Beneficiation and Purification of Tungsten and Cassiterite Minerals

Home , Cassiterite, Scheelite, Wolframite

Article Beneﬁciation and Puriﬁcation of Tungsten and Cassiterite Minerals Using Pb–BHA Complexes Flotation and Centrifugal Separation

Tong Yue, Haisheng Han *, Yuehua Hu, Zhao Wei, Jianjun Wang, Li Wang, Wei Sun *, Yue Yang , Lei Sun, Ruohua Liu and Khoso Sultan Ahmed School of Mineral Processing and Bioengineering, Key Laboratory of Hunan Province for Clean and Efﬁcient Utilization of Strategic Calcium-Containing Mineral Resources, Central South University, Changsha 410083, China; [email protected] (T.Y.); [email protected] (Y.H.); [email protected] (Z.W.); [email protected] (J.W.); [email protected] (L.W.); [email protected] (Y.Y.); [email protected] (L.S.); [email protected] (R.L.); [email protected] (K.S.A.) * Correspondence: [email protected] (H.H.); [email protected] (W.S.); Tel: +86-0731-8883-0482 (H.H. & W.S.) Received: 3 October 2018; Accepted: 27 November 2018; Published: 4 December 2018

Abstract: Pb–BHA complexes have been shown to be selective for the separation of tungsten and cassiterite minerals from calcium minerals. These minerals could be enriched synchronously to some extent using Pb–BHA complexes flotation. However, it is difficult to further improve the quality and recovery of the scheelite, wolframite, and cassiterite concentrate due to their different behavior in flotation, such as flotation rate and sensitivities to depressants. Moreover, the super fine particles create some challenges for the cleaning flotation process. In this study, advanced gravity separators for super fine particles were introduced for the cleaning process based on the slight difference in the specific gravity of scheelite, wolframite, and cassiterite. The new process featured pre-enrichment using Pb–BHA flotation, and upgrading using gravity separation, taking into account both the similarities and differences in floatability and density of the different minerals. The grades of WO3 and Sn in the concentrate of the new process reached to 61% and 2.89%, respectively, and the recovery of Sn was significantly improved. In addition, gravity separation is highly efficient, cost effective, and chemical-free, which is environmentally friendly. This study has proven that physical separation can be used for the purification of flotation products and provide some solutions for separation problems of complex refractory ores, which has, up until now, been rarely reported in the literature and/or applied in mineral processing.

Keywords: tungsten minerals; cassiterite; ﬂotation; gravity separation; Pb–BHA complexes

1. Introduction Tungsten, which is a hard, refractory, and rare metal, is important in many commercial and industrial applications. For example, key alloys of tungsten are widely used in the production of incandescent light bulb ﬁlaments, X-ray tubes, electrodes in welding, radiation shielding, and superalloys. Tungsten’s high strength, hardness, and density make it ideal for military applications in penetrating projectiles. Tungsten compounds are also often used as catalysts in many industrial processes, including dehydrogenation, isomerization, polymerization hydrocracking in the chemical industry, hydrodesulfuration and hydrodenitriﬁcation of mineral oil products, and removal of nitrogen oxides from combustion power plant stack gases by selective catalytic reduction with ammonia. Tungsten naturally occurs in the earth crust exclusively combined with other elements in chemical compounds as minerals, and is usually extracted from those minerals. Wolframite ((Fe,Mn)WO4)

Minerals 2018, 8, 566; doi:10.3390/min8120566 www.mdpi.com/journal/minerals Minerals 2018, 8, 566 2 of 12

and scheelite (CaWO4) are the main ore minerals of tungsten deposits that occur in sufficient abundance to be of economic significance. Those tungsten ores are generally subjected to physical beneficiation techniques such as gravity, flotation, magnetic and electrostatic separation. In particular, to extract tungsten from fine mineral particles, especially for mineral particles with a size of less than 10 to 20 microns, flotation methods using fine air bubbles are often adopted, as flotation is a surface-chemistry based process that takes advantage of the different wettability on mineral particle surfaces [1–4]. Research regarding floatability and reagents has gained great progress over the past 20 years. However, with the exploitation and consumption of tungsten deposits of high quality, the need of low-grade complex ores is increased. Those low-grade tungsten deposits are usually in finely disseminated form with a complicated composition, containing high calcium gangue minerals and multi valuable minerals, which create many challenges for tungsten extraction. Firstly, separation of scheelite from other calcium-bearing minerals by flotation is challenging due to their very similar surface properties [5–8]. Most attempts have only achieved limited success or specific application (related to the type or the location of the ore). In addition, some fine cassiterite minerals occur in nature in association with tungsten minerals, while neither recovering cassiterite from the flotation tailings of tungsten minerals, nor the synchronous extraction of them, are practicable because of the great differences between tungsten minerals and cassiterite. Our previous studies [9,10] indicated that Pb–BHA complexes had good selectivity for the separation of scheelite from calcium minerals by regulating the Pb/BHA ratio and pH value with the recovery of tungsten minerals being significantly improved. Tian et al. [11] proved that cassiterite can be separated efficiently from cassiterite–calcite binary mixed minerals by using Pb–BHA complexes as the collector, and carboxymethyl cellulose as the depressant. Therefore, it can be speculated that tungsten minerals and cassiterite minerals could be enriched together by Pb–BHA complexes, which will significantly improve the recovery of fine tungsten and cassiterite minerals. However, problems appear when it comes to the purification of tungsten and cassiterite minerals, as they possess different surface wettability and flotation rates. Also, the depressants used for gangue minerals depression, such as organic colloids (dextrine, starch), quebracho tannin, sodium phosphates, sodium silicate solutions containing polyvalent cations (hydrosols), etc. [7,12–14], depress tungsten minerals and cassiterite recovery to some extent. However, it is hard to gain good further separation of the tungsten and cassiterite minerals by flotation with multiple attempts. Therefore, new cleaning processes to further improve the concentrate grade and recovery of the tungsten minerals is needed. Gravity separation [15,16] has been proven to have several advantages over the other mineral processing techniques due to its excellent properties such as high efficiency, low capital and operating costs, no additional chemicals, and consequently no environmental concerns. The extensive use of gravity circuits, and the need to recover super fine particles, have led to the development of specific devices to recover particles which are too fine to be recovered efficiently using conventional spirals etc. Some enhanced gravity concentrators are designed for gravity separation at fine particle size ranges, such as the hang and vibrate of cone concentrator (a Multi-Gravity Separator) [17] and Falcon concentrator [18]. They are very selective for separation of fine-sized particles (typically −75 to 10 µm) and have very high upgrading ratios (typically 20 to 1 µm). These gravity separators introduce centrifugal force, fluid force, shear force, and so on, except for the gravity, which provide possibilities for improving the cleaning process of the fine rougher concentrates. In this study, floatability of scheelite, wolframite, and cassiterite using Pb–BHA complexes and Al-SiO3 complexes were well studied. The mineral composition and size distribution of the raw ore and rougher concentrates were analyzed to account for the low indexes of the twice cleaning flotation. Both Falcon Concentrator and the hang and vibrate of cone concentrator were used for the upgrading of the rougher concentrate to evaluate the feasibility of the cleaning process using gravity separation. A novel process was developed to further improve the recovery and quality of the concentrate. Minerals 2018, 8, 566 3 of 12

