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Ind. Eng. Chem. Res. 2000, 39, 488-493
Determination of the Optimum Conditions for Boric Acid Extraction with Carbon Dioxide Gas in Aqueous Media from Colemanite Containing Arsenic Osman Nuri Ata,† Sabri C ¸ olak,† Mehmet C ¸ opur,† and Cafer C ¸ elik*,‡ Departments of Chemical Engineering and Industrial Engineering, University of Atatu¨ rk, 25240 Erzurum, Turkey
The Taguchi method was used to determine optimum conditions for the boric acid extraction from colemanite ore containing As in aqueous media saturated by CO2 gas. After the parameters were determined to be efficient on the extraction efficiency, the experimental series with two steps were carried out. The chosen experimental parameters for the first series of experiments and their ranges were as follows: (i) reaction temperature, 25-70 °C; (ii) solid-to-liquid ratio (by weight), 0.091 to 0.333; (iii) gas flow rate (in mL/min), 66.70-711; (iv) mean particle size, -100 to -10 mesh; (v) stirring speed, 200-600 rpm; (vi) reaction time, 10-90 min. The optimum conditions were found to be as follows: reaction temperature, 70 °C; solid-to-liquid ratio, 0.091; gas flow rate, 711(in mL/min); particle size, -100 mesh; stirring speed, 500 rpm; reaction time, 90 min. Under these optimum conditions, the boric acid extraction efficiency from the colemanite containing As was approximately 54%. Chosen experimental parameters for the second series of experiments and their ranges were as follows: (i) reaction temperature, 60-80 °C; (ii) solidto-liquid ratio (by weight), 0.1000 to 0.167; (iii) gas pressure (in atm), 1.5; 2.7; (iv) reaction time, 45-120 min. The optimum conditions were found to be as follows: reaction temperature, 70 °C; solid-to-liquid ratio, 0.1; gas pressure, 2.7 atm; reaction time, 120 min. Under these optimum conditions the boric acid extraction efficiency from the colemanite ore was approximately 75%. Under these optimum conditions, the boric acid extraction efficiency from calcined colemanite ore was approximately 99.55%. Introduction Boron is never found free in nature but invariably occurs as its oxide, B2O3, in combination with the oxides of other elements, forming borates of complexity.1 Boron compounds, which are used in many branches of industry, are produced from boron-containing ores. Most of the world’s commercially recoverable reserves of borons are in the form of the hydrated borate minerals, such as pundermite, ulexite, tincal, and colemanite. It is estimated that about 54% of the known reserves are in Turkey, which has a substantial boron extraction industry. Boron is usually extracted in the form of boric acid by acid leaching of the borate mineral. In Turkey, colemanite (2CaO3‚3B2O3‚5H2O) is leached using sulfuric acid, but the disposal of the byproduct, gypsum (CaSO4‚2H2O), causes severe environmental problems.2 For the colemanite ore containing Fe and As, it is rather difficult to process this mineral by using this method. To obtain boron compounds from boron minerals, many studies have been performed.2-5 It is known that the emission of CO2 gas into the atmosphere will cause environmental problems, called the greenhouse effect.6 Thus, CO2 gas has been used as a leach reactive for the extraction of H3BO3 from ulexite in atmospheric pressure. The optimum conditions for the H3BO3 extraction from ulexite ore in aqueous media saturated by CO2 gas * Correspondence author. Tel: (+90) (0442) 2184120 (15 lines), ext. 3802. Fax: (+90) (0442) 2336961. E-mail: ccelik@ rocketmail.com. † Department of Chemical Engineering. ‡ Department of Industrial Engineering.
