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Heap leaching: Could it be an alternative to conventional gold extraction?

The process of heap leaching is not as simple as many would come to believe and it works really well for ores that are oxide in nature and are permeable.

Kinross Gold Corporation - Heap leaching: Could it be an alternative to conventional gold extraction?
The heap leach pad of Tujuh Bukit Mine in Indonesia holds about 8 million tonnes

I have always thought that the heap leach process was for deposits that were substandard and for those that cannot raise good capital for a 'decent' mining process. It was a recent discussion with an associate about the treatment of transitional and oxide ores in a low-capex manner that I decided to do some homework.

The conversation was pretty non-engaging and straightforward until he mentioned the Tujuh Bukit Mine in Indonesia. He told me that the heap leach pad has about eight million tonnes of material on it. That is one big pile of dirt.

Although, I have always known about this process as a cheap means of extracting gold, nickel, REE and copper ores, what I did not realise was the sophistication that has been applied, learned and developed over the years.

What little I did know was that the ores are mainly low-grade and must have the specifications attuned to the process. One of the main attributes required is the ability to be permeable, and preferably the ore is of an oxide nature.

Heap leaching is getting popular. (source: www.miningfeeds.com)

That was the moment that made me curious enough to realise that this is a serious process and not just for the likes of small-time miners.  And, of course, more reading has made me realise that this is more than a serious operation with the extensive leaching processes in the USA, Chile and Peru.

The heap leach process is so advanced that there are practically hundreds of articles written about all aspect of the process. The way they 'pile' up the ore is also sophisticated. 

In Chile, the world's largest copper mine Escondida has a leach pad for the sulphide ores and another for the oxide ores.

The diagram below gives some context of the size of these heap leach pads. I am not sure if the Escondida one is bigger than the featured image from the Tujuh Bukit Gold mine in Sumatra, Indonesia. Suffice to say these are excellent examples of the success of the heap leach process.

To give some context of what I mean by a giant mine, the photo below is that of the Escondida Copper Mine. Imagine the low-grade nature that is analogous with porphyry deposits and the associated large amount of ore that is treated with a heap leach process.  In some literature, the heap leach process is also described as being more environmentally friendly than traditional mineral processing methods.

Escondida Mine (source: www.bhp.com)

History of heap leaching

(source: A Brief Note on the Heap Leaching Technologies for the Recovery of Valuable Metals, 2019, Thriveni Thenepalli, Ramakrishna Chilakala, Lulit Habte, Lai Quang Tuan  and Chun Sik Kim; Sustainability 2019, 11, 3347; doi:10.3390/su11123347)

Heap leaching is one of the oldest and the most traditional mining process used to extract valuable metals from specific minerals.

This is a hydrometallurgical process in which the solution is applied for the dissolution of minerals from the ore that is used for the extraction of metals.

Originally, heap leaching was practised 500 years ago. Georgius Agricola published a book De Re Metallica in 1557 and reported that the heap leaching process was finished in a 40-day cycle.

Since the middle of the 16th century, heap leaching was practised in Hungary for copper extraction. In 1969, gold heap leaching began in Nevada (birthplace of modern heap leaching) and in the middle of the 20th century, the US Bureau of Mines began applying this technology.

Gold and silver heap leaching first began at Cortez in 1969. Currently, 37 different heap leaching operations are active worldwide for the production of gold, which is estimated to be around 198 tons per year.

What is heap leaching?

According to Wikipedia, it is a simple process.

The mined ore is usually crushed into small chunks and heaped on an impermeable plastic or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals.

While sprinklers are occasionally used for irrigation, more often operations use drip irrigation to minimise evaporation, provide more uniform distribution of the leach solution and avoid damaging the exposed mineral.

The schematic diagram for the atypical heap leach process. (source: 911 Metallurgist)

The solution then percolates through the heap and leaches both the target and other minerals. This process called the 'leach cycle', generally takes from one or two months for simple oxide ores (eg most gold ores) to two years for nickel laterite ores.

The leach solution containing the dissolved minerals is then collected, treated in a process plant to recover the target mineral and in some cases precipitate other minerals, and recycled to the heap after reagent levels are adjusted.