2. Materials and Methods

2.1. Materials High-purity wolframite, and scheelite samples were obtained from Shizhuyuan Mine, Hunan, China. Pure cassiterite samples were obtained from Yunnan Province in China. X-ray diffraction (XRD) and X-ray fluorescence (XRF) data confirmed that the purity of the samples was higher than 97%. The fine size fraction of the samples (less than 74 µm) was used for the flotation experiments. Analytical grade BHA, lead nitrate, and terpineol (frother) were purchased from Guangfu, Tianjin, China. Deionized water was used throughout the flotation experiment. Table1 is the particle analysis of the raw ore, showing that most of the tungsten and cassiterite mineral particles fell into the fine particle size range. Specially, most of the valuable minerals distribute in −0.074 mm fraction, and super fine particles (−0.019 mm) account for more than 30%.

Table 1. Particle analysis of the raw ores.

Yield (%) Grade (%) Distribution (%) Particle Size/mm Individual WO3 Sn CaCO3 WO3 Sn CaCO3 +0.074 23.68 0.27 0.08 12.80 17.98 12.90 25.82 −0.074~+0.037 18.54 0.31 0.11 13.41 16.17 13.89 21.18 −0.037~+0.019 27.76 0.40 0.16 11.58 31.23 30.26 27.38 −0.019 30.02 0.41 0.21 10.11 34.62 42.94 25.85 Total 100 0.36 0.15 11.74 100.00 100.00 100.00

2.2. Flotation Tests of Pure Minerals Pure minerals flotation tests were carried out in an XFG flotation machine with a 40 mL plexiglass cell. The procedure of the flotation experiments has been described in our previous work [10]. In brief, 2.0 g of pure minerals, including scheelite, wolframite, cassiterite, fluorite, and calcite, was dispersed into 30 mL DI water at an impeller speed of 1900 rpm. This was followed by the adjustment of the pH of the suspensions in the range of 3 to 12 using 0.1 mol/L HCl or NaOH solutions. After adding the Pb–BHA complexes, which was prepared by mixing the PbNO3 and BHA solutions with the molar ratio 5:3 (the concentration of Pb2+ was 2.5 × 10−4 mol/L and the concentration of BHA was 1.5 × 10−4 mol/L), as the collector and 12.5 µL/L of terpineol as the frother, the suspensions were homogenized for 3 min. The foams were collected for 3 min and then filtered and dried at 60 ◦C for 12 h. In the kinetic flotation, the collected time ranged from 0 to 300 s and the pH of the suspension was maintained at 9.5.

2.3. Gravity Separation A Falcon centrifugal concentrator (Model SB40) and hang and vibrate of cone concentrator were used to upgrade the rougher concentrates of scheelite, wolframite, and cassiterite. The optimum parameters for the gravity separation are obtained by single factor experiments, which are listed in Table2.

Table 2. The optimized parameters for the gravity separators.

Gravity Separator Parameters Feeding slurry Rotation bowl Fluidized water ﬂow Falcon centrifugal concentrator concentration (%) speed (rpm) rate (L/min) 32 800 5.0 Feeding slurry Rotation speed of Vibrational frequency Hang-vibrate of cone concentrator concentration (%) drum (rpm) (HZ) 32 2.0 32 Minerals 2018, 8, 566 4 of 12 Minerals 2018, 8, x FOR PEER REVIEW 4 of 12

ForFor Falcon Falcon centrifugal centrifugal concentrator concentrator [15,18], [15,18], fluidization fluidization water water was was introduced introduced into into the the bowl bowl (concentrate(concentrate cone) cone) through through a a series series of of fluidization fluidization holes holes at at the the top top of of its its wall. wall. The The feed feed slurry slurry was was thenthen introduced introduced through through the the stationary stationary feed feed tube tube at at the the bowl bowl center center and and into into the the concentrate concentrate cone. cone. WhenWhen the the slurry reached thethe bottombottom ofof the the cone, cone, it it was was forced forced outward outward and and up up the the cone cone wall wall under under the theinfluence influence of the of centrifugalthe centrifugal force. force. During During the separation the separation process, process, tailings tailings flowed flowed out the out top ofthe the top cone of theinto cone the tailingsinto the launder.tailings launder. After the After separation the separation was finished, was finished, the concentrates the concentrates were flushed were fromflushed the fromcone the into cone the concentrateinto the concentrate launder. launder. The tailing The and tailing concentrate and concentrate were settled, were collected, settled, collected, dried, weighed, dried, weighed,and chemically and chemically analyzed. analyzed. HangHang and and vibrate vibrate of of cone cone concentrator concentrator is a multi-gravity is a multi-gravity separator, separator, which combines which combines the centrifugal the centrifugalmotion of motion an angled of an rotating angled drum rotating (though drum not(though at such not aat high such speed) a high ofspeed) a Kelsey of a Kelsey jig or Falconjig or FalconConcentrator, Concentrator, with the with oscillating the oscillating motion ofmotion a shaking of a table,shaking to providetable, to an provide enhanced an gravityenhanced separation, gravity separation,particularly particularly suitable for suitable fine particles for fine separation. particles separation. The principle The ofprinciple the separation of the separation is based uponis based the uponabove-mentioned the above-mentioned forces that forces act on thatthe particles act on the in a particles slurry stream in a slurrybeing fed stream and are being distributed fed and onto are distributedthe inside ofonto the the drum’s inside surface. of the Withdrum’s the surface. aid of the With scrapers the aid and of washthe scrapers water, theand light wash fine water, particles the lightmigrated fine particles up the drum migrated to discharge up the drum over to the discharge drum’s topover lip, the while drum’s the top heavy lip, while large particlesthe heavy flowed large particlesslowly and flowed moved slowly to the and concentrate moved to launderthe concentrate with the launder rotation with of the the drum. rotation of the drum.

2.4.2.4. Pilot Pilot Scale Scale Tests Tests TheThe real real ore ore flotation flotation tests tests were were performed performed in in the the dressing dressing plant plant of of Shizhuyuan Shizhuyuan Mine Mine Group. Group. DuringDuring the the pilot pilot scale scale tests, tests, 200 200 tons tons of of real real ore ore was was treated treated per per day. day. The The mixed mixed solution solution of of BHA BHA (400(400 g/t) and PbNO PbNO33 (500 (500 g/t) g/t) was was used used as as the the collector, collector, and andterpineol terpineol (10 g/t) (10 was g/t) usedwas asused the frother.as the frother.The flotation The flotation flowsheet flowsheet was shown was inshown Figure in1 .Figure 1.

