Water Management and Pollution Control in Steel Plant
Water Management and Pollution Control in Steel Plant
Water is a crucial utility which is needed for the production of iron and steel in a steel plant. Cheap and plentiful, water was for centuries a production utility which the steel industry took for granted. But in the present scenario, water resources are becoming increasingly scarce due to the growing imbalance between fresh water availability and consumption, hence the access to clean and safe fresh water has become one of the major challenges of the modern society.
Water demand is keeping on increasing because of (i) increase of the population and migration to drought prone regions, (ii) rapid industrial development and increasing water use per capita, and (iii) climate change leading to changing weather patterns in the populated areas. This has resulted into steel industry entering into a new water-constrained era. Further, in the last three decades, concerns about environmental pollution have increased around the world and this has resulted in the promulgation of more restrictive environmental regulations. Since there is a threat of shortage in water resources in future, steel industry has changed its approach to water strategy and policy towards the reduction in fresh water consumption.
As fresh water availability and quality represent major concerns, water management is an important challenge faced by steel plant management and this is being faced for improving the sustainability of the production cycle. Water, like steel, can be reused and recycled. However, the increase of water recycling, after cleaning and cooling, can decrease the water quality. Hence the steel plant is to continuously improve the cleaning technologies for the reduction of water consumption. Furthermore, the water-related challenges (i.e., availability, seasonal shortage, competition with other users) depend on the plant location, it is important to have a local approach and a tailor-made solution. The life cycle of water is shown in Fig 1.
Fig 1 The life cycle of water
A large quantity of water is used in an integrated steel plant. However, the quantity consumed is not high, since most of the water is reused or returned to the source. Most of the consumed water is evaporated and around 90 % (on average, 88 % in an integrated plant and 94 % in an EAF-based plant) of the water is discharged after cleaning and / or cooling and frequently used by other utilities. The water which is not consumed in the steel plant is cleaned, cooled, and returned to the source. Water returned to rivers and other sources is frequently cleaner than when extracted at the intake pump.
A steel plant incorporates a number of processes right from raw material preparation to rolling of finish steel. All these processes need water. When the water is used in these processes, it gets contaminated with pollutants in various amounts and concentrations. Typical requirement of water in the various processes in steel plant ranges widely between steel plants and can range from 20 cubic meters per ton of crude steel to 150 cubic meters per ton of crude steel. Pumping capacities for these water quantities are needed for the functioning of various processes in the steel plant. The water consumption is considered as the water intake minus the water discharge. The overall water consumption depends on the water treatment and recirculation facilities in the steel plant and normally is in the range of 1.5 cubic meters per ton of crude steel to 4 cubic meters per ton of crude steel.
A steel plant uses huge quantities of fresh water for a variety of usage which includes cooling, dust suppression, cleaning, temperature control (heat treatment), transport of waste materials (ash, sludge, and scale etc.), and other usages. Water is an essential part of some of the steel plant processes such as water addition for controlling the moisture content of the coking coal blend, pelletizing of sinter mix, making of green pellets during the production of iron ore pellets, production of steam and hence power, and granulation of blast furnace slag etc. Use of large amount of water also generates considerable quantity of wastewater which is to be treated before its discharge.
Since large quantity of water is used in the steel plant, any technological improvement in the cooling technology leading to a small percent of reduction in the consumption of water, has a potential to spare a big quantity of water in the natural environment and minimize the water footprint. It is important to seek for ways and technologies, which allows for the minimization of the water consumption in the steel industry.
Water footprint is a new concept which allows quantification of fresh water appropriation. The water footprint methodology was introduced by Hoekstra as an indicator of fresh water appropriation, with the aim to quantify and map indirect water use and show the relevance of involving consumers and producers along the supply chains in the water management.
Water management strategies help steel plant to recover, reuse, and / or recycle water, which can in turn generate considerable cost savings by minimizing the demand for intake water and by reducing discharge volumes. The water management in an integrated steel plant primarily depends on the local conditions, availability of water, and regulatory requirements.
