Acid Pickling Line

Overview

An acid pickling line cleans the surface of hot-rolled steel strip before it goes to a cold mill or a coating line. When steel comes off the hot strip mill it carries a layer of mill scale, a hard oxide skin of wustite, magnetite and hematite that forms while the steel cools in air. Scale is abrasive and uneven, so it must be removed before the strip can be cold rolled to gauge or given a clean coating. The Acid Pickling Line dissolves that scale in hot hydrochloric acid, rinses the strip clean, dries it and rewinds it as a finished coil.

The line is continuous. Individual coils are welded head to tail so the strip runs through the acid as one unbroken ribbon, which keeps every part of the strip in the bath for the same time and avoids the streaks a stop-start process would leave. To stay continuous while coils are loaded and unloaded, the line uses strip accumulators at each end that store a long loop of strip and pay it out while the reels are stopped.

Entry and coil joining

A coil is set on the Coil Loading Car and loaded onto the Expanding Mandrel of the Uncoiler, which pays the strip off under back-tension from its Servo Motor. The Peeler Unit breaks the outer lap loose and the Preparation Pinch Roll feeds the head into the Coil Joining Welder. There the Crop Shear crops both coil ends square, the Weld Head joins them with a flash-butt or stitch weld, and the Weld Bead Trimmer dresses the bead flat so the joint runs through the rolls cleanly.

Once welded, the strip enters the Entry Accumulator. Its Accumulator Carriage carries a moving set of Accumulator Roll rolls that lengthen the stored loop while the line runs and shorten it to feed the process section when the reel stops for the next coil. The Entry Bridle sets the tension going into the scale breaker, and the Entry Edge Guide keeps the strip centered using its Edge Position Sensor feedback.

Scale breaking

Before the acid, the strip runs through the Scale Breaker. This is a tension leveler whose small Scale Breaker Roll rolls bend the strip back and forth over a short arc. Mill scale is hard and brittle while the steel underneath is ductile, so the bending cracks the scale into a dense network of fine fractures without harming the steel. Each slender bend roll is held against deflection by a Scale Breaker Backup Roll in the Scale Breaker Cassette, and the Breaker Intermesh Adjuster sets how deeply the rolls penetrate to control the bend severity. Cracking the scale lets the acid get under it and float it off, which can cut pickling time by half compared with attacking an intact layer.

Pickling

The Pickling Section is the chemical heart of the line. The strip passes through four shallow Pickling Tank tanks of hydrochloric acid held at 80 to 85 C, with free acid around 150 to 200 g/L. The tanks are shallow and the strip runs just below the surface, held down by Submerged Roll rolls. A Turbulence Nozzle Header header pumps acid across the strip face at high velocity so fresh acid always reaches the metal and the dissolved iron is swept away, which is what makes the reaction fast in a shallow turbulent bath rather than a deep still one.

Acid attacks steel, so the Pickling Tank is built with a Acid-Resistant Lining of soft rubber and acid brick over its Tank Shell. Acid cascades counter-flow from tank to tank over the Cascade Overflow Weir so the strip meets progressively fresher and stronger acid as it advances. Each tank carries a sealed Tank Cover with a Cover Seal Strip at the strip slots to contain the corrosive fumes, and a Bath Heater using a graphite Bath Heat Exchanger holds the bath temperature with indirect steam so the acid is never diluted. The Pickling Bridle pulls the strip through the tanks at steady tension.

Rinsing and drying

A pickled strip leaves the last tank wet with acid and iron chloride, so the Rinse Section washes it clean. Three Rinse Tank stages run counter-flow, the cleanest water meeting the strip last, and between stages a Wringer Roll Pair pair squeezes the water film off with its Squeegee Roll rolls so very little carries forward. The strip then runs through the Hot Air Dryer, where blowers and a Air Knife drive hot air across both faces to leave it dry, which matters because any trapped moisture would flash-rust the bare steel within minutes.

Acid recovery and fume control

Behind the tanks sits the Acid and Fume System. The Acid Recirculation Pump units, built with a Pump Casing and Pump Impeller in fluoropolymer, circulate hot acid from the tanks through the Acid Heat Exchanger and back through the nozzles. Fresh and spent acid are held in the Acid Storage Tank tanks, with spent acid routed off for regeneration where the iron chloride is roasted back to acid and iron oxide. The corrosive air drawn off the covered tanks runs through the Fume Scrubber, a packed column where scrubbing liquor washes the HCl out before the Fume Exhaust Fan vents the cleaned air, and a Mist Eliminator strips the last droplets.

Trimming, exit and control

After drying, the Side Trimmer slits both edges to final width and the Scrap Chopper cuts the trim into short scrap, then the Strip Oiler lays a thin rust-preventive film. The strip enters the Exit Accumulator, which lets the recoiler stop to divide coils, and the Dividing Shear cuts the strip so the Recoiler can drop a finished coil and start the next with its Belt Wrapper. The whole line is coordinated by the Drive and Control System system: a master Line PLC holds speed and tension matched across every section through the Tension Control Unit unit and its load cells, while the Process Monitor logs acid concentration, bath temperature and line speed against each coil. Everything in the wet section is carried on the corrosion-resistant Machine Frame and Platforms, whose Tank Support cradles and Maintenance Platform gratings are built to survive constant acid mist.