Recovery of vanadium from heavy oil and Orimulsion fly ashes

Hydrometallurgy 57 Ž2000. 141–149 www.elsevier.nlrlocaterhydromet

Recovery of vanadium from heavy oil and Orimulsion fly ashes Sandra Vitolo ) , Maurizia Seggiani, Sara Filippi, Cristina Brocchini Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali UniÕersita` di Pisa Via DiotisalÕi, 2 56126 Pisa, Italy Received 10 February 2000; accepted 5 April 2000

Abstract This work concerns a three-step process for the recovery of vanadium from heavy oil and Orimulsion combustion fly ashes. This consisted of acid leaching, oxidation and precipitation of the vanadium pentoxide, followed by washing of the precipitate. Preliminary tests were conducted to investigate the effect of some operating parameters for the various steps of the process. After these preliminary tests, the recovery of vanadium from the fly ash samples was performed on a laboratory scale and the overall yield of the process was determined. By washing the precipitate, it was possible to reduce the concentration of the impurities and to allow its use for the production of ferrovanadium alloys. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Heavy oils; Orimulsion; Fly ashes; Vanadium; Recovery

1. Introduction The fly ashes produced from the combustion of heavy oils are composed of unburned carbon and an inorganic fraction containing a low concentration of vanadium Ž1%–7%.. This concentration is, however, comparable to that found naturally in mineral resources. Even if vanadium is widely distributed throughout the lithosphere, it is not present in high concentrations in minerals and is very often recovered as a by-product during the extraction of other metals w1x. For this reason, the recovery of vanadium

) Corresponding author. Tel. q39-0-5051-1278; fax: q39-05051-1266. E-mail address: [email protected] ŽS. Vitolo..

from oil-fired fly ashes has received attention since the 1960s. Another reason for interest in this extraction process is that the disposal of this industrial waste may lead to environmental problems such as dusting and pollution of water with heavy metals. Several processes have been proposed to recover vanadium from fly ashes using hydrometallurgical processes. The recovery may be carried out directly on the fly ashes by acid w2–4,6,8,9,11,12x, alkaline w5,7,13–17x or water w10x leaching; this may be followed by an oxidation of vanadium Žusing air w10x, NaClO 3 w2,3,6x, NaClO w12x, H 2 O 2 w5x oxygen w17x or Cl 2 w11,13x.; vanadium may be separated from the leaching solution using ion exchange resins and eluted with acid solutions w7–9,11,14,16x; this may be followed by isolation and purification by solvent extraction w8,9x. Vanadium may also be directly separated from the leaching solution by sol-

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 9 9 - 2

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S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

vent extraction w13,16,18,19x. Whichever method is used, the vanadium is finally precipitated as ammonium vanadate or vanadium pentoxide. The recovery may also be carried out by roasting the fly ashes in air in the presence of alkali metal carbonates, sulphates, or chlorides, then leaching with an alkaline solution and precipitating the vanadium as ammonium vanadate or vanadium pentoxide w20–22x. Recently, Orimulsion Ža registered trade mark belonging to Bitumines Orinoco. has received increasing attention as an alternative fuel. Orimulsion is a stable bitumen-in-water emulsion made from pure natural bitumen occurring in the Orinoco Belt of Venezuela. The bitumen reserves are put at 186 = 10 9 tonnes, of which 45 = 10 9 are estimated to be recoverable. This fuel is competitive with coal and the reserves may supply one hundred 1000 MW power stations for over 250 years w23x. The fly ashes collected have an appreciable level of vanadium Žup to 12%. in a highly leachable form that makes its recovery of interest; the fly ashes are very fine and are characterised by a very low density that makes disposal very difficult. In this work, a simple three-step process for the recovery of vanadium from heavy oils and Orimulsion combustion fly ashes was studied. The recovery process consisted of an acid leaching, oxidation and precipitation of vanadium pentoxide. The use of extraction solvents was avoided and the purity of the precipitate was achieved through a specific washing of the precipitate in order to control the concentration of the impurities Žtypically Cu, Co, Na, K, S, and P. and to allow the use of the precipitate for the production of ferrovanadium alloys.

HNO 3 and 2.5 cm3 HF. in an autoclave at 1208C for 2 h. Vanadium content of the liquid phases was determined by a potentiometric titration w24x and the measurements were verified using ICP-AES. The leaching and oxidationrprecipitation stages were conducted at atmospheric pressure in a Pyrex stirred reactor, equipped with a reflux condenser and placed in a heated oil bath. The various solutions derived from leaching of the fly ashes, precipitation of the vanadium pentoxide, and washings of the cakes were all separated by vacuum filtration Ž0.7 atm..

