**2.2. Sulfur removal**

[39]. They continued their experiments to increase the recovery of coal by using three stage addition process, and for same substance, a product of 86% coal throughput involving 12% ash was obtained [40]. Within the matter of improving reverse flotation, they extended their studies and compared the laboratory column cell using synthetic feed mixture consisting coal and quartz. Thus, recovery capacity of coal was much better than in normal flotation [41].

Later, Ding and Laskowski took Stonestreet's studies step further by adding dolomite and calcite as gangue minerals and surveyed the effects of factors on separation. They used dodecyl trimethyl ammonium chloride (DTAC) as a collector whose properties of separation are good when it is used minimum 6 kg/t [2]. After, Ding further continued to study on reverse flotation using DTAC on subbituminous coal, too. DTAC consumption was dramatically descended from 6 to 1.375 kg/t by applying polyacrylamide (PAM) and zero-conditioning time method. Besides, in order to improve selectivity, dextrin was governed, and the addition of tannic acids as a dispersant improved the quality of concentrate. For the feed ash content of 34.6%,

On the other hand, Patil and Laskowski carried out their studies regarding to enhancing reverse flotation of coal. Patil drew on dodecyl trimethyl ammonium chloride (DTAB) in reverse flotation as collector, but used no depressant, first. Also, zero-conditioning time method was applied in their studies. Zero conditioning was accomplished by adding necessary quantity of DTAB in one step, immediately after system was exposed to the air. The logic behind the zero-conditioning time is that continuity of reverse flotation should not be interrupted in any case. Air bubbles formed from air introducing carried collectors, DTAB, during the flotation process. The entrainment of DTAB carried in air bubble demonstrated that reduction of ash from sub-bituminous coal (LS-26) from 34.7 to 22.9% with gangue yield of 36.8% by using any depressant. In the existence of depressant which was dextrin with 0.5 kg/t, the ash

substance of LS-26 was reduced from 34.7 to 16.5% at the clean coal yield 55% [46].

trimethyl ammonium bromide (HTAB) was oriented as collector [42].

Generally, researchers have been seeking the behavior of ash particles under the participation of different reagents. They mainly focused on low rank coal like lignite due to their hydrophilic properties. Vamvuka also studied on lignite and oxidized coals and used dodecyl amine (DDA) with kerosene in flotation. Ash reduction of 18% with coal recovery of 80% was achieved [47]. Ozturk also proceeded their studies on reverse flotation of Turkish lignite samples involving high ash and sulfur content. They used ionic collector (Aero 3477) and obtained clean coal product of 29.04% ash at a combustible yield of 78.14%, while with nonionic collector [kerosene], these values altered to 27.04 and 81.19%, respectively [27]. On the other hand, Zhang et al. also worked on reverse flotation, but used different reagents, as a collector dodecyl amine chloride (DAH), as a depressant corn starch, and as a further methyl isobutyl carbinol (MIBC), and observed the effect of particle size in the presence of soluble salt. When the highest reverse flotation performance was achieved, concentrate ash content of 11.30% was obtained with a combustible recovery of 65.29% [43]. Finally, Xia et al. applied reverse flotation on taixi oxidized coals. Dextrin was used as depressant, whereas hexadecyl

Alternatively, Li studied with sub-bituminous coal involving a significant amount of oxygen which causes to decreasing hydrophobicity. Due to being difficult to upgrade fine fraction

the concentrate of 16.7% ash at 50.4% yield was acquired [36].

146 Energy Systems and Environment

In the early age of 1960s, Eveson was the first person who took attention on reverse flotation by removing shale from bituminous coal. After, reverse flotation had become popular among other researchers, and they started to focus on desulfurization of coal by using this method [30–32, 49].

The presence of sulfur in coal might be found in three forms which can be categorized as organic sulfur, metallic sulfur, and sulfatic sulfur [9, 50–53]. Organic sulfur in coal originates from carbonated plant, while metallic and sulfatic ones' source is inorganic sulfur which exists among mineral compounds. The most common example of metallic sulfur is pyrite and it is called as pyritic sulfur. In addition to pyrite, other minerals may be involved in coal structure such as marcasite, galena, sphalerite, etc. For sulfatic ones, gypsum may be illustrated [54].

