**2.5. Effects of pH**

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

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 con-

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 float-

minerals have not been used commonly [72].

150 Energy Systems and Environment

tent in concentrate stimulates increase in fixed carbon [88].

able maceral, fusain, may be upgraded by using reverse flotation, too.

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

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

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

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 along the different pH ranges. Stonestreet performed studies on ash reduction by applying reverse flotation method on quartz coal mixtures with same amount. The result of their studies showed that maximum ash reduction was succeeded using talk water whose pH was equal to 7.6 [4]. After, Pawlik also touched upon ash reduction with different coal substances. Sub-bituminous, bituminous, and oxidized bituminous coal were objective of reverse flotation with different pH ranges. At first, bituminous coal were exposed to different pH ranges, and optimal results for normal and oxidized coal differed. For normal bituminous coal, the flotation yield went into decline for higher pH values than 9.5. However, sharp decrease was observed when pH level becomes higher than 4.0 for oxidized bituminous coal sample. Besides, different tests were performed for sub-bituminous coal/silica mixtures, and for these tests, optimum pH range was determined between 8.3 and 8.6 [25]. Moreover, Ozturk mainly focused on ash reduction by reverse flotation of lignite, and they achieved maximum level of reduction at neutral pH range around 8 [27]. Same with Ozturk, Zhang also implemented reverse flotation on lignite sample at neutral pH level [73]. Lastly, sub-bituminous coal/quartz mixture with ratio 7:3 was subjected to the test, and all tests were done at neutral pH level [48]. Some additional examples are given from different pH ranges to represent bad results. For example, the low ash content of concentrate was obtained around 10.5 pH value due to optimum flotation of calcite and dolomite around this pH [93].

As well as ash and pyrite, some other valuable minerals can also be regained by reverse flotation throughout different pH levels. Ding worked on sub-bituminous coal sample with 48.5% ash content and gangue minerals such as calcite dolomite and silica with the ratio of 7:1:1:1, respectively. These gangue minerals were intended to be floated. In first study, when pH was 10.4, these gangue minerals can be floated [35], whereas in second test, zero-time conditioning was also taken into consideration, and maximum yield was achieved at the 7.5–8.4 pH range [36]. Wang also tried to float calcite as well as vanadium, which has great value in industry. For calcite, pH level varied between 7 and 8, while it was 3.0 for vanadium [70].

Maximum performance was obtained for −74-micron fraction size with the combustible recovery of 65.29% and ash content of 11.30% after 20 minutes flotation. However, maximum flotation rate constant was achieved in 150–250-micron range, and the maximum reverse flotation index efficiency was attained for −425 microns. Hence, combustible recovery increased with increasing size fraction but meanwhile the concentrate ash content also increased particularly for finer particle sizes [73]. For finer fraction, slime problem appears. Because of that reason, there must be performed detailed studies for finer fraction flotation in order to obtain

Taixi oxidized coal 250–500, 250–125, 125–74, −74 Mineral matter [42] Lignite +300, 300–75, −75 Ash [47]

−38, 38–50, 50–74, 74–154, 154–300, +300 Carbon, vanadium [71]

**Coal type Particle size (micron) Floated minerals Reference**

−38 (fine coal), −200 (raw coal) −56 (fine silica), −200 (coarse silica) Lignite with 42.34% ash content −425, 250–425, 150–250, 150–74, 74–45, −45 Ash [43]

Mineral matter [50]

http://dx.doi.org/10.5772/intechopen.74082

Reverse Flotation

153

Ash [27]

Carbon, vanadium [70]

Sub-bituminous coal/quartz 150–200 (coarse coal), 74–120 (medium size coal)

Lignite 100 (86% of material finer mineral than 100 micron)

Low grade stone coal −38, 38–50, 50–74, 74–154, 154–600, 600–1500, +1500

**Table 1.** Particle size of minerals participating into the reverse flotation.

The purpose of using reagents is to change the surface properties of minerals to adjust which material is floated and depressed. In this concept, regulator reagents (pH adjustor, activator, depressants, and dispersants) are entrained into flotation process to improve quality of selectivity and separation. In **Table 2**, reagents are demonstrated in three groups: depressants, collectors, and frothers. Most of reverse flotation experiments are listed, and for each test,

Collectors are the reagents, which cause to arranging hydrophobicity of material. Collectors can be observed into two main topics that are ionic and non-ionic collectors. Non-ionic collectors are organic compounds formed from hydrocarbon chains having no neutral and polar groups, whereas ionic collectors are divided into two groups, anionic and cationic. The surface properties of minerals determine the reagents selections, and after necessary conditioning is done, flotation process starts. Some of the preferred collectors in reverse flotation are DTAB, HTAB, DTAC, and so on. On the other hand, PAM and ferric silicate were governed besides collector in order to increase activation of them. On the other hand,

optimum results.

