The Role of Gangue Mineralogy on Flowsheet Development in Fluorite Processing

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Article The Role of Gangue Mineralogy on Flowsheet Development in Fluorite Processing

Jestos Taguta 1,*, Kebone Carol Teme 2 and Portia Ngobeni 1

1 Mineral Processing Division, Mintek, Private Bag X3015, Randburg 2125, South Africa; [email protected] 2 Jubilee Metals Group, 7 Einstein St, Highveld Techno Park, Centurion 0157, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-011-709-4307

Received: 21 January 2020; Accepted: 20 February 2020; Published: 6 March 2020

Abstract: Fluorite, CaF2, is considered a strategically important mineral as it is a raw material for many strategic industries. Froth flotation is reported to be the most efficient and economically viable process for the production of an acid-grade product with a CaF2 content of at least 97%. Selective flotation of fluorite from gangue minerals, e.g., calcite and barite, is challenging because these minerals have similar physicochemical properties. This study employed batch flotation tests coupled with mineralogical analysis to design suitable customised flowsheets and efficiently optimised reagent regimes for optimum production of acid-grade fluorite concentrate from two different fluorite ores by the froth flotation process. The effect of pulp temperature on fluorite flotation was investigated in this study with the objective of lowering pulp temperatures in fluorite flotation. The results showed that an acid-grade CaF2 concentrate could be obtained from the flotation of both ores at ambient pulp temperatures. This eliminates the requirement for high-temperature pulp treatment, which would result in a significant reduction in thermal energy costs. This study showed that an understanding of gangue mineralogy is key to process optimisation for acid-grade CaF2 production. Although an acid-grade CaF2 concentrate could be produced from both ores, the flowsheets and reagent regimes were markedly different. For instance, the production of an acid-grade CaF2 product from a high quartz and calcite ore was achieved by employing a simple rougher–multiple cleaner flotation circuit using tannin and sodium silicate as calcite and quartz depressants, respectively. On the other hand, the production of an acid-grade CaF2 product from the flotation of the pyritic ore required a pre-sulphide flotation stage for upfront sulphur removal and the use of a sulphide depressant. Multiple stages of cleaning were required for improved selectivity in the flotation of both ores.

Keywords: froth ﬂotation; ﬂuorite; selectivity; depressant; gangue; mineralogy

1. Introduction

Fluorite, CaF2, is considered a strategically important mineral that is used in the production of hydrofluoric acid and as a flux agent in the steel making industry. Fluorite ores are beneficiated through different techniques, viz., froth flotation, and gravity and magnetic concentration depending on the end use of the fluorite. Froth flotation is reported to be the most efficient and economically viable process for the production of an acid-grade product with a CaF2 content of at least 97%. The mineralogical compositions of current fluorite deposits are becoming increasingly complex, making the selective flotation of fluorite from gangue even more challenging. Typical gangue minerals include calcite, quartz and barite [1]. Some sulphide gangue minerals, such as pyrite, galena and sphalerite, may also be present in fluorite ores. Selective flotation of fluorite from the gangue e.g., calcite and barite, is challenging because all these minerals have similar physicochemical properties

Minerals 2020, 10, 237; doi:10.3390/min10030237 www.mdpi.com/journal/minerals Minerals 2020, 10, 237 2 of 13 arising from Ca2+ surface cations. The Ca2+ cations react with the commonly used fatty acid collectors resulting in these minerals exhibiting a similar flotation response, translating to low-grade CaF2 concentrates. The calcite and barite depressants also inhibit fluorite flotation, thus careful depressant selection is central to selective fluorite flotation. The presence of sulphidic gangue minerals in fluorite ores results in a CaF2 concentrate with sulphur content higher than the market specification of 0.03%. It is therefore necessary to control the sulphidic gangue in fluorite flotation. Some fluorite processing plants are located in northern China, which is characterised by long cold winters, necessitating thermal heating of the ore slurry translating to high energy costs. Apart from the proper selection of depressants, high-temperature pulp treatment has also been shown to improve the selectivity in fluorite flotation [2–4]. There is therefore a need to come up with customised flowsheets and efficient optimised reagent regimes for improved selectivity and reduced energy costs in fluorite flotation [4–6]. In this study, the role of gangue mineralogy in fluorite flotation was investigated. It was proposed that flowsheet and reagent regime selection depend on the type and proportion of gangue minerals present in an ore. This paper presents results of batch scale flotation tests for two different fluorite ores sourced from different geographical locations. The effects of pulp temperature, reagent types and flowsheet on flotation of the two fluorite ores were systematically investigated. Optimisation of different reagent regimes was achieved by varying reagent dosages in order to improve fluorite grade-recovery relationships.

2. Materials and Methods

2.1. Sample Preparation, Mineralogical and Chemical Analysis Two fluorspar ores sourced from different geographical locations within Morocco and South Africa (thereafter referred to as A and B, respectively, in this manuscript) were investigated in this study. The ores were crushed to minus 1.7 mm for flotation tests using a jaw crusher. A rotary splitter was then used to divide the ores into 1 kg representative samples. The mineralogical analysis of the representative sized particles of the ores was conducted using a field emission gun automated quantitative electron scanning microscopy (QEMSCAN, FEI company, Brisbane, Australia) and X-ray diffraction (XRD). XRD was conducted to identify the mineral phases and composition in a given sample, the result of which was used to set up mineral identification parameters on the QEMSCAN. The particle mineral maps were produced using the particle map analysis (PMA) measuring mode, from which the bulk modal proportions of the minerals present, the liberation and association information of fluorine-bearing minerals were derived. The major elemental composition of the fluorite ores was determined using the X-ray fluorescence (XRF) method, which required the double Ca titration method for the removal of soluble Ca from carbonates, followed by the determination of Ca associated with fluorine.

