Abstract:
A process for recovering and purifying vanadium found in petroleum coke is disclosed. Coke containing V and sulfur is charged to a molten metal bath and dissolved to form a molten metal bath with dissolved carbon, dissolved V metal and dissolved sulfur. At least a majority of the dissolved sulfur is released as H 2 S by maintaining reducing conditions in the bed, by maintaining a high concentration of dissolved carbon or addition of steam or hydrogen rich hydrocarbon such as methane or some combination of these approaches.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of prior provisional application Low or no Slag Molten Metal Processing of Coke Containing Vanadium and Sulfur, filed Jun. 9, 1998, Serial No. 60/088,564. 
    
    
     Many refiners are now forced to process more difficult stocks, which are heavier and more metals contaminated. Many heavy crudes contain significant amounts of vanadium and sulfur and frequently with lesser amounts of Ni, Fe and other metal compounds. 
     Such heavy crudes are difficult to process catalytically, so an initial stage of thermal processing, usually some form of coking, is typically used to free distillable, relatively metals free products from vanadium containing coke. The coker gas oil and coker naphtha are essentially free of metals and may be processed by conventional catalytic upgrading processes to produce high quality transportation fuels and other hydrocarbon products. Unfortunately, coking of these difficult crudes produces large amount of coke, a solid product with an enhanced concentration of vanadium and sulfur and other metals present in the charge to the coker. 
     Many cokers produce coke which is so contaminated with metals and sulfur that it has little value as fuel. The sulfur generally precludes use of coke in cement kilns. Such materials usually have enough vanadium to cause processing problems but too low a vanadium concentration to make vanadium recovery economically attractive. 
     Some coking processes, such as fluid bed cokers, especially those employing gasifiers, can produce large amounts of fine, low density coke particles which is especially difficult to treat using conventional technology. 
     To illustrate just how difficult it is to process vanadium rich cokes, some representative prior art processes are reviewed below. 
     U.S. Pat. No. 4,203,759, Metrailer et al, PROCESS FOR THE PREPARATION OF A VANADIUM-CONTAINING METALLIC ASH CONCENTRATE, taught heating partially gasified coke with oxygen containing gas to produce low density ash. Most ash particles produced had a diameter of less than 1 micron. The fine ash was readily processed with sulfuric acid to recover vanadium. 
     U.S. Pat. No. 4,243,639, Haas et al, METHOD FOR RECOVERING VANADIUM FROM PETROLEUM COKE, taught gasifying the coke with steam in the presence of an alkali metal salt to improve V recovery during subsequent processing steps. 
     U.S. Pat. No. 4,345,990, Fahlstrom et al, METHOD FOR RECOVERING OIL AND/OR GAS FROM CARBONACEOUS MATERIALS, taught use of two molten baths to process a variety of carbon containing wastes. A lead or zinc first bath operated at 500C. to thermally devolatilize the carbonaceous material, while the second bath of molten iron operated at 1200 C. and operated with oxygen to gasify residual quantities of carbon. 
     U.S. Pat. No. 4,443,415, Queneau et al, RECOVERY OF V 205  AND NICKEL VALUES FROM PETROLEUM COKE, taught slurrying coke in an aqueous solution of sodium carbonate to increase V recovery. 
     U.S. Pat. No. 4,708,819, Vasconcellos et al, REDUCTION OF VANADIUM IN RECYCLE PETROLEUM COKE, taught the problems of high vanadium levels during partial oxidation (the vanadium forms eutectics with refractory walls, melting them). The vanadium content of recycled, unconverted coke was reduced by froth flotation treatment. 
     U.S. Pat. No. 4,816,236 Gardner, RECOVERY OF VANADIUM AND NICKEL FROM PETROLEUM RESIDUES, taught recovering vanadium from various Flexicoke residues with varying V contents and particle sizes. The patent reported that Flexicoke from the heater bed had 1-5 wt % vanadium pentoxide, while coke from the cyclone venturi fines had 8-12 wt % vanadium pentoxide. Coke from the wet scrubber had as much as 20 wt % vanadium pentoxide. The patentee taught mixing coke with NaOH, burning and then leaching to recover vanadium. A majority of the Ni was reported to be left in the solids after the leach step. 
     U.S. Pat. No. 5,259,864, Greenwalt, METHOD OF DISPOSING OF ENVIRONMENTALLY UNDESIRABLE MATERIAL AND PROVIDING FUEL FOR AN IRON MAKE PROCESS E.G., PETROLEUM COKE, taught using a sulfur and heavy metal containing petroleum coke in a melter. The coke was mostly burned to form a fluidized bed of coke which was then reacted with ore and oxygen to make molten iron or steel preproducts. A slag layer contained sulfur freed during combustion of the petroleum coke. 
