Many lower grade gold deposits have become economical as gold prices have risen. At current prices, the mining companies have been trying hard to increase the efficiency of their mining methods. This is where the heap leach mining technique was born. Heap leaching has emerged as the safest and most efficient ways to mine the lower grade gold that was uneconomical in the past.
It is a flexible and continuously evolving extraction technology which gained popularity in the 1980’s when gold prices rocketed from $35 to $800. Developers and miners took advantage of the higher prices and opened mines with grades as low as 2-3 grams per ton gold. Heap leach mining methods are generally used on open pit mines with ore that is near surface and has been oxidized.
Heap leaching involves a series of chemical processes through which the gold deposits are piled onto a pad or “heap”. Then a leach solution using a diluted alkaline cyanide solution is spread over the surface of the deposit to leach the gold from this heap. The cyanide is not as dangerous, as one might think in this operation, as it becomes benign when exposed to the atmosphere. There are other chemical processes used in heap leaching, but the cyanide process is most common.
This process can take several weeks or months to extract the gold. And the leaching time can also increase if the heap leach pad becomes too compacted. The cyanide must come in contact with the gold particles and sometimes it penetrates very slowly. The low percolating of the heap can lead to the creation of channels, where the solution generally tends to accumulate and miss much of the ore. This is similar to ruts in a dirt road where rain tends to follow the same path once a rut is started Once a heap leach pad is badly compromised, much of the ore is never exposed to the solution and the gold is stranded on the pad.
How Does Heap Leaching Process Work?
Gold ores that are near surface are often oxidized by nature and can be leached within 1 or 2 months. The oxidation process is usually caused by exposure to water sometime in the sedimentary process of the earth’s surface. For instance, Nevada was a large lake millions of years ago, so you see a lot of oxidized gold there. Oxidation is nature’s way of breaking down the sulfide crystal structures that usually encapsulate the gold.
The mined oxide gold ore is often crushed and then stacked over a leach pad which is lined with an impenetrable polyethylene or polypropylene material. Some mines use “run of mine” ore with no crushing at all. This is cheaper, but often leads to the compaction problems discussed above. Next, the leach pad is soaked with the cyanide solution under strict atmospheric conditions. The goal of this step is to dissolve the gold (and sometimes silver) into the solution. The pad is stacked in layers or “lifts”. Depending on the material a lift can be 10 or more meters and the pad can reach several hundred feet.
Most miners use a simple drip irrigation method to wet the pad. This reduces the risk of evaporation and also provides an even distribution of the leach solution. The cyanide then enters the heap and leaches the gold. Gold loves cyanide. Once the leach solution dissolves the gold, it continues percolating through the lower lifts until it reaches the liner located underneath the heap. Then it drains into a storage pond. Most economical heap leach mines achieve 70-80% recovery of gold and 30-50% of silver.
How is the Gold Recovered from the Cyanide?
The gold rich solution is run through plant to recover the metals. Most common is an Adsorption-Desorption-Recovery Plant or “ADR” plant. The ADR plant uses a series of tanks loaded with carbon particles to extract the gold from the cyanide solution. Gold loves carbon even more than cyanide.
After leaving the ADR plant and “loading the carbon”, the solution is recycled to the heap to recover any gold precipitated in the leach solution and to reduce cyanide costs. If not recycled, the solution is passed to an industrial water treatment facility.
The biggest breakthroughs in heap leach mining for gold recovery have been in the development of methods for chemically desorbing gold from “loaded carbon”. Today, many options are available to the recovery plant designer and operator. Each method has advantages and disadvantages, which should be evaluated when deciding which process to use. These procedures must allow the carbon to be recycled for overall gold recovery to be economical.
If the ore (and solution) has high amounts of silver or copper, the Merrill Crowe recovery method is the superior plant option. These plants are more expensive and use zinc to recover the metals. Merrill Crowe is used when the overall volume of metals is just too great for an ADR plant to efficiently recover the more valuable gold and carbon use can skyrocket.
What is an Economical Heap Leach Mine Today?
As a rule of thumb, one gram per ton of ore can make a profitable heap leach mine. At a 75% gold recovery, it takes 41 tons of ore on the pad to recover one ounce of gold. However, all the material in the mine is not ore. Most mines will have 2-3 tons of waste material to 1 ton of ore. This waste must be taken to a separate dump. This is called the “strip ratio” and is a very important aspect in developing and operating a heap leach mine. If a mine has a 3-1 strip ratio, 164 tons of material must be mined and moved to produce one ounce of gold. If the strip ratio is low, gold grades as low as ½ gram per ton can become a mine.
This article contains the author's opinions. These are not investment-related recommendations. Do not consider the article as any type of commercial solicitation or an investment product offer.
About the Company
ELY GOLD ROYALTIES INC.(TSXV:ELY; OTCQB:ELGYF) is a Vancouver-based emerging royalty company with assets focused in Nevada and the Western US. Its current portfolio includes 33 Deeded Royalties and 21 Properties being sold to retain royalties. Their portfolio includes three producing royalties and is currently generating significant revenue.
Ely Gold’s royalty portfolio includes producing royalties, fully permitted mines, mines under construction and development projects that are being permitted for mine construction.