2.2. Reagents The chemical reagents used in this study are shown in Table1. Reagent selection was conducted in close consultation with various ﬂotation reagent suppliers based on their database and experience on treatment of similar ore types. Flotinor 7243 is a mixture of lauryl polyglycol ether carboxylic acid (LCA) and nonylphenol polyglycol ether (NPE). A variety of gangue depressants were tested in this study based on the type of gangue minerals present in each ore (Section 3.1). All the reagents were dosed based on active content, and standard tap water was used as the medium in all ﬂotation experiments. Minerals 2020, 10, 237 3 of 13

Table 1. List of reagents used in this study.

Reagent Chemical Name/Formula Use Purity (%) Targeted Ore Supplier 15–25 LCA; Flotinor 7243 Fatty acid Fluorite collector A Clariant 5–15 NPE Sylfat FA 2 Tall oil fatty acid Fluorite collector 100 B Arizona Chemicals, USA Sodium ﬂuorite NaF Fluorite activator 100 A & B Hyperion chemicals & consulting

Sodium amyl xanthate C3H5NaOS2 Sulphide collector 99.9 B Senmin 75 NaHS; 3 Na S; Sodium hydrosulphide NaHS Sulphide activator 2 B Axis house 15 ppm Fe content

Soda ash Na2CO3 pH modiﬁer 100 A & B Hyperion chemicals & consulting Pionerra F250 - Sodium lignin sulphonate Barite and sulphide depressant 100 A & B Hyperion chemicals & consulting Tan XS Acacia mearnsii extract, bisulﬁted Calcite depressant 64 A & B Bondtite (Pty) Limited

Sodium silicate Na2SiO3 Silica depressant 40 A & B Hyperion chemicals & consulting Minerals 2020, 10, 237 4 of 13

Minerals 2020, 10, x FOR PEER REVIEW 4 of 13 2.3. Batch Flotation Tests The Denver D12 flotation machine was used for the batch flotation testwork at an impeller speed 2.3. Batch Flotation Tests of 1500 rpm. Milling tests were conducted on the ore to establish the milling time required to achieve targetedThe grinds Denver for D12 the twoflotation different machine ores was (Section used 3.2for). the The batch market flotation specification testwork at demands an impeller that speed particle sizeof distribution1500 rpm. Milling (PSD) tests of the were product conducted be 80% on passingthe ore to 65 establishµm. Accordingly, the millingthis time study required investigated to achieve the flotationtargeted response grinds for of samplesthe two different whose PSDs ores (Section were coarser 3.2). The than market 80% passing specification 65 µm. demands The market’s that particle product particlesize distribution size distribution (PSD) of (PSD) the prod specificationuct be 80% was passing 80% passing65 µm. Accordingly, 65 µm and hencethis study grinds investigated coarser than the 65 µmflotation were investigated response of in samples this study. whose High PSDs chrome were steel coarser rods than were 80% used passing as grinding 65 µm. media The andmarket’s milling wasproduct conducted particle at 50%size solids’distribution concentration (PSD) specification in the mill. was Batch 80% flotation passing tests 65 wereµm and conducted hence grinds on 1 kg feedcoarser samples than that 65 µm were were rod investigated milled to a in given this grind.study. TheHigh milled chrome slurry steel wasrods conditionedwere used as in grinding a Denver media and milling was conducted at 50% solids’ concentration in the mill. Batch flotation tests were D12 flotation machine, with a slurry density of 33% solids in the rougher and 18% in the cleaners. conducted on 1 kg feed samples that were rod milled to a given grind. The milled slurry was All flotation reagents were prepared at 1% strength with exception of the fatty acids, which were conditioned in a Denver D12 flotation machine, with a slurry density of 33% solids in the rougher added neat. A given amount of depressant was added followed by the addition of a given amount of and 18% in the cleaners. All flotation reagents were prepared at 1% strength with exception of the collector. Sodium carbonate was used as a pH modifier in the rougher, targeting a pH of 9.5. Pulp fatty acids, which were added neat. A given amount of depressant was added followed by the temperatureaddition of wasa given adjusted amount to of a collector. targeted Sodium value using carbonate a water was bath. used Theas a concentratespH modifier in and the therougher, tailings weretargeting dried separately.a pH of 9.5. ThePulp dry temperature mass of the was concentrates adjusted to anda targeted tailings value were using determined a water bath. to calculate The theconcentrates recoveries. and The the concentrates tailings were were dried assayed separately for. CaF The2 ,dry CaCO mass3, SiOof the2 and concentrates S usingthe and techniques tailings highlightedwere determined in Section to 3.1calculate. Fluorite the grade–recoveryrecoveries. The curvesconcentrates were usedwere asassayed primary for responses CaF2, CaCO to3 compare, SiO2 theand performances S using the oftechniques different highlighted flowsheets in and Section reagent 3.1. regimes. Fluorite grade–recovery curves were used as primary responses to compare the performances of different flowsheets and reagent regimes. 3. Results and Discussion 3. Results and Discussion 3.1. Mineralogical and Chemical Analysis 3.1.The Mineralogical head grade, and mineralogical Chemical Analysis composition and the mineral phase proportions of ores A and B are shownThe inhead Table grade,2 and mineralogical Figure1. Liberation composition characteristics and the mineral of both phase ores were proportions determined of ores at aA grind and B of 80%are passing shown in 150 Tableµm. 2 and Figure 1. Liberation characteristics of both ores were determined at a grind of 80% passing 150 µm. Table 2. Chemical analysis of the fluorspar ores A and B. Table 2. Chemical analysis of the fluorspar ores A and B. Ores CaF2 MgO Al2O3 SiO2 CaCO3 Fe S BaSO4 K2O MnO Ores CaF2 MgO Al2O3 SiO2 CaCO3 Fe S BaSO4 K2O MnO A 30.4 0.24 1.32 26.1 36.2 0.33 0.41 1.45 0.05 0.05 A 30.4 0.24 1.32 26.1 36.2 0.33 0.41 1.45 0.05 0.05 B 26.9 10.8 0.83 6.32 54.3 5.97 4.10 0.22 0.19 0.58 B 26.9 10.8 0.83 6.32 54.3 5.97 4.10 0.22 0.19 0.58

