Grinding and Classifying of Rare Earth Powders and their Use in Enhancing Quality in the Manufacture of Magnets

Neodymium-iron-boron-magnets (Nd-Fe-B) are so-called rare earth magnets distinguished by their very high energy densities. Thus, they are constantly gaining importance in the age of electric mobility and miniaturization and particularly in application areas in which strong magnetic fields at low volumes and low weights are necessary. An increasing focus is on resource conservation of raw materials, weight reduction of the drives and longer service lives of permanent magnets. A decisive factor for the quality and properties of permanent magnets is a narrow particle size distribution with the lowest possible fraction of finest- (< 2 µm) and coarsest particles (> 8 µm).

With jet mills and ultra-fine classifiers made by NETZSCH, sensitive Nd-Fe-B-compounds or other rare earth alloys can be ground reliably to fine powders under inert gas operation giving a narrow particle size distribution and defined upper particle size limit with reproducible results.

With NETZSCH M-JET, a combination of a spiral jet mill with a dynamic air classifier, highest reproducible finenesses independent of the load in the gas jets are obtained. A decisive advantage of the M-JET compared to fluidized bed jet mills or target mills is the possibility of the automatic rejection of components which are difficult to grind. This requires only a few seconds. The overpressure in the mill ensures that components which are difficult to grind are transported into the filter. In this way, there is absolutely no problem caused by contamination of the product-conveying piping with coarse product particles and/or components which are difficult to grind.

Due to the differences in design, the product content during grinding phase of an M-JET is 20 to 25 times lower than that of a correlated fluidized bed jet mill. Due to this fact, practically no fluctuations in throughput capacity and especially in the particle size distribution occur during start and stop of the plant. Furthermore, a selective grinding of individual alloy components does not take place.

The dynamic air classifier integrated in the spiral jet mill provides controlled maximum particle size of the ground product. In a subsequent step, undesirable finest particles are separated by classifying the ground material with a NETZSCH High-efficiency Fine Classifier M-CLASS (optimized version of CFS/HD-S) to obtain a powder with a defined, narrow particle size distribution.

Compared to a milled product with a d10 value of 1.54 µm, the d10 value of the subsequently classified product is 2.03 µm and the proportion of ultra-fine particles < 1 µm is almost 0.0 %. The d90/d10 value also improves significantly from 3.6 to 2.6 (each with d50 = 3.0 µm) after additional classifying. Compared to magnets made of ground powder only, rare earth magnets made of classified powder have a higher coercive field strength and significantly improved knee-field strength Hk and rectangularity R = Hk/Hcj.