Chemistry of inorganic compounds – Nitrogen or compound thereof – Carbon containing
Reexamination Certificate
1999-01-22
2001-03-13
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Carbon containing
C210S634000, C210S639000, C210S903000, C210S904000, C423S024000, C423S029000, C423S372000, C588S249000
Reexamination Certificate
active
06200545
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of processes for the recovery of cyanide from aqueous solutions, particularly waste solutions from gold ore processing operations.
BACKGROUND OF THE INVENTION
Cyanide solutions are widely used in a number of chemical processes. A common use of cyanide is in the leaching of gold. The chemistry by which cyanide leaches gold may be expressed as follows:
4
Au+
8
NaCN+O
2
+2
H
2
O−>
4
NaAu
(
CN
)
2
+4
NaOH
(1)
Gold is usually present in very low concentrations in naturally occurring ores and in concentrates derived from such ores. Typical gold concentrations are in the range of from about 1 g/tonne for some ores to about 1000 g/tonne for some concentrates. In the leaching process, cyanide is typically added to ores or concentrates at elevated pH to keep cyanide in solution and to thereby prevent the formation and evolution of highly toxic HCN gas. To help maximize the efficiency of leaching, cyanide is typically added in excess of the stoichiometric amount required for leaching in accordance with reaction 1. The excess cyanide is required in part because cyanide typically reacts with other minerals, is oxidized or volatilizes from the system.
Following leaching, gold may be recovered by a number of processes, such as zinc cementation or carbon adsorption, leaving a barren solution. The presence of excess cyanide in the barren solution at the end of the gold leaching and recovery operation creates a disposal problem for gold leaching plants. A variety of approaches may be taken to address this problem. The cyanide may be discharged to the environment if the cyanide concentration is sufficiently low. The cyanide may be destroyed using a chemical or biological treatment method, such as known methods of SO
2
/air treatment, alkaline chlorination, biological oxidation, hydrogen peroxide treatment or Caro's acid treatment. The cyanide may be recovered for recycle by known methods such as AVR (acidification, volatilization and re-neutralization), AFR (acidification, filtration and reneutralization), or MNR (Metallgeselshaft Natural Resources) processes, the Cyanisorb™ process, or the Augment™ process.
The AVR process has been of interest to gold processors for a long time. The process involves addition of acid to a waste cyanide solution followed by volatilization of HCN gas, reneutralization and scrubbing of volatile HCN from the stripping air, in accordance with the reactions 2, 3 and 4.
Acidification:
2CN
−
+H
2
SO
4
→2HCN(aq)+SO
4
2−
(2)
Volatilization:
2
HCN
(
aq
)
→
Airstripping
2
HCN
(
g
)
(
3
)
Reneutralization:
2HCN(g)+2NaOH(aq)→2NaCN(aq)+2H
2
O (4)
NaCN recovered from the reneutralization step of an AVR process may be returned to a leaching process. The AVR process may be particularly useful when cyanide is present as metal complexes such as copper cyanides. As shown in equation 5, two of the three cyanides in the copper cyanide complex may be recovered by this method while copper is recovered as a CuCN precipitate.
Cu(CN)
3
2−
+H
2
SO
4
→2HCN(aq)+SO
4
2−
+CuCN(s) (5)
The AVR method suffers from a number of potentially important drawbacks, including volatilization of dangerous HCN gas and inefficiencies that may raise costs. Volatile HCN is acutely toxic, which raises important safety concerns, particularly in areas where there is a higher risk of leaching plant disruption due for example to power outages. HCN is a soluble acid that is difficult to strip using air so that the size and cost (capital and operating) for volatilization operations may be significant. Also, the large volumes of gas typically used for volatilization must be scrubbed with NaOH to ensure good cyanide recovery and to minimize HCN loss in any discharged ‘tail’ gas, a process that may further raise costs.
A second known method of dissolved cyanide recovery is the MNR process. This process differs slightly from the AVR process in that NaSH (sodium hydrosulfide) is added during acidification. The NaSH is thought to maximize cyanide recovery by converting base metal cyanides to metal sulfides, as shown in equations 6 through 9.
Acidification and Sulfidization
2CN
−
+H
2
SO
4
→2HCN(aq)+SO
4
2−
(6)
2Cu(CN)
3
2−
+5/2H
2
SO
4
+NaSH→Cu
2
S+6HCN(aq)+5/2SO
4
2−
+Na
+
(7)
Volatilization
2
HCN
(
aq
)
→
Airstripping
2
HCN
(
g
)
(
8
)
Reneutralization
2HCN(g)+2NaOH(aq)→2NaCN(aq)+2H
2
O (9)
The MNR process suffers from many of the same drawbacks as the AVR process, since HCN is volatilized and scrubbed.
It is an object of the invention to provide a process for the recovery of cyanide from aqueous solutions that my be used as an alternative to known processes such as volatilization and reneutralization of HCN as part of AVR or MNR processes.
SUMMARY OF THE INVENTION
The invention provides a process for recovering hydrogen cyanide from an aqueous solution by extracting the hydrogen cyanide into an organic solvent phase. The organic solvent may comprise a neutral organophosphorous compounds, such as compounds selected from the group consisting of alkyl or aryl substituted phosphates, phosphonates and phosphine oxides. In alternative embodiments the organophosphorous compound is tri-butyl phosphate, di-butyl-butyl-phosphonate or tri-alkyl phosphine oxides. The organic solvent may be diluted in an organic diluent, such as an aliphatic or kerosene-type diluent. Alternative dilutions may be used, such as 75%, 50% or 25%. In some embodiments, the pH of the aqueous solution containing dissolved cyanide may be adjusted to between 2 and 8, or between 3 and 7, or between 4 and 6. The organic solvent may be contacted following extraction with a basic aqueous solution to strip cyanide from the organic solvent into a basic aqueous cyanide strip solution. The stripped organic solvent may then be returned to the loading process, to extract HCN from fresh aqueous solution.
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Alguacil, F.J. et al.
Griffin Steven P.
Medina Maribel
Townsend and Townsend / and Crew LLP
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