Claims:We Claim:
1. A system (100) for fluidized bed roasting of spent catalyst to recover metal values like molybdenum, vanadium, tungsten, the system comprises the following components:
a) a receiving area (1) for said raw spent catalyst mass comprising sieve to separate oversize inert material,
b) a material conveying area (2) for passing catalyst mass having particle size between 1 to 3 mm,
c) a fluidized bed roaster (4) configured to calcine spent catalyst mass at controlled oxidizing conditions in fluidized state comprising a separate inlets for receiving spent catalyst mass having particle size between 1 to 3 mm, inlet (8) for hot air and gas, distribution plate (12), outlet (7) to collect roasted spent catalyst, air discharge valve (6) and flue gas elimination duct (9),
d) a cyclone chamber (10, 10’) configured for separating the fine particles from flue gases and final outlet (11) for flue gas;
e) a two stage alkali liquid scrubber configured to outlet (11) for dissolving the SOx and generating the effluent stream;
f) a precipitation reactor where effluent stream having solubilized SOx is treated with lime slurry under air purging to oxidize sulphite to sulphate to initiate gypsum synthesis and regenerating alkali thereby reduction in alkali requirement by about 40-60% per kg of Sulfur in scrubbing step,
g) a two stage leaching reactors configured at the end of outlet (7) comprising an alkali solution for leaching out metals selected from molybdenum, vanadium, tungsten and mixture thereof from roasted spent catalyst mass.

2. The system (100) for fluidized bed roasting of spent catalyst as claimed in claim 1, wherein a waste heat recovery boiler system configured to flue gas elimination duct (9) for waste heat recovery.
3. A method for preparing a roasted spent catalyst to recover metal values like molybdenum, vanadium, tungsten, the method comprises the following steps:
Step 1) Sieving of spent catalyst mass to receive 1-3 mm material,
Step 2) charging of pre-roasted/inert material to back mixed fluid bed chamber which will be maintained at a temperature of 180-220 °C by using burner,
Step 3) Initializing continuous roasting of material collected in step 1) by starting continuous charging into fluidized bed roaster at a controlled feeding rate of 600 to 800 Kg/hr to maintain the roaster temperature range between 370 to 630 °C with retention time from 15 to 30 min, thereby using the inherent energy of the spent catalyst,
Step 4) Collecting a roasted spent catalyst through discharge valve and cooling to room temperature,
Step 5) Passing the flue gas though cyclone to collect fine calcined particles at the bottom,
Step 6) combining the collected particles from cyclone with step 5) material,
Step 7) passing the flue gas through waste heat recovery boiler followed by in two stage scrubber sequentially from Scrubber 1 to scrubber 2 consisting alkali configured for dissolving the SOx and generating the effluent stream,
Step 8) passing the effluent stream of step 7) into precipitation reactor where air is purged (to oxidize sulphite to sulphate) and lime slurry is added to precipitate out gypsum and regenerating alkali,
Step 9) Regenerated alkali in step 8) is recirculated in scrubber 1 and 2 alkali liquid scrubber thereby alkali quantity reduced by about 40-60% per kg of Sulfur in scrubbing step,
Step 10) grinding the material collected at step 6) to 100 mesh size and passing in two stage leaching reactor for leaching of metals selected from molybdenum, vanadium, tungsten and combination thereof with alkali solution.
4. The method for preparing a roasted spent catalyst to recover metal values as claimed in claim 3, wherein in step 3) the temperature preferably between 330-550 °C with a roasting time of 15-18 min.
5. The method for preparing a roasted spent catalyst to recover metal values as claimed in claim 3, the flue gas generated from step 3) is passed through waste heat recovery boiler where the temperature of flue gas is between 40-80 °C.
6. The method for preparing a roasted spent catalyst to recover metal values as claimed in claim 3, wherein in step 7) scrubber 1 and 2 alkali solution selected from Sodium hydroxide to neutralize SOx.
7. The method for preparing a roasted spent catalyst to recover metal values as claimed in claim 3, wherein step 8) lime solution has a concentration of 50% wt/vol and precipitation reaction temperature 70-80 °C to generate gypsum 6.4-7.1 kg/kg and sodium hydroxide regeneration 1-3 kg/kg of Sulfur removed.
8. The method for preparing a roasted spent catalyst to recover metal values as claimed in claim 3, the wherein step 8) the leaching reactors temperature is maintained at 80-90 °C.
, Description:FIELD OF INVENTION
The present invention relates to a system and method for preparing roasted spent catalyst. More particularly, the present invention provides an energy efficient system and method for oxidative roasting of waste catalysts bearing precious metals like molybdenum, vanadium, tungsten involving a removal of hydrocarbons, coke and sulfur, thereby generating roasted material for further processing to obtain consistent and high recovery of metals from said roasted spent catalysts. The said method has low fuel requirements, economically viable, environmentally friendly, SOX recovery as well as preparing gypsum by-product, alkali regeneration for SOx recovery and the said system provides a heat recovery from exhaust flue gases and process the spent catalyst with lesser space requirement.

