Extraction of Boron from Tourmaline Ore: Mechanism of Thermal Analysis of the Schorl

Abstract

Tourmaline is the chief boron-bearing mineral on the earth and is present in an excess amount in the crustal rocks. However, schorl is an iron-rich alkali that forms a solid solution with the magnesium-rich, alkali tourmaline, dravite. In this work, tourmaline (schorl variety) was treated along with soda ash, and its behavior was analyzed using electron probe microanalysis (EPMA), x-ray diffraction (XRD), scanning electron microscope, and energy dispersive spectrometer (SEM-EDS) analyses, thermogravimetric analysis (TGA), differential thermal analysis (DTA), in order to annotate the soda-ash activation of boron within the tourmaline ore. To extract boron from the sample, sodium carbonate powder was mixed with the schorl in 20% of the total weight of schorl powder. When the sample was treated with sodium carbonate, the sodium gets combined with the boron to form sodium borate at 566°C along with aegirine and aluminum oxides. This sodium borate can be treated with hydrochloric acid to get boron-oxide along with NaCl.

1. Introduction

Tourmaline is an important and complex boron-bearing mineral on the earth and is present in an excess amount in the crustal rocks. It is not a single mineral, but a group of isomorphs minerals with identical crystal lattices. The general formula of the tourmaline group is very complex: X1Y3Al6B3Si4O27 (OH) ₄ where X = Na, K, Ca and Y = Li, Fe, Mg, Mn, Al, Cr, Ti; some of the OH– ions are generally substituted by F. The boron concentration in the tourmaline has been found 3.40 ± 1% within the normal range (2.5–3.8%) reported worldwide [1]. Whereas, schorl is an iron-rich alkali tourmaline that forms a solid solution with the magnesium-rich, alkali tourmaline, dravite. It is reported that, economically sustainable deposits of borax have not been established in India so far. The only deposit of little economic importance is reported from Puga Valley in Leh district, Jammu & Kashmir. As per National Mineral Inventory data, based on the UNFC system, the total reserves/resources of borax as of 1.4.2015, have been estimated at 74,204 tons in Jammu & Kashmir. Occurrences are also reported from Surendranagar district, Gujarat, and Jaipur district, Rajasthan [2].

Boron does not occur in free state in nature. It occurs mainly in the form of the salts of boric acid. It mainly occurs in the form of boric acid (H₃BO₃) and borax (Na₂B₄O₇.10H₂O). Element boron and its compounds are widely used in the industrial and agricultural sectors due to their properties such as high hardness, wear resistance, high strength, fire inhibitor, heat resistance, high strength, wear resistance, and catalytic performance. In general, boron–iron separation and dissolution activity of boron-bearing minerals in alkaline liquor are the two key issues in the utilization of tourmaline ore, governing the boron recovery as well as operating cost [3]. On a large scale, boron is extracted from its minerals, borax Na₂B₄O₇ or colemanite Ca₂B₆O₁₁. The latter is first converted to borax by boiling with a solution of sodium carbonate in the requisite proportion. In the present study, attempt has been made to extract boron by thermal analysis method.

2. Materials and methodology

Tourmaline (Schorl) was obtained from the Kyanite, Sillimanite mine of Girola area, Sakoli tehsil, Bhandara. The samples were collected, crushed, ground, and separated to 80% passing through 200 sieve size (0.074 mm). The XRD analysis of the sample was completed on Panalytical X’Pert Pro (model-PW 3040/60) diffractometer with Cu Kα radiation (λ = 1.54 Å) produced at a voltage of 45 kV and current of 40 mA. Scanning was done at the 2θ angle of 10 to 100° with a scan step size and time per step of 0.01° and 15 seconds, respectively. The surface morphology was examined using a Scanning Electron microscope (SEM-JEOL 6830A). Before the study of surface morphology, to make the material electron-conducting it was coated with a thin platinum coat using an auto sputter (JOEL-JFC 1600 auto fine coater). Whereas, the elemental composition was studied using Energy Dispersive Spectroscopy (EDS).

