Abstract:
A method of removing organic contaminants from contaminated solids in which the solids are comminuted as necessary to resolve them into particles with a top size of typically not more than 15 cm. A slurry of the remaining particles and a contaminant treatment agent which is a mixture of an alcohol and an alkali hydroxide is agitated for a period which is sufficiently long to separate contaminants from the particles. After initial treatment, particles larger than 2 mm can be separated out and processed separately, typically by surficial cleaning. The slurry is thereafter resolved into a predominantly liquid phase containing fines and one or more less contaminated, separately treatable fractions in which solids predominate.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to novel, improved methods for removing halogenated organic contaminants from contaminated solids. 
     BACKGROUND OF THE INVENTION 
     The disposal of improperly stored and discarded hazardous solids is a major and worldwide environmental problem. Some of these improper disposal sites are extremely large with U.S. sites such as New Bedford Harbor and the Hudson River having millions of tons of polychlorinated biphenyl (PCB) and oil contaminated sediments. Worldwide, tens of thousands of sites are estimated to require cleanup with multimillion dollar per site cleanup costs being common. 
     This existence of large numbers of extensively contaminated sites has created a major need for low cost methods of treating contaminated solids. 
     Direct treatment methods such as incineration and dechlorination (Peterson U.S. Pat. No. 4,574,013, Rogers et al. U.S. Pat. No. 5,019,175) are very effective for cleaning up solids contaminated with organics. However, such methods are relatively expensive and generally operate at elevated temperatures (150° to 1500° C.). 
     To reduce the cost, contaminated solids may be pretreated before direct methods are applied. 
     Soil washing and solvent extraction are well-known methods for pretreating contaminated solids. Soil washing refers generally to processes using water as the primary solvent while solvent extraction implies a use of solvents other than water. In processing contaminated soils, soil washing has been extensively used in Europe for sandy solids; but &#34;treatment of high clay soils has not been proven&#34; (NTIS PB89-212656 &#34;Cleaning Excavated Soil using Extraction Agents: A State of the Art Review&#34;, p. 15) In general, the European soil washing procedures are limited to materials with a maximum of 20-30% fines (&lt;63 micron) (NTIS PB90-106428 &#34;Assessment of International Technologies for Superfund Applications: Technology Review and Trip Report Results&#34;). 
     Soil washing processes such as those of Trost (U.S. Pat. No. 4,783,263), Giguere (U.S. Pat. No. 4,336,136) and Yoshida (U.S. Pat. No. 4,555,345) generally use water amended with caustics and surfactants such as soaps to cause the oily contaminants to separate from the clean solids. The oil and contaminated fines are removed by froth flotation. Solvent extraction processes such as those of Weitzman (U.S. Pat. No. 4,662,948), Keane (U.S. Pat. No. 4,610,729), Steiner (U.S. Pat. No. 4,801,384) and Morris (U.S. Pat. No. 4,606,774) use volatile chlorinated solvents or light hydrocarbon solvents to dissolve contaminants; and they then use standard separation methods such as filtration to separate the contaminated liquids from the clean solids. 
     In soil washing and solvent extraction, large volumes of solvent or water, generally ten to fifteen times the volume of the solids, are required to achieve effective extraction. This is a major drawback of these processes since the amount of material handled strongly affects the overall processing costs. 
     Thus, there is an existing and continuing need for a low cost method of pretreating contaminated solids and thereby reducing the cost of subsequently treating those solids by direct methods such as incineration and dechlorination. Further needed is a decontamination process of that character which generates less waste than washing and solvent extraction techniques. 
     One process for separating organic contaminants from contaminated solids which meets these criteria is disclosed in copending application Ser. No. 07/797,778 filed November 25, 1991 for METHODS FOR REMOVING ORGANIC CONTAMINANTS FROM CONTAMINATED SOLIDS (I). That process employs a decontamination agent which contains an alkali metal hydroxide, an alcohol, and liquid sulfoxide. In some circumstances, handling problems associated with and/or the cost of the decontamination agent&#39;s sulfoxide constituent may be a concern. 
     Consequently, there is an also existing and continuing need for a solids decontamination process which has the attributes described above but is free of sulfoxides and comparable compounds. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are certain new and improved methods for decontaminating solids which use a combination of solvation and size separation to so decontaminate a volume of solids as to resolve the solids into a clean fraction and a dirty fraction. Instead of the 5-15 volumes of solvent per volume of solids required in soil washing and solvent extraction processes to achieve &gt;90% contaminant reduction in 90% of the original solids, the processes disclosed herein require not more than one volume of solvent to one volume of solids to provide much more efficient solids decontamination. 
     Thus, the amount of material that needs to be handled is materially reduced; and there is a corresponding reduction in processing costs. 
