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Flotation behaviour of weathered coal in mechanical and column flotation cell
Powder Technology 246 (2013) 689–694
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Flotation behaviour of weathered coal in mechanical and column flotation cell Shobhana Dey a,⁎, Gyana Manjari Paul a, Santosh Pani b a b
Mineral Processing Division, CSIR–National Metallurgical Laboratory, Jamshedpur, Jharkhand, India Centre for Minerals Research, Chemical Engineering Department, University of Cape Town, South Africa
a r t i c l e
i n f o
Article history: Received 30 January 2013 Received in revised form 3 June 2013 Accepted 11 June 2013 Available online 21 June 2013 Keywords: Oxidized coal flotation Pre-treatment of coal Column flotation Promoter Mechanical flotation
a b s t r a c t The low rank or oxidized coals show unpleasant flotation behaviour. The non-coking coal from Talcher, containing 26.8% ash, 4.9% moisture, 35.7% volatile matter and 1% oxygen was used for the investigation. The flotation performance of the weathered coal in a mechanical and column flotation cell was compared at low ash level of about 12%. The oxidized coals possess negative surface charge due to the surface functional groups like carboxylic (−COOH) and phenolic (−OH). The surface charge could be determined by zeta potential study. The flotation at different pH values and pre-treatment of coal fines using a modifier were studied in a mechanical and column flotation cell. This type of coal is very difficult to float even at higher concentration of collector due to less dispersion of the collector molecules on the coal surface. The surface charge determined by the zeta potential study appears to be negative. It designates the oxidation of the surface and makes the coal hydrophilic in nature. As a result, attachment of air bubbles with the particles gets reduced. Flotation performances, due to the modifier added in grinding mill, give encouraging results as it is inferred from the isoelectric point shifted towards the alkaline regime. Flotation studies carried out with promoter added in the mill followed by diesel oil in mechanical cell show that cleaning of the rougher concentrate is essential to reduce the ash content in the concentrate, whereas the single-stage flotation in a column is found to be better which yields 49.6% concentrate at 12% ash. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Froth flotation is a conventional technique used in the beneficiation of fine coal [1]. The flotation behaviour of coal depends on many factors, such as macerals, rank, origin, quality of impurities and degree of oxidation. The coal undergoes oxidation due to weathering [2,3]. Weathering also increases the brittleness of oxidized coal, lowers the pulp pH and increases the free cations in the coal pulp and thus decreases the surface hydrophobicity. Sun [4] stated that the low floatability of oxidized coal is due to an increase in the non-floatable components (oxygen containing components and coal ash content) together with a decrease in floatable components (carbon and hydrogen containing parts) of the coal. Quast and Readett [5] reported that sub-bituminous coals have an average oxygen content of 18% with carboxylic groups constituting about one third of this amount. The presence of functional groups like carboxylic (−COOH), ester and phenolic (OH) on the coal surface reduces the floatability as these surface functional groups are hydrophilic in nature [6,7]. The flotation of fine low rank or oxidized coal is difficult to achieve with the common coal flotation collectors like kerosene, fuel oil or diesel ⁎ Corresponding author. Fax: +91 6572345213. E-mail address: [email protected] (S. Dey). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.06.015
oil (oily collector). The amount of adhesion of oil droplets onto low rank coals is very small and the use of oil alone does not improve flotation [8]. Many researchers have carried out the flotation studies on oxidized/low rank coal. A change in the surface properties of the coal particles largely affects their attachment and detachment characteristics with other dispersed phases in flotation pulp. The addition of surfactant or oxygenated functional groups to the collector molecule markedly enhances the flotation of lower rank or oxidized coals due to the hydrogen bonding with the polar part of the coal surface and the reagent [9]. The promoters also act as surface modifiers and alter the hydrophobicity of the low rank coal [10–15]. The mode of addition of non-ionic surfactant with oily collector has significant role in the flotation response [16,17]. The low rank coal/oxidized coals can be effectively floated using blending of hydrocarbon and non-hydrocarbon collectors, such as copolymers, long chain amine, fatty acid, etc. [18–23]. Pre-treatments of low rank/ oxidized coals were also studied to enhance the flotation performance by improving the hydrophobicity. The most important methods of pretreatments are grinding, premixing/preconditioning, thermal, microwave, ultrasound and direct mixing of the reagents with dry coal before wetting for flotation [24,25]. The grinding pre-treatment improves the floatability due to the stripping of the surface of oxidized coal. Sokolovic et al. [26] applied the attrition at the high solid concentration to generate new or unoxidized surfaces on oxidized coal which improves floatability.
