U.S. patent application number 09/730026 was filed with the patent office on 2001-07-19 for method for the regeneration of sorbent materials.
Invention is credited to Robles, Antonio T..
Application Number | 20010008617 09/730026 |
Document ID | / |
Family ID | 23805222 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008617 |
Kind Code |
A1 |
Robles, Antonio T. |
July 19, 2001 |
Method for the regeneration of sorbent materials
Abstract
The process of the present invention is directed to the use of
sulfuric acid and/or an organic acid to regenerate spent sorbent
from a metal recovery process. The sorbent can be activated
carbon.
Inventors: |
Robles, Antonio T.;
(Kirkland Lake, CA) |
Correspondence
Address: |
SHERIDAN ROSS P.C.
1560 Broadway
Suite 1200
Denver
CO
80202-5141
US
|
Family ID: |
23805222 |
Appl. No.: |
09/730026 |
Filed: |
December 4, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09730026 |
Dec 4, 2000 |
|
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09454584 |
Dec 6, 1999 |
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Current U.S.
Class: |
423/22 ; 423/109;
423/24; 423/29; 502/180; 502/28; 75/711 |
Current CPC
Class: |
Y02P 10/20 20151101;
Y02P 10/234 20151101; C22B 3/24 20130101; C22B 11/042 20130101;
C22B 11/08 20130101; Y02P 10/214 20151101 |
Class at
Publication: |
423/22 ; 423/29;
423/24; 423/109; 502/180; 502/28; 75/711 |
International
Class: |
C22B 011/08 |
Claims
What is claimed is:
1. A process for recovering a metal from a metal-containing
material, comprising: (a) leaching the material to form a pregnant
leach solution containing at least a portion of the metal, (b)
contacting the pregnant leach solution with a sorbent to form a
loaded sorbent containing at least a portion of the metal, (c)
contacting the loaded sorbent with an eluant to form a loaded
eluant containing at least a portion of the metal in the loaded
sorbent and a spent sorbent, (d) contacting the spent sorbent with
an organic acid or sulfuric acid to form a regenerated sorbent, and
(e) recovering the metal from the loaded eluant.
2. The process of claim 1, wherein the sorbent is at least one of
carbon, and resin.
3. The process of claim 1, wherein step (a) is performed by
contacting a lixiviant with the material and wherein the lixiviant
includes at least of cyanide, thiosulfate, thiourea, thiocyanate
and mixtures thereof.
4. The process of claim 1, wherein the eluant includes at least one
of a carbohydrate, a hydroxide, a cyanide, an alcohol, a ketone, a
nitrile, acetone, a cyanate, a hydrazine, a hydroxy acid, a
carboxylic acid, a hydroxycarboxylic acid, a hydrocarbon, ammonia,
and mixtures thereof.
5. The process of claim 1, wherein the metal is a member of at
least one of Groups VIIIA, IB, or IIB of the Periodic Table of the
Elements.
6. The process of claim 1, wherein the organic acid is one or more
hydroxy acids.
7. The process of claim 6, wherein the hydroxy acid is one or more
hydroxycarboxylic acids.
8. The process of claim 1, wherein the regenerated sorbent is
reused in step (b).
9. The regenerated sorbent made in the process of claim 1.
10. The metal recovered in the process of claim 1.
11. A process for treating spent carbon, comprising contacting the
spent carbon with an organic acid.
12. The process of claim 11, further comprising: (a) leaching the
material with a lixiviant to form a pregnant leach solution
containing at least a portion of the metal, (b) contacting the
pregnant leach solution with a sorbent to form a loaded sorbent
containing at least a portion of the metal, (c) contacting the
loaded sorbent with an eluant to form a loaded eluant containing at
least a portion of the metal in the loaded sorbent and a spent
sorbent, wherein the lixiviant includes at least one of cyanide,
thiosulfate, and mixtures thereof, and the eluant includes at least
one of a carbohydrate, ammonia, a hydroxide, a cyanide, an alcohol,
a ketone, a nitrile, acetone, a cyanate, a hydrazine, a hydroxy
acid, a carboxylic acid, a hydroxycarboxylic acid, a hydrocarbon,
and mixtures thereof.
