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Leaching of manganese from low-grade manganese ore using oxalic acid as reductant in sulphuric acid solution
Hydrometallurgy 62 (2001) 157 – 163 www.elsevier.com/locate/hydromet
Leaching of manganese from low-grade manganese ore using oxalic acid as reductant in sulphuric acid solution R.N. Sahoo, P.K. Naik, S.C. Das* Regional Research Laboratory, Council of Scientific and Industrial Research, Bhubaneswar 751013, Orissa, India Received 29 June 2001; received in revised form 29 June 2001; accepted 7 September 2001
Abstract Leaching studies of low-grade Joda manganese ore containing 24.7% Mn and 28.4% Fe were carried out at high temperature and atmospheric pressure using oxalic acid as reductant in sulphuric acid medium. The experiments were designed according to 24 full factorial design, and regression equations for extraction of manganese, iron and aluminum were determined from the data. All the significant main and interaction effects on extraction of Mn, Fe and Al have positive effect, except oxalic acid concentration and time interaction for extraction of Al. Oxalic acid concentration has strongest effect on extraction of Mn, whereas temperature and time have strongest effect on extraction of Fe and Al, respectively. 98.4% Mn and 8.7% Fe were extracted from 150 + 105 mm ore with 30.6 g/l oxalic acid, 0.543 M sulphuric acid concentration at 85 C in 105 min. D 2001 Published by Elsevier Science B.V. Keywords: Manganese ore; Leaching; Oxalic acid; Factorial design and optimization
1. Introduction Manganese is a strategically important metal that has several industrial uses. The most important are in the manufacture of steel, some non-ferrous alloys, carbon – zinc batteries and some chemical reagents. The rich deposits with manganese concentrations in excess of 35% are preferred industrially, and lean deposits with manganese concentration less than 35% are presently ignored because extraction methods applied to rich manganese ore are not economically viable. India has manganese ore reserves in the order
*
Corresponding author. Tel.: +91-674-581-750; fax: +91-674581-750. E-mail address: [email protected] (S.C. Das).
of 176.5 million tons (IBM, 1995). In 1992 – 1993, the production of manganese ore was 1.90 million tons out of which 47% was low grade (IBM, 1995). The Keonjhar – Bonai region of the state of Orissa has a sizable portion of manganese resources. In this region, the deposits are found in small pockets and the ores occur in a wide variety of grades. The high-grade ores are mined by open cast method and low-grade ores are dumped at the mine site as reject. Manganese from these ores can be extracted selectively using hydrometallurgical techniques. As manganese dioxide ores are stable in acid or alkaline oxidising conditions, the extraction of manganese must be carried out in reducing condition. In aqueous reduction, SO2 (Naik et al., 2000), FeSO4 (Das et al., 1982), sucrose (Toro and Veglio, 1982), charcoal (Das et al., 1989), coal and lignite (Hanock and Fray, 1986), pyrite (Parida et
0304-386X/01/$ - see front matter D 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 1 9 6 - 7
158
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
al., 1990), etc. can be used as reducing agents. Oxalic acid can also be used as a reducing agent for manganese extraction from manganese dioxide ore. Leaching of manganese ore using oxalic acid-producing microorganisms has been reported (Stone, 1987). Chemical leaching of Mn ore using oxalic acid may give an insight into bioleaching. The dissolution of manganese is due to reduction of its dioxide by oxalic acid. The reduction (Ehrlich, 1980) between MnO2 and oxalic acid in acid medium may be given as follows. MnO2 þ HOOC COOH þ 2Hþ ! Mn2þ þ 2CO2 þ 2H2 O:
ð1Þ
Statistical design is an important tool to understand process behavior (Cochran and Cox, 1957; Daniel, 1976; Davis, 1978; Box et al., 1978; Datta et al., 1995). The statistical design concept has been successfully used in leaching studies (Veglio and Toro, 1994; Veglio et al., 1993; Parida et al., 1997). In the present work, a leaching study has been carried out on low-grade ore of Joda, Orissa using oxalic acid as a reductant in sulphuric acid medium. The experiments have been carried out using a statistical design.
