Ind. Eng. Chem. Res. 2000, 39, 2947-2953
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Surface Response Methodology and Preliminary Process Analysis in the Study of Manganiferous Ore Leaching by Using Whey or Lactose in Sulfuric Acid Solutions F. Veglio` ,*,† I. Volpe,‡ M. Trifoni,‡ and L. Toro‡ Dipartimento di Ingegneria Chimica e di Processo, Facolta` di Ingegneria, Universita` degli Studi di Genova, via Opera Pia, 15-16145 Genova (Albaro), Italy, Dipartimento di Chimica, Facolta` di S.M.F.N., Universita` degli Studi “La Sapienza”, P.le A. Moro, 5-00185 Roma, Italy
In the present work, the use of lactose and whey as reducing agents for the acid leaching of manganiferous ores was investigated. The target of the leaching tests performed by using whey was to simulate the possible use of the byproducts of cheese industries as reducing agents for metal extractions. The experimental results were analyzed by the “response surface methodology” (RSM) to find empirical models suitable to predict metal extraction yields as functions of the main operative conditions. A preliminary process analysis has been performed and a flow-sheet for MnSO4 has been proposed. Introduction Manganese represents an essential raw material for steel industries. The 90% of the total available manganese is used as deoxidizer and desulfurizer in iron and steel production, whereas the remaining 10% is employed in chemical industry (fertilizers, fungicide etc.), in battery manufacturing, in animal feed (as an additive or food integrator), and in the ceramic and glass industry. It has been estimated that the world manganese request could reach 18 million tons in the year 2000. For this reason, studies concerning the extractive processes from nonconventionally treatable minerals (e.g., by pyrometallurgical processes) may be particularly important. Many hydrometallurgical processes, both in acid and basic media, have been studied to recover manganese from ores.1-3 In particular, the MnSO4 production by leaching processes has been recently studied by using H2SO4 both in the presence and in the absence of reducing compounds, taking into consideration the oxidative state of the ores.4,5 As shown in previous works, a reductive-acid leaching process has been developed by using carbohydrates as reducing agents. By such a process, it has been possible to obtain large manganese extraction yields in a relatively short time by using sucrose or glucose as a source of reducing agents.4,6 It was demonstrated that the overall dissolution process can be described by the following reaction (when sucrose is employed as reducing agent):
C12H22O11 + 24MnO2 + 48H+ f 12CO2 + 24Mn2+ + 35H2O (1) Different carbohydrates (glucose, sucrose, fructose) have been used in the leaching tests,4,6 and several * To whom correspondence should be addressed. Tel.: 010/ 3532583. Fax: 010/3532586. E-mail:
[email protected]; veglio@ ing.univaq.it. † Universita ` degli Studi di Genova. ‡ Universita ` degli Studi “La Sapienza”, Rome.
studies have been carried out to evaluate the best experimental conditions in terms of metal extraction yields.6 Sulfuric acid and carbohydrate concentration, temperature, particle size, and mixing conditions were the factors taken into consideration in order to determine their influence on the leaching process. Temperature was found to be a very important factor, indicating a chemical control regime of the leaching process.6 On the basis of the results of those studies, a temperature of 70 °C was considered to be an acceptable compromise value, taking into consideration both the kinetic of the process and practical aspects such as the mechanical and chemical resistance of the construction materials to be selected in the pilot plant design study. The present work will focus on the study of the main process conditions for the manganesiferous ore leaching by using whey or lactose in sulfuric acid solutions at 70 °C. An experimentation work is considered necessary because previous work tests have been carried out by using synthetic compounds such as glucose or sucrose, but a real agro-industrial waste (such as whey or