U.S. patent application number 13/698492 was filed with the patent office on 2014-05-29 for stripping agent and method of use.
The applicant listed for this patent is Greggory W. Fuerstenau. Invention is credited to Maurice C. Fuerstenau, Carl C. Nesbitt, Thomas Joseph Seal.
Application Number | 20140147354 13/698492 |
Document ID | / |
Family ID | 44992043 |
Filed Date | 2014-05-29 |
United States Patent
Application |
20140147354 |
Kind Code |
A1 |
Fuerstenau; Maurice C. ; et
al. |
May 29, 2014 |
STRIPPING AGENT AND METHOD OF USE
Abstract
A method of removing mercury adsorbed onto activated carbon is
provided. The method includes treating an adsorbed mixture of metal
cyanide complexes on a carbon substrate with an acidic solution of
a stripping agent that is a weak acid. The method also eliminates
inorganic scalants from the carbon substrate. In precious metal
mining operations, the disclosed method reduces environmental
emissions of mercury during the gold elution and carbon
reactivation processes.
Inventors: |
Fuerstenau; Maurice C.;
(Reno, NV) ; Nesbitt; Carl C.; (Reno, NV) ;
Seal; Thomas Joseph; (Spring Creek, NV) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fuerstenau; Greggory W. |
Taylorville |
IL |
US |
|
|
Family ID: |
44992043 |
Appl. No.: |
13/698492 |
Filed: |
May 18, 2011 |
PCT Filed: |
May 18, 2011 |
PCT NO: |
PCT/US11/37002 |
371 Date: |
June 28, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61345769 |
May 18, 2010 |
|
|
|
61417133 |
Nov 24, 2010 |
|
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Current U.S.
Class: |
423/27 |
Current CPC
Class: |
B01J 20/3475 20130101;
Y02P 10/234 20151101; Y02P 10/20 20151101; C22B 3/24 20130101; B01J
20/3416 20130101; C22B 11/04 20130101 |
Class at
Publication: |
423/27 |
International
Class: |
C22B 3/00 20060101
C22B003/00 |
Goverment Interests
GOVERNMENT LICENSE RIGHTS
[0002] This invention was made with government support under
contract DE-FC26-02NT41607 awarded by the Department of Energy. The
government has certain rights in the invention.
Claims
1. A method of removing mercury from an adsorbed mixture comprising
mercury and gold that is adsorbed on a carbon substrate, the method
comprising: desorbing mercury from the carbon substrate by
contacting the adsorbed mixture with an acidic aqueous solution
comprising a stripping agent that is a weak acid.
2. The method of claim 1, wherein prior to desorbing mercury from
the carbon substrate, the method further comprises: adsorbing
mercury and gold on the carbon substrate to form the adsorbed
mixture, which includes mercury and gold.
3. The method of claim 1, wherein the weak acid comprises
phosphoric acid.
4. The method of claim 1, wherein the weak acid comprises an
organic acid.
5. The method of claim 1, wherein the weak acid comprises a
carboxylic acid.
6. The method of claim 5, wherein the carboxylic acid is a mono
acid.
7. The method of claim 6, wherein the mono acid is selected from
the group consisting of formic acid, acetic acid, and propionic
acid.
8. The method of claim 1, wherein the acidic solution further
comprises an alcohol.
9. The method of claim 1, wherein desorbing mercury from the carbon
substrate comprises contacting the adsorbed mixture with the acidic
aqueous solution at a temperature from about 40.degree. C. to about
120.degree. C.
10. The method of claim 1, wherein desorbing mercury from the
carbon substrate comprises contacting the adsorbed mixture with the
acidic aqueous solution at a temperature from about 60.degree. C.
to about 100.degree. C.
11. The method of claim 1, wherein desorbing mercury from the
carbon substrate comprises contacting the adsorbed mixture with the
acidic aqueous solution at a temperature from about 80.degree. C.
to about 90.degree. C.