     Furthermore, these novel decontamination processes employ decontamination agents which are free of sulfoxides and the like. Thus, the cost of this relatively expensive chemical is eliminated as are the handling problems appurtenant to liquid sulfoxides. 
     In the novel decontamination processes disclosed herein, the solids being treated are slurried with a decontamination agent which is a mixture of an alkali metal hydroxide and alcohol. After agitation for an appropriate period of time, the most contaminated light fraction is separated from the only slightly contaminated bottoms by settling or centrifugation. All that is typically required to reduce the contamination of the bottoms to an acceptable level is an aqueous rinse. The more contaminated light fraction--generally made up of liquids and fines--can be processed as described in Peterson U.S. Pat. No. 4,574,013 or in any other appropriate fashion. 
     The decontamination process just described is not quite as effective as the process disclosed in copending application Ser. No. 07/797,778is. However, the results are nonetheless almost always completely acceptable, especially in those cases in which the process is employed to pretreat contaminated solids because an up to 97% removal of contaminants from 90% of the soil being treated may be obtained in a single pass using as little as one volume of solvent per volume of solids. 
     From the foregoing, it will be apparent to the reader that one important object of the present invention is the provision of novel, improved methods for decontaminating solids. 
     Other important objects as well as additional features and advantages of the invention will be apparent to the reader from the foregoing and the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of one system in which solids can be decontaminated by methods employing the principles of the present invention; and 
     FIG. 2 is a graph comparing the results achieved by a process as disclosed herein with those of a conventional solids decontamination process and a second proprietary process for decontaminating solids. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first step in decontaminating solids by the novel technique disclosed herein is to comminute the solids to be treated if, and as, necessary to reduce the solids to a top size &lt;15 cm. 
     The solids and a liquid decontamination agent are then introduced into a mixer in proportions ranging from 0.5 to two volumes of decontamination agent to one volume of the solids to be treated and formed into a slurry. A rotary mixer which is capable of providing deagglomeration and further comminution by wet grinding such as a cement mixer is suitable, but other type of mixers may be used. 
     As discussed above, the liquid decontamination agent is a mixture of alkali metal hydroxide and an alcohol. 
     The preferred alkali metal hydroxide for the solids decontamination process disclosed herein because of its fast reaction rates coupled with its modest cost is potassium hydroxide. However, sodium hydroxide can also be employed as can lithium, cesium, and rubidium hydroxides. The last three hydroxides, though, are currently too expensive to be practical. 
     Both mono and dihydric alcohols can be used. Of the monohydric alcohols, benzyl alcohol is preferred because it is an extremely powerful reagent, yet has a relatively low molecular weight. Other aromatic alcohols can also be employed as can the higher aliphatic alcohols such as octanol although the latter does not perform as well as benzyl alcohol. Lower aliphatic alcohol--notably methyl alcohol--perform very poorly. Operable dihydric alcohols include ethylene glycol, propylene glycol, and polyethylene glycols (PEG&#39;s), particularly those having a molecular weight of 200-6000. 
     Decontamination agents with an alcohol to hydroxide mass ratio ranging from 7:3 to 99:1 are contemplated. 
     Preferred is a decontamination agent made up of potassium hydroxide and PEG in essentially equal mass proportions. 
     The solids and liquid are mixed for sufficient time to scrub the contaminants from the surfaces of the solids and to separate agglomerated particles by a mixture of wetting and mechanical attrition. This will typically take 0.5 to 2 hours. 
     The slurry is then passed through an appropriate sieve such as a conventional vibrating screen to remove all particles &gt;2 mm in size. The materials that don&#39;t pass thru the screen are generally too large to hold significant amounts of contaminant after the surficial cleaning in the mixer. Therefore, they may be rinsed, analyzed and discarded. 
     The liquid and solids &lt;2 mm are transferred from the sieve to a continuous mixing tank capable of sufficient agitation to maintain the solids in a suspended state. Contaminants are separated from the &lt;2 mm fines in this tank. 
     Decontamination agent, &lt;2 mm solids suspension is continuously pumped from the mixing tank into a gravity settling device or centrifuge or similar device. This results in the liquid/solids mixture being separated into a light fraction and a heavy fraction. The light fraction, consisting of contaminated liquid and very small contaminated particles (fines), is continuously removed from the separation device for further separation and treatment--by the technique disclosed in Peterson U.S. Pat. No. 4,544,013, for example. &#34;Clean&#34; decontamination agent is mixed with the heavy fraction to form a new slurry, which is pumped back to the continuous mixing tank. 
     One important advantage of the novel decontamination process just described is that a high proportion of the contaminants can be removed from contaminated solids at a comparatively modest cost. Another important advantage over many heretofore proposed processes for decontaminating solids is that it can be carried out over a wide, low temperature range of 20°-150° C. 
     FIG. 1 depicts an exemplary system 20 in which the novel solids decontamination process just described can be carried out. 
