Mineral processing: Processing, hydrometallurgy & pyrometallurgy

Depending on the scale of your operation, the size of your operation and the kind of mineralogy you've got; you've got lots of different routes. If you are operating a small mine in the Congo, you might at this point bag up your concentrate as a Cobalt concentrate or a coal ton concentrate and it gets banged up in your local town and then fix a price and it gets taken up by a trader and you've got people who are buying trucks and driving sacks of concentrate out. If you are in Niger the French from Cominak, they put a yellowcake, which is a Uranium concentrate, it’s Uranium oxide, they put it into these barrels and they seal them up and they drive them 2200km down the road to the port of Cotonou. If you're in one of the big, Endian copper mines, you're producing a concentrate, which you then don't dry it completely, you make it into the texture of almost slightly wet toothpaste, and you put it in a slurry pipeline. And you can get 60,000t of the stuff down that little toothpaste tube all the way to your port, and then it's shipped off for processing.

Quite often at the mine site, you just get to a concentrate and then you chuck it off for subsequent processing. Equally, there are other places where you can get to a metal on the mine site. This is the stage where you've done your liberation, you've done your separation, you've done your concentration, you've put your concentrated bagged up and at this point, you are ready to go into one of the 3 disciplines of either hydrometallurgy, electrometallurgy or pyrometallurgy. So it's more or less split in Hydro and Pyro. Hydro- water based aqueous solutions, leaching. Pyro- fire heat smelting reduction of hot processes. Now, given the Capex of these big bits of kit, quite often what you'll find is that they are built in places where there's good port access and there's cheap power and they act as central hubs, which is why the big pyrometallurgical complexes are either at the centre of a massive mining hub, up in Sudbury or they're on a port so they've got good access for shipping, which is a low-cost transport medium, and they act as central hubs. It's very rare for mineral deposits to have enough value to be able to pay back the cost of the smelter, and the processing. Typically, they get the economics from drawing on concentrate feed from a number of sources.

On hydrometallurgy, however, you can get away with sometimes the cost of it is actually low enough for it being economic to build your own little metal producing plant on site. At this juncture, it's quite fun to look back at the history of metallurgy because we've been doing this for 1000s of years, the Romans have been doing it and just looking at the history of hydrometallurgy itself because in the old days, it was gravity concentration and then heating it out to separate out the various portions of it. But the history of hydrometallurgy actually goes right back to the 7th and 8th Century and in the 8th century there's an Arab alchemist who discovered Aqua regia, which is the two assets of hydrochloric and nitric acid put together. It was the only known solvent for gold, we've now established others, but it's still used today in assaying and in Gold refining. In the 1500s in the Harz mountains in Germany and in Rio Tinto in Spain, the mining engineers or the metallurgists established heap leaching of Copper from Copper rich Pyrite or Copper sulphide and gradual progress has been made. I think there were two key developments. Well, I mean, goodness me there's so many developments that you've had the buyer process which started the precipitation of aluminium from bauxite and pressure leaching and so, you start to be able to kind of extract aluminium on commercial basis in the late 19th century. And then really, it was the kind of invention of the cyanidation process, which is the dissolution of Gold by using a dilute sodium cyanide solution and then subsequent precipitation of Gold from the solution to using Zinc so that there’s Merrill Crowe process. In the 20th century, it went through copper leaching from solution, in the 1960s, we had bio-leaching and then in the 1980s, you had huge advances in Gold metallurgy with activated carbon technology and the oxidation of refractory ores using various processes so that what was once a locked up mineral that you couldn't access the Gold, you couldn't leach the Gold out of it, you did a pre-treatment to the mineral, and then you could leach the Gold.

Let’s get back to Hydrometallurgy. So, you're effectively precipitating metal out, what you're trying to do is you're trying to put the metal into solution, and then you are going to precipitate out you can use an electric current for example, around the world, about 40% of Copper production is one through solvent extraction electrowinning. It started in the 1980s. What you do is you put your fine concentrate into a solution which preferentially upgrades the solution or concentrate into a solvent and so you get a pregnant liqueur, pregnant solution, which has preferentially selected and reacted and found with the Copper. What you then do is you take that solvent it reacts in a strong aqueous acid and in electrical current, it will deposit Copper onto cathodes. So if you go to a Copper mine which has got its own SX-EW plant, we have the tank house where they're producing Copper cathodes on a daily basis. You produce pretty much an LME grade Copper, which sometimes might need to go for refining but typically doesn't. Similarly, in Gold mines around the world, you'll have a carbon activated process cyanide leach or cyanide and pulp, you use the absorption properties of the cyanide and hydrogen peroxide into the concentrate with the in-solution you mix in this great VAT with the fine ground liberated concentrate of sulphide with the Gold in it. You mix in the cyanide and it leaches and preferentially get a pregnant liqueur which has got the Gold into it and then you go through to an extraction process. What that creates is a gold Doré once it's melted and that gold Doré is then transported out of your Gold mine, normally sell it at the mine gates to one of the bullion companies and that is often taken out by helicopter sometimes or by an armoured vehicle.

