A Review on the Beneficiation Methods of Borate Minerals

Review A Review on the Beneﬁciation Methods of Borate Minerals

Soehoe-Panhyonon Benedict Powoe 1,†, Varney Kromah 1,2,† , Mohammad Jafari 3,† and Saeed Chehreh Chelgani 4,*

1 Department of Geology and Mining Engineering, Faculty of Engineering, University of Liberia, Monrovia 1000, Liberia; [email protected] (S.-P.B.P.); [email protected] (V.K.) 2 School of Environmental Science and Engineering, Tianjin University, Tianjin 300350, China 3 School of Mining Engineering, College of Engineering, University of Tehran, Tehran 16846-13114, Iran; [email protected] 4 Minerals and Metallurgical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden * Correspondence: [email protected]; Tel.: +4-67-3073-7927 † These authors contributed equally to the study.

Abstract: The modern boron applications have adsorbed the mineral processors’ attention to improve typical boron mineral’s (BM) beneficiation methods. In this regard, dry treatment and pretreatment processes—such as magnetic separation and calcination as environmentally friendly methods, due to their minimal or zero adverse effect on the environment—need more consideration. Over the years, anionic flotation has become the main technique for beneficiation of friable BMs; however, there is a gap in the investigation of cationic flotation separation since BMs’ surface negatively charges in a wide pH range. At present, enriching BMs’ flotation via surface modification is taking center stage, which can also be considered for reprocessing long-forgotten BM tailings. As a comprehensive review, this work is going to provide a synopsis of the processes, techniques, optimum parameters, and conditions—such as size reduction, zeta potential, pH, and reagents—which have been employed in Citation: Powoe, S.-P.B.; Kromah, V.; the processing of BMs. Gaps in our understanding of BM’s flotation are presented in the context of Jafari, M.; Chehreh Chelgani, S. A Review on the Beneficiation Methods addressing the existing processes, considering possibilities and rooms for efficiency improvement. of Borate Minerals. Minerals 2021, 11, Considering these gaps may improve the performance of existing methods for processing fine and 318. https://doi.org/10.3390/ ultrafine BMs, and help in the development of new technologies to improve flotation recoveries. min11030318 Keywords: borax; ulexite; colemanite; flotation; magnetic separation; flocculation; calcination Academic Editor: Lev Filippov

Received: 12 February 2021 Accepted: 16 March 2021 1. Introduction Published: 19 March 2021 Over 150 Boron “B” minerals (BM) have been recorded, and the most frequent ones are borax or tyncal (Na2B4O5 (OH)4·10H2O), ulexite (NaCaB5O6 (OH)6·5H2O), colemanite Publisher’s Note: MDPI stays neutral (Ca2B6O115H2O), and kernite (Na2B4O7·4H2O) [1–3]. It was reported that boron com- with regard to jurisdictional claims in pounds are generally produced from brines rich in this element [4,5]. Typically, BMs are published maps and institutional affil- naturally found as hydrated oxides of alkali metals (especially sodium and calcium), which iations. lose crystallization water through the heating [6]. Worldwide, the estimated BM reserves exceed a billion metric tons, against a yearly production of about four million tons. Turkey, where the largest borate reserve globally is found, is followed by Russia, South America, and the United States. These four countries/regions account for an estimated 95% of the Copyright: © 2021 by the authors. world’s Boron reserves (data from Eti Maden AS). Simultaneously, countries such as China, Licensee MDPI, Basel, Switzerland. Italy, and Germany meet only the local demand due to their smaller deposits [7]. This article is an open access article BMs have been used in various applications dating from the eighth century when distributed under the terms and applied in the refining and assaying of gold and silver [8]. They have been used in conditions of the Creative Commons several other applications such as medicines, food preservatives, ceramic glazes, and metal Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ fluxes [9]. The modern applications of BMs and boron derivatives include microbicides, 4.0/). insecticides, wood treatment agents, washing products (e.g., detergents and soaps), special

Minerals 2021, 11, 318. https://doi.org/10.3390/min11030318 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 18

Minerals 2021, 11, 318 2 of 18 cides, wood treatment agents, washing products (e.g., detergents and soaps), special al- loys, fertilizers, fire retardants, fiberglass, and heat resistant glass (e.g., Pyrex) [10–13]. BMs also canalloys, be used fertilizers, in the metallurgical fire retardants, industries fiberglass, [2,14]. and Boron heat compounds resistant glass have (e.g., a con- Pyrex) [10–13]. BMs also can be used in the metallurgical industries [2,14]. Boron compounds have a siderable potential to be used in the energy sector where “B” as a hydrogen carrier in the considerable potential to be used in the energy sector where “B” as a hydrogen carrier form of BH4, has a higher hydrogen storage capacity than other hydrides and compounds; in the form of BH , has a higher hydrogen storage capacity than other hydrides and hence, it can be used as a hydrogen4 carrier. However, it was reported that the glass, ce- compounds; hence, it can be used as a hydrogen carrier. However, it was reported that the ramics, and agricultural sectors consume about 80% of refined borates and boric acid pro- glass, ceramics, and agricultural sectors consume about 80% of refined borates and boric duced in the world (Figure 1) [15]. acid produced in the world (Figure1)[15].

others, cleaning 20% and detergent, 2% glass, 48% ceramic industry, 15% agriculture, 15%

Figure 1. General applications of boron.

Although signiﬁcant attention has now been drawn to BMs, their Processing poses se-

vere challenges. BMs and their associated gangues are usually salt-type minerals, naturally Figure 1. Generalhydrophilic, applications soluble, of boron. and exhibit similar surface characteristics. For instance, colemanite and realgar (its associated valuable mineral) show similar water contact angles [16]. AlthoughUlexite, significant borax, attention and associated has now clay been gangue drawn minerals to BMs, exhibit their negativeProcessing zeta poses potentials at all severe challenges.practical BMs pH and [1,2 ,their11,17 associated]. Moreover, gangues BMs are are relatively usually friable;salt-type thus, minerals, their grinding natu- generates rally hydrophilic,fines, whichsoluble, enhance and exhibit slime similar coating surface and loss characteristics. of a sufficient amountFor instance, of valued cole- minerals to manite and therealgar tailings (its [associated18,19]. Furthermore, valuable mineral) the conventional show similar wet water mechanical contact attrition angles and wash- [16]. Ulexite,ing borax, processes and associated used for pretreatment clay gangue ofminerals BMs for exhibit further negative processing zeta promote potentials slime coating at all practicaland pH are [1,2,11,17]. environmentally Moreover, unfriendly BMs are [6 ,relatively20]. All of friable; these obstacles thus, their limit grinding the efficiencies of generates fines,beneficiation which enhance methods slime employed coating in and the processingloss of a sufficient of BMs. amount of valued minerals to the tailingsTherefore, [18,19]. removing Furthermor ganguee, mineralsthe conventional from BMs wet utilizes mechanical several attrition preprocessing tech- and washingniques processes such used as ultrasonic for pretreatment pretreatment of BMs [21 for], attrition-scrubbing/classificationfurther processing promote slime [22,23], and coating andthermal are environmentally decomposition/decrepitation unfriendly [6,20]. [All24]. of These these pretreatmentobstacles limit techniques the efficien- are followed cies of beneficiationby various methods beneficiation employed methods in the such processing as electrostatic of BMs. and magnetic separations [25], and Therefore,flotation removing [4,23 ,gangue26,27]. Theseminerals processes from BMs exhibit utilizes grave several limitations preprocessing and may tech- lead to loss of niques suchBMs as ultrasonic in the tailings pretreatment and cause [21], various attrition-scrubbing/classification environmental problems. Hence,[22,23], theyand need to be thermal decomposition/decrepitationcontrolled and optimized by[24]. size Thes reduction,e pretreatment surface techniques property modification,are followed by zeta potential various beneficiationand pH regulation, methods andsuch the as applicationelectrostatic of and appropriate magnetic reagent separations types. [25], However, and there are flotation [4,23,26,27].gaps in the These scientific processes literature exhibit concerning grave limitations BMs’ Processing and may as accentuatedlead to loss byof the limited BMs in the publicationstailings and cause before various 2010 on environmental BMs referenced problems. in this work. Hence, Therefore, they need this to be work aims to controlled andaddress optimized these gapsby size by reduction, systematically surface reviewing property multiple modification, beneficiation zeta potential methods for BMs to and pH regulation,present aspectsand the thatapplication can improve of appropriate their processing reagent from types. their However, resources there and are offer efficient gaps in the andscientific environmental literature concerning beneficiation BMs’ methods. Processing as accentuated by the limited publications before 2010 on BMs referenced in this work. Therefore, this work aims to address these2. Particlegaps by Sizesystematically reviewing multiple beneficiation methods for BMs ◦ to present aspectsCalcination that can improve at 450 Ctheir successfully processing processes from their BMs, resources especially and borax,offer effi- which causes cient and environmentalthe minerals beneficiation to expand in methods. size to a maximum degree. This is possible at up to 50 mm particle size. Fragmentation then occurs once the calcined BMs reach their full growth in size due to heat, with air classification utilized after that to separate the fine gangue