12. The method of claim 1, wherein the weak acid is present in a
concentration greater than 0% and less than about 30% by volume of
the acidic aqueous solution.
13. The method of claim 1, wherein the weak acid is present in a
concentration from about 5% to about 20% by volume of the acidic
aqueous solution.
14. The method of claim 1, wherein the weak acid is present in a
concentration from about 5% to about 10% by volume of the acidic
aqueous solution.
15. A method of removing an inorganic scalant from an adsorbed
mixture comprising the inorganic scalant, mercury, and gold that is
adsorbed on an activated carbon used in a precious metal recovery
processes, the method comprising: desorbing the inorganic scalant
from the carbon substrate by contacting the adsorbed mixture with
an acidic aqueous solution comprising a stripping agent that is a
weak acid.
16. The method of claim 15, wherein the weak acid comprises
phosphoric acid.
17. The method of claim 15, wherein the weak acid comprises an
organic acid.
18. The method of claim 15, wherein the weak acid comprises a
carboxylic acid.
19. The method of claim 18, wherein the carboxylic acid is a mono
acid.
20. The method of claim 19, wherein the mono acid is selected from
the group consisting of formic acid, acetic acid, and propionic
acid.
21. The method of claim 15, wherein the acidic solution further
comprises an alcohol.
22. The method of claim 15, wherein desorbing the inorganic scalant
from the carbon substrate comprises contacting the adsorbed mixture
with the acidic aqueous solution at a temperature from about
40.degree. C. to about 120.degree. C.
23. The method of claim 15, wherein desorbing the inorganic sealant
from the carbon substrate comprises contacting the adsorbed mixture
with the acidic aqueous solution at a temperature from about
60.degree. C. to about 100.degree. C.
24. The method of claim 15, wherein desorbing the inorganic sealant
from the carbon substrate comprises contacting the adsorbed mixture
with the acidic aqueous solution at a temperature from about
80.degree. C. to about 90.degree. C.
25. The method of claim 15, wherein the weak acid is present in a
concentration greater than 0% and less than about 30% by volume of
the acidic aqueous solution.
26. The method of claim 15, wherein the weak acid is present in a
concentration from about 5% to about 20% by volume of the acidic
aqueous solution.
27. The method of claim 15, wherein the weak acid is present in a
concentration from about 5% to about 10% by volume of the acidic
aqueous solution.
28. The method of claim 15, wherein the inorganic sealant includes
a calcium precipitate.
29. The method of claim 28, wherein the calcium precipitate is
calcium carbonate.
30. The method of claim 28, wherein the calcium precipitate is
calcium sulfate.
31. The method of claim 15, wherein desorbing the inorganic sealant
from the carbon substrate by contacting the adsorbed mixture with
the acidic aqueous solution further includes desorbing mercury from
the carbon substrate.
32. A method of reducing mercury emissions in precious metal mining
operations, comprising: washing an adsorbed mixture comprising
mercury and gold that is adsorbed on an activated carbon substrate
with an acidic aqueous solution comprising a stripping agent that
is a weak acid, wherein at least a portion of a first amount of
mercury is desorbed from the activated carbon substrate; removing
at least a portion of the gold from the activated carbon substrate;
and regenerating the activated carbon substrate by heating, wherein
a second amount of mercury remaining on the activated carbon
substrate is volatilized from the activated carbon substrate, the
second amount of mercury is less than the first amount of
mercury.
33. The method of claim 32, wherein the weak acid comprises
phosphoric acid.
34. The method of claim 32, wherein the weak acid comprises an
organic acid.
35. The method of claim 32, wherein the weak acid comprises a
carboxylic acid.
36. The method of claim 35, wherein the carboxylic acid is a mono
acid.
37. The method of claim 36, wherein the mono acid is selected from
the group consisting of formic acid, acetic acid, and propionic
acid.
38. The method of claim 32, wherein the acidic aqueous solution
further comprises an alcohol.
