Acid pickling of carbon steel

1. INTRODUCTION

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Chemical pickling is an industrial process to remove the surface layer of iron oxides that are formed in steel products exposed to atmospheric oxidation or in the different stages of carbon steel production so that it can then be coated and protected from corrosion.

When carbon steel is heated to air at temperatures between 575 and 1.370 °C, oxides can be formed on its surface (Turgoose and Bullough, 2013Turgoose, S., Bullough, W. (2013). Pickling in Acid. In Corrosion. Third Edition, Vol 2, pp. 14-23. https://doi.org/10.1016/B978-0-08-052351-4.50090-X.) during the different stages of the steel production, for example during casting, forging, hot rolling and annealing. This oxide is commonly called husk and consists of three well-defined layers, the thickness and composition of which depend on the duration and temperature of heating. In general, the layers, from the steel base outward, comprise a thick layer of wustite, which composition approximates the formula FeO, a layer of magnetite (Fe₃O_4), and a thin layer of hematite (Fe₂O₃₎ (Turgoose and Bullough, 2013Turgoose, S., Bullough, W. (2013). Pickling in Acid. In Corrosion. Third Edition, Vol 2, pp. 14-23. https://doi.org/10.1016/B978-0-08-052351-4.50090-X.). The husk tends to protect the metal from further oxidation at high temperature.

However, in practice, the oxide layer breaks, increasing the thickness of the film, which tends to peel, increasing at the rate of oxidation. High temperatures and high heating rates cause cracking and breakage in the oxide film sheets, although this can also occur at relatively low temperatures. Rapid temperature fluctuations can also lead to the breakage of the protective oxide layers (Fig. 1).

Figure 1. Mechanism of removal of scales with acid. (a) High temperature scale and (b) Low temperature scale.

The removal of the husk to apply a subsequent coating is carried out by pickling the steel in a solution of sulfuric or hydrochloric acid, then washing the steel with water to drag the remains of acid and dissolved iron. These waters are usually taken to a physicochemical treatment station to remove the remains of acid and iron. The surface oxides pass to the solution in the form of sulfate or chloride, depending on the stripper, integrating the residual pickling liquors.

In the pickling process when carbon steel is immersed in acid, this acid penetrates through the cracks and pores of the upper layers of the scale and quickly attacks the wustite layer, thus releasing the relatively insoluble layers of magnetite and hematite.

Iron dissolves to form ferrous ions, and magnetite is reduced, forming more ferrous ions. Since all three compounds are good electrical conductors, the resistance is so small that the dissolution rate of the wustite layer depends on the rate at which the acid diffuses through the cracks into it, and the rate at which the spent acid diffuses from it. A similar but slower action occurs between the exposed metal and the layers of magnetite and hematite that have not come off (Turgoose and Bullough, 2013Turgoose, S., Bullough, W. (2013). Pickling in Acid. In Corrosion. Third Edition, Vol 2, pp. 14-23. https://doi.org/10.1016/B978-0-08-052351-4.50090-X.).

Pickling waters from steel industry are listed as hazardous wastes by the EU Waste Framework (Directive UE 2018/851, 2018Directive UE 2018/851 (2018). European Union Directive 2018/851 amending Directive 2008/98/EC on waste, 109 (2018). https://www.eea.europa.eu/policy-documents/directive-eu-2018-851-of.) and most of the industrialized countries. Pickling waters corrosive nature is considered an important environmental problem. Many solutions have been used to avoid disposal; however, they are not always adequate or economically viable.

The objective of this work is to carry out a bibliographic review of the existing scientific literature on steel pickling, focusing mainly on those processes that are developed on an industrial scale.

2. CARBON STEEL PICKLING WITH CHLORIDIC ACID

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Numerous methods have been used to remove iron oxides from metal surfaces. These methods include abrasive jet cleaning, tubes, brushing, acid pickling, salt bath decalcification, alkaline decalcification, and acid cleaning. The preferred method in steel production is pickling in acid. A solution of hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) is usually used to remove the rust of carbon steel, while for stainless steel a combination of hydrofluoric and nitric acids is often used. Among all acids, hydrochloric acid is preferentially used commercially. Beginning in 1964, numerous steel pickling facilities switched from sulfuric acid to hydrochloric acid. Hydrochloric acid offers the following advantages over sulfuric acid and other acids (Hudson and Warning, 1982Hudson, R.M., Warning, C.J. (1982). Effect of Strip Velocity on Pickling Rate of Hot-Rolled Steel in Hydrochloric Acid. JOM 34 (2), 65-70. https://doi.org/10.1007/BF03339114.)

It provides shiny and smooth surfaces.
It carries a lower possibility of localized corrosion than with other acids.
Ferrous chloride salts do not act as process inhibitors until high concentrations.
It washes easily due to the high solubility of chlorides.
Galvanic coatings adhere better to metal, more easily and are more uniform.
Less energy is consumed because the required temperature is lower.

In addition, hydrochloric acid compared to other acids that have been used, for example, nitric acid and phosphoric acid, is safer for the environment, since it does not release effluents containing nitrogen and phosphorus (Agrawal and Sahu, 2009Agrawal, A., Sahu, K.K. (2009). An overview of the recovery of acid from spent acidic solutions from steel and electroplating industries. J. Hazard. Mater. 171 (1-3), 61-75. https://doi.org/10.1016/j.jhazmat.2009.06.099.) which can cause eutrophication problems in freshwater bodies.

The main drawbacks of pickling with HCl are the cost, higher than that of H₂SO₄, and the need to install steam control systems. (Hudson and Warning, 1969Hudson, R.M., Warning, C.J. (1969). Minimizing Fuming during Pickling with Hydrochloric Acid. Sheet. Metal. Ind. 46 (8), 523-525.).

