Effect of variation of pH on flocculation of coal/vitrinite slurries

Of late, flocculation, especially with polymeric flocculants, is being advantageously used in coal industry for retrieving the coal fines form the slurries of washery waste waters. This, in turn, helps to conserve the coal and to utilize it more efficiently. The present paper deals with the results of the investigations on the effect of variation of pH on flocculation of coal as well as vitrinite slurries, using both commercial (Preastol and Tru-Floc) and laboratory synthesized hydrolysed polyacrylamide (HPAM) flocculants. The coals selected for these investigations are the representatives of prime coking coals of Jharia coalfields, viz., Bhowrah XIV seam, Lodna XIV seam and Jharia XII seam coals. The flocculation behaviour of these coals as also the vitrinite was studied at different pH values (in the range 2-12) by determining the settling rates. It is found that the optimum flocculation is achieved in the pH range 6-7, the efficiency of the flocculants being in the order : HPAM > Praestol > True-Floc. However, at the optimum PH (6-7), the magnitude of sedimentation of the coals follows the order: Bhowrah XIV seam > Lodna XIV seam > Jharia XII seam coal, suggesting thereby that the higher the seam of coal, the greater is its settling rate. The maximum settling rate for the composite coal (including Vitrinite) is found to be greater in magnitude than for vitrinite alone. The observation of lower settling rates in case of vitrinite has been explained on the basis of charge formation on the particles. The low values ofsettling rates in both the acid and alkaline media are possibly due to the coiling of the polymer, which is known to take place in a medium other than neutral. The dependence of settling rate on the seam of the coal has been explained on the basis of ash content in different seams of coal