Characterization and Physical Properties of Hydrated Zinc Borates Synthesized from Sodium Borates

Main Group Met. Chem. 2016; 39(1-2): 59–66

Azmi Seyhun Kipcak, Fatma Tugce Senberber, Meral Yildirim, Sureyya Aydin Yuksel, Emek Moroydor Derun and Nurcan Tugrul* Characterization and physical properties of hydrated zinc borates synthesized from sodium borates

DOI 10.1515/mgmc-2016-0002 which enable them to be used as fire retardants in the Received January 12, 2016; accepted February 24, 2016; previously plastic and rubber industries. They also have applications published online March 25, 2016 as char promoters, as anti-arcing agents, in the preserva- tion of wood composites, as modifiers of electrical-optical Abstract: In this study, Zn B O ·3.5H O, a type of a zinc 3 6 12 2 properties and as after-glow suppressant additives (Shen borate hydrate, was synthesized from the sodium borate and Ferm, 1996; Ivankov et al., 2001; Tian et al., 2006; Shi mineral Na B O ·5H O. Two different zinc sources, i.e. 2 4 7 2 et al., 2008; Kilinc et al., 2010). One of the most common ZnSO ·7H O and ZnCl , were used in the hydrothermal syn- 4 2 2 commercial forms of zinc borate is 2ZnO·3B O ·3.5H O thesis. Products were characterized using X-ray diffraction 2 3 2 (Shen and Ferm, 1996). Particles of this material must be (XRD), Fourier transform infrared spectroscopy (FT-IR) and small in size to obtain an effective and uniform distribution. Raman spectroscopy. Product morphologies were studied Additionally, there are other forms of zinc borate hydrates using scanning electron microscopy (SEM). Then optical such as 2ZnO·2B O ·3H O, 4ZnO·B O ·H O, Zn B O ·3H O, absorption characteristics and electrical properties were 2 3 2 2 3 2 2 6 11 2 Zn B O ·7H O and Zn B O ·14H O (Shi et al., 2008; Tian investigated. Based on these results, Zn B O ·3.5H O was 2 6 11 2 3 10 18 2 3 6 12 2 et al., 2008; Gao and Liu, 2009; Gurhan et al., 2009; Zheng obtained under many synthetic conditions as a single phase et al., 2009; Bardakci et al., 2013; Gao et al., 2013). Hydro- with high reaction efficiencies and sub-micrometer (100 nm thermal synthesis is commonly preferred for the prepara- to 1 μm) particle sizes. The electrical resistivity and optical tion of zinc borates. In the hydrothermal procedure, the energy gap were found as 8.8 × 1010 Ω cm and 4.13 eV, respec- zinc and boron sources are dissolved in a liquid medium. tively. The novelty obtained in this study is the synthesis of They react due to the temperature increase and use of zinc borate hydrate compound with high crystallinity with- modification agents (Gurhan et al., 2009; Kilinc et al., out using any modification agent or organic solvent. 2010; Li et al., 2010; Tugrul and Acarali, 2011; Gao et al., Keywords: electrical properties; hydrothermal synthesis; 2013). The characterization of synthesized compounds optical properties; sodium borate; zinc borate hydrate. plays a significant role in scientific studies and industrial applications. The characterization of hydrated zinc borate minerals is commonly based on the determination of the Introduction hydrophobicity and thermal properties based on the ther- modynamic and dehydration behavior (Speghinia et al., Zinc borates are well-known members of the metal borate 2000; Gurhan et al., 2009; Kilinc et al., 2010; Li et al., family because of their high heat resistance properties, 2010; Tugrul and Acarali, 2011; Gao et al., 2013). Gao and Liu (2009) synthesized zinc borate hydrated minerals at the boiling point of the solvent and with a *Corresponding author: Nurcan Tugrul, Faculty of Chemical and reaction time of 11 h. Bardakci et al. (2013) synthesized Metallurgical Engineering, Department of Chemical Engineering, Davutpasa Campus, Davutpasa Street No.127, 34210 Esenler, zinc borate hydrate using the raw materials zinc oxide