Acid pickling with HCl is a chemical process for dissolving iron oxides from the metal surface without any significant attack on the steel itself. The need of the steel industry to provide a high quality of its products is evident, that is why they require a complete removal of rust on the surface that cannot tolerate insufficient pickling, nor too long a permanence of steel products in the pickling bath that can cause erosion of the surface itself. Both problems lead to more expenses or losses.

The oxidation layer composed of different types of iron oxides (Fe₂O_3, Fe₃O₄ and FeO) reacts with hydrochloric acid to form ferrous chloride through reactions (1-31, 2, 3) (Tang et al., 2012Tang, B., Su, W., Wang, J., Fu, F., Yu, G., Zhang, J. (2012). Minimizing the creation of spent pickling liquors in a pickling process with high-concentration hydrochloric acid solutions: Mechanism and evaluation method. J. Environ. Manage. 98 (1), 147-154. https://doi.org/10.1016/j.jenvman.2011.12.027.). Finally, hydrochloric acid penetrates the oxidation layer and attacks the underlying metal generating hydrogen, according to the reaction (4)

{F e}_{2} O_{3} + F e + 6 H C l \to 3 F e {C l}_{2} + {3 H}_{2} O

(1)

{F e}_{3} O_{4} + F e + 8 H C l \to 4 F e {C l}_{2} + {4 H}_{2} O

(2)

F e O + 2 H C l \to F e C l_{2} + H_{2} O

(3)

F e + 2 H C l \to F e {C l}_{2} + H_{2}

(4)

Due to these reactions, the oxidized layer detaches from the surface, which constitutes an important action in the pickling operation (Staley, 1926bStaley, H.F. (1926b). The theory of pickling of sheet iron and steel for enameling purposes. J. Am. Ceram. Soc. 9 (12), 787-796. https://doi.org/10.1111/j.1151-2916.1926.tb17958.x.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.). Although most of the hydrogen produced in the cathodic areas is released as a gas and helps to remove scale by reducing Fe³⁺ to Fe²⁺ within the solution, some of it diffuses into the steel in atomic form and can make it brittle (weakening of steel by the hydrogen action).

Hydrogen in solution, as stated above, can act as a reducer to dissolve and reduce Fe₃O₄ into Fe₂O₃ in the pickling solution (Özdemir et al., 2006Özdemir, T., Öztin, C., Kincal, N.S. (2006). Treatment of waste pickling liquors: Process synthesis and economic analysis. Chem. Eng. Commun. 193 (5), 548-563. https://doi.org/10.1080/00986440500192238.). However, the hydrogen formed adsorbed on the base surface can penetrate the metal layer, which can weaken the binding forces of the layer and reduce the deformation capacity of the metal (Paatsch, 2011Paatsch, W. (2011). Hydrogen determination and hydrogen embrittlement - Meaning and advantage. Galvanotechnik 102 (1), 48-55.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.) . Due to the reduced ductility, brittle fracture of metallic materials occurs when an external tensile load is applied, (Kuron, 1986Kuron, D. (1986). Hydrogen and corrosion (Issue Festschrift for Prof. Dr. Hubert Graefen on the occasion of his 60. anniversary. Wasserstoff und Korrosion. Festschrift zum 60. Geburtstag von Prof. Dr. Hubert Graefen. Germany, Mater. Corros. 37 (5), p. 294.; Treischel, 1919Treischel, C. (1919). The cause and control of blistering in sheetsteel enameling. J. Am. Ceram. Soc. 2 (10), 774-781. https://doi.org/10.1111/J.1151-2916.1919.TB17472.X.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.) this process is commonly known as hydrogen embrittlement.

One of the solutions to deal with this problem is the addition of inhibitors in the pickling solution. Inhibitors are specifically adsorbed in places where the base steel is exposed to form thin layers that prevent the attack of the metal by acid (Feser et al., 2002Feser, R., Friedrich, A., Scheide, F. (2002). The influence of long term use of inhibitors in hydrochloric acid pickling baths on hydrogen induced stress corrosion cracking. Mater. Corros. 53 (9), 637-646. https://doi.org/10.1002/1521-4176(200209)53:9<637::AID-MACO637>3.0.CO;2-P.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.). Another practical method is to reduce the immersion time of the metal in the pickling solution and thus reduce the absorption of hydrogen (Staley, 1926aStaley, H.F. (1926a). The Theory of Pickling of Sheet Iron and Steel for Enameling Purposes. J. Am. Ceram. Soc. 9 (12), 787-796. https://doi.org/10.1111/j.1151-2916.1926.tb17958.x.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.).

2.1. Acid volatility

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The main drawback of using hydrochloric acid is its high volatility under pickling conditions. Gaseous hydrochloric acid causes corrosion to the equipment during and after the pickling process and increases the consumption of pickling solution (Sittig, 1981Sittig, M. (1981). Handbook of toxic and hazardous chemicals. Noyes Data Corp., Park Ridge, NJ. https://www.osti.gov/biblio/5444143.; Evans et al., 2011Evans, C.D., Monteith, D.T., Fowler, D., Cape, J.N., Brayshaw, S. (2011). Hydrochloric acid: An overlooked driver of environmental change. Environ. Sci. Technol. 45 (5), 1887-1894. https://doi.org/10.1021/es103574u.;Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.). In addition, HCl gas emissions, if not contained, can cause acidification of the surrounding land

The temperature favors the pickling process. Pickling efficiency can be roughly doubled with an increase of 10 ºC (Garverick, 1994Garverick, L. (1994). Corrosion in the petrochemical industry. ASM international.; Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.). However, the volatilization of hydrochloric acid is accelerated with the increase in temperature due to the endothermic nature of the volatilization process and the loss of HCl in the bath decreases the efficiency of the layer. In addition, as the concentration of HCl in the solution increases, evaporation increases (Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.).

The existence of ferrous ions in the hydrochloric acid solution can effectively inhibit the volatilization of hydrochloric acid (Yang et al., 2020Yang, F., Wu, Y., Fang, X., Ma, L. (2020). Experimental and theoretical study on the behaviour of a pickling solution: The role of ferrous ions. J. Clean. Prod. 243, 118631. https://doi.org/10.1016/j.jclepro.2019.118631.).

