United States Patent (19) 11) 4,208,378 Heinen Et Al
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United States Patent (19) 11) 4,208,378 Heinen et al. 45) Jun. 17, 1980 54 DESORPTION OF GOLD FROMACTIVATED 58 Field of Search ....................... 423/23, 27, 25, 24; CARBON 75/118; 210/30 R, 37 B, 40; 252/414 75 Inventors: Harold J. Heinen; David G. Peterson; 56) References Cited Roald E. Lindstrom, all of Reno, U.S. PATENT DOCUMENTS Nebr. 3,920,403 11/1975 Ross ....................................... 75/118 (73) Assignee: The United States of America as 3,935,006 1/1976 Fischer ................................... 75/118 represented by the Secretary of the 3,970,737 7/1976 Davidson ............................... 423/27 Interior, Washington, D.C. Primary Examiner-Brian E. Hearn Attorney, Agent, or Firm-William S. Brown; Donald A. 21 Appl. No.: 786,739 Gardiner 22 Filed: Apr. 11, 1977 57 ABSTRACT Gold is desorbed from activated carbon by contacting, Related U.S. Application Data at a temperature of about 70 to 160" C., with a solution 63 Continuation-in-part of Ser. No. 660,942, Feb. 24, 1976, comprising about 20 to 30 percent by volume of a water abandoned. soluble alcohol and about 80 to 70 percent by volume of an aqueous solution of a strong base. 51) Int. Cl’................................................ CO1G 3/00 52 U.S. Cl. ......................................... 423/27; 423/25 5 Claims, 3 Drawing Figures d 2 O al a. O (f) U s C O O 2O ETHANOL CONCENTRATION, pct/vol. U.S. Patent Jun. 17, 1980 Sheet 1 of 3 4,208,378 OO 9 O 8 O 7 o O IO 20 3O 4-O 5O w ETHANOL CONCENTRATION, pct/vol. A/G. /. U.S. Patent Jun. 17, 1980 Sheet 2 of 3 4,208,378 OO r 90 8O 7O d Cl 60 2. h 50 O 40 s C O O - 3O 4O 5O 6O 7O 8O 90 OO TEMPERATURE, C A/6. 2. U.S. Patent Jun. 17, 1980 Sheet 3 of 3 4,208,378 6o NUMBER OF BED VOLUMES OF ELUENT 4,208,378 1. 2 base at a temperature of about 70 to 160 C, preferably DESORPTION OF GOLD FROM ACTIVATED about 80' to 90' C. CARBON BRIEF DESCRIPTION OF THE DRAWINGS This application is a continuation-in-part of applica- 5 FIG. 1 is an illustration of percentage of gold desorp tion Ser. No. 660,942, filed Feb. 24, 1976 and now aban tion vs. ethanol concentration as in Example 1. doned, FIG. 2 is an illustration of percentage of gold desorp Cyanidation is commonly employed for extraction of tion vs. temperature as in Example 2. gold from its ores. In this process, the crushed ore is FIG. 3 is an illustration of percentage of gold de dissolved in a dilute solution of sodium cyanide, or 10 sorbed vs. bed volumes of eluent. calcium cyanide, and a small amount of lime, in the As illustrated in the examples below, the use of the presence of oxygen, the gold dissolving in the form of elevated temperature results in a dramatic improvement the complex Au(CN)2. Recovery of the gold has con in efficiency of desorption of the gold. An even more ventionally been accomplished by treatment of the re unexpected result, however, is that the desorption effi sulting cyanide solution with zinc dust. More recently, 15 ciency at the elevated temperature is substantially as activated carbon has been found to be an efficient adsor good with the low alcohol concentration of the inven bent of the gold cyanide complex, and a variety of pro tion, i.e., 20 to 30 percent, as with the much higher cesses based on this reaction have been developed. Ef alcohol concentration of the prior art, whereby the fectiveness of these processes is, however, dependent economy of the process is greatly improved. Recovery on development of efficient means for desorbing the 20 of the gold from the stripping solution is readily accom gold from the loaded carbon. Stripping with hot caustic plished by conventional means such as electrolysis. sodium cyanide solution at atmospheric pressure to Activated carbon, suitable for use in the invention is desorb the gold, followed by electrolysis to win the a widely available material that is conventionally used metal values, has been employed. However, this in adsorption process, including gold adsorption. Typi method has the disadvantage that simultaneous strip 25 cally, it will have a particle size of about 0.15 to 2 mm ping and electrolysis is slow. Elevated pressure has also and a total surface area of about 600-900 M2/g. It may been employed, so that the temperature of the caustic be derived from any of a variety of sources such as coal, cyanide solution can be raised to about 250 F. without petroleum chars, coconut shell, or pulp mill black ash, boiling, thereby accelerating the desorption rate. This and is activated by conventional means such as heating 30 in a steam-air