Minerals 2021, 11, 318 3 of 18

minerals from the BMs. [6]. However, an upper threshold of around 9.5 mm was reported as the largest particle size for the efficient BM processes. Decreasing the BM particle sizes increases their solubility [28]. This increase is detrimental to selecting an efficient upgrading process. Traditional methods for concentrating BMs are scrubbing, washing, and classification, followed by the size reduction steps [9,29,30]. Nevertheless, these non- chemical processes are generally successful only at a high grade and coarse particle size fractions, and their performances diminish at fine particle ranges [30]. As BMs are friable, the mining and processing steps continuously magnify the fine particle production [18]. These fine particles are the roots of severe predicaments in storage and environmental pollution [18,19], discharging in tailings and losing a substantial amount of valuable minerals. Typically, for the physical upgrading processes, size fractions below 0.2–3 mm are designated as fines and challenge these types of processes [19,29]. It was reported if particle sizes finer than 150–200 µm is delivered to the tailings, it would lead to loss of around 20–26% of B2O3 [19,23,30]. Hence, significant attention has been turned to developing innovative methods for processing BM fines. For other processes such as magnetic separation, the smaller size limit is 75 µm [31,32]. It was documented that particle size of about 38 µm is generally used as the lower threshold limit to prevent slimes’ adverse effects for beneficiation by froth flotation [20,33].

3. Magnetic Separation Calcium and magnesium borates pose a severe challenge to wet method processing. Thus, dry methods—such as calcination, magnetic separation, and ultrasound techniques— could have a high potential for their processing. Calcination through decrepitation concentrates borates in the coarse fractions [7]. However, calcination cannot remove iron impurities. Several investigations have suggested that magnetic separation has a high potential of being used for processing diamagnetic BMs from their typical associated clay gangue phase [27,31,34–36]. This separation is possible because minerals of as low as 5 × 10−6 m3/kg are still capable of being separated at field gradients of about 50–500 T/m with a considerable high magnetic field around 0.8 T [31]. Generally, the magnetic susceptibility of clay gangues depends on their iron content. Therefore, they may respond to a high magnetic field (when they have adequately high susceptibilities). It was reported that a magnetic separator operating at 0.45 T using a steel ball matrix could reduce BM’s concentrate iron component to as low as 0.6%, which is within the market limit of 0.7%. Nevertheless, its recovery of 80% is not encouraging; hence, it can be considered as a pre-concentration technique [7]. The paramagnetic properties of the colemanite ore are contingent on magnetite [7]. These properties also depend on silicate minerals in their structure, including the smec- tite group, in which iron is distributed (iron readily experiences oxidation). It was well understood that the whole sample’s magnetic susceptibility is increased by the high mass fraction of iron-containing silicates [37]. Thus, samples with high colemanite content may turn out to have low susceptibilities. Reference [35] indicated that samples with ≥63 wt % colemanite content exhibited low susceptibilities. This lower attraction to magnet may lead to a selective separation of diamagnetic colemanite from its paramagnetic iron-bearing silicate gangue in a high magnetic field. Dry magnetic separation as a simple, environmentally acceptable beneficiating process is preferable to flotation and decrepitation for processing colemanite ores [2,26]. However, as mentioned, it has a size limitation, and for processing fine BMs, flotation has to be used.

4. Flotation Flotation has the highest potential for processing of the BM fine particles [11,12]. However, as ultrafine particles (i.e., slimes) have an unfavorable impact on the BMs’ flotation recovery and grade [30,38], various pretreatment methods can be implemented before flotation separation. It should be noted that these modification processes could stand alone in some instances. Minerals 2021, 11, 318 4 of 18

4.1. Pretreatment 4.1.1. Ultrasonic Treatment Ultrasound is a cost-effective and environmentally friendly pretreatment method that can disperse clay minerals before removing them and decreasing their BM coating rate [20,21,39]. The attachment of clay minerals on the surface of colemanite particles is a significant factor for decreasing their flotation efficiency. Hence, a series of flotation experiments have been conducted to determine ultrasonic treatment’s ability in dispersing clays from the colemanite surface [11]. Celik et al. reported that flotation could recover 5% of colemanite without ultrasonic pretreatment, while over 90% of the ultrasonically treated sample could be recovered by flotation separation [11]. In-situ ultrasound treatment detaches clay particles from the colemanite surfaces and increases anionic collector adsorption rate on their surfaces [6]. In other words, ultrasonic pretreatment improves reagents’ effectiveness due to the improvement of their uniform distributions and activities in the suspension (Table1)[ 40–43]. It was also reported that conditioning through in-situ sonica- tion requires a lower collector addition (several times lower than the prepared sample by conventional conditioning); hence, it serves as a cheaper separation method [11] (Figure2). The shearing forces of ultrasound waves are therefore responsible for the reduction in the quantity of collectors/reagents used.

Table 1. Inﬂuence of ultrasonic waves on the ﬂotation of borax minerals

Results Condition Grade (%) Recovery (%) Reference Ultrasound Size Collector Frequency Power (W) With Without With Without (kHz) Ultrasound Ultrasound Ultrasound Ultrasound [39] −250 + 38 (µm) Cytec R825 2000 mg/g 35 120 49 45 90 80 [21] −6 (mm) 35 250 35 25 69 67 [11] −150 + 74 (µm) Sodium dodecyl sulfate 90 5

Hence, the ultrasound pretreatment technique is superior to mechanical attrition and washing, as its application enhances reagent absorption on the surfaces of BMs, improving flotation (Table2)[ 20]. The utilization of ultrasonic waves, followed by the application of reagents, significantly increased the percentage grade of concentrate. This increase in grade is proportional to the reduction of calcined material, with the dispersion of clay particles being improved as particle size increased. The rise in the dispersion can be attributed to better penetration of the waves among coarser particles, resulting in cleaner concentrates [20]. The results showed that ultrasound could be used to obtain high-grade concentrates. However, this technique faced two significant challenges: the need for higher capacity tanks because of long residence time and the considerable initial capital cost. Nevertheless, these difficulties could be offset by low energy requirements and operating costs. Besides, high power ultrasonic generators could occasion superior results, and environmental degradation could be reduced [20]. Minerals 2021, 11, x FOR PEER REVIEW 5 of 18 Minerals 2021, 11, 318 5 of 18

(a)

Collector Borax Borax mineral mineral +

(b) Ultrasonic waves

Borax Borax mineral mineral +

FigureFigure 2. 2. ImpactImpact of of ultrasonic ultrasonic waves waves on on the the flotation ﬂotation of of borax borax mineral: mineral: (a ()a clay) clay minerals minerals coated coated the the surfacesurface of of borax borax and and more more collecto collectorr is needed is needed for forbeneficiation; beneﬁciation; (b) (ultrasonicb) ultrasonic waves waves cleaned cleaned the the surfacesurface of of borax borax and and lower lower quantity quantity of of collector collector is is required. required.

