The current study investigated the effects of novel hybrid polyacrylamide polymers as ash (slime) depressants in fine coal flotation to enhance combustible recovery and ash rejection. Coal samples at P 80 of approximately 45 um with ~25% ash content were floated in the presence of in-house synthesized hybrid aluminum hydroxide polyacrylamide polymers (Al(OH) 3-PAM, or Al-PAM). All flotation experiments were carried out in a 5-L Denver flotation cell. Various influencing factors were examined to optimize the flotation process in the presence of the Al-PAM polymers, including the Al-PAM dosage, Al-PAM conditioning time, impeller rotation speed and pulp pH. Comparative and synergistic studies were also performed using organic polyacrylamide polymers (PAMs), commercial dispersants and Al-PAM/dispersant system. Results showed a significant improvement in both combustible recovery and ash rejection at an Al-PAM dosage of 0.25 mg/L. The maximum combustible recovery obtained, at natural pH, with Al-PAM and Al-PAM/dispersant system was determined to be 70% and 66% at ash content of 7.74% and 7.4%, respectively. Zeta potential values of both the raw coal and concentrate products showed a large shift toward more positive values (from ˉ50 mV to ˉ13 mV), indicating a significant decrease in ash-forming minerals (slimes) when Al-PAM polymers were applied.
The effective liberation of coal particles from lower-grade coal deposits requires micron and submicron comminution processes that will generate a vast amount of coal fines. A large amount of coal fines and ultrafines can also be generated due to the adoption of modern full seam mechanized mining, ore handling and preparations [
Synthetic and naturally occurring polymers have been successfully used in mineral processing/coal preparation mainly as gangue depressants or dispersants to increase the combustible recovery of coal fines [
Coal samples used in this study were obtained from a mine located in Southern Illinois, USA. The as-received run-of-the-mine coal was first crushed in a jaw crusher (2 × 6 model, Sturtevant Inc., Hanover, MA, USA) and then further crushed in a roll crusher (8 × 5 model Sturtevant Inc., Hanover, MA, USA) to a size of 850 μm. Finally, a laboratory ball mill was used to grind the coal samples to a finer size. After crushing and grinding, the coal was screened to −75, +38 µm to be used as a feed in all flotation experiments. The particle size distribution of the flotation feed showed a P80 value of 45 um. The proximate analysis of the −75, +38 µm coal (flotation feed) is shown in
The hybrid polyacrylamide Al-PAM was synthesized in-house according to a procedure described elsewhere [
Proximate Analysis | As determined (%) |
---|---|
Moisture | 10.05 |
Ash | 25.12 |
Volatile matter | 22.94 |
Fixed carbon | 44.86 |
solution at a rate of 0.5 g/min. After adding approximately 36 - 37 g of the (NH4)2CO3 solution into 25 g of AlCl3 solution, the pump was stopped and the solution was gently stirred at a rate of 300 rpm for approximately 30 min to complete the reaction. The prepared Al(OH)3 suspension has an approximate particle size of 30 - 50 nm, and the measured zeta potential value was +27, −30 mV. The particle size and zeta potential studies were measured using a Zetasizer Nano ZS (Malvern, USA) The next step was the polymerization of acrylamide in an Al(OH)3 colloidal suspension to synthesize the Al-PAM polymer. The polymerization process was achieved according to the following procedure: approximately 4.5 g of acrylamide monomer was dissolved in 25.5 mL of previously prepared Al(OH)3 colloidal suspensions. The suspension was stirred and heated in an oil bath to 40˚C under nitrogen flow for nearly 30 min. Two milliliters of 1000 mg/L stock solutions of redox initiators (1 mL each) were slowly added to the prepared Al(OH)3 suspension containing acrylamide monomer to initiate the polymerization of acrylamide onto colloidal aluminum hydroxide particles. The polymerization reaction was kept running overnight. The formed Al-PAM gel was dissolved in MilliQ water to 10 wt%. The aqueous polymer colloidal suspension was then introduced into acetone drop-wise to precipitate Al-PAM. The resulting solid precipitates were dried under vacuum at 55˚C for 6 - 8 hours. Freshly prepared 1000 mg/L stock solutions of Al-PAM polymers in deionized water were used in all of the flotation experiments.
Polymer CharacterizationThe aluminum content by weight in Al-PAM polymer was measured using the PerkinElmer Inductively coupled plasma system 2000 DV instrument equipped with Optical Emission Spectrophotometer and WinLab32 for ICP version software for measurement of Aluminum at wavelength 396.153 nm. The RF power was used 1500 watts, Plasma flow was 15 L/min, Auxiliary Flow was kept at 0.2 L/min, and Nebulizer Flow was kept at 0.8 L/min, Pump Rate was kept at 2 ml/min. The calibration curve plotted using 7 different concentrations of solutions diluted in 1% HNO3 (0 mg/L, 0.1 mg/L, 0.5 mg/L, 1 mg/L, 5 mg/L, 10 mg/L, and 25 mg/L) made by using HIGH-PURITY STANDARDS purchased from Fisher-Scientific. The correlation coefficient was 0.9999. Sample measurements were repeated in 3 times. The sample contains 0.14 wt% of aluminum.
