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Leaching of synthetic scheelite by hydrochloric acid without the formation of tungstic acid
Hydrometallurgy 70 (2003) 131 – 141 www.elsevier.com/locate/hydromet
Leaching of synthetic scheelite by hydrochloric acid without the formation of tungstic acid J.I. Martins a,b,*, A. Moreira a, S.C. Costa a a
Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal b CISE, Departamento de Engenharia Electrote´cnica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal Received 8 November 2002; received in revised form 22 January 2003; accepted 5 March 2003
Abstract The kinetics of the leaching reaction of synthetic scheelite with hydrochloric acid was studied to determine the effect of pH and temperature in the formation of tungstic acid. Conversion data obtained from leaching in a temperature range of 28 – 100 jC and pH between 1.5 and 3.0 are interpreted in terms of the predominating tungsten species present in the leaching liquors. The results show that it is possible to digest the synthetic scheelite in the acidic hydrochloric medium without the formation of tungstic acid by monitoring and controlling the pH, and controlling the temperature in the domain of metatungstates. In this case, the experimental results match with the shrinking core model, and show that the solids leaching is kinetically under both chemical and mass transfer control. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Synthetic scheelite; Acid leaching; Isopolytungstates; Tungstic acid; Kinetics
1. Introduction Synthetic scheelite is an artificial product resulting from the interactions of sodium tungstates with calcium salts or calcium solutions. Historically, the production of this compound is related with the tungsten recovery from alkaline leaches produced by the digestion of wolframite concentrates or low-grade scheelite concentrates, respectively, by sodium hydroxide or sodium carbonate.
* Corresponding author. Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal. E-mail address: [email protected] (J.I. Martins).
In order to minimize tungsten losses in the physical beneficiation process, the mining industry is forced to produce less pure concentrates. It becomes apparent that intermediate purification has to be introduced in the global process flow sheet for the production of tungsten. As can be seen in Fig. 1 (Martins, 1983), synthetic scheelite appears naturally as one of the routes to refine the alkaline solutions coming from the leaching of tungsten concentrates. Pure tungstic acid can be then obtained from the leaching of synthetic scheelite by hydrochloric acid according to the following reaction: k0
CaWO4ðsÞ þ 2HC1ðaqÞ V H2 WO4ðsÞ þ CaCl2ðaqÞ
0304-386X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-386X(03)00053-7
k0V
ð1Þ
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Fig. 1. Generalized process flow sheet for the tungsten production.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
for which the equilibrium constant, K=[CaCl 2 ]/ [HCl]2, has the value of 104 and 1.5 104, respectively, at 20 and 100 jC (Meerson and Khavskii, 1961). Such values would suggest a high decomposition rate of the reaction. Conversely, the tungstic acid developed over the solid particles hinders the transport of hydrochloric acid inside and removal of the products out of the unreacted core and slowing down the leaching rate. The system stays under diffusion control. Some strategies are usually adopted to increase the reaction rate and consequently the corresponding yield: (a) working with fine particles ( < 44 Am) (Li and Wang, 1947; Fieberg and Coetzee, 1986), despite the grinding costs; (b) using a large excess of concentrated hydrochloric acid (200 – 350% of the stoichiometric amount) (Vezina et al., 1970; Yih and Wang, 1979), which requires expensive acid-resistant equipment independently of waste treatment costs; (c) utilizing heated ball mill reactors for desegregation of the H2WO4 ash layer, although with considerable difficulties in the manufacturing considering the corrosion by HCl and abrasive action of the grinding balls at relatively high temperature (Meerson and Khavskii, 1961; Mikhailov et al., 1985; Zelikman, 1985; Li et al., 1986); (d) promoting soluble compounds during the acid leaching with complex reagents, such as oxalic, tartaric, citric and phosphoric acids (General Electric, 1959; Hubbard et al., 1962; Xuin et al., 1986; Gu¨rmen et al., 1999); (e) the use of alcoholic solutions to solubilise the produced H2WO4 during the leaching (Forward and Vizsolyi, 1965; Dow Chemical, 1968). The oldest industrial process to produce ammonium paratungstate (APT) uses acid leaching as a method to digest the natural or synthetic scheelite. The process is described as follows: The raw material with particle size equal to or lower than 74 Am is charged to a digester vessel consisting of a steel shell lined with rubber. Commercial HCl is added at about twice or threefold the amount required by the stoichiometry of the leaching reaction. An amount of sodium nitrate at a rate of 0.05% of calcium tungstate is also added as an oxidizing agent, and the total mixture is agitated and maintained at 70 jC for 12 h or at 90 –100 jC for 5 h by steam jacket. The slurry is then diluted and allowed to settle. Nine washes are performed by
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settling and decantation in steel shell tanks with rubber lining, the first seven at 70 jC (from the third washes 1% HCl is used) and the last two at ambient temperature. The slurry of tungstic acid is then digested with approximately 100% excess of 28% ammonia at 60 jC for 2 h. The system generates a pressure (6 psi), so that the vessel must be closed to avoid loss of ammonia. Filtration of the final liquor separates the undissolved ore or gangue from the ammonium paratungstate solution. This solution is tested for calcium, the specific gravity fixed at about 1.26 or 1.12, respectively, according to Yih and Wang (1979) and Smithels (1936), and tungsten is recovered as pure APT by evaporation of solution. The operation continues until the specific gravity of the solution falls to the range 1.05 –1.07. The crystals are allowed to settle, the mother liquor decanted, the solids washed with water using the settling-decantation method, and then dried at 120 – 135 jC. The aim of this work is to show that by monitoring the pH of the reaction and controlling the temperature it is possible to leach the synthetic scheelite by hydrochloric acid without the formation of tungstic acid precipitate.
2. Chemistry of tungstates in solution Many of the chemical difficulties associated with understanding of the process in which tungsten in solution is extracted from its ores are related to the fact that the solution chemistry of tungsten is extremely complex. Upon acidification of a solution containing the ion WO42 , polyanions develop very quickly and the velocity increases remarkably with concentration (Souchay et al., 1972). One way of rationalising the formation of isopolytungstates is to regard them as being produced by the addition of acid to the orthotungstate ion. The general equation can be written as follow: ð2npÞ
þ nWO2 4ðaqÞ þ pHðaqÞ X Hx Wn O4n1=2ðpxÞðaqÞ
þ 1=2ðp xÞH2 OðaqÞ
ð2Þ
It deserves to be mentioned that some protons act as central atoms and cannot be exchangeable.
