Grind it up
The first step in processing raw ore is reducing the rocks and various chunks to small, uniform sizes. Comminution is the process of crushing and grinding the stone, often to particles the size of sand or dust, then screening, separating, and filtering the remains.
The next step is to separate valuable minerals from the rest of the ground up material (gangue). This is accomplished by taking advantage of the unique properties of the target ore, such as color, density, magnetic or electric properties or even flotation. Processes might include passing the ore through a stream of water that carries the lightweight gangue away, leaving the heavier ore behind. Magnetic separation involves passing a magnetic field over a thin layer of particles and, depending on the characteristics of the desired element, either retaining or discarding the remains.
Transporting metals and tailings along with large amounts of water to the leaching site is impractical and expensive, so most of the liquid used in separation is extracted beforehand. Ideally, this water is recycled to be used again in processing, which resolves the issue of dealing with even more potentially toxic liquids while significantly reducing the overall cost of water.
Ore sometimes needs to be treated before it is subjected to leaching; the primary step in extracting metal from the ore. Conversion can involve various forms of roasting (application of heat) and oxidation in preparation for the final step.
Leaching involves use of acids to convert target metals to salts and dissolving them within a solution. Depending on the ore type, sulfuric acid, ammonium hydroxide, cyanide or simple sodium chloride can be used. There are several leaching techniques, from leaching underground directly at the source (in situ leaching), to heap leaching. The latter is generally used for lower grade ores and involves heaping prepared ore onto pads and spraying the heaps with chemicals. As the acids trickle through the ore, they dissolve the valuable metals. The resulting “pregnant” solution is then drained in preparation for recovery
There are a variety of methods that can be used to restore dissolved metals to their solid, pure form. Electrolytic deposition passes a current through the leach solution and causes metal ions to deposit on a cathode. This method produces a pure product but is relatively expensive. Solvent extraction is a process that involves adding reagents that react with the leachate, isolating the metal in increasing concentrations with each set, and then discarding the previous (now empty) solution. Each attract-and-discard step leaves behind a richer sample. Chemical precipitation describes a range of chemical and physical processes, including adsorption, filtration and modification of the solution’s pH.