13. The process of claim 11, wherein the metal is a member of at
least one of Groups VIIIA, IB, OR IIB of the Periodic Table of the
Elements.
14. The process of claim 11, wherein the organic acid is one or
more hydroxy acids.
15. The process of claim 14, wherein the hydroxy acid is one or
more hydroxycarboxylic acids.
16. The process of claim 15, wherein the hydroxycarboxylic acid is
selected from the group consisting essentially of glycolic acid,
lactic acid, and mixtures thereof.
17. The process of claim 12, wherein the regenerated sorbent is
reused in step (c).
18. The regenerated sorbent made in the process of claim 11.
19. The metal recovered in the process of claim 11.
20. The process of claim 11, further comprising: contacting the
spent carbon with a metal-containing solution to form a metal
loaded carbon; contacting the metal loaded carbon with at least one
of an organic and inorganic acid to form a treated metal loaded
carbon; and contacting the treated metal loaded carbon with an
eluant.
21. Regenerated carbon for reuse in an elution process, comprising
an organic acid adsorbed in a plurality of pores of the carbon.
22. The regenerated carbon of claim 21, wherein the organic acid is
a hydroxy acid.
23. The regenerated carbon of claim 22 wherein the
hydroxycarboxylic acid is selected from the group consisting of
glycolic acid, lactic acid, and mixtures thereof.
24. The regenerated carbon of claim 22, wherein regenerated carbon
includes both the sulfuric acid and the organic acid.
25. The regenerated carbon of claim 22, wherein the regenerated
carbon includes a hydroxycarboxylic acid.
26. A process for eluting a metal from a sorbent, comprising:
contacting a metal-loaded sorbent with an organic acid or sulfuric
acid to form a treated metal-loaded carbon; and contacting the
treated metal-loaded carbon with an eluant to form a barren sorbent
and a metal-loaded eluant containing at least most of the metal
loaded onto the metal-loaded sorbent.
27. The process of claim 26, wherein the organic acid is a hydroxy
acid.
28. The process of claim 26, wherein the hydroxy acid is selected
from the group consisting of glycolic acid, lactic acid, and
mixtures thereof.
29. The process of claim 26, wherein the sorbent is at least one of
activated carbon, a resin, and mixtures thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation-in-part of U.S.
Patent Application entitled "Method for the Regeneration of
Activated Carbon" having Ser. No. 09/454,584, filed Dec. 6, 1999,
which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to the regeneration of
sorbents used for metal removal and specifically to the
regeneration of carbonaceous sorbents.
BACKGROUND OF THE INVENTION
[0003] Cyanidation is commonly employed for the extraction of gold
from its ores. In this process, the crushed ore is treated with a
dilute solution of sodium cyanide (NaCN), and a small amount of
lime (CaO) to maintain a pulp pH of >9. In the presence of
oxygen, gold dissolves forming gold cyanide complex.
[0004] Recovery of the gold is typically accomplished by adsorbing
the gold cyanide complex on activated carbon. The gold-loaded
carbon is subjected to an elution process whereby the gold is
eluted from the carbon by an eluant. The eluted (spent) carbon
contains a low concentration of gold and has its gold adsorption
capacity reduced. The spent carbon must therefore be regenerated
before reuse.
[0005] Various regeneration methods have been proposed and
practiced, but all of the methods are problematical in the areas of
cost, regeneration efficiency and maintenance of the mechanical
strength of the regenerated carbon. Therefore, development of an
effective and economical regeneration method has been eagerly
desired in the art.
[0006] Some conventional methods for the regeneration of activated
carbon are: (a) a low temperature heating method comprising heating
spent carbon at a temperature of 100-400.degree. C. with steam or
the like, (b) a high temperature heating method comprising heating
spent carbon at a temperature of 400-1000.degree. C., (c) a
regeneration method using a high temperature inert gas, (d) a
chemical regeneration method using acid or alkali, and (e) a wet
oxidation method, etc.
[0007] In the gold mining industry using Carbon-in-Pulp (CIP) or
Carbon-in-Leach (CIL) process, good carbon activity is important.