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ash in the concentrate is high due to the entrainment of the gangue in the froth phase. Therefore, this investigation highlights the performances of oxidized coal in a mechanical and column flotation cell to achieve the concentrate with low ash applying pre-treatment method. The objective of this investigation is to explore the flotation performance of the weathered coal in a mechanical and column flotation cell at a targeted ash level of 12% as the demand of the low ash clean coal is increasing. The recovered clean coal can be used for composite pellets of iron and coal for direct steel making.
Table 1 Characterization of as-received feed. Proximate analysis Heat value Petrographic analysis
Ash: 26.8%, volatile matter: 35.7%, fixed carbon: 37.5%, moisture: 4.9% 4525 kcal/kg Vitrinite: 39%, oxidized vitrinite: 12%, inertinite: 18% by volume Mineral matter: 23%
2. Experimental 2.1. Material characterization The investigation was performed using non-coking coal from Talcher coalfield. The characterization and the proximate analysis of the coal on moisture-free basis were carried out and is presented in Table 1. The ash is 26.8% with 35.7% volatile matter, which is quite high. The fixed carbon in the as-received coal is 37.5%, and moisture content is relatively high (4.9%). The petrographic constituents of the coal are the important factors for influencing the floatability [30]. The petrographic analysis of the coal sample reveals that it is dominated by vitrinite (39%) of which about 12% is in oxidized form. Inertinites is the second dominate maceral constituting 18% by volume and the mineral matter is 23% causing low heat value of the raw coal (4525 kcal/kg). The surface oxidation of the coal is characterized by measuring the zeta potential at different pH values (Fig. 1). The negative values of the zeta potential at various pHs signifies the presence of negative charge on the surface of coal and also indicates the surface oxidation. The schematic plan of work is shown in Fig. 2.
Fig. 1. Effect of Zeta potential at different pH.
Some researchers investigated the flotation behaviour of oxidized coal dry-ground with collector. The floatability of difficult-to-float coal was enhanced involving dry grinding of coal in presence of bituminous coal pitch followed by flotation with oily collectors [27–29]. The studies contribute to the knowledge of flotation of the oxidized coal. However, the
2.2. Methods The experiment was initiated with the − 1.68 mm sized coal for reducing the ash to a target level of 12%. This feed size of the coal is
Feed
Characterization
Petrography
Grinding for different feed size
Proximate analysis
Zeta potential
Particle size & ash distribution
Flotation study
Mechanical cell
Variables: pH, promoter-addition, feed size,
collector dosage
Fig. 2. Schematic plan of work.
Column flotation
Variables: Froth height, superficial velocity
S. Dey et al. / Powder Technology 246 (2013) 689–694
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Table 2 Size-wise ash distribution of particles at different grinding time. Grinding time
10 min
Size, μm
Cum pass wt%
Ash%
15 min Cum pass wt%
Ash%
Cum pass wt%
Ash%
−300 + 150 −150 + 100 −100 + 74 −74
100.0 83.2 58.9 48.1
23.45 22.92 22.49 30.86
100.0 94.0 79.8 66.1
23.21 21.85 21.59 29.13
100.0 99.2 91.0 76.9
24.13 23.73 21.02 28.12
inappropriate for treating in a Dense Media Concentrator (DMC) and is also too large for flotation studies as the flotation feed has an intense effect on the performance. From the characterization study of the coal, it appeared to be oxidized. Dry grinding is advisable to prevent further oxidation [27,28]. The size distribution at different grinding time is given in Table 2. The three feed sizes prepared were 80%, 90% and 100% passing of 150 μm. The size-wise ash distribution in different size fraction shows that at 100% passing of 150 μm, more fines were generated which resulted in a loss of carbon values in the ultrafine fraction. All the flotation tests were carried out in a mechanical flotation cell at constant pulp density of 10% solid and 2 cm froth height. The effect of pulp pH was studied with one feed size (90% passing of 150 μm) both at natural (4.3) and neutral (6.9) pH. Neutral pH was maintained using alkali (Fig. 3). Gray et al. studied the surface modification of the oxidized coal using short chain alcohol [31]. In this investigation, ethyl alcohol was used in two different conditions, viz. in a grinding mill and flotation cell as a promoter for surface modification and improvement in the flotation performance. The gangue minerals are mostly silicates which were depressed by sodium silicate (2.0 kg/ton). Diesel oil was used as collector and methyl isobutyl alcohol (0.5 kg/ton) as frother. The collector and frother were added in stages. Ash content in the concentrate was further reduced by cleaning the rougher concentrate, depending upon the requirement of the desired ash level. Column flotation is a widely adopted method used for processing of the fine particles, especially in a multi-stage cleaning circuit as it provides enhanced selectivity [32,33]. The flotation behaviour of the oxidized coal was studied in a column flotation cell of 3 m length and inner diameter of 75 mm using 80% passing of 150 μm feed. The slurry pulp density was kept constant at 10% solids and the wash water at 0.3 l/min. The same dosages of reagents viz. promoter (5 l/ton),
