Heap Leach Design & Operation of Heap Leaching in Gold or Copper
This heap leaching section seeks to provide an overview of important factors involved in the implementation of heap leachi echnology as a vehicle for gold extraction from its ores. To this end, discussions of the various elements important to this process are presented here. Subjects covered in this part include a history of the process, basic chemical reactions responsible for the process, characteristics of ores amenable to heap leaching, location of some heap leaching projects, heap leach components, methods and economic considerations. Most of these subjects are treated at much greater length in subsequent parts.
History of the heap leaching process.
Nowadays precious metal heap leach technology has largely developed over the past decade, although the principles of heap leaching per se, as well as those of gold extraction through Cyanidation have a long history. Nevertheless, it is in the last 20 years that heap leaching has developed into an efficient method of treating oxidized gold and silver ores. It has proven to be both an efficient way to extract precious metals from small, shallow deposits, as well as an attractive way to treat large, low grade, disseminated deposits. Heap leaching has several advantages compare to conventional milling (i.e., crushing, grinding, and agitating leaching). In general, these advantages include simplicity, lower capital and operating costs, shorter start-up times, and less intensive environmental regulatory concerns. A possible disadvantage of currently existing heap leaching technology is a potentially lower percentage of metal extraction being obtained from ore than would be the case with conventional milling.
The actual principle of heap leaching has a long history, as inferred previously. For example mines in Hungary recycled copper-bearing solutions through waste heaps in the mid-sixteenth century, and Spanish miners percolated acid solutions through large heaps of oxide copper ore on the banks of the Rio Tinto in 1752. By 1900, leaching operations were employing such techniques as leach/rest cycles to maximize copper recovery. Copper dump leaching is currently practiced worldwide for low grade ores. Additionally, heap leaching using both acid and alkaline solutions has been practiced by uranium producers since the late 1950’s.
Gold extraction through cyanidation became a commercial process through the pioneering work of two Scottish brothers, Doctors Robert and William Forrest, in conjunction with John MacArthur, a self-taught chemist. These men experimenting for years in a homemade laboratory in Scotland sought to find an improved process for recovering gold from complex ores. They succeeded in their quest and obtained a British patent. Subsequent U.S. patents 403,202 and 418,137 were issued to the three men in 1889. The process covered by these patents involved the agitation of pulp in the presence of air followed by precipitation with zinc from a separated gold cyanide solution. This process is considered milestone in the metallurgy of gold due the fact that a chemical process for treating gold ores had been discovered.
The contributions of MacArthur and the Forrest brothers were not without legal controversy, due to the lack of enthusiasm by gold industry leaders with regard to royalty payments to MacArthur. A very long and expensive court battle led to disallow the patents and saddled the inventors with litigation c osts of about $ 1 million. Although the MacArthur and Forrest brothers were deprived of financial gains from their discovery, they are considered to have provided a valuable advancement in the technology of gold processing.
Heap leaching of precious metal ores using cyanidation process was originally suggested by the U.S. Bureau of Mines in 1967. The first commercial application of heap leaching occurred in the late 1960’s by the Carlin Gold Mining Company in northern Nevada. Cortez Gold Mines started the first large scale operation in the early 1970’s by leaching marginal grade ore.
In mid-1970’s, heap leaching technology was improved in order to handle low grade, clayey deposits. The improvements, called agglomerating-heap leaching were prompted by increased exploration for low grade deposits as the price of gold increased dramatically. Many of the deposits discovered could not be processed by conventional heap leaching techniques because the clays or fines generated during crushing impeded uniform percolation of solution through the heaps of ore. Agglomeration-heap leaching technology is applicable to most ore, wastes, and milled tailings.
The results of the technological improvements which have occurred throughout the 70’s and into the 80’s can be seen in dramatic production level increases. By the mid of 80’s production of gold from heap leaching had increased to over 30 percent of total U.S. gold production from an estimated 6 percent at the beginning of 80’s.
As was mentioned previously, large low grade deposits can be processed using current heap leach technology. Gold deposits with grade as low as 0.02 oz/ton (0.65 g/mt) are currently being processed. It is the ability of the recently developed technology to process such low grade ores that is responsible, in large part, for the increased production rates discussed above. Figure 1.1 shows a plot of average ore grade as a function of ore reserves for selected gold ore bodies.