BACKGROUND AND PRIOR ART OF THE INVENTION
Mono-, di- or trimetallic oxide catalyst with Molybdenum and/or tungsten as a dopants on alumina support materials with Ni/Co as promoters are extensively used by the petroleum refining industry for crude oil purification with respect to removal of undesirable impurities such as S, N and metals by hydrodesulfurisation (HDS), hydrodenitrification (HDN) and hydrometalisation (HDM) reactions. Depending on the type of feed crude oil, catalyst performance cycle varies from 6 months to 2.5 years depending on the type of crude oil processed. After certain period of usage, the catalyst gets deactivated due to the deposition of coke and metals (especially Ni and V from crude) and considered as spent catalyst available for disposal to landfill site or regeneration or recycling process.
Spent hydro-desulfurisation catalysts or similar spent catalysts contain valuable metal ions like Mo, W, and V either in combination or separately along with Ni/Co for processing and recovery, which again depends on the process cost and products quality. Several methods such as chlorination, acid leaching, alkali leaching with oxidants, bioleaching, roasting with sodium salts and water leaching, oxidation roast - alkali leaching are some of the processes studied for the recovery of the valuable metals such as Mo, Ni/Co, V, W from spent hydro processing catalysts.
US Patent 2,367,506 (1945) by Kissock describes a process for the recovery of the molybdenum present in spent hydro processing catalysts with a support of alumina. The process involves immersing the spent catalyst in sodium carbonate solution and then heating at high temperature in a rotary furnace. Under these conditions, the alumina is left substantially insoluble and the sodium aluminate formed may then be dissolved in water, entraining only a small quantity of aluminum in the form of sodium aluminate. The process has the disadvantage of high temperature roasting (1000-1200 °C, preferably 1150 °C).
US Patent 4,075,277 (1978) of Castagna and coworkers describes a process for the recovery of high purity molybdic acid from spent catalyst materials. The process involves soaking of spent catalysts in sodium carbonate solution and heated to convert molybdenum compounds into sodium molybdate within temperature range of 600-800 0C, while avoiding substantial conversion of alumina into a water soluble compound. Molybdenum is then separated from alumina, cobalt and nickel by leaching with hot water. The drawback of the process is that soaking of spent catalyst will lead to the reduction in throughput and the removal of water from the soaked catalyst requires additional energy.
US Patent 4,087,510 (1978) describes a process, where the recovery of vanadium and molybdenum is achieved by mixing catalyst with solid sodium carbonate, and roasted at 800 0C to convert sulfur, vanadium and molybdenum into water soluble compounds and extracting with water. The drawback of the process is that high temperature roasting for fusion is required.
US Patent 3,773,890 (1973) by Fox and coworkers, disclose a process for utilizing spent hydro processing catalysts for metal recovery using a two step process wherein the catalysts were calcined at 725 °C followed by roasting with sodium chloride at 525-925 0C, which converts vanadium and molybdenum to a water soluble form. After water leaching, the vanadium and molybdenum solution is separated and the residue is treated with a sodium hydroxide in a pressure reactor in order to dissolve and recovery of alumina. The drawback of the process is sodium chloride generates chlorinated hazardous gases from roasting and it operates at high temperature for fusion to occur.
US Patent 4,145,397 (1979) reports an invention, wherein molybdenum, vanadium, cobalt and nickel are recovered. The spent catalyst was roasted at 600-950 °C, leached with a hot caustic alkali solution, which solubilise most of the vanadium, molybdenum and some of the aluminum. The drawback of the process is that high temperature roasting and poor energy efficiency. Also there is possibility of loss of molybdenum due to sublimation of molybdenum oxide at high temperature.
US Patent 3,567,433 (1971) of Gutnikov describes the invention, which involves oxidative roasting of spent hydro processing catalyst at 650 0C followed by hot ammonia - ammonium carbonate solution leaching in pressure reactor in order to dissolve molybdenum, vanadium and nickel. Only molybdenum is substantially solubilized by the procedure (~90%) while the rest of the metal recovery remain moderate (65-70%). Because the process utilizing a fixed bed or fluidized bed using oxygen containing gas with nitrogen to control the burning rate for roasting of spent catalyst.
US Patent 4,514,368 (1985) by Hubred discloses a process for removing nickel, cobalt, molybdenum, and vanadium from spent hydro processing catalyst particles by roasting the catalyst at 400-600 °C and leaching the catalyst particles with an aqueous solution of ammonia and an ammonium salt. This process extracts at least 85% of Mo, 70% of the V, 70% of Ni and 45% of Co in a given time. This process also uses nitrogen gas for controlling the roasting temperature.