3. Results and discussion 3.1. Fly ash samples Three samples of heavy oil combustion fly ashes were examined. These were collected from the flue gases of different oil-fired power plants. The Orimulsion combustion fly ash sample was collected from the flue gases of the 320 MW Fiume Santo power plant ŽNuoro, Italy.. This plant is the ENEL S.p.A. demonstration unit for the evaluation of Orimulsion as a competitive fuel. The composition of the fly ashes is reported in Table 1. Of the three oil-fired fly ash samples, sample C is characterised by a high carbonaceous fraction and by a lower vanadium content. The Orimulsion fly ash shows a much higher vanadium content than the oil fly ashes. Due to the

Table 1 Main constituents of the fly ashes Žwt.%. Oil fly ashes

2. Experimental Carbon content was determined by a Carlo Erba 1106 analyser. Sulphur content was determined by a LECO SC 432 DR I.R. detector. Moisture Ž1058C, in nitrogen atmosphere. of the fly ashes were determined by a METTLER TG 50 thermobalance. The composition of the mineral fraction of the fly ashes and of the precipitated vanadium pentoxide was determined by a PERKIN-ELMER PLASMA 400 ICPAES analyser on the solutions obtained by digesting the dried samples Ž0.5 g. with strong acids Ž7.5 cm3

Moisture C Žd.b.. V Žd.b.. Fe Žd.b.. Na Žd.b.. K Žd.b.. Ni Žd.b.. Si Žd.b.. Al Žd.b.. Ti Žd.b.. S Žd.b.. P Žd.b.. a

A

B

C

Orimulsion fly ashes

1.9 33.1 2.6 3.5 2.3 0.2 1.3 2.5 2.1 0.1 12.6 0.1

0.9 39.6 3.3 3.9 2.4 0.2 1.2 0.7 0.4 traces 11.3 0.1

1.5 79.1 1.3 4.0 0.2 0.1 0.7 0.8 0.6 traces 1.6 0.1

5.2 7.1 11.7 0.6 1.1 n.r.a 2.5 n.r. 3.2 0.1 13.2 traces

Not revealed.

S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

high level of sulphur in the Orimulsion, the addition of MgO in the combustion chamber is provided in order to neutralise the sulphur oxides to MgSO4 , which becomes a component of the fly ashes. 3.2. Preliminary tests 3.2.1. Vanadium leaching Some preliminary tests were conducted to set out the leaching operating conditions. The increase of the vanadium concentration in the extract was followed over 24 h. However, even in the tests conducted in the most unfavourable conditions Žwater leaching at room temperature., practically all the extractable vanadium was obtained within 30 min for all the fly ash samples. The preliminary tests disclosed also that alkaline leaching was not effective. Once the leaching time and the nature of the leaching solution was assessed, the effect of some operating parameters on the yield of vanadium extracted Žliquid-to-solid ratio ŽLrS., temperature, and acidity of leaching solution. was studied and is reported as follows. 3.2.1.1. Oil fly ashes. Fig. 1 reports the extraction yield obtained by varying LrS from 2 to 5 mLrg, using 0.5 M sulphuric acid as the leaching agent at both room and boiling temperatures. Both an increase in temperature and LrS ratio generally led to an increase in the extraction yield; however, a different behaviour was observed in the three samples. An increase in LrS from 2 to 3 mLrg had practically no effect on the extraction yield of sample A, whilst an increase in yield of 5% and 15.5% was observed for samples B and C, respectively. The yield did not increase significantly above an LrS ratio of 3 mLrg for all three samples. This result may be related to the wettability of the fly ashes, which decreases with carbonaceous fraction; ashes with a higher carbonaceous fraction Žsee Table 1. have a lower wettability. This means that a higher LrS ratio is necessary for ashes with a high carbonaceous fraction to ensure an adequate contact between the solid and the liquid phases. Fig. 2 reports the yield of vanadium extraction obtained by leaching with water and with solutions of H 2 SO4 at different concentrations, both at room