2.2. Acid Inhibitors

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During the pickling of carbon steel, the finer and more soluble scale is removed before the thicker and less soluble scale. Consequently, part of the exposed base metal is attacked before the pickling operation is completed. To minimize this acid attack, acid inhibitors are used. Its use also causes a lower consumption of acid, fewer marks appear on the surface of the pickled steel and the amount of acid aerosols and water vapor is reduced. In addition, pickling blister formations due to the existence of occluded hydrogen under the surface of the metal, roughness and pitting caused by excessive dissolution of the metal are reduced.

The addition of corrosion inhibitors in the acidic medium is the cheapest and most convenient. Although inhibitors reduce the attack of acids, they do not prevent the attack of oxidizing agents on the exposed base metal. Thus, the ferric ions resulting from the gradual dissolution of the detached magnetite and hematite attack the exposed steel even in the presence of an inhibitor, after this attack these ferric ions become ferrous.

Many acid-soluble or colloidally dispersible organic substances have been shown to have inhibitory properties. The reaction mechanisms of the inhibitor on the metal are complex. Mechanisms of organic-metal composite reaction based on electrostatic attraction between molecules and the charged metal have been described (Ostovari et al., 2009Ostovari, A., Hoseinieh, S.M., Peikari, M., Shadizadeh, S.R., Hashemi, S.J. (2009). Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid). Corros. Sci. 51 (9), 1935-1949. https://doi.org/10.1016/j.corsci.2009.05.024.) interaction of uncharged electron pairs of the organic molecule with the metal (Ostovari et al., 2009Ostovari, A., Hoseinieh, S.M., Peikari, M., Shadizadeh, S.R., Hashemi, S.J. (2009). Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid). Corros. Sci. 51 (9), 1935-1949. https://doi.org/10.1016/j.corsci.2009.05.024.) interaction of π-electrons with the metal (Ostovari et al., 2009Ostovari, A., Hoseinieh, S.M., Peikari, M., Shadizadeh, S.R., Hashemi, S.J. (2009). Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid). Corros. Sci. 51 (9), 1935-1949. https://doi.org/10.1016/j.corsci.2009.05.024.) and finally, combination of those of the first and third (Ostovari et al., 2009Ostovari, A., Hoseinieh, S.M., Peikari, M., Shadizadeh, S.R., Hashemi, S.J. (2009). Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid). Corros. Sci. 51 (9), 1935-1949. https://doi.org/10.1016/j.corsci.2009.05.024.).

3. RECYCLING AND REUSE OF SPENT ACID

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The corrosive nature and high concentration of acids and metals result in decay acid being considered special hazardous waste (Dahlgren, 2010Dahlgren, L. (2010). Treatment of Spent Pickling Acid from Stainless Steel Production: A review of regeneration technologies with focus on the neutralisation process for implementation in Chinese industry. Master of Science Thesis, Royal Institute of Technology, Stocholm.; Directive UE 2018/851, 2018Directive UE 2018/851 (2018). European Union Directive 2018/851 amending Directive 2008/98/EC on waste, 109 (2018). https://www.eea.europa.eu/policy-documents/directive-eu-2018-851-of.). In addition to the corrosivity, the main effects of this wastewater on the receiving medium are as follows:

Hydrolysis of ferrous salts to ferrous-ferric hydroxides, insoluble in water, give rise to an ochre-coloured sediment.
Iron reduces dissolved oxygen in water, which decreases the regenerative power of rivers.
Proliferation of ferruginous bacteria, together with the ferritic deposits affect the flora and fauna.
Concentrations of dissolved iron in water disqualifies its use for food, domestic and industrial purposes.

Increasingly stringent regulations on the discharge of acids and metals into the environment, and the increasing emphasis on the recycling or reuse of these effluents after treatment, have focused the interest of the research community in the development of new approaches to the recovery of acids and metals from industrial waste. The regeneration of acids from pickling allows for several benefits: the reduction of hazardous gas emissions, increased pickling productivity, greater environmental responsibility and promoting the principles of best available technology (BAT) (Regel-Rosocka, 2010Regel-Rosocka, M. (2010). A review on methods of regeneration of spent pickling solutions from steel processing. J. Hazard. Mater. 177 (1-3), 57-69. https://doi.org/10.1016/j.jhazmat.2009.12.043.; Tang et al., 2016Tang, J., Pei, Y. Hu, Q., Pei, D., Junpeng Xu, J. (2016) The Recycling of Ferric Salt in Steel Pickling Liquors: Preparation of Nano-sized Iron Oxide. Procedia Environ. Sci., 31, 778-784. https://doi.org/10.1016/j.proenv.2016.02.071. ).

3.1. Pickling processes with HCl and resulting waste

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3.1.1. Continuous pickling process

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The continuous pickling process is used to treat coils, rods, wires, and tubes, in the sense that the steel is connected from end to end and continuously passes through the pickling tanks (Fig. 2). The pickling of coils is done by joining the different bands by means of a welding machine, even a stapling machine or a much more basic system, coil to coil processing, also known as “push-pull”. The latter makes productions smaller due to its low productivity. After the continued pickling process there is usually a water cascade wash to clean the surface of traces of solution. This wastewater is sent along with the effluent from the scrubber to a physicochemical treatment plant to neutralize the acidity and precipitate the metals. Finally, the steel is dried before passing it to the next process either oiled, galvanized, painted.

Figure 2. Continuous pickling line scheme.

