The iron and steel industry needs to opt for technologies that help to either prevent or reduce the generation and discharge of process wastes. The various preventive measures to be adopted for reducing the environmental impacts are as follows:
1. Reduction of dust emissions at furnaces by covering iron runners and using nitrogen blankets during tapping of the blast furnace
2. Use of pneumatic transport, enclosed conveyor belts or self-closing conveyor belts, wind barriers and other dust suppression measures to reduce the formation of fugitive dust
3. Use of low-NOx burners to reduce NOx emissions from burning fuel in ancillary operations; use of dry SOx and dust removal systems in flue gases
4. Recycling of iron-rich materials such as iron ore fines, pollution control dust, and scale in a sinter plant
5. Recovery of thermal energy in the gas from the blast furnace before using it as a fuel; increasing fuel efficiency and reducing emissions by improving blast furnace charge distribution; recovery of energy from sinter coolers and exhaust gases
6. Use of a continuous process for casting steel to reduce energy consumption
The processes used in manufacturing steel products use a significant amount of water, and wastewater minimization is necessary both in terms of water use and pollutant discharge loadings. These technologies achieve these reductions by retarding pollutant buildup and improving water quality to allow greater reuse; reducing the volume of wastewater treated and discharged; prolonging process bath life, enabling sites to spend less on process bath makeup and reducing bath treatment and disposal costs; and improving treated effluent quality by enhanced wastewater treatment.19 The various types of water minimization techniques are given in the following sections.
High-rate recycle systems consist of a water recirculation loop that recycles 95% or more of the water from a process for reuse. They are used for product cooling, cleaning, and air pollution control, in operations like blast furnace iron making, sintering, basic oxygen furnace steel making, vacuum degassing, continuous casting and hot forming operations. However, during the recycling operation a portion of the water is discharged to prevent concentration buildup of contaminants within the system. These blowdown streams are either treated at an end-of-pipe treatment system or discharged to surface water or publicly owned treatment works (POTWs). The high-rate recycle consists of solids removal devices, cooling devices, and water softening technologies to improve water quality prior to reuse. Improvement in the water quality helps to increase recycle rates significantly. This in turn decreases the pollutant loading, blowdown discharges rates, and the amount of fresh water added as makeup.
Countercurrent cascade rinsing involves a series of consecutive rinse tanks in which water flows from one tank to another in the direction opposite to that of the product flow. Fresh water flows into the rinse tank located farthest from the process tank and overflows to the rinse tanks closer to the process tank. Over a certain period of time, the first rinse becomes contaminated with dragout solution and reaches a stable concentration much lower than the process solution. The second rinse stabilizes at a lower concentration, which enables less rinsewater to be used compared to a one-rinse tank. The greater the number of countercurrent cascade rinse tanks, the less will be the amount of rinsewater needed to adequately remove the process solution. This differs from a single, once-through rinse tank where the rinsewater is discharged without any recycle or reuse. Countercurrent cascade rinsing is used in steel finishing operations, including acid pickling, alkaline cleaning, electroplating, and hot dip coating, as the steel needs to be relatively contaminant-free for processing. However, such systems have a higher capital cost compared to once-through rinsing systems and require more space. Also, the relatively low flow rate through the rinse tanks require the use of air or mechanical agitation for dragout removal.
2.8.3 Acid Reuse, Recycle, and Recovery Systems
Acid reuse, recycle, and recovery systems are extensively used in the acid pickling industry. Typical industrial acid reuse and recovery systems include the following:
1. Fume scrubber water recycle. The steel finishing industry uses fume scrubbers to capture acid gases from pickling tanks. Scrubber water, which may contain a dilute caustic solution, is neutralized and recirculated continuously to adsorb the acid. Makeup water is added to replace water lost through evaporation and water that is blown down to end-of-pipe metals treatment.
2. Hydrochloric acid regeneration. This process is used to treat the spent pickle liquor containing free hydrochloric acid, ferrous chloride, and water that is obtained from steel finishing operations. The liquor is concentrated by heating to remove some of the water, followed by thermal decomposition in a "roaster" at temperatures (925 to 1050°C) sufficient for complete evaporation of water and decomposition of ferrous chloride into iron oxide (ferric oxide, Fe2O3) and hydrogen chloride (HCl) gas.19 The iron oxide is separated for offsite recovery or disposal. The hydrogen chloride gas is reabsorbed in water (sometimes rinsewater or scrubber water) to produce hydrochloric acid solution, which is reused in the pickling operation.
3. Sulfuric acid recovery. Recovery of sulfuric acid takes place by pumping the spent pickle liquor high in iron content into a crystallizer, where the iron is precipitated (under refrigeration or vacuum) as ferrous sulfate heptahydrate crystals. The water is removed as the crystals are formed and the free acid content of the solution increases to a level usable in the pickling operation. The byproduct ferrous sulfate heptahydrate, referred to as "copperas," is commercially marketable as a coagulant used for water and wastewater treatment. The Blow-Knox-Ruthner process may also be used for sulfuric acid recovery. In this process, the waste liquor is concentrated by evaporation and discharged to reactors where anhydrous hydrogen chloride gas is bubbled to react with the ferrous sulfate, producing sulfuric acid and ferrous chloride. Ferrous chloride is then separated from the sulfuric acid (which is returned to the pickling line) and converted to iron oxide in direct fired roaster. This is followed by the liberation of HCl, which may be recovered by scrubbing, stripping, and recycling to the reactor.8
4. Acid purification and recycle. This technology is used to process various acid pickling solutions such as the sulfuric acid and nitric/hydrofluoric acids used in stainless steel finishing mills. Acid is purified by adsorption on a bed of alkaline anion exchange resin that separates the acid from the metal ions. Acid is desorbed from the resin using water. The metal-rich, mildly acidic solution passes through the resin and is collected at the top of the bed. Water is then pumped downward through the bed and desorbs the acid from the resin. The purified acid solution is collected at the bottom of the bed and recycled back to the process. Acid purification and recycle reduces nitrate discharges and the overall volume of acid pickling wastewater discharged.
Prolonging the life of solutions reduces the additional investment of fresh process solutions and time spent replacing spent process solutions. The technologies to extend process solution life are as follows:
1. In-tank filtration. Steel finishing electroplating and alkaline cleaning operations use in-tank filters to extend process bath life by removing contaminants in the form of suspended solids. Solids are usually disposed of offsite. Devices such as granular activated carbon filters remove dissolved contaminants, such as organic constituents.
2. Magnetic separation of fines in cold-rolling solution. Magnetic separators are used to extend the life of cold-rolling solutions. The most effective systems use vertical or horizontal configurations of magnetic rods to remove metal fines.
3. Evaporation with condensate recovery. With this technology, steel finishing mills can recover electroplating chemicals such as chrome, nickel, and copper that are lost to electroplating rinsewater. There are two basic types of evaporators: atmospheric and vacuum. In a vacuum evaporator, evaporated water can be recovered as a condensate and reused on site. Generally deionized water is preferred as rinsewater to prevent unwanted contaminants from returning and accumulating in the electroplating process bath in addition to the dragout.
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