lactose to simulate the whey behavior) should be employed in order to reach acceptable process costs. At the same time, by using whey it is possible to degrade this kind of waste having about 50.000 mg/L as COD content mainly due to the presence of lactose (about 40-50 g/L). The obtained experimental results have been fitted by empirical models for a preliminary process analysis, simulation, and optimization purpose. These results has been used to evaluate as well the extraction of other elements such as Fe and Ca. These metals must present low concentrations in the MnSO4 solution when this compound is used as additive for animal feed (industrial data here not reported). For this reason, with respect to the other works where only manganese leaching was studied, the iron and calcium dissolution have been monitored during the process in order to determine the treatment selectivity in comparison with the manganese dissolution and to evaluate the possible purification process of the liquor leach. Empirical models can be used to find the best operative conditions (sulfuric acid and carbohydrate concen-
10.1021/ie990841i CCC: $19.00 © 2000 American Chemical Society Published on Web 07/19/2000
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Table 1. Elemental Composition of the Main Metals Present in the Investigated Ore elements
composition (%)
Mn Al Fe Si Mg Ca Na Zn
38.8 ( 0.9 10 ( 1 1.23 ( 0.04 19.8 ( 0.9 0.45 ( 0.06 1.8 ( 0.2 15.3 ( 0.9 0.08 ( 0.01
Table 2. Analytical Characteristics of the Used Powder Whey Obtained by Lyophilization components
composition (%)
lactose proteins humidity ash
70 12.3 6.44 9
tration) for obtaining the highest manganese concentration and extraction yield with respect to the other metals (mainly iron and calcium) in the investigated experimental conditions. A preliminary process analysis has been developed for the overall MnSO4 production process to perform a preliminary technical feasibility of the process. 2. Materials and Methods 2.1. Ore and Reducing Agent. The ore used in the present work comes from Italian mines of Casale Castiglione (Viterbo province). The chemical composition of the main elements that are present in the ore is shown in Table 1. Manganese is mainly present as MnO2 (pyrolusite). Other chemical-physical characterizations and the main mineralogical forms present in the ore are reported elsewhere.7 The whey used as reducing agent in the leaching tests was supplied by CRAB (Consorzio Ricerche Applicate alla Biologia, Avezzano, Italy). Its main characteristics are summarized in Table 2. Lactose (R-lactose monohydrate by Sigma) was also used to simulate the whey behavior in the leaching experiments. 2.2. Batch Tests. In each test, 10 or 30 g of mineral, lactose (or whey), and 100 mL of sulfuric acid solution were put in Erlenmeyer flasks and then placed in a Dubnoff shaker at constant mixing conditions (200 rpm) and temperature (70 °C). Two factors were investigated in the laboratory tests: sulfuric acid concentration and lactose concentration according to the selected experimental conditions. 2.3. Chemical Analyses. Several liquor leach samples were collected at different times during the leaching treatment and analyzed after centrifugation. Metal concentrations were detected by a Varian atomic absorption (Mod. SpectrAA-200) after an automatic calibration procedure. 2.4. Data Analysis. Empirical models have been considered suitable at this stage of the work, and the planning of the experimental tests was carried out using the response surface method.8 This methodology has shown to be very helpful to find a suitable relationship between the metal extraction yields (y) and the set of the selected independent variables: sulfuric acid (x1) and lactose concentration (x2).8
Table 3. Tested Experimental Conditions in the Comparison between Leaching Tests Carried out by Lactose and Wheya levels
factors
substoichiometric -20%
stoichiometric 0%
overstoichiometric +20%
lactose (g/L) H2SO4 (M) whey (g/L) H2SO4 (M)
8.3 0.58 11.9 0.58
10.0 0.73 14.2 0.73
12.5 0.88 17.8 0.88
a Temperature 70 °C; ore percentage 10% (w/v); mixing condition 200 rpm.
