5256
Ind. Eng. Chem. Res. 2003, 42, 5256-5260
SEPARATIONS Boron Recovery from Borax Sludge, Boron Industrial Waste, by Solid-Liquid Extraction Mine O 2 zdemir* and I4 lker Kıpc¸ ak Department of Chemical Engineering, Osmangazi University, 26480 Batı Mes¸ elik, Eskis¸ ehir, Turkey
The recovery of boron from borax sludge by solid-liquid extraction was investigated using 23 factorial experimental design. Distilled water and sulfuric acid were used as extraction solvents. The effects of solid-to-liquid ratio, reaction time, and reaction temperature were investigated for extraction with distilled water. The effects of solid-to-liquid ratio, reaction time, and solvent concentration were investigated for extraction with sulfuric acid. The interaction among the operating parameters and their relative significance were studied. Regression equations were established for both solvents, and the extraction was represented as a function of response variables. The accuracy of equations was verified by Fisher’s adequacy test. It was found that the most effective parameter for the extraction process was solid-to-liquid ratio for both of the solvents. Maximum boron oxide extraction efficiency was obtained under the following working conditions: solid-to-liquid ratio, 1/120 g/mL; reaction time, 60 min; reaction temperature, 75 °C for distilled water; and solid-to-liquid ratio, 1/120 g/mL; reaction time, 60 min; solvent concentration, 1%, v/v; reaction temperature, 75 °C for sulfuric acid. Introduction The boron minerals and compounds are used in various industries, and their usage increases gradually. Turkey has 803 million tonne of boron reserves, which comprise 63% of the total world boron reserves. Turkey is the second producer following the United States with 1.72 million tonne of boron minerals production/yr.1,2 In the Etibank Kırka Borax Plant, the production amounts are by 800 000 tonne/yr of concentrated tincal (Na2O2B2O3‚10H2O), 160 000 tonne/yr of borax pentahydrate, 60 000 tonne/yr of anhydrous borax, 17 000 tonne/yr of borax decahydrate. Annually 175 000 tonne of borax sludge forms during production in the borax concentration unit and borax pentahydrate unit of the plant. This waste, containing 19.44% B2O3, is discharged into the ponds having an area bigger than the plant area. If these waste ponds overflow, waste damages agricultural areas. Boron compounds in this waste pass to soil; they form some complexes with heavy metals so that the potential toxicity of heavy metals increases. Thus, boron compounds cause some serious health and environmental problems when the complexes pass to groundwater.3 Therefore, there is a necessity of making use of this waste in large amount to avoid the environmental problems. Furthermore, not using this waste containing boron oxide in a high concentration is an economical loss. Some studies concerning the utilization of industrial solid boron wastes were about the boron recovery from solid wastes; the production of cement, building mate* To whom correspondencd should be addressed. Tel.: +90 222 2393750 (3279). Fax: +90 222 2393613/2290535. E-mail:
[email protected].
rial, ceramic, glaze, and fertilizer.4-6 Water, water saturated by SO2 and CO2, and sodium carbonate and bicarbonate solutions were used as solvent in studies related to boron recovery from solid wastes by extraction. In these works, it was determined that 90% of B2O3 in wastes was recovered under various conditions. We have not noticed any work on boron recovery from borax sludge with sulfuric acid by solid-liquid extraction. Thus, the aim of this study is to determine the conditions under which boron oxide extraction from the solid part of borax sludge with maximum efficiency takes place and to investigate the effects and interactions of selected parameters and their relative importance by using experimental design. Statistical design of experiments is a useful analytical method for process characterization, optimization, and modeling. The experiments in which the effects of more than one factor on response is investigated are known as full factorial experiments. One of the advantages of full factorial experimental design is that the experimental cost is kept at a minimum level because of the smaller number of experiments and the shorter time. Its other advantages are that not only the effects of individual parameters but also their relative importance in given process are obtained and that the interactional effects of two or more variables can also be known. This is not possible in a classical experiment.7,8 Materials and Methods The borax sludge used in this study was provided from the Etibank Kırka Borax Plant in Turkey. The solid and liquid parts of this sludge were separated by filtrating. The solid part was dried at room temperature and then sieved by using ASTM standard sieves. The
10.1021/ie020653j CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5257 Table 1. Parameters and Their Levels for Boron Oxide Extraction with Distilled Water
Table 3. Experimental Design Matrix for Boron Oxide Extraction with Distilled Water
parameter
low level (-)
high level (+)
expt no.