20 min
depressant (2 kg/ton), collector (10 kg/ton) and frother (0.5 kg/ton), as those used in the mechanical cell were maintained. Except promoter, all reagents were added in a conditioning tank. The experimental setup for the column flotation studies is shown in Fig. 3. It is well known that mechanical entrainment of hydrophilic gangue minerals can be overcome by increasing the residence time of particles in the froth zone [34,35]. The mechanical entrainment of hydrophilic gangue minerals can be effectively controlled by column flotation as it enables different froth depths with generation of small bubbles yielding high recovery of selective minerals. In column flotation, there is counter current flow of the feed and bubbles. It operates under steady state and plug flow conditions with regulating degrees of axial dispersion with higher bubble particle contact time. Column flotation studies were carried out at different froth heights and airflow rate. Froth heights were adjusted using a PID controller. Targeted ash levels can be achieved in a single stage as the froth resident time and specific surface area of the bubbles is more than in the conventional flotation cell. The results so obtained are discussed in the results and discussion section. 3. Results and discussion 3.1. Flotation in mechanical cell The effects of pH on flotation tests, both at natural and neutral pH of pulp, are shown in Fig. 4. Neutral pH was found to be more effective. It seems that at natural pH, the flotation performance is less encouraging than at neutral pH. This infers that the negative surface charge on the coal surface resist the adsorption of the collector molecules in the low pH region. Therefore, all the flotation
Fig. 3. Experimental setup of Column flotation.
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Fig. 4. Flotation at natural and neutral pH.
tests were carried out at neutral pH maintained by alkali. Without promoter, the performance is not encouraging even at 18 kg/ton of collector added in stages (Fig. 5). As the coal surface is weathered, the promoter added for surface modification plays a significant role in enhancing the adsorption of oily collector. The promoter assists to disperse the oily collector into small droplets which adsorb on the coal surface and reduces the collector consumption [17]. The effect of promoter in a grinding mill and flotation cell is shown in Fig. 5. It reveals that promoter added during milling is more effective than in a flotation cell. When 3 l/ton ethyl alcohol was used, the flotation response is better than 10 l/ton of same added in the flotation cell; however, at 5 l/ton of ethyl alcohol in the mill, the recovery and grade of the concentrate is encouraging. This may be due to the attrition at a high solid concentration prior to introducing coal to the froth flotation process creates fresh unoxidized surfaces. Sokolovic et al. [36] studied the activation of the oxidized surface of anthracite waste coal. They found that attrition, in the viscous pulp with the solid content of 50%, lead to the mechanical cleaning of oxidized surface and activation of the surface of coal particles. In this investigation, it was observed that promoter added in a flotation cell and a mill requires 10 l/ton and 5 l/ton, respectively. This reveals that double the dosage of promoter is needed for surface modification in a flotation cell. The surface modification was manifested by the zeta potential study of the alcohol treated coal (Fig. 1). The IEP shifted from pH 3.8 to 6.5. It shows that the surface becomes more positive. The flotation study also shows that the collector consumption reduces by 10 kg/ton. The flotation results obtained under different conditions in mechanical cell are discussed below. The single-stage flotation studies with a different feed sizes are shown in Fig. 6. It shows that the feed having 90% passing of 150 μm was effective for achieving 12% ash with less yield; however, at higher ash level (20%), the feed of 80% passing 150 μm produces the same yield as from the finer feed. It is also obvious that yield at a low ash
Fig. 5. Effect of promoter in flotation.
level for any feed size is not encouraging in a single-stage flotation due to the loss of combustible matter in the tailing. In view of this, an attempt was made to recover the relatively high ash concentrate with the coarse feed in the first stage of flotation. The ash content in the concentrate could be reduced by the subsequent cleaning of rougher concentrate. It was observed that cleaning at two stages could produce 46.7% yield with 12.14% ash, while by single-stage cleaning, the yield is 38.5% with minimum ash of 13%. Results of mechanical flotation cell are shown in Fig. 7 and Table 2. Thus, the flotation of the oxidized coal for recovering the clean coal with the targeted ash level of 12% in a mechanical cell requires two stages of cleaning. In a mechanical flotation cell, the froth height can be adjusted only at 2 to 3 cm, and the fine gangue particles can easily get entrained into the froth zone under the influence of the slurry viscosity. The results of mechanical flotation cell become witness that it is very difficult to achieve the target ash level in single-stage flotation. In lieu of achieving the low ash concentrate from less hydrophobicoxidized coal, column flotation was attempted.
