Classifying – separating – and assorting solids – Electrostatic – Stratifying
Reexamination Certificate
2002-04-10
2004-09-28
Walsh, Donald R (Department: 3653)
Classifying, separating, and assorting solids
Electrostatic
Stratifying
C209S127100, C209S127400, C209S128000, C209S129000, C209S130000
Reexamination Certificate
active
06797908
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to an electrostatic separator for the benificiation or separation of particulate materials and, more particularly, to a high-tension electrostatic separator including a corona classifier section for classifying particulate materials according to size, and associated method.
2. Prior Art
Electrostatic separation is based upon the ability to electrically charge particulate materials having different conductive properties and then separate such particulate materials when an external electric field is applied thereto. Three main charging mechanisms applied to electrically separated particulate materials include induction, triboelectrification, and ion bombardment. Because the electrostatic force created by these mechanisms is proportional to the surface charge of the available surface area of the particulate materials and the intensity of the electric field, physical characteristics such as size, shape and specific gravity impact this process.
In general, particulate material sizes effectively separated by a high-tension electrostatic separator is coarser than approximately 100 &mgr;m. In practice, uniform feed particulate material size provides better separation efficiency. Therefore, effective sizing of the particulate materials should be addressed with high-tension electrostatic separation processes to render more effective results. Screening is one method of sizing particulate materials. However, the efficiency decreases rapidly for fine particulate materials. For particulate material sizes finer than 250 &mgr;m, sizing is normally performed by classification techniques. Size classification is based upon the velocity with which particulate materials fall through a medium such as air and water, for example.
In a conventional high-tension electrostatic separator, particulate materials are commonly introduced on top of a roll-type electrode. The position of a charging (corona) electrode and a static electrode, as well as the roll-rotation speed is influenced by the characteristic of particulate materials. For particulate materials having wider size distributions, the separation process requires several stages of retreatment to obtain satisfactory separation. Accordingly, from a processing point of view, it is necessary to classify such particulate materials into narrower size fractions, prior to separation, to obtain higher separation efficiency.
It is known in prior art that a high-tension electrostatic separation process has better separation efficiency with particulate materials having narrower size distributions. It has also been established that roll-type, high-tension separators are more suitable for separating finer particulate materials while plate-type, induction separators are more suitable for separating coarser particulate materials.
A significant problem with high-tension electrostatic roll-type, separators is that the fine conducting particulate materials remain on the roll outer drum surface and are misplaced with nonconducting particulate materials. This can be attributed to fine particulate materials having a higher surface charge, less inertia/centrifugal forces, as well as being susceptible to particle entrapment.
Fine particulate materials may acquire higher charges because their specific surface area is larger than the specific surface area of a coarse particulate material. Accordingly, the electrode arrangement used to separate fine particulate materials should provide a narrower corona field, less corona current, and a wider and stronger static field. In addition, higher roll-rotation speeds should be used to insure that fine conducting particulate materials leave the electrode outer drum surface as early as possible.
Alternately, coarse particulate materials have smaller specific charges. However, such coarse particulate materials have larger centrifugal forces acting thereon because their centrifugal forces are proportional to the cube of their radius. Therefore, for separating coarse particulate materials, a significant problem is that the coarse nonconducting particulate materials leave the roll-type electrode outer drum surface too early. Also, such coarse nonconducting particulate materials can be misplaced with conducting particulate materials if their surface charges are not sufficient. Consequently, the electrode arrangement used to separate coarse particulate materials should provide a wider corona field to enhance the charging thereof. In addition, the roll-rotation speed should be lower to minimize the negative effect from the centrifugal force acting on the coarse particulate materials.
Accordingly, to obtain optimal separation performance, finer and coarser fractions of particulate materials should be classified and subsequently separated with different types of electrostatic separators. However, size classification is such a task that people want to avoid unless it is necessary. Size classification by means of electrostatic techniques has been reported in literature. These techniques mainly deal with classifying dry, fine powder when conventional size classifying processes fail to provide satisfactory separation. For example, a prior art attempt to separate fine, dust-like particulate material is disclosed in U.S. Pat. No. 3,222,275 to Breakiron et al. According to this patent, very fine particulate materials that are of a mesh size of −200 are amenable to high-tension separation with a spray of mobile ions produced by a corona discharge.
Most techniques for classifying particulate materials employ the phenomenon that particulate materials become charged by means of induction when they are subject to a strong electric field. Size separation may thereby be achieved by passing charged particulate materials through electrified sieves. For example, U.S. Pat. No. 5,484,061 to Dunn discloses such an electrostatic sieving apparatus for classifying particulate materials according to size. U.S. Pat. No. 5,161,696 to Seider discloses an apparatus for separating shapes of abrasive grains by imposing a high-voltage corona induction charge to free-falling abrasive particulate materials.
In addition to particulate material size, operating parameters affect an electrostatic separator's performance. Such operating parameters are roll speed, number of corona electrodes and their corresponding position with respect to the grounded electrode, intensity and polarity of applied potential, particulate material rate, electrode surface cleaning, temperature of the particulate materials, and splitter positions.
BRIEF SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the invention to provide a high-tension electrostatic classifier and separator that may include a corona classification section for classifying feed particulate materials into a fine to middle size fraction and middle to coarse size fraction before such fractions are separated by a roll electrode separator and plate electrode separator, respectively. These and other objects, features, and advantages of the invention, are provided in a high-tension electrostatic separator for classifying and separating particulate materials based upon size and conductivity that may include a corona classifier that may have an elongated passageway having generally planar sidewalls defining a first end for receiving particulate materials and a second end for directing the particulate materials into two fractions according to size. The corona classifier may further include corona means located adjacent one of the sidewalls for providing ion bombardment in a horizontal direction to particulate materials dropping down the passageway so that middle to coarse size particulate materials travel in a more generally vertical direction and fine to middle size particulate materials travel in a less generally vertical direction, whi
Grey Thomas J.
McHenry Kevin R.
Yan Eric S.
Miller Jonathan R
Outokumpu Oyj
Walsh Donald R
Yeager Arthur G.
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