FigureFigure 1. 1. Flotation Flotation flowsheet flowsheet of tungsten of tungsten minerals minerals using Pb–BHA using complexes Pb–BHA incomplexes Shizhuyuan in dressing Shizhuyuan plant. 2.5. Analyticaldressing plant. Techniques

2.5. AnalyticalXRD (SIMENS Techniques D500, Bruker, Switzerland) analysis WAS used for semi-quantitative characterization of different crystalline phases. The composition of the samples was analyzed using XRF (AxiosmAX, XRD (SIMENS D500, Bruker, Switzerland) analysis WAS used for semi-quantitative Panalytical B.V., Almelo, The Netherlands) and AAS (ICE 3500, Thermo Fisher Scientific, Waltham, MA, characterization of different crystalline phases. The composition of the samples was analyzed using USA). A laser particle size analyzer (Malvern Instruments Ltd., Malvern, UK) was used for particle size XRF (AxiosmAX, Panalytical B.V., Almelo, The Netherlands) and AAS (ICE 3500, Thermo Fisher measurement of the raw ore and rougher concentrate. Scientific, Waltham, MA, USA). A laser particle size analyzer (Malvern Instruments Ltd., Malvern, UK) was used for particle size measurement of the raw ore and rougher concentrate.

Minerals 2018, 8, x FOR PEER REVIEW 5 of 12

Minerals 2018, 8, 566 5 of 12 3. Results and Discussions

3.1.3. Results Flotation and Tests Discussions in Lab Figure 2 shows the difference of floatability among scheelite, wolframite, cassiterite, calcite, and 3.1. Flotation Tests in Lab fluorite versus pH. The results indicate that scheelite, wolframite, cassiterite, and calcite could be well Figurecollected2 shows by Pb–BHA the difference complexes of floatabilityand the former among three scheelite, minerals wolframite, show a synchronous cassiterite, flotation calcite, behaviorand fluorite with versus pH change, pH. The while results fluorite indicate does that not. scheelite, Our previous wolframite, work cassiterite,indicated that and calcitethe colloidal could Pb–BHAbe well collected particles by werePb–BHA positively complexes charged and the in former weakly three alkaline minerals conditions, show a synchronous while the flotation surface potentialsbehavior with of scheelite, pH change, wolframite, while fluorite and does cassiterite not. Our were previous negative work [19]. indicated Therefore, that the the Pb–BHAcolloidal complexesPb–BHA particles could be were adsorbed positively on chargedthe mineral in weakly surfaces alkaline by electrostatic conditions, force, while leading the surface to the potentials similar variationof scheelite, trend wolframite, in the recovery and cassiterite of scheelite, were wolframite, negative [18 and]. Therefore, cassiterite the minerals Pb–BHA with complexes increasing could the pHbe adsorbedof the flotation on the mineralsystem. In surfaces fact, the by floatability electrostatic of force, scheelite, leading wolframite, to the similar cassiterite, variation and trend calcite in werethe recovery different of from scheelite, each wolframite,other. The recovery and cassiterite of scheelite minerals was with much increasing higher than the pHthat ofof the wolframite flotation andsystem. cassiterite, In fact, and the floatability the effective of pH scheelite, range wolframite, was also larger. cassiterite, Scheelite, and wolframite, calcite were and different cassiterite from couldeach other. be collected The recovery concurrently of scheelite using was Pb–BHA much highercomplexes than at that pH of 9 wolframiteto 10. The synchronous and cassiterite, collecting and the ofeffective those pHminerals range wasenhanced also larger. the recovery Scheelite, of wolframite, tungsten minerals and cassiterite and cassiterite could be collected from a polymetallic concurrently deposit,using Pb–BHA such as complexesthe Shizhuyuan at pH Mine. 9 to 10. At The this synchronous condition, the collecting calcite was of those also, mineralsinevitably, enhanced collected the to somerecovery extent, of tungsten creating minerals some difficulties and cassiterite for from the upgrading a polymetallic of the deposit, concentrate. such as the Figure Shizhuyuan 3 shows Mine. the differenceAt this condition, in the the flotation calcite ratewas also, of scheelite, inevitably, calcite, collected wolframite, to some extent, and cassiterite creating some using difficulties Pb–BHA complexes.for the upgrading The flotation of the concentrate.rate of wolframite Figure and3 shows cassiterite the difference were lower in the than flotation that of ratescheelite. of scheelite, It was difficultcalcite, wolframite, to get the and maximum cassiterite recovery using Pb–BHA and best complexes. grade of The different flotation mineralsrate of wolframite by flotation and simultaneously.cassiterite were lower than that of scheelite. It was difficult to get the maximum recovery and best gradeDepressants of different minerals for calcite by flotation have been simultaneously. widely reported in previous studies [12,19–21], and aluminumDepressants silicate for solutions calcite have (Al-SiO been3) widelyhave been reported proven in previousto be effective studies for [12 scheelite,18–20], andflotation aluminum with Pb–BHAsilicate solutions complexes (Al-SiO as the3 )collector have been [9,22,23]. proven Al-SiO to be3 effectivewas used foras the scheelite depressant flotation for the with upgrading Pb–BHA ofcomplexes the rougher as the concentrate collector [ 9 in,21 , this22]. research. Al-SiO3 was The used effect as of the Al-SiO depressant3 complexes for the on upgrading floatability of the of scheelite,rougher concentratewolframite, incassiterite, this research. and calcite The effectis shown of Al-SiO in Figure3 complexes 4. It can be on seen floatability that the ofrecovery scheelite, of allwolframite, the minerals cassiterite, was depressed and calcite at different is shown degrees, in Figure especially4. It can beat seenhigh thatAl-SiO the3 dosage. recovery Obviously, of all the Al-SiOminerals3 was was not depressed suitable atto different improve degrees, the upgrading especially of the at high concentrate. Al-SiO3 dosage.The inhibition Obviously, effect Al-SiO of the3 depressantwas not suitable on upgrading to improve is the a upgradingcommon issue of the in concentrate. flotation. As The is inhibition well known, effect floatability of the depressant of the mineralson upgrading greatly is adepends common on issue the surface in flotation. properties As is wellof the known, minerals, floatability such as ofthe the crystal minerals anisotropy, greatly whichdepends can on be the greatly surface propertiesinfluenced of by the the minerals, depressant such as[24–27]. the crystal Take anisotropy, scheelite whichas an canexample; be greatly the wettabilityinfluenced byof the depressant(001) plane [of23 –scheelite26]. Take decreased scheelite asslightly, an example; whilst the increasing wettability the of dosage the (001) of Al-SiO plane of3, whilescheelite the decreased wettability slightly, of the whilst (112) increasing plane decreased the dosage significantly. of Al-SiO3 , If while the (112) the wettability plane are of the the main (112) scheeliteplane decreased cleavage significantly. planes, the If the recovery (112) plane of scheelite are the main will bescheelite significantly cleavage affected planes, the by recovery the use of Al-SiOscheelite3 in will flotation be significantly [9]. affected by the use of Al-SiO3 in flotation [9].

