3.2 Analysis of ICP measurement results
After dissolving the mineral in water, its surface constituent ions detach from the mineral and migrate into the water. After the ions enter the solution system, they affect the composition of pulp ions and mineral surface atoms, which in turn affects the floatability of the mineral and the role of flotation agents on the mineral surface[17, 25]. An ICP instrument was used to measure the Mg2+ dissolved from brucite for different dissolution methods to clarify the characteristics of dissolved Mg2+ in case of brucite.
1. Relation between brucite content and dissolved Mg2+ concentration
Figure 4 shows that as the content of brucite increases, the dissolved Mg2+ concentration of brucite shows a gradual upward trend, but the dissolved Mg2+ concentration does not show a doubling pattern as the content of brucite. When the content of brucite is 0.5 g, the lowest dissolved Mg2+ concentration is 25.6 mg/L. When the content of brucite is 3 g, the highest dissolved Mg2+ concentration is 76.8 mg/L. Within the range of brucite content from 1.0 to 2.5 g, the increase in dissolved Mg2+ concentration is slow. The dissolution characteristics and trend of Mg2+ in brucite are consistent with previous studies[7].
2. Characteristics of dissolved Mg2+ concentration for brucite at different dissolution times
Figure 5 shows that the dissolved Mg2+ concentration of brucite increases gradually with increasing dissolution time. When the dissolution time is 1 min, the lowest concentration of dissolved Mg2+ is 58.8 mg/L. When the dissolution time is 30 min, the highest concentration of dissolved Mg2+ is 100 mg/L. Therefore, an increase in dissolution time promotes the transfer of Mg2+ dissolved on the surface of brucite to the aqueous solution and increases the content of Mg2+ in the solution.
3. Characteristics of dissolved Mg2+ concentration of brucite at different pulp pH values
Figure 6 shows that the concentration of Mg2+ dissolved in brucite gradually decreases with increasing pH value of the slurry. After the pH value of the slurry reaches 12, the dissolved Mg2+ concentration of brucite is considerably low and gradually approaches zero. Under this condition range, the effect of dissolved Mg2+ on mineral flotation is significantly weakened.
4. Characteristics of Mg2+ concentration in brucite under different dissolution modes
Figure 7 shows that when the dissolution modes is H2O dissolution, the lowest concentration of Mg2+ dissolved in brucite is 72.8 mg/L. After dissolution with HCl, the dissolved Mg2+ concentration of brucite is 100 mg/L. Furthermore, after dissolution with H2SO4, the Mg2+ concentration of brucite is 172 mg/L. Therefore, the effectiveness of the different modes in promoting the dissolution of Mg2+ from brucite follows the order of H2SO4 dissolution > HCl dissolution > H2O dissolution.
3.1 Flotation test result
3.1.1 Effect of collector concentration on the floatability of brucite
This floatability characteristics of magnesite were investigated using sodium oleate, sodium dodecyl sulfonate, and oxidized paraffin soap as collectors.
Figure 8 shows that at a slurry pH of 10.5, the collection capacities of the three collectors for brucite follows the order of sodium dodecyl sulfonate > sodium oleate > oxidized paraffin soap. The flotation recovery of brucite for all three collectors initially increases and then slightly decreases with increasing collector concentration. When the concentrations of sodium oleate, sodium dodecyl sulfonate, and oxidized paraffin soap are 160 mg/L, the maximum flotation recovery rates of brucite are 60%, 97%, and 46.5%, respectively.
3.1.2 Effect of pulp pH on the floatability of brucite
Figure 9 shows that the overall flotation recovery rate of brucite for the three collector systems follow the order of sodium dodecyl sulfonate > sodium oleate > oxidized paraffin soap. The flotation recovery rate of brucite for the sodium dodecyl sulfonate and oxidized paraffin soap systems initially increases and subsequently decreases with increasing pH value of the slurry; furthermore, the recovery rate gradually increases for the sodium oleate system. After the pH reaches 11.5, the ability of sodium oleate to capture brucite gradually increases, whereas that of sodium dodecyl sulfonate and oxidized paraffin soap to capture brucite gradually decreases.
3.1.3 Effect of acid dissolution on the floatability of brucite
1. Effect of HCl dissolution time on the floatability of brucite
Figure 10 shows that the floatability of brucite in the sodium dodecyl sulfonate system does not change considerably with increasing acid dissolution time of HCl. However, the flotation recovery rate for the oxidized paraffin soap and sodium oleate systems exhibits an upward trend, and the maximum recovery rate of brucite is achieved at a dissolution time of 20 min, but it is still lower than the flotation recovery rate without dissolution treatment. Thus, the acid dissolution of HCl is beneficial for the dissolution of brucite and decreases the flotation recovery rate of brucite.
