Helmholtz-Zentrum Dresden-Rossendorf (HZDR)

Role of sodium carbonate in scheelite flotation – a multi-faceted reagent

Kupka, N.; Rudolph, M.;

Originally published:

October 2018 Minerals Engineering 129(2018), 120-128

DOI: https://doi.org/10.1016/j.mineng.2018.09.005

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https://www.hzdr.de/publications/Publ-27114

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Role of sodium carbonate in scheelite flotation – a multi-faceted reagent 1 1,a 1 2 Nathalie Kupka ; Martin Rudolph 3 1 Helmholtz Institute Freiberg for Resource Technology, Helmholtz Zentrum Dresden-Rossendorf, Chemnitzer 4 Straße 40, 09599 Freiberg, Germany 5 a 6 [email protected] 7 8 Abstract 9 10 Even though sodium carbonate is a reagent frequently used in flotation, its role is mostly described as 11 a buffering pH modifier and a pulp dispersant. In the case of scheelite flotation, it has been hinted that 12 13 sodium carbonate improves both grade and/or recovery but the mechanism itself is ambiguous at best 14 or at least has not been distinctly reported in the literature. Furthermore, the addition of depressants 15 such as sodium silicate or quebracho could be triggering additional mechanisms. Through batch 16 flotation testwork on a skarn scheelite ore with high calcite content, single mineral flotation and 17 contact angle measurements, this article aims at demonstrating that sodium carbonate is a multi- 18 faceted reagent, which serves as a buffering pH modifier, a pulp dispersant precipitating calcium and 19 20 magnesium ions in suspension, a depressant for calcite and calcium silicates and also a promoter for 21 scheelite. It acts mostly synergistically and partially antagonistically with other depressants, notably 22 sodium silicate and quebracho. 23 24 Keywords: sodium carbonate, scheelite flotation, mechanism 25 26 1. Introduction 27 A typical reagent regime for scheelite flotation is called the “lime flotation process”, which involves 28 29 the addition of lime and sodium carbonate as co-pH modifiers, sodium silicate as a depressant and 30 oleic acid (or sodium oleate) as the main collector [1, 2]. 31 The presence of lime could be considered as ill-advised as it was commonly observed that calcium had 32 33 deleterious effects in fatty acid flotation systems [3]. Indeed, Arnold et al. [4] and Gao, Z. et al. [5] 2+ 2+ 34 showed that scheelite recovery could decrease drastically at very high concentrations of Ca or Mg 35 when using sodium oleate as a collector, especially since it is sensitive to water hardness [6]. 36 37 This is where sodium carbonate comes along: this reagent precipitates calcium and magnesium ions 38 into CaCO3 and MgCO3 if the pH is high enough, as well as heavy metal ions [7-12], limiting their 39 presence in the pulp. Additionally, sodium carbonate, namely its carbonate ion, is sensitive to pH and 40 can end up precipitating onto mineral surfaces [7, 9] such as fluorite [13, 14] or calcite [1]. This means 41 it could end up depressing said minerals. 42 43 Sodium carbonate (or soda ash) is therefore a buffering pH modifier [8, 10, 15], a pulp dispersant [7- 44 10] and a depressant at the same time [7]. In the case of scheelite flotation, it has been shown that 45 46 sodium carbonate improves both grade and/or recovery [16-19] but the mechanism itself is 47 ambiguous or at least has not been distinctly reported in the literature for scheelite. Furthermore, the 48 addition of depressants such as sodium silicate or quebracho could be triggering new mechanisms [3, 49 16, 17]. 50 51 As a consequence, this article aims at describing the effects of sodium carbonate in order to identify 52 or at least hypothesize its role in froth flotation of scheelite. 53 54 2. Material and Methods 55 56 Single minerals Scheelite and calcite were the feed materials for single mineral flotation 57 (microflotation). Scheelite came from the Mittersill mine, Austria, and was concentrated through 58 gravity separation, sulphide flotation and magnetic separation. The target size fraction was 32-63µm. 59 X-ray diffraction determined that the sample was composed of 96 wt% scheelite with pyrite, 60 molybdenite and other minerals as contaminants. Calcite came from Straßberg, Germany and the 63- 61 62 100µm fraction was selected. Purity of the calcite sample was determined by X-ray diffraction and 63 64 65 showed that it was composed of 97 wt% of calcite, 1wt% of quartz and minor amounts of fluorite, 1 ilmenite and others. 2 3 Microflotation was conducted in a Hallimond tube designed at the TU Bergakademie Freiberg. 