3. Results 3.1. Comparison between Lactose and Whey. Several preliminary leaching tests have been performed to evaluate the efficacy of the whey as reducing agent in the manganese dissolution. As mentioned above, this one is an agro-industrial waste that would constitute an environmental pollutant if it was dumped. Moreover, it offers an economic advantage because it costs much less than other reducing agents previously employed.4,6 The aim of these tests was to make a comparison between the reducing activity of lactose and whey in different stoichiometric condition leaching tests. Six replicated tests (three tests by using lactose and three by using whey) have been performed at different acid and carbohydrate concentrations in order to carry out a comparison between the two reductants. The selected experimental conditions are reported in Table 3. These conditions have been established considering the stoichiometry reaction (1) and defining three levels of reagent concentrations: a stoichiometric point, an over-stoichiometric point (+20% of whey or lactose with respect to the manganese dioxide content in the ore), and a sub-stoichiometric point (-20% of whey and lactose with respect to the manganese dioxide content in the ore). The three levels investigated were chosen by considering as stoichiometric amount of the reagent the quantity necessary to reduce all the manganese present in the ore and hypothesizing all the other ore compounds as not reacting. The whey amount was evaluated considering its lactose concentration (see Table 2): lactose content in the powdered whey is approximately 70% (w/w). The same experimental conditions have been utilized for both reducing agents (whey and lactose) as shown in Table 3. The H2SO4 amount was always employed with a lactose/H2SO4 molar ratio of 1:24 as reported in eq 1. Manganese extraction yields (iron and calcium dissolution are not here reported) have been calculated as a function of the treatment time. The experimental results are shown in Figure 1. Similar manganese extraction yields obtained by using both lactose and whey demonstrated that it is possible to use lactose in order to simulate the whey behavior during the leaching process. The tested over-stoichiometric reagent condition was chosen taking into consideration the complex chemical and mineralogical composition of the used ore. In fact, the presence of metals such as iron, calcium, aluminum, etc. in the ore gangue can cause a noticable acid consumption.9 As shown in Figure 1, the employed reagent excess did not succeed in obtaining a complete manganese extraction.
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2949 Table 4. Tested Experimental Conditionsa noncodified data
codified data
test no.
H2SO4 (M)
lactose (g/L)
H2SO4 (x1)
lactose (x2)
1 2 3 4 5 6 7 8 9 10 11 12
1.0 1.5 1.0 1.5 1.25 1.25 1.25 1.25 0.9 1.6 1.25 1.25
10.0 10.0 30.0 30.0 20.0 20.0 5.9 34.14 20.0 20.0 20.0 20.0
-1 1 -1 1 0 0 0 0 -1.414 1.414 0 0
-1 -1 1 1 0 0 -1.414 1.414 0 0 0 0
a Temperature 70 °C; ore percentage 10% (w/v); mixing condition 200 rpm.
Table 5. Tested Experimental Conditionsa noncodified data
Figure 1. Comparison between manganese extraction by using lactose and whey: ore percentage 10% (w/v), temperature 70 °C, mixing condition 200 rpm.
These experimental results underline that the reaction stoichiometry obtained by considering only the pyrolusite (MnO2) reduction does not allow to evaluate the necessary reagent amounts when a complex ore is studied. The development of empirical models relating the metal extraction yields to the reagent concentrations permit overcoming this hindrance. 3.2. Response Surface Methodology (RSM). Since lactose and whey showed a similar reducing effect, the following tests have been carried out by using only lactose. Two set of experiments, at different ore percentages (10% and 30% w/v), have been performed to find some empirical models suitable to predict metal extraction yields (Mn, Fe, and Ca) as functions of the selected operative conditions (sulfuric acid and lactose concentration).8 In this way, the real stoichiometry of the process can be evaluated and then used for process analysis purposes. The experimental tests were planned by using the central composite design.8 It is an orthogonal and rotatable design that may be built up from the first-order design by adding the axial points (coded as (1.414 notation). The central point was four-time replicated in order to evaluate the experimental error.8 According to the RSM, the empirical model suitable to fit the experimental results with two independent variables is a second-order polynomial that can be written as 2
2
y ) a + bx1 + cx2 + dx1x2 + ex1 + fx2
(2)
where
y ) metal extraction yield (Mn, Fe, Ca, %) x1 ) sulfuric acid concentration (in coded form) x2 ) lactose concentration (in coded form) a, b, c, d, e, f ) model parameters obtained by linear regression
codified data
test no.
H2SO4 (M)
lactose (g/L)
H2SO4 (x1)
lactose (x2)
1 2 3 4 5 6 7 8 9 10 11 12
3.00 4.50 3.00 4.50 3.75 3.75 3.75 3.75 2.69 4.81 3.75 3.75
30.0 30.0 90.0 90.0 60.0 60.0 17.9 102.4 60.0 60.0 60.0 60.0
-1 1 -1 1 0 0 0 0 -1.414 1.414 0 0
-1 -1 1 1 0 0 -1.414 1.414 0 0 0 0
a Temperature 70 °C; ore percentage 30% (w/v); mixing condition 200 rpm.
