After more than 30 years of application, heap leaching is by far the most attractive process for recovering gold at low operating costs. In gold recovery process, the largest plants and the greatest progress are found in heap leaching and the first step to evaluate the application of the process is to perform percolation or column tests. Technological improvements to meet the challenge of lower gold grade ores have taken the wholly predictable path towards improved testing procedures. Gold and silver can be recovered at acceptable levels by heap leaching and column tests have been the best route to evaluate the amenability of the ore to the process. Basically the column body is plastic tube, 3-6 m length with an inside diameter of 10-30 mm. the column must be of sufficient thickness to resist the ore weight. At the bottom of the column is placed a perforated plastic disc. The weight required to perform column leach tests is 100 to 500 kg. Column leach tests evaluate several variables such as particle size, ¼”- 2”; cyanide strength, 0.5-2.0 g/L; solution flow rate, 8-15 lt/hr/m2 and pH, 10-11.
Cyanide solution at alkaline pH irrigates the material by a dropping system. The solution dissolves gold form the material and transports it down and out of the column. The pregnant solution is passed through activated carbon to recover gold and prior to recycle the solution more cyanide is added to keep the initial solution cyanide strength. If everything is ok, results column leach results can be scaled without problems and the heap leaching design process can be performed. It is important to mention that column leach tests are heap leaches at lab scale. It has been noted that several comparisons of results from heap leaching operations and laboratory tests are indicated that real operation have a slightly different performance, but this difference can be predicted from laboratory test results. Heap leaching operations need three to seven times to reach laboratory results.
Column Leach Tests