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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

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.

GOVERNMENT LICENSE RIGHTS

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.

TECHNICAL FIELD

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

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.

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.

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.

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 (CaCO₃, CaSO₄) 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.

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.

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

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.

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.

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.

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

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.

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

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

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.

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.

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 m²/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 6×12 mesh).

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.

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 (CH₃COOH), proprionic acid (CH₃CH₂COOH), tannic acid, oxalic acid, citric acid, and the like. Exemplary carboxylic acids include mono acids, such as formic acid, acetic acid, and proprionic acid.

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.

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.

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° C. to about 120° 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° C. to about 110° C., from about 60° C. to about 100° C., from about 70° C. to about 100° C., or from about 80° C. to about 90° C.

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.

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.

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.

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° C.

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.

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.

General Experimental Details

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 KMnO₄ and K₂S₂O₈ 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° C. for 2 minutes; 2.5 ml of 5% Na₂S₂O₈ was added and heated at 95° C. for 30 minutes; and then 5 ml of 5% KMnO₄ was added and heated at 95° 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.

Gold Analysis: Gold-bearing solution was diluted with 5% HNO₃ 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.

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 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° 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

As shown in Table 1, using a 10 vol % of acetic acid stripping agent at 60° 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.

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° 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.

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.

Methanol/Ethanol

Methanol and ethanol can be used as eluants for selective stripping of mercury cyanide from Au(CN)₂⁻ 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 2
Stripping of mercury cyanide and
Au(CN)2− from carbon (100% alcohol).
	Temperature
	23° C.		52° 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 3
Stripping of mercury cyanide and Au(CN)2− from carbon with
various concentrations of methanol at 52° C.
	Methanol/H2O
	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
(%)

These results indicated that methanol was superior to ethanol as a stripping agent. However, selective separation of mercury cyanide species and Au(CN)₂⁻ was not observed with either of these reagents at 23° C. And while increased desorption of the metal cyano complexes was observed at higher temperatures, only modest selectivity was observed.

Effect of Various Acids

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/H₂O= 25/75 vol %); volume=30 ml; shaken for 1 hour at 60±1° C. in a water bath. The addition of the acids into the total volume of 30 ml is given in Table 4.

TABLE 4
Mercury cyanide and Au(CN)2− 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.
HNO3	0.36	0.74	18.7	0.22	85	60° C.
HCl	0.09	1.92	4.1	0.06	68	50° C.
H3PO4	0.36	1.17	35.4	0.124	285	60° C.
Perchloric	0.36	1.1	40.8	0.20	204	60° C.
Formic	1.5	2.2	36.7	0.107	343	60° C.
Acetic	1.5	3.3	34.4	0.104	303	60° C.
	3.0	3.2	41.0	0.13	315	60° C.
Tannic	1.5 g	3.7	39.2	0.28	140	60° C.
Oxalic	1.5 g	1.1	35.2	0.20	176	60° C.
Citric	1.5 g	2.4	24.3	0.10	243	60° C.

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.

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° C.

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

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.

Effect of Temperature

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° C. to 90° C. in which only about 2 wt % of the initial mercury was retained on the carbon. At the conventional elution temperature of 135° C., about 25% of the initial mercury remains on the carbon.

Without intending to be limited by theory, the optimal temperature range might be explained on the following basis. From room temperature to about 90° C., the kinetics of desorption increases with increasing temperature. Above about 100° 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.

Effective and selective stripping of mercury cyanide from Au(CN)₂⁻ 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.

Acid Washing

In typical gold processing operations, activated carbons loaded (adsorbed) with gold cyanide, are washed with dilute solutions of mineral acids, such as HCl or HNO₃, in order to remove certain inorganic scalants, such as CaCO₃. 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.

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.

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° C. to about 120° 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° C. to about 100° 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° C. to about 90° 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° C. to about 120° 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° C. to about 100° 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° C. to about 90° 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° C. to about 120° 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° C. to about 100° 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° C. to about 90° 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.