Abstract:
The present invention relates to a method for treating of oil contaminated, metal-containing solids, in particular spent catalysts. The solids are cleaned with a dense phase gas and then metal is recovered from the cleaned solids.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from International Patent Application Serial No. PCT/EP2006/003822, filed Apr. 25, 2006, which claims priority from European Patent Application No. 05009468.9, filed Apr. 29, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for treating of oil contaminated, metal-containing solids comprising a cleaning step wherein said oil is removed from said solids and a metal recovery step wherein metal is recovered from said cleaned solids. 
         [0003]    In the oil and petrochemical industry, large amounts of heterogeneous catalysts are employed. Often, metal salts are precipitated onto solid carriers, and the resulting product is used to facilitate or accelerate chemical reactions. As an example, silver oxide precipitated onto a solid ceramic such as aluminum dioxide or silicon dioxide catalyses the partial oxidation of ethylene to ethylene oxide. As a further example, metals and metal compounds such as nickel, vanadium, lanthanum, niobium, iron and many others facilitate, e.g., in the presence of hydrogen, the cracking of high molecular weight hydrocarbons to more desired low molecular weight fractions such as gasoline, diesel and the like. As yet another example, nickel, palladium and platinum are used as hydrogenation catalysts. 
         [0004]    For various reasons such as coking or sintering of active phases during use, the lifetime of heterogeneous catalysts is limited. This results in waste streams which demand proper, safe, environmentally acceptable and economic treatment and/or disposal options. In particular, it is desired to recover precious metals from spent catalysts. 
         [0005]    Challenges to proper treatment are given by the toxicity of products remaining on the catalyst particles after use, the pyrophoric nature of the contaminants, such as oil or hydrocarbons, and the need to provide a suitable form of the cleaned catalysts for further processes, in particular metal recovery schemes. 
         [0006]    Currently, spent catalysts are typically incinerated prior to disposal/landfilling or transfer to metal recovery. However, various problems are associated with the state-of-the-art handling of oil-contaminated solids such as catalysts. Incineration often proceeds without properly using the caloric energy of the contaminants. Furthermore, incineration leads to the formation of toxic compounds such as polyaromatic compounds and additional coking. Coking poses problems in later metal recovery schemes and additional waste problems. Sulfur and nitrogen, typical components on spent catalysts, are converted during incineration to sulfuric acid respectively nitrous oxides/acids which give rise to corrosion problems. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Thus, it is an object of the present invention to provide an improved method for treating of oil contaminated, metal-containing solids which ideally recovers the caloric energy of the contaminants such as oil and which ideally does not create additional toxic or corrosive or dangerous compounds. 
         [0008]    Further, it is an object to provide a method for treating of oil contaminated, metal-containing solids which ideally provides a suitable raw material for further processes such as metal recovery, and which ideally is safe in operation, economic in investment and operational costs and environmentally benign. 
         [0009]    This object is achieved by a method for treating of oil contaminated, metal-containing solids comprising a cleaning step wherein said oil is removed from said solids and a metal recovery step wherein metal is recovered from said cleaned solids, wherein said cleaning step comprises contacting said solids with a sub-critical dense phase gas. 
         [0010]    Hydrocarbons show a good solubility in dense phase gases such as liquid carbon dioxide. It has been found that a liquid dense phase gas at subcritical conditions shows a high solvation power which can be used to substitute the prior art incineration of oil contaminated solids by cleaning in that liquid dense phase gas. 
         [0011]    DE-OS-28 53 065 claims a method for extracting reaction products out of special catalysts by using super-critical carbon dioxide. Surprisingly, oil can be removed from solids by cleaning in carbon dioxide at sub-critical conditions. Compared to super-critical cleaning this has the advantage of cheaper cleaning machines. 
         [0012]    Surprisingly, it has been found that the thus cleaned solids are in a more preferable form as feedstock for recovery than solids which are cleaned in a conventional way. Without wishing to be bound by theory, it may be speculated that incineration leads to additional coking and formation of layers similar in chemical nature to bitumen which reduce the speed of the following metal recovery processes. 
         [0013]    The term dense phase gas is to be understood as a high pressure gas with a pressure higher than 30 bar, preferably higher than 50 bar and more preferably higher than 60 bar. 
         [0014]    The extraction temperature is case-dependent, but it is preferred to contact the solids to be cleaned with the dense phase gas at a temperature in the range of 10° C. up to 100° C. 
