Selective dissolution of critical metals from diesel and naptha spent hydrodesulphurization catalysts

resources,

ELSEVIER

Resources, Conservation and Recycling 13 (1995) 269-282

conservation and recycling

Selective dissolution of critical metals from diesel and naptha spent hydrodesulphurization catalysts T.N. Angelidis *, E. Tourasanidis, E. Marinou, G.A. Stalidis Laboratory of General and Inorganic Chemical Technology, Department of Chemistry, P.O. Box 114, Aristotle University, 54006 Thessaloniki, Greece Received 1 June 1994; accepted 1 October 1994

Abstract

The petroleum refining industry makes extensive use of catalysts, containing critical metals, such as, Mo, Co and Ni, for the desulphurization of various oil fractions. The selective recovery of these metals from two uncrushed and at low temperature calcined industrial hydrodesulphurization (MoC0/A1203 and Mo-Ni/A1203-SiO2) catalysts was studied, applying a two-step alkali-acid procedure. Fundamental kinetic aspects of the process, such as, reaction time, leaching reagents concentration and reaction temperature, were studied. Recoveries up to 97% for Mo and up to 92% for Co or Ni in separate solutions were achieved, using low cost and easily available reagents, such as sodium hydroxide and sulphuric acid. Keywords: Critical metal; Alkali-acid leaching; Spent catalyst; Petroleum refining

1. Introduction

The petroleum refining industry makes extensive use of catalysts for the desulphurization of the various oil fractions. Catalysts used for hydrodesulphurization contain critical metals, such as, Mo, Co and Ni, generally supported by alumina or silicoaluminates. The life time of a hydrodesulphurization catalyst, after several intermediate regeneration operations, is from 3 months to 6 years [ 1 ]. During hydrodesulphurization, the catalysts are deactivated by compounds of S, C, V, Fe, Ni, Si and traces of As and P [ 1]. The origin of these compounds is the oil fractions themselves or the equipment construction materials. Efforts of spent catalysts regeneration by selective removal of the metallic contaminants gave pure results [2]. The refiners are then confronted with the problem of disposing the spent catalysts. In the past, landfilling was a common method of disposal, and it is still * Corresponding author. 0921-3449/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0921-3449 ( 9 4 ) 0 0 0 4 9 - 2

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T.N. Angelidis et al./ Resources, Conservation and Recycling 13 (1995) 269-282

practiced in some countries. Other refiners prefer to stockpile the spent catalysts at the refineries until the price of metals warrants their sale to metal reclaimers. This practice, however, poses environmental problems to the refineries for storage and disposal of spent catalysts [3-5]. Spent hydrodesulphurization catalysts are a potential source of the contained critical metals [6]. A variety of processing approaches for recovering metals from spent catalysts has been proposed and most of the literature in this field is patented. After heating (to remove carbon and sulphur) and crushing, the spent catalysts are subjected to hydrometallurgical or hydropyrometallurgical treatment for metals recovery. In both cases, the metals are recovered as mixed solutions and then separated by conventional separation techniques (solvent extraction, selective precipitation, ion-exchange, etc.) [6]. Hydrometallurgical processes involve leaching with alkaline or acidic solutions [ 1,510]. Many reagents, such as NaOH, H2SO4, NH3, NHa-(NH4)2SO4, oxalic acid and SO2 have been tested. Hydropyrometallurgical processes involve roasting with Na2CO3, NaC1 or C12 gas (dry chlorination) [ 1,3,6,11,12]. An evaluation of the processing schemes, made by US Bureau of Mines, that are technically feasible, show that dry chlorination and a twostep alkali-acid leaching processe is economically feasible [ 1 ]. Generally, chlorination has disadvantages since is connected with corrosion and gas handling problems [6]. Pretreatment and especially the temperature of calcination for carbon and sulphur removal are critical in hydrometalllurgical processes. Calcination at temperatures higher than 500°C may cause [ 12] : sublimation of MOO3; and mixed oxides formation (molybdates, aluminates and silicoaluminates of Co or Ni) which are refractory in nature. The main effort of the present research work was to study basic kinetic aspects of critical metals recovery from two types of uncrushed and low temperature calcined industrial spent hydrodesulphurization catalysts. A two-step alkali-acid leaching process for the selective dissolution of Mo and Co or Ni was examined.

