SME 2006 Papers on Heap Leaching

SME Conference 2006
Heap Leach Pads

Jack A Caldwell

December 2006

Heap leach pad design, operation, and performance are the topic of six papers from the SME 2006 Meeting in St Louis. The big issue this year appears to be: do you or do you not force-aerate your pile? Here is my summary of the papers and a brief recap of their conclusions regarding the question of aeration versus non-aeration.

History and Development of Aeration in the Cerro Verde Mine Peru. G. Velarde describes extensive field and laboratory testing of leaching operations at the Cerro Verde Mine. He concludes that, on a site-specific basis, aeration may be beneficial and in particular may facilitate the use of greater lift heights.

History of Forced Aeration in Copper Sulfide Leaching. W. J. Schitt describes the history, theory, practice, and case studies of forced aeration of copper sulfide leaching operations. His conclusions: forced aeration boosts sulfide leach rates where the dump design impedes natural air flow; installation of an HDPE pipe system beneath a new pad is cheap insurance—if you need the air, the system is there, but if you do not, the cost was minor; design of a dump to promote natural air flow is the best and most cost-effective approach—avoid the need for force aeration if you can; blowing too much air into a pad can be counterproductive—the air coming out removes the moisture and the heat needed to keep the process going.

The Rewards of Patience. R.E. Scheffel concludes: Time rewards the patient. Based on a detailed examination of seven copper heap leach operations, he further concludes: “The common feature of all the commercial operations is that all can achieve 80% to 90% recovery of the acid and ferric soluble mineral content. This level of extraction is achieved by reaching a proper compromise on crush size and ample leaching time.” The author questions the conventional wisdom of forced aeration; his statement is a masterpiece of diplomatic-speak: “The results of these comparisons suggest it is highly questionable if forced aeration for ore grades up to 1.0% to 1.5% chalcocite provides a quantifiable advantage to extraction rates or terminal levels—at least for a leach cycle time necessary to achieve 85% recovery of the oxide and supergene mineral content under conditions of non-homogeneous solution wetting and/or flow.”

Modeling Chalcocite Leaching. G. Cooper and S. Dixon from the Phelps Dodge Mining Company, Safford, Arizona describe the use of the PD/PERI Heap2D Model to evaluate best management practices for operating a chalcocite leach pad. With the model they simulate and evaluate variations in raffinate application rate, aeration rate, raffinate iron grade, acid cures, lift height, crush size, bulk density, air injection spacing, and raffinate drip emitter spacing. Their results quantify the influence of crush size, bulk density, segregation and other operational factors on the heap’s air permeability hence air circulation hence leach efficiency. They conclude that for their operations aeration is necessary. It would be nice, I suspect, to have an analysis of your operations using their model before committing to the expense of aeration of you pad.

Geochemical Profiling of a Sulfide Leaching Operation: A Case Study. Guzman, Scheffel, and Flaherty note: (a) You don’t get it if you don’t wet it; (b) You can’t manage it if you can’t measure it; and (c) If we don’t have data from a particular process, all we have is our opinion. The theory and data presented by the authors justifies their contention that it is possible to manage heap leach operations as industrial reactors rather than as simple black boxes and to concentrate on the key issue of delivery of the reagent to the ore.

And taking a look at academic pursuits:

Bioleaching and Electrobioleching of Sulphide Minerals. Connor, Cho, and Yang, all from West Virginia University report on bench scale testing of electrobioleaching of chalcopyrite, sphalerite, and pyrite. I quote: “..an increase in Fe(II) concentration, which is a nutrient of the bacteria, by electrochemical reduction of Fe(III) would increase the bacterial population and would in turn increase the leaching of the material.” Electrobioleaching seems to work with the chalcopyrite and pyrite, but not with the sphalerite. I leave you to read the paper to glean the reasons for this performance. Suffice it to note that the authors conclude that the slow rate of electrobioleaching of chalcopyrite renders it useless for commercial application. Conversely they hold out hope for pyrite: “The application of electrobioleaching of pyrite could be extended to applications like coal desulferization and pretreatment of refractory gold ore in heap leaching where fine gold particles are entrapped inside the pyrite particles.”

Not from the SME St Louis Conference but included here for completeness is a summary of another paper by W. J. Schlitt. (See the February 2006 issue of Minerals & Metallurgical Testing published by SME.)

In the early 1990s Kennecott’s Bingham Canyon operation included 350 Mt of low grade material containing at least 0.15% Cu present as chalcopyrite. Milling would have displaced higher grade copper ore, so the decision was taken to carry out a heap leach test involving a full-sized heap of 960kt using truck-hauled, run-of-mine ore and an ancillary solvent extraction-electrowinning (SX-EW) pilot plant.

The test heap was located atop an old leach dump which was graded and capped with 150 mm of calcium-treated clays and fines containing electronic moisture sensor to detect leaks. Another 150-mm layer of minus 6-mm fines screened from run-of-mine materials was placed over the instrumented layer. Each of the three cells was lined with a different liner: 1.5 mm LDPE; 0.5 mm PVC; and 1.0 mm HDPE. 100-mm diameter corrugated and perforated HDPE pipes were laid at 15-m centers. The pipes were covered with 450 mm of minus screened quartzitic drainage material grading plus 6 mm to minus 50 mm. In-heap aeration pipes were placed over the drainage layer and cover with 1.5 m of run-of-mine ore before placement by truck dumping of the 16-m of heap materials.

Before construction of the second lift, the first lift was graded to slope and five evenly spaced shallow channels cut and lined with HDPE and two 50-mm diameter pipes. The intent of these was to intercept solution from the second lift before it percolated into the first lift. The height of the second lift varied from 11 to 14 m.

The paper provides comprehensive data about the leaching, the recovery, and the performance of the plant. Here is a summary:

· At least 30 percent of the copper was recovered.

· Supplemental forced aeration did not improve leach pad performance—there was sufficient natural convective airflow to provide oxygen.

· Heap temperatures ran at least 30 degrees Celsius above ambient.

· The pregnant leach solution underwent little attenuation as it percolated from the upper lift into the lower lift.

· Copper recovery from the first lift continued even after placement of the second lift.

· The SX-EW plant performed very well—achieving 95% extraction efficiency and producing high-quality cathode copper that was sold to offset the cost of the test.