Abstract:
An organic solution of a soluble molybdenum compound is ammonated with ammonia, ammonium hydroxide and/or an ammonium salt in order to significantly reduce the electrical resistance of the solution and a direct electric current is passed therethrough in order to collect a significant amount of the molybdenum in solid form at the negative electrode. Preferably, an epoxidation reaction product formed by the molybdenum catalyzed reaction of propylene with tertiary butyl hydroperoxide to provide propylene oxide and tertiary butyl alcohol is separated by distillation into a propylene fraction, a propylene oxide fraction, a tertiary butyl alcohol fraction and a heavy liquid distillation fraction containing substantially all of the dissolved molybdenum catalyst, the heavy liquid distillation fraction is saturated with ammonia to precipitate the molybdenum therefrom, the precipitated molybdenum is removed from the ammonated heavy distillation fraction and a direct electric current is passed through the remaining liquid portion of the ammonated fraction in an electrolytic cell in order to collect a significant portion of the remaining molybdenum residually dissolved therein at the cathode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improvement in a process for the molybdenum-catalyzed reaction of an olefinically unsaturated compound with an organic hydroperoxide. More particularly, this invention relates to an improvement in a process for the molybdenum-catalyzed reaction of an ethylenically unsaturated compound such as an ethylenically unsaturated alkyl, cycloalkyl, aryl, etc., compound with an alkyl, cycloalkyl, aryl, etc., hydroperoxide wherein the ethylenically unsaturated compound is converted to the corresponding epoxide. Still more particularly, this invention relates to an improvement in the process for resolving the reaction mixture that is formed in preparing propylene oxide and tertiary butyl alcohol by reacting propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in the presence of a soluble molybdenum catalyst. Even more particularly, this invention is directed to substantially removing molybdenum from the reaction mixture. 
     Molybdenum compounds are somewhat toxic to livestock and, therefore, solutions containing molybdenum must be handled with care. Also, the presence of molybdenum in liquid by-products presents a disposal problem because of the limited toxicity of molybdenum to livestock. 
     The epoxidation reaction mixture that is formed when an ethylenically unsaturated compound such as propylene is reacted with an organic hydroperoxide such as tertiary butyl hydroperoxide in the presence of a soluble molybdenum epoxidation catalyst will normally comprise, for example, unreacted propylene, propylene oxide, tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, the soluble molybdenum catalyst and impurities, including C 1  to C 4  lower aliphatic carboxylic acids. The reaction mixture is usually separated by distillation into a plurality of fractions including recycle propylene fraction, a propylene oxide product fraction, a tertiary butyl alcohol product fraction and a heavy liquid distillation fraction containing tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide and impurities, including substantially all of the dissolved molybdenum catalyst and a portion of the lower aliphatic carboxylic acid impurities. 
     In accordance with the present invention, the heavy liquid distillation fraction is charged to a molybdenum separation zone where it is treated with an amount of ammonia sufficient to precipitate the bulk of molybdenum. The filtrate, which has a greatly reduced electrical resistance as compared to the heavy liquid distillation fraction, is placed in an electrolytic cell and a direct electric current is then passed through the ammonated solution in order to collect a significant amount of the molybdenum in solid form at the negative electrode. In accordance with a preferred embodiment, the heavy liquid distillation fraction is first charged to a molybdenum precipitation reactor where it is brought into liquid phase contact with an excess of ammonia (based on the molybdenum content). Preferably, the heavy liquid distillation fraction is saturated with ammonia. As a consequence, the ammonia will react with substantially all of the molybdenum to form a molybdate precipitate that can be separated from the thus-treated heavy liquid distillation fraction in a precipitate separation zone. The separate precipitate will comprise a concentrated solid molybdenum product that may be processed for the recovery of molybdenum. The precipitate will contain a substantial amount, but not all of the molybdenum present in the initial reaction mixture and the liquid portion of the heavy liquid distillation fraction remaining after removal of the molybdenum precipitate will still contain a residual amount of molybdenum. In accordance the present invention, the liquid portion, comprising an ammonated solution of the residually dissolved molybdenum, is charged, as the electrolyte, to an electrolytic cell (a molybdenum collection zone) provided with spaced electrodes through which a direct electric current is passed in order to collect a significant portion of the residually dissolved molybdenum in solid form at the negative electrode (cathode) of the electrolytic cell. 
