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 xe2x80x9cslimes.xe2x80x9d 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 production even at applied electrode voltages that are much less than the nominal sparkover voltage. Further, the particles of precipitated layers formed of conductive materials tend to discharge into the electrode to become uncharged. These uncharged particles return to the separation stream, thus placing an upper limit on the purity of the separation process.
Accordingly, it is an object of the invention to provide a separator that provides improved particle separation while concomitantly providing increased durability and wear resistance.
It is another object of the invention to provide a separator that allows adjustment of the locations of the input ports.
It is yet another object of the invention to provide a separator that allows the electrostatic field to be varied as a function of time and spatial position.
These and other objects of the invention are attained by a separator for separating the particles of a mixture according to the teachings of the invention that employs at least two spaced-apart field element arrays to establish a field between the element arrays. The separator also includes an input for introducing the particles of the mixture into the space between the field elements. Further, two oppositely-rotating agitators, operably disposed in the space between the field elements, agitate the particles to charge them to one of two charge polarities. The charged particles move under the influence of the field such that particles of one charge polarity substantially accumulate in a region close to one field element array, and particles of opposite charge polarity substantially accumulate in a region in the proximity of the other field element array. The rotation of the agitators sweep the charged particles of opposite polarities in two different directions, and also agitate the uncharged particles to triboelectrically charge them.
According to one aspect of the invention, the separator can include output ports for collecting the separated particles. The invention configures the field element arrays and the output ports such that there is a region in the vicinity of the output ports that is substantially field free. As the agitators bring the particles into this field free region, some of the particles under the influence of external forces, such as gravity and/or centrifugal forces, leave the agitators and enter one of the output ports. Because the agitators can rotate in opposite directions, the charged particles accumulated near one agitator first encounter one output port as they enter the field-free region whereas the oppositely charged particles, accumulated near the counter-rotating agitator, first encounter another output port as they enter the field-free region. Accordingly, each output port substantially collects particles of one of two charge polarities, thus separating the particles.
In accordance with another aspect of the invention, at least one of the field element arrays includes a plurality of field generating electrodes arranged to form an annular disk. For example, one or both field element arrays can include a plurality of electrodes having narrow strips of a conductive material, such as copper, that are insulated from each other and from the opposed field element array by strips of an insulating material having a high breakdown voltage, such as a Kapton film. One practice of the invention employs a ceramic, such as alumina, to encapsulate the strips. An alternative practice of the invention encapsulates the strips by polyurethane and employs a layer of a ceramic, such as alumina, disposed on the polyurethane to provide a hard surface for the encapsulated strips. Such construction of one or both field element arrays provides some flexibility in configuring the spatial distribution of the electric field between the field element arrays. In particular, different voltages can be applied to different electrodes to effectuate a desired configuration of the electric field in the space between the field element arrays. Further, the invention can apply time-varying voltages to any number of the electrodes to produce a time-varying electric field in the space between the field element arrays.
According to one aspect of the invention, each electrode includes a mechanical substrate, to which a metallic conductor is bonded. The electrode further includes a high-voltage connector in electrical contact with the metallic conductor for application of an electrical potential to the conductor. An insulating layer covers the metallic conductor to electrically insulate the electrode, and a wear strip covers the insulating layer to provide mechanical protection. One preferred practice of the invention encloses the electrode partially with a conductor to provide a ground plate, and optionally employs non-conducting inserts to provide tie points for attaching selected mechanical structures to the electrode.
According to another aspect of the present invention, the separator can employ agitators in the form of two annular disks of non-conducting material that are operably disposed in the space between the electrodes, preferably co-axially with the two electrodes. A variety of non-conducting materials, such as plastic, ceramic, industrial glasses, or plastic-ceramic composites, can be employed to construct the agitators. In particular, ceramic materials having sufficient surface hardness, breakdown voltage, and temperature resistance are suitable for construction of the agitators. The annular disks preferably possess a common axis of rotation and have a number of openings therein to allow the passage of the particles of the mixture therethrough.
