Ind. Eng. Chem. Res. 2005, 44, 427-433
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APPLIED CHEMISTRY Recovery of Boron from Tincal Wastes Melike Sinirkaya,*,† M. Muhtar Kocakerı3 m,† Recep Boncukc¸ uogˇ lu,‡ O 2 zkan Ku 1 c¸ u 1 k,† and Salim O 2 ncel§ Engineering Faculty, Chemical Engineering and Environmental Engineering Departments, Ataturk University, Erzurum, Turkey, and Environmental Engineering Department, Gebze Institute of Technology, Gebze, Turkey
Boron ores are some of Turkey’s most important underground riches. Tincal (Na2B4O7‚10H2O), having the largest deposit, originally contains clay minerals such as illite and montmorillonite. One of the most important problems in refined borax production is separating tincal from the clay minerals, which are in a colloidal state in an aqueous solution and cannot be filtered. To separate clay particles, various additives are used as coagulants and cause both increasing production cost and borax loss. In this study, the reactor waste forming during borax production from tincal ore was dissolved in distilled water, CO2-saturated water, and SO2-saturated water to recover its B2O3 contents, to investigate dissolution kinetics, and to determine the filtration conditions in these dissolving media. The dissolution kinetics of tincal waste were investigated by choosing the temperature as a parameter, and it was found that activation energies and Arrhenius constants were 62.2 kJ mol-1 and 1.8 × 1010 for SO2-saturated water and 24.5 kJ mol-1 and 1.6 × 104 for CO2-saturated water, respectively. On the other hand, the effects of passing SO2 and CO2 gases through a tincal solution on spoiling the colloidal state of clay minerals were studied by using the temperature and stirring time for dissolution as parameters. It was seen that the colloidal state could be spoiled by passing SO2 or CO2 gases through the suspension. The filtration rate increased by increasing the temperature and stirring time for dissolution when SO2-saturated water was used, but filtration was difficult in CO2-saturated water and impossible in distilled water. Introduction Turkey has 60% of the world boron deposits in which its minerals are in the form of hydrated borates containing Ca, Mg, and Na. Tincal, the natural mineral of a chemical compound called borax decahydrate with the chemical formula Na2B4O7‚10H2O, is one of the most widespread boron minerals and has a monoclinal crystal structure. Boron ores are used in glass, ceramics, soap, and detergent industries, in metallurgical and nuclear processes, and in the production of various boron compounds used in different industrial areas. Boric acid, borax, boric oxide, and refined hydrated sodium borates and perborates are important boron compounds economically. Various studies have been carried out to produce boron compounds, especially boric acid and borax from boron ores. Gu¨lensoy and Kocakerim examined the dissolution of colemanite in CO2-saturated water and the geological occurrence of this mineral.1 The dissolution kinetics of pure and clay-containing colemanite were studied in SO2-saturated water, and it was found that dissolution processes were chemically controlled for * To whom correspondence should be addressed. Fax: +90 442 2360957. E-mail:
[email protected]. † Chemical Engineering Department, Ataturk University. ‡ Environmental Engineering Department, Ataturk University. § Gebze Institute of Technology.
both cases. However, the former had a larger apparent activation energy than the latter.2,3 In another study, the evaluation of the reactor waste in borax production was investigated to recover B2O3 and therefore to reduce the toxic effect of the waste.4 Also, some of these studies include the production of ammonium borate5 and boric acid6 from colemanite by CO2-saturated ammonia solutions, borax from colemanite by using NaCl and seawater,7 boric acid from sulfuric acid and borax,8 and boric acid and borax from hydrochloric acid and boron ore.9 In addition, the dissolution of colemanite and other boron ores has been carried out in various acid solutions.10-19 One of the most important problems in refined borax production is separating tincal from clay minerals. In the process applied at the present time, when tincal ore is dissolved in hot water, clay minerals in it swell and turn to the colloidal form. The suspension cannot be filtered in this form, and it is necessary to add some coagulants, which increase costs and the formation of solid waste containing about 5% B2O3. In this study, the dissolution of the solid waste obtained during the production of refined borax from tincal in SO2-saturated water, CO2-saturated water, and distilled water was investigated. Thus, studies were conducted to determine the conditions under which boron losses will decrease, the filtration will become easier, and boron pollution due to discharge to the environment of boron-containing wastes will be removed.