Hence, the ultrasound pretreatment technique is superior to mechanical attrition and Table 2. Comparisonwashing, between as ultrasonic its application beneficiation enhances and mechanical reagent absorption attrition and on washing the surfaces methods of [BMs,20]. improving flotation (Table 2) [20]. The utilization of ultrasonic waves, followed by the application Pretreatment Method Particle Size (mm) Weight (%) B2O3 (%) Distribution (%) of reagents, significantly increased the percentage grade of concentrate. This increase in Ultrasonic beneficiations −3.000 + 0.038 56.25 41.69 97.01 grade is proportional to the reduction of calcined material, with the dispersion of clay −0.038 + dissolved 43.75 1.65 2.99 particles being improved as particle size increased. The rise in the dispersion can be at- − Mechanical attrition and washingtributed to better3.000 + penetration 0.038 of the 62.16waves among coarser 33.01 particles, resulting 84.93 in cleaner −0.038 + dissolved 37.84 9.62 15.07 concentrates [20]. The results showed that ultrasound could be used to obtain high-grade concentrates. However, this technique faced two significant challenges: the need for higher4.1.2. Calcinationcapacity tanks because of long residence time and the considerable initial capital cost. Nevertheless, these difficulties could be offset by low energy requirements and op- The effect of heat on BMs and their gangues has been extensively investigated from eratingbeneficiation costs. Besides, results of high such power minerals ultrasonic [6,44]. It generators was documented could thatoccasion calcination superior can results, be used andas aenvironmental single processing degradation method or could a pretreatment be reduced technique [20]. for processing BMs. Moderate temperatures cause shrinkage of ulexite size [44,45] and decrepitation in colemanite [24,25]. Table 2. Comparison between ultrasonic beneficiation and mechanical attrition and washing Calcination successfully has been used to process borax when the unaltered gangue resides methods [20]. in the coarse fraction [7]. Unlike the reactions of ulexite and colemanite to heat, borax experiencedPretreatment expansion Method under Particle heating Size [6 (mm),25,44 ]. The Weight borax (%) processing B2O3 (%) via heat Distribution pretreatment (%) Ultrasonic beneficiations −3.000 + 0.038 56.25 41.69 97.01 resulted in a concentrate grade of 51.05% B2O3 with 93.73% recovery [6]. Piskin demon- strated thatborax starts to expand−0.038 + dissolved above 100 ◦C and43.75 reaches 1.65 300% expansion 2.99 at 450 ◦C Mechanical attrition and washing −3.000 + 0.038 62.16 33.01 84.93 −0.038 + dissolved 37.84 9.62 15.07

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(before shrinking to the original size, borax acquires a ﬂat shape and has greyish color). Temperature, heating time, and particle size can affect the growth of pure borax [44]. There is a positive correlation between the optimum expansion of borax and its optimum upgrading states; as a function of particle size, both growth and upgrading reach their optimum conditions over the 375–475 ◦C[44]. It was revealed that particle size controls heat access to the particles, and at a given time and temperature, coarse particles would be less affected than ﬁner ones [6]. The maximum concentrate grade (>52%) was obtained at a large particle size fraction (−12.6 + 4.76 mm) at 475 ◦C, but it was possible to arrive at a grade of above 50% with >85–90% recovery at a smaller particle size fraction at 375–425 ◦C (Table3). The gangue, mainly clay minerals, undergo changes as an increase in heat application gradually hardens the clay minerals, making them separate material within the borax matrix [6]. This phenomenon could lead to a selective separation of BMs from clay minerals [44]. Concerning grades, 15 min was reported as the optimum heating time for most fractions (Table3). However, an upper limiting particle size of 9.51 mm was required for obtaining over 80% recoveries. Therefore, investigators suggested this narrow particle size range enhances both grade and recovery in real plant operations [6].

Table 3. Effect of particle size, temperature, and heating time on borax recovery and grade [6].

Feed Concentrate Optimum Optimum Optimum Heating Optimum Heating Particle Size Weight %B2O3 Temp. Temp. %B2O3 ◦ %B2O3 ◦ %B2O3 Time %B2O3 Time (mm) (%) Grade Grade ( C) Recovery ( C) Grade (min) Recovery (min) +0.50–1.00 9.00 29.91 43.8 325 85.0 375 46.0 15 90.2 10 +1.00–3.00 17.51 54.15 49.6 375 90.0 375 55.0 15 86.1 10 +3.00–4.76 10.81 25.17 53.0 375 87.5 375 58.1 15 96.0 10 +4.76–9.51 14.93 22.71 53.5 425 88.0 375 57.3 15 90.3 10 +9.51–12.50 8.63 23.54 48.5 475 81.0 475 61.0 25 79.8 15 +12.50–25.00 21.58 23.92 52.4 475 90.0 475 60.1 15 70.3 15 +25.00–50.00 5.92 18.02 54.0 575 75.0 575 60.2 25 50.1 10

Calcination is environmentally friendly in comparison to wet methods. Calcination only utilizes the application of heat, wherein the minerals expand to such extend that the gangue material becomes easily separable from the desired BMs, without the use of reagents. The wet process encompasses the use of reagents, which, when not properly supervised, pose an environmental problem. Comparing the outcomes obtained from the currently operating industrial-scale wet process showed 33% grade and 83% recovery, while a pilot plant scale dry process resulted in 55% grade and 90% recovery of B2O3 [6]. Another signiﬁcant advantage of calcination (a dry scheme) over the wet methods is the product’s pelletization or briquette [46]. However, there is a need to conduct more studies into calcination as a beneﬁciation method for BMs, especially to ascertain the correlation between growth and optimum upgrading [6].

4.2. Surface Properties 4.2.1. Zeta Potential Zeta potential (ζ) and isoelectric points (IEPs) for a series of BMs in water and when conditioned with reagents have been reported. For instance, colemanite exhibits IEP at a pH of around 10.5 in distilled water (Figure3)[ 2,19,26]. Inderite (magnesium borate) and tunellite (strontium borate) show positive ζ potentials with no clear IEP throughout the pH range 7 to 11 [25]. Similarly, ulexite [2,25] and borax [17] exhibit no IEP down to about pH 7 but possess negative ζ potentials. Minerals 2021, 11, x FOR PEER REVIEW 7 of 18

Zeta potential (ζ) and isoelectric points (IEPs) for a series of BMs in water and when conditioned with reagents have been reported. For instance, colemanite exhibits IEP at a pH of around 10.5 in distilled water (Figure 3) [2,19,26]. Inderite (magnesium borate) and tunellite (strontium borate) show positive ζ potentials with no clear IEP throughout the pH range 7 to 11 [25]. Similarly, ulexite [2,25] and borax [17] exhibit no IEP down to about Minerals 2021, 11, 318 pH 7 but possess negative ζ potentials. 7 of 18

Figure 3. Zeta potential of boron minerals in the presence of clay and some flotation reagents [11,17,47]. Figure 3. Zeta potential of boron minerals in the presence of clay and some flotation reagents [11,17,47]. Divalent ion (Ca2+) (for colemanite), the anion B O −7, and the H+ and OH− ions, 2+ 4−7 2 + − Divalent ion (Ca ) (for colemanite),2− the anion B4O2 , and the H and OH ions, which which control the HCO3/CO3 ratio, are the constituent lattice members for BMs and control the HCO3/CO32− ratio, are the constituent lattice members for BMs and determi- determinants for the ζ potentials [25]. It has been proven that ionic strength affects the ζ potentialsnants for the of colemaniteζ potentials and [25]. ulexite It has been [2]. In prov detail,en that monovalent ionic strength ions decreaseaffects the the ζ potentials charge of colemaniteof colemanite due and to theulexite electrical [2]. In double detail, layermonovalent compression; ions decrease however, the they charge oppositely of colemanite affect ulexitedue to the because electrical of the double decrease layer in calciumcompression; [2]. The however, release ofthey Ca oppositely2+ ions in solution affect ulexite depends be- 2+ oncause the of amount the decrease of solid in percentage, calcium [2]. suggesting The release that of the Caζ potential ions in solution of colemanite depends is directly on the proportionalamount of solid to changespercentage, in the suggesting solid concentration that the ζ potential [25]. Therefore, of colemanite solid concentrationsis directly pro- ofportional more than to changes 1% or prolonged in the solid conditioning concentration time to[25]. release Therefore, all Ca2+ solidions concentrations produce reliable of zetamore potential than 1% resultsor prolonged [29]. For cond instance,itioningζ timepotential to release studies all Ca have2+ ions shown produce that clay reliable content zeta aspotential low as results 1% can [29]. reverse For instance, the colemanite ζ potential surface studies charge have from show positiven that clay to negative. content as This low reversalas 1% can indicated reverse thatthe colemanite the strong affinitysurface betweencharge from the positive negatively to chargednegative. clay This minerals reversal andindicated positively that chargedthe strong colemanite affinity between surfacethrough the negatively electrostatic charged attraction clay minerals is responsible and posi- for slimetively coatingcharged during colemanite flotation surface [11]. through electrostatic attraction is responsible for slime coatingDodecylamine during flotation hydrochloride [11]. (DAH) can increase the colemanite IEP to around 11.6, whileDodecylamine sodium dodecyl hydrochloride sulfate (SDS) (DAH) negligibly can increase decreases the itcolemanite to around IEP 10 (Figureto around3). The11.6, SDSwhile and sodium Na-oleate dodecyl adsorption sulfate (SDS) ensues negligibly with the decreases formation it ofto Ca(DS)around2 10and (Figure Ca(Oleate) 3). The2 onSDS colemanite and Na-oleate surfaces, adsorption making ensues its zeta with potential the formation mostly negativeof Ca(DS) at2 and pH Ca(Oleate) values higher2 on thancolemanite 10.2 [13 surfaces,]. The electrokinetic making its zeta experiments potential resultsmostly reveal negative that at both pH sodiumvalues higher oleate than and SDS10.2 engendered[13]. The electrokinetic only small experiments changes to theresu zetalts reveal potential that ofboth colemanite sodium oleate and its and clayey SDS gangues,engendered as only concentration small changes increases to the evenzeta potential though there of colemanite were charge and its reversals clayey gangues, in some instancesas concentration(Figure increases4) . Furthermore, even though the there carboxylate were charge seems reversals to adsorb in some more instances significantly (Fig- onure the 4). Furthermore, clays than on the colemanite. carboxylate However, seems to there adsorb were more substantial significantly affinities on the for clays sulfate than adsorptionon colemanite. on both However, minerals. there These were adsorptions substantial indicateaffinities that for thesesulfate two adsorption reagents on are both not reliableminerals. for These selective adsorptions separation. indicate On the that other thes hand,e two sulfonatesreagents are (R801 not andreliable R825) for exhibited selective extensiveseparation. changes On the in other the zeta hand, potential sulfonates of colemanite (R801 and with R825) large exhibited negative extensive values after changes charge in reversals and signifying effective adsorption onto the mineral surface. The R801 adsorption seems more prominent than R825. The difference in electrical charge on the surface of colemanite and clay in the presence of sulfonates implies that they could enhance selective separation [19]. Minerals 2021, 11, x FOR PEER REVIEW 8 of 18