A simple flowchart for a gold heap leaching process. (Source: modified from Donald, I.B. Estimated Water Requirements for Gold Heap-Leach Operations. (ver.1.1,11 December2012): U.S. Geological Survey Open-File Report 2012–1085. pp. 1–17. Available online: http://pubs.usgs.gov/of/2012/1085 - accessed on 6 June 2019)

Ultimate recovery of the target mineral can range from 30% of contained run-of-mine dump leaching sulphide copper ores to over 90% for the ores that are easiest to leach, some oxide gold ores.

The significant advantage of the heap leaching method over conventional leaching and recovery techniques is that heap leaching consumes less than 0.3 tons of water for one ton of ore.

What do you leach?

(Source: Wikipedia)

Heap leaching is an industrial mining process used to extract precious metals, copper, uranium and other compounds from ore using a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials.

Similar to in situ mining, heap leach mining differs in that it places ore on a liner, then adds the chemicals via drip systems to the ore, whereas in situ mining lacks these liners and pulls pregnant solution up to obtain the minerals.

Heap leaching is widely used in modern large-scale mining operations as it produces the desired concentrates at a lower cost compared to conventional processing methods such as flotation, agitation and vat leaching.

Additionally, dump leaching is an essential part of most copper mining operations and determines the quality grade of the produced material along with other factors.

Due to the profitability that the dump leaching has on the mining process, ie it can contribute substantially to the economic viability of the mining process, it is advantageous to include the results of the leaching operation in the economic overall project evaluation.

This, in effect, requires that the key controllable variables, which have an impact on the recovery of the metal and the quality of solution coming from a dump leaching process.

The process has ancient origins; one of the classical methods for the manufacture of copper as (iron sulphate) was to heap up iron pyrite and collect the leachate from the heap, which was then boiled with iron to produce iron(II) sulphate.

Examples of major heap leach pads

Several major projects around the world are using this method of extraction. As mentioned, the major proponents of this process are miners of gold and copper porphyry deposits and these are in the USA, Chile, Peru and Indonesia. These deposits are mainly lower grade and have a large tonnage.

Interestingly, the other gold mining countries such as Australia, South Africa and Canada do not seem to practice this form of processing as much. In terms of weather, the dry arid nature of Australia would be ideal, especially the low rainfall parts of the drier parts of Australia.

Tujuh Bukit project

Tujuh Bukit Heap Leah Project (source: Merdeka Copper Gold. Approximately 3MT on the pad as working inventory. (source Merdeka Gold Copper)

According to the company website, the Tujuh Bukit Project consists of two distinct mining development opportunities:

  • Firstly, the low capital cost, low operating cost and low technical risk Heap Leach Project, which entered the production at the beginning of 2017; and
  • Secondly, the potential future development of the world-class Porphyry Project subject to the successful completion of required technical, environmental and social studies.

The heap leach pad working inventory is 3mt at 0.72 g/t gold and 8 g/t silver containing 700,000 ounces of gold and 800,000 ounces of silver.

The heap leach pad is considered to be a working inventory because the active ore placement lift has a planned 150-day leach cycle to realise 100% of the estimated total recovery. In addition to this planned leach cycle, there is potential for additional recovery of metal as the leach solution percolates through the underlying lifts.

As of 31 December 2017, the Heap Leach Oxide Project mineral resources are 106mt at 0.73 g/t gold and 24 g/t silver containing 2.5 million ounces of gold and 80 million ounces of silver.

Escondida Copper Mine

(source: Extremophiles in Mineral Sulphide Heaps: Some Bacterial Responses to Variable Temperature, Acidity and Solution Composition – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Run-of-mine-dump-bioleach-at-Escondida-Chile-showing-a-surface-with-irrigation-lines_fig1_281767633 - accessed 25 Nov, 2019)

(Source: Wikipedia)

Escondida is a copper mine in the Atacama Desert in Antofagasta Region, Chile. The Escondida deposit is one of a cluster of porphyry coppers in an elongated area about 18km north-south and 3km east-west. It is associated with the 600km-long West Fissure (Falla Oeste) system, which is in turn associated with most of the major Chilean porphyry deposits.

A barren, leached cap, in places up to 300 metres thick, overlies a thick zone of high-grade secondary supergene mineralisation of the main ore body, largely chalcocite and covellite, which in turn overlies the unaltered primary mineralisation of chalcopyrite, bornite and pyrite.

