Investigation of Copper Recovery from a New Copper Ore Deposit (Nussir) in Northern Norway: Dithiophosphates and Xanthate-Dithiophosphate Blend As Collectors

minerals

Article Investigation of Copper Recovery from a New Copper Ore Deposit (Nussir) in Northern Norway: Dithiophosphates and Xanthate-Dithiophosphate Blend as Collectors

Priyanka Dhar *, Maria Thornhill and Hanumantha Rao Kota

Department of Geoscience and Petroleum, Norwegian Institute of Science and Technology, 7032 Trondheim, Norway; [email protected] (M.T.); [email protected] (H.R.K.) * Correspondence: [email protected]

Received: 12 January 2019; Accepted: 21 February 2019; Published: 28 February 2019

Abstract: The Norwegian mining industry is currently showing increasing interest in the production of metals. Recent research has demonstrated promising results identifying the high potential of the Nussir deposit for the production of copper and other valuable minerals. Mineralogical characterization for Nussir ore samples and their flotation concentrates was performed with optical microscopy and Zeiss automated mineralogy (Mineralogic) where the fine copper sulphide middlings were not completely recovered with a traditional sodium isobutyl xanthate (SIBX) collector. In the current study, dithiophosphate and a mixture of xanthate and dithiophosphate collectors’ interaction on copper and other gangue mineral components of the ore sample were investigated with zeta potential, quantitative adsorption, FTIR studies and Hallimond tube flotation. All the results for single mineral experiments confirmed the feasibility of selective copper sulphide flotation by disecondary butyl dithiophosphate (DBD) as collector. The blend of xanthate and dithiophosphate was chemically adsorbed as individual entities on the surface of the copper minerals via competitive adsorption. A systematic study with DBD and a mixed collector (SIBX and DBD) system was conducted on the coarse grind (−105 µm) of the Nussir ore sample, and the results showed a synergistic interaction between the two reagents. The beneficiated copper concentrate using this mixture of collectors is indeed of improved copper grade and recovery. The highest copper recovery in bench scale flotation was 95.3% with a concentrate grade of 19.4% Cu for DBD collector, whereas mixtures of dithiophosphate and xanthate collectors in the ratio of 3:1 resulted in the highest copper grade (24.7%) and recovery (96.3%).

Keywords: Nussir ore; chalcopyrite; chalcocite; bornite; calcite; quartz; zeta potential; hallimond ﬂotation; bench scale ﬂotation

1. Introduction Nussir ASA, located in the Kvalsund area of Finnmark, is exploring the Nussir and Ulveryggen deposits, and production is anticipated to begin in the near future. With a metal endowment totalling ca. 6% copper, Nussir is one of the largest deposits in Norway. Nussir copper ore is comprised within a relatively simple deposit with very high-grade copper minerals, and the presence of other iron sulphide impurities are negligible in the ore. The latest mineral resource estimation is from July 2014, which states that Nussir consists of 5.8 million tonnes of indicated resources and 60.2 million tonnes of inferred resources, giving 66 million tonnes of copper ore. This ore contains approximately from 0.5% to 6% copper, which varies signiﬁcantly based on geographical location. The copper mineralogy mainly consists of chalcopyrite, chalcocite and bornite [1], and the ore has very little inclusion of iron

Minerals 2019, 9, 146; doi:10.3390/min9030146 www.mdpi.com/journal/minerals Minerals 2019, 9, 146 2 of 23 sulphides. The deposit also contains some traces of economically interesting elements such as silver, gold and platinum group elements (PGE). The traditional method for processing copper sulphide ores is by flotation, which is both economical and suitable as per the environmental standards. The most important factor in flotation of sulphides is the choice of reagents for optimum flotation. A priori, the advanced mineralogy and surface properties of these minerals in this ore deposit are necessary for a judicious choice of the collector for flotation to recover valuable minerals. Automated mineralogical characterization using energy-dispersive scanning electron microscopy (SEM–EDS) is gaining significance in mineral processing operations because of its ability to collect statistically valid quantitative data on mineral distributions, liberation and associations. This data adds precise information on particulates that have traditionally been characterized by bulk mineralogical and chemical methods and optical microscopy. Dhar et al. [2] found that the percentage of liberation of Cu sulphides is approximately 80% for the Nussir ore sample at −105 µm. This study shows that a major part of the copper is present in middlings, which were difficult to recover using the traditional xanthate collector. Earlier, in the development of ore processing flotation, xanthates were almost always employed as collectors for flotation performed on metal sulphides. Due to the high selectivity of dithiophosphates, these collectors were also employed for selective copper sulphide flotation, and a few studies explained the interaction of dithiophosphates with pure copper minerals [3–7]. The mechanism of copper mineral flotation with dithiophosphates is believed to be the chemical adsorption of the dithiophosphate collector by the formation of cuprous dithiophosphate in neutral and slightly alkaline conditions. Recently, Zhong et al. [8] indicated that the recovery of pure chalcopyrite was high in the presence of dithiophosphinate even at natural pH 8 (of the ore) and further showed optimum flotation results of bench scale flotation of a copper ore at the same pH. In the current study, the adsorption and interaction mechanism of disecondary butyl dithiophosphate (DBD) was investigated using pure minerals and bench scale flotation tests which were performed on Nussir ore samples as a function of pH and collector concentration. Additionally, the response of the DBD collector towards chalcopyrite, chalcocite and bornite has been presented and compared with xanthate as a collector. In the past, Glembotskii [9] demonstrated that when a weaker collector is added to a stronger collector, the mixture results in an increase in recovery as compared to the individual strong collector system. Later, Bradshaw et al. [10,11] and Lotter and Bradshaw [12] reviewed the use of various mixtures of collectors in sulphide flotation where the selectivity and, thereby, the recovery and grade increase depending on the proportions of constituents of the collector blend. Additionally, the same authors indicated that the recoveries of middlings could be increased with the usage of mixtures of collectors in optimum ratios. A synergistic interaction of the collectors results in a greater adsorption, which increases the hydrophobicity of the desired mineral. The synergism of the collectors in blends is generally explained with bench scale flotation or microflotation studies; only a few studies were performed on the basis of adsorption results. In 1980, Wakamatsu et al. [13] reported the co-adsorption of collectors at low concentrations and the competitive adsorption at high concentrations of ethyl xanthates (EX) and dibutyl dithiophosphate (DBDTP) collectors in the region of pH 6–7 for galena. Adsorption studies were performed with the solution depletion/abstraction method, and they stated that the adsorption of xanthates increased and DBDTP decreased for the equimolar mixtures of both the collectors at higher collector concentrations. Alternatively, Corin et al. [14] found no evidence of a synergistic effect between diethyl dithiophosphate (DEDTP) and isobutyl xanthate (IBX) for a platinum group ore. The authors reported that the dithiophosphate led to the stabilization of the froth phase and did not contribute to enhancing the hydrophobicity of the mineral surface. However, McFadzean et al. [15,16] showed that a significant increase in pure mineral flotation was observed in the presence of DEDTP and IBX collector mixtures for galena. They clarified that the improvement in the flotation performance relies on the sequence of collector addition. Additionally, the same authors, in 2013, observed synergism in the batch flotation of pyrite with a mixture of IBX and DEDTP. Minerals 2019, 9, 146 3 of 23

Woods [7] studied xanthates and dithiocarbamate mixtures and explained that the chemisorbed collector species were more evenly distributed on the mineral surface than physisorbed species. A mixture may result in a balance of better distributed, more strongly held chemisorbed species and more hydrophobic neutral physisorbed dithiolates, and this could result in the formation of a multilayer surface product that is more strongly attached to the mineral surface. Bagci et al. [17] and Critchley and Riaz [18] proposed that the adsorption is improved by the addition of a weaker but more selective collector (dithiophosphate) before the less selective collector (SIBX). This paper represents the first study on processing characteristics of the Nussir ore to date. In the process of developing a flowsheet for Cu-recovery, dithiophosphate and its blend with xanthate are examined as candidates for the flotation collector to beneficiate the ore. Since the Nussir ore is free from iron impurities, a successful collector selection would be of benefit to future copper deposits of similar mineral constitution. Based on the literature, the mixture of xanthate and dithiophosphate collectors in different proportions is studied in this paper to find the best reagent scheme for Nussir ore flotation. This collector blend is capable of reducing production costs and potentially providing an increase in efficiency in terms of copper recovery and grade. The information from this study will then contribute and assist the detailed flowsheet development being performed in parallel in collaboration with SGS mining and Woodgrove, Canada. Prior to this, the adsorption of both the individual collectors in the collector mixture (SIBX-DBD) on copper sulphide minerals were evaluated using adsorption studies. To our knowledge, no studies have been reported explaining the nature and quantity of adsorption of dithiophosphates and xanthates on copper sulphides. An attempt has been made to demonstrate the mixed dithiophosphate and xanthate adsorption on the copper minerals with quantitative and qualitative adsorption studies together with zeta potential, pure mineral and bench scale flotation studies.

2. Materials and Methods Pure mineral rock specimens of chalcopyrite (>92% grade), chalcocite (>93% grade) and bornite (>91% grade) were purchased commercially. The samples were obtained from Sweden, England and Austria respectively. Pure crystalline calcite and quartz samples used in these experiments were obtained from Norway. The sample preparation procedure for zeta potential measurements, FTIR studies, adsorption studies and Hallimond flotation tests was a three-fold process: (i) crushing using a jaw crusher with 3 mm discharge-opening, (ii) milling using a ball mill followed by sieving to collect the size range from −150 to +45 µm to be used in the Hallimond flotation (iii) for the zeta potential, adsorption studies and FTIR experiments. A portion of the −45 µm sample was further milled in a Fritsch Pulverisette 6 (Idar-Oberstein, Germany) planetary mono mill and wet screened to −10 µm. Precautions were taken during the whole milling and sieving process until the initial conditioning phase for the Hallimond flotation or zeta potential experiments to ensure that surface oxidation did not affect the results. The Nussir ore (Figure1) material was collected from two different parts (panels) of the mine, Nussir North East (N-NE) and Nussir Old West (N-OW). For bench scale flotation tests, the samples were thoroughly mixed, crushed and milled to −105 µm (coarse grind). Cytec Solvay group supplied the reagents used in these experiments. The molecular weight of DBD used in the preparation of collector solutions was 264 g·mol−1, (as specified by the manufacturer). Methyl isobutyl carbinol (MIBC) was employed as the frother. The pH was adjusted with dilute solutions of reagent-grade milli Q water of HCl and NaOH. Deionized water was used in all the studies with a pure mineral system, whereas tap water was used for the bench scale flotation tests. Additionally, a standard quartz suspension was used for calibrating the zeta potential instrument. Copper nitrate was employed to prepare the Cu-precipitates with a DBD collector for FTIR analysis. The Brunauer–Emmett–Teller (BET) surface areas for the −10 µm size fraction of chalcopyrite, chalcocite and bornite were 0.98, 1.31 and 1.01 m2·g−1 respectively. MineralsMinerals2019, 20199, 146, 9, x FOR PEER REVIEW 4 of 234 of 23

(a) (b)

FigureFigure 1. (a 1.) The(a) The Nussir Nussir drill drill hole hole specimen: specimen: VeinletsVeinlets of of chal chalcocitecocite and and bornite bornite on a on carbonate a carbonate structure; structure; (b) the(b) crushed the crushed ore ore sample sample received received from from Nussir Nussir ASA.ASA.

