Recycling Cooling Water Protects The Environment

The mini mill of Slovakia Steel Mills in Strážské is designed to produce around 600,000 tonnes of structural steel per year. It incorporates the complete process chain consisting of an electric arc furnace, ladle metallurgy facility, a three-strand continuous casting plant and a rolling mill. Steel production requires large quantities of treated water for direct and indirect cooling, so the central Siemens treatment plant has to supply some 7,000 cubic metres of treated water every hour. The technical equipment includes cooling towers, pumping stations, pressure sand filters, metering stations and sludge treatment facilities.

Heavily polluted water returning from the direct cooling system of the casting plant and rolling mill is first fed into a cyclone separator to remove scale. Oil is then removed by a skimmer and the remaining solids are filtered out by downstream pressure sand filters. The temperature of the water is then lowered by 10 to 15°C in a cooling tower before it is fed back into the cooling systems. The less heavily polluted water from the secondary cooling system passes through bypass filtration. This reduces the concentration of suspended solids before the water is pumped into the cooling towers.

The backwash water from the sand filters, still contaminated with sludge, then passes through the sludge treatment plant. Suitable polymers are added to the sludge to make it easier to separate the solids from the water in a lamella separator. The pre-clarified water passes through a sand filter again, after which it can also be fed back into the cooling system. The remaining sludge is thickened and dewatered in a filter press. Part of the metalliferous sludge can be recycled back into the production.

The entire water treatment plant is automated by Simatic type S7 300 programmable logic controllers. Visualization is provided by a WinCC HMI system. Siemens is also supplying the process instrumentation equipment for monitoring the water quality. This includes instruments to measure flow rates, pressures, temperatures and levels. All the pump drives are equipped with frequency converters, enabling the pumps to be run according to demand and thus saving energy.