APPARATUS AND METHOD FOR MAGNETIC SEPARATION

Abstract: A method for the selective removal of non-magnetic hydrophobic substances from suspension or solution by adsorption on the surface of a particulate, hydrophobic magnetic material, characterised in that the solution or suspension is passed through a plug of the magnetic material, said plug being formed and maintained by a magnetic field designed to retain the magnetic particles against both viscous drag and gravitational forces and to give a uniform packing of the magnetic particles across the direction of flow of the solution or suspension.

Description: - ι - "APPARATUS AND METHOD FOR MAGNETIC SEPARATION" This invention relates to an improved apparatus and method for magnetic separation. The invention is especially concerned with the use of adsorption and diffusion effects to assist in the selective recovery of non-magnetic materials by the use of magnetic materials. In the past, magnetic separators have generally been designed around one of two principles. The first and most generally used design is the single-surface- type magnetic separator, i.e., in which the magnetic field-producing elements also directly produce the field gradients. Examples of these single-surface separators includes grade suspended magnets, pulleys and drums. A drum separator may have permanent magnets or electromagnets to produce both the magnetic fields and the gradients on the trapping surface of the drum. The second most common design is the matrix-type magnetic separator wherein an electromagnetic coil or coil—iron combination is utilised to produce a relatively uniform magnetic field in an open working - volume. The working volume contains a ferro-magnetic structure which' is magnetised to produce strong, magnetic-field gradients on the sharp edges or small radius-of-curvature elements of the structure. Further details•of separators ' of both of these types may be found in the literature, for example, in irk-Othmer "Encyclopaedia of Chemical Technology", Third Edition, Volume 14, John Wiley & Sons. The method as described in the literature are usually employed to separate magnetic particles from non-magnetic particles. It is also known, however, to use magnetic particles as adsorbents for non-magnetic material, e.g. , where the surface of the particles is able to exert some attractive effect on the other material. Examples include the adsorption of oil onto the hydrophobic surface of magnetite. Also in the prior art, magnetic- particles or fibres have been used to bring about iltration by entrainment of non-magnetic particles in a mess of the ' magnetic particles or fibres " which can be caused to " contract and expel solvent or supporting liquid. The present invention is not concerned with the normal processes of magnetic separation and filtration. Rather, the invention is concerned with facilitating the handling and treatment of hydrophobic * non-magnetic substances by the use of magnetic materials, particularly those having hydrophobic surface properties. Ϊ3U- / According to one aspect of the present invention, there is provided a method for the selective removal of non-magnetic.hydrophobic substances from suspension or solution by adsorption- on. the surface of a 5 particulate, hydrophobic magnetic material, which method comprises passing the solution or suspension through a plug of the magnetic material, said plug being formed and maintained by a magnetic field designed to retain the magnetic particles against both 10 viscous drag and gravitational forces and to give a uniform packing of the magnetic particles across the . direction of flow of the solution or suspension. . * It will usually * be convenient to form the plug by mixing the magnetic material with the suspension or 15 solution and thus, in accordance .with a further aspect of the invention, there is provided a method for the selective removal of non-magnetic hydrophobic substances from solution or suspension by adsorption on the surface of a particulate hydrophobic material, 20 which method comprises mixing the -magnetic material with the suspension or solution and passing the mixture through a magnetic field as defined above. When used in this way the magnetic field causes the progressive building up of magnetic particles into 25 a porous plug.. The solution or suspension is thus forced to take tortuous . flow path through the plug and thereby ensure intimate contact between the non-magnetic material in the suspension or solution and the magnetic particles. Thus, the effects of 30. surface adsorption can be fully realised and thereby ensure optimum adsorption of the non-magnetic substance onto the magnetic material. The design of the magnetic field is an essential part of the invention and preferably consists of a 5 field which is extended in the direction of flow of the solution or suspension and is configured in such a way that over at least a substantial portion of the length of the field, the product of the magnetic field intensity and the gradient of the field is constant. 