Table 2 The 24 factorial design for Mn leaching: size of ore 150 + 105 mm; solid-to-liquid ratio (w/v), 1:20 Variables X1 X2 X3 X4
Oxalic acid (g/l) Sulphuric acid concentration (M) Temperature (C) Duration of leaching (min)
Low level 2.5 0.2 35 180
Base level 3.75 0.3 45 270
High level 5.0 0.4 55 360
with some cryptomelane and manganite. Braunite also occurs in minor quantities (IBM, 1974). Samples of reject low-grade manganese ore were collected from the Joda area. The collected sample was ground to below 150 mm and sieved using a 105-mm sieve. The factorial experiments were conducted using near monosize ( 150 + 105 mm) particles to reduce the effect of size. One gram of ore was digested in aquaregia and filtered with Whatman No. 542 (ash less) filter paper. The filter paper was burned and weighed to find acid insoluble. The filtrate was analyzed for the different elements. Chemical analysis of the ore is given in Table 1. The oxalic acid used for leaching was laboratory grade reagent supplied by ‘QUALIGENS’ SQ, India. 2.2. Leaching tests
2. Experiments 2.1. Materials The main manganese minerals of the Singhbhum – Keonjhar –Bonai belt are psilomelene and pyrolusite Table 1 Chemical analysis of manganese ore ( 150 + 105 mm) Element
% (w/w)
Mn Fe K Al Co Zn Ni Mg Ca Cu Acid insoluble
24.67 28.39 0.9 4.05 0.05 0.02 0.02 0.06 0.01 0.03 16.17
A 250-ml iodine flask was taken. One hundred milliliters of sulphuric acid solution and oxalic acid of required amount was poured in to it. The flask was then magnetically stirred and heated. When the solution attained the pre-required temperature, 5 g of ore was added. The rate of agitation was kept constant for all the experiments. A condenser was fitted to the flask to prevent evaporation. After each experiment, the slurry was filtered and the residue was washed with distilled water. The leach liquor along with washing was analyzed for different elements using AAS and ICP. 2.3. Experimental planning In the present work, 24 full factorial design was chosen for conducting the leaching experiments (Akhanazarova and Kafarov, 1982). The variables selected were quantity of oxalic acid, sulphuric acid concentration, temperature and time. The levels of variables
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
are given in Table 2. In addition, some experiments were carried out for qualitative analysis.
Table 3 Results of 24 full factorial experiments N
3. Results and discussion Base level experiments were carried out to test the significance of the coefficients and adequacy of the models. The coded variables for 24 full factorial design and the results showing recovery of Mn, Fe and Al are given in Table 3. The regression equation for the above matrix may be represented as: Y ¼ b0 þ b1 X1 þ b2 X2 þ b3 X3 þ b4 X4 þ b12 X1 X2 þ b13 X1 X3 þ b14 X1 X4 þ b23 X2 X3 þ b24 X2 X4 þ b34 X3 X4 þ b123 X1 X2 X3 þ b124 X1 X2 X4 þ b134 X1 X3 X4 þ b234 X2 X3 X4 þ b1234 X1 X2 X3 X4
ð2Þ
where Y = the percentage of metal extracted; b = empirical model coefficients; X1, X2, X3, X4 = dimensionless coded factors for oxalic acid concentration, sulphuric acid concentration, temperature and time, respectively. The relations between the coded and actual values are given as:
159
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
X1 + + + + + + + + 0 0 0 0 0 0
X2 + + + + + + + + 0 0 0 0 0 0
X3 + + + + + + + + 0 0 0 0 0 0
X4 + + + + + + + + 0 0 0 0 0 0
Response Mn
Fe
Al
9.97 20.02 9.890 19.49 10.29 20.75 12.40 24.32 9.76 21.28 10.14 20.07 10.35 21.07 12.49 24.82 14.21 13.82 13.66 14.53 14.15 13.99
0.70 0.80 0.83 0.90 1.84 1.94 2.42 3.30 0.70 0.75 0.97 1.20 1.89 2.07 2.65 3.36 1.03 0.96 0.92 1.11 1.77 0.99
4.44 6.45 4.77 5.93 8.31 11.28 11.40 16.34 11.41 12.0 14.66 13.68 20.34 22.58 27.30 29.87 9.89 8.79 8.46 10.13 9.45 9.12
X1: oxalic acid (g/l); X2: sulphuric acid concentration (M); X3: temperature (C); X4: duration of leaching (min); : low level; +: high level; 0: base level.