The two independent variables (x1, x2) have been expressed in a coded manner using the following relations
x1 )
C1 - C h1 ∆C1
x2 )
C2 - C h2 ∆C2
where
C1 ) sulfuric acid concentration (M) C2 ) lactose concentration (g/L) C h 1, C h 2, ∆C1, ∆C2 are selected as reported elsewhere8 The investigated treatments which have been used are reported in Tables 4 and 5. The results, expressed as percentage yields, have been plotted as a function of the leaching time in Figures 2-7. These data (variable values and obtained responses) have been analyzed by using a linear regression method, and the parameters of eq 2 have been estimated for a selected leaching time. The regression analysis results are shown in Table 6 for a leaching time of 4 h. This leaching time was selected taking into consideration the extraction behavior plotted in Figures 5-7 showing no large improvement in terms of maximum manganese extraction yield after 4-5 h of leaching in the selected experimental conditions. The good agreement between
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Figure 2. Manganese extraction curves vs time: ore percentage 10% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of the replicated tests 5, 6, 11, and 12 (see Table 4).
Figure 4. Calcium extraction curves vs time: ore percentage 10% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of tests 5, 6, 11, and 12 (see Table 4).
Figure 3. Iron extraction curves vs time: ore percentage 10% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of tests 5, 6, 11, and 12 (see Table 4).
Figure 5. Manganese extraction curves vs time: ore percentage 30% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of tests 5, 6, 11, and 12 (see Table 5).
the experimental data and those estimated by the empirical models is shown in Figure 8 as an example. Yield contour plots have been used to give graphic representation of the metal extraction yields. Figure 9 shows a contour plot of the manganese extraction yield when 10% of the ore is employed. Considering these results, it can be observed that: (1) In the case of manganese extraction by using a content of pulp of 10% the parameters a, b, c, and f were found to be significant. Both acid and carbohydrate concentration affect the metal recovery. When 30% of ore was employed, only the parameters a and f were found to be significant. The experimental evidence collected for the 30% of ore concentration shows that the manganese extraction yield depends only on the lactose concentration (parameter f), but this experimental evidence does not mean that the sulfuric acid is
not an important factor for the metal extraction: its concentration is not influential only in the range of the experimental condition investigated (in other words, there is a stoichiometric excess of H2SO4). (2) The iron extraction yield depends only on H2SO4 (by using a pulp content of 10%), whereas in the tests carried out with an ore percentage of 30% this metal extraction is not affected by lactose concentration or by acid amount. A previous work9 showed that the carbohydrate and acid presence may improve the iron recovery mainly when its oxidation state is +3 (for example, as Fe2O3). In the case of the tested conditions, these effects may be not evidenced because high reagent concentrations are used with respect to the stoichiometric conditions necessary for the iron(III) reduction. (3) The calcium extraction yield result to be affected by sulfuric acid concentration both for 10% and 30%
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2951
Figure 6. Iron extraction curves vs time: ore percentage 30% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of tests 5, 6, 11, and 12 (see Table 5).
Figure 8. Scatter diagram of the estimated extraction yields vs experimental extraction yields: ore percentage 10% (w/v).
Figure 9. Contour plot of the manganese extraction yield as a function of sulfuric acid and lactose concentration: ore percentage 10% (w/v). Figure 7. Calcium extraction curves vs time: ore percentage 30% (w/v), temperature 70 °C, mixing condition 200 rpm. *Average value of tests 5, 6, 11, and 12 (see Table 5).
of pulp. As shown in Figures 4 and 7, its recovery decreases after 4 h of leaching treatment. By using large sulfuric acid concentrations the calcium sulfate precipitation can take place after extraction times higher than 4 h (common ions effect). SEM analysis performed on the white precipitate recovered in the liquor leach confirmed this supposition (data here not reported). Moreover, the kinetic of this salt precipitation may be influenced by the presence of other metals as reported elsewhere.10 Although empirical relations for the calcium leaching have been obtained, studies of solubility for the system Mn2+, Fe3+, Ca2+/SO42- are in progress. The empirical models obtained can be useful to evaluate the suitable process conditions in order to gain a satisfactory manganese extraction yield and a low amount of impurities (iron and calcium in the case studied).