     2. Prior Art 
     It is known to react an olefinically unsaturated compound such as propylene with an organic hydroperoxide such as tertiary butyl hydroperoxide in the presence of a soluble molybdenum catalyst to provide a reaction product comprising propylene oxide and tertiary butyl alcohol. See, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635, and Russell U.S. Pat. No. 3,418,340. 
     It is also known to prepare soluble molybdenum catalysts to catalyze the reaction as disclosed, for example, in Bonetti et al. U.S. Pat. No. 3,480,563, Shum et al. U.S. Pat. No. 4,607,113, Marquis et al. U.S. Pat. No. 4,626,596, Marquis et al. U.S. Pat. No. 4,650,886, Marquis et al. U.S. Pat. No. 4,703,027, etc. 
     Kollar U.S. Pat. No. 3,860,662 is directed to an improvement in his basic process relating to the recovery of alcohols from the reaction product, which product is stated to be of an acidic nature, wherein a basic material such as an alkali metal or alkaline earth metal compound is added to the reaction mixture. Kollar U.S. Pat. No. 3,947,500 discloses a method for treating the reaction product formed by the reaction of an organic hydroperoxide with an olefin wherein an organic alcohol is formed as a by-product. It is stated that the alcohol tends to dehydrate and that to at least partially overcome this problem the oxidation reaction product is treated with an alkali metal or an alkaline earth metal compound. Kollar states that the alkali metal or alkaline earth metal compound can be added to the epoxidation reactor or to the reaction product. 
     Sorgenti U.S. Pat. No. 3,573,226 discloses a method wherein a molybdenum-containing catalyst solution is prepared by incorporating metallic molybdenum into the distillate bottoms fraction of an epoxidation reaction product followed by heating of the resultant mixture in order to form a soluble molybdenum-containing reaction product which can be used to catalyze the epoxidation reaction. 
     Maurin U.S. Pat. No. 3,931,044 is directed to a method for recovering molybdenum catalyst values from a epoxidation reaction product for recycle. Maurin discloses one of three techniques. In accordance with the first embodiment, the residue fraction is calcined to provide molybdenum trioxide which is then used to prepare a soluble molybdenum compound by reaction with aqueous ammonia. In a second embodiment, the molybdenum-containing fraction is treated with aqueous ammonia without calcining to form an ammonium molybdate which is treated with a polyalcohol to give a molybdic ester. In a third embodiment, the molybdenum-containing fraction is treated with gaseous ammonia in order to form an ammonium molybdate precipitate which can be recovered by filtration. 
     Harvey U.S. Pat. No. 3,449,217 is directed to a process for the recovery of tertiary butyl hydroperoxide from a mixture comprising tertiary butyl hydroperoxide, tertiary butyl alcohol and organic acids and esters resulting from the liquid phase oxidation of isobutane by a process which minimizes hydroperoxide decomposition. This is done by accomplishing the distillation while the product has an effective pH of below about 9. The patentee teaches the treatment of the reactor effluent with a neutralizing agent such as an alkali metal or an alkaline earth metal hydroxide. 
     Levine U.S. Pat. No. 3,819,663 is directed to a method for treating a heavy distillation fraction of this nature in order to recover the molybdenum in the concentrated bottoms fraction for recycle to the epoxidation reaction zone as makeup catalyst. 
     Levine conducts his wiped-film evaporation process under conditions including a temperature of about 550°-650° F. (about 273° to about 330° C.) at atmospheric pressure to obtain his desired residual fraction for recycle as catalyst makeup and a distillate fraction comprising about 85% or more of the heavy distillation fraction. Levine states that the distillate fraction that is thus obtained can be used as a furnace fuel or can be worked up for recovery of the individual components contained therein. However, Levine et al. does not contain any teaching as to how the individual components in the fraction would be obtained. 
