Expanded markets for copper in the past few years and a consequent search for new ore bodies have revitalized the widely known but seldom applied method of producing copper called heap leaching. This term should be differentiated from dump leaching in that the latter is applied to dumps of mixed oxide and sulfide ores (although either may predominate) that have been too low grade to beneficiate and have, therefore, been stripped off the higher grade underlying sulfide ores. Heap leaching is defined here as the process applied to oxide ores which have been mined solely for the purpose of leaching.
Although heap leaching does reduce capital expenditures, it also requires extensive test work and sound engineering to produce a successful operation. A company contemplating heap leaching must insist on a definite, well planned program of investigation. Adequate exploration is a necessity to assure that high acid consuming sections are not present in the ore body and test work must be done on drill cores to determine if acid attack on the host rock will result in physical degradation which would blind a leach heap. Items such as acid consumption, seepage and evaporation losses, leaching rates and anticipated ultimate recovery must be established.
The initial leaching rate of the ore might, therefore, be of more importance than one might at first believe if funds are limited. However, after leaching is established and copper is being produced, the initial leaching rate of the freshly mined ore is not of great importance to the cash flow of the operation as long as the period for recovering the majority of the leachable copper does not amount to six months or a year.
Recovery of copper during heap leaching, as stated above, cannot be accurately forecast by presently available technology. As a part of the testwork and an aid in predicting recovery, we believe a test leaching column 16 to 20 feet high and 4 to 5 feet in diameter offers the best possibilities for most accurate predictions.
Capital charges for heap leaching and cementation operations are relatively low and, therefore, we must look very carefully at operating costs. A minor portion of these are relatively fixed in amount (labor, power, water, supplies and scrap iron) while the major costs can vary substantially (mining and sulfuric acid).
Mining of oxide ore for heap leaching can represent a very substantial cost, particularly if a good portion of the ore is hard enough to need blasting, however, the physical nature of these ores generally permits mining by ripping and scraping thus taking advantage of these low cost methods of earth moving. In order to keep capital investment to a minimum, it may be advantageous to contract the mining operation but in this case, extra care must be taken so that overburden or high acid consuming ore is not put on the leach heaps.
The largest cost variable in heap leaching is the amount and particularly the cost of sulfuric acid. The amount of acid used is, of course, dictated by the mineralogy of the ore but it would appear that at the present time the cost of acid depends greatly on local scarcity or surplus.
A discussion of heap leaching economics would not be complete if it ended at the production of cement copper which still is subject to smelting charges before it may be sold. To complete the picture, the production of cathode copper through solvent extraction (liquid ion exchange) and electrowinning should be thoroughly investigated. Investing in a smelter for a small tonnage of copper concentrates obtained from conventional milling of a small sulfide ore body would, of course, be out of the question.