Abstract:
An apparatus and process for producing a homogeneous analytical sample from a heterogenous feedstock by: providing the mixed feedstock, reducing the temperature of the feedstock to a temperature below a critical temperature, reducing the size of the feedstock components, blending the reduced size feedstock to form a homogeneous mixture; and obtaining a representative sample of the homogeneous mixture. The size reduction and blending steps are performed at temperatures below the critical temperature in order to retain organic compounds in the form of solvents, oils, or liquids that may be adsorbed onto or absorbed into the solid components of the mixture, while also improving the efficiency of the size reduction. Preferably, the critical temperature is less than 77 K (−196° C.). Further, with the process of this invention the representative sample may be maintained below the critical temperature until being analyzed.

Description:
The United States Government has rights in this invention pursuant to the employer-employee relationship of the U.S. Department of Energy and the inventor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for homogenizing and sampling heterogeneous feedstock. More particularly, this invention relates to a method for providing a representative sample from heterogeneous feedstock containing solids, liquids, and volatile compounds that vaporize under standard atmospheric conditions. 
     2. Description of Related Art 
     Hazardous and/or radioactive materials are typically present in two forms, primary sources of contamination and secondary hazardous waste. Examples of primary sources of contamination are radioactive, chemical or biological materials. Primary sources of contamination by their nature present an increased risk to human health and the environment upon exposure and must be maintained and stored in such a manner as to prevent contact with both. 
     The handling of primary sources of contamination results in the generation of secondary wastes. Typically, secondary waste is a heterogeneous mixture of materials, such as protective clothing, polyethylene air canisters, handling equipment, sample bags, and laboratory sampling accessories (e.g., laboratory wipes, glassware, and swabs). Protective clothing may be in the form of full body suits, goggles, face masks, boots, and gloves. This secondary heterogeneous waste may be made from materials such as paper, cloth, vinyl, rubberized material, Tyvek®, or metal. This secondary heterogeneous waste may be contaminated with organic compounds in the form of solvents, oils, liquids or in combination with each other and/or inorganic compounds or elements. The proper storage, treatment and disposal of the secondary heterogeneous waste depend on the type of contamination. 
     Typically, secondary heterogeneous waste is retained after use in appropriate containment vessels until a sufficient quantity of material is collected for disposal. Under parts 261 and 268 of the Resource Conservation and Recovery Act (RCRA), prior to final treatment or disposal, the secondary heterogeneous waste must be characterized to determine the proper treatment and disposal regime. The current procedure for obtaining samples involves personnel dressed in appropriate personal protective equipment opening the containers, reaching into the secondary heterogeneous waste matrix with a knife or scissors, cutting randomly-selected pieces of soft waste materials at different levels within the container, depositing them into sample jars, labeling them and sending them for appropriate analysis in accordance with Environmental Protection Agency (EPA) Publication SW-846. Due to the variability of the material that is contained in each vessel it may be difficult to generate a statistically representative sample of a small enough size, for later chemical and physical analysis. Normally, with this sampling method, items within each vessel may be missed, thereby providing inaccurate information for storage, treatment or disposal compliance purposes. Further, this method also exposes the sample taker unnecessarily to radiation and/or chemical hazards. 
     Another problem associated with sampling the secondary heterogeneous waste is how to accurately capture volatile organic compounds at the detection limits required under the regulations. Oftentimes, hazardous chemical constituents adsorbed onto, or absorbed into the soft waste matrices are not properly detected in the samples. This may be due to matrix effects, sampling error, the inherent inaccuracy of the current sampling method, or a combination of any or all of these. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of this invention is to provide a representative sample from heterogenous feedstock. 
     Another object of this invention is to provide a process that will retain volatile organic compounds contained within a feedstock during the sampling procedure. 
     Another object of this invention is to provide a process that is consistent with the hazardous waste regulations. 
     Another object of this invention is to provide a process that reduces the number of samples required to adequately characterize a population of heterogeneous waste. 
     Another object of this invention is to provide a contained apparatus such that the apparatus is maintained at cryogenic operating conditions and minimize the spread of radioactive and/or hazardous material and personnel exposure. 
