Pickling of Hot Rolled Strip of Carbon Steel in Pickling Lines
Pickling of Hot Rolled Steel Strip of Carbon Steel in Pickling Lines
Pickling is carried out in order to prepare (remove scale or oxides) the steel surface for the next process of cold rolling. The oxide scale is required to be completely removed from hot rolled steel strip before subsequent cold rolling to prevent wear on the rolls and to avoid surface defects in the cold rolled product. The thickness of the scale depends mainly on the processing parameters of the hot rolling mill, the main factor being the coiling temperature, although the rolling process itself also has a marked influence.
The oxide scale originates during the hot rolling of steel, when the surface of the steel reacts with oxygen (O2) in the air to form oxides of iron (Fe). The oxide layer is known as mill scale. The mill scale actually consists of three layers of iron oxides with different proportions of FE and O2 (Fig 1). Hematite, Fe2O3, which contains 30.1 % O2, is the outermost oxide in the scale layer, whereas wustite, FeO, with 22.3 % O2, is the innermost oxide. Magnetite, Fe3O4, which is the middle, contains 27.6 % O2. When all oxides are present, the middle layer in the scale is magnetite. At temperatures above 566 deg C, wustite is the predominant oxide, but during cooling below 566 deg C, a portion of it is transformed to iron and magnetite (4FeO = Fe3O4 + Fe). In cases of rapid cooling, which can occur with rod and bar rolling, substantial amounts of wustite are retained in the cooled product. When cooling after hot rolling is relatively slow, as it is with coiled strip, magnetite is the main oxide constituent of the scale in the cooled product.
Fig 1 Scale layer on the surface of hot rolled strip
Pickling is the most common of several processes used to remove the scale from steel surfaces. The term pickling refers to the chemical removal of scale by immersion in an aqueous acid solution. The process originated in the late 1700s, when sheets of steel were descaled by immersion in vats of vinegar. Wide variations are possible in the type, strength, and temperature of the acid solutions used, depending on time constraints (batch against continuous operations), as well as the thickness, composition, and physical nature (cracks) of the scale. Fig 2 shows major inputs and outputs of the pickling process.
Fig 2 Major inputs and outputs of the pickling process
The surfaces of hot rolled steel strip and pickled steel strip are shown in Fig 3.
Fig 3 Surfaces of hot rolled steel strip and pickled steel strip
For carbon (C) steel, sulphuric (H2SO4) acid is used in most batch pickling operations, whereas hydrochloric (HCl) acid has become the pickling agent of choice, as of 1994, for continuous operations with hot rolled strip. Pickling with HCl acid started in 1964 and slowly many picking installations have switched over to HCl acid pickling. Mixtures of HCl and H2SO4 acids have also been used in batch pickling, often by adding rock salt (NaCl) to a H2SO4 acid pickling bath. Such practices are expected to give the bright, pickled steel surface characteristics associated with HCl acid and to increase pickling rates, but not without some drawbacks. The proportion of HCl to H2SO4 acids which is needed to achieve the rapid scale removal rate that is possible with HCl alone is too high to be economical, and the mixed acids cannot be properly handled by many of the spent pickle liquor disposal methods now in use.
The advantages of H2SO4 acid pickling are (i) acid can be renewed more frequently, (ii) raising temperature allows lower acid concentrations to pickle effectively, (iii) ease of recovering iron sulphate (FeSO4), and (iv) the rate of pickling can be controlled by varying the temperature. The disadvantages of H2SO4 acid pickling are (i) greater acid attack on base steel, (ii) greater H2 diffusion into the steel, (iii) pickling residues are more adherent, and (iv) acid solution is to be heated.
The advantages of HCl acid pickling are (i) reduction in heating costs since pickling solutions are used at room temperature, (ii) more extensive removal of scale, (iii) lesser penetration of H2 by diffusion, and (iv) lesser deposition of Fe salts on the pickled surface. The disadvantages of HCl acid pickling are (i) generation of fumes when heated above ambient temperatures, (ii) acid recovery systems are expensive, (iii) more corrosive toward equipment, and (iv) higher disposal costs than H2SO4 acid. Tab 1 shows the comparison of pickling by H2SO4 acid and HCl acid in continuous pickling lines.
|Tab 1 Comparison of pickling by H2SO4 acid and HCl acid|
|H2SO4 acid||HCl acid|
|1||Weight of output strip||tons||0.99||0.98|
|5||Make up acid||kg|
|6||H2SO4 (25 % solution)||85|
|7||HCl (17 % solution)||140|
|9b||Sulphate in spent acid||kg||18|
|9c||Chloride in spent acid||kg||22|
Acids other than HCl or H2SO4 have been used to remove rust and scale from carbon steel. Citric acid, oxalic acid, formic acid, hydrofluoric acid, fluoboric acid, and phosphoric acid are all capable of removing mill scale from steel, but the rates of removal are generally not regarded as useful or economical for most commercial applications, especially continuous operations.