Low grade stone coal (mass

fraction)

**2.7. Reagents (collectors, depressants, and frothers)**

available used reagents are indicated (**Table 3**).

Therefore, importance of pH is clearly explained by studies, and more research should be carried out to take a step further in reverse flotation.

#### **2.6. Effects of particle size**

The most important criteria of mineral processing is associated to size of liberated mineral particle. The performance of flotation process depends on the degree of liberation of mineral in fine fraction. For this reason, flotation is applied to minerals that are intended to recover or remove from coal. Coal has a complex structure and possesses various minerals. Although the particle size of coal is generally less than 0.5 mm, liberation may not be achieved in that fraction size, so grinding may become inevitably for well separation. Hence, it is compulsory to apply flotation on coal due to liberation in fine fraction phase. However, finer fraction does not always mean that every parameter related with flotation is obtained in well range. There is a trade of between fraction size and performance of flotation. In **Table 1**, some studies with different coal type associated with various fraction size are demonstrated.

Zhang et al. studied on lignite which possessed 42.34% ash content by applying reverse flotation. Recovery amount and flotation performance can differ with respect to particle size.


**Table 1.** Particle size of minerals participating into the reverse flotation.

along the different pH ranges. Stonestreet performed studies on ash reduction by applying reverse flotation method on quartz coal mixtures with same amount. The result of their studies showed that maximum ash reduction was succeeded using talk water whose pH was equal to 7.6 [4]. After, Pawlik also touched upon ash reduction with different coal substances. Sub-bituminous, bituminous, and oxidized bituminous coal were objective of reverse flotation with different pH ranges. At first, bituminous coal were exposed to different pH ranges, and optimal results for normal and oxidized coal differed. For normal bituminous coal, the flotation yield went into decline for higher pH values than 9.5. However, sharp decrease was observed when pH level becomes higher than 4.0 for oxidized bituminous coal sample. Besides, different tests were performed for sub-bituminous coal/silica mixtures, and for these tests, optimum pH range was determined between 8.3 and 8.6 [25]. Moreover, Ozturk mainly focused on ash reduction by reverse flotation of lignite, and they achieved maximum level of reduction at neutral pH range around 8 [27]. Same with Ozturk, Zhang also implemented reverse flotation on lignite sample at neutral pH level [73]. Lastly, sub-bituminous coal/quartz mixture with ratio 7:3 was subjected to the test, and all tests were done at neutral pH level [48]. Some additional examples are given from different pH ranges to represent bad results. For example, the low ash content of concentrate was obtained around 10.5 pH value due to

As well as ash and pyrite, some other valuable minerals can also be regained by reverse flotation throughout different pH levels. Ding worked on sub-bituminous coal sample with 48.5% ash content and gangue minerals such as calcite dolomite and silica with the ratio of 7:1:1:1, respectively. These gangue minerals were intended to be floated. In first study, when pH was 10.4, these gangue minerals can be floated [35], whereas in second test, zero-time conditioning was also taken into consideration, and maximum yield was achieved at the 7.5–8.4 pH range [36]. Wang also tried to float calcite as well as vanadium, which has great value in industry.

Therefore, importance of pH is clearly explained by studies, and more research should be car-

The most important criteria of mineral processing is associated to size of liberated mineral particle. The performance of flotation process depends on the degree of liberation of mineral in fine fraction. For this reason, flotation is applied to minerals that are intended to recover or remove from coal. Coal has a complex structure and possesses various minerals. Although the particle size of coal is generally less than 0.5 mm, liberation may not be achieved in that fraction size, so grinding may become inevitably for well separation. Hence, it is compulsory to apply flotation on coal due to liberation in fine fraction phase. However, finer fraction does not always mean that every parameter related with flotation is obtained in well range. There is a trade of between fraction size and performance of flotation. In **Table 1**, some studies with

Zhang et al. studied on lignite which possessed 42.34% ash content by applying reverse flotation. Recovery amount and flotation performance can differ with respect to particle size.

For calcite, pH level varied between 7 and 8, while it was 3.0 for vanadium [70].

different coal type associated with various fraction size are demonstrated.

optimum flotation of calcite and dolomite around this pH [93].

ried out to take a step further in reverse flotation.

**2.6. Effects of particle size**

152 Energy Systems and Environment

Maximum performance was obtained for −74-micron fraction size with the combustible recovery of 65.29% and ash content of 11.30% after 20 minutes flotation. However, maximum flotation rate constant was achieved in 150–250-micron range, and the maximum reverse flotation index efficiency was attained for −425 microns. Hence, combustible recovery increased with increasing size fraction but meanwhile the concentrate ash content also increased particularly for finer particle sizes [73]. For finer fraction, slime problem appears. Because of that reason, there must be performed detailed studies for finer fraction flotation in order to obtain optimum results.

