Process for eluting precious metals from activated carbon

Details Process for eluting precious metals from activated carbon Process for eluting precious metals from activated carbon

1999-08-13

2001-03-13

King, Roy V. (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Free metal or alloy reductant contains magnesium

C423S024000, C502S022000, C502S418000, C502S421000, C502S423000, C210S670000, C210S671000, C210S672000, C210S673000, C210S674000, C210S675000, C210S676000, C210S677000, C210S678000

Reexamination Certificate

active

06200364

ABSTRACT:

BACKGROUND—FIELD OF INVENTION
This invention relates to the recovery of precious metals from activated carbon, specifically to an improved elution process.
BACKGROUND—DESCRIPTION OF PRIOR ART
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.
Recovery of the gold is accomplished by adsorbing the gold cyanide complex on activated carbon. A variety of processes based on this reaction have been developed. Effectiveness of these processes is, however, dependent on the development of an efficient means of eluting the gold from the gold-loaded carbon.
The most common commercial methods for the elution of gold cyanide from activated carbon are the Zadra (U.S. Pat. No. 2,579,531, issued Dec. 25, 1951) and Anglo processes.
In the latest Zadra elution process, hot solution of 1% weight/volume (w/v) sodium hydroxide (NaOH) and 0.2% w/v NaCN are recycled through a gold-loaded activated carbon bed for up to 72 hours at 95-100° C. to elute gold cyanide. A modified Zadra process operating at 140° C. in a pressurized system reduces elution time to 10-12 hours.
In the Anglo elution process, gold-loaded activated carbon is soaked in a solution containing 3-5% w/v NaCN and 1% w/v NaOH followed by elution for 8-12 hours with deionized water at 100-120° C. The eluant solution is not recirculated; therefore, it is a once-through process.
The recycling of the weak gold-loaded eluant from the “tail-end” of an elution cycle to the beginning of the next elution cycle is practiced successfully both in the Zadra and Anglo processes. However, in the Zadra, high gold value in the recycled eluant slows down the elution process.
While both the Zadra and Anglo processes are effective in eluting gold from activated carbon, these processes suffer from high energy consumption, high capital costs for pressurized operations, and a long elution period. Although, conducting the elution under pressure or modifying the eluant with organic compounds also improves the rate of elution, these processes are complicated to implement.
I will now relate other attempts to elute gold from activated carbon involving the use of temperatures lower than those used in the Zadra or the Anglo elution process. D. M. Muir tried to elute gold by pretreating gold-loaded activated carbon with a solution of sodium cyanide and sodium hydroxide, and then eluting the carbon with methanol, ethanol, or acetonitrile vapors and condensate at 65-80° C. Using this process, he eluted gold cyanide in 4-6 hours. However this process requires an expensive sealed system to minimize (1) fire hazards of electrowinning due to the flammable organic solvent and, (2) solvent losses due to evaporation.
F. Espiell tried to elute gold from activated carbon using mixtures of NaOH (20 g/L) and 50% aqueous organic solvents at 30° C. He found that this acetone-water-hydroxide method was most efficient at gold desorption with over 90% of the gold being eluted in less than 40 minutes. However, a loss in gold-binding activity resulted over several loading/eluting cycles. This loss resulted from the failure of the acetone solvent system to elute the gold most strongly adsorbed to the activated carbon.
Heinen et al., U.S. Pat. No. 4,208,378, tried to elute gold at 70-160° C. with a solution of about 20-30% v/v water soluble alcohol and about 80-70% aqueous solution with a strong base of sodium or potassium hydroxide.
Parker et al., U.S. Pat. No. 4,427,571, tried to elute gold from activated carbon using at least 20% v/v polar organic solvents or mixture of polar organic solvents, preferably, nitrites containing sodium cyanide or sodium thiocyanate.