2.1. Zeiss2.1. Zeiss Mineralogical Mineralogical Mining Mining Method Method All samplesAll samples were were initially initially examined examined by by opticaloptical microscopy. Subsequent Subsequent automated automated mineralogy mineralogy analysesanalyses were were performed performed with with the the Zeiss Zeiss Mineralogic Mineralogic Mining Mining System System at atthe the Department Department of Matériaux of Matériaux et Environnement, Université de Liège, Belgium. et Environnement, Université de Liège, Belgium. The Zeiss Mineralogic Mining System is a new generation of Automated Mineralogy systems The Zeiss Mineralogic Mining System is a new generation of Automated Mineralogy systems designed to provide information on the modal mineralogy (by area % and wt %), morphology of designedanalysed to provide grains/particles, information mineral on liberation, the modal locking mineralogy characteristics, (by area mineral % and association, wt %), morphology chemical of analysedassay grains/particles, and element deportment. mineral Data liberation, acquisition locking by the characteristics, Mineralogic was mineral performed association, using a Zeiss chemical assaySigma and element 300 field deportment. emission gun Data equipped acquisition with two by theBruker Mineralogic EDX detectors was performed (XFlash® 630 using SDD a Zeissenergy Sigma 300 fielddispersive emission spectrometer). gun equipped The withoperating two Brukerconditio EDXns used detectors for the (XFlash data acquisition®630 SDD were energy a 20 dispersive kV spectrometer).acceleration The voltage, operating a current conditions of 20 nA, used a 8.5 for mm the working data acquisition distance and were a “full a 20 mapping” kV acceleration analytical voltage, a currentmode. of Analyses 20 nA, a were 8.5 mm conducted working on distance selected andrepres a “fullentative mapping” areas of analytical the wholemode. specimen Analyses with a were conductedmapping on step selected size of representative 3 µm and from areas0.025 to of 0.03 the seconds whole specimen of dwelling with time. a mapping step size of 3 µm For the microscopic observations, approximately 1 g of representative sample was embedded in and from 0.025 to 0.03 seconds of dwelling time. a mixture made with 0.4 g of PRINTEX® carbon black powder, 15 ml of epoxy resin and 2 ml of Forhardener the microscopic to obtain blocks observations, of 3 cm diameter. approximately The addition 1 of g carbon of representative black avoids samplethe grain was settlement embedded in a mixtureby density made into resin with and 0.4 gallows of PRINTEX a good spatial®carbon dispersion black powder,of particles 15 [19]. mL of epoxy resin and 2 mL of hardener to obtain blocks of 3 cm diameter. The addition of carbon black avoids the grain settlement by density2.2. Zeta into Potential resin Measurements and allows a good spatial dispersion of particles [19]. The zeta potential measurements were performed using a DT-300 system instrument from 2.2. Zeta Potential Measurements Quantachrome/Dispersion Technologies, which works on the electroacoustic techniques principle. TheThe chalcopyrite, zeta potential chalcocite, measurements bornite, quartz were and performed calcite samples using were a DT-300 first analysed system in instrument the particle from Quantachrome/Dispersionsize analyser to obtain the Technologies,mean particle diameter which. works A 2 g sample on the of electroacoustic pure mineral was techniques transferred principle. into The chalcopyrite,a 100 ml beaker chalcocite, containing bornite, 50 ml solution quartz andof a desired calcite samplespH and concentration were first analysed of DBD. in Conditioning the particle size was performed in the following order before recording the titration experiments in the pH range from analyser to obtain the mean particle diameter. A 2 g sample of pure mineral was transferred into a 2 to 12: (i) a pH adjustment for 5 min and (ii) collector addition for 5 min. During titration, the pH 100 mL beaker containing 50 mL solution of a desired pH and concentration of DBD. Conditioning was adjusted using 1 M HCl and 1 M NaOH. The conductivity and pH of the suspension were was performedmonitored continuously in the following during order the beforemeasurement, recording and the the titrationambient experimentstemperature was inthe maintained pH range at from 2 to 12:22 ± (i) 0.5 a pH°C. All adjustment experiments for were 5 min performed and (ii) collector in triplicate, addition and an for average 5 min. result During was titration, reported.the pH was adjusted using 1 M HCl and 1 M NaOH. The conductivity and pH of the suspension were monitored continuously2.3. Adsorption during Measurements the measurement, and the ambient temperature was maintained at 22 ± 0.5 ◦C. All experimentsAdsorption were experiments performed of in DBD triplicate, and mixed and anDBD-SI averageBX collectors result was on reported.copper sulphide surfaces were performed with a UV-2401PC (Shimadzu, Kyoto, Japan) spectrometer. A calibration curve was 2.3. Adsorptionestablished Measurements using the UV-spectra taken for 50 ml of different aqueous DBD solutions (from 1 × 10−7 to −3 Adsorption2 × 10 M). The experiments pH of the solution of DBD was and adjusted mixed to DBD-SIBX approximately collectors 8.2 with on 1 M copper HCl and sulphide 1 M NaOH. surfaces To these collector solutions, 2 g of −10 µm each of chalcopyrite, chalcocite and bornite was added and were performed with a UV-2401PC (Shimadzu, Kyoto, Japan) spectrometer. A calibration curve was stirred for 30 min. The suspensions were further filtered, and the supernatant solutions were × −7 established using the UV-spectra taken for 50 mL of different aqueous DBD solutions (from 1 10 to 2 × 10−3 M). The pH of the solution was adjusted to approximately 8.2 with 1 M HCl and 1 M NaOH. To these collector solutions, 2 g of −10 µm each of chalcopyrite, chalcocite and bornite was added and stirred for 30 min. The suspensions were further filtered, and the supernatant solutions were Minerals 2019, 9, 146 5 of 23 characterised in a UV-Visible spectrometer (with required dilution). The amount of dithiophosphate adsorbed was calculated from the difference between initial and residual concentration of the solutions. Another set of experiments were performed using mixtures of dithiophosphate and xanthate collectors in 1:1 ratio under similar conditions. The characteristic peaks of SIBX and DBD were used for the estimation of the amount of dithiophosphate and xanthate adsorbed.

2.4. FTIR Measurements Mineral samples conditioned with the desired collector(s) concentration and speciﬁed pH from the quantitative adsorption studies were dried and analysed with a Bruker Vertex 80v Infrared spectrometer. A typical FTIR spectrum was an average of 200 scans measured at a 4 cm−1 resolution with a narrow band liquid nitrogen cooled MCT detector. The pure mineral powder was used as a reference while recording the spectra of the reagent-treated mineral samples. The spectra of copper sulphides conditioned with 1:1 xanthate and dithiophosphate were also analysed by an infrared spectrometer. Additionally, the spectra of the minerals, reagent and Cu-dithiophosphate precipitates were noted for reference. The atmospheric water was always subtracted from the sample spectrum. The area under the alkyl chain bands and the intensities of functional group bands were measured with the facility available within spectral manipulation.

2.5. Hallimond Tube Flotation Experiments The Hallimond flotation tests were conducted with 2 g from −150 to +50 µm size chalcopyrite, chalcocite, bornite, calcite and quartz samples in a 100 mL Hallimond tube flotation cell. The first step was to adjust the desired pH level using HCl and NaOH. Afterwards, the collector and frother were added and conditioning was performed for 5 and 2 min respectively in a 100 mL standard volumetric flask. The sample was conditioned with a magnetic stirrer for the respective time and subsequently transferred to the Hallimond flotation cell. Air was supplied at the rate of 8 ml·min−1, and flotation time was 1 min. The floated and tailing fractions were collected separately, dried and weighed. The flotation cell was thoroughly washed using ethanol and subsequently rinsed with deionized water after each experiment. All experiments were performed in triplicate, and an average result was reported.

2.6. Bench Scale Flotation Tests The bench scale flotation experiments were performed with 200 g (−105 µm size fraction) of ore, wet ground with tap water in a steel mill with a 1 kg grinding medium followed by flotation in a Maelgwyn bench scale flotation cell of 1 L capacity. The following reagents were added in sequence: a pH regulator, collectors and a frother, the dosages of which varied from one experiment to another. The impeller speed and airflow rate were fixed at 1200 rpm and 3 L·min−1, respectively. The flotation was performed in two stages, where the total flotation time was 4 min. The resulting purified copper sulphide concentrates were filtered, dried and chemically analysed with a Bruker AXS S8 Tiger Wavelength Dispersive XRF.

3. Results

3.1. Mineralogy The ore consists primarily of chalcopyrite, chalcocite and bornite with a minimal amount of covellite. A SEM image is presented in Figure2, which represents all the copper sulphides in the ore. The presence of economically valuable precious metal minerals in low quantities was also noted. Mineralogical characterisation revealed that the copper minerals from the panel N-NE are dominated by bornite and chalcocite with a small amount of chalcopyrite, while the N-OW composite is rich in bornite and chalcopyrite with small amounts of chalcocite. In our previous investigations, the flotation experiments performed with xanthate as a collector showed that the maximum grade and recovery were 14% and 91% Cu, respectively [20]. Minerals 2019, 9, x FOR PEER REVIEW 6 of 23 Minerals 2019, 9, 146 6 of 23 flotation experiments performed with xanthate as a collector showed that the maximum grade and recovery were 14% and 91% Cu, respectively [20]. Before and after the bench scale flotation of the ore with xanthate, the feed and float materials analyzed with Mineralogic revealed that a significant quantity of copper sulphides in the feed material are associated with other copper sulphides, silicate and carbonate minerals. The middlings percentage in both the samples for the −105 µm range is approximately 10%–17%. Although the ore is relatively simple, with little or no intrusion of iron sulphides, the silicate and carbonate gangue minerals are finely Minerals 2019, 9, x FOR PEER REVIEW 6 of 23 disseminated within the matrix of ore samples. The overall distribution of the various associations of copperflotation sulphides experiments before performed and after flotation with xanthate with xanthate as a collector and dithiophosphate showed that the andmaximum these two grade collectors and inrecovery the blend were (1:1) 14% is presented and 91% Cu, in Figure respectively3. [20].

Figure 2. A photomicrograph of the copper sulphides present in the ore. Cc—chalcocite; Bn—bornite; Cp—chalcopyrite; and Cv—covellite.

Before and after the bench scale flotation of the ore with xanthate, the feed and float materials analyzed with Mineralogic revealed that a significant quantity of copper sulphides in the feed material are associated with other copper sulphides, silicate and carbonate minerals. The middlings percentage in both the samples for the −105 µm range is approximately 10%–17%. Although the ore is relatively simple, with little or no intrusion of iron sulphides, the silicate and carbonate gangue minerals are finely disseminated within the matrix of ore samples. The overall distribution of the variousFigureFigure associations 2. 2.A A photomicrograph photomicrograph of copper ofsulphidesof thethe copper before sulphides and after pres presentent flotation in in the the ore. with ore. Cc—chalcocite; Cc—chalcocite;xanthate and Bn—bornite; dithiophosphate Bn—bornite; andCp—chalcopyrite;Cp—chalcopyrite; these two collectors Cv—covellite. and Cv—covellite.in the blend (1:1) is presented in Figure 3.

Before and after the bench scale flotation of the ore with xanthate, the feed and float materials analyzed with Mineralogic revealed that a significant quantity of copper sulphides in the feed material are associated with other copper sulphides, silicate and carbonate minerals. The middlings percentage in both the samples for the −105 µm range is approximately 10%–17%. Although the ore is relatively simple, with little or no intrusion of iron sulphides, the silicate and carbonate gangue minerals are finely disseminated within the matrix of ore samples. The overall distribution of the various associations of copper sulphides before and after flotation with xanthate and dithiophosphate and these two collectors in the blend (1:1) is presented in Figure 3.

FigureFigure 3. The3. The distribution distribution of of various various associations associations ofof coppercopper sulphides in in the the Old Old West West (N-OW; (N-OW; P1) P1) andand Nussir Nussir North North East East (N-NE; (N-NE; P2) P2) samplessamples at collector concentration concentration 3 × 3 10×−510 M− 5andM pH and 8.2. pH P1 8.2. and P1 andP2—Feed; P2—Feed; F—Flotation F—Flotation product; product; X—sodium X—sodium isobutyl isobutyl xanthate xanthate (SIBX); (SIBX); D—disecondary D—disecondary butyl butyl dithiophosphatedithiophosphate (DBD); (DBD); M—SIBX:DBD M—SIBX:DBD = = 1:1 1:1 ratio ratio (Liberated:(Liberated: >80% liberation liberation;; Free: Free: Fully Fully liberated; liberated; andand Middling: Middling: <80% <80% liberation). liberation).

