Hydrometallurgy, 22 (1989) 3-24 3 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Process ing of Petroleum Coke for Recovery of Vanadium and Nicke l
P.B. QUENEAU, R.F. HOGSETT, L.W. BECKSTEAD and D.E. BARCHERS
Hazen Research, Inc., 4601 Indiana Street, Golden, CO 80403 (U.S.A.)
(Received February 26, 1988; revised and accepted August 18, 1988)
ABSTRACT
Queneau, P.B., Hogsett, R.F., Beckstead, L.W. and Barchers, D.E., 1989. Processing of petroleum coke for recovery of vanadium and nickel. HydrometaUurgy, 22: 3-24.
Certain South and North American oils and tars contain substantial quantities of both vana- dium and nickel that are concentrated during refining in the by-product petroleum coke. A pres- sure oxidation process has been developed in the laboratory for solubilization of vanadium from the coke as sodium vanadate. The process operates at ~ 300 ° C and a pH of ~ 9.5 using industrial wet-oxidation procedures in proven commercial reactors. Sufficient steam is generated via the exothermic digestion reactions to provide process power needs. Nickel concentrates in the ash as basic nickel carbonate. Conventional solvent extraction-crystallization technology is recom- mended for conversion of the sodium vanadate to high-purity fused V20~. The process has the potential to operate in conjunction with oilfield injection of C02 and steam for liberation of heavy- metal-rich tars. A portion of the operating and capital costs for recovery of the metal values would then be shared with the oil recovery operation.
INTRODUCTION
During the coming decades, increasingly large quantities of oil will probably be extracted from the metal-rich crudes, tars, and sands of North and South America. The principal metallic impurities are vanadium and nickel [ 1 ]. These metals become concentrated primarily in the petroleum coke by-product of the oil-refining process. Where a utility fires its boilers with residual fuel oil which has high vanadium content, the metal values are concentrated in a marketable by-product ash [2]. In the mid-1960s, Canadian Petrofina Ltd. (Point-aux- Trembles, Que. ) built a plant for recovering V205 flake from ash via acid leach- ing [3 ]. This plant has since been shut down. During the past decade, a number of alternative technologies for vanadium and nickel recovery from fly ash have been published [4-10].
One method for upgrading heavy-metal-contaminated petroleum coke is 'Flexicoking', as developed by the Exxon Research and Engineering Company
(Florham Park, N.J.). Flexicoking, an extension of Exxon's fluid coking pro- cess, concentrates metal values by a factor of ~ 20 [11 ]. The process adds a coke gasifier to the conventional fluid coking apparatus, yielding a light hy- drocarbon gas and residual low-sulfur coke. Exxon calls this metal-rich by- product 'Flexicoke'. We describe here the laboratory development of a wet- oxidation process for recovery of V20~ flake and basic nickel carbonate from Flexicoke. The work was carried out at AMAX's Research & Development Center in Golden, Colo. [12 ].
The Flexicoking process generates three Flexicoke fractions: purge coke, cy- clone underflow coke, and venturi underflow coke. Purge coke is formed when residuum hydrocarbons are sparged into a gasifier containing fluidized coke. During the resulting 'cracking' process, lighter hydrocarbons exit overhead, and coke deposits on the surfaces of the 'seed' coke in the fluid bed. Thus, a series of concentric shells, each ~ 5/~m thick, are deposited around the initial seed [12]. Vanadium, as well as other metallic and non-metallic impurities, are occluded during this carbon deposition process. Removal of excess coke from the fluid bed as 'purge coke' maintains a constant solids inventory in the reactor.
Purge from the fluid bed constitutes half of the Flexicoke tonnage. The other half of the Flexicoke is collected overhead as - 15/~m dust. Of this dust, half is recovered dry as cyclone underflow containing an intermediate vanadium concentration (i.e., about twice the level of the gasifier bed). The cyclone ov- erflow proceeds to a venturi scrubber, which generates an ~ 3% solids slurry of venturi underflow which has twice the V205 content of the cyclone under- flow. Most of the vanadium in the venturi fines, as well as the other two coke fractions, is present as discrete particles of V (III) oxide. One Flexicoking unit generates 100-120 short ton day-1 of Flexicoke. Typical analyses of the prod-
TABLE 1
Characterization of Flexicoke generated from a typical Venezuelan residuum [ 13 ]
uct generated when processing Boscan crude are given in Table 1. Also shown are weight and particle size distributions.
LABORATORY PROCEDURES AND RESULTS
Preliminary work was performed using Flexicoke collected as cyclone un- derflow at the Exxon pilot plant in Baytown, Tex., during 1975 [11]. During this period, the feedstock to Exxon's pilot unit was Boscan crude from Vene- zuela. The cyclone underflow, so-called 'tertiary fines', assayed 15% V205, 0.85% Ni, 78% C, 0.36% Fe, and 2.0% S. The material had a chemical oxygen demand of ~ 1.7 g O2/g coke. Wet oxidation experiments were carried out in a Parr 2-1 stainless-steel autoclave agitated at 1000 r.p.m. (2~-in axial turbine impeller). Four exploratory tests used 10% sulfuric acid, water, or caustic soda solutions to digest 200 g l - 1 of coke at 250 ° C. An oxygen tank provided 50 psig 02 over- pressure. Carbon dioxide was bled from the autoclave every 15 min. Results are summarized in Table 2.