X2 ¼ ðx2 0:3Þ=0:1
t-test method at 95% confidence level. The significant coefficients for Mn, Fe, and Al extraction are compared in Fig. 1. On deleting the coefficients not significant at 95% confidence level, the regression Eq. (2) becomes:
X3 ¼ ðx3 45Þ=10
YMn ¼ 16:0 þ 5:43X1 þ 0:61X2 þ 1:01X3
X1 ¼ ðx1 3:75Þ=1:25
X4 ¼ ðx4 270Þ=90 The regression coefficients were estimated by: X Yi =N ; b0 ¼ bj ¼
X
bnj ¼
Xj Yi =N ;
X
Xni Xji Yi =N ; etc:
þ 0:29X1 X2 þ 0:79X2 X3 þ 0:3X1 X2 X3 YFe ¼ 1:65 þ 0:15X1 þ 0:31X2 þ 0:79X3 þ 0:05X4 þ 0:09X1 X2 þ 0:09X1 X3 þ 0:19X2 X3 þ 0:07X1 X2 X3
ð4Þ
YAl ¼ 13:8 þ 0:97X1 þ 1:7X2 þ 4:63X3 þ 5:18X4 þ 0:64X1 X3 0:42X1 X4 þ 1:1X2 X3 þ 0:7X2 X4 þ 1:41X3 X4
The regression equations for extraction of Mn, Fe, and Al were developed and the main interaction coefficients were tested for significance by Student’s
ð3Þ
ð5Þ
where YMn, YFe and YAl are the percentages of Mn, Fe and Al extracted, respectively.
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Fig. 1. Significant main and interaction empirical coefficients for Mn, Fe and Al extraction yields.
The goodness of fit of Eqs. (3), (4) and (5) were tested by the test (Akhanazarova and Kafarov, 1982). The comparison of experimentally obtained variance ratio and tabulated value of F (Akhanazarova and Kafarov, 1982), for a = 0.05 (95% confidence level), is given in Table 4. Since F < F1 a (c1 and c2 — df), the estimated regression Eqs. (3), (4) and (5) fit the experimental data adequately. The residual analysis for manganese, iron and aluminium is given in Fig. 2. Table 4 Comparison of experimentally obtained variance ratio and tabulated value of Fisher’s F (Akhanazarova and Kafarov, 1982); c1, total number of observations minus number of significant coefficient in the regression equation; c2, number of repeat test minus one Regression
Variance
Experimental
Tabulated value
equation no. c 1
df c2
ratio
value
of Fisher’s F
Eq. (3), YMn Eq. (4), YFe Eq. (5), YAl
5 5 5
FMn FFe FAl
2.067 1.064 0.454
3.5 4.0 4.4
9 7 6
All the main and interaction effects, significant at 95% confidence level, have positive effect on extraction of Mn, Fe and Al, except the oxalic acid and the time interaction effect for extraction of aluminum which has negative effect. Oxalic acid concentration has strongest effect on extraction of Mn followed by temperature and sulphuric acid concentration. In the case of iron extraction, temperature has strongest effect followed by sulphuric acid concentration, oxalic acid concentration and time. Time has maximum effect on extraction of aluminum followed by temperature, sulphuric acid concentration and oxalic acid concentration.
4. Optimization study The main objective was to maximize extraction of manganese. Therefore, the next set of experiments was conducted by the steepest ascent method (Akha-
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
161
Fig. 2. Residual analysis for Mn, Fe and Al.
nazarova and Kafarov, 1982) to arrive at optimized parameters for extraction of manganese. The optimized process conditions were evaluated from Eq. (3) and increments were decided (Table 5). In this set of experiments, oxalic acid and sulphuric acid concentration and temperature were increased as they have positive effect, whereas time was kept at low
level since it has no effect on the extraction of manganese. The operating conditions and the corresponding extraction of Mn, Fe and Al are given in Table 6. The Experiment No. 26 was considered optimum because in the next experiment (Experiment
Table 6 Optimization study No.