3.1.2. Batch pickling process

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The batch process is used to descale rods or wires into coils, pipes, and metal parts. The material is immersed in the pickling tank for a certain period until the scale dissolves (Fig. 3). Each pickling process is followed by a rinsing stage to remove the remains of solution from the metal surface, which in turn is treated in a physicochemical treatment plant like that is used for the washing waters of the continuous pickling process. This process is mainly used as a pre-treatment in the galvanizing process of already formed steel products. Initially these products are degreased to remove traces of oils and oil emulsions. Once the organic impurities are removed, the steel is pickled to remove the oxides from the surface. Finally, just before galvanizing, they are immersed in a chemical flux composed of a double salt of zinc chloride and ammonium chloride, which is designed to chemically remove the last traces of oxides and produce an intense cleaning of the metal surface. These salts favor the wetness of the steel surface by molten zinc, improving the iron-zinc alloy reaction.

Figure 3. Batch pickling line schematic.

In addition to the pickling solution, this process generates numerous wastes such as sludge, which originates in wastewater treatment and flux regeneration plants that are sent to landfill as hazardous waste, zinc ash and dross that can be recycled into metallic zinc and zinc oxide and finally galvanizing powders that are collected in filter sleeves and also sent to landfill as hazardous waste.

The residue resulting from pickling with hydrochloric acid, both in continuous and batch processes, is an acidic solution of ferrous chloride that is classified as hazardous waste (Directive UE 2018/851, 2018Directive UE 2018/851 (2018). European Union Directive 2018/851 amending Directive 2008/98/EC on waste, 109 (2018). https://www.eea.europa.eu/policy-documents/directive-eu-2018-851-of.) mainly because of its high corrosivity, but also because it can be classified as hazardous due to its metal content. There are mainly two types of solutions that correspond to the types of pickling processes already mentioned:

Continuous pickling process liquor: the continuous pickling process is done in closed tanks and at temperatures between 50-80 ºC. This means that the main content of the spent acid is a mixture of FeCl₂ in solution, together with the HCl that has not reacted with the scale.

Batch pickling process liquor: The pickling of carbon steel parts is done in open tanks and at room temperature and constitutes a process prior to galvanizing. In this case, the liquors are known as galvanic pickling liqueurs and not only contain ferrous ions, but also other metal ions such as Zinc. Zinc contamination enters the pickling line through hooks, insoles, cages, and baskets that are used to suspend batches of carbon steel parts during immersion. These are galvanized, sent back to the head of the plant for reuse, and stripped during pickling of the next workpiece, thus contaminating the galvanic pickling liquor with zinc. This liquor is, therefore, a solution containing various concentrations of HCl, and salts in solution of FeCl₂ and ZnCl₂.

3.2. Technological solutions to reuse or recycle used pickling solutions with HCl

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There are many studies aimed at the recovery of value contents in pickling waters. These studies are based on two principles the recovery of the existing free acid waters and metals recovery.

3.2.1. Ferric chloride production

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The most used method to obtain ferric chloride is based on the use of pickling liquors. It is a more cost-effective method than dissolving ferrous scrap in hydrochloric acid.

The first step in the production of ferric chloride from pickling waters is the removal of free acid by adding scrap. After this reaction, the resulting solution without free acid is chlorinated with chlorine gas to form ferric chloride through reaction (5) (Inguru Consultores, 2006Inguru Consultores (2006). Estudio de impacto ambiental proyecto de ampliación de las instalaciones de producción de cloruro férrico - ACIDEKA, S.A., P.I. Lantarón (Álava). ):

2 F e {C l}_{2} + {C l}_{2} \to 2 F e {C l}_{3}

(5)

Once the solution reaches 40% concentration of FeCl_3, the process is concluded and ready to be marketed in the form of a solution. The most common end use is coagulant of contaminants in wastewater treatment.

3.2.2. Recuperation of free hydrochloric acid and other by-products for pickling operations

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Various technologies have been proposed to recover free acid. HCl is a volatile compound and the liquid-vapor balance is influenced both by temperature and by the concentration of acid and ferrous salts in the solution (Tomaszewska et al., 2001Tomaszewska, M., Gryta, M., Morawski, A.W. (2001). Recovery of hydrochloric acid from metal pickling solutions by membrane distillation. Sep. Purif. Technol. 22-23, 591-600. https://doi.org/10.1016/S1383-5866(00)00164-7.) which means that its recovery requires technologies that can involve a high investment cost. The most outstanding technologies are:

Acid Retardation or Ion Exchange Resins is a process in which metal ions are separated from acids by their mobility difference. The spent pickling solution passes through an anionic-type ion exchange resin that favors the absorption of strong acids. The movement of acids in the ion exchange resin is delayed relative to the movement of salt and water flowing faster (Hatch and Dillon, 1963Hatch, M., Dillon, J. (1963). Acid Retardation. Simple Physical Method for Separation of Strong Acids from Their Salts. Ind. Eng. Chem. Process Des. Dev. 2 (4), 253-263. https://doi.org/10.1021/i260008a001.; Bood, 2020Bood, C. (2020). Acid retardation: recovery and recycling of acid and metal. Degree Project, Luleå University of Technology.). The acid is desorbed from the ion exchange resin by backwashing with water. Due to its simplicity, reliability, and relatively low cost, acid retardation is one of the most common purification techniques for spent pickling solutions (Naushad and Al-Othman, 2013Naushad, M., Al-Othman, Z.A. (2013). A Book on Ion Exchange, Adsorption & Solvent Extraction. King Saud University, Riyadh, Saudi Arabia.; Bood, 2020Bood, C. (2020). Acid retardation: recovery and recycling of acid and metal. Degree Project, Luleå University of Technology.). The main disadvantage is the iron scales or some other impurities that can be produced in resins. On the other hand, this type of membranes only absorbs a part of the acid, so the problem is only partially reduced (Palatý and Žáková, 2006Palatý, Z., Žáková, A. (2006). Competitive transport of hydrochloric acid and zinc chloride through polymeric anion-exchange membrane. J. Appl. Polym. Sci. 101 (3), 1391-1397. https://doi.org/10.1002/app.22748.).
Diffusion dialysis (DD) is a membrane process that operates under some of the same principles as ion exchange resins (acid delay) using ion-selective membrane material. Clean water (dialyzed) is introduced countercurrent into the permeate side of the membrane to absorb the acid passing through the semipermeable surface. Diffusion dialysis is an ongoing process. Compared to other membrane technologies (e.g. electrodialysis or reverse osmosis). In addition, the diffusion dialysis process does not work with electrical potential or pressure difference across the membrane. For the pickling system with HCl, iron chlorides were found to provide additional chlorides in solution that improve acid passage (Luo et al., 2013Luo, J., Wu, C., Wu, Y., Xu, T. (2013). Diffusion dialysis of hydrochloric acid with their salts: Effect of co-existence metal ions. Sep. Purif. Technol. 118, 716-722. https://doi.org/10.1016/j.seppur.2013.08.014.; Gueccia et al., 2019Gueccia, R., Randazzo, S., Chillura Martino, D., Cipollina, A., Micale, G. (2019). Experimental investigation and modeling of diffusion dialysis for HCl recovery from waste pickling solution. J. Environ. Manage. 235, 202-212. https://doi.org/10.1016/j.jenvman.2019.01.028.; Gueccia et al., 2020Gueccia, R., Aguirre, A.R., Randazzo, S., Cipollina, A., Micale, G. (2020). Diffusion dialysis for separation of hydrochloric acid, iron and zinc ions from highly concentrated pickling solutions. Membranes 10 (6), 129. https://doi.org/10.3390/membranes10060129.; Gueccia et al., 2021Gueccia, R., Winter, D., Randazzo, S., Cipollina, A., Koschikowski, J., Micale, G.D.M. (2021). An integrated approach for the HCl and metals recovery from waste pickling solutions: pilot plant and design operations. Chem. Eng. Res. Des. 168, 383-396. https://doi.org/10.1016/j.cherd.2021.02.016.) In contrast, zinc ions in acid solution are mainly present as negative chlorine complexes, which act as competitors to acid recovery, thus reducing the passage of acid (Jung et al., 2000Jung Oh, S., Moon, S.H., Davis, T. (2000). Effects of metal ions on diffusion dialysis of inorganic acids. J. Membr. Sci. 169 (1), 95-105. https://doi.org/10.1016/S0376-7388(99)00333-6.).
Pyro-hydrolysis is a roasting process in which acidic ferrous chloride is burned to produce iron oxide and gaseous hydrogen chloride. It is used as an effective way to fix iron as an oxide, for sale as a by-product or for safe disposal and to produce hydrogen chloride gas that reacts with incoming water (usually washing water from the pickling process) to form almost azeotropic hydrochloric acid. There are mainly two technologies available: fluidized bed and spray roasting (Lee et al., 2006Lee, C., Small, D., Adham, Kamal (2006). Energy consumption for iron chloride pyrohydrolysis: A comparison between fluidized beds and spray roasters. Conference COM 2006 Conference of Metallurgists, Montreal Quebec Canada: Proceedings of the COM 2006. Canadian Institute of Mining Metallurgy and Petroleum.). The main drawback of pyrohydrolysis is that it consumes a lot of energy (Stocks et al., 2005Stocks, C., Wood, J., Guy, S. (2005). Minimisation and recycling of spent acid wastes from galvanizing plants. Resour., Conserv. Recycl. 44 (2), 153-166. https://doi.org/10.1016/j.resconrec.2004.11.005.; Devi et al., 2014Devi, A., Singhal, A., Gupta, R., Panzade, P. (2014). A study on treatment methods of spent pickling liquor generated by pickling process of steel. Clean Techn. Environ. Policy 16 (8), 1515-1527. https://doi.org/10.1007/s10098-014-0726-7.; Gueccia et al., 2021Gueccia, R., Winter, D., Randazzo, S., Cipollina, A., Koschikowski, J., Micale, G.D.M. (2021). An integrated approach for the HCl and metals recovery from waste pickling solutions: pilot plant and design operations. Chem. Eng. Res. Des. 168, 383-396. https://doi.org/10.1016/j.cherd.2021.02.016.) and suffers from the presence of zinc in solution, due to the formation of low melting compounds at the high working temperature of the process (Kerney, 1994Kerney, U. (1994). Treatment of spent pickling acids from hot dip galvanizing. Resour., Conserv. Recycl. 10 (1-2), 145-151. https://doi.org/10.1016/0921-3449(94)90047-7.). This technology was widely use in past but the drop in the price of HCl and the high energy cost made it unattractive.

In spray roasting units, the exhaust gases and oxides leave the roaster in counter current flow at temperatures in the range of 400 °C to 500 °C. Due to the counter current flow, the exit temperature in the spray roaster is lower than the temperature of the reaction zone. The residence time of the sprayed particles in the high-temperature reaction zone is very short; therefore, very small liquid droplets must be created by atomisation, which can be heated quickly. Rapid heating results in the formation of a solid metal oxide crust on the surface of each droplet. As the bulk of the droplet is heated, the water content vaporises and breaks through the oxide layer. Therefore, spray-roasted oxides often consist of very fine hollow spheres.