Each mining operation has to match carbon activity with process
requirements. When thermal carbon regeneration is used to
regenerate spent carbon, it is a general rule that the higher the
carbon activity the weaker the carbon and thus higher carbon losses
in these processes. Therefore, it is common practice in the
industry to thermally regenerate 50% or less of the total process
carbon to save on regeneration costs and to reduce carbon
losses.
[0008] Chemical methods of regenerating carbon achieve partial
regeneration of the carbon and can reduce the frequency of thermal
regeneration. It is employed to remove specific solvents and/or
compounds from the carbon. The most common of the chemical methods
are; (a) acid washing the spent carbon with hydrochloric or nitric
acid solutions, and (b) caustic washing with sodium hydroxide
solution to remove organic contaminants.
[0009] Acid washing using nitric or hydrochloric acid at room
temperature after elution may restore carbon activity to some
extent, but the improved carbon activity is small compared with
that of thermal regeneration.
[0010] Carbon activation with sulfuric acid by impregnation and
heating at 250.degree. C. for 4-6 hours was reported to improve
adsorption of ammonia by active carbon fibers.
[0011] Davidson et al., U.S. Pat. No. 4,267,069, tried to increase
carbon gold adsorption capacity by contacting the carbon with an
aqueous copper salt solution at a pH less than 6 in the presence of
carbonic acid, carbonate or bicarbonate.
[0012] Hanceford et al., U.S. Pat. No. 4,407,725, tried to
regenerate activated carbon with caustic wash and an optional
periodic acid wash.
[0013] Lambert et al., U.S. Pat. No. 5,087,374, tried to regenerate
activated carbon used to purify wastewater with an aqueous bath
containing a surface-active agent while ultrasonic vibration is
applied to the aqueous bath.
[0014] Activated carbon used for liquid phase, however, cannot be
regenerated unless it is subjected to the high temperature heating
method, namely, a re-activation or a thermal regeneration method in
which regeneration is effected under the same conditions as adopted
for production of activated carbon. Further, regeneration
conditions, such as the amount of steam and the residence time in
the regenerating furnace vary depending on amount and properties of
activated carbons. Therefore, regeneration loss is increased by
prolongation of the residence time, excessive incorporation of air
and the like, and simultaneously, the mechanical strength is
reduced in the regenerated activated carbon.
[0015] Furthermore, in some cases the exhaust gas from the
regenerating furnace causes secondary air pollution. On the other
hand, the chemical regeneration method in which high temperature
heating is not conducted is advantageous in that the regeneration
loss can be reduced, but the degree of regeneration is insufficient
and lower than in the thermal regeneration method and both the
waste coming from the spent carbon and the chemicals used for
regeneration should be treated completely.
[0016] In summary, prior methods of regenerating activated carbon
had disadvantages including the following:
[0017] high energy costs of thermal regeneration;
[0018] weakened carbon and generation of carbon fines;
[0019] inadequate regeneration;
[0020] expensive reagents;
[0021] complex process; or
[0022] expensive equipment.
[0023] Accordingly, the development of a regeneration method that
will be able to overcome these defects involved in the conventional
methods and to perform regeneration at an optimum regeneration rate
with lessened regeneration loss has been highly demanded in the
art.
[0024] As a result of intensive investigation made on regeneration
of activated carbon, a novel activated carbon regeneration process
has been discovered that satisfies the above-mentioned demands held
in the art.
SUMMARY OF THE INVENTION
[0025] These and other needs are addressed by the process of the
present invention. Generally, the present invention provides a
methodology for regenerating a sorbent, such as activated carbon,
for reuse in a metal adsorption, absorption, or entrapment process.
As used herein, a "sorbent" refers to any substance that absorbs,
adsorbs, or entraps another material. The methodology utilizes an
organic acid and/or sulfuric acid during sorbent regeneration.
[0026] In one embodiment, the process includes the following
steps:
[0027] (a) leaching (such as with a lixiviant) a metal-containing
material to form a pregnant leach solution containing at least a
portion of the metal,
[0028] (b) contacting the pregnant leach solution with a sorbent to
form a loaded sorbent containing at least a portion of the metal
dissolved in the pregnant leach solution,
[0029] (c) contacting the loaded sorbent with an eluant to form a
loaded eluant containing at least a portion of the metal in or on
the loaded sorbent and a spent sorbent,
[0030] (d) contacting the spent sorbent with an organic acid to
form a regenerated sorbent, and
[0031] (e) recovering the metal from the loaded eluant.
