**1. Introduction**

Coal is a type of sedimentary rock, which exists in nature in the form of black-dark gray and brown-black color and consumed as fossil fuel. Besides, coal possesses carbonated plants, and its proportions in terms of weight and volume are more than 50 and 70%, respectively [1].

Raw coal is enriched by different beneficiation methods that are mainly gravity-based separation and flotation. In gravity-based separation, undesired substances, which are not compound of coal, might be reduced but it could not be effective on ingredients being in internal

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

structure of coal. It is dangerous to consume raw coal as fossil fuel for environment due to high amount of sulfur ingredients, because impurities like sulfur produces harmful gases after heating process. Therefore, in order to remove these undesired contents, flotation is applied as an enrichment method.

due to surface of minerals with low surface hydrophobicity [23]. For this reason, collector consumption is much more in flotation of low rank coal compared with high rank ones [25]. In low rank coal flotation, in order to increase performance, parameters like grain size, pulp density, reagents type and dosage, pulp pH, flotation and time conditioning, and air entrainment rate should be investigated, and [9, 22, 26, 27] most proper condition should be determined with respect to results of them. In addition to conventional flotation method, reverse flotation

Reverse Flotation

145

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

The reverse flotation was commenced to investigate in 1950s [28, 29]. In following studies after 1950s, different researchers also continued to work on that concept [30–34] and still have been proceeding [4, 25, 27, 35–43]. However, researchers have not understood completely how particles inside the pulp interact with each other, yet because there are many uncertain factors that influence the surface of minerals and coal existing in pulp. Foremost among these factors is that coal possesses different type of elements and behavior of these elements has not been identified; thus, theoretical and experimental verification of interaction between elements is difficult. For that reason, more researches should be performed in that regard. Studies until current time are generally associated with reduction of pyrite and ash. However, recovery of valuable elements in coal should also be taken into consideration in the environmental aspect as well, so the objective of this study is to discuss the benefits of reverse flotation of coal for

Ash content is important for both environment and flotation efficiency. Ash content in coal can be eliminated by both physical methods and flotation process, but the presence of ash in internal structure of coal particle could not be removed by physical methods. To be able to remove impurities inside coal particle, reverse flotation is applied on coal. In reverse flotation,

Ash content is crucial for coal flotation efficiency. Froth is the key element of determining flotation concentrate, and between concentrate froth and the ash content, there is a strong

Ash reduction in reverse flotation of coal subjects have been developed for 30 years by some important researchers. Stonestreet, Pawlik, and Ding have performed intensive studies on ash reduction in reverse coal flotation. Stonestreet and Pawlik prepared their PhD thesis on reverse flotation of coal separately [4, 45] and continued his studies with Franzidis in advance [39–41]. In their experiment, clean coal with 7% ash and silica were mixed as a feed, and quantity of each element was same. From the results of experiments, reverse flotation process, 92% ash reduction was achieved from feed in which ash content was 54%. In depressed coal, ash content was 12%, whereas the recovery of coal is 27%. Results of experiment show that recovery of coal was not good even it was achieved that high ash reduction was obtained

tailing is taken as clean coal, and concentrate is accepted as gangue minerals.

is also applied on enrichment of minerals.

environment on the behalf of past researches.

**2. Process of reverse flotation**

**2.1. Ash reduction**

relation [44].

Flotation, as the name implies, is expressed like separation of substance in compound form by floating process. Floatability of material is crucial aspect for efficiency of flotation. For coal mineral, some of them have natural floatable properties because of its nature, but some others do not possess an inherent floatability because of its internal structure properties [2–4]. The surface texture of coal particle may involve both hydrophobic and hydrophilic zones. Thus, domination of this zones is one of the criteria which decides whether coal is floatable or not [5, 6].

Another criterion for defining floatability of coal particle is moisture content. For example, lignite, which involves 70–80% carbon, has high moisture content and extremely less hydrophobic disposition. The chemical structure of lignite is altering due to elimination of polar groups like hydroxyl and carboxyl groups, and natural moisture content decreases during the transition from lignite to bituminous coal. As a result, coal comes in position of more hydrophobic. Moreover, the content of carbon is in relation with hydrophobicity of coal, too. In the range of 81–89% of carbon content, polar character loses its influence, and coal becomes more hydrophobic. Hydrophobicity of coal reaches its maximum level in 89% of carbon content, and it decreases slightly when the carbon content climbs up from this level. The flotation is directly related with the floatability of particles, so higher carbon content makes conventional flotation process easier [7, 8].

In addition, flotation efficiency is directly related with the properties of the inorganic and organic mineral impurities existed in coals and amount and dispersion of gangue mineral inclusion. It is not possible to remove these finely dispersed impurities inclusion by applying physical methods [9], so flotation should be taken in consideration inevitably in the manner of protecting environment and recovering valuable minerals.

In this respect, flotation is playing significant role in supplying raw materials for various industries. More than 2 billion tons of minerals and fine coals are being processed annually by using flotation in worldwide [10]. For this reason, flotation becomes one of the most important methods for enrichment of minerals and is commonly used in the world [10–13]. At the same time, flotation is utilized for finely-grain-sized coal upgrading [14–18], fly ash decarburization [19], and wastewater treatment [20]. Since coals and ore are liberated in fine grain size, tendency to flotation in mineral processing increases [21].

For environment, sulfur and ash content of coal is too important because heating process leads to propagate harmful gases to the environment. Flotation is one of the effective methods for desulfurization and deashing of raw coal having high ash and sulfur content [22]. Flotation properties depend on the surface texture and other features of particle, so to make particle float or depress in pulp, some different reagents are governed. In conventional flotation, oily collector and frother are used, and these reagents are conditioned in a period of time [23, 24]. In flotation of low rank coal, it is difficult to obtain good result using oily collector due to surface of minerals with low surface hydrophobicity [23]. For this reason, collector consumption is much more in flotation of low rank coal compared with high rank ones [25]. In low rank coal flotation, in order to increase performance, parameters like grain size, pulp density, reagents type and dosage, pulp pH, flotation and time conditioning, and air entrainment rate should be investigated, and [9, 22, 26, 27] most proper condition should be determined with respect to results of them. In addition to conventional flotation method, reverse flotation is also applied on enrichment of minerals.

The reverse flotation was commenced to investigate in 1950s [28, 29]. In following studies after 1950s, different researchers also continued to work on that concept [30–34] and still have been proceeding [4, 25, 27, 35–43]. However, researchers have not understood completely how particles inside the pulp interact with each other, yet because there are many uncertain factors that influence the surface of minerals and coal existing in pulp. Foremost among these factors is that coal possesses different type of elements and behavior of these elements has not been identified; thus, theoretical and experimental verification of interaction between elements is difficult. For that reason, more researches should be performed in that regard. Studies until current time are generally associated with reduction of pyrite and ash. However, recovery of valuable elements in coal should also be taken into consideration in the environmental aspect as well, so the objective of this study is to discuss the benefits of reverse flotation of coal for environment on the behalf of past researches.
