Acid-Assisted Recycling of Iron Hydroxide Sludge as a Coagulant for Metalworking Fluid Wastewater Treatment

Abstract

Coagulation is capable of treating metalworking fluid (MWF) wastewater after MWF oil recovery, as an alternative fuel by dissolved-air flotation, but it yields a substantial amount of oil-contaminated sludge. The oil-contaminated sludge is a resource of coagulant recovery, which reduces the sludge disposal and coagulant cost. This study examined the acid recovery and reusability of iron coagulant for MWF wastewater treatment. Ferric chloride (FeCl3) was used as a coagulant, while hydrochloric acid (HCl) was used in the solubilisation process. Without MWF contamination, the iron hydroxide (Fe(OH)3) to HCl ratio of 3:1 was required to completely dissolve the Fe(OH)3 sludge within 5 min. The presence of MWF oil on the sludge substantially decreased the acid recovery of Fe(OH)3. Even at 30 min, the Fe(OH)3 to HCl ratio of 3:1 could recover only 87.07% by mass of iron. The use of anionic polymer (polyacrylate) as a coagulant aid for MWF treatment made the acid recovery of iron coagulant less effective. Acid-recovered iron coagulant can be reused for four cycles to treat the MWF wastewater because increasing the cycles of reuse lowers the treatment efficiency. The use of HCl waste in this approach reduces both the operational cost and potential global warming effect.