heap mining efficiently leaches lower grade gold from s
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 1980s 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.
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 earths surface. For instance, Nevada was a large lake millions of years ago, so you see a lot of oxidized gold there. Oxidation is natures 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.
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.
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 gram per ton can become a mine.
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.
heap leaching a stack of gold - miningfeeds
Heap leaching is used to extract gold, copper, silver, uranium, and iodine. This method was first employed to extract copper at the Bluebird mine in the US in the early 1960s, and then by several gold mines in the western parts of the US a few years later. Since then, heap leaching has been adopted successfully by many mines across the world.
Heap leaching occurs after the mining and crushing of low-grade ores, usually in an opencast mine. High-grade ores and ores not amenable to cyanide leaching at coarse particle sizes require further processing to recover thegoldvalues. These processing methods can include further grinding, concentration, pressure oxidation, and roasting, which is used to treat these ores to expose the gold particles prior to cyanidation.
The crushed ore undergoes an agglomeration stage, after which the agglomerated ore is deposited onto the heap leach pad. The heaped ore is irrigated with a lixiviant (a liquid medium) to dissolve the metals and generate the leachate. The lixiviant will depend on the target metal being extracted.
The pad is compacted and then lined with a high-density polyethylene membrane, which prevents toxic compounds and elements (such as cyanide and the leachate solution) from entering the groundwater system.
The leachate is collected in a pond or tank, and it is referred to as a pregnant or value-bearing solution. The solution is then processed to recover the metals. In gold operations, recovery is affected through carbon adsorption or the Merrill-Crowe process. The barren solution, together with additional lixiviant, is recycled back to the heap.
Heap leaching can take anything from a couple of months to several years. In the case of gold recovery, heap leaching generally requires 60 to 90 days to leach the ore, compared to the 24 hours required by a conventional agitated leach process. Gold recovery is also usually only 70% compared with 90% recovery in an agitated leach plant. Other metals, such as copper, use solvent extraction and electrowinning to extract the target metal from the solution.
According to Phil Bundo, process engineering director at Senet, mines need large reserves, a large resource, and significant real estate if they want to employ the heap leach method. To build a big heap and accommodate all the associated equipment, a large space is required; and to fill that space, a mine needs to produce enough ore, says Bundo. Bundo adds that climatic conditions like rain can negatively affect a heap leach operation, although this would not ordinarily be enough cause to discard heap leaching as a processing method. Resources and reserves play a much bigger role in determining whether heap leaching will be viable.
Heap leaching is not that popular in South Africa because it is more applicable to shallow opencast mining, and South African gold mines are mostly underground operations. The method is, however, being used on a large scale in the copper mines of South America and, according to Bundo, it will be used more and more on the African continent in future.
According to Bundo, there are other recovery methods, besides heap leaching, to consider when mining gold. These include gravity concentration, carbon in pulp (CIP), and carbon in leach (CIL). The methodology selected is a function of the mineralogy of the ore. If the gold is associated with oxides, for example, it can be amenable to heap leaching. The grade also plays a key role. Heap leaching is used for low-grade oxides, while high-grade ore (with or without oxides) is better suited to CIP or CIL methods, says Bundo.
Bundo explains that while heap leaching is not as costly as CIL or CIP, the recovery achieved is also not as efficient. CIP and CIL are costlier in terms of initial capital and operational costs, but they provide the benefit of high recovery, says Bundo. In cases where the oxides are amenable to heap leaching and cannot economically justify the construction of a CIL or CIP plant, then the operations start off with heap leaching to generate sufficient capital to finance the CIP or CIL circuits when mining more refractory ore.
It is also important to remember that if the gold is associated with sulphides or other minerals, heap leaching cannot be effectively employed as recoveries are generally low. In such cases, it is advisable to install a CIP or CIL system from the beginning, says Bundo.