Electron-probe microanalysis (EPMA) of the sample was executed at the National Centre of Excellence in Geoscience Research, GSI, Bangalore using a CAMECA SX-100 electron microprobe analyzer. The spectra were collected for each sample with a wavelength-dispersive spectrometer (WDS) and with WDS 1 (TAP crystal), WDS 4 (TAP crystal), WDS 3 (LPET crystal), WDS 2 (PET crystal), and WDS 5 (LIF crystal) spectrometers. The spectra were collected using column condition of an acceleration voltage of 15 keV, beam current of 15 nA, and beam size of 1 μm. Calibration, quantification, and overlap correction were executed using CAMECA SX- 100 Peak Sight-Geo Quanta software package.

However, to extract boron from the sample, sodium carbonate powder was mixed with the schorl in 5, 10, 15, and 20% of the total weight of schorl powder. The virgin, as well as prepared samples, were roasted to 1000°C using a tubular furnace [4], and analyses were done. However, suitable boron reach compound was found to be in 20% proportion of sodium carbonate powder, hence further analysis was done of this composition by leaching out with HCl (Figure 1). In order to achieve the boron in pure form, acid leaching was done multiple times. However, by further synthesis iron can be removed from the compound, followed by the recovery of aluminum.

Figure 1.

Flow chart showing the extraction process.

3. Results

3.1 EPMA

The EPMA study was carried out for the extracted sample. From the EPMA study of the sample (Table 1), the mineral shows the highest content of Al₂O₃ of 25.88 to 61.9 wt. %, SiO₂ ranges from 30.01 to 44.89 wt. %, TiO₂ ranges from 0 to 1.82 wt. %, MnO content is 0 to 9.59 wt. %, MgO ranges from 0 to 14.07 wt. %, CaO ranges from 0 to 6.92%, Na₂O content is 0 to 0.95 wt. %, and K₂O ranges from 0 to 0.93 wt. %. The mineral shows a high percentage of Al and considerable x-site vacancy, an intermediary between schorl, dumortierite, and dravite.

	1	2	3	4	5	6	7	8	9	10
SiO2	30.74	32.73	33.58	32.92	33.13	34.04	36.5	37.22	37.68	31.43
TiO2	0	2.87	1.63	0.54	2.19	0.39	0	0.07	0.03	0.02
Al2O3	1.4	16.26	30.62	30.7	23.38	27.33	40.07	38.03	33.14	46.53
Fe2O3	43.89	14.78	0.86	12.37	5.44	7.63	0	0	0	0
FeO	2.69	11.24	12.65	5.98	8.66	4.91	0.22	0.1	1.95	0.05
MnO	0	0.1	0.06	0.11	0.07	0	3.07	4.06	0	0.02
MgO	6.45	4.66	2.69	0.16	7.8	7.34	0	0.18	9.71	0
CaO	0	0.04	0	0.2	3.3	0.99	0.2	2.63	0.03	1.74
Na2O	2.12	2.71	2.84	2.49	1.16	2.35	2.15	1.4	1.42	1.33
K2O	1.04	0.19	0.06	0.07	0.05	0	0	0.02	0	0
B2O3	9	9	10	10	10	10.83	11.56	0	0	16.2
H2O	2.56	3	3	1	3	3.18	3.13	3.17	0	3.25
Total	100.1	98.16	98.571	97.594	98.66	99	100.832	101.859	100.434	101.849
Unit formula normalized to 31 anions
Si	5.878	5.956	5.785	5.644	5.771	5.792	5.877	5.941	5.995	5.149
Ti	0	0.393	0.211	0.07	0.287	0.05	0	0.008	0.004	0.002
Al	0.316	3.487	6.217	6.204	4.8	5.481	7.604	7.154	6.214	8.984
Fe3+	6.315	2.024	0.111	1.596	0.713	0.977	0	0	0	0
Fe2+	0.43	1.711	1.823	0.857	1.262	0.699	0.03	0.013	0.259	0.007
Mn	0	0.015	0.009	0.016	0.01	0	0.419	0.549	0	0.003
Mg	1.839	1.264	0.691	0.041	2.026	1.862	0	0.043	2.303	0
Ca	0	0.008	0	0.037	0.616	0.18	0.035	0.45	0.005	0.305
Na	0.786	0.956	0.949	0.828	0.392	0.775	0.671	0.433	0.438	0.422
Li	0	0.004	0.022	0	0	0.003	1.041	1.194	0	0.369
B	3.027	3	3.081	3.001	3.121	3.181	3.213	3	3	4.581
Name	Schorl	Schorl	Oxy-Schorl	Dumortierite	Dumortierite	Dravite	Schorl	Schorl	Dumortierite	Schorl

