Recovery of boron from wastewater using 2, 2, 4-trimethyl-1,3-pentanediol in carbon tetrachloride

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1 Indian Journal of Chemical Technology Vol. 15, September 2008, pp Recovery of boron from wastewater using 2, 2, 4-trimethyl-1,3-pentanediol in carbon tetrachloride Debasish Mohapatra 1*, Gautam Roy Chaudhury 2 & Kyung Ho Park 3 1 DAEIL Development Co. Ltd. R&D Centre, Ansan , Korea 2 Institute of Minerals and Materials Technology (IMMT), Bhubaneswar , India 3 Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon , Korea m_debasish2001@yahoo.com Received 16 August 2007; revised 20 June 2008 A process was developed to recover boron from the waste stream of liquid crystal display (LCD) industry using 2,2,4- trimethyl-1,3-pentanediol dissolved in carbon tetrachloride as an extractant. Several parameters such as contact time, boron concentration in the aqueous phase, extractant concentration, equilibrium ph and phase ratio were studied for process optimization. Equilibrium conditions for boron recovery were optimized from the batch test results as: 2 stages of extraction and 2 stages of stripping with an organic: aqueous (O:A) ratio of 1. Maximum boron extraction was achieved at a ph range of Overall 95.6% boron recovery was achieved under the best experimental conditions. Keywords: LCD industry, 2,2,4-Trimethyl-1,3-pentanediol, Boron extraction, Solvent extraction Due to rapid growth in liquid crystal display (LCD) manufacturing industry, several million tons of wastewater is generated annually. More specifically, the LCD polarization process generates a wastewater that contains significant amount of iodine and boron. Because of appreciable concentration of these elements, it is economically viable to recover them prior to treatment in wastewater plant for final disposal. With this view, the present investigation examines the feasibility of boron recovery from such wastewater using solvent extraction technique. Several boron removal/recovery methods have been proposed in the literature. However, most of these processes are unsuitable for industrial application due to various factors. Boron precipitation is not practicable because of unquantitative precipitation of different boron compounds 1. Coprecipitation method using metal hydroxides (i.e. Fe, Al, Mg, Co, Ni and Zn) is an inefficient and environmentally ineffective process due to several disadvantages such as low removal rate, large quantity of metal hydroxide requirement and large amount of unrecyclable wastes 2. Adsorption method using some low cost natural minerals i.e., red mud and fly ash 3,4, synthetic materials such as hydrotalcite-like compound 5 are mostly used for boron removal purposes (not for recovery purpose). The ion exchange method is rather uneconomical in removing boron compounds from wastewater containing several hundred to thousand ppm of boron since expensive ion exchange resins are used 6. On the other hand, solvent extraction of boron is considered promising, especially for treating liquor containing high boron concentration. Generally, diols, triols and other polyols with relatively long aliphatic chains having low solubility in water have been proposed for boron extraction Based on those studies, boron extraction mechanism has been classified into three groups, (i) physical extraction, (ii) extraction by forming nonionic ester complex, (iii) extraction by forming borate salt complex 13. The extractants belonging to groups (ii) and (iii) are suitable for boron extraction from acidic and alkaline solutions, respectively. Although several extractants have been tested, hydroxyl compounds, especially 1,3-diols are considered most effective for boron extraction. But the extraction process suffers from the loss of extractants because of the solubility of these diols in water. So, it is a challenge for researchers worldwide to find out a suitable extractant for boron extraction which is insoluble in aqueous medium with a good extraction capacity.