45 40 35 30 25 20 15 % % Abundance 10 5 0

A B

FigureFigure 1. 1.Bulk Bulk mineralogical mineralogical analysis of fluorspar ﬂuorspar ores ores A A and and B Bin in mass mass %. %.

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It is clear from Table 2 and Figure 1 that the CaF2 grades were relatively similar in both ores. However,It is clear there from were Table 2significant and Figure differences1 that the CaFin terms2 grades of the were type relatively and proportion similar inof boththe gangue ores. However,minerals there present were in significant both ores. di ffOreerences A had in termshigher of qu theartz type and and calcite proportion contents of thecompared gangue mineralsto ore B. In presentfact, inthe both quartz ores. and Ore calcite A had contents higher quartz of ore andA were calcite approximately contents compared eleven totimes ore B.and In twice fact, the those quartz of ore andB, calcite respectively. contents On of the ore other A were hand, approximately ore B had higher eleven pyrite times andand twicedolomite those contents of ore B,compared respectively. to ore OnA. the It otheris clear hand, that oreore BB had higher30 times pyrite the pyrite and dolomite content of contents ore A. The compared dolomite to orecontent A. It in is ore clear B thatwas 13 oretimes B had that 30 timesin ore theA. The pyrite differences content of inore the A.mineralogi The dolomitecal contents content of in the ore two B was ores 13 are times expected that in to oreaffect A. Theflowsheet differences and reagent in the mineralogicalregime selection contents to process of the both two ores. ores are expected to affect flowsheet and reagent regime selection to process both ores. 3.2. Flowsheet Development 3.2. Flowsheet Development The development of suitable flowsheets and optimised reagent regimes for the processing of oresThe A development and B was based of suitable on the flowsheetsores’ mineralogical and optimised characteristics. reagent regimes Preliminary for the reagent processing scouting of ores tests A andshowed B was that based Flotinor on the ores’7243 mineralogicaland Arizona characteristics.Sylfat FA2, both Preliminary fatty acid reagent collectors, scouting produced tests showed superior thatmetallurgical Flotinor 7243 results and Arizona for the Sylfatprocessing FA2, bothof ore fatty A an acidd B, collectors, respectively. produced The results superior were metallurgical not presented resultsnot only for the for processing the brevity of of ore the A andmanuscript B, respectively. but also The because results this were paper not presentedfocussed only not onlyon the for role the of brevitygangue of themineralogy manuscript on butflowsheet also because development this paper for focussedfluorite processing. only on the Preliminary role of gangue flotation mineralogy testwork onalso flowsheet showed development that six cleaning for fluorite stages processing. were required Preliminary to produce flotation an acid-grade testwork CaF also2 product showed thatfrom six both cleaningores. stages were required to produce an acid-grade CaF2 product from both ores.

3.2.1.3.2.1. Flowsheet Flowsheet for for Ore Ore A A TheThe mineralogical mineralogical and and chemical chemical analysis analysis results results depicted depicted in Table in Table2 and 2 Figure and Figure1 indicate 1 indicate that for that orefor A toore be A upgraded, to be upgraded, quartz and quartz calcite and minerals calcite areminerals to be separatedare to be fromseparated the fluorite from the during fluorite flotation. during Tanninsflotation. have Tannins been shown have to been be strong shown calcite to be depressants strong calcite [6,7 depressants], while sodium [6,7], silicate while is sodium reported silicate to be is anreported effective silicato be depressantan effective [ 8silica,9]. In depressant terms of liberation [8,9]. In terms characteristics of liberation at a grindcharacteristics of 80% passing at a grind 150 of µm,80% 90% passing of the fluorite150 µm, grains 90% of were the fullyfluorite liberated, grains withwere lessfully than liberated, 15% of with fluorite less grains than 15% showing of fluorite an intimategrains association showing an with intimate calcite association and other silicatewith calcit ganguee and minerals. other silicate Therefore, gangue a grind minerals. of 80% Therefore, passing a 150grindµm was of 80% assumed passing suitable 150 µm to was achieve assumed adequate suitable liberation to achieve of selectiveadequate fluorite liberation flotation. of selective A simple fluorite flotationflotation. flowsheet, A simple shown flotation in Figure flowsheet,2, was proposedshown in forFigure processing 2, was oreproposed A. Flotation for processing times were ore as A. follows:Flotation 4 min times for thewere rougher; as follows: 4 min 4 formin cleaners for the 1roughe and 2;r; 3 4 min min for for cleaners cleaners 3 and1 and 4; 22; min3 min for for cleaners cleaners 5 and3 and 6. 4; 2 min for cleaners 5 and 6.

