Water Management and Pollution Control in Steel Plant
Water Management and Pollution Control in Steel Plant
An integrated 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 various processes in steel plant needs 100 to 200 Cum per ton of steel. Pumping capacities for this amount of water is needed in the steel plant. The requirement of makeup water depends on the water treatment and recirculation facilities in the plant and usually is in the range of 2 to 4 Cum per ton of steel.
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 makeup 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.
Use of water
Water has many varied uses in the iron and steel plant and the quality of water needed for each use also vary. Further there may be one through systems and there are systems with either partial or full recirculation of water. 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 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 required 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.
Terms used for water
- Process water – It is the water that comes into contact with an end product or with materials incorporated into an end product.
- Cooling water – Water used exclusively for cooling.
- Boiler feed water –It is demineralised water used in boilers for conversion into steam.
- Fire fighting water – It is the water used for extinguishing of fires
- Intake water – It is the water added to a system.
- Make up water – It is the water which is added to the system to compensate for the losses.
- Effluent water – It is the waste water taken out of the system.
- Domestic water – It is also known as sanitary water and used for drinking and wash rooms.
- Service water – It is the water used for shop floor cleaning and gardening and other such uses.
Pollutants in effluent water
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 will cause pollution at the place of discharge and hence adequate treatment of effluent water is necessary before its discharge from the steel plant.
Processes for effluent treatment
The following are some of the major methods commonly 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 withdrawl 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, 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 between 6.5 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 avilable then addition of this sewage helps in activated sludge process.
A typical flow sheet for water pollution control is given in Fig 1
Fig 1 Typical flow sheet of water pollution control