Separating Metals from Mud - Mining Technologies
Diana - An important resource that we find in deep dark places are the metals and minerals we need for industry and everyday life, which means that mining minerals like copper and platinum is a multi-billion pound industry. Dave and Meera have been out exploring how you separate the metal we do want from the rocks we don't...
Meera - This week, Dave and I are exploring the process of mining, so where our metals and our minerals actually come from. Dave, it's not as simple as just digging them out of the ground, is it?
Dave - That's right. When you normally think about mining, of course you think about digging stuff out of the ground and then you learn in school about taking those ores, smelting them, and turning them into a useful metal we can use every day. However, there's a big, really important process inbetween that. The ore which we dig out of the ground is a combination of rock and the useful minerals we want. The problem is the proportion of the useful minerals can be only 1 or 2 percent, and that's still worth extracting. And this process of extracting the valuable minerals out of the rock is incredibly difficult. Originally, this was done by processes like panning, you swirl water over a mixture of sands and the densest ones fall to the bottom, but doing this on an immense scale is a very, very difficult process.
Meera - Well one scientist looking into this separation process is Professor Jan Cilliers from Imperial College London. We've come down to the Royal School of Mines in Central London to find out more about this.
Jan - I think maybe we should take a step back and just look at the scale of the mines we're dealing with nowadays. The copper mines we're talking about treat literally tens of millions of tonnes of rock a year to produce hundreds of thousands of tonnes of copper. But within the mine, maybe only 2 percent of the rock we mine is valuable mineral and the other 98 percent is waste. In order to do the separation between the valuable minerals and the waste minerals, we need to grind them very finely down to, let's say less than a tenth of a millimetre before we can do the separation. This is called liberation. Once we've ground up the rocks and they're liberated from each other now, we actually want to do the separation from the valuable minerals to the non-valuable ones. The work horse of mineral processing nowadays where we have to separate out these very small amounts of minerals from each other is a technique called froth flotation which depends on the hydrophobicity differences between different minerals. In this case, we make the one mineral we do want hydrophobic, meaning water-hating, blow air through this mixture, a slurry, or almost a mud of the mixture of minerals, the hydrophobic particles stick to the bubbles, float to the surface, and form a continuously overflowing froth that contains all our valuable minerals.
Dave - So are the valuable minerals chemically different so that your additive can make them hydrophobic?
Jan - They have to be and in this case, nature has made it so, that the valuable minerals are sulphide minerals, they are copper iron sulphide, or copper sulphide, something like a chalcopyrite as it's called versus an oxide mineral which is our gangue or our waste minerals, the other 98 percent is that. So, it's quite straightforward for this chemical to attack the one mineral versus the other one, and make it hydrophobic, whereas it ignores the other ones and leaves them hydrophilic, or water-loving.
Meera - What is the chemical used to make them hydrophobic?
Jan - We use two chemicals commonly. The first ones are the xanthates and the second ones are the diathocarbonates, and they both are long hydrocarbon chains with a reactive front end that looks for sulphur and it attaches to the sulphur, or chemically attaches to the sulphur, leaving a long hydrocarbon chain in the solution that looks oily and makes the mineral look hydrophobic and act like it hates being in water.
Meera - So what types of minerals is this process used to extract?
Jan - Pretty much all the base metals; copper, lead, zinc primarily. Many of the other metals are associated with the base metals, so platinum comes up as a nickel-copper type deposit, silver often comes with lead, and in all the major gold mines in the world now, the gold is actually a by-product of the copper production.
Jan - At the moment, the flotation process for copper accounts for about 70 percent of the world's production. If you look at a typical mine in North America or in Chile where the mass of copper mines are nowadays, a typical mine would maybe treat 10,000 tonness of rock an hour. That's a cube of rock 15 metres by 15 metres by 15 metres every single hour. So, the scale and the tonnage is just unimaginably large.
Meera - And the tanks themselves then that the rock is in and the bubbles are being pushed through...
Jan - The biggest tanks nowadays are 300 to 400 cubic meters each and a typical mine would have 60 to 70 of these tanks in a row. So yes, we're talking about aircraft hangers of froth.
Meera - This process has been used for about 100 years now. We're in your lab where you study the foam and flotation process. What aspects of this are you studying to further it or enhance it?
Jan - We've been looking at the physics of the froth; the bubble size, the velocity at which it flows, and what variables affect that.
Meera - We've got a large water bucket/tank in front of us with a gutter around it, with many tubes going in, many tubes coming out, laser monitors on top, as well as cameras attached. So, I imagine you initiate a foam here, but what are you actually monitoring and how?
Jan - As you see, the tank is containing our slurry. The froth is forming inside them, overflowing the edge into what you call the gutter or we call the launder, and then overflows into a re-circulation tank, and through the pump as you can see. What we're measuring is two things. The one is the height of the foam overflowing the edge of the tank and the velocity which we measure with a video camera, you can see in the back there. We are trying to measure the volume of the froth overflowing, so we need a velocity and we need a height, and of course, we know what the length of the lip is. If we multiply those three together, we get the volume of the froth.
Dave - I guess the depth at which the froth is overflowing is quite important because if it hasn't had time to separate out, and you're sort of pumping froth through too quickly, you're going to get lots of other minerals mixed in and if you go too slowly, you're just not going to get enough production from your system.
Jan - What we've discovered over our years of research is that the rate at which the bubbles are bursting on the top surface is the critical parameter for controlling this process.
Dave - So if the bubbles are bursting, you're essentially losing the separation which the bubbles have done already for you.
Jan - If there's too much bursting, we lose the mineral we want. If there's too little bursting, we collect too much of the minerals we don't want and it's that delicate balance that we're trying to optimise. The main variable to control the rate of bursting is how much air we put into the tank.
Meera - Is this something that's controlled and monitored on industrial mines at the moment?
Jan - In some mines, it is. It's not necessarily controlled although there is move towards that. We'd like to see that implemented far more widely and that this technique of monitoring the volume of froth overflowing is used as a control measure. If you look at a typical copper mine, we recover about 90 percent of the minerals that come in to our process and we lose about 10 percent. If we push that to 91 percent, that extra percent is worth, say, $20 million per annum. So there's big incentive for us to get this right.
Diana - What a lot of money! That was Jan Cilliers, Professor of Minerals Processing at Imperial College London, taking Meera and Dave through the copper separation process.