Istanbul, Turkey, e-mail: [email protected] (ZnO) and boric acid (H3BO3) at 95°C for 2 h. Tugrul and Azmi Seyhun Kipcak, Fatma Tugce Senberber, Meral Yildirim and Acarali (2011) studied the effects of different modification Emek Moroydor Derun: Faculty of Chemical and Metallurgical agents to obtain hydrophobic zinc borate hydrate min- Engineering, Department of Chemical Engineering, Davutpasa erals. Zheng et al. (2009) prepared 4ZnO·B O ·H O after Campus, Davutpasa Street No.127, 34210 Esenler, Istanbul, Turkey 2 3 2 Sureyya Aydin Yuksel: Faculty of Arts and Sciences, Department 7 h using phosphate esters as a modification agent. The of Physics, Davutpasa Campus, Davutpasa Street No. 127, 34210 mineral 2ZnO·3B2O3·3.5H2O was produced using oleic acid Esenler, Istanbul, Turkey as a modification agent at 95°C and 4 h (Li et al., 2010). 60 A.S. Kipcak et al.: Characterization of hydrated zinc borates

Gao et al. (2013) investigated the effects of different modi- are given in Table 1 [XRD scores can be explained as; when fication agents on the morphological properties of syn- all of the peak intensities (%) and peak locations matched thesized zinc borate hydrates. Mergen et al. (2012) studied perfectly with the pdf card number of reference mineral, the effects of zinc borate addition to PVC to strengthen the the XRD score of analyzed mineral is equal to 100 (İbroşka flame retardant features. Additionally, the electrical and et al., 2015)]. The type of synthesized zinc borate was optical characterizations of the zinc borate compounds found to be zinc oxide borate hydrate (Zn3B6O12·3.5H2O), have only been studied in the dehydrated forms (Longo with pdf number 00-035-0433. The same zinc borate com- et al., 1998; Speghinia et al., 2000; Ivankov et al., 2001). pound was synthesized in the studies of Bardakci et al. As observed in the literature, the synthesis of zinc (2013), Kipcak et al. (2014a,b) and Kipcak et al. (2015). The borate particles with higher crystallinities requires high XRD scores obtained from the reactions of ZnSO4·7H2O, reaction temperatures ( ≥ 95°C), long reaction times ( ≥ 2 h) Na2B4O7·5H2O, H3BO3 (ZS-T) and ZnCl2, Na2B4O7·5H2O, H3BO3 and modification agents (i.e. oleic acid) to obtain homo- (ZC-T) were plotted against the reaction time and tempera- geneous particle size distributions (Longo et al., 1998; ture using StatSoft Statistica software, where the y-axis Speghinia et al., 2000; Ivankov et al., 2001; Shete et al., shows the XRD score, the z-axis shows the reaction tem- 2004; Tian et al., 2008; Gao and Liu, 2009; Gurhan et al., perature and the x-axis shows the reaction time; this plot 2009; Zheng et al., 2009; Kilinc et al., 2010; Li et al., 2010; is given in Figure 1. The results shown in Figure 1 indicate Tugrul and Acarali, 2011; Mergen et al., 2012; Bardakci that the XRD scores of the zinc borates increased with et al., 2013; Gao et al., 2013). There are two novel contribu- increasing time and temperature. tions of this study. The first contribution is the preparation In the ZS-T experiments, zinc borate was formed of zinc borate hydrate minerals with high crystallinities using a reaction temperature of 70°C and a reaction time under moderate conditions without using any type of of 4 h. In contrast, at the same temperature, the formation organic solvent or modification agent. Second, the effects of zinc borate did not occur in the ZC-T experiments. The of the reaction temperature and reaction time on the zinc borates were coded in the format ‘set code-reaction morphologies of the synthesized minerals were investi- temperature-reaction time’, where the set codes were ‘ZS-T’ gated. Hence the electrical and optical characterizations and ‘ZC-T’. For example, the product synthesized at 80°C of zinc borate compounds have been studied only in the and 4 h using ‘ZnSO7·7H2O-Na2B4O7·5H2O-H3BO3’ was coded dehydrated form, the investigation of the electrical and as ‘ZS-T-80-4’. At a reaction temperature of 80°C, the zinc optical properties of hydrated zinc borate compounds is borate formation started after 1 h of reaction, but complete the second novel contribution of the study. formation was only achieved after 4 h of reaction in the ZS-T set. In contrast, in the ZC-T set after 1 h of reaction, amorphous zinc borate was obtained. Crystalline zinc borate Results and discussion formed after 2 h of reaction, and better XRD scores were obtained after 3 and 4 h of reaction. At reaction temperatures of 80°C and 90°C in the ZS-T set, the formation of zinc Results of the raw material characterization borate began after 1 h of reaction. Complete formation was