US patent 9273397B2 (2016) by Grimley describes a high temperature thermal oxidation process for roasting of spent HDS catalyst at 900°C to produce ashes. Ashes are leached with alkali and H2O2 as oxidizer and recovering Mo and V as their salts. This process uses high temperature roasting where Mo and V compounds formation with others cannot be avoided and the Mo loss due to sublimation of Mo would be there as the sublimation temperature of molybdenum oxide is above 700 oC, there by difficult dissolution of metals during leaching.
But, most of the spent catalyst roasting processes (in a conventional rotary kiln) suffer from commercial viability either due to high process cost, low recovery of molybdenum during post processing stage, large number of purification steps and loss of molybdenum, generation of side products and their disposal, increased safety requirements and production of low quality products.
In other hand, US patent 3,941,867 (1976) of Wilkomirsky and coworkers describes oxidation roasting of finely divided flotation concentrate to molybdenum trioxide in a fluidized bed roaster, maintained at a high recycle ratio of oxidized calcine to feed to avoid sticking and deposition problems. The disadvantages of this disclosure are: (i) sintering problems, refractory inert materials like silica having over size than the concentrate used in this investigation, which contribute to decrease of calcine product output. Further, separation of inert and calcine concentrate involves investment of equipment, efficient separation step. (ii) Contamination of roasted molybdenite concentrates in the present invention be materials like silica cannot be avoided, thereby restricting direct applications to chemical grade molybdenum compounds preparation. (iii) retention times are longer, thereby decreasing the roasted calcine out put (iv) agglomeration/ sintering of material in roaster bed, there by disturbing the fluidization velocity during roaster operations (v) heavy loss of molybdenite concentrate of 40 to 55 % wt% ratio of charge with S values from 2-4%, thereby increased investments on size and number of cyclone scrubbers (vi) use of wet scrubbers after cyclone scrubber to capture concentrate fines result in wet cake, there by adding solids separation step (vii) mixing of wet cake with feed concentrate and charging to the fluid bed roaster results in problems like feeder choking and change of oxygen potential within the roasted.
US patent 6190625 (2001) of Jha and co-workers describes fluidized bed roasting of molybdenite concentrate. This invention is carried out to overcome the disadvantages observed by Wilkomirsky et.al ( US patent 3,941,867 (1976)) using a plug flow fluid bed reactor to avoid problems like sintering, recycle of the particulate solids, minimize dust entrainment in the roster off gas.
But, the process or system disclosed in both the above patents (US patent 3,941,867 (1976) of Wilkomirsky and co-workers and US patent 6190625 (2001) of Jha and co-workers) are not useful for roasting of spent HDS catalyst as feed materials in the fluid bed roasting reactor.
Because, the component of spent catalysts are greatly different than that of molybdenite concentrate.
- Particle size of spent catalyst materials vary from 1mm-6mm length with shapes like trilobe, tetralobe, cylindrical, spherical ball etc.
- Where as in above reported patents the material is a fine size molybdenum concentrates with -325 mesh size and can be further milled for better size reduction below nm range.
- As received spent catalyst materials used without size reduction.
- Spent catalyst material contains only 15-20% molybdenum sulfide as compared with normal molybdenite concentrate, containing 50-100% molybdenum sulfide.
- Further, the waste catalyst is basically a mass of an Alumina/silica based spent catalyst of petroleum industries, which also contains coke and hydrocarbons, Sulfur that varies from 5-8% and, carbon/Hydrocarbon varies from 8-25%.
Thus, the components of the waste catalyst are different apparently, which results in the different treatment of the waste catalyst for roasting in fluidized bed roaster. The process of the oxidative roasting disclosed in prior art is just suitable for treating the molybdenite concentrate (very fine powder having high molybdenite) but no other impurities like coke, hydrocarbon, sulphur etc.
Hence, there is need in the art to develop an energy efficient roasting system and method thereof for the preparation of roasted spent catalyst for recovering precious metals like molybdenum, vanadium and tungsten to satisfies cost advantage and environmental friendly approach. The present invention is the first to report a system and method on the oxidation roasting of spent hydrodesulfurisation catalysts in a fluid bed reactor with or without recycling, wherein the present invention overcomes the existing limitation of rotary kiln processing of spent catalyst involving high energy consumption, longer residence time for removal of sulfur and volatile, formation of alloy phases and low recovery of molybdenum.
Accordingly, the inventors of the present invention developed a system for roasting a molybdenum spent catalyst or similar spent catalysts bearing vanadium or tungsten, which is novel, energy efficient and cost effective compared to conventional oxidative roasting of spent catalysts done either in rotary kiln. Finally, the roasted catalyst obtained from present invention system is further process for recovery of metals (molybdenum/ vanadium/ tungsten), which can utilized to produce high purity salts for specialty applications like pigments, fertilizer industry, water treatments chemicals and catalysts preparation.

