SOLVENT EXTRACTION OF MOLYBDENUM ( VI ) FROM DILUTED AND CONCENTRATED HYDROCHLORIC ACID SOLUTIONS WITH TRIN-BUTYL PHOSPHATE

The solvent extraction of Mo (VI) from diluted and concentrated HCl solutions with tri-n-butyl phosphate (TBP) has been studied. The percentage Mo (VI) extraction (E%) reaches maximal levels of 49.60% and 51.20% at pH 1.0 and 4.0 respectively, however, it is much higher (E% = 85.60 –90.80%) in concentrated HCl (2.0 – 7.0M) solutions and decreases with temperature. The mechanism of extraction appears to be through the formation of condensed molybdic acid n(MoO3.2H2O). mTBP at low pH but involves molybdenyl species MoO2Cl2. (H2O) 2. mTBP in concentrated acid medium and pH 4-6. Electronic and infrared spectra data have been used to deduce the nature of the extracted species.


INTRODUCTION
Molybdenum has extensive industrial applications and is regarded as one of the vital strategic metals in modern technology.An outstanding application of this element is in the manufacture of anticathode for X-ray tubes.Molybdenum wire is used extensively for the support of tungsten filament, and also as a component of hydrodesulphurization catalyst employed in the petrochemical industry.Due to molybdenum's widespread use this natural resource has become depleted and therefore, its liquid-liquid extraction and purification from ores and very recently wastes which avoids a sole dependence on primary resources that would impact nature by increasing the dispersion of the metal in ecosystems, is regarded to be of utmost importance.(Saberyan et al, 2004;Wernick and Themelis, 1998;Spevack and McIntyre, 1993).
The extraction of molybdenum (VI) from aqueous medium containing hydrochloric, nitric and sulphuric acids using organophosphates or organophosphoric acid has been successfully effected (Kolarik, 1973;Cruywagen and Mckay 1970;Hirai et al, 1995).
Chelating extractants such as 8-hydroxyquinoline and hydroxiquinaldine have also been used to recover molybdenum as cationic complexes from weakly acidic solutions (Trujillo, 1987).
Considerable success has also been recorded in the use of anionic extractants to recover molybdenum either isolatedly or from multimetal solutions (Hirai et al, 1993;Zhang et al, 1997;Cosar and Ziyadangullari, 1998;Chen et al, 2006).Because most of the works we have found in literature have been done at millimolar (10 -3 M) or lower molybdenum concentration levels due to complexities in the nature of the polymeric or condensed species formed at high concentrations, we have nevertheless carried out this work at high Mo(VI) concentration that simulated typical industrial conditions.In the present study, the recovery of molybdenum (VI) from diluted and concentrated HCl solutions with tri-n-butyl phosphate (TBP) dissolved in n-hexane is presented.

Materials
The chemicals, MoO 3 , HCl, NaOH, TBP and di (2ethylhexyl) phosphoric acid (D2EHPA) were obtained from the British Drug House Limited (BDH).All the chemicals are of analytical grade except TBP and D2EHPA and were used without further purification.

Preparation of 0.025 M Mo (VI) stock solutions
Molybdenum (VI) oxide (3.60 g, 0.025 mol) and KCl (7.48 g, 0.1 mol) were dissolved in fairly concentrated NaOH and finally made up to the 1-litre mark by adding HCl solution.The adjustment of pH was effected by adding appropriate amounts of NaOH or HCl.

