Introduction

In recent years, there has been an increasing demand for the borate family for technological applications. Inorganic borates exist in numerous structural types. Among them, potassium pentaborate single crystals are significant compounds that can be used in industry. Containing approximately 72% of the world's boron reserves, Turkey is the country that has the largest boron deposits. Boron element, which is never found in free form, only forms complex borate structures as a result of the combination of boron oxide (\({\text{B}}_{2} {\text{O}}_{3}\)) with the oxides of other elements (Pye et al. 2012). Borate compounds can generally be grouped into different subclasses such as calcium borates, calcium-sodium borates, sodium borates, and magnesium borates (Küçük et al., 2002). The most important of these and the most preferred in industry are colemanite (\({\text{Ca}}_{2} {\text{B}}_{6} {\text{O}}_{{11}} \cdot 5{\text{H}}_{2} {\text{O}}\)), tincal (\({\text{Na}}_{2} {\text{B}}_{4} {\text{O}}_{7} \cdot 10{\text{H}}_{2} {\text{O}}\)), and ulexite (\({\text{NaCaB}}_{5} {\text{O}}_{9} \cdot 8{\text{H}}_{2} {\text{O}}\)) (Karagöz et al., 2018). However, many synthetic borate compounds have been selected and used in the industry apart from those found in nature. One of them, potassium pentaborate tetrahydrate (\({\text{KB}}_{5} {\text{O}}_{8} \cdot 4{\text{H}}_{2} {\text{O}}\)), which contains 4 mol of structural water per mole of crystal, is one of the important borate compounds and is generally shown in the literature with the formula “\({\text{KB}}_{5}\)” rather than potassium pentaborate tetrahydrate (Yang et al., 2005). \({\text{KB}}_{5}\) crystals are uncolored, chemically stable and optically biaxial compounds. Potassium pentaborate can be used in welding, insulation, lubricating oil additives, metal refining, cement, textile, fiberglass, and nonlinear materials. Latest research on effective nonlinear optical (NLO) materials shows that \({\text{KB}}_{5}\) crystals are superior to other more commonly used NLO materials. That is why potassium pentaborate is successfully used to convert significant laser radiation and inorganic NLO crystals into UV and vacuum UV wavelength regions (Kamatchi et al., 2018; Kıpçak et al., 2015; Kumar et al., 2013; Xue et al., 2000).

KB5 crystals can usually be obtained by reacting boric acid and KOH in an aqueous medium by keeping the mole ratio of B2O3/K2O around 5 (Gürbüz et al., 2005). The chemical equation for the synthesis of \({\text{KB}}_{5}\) crystals using the traditional method is given in Eq. 1.

$${\text{KOH}}\left( {{\text{aq}}} \right) + 5{\text{H}}_{3} {\text{BO}}_{3} \left( {{\text{aq}}} \right) \to {\text{KB}}_{5} {\text{O}}_{8} .4{\text{H}}_{2} {\text{O}}\left( {{\text{aq}}} \right) + 4{\text{H}}_{2} {\text{O}}\left( {\text{s}} \right)$$
(1)

Owing to the chemical and optical significance of potassium borates, many investigators displayed an interest in their synthesis and application in different fields. There are some studies in the literature where potassium pentaborate is produced by changing the potassium and boron sources. The literature summary of these studies is given below.