Sulfuric acid proved to be an effective solvent for vanadium, provided the vanadium was kept in a reduced form via exclusion of oxygen from the auto- clave (see the first two tests in Table 2). Water alone solubilized only 3% of the vanadium. Digestion in caustic soda gave 90% V205 extraction. Subsequent leaching studies were restricted to alkaline media, specifically soda ash, for several reasons: ( 1 ) The CO2 product of oxidation immediately reacts with the NaOH to form
Na2C03. Soda ash, which is generally less expensive than caustic soda, was therefore used directly.
(2) Soda ash is generally less corrosive to alloy-steel pressure vessels than is either caustic soda or sulfuric acid. Also, alkaline medium provides a pre- liminary vanadium (soluble)/nickel (insoluble) separation.
(3) Alkaline medium allows the coke to be oxidized while solubilizing vana- dium. The heat from this combustion generates process steam, and also produces a potentially valuable C02 by-product.
(4) The petroleum coke contains environmentally hazardous components,
TABLE 2
Effect of reagent type on V205 solubilization from 200 g l- 1 of tertiary fines
Reagent type Temp. Time Oxygen V205 extraction (°C) (h) (psig) (%)
par t icu la r ly cyanide and phenol . Wet ox ida t ion des t roys these chemicals , faci l i ta t ing subsequent t r e a t m e n t of the nickel- r ich ash.
Effect of temperature
Pre l imina ry tes ts were conduc ted in which 10 g 1-1 of t e r t i a ry fines were oxidized for 2 h at 300 psig 02 in 75 g l - 1 Na2CO3; 99 + % VeO~ ex t rac t ion was ob ta ined at 200-270 °C (see Table 3). Coke weight loss as a resul t of wet oxi- da t ion was 40-95%, depending on reac t ion t empera tu re . In the case of 97% weight loss, the residual 3% weight represen ts the nickel, i ron and ash co n t en t of the coke. Burn ing of the coke was incomple te at 200 and 230 ° C, yet V205 ex t rac t ion was complete . Addi t ional da ta on the effect of t e m p e r a t u r e on the rate of wet oxidat ion of cyclone coke are given in a sect ion below, en t i t l ed 'Ex i t gas composi t ion and s to ich iometry ' .
Effect of Na2C03 concentration
T h e effect of Na2CO3 concen t r a t i on on dissolut ion of vanad ium at 250 and 270 °C f rom 100 g l-1 of t e r t i a ry fines is given in Table 4. Vanad ium solubili- zat ion was 95-97% for the condi t ions listed. Increas ing the average Na2CO3
TABLE 3
Effect of temperature on V20~ solubilization from 10 g 1-1 of tertiary fines (300 psig 02; 2 h)
~'Evaporation of liquid during the 2-h experiments increased the effective concentration of NaeCO~.
concentration from 60 to 160 g 1-1 increased V205 extraction by 2%. The weight loss data in Table 4 indicate that higher carbonate concentration and higher temperature significantly increase the rate of wet oxidation of the coke.
Effect of high metals concentration
Table 5 shows the negative effect of high levels of coke addition on V20~ extraction; 99 + % V20~ extraction was at tained when burning 10 g l - 1 coke, whereas 93% V205 resulted when treating 200 g 1-1 coke. This decrease in solubilization is related not only to incomplete oxidation of the coke at the relatively low (250°C) oxidation temperature, but also to the formation of a sparingly soluble vanadium compound, probably Fe (VQ)3 . The higher the percentage of solids, the greater is the concentration of both iron and vana- dium in the leachant; the result is precipitation of Fe (V03)3.
The 150- and 200-g l - 1 coke experiments shown in Table 5 gave higher than expected final Na2CO3 and V205 concentrations because of removal of water in the saturated gas phase leaving the reactor. This problem was later cor- rected, either by not venting, or by saturating the feed gas with water. This evaporation is used in the continuously operated commercial reactor to remove heat from the oxidizing coke slurry. The steam generated drives a condensing turbine.
Exit gas composition and stoichiometry
A Hewle t t -Packard Model 5840A gas chromatograph was used to measure the composition of gases leaving the autoclave during several wet-oxidation experiments. The feed coke was tertiary fines. Results of a 250 ° C test are shown in Fig. 1A. The only product gases detected were CO2, CO, and residual 02. The volume ratio of C O J C O evolution averaged ~ 12/1. Oxidation of the car- bon was essentially complete after 2 h; carbon weight loss was 97% after 3 h.
TABLE 5
Effect of percentage of solids on V205 solubilization for tertiary fines (250 ° C; 300 psig, 2 h)
aOxygen addition during this experiment was insufficient.
VOLUME -%
I 0 0 = , ' ' I
|
0 4 0 eo 12o 16o 180
TIME - minutes
O0 t
60
40
2oE l
0 ~ 0
5' 270°I
40 so izo 16o teo TIME -minutes
Fig. 1. C02, 02, and CO (as vol.% ) in the exit gas from wet oxidation of 100 g/1 of tertiary fines at 250°C (A) and 270°C (B), using 220 psig initial oxygen partial pressure and 75 g 1-' initial Na2CO.~ concentration.