2. Solubilities and chemical reactions

A two-step leaching process is based on the difference of the solubility of MoO3 and CoO or NiO in acidic and basic media. As shown in Figs. 1 and 2, at higher pH values CoO and NiO are almost insoluble, while M o O 3 is readily soluble. According to the above observations a first-stage leaching procedure with an alkali is going to dissolve MOO3, leaving almost unaffected the NiO or CoO, the later may be dissolved in a second acidic media leaching step. A selective MoO3 and CoO or NiO is going to be succeeded, simplifying the subsequent separation procedure. In both cases a quantity of the substrate is going to be dissolved, since A1203 is soluble in acidic as well as in basic media having its lower solubility at a pH value of about 5 [ 16]. NaOH and H2SO4 were selected as the leaching reagents due to their availability and low cost. The simplified reactions taking place are: (a) Basic media:

T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

271

MoO3 + 2NaOH--* Na2MoO4 + H20

(1)

A1203 + 2 N a O H ~ 2NaA102 + H 2 0

(2)

CoO + 2NaOH --* Na2CoO2 + H20

(3)

NiO + 2NaOH --* Na2NiO2 + H20

(4)

(b) Acidic media: CoO + H2SO4 --* COSO4 + H20

(5)

NiO + H2SO4 --->NiSO4 + H20

(6)

A1203 + 3H2SO4 ~ Ale (SO4)3 + 31-120

(7)

^2 S

'Tr~ CoO ~-2:E O" 0 0

2 0

Co+2

{-4"

'l \

0

b - 60

£

HMo04- i MoO~-2 ~ /HCoO~ -8 °

,

i/

4- 6 8 10 1'2 pH

Fig. 1. Comparison of M o O 3 and CoO solubilities at various pH values (thermodynamic data from [ 13,14] ).

2 6

0"

Ni0

"~--2" >

~-4"

Ni+2 /

"5

/ /

b - 6-

£

/ i !

HMoO4- i MoO¢-2t~ ,,/HNiOa-8

pH Fig. 2. Comparison of MoO3 and NiO solubilities at various pH values (thermodynamic data from [ 13,15] ).

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T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

3. Experimental p r o c e d u r e s

3.1. Catalysts characterization

The spent catalysts used were extrudes of a Co-Mo/A1203 (HDS-22T) diesel desulphurization catalyst and a Ni-Mo/AI203/SiO2 (KF-842) naptha desulphurization catalyst derived from the EKO refinery in Thessaloniki region (Greece). Complete chemical analyses of the examined samples are shown in Table 1. The near surface elemental analyses of the catalyst (SEM-EDS) after carbon and sulphur removal by calcination at 500°C is shown in Figs. 3 and 4. The main contaminants were carbon and sulphur deposits (up to 30% w / w in the spent cobalt based catalyst) and iron. Arsenic, phosphorous and remaining sulphur were detected at the surface by SEM-EDS analysis, but their concentration was negligible in bulk analysis. Remaining sulphur may be attributed to the presence of a small quantity of cobalt or nickel sulphides [ 12 ]. Although vanadium is not in the original catalyst material, it is commonly removed from some crude oils or heavy oil fractions [ 11 ]. The vanadium concentration of the studied catalysts was not detectable due to their use for the desulphurization of light oil fractions. Table 1 Chemical characteristics of the selected samples (composition%) Catalyst

NiO

CoO

MoO3

A1203

SiO2

Fe203

C&Sa

Humidity b

Ni-Mo Co-Mo

2.5 -

3.1

12.1 10.4

48 51

14 -

1.5 0.9

15.0 24.7

4.0 8.0

aWeightloss after calcination of a dry sample at 500°C for 1 h. bWeightloss after heating at 120°Cfor 6 h. ELEMENTALCOMPOSITION, EO Co-MoOAl~LY81"

70 60 6040 30 20I00

Mo

Na

go

8

AI

k8

P

Co

Fig. 3. Near surfaceelemental composition (%) of the Mo-Co/A1203spgntcatalyst, after heat treatment, derived by SEM-EDSanalysis.