39. The method of claim 32, wherein the mercury is desorbed from
the carbon substrate by washing the adsorbed mixture with the
acidic aqueous solution at a temperature from about 40.degree. C.
to about 120.degree. C.
40. The method of claim 32, wherein the mercury is desorbed from
the carbon substrate by washing the adsorbed mixture with the
acidic aqueous solution at a temperature from about 60.degree. C.
to about 100.degree. C.
41. The method of claim 32, wherein the mercury is desorbed from
the carbon substrate by washing the adsorbed mixture with the
acidic aqueous solution at a temperature from about 80.degree. C.
to about 90.degree. C.
42. The method of claim 32, wherein the weak acid is present in a
concentration greater than 0% and less than about 30% by volume of
the acidic aqueous solution.
43. The method of claim 32, wherein the weak acid is present in a
concentration from about 5% to about 20% by volume of the acidic
aqueous solution.
44. The method of claim 32, wherein the weak acid is present in a
concentration from about 5% to about 10% by volume of the acidic
aqueous solution.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/345,769, filed May 18, 2010; and U.S.
Provisional Application No. 61/417,133, filed Nov. 24, 2010, which
are hereby incorporated by reference herein in their entirety.
TECHNICAL FIELD
[0003] The present disclosure relates to stripping agents and their
uses, including removing mercury and inorganic sealants from
activated carbon used in mining or metal recovery operations.
BACKGROUND
[0004] Precious metal production has evolved over the past several
decades, and is based principally on the ability to dissolve
precious metals using a lixiviant, such as a basic aqueous cyanide
solution, to form soluble metallocyanide complexes. Gold and silver
are most notably recovered, but often the precious metal ores
contain other metallic minerals, which are also dissolved in the
basic cyanide medium. The processes to recover the gold and silver
cyanide complexes vary with the type of ore being treated, and the
quantity of other metals in the solution. One of the simplest (and
most widespread) techniques is to adsorb the metallocyanide
complexes onto activated carbon substrates, such as an activated
carbon derived from coconut shells. The carbon continues to adsorb
the metallocyanide complexes until it reaches its ultimate loading,
afterwards an elution process can be used to recover the precious
metals in a more concentrated solution. There are several methods
by which the carbon can be applied, such as carbon in pulp (CIP),
carbon in leach (CIL) and carbon in column (CIC) operations. While
this technique is quite efficient and has been used widely in the
mining industry for over 50 years, it is not without its
problems.
[0005] The most notable problem arises from the fact that ores
often contain other substances, such as metals or scalants, which
can also be adsorbed with the favored precious metal metallocyanide
complexes. Specifically, cyano-complexes of mercury are also
adsorbed on the activated carbon together with the gold and silver
cyanide complexes. This "contaminant" is a problem from many
standpoints. First, the adsorbed mercury cyanide complexes occupy
space on the activated carbon, thereby reducing the space available
for adsorbing the favored precious metals. Secondly, the mercury
generally follows the precious metals in subsequent processes,
requiring additional and expensive processing steps to remove and
recover the mercury separately from the gold and silver. Thirdly,
mercury is a strictly regulated "toxic substance" that must be
handled with expensive processes to minimize or eliminate its
release to the environment.
[0006] Elution (stripping) of the gold and silver cyanide complexes
from the activated carbon for recovery of these precious metals may
be accomplished by treating the activated carbon with a stripping
agent. Generally, the adsorbed mixture of precious metal cyanide
complexes adsorbed on the activated carbon is treated with a sodium
cyanide/sodium hydroxide stripping agent solution at elevated
temperatures. When mercury cyanide complexes are also adsorbed,
some of the mercury will be eluted with the gold and silver.
However, a significant percentage of the mercury will remain
affixed to the carbon after elution, reducing the effectiveness of
the carbon as it is recycled to process more solution.