The mechanism of scale removal
Pickling by mineral acids involves the penetration of acid through cracks in the scale, followed by the reaction of the acid with the innermost scale layer and base steel. The presence of hydrogen (H2) gas, which forms when the acid reacts with the base steel, and the dissolution of FeO help the detachment of the outer scale layer from the steel surface.
The reactions of H2SO4 acid with FeO or with scale which is substantially Fe3O4 mixed with Fe form ferrous sulphate (FeSO4) and water are given by the equations (i) FeO + H2SO4 = FeSO4 + H2O, and (ii) Fe3O4 + Fe + 4H2SO4 = 4FeSO4 + 4H2O. The reaction of H2SO4 acid with base steel forms FeSO4 and H2 gas as per the equation Fe + H2SO4 = FeSO4 + H2 (g).
In case of pickling by H2SO4 acid, the acid finds its way to the steel surface through the cracks in the mill scale and dissolves the surface iron. The process forms H2 bubbles. The scale is loosened by the H2. The dissolving of scale in the acid is a slow process and hence it falls down in the bath and slowly gets dissolved. Both the reactions of pickling are exothermic reactions but does not compensate for the heat loss associated with the heating of the cold strip and heat losses to the surrounding atmosphere and hence bath heating is needed. The heating is done by steam. Since the pickling is carried through attack of acid on steel, the chances of over pickling are high in the process. The product of pickling is FeSO4 which is green in colour and is generally recovered during the regeneration of the H2SO4 acid. H2SO4 acid pickling is dependent on concentration and temperature of the acid. The pickling rate goes up proportionately as the concentration of the acid is increased from 0 % to 25 %. Above 25 % the increase in the pickling rate is slow. The effect of temperature on the pickling rate is that the pickling rate is doubled for each rise of 6 deg C to 8 deg C between temperatures of 25 deg C and 95 deg C.
With HCl acid, removal of scale primarily involves direct attack on the oxides. However, the penetration of acid through cracks in the scale does contribute to the scale removal process, although the magnitude of the effect resulting from enhanced scale cracking is somewhat less than it is with H2SO4 acid. The reactions of HCl with FeO or with scale which is substantially Fe3O4 mixed with Fe form ferrous chloride (FeCl2) and water are given by the equations (i) FeO + 2HCl = FeCl2 + H2O, and (ii) Fe3O4 + Fe + 8HCl = 4FeCl2 + 4H2O. The reaction of HCl acid with base steel forms FeCl2 and H2 gas as per the equation Fe + 2HCl = FeCl2 + H2 (g).
The pickling by H2SO4 acid produces satisfactory results when used for batch pickling of C steel rod and wire (upto 0.60 % C) and for continuous cleaning, if the Fe concentration in the bath is less than 8 grams per 100 cubic centimeters (g/100 cc). Commercial H2SO4 acid is normally supplied at a concentration level of 93 %, whereas HCl acid is supplied at concentrations of 31 % or 35 %. An advantage of using H2SO4 acid is less fuming over pickling solutions. Disadvantages include darker surfaces and the production of dirt, particularly on high C steel, as well as a greater inhibiting effect on the H2SO4 acid of Fe salts in the bath.
Emissions from H2SO4 acid pickling can include a spray (droplets of pickling solution resulting from acid attack on base steel which generates H2 gas). Hence, adequate ventilation is to be provided to prevent localized corrosion of equipment and unsatisfactory working conditions.
The pickling by the HCl acid is preferred for the batch pickling of hot rolled high C steel. Continuous pickling operations also use HCl acid to produce the very uniform surface characteristics required for both low C steel and high C steel. The possibility of over-pickling is minimized in these short time operations.