3.2. Flotation study in column Column flotation studies carried out at different froth heights and airflow rate are shown in Figs. 8 and 9, respectively. At high froth height of 55 cm, the froth residence time increases and the gangue minerals detach from the froth zone. Thus, the mechanical entrainment and entrapment of the mineral particles could be defeated in column flotation [37]. The effect of superficial air velocity on the concentrate ash and yield shown in Fig. 9 reveals that at 1 cm/s, the maximum yield is 49% at 12.4% ash. Coal being light in nature, low superficial air velocity is preferable in achieving the low ash concentrate. Smaller bubbles produced at a lower superficial air velocity can only carry the lighter particles to the froth phase and thus provides better selectivity with less mechanical entrainment of gangue minerals.
3.3. Comparison of the flotation performance The performance of the oxidized coal in a mechanical and column flotation cell are compared in Table 3 and presented in Fig. 10. The minimum ash in the concentrate recovered without cleaning from mechanical flotation is 13.2% with a yield of 8.2%. The single-stage cleaning of the rougher concentrate reduces the ash to 13% with 38.7% yield. The two stages of cleaning significantly improve the yield to 46.7% and 62.7% at 12.14% and 13.27% ash, respectively. The concentrate achieved from the column flotation in a single stage at low superficial air velocity (1 cm/s) and moderate froth height (40 cm) is 49.6% yield with 59.2% recovery of combustible at 12.2% ash. It appears that single-stage column flotation results are encouraging for producing low ash concentrate.
Fig. 6. Flotation studies in mechanical cell at different conditions.
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Tail 17.9 Rougher flotation
Feed 100 26.8
693
61.7
Conc 82.1 18.8
Single stage Cleaner flotation
Cleaner flotation
Two stage Cleaner flotation Tail 43.6
23.9
Conc 38.5
13.0
Conc 46.7 Stream Yield%
12.14
Tail 35.4
27.5
Ash%
Fig. 7. Results of flotation in mechanical cell to achieve concentrate with 12% ash.
In general, after gravity separation, the reject stream from the HMC is dumped. As a result, weathering reduces the floatability of the fine coal. Sometimes, they are also used in thermal power plants. This coal can be used for metallurgical purpose as the availability of the good quality coal is depleting and the demand for the low ash coal is increasing steadily
due to the quantum expansion of the steel industries. However, this type of coal fines needs beneficiation before end use. Nowadays, pellets are used in DRI plants as they improve the productivity. Therefore, in this investigation, a challenge was taken to recover the low ash clean coal to a greater extent. About 50% of the clean coal yield with low ash (about 12%) can be recovered using column flotation technique. 4. Conclusions The oxidation of the coal surface alters both physical and chemical properties and reduces the floatability. The following inferences can be drawn from the above investigation.
Fig. 8. Effect of froth height in column flotation.
• The results of zeta potential show that the surface of coal particles becomes negatively charged due to oxidation. • The addition of promoter in a mill significantly reduces its consumption due to surface attrition and creation of the new surfaces. It adsorbs on the fresh surface, modifies the surface and hence increases the hydrophobicity. • At natural pH, the adsorption of the collector molecules is hindered due to the negative surface charge where as the neutral pH makes the flotation more effective. • Flotation studies carried out in a mechanical flotation cell shows that cleaning of the rougher concentrate is necessary to reduce the ash content in the concentrate. • The single-stage flotation in a column is found to be better than the mechanical cell as the process variables are more adjustable. Low superficial air velocity is favorable to produce low ash concentrates. • In view of the demand of low ash clean coal for steel making, it is inevitable to recover the combustible matter from coal fines. The product can be used for making composite pellet of coal fines and concentrate of iron ore fines.
Table 3 Comparison of flotation performance of oxidized coal in mechanical and column flotation cell.
Fig. 9. Effect of air flow rate on concentrate and ash in column flotation.
Flotation cell
Concentrate
Mechanical cell
Yield (%)
Ash (%)
RC (%)
Yield (%)
Ash (%)
RC (%)
No cleaning Single-stage cleaning Two-stage cleaning Column flotation
8.2 38.5 46.7 49.6
13.2 13.0 12.14 12.22
9.7 45.8 56.1 59.2
91.8 61.5 53.3 50.4
28.2 35.2 39.6 40.5
90.3 54.2 43.9 40.8
RC: recovery of combustibles.
Tailings
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Fig. 10. Flotation performance in mechanical and column flotation cell.