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Besides the common tungstates, the paratungstates 5 ( p/n = 7:6, HW6O21 ) and metatungstates ( p/n = 3:2, 6 H2W12O40 ) have been known for a long time. Since the tungstic acid is obtained only for more acidic solutions ( p/n = 2:1), it provides a suggestion to perform the leaching reaction of synthetic scheelite avoiding its formation by controlling the pH below this value. Thus, the reaction rate remains the same throughout leaching, in contrast to the classic leaching by hydrochloric acid.
t ¼ 1 ð1 X Þ2=3 s
ð6Þ
with s¼
qs R2 2DCA0
ð7Þ
If the chemical relation controls the reaction, ksbD, then Eq. (3) becomes:
3. Noncatalytic solid –liquid reaction In the desired solid –liquid reaction, both products are soluble (calcium chloride and isopolytungstates) and so the solid particles become smaller during the leaching. Therefore, the reaction surface moves from outside towards the centre and is progressively reduced. The shrinking core model (Levenspiel, 1972) seems to be a suitable mathematical model to describe the kinetics of the process. Considering the approximation to the pseudostationary state under laminar regime, the evolution of the particle size with time for first- and zeroorder reactions, respectively, to the leaching agent and to the solid particles, is given by the following relation: t bks R2 ¼1 ð1 X Þ2=3 s R2 bks þ 2DR 2DR ð1 X Þ1=3 R2 bks þ 2DR
ð3Þ
or, t ¼ 1 Fm ð1 X Þ2=3 Fc ð1 X Þ1=3 s
ð4Þ
t ¼ 1 ð1 X Þ1=3 s
ð8Þ
with s¼
qs R bks CA0
ð9Þ
4. Experimental details 4.1. Materials The synthetic scheelite was supplied by Instituto Geolo´gico Mineiro. The reagents used in the leaching experiments were commercial grade. The reference solutions of isopolytungstates for UV/VIS analysis were prepared with reagent-grade ammonium metatungstate hydrate and ammonium paratungstate, obtained from Fluka. Chemical analysis by atomic absorption spectroscopy of raw material shows a content of 78% WO3 and 1% of Ca(OH)2. Particle size distribution analysis of synthetic scheelite shows a size range between 1 and 8 Am with an average size of about 4 Am. 4.2. Equipment
where 2
bks R ; s þ 2DR q R2 qR s¼ s þ s 2DCA0 CA0 bks Fm ¼
If the fluid film resistance controls the reaction rate, Dbks, then Eq. (3) becomes:
R2 bk
Fc ¼
2DR ; s þ 2DR
R2 bk
and ð5Þ
The equipment used consisted mainly of a fourneck Pyrex Quickfit vessel (Fig. 2) with a heating jacket, containing the leaching solution stirred by a magnetic agitator. A spiral condenser was utilized to eliminate the water losses from the system. A set of electrodes was used to measure the pH of the solution.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
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Fig. 2. Diagram of experimental equipment. A—Pyrex Quickfit vessel; B—thermostatic bath; C—magnetic stirrer; D—double electrode (CTWL/S7); E—pH meter (Crison 2002); F—microburette (Crison BU 2031); G—HCl solution; H—computer (Amstrad PC 2086/30); I— condenser; J—thermometer.
The glass electrode was connected to a pH meter (Crison 2002) controlled by computer, which ordered a microburette (Crison BU 2031) to add the amount of acid required to keep the pH at a preset value. The acid consumption during the reaction tends to increase the pH of the solution. UV spectroscopy (JASCO V-560 UV/VIS Spectrophotometer) was used as support to obtain information about the isopolytungstates present in leaching solutions.
preset pH value. A 3 M hydrochloric acid solution was used to control the pH along the reaction.
5. Results The plotting of the conversions of WO3 vs. time, for the pH range between 1.5 and 3, and temper-
4.3. Procedure Leaching tests were performed using 180 mL of the hydrochloric acid solution at a preset pH value in a 300-mL vessel with magnetic stirring and heating to the required temperature. When the conditions were attained, the reaction was initiated with the addition of 30 g of synthetic scheelite. Samples (15 mL) were quickly collected and filtered with Whatman paper no. 42 at selected intervals and then 10 mL of the solution was taken for WO3 analysis by the cinchonin (Li and Wang, 1947) method. The volume of the solution was maintained constant by the addition of the same volume of HCl solution at the
Fig. 3. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 1.5.
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Fig. 4. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 1.9.
Fig. 6. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 2.5.
atures between 28 and 100 jC, is presented in Figs. 3– 7. The results show that if the temperature is equal to or higher than 85 jC, the formation of tungstic acid is induced, since the yields of WO3 are lower than those obtained at 80 jC. In these cases the reaction will be controlled by the diffusion through the ash layer of tungstic acid. However, this solid is partially converted into soluble tungsten compounds (isopolytungstates), as is inferred by the contents of WO3 obtained by chemical analysis of the filtered acidic solutions. Indeed, as an example, the filtered solution analysed for the experiment performed at pH = 2.2 and 85 jC gives a concentration of about 67 g/L of WO3 after 7 h of leaching. Meanwhile, the results of El Wakkad and Rizk (1956) show that the maximum solubility for the
tungstic acid in the range of 0.2 –0.3 M of hydrochloric acid is only about 0.23 g/L, which confirms our last assertion. Increasing the temperature from 80 jC and the hydrochloric acid concentration the tungsten content in the acidic solutions is reduced, unless there is a break in the tungstic acid film (curve at 90 jC and pH = 1.9). According to published results, isopolytungstates in acidic solutions decompose on heating giving tungstic acid (Busev et al., 1976; Palant et al., 1980). The leaching performed at 100 jC and pH = 2.5 shows a decrease of tungsten content in the leach from 19.3% to 4.6% after the first 3 h. This is associated with the homogeneous reaction of isopolytungstate to yellow
Fig. 5. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 2.2.
