Evaluation of L-A rginine as an Eco-Friendly Activator for Malachite Sulfidization Flotation

: Sulfidization xanthate flotation remains the most promising method for the beneficiation of malachite. In this study, L-arginine (LA) was first used to modify the malachite surface and improve the efficiency of sulfidization flotation. The performance of LA was evaluated by the flotation experiments. The mechanism of interaction between LA and the malachite surface was investigated by adsorption experiments, zeta potential measurements, scanning electron microscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) analysis. Flotation experiments showed that LA had a significantly promoting effect on malachite sulfidization flotation. Adsorption experiments and SEM-EDS results indicated that LA improved the adsorption of S (II) species into the malachite surface and promoted the formation of sulfides. This finding was further confirmed by the XPS analysis. The XPS measurements results determined that S (II) species reacted with Cu (II) on the mala-chite surface and form polysulfides, adding LA promoted the reaction. The zeta potential measurements showed that LA increased the positive electrical properties of the mineral surface, which was conducive to S (II) species adsorption and the sulfidization reaction. This work sheds new light on the development of sulfidization activation.


Introduction
Copper metal has a wide range of applications in electrical, electronic, military, machinery manufacturing, and transportation industries [1,2].Copper sulfide mineral (Cu2S) is the common mineral of copper.With the increasing demand for copper, copper sulfide resources cannot meet the needs.The exploitation and utilization of other copper resources is the key to solving the crisis.For example, Kasongo et al used a 2 4 -factor design experiment to explore the influencing factors of flotation in order to recover copper metal as much as possible [3].The utilization of copper oxide also attracts more and more attention.Oxidized copper reserves deposits are relatively abundant in the world, and oxidized mineral belts are also found above most sulfide deposits [4].Copper oxide minerals include malachite (Cu2(OH)2CO3), cuprite (Cu2O), chrysocolla (Cu,Al)2H2Si2O5(OH)4•nH2O), azurite (Cu3(CO3)2(OH)2), tenorite (CuO), and atacamite (Cu2Cl(OH)3) [5][6][7].Malachite is a representative copper oxide mineral, which has received increasing attention [8].Copper oxide minerals has strong hydrophilicity and poor floatability, which makes it difficult to recover [5].The efficient recovery of copper oxide minerals is a major problem.
Flotation is an economical method for malachite beneficiation [9].Some studies have shown that the surface tension of flotation solution can be changed by adding a collector, and the change of surface tension of the solution will affect the wettability of mineral particles in the solution [10].For malachite, the surface can strongly interact with water molecules to form a hydration layer, which makes the surface hydrophilic and the collector molecules cannot effectively adsorb [11].Direct flotation cannot effectively recover malachite.Sulfidization-xanthate flotation is an effective strategy for malachite beneficiation, and it has been extensively used in industrial applications [12][13][14].The basic idea of sulfidization-xanthate flotation is as follows: first, sulfur ions reach the hydration layer of the surface of malachite, adsorb onto the surface, and react with the surface to form a sulfide layer, which provides suitable conditions for the absorption of the collector; second, xanthate is used as a collector to achieve the beneficiation of sulfation malachite [15].Compared with direct flotation, sulfidization flotation has the advantages of better selectivity and high recovery.However, controlling sulfidization is not a simple task [16].In the presence of excessive sulfide, colloidal copper sulfide species are formed in the solution, which will depress malachite flotation [17,18].Surface sulfidization is the key stage of sulfidization flotation [19].In improving the efficiency of sulfidization flotation, many scholars have focused on finding an activator to improve the efficiency of sulfidization flotation.Ammonium sulfate [20], ethylenediamine phosphate [8], and 8-hydroxyquinoline [21] have been used to promote the sulfidization flotation of malachite.Among them, ammonium sulfate and ethylenediamine phosphate are the most common activators.Liu et al [22] investigated the activation mechanism of ammonium sulfate and proposed that the addition of ammonium sulfate can reduce the inhibitory effect caused by excess S ions and accelerate the speed of sulfidization.Liu et al [20] and Shen et al [7] further confirmed that ammonium sulfate can eliminate the inhibitory effect of excess S ions.Furthermore, Liu et al [23] inferred that ammonium ions may mediate the nucleation and growth of a sulfidization product during malachite sulfidization.Feng et al [8] found that an ethanediamine-modified mineral surface adsorbed with more sulfide ion species and enhanced the reactivity of the sulfidization products.
Although organic activators can significantly promote the sulfidization of malachite, these options have their shortcomings.Most of these compounds have relatively high levels of toxicity.Once discharged into the environment, they cause serious pollution of water bodies.Finding new, highly efficient, and non-toxic reagents is important.
As a natural amino acid, L-arginine (LA) is abundantly present in meat protein, which possesses the advantages of being inexpensive and environmentally friendly.The molecular model of LA is shown in Figure 1.LA with a molecular formula of C6H14N4O2 and a molecular weight of 174.2 is easily soluble in water and slightly soluble in ethanol, can be used as a non-toxic, easily biodegradable [24], and widespread activator.In this study, LA was first introduced as a sulfidization activator to enhance the sulfidization flotation of malachite.The effectiveness of LA as the activator was determined by the flotation experiments.In addition, the activation mechanism was explored by adsorption experiments, zeta potential measurements and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) analyses.