Based on the X-ray diffraction (XRD) results of the start- ing materials, the zinc source of zinc sulfate heptahydrate Table 1: XRD scores of the synthesized zinc borates. (ZnSO ·7H O) was identified as a mixture of Bianchite 4 2 Temperature (°C) Time (h) Set 1 (ZS-T) Set 2 (ZC-T) (ZnSO4·7H2O, powder diffraction file (pdf) number 01-075- 90 1 6a – 0949) and Goslarite (ZnSO4·6H2O, pdf: 00-009-0395). The a other zinc source was determined to be a mixture of two 2 63 72 3 72 76 different zinc chlorides (ZnCl ) with pdf numbers ‘01-074- 2 4 77 80 0519’ and ‘01-070-1284’. The boron sources were identi- 80 1 6a – a fied as sassolite (H3BO3, pdf: 01-073-2158) and tincalconite 2 27 70 3 68a 75 (Na2B4O7·5H2O, pdf: 00-007-0277). 4 72 77 70 1 14a 23a XRD results of the synthesized zinc borates 2 26a 32a 3 40a 48a 4 69 68a The XRD scores of the zinc borates, which were synthesized at different temperatures and different reaction times aCrystal formation was not completed. A.S. Kipcak et al.: Characterization of hydrated zinc borates 61

ZS-T ZC-T

100 100

80 80

60 60 XRD score 40 40 XRD score 20 20

4.0 90 4.0 90 3.5 3.5 85 85 3.0 7 Reaction time (h) Reaction3.0 time (h) 2.5 80 2.5 80 2.0 2.0 75 75 1.5 1.5 1.0 70 Reaction temperature (°C) 1.0 70 Reaction temperature (°C)

Figure 1: XRD score graph of the synthesized zinc borates according to the reaction time and reaction temperature. achieved after 3 h of reaction, and the XRD score increased According to Figure 2, the characteristic peaks and for 4 h. The increase in the reaction temperature decreased respective miller indices (h k l) and d spacing [Å] of synthe- the reaction time by 1 h for the ZS-T set. At the same tem- sized zinc borates were determined to be 17.7° (1 1 -1) [5.88 Å]; perature in the ZC-T set, amorphous zinc borate was again 18.1° (0 2 0) [4.90 Å]; 20.6° (1 0 1) [4.30 Å]; 21.8° (1 2 0) [4.08 Å]; obtained after 1 h of reaction. Crystalline zinc borate was 23.7° (1 2 -1) [3.74 Å]; 25.8° (2 1 0) [3.44 Å]; 28.8° (0 1 2) [3.10 Å]; formed after 2 h of reaction, and better XRD scores were 30.2° (2 2 1) [2.95 Å]; and 36.8° (2 3 0) [2.44 Å]. obtained as the reaction time increased to 3 and 4 h. The XRD scores of the ZC-T set were found to be higher than the ZC-T set. Some selected zinc borate XRD patterns are shown FT-IR and Raman spectral analysis results for in Figure 2. In optimal zinc borate, formations were given the synthesized products as 95°C after 2 h, 100°C after 2 h and 90°C after 2 h in the studies of Bardakci et al. (2013), Kipcak et al. (2014a,b) and Fourier transform infrared spectroscopy (FT-IR) spectra Kipcak et al. (2015), respectively. It is seen that the XRD of selected zinc borates are shown in Figure 3. In results were in agreement with the literature. the FT-IR spectra, the asymmetrical [νas(B(3)-O)] and

