Confirmation of Interlayer Sulfidization of Malachite by TOF-SIMS and Principal Component Analysis

In this work, a sulfidization mechanism of malachite was confirmed based on the depth profile product, principal component, and depth profile curve analyses of time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results showed that Cu/S species, including fragment ion peaks of Cu2S, Cu3S, S−, HS−, S2, CuS2, and CuS3, were present in the inner layers of sulfidized malachite in the positive and negative spectral ranges 75–400 and 30–470 m/z. Na2S reacted with the surface and inner atoms, causing simultaneous sulfidization of malachite on the surface and in the inner layers. The inner layer mainly contained positive fragment ions with large Cu/S ratios. In summary, the interlayer sulfidization phenomenon was confirmed and the differences in sulfidization products between the surface and inner layers were determined.


Introduction
Malachite is a copper-bearing carbonate mineral.It is one of the most important and common copper-bearing minerals from which copper is extracted [1][2][3].For many years, flotation has been the main method for recovering malachite.In essence, the malachite is pre-sulfidized with sulfidizing agents such as sodium sulfide or sodium hydrosulfide [4].A xanthate collector is then added for flotation [4][5][6].After the addition of sodium sulfide, the surface of the oxidized mineral quickly adsorbs S 2− or HS − to form a metal sulfide film [7,8].Interactions between malachite and the collector are promoted, which improves the flotation recovery of malachite.
The process and mechanism of malachite sulfidization has been examined in previous research studies.Zhou and Chander [9] investigated the kinetics of malachite sulfidization, with sodium hydrosulfide and tetrasulfide as the sulfidizing agents.The sulfidization reaction involves the formation of a primary sulfidized layer and two secondary reactions [9].One is the precipitation of copper ions, which diffuse through the primary layer; the other is the oxidation of the sulfidizing agent to oxysulfide species [9].Feng et al. [10] studied the copper sulfide species formed on malachite surfaces in relation to flotation.Their results demonstrated that the flotation recovery of malachite was correlated with the contents of sulfidization products and their active components, as well as the residual sulfide ion species in pulp solutions [10].The sulfidization product was composed of cuprous monosulfide, cuprous disulfide, and cuprous polysulfide.Among them, disulfide and polysulfide Minerals 2019, 9, 204 2 of 10 positively contributed to the activity of the product [10].Castro et al. [11] discussed the chemical factors in the sulfidization of copper oxide and found that besides the sulfidization with the formation of a copper sulfide layer, substantial oxidation of sulfide ions takes place at the surface of the solid.Burkitseterkyzy et al. [12] studied the material composition of raw materials and the thermodynamic analysis of ore sulfidization.It was shown that in the temperature range 298.15-500K, the formation of copper (II) sulfide and sulfur (IV) oxide in the interaction of oxidized copper minerals with elemental nanosized sulfur is most likely [12].Kinetic data show that the sulfidization process is controlled by a mixture of diffusion and oxidation reactions [9].Other scholars [11,[13][14][15] proposed some hypotheses regarding the sulfidization mechanism of malachite based on the phenomena they found.
Previous researchers considered that sulfidization reactions of malachite occurred mainly at the surface of malachite.However, in our earlier work, results of density functional theory (DFT) calculations indicated that sulfur ions could enter the interlayer of malachite, which produced the interlayer sulfidization phenomenon [16].However, this finding needs to be further confirmed by other research methods.Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a sensitive technique for detection of all elements of the periodic table [17,18].TOF-SIMS has a detection limit of parts per million or parts per billion for most substances [19].The conclusions of S. Chehreh Chelgani and B. Hart [19] indicated that TOF-SIMS, as a unique surface analysis technique, can potentially provide direct determination of the parameters which control surface reactivity and, consequently, plays an important role in determining the flotation behavior of minerals [20][21][22].In this work, interlayer sulfidization of malachite was confirmed on the basis of TOF-SIMS depth profile product, principal component, and depth profile curve analyses.