Table 6. Estimated Parameter Values of the Empirical Models for the Extraction Percentage Yields (after 4 h) of Manganese, Iron, and Calciuma parameter element
a
b
c
d
e
f
Mn Fe Ca
Ore Percentage ) 10% (w/v) 76 ( 1 3(1 9 ( 1 n.s. 28.2 ( 0.3 3.6 ( 0.4 n.s. n.s. 63 ( 1 6(2 -6 ( 2 n.s.
n.s. n.s. n.s.
-4 ( 1 n.s. n.s.
Mn Fe Ca
Ore Percentage ) 30% (w/v) 90 ( 4 n.s. n.s. n.s. 34 ( 2 n.s. n.s. n.s. 10.3 ( 0.3 -1.0 ( 0.3 n.s. n.s.
n.s. n.s. n.s.
-8 ( 4 n.s. n.s.
a
n.s. ) not significant.
Process Analysis On the basis of the obtained experimental results and on those reported elsewhere,6,11 it is possible to develop a preliminary flow-sheet of the process. The MnSO4
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Figure 11. Simulation plot. Metal amounts in the liquor leach as a function of H2SO4 concentration. Dotted line: 10% of pulp, 20 g/L of lactose. Solid line: 30% of pulp, 60 g/L of lactose. Manganese extraction yield: 69-78% for 10% of pulp and 90% for 30% of pulp.
Figure 10. Flow-sheet of the MnSO4 production process.
production has been considered in this case. The leaching process was demonstrated to be suitable for different ores,7 and some preliminary purification tests of the liquor leach has been carried out.11 Moreover, a recycling of the exhaust ore (mainly constituted by aluminum silicate7), after a washing step by water to eliminate sulfates, can be supposed for the lightweight building material production (celloblocks).12 In this way, the overall process can be summarized in the flow-sheet shown in Figure 10. After the leaching step, a filtration unit permits the separation of the liquor leach from the exhaust ore. This last is washed by water to eliminate sulfates that must be absent for building material manufacturing.12 The washing water containing MnSO4, etc. can be reused to save water in the leaching solution preparation. A preliminary liquor leach purification can be performed by adding NaOH to precipitate metals such as iron, aluminum, etc. However, other solutions may be possible for iron purification.13-15 After this purification step, a concentration by evaporation is necessary if the MnSO4 concentration is not at the target value required by industries (about 300 g/L in solution). A drying step is eventually necessary if MnSO4 powder must be produced too. It is obvious that the increasing of the ore concentration in the leaching step is possible to have large Mn2+ concentration and reduce the cost of the evaporation step. By using the empirical equations reported above (see Table 6), a simulation study was carried out in order to evaluate the effect of the ore concentration on the Mn, Fe, and Ca amount in the liquor leach as a function of sulfuric and lactose content. Figure 11 shows a simulation as an example: the lactose concentrations for the two ore percentages (10 and 30%) were 20 and 60 g/L, respectively. From the analysis of these results, it is possible to conclude that between the two tested ore concentrations the largest one may be considered much more suitable for the leaching condition. Moreover, in this experimental con-
dition the calcium concentration is much lower than that reached by using 10% of ore amount. The calcium dissolution was considered more critical with respect to the iron one because the liquor leach purification from iron may be easily obtained by the pH solution change. Moreover, for large ore concentrations, also the Mn/Fe and Mn/Ca molar ratio is much larger and, at the same time, the manganese concentration in the liquor leach may be improved with obvious advantage for the overall process. By considering both the purification step and the economical constrains, it is clear that only a global process optimization can be suitable to find the best process conditions. The proposed process flow-sheet and the obtained experimental results suggest further achievements for this research. In fact, experimental and theoretical studies are in progress in order to evaluate (1) the equilibrium and the kinetic of the precipitation step (calcium and iron removal from liquor leach); (2) the technical feasibility of the solid waste recycling for the celloblocks production; (3) the optimization of the overall process