Michael Drozd, Vice President of Operations, Prophecy Development Corp.
Date April 10, 2019
Heap Leaching is a process that is not always understood. This is understandable coming from an industry that names its projects, Jackspile (New Mexico), Shitemaring (Utah) and Cerro Mojon (Nicaragua). For clarity, it should be noted that I have worked at all of the above colorfully named projects.
Heap leaching involves taking mined material and either loading it directly on a containment area or crushing that mined ore and loading that ore onto a containment area, under some circumstances an additional step of agglomerating (a step I will describe with more detail below) the crushed ore is added before loading onto the containment area.
Why would crushing the ore and adding it directly to a containment be preferable to than crushing the ore, grinding the ore and putting it in a tank leach in order to recover the metal being leached be a better option? The answer is cost. By taking crushed ore and placing it on the containment facility you can eliminate up to 90% of the processing costs. And since the disposal area of a heap leach (except for something called an “on-off” pad) is the same area as the processing area, you eliminate the need for a tailing facility; what you put on the heap stays there.
Saving cost sounds great, but how does it work? It is quite simple; the main motive force for the heap leach pad leaching is gravity. After the ore is crushed (or when the ROM is delivered to the pad) the material is stacked in layers called lifts. Once the lift is sufficiently completed to leach, a series of distribution pipes are added to the top of the lift and leach solution is added to the lift surface by means of drippers or sprinklers. These devices are the same as those used in the agricultural industry. The solution application is designed to add solution evenly over the pad at a surficial application rate equivalent to a precipitation rate of approximately 0.25” to 0.5” per hour. The leach solution contains the chemical required to leach the metals present and that chemical is called a lixiviant. In the case of gold and silver the lixiviant is cyanide and for copper, nickel, cobalt, or vanadium the lixiviant is sulfuric acid. The leach solution percolates down through the ore and the metal is dissolved in the leach solution. The pregnant leach solution (“PLS”) flows through the heaped material to the containment liner and the flows by gravity to the containment pond called the PLS pond. This PLS solution is then pumped to the processing area where the metals are recovered and the spent PLS is then designated barren solution for gold and silver or raffinate for the other metals. This solution is, then pumped back to the distribution system and the process starts again (your typical wash, rinse, repeat scenario). The solution is returned be recovered and then the pad is allowed to drain and then it is closed according to the procedures (“Closure”).
The process is relatively simple, which is exactly why it reduces costs so dramatically and is the point to using heap leaching on low grade ores. So what do you do, just load up a pad, throw some solution in the air, get an easy chair and watch as the metal rolls of the pad?
Intuitively this is true, but in order to make it reality there are technical aspects like making sure there is adequate solution percolation through the material, the ore is properly stacked at the proper angle for a stable pile and control the solution addition to ensure that the material does not saturate with solution and the solution flows evenly through the ore.
Over the years, the heap leaching industry has trained engineers and operators to understand the operating requirements to have a successful heap leach. These professionals and the new people coming into the industry ensure that the proper conditions are maintained for a successful heap operation. But as long as the proper lixiviant concentration is maintained the metals will be extracted from the ore and then recovered in the recovery facility.
Because proper percolation of the leach solution through the heap is important, the process called agglomeration was developed to take high clay ores that have low percolation rates and improve the material percolation to the equivalency of material that does not have clay (or silts) in the ore. Agglomeration is the process where a binder (cement to alkaline leaching and polymer for acid leaching) is added causing the fine particles to be bound to the coarse particle therefore increasing the material permeability. Many heap leach ores are located in high precipitation area or the ore was deposited during periods of high rainfall where the ore structure is changed to a clay. Clays are “finely-grained natural rock or soil that contains one or more clay minerals with possible traces of silica, metal oxides and organic matter.” Normally clay particles are considered to have particles 2 microns or less in size while silts are considered to have particles larger than 2 microns. These very fine particles decrease the materials permeability due to a phenomenon called capillary action. This is that the channels between the particles are so small that the water has trouble flowing through the space allowed. Agglomeration is used to increase the pore space of the material allowing free flow of solution through the ore.