The sulfur content is varying by different types of coal, and sulfur content accounts for pyritic and organic sulfur amount in coals. Even coal substances might be extracted from same ore bed, they possess different sulfur content. Pyritic sulfur may represent 20–80% of total sulfur content [38, 50]. Like ash, pyrite particles may also exist in internal structure of coal, so physical methods are becoming nonfunctional in removal of pyrite from coal substance [28]. Flotation was started to be applied to achieve desulfurization as well.

Each type of pyrite mineral shows different floatability properties, and the reason of that was investigated by some important researchers. Feurstenau considered that the cause of that variation is related with the formation of elemental sulfur. Because of being naturally hydrophobic [37, 55, 56], elemental sulfur can conduct with the surface of pyrite, and then, it may behave like a collector for pyrite. Oxidation of pyrite under proper condition forms elemental sulfur, and it is frequently observed in weathered coal, not in fresh coal [57]. Two basic reactions are standing below for expressing elemental sulfur formation from pyrite. These are as follows:

$$\text{\textbullet } Fe^{\ast 3}(aq) + FeS\_2 = \text{\textbullet } Fe^{\ast 3}(aq) + \text{\textbullet } S\_{\text{\textbullet }} \tag{1}$$

Eq. (1) accounts for formation of elemental sulfur under microbial oxidation of pyrite, i.e. pyrite oxides in moist. To proceed the process of oxidation, acidic environment is necessary because pyrite-oxidizing bacteria can grow under this circumstance [37, 38]; that means, during the pyrite oxidation, iron sulfates, type of salts, constitutes, and these are known as flotation depressant [55]. These depressants can only be dissolved when acidic conditions are satisfied in pulp. On the other hand, unlike formation of sulfate format, if elemental sulfur formation is obtained at the end of oxidation, it is assumed that reaction of coal pyrite is similar to reaction of mineral pyrites, and flotation can be carried out on neutral pH range [37]. Second oxidation reaction may take place with water [58], and it is expressed in the form of;

$$FeS\_2 + 3\, H\_2O = Fe\, (OH)\_3 + 2\, S^\circ + 3\, H^+ + 3\, e^- \tag{2}$$

Stone coal was exposed to the two stages of flotation processes to recover vanadium microelement. Mineral composition of coal was calcite, barite, quartz, and V-minerals. Reagents were sulfuric acid (pH regulator), oleic acid (Ca minerals collector), sodium silicate (dispersant), sodium fluorosilicate (SFF and depressant), melamine (EA and V minerals collector), dodecyl amine (DDA and V minerals collector), octadecylamine (DC and V minerals collector, terpenic oil (frother)). pH was kept between 7 and 8, and water glass was used as depressant

At the end of this study, selective separation of vanadium-bearing minerals can be achieved

recovery rate of 76.58% are obtained by desliming-flotation process and 72.51% of the raw ore is rejected as tailings [70]. Also, results of other tests demonstrate that grade and recovery of

Although vanadium recovery from stone coal is exploring recently by researchers, studies have already been demonstrated how well vanadium is recovered and obtained clean coal

Coal quality is determined by many properties, but major factor is coal rank. The rank of coal is identified by the percentage of fixed carbon, moisture (water), volatile matter, and calorific value in British thermal units after sulfur and mineral matter content have been subtracted. Coal types that might be ordered from lowest to the highest rank are lignite, sub-bituminous coal, bituminous coal, and anthracite [72]. Rank directly influences floatability of the coal since chemical structure changes due to elimination of polar groups during coalification process. At the end of this process, carbon content increases, and result in increasing hydrophobicity [73, 74]. Although, rank and floatability are directly proportional, highest hydrophobicity is achieved in bituminous coal, not anthracite, which has highest coal rank, but difference in floatability is not significant between them [75, 76]. Bituminous coals are enough hydrophobic to be floated in further without collector, but in order to improve coal recovery, collector oil is