Figure 2. Flowsheet for the processing of ore A. Figure 2. Flowsheet for the processing of ore A. 3.2.2. Flowsheet for Ore B 3.2.2.The Flowsheet beneﬁciation for ofOre ore B B required the depression of pyrite, calcite and dolomite. Similar to ore A, tanninsThe and beneficiation sodium silicate of ore were B required used as calcitethe depression and silica of depressants pyrite, calcite [6 –and9]. Lignindolomite. sulphonate Similar to is ore reportedA, tannins to be and an esodiumﬀective silicate pyrite depressantwere used as [10 calcite]. In terms and silica of liberation depressants characteristics [6–9]. Lignin at sulphonate a grind of is

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80% passing 150 µm, 25% of fluorite grains were fully liberated with 75% of fluorite grains reporting as middlings and 42% fluorite being associated with major gangue. Because of the poor degree of liberation, ore B might require concentrate regrinding to liberate the fluorite grains as opposed to ore A. To determine the optimum grind for ore B, the effect of grind on the grade–recovery relationship in rougher flotation was investigated and the results are shown in Table3.

Table 3. Eﬀect of grind on CaF2 grade–recovery relationship for ore B using Arizona Sylfat FA2 collector in rougher cells.

P80 Grind (µm) Mass Pull (%) CaF2 Grade (%) CaF2 Recovery (%) 125 46.6 32.8 86.5 106 37.1 41.9 86.0 90 44.1 34.0 89.6 75 52.3 30.2 88.7

Table3 shows that the rougher CaF 2 recoveries for P80 grinds of 125 µm and 106 µm were similar at 86%, while for P80 of 90 µm and 75 µm the rougher CaF2 recoveries were similar at 89%. The rougher CaF2 grades decreased in the following order: P80 = 106 µm > P80 = 90 µm > P80 = 125 µm > P80 = 75 µm. Because it produced the highest CaF2 grade at reasonably high CaF2 recoveries, the 80% passing 106 µm grind was selected as the optimum grind for further flotation testwork with a view to improving metallurgical performance. Different process routes were considered for the removal of the pyritic gangue in ore B. The first route was through the selective depression of the pyritic gangue using the lignin sulphonate depressant, employing the simple flowsheet shown in Figure2. The second route was an upfront removal of the pyritic gangue using a pre-sulphide flotation cell as shown in Figure3. The second route used NaHS as a sulphide activator [2,11] at a low dosage of 15 g/t and potassium amyl xanthate (PAX) as a sulphide collector [12] in a pre-sulphide flotation cell. A higher NaHS dosage would be detrimental as it would depress the pyrite. NaHS functions by either cleaning or sulphidising the pyrite surface and hence improving the pyrite floatability. Flotation times were as follows: 6 min for the pre-sulphide rougher; 4 min for the rougher; 4 min for cleaners 1 and 2; 3 min for cleaners 3 and 4; 2 min for cleaners 5 and 6. The metallurgical performance of the two process routes was evaluated and the results are presented in Table4. Table4 shows, that although both process routes produced an acid-grade CaF 2 product, the pre-sulphide flotation route produced 50% less sulphur content compared to the selective depression route. The pre-sulphide flotation route produced higher CaF2 recovery and grade than the selective depression route. It is clear that the pre-sulphide flotation route was the most effective means of removing the pyritic gangue from the fluorite ore and hence the flowsheet in Figure3 was used for further optimisation tests.

Table 4. Comparison of the metallurgical performance different process routes in terms of pyrite removal using lignin sulphonate depressant at 170 g/t dosage for ore B.

Grade (%) Recovery (%) Process Route CaF2 S CaF2 No pre-sulphide ﬂoat 96.8 0.14 75.1 Pre-sulphide ﬂoat 97.0 0.07 78.9

Although the pre-sulphide flotation route was successful in reducing the pyritic gangue reporting to the CaF2 concentrate, the sulphur content was still higher than the market specification of at most 0.03%. Therefore, optimisation tests were conducted to reduce the sulphur content in the CaF2 concentrate. Minerals 2020, 10, x FOR PEER REVIEW 7 of 13

Minerals 2020, 10, 237 7 of 13 Minerals 2020, 10, x FOR PEER REVIEW 7 of 13

Figure 3. Flowsheet for the processing of ore B, which includes a pre-sulphide flotation stage, fluorite roughing and sequential cleaning. Figure 3. Flowsheet for the processing of ore B, which includes a pre-sulphide flotation stage, fluorite Figure 3. Flowsheet for the processing of ore B, which includes a pre-sulphide flotation stage, fluorite 3.3.roughing Effect of and Pulp sequential Temperature cleaning. roughing and sequential cleaning. 3.3. EffectFigure of Pulp 4 Temperatureshows the effect of pulp temperature on the CaF2 grade–recovery relationship for ore A 3.3. usingEffect theof Pulp Flotinor Temperature 7243 collector at 300 g/t dosage. The objective was to evaluate the affinity and the Figure4 shows the e ffect of pulp temperature on the CaF2 grade–recovery relationship for ore subsequent efficiency of the collector towards different minerals within the ore as a function of pulp A usingFigure the 4 Flotinor shows the 7243 effect collector of pulp at temperature 300 g/t dosage. on the The CaF objective2 grade–recovery was to evaluate relationship the affi fornity ore and A temperature. Figure 4 shows that as the pulp temperature increases, the CaF2 recovery increases, theusing subsequent the Flotinor effi 7243ciency collector of the collectorat 300 g/t towards dosage. diThefferent objective minerals was to within evaluate the orethe asaffinity a function and the of while the CaF2 grade decreases. It is clear from Figure 3 that although the CaF2 recovery was low pulpsubsequent temperature. efficiency Figure of the4 shows collector that towards as the pulp different temperature minerals increases, within the CaFore as2 recoverya function increases, of pulp around 56%, the product grade met the market specification of 97% CaF2 at 20 °C. However, it was whiletemperature. the CaF Figure2 grade 4 decreases. shows that It as is clearthe pulp from temperature Figure3 that increases, although the the CaF CaF2 2recoveryrecovery increases, was low not possible to produce the acid-grade concentrate at 40 °C. In order to investigate whether the aroundwhile the 56%, CaF the2 grade product decreases. grade met It theis clear market from specification Figure 3 that of 97% although CaF2 at the 20 ◦CaFC. However,2 recovery it was was low not observed results were ore-specific or not, the effect of pulp temperature was also investigated for ore possiblearound 56%, to produce the product the acid-grade grade met concentrate the market at specification 40 ◦C. In order of 97% to investigate CaF2 at 20 whether °C. However, the observed it was B using a different fatty acid collector, Sylfat FA2 at 300 g/t dosage, and the results are presented in resultsnot possible were ore-specificto produce or the not, acid-grade the effect ofconcentrate pulp temperature at 40 °C. was In alsoorder investigated to investigate for orewhether B using the a Figure 5. diobservedfferent fatty results acid were collector, ore-specific Sylfat or FA2 not, at 300the effect g/t dosage, of pulp and temperature the results was are presentedalso investigated in Figure for5. ore B using a different fatty acid collector, Sylfat FA2 at 300 g/t dosage, and the results are presented in Figure 5. 100