was determined by using 2n factorial design. Optimum working conditions were determined to be as follows: reaction temperature, 72-75 °C; stirring speed, 500 rpm; solid-to-liquid ratio, 1 to 4; reaction time, 140150 min.7 CO2 gas was used as a leach reactive in this study for the extraction of H3BO3 from colemanite containing As because As dissolves in trace quantities in this medium. The reaction between colemanite and CO2 may be represented as follows:
2CaO‚3B2O3‚5H2O + 2CO2 + nH2O S 2CaCO3 + 6H3BO3 + (n - 4)H2O (1) To the authors knowledge, no experimental research on the determination of the optimum conditions for the boric acid extraction from colamanite containing As in aqueous media saturated by CO2 gas exists in the related literature. Thus, it is the aim of the present study to determine optimum working conditions for the boric acid extraction from the colemanite ore containing As in aqueous media saturated by CO2 gas by using the Taguchi method. One of the advantages of the Taguchi method over conventional experimental design methods, in addition to keeping the experimental cost at a minimum level, is that it minimizes the variability around the target when bringing the performance value to the target value. Another advantage is that optimum working conditions determined from the laboratory work can also be reproduced in the real production environment. Because the aim of this study is not the Taguchi method, it will not be explained here. However,
10.1021/ie990314z CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 489 Table 1. Chemical Composition of Colemanite Ore Used in the Study main mineral
%
CaO B2O3 H2O As2O3 SiO2 others
21.6 40.5 20 4.5 5 4.4
those readers who are interested in the method are referred to refs 8-12. Material and Methods The colamanite ore containing arsenic used in the experiments was provided from Bandirma Plant of Etibank, Turkey. After the ore was crushed and ground, it was sieved by using ASTM standard sieves and separated into the proper fractions. The chemical composition of the ore was determined by volumetric and gravimetric methods. The result obtained from the sample is given in Table 1. The first series of extraction experiments were carried out in a glass reactor of 250 mL volume equipped with a mechanical stirrer having a digital controller unit and timer, a thermostat, and a backcooler. The temperature of the reaction medium could be controlled within (1 °C. First 200 mL of H2O was put into the reactor, and CO2 gas at a given flow rate was fed into the reactor; then when a desired temperature of the reactor content was reached, a predetermined amount of the ore was added into the solution while the contents of the vessel was stirred at a certain speed. At the end of the reaction period, the contents of the vessel was filtered and the filtered solution was then analyzed volumetrically for H3BO3.13-14 Experimental parameters and their levels to be studied in the first series of experiments were determined by preliminary tests and are given in Table 2a. The second series of extraction experiments were carried out in a glass reactor of 500 mL volume equipped with a magnetic stirrer and thermostat. The reactor pressure which could be controlled within (0.005 atm was provided by feeding CO2 into the reactor. Experimental parameters and their levels which were determined in light of the first series of experiments are given in Table 2b. The experiments were carried out as the first series of experiments. The orthogonal array (OA) experimental design method was chosen to determine experimental plan L25 (56) for the first series of experiments (Table 3a), because it is the most suitable for the conditions being investigated (six parameters each with five values), and L9 (34) for the second series of experiments (Table 3b), because it is the most suitable for the conditions being investigated (four parameters each with three values).12 To observe the effects of noise sources on the extraction process, each experiment was repeated twice under the same conditions at different times. The performance statistics was chosen as the optimization criteria. The performance statistics was evaluated by using the following equation:9
( )
ZB ) -10 log
1
n
∑ ni)1
1
Yi2
(2)
where ZB is performance statistics, n the number of repetitions done for an experimental combination, and Yi the performance value of the ith experiment. In the Taguchi method the experiment corresponding to optimum working conditions might not have been done during the whole period of the experimental stage. In such cases, the performance value corresponding to optimum working conditions can be predicted by utilizing the balanced characteristic of OA. For this the additive model may be used:15
Yi ) µ + Xi + ei
(3)
where µ is the overall mean performance value, Xi the fixed effect of the parameter level combination used in the ith experiment, and ei the random error in the ith experiment. As a general rule, the optimum performance will be calculated using eq 3. To illustrate with one example from Table 2a, assume the A:5, B:5, C:5, D:1, E:4, F:5 treatment condition (for the first series of experiments) is to be estimated. Then
Yi ) average performance + (contribution of A:5) + (contribution of B:5) + (contribution of C:5) + (contribution of D:1) + (contribution of E:4) + (contribution of F:5) + ei average performance ) 16.7, A:5 ) 8.74, B:5 ) 5.65, C:5 ) 4.67, D:1 ) 7.98, E:4 ) 3.43, and F:5 ) 7.66 Yopt ) 16.87 + 8.74 + 5.65 + 4.67 + 7.98 + 3.43 + 7.66 ) 55% Because eq 3 is a point estimation, which is calculated by using experimental data to determine whether results of the confirmation experiments are meaningful or not, the confidence interval must be evaluated. The confidence interval at chosen error level may be calculated by10
Yi (
x
FR;1,DFMSeMSe
(