1g of -2 4 the single minerals is added to an aqueous solution of 10 M KCl, stirring on a magnetic stirrer with a 5 rotation speed of 400rpm. 3 to 5 minutes are allocated to pH conditioning with HCl and NaOH. After 6 achieving stable pH, the depressant is added before the collector with a conditioning time of 2 and 3 7 minutes respectively. The flotation suspension is transferred to the Hallimond tube and agitated for 8 9 another 3 minutes with a rotational speed of 800rpm using a magnetic stirrer. Airflow rate is set to 20 10 cm³/min and microflotation lasts 2 minutes. The filtered flotation products are dried in an oven 11 overnight at 50-60°C and flotation recovery and loss are calculated based on said dried weights. All 12 experiments were conducted at least twice, up to five times, the number of repetitions being driven 13 by the mass loss, which had to be strictly inferior to 10%. 14 15 Batch flotation The ore used for batch flotation tests contains 0.51% scheelite, 1.70% calcite, 0.28% 16 apatite and various silicates (quartz, micas, plagioclases and hornblende). The ore was ground with a 17 rod mill and has a d of 63µm after milling. Froth flotation tests were conducted with 500g of ore at 18 80 19 33% pulp density in a Magotteaux® bottom-driven flotation cell and at constant pH of 9.5. Each had 20 only a rougher stage, lasted seven minutes and involved three concentrates with a 5 second scraping 21 rate, 450 rpm impeller speed and an air flowrate of 5L/min. The filtered flotation products are dried in 22 an oven overnight at 50-60°C and mass pull is calculated based on said dried weights. All samples were 23 split and send to ICP-MS and XRF for chemical assays (done by ALS Global®). It should be noted that all 24 25 calcium grades given in this article have been recalculated by removing calcium contained in scheelite. 26 The feed and tailings water samples were sent to ICP-OES for cation assays (done by the Technische 27 Universität Bergakademie Freiberg, Germany). Finally, automated mineralogy conducted on the feed 28 of flotation (after milling) showed that minerals are well liberated, with a surface liberation of 96% for 29 scheelite. 30 31 Contact angles Contact angle measurements were conducted on epoxy resin grain mounts of scheelite 32 and calcite with a Dataphysics OCA 50 instrument equipped with the SCA 20 software. The pure 33 scheelite crystal came from Malakka, India whereas the pure calcite crystal came from Mittersill, 34 -2 35 Austria. The single crystal is conditioned in a background solution of 10 mol/l KCl and the reagents 36 (also prepared with KCl) are added one after the other after setting the pH at 9 with NaOH. The grain 37 mount is then dried with an air blower and placed on the instrument. Measurement is done with the 38 sessile drop method with water droplets of a constant volume of 1µL. Once the measurement is 39 complete, the substrate is polished and cleaned in a plasma cleaner before being used again. Each 40 point displayed on the diagrams corresponds to at least 12 to 20 measurements unless the angle is 41 42 lesser than 10°, then it corresponds to 5 measurements. 43 Reagents Sodium oleate (C H NaO ) of >90% purity from Carl Roth® was used as a collector at a 44 18 33 2 45 single dosage of 200g/t in order to avoid multicomponent technical grade collectors with effects 46 difficult to evaluate. A complex mixture of glycols, Flotanol 7197® from Clariant Mining Solutions®, was 47 used as a frother at 20g/t. Four pH modifiers were used: analytical grade sodium hydroxide (NaOH), 48 lime (CaO), and HCl from Carl Roth® and analytical grade sodium carbonate (Na2CO3) from Alfa Aesar®. 49 Analytical grade sodium chloride (NaCl) from Carl Roth® was used for microflotation. Depressants 50 included analytical grade of sodium metasilicate nonahydrate (Na O Si.9H O) from Aldrich Chemistry® 51 2 3 2 52 and extract of quebracho from Baeck Gmbh & Co. 53 54 3. Impact as a co-pH modifier 55 Different pH modifiers can be used in scheelite flotation, such as sodium hydroxide, sodium carbonate 56 or calcium oxide. As described before, sodium carbonate can be considered a co-pH modifier, even 57 58 though its addition fulfills several roles. In a series of scheelite batch flotations, the authors compared 59 the impact of the pH modifier type by using NaOH and CaO alone or in combination with sodium 60 carbonate. The authors chose not to use sodium carbonate alone because: 61 62 63 64 65  Vazquez et al. [1] note in their scheelite flotation that sodium carbonate alone (without lime 1 or NaOH) did not precipitate any dissolved calcium in their case and that these calcium ions 2 did not have any impact on either grade or recovery anyways. 3 4  By analyzing De Castro et al. [20]’s data, it appeared that sodium carbonate alone compared 5 to sodium hydroxide did not offer higher grades or higher selectivity against calcite in the 6 flotation of celestite (SrSO4). 7 8 3.1. General observations 9 10 When looking strictly at mass pull and water pull curves (Figure 1, top left), it seems the type of pH 11 modifier is irrelevant. The addition of sodium carbonate appears to slow down the flotation rate. It 12 could be therefore considered that sodium carbonate does not interact with the froth whatsoever. 13 The influence is much greater when considering the grade-recovery curves and the tungsten- calcium 14 selectivity curves (Figure 1, top right, bottom left). 15 16 23.0 2.0 17 18 21.0 1.8 19 1.6

