All Eriez Magnetic’s Wet Drum Magnetic Separators operate on the same basic principle of a drum rotating around shaft mounted, permanent magnetic elements encased in stainless steel canisters. The drum containing the magnetic elements is mounted in a stainless steel tank which receives the feed slurry. The slurry enters the magnetic field generated by the magnetic elements and the ferro-magnetics are attracted against the drum which rotates the ferro-magnetics out of the slurry stream and discharge as they leave the magnetic field.
In a wet drum separator, the magnetic force acting on a ferromagnetic particle is predominately opposed by hydrodynamic drag force. This feature, when properly applied, provides the vehicle of separation washing away the non-magnetic particles while the ferromagnetic particles are collected in the magnetic field. The hydrodynamic drag force is also responsible for any losses of ferromagnetics.
The variables affecting the collection of ferromagnetics in a wet drum magnetic separator are:
1) Magnetic field strength. The magnetic field strength must be sufficient to effectively collect ferromagnetic minerals.
2) Hydraulic capacity. Ferromagnetic recovery is directly related to the flow rate through the separator. As the flow rate increases, the slurry velocity and consequently the fluid drag force increases which tends to detach more magnetite particles from the opposing magnetic field.
3) Percent solids. The percent solid in the feed directly affects the selectivity of the separation. As the percent solids increases, the slurry becomes more viscous minimizing the effects of the fluid drag to assist in the separation of the silica.
4) Ferromagnetic content. Any given wet drum magnetic separator has the characteristic of removing a limited amount of ferromagnetics based on the diameter of the drum, peripheral speed, and the magnetic field strength. This is referred to as the “magnetic loading”. Exceeding the limits of this magnetic loading will result in increased magnetite losses.
Eriez Magnetics offer three styles of wet drum magnetic separators:
1. Counter Rotation
2. Counter Current (“Steffenson”)
The drum rotates against the slurry flow in the counter rotation tank style. Feed enters the feed box and flows directly into the magnetic field generated by the drum.
The magnetite is attracted by the magnetic field, and thus the magnetic particles are held to the drum surface and pass through an opening between the drum shell and the “squeeze pan” where excess water is forced out.
The relatively long travel of magnetic material through the water while being agitated by the special magnet design gives a good washing action and produces a clean magnetic concentrate.
The feed is introduced near the bottom of the drum and the magnetic particles are picked up by the drum and agitated by wash water jets. The tailings discharge through an overflow at the opposite end of the tank, which also acts as a pulp level control. The counter current design ensures a high recovery of magnetic material and a high quality concentrate.