Classifying – separating – and assorting solids – Electrostatic – Free fall type particle charging
Patent
1996-12-02
1998-12-08
Terrell, William E.
Classifying, separating, and assorting solids
Electrostatic
Free fall type particle charging
2091272, B03C 710
Patent
active
058457836
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
THIS INVENTION is concerned with an apparatus and method for the electrostatic separation of mixtures of particulate materials possessing differing electrical properties and in particular to separation of mixtures of substantially electrically conductive and substantially nonconductive materials.
BACKGROUND OF THE INVENTION
The apparatus and method of the invention are particularly although not exclusively directed to the separation of carbonaceous materials from fly ash obtained from combustion or incineration processes typically employed in coal fired power generators, brick kilns and ore roasting/calcining kilns as well as municipal waste incinerators.
Fly ash is obtained in large quantities from coal burning electric power generators and generally this recovered fly ash is used as a replacement or supplement for cement powder in the manufacture of concrete.
Depending upon the quality of the coal employed as a fuel and the efficiency of the combustion process, the recovered fly ash may contain varying amounts of partially combusted carbon particles up to about 10-12% by weight.
Internationally accepted standards for pozzolans, in particular, fly ash in the manufacture of concrete generally limit the amount of uncombusted carbon in the fly ash to below 4% and in consequence, fly ash from many potential sources cannot be employed in concrete manufacture.
With increasing environmental concerns and regulations relating to NO.sub.x and S.sub.x o emissions from coal fired furnaces, furnace practice or operating conditions have been changed to reduce these emissions with the result that the carbon content of fly ash has increased thereby precluding previously acceptable sources.
There are many economic benefits to be obtained from the continued use of fly ash in cement powder production and accordingly there exists a need to remove excessive quantities of carbon from fly ash with an economically viable process.
Electrostatic separation of particulate materials having differing electrical properties is well known and generally falls into four categories--Electrophoresis, Conductive Induction, Contact Charging and Dielectrophoresis.
In electrophoretic separation, mixtures of conductive and non conductive particles are ionised in a corona discharge field such that all particles acquire a like surface charge. The charged particles are initially attracted to the surface of a grounded rotating metal roller or a stationary inclined metal plate, also grounded, having a convexly curved surface.
The grounded roller or plate allows the charge on conductive particles to dissipate quickly and as the particles either rotate with the metal roller or slide over the convex surface of the stationary plate, a combination of gravitational and centrifugal forces are applied to the particles. The conductive particles, being substantially discharged leave the surface of the roller or plate first under the influence of the forces applied whilst the charged non conductive particles cling to the surface for a longer period until gravitational forces exceed the attractive forces between the charged particles and the grounded surface over which they move. A splitter directs conductive and non conductive particles travelling through different trajectories to respective collection regions.
Conductive induction involves transportation of a mixture of conductive and non conductive particles on a grounded metal roller or curved, inclined metal plate through an electrostatic field generated by a spaced electrode having an opposite charge to the roller or plate.
Conductive particles on the transport surface acquire a charge of like sign to the transport surface both by conduction from the transport surface and induction by the spaced electrode of opposite charge. When the conductive particles become charged they are attracted towards the electrode and in a manner similar to that described above, the charged and uncharged particles follow differing trajectories as they leave the surface of the transportation means to f
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patent: 2361946 (1944-11-01), Johnson et al.
patent: 3426895 (1969-02-01), Bullock
patent: 3998727 (1976-12-01), Giffard
patent: 4557827 (1985-12-01), Fricke et al.
patent: 5484061 (1996-01-01), Dunn
Mackey Patrick
Pozzolanic Engerprises Pty Ltd
Terrell William E.
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