Getting metals or other useful commodities out of the rocks we find them in is often complicated and difficult.
Difficult = Expensive.
You can get almost any metal out of almost any rock if you apply enough acid, heat, and pressure to it. But the more you do, the more it costs.
The key to profiting as a miner is finding cheap ways to recover minerals from ores.
How to do that depends on the chemical and physical characteristics of minerals being recovered and of the rock from which they’re being recovered. I’m sure it won’t surprise you for me to say that this varies a lot from mineral to mineral and from rock to rock. If you’re reading this, it’s probably precisely because there is such a variety of processes, reagents, equipment types, and other considerations. Trying to parse all this in company press releases can be bewildering.
A complex subject like this deserves a book that, frankly, I’m not qualified to write. However, the definitions and considerations in this cheat sheet may help you make sense of what mining companies are saying—or trying to obscure—in related press releases.
From simplest (generally cheapest) to most difficult (generally most expensive), here’s my take on the main types of mineral processing.
Gravity
Gravity separation is a term that covers several kinds of mechanical separation of metal from host rock. Since most metals are reactive and occur in oxide or sulfide combinations, this is mostly used for gold, which is unreactive and more often found in its pure metallic form. The key is that gold is much heavier than most rock. So, if you crush the ore and flow the result in water in various ways, you can get a lot of the gold out with no other chemicals needed. Sometimes very little crushing is needed. Other times, more crushing and some (costly) grinding is needed as well. Common equipment includes vibrating tables, water spirals, and continuous-centrifuge concentrators. Dredging rivers and such for gold is also a form of gravity processing. This is basically panning for gold on an industrial scale.
This simple, relatively benign form of processing makes permitting easier (no scary cyanide), but recoveries tend to be lower. I’ve heard of small, high-grade mines with a lot of granular gold getting up to 80% recovery by gravity. It’s usually much lower than that. Large, high-grade mines may get 30–60% gravity recovery. That’s great because it means much less material needs to be processed by more intensive means. But such recoveries by gravity alone would mean that a lot of the gold is not recovered. (This is why some miners today can make money reprocessing waste dumps and tailings left behind by less efficient miners in previous centuries.)
Many major gold-silver mining operations have no gravity circuits at all, or it’s only a minor processing component. Where it can be added, it’s generally a plus.
Things to watch for include:
- Low recoveries in projects that aim to use gravity only.
- High crushing and grinding costs may make this more expensive than usual.
- Alluvial (“Placer” = dredging/panning) operations that may show good gravity recovery but with poorly-defined resources, and produce inconsistently.
Heap Leach
Heap leach processing doesn’t run the ore through a processing plant at all. Instead, the ore is stacked on a pad with a thick plastic liner and sprinkled with some reagent solution that dissolves the metal. The solution trickles out the bottom and collects in a “pregnant solution” pond. The metal is then extracted in a relatively small plant, usually by electrolysis, followed by pouring ingots or doré bars from a smelting furnace.
The best-case scenario for this is when the metal recovers so easily from the rock, it can just be blasted and stacked on the leach pad with no crushing or grinding. This is called run-of-mine (ROM) processing. (Getting high recoveries with ROM processing is a very good thing.) Even a modest recovery of 60–70% can pay nicely in a low-cost ROM operation.
The next-best case would be heap leach with some crushing, but no grinding. Grinding is much more energy-intensive and uses up mill balls (steel balls put in the mill to help grind the rock) and liners. Most heap leach operations require some crushing and agglomeration with cement or lime to hold the materials together and keep the heap open for the solution to percolate.
Heap leaching gold uses a cyanide solution. Other metals like copper and uranium leach with sulfuric acid.
Things to watch for include:
- Carbonaceous material in gold ores (often looks black in the drill core or pit wall) can "steal" gold from pregnant solutions.
- A bit of copper in a gold deposit can interfere with leach recoveries, increasing costs.
- High use of consumables (cyanide, acid, lime, etc.) can drive up costs.