     BACKGROUND INFORMATION 
     It is known, as shown for example in Kollar U.S. Pat. No. 3,351,365, to react an ethylenically unsaturated compound with a hydrocarbon hydroperoxide in the presence of a soluble molybdenum compound, whereby the ethylenically unsaturated compound is converted to the corresponding epoxide and whereby the hydroperoxide is converted to the corresponding alcohol. 
     Olefinically unsaturated compounds that may be used as feedstocks include substituted and unsubstituted aliphatic and alicyclic olefins such as olefinically unsaturated hydrocarbons, hydrocarbon esters, hydrocarbon alcohols, hydrocarbon ketones, etc., which preferably contain about 3 to 30 carbon atoms and include compounds such as propylene, normal butylene, isobutylene, the pentenes, the methyl pentenes, hexenes, octenes, dodecenes, cyclohexenes, methyl cyclohexene, butadiene, styrene, methyl styrene, vinyl toluene, vinyl cyclohexene, allyl alcohol, methallyl alcohol, diallyl ether, methyl methacrylate, methyl oleate, methyl vinyl ketone, allyl chloride, etc. 
     The organic hydroperoxide is suitably a hydroperoxide having the formula: 
     
         R--COOH 
    
     where R represents an alkyl, a cycloalkyl or an aryl group. Illustrative hydroperoxides include compounds such as cumene hydroperoxide, ethylbenzene hydroperoxide, tertiary butyl hydroperoxide, tertiary amyl hydroperoxide, tetralin hydroperoxide, methyl cyclohexene hydroperoxide, etc. 
     When an alkyl, cycloalkyl, or aryl olefinically unsaturated compound is reacted with an alkyl, cycloalkyl, or aryl, hydroperoxide in the presence of a soluble molybdenum compound, the ethylenically unsaturated compound is converted to the corresponding epoxide and the hydroperoxide is converted to the corresponding alcohol. 
     For example, when propylene is reacted with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in an epoxidation reaction zone in the presence of a soluble molybdenum catalyst to form propylene oxide and additional tertiary butyl alcohol, an epoxidation reaction mixture is formed which will contain not only unreacted feed components and the desired propylene oxide and tertiary butyl alcohol, but also impurities including the dissolved molybdenum catalyst, oxygen-containing impurities such as ditertiary butyl peroxide, lower aliphatic C 1  to C 4  carboxylic acids such as formic acid, acetic acid, isobutyric acid, etc., alcohols such as methanol, isopropyl alcohol, tertiary butyl alcohol, etc., esters such as methyl formate, methyl acetate, methyl isobutyrate, etc., ketones such as acetone, etc., aldehydes such as isobutyraldehyde, etc., and hydrocarbon impurities resulting from undesired side reactions of the propylene, such as hydrocarbons containing 6 or more carbon atoms. 
     Although most of the impurities are originally present in the epoxidation reaction mixture in trace quantities, as the epoxidation reaction mixture is resolved by distillation into a propylene recycle fraction, a propylene oxide product fraction and a tertiary butyl alcohol product fraction, all of which are distillate fractions, the heavier impurities are progressively concentrated in a heavier distillation fraction, such as a distillation fraction having the composition generally set forth in Table I. 
     
                       TABLE I______________________________________COMPOSITION OF HEAVY DISTILLATION FRACTIONSComponent          Concentration, Wt. %______________________________________Impurities lighter than TBA              0.1-2Tertiary butyl alcohol              70-90Impurities heavier than TBA              1-4but lighter than TBHPTertiary butyl hydroperoxide               2-20Impurities heavier than TBHP               3-12Molybdenum concentration              500-5,000 ppm______________________________________ 
    
     SUMMARY OF THE INVENTION 
     Hydrocarbon solutions of molybdenum compounds are characterized, in part, by strong resistance to the flow of electrical current therethrough. In accordance with the present invention, however, it has been discovered that hydrocarbon solutions of molybdenum compounds that are ammonated by treatment with ammonia are sufficiently conductive of electrical current to make the recovery of molybdenum from such solutions a feasible operation, particularly in respect of the recovery of molybdenum from hydrocarbon solutions of molybdenum compounds used in the molybdenum-catalyzed epoxidation of an olefin with a hydrocarbon hydroperoxide. 
