From threat to an asset: water in steelworks. How modern steelworks can improve water related performance via benchmarking and development of High Density Sludge (HDS) Process

The Water Framework Directive (WFD) 2000/60/EC is set to overhaul the management of the water environment within the EU. Following its enforcement in 2015, changes are expected to the current water related regulations and water intensive industries, including steelworks, ought to prepare themselves for changes. In 2007 Corus Group was taken over by Tata Steel, now one of the World’s top 10 steel producers with its production of 31 MTPA (million tonnes per annum of crude steel). Tata Steel Port Talbot Integrated Steelworks is one of Tata Steel’s main sites, currently producing some 4.33 MTPA (in 2007) of crude steel (slab) and is a major user of water with its 8 production facilities and supporting functions. From 2007 to 2011 the author worked as a core member of the World Steel Association Water Management Project. The project included development of a survey to gather water-related data from the World’s steelworks. 29 steelworks took part in the survey and using the data, an extensive assessment of water related performance in steelworks around the World has been carried out. The findings show that water performance related figures, including water use and effluent generation, vary from under 1 to near 150 m3/ts. The average consumption figure being 28.4 m3/ts with once-through cooling using an average 82% of this water. The average effluent discharge figure is 25.4 m3/ts. For Port Talbot Steelworks these figures are 33.8 m3/ts and 28.8 m3/ts respectively. An investigation into effluent treatment technologies and efficiencies included carrying out chemical precipitation and co-precipitation titration experiments, especially looking at zinc, in order to better understand the behaviour of relevant metals during hydroxide precipitation reactions. The experimental results were compared against PHREEQCi theoretical geomodelling precipitation prediction data and PHREEQCi 2 indicated minimum zinc solubility is received at pH 9.5. Laboratory experiments support this. Iron enhances zinc precipitation strongly via co-precipitation. A similar effect, although to a lesser extent, is achieved for zinc co-precipitation with nickel and lead. The author’s study of the Port Talbot water systems established that the chemical precipitation processes in operation leads to the generation of voluminious sludge that is hard to dewater further. This prompted the initiation of an investigation into the suitability of the High Density Sludge (HDS) process in treating high volume, non-acidic low metal concentration effluents, such as steelworks final effluent. Prior to this research the HDS process has been used mainly for the treatment of mine effluents and its suitability in treating non-acidic, low metal concentration effluent has not been fully explored. During the trial, a 10 L/h influent feed rate was aimed for with a half hour retention time at the first two reactors. The flocculant feed rate was around 2.5-3 mg/l of treated effluent throughout the trial. At the end of the trial the sludge concentrations exceeded 17% (w/w), while the treatment efficiencies of zinc and other metals stabilised and improved. Furthermore, the sludge was behaving as HDS sludge achieving high settling rates in excess of 22 m/h at 5% (w/v). Solids concentrations and sludge filterability had improved with the specific cake resistance reducing from the ‘single pass’ precipitation sludge near 35,000 Gm/kg to the 777 Gm/kg after 2 weeks of trial to a mere 169 Gm/kg at the end of the HDS trial