TheThe liberated liberated copper copper sulphides sulphides were were clearlyclearly floatedfloated when using SIBX SIBX as as a a collector, collector, while while a a significantsignificant amount amount of of middlings middlings did did notnot floatfloat with with xanthate xanthate at at collector collector concentration concentration 3 × 10 3 ×−5 M.10 −It5 isM. It is noticeable that the flotation of copper sulphide–carbonate/silicate blends (CuS-carb/CuS-Qtz) is equallyFigure efficient 3. The distribution as the copper of various sulphide–copper associations of sulphidecopper sulphides (CuS–CuS) in the Old blends West in (N-OW; the presence P1) of −5 xanthateand collectors. Nussir North Apparently, East (N-NE; at P2) the samples same collectorat collector dosage, concentration a considerable 3 × 10 M amountand pH 8.2. (ca. P1 8%–10%) and of P2—Feed; F—Flotation product; X—sodium isobutyl xanthate (SIBX); D—disecondary butyl dithiophosphate (DBD); M—SIBX:DBD = 1:1 ratio (Liberated: >80% liberation; Free: Fully liberated; and Middling: <80% liberation).

The liberated copper sulphides were clearly floated when using SIBX as a collector, while a significant amount of middlings did not float with xanthate at collector concentration 3 × 10−5 M. It is

Minerals 2019, 9, 146 7 of 23 Minerals 2019, 9, x FOR PEER REVIEW 7 of 23 coppernoticeable sulphide that middlings the flotation (specifically of copper CuS–CuS sulphide–carbonate/silicate blends) were able toblends float with(CuS-carb/CuS-Qtz) a DBD collector; is less flotationequally of CuS-carb/CuS-Qtz efficient as the copper is sulphide–copper observed. Moreover, sulphide a substantial (CuS–CuS) amountblends in ofthe associated presence of mixed xanthate copper collectors. Apparently, at the same collector dosage, a considerable amount (ca 8%–10%) of copper sulphide particles were floated, together with CuS-carb/CuS-Qtz blend minerals in the presence of sulphide middlings (specifically CuS–CuS blends) were able to float with a DBD collector; less the mixture of SIBX and DBD collector. The results are in agreement with the previous findings flotation of CuS-carb/CuS-Qtz is observed. Moreover, a substantial amount of associated mixed that SIBXcopper is sulphide a stronger particles (more were reactive) floated, collector together and with that CuS-carb/CuS-Qtz DBD is more selectiveblend minerals towards in the copper sulphidespresence [10]. of Earlier, the mixture a few of researchers SIBX and DBD [11 ]collector. also observed The results improvement are in agreement in coarse with particle the previous recovery and middlingsfindings that recovery SIBX is a with stronger collector (more mixtures reactive) collector on Cu/Pt and ores. that DBD Therefore, is more dithiophosphate selective towards and dithiophosphate-xanthatecopper sulphides [10]. Earlier, blends a were few studiedresearchers as collectors,[11] also observed and an improvement extensive investigation in coarse particle has been maderecovery to assess and the middlings interaction recovery of these with collectors collector mi withxtures the on pure Cu/Pt copper ores. Therefore, sulphide dithiophosphate minerals. Detailed benchand scale dithiophosphate-xanthate flotation results as a function blends were of pH studied and collectoras collectors, concentration and an extensive are presented investigation later has in the flotationbeen section. made to assess the interaction of these collectors with the pure copper sulphide minerals. Detailed bench scale flotation results as a function of pH and collector concentration are presented 3.2. Zetalater Potential in the flotation Studies section.

The3.2. zetaZeta Potential potential Studies values of chalcopyrite, chalcocite, bornite, calcite and quartz were measured as a functionThe of pHzeta in potential aqueous values solution, of chalcopyrite, and the results chalco arecite, presented bornite, calcite in Figure and quartz4. The were isoelectric measured points (IEPs)as of a chalcopyrite function of pH and in chalcocite aqueous solution, are located and atthe pH results 4 and are 4.3 presented respectively. in Figure The IEPs4. The of isoelectric bornite are at two pHpoints values, (IEPs) 2.8 of and chalcopyrite 6. Zhao and et al. chalcocite [21], Kelebek are located and Smithat pH [422 and], and 4.3 Fullstonrespectively. et al. The [23 IEPs] reported of similarbornite results. are Theat two IEP pH of values, gangue 2.8 minerals,and 6. Zhao calcite et al. and[21], quartz,Kelebek areand atSmith pH 8.2[22], and and 2.1.Fullston These et valuesal. are in[23] agreement reported with similar previously results. The reported IEP of gangue results minerals, by Somasundaran calcite and [quartz,24]. In are Figures at pH4 and8.2 and5, the 2.1. zeta potentialThese values values decrease are in agreement with increasing with previously pH values repo inrted the results presence by Somasundaran and absence of [24]. the In collector. Figures There4 is a shiftand towards 5, the zeta negative potential zeta values potential decrease values with afterincreasing the addition pH values of thein the anionic presence DBD and collector, absence of which the collector. There is a shift towards negative zeta potential values after the addition of the anionic signiﬁes the adsorption of the reagent on the copper sulphide surface and the mode of adsorption DBD collector, which signifies the adsorption of the reagent on the copper sulphide surface and the is chemisorption on Cu-sites, which is later shown by the FTIR studies. It is evident that the zeta mode of adsorption is chemisorption on Cu-sites, which is later shown by the FTIR studies. It is potentialevident values that of the copper zeta sulphidespotential values also decreased of coppe withr sulphides the subsequent also decreased addition with of the SIBX subsequent [2]. However, the copperaddition sulphides of SIBX [2]. conditioned However, the with copper DBD sulp showhides more conditioned negative with potentials DBD show (in themore pH negative range 4–9) as comparedpotentials to (in those the pH conditioned range 4–9) withas compared SIBX. Overall, to those conditioned it is observed with that SIBX. the Overall, zeta potential it is observed values of copperthat sulphides the zeta potential decrease values signiﬁcantly of copper in sulphides the acidic decrease and neutral significantly pH region, in the acidic whereas and inneutral alkaline pH pH, thereregion, is a minimal whereas decrease in alkaline in zetapH, there potential is a minimal values. decrease in zeta potential values.

FigureFigure 4. The 4. The zeta zeta potential potential of of copper copper minerals minerals andand gangue minerals minerals in inaqueous aqueous solution. solution.

Minerals 2019, 9, 146 8 of 23 indicating an adsorption of both the collectors on the surface of the copper minerals. It is also noted that the behaviour of different copper sulphides is largely dependent on the composition of the reagents employed in the experiments. Less variation in zeta potential values in the presence of different compositions of DBD-SIBX mixtures is observed as compared to bornite and chalcopyrite (Cu–Fe sulphides).Figure The 5. presence The zeta potentials of different of minerals active sites as a offunction Cu and of FepH would at DBD be concentration achieved due 5 × to10− the5 M. different degrees of their oxidation sites. The ﬂotation behaviour of these copper sulphide minerals in the The zeta potential values of copper sulphides conditioned with mixtures of the collector (total presenceMinerals of DBD2019, 9, and x FOR collector PEER REVIEW blends is further examined with Hallimond tube ﬂotation experiments.8 of 23 collector concentration: 3 × 10−5 M) are shown in Figure 6. A major shift towards negative values in the zeta potentials of copper sulphides is observed in the pH range 6–10. It is noted that when the fraction of DBD is greater in the reagent mixture, the values of zeta potential are more negative. This is consistent with the single collector systems, where values of zeta potential were more negative for copper sulphides conditioned with DBD as compared to SIBX [2]. In addition, the negative potentials of copper sulphides are higher in the mixed reagent system compared to a single reagent system, indicating an adsorption of both the collectors on the surface of the copper minerals. It is also noted that the behaviour of different copper sulphides is largely dependent on the composition of the reagents employed in the experiments. Less variation in zeta potential values in the presence of different compositions of DBD-SIBX mixtures is observed as compared to bornite and chalcopyrite (Cu–Fe sulphides). The presence of different active sites of Cu and Fe would be achieved due to the different degrees of their oxidation sites. The flotation behaviour of these copper sulphide minerals in the presence of DBD and collector blends is further examined with Hallimond tube flotation experiments. −5 FigureFigure 5. The 5. zeta The potentialszeta potentials of minerals of minerals as as a functiona function of of pHpH atat DBD concentration concentration 5 × 5 10× 10 M.− 5 M.

The zeta potential values of copper sulphides conditioned with mixtures of the collector (total collector concentration: 3 × 10−5 M) are shown in Figure 6. A major shift towards negative values in the zeta potentials of copper sulphides is observed in the pH range 6–10. It is noted that when the fraction of DBD is greater in the reagent mixture, the values of zeta potential are more negative. This is consistent with the single collector systems, where values of zeta potential were more negative for copper sulphides conditioned with DBD as compared to SIBX [2]. In addition, the negative potentials of copper sulphides are higher in the mixed reagent system compared to a single reagent system, indicating an adsorption of both the collectors on the surface of the copper minerals. It is also noted that the behaviour of different copper sulphides is largely dependent on the composition of the reagents employed in the experiments. Less variation in zeta potential values in the presence of different compositions of DBD-SIBX mixtures is observed as compared to bornite and chalcopyrite (Cu–Fe sulphides). The presence of different active sites of Cu and Fe would be achieved due to the Mineralsdifferent 2019, 9, xdegrees FOR PEER of REVIEWtheir oxidatio n sites. The flotation behaviour of these copper sulphide minerals9 of 23 in the presence of DBD(a) and collector blends is further examined with Hallimond(b) tube flotation experiments.

(c) (a) (b) Figure 6.6. The zeta potentials of (a)) chalcopyrite,chalcopyrite, ((b) chalcocite,chalcocite, and ((c) bornitebornite asas aa functionfunction ofof pHpH inin presencepresence ofof collectorcollector mixturemixture (SIBX:DBD).(SIBX:DBD).

3.3. Hallimond Flotation Studies The floatability of chalcopyrite, chalcocite, bornite, calcite and quartz in the presence of DBD is investigated as a function of pH (Figure 7). In this figure, the highest recoveries for all the three sulphides were observed for pH ≈6, while the recoveries are also considerably high at pH 8–9. At acidic pH values, the recoveries of chalcocite, chalcopyrite and bornite are low, while the recoveries are consistently more than 70% above pH 5. However, the recovery of copper sulphides, specifically chalcopyrite, is relatively less at pH 12. It is known that the copper-iron sulphides are oxidised more in alkaline solutions as compared to acidic or neutral solutions. In fact, the precipitation of metal species as hydroxides is predominantly prevalent in alkaline pH values. Therefore, recovery decreased in alkaline pH solutions and is due to the heavy surface coating hydrophilic metal hydroxides [5,25]. The high recovery of copper sulphides signifies a high selectivity of DBD in a wide pH range (pH 4–11).

Figure 7. The Hallimond flotation recovery as a function of pH at collector concentration 5 × 10−5 M.

Figure 8 depicts the recovery of pure mineral flotation as a function of collector concentration at a natural pH. The results are very similar to our earlier findings on flotation of copper sulphides with SIBX as the collector. However, it is noticeable that a lesser amount of DBD is required as compared to SIBX for a higher flotation of copper sulphides. Chalcopyrite and bornite show a sudden rise in recovery at collector concentration 1 × 10−5 M, whereas chalcocite shows a steady increase in recovery. In general, the flotation response of chalcopyrite and bornite is slightly higher than chalcocite. These

Minerals 2019, 9, x FOR PEER REVIEW 9 of 23

(c) Minerals 2019, 9, 146 9 of 23 Figure 6. The zeta potentials of (a) chalcopyrite, (b) chalcocite, and (c) bornite as a function of pH in presence of collector mixture (SIBX:DBD).

