The flotation of low-quality lignite coal in the presence of NaCl, MgCl2 and CaCl2 inorganic salts with/without the use of kerosene and MIBC as flotation chemicals was investigated in this study. In addition, the zeta potential and contact angle measurements were performed. The lignite sample exhibited a negative surface charge over a broad pH range and had no isoelectric point (iep). It was determined that the magnitude of negative charge significantly decreased above 10-2 M concentration of the salts and the flotation of lignite without flotation reagents also started to take place at concentrations higher than the same value. The contact angles measured on the lignite surfaces also increased with increasing salt concentration. The lignite sample was hydrophilic, and therefore very difficult to float. Consequently, the flotation of lignite with collector and frother was not achieved to the desired extent, even using high reagent concentrations. In the absence of flotation chemicals, high concentrations of these salts also provided low flotation performances. However, the flotation of lignite could be achieved with kerosene and MIBC in the presence of inorganic salts. The flotation experiments of lignite containing ash of 25.02% showed that a concentrate with an ash content of 19% was obtained with a combustible recovery of 51.2% at 10-1 M MgCl2 and 8 g/dm3 kerosene concentrations. In addition, it was determined that MgCl2 and CaCl2 salts were more effective on the lignite flotation than NaCl salt.
Flotation is one of the most important physicochemical separation processes, used largely in mineral separation operations. This technique is used not only in mineral processing and recovery operations, but also in water treatment and purification, the recycling of secondary materials, and the recovery of ionic and colloidal materials from aqueous solutions .
Coal flotation also is a complex process involving several phases (particles, oil droplets and air bubbles). These phases simultaneously interact with each other and with other chemicals such as the molecules of collectors, frothers and dissolved ions in water. These physical and chemical interactions determine the efficiency of the flotation process . The hydrophobicity of coals depends strongly on its rank and the natural hydrophobic coals with high rank float easily. Lower rank coals such as lignite are more hydrophilic, and therefore difficult to float in the traditional manner, even using a high reagent dosage [3–5]. In this case, the reverse flotation technique is likely better suited for the flotation of lignite coals [5,6]. On the other hand, it has been stated that the floatability of naturally hydrophobic solids such as coal can be improved significantly by the addition of inorganic electrolytes [7–11]. High salt concentrations have a significant effect on bulk and interfaces, and change colloidal interactions between bubbles and particles hence affect flotation of minerals and coals [9–11].
The accomplishment of the flotation of lignite coal with low floatability in the presence of NaCl, MgCl2 and CaCl2 salts with/without the use of collector and frother as flotation chemicals was aimed in this study. On the other hand, although there have been much data on the flotation behavior and characteristics of high rank coals in the presence of inorganic salts, there has been limited study on the flotation properties of lignites with salts in the literature. Therefore, this paper also aims to determine those characteristics experimentally and contributes to complete the lack of data in the area of interest.
The flotation experiments were conducted using a low-quality lignite sample from Konya-Ilgin district, Turkey. Analysis of the lignite sample on dry basis was: 25.02% ash, 16.9% fixed carbon and 14.1 MJ/kg in the gross calorific value, and also 24.5% moisture on air-dried basis. The lignite sample was dry-ground in a rod mill and sieved to –212 μm particle size fraction. The particle size distribution of the ground sample is given in Fig. 1. As can be seen, the prepared sample has 80% passing at 140 μm, based on wet screening results. Sodium chloride (NaCl), calcium chloride (CaCl2·2H2O) and magnesium chloride (MgCl2·6H2O), purchased from Merck Company, were used as inorganic salts. Kerosene and MIBC (methyl isobutyl carbinol) in the flotation of lignite were also employed as collector and frother, respectively. Sodium silicate used as dispersant/depressant was purchased from Merck. Sodium hydroxide and hydrochloric acid (Merck) were employed for pH adjustments, and the control of pH of the suspensions was provided by a digital pH meter. All of these chemicals were analytical grade and distilled water was employed for all experimental work.
The flotation experiments of lignite were carried out in a 1 dm3 Denver flotation cell. All experiments were performed at room temperature and natural pH (7.3) of the coal suspension. For each experiment, 50 g of lignite sample was first mixed with the desired salt solution in the cell, and then agitated for 5 min at an impeller rotation speed of 1350 rpm. Next, kerosene and MIBC were added to the suspension. The condition times for kerosene and MIBC were 5 and 3 min, respectively. Subsequently, air was introduced into the cell at a flow rate of 3.5 dm3/min and the suspension was floated for 3 min. The collected froth was filtered, dried, weighed, and ash content was determined by applying the ASTM procedure. The combustible recovery was calculated using the following formula: combustible recovery, % = [C x (100 – c) / F x (100 – f)] x 100 (1) where C and F are the weights of concentrate and feed, respectively. c and f are the ash contents of concentrate and feed, respectively.