[0007] As stated above, metallocyanide complexes are not the only
substances that are adsorbed on the carbon. In fact, inorganic
and/or organic fouling is a recurring problem in gold and silver
production facilities. Inorganic scalants include various forms of
lime scale (CaCO.sub.3, CaSO.sub.4) and adsorb and blind large
areas of the carbon. These inorganic scalants can remain even after
the precious metals are eluted from the carbon, which is typically
accomplished using a basic aqueous cyanide solution eluent.
However, an acid rinse with a strong acid, such as hydrochloric
acid, may be used to dissolve the inorganic scalants prior to
eluting the precious metal complexes from the carbon.
[0008] Oils, greases and other volatile organic compounds are also
readily adsorbed by activated carbon. But these volatile organic
compounds may be removed from the carbon after the precious metals
have been stripped by heating the carbon to elevated temperatures
using "in-house" regeneration kilns prior to the carbon being
returned to process more solution. However, any mercury that is not
desorbed from the activated carbon can also become volatilized from
the carbon in the high-temperature regeneration (or reactivation)
process, and may be potentially emitted to the environment.
[0009] Accordingly, a need exists for new methods for desorbing
mercury and/or inorganic scalants from an activated carbon, such as
when used in mining or metal recovery operations.
SUMMARY OF THE INVENTION
[0010] Certain aspects of the present disclosure are described in
the appended claims. There are additional features and advantages
of the subject matter described herein. They will become apparent
as this specification proceeds. In this regard, it is to be
understood that the claims serve as a brief summary of varying
aspects of the subject matter described herein. The various
features described in the claims and below for various embodiments
may be used in combination or separately. Any particular embodiment
need not provide all features noted above, nor solve all problems
or address all issues noted above.
[0011] According to an embodiment of the invention, a method of
removing mercury from an adsorbed mixture comprising mercury and
gold that is adsorbed on a carbon substrate is provided. The method
includes desorbing mercury from the carbon substrate by contacting
the adsorbed mixture with an acidic aqueous solution comprising a
stripping agent that is a weak acid.
[0012] According to another embodiment of the invention, a method
of removing an inorganic scalant from an adsorbed mixture
comprising the inorganic scalant, mercury, and gold that is
adsorbed on an activated carbon used in a precious metal recovery
process is provided. The method includes desorbing the inorganic
scalant from the carbon substrate by contacting the adsorbed
mixture with an acidic aqueous solution comprising a stripping
agent that is a weak acid.
[0013] According to another embodiment of the invention, a method
of reducing mercury emissions in precious metal mining operations
is provided. The method includes washing an adsorbed mixture
comprising mercury and gold that is adsorbed on an activated carbon
substrate, with an acidic aqueous solution comprising a stripping
agent that is a weak acid, wherein at least a portion of a first
amount of mercury is desorbed from the activated carbon substrate.
The method further includes removing at least a portion of the gold
from the activated carbon substrate, and regenerating the activated
carbon substrate by heating, wherein a second amount of mercury
remaining on the activated carbon substrate is volatilized from the
activated carbon substrate, the second amount of mercury is less
than the first amount of mercury.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with a general description of the
invention given above, and the detailed description of the
embodiments given below, serve to explain the principles of the
invention.
[0015] FIG. 1 is a graph of the percentage of mercury stripped and
Hg/Au stripping ratio versus number of stripping steps according to
an embodiment of the present disclosure; and
[0016] FIG. 2 is a graph of the amount of mercury that remained
adsorbed on activated carbon (percent) versus elution temperature
using sodium hydroxide and sodium cyanide over twenty-four hours of
elution.
DETAILED DESCRIPTION OF EMBODIMENTS
[0017] Unless otherwise explained, all technical and scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which this disclosure belongs.
In case of conflict, the present specification, including
explanations of terms, will control. The singular terms "a," "an,"
and "the" include plural referents unless context clearly indicates
otherwise. Similarly, the word "or" is intended to include "and"
unless the context clearly indicates otherwise. The term
"comprising" means "including;" hence, "comprising A or B" means
including A or B, as well as A and B together.