In fluidised beds, the gas and solids are completely mixed, and their outlet temperature is normally 800 °C to 900 °C, which is the same as that of the reaction zone. The hot flue gas is normally produced by submerged nozzles that directly inject air and fuel into the bottom of the fluidised bed. The hot gas flows upwards and fluidises the particulate bed. As the flue gas flows through the well-stirred oxide bed, it quickly reaches thermal equilibrium with the bed. The liquid dosage of solution is not sprayed but poured directly into the oxide bed. The solution wets the outer layer of the hot oxide particles and quickly evaporates to form a layer of new solid oxide on top of the existing oxide, thus producing dense homogeneous particles.
Evaporative recovery by which the solution of HCl, water and metal salt enters a vacuum evaporator and is heated above the boiling point of the HCl (108.6 ºC) (Cullivan, 2020Cullivan, B. (2020). Technology Profile: Acid Recovery and Recycle Technologies. Beta Control Systems. http://www.betacontrol.com/sites/default/files/uploads/file/HCl_literature/Acid%20Recovery%20and%20Recycling%20Technologies_Beta%20Control%20Systems%20Z.pdf.) To keep the heat sensitive and ensure a more efficient heat cycle, the forced circulation of the spent pickling solution generates a slight back pressure. With the rest of the process operating under vacuum, the pressurized and heated solution is sprayed into a separator where both water and hydrochloric acid evaporate simultaneously (Cullivan, 2020). The remaining metal chloride salt solution is concentrated as more spent acid enters the evaporator and the HCl/H₂O solution vaporizes (Cullivan, 2020Cullivan, B. (2020). Technology Profile: Acid Recovery and Recycle Technologies. Beta Control Systems. http://www.betacontrol.com/sites/default/files/uploads/file/HCl_literature/Acid%20Recovery%20and%20Recycling%20Technologies_Beta%20Control%20Systems%20Z.pdf.). When the specific gravity of the metal salt reaches the target, the metal chloride concentrate is pumped into a storage tank for transport to an end user. HCl/H₂O vapor travels through a rectifier filled with an acid-resistant material, where the water descends from the top and mixes with the HCl vapor as it descends. Condensed steam droplets increase their acid concentration as they descend and approach the azeotrope of HCl at 20%. The remaining water vapor goes up through the filler, into a water-cooled condenser. The steam condenses into reusable process water.

To be more efficient, evaporative recovery works under partial vacuum. This reduces the boiling point of HCl from 108.6 ºC to less than 80 ºC. Vacuum processing reduces energy consumption, construction cost and the potential for chemical excursions (Cullivan, 2020Cullivan, B. (2020). Technology Profile: Acid Recovery and Recycle Technologies. Beta Control Systems. http://www.betacontrol.com/sites/default/files/uploads/file/HCl_literature/Acid%20Recovery%20and%20Recycling%20Technologies_Beta%20Control%20Systems%20Z.pdf.).

3.2.3. Selective metal recovery technologies

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The water treatment industry uses large amounts of ferric chloride as a flocculant in treated wastewater treatment before discharge into surface water. Ferric chlorides do not occur naturally and must be manufactured as indicated in section 3.1 from spent pickling solutions. However, in order to reuse this pickling solution it is always necessary that the iron chloride is at a high level (>130 g·l^-1 of Fe) and the zinc chloride is below a specified level (<1 g·l^-1 of Zn) (Stocks et al., 2005Stocks, C., Wood, J., Guy, S. (2005). Minimisation and recycling of spent acid wastes from galvanizing plants. Resour., Conserv. Recycl. 44 (2), 153-166. https://doi.org/10.1016/j.resconrec.2004.11.005.).

There are several chemical separation techniques that could achieve the separation of iron and zinc from these acid wastes, such as chemical precipitation, ion exchange and solvent extraction, but also includes crystallization that, although it is not valid for the separation of metals if it is to recover metals in the form of salts.

Chemical precipitation: consists of the removal of iron in the form of Hydroxides Fe (OH)₃. However, in galvanic pickling waters, no chemical can be found that selectively precipitates Zn from solutions high in Fe⁺² without contaminating the precipitate with Fe and leaving the residual Zn dissolved (Kerney, 1994Kerney, U. (1994). Treatment of spent pickling acids from hot dip galvanizing. Resour., Conserv. Recycl. 10 (1-2), 145-151. https://doi.org/10.1016/0921-3449(94)90047-7.). This technique is usually carried out after having recovered or spent almost all the hydrochloric acid from the pickling solution, since the following steps require oxidizing FeCl₂ to FeCl₃ and then precipitating the iron increasing the pH of the solution. If the pickling solution contains ZnCl₂ in addition to FeCl₂ the increase in pH is made with the addition of ammonium hydroxide. Zinc ions remain in solution along with ammonium chlorides, due to their higher precipitation pH values (Gueccia et al., 2019Gueccia, R., Randazzo, S., Chillura Martino, D., Cipollina, A., Micale, G. (2019). Experimental investigation and modeling of diffusion dialysis for HCl recovery from waste pickling solution. J. Environ. Manage. 235, 202-212. https://doi.org/10.1016/j.jenvman.2019.01.028.; Gueccia et al., 2021Gueccia, R., Winter, D., Randazzo, S., Cipollina, A., Koschikowski, J., Micale, G.D.M. (2021). An integrated approach for the HCl and metals recovery from waste pickling solutions: pilot plant and design operations. Chem. Eng. Res. Des. 168, 383-396. https://doi.org/10.1016/j.cherd.2021.02.016.). Once the iron is removed, the resulting solution would form a double salt of NH₄Cl and ZnCl₂ very similar to that used as a flux in the galvanizing process, although it should be adjusted to be reused. The precipitate of iron hydroxides is difficult to market, so its destination will most likely be the landfill.
Separation of ion exchange resins: it is an effective procedure for removing metals present in industrial effluents (Bolto and Pawlowski, 1987Bolto, B.A., Pawlowski, L. (1987). Wastewater treatment by ion-exchange. L. Pawlowski (ed.), Ed.Spon Press. 300 pp. ISBN-10: 0419133208.; Meltzer, 1990Meltzer, M. (1990). Metal Bearing Waste Streams: Minimizing, Recycling and Treatment. William Andrew Publishing. USA, 426 pages. ; Marañón et al., 2000Marañón, E., Fernández, Y., Súarez, F.J., Alonso, F.J., Sastre, H. (2000). Treatment of acid pickling baths by means of anionic resins. Ind. Eng. Chem. Res. 39 (9), 3370-3376. https://doi.org/10.1021/ie0000414.), since dissolved metal ions are retained in the resin bed by a reversible reaction. When this bed becomes saturated, it must regenerate, passing the ions to the regenerated solution. The regenerated volume is much smaller than the volume of treated effluent, so the metals are in a much higher concentration in the regenerated liquid and can be properly treated by precipitation, distillation or other recovery processes (Marañón et al., 2000Marañón, E., Fernández, Y., Súarez, F.J., Alonso, F.J., Sastre, H. (2000). Treatment of acid pickling baths by means of anionic resins. Ind. Eng. Chem. Res. 39 (9), 3370-3376. https://doi.org/10.1021/ie0000414.). The affinity of zinc for resins allows iron to be removed in the presence of zinc in the solution (Marañón et al., 2000Marañón, E., Fernández, Y., Súarez, F.J., Alonso, F.J., Sastre, H. (2000). Treatment of acid pickling baths by means of anionic resins. Ind. Eng. Chem. Res. 39 (9), 3370-3376. https://doi.org/10.1021/ie0000414.). However, it is also necessary to initially oxidize Fe⁺² to Fe⁺³ which then forms an anionic complex that helps not to interfere with zinc (Marañón et al., 2000Marañón, E., Fernández, Y., Súarez, F.J., Alonso, F.J., Sastre, H. (2000). Treatment of acid pickling baths by means of anionic resins. Ind. Eng. Chem. Res. 39 (9), 3370-3376. https://doi.org/10.1021/ie0000414.).