- Some ores require cement to stack properly or agglomeration to leach properly. This is an expense to monitor.
In Situ Leaching
An extraction method that can be even cheaper than heap leaching is in-situ leaching, or "solution mining." This involves leaving the ore right where it is in the ground and drilling holes to pump acid or other reagents through it. This dissolves the metals in place (in situ), bringing only a pregnant solution to surface, from which the contained metals can be recovered by a variety of means.
This obviously saves a great deal of capital and operating expense, but not all ores are amenable to this sort of processing. You need good porosity and other physical and chemical characteristics. But where it does work, this can be the very cheapest way to mine uranium, copper, or any soluble mineral.
Things to watch for include:
- High natural water flow diluting reagents.
- Local populations afraid of pumping acid through the ground.
- Unexpected chemical interactions that prevent the metals from dissolving as expected.
- Unexpected minerals that clog up the pumping wells.
Tank Leach
If you can’t heap leach an ore, sometimes you can grind it and leach it in a tank with the appropriate reagents. There are several forms of this, including carbon in leach (CIL) and carbon in pulp (CIP) for gold, and agitated leach for copper. I won’t get into the details, but the key points to remember are that this involves crushing and grinding expense, and it’s done in a tank rather than a leach pad. It still results in some sort of final metal-rich liquid that goes for final processing (though in rare cases, loaded carbon is shipped as a final product).
As you can see from the process names, carbon likes gold dissolved in cyanide. That’s why carbonaceous material in the ore can be such a problem.
Things to watch for include:
- If the rock/ore is very hard, it takes more energy to grind and uses up mill balls and liners faster. Watch for a high Bond Work Index on this front (over 18kWh/t).
- High reagent consumption drives up costs.
- Leach pads can cover acres. Leach tanks are much smaller (and the bigger they are, the more expensive they are), so it generally takes higher-grade ore to pay for tank leaching than heap leaching. However, recoveries are usually higher in CIP/CIL.
Flotation
I see flotation as a step up from heap leaching, not tank leaching. It too involves crushing and grinding, but the pulverized ore is put in special cells with reagents (essentially soap) and nozzles that blow bubbles in the mix.
Sound crazy?
I can imagine some old miner 120 years ago taking a soak after a long, hard day, and noticing metallic-looking particles sticking to the soap bubbles in his bath. I don’t know how the process was actually discovered, just that metal sulfides stick to soap bubbles—and you can control which sulfides by using different kinds of “soap.”
Note that this does not work for oxide gold, which is generally leached. But various other oxides and sulfides float quite nicely on the right bubbles. They can then be mechanically scraped off the top of the cell while more ground ore is added and more sulfides bubble up. The thus concentrated material is thickened and dried off. They’re usually sold in that form: a bulk sulfide concentrate. This “con,” as it’s called, looks like dark, sparkly sand. It gets sold to a smelter for final processing.
One can float the ore slurry sequentially to recover, say, silver-rich lead sulfides (galena) first, and then gold-rich copper sulfides (chalcopyrite) next. Or the reverse, depending on priorities. Gold often occurs with pyrite (iron sulfide, aka “fool’s gold”), so I’ve seen mines where a fool’s gold concentrate is the final product. Other times, the pyrite is barren and just gets in the way. With the right mix in the flotation cells, you can suppress the pyrite and float the galena, chalcopyrite, sphalerite (zinc sulfide), or whatever you want.
Notice that flotation doesn’t use generally cyanide or even acids. It’s a relatively benign and easier to permit form of processing, but you get a concentrate instead of metal ingots at the end.
Things to watch for include:
- Presence of deleterious elements (“nasties”) like mercury and arsenic in the con.
- Lower value of the product; a net smelter return (NSR) on a con is typically 70-80%, compared to 99.95% for Doré.
- Naturally hydrophobic minerals that contaminate the con.
- High Bond Work Index.
- Low recoveries.
- High reagent cost.
- Contamination of barren minerals (like plain pyrite) in the con.