Materials
Pure malachite samples were collected from the Dongchuan copper deposit, Yunnan Province, China and subjected to manual crushing and handpicking.The isolated high-purity samples were dry ground in an agate mortar and dry-screened to obtain fractions with different particle sizes.In order to determine the purity of the sample, chemical analysis was performed on the malachite, and Table 1 shows the results.There were lower levels of impurities, such as SiO 2 , MgO, CaO, Fe, and Al 2 O 3 .The copper grade of the sample was 56.02%, which is similar to the theoretical grade of copper in malachite (57.57%).Moreover, the malachite had a high purity of about 97%.The X-ray diffraction analysis (XRD) patterns and crystal structure of malachite are shown in Figure 1.Globally, layered malachite is widely distributed [23][24][25].It is easier now to synthesize various types of mineral crystals, and to determine their properties [26][27][28].Layered malachite has been artificially synthesized [15,29,30]

Procedure of Sulfidization
The pure malachite sample was broken into pieces of approximately 1 × 0.8 × 0.5 cm 3 , which were polished using a polishing machine.The mineral sample was placed in a 50 mL beaker with deionized water (40 mL) and washed with an ultrasonic cleaner for 5 min.The upper cleaning fluid

Procedure of Sulfidization
The pure malachite sample was broken into pieces of approximately 1 × 0.8 × 0.5 cm 3 , which were polished using a polishing machine.The mineral sample was placed in a 50 mL beaker with deionized water (40 mL) and washed with an ultrasonic cleaner for 5 min.The upper cleaning fluid was removed after static clarification, and then washed three times with deionized water.The pH of the suspension was adjusted with NaOH to pH 9, and then Na 2 S solution of concentration 5 × 10 −4 mol/L was added to the pulp solution and absorbed for 15 min.A control experiment without Na 2 S was also performed.

TOF-SIMS Analysis
A TOF-SIMS V (IONTOF GmbH, Münster, Germany) instrument was used to acquire depth profiles of the multilayer films [33,34].TOF-SIMS depth profile analysis of malachite reacted with Na 2 S for 15 min and a malachite control sample was performed at pH 9. A 15 keV Bi 3+ source was used as the main gun and an oxygen source was used as the sputter gun.It is worth noting that, the use of a O 2 gun as an etching gun may cause lattice damage to the surface, preferential sputtering, and surface atom mixing.Therefore, a relatively low O 2 energy of 0.5 keV was used.This "knock-on" effect is weaker compared to other typical O 2 guns of 1000 and 2000 keV.An electron neutralization gun was used to neutralize any excess charge generated during sputtering.The analytical procedure was as follows.First, the main gun was used to scan a surface area of 100 × 100 µm 2 for 6.25 s.The main gun was then stopped and the sputter gun was loaded.The sputter gun was used to etch an area of 400 × 400 µm 2 at an etching rate of about 0.1964 nm/s.After etching for 2 s, sputtering was suspended.The electron neutralizing gun was then used to electronically neutralize the sputtered surface for 0.5 s.The process was repeated and the procedure was stopped after 32 scans with the main gun.The scanning time was 200 s.Data acquisition and subsequent data processing were performed using SurfaceLab (6.7, ION-TOF GmbH, Münster, Germany).

Determination of Sulfidization Species from TOF-SIMS Depth Profile
The aim of TOF-SIMS depth profile analyses was to determine the sulfidization products inside the malachite.Mass spectra of sulfidized and unsulfidized malachite at different depths were obtained, including cumulative mass spectra from scans 1-8, 9-16, 17-24, and 25-32.The total scanning time for each cumulative mass spectrum was 50 s.Figure 2 shows the cumulative mass spectra at different depths.
The mass spectra in Figure 2a-c  CuS4O2H − , and CuS3 − were detected.These results show that sulfur ions reacted with the surface and inner atoms of malachite, resulting in simultaneous sulfidization of the surface and inner layers of malachite.This confirms that interlayer sulfidization occurred.Sulfidized malachite has similar positive or negative spectra at different depth ranges, which indicates that the sulfidization products of malachite are similar at different depth ranges.It was concluded that external sulfide ions synchronously enter different inner regions of malachite.