Initial oxygen partial pressure was ~ 220 psig, and total pressure was main- tained at ~ 800 psig. Rate of oxygen addition was ~ 1.7 1 (STP) min-1. The initial surge of oxygen into the reactor at t ime zero accounted for the initially high oxygen content of the exit gas. Sufficient oxygen had been added after ~ 60 min to supply stoichiometric requirements for oxidation of C, S, and V205 in the coke. The C02-CO analyses indicate that oxidation of the coke was ~ 90% complete after 100 rain. Vanadium solubilization was 97% after 180 min. It is noteworthy that, even after oxidation of the coke is complete, CO2 continues to leave the autoclave as a result of the buildup of gas-phase inventory.
Figure 1B is similar to Fig. 1A, except that the results are shown for a test conducted at 270°C rather than 250°C. Again, the only gases observed were C Q , CO, and residual 02. Sufficient oxygen was added after ~ 60 min to pro- vide the stoichiometric requirements of this gas. Initial oxygen pressure was 220 psig, and total pressure was maintained at ~ 1000 psig. At 270 ° C, the ox- idation of the carbon was 90% complete after 70 min, rather than after 100 min as at 250 ° C. Solubilization of V205 remained the same (i.e., 97% ), whereas coke weight loss was 97%. The average Na2CO~ concentration during both of these tests was ~ 110 g 1-L The increase in concentration from the initial 75 g l - 1 Na2CO3 was a result of removal of water by the saturated exit gas.
Use of 6% excess air
Additional experiments were conducted at Zimpro, Inc., in Rothschild, Wisc. Work was performed using 750-ml "autoclave bombs" for which agitation was provided by shaking the entire autoclave. Operating with 7-22 g tertiary fines per liter, over 99% V20~ extraction was at tained in 1 h using 75 g 1-1 Na2CQ. Oxygen was initially added as compressed air to give a 6% stoichiometric ex-
cess over tha t needed to oxidize completely the contained carbon, sulfur, and vanadium. Vanadium extraction increased from 99.1 to 99.8% when the tem- perature was raised from 270 to 310°C. It should be noted tha t these high extractions were at tained without venting the CO2 reaction product. Flexicoke weight loss was complete. Details of additional work performed by Zimpro are given in the S. Sit patent [ 13 ].
Treatment of overall coke blend
In addition to the cyclone tert iary fines, and Exxon Flexicoker produces two other coke products: purge coke and venturi scrubber fines. The tert iary fines represent only 25% of the weight of the total coke produced. The purge coke represents 50% of the weight, and the venturi scrubber fines constitute the remaining 25%. Samples of these fractions as generated by Exxon in its 750 bbl /day prototype Flexicoker at Baytown, Tex., during 1975 were given to AMAX Extractive R&D. Boscan residuum (having a very high metals con- tent) was used as the feed to the Flexicoker. The purge coke and tertiary cy- clone fines of the Baytown samples were received dry. The venturi scrubber fines arrived as an aqueous slurry that had no noticeable odor of H2S; the slurry was dried at 110 ° C. Data in Table 6 characterize each of the coke fractions.
Several autoclave digestion experiments were conducted using the PTV blend, which simulates the net output of an Exxon Flexicoker processing V205-
TABLE6
Characterization of coke fractions produced at the Exxon Prototype Flexicoker at Baytown, Tex.
Analysis Purge coke Tertiary fines Venturi fines PTV blend a (%)
C 87.7 75.2 60.1 78.7 V~O~ 6.2 16.4 28.1 14.2 S 2.3 2.1 2.1 2.2 Ash 6.4 20.0 33.9 16.7 Ni 0.41 1.0 1.6 0.86 Fe 0.11 0.20 1.3 0.43 P20~ NA NA NA NA SiO2 0.09 0.13 0.18 0.12 Al2Oa 0.02 0.02 0.03 0.023 C1 < 0.01 < 0.01 < 0.01 < 0.01 Mo 0.017 0.034 0.066 0.034 Co Nil Nil Nil Nil
aPTV blend=2 parts purge coke + 1 part tertiary fines + 1 part venturi fines, by weight.
10
rich Boscan crude. The results of the tests are shown in Table 7. The feed gas was tank oxygen that was saturated with water at 275 °C before its use for oxidation of the coke slurry; thus, water removed from the wet oxidation au- toclave was replaced by water in the incoming air. The first test shows that 98% V20~ solubilization can be obtained from the - 100-mesh PTV coke blend after 1 h at 275°C using 85 g 1-1 Na2CQ. It should be noted tha t the V20~ concentrat ion of the pregnant liquor was low (i.e., 4 g l-1 V20~).
A commercial continuous autoclave operating at steady state would contain a high V205 concentrat ion (50-100 g l - 1 V20~) and a low percentage of solids. Most of the coke would oxidize during the first few minutes in the reactor. The last four experiments in Table 7 simulate this steady-state condition of high V205 concentration. Extract ion decreased to ~90%, regardless of retent ion time (0.5-2 h), grind ( - 100 to - 270 mesh), and Na2C03 concentrat ion (85- 200 g 1). Sodium sulfate was added to simulate buildup of this component, which results from oxidation of the 2% sulfur content of the coke. The color of the leach residues from Runs 1-3 was yellow-brown, which indicates almost complete oxidation of the carbon. The residues from Runs 4 and 5 were brown- black, indicating residual carbon. In Exper iment 4, the reaction time was in- sufficient for complete carbon oxidation. In Exper iment 5, the high initial coke concentrat ion probably resulted in incomplete carbon oxidation. Emission spectrographic analysis of the ash showed 0.1-0.5% each of Ba, Ca, Cr, Mn, and Na, and traces of B, Co, Cu, Mo, Pb, and Sr.