[0032] The process is effective for a variety of metals and metal
complexes. Preferably, the metal is a member of any one of Groups
VIIIA, IB, or IIB, of the Periodic Table of the Elements. More
preferably, the metal is gold, silver, platinum, copper, nickel,
cobalt, mercury, and/or mixtures thereof. The metal can be
complexed when in the pregnant leach solution, eluant, and/or in or
on the loaded sorbent, with a variety of functional groups,
including cyanide, thiocyanate, thiosulphate, and mixtures thereof.
Typically, the metal is complexed with cyanide.
[0033] The lixiviant in step (a) can be any suitable material for
dissolving the metal in the metal-containing material. Preferably,
the lixiviant is one or more of cyanide, thiosulfate, thiourea,
thiocyanate, and mixtures thereof. The leaching step can be
performed by contacting the lixiviant with the material in a
stirred or agitated or unstirred or unagitated vessel such as an
open vessel or a closed vessel such as an autoclave or by applying
the lixiviant to a pile or heap of the material or by applying the
lixiviant to the material in situ.
[0034] The pregnant leach solution is contacted with the sorbent in
any suitable vessel by any suitable technique. Preferably, the
pregnant leach solution is contacted with the sorbent in an open or
closed stirred or agitated or unstirred or unagitated vessel or by
passing the pregnant leach solution through a bed of the sorbent in
either a fluidized or fixed bed configuration.
[0035] The sorbent can be any suitable material for absorbing,
adsorbing, or otherwise entrapping the metal. Preferably, the
sorbent is organic and more preferably the sorbent is carbon (e.g.,
activated carbon), resins, and/or mixtures thereof.
[0036] As in the case of the pregnant leach solution, the eluant
can be contacted with the loaded sorbent in any suitable vessel by
any suitable technique. Preferably, the pregnant leach solution is
contacted with the sorbent in a stirred or agitated or unstirred or
unagitated vessel or by passing the pregnant leach solution through
a bed of the sorbent in either a fluidized or fixed bed
configuration.
[0037] The eluant can include any suitable eluting materials, such
as ammonia, carbohydrates (a compound having the formula
C.sub.x(H.sub.2O)y where x and y are whole numbers), inert gases
such as molecular nitrogen, a metal hydroxide (a compound
containing one or more OH-- groups), cyanides (a compound
containing one or more --CN radicals), alcohols (an organic
compound containing one or more hydroxyl groups), ketones (a
compound in which the carbonyl group is attached to two alkyl
groups), nitrites (an organic compound containing the --C.dbd.N
grouping), acetone, cyanates (a salt of cyanic acid containing the
radical --CNO), hydrazines, hydroxy acids, carboxylic acids,
hydrocarbons (a compound consisting only of C and H), and mixtures
thereof. The eluant typically includes a solvent for the eluting
material. To avoid competing with the eluting material in
displacement of the metal or metal complex from the sorbent, the
solvent in a preferred embodiment preferably is a polar or
substantially polar compound and the eluting material is nonpolar
or substantially nonpolar. Preferred solvents include water,
alcohols, and mixtures thereof. The preferred elution process is
described in U.S. patent application Ser. No. 09/372,025, filed
Aug. 13, 1999, and Ser. No. 09/632,386, filed Aug. 3, 2000, both of
which are incorporated herein by this reference.
[0038] The regeneration of the spent sorbent is preferably
performed using an inorganic acid such as a mineral acid (e.g.,
sulfuric acid) and/or an organic acid such as a hydroxy acid (e.g.,
an acid containing one or more hydroxy groups such as glycolic acid
and/or lactic acid) and/or a carboxylic acid (e.g., an acid
containing one or more carboxylic groups such as acetic acid (or
methanecarboxylic acid), formic acid, caproic acid, oxalic acid,
cerotic acid, aromatic acids such as benzoic acid and salicylic
acid, alicyclic acids such as abietic acid and chaulmoogric acid
and/or amino acids), with organic acids particularly hydroxy acids
being preferred and hydroxycarboxylic acids (particularly glycolic
acid) being even more preferred. The operating parameters for the
regeneration are important to realize the optimum degree of
regeneration. The pH at which the regeneration is performed
typically ranges from about pH 0.5 to about pH 3 and the
temperature typically from about ambient to about 95.degree. C. The
concentration of the acid in the regeneration or acid wash solution
contacted with the spent sorbent preferably ranges from about 3 to
about 10% weight. Regeneration can be performed in any suitable
stirred or unstirred, agitated or unagitated vessel or by passing
the acid wash solution through the spent sorbent in a fluidized or
fixed bed configuration with the latter being preferred.
