Cyanide detoxification process

Details Cyanide detoxification process Cyanide detoxification process

2000-10-27

2003-04-22

Cintins, Ivars (Department: 1724)

Liquid purification or separation

Processes

Ion exchange or selective sorption

C210S667000, C210S724000, C210S726000, C210S904000

Reexamination Certificate

active

06551514

ABSTRACT:

BACKGROUND OF THE INVENTION
The principal method used in the extraction of gold is cyanidation. The basic principle of the cyanidation process is that cyanide solutions have a preferential dissolving action for the precious metals contained in an ore (Marsden J., House I., The Chemistry of Gold Extraction, Chapter 6, published by Ellis Horwood Limited, 1992, page 259-308). The mechanism of cyanidation can be demonstrated by reaction (1) (Tukel, C., Celik, H., Ipekoglu, U., Tanriverdi, M., & Mordogan, H., Leaching of Ovacik gold ore with cyanide and thiourea,
Changing Scopes in Mineral Processing,
1996, page 567-572).
4Au+8CN
−
+O
2
+2H
2
O
4Au(CN)
2
−
+4OH
−
  (1)
The gold dissolution rate is believed to be dependent on the concentration of NaCN and the alkalinity of the solution. For efficient leaching, the gold should occur as free, fine-size, clean particles in an ore that contains no cyanicides or impurities that might destroy cyanide or otherwise inhibit the dissolution reaction.
The cyanidation process for the extraction of gold from different gold ores has been employed for nearly a century. Cyanide is a powerful lixiviant for gold and silver extraction from ores. But at the same time cyanide forms complexes with other metals, such as mercury, zinc, copper, iron, nickel and lead, which partially account for the consumption of cyanide in gold extraction. Also free cyanide as well as cyanide complexes of heavy metals are discharged to the tailings pond. Because of the toxicity of cyanide it is necessary to detoxify cyanide from the process stream before discharge.
The toxicity of free cyanide and metal cyanide complexes can manifest itself in either an acute or chronic manner. The period of acute toxicity ranges from a few minutes to several days. Although cyanide is not an accumulative toxicant, the metals bound to cyanide can bioconcentrate or bioaccumulate, resulting in permanent physiological damage.
In the western United States, gold operations are generally permitted with zero discharge from the processes. The state of Nevada which has the largest gold extraction operations in the United States, requires that operations not degrade the quality of the groundwater of the state. United States, requires that operations not degrade the quality of the groundwater of the state. Nevada sets the surface water quality at concentrations equal to state and federal drinking water standards with cyanide concentrations not to exceed 0.2 milligrams weak and dissociable (WAD) cyanide per liter. (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes,
Mining Journal Books Limited
, London, 1991; Bucknam C. H., Cyanide Solution Detoxification Jar Tests,
The Minerals Metals & Materials Society,
1997, page 191-204).
In addition to the mining industries, cyanide wastes are also produced by the electroplating, chemical, petrochemical, metallurgical processing and tin production industries, for example. The toxicity of cyanide in wastewaters is related to its form and concentration. The chemistry of cyanide is complex and many forms of cyanide exist in mining solutions (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes,
Mining Journal Books Limited
, London, 1991). The major categories of cyanide compounds which are important from a toxicity viewpoint include: free cyanide; iron cyanide; weak acid dissociable (WAD) cyanides; and cyanide related compounds.
Mineral wastes are highly complex because of the chemical interactions occurring in the metallurgical processes, ore geochemistry, leaching reagents, meteorological factors, and site hydrology. The pH of cyanide leached slurries and process solutions are generally in the range of 9-11 and may contain high concentrations (beyond dischargeable limits) of cyanide, heavy metals and species which can form anionic complexes at high pH values viz., As, Mo and Se. Thiocyanate, cyanate and ammonia may also be present at levels of concern (Ritcey, G. M., Tailings Management: Problems and Solution in The Mining Industry. Process Metallurgy 6.
Energy, Mines and Resources
Canada, CANMET. Elsevier, 1989 pp 970).
Acid solutions favor the presence of HCN and at pH values below 7 essentially all of the free cyanide is present in CN
−
form. At a pH value of 9.36 (equal to the pK), the concentrations of HCN and CN ion are equal. At lower pH values, and at 20° C., most cyanide exists as molecular HCN: 69.6 percent at pH 9; 95.8 percent at pH 8; and greater than 99 percent at pH 7. It is apparent that most of the “free cyanide”—the sum of molecular hydrogen cyanide and the cyanide ion in natural waters would be in the form of HCN (Marsden J., House I., The Chemistry of Gold Extraction, Chapter 6, published by Ellis Horwood Limited, 1992, page 259-308).
Simple cyanides are represented by the formula A(CN)
x
, where A is an alkali (sodium, potassium, ammonium) or metal, and x, the valence of A, represents the number of cyano groups present in the molecule (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing,
Proceedings of a Workshop
, Salt Lake City, Utah, 1982). Soluble compounds, particularly the alkali cyanides, ionize to release cyanide ions. The solubility is influenced by pH and temperature.
The complex alkali-metallic cyanides can generally be represented by the formula A
y
M(CN)
x
, where A is the alkali, y is the number of alkalies, M is the heavy metal (ferrous or ferric iron, cadmium, copper, nickel, silver, zinc, or others) and x is the number of CN groups. The value of x is equal to the valence of A taken y times, plus the valence of the heavy metal. The soluble complex cyanides release the radical or complex ion M(CN)
x
rather than the CN group. The complex ion may then undergo further dissociation releasing cyanide ion. Metal cyanide complex ions may be considered as the soluble products of the reaction between the corresponding insoluble simple cyanide and excess cyanide ion.
In general, the complex ion is more stable than the original compound and thus subsequent dissociation is relatively minor. There are 18 elements that form complex cyanide compounds, and there are more than 64 oxidation states of these metals capable of forming complex cyanides under certain conditions (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes,
Mining Journal Books Limited
, London, 1991). Two factors influencing the rate of dissociation at a given temperature are the pH of the medium and the concentration of the complex ion. In general, dissociation rates increase with either decreasing pH or decreasing total cyanide concentration. As expected, the rates of dissociation differ among the complex cyanides.
Not only is the rate of dissociation dependent upon pH and concentration, but the degree of dissociation also depends strongly on pH and concentration, i.e., the production of free cyanide from the dissociation of complex cyanides. At acid pH, the percentages of free cyanides are high and decrease as pH is increased to neutral and alkaline values.
Some of the heavy metals present in tailings solution as a result of mineral dissolution or flotation reagent addition, notably iron, copper, and zinc, can be precipitated from simple salts as their hydroxides and settle within the tailing mass. Soluble cyano complexes of these metals, however, are not readily precipitated as hydroxides and remain in solution. Their removal generally requires more sophisticated treatment than simple hydroxide precipitation.
Table 1 describes the relative stabilities of cyanide complexes (Scott, J. S., An overview of Cyanide Treatment Methods for Gold Mill Effluents,
Cyanide and the Environment Proceedings of a Conference
, Tucson Ariz. 1984; Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes,
Mining Journal Books Limited
, London, 1991). Total cyanide includes all the species of cyanide present in the solution.
TABLE 1
Relative Stabilities of Cyanide Complexes in Wa

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