100 90

90 80

80 70 Grade (%) 2

70CaF

Grade (%) 60 20°C 2 40°C

CaF 60 50 20°C 5040°C 60 70 80

50 CaF2 Recovery (%)

Figure50 4. Eﬀect of pulp temperature 60 on the grade–recovery 70 relationship 80 for ore A. Figure 4. Effect of pulp temperatureCaF2 Recovery on the grade–recovery (%) relationship for ore A. The results in Figure5 show that an acid-grade concentrate with a grade of 97% CaF 2 can be attainedThe at 75% results and in 85% Figure recoveries 5 show using that thean acid Sylfat-grade FA2 concentrate collector at 20with◦C a and grade 40 ◦ofC, 97% respectively. CaF2 can be Figure 4. Effect of pulp temperature on the grade–recovery relationship for ore A. A furtherattained increase at 75% of and the 85% pulp recoveries temperature using to 60the◦ CSylfat resulted FA2 incollector a signiﬁcant at 20 increase°C and 40 in °C, CaF respectively.2 recovery, A further increase of the pulp temperature to 60 °C resulted in a significant increase in CaF2 recovery, The results in Figure 5 show that an acid-grade concentrate with a grade of 97% CaF2 can be attained at 75% and 85% recoveries using the Sylfat FA2 collector at 20 °C and 40 °C, respectively. A further increase of the pulp temperature to 60 °C resulted in a significant increase in CaF2 recovery,

Minerals 2020, 10, 237 8 of 13 Minerals 2020, 10, x FOR PEER REVIEW 8 of 13 butbut at at the the expense expense of of CaF CaF2 grade.2 grade. It wasIt was therefore therefore not not possible possible to produceto produce an acid-gradean acid-grade concentrate concentrate at 60at◦ C60 using °C using the Sylfatthe Sylfat FA2 FA2 collector. collector.

100

Grade (%) 60 2

CaF 50 20°C 40°C 40 60°C 30 60 70 80 90 100 CaF Recovery (%) 2 Figure 5. Eﬀect of pulp temperature on CaF grade–recovery relationship for ore B. Figure 5. Effect of pulp temperature on CaF2 2 grade–recovery relationship for ore B.