19.0 20 1.4 21 17.0 22 1.2 15.0 23 1.0 24 13.0

25 0.8 Mass pull Mass (%)

26 11.0 NaOH 0.6 NaOH Tungsten grade Tungsten (%) 27 NaOH + SC NaOH + SC 28 9.0 0.4 CaO CaO 29 7.0 0.2 CaO + SC CaO + SC 30 0.0 31 5.0 40.0 50.0 60.0 70.0 80.0 90.0 32 3.0 8.0 13.0 18.0 23.0 28.0 33 Water pull (%) Tungsten recovery (%) 34 95.0 35 90.0 36 90.0 37 80.0

38 70.0 85.0 39 40 60.0 80.0 41 42 50.0 75.0 43 44 40.0 70.0 45 Scheelite (Ref-NaOl) NaOH Recovery (%) 65.0

Tungsten recovery recovery Tungsten (%) 30.0 46 NaOH + SC Scheelite (SC) 47 CaO Calcite (Ref-NaOl) 20.0 60.0 48 CaO + SC Calcite (SC) 49 No selectivity 10.0 55.0 50 10.0 20.0 30.0 40.0 1.00E-05 1.00E-04 1.00E-03 1.00E-02 51 Calcium recovery (%) SC concentration (mol/l) 52 53 Figure 1: top left – mass pull against water pull depending on the pH modifiers in use, top right – grade-recovery curves, 54 bottom left – selectivity curves and bottom right – Single-mineral flotation results for scheelite and calcite, pH is set at 9 with 55 NaOH, sodium oleate is added at a dosage of 1x10-4 mol/l (Ref-NaOL are experiments without sodium carbonate) 56 57 The difference between CaO and NaOH is limited, using CaO does improve the grade but does not 58 impact the recovery, which has been observed by Vazquez et al. [1] in the seventies. These authors 59 had also stated that using NaOH produced a poor flotation response, which is not the case here. The 60 presence of sodium carbonate has a deep impact on the grade and recovery of tungsten. It seemingly 61 erases the difference between NaOH and CaO while improving the flotation performance and 62 63 64 65 selectivity against calcium-bearing minerals (except apatite which is not represented here) as well as 1 silicates (also not represented here). 2 3 Taking into account the fact that in this ore, most of the calcium is contained in silicate minerals, it 4 could be considered that sodium carbonate increases selectivity against calcium-containing silicates 5 and have no interaction with calcite. However, single-mineral flotation showed this was not the case 6 (Figure 1, bottom right) as sodium carbonate promotes scheelite and depresses calcite specifically. 7 This additional selectivity against calcite between NaOH and sodium carbonate was signaled by 8 9 Klassen and Mokrousov [21] in the sixties without specifying for which flotation type. 10 These microflotation results are the exact opposite of those obtained by Patil and Nayak [22], whose 11 scheelite single-mineral flotation is affected in the presence of high concentrations of sodium 12 13 carbonate. In their case, calcite recovery remained unaffected. They worked however at higher pH 14 (10), lower collector dosage and shorter flotation times. 15 16 3.2. Calcium and magnesium ions in the pulp 17 Figure 2 shows the enrichment rate in calcium and magnesium ions between the feed water and the 18 tailings water depending on the pH modifier in use. Iron has also been measured but is ignored as the 19 values were too close to zero to be reliable. It has been stated before that sodium carbonate limits 20 calcium and magnesium ions in the pulp. Comparing NaOH to NaOH + SC, it appears clearly that the 21 2+ 2+ 22 Ca and Mg have been reduced, calcium more intensely so. This had been observed by Dean and 23 Schack [23] in the batch flotation of five different calcareous scheelite ores from the USA, who stated 24 that “sodium carbonate was more effective than sodium hydroxide as an overall pulp-regulating 25 reagent”. Between CaO and NaOH, it is clear that calcium is much more reduced since the input of 26 calcium ions is higher when using lime. In their flotation of Co-Mo sulphide ores, Jeldres et al. [24] 27 28 wrote that lime as an alkalinizing reagent is a “promoter of magnesium precipitation” whereas sodium 29 carbonate precipitates calcium but the magnesium reduction is “conservative”. In the present case, 30 when comparing CaO to CaO + SC, there indeed has been a magnesium reduction but the presence of 31 sodium carbonate resulted in lower calcium reduction. 32 33 1 34 0.9 35 0.8 36 37 0.7 38 0.6 39 0.5 40 0.4 41 42 0.3 43 0.2 44 0.1 45 0 46 NaOH NaOH+SC CaO CaO+SC 47 Ca reduction Mg reduction 48 49 Figure 2: Calcium and magnesium ions reduction in the tailings water (SC being the abbreviation of sodium carbonate) 50 51 As a whole, these ions do not seem to play a role in the present flotation experiments and the pulp 52 dispersing properties of sodium carbonate are not of primary importance here. 53 54 3.3. Flotation kinetics 55 Finally, the experimental flotation rate k and the associated maximum recovery at infinite flotation 56 57 time Rmax for tungsten, calcium and silica has been calculated based on the classical first order kinetics 58 equation [25] (Figure 3). Interestingly, if the presence of sodium carbonate clearly slows down the 59 flotation rate for calcium, it does not impact the Rmax. This could mean that sodium carbonate first 60 depresses calcite, and that the dosage is not enough to depress calcium silicates, which will end up in 61 the slow-floating fraction or be entrained. For silica, Rmax logically increases, because the non-floated 62 63 64 65 calcium-bearing minerals are replaced with the silica gangue, while the flotation rate constant still 1 decreases because calcium silicates are slowed down. For tungsten, if sodium carbonate does improve 2 the Rmax, it either barely impacts its flotation rate or slows it down. The effect here is unclear. Its Rmax 3 appears correlated with the one from silicates, but this is only a coincidence since MLA (Mineral 4 Liberation Analysis) conducted on the feed of flotation showed that scheelite was ~ 96% liberated. 5 6 1.2 100 7 8 90 1

9 80 1)

10 - 70 11 0.8 12 60 13 0.6 50 14

15 40 (%) Rmax 0.4 16 30 17 Experimentalk (min 20 18 0.2 19 10 20 0 0 21 NaOH NaOH + SC CaO CaO + SC 22 23 k (WO3) k (Ca) k (SiO2) Rmax (WO3) Rmax (Ca) Rmax (SiO2) 24 25 26 Figure 3: Flotation kinetics - experimental k and Rmax of tungsten, calcium and silica 27 3.4. Short summary 28 29 As a summary, in the absence of depressants, the presence of sodium carbonate yields the following 30 observations: 31 32  The absence of impact on mass and water pull would support the idea that sodium carbonate 33 does not interact with the froth in any way; 34  It increases tungsten recovery and grade, clearly showing a promoting effect for scheelite. It 35 also increases selectivity against calcium-bearing minerals, notably calcite, and silicates. This 36 37 increase in selectivity could be linked to the fact that sodium carbonate delays the flotation of 38 calcium minerals by depressing calcite and only a part of the calcium silicates, which will end 39 up either entrained or in the slow floating fraction. 40 41  Magnesium reduction always take place and is intensified by sodium carbonate. The behavior 42 of calcium ions is inconclusive. Besides, their impact in the present flotation is virtually non- 43 existent while using sodium carbonate. Between CaO and NaOH alone however, calcium ions 44 in suspension might be improving concentrate grade. 45 46 47 4. Interaction with depressants: example of sodium silicate and quebracho 48 Sodium carbonate alone does behave as more than a pH modifier, it could be both a promoter for 49 50 scheelite and a depressant for calcium-bearing minerals, especially calcite but excluding apatite. Yet, 51 these effects might vary in the presence of additional depressants. In order to test this hypothesis, 52 two classic depressants for scheelite flotation have been used, namely sodium silicate and quebracho, 53 sodium silicate being the primary focus of this article and quebracho allowing for a comparison. 54 55 A flotation testwork was set up using DesignExpert® software (Version 10.0.5.0) from Stat-Ease, Inc, 56 with three parameters with several levels: the type of pH modifier (4 levels: NaOH, lime and both with 57 sodium carbonate), the type of depressant (2 levels: sodium silicate and quebracho) and the dosage of 58 the depressant (3 levels). The dosages of water glass and quebracho were adapted based on previous 59 60 flotation testwork conducted on the ore in use: dosages of sodium silicate were 500, 1000 and 61 62 63 64 65 2000g/t and quebracho 15, 30 and 60g/t, while sodium carbonate was added at a constant dosage of 1 100g/t. 2 3 This flotation program bore in mind that Crozier et al. [26] considered that two levels for a variable 4 would detect trends only, three would detect the existence of a response curve but not its shape, and 5 more than three levels would progressively refine the shape. The testwork includes repetitions of the 6 experiments. DesignExpert® was then used to analyze the data through ANOVAs and create response 7 models with predictive properties, as can be seen in the following figures. 8 9 4.1. Mass pulls 10 First, in terms of mass pull (Figure 4), the presence of sodium silicate leads to an intense decrease in 11 12 mass collected in the concentrates which is expected, similarly to quebracho, where the decrease is 13 more moderate but still marked. Water pulls show the same trends and froth effects cannot be 14 ignored. The trends are almost identical for both depressants: using CaO as a pH modifier is at the 15 beginning less affecting the mass pull than NaOH is but then brings about a greater decrease in mass 16 pull with increasing dosage of depressants. This difference between CaO and NaOH fades completely 17 18 in the presence of sodium carbonate. 19 20 25 25 21 22 23 20 20