Rajasekar et al. (2003) successfully obtained \({\text{KB}}_{5}\) by dissolving \({\text{K}}_{2} {\text{CO}}_{3}\) and \({\text{H}}_{3} {\text{BO}}_{3}\) in suitable proportions in double-distilled water. Growth conditions and surface morphology of the crystals of pure and magnesium-, calcium-, barium-, and copper-doped \({\text{KB}}_{5}\) were optimized, and magnified crystals were verified by XRD. Prabha et al. (2013) used the low-temperature slow solvent evaporation method and characterized pure and 1, 10 phenanthroline-doped \({\text{KB}}_{5}\) and determined their potential as a nonlinear optical material. Yıldırım et al. (2015) aimed to produce potassium borate compounds by the hydrothermal method. In the study where the reaction temperature and reaction time were determined as 80 °C and 1 h, \({\text{KB}}_{5}\) was produced by mixing different moles of \({\text{KNO}}_{3}\) as the potassium source, NaOH as the hydroxide ion source and \({\text{H}}_{3} {\text{BO}}_{3}\) as the boron source. Kıpçak et al. (2015) used KCl as a potassium source, NaOH as a hydroxide ion source, and \({\text{B}}_{2} {\text{O}}_{3}\) as a boron source and reacted them in different mole ratios. As a result of the experiment, they achieved optimum efficiency with a mole ratio of 1:1:7 (KCl:NaOH:\({\text{B}}_{2} {\text{O}}_{3}\)). Vigneshwaran et al. (2016) reacted \({\text{K}}_{2} {\text{CO}}_{3}\) and \({\text{H}}_{3} {\text{BO}}_{3}\) in the ratio of 1:10 mol and synthesized \({\text{KB}}_{5}\) crystals. The obtained crystals were kept in a 32 °C constant temperature bath for 50–60 days using pure water. Thus, the crystals were grown by a slow evaporation solution growth technique. Asensio et al. (2016) managed to obtain \({\text{KB}}_{5}\) crystal with 84.88% and 95.11% yield using \({\text{K}}_{2} {\text{CO}}_{3}\), \({\text{H}}_{3} {\text{BO}}_{3}\), and \({\text{B}}_{2} {\text{O}}_{3}\). The researchers, who found that the crystal they obtained lost the crystal water at 500–450 °C, also examined the crystalline thermal dehydration kinetics and characterized the crystals by using thermal gravimetry and differential thermal analysis. Derun (2018) used potassium chloride (KCl) as a source of potassium, and tincalconite, borax \({\text{~}}\left( {{\text{Na}}_{2} {\text{B}}_{4} {\text{O}}_{7} \cdot 10{\text{H}}_{2} {\text{O}}} \right)\), boric acid (\({\text{H}}_{3} {\text{BO}}_{3}\)), and boron oxide \({\text{~}}\left( {{\text{B}}_{2} {\text{O}}_{3} } \right)\) as sources of boron.

KB5 was synthesized at temperatures above 200 °C and in more than 1 week using the conventional hydrothermal method in the literature. In these studies, researchers generally preferred pure boric acid as a boron source and \({\text{KOH}},{\text{~KNO}}_{3} ,{\text{~or~K}}_{2} {\text{CO}}_{3}\) as a source of potassium (Apagyi and Csetenyi, 2001; Arivuselvi and Babu, 2018; Job, 2015; Joseph et al., 2003; Lakshmipriya et al., 2013; Şenberber 2018; Vigneshwaran et al., 2016). But boric acid is also a product and is more expensive than unprocessed raw ore.

In this study, a different potassium source (\({\text{KH}}_{2} {\text{PO}}_{4}\) solution) and different boron source (raw colemanite ore) were used for the first time in literature in order to obtain low cost \({\text{KB}}_{5}\) single crystals under the conditions of low-temperature and short time. The optimum dissolution conditions were taken from our previous studies (Karagoz and Kuslu 2017; Karagöz and Kuşlu, 2017). Various slow evaporation times were trialed to obtain the best \({\text{KB}}_{5}\) single crystal.

Materials and methods

The boron source of colemanite was obtained from Emet Mine in Kütahya, Turkey. The potassium source of potassium dihydrogen phosphate (Merck reagent of 99.9% purity) was used in the experiments. The colemanite samples were crushed (Retsch BB2/A) and ground (Brook Crompton Series, 2000). Then sieved by using ASTM standard sieves (Retsch5657, Haan, Germany) to obtain 165 µm average size fraction. The chemical composition of the ore was determined by volumetric and gravimetric methods. The purity of the colemanite ore was found to be 87%. The chemical analyses of colemanite ore are given in Table 1.

Table 1 Chemical analysis of colemanite ore
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XRD analyses were performed with a PANalytical Empyrean X-ray diffractometer (XRD) by using Cu-Kα radiation. Operating parameters of the device were 2θ range of 10°–90°, 45 kV and 40 mA (λ = 1.53 nm). X-ray diffraction was used on the original sample. X-ray diffraction of colemanite ore can be seen in Fig. 1. A Scanning Electron Microscope with Sistem Zeiss brand and Sigma 300 model was used to take SEM images. Fourier Transform Infrared Spectroscopy (FT-IR) with Bruker VERTEX 70w was used for FT-IR analyses and WTech alpha 300 R brand Raman spectroscopy was used for RAMAN analysis. The characteristic peaks of borate compounds were found in the range of 500–1500 cm−1 from researching the literature. For this reason, the 400–1800 cm−1 spectral range was studied with FT-IR spectroscopy and RAMAN spectroscopy (Asensio et al., 2016; Kıpçak et al., 2015).

