The liquid effluents from a coke plant in an integrated steel manufacturing facility usually contain ammonium cation, thiocyanate and cyanide anions, phenols and suspended solids. Cyanides, cyanates and thiocyanates are hazardous substances with a negative effect on living species and the environment. The generation of these pollutants is inherent to the manufacturing process, and the wastewaters containing them cannot be discharged without treatment, according to the current regulations. Chemical treatment, incineration and biological digestion are the most common treatment methods. However, when the thiocyanate ion concentration exceeds 400 ppm, the microbial activity is hindered, and thus, an additional chemical pre-treatment is needed. Incineration can also be used to treat concentrated thiocyanate effluents, but at a high cost.
Reverse osmosis (RO) has proved to be a feasible technology in the recovery of thiocyanate from the aforementioned condensates. Several experiments have been carried out at laboratory scale to choose a suitable membrane and to gather experimental data which would allow developing a model to design and scale up the filtration process. Commercially available RO spiral wound membranes were selected. The membrane showing the best behaviour was the so-called SW30-HR membrane from DOW FILMTECTM as it gave adequate values of rejection and permeability. The effect of process parameters has also been addressed and recommendations for plant operation have been proposed.
Onsite pilot plant runs have also been conducted in order to evaluate RO performance when operating longer term with the actual industrial stream. A pilot unit equipped with two 4” spiral wound modules were used. The results have been in close agreement with those obtained at laboratory scale.
A mathematical model which represents the process has then been proposed, validated and used to design a full scale plant which accomplishes the desired separation. Coking wastewaters effluents usually have thiocyanate concentrations between 1000 and 3000 ppm, but eventually, it is possible to have condensates that reach 9000 ppm. Therefore, several case studies and sensitivity analysis have been carried out based on both these typical and extreme values. An economic evaluation is also included.
Financial support by PCTI (RIA Project, PEST08, Principality of Asturias, Spain)
See more of this Group/Topical: Topical 1: Water Technology for Developed and Developing Countries (see also Separations Division)