THE NEW MEDIUM-INTENSITY DRUM-TYPE PERMANENT MAGNETIC SEPARATOR PERMOS (C) AND ITS PRACTICAL APPLICATIONS FOR THE PROCESSING OF INDUSTRIAL MINERALS AND MARTITIC IRON ORES

: KHD Humboldt Wedag AG developed an improved version of a drum-type magnetic separator PERMOS with special magnetic field configuration for the NdFeB permanent magnets which is giving the magnetic field strength of 0.7 Tesla and a wide working range outside the drum. The magnet system within the P ERMOS unit is no longer made of relatively large magnet blocks of alternate radial magnetisation; the system is now made up of a multitude of small NdFeB bars with a specially oriented magnetisation. This open-gradient magnetic separator can be used for dry magnetic separations in "top-feed" or "free-fall" feed configuration as well as in the conventional wet "drum-type" feed mode. It allows high throughput rates of up to 40 tonnes of solids per hour per meter of the drum length without any dogging problems in case of higher magnetite content in the feed material. Important design features and the actual machine characteristics of the improved PERMOS units for dry and wet magnetic separation are described and typical examples for their commercial application for industrial minerals and for martitic iron ores are presented. the project manager for mineral dressing, pelletizing joined KHD Humboldt 1980 as the product manager for mineral dressing, equipment and systems and is now the head of the Process Department of the Mineral Dressing Division.


INTRODUCTION
In the past magnetic separators for the beneficiation of ores and minerals were generally divided into two categories" -low-intensity magnetic separators for the upgrading of strongly magnetic The low-intensity magnetic separators used in commercial operation are mainly drum-type open-gradient separators (OGMS) in which systems of permanent magnets made of barium ferrites generate magnetic field intensities of less than 0.2 Tesla in the separation zone. These drum-type low-intensity units are used mainly for the upgrading of magnetite iron ores in wet or dry processes and for the recovery of dense media in heavy medium plants.
The high-intensity magnetic separators used on commercial scale are mainly wet---operating matrix-type separators using high intensities (WHIMS) and/or high gradients (HGMS). They provide high throughput rates at magnetic field intensities of up to 1.5 Tesla in the separation zone with electromagnetic coils, and more than 3 Tesla with superconducting coils. Their main field of practical applications is processing of fine-grained, feebly magnetic iron ores, ilmenite ores and beach sands, the removal of iron-bearing contaminants from fine-grained industrial minerals, including improving the brightness of kaolin [1,2].
Meanwhile, the required medium field strength of up to 1 Tesla can also be made available by using the new permanent high-duty magnetic materials, particularly neodymium-iron-boron (NdFeB) compounds. This material has the remanence of up to 1.2 Tesla, and the coercive force dose to 10 A/m, with the energy product of up to 300 kJ]m . These properties allow relatively high degree of feedom in shaping individual magnet blocks to the desired optimum magnet system.
For efficient performance of drum separators, only forces acting in the radial direction are useful because they attract particles to the surface of the drum. Once being on the surface, the particles must be transported tangentially to the outlet for the magnetic product, by means of friction on the drum surface. Tangentially-acting magnetic forces disturb this more or less smooth movement of the magnetic particles to the outlet. Therefore, it is very important that the magnetic field strength decreases strongly radially (high radial forces), but remains as uniform as possible along the circumference of the drum (low tangential forces) [3].  Fig. 1 [4]. The magnet structure of the PERMOS separators It does not comprise any more large blocks of magnets magnetised radially, but contains a plurality of small magnet blocks whose direction of magnetisation changes in small steps. For this system, the number of poles is not the same as the number of magnetic blocks. An arcuate magnet system with about 50 blocks consisting of small NdFeB elements magnetised in different directions and covering an angle ranging from 90 to 120 degrees of the total circumference was selected for commercial PERMOS separators. Such a design produces the magnetic field intensity of approximately 0.7 Tesla on the drum surface, and more than 0.3 Tesla at the distance of 8 mm from the drum surface, with minimum tangential forces, as is shown in Fig. 2. The PERMOS separator is thus described as the medium-intensity magnetic separator with medium working range. The measured pattern of the magnetic of the PERMOS separator The NdFeB magnet system is fixed in a certain position within the revolving stainless steel drum on which separation of strongly and feebly magnetic material from the non-magnetic components takes place. This process is identical to that used in conventional low-intensity drum magnetic separators which are in extensive commercial use for dry as well as for wet separation.