2. Effect of H2SO4 dissolution time on the floatability of brucite
Figure 11 shows that the overall flotation recovery rate of magnesite for the three collector exhibits an upward trend with increasing acid dissolution time of H2SO4. When the dissolution time is 15 min the maximum recovery rate for sodium oleate is 21%. When the dissolution time is 20 min, the maximum recovery rate for oxidized paraffin soap is 56%. Furthermore, the maximum recovery rate for sodium dodecyl sulfonate is 94.5% when the dissolution time is 20 min. However, the maximum flotation recovery obtained after acid dissolution of the three collectors is still lower than that obtained without dissolution treatment, indicating that H2SO4 dissolution treatment is also not conducive to the flotation recovery of brucite.
3.1.4 Effect of acid leaching on the floatability of brucite
1. Effect of HCl leaching treatment on the floatability of brucite
Figure 12 shows that after HCl leaching treatment, the flotation recovery of brucite for sodium dodecyl sulfonate and oxidized paraffin soap systems initially increases and then decreases, while the flotation recovery gradually decreases for the sodium oleate system. When the concentration of HCl leaching is > 0.5 mol/L, the flotation recovery of brucite for the three collectors gradually decreases. Afterwards, the acid leaching concentration continues to increase, which will have a certain inhibitory effect on the flotation of brucite.
2. Effect of H2SO4 leaching treatment on the floatability of brucite
Figure 13 shows that after H2SO4 leaching treatment, the flotation recovery rates of brucite for all three collector systems initially increases and then decreases. At an H2SO4 leaching concentration of 0.1 mol/L, the highest flotation recoveries of 64.5%, 94.5%, and 53.5% are achieved, respectively. Afterward, the flotation recoveries of the three minerals increase, and the higher the concentration of H2SO4 leaching treatment, the more the flotation recovery rate decreases. Thus, the H2SO4 leaching treatment is not conducive to the flotation recovery of brucite for the three collector systems.
3.3 Analysis of zeta potential detection results
Figure 14 shows that dissolution causes a positive shift in the zeta potential of brucite, with the shift degree following the order of H2SO4 dissolution > HCl dissolution > H2O dissolution. These results indicate that the ability of dissolved Mg2+ to promote the increase in the surface potential of brucite follows the order of H2SO4 dissolution > HCl dissolution > H2O dissolution.
Figure 15 shows that after the addition of MgCl2, the zeta potential values of brucite for different dissolution methods exhibit a positive shift. Compared with the results presented in Fig. 14, the increase in zeta potential of brucite follows the order of H2SO4 dissolution > HCl dissolution > H2O dissolution > raw brucite ore[21].
3.4 Adsorption quantity analysis
To better investigate the effect of surface dissolution on the adsorption performance of brucite, the adsorption characteristics of brucite were measured for three collector systems; the results are shown in Table 3.
Table 3
Test results of surface adsorption quantity of brucite
Sample
|
Adsorption quantity (mg/L)
|
Sodium oleate
|
Sodium dodecyl sulfonate
|
Oxidized paraffin soap
|
Brucite
|
156.102
|
158.488
|
149.150
|
H2O-dissolved brucite
|
152.293
|
156.574
|
146.855
|
HCl-dissolved brucite
|
151.505
|
153.467
|
142.513
|
H2SO4-dissolved brucite
|
150.858
|
151.953
|
136.562
|
Table 3 shows that the ability of the three collectors to adsorb brucite follows the order of sodium dodecyl sulfonate > sodium oleate > oxidized paraffin soap. The adsorption performance for different treatment methods of brucite for three types of collectors follows the order of raw brucite ore > H2O dissolution > HCl dissolution > H2SO4 dissolution. This adsorption feature further confirms that dissolution is not beneficial for the adsorption of the three collectors on the surface of brucite.
3.5 Analysis of SEM results
The morphological characteristics of different treatment methods of brucite were characterized through SEM measurements. Subsequently, the surface characteristics of brucite were measured and analyzed for the content of C, O, and Mg. The results are shown in Figs. 16–19.
Figure 16 shows three characteristic peaks of C, O, and Mg in case of the raw brucite ore, with percentage contents of 32.0%, 47.6%, and 20.4%, respectively. The content of C corresponds to the red area in the figure, indicating that the content of C in the brucite ore is considerably low.
Figure 17 shows that H2O-dissolved brucite also exhibits three characteristic peaks of C, O, and Mg, with percentage contents of 36.9%, 45.4%, and 17.7%, respectively. Compared with the C, O, and Mg contents of the raw brucite ore, the content of C increases by 4.9%, whereas those of O and Mg decrease by 2.2% and 2.7%, respectively. At this point, the red area corresponding to C slightly increases and the content of Mg in the sample decreases after removing dissolved ions through H2O dissolution.
Figure 18 shows that HCl-dissolved magnesite also exhibits three characteristic peaks of C, O, and Mg, with percentage contents of 41.4%, 43.0%, and 15.6%, respectively. Compared with the C, O, and Mg contents of the raw brucite ore, the content of C increases by 9.4%, whereas those of O and Mg decreases by 4.6% and 4.8%, respectively. At this point, the red area corresponding to C continues to increase. Owing to the stronger dissolution ability of HCl than H2O, the content of Mg continues to decrease after removing the dissolved ions from the sample.