100

Scheelite 80 Wolframite Cassiterite Calcite 60 Fluorite

40 Recovery /%

0 4 6 8 10 12 pH Figure 2. The ﬂoatabilityfloatability of of scheelite, scheelite, wolframite, wolframite, cassiterite, cassiterite, ﬂuorite, fluorite, and and calcite calcite minerals minerals with with Pb–BHA Pb– −4 −4 BHAcomplexes complexes as the as collector the collector (CBHA (C=BHA 1.5 =× 1.510 × 10mol/L,−4 mol/L, CPb C=Pb 2.5= 2.5× ×10 10−4 mol/L,mol/L, C terpineol = =12.5 12.5 μL/L).µL/L).

Minerals 2018, 8, x 566 FOR PEER REVIEW 66 of of 12 Minerals 2018, 8, x FOR PEER REVIEW 6 of 12 100 100

80 80

60 60

40 Recovery /% Recovery 40 Recovery /% Recovery Scheelite ScheeliteCalcite 20 Calcite 20 Wolframite WolframiteCassiterite Cassiterite 0 0 0 30 60 90 120 150 180 210 240 270 300 0 30 60 90 120 150 180 210 240 270 300 Time /s Time /s Figure 3. Difference in the flotation rate of scheelite, calcite, wolframite, and cassiterite using Pb– FigureFigure 3. Difference in in the the ﬂotation flotation rate rate of of scheelite, scheelite, calcite, calcite, wolframite, wolframite, and and cassiterite cassiterite using using Pb–BHA Pb– BHA complexes (CBHA = 1.5 × 10−4 mol/L, and CPb = 2.5 × 10−4 mol/L, pH = 9.5). complexes (C = 1.5 × 10−4 mol/L, and C = 2.5 × 10−4 mol/L, pH = 9.5). BHA complexesBHA (CBHA = 1.5 × 10−4 mol/L, and PbCPb = 2.5 × 10−4 mol/L, pH = 9.5). 100 100 Scheelite 90 Scheelite 90 Wolframite 80 WolframiteCalcite 80 CalciteCassiterite 70 CassiteriteFluorite 70 60 Fluorite 60 50 50 40

Recovery /% Recovery 40

Recovery /% Recovery 30 30 20 20 10 10 0 0 0 50 100 150 200 0 50Al-SiO complexes 100 mg/L 150 200 3 Al-SiO3 complexes mg/L Figure 4. Figure 4.Effects Effects of ofAl-SiO Al-SiO3 complexes3 complexes onthe on flotation the flotation of different of different minerals mineralsusing Pb–BHA using complexes Pb–BHA −4 −4 Figureas the collector 4. Effects (C of Al-SiO= 1.5 ×3 10 complexesmol/L, −4 on and the C flotation= 2.5 × 10 of differentmol/L,−4 pH minerals = 9.5). using Pb–BHA complexes as the collectorBHA (CBHA = 1.5 × 10 mol/L,Pb and CPb = 2.5 × 10 mol/L, pH = 9.5). complexes as the collector (CBHA = 1.5 × 10−4 mol/L, and CPb = 2.5 × 10−4 mol/L, pH = 9.5). 3.2. Flotation in Pilot Scale Tests 3.2. Flotation in Pilot Scale Tests 3.2. FlotationTable3 showsin Pilot the Scale flotation Tests process and the recovery of scheelite, wolframite, and cassiterite in Table 3 shows the flotation process and the recovery of scheelite, wolframite, and cassiterite in different flotation units of the Figure1, respectively. Scheelite, wolframite, and cassiterite can be well differentTable flotation 3 shows unitsthe flotation of the Figureprocess 1, and respectively. the recovery Scheelite, of scheelite, wolframite, wolframite, and cassiteriteand cassiterite can bein collected in the roughing flotation with the recovery rate at 81.81%, 78.48%, and 68.86%. The recovery differentwell collected flotation in the units roughing of the Figureflotation 1, withrespectively. the recovery Scheelite, rate at wolframite, 81.81%, 78.48%, and cassiterite and 68.86%. can The be rate decreased significantly in the cleaning flotation due to the use of depressants, especially for wellrecovery collected rate in decreased the roughing significantly flotation inwith the the cleaning recovery flotation rate at 81.81%, due to 78.48%, the use and of depressants,68.86%. The wolframite and cassiterite, which is consistent with the previous results. The cassiterite recovery of recoveryespecially rate for decreased wolframite significantly and cassiterite, in the which cleaning is consistent flotation duewith to the the previous use of depressants, results. The cleaning flotation I and II were only 71.34% and 58.82%. No improvement was found for the grade of especiallycassiterite recovery for wolframite of cleaning and flotation cassiterite, I and which II were is consistentonly 71.34% with and the 58.82%. previous No improvement results. The the cassiterite, indicating that most of the cassiterite minerals were depressed in the cleaning process. cassiteritewas found recovery for the gradeof cleaning of the flotation cassiterite, I and indicating II were only that 71.34% most of and the 58.82%. cassiterite No mineralsimprovement were Overall, synchronous flotation of tungsten minerals and cassiterite seems to be not efficient in the wasdepressed found infor the the cleaninggrade of process.the cassiterite, Overall, indicating synchronous that most flotation of the of cassiterite tungsten minerals minerals were and cleaning process. depressedcassiterite seems in the to cleaningbe not efficient process. in the Overall, cleaning synchronous process. flotation of tungsten minerals and cassiterite seems to be not efficient in the cleaning process.