In recent times, water management has become one of the most important parts of sustainable steel. Management of water sustainably is very important for the steel plant management because of water’s essential value to society. It is necessary for the steel plant management to take water management seriously. The management is required to constantly evaluate how best to use water, and find improvements both in its conservation and its reuse.
As In the case of steel, water can be reused and recycled. This improves efficiency of the water use and reduces its demand as well as the cost. By increasing the water recycling and cascading water use from higher to lower quality, the use and consumption of water can be reduced considerably at the steel plant. However, it is necessary to consider the potential increase in energy use before implementing any water reuse management system.
The most effective way to reduce water intake is by reusing it. This normally involves cleaning and cooling water flows between each use. Some of these treatments, such as cooling, need large amounts of energy and can lead to increased rates of water consumption due to higher evaporation. The additional processes needed are always nearly in conflict with objectives to reduce energy consumption which means higher emissions of carbon di-oxide. Hence, it is necessary to assess the effectiveness of water reuse in an integrated manner, taking all environmental aspects into consideration.
Hence energy efficiency is very important aspect in the water management in the steel plant. Water reduction measures are to be coupled with the use of the energy efficient pumps and heat recovery facilities. Energy efficient pumps means less energy needed for water extraction, water treatment, and water circulation. Further, water usage efficiency is to consider actual consumption i.e. difference in intake and discharge (of the same or better quality), as well as availability aspects and influences on other resource categories of the steel plant such as energy. Also, the reuse of water in a high-temperature industry such as a steel plant can increase water consumption.
The water consumption in the steel plant varies very widely and is dependent on several factors such as (i) availability of water, (ii) technology employed, (iii) age and condition of the plant and equipments, (iv) types of the production processes, and (v) plant operating procedures. The three basic factors which are crucial for efficient water management are (i) quality of make-up water, (ii) climatic conditions at the location of the steel plant, and (iii) regulatory requirements.
The main water related issues which have effects on the water management in a steel plant are (i) quality of process water needed (ii) quality of wastewater, (iii) recycling of water and minimizing of its consumption, (iv) pollutants in the water, (v) implementation of new technologies for water management, and (vi) implementation of wastewater treatment technologies and their cost-effectiveness.
Since due to the increased consumption, additional energy needed, and the need to treat / dispose of generated by-products, it is crucial to have a holistic approach taking all environmental aspects into account when considering the introduction of different water reduction and treatment technologies with the aim to get zero effluent discharge. Hence, it is crucial to have a holistic view of the overall impact on water resources. Fig 2 shows a holistic approach to water management.
Fig 2 Holistic approach to water management
Uses of water
Water has many uses in the iron and steel plant and the quality of water needed for each use also vary. Further there can be one through systems and there can be systems with either partial or full recirculation of water.
Most of the water in the iron and steel plant is used for cooling, to protect equipment, and to improve the working conditions at the workplaces. A smaller, but still considerable, quantity of water is used as process water. A small amount of water is used as boiler-feed water and as domestic and service water.
Water is used not only in cooling operations, but also for other purposes. There are different qualities of water needed to meet the water requirements at various places in the plant. Further, fresh water is mainly used for processes and direct and indirect cooling, while seawater is normally used for once-through cooling after an anti-fouling pretreatment. The use of freshwater is normally considered as a performance indicator. In direct cooling, water comes in contact with the material and equipment while in indirect cooling water does not come into contact with the material or equipment.
Water from sea is mainly used for once-through cooling without pre- treatment or post-treatment. It does not come into contact with the material or equipment. Fresh water is mainly used for processes and cooling. It comes into contact with the material and equipment and is treated before reuse or discharge. The important uses of water in an integrated steel plant are given below.
