Patent Number: 053002580
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The need for the present invention was premised on the belief that certain contaminants, particularly those adherent to resins which are or tend to be in contact with soil, present a difficult problem for removal through typical soil-washing processes. The method of the present invention for separation of contaminated resin from soil utilizes the fact that the resins generally have a specific gravity (approximately 1.1 for organic ion exchange resins, e.g.) much lower than that of soil (typically about 2.8). By passing a fluid upflow through a bed of soil and resin, the lower density resin can be readily separated from the soil particles of the same size. Using Stoke's Law, the fluid velocity (i.e., terminal velocity) required to entrain a particle of a particular size can be calculated. During fluid upflow in a solid particle bed, such as in a mineral jig, there occurs fluidization of the solid particles given sufficient fluid velocity. If the fluid velocity exceeds the terminal velocity of the particle, the particle is entrained in the fluid and removed from the bed. The terminal velocity, defined as the velocity eventually attained by a solid particle as it is allowed to fall through a sufficiently high column of a fluid, can be estimated using Stoke's Law: EQU U.sub.t =(p.sub.s -p.sub.f)*g*d.sup.2 /(18*u) where U.sub.t =terminal velocity, cm/sec PA1 p.sub.s =solid density, g/cm.sup.3 PA1 p.sub.f =fluid density, g/cm.sup.3 PA1 g=gravitational acceleration, cm.sup.2 /sec PA1 d=particle diameter, cm PA1 u=fluid viscosity, g-cm/sec for Re.sub.t =U.sub.t *d*p.sub.f /u&lt;0.3 where Re.sub.t is the particle Reynold's number evaluated at the terminal velocity. For Re.sub.t greater than 0.3 and less than 1000, the following modified expression of Stokes's Law can be used: EQU U.sub.t =0.153*d.sup.1.14 *g.sup.1.71 *(p.sub.s -p.sub.f).sup.0.71 /(u.sup.0.43 *p.sub.f.sup.0.29) These equations, while only strictly applicable to spherical particles, are used herein to estimate the terminal velocity for soil and resin particles. The estimated terminal velocity as a function of particle size and particle density are given in Table 1 hereinafter. A comparison of the fluid velocities required for entrainment or fluidization of resin and soil in water as a function of particle size is shown in FIG. 1. It is readily apparent that the resin can be separated from the same size soil particles. However, while removing a particular size resin bead, smaller size soil particles will also be removed. For example, while removing 250 micron resin beads, soil particles 44 micron and smaller will also be removed. Based on the significant size difference between the entrained soil and resin, the overflow stream can be screened to collect the resin beads while allowing the smaller size soil particles to pass through. The method of the present invention thus involves the combination of fluidization of contaminated resin particles and at least a portion of the soil particles at a controlled fluid velocity, and the selective screening of the fluidized effluent. A unique way of achieving this selective separation is with a mining apparatus called a mineral jig (see FIG. 2). The mineral jig as used in the present invention, is operated in a manner which is contrary to its standard use. In a typical ore processing operation, for which the mineral jig is designed, only a relatively minor amount of the highest density fraction of the feed, which is the mineral of interest, is collected in the bottom, or hutch, of the jig. In this normal use, the jig is fed with a slurry through the top, and the jig is pulsed, which induces a pulse on the slurry. Water flows upward through the jig, normally when the pulse is on the down stroke. This pulsing action causes the densest particles to settle more quickly, allowing the lighter, less dense particles to be carried away by the water upflow. TABLE 1 __________________________________________________________________________ Terminal Velocity Estimation for Resin and Soil Particles as a Function of Particle Size Terminal Velocity Solid Particle Screen Rev. No. U.sub.t at Re &lt; 0.3 U.sub.t for 0.3 &lt; Re &lt; 1000 Density Diameter Size Re U.sub.t, U.sub.t, (ps) g/ml (d) (cm) Mesh (IU*d*p.sub.f /u) cm/sec GPM/ft.sup.2 cm/sec GPM/ft.sup.2 __________________________________________________________________________ SOIL 2.8 0.118 16 226.4 19.2 282.5 132.7 1953.8 2.8 0.071 25 76.3 10.8 158.3 18.0 707.3 2.8 0.060 30 53.2 8.9 130.7 34.3 505.1 2.8 0.025 60 8.2 3.3 48.2 6.0 87.1 2.8 0.015 100 2.7 1.8 26.9 2.1 31.6 2.8 0.009 170 0.9 1.0 15.0 0.8 11.4 2.8 0.005 325 0.2 0.5 6.8 0.2 2.8 RESIN 1.1 0.118 16 29.7 2.5 37.0 7.6 111.6 1.1 0.071 25 10.0 1.4 20.7 2.7 40.4 1.1 0.060 30 7.0 1.2 17.1 2.0 28.9 1.1 0.025 60 1.1 0.4 6.3 0.3 5.0 1.1 0.015 100 0.4 0.2 3.5 0.1 1.8 1.1 0.009 170 0.1 0.1 2.0 0.0 0.6 1.1 0.005 325 0.0 0.1 0.9 0.0 0.2 __________________________________________________________________________ NOTES: a) To calculate U.sub.t at Re &lt; 0.3, U.sub.t = 0.153*d.sup.