Minerals 2018, 8, 566 7 of 12

Minerals 2018, 8, x FOR PEER REVIEW 7 of 12 Table 3. Grade and recovery of scheelite, wolframite, and cassiterite in different flotation units. Table 3. Grade and recovery of scheelite, wolframite, and cassiterite in different flotation units. Grade in each Flotation Unit/% Recovery in Each Flotation Unit/% Grade in each Flotation Unit/% Recovery in Each Flotation Unit/% Scheelite Wolframite Cassiterite Scheelite Wolframite Cassiterite Scheelite Wolframite Cassiterite Scheelite Wolframite Cassiterite Roughing flotation 12.12 6.18 0.56 81.81 78.48 68.86 Roughing flotation 12.12 6.18 0.56 81.81 78.48 68.86 Cleaning flotation I 32.86 13.83 1.24 91.23 86.38 71.34 CleaningCleaning flotation flotation I II 32.86 41.20 13.83 16.86 1.241.02 88.69 91.23 81.76 86.38 58.82 71.34 CleaningScavenging flotation flotation II I 41.20 0.43 16.86 0.31 1.020.27 37.66 88.69 41,38 81.76 45.22 58.82 ScavengingScavenging flotation flotation I II 0.43 0.11 0.31 0.12 0.270.19 27.72 37.66 32.16 41,38 39.34 45.22 ScavengingScavenging flotation flotation II III 0.11 0.08 0.12 0.10 0.190.11 12.5 27.72 16.44 32.16 18.98 39.34 Scavenging flotation III 0.08 0.10 0.11 12.5 16.44 18.98 3.3. Mineral Composition and Characters of the Rougher Concentrate 3.3. Mineral Composition and Characters of the Rougher Concentrate Table4 and Figure5 show the mineral composition and phases of the rougher concentrate. Most of Table 4 and Figure 5 show the mineral composition and phases of the rougher concentrate. the scheelite, wolframite, and cassiterite minerals were recovered with a high enrichment ratio in the Most of the scheelite, wolframite, and cassiterite minerals were recovered with a high enrichment roughing flotation. The selectivity of Pb–BHA complexes on calcium minerals was confirmed by the ratio in the roughing flotation. The selectivity of Pb–BHA complexes on calcium minerals was lower enrichment efficiency of calcite and fluorite. Quartz, the main gangue mineral in the rougher confirmed by the lower enrichment efficiency of calcite and fluorite. Quartz, the main gangue concentrate, was not a concern here as it can be separated easily. XRD results show that quartz, chlorite, mineral in the rougher concentrate, was not a concern here as it can be separated easily. XRD results muscovite, feldspar, and amphibole also exist in the rougher concentrate, which could be due to the show that quartz, chlorite, muscovite, feldspar, and amphibole also exist in the rougher concentrate, entrainment of super fine particles in the foam. The existence of those minerals made it easier for the which could be due to the entrainment of super fine particles in the foam. The existence of those grinding process to get muddy because of their lower hardness [27–29], posing great challenges for the minerals made it easier for the grinding process to get muddy because of their lower hardness [28– separation of them from the valuable minerals by flotation. As shown in Figure6,D 50 of the rougher 30], posing great challenges for the separation of them from the valuable minerals by flotation. As concentrate was 0.021 mm, which falls into the fine particle size range, causing problems for further shown in Figure 6, D50 of the rougher concentrate was 0.021 mm, which falls into the fine particle size improvement of the concentrate quality. range, causing problems for further improvement of the concentrate quality. Table 4. Multi-elementary analysis results of the rougher concentrate/%. Table 4. Multi-elementary analysis results of the rougher concentrate/%. Elements WO3 Sn Fe Mn Ti Zn Zr Pb Elements WO3 Sn Fe Mn Ti Zn Zr Pb Content (%) 10.58 0.54 7.94 0.99 0.39 0.18 0.02 0.17 Content (%) 10.58 0.54 7.94 0.99 0.39 0.18 0.02 0.17 Elements Mo Ca(CaF ) Ca(CaCO ) SiO Bi As Cr Else Elements Mo Ca(CaF2)2 Ca(CaCO3 3) SiO2 2 Bi As Cr Else ContentContent (%) (%) 0.13 0.13 14.68 14.68 18.54 18.54 29.27 29.27 0.09 0.09 0.05 0.05 0.04 0.0416.39 16.39

FigureFigure 5. X-ray 5. X-ray diffraction diffraction (XRD) (XRD) analysis analysis of the of the raw raw ores ores (a ()a )and and the the rougher rougher concentrateconcentrate (b ().b).

Minerals 2018, 8, 566 8 of 12 Minerals 2018, 8, x FOR PEER REVIEW 8 of 12

10 Raw ore 9 Rough concentrate 8

4 Volume /%

0 0.1 1 10 100 1000 Particle size /mm Figure 6. TheThe particle particle size size distribution of the raw ores and the rougher concentrate. 3.4. Flotation and Gravity Separation Combination in Pilot Scale Tests 3.4. Flotation and Gravity Separation Combination in Pilot Scale Tests Roughing flotation using Pb–BHA complexes has been proven to be effective in the recovery of Roughing flotation using Pb–BHA complexes has been proven to be effective in the recovery of scheelite, wolframite, and cassiterite minerals, which takes full advantage of their similar floatability. It scheelite, wolframite, and cassiterite minerals, which takes full advantage of their similar should be noted that scheelite, wolframite, and cassiterite are heavy minerals with a density of 6.1, 7.2 to floatability. It should be noted that scheelite, wolframite, and cassiterite are heavy minerals with a 7.5, and 6.8 to 7.1 g/cm3, respectively, which are much heavier than that of most gangue minerals, such density of 6.1, 7.2 to 7.5, and 6.8 to 7.1 g/cm3, respectively, which are much heavier than that of most as quartz, calcite, and fluorite. With great progress in new gravity separation equipment development gangue minerals, such as quartz, calcite, and fluorite. With great progress in new gravity separation over the last 20 years, gravity separation is always a preferred technique [15,30]. Some advanced gravity equipment development over the last 20 years, gravity separation is always a preferred technique concentrators have been developed for the gravity separation of fine mineral particles. The combination [15,31]. Some advanced gravity concentrators have been developed for the gravity separation of fine of flotation and gravity separation, which depends on the surface chemical and physical properties of mineral particles. The combination of flotation and gravity separation, which depends on the surface the minerals, is considered to be effective in further improving the recovery of the minerals. chemical and physical properties of the minerals, is considered to be effective in further improving Flotation and gravity separation using falcon concentrator or hang and vibrate of cone the recovery of the minerals. concentrator were conducted for the cleaning process. It can be seen in Table5 that cleaning process Flotation and gravity separation using falcon concentrator or hang and vibrate of cone using flotation shows the highest recovery of tungsten minerals, while the grade of the concentrate concentrator were conducted for the cleaning process. It can be seen in Table 5 that cleaning process was about 55%, which was lower than that of gravity separation. The recovery of cassiterite was only using flotation shows the highest recovery of tungsten minerals, while the grade of the concentrate at 29.18% by flotation, indicating a great loss of cassiterite in the tailings. Gravity separation, especially was about 55%, which was lower than that of gravity separation. The recovery of cassiterite was only when using hang and vibrate of cone concentrator, was found to be effective for the recovery of both at 29.18% by flotation, indicating a great loss of cassiterite in the tailings. Gravity separation, tungsten minerals and cassiterite, with the quality of the concentrate significantly improved. especially when using hang and vibrate of cone concentrator, was found to be effective for the recoveryTable of 5. bothResults tungsten of cleaning minerals process and using cassiterite, flotation andwith gravity the quality separation of the by Falconconcentrate concentrator significantly or improved.hang and vibrate of cone concentrator.

