How magnets are made: mine to magnet

Rare earths are mined like most other metals. Hard-rock deposits such as bastnaesite and monazite are drilled, blasted, and hauled out of an open pit. The ore is not remarkable to look at; the rare earth minerals are a small fraction of the rock, and the rest is waste that will be removed in the next step.

Ionic clay deposits are the exception. The rare earths are held loosely on the surface of fine clay particles, so there is no rock to crush. The clay is already in a form ready for extraction, and its element mix tends to be more favorable for magnet making, with a larger share of the heavy rare earths that magnets need.

The ore is crushed and milled to a fine powder to liberate the rare earth minerals from the surrounding rock. The powder is then run through flotation: chemicals are added that make the rare earth minerals attach to air bubbles, so they float to the surface and are skimmed off while the waste rock sinks.

The result is a concentrate. Where the ore ran around 7% rare earths, the concentrate runs around 60%. Nothing has been chemically changed yet; the same minerals are simply present in far greater proportion, and the great majority of the mined mass has been left behind as tailings.

The concentrate is roasted and leached in acid, which takes the rare earths into solution and leaves the remaining rock behind. What comes out of the leach is a mixed solution of all the rare earths together, and they have to be separated into individual elements before any of them is useful.

The rare earths are chemically very similar to each other, so separation is done by solvent extraction: the solution is passed, stage after stage, against an organic liquid that pulls some elements across slightly more readily than others. Repeated over hundreds of stages, the small preference becomes a clean split. Neodymium and praseodymium come out together as a pair, and are precipitated and calcined into an oxide powder. That oxide is the feedstock for everything that follows.

An oxide is a metal that has already reacted with oxygen, and it is chemically very stable. To get the metal back, the oxygen has to be stripped off. For neodymium this is done by electrolysis in a bath of molten fluoride salts, running continuously at high temperature.

The oxide is fed onto the surface of the bath, where it dissolves. A direct current is passed between graphite anodes and a tungsten cathode. Neodymium metal forms at the cathode and drips into a crucible below, while the oxygen reacts with the graphite at the anode and leaves as gas. The crucible is periodically tapped and the metal is cast into ingots. A neodymium cell runs around the clock, and the metal it produces is the single most valuable input to the magnet.

The magnet is not made of neodymium alone. Neodymium, iron, boron, and a set of minor additions are melted together under vacuum in an induction furnace to form the magnet alloy. The chemistry set here is the single largest determinant of what the final magnet can be.

The molten alloy is poured through a tundish onto a spinning, water-cooled copper wheel. It freezes in a fraction of a second into a thin flake, well under a millimeter thick, that peels off the wheel. Because the flake is so thin, all of the material solidifies under the same conditions, and the desired microstructure forms throughout: fine columnar grains of the magnetic phase, each wrapped in a thin neodymium-rich layer. That structure is what every later step is built to preserve.

The flake is loaded into a rotating kiln and exposed to hydrogen. Hydrogen is absorbed preferentially by the neodymium-rich layer between the grains, and that layer swells as it takes the hydrogen up. The grains themselves barely change, so the swelling cracks the flake apart along exactly the boundaries that were formed on the wheel.

The flake crumbles, without any mechanical force, into a coarse powder around 200 microns in size. The kiln is then evacuated and heated to drive most of the hydrogen back out. This step is elegant because it uses the microstructure to break the material where we want it broken, and it produces a powder that the next step can mill efficiently.

The coarse powder is fed into a fluidized bed jet mill. High-velocity jets of nitrogen accelerate the particles into one another, and they fracture on impact. There is no grinding media and nothing touches the powder but gas, which keeps it clean. A classifier wheel at the top of the mill lets only particles below a set size leave; anything larger is thrown back into the bed to be milled again.

The result is a powder around three microns in size, tightly controlled, in which each particle is a single grain of the magnetic phase. From this point until the material is sintered, the powder is self-heating: with this much surface area, it will oxidize and warm on contact with air. It is kept under a protective atmosphere throughout.