143

and boiling temperatures and with an LrS ratio of 3 mLrg. Generally, the yield of extraction increased with the acidity of the leaching solution. While this effect is particularly significant for samples B and C, the lower sensitivity of sample A may be partially explained by its higher intrinsic acidity as indicated by the pH of the extract solution obtained using water at room temperature ŽpH s 1.48, 1.98, and 2.88 for samples A, B, and C, respectively., its higher wettability and by the possible presence of easier leachable vanadium compounds. 3.2.1.2. Orimulsion fly ash. Following the results obtained with the heavy oil fly ashes, the process was conducted at the boiling temperature of the solution. In Fig. 2, the extraction yield obtained by leaching with acidic solutions with an LrS of 3 mLrg is reported. The yield of extraction of vanadium increases with the acidity of the leaching solution. The effect of the liquid-to-solid ratio was studied in the range LrS from 3 to 5 by leaching with sulphuric acid Ž2 M.. As reported in Fig. 1, the increase of the extraction yield was obtained by raising the LrS from 3 to 4 mLrg. Compared to the heavy oil ashes, a higher acidity of the leaching solution is required for the Orimulsion ash, owing to a much lower intrinsic acidity, as indicated by a pH 6.1 of the extract solution obtained using water ŽLrS 5. at room temperature. This lower acidity is due to the neutralisation of the sulphur oxides caused by the addition of MgO in the combustion chamber. 3.2.2. Vanadium oxidatiÕe precipitation The oxidative precipitation of the vanadium from the extract solution was performed under the conditions suggested by Giorgini et al. w6x, using NaClO 3 Ž25% excess. as the vanadium oxidating agent at the solution’s boiling temperature, keeping the pH at about 2.3. In this condition of pH and at the vanadium concentration of the solutions used for precipitation, vanadium pentoxide is the major component in solution w25x. Since the oxidation of V ŽIV. to V ŽV. is accompanied by a release of Hq, Na 2 CO 3 was added to achieve and maintain the ideal pH during precipitation. The precipitation of vanadium Žin the form of complex hydrates and vanadates, mainly accompanied by iron compounds. was practically

144 S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

Fig. 1. Extraction yield of vanadium versus liquidrsolid ratio at room Ž`. and boiling temperature Žv . using H 2 SO4 0.5 M for the heavy oil fly ash samples and H 2 SO4 2 M for the Orimulsion fly ash sample as leaching solution.

S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

Fig. 2. Extraction yield of vanadium versus the molarity of the H 2 SO4 leaching solution at room Ž`. and boiling temperature Žv . with a liquidrsolid ratio of 3 mLrg.

145

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S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

complete in 1 h. The precipitate obtained in the above conditions has, however, a relatively high sodium content, which may preclude its use in the preparation of ferrovanadium alloys. 3.2.3. Precipitate washing Preliminary studies ŽM. Giorgini, private communication. have shown that the sodium contained in the raw precipitate may be reduced by up to 95% through repeated washings of the precipitate with acidic solutions. The increased effectiveness of the acidic solutions with respect to water is due to the greater capacity of dissolving the sodium vanadates present in the precipitate. 3.3. Vanadium pentoxide recoÕery process By adopting the operating conditions that were set out in the preliminary tests, the three-step process

Žleaching, oxidative precipitation, washing of the precipitate. of vanadium recovery was performed and the overall yield was determined. The block diagram of the process is reported in Fig. 3. The leaching stage was conducted on 50 g of fly ash. According to the preliminary tests, the extraction was performed at the solution’s boiling temperature for all the fly ash samples under the following leaching conditions: the heavy oil fly ashes were leached with H 2 SO4 Ž1 M. and LrS 3 mLrg, the Orimulsion fly ashes were leached either with H 2 SO4 Ž2 M. and LrS 4 or with H 2 SO4 Ž3 M. and LrS 3. The extract was separated from the carbonaceous residue by filtration and the residue was washed three times with water Ž150 mLrstage. to recover the extract solution retained by the cake. After the oxidative precipitation, the precipitate was separated from the exhaust solution by filtration and was washed three times with dilute H 2 SO4 with a pH of 2 ŽLrS s 10 mLrg..

Fig. 3. Block diagram of the process.

S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

147

Table 2 Recovery and loss of vanadium during the various stages of the process, and elution of V, Fe, Na in the raw red cake washing for the three heavy oil fly ash samples

Sample

Fly ash leaching Leaching filtrate Carbonaceous residue washing 1st washing filtrate 2nd washing filtrate 3rd washing filtrate

V recovery, wt.%

V loss, wt.%

A

A

B

C

68.1

57.5

26.7

18.8

22.9

49.3

V2 O5 precipitation Residual solution Raw red cake washing 1st washing filtrate 2nd washing filtrate 3rd washing filtrate Washed red cake

63.3

57.8

B

Elution, wt.% of raw red cake content C

0.9 `

1.6 0.1

5.9 0.3

17.2

20.5

25.4

0.9 0.5 0.3 1.7

2.3 0.7 0.3 3.3

2.6 0.5 0.1 3.2

V

Fe

Na

A

B

C

A

B

C

A

B

C

1.4 0.7 0.5 2.6

3.9 1.1 0.6 5.6

5.4 0.9 0.3 6.6

4.5 1.5 0.7 6.7

5.0 1.6 1.6 8.2

6.6 1.1 0.4 8.1

75.7 20.2 1.8 97.7

83.6 9.5 1.1 94.2

82.6 11.4 3.6 97.6

45.4

Table 3 Recovery and loss of vanadium during the various stages of the process, and elution of V, Fe, Na in the raw red cake for the Orimulsion fly ash sample