The higher capital and operational investment in methods such as CIP and CIL are due to the more aggressive techniques used to recover the gold. A CIP process will, for example, involve crushing, milling, and agitator tanks. Heap leaching requires less aggressive techniques: a chemical solution is simply sprayed on heap ore. As the solution percolates through the heaped ore, it dissolves the gold. The solution is then collected and treated further by adsorption.
The heap leach stack must be porous enough to allow the solution to drip or to drain through the stack. There are potential recovery failures due to the inability to obtain the optimal flow solution. High clay content is achieved by agglomeration prior to stacking the piles.
Further research is underway to both increase the recovery of metals and reduce the risks of the solutions used and generated during the process. Heap leaching has the potential of extending the life of mine or bringing mines under care-and-maintenance back into production by reprocessing the tailing or fine-residue dumps.
Part of the preparation stage is known as agglomeration, in which an agglomeration drum is used. Although heap leaching is efficient on its own, its efficiency is greatly improved by adding an agglomeration drum. Agglomeration drums are also called ore drums, agglomerators, and heap leaching drums. Agglomeration is based on a rotary drum design that tumbles ore fines, in the presence of the leachate, through its interior to promote uniformity and to mix the leachate and fines. The agglomeration step happens after the ore is crushed, but before it is heaped.
Using an agglomeration drum to agglomerate the ore fines will ensure that the crushed ore particles are more uniform, making it easier for the leaching solution to travel through the channels between the particles to help maximise recovery (increased percolation). In addition, adding the agglomeration drum allows the leaching solution to mix with the ore fines. As the ore fines are agglomerated, the solution is sprayed throughout the drum and mixed thoroughly with the ore fines, which ultimately makes the process more efficient.
The agglomeration drum is the start of the heap leach stacking process, which ends at the leaching pad, says Theo Winterbach, mechanical and materials handling manager at Senet. From the agglomeration drum, the material is transferred via an intricate conveyor system to the heap leach pad. The heap leach pad covers a large area and the product has to reach the entire pad. The system thus requires multiple conveyors to transfer the material to the furthest point of the pad, says Winterbach. The agglomerate should be transferred as smoothly as possible, as it is important to keep it intact. One benefit of having multiple and mobile equipment is that a company will not have to lay out all the capital in the beginning. Once the team has decided where the pad will be located, they can acquire only the necessary equipment until production starts ramping up. The added advantage is that the mobile equipment allows for a certain amount of flexibility; therefore, the entire system can be moved to ensure that the conveyors feed the stacker at the end of the chain, says Winterbach. A typical heap is between eight and 10 metres high. The normal flow of material is from the agglomeration drum to the grasshoppers and then to the linear index conveyor, which delivers it to the stacker machine.
The linear index and stacker machine need to move backwards as the stockpile is stacked in preparation for the leaching. At times (depending on the size of the heap leach pad), the entire set of machines has to be fully retracted and repositioned to a new position on the heap leach pad before the process is repeated. When the entire system needs to be adjusted, front-end loaders or other applicable equipment can be used to move the equipment, as most of the equipment, including the stacker, is wheel or track driven. A combination of wheels and tracks is used to minimise the bearing pressure on the pad.
The stacker can slew and move backwards and forwards in a linear fashion. Heap leach pads require special preparation in construction. According to Bundo, a pad should slope by one or two degrees so that the solution can gravitate to the pond, and there should be sufficient aggregates underlying the surface. Moreover, the high-densitypolyurethane(HDPE) liners that cover the surface are critical to prevent the chemical solution from seeping into the groundwater. Special collection pipes are installed underneath the pad to direct the solution to the relevant ponds.
For Breton Scott, managing director of Bowline Professional Services, the most important advantage of heap leaching is that it lowers the capital and operating expenses relative to other traditional methods like flotation, agitation, and vat leaching, especially where low-grade ores and tailings are present. It also has a potentially rapid payback period.
Heap leaching further eliminates some environmental concerns and restraints. The main benefit, in terms of the environmental impact, is that it requires less energy and water, says Scott. Moreover, the method has uncomplicated design and equipment requirements, and the construction phase is a lot faster than other treatment methods.