     U.S. Pat. No. 5,277,795, Thornhill et al, PROCESS AND APPARATUS FOR RECOVERING HEAVY METAL FROM CARBONACEOUS MATERIAL, taught burning petroleum coke to produce ash then processing the ash to extract metallic compounds. 
     U.S. Pat. No. 5,324,341, Nagel et al, METHOD FOR CHEMICALLY REDUCING METALS IN WASTE COMPOSITIONS, taught molten metal processing of metal containing wastes. Multiple reducing agents reduced oxygen-containing metals compounds in a metal-containing waste composition. Claim  24  mentioned use of an oxide of V as an oxidizing agent. 
     U.S. Pat. No. 5,427,603, Samant et al, METHOD OF TREATING A VANADIUM-CONTAINING RESIDUE, taught processing petroleum coke with air at 850C. in a fluidized bed with an inert bed of granular iron oxide with an oxygen partial pressure between 10-4 bar and 10-6 bar to produce ash. The ash was then processed over a magnetic separator to recover the inert iron oxide for recycle. 
     U.S. Pat. No. 5,484,554 Vuoung et al, OXIDANT INJECTION FOR IMPROVED CONTROLLED OXIDATION, taught processing of coke in a partial oxidation reactor. The V in the coke forms a V rich slag in the reactor. 
     While not related directly to processing coke, U.S. Pat. No. 4,071,355, Staggers, RECOVERY OF VANADIUM FROM PIG IRON, taught removal of vanadium from pig iron to produce a vanadium rich slag by oxidizing molten pig iron at 2600-2900 F. 
     The teachings of these patents, which are incorporated by reference, could be summarized as follows. 
     Vanadium in petroleum containing coke is difficult to recover directly, that is, from the coke, because it is in a form which does not lend itself to conventional leaching approaches. The carbon, probably present in the form of condensed chelating structures, effectively shields significant portions of the metal from conventional leaching solutions. 
     Vanadium in coke can be recovered much more readily after combustion, because the vanadium in the ash produced is more susceptible to leaching, though even here some treatment, such as with sodium carbonate or sodium hydroxide was needed to improve metal recovery during leaching. 
     Vanadium in coke is always found with sulfur. The conventional way to deal with sulfur in steel making is to make slag with the sulfur. 
     We discovered that a molten metal process, originally developed to produce high purity hydrogen or synthesis gas from various waste streams, could be modified to process vanadium containing coke, dissolve the vanadium in its metallic state into the iron bath and reject much, but not all of the sulfur in the coke as H2S which could be processed in a Claus unit to recover elemental sulfur. We also discovered a way to enhance sulfur removal, by adding various agents which created a reducing atmosphere in the bath and promoted removal of dissolved sulfur as H2S. It was even possible to reduce or eliminate the need for sulfur capture agents and an accompanying sulfur rich slag, while processing V and S containing coke. 
     Details of the basics or a preferred molten metal process are disclosed in one or more of the following patents, which are incorporated by reference. 
     U.S. Pat. No. 5,755,839, MOLTEN METAL REACTOR SWING SYSTEM AND PROCESS. 
     U.S. Pat. No. 5,645,615, MOLTEN DECOMPOSITION APPARATUS AND PROCESS. 
     U.S. Pat. No. 5,577,346 MULTI-ZONE MOLTEN-METAL HYDROGEN AND FUEL GAS GENERATION PROCESS. 
     U.S. Pat. No. 5,435,814, MOLTEN METAL DECOMPOSITION APPARATUS. 
     The process defined by the above four patents could tolerate a great many feeds, including coal and trash, but was primarily directed to production of relatively pure hydrogen gas at superatmospheric pressure. This work was not directed toward vanadium recovery from petroleum coke, ignored the problem of S/V containing coke, and did not suggest use of reducing conditions in the bath during V capture, nor use of gaseous reducing agents to continuously, or at the end of a cycle, strip sulfur from the molten metal bath as H2S. 
     Our process used a special form of molten metal processing to dissolve the carbon, V and sulfur present in the coke. Much of the carbon is present in the form of a collapsed metalo-porphyrin surrounding an atom of vanadium metal. The carbon dissolves readily in the molten iron bath, exposing the vanadium and permitting rapid and complete dissolution of the vanadium and other metals found in the petroleum coke in the molten iron bath. Sulfur dissolves relatively rapidly in the molten metal bath, the important variable is to maintain relatively reducing conditions in the bath, e.g., by maintaining high carbon levels in the bed, to prevent oxidation of sulfur. 