1+m 1 + N ni
)
(4)
where F is the value of F table, R the error level, DFMSe the degrees of freedom of mean square error, m the degrees of freedom used in the prediction of Yi, N the number of total experiments, and ni the number of repetitions in the confirmation experiment. If experimental results are in percentage, before eqs 3 and 4 are evaluated first Ω transformation of percentage values should be applied using the equation
(P1 - 1)
Ω(db) ) -10 log
(5)
where Ω(db) is the decibel value of percentage value subject to Ω transformation and P percentage of the product obtained experimentally. Once the statistical analysis is done, the percentage values can be recovered by inverting the Ω transformation.11 The order of the experiments was obtained by inserting parameters into columns of OA, L25 (56), and L9 (34), chosen as experimental plans given in Table 3a,b. However, the order of experiments was made random to avoid noise sources which had not been considered
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Table 2. Parameters and Their Values Corresponding to Their Levels To Be Studied level parameter
1
2
A B C D E F
reaction temperature (°C) solid-to-liquid ratio (by weight) gas flow rate (mL/min) particle size (mesh) stirring sped (rpm) reaction time (min)
a. First Series of Experiments 25 35 0.333 0.200 66.7 208.8 -100 -70 200 300 10 20
A B C D
reaction temperature (°C) solid-to-liquid ratio (by weight) CO2 pressure (atm) reaction time (min)
b. Second Series of Experiments 60 70 0.167 0.125 1.5 2.2 45 90
Table 3. Experimental Plan Table parameters and their levels experiment no.
A
B
C
D
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
a. First Series of Experiments L25 (56) 1 1 1 1 1 2 2 2 1 3 3 3 1 4 4 4 1 5 5 5 2 1 2 3 2 2 3 4 2 3 4 5 2 4 5 1 2 5 1 2 3 1 3 5 3 2 4 1 3 3 5 2 3 4 1 3 3 5 2 4 4 1 4 2 4 2 5 3 4 3 1 4 4 4 2 5 4 5 3 1 5 1 5 4 5 2 1 5 5 3 2 1 5 4 3 2 5 5 4 3
1 2 3 4 5 6 7 8 9
b. Second Series of Experiments L9 (34) 1 1 1 1 1 2 2 2 1 3 3 3 2 1 2 3 2 2 3 1 2 3 1 2 3 1 3 2 3 2 1 3 3 3 2 1
E
F
1 2 3 4 5 4 5 1 2 3 2 3 4 5 1 5 1 2 3 4 3 4 5 1 2
1 2 3 4 5 5 1 2 3 4 4 5 1 2 3 3 4 5 1 2 2 3 4 5 1
initially and which could take place during an experiment and affect results in a negative way. The interactive effects of parameters were not taken into account in the theoretical analysis because some preliminary tests showed that they could be neglected. The validity of this assumption was checked by confirmation experiments conducted at the optimum conditions. Results and Discussion The collected data were analyzed by an IBM-compatible PC using an ANOVA-TM computer software package for evaluation of the effect of each parameter on the optimization criteria. The results obtained are given in Figures 1 and 2. The order of graphs in Figures 1 and 2 is according to the degree of the influence of param-
3
4
5
45 0.143 288.8 -40 400 30
55 0.111 444.3 -20 500 45
70 0.091 711.0 -10 600 90
80 0.100 2.7 120
eters on the performance statistics. At first sight it is difficult and complicated to deduct experimental conditions for graphs given in these figures. We will try to explain it with an example. Let us take Figure 1, which shows the variation of the performance statistics with reaction temperature. Now let us try to determine experimental conditions for the first data point. The reaction temperature for this point is 25 °C which is level l for this parameter. Now let us go to Table 3a and find the experiments for which the reaction temperature level (column A) is 1. It is seen in Table 3a that experiments for which column A is 1 are experiment nos. 1-5. The performance statistics value of the first data point is thus the average of those obtained from experiment nos. 1-5. Experimental conditions for the second data point thus are the conditions of the experiments for which column A is 2 (i.e., experiment nos. 6-10), and so on. The numerical value of the maximum point in each graph marks the best value of that particular parameter and is given in Table 4a for each parameter. That is, parameter values given in Table 4a are the optimum conditions for the first series of experiments. If the experimental plan given in Table 3a is studied carefully together with Table 2a, it can be seen that experiments corresponding to optimum conditions (A:5, B:5, C:5, D:1, E:4, F:5; see Table 4a) have not been carried out during the experimental work. Thus, it should be noted that the extraction percentage given in Table 4a predicted results by using eqs 3 and 5. Also, a 95% significance level confidence interval of prediction is given in Table 4a. To test predicted results, confirmation experiments were carried out twice at optimum working conditions. From the fact that the extraction percentages obtained from confirmation experiments (55% and 53%) are within the calculated confidence intervals (see Table 4a), it can be said that experimental results are within (5% error. This proves that interactive effects of parameters are indeed negligible. The second series of experiments was decided because the obtained extraction efficiency (54%) was not good enough industrially. In the first series of experiments, it has been found that the flow rate of CO2 and the stirring speed have no sensible effect within the range worked. Because the system was open to the atmosphere, once maximum saturation of CO2 is achieved at the given temperature, the increasing flow rate of CO2 would not give any sensible effect on the solubility of CO2. For this reason, in the second series of experiments, a system that was close to the atmosphere was used instead of an open one. Because the solubility of CO2 increases with
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 491
Figure 1. Effect of each parameter on the H3BO3 extraction from colemanite for the first series of experiments.
