J.Cent.South Univ.(2025) 32: 977－990
Graphic abstract:
1 Introduction
Magnesite, an important magnesium mineral resource, is often used as a raw material for extracting metallic magnesium and producing magnesium compounds [1, 2]. Its chemical composition is MgCO3, and it is a carbonate mineral belonging to the cubic crystal system [3]. It is often used in the form of rhombic hexahedra, columnar, tabular, granular, dense, earthy, and fibrous shapes, with colors such as white, gray, yellow, brown, and black. Magnesite ore is mainly composed of magnesite, often containing small amounts of dolomite, calcite, quartz, and carbonaceous substances [4]. Meanwhile, dolomite is the primary gangue mineral of magnesite, and its crystal belongs to the carbonate mineral of the cubic crystal system, with a chemical composition of CaMg(CO3)2 [5]. Due to the fact that magnesite and dolomite are both alkaline earth carbonate minerals with similar crystal structures and chemical properties, it is difficult to separate dolomite from magnesite [6, 7].
Among the separation methods for magnesite and dolomite, foam flotation is a feasible method, which is based on the physical and chemical properties of the mineral and realizes separation through the difference of surface properties [8, 9]. Based on current research, it is difficult to separate magnesite from dolomite by relying on the collector performance [10]. Instead, the use of adjustment agents is required to achieve this sorting goal. If the effect of the regulator on magnesite and dolomite is similar and both are either activated, inhibited, or not activated simultaneously, the separation of the two minerals cannot be achieved [11]. Therefore, it is necessary to choose a regulator that has a selective effect on a single mineral, which achieves the purpose of separating the two minerals by expanding the surface property difference between magnesite and dolomite [12]. However, it cannot be ignored that the interaction between magnesite and dolomite during the flotation process will further deteriorate regulator effectiveness and increase the difficulty in separating the two minerals [3, 13].
Research has shown that surface pretreatment of minerals before flotation can selectively alter surface properties as well as increase flotation differences between minerals, thereby improving the selectivity of the separation process, and to some extent, weakening the interaction between the two minerals [14, 15]. IRANNAJAD et al [16] summarized the pretreatment methods before flotation and found that auxiliary pretreatment of minerals can improve separation selectivity; surface dissolution is a pretreatment process that modifies the surface properties of minerals. Other scholars have also performed pretreatment before flotation and have achieved satisfactory results and conclusions [17, 18].
Regarding chemical pretreatment, DU et al [19] found that under the sodium oleate (NaOL) system, the floatability of the three types of ilmenites pretreated with sulfuric acid surface greatly improved. Meanwhile, LI et al [20] have found that oxalic acid pretreatment is beneficial for NaOL selective adsorption on the diaspore surface rather than kaolinite. SHENG et al [21] found that H2O2 oxidation pretreatment improved Na2S and NaBX adsorption on the chalcopyrite surface as well as its flotation performance. Meanwhile, SUN et al [22] found that after surface modification with lead-ion pretreatment, the grade of fluorite concentrate considerably increased. During the modification process, a large amount of lead ions were adsorbed on the calcite surface, forming Pb3(CO3)2(OH)2 and Pb(OH)2 precipitates, thereby enhancing calcite adsorption on collectors and achieving reverse flotation separation for fluorite and calcite.
In terms of other pretreatment methods, TENG et al [23] used silicate bacteria to dissolve silicate minerals in magnesite to ensure that reverse flotation desilication was achieved using dodecylamine as the collector. Moreover, PENG et al [24] achieved efficient flotation separation of chalcopyrite and molybdenite using electrocatalytic oxidation pretreatment. Furthermore, ZHENG et al [25] demonstrated that ultrasonic pretreatment can improve the flotation performance of coking intermediate products, improve the combustible recovery rate, and reduce the ash value of flotation tails. YANG et al [26] demonstrated that using high-temperature air oxidation for treating minerals can effectively reduce chalcopyrite hydrophobicity to achieve efficient flotation separation of chalcopyrite and galena.
Chemical pretreatment, physical pretreatment, biological pretreatment, and other pretreatment methods can have a certain improvement effect on the flotation performance and sorting effect of minerals. Although research and application of pretreatment methods are gradually increasing, they are rarely used to separate magnesite and dolomite. Therefore, herein, tartaric acid, citric acid, and tannic acid (TA) were selected to conduct pretreatment research on magnesite and dolomite as well as mixed ore validation experiments and mechanism analysis on TA with notable effects. Further development of this study can provide new ideas and directions for the flotation separation and purification of magnesite and dolomite.