More than 98% V205 extract ion was at tained when using low V20~-Na2SO4 concentrat ion. The 89-91% V20~ extract ion (Table 7) in the presence of high
~275°C; 300 psig 02; feed coke was the PTV blend described in Table 6.
11
VzQ-NaeSOt concentration, regardless of grind time or Na2C03 addition, in- dicates the formation of an insoluble vanadium compound. The probable com- pound is Fe(VO3)3, which forms a compound that is usually amorphous to X- ray diffraction [ 11, 14 ].
To resolve this problem, Zimpro, Inc., used its 750-ml "shaking autoclave" to oxidize 10 g of PTV blend in 200 ml of liquor containing 96 g l - 1 VeQ, 90 g 1-' NazSO4, and 75 g 1-1 NazCO3. Test conditions were 300°C for 1 h using 6% stoichiometric excess of air. The product ash weighed 0.55 g, and assayed 36% V205, 15% Ni, 7.8% Fe, 5.0% Ca, and 0.76% Mg. Vanadium extraction, based on the residue assay, was 84%. However, the only compound tentatively iden- tified by. X-ray diffraction of the ash was Ca3NaNizV3012 and/or Ca3NaMg2V30~2. The Ca and Mg content of the ash was too low to compound with more than one-quarter of the vanadium content of the residue as Ca3NaNi2V3012 and/or Ca.~NaMg2V30~2. The stoichiometry for Fe (V03)3 was much closer. The V2OJFe weight ratio in the ash was 4.6. For Fe(VQ.~)3, a V2Os/Fe weight ratio of 4.9 would be expected.
AUTOCLAVE HEAT AND WATER BALANCE
An important step in sizing a commercial autoclave for carrying out a wet- oxidation process is calculation of a series of heat and mass balances that char- acterize various operating conditions [ 15 ]. Laboratory data show that wet ox- idation of the PTV Flexicoke blended at ~ 300 ° C in aqueous soda ash solubi- lizes 85-99% of the contained V205 values. The absolute level of vanadium extraction depends on the VeO5 concentration of the pregnant liquor. Nickel and residual vanadium concentrate in the by-product ash, probably as basic nickel carbonate and ferric vanadate. The commercial autoclave to which the mass and heat balance must relate will generate flashed slurry, plus steam- laden vent gas. Because most of the mass and energy leaves with the vent gas, energy loss by direct condensation in the flash tank is not great.
Table 8 summarizes the results of three computer mass balances for three grades of Flexicoke. All three cases treat 4500 kg h - 1 of coke at 304 ° C and 2500 psig total pressure using 5% excess air. The analyses of the three coke feeds are shown in Table 9.
Heat loss to the surroundings was set at 500 kcal h - ' . Although the amount of gas leaving the autoclave decreased with decreasing carbon content of the coke (see bottom of Table 8), the composition of the exit gas remained quite constant: 61 vol.% H20, 30 vol.% N2, and 7.3 vol.% CO2. It is noteworthy that the fuel value of the coke decreases as its V20~ content increases. The more V2Q in the coke, the lower is its carbon content, and the less air is required for combustion. When feeding the 9.3 and 15% VzQ coke, the heat balance allows 100 g l-1 V20~ flash liquor to be attained. The flashed steam is con- densed into the slurry leaving the autoclave. When feeding 5.0% V205 coke,
12
TABLE 8
Process conditions for wet oxidation of 4500 kg h - 1 of Flexicoke containing 5-15% V205
V205 in feed coke (%)
5.0 9.3 15.0
Temperature ( ° C) 304 304 304 Pressure (psig) 2000 2500 2500 Excess of air (%) 5 5 5 Coke (kg h - ' ) 4500 4500 4500 Coke (kcal kg- ' ) 7100 6800 6400 Air (kg h - 1 ) 45,000 43,100 40,700 Na2C03 (kg h 1) 690 780 1050 Feed slurry solids (%) 8.3 8.6 8.6 Feed slurry temp ( ° C ) 30 30 48 Exit V205 (aq) (aq g 1-1) 50 98 100 Exit Na2SO4(aq) (g 1- ') 110 114 72 Exit Na2C03 (aq) (g 1- ') 60 60 60 HP steam (kg h - ' ) 45,000 43,100 40,700 Exit gas (kg h - ~ ) 94,000 90,000 85,000
the product liquor contains only 50 g 1-1 V205" In each case, the exit Na2C03 concentration was limited to 60 g l- 1. The pregnant liquor sodium sulfate con- centration ranged from 72 to 114 g l-1, depending on the V20~ concentration of the feed and exit streams.
The computer-calculated mass balance was limited to the seven reactions given below. Carbon as elemental carbon was 100% oxidized to generate C02 and CO in a 12/1 ratio. Sulfur, assumed to be in the elemental state, was oxi- dized totally to sulfate via reaction (3). Vanadium as V203 was 100% oxidized to aqueous metavanadate (reaction (4)), 10% of which precipitated as ferric vanadate (reaction (5) ). Residual iron converted to ferric hydroxide, as shown in reaction (6). Nickel was 100% converted to nickel carbonate (reaction (7) ):
Figure 2 illustrates the sensitivity of the wet air-oxidation (WAO) process to both the fuel value of the coke and the temperature of the feed slurry. The higher the fuel value (keeping the pressure, temperature, and air input con- stant), the more heat must be dissipated from the system. Because pressure- temperature-air are fixed, the amount of heat leaving overhead is fixed. The only convenient way left to return the system to equilibrium is to dilute the feed slurry. Thus, the higher the coke fuel value (or the hotter the feed slurry), the more dilute one must make the feed slurry. Adding more air to oxidize added fuel value will cause the curves in Fig. 2 to become more horizontal, reducing the sensitivity of the system to this variable.
