1 Introduction

Magnesium is listed as a key critical raw material according to a report by the European Commission which evaluated raw materials based on supply risk versus economic importance [1]. It has great potential for many engineering applications because of its extraordinary properties including its non-toxicity, inertness, high melting point, stability, low weight, chemical resistance, electrical insulation, optical transparency, corrosion-resistant behavior, and refractory nature. It has been widely used in agricultural, environmental (antibacterial [2], water purification [3,4,5,6]), electronic, automotive, drug delivery [7], ceramic [8], and construction (as filler in brick [9] and concrete production (magnesium oxychloride [10], magnesium oxysulfate [11])] applications. Furthermore, the use of MgO-based sorbents has been investigated at low temperatures (20–300 °C) for CO2 capture in the form of an inorganic carbonate, which is thermodynamically stable at ambient conditions [12,13,14,15]. However, the production conditions (calcination temperature, and duration) needed to produce MgO have a strong effect on its use in the above-mentioned applications. MgO particles are produced via the calcination of magnesite ore at different temperatures depending on the desired quality of the product, which is classified as fused (2800 °C), dead-burned (>1400 °C), hard-burned (1000–1400 °C), or light-burned MgO (700–1000 °C) [16]. However, the global availability of magnesite ore is limited. As of 2017, large magnesite reserves were located mainly in Russia, North Korea, China, Brazil, Australia, and Greece [17].

Therefore, new alternative sources including seawater, natural brine, desalination reject water [16, 18], dolomite [19], and serpentine [20] have been investigated by various researchers for the production of MgO or Mg(OH)2 with desired properties. Methods including chemical precipitation [16, 18], sol–gel [21], hydrothermal [22], chemical combustion [23], flame spray pyrohydrolysis [24, 25], and a combination of leaching-precipitation-calcination [19, 26,27,28] have been used to produce MgO particles from these sources. Compared with the traditional calcination route [29], these processes have the advantage of being able to produce MgO particles with specific properties for use in the desired applications.

Dong et al. [30] evaluated the properties of MgO particles obtained from desalination reject water via the addition of different bases including NH4OH [16] and NaOH [18], which revealed that alkali type was an important parameter, along with calcination time and duration, significantly affecting the properties of MgO particles. The use of different alkali types resulted in the production of MgO particles with different specific surface area (SSA), reactivity, and microstructural properties. MgO particles with an SSA of 51.4 m2/g were obtained from Mg(OH)2 generated using NaOH, while the use of NH4OH as the alkali source led to the production of MgO particles with an SSA of 78.8 m2/g obtained at a temperature of 500 °C for 2 h. In a follow-up study [31], an SSA value of 47.3 m2/g was found for MgO particles obtained from dolomite ore at a temperature of 900 °C for 40 min, which was considerably lower than the SSA value of MgO particles obtained from magnesium salts (in the range of 105–137.6 m2/g). This can be explained by the primary particle size of dolomite ore. Many studies have also reported in detail on the production of MgO particles from dolomite ore [19, 26, 27].

It is believed that dolomite ore has good potential for use in producing MgO due to the presence of substantial reserves (up to 22 billion tons, apparent reserve) in Turkey, with MgO content in the range of 15–20% [32], while total magnesite reserves are about 0.1–0.15 billion tons, with MgO content of 41–48% [33]. Therefore, in terms of MgO content, dolomite reserves in Turkey represent a considerably higher source than magnesite reserves. The use of dolomite as a raw material for producing MgO particles would be of great interest to the production industry in the near future in Turkey and other countries (USA, Iran, India, etc.) with high dolomite reserves. Thus, the main goal of this study was to produce MgO particles with different properties from dolomite ore using a leaching-precipitation-calcination process. The method adopted here differs from that of previous studies [19, 28, 34], which used Mg(OH)2 as an alkali source to produce MgO particles from dolomite ore. In the precipitation stage, different bases (NaOH, KOH, and NH4OH) were used as precipitant to remove impurities (pH: 8.1 ± 0.05) and obtain magnesium ions (pH: 10.5 ± 0.05) in the form of hydroxide, respectively. The influences of different bases and calcination conditions (temperature and duration) on the properties of selected MgO particles were determined using instrumental techniques including scanning electron microscopy (SEM), Brunauer–Emmet–Teller (BET), X-ray diffraction (XRD), and atomic force microscopy (AFM) analyses. The results obtained were compared with each other and also with the literature.