the zeta potential of colemanite with large negative values after charge reversals and signifying effective adsorption onto the mineral surface. The R801 adsorption seems more prominent than R825. The difference in electrical charge on the surface of colemanite and Minerals 2021, 11, 318 clay in the presence of sulfonates implies that they could enhance selective separation [19].8 of 18

Figure 4. Effect of reagent concentration on the zeta potential of colemanite and its associated clayey gangues in the vicinity of a natural buffer pH (9.1–9.4) [19]. Figure 4. Effect of reagent concentration on the zeta potential of colemanite and its associated clayey4.2.2. gangues Contact in Anglethe vicinity of a natural buffer pH (9.1–9.4) [19]. Colemanite should be considered as a highly hydrophilic mineral, primarily due to its 4.2.2.chemical Contact structure Angle and relatively high solubility. Realgar also exhibits low contact angles at itsColemanite interface with should water. be Increasing considered pH as can a highly cause a hydrophilic negligible change mineral, in theprimarily contact due angle to of ◦ ◦ itscolemanite chemical structure (34–32 ) andand realgarrelatively surface high (26–27solubility.)[16 Realgar]. Anionic also and exhibits cationic low collectors contact affectan- glesthe at colemanite-liquid its interface with interfaces’water. Increasing contact pH angles can (Tablecause 4a)[ negligible16,19]. Measurements change in the indicated contact anglethat of R801, colemanite as an anionic (34–32°) collector, and realgar occasioned surface the(26–27°) greatest [16]. hydrophobicity Anionic and cationic on the collec- surface torsof affect colemanite the colemanite-liquid at a low concentration interfaces’ range contact than angles any other (Table examined 4) [16,19]. collectors, Measurements with a ◦ indicatedcontact anglethat R801, of up as to an 73 anionic. This observationcollector, occasioned suggested the that greatest R801 (sulfonates)hydrophobicity can generateon the surfacethe required of colemanite hydrophobicity at a low concentration for colemanite range ﬂotation than [19any]. other Moreover, examined the BMs’ collectors, contact withangle a contact can increase angle of with up ato rise 73°. in This sodium observation oleate dosage suggested (a carboxylate) that R801 (sulfonates) at pH 9 (beyond can generatethis point, the therequired contact hydrophobicity angle slightly decreases).for colemanite Armac-T flotation (amine), [19]. Moreover, as a cationic the collector, BMs’ ◦ contactin proportions angle can up increase to 0.10% with generated a rise in a risesodium in colemanite oleate dosage contact (a carboxylate) angle from 32 at to pH 47 9at (beyondpH 9 [16 this]. point, the contact angle slightly decreases). Armac-T (amine), as a cationic collector, in proportions up to 0.10% generated a rise in colemanite contact angle from 32 Table 4. Effects of anionic reagents on colemanite and realgar contact angles at pH 9 [16,19]. to 47° at pH 9 [16]. [19][16] Table 4. Effects of anionic reagents on colemanite and realgar contact angles at pH 9 [16,19]. Colemanite Colemanite Realgar Reagent Conc. (M) [19] [16] Reagent Conc. (M) Contact Angle (◦) Contact Angle (◦) Colemanite Colemanite Realgar −5 −4 Reagent Conc.−4 (M)− 2 ReagentR801 Conc.E (M)to E M Contact67.0–73.0 Angle (°) KAX E to E M 32.0–34.0 Contact Angle (°) 25.0–62.0 −4 −3 −4 −2 R801R825 E−5 Eto E−4to ME M 67.0–73.057.0–58.0 KAX KEX E−4 toto E−2 E M M 32.0–34.032.0–36.0 25.0–62.0 30.0–63.0 −4 −3 R825SDS E−4 Eto E−3to ME M 57.0–58.053.0–56.0 KEX R825 E− 0.01–0.50%4 to E−2 M 32.0–36.0 32.0–35.0 30.0–63.0 32.0–47.0 − − SDSNa-oleate E−4 Eto E4−3to ME 3 M 53.0–56.050.0–51.0 R825 R840 0.01–0.50% 0.01–0.05% 32.0–35.0 36.0–43.0 32.0–47.0 25.0–39.0 Na-oleate E−4 to E−3 M 50.0–51.0 Armac-T R840 0.01–0.05% 0.01–0.10% 36.0–43.0 44.0–48.0 25.0–39.0 32.0–43.0 Armac-TNa-silicate E0.01–0.10%−4 to E− 2 M 44.0–48.027.0–34.0 32.0–43.0 26.0–42.0

Na-silicate E−4 to E−2 M 27.0–34.0 26.0–42.0

Contact angle measurement on the surface of realgar indicated an increase with in- Contact angle measurement on the surface of realgar indicated an increase with increasing the concentrations of collectors. Measurements showed high contact angle values creasing the concentrations of collectors. Measurements showed high contact angle values (62◦), at a concentration of 10−2 M of KAX and KEX at pH 9 (Table4). These collectors (62°), at a concentration of 10−2 M of KAX and KEX at pH 9 (Table 4). These collectors engendered the highest contact angles for realgar and generated the widest difference in hydrophobicities between colemanite and realgar. Whereas colemanite generally exhibited higher contact angles with the sulfonates than the xanthate counterparts, the latter exceedingly surpassed the former as per increasing the contact angle of realgar. These observations indicated that these particular xanthates did not chemically adsorb onto the colemanite surface even at relatively high concentrations. This suggestion agreed with the Minerals 2021, 11, x FOR PEER REVIEW 9 of 18

engendered the highest contact angles for realgar and generated the widest difference in hydrophobicities between colemanite and realgar. Whereas colemanite generally exhibited higher contact angles with the sulfonates than the xanthate counterparts, the latter exceedingly surpassed the former as per increasing the contact angle of realgar. These Minerals 2021, 11, 318 observations indicated that these particular xanthates did not chemically adsorb onto9 of the 18 colemanite surface even at relatively high concentrations. This suggestion agreed with the presented electrokinetic potential measurements. The addition of the sodium silicate as a presented electrokinetic potential measurements. The addition of the sodium silicate as a depressant to the flotation separation changed the contact angles of colemanite and real- depressant to the ﬂotation separation changed the contact angles of colemanite and realgar gar to 27° and 42°, respectively [16]. to 27◦ and 42◦, respectively [16].