Table 1.

Showing the composition of virgin sample.

However, the EPMA analysis of the extracted sample (Table 2) shows the highest range of SiO₂ from 41.75 to 42.8 wt. %, MgO ranges from 14.3 to 15.27 wt. %, CaO 11.15 to 11.19 wt. %, and Al₂O₃ 9.33 to 9.49 wt. %. The content shows the presence of aegirine mineral [5].

Oxides	1	2	3	4	5	6	7	8
SiO2	42.63	42.83	42.8	41.75	42.83	42.8	42.63	41.75
TiO2	1.21	1.12	1.1	1.23	1.12	1.1	1.21	1.23
Al2O3	9.49	9.39	9.33	9.39	9.39	9.33	9.49	9.39
Fe2O3	7.24	7.15	7.2	7.24	7.15	7.2	7.24	7.24
FeO	7.06	7.1	7.13	7.1	7.1	7.13	7.06	7.1
MgO	14.32	14.27	14.3	15.27	14.27	14.3	14.32	15.27
CaO	11.19	11.15	11.17	11.15	11.15	11.17	11.19	11.15
Na2O	1.35	1.25	1.29	1.25	1.25	1.29	1.35	1.25
K2O	0.11	0.16	0.19	0.16	0.16	0.19	0.11	0.16
H2O (+)	4.87	4.69	4.7	4.89	4.69	4.7	4.87	4.89
H2O (−)	0.12	0.17	0.2	0.15	0.17	0.2	0.12	0.15
Total	99.59	99.28	99.41	99.58	99.28	99.41	99.59	99.58
Fe2O3/FeO	1.03	1	1	1.01	1	1	1.03	1.01
Fe2O3/Al2O3	0.76	0.76	0.77	0.77	0.76	0.77	0.76	0.77
Name	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine	Aegirine

Table 2.

Showing the composition of extracted sample.

3.2 XRD

In order to explain borate formation, an XRD analysis of the samples was done. Figure 2 shows the XRD patterns of the untreated and treated samples. In Figure 2a, peak of schorl (NaFe²⁺₃Al₆ (Si₆O₁₈) (BO₃)₃(OH) 3(OH)) [6], dumortierite (Al₇ BO₃ (SiO₄)3O₃) [7] and SiO₂ [8] was found. However, after the extraction (Figure 2b), the identified peaks correspond to sodium borate (Na₂B₄O₇) [9], aegirine (NaFeSi₂O₅) [10], and Al₂CO₃ [11]. However, after multiple times acid leaching of the extracted sample, boron oxide was recovered (Figure 2c).

Figure 2.

XRD graph of (a) virgin (K- Kyanite, D-Dumortierite, S-Schorl) (b) extracted sample (A-Aegirine, T-sodium borate, C- Aluminum oxide) (c) boron recovery sample.

3.3 SEM

The surface morphology of the sample has been shown in Figure 3. The formation of orthorhombic-shaped dumortierite crystals [12] has been observed in SEM (Figure 3a). However, schorl is found in well-crystallized hexagonal-shaped along with kyanite (Figure 3b). The dumortierite crystals are formed after the disintegration of the tourmaline grain. However, in the extracted sample, the formation of sodium borate along with aegirine was observed. From this, it can be stated that, at a particular temperature, schorl along with sodium carbonate can form sodium borate [13] (Figure 3c) and aegirine ((Figure 3d). The acid leaching helps in the extraction of boron oxide from the mineral (Figure 3e and f), as the sodium borate reacts with HCl forming salts and boron-oxide. The Fe and Al residue gets dissolved in the solute, which can be extracted by further synthesis.