2 Experimental Procedure

The ore sample used in this study was collected from Hatay province, Turkey. The chemical composition of the sample was as follows: 21.22% MgO, 30.01% CaO, 0.27% Fe2O3, 0.44% Al2O3, 1.77% SiO2, 0.02% P2O5, along with loss on ignition (LOI) of 46.17%. The sample was identified as dolomite according to the XRD analysis [PDF card no: 01-073-2409]. Chemicals (NaOH, KOH, NH4OH, and HCl) were purchased from Merck and used without further purification. The experimental procedure comprises four stages including sample preparation, HCl leaching, precipitation, and calcination. These four stages are explained below.

  • Sample preparation (S1): The sample was crushed into particles smaller than 1 mm in a laboratory for use in the following stage.

  • HCl leaching (S2): The experimental parameters conducted in this study are as follows: stirring speed of 400 rpm, acid/dolomite ratio of 4.50, particle size of <1 mm, solid-to-liquid ratio of 1:3, reaction time of 14 min, and temperature of 25 °C. These parameters were almost identical to those of a previous study [35], except for stirring speed and particle size, which accelerate the dissolution rate of dolomite. At the end of a predetermined reaction time, insoluble impurities (i.e. SiO2) were removed from a leachate solution using Whatman filter paper. Finally, 10 L of the leachate solution containing 2.11 M Mg2+, 2.14 M Ca2+, and 0.135 M Fe2+ was prepared and used as a stock solution in the subsequent stage.

  • Purification and precipitation (S3): 100 mL of the leachate solution obtained in the previous stage was used in each stage. To remove Fe2+ ions in the form of hydroxide, the pH value of the leachate solution stirred at 250 rpm was increased up to 8.1 ± 0.05 via the addition of different bases which were prepared as 1 M (NaOH, KOH, and NH4OH). The solution pH was monitored via a pH meter (WTW 3110), and the solution was stirred for 20 min after reaching a pH of 8.1 ± 0.05. The iron precipitate was removed via filtration, and the purified colorless solution was obtained. Next, a different base was added to the purified solution and stirred at 250 rpm and maintained until reaching a pH of 10.30. The solution was then stirred for 20 min in order to obtain all Mg2+ ions in the form of Mg(OH)2. The latter precipitates obtained in this stage were obtained via filtration and dried at 105 °C in an oven for 4 h in preparation for the calcination and characterization tests.

  • Calcination (S4): The precipitates obtained from the previous stage via the addition of three different bases were calcined at temperatures of 600, 800, and 1000 °C for different predetermined times (1, 3, and 5 h). In total, 27 samples were prepared under different calcination conditions. Therefore, the calcination conditions that were conducted to produce MgO from Mg(OH)2 were designated as MgO_X-Y-Z, where X is the name of base (Na, K, or NH) used for producing Mg(OH)2, Y is the calcination temperature (600, 800, and 1000 °C), and Z is the calcination duration (1, 3, and 5 h). For example; the notation MgO_Na-800-5 indicates that MgO particles were produced at a calcination temperature of 800 °C for 5 h from Mg(OH)2 generated via the addition of NaOH.

2.1 Characterization

The chemical composition of each product was determined by EDTA titration and atomic absorption spectrometry (AAS, Perkin Elmer PinAAcle 900H). The phase properties of each product obtained at different stages were identified using a Rigaku X-ray diffractometer equipped with Cu-Kα radiation in the 2θ range of 15°–85° with a step size of 0.02, and the patterns were evaluated using PDXL software for mineral identification (Rigaku). SEM analysis (Quanta 650 FEG) was further performed to evaluate the surface morphologies of the selected product. The SSA of the product was measured using the BET method (Costech Sorptometer Kelvin 1042). The surface topography of the selected product was determined on a 2.5 μm × 2.5 μm area by AFM (Park NX10, Park Systems) which was used in non-contact dynamic force mode (DFM).

3 Results and Discussion

3.1 Leaching Tests

First, dolomite ore (3 mol) was dissolved in HCl under the predetermined conditions to obtain a stock leachate solution, which was used in subsequent stages. However, the solution obtained was quite acidic, and its pH value was below 0. The pH of the leachate solution (100 mL) stirred at 250 rpm was increased up to 8.1 ± 0.05 by the addition of different bases (NaOH, KOH, and NH4OH) to precipitate iron impurities as hydroxide, and the precipitate was removed via filtration. The pH of the purified solution increased via the addition of different bases up to 10.5 ± 0.05, and the solution was then stirred at 250 rpm for 30 min to precipitate all magnesium ions in the form of magnesium hydroxide [Mg(OH)2], which is referred to as Na-based, K-based, and NH-based Mg(OH)2 depending on the base used. Na-based Mg(OH)2 was precipitated at an NaOH/Mg2+ molar ratio of 2, whereas the required molar ratios of KOH/Mg2+ and NH4OH/Mg2+ for producing Mg(OH)2 were 2.70 and 8, respectively. The purity of each product obtained from the solid–liquid separation was quite high, and there were no impurities present according to the chemical analysis results. In addition, the obtained solution contained high calcium ion concentrations, which will be evaluated to produce precipitated calcium carbonate (PCC) particles via the addition of different carbonates using a chemical precipitation method in a study in the near future.

