Classifying – separating – and assorting solids – Fluid suspension – Liquid
Patent
1988-01-27
1989-10-31
Rolla, Joseph J.
Classifying, separating, and assorting solids
Fluid suspension
Liquid
209 2, 209 20, 209590, 209210, 210748, B03B 302
Patent
active
048775169
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the manipulation of particulate matter in a fluid medium by the use of ultrasonic wave energy, including the segregation of dissimilar particles from a mixture of particles.
Standing waves generated by acoustic energy sources have been used in a variety of ways to influence the behaviour of particles suspended in fluids, it being known that particles can be attracted to the nodes of the standing wave. In essence the attracted particles becomes concentrated at the nodal planes lying normal to the axis of propagation of the standing wave. At this juncture it should be said that the detailed theory underlying the observed phenomenon of standing waves and their effect of particles is not fully understood, in particular the factors influencing whether any given particle type tends to accumulate at the nodes or at the antinodes are unclear. However, this lack of theoretical understanding has no bearing on the practical application of the present invention and throughout this specification the terms nodes and nodal planes are used to include references to both nodes and antinodes, because in practice it does not appear to matter whether any given particle type collects at one or the other.
If a standing wave is moved along its axis, the particles attracted to the nodes can be carried through the fluid while they remain attached to the standing wave: this is proposed, for example, in G.B. 2098498A. In addition, it is found that particles with different acoustic properties, in particular particles with different sizes and densities, can be differently affected by the acoustic energy of a standing wave and it has been proposed to use ultrasonic frequencies to manipulate particles in a chromatographic analytical procedure, to separate different particle types. In U.S. Pat. No. 4,280,823 it is suggested that a standing wave can function as the plates of a conventional chromatographic column by retaining different particles at its nodes for different times, the particles being carried through the standing wave by a gas flow directed axially along it.
There may also be mentioned the apparatus described in U.S. Pat No. 4,523,632 (Barmetz et al) in which particles of different types are separated to some degree as they are carried by a liquid flow along the length of a horizontal chamber in which a standing wave is established with a wavelength that is half the height of the chamber. This results in a so-called force potential well in the mid-height region of the chamber in which the particles will tend to concentrate. The gravity forces on the particles cause some degree of vertical separation of the different types of particle, i.e. each type of particle will be located at a particular height representing an equipotential plane, dependent upon the influence of gravity on that particle type.
It is a characteristic of all these earlier proposals that controls or manipulation of particles attracted to the nodes relies on the acoustic forces acting on the particles in dependence upon the distance from each node, i.e. axially along the standing wave. One of the difficulties which exists in such a system and which becomes particularly acute at the higher ultrasonic frequency ranges suitable for processing smaller particles is that attenuation occurs in the energy propagated from the ultrasound source. Divergence of the beam accentuates this effect. There is therefore an energy density gradient experienced by the fluid as a undirectional force, in effect a radiation pressure, which above a minimal value, produces movement of the fluid away from the radiation source. This movement of fluid, referred to herein as acoustic streaming, clearly can have a disturbing influence on any attempt to control the movement of particles by means of the acoustic forces acting directly on them, and especially if those acoustic forces are employed to discriminate between different particle types.
If a standing wave is formed in a body of fluid by a coaxial reflection of ultrasonic radiation from a single source, there w
REFERENCES:
patent: 3826740 (1974-07-01), Jewett
patent: 4055491 (1977-10-01), Porath-Furedi
patent: 4523682 (1985-06-01), Barmatz et al.
patent: 4759775 (1988-07-01), Peterson et al.
Translation of Lierke DE No. 32 18 487 A1.
Copy of NASA Technical Brief, "Acoustical-Levitation Chamber for Metallurgy", 2301 N.T.I.S. Tech Notes, (1984) Apr., A-K, Springfield, VA, USA.
National Research Development Corporation
Rolla Joseph J.
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