A New Way of Mining?

One of the bigger problems our economies face is obtaining metals. Apparently the price of metals used in lithium-ion batteries is soaring because supply cannot expand sufficiently, and there appears to be no way current methodology can keep up.

 Ores are obtained by physically removing them from the subsurface, and this tends to mean that huge volumes of overburden have to be removed. Global mining is estimated to produce 100 billion t of overburden per year, and that usually has to be carted somewhere else and dumped.  This often leads to major disasters, such as mine tailing causing dams, and then collapsing, thus Brazil has had at least two such collapses that led to something like 140 million cubic meters of rubble moving and at least 256 deaths. The better ores are now worked out and we are resorting to poorer ores, most of which contain less than 1% is what you actually want. The rest, gangue, is often environmentally toxic and is quite difficult to dispose of safely. The whole process is energy intensive. Mining contributes about 10% of the energy-related greenhouse gas emissions. Yet if we take copper alone, it is estimated that by 2050 demand will increase by up to 350%. The ores we know about are becoming progressively lower grade and they are found at greater depths.

We have heard of the limits to growth. Well, mining is becoming increasingly looking like becoming unsustainable, but there is always the possibility of new technology to get the benefit from increasingly more difficult sources. One such possible technique involves first inserting acid or lixiviant into the rock to dissolve the target metal in the form of an ion then use a targeted electric field to transport the metal-rich solution to the surface. This is a variant of a technique used to obtain metals from fly ash, sludge, etc.

The objective is to place an electrode either within or surrounding the ore, then the acid is introduced from an external reservoir. There is an alternative reservoir with a second electrode with opposite charge to that of the metal-bearing ion. The metal usually bears a positive charge in the textbooks, so you would have your reservoir electrode negatively charged, but it is important to keep track of your chemistry. For example, if iron were dissolved in hydrochloric acid, the main ion would be FeCl4-, i.e. an anion.

Because transport occurs through electromigration, there is no need for permeability enhancement techniques, such as fracking. About 75% of copper ore reserves are as copper sulphide that lie beneath the water table. The proposed technique was demonstrated on a laboratory scale with a mix of chalcopyrite (CuFeS2) and quartz, each powdered. A solution of ferric chloride was added, and a direct current of 7 V was applied to electrodes at opposite ends of a 0.57 m path, over which there was a potential drop of about 5V, giving a maximal voltage gradient of 1.75 V/cm. The ferric chloride liberated copper as the cupric cation. The laboratory test extracted 57 weight per cent of the available copper from a 4 cm-wide sample over 94 days, although 80% was recovered in the first 50 days. The electric current decreased over the first ten days from 110 mA to 10 mA, suggestive of pore blocking. Computer simulations suggest that in the field, about 70% of the metal in a sample accessed by the electrodes could be recovered over a three year period. The process would have the odd hazard, thus a 5 meter spacing between electrodes employed, in the simulation, a 500 V difference. If the ore is several hundred meters down, this could require quite a voltage. Is this practical? I do not know, but it seems to me that at the moment the amount of dissolved material, the large voltages, the small areas and the time taken will count against it. On the other hand, the price of metals are starting to rise dramatically. I doubt this will be a final solution, but it may be part of one.