10 • in practice, this can be achieved by providing at least two pairs of magnetic poles, the poles of each pair being on opposing sides of the flow path of the suspension or solution, the pairs being of opposite polarity, i.e., . N-S for the first pair and S-N for the 15. second pair. These poles may be constituted by the poles of- a permanent magnet but generally it will be more convenient to use electromagnets. It will usually be necessary in using the method of the invention to draw off some-of the magnetic 20 particles from the bottom of the plug and continuously regenerate the plug from above. This can be done by adjusting the magnetic field " strength in order to allow a continuous bleed of the magnetic particles from the bottom of the plug at a rate equal to the 5 addition of particles from the top. Alternatively, after an adequate depth of plug has been generated and sufficient time has been allowed for adsorption to take place, the magnetic field may be removed, e.g., by temporarily switching off or reducing the exciting 0 . current in the windings of the electromagnets generating the field. This process allows the plug to move downwards a small distance whereupon the magnetic field is again applied and a new plug created on top of the initial plug while, magnetic particles with the entrained or adsorbed non-magnetic substance, are lost from the plug into the effluent liquid stream. The magnetic particles can then be removed from the effluent stream by any suitable means, usually and most desirably, a magnetic separator of conventional or any other type. The invention also provides apparatus for carrying out " .the above method,, the said apparatus comprising a conduit for passage of liquid and . magnetic field generating means on opposing sides of the conduit, said field generating means comprising at ■ -least two- airs of opposed magnetic poles, said pairs being of opposite polarity and being arranged in spaced relationship so as to produce a magnetic field which is extended in the longitudinal direction of the conduit and in which, over at least a substantial proportion of its length, the product of-the magnetic field intensity and gradient is constant. More than two pairs of magnetic poles can be used if desired thereby providing a still further extended field. The poles, are preferably the poles of electromagnets. Preferably also the poles have a general form of a cusp generated by the intersection of two circles or similarly curved figures with the apex of the cusp adjacent to conduit. Other pole shapes are, however, possible, the only limitations being that the field must have the specified characteristics. The method and apparatus of the invention are of ' particular value fox the recovery by adsorption of bacterial or algal cells which are adsorbable onto magnetite or other magnetic materials by hydrophobic effects and which are present at low concentrations in aqueous suspensions.. One application of particular importance is the harvesting from saline waters of algae of the genus Dunalielia for. subsequent recovery from the algae of valuable -substances including beta-carotene and glycerol, for example as described in Australian Patent Application No. PF 1093/81. Magnetite is a particularly suitable material for this purpose but . other magnetic minerals such as haematite may also be used. Apart from the specific applications just referred to, the method of the invention is also applicable to a wide range of other uses. For example, hydrophobic substances adsorbed on magnetic substrates according to the " teaching of the invention may be subjected _in situ to various physical and chemical treatments before being desorbed from the substrate. These and many other possible modifications will be obvious to those persons skilled in the art. The invention is further described and illustrated by reference to the accompanying drawing. The drawing shows a cross section of the apparatus. The conduit 1 is of elongated rectangular cross section (as shown in the drawing, its widest dimension extends vertically through the plane of the paper) . On either side of the conduit is a shaped low carbon iron block 3. The blocks 3 constitute the armatures of an electromagnet and define longitudinally- extending channels housing the windings 5 of the electromagnets. These channels partly define the pole pieces of the electromagnets which, as shown in the drawing, are generally cusp shaped at their ends with the apexes of the cusps adjacent to conduit. The windings are connected in such a fashion that, when the magnet is energised the two " se ' ts of pole pieces are of opposite polarity, as shown in the drawing. Bands 4 extend the. length of the blocks and are joined across the faces of the blocks to complete the magnetic circuit. The magnetic field thus generated is such that the product of the magnetic field-intensity and * the * field gradient (i.e., the differential of the magnetic field intensity) is constant along that part of the conduit defined by the two " sets of poles except for a * singularity on the axis of symmetry 2 of the poles. When the field is energised and a liquid stream containing magnetic particles is passed downwards through the conduit, the magnetic particles form threads extending between the singularities 2 and thereby form a column or plug of the magnetic particles within the conduit. The thus formed plug is - of remarkably uniform density throughout its width and length. An experimental magnet .(used in the following examples) had the following dimensions : Width (each block) approx. 1.5" - (38mm) Height " 3 " ( 76mm) Thickness " 1" ( 25mm) Coil channels (diameter) " 0.5" (12 .5mm) Radii of pole piece faces" 0 .75 " (19mm) Spacing of poles (vertically) - approx. 