It is possible to buy most of the elements on the periodic table. Some of them come from mineral deposits rich enough to justify devoting a whole mine to that element. Iron, gold, molybdenum, copper, titanium, lead, zinc, aluminum, and a few more are extracted from mining operations devoted to that element. Many of these mines also produce side streams or even flue dusts that are enriched in other elements valuable enough to justify extraction.
Gold mining, for instance, may produce significant quantities of silver. Silver mining may produce significant quantities of gold. Copper mining produces streams containing zinc, bismuth, and lead, as well as some amount of gold and silver. Platinum ores also produce rhodium, iridium, palladium, ruthenium, and osmium. There may also be associations with chromium. Aluminum side streams produce gallium. Uranium ores may have important levels of rare earth elements present.
What is worthy of note is the fact that the economics of mining elements like gallium, tellurium, or rare earths may be so meager as to void any real prospect of recovery on their own merits. Some rare earth occurences like Mountain Pass in California are so rich that they can serve as the primary product. In a recent discussion I had with a mining engineer familiar with that deposit I learned that the original metal of interest was barium. Barium minerals were sought after as a component of drilling mud. The high density of the barium minerals contributed to the density of the drilling mud and consequently to the weight of a column of drilling mud. This property added to the mud’s utility in controlling gas and oil well pressures during drilling. But eventually, the ore body became more valuable for the REE’s.
This is where waste streams come into view. A waste stream from any given extraction process is a highly disturbed material. The ore feeding an extraction process is subjected to what are often severe conditions of temperature, pressure, and corrosive reagents. This treatment frees the desired element and discharges a material that may be then significantly enriched in minor components. Elements that were present in the ore in tens of ppm’s or less may be enriched into the hundreds or thousands of ppm in the waste stream. That enrichment, as well as the chemical transformation that occured to extract the principal element, may be sufficient to render extraction of the minor element economical.
Mineral extraction processes are devised so as to make the desired metal easily separable by inexpensive reagents. In minerals rich in refractory or relatively inert matrices like silicates and aluminates, much effort must go into disrupting the matrix. The disruption process very often involves crushing to produce maximum surface area. This comminuted material may then be roasted in air, chlorine, sulfur dioxide, or exhaust gases to disrupt the chemical structure of the matrix so that some kind of differential solubility can be exploited, for instance.
Alternatively, the finely powdered mineral may be subjected to froth flotation where its affinity or lack therein to a flotation agent is exploited to effect a physical separation of mineral components. Flotation is a very important and common process.
Without metallurgical process waste sidestreams, many elements would not be economically viable to isolate on their own or too expensive to use.