increasing pressure and also the main reaction is the reaction between solid and liquid, the pressure was chosen as a new parameter. Because the process was a
chemically controlled one, the value of the stirring speed was not very important within the range worked.5 Also, in the second series of experiments, the stirring speed
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Figure 2. Effect of each parameter on the H3BO3 extraction from colemanite for the second series of experiments. Table 4. Optimum Working Conditions and Predicted Extracted Quantities of H3BO3 parameter A B C D E F
A B C D
value
level
a. First Series of Experiments reaction temperature (°C) 70 5 solid-to-liquid ratio (g/mL) 0.091 5 gas flow rate (mL/min) 711 5 particle size (mesh) -100 1 stirring speed (rpm) 500 4 reaction time (min) 90 5 predicted dissolved quantity (%) 53.45 predicted confidence interval (%) 50.93-57.90 b. Second Series of Experiments reaction temperature (°C) 70 solid-to-liquid ratio (g/mL) 0.100 CO2 pressure (atm) 2.7 reaction time (min) 120 predicted dissolved quantity (%) 75 predicted confidence interval (%) 70-80
2 3 3 3
remained constant in the optimum value of 500 rpm, which was determined in the first series of experiments. The particle size remained constant at -100 mesh, which was found as the optimum value in the first series of experiments. Besides, the parameters temperature,
solid-to-liquid ratio, and time, which were found to be effective in the first series of experiments, were examined in detail around their optimum values found in the first series of experiments. In the second series of experiments, the L9 (34) experimental plan was chosen, each experiment was repeated twice at different times, and extraction values obtained are shown in Figure 2. Graphics and statistical analysis were evaluated, and optimum working conditions were determined and were given in Table 4b for each parameter. Under optimum conditions determined at the end of the second series of experiments, confirmation experiments were performed, but a statistically estimated extraction efficiency could not be obtained. When the reason was researched, it was observed that there were significant interactive effects between pressure and temperature. The solubility of CO2 decreases with increasing temperature. The solubility of CO2 may be a limiting factor on the extraction yield. Other parameters were kept constant, the relationship between pressure and reaction temperature was examined, and the results obtained are shown in Figure 3.
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 493
Figure 3. Interactive effects between pressure and temperature.