#### **2.7. Reagents (collectors, depressants, and frothers)**

The purpose of using reagents is to change the surface properties of minerals to adjust which material is floated and depressed. In this concept, regulator reagents (pH adjustor, activator, depressants, and dispersants) are entrained into flotation process to improve quality of selectivity and separation. In **Table 2**, reagents are demonstrated in three groups: depressants, collectors, and frothers. Most of reverse flotation experiments are listed, and for each test, available used reagents are indicated (**Table 3**).

Collectors are the reagents, which cause to arranging hydrophobicity of material. Collectors can be observed into two main topics that are ionic and non-ionic collectors. Non-ionic collectors are organic compounds formed from hydrocarbon chains having no neutral and polar groups, whereas ionic collectors are divided into two groups, anionic and cationic. The surface properties of minerals determine the reagents selections, and after necessary conditioning is done, flotation process starts. Some of the preferred collectors in reverse flotation are DTAB, HTAB, DTAC, and so on. On the other hand, PAM and ferric silicate were governed besides collector in order to increase activation of them. On the other hand,


Dowfroth 200 follows it. On the other hand, the use of frothers is not compulsory for reverse

• DC, Octadecyl amine • CDBAC, Cetyl dimethyl benzyl ammonium chloride • FAA, Fatty amine acetates • Dowfroth 200, A polypropylene glycol methyl ether

• DDA, Dodecyl amine

• TTAB, Myristyl trimethyl ammonium bromide • HPYC, Hexadecyl pyridinium chloride • SDS, Sodium dodecyl sulfate • HPYB, Hexadecyl pyridinium bromide • EA, Melamine • CTAC, Cetyltrimethylammonium chloride • DAH, Dodecylamine hydrochloride • CTAB, Cetyltrimethylammonium bromide

Alternatively, Yi et al., stated that waste cooking oil (WCO) can be converted into a bio-flotation agent (BFA), which can be replaced with diesel improves a new coal flotation agent with Zr-SBA-15 catalyst. Pilot program data demonstrated that WCO to BFA brings saving energy

sel use [96]. As a new trend, environmental aspects should be considered so that less harmful collectors should be employed within the manner of reducing the damage to environment, and in this respect, more studies should be handled to overcome environmental issues.

Flotation was developing at the end of 1800s, and reverse flotation was first tried in 1960s. Although not much researcher paid attention on reverse flotation issue, they contributed to literature significantly through past 50 years. With respect to these results, it is inevitable to

 gases are the main triggering factor of acid rains due to propagating toxic gases after burning treatment. Sulfur gases may be found in coal in the form of organic and mineral sulfur

). In order to restrain environment and habitat from unhealthy gases, before burning treatment of coal, they should be removed from coal. Besides, Hg and U spread to the air by burning treatment. On the other hand, minerals like Be, Cr, Ni, As, Cd, Co, Ni, Sb, Se, Pb, Co, Cl, Be, Ba, etc. may involve in water and vegetation cover and lead to great damage for habitat. These heavy minerals are also dangerous for human body because human body could not get rid of these minerals easily and cause to irreversible damages. Because of that, these minerals should be removed from coal in prior to burning treatment. Another issue about coal is about

tat of artificial trees should be constituted especially in the area where coal is being consumed

<sup>⋅</sup>2H<sup>2</sup>

, which causes greenhouse effects. To achieve this, beech

gases in photosynthesis process and the habi-

O and Barite—

Reverse Flotation

155

http://dx.doi.org/10.5772/intechopen.74082

(pyrite, marcasite, galena, and sphalerite) and sulfate (Gypsum—CaSO<sup>4</sup>

emission by 76% as well as production cost when compared with petro-die-

flotation because some collectors possess foaming agents.

by 13%, and CO<sup>2</sup>

**Table 3.** Abbreviations.

**2.8. Conclusion**

SO<sup>2</sup>

BaSO<sup>4</sup>

reach success by reverse flotation.

• MIBC\* indicates that it was not used for all

• LPC, Lauryl pyridinium chloride

experiments

eliminating the harmful effects of CO<sup>2</sup>

as a main energy source.

trees which has great potential of consuming CO<sup>2</sup>

**Table 2.** Use of reagents in reverse flotation.

depressants are the reagents which are added to the pulp to make mineral surface more hydrophobic. As depressants, commonly used reagents in coal reverse flotation are dextrin, humic acids, and corn starch. Frothers are utilized for forming small size bubbles and durable forth which can bear floated minerals without getting any damage during transportation process. In reverse flotation, most common reagents are MIBC, and pine oil, terpenic oil, and


**Table 3.** Abbreviations.

Dowfroth 200 follows it. On the other hand, the use of frothers is not compulsory for reverse flotation because some collectors possess foaming agents.