2 Experimental
2.1 Materials
Magnesite and dolomite block-like ore samples were collected from the Dashiqiao mining area in Liaoning Province, China. Figure 1 shows that the measured spectra of magnesite and dolomite are consistent with the standard spectra. The purity of magnesite and dolomite is 98.56% and 98.42% (Table 1), respectively. An ore sample with size of 38-74 μm was used for the flotation test, where the sample was treated with 25% regulator solution, and the pretreatment sample was obtained after cleaning and filtering.
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| Mineral | MgO | CaO | Purity |
|---|---|---|---|
| Magnesite | 47.12 | 0.14 | 98.56 |
| Dolomite | 20.95 | 29.92 | 98.42 |
The magnesium containing minerals used for rough selection were taken from Liaoyang City, Liaoning Province, China. Chemical element analysis showed that the main components of the ores were 42.33% MgO, 1.24% CaO, 6.08% SiO2, 0.27% Al2O3 and 0.21% total Fe. In addition, the XRD results (as shown in Figure 2) indicated that magnesium containing minerals mainly exist in the form of magnesite, with a small amount of dolomite and quartz.
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In flotation experiments, HCl and NaOH are used to adjust the slurry pH value; NaOL is used as the collector; TA, tartaric acid, and citric acid are used as the pretreatment regulators for magnesite and dolomite.
2.2 Micro-flotation test
A pretreatment experiment was conducted in a 500-mL beaker, with 100 g of mineral and 400 mL of the acid-leaching solution of the adjusting agent added for each experiment.
An XFG-II hanging trough flotation machine was used at a speed of 1992 r/min, and 2.0 g of ore sample and 20 mL of deionized water were added to a 30-mL flotation cell for each experiment [27]. After drying and weighing, the flotation recovery of artificial mixed ore was calculated by combining the yield and CaO content in tailings [28, 29].
The selectivity index (SI) was used as the evaluation standard to describe the flotation selectivity of TA pretreatment for separating dolomite from magnesite, as shown below [30]:
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where ε (%) represents mineral recovery in the concentrates (expressed as subscript c) and tailings (expressed as subscript t); and superscripts M and D denote magnesite and dolomite, respectively.
2.3 Characterization
The particle morphology characteristics of magnesite and dolomite samples before and after TA pretreatment was analyzed by scanning electron microscopy (SEM, TESCAN MIRA LMS) [31]. The pore size and specific surface area of the samples were analyzed using a fully automated surface area and porosity analyzer (BET, Kantar Instruments Inc., USA) [32]. The adsorption quantity of NaOL on the sample surface was measured by UV-visible photometer (Shanghai Aoyi Scientific Instrument Co., Ltd.) [33]. The static contact angle of deionized water on the mineral surface was measured using the JC2000A contact angle measuring instrument (Shanghai Xuanyi Chuangxi Industrial Equipment Co., Ltd.) [34]. The XPS spectra of the samples were measured using ESCALAB 250Xi spectrometer (Thermo Scientific, USA), and Thermo Avantage software was used for result processing and analysis [35].
2.4 Batch flotation experiments
The rough flotation test of actual magnesite ore was carried out using a flotation machine equipped with a 750-mL flotation cell, with the impeller speed and gas flowrate fixed at 1992 r/min and 0.22 m3/h, respectively. Before each flotation test, add 250 g (75% pass through 74 μm) of ore sample pre-treated with TA to the flotation cell and conduct the test according to the pure mineral flotation process. Subsequently, collect flotation products, filter and dry them, weigh the concentrate and tailings separately, and determine the content of MgO and CaO. Each experiment is repeated three times, and the reported result is the average of these experiments.
3 Results and discussion
3.1 Flotation test results
At pH 10, the effect of NaOL concentration on the floatability of magnesite and dolomite before and after pretreatment was investigated (Figure 3).
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Within the NaOL concentration range of 60-120 mg/L (Figure 3), the magnesite and dolomite minerals exhibited good floatability. However, the difference in flotation between the two minerals was small, indicating that relying solely on the collection capacity of NaOL cannot separate the two minerals. After pretreatment with citric acid, the flotation difference between magnesite and dolomite initially decreased and then decreased. However, the maximum flotation difference between the two minerals was 46.45%, which was still insufficient for separating the two minerals. After pretreatment with tartaric acid, the overall difference in floatability between magnesite and dolomite increased; however, the maximum difference in floatability between the two minerals was 29.03%, which was fairly small. After pretreatment with TA, there was a notable difference in the floatability of magnesite and dolomite, especially in the NaOL concentration range of 60-120 mg/L. At a NaOL concentration of 110 mg/L, magnesite achieved a maximum flotation recovery of 92.05% and the two minerals achieved a maximum flotation difference of 87.1%. Hence, at the NaOL concentration of 110 mg/L, the effect of slurry pH on the floatability of magnesite and dolomite before and after TA pretreatment was further investigated.