x1
x2
x3
x1x2
x1x3
x2x3
x1x2x3
solid-to-liquid ratio (g/mL) reaction time (min) reaction temperature (°C)
1/120 5 25
10/120 60 75
1 2 3 4 5 6 7 8
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
Table 2. Parameters and Their Levels for Boron Oxide Extraction with Sulfuric Acid parameter
low level (-)
high level (+)
solid-to-liquid ratio (g/mL) reaction time (min) solvent concentration (% v/v)
1/120 5 0.1
10/120 60 1.0
particles smaller than 100 mesh were used in the experiments. The chemical analysis of the solid part used was carried out by using XRF ARL 8680. The chemical analysis for boron oxide content was achieved by volumetric method.9 The chemical composition of the solid waste is as follows: 19.44% B2O3, 16.85% CaO, 13.01% MgO, 9.82% SiO2, 10.30% Na2O, 1.30% Al2O3, 1.49% Fe2O3, 1.01% K2O, and 26.78% H2O. Extraction experiments were carried out in a batch-type heaterjacketed reactor of 150 mL capacity. The reactor was kept at the desired temperatures by circulating water from a MGW Lauda model constant-temperature bath. A Heidolph MR3001 model magnetic stirrer was used for constant stirring. In the experiments, 120 mL of solvent (distilled water or H2SO4 solution) was added to the reactor, which was then heated to the reaction temperature. Then a definite amount of the solid waste was fed into the reactor. The reactor content was stirred at 1250 rpm for a certain reaction period. At the end of the reaction time, the reactor content was filtered. The amount of boron oxide in the filtrate was analyzed by volumetric method. The extraction percentage of boron oxide was calculated in terms of boron oxide in the original solid waste. Furthermore, at the conditions under which the maximum boron oxide extraction was obtained, the amount of MgO and CaO in the filtrate was analyzed by volumetric method. Ca2+ and Mg2+ in the filtrate were precipitated as Ca3(PO4)2 and MgNH4PO4‚6H2O by using (NH4)2HPO4 solution. Then, the remaining solution containing boron oxide was precipitated. Chemical analysis and X-ray diffractometry were applied to the obtained precipitate and the remaining solid part in order to observe the structural changes that occurred in the solid waste after extraction. Results and Discussion Design of Experiments. Boron recovery from borax sludge was investigated by solid-liquid extraction using 23 full factorial experimental design. Three parameters and two levels were chosen. Parameters and their levels are given in Tables 1 and 2. The higher level was designated as (+), and the lower level was designated as (-). The total number of experiments needed for investigation is 23. Each experiment was carried out twice by parallel. If Y, the extraction efficiency of boron oxide, is the response variable, then the regression equation with three parameters and their interaction with each other is given by
Yi ) b0 + b1x1i + b2x2i + b3x3i + b12x1ix2i + b13x1ix3i + b23x2ix3i + b123x1ix2ix3i (1)
Y (%) 66.73 58.58 75.02 60.20 84.71 69.57 92.33 68.00
68.98 58.83 77.29 60.42 86.65 71.26 93.25 68.50
Table 4. Experimental Design Matrix for Boron Oxide Extraction with Sulfuric Acid expt no.