Principal Component Analysis
The analytical results in Section 3.1 show that the inner layers of malachite are simultaneously sulfidized by Na2S.The intensities of important fragment ion peaks by principal component analysis in multivariate statistical analysis were analyzed [34,35].Principal component analysis is used to enrich the information obtained from multivariable systems [17].The minimum number of newly generated variables were selected to reflect most of the information in the original variables.Among independent principal components, the first principal component has the largest variance contribution rate.The proportions of principal components in the total variance decrease in descending order.The TOF-SIMS data contain tens of thousands of fragment ion peaks, and principal component analysis is therefore an effective method for analyzing mass spectroscopic data [36,37].
In the present study, 32 independent mass spectra-in depth order from the total mass spectrum obtained from 32 scans-were read.The scan analysis time for each mass spectrum was 6.25 s, with a depth interval of about 0.3928 nm between the mass acquisition surfaces.For the first and 32nd

Principal Component Analysis
The analytical results in Section 3.1 show that the inner layers of malachite are simultaneously sulfidized by Na 2 S. The intensities of important fragment ion peaks by principal component analysis in multivariate statistical analysis were analyzed [34,35].Principal component analysis is used to enrich the information obtained from multivariable systems [17].The minimum number of newly generated variables were selected to reflect most of the information in the original variables.Among independent principal components, the first principal component has the largest variance contribution rate.The proportions of principal components in the total variance decrease in descending order.The TOF-SIMS data contain tens of thousands of fragment ion peaks, and principal component analysis is therefore an effective method for analyzing mass spectroscopic data [36,37].
In the present study, 32 independent mass spectra-in depth order from the total mass spectrum obtained from 32 scans-were read.The scan analysis time for each mass spectrum was 6.25 s, with a depth interval of about 0.3928 nm between the mass acquisition surfaces.For the first and 32nd mass spectra, the acquisition depths were 0 and 12.1768 nm.After uniform mass calibration of the mass spectra, the important fragment ion peaks were selected for principal component analysis.The raw data were usually preprocessed before principal component analysis.We divided the intensity of each selected fragment ion peak by the total intensity of fragment ion peaks for normalization and standardization.The intensity variance of a treated fragment ion peak is equal to 1. Figure 3 shows the principal component analysis results for the positive spectra of sulfidized malachite at 32 different depths.PC1 and PC2 respectively represent the loadings or scores in the transverse and longitudinal directions in Figure 3. between the surface and inner layers.The results also show that scans 1-3 had strong correlations with CuHS + , Cu2HS + , and Cu2S2 + , while scans 4-22 were strongly correlated with Cu3S2 + , Cu4S2 + , Cu5S2 + , Cu2S + , Cu3S + , Cu3H2S + , Cu4SO + , Cu2SO + , and SO + .The copper-oxygen-containing substance had a small positive score in PC2, while scans 23-32 had negative scores in PC2.This shows that scans 23-32 have strong negative correlations with positive fragment ions such as Cu3S2 + , Cu4S2 + , Cu5S2 + , Cu2S + , Cu3S + , and Cu3H2S + .The copper-sulfur-containing and copper-oxygen-containing positive fragment ions have negative and positive loadings, respectively, in PC1, which is consistent with the fact that the sulfidized malachite consists of copper-sulfur-containing compounds.PC1 therefore represents the sulfidization degree of malachite.The sulfur-containing positive fragment ions have different loadings in PC2, indicating that these sulfur-containing fragment ions originate from different compounds.PC2 therefore represents the difference of sulfidization products.
Figure 3b shows that the 32 positive spectra are well separated in a score plot.The scores for scans 1-32 in PC1 gradually increase from negative to positive.Figure 4a indicates that the sulfidization degree of malachite decreases with increasing etching depth.This is in line with the characteristics of the sulfidization process in that sulfur-containing components can easily diffuse from the solution to the surface and interlayers of malachite.However, it become difficult for the sulfur-containing components to diffuse deeper into the interlayers.A smaller distance from the surface or cross section leads to a higher sulfidization degree of malachite.This is consistent with the findings of Castro et al. [11,38]. .The first and second types of negative fragment ions have positive and negative loadings, respectively, in PC1.Therefore, PC1 represents the sulfidization degree of malachite.However, there are significant differences between the loadings of sulfur-containing negative fragment ions in PC2, indicating that these fragments originate from different compounds.Therefore, PC2 represents the difference between the sulfidization products.
