Classifying – separating – and assorting solids – Plural – diverse separating operations – Sifting and stratifying
Reexamination Certificate
2001-03-01
2003-05-13
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Plural, diverse separating operations
Sifting and stratifying
C209S013000, C209S012100, C209S017000, C209S018000, C209S454000, C209S455000
Reexamination Certificate
active
06561359
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a system for sorting particulate material. More particularly, this invention pertains to a method and an apparatus for separating a mixture of particles according to specific gravity.
BACKGROUND OF THE INVENTION
One specific purpose of the invention is to remove lightweight deleterious impurities such as particles of coal from sand destined for use in concrete and mortar and bring the level of such deleterious material down to an acceptable level. There is an ongoing need for reasonably clean sand in order to provide essential ingredients for concrete, mortar and asphalt products. Typically medium sized sand is used for concrete, a finer mix for mortar and a combination of sizes for asphalt. When sand is quarried from deposits in a raw form, it usually contains unwanted components including rocks of various sizes, vegetable matter, clay, silt, slime and various lightweight materials inter alia coal (anthracite and lignite) and shale. All these materials generally need to be removed. It is particularly important, for several reasons, to remove lightweight particles such as coal when the sand is destined for use in concrete and mortar. The primary reason is that coal is a significantly softer material than sand. If not removed, the hardened concrete or mortar will have the coal particles therein, and these particles quickly erode and degrade resulting in voids within or on the surface of the finished concrete or mortar. These voids fill with water, and freezing and melting cycles can quickly degrade the concrete or mortar. The coal is also more porous than sand and tends to absorb water even before degrading, thus promoting destructive freezing and melting cycles and early damage to the concrete or mortar. Additionally, the coal also tends to smear when the concrete is worked and tooled, causing unsightly black streaking of the concrete or masonry surface. These problems make it important to remove the coal during the cleaning cycle.
State of the art sand processing begins with the quarried, raw sand being run through one or more conventional, superposed screens having decreasing mesh openings in which rocks of various sizes, vegetable matter, and any other fairly large impurities are separated from the raw sand. During this screening, a working fluid is often added to the raw sand to form a slurry. With sand processing, the working fluid will be water. The slurry produced during this preliminary step is next directed to a classification tank in which the raw sand is allowed to settle to the bottom of the classification tank and the lighter clay, silt, and slime is washed away with the water.
Generally, classification tanks are rectangular and include an inlet end, an outlet end and a plurality of settling stations (typically 7-11) positioned between the inlet and outlet ends along a bottom surface. There are usually no physical dividers between these stations, and the first two to three stations at the inlet end will be spaced more closely to each other than will the last stations at the outlet end of the tank. The raw sand, after being preliminarily screened and washed as described above, will be directed into the classification tank in a slurry form, entering the tank at a predetermined rate. As the slurry moves along the classification tank, the raw sand carried by the slurry is transported along the classification tank with the heaviest sand particles dropping to the bottom at the first two or three settling stations and the medium and smaller sand settling at subsequent settling stations. Clay, slime and silt tend to remain in suspension and are carried away by the water flow as it exits the outlet end of the classification tank.
Each settling station is usually operatively connected to three drainpipes, and each of these pipes discharge to one of three distinct and adjacent flumes positioned beneath the tank. The drainpipes at these settling stations are normally kept closed by remotely operated valves and the valves are opened at predetermined times to discharge the accumulated sand particles of that station. A control system determines which of the three valves at each settling station to open, and as a result the collected sand at that station may be directed to a particular flume. For example, the coarsest particles may be directed to the first flume, the medium particles directed to the second flume and the fine particles directed to the third flume.
Until the present invention, it was quite challenging to remove lightweight impurities from the slurry, and previous attempts to do so proved less than satisfactory. In one attempt, a jig that was used to separate lightweight particulate matter from larger, stone sized particles was modified for use with smaller, sand sized particles. A typical jig consists of a nearly horizontal bed over which a very shallow flow of slurry is directed. As the slurry traverses the bed, the bed is vibrated, and this vibration causes the lightweight material to rise to the top of the slurry. When the slurry reaches the end of the bed, the lightweight material is skimmed off from the upper portion of the slurry by a knife the desirable particles are allowed to pass beneath. The knife adaptation of this device for sand sized particles has met with only limited success. For example, the capacity of such a modified jig is inherently limited because if the slurry is too deep, the lightweight particles will not be able to be brought up to the surface by the vibrating bed. Conversely, it is not very efficient to have a relatively shallow slurry depth because only small amounts of material may be processed. Moreover, such a modified jig is a complicated and expensive piece of equipment that is costly to operate. Additionally, and more importantly, such a jig is unable to effectively fractionate the raw sand feed material and process only that portion in which lightweight particles are concentrated.
Another apparatus known to the art to separate lightweight particles from denser particles used a density separator. A density separator is a device that also operates on the principle of floatation. A typical density separator is a large, square, open-top tank having a bottom with a single discharge valve. The open top of the tank includes weirs over which a working fluid and suspended material is permitted to flow. A slurry is introduced into the tank and fills it to overflowing. By adjusting the flow rate, it is possible to suspend and remove lightweight material as the upwelling fluid flow passes over the weirs. This overflow is typically passed over a static screen to remove larger lightweight materials, and the finer material and working fluid which pass through the screen may be directed to other areas for further processing. The coarser material settles to the bottom of the tank and is periodically directed to other areas for further processing. As with the aforementioned modification of an existing jig, use of a density separator to separate and remove lightweight materials from smaller, sand sized particles has met with limited success. Capacity is limited. For example, if the slurry is inputted in to the tank at too great a rate, both undesirable and desirable material may flow over the weirs and onto the screen. All of this discharge tends to clog the screen and reduce its efficiency. Moreover, not all of the lightweight material is effectively separated from the sand particles, and lightweight material can become trapped along with the coarser particles which settle at the bottom of the tank. If the slurry is inputted too slowly, it will not be possible to induce all of the lightweights to flow over the weirs and excessive lightweight material settles at the bottom of the tank along with the coarser sand particles. This is not only limiting, it is also inefficient. Another inefficiency occurs at the single screen through which all of the overflow material is directed. And, as with the modified, aforementioned jig, this device is unable to fractionate the raw feed material and proce
Anderson Stephen
Egge Alan R.
Astec Industries Inc.
Miller Jonathan R
Moore & Hansen
Walsh Donald P.
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