Operating conditions for batch pickling in HCl acid solutions typically involve acid concentrations of 8 g/100 cc to 12 g/100 cc, temperatures of 38 deg C to 40 deg C, and immersion times of 5 minutes (min) to 15 min, with a maximum allowable iron concentration of 13 g/100 cc. In the pickling with the HCl acid normally a chemical inhibitor is used to reduce the attack of the acid on the base steel.
HCl acid offers a number of advantages, when compared with H2SO4 and other acids. It consistently produces a uniform light-gray surface on high C steel. The possibility of over pickling is less. Effective pickling can be obtained with Fe concentrations as high as 13 g/100 cc. Rinsing is facilitated because of the high solubility of chlorides. The main disadvantage of HCl acid is the necessity for a good fume control system. Emissions from HCl acid pickling include HCl gas which is to be adequately vented to prevent localized corrosion of equipment and unsatisfactory working conditions.
The rate of pickling is affected by several variables, including the base steel constituents, the type of adherence of oxides, acid concentration and FeSO4 or FeCl2 concentration in the solution, temperature of the solution, agitation, time of immersion, and the presence of inhibitors. Pickling rate increases as acid concentration or temperature increases. As pickling continues, free acid (H2SO4 or HCl depletes and the Fe salt builds up in the pickle liquor to an extent that pickling cannot be accomplished effectively and the quality of the treated steel surface deteriorates. At that point, the pickle liquor is discharged from the pickling tank to a storage tank, and the pickling tank is replenished with fresh acid solution. Acid transfer is done either continuously or in a batch mode.
Excessive contamination of the pickling bath by oiled steel results in non uniform pickling and staining of the steel. To avoid this problem, oiled steel is to be degreased before pickling. When pickling either oiled or degreased steel, the use of a wetting agent in the acid solution increases the effectiveness and efficiency of the bath, thereby reducing immersion time. Many commercial pickling inhibitors are formulated with a wetting agent.
Pickling lines often report acid and Fe salt concentrations in weight/ volume (w / v) units of g/100 CC. Although these units are sometimes loosely referred to as ‘percent’, concentrations in g/100 cc is to be divided by the density of the solution in g/cc to convert to true weight percent (weight/weight, or w/w, units). For this purpose, approximate equations for calculating densities have been developed from published data on H2SO4-FeSO4 solutions and on HCl-FeCl2 solutions, These equations are D = 0.9971 + (0.00633) x C(H2SO4) + (0.0099) x C(FeSO4) and D = 0.9971 + (0.00446) x C(HCl) + (0.00815) x C(FeCl2), where D is expressed in g/cc at 25 deg C and the concentrations C(H2SO4), C(FeSO4), C(HCl), and C(FeCl2) are expressed in g/100 cc.
Inhibitors are added to acid pickling solutions in order to (i) minimize acid attack on the base steel with excessive loss of Fe, (ii) avoid pitting associated with over-pickling, which contributes to poor surface quality, (iii) reduce acid solution spray resulting from H2 which forms when acid attacks steel, (iii) lower acid consumption, (iv) minimize the risk of H2 embrittlement. When used at appropriate concentrations, inhibitors do not appreciably affect the rate of scale removal. A number of additives have been used in pickling solutions to inhibit acid attack on steels. Natural products, such as bran, gelatin, glue, byproducts from petroleum refining and coal coking, and wood tars were initially used. Modern inhibitors are largely formulations of wetting agents with mixtures of active synthetic materials, including nitrogen (N2) base compounds (pyridine, quinidine, hexamethylene tetramine, and other amines or polyamines), aldehydes and thioaldehydes, acetylenic alcohols, and sulphur (S) containing compounds such as thiourea and thiourea derivatives.
Frequently, two or more active ingredients provide a synergistic effect, whereby the mixture is more effective than the additive effect of the individual components. A good inhibitor is not to show ’breakout’, which is sludge that deposits on the work, a characteristic of many of the natural products formerly used. It is to be stable at the temperature of the pickling bath and is not to emit offensive odours. Modern inhibitors used with H2SO4 acid often contain thiourea or a substituted thiourea with an amine. Most of the newer inhibitors which have been developed for use with HCl acid contain amines or heterocyclic N2 compounds as active ingredients. In H2SO4 acid pickling, the FeSO4 buildup in a worked pickling bath also inhibits the activity of the acid and reduces the effectiveness of the solution for cleaning and brightening the steel. Most steels are reactive with acid and require inhibited solutions.
