching process
under the aforementioned conditions is to maximise the leaching of
the sulphides in the concentrate and to maximise mass loss, and to
minimise the precipitation in pentavalent form of arsenic which may
be present in solution. The product from the primary bioleach
residue may contain high concentrations of elemental sulphur due to
the maximised bioleaching conditions.
[0016] In the metal recovery process toxic silver may be removed
from the primary bioleaching residue. The silver may be removed
using a brine leaching process.
[0017] The thermophilic secondary bioleaching process may be
carried out in a series of continuously stirred tank reactors at a
temperature of from 65.degree. C. to 80.degree. C. in the presence
of active quantities of extreme thermophilic microorganisms.
[0018] The method may include the step of controlling the pH of the
pulp in the thermophilic reactors at a value of from 1,0 to 1,7.
This may be achieved by the addition of limestone or raffinate
produced in the metal recovery step. Oxygen & carbon dioxide
may be supplied to the reactors in the form of enriched gas
containing from 95% to 98% oxygen and 1% to 5% carbon dioxide.
[0019] An objective of operating the thermophilic secondary
bioleaching process under the aforementioned parameters is to
maximise the oxidation of sulphides minerals and mass loss, and to
minimise the precipitation in pentavalent form of arsenic which may
be present in solution.
[0020] Furthermore sulphur oxidation at thermophilic temperature
conditions is maximised and thus any elemental sulphur produced
during the proceeding preleach and primary bioleach may be fully
oxidised. This is very important if further treatment of the
thermophilic secondary bioleach residue is required for precious
group metals (PGM's) recovery like gold. Elemental sulphur
increases cyanide consumption during cyanidation to recover gold
and thus contributes significantly to the increase in cyanidation
costs. Additionally, elemental sulphur not oxidised decreases the
acid produced in the bioleach solution and thus may decrease the
effectiveness of any preleach step using recycled bioleach
solution.
[0021] As indicated the at least one base metal is recovered from
the leach solutions produced by the bioleaching processes.
Preferably the pH of the solution produced in the preleaching step
is adjusted to maximise recovery of the at least one base metal
using solvent extraction techniques.
[0022] Arsenic present in the solution may be caused to precipitate
as ferric arsenate by increasing the pH of the solution to at least
2.
[0023] The pH of the solution may be increased by the addition of
limestone slurry to the solution.
[0024] The pH adjustment may be carried out in a series of
continuously stirred tank reactors which are operated at a
temperature of from 60.degree. C. to 80.degree. C.
[0025] The at least one base metal, eg. copper, may be recovered by
concentrating the copper and stripping, followed by cathode
production by electrowinning.
BRIEF DESCRIPTION OF THE DRAWING
[0026] The invention is further described by way of example with
reference to the accompanying drawing which is a flow sheet
illustrating various steps in a method of treating a concentrate
obtaining at least one base metal in accordance with the principles
of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
[0027] In the method of the invention a concentrate 10 which
contains a base metal such as copper and which may have a high
arsenic content is subjected to a preleaching step 12. In this step
the fresh concentrate is contacted with bioleach overflow solutions
14 and 16 respectively produced in subsequent primary and
thermophilic secondary bioleaching stages 18 and 20.
[0028] The solutions 14 and 16 are rich in ferric and remove easily
leachable copper from the feed 10. This ensures a lower residual
copper tenor in the bioleaching tanks in the stages 18 and 20.
[0029] The product 22 of the preleaching stage is subjected to
solid/liquid separation 24. An overflow solution 26 from the
separation step 24 is directed to a pH adjustment stage 28 while
the underflow 30, diluted with water 32 and raffinate 34 from a
solvent extraction section 36, is fed to the primary bioleaching
stage 18.
[0030] The purpose of the primary bioleaching stage 18 is to
oxidise sulphide minerals in the feed and release base metals of
interest into solution. The bioleaching is carried out in a series
of continuously stirred tank reactors which are operated at a
temperature of 35.degree. C. to 50.degree. C. in the presence of
active quantities of mesophilic and moderate thermophilic
microorganisms.
[0031] The pH of the pulp of the reactors in the primary
bioleaching stage is controlled at a value of from 1,2 to 1,7 by
the addition of limestone 40 or raffinate 34. Oxygen 42, required
for the oxidative reaction, is supplied in the form of enriched air
with an oxygen content of from 95% to 98%.
[0032] By operating the primary bioleaching stage 18 under the
aforementioned conditions the oxidation of the sulphide minerals
and the mass loss are maximised while, if arsenic is present in the
feed, the precipitation thereof in pentavalent form is
minimised.
[0033] The product 44 of the primary bioleaching section 18 reports
to bioleach thickening and washing 46. As has been indicated the
overflow solution 14 is fed to the preleaching step 12 while the
underflow 48 is the feed to a metal recovery section 50.