Figure 19 shows that H2SO4-dissolved magnesite also exhibits three characteristic peaks of C, O, and Mg, with percentage contents of 45.8%, 41.0%, and 13.2%, respectively. Compared with the C, O, and Mg contents of the raw brucite ore, the content of C increases by 13.8%, whereas those of O and Mg decreases by 6.6% and 7.2%, respectively. At this point, the red area corresponding to C increases considerably. Owing to the stronger dissolution ability of H2SO4 than HCl, the content of Mg in the test sample decreases substantially.
3.6 XPS results analysis
XPS detection is a surface analysis method that tests the composition and relative content of mineral surface elements. The influence of surface dissolution on the surface properties of brucite can be further identified by analyzing the composition and characteristic peak positions of brucite elements before and after surface dissolution[26].
Figure 20 shows several characteristic peaks, such as Mg 1s, O 1s, Mg KLL, C 1s, Mg 2s, and Mg 2p, for the surface of brucite before and after surface dissolution. Further observation reveals that the peak heights corresponding to Mg and O in the raw brucite ore are the highest, followed by H2O-dissolved brucite, HCl-dissolved brucite, and H2SO4-dissolved brucite. From this, it can be concluded that the more dissolved Mg and O, the lower the strength of Mg and O elements on the surface of the brucite sample[27].
Table 4
XPS detection results of surface element composition of brucite
Sample
|
Elements (atomic %)
|
C 1s
|
O 1s
|
Mg 1s
|
Brucite raw ore
|
23.42
|
60.93
|
15.65
|
H2O dissolved brucite
|
24.36
|
60.53
|
15.11
|
HCl-dissolved brucite
|
26.42
|
59.36
|
14.22
|
H2SO4 dissolved brucite
|
27.12
|
59.20
|
13.68
|
Table 4 shows the changes in the relative content of the surface elements of brucite before and after dissolution. The content of C on the surface of brucite considerably increases after dissolution, with this increase following the order of H2SO4 dissolution of brucite > HCl dissolution of brucite > H2O dissolution of brucite. The relative contents of O and Mg increase, with this increase following the order of H2SO4-dissolved brucite > HCl-dissolved brucite > H2O-dissolved brucite. Therefore, surface dissolution decreases the surface characteristic Mg and O contents of brucite.
To further investigate the effect of surface dissolution on the flotation performance of brucite, the fine spectra of characteristic Mg peaks of brucite before and after dissolution were analyzed; the results are shown in Figs. 21 and 22.
As shown in Fig. 21, after H2O dissolution, the Mg 2p peak of brucite shifted by 0.02 eV. Furthermore, after HCl and H2SO4 dissolution, the Mg 2p peak of brucite shifted by 0.09 and 0.19 eV, respectively[28]. The degree of shift of the peak positions shows that the degree of influence of dissolution on the characteristic peak positions of brucite follows the order of H2SO4 dissolution > HCl dissolution > H2O dissolution.
As shown in Fig. 22, after H2O dissolution, the Mg 1s peak of brucite shifted by 0.04 eV. Furthermore, after HCl and H2SO4 dissolution, the Mg 1s peak of brucite shifted by 0.09 and 0.12 eV, respectively. The migration trend of the Mg 1s peak position of brucite with respect to dissolution is consistent with that of the Mg 2p peak position, indicating that the promotion of the dissolution ability of brucite follows the order of H2SO4 > HCl > H2O.
3.7 Analysis of dissolution and adsorption mechanism
After dissolution, the flotation recovery rate of brucite in the three anionic collectors decreases, which is intuitively manifested as a decrease in the adsorption capacity of brucite on the three collectors. SEM and XPS analysis showed that after dissolution, the relative content of Mg and O elements on the surface of brucite decreased, and its surface active sites decreased, which was not conducive to the adsorption of the collector. Therefore, a dissolution and adsorption mechanism diagram for brucite is shown in Fig. 23.
3.8 Shortcomings and Prospects
The Mg2+ generated by the dissolution of brucite enters the slurry, reducing the Mg sites on the surface of brucite and reducing the adsorption performance of the three anionic collectors. However, according to the changes in the flotation recovery rate of dissolution tests, it can be found that as the dissolution time continues to increase, the flotation recovery rate of brucite shows an upward trend, only smaller than the flotation recovery rate without dissolution; In the acid leaching experiment for removing dissolved ions from brucite, the flotation recovery rate of brucite also showed a brief increase, both of which pointed to the adsorption characteristics of dissolved Mg2+ on the surface of brucite. Therefore, whether the dissolution of Mg2+ in brucite can be re adsorbed on the mineral surface is a direction that we will continue to investigate in depth.