Coagulation and flocculation process for removal of heavy metal from electroless plating process industrial raw effluent via hydroxide precipitation versus sulphide precipitation

Electroless plating is deposition of thin protective layer by using chemical reaction. Opposite to the electroplating solution, electroless plating solutions does not requires any source of current to plate. Purpose of plating is to protect the metal surface and prevents it from corrosion. However, effluent produce from electroless plating industrial contains several heavy metals such as nickel, chromium, zinc, copper, cadmium and lead [4]. Electroless plating effluent was chosen in this research because it contains several types of metals compare to other industries. Hydroxide precipitation method does not suitable for mixed metals effluents because it cannot remove multiple metals at single pH because each metal hydroxide has different pH to precipitate. Hydroxide precipitates tend to resolubilize if the solution pH is changed. Sulphide precipitation has advantages over hydroxide precipitation. It can remove multiple metals at single pH due to the lower solubility of metal sulphide over a broad pH range. However, sulphide precipitation is still not used as widely because the production of hydrogen sulphide (H2S) gas. In this study, we will focus on zinc, copper and nickel removal. Two samples are tested in this study. Samples are obtained from two different electroless plating companies. First sample is acid rinse from electroless copper plating process in Company A while second sample is nickel rinse from electroless nickel plating process in Company B. First, optimum pH and coagulant dosage for both samples are determine using jar test method. The optimum pH for acid rinse sample and nickel rinse sample are pH 8 and pH 10 respectively. At the optimum pH, the turbidity value is the lowest. The optimum coagulant dosage for acid rinse sample using hydroxide precipitation and sulphide precipitation are 0.4 ml and 0.6 ml respectively. The optimum coagulant dosage for nickel rinse sample using hydroxide precipitation and sulphide precipitation are 0.6 ml and 0.4 ml respectively. Sludge production for acid rinse sample using hydroxide precipitation and sulphide precipitation are both 0.02 g. Sludge production for nickel rinse sample using hydroxide precipitation and sulphide precipitation are 0.01 g and 0.02 g respectively. Sulphide precipitation remove metal more efficient. For acid rinse sample, 95.32% of nickel is remove using sulphide precipitation while 76.66% removal using hydroxide precipitation. 100% of copper is remove for both hydroxide and sulphide precipitation. For nickel rinse sample, 93.75% of zinc is remove using sulphide while 68.75% removal using hydroxide. 65.79% of nickel is remove using both sulphide and hydroxide. While 100% removal of copper for both sulphide and hydroxide precipitation.