24

25 26 15 15 27 28 29

10 10 NaOH Mass pull Mass (%) 30 pull Mass (%) NaOH + SC 31 CaO 32 CaO+SC 33 5 5 Ref-NaOH 34 Ref-NaOH+SC Ref-CaO 35 Ref-CaO+SC 36 0 0 37 500 1000 1500 2000 2500 15 25 35 45 55 65 75 38 Sodium silicate dosage (g/t) Quebracho dosage (g/t) 39 40 Figure 4: Mass pull depending on the type of pH modifier and the depressant dosage with left sodium silicate and right 41 quebracho, as a result of statistical analyses of the experimental results, legend is the same for both diagrams, Ref is the 42 reference value of the mass pulls for tests without depressants 43 4.2. Tungsten grade and recovery 44 45 Logically, when the mass pull decreases massively, the recoveries of minerals will follow. Therefore, it 46 is not surprising to see that sodium silicate induces an intense decrease in tungsten recovery with 47 higher dosages (Figure 5). This decrease is much more limited for quebracho. In the case of sodium 48 silicate, the tungsten grade is doubled to quadrupled for the same recovery as the reference tests. 49 Interestingly, even though quebracho is often regarded as a better depressant in scheelite flotation 50 than sodium silicate, it does not seem to be superior to sodium silicate in its lower dosages for this 51 52 ore. If there is a slight improvement in grade, it’s mostly in the fact that higher dosages do not impact 53 the tungsten recovery as much that quebracho might be considered better. In the absence of sodium 54 carbonate, NaOH appears as the better pH modifier compared to CaO, which is the opposite result of 55 when no depressant is in play. 56 57 58 59 60 61 62 63 64 65 3 3 1 NaOH NaOH+SC 2 CaO CaO+SC 3 2.5 2.5

4 Ref-NaOH Ref-NaOH+SC

5 Ref-CaO Ref-CaO+SC 2 2 6 7 2500g/t 8 1.5 1.5 9 500g/t 10

11 1 1 75g/t Tungsten grade Tungsten (%) 12 grade Tungsten (%) 13 15g/t 14 0.5 0.5 15 16 0 0 17 30 40 50 60 70 80 90 30 40 50 60 70 80 90 18 Tungsten recovery (%) Tungsten recovery (%) 19 20 Figure 5: Tungsten grade-recovery curves depending on the type of pH modifier and the depressant dosage with left sodium 21 silicate (dosages from 500 to 2500g/t increasing from l to left) and right, quebracho (dosages from 15 to 75g/t increasing 22 from right to left), as a result of statistical analyses of the experimental results, legend is the same for both diagrams, Ref is 23 the reference value of the tungsten grade-recoveries for tests without depressants 24 25 Single mineral flotation might provide part of an explanation to these observations (Figure 1 bottom 26 right and Figure 6). When tested with only sodium oleate and sodium silicate, scheelite was slightly 27 depressed by sodium silicate whereas calcite was almost completely depressed. For sodium oleate 28 and sodium carbonate, scheelite is promoted while calcite is slightly depressed with increasing dosage. 29 It’s in presence of both sodium silicate and sodium carbonate that flotation seems most effective: on 30 31 one hand, calcite flotation improves slightly and seems to reach an average between SS and SC alone 32 at high sodium carbonate dosages but is yet depressed. On the other hand, any depression effect on 33 scheelite disappears and the mineral is actually promoted. It would seem that sodium carbonate 34 largely prevents interaction between scheelite and sodium silicate and more moderately between 35 calcite and sodium silicate. These microflotation tests have not been conducted with quebracho. 36 37 38 90.0 39 80.0 40 70.0 41 42 60.0 43 50.0 44 45 40.0 46 30.0 47

48 20.0 Recovery Recovery (%) 49 10.0 50 51 0.0 52 1.00E-05 1.00E-04 1.00E-03 1.00E-02 53 Sodium carbonate concentration (mol/l) 54 Scheelite (Ref-SS) Scheelite (SC) Scheelite (SC+SS) 55 Calcite (Ref-SS) Calcite (SC) Calcite (SC+SS) 56 Figure 6: Single-mineral flotation results for scheelite and calcite, pH is set at 9 with NaOH, sodium oleate is used as a collector 57 -4 -2 58 at a dosage of 1x10 mol/l, sodium silicate is used as a depressant at a dosage of 1x10 mol/l (Ref-SS referring to experiments without sodium carbonate and with sodium silicate) 59 60 61 62 63 64 65 4.3. Selectivity against calcium, phosphate and silica 1 Tungsten-calcium selectivity curves (Figure 7) provide additional information to the tungsten grade- 2 recovery curves. The difference between NaOH and CaO is less marked in the selectivity against 3 calcium when using sodium silicate than quebracho but follows the same trend of a moderate 4 5 superiority of NaOH. According to Iskra et al. [27], quebracho adsorbs onto scheelite, fluorite and 6 calcite surfaces via lattice and pulp calcium ions in different proportions depending on the mineral and 7 therefore competing with the collector on the mineral surface. They consider therefore that excess 8 calcium ions in the system will cause quebracho to bind to scheelite as well, which would fit with the 9 present results with CaO. 10 11 As a consequence of the previous observations, it is only logical that the selectivity against calcium is 12 much higher for sodium silicate than quebracho, as the mass pull and the tungsten grade-recoveries 13 are much lower. Finally, as previously stated, the presence of sodium carbonate erases any difference 14 15 between CaO and NaOH and improves the overall performance. At low dosages, sodium silicate 16 performs better than quebracho but with increasing dosage, quebracho allows higher selectivity 17 without losing any tungsten. 18 19 Selectivity against phosphate (represented only by apatite in this ore) has also been investigated. 20 Similarly to the absence of depressant, regardless of the pH modifier and the depressant in play, 21 selectivity against apatite is non-existent. The presence of sodium carbonate actually promotes 22 apatite. 23 24 Selectivity against silica shows the same trends as with selectivity against calcium. Silicates, notably 25 quartz, are activated in the presence of calcium ions on the basis of exchanges between Ca2+ and 26 hydrogen ions on the silicates surface [19]. This would explain why selectivity is lower when using CaO 27 28 on its own. Sodium silicate is much more selective against silicates than quebracho, which is logical 29 based on the fact that sodium silicate is originally a depressant for silicates. 30 31 90 90 32 33 80 80