Fig. 7. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 3.0.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
137
Fig. 8. The ratio of the acid moles to tungsten moles during the reaction time: (a) T = 70 jC, pH = 1.9; (b) T = 80 jC, pH = 1.5; (c) T = 80 jC, pH = 1.9; (d) T = 85 jC, pH = 2.2; (e) T = 85 jC, pH = 3; (f) T = 90 jC, pH = 1.9; (g) T = 90 jC, pH = 2.2; (h) T = 90 jC, pH = 2.5; (i) T = 90 jC, pH = 3; (j) T = 100 jC, pH = 2.5.
tungstic acid. From this point onwards, leaching the tungsten content in the acidic solution is at constant rate, which means a reaction blocking. We have observed in the trials that the microburette stopped the addition of acid. So, we conclude that the lower acid concentration cannot promote the diffusion of hydrogen ion through the yellow tungstic acid. Fig. 8 shows the ratio of the acid moles to tungstate moles vs. time for some of the experiments. This ratio has the tendency to assume initially a value of the order of 2, which is consistent with the production of a compound with the chemical formula H2WO4. The high p/n ratios can be explained from the fact that we have analysed only the WO3 content present in the solution. So, not taking into account the WO3 in the form of tungstic acid formed on the solid particles or in
the solution, the p/n ratio increases and more significantly for the lower yields. Depending on temperature and hydrochloric acid concentration, the reaction can proceed with the formation of the structural lattice of the solid tungstic acid ( p/n z 2), or with the formation of metatungstates (1.5 V p/n V 1.7). El Wakkad and Rizk (1957) have found it necessary to add 1.8 mol HCl/mol WO42 to form the metatungstate ion in the case of 0.10 M sodium tungstate solutions, and 1.9 mol in the case of 0.005 M solutions. At the considered instants, the plotting of logarithm of the rate of the reaction as a function of logarithm of concentration allows the determination of the reaction order, while the Arrhenius plots give the activation energy. The apparent reaction order for the hydrochloric acid and the process activation energy calcu-
Table 1 Apparent order reaction (n), at temperatures of 70 and 80 jC and different reaction times, for the leaching of synthetic scheelite by hydrochloric acid
Table 2 Apparent activation energy (kJ/mol), at pH = 1.5 and 2.2 and different reaction times, for the leaching of synthetic scheelite by hydrochloric acid
Temperature (jC)
pH
70 80
Reaction time (h) 1
2
3
4
0.75 0.84
0.84 0.94
0.82 0.94
0.88 0.96
1.5 2.2
Reaction time (h) 1
2
3
4
23 –
19 22
17 27
16 28
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lated for the experiments results performed at temperatures of 70 and 80 jC are shown, respectively, in Tables 1 and 2. The values suggest a first-order reaction to the leaching agent and a process that falls within the range of rate control by both reaction and mass transfer. Mass transfer control has its origin in the stirring system used. Indeed, the imposed stirring rate was only enough to avoid the deposition of solid particles on the bottom of the reactor vessel.
temperature on the reaction is given by the slope of the straight lines; its increasing value corresponds to a reduction in time for the total conversion. This good correlation implies that the effective diffusivity of the leaching agent has to have an order of 5 10 13 m2/s. This value is relatively small regarding the usual order of magnitude expected for ionic species in aqueous media. However, we believe that is possible to justify it considering the following mechanism for the reaction:
6. Discussion
1. The HCl begins to attack the solid particles of synthetic scheelite by the adsorption of the hydrogen ion on its surface:
At the initial moments of the leaching, it is possible to accept that the system is only under chemical reaction control. Then, the rate constant of the reaction can be calculated by the expression resulting from the differentiation of Eq. (8): ks ¼
qs R dX ð1 X Þ2=3 3bCA0 dt
ð10Þ
Using the experimental results of Meerson and Khavskii (1957), the rate constant of the acid reaction at 80 jC is of the order of 10 5 cm/s. The Fm/ Fc ratio practically does not change in the temperature range of 40– 80 jC, considering the average activation energy calculated (22 kJ/mol) for the reaction. Thus, taking the value of 0.2 for Fm/Fc ratio and plotting the function [1 Fm(1 X)2/3 Fc(1 X)1/3] vs. t, for results at pH = 1.9, a linear relationship is obtained (Fig. 9). The influence of
k1
þ SolidðsÞ þ Hþ ðaqÞ V Solid HðadsÞ k1V
2. The next step will be the calcium ion desorption.
k2
2þ Solid Hþ ðadsÞ V Solid HWO4ðadsÞ þ CaðaqÞ k2V
3. A new hydrogen ion bonding of anionic ions adsorbed on the solid surface:
k3
þ Solid HWO 4ðadsÞ þ HðaqÞ V Solid H2 WO4ðadsÞ k3V
4. The reaction for concentrated acid solutions (pH V 1.2), according to Kim et al. (1986), should proceed with the formation of the structural lattice of tungstic acid on the solid particles:
k4
Solid H2 WO4ðadsÞ V Solid H2 WO4ðsÞ k4V
Fig. 9. Application of the shrinking core model to the leaching of synthetic scheelite with hydrochloric acid at pH = 1.9 in the temperature range of 40 – 80 jC.
Our results show that this same reaction also happens when temperatures are equal to or higher than 85 jC, in the range of pH between 1.5 and 3.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
Outside the reported domain for the formation of solid H2WO4, the next steps of leaching would be regarded as a process accompanied by multiple reac-
139
tions according to the following scheme (Szarvas and Kurki, 1960):
ψ
Intermediates species before the formation of paratungstate A, [W 2 O 7 (OH)] 3 , [W 4 O 12 (OH) 4 ] 4 , [HW4O12(OH)4]3 , [W6O20(OH)2]6 (Cordis et al., 1975), would be present at all times, but their concentrations are believed to be low. However, there is some controversy between the mechanism schemes suggested by Szarvas and Kurki (1960) and Cordis et al. (1975). Effectively, the second one considers that paratungstate A gives after some minutes the c-metatungstate and only after some weeks is the metatungstate formed. Our results have been analysed by UV/VIS and, as an example, the spectrum recorded at pH = 1.9 at 70 jC after 1.5 h of leaching is put in comparison with the spectra obtained from paratungstate and meta-
tungstate solutions (Fig. 10). We observe in the curve a peak at about 256 nm identical to the metatungstate solution, while the paratungstate solution has one only at 318 nm. Thus, this result supports the mechanism scheme of Szarvas and Kurki (1960). The proposed mechanism for the digestion of synthetic scheelite by hydrochloric acid shows a sequence of multiple reactions where the hydrogen ion is involved in homogeneous and heterogeneous reactions. Indeed, these reactions occur on and near the boundary of the solid particles. Thus, it is reasonable to consider the film surrounding the solid particles as a ‘‘packed bed’’ with a small permeability, which is responsible for the lower effective diffusion of hydrogen ion. At present we are developing research on natural scheelite, considering the advantages using this new technique of acid leaching in the industry.
7. Conclusions
Fig. 10. Ammonium metatungstate hydrate (A) and ammonium paratungstate reference spectra (B) with 3.87 10 5 and 9.87 10 5 M concentrations, respectively. Solution spectrum corresponding to the experiment performed at pH = 1.9 and 70 jC after 1.5 h of leaching with a dilution of 1:1500 (C).