500 1000 ZS-T-90-4 ZC-T-90-4

0 0

500 1000 ZC-T-90-3 ZS-T-90-3 0 0 Intensity (a.u.) Intensity (a.u.)

500 1000 ZC-T-80-4 ZS-T-80-4 0 0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Position (°2θ) (Copper (Cu)) Position (°2θ) (Copper (Cu))

Figure 2: XRD patterns of selected zinc borates. 62 A.S. Kipcak et al.: Characterization of hydrated zinc borates

ZS-T-90-4 ZC-T-90-4

1338 1252 1188 1406 1337 1252 1187 1406 1109 1291 1060 1292 11071061 792 656 856 792 655 856 749 750 ZS-T-90-3 916 ZC-T-90-3 918

1188 1406 1338 1252 1406 1252 1186 1340 1110 1109 1292 1060 792 656 1291 1062 855 855 791 ansmittance (%) 749 ansmittance (%) 749 657

Tr ZS-T-80-4 917 Tr ZC-T-80-4 919 656 792 1339 1188 856 1406 1337 1252 1188 856 750 1407 1251 1111 917 1108 919 1291 1059 749 1292 1061 792 656

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 650 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 650 Wavenumber (cm-1) Wavenumber (cm-1)

Figure 3: FT-IR spectra of selected zinc borates.

-1 symmetrical [νs(B(3)-O)] stretching bands of three-coor- observed at approximately 755 cm . The peaks at approxi- -1 dinate boron to oxygen were observed in the wavenum- mately 664 cm coincided with γ(B(3)-O), and the peaks at -1 -1 bers of 1407–1251 and 919–916 cm , respectively. The 578 cm belonged to the δ(B(3)-O)/δ(B(4)-O) bonds. In the IR peaks in the range of 1062–1059 cm-1 corresponded range of 437–298 cm-1, the observed peaks were due to to the asymmetrical stretching of the four-coordinate δ(B(4)-O) bonds. boron to oxygen bands [νas(B(4)-O)]. The symmetrical Both FT-IR and Raman spectra of synthesized zinc stretching bands of four-coordinate boron to oxygen borates are in agreement with the previous studies and lit- -1 [νs(B(4)-O)] were observed between 856 and 791 cm . The erature data (Yongzhong et al., 2000; Bardakci et al., 2013; bending mode of boron-oxygen-hydrogen [δ(B-O-H)] Kipcak et al., 2014a,b; Kipcak et al., 2015). was observed between 1188 and 1107 cm-1. The peaks at -1 4- approximately 750 cm were attributed to the [νp(B(OH) )] band, while the bending mode of the three-coordinate Surface morphologies and particle sizes of γ boron to oxygen bond [ (B(3)-O)] was observed at approxi- the synthesized zinc borates mately 656 cm-1. In Figure 4, the Raman spectra of selected zinc borates The morphologies of the synthesized zinc borate miner- are given. In Raman spectra, the bending of δ (B-O-H) was als were investigated using scanning electron microscopy -1 observed in the range of 1191–1186 cm . νas(B(4)-O) and (SEM) analysis. The SEM results are shown in Figure 5. The

νs(B(4)-O) bands were observed in the ranges of 1050– surface morphologies differed due to the changes in the 1048 cm-1 and 851–848 cm-1, respectively. The peaks in reaction temperature and time for both ZS-T and ZC-T. In the range of 932–924 cm-1 corresponded to the bending both sets, the particle sizes of the synthesized minerals - of νs(B(3)-O), and vibrations of the νp(B(OH)4 ) bonds were decreased with increasing reaction temperature and time.