2 484 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2008 The objective of the present study is to examine the possibility of separating boron from wastewater using 2,2,4-trimethyl-1,3-pentanediol as extractant dissolved in carbon tetrachloride. The effects of the aqueous ph, contact time, presence of cations/anions, extractant concentration, feed boron concentration and phase ratio on the extraction of boron have been investigated to obtain the optimum boron extraction conditions. Experimental Procedure Reagents and Apparatus The wastewater used for this investigation was procured from DWHI Chemistry Company, South Korea. After iodine removal the wastewater was used as feed for solvent extraction of boron. The physico/chemical composition of the wastewater after iodine removal is presented in Table 1. The commercial extractant, 2,2,4-trimethyl-1,3- pentanediol (C 16 H 30 O 4 ) was procured from Dayang Chemicals, P.R. China, and used as such without further purification. The extractant, 2,2,4-trimethyl- 1,3-pentanediol has an average molecular weight of , density (24 o C): g/cm 2, melting point - 70 o C with >99.0 wt% assay concentration. Extra pure carbon tetrachloride supplied by Daihan Chemicals, Korea was used as the diluent. A Varian 720-ES model inductively coupled plasma optical emission spectrometer (ICP-OES) and Orion expandable ion analyzer EA 920 equipments were used to determine the metal concentrations and the ph of the aqueous phase, respectively. Anion measurement was carried out by ion chromatography (761, Compact IC, Metrohm). The concentration of 2,2,4-trimethyl-1,3- pentanediol in the organic phase was determined by GC-MS (Hewlett Packard, 6890N series) to estimate Table 1 Physico/chemical composition of wastewater after iodine removal Physico/Chemical constituents Value ph 0.9 Density, kg/l 1.12 SO 4 2-, g/l 118 Na, mg/l 40 K, mg/l 3000 Ca, mg/l 8.0 B, mg/l 820 I -, mg/l 3.0 the loss of the organic extractant into the aqueous phase. All other chemicals used were analytical grade procured from Daihan Chemicals, Korea. Solvent extraction Equal volumes of aqueous and organic phases (20 ml each) were equilibrated by manually shaking in a 100 ml separatory funnel for 20 min. The ph adjustment of the aqueous phase was done by adding small amount of NaOH/ H 2 SO 4 solution. After phase disengagement, the aqueous phase was separated and its equilibrium ph was measured. The metal concentrations in the aqueous phase were estimated directly by ICP-OES after suitable dilutions. The concentrations of metal in the organic phase were calculated from the difference between the metal concentration in the aqueous phase before and after extraction. Stripping of boron from the organic phase was conducted using sodium hydroxide of different concentrations as the stripping agent. All experiments were carried out at a constant temperature of 23±1 o C. Reproducibility of results were checked by repeating several experiments and found to be within ±3%. The distribution coefficient, D, was calculated as the concentration of metal present in the organic phase to that part in the aqueous phase at equilibrium. From the D values, the percentage extraction was calculated using the equation: Percentage extraction = D x 100/ [D + (V aq /V org )] where V aq and V org are the volumes of aqueous and organic phases, respectively. Results and Discussion Effect of contact time The kinetics of boron extraction was evaluated by studying the extraction percentage as a function of the time. It was observed that higher the mixing time better the boron extraction efficiency. About 50% of the total extractable amount was transferred to the organic phase within first 3 min. Then the extraction rate was slow and reached equilibrium at about 20 min. There was no appreciable increase in boron extraction up to 30 min. It was also observed that, boron extractability was independent of the initial boron concentration and initial extractant concentration. So, all further extraction experiments were carried out for 20 min contact time irrespective of other conditions.

3 MOHAPATRA et al.: RECOVERY OF BORON FROM WASTEWATER 485 Effect of equilibrium ph The extraction characteristics of boron was examined by contacting equal volumes of 0.5 M 2,2,4-trimethyl-1,3-pentanediol dissolved in carbon tetrachloride and an aqueous solution containing 820 mg/l boron at various equilibrium ph values between 1.0 to 9.0. The percentage extraction of boron as a function of the equilibrium ph is shown in Fig. 1. It was found that maximum boron extraction was achievable at a ph range of 1.0 to 6.0. Above ph 6.0, the extent of boron extraction decreased, due to the solubility of boron/organic complex and subsequently release of boron from its complex form. Boron extraction almost remained constant at ~59% between ph 1.0 and 6.0. Above ph 6.0 the boron extraction started decreasing and reached 42.6% at equilibrium ph 8.0. This is similar to boron extraction ph