WO CaCO CaF Sn Table 5. Results ofProduct cleaning process Yield using flotation3 and gravity3 separation by Falcon2 concentrator or hang and vibrate of cone concentrator.Grade Recovery Grade Recovery Grade Recovery Grade Recovery Concentrate 16.18 55.64 87.83 1.98 1.79 6.99 8.63 1.01 29.18 Flotation Tailing 83.82WO 1.493 12.17 20.99 CaCO3 98.21 14.28 CaF2 91.37 0.47 Sn 70.82 Product Yield 100.00 10.25 100.00 17.91 100.00 13.10 100.00 0.56 100.00 Grade Recovery Grade Recovery Grade Recovery Grade Recovery Concentrate 12.86 60.88 76.38 0.91 0.65 5.82 5.71 2.62 60.17 Falcon ConcentratorConcentrateTailing16.18 87.14 55.64 2.78 87.83 23.62 1.98 20.42 1.79 99.35 6.99 14.17 8.6394.29 0.26 1.01 39.83 29.18 Flotation Tailing 83.82 100.00 1.49 10.25 12.17 100.00 20.99 17.91 98.21 100.00 14.28 13.10 91.37 100.00 0.56 0.47 100.00 70.82 Concentrate100.00 13.9210.25 61.35100.00 83.1517.910.87 100.00 0.68 13.10 4.81 100.00 5.13 2.890.56 74.50100.00 Hang-vibrate of Tailing 86.08 2.01 16.85 20.70 99.32 14.38 94.87 0.16 25.50 cone concentratorConcentrate 12.86 60.88 76.38 0.91 0.65 5.82 5.71 2.62 60.17 Falcon 100.00 10.27 100.00 17.94 100.00 13.05 100.00 0.54 100.00 Tailing 87.14 2.78 23.62 20.42 99.35 14.17 94.29 0.26 39.83 Concentrator 100.00 10.25 100.00 17.91 100.00 13.10 100.00 0.56 100.00 It shouldConcentrate be noted13.92 that gravity 61.35 separation 83.15 was 0.87 not shown 0.68 to 4.81 be efficient 5.13 for the 2.89 separation 74.50 Hang-vibrate of of ultrafine particlesTailing with 86.08 a size 2.01 of less 16.85 than 10 µ 20.70m. As 99.32 shown in 14.38 Table 5, 94.87 the recovery 0.16 of WO25.50 cone concentrator 3 in the gravity separations 100.00 was 10.27lower than 100.00 that of 17.94 the cleaning 100.00 flotation. 13.05 100.00 As a result 0.54 of that, 100.00 the collecting ability of gravity separations for the ultrafine mineral particles (<10 µm) was worse than It should be noted that gravity separation was not shown to be efficient for the separation of ultrafine particles with a size of less than 10 μm. As shown in Table 5, the recovery of WO3 in the

Minerals 2018, 8, x FOR PEER REVIEW 9 of 12 Minerals 2018, 8, 566 9 of 12 gravity separations was lower than that of the cleaning flotation. As a result of that, the collecting ability of gravity separations for the ultrafine mineral particles (<10 μm) was worse than that of that of flotation. Although, the optimum particle size in flotation ranges from 10 to 100 µm [31,32]. flotation. Although, the optimum particle size in flotation ranges from 10 to 100 μm [32,33]. Ultrafine Ultrafine particles tend to be caught in streamlines created by the rising bubbles instead of attachment particles tend to be caught in streamlines created by the rising bubbles instead of attachment with with the bubbles. This problem would be solved if some coarse particles could serve as the carrier for the bubbles. This problem would be solved if some coarse particles could serve as the carrier for the the fine particles (defined as carrier flotation) or the fine particles could aggregate with each other to fine particles (defined as carrier flotation) or the fine particles could aggregate with each other to form a larger particle, (defined as flocculation flotation). It has been reported that hydrophobic fine form a larger particle, (defined as flocculation flotation). It has been reported that hydrophobic fine particles could aggregate with each other or a large particle to form larger particles, thereby improving particles could aggregate with each other or a large particle to form larger particles, thereby the flotation recovery [33]. The mechanisms of shear-flocculation and carrier flotation are governed improving the flotation recovery [34]. The mechanisms of shear-flocculation and carrier flotation are by physical, chemical, and geometrical variables, which have been investigated comprehensively governed by physical, chemical, and geometrical variables, which have been investigated in Subrahmanyam’s review paper [34]. After research for almost three decades, it has been widely comprehensively in Subrahmanyam’s review paper [35]. After research for almost three decades, it accepted that carrier flotation and shear-flocculation can be used for pre-treatment of fine particles in has been widely accepted that carrier flotation and shear-flocculation can be used for pre-treatment flotation. Numerous studies have reported improvements in the recovery of fine particles by using this of fine particles in flotation. Numerous studies have reported improvements in the recovery of fine technique, with the recovery rate increased from 20% to 50% compared to the conventional flotation. particles by using this technique, with the recovery rate increased from 20% to 50% compared to the Feeding the tailings of gravity separation and fine particles to the roughing flotation for a further conventional flotation. Feeding the tailings of gravity separation and fine particles to the roughing recoveryflotation isfor thus a further proposed. recovery The effect is thus of the proposed. stirring The rate oneffect the of shear the flocculation stirring rate was on studied the shear in the lab and the results are shown in Figure7. Initially, the recovery of tungsten minerals increased flocculation was studied in the lab and the results are shown in Figure 7. Initially, the recovery of significantlytungsten minerals when increased increasing significantly of the stirring when rate. increasing Where a downwardof the stirring trend, rate. followed Where a by downward optimum tungsten minerals recovery was observed, this was more noticeable for coarse particles. Aggregation trend, followed by optimum tungsten minerals recovery was observed, this was more noticeable for ofcoarse fine particles. particles promotesAggregation the of flotation fine particles of the gravitypromotes tailings, the flotation with the of optimumthe gravity recovery tailings, iswith said the to beoptimum around recovery 78%. When is said the to gravity be around tailings 78%. were When fed the back gravity to the tailings raw ores were containing fed back coarse to the particles,raw ores thecontaining recovery coarse was improved particles, dramatically. the recovery Both was carrier improved flotation dramatically. and flocculation Both floatation carrier flotation contributed and toflocculation the recovery floatation of fine particles.contributed Moreover, to the recovery the increased of fine tungsten particles. recovery Moreover, with the the increased increased tungsten stirring mayrecovery also with result the from increased increased stirring probability may also of result collision from between increased fine probability particles and of collision bubbles between and the increasedfine particles entrainment. and bubbles As shown and the in Figure increased7, the improvedentrainment. concentrate As shown grade in with Figure the 7, increasing the improved of the stirringconcentrate indicates grade that with it isthe not increasing due to the of increased the stirring entrainment indicates but that from it carrieris not due flotation, to the flocculation increased floatationentrainment and but increased from carrier ore collision flotation, between flocculation bubbles floatation and particles. and increased However, ore the collision decreasing between of the gradebubbles may and from particles. an increasing However, entrainment. the decreasing of the grade may from an increasing entrainment.

Figure 7. Flocculation and carrier flotation ﬂotation of fine ﬁne particles.