1" ( 25mm) The energizing coils each contained 100 turns. The following examples illustrate the invention. EXAMPLE 1 The apparatus used included the experimental electromagnet assembly described above. The conduit used as the- column,was a rectangular tube with an outside cross section of approximately 7/8" x 1/4" ' (22.2 x 6.4mm) and inside measurements of approximately 3/8" x 1/16" (9.5 x 1.6mm). The length of the column was approximately 9" (230mm) . The column was arranged between the poles of the magnet assembly and the top connected to a. container with a stirrer in which the test suspension was placed. ■ '"- ' The magnetic material used was silanised magnetite prepared as described in our copending Australian patent application No. F 1093/81. Dried 120 mesh (BSS) magnetite was treated with a 1% 5 solution of dichlorodi ethyl silane in petroleum ether at ambient temperature for three hours. The treated magnetite was separated from the solvent by decantation and draining with the aid of a magnet, dried for half an hour and then demagnetised. 10. A mixture of .the silanised magnetite (lg) in 1 litre of brine containing Dunaliella salina (from a .salt lake) .were stirred together in the container and . then allowed to pass down the column at a rate of 15 ml/min. . 5 With the -magnet 'energized with an exciting current of 1 Amp. the.magnetite formed into, and was retained in the column as, a plug in the region of the magnet assembly while the saline water percolated through. The flow rate was sufficient for some 0 magnetite to be * lost from the lower end of. the plug- at a rate equal to the inflow of magnetite from the top. The magnetite emerging with " the effluent from the column was continuously recovered with a stationary magnet. Cell debris and other unwanted materials 5 remained in the liquid effluent. The recovered magnetite was analysed for beta-carotene and the analytical results indicated an 82% recovery of the total beta-carotene content of the algae. EXAMPLE 2 The above experiment was repeated expect that the current through the magnet coils was 2. Amp. and the. field was pulsed by turning off the current for l/lOOth second every 1/10 second. Analysis of the recovered magnetite showed an 80% recovery rate for the beta-carotene.

Claims: 11 CLAIMS : 1. A method for the selective removal of non-magnetic hydrophobic substances from suspension or solution by adsorption on the surface of a particulate, hydrophobic magnetic material, characterised in that the solution or suspension is passed through a plug of the magnetic material, said plug being formed and maintained by a magnetic ield designed to retain the magnetic particles against both viscous drag and gravitational forces and to give a uniform packing of the magnetic particles across ^ the direction of flow of the solution or " suspension. 2. A method * as claimed in Claim 1, characterised in that the magnetic material is mixed with the s spension or solution and the mixture is passed through the magnetic field thereby to form the said plug. 3. A method as claimed in Claim or Claim 2, characterised in that the field is extended in the direction of flow of the solution or suspension and is configured in such a way that over at least a substantial portion of the length of the field, the product of the magnetic field intensity and. the gradient of the field is constant. 4. A method as claimed in Claim 3 , characterised in that the field is produced by at least two pairs of magnetic poles, the poles of each pair being on opposing sides of the flow path of the suspension or solution and the pairs being of opposite polarity. / 5. A method as claimed in any one of the preceding claims, characterised in that the magnetic field strength is varied in order to allow a continuous bleed of the magnetic particles from the bottom of the plug and the addition of particles to the to " of the plug. 6. A method as claimed in any one of Claims 1 to 4, characterised in that the magnetic field is cyclically reduced or removed to allow the plug to move downwards a small distance whereupon the magnetic field is again applied and a new plug created on top of the initial- plug while magnetic particles with the entrained or adsorbed non-magnetic substance, are lost from the bottom of the plug into the effluent liquid stream. 7. A method as claimed in any one of the preceding claims, characterised in that the magnetic particles are removed from the effluent stream by any suitable means. 8. * A method as claimed in any one of the preceding claims or the recovery of Dunaliella spp from saline waters. 9. Apparatus for carying out the method of any one of the preceding claims, the said apparatus comprising a conduit for passage of liquid and magnetic field generating means on opposing sides of the conduit, charasterised in that said field generating means comprises at least two pairs of " opposed magnetic poles, said pairs being of opposite polarity and being arranged in spaced relationship so as to produce a magnetic field which is extended in the longitudinal direction of the conduit and in which, over at least a substantial proportion of its length, the product of the magnetic field intensity and gradient is constant. 10. Apparatus as claimed in Claim 9, characterised in that the poles have a general form of a cusp generated by the intersection of two circles or similarly curved figures with the apex of the cusp adjacent to conduit.

Inventors: SNOOK HARVEY, SNOOK, HARVEY
Applicants: COCKAJEMMY PTY LTD, COCKAJEMMY PTY. LTD.
Application year: 1982
Main classification: B03C1
IPC codes: B01D35, B03C1