Table 5 Evaluation of optimized process conditions
Base level (Zj) Increment (DZj) Coefficient (bj) bjDZj Normal steps
Oxalic acid (g/l)
Sulphuric acid (M)
Temperature (C)
3.75 1.25 5.43 6.78 6.71
0.3 0.1 0.61 0.061 0.061
45 10 10 10 10
23 24 25 26 27
Oxalic acid (g/l)
10.46 17.17 23.88 30.59 37.30
Sulphuric acid (M)
0.361 0.421 0.482 0.543 0.604
Temperature (C)
55 65 75 85 95
Extraction of metals (%) Mn
Fe
Al
38.7 61.3 83.6 98.2 99.4
1.7 3.0 4.8 9.5 24.3
12.8 20.0 28.4 39.9 44.6
Ore concentration solid-to-liquid, 1:20 (w/v); 180 min.
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Fig. 3. Effect of duration of leaching on extraction of Mn, Fe and Al. Particle size: 150 + 105 mm; ore: 5 g; oxalic acid: 30.6 g/l, sulphuric acid: 0.528 M; temperature: 85 C.
No. 27), there is large (14.8%) increase in extraction of iron which is a major impurity in leach liquor whereas there is marginal (1.3%) increase in extraction of manganese. Time has no effect on extraction of Mn after 3 h (Eq. (3)). Therefore, the effect of time on extraction of Mn including Fe, K and Al was studied varying time from 30 to 180 min (Fig. 3). Most of the reactions are complete in the first 30 min (Fig. 3). In 105 min, 98.4% Mn, 8.7% Fe, 96.4% K and 36.4% Al were extracted.
5. Conclusions (i) Manganese can be extracted from low-grade Joda ore using oxalic acid as reductant at 85 C in sulphuric acid medium.
(ii) From the statistical analysis, it was found that all the main and interaction terms significant at 95% confidence level have positive effect on the extraction of Mn, Fe and Al, except oxalic acid concentration and time interaction for extraction of Al which has negative effect. (iii) The oxalic acid concentration has strongest effect on the extraction Mn followed by temperature and sulphuric acid concentration. (iv) Temperature has maximum effect on the extraction of iron followed by sulphuric acid and oxalic acid concentrations. (v) Time has strongest effect on the extraction of Al followed by temperature, sulphuric acid concentration and oxalic acid concentration. (vi) 98.4% Mn could be extracted from low-grade manganese ore of Joda with oxalic acid concentration
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
30.6 g/l, sulphuric acid concentration of 0.543 M at 85 C and leaching time of 105 min.
Acknowledgements The authors are thankful to Dr. V.N. Misra, Director, Regional Research Laboratory, Bhubaneswar, Orissa, India for giving permission to publish this paper.
References Akhanazarova, S., Kafarov, V., 1982. Experiment Optimization on Chemistry and Chemical Engineering. Mir Publishers, Moscow, p. 312. Box, G.E.P., Hunter, W.G., Hunter, J.S., 1978. Statistics for Experiments. Wiley, New York. Cochran, W.G., Cox, G.M., 1957. Experimental Designs, 2nd edn. Wiley, New York. Daniel, C., 1976. Application of Statistics to Industrial Experimentation. Wiley, New York. Das, S.C., Sahoo, P.K., Rao, P.K., 1982. Extraction of manganese from low grade manganese ore by FeSO4 leaching. Hydrometallurgy 8, 35 – 47. Das, S.C., Anand, S., Das, R.P., Jena, P.K., 1989. Sulphuric acid leaching of manganese nodules in presence of charcoal. Aust. IMM Bull. Proc., 294 (IF). Datta, P., Ray, H.S., Tripathy, A.K., 1995. Application of statistical design of experiments in process investigation. In: Tripathy, A.K., Datta, P., Ray, H.S. (Eds.), Quantitative Approaches in Process Metallurgy. Applied Publishers, pp. 266 – 296.