     The initial component in system 20 is a mixer 22 into which the solids to be decontaminated--previously comminuted if, and as, necessary to a &lt;15 cm top size--are introduced, typically on an equal volume basis. In mixer 22: (1) the solids are wet ground to an extent which resolves any agglomerates into individual particles and perhaps effects some reduction in particle size; and (2) the solids and decontamination agent are intimately mixed, which results in the separation of contaminants from the solids, particularly those particles of larger size. 
     Once the mixing process is completed, the solvents/decontamination agent slurry is transferred to a sizing device 24 such as the illustrated screen or sieve. This device resolves the slurry into &gt;2 mm solid particles, which are trapped on the screen, and a mixture of liquid and smaller particles. 
     As indicated above, the process carried out in mixer 22 is particularly effective in removing contaminants from the larger particles of solids. Typically, therefore, rinsing with water or a water/detergent solution is all that is required to reduce the contamination of these larger sized particles to an acceptable level. 
     The mixture of liquid and smaller particles is transferred from screening device 24 to a mixing tank 26 equipped with a conventional, motor driven agitator or impeller 28. Agitator 28 is operated at a sufficiently high speed to keep the solid particles suspended in the liquid phase in tank 26. This results in additional contaminants being separated from the smaller particles in that tank. 
     Suspension is continuously pumped from mixing tank 26 into a unit 30 which includes a settling or separation device 32 such as a centrifuge and a reslurry section 34. Here, the suspension is separated into a light fraction consisting of: (1) contaminated decontamination agent, fines, and, typically, an oil in which the contaminant(s) are dissolved or suspended; and (2) a heavy fraction consisting principally of &lt;2 mm particles. 
     The mixture of fines, decontaminating agent, and (typically) oil is transferred elsewhere for separation and treatment as disclosed, for example, in Peterson U.S. Pat. No. 4,574,013. The heavier fraction(s) or bottoms are mixed with &#34;clean&#34; decontamination agent in the reslurry section 34 of unit 30 and recirculated to mixing tank 26 for further separation of contaminants from those solids. 
     Periodically, the status of the solids in mixing tank 26 is ascertained, typically by withdrawing a sample of solids through discharge line 36 and analyzing those solids for contamination. Once the contamination has been reduced to an acceptable level: (1) the now decontaminated solids are discharged from mixing tank 26 through line 36, (2) any liquids associated with those solids recovered, and (3) a new batch of contaminated solids is introduced into the mixing tank 26 from mixing/wet grinding unit 22 by way of sieve unit 24. 
     The utility of the solids decontamination process disclosed herein was verified by tests employing authentic contaminated solid samples obtained from contaminated sites. Equal 200 g. samples of solids and liquid were placed in a tall graduated cylinder. The cylinder was held in a water bath to maintain constant temperature. The solids and liquid were mixed for a period of 10 minutes. Thereafter, the samples were allowed to settle for 80 minutes. The column was then divided into one light fraction or liquid layer (top) and four equal volume solid layers (bottom). Aliquots of each layer were analyzed by standard methods to determine the levels of contamination. 
     PCB soil samples from a Superfund site were treated using the protocol described above and three different solvents: water with surfactant (conventional approach). KOH/polyethylene glycol (PEG)/dimethyl sulfoxide (DMSO) (copending application Ser. No. 07,797,778, and KOH/methoxy ethoxy ethanol (MEE). The results of testing are tabulated below 
     
         ______________________________________Water/surfactant       KOH/MEE        KOH/PEG/DMSO% SOIL % PCB    % soil   % PCB   % soil % PCB______________________________________48       5      32       0.44    34     0.2789      16      59       0.75    63     0.5196      25      85       1.7     89     0.72100     33      100      2.5     100    0.88______________________________________ 
    
     The data showing the effect of the three decontamination agents on the contaminated solids is shown in graphical form in FIG. 2. 
     Each succeeding entry in the data tabulated above identifies the amount of PCB found in that percentage of the processed soil indicated in the entry. For example, the third entry in the middle column shows that 85 percent of the soil decontaminated in accord with the principles of the present invention contained only 1.7 percent of the PCBs with which the original soil was contaminated. The remaining 98.3 percent was found in the liquid and the fines. This means that at least 85 percent of the solids could most likely be decontaminated to an acceptable level with a water rinse. This is not true of a conventional soil washing decontaminant process employing a comparable amount of decontamination agent. In the tests described in this Example in which conventional washing was used, for instance, 89 percent of the solids contained 16 percent of the PCBs and would require extensive additional processing. 
     The invention may be embodied in forms other than those described above without departing from the spirit or essential characteristics of the invention. The specifically disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and the drawings. All changes which come within the meaning and the range of equivalency of the claims are intended to be embraced therein.