[0018] According to the present disclosure, methods of removing
mercury from an activated carbon substrate are provided. The
process also has the added benefit of eliminating calcium salts and
other inorganic scalants that can accumulate on activated carbon
used to recover precious metal cyanide complexes from leach
solutions. These methods, which upon their application to precious
metal mining, operations, advantageously also provides separating
mercury from precious metals, such as gold, and also reduces
atmospheric emissions of mercury, as discussed below. The
procedures disclosed can be substituted into the processing stream
without adding unit operations or unit processing steps to the
current physical plant of a mining operation.
[0019] The starting materials for the methods described herein can
include activated carbon, such as for use in precious metal mining
operations to concentrate and recover precious metal cyanide
complexes from leach solutions. Carbon substrates suitable for use
with the described methods include those activated carbons
generally used in the precious metal mining industry, and can
include those carbon substrates having high porosity and
superficial area of more than 1000 m.sup.2/g. In one example, the
pores may have diameters of about 10-20 Angstroms. One commonly
used activated carbon substrate is available from Carbon Activated
Corp. of Compton, Calif. (item number 004-C activated carbon,
coconut shell 6.times.12 mesh).
[0020] The ores suitable for the methods described herein are not
particularly limited to any specific type of precious
metal-containing ore. However, gold ores found in the state of
Nevada in the United States of America are exemplary of ores that
also contain significant amounts of mercury.
[0021] According to embodiments of the invention, an acidic aqueous
solution that includes a stripping agent of a weak acid is used to
desorb mercury from the adsorbed mixture of the metal cyanide
complexes on the carbon substrate. As used herein, a weak acid is
an acid that dissociates incompletely and therefore has a higher
pKa than a strong acid, such as hydrochloric acid, which
effectively releases substantially all of its acidic proton(s) when
dissolved in water, i.e., completely dissociates. Examples of weak
acids include some inorganic acids, such as phosphoric acid, and
organic acids, such as carboxylic acids. Suitable organic acids
include formic acid (HCOOH), acetic acid (CH.sub.3COOH), proprionic
acid (CH.sub.3CH.sub.2COOH), tannic acid, oxalic acid, citric acid,
and the like. Exemplary carboxylic acids include mono acids, such
as formic acid, acetic acid, and proprionic acid.
[0022] The concentration of the stripping agent in the acidic
aqueous solution may range from greater than 0% to about 30 percent
by volume. For example, the stripping agent concentration may be
about 5%, 10%, 15%, 20%, 25%, or 30% by volume. According to
various embodiments, the stripping agent concentration may be a
dilute concentration, such as from about 0.5% by volume to about
10% by volume, from about 2% by volume to about 8% by volume, from
about 3% by volume to about 7% by volume, from about 4% to about 6%
by volume, or from about 4.5% to about 5.5% by volume.
[0023] In addition to water, the acidic aqueous solutions may also
include one or more co-solvents such as alcohols. For example,
methanol, ethanol and the like may be used as a co-solvent.
[0024] The acidic aqueous solutions, which include the stripping
agent, and the adsorbed mixture may be intermixed under a variety
of contacting temperatures and conditions. According to embodiments
of the invention, the contacting temperature may range from about
40.degree. C. to about 120.degree. C. to affect about 75%
desorption of the available mercury from the activated carbon
substrate over a 24 hour period, as shown in FIG. 2. According to
certain embodiments, the contacting temperature may be from about
50.degree. C. to about 110.degree. C., from about 60.degree. C. to
about 100.degree. C., from about 70.degree. C. to about 100.degree.
C., or from about 80.degree. C. to about 90.degree. C.
[0025] The acidic aqueous solutions and the adsorbed mixture of
metal cyanide complexes and activated carbon substrate may be
contacted under batch or flow conditions. In batch operations, the
combined mixture of the acidic aqueous solutions and the adsorbed
mixture may be mixed or agitated by any known manner, such as
stirring or shaking. In flow operations, various parameters, such
as flow rate, column dimensions, flow configuration, pressure, and
the like may be optimized to affect the desired desorption
results.
