The process of steel production depends largely on water. Owing to its high specific heat capacity, water is one of the most desirable mediums for heat transfer in industrial processes.
Huge quantities of water are used in steel and rolling mills either indirectly or directly to cool the furnaces, descaling the hot-rolled products, spraying on the cold-rolled products, and many more purposes.
Since water is easily available at a very low cost, these mills depend heavily on water-intensive processes. However, the industry recognizes the need of managing water, and a lot of it is returned to the source.
So, how do steel and rolling mills manage cooling water? What are cooling towers in steel plants? Let’s dive right into it, shall we?
Direct Cooling Water Circuits
In hot strip mills, water is sprayed directly onto the hot-rolled coils or plates with high pressure while they move for descaling. It is also sprayed to cool the bearings and rollers.
The hot return water is heavy with lubricating oil and mill scale. This water is drained into scale pits after use. Mills do not waste all of this water; it is recycled.
Mill scales are sent to sintering plants for recycling to the blast furnace. Here, the water is filtered for further use.
The overflow with the fine mill scale and oil is sent to a clarifier or a sand filter to eliminate any suspended solids.
The filtered and clarified water then gets recirculated back to the system via a steel cooling tower. Since a significant amount of water is lost due to direct evaporation, the tower operates with low cycles of concentration.
Indirect Cooling Water Circuits
Let us break down how a continuous castor at a steel mill functions. First, molten metal is poured into a ladle and cast into slabs, blooms, or billets via a tundish along with an adjustable discharge device and into a water-cooled copper mold.
It is the shape of this mold that defines the shape of the steel. Intensive cooling processes then solidify the metal completely.
Water is used here for the sprays and cooling the molds. This water needs to be demineralized or soft, equipped with a copper corrosion inhibitor. It is recycled into the system in a closed circuit.
The heat from the system is removed through a secondary cooling circuit with a steel cooling tower. Moreover, filtered and clarified water is added as make-up to the secondary cooling water circuit.
The water that is used for direct spray has to be completely free of suspended particles. Failure to ensure so might result in clogging of the sprays, leading to unequal surface cooling. Unequal cooling can cause rejection of the material and might seriously damage the equipment.
On the other hand, the water used for spraying is gathered from the bottom trough to decant the oil and get rid of scales. The overflow is then sent to the closest clarifier and filter before returning to the steel cooling tower.
Other Uses of Water in Steel and Rolling Mills?
Here are a few other instances where cooling water is required in steel and rolling mills.
Coke Oven and By-Product Plant
Coke ovens and by-product plants require coke mostly for indirect cooling. The moisture of the coking coal, which is used for coke production and condensation, makes the by-product process a significant generator of process water.
Water is used in sinter plants to control the moisture content in the pre-sinter mix, ensure sinter product cooling and dust control.
Vacuum Degassing Unit
In a vacuum degassing unit or a gas cleaning system, a vacuum is created by adding steam through multi-point injectors, which leads to an atmospheric condenser. When the steel exhaust emissions come in contact with the steam, the particulate gets wetted and is collected in the condenser water.
Water is clearly an indispensable part of steel and rolling mills. However, it is a precious resource that needs to be recycled.
Mills do their bit by ensuring that the water used in their processes is cooled down via steel cooling towers and released for further use.
If the mills want to remain competitive and continue making profits, they cannot discard the recycling process. Numerous improvements are being made to improve how efficiently steel and rolling mills manage their cooling water.