Materials
The malachite sample (the purity is about 96%) used in this study was collected from Yunnan Province, China.After manual cleaning, the malachite ore was ground in a porcelain ball mill to obtain a mineral of −0.074 mm size fraction.Then, the ground malachite sample was collected in a jar, and used for the flotation experiments and characterization studies.Based on X-ray diffraction (XRD) (shown in Figure 2) results, the sample was of high purity.Analytical-grade LA, ammonium sulfate ((NH4)SO4), and sodium sulfide (Na2S•9H2O) were used as the activators and sulfidization agent, respectively.Industrialgrade amyl xanthate (AX) and butyl xanthate (BX) (the purity is around 86%) were used as the collector.Moreover, industrial-grade terpineol (the purity is around 80%) was used as a frother.Analytically pure hydrochloric acid (HCl) (38% concentration) and analytically pure sodium hydroxide (NaOH) were used as pH regulators, and deionized (DI) water was used in all the tests.

Flotation Experiments
The flotation experiments were performed using an XFG-type flotation machine (Shun ze mining metallurgy machinery manufacturing co., LTD, Changsha, China) at a spindle speed of 2000 rpm.For each test, 6.0 g of malachite was placed in a 150 mL flotation cell and mixed with 140 mL DI water (the solid-liquid ratio was about 4.3%), stirred for 1 min.Next, the activator solution was added to the suspension and stirred for 1 min.Subsequently, the prepared Na2S solution was added to sulfurize the malachite surface for 2 min.The pH of the pulp was adjusted to 9.6 (or a specified value) using HCl and NaOH, as necessary.Next, the collector was freshly added, and the mixture was conditioned for 1 min.Finally, terpineol was added as the frother, and the mixture was floated for 4 min.The floating particles were collected, dried, and assayed to measure the recovery.All the tests were repeated three times under the same conditions.

Adsorption Experiments
An inductively coupled plasma (ICP) atomic emission spectrometer (Spectro Blue Ⅱ, Speck Instruments, Roth, Germany) was used to measure the residual S species in the solution.The UV spectrophotometer (UV2600, Shimadzu Corporation, Kyoto, Japan) at a wavelength (λ) of 300.5 nm was used to determine the residual xanthate in the solution.The adsorption amount of the xanthate was calculated using Equation (1).

= ( − ) ×
(1 Where Γ is the adsorption amount (mg/g); and are the initial concentration (mg/L) and residual concentration in solution (mg/L), respectively.V is the pulp volume (L), and m is the weight of the mineral sample (g).The samples were prepared as follows: 1.0 g of malachite was placed into the beaker and 30 mL of DI water was added.Then, the activator was added (if necessary), and the mixture was conditioned for 1 min.Subsequently, the Na2S solutions were added, and then the pH was adjusted to 9, and stirred for 2 min.Finally, BX (if necessary) was added.After the adsorption experiments, the solution was collected and filtered by syringe and filter.The final clear solution was used to measure the concentration of S or BX in the solution.

SEM-EDS Analysis
Scanning electron microscopy (SEM, TESCAN MIRA3 LMU, TESCAN, Brno, Czechia) coupled with energy-dispersive spectroscopy (EDS; Oxford X-Max20, Oxford Instruments, Abingdon, UK) was used to observe the surface topography of malachite.The mineral samples were prepared as follows: 1 g of malachite was added to 30 mL of an aqueous solution.Then, the activator was added to the slurry (if necessary) and stirred for 1 min.Next, Na2S solution was added (if necessary) to modify the mineral surface and adjust the pH to 9, and stirred for 2 min.After the reaction, the solid particles were collected and dried in a vacuum drying oven at 40 °C.Then, a dry sample was used for SEM-EDS analysis.