[0026] According to embodiments of the invention, the acidic
aqueous solution with its stripping agent selectively desorbs and
removes mercury from the activated carbon substrate, while
substantially leaving the precious metals such as gold adsorbed on
the activated carbon substrate. In one example, the acidic aqueous
solution with its stripping agent removed about 35.4 wt % of the
total adsorbed mercury on the adsorbed mixture, while only removing
about 0.124% of the total adsorbed gold from the adsorbed mixture,
which provides a selectivity of (35.4)/(0.124) or 285 Hg:Au
stripped ratio. According to one embodiment of the invention, the
Hg:Au stripped ratio is about 100 or more, about 200 or more, about
300 or more, about 400 or more, about 500 or more, or about 600 or
more. In another example, the Hg:Au stripped ratio can range from
about 100 to about 700.
[0027] Another advantage of the disclosed methods is the reduction
of inorganic scalants, such as calcium carbonate and/or calcium
sulfate, that can also be adsorbed onto the activated carbon
substrate. The acidic aqueous solution with its stripping agent can
dissolve these foulants and thereby obviate or substantially reduce
the amount of acid washing generally used in many reactivation
procedures, as discussed below.
[0028] After the desired amount of mercury has been desorbed and
removed from the adsorbed mixture, the adsorbed precious metals may
be removed by any suitable method, e.g., elution with 2.5 wt % NaCN
and 2.5 wt % NaOH at 130.degree. C.
[0029] After the precious metals have been sufficiently desorbed
and removed from the carbon substrate of the adsorbed mixture, the
carbon generally needs to be reactivated, e.g., by heating at
elevated temperatures in a reducing atmosphere. Therefore, the
elimination or substantial reduction of mercury content remaining
on the carbon substrate minimizes the amount of mercury that will
be volatilized during the kilning process. As such, the methods
disclosed herein allow for the reduction in mercury emissions to
the environment during precious metal mining operations.
[0030] Although methods and materials similar or equivalent to
those described herein can be used in the practice or testing of
the present disclosure, suitable methods and materials are
described herein. The disclosed materials, methods, and examples
are illustrative only and not intended to be limiting.
[0031] General Experimental Details
[0032] Mercury Analysis: An Atomic Absorption/Mercury Cold Vapor
Technique was used. To obtain metallurgical balances, the amount of
mercury contained in the carbon was first established. Oxidants
used were KMnO.sub.4 and K.sub.2S.sub.2O.sub.8 and aqua regia was
used as lixiviant. The technique developed is as follows: after
filtration and water rinsing and drying, 1, 2 or 3 grams of carbon
was digested in 10 ml of aqua regia at 95.degree. C. for 2 minutes;
2.5 ml of 5% Na.sub.2S.sub.2O.sub.8 was added and heated at
95.degree. C. for 30 minutes; and then 5 ml of 5% KMnO.sub.4 was
added and heated at 95.degree. C. for 30 minutes. The supernatant
was poured off and analyzed using a SpectraAA-200 Atomic Absorption
Spectrophotometer, manufactured by Varian. This procedure
represented one stage of digestion. After five stages,
approximately 90% of the mercury was desorbed.
[0033] Gold Analysis: Gold-bearing solution was diluted with 5%
HNO.sub.3 to a selected volume so that its gold concentration was
within the range of 0-2 ppm, and then analyzed with the
SpectraAA-200 spectrophotometer.
[0034] Evaluation of acetic acid as a stripping agent: The addition
of acetic acid as a stripping agent selectively desorbs mercury
cyanide from activated carbon leaving gold cyanide adsorbed on the
carbon. Results from a method according to the present disclosure
are provided in Table 1 and FIG. 1.
TABLE-US-00001 TABLE 1 Effect of concentration of acetic acid.
Conditions: 2.0 g carbon loaded with 4.3 mg Au/g C and 4.5 mg Hg/g
C; 37.5 ml water plus acetic acid, stripping - 1 hour at 60.degree.