34

35 70 500g/t 70 36 15g/t 37 60 60 38 2500g/t 39 50 50 75g/t NaOH 40 NaOH+SC 41 40 40 CaO

42 CaO+SC Tungsten recovery recovery Tungsten (%) 43 30 recovery Tungsten (%) 30 No selectivity 44 Ref-NaOH Ref-NaOH+SC 45 20 20 Ref-CaO 46 Ref-CaO+SC 47 10 10 48 10 15 20 25 30 35 40 10 20 30 40 49 Calcium recovery (%) Calcium recovery (%) 50 51 Figure 7: Selectivity curves of tungsten against calcium depending on type of pH modifier and the depressant dosage with left 52 sodium silicate (dosages from 500 to 2500g/t increasing from right to left) and right, quebracho (dosages from 15 to 75g/t 53 increasing from right to left), as a result of statistical analyses of the experimental results, legend is the same for both 54 diagrams, Ref is the reference value of the selectivity curves without depressants 55 56 4.4. Calcium and magnesium ions in the pulp 57 Understandably, calcium ions behave differently in the presence of depressants. In the case of sodium 58 silicate, the higher the dosage, the greater the calcium ion concentration in the tailings water, 59 regardless of the pH modifiers in use. The concentration increases so greatly, that without any sodium 60 carbonate, calcium ions are actually enriched instead of being reduced and if they are reduced, it’s at 61 62 63 64 65 a higher rate than in the absence of depressants. It seems therefore that sodium silicate increases the 1 solubility of calcium-bearing minerals, which could very well be the calcium-silicates. 2 3 As a consequence, the pulp dispersing properties of sodium carbonate play a greater role here and the 4 possibility that it can reduce calcium ions is actually visible, especially in the case of CaO+SC, where 5 the reduction is increased compared to CaO on its own or without depressant. It is also possible that 6 sodium carbonate does not precipitate the calcium ions in suspension but simply prevents sodium 7 silicate from increasing the solubility of calcium-bearing minerals. It could simply be a combination of 8 9 both. 10 The behavior of calcium ions is entirely different with quebracho. Without sodium carbonate, calcium 11 ions are slightly more or slightly less reduced for NaOH and CaO alone respectively. This reduction is 12 13 unchanged throughout the dosage increase. It could be considered that either quebracho does not 14 interact at all with ions in the pulp, hence the almost flat lines, or, based on the theory of Iskra et al. 15 [27], quebracho consumes the calcium ions arising from solubility in a stable manner, which would 16 assume that at higher dosages not all the quebracho is consumed or it consumes calcium ions (this is 17 unlikely). Interestingly, the fact that the calcium reduction remains unchanged with the dosage also 18 19 means that sodium oleate does not interact with calcium ions at all, as they are not floated with the 20 concentrate. Sodium carbonate causes a moderate intensification of the reduction, greater for CaO 21 than for NaOH. Similarly to the case with sodium silicate, there are three possibilities: either sodium 22 carbonate also competes on the mineral surface with quebracho, preventing its absorption and 23 forcing its precipitation with ions in the pulp, or it reduces calcium ions directly or it does a bit of both. 24 25 1.4 1.4

26 NaOH

27 NaOH+SC 28 1.2 1.2 CaO 29 CaO+SC 30 1 1 Ref-NaOH 31 Ref-NaOH+SC 32 Ref-CaO 0.8 0.8 Ref-CaO+SC 33 intailings water 34 35 0.6 0.6 36 37 0.4 reduction 0.4 38

39 Calcium reduction Calcium in tailings water 0.2 Calcium 0.2 40 41 42 0 0 43 500 1000 1500 2000 2500 15 25 35 45 55 65 75 44 Sodium silicate dosage (g/t) Quebracho dosage (g/t) 45 Figure 8: Calcium ions reduction in tailings water compared to flotation feed water depending on the type of pH modifier and 46 the dosage of depressant with left, sodium silicate and right, quebracho, as a result of statistical analyses of the experimental 47 results, legend is the same for both diagrams, Ref is the reference value of the calcium reduction in tailings water for tests 48 without depressants 49 50 It could be expected that the behavior of magnesium ions in the presence of depressants would be 51 similar if not the same like calcium ions (Figure 9). In the case of quebracho, it almost holds true. With 52 the fact that without sodium carbonate the curves remain flat and very close to those without 53 54 quebracho, it can be safely concluded that quebracho does not interact with magnesium ions in the 55 pulp and that these do not float with the concentrate or interact with sodium oleate. The main 56 difference resides in the fact that both combinations of CaO+SC and NaOH+SC behave similarly, which 57 implies a much greater magnesium reduction in the presence of quebracho as compared to CaO+SC 58 without depressant and an increase then a decrease in magnesium reduction in the presence of 59 60 quebracho as compared to NaOH+SC without depressant. This would imply that sodium carbonate 61 does reduce magnesium ions in the pulp. 62 63 64 65

1 1

1 2 3 0.9 0.9 4 5 6 0.8 0.8 7 8 0.7 0.7 9 10 NaOH 11 NaOH + SC 0.6 0.6 CaO 12 CaO+SC 13 Ref-NaOH 14 0.5 0.5 Ref-NaOH+SC