10.1021/ie049705q CCC: $30.25 © 2005 American Chemical Society Published on Web 12/29/2004
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Figure 1. X-ray diffractogram of the sample.
Figure 2. Effect of the temperature on the dissolution of the B2O3 contents in the reactor waste in SO2-saturated water.
Material and Method Reactor waste used in the study was assured from Bandirma Borax and Acid Factories in Turkey and composed of the undissolved part of tincal ore in the tincal reactor. After this sample was dried under ambient conditions, it was crushed, ground, and divided into fractions by sieving with ASTM standard sieves. The chemical analysis showed that the sample contained 5.5% B2O3, 38.5% CaCO3, 32.3% MgCO3, and 23.7% clay minerals. An X-ray diffractogram of the sample, obtained by a Rigaku DMAX 2000 series X-ray diffractometer, is given in Figure 1. It is seen that the sample contains tincalconite, dolomite, and clay minerals pertaining to the montmorillonite group. Dissolution experiments were carried out in a 500mL jacketed glass reactor under atmospheric pressure. The particle size due to the comminating form of the waste, stirring speed, and gas flow rate because of their ineffectiveness and solid-to-liquid ratio were kept constant. Their values were 250-180 µm, 400 rpm, 200 mL min-1, and 5 g/300 mL, respectively. The reactor was equipped with a mechanical stirrer and a tachometer to control the stirring speed and with a constant-temperature circulator to control the reaction temperature. The reactor containing 300 mL of distilled water was fed by SO2 or CO2 gases, which were dried by being
Figure 3. Effect of the temperature on the dissolution of the dolomite contents in the reactor waste in SO2-saturated water.
passed through a gas washing bottle and drying tower filled with active carbon. After the solution was saturated with these gases and the solid sample was added to this saturated solution, passing of CO2 or SO2 gases through the solution continued during the dissolution process. In addition to these, the experiments were repeated with only water without using SO2 and CO2 gases also. Dissolution experiments in which the temperature was chosen as a parameter were carried out at 5, 11, 25, 35, and 50 °C. After each experiment, the reactor content was filtered, and B2O3, Ca2+, and Mg2+ analyses were made in the filtrate.20,21 Dissolution fractions of B2O3 were used in the prediction of dissolution kinetics. Filtration was carried out by 2.5-µm-pore-diameter cellulose filters under atmospheric pressure conditions. In these experiments, the temperature and stirring time were used as parameters. Filtration rates were compared with each other to determine the filtration properties of all three suspension cases. Results and Discussion Distilled water, CO2-saturated water, and SO2saturated water were used as dissolving media to
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Figure 4. Effect of the temperature on the dissolution of the B2O3 contents in the reactor waste in CO2-saturated water.
