Industries, especially in the chemical sector that face the problem of getting rid of waste water, can look forward to some productive options. Thanks to an indigenous technology developed by scientists at the CSIR-IICT (Indian Institute of Chemical Technology), Hyderabad.
It’s a simple nanofiltration technique that helps recover water from industrial waste, which then could be recycled for specific uses. A set of low cost membranes ensure the separation of harmful chlorides, cyanide and water from the polluted /contaminated water generated in certain chemical industries. The super thin membrane is economical and the process is low pressure, which can substitute the regularly used Reverse Osmosis for specific applications, say the scientists of the IICT. They have demonstrated the technique in Tata Steel’s Jamshedpur plant last year.
In the manufacture process of steel, coke is an important ingredient. It is a solid carbon source used to melt and reduce iron ore. Coke production begins with pulverisation of bituminous coal, which is fed into a coke oven and heated to very high temperatures. After the coke is finished, it is moved to a quenching tower where it is cooled by spraying water. Once cooled, the coke is moved directly to an iron melting furnace for steel production.
The effluent from this quenching tower contains excess chloride and some cyanide, which needs to be removed before the water can be recycled. In January 2017, S Sridhar and his team from the IICT successfully installed and commissioned a nanofiltration pilot plant of capacity 5.5 M3/h capacity (1.1 lakh litres per day) for the removal of excess chloride from steel quenching tower effluent at Haldia, West Bengal.
The trials were performed at an applied pressure of 5 to 7.5 kg/cm2 pressure to achieve separation of chloride from 1500 mg/L in the effluent to an acceptable level of less than 400 mg/L with a water recovery of 75 per cent. In comparison, the Reverse Osmosis will not be able to achieve such high water recovery. The operating cost would be higher at 5 paise per litre for nanofiltration instead of 3 paise per litre.
Operation is full-fledged at the plant and its success could result in replication of similar plants of higher capacity (commercial scale of 150-200 M3/hr) for the steel industry to facilitate zero liquid discharge enforced by State pollution control boards, to minimise environmental contamination, says Sridhar.
Explaining the process, he says, during one of the critical steps of its manufacture, steel from blast furnace is quenched in a tower, which results in the release of excessive chloride and cyanide into the aqueous stream. Chloride levels above 800 mg/L cause corrosion in the blast furnace. These two issues pose a major challenge. The team developed a high flux, low fouling nanofiltration membrane, which provides high water recovery and sufficient chloride separation.
After the trial run for Tata’s on a laboratory scale, the design of a pilot plant was taken up under a sponsored project, for prospective installation at Tata Steel’s Haldia Metcoke Division. The capital investment for the pilot plant is ₹12 lakh and operating cost including power consumption, filter replacement, chemicals for maintenance among others costs only ₹20 per cubic meter
In times of scarcity of water not just for drinking but for industrial use, this is an encouraging news indeed.
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