15 Ref-CaO Magnesium Magnesium enrichment tailings in water 16 Magnesium enrichment tailings in water Ref-CaO+SC 17 0.4 0.4 18 500 1000 1500 2000 2500 15 25 35 45 55 65 75 Sodium silicate dosage (g/t) Quebracho dosage (g/t) 19 20 Figure 9: Magnesium ions reduction in tailings water compared to flotation feed water depending on the type of pH modifier 21 and the dosage of depressant with left, sodium silicate and right, quebracho, as a result of statistical analyses of the 22 experimental results, legend is the same for both diagrams, Ref is the reference value of the magnesium reduction in tailings 23 water for tests without depressants 24 25 In the case of sodium silicate, the behavior of magnesium ions does not correspond to that of calcium 26 ions. Magnesium reduction is intensified in the presence of sodium silicate but the intensity decreases 27 with increasing dosage until it doesn’t seem to affect it anymore above 2000g/t. With the presence of 28 sodium carbonate, magnesium reduction is intensified only for CaO+SC and even though NaOH+SC 29 behaves quite similarly, it is too close to the point without depressant to draw any conclusions. 30 31 4.5. Short summary 32 In the presence of depressants, very different observations are collected: 33 34  Sodium silicate impacts the mass pull much more intensely than quebracho, yielding poorer 35 36 recoveries but higher tungsten grades and a greater selectivity against calcium. Any 37 interaction with the froth cannot be excluded based on the current flotation tests. 38  Sodium silicate only outperforms quebracho in its lower dosages, any increase in dosage 39 clearly demonstrates the superiority of quebracho as a depressant for this ore. 40 41  Contrary to the absence of depressants where CaO improved tungsten grade, with 42 depressants, NaOH performed better as a pH modifier, allowing a higher tungsten grade and 43 even a higher selectivity when used with quebracho. This could be linked to the fact that in 44 45 calcium-enriched pulps, quebracho may bind to scheelite surfaces, ending up in its 46 depression. 47  The presence of sodium carbonate limits or prevents any loss in tungsten while improving 48 tungsten grade in the case of quebracho and also erases any previous differences between 49 50 CaO and NaOH especially for tungsten-grade recoveries, selectivity against calcium and silica, 51 and magnesium ions reduction, with the exception of calcium ions reduction. 52  Sodium silicate causes an enrichment in calcium ions in the tailings water, which could mean 53 54 that it increases the solubility of calcium-minerals. Based on literature, fluorite and calcite 55 would not be concerned but scheelite. It is highly possible that sodium silicate increases the 56 solubility of calcium silicates in particular. 57  58 Quebracho on its own does not interact with calcium or magnesium ions in the pulp. 59 60 61 62 63 64 65

1 2 5. Mechanisms of sodium carbonate, a multi-faceted reagent 3 Sodium carbonate is clearly a crucial reagent in scheelite flotation, with a large range of combined 4 5 impacts beyond pH setting. The role of sodium carbonate is therefore not limited to precipitating and 6 limiting calcium and magnesium ions in the pulp or to buffering pH, and not just depressing gangue 7 but also promoting scheelite. 8 9 A flotation involving semi-soluble salt type minerals such as scheelite, calcite, fluorite and calcium 10 silicates, and calcium and magnesium ions in the pulp coupled to the use of several reagents leads to a 11 very complex speciation system. This chapter aims at identifying the potential mechanisms of sodium 12 carbonate in such a flotation environment, based on thermodynamics calculations available in the 13 literature and the observations made throughout this article’s testwork. The authors remind that all 14 15 flotation tests were conducted at a constant pH of 9.5. This affects the interpretation of the 16 mechanisms and said mechanisms might vary at lower and higher pH values. 17 18 5.1. Sodium carbonate on its own 19 Sodium carbonate has a distinct depressing effect on calcium minerals of the gangue, which would 20 include not only calcite but also calcium silicates. The most logical mechanism seems to be surface - 21 precipitation of sodium carbonate through its anion HCO3 , as it is its main anion in solution at pH 9.5 22 according to Feng and Luo [11] (Figure 10, left) and Fuerstenau, D. W. et al. [28] and it was stated that 23 2- 24 it specifically adsorbs at the calcite/ aqueous solution interface [28].The CO3 anion would also have 25 this ability. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