T.N. Angelidis et al. / Resources, Conservationand Recycling 13 (1995) 269--282

273

ELEMENTAl. COMPOEIlTION,*k 7'0NI-Mo OAI"ALYOT

805040SO 20 10

Mo

NI

Na

Fe

8

AI

/~

81

P

Fig. 4. Near surfaceelementalcomposition(%) of the Mo-Ni/AI203-SiO2 spent catalyst,after heat treatment, derivedby SEM-EDSanalysis. 3.2. Pretreatment and leaching procedure

Calcination for carbon and sulphur removal was carried out at 450°C for 2 h to prevent mixed oxides formation and MoO3 sublimation. After calcination the catalysts are normally crushed to increase the reaction surface during the leaching procedure. Crushing is an energy-consuming procedure and increases significantly the recovery cost. Since the catalyst extrudes were of small size (average length 2-4 nun and average diameter 1.15-1.35 mm after calcination), uncrushed samples were used for the subsequent leaching tests. The analysis of the calcined catalysts for the elements of interest (Mo, Co or Ni and A1) gave the following results (% w/w composition, dry base): Mo-Co CATALYST: 10.3% Mo, 3.5% Co, 39.7% A1; Mo-Ni CATALYST: 10.0% Mo, 2.4% Ni, 31.7% A1. Analytical grade sodium hydroxide and sulphuric acid were used for the preparation of the leaching solutions. Sodium hydroxide leaching was carried out at atmospheric pressure in pyrex conical flasks, equipped with a reflux condenser at higher temperatures, and agitated in a thermostatted water bath. Samples of 5 g of the pretreated waste catalyst were leached with 50 ml of NaOH solution. 5-ml aliquots were withdrawn at certain time intervals for chemical analysis. An equal quantity of fresh NaOH solution was added after each aliquot removal to keep the liquid volume constant. Sulphuric acid leaching tests were carried out using an alkali preleached catalyst sample. The alkali preleaching stage was carded out using a fixed-bed laboratory glass reactor filled with spent catalyst. The leaching solution ( 10 g/1NaOH at 80°C) was pumped continuously through the catalyst bed at a rate of 10 ml/min from the bottom of the reactor (up-flow). The procedure was stopped when the thiocyanate test for molybdenum in the leaching solution [ 17] was negative. The analysis of the as above preleached catalysts for the elements of interest (Mo, Co or Ni and AI gave the following results (% w/w composition, dry base): Mo-Co CATALYST: 0.42% Mo, 4.49% Co, 46.14% A1, weight loss 26%, these values represent 97% Mo, 5% Co and 14% AI removal. Mo-Ni CATALYST: 0.26% Mo,

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T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

2.95% Ni, 36.17% AI, weight loss 22%, these values represent 98% Mo, 4% Ni and 11% A1 removal. 5 g samples of the preleached catalyst were subjected to acid leaching following the same procedure as in alkali leaching. The collected samples were analyzed for cobalt, nickel, aluminum and iron by an atomic absorption spectrophotometer (Perkin Elmer 2380) and molybdenum by the thiocynate method [ 18] using an Hitachi U-2000 spectrophotometer. The measured concentrations were corrected to take into account aliquot removals and fresh solution additions. The results are presented as % removal of the initial quantity of the respective element present in the calcined samples.