Figure 3.

SEM images of (a, b) virgin sample (c, d) extracted sample, where S- schorl, D- dumortierite, T- sodium borate, and A- aegirine, and (e, f) extracted BO_3.

3.4 TGDTA

Three major weight losses are seen in Figure 4a. The first weight loss up to 85°C accounts for the dehydration of the untreated sample, which produced an endothermic peak. The second weight loss and an exothermic peak are seen at 320°C. This exothermic peak corresponded to the crystallization of FeAlO₃, oxidation of Fe³⁺, and oxidation of Fe²⁺ to Fe³⁺. Further, an increase in temperature also triggered the decomposition of the anhydrous salt, contributing to the second weight loss. The third weight loss up to 857°C accounts for the decomposition of residual nitrate and the phase transformation of tourmaline. Therefore, the temperature of 650°C was selected as the heat-treatment temperature to protect the crystalline structure and improve the far-infrared emission of the tourmaline [14].

Figure 4.

TGDTA graph of (a) virgin (b) extracted sample.

However, the TGDTA peak of a treated sample (Figure 4b). First exothermic peak at 566°C was due to the crystallization of anhydrous sodium borate (Na₂ O (B₂O₃)₂) from the amorphous phase [9, 15]. Another endothermic peak observed at 742°C was due to the melting of the crystalline anhydrous sodium borate phase.

4. Discussion

In order to extract boron from the tourmaline sample, it was treated with 20% of sodium carbonate power, in order to form a boron compound that will ease the element to get out of the complex tourmaline structure. On heating, the virgin, as well as extracted samples up to 1000°C following conclusions, were observed:

1. On heating the virgin sample, iron-aluminum oxides were formed at 320°C, and at the higher temperature, a transformation of tourmaline was observed. Hence, no liberation of the boron compound was found in this case.

2. Whereas, for the extracted sample, following reactions 1 and 2 were observed, which can state the formation of sodium borate and boron oxides, respectively.

$$\begin{array}{l}F{e}^{2+}{}_{3}A{l}_6\left(S{i}_6{\text{O}}_{18}\right){\left(B{O}_3\right)}_3{\left(OH\right)}_3\left(OH\right)+\text{2Na}{\left(C{O}_3\right)}_3\\ \to N{a}_2{\text{B}}_4{\text{O}}_5\cdot 8{\text{H}}_2\text{O}+{\text{NaFeSi}}_2{\text{O}}_6+{\text{AlCO}}_3\end{array}$$E1

$$\begin{array}{l}N{a}_2{\text{B}}_4{\text{O}}_5\cdot 8{\text{H}}_2\text{O}+{\text{NaFeSi}}_2{\text{O}}_6+{\text{AlCO}}_3+4HCl\\ \to 4\text{B}{\left(OH\right)}_3+4\text{NaCl}+{\text{8H}}_2\text{O}+{\text{FeCO}}_3\uparrow +A{l}_2{\text{O}}_3\cdot Si{O}_2+4\text{H}\end{array}$$E2

As, when the sample was treated with sodium carbonate, the sodium gets combined with the boron to form sodium borate at 566°C along with aegirine and aluminum carbonate (Eq. (1)). Moreover, the sodium borate when treated with hydrochloric acid forms boron-oxide along with NaCl (Eq. (2)). However, further synthesis process can be followed to remove the Fe and Al recovery from the leachate.

5. Concluding remarks

In order to extract boron from the tourmaline sample, it was treated with 20% NaCO₃ and heated up to 1000°C. It was found that the sodium from the NaCO₃ powder gets combined with the boron and forms sodium borate. However, the boron-oxide can be extracted by treating the sodium borate with hydrochloric acid. Extraction of boron by this method is cost-effective and can fulfill the need for boron elements in various industries.