Figure 4b shows that the 32 negative spectra are well separated in the score plot.From scan 1 to scan 32, the PC1 score gradually decreased from positive to negative.The combined results in Figure 4a,b led to the conclusion that the sulfidization degree of malachite decreased with increasing etching depth.Similarly, the scores for scan 1 to scan 32 for PC2 gradually change from positive to negative, and then to positive.This indicates that sulfidized malachite contains different sulfidization species at different depths.The first scan (depth = 0 nm) had the largest positive score and the largest distance from other points in PC2.Therefore, there is a significant difference of sulfur-containing compounds between the surface and inner of sulfidized malachite.According to the results in Figure 4a, we can see that scans 1-3 had strong correlations with S3 − , CuS4 − , S4 − , HS − , CuS3 − , CuS4O2H − , Cu2S3 − , S − , and S2 − ; scans 4-21 were strongly correlated with Cu2S2 − , CuS2 − , CuSOH − , S2O − , CuS − , and SOH − , and scans 22-32 had strong negative correlations with negative fragment ions such as Cu2O3H − , O − , CO3 − , OH − , CuO2 − , CuSO − , and CuO − .Combining the results of principal component analysis of positive and negative ion fragments, we also found that fragment ions with small Cu/S ratios dominate in the superficial layer of malachite (i.e., scans 1-3, depth: 0-0.7856 nm).In the middle layer, i.e., scans 4-21 (depth: 1.1784-7.856nm), the fragment ions with large Cu/S ratios and those containing sulfur and oxygen affect the results more, with the greatest effect observed for scan 9 (depth = 3.1424 nm).In the deep layer, i.e., scans 22-32 (depth: 8.2488-12.1768nm), mass spectrometry information was mainly affected by fragment ions   The scores for scans 1-32 in PC2 gradually change from negative to positive, and then to negative again.This indicates that different sulfides are formed at different depths during interlayer sulfidization.The first scan (depth = 0 nm) had the smallest negative score and the largest distance from other points in PC2.Hence, there is a significant difference in the sulfur-containing compounds between the surface and inner layers.The results also show that scans 1-3 had strong correlations with CuHS + , Cu Figure 4 shows the principal component analysis results of the negative fragment ions.The results show that the negative fragment ions were more discrete than the positive fragment ions in the load plot.On the basis of the degree of dispersion, most of the negative fragment ions can be divided into two types.The first is negative fragment ions produced from sulfidization products: S and CuO − .The first and second types of negative fragment ions have positive and negative loadings, respectively, in PC1.Therefore, PC1 represents the sulfidization degree of malachite.However, there are significant differences between the loadings of sulfur-containing negative fragment ions in PC2, indicating that these fragments originate from different compounds.Therefore, PC2 represents the difference between the sulfidization products.
Figure 4b shows that the 32 negative spectra are well separated in the score plot.From scan 1 to scan 32, the PC1 score gradually decreased from positive to negative.The combined results in Figure 4a,b led to the conclusion that the sulfidization degree of malachite decreased with increasing etching depth.Similarly, the scores for scan 1 to scan 32 for PC2 gradually change from positive to negative, and then to positive.This indicates that sulfidized malachite contains different sulfidization species at different depths.The first scan (depth = 0 nm) had the largest positive score and the largest distance from other points in PC2.Therefore, there is a significant difference of sulfur-containing compounds between the surface and inner of sulfidized malachite.According to the results in Figure 4a Combining the results of principal component analysis of positive and negative ion fragments, we also found that fragment ions with small Cu/S ratios dominate in the superficial layer of malachite (i.e., scans 1-3, depth: 0-0.7856 nm).In the middle layer, i.e., scans 4-21 (depth: 1.1784-7.856nm), the fragment ions with large Cu/S ratios and those containing sulfur and oxygen affect the results more, with the greatest effect observed for scan 9 (depth = 3.1424 nm).In the deep layer, i.e., scans 22-32 8.2488-12.1768nm), mass spectrometry information was mainly affected by fragment ions produced by malachite itself, with a low sulfidization degree.