Harvey et al., U.S. Pat. No. 5,769,925, tried to elute gold by adding a powerful reducing agent, such as hydrazine monohydrate, to standard eluants, such as NaOH/NaCN with or without alcohol.
Belsak et al., U.S. Pat. No. 4,968,346, tried to elute gold using an eluant of about 2-3% v/v alcohol and 97-98% v/v deionized water. This approach involves adding to the eluant at least 2.5% w/w of a strong base (sodium or potassium hydroxide) and at least 0.3% w/w sodium or potassium cyanide.
Fuller et al., U.S. Pat. No. 5,073,354, tried to elute gold using as an eluant a compound containing the carboxylate functionality, selected from benzoic or substituted benzoic acids and polyacrylic acids of less than about 100,000 M. W.
Fisher, U.S. Pat. No. 3,935,006, tried to elute gold using, as eluants, water-soluble alcohols or ketones alone or with their aqueous solutions. Adding a strong base of sodium or potassium hydroxide facilitate elution.
In prior art, sodium cyanide and sodium hydroxide are universally used in the elution process. The importance of these two reagents in the elution process is illustrated in numerous studies on the mechanism of adsorption and elution of gold cyanide from activated carbon. Thus, Davidson established that the addition of “spectator cations” could enhance appreciably the gold adsorption following the sequence, Ca
2+
>Mg
2+
>H
+
>Li
+
>Na
+
>K
+
. He proposed a mechanism involving the adsorption of gold as ion pair M
n+
[Au(CN)
2
−
]
n
, and the use of the cations to preserve electroneutrality as counter ions in the electrical double layer.
According to Van Deventer, the presence of spectator cations (Mn
n+
) enhances the formation of Mn
n+
[Au(CN)
2
−
]
n
ion pairs on the carbon, which in turn suppresses the elution of gold cyanide. When the concentration of cations in the eluant is high and cyanide is absent from the solution or the carbon, very little desorption of gold is observed. Free cyanide in the eluant, which causes some competitive adsorption of cyanide with gold cyanide, plays a minor role at the elevated temperatures used in the industry. A more important effect of cyanide is its reaction with functional groups on the carbon, the products of which passivate the surface for adsorption of gold cyanide, with cyanide enhancing the elution of gold cyanide. The degree of passivation, which is determined to a large extent by the temperature of pretreatment, also affects the elution of cations and the degradation/adsorption of cyanide itself.
As the following table shows, conditions that enhance elution hinder adsorption.
Conditions Favoring Adsorption
Conditions Favoring Elution
Low temperatures
High temperatures
Low cyanide concentrations
High cyanide concentrations
Low Alkalinity
High Alkalinity
High ionic strength medium
Low ionic strength medium
Presence of Ca
+2
, Mg
+2
Absence of Ca
+2
, Mg
+2
Jia observed that ethanol and butanol, adversely affect gold adsorption. He also observed that low pH increased adsorption of gold and silver cyanide whereas organic solvents and high temperatures decreased gold and silver adsorption.
In summary, prior methods of eluting gold cyanide from activated carbon called for:
(a) A high temperature/pressure pre-soak of the loaded carbon with NaCN/NaOH solution followed with hot deionized water;
(b) A high temperature/pressure elution with aqueous NaCN/NaOH;
(c) Organic solvents or compounds which contain aqueous NaCN/NaOH; or
(d) Relatively complex unit operations involving distillation with aqueous NaCN/NaOH, such as the Micron process.
These prior methods had the following disadvantages:
(a) high energy costs because of elution at high temperatures;
(b) lengthy elution period;
(c) requirement for high quality water (Anglo process);
(d) expensive organic solvents or compounds, or both;
(e) fire hazards associated with organic solvents; or
(f) large volumes of gold-loaded eluant.
Thus, there is a need for a fast, safe, low-temperature, and efficient process for eluting gold cyanide from activated carbon for the recovery of metallic gold from aqueous solutions containing gold cyanides. My invention fills

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