The leaching of synthetic scheelite by hydrochloric acid is strongly dependent on pH and temperature. The main conclusions for monitoring the pH (1.5 – 2.2) in the domain of metatungstates formation and temperature control (70 – 80 jC) of the synthetic scheelite leaching by hydrochloric acid are the following: 1. Decrease of leaching agent consumption to about 20%, since the production of metatungstates ions
140
2. 3. 4.
5. 6. 7.
8.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
( p/n = 1.5 – 1.7) occurs instead of solid tungstic acid ( p/n = 2), but much more a 20% if we consider the excess commonly used to overcome the resistance imposed by the ash of tungstic acid (about 200%). Decrease of the reaction temperature from the usual value in the traditional acid leaching of 100 to 80 jC. Decrease of the reaction time to 45 min (at pH = 1.5 and T = 80 jC). Use of test equipment material without rubber lining that reduces the heating transfer global coefficient, because the level of the chloride ions (1.15 g/L) enables the use of a titanium alloy ATi 24 (Avesta, 1972) or a 42% Ni – 21.5% Cr alloy with copper and molybdenum (Polar, 1961). Elimination of the solid –liquid separation costs, since the yellow H2WO4 is not produced. Reduced costs of waste treatment. Reduced costs of APT production, since a smaller amount of ammonium hydroxide is used to set the solution in the suitable pH range to initiate the crystallization. Leaching operation amenable for continuous operation.
List of symbols b stoichiometric factor of solid particle in the reaction C fluid concentration (kmol/m3) D effective diffusivity of fluid (m2/s) E energy activation (kJ/mol) k specific reaction rate for surface reaction (m/s) R initial radius of spherical particle (m) t reaction time (s) s time for the total conversion (s) T temperature (K) X fractional degree of conversion qs molar density of solid reactant (kmol/m3) Fm coefficient of mass transfer Fc coefficient of chemical reaction Subscripts aq aqueous solution A0 initial liquid reactant s solid particle; interface between liquid and solid
Acknowledgements Funding from Fundacß a˜o para a Cieˆncia e a Tecnologia, POCTI/36287/ECM/1999, is gratefully acknowledged.
References Avesta, J.A., 1972. Tableaux de corrosion pour l’acier inoxydable et le titane. Carlson Press Boktryckeri, Stockholm. Busev, A.I., Ivanov, V.M., Sokolova, T.A., 1976. Analiticheskaya khimiya vol’frama Izd. Nauka, Moscow. ¨ ber isopolywolframatCordis, V., Tytko, K.H., Glemser, O., 1975. U feststoffe und deren beziehung zn ispolywolframationen in waßringer lo¨sung. Z. Naturforsch. 306 (11/12), 834 – 841. Dow Chemical, 1968. Process for recovery of tungsten values, London Patent 1,137,518. El Wakkad, S.E.S., Rizk, H.M., 1956. The solubility of tungsten trioxide in hydrochloric acid. J. Chem. Soc. 3248. El Wakkad, S.E.S., Rizk, H.M., 1957. The polytungstates and the colloidal nature and the amphoteric character of tungstic acid. J. Chem. Soc. 61, 494 – 497. Fieberg, M.M., Coetzee, C.F.B., 1986. The recovery of tungsten from South African low-grade scheelite concentrates. MINTEK, report no. M164D, published by the Council for Mineral Technology, pp. 1 – 22. Forward, F.A., Vizsolyi, A.I., 1965. Process for the production of tungstic acid. U.S. Patent 3,193,347. General Electric, 1959. London Patent 818,117. Gu¨rmen, S., Timur, S., Arshn, C., Duman, I., 1999. Acid leaching of scheelite concentrate and production of heteropoly-tungstate salt. Hydrometallurgy 51, 227 – 238. Hubbard, H.M., Acres, G., Kennedy, J.H., 1962. Process for the extraction of tungsten values from materials containing same. U.S. Patent 3,047,361. Kim, T.K., Mooney, R.W., Chiola, V., 1986. Study of soluble tungstate species by solvent extraction. Sep. Sci. 3 (5), 476 – 478. Levenspiel, J.A., 1972. Chemical Reaction Engineering, 2nd ed. Wiley, New York. Li, K.C., Wang, C.Y., 1947. Tungsten, 2nd ed. Reinhold Publishing, New York. Li, H., Lin, M., Shi, Z., Dai, C., He, Y., Li, J., He, K., 1986. Faming Zhuanli Shengning Gongkai Shuomingshn CN 85,100,350. Martins, J.I., 1983. The hydrometallurgy in the valorisation of tungsten ores. PhD thesis, FEUP/Univ. Porto. Meerson, G.A., Khavskii, N.N., 1957. Treatment acid of scheelite. Tsvet. Met. 30 (3), 41 – 47. Meerson, G.A., Khavskii, N.N., 1961. Hydrometallurgic heterogeneous reactions happened during the grinding. Izv. Akad. Nauk SSSR, Otd. Tekh. Nauk, Metall. Topl. 5, 36 – 42. Mikhailov, V.A., Pomoshmikov, E.E., Vosse’l, S.V., Marova, I.V., Vasenin, N.T., Makarov, E.P., Eremenko, S.I., Dulepov, E.V., Amfrienko, E.V., 1985. Investigation of physical and chemical
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141 properties of mechanically-activated tungsten concentrates. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 4, 29 – 34. Palant, A.A., Reznichenko, V.A., Nikiforova, S.E., 1980. Influence of temperature on the extraction of tungsten (VI) by amines. Russ. J. Inorg. Chem. 25 (3), 480 – 482. Polar, J.P., 1961. A Guide to Corrosion Resistance. Climax Molybdenum, New York. Smithels, J.C., 1936. Tungsten: A Treatise on its Metallurgy Properties and Applications, 2nd ed. Chapman & Hall, London. Souchay, P., Boyer, M., Chauveau, F., 1972. Advances in the solution chemistry of tungstates. Trans. R. Inst. Technol. Stockh. 259, 160 – 172.
141
¨ ber die kinetic der bildung von Szarvas, V.P., Kurki, E.Cs., 1960. U polywolframaten. Z. Anorg. Allg. Chem. 305, 55 – 63. Vezina, J.A., Gow, W.A., Lalibert, J.J., 1970. Continuous recovery of tungstic trioxide. European Patent 836,441. Xuin, G.H., Yu, D.Y., Su, Y.F., 1986. Leaching of scheelite by hydrochloric acid in the presence of phosphate. Hydrometallurgy 16, 27 – 40. Yih, S.W.H., Wang, C.T., 1979. Tungsten: Sources, Metallurgy, Properties, and Applications, 2nd ed. Chapman & Hall, London. Zelikman, A.N., 1985. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 4, 23 – 29.