Figure 3 shows the sensitivity of the WAO operation to changes in temper- ature and pressure. The higher the pressure, the lower is the water content of the exit gas, and the lower the vanadium content of the pregnant liquor. The inverse is true as temperature is increased. The steepness of the curves in Fig.
,2° I I00
8O
v2o5- g t -~ 6O
40
20
I T$1urry = 9 0 o c =
( V~,OS}Feed = 9,3%
' ' 6' 6000 6800 7 O0
FUEL VALUE OF COKE-Kcat kg -1
Fig. 2. Effect of feed temperature of the Flexicoke (9.3% V20~) feed slurry on final aqueous V205 concentration as a function of coke fuel value when oxidizing at 304 ° C and 2500 psig using con- stant air input.
14
L201 (V2Os)F~d ;93%
oo / 8O
V205_g i -1 2ooo pslg ]
zaso 250oj
' Z 40 L
i
I i I o 280 2;5 290 295 ~o
OPERATING TEMPERATURE-°C
Fig. 3. Effect of autoclave operating pressure on final aqueous V20s concentration as a function of autoclave slurry temperature when oxidizing Flexicoke (9.3 % V20.5) using 5% excess air.
3 is the result of the high ratio of water carried by the exit gas to the water in the exit slurry. As the amount of water in the exit gas approaches the amount of water in the feed slurry, the level of V205 in the pregnant liquor increases rapidly.
FLOWSHEET DEVELOPMENT
Block diagrams of the flowsheet proposed for recovery of high-purity V205 from the pregnant liquor using conventional vanadium refining technology are shown in Fig. 4A and 4B. The handling of recycle streams is shown in Fig. 5. A block diagram illustrating the recovery of nickel carbonate from the leach residue (i.e., the 'ash') is given in Fig. 6. The overall process involves the fol- lowing steps:
(1) An ~ 8%-solids slurry of - 200 mesh Flexicoke blend is reacted in aqueous soda ash at 305 ° C and 2500 psig total pressure. Carbon is oxidized to C02, 85- 90% of the contained V20~ is solubilized, and nickel concentrates in the leach residue.
(2) The oxidized slurry is flashed, and a solid-liquid separation made. The gas leaving from the top of the autoclave is processed for energy recovery. The residual ash is treated for recovery of nickel carbonate and recycle of vanadium.
(3) Silica and phosphorus are precipitated from the pregnant liquor, fol- lowed by solvent extraction to convert the vanadium from a sodium species to
Fig. 4A. Block diagram for wet oxidation of Flexicoke, including SiO2-P20,~ removal, and neu- tralization to pH 3 in preparation for solvent extraction.
N oVO-5 + Nmz,SO 4 SX FEED
. . . . ~ _ _ _ ~ H 2 S 0 4 I ,~VENT EXTRACTIO~N ~ 3 t H20 'NH3
N°2S04 - - ~ L I (NH4)2S04 /XL,Z, Raffinate ~ ~V ~ -B{eed
T PPTN ' H2S04 + J Mother L ~\\ XLIZE / LiquOr i ~~/~
(NH4)2SO 4
Fue l~ . DEAMMONIATOR NH3
~ V205
FueI~--FUSION & CASTING ] ~ V 2 0 5 FLAKE
Fig. 4B. Block diagram for production of high-purity V20~ by solvent extraction, crystallization, deammoniation, and fusion casting.
16
L,QuOr
Lime
-T J NE UTRALIZATIONI,~
J Gypsum
I__ Ammon,o
H20 CO 2
~I ACIDULATION 19
,,~ VANADIUM I _ EXTRACTION ]-
VANADIUM J_ STRIPPING F"
I A f~-
CRYSTALLIZ ATIO~ I
BASKET l CENTRIFUGE
DRYER I DE AMMONIATOR J
I F-I FUSION J
FURNACE
CASTING I WHEEL
V205 Floke
H2SO4j~
Ammon,o
No S04 Ammonm Sulfole Effluent Fertdtzer
Sutfuric Acid
I SCAVENGER I J EXTRACTION
I ' A
_ J AMMONIUM J - J SULFATE
EVAP./CRYST.
Fig. 5. Handling of recycle streams during the refining of vanadium by solvent extraction.
an ammonium species. Crystallization of ammonium metavanadate (AMV) follows.
(4) The AMV is calcined to V20~, evolving ammonia gas. The V20~ is finally fused, then poured on a chilled wheel to generate V205 flake.