[0039] While not wishing to be bound by any theory, it is believed
that the shorter chain organic acids, particularly glycolic acid,
penetrate deeply into the sorbent matrix, particularly for organic
sorbents such as carbonaceous sorbents. The depth of acid
penetration into the matrix is believed to be dependent upon
factors such as the shape of the acid molecule and the charge
distribution on the molecule. The deep penetration of the acid
inhibits removal and/or neutralization of the acid when the sorbent
is water rinsed (after the regenerative acid wash) to remove excess
acid. Unremoved (excess) acid will consume cyanide and other basic
leaching agents when the sorbent is again contacted with the
pregnant leach solution. During the contacting step (b) above, the
unremoved acid in the sorbent matrix is believed to cause a lower
pH to exist in or near the sorbent surface and/or pores than exists
in the surrounding pregnant leach solution. The lower pH causes the
metal to be more strongly attracted to the sorbent, thereby causing
a higher amount of metal collection in the sorbent than has been
previously realized with acids such as nitric acid and hydrochloric
acid, which are not believed to penetrate deeply into the sorbent
matrix. Organic sorbents such as activated carbon are believed to
adsorb organic acids better than inorganic acids such as nitric
acid and hydrochloric acid.
[0040] In one configuration that is particularly applicable to
carbon-containing sorbents, a first portion of the spent carbon is
contacted with the organic acid and a second portion of the spent
carbon is contacted with sulfuric acid. The first portion of the
spent carbon is subsequently thermally reactivated (e.g., in a kiln
or other suitable reactor or heater typically at a temperature
ranging from about 600 to about 800.degree. C.) before being
recycled to step (b) above and the second portion is not
subsequently thermally reactivated before being recycled to step
(c) above. The presence of an excessive amount of the sulfur atom
in or on the second portion may cause oxidation and loss of carbon
from carbon-containing sorbents during thermal reactivation.
[0041] The regenerated sorbent will typically contain some of the
acid on the surface of or inside the pores of the sorbent. For
example, regenerated carbon typically includes sulfuric acid and/or
the organic acid adsorbed in a plurality of pores of the
carbon.
[0042] The process, in one or more of its many embodiments, can
have a number of advantages. Treating the activated carbon with a
solution containing organic acids, particularly glycolic acid, can
restore significant gold adsorption capacity to the spent sorbent.
The process can have low capital and operating costs yet high
regeneration efficiency and can maintain the mechanical strength of
the regenerated sorbent. Although the process uses thermal
reactivation techniques in one embodiment, other embodiments employ
no thermal reactivation and are conducted at ambient to below about
100.degree. temperatures, which lowers operating (energy) costs, is
energy efficient, and avoids compromising the structural integrity
of the carbon (thereby generating few if any carbon fines and
reducing loss of carbon). Even when thermal reactivation is
employed, the spent sorbent is typically heated to relatively low
temperatures, which will not seriously compromise the mechanical
strength of the regenerated sorbent. The regeneration techniques of
the present invention can be fast and highly effective, thereby
decreasing the risk of inadequate regeneration. The process can use
relatively inexpensive reagents such as sulfuric acid and glycolic
acid at optimum regeneration rates. The process can be relatively
simple and can use relatively inexpensive equipment. In some
embodiments, the regeneration process is performed in situ, thereby
requiring less carbon handling. The process can be readily adapted
to existing plants and infrastructures. The description and
drawings below show additional objects and advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 shows the major components and flow directions of a
typical elution process incorporating the chemical regeneration
process of the eluted (spent) carbon.
[0044] FIG. 2 shows another embodiment of a regeneration process
that may be employed with any suitable elution process;
[0045] FIG. 3 shows the plot of gold adsorption rate of carbons
subjected to different carbon regeneration treatment.