4.2.3. Effect of Ions BMs containcontain monovalentmonovalent salts salts in in their their lattice lattice structure structure (for (for instance, instance, sodium sodium being being the prominentthe prominent one one in borax).in borax). Monovalent Monovalent ions ions compress compress the the double double layers layers of of mineralsminerals andand change the surface activity for collector adsorption by modifying the bulk water structure and micellization of collectors. Some investigations showed that monovalent cationscations andand anions change the bulk structure of water and transport the same operation to the surface, depending on on their their water-breaking water-breaking and and water-ma water-makingking structures. structures. It was It was also also stressed stressed that thatmonovalent monovalent ions are ions adsorbed are adsorbed at the at solid the solidsurfaces surfaces and interact and interact with water with water[5]. The [5 ].mech- The mechanismanism of BM of flotation BM ﬂotation was wasinvestigated investigated using using the chlorides the chlorides of monovalent of monovalent ions ions Na+ Na, K++, Kand+, andCs+ in Cs the+ in presence the presence of SDS of and SDS DAH and DAH surfactants. surfactants. NaCl NaCl and KCl and can KCl be can structure be structure mak- makersers or breakers, or breakers, while while CsCl CsCl is a is potent a potent structure structure breaker breaker [28,48]. [28,48]. High High dosages dosages of these monovalent salts (≥0.10.1 M) M) make make electrostatic electrostatic interactions interactions ineffective ineffective (Figure (Figure 55))[ [2,6,49].2,6,49].

100 90 80 70 60

Floated (%) 50 40 SDS/KCl SDS/NaCl SDS/CsCl 30 DAH/KCl DAH/NaCl DAH/CsCl 20 01234 Concentration (M)

−5 Figure 5. Effect of monovalent salts salts on on colemanite colemanite flotation ﬂotation with with 4×10 4 × −105 M ofM anionic of anionic (SDS) (SDS) and and cationic (DAH) collectors,collectors, pHpH 9.39.3 ±± 0.1.0.1. Colemanite Colemanite was was conditioned conditioned in in the the reagents reagents for 10 min and ﬂoatedfloated for 1 min [[11]11]..

2+ Multivalent ions, ions, especially especially Ca Ca2+, can, can play play different different roles roles in BMs’ in BMs’ flotation ﬂotation separation separation[26,29]. [26 ,In29 ].the In presence the presence of SDS, of SDS,three three areas areas characterized characterized the reaction the reaction of colemanite of colemanite with withcations: cations: (1) the (1) adsorption the adsorption of divalent of divalent ions onto ions the onto surface the surfaceof colemanite; of colemanite; (2) the adsorp- (2) the adsorptiontion of an anionic of ananionic surfactant surfactant on the surface; on the surface; and (3) the and bulk (3) the precipitation bulk precipitation of metal ofdodecyl metal dodecylsulfate onto sulfate the ontosurface the of surface colemanite of colemanite (surface precipitation (surface precipitation occurs before occurs bulk before precipita- bulk precipitation)tion) [50]. Relative [50]. to Relative their solubility, to their solubility, multivalent multivalent ions react ions with react collectors; with collectors; for instance, for instance,with dodecyl with sulfate dodecyl (DS), sulfate the product (DS), the strength products of strengths collector ofsalts collector show the salts following show the order following order Ba(DS) < Ca(DS) < Mg(DS) [26]. As Figure6 showed, the least soluble Ba(DS)2 < Ca(DS)2 < Mg(DS)2 2 [26].2 As Figure2 6 showed, the least soluble Ba(DS)2 activates Ba(DS) activates colemanite most; the most soluble Mg(DS) activates it the least. It was colemanite2 most; the most soluble Mg(DS)2 activates it the least.2 It was also reported that also reported that Ca(DS)2 unavoidably precipitates at a high salt concentration [6,51]. Ca(DS)2 unavoidably precipitates at a high salt concentration [6,51]. Flotation experiments Flotation experiments on colemanite samples from Emet mines in Turkey indicated that 2+ 2+ increasing Ca and Mg ion concentrations from 0 to 300 ppm reduced B2O3% recovery values from 99–65% for combined usage of Ca and Mg ions. The addition of these ions

increased the tailings grade values by 18–25% [52]. Minerals 2021, 11, x FOR PEER REVIEW 10 of 18

on colemanite samples from Emet mines in Turkey indicated that increasing Ca2+ and Mg2+ ion concentrations from 0 to 300 ppm reduced B2O3% recovery values from 99–65% for Minerals 2021, 11, 318 combined usage of Ca and Mg ions. The addition of these ions increased the tailings10 grade of 18 values by 18–25% [52].

100 Ba (II) Ca (II) Mg (II)

60 Floated %

30 0.0001 0.001 0.01 0.1 Concentration of ion (M)

Figure 6. Effect of multivalent ion concentration on the activation of colemanite at pH 9.3 ± 0.1 [11]. Figure 6. Effect of multivalent ion concentration on the activation of colemanite at pH 9.3 ± 0.1 [11]. The role of multivalent ions on the surface of these BMs is dependent on pH. At pH 9.3, multivalent ions depressed the floatability of ulexite, while at higher pHs, the same The role of multivalent ions on the surface of these BMs is dependent on pH. At pH multivalent ions enhance its floatability [6,29,50]. At the pH range of 4.5–8.5, precipitation 9.3, multivalent ions depressed the floatability of ulexite, while at higher pHs, the same of Al(OH) on the ulexite surface increases its positive charge, whereas the electrical charge multivalent3 ions enhance its floatability [6,29,50]. At the pH range of 4.5–8.5, precipitation is reversed above this [53]. It was suggested that particles become hydrophobic where of Al(OH)3 on the ulexite surface increases its positive charge, whereas the electrical metal ions in the solution react with the collectors and precipitate them [6,11]. charge is reversed above this [53]. It was suggested that particles become hydrophobic 4.3.where Flocculation metal ions in the solution react with the collectors and precipitate them [6,11]. Flocculation of BMs, especially colemanite, effectively enhances the efficiency of 4.3. Flocculation flotation separation. Colemanite particles in the suspension agglomerate due to the hydro- carbonFlocculation chain association of BMs, ofespecially the surfactant. colemanite, The surfactanteffectively adsorbsenhances on the the efficiency particle layersof flo- andtation contacts separation. with Colemanite each other. Theparticles hydrocarbon in the suspension chain association agglomerate depends due onto thethe carbonhydro- numbercarbon chain of surfactants association (long-chain of the surfactant. surfactants Th aree surfactant more effective adsorbs than on shorter the particle ones). layers Shear flocculationand contacts closelywith each correlates other. The with hydrocar the particle’sbon chain hydrophobicity association depends in suspension on the [54carbon–56]. Increasingnumber of thesurfactants surfactant (long-chain concentration surfactants can lead are to more an increase effective in colemanite’sthan shorter shearones). floccu- Shear lation,flocculation thereby closely improving correlates the particles’ with the surface particle’s hydrophobicity. hydrophobicity The in energy suspension barrier [54–56]. should increaseIncreasing as the negative surfactant zeta potentialconcentration increases, can lead and hinderingto an increase particle in colemanite’s flocculation. shear However, floc- anculation, increase thereby in surface improving charge due the toparticles’ the surfactant’s surface adsorptionhydrophobicity. on the colemaniteThe energy surfaces barrier didshould not increase decrease as the negative colemanite zeta flocculation potential increases, [13]. and hindering particle flocculation. However,Sodium an oleateincrease (C in17 Hsurface33COONa) charge and due sodium to the surfactant’s dodecyl sulfate adsorption (C12H25 onSO the4Na) coleman- in the roleite surfaces of flocculants did not can decrease positively the colemanite influence the flocculation aggregation [13]. of colemanite. Sodium oleate provedSodium to be oleate more (C efficient17H33COONa) than sodium and sodium dodecyl dodecyl sulfate sulfate in the (C12 shearH25SO flocculation4Na) in the role of colemaniteof flocculants [54 ,57can]. positively It was also influence documented the thataggregation Aero 801 of performs colemanite. better Sodium than SDSoleate in flocculatingproved to be colemanite more efficient [18 ,19than]. Ucbeyiaysodium dode andcyl Ozkan sulfate (2014) in the showed shear flocculation two-stage floccula- of cole- tionmanite could [54,57]. provide It was higher also concentratedocumented grades that Aero for shear801 performs flocculation better and than column SDS in flotation floccu- methods.lating colemanite They indicated [18,19]. theUcbeyiay use of sodiumand Ozkan pyrophosphate (2014) showed (SPP) two-stage and sodium flocculation hexam- etaphosphatecould provide (SHMP)higher concentrate as dispersants grades enhanced for shear the flocculation shear flocculation and column of colemanite flotation inmeth- the presenceods. They of indicated Aero 801 the and use SDS of [sodium18]. pyrophosphate (SPP) and sodium hexametaphos- phateStirring (SHMP) speed as dispersants and flocculation enhanced time the are shea criticalr flocculation parameters of colemanite in the flocculation in the pres- of colemanite.ence of Aero An 801 optimized and SDS stirring[18]. speed ruptures the flocs while stirring rates below their optimum value may reduce particles’ collision efficiency in the suspension and reduce the suspension’s BMs’ flocculation. Studies showed that a stirring speed of 500 rpm and conditioning for 3 min affects the highest degree of flocculation with sodium oleate and SDS (Figure7). Shear flocculation, a slow process [ 54], has shown high flocculation degrees at low flocculation times for some minerals. There was no significant decrease resulting from a floc rupturing process at flocculation times above the 3 min when colemanite is aggregated Minerals 2021, 11, x FOR PEER REVIEW 11 of 18

