Water is used in every shop of a steel plant and for practically all the functions of water. A steel plant cannot function without water. Because of this steel plants are normally built near to ample sources of fresh water to ensure the availability and quality of water needed by the steel plant. However these days, greater attention is given to the management of available water resources in the steel plant environment, particularly in terms of water quality, quantity, and how it is used.
A steel plant uses large quantity of water for steam generation, cooling, waste transfer, and dust control etc. The processes of the plant cannot take place without the availability of water. Enormous quantity of water is needed at every stage of production. Less than 10 % of this water is actually consumed and balance water is usually is returned to the system.
Several factors make water a versatile material. It is normally easy to handle, readily available, and inexpensive. It can carry large amounts of heat per unit volume (high specific heat). It neither expands nor compresses significantly within ambient temperature ranges. It does not decompose. It can dissolve, entrain, suspend, and subsequently transport other materials.
In spite of the tremendous importance of water in the steel plants (Fig 1), the way water is used is not standardized as are the steel production processes and there is no ‘one size fits all’ strategy or technology to use water in each particular context. There are a number of aspects related to water and water related technologies which are important for a steel plant. These are given below.
- Water is a medium for heat transfer and thus is related to energy efficiency
- Importance of water sourcing, in terms of quality and quantity
- Treatment of water, its recycling and recirculating systems
- Water-related costs
- Quality of water depends on the process where it is used
- Trends for discharge of water effluents (chemical oxygen demand, suspended solids, biological oxygen demand etc.) are evolving towards more demanding standards
- Water softening needs are to be optimized on a process by process basis
Fig 1 Importance of water in a steel plant
Water usage can generate specific health and safety problems, such as legionella proliferation, as well as environmental problems
Water is used for direct contact cooling and cleaning of the steel in process, for cooling the process off-gases, for product rinsing, and for process solution make-up etc. A large quantity of water is used for indirect non-contact cooling of processing equipment. Water is also used for steam and power generation, potable uses, and dust and moisture control. In fact, in a steel plant, water carries out a number of functions. Major functions of water in a steel plant are given below.
- Most of the water used in a steel plant is for cooling. Examples are coke quenching, cooling of blast furnace shell, oxygen lance in converter, cooling in continuous casting machines and hot rolling mills. Virtually any hot spot is cooled mainly by using water, one way or another.
- Water is used for cleaning off-gas, in coke ovens, blast furnaces, and basic oxygen furnace etc.
- Water is used for descaling in continuous casting and in hot rolling operations. It is also used as conveying media for transporting scales to the scale pit.
- Water is used for on line heat treatment (thermo mechanical treatment) in the hot rolling mills.
- Water serves as a resource to produce steam.
- Water is also used as part of chemical treatments, such as solvent in acidic pickling, matrix for generation of emulsions for rolling, cleaning, degreasing or rinsing steel sheet surface, etc.
- Water is used for electrochemical treatments, such as electro-galvanizing or tin plating etc.
- Water is used for dust suppression
Depending on the use and the quality of water, water carry different terminology in different places of the steel plant These are (i) raw water, (ii) intake water, (iii) rain water, (iv) treated water, (v) process water, (vi) recirculating water, (vii) direct cooling water, (viii) indirect cooling water, (ix) boiler feed water, (x) DM (demineralized) water, (xi) soft water, (xii) distilled water, (xiii) make up water, (xiv) water condensate, (xv) drinking water, (xvi) fire- fighting water, (xvii) domestic water, (xviii) contaminated water, (xix) phenolic water, (xx) waste water, and (xxi) water effluent etc.
Raw water is that water which is available in the natural sources such as (i) surface or ground water sources (river, lake, reservoir, and sea etc.) and (ii) underground water sources. Sea water is having high salinity and need treatment like reverse osmosis etc. for its use in the steel plant. Ground water is also required to be tested since it may be brackish. Raw water collected from sources other than sea also needs treatment before it can be used in the steel plant. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water. Water after treatment is called treated water.