[0046] FIG. 4 shows carbons that have been subjected to further
gold adsorption cycles to increase its gold loading capacity.
DETAILED DESCRIPTION
General Discussion
[0047] Although the process embodiments discussed below
specifically refer to the elution of gold from gold-loaded
activated carbon generated from an extractive metallurgical process
using a cyanide-containing lixiviant and a carbohydrate-containing
eluant, it is to be understood that the processes are equally
applicable for metals other than gold for eluants that do not
contain carbohydrates, and for processes other than extractive
metallurgy. For example, the processes can be used on other
precious metals and base metals, to name but a few, and may be used
to elute or remove metals/metal complexes from sorbents generated
in water treatment and/or purification processes.
[0048] The acid wash solution can include any suitable mineral
acid, such as nitric acid, hydrochloric acid, sulfuric acid, and/or
an organic acid such as glycolic acid, and preferably has a pH
ranging from about pH 0.1 to about pH 5 and more preferably from
about pH 0.5 to about pH 2. The concentration of the acid in the
acid wash solution preferably arranges from about 0.5 to about 25
vol. % and more preferably from about 3 to about 5 vol %.
[0049] While not wishing to be bound by any theory, it is believed
that the eluting or blinding agent, in the presence of the acidic
pH, is at most only weakly attracted to the sorbent, thereby
causing a large fraction (typically at least about 90%) of the
blinding agent into the solution from the sorbent surface. In other
words, in the presence of a basic pH, the equilibrium between the
blinding agent in the solution and the blinding agent on the
surface of the sorbent strongly favors the attachment of the
blinding agent to the sorbent surface. In contrast in the presence
of an acidic pH, the equilibrium between the dissolved blinding
agent and the blinding agent on the surface of the sorbent strongly
favors the dissolution (or hydrolysis) of the blinding agent.
[0050] The temperature of the acid wash typically ranges from about
80 to about 120.degree. C.
[0051] After the solvent has been contacted with the acid wash for
a time typically ranging from about 1 to about 2 hours, the
regenerated sorbent can be rinsed with water to remove residual
acid solution.
[0052] The sorbent can then be reused in a variety of known
processes to remove dissolved metal/metal complexes from pregnant
lixiviant solutions, industrial affluents, waste waters, etc., to
form metal/metal complex loaded sorbent for use as the feed
material in any elution processes including those described
above.
PREFERRED EMBODIMENT
Description
[0053] A preferred process involving selected major operations is
shown in FIG. 1. The gold-loaded carbon is soaked in the carbon
elution column 10 at a temperature preferably ranging from about 80
to about 100.degree. C. and more preferably about 95.degree. C.
with an organic acid wash solution 14 (having about 2% w/w organic
acid) typically for at least about one hour. The acid wash solution
14 is drained out of the carbon column 10 and the carbon is
thereafter neutralized with a suitable base 18 such as ammonia
water (having about 2% w/w base) by passing the base 18 through the
column 10. The gold-loaded carbon is then eluted in the column 10
with an eluant 22 in column 10 to strip the gold from the carbon
and form gold-loaded eluant 26. After the elution process, the
spent carbon in column 10 is washed with hot (e.g., about
95.degree. C.) acid wash solution 30. As will be appreciated, the
organic acid wash solutions 14, 30 can be stored in the same vessel
or in separate vessels. Heater 34 and heat exchanger 38 are used to
heat the various solutions as desired.
[0054] The reagents used in this process, e.g., glycolic acid and
ammonia solutions, may be reused if necessary by draining each
solution back to their respective holding tank after each batch of
carbon is treated by the solution. The solution can be made up to
strength with fresh reagents before reuse.
[0055] The pre-elution treatment using organic acid solution 14 and
base 18 hastens the elution process as well as helps in the
regeneration of the spent carbon in the post-elution treatment with
organic acid solution 30.
Operation
[0056] This part describes how my invention operates in reference
to FIG. 1.