     These and other objectives of the invention, which will become apparent from the following description, have been achieved by a novel apparatus and process for providing a homogeneous analytical sample comprising: providing a heterogenous feedstock having an average initial particle size; reducing the temperature of the heterogeneous feedstock to a temperature below a critical temperature; conveying the heterogeneous feedstock (rubber, latex, plastic, paper or wood) to a size reduction device; reducing the size of the feedstock components; blending the reduced size feedstock to form a homogeneous mixture; obtaining a representative sample of the homogeneous mixture. Critical temperature is used herein to mean a temperature below which a significant portion of the feedstock is embrittled such that it will break or fracture upon bending, as opposed to flexing, as well as retain the volatile organic compounds. Preferably, the critical temperature is less than about 100 K (−173° C.). Herein, heterogeneous feedstock refers to a mixture of materials having a wide variability in size (e.g., supplied air suits which are 6 feet×2 ft to paper tissue which is 1 in×2 inches) and chemical compositions. The heterogeneous feedstock further comprises rigid solids, flexible solids, liquids, and volatile organic compounds. The volatile organic compounds may include, but are not limited to, methyl ethyl ketone (MEK), carbon tetrachloride, benzene, toluene and vinyl chloride. Preferably, the process of this invention maintains the representative sample below the critical temperature prior to being analyzed. Preferably, the size reduction and blending step are maintained at or below the critical temperature. Preferably, the process of this invention maintains the representative sample below 100 K (−173° C.) and more preferably 77 K (−196° C.) prior to being analyzed. The process is such that volatile organic compounds present in the feedstock are retained in the representative sample. Preferably liquid nitrogen is used to reduce the temperature of the feedstock below the critical temperature and maintain the temperature below the critical temperature. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS 
     With this description of the invention, a detailed description follows with reference being made to the accompanying figures of drawings which form part of the specification, in which like parts are designated by the same reference numbers, and of which: 
     FIG. 1 is a schematic view of the process of this invention; 
     FIG. 2 is a front-plan view of the device of this invention; 
     FIG. 3 is a side-plan view of the device of this invention; 
     FIG. 4 is a top-plan view of the shredder of this invention; 
     FIG. 5 is an enlarged view of region  5  of FIG. 4; 
     FIG. 6 is a detailed view of one shredder blade; 
     FIG. 7 is a partial cross-sectional view of the shredder bearings for use with this invention; 
     FIG. 8 is a partial cut away front-plan view of the mixing device of this invention; 
     FIG. 9 is a partial cut away side-view illustrating the mixing paddles and blade assembly for use with this invention; 
     FIG. 10 is a side-plan view of the sampling device for use with this invention; and 
     FIG. 11 is a front-plan view of the preferred embodiment of this invention. 
    
    
     The invention is not limited in its application to the details and construction and arrangement of parts illustrated in the accompanying drawings since the invention is capable of other embodiments that are being practiced or carried out in various ways. Also, the phraseology and terminology employed herein are for the purpose of description and not of limitation. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Description of the Preferred Embodiment(s) 
     The process of this invention is shown generally at  10  in schematic form in FIG.  1 . Typically, the feedstock of heterogeneous material  12  is stored in vessels  14 , e.g., 55 gallon steel drums. The feedstock may contain protective bodysuits, boots, goggles, rags, laboratory tissues (commonly known as KIMWIPES®) and various protective clothing and accessories. The heterogeneous feedstock contains a mixture of materials, such as, but not limited to paper, vinyl, rubber, plastic, and wood, which may be contaminated with a wide range of inorganic and organic compounds and/or radioactive components. The compounds from which the feedstock is made include a wide range of chemical compounds (straight chain polymers, branched chain polymers, cyclic organic, cellulosic, and aromatics). The feedstock has a wide variability in size (e.g., supplied air suits which are 6 feet×2 ft to paper tissue which is 1 in×2 inches). The heterogeneous material  12  is placed in a vat  16  and chilled to a temperature less than a critical temperature. Critical temperature is used herein to mean a temperature below which a significant portion of the feedstock is embrittled such that it will break or fracture upon bending, as opposed to flexing, and retains the volatile organic compounds. The temperature of the feedstock may be reduced by exposing it to cryogenic coolants such as chilled gases or liquids. Preferably, the feedstock is chilled by immersion in liquid nitrogen at atmospheric pressures, which is maintained at about 77 K (−196° C.). Other gases and liquids such as a liquid or chilled gaseous carbon dioxide, or similar compounds may be used. It is important that the gas or liquid selected does not chemically react with the feedstock and maintains the temperature low enough to retain organic compounds. The heterogeneous material  12  is reduced to a second smaller sized material  18  by size reduction equipment. The sized material  18  is blended to generate a homogenized mixture  20 , which is then sampled to produce a representative sample  22  for analysis. The sized material  18 , homogenized material  20 , and representative samples are maintained below the critical temperature. 