It has been observed from Figures4 and5, that as the pulp temperature increases, the CaF 2 It has been observed from Figures 4 and 5, that as the pulp temperature increases, the CaF2 recovery increases, while the CaF2 grade decreases for both ores. Thus, the results are not ore-specific. recovery increases, while the CaF2 grade decreases for both ores. Thus, the results are not ore-specific. Similar observations were made by [4], who showed that better fluorite selectivity in carbonate-fluorite Similar observations were made by [4], who showed that better fluorite selectivity in carbonate- ores is achievable at low temperatures. The same finding was made by [3] in their investigation on fluorite ores is achievable at low temperatures. The same finding was made by [3] in their a fluorite ore from China using sodium naphthenate and salted copper sulphate as collector and investigation on a fluorite ore from China using sodium naphthenate and salted copper sulphate as depressant of phosphate gangue minerals, respectively. The results presented in both Figures4 and5 collector and depressant of phosphate gangue minerals, respectively. The results presented in both suggestFigures that 4 and operating 5 suggest at lowerthat operating temperatures at lower would temperatures be more favourable would be more to produce favourable an acid-grade to produce fluoritean acid-grade product. fluorite The results product. therefore The results show thattherefor bothe Flotinorshow that 7243 both and Flotinor Sylfat 7243 FA2 canand beSylfat effectively FA2 can usedbe effectively as collectors used in the as flotationcollectors of in ore the A andflotation B, respectively, of ore A and to produce B, respectively, acid-grade to CaF produce2 concentrate acid-grade at low pulp temperatures. Besides the benefit of increased selectivity, operating at low pulp temperatures CaF2 concentrate at low pulp temperatures. Besides the benefit of increased selectivity, operating at wouldlow pulp result temperatures in a significant would reduction result ininenergy a significan costst in reduction fluoriteflotation. in energy This costs is in especially fluorite trueflotation. for mostThis fluorite is especially processing true plants for most located fluorite in northern processing China, plants which located are characterised in northern by longChina, cold which winters. are Incharacterised such plants, energyby long consumptioncold winters. isIn highsuch dueplants, to heatingenergy consumption of the ore slurry is high in flotation due to heating circuits of for the effiorecient slurry separation. in flotation The circuits use of thefor Flotinorefficient 7243separation. and Sylfat The FA2use collectorsof the Flotinor therefore 7243 eliminatesand Sylfat theFA2 needcollectors for high-temperature therefore eliminates pulp treatment,the need for and high-tem thus a significantperature pulp reduction treatment, in energy and thus costs. a significant reductionTo understand in energy the costs. reason behind the loss of selectivity as pulp temperature increases for ore B, the behaviourTo understand of gangue the minerals reason behind as a function the loss of of pulp select temperatureivity as pulp was temperature investigated, increases and the for results ore B, arethe presented behaviour in of Table gangue5. Table minerals5 shows as thata function the calcite of pulp contamination temperature increased was investigated, with increasing and the pulp results temperature.are presented The in Table increase 5. Table in CaF 5 2showsrecoveries that the with calcite increasing contamination pulp temperature increased couldwith increasing be due to pulp an increase in the hydrophobicity and subsequent floatability of CaF2 as pulp temperature increases. This temperature. The increase in CaF2 recoveries with increasing pulp temperature could be due to an could be a result of improved dispersion of the collector throughout the pulp at high pulp temperatures. increase in the hydrophobicity and subsequent floatability of CaF2 as pulp temperature increases. This could be a result of improved dispersion of the collector throughout the pulp at high pulp Table 5. Effect of temperature on mass pull and gangue (calcite) contamination. temperatures. Grade (%) TemperatureTable (5.◦C) Effect Massof temperature Pull (%) on mass pull and gangue (calcite) CaFcontamination2 Recovery. (%) CaF2 CaCO3 20 11.8 97.4Grade 1.34(%) 69.5 Temperature (°C) Mass Pull (%) CaF2 Recovery (%) 40 12.9 97.8CaF2 CaCO 2.633 65.4 6020 19.111.8 80.397.4 10.31.34 76.969.5 40 12.9 97.8 2.63 65.4 60 19.1 80.3 10.3 76.9

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Improved collector dispersion and increased rate of mineral–collector interactions at high temperatures may promote the unselective adsorption of the fatty acid collector onto the gangue minerals. This renders the gangue minerals floatable, and hence the observed increase in gangue dilution of the CaF2 concentrate as pulp temperature increases. A similar finding was made by [13] in ilmenite flotation, where they observed the improved floatability of feldspar and biotite gangue minerals as pulp temperature was increased, resulting in reduced low-grade ilmenite concentrates.

3.4. Effect of Fluorite Activation on Fluorite Flotation The addition of sodium fluoride, NaF, as a fluorite activator on the flotation performance of both ores A and B was investigated in the rougher flotation cell and the results are shown in Table6. It is clear from Table6 that increasing NaF dosage from 0 to 500 g /t resulted in a slight increase in the recovery of CaF2 but a significant increase in the concentrate grade for ore A. In fact, the concentrate grade was increased by more than 12%. The significant increase in CaF2 grade was accompanied by around a 50% decrease in silica and calcite gangue content. However, a further 50% increase in the NaF dosage resulted in a 3% drop in the CaF2 grade. This was clearly due to the increase in the recovery of gangue at high NaF dosages. It is also clear from Table6 that CaF 2 recovery dropped by almost 50% when NaF was added to the roughers for ore B. Furthermore, gangue content increased with the addition of NaF. This shows that the use of NaF at 500 g/t dosage had a detrimental effect on fluorite recovery from ore B.

Table 6. Eﬀect of ﬂuorite activation using NaF on CaF2 grade and recovery for ore A and B.

Grade (%) Ore Type NaF Dosages (g/t) Mass Pull (%) CaF2 Recovery (%) CaF2 SiO2 CaCO3 0 33.0 84.0 2.46 1.42 83.0 A 500 29.2 96.4 1.41 0.75 85.0 750 31.0 93.8 2.99 1.30 86.0 0 22 95 0.32 2.29 80.6 B 500 14 93 0.84 3.23 48.0

The increase in the CaF2 grade with increasing NaF dosage observed for ore A is consistent with the findings made by [4], who concluded that NaF improved fluorite selectivity in the rougher cells. The authors [4] proposed that the NaF and collector interact, forming fluorine-bearing associates which have a higher affinity for fluorite than gangue minerals, e.g., carbonates and sulphates. They also suggested that fluorite activation is a result of the restructuring of the fluorite surface layer under the influence of the excess F− ions generated from the NaF addition. Both mechanisms should result in increased fluorite recovery and selectivity. However, the results presented in Table6 suggest that there was an optimum dosage (500 g/t NaF) at which the CaF2 grade was maximum and gangue contamination was minimum for ore A. The results in Table6 also indicate that NaF depresses the gangue at low dosages. This is evidenced by the drop in the gangue content, coupled with a significant increase in the CaF2 product grade by the addition of 500 g/t NaF. It can be therefore concluded that NaF functions both as a fluorite activator and gangue depressant at low dosages. For ore B, the CaF2 grade just dropped by the addition of 500 g/t of NaF. The results seem to suggest that the 500 g/t NaF dosage was already higher than the optimum NaF dosage. It is therefore recommended that lower NaF dosages be investigated to establish the optimum NaF dosage that would give the best metallurgical benefits in terms of CaF2 grade and recovery for ore B. At high NaF dosages, the excess fluorine-bearing associates formed from the interaction between the excess NaF and collector may unselectively adsorb onto the gangue minerals. Thus, the gangue minerals become floatable, hence the observed increase in silica and calcite contamination. Neutralisation or passivation of the Ca sites on the fluorite mineral surface by excess F− may also occur, such that there Minerals 2020, 10, 237 10 of 13 are no longer any Ca sites available on the fluorite mineral surface to bond with the collector on the fluorite surface, hence the loss in CaF2 recovery at high NaF dosages.

