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metallurgy notes,mineral dressing,mod 4,Lecture 7
 

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    metallurgy notes,mineral dressing,mod 4,Lecture 7 metallurgy notes,mineral dressing,mod 4,Lecture 7 Presentation Transcript

    • MINERAL DRESSING METALLURGY , SEM 3 MODULE 4 LECTURE 7 Ref: Chapter 14 , “ Mineral processing”, S.K.JAIN, CBS PUBLISHERS & DISTRIBUTORS P Ltd, N Delhi, 2012. Chapter 17, “Principles of Mineral Dressing” , A. M. Gaudin, Tata Mc Graw Hill, 1993 . 1
    • MAGNETIC SEPARATION 2
    • MAGNETIC SEPARATION. MAGNETIC SEPARATION is a process in which magnetically susceptible material is extracted from a mixture using a magnetic force. 3
    • MAGNETIC SEPARATION. MAGNETISM. Concentration of certain minerals can be done using MAGNETIC FORCES. Different minerals have different magnetic properties. By combining magnetic forces with gravitational or frictional forces, mineral particles can be separated. Three products are obtained – CONCENTRATES, MIDDLINGS and TAILINGS. MAGNETIC FIELD is exerted by a magnet around it. When a material is kept in this field, it experiences ATTRACTION or REPULSION. 1. Paramagnetic (weakly magnetic)- attracted along the lines of force of a magnet to the point of greater magnetic field intensity( Mn, Cr Ni, Co., Ce, Ti ) Ferromagnetism (strongly magnetic) - A special case of paramagnetism-(permanent)- minerals having permanent magnetism if the temperature is below the Curie Temperature-( the temperature above which a ferromagnetic substance loses its ferromagnetism and becomes paramagnetic ). These materials will become magnetized when placed in a magnetic field and will remain magnetic after the external field is removed (REMNANCE). Eg:- magnetite, hematite, pyrrhotite, ferro-silicon etc. 4
    • MAGNETIC SEPARATION. 2. Diamagnetic ( slightly repelled along the lines of force of a magnetic field) . Metallic Bi faces repulsion in a magnetic field. Magnetic minerals result from properties that are specific to a number of elements. Minerals that do not have these elements and thus have no magnetism are called diamagnetic .Eg:- quartz, calcite, apatite. Magnetic force F ( dynes ) = ( m1m2 )/ μd2. m1 , m2 = strength of magnetic poles , d = distance between poles ( cm ), μ = constant depending upon the medium = = = = Mag . permeability 1 for a vacuum, Near to 1 for most materials, very large for ferromagnetic 5 materials.
    • MAGNETIC SEPARATION. MECHANISM OF MAGNETIC SEPARATIONThe strongly magnetic material is separated from the weaker one using their degrees of SUSCEPTIBILITY – Ratio of the intensity of magnetisation to the magnetic field of the object. It is a dimensionless number - indicates the degree of magnetization of a material in response to an applied magnetic field. fe e d N Materials, which are repelled from the magnetic field, are called diamagnetics. They have negative values of the magnetic susceptibility. S c o n c e n tra te ( par amagnet i c par t i cl es) ta i l i n g ( di amagnet i c par t i cl es) Particles attracted towards greater intensity of the magnetic field are called paramagnetics Magnetisation / unit mass = ς = ( B – H ) / πρ . B = induced flux /unit area ( Gauss ), H = Flux intensity of the existing field, ρ = Sp.gr. Of the particle. Magnetic susceptibility X = (magnetisation intensity / magnetic field strength) in e.m.u. = ς / H. 6
    • MAGNETIC SEPARATION. A mineral particle is “ SATURATED “ on Magnetisation. Ferromagnetic materials are easily saturated. Non magnetic materials are not saturated. Degree of saturation of other materials lie in between these two. Hence they can be easily separated magnetically. 7
    • MAGNETIC SEPARATION. m a g netiza tio n,  w H   ferro m a g netics ferri- a nd a nty ferro m a g netics true pa ra m a g netics 0 dia m a g netics m a g netic field, H m a g netic susceptibility Influence of magnetic field on magnetization of materials ferro m a g n etics tru e p a ra m a g n etics Cu rie p o in t Né e l p o in t a n ty ferro m a g n etics d ia m a g n etics tem pera ture Influence of temperature on magnetic susceptibility of materials 8
    • MAGNETIC SEPARATION. MAGNETIC METHOD IS USED TO SEPARATE 1. “tramp” iron from minerals ( mechanically held impurities) – to safe guard the machines, 2. concentration of Magnetite, 3. concentration of iron minerals made magnetic – roasted hematite ,siderite, limonite. 4. remove small quantities of iron from ceramic raw materials, 5. cleaning magnetic material ( Ferro Silicon Fe-Si) used in HMS etc. REQUIREMENTS FOR EFFICIENT MAGNETIC SEPARATION1.Separation in a moving stream (of particles). 2. The stream moves through consecutive magnetic fields. 3. Polarity of magnets reversed for alternate magnets- the ferromagnetic materials passing under rotate through 180 0 , thus freed from gangue material. 4. Successive magnetic fields should be increasingly stronger- Removes strongly magnetic particles first and weaker by subsequent magnets. 5. The material should be spread over- gangue should not mask. 6. Converging field should be provided at each point of separation. 9 7. The material to be separated should be moisture free.