Depth Profile Curve Analysis
As discussed in Section 3.2, principal component analysis shows that the sulfidized malachite had different sulfidization compositions at different depths, therefore, the variations of various fragment ions with depth using depth profile curve analysis of TOF-SIMS was investigated.
The depth profile curves of the positive fragment ions in Figure 5a,b show that the curves can be broadly divided into three categories.The blue line represents the first type: CuHS + , Cu interlayer sulfidization mode.The transition layer and interlayer sulfidization give positive fragment ions with large Cu/S ratios: Cu3S2 + , Cu4S2 + , Cu5S2 + , Cu2S + , Cu3S + , and Cu3H2S + .Steric hindrance exists during the diffusion of sulfur-containing components from the surface and cross sections to the interior of malachite.The intensity of the second type of positive fragment ion therefore decreases after initially increasing.Due to a decrease in the number of copper-containing sites and the increased steric hindrance, sulfidization of the same layer weakens with increasing depth, and the intensity of the first type of positive fragment ion therefore decreases with increasing depth.The depth profile curves of negative fragment ions in Figure 6 show that the intensities of different negative fragment ions vary with changes in depth.This is in accordance with the fact that negative fragment ions are more discrete in Figure 5a, which is similar to the results of the analysis of positive ion depth profiles.The species represented by red lines, i.e., the negative fragment ions, including S3 − , CuS4 − , S4 − , HS − , CuS3 − , CuS4O2H − , Cu2S3 − , and S2 − , are mainly derived from surface sulfidization products.The species represented by blue and pink lines, i.e., S − , CuS2 − , Cu2S2 − , CuSOH − , CuS − , and SOH − , possibly originate from both the same layer and interlayer sulfidization products, while S2O − mainly comes from interlayer sulfidization products.
For Cu4SO + , Cu2SO + , and SO + in Figure 4a, and CuSO − in Figure 5a, the intensities increase with increasing depth.This type of fragment ion may represent another interlayer sulfidization mode, The above analysis suggests that the sulfidization mechanism at the surface is different from that in the inner layers.The malachite discussed in this study had a layered structure.The minimum distance between copper atoms in adjacent layers of malachite is 0.393 nm, which is larger than the diameter of S 2− , i.e., 0.368 nm; S 2− can therefore theoretically enter the interlayers of malachite [16] At the malachite surface, the sulfur components in the solution react with the copper atoms in the same layer to generate copper-sulfur-containing compounds.These substances produce positive fragment ions with small Cu/S ratios-CuHS + and Cu 2 HS + -which is defined as surface sulfidization.The sulfur-containing component that diffuses from the surface to the interlayer simultaneously reacts with copper atoms in upper and lower layers to form another copper-sulfur-containing compound, which is defined as the transition layer sulfidization.The sulfur component intercalates into the interlayer from the cross section of the lamellar structure, which can be defined as the interlayer sulfidization mode.The transition layer and interlayer sulfidization give positive fragment ions with large Cu/S ratios: Cu 3 S 2 + , Cu 4 S 2 + , Cu 5 S 2 + , Cu 2 S + , Cu 3 S + , and Cu 3 H 2 S + .Steric hindrance exists during the diffusion of sulfur-containing components from the surface and cross sections to the interior of malachite.The intensity of the second type of positive fragment ion therefore decreases after initially increasing.Due to a decrease in the number of copper-containing sites and the increased steric hindrance, sulfidization of the same layer weakens with increasing depth, and the intensity of the first type of positive fragment ion therefore decreases with increasing depth.The depth profile curves of negative fragment ions in Figure 6 show that the intensities of different negative fragment ions vary with changes in depth.This is in accordance with the fact that negative fragment ions are more discrete in Figure 5a, which is similar to the results of the analysis of positive ion depth profiles.The species represented by red lines, i.e., the negative fragment ions, including S  4a, and CuSO − in Figure 5a, the intensities increase with increasing depth.This type of fragment ion may represent another interlayer sulfidization mode, namely, Cu top -S middle -O bottom sulfidization.This mode may be more incidental than Cu top -S middle -Cu bottom sulfidization.Therefore, the intensities of these fragment ions increase with increasing etching depth.However, it cannot be ignored that O 2 was used as the etching gas in this study [39,40].As the etching depth increases, the amount of oxygen-containing fragment ions from malachite increases.Consequently, oxygen-containing, sulfur-containing, and copper-containing fragment ions may recombine to form new Cu-S-O fragment ions.
namely, Cutop-Smiddle-Obottom sulfidization.This mode may be more incidental than Cutop-Smiddle-Cubottom sulfidization.Therefore, the intensities of these fragment ions increase with increasing etching depth.However, it cannot be ignored that O2 was used as the etching gas in this study [39,40].As the etching depth increases, the amount of oxygen-containing fragment ions from malachite increases.Consequently, oxygen-containing, sulfur-containing, and copper-containing fragment ions may recombine to form new Cu-S-O fragment ions.