Moreover, degree of the oxidation of the coal surface is essential for hydrophobicity. It leads coal to act like lower rank coal whose hydrophobicity is lower [4, 79, 80]. However, there are some cases that oxidation increases in the floatability of coal like freshly cleaved coal surfaces. Short-time air exposure may increase hydrophobicity due to drying of the coal surfaces which

Coal is composed of many different minerals that influence type of beneficiation method and its applications. These materials cannot be removed from coal totally by using conventional method [81]. Coal has heterogeneous structure, but is mainly formed from inorganic materials such as clay, quartz, sulfides, and sulfates [82, 83]. Mineral content determines the coal grade, and its rate should be less than 50% to be accepted as coal [54]. There are more than

 concentrate are 1.32% and 88.38, respectively, and tailing yield is 38.36%. On the other hand, recovery and grade of carbon mineral which may be used as fossil fuels are 75.10 and

O5

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grade of 1.88% and

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(2000 g/t). Besides, oleic acid was taken 200 g/t as collector [70].

V2 O5

30.08%, respectively [71].

**2.4. Effects of rank, mineral, and maceral**

governed [77] even it reduces the coal rank [78].

then becomes more difficult to wet [9].

simultaneously.

in pH 3 using melamine (EA). The final vanadium concentrates with V<sup>2</sup>

In both cases, if conditions are suitable, substantial quantity of elemental sulfur formation on pyrite sulfur can be observed even the elemental sulfur is normally an intermediate product in a series of reactions that are over with producing sulfate (SO<sup>4</sup> −2 ) [37].

Pyrite has small hydrophobic tendency [55, 59–61] when its surface is unoxidized. With the presence of water, oxidation takes place on surface of pyrite and forms ferric hydroxide, which leads to decreasing hydrophobicity [62–64]. In pH range of 4.5–6.9, oxidation products of pyrite act like strong depressant [60], as it was mentioned before. In order to enhance hydrophobic tendency of pyrite, it is required to add some collector like xanthate [65] since floatability of pyrite intimately depends on pH, and highest floatability might be obtained in acidic pulp [38].

Kawatra performed experiments on fresh coal and 1-year aged coal for different conditions. Fresh coal substances were exposed to different pH levels, and the percentage of froth weight being in directly proportional with floatability of pyrite were investigated. pH levels were defined as 8.3, 7.5, 2.3,2.2, and 2.0, and froth weights were found as 5, 4, 98, 98, and 99%, respectively. On the other hand, same procedure was applied on 1-year aged coal at -15°C, and lowest weight was attained around 37% when pH was almost equal to 6.8, whereas pH was dropped to 2.0, achieved weight dramatically increased, and equal to 92%. Lastly, 1-year aged coal substances were heated up to 100°C and tested in pH level 6.2 and 2.0, and results were 7% and 82%, respectively. From the results, it can be understood that freshly ground mineral pyrite is not readily floatable at neutral pH despite of being highly floatable in acidic pH. On the other hand, pyrite oxidation concludes with sulfate formation, which is not hydrophobic. If necessary conditions might be satisfied for forming elemental sulfur, pyrite can be floated. Also, it is shown that under certain condition, pyrite can be floated at neutral pH, but that is not a normal case. [37]

#### **2.3. Vanadium recovery**

Vanadium is strategic metal and has been extensively used in the field of steel and alloy industry. Tensile strength capacity of vanadium is too high, so 80% of vanadium are utilized for alloy steels, whereas remaining portion is applied in chemical industry [66–68].

Vanadium is another element that might be recovered from coal by using reverse flotation. Coal vanadium element can be found in some coal minerals such as illite, muscovite, roscoelite, and kaolin in the form of isomorphism, whereas tantalite and garnet are appeared in the form of absorption [69]. In addition, quartz, calcite, and carbonaceous are found to be main gangues in stone coal [68]. For many studies, flotation has been popular topic for many years, but there are not much more available studies on pre-concentration of vanadium in low-grade coal by the method [70].

Stone coal was exposed to the two stages of flotation processes to recover vanadium microelement. Mineral composition of coal was calcite, barite, quartz, and V-minerals. Reagents were sulfuric acid (pH regulator), oleic acid (Ca minerals collector), sodium silicate (dispersant), sodium fluorosilicate (SFF and depressant), melamine (EA and V minerals collector), dodecyl amine (DDA and V minerals collector), octadecylamine (DC and V minerals collector, terpenic oil (frother)). pH was kept between 7 and 8, and water glass was used as depressant (2000 g/t). Besides, oleic acid was taken 200 g/t as collector [70].