     The soft waste and size reduction apparatus for use with this invention is shown in FIG.  2  and FIG.  3 . generally at  30 . Material to be sampled (not shown) is pre chilled to the critical temperature in the precooling vessel  32  in a cryogenic coolant, preferably liquid nitrogen, prior to processing by this device. Material is delivered into the shredder  34  via feed hopper  36  which is cooled by the periodic delivery of a cryogenic coolant to reservoir  38  provided through conduit  40  from the liquid nitrogen storage tank (Not shown). Reduced material  42  from the shredder  34  is transferred to the mixer  44  via hopper  46 . The feed hopper  36 , shredder  34 , and mixer  44  may be cooled, as needed, by a feed of cryogenic coolant via conduits  48 ,  40  (noted hereinabove),  50 , and  52  respectively. Only the equipment is cooled. The pre chilling of the feedstock minimizes the need to spray cryogenic fluid onto feedstack during operation of the process. The blend material is sampled from the mixer  44  by access obtained through mixer opening  54  after the mixer  44  is moved from its normally operating position below the shredder  34  to a second position, by means of roller  56  and track system  58  where access to the mixer  44  can be provided. 
     The soft waste and size reduction apparatus  30  (as shown in FIGS. 2 and 3) is maintained at a temperature below ambient temperature and preferably at a temperature below the critical temperature. Preferably the critical temperature is liquid nitrogen temperatures 77 K (−196° C.). A cryogenic chamber/cold box  60  as shown in FIGS. 2,  3  surrounds the soft waste and size reduction apparatus  30 . Further, each process component of the apparatus  30  is isolated from the ambient environment and the motors used to drive the equipment so that the cold box  60  is maintained at or below ambient temperature. The shredder shaft  62  is coupled via a flexible coupling  64 , such as a hex joint, to a low thermal conductivity shaft  66 . The low-conductivity shaft  66  is typically constructed from a hollow tube to further reduce the thermal conductivity. The second end of the shredder low-conductivity shredder shaft  66  is coupled by means of a second flexible coupling  68  (U-joint, FIG. 4) to a gear reduction box  70  and motor  72 . A counter torque tube  74  connects the gearbox front plate  76  to the shredder support frame  78  to oppose the torque conducted through the shredder shaft  66 . 
     The shredder  34 , the rollers, and the associated drive assembly are shown in a top view in FIG.  4 . The shredder  34  for use in the fine or final shredding of the feedstock has been designed for operation at below ambient temperature and in particular, operation at cryogenic temperatures, which preferably is at about liquid nitrogen temperatures. The shredder  34  as shown is constructed of two counter rotating shredder shafts  62  held by bearings  80  held in bearing assemblies  82 . The detail construction of the bearing assemblies  82  and cooling system will be discussed later. The cutter blades  84  as shown in FIGS. 4 and 5 are constructed of Maraging 200 steel, or similar material to retain the requisite hardness, yet withstand operation at cryogenic temperatures. The material from which the cutter blades are made must be sufficiently hard to cut brittle polymers and other solids while remaining sufficiently ductile, at temperatures as low as 77 K (−196° C.), to avoid excessive wear or possible shattering. The configuration of the cutter blades  84  for use in the final size reduction of the feedstock is typically such that the thickness  86  and spacing  88  requires the use of narrow gauge materials. Due to the mechanical stresses placed on the cutter blades  84  it is impractical from an operation standpoint to rely on the narrow thickness  86  (Typically on the order of 3 mm) of the cutter blades  84  to conduct the torque from the shredder shaft  62  to the edge  90  of the cutter blade  84 , as shown in FIG.  6 . Therefore, the cutter blades  84  are constructed in units of 8 or 10 blades that are attached to a central collar  92 , (FIG.  6 ). The collar  92  is keyed  94  to engage with the shredder shaft  62  to provide for the effective transfer of torque from the shredder shaft  62  to the cutter blades  84 . 