Molecular weight of Al-PAM was determined using a Zetasizer Nano (Malvern Instruments Ltd., USA) by employing static light scattering (SLS). Zetasizer Nano software measures the intensities of the scattered light of the polymer sample and automatically calculates the molecular weight by applying the Rayleigh Equation (1) below [
where Rθ is Rayleigh ratio which is the ratio of scattered light to incident light; MW is the molecular weight; A2 is the 2nd Virial Coefficient which represents the interaction strength among the polymer chains and has been correlated with solubility [
where NA is Avogadro’s constant; lo is the wavelength of laser used; no is the refractive index of the solvent and dn/dc is the differential refractive index increment which is the change in refractive index as a function of the change in concentration. Therefore, a plot of KC/Rθ versus C is expected to be linear with an intercept equivalent to 1/MW and a slope equal to the second virial coefficient A2. This plot is known as “Debye plot”. To establish Debye plot of Al-PAM, samples of different concentration; 15, 13.5, 10.5 and 7.5 mg/ml were prepared. The calibration was made by using pure (>99.5%) toluene. All measurements were conducted at 25˚C. The Debye plot for the Al-PAM data is shown in
An XRD analysis was used to reveal qualitative information on the mineral composition of raw coal samples (−75 µm +35 µm) used in this study. The XRD analysis was performed using a PANalytical X’Pert Pro Multi-Purpose Diffractometer (MPD; PANalytical, Inc., MA, USA) system with a Cu (k-alpha)-source at a tube voltage of 40 kV.
Content | Wt.(%) |
---|---|
Amorphous | 76.2 |
FeS2 (Pyrite) | 1.0 |
Quartz | 5.9 |
Kaolinite Al2(Si2O5)(OH)4 | 4.6 |
Corundum, Al2O3 | 10.2 |
Calcite, Ca(CO3) | 2.1 |
by proximate analysis. The major crystalline phases observed were FeS2 (pyrite), quartz, kaolinite Al2(Si2O5)(OH)4, corundum Al2O3 and small amounts of calcite Ca(CO3).
An RA is the analogous counterpart in coal froth flotation to the float and sink methods in coal gravity concentration. The major objective of an RA is to obtain the best possible separation performance by any froth flotation process [
The zeta potential of the coal samples were measured using a Zetasizer Nano ZS instrument (Malvern Instruments, Inc., Westborough, MA, USA). Data on the interactions of coal samples with ash-forming minerals in the flotation pulp were obtained by measuring the zeta potential of the raw coal samples and the froth obtained from the polymer-assisted flotation process and comparing the results to the zeta potential of clean coal obtained from the RA. The experiments were conducted using a 0.01 M KCI electrolyte solution at natural pH. Zeta potential measurements were performed for raw coal (i.e., the flotation feed), the first concentrate produced from the RA (herein referred to as clean coal with an ash content of 3.91%) and froth collected from the polymer-assisted flotation. Sample measurements were repeated 3 times. Isoelectric graphs of the clean and raw coal were obtained using MPT-2 autotitrator which is part of the Zetasizer Nano. The solution pH was adjusted using HCl and NaOH. Coal suspensions were prepared at 1 wt.% coal in a 0.01 M KCI solution. Each prepared coal suspension was agitated using an IKA RW 20 mechanical stirrer for approximately 30 min at a constant agitation rate of 300 rpm. The suspension was then allowed to settle for 5 - 10 min. The upper portion of the supernatant was considered for the zeta potential distribution measurements.
