Steel manufacturing involves a range of interlinked chemical processes. The end product is a composite of successive stages of chemical pretreatment for removal of impurities, right from raw materials processing to rolling, pickling and color coating. Economies of scale call for a high level of operational efficiency and the aim of technological innovation is the valorisation of value – both in quantitative and qualitative terms.
Waste Acid: An Eco Hazard?
In steel mills, the hot rolled steel coil (HRC) requires pre-treatment before undergoing various other processes like cold rolling and galvanizing. Chemical pre-treatment is aimed at removing various oxide layers, rust or scale from the steel surface by passing the workpiece, as it is called, through a vat of pickle liquor. The pickling process generates large amounts of pickling sludge containing acidic rinse water, dissolved metal salts of iron, iron chloride, zinc and waste acid. The spent pickle liquor is considered hazardous waste and, therefore, environmental considerations – over and above economic concerns related to regeneration and reuse – stand in the way of instant disposal.
Pickling of carbon steel is a process in which a thin layer of oxidized iron, or scale, is removed from the surface of the steel. The reaction of alloying metals with oxygen at high temperatures during hot rolling leads to the formation of scales. Pickling involves placing the HRC on an uncoiler and the strip is pulled through the acid bath section, which comprises four tanks in a row containing hydrochloric acid, at a specific speed. The rubber scrubber scrubs the surface to remove the impurities. The strip is subsequently passed to a water tank where it is rinsed with water spray and air-dried, leaving a dull silver luster to the metal. At the end of the pickling line, the strips recoil.
Pickling Processes
This story is from the January 2020 edition of Steel 360.
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This story is from the January 2020 edition of Steel 360.
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