- Water is needed to meet the process needs. Examples are adjustment of moisture in coal blend, wet quenching of coke, addition in sinter mix for pelletization in pelletizing drum, spraying of water at blast furnace (BF) top for controlling top temperature, granulation of liquid BF slag, pig casting, continuous casting, descaling of steel before hot rolling, on line heat treatment and control of temperature in hot rolling mills and adjustment of strength of pickling acid etc.
- Water is used for conditioning of the materials for the ease in handling and transport. Examples are transport of scale in continuous casting and hot rolling, addition of water in raw materials for dust suppression during their transport and storages, making of slurry for pipeline transport, transport of fly ash to ash pond and its storage there etc.
- Various type of gases generated during different processes are cooled, cleaned, and conditioned with water.
- Water is used for heat transfer. Various equipments, rolls, and shell of furnaces are cooled by water. Both direct and indirect cooling methods are employed for this purpose.
- Production of steam for process needs and for power generation purpose needs demineralised water.
- Water is used for miscellaneous cooling such as furnaces and refractories are cooled with water before taking up of repairs and / or relining. Metal and slag spillages are cooled with water before their transportation.
- Water based mortars, runner mass and clays need water in measured quantity.
- Sewage and water treatment processes need addition of water for efficient treatment and better reaction. Water is also needed for neutralizing of acidic effluents.
- Water is used for cleaning and washing of shop floors so that settled dust on the shop floors can be cleaned.
- Electric control rooms and tunnels are ventilated with moist air for which water sprayers are used.
- Water is used for fire fighting purpose in a steel plant. Water is also used for drinking water, cooler, canteens and wash rooms.
- Steel plants are to maintain greenery inside the plant for the purpose of environment. This activity needs water.
Water flow diagram and the terms used for water
The water flow diagram in a steel plant is shown in Fig 3. The terms used for different types of water in the diagram are described below.
Fig 3 Water flow diagram of a steel plant
Intake water – It is the water pumped from the water body at the intake pump house and added to the water system of the steel plant.
Make-up water – It is the water which is added to the water system. Make-up water is frequently needed in systems that use recirculating water. It is that water which is added to the system to compensate for the water (i) lost in the process, (ii) lost by the evaporation, and (iii) lost due to the leakages.
Process water – It is the water which comes into contact with an end product or with materials incorporated into an end product. Process water broadly includes streams which make contact with the raw materials or end products of steel production, or which becomes a part of the final product. There are many process water applications, including physical separation of ore constituents, quench hardening, descaling, galvanizing, and plating. Process water is also a component of solvents, acids, and emulsions used for cleaning, degreasing, and rinsing steel surfaces.
Cooling water – It is the water which is exclusively used for cooling. Cooling water refers to streams which are used to dissipate excess heat from steel products and associated equipments. Cooling applications account for the bulk of water used in a steel plant, and include contact cooling in hot and cold rolling processes, off-gas cooling and containment, cooling of ovens and furnaces, and other applications.
The amount of water drawn and discharged by a facility varies significantly depending upon the type of cooling system used. Once-through cooling systems have the highest demand for water, while the recirculating cooling systems retain nearly all water for repeated cooling cycles.
Demineralized water – Demineralized water is that water where are the minerals of the water has been removed. Demineralized water is also used as boiler feed water. The demineralization process is normally done when the water is to be used for chemical processes and the minerals present can interfere with the other chemicals. With the demineralization process, the water is softened by the removal of the undesired minerals. Demineralized water has a higher conductivity than deionized water.
Boiler feed water – Demineralized water is used as boiler feed water. Boiler feed water is used to generate process steam used to power many processes involved in steel production. Boilers are used to produce energy for process heating and cooling, as well as powering mechanical drive systems and pressure control systems. Boilers need high water quality standards to ensure safe function under their typically high temperature and pressure operating conditions. With proper pre-treatment, however, water from various sources can be recycled for use as boiler makeup water.
Fire fighting water – It is the water used for extinguishing of fires in the steel plant.