Batch flotation experiments were conducted to study the effects of novel organic/inorganic (hybrid) polyacrylamide polymers in enhancing the combustible recovery and ash rejection of coal flotation under various operating parameters. All flotation experiments were conducted 3 times as indicated by the error bars in the figures. In the study, various parameters, such as polymer dosage, impeller speed, pH and conditioning time, were assessed individually. All of the flotation experiments were run in a 5-L D12-Denver flotation laboratory cell at natural pH unless otherwise stated. The air flow rate and solids concentration were kept constant at 6 L pm and 5%, respectively, in all experiments. In a typical flotation test, 253 g of coal and 4800 mL of tap water were loaded in the 5-L cell, and the pulp was conditioned to allow wetting of the coal for 5 min prior to any reagent addition. Collector (i.e., kerosene) was then added at a predetermined dosage, and the suspension was conditioned for an additional 3 min. A desired dosage of Al-PAM was added after the slurry was conditioned with the collector, and the pulp was agitated for another 3 min. When the dispersant was used, it was first added before adding the collector. The polymer and dispersant dosages are expressed in reference to the total volume of the feed slurry (i.e., the combined volume of coal and water). The frothing agent (MIBC) was added at a fixed amount of 200 μL/ton on a mass basis relative to the dry feed mass. The pulp was further conditioned for 2 min before air was introduced. The resultant froth was collected at 1-min time intervals. The concentrate fractions were washed, filtered and dried in an oven overnight at 80˚C. After drying, the ash contents of the concentrates were determined according to ASTM D3174-73 [
Flotation kinetic tests were conducted to determine the effects of Al-PAM on the flotation rate constants. All experiments were conducted in the presence or absence of Al-PAM at the optimum dosages for comparison purposes. In each test, the froth product was collected at 1-min time intervals; 5 fractions were collected. The collected fractions were washed, filtered and dried in an oven overnight at 80˚C. After drying, the ash contents of the concentrates were determined according to ASTM D3174-73.
A series of flotation experiments were conducted to determine the effects of Al-PAM at various dosages (i.e., 0.04, 0.08, 0.16, 0.25, 0.5, 1, 2, and 3 mg/L) on the combustible recovery of coal and product ash. In this set of experiments, a collector, Al-PAM polymer and a frothing agent were sequentially added to coal slurries. Baseline experiments without the Al-PAM polymer were performed periodically to define a base recovery and product ash vs. dosage relationship to ensure the reproducibility of the results and a congruity of the experiment parameters. As shown in
forces and/or hydrogen bonding. Coagulation would result in the settling of coal particles and thereby decrease the recovery. Additionally, the product ash content also increased from 7.9% to 13.7% as the Al-PAM dosage increased from 0.5 to 3 mg/L.
To better understand the effect of hydrodynamics on the flotation performance, coal flotation experiments were carried out at various impeller speeds (i.e., 1200, 1500, 1800 and 2100 rpm) in the presence or absence of Al-PAM polymers. First, a set of baseline flotation experiments were conducted without Al-PAM.
Reagent conditioning plays a dominant role in the overall performance of the flotation process and has been recognized as an important method for improving the performance of the flotation process. Kalyani et al. concluded that reagent conditioning
enables regents to be uniformly distributed within the suspension and thereby improves the collision and adhesion probability of distributed reagents with coal particles [
Solution pH is one of the most important factors controlling slime coating due to the possible Van Der Waals attractive forces between ash-forming minerals and coal particles. Flotation experiments were conducted at a slightly acidic pH of 5 and a moderately alkaline pH of 10.2 to further explore the role of Al-PAM in coal flotation. First, baseline flotation experiments (collector and frothing agents only) were conducted at pH values of 5, 7.8 and 10.
closer to its isoelectric points, i.e., at pH 3 - 5 [
The influence of Al-PAM on fine coal flotation can be partially explained by the flotation rate constants. Batch flotation in a mechanical cell is based on the first-order flotation rate Equation (3) [
where C is the concentration of hydrophobic particles in the cell at any time, t is time (representing the duration of the flotation test) and k is the flotation rate constant. Solving Equation (3) for the flotation rate constant results in Equation (4):
where R is the flotation recovery, which is defined as the ratio of the concentration of hydrophobic particles at time t (C(t)) to the original concentration of hydrophobic particles (C0). The data collected from the experiments were then used to estimate rate constant values, enabling the better evaluation of the flotation performance with or without Al-PAM. Kinetic rates were conducted at a slightly acidic pH of 5 and at neutral pH. The flotation recovery as a function of time in the presence and absence of Al-PAM at pH 5 and 7.8 is shown in
For comparison, coal flotation experiments were conducted in the presence of commercially available polyacrylamide (with a molecular weight of ~5 × 106 Dalton) to determine the effects of polyacrylamide on combustible recovery and product ash. Flotation experiments were first conducted by varying the dosage of PAM at a constant natural pH of 7.8. The effects of PAM on the combustible recovery of coal and product ash at pH 7.8 as a function of PAM dosage are shown in
The results obtained in this study correlated with those obtained by Moudgil [
Al-PAM Present | No Al-PAM | |
---|---|---|
Parameter 1: k(min-1) | ||
pH 5 | 0.36 | 0.28 |
pH 7.8 | 0.25 | 0.18 |
Parameter 2: R(%) | ||
pH 5 | 78.5 | 70.5 |
pH 7.8 | 68 | 59.4 |
PAM increased the amount of ash content in the concentrate. The product ash obtained when no PAM was added was lower at an ash content of 8.2%. However, the ash content in the concentrate increased considerably when PAM was added to the flotation pulp. The overall ash content in the product ash increased from 8.2% to 19.62%.