Acknowledgements The authors would like to thank the Ministry of Steel, Government of India, for receiving the financial assistance for the research work.
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Flotation behaviour of weathered coal in mechanical and column flotation cell Shobhana Dey a,⁎, Gyana Manjari Paul a, Santosh Pani b a b
Mineral Processing Division, CSIR–National Metallurgical Laboratory, Jamshedpur, Jharkhand, India Centre for Minerals Research, Chemical Engineering Department, University of Cape Town, South Africa
a r t i c l e
i n f o
Article history: Received 30 January 2013 Received in revised form 3 June 2013 Accepted 11 June 2013 Available online 21 June 2013 Keywords: Oxidized coal flotation Pre-treatment of coal Column flotation Promoter Mechanical flotation
a b s t r a c t The low rank or oxidized coals show unpleasant flotation behaviour. The non-coking coal from Talcher, containing 26.8% ash, 4.9% moisture, 35.7% volatile matter and 1% oxygen was used for the investigation. The flotation performance of the weathered coal in a mechanical and column flotation cell was compared at low ash level of about 12%. The oxidized coals possess negative surface charge due to the surface functional groups like carboxylic (−COOH) and phenolic (−OH). The surface charge could be determined by zeta potential study. The flotation at different pH values and pre-treatment of coal fines using a modifier were studied in a mechanical and column flotation cell. This type of coal is very difficult to float even at higher concentration of collector due to less dispersion of the collector molecules on the coal surface. The surface charge determined by the zeta potential study appears to be negative. It designates the oxidation of the surface and makes the coal hydrophilic in nature. As a result, attachment of air bubbles with the particles gets reduced. Flotation performances, due to the modifier added in grinding mill, give encouraging results as it is inferred from the isoelectric point shifted towards the alkaline regime. Flotation studies carried out with promoter added in the mill followed by diesel oil in mechanical cell show that cleaning of the rougher concentrate is essential to reduce the ash content in the concentrate, whereas the single-stage flotation in a column is found to be better which yields 49.6% concentrate at 12% ash. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Froth flotation is a conventional technique used in the beneficiation of fine coal [1]. The flotation behaviour of coal depends on many factors, such as macerals, rank, origin, quality of impurities and degree of oxidation. The coal undergoes oxidation due to weathering [2,3]. Weathering also increases the brittleness of oxidized coal, lowers the pulp pH and increases the free cations in the coal pulp and thus decreases the surface hydrophobicity. Sun [4] stated that the low floatability of oxidized coal is due to an increase in the non-floatable components (oxygen containing components and coal ash content) together with a decrease in floatable components (carbon and hydrogen containing parts) of the coal. Quast and Readett [5] reported that sub-bituminous coals have an average oxygen content of 18% with carboxylic groups constituting about one third of this amount. The presence of functional groups like carboxylic (−COOH), ester and phenolic (OH) on the coal surface reduces the floatability as these surface functional groups are hydrophilic in nature [6,7]. The flotation of fine low rank or oxidized coal is difficult to achieve with the common coal flotation collectors like kerosene, fuel oil or diesel ⁎ Corresponding author. Fax: +91 6572345213. E-mail address: [email protected] (S. Dey). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.06.015
oil (oily collector). The amount of adhesion of oil droplets onto low rank coals is very small and the use of oil alone does not improve flotation [8]. Many researchers have carried out the flotation studies on oxidized/low rank coal. A change in the surface properties of the coal particles largely affects their attachment and detachment characteristics with other dispersed phases in flotation pulp. The addition of surfactant or oxygenated functional groups to the collector molecule markedly enhances the flotation of lower rank or oxidized coals due to the hydrogen bonding with the polar part of the coal surface and the reagent [9]. The promoters also act as surface modifiers and alter the hydrophobicity of the low rank coal [10–15]. The mode of addition of non-ionic surfactant with oily collector has significant role in the flotation response [16,17]. The low rank coal/oxidized coals can be effectively floated using blending of hydrocarbon and non-hydrocarbon collectors, such as copolymers, long chain amine, fatty acid, etc. [18–23]. Pre-treatments of low rank/ oxidized coals were also studied to enhance the flotation performance by improving the hydrophobicity. The most important methods of pretreatments are grinding, premixing/preconditioning, thermal, microwave, ultrasound and direct mixing of the reagents with dry coal before wetting for flotation [24,25]. The grinding pre-treatment improves the floatability due to the stripping of the surface of oxidized coal. Sokolovic et al. [26] applied the attrition at the high solid concentration to generate new or unoxidized surfaces on oxidized coal which improves floatability.