437 578 578 437 363 363 755 664 299 664 319 318 755 298 ZS-T-90-4 1049 848 405 403 1191 926 ZC-T-90-4 1291 1189 1048 932 849

437 363 436363 578 578 318 754 664 754 664 318 403 299 403 298 851 ZS-T-90-3 ZC-T-90-3 924 1189 1048 932 850 1186 1048

436 363 Intensity (a.u.) 437 363 Intensity (a.u.) 578 578 319 664 755 665 298 ZS-T-80-4 754 318 401 299 ZC-T-80-4 402 1292 930 1188 1049 933 848 1050 850

1800 1600 1400 1200 1000 800 600 400 250 1800 1600 1400 1200 1000 800 600 400 250 Raman shift (cm-1) Raman shift (cm-1)

Figure 4: Raman spectra of selected zinc borates. A.S. Kipcak et al.: Characterization of hydrated zinc borates 63

Figure 5: SEM surface morphology and particle sizes of the zinc borate hydrate compounds.

In the ZS-T experiments, smooth-edged crystal forma- 100 99.7 99.6 99.6 ZS-T ZC-T tion was determined on the sub-microscale. The shapes of 96.5 96.1 95 the resulting particles were not uniform at 80°C, and the 92.7 particle sizes were in the range of 525 nm to 2.00 μm. More 90 87.7 87.1 uniform particle sizes and crystal shapes were observed as 86.2 86.5 the temperature increased. At the reaction temperature of 85 90°C, the particle sizes were in the range of 380 nm to 1.77 80 μm after a reaction time of 3 h and 332 nm to 1.70 μm after Reaction yield (%) a reaction time of 4 h. 75 In the ZC-T experiments, sharp-edged crystal forma- 70 tion was determined on the sub-microscale. The mineral 90°C-4 h 90°C-3 h 90°C-2 h 80°C-4 h 80°C-3 h 80°C-2 h70°C-4 h synthesized at 80°C was a mixture of different crystal sizes. Reaction condition At this reaction temperature, the particle size was in the Reaction yields of the synthesized zinc borates from a. range of 327 nm to 1.38 μm. Smaller particle formation may Figure 6: ZS-T, b. ZC-T. have occurred due to cracks in the crystals. As the temperature increased, the particle sizes increased to 485 nm to 2.50 μm. The best particle formation was observed in the and 70°C. The reaction yields were calculated to be at experiment at 90°C with a reaction time of 4 h. 86.5–99.7% and 86.2–99.6% for the ZS-T and ZC-T sets, respectively. The highest reaction yields were calculated for the reaction temperature of 90°C and reaction time of Yield calculation of synthesized zinc borate 4 h as 99.7±0.2% and 99.6±0.2% for the ZS-T and ZC-T sets, hydrates respectively. The yields of the ZS-T sets were higher than those of the ZC-T sets. Reaction yields were higher in this Figure 6 shows the reaction yields of the synthesized study than in the studies given in the literature obtained; zinc borate hydrates; the reaction yields increased with the reaction yields in the optimum formations were found increasing temperature and reaction times for both sets. to be 86.78% in Bardakci et al. (2013), 95.7% in Kipcak In the ZS-T set, higher reaction yields were observed at et al. (2014a,b) and between 96.1 and 97.8% for different 90°C and lower reaction yields were observed at 80°C raw materials in Kipcak et al. (2015). 64 A.S. Kipcak et al.: Characterization of hydrated zinc borates