Roasting
Sometimes you get an ore that just won’t leach, float, or come out of the rock at all without more intensive processing. This includes, but is not limited to, “refractory” ores where the desired mineral is bound up with or encapsulated in worthless or nasty minerals. This can often be dealt with by roasting the ore (sometimes in an autoclave, which adds pressure) or such device that turns it into an oxide that can be leached.
This is tried and true technology that has worked well for over a century. (I’ve been told that the first environmental protests in the world were back in the 1800s in response to the original Rio Tinto mine in southern Spain using gigantic pyres kilometers long to roast ore by the megatonne.) Roasting’s main drawback is that it requires special (expensive) equipment and consumes huge amounts of energy. In some cases, the process is energy-positive (through heat generated burning sulfur), but you still have to scrub gaseous products. In other words, it adds both to a project’s capital expenditures (capex) and operating expenses (opex).
Things to watch for include:
- Capex, opex, and return on investment.
- Tough ores that don’t recover well despite roasting.
- Remote projects far from cheap energy supplies.
BIOX
Sometimes you get a difficult sulfide ore, but roasting doesn’t work or is too expensive. You can still convert it to an oxide form using special bacteria. The oxide output is usually then leached in tanks. This bio-oxidation (BIOX) processing is much cheaper than roasting, but not all sulfide ores are amenable. Note that BIOX is a trademarked, proprietary process.
A key point is that the bugs that do the work must be kept happy. Their process is exothermic (gives off heat), which is great in cool climates, but can require money for cooling in hot climates.
This is a well-established technology these days, but it’s new compared to flotation or roasting, and that makes some investors nervous. My view is that, done well, it’s fine. But it needs to be done well, for which I like to see experienced teams and contractors—on the ground, not just the C-suite.
Finally, while sometimes cheaper than roasting, it’s not as cheap as flotation or leaching.
Things to watch for include:
- Complications keeping the bacteria alive and productive.
- Capex/opex issues.
Other General Considerations
More general things to watch for in metallurgical test results and mine-plan press releases include:
- As discussed in my equivalency cheat sheet, polymetallic ores are tricky. It’s very rare to get high recoveries of all the metal in ores that include more than two. Even in deposits with just gold and silver, it’s common to see recoveries for silver lower than gold (typically 85-98% for gold and 30-70% for silver).
- Rare earth elements (REEs) are basically l’enfant terrible of polymetallic ores. They usually occur as a complex bunch of many metal oxides that require intensive acid leaching and are subject to random problems like slime fouling. Even a full feasibility study doesn’t guarantee that REEs will be recovered as advertised. After decades of watching REE projects struggle, my sad conclusion is that you never know what you’ll get until you build a plant, shove a bunch of REE-rich dirt through it, and see what comes out.
- It’s usually carbon for gold, but other factors like clays and sulfide materials can prevent recovery of metals from a heap leach operation’s pregnant solution pond. We call this preg-robbing. Beware.
- As above, rock hardness is a huge cost factor when it comes to grinding. Watch for a high Bond Work Index as a cost factor.
- Remoteness generally makes everything more expensive, but the more intensive the processing an ore requires, the more severe the consequences are from being far from power, water, roads for reagent delivery, etc.
- Altitude is like remoteness—but on steroids. Everything is more expensive at high elevations. Just breathing is a chore for workers, making them less efficient. Same goes for pumps, generators, etc. Ideally, ore high in a mountain can be conveyed downhill to where processing is cheaper and more efficient. (Sometimes that even generates electricity.)
- Water is key for all of these processes but is used in larger quantities in heap leach operations. Beware of plans for heap leach operations in deserts. Make sure a company secures enough water rights early in the process. Not enough water = no mine. That simple.
In Closing
I want to stress that I’m not a metallurgist. This cheat sheet should be seen as an alpha, not the omega of what one should consider when evaluating a mining project. My object is not to answer all questions, but to give investors an idea of the issues they should be on the lookout for.
I hope you find it useful.
L
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