Chemical and economic optimization of the coagulation-flocculation process for silver removal and recovery from industrial wastewater

Silver is a crucial element in several fields of industry. Its removal from aqueous waste streams before discharging is essential for environmental protection. Due to the environmental toxicity of Ag, stringent discharge limits are imposed by governments. Several removal techniques exist of which coagulation flocculation is a favourable option for reasons of robustness and limited investment costs. The objective of the present work is to optimize the coagulation-flocculation process to remove silver from industrial wastewater by a profound screening and Central Composite Design. More specifically, this study focuses on maximizing the removal of silver on the one hand, and reducing the cost of operation on the other hand. Use of Poly Aluminium Chloride (PAC) coagulant in combination with a pH-correction and an anionic polyelectrolyte as flocculant led to decreasing the wastewater's Ag concentration from 2.50 mg L-1 to a residual Ag concentration of 0.027 mg L-1. Application of sodium poly-acrylated aluminium chloride (Magnasol 5155) as coagulant could even reduce the Ag concentration further to 0.004 mg L-1. Both coagulants are able to achieve the stringent Ag discharge standard in Flanders, Belgium (0.08 mg L-1), however the latter one corresponds to a higher price sensitivity, meaning that a larger removal of Ag was achieved against the same cost of chemicals used in the recovery process. Moreover, it facilitated the formation of a sludge that is 42.4 +/- 3.4 m% rich in Ag, which is extremely beneficial when recovery of the precious metal is targeted. (C) 2017 Elsevier B.V. All rights reserved.