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Davis, O.L. (Ed.), 1978. The Design and Analysis of Industrial Experiments, 2nd edn. Longman Group, London. Ehrlich, H.L., 1980. In: Tridinger, P.A., Watler, M.R., Ralph, B.J. (Eds.), Biogeochemistry Ancient and Modern Environments. Australian Academy of Sciences and Verlog, Canberra, p. 609. Hancock, H.A., Fray, D.O., 1986. Use of coal and lignite to dissolve manganese oxide in acidic solutions. Trans. Inst. Min. Metall., Sect. C 95, C27 – C47. IBM, 1974. Minerals Facts and Problems MANGANESE ORE, Monograph No. 4, Publication Cell, Indian Bureau of Mines, Nagpur, July 1974, 73. IBM, 1995. Indian Minerals Year Book, Indian Bureau of Mines, Ministry of Steel Mines, Nagpur, vol. 2, pp. 497 – 514. Naik, P.K., Sukla, L.B., Das, S.C., 2000. Aqueous SO2 leaching studies on Nishikahal manganese ore through factorial experiment. Hydrometallurgy 54, 217 – 228. Parida, K.M., Nayak, B.B., Rao, K.K., Rao, S.B., 1990. Kinetics and mechanism of the reductive leaching of manganese ores by iron pyrites in mild acidic solutions. In: Gaslall, D.R. (Ed.), EPD Congress. The Minerals Metals and Materials Society, pp. 217 – 228. Parida, K.M., Das, J., Datta, P., 1997. Application of statistical design of experiments of geothite (a-FeOOH) in hydrochloric acid in the presence of ascorbic acid. Hydrometallurgy 46, 271 – 275. Stone, A.T., 1987. Microbial metabolites and the reductive dissolution of manganese oxides: oxalate and pyruvate. Geochim. Cosmochim. Acta 51, 919. Veglio, F., Toro, L., 1994. Fractional factorial experiments in the development of manganese dioxide leaching by sucrose in sulphuric acid solutions. Hydrometallurgy 36, 215 – 230. Veglio, F., Pagliarini, A., Toro, L., 1993. Factorial experiments for the development of kaolin bleaching process. IJMP 39, 87 – 99.
Leaching of manganese from low-grade manganese ore using oxalic acid as reductant in sulphuric acid solution R.N. Sahoo, P.K. Naik, S.C. Das* Regional Research Laboratory, Council of Scientific and Industrial Research, Bhubaneswar 751013, Orissa, India Received 29 June 2001; received in revised form 29 June 2001; accepted 7 September 2001
Abstract Leaching studies of low-grade Joda manganese ore containing 24.7% Mn and 28.4% Fe were carried out at high temperature and atmospheric pressure using oxalic acid as reductant in sulphuric acid medium. The experiments were designed according to 24 full factorial design, and regression equations for extraction of manganese, iron and aluminum were determined from the data. All the significant main and interaction effects on extraction of Mn, Fe and Al have positive effect, except oxalic acid concentration and time interaction for extraction of Al. Oxalic acid concentration has strongest effect on extraction of Mn, whereas temperature and time have strongest effect on extraction of Fe and Al, respectively. 98.4% Mn and 8.7% Fe were extracted from 150 + 105 mm ore with 30.6 g/l oxalic acid, 0.543 M sulphuric acid concentration at 85 C in 105 min. D 2001 Published by Elsevier Science B.V. Keywords: Manganese ore; Leaching; Oxalic acid; Factorial design and optimization
1. Introduction Manganese is a strategically important metal that has several industrial uses. The most important are in the manufacture of steel, some non-ferrous alloys, carbon – zinc batteries and some chemical reagents. The rich deposits with manganese concentrations in excess of 35% are preferred industrially, and lean deposits with manganese concentration less than 35% are presently ignored because extraction methods applied to rich manganese ore are not economically viable. India has manganese ore reserves in the order
*
Corresponding author. Tel.: +91-674-581-750; fax: +91-674581-750. E-mail address: [email protected] (S.C. Das).