     Thus, the recovery method of the present invention can be effectively practiced in a process where an ethylenically unsaturated compound is reacted with an alkyl, cycloalkyl, or aryl, hydroperoxide in the presence of a soluble molybdenum compound, wherein the ethylenically unsaturated compound is converted to the corresponding epoxide, wherein the hydroperoxide is converted to the corresponding alcohol and wherein the thus-formed reaction mixture is resolved by distillation into a plurality of distillation fractions, including a heavy distillation fraction containing substantially all of the dissolved molybdenum catalyst. 
     In accordance with the present invention, the heavy distillation fraction is treated with an amount of ammonia sufficient to significantly reduce the electrical resistance of the said heavy distillation fraction, and the thus-ammonated distillation fraction is charged, as the electrolyte, to an electrolytic cell through which a direct electric current is passed in order to collect a significant amount of the molybdenum in solid form at the negative electrode. 
     In accordance with a preferred embodiment of the present invention, an epoxidation reaction mixture is formed by the molybdenum-catalyzed reaction of a olefinically unsaturated compound such as propylene with an organic hydroperoxide such as tertiary butyl hydroperoxide and will comprise, for example, the unreacted olefinically unsaturated compound, the corresponding epoxide formed therefrom, the unreacted organic hydroperoxide, the corresponding alcohol formed therefrom, by-products and dissolved molybdenum catalyst. Typically, the reaction mixture is charged to a distillation zone where it is resolved into a plurality of distillation fractions, such as recycle fractions for the unreacted feed components, an epoxide product fraction, an alcohol product fraction and a heavy distillation fraction comprising the dissolved molybdenum catalyst, by-products of the reaction, etc. In general, the heavy distillation fraction will contain from about 0.4 to about 0.8 wt. % of dissolved molybdenum and, when the starting materials are propylene and tertiary butyl hydroperoxide will also contain lower aliphatic carboxylic acids resulting from the removal of propylene, propylene oxide and tertiary butyl alcohol from an epoxidation reaction mixture. 
     In accordance with the present invention, the ammonated heavy distillation fraction, or an ammonated fraction thereof, is charged to an electrolytic cell comprising a spaced cathode and a spaced anode through which a direct electric current is passed. As a consequence, a significant portion of the molybdenum will collect at the cathode. 
     Preferably, however, the heavy distillation fraction is charged to a precipitation zone which may suitably comprise a reactor, such as an autoclave, provided with suitable agitating means (e.g., an impeller), temperature control means such as a jacket or coils through which a liquid heat exchange medium can be circulated, charge lines for the heavy distillation fraction and for the ammonia and a discharge line for withdrawal of the treated product. Within the precipitation zone the ammonia will react with the molybdenum compounds present in the heavy distillation fraction to form a reaction product comprising a molybdenum-containing precipitate that can be withdrawn from the precipitation zone. The precipitate can be removed in any desired manner in a precipitate removal zone (e.g., by filtration, centrifugation, etc.). The precipitate can be recovered for disposal in any suitable environmentally acceptable manner, such as for example by treatment in a metals-reclaiming plant for the recovery of the molybdenum. 
     When the heavy distillation fraction contains only about 0.8 wt. % or less of molybdenum (e.g., 0.4 to 0.8 wt. %), the precipitation of the molybdenum compounds will be essentially complete in that that precipitate will contain substantially all of the molybdenum charged to the precipitation zone. 
     However, a residual amount of molybdenum will remain in the liquid portion of the ammonated heavy distillation fraction after the separation of the precipitate. Normally the precipitate-free ammonated heavy distillation fraction will contain from about 20 to about 100 ppm of dissolved molybdenum. It is desirable to approach an absolutely molybdenum free liquid product as closely as possible. A further significant reduction in molybdenum concentration can be achieved by the process of the present invention, such as 40% to 60% reduction in the residual dissolved molybdenum content. 