A new closed-circuit process, pre-enrichment using flotationflotation with Pb–BHA complexes, upgrading using gravity separation and feeding the tailing of gravity separation back to rougher flotation,flotation, was was developed developed to furtherto further improve improve the recovery the recovery and grade and of grade tungsten of tungsten minerals and minerals cassiterite. and Acassiterite. pilot-scale A experimentpilot-scale wasexperiment conducted was with conducted Figure8 withillustrating Figure the8 illustrating flotation process. the flotation Table5 process. shows thatTable a 5 concentrate shows that with a concentrate 61% WO3 withand 2.89%61% WO Sn3can and be 2.89% obtained, Sn can and be theobtained, recovery and of the cassiterite recovery was of significantlycassiterite was improved. significantly This concentrateimproved. This meets concentrate the requirements meets forthe therequirements following metallurgy for the following process. Cassiteritemetallurgy inprocess. the concentrate Cassiterite could in the be concentrate further enriched could and be recoveredfurther enriched either byand mineral recovered processing either by or metallurgymineral processing process. Moreover, or metallurgy gravity process. separation Moreover, is highly gravity efficient, separation cost effective, is highly and chemical-free.efficient, cost effective, and chemical-free.

Minerals 2018, 8, 566 10 of 12 Minerals 2018, 8, x FOR PEER REVIEW 10 of 12

Figure 8. New flotation flowsheet of tungsten minerals in Shizhuyuan dressing plant. Figure 8. New flotation flowsheet of tungsten minerals in Shizhuyuan dressing plant. 4. Conclusions 4. Conclusions For the flotation of the polymetallic minerals in the Shizhuyuan Mine Group, Pb–BHA complexes can effectivelyFor the flotation collect scheelite, of the polymetallic wolframite, minerals and cassiterite in the in Shizhuyuan the rougher Mine flotation Group, synchronously. Pb–BHA Butcomplexes in was hard can to effectively increase the collect grade scheelite, of these mineralswolframite, in the and concentrate cassiterite to in the the expectations rougher flotation following twice-cleaningsynchronously. flotation. But in was The hard XRD to and increase size distribution the grade resultsof these revealed minerals that in thethe rougherconcentrate concentration to the expectations following twice-cleaning flotation. The XRD and size distribution results revealed that included the fine gangue particles, which was easier to be entrained into the concentrates in the the rougher concentration included the fine gangue particles, which was easier to be entrained into cleaning flotation. The advanced gravity separation equipment was applied to deal with the rougher the concentrates in the cleaning flotation. The advanced gravity separation equipment was applied concentrates. Comparing with the grade and recovery of WO3 (55.63% and 87.83%) and Sn (1.01% and to deal with the rougher concentrates. Comparing with the grade and recovery of WO3 (55.63% and 29.18%) in the cleaning flotation, the grade and recovery were 60.88% and 76.38% for WO and 2.62% 87.83%) and Sn (1.01% and 29.18%) in the cleaning flotation, the grade and recovery were3 60.88% and 60.17% for Sn using the falcon concentrator, while the results were 61.35% and 83.15% for WO and 76.38% for WO3 and 2.62% and 60.17% for Sn using the falcon concentrator, while the results 3 and 2.89% and 74.50% for Sn using the hang and vibrate of cone concentrator. Theses process indexes were 61.35% and 83.15% for WO3 and 2.89% and 74.50% for Sn using the hang and vibrate of cone indicatedconcentrator. that theTheses advanced process gravity indexes separation indicated dealingthat the with advanced the rougher gravity concentrate separation coulddealing effectively with enhancethe rougher the gradeconcentrate of WO could3 and effectively Sn and the enhance recovery the ofgrade Sn, of with WO a3 slightand Sn decrease and the recovery in the recovery of Sn, of WOwith3. Ina slight addition, decrease the results in the ofrecovery hang and of WO vibrate3. In ofaddition, cone concentrator the results of are hang comprehensively and vibrate of superiorcone toconcentrator that of Falcon are concentrator.comprehensively Therefore, superior theto that advanced of Falcon gravity concentrator. separation Therefore, is a high-efficiency the advanced and low-costgravity method separation to displace is a high-efficiency the traditional and cleaning low-cost flotation method to to deal displace with the the complexed traditional polymetallic cleaning rougherflotation concentrates. to deal with the complexed polymetallic rougher concentrates.

ReferenceAuthor Contributions: Conceptualization, H.H. and W.S.; Methodology, T.Y.; Software, T.Y.; Validation, H.H., W.Z. and Y.H.; Formal Analysis, T.Y.; Investigation, J.W.; Resources, Y.H.; Data Curation, L.W. and K.S.A; Writing–Original Draft Preparation, T.Y.; Writing–Review & Editing, Y.Y.; Visualization, L.S.; Supervision, R.L.; Author Contributions: Conceptualization, H.H. and W.S.; Methodology, T.Y.; Software, T.Y.; Validation, H.H., W.Z.Project and Administration, Y.H.; Formal W.S.; Analysis, Funding T.Y.; Acquisition, Investigation, W.S. J.W.; Resources, Y.H.; Data Curation, L.W. and K.S.A; Writing–OriginalFunding: This work Draft was Preparation, supported by T.Y.; National Writing–Review Natural Science & Editing, Foundation Y.Y.; Visualization, of China (NSFC) L.S.; (No. Supervision, 51804340), R.L.; Project Administration, W.S.; Funding Acquisition, W.S. Innovation Driven Plan of Central South University (No. 2015CX005), National 111 Project (No. B14034), and Funding:CollaborativeThis Innovation work was supportedCenter for Clean by National and Efficient Natural Utilization Science Foundationof Strategic Metal of China Mineral (NSFC) Resources, (No. 51804340), Key InnovationLaboratory Driven of Hunan Plan Province of Central for Clean South and University Efficient (No. Utilization 2015CX005), of Strategic National Calcium-containing 111 Project (No. Mineral B14034), andResources Collaborative (No. 2018TP1002). Innovation Center for Clean and Efﬁcient Utilization of Strategic Metal Mineral Resources, Key Laboratory of Hunan Province for Clean and Efﬁcient Utilization of Strategic Calcium-containing Mineral Resources (No. 2018TP1002).

