When it comes to spent pickle liquor, steel makers need to embrace innovative solutions
to reduce environmental impact & increase efficiency
By NIRMALYA DEB
Steel manufacturing involves a range of interlinked chemical processes. The end product is a composite of successive stages of chemical pre-treatment for removal of impurities, right from raw materials processing to rolling, pickling and colour coating. Economies of scale call for a high level of operational efficiency and the aim of technological innovation is 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 galvanising. 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 a 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 oxidised 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 lustre to the metal. At the end of the pickling line the strips are recoiled.
Tenova, which offers tried-and-tested technology packages for the mines and metals industries, owns the Tenova Italimpianti brand specialising in downstream operations such as reheating furnaces and strip processing lines. The company’s continuous, semi-continuous and push-pull pickling lines are designed with advanced technological solutions.
Tenova supplies high-capacity and fully automatic continuous pickling lines for carbon steel featuring capacity of up to 2,200,000 tonnes per year and process speed of up to 350 metres per minute.The continuous pickling lines for carbon steel ensure accurate strip and pickling control, stable processes and precise strip tracking. These lines are equipped with a fully automatic coil handling system. “The innovative process equipment consists of a tension leveller or scale breaker for more efficient pickling and strip shape improving,” informs Giovanni Astengo, R&D Manager, Strip Processing, Tenova in an exclusive chat with Steel360. “The plastic or steel rubber lined pickling tanks of shallow design ensure high efficiency and reliability and the multi-cascade rinsing system is an added advantage,” he says.
“Tenova’s push pull pickling lines provide a capacity of up to 750,000 tonnes per year. Some of the most important advantages of such pickling lines are the extra wide production flexibility, process speed of to 180 metres per minute and single or double entry sections. The latest improvements to have been incorporated are optimal scale removal, low-waste plant, turbulence technology, simple acid re-circulating systems, online acid management system,” says Astengo. The lines are equipped with simple but state-of-the-art automation providing for online acid control and with a scale breaker for more efficient pickling. Tenova is able to combine push-pull and semi-continuous pickling lines into one solution called wide range pickling line which optimises the advantages of the two line types providing high productivity and pickled quality for both thin and thick gauges. In the push-pull mode the line satisfies requirements for high strip thickness and strength ranges.
“The strip passes from uncoiling sections directly to the process tanks and then to the trimming machine and the tension reel without passing through the looper and relevant bridles (as in conventional push-pull lines). For thinner gauges the line is used in semi-continuous mode and coil ends are joined by a stitcher without stopping the strip in the process section thanks to a horizontal looper,” says Astengo.
The main advantages of these pickling lines are:
a) Fully automatic process control with largely intuitive video pages;
b) Accurate control of effluents and fumes; and
c) Potentially zero waste in combination with an acid regeneration plant.
During the pickling process, large amounts of hydrochloric acid are used in order to remove impurities. Direct disposal of the contaminated acid is not only environmentally unfeasible but also depletes the quantity of fresh hydrochloric acid. Thus, to eliminate the need for and cost of disposal of spent acid and the cost of replacement of hydrochloric pickle liquor, the spent acid from the continuous pickling line is regenerated in the acid regeneration plant.
The waste pickling liquor is transferred to the acid regeneration plant through high pressure pipelines and is stored in a tank. Acid regeneration plants consist of four tanks: the first tank is stuffed with waste pickling liquor and the other three tanks contain water, alkaline solutions and oxygen.
“By a method known as pyrohydrolysis we can regenerate hydrochloric acid and recover a metal-free acid solution. We also obtain high-quality iron oxide either as pellets or fine powder which is used by various industries. Regeneration processes are, by definition, such processes by which the total amount of chlorides in a metal chloride solution is recovered. Pyrohydrolysis is a process occurring in the presence of water (oxygen) vapour above 300°C,” quips Astengo.
The most important application of hydrogen chloride (HCl) is the recovery of waste acid from steel pickling. Industrially, two regeneration processes are used: spray roasting and fluidised bed. Both are based on the same chemical reaction, but with different reactor design, while other process parts are practically identical. Both of them rely on direct heat transfer and the combustion of hydrocarbon fuels to maintain the required elevated temperature for evaporation and reaction in a reactor. The absorption and off-gas treatment is practically the same. The recovery rate is almost 100% of the total chloride content. Although energy consumption and investments are high compared to alternative processes, above 4.000 m³/yield of waste acid the technology becomes highly economic, adds Astengo.
Optimised Tech Solutions
“Tenova can add the acid regeneration process to its proven technologies in continuous or push-pull pickling lines and provide all customers with optimised, tailored and modern pickling lines combined with regeneration plants, with the aim to reduce environmental impact and increase the efficiency of the pickling process,” he says.
The fluidised bed acid regeneration technology permits the formation of sintered oxide at a high temperature forming granules from 0.2mm to 2 mm in size. The sintered granules are free of dust and contain no chlorides. They are easy to store and can either be recycled for steel production (e.g. in the electric arc furnace) or sold as a by-product for which there is high demand for such applications as production of ferrites, pigments, refractories and ceramics. The iron oxide so obtained is pure and achieves the best prices on the market.
“Using the spray roaster regeneration technology, at a lower temperature compared to the fluidised bed, oxide is obtained as powder. Many years of experience provide Tenova with specialised know-how in spray roaster technologies to provide optimised and updated solutions to customers,” claims Astengo.
Tenova acid regeneration plants are fitted with the state-of-the-art BLUEdriven Technology including the ZEROWaste Process. The relative advantages include a unique automation algorithm for higher stability in operation and for reduction of energy consumption, low emissions and optimised consumption values, upgrade availability for future extensions and long term after sales service and support. With close to 50 installations in 26 different countries, the acid regeneration plants guarantee acid recovery efficiency and optimised operation by reducing maintenance time and enhancing oxide quality. Thane-based Tenova India Pvt. Ltd. supplies the regeneration plants to steel mills in India.