XPS Analysis
A PHI 5000 VersaProbe II system (ESCALAB250Xi, ThermoFisher-VG Scientific, Waltham, Massachusetts, USA) with a monochromatic Al K X-ray source was used for XPS measurements.Casa software (CASA SOFTWARE LIMITED, UK) was used for analysis.The samples were prepared using the method described in Section2.2.3.

Zeta Potential Measurements
The zeta potential measurements were carried out using a Zetasizer Nano Zs90 potential analyzer (Malvern Panalytical, Malvern, UK).Before the measurement, the mineral sample was ground to a particle size of 2 μm.Then, 1.0 g of the sample was added to a 100 cm 3 beaker filled with 50 mL of DI water containing 1 × 10 −3 mol/L KNO3 as a background solution and then stirred with a magnetic stirrer for 2 min.The pH was adjusted using HCl or NaOH.At the pH of the slurry, the flotation reagent was added and stirred for 1 min.The suspension was allowed to stand for the coarse particles settle down for 4 min, and the supernatant was collected for testing.Each sample was measured three times.

Flotation Experiments
The flotation recovery of malachite as a function of Na2S concentration is shown in Figure 3a.The flotation results showed that the malachite flotation recovery varied for both collectors; it initially increased and then decreased with the increase of Na2S dosage.The recovery reached its peak when the Na2S concentration was 3-5 × 10 −3 mol/L.Afterwards, the flotation recovery of malachite decreased with the increase of Na2S concentration.This phenomenon was attributed to the inhibitory effect of excessive Na2S on the malachite surface [7,12,25].When BX was used as the collector, the maximum flotation recovery of malachite reached with 3.0 × 10 −3 mol/L of Na2S.Therefore, the Na2S dosage of 3.0×10 −3 mol/L was selected as an optimum dosage to study the activation behavior of LA.At pH 9.6, the flotation recovery of malachite in the absence and presence of Na2S as a function of the collector (BX) concentration is shown in Figure 3b.The flotation recovery showed a positive correlation with the collector concentration.In the absence of Na2S, the recovery of malachite was low.This result can be attributed to the malachite surface forming a hydration film, making it difficult for the collector xanthate to adsorb [26].When 3.0 × 10 −3 mol/L of Na2S was added to modify the mineral surface, the flotation recovery of malachite sharply increased from 51.30% to 70.50% as BX dosage increased to 1.0 × 10 −4 mol/L.The recovery then remained nearly constant with the increase of BX.After sulfidization, the malachite surface could easily interact with xanthate, which improved the hydrophobicity of the mineral surface [27].This result indicated that the sulfidization could promote the flotation of malachite.
In the previous experiment, the optimal dosage of BX was determined to be 1.0 × 10 −4 mol/L.Under this condition, the effect of the activator concentration on the malachite flotation was studied at pH 9.6, and the corresponding flotation results are shown in Figure 4. Figure 4 shows that the flotation recovery of malachite increased with the increase of LA dosage, and a maximum flotation recovery of 87% was achieved with 6.0 × 10 −5 mol/L of LA.Furthermore, the flotation recovery of malachite decreased when the dosage of LA was further increased.It was hypothesized that in the presence of an excessive amount of LA, the covering effect of LA makes it difficult for S species to contact the mineral surface, thereby hindering surface sulfidization.This results in a decrease in the hydrophobicity of the mineral surface.After adding ammonium sulfate, the flotation recovery of malachite also showed a similar trend, which initially increased and then decreased.The maximum recovery of 82% was achieved with 4.0 × 10 −4 mol/L of ammonium sulfate.Compared with ammonium sulfate, LA as the activator showed a better promoting effect at low concentrations.
The flotation behavior of malachite as a function of pH is shown in Figure 5.The recovery of malachite without Na2S increased as pH increased from 3 to 9, and after that the recovery slightly decreased as the pH increased from 9 to 11.This result indicates that malachite does not respond well to traditional copper sulfide collectors.When 3 × 10 −3 mol/L of Na2S was added, the flotation recovery initially decreased and then increased, whereas at pH 9, it reached a maximum of 75%.At this pH, the main S species in the solution was HS − , and the presence of a large amount of HS − is conducive to the sulfidization of the malachite surface [28].These results were similar to those in the previously reported literature.Notably, adding 6.0 × 10 −5 mol/L of LA in sulfide, the flotation recovery of malachite significantly improved in the whole pH range.At pH 3, the recovery rate exceeded 75%.This phenomenon can be explained as surface sulfidization involves ionexchange reaction, surface adsorption, and redox reaction [14,29,30], although the content of HS − in the solution is little under acidic conditions, leading to a decrease in the ionic adsorption of HS − on the mineral surface, H2S still undergoes redox reactions with mineral surfaces.The addition of LA can promote the redox reaction and promote the formation of sulfide, which can improve the sulfide flotation of malachite.Furthermore, the recovery was highest at pH 7.This result indicates that LA has a positive effect on the sulfidizationxanthate flotation of malachite in the pH range of the experiment.

