Cleaner technology systems for surface finishing: evaporative coolers for close circuiting low temperature plating process

Abstract

Abstract In the electroplating process, the rinse system generates large quantities of wastewater requiring treatment prior to disposal to municipal systems. The use of conventional water treatment systems is a challenge due to the presence of hazardous components. In addition, this does not solve the problem of the generation of rinse wastewater, but only treats it. Thus the focus was on point-source reduction technologies, specifically the application of a three-stage low flow counter current rinse for recovery of the rinse water in the plating bath, enabling close circuiting of the plating bath rinse system. However, recovery of the rinse water into the plating bath is impeded by the low rates of evaporation, especially in the low temperature plating baths. Alternative methodologies to heating were investigated to facilitate evaporation, with evaporative cooling being identified as the most feasible option. Evaporative cooling facilitates evaporation, whilst maintaining the plating bath temperature within the operational limits. For the recovery of the rinse water in the plating bath, the rate of evaporation in the plating bath must be equivalent to the fresh make-up water requirements of the rinse tanks. The Closed Circuit Plating System (CCPS) model was developed to enable the proper design and/or implementation of an evaporative cooler; whereby the user specified inputs are evaluated in achieving the required evaporation rates for the recovery of the rinse water in the plating bath. The key characteristic of the CCPS model is the minimum requirement of proprietary plating solution specific information. The inputs for the model are chemical composition of the plating solution, flowrates, temperature and height of the cooling tower. The outputs from the model are evaporation rates and equilibrium temperatures of the plating bath and cooling tower. The primary limitation of the CCPS model is that it is based on an air–water system. Single and multiple variable sensitivity analyses were performed on the plating plant operational parameters to determine their influence on close circuiting of the rinse plating system: plating solution composition and operational temperature; ambient air temperature; air flow rate and the surface area of the packing in the cooling tower. The results from the model indicated the upper limit plating solution operational temperature, high air flow rates, low ambient air temperature and large surface area of packing facilitated water evaporation rates and lower equilibrium temperatures in the plating bath and cooling tower. The sensitivity analyses will allow the electroplater to optimise the operating conditions to achieve the required evaporation rates for recovery of the rinse water into the plating bath, while simultaneously maintaining the outputs of the electroplating plant and reducing the rinse wastewater generation to almost zero.