3.2 Characterization of Mg(OH)2 and MgO Particles

Figure 1 shows the XRD patterns of Mg(OH)2 obtained using different bases and selected MgO particles produced as a result of the calcination of Mg(OH)2 particles under different conditions.

Fig. 1
figure 1

XRD patterns of Mg(OH)2 generated using different bases and selected MgO products: a MgO_Na-600-1, b MgO_Na-800-1, c MgO_K-800-1, d MgO_Na-800-5, e MgO_K-800-5, f MgO_Na-1000-1, g MgO_NH-1000-1, and h MgO_NH-1000-5

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The characteristic peaks of each product obtained via the addition of different bases are well-matched with the standard pattern of brucite [PDF card no: 00-007-0239]. Each product obtained from the calcination process was identified as periclase (MgO), and the obtained patterns are in line with different standard patterns of MgO (PDF card no: 01-071-1176 and 01-076-6599). No characteristic peaks representing brucite mineral were observed in the XRD patterns of any of the MgO particles, which indicates their high quality. However, the intensity of the characteristic peaks representing the periclase (MgO) mineral differed, which can be explained by the experimental factor involved in the calcination process (base type, calcination temperature, and duration). It was clearly seen in XRD patterns that an increase in the calcination duration from 1 to 5 h at a temperature of 1000 °C led to an increase in the intensity values of characteristic peaks of MgO (2θ = 36.78°, 42.76°, 62.14°, 74.54°, and 78.46°) (Fig. 1g and h), but a decrease in the full-width half-maximum (FWHM) value of peak (42.76°) along with the crystallite size of MgO particles. This resulted in an increase in the SSA of MgO particles (given in the following section) that was consistent with those of previous studies [34, 36], which have reported that an inverse relationship was observed between crystallite size and SSA properties of the produced MgO particles.

3.2.1 Specific Surface Area Properties of Mg(OH)2 and MgO Particles

Figure 2 shows the SSA values of Mg(OH)2 particles and some selected MgO particles obtained as a result of the Mg(OH)2 calcination procedure, which was performed at a temperature of 600–1000 °C for 1 and 5 h. Experimental findings indicate that the SSA value of Mg(OH)2 particles was in the range of 18.152–60.235 m2/g, depending on the base type, whereas that for MgO particles varied between 4.493 and 44.547 m2/g. As can be seen in Fig. 2, an increase in the calcination temperature and time resulted in a decrease in the SSA values of MgO particles.

Fig. 2
figure 2

SSA values of Mg(OH)2 particles and some selected MgO particles

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The SSA value of MgO particles produced from the calcination of Na-based Mg(OH)2 at a temperature of 600 °C for 1 h was found to be 44.547 m2/g, which was the highest value. However, this value decreased to 38.861 m2/g after conducting the calcination process for 5 h. Similar results were observed with the use of other bases (NH4OH and KOH), and the SSA values of MgO particles obtained from the NH-based and K-based Mg(OH)2 at a temperature of 600 °C for 1 h were 21.336 and 34.282 m2/g, respectively. Considering all experimental results, the effects of base type on the SSA value of MgO particles followed the order NaOH, KOH, and NH4OH. However, these findings are not consistent with those of a previous study [30], which indicated that MgO particles with higher SSA (78.8 m2/g) could be produced via the use of NH4OH, compared to the SSA of MgO particles (51.4 m2/g) prepared via the addition of NaOH.

The SSA values of MgO obtained using three different bases (Na, K, and NH) showed a similar decreasing trend depending on the calcination temperature and time. The SSA values of MgO obtained from Na-, K-, and NH-based Mg(OH)2 particles at a temperature of 1000 °C for 5 h were found to be 7.318, 5.793, and 4.493 m2/g, respectively. These values indicated an increase in the crystallite size of MgO particles due to the inverse relationship between their SSA and crystallite size properties. Also, higher calcination temperatures resulted in a sintering behavior of MgO, which led to an increase in the crystallite size of MgO due to its agglomeration characteristics [37, 38].

3.2.2 SEM Images of Selected MgO Particles

Figure 3 shows the SEM images of the selected MgO particles produced under different conditions, and it is clear that the findings given in the previous section are in good agreement with the SEM images. The MgO particles obtained from Na-, K-, and NH-based Mg(OH)2 particles at a temperature of 600 °C or 800 °C for 3 h were amorphous and not well-crystallized, comprising agglomerated nanoparticles, but an increase in the calcination temperature to 1000 °C produced MgO particles with well-shaped and smooth crystals. The SEM images of the MgO particles using different bases were almost identical, and therefore it can be said that the morphology of the MgO particles was not strongly dependent on the type of base, but calcination temperature played a crucial role in the preparation of amorphous or crystalline MgO particles.