     The process was fast and simple—no special processing of the coke was needed, though preferably it was dry. Heat/utility requirements were low, in fact the worse the feed in terms of % V in the coke, the more heat the process generated. The process was tolerant of many other impurities found in coke containing feed, such as Ni and S compounds. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a process for dissolving coke containing both V and sulfur in a molten metal bath, preferably a molten iron bath to produce a molten metal bath containing dissolved carbon and dissolved vanadium metal and dissolved sulfur, exothermically oxidizing at least a majority of the net amount of carbon dissolved in said bath to produce carbon oxides and maintain said bath in a molten state and maintaining reducing conditions and a pre-determined amount of carbon in said bath sufficient to maintain at least a majority of said V and said sulfur dissolved in said molten metal bath in an elemental state, allowing said sulfur to accumulate in said bath and removing at least a portion of said dissolved sulfur as H2S which is removed as a product of the process. 
     Preferably, at least periodically there is added to said bath steam or hydrogen or a hydrocarbon in an amount sufficient to promote production of H2S from said bed, which produced H2S is removed as a product of the process. 
     Preferably, the bath is maintained in a relatively reducing mode of operation, so that little of the vanadium and/or sulfur is oxidized. The vanadium level is allowed to increase until the bath contains 5 wt % or 10 wt % V, more preferably at least 20 wt % V, and most preferably 40 wt % V. After sufficient Vanadium has accumulated, it may be beneficial to strip sulfur from the molten metal bath by addition of steam or methane or other hydrogen rich hydrocarbon, to desulfurize the bath. After the desired amount of sulfur has been removed from the bath, it is possible to conduct further processing of the dissolved elemental vanadium, as by addition of oxygen or other oxidizing agent, to promote oxidation of vanadium. This can be done by intermittently stopping addition of coke and/or increasing the addition of oxygen or oxygen containing gas until the desired amount of vanadium has been oxidized and removed from the molten iron bath as a slag product. 
     The bath may be continuously or intermittently replenished with fresh iron. 
     In preferred embodiments, the bath is run at superatmospheric pressure, preferably 2 to 200 atm. High pressure operation allows higher feedstock rates without excessive carryover of dust. Higher pressures also increases the rate at which carbon in feedstock dissolves in the molten metal bath. 
     Presence of large amounts of carbon in the bath (i.e., reducing conditions) permits processing of sulfur rich coke with much or essentially all of the sulfur content released as H2S, which can be readily processed in a refinery Claus unit or other H2S recovery process. If desired, especially at the end of a cycle when the last traces of sulfur must be removed, it is possible to operate with oxidizing conditions in the molten metal bath which will produce oxides of sulfur which may require sulfur capture with conventional slag forming agents or stack gas processing. Thus some slag can be formed, but only for a fraction of the cycle and the slag production will be only a fraction of that required by stoichiometry had the entire bath of coke been processed in a molten metal bath at relatively oxidizing conditions. 
     When desired, multiple zone processing of the coke, alone or admixed with a hydrocarbon, may be practiced to permit recovery of a relatively pure hydrogen stream. 
     Our process is generic as to the bath used. Circulating baths, pressurized single zone baths, multiple zone baths, and the like can all be used. Thus while a high pressure design, such as that developed by Ashland and disclosed in the above four patents discussed above which were assigned to Ashland, may be used it is also possible to use other molten metal bath designs such as the Molten Metal Technology reactor or other molten bath designs now existing or hereafter developed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified schematic drawing of a preferred embodiment wherein coke containing sulfur and V is processed in a molten metal reactor to produce a vanadium containing molten metal bath and H2S, with greatly reduced or eliminated slag formation due to sulfur. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The petroleum coke feedstock may be any metal containing coke. Usually the coke will contain vanadium, nickel, and sulfur. Such materials are well known and widely available, they are produced wherever coking is used as an upgrading step for heavy Venezuelan crudes. 
     The coke may be sponge coke from a delayed coker, shot coke, fines from fluid coker, and the like. Some fluidized coking units partially burn some of the coke, yielding a coke product of enhanced vanadium concentration but generally still classified as coke despite the oxidative treatment. 
     In addition to coke, the feed may also comprise, or consist essentially of, liquid hydrocarbons. Thus the process may be used to process Venezuelan or other crudes which are contaminated with vanadium, or liquid or semi-solid fractions derived from such crudes. 
