Storage Tanks for Acid. Hydrochloric acid can be stored in rubber-lined steel tanks or glass-fiber-reinforced polyester-resin tanks. Polyester-glass linings in steel tanks are not recommended, because permeation of acid through pinholes in the lining could result in attack on the steel. Hydrofluoric acid (70%) is usually stored in plastic-lined tanks, although low-carbon steel can be used at temperatures up to 38 °C (100 °F) if properly passivated. Concentrated nitric acid (94.2% or higher) is best stored in tanks made of 3003 aluminum. For less concentrated nitric acid, tanks should be of type 304L stainless steel (type 304 annealed stainless for nonwelded construction) or 15 to 16% high-chromium iron. Dilute sulfuric acid is more corrosive to iron and steel than more concentrated acid. A 93.2% sulfuric acid can be stored in iron tanks over a wide range of temperatures, but a 77.7% sulfuric acid must be stored below 38 °C (100 °F). A < 74.4% sulfuric acid cannot be stored in unlined iron tanks or carried through unlined iron pipes. A glass or phenolic lining, among others, can be used.

Batch Pickling Tanks and Auxiliary Equipment. Construction materials for pickling tanks include wood, concrete, brick, plastic, and steel. Acid-resistant linings provide protection for the outer shell of the tank and are commonly made from natural, pure gum, or synthetic rubber. Acid-resistant brick is used to line the sides and floor of the tank. Bricks are mortared with poured sulfur cement or an acid-resistant resinous cement. Drainage lines should be made of vitrified tile caulked with an acid-resistant cement. Figure 1 shows the materials used in constructing a 3.5 to 4.4 Mg (3.9 to 4.9 ton) capacity tank for the pickling of coiled steel. In a batch pickling operation, bar product to be pickled is placed in tiers on crates and racks made of an acid-resistant material, such as Monel metal. Coils of strip, rod, or wire are pickled by passing a C-hook or chain through the open center. The holders are raised and lowered by an overhead crane.

Fig. 1 Materials used in construction of 3.5 to 4.4 Mg (3.9 to 4.9 ton) capacity tanks for pickling coils of steel

After pickling, the product should be rinsed with high-pressure cold-water sprays to remove excess acid solution. It should then be placed in a hot-water rinse tank with sufficient overflow to ensure that the pH is not less than 5 or 6.

Heating Methods and Temperature Control. In the past, the most widely used method of heating pickling solutions was the direct injection of live steam through steam jets. Although steam so introduced does provide some agitation of the solution, the steam condensate dilutes the pickling solution, requiring higher amounts of acid to be added to maintain concentration. Steam sparging also increases the volume of spent pickle liquor. Better heating methods include coil steam heat, heat interchangers, and, for small installations, electric immersion heaters. Heating equipment must be made of acid-resistant materials, such as carbon, lead alloy, stainless steel, or zirconium for use with sulfuric acid. Carbon and polytetrafluoroethylene-covered heat exchangers should be used with hydrochloric acid. Acid-resistant indicating and regulating temperature-control instruments are available for pickling solutions. Heaters can be centrally located or, in the case of continuous pickling, placed at strategic intervals along the tank.

Continuous-Strip Pickling Lines. A few pickling lines make use of vertical towers in which one or two hydrochloric acid spray columns are used (Ref 14, 15). The acid spray columns are assembled and sealed in sections made of fiberglass-reinforced polyester, with a tower height of 21.3 to 45.7 m (70 to 150 ft). The tank sections are made from rubber-lined steel. After use, acid flows into a sump and is returned to the circulating tank. The composition of the acid in the recirculation tank is typically maintained at 11 g/100 mL HCl and 13% FeCl2. It is passed through a carbon-block heat exchanger and delivered to the sprays at 77 °C (170 °F). Most lines of this type have acid-regenerating facilities. Entry and exit coil handling are similar to the more common horizontal lines.