     The heavy distillation fraction will normally contain less than about 1 wt. % of water. Anhydrous ammonia can be used, but an aqueous ammoniacal solution may be used in order to further improve the electrical conductivity of the heavy distillation fraction or the portion thereof that comprises the electrolyte of the electrolytic cell. The ammonia should preferably be used in an amount which is equivalent to the amount of molybdenum in the heavy distillation fraction and, preferably, an excess of ammonia will be used, such as about 1 to about 200 moles of ammonia per gram atom of molybdenum present in the heavy distillation fraction. Preferably, the heavy distillation fraction is saturated with ammonia. 
     The precipitation reaction can be conducted under ambient conditions of temperature and pressure, although somewhat higher temperatures and pressures may be used, if desired, such as temperatures within the range of about 20° to 250° C. and pressures within the range of about 0 to 3,000 psig. The contact time should be sufficient to insure that the precipitation reaction goes to completion (e.g., 0.2 to 2 hours). 
     After the precipitation reaction is completed, the precipitate is removed in any desired manner, e.g., filtration, centrifugation, evaporation, etc. Since the precipitate constitutes only a minor amount of the mixture of precipitate and treated heavy distillation fraction, filtration is preferred. 
     The precipitate, which will normally contain about 40 to about 58 wt. % of molybdenum can be disposed of in an environmentally acceptable manner. For example, it can be used as a feedstock in a metals-reclaiming plant or used as a raw material for the preparation of an additional amount of fresh molybdenum catalyst solution. 
     The filtrate obtained by the practice of the present invention will contain only a residual amount of molybdenum (e.g., from 20 to 100 ppm). 
     In accordance with a preferred embodiment of the present invention, the filtrate, comprising an ammonated solution of the residually dissolved molybdenum, is charged, as the electrolyte, to an electrolytic cell provided with spaced electrodes through which a direct electric current is passed in order to collect a significant portion of the residually dissolved molybdenum in solid form at the negative electrode (cathode) of the electrolytic cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, the figure is a schematic drawing of a preferred reaction and purification sequence that may be used in the practice of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawing, there is shown a schematic flowsheet illustrating a preferred method of practicing the process of the present invention. 
     An epoxidation reaction zone 10 is provided and propylene is charged thereto by a line 12 together with a soluble molybdenum catalyst charged by a line 14. A solution of tertiary butyl hydroperoxide in tertiary butyl alcohol is charged by a line 16. 
     The epoxidation reaction is an epoxidation reaction of the type disclosed by Kollar U.S. Pat. No. 3,351,653 as further elaborated upon, for example, in British patent specification No. 1,298,253 wherein propylene is reacted with tertiary butyl hydroperoxide under reaction conditions including a reaction temperature within the range of about 180° to about 300° F., a pressure of about 300 to about 1000 psig. and, more preferably, a temperature of about 220° F. to about 280° F. and a pressure of about 500 to about 800 psig. As another example, the epoxidation of propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in the presence of a soluble molybdenum catalyst is disclosed in Marquis et al. U.S. Pat. No. 4,891,437. See also, Marquis et al. U.S. Pat. No. 4,845,251. 
     The soluble molybdenum catalyst charged to the epoxidation reaction zone by the line 14 may be an epoxidation catalyst of the type known in the art such as those disclosed by the Kollar patent or the British patent or by Marquis et al. U.S. Pat. No. 4,626,596, U.S. Pat. No. 4,650,886, U.S. Pat. No. 4,654,427, U.S. Pat. No. 4,703,027, or U.S. Pat. No. 4,758,681. The Marquis et al. patents are directed to molybdenum/alkanol complexes such as solutions of molybdenum compounds in ethylene glycol which contain a high concentration of molybdenum and are particularly useful as catalysts in the epoxidation reaction. Marquis et al. teach, for example, the epoxidation of propylene with tertiary butyl hydroperoxide with their catalyst under epoxidation conditions including a temperature of 50° to 180° C. and a use of propylene to tertiary butyl hydroperoxide ratios within the range of about 0.9:1 to about 3.0:1. 