Alternatively, Yi et al., stated that waste cooking oil (WCO) can be converted into a bio-flotation agent (BFA), which can be replaced with diesel improves a new coal flotation agent with Zr-SBA-15 catalyst. Pilot program data demonstrated that WCO to BFA brings saving energy by 13%, and CO<sup>2</sup> emission by 76% as well as production cost when compared with petro-diesel use [96]. As a new trend, environmental aspects should be considered so that less harmful collectors should be employed within the manner of reducing the damage to environment, and in this respect, more studies should be handled to overcome environmental issues.

#### **2.8. Conclusion**

depressants are the reagents which are added to the pulp to make mineral surface more hydrophobic. As depressants, commonly used reagents in coal reverse flotation are dextrin, humic acids, and corn starch. Frothers are utilized for forming small size bubbles and durable forth which can bear floated minerals without getting any damage during transportation process. In reverse flotation, most common reagents are MIBC, and pine oil, terpenic oil, and

**Coal type Depressant Collector Frother Reference**

CDBAC, LPC

Subbituminous coal Not used DTAB MIBC\* [45]

Sub-bituminous coal Dextrin DTAC not used [36] Sub-bituminous coal Dextrin Lilaflot D817M MIBC [48]

Taixi oxidized coal Dextrin HTAB – [42, 95]

Lignite coal Not used Aero-3477 (anionic collector), kerosene Pine oil [27] Mineral pyrite Not used Fuel oil MIBC [37] Pittsburgh coal Not used Fuel oil Dowfroth 200 [37] Mineral pyrite, coal Not used Fuel oil Dowfroth 200 [48]

Lignite coal Not used (Cationic; DDA, TTAB) MIBC [47]

Lignite coal Corn starch DAH MIBC [43]

Alimine

Low grade stone coal Sodium silicate oleic acid, EA, DDA, DC, Mixed

Humic acids (HA)

Anionic; SDS, non-ionic (2-ethyhexanol)/kerosene

Octylamine, CTAC, FAA, CTAB,

Dextrin DTAB, PAM DTAB [94]

Dextrin DTAB MIBC [46]

Dextrin DTAC Not used [35]

Dextrin DTAB MIBC [39, 40]

DTAB MIBC\* [45]

HTAB, HPYC [39, 40]

HPYB [39]

DTAB MIBC\* [25]

MIBC [29]

Terpenic oil [70, 71]

Bituminous coal Inorganic

154 Energy Systems and Environment

Bituminous & subbituminous coal

Bituminous & subbituminous coal

Clean coal and silica

Calcite, dolomite, silica,

Quartz and clean cooking

Quartz and clean cooking

Quartz and clean cooking

mixture

and raw coal

coal mixture

coal mixture

coal mixture

Silica and raw coal

**Table 2.** Use of reagents in reverse flotation.

mixture

oxidants

Humic acids (HA)

> Flotation was developing at the end of 1800s, and reverse flotation was first tried in 1960s. Although not much researcher paid attention on reverse flotation issue, they contributed to literature significantly through past 50 years. With respect to these results, it is inevitable to reach success by reverse flotation.

> SO<sup>2</sup> gases are the main triggering factor of acid rains due to propagating toxic gases after burning treatment. Sulfur gases may be found in coal in the form of organic and mineral sulfur (pyrite, marcasite, galena, and sphalerite) and sulfate (Gypsum—CaSO<sup>4</sup> <sup>⋅</sup>2H<sup>2</sup> O and Barite— BaSO<sup>4</sup> ). In order to restrain environment and habitat from unhealthy gases, before burning treatment of coal, they should be removed from coal. Besides, Hg and U spread to the air by burning treatment. On the other hand, minerals like Be, Cr, Ni, As, Cd, Co, Ni, Sb, Se, Pb, Co, Cl, Be, Ba, etc. may involve in water and vegetation cover and lead to great damage for habitat. These heavy minerals are also dangerous for human body because human body could not get rid of these minerals easily and cause to irreversible damages. Because of that, these minerals should be removed from coal in prior to burning treatment. Another issue about coal is about eliminating the harmful effects of CO<sup>2</sup> , which causes greenhouse effects. To achieve this, beech trees which has great potential of consuming CO<sup>2</sup> gases in photosynthesis process and the habitat of artificial trees should be constituted especially in the area where coal is being consumed as a main energy source.