Stirring speed and flocculation time are critical parameters in the flocculation of colemanite. An optimized stirring speed ruptures the flocs while stirring rates below their optimum value may reduce particles’ collision efficiency in the suspension and reduce the suspension’s BMs’ flocculation. Studies showed that a stirring speed of 500 rpm and conditioning for 3 min affects the highest degree of flocculation with sodium oleate and SDS Minerals 2021, 11, 318 (Figure 7). Shear flocculation, a slow process [54], has shown high flocculation degrees11 of at 18 low flocculation times for some minerals. There was no significant decrease resulting from a floc rupturing process at flocculation times above the 3 min when colemanite is aggre- gatedin a suspensionin a suspension treated treated with with oleate oleate and and SDS. SDS. Therefore, Therefore, 3 3 min min proved proved toto be sufficient sufficient timetime for for colemanite colemanite flocculation. flocculation. Flocculation Flocculation time time depends depends on the on surface the surface charge, charge, the par- the ticleparticle size, size, the theparticle particle concentration, concentration, and and elec electrictric charges charges on on the the surface surface generate generate kinetic kinetic energyenergy barrier barrier [52,55]. [52,55 ].Overcoming Overcoming this this barrier barrier requires requires appropriate appropriate stirring stirring strength. strength. De- creasingDecreasing the thesurface surface charge charge decreases decreases the the critic criticalal shear shear rate rate needed needed for for shear shear flocculation flocculation ofof minerals. minerals. AA highhigh surface surface charge charge requires requires prolonged prolonged flocculation flocculation time, time, while while a low chargea low chargemay cause may acause rapid a agglomeration.rapid agglomeration. In general, In general, coarser coarser particles particles require require lower lower rates rates than thanfiner finer particles particles [52,54 [52,54,55].,55].

100

Shear flocculation (%) flocculation Shear SDS/Time 60 Na-oleate/Time SDS/Stirring Speed Na-oleate/Stirring Speed 50 01234567 - Flocculation time (min) - Stirring speed (× 100 rpm)

FigureFigure 7. 7. EffectEffect of of flocculation flocculation time time and and stirring stirring sp speedeed on on the the degree degree of of shear shear flocculation flocculation [54]. [54]. Multivalent ions have been studied for their effects on colemanite shear flocculation Multivalent ions have been studied for their effects on colemanite shear flocculation (Figure8). Multivalent ions such as Mg 2+, Ba2+, and Fe3+ enhance BMs’ flocculation as (Figure 8). Multivalent ions such as Mg2+, Ba2+, and Fe3+ enhance BMs’ flocculation as a a function of pH [13]. They reduce the BMs’ negative surface charge and enhance their function of pH [13]. They reduce the BMs’ negative surface charge and enhance their co- coagulation in the suspension [39,58,59]. Celik et al. [11] indicated that barium ion is more agulation in the suspension [39,58,59]. Celik et al. [11] indicated that barium ion is more effective in colemanite flocculation than magnesium ion. The difference in their product effective in colemanite flocculation than magnesium ion. The difference in their product solubility with SDS was suggested as the main reason [11]. However, significant decreases solubility with SDS was suggested as the main reason [11]. However, significant decreases in colemanite flocculation were observed at concentrations higher than these ranges [13]. in colemanite flocculation were observed at concentrations higher than these ranges [13]. Minerals 2021, 11, x FOR PEER REVIEWIn the presence of multivalent ions, the maximum degrees of colemanite12 of 18 flocculation was In the presence of multivalent ions, the maximum degrees of colemanite flocculation was reached at pH between 9–11.5 [11,13]. reached at pH between 9–11.5 [11,13].

% Shear flocculation % Shear Mg2+/Na-oleate Ba2+/Na-oleate Al3+/Na-oleate Fe3+/Na-oleate Mg2+/SDS Ba2+/SDS 30 0.E+0 1.E-5 1.E-4 1.E-3 1.E-2 Concentration of ions (M) Figure 8. Effect 8. Effect of multivalent of multivalent ions on the ions shear on floccu thelation shear of colemanite ﬂocculation with of60 mg/L colemanite of collec- with 60 mg/L of collector tor at pH 11.5 [13]. at pH 11.5 [13]. 4.4. Direct Flotation Nevertheless, the most crucial reagent in the flotation process is the collector. Both cationic and anionic collectors can be considered to recover BMs through direct flotation [11,29]; however, anionic reagents are generally the collector of choice for BMs. The most prominent are the sulfonates, xanthates, carboxylates (sodium oleate or oleic acid), and sulfate (SDS) (Table 5). It is essential to restate here that although KAX and KEX are capable of floating realgar, they were not effective in floating colemanite [16]. In general, sulfonates, especially petroleum sulfonates, have proven to be potent collectors for BMs’ flotation. For instance, experiments at pH 10 with a combination of anionic collectors (R-801, R-825, and Hoechst F-698) through the colemanite flotation resulted in the B2O3 recovery and grade of 86.1% and 44.5%, respectively [23]. Other investigations have also shown that petroleum sulfonates performed better in floating colemanite than SDS, olefin sulfonates, sodium oleate, and even cationic collectors [16,19,26]. Their higher efficiencies could be due to their higher molecular weight and the corresponding hydrocarbon chain length.

Table 5. Conditions used for the direct flotation separation of boron minerals

Recovery Grade Collector Collector Type BM Gangue Depressant pH Reference (% B2O3) (% B2O3) Ke-1365, dextrine, R-801, R-825 and Clays and Sulfonate, anionic Colemanite Na2SiO3, Cataflot 10 86.1 44.5 [23] Hoechst F-698 Carbonates P-40 R801 Sulfate, anionic Colemanite Clay 7 98.47 45.42 [30] Dodiflood Petroleum sulfonate, Colemanite Clay 7 96% V3622; MW, 383 anionic [26] Flotigam T Amine, cationic Colemanite Clay 10 95% SDS Sulfate, anionic Colemanite Clay 8.3 59 [2] Clay, quartz, R801 sulfonate, anionic Colemanite Corn starch 9.3 99 46 calcite Clay, quartz, R825 sulfonate, anionic Colemanite Corn starch 9.3 70 40 calcite [19] Clay, quartz, SDS Sulfate, anionic Colemanite Corn starch 9.3 85 38 calcite Clay, quartz, Sodium oleate Carboxylate, anionic Colemanite Corn starch 9.3 98 38 calcite SDS Sulfate, anionic Ulexite Clay 8 45 [2] DAH Amine, cationic Ulexite Clay 9.3 97

Minerals 2021, 11, 318 12 of 18

4.4. Direct Flotation Nevertheless, the most crucial reagent in the flotation process is the collector. Both cationic and anionic collectors can be considered to recover BMs through direct flotation [11,29]; however, anionic reagents are generally the collector of choice for BMs. The most prominent are the sulfonates, xanthates, carboxylates (sodium oleate or oleic acid), and sulfate (SDS) (Table5). It is essential to restate here that although KAX and KEX are capable of floating realgar, they were not effective in floating colemanite [16]. In general, sulfonates, especially petroleum sulfonates, have proven to be potent collectors for BMs’ flotation. For instance, experiments at pH 10 with a combination of anionic collectors (R-801, R-825, and Hoechst F-698) through the colemanite flotation resulted in the B2O3 recovery and grade of 86.1% and 44.5%, respectively [23]. Other investigations have also shown that petroleum sulfonates performed better in floating colemanite than SDS, olefin sulfonates, sodium oleate, and even cationic collectors [16,19,26]. Their higher efficiencies could be due to their higher molecular weight and the corresponding hydrocarbon chain length.