As the pH value of the slurry increased (Figure 4), the float difference between magnesite and dolomite without TA pretreatment was very small and the two minerals could not be separated. After TA pretreatment, the difference in flotation difference between the two minerals showed a trend of first increasing and then decreasing. When the slurry pH value was 10, the difference in flotation between the two minerals was the highest, which is most conducive to flotation separation.
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Under the optimal separation conditions of a single mineral at a NaOL concentration of 110 mg/L and slurry pH of 10, a flotation validation test was conducted on 2 g of binary mixed ore (magnesite: dolomite=8:2), with the results shown in Figure 5.
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The recovery of magnesite and dolomite in the concentrate were 88.77% and 82.72%, respectively (Figure 5). After TA pretreatment, the recovery of magnesite in the concentrate was 86.02%, while the recovery of dolomite in the concentrate was only 34.04%. This indicated that the floatability of dolomite in the concentrate greatly reduced after TA pretreatment. Meanwhile, after TA pretreatment, the MgO grade in the concentrate increased by 2.58% and the CaO grade in the concentrate decreased by 2.94%, indicating that the magnesite content in the concentrate after TA pretreatment considerably increased, whereas the dolomite content considerably decreased. In addition, after TA pretreatment, the SI value increased from 1.65 to 3.46, which is an increase of 1.81, indicating that TA pretreatment improved the separation efficiency of magnesite and dolomite, achieving the flotation separation of the two minerals.
3.2 SEM and BET analysis
This section characterizes the surface morphology characteristics of magnesite and dolomite before and after TA pretreatment as well as the specific surface area and pore size characteristics of mineral particles through SEM and Brunauer-Emmett-Teller (BET) measurement; the analysis results are shown in Figure 6.
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Based on the graph shown in Figure 6, a certain number of small particles were dispersed on the magnesite and dolomite surfaces before TA pretreatment, while the surface morphology of the two minerals was not notably different. After TA pretreatment, the magnesite and dolomite surfaces became relatively flat. However, after pretreatment, there were many obvious concave pores on the dolomite surface, indicating that TA pretreatment considerably changed the morphological characteristics of dolomite.
To better explain the effect of TA on the specific surface area and pore size of magnesite and dolomite surface morphologies, BET analysis was conducted to measure the relevant parameters of the two minerals before and after TA pretreatment; the analysis results are presented in Table 2.
| Sample | Surface area/ (m2·g-1) | Total pore volume/ (cm3·g-1) | Average pore diameter/nm |
|---|---|---|---|
| Magnesite | 0.605 | 0.001775 | 11.7413 |
| TA+magnesite | 0.599 | 0.001840 | 12.2887 |
| Dolomite | 0.370 | 0.0009877 | 10.6845 |
| TA+dolomite | 0.668 | 0.002496 | 14.9578 |
Based on the calculation presented in Table 2, the specific surface area of magnesite decreased by 0.006 m2/g, total pore volume increased by 0.000065 cm3/g, and average pore size increased by 0.5474 nm before and after TA pretreatment. Meanwhile, the specific surface area of dolomite increased by 0.298 m2/g, total pore volume increased by 0.0015083 cm3/g, and average pore size increased by 4.2733 nm. Compared with magnesite, TA pretreatment considerably changed the pore volume, pore size, and specific surface area characteristics of dolomite. Thus, TA had a stronger effect on the dolomite surface.
By comparing the changes in the pore size and cumulative specific surface area of magnesite and dolomite before and after TA pretreatment, the impact characteristics of TA pretreatment on the surface morphology and properties of magnesite and dolomite can be further analyzed. The obtained cumulative specific surface area distribution curve is shown in Figure 7.