4. Results and discussion 4.1. Alkali leaching

A series of experiments were carded out to examine the influence of time and NaOH concentration on Mo recovery for the two types of catalysts at 25°C. The results are shown in Figs, 5 and 6. Maximum Mo recovery, in all cases, is accomplished within 2 h from the beginning of the experiments and then remains constant. NaOH concentration does not seem to significantly affect recovery yield. In Figs. 7 and 8, the results of the above experiments for Mo recovery, Co or Ni and aluminum dissolved are summarized, as measured at the above-mentioned optimum reaction time (2 h). Only Mo and A1 seem to dissolve in considerable quantifies at these conditions, a small quantity of cobalt or nickel was detected, while the presence of iron was negligible. A small increase of Mo recovery yield is connected with additional AI dissolution as NaOH concentration increases. The maximum achieved Mo recovery yield is about 60% at 25°(2. A considerable quantity of Mo (about Re(Uo), ~,

80

×

40

.~

U7 ~

'°o

'

WRier

~

NILOH 4 0 0 / L

-X-- NaOH 80g/L

m

I

I

o

so

too

NIIOH 100/L

I

~

I

15o 2oo TIME (mln)

NllOH

I

26o

.

209/L

.l

3oo

f

as0

Fig. 5. Molybdenumdissolutionrecovery% fromthe Mo-Co/A1203spentcatalystas a functionof timefor various NaOH concentrations (temperature25°C, solid/liquid = 1:20g/cm3).

T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

80

275

Re(Mo),

/i; ~

40

.....

.o.,0.,L

. . o . °00,,

I p

0

50

100

lifo

200

280

300

350

T I M E (rain)

Fig. 6. Molybdenum dissolution recovery % from the Mo-Ni/A1203-SiO2 spent catalyst as a function of time for various NaOH concentrations (temperature 25°C, solid/liquid = 1:20 g/cm3).

30%) is dissolved even in pure water, showing the environmental leachate problems connected with the storage of the spent catalysts in open areas. A second series of experiments were carried out increasing the reaction temperature and keeping the NaOH concentration constant ( 10 g/l). The results are shown in Figs. 9 and 10. Mo recovery yield in both cases increases considerably as temperature increases and reaches a maximum value of 96% at 100°C. In Table 2, the results of the above experiments for Mo recovery, Co or Ni and aluminum dissolved are summarized, as measured at the above-mentioned optimum reaction time (2 h). As shown in Table 2 increased Mo recovery during increasing temperature experiments is directly connected with excess AI dissolution, 80

Re,%

[-~-MOLYIIDENUM -dI-ALUMINUM -a[-COEALT} oo

. ~

-,

.

o

20'

f"""

0 0

10

"~ . . . . . . . . ' 20 30

X 40

i 80

... , 60

i 70

80

NaOH, g/L

Fig. 7. Molybdenum, aluminum and cobalt dissolution recovery % from the Mo-Co/A1203 spent catalyst as a function of NaOH concentration (reaction time 2 h, temperature 25°C, solid/liquid = 1:20 g/cm3).

276

T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282 I~,ek

8O

{-O'-MOUrllOENVM

-,~-ALUMINUM

20

-'~-NICKEL

°...--""

G 0

"~" . . . . . . . . [ 20 30

10

I

..........

Z x 40 60 NaOH, g / I .

60

70

80

Fig. 8. Molybdenum, aluminum and nickel dissolutionrecovery % from the Mo-Ni/A1203-Si02spent catalyst as a function of NaOH concentration (reaction time 2 h, temperature 25°C, solid/liquid = 1:20 g/era3). which liberates additional M o O 3 quantities trapped in blocked catalyst pores. Only small quantities of Co and Ni were dissolved, while Fe was not detectable in the leaching solutions. A n increase of reaction time m a y improve the M o recovery yield but it will cause excess leaching reagent consumption mainly by additional A1 dissolution. 4.2. A c i d leaching

The alkali-treated catalyst samples were used in the acid leaching experiments. A series of experiments were carried out to examine the influence of reaction time and sulphuric tO0

l~(Mo), ~,

8O

6o

2O ~

0

i

i

10

20

i

30

40

~

I

I

60 80 70 TIME (mln)

I

I

80

90

I

I

100 110 120

Fig. 9. Molybdenumdissolutionrecovery % from the Mo-Co/AI203spent catalyst as a function of time for various temperatures (NaOH concentration 10 g/l, solid/liqnid= 1:20 g/cm3).

T.N. Angelidis et al. /Resources, Conservation and Recycling 13 (1995) 269-282

100

277

l~(Mo),

~

n.