[0034] The purpose of the step 46 is to separate the liquids and
the solids so that the base metals of interest and arsenic, if
present, report to the pH adjustment section 28 via the preleaching
step 12. In the metal recovery step 50 toxic silver 52 is removed
from the primary bioleaching residue 48 using a brine leaching or
other suitable method.
[0035] The residue 54 from the metal recovery step is repulped with
water 56 and raffinate 34 and the resulting slurry is fed to the
thermophilic secondary bioleaching stage 20.
[0036] The purpose of the stage 20 is to oxidise, to the extent
possible, the sulphide minerals and the elemental sulphur which
were not leached in the primary bioleaching stage 18. The base
metals of interest are thereby released into solution. The
thermophilic secondary bioleaching process is carried out in a
series of continuously stirred tank reactors which are operated at
a temperature of from 65.degree. C. to 80.degree. C. in the
presence of active quantities of extreme thermophilic
microorganisms.
[0037] The pH of the pulp in the thermophilic reactors is
controlled at a value of from 1,0 to 1,7 by the addition of
limestone 40 or raffinate 34. Oxygen 42 required for the oxidative
reactions is supplied in the form of enriched gas with an oxygen
content of from 95% to 98%. Carbon dioxide 57 may be required for
improved thermophilic cell growth is supplied in the form of
enriched gas with a carbon dioxide content of 1% to 5% by volume.
By operating the thermophilic secondary bioleaching sections under
these conditions the oxidation of the sulphide minerals and the
mass loss are maximised while the precipitation of arsenic which
may be present in the slurry 54, in the form of pentavalent
arsenic, is minimised.
[0038] The product 60 of the thermophilic bioleaching section 20
reports to a bioleach thickening and washing step 62. The overflow
solution 16 is fed to the preleaching section 12 while the
underflow 64 is directed to a tailings pond 66 for disposal. If the
underflow 64 contains PGM's then the underflow is directed to a
metal recovery step 67 where the metal is removed from the
underflow using cyanide as a leaching process for gold or other
suitable method.
[0039] The purpose of the step 62 is to separate liquid and solids
so that base metals of interest and arsenic, if present, are
reported in solution to the pH adjustment section 28 via the
preleaching stage 12.
[0040] The pH adjustment section 28 includes a series of
continuously stirred tank reactors which are operated at a
temperature of from 60.degree. C. to 80.degree. C. The pH of the
solution 26 is increased to a required level using limestone 40 or
any other suitable neutralising agent. The product 70 of the pH
adjustment section is then thickened in a step 72. The thickener
underflow 74, which contains precipitated ferric arsenate, is
directed to a tailings pond 76 for disposal. The overflow from the
thickener step reports as pregnant leach solution (PLS) 80 to the
solvent extraction section 36.
[0041] The purpose of the pH adjustment section 28 is to increase
the pH of the pregnant leach solution, which is fed to the solvent
extraction section 36, to above 2,0 so that the solvent extraction
efficiency is maximised. Arsenic which is present in the solution
26 is caused, by the increase in the pH, to precipitate primarily
as ferric arsenate which is not readily dissolved. The ferric
arsenate passes EPA limits and is safe for land disposal.
[0042] In the solvent extraction section 36 dissolved copper is
recovered from the pregnant leached solution. The copper is
stripped followed by cathode production (84) by electrowinning.
[0043] In the process of the invention the base metal containing
concentrate is subjected to primary mesophilic and or moderate
thermophilic leaching, metal recovery and thermophilic secondary
leaching in combination so that secondary sulphides are
successfully and economically leached in the primary section, toxic
silver is removed in the metal recovery section, and a residue
containing unleached primary sulphides and elemental sulphur is
leached to completion successfully and economically in the
thermophile secondary section. If arsenic is present in the
concentrate the primary and thermophilic secondary sections are
operated so that the redox potential of the solutions produced
result in the natural oxidation of As(III) to As(V). Arsenic
precipitation in the bioleaching sections is intentionally
minimised so that the arsenic is precipitated externally in the pH
adjustment section 28. This avoids the production of a bioleach
residue contaminated with arsenic.
[0044] It is cost effective to reduce the arsenic reporting to the
thermophilic stage 20 by causing the arsenic to precipitate in a
separate dedicated process step ie. the pH adjustment section 28.
By minimising precipitation in the mesophilic stage 18 the mass
loss throughout the process is maximised. This reduces the capital
and operating cost of the downstream processes including the
thermophilic section 20.
[0045] Furthermore sulphur oxidation at thermophilic temperature
conditions is maximised and thus any elemental sulphur produced
during the proceeding preleach and primary bioleach may be fully
oxidised. This is important if further treatment of the
thermophilic secondary bioleach residue is required for precious
group metals (PGM's) recovery like gold. Elemental sulphur
increases cyanide consumption during cyanidation to recover gold
and thus contributes significantly to the increase in cyanidation
costs. Additionally, elemental sulphur not oxidised decreases the
acid produced in the bioleach solution and thus may decrease the
effectiveness of any preleach step using recycled bioleach
solution.
* * * * *