of 176.5 million tons (IBM, 1995). In 1992 – 1993, the production of manganese ore was 1.90 million tons out of which 47% was low grade (IBM, 1995). The Keonjhar – Bonai region of the state of Orissa has a sizable portion of manganese resources. In this region, the deposits are found in small pockets and the ores occur in a wide variety of grades. The high-grade ores are mined by open cast method and low-grade ores are dumped at the mine site as reject. Manganese from these ores can be extracted selectively using hydrometallurgical techniques. As manganese dioxide ores are stable in acid or alkaline oxidising conditions, the extraction of manganese must be carried out in reducing condition. In aqueous reduction, SO2 (Naik et al., 2000), FeSO4 (Das et al., 1982), sucrose (Toro and Veglio, 1982), charcoal (Das et al., 1989), coal and lignite (Hanock and Fray, 1986), pyrite (Parida et
0304-386X/01/$ - see front matter D 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 1 9 6 - 7
158
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
al., 1990), etc. can be used as reducing agents. Oxalic acid can also be used as a reducing agent for manganese extraction from manganese dioxide ore. Leaching of manganese ore using oxalic acid-producing microorganisms has been reported (Stone, 1987). Chemical leaching of Mn ore using oxalic acid may give an insight into bioleaching. The dissolution of manganese is due to reduction of its dioxide by oxalic acid. The reduction (Ehrlich, 1980) between MnO2 and oxalic acid in acid medium may be given as follows. MnO2 þ HOOC COOH þ 2Hþ ! Mn2þ þ 2CO2 þ 2H2 O:
ð1Þ
Statistical design is an important tool to understand process behavior (Cochran and Cox, 1957; Daniel, 1976; Davis, 1978; Box et al., 1978; Datta et al., 1995). The statistical design concept has been successfully used in leaching studies (Veglio and Toro, 1994; Veglio et al., 1993; Parida et al., 1997). In the present work, a leaching study has been carried out on low-grade ore of Joda, Orissa using oxalic acid as a reductant in sulphuric acid medium. The experiments have been carried out using a statistical design.
Table 2 The 24 factorial design for Mn leaching: size of ore 150 + 105 mm; solid-to-liquid ratio (w/v), 1:20 Variables X1 X2 X3 X4
Oxalic acid (g/l) Sulphuric acid concentration (M) Temperature (C) Duration of leaching (min)
Low level 2.5 0.2 35 180
Base level 3.75 0.3 45 270
High level 5.0 0.4 55 360
with some cryptomelane and manganite. Braunite also occurs in minor quantities (IBM, 1974). Samples of reject low-grade manganese ore were collected from the Joda area. The collected sample was ground to below 150 mm and sieved using a 105-mm sieve. The factorial experiments were conducted using near monosize ( 150 + 105 mm) particles to reduce the effect of size. One gram of ore was digested in aquaregia and filtered with Whatman No. 542 (ash less) filter paper. The filter paper was burned and weighed to find acid insoluble. The filtrate was analyzed for the different elements. Chemical analysis of the ore is given in Table 1. The oxalic acid used for leaching was laboratory grade reagent supplied by ‘QUALIGENS’ SQ, India. 2.2. Leaching tests
2. Experiments 2.1. Materials The main manganese minerals of the Singhbhum – Keonjhar –Bonai belt are psilomelene and pyrolusite Table 1 Chemical analysis of manganese ore ( 150 + 105 mm) Element
% (w/w)
Mn Fe K Al Co Zn Ni Mg Ca Cu Acid insoluble
24.67 28.39 0.9 4.05 0.05 0.02 0.02 0.06 0.01 0.03 16.17
A 250-ml iodine flask was taken. One hundred milliliters of sulphuric acid solution and oxalic acid of required amount was poured in to it. The flask was then magnetically stirred and heated. When the solution attained the pre-required temperature, 5 g of ore was added. The rate of agitation was kept constant for all the experiments. A condenser was fitted to the flask to prevent evaporation. After each experiment, the slurry was filtered and the residue was washed with distilled water. The leach liquor along with washing was analyzed for different elements using AAS and ICP. 2.3. Experimental planning In the present work, 24 full factorial design was chosen for conducting the leaching experiments (Akhanazarova and Kafarov, 1982). The variables selected were quantity of oxalic acid, sulphuric acid concentration, temperature and time. The levels of variables
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
are given in Table 2. In addition, some experiments were carried out for qualitative analysis.