3.5. Effect of Sodium Silicate Dosage for Ore A The effect of adding sodium silicate as a silicate depressant in an attempt to reduce silica content to below 1% in the concentrate was investigated in the rougher flotation cell and the results are shown in Table7. It is clear from Table7 that only a slight improvement in silica depression was achieved by increasing sodium silicate dosage from 0 to 310 g/t. A further increase in the sodium silicate dosage to 390 g/t resulted in the targeted silica contamination of the less than 1% in the concentrate. However, this this was accompanied by significant loss in CaF2 recovery. The CaF2 recovery loss at a high sodium silicate dosage could be attributed to both reduced mass pull and the depression of the fluorite grains that are associated with silica gangue minerals, as revealed by the mineralogical analysis as shown in Table8. The lower sodium silicate dosage was therefore not strong enough to depress the silica associated with the fluorite particles. Mineral association data are derived from shared boundaries amongst the identified mineral grains, therefore the higher the associated percentage is, the greater the degree of boundary-sharing between mineral species. In Table8, it is clear that there exists some fluorite association with silicate minerals such as calcite, dolomite, pyroxene, K-feldspar and quartz, which ultimately resulted in CaF2 recovery loss when the depressant dosage was increased. This result indicates that there is an opportunity to explore other silicate depressants in an attempt to maximise the CaF2 recovery to the product.

Table 7. Eﬀect of sodium silicate on CaF2 grade and recovery.

Grade (%) Na2SiO3 Dosage (g/t) Mass Pull (%) CaF2 Recovery (%) CaF2 SiO2 CaCO3 0 29.2 96.4 1.63 1.49 83.0 310 28.2 96.5 1.41 0.75 85.0 390 24.5 97.4 0.92 0.60 73.2

Table 8. Mineral associations between ﬂuorite and gangue minerals in mass %.

Mineral Name Fluorite Calcite Dolomite Free surface 66.23 60.69 13.24 Fluorite 0.00 14.98 2.46 Pyrite 0.02 0.04 0.01 Barite 0.10 0.01 0.02 Pyroxene 2.79 2.41 0.89 K-Feldspar 0.15 0.18 0.03 Biotite 0.10 0.11 0.03 Quartz 0.83 0.76 0.27 Fe oxides 0.29 0.04 0.04 Calcite 28.22 0.00 82.93 Dolomite 1.16 20.69 0.00 Others 0.10 0.10 0.08

3.6. Effect of Concentrate Regrind for Ore B The results so far have demonstrated that it is possible to produce an acid-grade product with at least 96% CaF2. However, the S content in the CaF2 concentrate was well above the market specification of 0.03%. The high S content could be attributed to the association of fluorite grains with sulphide minerals as shown from the mineralogical analysis of the CaF2 concentrate presented in Table9. Minerals 2020, 10, 237 11 of 13

Minerals 2020, 10, x FOR PEER REVIEW 11 of 13 Table 9. Fluorite concentrate sulphide mineral association in mass %.

Table 9. FluoriteMineral concentrate sulphide Sulphide mineral Mass association % in mass %. Free surfaceMineral Sulphide 51.2Mass % SerpentineFree surface 51.2 0.22 PyroxeneSerpentine 0.22 0.63 FeldsparPyroxene 0.63 0.10 MicaFeldspar 0.10 0.47 QuartzMica 0.47 0.79 Fe oxides 2.88 Quartz 0.79 Fluorite 23.7 Fe oxides 2.88 Carbonates 13.1 Fluorite 23.7 Apatite 0.10 OthersCarbonates 13.1 6.80 TotalApatite 0.10 100 Others 6.80 Total 100 Table9 shows that 24% of the sulphides were associated with fluorite grains and hence the e ffect of concentrateTable 9 shows regrinding that 24% on theof the sulphur sulphides content were in associated the CaF2 withconcentrate fluorite grains was investigated and hence the and effect the resultsof concentrate are shown regrinding in Figure6 onand the Table sulphur 10. content in the CaF2 concentrate was investigated and the resultsTo avoid are shown grinding in Figure finer than6 and the Table market’s 10. product PSD specification of 80% passing 65 µm, a millingTo curve avoid was grinding constructed, finer than which the showedmarket’s that product the regrindPSD specification time of 8 minof 80% produced passing a65 product µm, a withmilling a PSD curve coarser was than constructed, a P80 of which 65 µm. showed The milling that th curvee regrind showed time of that 8 min a regrind produced time a product of 1 min with was requireda PSD tocoarser achieve than a Pa 80P80of of 90 65µ µm.m. TwoThe milling cleaner curve concentrates showed werethat a therefore regrind time independently of 1 min was ground required for 1 andto achieve 2 min. Figurea P80 of6 90shows µm. Two that concentratecleaner concentrates regrinding were reduced therefore both independently the CaF 2 recoveries ground for and 1 grades.and 2 Themin. drop Figure in CaF 6 2showsrecoveries that concentrate could be due regrinding to the slower reduced floating both the finer CaF particles2 recoveries generated and grades. during The the drop in CaF2 recoveries could be due to the slower floating finer particles generated during the concentrate regrind. The drop in the grade of the CaF2 concentrate grade could be due to both the increasedconcentrate liberation regrind. of floatableThe drop gangue, in the grade such asof calcitethe CaF and2 concentrate entrainment grade of finecould gangue be due particles. to both the The resultsincreased in Table liberation9 also indicateof floatable that gangue, concentrate such as regrinding calcite and resultedentrainment in reduced of fine gangue S content particles. reporting The results in Table 9 also indicate that concentrate regrinding resulted in reduced S content reporting to to the CaF2 concentrate. However, the S content was still above the market specification of 0.03%. Overall,the CaF concentrate2 concentrate. regrinding However, had the a detrimentalS content was eff ectstil ofl above concentrate the market dilution spec byification calcite of as 0.03%. well as Overall, concentrate regrinding had a detrimental effect of concentrate dilution by calcite as well as reduced CaF2 recovery. It was also observed that with longer regrinding time, the total sulphur content reduced CaF2 recovery. It was also observed that with longer regrinding time, the total sulphur increased. It is therefore clear that the processing of ore B without regrind was still the best, though content increased. It is therefore clear that the processing of ore B without regrind was still the best, sulphur content was still off-spec. though sulphur content was still off-spec.