[0057] As indicated in FIG. 1, the gold-loaded carbon in column 10
is prepared for elution by washing with about 1.5 to 2 Bed Volumes
(BV) of about 2% weight by weight ratio (w/w) organic acid wash
solution 14 at about 95.degree. C. for one hour. The organic acid
solution 14 is drained and the carbon is neutralized in column 10
with about 1.5 to 2 BV of 2% w/w base or basic solution 18 for
another hour. This pre-elution treatment is meant to: (a) dissolve
carbonate scale and reduce levels of calcium, magnesium, aluminum,
and other metals, (b) reduce overall contaminant levels in the
pores of the carbon, (c) hasten the elution process, and (d) help
in the chemical regeneration of the carbon.
[0058] Eluant 22 is then contacted with the loaded carbon in column
10 to form the gold-loaded eluant 26. After the elution is at least
substantially completed, the eluant 22 is drained from the carbon
column_10. The spent carbon is then soaked in column 10 with about
2% w/w organic acid solution 30 at about 95.degree. C. to
regenerate the spent carbon. The acid wash treatment is repeated at
about 1.5 to 2 BV per hour for at least about one hour. The organic
acid solution 30 is then drained from column 10 for reuse.
ALTERNATIVE EMBODIMENT
In-Situ Regeneration of Carbon
Description
[0059] An additional embodiment conducts the alternating process of
metal (e.g., gold) adsorption onto the sorbent and organic acid or
sulfuric acid washing on the organic sorbent (e.g., carbon) to
increase its metal loading capacity.
[0060] This embodiment relates to the regeneration of the organic
sorbent outside of the elution process to increase gold loading
capacity.
In-Situ Regeneration of Carbon
Operation
[0061] In the process of recovering the metal (particularly gold)
from a pregnant leach solution (particularly a cyanide solution)
using an organic sorbent or sulfuric acid (particularly activated
carbon), the loading capacity of the sorbent carbon can be
increased by the alternating process of metal adsorption and
organic acid washing.
[0062] In one configuration, the operation involves the following
steps:
[0063] Drain the gold cyanide solution from the carbon column 10
after at least a portion of the gold (or other metal) is loaded
onto the carbon;
[0064] Rinse the gold loaded carbon with water to reduce the
cyanide concentrations on the carbon;
[0065] Drain the rinse water from the column 10 and soak the carbon
in the organic acid solution in column 10;
[0066] Drain the organic acid or sulfuric acid solution from column
10 and rinse with at least about one bed volume of water to reduce
acidity; and
[0067] Put the carbon column into operation (e.g., proceed with
eluting the gold from the carbon and/or regenerating the
sorbent).
[0068] The process can be done in any carbon vessels and at any
temperatures. The organic or sulfuric acids and basic solutions
preferably use the concentrations noted above. As will be
appreciated, the process can be used for other metals and other
sorbents described herein.
[0069] As will be appreciated, higher acid washing temperatures
will achieve faster carbon regeneration.
[0070] In another embodiment of the present invention both a first
acid wash solution containing an organic acid and a second acid
wash solution containing an inorganic acid or a first acid wash
solution containing a first organic acid and a second acid wash
solution containing a second organic acid different from the first
organic acid are employed to acid wash first and second portions,
respectively, of the spent sorbent. One or both of the separately
washed portions can be further subjected thermal reactivation to
provide a high degree of sorbent activity.
[0071] In another embodiment of the present invention, glycolic
acid, sulfuric acid, or both, is used to reactivate spent sorbent
that may or may not undergo thermal regeneration before reuse. FIG.
2 shows this process according to this embodiment when the spent
sorbent is generated using a less common continuous elution
process. Most spent carbon is produced batchwise using a batch
elution process; therefore the alternating batches of spent sorbent
are thermally reactivated or not thermally reactivated. To further
save on cost, spent sorbent that will be thermally regenerated is
typically not acid washed. In this configuration, only spent
sorbent that will not be thermally regenerated is acid washed with
glycolic or sulfuric acid by a hot; preferably acid wash.