Although Scott says that the heap leach method is not seriously affected by climate, he mentions that a lower efficiency has been noted at low temperatures. High rainfall areas may also dilute the solution, requiring additional monitoring, he says.
The risks associated with heap leaching are mainly related to environmental concerns, should the pad construction process not be done correctly from the design stage. Potential issues with the regional water balance are highlighted as a risk, along with the possible exposure of the solutions used to the surrounding areas. Heap leaching does, however, have a much lower potential of acid mine drainage. The costs associated with pollution control and closure efforts are one of the main continual expenses in such operations.
The drilling of water-monitoring boreholes and regular testing of the groundwater by an accredited water-quality laboratory would be required if the heap leach method is used, says water laboratory analyst, Ben Steyn. Tests would generally include pH, dissolved solids, and heavy metals.
The biggest question a mine needs to ask itself is whether it has an ore body that is amenable to heap leaching. Senet prefers to get involved in a project from the test work phase, which enables us to prove that heap leaching, as a processing method, will work for the project. We not only consider heap leaching, but also all the other options available. It is always a trade-off between the capital investment and recovery, Bundo concludes.
Leon specializes in African affairs and doing business in Africa, and has been writing about mining in Africa for 8 years. He was born in Johannesburg, South Africa, and has traveled Africa extensively, especially southern Africa. He has a BA degree with a specialization in African studies and an honours degree in Africa Politics. He also have a national diploma in Nature Conservation and an honours degree in Environmental Management. It is is passion to promote business in Africa and I can assist companies that are interested in doing business in African countries.
heap leach: mining's breakthrough technology
According to the World Economic Forums Mining & Metals scenarios to 2030 report,global population growth together with upward trends in urbanization and industrialization, particularly in emerging economies has led to a strong increase in demand for commodities from the mining and metals industries. This growth is highly likely to continue and will place pressure on the demand of resources.
While demand is expected to rise steadily, supply has not always kept pace. Grades of metals in ore of metallic deposits decreased significantly over the past decades (Figure 1). World class deposits discovery rates are falling and a growing number of new mines use higher-cost and technically challenging underground mining methods.
All these factors, combined with growing resource nationalism, global water supply issues, as well as stricter environmental and other permitting regulations, raise great concerns over the sustainability of future supplies of primary metals.
Under these circumstances, mining companies are proactively seeking advanced technologies that could allow them to reduce costs and increase performance. Producers are also looking for ways to gain access to resources that at present are considered abandoned or uneconomic.
Heap leaching (HL) is a flexible and constantly developing mineral processing and extraction technology that is gaining popularity and recognition for existing miners and developers. HL has solid advantages over traditional metallurgical methods, where economically feasible options have become limited.
Due to abovementioned economic benefits, the number of HL operations has experienced an impressive and sustained growth over the lastdecades, with precious metal operations benefitting the most from the technology.
In 2014 about 150 major gold-silver mines worldwide utilized HL technology and recovered about 15 million troy ounces of gold, roughly 17% of global gold production (~89 million troy ounces, according to USGS data).
With 970,000 troy ounces of gold recovered in 2014, Newmont/Buenaventuras Yanacocha mine is the leader in HL of gold, followed by Barricks Veladero (722,000 troy ounces of gold) and Lagunas Norte (582,000 troy ounces of gold) operations (Table 1).
In 2014, more than 50 major HL-SX-EW operations worldwide recovered approx. 3 million tonnes of copper, which represents roughly 16% of total copper production. With about 327 kt produced in 2012, CODELCOs Radomiro Tomic mine is believed to takes first place in terms of HL copper output, followed by BHP Billitons Escondida and Spence mines (Table 2).
As can be seen in Table 2, the five biggest copper HL production facilities are located in Chile. There is no coincidence, since this is the region where the worlds biggest low-grade porphyry copper deposits are located.