Coal flotation was conducted in the presence of an ash dispersant to investigate whether an ash dispersant would optimize the flotation performance. Sodium metasilicate was selected as the dispersant because it has been reported to be one of the most effective dispersants for coal flotation [
When the sodium metasilicate dispersant dosage increased from 1.2 to 1.5 mg/L, the combustible recovery of coal decreased but the product ash content increased. These findings were not surprising because the limited literature on fine coal flotation in the presence of sodium metasilicate has largely indicated that moderate to high levels of coal depression could be expected when sodium metasilicate is added in excess. However, more recent studies have shown that the depression of coal during flotation in the presence of sodium metasilicate could be minimized by adding relatively low dosages of dispersing agents. In this study, the product ash was slightly contaminated in the presence of sodium metasilicate. Similar results have been reported in the literature. Zolghadri et al. concluded that froth contamination was likely because the dispersion of coagulates consisted of higher levels of non-hydrophobic ash particles and lower levels of hydrophobic ash particles [
Experiments were conducted to evaluate the use of a dual Al-PAM-dispersant system to improve the combustible recovery of coal and ash depression during coal flotation. First, floatation experiments were conducted using Al-PAM alone at various dosages. These experiments were mainly conducted to determine the optimum dosage of Al-PAM, as shown in
As shown in
To further investigate the dual use of the Al-PAM-dispersant system, flotation experiments were conducted at a slightly acidic pH of 5 and an alkaline pH of 10. In this set of experiments, the polymer dosage, dispersant dosage, impeller speed and polymer conditioning time were kept constant at 0.25 mg/L, 0.8 mg/L, 1800 rpm and 6 min, respectively.
The role of Al-PAM in fine coal flotation was fundamentally explored by examining the surface properties of raw coal and froth after polymer-assisted flotation. Zeta potential measurements were conducted for clean coal (4% ash), raw coal and concentrates from froth flotation experiments in the absence or presence of Al-PAM. Froth products obtained from the use of the dual Al-PAM-dispersant system were also tested for comparison. The zeta potential values of clean coal (4% ash) and raw coal as functions of increasing and decreasing pH were determined and are shown in
(4% ash) exhibited a positive charge at pH levels below 3, whereas raw coal has a negative charge over the pH range of 2 - 11. As the pH increased from 3 to 11, the zeta potential of clean coal (4% ash) became more negative. The isoelectric point of clean coal is approximately pH 3.5, which indicates that as pH decreased below 3, the amount of adsorbed hydronium (H+) ions increased on the surface of the coal, positively charging the surface of the coal. Conversely, when the pH increased above 3, the amount of hydroxyl (OH−) ions increased and adsorbed on the surface of the coal particles, replacing the hydronium ions and rendering the surface negatively charged. This indicates that changing the concentration of the hydronium or hydroxyl ions changes the magnitude and sign of the zeta potential. As the pH increased from 2 to 11, the zeta potential of the clean coal (4% ash) was the most negative, whereas that of the raw coal was the least negative. The less negative surface charge of raw coal (compared with clean coal) under increasing pH is attributed to the adsorption of positively charged metal ions, such as Mg2+, Ca2+ and Fe2+, on the negatively charged surface of raw coal particles [
Results for the zeta potential distribution peaks for clean coal, raw coal, concentrates obtained from froth flotation experiments using Al-PAM, the dual Al-PAM-dispersant system and collector or frothing agent alone are shown in
The laboratory batch flotation results obtained in this study demonstrated the positive impacts of hybrid polyacrylamide polymers (Al-PAMs) on the combustible recovery of coal and ash reduction. Results indicate that the use of a dual Al-PAM-dispersant system provided an attractive means for improving the overall flotation performance. An optimal separation was obtained when 0.25 mg/L of Al-PAM and 0.8 mg/L of sodium metasilicate were added to the flotation pulp. The study also shows that the depression of coal during flotation in the presence of sodium metasilicate can be minimized by adding relatively low dosages of dispersing agents. However, moderate-to-high levels of coal depression may be expected when sodium metasilicate is added in excess. Furthermore, this work demonstrates that an organic/inorganic (hybrid) polyacrylamide polymer is a better ash-forming depressant than commercially available organic polyacrylamide (PAM).
The authors would like to thank Dr. Mary R. Reidmeyer and Dr. Ronald J. OMalley from the department of Materials Science and Engineering at Missouri University of Science and Technology for allowing the use of mineral processing facilities.
Molatlhegi, O. and Alagha, L. (2016) Ash Depression in Fine Coal Flotation Using a Novel Polymer Aid. International Journal of Clean Coal and Ener- gy, 5, 65-85. http://dx.doi.org/10.4236/ijcce.2016.54006