At the end of this study, selective separation of vanadium-bearing minerals can be achieved in pH 3 using melamine (EA). The final vanadium concentrates with V<sup>2</sup> O5 grade of 1.88% and recovery rate of 76.58% are obtained by desliming-flotation process and 72.51% of the raw ore is rejected as tailings [70]. Also, results of other tests demonstrate that grade and recovery of V2 O5 concentrate are 1.32% and 88.38, respectively, and tailing yield is 38.36%. On the other hand, recovery and grade of carbon mineral which may be used as fossil fuels are 75.10 and 30.08%, respectively [71].

Although vanadium recovery from stone coal is exploring recently by researchers, studies have already been demonstrated how well vanadium is recovered and obtained clean coal simultaneously.

#### **2.4. Effects of rank, mineral, and maceral**

depressant [55]. These depressants can only be dissolved when acidic conditions are satisfied in pulp. On the other hand, unlike formation of sulfate format, if elemental sulfur formation is obtained at the end of oxidation, it is assumed that reaction of coal pyrite is similar to reaction of mineral pyrites, and flotation can be carried out on neutral pH range [37]. Second oxidation

In both cases, if conditions are suitable, substantial quantity of elemental sulfur formation on pyrite sulfur can be observed even the elemental sulfur is normally an intermediate product

Pyrite has small hydrophobic tendency [55, 59–61] when its surface is unoxidized. With the presence of water, oxidation takes place on surface of pyrite and forms ferric hydroxide, which leads to decreasing hydrophobicity [62–64]. In pH range of 4.5–6.9, oxidation products of pyrite act like strong depressant [60], as it was mentioned before. In order to enhance hydrophobic tendency of pyrite, it is required to add some collector like xanthate [65] since floatability of pyrite intimately depends on pH, and highest floatability might be obtained in acidic pulp [38]. Kawatra performed experiments on fresh coal and 1-year aged coal for different conditions. Fresh coal substances were exposed to different pH levels, and the percentage of froth weight being in directly proportional with floatability of pyrite were investigated. pH levels were defined as 8.3, 7.5, 2.3,2.2, and 2.0, and froth weights were found as 5, 4, 98, 98, and 99%, respectively. On the other hand, same procedure was applied on 1-year aged coal at -15°C, and lowest weight was attained around 37% when pH was almost equal to 6.8, whereas pH was dropped to 2.0, achieved weight dramatically increased, and equal to 92%. Lastly, 1-year aged coal substances were heated up to 100°C and tested in pH level 6.2 and 2.0, and results were 7% and 82%, respectively. From the results, it can be understood that freshly ground mineral pyrite is not readily floatable at neutral pH despite of being highly floatable in acidic pH. On the other hand, pyrite oxidation concludes with sulfate formation, which is not hydrophobic. If necessary conditions might be satisfied for forming elemental sulfur, pyrite can be floated. Also, it is shown that under certain condition, pyrite can be floated at neutral pH, but that is not a normal case. [37]

Vanadium is strategic metal and has been extensively used in the field of steel and alloy industry. Tensile strength capacity of vanadium is too high, so 80% of vanadium are utilized

Vanadium is another element that might be recovered from coal by using reverse flotation. Coal vanadium element can be found in some coal minerals such as illite, muscovite, roscoelite, and kaolin in the form of isomorphism, whereas tantalite and garnet are appeared in the form of absorption [69]. In addition, quartz, calcite, and carbonaceous are found to be main gangues in stone coal [68]. For many studies, flotation has been popular topic for many years, but there are not much more available studies on pre-concentration of vanadium in low-grade

for alloy steels, whereas remaining portion is applied in chemical industry [66–68].