OBJECTIVES OF THE INVENTION
• The primary objectives of the present invention is to develop an energy efficient, cost effective and environmental friendly roasting technology for the removal of volatile organics, coke and sulfur from spent HDS catalysts/ or any similar raw materials so as to make the calcined catalyst suitable for further processing with a maximum doped metals recovery.
• Another objective of the roasting process is that the spent catalyst particles are maintaining at proper fluidization by passing required quantity of air.
• Another objective is to roast the material for the removal of HC, C and S utilizing the inherent fuel of the spent catalyst in the presence of air and exothermic reaction heat generated during sulfur oxidation reaction by a novel fluid bed roaster (FBR) technology on continuous scale maintaining a proper temperature for almost complete removal of HC, C and S.
• Another objective of the present invention is that the temperature of roaster is controlled by feeding the raw material at controlled rate through an automatic controlled mechanism.
• Another objective of the present invention is that the temperature of roaster is controlled below the sublimation temperature of molybdenum oxide which is above 700 oC.
• Another objective is that the reaction kinetics of spent catalyst roasting in the presence of excess air in FBR is fast as the input catalyst particles are in proper fluidized conditions within the roaster and maintaining roaster temperature at the desired level.
• Another objective is that roasted catalyst particles are instantaneously separated from the fresh catalyst input particles due the difference in Sp.gr.
• Another objective of the present invention is that the temperature of roaster is controlled so that the formation of single phase of molybdenum oxide, vanadium oxide and tungsten oxide is maximized and at the same time minimizing the compounds formation of said metals (molybdenum/vanadium/tungsten) with other constituents of spent catalyst, which results in high recovery of molybdenum/ vanadium/tungsten during down steam processing of calcine catalyst.
• Another objective is to clean the flue gases leaving the roaster for particulate matter by passing through a dual stage cyclone chamber, which collects all the particulate matters.
• Another objective is to recover waste heat of flue gas by the configuration of waste heat boiler system.
• Another objective is the utilization of two stage alkali scrubber for flue gases (containing SOx’s, CO2 gases) cleaning in order to meet the environmental norms of gas emissions thereby making the roasting process meeting environmental compliances and reducing the process cost.
• Another objective is to pass the clean flue gases leaving the cyclone chambers passed through a 2 stage alkali scrubbing unit operated counter current mode to scrub SOx gases and finally to a storage tank.
• Another objective is to recover the alkali from the alkali SOx solutions from the scrubber tank by transferring to another reactor and reacted with lime slurry there by generating alkali for reuse as scrubbing medium of SOx flue gases in scrubber 1 and 2.
• Another objective is reducing alkali consumption scrubbing step thereby reducing roasting cost and at the same time generating gypsum as value added product, which can find application in cement industry or any other suitable applications.
• Another objective of the present invention that alkali quantity reduced by about 40-60% per kg of S in scrubbing step.
• Another objective of the present invention of roasting process is novel and new in the processing of spent catalyst as it requires nominal energy, produces consistent quality calcined catalyst with respect to S, HC and coke removal and at the same time regenerates alkali during SOx gases scrubbing stage there by making roasting step more cost effective as it is energy efficient and less cost compared to conventional rotary roasters.
• Another objective is discharging the hot calcined catalyst material through an automatic discharge valve of FBR maintaining the material at a fixed bed volume in the roster thereby maintaining almost same fluidization velocity, smooth operation efficiency of roaster and achieving consistence in the quality and quantity output of calcined catalyst material for further processing.
• Another objective is maintaining the roaster conditions in such a way that the formation of compounds of Mo with other components of the catalyst minimized in the calcined catalyst thereby promoting maximization of leaching efficiency of molybdenum with alkali solutions.
• Another objective is discharging the hot calcined material with low Sulfur values (0.3-0.6%) and HC through a rotary cooler.

