From Cyanide Leach Solutions
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It has been known that gold in a cyanide leach solution would adsorb to activated carbon for over 70 years, and many papers have been written on the subject. Carbon, as a media of recovery for gold from leach solutions, did not gain wide acceptance in the gold industry until the 1970's, with the explosion of heap leaching of vast quantities of low grade gold ores, and subsequent production of many gallons of pregnant leach solution. It is now the most common method used for the recovery of gold from a cyanide leach solution. Two primary methods, Carbon In Pulp (CIP) and Carbon Columns probably comprise the most widely utilized techniques for extracting gold, now days. In CIP, the carbon is mixed with the cyanide leach solution, and agitated in the leach tanks, and agitated with the ore while the gold is being dissolved. This assures a rapid interface between the carbon and the gold loaded cyanide solution. Granular, hard carbon is used, in the size range of 10-16 mesh. The carbon is then removed by screening, across a carbon screen, allowing the liquid and finely ground ore (-100 mesh) to pass through easily, and retaining the carbon on the screen. The carbon is then generally subjected to a heated solution (200 deg to 250 deg F) of (1%) sodium hydroxide and (0.1%) cyanide and the gold is re-dissolved and removed from the carbon surface. At this point, the NaOH/CN solution is sent to electrowinning cells, where the gold is plated onto stainless steel cathodes. The gold is removed from the cathodes by either using a high pressure water spray, sometimes shaking the cathode, and in some cases, the stainless steel cathode is placed into a melting furnace and heated to 2,000 degrees F, to melt the gold from the cathode. The stainless steel cathode, of course, does not melt at this temperature, but such temperatures reduce the life of the cathodes, and most will wash the gold "sponge" from the cathodes. The fine, powdered gold is then melted and poured into dore bars. 
The carbon in pulp method is only used where tank leaching is utilized for the recovery of gold from the ores and this is not the case in most large gold mines today, since heap leaching is the lowest cost method for recovering gold from ores containing from a few grams to 5 or 6 grams per ton. If the ore is a high grade, it may be feasible to tank leach it. By high grade, it would probably be in the range of ¼ ounce per ton (Au) range or greater. The cost of leaching by tank is considerably greater than leaching in a heap. This is due to the requirement to crush the ore to -100 mesh (at least), and to provide mechanical equipment to store and agitate the ore for extended periods of time (4-6 hours). Occasionally, carbon columns will be used with tank leaching operations, also, especially if the mine has a dual leach system, heap leach of low grade and tank leach of high grade. Carbon columns consist of large columns (5 to 8 ft diameter) and 15 to 20 feet in height, charged with activated carbon. The minimum load of carbon rule of thumb is one pound of activated carbon per 0.1 ounce of gold present in the solution. Usually, it is several pounds per 1/10 ounce, though, as the cost of carbon is relatively insignificant compared to timely recovery of gold. 
The pregnant cyanide solution flows through the carbon columns, which are generally situated in step fashion, so that the overflow from the highest column will gravity flow to the next column, alleviating the requirement for pumping. The gold adsorbs onto the surface of the carbon. The loaded carbon is periodically removed from the columns, and sent to the stripping circuit. The carbon is then generally subjected to a heated solution (200 deg to 250 deg F) of (1%) sodium hydroxide and (0.1%) cyanide and the gold is re-dissolved and removed from the carbon surface. At this point, the NaOH/CN solution is sent to electrowinning cells, where the gold is plated onto stainless steel cathodes. The gold is removed from the cathodes by either using a high pressure water spray, sometimes shaking the cathode, and in some cases, the stainless steel cathode is placed into a melting furnace and heated to 2,000 degrees F, to melt the gold from the cathode. The stainless steel cathode, of course, does not melt at this temperature, but such temperatures reduce the life of the cathodes, and most will wash the gold "sponge" from the cathodes. The fine, powdered gold is then melted and poured into dore bars. Any silver present in the ore, will also be leached in the cyanide solution, and will be present in the dore bar, as well. Most mines do not have the refining capacity to separate the silver from the gold and the bars are sent to a commercial refinery for producing the 99.99% gold and silver ingots that are sold on the commercial gold market. Once the carbon has been stripped, it can be reused after recharging it in a kiln, by heating it to around 1300 degrees F. The carbon is then cooled, screened to remove fines and is ready for reuse. Fines are not desirable in carbon circuits, since they will escape through the carbon screens and end up in the leach, capturing gold that will not be recovered. For this reason, high quality and hard carbon is generally utilized for these applications. Generally, the size range of the carbon is 10 mesh to 16 mesh and almost always no finer than 20 mesh. Coconut shell carbon is generally the preferred carbon for use in recovering gold from leach solutions. |