Water Management and Water Purification in Iron and Steel Works
Iron and steel works, from the viewpoint of water management, can still roughly be classified into "wet" and "dry" works. In the first category, usable water is available in unlimited quantities, which means that, dependent upon the quality of the water, the consumption may approach the usage, the latter amounting to 150—200 m3/t of ingot steel, excluding power generation, for a modern integrated works. "Wet" works can only be found alongside rivers and freshwater lakes, being the only situations affording a "once-through" waterhousehold. "Dry" works have to re-use their water to a very high degree, resulting in a water consumption of only 2.5-3 m3/t of ingot steel, being roughly 2% of the usage needed. In extreme cases, these figures may be even lower. Many intermediate situations occur. Coastal works, for instance, still often apply seawater of brackish water for once-through cooling or gaswashing in cases where the resulting corrosion problems can be mastered at acceptable costs. There is, however, a clearly recognizable trend towards re-use of cooling water and process water in the steel industry. The main reason for this development arises from the need for environmental care. As in all process industries, decrease of water usage and water consumption improves the technical and economical possibilities for cleanup of the water and favours reduction of the thermal load. Under the pressure of national and international regulations against water pollution, an economical balance has to be struck. It seems logical that the outcome tends to decreasing differences in methods of water management between "wet" and "dry" works. Neither river basins and lakes, nor even sea basins and coastal waters offer the possibility of accepting the contamination caused by human activities unlimited. Although the iron and steel industry does not belong to the biggest water-polluters among the process industries, there are some typical contaminants needing attention; cyanides, oil and some non ferrous metals being the most important. Zero pollution of surface water is technically not feasible in the iron and steel industry. It can, however, be approached to an extent that does not seriously impede with ecological conditions in the receiving water. Waste technology has to be further developed to treat the various sludges resultingfromwastewater purification in order to create economic feasibility as well.