<sup>3</sup> + 2 *So* + 3 *H*<sup>+</sup> + 3 *e*<sup>−</sup> (2)

−2 ) [37].

reaction may take place with water [58], and it is expressed in the form of;

in a series of reactions that are over with producing sulfate (SO<sup>4</sup>

*FeS*<sup>2</sup> + 3 *H*<sup>2</sup> *O* = *Fe* (*OH*)

148 Energy Systems and Environment

**2.3. Vanadium recovery**

coal by the method [70].

Coal quality is determined by many properties, but major factor is coal rank. The rank of coal is identified by the percentage of fixed carbon, moisture (water), volatile matter, and calorific value in British thermal units after sulfur and mineral matter content have been subtracted. Coal types that might be ordered from lowest to the highest rank are lignite, sub-bituminous coal, bituminous coal, and anthracite [72]. Rank directly influences floatability of the coal since chemical structure changes due to elimination of polar groups during coalification process. At the end of this process, carbon content increases, and result in increasing hydrophobicity [73, 74]. Although, rank and floatability are directly proportional, highest hydrophobicity is achieved in bituminous coal, not anthracite, which has highest coal rank, but difference in floatability is not significant between them [75, 76]. Bituminous coals are enough hydrophobic to be floated in further without collector, but in order to improve coal recovery, collector oil is governed [77] even it reduces the coal rank [78].

Moreover, degree of the oxidation of the coal surface is essential for hydrophobicity. It leads coal to act like lower rank coal whose hydrophobicity is lower [4, 79, 80]. However, there are some cases that oxidation increases in the floatability of coal like freshly cleaved coal surfaces. Short-time air exposure may increase hydrophobicity due to drying of the coal surfaces which then becomes more difficult to wet [9].

Coal is composed of many different minerals that influence type of beneficiation method and its applications. These materials cannot be removed from coal totally by using conventional method [81]. Coal has heterogeneous structure, but is mainly formed from inorganic materials such as clay, quartz, sulfides, and sulfates [82, 83]. Mineral content determines the coal grade, and its rate should be less than 50% to be accepted as coal [54]. There are more than 120 minerals involved in coal, and primary ones regarding their degree of presence are quartz, kaolinite, illite, montmorillonite, chlorite (may contain Mn), clays (may also contain Be, Ni, and other trace elements), pyrite (may contain As, Cd, Co, Hg, Ni, Sb, and Se), calcite, and siderite (may contain Mn); not common ones are analcime, apatite, barite, chalcopyrite, clausthalite, crandallite, floricide, gorseksit, goyasite, dolomite, feldspar, galena, marcasite (may contain same elements as pyrite), monazite, rutile, sphalerite (may contain Cd), xenotime, and zircon; and rare ones are chromite, gypsum, gold, gibbsite, rock salt, magnetite, and muscovite [72]. Seventy-six elements of periodic table can be found in coal substance. Some of these are trace elements and their ratios are expressed with ppm. Some trace elements may be concentrated in specific coal bed, which make that bed a valuable resource for those elements such as silver, zinc, or germanium [84]. However, some elements have potential to damage environment like cadmium or selenium if their concentrates are more than trace amounts. Trace elements associated with clays or pyrite are removed from coal by flotation process, and it is significant to dispose all trace element with in the manner of environment and recovery of valuable elements. Nevertheless, except for gypsum, the various forms of ash and germanium, recovered minerals have not been used commonly [72].

**2.5. Effects of pH**

The pH has great importance in flotation because pH of liquid phase influences the surface characteristics and behavior of mineral and induces minerals to absorb all types of reagents on the surface. Response of reagents to the pH is essential for flotation, and there are no standard pH values for particular minerals in flotation. Instead, it is generally expressed with range for flotation of specific minerals, and it may differ according to participating reagents. For this reason, this may become complicated and needed to perform sensitive when highly selective products are required. Like coal reverse flotation, effects of hydroxide ions (OH−) and hydrogen ions (H+) ions are not only important for floating mineral matter but also important for suppressing coal [89]. Mineral surface can be altered with adjusting these ions in pulp. Minerals in pulp can be charged positively or negatively by arranging pH regarding the isoelectric point (IEP). When the pH is higher than IEP, minerals charge negatively, on the contrary, opposite actions will take place in mineral charging. Zeta potential is related with

