2.9.1 Effluent-Free Pickling Process with Fluid Bed Hydrochloric Acid Regeneration
This pickling process is operated such that no wastewater is discharged from a hydrochloric acid pickling line. Spent pickle liquor is fed via a settling tank and venturi loop into the fluidized bed reactor. The fluidized bed consists of granulated iron oxide. Residual acid and water are evaporated at 850°C and the iron chloride is converted to hydrochloric acid gas. Growth and the new formation of iron oxide grains in the fluidized bed are controlled so that a dust-free granulated product is obtained. Because the fluidized bed process operates at approximately 850°C, rinse and scrubber water from the pickle line is used at the regeneration plant to cool fluidized bed off-gases, which contain hydrochloric acid vapor and a small amount of iron oxide dust. The off-gases are cooled to approximately 100°C in a venturi scrubber. The thermal energy of the off-gases helps to concentrate the pickling liquor by evaporation before it is fed to the reactor.19
From the venturi scrubber, the cooled gas stream goes to the absorber, where hydrogen chloride is absorbed with rinsewater from the pickling line and fresh water to produce hydrochloric acid. The acid is recycled directly to the pickling process or placed in a storage tank for later use. Having passed through the scrubbing stages and mist collector, the fluidized bed off-gases are virtually free of hydrochloric acid and are released to the atmosphere.
Nitric-acid-free pickling requires the same equipment as conventional acid pickling processes and is also compatible with acid regeneration. This technology uses a nitric-acid-free solution that contains an inorganic mineral acid base, hydrogen peroxide, stabilizing agents, wetting agents, brighteners, and inhibitors.19
Wet air pollution control (WAPC) devices are used to treat exhaust gases from stainless steel pickling operations, thereby generating wastewater, which are treated using the selective catalytic reduction (SCR) technology in which anhydrous ammonia is injected into the gas stream prior to a catalyst to reduce NOx to nitrogen and water. The most common types of catalysts are a metal oxide, a noble metal, or zeolite.
2.9.4 Elimination of Coke with Cokeless Technologies
Some cokeless technologies in use or under development include the Japanese direct iron ore smelting (DIOS) process, in which molten iron is produced directly with coal and sinter feed ore, the HIsmelt process, where ore fines and coal are used to achieve a production rate of 8 t/h using ore directly in the smelter, and the Corex process, which has an integral coal desulfurizing step, making it amenable to a variety of coal types.14
These technologies in use or under development reduce the quantity of coke needed by changing the method by which coke is added to the blast furnace or by substituting a portion of the coke with other fuels, thereby reducing coking emissions. Pulverized coal injection substitutes pulverized coal for about 25 to 40% of coke in the blast furnace. Nonrecovery coke battery allows the combustion of the gases from the coking process, thus consuming the byproducts that are usually recovered. The Davy Still Autoprocess is a precombustion cleaning process in which coke oven battery process water is utilized to strip ammonia and hydrogen sulfide from coke oven emissions. Another option involves the use of alternative fuels such as natural gas, oil, and tar/pitch instead of coke into the blast furnace.14
Various treatment technologies are used at the iron and steel plant for recycle system water treatment prior to recycle and reuse, or end-of-pipe wastewater treatment prior to discharge to surface water or a POTW. The physical/chemical treatment technologies extensively used include equalization, tar removal, free and fixed ammonia stripping, cooling technologies, cyanide treatment technologies, oily wastewater treatment technologies, carbon dioxide injection, metals treatment technologies, solids separation technologies, and polishing technologies.
Ammonia stripping also removes cyanide, phenols, and other VOCs typically found in coke-making wastewater. Phenols may also be removed by conversion into nonodorous compounds or into crude phenol or sodium phenolate by either biological means (phenol concentration <25 mg/L) or by physical processes.21 However, the Koppers dephenolization process is considered to be quite effective as it lowers the phenol content by 80 to 90% in ammonia still wastes. In this process a stream stripping process followed by mixing in a solution of caustic soda results in renewal of pure phenol with the flue gas.8
Blast furnace, vacuum degassing, continuous casting, and hot forming operations use cooling methods in recirculation systems. Byproduct recovery coke-making plants commonly use cooling prior to biological treatment systems, because high temperatures are detrimental to the biomass. Cyanide treatment technologies include alkaline and breakpoint chlorination using sodium hypo-chlorite or chlorine gas in a carefully controlled pH environment to remove cyanide and ammonia. In cyanide precipitation, cyanide combines with iron to form an insoluble iron-cyanide complex that can be precipitated and removed by gravity settling. Ozone oxidation results in the conversion of cyanide to cyanate. Oily wastewaters from hot forming and cold rolling operation are treated by gravity flotation, oil/water separation, emulsion breaking, followed by dissolved air flotation and ultrafiltration.21-23 Carbon dioxide injection is one method of removing scale-forming metal ions that accumulate in water recirculation systems from BOF recycle water.
Strong reducing agents such as sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate are used in the iron and steel finishing sites to reduce hexavalent chromium to the triva-lent form, which allows the metal to be removed from solution by chemical precipitation.21-23 Metal-containing wastewaters may also be treated by chemical precipitation or ion-exchange.
Solid wastes, including scale, biosolids, precipitate from cyanide and chemical precipitation systems, and solids from filtration backwash may be treated using scale pits, classifiers, clarifiers, and the microfiltration technique.19,24,25 Polishing technologies include multimedia filters following clarification to remove small concentrations (<20 mg/L) of entrained suspended solids, or carbon adsorption to remove trace concentrations of organic pollutants remaining in coke-making wastewater following biological treatment. Biological denitrification (anaerobic) can be used to treat coke-making wastewater following biological nitrification. Steel mill sludge thickening and dewatering may be accomplished using gravity thickeners, rotary vacuum filters, centrifugation, sludge drying, belt and pressure filters. However, it has been identified that rolling mill sludges are not amenable to vacuum filters and centrifuges.9,24,25
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