in terms of minimization of costs and waste (approaching a “zerowaste” process). The experimental results have shown the technical feasibility of the process for the leaching section: an industrial waste, such as whey, can be used as reducing agent in the manganese dissolution process. Conclusions The experimental results have shown the technical feasibility of this process and the application of the whey (a non-environmentally friendly waste of the cheesefactories) as ra educing agent in the manganiferous ore leaching process. Moreover an empirical model, suitable to fit experimental results of metal extraction yields, has been studied by using the response surface methodology. Yield contour plots helpful for a successive process analysis have been built. In this way it is possible to find (in the range of the investigated experimental conditions) the sulfuric acid and the whey concentration suitable to obtain metal concentrations as close as possible to those required by market. A
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preliminary process analysis has been carried out and indications for further improvements in the process development have been indicated. Acknowledgment This research was carried out with the financial support of CNR. The authors thank Mr. Marcello Centofanti for his helpful technical collaboration in this work. Literature Cited (1) Kanungo, S. B.; Das, R. P. Extraction of metals from manganese nodules of the Indian ocean by leaching in aqueous solutions of sulphur dioxide. Hydrometallurgy 1988, 20, 135. (2) Paixao, J. M. M.; Amaral, J. C.; Memorial L. E.; Freitas L. R. Sulphation of Carajas manganese ore. Hydrometallurgy 1995, 39, 215. (3) Momade, F. W. Y. Sulphuric acid leaching of the Nsuta manganese carbonate ore. Hydrometallurgy 1996, 49, 123. (4) Veglio`, F.; Toro, L. Reductive leaching of a concentrate manganese dioxide ore in acid solution: stoichiometry and preliminary kinetic analysis. International Journal of Mineral Processing 1994, 40, 257. (5) Veglio`, F.; Centofanti, M.; Beolchini, F.; Ubaldini, S.; Toro, L. Manganese dioxide reductive leaching by sulphuric acid solutions containing carbohydrates. In Innovation in Mineral and Coal Processing; Atak, S., Onal, G., Celik, M. S., Eds.; Balkema: Rotterdam, 1998; p 457. (6) Veglio`, F.; Toro, L. Fractional factorial experiments in the development of manganese dioxide leaching by sucrose in sulphuric acid solutions. Hydrometallurgy 1994, 36, 215.
(7) Trifoni, M.; Veglio`, F.; Taglieri, G.; Toro, L. Acid leaching process by using glucose as reducing agent: a comparison among the efficiency of different kinds of manganiferous ores. Minerals Eng. 2000, 13(2), 217. (8) Montgomery, D. C. Design and Analysis of Experiments; John Wiley & Sons: New York, 1991. (9) Veglio`, F.; Recinella, M.; Massacci, P.; Toro, L. Screening tests in the study of the iron oxide leaching by sucrose in sulphuric acid solutions using statistical methods. Hydrometallurgy 1994, 35, 293. (10) Hamdona, S. K.; Nessim, R. B.; Hamza, S. M. Spontaneous precipitation of calcium sulphate dihydrate in the presence of some metal ions. Desalination 1993, 94, 69. (11) Trifoni, M.; Veglio`, F.; Volpe, I.; Toro, L. Reductive acid leaching process of manganiferous ores: manganese sulphate production and its purification. ICheaP-4, Florence, May 1999. (12) Volpe, I.; Centofanti, M.; Trifoni, M.; Di Benedetto, G.; Taglieri, G.; Veglio`, F.; Toro, L. Processo innovativo di lisciviazione chimica di minerali manganiferi: sviluppo di processo mediante un approccio “zero-waste”. 2nd National Congress Valorization and Recycling of Industrial Wastes; L′Aquila, July 1999. (13) Chen, J. C.; Yu, S.; Liu, H.; Meng, X.; Wu, Z. New mixed solvent systems for the extraction and separation of ferric iron in sulphate solutions. Hydrometallurgy 1992, 30, 401. (14) Tait, B. K.; Mdlalose, K. E.; Taljaard, I. The extraction of some metal ions by LIX 1104 dissolved in toluene. Hydrometallurgy 1995, 38, 1. (15) Das, R. P.; Anad, S. Precipitation of iron oxides from ammonia-ammonium sulphate solutions. Hydrometallurgy 1995, 38, 161.
Received for review November 19, 1999 Revised manuscript received March 22, 2000 Accepted April 23, 2000 IE990841I