    • MAGNETIC SEPARATION. EDISON SEPARATOR Edison separator consists of a bar magnet . The ore as a thin stream falls in front of the poles. Susceptible particles are deflected inwards. Nonsusceptible particles continue to fall undeflected. This separator was not wholly successful due to its inability of control flow of solid in a thin sheet and lack of control over the speed of the falling particles. 10
    • MAGNETIC SEPARATION. MAGNETIC SEPARATORS1. Converging field should be produced. 2. Intensity should be able to be controlled. 3. Uniform feeding arrangement. 4. Speed should be controlled. 5. Non magnetic material should not floc and mask the magnetic poles. 6. Disposing separated particles as concentrates, middlings and tailings. DESIGN OF MAGNETIC POLESCONVERGING FIELD NEEDED FOR SEPARATION OF MAGNETIC MATERIALS. 1. The field of a magnet having two point poles ( horse shoe magnet) is STRONGLY CONVERGENT near the poles and UNIFORM at the equator . The field near EACH POLE ONLY IS SUITABLE. 2.If there are two flat poles nearby – UNIFORM FIELD – UNSUITABLE. 3. A pointed pole near a flat pole- CONVERGING FIELD- SUITABLE. 11
    • MAGNETIC SEPARATION. DESIGN OF MAGNEIC POLES A steep magnetic flux gradient is essential for efficient separation of minerals. WEDGE SHAPED attracting poles are used. The lines of magnetic force start from poles normal to the surface. The greatest attraction occurs along the sharp edge of the wedge shaped pole. Concentrate is deflected from the ore path at d1 Middlings are deflected from the ore path at d2 12
    • MAGNETIC SEPARATION. MAGNETIC SEPARATORS1. Electromagnets or permanent magnets ( ALNICO, Ceramics ) can be used. 2. The system may be DRY or WET. 1. DRY MAGNETIC SEPARATORLow and high intensity can be used. The ROM ore is subjected to coarse comminution. Rough concentrate is made first. Subsequent concentration is done by further comminution and magnetic separation. AdvantagesNo water used – more economical. Good for separation after dry autogenous grinding. The concentrate will not get damaged by water under freezing climatic conditions. 13
    • MAGNETIC SEPARATION. DRY MAGNETIC SEPARATORS1.Dry belt magnetic ( WETHERILL ) separator. A high powered machine. Material is fed to the feed roller through a hopper. The material is uniformly spread over the entire width of an endless conveyor belt running over the feed and discharge rollers. It passes through magnets. The poles are arranged one above the other. The poles of the upper magnets have pointed edges . Lower poles are flat. Cross belts prevent adherence of magnetic particles to the poles . They remove the separated particles. 14
    • MAGNETIC SEPARATION. 2. Dry drum separator. It has a series of oppositely charged magnets. They are held inside a rotating drum. The magnets are stationary. The feed is introduced at the top of the drum. Magnetic particles are attracted and held to the drum. Non magnetic particles fall down. The rotating drum brings them to the place where no magnets are placed. They are released . 15
    • MAGNETIC SEPARATION. 3. High Intensity Dry Magnetic separator. Used for weakly magnetic fine sized materials. Two drums made of non magnetic material are surrounded by fixed , permanent magnets The rolls are laminated – alternate lamina of permeable and non permeable materials. So the Magnetic field is strongly convergent towards the rolls. Magnetic materials are separated on the drums. 16
    • MAGNETIC SEPARATION. High Intensity Dry Magnetic separator. 17
    • MAGNETIC SEPARATION. WET MAGNETIC SEPARATORS1. Drum type wet magnetic separator. A drum with fixed assembly of permanent magnets inside dips into a tank holding the pulp. Magnetic particles are lifted from the pulp. Non magnetic particles are left out. The water current keeps the pulp in suspension. The concentrate is removed from the magnets by water sprays / flushing water. Feed A system with TWO DRUMS too can be used. The concentrates from the first drum are treated ( Scavenged ) in the second one. Concentrate Concentrate, middlings and tailings are separated out. Tailings Middlings 18
    • MAGNETIC SEPARATION. 19
    • MAGNETIC SEPARATION. WET MAGNETIC SEPARATORS 20
    • MAGNETIC SEPARATION. ADVANCES IN MAGNETIC SEPARATION. 1. High intensity Wet Magnetic Separation. Can be used to separate weakly magnetic materials ( hematite, ilmenite, Cr, Mn, Ni,Mo etc ) from non magnetic materials. It has 1. an electromagnetic coil of a series of permanent magnets. 2. a rotating ring to converge the flux, 3. feed and wash water entry, 4. discharge chutes for magnetic and non magnetic materials. 21
    • MAGNETIC SEPARATION. High intensity Wet Magnetic Separation. 22
    • MAGNETIC SEPARATION. 2. A superconducting magnet An electromagnet made from coils of superconducting wire. The superconducting wire has NO ELECTRICAL RESISTANCE, at LOW TEMPERATURES ( Eg:- NbTi in Liquid He at 4.2 K) . They must be cooled to cryogenic temperatures ( < −150 °C ) during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire. It creates INTENSE MAGNETIC FIELDS. Superconducting magnets can produce greater magnetic fields . It is cheaper to operate as NO ENERGY IS DISSIPATED AS HEAT IN THE WINDINGS. 23
    • MAGNETIC SEPARATION. DEMAGNETISATION OF THE CONCENTRATE. The Susceptible minerals ( concentrate ) retain residual magnetism after magnetic concentration. It hinders the further processing of the concentrates. A demagnetiser demagnetises the material. It has a A.C. Coil. The AC causes change in direction of magnetic field induced on the material passing through it. This removes the remnant magnetism in the material. The dry material / wet pulp flows through it. The coil has gradually reducing diameter. This results in gradually reducing intensity. 24
    • Comminuted ore Comminuted ore 25
    • THANK YOU 26