According to this, optimum working conditions were determined as follows: reaction temperature, 70 °C; pressure, 2.7 atm; solid-to-liquid ratio, 1 to 10; reaction time, 120 min. If the experimental plan given in Table 3b is studied carefully together with that of Table 2b, it can be seen that experiments corresponding to optimum conditions (A:2, B:3, C:3, D:3; see Table 4b) have not been carried out during the experimental work. Thus, it should be noted that the extraction percentage given in Table 4b predicted results by using eqs 3 and 5. Also, a 95% significance level confidence interval of prediction is given in Table 4b. To test predicted results, confirmation experiments were carried out twice under optimum working conditions. From the fact that the extraction percentages obtained from confirmation experiments (74% and 76%) are within the calculated confidence intervals (see Table 4b), it can be said that experimental results are within (5% error. In the experiments carried out in these conditions, as expected, a 75% extraction efficiency was reached. In the present laboratory conditions, to obtain over 75% extraction efficiency, after calcining the ore at 500 °C for 4 h,2 under the optimum conditions determined at the end of the second series of experiments with calcined ore, 99.55% extraction efficiency was obtained. It was observed that the parameters in both works, Yapı´cı´’s7 and the present work, have similar effects on the process when a comparison was made between the results at the present work and Yapı´cı´’s work. However, in this work, the effects of particle size, pressure, and CO2 flow rate were also investigated additionally and there are some differences between these works. Yapı´cı´ et al. treated ulexite mineral in high purity while the present used colemanite ore. The optimization technique is also different for both studies. On the other hand, the present work has the advantage of not having Na+ ion, which is difficult to separate from the solution. Conclusions The optimum conditions for the H3BO3 extraction from colemanite ore containing arsenic in aqueous
media have been determined. The major conclusions derived from the present work are as follows: (a) The most effective parameters are found as follows, respectively: for the first series of experiments, reaction temperature, reaction time, solid-to-liquid ratio, and particle size; and for the second series of experiments, CO2 pressure, reaction time, and solid-to-liquid ratio. Also, it was found that there was a significant interaction effect between temperature and pressure. (b) Extraction efficiencies of 75% with the original ore and 99.55% with the calcined ore were reached. (c) According to these data, it might be suggested that CO2 is a suitable reactive for extraction of H3BO3 from colemanite ore containing arsenic, because arsenic dissolves in trace quantities in water saturated by CO2 gas. Because the optimum conditions determined by the Taguchi method in a laboratory environment are reproducible in real production environments as well, the findings of the present laboratory-scale study may be very useful for production of H3BO3 in industrial scale. Literature Cited (1) Kemp, H. P. The Chemistry of Borates, Part I. Borax Consolidated; London, 1956. (2) Davies, T. W.; C¸ olak, S.; Hooper, K. M. Boric Acid Production by the Calcination and Leaching of Powdered Colemanite. Powder Technol. 1991, 65, 433. (3) Imamutdinova, V. M. Mechanism of Solution of Native Borates in HCI Solutions. Zh. Prikl. Khim. 1963, 37 (5), 1095. (4) Kononova, G. N.; Nozhko, G. S. Nature of the Sulphuric Acid Dissolution of Magnesium Borates. Zh. Prikl. Khim. 1981, 4 (2), 379. (5) Alkan, M.; Kocakerim, M. M.; C¸ olak, S. Dissolution Kinetics of Colemanite in Water Saturated by Carbon dioxide. J. Chem. Technol. Biotechnol. 1985, 35A, 382. (6) Berkers, F.; Kis¸ laliogj lu, M. Ecology and Environment; Remzi Press: Istanbul, Turkey, 1993. (7) Yapı´cı´, S.; Kocakerim, M. M.; Ku¨nku¨l, A. Optimization of Production of H3BO3 from Ulexite. Turkish J. Eng. Environ. Sci. 1994, 18, 91. (8) Kackar, R. N. Off-Line Quality Control, Parameter Design and Taguchi Methods. J. Qual. Technol. 1985, 17, 4, 176. (9) Pignatiello, J. J. J. An Overview of the Strategy and Tactics of Taguchi. IEE Trans. 1988, 20 (3), 247. (10) Ross, P. J. Taguchi Techniques for Quality Engineering; McGraw-Hill: New York, 1987. (11) Taguchi, G. System of Experimental Design; Unipub: New York, 1987. (12) Phadke, M. S. Quality Engineering Using Robust Design; Prentice Hall: Englewood Cliffs, NJ, 1989. (13) Perry, R. H. Perry’s Chemical Engineers Handbook, 6th ed.; McGraw-Hill: New York, 1984. (14) Furman, N. H. Standard Method of Chemical Analysis, 6th ed.; D. Van Nostrand Comput. Inc.: New York, 1963. (15) Phadke, M. S.; Kackar, R. N.; Speeney, D. V.; Grieco, M. J. Off-Line Quality Control in Integrated Circuit Fabrication Using Experimental Design. Bell Syst. Technical J. 1983, 62 (5), 1273.
Received for review May 4, 1999 Revised manuscript received September 22, 1999 Accepted September 22, 1999 IE990314Z