Table 3 Results of 24 full factorial experiments N
3. Results and discussion Base level experiments were carried out to test the significance of the coefficients and adequacy of the models. The coded variables for 24 full factorial design and the results showing recovery of Mn, Fe and Al are given in Table 3. The regression equation for the above matrix may be represented as: Y ¼ b0 þ b1 X1 þ b2 X2 þ b3 X3 þ b4 X4 þ b12 X1 X2 þ b13 X1 X3 þ b14 X1 X4 þ b23 X2 X3 þ b24 X2 X4 þ b34 X3 X4 þ b123 X1 X2 X3 þ b124 X1 X2 X4 þ b134 X1 X3 X4 þ b234 X2 X3 X4 þ b1234 X1 X2 X3 X4
ð2Þ
where Y = the percentage of metal extracted; b = empirical model coefficients; X1, X2, X3, X4 = dimensionless coded factors for oxalic acid concentration, sulphuric acid concentration, temperature and time, respectively. The relations between the coded and actual values are given as:
159
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
X1 + + + + + + + + 0 0 0 0 0 0
X2 + + + + + + + + 0 0 0 0 0 0
X3 + + + + + + + + 0 0 0 0 0 0
X4 + + + + + + + + 0 0 0 0 0 0
Response Mn
Fe
Al
9.97 20.02 9.890 19.49 10.29 20.75 12.40 24.32 9.76 21.28 10.14 20.07 10.35 21.07 12.49 24.82 14.21 13.82 13.66 14.53 14.15 13.99
0.70 0.80 0.83 0.90 1.84 1.94 2.42 3.30 0.70 0.75 0.97 1.20 1.89 2.07 2.65 3.36 1.03 0.96 0.92 1.11 1.77 0.99
4.44 6.45 4.77 5.93 8.31 11.28 11.40 16.34 11.41 12.0 14.66 13.68 20.34 22.58 27.30 29.87 9.89 8.79 8.46 10.13 9.45 9.12
X1: oxalic acid (g/l); X2: sulphuric acid concentration (M); X3: temperature (C); X4: duration of leaching (min); : low level; +: high level; 0: base level.
X2 ¼ ðx2 0:3Þ=0:1
t-test method at 95% confidence level. The significant coefficients for Mn, Fe, and Al extraction are compared in Fig. 1. On deleting the coefficients not significant at 95% confidence level, the regression Eq. (2) becomes:
X3 ¼ ðx3 45Þ=10
YMn ¼ 16:0 þ 5:43X1 þ 0:61X2 þ 1:01X3
X1 ¼ ðx1 3:75Þ=1:25
X4 ¼ ðx4 270Þ=90 The regression coefficients were estimated by: X Yi =N ; b0 ¼ bj ¼
X
bnj ¼
Xj Yi =N ;
X
Xni Xji Yi =N ; etc:
þ 0:29X1 X2 þ 0:79X2 X3 þ 0:3X1 X2 X3 YFe ¼ 1:65 þ 0:15X1 þ 0:31X2 þ 0:79X3 þ 0:05X4 þ 0:09X1 X2 þ 0:09X1 X3 þ 0:19X2 X3 þ 0:07X1 X2 X3
ð4Þ
YAl ¼ 13:8 þ 0:97X1 þ 1:7X2 þ 4:63X3 þ 5:18X4 þ 0:64X1 X3 0:42X1 X4 þ 1:1X2 X3 þ 0:7X2 X4 þ 1:41X3 X4
The regression equations for extraction of Mn, Fe, and Al were developed and the main interaction coefficients were tested for significance by Student’s
ð3Þ
ð5Þ
where YMn, YFe and YAl are the percentages of Mn, Fe and Al extracted, respectively.
160
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
Fig. 1. Significant main and interaction empirical coefficients for Mn, Fe and Al extraction yields.
The goodness of fit of Eqs. (3), (4) and (5) were tested by the test (Akhanazarova and Kafarov, 1982). The comparison of experimentally obtained variance ratio and tabulated value of F (Akhanazarova and Kafarov, 1982), for a = 0.05 (95% confidence level), is given in Table 4. Since F < F1 a (c1 and c2 — df), the estimated regression Eqs. (3), (4) and (5) fit the experimental data adequately. The residual analysis for manganese, iron and aluminium is given in Fig. 2. Table 4 Comparison of experimentally obtained variance ratio and tabulated value of Fisher’s F (Akhanazarova and Kafarov, 1982); c1, total number of observations minus number of significant coefficient in the regression equation; c2, number of repeat test minus one Regression
Variance
Experimental
Tabulated value
equation no. c 1
df c2
ratio
value
of Fisher’s F
Eq. (3), YMn Eq. (4), YFe Eq. (5), YAl
5 5 5
FMn FFe FAl
2.067 1.064 0.454
3.5 4.0 4.4
9 7 6
All the main and interaction effects, significant at 95% confidence level, have positive effect on extraction of Mn, Fe and Al, except the oxalic acid and the time interaction effect for extraction of aluminum which has negative effect. Oxalic acid concentration has strongest effect on extraction of Mn followed by temperature and sulphuric acid concentration. In the case of iron extraction, temperature has strongest effect followed by sulphuric acid concentration, oxalic acid concentration and time. Time has maximum effect on extraction of aluminum followed by temperature, sulphuric acid concentration and oxalic acid concentration.