Table 5. Conditions used for the direct ﬂotation separation of boron minerals

Recovery Grade Collector Collector Type BM Gangue Depressant pH Reference (% B2O3) (% B2O3) Ke-1365, R-801, R-825 and Clays and dextrine, Sulfonate, anionic Colemanite 10 86.1 44.5 [23] Hoechst F-698 Carbonates Na2SiO3, Cataﬂot P-40 R801 Sulfate, anionic Colemanite Clay 7 98.47 45.42 [30] Dodiﬂood V3622; Petroleum Colemanite Clay 7 96% MW, 383 sulfonate, anionic [26] Flotigam T Amine, cationic Colemanite Clay 10 95% SDS Sulfate, anionic Colemanite Clay 8.3 59 [2] Clay, quartz, R801 sulfonate, anionic Colemanite Corn starch 9.3 99 46 calcite Clay, quartz, [19] R825 sulfonate, anionic Colemanite Corn starch 9.3 70 40 calcite Clay, quartz, SDS Sulfate, anionic Colemanite Corn starch 9.3 85 38 calcite Carboxylate, Clay, quartz, Sodium oleate Colemanite Corn starch 9.3 98 38 anionic calcite SDS Sulfate, anionic Ulexite Clay 8 45 [2] DAH Amine, cationic Ulexite Clay 9.3 97

Anionic collectors showed different adsorption mechanisms on the surface of BMs. A study showed that R825 was not chemically adsorbed on the colemanite surface, as were sodium oleate and R840 [16]. R801 has an electrostatic and chemical in nature interaction by the surface of BMs because of the existence of Ca2+ in the solution [60,61], which may involve micellization and precipitation of surface complexes such as Ca(OH)RSO3 [51,62]. The flotation recovery of colemanite by anionic collectors decreased by increasing pH and was very low at the IEP. These phenomena indicated a coulombic attraction between the anionic collectors and the positively charged colemanite surface. Celik et al. [11] indicated that the recovery of colemanite, floated with SDS, decreased by increasing pH; though its recovery was regained at the high collector concentrations. These observations could be ascribed to the better collector performance in its competition with clay particles for the colemanite surface at higher collector concentrations [11]. From the perspective of impact on shear flocculation, it was earlier stated that sodium oleate was better than SDS for colemanite. Like petroleum sulfonate, this performance can be attributed to the longer chain of the oleate radical. This assertion agrees with the previous conclusion that longer chains are more effective because alkyl chain interaction is contingent on the carbon number of adsorbed surfactant chains [54,57]. Although sodium oleate is a common collector for floating many salt-type minerals [63,64], its selectivity is low, and it is highly sensitive to water hardness and flotation temperature [65]. Minerals 2021, 11, 318 13 of 18

Due to the relatively low selectivity of anionic fatty acids and their derivatives, cationic collectors are also used in the flotation of these minerals with modification of parameters and the use of modifiers/depressants [64]. They include Flotigam T, DAH, Armac-T, and DTAC. Generally, they are capable of floating colemanite and ulexite. For example, Armac-T and DAH can float both colemanite and realgar in a similar manner [2,12]. Since ulexite and the clay minerals associated with colemanite carry the same surface charge and properties, cationic collectors may not selectively separate colemanite from them. The most efficient colemanite recovery with an amine collector could be achieved at pH around 10, where the highest amounts of ion-molecular complexes form [5,26]. The absence of difference in adsorption bands observed with colemanite and cationic collector indicates the only adsorption would be physical (electrostatic) [16]. Celik et al. revealed that primary amines could float colemanite at lower concentrations than quaternary amines and that the interaction with the former could involve hydrogen bonding. The better performance of primary amines can be ascribed to the structural arrangement of their functional groups [11].

4.5. Reverse Flotation BMs are generally recovered by direct flotation; however, some conditions render direct flotation inadequate for their selective separations. Borax and its associated montmorillonite clay can both be floated equally by collectors because of their similar surface properties [40]. In light of this, reverse flotation (the direct flotation of the less valued mineral) is applied. Reverse flotation is also recommended to recover fine BMs from tailing dams because it can be more selective than direct flotation (based on the mineralogy of gangue phases). Fortunately, a combination of various reagents can improve the efficiency of the process (Table6).

Table 6. Flotation conditions for the reverse ﬂotation of BMs.

Gangue Collector Borate Recovery Grade Collector Depressant pH Reference Mineral Type Mineral (% B2O3) (% B2O3) Xanthate, Sodium Realgar KAX Colemanite 9 96.99 33.93% [16] anionic silicate Calcite, Sodium Carboxylate, Sodium dolomite, Borax 8 81.78 23.47 [40] oleate anionic silicate and clay Xanthate, Clay R801 Colemanite Tannic acid 9.3 2.5 (ﬂoated) [26] anionic

Calcite serves as a hindrance to the selective flotation of borax. Therefore, sodium silicate was used to depress borax. The sodium oleate’s maximum chemisorbed on the calcite surface can occur at pH ranges 10–11 [66,67]. Though, Çilek and Üresin showed that sodium oleate dosage is the essential variable affecting both the grade and recovery of BMs in the concentrate (Figure9)[40]. Koca et al. (2003) investigated the effects of potassium amyl xanthate (KAX) on flotation separation of realgar from colemanite since the xanthates do not efficiently float colemanite [12]. However, they are one of the best collectors for realgar. KAX was employed for the reverse flotation recovery of colemanite from its ore-containing realgar. After successful trials with two artificial samples, a concentrate recovery of 96.99% B2O3 and a grade of 33.93% B2O3 was achieved. This result represented a reduction in arsenic content by 31.20% [12,16]. Minerals 2021, 11, x FOR PEER REVIEW 14 of 18

Calcite serves as a hindrance to the selective flotation of borax. Therefore, sodium silicate was used to depress borax. The sodium oleate’s maximum chemisorbed on the Minerals 2021, 11, 318 calcite surface can occur at pH ranges 10–11 [66,67]. Though, Çilek and Üresin showed 14 of 18 that sodium oleate dosage is the essential variable affecting both the grade and recovery of BMs in the concentrate (Figure 9) [40].

100

40 pH=8/Na-Oleat Recovery(%) 20 pH=9/Na-Oleat pH=10/Na-Oleat 0 50 100 150 200 Collector dosage (g/t)