BRIEF DESCRIPTION OF FIGURES
For more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which:
Figure No. 1:Illustrates a system for processing a spent catalysts for removal of sulphur, volatiles, hydrocarbons and prepare a roasted catalyst mass for recovery of molybdenum/vanadium as per present invention.
Figure No. 2: Illustrates a flow diagram of method for processing a spent catalysts for removal of sulphur, volatiles, hydrocarbons and prepare a roasted catalyst mass for recovery of molybdenum/ vanadium as per present invention.

DETAILED DESCRIPTION OF THE INVENTION AND FIGURE
The term "spent catalyst" used herein is synonymous with "used catalyst” and “waste catalyst".
The term "metals" as used herein generally refers to a single or mixture of any transition metals, as is conventionally known in the art and not limiting only to molybdenum, vanadium and tungsten.
The present inventions relates to the development of energy efficient and cost effective process for the removal of hydrocarbon, Coke and Sulfur from molybdenum/ vanadium based spent catalysts used in petroleum industry/ or similar raw materials by oxidative roasting of spent catalysts/ or any similar raw materials in a fluidized bed roaster equipment, there by generating calcined catalysts suitable for further processing to recover pure molybdenum and/or vanadium without loss of traces of active metals or sulfur.
The catalyst used for any hydrodesulfurisation(HDS), hydrodenitrification(HDN) and hydrometalisation (HDM) reactions are basically consist of major alumina support and doped with active metals like molybdenum (Mo), Nickel (Ni) and/or Cobalt (Co) and sometimes tungsten (W) by impregnation/precipitation over the support for the removal of S, Ni and V from crude oil refining.
In view of the above-described objectives, it is one objective of the invention to provide a system for preparing roasted spent catalyst obtained as a waste from petroleum industry to recovered adsorbed traces of metals. The system features an energy efficient, cost effective as it should maintain fast reaction kinetics, avoiding unnecessary molybdenum compounds formation there by making the calcined catalysts suitable for downstream processing with high molybdenum recovery with high purity.
To achieve the above objective, in accordance with one embodiment of the invention, there is provided a system (100) (Fig.1) for preparing a roasted molybdenum based material from a spent catalyst of hydrodesulfurisation, the system comprises the following components:
a) a receiving area comprising sieve to separate oversize inert material from said raw spent catalyst mass,
b) a material conveying area (1 feeding system, 2 screw conveyor) to pass only sieved spent catalyst,
c) a fluidized bed roaster (4) configured to calcine spent catalyst under controlled oxidizing conditions in fluidized state comprising a separate inlets for sieved spent catalyst, inlet (8) for hot air and gas, distribution plate (12), outlet (7) to collect roasted spent catalyst and flue gas elimination duct (9),
d) a cyclone chamber (10,10’) configured for separating the fine particles from flue gases,
e) a primary alkali liquid scrubber configured for dissolving the SOx and releasing the effluent stream,
f) a precipitation reactor comprising of lime slurry for reacting effluent stream having solubilized SOx to initiate gypsum synthesis.
g) a waste heat recovery boiler system configured for waste heat recovery.
h) a two stage leaching reactors comprising an alkali solution for leaching out molybdenum from calcined catalyst mass.
The said system further comprises of viewing glass (3), Temperature sensor (5) roasted catalyst continuous discharge valve (6), material drain valve (7) and final outlet (11) for flue gas.

