Effect of PAM and surfactant on agglomeration of flotation clean coal particles under different pH conditions

pH affected the aggregation performance by altering the interactions between coal particles and reagents. To reveal the influence of pH on particle aggregation, this study calculated single-molecule adsorption energy, electrostatic potential energy, and Mulliken charge, and combined these with extended derjaguin–landau–verwey–overbeek (EDLVO) theory and focused beam reflectance measurement (FBRM) particle size distribution analysis to investigate the effects of pH variation on reagent adsorption behavior on the coal surface, particle interactions, and floc size. The results showed that H+ increased the electrostatic repulsion between cationic polyacrylamide (CPAM) monomers and coal molecules, and converted the electrostatic attraction between coal molecules and nonionic polyacrylamide (NPAM) into electrostatic repulsion. The addition of OH− weakened the repulsion between CPAM and coal molecules, and enhanced the electrostatic attraction between coal molecules and NPAM. H+ increased the electrostatic repulsion between polyoxyethylene alkyl ether (AEO-23) and coal molecules, whereas OH− enhanced their electrostatic attraction. Both H+ and OH− increased the electrostatic repulsion between sodium dodecyl benzene sulfonate (SDBS) and coal molecules, while both ions enhanced the electrostatic attraction between dodecyl dimethyl benzyl ammonium chloride (DDBAC) and coal molecules. EDLVO theory analysis indicated that at a fixed interparticle distance, van der Waals energy varied little with increasing pH, whereas electrostatic and hydrophobic interaction energies increased markedly, leading to an overall increase in interaction energy (i.e. stronger repulsion), which was unfavorable for aggregation. Under neutral and alkaline conditions, all three types of polyacrylamide (PAM) maintained coal particles predominantly in small-size fractions. Acidic conditions compressed the electrical double layer, thereby reducing electrostatic repulsion, amplifying surfactant adsorption, and ultimately resulting in distinct adsorption modes and floc structures.