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The cumulative specific surface areas of magnesite before and after TA pretreatment were 0.36635 and 0.4202 m2/g, respectively (Figure 7), with an increase of 0.05385 m2/g in cumulative specific surface area. Meanwhile, the cumulative specific surface area of dolomite before and after TA pretreatment was 0.22849 and 0.41465 m2/g, respectively, with an increase of 0.18616 m2/g in cumulative specific surface area. Furthermore, the TA pretreatment had a greater effect on the cumulative specific surface area of dolomite than on the cumulative specific surface area of magnesite. Therefore, TA pretreatment changed the surface morphology characteristics of magnesite and dolomite, including pore size and specific surface area, as well as considerably changed the surface characteristics of dolomite, thereby strongly altering its surface morphology properties.
3.3 Adsorption quantity analysis
The adsorption capacity of minerals on collectors can reflect their flotation performance. Figure 8 shows the adsorption capacity of NaOL on magnesite and dolomite surfaces before and after TA treatment.
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Without TA pretreatment, the adsorption capacity of NaOL on the magnesite and dolomite surfaces was 0.8632 and 0.9567 mg/g, respectively (Figure 8). The adsorption capacity of NaOL on the two mineral surfaces is relatively large and similar. After TA pretreatment, the adsorption capacity of NaOL on the magnesite and dolomite surfaces was 0.4153 and 0.1558 mg/g, respectively. It was observed that after TA pretreatment, the adsorption capacity of NaOL on the magnesite surface was large and that of NaOL on the dolomite surface was minimal. Therefore, TA pretreatment can selectively and considerably reduce the adsorption of NaOL on the dolomite surface while slightly affecting the adsorption of NaOL on the magnesite surface.
3.4 Contact angle analysis
The difference in the floatability of minerals is directly related to the surface wettability characteristics of the minerals after flotation agent modification, which is intuitively reflected by the contact angle, i.e., the larger the contact angle of a mineral, the stronger its hydrophobicity.
The contact angles of magnesite and dolomite were 27.2° and 29.1°, respectively (Figure 9). The contact angles of the two minerals and their floatability were similar. After TA pretreatment, the contact angles of magnesite and dolomite were 42.6° and 45.9°, respectively, indicating that the floatability of the two minerals was still similar. When NaOL acted alone on the surfaces of the two minerals, the floatability of magnesite and dolomite greatly improved, reaching 79.7° and 98.1°, respectively. When NaOL acted on the surfaces of two minerals pretreated with TA, the magnesite contact angle decreased to 59.9° (a 19.8° decrease) and the dolomite contact angle decreased to 42.3° (a 55.8° decrease). The magnesite contact angle slightly decreased after TA pretreatment, and the dolomite contact angle considerably decreased. Therefore, the adsorption capacity of NaOL after pretreatment on the magnesite surface was good, whereas it was weak on the dolomite surface. This adsorption difference is conducive to the flotation separation of magnesite and dolomite.
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3.5 XPS analysis
The measurement of the adsorption capacity and contact angle can visually demonstrate that the adsorption capacity of NaOL on the dolomite surface is considerably weaker than that on the magnesite surface after TA pretreatment. However, it cannot explain the selective mechanism of TA pretreatment on the surfaces of the two minerals. Therefore, this section characterizes the selective action characteristics of TA pretreatment on the magnesite and dolomite surfaces through XPS detection. Figure 10 shows the XPS full spectrum of magnesite and dolomite before and after TA pretreatment.
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As shown in Figure 11, three characteristic peaks of Mg 1s, O 1s, and C 1s appeared on the magnesite surface before and after pretreatment, while four characteristic peaks of Mg 1s, O 1s, Ca 2p and C 1s appeared on the dolomite surface. After TA pretreatment acted on the surfaces of the two minerals, the types of elements on the surfaces of the two minerals did not change, reflecting TA adsorption characteristics with elemental compositions of C, H, and O on the surfaces of the two minerals [36]. To better observe the effect of pretreated TA on the surfaces of magnesite and dolomite, the relative content characteristics of surface elements of the two minerals before and after pretreatment were measured, and the results are shown in Table 3.