High-pressure oxidation
The autoclave is a vertical cylinder with hemispherical pressure heads; the interior diameter is 6 ft, and the height is 60 ft. Wall thickness is about 6 in, which includes a nickel-alloy lining for corrosion resistance, plus a backup carbon steel shell to accommodate the 2500-psig operating pressure. Coke feed slurry is continuously pumped into the bottom of the reactor (autoclave). A 5% excess of oxygen is provided as compressed air. The air not only provides
17
NLCOs- FeNO3) 3 ASH
H20 ( NaOH H20 H2S04 F F
I L Ac. ---CO2 I "EAt" [p. 5
H2 i "~F L'~TER F~iCltl) 3 C F,LTER )H20 - Res,due Ni(OH) 2 " ~ F Fe(OH)3
No2CO3 I AQ
(.o3vo4,,o I Ip., (Recycle To
AuloclQvI Feed) ~ Y F H20
NiCO3~- ~ FILTER )
= (No2SO4 Ao
Fig. 6. Block diagram showing the conversion of WAO ash to NiCO:~ product, and Na3VQ for recycle.
the oxygen required to burn the coke and oxidize the vanadium, but also agi- tates and suspends the slurry. The air is supplied by a centrifugal compressor at a rate of 50 ton h-1 at 2500 psig. Liquid level in the autoclave is ~ 50 ft, which allows 10 ft at the top for gas-liquid separation. An internal overflow tube removes slurry from the reactor; the saturated gases leave at the top. The reactor has internal baffles to limit back-mixing. Retention time of the slurry in the reactor is ~2 h. The gas velocity is ~ 18 ft min -1, which allows 3 min retention for the gas phase. At the operating condition of 304 ° C, the pressure due to water vapor alone is ~ 1300 psig; the remaining 1200 psig is gas over- pressure, which controls the velocity of the gas phase and limits the amount of water evaporated.
In the autoclave, the oxidation of coke is very rapid: > 90% of the coke is consumed in the bottom 10 ft of the reactor height. Concurrent with the coke oxidation, the vanadium is oxidized, and reacts with the sodium carbonate to form sodium metavanadate. The slurry leaving the reactor contains ~ 2-3% solids (Fe(VO3)3, NiCO3, and ash), and ~ 100 g 1-1 V20~ as NaVO3. This slurry is passed through a choke, and steam is flashed off in stages. Relatively little energy is contained in the slurry stream, but this steam energy can be recovered to supply the ammonium sulfate crystallizer and other low-pressure process heating requirements. Excess steam is condensed and used to slurry the reactor feed coke. Following letdown to atmospheric pressure, the slurry is
18
pressure-filtered for separation of the ash from the solution phase. The ash goes to a treatment step for recovery of nickel and residual vanadium; the filtrate goes on to the SIO2-P205 removal step.
The 5% excess of oxygen fed to the autoclave is based on process control requirements, rather than reaction kinetics. F. Charest has shown that carbon oxidizes very rapidly in aqueous media above 270 °C and 400 psig 02 [16], and he also established that:
(1) At low agitation rates, the oxidation is partially mass-transfer controlled as a result of the influence of diffusional resistances at both the gas-liquid and gas-particle interfaces. At intermediate agitation (400-600 r.p.m, in a 1-1 au- toclave), mass transfer is not rate limiting. At high agitation, the carbon par- ticles have some direct contact with the gas, diminishing the effective carbon- liquid interracial area; the overall oxidation rate decreases.
(2) The reaction rate is first order with respect to oxygen partial pressure. The activation energy obtained from the intrinsic kinetics is quite low (8.4 kcal mol- 1 ), suggesting at least partial reaction rate control by a free-radical mechanism in the liquid phase. Similar mechanisms are hypothesized for the liquid-phase oxidation of hydrocarbons.
Energy recovery from autoclave exit gases
The bulk of the reactor output energy is contained in the exit gases. This energy is recovered in two steps [17]. The gases first pass through a heat ex- changer, which is supplied with boiler-quality feed waste (see Fig. 7). Latent heat is removed from the gas-water vapor phase in the heat exchanger, and 75 000 lb h-1 of high-pressure (400-500 psig), high-quality steam is generated. This steam passes to a condensing turbine, where it is transformed to mechan- ical energy. The condensate is clean, and is returned to the heat exchanger. After the gases pass through the heat exchanger, they go through a separator, where process condensate is recovered. The dry inert gas (125 000 lb h -1 at 2500 psig) goes to a gas expander, where much of the remaining energy is transformed into mechanical energy. The turbine and gas expander are both mechanically connected to a motor/generator, which in turn is connected to the centrifugal air compressor. In this way, ~ 6000 kWh is recovered in both the turbine and expander (i.e., 12,000 kWh), and directly coupled to the air compressor, which requires ~ 10,000 hp. The remaining 2000 hp is removed from the mechanical couple by the motor/generator and converted to electric- ity for distribution to other process steps. During start-up, the air compressor is driven by outside electricity using the motor/generator. Overall, the process is autogenous; energy generated in the oxidation of coke is recovered and sent back to the process as steam, electricity and mechanical energy. (The efficien- cies of the energy recovery devices, and the loss of low-quality waste heat, have
19
120 TPD FLEXICOKE 1200 TPD AIR
N ° 2 C 0 3 ~ ' ~ WET OXIDATION ] _ H O T - - 3 0 5 " C - 2500psig / S L U R R Y H20 - "I
I0,000 KWH ~ ZOO0 KWH PLANT POWER ,- REQUIREMENT I ~ EXCESS POWER
Fig. 7. Distribution of energy output from soda ash processing of Flexicoke via wet oxidation ( TPD = tons per day).
been taken into account. ) There still remains excess energy that can be dis- tributed as either steam or electricity.
