Liquid purification or separation – Processes – Using magnetic force
Patent
1998-06-11
2000-04-04
Walker, W. L.
Liquid purification or separation
Processes
Using magnetic force
96 1, B01D 3506
Patent
active
060457058
DESCRIPTION:
BRIEF SUMMARY
This invention relates to magnetic separation.
There currently exist a plurality of methods for the magnetic separation of various different articles. However, these methods all suffer from common disadvantages that limit their industrial utility.
High gradient magnetic separation is one of these processes in which magnetisable particles are extracted onto the surface of a fine ferromagnetic wire matrix which is magnetised by an externally applied magnetic field. The process, which is used to improve kaolin clay, was developed for and in conjunction with the kaolin industry in the United States of America. This process allows weakly magnetic particles of colloidal size to be manipulated on a large scale at high processing rates.
In addition to the clay industry, there are a large number of potential applications in fields as diverse as the cleaning of human bone marrow, nuclear fuel reprocessing, sewage and waste water treatment, industrial effluent treatment, industrial and mineral processing and extractive metallurgy.
Generally, these processes adopt one of a number of ways in which magnetic separation can be achieved, namely, separated is sufficiently large to enable the separation of strongly magnetic particles from weakly or non-magnetic particles; to something which is sufficiently magnetic for separation to be achieved, or biochemical treatment may be utilised to produce a magnetic precipitate which can either be extracted itself or attached to a magnetic particle.
Generally, in prior art methods of magnetic separation, electromagnets in conjunction with an iron circuit have been used to generate a magnetic field in the gap between the poles. Field gradients within the gap may be produced by shaping the poles or by using secondary poles.
Secondary poles consist of pieces of shaped ferromagnetic material which have been introduced into the gap. The magnetic induction produced in the gap in an iron circuit is limited to about 2 Tesla if the separation zone is reasonably large compared to the volume of the iron in the magnetic circuit.
The magnetisable particles processed by these prior art machines are separated by being deflected by the magnetic field configuration or they are captured and held by the secondary poles. The particles are released from the secondary poles by either switching off the magnetic field or by removing the secondary poles from the field mechanically. With particles which are large or strongly magnetic, separation can be accomplished with electromagnets which consume modest amounts of electric power.
Magnetic separation is achieved by a combination of a magnetic field and a field gradient which generates a force on magnetisable particles such that paramagnetic and ferromagnetic particles move towards the higher magnetic field regions and diamagnetic field particles move towards the lower field regions. The force F.sub.m on a particle is given in equation (1), below: ##EQU1## where .chi. is the magnetic susceptibility of a particle with volume V.sub.p,
High gradient magnetic separation (HGMS) suffers from a number of disadvantages and problems when used for industrial purposes. For example, when a high particle recovery rate is required, a loss of recovered particle grade and mechanical entrainment of unwanted particles on the matrix may be observed. Furthermore, if the velocity of the slurry flow is increased to optimise the process, so the quantity of material trapped decreases. Furthermore, as the fluid velocity is increased the duty factor, ie the quantity of time for which the matrix is operable before it has to be cleaned, is dramatically reduced.
Finally, the parameter under which selection takes place in HGMS is .chi.b.sup.2 where .chi. is the magnetic susceptibility and b is the particle radius. HGMS is not selective for .chi. and this problem becomes worse as the particle size decreases and capture is dominated by size rather than .chi..
A relatively new technique entitled Vortex Magnetic Separation (VMS) solves some of these problems. Watson and Li, in an article enti
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Watson, et al.; "The Effect of the Matrix Shape on Vortex Magnetic Separation", Minerals Engineering, vol. 8, No. 4-5, pp. 401-407, 1995.
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Li Zhengnan
Watson James Henry Peter
Cecil Terry K..
University of Southampton
Walker W. L.
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