Leaching of synthetic scheelite by hydrochloric acid without the formation of tungstic acid J.I. Martins a,b,*, A. Moreira a, S.C. Costa a a
Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal b CISE, Departamento de Engenharia Electrote´cnica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal Received 8 November 2002; received in revised form 22 January 2003; accepted 5 March 2003
Abstract The kinetics of the leaching reaction of synthetic scheelite with hydrochloric acid was studied to determine the effect of pH and temperature in the formation of tungstic acid. Conversion data obtained from leaching in a temperature range of 28 – 100 jC and pH between 1.5 and 3.0 are interpreted in terms of the predominating tungsten species present in the leaching liquors. The results show that it is possible to digest the synthetic scheelite in the acidic hydrochloric medium without the formation of tungstic acid by monitoring and controlling the pH, and controlling the temperature in the domain of metatungstates. In this case, the experimental results match with the shrinking core model, and show that the solids leaching is kinetically under both chemical and mass transfer control. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Synthetic scheelite; Acid leaching; Isopolytungstates; Tungstic acid; Kinetics
1. Introduction Synthetic scheelite is an artificial product resulting from the interactions of sodium tungstates with calcium salts or calcium solutions. Historically, the production of this compound is related with the tungsten recovery from alkaline leaches produced by the digestion of wolframite concentrates or low-grade scheelite concentrates, respectively, by sodium hydroxide or sodium carbonate.
* Corresponding author. Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Oporto, Portugal. E-mail address: [email protected] (J.I. Martins).
In order to minimize tungsten losses in the physical beneficiation process, the mining industry is forced to produce less pure concentrates. It becomes apparent that intermediate purification has to be introduced in the global process flow sheet for the production of tungsten. As can be seen in Fig. 1 (Martins, 1983), synthetic scheelite appears naturally as one of the routes to refine the alkaline solutions coming from the leaching of tungsten concentrates. Pure tungstic acid can be then obtained from the leaching of synthetic scheelite by hydrochloric acid according to the following reaction: k0
CaWO4ðsÞ þ 2HC1ðaqÞ V H2 WO4ðsÞ þ CaCl2ðaqÞ
0304-386X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-386X(03)00053-7
k0V
ð1Þ
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Fig. 1. Generalized process flow sheet for the tungsten production.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
for which the equilibrium constant, K=[CaCl 2 ]/ [HCl]2, has the value of 104 and 1.5 104, respectively, at 20 and 100 jC (Meerson and Khavskii, 1961). Such values would suggest a high decomposition rate of the reaction. Conversely, the tungstic acid developed over the solid particles hinders the transport of hydrochloric acid inside and removal of the products out of the unreacted core and slowing down the leaching rate. The system stays under diffusion control. Some strategies are usually adopted to increase the reaction rate and consequently the corresponding yield: (a) working with fine particles ( < 44 Am) (Li and Wang, 1947; Fieberg and Coetzee, 1986), despite the grinding costs; (b) using a large excess of concentrated hydrochloric acid (200 – 350% of the stoichiometric amount) (Vezina et al., 1970; Yih and Wang, 1979), which requires expensive acid-resistant equipment independently of waste treatment costs; (c) utilizing heated ball mill reactors for desegregation of the H2WO4 ash layer, although with considerable difficulties in the manufacturing considering the corrosion by HCl and abrasive action of the grinding balls at relatively high temperature (Meerson and Khavskii, 1961; Mikhailov et al., 1985; Zelikman, 1985; Li et al., 1986); (d) promoting soluble compounds during the acid leaching with complex reagents, such as oxalic, tartaric, citric and phosphoric acids (General Electric, 1959; Hubbard et al., 1962; Xuin et al., 1986; Gu¨rmen et al., 1999); (e) the use of alcoholic solutions to solubilise the produced H2WO4 during the leaching (Forward and Vizsolyi, 1965; Dow Chemical, 1968). The oldest industrial process to produce ammonium paratungstate (APT) uses acid leaching as a method to digest the natural or synthetic scheelite. The process is described as follows: The raw material with particle size equal to or lower than 74 Am is charged to a digester vessel consisting of a steel shell lined with rubber. Commercial HCl is added at about twice or threefold the amount required by the stoichiometry of the leaching reaction. An amount of sodium nitrate at a rate of 0.05% of calcium tungstate is also added as an oxidizing agent, and the total mixture is agitated and maintained at 70 jC for 12 h or at 90 –100 jC for 5 h by steam jacket. The slurry is then diluted and allowed to settle. Nine washes are performed by
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settling and decantation in steel shell tanks with rubber lining, the first seven at 70 jC (from the third washes 1% HCl is used) and the last two at ambient temperature. The slurry of tungstic acid is then digested with approximately 100% excess of 28% ammonia at 60 jC for 2 h. The system generates a pressure (6 psi), so that the vessel must be closed to avoid loss of ammonia. Filtration of the final liquor separates the undissolved ore or gangue from the ammonium paratungstate solution. This solution is tested for calcium, the specific gravity fixed at about 1.26 or 1.12, respectively, according to Yih and Wang (1979) and Smithels (1936), and tungsten is recovered as pure APT by evaporation of solution. The operation continues until the specific gravity of the solution falls to the range 1.05 –1.07. The crystals are allowed to settle, the mother liquor decanted, the solids washed with water using the settling-decantation method, and then dried at 120 – 135 jC. The aim of this work is to show that by monitoring the pH of the reaction and controlling the temperature it is possible to leach the synthetic scheelite by hydrochloric acid without the formation of tungstic acid precipitate.
2. Chemistry of tungstates in solution Many of the chemical difficulties associated with understanding of the process in which tungsten in solution is extracted from its ores are related to the fact that the solution chemistry of tungsten is extremely complex. Upon acidification of a solution containing the ion WO42 , polyanions develop very quickly and the velocity increases remarkably with concentration (Souchay et al., 1972). One way of rationalising the formation of isopolytungstates is to regard them as being produced by the addition of acid to the orthotungstate ion. The general equation can be written as follow: ð2npÞ
þ nWO2 4ðaqÞ þ pHðaqÞ X Hx Wn O4n1=2ðpxÞðaqÞ
þ 1=2ðp xÞH2 OðaqÞ
ð2Þ
It deserves to be mentioned that some protons act as central atoms and cannot be exchangeable.
