Hydrogen Decrepitation Plant
HD 200/300
H2-Technologie for Rare Earth-Transition Metal Alloys

Embrittlement and Crushing of NdFeB Permanent Magnet Alloys

The Hydrogen Decrepitation plant, HD 200/300, has been designed for premilling ingot material without mechanical forces when manufacturing permanent magnets from alloys based on the intermetallic compound Nd2Fe14B. This replaces with the mechanical crushing process of the ingot material.

These alloys react strongly with Hydrogen to form hydrides. As a result of this reaction the ingot material, which is normaly loaded as large lumps, embrittles and decrepitates to form a powder with grains less than 1 mm.
The HD treated material may be subjected to a sieving operation in order to scalp off the Fe rich particles which were uneffected by the Hydrogen.
The application of the HD process prior to milling, increases the production rate of a jet mill by a factor of 3 to 4.

The plant consists of

- vacuum vessel
- vacuum pumping unit
- rotation drive unit with magnetic coupling
- gas supply system for H2, N2, Ar
- full safety interlock system for gas handling
- resistance heating elements
- recirculation gas cooling
- PLC process controller

The HD plant is operated fully automatically. Operating lemperatures upto 600 C are standard.The integrated heating system allows partial degassing after the hydrogenation step.

Constructional Features of HD 200/300

The main feature of the plant is a rotary movement of the alloy during the process.
The material is loaded into specially designed drums, which are placed on roller bearings inside the vacuum vessel. The rotation of the drum during the process results in optimum homogeneity of the powder yields.
The integrated heating system allows for partial degassing after the hydrogenation step. Operating temperatures of up 600 °C are standard. Temperatures ranging between 500 and 600 °C are sufficient for degassing the magnetic 14/2/1 phase under vacuum conditions.
All safety precautions, as required when using hydrogen gas in closed containers have been conformed to, and doubled for safety back up.
The standard plants have been designed for 200 kg batch sizes. Higher loads or specifications changes are possible and can be negotiated.

Process Cycle
The automatic cycle comprises six steps:

1. Evacuation
After loading the drum with the material into the reactor, the process is started and the reactor is evacuated

2. Safety Check
An over pressure test followed by a leakage test are preconditional for the release of the hydrogen supply valves

3. Hydriding - Decrepitation Phase
Hydrogen absorption starts with an incubation time which can be between 5 and 20 minutes depending on composition and microstructure of the ingot material. During the absorption the temperature of the material increases due to the exothermic character of the reaction.

4. Degassing
The 14/2/1 phase is degassed by heating up under vacuum conditions. Degassing is temperature and pressure controlled.

5. Cooling
The degassed powder is cooled down in recirculated Ar gas which transmits the heat to the water cooled reactor wall.

6. Purging - End of Cycle
The proces cycle ends with evacuation and purging the reactor with Ar. The reactor is ready for unloading.
The drum rotates during the hydriding, degassing and cooling phases in order to optimize heat transfer and material homogeneity within the container. Gas pressures are controlled at an over pressure of 50 mbar typically.

Advantages of the HD 200/300 in comparision to mechanical pre-milling of NdFeB magnet powders

- increased capacity in the subsequent fine milling step, milling rates 3-4 times faster are typical
- excellent homogeneity of the powder due to the rotary principle
- narrow grain size distribution
- short cycle time
- partial degassing yields reduced sinter cycle times
- lower investment and production costs