On the pyrometallurgy, again, typically this is not done at a single mine site unless you're dealing with an absolutely massive mine. Really what you're doing here is a very high temperature process it's smelting and you're converting and then refining of the metal. You've got 3 types, you've got oxidising where you take your sulphide, for example, a Lead sulphide, you add an oxygen, you produce a lead-oxide and sulphur-dioxide. You've got a volatilizing smelter where you remove oxide, and you've got a chloritizing process where you convert metal to chlorides for later reduction. It's not something that most mineral companies have to deal with intimately, it's not something they have to build. It's a cost that they pay for down the line. And it's very important to understand your smelter charges, your treatment charges and your refining charges, because it affects how much of your payable metal you get. It has a huge impact on what constitutes or not because for example, if you're dealing with Zinc process, the smelter will take 7 or 8% of the value. So your grade and your Zinc project has to be higher than it would be for a Copper one because the smelter takes more of the Zinc than it does out the Copper in terms of value. That was a bit of a gap around the main processes.

What you will see in the Feasibility Study, and the Pre-Feasibility studies will be a focus on the mineral processing side down to a concentrate, sometimes they will go on and talk about the metal extraction. But that's not always the case. It's worth remembering that your original metallurgy can be established quite early on from doing a very simple petrographic analysis. You take some thin sections of your rocks, and you can look at it down a microscope and you can see the grain size and the way that the value mineral is encapsulated or entrained with the waste minerals. And that can tell you a lot of whether this is going to be refractory or not. That's another thing that people often use the word refracturing. What do they mean by that? It just means really how easy it is to liberate or how easy it is to leach.

I haven't really spoken about in-situ leaching or heap leaching but essentially these are part of hydrometallurgy, where you're trying to get access of the reactive solution with the reactive value mineral so you can get the value mineral into solution and then you can collect it and then you can extract the metal later. Heap leaching is all about flow dynamics, which is a function of porosity and permeability. You don't want too much clay in there. And again, it's reaction time how long does an active leach solution have in contact with the metal portion of your ore because remember, in a heap leach, you haven't gone through that extensive liberation, separation and comminution process, you're dealing with just the natural flow dynamics of fluid through the original or rather than through a separated or liberated concentrate. The mineral processing side of things is absolutely fascinating. And it is something that you can get a grasp of early through, as I said, very simple petrographic study will give you most of the indications of where this thing is going to behave metallurgically speaking, for example, I've worked on a Gold project where on drillhole 4 I mean, even before we knew how to deposit a commissioned a Petrographic study, we went to a really good guy who rushed up and he effectively gave us an indication of what kind of things we could expect from the challenges through the metallurgy or the opportunities. And that guy did a lot of the mineral processing metallurgical test work that we did later on and he was absolutely spot on. That's not to say that you don't have to do the mineral processing and the test work. It's absolutely vital, and you really do have to do it but it can certainly help you out.

So this is a really, really key thing is that quite often, the team that will work on the process flow sheet in the design doesn't really understand the geology. And you can have these variations in your ore deposit, which are not picked up by the metallurgist but have a huge impact on the recoverability of your material going through. So, it's absolutely crucial that is not just grade mapping that you do when you are evaluating your resources, absolutely vital that you do metallurgical mapping. In some places, the grade is not the driving factor. For example, if you're in a leaching environment, your Copper grade may stay constant but the leachability of it may vary enormously through various chemical or physio-chemical changes through a deposit. The question of ‘have you done representative sampling on your geology before your metallurgical test work is really crucial’. And I say time and again, because quite often, what happens is that the geologists will select what they think of the best examples of the mineralization but actually, because of the inverted commas ‘the best’, it actually can be high-grade.

So what happens is that the metallurgical test work is done on high-grade material and think, oh, great, we've got this accurate, granular geology that mineralogy that leeches perfectly and we're going to use this process and the recoveries are going to be X or Y. And then when it comes to it, you don't have that perfect mineralogy put in the plant. And so you often end up with very, very unhappy investors and shareholders as your plant loses money, and then you could redesign things. So, it's the proper metallurgical test work. It's not so much the theory is to practice, you know, is this all representative? You've got to do a lot of tests where it's probably wise to do bulk samples, take it from different areas. And then of course, you've got this whole thing about the varying between the surface and deep. How does the mineral change in the process plant when it's been oxidised? When it's got some sulphides in the ores, that's kind of a transition ore, and or you're going deep into fresh rock. So that is the trick is to make sure that you match up the test work, and you've got an appropriate mineral processing facility for the deposit and that you know, what's going to be going through it. There are some real horror stories out there on things going wrong. But I hope that this guide around mineral processing side of the business has been interesting for you. Thank you very much.

‍