Extraction and Analytical Procedure
Different concentrations of TBP and TBP-D2EHPA mixtures were prepared by mixing appropriate amounts of the extractants with n-hexane.15 cm 3 portions of TBP or the mixed extractants were equilibrated with 15 cm 3 portions of Mo (VI) solutions (preliminary experiments have shown that variation in Mo(VI) concentration between 0.025 M-0.01M have no significant effect on percentage Mo(VI)extraction) at the appropriate pH and temperature in a separating funnel, swirled manually and later on a mechanical shaker operated at 120 rev/min for 3mins (found optimum for equilibration).The layers were allowed to separate, the lower aqueous layer ran into a conical flask and the upper organic layer removed from the top of the separating funnel to avoid contamination.The amount of molybdenum extracted was determined by spectrophotometry using ultraviolet -visible spectrophotometer.Biochom 4060 model, which involves measuring the absorbance of the red complex formed between Mo(VI) and thiocyanate ions in isopentyl alcohol at 465 nm.The electronic spectra was recorded on the same instrument while the infrared spectra was recorded on infrared spectrophotometer, Buck scientific model 500.The observation of different peaks and linear portions on the curve suggest that different Mo (VI) species were involved in the extraction.(Cruywagen and Mckay,1970).The percentage Mo (VI) extraction (E%) reaches maximal levels of 49.60% and 59.20% at pH of 1.0 and 4.0 respectively, and falls subsequently.This could probably be explained by the mechanism below.
The molybdenyl cation may then combine with Cl -ion to form extractable neutral chloro complex according to equation 3.
Since a low amount of Mo (VI) is extracted at the pH region, it could be infered that the reaction expressed by equation 1 is more(that is MoO 2 2+ is available in low concentration) favoured than equation 2.
At pH 3.5-4.0,the reactions will probably be The MoO 2 2+ will be converted to the neutral complex as expressed by equation 3.Both the neutral complex and molybdic acid can subsequently be extracted by TBP.Further works were carried out at pH of 1.0 because of the inherent little risk of hydrolysis and a probable less interference from anionic species.The quantitative extraction observed at 7.0 M HCl might be due to the ease with which MoO 2 2+ is formed at high acid concentration and the large presence of Cl -ions required for the formation of an extractable complex.The complex might be further solvated by HCl molecules thereby increasing its extractability.

Effect of TBP concentration
The effect of TBP concentration on percentage molybdenum (VI) extraction is presented in Table 1 and Fig. 3.A dded D2EHPA

Effect of Temperature
The effect of temperature on the distribution coefficient of Mo (VI) is presented in Fig. 6.The percentage Mo(VI) extraction (as presented in Table 2) and distribution ratio both decrease with increase in temperature, suggesting an exothermic extraction process and a possible Mo(VI) overall mass transfer rate that is diffusion controlled.The E% varies from 49.60% at 301 K to 28.01% at 321K while distribution coefficient (D) at both temperatures are 0.984 and 0.389 respectively.

Electronic and Infrared Spectra
The electronic spectra for the TBP extract of Mo(VI) are shown in Table 3.The absorption bands observed at 44444 cm -1 , 40000 cm -1 and 30769 cm -1 have been assigned to ligand -metal -charge transfer (LMCT) of condensed molybdic acid while the band at 28571 cm -1 probably involves the molybdenyl species undergoing LMCT (Murata and Ikeda, 1970).The band at 1278 cm -1 assigned to ν (P=O) vibration in the ligand shifts to 1271cm -1 in the complex while the ν (P-O-C) band at 1028 cm -1 shifts to 1034 cm -1 in the complex indicating the involvement of the phosphoryl group in coordination.
The weak band observed at 990 cm -1 in the complex and which is absent in the ligand has been assigned to ν ( Mo=O) vibration .And finally, the presence of a very broad band at 3447 cm -1 assigned to ν (O-H) vibration confirms coordination or solvation by water molecules.(Ojo, 2003).

CONCLUSIONS
The extraction of Mo (VI) at moderately high concentration employed in this work poses no hinderance to its extraction and could be employed at any scale.The extraction of Mo(VI) from HCl with TBP could be carried out at the pH of 1.0 and 4.0 or concentrated HCl to attain a high extraction efficiency, but heating should be discouraged, since the extraction efficiency decreases with increase in temperature.Inspite of the higher E% (85.60-90.80%)obtained for Mo(VI) in concentrated HCl than dilute solutions (pH 1.0 and 4.0), we would recommend its large scale extraction at pH 1.0 because of the risk of TBP degradation already reported at high HCl concentration and more so when the E % (49.60) at pH 1.0 is high enough for a counter-current large scale process.The possible absence of anionic products serves also as an added advantage over pH of 4.0.

FIG. 6 :
FIG. 6: Effe ct of Variation of Te m pe rature on Dis tributionCoe fficie nt of M o (VI)

Table 1 :
Effect of TBP concentration on percentage Mo(VI)

Table 3 :
The electronic spectra data (cm -1 ) and assignment for the complexes.The infrared spectra of the free TPB ligand and TPB-Mo (VI) extract are shown in Table4.

Table 4 :
The infrared spectra data (cm -1 ) and Assignments for TBP and complexes.