     Bearing Assembly 
     To remove heat generated by friction within the bearing assembly  82  and to maintain the bearings  80  at the critical temperature a cryogenic cooling system is proposed and is illustrated in FIG. 7. A cryogenic coolant, preferably liquid nitrogen, is fed through inlet conduit  96  to bearing chamber  98 . The cryogenic coolant passes around bearings  80  held between inner bearing raceway  100  and outer bearing raceway  102  into reservoir  104 . The cryogenic coolant exits the bearing assembly  82  through exhaust conduit  106  and is returned to a cryogenic coolant feed tank (not shown). A drive shaft seal  108  and a cutter shaft seal  110  seal the bearing assembly  82  to prevent the loss of the cryogenic coolant. For the bearings  80  to operate properly at cryogenic temperatures, the bearings must be chilled to cryogenic temperatures then warmed gradually to ambient temperatures before final finishing in order to avoid seizing during operation at cryogenic temperatures, due to a Martensitic transformation. 
     Mixer Assembly 
     The reduced material  42  exiting the shredder  34  passes through hopper  46  into the mixer  44 , which is shown in detail in FIGS. 8 and 9. Reduced material  42  passes through mixer opening  54  of mixer  44  and enters the mixing chamber  112  where it is agitated by mixing paddles  114  affixed to mixer shaft  116 . The mixing paddles  114  comprises a paddle arm  118  that is substantially perpendicular to the mixing shaft  116 , a rotated paddle section  120  at an angle α from the paddle arm  118 , and a paddle base  122  substantially perpendicular to the rotated paddle section  120 . The angle α is from about 30° to about 60° and preferably 45°. The mixing paddles  114  are oriented in such a manner that the reduced material  42  is swept toward the center of the mixing chamber  112 . For example, for the mixer  44  shown in FIGS. 8 and 9, the rotated paddle sections  120  located in the first section  124  of the mixer  44  are rotated clockwise relative to the paddle arms  118 , while the rotated paddle sections  120  located in the second section  126  of the mixer  44  are rotated counter clockwise relative to the paddle arms  118 . The reduced material  42  moves to the center of the mixing chamber  112  where it strikes the fixed blade  128  that is affixed to the mixer inner wall  130  adjacent to the mixer opening  54  near the top of the mixer  44 . The action of the reduced material  42  impacting on the fixed blade  128  breaks up agglomerations of the reduced material  42 , thereby improving the mixing efficiency by providing both axial and radial blending. 
     Sampling 
     Representative samples of the reduced material are removed from the mixer  44 . The mixer  44  is moved from its operating position  130 , as shown in FIG. 2, to the sampling position  132 . A representative sample is withdrawn from the mixer  44  with tongs or a clamping device (not shown) and placed in sample bottle  134  as shown in FIG. 10. A funnel  136  directs the reduced material  42  into the sample bottle  134 . A manipulator  140  permits handling of the reduced material  42  sample and sample bottle  134  while maintaining the two at cryogenic temperatures. The sample bottle  134  can be clamped into the manipulator  140  with clamp  142  by moving transfer rod  144  within guide tube  146 . The sample bottle  134  can then be placed into an insulated storage container (not shown) where it can be maintained at the desired temperature until analyzed. Preferably the sample bottle  134  is maintained at at least the temperature of dry ice (solid CO 2 ) or at liquid nitrogen temperature 77 K (−196° C.). 
