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Rare earths tend to come grouped together, with several present in the host rock\u2014Mountain Pass\u2019s bastnaesite ore contains fifteen of the seventeen rare earths, all but scandium and promethium\u2014but they make up only a small fraction of that rock, and the concentrations vary widely. Mountain Pass\u2019s rock is 7 to 8 percent rare earths, on average. That\u2019s a very high concentration by industry standards and one of the reasons the mine is so promising. Still, separating the valuable rare earths from the surrounding junk rock takes a lot of muscle, as well as a lot of finesse.\u00a0<\/p>\n\n\n\n
The process of converting raw, earthbound rock into pure metals often starts with blowing it up. The American mining industry alone uses well over one million tons of explosives each year. To prepare for the blast I saw, engineers examined a roughly rectangular outcropping of solid rock at the pit\u2019s bottom and identified its fissures and weak points. They then drilled 236 holes into the rock, each about thirty-four feet deep, packed them with ammonium nitrate, and detonated all of them simultaneously. When the dust cleared, the rock slab was\u2026still there, more or less. Only now, instead of one solid, flat-topped mass, it had been shattered into boulders and loose stones that were stacked together like a bunch of misshapen building blocks. <\/p>\n\n\n\n
All those drill holes were carefully calibrated to break up the rock without sending it flying off in all directions. That makes it much easier to haul the ore up out of the pit. \u201cThe goal is to have as little horizontal motion as possible. You just kind of shock it into cracking,\u201d explains Rosenthal. \u201cEverything should basically be exactly where it was, just now loose.\u201d <\/p>\n\n\n\n
Earth-moving machines then load the loose rock into hundred-ton dump trucks, which haul it out of the pit to the surface and over to a crushing machine. In the crusher\u2019s maw, the boulders are shattered into pebbles small enough to fit in a thimble. A dusty conveyor belt carries the pebbles up some thirty feet high, then dumps them into a huge, ever-shifting pile. <\/p>\n\n\n\n
Those pebbles are then trucked down to a large building crammed with pipes, ducts, walkways, and holding tanks. Inside, the building reverberates with a multilayered industrial cacophony of motors, conveyor belts, and clangor from the key piece of gear: the spinning ball mill. The ball mill is a studded metal cylinder as long as a semitrailer truck filled with hundreds of two-inch steel balls. Inside the mill, the pebbles are mixed with water and spun around and around, the steel balls bashing and smashing them down into a powdered slurry. <\/p>\n\n\n\n
The next step is to separate out and concentrate the grains of powder that contain bastnaesite\u2014the ore that contains the rare earths\u2014from the grains that are commercially worthless mineral junk. The slurry is run through a series of tanks in which it is treated with chemicals that make the bastnaesite hydrophobic\u2014literally translated: afraid of water. Air is piped through the mixture, and the particles of bastnaesite, desperate to escape the terrifying water around them, latch on to the air bubbles, which carry them to the top of the tank, where they emerge amid a thick, bubbling goop. Heavy mechanical presses then squeeze the water from the goop. What\u2019s left after that first separation phase is a powder that is approximately 80 to 90 percent pure bastnaesite. Also left over: boatloads of wastewater. <\/p>\n\n\n\n
Mines of all sorts all over the world use similar procedures to process the stone they dig up. Handling the resulting wastewater is a major issue. Typically, mining companies dump wastewater into what are known as tailings ponds\u2014artificial lakes loaded with crushed rock and chemicals left over from this process. Over time, if all goes well, the solids settle to the bottom of the pond, and the water evaporates or is recycled. These ponds are supposed to be engineered to keep their toxic contents safely away from everything else, but there\u2019s a constant risk that polluted water will seep out of the pond and into the local groundwater. Toxic wastewater leakage is what got the Mountain Pass mine shut down in the first place. <\/p>\n\n\n\n