Since 2005, the following notable gold and copper HL operations have been commissioned: Veladero (Argentina), Lagunas Norte (Peru), Kisladag (Turkey), Los Filos (Mexico), Copler (Turkey), Porvenir (Mexico), Karma (Burkina Faso), Caserones (Chile), and many others.
It is important to note that the mining industry has managed to implement a number of important advancements which allow producers to push HL boundaries to new more challenging environments. For example, there were new HL operations successfully commissioned in recent years throughout the world aimed to treat material accumulated in past decades in tailings and waste management facilities.
Performance indicators shows stunning success of these kind of operations so far, with 62.4% EBITDA margin achieved at Kounrad HL waste dumps facility in Kazakhstan, and a cash cost of $4.29 /oz Ag at the Parral tailings HL project in Mexico.
HL technology has all the chances to succeed in the future, with plenty of opportunities, including HL application to extract massiveamounts of precious and base metals sitting in abandoned tailings and waste management sites. Hopefully, recent developments will also allow theuse HL to process primary sulfide ores. This would result in a dramatic expansion of the technology as ~80% of total ore types are sulfide in nature.
Heap Leaching, at first sight, seems to be a relatively simple technology, but is in fact, quite a sophisticated process. Many disciplines and specialists have to be involved for HL to succeed, including mining engineers, geotechnical engineers, geologists, environmentalists, analytical specialists, processing chemists, metallurgists, hydrologists and other specialists.
The upcoming Heap Leach Solutions 2015 Conferenceto beheld September 12-16, 2015in Reno, Nevada, is a perfect opportunity for all interested stakeholders around HL technology to get together and discuss recent developments, insights and trends in HL.
IntelligenceMine is global mining market intelligence for Researchers, Investors and Suppliers. Get access to more than 45,000 company and property profiles, a powerful multi-faceted search with comparative result grids, sorting and download capabilities, an online interactive mapper and much more. Find out more at www.IntelligenceMine.com.
Good summary of HL features. Downsides of the process are 1) low predictability of process performance as hidden factors abound in comparison with milling- CIL/CIP, 2) lower recovery rates not good for high-grade material, 3) closure plan, which tends to be expensive, especially in rainy areas (reshaping of slopes, rehandling of material, long-term water monitoring, etc), and 4) larger footprint required (land control, social impact)
As any other technology, HL has its own pros and cons. A the same time, if you do all pre-production steps (all laboratory tests, trial leaching tests, etc.) rigorously, you can, at least, minimize most of the issues related to HL. Thank you for you feedback.
I am not an expert but what does less environmental concerns mean? When the operation finishes they just bury the stuff in the ground and leave all the chemicals in there. After year all these mines will be surrounded of heaps full of chemicals. Isnt it so?
how gold-mining operations benefit from heap leaching extraction
Building and permitting a mill to process mined resources can take at least a decade, not to mention a large outlay of capital. Heap leaching is a hydrometallurgical technique with lower operational costs than more conventional processing technologies. The technique offers gold producers a user-friendly extractive solution with the ability to significantly improve recovery rates and fast-track a property into production. This well-proven and cost-effective approach to precious metals extraction has been used by majors including Barrick Gold (TSX:ABX,NYSE:GOLD) and Newmont (TSX:NGT,NYSE:NEM), and is increasingly being considered both in the design of new mines and the expansion of existing operations.
The heap leaching process for precious metals typically begins with low-grade surface ore that has been separated through crushing. To maximize recoveries, this crushed ore is then agglomerated prior to leaching. This involves mixing the ore with lime or portland cement to produce larger masses of material that are more uniform and easier to leach than fine particulate matter which can clog and slow down the flow of solution as it percolates through the heap. Once the agglomeration stage is complete, the material is then spread over high-density polyethylene membrane-lined leaching pads specially designed to prevent contaminants from entering the soil and groundwater. Next, a leaching solvent such as sulfuric acid or cyanide is sprayed over the ore, dissolving the gold and silver as it passes through the heap. This creates a solution pregnant with valuable minerals that can be recovered via carbon absorption or the Merrill-Crowe process.