mixture at a temperature of about 850 C. technique, however, requires the use of a boiler-pres The activated carbon adsorbent is initially loaded by sure reactor, which makes the process more compli conventional means, i.e., by contacting a gold-cyanide cated. solution e.g., a cyanide plant effluent, with the activated A recent innovation, described in U.S. Pat. No. carbon for a time sufficient to permit adsorption of a 3,935,006, issued Jan. 27, 1976, employs caustic-alcohol 35 major amount of the gold-cyanide complex. This may water mixtures, containing over 75 percent alcohol by be accomplished by any conventional means for con volume, at ambient temperature and pressure for de tacting liquids with solid adsorbents, e.g., bypassing the sorbing gold from activated carbon. This method is gold-cyanide solution through a columnar unit contain very effective for desorption of gold from activated ing a fixed bed of the activated carbon. carbon loaded by treatment with synthetic NaAu(CN)2 The gold is then desorbed from the loaded carbon, solutions. However, as illustrated in the examples be according to the process of the invention, by treatment low, it has not been found to be effective for desorption with a solution comprising a water soluble alcohol and of gold from activated carbon loaded by treatment with an aqueous solution of a strong base, at a temperature of cyanide plant effluents, particularly when the effluents about 70 to 160° C., preferably about 80 to 90° C. As contain the gold in the form of CaAu(CN)2, i.e., when 45 discussed above, this temperature range has been found the effluent is formed by treatment of ores with calcium to be essential to achievement of efficient desorption of or sodium cyanide and lime. Investigations have shown the gold, particularly where the gold-cyanide solution is that there is a significant difference in the behavior of a cyanide plant effluent formed by treatment of ores sodium and calcium ions regarding carbon adsorption with calcium or sodium cyanide and lime. of gold, the aurocyanide complex being much more 50 The water soluble alcohol is preferably a lower ali strongly adsorbed when calcium is employed as the phatic alcohol such as methanol, ethanol, propanol, or cation. Since lime is generally employed to provide isopropanol. As mentioned above, the use of an elevated protective alkalinity in conventional cyanidation pro temperature permits the use of a relatively small pro cesses, an efficient process for desorption of the portion of alcohol, preferably about 20 to 30 percent by aurocyanide complex in the presence of calcium ion is 55 volume of the stripping solution. essential for economical recovery of gold by means of The balance of the stripping solution consists essen activated carbon adsorption. In addition, soluble extra tially of a water solution of a strong base. Sodium hy neous matter, such as organics, silicates, and ferrous droxide is the preferred base, but potassium hydroxide iron salts, in the pregnant cyanide effluents may play a may also be used. The base is employed in an amount of significant role in the desorption process. 60 about 1 to 2 percent by weight of the water solution, or It has now been found, according to the process of an amount sufficient to provide the stripping solution the invention, that gold in the form of the aurocyanide with a pH of about 11 to 14, preferably about 13 to 14. complex, and particularly the calcium aurocyanide Use of a high pH is essential since the activated carbon complex, is very efficiently desorbed from activated has a substantially decreased capacity for adsorption of carbon by treatment of the loaded carbon with a strip- 65 the gold-cyanide complex at higher values of pH. ping solution comprising about 20 to 30 percent by The stripping solution may also contain a small volume of a water soluble alcohol and about 80 to 70 amount of sodium cyanide, e.g., about 0.02 to 0.1 per percent by volume of an aqueous solution of a strong cent by weight of the water solution, particularly where 4,208,378 3 4. the stripping solution is recycled after removal of the operating temperatures were employed. Results are desorbed gold. Such recycled solutions often contain shown graphically in FIG. 2. It will be seen that the small amounts of sodium cyanide as a contaminant from gold desorption is highly temperature dependent and the stripping operation. Alternatively, the sodium cya that a temperature of about 80 C. or above is essential nide may be added to the stripping solution and may 5 for efficient desorption. Temperatures above about 90 result in somewhat increaseed efficiency in some cases.