4. Optimization study The main objective was to maximize extraction of manganese. Therefore, the next set of experiments was conducted by the steepest ascent method (Akha-
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
161
Fig. 2. Residual analysis for Mn, Fe and Al.
nazarova and Kafarov, 1982) to arrive at optimized parameters for extraction of manganese. The optimized process conditions were evaluated from Eq. (3) and increments were decided (Table 5). In this set of experiments, oxalic acid and sulphuric acid concentration and temperature were increased as they have positive effect, whereas time was kept at low
level since it has no effect on the extraction of manganese. The operating conditions and the corresponding extraction of Mn, Fe and Al are given in Table 6. The Experiment No. 26 was considered optimum because in the next experiment (Experiment
Table 6 Optimization study No.
Table 5 Evaluation of optimized process conditions
Base level (Zj) Increment (DZj) Coefficient (bj) bjDZj Normal steps
Oxalic acid (g/l)
Sulphuric acid (M)
Temperature (C)
3.75 1.25 5.43 6.78 6.71
0.3 0.1 0.61 0.061 0.061
45 10 10 10 10
23 24 25 26 27
Oxalic acid (g/l)
10.46 17.17 23.88 30.59 37.30
Sulphuric acid (M)
0.361 0.421 0.482 0.543 0.604
Temperature (C)
55 65 75 85 95
Extraction of metals (%) Mn
Fe
Al
38.7 61.3 83.6 98.2 99.4
1.7 3.0 4.8 9.5 24.3
12.8 20.0 28.4 39.9 44.6
Ore concentration solid-to-liquid, 1:20 (w/v); 180 min.
162
R.N. Sahoo et al. / Hydrometallurgy 62 (2001) 157–163
Fig. 3. Effect of duration of leaching on extraction of Mn, Fe and Al. Particle size: 150 + 105 mm; ore: 5 g; oxalic acid: 30.6 g/l, sulphuric acid: 0.528 M; temperature: 85 C.
No. 27), there is large (14.8%) increase in extraction of iron which is a major impurity in leach liquor whereas there is marginal (1.3%) increase in extraction of manganese. Time has no effect on extraction of Mn after 3 h (Eq. (3)). Therefore, the effect of time on extraction of Mn including Fe, K and Al was studied varying time from 30 to 180 min (Fig. 3). Most of the reactions are complete in the first 30 min (Fig. 3). In 105 min, 98.4% Mn, 8.7% Fe, 96.4% K and 36.4% Al were extracted.
5. Conclusions (i) Manganese can be extracted from low-grade Joda ore using oxalic acid as reductant at 85 C in sulphuric acid medium.
(ii) From the statistical analysis, it was found that all the main and interaction terms significant at 95% confidence level have positive effect on the extraction of Mn, Fe and Al, except oxalic acid concentration and time interaction for extraction of Al which has negative effect. (iii) The oxalic acid concentration has strongest effect on the extraction Mn followed by temperature and sulphuric acid concentration. (iv) Temperature has maximum effect on the extraction of iron followed by sulphuric acid and oxalic acid concentrations. (v) Time has strongest effect on the extraction of Al followed by temperature, sulphuric acid concentration and oxalic acid concentration. (vi) 98.4% Mn could be extracted from low-grade manganese ore of Joda with oxalic acid concentration
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30.6 g/l, sulphuric acid concentration of 0.543 M at 85 C and leaching time of 105 min.
Acknowledgements The authors are thankful to Dr. V.N. Misra, Director, Regional Research Laboratory, Bhubaneswar, Orissa, India for giving permission to publish this paper.
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