C. Conc. of Acetic Acid Stripped (%) Stripped (Vol %) Hg Au Hg/Au
Ratio 5 30.9 0.046 672 10 42.7 0.063 678 20 43.6 0.10 436 30 48.6
0.16 304
[0035] As shown in Table 1, using a 10 vol % of acetic acid
stripping agent at 60.degree. C. provided that 42.7% of the
adsorbed mercury was stripped from the carbon after one hour of
elution, while only 0.063% of the gold was desorbed, which gave a
678 Hg:Au stripped ratio.
[0036] Mercury stripping was also conducted with five 1-hour
stripping stages (total of five hours). Stripping conditions were:
2.0 g carbon loaded with 4.3 mg Au/g C and 4.5 mg Hg/g C. The
solution volume was 37.5 ml, the temperature was 80.degree. C., and
the solution was 10 vol % acetic acid. Results are shown in FIG. 1.
The stripping stages were conducted with fresh 10 vol % acetic acid
in each case.
[0037] Under these conditions 51.8% of the mercury was stripped in
one stage of stripping. After five stages of stripping, 85.1% of
the mercury was eluted from the activated carbon.
[0038] Methanol/Ethanol
[0039] Methanol and ethanol can be used as eluants for selective
stripping of mercury cyanide from Au(CN).sub.2.sup.- when both
cyano complexes are adsorbed on activated carbon. Results from
using this method are shown in Tables 2 and 3. Conditions for the
stripping were: 1.00 g carbon, loaded with 4.7 mg Au and 4.2 mg Hg;
solvent volume 15 ml; these substances were placed in a 250-ml
Erlenmeyer flask with a rubber stopper seal, and shaken for 5
seconds every 10 minutes for 1 hour.
TABLE-US-00002 TABLE 2 Stripping of mercury cyanide and
Au(CN).sub.2.sup.- from carbon (100% alcohol). Temperature
23.degree. C. 52.degree. C. Desorbed Metal Hg (%) Au (%) Hg (%) Au
(%) Methanol 41.4 39.6 69.6 38.7 Ethanol 15.7 16.3 24.8 21.7
TABLE-US-00003 TABLE 3 Stripping of mercury cyanide and
Au(CN).sub.2.sup.- from carbon with various concentrations of
methanol at 52.degree. C. Methanol/H.sub.2O 100/0 50/50 25/75 0/100
Desorbed Metal Hg Au Hg Au Hg Au Hg Au Desorbed Amount 69.6 39.1
31.7 28.3 18.8 17.7 2.0 0.04 (%)
[0040] These results indicated that methanol was superior to
ethanol as a stripping agent. However, selective separation of
mercury cyanide species and Au(CN).sub.2.sup.- was not observed
with either of these reagents at 23.degree. C. And while increased
desorption of the metal cyano complexes was observed at higher
temperatures, only modest selectivity was observed.
[0041] Effect of Various Acids
[0042] Stripping efficiency of mercury cyanide was evaluated in the
presence of various acids in the presence and absence of methanol.
The conditions used were 2.00 g carbon loaded with Au4.3 mg/g and
Hg4.5 mg/g; stripping solution (methanol/H.sub.2O= 25/75 vol %);
volume=30 ml; shaken for 1 hour at 60.+-.1.degree. C. in a water
bath. The addition of the acids into the total volume of 30 ml is
given in Table 4.
TABLE-US-00004 TABLE 4 Mercury cyanide and Au(CN).sub.2.sup.-
desorption with various acids in the presence of 25 vol % methanol.
Dose Stripped % Stripped Acid (ml or g) pH Hg Au Ratio (Hg/Au)
Temp. HNO.sub.3 0.36 0.74 18.7 0.22 85 60.degree. C. HCl 0.09 1.92
4.1 0.06 68 50.degree. C. H.sub.3PO.sub.4 0.36 1.17 35.4 0.124 285
60.degree. C. Perchloric 0.36 1.1 40.8 0.20 204 60.degree. C.
