Classifying – separating – and assorting solids – Electrostatic
Reexamination Certificate
1999-10-19
2002-04-02
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Electrostatic
C209S128000, C209S129000, C209S130000
Reexamination Certificate
active
06365856
ABSTRACT:
BACKGROUND
The present invention relates to apparatus and method for separating particle constituents of a mixture. In particular, the invention relates to triboelectrically charging the particles and separating them under the influence of a field.
There is a need for separating various constituents of a mixture in many technological and scientific fields. For example, sulfur-bearing constituents of coal reduce its burning efficiency, and also contribute to the pollution of the environment. Thus, it is desirable to remove such sulfur-containing constituents from pulverized coal. There is a similar need for separation processes to recover phosphate rock from phosphate ores which are mined in a matrix that includes a mixture of phosphate rock and silica in a clay-like material known as “slimes.” Separation processes can also be profitably utilized in separating various constituents of a frozen aqueous solution. Such separation processes for liquids find applications in preparation of concentrate foods. For example, removal of ice crystals from a frozen fruit juice concentrates the fruit juice.
A number of electrostatic separators are known in the art. For example, U.S. Pat. No. 4,274,947 describes a separator that includes an elongated enclosure in which a mixture of particles are triboelectrically charged by mechanical agitation and the motions induced by air flow in a fluidized bed. An electrical potential applied to a horizontal electrode above the mixture and the grounded base of the enclosure establish an electric field within the enclosure, which by differential attraction of the charged particles, induces vertical migration and stratification of the charged particles. Paddles attached to endless chains move the charged particles in the lowest stratum toward one end of the enclosure, and buoyant forces cause the charged particles in the upper stratum to move toward the opposed end of the enclosure. The oppositely charged particles are removed from the opposite ends of the enclosure. The stratification of the particles in this type of separator is partially determined by the sizes of the particles and their degree of buoyancy in the fluidized bed, thus restricting the types of mixtures that can be separated.
U.S. Pat. No. 4,194,971 also employs paddles and drive chains, substantially submerged in the particle stream, for moving inputted particles that have been triboelectrically charged. The paddles drive charged particles of one polarity in a lower stratum in one direction, and drive the oppositely charged particles in an upper stratum in an opposite direction, thereby producing two current flows. An intermediate shear zone separates these two current flows. In this separator, the mechanical properties of the particles, such as size, mass, and buoyancy, rather than their triboelectrically charging properties, solely determine the stratification of the charged particles, and hence their separation.
In another example, U.S. Pat. No. 4,839,032 describes an electrostatic separator that employs two electrodes having opposite voltages to create an electric field between the electrodes. A perforated dielectric sheet, placed in the space between the electrodes, provides areas that exhibit electric fields and areas that do not exhibit electric fields. Particle charging due to contact occurs in the former, and particle separation occurs in the latter. This patent asserts that an endless belt moves the particles of the mixture continuously in a direction transverse to the field to allow triboelectrically charging of the particles and separation of the particles in field-free areas. One disadvantage of this type of separator is that the belt tends to wear out quickly due to the abrasive environments in which it operates. Thus, periodic monitoring and repairing of the belt is needed. Such maintenance is time-consuming, and also adds to the operating costs. In addition, many of such separators do not provide any structures for the introduction of the mixture into the space between the electrodes at a number of different positions. Although some conventional separators include multiple input ports, the locations of the input ports of such separators are fixed, and can not be spatially varied in order to optimize the separation process.
A number of beltless electrostatic separators are also known in the art. A class of such prior art beltless electrostatic separators employ rubbing contact of the particles of a mixture with a surface, while the particles are moving at high velocities, to triboelectrically charge the particles. Such contact imparts either positive or negative charge to particular particles of the mixture depending on the charge-bearing properties of the particles. One such separator blows the particles of a mixture at high velocity through a sinuous path. The impact of the particles with the inner walls of the path results in charging of the particles. Upon leaving the sinuous path, the passage of the charged particles through a space between two charged electrodes separates the particles. The impact of the particles with the walls of the sinuous path can result in disintegration of some of the particles into smaller components. Such disintegration may not be desirable, thus limiting the number of applications of such a system for separating the particles of a mixture. Further, the impact of the particles having high velocities with various parts of such separators typically results in a rapid wear of the parts. In addition, such separators typically have a low throughput, and hence are not suitable for industrial-scale separation processes. Further, similar to the separators having belts, the beltless separators typically do not allow adjusting the locations of the input and output ports.
Further, many prior art separators for bulk processing applications employ uninsulated metallic or uninsulated conductive ceramic electrodes mainly because of difficulties in insulating high voltage electrodes, susceptibility of insulating surfaces to wear, and reduction in electric field strength in the separation region as a result of voltage drop across the insulating material. The use of uninsulated electrodes can result in a sparkover voltage between the electrodes, which can cause an electrical arc if the electrodes are not current limited, and if the high voltage power supply can sustain the required power.
Such an electrical arc causes the voltage between the positive and the negative electrodes to drop below a voltage required for particle separation. Further, such an electrical arc can cause damage to the electrodes. Many prior art separators employ current-limiting resistors, in the megaohm range or higher, connected in series with the electrodes to guard against formation of electrical arcs. Such resistors, however, do not eliminate the occurrence of a sparkover voltage. Sparkover voltages can result in formation of streamers, i.e., sustained electrical discharges in the microampere to milliampere range.
The passage of currents of such magnitudes through the current limiting resistors cause power dissipation in the range of tens to hundreds of watts, rendering safe design and construction of the resistors difficult. Further, the streamers can erode metallic portions of the separator, and can cause fires in the separation region.
Further, sparkover voltages can cause material erosion, such as erosion of conductive ceramic tiles of the electrodes employed in many electrostatic separators. Further, electrical arcs between the electrodes can potentially cause explosions when the particle mixture includes combustible or flammable materials. This leads to difficult and costly design and construction methods to guard against such explosion hazards.
Uninsulated or conductive electrodes can also lead to formation of precipitated layers of material on the electrodes when the removal mechanism fails to completely remove the material accumulated on the electrodes. Precipitated layers, formed of non-conductive materials, can disadvantageously lead to local microsparking and ion productio
Lahive & Cockfield LLP
Miller Jonathan R.
Walsh Donald P.
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