Classifying – separating – and assorting solids – Fluid suspension – Liquid
Reexamination Certificate
1999-12-03
2001-07-24
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Fluid suspension
Liquid
C209S155000, C209S172000, C209S172500, C209S173000
Reexamination Certificate
active
06264040
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable
FIELD OF THE INVENTION
The invention relates to the use of a separator to partition a particulate assemblage into various constituents based on a difference in the settling velocity of particles having similar or different densities in a fluid medium.
BACKGROUND OF THE INVENTION
Hydraulic classification devices are used extensively throughout the minerals industry to produce differently sized products from a particulate assemblage consisting of a full distribution of particle sizes. Although numerous devices have been developed over the years, a technique currently popular is hindered/fluidized-bed separators. These devices work well for mineral classification if the particle size and density ranges are within acceptable limits.
A great deal of research has been devoted to the study of fluidized-beds and their use in gas/solid contacting and in liquid/solid applications. Studies describing the latter having typically focused on the classification aspects of fluidized-bed separators, although recent work has shown that these devices can also be effectively used for mineral concentration. A hindered-bed separator is a vessel in which water is evenly introduced across the base of the device and rises upward. The separator typically has an aspect ratio of two or more and is equipped with a means of discharging faster settling solids through the bottom of the unit. Rising water and light solids flow over the top of the separator and are collected in a launder. Solids are introduced in the upper portion of the vessel and begin to settle at a rate defined by the particle size and density. The coarser solids settle at a rate that exceeds that of the rising water. A restricted orifice in the base of the separator regulates the discharge of the coarse solids. As a result, a teetering, high-density bed of particles is maintained within the separator. The small interstices within the teeter bed create high interstitial liquid velocities that resist penetration of the finer particles. The fines, therefore, are maintained in the upper portion of the separator and discharge over the top into a collection launder.
It is obvious from the above description that quiescent flow conditions must exist within the separator to maintain a high efficiency. Excessive turbulence and/or changes in flow conditions can result in misplacement of particles. Unfortunately, current teeter-bed separators utilize a feed injection system that discharges directly into the main separation chamber. These systems typically consist of a vertical pipe that terminates approximately one-third of the way into the main separator body. The pipe discharge is usually equipped with a dispersion plate to laterally direct the feed slurry (a mixture of solids and water). This approach creates turbulence within the separator. Additionally, the water that is injected with the feed must also report to the overflow launder. As a result, the rise velocity of the water is substantially increased at the feed injection point. Above the feed point, the liquid rise velocity is the sum of the teeter water and the feed water. This discontinuity often results in a second teeter interface within the separator. In fact, at higher feed rates the volume of water associated with the feed slurry is greater than the volume of teeter water; thus severely affecting unit performance.
Maintenance and reliability are also crucial to long-term separator performance. Conventional teeter-bed separators use a series of lateral pipes located in the base of the separation zone. These pipes are perforated at regular intervals with small diameter holes. Teeter water is injected through these numerous holes over the entire cross-section of the separator. The large water flow rates and small injection hole diameter leave the device susceptible to frequency blockage and plugging due to contaminates in the process water. When several orifices become blocked a dead zone occurs in the fluidization chamber resulting in a loss of performance in this area. It can be seen, therefore, that the conventional hindered-bed separator has inherent design features that limit the capacity and efficiency of the separator. Design modifications are presented that have been demonstrated to provide a higher unit capacity and minimize maintenance aspects of the separator.
SUMMARY OF THE INVENTION
From the discussion presented above, it is apparent that modifications should be incorporated into new devices to correct the inefficiencies associated with conventional hindered-bed separators. Typically, the feed slurry composition cannot be modified. Likewise, it is difficult and expensive to pre-treat the process (teeter) water to remove contaminants. Therefore, a different approach must be considered. Modification of the separator is needed to overcome existing deficiencies. Various approaches have been evaluated such as introducing the feed and teeter-water tangentially into a cylindrical tank.
One of the distinctive features of the present invention is the feed introduction system. A feed delivery device has been developed that minimizes the flow velocity and gently disperses the feed slurry across the top of the separator. As previously stated, the velocity of the feed slurry as it enters the separator significantly impacts performance. Therefore, it is desirable to minimize the feed velocity. This is achieved by providing a feed transition box and a tangential feed introducer. The transition box redirects the feed slurry over a larger cross-sectional area. It is well known that the product of the flow velocity and cross-sectional area represents volumetric flow. It is obvious, therefore, that increasing the cross-sectional area will reduce the flow velocity. In the feed transition box, the flow area is increased to the full width of the separator. As such, the slurry velocity is minimized.
A second feature of the invention is a tangential feed introducer. The feed introducer is located at the top of the separator and provides a smooth conveyance of the slurry from the transition box to the separator. The J-shaped introducer has a horizontal approach allowing the feed slurry to travel across the top of the separator to the overflow launder. As a result, performance is not affected since the teeter water velocity remains constant throughout the separation chamber. A baffle plate is also located at the discharge end of the feed introducer to prevent short-circuiting of solids directly to the overflow launder.
The cross-flowing concept has been tried before using short, horizontal tanks. These earlier systems, however, did not incorporate adequate water introduction and underflow discharge systems. As a result, they behaved as free-settling separators rather than hindered-bed devices. It is important to recognize the fundamental difference between free- and hindered-settling conditions. Hydraulic classifiers fall into one of two categories; free settling or hindered settling. Under free settling conditions, individual particles do not affect the settling behavior of adjacent particles and, as such, the pulp has the rheological characteristic of the fluid. Furthermore, the settling velocity is determined by particle size and density as dictated by Stokes' law. These devices typically require more elutriation water, have a lower unit capacity and are less efficient.
Hindered settling is fundamentally different. At high solids concentrations, adjacent particles collide with each other influencing the settling characteristics. The settling path is greatly obstructed reducing particle velocity. Additionally, the high solids concentration increases the apparent viscosity and specific gravity of the pulp, thus further reducing particle settling. As a result, the acceleration of particles becomes more important than the terminal velocity. This collision phenomenon is the most important aspect of hindered settling and provides a degree of efficiency that can not be achieved in a free-settling environment.
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Luttrell Gerald H.
Mankosa Michael J.
Beauchaine Mark J.
Lovercheck & Lovercheck PC
Walsh Donald P.
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