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ash in the concentrate is high due to the entrainment of the gangue in the froth phase. Therefore, this investigation highlights the performances of oxidized coal in a mechanical and column flotation cell to achieve the concentrate with low ash applying pre-treatment method. The objective of this investigation is to explore the flotation performance of the weathered coal in a mechanical and column flotation cell at a targeted ash level of 12% as the demand of the low ash clean coal is increasing. The recovered clean coal can be used for composite pellets of iron and coal for direct steel making.
Table 1 Characterization of as-received feed. Proximate analysis Heat value Petrographic analysis
Ash: 26.8%, volatile matter: 35.7%, fixed carbon: 37.5%, moisture: 4.9% 4525 kcal/kg Vitrinite: 39%, oxidized vitrinite: 12%, inertinite: 18% by volume Mineral matter: 23%
2. Experimental 2.1. Material characterization The investigation was performed using non-coking coal from Talcher coalfield. The characterization and the proximate analysis of the coal on moisture-free basis were carried out and is presented in Table 1. The ash is 26.8% with 35.7% volatile matter, which is quite high. The fixed carbon in the as-received coal is 37.5%, and moisture content is relatively high (4.9%). The petrographic constituents of the coal are the important factors for influencing the floatability [30]. The petrographic analysis of the coal sample reveals that it is dominated by vitrinite (39%) of which about 12% is in oxidized form. Inertinites is the second dominate maceral constituting 18% by volume and the mineral matter is 23% causing low heat value of the raw coal (4525 kcal/kg). The surface oxidation of the coal is characterized by measuring the zeta potential at different pH values (Fig. 1). The negative values of the zeta potential at various pHs signifies the presence of negative charge on the surface of coal and also indicates the surface oxidation. The schematic plan of work is shown in Fig. 2.
Fig. 1. Effect of Zeta potential at different pH.
Some researchers investigated the flotation behaviour of oxidized coal dry-ground with collector. The floatability of difficult-to-float coal was enhanced involving dry grinding of coal in presence of bituminous coal pitch followed by flotation with oily collectors [27–29]. The studies contribute to the knowledge of flotation of the oxidized coal. However, the
2.2. Methods The experiment was initiated with the − 1.68 mm sized coal for reducing the ash to a target level of 12%. This feed size of the coal is
Feed
Characterization
Petrography
Grinding for different feed size
Proximate analysis
Zeta potential
Particle size & ash distribution
Flotation study
Mechanical cell
Variables: pH, promoter-addition, feed size,
collector dosage
Fig. 2. Schematic plan of work.
Column flotation
Variables: Froth height, superficial velocity
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Table 2 Size-wise ash distribution of particles at different grinding time. Grinding time
10 min
Size, μm
Cum pass wt%
Ash%
15 min Cum pass wt%
Ash%
Cum pass wt%
Ash%
−300 + 150 −150 + 100 −100 + 74 −74
100.0 83.2 58.9 48.1
23.45 22.92 22.49 30.86
100.0 94.0 79.8 66.1
23.21 21.85 21.59 29.13
100.0 99.2 91.0 76.9
24.13 23.73 21.02 28.12
inappropriate for treating in a Dense Media Concentrator (DMC) and is also too large for flotation studies as the flotation feed has an intense effect on the performance. From the characterization study of the coal, it appeared to be oxidized. Dry grinding is advisable to prevent further oxidation [27,28]. The size distribution at different grinding time is given in Table 2. The three feed sizes prepared were 80%, 90% and 100% passing of 150 μm. The size-wise ash distribution in different size fraction shows that at 100% passing of 150 μm, more fines were generated which resulted in a loss of carbon values in the ultrafine fraction. All the flotation tests were carried out in a mechanical flotation cell at constant pulp density of 10% solid and 2 cm froth height. The effect of pulp pH was studied with one feed size (90% passing of 150 μm) both at natural (4.3) and neutral (6.9) pH. Neutral pH was maintained using alkali (Fig. 3). Gray et al. studied the surface modification of the oxidized coal using short chain alcohol [31]. In this investigation, ethyl alcohol was used in two different conditions, viz. in a grinding mill and flotation cell as a promoter for surface modification and improvement in the flotation performance. The gangue minerals are mostly silicates which were depressed by sodium silicate (2.0 kg/ton). Diesel oil was used as collector and methyl isobutyl alcohol (0.5 kg/ton) as frother. The collector and frother were added in stages. Ash content in the concentrate was further reduced by cleaning the rougher concentrate, depending upon the requirement of the desired ash level. Column flotation is a widely adopted method used for processing of the fine particles, especially in a multi-stage cleaning circuit as it provides enhanced selectivity [32,33]. The flotation behaviour of the oxidized coal was studied in a column flotation cell of 3 m length and inner diameter of 75 mm using 80% passing of 150 μm feed. The slurry pulp density was kept constant at 10% solids and the wash water at 0.3 l/min. The same dosages of reagents viz. promoter (5 l/ton),