Continuous-strip pickling lines with horizontal pickling tanks are capable of handling coils that are welded head to tail. The entry section comprises a coil conveyer, one or two uncoilers, one or two processors, one or two shears, and a welder. Processors are integral with the uncoiling equipment and consist of a mandrel, hold-down roll, and a series of smaller-diameter rolls. As the strip is flexed through the processor, some cracking occurs in the scale layer, although not nearly as much as that imparted by a temper mill. Proper welding and weld trimming is essential to avoid strip breaks in the line. The section prior to the pickling tanks uses bridles for tensioning the strip; a strip accumulator, either in the form of wet looping pits or, for more modern lines, a coil-car accumulator; and, for many lines, a temper mill to crack the scale on the surface of the strip. A stretch leveler can replace the temper mill and not only effectively cracks the scale, but also contributes to superior strip shape.

The pickling section usually contains three or more tanks. So-called "deep tanks" are typically 1.22 m (4 ft) in depth and up to 31.3 m (90 ft) in length. Acid tanks are steel shells with layers of rubber bonded to the steel. The rubber is protected from abrasion by a lining of silica-base acid-proof brick. Most lines have a cascade flow of pickling solutions countercurrent to the direction of strip movement. When fresh acid is added to the last tank, it will contain the highest concentration of acid. Acid concentrations will decrease from the last tank to the first tank, from which the spent pickle liquor is discharged. A rinse section follows the pickling section.

An especially effective rinsing method used on many continuous lines is the cascade rinse system (Ref 16). Several rinse compartments are used, and fresh water is added to the last compartment. The solution in that compartment cascades over weirs into the preceding compartments. The excess overflows from the first compartment and is sent to the waste-water treatment plant (a portion can be used for makeup water in the pickle tanks). Each compartment contains less acid than the previous compartment. At the exit end of the line, there are usually an exit strip accumulator, steering rolls, a strip inspection station, dual side trimmers, an oiler, and two coilers. Pickling lines must have fume scrubbers to capture emissions/spray from the pickle tanks.

In some modern lines, the pickling solution is contained in shallow tanks with liquid depths of approximately 0.41 m (16 in.) and lengths up to approximately 36 m (118 ft). Although they involve a cascade system, the solution in each tank is recirculated through a heat exchanger. During a line stop, the pickling solution can be rapidly drained from shallow tanks into individual storage tanks and then pumped back when the line starts up. Lines with deep tanks usually have strip lifters provided to remove the strip from the acid solution during an extended line stop. Tank covers may be made from fiberglass or polypropylene. Some lines have squeegee rolls, covered with acid-resistant rubber, located above and below the strip at each tank exit to minimize acid carryover from one tank to another.

Maximum speeds in modern lines in the pickling section can be as high as 305 to 457 m/min (1000 to 1500 ft/min). Although sustained operation at such speeds is limited by other aspects of coil handling, the selection of pickling tank acid concentrations and temperatures must be such that complete scale removal is achieved during periods of high-speed operation. The combination of a pickling line and a cold reduction mill in tandem represents a new state of the art in continuous processing facilities (Ref 17). Another type of strip pickling line suitable for plants with moderate production requirements is the push-pull type, which has many of the features of the continuous-type lines, but no welder (Ref 18). Turbulent-flow, shallow-tank, continuous-strip lines that claim to provide more effective pickling action than conventional lines have been developed (Ref 19).

References cited in this section

14. Spray Tower Pickles Steel, Iron Age, Vol 190, 4 Oct 1962, p 78-79

15. D.E. Poole, Hydrochloric Acid Pickling of Steel Strip, J. Met., Vol 17, 1965, p 223-224

16. J.B. Hodsden and W.L. Van Kley, Cascade Rinse System at Inland's 63-in. Continuous Strip Pickling Line, Iron Steel Eng., Vol 51 (No. 3), 1974, p 49-53

17. N.L. Samways, Modernization at Pittsburg to Make USS-POSCO More Competitive, Iron Steel Eng., Vol 64 (No. 6), 1987, p 21-29

18. G. Kuebler, Pushing for Pickling Productivity, 33 Metal Producing, Vol 30 (No. 9), 1992, p 28-31

19. F.G. Pempera and F.W. Delwig, Turbulent Shallow-Type Pickling Lines, Iron Steel Eng., Vol 65 (No. 3), 1988, p 33-36

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