x1
x2
x3
x1x2
x1x3
x2x3
x1x2x3
1 2 3 4 5 6 7 8
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
+ + + +
Y (%) 87.10 74.80 93.97 76.06 91.08 78.34 99.76 79.83
87.40 75.25 95.01 76.84 91.82 79.30 99.90 79.95
The regression coefficients are calculated by using the following equations:
∑Yi/N bj ) ∑xjiYi/N bnj ) ∑(xnjxji)Yi/N b0 )
(2) (3) (4)
where xji values (j ) 1, 2, 3; i ) 1, 2, 3, ....., 16) represent the corresponding parameters in their coded forms. The experimental design matrix and the values of boron oxide extraction percentages are shown in Tables 3 and 4. b0 gives the average value of the results; b1-b3 are the linear coefficients; b12, b13, b23 and b123 are the interaction coefficients. The coefficient b0 gives the average value of the results obtained for the extraction percentages of boron oxide in Tables 3 and 4. For boron oxide extraction by using distilled water, b1-b3 show the effects of solidto-liquid ratio, reaction time, and reaction temperature, respectively; and coefficients b12, b13, and b23 show respectively the interacting effects of solid-to-liquid ratio-reaction time, solid-to-liquid ratio-reaction temperature, and reaction time-reaction temperature; and b123 shows the interacting effect of solid-to-liquid ratioreaction time-reaction temperature. If sulfuric acid is used as solvent, b1-b3 represent the effects of solidliquid ratio, reaction time, and solvent concentration, respectively; b12, b13, and b23 represent the interacting effects of two variables; and b123 represents the interacting effect of all three variables. The values of regression coefficients are given in Tables 5 and 6. The regression equation for boron oxide extraction by using distilled water, after substituting in eq 1 values of all coefficients, is as follows:
Y ) 72.52 - 8.10x1 + 1.856x2 + 6.764x3 1.996x1x2 - 1.85x1x3 - 0.62x2x3 - 0.323x1x2x3 (5) The importance of each coefficient was determined by the F test method, and the unimportant terms were neglected from eq 5. The regression equation was tested to see how it fitted with the observations, using Fisher’s adequacy test at the 95% confidence level. The F ratios
5258 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 5. Values of Coefficients, F Ratios, and Decisions for Boron Oxide Extraction with Distilled Water coefficients
values
source of variation
F ratio
decision
b0 b1 b2 b3 b12 b13 b23 b123
72.520 -8.100 1.856 6.764 -1.996 -1.850 -0.620 -0.323
x1 x2 x3 x1x2 x1x3 x2x3 x1x2x3
930.93 48.89 649.11 56.54 48.63 5.45 1.48
effective effective effective effective effective effective ineffective
Table 6. Values of Coefficients, F Ratios, and Decisions for Boron Oxide Extraction with Sulfuric Acid coefficients
values
source of variation
F ratio
decision
b0 b1 b2 b3 b12 b13 b23 b123
85.400 -7.854 2.264 2.097 -1.641 -0.288 0.098 -1.187
x1 x2 x3 x1x2 x1x3 x2x3 x1x2x3
4530.65 376.56 322.91 197.68 6.10 0.71 2.56
effective effective effective effective effective ineffective ineffective
were calculated according to variance analysis of data in Tables 3 and 4. The F ratios obtained were compared with Fisher’s F value [F0.05(1.8)]: 5.32. The F ratios and decisions are given in Tables 5 and 6. Thus, it was observed that the following equation was adequate:
Y ) 72.52 - 8.10x1 + 1.856x2 + 6.764x3 1.996x1x2 - 1.85x1x3 - 0.62x2x3 (6) Similar studies were also carried out by using sulfuric acid. The following regression equation was obtained:
Y ) 85.40 - 7.854x1 + 2.264x2 + 2.097x3 1.641x1x2 - 0.288x1x3 (7) A negative value for the effect indicates that the measured value decreased as the factor was changed from its low level to its high level.7,8 Regression equations (eqs 6 and 7) are adequate for experimental observations. Effects of Parameters. As seen in eq 5 and Table 5, solid-to-liquid ratio is the most important parameter affecting the boron oxide extraction by using distilled water, followed by reaction temperature. The reaction time has the least effect. By using sulfuric acid solution as solvent, the most important parameter on the boron oxide extraction is solid-to-liquid ratio. Also the reaction time and solvent concentration have significant effects, as given in eq 7 and Table 6. For both solvents, all the three parameters selected have an influence on the boron oxide extraction. Solid-to-liquid ratio shows a negative effect on the extraction process (b1 in Tables 5 and 6). Extraction efficiency of boron oxide decreases with increasing solidto-liquid ratio for both solvents as shown in line 2 of Tables 3 and 4. This can be explained by the fact that the amount of solvent in medium is not enough to extract boron oxide from solid waste when solid-to-liquid ratio is high. Reaction temperature positively affects boron oxide extraction for distilled water as solvent (b3 in Table 5). As the reaction temperature increases, boron oxide extraction efficiency increases (line 5 in Table 3). This