         [0015]    Preferably, said contaminated solids are contacted with dense phase carbon dioxide at sub-critical conditions. Carbon dioxide shows good solvation characteristics. Further, dense phase carbon dioxide diffuses and flows like a gas and hence reaches even the smallest pores. 
         [0016]    Depending on the type of contamination it is preferred to add additives to the dense phase gas. As the case may be, additives in gaseous or liquid form may be employed. Preferred gaseous additives are for examples paraffins such as C 3  (propane) to C 8  (octane). It is further advantageous to use liquid additives such as alcohols, especially isopropanol, or suitable cosolvents such as acetone or methylethylketone which easily can be recovered from the product or cosolvents such as toluene or surfactants such as nonionic surfactants which are acceptable as component in the liquid oil fraction. 
         [0017]    A preferred composition for cleaning the oil contaminated solids comprises supercritical or near-critical carbon dioxide together with additives and detergents as mentioned above. 
         [0018]    The preferred field of application is the treatment of spent catalysts, especially catalyst particles. More preferred, such particulate material has an average size of less than 1 mm up to several centimeter, most preferred an average size between 0.5 mm and 10 mm. The present invention provides a proper treatment of catalysts wherein after the cleaning step in the dense phase gas the catalyst particles are in a suitable form for further processes, in particular metal recovery schemes. 
         [0019]    The present invention can for example be used to clean catalyst particles from oil or lubricating oil which had been added to increase the capability of the catalyst particles to flow. 
         [0020]    The inventive cleaning step may also be used to clean sludge such as oil- and metal-contaminated earth under petrol filling stations, or from other industrial sites, for example leather industries or cable factories. 
         [0021]    A preferred embodiment for treating oil contaminated solids comprises one or more of the following steps: 
         [0022]    The oil contaminated material, in particular granulate material, is placed into and stored in optionally inerted or gas-blanketed enclosure. For example, the contaminated material is stored in bins or containers under exclusion of ambient air. That means, all handling of dangerous, for example toxic and/or pyrophoric, oil contaminated material is carried out under inert gas protection and blanketing. 
         [0023]    An appropriate amount of that oil contaminated material is then transferred to a cleaning apparatus under exclusion of air, in particular under inert gas atmosphere, such that the risk of fire/ignition is minimized. The transfer may preferably be carried out using a pneumatic system or a transport band. 
         [0024]    The cleaning apparatus comprises at least one reactor, and at least one attached vessel for separating the gaseous solvent from any removed contaminants, such as oil. The attached vessel is preferably designed to separate the gaseous solvent from the contaminants by distillation under more than atmospheric pressure. The cleaning apparatus may further comprise additional components such as intermediate gas tanks. 
         [0025]    The contaminated solids are preferably reloaded from the inerted enclosure into compartments or cassettes which comprise gas-permeable walls such as metal nets or other perforated materials which allow an efficient flux of gas/liquid through the cassette. These cassettes are then placed into the cleaning reactor. Preferably, after cleaning the cassettes can be emptied through the bottom, or through suction from the top without having to move the cassettes. 
         [0026]    The cleaning reactor preferably comprises means to agitate the solids and/or the dense phase gas during the cleaning process. Such means include the option to rotate the cassette with the solids, or to actively pump or move the dense phase gas through the reactor, e.g., by propellers, pumps and the like. 
         [0027]    Rotation may be at any angle relative to horizontal, however, close to 90 degree relative to horizontal is preferred as this mode allows top loading of the reactor cassette, and in operation a closing device or lid for the cassette is not needed. 
         [0028]    The cleaning reactor may operate in a batch manner either in the way that a dense phase gas is pumped continuously through the reactor, alternatively in the way that sufficient amounts are kept in the reactor until the dense phase gas is close to saturation with contaminants whereupon said amount of dense phase gas is emptied from the reactor and replaced with fresh dense phase gas. 
         [0029]    The cleaning reactor is preferably inerted, especially filled with an amount of gaseous carbon dioxide, prior to charging with contaminated solid. The gas displaced during filling is ventilated off. 
         [0030]    During the cleaning, the pressure in the reactor may be varied in order to increase the gas flux especially into porous catalyst carriers, with the purpose to extract contaminants entrained within pores. 
         [0031]    Finally the cleaned solids or particles are discharged from the cleaning reactor and used as an excellent raw material for metal recovery, for example through known acid or alkali leaching processes. 