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Besides the common tungstates, the paratungstates 5 ( p/n = 7:6, HW6O21 ) and metatungstates ( p/n = 3:2, 6 H2W12O40 ) have been known for a long time. Since the tungstic acid is obtained only for more acidic solutions ( p/n = 2:1), it provides a suggestion to perform the leaching reaction of synthetic scheelite avoiding its formation by controlling the pH below this value. Thus, the reaction rate remains the same throughout leaching, in contrast to the classic leaching by hydrochloric acid.
t ¼ 1 ð1 X Þ2=3 s
ð6Þ
with s¼
qs R2 2DCA0
ð7Þ
If the chemical relation controls the reaction, ksbD, then Eq. (3) becomes:
3. Noncatalytic solid –liquid reaction In the desired solid –liquid reaction, both products are soluble (calcium chloride and isopolytungstates) and so the solid particles become smaller during the leaching. Therefore, the reaction surface moves from outside towards the centre and is progressively reduced. The shrinking core model (Levenspiel, 1972) seems to be a suitable mathematical model to describe the kinetics of the process. Considering the approximation to the pseudostationary state under laminar regime, the evolution of the particle size with time for first- and zeroorder reactions, respectively, to the leaching agent and to the solid particles, is given by the following relation: t bks R2 ¼1 ð1 X Þ2=3 s R2 bks þ 2DR 2DR ð1 X Þ1=3 R2 bks þ 2DR
ð3Þ
or, t ¼ 1 Fm ð1 X Þ2=3 Fc ð1 X Þ1=3 s
ð4Þ
t ¼ 1 ð1 X Þ1=3 s
ð8Þ
with s¼
qs R bks CA0
ð9Þ
4. Experimental details 4.1. Materials The synthetic scheelite was supplied by Instituto Geolo´gico Mineiro. The reagents used in the leaching experiments were commercial grade. The reference solutions of isopolytungstates for UV/VIS analysis were prepared with reagent-grade ammonium metatungstate hydrate and ammonium paratungstate, obtained from Fluka. Chemical analysis by atomic absorption spectroscopy of raw material shows a content of 78% WO3 and 1% of Ca(OH)2. Particle size distribution analysis of synthetic scheelite shows a size range between 1 and 8 Am with an average size of about 4 Am. 4.2. Equipment
where 2
bks R ; s þ 2DR q R2 qR s¼ s þ s 2DCA0 CA0 bks Fm ¼
If the fluid film resistance controls the reaction rate, Dbks, then Eq. (3) becomes:
R2 bk
Fc ¼
2DR ; s þ 2DR
R2 bk
and ð5Þ
The equipment used consisted mainly of a fourneck Pyrex Quickfit vessel (Fig. 2) with a heating jacket, containing the leaching solution stirred by a magnetic agitator. A spiral condenser was utilized to eliminate the water losses from the system. A set of electrodes was used to measure the pH of the solution.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
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Fig. 2. Diagram of experimental equipment. A—Pyrex Quickfit vessel; B—thermostatic bath; C—magnetic stirrer; D—double electrode (CTWL/S7); E—pH meter (Crison 2002); F—microburette (Crison BU 2031); G—HCl solution; H—computer (Amstrad PC 2086/30); I— condenser; J—thermometer.
The glass electrode was connected to a pH meter (Crison 2002) controlled by computer, which ordered a microburette (Crison BU 2031) to add the amount of acid required to keep the pH at a preset value. The acid consumption during the reaction tends to increase the pH of the solution. UV spectroscopy (JASCO V-560 UV/VIS Spectrophotometer) was used as support to obtain information about the isopolytungstates present in leaching solutions.
preset pH value. A 3 M hydrochloric acid solution was used to control the pH along the reaction.
5. Results The plotting of the conversions of WO3 vs. time, for the pH range between 1.5 and 3, and temper-
4.3. Procedure Leaching tests were performed using 180 mL of the hydrochloric acid solution at a preset pH value in a 300-mL vessel with magnetic stirring and heating to the required temperature. When the conditions were attained, the reaction was initiated with the addition of 30 g of synthetic scheelite. Samples (15 mL) were quickly collected and filtered with Whatman paper no. 42 at selected intervals and then 10 mL of the solution was taken for WO3 analysis by the cinchonin (Li and Wang, 1947) method. The volume of the solution was maintained constant by the addition of the same volume of HCl solution at the
Fig. 3. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 1.5.
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Fig. 4. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 1.9.
Fig. 6. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 2.5.
atures between 28 and 100 jC, is presented in Figs. 3– 7. The results show that if the temperature is equal to or higher than 85 jC, the formation of tungstic acid is induced, since the yields of WO3 are lower than those obtained at 80 jC. In these cases the reaction will be controlled by the diffusion through the ash layer of tungstic acid. However, this solid is partially converted into soluble tungsten compounds (isopolytungstates), as is inferred by the contents of WO3 obtained by chemical analysis of the filtered acidic solutions. Indeed, as an example, the filtered solution analysed for the experiment performed at pH = 2.2 and 85 jC gives a concentration of about 67 g/L of WO3 after 7 h of leaching. Meanwhile, the results of El Wakkad and Rizk (1956) show that the maximum solubility for the
tungstic acid in the range of 0.2 –0.3 M of hydrochloric acid is only about 0.23 g/L, which confirms our last assertion. Increasing the temperature from 80 jC and the hydrochloric acid concentration the tungsten content in the acidic solutions is reduced, unless there is a break in the tungstic acid film (curve at 90 jC and pH = 1.9). According to published results, isopolytungstates in acidic solutions decompose on heating giving tungstic acid (Busev et al., 1976; Palant et al., 1980). The leaching performed at 100 jC and pH = 2.5 shows a decrease of tungsten content in the leach from 19.3% to 4.6% after the first 3 h. This is associated with the homogeneous reaction of isopolytungstate to yellow
Fig. 5. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 2.2.
Fig. 7. Effect of temperature on the WO3 recovery by monitoring the HCl leaching at pH = 3.0.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
137
Fig. 8. The ratio of the acid moles to tungsten moles during the reaction time: (a) T = 70 jC, pH = 1.9; (b) T = 80 jC, pH = 1.5; (c) T = 80 jC, pH = 1.9; (d) T = 85 jC, pH = 2.2; (e) T = 85 jC, pH = 3; (f) T = 90 jC, pH = 1.9; (g) T = 90 jC, pH = 2.2; (h) T = 90 jC, pH = 2.5; (i) T = 90 jC, pH = 3; (j) T = 100 jC, pH = 2.5.