     Suitably, the tertiary butyl hydroperoxide that is charged to the epoxidation reaction zone 10 by way of line 16 is about a 40 to about 75 wt. % solution of tertiary butyl hydroperoxide in tertiary butyl alcohol. The catalyst is charged to the epoxidation reaction zone 10 by the charge line 14 in an amount such as to provide from about 50 to about 1000 ppm of molybdenum, based on the total of the reactants charged and, more preferably, from about 200 to 600 ppm. The reaction is preferably conducted at superatmospheric pressure such as a pressure of about 300 to 1000 psig. 
     When the reaction is conducted on a continuous basis, as illustrated in the drawing, the feed materials are charged to the epoxidation reaction zone 10 through the lines 12, 14 and 16 at rates sufficient to maintain the desired concentration of reactants and an equivalent volume of epoxidation reaction mixture is withdrawn from the epoxidation reaction zone 10 by way of a discharge line 18. The reaction product discharged by the line 18 will normally comprise unreacted propylene, a minor amount of unreacted tertiary butyl hydroperoxide, propylene oxide, tertiary butyl alcohol, including tertiary butyl alcohol formed by the reaction of the tertiary butyl hydroperoxide with propylene, the molybdenum catalyst and impurities such as propane, propionaldehyde, acetone, methanol, isopropanol, water, acetaldehyde, methyl formate, acetic acid, formic acid, isobutyric acid, hydrocarbons containing 6 or more carbon atoms and high boiling residue components. 
     The reaction product 18 is charged to an epoxidation reaction product distillation zone 20 where it is separated by distillation into desired fractions in accordance with methods known to those skilled in the art. For example, the distillation sequence disclosed in British Patent No. 1,298,253 may be used. 
     One of the distillate products that is recovered in the zone 20 is a propylene fraction which is discharged by a line 22 controlled by a valve 24 and provided with a branch line 26 controlled by a valve 28 in order to permit the recycle of unreacted propylene to the epoxidation reaction zone 10 through the propylene charge line 12. 
     Another distillate fraction that is obtained is a propylene oxide product fraction 30 which is discharged by the line 30. 
     The propylene oxide fraction may be purified in a propylene oxide purification zone (not shown) by known techniques such as, for example, those disclosed in Burnes et al. U.S. Pat. No. 3,715,284, Schmidt et al. U.S. Pat. No. 3,909,366, Schmidt U.S. Pat. No. 3,881,996, Jubin U.S. Pat. No. 3,607,669, Schmidt U.S. Pat. No. 3,843,488 or Schmidt U.S. Pat. No. 4,140,588. 
     Another product that is recovered from the epoxidation reaction product distillation zone 20 is a tertiary butyl alcohol distillate product 40 which may be further purified, if desired, to remove oxygenated impurities therefrom by catalytic treatment as disclosed, for example, in Sanderson et al. U.S. Pat. No. 4,704,482, Sanderson et al. U.S. Pat. No. 4,705,903 or Sanderson et al. U.S. Pat. No. 4,742,149. 
     A heavy distillation fraction 50, usually a bottoms fraction, is also discharged from the epoxidation reaction product distillation zone 20. As described by Levine U.S. Pat. No. 3,819,663 and Sweed U.S. Pat. No. 4,455,283, the heavy distillation fraction will contain substantially all of the molybdenum catalyst initially charged to the epoxidation reaction zone 10 by way of the line 14. The heavy distillation fraction 50 will contain other products such as tertiary butyl hydroperoxide, tertiary butyl alcohol and impurities including oxygenates lighter than tertiary butyl alcohol such as acetaldehyde, acetone, isopropyl alcohol, etc., oxygenates heavier than tertiary butyl alcohol but lighter than tertiary butyl hydroperoxide, and residue components heavier than tertiary butyl hydroperoxide such as propylene glycol tertiary butyl ethers, etc. As indicated, the heavy distillation fraction 50 will also contain carboxylic acids such as formic acid, acetic acid and isobutyric acid. 