situation can be attributed to the increasing solubility of boron oxide in solid waste with increasing reaction temperature. Solvent concentration has a positive effect on the extraction process by using sulfuric acid as the solvent (b3 in Table 6). While the solvent concentration increases, the extracted boron oxide amount increases because of using sulfuric acid in a higher amount than the stoichiometric amount (line 5 in Table 4). Reaction time shows a positive effect as presented in b2 of Tables 5 and 6. Extraction Reactions. According to chemical composition, the chemical formula of solid waste can be given as Na2O1.7B2O39.1H2O2MgO1.8CaOSiO2. According to chemical analyses, the reactions taking place in the medium can be written as follows when solid waste is added into the distilled water and the sulfuric acid, respectively:
Na2O1.7B2O39.1H2O2MgO1.8CaOSiO2(s) + H2O(aq) f 2Na+(aq) + 2B(OH)4-(aq) + 1.4H3BO3(aq) + 8H2O(aq) + 2MgO1.8CaOSiO2(s) (8) Na2O1.7B2O39.1H2O2MgO1.8CaOSiO2(s) + 4.8H2SO4(aq) f 3.4H3BO3(aq) + 2Na+(aq) + 2Mg2+(aq) + 1.8Ca2+(aq) + 4.8SO42-(aq) + 8.8H2O(aq) + SiO2(s) (9) Boron exists in the form of boric acid in acidic and neutral media. At alkali medium, boron begins to transform to borate ion, and it becomes 100% borate at pH 12, according to the following reaction as can be seen in the literature:10,11
B(OH)3 + H2O a B(OH)4- + H+
(10)
Therefore, both B(OH)4- and B(OH)3 are present in the medium when equilibrium is established according to eq 10. After the extraction of boron oxide with distilled water and sulfuric acid solution from solid waste, the pH values of the remaining solution were measured as 9.5 and 0.6, respectively. So, boron is in the form of boric acid and borate ion in distilled water and in the form of boric acid in sulfuric acid solution as can be seen in eqs 8 and 9. When determining the amount of CaO, MgO, and H3BO3 in the solution obtained by extraction, it was found that the concentrations of the species were in accordance with the reaction stoichiometry. It was determined that 4.16% of CaO and 3.87% of MgO were extracted by distilled water and that 92.62% of CaO and 93.92% of MgO were extracted by sulfuric acid solution. The removal percentages of these oxides are very low since these are less soluble in water. Whereas, the removal percentages of these by H2SO4 solution are higher because of their high solubility in sulfuric acid. These data are suitable to ones obtained by chemical analysis (Table 7). While the remaining solid part after the extraction with distilled water consists of 21.36% CaO, 20.01% MgO, and 14.64% SiO2, that of sulfuric acid consists of 0.81% CaO, 2.83% MgO, and 68.70% SiO2. The compositions of the precipitates formed with distilled water and sulfuric acid are 45.76% B2O3, 19.59% Na2O, and 31.82% H2O, and 13.74% B2O3, 51.24% SO3,
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5259
Figure 1. X-ray diffractogram of precipitate from extraction solution.
Figure 2. X-ray diffractogram of the remaining solid part in the extraction.
and 30.90% H2O, respectively. The boron compound containing about 77% B2O3 can be produced by calcination of the second precipitate. X-ray diffractogram of the precipitate formed after the extraction with sulfuric acid
shows the formation of amorphous borax, and that of the remaining solid part shows the formation of amorphous structure and feldspar [(K,Na)(AlSi3O8)] (Figures 1 and 2). X-ray diffractograms are in conformity with
5260 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 7. Chemical Compositions of Precipitates and Solid Parts Obtained from Extraction with distilled water component (%, w/w)
precipitate
B2O3 CaO MgO SiO2 Na2O Al2O3 Fe2O3 K2O SO3 LOI
remaining solid part
45.76 1.27 0.94 0.63 19.59
1.28 21.36 20.01 14.64 0.72 3.20 1.71 0.63
31.82
36.45
with sulfuric acid precipitate 13.74 0.86 0.13 0.52 2.63
51.24 30.90
remaining solid part 1.11 0.81 2.83 68.70 0.64 6.40 1.22 2.15 4.75 11.40
