Optimization of the operational variables of a medium-scale reactor for metal-containing wastewater purification by ferrite formation

A medium-scale (5 l) reactor was designed for the purification of metal-containing wastewater discharged by academical chemical laboratories. The process is based on the precipitation of metals in alkaline solution as metal-bearing ferrites of high magnetic permeability, which can be later used in a variety of applications based in their magnetic properties. Chemical variables affecting the process efficiency were optimized in a previous work and fixed here at their optimum values (pH, 10; Fe(II):Total metal molar ratio, 15). Operational variables or control factors were optimized using a Taguchi parameter design in order to achieve maximum efficiency of the purification process (or minimum total remnant cationic concentration, TRCC), and a solid waste (ferrite sludge) of maximum magnetic permeability, which allows its use in a variety of applications. The interference caused by metal-binding organic matter present in the wastewater was investigated introducing in the optimization experiment a three-level noise factor in order to find control factor levels that yield optimum process performance, insensitive to the uncontrollable variation of this noise. EDTA (disodium salt) was used as model for the noise factor. The effects of temperature, air flow and stirrer rate were moderate. It was observed that the presence of high concentrations of metal-binding organic matter hinders metal precipitation in such a way that EDTA concentrations higher than 10−3 M decrease purification efficiencies below 99%. Under the optimized conditions (reaction time, 1 h; temperature, 60°C; air flow, 30 l/min; and stirrer rate, 560 rpm), and for concentrations of metal-binding organic matter below 10−3 M, a purification efficiency of 99.9% and a chemically stable (inert) ferrite sludge of relative magnetic permeability 83.60 (66.6% of the MP value for pure magnetite) are obtained. The procedure is therefore recommended for purification of metal-containing wastewater. The chemical reactions involved in the process are discussed.