From the information above we can see what is involved in a heap leach and why we would think that heap leaching is a viable technology for vanadium recovery, which utilizes an acid lixiviant.
Acid heap leach has been known for quite some time. Georgius Agricola in 1557 illustrated a heap leach for alum recovery surprisingly similar to present day heap leaching facilities. It has been shown that acid heap leaching has been used in Spain since 1700 for copper. The first gold heap leach is attributed to the Cortez Project in Nevada in 1969, but in 1967 over 200,000 tons of copper was recovered from leach dumps. The first copper dump leach can be traced back to 1921. Heap leaching and especially acid heap leaching has a long history in the mineral industry.
The viability of a vanadium heap leach is supported by the long copper heap leaching history. We know vanadium-bearing rocks from the Gibellini Vanadium Project (which is operated by Prophecy Development Corp. (TSX: PCY, OTC: PRPCF) are a viable heap leach material because we have used the well-established copper process vetting procedures and testing procedures to screen the projects material for heap leaching. The factors that show that heap leaching applies to the Gibellini rocks are:
- Sulfuric acid will dissolve the vanadium as crushed material;
- Polymer agglomeration (used in the copper industry) will allow the clays present in the crushed material to be controlled so the leach solutions will flow freely through the medium;
- Stability testing has been done on leached material from column testing and sufficient permeability and material stability has been maintained to a heap height of 200 feet;
- The PLS solution can be sent to standard recovery processes (solvent extraction followed by ammonia precipitation and then ammonia calcining from the precipitated material to produce a V2O5 product);
- The testing data has been collected by engineering companies and the process has been designed for an economic mining and processing of the Gibellini rocks.
The Gibellini rocks are relatively unique as a vanadium-bearing material as 50% of the vanadium is contained in acid soluble oxides as well as 35 to 40 percent of the vanadium is also contained in alumino-silicates and iron oxides that also breakdown with acid leaching. This unique combination of minerals gives the Gibellini mineralization its amenability to heap leach processing. These characteristics are similar to many of the black shale type vanadium ores of the world except for this unusually high vanadium content attributed to the oxide minerals. But otherwise the Gibellini is just the same as the other black shale deposits.
Standard black shale processing scheme is to mine the ore, crush it, grind it, filter it, roast the ore with salt and sulfuric acid (about 200 lbs per ton acid consumption versus the 70 lbs per ton that Prophecy’s Gibellini uses), tank leach the ore, do a liquid- solid separation of the solution from the ore, recovery the vanadium by solvent extraction, precipitate an ammonia compound and calcine that material to form a V2O5 product. The heap leaching process eliminates a majority of the capital intense equipment and the energy intensive recovery processes. This allows the project to treat a material that is 0.3% V2O5 while conventional projects require a grade of around 1%. Either processing scheme produces about 60 to 70% final recovery of vanadium. The main difference is time, with the standard process method takes about 4 days from start to finish while the heap leaching process take about 200 days, with the same types of minerals breaking down to supply the vanadium. The heap leach process just accomplishes that over a longer period of time.
Since the amount of capital investment is less for the heap leach and the amount of power, chemicals and manpower are less than a standard process plant, the mineralized grade can be significantly less for a heap leach project while still producing vanadium at a price that is equal or less than the standard processing facility.
Mr. Drozd is the Vice President of Operations at Prophecy Development Corp (TSX: PCY, OTC PRPCF) which is developing America’s first primary vanadium mine. Mr. Drozd specializes in metallurgy with 40 years of experience in the mining industry with firms such as Barrick and AMEC. Mr. Drozd has authored publications in gold flotation, gold processing, heap leach operations, cyanide detoxification, and carbon absorption technology. He also holds patents in molybdenum flotation, cyanide detoxification and vanadium recovery.
If you have any questions regarding this article, please email us at info@prophecydev.com
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