Netzsch Trockenmahltechnik GmbH, Hanau, Germany, states that it has developed a new grinding process for the production of raw material for the manufacture of neodymium-iron-boron-magnets.
In order to produce raw materials which give the highest quality and properties for permanent magnets, Netzsch offers a patent pending grinding process with downstream classifying of rare earth powders. The system includes newly-developed jet mills and ultra-fine classifiers with which sensitive Nd-Fe-B-compounds or other rare earth alloys can reportedly 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, the highest reproducible finenesses independent of the load in the gas jets can be obtained. One advantage offered by the M-Jet, compared to fluidised 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 and takes place during operation of the M-Jet.
By automatically rejecting components which would be too difficult to grind and transporting them into the filter, the M-Jet avoids problems which could be caused by contamination of the product-conveying piping with coarse product particles and/or problematic components. Furthermore, the product content during the grinding phase of the M-Jet is said to be 20–25 times lower than that of a fluidised bed jet mill with the same amount of grinding gas.
This high product content results in a very low level of product loss during product change, due to the smaller volume of the grinding chamber. In addition, it is said that almost no fluctuations in throughput capacity, and especially in the particle size distribution, occur during starting and stopping of the plant. It is further noted that selective grinding of individual alloy components does not take place during the M-Jet’s grinding operations.
The dynamic air classifier integrated into the M-Jet is designed to ensure a clearly defined maximum particle size of the ground product. In a subsequent step, undesirable finest particles are separated out by classifying the ground material with a NETZSCH High-efficiency Fine Classifier M-Class to obtain a powder with a defined, narrow particle size distribution.
Compared to a ground 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 is also said to improve significantly from 3.6 to 2.6 (each with d50 = 3.0 μm) after additional classifying.
The higher grade of the raw materials produced using Netzsch’s new process is reported to be reflected in the quality of the magnets manufactured with them. Compared to magnets produced using ground powder only, rare earth magnets produced using classified powder are said to offer a higher coercive field strength and significantly improved knee-field strength. This could aid their ability to meet future challenges for the magnet industry, including progressively increasing miniaturisation at constant throughput.