In accordance with second embodiment of the invention, there is provided a method for preparing a roasted molybdenum based material from a spent catalyst of hydrodesulfurisation, the system comprises the following steps:
1) Sieving of spent catalyst mass to receive 1-3 mm material,
2) Charging the pre-roasted material (Collected from conventional roasting)/inert material in a fluidized bed roaster in the proportion of 20% the total bed volume. Bed temperature is raised to 180-220 °C by heating through hot air keeping material under fludization. In accordance to important embodiment of present invention, the charging of pre-roasted/inert material to back mixed fluid bed chamber which will be maintained at a temperature of 180-220 °C by using burner. At this stage the material is ready for self ignition. The sizing/hold up in this part is designed such that the sensible heat required by the feed material will be equal to the heat release rate. Then charging of fresh feed material at a controlled rate (600 to 800 Kg/hr) by temperature feedback from the bed temperature,

3) After attaining the temperature between 180-220 °C, charged the fresh feed at 600-800 kg/hr rate to maintain the temperature preferably between 330-550 °C with a roasting time of 15-18 min.
4) Collecting a roasted spent catalyst through discharge valve and cooling to room temperature,
5) Passing the flue gas though cyclone separator to remove fine calcined particles from flue gas going to scrubber-1 and 2.
6) Combining the collected particles from cyclone with step 5) material,
7) Passing the flue gas in two stage scrubber (Scrubber 1 and 2) consisting alkali configured for dissolving the SOx and releasing the effluent stream. The alkali solution is NaOH and has a concentration of between 40-100 g/L (or 1-2.5mol/L) and by passing the flue gas scrubber 1 and 2, the temperature rises to 40-80 °C,
8) Passing the effluent stream of step 7) into precipitation reactor consisting lime solution to precipitate out gypsum and regenerating an alkali. The lime solution has a concentration of 50% wt/vol and lime is used as slurry (50% wt/vol) , slurry is at room temp. Scrubbing reaction temperature is 70-80 °C,
9) Recirculating the regenerated alkali in step 8) in scrubber 1 and 2 alkali liquid scrubber,
10) Grinding the material collected at step 6) to 100 mesh size and passing in two stage leaching process for leaching of molybdenum with alkali solution. The leaching reactors temperature is maintained at 80-90 °C, the alkali solution is added to maintain the pH 9.5-10.5.
Advantages of the present disclosure are summarized as follows.
- No need to add inert refractory material in a continuous manner with the charge mix input in the roaster as reported in prior art fluidized bed roasting technique,
- No external heat energy is required as the raw material (Spent catalyst) contains sufficient fuel after initialization step at roasting up to 250-300°C using blower hot air,
- Once the spent catalyst material reaches roasting initiation temperature, temperature of roaster maintained more preferably around 550-650°C by controlled feed input to the roaster bed. It is a self sustaining reaction.
- Spent catalyst roasting time is less with retention time of 15-18 Mins.
- No agglomeration in catalyst mass in present invention as less retention time,
- Minimum dust entrainment loss in the roaster off gases as big size catalyst charge compared to fine size molybdenum concentrate,
- Less load on cyclone scrubber,
- Sulphur values in the roasted spent catalyst reached below 1% in single step roasting without any recycle of calcine or without multistage roasting operation,
- As the roaster off gas is lean with respect to SOx, dual alkali scrubber introduced to convert SOx gases to gypsum product and recovery of alkali,