3.3. Hallimond3.3. Hallimond Flotation Flotation Studies Studies The floatabilityThe floatability of chalcopyrite,of chalcopyrite, chalcocite,chalcocite, bornite, bornite, calcite calcite and and quartz quartz in the in presence the presence of DBD of is DBD is investigatedinvestigated as as a a function function ofof pHpH (Figure(Figure 77).). In In th thisis figure, figure, the the highest highest recoveries recoveries for forall the all thethree three sulphidessulphides were were observed observed for pHfor ≈pH6, while≈6, while the the recoveries recoveries are are also also considerably considerably high high at at pH pH 8–9. 8–9. At At acidic pH values,acidic pH the values, recoveries the recoveries of chalcocite, of chalcocite, chalcopyrite chalcopyrite and and bornite bornite are are low, low, while while thethe recoveries are consistentlyare consistently more than more 70% than above70% above pH pH 5. However,5. However, the the recoveryrecovery of of copper copper sulphides, sulphides, specifically specifically chalcopyrite,chalcopyrite, is relatively is relatively less less at pHat pH 12. 12. It isIt is known known that that the the copper-ironcopper-iron sulphides sulphides are are oxidised oxidised more more in in alkaline solutions as compared to acidic or neutral solutions. In fact, the precipitation of metal alkaline solutions as compared to acidic or neutral solutions. In fact, the precipitation of metal species species as hydroxides is predominantly prevalent in alkaline pH values. Therefore, recovery as hydroxides is predominantly prevalent in alkaline pH values. Therefore, recovery decreased in decreased in alkaline pH solutions and is due to the heavy surface coating hydrophilic metal alkalinehydroxides pH solutions [5,25]. andThe high is due recovery to the heavyof copper surface sulphides coating signifies hydrophilic a high selectivity metal hydroxides of DBD in a [ 5wide,25]. The high recoverypH range (pH of copper 4–11). sulphides signifies a high selectivity of DBD in a wide pH range (pH 4–11).

FigureFigure 7. The 7. The Hallimond Hallimond ﬂotation flotation recovery recovery as as a a functionfunction of pH pH at at collector collector concentration concentration 5 × 510×−5 M.10 −5 M.

FigureFigure8 depicts 8 depicts the recoverythe recovery of pureof pure mineral mineral flotation flotation as as aa functionfunction of of collector collector concentration concentration at at a naturala natural pH. The pH. results The results are veryare very similar similar to to our our earlier earlier findingsfindings on on flotation flotation of ofcopper copper sulphides sulphides with with SIBXSIBX as the as collector.the collector. However, However, it isit is noticeable noticeable thatthat a lesser amount amount of ofDBD DBD is required is required as compared as compared to SIBXto SIBX for a for higher a higher flotation flotation of of copper copper sulphides. sulphides. Chalcopyrite and and bornite bornite show show a sudden a sudden rise in rise in recovery at collector concentration 1 × 10−5 M, whereas chalcocite shows a steady increase in recovery. recovery at collector concentration 1 × 10−5 M, whereas chalcocite shows a steady increase in recovery. In general, the flotation response of chalcopyrite and bornite is slightly higher than chalcocite. These In general,Minerals the2019 flotation, 9, x FOR PEER response REVIEW of chalcopyrite and bornite is slightly higher than chalcocite.10 of 23 These resultsresults are consistent are consistent with with the the zeta zeta potential potential results. results. The recovery recovery of of calcite calcite and and quartz quartz also also increases increases −4 to 20%–30%to 20%–30% at a highat a high collector collector concentration: concentration: ca. ca1 1× × 10−4 M.M. All All the the comparisons comparisons with with xanthate xanthate are are performedperformed based based on ouron our earlier earlier investigation investigation [ 20[20].].

FigureFigure 8. Hallimond 8. Hallimond ﬂotation flotation recovery recovery as as a a functionfunction of collector collector concentration concentration at a at natural a natural pH. pH.

The floatability studies of pure minerals with collector mixtures at a natural pH and total collector concentration 3 × 10−5 M is shown in Figure 9. The recoveries of pure chalcopyrite and bornite minerals have two maxima, at pH 6 and 9, when SIBX:DBD is 1:2. However, when SIBX:DBD = 1:2, the recovery of chalcocite showed a maximum recovery at pH 8/9. These results are in agreement with the outcomes of individual reagent systems, where chalcocite indicated higher recoveries at pH 8, whereas chalcopyrite and bornite indicated a maximum recovery at pH 6. The recoveries of calcite and quartz are significantly lower for all the three different ratios. In general, it is observed that the synergistic action of both the reagents results in better recoveries than an individual collector scheme for all the single pure minerals. The major advantage of using collector mixtures is the total concentration of collectors required for flotation of copper sulphides is relatively lower than the single collector system. The improvements in the SIBX:DBD mixture are understandably due to a simple case of competitive adsorption (explained later in Section 3.4 adsorption studies) where DBD adsorbs on the higher active sites followed by the adsorption of the more reactive xanthate collectors on the remaining low activity sites.

(a) (b)

Minerals 2019, 9, x FOR PEER REVIEW 10 of 23 results are consistent with the zeta potential results. The recovery of calcite and quartz also increases to 20%–30% at a high collector concentration: ca 1 × 10−4 M. All the comparisons with xanthate are performed based on our earlier investigation [20].

Minerals 2019, 9, 146 10 of 23 Figure 8. Hallimond flotation recovery as a function of collector concentration at a natural pH.

The floatabilityfloatability studiesstudies of of pure pure minerals minerals with with collector collector mixtures mixtures at a natural at a natural pH and pH total and collector total collectorconcentration concentration 3 × 10− 35 ×M 10 is−5 shownM is shown in Figure in Figure9. The 9. The recoveries recoveries of of pure pure chalcopyrite chalcopyrite and and bornite bornite minerals have two maxima, at at pH 6 and 9, when SI SIBX:DBDBX:DBD is 1:2. However, when SIBX:DBD = 1:2, 1:2, thethe recovery of chalcocite showed a maximummaximum recoveryrecovery atat pHpH 8/9.8/9. These These results results are are in agreement with the the outcomes outcomes of of individual individual reagent reagent systems, systems, wh whereere chalcocite chalcocite indicated indicated higher higher recoveries recoveries at pH at 8,pH whereas 8, whereas chalcopyrite chalcopyrite and bornite and bornite indicated indicated a maximum a maximum recovery recovery at pH at6. pHThe 6.recoveries The recoveries of calcite of andcalcite quartz and are quartz significantly are significantly lower for lower all the for three all the diff threeerent different ratios. In ratios. general, In general,it is observed it is observed that the synergisticthat the synergistic action of action both the of bothreagents the reagentsresults in results better inrecoveries better recoveries than an individual than an individual collector collector scheme forscheme all the for allsingle the singlepure minerals. pure minerals. The major The major adva advantagentage of using of using collector collector mixtures mixtures is is the the total concentration ofof collectorscollectors required required for for flotation flotation of copperof copper sulphides sulphides is relatively is relatively lower lower than thethan single the singlecollector collector system. system. The improvements The improvements in the SIBX:DBDin the SIBX:DBD mixture mixture are understandably are understandably due to due a simple to a simplecase of case competitive of competitive adsorption adsorption (explained (explained later in Sectionlater in Section3.4 adsorption 3.4 adsorption studies) studies) where DBD where adsorbs DBD adsorbson the higher on the active higher sites active followed sites followed by the adsorptionby the adsorption of the more of the reactive more reactive xanthate xanthate collectors collectors on the onremaining the remaining low activity low activity sites. sites.

Minerals 2019, 9, x FOR PEER REVIEW 11 of 23 (a) (b)

(c)

FigureFigure 9.9. TheThe HallimondHallimond ﬂotationflotation recoveryrecovery ofof ((aa)) chalcopyrite,chalcopyrite, ((bb)) chalcocite,chalcocite, andand ((cc)) bornitebornite inin thethe presencepresence ofof aa collectorcollector blendblend as as a a function function of of pH pH at at collector collector concentration concentration 3 3× × 1010−55 M.M.

3.4. Adsorption Studies 3.4. Adsorption Studies The adsorption of DBD from aqueous solution onto copper minerals has been studied by the The adsorption of DBD from aqueous solution onto copper minerals has been studied by the solution depletion method using a UV-Visible spectrometer. The high intensity characteristic peak solution depletion method using a UV-Visible spectrometer. The high intensity characteristic peak of of DBD was observed at 226 nm [25]. The adsorption mechanism of a mixture of the collectors was DBD was observed at 226 nm [25]. The adsorption mechanism of a mixture of the collectors was also also investigated at a 1:1 ratio of SIBX and DBD by using the same spectroscopy method at pH 8. The investigated at a 1:1 ratio of SIBX and DBD by using the same spectroscopy method at pH 8. The characteristic peaks at 301 nm and 226 nm of SIBX and DBD respectively were present in the spectra, characteristic peaks at 301 nm and 226 nm of SIBX and DBD respectively were present in the spectra, allowing the adsorption of individual collectors from the mixture. A feeble dixanthogen peak was allowing the adsorption of individual collectors from the mixture. A feeble dixanthogen peak was observed in the spectra (only at high concentrations), but the dimer of dithiophosphates was not present in the UV spectra because this dithiophosphate dimer has far lower solubility, 5 × 10−7 mol·dm−3, as compared to dixanthogen (1.25 × 10−5 mol·dm−3 [6]). The adsorption isotherms of DBD on chalcopyrite, chalcocite and bornite at pH 8 are shown in Figure 10. This figure shows that the adsorption of DBD on chalcopyrite and bornite begins at a lower equilibrium concentration while the onset of adsorption for chalcocite is found to be 9.2 × 10−6 M. Initially, when a copper sulphide is introduced to the reagent solution, the mineral surface is hydrated due to the faster diffusion of water molecules compared to bulk ligand molecules (Raju and Forsling [26]). Further, the chemical adsorption of DBD on the Cu2+ sites of copper minerals occurs which is also observed from the FTIR spectra, resulting in a hydrophobic surface. Considering the cross-sectional area of a DBD molecule as 35.4 Å2 (as calculated by Matsuoka and Ichikoku [27]), a monolayer is anticipated to form at an adsorption density of 4.6 µmol·m−2. In Figure 11, the isotherm of chalcopyrite attains the plateau value at 3.6 µmol·m−2, thus indicating a 0.8 monolayer. Previously, Petrus et al. [28] observed less than a monolayer coverage of dithiophosphate on chalcopyrite at an equilibrium concentration 1.54 × 10−5 M. Our findings are in accordance with these studies, indicating less than a monolayer adsorption on the chalcopyrite surface followed by a sudden increase in adsorption density above concentration 1.87 × 10−4 M. The increase after a plateau region can be attributed to precipitate formation in the bulk of solution, or another assertion can be multilayers of surface product are adsorbed on the chemisorbed monolayer [7,17,25]. The chalcocite-xanthate adsorption isotherm indicates a marginal increase in adsorption density, followed by a steep rise in the slope which corresponds to the onset of adsorption due to the accumulation of the collector at equilibrium concentration 9.2 × 10−6 M. These results show a less than monolayer coverage of DBD on bornite where the adsorption density is merely 3.2 µmol·m−2. The bornite isotherm also depicts less than monolayer coverage, and the isotherm levels off at 2.8 µmol·m−2. In addition, the chalcocite and bornite isotherms also indicate an increasing adsorption density at a high concentration of DBD (≈1 × 10−3 M) corresponding to an increase in adsorption in the form of Cu(DBD)+(DBD)2 species of collector on the chemisorbed layer [4,10]. Figure 11 shows the area under the alkyl chain region between 2600–2900 cm−1 (the spectra are presented later in Section 3.5 FTIR) for all the three copper minerals with respect to DBD concentration.