100

No regrind 90

1 min. 80 regrind 2 min. 70 regrind Grade (%) 2

CaF 60

40 70 75 80 85 90 95 100

CaF2 Recovery (%)

Figure 6. Eﬀect of concentrate regrind on CaF2 grade and recovery. Figure 6. Effect of concentrate regrind on CaF2 grade and recovery.

Minerals 2020, 10, 237 12 of 13

Table 10. Eﬀect of concentrate regrind on CaF2 grade and recovery.

Grade (%) Regrind Time (minutes) CaF2 Recovery (%) CaF2 CaCO3 S 0 (no regrind) 97.2 3.51 0.07 78.9 1 93.4 3.72 0.06 77.6 2 91.3 4.26 0.59 72.4

3.7. Effect of Lignin Sulphonate (Pionerra F250) Dosage on Sulphur Reduction for Ore B Further flotation testwork was conducted using lignin sulphonate (Pionerra F250) as a sulphide depressant with no regrind with a view to reduce the S content to the industry specification of 0.03%. The depressant dosages were varied from 170 to 210 g/t and the results are presented in Table 11. It is clear from Table 11 that an increase in lignin sulphonate dosage resulted in a decrease in the total sulphur in the CaF2 concentrate from 0.07% to 0.02%. Therefore, the acid-grace concentrate specifications were met in terms of both CaF2 and S grades, although this was accompanied by a 7% loss in CaF2 recovery. It can be concluded that the stage addition sulphide depressants in the rougher and cleaning stages in the processing of fluorite ores having high sulphur content is necessary to control the sulphur content in the concentrate.

Table 11. Eﬀect of lignin sulphonate depressant on S grade as well as CaF2 grade and recovery.

Grade (%) Lignin-Sulphonate (g/t) Mass Pull (%) CaF2 Recovery (%) CaF2 SiO2 S 170 23 97.1 0.25 0.07 78.9 210 20 96.5 0.80 0.02 72.0

The observed loss in CaF2 recovery could be attributed to the depression of the sulphide minerals that are associated with the fluorite. To investigate this, mineralogical analysis was conducted on the concentrate to further understand the association of the minerals present in the ore, and the results are shown in Table9. Table9 shows that 37% of the sulphides were associated with fluorite and carbonates, while 7% of the sulphides were associated with other minerals. This observation shows that CaF2 recovery loss was inevitable if the S content in the final concentrate is to be reduced using a sulphide depressant.

4. Conclusions This study investigated the role of gangue mineralogy on flowsheet and reagent regime selection on the processing of two different fluorite ores. One ore had higher quartz and calcite contents while the other had higher pyrite and dolomite contents. Customised flowsheets and efficiently optimised reagent regimes were investigated for the optimum production of acid-grade fluorite for each ore by the froth flotation process. An increase in pulp temperature resulted in increased CaF2 recovery but decreased CaF2 grade for both ores. The results therefore show that the fatty acid collectors can be effectively used in fluorite flotation to produce acid-grade CaF2 concentrate at ambient pulp temperatures. High selectivity at low pulp temperatures eliminates the need for high-temperature pulp treatment and thus translates to a significant reduction in thermal energy costs. This study showed that gangue mineralogy plays a very important role in flowsheet and reagent regime selection on the processing of the different fluorite ores. Although an acid-grade CaF2 concentrate could be obtained from both ores, the flowsheets and reagent regimes were markedly different. Multiple stages of cleaning were required for improved selectivity in the flotation of both ores. Production of an acid-grade CaF2 product from a high quartz and calcite ore was achieved by employing a simple rougher–multiple cleaner flotation circuit using tannin and sodium silicate as calcite and quartz depressants, Minerals 2020, 10, 237 13 of 13

respectively. On the other hand, the production of an acid-grade CaF2 product flotation of the pyritic ore required a pre-sulphide flotation stage for upfront sulphur removal and a sulphide depressant during cleaning to meet the market specification with regards to sulphur content. The results showed that an understanding of gangue mineralogy is key to process optimisation for acid-grade CaF2 production.

Author Contributions: J.T., K.C.T., and P.N. conceived the research idea and designed the experiments; K.C.T. and P.N. conducted the experiments; J.T., K.C.T., and P.N. analysed the results; J.T. interpreted the data; J.T. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge Mintek’s commercial clients for granting the permission to publish information gained from the testwork. Support from the Mineral Processing Division’s technical team at Mintek is greatly appreciated. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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