Minerals 2018, 8, 566 11 of 12

Acknowledgments: We very appreciate the support of Shizhuyuan Mine (Hunan, China) and the help of the staff to this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Yin, W.-Z.; Wang, J.-Z. Effects of particle size and particle interactions on scheelite flotation. Trans. Nonferr. Met. Soc. China 2014, 24, 3682–3687. [CrossRef] 2. Sun, W.; Ma, L.; Hu, Y.; Dong, Y.; Zhang, G. Hydrogen bubble flotation of fine minerals containing calcium. Min. Sci. Technol. China 2011, 21, 591–597. [CrossRef] 3. Gao, Z.; Bai, D.; Sun, W.; Cao, X.; Hu, Y. Selective flotation of scheelite from calcite and fluorite using a collector mixture. Min. Eng. 2015, 72, 23–26. [CrossRef] 4. Gao, Z.; Sun, W.; Hu, Y. New insights into the dodecylamine adsorption on scheelite and calcite: An adsorption model. Min. Eng. 2015, 79, 54–61. [CrossRef] 5. Deng, L.; Zhong, H.; Wang, S.; Liu, G. A novel surfactant N-(6-(hydroxyamino)-6-oxohexyl) octanamide: Synthesis and flotation mechanisms to wolframite. Sep. Purif. Technol. 2015, 145, 8–16. [CrossRef] 6. Deng, L.; Zhao, G.; Zhong, H.; Wang, S.; Liu, G. Investigation on the selectivity of N-((hydroxyamino)-alkyl) alkylamide surfactants for scheelite/calcite flotation separation. J. Ind. Eng. Chem. 2016, 33, 131–141. [CrossRef] 7. Bo, F.; Luo, X.; Wang, J.; Wang, P. The flotation separation of scheelite from calcite using acidified sodium silicate as depressant. Miner. Eng. 2015, 80, 45–49. [CrossRef] 8. Gao, Y.; Gao, Z.; Sun, W.; Yin, Z.; Wang, J.; Hu, Y. Adsorption of a novel reagent scheme on scheelite and calcite causing an effective flotation separation. J. Colloid Interface Sci. 2018, 512, 39–46. [CrossRef] 9. Han, H.; Hu, Y.; Sun, W.; Li, X.; Cao, C.; Liu, R.; Yue, T.; Meng, X.; Guo, Y.; Wang, J. Fatty acid flotation versus BHA flotation of tungsten minerals and their performance in flotation practice. Int. J. Miner. Process. 2017, 159, 22–29. [CrossRef] 10. Han, H.-S.; Liu, W.-L.; Hu, Y.-H.; Sun, W.; Li, X.-D. A novel flotation scheme: Selective flotation of tungsten minerals from calcium minerals using Pb–BHA complexes in Shizhuyuan. Rare Met. 2017, 36, 533–540. [CrossRef] 11. Tian, M.; Hu, Y.; Sun, W.; Liu, R. Study on the mechanism and application of a novel collector-complexes in cassiterite flotation. Colloids Surf. A Physicochem. Eng. Asp. 2017, 522, 635–641. [CrossRef] 12. Shi, Q.; Feng, Q.; Zhang, G.; Deng, H. A novel method to improve depressants actions on calcite flotation. Miner. Eng. 2014, 55, 186–189. [CrossRef] 13. Veloso, C.; Filippov, L.; Filippova, I.; Ouvrard, S.; Araujo, A. Investigation of the interaction mechanism of depressants in the reverse cationic flotation of complex iron ores. Miner. Eng. 2018, 125, 133–139. [CrossRef] 14. Yu, J.; Ge, Y.; Guo, X.; Guo, W. The depression effect and mechanism of NSFC on dolomite in the flotation of phosphate ore. Sep. Purif. Technol. 2016, 161, 88–95. [CrossRef] 15. Falconer, A. Gravity Separation: Old Technique/New Methods. Phys. Sep. Sci. Eng. 2003, 12, 31–48. [CrossRef] 16. Mukherjee, A.K. A new method for evaluation of gravity separation processes. Miner. Process. Extr. Metall. Rev. 2009, 30, 191–210. [CrossRef] 17. Qin, G.-L.; Wang, C.-X.; Yang, B. Test research on fine slime tin tailing of changpo plant china tin group with using hang and vibrate of cone concentrator. Min. Metall. 2011, 2, 33–36. 18. El-Midany, A.A.; Ibrahim, S.S. Does calcite content affect its separation from celestite by Falcon concentrator? Powder Technol. 2011, 213, 41–47. [CrossRef] 19. Liu, C.; Feng, Q.; Zhang, G.; Chen, W.; Chen, Y. Effect of depressants in the selective flotation of scheelite and calcite using oxidized paraffin soap as collector. Int. J. Min. Process. 2016, 157, 210–215. [CrossRef] 20. Khayar, M.; Bessière, J. Study of the adsorption of collectors and depressants onto calcite by high frequency dielectric measurements. Int. J. Min. Process. 2017, 166, 1–6. [CrossRef] 21. Yue, T.; Han, H.; Hu, Y.; Sun, W.; Li, X.; Liu, R.; Gao, Z.; Wang, L.; Chen, P.; Zhang, C. New Insights into the Role of Pb–BHA Complexes in the Flotation of Tungsten Minerals. JOM 2017, 69, 2345–2351. [CrossRef] 22. Wei, Z.; Hu, Y.; Han, H.; Sun, W.; Wang, R.; Wang, J. Selective flotation of scheelite from calcite using

Al-Na2SiO3 polymer as depressant and Pb–BHA complexes as collector. Miner. Eng. 2018, 120, 29–34. [CrossRef] Minerals 2018, 8, 566 12 of 12

23. Gao, Z.; Hu, Y.; Sun, W.; Drelich, J.W. Surface-charge anisotropy of scheelite crystals. Langmuir 2016, 32, 6282–6288. [CrossRef][PubMed] 24. Gao, Z.; Li, C.; Sun, W.; Hu, Y. Anisotropic surface properties of calcite: A consideration of surface broken bonds. Colloids Surf. A Physicochem. Eng. Asp. 2017, 520, 53–61. [CrossRef] 25. Gao, Z.-Y.; Wei, S.; Hu, Y.-H.; Liu, X.-W. Surface energies and appearances of commonly exposed surfaces of scheelite crystal. Trans. Nonferr. Met. Soc. China 2013, 23, 2147–2152. [CrossRef] 26. Gao, Z.-Y.; Wei, S.; Hu, Y.-H.; Liu, X.-W. Anisotropic surface broken bond properties and wettability of calcite and fluorite crystals. Trans. Nonferr. Met. Soc. China 2012, 22, 1203–1208. [CrossRef] 27. Rao, D.S.; VijayaKumar, T.V.; Rao, S.S.; Prabhakar, S.; Raju, G.B. Effectiveness of sodium silicate as gangue depressants in iron ore slimes flotation. Int. J. Miner. Metall. Mater. 2011, 18, 515. [CrossRef] 28. Filippov, L.; Severov, V.; Filippova, I. An overview of the beneficiation of iron ores via reverse cationic flotation. Int. J. Miner. Process. 2014, 127, 62–69. [CrossRef] 29. Fornasiero, D.; Ralston, J. Cu (II) and Ni (II) activation in the flotation of quartz, lizardite and chlorite. Int. J. Miner. Process. 2005, 76, 75–81. [CrossRef] 30. Roy, S. Recovery improvement of fine iron ore particles by multi gravity separation. Open Miner. Process. J. 2009, 2, 17–30. [CrossRef] 31. Laskowski, J.; Xu, Z.; Yoon, R. Energy barrier in particle-to-bubble attachment and its effect on flotation kinetics. Ind. Miner. Mines Et Carr. Les Tech. 1992, 95. 32. Ralston, J. The influence of particle size and contact angle in flotation. In Developments in Mineral Processing; Elsevier: Amsterdam, The Netherlands, 1992; Volume 12, pp. 203–224. 33. Sadowski, Z.; Polowczyk, I. Agglomerate flotation of fine oxide particles. Int. J. Miner. Process. 2004, 74, 85–90. [CrossRef] 34. Subrahmanyam, T.; Forssberg, K.E. Fine particles processing: Shear-flocculation and carrier flotation—A review. Int. J. Miner. Process. 1990, 30, 265–286. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).