PERMOS DESIGN FOR WET MAGNETIC SEPARATION
The development work for wet medium-intensity separation was performed with an improved version of this PERMO$ WET prototype, according to Figs. 3 and 4, with the diameter of the drum of 600 mm and the drum length of 600 ram. The separator had the following features: -pulp tank made of stainless steel with semi-counter-current flow tank design and adjustable discharge gap at the discharge end of the magnetics, vertical non-magnetic discharge pipe with a valve for flow rate adjustment and lateral pulp overflow lip Top view of the PERMOS permanent magnet separator Further successful testwork was carried out with the removal of the iron-bearing contaminants from slimes originating from the polishing of optical glasses. The results are summarised in Table I. It should be noted that presently the total amount of this feed material must be deposited as the critical waste in special waste deposits. Since the two-stage magnetic separation using PERMOS reduces the concentration of FeO to less than 0.6 %, it will be most probably possible to recycle more than 90% by mass of the polishing waste back into the feed blend for the glass melting charge, and send only about 10% by mass to the special waste deposit. Complex martitic iron ores are characterised by a wide range of magnetic susceptibilities and for their magnetic separation the magnetic field strength and configuration of the separators must be adapted according to the concentration of martite. Table II Magnetic separation of the martitic iron ore using the   Table II, this ore with 58% Fe feed grade could have been upgraded to the pellet feed Of 66.8% Fe and less than 3.5% SiO by a simple wet separation using PERMOS followed by the JONES WHIMS. In spite of very low magnetite equivalent, the PERMOS separator itself produced already 35% by mass of the concentrate with excellent grade of 68.75% Fe, and only 1.0% SiO.
In the capacity test series performed so far for wet magnetic separation of martitic iron ores from Brazil, Venezuela, Iran and others, the solids throughput rates of up to 40 t]h per meter length of the drum, and the pulp throughput rates up to 160 m]h were obtained without apparent changes in the metallurgical results.

PERMOS DESIGN FOR DRY MAGNETIC SEPARATION
The essential parts of the standard PERMOS unit for dry separation ,with 600 mm diameter are illustrated in Fig.5 Cross-section of the PERMOS DRY separator with horizontal feeding onto the drum This unit can also be used as the so--called "free-fall" magnetic separator, if the vibrating feed chute is positioned in the opposite direction, as shown in Fig. 6. In this case the revolving drum will be fed by the vertical free fall of the material directly in a small horizontal distance from the drum shell. The vertical fall of the non-magnetic feed material is thus not hindered and only the magnetically susceptible components of the feed are attracted onto the drum, or their trajectories are at least directed towards the revolving drum, so that they can be separated from the non-magnetics by the splitters. The main application for the PERMO$ DRY separators is the removal of iron-bearing impurities from abrasives and other primary industrial minerals as well as dry preconcentration of martitic and hematite iron ores in those cases where no water or not enough water for wet processing is available. The results of a typical possible application are shown in Table IIl, for the final cleaning stage by magnetic separation after preconcentation by the DE$CO$ superconducting magnetic separator and by subsequent scrubbing. Feebly magnetic gangue mineral serpentinite is more or less completely removed over the entire size range fraction, PERMOS PERMANENT MAGNETIC SEPARATOR 211 producing a concentrate with the silica content of less than 0.5%, to obtain a very high-grade and high-quality sinter magnesia in a rotary kiln plant in Turkey. In the capacity test series performed so far for dry magnetic separation, the feed rates of up to 50 t/h per meter of the drum length were obtained, with different feed materials without apparent change in the metallurgical separation results., for the direct horizontal feeding onto the drum, as well as fo the free fall feed mode.
At higher throughput rates, a reduction on the selectivity of separation with the increasing feed rates was observed.