Several research groups have spent considerable time investigating coarse ore leaching. Coarse ore leaching includes both heap and dump leaching. The terra “heap leaching” will be used in this paper to mean any type of coarse ore leaching. The results of their work are generally in agreement: the progress of leaching can be described by the shrinking core model and its associated mathematics.
A software package has been developed to preform the calculation necessary for both gold heap leaching and copper heap leaching. This software program is called LEACH.
Data Entry Module
The data entry module is designed to create a disk file containing values for all the input variables needed by the calculations module, There are two methods of creating this data file. The first is to enter the value for each variable from the key board at the prompt identifying the variable. The second is to retrieve an existing input file and modify the values for those variables which you want to change.
After a data file containing all the necessary information has been created by the use of data entry module, the calculations module is used to calculate the performance of the heap. The calculations module divides the heap into 10 equal vertical segments and calculates the recovery for each of the particle size fractions specified by the data file, the preg solution metal content, and the reagent remaining in the leach solution as the solution passes to the next lower heap segment.
It should be emphasized that the results shown in all the examples are for demonstration purposes only and the response of an ore’s leach performance to changes in the operating parameters are specific to each ore and a function of the “Leachability” and reagent consumption of the ore.
The time of leaching is measured from the time at which leach solution is started on the top surface of the heap.
LEACH is used in two stages. First, the “Leachability” of an ore is determined from the results of a laboratory column test or a field test heap. Second, this “Leachability” is used together with the proposed heap leaching operating parameters to calculate the recovery/time curve and preg grade/time curve.
At times some of the ore characteristics or operating parameters may have been entered incorrectly or they may have a large experimental error. This may cause difficulty in arriving at a numerical value for the “leachability” for which the predicted curve matches the experimental curve. Knowledge of the affect each of the variables has on the recovery/time curve will help to identify what your next “guess” for the numerical value for the “leachability” should be or if one of the values used for one of the other input parameters should be examined more closely.
The retention time of the solution in the heap has not been sufficient to allow all of the reagent to react with the ore. The preg grade now begins to drop with time and the residual reagent concentration of the leaching reagent increases with time. During this phase of leaching the rate of leaching is controlled by diffusion of the leaching reagent through the leach shell around each particle. The rate of leaching is now a function of all the variables listed above plus the “leachability”, the size distribution of the ore in the heap, the ore grade and the percent saturation, percent voids.
The position of break point (both the time and recovery at which it occurs) between the two segments of the recovery/time curve is a function of all the variables and although it cannot be simply calculated its movement can be predicted as the ore characteristics or heap operating parameters are varied.
The curved portion of the recovery/time curve, designating that diffusion is controlling the metal production from the heap, is more difficult to characterize. It starts asymptotically from the straight line portion of the curve and approaches some ultimate recovery value slightly faster than asymptotically. This ultimate recovery value can be 100 percent but more usually is lowered by an insoluble metal content due to mineralogy or encapsulation of the mineral in a silicate completely impervious to the leach solution.