     Preferred Embodiment 
     In the preferred embodiment, as shown in FIG. 11, feedstock is introduced into the Soft Waste Processing System  150 , through air lock  152 . The feedstock introduced into the soft waste processing system  150  would normally be the entire contents of one storage vessel (not shown), for example a 55 gallon storage drum. Access to the air lock  152  is obtained through exterior door  154 . The air lock  152  is purged with an inert gas, such as nitrogen, argon, or helium, preferably nitrogen, through gas purge inlet  155 . The exhaust gas exits through exhaust port  156  and enters a filter system prior to release to the atmosphere. The feedstock introduced into the soft waste process system  150  via air lock  152  passes through interior door  158  where it falls into the pre-chilling vessel  160  that contains the cryogenic fluid, preferably liquid nitrogen. After the feedstock is cooled to below the critical temperature, the feedstock is raised to the discharge position  162  with the aid of moveable screen  164 . The material from which the moveable screen  164  is fabricated can be any suitable material that remains flexible and stable at the critical temperature, such as, but not limited to stainless steel or copper. The feedstock falls into the primary shredder feed hopper  166 , where it is held prior to introduction into primary shredder  168 . The primary shredder  168  reduces the feedstock size from the initial variable size (e.g., 6×2 to 2 in×1 in) to about ½ in×4 in. of the initial average feed size. The feedstock in the primary shredder  168  is maintained at or below the critical temperature by periodic cooling with cryogenic fluid, as needed, through nozzles  170 . The primary shredder discharge falls into secondary shredder hopper  172  from which it is fed into the secondary shredder  174 , such as a twin-shaft, low speed, high torque shredder. The secondary shredder  174  is similar in design and features to shredder  34  discussed hereinabove. The discharge from the secondary shredder is reduced from about a 4:1 to about 32:1 (final size is on the order of an ⅛ inch square) of the particle size of the feed material fed into the secondary shredder  174 . 
     The feedstock discharged from the secondary shredder  174  enters the mixer  176 . After all the feedstock from one vessel has passed through the primary shredder  168  and the secondary shredder  174  and into the mixer  176  the contents would be allowed to mix for a sufficient period of time to permit the contents to stabilize as a homogenous mix. The mixer  176  is then moved from the processing position  178  (indicated by arrows) to the sampling position  180  by means of rail system  182 . A representative sample is taken from mixer  176  with sample vessel  184 . The representative samples are moved to an insulated storage container (not shown) for retention until analysis of the samples. The samples are preferably maintained at 100 K (−173° C.) or more preferably 77 K (−196° C.) until analyzed. After sampling the mixer  176  is moved to discharge position  186  where it is delivered into discharge hopper  188  and fed into storage vessel  190 . The storage vessel  190  may be covered with a glove box system  192  to permit safe handling of the processed secondary waste. The pre-chilling vessel  160 , primary shredder  168 , secondary shredder  174 , mixer  176  and sampling zone  194  are collectively defined as the sample processing zone  196 . 
     The sample processing zone  196  is contained within an insulated chamber  198  in order to maintain the process and equipment at or below the critical temperature. The transport zone  200  located above the sample processing zone  196  may be maintained at a temperature above the critical temperature. Preferably the transport zone  200  is maintained at about ambient temperature and at a slight negative pressure to limit the release of gasses or toxic material from the process. Exhaust is provided through port  202  through a HEPA filter system (not shown) and other gas processing systems. The interface  204  at the top of the sample processing zone  196  is open to the transport zone  200 . Additional access to the sample processing zone  196  and the transport zone  200  is provided by glove ports  206  and  208  respectively. 
     The primary shredder  168  can be a shredder similar to shredder  34 , such as a twin-shaft, low speed, high torque shredder or similar shredder. The primary shredder  168  should be constructed from stainless steel or similar material having stability at cryogenic temperatures, in addition to facilitating easy for clean-up and decontamination. The secondary shredder  174  described hereinabove is similar in construction and features to shredder  34  discussed previously. Preferably, the secondary shredder  174  and mixer  176  are constructed from stainless steel or similar material except where noted. The design of mixer  176  is similar to mixer  44  discussed hereinabove. 
     Thus, in accordance with the invention, there has been provided a process that will retain volatile organic compounds contained within a feedstock during the sampling procedure. There has also been provided a representative sample from heterogenous feedstock. There has also been provided a process that is consistent with the hazardous waste regulations. There has also been provided a process that reduces the number of samples required to adequately characterize a population of heterogeneous waste. Additionally, there has been provided a contained apparatus such that the apparatus is maintained at cryogenic operating conditions and minimizes the spread of radioactive and/or hazardous material and personnel exposure. 
     With this description of the invention in detail, those skilled in the art will appreciate that modification may be made to the invention without departing form the spirit thereof. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments that have been illustrated and described. Rather, it is intended that the scope to the invention be determined by the scope of the appended claims.