Gold projects that host near-surface mineralization associated with low-grade oxides that are also capable of supporting a large-scale mining project are more amenable to the heap leach process. The process can also be used to recover economical amounts of valuable metals from tailings and waste stockpiles. High grade, sulphide gold often requires more sophisticated processing techniques with more expensive equipment. In deposits that contain both oxides and sulfides , heap leaching the oxide resources can move a project to cash flow at a quicker pace and generate the capital required to finance the more expensive processing facilities required to tackle a sulphide orebody.
Exploration companies like CANEX Metals (TSXV:CANX) have the potential to leverage on heap leaching extraction systems as a low cost mining technology. CANEX is currently developing a new gold discovery at its flagship Gold Range project in Arizona. Were right on the edge of the discovery curve. We have sampled widespread gold mineralization over a 5 by 3 kilometre area. We have mapped key controlling structures with geophysics and identified several targets across our Gold Range property. Our current focus is on the Eldorado oxide gold target which has a potential size of 200m x up to 3km. While it is early days, RC drilling to date is giving us some good bulk tonnage grades with good continuity. The preliminary metallurgical (cyanide soluble gold results from Eldorado) confirm the potential as a bulk tonnage heap leach target and further de-risk the exploration concept. The average cyanide soluble gold value of 79.5% compares very well to current heap leach producers and development projects in western North America. Were really excited to see this opportunity develop as the market is in need of high value, heap leach, low-cost mining targets, said CANEX Metal CEO Dr. Shane Ebert.
The World Gold Council has reported that new gold discoveries have declined over the past three decades and the average grade of new discoveries is also in decline. In a world where new gold discoveries are harder to find and gold grades are declining, heap leaching technology offers miners the ability to recover more gold from lower grade material at a lower cost. Heaping leaching can improve the economic viability of new projects, breathe new life into old mines and extend the life of existing mines.
Northern Vertex Mining (TSXV:NEE) owns and operates the nearby Moss gold mine, currently the largest precious metals mine in Arizona with open pit mine and heap leach processing. Argonaut Gold (TSX:AR) has successfully developed and operated several open pit heap leach mines. The companys latest project is the Nevada-based oxide gold heap leach project Florida Canyon.
Heap leaching has proven successful for large-scale use in gold projects for decades. Heap leaching technology was a major breakthrough because it significantly reduced gold recovery costs for low-grade but high-tonnage Carlin deposits such as those found in Nevada, according to Kitco contributor Jack Graham. The heap leaching technique was first used for precious metals recovery in 1969 at the Cortez gold mine where it proved highly-efficient at improving recoveries from near-surface oxidized ore, notes Graham. Today, Barrick Golds Cortez property includes the Pipeline complex open-pit operation and the recently discovered Cortez Hills deposit, which is Nevadas longest-running gold mine.
This INNSpired article is sponsored by CANEX Metals (TSXV:CANX). This INNSpired article provides information which was sourced by the Investing News Network (INN) and approved by CANEX Metals in order to help investors learn more about the company. CANEX Metalsis a client of INN. The companys campaign fees pay for INN to create and update this INNSpired article.
INN does not provide investment advice and the information on this profile should not be considered a recommendation to buy or sell any security. INN does not endorse or recommend the business, products, services or securities of any company profiled.
The information contained here is for information purposes only and is not to be construed as an offer or solicitation for the sale or purchase of securities. Readers should conduct their own research for all information publicly available concerning the company. Prior to making any investment decision, it is recommended that readers consult directly with CANEX Metalsand seek advice from a qualified investment advisor.
Please remember that by requesting an investor kit, you are giving permission for those companies to contact you using whatever contact information you provide.
If you want more than 20 investor kits, you need to make multiple requests. Select 20, complete the request and then select again.