Minerals 2019, 9, 146 11 of 23 observed in the spectra (only at high concentrations), but the dimer of dithiophosphates was not present in the UV spectra because this dithiophosphate dimer has far lower solubility, 5 × 10−7 mol·dm−3, as compared to dixanthogen (1.25 × 10−5 mol·dm−3 [6]). The adsorption isotherms of DBD on chalcopyrite, chalcocite and bornite at pH 8 are shown in Figure 10. This ﬁgure shows that the adsorption of DBD on chalcopyrite and bornite begins at a lower equilibrium concentration while the onset of adsorption for chalcocite is found to be 9.2 × 10−6 M. Initially, when a copper sulphide is introduced to the reagent solution, the mineral surface is hydrated due to the faster diffusion of water molecules compared to bulk ligand molecules (Raju and Forsling [26]). Further, the chemical adsorption of DBD on the Cu2+ sites of copper minerals occurs which is also observed from the FTIR spectra, resulting in a hydrophobic surface. Considering the cross-sectional area of a DBD molecule as 35.4 Å2 (as calculated by Matsuoka and Ichikoku [27]), a monolayer is anticipated to form at an adsorption density of 4.6 µmol·m−2. In Figure 11, the isotherm of chalcopyrite attains the plateau value at 3.6 µmol·m−2, thus indicating a 0.8 monolayer. Previously, PetrusMinerals et2019 al., 9, [ x28 FOR] observed PEER REVIEW less than a monolayer coverage of dithiophosphate on chalcopyrite12 of at23 an equilibrium concentration 1.54 × 10−5 M. Our ﬁndings are in accordance with these studies, indicatingThe area under less than the a alkyl monolayer chain adsorptionbands was onmeasured the chalcopyrite with the surface facility followed available by within a sudden the increasespectral inmanipulation adsorption densityfeature. above Adsorption concentration begins 1.87at low× 10 concentration−4 M. The increase of collector after a plateaufor chalcopyrite region can and be attributedbornite, while to precipitate the onset of formation adsorption in thefor chalcocite bulk of solution, was observed or another at 1 assertion× 10−5 M. These can be results multilayers are in ofagreement surface product with the are quantitative adsorbed on adsorption the chemisorbed results.monolayer The area under [7,17,25 the]. The alkyl chalcocite-xanthate chain values for adsorptionchalcopyrite, isotherm bornite indicatesand chalcocite a marginal is approximately increase in adsorption constant above density, collector followed concentration by a steep riseof 2 in × the10−5 slope M, 8 which× 10−5 correspondsM and 2 × 10 to−5 theM respectively. onset of adsorption The same due figure to the also accumulation depicts the ofintensities the collector of the at equilibriumcharacteristic concentration functional group 9.2 × band10−6 ofM. DBD These as results a function show of a collector less than concentration. monolayer coverage The intensities of DBD onof DBD bornite absorbance where the bands adsorption at 964 densityand 953 iscm merely−1 for chalcopyrite 3.2 µmol·m −and2. The chalcocite bornite respectively isotherm also are depicts nearly lessconstant than monolayerabove 2 × 10 coverage,−5 M. The and intensity the isotherm of the levels characteristic off at 2.8 µDBDmol· mband−2. Infor addition, bornite theat 949 chalcocite cm−1 is andapproximately bornite isotherms constant also above indicate 1 × an10− increasing4 M in accordance adsorption with density the quantitative at a high concentration adsorption ofresults. DBD −3 (Although≈1 × 10 notM) shown corresponding in this tocommunication, an increase in adsorptionthe FTIR spectra in the form recorded of Cu(DBD)+(DBD) at higher concentrations2 species of collectorclearly showed on the chemisorbedthe presence of layer randomly [4,10]. oriented collector species on the top of chemisorbed layers.

FigureFigure 10.10. The adsorption ofof dithiophosphatedithiophosphate onon thethe coppercopper sulphidesulphide surfacesurface atat pHpH 8.8.

Figure 11 shows the area under the alkyl chain region between 2600–2900 cm−1 (the spectra are presented later in Section 3.5 FTIR) for all the three copper minerals with respect to DBD concentration. The area under the alkyl chain bands was measured with the facility available within the spectral manipulation feature. Adsorption begins at low concentration of collector for chalcopyrite and bornite, while the onset of adsorption for chalcocite was observed at 1 × 10−5 M. These results are in agreement with the quantitative adsorption results. The area under the alkyl chain values for chalcopyrite, bornite and chalcocite is approximately constant above collector concentration of 2 × 10−5 M, 8 × 10−5 M and 2 × 10−5 M respectively. The same ﬁgure also depicts the intensities of the characteristic functional

Figure 11. The intensities of the characteristic DBD band and the area under the alkyl chain bands for chalcopyrite, chalcocite and bornite.

Figures 12 and 13 shows the adsorption isotherms of SIBX and DBD on copper minerals in the presence of both these collectors’ mixture (1:1) at pH 8. In general, both the collectors in the mixture are adsorbed on all the three copper minerals. However, the differences in the functional groups of SIBX and DBD influence the chemical reactions between the collector and mineral surface. The independent isotherms of SIBX and DBD depicts less than a monolayer coverage (θ) of the individual reagent on chalcopyrite. The adsorption corresponds precisely to 0.24 θ of SIBX and 0.6 θ of DBD on

Minerals 2019, 9, x FOR PEER REVIEW 12 of 23

The area under the alkyl chain bands was measured with the facility available within the spectral manipulation feature. Adsorption begins at low concentration of collector for chalcopyrite and bornite, while the onset of adsorption for chalcocite was observed at 1 × 10−5 M. These results are in agreement with the quantitative adsorption results. The area under the alkyl chain values for chalcopyrite, bornite and chalcocite is approximately constant above collector concentration of 2 × 10−5 M, 8 × 10−5 M and 2 × 10−5 M respectively. The same figure also depicts the intensities of the characteristic functional group band of DBD as a function of collector concentration. The intensities of DBD absorbance bands at 964 and 953 cm−1 for chalcopyrite and chalcocite respectively are nearly constant above 2 × 10−5 M. The intensity of the characteristic DBD band for bornite at 949 cm−1 is approximately constant above 1 × 10−4 M in accordance with the quantitative adsorption results. Although not shown in this communication, the FTIR spectra recorded at higher concentrations clearly showed the presence of randomly oriented collector species on the top of chemisorbed layers.

Minerals 2019, 9, 146 12 of 23 group band of DBD as a function of collector concentration. The intensities of DBD absorbance bands at 964 and 953 cm−1 for chalcopyrite and chalcocite respectively are nearly constant above 2 × 10−5 M. The intensity of the characteristic DBD band for bornite at 949 cm−1 is approximately constant above 1 × 10−4 M in accordance with the quantitative adsorption results. Although not shown in this communication, the FTIR spectra recorded at higher concentrations clearly showed the presence of randomly orientedFigure 10. collector The adsorption species of on dithiophosphate the top of chemisorbed on the copper layers. sulphide surface at pH 8.

Figure 11. The intensities of the characteristic DBD band and the area under the alkyl chain bands for chalcopyrite, chalcocite and bornite.

Figures 12 and 13 shows the adsorption isotherms of SIBX and DBD on copper minerals in the presence ofof bothboth thesethese collectors’ collectors’ mixture mixture (1:1) (1:1) at at pH pH 8. 8. In In general, general, both both the the collectors collectors in thein the mixture mixture are adsorbedare adsorbed on allon the all threethe three copper copper minerals. minerals. However, Howeve ther, differences the differences in the in functional the functional groups groups of SIBX of andSIBX DBD and influenceDBD influence the chemical the chemical reactions reactions between thebetween collector the and collector mineral and surface. mineral The surface. independent The isothermsindependent of SIBXisotherms and DBD of SIBX depicts and DBD less thandepicts a monolayer less than a coveragemonolayer (θ )coverage of the individual (θ) of the reagentindividual on chalcopyrite.reagent on chalcopyrite. The adsorption The correspondsadsorption corresponds precisely to 0.24preciselyθ of SIBX to 0.24 and θ 0.6of SIBXθ of DBDand 0.6 on θ chalcopyrite. of DBD on Similarly, based on the two plots, the proportion of DBD adsorbed on chalcocite is more than SIBX, and both reagents show less than a monolayer coverage. In the case of both chalcopyrite and chalcocite, the onset of adsorption for DBD is at a relatively lower concentration than SIBX, indicating competitive adsorption of the collectors. Alternatively, for bornite, the commencement of adsorption of SIBX is at a comparatively lower concentration than DBD. However, the quantity of adsorption of both the collectors is comparable above the equivalent concentration 5.2 × 10−5 M. The stability of the resulting species at the surface is also dependent on the differences in the proportions of reagents in the collector blend, which is not covered in this study. Bagci et al. [17] indicated that dithiophosphinates are more selective to copper sulphides as compared to xanthates, based on the electronegativities of the functional groups. In this study, DBD preferentially adsorbs on chalcopyrite and chalcocite in accordance with Bagci et al. [17]. However, the preferential adsorption of SIBX on bornite is probably due to the presence of Fe in the lattice, which could be a result of different oxidation sites. Previously, Ackerman et al. [29] also reported a decrease in the floatability of bornite in the alkaline pH region, whereas the flotation response of chalcopyrite and chalcocite was high. Therefore, the decrease in adsorption density may lead to a decrease in floatability; similarly, in our pure mineral flotation results, a small decrease in flotation response of bornite is observed. Unlike Wakamatsu et al. [13], our study shows the co-adsorption of collectors at high concentrations; xanthates being more reactive shows a greater adsorption than DBD. Minerals 2019, 9, x FOR PEER REVIEW 13 of 23 Minerals 2019, 9, x FOR PEER REVIEW 13 of 23 chalcopyrite.chalcopyrite. Similarly,Similarly, basedbased onon thethe twotwo plots,plots, thethe proportionproportion ofof DBDDBD adsorbedadsorbed onon chalcocitechalcocite isis moremore thanthan SIBX,SIBX, andand bothboth reagentsreagents showshow lessless thanthan aa monolayermonolayer coverage.coverage. InIn thethe casecase ofof bothboth chalcopyritechalcopyrite andand chalcocite,chalcocite, thethe onsetonset ofof adsorptionadsorption forfor DBDDBD isis atat aa relativelyrelatively lowerlower concentrationconcentration thanthan SIBX,SIBX, indicatingindicating competitivecompetitive adsorptionadsorption ofof thethe collectors.collectors. Alternatively,Alternatively, forfor bornite,bornite, thethe commencementcommencement ofof adsorptionadsorption ofof SIBXSIBX isis atat aa comparativelycomparatively lowerlower concentrationconcentration thanthan DBD.DBD. However,However, thethe quantityquantity ofof −5 adsorptionadsorption ofof bothboth thethe collectorscollectors isis comparablecomparable aboveabove thethe equivalentequivalent concentrationconcentration 5.25.2 ×× 1010−5 M.M. TheThe stabilitystability ofof thethe resultingresulting speciesspecies atat thethe surfacesurface isis alsoalso dependentdependent onon thethe differencesdifferences inin thethe proportionsproportions ofof reagentsreagents inin thethe collectorcollector blend,blend, whichwhich isis notnot coveredcovered inin thisthis study.study. BagciBagci etet al.al. [17][17] indicatedindicated thatthat dithiophosphinatesdithiophosphinates areare moremore selectiveselective toto coppercopper susulphideslphides asas comparedcompared toto xanthates,xanthates, basedbased onon thethe electronegativitieselectronegativities ofof thethe functionalfunctional groups.groups. InIn thisthis study,study, DBDDBD preferentiallypreferentially adsorbsadsorbs onon chalcopyritechalcopyrite andand chalcocitechalcocite inin accordanceaccordance withwith BagciBagci etet al.al. [17][17].. However,However, thethe preferentialpreferential adsorptionadsorption ofof SIBXSIBX onon bornitebornite isis probablyprobably duedue toto thethe presencepresence ofof FeFe inin thethe lattice,lattice, whichwhich couldcould bebe aa resultresult ofof differentdifferent oxidationoxidation sites.sites. Previously,Previously, AckermanAckerman etet al.al. [29][29] alsoalso reportedreported aa decreasedecrease inin thethe floatabilityfloatability ofof bornitebornite inin thethe alkalinealkaline pHpH region,region, whereaswhereas thethe flotationflotation reresponsesponse ofof chalcopyritechalcopyrite andand chalcocitechalcocite waswas high.high. Therefore,Therefore, thethe decreasedecrease inin adsorptionadsorption densitydensity maymay leadlead toto aa decreasedecrease inin floatability;floatability; similarly,similarly, inin ourour purepure mineralmineral flotationflotation results,results, aa smallsmall decreasedecrease inin flotationflotation responseresponse ofof bornborniteite isis observed.observed. UnlikeUnlike WakamatsuWakamatsu etet al.al. [13],[13], ourour studystudy showsshows thethe co-adsco-adsorptionorption ofof collectorscollectors atat highhigh concentrations;concentrations; Minerals 2019, 9, 146 13 of 23 xanthatesxanthates beingbeing moremore reactivereactive showsshows aa greatergreater adsorptionadsorption thanthan DBD.DBD.