         [0032]    The present invention has several advantages compared to the state of the art solutions. As is apparent from the description above, the invention allows the cleaning of contaminated solids in a manner which is characterized by:
       Safe handling of dangerous, e.g., pyrophoric and/or toxic goods   High degree of automatization possible, resulting in the double benefit of low operational costs and low risk of exposure of dangerous chemicals to workers   Almost total recovery of oil contaminants—which are useful as refinery feedstock   No additional coking and/or formation of dangerous or toxic byproducts from combustion   Cleaned solids are excellent raw material for metal recovery       
 
         [0038]    The invention as well as additional details of the invention shall now be explained by means of a preferred embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0039]    The FIGURE shows the results of four consecutive cleaning batches according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    The inventive method is preferably used to treat oil contaminated catalyst granulates, for example catalyst particles having an average diameter of about 5 mm. Spent catalyst is contaminated with oil, hydrocarbons and other substances which are often pyrophoric. Further, several toxic products are stuck to the surface of the catalyst which require a proper handling. 
         [0041]    The catalyst granulate is filled into a metal basket which is then enveloped by a plastic foil. Since the catalyst material is contaminated with oil and other highly inflammable substances that filling process includes a certain risk of ignition. However, the loading of the catalyst granulate into the metal baskets is carried out at the refinery or petrochemical plant which is equipped with appropriate fire extinguishing means. 
         [0042]    Then the enveloped catalyst granulate is transferred to a carbon dioxide cleaning installation. During the transport the plastic foil excludes any air from the inflammable catalyst material and thus there is no risk of ignition, fire or explosion. At the cleaning installation the plastic foil is removed from the metal basket. For safety reasons that removal is preferably carried out under inert gas atmosphere. 
         [0043]    The metal baskets filled with contaminated material are placed into the cleaning reactor of the carbon dioxide cleaning installation. Again for safety reasons, the cleaning reactor is initially filled with gaseous carbon dioxide. Thereby it is assured that the contaminated catalysts are handled in inert atmosphere all the time. 
         [0044]    Then the cleaning reactor is closed and filled with liquid carbon dioxide. During filling surplus gaseous carbon dioxide is ventilated off the cleaning reactor. The liquid carbon dioxide is pressurized to 60 bar at a temperature of 15° C. and the cleaning operation in carbon dioxide is carried out for 75 minutes. 
         [0045]    The FIGURE shows the result of a cleaning operation which has been repeated four times at the same conditions, that is at a pressure of 60 bar and a temperature of 15° C. After the first cleaning batch a weight reduction of about 8% has been obtained. The reduction in weight of the metal basket filled with the initially contaminated catalyst is equivalent to the weight of oil removed from the catalyst. As shown in the FIGURE, the weight reduction after four batches is already 16%. 
         [0046]    Between two cleaning batches, and/or alternatively continuously during the cleaning process, the dense phase carbon dioxide loaded with contaminants is withdrawn form the cleaning reactor and passed to a distillation vessel where the contaminants are separated from the carbon dioxide by distillation. The carbon dioxide is preferably filtered to collect insoluble small particles such as made of graphite, coke or ceramics. The carbon dioxide is condensed and passed to a storage tank for later use. The recovered oil contaminants are useful as refinery feedstock. 
         [0047]    After the cleaning step has been finished the metal basket is taken out of the cleaning reactor, and the cleaned catalyst material is unloaded from the metal basket and transferred to a metal recovery unit for further processing. 
         [0048]    It shall be pointed out that the speed and efficiency of cleaning can be further increased when one or more of the following measures are carried out:
       Use of surfactants and additives,   Variations in pressure and/or temperature in order to force liquid and gas in and out of pores of the catalyst,   Enforced agitation of the dense phase gas by jet streams, propellers or by pumping dense phase gas through the cleaning reactor,   Mechanical agitation of the solids, for example by rotating the metal basket within the cleaning reactor.       
 
         [0053]    According to another embodiment of the present invention the oil contaminated catalyst granulate is loaded into bins or containers which are preferably inerted. At the cleaning installation the contaminated material is either transfilled into metal baskets or cassettes which comprises metal nets or other perforated material which allows gas and liquid to pass into the interior of the cassettes. The cleaning operation is then carried out as described above. If the cleaning reactor is provided with a rotating basket it is also possible to directly fill the contaminated granulate into that rotating basket.