According to a further aspect of the present invention, the separator can include an input that is formed in one of the field element arrays to introduce the particles of the mixture into the space between the field element arrays. For example, an opening in the solid portion of one of the annular field elements can provide the input port for introducing the particles of a mixture into the separator. The input port is preferably adjustable so that the particles can be introduced into the space between the field element arrays at a number of different locations. In particular, it is preferable that the position of the input port can be varied continuously and in real-time while the separator is operating.
During operation of the separator, the inputted particles collide with the agitators and with each other and become triboelectrically charged to one of two polarities. The electrostatic field between the field element arrays exerts a force in the direction of the field on the positively charged particles, and exerts a force in the opposite direction on the negatively charged particles. Thus, the positively charged and negatively charged particles drift in opposite directions, and accumulate substantially in regions close to the field element arrays. For example, the positively charged particles substantially accumulate in the proximity of the negative field element array, and the negatively charged particles substantially accumulate in the proximity of the positively charged field element array.
In accordance with an alternative embodiment of the invention, the separator employs two spaced-apart cylindrical field element arrays, one disposed within the other, to establish a field in the space between the two cylinders. The cylindrical field element arrays are preferably co-axial, and can be constructed in a number of different ways. For example, one or both field element arrays can be formed of a plurality of field generating electrodes, having strips of conductive material such as copper, arranged to create a cylindrical or semi-cylindrical surface, and are electrically insulated from each other and the outside environment by an insulating material having a high breakdown voltage, such as Kapton film. Alternatively, the field element arrays can be constructed by utilizing a conductive material that is shaped into a cylindrical or a semi-cylindrical surface.
In one practice of the invention, time-varying electrical potentials are applied to the field element arrays to produce a time-varying electric field in the space between the field element arrays. For example, a temporary reversal of the polarity of the electric field between the field element arrays can reduce accumulation of material on the surfaces of one or both of the field element arrays. Further, a change in the magnitude and/or polarity of the electric field at selected locations in the space between the field element arrays can stimulate additional mixing of the particles which in turn can enhance the purity and/or the yield of the separation process. Such enhancements can allow construction of a separator with shorter separation zones having a performance comparable with a larger separator.
One aspect of the invention relates to providing easy access to various components of a separator for their replacement and/or repair. For example, one embodiment of a cylindrical separator according to the invention constructs the outer cylindrical field element array by connecting two semi-cylindrical segments together. One such connection can be a hinge that allows rotation of one semi-cylindrical segment with respect to the other to provide access to the agitators and the inner field element array disposed within the outer field element array. In addition, the invention can construct the agitators in a similar manner to provide easy access to various components of the separator.
In another aspect, the present invention provides a separator that includes two spaced-apart annular field element arrays for establishing an electric field in the space between the arrays. Further, the separator includes an input port for introducing the particles of the mixture into the space between the arrays. Two spaced-apart annular agitators, configured to rotate in the same direction, agitate the particles of the mixture, to triboelectrically charge them to one of two charge polarities. The agitators include openings therein to allow passage of the particles of the mixture. The separator further includes two output ports, and includes a plate disposed in the space between the agitators extending substantially over one of the input ports. The plate prevents entry of charge particles having one polarity into the output port over which it extends. Hence, the charged particles of one polarity enter one of the output ports, and those having an opposite polarity enter the other output port.
Another aspect of the invention relates to providing a separator that includes two annular spaced-apart field element arrays for establishing a field in the space between the arrays. The separator further includes an input port for introducing the particle mixture into the space between the arrays. Two agitators are disposed in the space between the field element arrays and are configured for rotation. Each agitator includes a ring from which a plurality of impellers are cantilevered. The impellers triboelectrically charge the particles into two charge polarities, where particles having one charge polarity substantially accumulate in the vicinity of one of the field element arrays, and the particles having the opposite charge polarity substantially accumulate in the vicinity of the other field element array.