Figure 12. The adsorption of xanthate from a mixture of xanthate and dithiophosphate on the copper FigureFigure 12.12. TheThe adsorptionadsorption ofof xanthatexanthate fromfrom aa mixturemixture ofof xanthatexanthate andand dithiophosphatedithiophosphate onon thethe coppercopper sulphide surface at pH 8. sulphidesulphide surfacesurface atat pHpH 8.8.

FigureFigure 13. 13.The The adsorptionadsorption ofof dithiophosphatedithiophosphate fromfrom a a mixturemixture of of xanthate xanthate and and dithiophosphate dithiophosphate on on the the Figure 13. The adsorption of dithiophosphate from a mixture of xanthate and dithiophosphate on the coppercopper sulphide sulphide surface surface at at pH pH 8. 8. copper sulphide surface at pH 8. Figure 14 shows the intensities of characteristic SIBX and DBD bands for all the three copper minerals in the presence of mixtures of both the collectors. The intensities of the distinctive DBD peak −1 −1 at 985 cm are more than the SIBX peak at 1065 cm for chalcopyrite. In the case of chalcocite, the intensities of the characteristic DBD peak at 895 cm−1 are greater than the SIBX peak at 1192 cm−1. For bornite, the SIBX peak at 1206 cm−1 is of a higher intensity than that of the DBD peak at 745 cm−1. These results corroborate the quantitative adsorption studies shown in Figures 11 and 12. SIBX preferentially adsorbed on bornite and the quantity of adsorption are comparable to DBD. The area under the alkyl chain for both the collectors is represented in Figure 15, and the results show that the areas under the alkyl chain for all the three copper minerals are approximately constant above cumulative concentration 1 × 10−5 M. Future works within this study will investigate the various proportions of collectors in the blend. In general, the surfaces of the copper sulphides comprise various degrees of surface-active sites. It is understood that when the collector mixture is introduced in the mineral solution, the selective collector first adsorbs on the strong active sites [7]. The less selective collector there will adsorb on the residual weak sites. The different molecular structures of the functional groups of xanthates and dithiophosphates influences the reactions between the collector and mineral surface and, thus, the stability of the species at the surface. Donor atoms are crucial in the interaction between collectors and the mineral surface. Collectors having highly electronegative donor atoms possess a higher affinity for adsorption and are considered as less selective collectors. Thus, the selectivity of the collectors can be Minerals 2019, 9, x FOR PEER REVIEW 14 of 23

Minerals 2019, 9, x FOR PEER REVIEW 14 of 23 Figure 14 shows the intensities of characteristic SIBX and DBD bands for all the three copper mineralsFigure in 14the shows presence the of intensities mixtures of bothcharacterist the collectors.ic SIBX The and intensities DBD bands of thefor distinctiveall the three DBD copper peak −1 −1 mineralsat 985 cm in theare presence more than of mixturesthe SIBX ofpeak both at the 1065 collectors. cm for Thechalcopyrite. intensities In of thethe casedistinctive of chalcocite, DBD peak the −1 −1 atintensities 985 cm−1 ofare the more characteristic than the SIBX DBD peak peak at at 1065895 cm cm− 1are for greater chalcopyrite. than the In SIBX the case peak of at chalcocite, 1192 cm . theFor −1 −1 intensitiesbornite, the of SIBXthe characteristic peak at 1206 DBD cm peak is of at a 895higher cm −intensity1 are greater than than that the of SIBX the DBD peak peakat 1192 at cm745−1 .cm For . bornite,These results the SIBX corroborate peak at 1206 the cmquantitative−1 is of a higher adsorp intensitytion studies than shown that of inthe Figures DBD peak 11 and at 745 12. cmSIBX−1. Thesepreferentially results corroborateadsorbed on the bornite quantitative and the quantityadsorption of adsorptionstudies shown are comparable in Figures to11 DBD.and 12.The SIBX area Minerals 2019, 9, 146 14 of 23 preferentiallyunder the alkyl adsorbed chain for on both bornite the collectors and the quantity is represented of adsorption in Figure are 15, comparable and the results to DBD. show The that area the underareas theunder alkyl the chain alkyl for chain both thefor collectorsall the three is represented copper minerals in Figure are 15, approximately and the results constant show that above the −5 explainedareascumulative under with concentrationthe their alkyl respective chain 1 ×for electro10 all M. the negativities. Future three workscopper Nagraj within minerals [30 ]this stated are study thatapproximately will the electronegativitiesinvestigate constant the variousabove (EN) ofcumulativeproportions O and C atom concentration of collectors in xanthate in 1 molecules the× 10 blend.−5 M. are Future higher works than thewithin P atoms this instudy the structure will investigate of dithiophosphates; the various thus,proportions the selectivity of collectors of DBD in the is higher blend. than SIBX.

Figure 14. The intensities of characteristic SIBX and DBD bands for chalcopyrite, chalcocite and bornite. Figure 14.14.The The intensities intensities of of characteristic characteristic SIBX SIBX and DBDand DBD bands bands for chalcopyrite, for chalcopyrite, chalcocite chalcocite and bornite. and bornite.

FigureFigure 15. 15.The The areaarea underunder thethe alkylalkyl chainchain bandsbands forfor chalcopyrite,chalcopyrite, chalcocitechalcocite and and bornite bornite in inthe the presencepresence of SIBX and DBD blend (1:1). Figureof SIBX 15. and The DBD area blend under (1:1). the alkyl chain bands for chalcopyrite, chalcocite and bornite in the presence of SIBX and DBD blend (1:1). OurIn general, adsorption the surfaces study demonstrates of the copper that sulphides DBD is adsorbedcomprise ﬁrstvarious on the degrees chalcopyrite of surface-active and chalcocite sites. surfaces followed by SIBX on the weak sites. A similar competitive adsorption is observed for bornite, It is Inunderstood general, the that surfaces when theof the collector copper mixture sulphides is introducedcomprise various in the degreesmineral ofsolution, surface-active the selective sites. where SIBX adsorbs ﬁrst and, subsequently, DBD adsorbs, which is due to the different degrees Itcollector is understood first adsorbs that when on the the strong collector active mixture sites [7].is introduced The less selective in the mineral collector solution, there will the adsorb selective on of oxidation. collectorthe residual first weak adsorbs sites. on The the differentstrong active molecular sites [7].stru Thectures less of selective the functional collector groups there of will xanthates adsorb andon dithiophosphates influences the reactions between the collector and mineral surface and, thus, the 3.5.the residual FTIR Studies weak sites. The different molecular structures of the functional groups of xanthates and dithiophosphatesstability of the species influences at the the su rface.reactions Donor between atoms theare collectorcrucial in and the mineral interaction surface between and, collectorsthus, the stabilityand Thethe ofmineral DRIFT the species spectrasurface. at oftheCollectors puresurface. copper havingDonor sulphide atomshighly are minerals,electronegative crucial in SIBX, the donorinteraction DBD andatoms thebetween possess Cu-precipitate collectors a higher (CuDBDand the mineral + Cu(DBD) surface.2) as Collectors reference having spectra highly are shown electronegative in Figure 16donor. The atoms dithiophosphate possess a higher and −1 copper-dithiophosphate precipitate spectra indicated a band structure especially below 980 cm (in the region 500–970 cm−1). The vibrational bands between 510–575 cm−1 represent P–S stretching bands and between 630–830 cm−1 correspond to P–O–C symmetric stretching vibrations [31–33]. The bands between 900 and 1100 cm−1 are attributed to P–O–C asymmetric stretching vibration [32,33]. In the collector spectrum, the bands at 735, 787, 828 and 860 cm−1 represent P–O–C symmetric stretching. The bands at 531 and 614 cm−1 indicate P–S stretching and P=S stretching modes respectively; Minerals 2019, 9, x FOR PEER REVIEW 15 of 23 affinity for adsorption and are considered as less selective collectors. Thus, the selectivity of the collectors can be explained with their respective electro negativities. Nagraj [30] stated that the electronegativities (EN) of O and C atom in xanthate molecules are higher than the P atoms in the structure of dithiophosphates; thus, the selectivity of DBD is higher than SIBX. Our adsorption study demonstrates that DBD is adsorbed first on the chalcopyrite and chalcocite surfaces followed by SIBX on the weak sites. A similar competitive adsorption is observed for bornite, where SIBX adsorbs first and, subsequently, DBD adsorbs, which is due to the different degrees of oxidation.

3.5. FTIR Studies The DRIFT spectra of pure copper sulphide minerals, SIBX, DBD and the Cu-precipitate (CuDBD + Cu(DBD)2) as reference spectra are shown in Figure 16. The dithiophosphate and copper- dithiophosphate precipitate spectra indicated a band structure especially below 980 cm−1 (in the region 500–970 cm−1). The vibrational bands between 510–575 cm−1 represent P–S stretching bands and between 630–830 cm−1 correspond to P–O–C symmetric stretching vibrations [31–33]. The bands between 900 and 1100 cm−1 are attributed to P–O–C asymmetric stretching vibration [32,33]. In the collectorMinerals spectrum,2019, 9, 146 the bands at 735, 787, 828 and 860 cm−1 represent P–O–C symmetric stretching.15 of 23 The bands at 531 and 614 cm−1 indicate P–S stretching and P=S stretching modes respectively; these −1 −1 peaksthese are peaks attributed are attributed to the PS to2 group the PS of2 groupDBD. The of DBD. broad The peak broad at 984 peak cm at signifies 984 cm P–O–Csigniﬁes asymmetric P–O–C stretchingasymmetric vibration. stretching The vibration.copper precipitate The copper spectrum precipitate indicates spectrum characteristic indicates characteristic peaks at 523 peaks cm−1at and 642523 cm cm−1 which−1 and correspond 642 cm−1 which to P–S correspond stretching to P–Sand stretchingP=S stretching and P=S modes stretching respectively. modes respectively. Additionally, − thisAdditionally, spectrum shows this spectrum a peculiar shows sharp a peculiar peak at sharp 970 peakcm−1 atwhich 970 cm refers1 which to P–O–C refers toasymmetric P–O–C asymmetric stretching vibration.stretching vibration.

FigureFigure 16. 16. TheThe DRIFT DRIFT spectra spectra of of xanthate, xanthate, dithiophosph dithiophosphateate andand coppercopper dithiophosphate dithiophosphate precipitates. precipitates.

