Classifying – separating – and assorting solids – Fluid suspension – Liquid
Reexamination Certificate
1999-03-25
2002-07-30
Lithgow, Thomas M. (Department: 1724)
Classifying, separating, and assorting solids
Fluid suspension
Liquid
C209S166000, C209S170000, C209S158000, C209S159000, C209S454000, C209S474000
Reexamination Certificate
active
06425485
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the use of a separator to partition a particulate assemblage into various constituents based on a difference in particle mass and more particularly to partition a particulate assemblage into various constituents based on a difference in particle mass after the specific gravity of one or more of the components in the assemblage has been decreased by the selective attachment of air bubbles.
BACKGROUND OF THE INVENTION
Gravity concentration devices are used extensively throughout the minerals industry to concentrate high-density particles from a mixture of high- and low-density material. Although many devices have been developed over the years, a technique gaining in popularity is hindered/fluidized-bed separators. These separators, traditionally used for classification, work reasonably well for mineral concentration if the particle size range and density difference 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 have typically focused on the classification aspects of fluidized-bed separators and less so on mineral concentration. Recent work has shown that fluidized-bed separators can be used to effectively separate mineral assemblages that have components with different densities. For instance, coal and the ash forming components (rock), silica and iron ore, and silica from various heavy minerals such as zircon and ilmenite. Results from these studies indicate that efficient concentration can be achieved if the particle size ration (top size to bottom size) is less than 3 or 4 to 1 and in a range from 200 mesh to several millimeters. Unfortunately, this is seldom the case and, as a result, the separation efficiency is poor. To correct this shortcoming, the valuable component (i.e., coal, iron ore, ilmenite and zircon) frequently must be reprocessed to achieve the desired quality.
A hindered-bed separator is a vessel in which water is evenly introduced across the base of the separator and rises upward. The separator typically has an aspect ratio of two or more and is equipped with a means of discharging solids through the bottom of the unit. Rising water and solids flow over the top of the separator and are collected in a launder. Solids are typically introduced in the upper portion of the vessel and begin to settle at a rate defined by the particle size and density. The coarse, higher density particles settle against the rising flow of water and build a bed of teetering solids. This bed of high-density solids has an apparent density much higher than the teetering fluid (water). Since particle settling velocity is driven by the density difference between the solid and liquid phase, the settling velocity of the particles is reduced by the increase in apparent density of the teetering bed. As a result, the low-density component of the feed resists penetrating the bed and remains in the upper portion of the separator where it is transported to the overflow launder by the rising teeter water.
Coarse, low-density particles, however, tend to gather at the interface between the high and low density particles because the teeter water velocity is not sufficient to transport this material to the overflow launder. These particles continue to gather at the bed interface and eventually migrate into the teeter bed, thus reporting with the high-density product. This inherent inefficiency can be partially corrected by increasing the teeter water velocity to convey the coarse, low-density solids to the overflow. Unfortunately, this approach will also cause the fine, high-density solids to be misplaced to the overflow launder resulting in a loss of efficiency. It can be seen, therefore, that a conventional hinder-bed separator has inherent inefficiencies when treating a mineral assemblage that has a Wide particle size distribution and/or a narrow density distribution.
Applicant is aware of the following U.S. Pat. No. 2,758,714 to Hollingsworth; U.S. Pat. No. 4,396,396 to Mainwaring; U.S. Pat. No. 4,822,493 to Barbery; U.S. Pat. No. 5,307,937 to Hutwelker and U.S. Pat. No. 5,456,362 to Laskowski.
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 particle size and density distribution of the feed cannot be modified. Therefore, a different approach must be considered. The most obvious means is to further enhance the density difference between the low- and high-density particles in the feed. Similar approaches have been evaluated such as introducing a second lower density liquid that has an affinity for a particular species of particles. A subsequent liquid phase separation is used to concentrate the solids. The use of this oil agglomeration technique has been successfully demonstrated for the separation of coal and ash forming impurities. Unfortunately, this technique suffers from high operating costs and low process capacities.
A distinctive feature of the present invention is aeration of a hindered-bed of solids to modify the effective density of one or more of the species. Aeration is achieved by introducing fine air bubbles with the teeter water supply. The bubbles rise with the upward current, impinge upon the particles and selectively attach to the surface of a particular species. Attachment depends upon the surface characteristics of the particle. For instance, coal is naturally hydrophobic and will spontaneously attach to an air bubble. Applications such as iron ore (with a silica contaminant) require chemical activation of the silica to promote bubble/particle attachment. The method for chemical activation is well known and is routinely used for flotation of fine particles (less than 0.2-0.3 mm).
The concept of bubble-particle attachment in a rising current separator (flotation column) has been previously demonstrated. Unfortunately, the approach uses an open-column reactor operating in the free, not hindered, settling regime. As a result, this configuration does not have the advantages associated with a hindered-bed separator. The distinctive advantage of the present invention is the synergy offered by the combination of a hindered-bed separator and a rising current, open-column flotation cell. The approach combines the pre-concentration of a hindered-bed with the further enhancement of bubble-particle attachment to modify the density of one of the feed components. As a result, separation of coarse, low-density material is greatly enhanced through the addition and attachment of air bubbles. Furthermore, since any particulate species can be chemically rendered hydrophobic, applications are not limited to materials having different densities. In fact, in some instances, the high specific gravity mineral can be enhanced through air addition and will report to the overflow as the light species. The efficiency of the new separator surpasses that which can be achieved by either individual technique.
To recognize the advantages of the invention the fundamental difference between free and hindered-settling conditions must be examined. Separators are generally recognized as falling 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 Theological characteristic of the fluid. Furthermore, the settling velocity is determined by particle size and density as dictated by Stokes' law. Under such conditions, less selective concentration of minerals is achieved.
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
Luttrell Gerald H.
Mankosa Michael J.
Eriez Magnetics
Lithgow Thomas M.
Lovercheck Wayne
Thomson Richard K.
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