Conclusions
In this study, TOF-SIMS depth profile product analysis, principal component analysis, and depth profile curve analysis were used to confirm a novel interlayer sulfidization of malachite.The conclusions are as follows.
(1) The depth profile product analysis showed that there were strong fragment ion peaks from Cu2S + , Cu3S + , and Cu5H2S2 + in the 75-400 m/z positive segment of the sulfidized malachite spectrum throughout the entire depth range.In the 30-470 m/z negative segment, fragment ion peaks, including S − , HS − , S2 − , CuS2 − , CuS4O2H − , and CuS3 − , were detected.These results showed that sulfur ions reacted with the surface and inner atoms of malachite, resulting in simultaneous sulfidization at the surface and inner layers.
(2) Principal component analysis showed that the sulfidized malachite products consisted of copper-sulfur-containing compounds, while the unsulfidized malachite consisted of copper-oxygencontaining compounds.There were significant differences in the sulfidization compounds between the surface and inter layers of malachite.
(3) The depth profile curve analysis showed that there were different sulfide compositions at different depths.Fragment ions with small Cu/S ratios dominated in the superficial layer, while fragment ions with large Cu/S ratios and those containing sulfur and oxygen were more prominent in the inner layers.
In summary, in this study, the interlayer sulfidization phenomenon was confirmed and the differences in sulfidization products between the surface and inner layers were determined.

Figure 3 .
Figure 3.The principal component analysis results for positive spectra of sulfidized malachite at 32 different depths.(a): Loadings plot of PCs 1 and 2; (b): Scores plot of PCs 1 and 2.

Figure 4 Figure 3 .
Figure4shows the principal component analysis results of the negative fragment ions.The results show that the negative fragment ions were more discrete than the positive fragment ions in

Figure 4 .
Figure 4. Principal component analysis results for TOF-SIMS negative spectra of sulfidized malachite at 32 different depths.(a): Loadings plot of PCs 1 and 2; (b): Scores plot of PCs 1 and 2.

Figure 4 .
Figure 4. Principal component analysis results for TOF-SIMS negative spectra of sulfidized malachite at 32 different depths.(a): Loadings plot of PCs 1 and 2; (b): Scores plot of PCs 1 and 2.

Figure 5 .
Figure 5.The depth profile curves for positive ions of sulfidized malachite: (a) sulfur containing positive fragment ions and (b) oxygen-containing positive ion and Cu + fragments.

Figure 5 .
Figure 5.The depth profile curves for positive ions of sulfidized malachite: (a) sulfur containing positive fragment ions and (b) oxygen-containing positive ion and Cu + fragments.

Figure 6 .
Figure 6.Depth profile curves for negative ions of sulfidized malachite: (a) sulfur-containing negative fragment ions and (b) oxygen-containing negative ions and Cu + fragments.

Table 1 .
Chemical composition of the sample.
. The characteristics of fragment ions such as Cu 3 S 2 + , Cu 4 S 2 + , Cu 5 S 2 + , Cu 2 S + , Cu 3 S + , and Cu 3 H 2 S + indicate that S 2− or HS − is probably connected to the copper atoms in the upper and lower layers, to form Cu top -S middle -Cu bottom .
The species represented by blue and pink lines, i.e., S − , CuS 2 − , Cu 2 S 2 − , CuSOH − , CuS − , and SOH − , possibly originate from both the same layer and interlayer sulfidization products, while S 2 O − mainly comes from interlayer sulfidization products.For Cu 4 SO + , Cu 2 SO + , and SO + in Figure