MineralsFiguresFigures 2019 , 17,9 , 17x FOR–1819 PEERandrepresent REVIEW19 represent the FTIR spectrathe FTIR of chalcopyrite,spectra of chalcopyrite, bornite and chalcocitebornite and conditioned chalcocite16 of 23 conditionedwith various with concentrations various concentrations of DBD. After of the DBD. interaction After the with interaction DBD, the characteristicwith DBD, the peaks characteristic for pure −1 −1 −1 peakschalcopyrite685 forcm pure peaks become chalcopyrite can be weak noted, and become which the characteristic weakcorrespond and peak theto P–O–Ccharacteristic (984 cm stretching,) of DBD peak and becomes (984 the cmband evident.−1) belowof DBD The 600 spectrabecomes cm of chalcopyrite in Figure 17 reveals the presence of a peak at 964 cm−1 at and above a 1 × 10−5 M evident.refers Theto P=S spectra stretching of chalcopyrite vibration (dithiolate in Figure formation).17 reveals theThe presence shift in P–O–Cof a peak stretching at 964 cmand−1 atP=S and concentration,stretching vibration which is corresponds related to the to thechemisorption asymmetric of P–O–C DBD stretchingon the chalcocite of DBD. surface. The bands All the at 777 changes cm−1 above a 1 × 10−5 M concentration, which corresponds to the asymmetric P–O–C stretching of DBD. andsuggested 531 cm that−1 refer the S to and P–O–C O atoms symmetric in the functional stretching gr andoup P–S of DBD stretching might vibrations.have taken Thepart shiftin the in reaction P–O–C The bands at 777 cm−1 and 531 cm−1 refer to P–O–C symmetric stretching and P–S stretching andwith P–S these vibrations ions. indicate the chemical adsorption of the collector on Cu-sites of chalcopyrite. vibrations. The shift in P–O–C and P–S vibrations indicate the chemical adsorption of the collector on Cu-sites of chalcopyrite. Similarly, the spectra of bornite and chalcocite conditioned with various concentrations of DBD are shown in Figures 18 and 19. For bornite, a shift is observed in the P–O–C symmetric and asymmetric stretching vibrations of DBD to 814 and 949 cm−1 respectively. The peak at 623 cm−1 which is related to P=S stretching vibrations is observed at and above a 8 × 10−5 M concentration of the collector, indicating the chemisorption of the collector on Cu sites. In the case of chalcocite, 953 and

Figure 17. 17. TheThe DRIFT DRIFT spectra spectra of dithiophos of dithiophosphatephate adsorption adsorption on chalcopyrite on chalcopyrite with increasing with increasing collector collectorconcentration. concentration.

Similarly, the spectra of bornite and chalcocite conditioned with various concentrations of DBD are shown in Figures 18 and 19. For bornite, a shift is observed in the P–O–C symmetric and asymmetric stretching vibrations of DBD to 814 and 949 cm−1 respectively. The peak at 623 cm−1 which is related

Figure 18. The DRIFT spectra of dithiophosphate adsorption on bornite with increasing collector concentration.

Minerals 2019, 9, x FOR PEER REVIEW 16 of 23

685 cm−1 peaks can be noted, which correspond to P–O–C stretching, and the band below 600 cm−1 refers to P=S stretching vibration (dithiolate formation). The shift in P–O–C stretching and P=S stretching vibration is related to the chemisorption of DBD on the chalcocite surface. All the changes suggested that the S and O atoms in the functional group of DBD might have taken part in the reaction with these ions.

Minerals 2019, 9, 146 16 of 23 to P=S stretching vibrations is observed at and above a 8 × 10−5 M concentration of the collector, indicating the chemisorption of the collector on Cu sites. In the case of chalcocite, 953 and 685 cm−1 peaks can be noted, which correspond to P–O–C stretching, and the band below 600 cm−1 refers to P=S stretching vibration (dithiolate formation). The shift in P–O–C stretching and P=S stretching vibration is relatedFigure to 17. the The chemisorption DRIFT spectra of of DBDdithiophos on thephate chalcocite adsorption surface. on chalcopyrite All the changes with increasing suggested collector that the S and Oconcentration. atoms in the functional group of DBD might have taken part in the reaction with these ions.

FigureFigure 18. TheThe DRIFT DRIFT spectra spectra of dithiophosphate of dithiophosphate adsorpti adsorptionon on bornite on bornitewith increasing with increasing collector Mineralscollectorconcentration. 2019, 9, concentration. x FOR PEER REVIEW 17 of 23

FigureFigure 19. TheThe DRIFT DRIFT spectra spectra of dithiophosphate of dithiophosphate adsorption adsorption on chalcocite on chalcocite with increasing with increasing collector collectorconcentration. concentration.

FigureFigure 2020 depicts depicts the the spectra spectra of of adsorption adsorption of aof mixed a mixed xanthate xanthate and and dithiophosphate dithiophosphate collector collector (1:1) −1 at(1:1) pH at 8 onpH the 8 on three the copper three minerals.copper minerals. The characteristic The characteristic xanthate bandsxanthate are bands between are 900 between and 1300 900 cm and, −1 and1300 the cm dithiophosphate−1, and the dithiophosphate bands are betweenbands are 600 between and 1000 600 cmand 1000. This cm indicates−1. This indicates adsorption adsorption of both theof both collectors the collectors on all the on three all the copper three mineralcopper minera surfaces.l surfaces. However, However, the spectrum the spectrum of chalcopyrite of chalcopyrite shows −1 twoshows strong two peaksstrong at peaks 883 and at 985883 cm and ,985 characterising cm−1, characterising P–O–C symmetric P–O–C symmetric and asymmetric and asymmetric stretching −1 vibrationstretching respectively. vibration respectively. Additionally, Additionally it also indicates, it peaksalso indicates at 1065 and peaks 1273 at cm 1065, corresponding and 1273 cm to−1, −1 C=Scorresponding and C–N stretching to C=S and respectively. C–N stretching Similarly, respective the chalcocitely. Similarly, spectrum the depicts chalco peakscite spectrum at 895 cm depictsand −1 764peaks cm at 895, implying cm−1 and P–O–C 764 cm symmetric−1, implying stretching P–O–C symmetric vibrations. stretching A semi-broad vibrations. peak A is alsosemi-broad observed peak at is also observed at 1192 cm−1, indicating C–O–C asymmetric stretching. However, bornite indicates two strong peaks at 1208 and 1351 cm−1: C–N stretching and C–O–C asymmetric stretching respectively. The characteristic peaks related to DBD were also observed at 633 and 745 cm−1. These spectra reveal that DBD is more strongly adsorbed than the SIBX collector on chalcopyrite and chalcocite. Similarly, the adsorption of the SIBX collector is stronger for bornite as compared to DBD. The infrared spectroscopy results are consistent with the zeta potential and Hallimond flotation results. All the information regarding the bands and peaks are summarized in Table 1.

Figure 20. The DRIFT spectra of the dithiophosphate and xanthate collector blend (1:1) adsorption on copper sulphides at pH 8 and a total concentration of collector 3 × 10−5 M.

Minerals 2019, 9, 146 17 of 23

1192 cm−1, indicating C–O–C asymmetric stretching. However, bornite indicates two strong peaks at 1208 and 1351 cm−1: C–N stretching and C–O–C asymmetric stretching respectively. The characteristic peaks related to DBD were also observed at 633 and 745 cm−1. These spectra reveal that DBD is more strongly adsorbed than the SIBX collector on chalcopyrite and chalcocite. Similarly, the adsorption of the SIBX collector is stronger for bornite as compared to DBD. The infrared spectroscopy results are consistent with the zeta potential and Hallimond ﬂotation results. All the information regarding the bands and peaks are summarized in Table1.

Table 1. The FTIR bands of dithiophosphate, Cu-dithiophosphate precipitate, xanthate and chalcopyrite, chalcocite and bornite (before and after adsorption of dithiophosphate/dithiophosphate-xanthate blend, 1:1).

Spectrum Name Bands (cm−1) Signiﬁcance References

531 vs (P–S) [32,33] 614 vs (P=S) [32,33] 735 vs (P–O–C) [28,32] 787 vs (P–O–C) [32,33] Dithiophosphate (DBD) 828 vs (P–O–C) [28,32] 860 vs (P–O–C) [32] 907 vas (P–O–C) [32] 984 vas (P–O–C) [32] 1036 vas (P–O–C) [32]

523 vs (P–S) [32,33] 642 vs (P=S) [32] 746 v (P–O–C) [33] Cu-dithiophosphate s 833 vs (P–O–C) [32,33] 970 vas (P–O–C) [32,33] 1168, 1255 vas (P–O–C) [32]

1047 vs (C=S) [34] 1065 vs (C=S) [32,34] 1086 v (C=S) [32,33] Xanthate (SIBX) s 1169 vas (C–O–C) [32,33] 1254 v(C–N) [32,34] 1508 v(C–N) [34]

530 vs (P–S) [32,33] 571 vs (P–S) [32] Chalcopyrite + DBD 692 vs (P–O–C) [32,34] 777 vs (P–O–C) [32,33] 964 vas (P–O–C) [32,33]

486 vs (P–S) [32,33] 623 vs (P=S) [32,34] Bornite + DBD 635 vs (P=S) [32–34] 814 vs (P–O–C) [32,33] 949 vas (P–O–C) [32,33]

625 vs (P=S) [32] 683 vs (P–O–C) Chalcocite + DBD 773 vs (P–O–C) [32,33] 953 vas (P–O–C) [32,33] 1024 vas (P–O–C) [32,33]

584 vs (P–S) [32] 883 v (P–O–C) [32–34] Chalcopyrite + DBD + SIBX s 985 vas (P–O–C) 1065 vas (C–O–C)

589 vs (P–S) [32–34] 663 vs (P–O–C) [32] Bornite + DBD + SIBX 1208 v(C–N) [33] 1351 v(C–N) [34] 1428 v(C–N) [32,33]

717 vs (P–O–C) [32,33] 848 vs (P–O–C) [32,33] Chalcocite + DBD + SIBX 935 vas (P–O–C) [32,33] 1186 vas (C–O–C) [32–34] Minerals 2019, 9, x FOR PEER REVIEW 17 of 23

Figure 19. The DRIFT spectra of dithiophosphate adsorption on chalcocite with increasing collector concentration.

Figure 20 depicts the spectra of adsorption of a mixed xanthate and dithiophosphate collector (1:1) at pH 8 on the three copper minerals. The characteristic xanthate bands are between 900 and 1300 cm−1, and the dithiophosphate bands are between 600 and 1000 cm−1. This indicates adsorption of both the collectors on all the three copper mineral surfaces. However, the spectrum of chalcopyrite shows two strong peaks at 883 and 985 cm−1, characterising P–O–C symmetric and asymmetric stretching vibration respectively. Additionally, it also indicates peaks at 1065 and 1273 cm−1, corresponding to C=S and C–N stretching respectively. Similarly, the chalcocite spectrum depicts peaks at 895 cm−1 and 764 cm−1, implying P–O–C symmetric stretching vibrations. A semi-broad peak is also observed at 1192 cm−1, indicating C–O–C asymmetric stretching. However, bornite indicates two strong peaks at 1208 and 1351 cm−1: C–N stretching and C–O–C asymmetric stretching respectively. The characteristic peaks related to DBD were also observed at 633 and 745 cm−1. These spectra reveal that DBD is more strongly adsorbed than the SIBX collector on chalcopyrite and chalcocite. Similarly, the adsorption of the SIBX collector is stronger for bornite as compared to DBD. MineralsThe2019 infrared, 9, 146 spectroscopy results are consistent with the zeta potential and Hallimond flotation18 of 23 results. All the information regarding the bands and peaks are summarized in Table 1.