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| Sample | C 1s | O 1s | Mg 1s | Ca 2p |
|---|---|---|---|---|
| Magnesite | 20.41 | 69.59 | 10.10 | |
| TA pretreated magnesite | 20.70 | 69.63 | 9.67 | |
| Dolomite | 32.43 | 58.14 | 5.03 | 4.40 |
| TA pretreated dolomite | 33.64 | 58.85 | 5.01 | 2.50 |
Based on Table 3, it can be seen that after TA pretreatment, the relative content of Mg element on the magnesite surface decreased from 10.10% to 9.67%, the relative content of Mg element decreased by 0.43%, and the relative content of O element on the magnesite surface increased by 0.04%. Meanwhile, the relative content of Mg element on the dolomite surface decreased from 5.03% to 5.01%, and the Mg element relative content decreased by 0.02%. Moreover, the relative content of the Ca element on the dolomite surface characteristics decreased from 4.40% to 2.50%, while the relative content of the Ca element decreased by 1.90%. The relative content of O element on the dolomite surface increased by 0.71%. Additionally, the influence of pretreated TA on the Mg element of the two minerals was much smaller than that on the Ca element of dolomite. Moreover, an increase in the O element relative content on the dolomite surface was considerably greater than that on the magnesite surface, indicating that the pretreated TA can selectively act on the dolomite surface through the O element.
To further analyze the action characteristics of TA pretreatment on the magnesite and dolomite surfaces, the fine spectra of magnesite and dolomite before and after TA pretreatment were analyzed, and the results are shown in Figure 11.
After TA pretreatment, the Mg 1s peak position of magnesite increased from 1305.11 to 1305.27 eV and the O 1s peak position changed from 532.22 to 532.03 eV, increasing by 0.16 and 0.19 eV, respectively (Figure 11). Due to the change in peak position being less than 0.20 eV, it can be considered that the adsorption ability of TA on the magnesite surface is weak and the adsorption method is physical adsorption [37].
After TA pretreatment, the Mg 1s peak position of dolomite increased from 1304.07 to 1304.14 eV, the peak position increased by 0.07 eV, the O 1s peak position changed from 531.49 to 531.71 eV, the peak position increased by 0.22 eV, and new characteristic peaks appeared at 533.17 eV (Figure 12). This indicates that TA pretreatment has a weak ability to act on the Mg peak position of dolomite and the adsorption method is still physical adsorption. Moreover, TA pretreatment has a strong ability to act on the O peak position of dolomite, which may be due to TA acting on the Ca site of dolomite through the O peak position.
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Before TA pretreatment, the Ca 2p peaks of dolomite appeared at 347.15 and 350.75 eV (Figure 13). After TA pretreatment, the Ca 2p peaks of dolomite appeared at 347.39 and 350.99 eV, with a change of 0.24 eV. Moreover, new Ca 2p peaks appeared at 351.41 and 355.51 eV. This indicates that the pretreated TA strongly adsorbed on the Ca sites on the dolomite surface. Based on the changes in O and Ca sites on the dolomite surface, it can be inferred that the pretreated TA is chemically adsorbed onto the Ca sites on the dolomite surface through O elements, thereby achieving selective adsorption of characteristic Ca sites on the dolomite surface.
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3.6 Batch flotation results
The above experiments and analysis results indicate that the TA pretreatment method achieves efficient flotation separation of magnesite and dolomite by selectively inhibiting the collecting of NaOL on dolomite. In order to further evaluate the effectiveness of TA pretreatment method, batch flotation (i.e. rough flotation) experiments were conducted on magnesite ore under pH 10 conditions, and the results are shown in Figure 14.
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As shown in Figure 14, in the actual ore of untreated magnesite, over 78.08% of magnesite and dolomite in the concentrate are floated by NaOL. As the pre-treatment concentration of TA increased from 0 to 0.75 kg/t, the recovery of dolomite significantly decreased by 54.97%, and the recovery of magnesite slightly decreased by 11.44%. Under the condition of a TA concentration of 0.75 kg/t, after rough flotation, a magnesite concentrate with a MgO grade of 45.49% and a CaO grade of 0.75% was obtained, in which 73.65% of magnesite was recovered, while only 23.11% of dolomite was floated. The above results indicate that the TA pretreatment method can effectively remove dolomite, thereby achieving flotation separation of magnesite and dolomite.
4 Conclusions
1) TA is a pretreatment regulator for efficient flotation separation of magnesite and dolomite.
2) TA pretreatment can selectively considerably increase the specific surface area and pore size of dolomite, slightly alter the specific surface area and pore size of magnesite, and exhibit the characteristic of easy adsorption on the dolomite surface.
3) Contact angle measurement and adsorption quantity measurement intuitively indicated that after TA pretreatment, magnesite had a good adsorption capacity for NaOL and dolomite had a very poor NaOL adsorption capacity.
4) XPS detection demonstrated that the adsorption of TA on magnesite is mainly physical adsorption, with weak adsorption ability. Moreover, adsorption on the dolomite surface is mainly strong chemical adsorption, which interacts with the Ca site of dolomite through the O element.
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