Magnettechnologie in der Verfahrenstechnik wässriger Medien

Abstract

Magnetic separation, in particular the so-called high-gradient magnetic separation (HGMS), represents a relatively new method for solid/liquid separation and an interesting alternative to conventional methods, such as sedimentation or depth filtration. The present professorial dissertation therefore describes the theory of magnetic separation, process technology aspects, the production of magnetic additives, types of magnetic separators as well as applications in industry and the municipal sector. Hence, it deals with the current state of theory and practice of the use of this method in water technology. The comprehensive theoretical section first explains the assumptions and simplifications that lead from the exact, but often complex, basic physical equations to the relationships used in the calculation models. As far as magnetic properties of the separation matrix and the particles to be separated are concerned, it is found that the phenomena of strongly magnetic substances, e.g. self-demagnetization and magnetic saturation, have to be observed. Paramagnetic particles, however, do not exhibit the phenomenon of magnetic saturation in the range of practically usable flux densities. Apart from the discussion of the magnetic field effect, the theoretical part contains a critical review and comparison of the hydrodynamic model conceptions used in the calculation models. For the model of viscous flow around a cylinder, a generally valid approximative solution is presented for the first time. In spite of the extended and verified description of hydrodynamic conditions in an HGMS, however, a calculation model with fixed values for particle size and particle susceptibility cannot describe the results of experiments regarding the magnetic separation of magnetite-containing, amorphous precipitation products. In contrast to this, introduction of a separately determined floc density function allows for a satisfactory prediction of experimental data to be obtained by microscope observation and video evaluation. The following sections present two synthesis paths for the production of magnetite suspensions with particle sizes of the primary magnetite crystals being in the range of about 10 - 30 nm only. These particles are a far better suited additive to increase the magnetic momentum of precipitation products than e.g. natural magnetite of 2 - 3 μm in size. The section dealing with the types of magnetic separators gives a survey of potential sources of magnetic fields as well as of their use in drum-type magnetic separators, high-gradient magnetic separators (HGMS), and open-gradient magnetic separators (OGMS). Particular emphasis is put on carrousel-type magnetic separators based on permanent magnets or the so-called magnetic barrier, a continuously operating OGMS system, which has been specially developed by the author for use in the field of water technology. As examples of potential applications of magnetic processes in water technology, the ferrite process as well as heavy metal and phosphate elimination by high-gradient magnetic separation using a magnetic additive are presented. The section dealing with the applications of the latter method is completed by a survey of additional fields of use of magnetic separation in connection with aqueous media and the results of corresponding tests at Forschungszentrum Karlsruhe. These fields of use include the separation of finest metal particles from cleaning baths and rinsing waters of steel and automotive industry. The professorial dissertation ends with a detailed outlook, in which the problems to be dealt with in the future are identified on the basis of the present state of the art of magnetic technology in the processing of aqueous media.