42 Figure 10: Right - distribution coefficients of various species in Na2CO3 solution as a function of pH modified from [11] and left 43 - Species distribution diagram of calcium (for a calcium concentration of 2x10-4 mol/l, in the present flotation testwork, 44 calcium concentration varied between 7.73x10-4 mol/l in the feed water to 4.82x10-4 mol/l in the tailings water) modified from 45 [29] 46 47 Vazquez et al. [1] observed that adding lime would tend to reverse the surface charge of fluorite, 48 calcite and scheelite through the adsorption of Ca2+ which would be followed by increased 49 precipitation of CaCO3 when adding sodium carbonate to adjust pH. More importantly, the surface 50 charge would not be affected in the same way, as scheelite’s surface remains negative unless the lime 51 dosage becomes too important, inducing calcite precipitation onto scheelite. This would be 52 53 compatible with the results of Arnold and Warren [30], who showed that calcium ions and sodium 54 carbonate had little effect on the zeta potential of synthetic scheelite over a pH range of 4 to 12 55 whereas magnesium ions would lead to a sign reversal above pH 10 (which is outside the current 56 scope of the article). Furthermore, calcium silicates in this article’s ore include mostly anorthite and 57 titanite and both have a positive surface charge at pH 9 [31], so sodium carbonate could also 58 59 precipitate onto them, if it was only a question of charge. 60 In the present testwork however, this depression occurs regardless of the main pH modifier in use, 61 NaOH or CaO. This concurs with Rahimi et al. [32] who also showed surface precipitation of sodium 62 63 64 65 carbonate onto negatively-charged calcite through zeta potential, contact angle and FT-IR 1 measurements while setting the pH with NaOH in their cationic flotation of pyrolusite. This does 2 partially contradict Vazquez et al. [1] since surface charge positivity does not seem to be a 3 requirement but could be a facilitating parameter. 4 5 This would mean that sodium carbonate already selectively precipitates on other calcium minerals 6 than scheelite. Calcite and fluorite (anecdotal in this ore) exhibit a stronger reactivity and higher 7 calcium surface site density at its mineral surface than scheelite [33-35] meaning that it can 8 accommodate the adsorption of more sodium carbonate, “neutralizing” the mineral by precipitating 9 – 2- 10 as CaCO3 through HCO3 (and CO3 ). Based on the kinetics, calcite would be the first mineral to be 11 depressed, followed by calcium silicates but to a lesser extent (calcium surface site density for these 12 minerals is unknown). Additionally, calcium ions can activate silicates on their surface through Ca(OH)+ 13 [19], which exists at pH 9.5 (Figure 10, right) in smaller proportions than Ca2+. Sodium carbonate could 14 15 prevent this not only precipitating onto the silicates surface into CaCO3 as aforementioned but also by 16 neutralizing calcium ions in the pulp. 17 Lastly, since other calcium mineral surfaces are not reactive anymore, sodium oleate absorbs 18 - 19 preferentially onto scheelite surfaces through its oleate anion RCOO ([36] in [18]), which is its main 20 species in solution along with the oleate homodimer (Figure 11). The impact of other anionic oleate 21 species is unknown or was not reported on. Aqueous species such as Ca(OH)(RCOO)(aq), which could be 22 resulting from the interactions between the pulp calcium species and the oleate anion, may be 23 responsible for quartz flotation for example [37]. The effects are therefore multiple. Of course, 24 25 adsorption of sodium carbonate onto scheelite is still possible, otherwise its overdosage would not 26 conduct to tungsten depression as reported by Dean and Schack [23] and Wu et al. [38]. Apatite has a 27 similar calcium density at its surface as scheelite [35], which would also fit with the fact that apatite 28 could not be depressed regardless of the reagent regime. 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Figure 11: Species distribution diagram of sodium oleate at a concentration of 5.26x10-4 mol/l modified from [39] (in the 47 -4 48 flotation testwork, concentration of sodium oleate at the beginning of the tests was of 2.65x10 mol/l) 49 The depression mechanism of sodium carbonate would then be clear. It however does not explain 50 51 why the reagent also promotes scheelite compared to using the collector on its own. It could be 52 simply considered that because sodium carbonate depresses calcite and calcium silicates, more 53 sodium oleate is available to float scheelite and therefore, tungsten is indirectly promoted in flotation. 54 Yet, the mechanism cannot be as simple, because single mineral flotation (Figure 1 bottom right) as 55 well as contact angle measurements performed on scheelite (Figure 12) show that sodium carbonate 56 interacts with scheelite and impacts its contact angle, hence its wettability. This promotion effect 57 58 would also happen if calcite is the only mineral available (Figure 13). 59 There is a clear inflection point at 1x10-3 mol/l of sodium carbonate for scheelite, with or without 60 61 sodium silicate, which shows consistency. Atademir et al. [40] stated that above this concentration, 62 63 64 65 sodium carbonate starts precipitating onto scheelite to form calcium carbonate. It could be that part 1 of the scheelite surface turns into calcium carbonate and starts behaving like calcite, which yields a 2 new increase in wettability with a further increase in the sodium carbonate concentration, since the 3 contact angle at 1x10-2 mol/l sodium carbonate is similar for calcite and scheelite. 4 5 Ratio of sodium carbonate to sodium silicate for the SC+SS range 6 1:100 1:10 1:1

7 100

)

8 ° 90 9 10 80 11 70 12 60 13 14 50 15 40 Scheelite (Ref-NaOl) 16 30 17 Scheelite (SC) 20 Scheelite (SS) 18 Scheelite (SC + SS)

19 10 Average medianAverage contact angle( 20 0 21 1.00E-05 1.00E-04 1.00E-03 1.00E-02 22 23 Sodium carbonate (SC) or sodium silicate (SS) concentration (mol/l) 24 Figure 12: Contact angle measurements on scheelite depending on the sodium carbonate concentration (sodium oleate 25 concentration is fixed at 1.10-4 mol/l, which is the same as in single mineral flotation, sodium silicate concentration is fixed at 26 1.10-2 mol/l for the SC+SS range, pH is 9 and fixed with NaOH) 27 28 Ratio of sodium carbonate to sodium silicate for the SC+SS range 29 1:100 1:10 1:1

30 100

) 31 ° 90 32 80 33 34 70 35 60 36 Calcite (Ref-NaOl) 50 Calcite (SC) 37 Calcite (SS) 38 40 Calcite (SC+SS) 39 30 40 20 41 10