The process conditions are strictly controlled in the present disclosure, so that the resulting product has high recovery rate and high purity, and can be directly used for the industrial applications. The method of the present disclosure has simple operation, simple flow path, and low requirements on devices and process condition, and thus is suitable for large-scale applications.
In the present invention, spent catalyst materials are calcined in a fluid bed roaster utilising the fuel available within the catalyst as heat source for removal of deposited S, Coke, and HC from catalyst surface. The flue gases from the roaster are passed through cyclone chamber to capture the fine size catalyst particles. Further, allowed to pass through a two stage alkali scrubber under circulation through a scrubber tank (Fig.2; scrubber 1 and 2) maintained at basic pH to remove SOx gases completely and meeting pollution norms of the stack gases venting out through a chimney. The solution from scrubber 1 and 2 tank is further reacted with lime slurry in another precipitation reactor to precipitate out soluble SOx’s as gypsum and regenerating alkali for reuse. Finally, the calcined catalyst fine materials are leached with alkali solution at suitable operating conditions to maximise molybdenum recovery (Fig.2).
The present invention of spent catalyst raw materials roasting in the FBR equipment is novel as it is energy efficient by utilising the available fuel of the spent catalysts, cost effective by introducing dual alkali scrubber philosophy for alkali regeneration and generate consistent quality calcined catalyst as feed for further processing compared to conventional rotary kiln roasters or multihearth furnaces. Heat recovery from the furnace outlet gases is possible as the gases come out at high temperatures by connecting to proper heat recovery systems.
For further illustrating the invention, experiments detailing a method for preparing a roasted catalyst from a spent catalyst of used in petroleum crude oil refining industries are described below. It should be noted that the following examples are intended to describe and not to limit the invention.
Examples
Example: 1: Method as per present invention:
Roasting of spent catalyst materials in fluid bed roaster for the removal of sulfur, coke, hydrocarbons and recovery of molybdenum from roasted spent catalyst materials with alkali involves series of steps as presented in fig. 2 and description of processing steps are as follows:
Spent catalysts generated in petroleum crude refining industry contain oversize inert materials such as silica, refractory materials( size from 6-10 mm) as balls and spent catalysts in the form of trilobe, cylindrical ( 0.5mm x 3-5mm), balls( 1-3 mm) shape. The first step of processing is to separate oversize inert materials in vibratory screener. The undersized material is mostly spent catalyst contains Mo. The composition of spent catalyst materials from different sources is given in table 1.
Table 1. Chemical analysis of spent catalyst raw materials from different sources

The operating procedure of roaster is that once roasted spent catalyst is charged first through charging valve (Fig.1) to fill the roaster to discharge valve level. After that, air is passed through root blower line to keep the material in fluidization condition and then raised temperature of material to about 180- 220 oC by putting on the burner so as to pass hot air to reach roaster initiation temperature. Once initiation temperature is reaching, fresh material charging started through a PLC operated automatic charging system and maintained roasting temperature conditions between 370 and 630 oC as per table 2, not exceeding 650 oC to avoid Mo loss due to vaporization of MoO3. The weight loss of spent catalyst varies about 25-30 % depending on S, coke and HC. The discharge valve operates as per set times to discharge the hot roasted spent catalyst continuously through the rotary cooler. The S content of roasted spent catalyst is reduced to 0.3- 0.6% (Table 3). The output of roasted spent catalyst from roaster depends on the fuel content of material.
Roasting reactions in Fluidized bed
MoS2 + 4 O2 --- > MoO3 + 2 SO2 + 1/2O2
Table 2. Spent catalyst raw materials roasting conditions

Table 3. Chemical analysis of roasted spent catalyst samples

Example: 2: Metal Leaching procedure from roasted spent catalyst prepared as per present invention:
Roasted spent catalyst from roaster (example 1) is grinded to -100 mesh size for Mo recovery testing as per conditions given in Table 4. 40 gm of finely ground RSC and 200 ml of DI water added to 500 ml beaker. The contents are kept stirring on hot plate cum magnetic stirrer and adjusted to 5.0-5.5 with H2SO4/NaOH, then H2O2 added in required quantity to oxidize S content of roasted spent catalyst and increased temperature to 50-60 oC and allowed for 30 min for reaction completion. Further, the pH of slurry raised to 9.5 -10.5 with alkali and carried 2 stage leaching (1st stage maintained at 9.5 pH and 2nd stage maintained at 10.5 pH) at 80-90 oC for 4 hrs. Then the slurry temperature reduced to 60-70 oC, decreased pH to about 7.5 using 98% H2SO4 and held for 30 min and filtered to separate solids from liquid (leach liquor, Table 4).

Example: 3: Metal Leaching procedure from roasted spent catalyst prepared as per conventional method:
The same feed material was subjected to oxidative roasting in muffle furnace at 650 °C for 3 hrs. Grinded to -100 mesh and studied two stage leaching process to check the recovery of precious metals. Data presented in Table No.4.