determine dissolution kinetics of borax in the solid waste and to recover borax. The solid-to-liquid ratio was kept constant at 5 g/300 mL, the particle size 250-180 µm, the stirring speed 400 rpm, and the gas flow rate 200 mL min-1. Reactions. When SO2 was used, dolomite (CaCO3‚ MgCO3) passes to the solution by the following reactions:
CaCO3‚MgCO3(s) + 4SO2(aq) + 2H2O f Ca(HSO3)2(aq) + Mg(HSO3)2(aq) + 2CO2(g) (1) Ca(HSO3)2(aq) + (x - 1)H2O f CaSO3‚xH2O(s) + SO2(g) (2) Mg(HSO3)2(aq) + (y - 1)H2O f MgSO3‚yH2O(s) + SO2(g) (3) However, when CO2 was used, dolomite does not dissolve or dissolves in a very small amount. B2O3 dissolves in this medium because it is in the form of borax. Dissolution. Dissolution temperatures were 5, 11, 25, 35, and 50 °C in the experiments in which B2O3 was recovered by SO2. The results are given in Figure 2. As seen in the figure, the dissolution rate increases with an increase in the temperature. In a 60-min experiment
at 5 °C, 73.3% of the B2O3 contents of waste passed to the solution, and for 30 min at 50 °C, 99%. A similar tendency was seen for dolomite passing to the solution also (see Figure 3). The experiments in which CO2 was used were carried out under the same conditions as those for experiments in which SO2 was used. The results given in Figure 4 show that the amounts of B2O3 passing to the solution increased with an increase in the reaction temperature. However, under these conditions, dissolution fractions of CaCO3 were too small (2 × 10-3-8 × 10-3) because of the very small dissolution of CaCO3. The behavior of MgCO3 was the same as that of CaCO3 also. In the experiments in which B2O3 was recovered by using distilled water, although the reactor conditions were the same as those of CO2 and SO2, satisfying results could not be obtained because of filtration problems. Dissolution Kinetics. The rate of a noncatalytic reaction between a solid and a fluid can be expressed by that of an unreacted-core model and a progressive conversion model.22,23 In an unreacted-core model, the rate may be controlled by diffusion through a fluid film, by diffusion through a product (ash) layer, or by chemical reaction. In a progressive conversion model, the rate may be shown by pseudohomogeneous models such as pseudo-first-order and pseudo-second-order models. In these models, fractional conversion (X) is given as a function of the reaction time. This function, which differs for each model, is
X ) t/t*
for diffusion through a fluid film (4) 2/3
1 - 3(1 - X) + 2(1 - X) ) t/t* for diffusion through a product (ash) layer (5) 1 - (1 - X)1/3 ) t/t*
for chemical reaction (6)
-ln(1 - X) ) ktm or -ln(1 - X) ) kt for pseudo-first-order reaction (7) X X ) ktm or ) kt 1-X 1-X for pseudo-second-order reaction (8) Dissolution kinetics of the tincal reactor waste were investigated in SO2- or CO2-saturated solutions. For this
Figure 5. Dissolution of the tincal waste in SO2-saturated water. Plots of X/(1 - X) versus different temperatures (particle size, 250180 µm; solid-to-liquid ratio, 5 g/300 mL; stirring speed, 400 rpm; gas flow rate, 200 mL min-1).
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Figure 6. Dissolution of the tincal waste in CO2-saturated water. Plots of X/(1 - X) versus different temperatures (particle size, 250180 µm; solid-to-liquid ratio, 5 g/300 mL; stirring speed, 400 rpm; gas flow rate, 200 mL min-1).
Figure 7. Experimental versus predicted values concerning dissolution of the tincal waste in SO2-saturated water.
purpose, the above reaction models were tested statistically. As a result, it was found that the experimental data fit the pseudo-second-order reaction model for both cases and the dissolution could be presented by a model as
X/(1 - X) ) k0e-E/RTtm
(9)
Also, X/(1 - X) versus t graphs were linear, as seen in Figures 5 and 6. Predicted kinetic parameters are given in Table 1. On the other hand, to test the agreement between experimental X values (Xexp) and values predicted from eq 9 (Xprd), plots of Xexp versus Xprd were drawn and are given in Figures 7 and 8, and the agreement between the experimental and predicted conversion values was found to be very good. Filtration. The temperature and stirring time were taken as parameters in the filtration studies in which a 2.5-µm-pore-diameter cellulose filter was used. In the studies carried out with 250-180-µm waste, the solidto-liquid ratio was chosen as 5 g mL-1, the stirring speed
400 rpm, and gas flow rate 200 mL min-1. In the studies with SO2, the solution was filtered up to 200 mL and the filtration times were registered for each 50 mL. Filtration times versus filtration volumes were plotted for various dissolution times at 11 °C, as given in Figure 9. In the examination of this plot, it was seen that, as the stirring time increased, the particles became large because of flocculation, the cake resistance became less, and the suspensions obtained were filtered more easily. In the plot of filtration time versus stirring time, it appeared that the filtration time lessened with an increase in the temperature and stirring time for the first 50-mL suspension (see Figure 10). The same case was valid for CO2-saturated water and distilled water also. When the filtration times of suspensions obtained in SO2- and CO2-saturated water and distilled water under the same experimental conditions were compared with each other, it was seen that the filtration times obtained with SO2 were shorter than those obtained with CO2. Also, when distilled water was used for dissolution,
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Figure 8. Experimental versus predicted values concerning dissolution of the tincal waste in CO2-saturated water.