Physical properties of the Conclusion synthesized zinc borates The present study was evaluated and showed that Zn B O ·3.5H O type zinc borates were synthesized by The UV-vis spectra of selected zinc borates are shown in 3 6 12 2 varying several reaction parameters without using any Figure 7 (i.e. ZC-T-90-4 h and ZS-T-90-4 h). They have the type of agent or solvent. Based on the results, the for- highest reaction yields at the reaction temperature of 90°C mation of crystalline zinc borate increased with increas- and reaction time of 4 h. The spectra were measured in the ing reaction temperature and reaction time in both ZS-T wavelength range of 200–1100 nm at room temperature. and ZC-T. The XRD scores of the ZC-T set were found to be The optical energy gaps of the zinc borate compounds, i.e. higher than those of the ZS-T set. The XRD results showed ZC-T-90-4 h and ZS-T-90-4 h, were determined by extrapo- that the zinc borates could be synthesized via ZS-T reac- lating the high-energy portions of the absorption spectra tions at 70°C for 4 h, at 80°C for 4 h and at 90°C for 3 h. and were about the same at 4.13 eV. Similarly, zinc borates could be synthesized via ZC-T reac- Figure 8 shows the current voltage characteristics of tions at 80°C for 2 h and at 90°C for 2 h. The SEM results the ZC-T and ZS-T zinc borate minerals synthesized with showed that the smoothness and homogeneity of the a reaction temperature of 90°C and a reaction time of 4 h. products increased with increasing temperature. The The resistivities of ZC-T-90-4 h and ZS-90-4 h were deter- highest yields were obtained at 90°C and 4 h for both ZS-T mined to be about the same at 8.8 × 1010 Ω cm. These data and ZC-T with yields of 99.7% and 99.6%, respectively. were obtained from the current voltage curves. The electrical resistivities and optical energy gaps of the minerals synthesized at 90°C and 4 h for both ZS-T and ZC-T were found to be approximately 8.8 × 1010 Ω cm and 4.13 eV, respectively.

Experimental section

Characterization of the raw materials

The synthesis of zinc borate was performed using ZnSO4·7H2O and

ZnCl2 as the zinc sources, which were obtained from Sigma Aldrich ® Figure 7: The optical absorption spectra of ZC-T-90-4 h and ZS-T- Reagent Plus , Taufkirchen, Germany ( ≥ 99.0% purity). Na2B4O7·5H2O

90-4 h. and H3BO3 were supplied by Bandırma Boron Works (Eti Maden, Balıkesir, Turkey) with a purity of 99.9%. Boric acid was sieved through a Fritsch analysette 3 Spartan pulverisette 0 vibratory sieve- shaker (Fritsch, Idar-Oberstein, Germany) (particle size, < 70 μm) 3.0×10-8 ZS-T-90-4 h after being subjected to crushing and grinding by Retsch RM 100 ZC-T-90-4 h (Retsch GmbH & Co KG, Haan, Germany) when the other raw materi- 2.0×10-8 als were used without any pretreatment. The identification studies of ) 2 raw materials were performed using a Philips PANalytical Xpert Pro -8 1.0×10 (PANalytical B.V., Almelo, The Netherlands) XRD with the working parameters of 45 kV and 40 mA (Cu Kα radiation). 0.0

-1.0×10-8 Zinc borate synthesis Current density (A/cm -2.0×10-8 To determine the optimum synthesis parameters, some preliminary -8 -3.0×10 experiments were conducted using different ZS-T and ZC-T molar ratios. ZS-T and ZC-T molar ratios of 1:1:3 (Kipcak et al., 2014b) and -100 -50 0 50 100 1:1:2, respectively, were selected for further experiments. Voltage (V) The expected reactions are shown in Eqs. (1) and (2):

+⋅++→⋅ Figure 8: The current voltage characteristics of ZC-T-90-4 h and ZnSO42·7HO Na24BO725H O3HB33OHx 23O13ZnB61O322.5(H O)

ZS-T-90-4 h. ++Na24SO 5H33BO +yHO2 (1) A.S. Kipcak et al.: Characterization of hydrated zinc borates 65