FigureFigure 20. The 20. DRIFTThe DRIFT spectra spectra of theof the dithiophosphate dithiophosphate andand xanthatexanthate collector collector blend blend (1:1) (1:1) adsorption adsorption on on −5 coppercopper sulphides sulphides at pH at 8pH and 8 and a total a total concentration concentration of of collector collector 3 × 1010−M.5 M. Minerals 2019, 9, x FOR PEER REVIEW 19 of 23

3.6. Correlation among the Pure Mineral Studies with Respect to Collector Concentration 3.6. Correlation Among the Pure Mineral Studies with Respect to Collector Concentration Figure 21 shows the correlation between the Hallimond flotation recovery, Zeta Potential results Figure 21 shows the correlation between the Hallimond flotation recovery, Zeta Potential results and adsorption densities for the mixed collector system (SIBX:DBD = 1:1) on the three copper minerals and adsorption densities for the mixed collector system (SIBX:DBD = 1:1) on the three copper minerals at collector concentration 3 × 10−5 M at pH 8. Figure 21a–c indicates a significant decrease in the at collector concentration 3 × 10−5 M at pH 8. Figure 21a–c indicates a significant decrease in the zeta × −5 zeta potentialpotential values at at the the total total collector collector concentration concentration (3 (3× 10−105 M); M);at the at thesame same concentration, concentration, the the floatabilityfloatability of the of the Cu-minerals Cu-minerals increased. increased. TheThe adsorptionadsorption of of DBD DBD is isapproximately approximately constant constant above above −5 collectorcollector concentration concentration 3 × 310 × 10−5M. M. XanthateXanthate adsorption adsorption is isless less pron pronouncedounced at this at thisconcentration. concentration. Thus, Thus, a gooda good correlation correlation is observed is observed among among all all the the pure pure mineralmineral results results on on all all the the three three copper copper minerals. minerals.

(a) (b)

(c)

FigureFigure 21. 21.The The correlation correlation between between thethe HallimondHallimond flotation ﬂotation recovery, recovery, zeta zeta potential potential results results and and adsorptionadsorption densities densities for ( afor) chalcopyrite, (a) chalcopyrite, (b) chalcocite, (b) chalcocite, and (c )and bornite (c) atbornite a total at collector a total concentrationcollector 3 × 10concentration−5 M (SIBX:DBD 3 × 10−5 ratio M (SIBX:DBD 1:1) at natural ratio 1:1) pH. at natural pH.

3.7. Bench Scale Flotation The natural ore flotation was carried out using DBD as the collector and MIBC as frother as a function of pH. Lime or any other additive was not employed in the current study because the ore does not contain iron sulphide gangue minerals. The feasibility of copper sulphide flotation with the collector DBD and mixtures of DBD and SIBX for the coarse grind samples (−105 µm) were tested and are reported in this study. As per the mineralogical studies, the copper sulphides are liberated (>80%) in this size range; the major middlings’ percentage are copper sulphide-copper sulphide associations. The results are presented in Table 2. At pH 4, recovery is considerable and close to 90% with a total concentrate grade of 17%. In the alkaline region, at pH 12, the grade is low, approximately 10.7%, but the recovery is above 90%. At natural pH (pH 8), the recovery is maximum, approximately 95% with a concentrate grade of 19.4%. The flotation response of Nussir ore with SIBX as a reagent produced a highest concentration grade of 14% [19], Thus, the copper concentrate grade is significantly improved by using DBD as a collector due to the high selectivity of the collector towards copper minerals. The results are consistent with the pure mineral studies. Table 3 represents the

Minerals 2019, 9, 146 19 of 23

3.7. Bench Scale Flotation The natural ore flotation was carried out using DBD as the collector and MIBC as frother as a function of pH. Lime or any other additive was not employed in the current study because the ore does not contain iron sulphide gangue minerals. The feasibility of copper sulphide flotation with the collector DBD and mixtures of DBD and SIBX for the coarse grind samples (−105 µm) were tested and are reported in this study. As per the mineralogical studies, the copper sulphides are liberated (>80%) in this size range; the major middlings’ percentage are copper sulphide-copper sulphide associations. The results are presented in Table2. At pH 4, recovery is considerable and close to 90% with a total concentrate grade of 17%. In the alkaline region, at pH 12, the grade is low, approximately 10.7%, but the recovery is above 90%. At natural pH (pH 8), the recovery is maximum, approximately 95% with a concentrate grade of 19.4%. The flotation response of Nussir ore with SIBX as a reagent produced a highest concentration grade of 14% [19], Thus, the copper concentrate grade is significantly improved by using DBD as a collector due to the high selectivity of the collector towards copper minerals. The results are consistent with the pure mineral studies. Table3 represents the recovery and grade results for the mixed collector systems at natural pH. The table indicates that all results of the collector combinations are better as compared to a single collector system. Thus, the results are in agreement with the previous findings in the literature [35], understandably due to the synergistic action of reagents which leads to higher flotation of copper sulphides. Overall, the best results were obtained for SIBX:DBD = 1:3 at natural pH, which showed a recovery of 96.3% with a concentrate grade of 24.7%. The results can be correlated with the pure mineral studies. It was observed in the microscopic analysis that the ore has bornite in major quantity. Therefore, the mixture with DBD at a higher proportion in the reagent mixture is the best suited reagent scheme proposed for this ore. The reagent mixture, with total concentration 3 × 10−5 M, produced the best grades and recoveries. Thus, the reagent concentration can be reduced by using the mixed collector system with improved metallurgical results.

Table 2. The grade and Recovery results with respect to varying pH at collector concentration 5 × 10−5 M. Feed grades 2.2%–2.5%.

Samples Conc/Tail pH Copper Grade (%) Copper Recovery Conc_1 34 86.7 Conc_2 1.3 3.3 Tail4 0.2 9.4 Head Bal. 17.4 90.0 Total Mass Bal. 99.4 Conc_1 42.6 93.3 Conc_2 1.1 2.0 Tail6 0.1 4.5 Head Bal. 19.4 95.3 Total Mass Bal. 99.8 Conc_1 32.2 92.8 Conc_2 0.9 1.8 Tail8 0.1 4.72 Head Bal. 19.4 94.6 Total Mass Bal. 99.3 Conc_1 46 83.1 Conc_2 1.1 7.3 Tail12 0.1 8.6 Head Bal. 10.7 90.4 Total Mass Bal. 99.0 Bal., Balance. Minerals 2019, 9, 146 20 of 23

Table 3. The grade and Recovery results with respect to varying collector concentration. Feed grades 2.2%–2.5%.

Samples Conc/Tail Collector Conc. Copper Grade (%) Copper Recovery Conc_1 42.8 93.4 Conc_2 1.05 2.41 2 × 10−5 M(D) Tail 0.1 3.3 1 × 10−5 M(S) Head Bal. 21.4 95.8 Total Mass Bal. 99.2 Conc_1 30.1 90.4 Conc_2 2.21 3.95 1 × 10−5 M(D) Tail 0.1 4.7 2 × 10−5 M(S) Head Bal. 19.6 94.4 Total Mass Bal. 99.1 Conc_1 47.3 93.5 Conc_2 1.5 3.01 2.25 × 10−5 M(D) Tail 0.07 3.01 0.75 × 10−5 M(S) Head Bal. 24.7 96.3 Total Mass Bal. 99.4 Conc_1 27.9 94.8 Conc_2 2.1 1.91 0.75 × 10−5 M(D) Tail 0.1 4.42 2.25 × 10−5 M(S) Head Bal. 22.3 96.6 Total Mass Bal. 99.3 D, DBD; S, SIBX; Bal., Balance.

4. Conclusions The copper mineralogy of Nussir copper ore primarily comprises chalcopyrite, chalcocite and bornite. Microscopic analyses showed the copper sulphides are more than 80% liberated in the −105 µm size fraction samples. It was also revealed that approximately 10%–17% of copper sulphides are associated with each other or other gangue minerals and that these mineral mixtures were difficult to float with xanthate collectors. Therefore, an attempt was made to employ a dithiophosphate collector (DBD) and a mixture of DBD-SIBX collectors to float these copper minerals from the ore. The zeta potential studies of copper minerals after interaction with DBD showed that the mineral surfaces acquired a comparatively more negative charge due to the chemical adsorption of the collector on the surface, chiefly on the Cu-sites. In the case of chalcopyrite, chalcocite and bornite, the zeta potential values significantly decrease with a higher proportion of DBD in the reagent mixture. Another important observation was that at alkaline pH values, the zeta potential values of copper minerals were less influenced by increases in the collector concentration; this is due to the prevalence of metal hydroxide, which diminishes the adsorption of collector on the copper minerals. Single mineral flotation results indicate a low recovery of all the three copper minerals at pH 4 while the best recovery was found in the pH range 6 to 10 and the recovery decreases marginally at very alkaline pH (>10). The recovery of chalcopyrite was consistently higher until pH 10, which decreases insignificantly beyond this pH value. However, the flotation response of gangue minerals (calcite and quartz) are negligible at natural pH. Based on data from the pure mineral studies, it can be stated that the copper sulphides can be floated in the natural pH region at a DBD concentration of 5 × 10−5 M. When xanthate and dithiophosphate mixtures were employed as collectors, the recovery was always higher than a single collector system. The highest recoveries of all the three copper minerals were observed for SIBX:DBD at 1:2, when the proportion of DBD was greater in the mixture. The FTIR spectral studies indicated that the interaction of dithiophosphate on copper sulphides was due to chemical reactions. The adsorption results obtained with chalcopyrite, chalcocite and bornite indicate the surface species formed to be Cu-(DBD). The spectra of copper sulphides Minerals 2019, 9, 146 21 of 23 conditioned with a mixed collector (1:1) show both the xanthate and dithiophosphate characteristic bands on the three copper minerals. However, strong peaks of the more selective collector DBD are observed on the chalcopyrite and chalcocite spectra, while strong peaks of SIBX are observed on the bornite spectrum. The quantitative adsorption results showed that DBD was adsorbed on the chalcopyrite and bornite surface at far lower concentrations (1 × 10−6 M), whereas the onset of adsorption on chalcocite was at a relatively higher concentration (1 × 10−5 M). All the three isotherms depict less than monolayer coverage; however, a higher concentration of collector was required for bornite to achieve saturation. The adsorption of the mixture of dithiophosphate and xanthate in the ratio 1:1 on the three copper sulphides indicated that DBD adsorbs preferentially on chalcopyrite and chalcocite surfaces, whereas SIBX adsorbs preferentially on the bornite. The results were substantiated by the area under the alkyl chain region and the intensity of characteristic peaks of DBD and SIBX in the FTIR spectra. Thus, it is clear from these results that the more selective collector adsorbs first on the copper mineral surface followed by the less selective collector. The bench scale flotation results of a Nussir ore sample with a DBD collector indicated that at natural pH and collector concentration 5 × 10−5 M, the optimum cumulative concentrate grade obtained was 19.6% copper with a recovery of 94.9%. The bench scale flotation with a mixed collector scheme showed the synergistic action of the collectors and the grade is improved for the various proportions of collector mixture at lower total collector concentration. A higher proportion of DBD in the collector blend resulted in a higher cumulative recovery (96%) and grade (24.7%) of copper. The pure mineral studies are consistent with the bench scale flotation results. In summary, the copper minerals in the Nussir deposit could be exploited with better metallurgical results using dithiophosphates and xanthate collector mixtures.

Author Contributions: P.D. carried out the experimental part, interpreted the data and wrote the manuscript. M.T. edited the manuscript, made the necessary English corrections and contributed in the final formatting. H.R.K. also contributed in the interpretation of data and reviewed the manuscript. H.R.K. also provided the resources and project administration and reviewed and edited the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge financial support from Norwegian University of Science and Technology (NTNU). Laurentius Tijhuis and Torill Sørløkk at the Department of Geoscience and Petroleum, NTNU, Norway must be thanked for characterising the samples by XRD and XRF. We would also acknowledge the Department of Matériaux et Environement, Université de Liège, Belgium for their guidance when using the Zeiss Mineralogic mining equipment. One of the authors, Priyanka Dhar, wishes to express her gratitude to Nussir ASA for providing the ore samples for this work. Conflicts of Interest: The authors declare no conflict of interest.

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