20 min
depressant (2 kg/ton), collector (10 kg/ton) and frother (0.5 kg/ton), as those used in the mechanical cell were maintained. Except promoter, all reagents were added in a conditioning tank. The experimental setup for the column flotation studies is shown in Fig. 3. It is well known that mechanical entrainment of hydrophilic gangue minerals can be overcome by increasing the residence time of particles in the froth zone [34,35]. The mechanical entrainment of hydrophilic gangue minerals can be effectively controlled by column flotation as it enables different froth depths with generation of small bubbles yielding high recovery of selective minerals. In column flotation, there is counter current flow of the feed and bubbles. It operates under steady state and plug flow conditions with regulating degrees of axial dispersion with higher bubble particle contact time. Column flotation studies were carried out at different froth heights and airflow rate. Froth heights were adjusted using a PID controller. Targeted ash levels can be achieved in a single stage as the froth resident time and specific surface area of the bubbles is more than in the conventional flotation cell. The results so obtained are discussed in the results and discussion section. 3. Results and discussion 3.1. Flotation in mechanical cell The effects of pH on flotation tests, both at natural and neutral pH of pulp, are shown in Fig. 4. Neutral pH was found to be more effective. It seems that at natural pH, the flotation performance is less encouraging than at neutral pH. This infers that the negative surface charge on the coal surface resist the adsorption of the collector molecules in the low pH region. Therefore, all the flotation
Fig. 3. Experimental setup of Column flotation.
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Fig. 4. Flotation at natural and neutral pH.
tests were carried out at neutral pH maintained by alkali. Without promoter, the performance is not encouraging even at 18 kg/ton of collector added in stages (Fig. 5). As the coal surface is weathered, the promoter added for surface modification plays a significant role in enhancing the adsorption of oily collector. The promoter assists to disperse the oily collector into small droplets which adsorb on the coal surface and reduces the collector consumption [17]. The effect of promoter in a grinding mill and flotation cell is shown in Fig. 5. It reveals that promoter added during milling is more effective than in a flotation cell. When 3 l/ton ethyl alcohol was used, the flotation response is better than 10 l/ton of same added in the flotation cell; however, at 5 l/ton of ethyl alcohol in the mill, the recovery and grade of the concentrate is encouraging. This may be due to the attrition at a high solid concentration prior to introducing coal to the froth flotation process creates fresh unoxidized surfaces. Sokolovic et al. [36] studied the activation of the oxidized surface of anthracite waste coal. They found that attrition, in the viscous pulp with the solid content of 50%, lead to the mechanical cleaning of oxidized surface and activation of the surface of coal particles. In this investigation, it was observed that promoter added in a flotation cell and a mill requires 10 l/ton and 5 l/ton, respectively. This reveals that double the dosage of promoter is needed for surface modification in a flotation cell. The surface modification was manifested by the zeta potential study of the alcohol treated coal (Fig. 1). The IEP shifted from pH 3.8 to 6.5. It shows that the surface becomes more positive. The flotation study also shows that the collector consumption reduces by 10 kg/ton. The flotation results obtained under different conditions in mechanical cell are discussed below. The single-stage flotation studies with a different feed sizes are shown in Fig. 6. It shows that the feed having 90% passing of 150 μm was effective for achieving 12% ash with less yield; however, at higher ash level (20%), the feed of 80% passing 150 μm produces the same yield as from the finer feed. It is also obvious that yield at a low ash
Fig. 5. Effect of promoter in flotation.
level for any feed size is not encouraging in a single-stage flotation due to the loss of combustible matter in the tailing. In view of this, an attempt was made to recover the relatively high ash concentrate with the coarse feed in the first stage of flotation. The ash content in the concentrate could be reduced by the subsequent cleaning of rougher concentrate. It was observed that cleaning at two stages could produce 46.7% yield with 12.14% ash, while by single-stage cleaning, the yield is 38.5% with minimum ash of 13%. Results of mechanical flotation cell are shown in Fig. 7 and Table 2. Thus, the flotation of the oxidized coal for recovering the clean coal with the targeted ash level of 12% in a mechanical cell requires two stages of cleaning. In a mechanical flotation cell, the froth height can be adjusted only at 2 to 3 cm, and the fine gangue particles can easily get entrained into the froth zone under the influence of the slurry viscosity. The results of mechanical flotation cell become witness that it is very difficult to achieve the target ash level in single-stage flotation. In lieu of achieving the low ash concentrate from less hydrophobicoxidized coal, column flotation was attempted.