Zeta potential and contact angle measurements
The zeta potential measurements were made by a ZetaPlus apparatus from Brookhaven. The lignite sample for the zeta potential measurements was ground to below size fraction 38 μm. A 10 g of the prepared sample was added into 1 dm3 of water or the desired salt solution, and the suspension was stirred for 30 min after adjustment of pH. Then the suspension was kept still for 20 min to let larger particles settle. Thereafter, a sample of supernatant was taken out and transferred into the clear disposable cell and the cell was placed to the zeta potential analyzer. An average of 10 runs was recorded for the measurement of zeta potential of each sample and the average values were reported.
The contact angles were also determined using a KSV CAM 101 contact angle goniometer. The pellets from lignite sample were prepared by using a hydraulic press and the obtained surfaces were highly smooth. A drop of solution containing inorganic salt was formed on the pellet surface by means of a special syringe and the resulting contact angle was determined by the goniometer. Each contact angle data presented in this paper were the average values obtained from at least four measurements.
Results and Discussion
The zeta potential of the lignite sample as a function of pH is shown in Fig. 2. As can be seen, the lignite sample in water acquired a negative surface charge and maintained this nature even in very acidic conditions. Also, the zeta potential of the particles became more negative with the increase of suspension pH and eventually decreased to approximately –37 mV at a pH of 12. The isoelectric point (iep) of the lignite sample was not determined. These findings are consistent with the results of Ozdemir et al. .
The effect of kerosene concentration in the absence of inorganic salts on the floatability of lignite is given in Fig. 3. As shown in Fig. 3, the flotation of the lignite sample was not achieved below 2 g/dm3 concentration of kerosene. However, the floatability of lignite could be poorly obtained at the higher concentrations of collector. The low-quality lignite coal used in the experimental study had a hydrophilic character, and hence its floatability was fairly low. This was due to the presence of higher oxygen-containing functional groups of lignite, such as hydroxyl, carbonyl and carboxyl [3,4,12].
The effect of concentration of different inorganic salts on the floatability of lignite without any flotation chemicals. As seen from Fig. 4, the flotation of lignite sample in the absence of collector and frother could be obtained at concentrations higher than 10-2 M for all the salts. have been stated that the floatability of bituminous coal decreased with increase in NaCl concentration at low concentrations and increased at high concentrations. The work of Harvey et al. , where the effect of electrolyte (NaCl and MgCl2) concentrations on coal flotation was investigated, supported this finding. The flotation behavior of bituminous and lignite coals in salty water was also investigated by Ozdemir et al. . Their results showed that it was possible to float the natural hydrophobic coal in different salt (NaCl, KCl, MgCl2 and CaCl2) solutions in the absence of collector and frother. However, it was stated that the flotation of lignite coal was generally difficult. Also, it was reported for both the coals that MgCl2 and KCl solutions showed the highest and the lowest flotation performance improvements, respectively.
The zeta potential of lignite as a function of inorganic salt concentration in the absence of flotation chemicals is shown in Fig. 5. As seen in Fig. 5, the negative values of zeta potential of lignite sample significantly decreased above 10-2 M concentration for all the salts and the flotation of lignite also started to occur at concentrations higher than this value (see also Fig. 4). It is known that the presence of electrolytes compresses the electrical double-layer which leads to the reduction of zeta potential of both bubbles and particles . This decrease also facilitates the bubble-particle attachment process in flotation system . Although there are some studies reporting that the flotation recovery reaches a maximum at minimum zeta potentials [9,15] other investigations showed that the maximum flotation response for coals occurs close to its isoelectric point [16,17]. It has also been stated that the enhanced floatability with salt addition results from the destabilization by the salt of the hydrated layers surrounding the particles . The destabilization makes the coal more hydrophobic and improves the bubble-particle adhesion process in flotation . On the other hand, the compression of the electrical double-layer by the added electrolyte, which can subsequently cause thinning and rupture of the wetting film between bubbles and particles, has also been proposed as a reason for the inorganic salt effects on flotation . Additionally, Ozdemir et al.  stated that the bubble size of the froth phase varied with the increase in salt concentration. In the case of reduced bubble size, this can be suggested as a possible reason for the enhanced floatability . Meanwhile, Craig et al.  indicated that the bubble coalescence was inhibited in the presence of some salt solutions. Hence smaller bubbles occur in the suspension and lead to more stable froth. The findings reached by Zhang and Liu  also supported these results.