The use of ion exchange resins for metal separation only works with low concentrations of iron and zinc (Marañón et al., 2000Marañón, E., Fernández, Y., Súarez, F.J., Alonso, F.J., Sastre, H. (2000). Treatment of acid pickling baths by means of anionic resins. Ind. Eng. Chem. Res. 39 (9), 3370-3376. https://doi.org/10.1021/ie0000414.). Therefore, a large volume of resin, with high investment costs, would be required to treat the currently generated spent acid, with its high and unpredictable levels of zinc.
Extraction with organic solvents (liquid/liquid separation): it is the most consolidated separation technology for the recycling of waste that needs to separate metals. Solvents are becoming increasingly selective, allowing specific molecules to be separated from the aqueous phase while others are preserved. Kerney, 1994Kerney, U. (1994). Treatment of spent pickling acids from hot dip galvanizing. Resour., Conserv. Recycl. 10 (1-2), 145-151. https://doi.org/10.1016/0921-3449(94)90047-7. and Agrawalk et al. (2011)Agrawal, A., Kumari, S., Sahu, K.K. (2011). Studies on solvent extraction of iron(III) as a step for conversion of a waste effluent to a value added product. J. Environ. Manage 92 (12), 3105-3111. https://doi.org/10.1016/j.jenvman.2011.07.026. demonstrated that solvent extraction was a viable treatment process for separating FeCl₂ and ZnCl₂ from HCl.

For the recovery of Zn (II), Fe (II), Fe (III) and HCl from spent pickling solution, various extraction agents such as D2EHPA, DBBP, TBP, Cyanex 921, Cyanex 923, Cyanex 301, Cyanex 302, Cyanex 272, Alamine 336, Alamine 308, Aliquat 336, TEHA and TiOA have been applied exhaustively (Kerney, 1994Kerney, U. (1994). Treatment of spent pickling acids from hot dip galvanizing. Resour., Conserv. Recycl. 10 (1-2), 145-151. https://doi.org/10.1016/0921-3449(94)90047-7.). The above agents simultaneously extract Zn(II) and Fe(III) from the pickling solution. The Zn (II) and Fe (III) present in the organic phase are subsequently separated in the re-extraction stage (Sinha et al., 2016Sinha, M.K., Pramanik, S., Sahu, S.K., Prasad, L. B., Jha, M.K., Pandey, B.D. (2016). Development of an efficient process for the recovery of zinc and iron as value added products from the waste chloride solution. Sep. Purif. Technol. 167, 37-44. https://doi.org/10.1016/j.seppur.2016.04.049.) The re-extraction of the metals extracted in the organic phase is conventionally carried out with acid/base reagents, recovering through precipitation, crystallization or electrodeposition.