Fig. 1
figure 1

X-ray diffraction pattern of colemanite ore

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Experimental procedure

The kinetic mechanism and optimum dissolution conditions of colemanite in \({\text{KH}}_{2} {\text{PO}}_{4} {\text{~}}\) solution were determined in our previous reports (Karagoz and Kuslu 2017; Karagöz and Kuşlu, 2017). These optimum conditions are given as follows: reaction temperature; 333 K, grain size; 165 µm, solid/liquid rate; 0. 02 g/mL, \({\text{KH}}_{2} {\text{PO}}_{4}\) concentration; 2. 0 M, and time; 60 min. Firstly, 200 mL of \({\text{KH}}_{2} {\text{PO}}_{4}\) solution and colemanite was dissolved in deionized water at ambient temperature by using a magnetic stirrer at about 300 rpm. Then, 98% of \({\text{B}}_{2} {\text{O}}_{3} {\text{~}}\) was dissolved as a result of the verification experiment carried out under the mentioned conditions. The experimental setup is shown in Fig. 2.

Fig. 2
figure 2

The experimental setup

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In the second stage, the solution of the verification experiment was used to synthesize the potassium pentaborate tetrahydrate (\({\text{KB}}_{5} {\text{O}}_{8} \cdot 4{\text{H}}_{2} {\text{O}}\)) product. This solution obtained under optimum conditions was crystallized for different durations at 25 °C. In this study, the chemical Eqs. (2–3) governing the reaction are given as follows:

$$2{\text{CaO}}.3{\text{B}}_{2} {\text{O}}_{3} .5{\text{H}}_{2} {\text{O}}_{{\left( {\text{k}} \right)}} + 4{\text{KH}}_{2} {\text{PO}}_{{4\left( {\text{k}} \right)}} + 6{\text{H}}_{2} {\text{O}}_{{\left( {\text{s}} \right)}} \to 2[{\text{CaHPO}}_{4} .2{\text{H}}_{2} {\text{O}}]_{{\left( {\text{k}} \right)}} + 6{\text{H}}_{3} {\text{BO}}_{{3\left( {{\text{aq}}} \right)}} + 2{\text{K}}_{2} {\text{HPO}}_{{4\left( {{\text{aq}}} \right)}}$$
(2)
$$\frac{1}{2}{\text{K}}_{2} {\text{HPO}}_{{4\left( {{\text{aq}}} \right)}} + 5{\text{H}}_{3} {\text{BO}}_{3} \left( {{\text{aq}}} \right){\text{~}} \to {\text{K}}({\text{H}}_{4} {\text{B}}_{5} {\text{O}}_{{10}} ).2{\text{H}}_{2} {\text{O}}\left( {{\text{aq}}} \right) + \frac{7}{2}{\text{~H}}_{2} {\text{O}}\left( {\text{l}} \right) + \frac{1}{2}{\text{HPO}}_{3} \left( {{\text{aq}}} \right)$$
(3)

As a result of the dissolution,\({\text{K}}({\text{H}}_{4} {\text{B}}_{5} {\text{O}}_{{10}} ) \cdot 2{\text{H}}_{2} {\text{O}}({\text{aq)}}\) was obtained in an aqueous medium as in Eq. (3). Hot filtering was done at 333 K with Whatman filter papers to remove impurities. The filtered solution was covered with porous papers and kept in a dust-free environment for crystallization. The effect of time on crystallization was examined. The solution containing the remaining \({\text{KB}}_{5}\) was crystallized at 25 °C using the slow evaporation solution technique for 6, 12, and 18 h.