Adsorption Behavior of S Species onto Malachite Surfaces in the Presence and Absence of LA
During the sulfidization flotation, a large amount of S species was transferred onto the mineral surface.Figure 6 shows the adsorption behavior of S species on the malachite surface as a function of Na2S dosage in the absence and presence of 6 × 10 −5 mol/L of LA.As shown in Figure 6, the adsorption amount increased as the dosage of Na2S increased.Furthermore, after adding LA, the adsorption amount of S species on the surface of malachite was larger than that without LA.This phenomenon could be attributed to LA increasing the adsorption of S species on the surface of minerals.

Adsorption Behavior of S Species onto Malachite Surfaces as a Function of LA Dosage
When adding 3 × 10 -3 mol/L Na2S, the adsorption behavior of BX as a function of BX dosage is shown in Figure 7. Without LA, the adsorption capacity of BX increased rapidly at first, and then the growth rate tends to be gentle.In the presence of LA, the adsorption of BX on the malachite surface is greater than that in the absence of LA.This was owing to the fact that the LA increases the sulfidization on the malachite surface and thus increases the adsorption of BX on the surface.However, when the concentration of the collector is too large, LA inhibits the adsorption of xanthate.This phenomenon is consistent with the flotation results.

SEM-EDS Measurements
The surface microtopography (SEM) of malachite particles before and after treatment is shown in Figure 8.The EDS results are presented in Table 1.The SEM images show that the surface of raw malachite particles was smooth and flat, and a small number of grains were found on malachite surfaces.The EDS results indicate the absence of an S element on the surface of raw malachite.The weight and atom percentages (wt% and at%) of Cu on the surface of raw malachite were only 46.62% and 16.89%, respectively.After the treatment with Na2S, the surface of malachite developed a layer of adhering material, particularly at the faults, which made the edges and corners blunt.Based on the EDS results, it was found that the S element appeared on the surface of malachite, indicating that sulfides were attached to the surface of malachite.Moreover, adding LA and Na2S, more attachments were found on the surface of malachite, and the edges and corners of the fracture layers were less apparent.The atom percentage of S increased from 0.29% to 0.39%, indicating that the addition of LA promoted the formation of surface sulfides.

XPS Analysis
The activation mechanism of LA was investigated by XPS analyses.The XPS fullrange spectra of treated or untreated malachite are shown in Figure 9, in which the surface of malachite comprised C, O, S, and Cu; their corresponding atomic surface concentrations are summarized in Table 2.In the spectra, N was not detected, which demonstrated that LA did not stably adsorb or form compounds on the surface of malachite.LA adsorbed on the malachite surface through hydrogen bonding.After the treatment with Na2S, peaks representing S 2s or S 2p were observed.Table 2 shows that with the addition of LA, the atomic concentration of S on the malachite surface was increased.This finding suggests that LA promotes the sulfidization efficiency of the malachite surface, which is consistent with the SEM-EDS results.10 and 11.The Cu 2p spectra of malachite samples before and after treatment show that the Cu 2p spectra consisted of peaks representing Cu 2p3/2 and Cu 2p1/2 [21,31,32].In the same chemical state, the Cu 2p3/2 peak was consistent with the Cu 2p1/2 peak.In this study, the Cu 2p3/2 peak was used to explore changes in the chemical state of the malachite surface.For the untreated malachite, the peak assigned to Cu (II) (cupric) appeared at 935.23 eV [11,33].After treatment with 3×10 −3 mol/L of Na2S, the peak representing Cu (I) (cuprous) [34][35][36][37] appeared in the spectrum, which indicated that a reduction reaction occurred on the surface of malachite.S(II) species will interact with the malachite surface through redox reaction.Notably, the addition of LA changed the intensity of the peak representing Cu (I).Table 3 shows that after adding LA, the percentage of Cu (I) in the Cu species increased from 9.59% to 19.53%.LA promoted the chemical reaction of S (II) species and Cu (II) on the mineral surface.However, if only LA was added, the chemical state of Cu in the malachite remained unchanged.LA did not react with copper on the malachite surface, which confirmed our conjecture.The S 2p spectrum was further analyzed to elucidate the effect of LA on the sulfidization products, and the results are shown in Figure 11.The S 2p spectrum of malachite treated with Na2S was divided into four pairs of spin-orbit peaks, which were attributed to the S 2p3/2 and S 2p1/2 doublet [38].The S 2p peaks with binding energies ranging from 161.2 to 162.3 eV represent divalent sulfide ions (S 2− ) [39,40]; peaks with high binding energies (162.4-164.3eV) [25,41] represent polysulfide ions (Sn 2− , n ≥ 2), and peaks with binding energies ranging from 164.5 to 167.1 eV correspond to SO3 2− ions [33,34].Furthermore, the peaks with binding energies of 167.8 and 169.9 eV correspond to the sulfate ion (SO4 2− ) [42][43][44].After adding LA, the peak intensities were altered.Table 4 shows that the percentage of S 2− decreased and that of Sn 2− increased, indicating that more low-valent states of S species were oxidized.These results indicate that LA promotes the redox efficiency of the malachite surface, thereby increasing the proportion of polysulfide species on the malachite surface.