The contact angle value of the lignite obtained with distilled water was equal to 0°; that is, the water spread completely on the lignite surfaces. Fig. 6 also shows the contact angle of lignite as a function of salt concentration in the absence of flotation chemicals. Although it was not much expected that inorganic salts had a significant effect on the hydrophobicity of coal, the contact angles measured on the lignite sample increased with increasing salt concentration. It is well- known that the addition of most inorganic salts to water raises the surface tension of solution with salt concentration [1,20,21]. Therefore, it can be said that the raised surface tension might lead to an increase in the contact angle values measured on the lignite surfaces. A similar result was also found for a different lignite coal in our previous study . Consequently, the increase in the solution surface tension with salt concentration should also be taken into account, in addition to the salt effects proposed in the literature. In this case, it can be stated that this was an additional possible reason for the floatability of lignite coal obtained at concentrations higher than 10-2 M in Fig. 4. In addition, it was stated that salt ions destabilized the hydrated layers surrounding the coal particles, hence reduced the surface hydration and made the coal more hydrophobic, as mentioned above. However, it was reported in some studies that the contact angles did not vary with increasing concentration of salt [9,18]. Ozdemir et al.  also stated that contact angle values of hydrophobic coal in bore water, which contained mostly NaCl, did not change compared to that in distilled water. On the other hand, Sghaier et al.  found that while the contact angles significantly increased with NaCl concentration on hydrophilic solid surfaces, the contact angle variations were small on hydrophobic solid surfaces. Aslan et al.  also reported for various minerals that there were a decrease in the contact angles as NaCl concentration increased and then an increase with a further concentration increase. In addition, it can also be noted that MgCl2 and CaCl2 salts on both the zeta potential and contact angle values of lignite were more effective than NaCl salt.
The effect of MgCl2 salt concentration on the lignite flotation with flotation chemicals (2 g/dm3 of kerosene and 50 g/t of MIBC) is shown in Fig. 7. As seen in Fig. 7, the addition of MgCl2 salt provided the flotation of lignite at the kerosene concentration (2 g/dm3) at which flotation was not possible (see also Fig. 3). While the combustible recovery value increased with increasing MgCl2 salt concentration, the concentrate ash content was not much affected by the concentration changes. A concentrate with an ash content of 22.9% was obtained with a combustible recovery of 55.6% at 1 M MgCl2 concentration. The similar results were also obtained for NaCl and CaCl2 salts. It can also be noted that the combustible recovery in the presence of MgCl2 and CaCl2 salts reached higher values compared to that in the presence of NaCl salt. Fig. 8 also shows the effect of kerosene concentration on the lignite flotation at 10-1 M MgCl2 salt. As can be seen, as the combustible recoveries improved with increasing kerosene concentration, the concentrate ash contents did not change much. The flotation results showed that a concentrate with an ash content of 19% was achieved with a combustible recovery of 51.2% at 10-1 M MgCl2 and 8 g/dm3 kerosene concentrations.
The effects of MIBC and sodium silicate concentrations on the lignite flotation are shown in Fig. 9 and 10, respectively. As seen in these figures, while the combustible recovery slightly increased with the increase in the frother concentration, the concentrate ash contents did not vary much with the concentration of MIBC and sodium silicate.
It was determined that the floatability of the low-quality lignite coal used in this study was fairly weak due to its strongly hydrophilic character and therefore its flotation using only collector and frother was not possible to the desired extent, even using high reagent concentrations. The lignite sample acquired a negative surface charge in the pH range of 2–12 and its zeta potential became more negative with increasing suspension pH. The isoelectric point (iep) of the lignite sample was not found in this pH range. It was also found that the magnitude of negative charge significantly decreased above 10-2 M concentration of NaCl, MgCl2 and CaCl2 salts and the flotation of lignite without flotation reagents also started to occur above this salt concentration. The contact angle values measured on the lignite surfaces exhibited an increasing trend possibly due to the increase in the surface tension of solution with increasing salt concentration. In addition to the salt effects proposed in the literature, this can be another possible reason for the flotation of lignite coal obtained at salt concentrations higher than 10-2 M. In the absence of flotation chemicals, the high concentrations of inorganic salts provided low flotation performances. However, the flotation of lignite could be improved with collector and frother in the presence of inorganic salts. Even the addition of inorganic salts provided the flotation of lignite at the kerosene concentration at which flotation was not possible. While the combustible recovery values increased with the increase in the concentration of salt, collector and frother, the concentrate ash contents were not much affected by these changes. The flotation results of lignite with the ash content in the feed of 25.02% showed that a concentrate with an ash content of 19% was achieved with a combustible recovery of 51.2% at 10-1 M MgCl2 and 8 g/dm3 kerosene concentrations. At a high MgCl2 concentration of 1 M, a concentrate with an ash content of 22.9% was also obtained with a combustible recovery of 55.6% at a kerosene concentration of 2 g/dm3. Furthermore, it was found that MgCl2 and CaCl2 salts were more effective on the lignite flotation than NaCl salt.
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