3.2. Flotation study in column Column flotation studies carried out at different froth heights and airflow rate are shown in Figs. 8 and 9, respectively. At high froth height of 55 cm, the froth residence time increases and the gangue minerals detach from the froth zone. Thus, the mechanical entrainment and entrapment of the mineral particles could be defeated in column flotation [37]. The effect of superficial air velocity on the concentrate ash and yield shown in Fig. 9 reveals that at 1 cm/s, the maximum yield is 49% at 12.4% ash. Coal being light in nature, low superficial air velocity is preferable in achieving the low ash concentrate. Smaller bubbles produced at a lower superficial air velocity can only carry the lighter particles to the froth phase and thus provides better selectivity with less mechanical entrainment of gangue minerals.
3.3. Comparison of the flotation performance The performance of the oxidized coal in a mechanical and column flotation cell are compared in Table 3 and presented in Fig. 10. The minimum ash in the concentrate recovered without cleaning from mechanical flotation is 13.2% with a yield of 8.2%. The single-stage cleaning of the rougher concentrate reduces the ash to 13% with 38.7% yield. The two stages of cleaning significantly improve the yield to 46.7% and 62.7% at 12.14% and 13.27% ash, respectively. The concentrate achieved from the column flotation in a single stage at low superficial air velocity (1 cm/s) and moderate froth height (40 cm) is 49.6% yield with 59.2% recovery of combustible at 12.2% ash. It appears that single-stage column flotation results are encouraging for producing low ash concentrate.
Fig. 6. Flotation studies in mechanical cell at different conditions.
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Tail 17.9 Rougher flotation
Feed 100 26.8
693
61.7
Conc 82.1 18.8
Single stage Cleaner flotation
Cleaner flotation
Two stage Cleaner flotation Tail 43.6
23.9
Conc 38.5
13.0
Conc 46.7 Stream Yield%
12.14
Tail 35.4
27.5
Ash%
Fig. 7. Results of flotation in mechanical cell to achieve concentrate with 12% ash.
In general, after gravity separation, the reject stream from the HMC is dumped. As a result, weathering reduces the floatability of the fine coal. Sometimes, they are also used in thermal power plants. This coal can be used for metallurgical purpose as the availability of the good quality coal is depleting and the demand for the low ash coal is increasing steadily
due to the quantum expansion of the steel industries. However, this type of coal fines needs beneficiation before end use. Nowadays, pellets are used in DRI plants as they improve the productivity. Therefore, in this investigation, a challenge was taken to recover the low ash clean coal to a greater extent. About 50% of the clean coal yield with low ash (about 12%) can be recovered using column flotation technique. 4. Conclusions The oxidation of the coal surface alters both physical and chemical properties and reduces the floatability. The following inferences can be drawn from the above investigation.
Fig. 8. Effect of froth height in column flotation.
• The results of zeta potential show that the surface of coal particles becomes negatively charged due to oxidation. • The addition of promoter in a mill significantly reduces its consumption due to surface attrition and creation of the new surfaces. It adsorbs on the fresh surface, modifies the surface and hence increases the hydrophobicity. • At natural pH, the adsorption of the collector molecules is hindered due to the negative surface charge where as the neutral pH makes the flotation more effective. • Flotation studies carried out in a mechanical flotation cell shows that cleaning of the rougher concentrate is necessary to reduce the ash content in the concentrate. • The single-stage flotation in a column is found to be better than the mechanical cell as the process variables are more adjustable. Low superficial air velocity is favorable to produce low ash concentrates. • In view of the demand of low ash clean coal for steel making, it is inevitable to recover the combustible matter from coal fines. The product can be used for making composite pellet of coal fines and concentrate of iron ore fines.
Table 3 Comparison of flotation performance of oxidized coal in mechanical and column flotation cell.
Fig. 9. Effect of air flow rate on concentrate and ash in column flotation.
Flotation cell
Concentrate
Mechanical cell
Yield (%)
Ash (%)
RC (%)
Yield (%)
Ash (%)
RC (%)
No cleaning Single-stage cleaning Two-stage cleaning Column flotation
8.2 38.5 46.7 49.6
13.2 13.0 12.14 12.22
9.7 45.8 56.1 59.2
91.8 61.5 53.3 50.4
28.2 35.2 39.6 40.5
90.3 54.2 43.9 40.8
RC: recovery of combustibles.
Tailings
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Fig. 10. Flotation performance in mechanical and column flotation cell.
Acknowledgements The authors would like to thank the Ministry of Steel, Government of India, for receiving the financial assistance for the research work.
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