ranges reported by Tural et al. 7 and Karakaplan et al. 12 using different extractants. Since the feed solution used for boron extraction is of ph ~1.0, there is no need of any additional step for ph adjustment before the solvent extraction process. Effect of initial extractant and boron concentration The effects of initial extractant concentration ( M) and boron concentrations ( M) were studied at an equilibrium ph of 1.6 and A:O ratio of 1. The relationship between the initial extractant concentration and the distribution coefficient of boron at various initial concentrations of boron are shown in Fig. 2. With an initial boron concentration of M the boron distribution coefficients were high and remained almost constant irrespective of initial extractant concentration. On the other hand, boron distribution coefficient decreased with increasing initial boron concentration. This was assumed to be due to the dissolution of 2,2,4- trimethyl-1,3-pentanediol into the aqueous solution at high boron concentrations. Also boron distribution coefficient increased with increasing extractant concentration, as expected. Extractant concentration above 1.5 M is not desirable because of third phase formation. Also it was observed that, with increasing extractant concentration the phase separation time also increased. Phase separation time increased from 0.75 to 3.5 min when extractant concentration increased from 0.5 to 1.5 M. In order to clarify the reason for the decrease in the boron distribution ratio, the equilibrium concentration of extractant was measured after each experiment. It was observed that, the distribution coefficient of 2,2,4-trimethyl-1,3- pentanediol decreased with an increase in the equilibrium concentration of boron in the aqueous phases. Ester formation of boric acid and 2,2,4- trimethyl-1,3-pentanediol in aqueous media as well as in organic media is considered as the reason for extractant loss into the aqueous phase 10. This is a problem in continuous run and more research is needed to minimize the solvent loss. Some preliminary study shows that, with increasing ionic strength solvent loss into the aqueous phase decreased significantly. Effect of cations/anions The roles of cations (Na +, K +, Ca 2+ ) and anions (SO 4 2-, Cl -, NO 3 - ) were studied to evaluate their effect on boron extraction. As mentioned earlier the feed solution for boron extraction was generated from LCD industry wastewater after iodine recovery. In the Fig. 1 Effect of equilibrium ph on boron extraction (organic phase = 0.5 M, boron in aqueous phase = M, phase ratio = 1:1, time =20 min) Fig. 2 Effect of exractant concentration on boron extraction (time = 20 min, boron in aqueous phase = M, phase ratio = 1:1, equilibrium ph = 1.6)

4 486 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2008 iodine recovery process strong mineral acids were used for acidification. So the presence of these acids was expected in the feed solution for boron extraction. It was found that, the effects of SO 4 2-, Cl - and NO 3 - were negligible on boron extraction when 2,2,4-trimethyl-1,3-pentanediol was used as an extractant. In all cases about 59% boron extraction was achieved at equilibrium ph of 1.6 and 0.5 M extractant concentration. This gives a wide choice of acids which can be used for pretreatment unlike in case of BEPD where sulphuric acid reportedly enhanced the extractability of boron 11. The coextraction of cations such as Na +, K + and Ca 2+ to the organic phase along with boron is presented in Fig. 3. In general the co-extractions of Na +, K + and Ca 2+ were negligible at lower ph range and increased with increasing equilibrium ph. For example, at equilibrium ph of 1.6, 4 mg/l Na +, 49 mg/l K + and 2 mg/l Ca 2+ were co-extracted to the organic phase, whereas, at equilibrium ph 9.0, 40 mg/l Na + (100%), 267 mg/l K + (13.3%) and 8 mg/l Ca 2+ (100%) were co-extracted to the organic phase. Boron was preferably extracted in presence of these cations and no negative effect on boron extraction was observed. When these cations are present in relatively large amount in the feed solution, it is clear that they will substantially affect the operation of a solvent extraction process. A scrubbing stage might be required to remove these unwanted metals to obtain the desired purity of the boron product. No attempt was made in the present study to scrub the undesired metals. Extraction isotherm To determine the number of stages required at a chosen volume phase ratio, the extraction isotherm was obtained by contacting the leach solution with a fixed concentration of extractant at different A:O ratios from 1 to 5 and O:A ratios from 1 to 5 (Fig. 4). From the extraction isotherm, it was observed that at A:O ratio of 1, quantitative extraction of boron was achieved in two stages. Considering the percent extraction, phase ratio, minimum stages required for complete removal of boron, A:O ratio of 1 was selected. To confirm McCabe-Thiele predictions, a two