Steels with high phosphorus contents (0.03 % or above) are particularly prone to over-pickling. Inhibited acid solutions are generally used in continuous strip lines to clean the internal surfaces of pipes. Although the immersion times during continuous strip pickling are substantially shorter than in batch operations, an excessive loss of base steel occurs during a line stop, if inhibitors are not used. This is not only being objectionable because of the roughened over-pickled surface, but also because of the effect on critical final gauge requirements of the product.
Additions are best made proportional to the acid additions to pickling tanks or to the acid volume in large storage tanks. A poor method of introducing inhibitor to pickling solutions is by adding inhibitor to the bath at certain time intervals which are not related to actual acid additions. Before inhibitor additions are made, the bath is to be under inhibited, and just after additions are made, the bath might be over inhibited.
It is generally agreed that the primary step in the action of inhibitors in acid solutions is adsorption onto the steel surface. The adsorbed inhibitor then acts to retard the cathodic and/or anodic electrochemical processes of the corrosion. When inhibitor concentrations are much below recommended levels, the adsorbed layer of inhibitor on the steel surface can be incomplete, which can result in preferential attack on unprotected areas.
For plain C steels containing less than 0.40 % C, and for batch pickling baths which contain 10 % to 14 % H2SO4 acid (specific gravity 1.82) and operate at 70 deg C or higher, strong inhibitors are used at concentrations of 0.25 % to 0.50 % raw acid in the tank. When the concentration of FeSO4 reaches 30 %, the solution is to be discarded, since this level of Fe salt slows down the pickling process and can cause dirt to form on the surface of the product. When Fe levels approach this concentration in batch pickling with HSO4 acid, further additions of inhibitor is not required. Plain C steels containing 0.40 % C or more are pickled in similar baths with somewhat lower temperatures (60 deg C to 66 deg C) and with FeSO4 concentrations of less than 20 %.
With HCl acid, strong inhibitors are used at concentrations of 0.125 % to 0.25 % of raw acid. Because pickling rates in both H2SO4 acid and HCl acid tend to decrease when the pickling solution contains high levels of Fe (higher levels are tolerable with HCl), especially when coupled with low acid concentration, commercial pickling bath additives, or accelerators, are sometimes used to enhance pickling rates. These proprietary materials are usually formulated with inhibitors to prevent excessive base-metal attack by the acid during scale dissolution.
Uninhibited acid solutions are often used for pickling high-alloy steels, because more chemical action is required to remove the oxide. If an inhibitor is used when pickling alloy steels, concentrations which are somewhat less than those recommended for plain C steels are suggested.
Continuous strip pickling lines
Continuous strip pickling lines with horizontal pickling tanks are capable of handling coils which are welded head to tail. The heart of the pickling line is its acid baths which generally consists of three or four tanks in a row and which contains the pickling acid. The entry section comprises a coil conveyor, one or two uncoilers, one or two processors, one or two shears, and a welding machine. 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. Proper welding and weld trimming is essential to avoid strip breaks in the line.
The continuous pickling line needs the coils to be joined together, head to tail. To achieve this, hydraulic shears at the entry end cut a section of the strip from each end of the coil, squaring up the ends of the coil and removing damaged outer wraps of the coil. To expedite the preparation of each coil, the head end is sheared shortly after the strip is cut, before it is even charged onto the line. The head of the next coil to be charged is butted up against the tail of the last coil, and high voltage (and current) is applied across the seam, melting the two ends. The two strips are then forced together (upset) hydraulically, joining them together with what is called a ‘butt-weld’. Cutting tools immediately after the welder trim the flash that is forced out of the seam during the upset. The soundness of the weld is very important since strip breaks on the line needs rethreading which is time consuming.
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 a stretch leveler which 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 1.2 m in depth and upto 32 m 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 counter-current to the direction of strip movement. When fresh acid is added to the last tank, it has the highest concentration of acid. Acid concentrations decrease from the last tank to the first tank, from which the spent pickle liquor is discharged. A rinse section follows the pickling section.
In some modern lines, the pickling solution is contained in shallow tanks with liquid depths of around 0.4 m and lengths upto around 36 m. 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 can 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 carry over from one tank to another. Turbulent-flow, shallow-tank continuous-strip line which claims to provide more effective pickling action than conventional lines are also been used.