Results and discussion

The solutions obtained three times by dissolving colemanite in \({\text{KH}}_{2} {\text{PO}}_{4} {\text{~}}\) solutions were crystallized at 6, 12, and 18 h to ensure the formation of \({\text{KB}}_{5} {\text{~}}\) single crystals which were named KB5-6 h, KB5-12 h, and KB5-18 h. The powder XRD patterns of \({\text{KB}}_{5}\) crystals obtained at different times can be seen in Fig. 3. When XRD patterns are examined thoroughly, characteristic XRD peaks of potassium borates cannot be seen until the time reached 18 h. According to XRD results, the produced inorganic borate minerals are potassium pentaborate (\({\text{KB}}_{5} {\text{O}}_{8} \cdot 4{\text{H}}_{2} {\text{O}}\)) with ICSD: 96-026-1927 pdf code at 18 h. The powder XRD pattern of the \({\text{KB}}_{5}\) crystal obtained in 18 h can be seen clearly and in detail in Fig. 4.

Fig. 3
figure 3

Powder XRD patterns of KB5 crystals obtained at different reaction times

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Fig. 4
figure 4

Powder XRD pattern of KB5 crystal obtained at 18 h

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SEM images obtained for the product crystallized by the slow evaporation method for 18 h were taken from different parts of the product. SEM images of \({\text{KB}}_{5}\)-18 h product are given in Fig. 5 at different magnifications (a) 200 nm, (b) 1 μm, (c) 2 μm, and (d) 10 μm. As can be seen from Fig. 5, potassium borate particles are in the form of homogeneous, angularly arranged, and polyhedral aggregates. The images in the study by Asensio et al. (2016) confirm this sequence.

Fig. 5
figure 5

SEM images of KB5-18-h product a 200 nm, b 1 μm, c 2 μm, and d 10 μm size

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The FT-IR spectrum of potassium borates synthesized as KB5-18-h product is given in Fig. 6. According Fig. 6, the IR peak at around 1360 cm−1 is assigned to asymmetric stretching of B–O. The peak appearing at 1250 cm−1 was attributed to the bending vibration of the B–O bond. The peaks at wavenumber 922 cm−1 belong to the ring stretching of B–O. The O–B–O terminal bending appears at 592 cm−1. In-plane O–B–O ring bending of B–O is observed at 510 cm−1. The Raman spectrum of \({\text{KB}}_{5}\)-18 h is given in Fig. 7. FT-IR and Raman spectra of the products appear to give typical potassium borate peaks in both the infrared and visible regions (Jun et al., 1995).

Fig. 6
figure 6

FT-IR spectrum of potassium borates synthesized for KB5-18-h product

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Fig. 7
figure 7

Raman spectrum of KB5-18 h

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Finally, at the end of 6 h and 12 h, it was observed that the crystals were not fully formed, and the particle size was small. However, after 18 h, it was observed that the crystallization efficiency increased with increasing particle size. In addition, it was seen that the crystalline particle size distribution obtained at the end of 18 h was more uniform than 6 h and 12 h.

Conclusion

  • Raw boron ore colemanite was dissolved in \({\text{KH}}_{2} {\text{PO}}_{4} {\text{~}}\) solution under optimum dissolution conditions.

  • \({\text{~K}}_{2} {\text{HPO}}_{{4\left( {{\text{aq}}} \right)}}\) and \({\text{H}}_{3} {\text{BO}}_{{3\left( {{\text{aq}}} \right)}}\), which were dissolved in the solution, were crystallized at different times (6, 12, and 18 h) by the slow evaporation solution method, and a single crystal of \({\text{KB}}_{5}\) was obtained.

  • Prepared samples were identified using the of XRD technique. XRD analysis results of \({\text{KB}}_{5}\) crystals obtained at different times showed that the best peaks were obtained with crystallization for 18 h. The same sample was characterized using the FT-IR and Raman spectroscopic methods. Morphological properties were investigated with SEM.

  • The analyses were completed and the results according to the XRD, FT-IR, RAMAN, and SEM images confirmed that the synthesized product was potassium pentaborate (\({\text{KB}}_{5} {\text{O}}_{8} \cdot 4{\text{H}}_{2} {\text{O}}\)) mineral with ICSD: 96-026-1927 pdf code.

  • The cost was reduced by using raw boron ore, and \({\text{~KB}}_{5} {\text{~}}\) was obtained in a shorter time and at a lower temperature.