'°

20

o

lo

2o

so

4o

so ao ro TIME (rain)

so

9o

~oo 11o 12o

Fig. 10. Molybdenum dissolutionrecovery % from the Mo-Ni/A1203-SiO2spent catalyst as a function of time for various temperatures (NaOH concentration 10 g/l. solid/liquid = 1:20 g/cm3). acid concentration on Co or Ni recovery for the two types o f catalysts at 25°C. The results are shown in Figs. 11 and 12. M a x i m u m metal recovery was achieved within 2 h. The influence o f sulphuric acid concentration is negligible to Co recovery, while it is significant in the case o f Ni recovery. The difference in the behaviour o f the two catalysts may be attributed to the different catalysts substrates and mainly to the presence of silica in the MoNi based catalyst. The presence of Ni silicates or aluminosilicates [ 12], is possibly responsible for this behaviour. In Figs. 13 and 14, the results o f the above experiments for Co or Ni recovery, M o and AI dissolved are summarized for a reaction time o f 2 h. Only Co or Ni and AI seems to dissolve considerably, and only small quantities of remaining M o were detected in the leachate. The quantity o f dissolved A1 was considerably higher than the one achieved during alkali leaching experiments, since alumina is readily soluble in acids. Almost all of the Fe ( > 99%) present as contaminant in both catalysts was determined in the acid leaching solutions after 2 h o f reaction. A considerable quantity o f Co (up to 22%) Table 2 Dissolution (%) of molybdenum, aluminum and cobalt or nickel, during the first leaching step, at various temperatures Temperature (0(2)

Catalyst Mo-Co

25 50 80 100

Mo-Ni

Mo

A1

Co

Mo

AI

Ni

57.6 76.1 84.8 96.0

1.0 7.2 9.7 14.1

0.03 0.05 0.08 0.09

56.5 75.0 87.0 97.0

0.8 6.5 7.9 11.6

0.06 0.06 0.07 0.08

NaOH concentration 10 g/l; reaction time 2 h.

278

T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282 Re(Co), 80

WMer "q-- 8ulph. Aold 89/L-'~" 8ulph. AOIdIO01L 8ulph. Aold ROg/L-X- 8ulph. AOId40g/L-~- 8ulph. Mid 800/L eO

40

•

20

;:

i

t

0

80

100

f

, i 180

200

TIME (rain) Fig. 11. Cobalt dissolution recovery % from the Mo-Co/Al2Oa spent catalyst as a function of time for various sulphuric acid concentrations (temperature 25°(2, solid/liquid = 1:20 g/cm3).

and Ni (up to 20%) were dissolved even in pure water, causing environmental leachate problems during storage in open areas. A second series of experiments were carded out increasing the reaction temperature and keeping constant the sulphuric acid concentration ( 10 g/l). The results concerning Co and Ni recovery are shown in Figs. 15 and 16. Co and Ni recovery increases considerably as temperature increases and reaches a maximum value of 93% for Co and 90% for Ni at 100°C. In Table 3, the results of the above experiments for Co or Ni recovery and A1 and Mo dissolved are summarized, as measured at the above conditions after 2 h of reaction. Again, increased Co or Ni recovery was connected with excess A1 dissolution (up to 55%

.oj

Re(NO, % I

-It- wMer - ~ 8ulph. #,old 6g/l.-'~- 8ulph. Acid tOg/l. 1 8uIph. AOM 20(V1.")'(- $ulph, Mid 4011/L"O- Oulph. Mid 80glL

I

eo

,0

.......

O~

0

I

I

I

60

100

180

200

TIME (mln) Fig. 12. Nickel dissolution recovery % from the Mo-Ni/AI203-SiO 2 spent catalyst as a function of time for various sulphuric acid concentrations (temperature 25°(2, solid/liquid = 1:20 g/cm3).