The equipment used in the pressure leaching and energy recovery portions of the process operates at conditions that appear severe by conventional me- tallurgical standards, but are routine in other technological fields. Over the past 20 yr, Zimpro, Inc., has built over 65 reactor systems for t rea tment of organic-containing slurries that operate at pressures > 1000 psig. Two of these facilities operate successfully at 320 °C and 3000 psig, processing Na2CO3-based pulping slurries from kraft paper plants [17]. This experience has given Zim- pro a high degree of confidence in designing the reactor/energy recovery sys- tem for use in pressure oxidation of Flexicoke. However, the hot-gas expander has shown a high maintenance and lower reliability history in the Zimpro ap- plications. Zimpro has tried different expander designs from a number of man- ufacturers, and has experienced blade wear, balance problems, and encroach- ment of condensate into the bearings. Experience at Zimpro Passavant (Rothschild, Wisc. ) is such that it does not frequently incorporate this energy recovery feature into the wet air oxidation process.
The Zimpro cylindrical diaphragm pump, reactor vessel, slurry letdown
20
equipment, heat exchanger, and condensing turbine are all proven designs that operate efficiently and with a high degree of reliability. In the case of the air compressor, a centrifugal unit is recommended rather than the reciprocating design because of better reliability, although the efficiency is not as good. Ex- cess energy is most conveniently removed from the process as high-pressure steam rather than as electricity, as the electric power requires careful regula- tion and distribution. Excess steam is removed ahead of the condensing tur- bine and distributed through pressure-reducing stations as required. Zimpro has not experienced any significant design or operating problems in connecting rotating equipment together through a common shaft and gearing.
Purification of Na V03 solution and ash treatment
The NaV03 solution leaves the pressure letdown step at 100 ° C, pH =9.5, 135 g 1-1 V205, and at a rate of 14 gal min -1. The main impurities in the solution are SiO2, P20~, and excess Na2CO3. Epsom salt and ammonium sul- fate are added to precipitate SiO2 and P205 from the pH -- 9.5 pregnant liquor [ 18, 19 ], as shown in Fig. 4A. A pressure leaf filter is used to remove the pre- cipitated solids. After water washing, these solids are discarded. Additional sulfuric acid is added to the filtrate, and the pH is lowered to three, which converts the metavanadate to decavanadate. During the acidulation step, ex- cess Na2CO3 reacts with the acid to form Na2SO4 and CO2, which is vented off. Following acidulation, the solution is diluted to 20 g l- 1 V205, and is cooled to 30 ° C in preparation for solvent extraction.
The ash solids removed from the reactor slurry are treated on a batch basis for recovery of nickel and recycle of residual vanadium, as shown in Fig. 6. The equipment consists of a number of reaction tanks, generating precipitates that are filtered and washed using pressure leaf filters. In the first step, the ash is repulped with hot process condensate. Caustic soda is then added to solubilize vanadium as the orthovanadate [20-22], which is recycled along with excess caustic soda back to the high-pressure reactor. Nickel and iron remain in the solids as hydroxides. The hydroxide filter cake is then repulped with water and acidified to pH 5 with H2S04, which solubilizes the nickel as NiS04 and leaves the iron in the solids as Fe (OH) 3. The Fe (OH) 3 is washed and discarded. The NiSO4 solution is finally neutralized with Na2C03, precipitating basic nickel carbonate, which is washed and recovered. The remaining Na2SO4 solution is discarded.
Solvent extraction and crystallization
The solvent extraction step operates at 30 ° C, and converts sodium decavan- adate solution at 20 g l-1 V205 to ammonium decavanadate at 35 g l-1 V20~. The solvent mix consists of 2-3% tertiary amine (Alamine 336), 2-3% modi-
21
tier (isodecanol or TBP), and the remaining percentage is the diluent (no. 2 fuel oil) [22, 23]. The aqueous sodium decavanadate feed is in contact with barren organic phase in three extraction stages. The vanadium is loaded and Na2S04 passes out in the raffinate. Then the loaded organic is washed in one or two stages using demineralized water, which removes entrained sodium and sulfate. The organic phase is then stripped in three stages with aqueous am- monia to form ammonium decavanadate. The strip liquor is heated to 60 ° C, and ammonia is added to raise the pH to 7.5. This operation converts ammo- nium decavanadate to ammonium metavanadate, which is relatively insoluble. About 2 g l- 1 V205 remains in solution. The slurry then advances to a cooling- type crystallizer (25 ° C ), which lowers the concentration of vanadium to < 0.5 g 1-1 V2Q.
Magma is removed from the crystallizer in small, semi-continuous batches, and passed through an automated basket centrifuge. The ammonium meta- vanadate crystals are discharged to a dryer, and the mother liquor recirculates back to the solvent extraction strip section (see Fig. 5). During the extraction stages, some sulfate loads on the organic, and eventually concentrates in the mother liquor. A bleed stream is used to remove ammonium sulfate from the recirculating mother liquor/strip liquor circuit. The bleed stream is sent to a single-stage scavenger circuit to load the residual metavanadate on barren or- ganic phase ahead of the extraction stages. The ammonium sulfate passes through the scavenger circuit as a separate raffinate stream, and is later crys- tallized into ammonium sulfate.
Separation of impurities
The principal impurities in Flexicoke are carbon, sulfur, nickel, iron, silica, phosphorus, aluminium, and molybdenum. Cyanide and phenol in the venturi scrubber liquor, as well as any alkali metals added during refining of the coke, are also of concern.