At mid-2007, Escondida had total proven and probable reserves of 34.7 million tonnes of copper, of which 22.5 million tonnes is estimated to be recoverable. Total resources, including reserves, were 57.6 million tonnes of copper, of which 33 million tonnes should be recovered. Exploration continues.

Sulphide ore, which contributes 77% of the recoverable copper reserve, is crushed and milled in one of the two concentrators and the copper concentrate is separated out using froth flotation. Approximately 86% of the copper is recovered. It is piped down to the port of Coloso, where it is dewatered before shipping.

Oxide ore, 4% of recoverable copper, is crushed, agglomerated and then acid leached in large heaps and the copper is recovered from the leach solutions as copper cathode in a solvent extraction/electrowinning (SX/EW) plant. Recovery is 68%.

The low-grade sulphide ore contributes 19% of recoverable copper. It is also crushed and dumped on large heaps, but here the leaching occurs through oxidation induced by microorganisms. The copper is also recovered by SX/EW.

Is heap leaching something of the past?

In July 2019, Kinross Gold Corporation (TSX:K) (NYSE:KGC) announced an agreement with N-Mining Limited to acquire Chulbatkan, a high-quality, heap leach development project for $283 million, including approximately $113 million in cash and approximately $170 million in Kinross shares.

In addition, N-Mining will have the right to economic participation equivalent to a 1.5% net smelter return (NSR) payment and contingent consideration linked to future reserve additions (www.globenewswire.com).

Chulbatkan is in the industrialised district of Khabarovsk Krai, Far East Russia, about 15km southwest of Udinsk, a settlement on the Amgun River and has year-round transportation access.

Chulbatkan Mine location. (source: Kinross Gold)

The projects have indicated resources of approximately 3 million ounces and have an estimated AISC of USD$550oz. Capex is expected to be in the US$500 million mark. The mine is expected to be low-cost and the processing is focused on a heap leach process.

Chulbatkan mining operations. (source: Kinross Gold)

The examples I have given above are very big, but on a smaller scale, there are also participants looking at a cheap alternative to mining. An example is Tartana Resources, a company at the time of research was quoted as looking to list on the ASX.

They have a smaller heap leach process that is looking to extract copper from oxide ores.

In my time in the Eastern Goldfields, I had seen many such operations. In 2015, Phoenix Gold Limited, taken over by Evolution Mining Ltd (ASX:EVN), announced a feasibility study to progress with a heap leach strategy with their Castle Hill project. They reported an AISC of A$913, and in essence, that is a good result for very low-grade ore.

Tartana heap leach pad. (source; www.i-g.net.au)

Conclusion

This has been an interesting article to write as there is more to the topic than I realised. Although I am not in the mining world, the whole topic is intriguing and shows that perception can be very deceiving.

I think most people who read or invest in this industry are in the same situation. We know enough to be dangerous is a phrase I use to say when we know just enough to make a fool of ourselves.

In a simple way, heap leach works really well for ores that are oxide in nature and are permeable. The way the pile is constructed or rather the design of the leach pad is the next most important aspect.

Recovery is normally lower than your conventional treatment methods, but the cost, the capital cost of the process, is also significantly lower. The operational cost would also be lower as you require less water, energy and chemicals.

As is always the case, when you learn something about what you think is little known or little publicised, it becomes so in your face you wonder why you never picked up on it.

As most people would have noticed, the mining grade is lowering and mines are going deeper. As I was promoting in 2009, the time for a revaluation of gold is coming as mines get deeper and the cost of production follows that curve. 

When you look at my recent article, Gold in the Pilbara - The Players and their Story, there are many deposits around the world that are isolated due to the fact that it is too expensive to process.

Alternative means of processing have to come into play soon or the gold price would be ascending very quickly.

World and economic situations around the world are already pointing to a rising gold price. There is no alternative to hedging against inflation at the moment. If paper money becomes too inflationary, how would you flock to safety? Apart from gold, what would be safe? Gold has no alternative to safety unlike how you would have an alternative to fossil fuel, electric vehicles and hydrogen cells, etc.

A rising gold price will help create alternatives, but I think if one can find projects that are amenable to heap leaching, profitability will be assured. With that comes happy investors and that leads to happy management. And we all know what that leads to, a happy spouse and a happy world.

www.samso.com.au

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