tungstic acid. From this point onwards, leaching the tungsten content in the acidic solution is at constant rate, which means a reaction blocking. We have observed in the trials that the microburette stopped the addition of acid. So, we conclude that the lower acid concentration cannot promote the diffusion of hydrogen ion through the yellow tungstic acid. Fig. 8 shows the ratio of the acid moles to tungstate moles vs. time for some of the experiments. This ratio has the tendency to assume initially a value of the order of 2, which is consistent with the production of a compound with the chemical formula H2WO4. The high p/n ratios can be explained from the fact that we have analysed only the WO3 content present in the solution. So, not taking into account the WO3 in the form of tungstic acid formed on the solid particles or in
the solution, the p/n ratio increases and more significantly for the lower yields. Depending on temperature and hydrochloric acid concentration, the reaction can proceed with the formation of the structural lattice of the solid tungstic acid ( p/n z 2), or with the formation of metatungstates (1.5 V p/n V 1.7). El Wakkad and Rizk (1957) have found it necessary to add 1.8 mol HCl/mol WO42 to form the metatungstate ion in the case of 0.10 M sodium tungstate solutions, and 1.9 mol in the case of 0.005 M solutions. At the considered instants, the plotting of logarithm of the rate of the reaction as a function of logarithm of concentration allows the determination of the reaction order, while the Arrhenius plots give the activation energy. The apparent reaction order for the hydrochloric acid and the process activation energy calcu-
Table 1 Apparent order reaction (n), at temperatures of 70 and 80 jC and different reaction times, for the leaching of synthetic scheelite by hydrochloric acid
Table 2 Apparent activation energy (kJ/mol), at pH = 1.5 and 2.2 and different reaction times, for the leaching of synthetic scheelite by hydrochloric acid
Temperature (jC)
pH
70 80
Reaction time (h) 1
2
3
4
0.75 0.84
0.84 0.94
0.82 0.94
0.88 0.96
1.5 2.2
Reaction time (h) 1
2
3
4
23 –
19 22
17 27
16 28
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lated for the experiments results performed at temperatures of 70 and 80 jC are shown, respectively, in Tables 1 and 2. The values suggest a first-order reaction to the leaching agent and a process that falls within the range of rate control by both reaction and mass transfer. Mass transfer control has its origin in the stirring system used. Indeed, the imposed stirring rate was only enough to avoid the deposition of solid particles on the bottom of the reactor vessel.
temperature on the reaction is given by the slope of the straight lines; its increasing value corresponds to a reduction in time for the total conversion. This good correlation implies that the effective diffusivity of the leaching agent has to have an order of 5 10 13 m2/s. This value is relatively small regarding the usual order of magnitude expected for ionic species in aqueous media. However, we believe that is possible to justify it considering the following mechanism for the reaction:
6. Discussion
1. The HCl begins to attack the solid particles of synthetic scheelite by the adsorption of the hydrogen ion on its surface:
At the initial moments of the leaching, it is possible to accept that the system is only under chemical reaction control. Then, the rate constant of the reaction can be calculated by the expression resulting from the differentiation of Eq. (8): ks ¼
qs R dX ð1 X Þ2=3 3bCA0 dt
ð10Þ
Using the experimental results of Meerson and Khavskii (1957), the rate constant of the acid reaction at 80 jC is of the order of 10 5 cm/s. The Fm/ Fc ratio practically does not change in the temperature range of 40– 80 jC, considering the average activation energy calculated (22 kJ/mol) for the reaction. Thus, taking the value of 0.2 for Fm/Fc ratio and plotting the function [1 Fm(1 X)2/3 Fc(1 X)1/3] vs. t, for results at pH = 1.9, a linear relationship is obtained (Fig. 9). The influence of
k1
þ SolidðsÞ þ Hþ ðaqÞ V Solid HðadsÞ k1V
2. The next step will be the calcium ion desorption.
k2
2þ Solid Hþ ðadsÞ V Solid HWO4ðadsÞ þ CaðaqÞ k2V
3. A new hydrogen ion bonding of anionic ions adsorbed on the solid surface:
k3
þ Solid HWO 4ðadsÞ þ HðaqÞ V Solid H2 WO4ðadsÞ k3V
4. The reaction for concentrated acid solutions (pH V 1.2), according to Kim et al. (1986), should proceed with the formation of the structural lattice of tungstic acid on the solid particles:
k4
Solid H2 WO4ðadsÞ V Solid H2 WO4ðsÞ k4V
Fig. 9. Application of the shrinking core model to the leaching of synthetic scheelite with hydrochloric acid at pH = 1.9 in the temperature range of 40 – 80 jC.
Our results show that this same reaction also happens when temperatures are equal to or higher than 85 jC, in the range of pH between 1.5 and 3.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
Outside the reported domain for the formation of solid H2WO4, the next steps of leaching would be regarded as a process accompanied by multiple reac-
139
tions according to the following scheme (Szarvas and Kurki, 1960):
ψ
Intermediates species before the formation of paratungstate A, [W 2 O 7 (OH)] 3 , [W 4 O 12 (OH) 4 ] 4 , [HW4O12(OH)4]3 , [W6O20(OH)2]6 (Cordis et al., 1975), would be present at all times, but their concentrations are believed to be low. However, there is some controversy between the mechanism schemes suggested by Szarvas and Kurki (1960) and Cordis et al. (1975). Effectively, the second one considers that paratungstate A gives after some minutes the c-metatungstate and only after some weeks is the metatungstate formed. Our results have been analysed by UV/VIS and, as an example, the spectrum recorded at pH = 1.9 at 70 jC after 1.5 h of leaching is put in comparison with the spectra obtained from paratungstate and meta-
tungstate solutions (Fig. 10). We observe in the curve a peak at about 256 nm identical to the metatungstate solution, while the paratungstate solution has one only at 318 nm. Thus, this result supports the mechanism scheme of Szarvas and Kurki (1960). The proposed mechanism for the digestion of synthetic scheelite by hydrochloric acid shows a sequence of multiple reactions where the hydrogen ion is involved in homogeneous and heterogeneous reactions. Indeed, these reactions occur on and near the boundary of the solid particles. Thus, it is reasonable to consider the film surrounding the solid particles as a ‘‘packed bed’’ with a small permeability, which is responsible for the lower effective diffusion of hydrogen ion. At present we are developing research on natural scheelite, considering the advantages using this new technique of acid leaching in the industry.
7. Conclusions
Fig. 10. Ammonium metatungstate hydrate (A) and ammonium paratungstate reference spectra (B) with 3.87 10 5 and 9.87 10 5 M concentrations, respectively. Solution spectrum corresponding to the experiment performed at pH = 1.9 and 70 jC after 1.5 h of leaching with a dilution of 1:1500 (C).
The leaching of synthetic scheelite by hydrochloric acid is strongly dependent on pH and temperature. The main conclusions for monitoring the pH (1.5 – 2.2) in the domain of metatungstates formation and temperature control (70 – 80 jC) of the synthetic scheelite leaching by hydrochloric acid are the following: 1. Decrease of leaching agent consumption to about 20%, since the production of metatungstates ions
140
2. 3. 4.