Formic 1.5 2.2 36.7 0.107 343 60.degree. C. Acetic 1.5 3.3 34.4
0.104 303 60.degree. C. 3.0 3.2 41.0 0.13 315 60.degree. C. Tannic
1.5 g 3.7 39.2 0.28 140 60.degree. C. Oxalic 1.5 g 1.1 35.2 0.20
176 60.degree. C. Citric 1.5 g 2.4 24.3 0.10 243 60.degree. C.
[0043] Of these acids, nitric and hydrochloric acids were somewhat
less effective in selectively stripping mercury cyanide from gold
cyanide in the presence of 25 vol % methanol.
[0044] Propionic acid was also evaluated as a stripping agent.
Table 5 shows Hg desorption data using propionic acid. 1.00 gm
carbon was loaded with 1.0 mg Hg/g C. Stripping with various total
solution volumes of 10 vol % propionic acid for 6.0 hrs at
80.degree. C.
TABLE-US-00005 TABLE 5 Hg desorption data using propionic acid.
Solution volume, Hg desorbed, Carbon, g ml mg Desorbed, % 1.0 10
0.607 60.7 1.0 25 0.840 84.0 1.0 80 0.840 84.0
[0045] Propionic acid functions as an effective stripping agent for
mercury cyanide from activated carbon. Under the experimental
conditions studied, up to 84 percent of the adsorbed Hg desorbed
from the carbon after stripping with 25 ml of 10 vol % propionic
acid for 6 hours.
[0046] Effect of Temperature
[0047] The effects of temperature and time on mercury and gold
elution from carbon with sodium cyanide and sodium hydroxide were
evaluated in detail. In these methods, 3.33 g of carbon was loaded
with 4.1 mg Hg/g C and 4.7 mg Au/g C. As shown in FIG. 2, optimal
elution temperature ranges from 80.degree. C. to 90.degree. C. in
which only about 2 wt % of the initial mercury was retained on the
carbon. At the conventional elution temperature of 135.degree. C.,
about 25% of the initial mercury remains on the carbon.
[0048] Without intending to be limited by theory, the optimal
temperature range might be explained on the following basis. From
room temperature to about 90.degree. C., the kinetics of desorption
increases with increasing temperature. Above about 100.degree. C.,
the mercury cyanide complexes become unstable, and mercuric
hydroxide forms. Conditions used for this example of the method
were: carbon 3.3 g; elution solution: 500 ml; (NaCN)=(NaOH)=2.5 wt
%; Au loading 4.7 mg/g; Hg loading 4.1 mg/g.
[0049] Effective and selective stripping of mercury cyanide from
Au(CN).sub.2.sup.- can be accomplished using acetic acid when both
species are adsorbed on activated carbon. In some cases, 95% or
greater desorption of the mercury from the carbon can be
accomplished while leaving virtually all of the gold cyanide on the
carbon.
[0050] Acid Washing
[0051] In typical gold processing operations, activated carbons
loaded (adsorbed) with gold cyanide, are washed with dilute
solutions of mineral acids, such as HCl or HNO.sub.3, in order to
remove certain inorganic scalants, such as CaCO.sub.3. It was
unexpectedly found that washing the loaded carbon with a dilute
solution of an organic acid, such as acetic acid, removes inorganic
scalants, and also removes mercury cyanide complexes from the
substrate (carbon), without substantially removing valuable
precious metal cyanide complexes, such as cyanide compounds of gold
or silver, from the substrate. After washing has been carried out
to a desired degree, the carbon is moved to the stripping
operation.
[0052] It is to be understood that the above discussion provides a
detailed description of various embodiments. The above descriptions
will enable those skilled in the art to make many departures from
the particular examples described above to provide apparatuses
constructed in accordance with the present disclosure. The
embodiments are illustrative, and not intended to limit the scope
of the present disclosure. The scope of the present disclosure is
rather to be determined by the scope of the claims as issued and
equivalents thereto.
* * * * *