stage counter-current study with 1.0 M 2,2,4- trimethyl-1,3-pentanediol was carried out at an A:O ratio of 1. In that case about 99% extraction efficiency was achieved, which resulted in a LO containing 0.81 g/l boron. Fig. 3 Effect of Na +, K + and Ca 2+ on boron extraction (organic phase = 0.5 M, aqueous phase boron = M; Na = 40 ppm; K = 2000 ppm; Ca = 8 ppm; phase ratio = 1:1, equilibrium ph = 1.6) Fig. 4 McCabe-Thiele plot for boron extraction (organic phase = 1.0 M, boron in aqueous phase = M, phase ratio = 1:1, equilibrium ph = 1.6, contact time = 20 min) Stripping study Stripping is the reverse of the extraction, so it should be promoted by those factors that affect extraction negatively, such as acidic, basic and salt media. Boron stripping from 1.0 M loaded organic containing 0.81 g/l boron was investigated using NaOH as stripping agent. The effects of phase ratio and NaOH concentration on boron stripping are presented in Tables 2 and 3, respectively. Unlike boron extraction, which is found to be a slow process, boron stripping from loaded organic is reasonably fast. Initial experiments conducted to determine the effect of time showed that over 5 min, time did not influence the amount of boron stripped. About 88% boron was stripped from the loaded organic phase using 0.5 M NaOH at an A:O ratio of 1. Theoretically, two stages are required to quantitatively strip boron to the aqueous phase. As

5 MOHAPATRA et al.: RECOVERY OF BORON FROM WASTEWATER 487 Table 2 Effect of phase ratio on boron stripping from loaded organic (LO = 0.81 g/l Boron, NaOH = 0.5 M) Phase ratio (A:O) [B] aq, g/l [B] org, g/l Stripping, % Theoretical stages for complete stripping 1: : : : : : Table 3 Effect of NaOH concentration on boron stripping (LO = 0.81 g/l, phase ratio (A:O) = 1:1) [NaOH] [B] aq, g/l [B] org, g/l Stripping, % shown in Table 3, boron stripping efficiency increased with increase in NaOH concentration and was about 97.3% when 0.7 M NaOH was used as stripping agent. It would be possible to enrich boron in the strip solution either by increasing O:A ratio or by increasing concentration of stripping agent. Overall the recovery of boron using 2,2,4-trimethyl- 1,3-pentanediol as extractant in carbon tetrachloride was 95.6%. The purified strip liquor, containing the boron ion was passed through activated carbon to remove traces of organic prior to recovery of a pure boron product by conventional methods. Based on the above findings it was assumed that 2,2,4-trimethyl- 1,3-pentanediol/ carbon tetrachloride system could be employed for boron recovery from acidic wastewater with the advantage of good extrability and a wide ph range to choose upon. However, there is concern of solvent loss, which is about 2.6% at the best extraction conditions. In a continuous system the problem can be resolved by periodical addition of extra solvent to accumulate the solvent loss. Conclusions The studies showed that, boron recovery is feasible by solvent extraction using 2,2,4-trimethyl-1,3- pentanediol dissolved in carbon tetrachloride as extractant. Batch test results demonstrated that, 99% boron extraction and 97% stripping efficiency was achievable. The boron extraction kinetics was slow and reached equilibrium at about 20 min. Maximum boron extraction was achieved at a broad ph range of 1.0 to 6.0. Extractant loss into the aqueous phase increased with increase in initial boron concentration. Ester formation of boric acid and 2,2,4-trimethyl-1,3- pentanediol in aqueous media as well as in organic media was assumed as the cause of extractant loss. The role of anions such as SO 4 2-, Cl - and NO 3 - were negligible on boron extraction, whereas, the coextraction of Na +, K + and Ca 2+ were negligible at lower ph range and increased with increasing equilibrium ph. In general, the system is feasible in terms of boron extractability, ph range, economical factors and unsuitable with respect to solvent loss and presence of cations such as Na +, K + and Ca 2+. References 1 Remy P, Muhr H, Plasari E & Querdiane I, Environ Prog, 24 (2004) Turek M, Dydo P, Trojanowska J & Campen A, Desalination, 205 (2007) Cengeloglu Y, Tor A, Arslan G, Ersoz M & Gezgin S, J Hazard Mater, 412 (2007) Ozturk N & Kavak D, J Hazard Mater, 127 (2005) Ferreira O P, demoraes S G., Duran N, Cornejo L & Alves O L, Chemosphere, 62 (2006) Badruk M, Kabay N, Demircioglu M, Mordogan H & Ipekoglu U, Sep Sci Technol, 34 (1999) Tural B, Tural S & Hosgoren H, Turk J Chem, 31 (2007) Tsuboi I, Kunugita E & Komasawa I, J Chem Eng Japan, 23 (1990) 480 (Japanese). 9 Kenan Poslu A & Dudeney W L, Hydrometallurgy, 10 (1983) Kwon T, Hirata M, Hano T & Yamagishi T, Sep Sci Technol, 40 (2005) Kwon T, Hirata M, Sakuma S & Hano T, Solvent Extr Ion Exch, 23 (2005) Karakaplan M, Tural S, Tural B, Turgut Y & Hogoren H, Solvent Extr Ion Exch, 22 (2004) Matsumoto M, Kondo K, Hirata M, Kokubu S, Hano T & Takada T, Sep Sci Technol, 32 (1997) 983.

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