Soft water – Soft water is that water which contains low concentrations of ions and in particular is low in ions of calcium and magnesium. The term is used to describe water which has been produced by a water softening process. Soft water can also contain elevated levels of sodium and bicarbonate ions. Soft water produces no calcium deposits in water heating systems and indirect water cooling system.
Effluent water – It is the wastewater discharged from the water system. Effluent water from a plant is discharged to a waste disposal system. However, within a plant, effluent from one unit can be used as intake for another unit. In most places the amount of water consumed is very difficult to determine and is normally considered to be the difference between intake and effluent or is estimated by the plant operator. In once-through systems, effluent normally is not measured.
Domestic water – It is also known as sanitary water and used for drinking and in wash rooms.
Service water – It is the water used for general shop floor cleaning, suppression of dusts, and flushing of wastes etc.
Other uses of water – Other uses of water are those uses of water which are not covered under above headings. Examples of other uses are water used in ventilation systems, transportation of waste materials such as slurries, and gardening etc.
Pollutants in effluent water
Water management in an integrated steel plant is very important for its successful operation since the integrated steel plant needs a large amount of water for its operation. During the operation of the plant this water gets polluted. It is very difficult to separate water pollution control from water management in the plant. A good water management leads to a decrease in pollution control of water while at the same time pollution control of water needs good water management. Good water management requires zero or minimum water discharge from the plant. This is achieved by incorporating elaborate water treatment systems for recirculation of water. This reduces water consumption since there is a reduction in the requirement of make-up water. It also reduces the water pollution since the water discharge from the plant boundary becomes nil or negligible. Further it reduces the necessity and cost of treatment of polluted water before its discharge from the plant boundary. The following pollutants are present in the effluent water.
- Colloidal and suspended solids – Ore and flux fines, scale, sand, fly ash, coal and coke fines etc.
- Temperature and heat – Cooling water, boiler blow down etc.
- Oil – Rolling mill oils, lubricants, hydraulic oils, quenching oils, fuel oils, solvents, tar and pitch etc.
- Chemicals – These consist of pickling liquor, acid sludge, caustic wash, lime, spent ammoniacal liquor, boiler blow down and toxic chemicals etc.
- Sanitary wastes – Waste from canteen, wash rooms, change rooms and toilets etc.
Discharge of effluent water without treatment causes pollution of the water bodies at the place of discharge and hence adequate treatment of effluent water is necessary before its discharge from the steel plant. The following are some of the major methods normally used for treatment of effluents.
- Settling tanks – A conventional settling process in rectangular or circular tanks is used for sedimentation of suspended solids. Considering the outputs, flocculation is sometimes used. The withdrawal of sludge is done by means of pumps or by means of a crane with grab.
- For removal of suspended solids of very fine size and varying size thickeners, clarifiers and clariflocculators are used individually or in combination. Coagulants such as alum, ferric chloride, ferric sulphates, and poly-electrolytes are used for coagulation of the suspended solids.
- For removal of heat and reduction of temperature. Hot water is treated in cooling pond, spray basin or cooling towers.
- Neutralization – Waste water pH value is to be controlled between6.5 to 7.5. For this acid water with low pH values are treated with alkalis while the alkaline water with high pH values are treated with acids.
- Effluent containing oils – For collecting oils from effluents, conventional process of natural floatation and skimming is used. The collection of skimmed oil is done by mobile troughs.
- Waste water from coke ovens with high biological oxygen demand (BOD) are treated either in trickling filters or by activated sludge process. Activated sludge process with extended aeration is more effective in reduction of BOD values. For proper microbial decomposition, coke oven waste water needs supplementation of phosphorus to make it balanced for nutrients. If raw sewage is available then addition of this sewage helps in activated sludge process.
A typical flow sheet for the treatment of sludge is shown in Fig 4.
Fig 4 Flow sheet for treatment of sludge