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The pH plays important role in pyrite removal, which is hazardous mineral for environment. Mineral pyrite and coal pyrite act different. Inherently, mineral pyrite is floatable, and it loses its floatability when pH is greater than 5.0. When the pH range is between 5 and 9, the recovery of mineral pyrite is not noticeable even neutral oil collectors are utilized to render mineral pyrite floatable. Although same fashion is used for coal pyrite, it does not act as mineral pyrites. In the pH range of 2.2–8.8, coal pyrite can be recovered 31–43%, whereas mineral matter pyrite can be achieved to regain 99% over the same pH range. Kawatra carried out microscopic examination of coal pyrite flotation and resulted in floated materials that were coal and locked coal/pyrite particles. Therefore, it is assumed that coal pyrite was floated due to attachment to coal [38]. Chander and Aplan performed studies to prove that pyrite is inherently less floatable due to exposed oxidation during purifying from coal which may result in destroying floatability of pyrite [34]. The studies show that coal pyrite may be floated due to locked or entrained particles [38]. Some experiments were handled by Kawatra to examine the effects of pH with using different reagents. In the first experiment, the pulp pH was arranged lower than 4.0, and fuel oil was used as collector for mineral pyrite. Flotation could be achieved with the range of that pH, but native floatability was entirely lost with higher pH values. In second trial, coal pyrite was tried to be floated. However, coal pyrite may behave like mineral pyrite,

The effects of surface and solution chemistry of Fe(II) and Fe (III) ions on the flotation of both mineral and coal pyrite with xanthate were investigated based on flotation output, zeta point measurements, and thermodynamic calculations. The results showed that existence of ferrous and ferric ions induced pyrite depress in pH range 6–9.5. Coal pyrite was recognized non-floatable above pH 6 due to large number of ferrous ions resulted from pyrite oxidation. Moreover, thermodynamic calculations demonstrated that formation of ferric hydroxyl xan-

On the other hand, some additional experiments have been performed for different types of minerals existing in coal structure. As it is well known that materials vary between each other with respect to their properties. Like pyrite, ash also can be recovered by reverse flotation

thate leads to reducing floatability of pyrite when the pH is greater than 6.0 [92].

absolute changing in pH with respect to IEP, so it increases slowly [90, 91].

and it was not recovered at neutral pH range [38].

In addition to rank and mineral matter, maceral also affects the flotation of coal since coal hydrocarbon structure and hydrophobicity are influenced by maceral content [78, 85]. The macerals consist of lithotypes, and their proportions vary. The properties of lithotypes also differ from each other. Macerals are classified into groups according to dominant components, which are vitrine, inertinite, or liptinite, and has different hydrophobicity. For example, the lithotype fusain involving inertinite group macerals is generally the least floatable, whereas vitrian involving vitrinite group macerals is the most floatable [75]. Studies associated with maceral recovery proved that it is possible to regain good volume macerals without much loss of combustible value [83]. The studies on floatability of coal macerals were appeared at the beginning of 1950 by Horsley [8] and Sun [86]. According to Burdon, maceral content changed with increasing time flotation [87]. Even in same flotation cell, maceral content could vary when samples were taken from different place of it [3] because the macerals are the basic microscopic, physically distinct, and chemically different constituents of the carbonaceous matter in coal, which originate from material deposited in the primeval swamps [83]. Due to variance in maceral content, it is essential to define coal nature and response each maceral during the flotation. Based on maceral content in clean coal, the use of coal can be optimized. For example, high liptinite content increases the calorific value, whereas high inertinite content in concentrate stimulates increase in fixed carbon [88].

Rank, mineral content and macerals are influencing the flotation performance and type because these are essential parameters for floatability of coal. Reverse flotation can be optimized for low rank coal because low rank coal can be suppressed more easily since hydrophobicity of low rank coals are more less due to its polar structures. In order to recover valuable minerals and remove hazardous minerals at the same time, reverse flotation is again proper way because mineral content of low rank coal is more compared to high rank coal, so minerals might be floated without taking much more effort. Lastly, macerals can be divided into some groups according to compounds content as it was mentioned before. Coal with lowest floatable maceral, fusain, may be upgraded by using reverse flotation, too.