Fig. 3
figure 3

SEM images of some selected MgO particles produced under different conditions

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3.2.3 AFM Images of Selected MgO Particles

The surface topographies (scan area 2.5 μm × 2.5 μm) of selected MgO particles imaged by AFM are shown in Fig. 4. It is clearly seen in the AFM images that the surface properties of the MgO particles were strongly influenced by the production conditions. Therefore, it is possible to produce MgO particles with different surface roughness depending on calcination temperature and durations.

Fig. 4
figure 4

Surface topography (2.5 μm × 2.5 μm) of selected MgO particles produced under different conditions

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Figure 4 reveals that MgO_Na-600-1 particles were composed mainly of small grains, but an increase in the calcination temperature and duration resulted in MgO particles with large grains due to their sintering tendency, which was in good agreement with the BET analysis results. However, MgO_K-600-1 and MgO-NH-600-1 particles had larger granular structures than MgO_Na-600-1 particles. The number of grains in the surface of MgO particles obtained from K- and NH-based Mg(OH)2 increased under the calcination condition of 1000 °C for 5 h, and these grains made their surface roughness higher.

The 2D images of selected MgO particles and their distribution can be seen in the Supplementary File (Fig. S1). The average surface roughness value obtained from the 2D images was found to be 3.912 nm for MgO_Na-600-1, 6.490 nm for MgO_Na-1000-5, 3.831 nm for MgO_K-600-1, 7.810 nm for MgO_K-1000-5, 5.810 nm for MgO-NH-500-1, and 21.304 nm for MgO-NH-1000-5. The average surface roughness values of MgO particles tended to increase by increasing calcination temperature, but the BET value of MgO particles started to decrease. These obtained findings are in good agreement with those of a previous study [39], which indicated that temperature had a strong effect on the surface topography of materials. In another work [17], an increase in the calcination temperature to 1000 °C from 700 °C led to a decrease in the surface roughness of MgO particles to 4 nm from 45 nm, which is inconsistent with the results obtained in this study. This difference can be explained by the sintering of small grains resulting in the growth of grain, which can be seen clearly in each AFM image (Fig. 4). The gaps with high depths were observed in the surface points of MgO particles produced at a temperature of 1000 °C for 5 h [Fig. S1(d, e, f)]. The growth of these grains resulted in an increase in the surface roughness of MgO particles and therefore large grains were observed, which were in line with the SEM images given in Fig. 3. Furthermore, the distribution of surface roughness in the 2D images of MgO particles lined with red color was evaluated by using surface skewness (Rsk) and kurtosis (Rku) values. The obtained statistical values are listed in Table 1.

Table 1 Surface properties of the selected MgO particles
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The Rsk value of MgO particles obtained from Na-based Mg(OH)2 particles changes from negative to positive values with an increase in the calcination temperature and duration, but these values are close to zero. Similar values were observed in the AFM images of MgO-K and MgO-NH, which were produced under different conditions. These values mean that the distributions of surface roughness for each MgO particles are symmetrical, indicating that MgO particles with uniform surface properties were produced. However, the Rku of each MgO particles varied between 2.188 and 4.882, which indicates that variability in the surface points of MgO particles was higher.

4 Conclusion

Dolomite ore is one of the most important sources for producing MgO particles. Therefore, this study demonstrated the production of MgO particles with different properties from dolomite ore using the HCl leaching-precipitation-calcination method. The effects of base types on the production of Mg(OH)2 particles and their transformation into MgO particles under different calcination conditions were studied. The required molar ratio of base/Mg2+ to produce Mg(OH)2 from the leachate solution was ordered as follows: NH4OH > KOH > NaOH. All MgO particles produced were identified as periclase mineral. SEM and AFM observations revealed that the morphology and surface properties of the MgO particles were strongly dependent on the production conditions (base type, calcination temperature, and duration). The experimental findings showed that it was possible to produce MgO particles with different SSA and surface roughness properties via the addition of different bases. For example, MgO particles with the highest SSA (44.54 m2/g) were produced from the Na-based Mg(OH)2 particles at a temperature of 600 °C for 1 h, while the NH-based Mg(OH)2 particles converted into MgO particles with the lowest SSA (4.49 m2/g) at a calcination temperature of 1000 °C for 5 h.

It can be summarized that the type of base used as precipitant (along with calcination temperature and conditions) plays a key role in producing MgO particles from dolomite ore, and each precipitant can be used to produce MgO particles with different properties, which strongly influence their use in industry.