T.N. Angelidis et aL / Resources, Conservation and Recycling 13 (1995) 269-282

279

Re(Co), ~, 100

80

00 40

2O

V o

I-'-e6"c "+-so"¢ '~-8o°¢ -B-Ioo°cl I

l

I

I

lo

eo

ao

40

I

I

I

so eo ro TIME (rain)

I

I

80

90

I

I

~oo .o

12o

Fig. 13. Cobalt dissolution recovery % from the Mo-Co/AI203spent catalyst as a function of time for various temperatures (sulphuric acid concentration 10 g/l, solid/liquid= 1:20g/cm3). 100

ReINI),

8o

6o

4O

20

I~

I

]

I

I

1

I

I

I

I

10

20

30

40

50

60

70

80

90

I

I

100 110 120

TIME (rain)

Fig. 14. Nickeldissolution recovery% from the Mo-Ni/AI203-SiO2spent catalystas a functionof time for various temperatures (sulphuric acid concentration 10 g/I, solid/liquid= 1:20g/cm3). or 65%, respectively). Mo concentration in the leaching solutions was almost negligible, while all iron present was dissolved. An increase of reaction time may improve recovery yield but will cause excess leaching reagent consumption through additional alumina dissolution. The solid residue of the fixed-bed procedure and acid leaching at 100°C was analyzed for the elements of interest. The chemical analysis gave the following results (% w / w composition, dry base: Mo-Co CATALYST: 1.0% Mo, 0.5% Co, 45.0% A1, total weight loss 72% Mo-Ni CATALYST: 0.6% Mo, 0.4% Co, 21.0% A1, total weight loss 65%

280

T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

The above-listed results are in good agreement with the ones expected from the alkalileached residue and the concentrations of the acid-stage leachate: Mo-Co CATALYST: Expected removal (%) Based on residue (%) Mo-Ni CATALYST: Expected removal (%) Based on residue (%)

Mo 97.03 97.30 Mo 98.01 97.90

Co 96.10 96.00 Ni 94.00 94.20

A1 68.30 68.20 A1 76.70 76.80

Re,

§0 1

{ -~- MOLYBDENUM . A. ALUMINUM -K- COBALT ]

80 /k ...........o---

40

.....

"

.,.-

20

,

,,.

!21 .-j 0

10

20

I

•v

I

30 40 60 8ulphurio aoJd, g/k

I

I

eO

70

80

Fig. 15. Molybdenum, aluminum and cobalt dissolution recovery % from the Mo-Co/A1203 spent catalyst as a function of sulphuric acid concentration (reaction time 2 h, temperature 25°C, solid/liquid = 1:20 g/cm3). Fie,

I

-O- MOLYBDENUM

-r.. ALUMINUM

-K- NICKEL

I .4

8O . o..-'°'°'"

,

' ro

8o

2O

0 "'~':'-o IO

O 2o

' . ' 80 40 60 8ulphurl¢ acid, glL

' eo

Fig. 16. Molybdenum, aluminum and nickel dissolution recovery % from the Mo-Ni/A1203-SiO2 spent catalyst as a function of sulphuric acid concentration (reaction time 2 h, temperature 25°C, solid/liquid= 1:20 g/cm3).

T.N. Angelidis et aL ~Resources, Conservation and Recycling 13 (1995) 269-282

281

Table 3 Dissolution (%) of molybdenum, aluminum and cobalt or nickel, during the second leaching step, at various temperatures Temperature C

Catalysts Mo-Co

25 50 80 100

Mo-Ni

Mo

A1

Co

Mo

AI

Ni

nd 0.01 0.02 0.03

8.8 24.5 32.7 54.3

53.5 61.4 79.2 91.1

nd nd 0.01 0.01

11.0 27.4 48,6 65.7

39.4 55.4 75.8 90.0

Sulphuric acid concentration 10 g/l; reaction time 2 h; nd= not detected. Dissolution yield (%) refers to the initial quantities of the respective element before first-step leaching. 5. C o n c l u s i o n s

A two-step alkali-acid leaching process gave good results for the recovery of critical metals (Mo, Co and Ni) from two uncrushed, calcined at low temperature, M o - C o / A I 2 0 3 and Mo-Ni/AI203-SiO2, industrial hydrodesulphurization catalysts. Recoveries up to 97% for Mo and 90-93% for Co and Ni were achieved. The main advantages of the method are: - The selective dissolution of the critical metals by the production of two separate solutions, one basic containing mainly Mo and A1 and one acid containing mainly Co or Ni, A1 and Fe. The above procedure simplifies the subsequent separation process. - The use of low cost and easily available leaching reagents such as sodium hydroxide and sulphuric acid. Work is in progress concerning further improvement of the recovery yield and the development of a separation procedure of the critical metals from the leaching solutions.