     Although the molybdenum catalyst is present in the epoxidation reaction zone 10 in an amount in the range of about 50 to 1,000 ppm, and usually 200 to 600 ppm, it is progressively concentrated in the epoxidation reaction product distillation zone 20 and is normally present in the heavy distillation fraction 50 in an amount in the range of about 0.4 to 0.8 wt. % (about 4,000 to 8,000 ppm). 
     The molybdenum-contaminated heavy distillation fraction 50, in accordance with the present invention, is charged to a precipitation zone60 which may comprise a reaction vessel such as an autoclave which is equipped with suitable agitation means (e.g., an impeller) and suitable temperature control means such as an external jacket or internal coils through which a heat exchange medium can be circulated. Within the precipitation zone the heavy distillation fraction 50 is brought into contact with ammonia which is charged by an ammonia charge line 52 in at least an equimolar amount, based on the molybdenum content of the heavy distillation fraction 50 and, preferably, in a molar excess. The ammonia is preferably used in the form of anhydrous ammonia in order to minimize the water content of the treated heavy distillation fraction 50. The ammonia is suitably brought into contact with the heavy distillation fraction 50 under ambient temperature and pressure conditions, although higher temperatures and/or pressures may be used, such as temperatures within the range of about 20° to 250° C. and pressures within the range of 0 to about 3,000 psig. The contact time should be sufficient to ensure as complete a reaction of the ammonia with the molybdenum as is reasonably possible and to ensure substantially complete precipitation of the product, such as a contact time of about 0.2 to 2 hours. 
     The thus-formed slurry of precipitate in the treated heavy distillation fraction 50 is discharged from the precipitation zone 60 by a slurry discharge line 62 leading to a precipitate separating zone, such as a filtration zone 70 where the slurry is resolved into a precipitate that is removed by a discharge line 72 and a filtrate fraction that is discharged by a filtrate discharge line 74. 
     The filtrate 74 is charged to a molybdenum collection zone comprising an electrolytic cell 76 of any suitable construction containing an electrolyte (the filtrate 74) and immersed, spaced cathodes and anodes (not shown). A direct electric current from a suitable source (not shown) is passed through the electrolytic cell 76 and this will cause a significant portion of the residual dissolved molybdenum in the filtrate will collect in solid form on the cathode. The thus-treated electrolyte can then be discharged from the electrolytic cell 76 by a discharge line 78 for disposal in any suitable manner. For example, the electrolyte will normally meet environmental guidelines and normally can be used as a boiler fuel. 
     EXAMPLES 
     The invention will be illustrated by the following specific examples which are given by way of illustration and not as limitations on the scope of this invention. 
     Use of Ammonia to Preliminarily Remove Most of the Molybdenum 
     In accordance with a preferred embodiment of the present invention, the heavy distillation fraction is initially treated with ammonia to remove most of the dissolved molybdenum contained therein. 
     These experiments were conducted in a one-liter batch stirred reactor under the conditions as noted in Table II. For example, in Experiment 6547-62, the ammonia was reacted with a molybdenum-containing catalyst solution for one-half hour at 50° C. and a pressure of 50 psig. The reactor was cooled and then vented and the remaining contents filtered. The dried precipitate and the filtrate were analyzed for molybdenum content. In Experiment 6547-62, one part of NH 3  (1.0 gram) or 0.0588 mole of ammonia was reacted with six parts (6.0 grams) of a catalyst solution that had a molybdenum content of 0.56 wt. % (6×0.0056, i.e. 0.0336 grams of molybdenum). Since the weight of a gram atom of molybdenum is 95.95 grams, this amounts to 0.00035 gram atoms of molybdenum or a ratio, for Experiment 6547-62, of 168 moles of ammonia per gram atom of molybdenum. For Experiment 6547-62, the dried precipitate was found to contain 55 wt. % of molybdenum and the filtrate was found to contain 52 ppm of molybdenum. 
