Abstract
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 P80 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.
Highlights
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
The use of Al-PAM increased the overall combustible recovery of coal by 7.2%. This increase in combustible recovery was observed for the Al-PAM dosage range of 0.04 - 0.25 mg/L
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
Summary
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. The coal flotation process has been challenging, problematic and costly due to the slime coating of colloidal mineral matter on the surfaces of coal particles and air bubbles These slime coatings likely reduce the effectiveness of flotation recovery, especially when fine grinding is required. Several investigators have demonstrated that the use of tri-block copolymers of polyethylene oxide (PEO) and polypropylene oxide (PPO) as flotation collectors could increase the hydrophobicity of coal and significantly increase the overall recovery These polymeric surfactants were unable to reduce the ash content [7]. A novel organic-inorganic hybrid polyacrylamide polymer, Al-PAM, was synthesized and used for the selective depression of ash-forming mineral matter in fine coal flotation. The effect of commercially available polyacrylamide (PAM) polymer and sodium metasilicate dispersant on the flotation process were examined for comparison
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