An especially effective rinsing method used on many continuous lines is the cascade rinse system. 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 make-up water in the pickle tanks). Each compartment contains less acid than the previous compartment. After rinsing, the strip is air dried and leaves the air dryer with a dull silver structure.
At the exit end of the line, there is normally an exit strip accumulator. Both in the entry and exit ends accumulators are large strip accumulators. They are also called loopers and are installed both in the entry and exit ends of the acid tanks to keep the strip moving through the pickle and rinse tanks at constant speed when the entry and exit ends are stopped for change of coils. This is important not only from productivity point of view but also to avoid stains that can occur when the strip stops between the acid tanks and the air dryer.
Other equipments at exit end of the line are steering rolls, a strip inspection station, dual side trimmers, an oiler, and one or two coilers. At the exit end of the line, the edges of pickled strip are trimmed by rotary shear ‘knives’ (dual side trimmers) when needed, resulting in a more uniform width and edge condition. Typically, shearing the edges at the pickle line removes around 30 mm to 50 mm of ‘side-trim’ from the width. Oil is applied to the surface of the strip just before it is coiled. The oiler applies oil with S additives to improve the cleanliness of the final product. Galvanized products typically are not oiled after pickling. The pickled steel is then coiled. Pickling lines are to have fume scrubbers to capture emissions / spray from the pickle tanks.
Maximum speeds in modern lines in the pickling section can be as high as 300 meters per minute (m/min) to 460 m/min. Although sustained operation at such speeds is limited by other aspects of coil handling, the selection of pickling tank acid concentrations and temperatures are to be such that complete scale removal is achieved during periods of high-speed operation. A schematic diagram of a typical continuous pickling line is at Fig 3.
Fig 4 Schematic diagram of a typical continuous pickling line
A few pickling lines make use of vertical towers in which one or two HCl acid spray-columns are used. The acid spray columns are assembled and sealed in sections made of fiber-glass reinforced polyester, with a tower height of 21 m to 46 m. 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 cc HCl acid and 13 % FeCl2. It is passed through a C-block heat exchanger and delivered to the sprays at 77 deg C. Most lines of this type have acid-regenerating facilities. Entry and exit coil handling are similar to the more common horizontal lines.
Types of pickling lines
Basically there are three types of pickling lines. These are (i) push and pull type pickling line, (ii) semi-continuous pickling line, and (iii) continuous pickling line. Push and pull type pickling lines – These types of lines are used normally for small and medium production capacities. In these lines preferred thickness of hot strip is more than 1.5 mm. In these types of lines, the strip is neither welded nor stitched but is pushed or pulled through the line strip by strip.
The advantage of the push and pull pickling line is its high flexibility combined with high productivity at low investment costs. Cost-intensive equipment, like welding machines, loopers, and bridle rolls, is not required. The shallow pickling tank design ensures high turbulence and reduced pickling time.
Push and pull pickling lines are capable of processing strip thicknesses from 1 mm to 16 mm and strip widths of upto 2,100 mm, with different steel grades and dimensions, coil by coil, and in direct succession. These lines for C steel can have a maximum capacity of upto 1.2 million tons per annum (Mpta). The optimized pickling process, with individual circulation and heating system, ensures minimized consumption of utilities, while delivering a fully pickled and defect-free strip surface. Schematic drawing of this line is shown in Fig 5.
Fig 5 Schematic drawing of a push and pull type pickling line
Semi continuous pickling lines – Outstanding features of semi-continuous pickling lines are the small loopers which ensure that the strip in the process section does not have to come to a complete stop while the strips are being joined by a stitcher in the entry area. As a result, it is not necessary to re-thread every length of strip. These lines are suitable for small or medium sized production capacities. These lines are suitable for thin and ultra thin strip thicknesses (less than 3 mm). An added advantage is that they can be subsequently upgraded to continuous models. Schematic drawing of a semi-continuous pickling line is shown in Fig 6.
Fig 6 Schematic drawing of a semi continuous pickling line
Continuous pickling lines – These lines are meant for medium to high capacities and also for thin to medium strip thicknesses. In these lines, a welding machine at the entry of the continuous pickling line joins individual strips into endless strip and then horizontal loopers ensure continuously high speed in the processing section. This means, it is possible to achieve top quality standards at very high capacities. Continuous pickling lines can also be coupled to cold rolling mills. Schematic drawing of a continuous pickling line is at Fig 7.
Fig. 7 Schematic drawing of a continuous pickling line