     As another example, in Experiment 6270-90, 1 part ammonia was reacted with about 6 parts by weight of a 0.56 wt. % molybdenum catalyst solution (this material was over two years old) at conditions of 100° C., 190 psig. for one hour. The reactor was cooled then vented and the remaining contents filtered. The dried precipitate contained 31 wt. % molybdenum while the filtrate contained 350 ppm molybdenum. 
     The next six experiments of Table II represented the same recently produced starting material so that the results could be compared directly. In Experiments 6547-63 and 64, gaseous ammonia was used. In Experiments 6547-60, 61, 62 and 66, ammonia and catalyst were charged to the batch stirred reactor. Amounts were added so that the reactor would be about 90% liquid full at operating temperature. It appears that lower levels of molybdenum in the filtrate were achieved with liquid phase reaction of ammonia. 
     Experiment 6547-62 represented the single stage reaction with ammonia. Experiments 6547-70/76 represented the first and second stage reactions with a more concentrated molybdenum starting material. Molybdenum concentrations are higher in the solid and lower in the filtrate with the single reaction stage as compared to the two stage reaction sequence. Furthermore, milder operating conditions are utilized with the single stage reaction. 
     When starting materials with higher molybdenum concentrations are reacted with ammonia (Experiments 6547-69 and 73), the filtered solids contained lower molybdenum values whereas the filtrates contained higher levels of molybdenum as compared to a more dilute molybdenum solution starting material. 
     Experiments 6547-70 and 76 represented a two-stage reaction sequence. In the first stage the molybdenum content was reduced from 15,000 to 3,000 ppm. This filtrate was subjected to a secondary reaction with ammonia in which the level of molybdenum in the filtrate was reduced to 1,900 ppm. 
     There does not appear to be an advantage in concentrating the molybdenum solution followed by a two-stage reaction sequence over that of a single reaction stage starting with a more dilute molybdenum solution. 
     
                                           TABLE II__________________________________________________________________________Reaction of Mo Catalyst Solution with Ammonia g NH.sub.3 / g Cat.   Press              t   Init Precip                             FiltReference Soln.      T(C)          (psig)              (hr)                  ppm Mo                       wt % Mo                             ppm Mo__________________________________________________________________________6270-90 0.16 100 190 1   5,600                       31      3506547-63 0.21  28 Atm 1   5,600                       46      916547-64 0.21  55 Atm 1   5,600                       50      1006547-60 0.05  55  25 0.5 5,600                       48      616547-61 0.10  50  55 0.5 5,600                       48      706547-62 0.16  50  50 0.5 5,600                       55      526547-66 0.32  50 200 0.5 5,600                       47      536547-69 0.21 200 2,600              2   25,000                       43    2,5006547-73 0.20 180 830 1   18,000                       44    12,0006547-70 0.40 200 2,500              2   15,000                       42    3,0006547-76 0.83 180 1,980              1   3,000                       NES.sup.1                             1,900__________________________________________________________________________ .sup.1 Not Enough Sample for Testing 
    
     II 
     Electroytic Removal of Dissolved Molybdenum 
     In an initial experiment (Table II), solution resistance was reduced from 300,000 to 6,000 ohms (electrodes 1&#34; apart) after pretreatment with ammonia. In another experiment, a 2.3 wt. % molybdenum solution pretreated with ammonia was placed in an electrochemical cell (6V DC) for a period of about seven days. When removed the negative electrode had a solid-like material deposited on it which contained 33 wt. % molybdenum. 
     In a third experiment, a 52 ppm molybdenum solution pretreated with ammonia (Experiment 6547-62) was reduced to 27 ppm in an electrochemical cell. The cell consisted of a 3V continuous DC power supply with bronze scouring pads as electrodes The current across the cell ranged from 18-25+mA and the cell was operated over the period of 62 hours. 
     In a fourth experiment, a 70 ppm molybdenum solution pretreated with ammonia (Experiment 6547-61) was reduced to 37 ppm in an electrochemical cell. The cell consisted of a 3V continuous DC power supply with 6 foot sections of brass chain as electrodes. The contents of the cell was continuously agitated to improve mass transfer. The current across the cell ranged from 7-8 mA and the cell was operated for a period of 4 hours.