[0072] Referring to FIG. 2, the spent sorbent 100 in the embodiment
using continuous elution is first divided 104 into separate first
and second portions 108, 112 by any suitable technique and in any
proportion. The first portion 108 is subjected to a first acid wash
116 using a first acid wash solution 120 containing an organic acid
such as glycolic acid to form an acid washed first portion 124. The
acid washed first portion 124 is then thermally reactivated 128
typically at a temperature ranging from about 600 to about
800.degree. C. in a suitable reactor or heater for a time ranging
from about 30 to about 60 min. to form thermally reactivated first
portion 132. The second portion 112 is acid washed 136 with a
second acid wash solution 140 containing an inorganic acid such as
sulfuric acid to form acid washed second portion 144. The acid
washed second portion 144 and thermally reactivated first portion
132 are recombined 148 by suitable techniques to form the
regenerated sorbent 152, which may be reused in the
sorption/elution process.
[0073] As noted, FIG. 2 reflects a process utilizing the continuous
elution of spent sorbent. For a batchwise process, FIG. 2 would be
modified such that alternating batches of eluted or spent sorbent
would constitute the first and second portions 108, 112. The
dividing step 104 and recombining step 148 would be absent from the
flow schematic in this embodiment as the first and second portions
would be discrete from each other. As will be appreciated, when
alternating batches of spent sorbent 100 are regenerated
sequentially, a separating or dividing step would not be utilized.
As noted, the first portion 108 may not be acid washed before
thermal reactivation to reduce costs, and the first portion 108 and
second portion 112 are sequentially and alternative respectively
thermally reactivated and not thermally reactivated.
Example 1
[0074] A method for comparing carbon activity was used to compare
the following carbon regeneration treatments:
[0075] Spent carbon, with no regeneration treatment;
[0076] Spent carbon, acid washed at 95.degree. C. with nitric acid
(HNO.sub.3);
[0077] Spent carbon, acid washed at 95.degree. C. with hydrochloric
acid (HCl);
[0078] Spent carbon, acid washed at 95.degree. C. with glycolic
acid (GA); and
[0079] Spent carbon, with thermal regeneration.
[0080] Methodology involves presoaking one gram of carbon sample in
"tailings" slurry for one hour before adding the carbon to one
liter of standard gold cyanide solution containing about 10 ppm
gold and buffered to pH 10. The purpose of presoaking the carbon in
tailings slurry was to neutralize the "pH effect" of the acid
treatment on carbon activity. The carbonsamples were screened to
minus 8-mesh and plus 10-mesh. At 15 minutes time intervals a small
aliquots of test solution were removed to determine the remaining
gold concentration. A plot is prepared for each carbon sample as
shown in FIG. 3.
[0081] The data shows no significant improvement in carbon activity
was attributed to the use of nitric and hydrochloric acid wash.
Conventional acid washing allows for the removal of minerals
containing calcium, magnesium and aluminum. The removal of these
minerals proves helpful in improving thermal regeneration of the
carbon.
[0082] On the other hand, washing with glycolic and sulfuric acid
solution, significantly improved carbon activity. It is possible
that thermal regeneration can be significantly reduced or
eliminated in gold-adsorption processes such as CIL or CIP.
[0083] Maximum carbon regeneration was achieved by thermal
regeneration at a price of high carbon losses. It is not always
necessary to have a carbon that has very high activity. The gold
adsorption process will dictate the required carbon activity for
optimum operation.
Example 2
[0084] FIG. 4 and Table 1 show that alternating gold adsorption and
glycolic acid washing of the loaded carbon can increase the gold
loading capacity of the carbon. Two spent carbons, A and B, each
initially containing 300-ppm gold were subjected further to four
periods of gold adsorption. Carbon A was not acid washed between
periods while Carbon B was acid washed with glycolic acid and water
rinsed between periods.
[0085] The methodology involves two grams of carbon sample added to
100 mL of 127 ppm gold solution. The gold solution contains 0.5 g/L
sodium cyanide and 1 g/L calcium hydroxide. The gold adsorption
period was 2 hours and the acid washing period was 15 minutes with
water rinse. After each gold adsorption period the gold solution
was analyzed to determine the amount of gold adsorbed by the
carbon. The carbon was then put into fresh gold solution for the
next gold adsorption period.
1 TABLE 1 Gold loading (ppm gold) in the Carbon after each cycle
Cycles 1 2 3 4 Carbon A 3820 6010 7615 9135 Carbon B 3990 7635
10785 13530
[0086] While various embodiments of the present invention have been
described in detail, it is apparent that further modifications and
adaptations of the invention will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
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