Of particular interest is the use of Cyanex 923 as an extraction agent for the selective recovery of Zn(II) and Fe(III) (Saji John et al., 1999Saji John, K., Saji, J., Reddy, M.L.P., Ramamohan, T.R., Rao, T.P. (1999). Solvent extraction of titanium(IV) from acidic chloride solutions by Cyanex 923. Hydrometallurgy 5 (1), 9-18. https://doi.org/10.1016/s0304-386x(98)00066-8.; Alguacil and Martínez, 2001Alguacil, F.J., Martínez, S. (2001). Solvent extraction of Zn(II) by Cyanex 923 and its application to a solid-supported liquid membrane system. J. Chem. Technol. Biotechnol. 76 (3), 298-302. https://doi.org/10.1002/jctb.369.; Regel et al., 2001Regel, M., Sastre, A.M., Szymanowski, J. (2001). Recovery of Zinc(II) from HCl Spent Pickling Solutions by Solvent Extraction. Environ. Sci. Technol. 35 (3), 630-635. https://doi.org/10.1021/es001470w.; Agrawal and Sahu, 2010Agrawal, A., Sahu, K.K. (2010). Treatment of chloride waste pickle liquor by solvent extraction for the recovery of iron. Miner. Process. Extr. Metall. Rev. 31 (3), 121-134. https://doi.org/10.1080/08827501003727006.; Mishra et al., 2010Mishra, R.K., Rout, P.C., Sarangi, K., Nathsarma, K.C. (2010). A comparative study on extraction of Fe(III) from chloride leach liquor using TBP, Cyanex 921 and Cyanex 923. Hydrometallurgy 104 (2), 298-303. https://doi.org/10.1016/j.hydromet.2010.07.003.; Sinha et al., 2016Sinha, M.K., Pramanik, S., Sahu, S.K., Prasad, L. B., Jha, M.K., Pandey, B.D. (2016). Development of an efficient process for the recovery of zinc and iron as value added products from the waste chloride solution. Sep. Purif. Technol. 167, 37-44. https://doi.org/10.1016/j.seppur.2016.04.049.; Regel-Rosocka et al., 2002Regel-Rosocka, M., Sastre, A.M., Szymanowski, J. (2002). Zinc (II) extraction from hydrochloric acid solutions with basic and solvating extractants. XVII-Th ARS SEPARATORIA-Borówno.). In addition, Cyanex 923 is a stable reagent under highly acidic conditions.
Crystallization: the recovery of residual pickling acids can also be achieved by crystallization, since the lower the temperature, the lower the solubility of ferrous chloride in the recovered acid stream due to the inverse relationship of ferrous chloride solubility to temperature (Özdemir et al., 2006Özdemir, T., Öztin, C., Kincal, N.S. (2006). Treatment of waste pickling liquors: Process synthesis and economic analysis. Chem. Eng. Commun. 193 (5), 548-563. https://doi.org/10.1080/00986440500192238.) . Some processes have been developed on the regeneration of spent pickling liquids, including the recovery of acids and metals (Agrawal and Sahu, 2009Agrawal, A., Sahu, K.K. (2009). An overview of the recovery of acid from spent acidic solutions from steel and electroplating industries. J. Hazard. Mater. 171 (1-3), 61-75. https://doi.org/10.1016/j.jhazmat.2009.06.099.; Regel-Rosocka, 2010Regel-Rosocka, M. (2010). A review on methods of regeneration of spent pickling solutions from steel processing. J. Hazard. Mater. 177 (1-3), 57-69. https://doi.org/10.1016/j.jhazmat.2009.12.043.) and new process options based on crystallization have been proposed as alternative methods (Özdemir et al., 2006Özdemir, T., Öztin, C., Kincal, N.S. (2006). Treatment of waste pickling liquors: Process synthesis and economic analysis. Chem. Eng. Commun. 193 (5), 548-563. https://doi.org/10.1080/00986440500192238.; Al-Othman and Demopoulos, 2009Al-Othman, A., Demopoulos, G.P. (2009). Gypsum crystallization and hydrochloric acid regeneration by reaction of calcium chloride solution with sulfuric acid. Hydrometallurgy 96 (1-2), 95-102. https://doi.org/10.1016/j.hydromet.2008.08.010.; Leonzio, 2016Leonzio, G. (2016). Recovery of metal sulphates and hydrochloric acid from spent pickling liquors. J. Clean. Prod. 129, 417-426. https://doi.org/10.1016/j.jclepro.2016.04.037.) while separation techniques, such as evaporation (Anand et al., 2008Anand, S., Atul, N.V., Manukoda, S.K., Purushottam, K. (2008). An improved process for beneficiation of pickling wastewater with simultaneous recovery of hydrochloric acid and ferrous sulfate (Patent No. 216825). https://www.allindianpatents.com/patents/216825-an-improved-process-for-beneficiation-of-pickling-wastewater-with-simultaneous-recovery-of-hydrochloric-acid-and-ferrous-sulfate.) seem to be more appropriate for treating pickling liquids when the metal composition is quite complex. Although when the metal composition is homogeneous, without the presence of zinc, the recovery of acid by evaporation and the crystallization of FeCl₂+4H₂O can be combined upon reaching the saturation point of salt as indicated by the Kleigarn curve (Kleingarn, 1990Kleingarn, J.-P. (1990). Pickling in hydrochloric acid. Galvano-Organo-Traitements de Surface, 59 (605), 325-331.)

The DECAPVALUE project carried out at the National Center for Metallurgical Research of the Higher Council for Metallurgical Research (CENIM-CSIC) funded by HIASA Contract ID 20200543, has obtained ferrous chloride crystals with FeCl₂. 4H₂O in a pilot evaporation-crystallization plant using the company’s pickling liquor. HIASA (Asturias, Spain). Thus, all the results obtained reveal that it is possible to obtain crystals of FeCl₂. 4H₂O of high purity and high stability from pickling liqueurs (Alcaraz et al., 2021Alcaraz, L., Sotillo, B., López, F.A., Alguacil, F.J., Marco, J.F., Fernández, P. (2021). Obtention and Characterization of Ferrous Chloride FeCl₂·4H₂O from Water Pickling Liquors. Materials 14 (17), 4840. https://doi.org/10.3390/ma14174840.)

The crystallization technique is standard for the regeneration of pickling solutions with sulfuric acid, since at low temperature ferrous sulfate crystallizes very easily. Another alternative offered by crystallization is to add sulfuric acid to the spent pickling solution. Sulfuric acid reacts with ferrous chloride, thus releasing free hydrochloric acid into the solution together with ferrous sulfates according to the reaction (6):

F e {C l}_{2} + H_{2} {S O}_{4} \to 2 H C l + F e {S O}_{4}

(6)

The mixed acid is cooled below the freezing point of the water, which causes the crystallization of the ferrous sulfate heptahydrate (Brown, 2006Brown, C.J. (2006). Regeneration of hydrochloric acid pickle liquors by crystallization. Canadian Institute of Mining, Metallurgy and Petroleum, Chemionex Inc.). The pickling solution is now enriched with hydrochloric acid, although it contains traces of sulfuric acid, then separated from the ferrous sulfate crystals heptahydrate that can be easily marketed and the acid reused (Brown, 2006Brown, C.J. (2006). Regeneration of hydrochloric acid pickle liquors by crystallization. Canadian Institute of Mining, Metallurgy and Petroleum, Chemionex Inc.)

4. CONCLUSIONS

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The technologies with greater maturity used for the recycling of carbon steel pickling liquors correspond to pyrolysis (fluidized bed and spray roasting), in addition to those oriented to the production of FeCl₃. However, none of these technologies are valid for galvanic liquors and these must be treated with other less mature technologies. In this case, ion exchange resins or solvent extraction must be chosen, but in both cases the metal concentrations and the limitations of each technology must be considered.

REVIEW