Figure 9. Filtration times of the suspension obtained with SO2-saturated water at 11 °C for various dissolution times (filtration was carried out at ambient temperature).
filtration times were too long. This situation is seen in Figure 11 for 50 °C. Conclusions The main results of this study are as follows: The dissolution rate increases with an increase in the temperature. Also, it was found that all of the B2O3 contents could be recovered with an increase in the stirring time. Especially, the dissolution studies with SO2 gave satisfactory results. In the dissolution experiments with SO2, all of the B2O3 contents could be recovered at 50 °C and 30 min of stirring time. The results obtained with CO2-saturated and distilled water did not satisfactorily recover all of the B2O3 contents. Dissolution data showed that, in dissolution experiments with SO2 and CO2, the dissolution rates fit the pseudo-second-order reaction model. The activation energies were 62.2 kJ mol-1 for SO2 and 24.5 kJ mol-1 for CO2. It was seen in filtrations of suspensions using SO2 that the colloidal particles coagulated and could be
filtered easily. In the case in which CO2-saturated water was used, the filtrations of the suspensions obtained were too difficult and almost impossible for dissolutions with distilled water. The high performance of the SO2 system in comparison to the CO2 system and distilled water was attributed to the facts that SO2-saturated water was a stronger acid than CO2-saturated water and clay minerals coagulated more easily at the pH of SO2-saturated water. On the other hand, it was determined that the filtration became easier with an increase in the suspension temperature and stirring time. Under studied conditions, the shortest filtration time was obtained with SO2-saturated water at 50 °C. This situation showed that suspensions coagulated best in SO2saturated water at 50 °C for 60 min of stirring time and could be filtered more easily; thus, all of the B2O3 contents could be recovered. As a result, it was determined that the colloidal state of clay minerals could be spoiled, boron loss could be minimized, and some boron minerals remaining undis-
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Figure 10. Effect of the temperature on filtration times of 50-mL suspensions in SO2-saturated water for various dissolution times (filtration was carried out at ambient temperature).
Figure 11. Filtration times of 50-mL suspensions in SO2- or CO2-saturated water and distilled water at 50 °C for various dissolution times (filtration was carried out at ambient temperature).
List of Symbols
Table 1. Values of Predicted Kinetic Parameters kinetic parameter reactive
k0
E (kJ mol-1)
m
SO2 CO2
1.8 × 1010 1.6 × 104
62.2 24.5
1.17 0.31
solved in water could be dissolved. It was found also that the boron contents of the sample studied passed to the solution in short-term dissolution. In addition, as a result, it was possible to filtrate quickly by using this method, without the coagulant and with less boron loss. When the process proposed was compared with the present process, it was seen that there was no B2O3 loss in this new process. In addition, some minerals such as colemanite, ulexite, etc., which do not dissolve in water and may be present in tincal ore in very small amounts, dissolve in this medium and cause additional boron recovery. Filtration is cheaper and easier.
X ) fractional conversion t ) time (min) t* ) time for complete conversion of a single solid particle (min) E ) activation energy (kJ kmol-1) R ) gas constant (8.314 kJ kmol-1 K-1) k0 ) Arrhenius constant k ) rate constant m ) constant
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Received for review April 13, 2004 Revised manuscript received October 4, 2004 Accepted November 2, 2004 IE049705Q
