High Rate Clarification for Treatment of Mine Water

Abstract

Two high-rate clarification processes are often used in mine water treatment, the high density sludge (HDS) process and the ballasted flocculation process. The purpose of this research project was to compare treatment outcomes of HDS and ballasted flocculation to the conventional active lime treatment process in terms of water and sludge quality. Additionally, the effect of replacing lime, which is typically used in mine water treatment for pH adjustment, with cement kiln dust (CKD), an alkaline by-product of cement manufacture, in these processes was investigated. Initially, a comparison between the conventional, HDS, and ballasted flocculation processes for treatment of acid mine drainage (AMD) using lime as the alkali was undertaken. It was found that the ballasted flocculation process resulted in settled water with lower concentrations of arsenic (As) and zinc (Zn) and lower turbidity compared to the other two treatment processes, confirming anecdotal evidence. This was achieved even with the considerable difference in sedimentation times for the processes, 60 minutes for conventional and HDS versus 3 minutes for ballasted flocculation. The HDS process, as its name suggests, produced the highest density sludge (i.e., lowest sludge volume and sludge volume index and highest % wet and dry solids). Further investigations into the HDS and ballasted flocculation processes, and the replacement of lime with CKD therein, revealed that solids loading had the greatest impact on floc density, settling rate, and sludge density. No difference in iron concentrations or turbidity measurements was found in ballasted flocculation-treated and clarified samples using various ballast materials (i.e., microsand, glass beads, and magnetite) as well as no ballast at all, using either lime or CKD as the alkali. Therefore it was concluded that the increased contaminant removal found with the ballasted flocculation process compared to the conventional or HDS processes was due to the higher flocculation speed used, not the addition of ballast material. Finally, a two-stage ballasted flocculation treatment process tested for remediation of high-As neutral mine drainage (NMD) was able to decrease As concentrations to below 0.10 mg/L, whereas a single-stage process could not.