Each particle of powder is a single grain, and each grain has one direction along which it prefers to be magnetized. To make a strong magnet, all of those directions have to point the same way. The powder is placed in a die, a magnetic field is applied, and the particles physically rotate to line up with it. While they are held in alignment, they are pressed into a solid, fragile shape called a green body, at roughly 70% of full density.

There are two ways to do this. In die pressing, the powder is compacted between punches, with the field applied across the direction of pressing; this is the workhorse process for high-volume parts. Pressing along the field is also possible, but it costs magnet performance. In isostatic pressing, the powder is sealed in a flexible bag, aligned in a pulse coil, and then squeezed by fluid pressure from every side at once. Because nothing pushes the particles out of line, isostatic pressing gives the best alignment and the highest-performing material, at the cost of throughput.

The green bodies are loaded into a vacuum furnace and heated until the neodymium-rich phase between the grains melts, while the grains themselves stay solid. That thin liquid pulls the grains together, the pores between them close, and the part shrinks by roughly 15% in each dimension to become a fully dense metal block. The alignment set in the press is preserved.

The block does not yet perform as a magnet. A second, lower-temperature heat treatment, called aging, follows, and takes approximately one day. Aging re-forms the neodymium-rich phase into a thin, continuous layer around every grain, which is what allows each grain to hold its magnetization independently of its neighbors. The sintering and aging cycle is where the magnetic properties are actually created, and it is the most closely held know-how in the process.

Sintered blocks are made oversize and in simple shapes, because the shrinkage in the furnace cannot be held to a finished tolerance. The block is sliced into individual parts, usually with a multi-wire saw, and each part is ground to its final dimensions. The material is hard and brittle, so it is cut with diamond abrasives and cannot be machined the way a steel part can.

Every cut removes material, and that material is lost. Between the saw kerf, the grinding stock, and parts that chip or crack, machining typically consumes a quarter of the sintered block. This is the largest single yield loss in the plant after the mine, and it is why part geometry and block design matter so much to the economics of a magnet.

The heavy rare earths dysprosium and terbium make a magnet more resistant to demagnetization at high temperature, which is what motors and generators demand. Historically they were added to the melt, which put them everywhere, including inside the grains where they do little good and reduce the strength of the magnet. They are also scarce and expensive.

Grain boundary diffusion puts them only where they are needed. A source of dysprosium or terbium is applied to the surface of the finished part, and the part is heated. The heavy atoms travel inward along the neodymium-rich boundaries, which are far easier paths than the grains themselves, and enrich the outer edge of each grain they pass. The effect is strongest at the surface and tapers off quickly with depth, so the process works best on thin parts. It delivers the high-temperature performance of a heavily alloyed magnet with a fraction of the heavy rare earth, and no loss of strength. The specific methods of applying the source are the most heavily patented territory in the industry.

Neodymium magnets corrode readily. The neodymium-rich phase at the grain boundaries, which every step so far has been working to create, is also the part of the magnet that reacts most easily with moisture. Left bare, the surface will oxidize and the magnet will shed material and lose performance.

The finished parts are coated. Electroless nickel is the common choice: the parts are tumbled in a perforated barrel through a sequence of baths, and the nickel bath deposits metal by a chemical reaction rather than an electric current, so the layer builds at the same rate on every surface, including holes and edges, and the thickness is uniform. Other coatings, from zinc to epoxy, are chosen when the application calls for them. Whichever is used, the coating is a barrier and nothing more; it adds no magnetic performance, and it must be intact for the magnet to survive its service life.

Until this point, the part is not a magnet. Every grain has its preferred direction lined up with its neighbors, but the grains are magnetized in a mix of directions that cancel each other out, and the part will not stick to a refrigerator. This is deliberate: an unmagnetized part can be machined, coated, handled, and shipped without pulling on everything ferrous around it.

To magnetize it, the part is placed in a fixture and a capacitor bank is discharged through a coil around it. The pulse lasts a few thousandths of a second and produces a field far stronger than the magnet itself will ever generate. Inside the material, every grain snaps into the same direction along the axis set at the press, and stays there. The part is now a magnet, and will remain one for decades. Its performance is described by a single curve, the demagnetization curve, which is the culmination of every step before it.