+⋅++→⋅performed. The analysis parameters for XRD were arranged as speci- ZnCl22Na BO475H23O2HBOH32x O13ZnB36O312 .5(H2O) fied in the Characterization of the raw materials section. The XRD ++2NaCl4HBOH+y O (2) 33 2 data were collected from 10° < 2θ < 70° with a scan step size of 0.03°. For FT-IR analyses, PerkinElmer Spectrum One FT-IR (Perki- To determine the effects of the reaction temperature and reaction nElmer, MA, USA) was used with universal attenuation total reflec- time on the synthesized product, the temperatures and reaction times tance (ATR) sampling accessory (Diamond/ZnSe crystal). For Raman were varied between 70°C and 90°C and from 1 to 4 h, respectively. analyses, Perkin Elmer Raman Station 400F (PerkinElmer, CT, USA) -1 For experiments with ZS-T, 0.0189 mol Na2B4O7·5H2O and 0.0566 mol was used. The scan ranges were selected at 650–1800 cm and 250– -1 H3BO3 were dissolved in 25 mL of pure water obtained from GFL 2004 1800 cm for FT-IR and Raman, respectively. Based on the literature, (Gesellschaft für Labortechnik, Burgwedel, Germany), and 0.0189 for FT-IR and Raman analyses, the spectral ranges were set between -1 -1 mol ZnSO4·7H2O was then added to the reactor. For better crystalliza- 600 cm and 1800 cm (Kipcak et al., 2014a,b). tion, a commercial zinc borate (Zn3B6O12·3.5H2O), which was retrieved The morphological structures and particle sizes of the synthe- from a local supplier (Melos A.Ş., Istanbul, Turkey), was also added sized zinc borate minerals were investigated using a CamScan Apollo to the reactor (0.5% w/w as Na2B4O7·5H2O+H3BO3). In the ZC-T experi- 300 Field-Emission SEM (CamScan, Oxford, UK). The zinc borate ments, the amounts of ZnCl2, Na2B4O7·5H2O and H3BO3 were 0.0220, samples were coated with platinum-gold (Pt-Au) to improve the con- 0.0220 and 0.0439 mol, respectively. The raw materials were reacted ductivity. in closed temperature-controlled vessels. After the determined reaction time, the solution was filtered through a blue ribbon filter paper (Chmlab, Barcelona, Spain), and the crystallized products on the fil- Determination of the physical properties of zinc borate ter paper were washed three times with pure water (approximately 1000 mL) at 50–60°C to remove unreacted reagents and Na SO and 2 4 The synthesized zinc borate minerals were pressed under a pres- NaCl by-products. Then the washed products were dried in an Eco- sure of 20 MPa into pellets with 13-mm diameters and approximately cell LSIS-B2V/EC55 model incubator (MMM Medcenter Einrichtun- 0.4- to 0.5-mm thicknesses. The electrical resistivity measurements gen GmbH, Planegg, Germany) at 105°C for 24 h. The experimental of the synthesized zinc borate minerals with the highest crystalline method is shown schematically in Figure 9. yields were performed using standard current voltage measurement at room temperature on a Keithley 2400 in the dark with thermally evaporated silver contacts on two surfaces of the pellets. The ultra- Zinc borate characterization studies violet-visible (UV-vis) absorption spectra of the zinc borate minerals were measured using a Perkin Elmer Lambda 35 UV-vis spectropho- To identify and determine the characteristics of the produced tometer in the wavelength range of 200–1100 nm at room tempera- samples, XRD, FT-IR and Raman spectroscopy techniques were ture. In these measurements, the zinc borate powders were dispersed

Equipments & processes 1. Batch reactor 2. Filtration 3. Incubator 1

Streams 1. ZS (s) of ZC (s) 2. T (s)+H (s) 3. ZB (s)+H (aq)+(NS (aq) or NC (aq))+water (I) 4. ZB (s)+water (I) 5. H (aq)+(NS (aq) or NC (aq))+water (I) 6. Water (g) 7. ZnB (s)

Abbreviations 6 ZS: Zinc sulfate heptahydrate ZC: Zinc chloride T: Tincalconite 3 7 H: Boric acid 3 ZB: Zinc borate 4 NS: Sodium Sulfate NC: Sodium Chloride 2 s: solid, aq: aqueous I: liquid, g: gas 5

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