(1.14) *g.sup.(0.71) *p(p - p.sub.f.sup.(0.71) /u.sup.(0.43) *p.sub.f.sup.(0.29) b) To calculate U.sub.t for 0.3 &lt; Re &lt; 1000, U.sub.t = (p.sub.s - p.sub.f)*g*d; &lt; (18*u). c) It is assumed that only half of the jig area is available to flow (i.e., 50% screen area). d) pf = fluid density = 1 g/ml; g = gravitational acceleration = 980 u = fluid viscosity = 0.01 gcm/sec The operation of the jig for the resin segregation application of the present invention, however, is modified so that a majority of the soil passes downflow through the jig, and only the resin and soil fines are carried over. This is accomplished by setting a relatively long stroke in the jig, giving the particles more time to settle before the next pulse and by minimizing the bed depth in the jig, preferably using oversized particles in the bed, and using a continuous upflow. In this way, it is actually possible to entrain the resin particles at a fluid velocity lower than the theoretical entrainment velocity. The overflow containing resin and fines is then screened to separate the resin from the fines. Referring to FIG. 2, a preferred method of practicing the invention is illustrated. A mixture of contaminated soil, which contains resin, generally 10, is slurried in any known manner, for example, by blending the mixture with water in a slurry tank 11. This resin/soil mixture may be the product of an accidental resin spill or may be the product of resin which has been intentionally introduced to the soil to remove contaminants from the soil. Fluids other than water could of course be used to form the slurry, such as other liquids (oil, e.g.). Also, gases (such as air, e.g.), can be used to fluidize the resin. The slurry is fluidized such that the slurry achieves a velocity required to entrain substantially all of the resin particles and a portion of the soil particles, generally the fines. However, the fluid velocity should not be so great as to entrain the entire mixture of soil and resin, or the advantages of separation according to the present invention will be compromised. Over-entrainment would further result in wasted energy. The slurried mixture may be entrained in any known way, provided the desired terminal or entrainment velocity is reached. Entrainment methods may include the use of pumps, gravity (by developing sufficient head to provide the desired terminal velocity downstream), stirrers, blowers, etc. The entrained resin and soil are separated from those soil particles which have not been entrained in the fluid. The simplest way of doing this is to allow the soil particles which have not been entrained to settle out by gravity and be collected. This is illustrated schematically in FIG. 2, by the slurried mixture, 12, entering the jig, 13, which is fed with jig water 14. The soil particles which have not been entrained by the jig 13, settle out 15, and are collected as clean soil in a product carboy 16. Meanwhile, the overflow 17, which passes upflow from the jig 13, has achieved terminal or entrainment velocity and has entrained the resin and at least a portion of the soil, typically fines, is passed through particle size separation means sized to recover the resin, such as a 60 mesh screen 18. The contaminated resin 19 is removed for disposal, thermal destruction, oxidation of contaminants, recovery of heavy metals and the like. The soil-containing stream 20 passes through the particle size separation means resin free and is collected in a hopper 21 for return to the site or further processing. The ability to accomplish the desired resin segregation using this approach is demonstrated in the following example. EXAMPLE Remediation studies on a soil from a uranium solution mining site in Bruni, Tex. had shown that resin contamination was present in certain soil samples. Extractants that were successful at removing the