Leaching

Fly ash leaching Leaching filtrate Carbonaceous residue washing 1st washing filtrate 2nd washing filtrate 3rd washing filtrate

V recovery, %

V loss, %

Elution, wt.% of raw red cake content

1a

1

V

2b

88.5

88.7

6.1

7.7

V2 O5 precipitation Residual solution Raw red cake washing 1st washing filtrate 2nd washing filtrate 3rd washing filtrate

Washed red cake a b

84.2

H 2 SO4 Ž2M. and LrS ratio of 4 mLrg. H 2 SO4 Ž3M. and LrS ratio of 3 mLrg.

82.4

2

0.5 0.3

1.0 0.5

7.3

6.4

1.3 0.6 0.9 2.9

2.1 1.3 1.9 5.3

Fe

Na

1

2

1

2

1

2

1.5 0.7 1.0 3.2

2.4 1.5 2.1 6.0

11.1 0.3 0.2 11.6

27.3 13.7 4.4 45.4

79.5 8.8 2.9 91.2

66.3 16.3 12.0 94.6

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S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

Fig. 4. Precipitation yield versus vanadium concentration.

Tables 2 and 3 report the recovery and loss of vanadium at the various stages of the process for the heavy oil fly ashes and the Orimulsion fly ash, respectively. After leaching, the vanadium in solution retained by the carbonaceous residue increased with the carbon content of the fly ash. In the first stage of washing, most of the vanadium retained was recovered for all the samples, with the exception of the carbonaceous residue C, which released the vanadium retained more gradually in the subsequent washings. The Orimulsion fly ash, characterised by the lowest carbon content, displayed the best leachability. The solution obtained from the first stage of washing was mixed with the extract, and used for the precipitation. The vanadium concentration of the solutions used in the oxidative precipitation, depending upon the vanadium content of the samples and the leaching yield, fell in the range 2.5–6.1 grL for the heavy oil fly ashes, and fell in the range 13.7–18.1 grL for the Orimulsion fly ash. As reported in Tables 2 and 3 and in Fig. 4, the precipitation of vanadium occurred with higher yields from the more concentrated solutions, in agreement with Giorgini et al. w6x. Table 3 also reports the elution of V, Fe, Na in the raw red

cake washing stages. The sodium elution was slightly higher for the heavy oils Ž94.2%–97.7% of Na removed. in comparison with the Orimulsion red cakes Ž91.2%–94.6% of Na removed.. The higher percentTable 4 Composition of the washed red cake Žwt.%. Oil fly ashes

Weight loss at 1058C V Žd.b.. Cu Žd.b.. Co Žd.b.. Ni Žd.b.. Ti Žd.b.. Fe Žd.b.. Na Žd.b.. K Žd.b.. Si Žd.b.. Al Žd.b.. S Žd.b.. P Žd.b.. a b

Orimulsion fly ashes

A

B

C

1

2

69.9

63.8

58.2

62.2

70.7

37.3 n.r.a traces n.r. 0.5 11.3 0.4 n.r. traces 0.1 0.1 0.3

29.5 n.r. traces n.r. 0.2 9.0 0.8 n.r. n.d.b 1.1 0.1 0.5

22.5 n.r. traces n.r. 0.3 19.2 0.3 n.r. 1.1 0 0.2 1.3

40.6 n.r. traces 0.1 0.3 0.5 1.1 n.r. n.r. n.r. n.r. 0.1

43.3 n.r. traces 0.1 0.3 0.2 1.5 n.r. n.r. 1.1 traces 0.1

Not revealed. Not determined.

S. Vitolo et al.r Hydrometallurgy 57 (2000) 141–149

age removal of iron from the Orimulsion red cake may be related to the much lower iron content of the fly ash. The vanadium losses due to the red cake washing were acceptable Žbelow 6.6%.. The composition of the precipitate after washing, reported in Table 4, indicates that by using the Orimulsion fly ash it is possible to obtain a red cake having a higher vanadium content and a much lower percentage of iron in comparison with the heavy oil fly ashes.

4. Conclusions The recovery of vanadium from the fly ashes of heavy oil and Orimulsion combustion was performed using a three-step process, which consisted of an acid leaching, oxidation and precipitation of the vanadium pentoxide, followed by washing of the precipitate. The overall yield of vanadium recovery was higher for the Orimulsion fly ash and must be attributed to the higher yields obtained both in the leaching and in the precipitation stage. By washing the precipitate it was possible to reduce the concentration of the impurities Žtypically Cu, Co, Na, K, S, and P. and to allow its use for the production of ferrovanadium alloys.

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