Zeta Potential Measurements
The zeta potential measurements were carried out to estimate the surface electrical property of the particle in suspension.The adsorption of ions onto the malachite surface significantly affected the zeta potential.As shown in Figure 12, the zeta potential of malachite decreased with the increase of pH values.As a typical copper oxide mineral, malachite has certain hydrophilicity, and it can easily adsorb hydroxide ions in the solution, thereby forming a surface hydration film.This phenomenon leads to a decrease in the zeta potential.After adding LA, the zeta potential of malachite became more positive in the absence of LA, and the IEP value also increased [45,46].With the addition of Na2S, the zeta potential of malachite was significantly and negatively charged.This result indicated that S(II) species in the pulp solution were adsorbed onto the mineral surface.Moreover, the addition of LA and Na2S modified the malachite surface, and the zeta potential became more negative than that treated with Na2S.This means that more S(II) species is adsorbed on the surface of malachite.Based on the previous results, it was hypothesized that LA produces a hydrogen bond with OH − adsorbed on a malachite surface through its N atom, resulting in a decrease of hydroxide ion in the surface and an increase of surface potential.The positive electrical properties of the mineral surface are conducive to the adsorption of S(II) species, which thereby increases the contact of S(II) species with the mineral surface, promotes the reaction of S with the mineral surface, and improves surface sulfidation efficiency.The activation mechanism of LA is shown in Figure 13.

Conclusions
The activation effects and mechanism of LA on malachite sulfidization flotation were investigated by flotation, adsorption experiments, SEM-EDS and XPS analyses and the zeta potential measurements.The main findings of this study are as follows: (1) The flotation experiments demonstrated that using LA as the activator can significantly improve the recovery of malachite sulfidization flotation.Furthermore, LA showed a higher promoting effects and lower reagent consumption in comparison with ammonium sulfate.(2) Based on the adsorption experiments and SEM-EDS results, it was concluded that LA promoted the adsorption of sulfur ions on the surface of malachite and improved the sulfidization efficiency of the malachite surface.
(3) The zeta potential measurements that indicated that LA enhanced the positive charge on the malachite surface.More positive surface was conducive to the adsorption of S(II) species and facilitation of the contact of S(II) species with the mineral surface, thereby promoting the reaction of S with the malachite surface, and improving the surface sulfidation efficiency.XPS analysis further confirmed this phenomenon.

Figure 6 .
Figure 6.The adsorption amount of S in the solution as a function of Na2S dosage in the absence and presence of LA (c(LA) = 6.0 × 10 −5 mol/L; pH = 9).

Figure 13 .
Figure 13.Proposed model of LA to promote malachite sulfidization flotation.

Table 2 .
Relative contents of representative elemental constituents of malachite surface.Cu and S peak fitting were conducted to reveal the chemical state of each element on the malachite surface, and the results are shown in Figures

Table 3 .
Contents and percentages of different Cu species in total Cu on the malachite surface.

Table 4 .
Contents and percentages of different S species in total S on the malachite surface.