Carbon and sul[ur Wet oxidation rejects carbon into the wet oxidation exhaust gases as C02
and CO in a 12/1 mole ratio. The sulfur is quantitatively oxidized to sulfate, which is separated from the vanadium in the ammonium metavanadate crys- tallization step (Fig. 4B).
Nickel and iron Nickel and iron remain in the wet-oxidation ash, along with ~ 10% of the
vanadium. The filtrate after wet oxidation (pH 9.5 ) typically assays 100 g l- 1 V20~, 0.01 g 1-1 Ni, and 0.05 g 1-1 Fe. The iron and nickel components of the residue are believed to be xNiCOa-yNi (OH)e" 4H20, Fe (VOs)a, and Fe (OH)3. Ferric iron is known to form a mixture of ferric hydroxide and ferric vanadate
22
when added to sodium vanadate solution [24]. The proposed treatment of the wet oxidation ash for recovery of nickel values and recycle of residual vana- dium is shown in Fig. 6.
Silica and phosphorus Silica and phosphorus can be removed from the pregnant vanadate liquor
by addition of Epsom salt, plus a small quantity of ammonium sulfate, at pH = 9.5. The magnesium reacts with the carbonate in solution to form insol- uble MgNH4PO4 precipitate. Alternatively, silica and phosphorus can be sep- arated from the vanadium at p H = 7 by addition of alum [25]. The slurry is held at pH -- 6.5-7.5 at 70°C for 1 h using just enough agitation to keep the aluminum hydroxide in suspension.
Removal of molybdenum Fused black (i.e., V20~) suitable for production of ferrovanadium requires a
high degree of purity with respect to molybdenum. The Boscan Flexicoke PTV blend (Table 7 ) contains a 420/1 ratio of V2OJMo; molybdenum removal will thus probably be required. Molybdenum follows vanadium into the AMV crys- tallizer. The NH4VQ mother liquor is therefore bled at a rate that maintains the Mo concentration at 0.1 g 1-1. This concentration is sufficiently low to allow production of AMV crystals having a V2Os/Mo ratio that exceeds 750. The molybdenum-contaminated NH4VQ mother liquor can be treated by neu- tralization to pH = 3, followed by processing through a D2EHPA solvent ex- traction circuit [26]. The vanadium is then selectively stripped with 10% H2SO4, which is recycled to neutralize the advancing alkaline tertiary amine- SX feed liquor. The molybdenum load in the D2EHPA system is finally stripped with NH4OH- (NH4) 2MOO4 solution.
Alkali metals Sodium and potassium are detrimental to many end-users of V20~. The flow-
sheet shown in Fig. 4B rejects alkali metals into the solvent extraction raffinate. Phenol and cyanide are not a particular problem to a 305 °C wet-oxidation
process. These substances are being routinely destroyed in Zimpro wet-oxi- dation plants associated with processing wastes from the manufacture of acryl- onitrile, metallurgical coke, and petrochemicals [27-29]. For an update on recent developments in wet oxidation at Zimpro Passavant, see refs. [30-32].
CONCLUSIONS
(1) Wet oxidation of Flexicoke at ~ 300 ° C in aqueous soda ash solubilizes 85-99% of the contained V20~ values. The absolute level of vanadium extrac- tion depends on the pregnant liquor V2OJFe ratio in the Flexicoke feed.
(2) Nickel values, along with residual ferric vanadate, are concentrated in
23
the ash from the wet-oxidation process. Carbon and environmentally hazard- ous feed components, particularly cyanide and phenol, are oxidized. Carbon leaves the autoclave as C02 and CO in an ~ 12/1 mole ratio.
(3) The wet-oxidation autoclave generates sufficient high-pressure steam to meet power requirements for the Flexicoke refining process. This power requirement, which includes processing both the autoclave discharge liquor and the by-product ash, is -~ 10 000 kWh day-1 day for treating 110 ton day-1 of Flexicoke.
(4) Sulfur in the Flexicoke feed exists as aqueous sodium sulfate. Wet oxi- dation of Flexicoke requires high temperature ( > 270 ° C ), and proceeds well at an intermediate level of Na2CO3 concentration (50-150 g 1-1 ). Only a small excess of oxygen over stoichiometric is required.
(5) The higher the fuel value of the coke, keeping pressure and temperature constant, the more the feed slurry must be diluted to maintain thermal equi- librium. As the total pressure in the autoclave is increased at constant tern- perature, the feed slurry must be further diluted. The inverse is true if temper- ature is increased while maintaining total pressure constant.
(6) Vanadium can be recovered from the pregnant liquor by precipitating silica and phosphorus, acidifying to pH 3, converting from the sodium to the ammonium system via solvent extraction, crystallizing ammonium metavan- adate from the strip liquor, and calcining to generate V205. Nickel values are recovered by processing the wet-oxidation ash.
ACKNOWLEDGEMENTS
Technical assistance given to the authors by C.L. Soukup (formerly Man- ager of Business Development, Zimpro, Inc., Rothschild, Wisc.) and J.E. Litz (Vice-President, Hazen Research, Inc., Golden, Colo.) was invaluable in for- mulating autoclave design and vanadium-liquor refining technology, respec- tively. The authors extend special thanks to AMAX Inc. for permission to publish this work, and to Exxon Research and Development Laboratories (Ba- ton Rouge, La. ) for providing coke samples from their prototype Flexicoker.
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