Modelling of an ozonation process for cyanide removal from blast furnace gas-washing water and analyses of process behaviour in different scenarios

In the steel sector, the control of cyanide in the wastewater of the gas washing system of blast furnaces is one of the main issues related to the wastewater quality. Although established treatment processes exist, the relevant costs, the stringent environmental regulation and the extremely complex and variable composition of gas washing water lead to investigate modifications of current treatments and to develop new ones. A cyanide treatment based on ozonation is one of the proposed and experimented methods within the European project “DynCyanide”. Starting from experimental data, a process model was developed through Aspen Plus® v.8.6, in order to test a wide range of operating conditions and wastewater features and to prove the robustness of the proposed process. The paper focuses on the process modelling and simulation phases. Water stream and ozonation are modelled through a number of theoretical reactors, by considering main and collateral reactions and a genetic algorithm is applied in order to reproduce at best the kinetics of these reactions. The validated model was applied for scenario analyses. Process knowledge was enhanced and, after finding the parameters and the compounds that reduce the ozonation efficiency, suggestions were obtained to improve the treatment performances. A temperature of 30°C and an initial pH value of 10 favour the cyanide ozonation; the batch mode of the treatment allows the removal of cyanide after 4.5 hours (3 cycles) using a limited ozone amount. If the wastewater contamination level is higher, a further cycle or a higher amount of ozone are needed.