Acknowledgements

The authors are grateful to the authorities of the EKO refinery (Thessaloniki, Greece) for financial support and provision of the catalysts samples.

References

[ 1] Jong, B.W., Rhoads, S.C., Stubbs, A.M, and Stoelting, T.R., 1989. Recovery of Principal Metal Values from Waste Hydroprocessing Catalysts, US Bureau of Mines, US Department of Interior, RI 9252, August 1989. [ 2] Marafi, M. and Stanislaus, A., 1989. Regeneration of spent hydroprocessing catalysts: metals removal. Appl. Catalysis, 47: 85-96. [3] Recyclers Try New Ways to Process Spent Catalysts. Chemical Engineering, 16 February 1987, pp. 25-30. [4l Habermehl, R., 1988. Safe Handling and Disposal of Spent Catalysts. Chemical Engineering Progress, February 1988, pp. 16-19.

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T.N. Angelidis et al. / Resources, Conservation and Recycling 13 (1995) 269-282

[5] Corbett, R.A., 1990. Refiners, petrochem plants focus on new waste challenges. OGJ SPECIAL. Oil and Gas J., Marc. 5: 33-38. [6] Siemens, R.E., Jong, B.W. and Russel, J.H., 1986. Potential of spent catalysts as a source of critical metals. Conservation and Recycling, 9(2): 189-196. [7] Biswas, R.K., Wakihara, M. and Taniguchi, M., 1986. Characterization and leaching of the heavy oil desulphurization waste catalyst. Bangladesh J. Sci. Ind. Res., XXI, Nos. (1-4): 228-237. [8] Raisoni, P.R. and Dixit, S.G., 1988. Leaching of cobalt and molybdenum from a Co-Mo/y-A1203 hydrodesulphurization catalyst waste with aqueous solutions of sulphur dioxide. Minerals Engin., 1(3): 225-234. [9] Wiewiorowski, E., Tinnin, R. and Crnoievich, R., 1988. A cyclic process for recovery of metals from spent catalysts. Presented at the SME Annual Meeting, Phoenix, AZ, 25-28 January 1988, Preprint No. 88-168. [10] McPartland, J.S. and Bantista, R.G., 1990. Extraction of cobalt from hydrotreating catalysts using supercritical ammonia. Separation Sci. Technol., 25 (13-15): 2045-2055. [ 11 ] Biswas, R.K., Wakihara, M. and Taniguchi, M., 1985. Recovery of vanadium and molybdenum from heavy oil desulphurization waste catalyst. Hydrometallurgy, 14: 219-230. [ 12] Llanos, Z.R., Lacave, J. and Deering, W.G., 1986. Treatment of spent hydroprocessing catalysts at Gttlf Chemical and Metallurgical Corporation, Presented at the SME Annual Meeting~ New Orleans, Louisiana, March 2-6, 1986, Preprint No. 86--43. [13] Deltombe, E., de Zoubov, N. and Pourbaix, M., 1974. Molybdenum, In: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutfions, NACE, pp. 272-279. [ 14] Deltombe, E. and Pourbaix, M., 1974. Cobalt, In: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, pp. 322-329. [ 15 ] Deltombe, E., de Zoubov, N. and Pourbaix, M., 1974. Nickel, In: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, pp. 330-342. [ 16] Deltombe, E., Vanleugenhaghe, G. and Pourbaix, M., 1974. Aluminium, In: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, pp. 168-176. [17] Treadwell, F.P., 1963. Analytical Chemistry, Vol. I, Qualitive Analysis, 9th edn., J. Wiley & Sons, New York, 506 pp. [ 18] Marczenko, Z., 1976. Spectrophotometric Determination of Elements, J. Wiley & Sons, New York, 362 pp.