chemical compositions. All of these explanations support the occurrence of the extraction reactions given above. When the undesirable components such as calcium and magnesium in the extraction solution are precipitated by multistage precipitation with adequate flocculents or when the extraction solution is treated with a chelating resin for selective adsorption of boron, a purified solution for the production of boron compounds can be obtained.12 Then, boron compounds can be crystallized by providing suitable conditions. Conclusions The conditions for boron oxide extraction from the solid part of borax sludge with maximum efficiency and the effects and interactions of selected parameters and their relative importance were determined by using experimental design. The major conclusions derived from the present work are as follows: Statistical design of experiments for boron oxide extraction is an efficient technique to quantify the effect of variable parameters. It was determined that the most effective parameter for the extraction process was solid-to-liquid ratio for both solvents. It was found that the order of the effects of parameters was as follows: solid-to-liquid ratio > reaction temperature > solid-to-liquid ratio and reaction time together > reaction time > solid-to-liquid ratio and reaction temperature together > reaction time and reaction temperature together when distilled water was used as solvent. It was found that the order of the effects of parameters was as follows: solid-to-liquid ratio > reaction time > solvent concentration > solid-to-liquid ratio and reaction time together > solid-to-liquid ratio and solvent concentration together when sulfuric acid solution was used as solvent. Regression equations were established, and boron oxide extraction efficiency was able to be obtained as a function of the operating parameters. The working conditions under which maximum boron oxide extraction efficiency was obtained were determined as follows: solid-to-liquid ratio, 1/120 g/mL; reaction time, 60 min; reaction temperature, 75 °C in the case of extraction with distilled water and solid-toliquid ratio, 1/120 g/mL; reaction time, 60 min; solvent concentration, 1% (v/v) at the reaction temperature of
75 °C extraction with sulfuric acid solution. Approximately, extraction efficiencies of 90% with distilled water and 100% with sulfuric acid were reached. A precipitate containing about 46% B2O3 was formed by precipitation of the solution obtained by the extraction with water, and a precipitate containing about 77% B2O3 was formed by precipitation of the solution obtained by the extraction with sulfuric acid solution. By this way, economical loss and the soil and underground water pollution sourced from the borax sludge can be prevented by the recovery of boron. When extraction solutions are purified by multistage precipitation or selective adsorption, pure boron compounds can be crystallized and formed. The remaining solid part containing feldspar after the extraction can be used in the industries such as ceramic and glass industries. Acknowledgment We thank the Osmangazi University Research Foundation (Project 2000/6) for financial support and the Etibank Kırka Borax Plant for the solid boron wastes. Literature Cited (1) DPT, The State Planning Organization. Report 2414, Turkey, 1995. (2) DPT, The State Planning Organization. Report 2427, Turkey, 1996. (3) Soiler, H. G.; Sigiel, H.; Sigiel, A. Handbook of Toxicity of Inorganic Compounds; Marcel Dekker: New York, 1988; p 1024. (4) Boncukcuogˇlu, R.; Kocakerim, M. M.; Alkan, M. Borax Production from Borax Slime. An Industrial Waste. Water, Air Soil Pollut. 1998, 104, 103. (5) O ¨ zdemir, M.; O ¨ ztu¨rk, N.; Kıpc¸ ak, I˙ .; Bektas¸ , T. E. Kırka Boraks Tesisi Kil Pestili Atıklarının C¸ imento U ¨ retiminde Kullanılabilirligˇinin Aras¸ tırılması (Investigation of Utilization of Solid Wastes, Pressed Clays from Etibank Borax Plants, In Cement Production); The First National Solid Waste Conference, I˙ zmir, Turkey; 2001, 7, 1. (6) O ¨ zdemir, M.; O ¨ ztu¨rk, N.; Kıpc¸ ak, I˙ .; Bektas¸ , T. E.; Kavak, D. Sularda Adsorpsiyonla Bor Giderimi ve Bor Tu¨revleri Tesisleri Katı Atıklarının Degˇerlendirilmesi (Boron Removal from Water By Adsorption and Utilization of Solid Wastes, Boron Plants); Project 2000/6, Osmangazi University Research Foundation: Eskis¸ ehir, Turkey, 2001. (7) Montgomery, D. C. Design and Analysis of Experiments; John Wiley & Sons: New York, 1996; p 704. (8) C¸ o¨mlekc¸ i, N. Deney Tasarımı ve C¸ o¨ zu¨ mlemesi (Design and Analysis of Experiments); Anadolu University: Eskis¸ehir, Turkey, 1988; p 292. (9) Gu¨lensoy, H. Kompleksometrinin Esasları ve Kompleksometrik Titrasyonlar (Principles of Complexometry and Complexometric Titrations); I˙ stanbul University: I˙ stanbul, Turkey, 1984; p 259. (10) Nadav, N. Boron Removal from Seawater Reverse Osmosis Permeate Utilizing Selective Ion Exchange Resin. Desalination 1999, 124, 131. (11) Muetterties, E. L. The Chemistry of Boron and Its Compounds; John Wiley & Sons: New York, 1967; p 699. (12) Tsuboi, I.; Kunugita, E.; Komasawa, I. Recovery and Purification of Boron from Coal Fly Ash. J. Chem. Eng. Jpn. 1990, 23, 480.
Resubmitted for review March 10, 2003 Revised manuscript received August 8, 2003 Accepted August 11, 2003 IE020653J