5. 6. 7.
8.
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141
( p/n = 1.5 – 1.7) occurs instead of solid tungstic acid ( p/n = 2), but much more a 20% if we consider the excess commonly used to overcome the resistance imposed by the ash of tungstic acid (about 200%). Decrease of the reaction temperature from the usual value in the traditional acid leaching of 100 to 80 jC. Decrease of the reaction time to 45 min (at pH = 1.5 and T = 80 jC). Use of test equipment material without rubber lining that reduces the heating transfer global coefficient, because the level of the chloride ions (1.15 g/L) enables the use of a titanium alloy ATi 24 (Avesta, 1972) or a 42% Ni – 21.5% Cr alloy with copper and molybdenum (Polar, 1961). Elimination of the solid –liquid separation costs, since the yellow H2WO4 is not produced. Reduced costs of waste treatment. Reduced costs of APT production, since a smaller amount of ammonium hydroxide is used to set the solution in the suitable pH range to initiate the crystallization. Leaching operation amenable for continuous operation.
List of symbols b stoichiometric factor of solid particle in the reaction C fluid concentration (kmol/m3) D effective diffusivity of fluid (m2/s) E energy activation (kJ/mol) k specific reaction rate for surface reaction (m/s) R initial radius of spherical particle (m) t reaction time (s) s time for the total conversion (s) T temperature (K) X fractional degree of conversion qs molar density of solid reactant (kmol/m3) Fm coefficient of mass transfer Fc coefficient of chemical reaction Subscripts aq aqueous solution A0 initial liquid reactant s solid particle; interface between liquid and solid
Acknowledgements Funding from Fundacß a˜o para a Cieˆncia e a Tecnologia, POCTI/36287/ECM/1999, is gratefully acknowledged.
References Avesta, J.A., 1972. Tableaux de corrosion pour l’acier inoxydable et le titane. Carlson Press Boktryckeri, Stockholm. Busev, A.I., Ivanov, V.M., Sokolova, T.A., 1976. Analiticheskaya khimiya vol’frama Izd. Nauka, Moscow. ¨ ber isopolywolframatCordis, V., Tytko, K.H., Glemser, O., 1975. U feststoffe und deren beziehung zn ispolywolframationen in waßringer lo¨sung. Z. Naturforsch. 306 (11/12), 834 – 841. Dow Chemical, 1968. Process for recovery of tungsten values, London Patent 1,137,518. El Wakkad, S.E.S., Rizk, H.M., 1956. The solubility of tungsten trioxide in hydrochloric acid. J. Chem. Soc. 3248. El Wakkad, S.E.S., Rizk, H.M., 1957. The polytungstates and the colloidal nature and the amphoteric character of tungstic acid. J. Chem. Soc. 61, 494 – 497. Fieberg, M.M., Coetzee, C.F.B., 1986. The recovery of tungsten from South African low-grade scheelite concentrates. MINTEK, report no. M164D, published by the Council for Mineral Technology, pp. 1 – 22. Forward, F.A., Vizsolyi, A.I., 1965. Process for the production of tungstic acid. U.S. Patent 3,193,347. General Electric, 1959. London Patent 818,117. Gu¨rmen, S., Timur, S., Arshn, C., Duman, I., 1999. Acid leaching of scheelite concentrate and production of heteropoly-tungstate salt. Hydrometallurgy 51, 227 – 238. Hubbard, H.M., Acres, G., Kennedy, J.H., 1962. Process for the extraction of tungsten values from materials containing same. U.S. Patent 3,047,361. Kim, T.K., Mooney, R.W., Chiola, V., 1986. Study of soluble tungstate species by solvent extraction. Sep. Sci. 3 (5), 476 – 478. Levenspiel, J.A., 1972. Chemical Reaction Engineering, 2nd ed. Wiley, New York. Li, K.C., Wang, C.Y., 1947. Tungsten, 2nd ed. Reinhold Publishing, New York. Li, H., Lin, M., Shi, Z., Dai, C., He, Y., Li, J., He, K., 1986. Faming Zhuanli Shengning Gongkai Shuomingshn CN 85,100,350. Martins, J.I., 1983. The hydrometallurgy in the valorisation of tungsten ores. PhD thesis, FEUP/Univ. Porto. Meerson, G.A., Khavskii, N.N., 1957. Treatment acid of scheelite. Tsvet. Met. 30 (3), 41 – 47. Meerson, G.A., Khavskii, N.N., 1961. Hydrometallurgic heterogeneous reactions happened during the grinding. Izv. Akad. Nauk SSSR, Otd. Tekh. Nauk, Metall. Topl. 5, 36 – 42. Mikhailov, V.A., Pomoshmikov, E.E., Vosse’l, S.V., Marova, I.V., Vasenin, N.T., Makarov, E.P., Eremenko, S.I., Dulepov, E.V., Amfrienko, E.V., 1985. Investigation of physical and chemical
J.I. Martins et al. / Hydrometallurgy 70 (2003) 131–141 properties of mechanically-activated tungsten concentrates. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 4, 29 – 34. Palant, A.A., Reznichenko, V.A., Nikiforova, S.E., 1980. Influence of temperature on the extraction of tungsten (VI) by amines. Russ. J. Inorg. Chem. 25 (3), 480 – 482. Polar, J.P., 1961. A Guide to Corrosion Resistance. Climax Molybdenum, New York. Smithels, J.C., 1936. Tungsten: A Treatise on its Metallurgy Properties and Applications, 2nd ed. Chapman & Hall, London. Souchay, P., Boyer, M., Chauveau, F., 1972. Advances in the solution chemistry of tungstates. Trans. R. Inst. Technol. Stockh. 259, 160 – 172.
141
¨ ber die kinetic der bildung von Szarvas, V.P., Kurki, E.Cs., 1960. U polywolframaten. Z. Anorg. Allg. Chem. 305, 55 – 63. Vezina, J.A., Gow, W.A., Lalibert, J.J., 1970. Continuous recovery of tungstic trioxide. European Patent 836,441. Xuin, G.H., Yu, D.Y., Su, Y.F., 1986. Leaching of scheelite by hydrochloric acid in the presence of phosphate. Hydrometallurgy 16, 27 – 40. Yih, S.W.H., Wang, C.T., 1979. Tungsten: Sources, Metallurgy, Properties, and Applications, 2nd ed. Chapman & Hall, London. Zelikman, A.N., 1985. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 4, 23 – 29.