uranium contamination from just soils, were no longer effective on the soils that contained the resin. This was due to the fact the resin (DOWEX 21K, Rohm and Haas, Philadelphia, Pa.) has a high affinity for the uranium, which could not be mobilized by extractants. Chemical analysis of the soil and resin mixture showed the uranium content to be approximately 90 ppm, which is above the required remediation level of 42 ppm. Further analysis showed that a majority of the contamination was associated with the resin. The resin, which represented about 1 weight percent of the soil mixture, contained 7000 ppm uranium. The soil itself contained less than 30 ppm uranium. As shown in FIG. 3, to achieve the desired uranium level in the soil it was thus necessary to segregate at least 70% of the resin from the soil. A particle size analysis of the resin showed that a majority of the resin was greater than 250 microns. Tests were thus run to determine if the resin could be segregated from the soil using a mineral jig to fluidize the resin and a 250 micron screen to collect the resin from the overflow. The results of the tests, summarized in Table 2, show that under conditions which are run to maximize solid downflow through the jig (Test A), only about 40% of the resin was removed from the soil. Subsequent testing showed that by adding a bed of oversized soil particles the segregation could be greatly improved. The oversized bedding soil was sized (0.19 to 0.25 inch diameter) to prevent its discharge from the bed by entrainment in the overflow stream, but to still allow adequate pulsing of the bed and thus allow the soil being processed to pass through the interstices in the bed. The bedding provides a more tortuous path for the resin to travel to the bottom of the jig, and thus provides much greater opportunity for the resin to be fluidized from the soil by the upflow stream. The bedding also results in better distribution of the solution flow up through the jig, thus minimizing channelling. The results of Table 2 show that with .the use of oversized bedding material, resin segregation of 80% and greater was achieved. The mineral jig may have a stroke length of up to 0.75 inch with a frequency of 800 rpm. The fluidizing zone of the jig may have dimensions of about 4".times.6" to 4 feet.times.6 feet in surface area with a height of up to about a foot. The important variable for fluidization is flow rate per unit surface area of the zone. The results of Table 2 also show that the resin segregation can be increased from 80% (Test B) to greater than 90% (Test C) by increasing the upflow rate from 1.6 to 3.2 GPM/ft.sup.2. Increasing the flow further to 4.8 GPM/ft.sup.2 (Test D) did not significantly increase the resin removal. According to Stoke's Law, fluid velocities of at least 5 GPM/ft.sup.2 should have been required to entrain this resin. The pulsing action of the jig and the short fluidization zone in the jig is believed to result in the lower fluid flow rates being required for resin entrainment. Other separation devices (e.g., fluidized beds) will require greater flow rates to achieve the same degree of resin segregation. Analysis of the soil products, which are the jig bottoms and the jig overflow which passed through the screen (&lt;250 microns), contained less than 30 ppm uranium. These streams represented .about.99% of the feed; thus the contamination was effectively concentrated in 1% of the feed which was collected in the overflow 250 micron screen. TABLE 2 ______________________________________ Resin Segregation Particle % Resin Test # upflow Rate Bed* Removed ______________________________________ A 4.8 GPM/ft.sup.2 No 43% B 1.6 GPM/ft.sup.2 Yes 80% C 3.2 GPM/ft.sup.2 Yes +90% D 4.8 GPM/ft.sup.2 Yes +90% ______________________________________ *Layer of solids in particle bed comprise 0.19-0.25 inch diameter solids. It will, of course, be appreciated by those skilled in the art that variations to the method of the invention disclosed herein may be practiced without departing from the spirit of the invention as set forth in the following claims. For example, the method of the invention may be used to remove any type of resin containing contaminants from soil, including those resins containing anions. Such anions may include, for example, complexes of uraninium, arsenic and/or chromium, which tend to carry an anionic charge. Of course, cationic exchange resins may also be removed from the soil according to the present invention.