12/06/2004 • Mechanical process technology • Process technology / Maintenance • Systems and Components

Insuring Surface Quality by means of high gradiant magnetic filtration

The steel industry is worldwide booming. Steel sheets with high surface properties are even more important tomorrow then today. Additional environmental and economical operation procedure plays a major role in fulfilling the operational targets. Efficient treatment of operating liquids is one key factor for the achievement of all mentioned points above. Car and food industries are enforced to work on this fields more and more.

Before coating the surface has to be free of particles ad free of grease. The relevant decreasing liquid is circulating in the process and within that treated. Are present processes mainly treated by settling processes in the tank and in some cases with magnetic chain filters the process tanks has to be cleaned manually in specific time distances. Discharging the liquid continuously often does the grease removing; economical and ecological an expensive way.

Nowadays, in some very innovative plants in Europe the steel producer are implementing additional equipment by means of membrane filters for the grease removal and magnetic filters for the solid removal. Both processes had left the field of testing and research. The latest development is the solid filtration by high gradient magnetic filter HGF. A most modern magnetic filtration system was implemented two yeas ago and is successfully working since nearly two years. (Pic 1)

Under the influence of a high gradient magnetic field generated by a permanent magnetic system (pic. 3a), very small particles get attached to a filter matrix (pic. 3b). After being loaded, the magnetic field is switched off by turning the magnet rotor and the matrix is rinsed with a flush. The magnetic field is then nearly zero Tesla and therefore the particles can easily be removed from the matrix.

In the HGF, ferrous particles are separated at a finely meshed wire matrix. This wire matrix consists of wires as fine as 100 – 200 _m which are arranged regularly. In the area of a magnetic field with an approximate strength of 0.4 Tesla they generate the high field gradients strictly required for the separation. The filter matrix is rinsed by the particle containing liquid. The filter matrix is surrounded by a so-called iron yoke, similar to a horse-shoe magnet. On the opposite side of the matrix, the iron yoke is interrupted by a large and strong permanent magnetic rotor .
When switched on, the permanent magnet is arranged in a way that a magnetic flux through the iron yoke is produced.
In this position, the ferrous particles are separated at the matrix.

If the filter matrix is loaded sufficiently with ferrous particles, the liquid input is interrupted and simultaneously, the permanent magnetic rotor is turned by 90 degrees by a gear motor. The poles are magnetically short-circuited, therefore the magnetic flow in direction of the filter matrix is interrupted as well.

In this state of the magnetic system, the rinsing valves are opened for a few seconds, and a short and intense flush rinses the filter matrix. This concentrate has a solid content of approximately 5 g per litre, after a usual input concentration of 100 – 500 milligram per litre.
Depending on the conditions prevailing in the plant, the further concentrate can be treated in the industrial waste water treatment or can be enriched to an even higher concentration up to a viscous sludge may be achieved by means of a magnetic wet drum separation. The magnetic wet drum separator NTS works with a stainless steel drum with a diameter of slightly more than 110 cm. In its interior, there is a neodymium permanent magnet arranged eccentrically. The preconcentrate is introduced evenly over a dam into the half circular sorting area below the drum. The ferrous particles are taken up by the drum surface. By rotation of the drum shell contrary to the liquid flow, the separated solid material is transported to the upper, non-magnetic part of the drum. In the area without a magnetic field, a discharge plate takes over the filter cake and removes it. The purified liquid is introduced once more into the process. Of course, this applies only if the HGF has been rinsed before with its own medium.

With a success of about 90 % for the removal of the very fine particles, even below one micron, the most modern High Gradient Magnetic Filter HGF has increased the economical benefit of liquid treatment technology. The particle content could be continuously reduced. Other continuous liquid treatment processes such as ultra filtration has become much more effective and economical. The removed very fine particles are not destroying the sensitive membranes anymore. The membranes are much longer in use.

Beside the continuous high separation efficiency the advantages of the HGF-process lies in the secured operation: no moving parts are in the liquid, no wear, no mechanical cleaning of the filter, permanent magnets avoid electrical power supply and heat emissions, nearly continuous operation with e.g. hours of filtration and 15 seconds for filter cleaning.

Newest experiences have shown, that even in the field of cooling liquids in steel rolling mills with their tremendous volume can be improved by implementing an high gradient magnetic filter. The flocculants can be reduced by more than 70 % and the remaining solid content is ensured at an low level as well. Again economical and ecological reasons are the force behind this development. Matching both environmental and financial targets are the base of process integrated environmental technology.

The technology, developed with research institutes, is well tested, approved and operating in the industry even in the tin and zinc plating industry or industrial car paint shops in Europe. A test unit is available to run tests on the customers plant and with its material.
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STEINERT Elektromagnetbau GmbH

Widdersdorfer Str. 329-331
50933 Köln

Phone: +49/(0)221/4984-0
Fax: +49/(0)221/4984-102