Table 4. Atmospheric alkali leaching conditions of roasted spent catalyst samples generated from FBR and conventional roasting
(Leaching Condition: PD 20%)

Example: 4: Gypsum Preparation procedure: The scrubbed solution from scrubber1 and 2 contains mixture of Na2SO3 and Na2SO4. This solution is subjected to the treatment with lime slurry (50% wt/vol) under sparging of air in precipitation reactor to form gypsum. The pH of the precipitation reactor raises to >12 after adding lime slurry. The mechanism of gypsum formation is shown as reaction in precipitation. The consumption of NaOH in scrubbing and generation of gypsum is presented in Table 5.
Reaction in Scrubber -1 and 2:

SO2(g) + 2NaOH(l) --- >Na2SO3(l) + H2O(l)

SO3(g) + 2NaOH(l) --- > Na2SO4(l) + H2O(l)

Reaction in precipitation:

Na2SO3+ Ca (OH)2 + ½ O2--- > CaSO4 + 2NaOH

Na2SO4 + Ca (OH)2 --- > CaSO4 + 2NaOH

Table 5. Consumption of NaOH in Scrubbing and Gypsum generation and norms

Accordingly, the present invention has application in recovering the pure metals from spent catalyst. Most of spent catalysts generated worldwide by petroleum crude oil refining industries are considered as good secondary source of raw materials for the recovery of molybdenum as well as vanadium and tungsten in some cases, provided the process satisfies cost advantage and environmental friendly. The first step spent catalyst industry operated is either oxidative roasting /or soda roasting step, utilizing rotary kiln which is energy intensive and applied in most of the spent catalyst processing industries as per literature information.

Hence, there a is need in the art for inventions related to roasting of spent catalysts that can meet :
(i) utilization of fuel values of spent catalysts,
(ii) selection of suitable equipment to handle heat generated and exothermic reactions of spent catalysts,
(iii) produce faster reaction kinetics,
(iv) mechanism of temperature control of roaster to avoid molybdenum losses and sintering of phases,
(v) reduce the requirement of alkali for scrubbing of SOx flue gases,
(vi) conversion of SOx gases of alkali scrubber step through regeneration of alkali and
(vii) conversion of alkali scrubbed SOx’s to a product as value addition,
(viii) consistent quality roasted catalyst samples for further processing to recover molybdenum.

The present invention discloses methodologies to fulfill these all above important objectives.
Fast the kinetics of roasting and minimizing molybdenum/vanadium/tungsten alloy formation, with comparable recovery of pure metal. Because of the faster reaction kinetics, throughput is higher compared to conventional methods of roasting.
The invention has high merits for implementation as it requires low capital investment or operating cost, space, apart from faster reaction kinetics and high recoveries. Heat recovery from exhaust flue gases by incorporating proper heat recovery system in place.
The roasting of spent catalyst or similar raw materials operates itself without any external fuel requirements in a closed temperature range thereby getting consistent & high molybdenum recovery. Flue gases coming out of furnace are scrubbed with alkali solution to convert SOx gases to solubilize as Na2SOx solution. Further, the process also recovers alkali used for SOX flue gases scrubbing by contacting Na2SOx scrub solution with lime slurry and generates by-product gypsum.
Spent HDS catalysts generally contain hydrocarbons, coke, sulfur deposited on the catalyst surface and considered as source of fuel. In the present invention, spent catalysts are subjected to oxidation roasting by a novel and cost effective fluid bed roaster technology maintained at a proper temperature utilizing the fuel present in spent catalyst and exothermic reaction of sulfur without any external supply of energy except heat supplied for initiation of roasting process. The roasting process reaction kinetics for coke and S removal is fast as the input catalyst particles are kept under proper fluidization conditions in the presence of excess air and maintained at a suitable temperature. In addition to this, the present invention also provides a method to recover SOx, wherein flue gases from the roaster out let containing SOx and CO2 gases are passed through a cyclone chamber for removal and recovery of particulate matters and further to a waste heat recovery boiler and then to an alkali scrubber tank. Dual alkali steps of SOX gases scrubbing and alkali regeneration step for further use is incorporated.
Advantages of Present invention over above cited prior arts:
• Roasting time is less as kinetics is very fast compare to soda roasting
• Through put is high due to lesser roasting time and mixing of soda/salt.Soaking time not required.
• Scrubbing cost is less compare to conventional. Due to reuse of regenerated alkali for alkali scrubbing.