Magnetic Techniques for the Treatment of Materials

Transcription

1 Magnetic Techniques for the Treatment of Materials by Jan Svoboda Formerly Research Programme manager and Principal Technology leader, De Beers Diamond Research Laboratory, De Beers Consolidated Mines (Pty.) Ltd., Johannesburg, South Africa KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON / LONDON

2 Contents Contents Preface v xi 1 Principles of Magnetic Treatment Magnetic separation and innovation Principles of magnetic separation Magnetic force on a magnetizable particle Competing forces in a magnetic separator Fundamental quantities of magnetism Magnetic field and magnetization Magnetic susceptibility and permeability Magnetic units Magnetic properties of materials Diamagnetism Paramagnetism Ferromagnetism Antiferromagnetism Ferrimagnetism Demagnetization Mixtures containing ferromagnetic (s.l.) materials Magnetic properties of minerals The classification of minerals according to their magnetic properties Magnetite Titanomagnetites Ilmenohematites Maghemite Hematite Goethite Siderite Iron sulphides Chromites ^jft." Measurement of magnetic properties jf

3 CONTENTS Measurement of the magnetic field strength Measurement of magnetic susceptibility and magnetization Sources of magnetic field Permanent magnets Iron-core electromagnets Solenoid electromagnets Superconducting magnets Generation of the magnetic field gradient 59 Review of Magnetic Separators Dry low-intensity magnetic separators Magnetic pulleys Plate magnets Grate magnets Suspended magnets Drum magnetic separators Wet low-intensity drum magnetic separators Concurrent tank " Counter-rotation tank Counter-current tank Design of the drum Dry high-intensity magnetic separators Cross-belt magnetic separator Induced magnetic roll separator Permanent magnetic roll separator High-intensity drum separators Wet high-intensity magnetic separators Dry separation versus wet separation Iron core-based wet high-intensity separators Solenoid-based magnetic separators Superconducting magnetic separators Cyclic superconducting HGMS Reciprocating superconducting HGMS Open-gradient superconducting magnetic separators Laboratory magnetic separators Davis Tube Frantz isodynamic separator Separators with a rotating magnetic field Eddy-current separators Separators with magnetic fluids Basic concept Ferrohydrostatic separators Magstream separator 159

4 CONTENTS vii 3 Theory of Magnetic Separation The forces and the equations of particle motion Particle motion in drum and roll separators Motion of a particle on the drum or roll surface Motion of the particle after it leaves the roll Separation of particles by a suspended magnet Magnetic force in a suspended magnet Torque on a magnetizable body Demagnetization factors The effect of the burden High-gradient magnetic separation Theory of particle capture by a single-collector Capture of particles in multi-collector matrices Linear open-gradient magnetic separation Magnetic flocculation Magnetic flocculation of strongly magnetic materials Magnetic flocculation of weakly magnetic minerals Magnetic separation by particle rotation RMF rotation separation Magnetic torque on a particle and condition for separation by rotation Rotation of particles in a liquid Separation in magnetic fluids Apparent density of a magnetic fluid A particle suspended in a magnetic fluid The effect of the hydrodynamic drag The effect of particle magnetic susceptibility Motion of particles in a ferrofluid Separation in a rotating ferrofluid Eddy-current separation Static ramp eddy-current separator Rotating drum eddy-current separators Eddy-current separation by particle rotation Design of Magnetic Separators Introduction Design of circuits with permanent magnets Basic concepts and figures of merit The static gap design Leakage factors Determination of permeance and leakage factors Magnetic field around a permanent magnet Magnetic forces Soft iron components of the magnetic circuits Stability of permanent magnets Magnetization of permanent magnets 269

5 viii CONTENTS 4.3 Design of iron-core electromagnets Calculation of the magnetic field Shape of iron parts of the circuit Practical procedure for design of the poles Design of solenoid magnets The power requirements The choice of the conductor The cooling of coils Thickness of the iron cladding Design of drum magnetic separators The basic magnet arrangements Geometry of the magnet assembly Electromagnetic modelling of the design Design of a magnetic roll Basic concepts of design Design parameters Design of a suspended magnet Design of a ferrohydrostatic separator FHS separators with a wedge-shaped working gap Separators with hyperbolic pole tips Calculation of profiles of pole tips in FHS Scale-up of ferrohydrostatic separators Practical Aspects of Magnetic Methods Selection of magnetic separation technique Dry magnetic separation Electrostatic phenomena in dry separation Density and magnetic tracers Comparison of dry magnetic separation techniques Wet magnetic separation Wet low-intensity drum magnetic separation Wet high-intensity magnetic separation : Magnetic field in high-intensity magnetic separators Matrix in high-gradient magnetic separators Flow velocity of the slurry Slurry density Feed rate The effect of particle size Pre-treatment of slurry The selectivity of separation Comparative tests with wet high-intensity magnetic separators Magnetic flocculation Magnetic flocculation of ferromagnetic materials Magnetic flocculation of weakly magnetic minerals Demagnetization 422

6 CONTENTS ix 5.6 Separation in magnetic fluids Magnetic fluids Ferrohydrostatic separation Ferrofluid recovery and recycling Separation in a rotating ferrofluid Magnetism in other areas of material handling Magnetic cyclone Magnetic gravity separation, flotation and vacuum filtration Magnetic jigging Industrial Applications Treatment of minerals Recovery of grinding ball fragments from SAG and ball mills Heavy medium recovery circuits Beneficiation of iron ores Beneflciation of manganese ores Beneficiation of ilmenite ores Beneficiation of ores of non-ferrous metals Mineral sands Industrial minerals Magnetic beneficiation of coal Nuclear industry Removal of solids from fuel dissolver liquor Removal of ferric hydroxide floes Soil decontamination Waste water treatment Steel industry Thermal power plants Magnetic carrier techniques General principles Effluent processing and metal ion removal Biomagnetic extraction of heavy metals Magnetic carriers and separation in biosciences Principles of biomagnetic separation techniques Magnetic separators in biosciences The application of magnetic separation Separation of red blood cells by HGMS Recovery of metals from wastes The separation processes in metal recycling Sorting of heavy non-ferrous metals The applications of ferrohydrostatic separation Recovery of gold and other platinum-group metals Applications in the diamond winning industry The fractionation analysis of coal 546

7 x CONTENTS 7 Innovation and future trends Introduction Science and technological innovation Magnetic separation and innovation Reliance on an empirical approach? A walk through innovation in magnetic separation The current status of magnetic separation technology What the future holds? Mineral processing Biomedical applications Recycling and waste processing Prospects for superconductivity in magnetic separation Research and development needs 573 List of Symbols 577 Bibliography 583 Index 629