Intake water is the water which is pumped at the intake pump house located at the raw water source. Factors determining the location of intake pump house are as follows.
- As far as possible, the site should be near the treatment facilities usually situated in the steel plant so that the cost of conveying water to the plant is less.
- The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.
- The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.
- The site should be such as to permit greater withdrawal of water, if required at a future date.
- The intake must be located at a place from where it can draw water even during the driest period of the year.
- The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
Rain water of the steel plant is harvested these days and pumped to the treatment plant. This helps in the reduction of the intake water and in turn helps in the conservation of the natural resources.
Process water is used in some processes for the purpose of process moisture control, process temperature control, and for steel heat treatment. Process water is also used for granulation of liquid blast furnace slag. Another use of process water is for pickling, cleaning, and coating operations in the cold rolling mills.
The major use of water in the steel plant is for the purpose of cooling. Cooling water is generally classified as either non-contact or indirect cooling water and contact or direct cooling water. Indirect cooling water application is the use of water for general cooling purposes where only equipment is being cooled and the water does not contact the material in process, off gases or fluids. Direct cooling water includes all applications of water where water comes in direct contact with material in process, process gas cleaning applications, process fluid applications (oils, cleaners, etc.) and rinse applications. Direct cooling waters are subject to treatment for recycling.
Boiler feed water is used for steam generation in a boiler. Raw waters must be pretreated to meet the water quality requirements needed by the boiler for steam making. Such treatment typically involves filtration, hardness removal (softening) and/or silica reduction, deaeration, and pH/alkalinity adjustment. If improperly conditioned, water-related problems, including mineral precipitation and corrosion, can result in permanent damage to the steam generating unit along with energy inefficient operation.
DM water is that water where are the minerals of the water has been removed. DM water is also used as boiler feed water. The demineralization process is usually done when the water is to be used for chemical processes and the minerals present may interfere with the other chemicals. With the demineralization process, the water is softened by the removal of the undesired minerals. DM water has a higher conductivity than deionized 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 that has been produced by a water softening process. Soft water may also contain elevated levels of sodium and bicarbonate ions. Soft water produces no calcium deposits in water heating systems and indirect water cooling system . Water that is not soft is referred to as hard water.
Distilled water is that water which is produced by distillation process that is first evaporating the treated water and then condensing the water vapours. Distilled water is the purest form of water and normally used in laboratories for the wet chemical analysis of the materials.
Make up water is frequently required 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.
Water condensate is that water which is produced during the condensation of steam in an industrial application. If there is no contamination of water condensate then the water condensate can be directly added to the boiler feed water.
Drinking water is that water which is used in various production shops and canteens of the steel plant for drinking purpose. The quality of drinking water is to be maintained at the level which is fit for human consumption.
Fire-fighting water is the most common fire-extinguishing agent. The quality of water required for fire-fighting purposes is much lower than the drinking water. The quality of water required for the fire-fighting has never been adequately defined but an obvious requirement is the need to limit the concentration of suspended material to a size that will pass through the various constrictions and orifices found in fire service pumping equipment.
Domestic water is the water which is used for washing and in wash rooms. In steel plants usually drinking water is used as domestic water.
Contaminated water is the water that gets contaminated with dissolved minerals and suspended solid particles after its use.
Phenolic water is generated in the by- product plant of coke oven battery and has substantial quantity of dissolved phenol in it. In the steel plant with wet quenching, this water is used for quenching of hot coke. In plant with dry quenching, this water is treated in mechanical, biological, chemical treatment plants before its discharge.
Waste water is the water which comes out of the direct cooling process. This water is usually treated in the water treatment plant and is recycled back after adding the make-up water.
Water effluents are those waters which cannot be recycled back even after treatment. Such water is discharged from the plant boundary after correcting its quality to the levels required as per the statutory norms. These days normally steel plants aim for zero discharge of water effluents. These plants normally treat water effluents to enhance its quality levels so that it can be used for gardening inside the plant or can be used in raw material storages for dust suppression.