Figure 9. Influence 9. Influence of different of different Na-Oleat dosages Na-Oleat on boron dosages mineral on recovery boron in mineral various pHs recovery (re- in various pHs (reverse verse flotation) [40]. flotation) [40]. Koca et al. (2003) investigated the effects of potassium amyl xanthate (KAX) on flotation separationIn the of context realgar from ofseparating colemanite since BMs the xanthates from each do not other efficiently using float cole- the flotation process, the manitemethod [12]. can However, be termed they are directone of the flotation best collectors or reverse for realgar. flotation KAX was ofemployed the BMs, depending on the for the reverse flotation recovery of colemanite from its ore-containing realgar. After suc- cessfulBM being trials with floated two artificial or depressed. samples, a concentrate Celik and recovery Bulut of (1996) 96.99% indicatedB2O3 and a grade that DAH is not selective offor 33.93% separating B2O3 was colemaniteachieved. This fromresult represented ulexite in a equalreduction amounts in arsenic ofcontent the collectorby [12]. However, 31.20%ulexite [12,16]. is only marginal floated by anionic collectors such as SDS. Therefore, an anionic In the context of separating BMs from each other using the flotation process, the methodreagent can would be termed be direct more flotation appropriate or reverse for flotation selective of the colemaniteBMs, depending and on the ulexite [2]. BM being floated or depressed. Celik and Bulut (1996) indicated that DAH is not selective for4.6. separating Depressants colemanite from ulexite in equal amounts of the collector [12]. However, ulexite is only marginal floated by anionic collectors such as SDS. Therefore, an anionic reagentVarious would be ganguemore appropriate minerals, for selective in varying colemanite amounts, and ulexite are [2]. usually associated with BMs. The clays, carbonates, and arsenic minerals are prominent [12,16,19,23]. Montmorillonite-type 4.6.clay Depressants minerals are transported into the concentrate by mechanical entrainment. The car- bonateVarious mineral gangue surfaceminerals, becomesin varying amounts, hydrophobic are usually using associated the Na-oleatewith BMs. The collector in the alkaline clays, carbonates, and arsenic minerals are prominent [12,16,19,23]. Montmorillonite-type claycondition minerals [are40 ].transported The arsenic into the minerals concentrate are by floated mechanical in entrainment. equal proportions The car- as the BMs with the bonateconventional mineral surface collectors becomes [ 12hydrophobic]. Hence, using separating the Na-oleate gangues collector from in the thealkaline valued minerals requires conditiondifferent [40]. additives The arsenic (modifiers) minerals are floated in the in flotation equal proportions systems as the [63 BMs,68 ,with69]. the conventional collectors [12]. Hence, separating gangues from the valued minerals requires differentDepressants additives (modifiers) play a in prominent the flotation rolesystems in [63,68,69]. the selective flotation process. They are generally usedDepressants to suppress play a clay prominent minerals role in as the the selective main flotation gangue process. phases. They The are gener- ones applied in depressing allyminerals used to suppress in the recoveryclay minerals of as BMsthe main include gangue phases. Ke-1365, The ones dextrin, applied Cataflot in de- P-40 [23], sodium pressing minerals in the recovery of BMs include Ke-1365, dextrin, Cataflot P-40 [23], sodiumsilicate silicate [12 [12,16,23,40],,16,23,40], sodium sodium pyrophosphate, pyrophosphate, and sodium and hexametaphosphate, sodium hexametaphosphate, and and corn cornstarch. starch. However, However, starch starch and andsodium sodium silicate are silicate typical depressants are typical used depressants for the di- used for the direct rectand and reverse reverse flotationsflotations of BMs of BMs to decrea tose decrease gangue minerals’ gangue presence minerals’ in the concen- presence in the concentrate (Tablestrate (Tables5 and 5 and6 )[6) [16,19].16,19 Sodium]. Sodium silicates’ silicates’ effects on effectsboron minerals on boron recovery minerals depend recovery depend on Minerals 2021, 11, x FOR PEER REVIEWon three essential parameters: pH, collector dosage, and depressant dosage (Figure15 of 10) 18 [40].three essential parameters: pH, collector dosage, and depressant dosage (Figure 10)[40].

FigureFigure 10. 10.EffectEffect of sodium of sodiumsilicate dosage silicate on the dosage recovery on of the the boron recovery minerals of [40]. the boron minerals [40].

5. Summary and Conclusions Because of widespread Boron applications, beneficiation methods for increasing BMs’ concentration from their primary resources have adsorbed considerable attention, while these processes pose significant challenges. Friability, solubility, hydrophilicity, and having similar surface properties to their associated minerals can be counted as the main causes of complexity through BMs’ processing. Although scrubbing, washing, and classification were traditional techniques for concentrating BMs, they mainly deal with high- grade and coarse particles. By decreasing BMs’ grade in their resources, non-chemical processes are not efficient and can be performed as pretreatment methods. Differences between BMs’ magnetic susceptibilities and their associated clay minerals could introduce the magnetic separation as a successful pretreatment technique that could even reduce the Fe content of BM’s concentrate within the market limit (as low as 0.6%). Ultrasound as an environmentally friendly pretreatment technique could disperse clay minerals, reduce their BM coating rate, improve collector adsorption, and in some cases increase recovery of BMs from 5% to 90%. Various responses of BMs and their gangue phases to heat introduced the calcination even as a method that can be used individually to upgrade BMs. Temperature, heating time, and particle size can be named the most important factors for BM ores’ calcination. Calcination as another eco-friendly method could briquette products and result in a concentration with 55% grade and 90% recovery of B2O3. Although magnetic separation, ultrasound, and calcination techniques as dry pretreatment techniques have several advantages for processing BMs, few studies have been conducted in their applications, and there is substantial room for implementing investigations in these areas. Since a massive amount of BM fines would be generated during their mining and processing, flotation separation is the most known processing method for their beneficiations. The IEP of pure BMs occurred at the alkaline conditions (pH > 8). Clay coating minerals render their surface charge negative for approximately all pH ranges. Moreover, multivalent ions in the pulp can play a significant role through the flotation separation BMs. Based on their solubility, they can make new compounds when reacted with collectors. The least soluble can activate BM’s surface most (mainly Ba + collector), and the most soluble (Mg + collector) activates it the least. Therefore, their existence can improve or

Minerals 2021, 11, 318 15 of 18

5. Summary and Conclusions Because of widespread Boron applications, beneficiation methods for increasing BMs’ concentration from their primary resources have adsorbed considerable attention, while these processes pose significant challenges. Friability, solubility, hydrophilicity, and having similar surface properties to their associated minerals can be counted as the main causes of complexity through BMs’ processing. Although scrubbing, washing, and classification were traditional techniques for concentrating BMs, they mainly deal with high-grade and coarse particles. By decreasing BMs’ grade in their resources, non-chemical processes are not efficient and can be performed as pretreatment methods. Differences between BMs’ magnetic susceptibilities and their associated clay minerals could introduce the magnetic separation as a successful pretreatment technique that could even reduce the Fe content of BM’s concentrate within the market limit (as low as 0.6%). Ultrasound as an environmentally friendly pretreatment technique could disperse clay minerals, reduce their BM coating rate, improve collector adsorption, and in some cases increase recovery of BMs from 5% to 90%. Various responses of BMs and their gangue phases to heat introduced the calcination even as a method that can be used individually to upgrade BMs. Temperature, heating time, and particle size can be named the most important factors for BM ores’ calcination. Calcination as another eco-friendly method could briquette products and result in a concentration with 55% grade and 90% recovery of B2O3. Although magnetic separation, ultrasound, and calcination techniques as dry pretreatment techniques have several advantages for processing BMs, few studies have been conducted in their applications, and there is substantial room for implementing investigations in these areas. Since a massive amount of BM fines would be generated during their mining and processing, flotation separation is the most known processing method for their beneficiations. The IEP of pure BMs occurred at the alkaline conditions (pH > 8). Clay coating minerals render their surface charge negative for approximately all pH ranges. Moreover, multivalent ions in the pulp can play a significant role through the flotation separation BMs. Based on their solubility, they can make new compounds when reacted with collectors. The least soluble can activate BM’s surface most (mainly Ba + collector), and the most soluble (Mg + collector) activates it the least. Therefore, their existence can improve or decrease the process efficiency depends on the type of multivalent ions. As a function of pH, these ions can enhance the flocculation of BMs. Direct flotation by cationic and anionic collectors can be considered the main flotation methods to recover BMs. Their reverse flotation benefits are mainly used when carbonates are the dominant associated minerals. The anionic collectors were the main focus of investigations, and few studies have been conducted on the cationic collectors. Since the surface charge on the BMs is mostly negative, anionic collectors generally have chemical adsorption. Among anionic collectors, sulfonates (high molecular weight) showed the highest efficiency for separating BMs from their associated gangue phases (pH ~ 10), although, by increasing pH, the BM flotation recoveries would be decreased. Xanthates are indicated as successful collectors for the reverse selective separation of colemanite from realgar. Starch and sodium silicate are the typical depressants used for the direct and reverse flotations of BMs. Therefore, there is substantial room for studying the generation of new reagents for the selective separation and surface properties of BMs in different conditions. Using high-tech surface analysis instruments—such as “time of flight secondary ion mass spectrometry (TOF-SIMS)”, “x-ray photoelectron spectroscopy (XPS)”, and “atomic force microscopy (AFM)”—can fill this gap and improve our understanding of various surfactant adsorptions on the BM surfaces and their mechanisms.

Author Contributions: S.-P.B.P., V.K., and M.J. read articles, summarized information, made tables, and interpreted subjects. S.C.C. gathered information, planned, trained and managed the group, generated ﬁgures, read summaries, discussed outputs, and wrote the manuscript. All authors have read and agreed to the published version of the manuscript. Minerals 2021, 11, 318 16 of 18

Funding: This research received no external funding. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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