42 medianAverage contact angle( 43 0 44 1.00E-05 1.00E-04 1.00E-03 1.00E-02 45 Sodium carbonate (SC) or sodium silicate (SS) concentration (mol/l) 46 47 Figure 13: Contact angle measurements on calcite depending on the sodium carbonate concentration (sodium oleate 48 concentration is fixed at 1.10-4 mol/l, which is the same as in single mineral flotation, sodium silicate concentration is fixed at 49 1.10-2 mol/l for the SC+SS range, pH is 9 and fixed with NaOH) 50 51 Single mineral flotation experiments with various concentrations of sodium chloride have been 52 conducted on scheelite and show absolutely no impact of the sodium ion on scheelite recovery. This 53 would mean interactions between sodium carbonate and scheelite are through the carbonate species. 54 The promotion effect for scheelite is unclear and needs to be further investigated. 55 56 5.2. Sodium carbonate and depressants 57 The addition of depressants adds a new layer of complexity to sodium carbonate’s role. Indeed, 58 synergetic and antagonistic effects between sodium silicate or quebracho with sodium carbonate are 59 60 apparent. As sodium carbonate can precipitate onto mineral surfaces bearing calcium, it is likely that 61 sodium carbonate competes with the depressants on the mineral surfaces, preferentially on calcite or 62 63 64 65 calcium silicates and even general silicates in the case of sodium silicate. This could be seen in single 1 mineral flotation (Figure 6). 2 3 Sodium silicate, in solution, yields colloidal silica, which can depress calcite [19, 41], as well as - 4 SiO(OH)3 and Si(OH)4, which can depress calcite, fluorite and dolomite [42-44]. Azizi and Larachi [42] - 5 determined that SiO(OH)3 absorbs through covalent or hydrogen bonding whereas Si(OH)4 only 6 - absorbs through hydrogen bonding, SiO(OH)3 being the main adsorbate. Sodium silicate logically also 7 yields Na+, which increases the zeta potential of quartz, and in turn, the silicates form insoluble salts 8 9 with ions replaced from the mineral surface [45]. 10 Based on the observations, it was possible that sodium silicate increases the solubility of calcium- 11 bearing minerals. In literature however, it is stated that sodium silicate reduces the solubility of calcite 12 13 and fluorite as well as their zeta potential, e.g. makes it more negative, whereas it increases the 14 solubility of scheelite and at a high pH of 9-11, silicate ions adsorb on scheelite, also increasing its 15 negative charge [40, 46, 47]. Sodium silicate could nevertheless increase the solubility of calcium 16 silicates by exchanging Na+ ions with Ca2+ ions, which is likely especially in the case of anorthite, as it is 17 part of the plagioclases series where impoverishment in calcium always is compensated by 18 enrichment in sodium. By precipitating onto silicate surfaces, sodium silicate could also prevent their 19 + 20 activation with Ca(OH) , leaving more calcium ions in the pulp. In these two mechanisms that are 21 complementary and as they both possess the sodium ion, sodium carbonate synergistically enhances 22 sodium silicate depressing effects. The sodium carbonate anions left in the pulp can then precipitate 23 the pulp calcium and magnesium. 24 25 For calcite and scheelite, both reagents would again react synergistically, as it is also known that 26 sodium carbonate increases the solubility of scheelite at high dosages [40]. On the basis of calcium 27 surface site density, sodium silicate anions would, similarly to sodium carbonate, preferentially 28 29 depress calcite over scheelite. Therefore, when sodium carbonate precipitates CaCO3 at the silicates 30 surface, it increases yet again sodium silicate’s depressing effect. This synergy is confirmed by contact 31 angles measurements made on calcite (Figure 13), which show that a minimal addition of sodium 32 carbonate to sodium silicate is already sufficient to strongly decrease the wettability of the mineral. 33 34 In single mineral flotation (Figure 6), it appeared that sodium carbonate prevented sodium silicate 35 from depressing scheelite and also calcite to a certain extent, even though the first was added after 36 the second. This had also been observed in single mineral flotation of celestite and calcite by de Castro 37 et al. [48]. This would mean that sodium carbonate can displace sodium silicate from the scheelite 38 39 surface and maintain a high recovery. Nonetheless, if sodium silicate is in too high dosages, it will still 40 depress scheelite as sodium carbonate loses progressively the competition against sodium silicate on 41 the scheelite surface. The effects are antagonistic in this case, which is also apparent from the contact 42 angle measurements made on scheelite (Figure 12). When scheelite starts behaving like calcite at 43 1x10-3 mol/l of sodium carbonate, the depressing effect of sodium silicate is enhanced by sodium 44 -2 45 carbonate, but as the concentration increases to 1x10 mol/l (so for a ratio of 1:1 of both reagents), 46 the wettability increases in a similar way than if only sodium carbonate was being used. 47 The case of quebracho seems much simpler. Similarly to sodium carbonate and sodium silicate, 48 49 quebracho will preferentially depress calcite over scheelite based on the calcium surface site density. 50 This depressing effect is enhanced by sodium carbonate as the latter precipitates as calcite onto the 51 mineral surfaces. Even if they compete on the surface of such minerals, sodium carbonate will mostly 52 enhance quebracho’s depressing power. Quebracho is not as selective as sodium silicate against 53 silicates, because it lacks the sodium ion. It should only depress silicates when those are activated by 54 55 calcium ions in the pulp or carbonated by sodium carbonate, which is why concentrate tungsten 56 grades with quebracho are lower than with sodium silicate. This also explains why much lower 57 dosages of quebracho are required to achieve similar to better results than sodium silicate, especially 58 in terms of tungsten recovery. And because such low dosages are used, the risk of depressing 59 scheelite is limited. 60 61 62 63 64 65 6. Conclusion 1 2 Sodium carbonate appears as a crucial reagent in scheelite flotation, increasing tungsten grade and 3 recovery as well as selectivity against other calcium-bearing minerals. Through batch flotation 4 testwork on a skarn scheelite ore with high calcite content, single mineral flotation and contact angle 5 measurements, robust hypotheses on its mechanisms are drawn: 6 - 7 1) Sodium carbonate precipitates onto the surface of calcium-bearing minerals through its HCO3 , 8 forming CaCO3 layers on said minerals. Additionally, silicates may be activated by calcium ions 9 10 in the pulp, which is either prevented by pulp precipitation or hindered by surface 11 precipitation. Sodium carbonate will preferentially interact with calcite over scheelite, as its 12 calcium surface site density is much higher and allows for more absorption of the reagent. 13 2) Sodium carbonate has a promoting effect on scheelite, increasing its wettability. The exact 14 15 mechanism remains to be investigated. 16 3) Sodium carbonate interacts mostly synergistically with sodium silicate, as they both possess 17 the sodium ion, which increases the solubility of calcium silicates, depressing them, while the 18 excess calcium ions in the pulp are precipitated by the carbonate anion of sodium carbonate. 19 20 4) Sodium carbonate may also counter sodium silicate depressing effect on scheelite by 21 displacing it from the mineral surface. 22 5) Sodium carbonate and quebracho act entirely in synergy as the surface carbonation of 23 24 silicates and calcium minerals by sodium carbonate increases the depressing power of 25 quebracho. However, lacking the sodium ion, quebracho will not depress non-calcium-bearing 26 silicates without the presence of sodium carbonate. This explains why higher grades are 27 achieved with sodium silicate but also why quebracho is a superior depressant, as it will 28 29 increase tungsten recovery even at lower dosages. 30 31 Sodium carbonate is therefore a multi-faceted reagent, which serves as a buffering pH modifier (which 32 should not be used on its own), a pulp dispersant precipitating calcium and magnesium ions in 33 suspension, a gangue depressant and a promoter for scheelite. It acts mostly synergistically and 34 partially antagonistically with other depressants, notably sodium silicate and quebracho. 35 36 7. Acknowledgements 37 38 The authors would like to acknowledge Dashjargal Chinbat, Adrian Van Hall and Ana Paula Resende for 39 conducting the single mineral flotations and the contact angle measurements, as well as Denis Bauer 40 for the flotation tests and Yvonne Volkmar for the ICP-OES analyses. 41 42 This paper was written thanks to the financial support of the OptimOre project. 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