     PROCESS DESCRIPTION 
     Referring to the FIGURE, a coke feed, typically a fine particulate feed having an average particle size less than 80 microns or even much smaller, is charged via line  5  to feed hopper  10 . Coke is discharged via rotary valve  15  through open swing valve  20  into upper hopper  25 . Coke is discharged down through swing valve  30  into pressurized hopper  35 , from which it is continuously or intermittently discharged via rotary valve  40  into a flowing stream of steam from line  45 . Other fluids may be used besides steam, but steam is preferred due to its ready availability and relative safety. The steam/coke mixture flows through line  50 , is mixed with a preferred but optional recycle fines stream from line  150 , and charged via line  70  into the HyMelt reactor  60 . The coke/steam mixture is preferably discharged down via outlet  76  towards molten metal bath  80 . Preferably the nozzle is close enough to the surface of metal bath  80  and is discharged with sufficient force to penetrate the metal bath. 
     Oxygen or oxygen containing gas is preferably co-fed with the coke. This allows a significant amount of pre-heating of the coke during its passage through the piping to nozzle outlet  76 . A portion of the bed contents may be continuously or intermittently withdrawn via line  65  as a product of the process. Preferably essentially all of the vanadium in the coke accumulates in the bed in the form of elemental vanadium metal along with dissolved elemental sulfur. 
     Off gas, which usually will contain entrained droplets of iron/vanadium and/or perhaps some slag droplets is removed via line  82 , quenched with relatively cool recycle gas from recycle gas line  84  and charged successively through heat exchanger  86  which produces high pressure steam and heat exchanger  88  which produces lower pressure ;steam. The temperature of the material withdrawn from reactor  60  is typically around 2800 F., while the temperature of the withdrawn vapor is reduced to 1000 F. and 350 F. respectively by passage through exchangers  86  and  88 . 
     The reactor off gas and entrained solids are charged to cyclone  92  which recovers a low particulate vapor via vapor outlet line  96 . A solids rich, dense phase fluidized phase is discharged via rotary valve  94 , though the cyclone solids rich phase outlet may be sealed by other conventional means such as a flapper valve or immersion of the cyclone dipleg in a dense phase fluidized bed of particulates. The cyclone vapor phase is charged via line  96  to bag filter  200 , which may be a conventional bag house or other gas/particulate separation means such as a third stage separator, electrostatic precipitator, or the like. A solids phase is continuously or intermittently removed via rotary valve  205  and charged via line  210  to admix with the cyclone  92  solids phase and pass via swing valve  120  into hopper  125  and swing valve  130  into recycle fines pressurized hopper  135 . Pressurized fines are discharged via rotary valve  140  into flowing steam in line  145  to be recycled, with fresh fluidized coke feed, via line  70  to the HyMelt reactor. 
     A portion of the net addition of V to the molten metal bath may be withdrawn as either a coarse dust product from the cyclone separator via line  94  and collection means  98  or as a finer dust product from the bag house via line  207  and collection means  208 . 
     The relatively particulate free vapor withdrawn via line  220  from bag filter means  200  may be further cooled using fin fan coolers, heat exchange with other process streams, or cooling water in cooler  230  to produce cooled vapor. A portion of cooled vapor is charged via line  235  to the inlet of recycle gas compressor  240  which discharges compressed recycle gas via line  84  to serve as quench stream. The remainder of the particulate free vapor is preferably charged through acid gas scrubber  260 . Lean solvent in line  280  from solvent regenerator  270  is charged to an upper portion of the scrubber to contact acid gas. A relatively sweet gas stream is withdrawn via line  290  and charged via line  300  into ZnO treater  300  or equivalent clean up means to produce a purified gas stream which may be used as fuel or as a hydrogen rich syngas removed via line  310 . 
     The rich solvent, with absorbed acid gas species, is removed via line  265  and recycled to solvent regenerator  270  which preferably recovers at least a portion of absorbed acidic sulfur containing gas species as H2S, which may be converted into elemental sulfur via a conventional Claus unit, not shown. 
     SULFUR CONTROL 
     The molten metal bath may be made sufficiently reducing by operating with large amounts of carbon that most and even essentially all of the sulfur in the coke will remain in the form of elemental sulfur dissolved in the molten metal bath. To further reduce the sulfur levels in the bath, as at the end of a cycle, it may be desirable to inject steam or methane into the molten metal bath to create reducing stripping gas which promotes release of sulfur form the bed as H2S. This may be done continuously or intermittently, and preferably is done after the molten metal bath has accumulated the desired amount of vanadium and it is important to desulfurize the bath. 
     The bath may be maintained significantly reducing by operating with at least 0.5 wt % carbon dissolved in the bath, and preferably contains more than 1 wt % carbon. Even higher carbon levels may be used with good results, such as 2, 3 or 4 wt % carbon. 
     The upper limit on carbon is usually set by solubility and sooting.