Methods for Recovering Hydrochloric Acid from Waste Solutions

During thermal treatment processes applied in steel production, iron reacts with oxygen in the air, and as a result of this reaction, an oxide layer forms on the steel surface. This oxide layer can be in the forms of FeO, Fe2O3, Fe3O4 depending on the cooling rate of the metal. In order to apply surface treatment processes in the subsequent steps of production more efficiently, the oxide layer formed on the steel surface must first be removed. Hydrochloric acid and sulfuric acid are used to remove iron oxide layers on carbon steel material surfaces; phosphoric acid, nitric acid, hydrofluoric acid, and sulfuric acid are used together in surface cleaning of stainless steel material. This article will discuss commonly applied methods for recovering HCl from waste solutions generated as a result of acid surface cleaning of carbon steel material, in addition to the surface cleaning process itself. In the initial phase of the surface cleaning process, pools are filled with acid solution containing 15-22% HCl by weight. As a result of steel material contact with acid, the following main reactions occur depending on the type of metal oxide [1]. During the surface cleaning process, HCl concentration in the solution decreases over time while iron concentration increases. When HCl concentration reaches approximately 5% (w/w) and iron concentration reaches 250 g/L FeCl2 (w/v), the cleaning process slows considerably and becomes inefficient. In this case, the pools are emptied and refilled with fresh acid solution. The solution withdrawn from the pools is neutralized and then sent to waste disposal facilities. The drawbacks of this method are listed below: • Labor, time and production losses required for pool emptying, • Chemical costs required for neutralization, • Waste disposal costs, • Not an environmentally friendly system [1, 2] A sustainable approach to eliminate all the above disadvantages is to recover the waste solution and recycle it back into the system. Various recovery methods are found in scientific and technological literature. With these recovery methods, the disadvantages mentioned above are eliminated, and by maintaining HCl and iron concentrations in the solution at levels necessary for effective surface cleaning, a continuous and efficient process can be achieved. Commonly applied methods for recovering HCl from waste solutions resulting from surface cleaning processes are briefly reviewed below.

1- Pyrometallurgical Methods

Pyrometallurgical methods are among the commonly used methods for waste solution recovery. In this method, the solution after filtering solid particles is fed to a reactor which can be of the spray roasting or fluidized bed type, and is brought into direct contact with high-temperature combustion gas. In the reactor, HCl and iron (III) oxide are formed through the following reaction. Fe2O3 is removed from the reactor as a solid. The HCl produced is in the vapor phase and is fed to a gas absorption column operating with water, and is removed from the system as an aqueous solution. The acidic solution obtained as a result of this process contains approximately 18% HCl and is recycled back to the surface cleaning pools (see Figure 1). This reaction is carried out in a spray roasting reactor at a temperature range of 350-550 degrees, while in a fluidized bed reactor it occurs at 820-880 degrees. The energy required for the reaction is obtained from the hot combustion gases, and the oxygen required is obtained from excess air fed with the combustion gas. [3, 4, 5] Fe2O3 obtained through pyrometallurgical methods at high purity is a by-product with commercial value and can be used in the industry as a colorant material. Pyrometallurgical methods are high energy-consuming processes (600-1200 kcal/L). At the same time, since work is carried out with a corrosive substance at high temperatures, maintenance costs are also high. Due to high investment and operating costs, it is not suitable for low-capacity systems. It is more suitable for high-capacity systems. [3, 6] [caption id="attachment_123238" align="aligncenter"] Figure 1. HCl Recovery by Spray Roasting Method [3].[/caption]  Another point to note is that when the zinc concentration in the waste solution is higher than 0.5 g/L, this method cannot be applied. The main reason is that zinc adheres to the reactor surface at high temperatures. At the same time, the purity of the by-product obtained as a result of the reaction is also affected and the commercial value of the product decreases. For this reason, zinc must be removed from the solution prior to application. Solvent extraction method can be used to separate zinc in the solution. In this method, mutually immiscible solutions and solvents (like water and oil) are mixed and selective transfer of zinc ions from the aqueous phase to the solvent phase is achieved. In the solvent extraction method, Tributyl Phosphate (TBP) and CYANEX 302 chemicals can be used as active agents. The acid solution separated from zinc ions is fed to the recovery process. As a second step, the solution containing solvent is mixed with water and zinc ions are transferred to water. The zinc-containing solution can be used in the galvanizing process. The solvent separated from zinc ions is used again in the extraction process. Recovery of solvent is a complementary step that determines the economics of the process. [7, 8, 9]

2. Evaporation

In the evaporation method, waste solution withdrawn from pools is filtered and then fed to the evaporator. From the top stream of the evaporator, HCl-H2O vapor mixture is obtained, while from the bottom stream, concentrated FeCl2 solution is obtained. If larger quantities of liquid are evaporated, FeCl2O crystals can also be obtained. The vapor exiting the evaporator is fed to an enrichment column through distillation, where HCl and H2O separation is performed. As a result of the process, HCl solution can be obtained up to 18% (w/w) concentration. Water vapor exiting the column is condensed, removed from the system, and sent back to the surface treatment process for reuse (see Figure 2). [2] Compared to pyrometallurgical methods, the operating costs of the evaporation method are approximately 50% lower. [4] [caption id="attachment_123240" align="aligncenter"] Figure 2. HCl Recovery by Evaporation Method [2][/caption] 

3. Ion Exchange

In this method, HCl in the solution passed through a column filled with anion exchanger resin is retained by the resin, while FeCl2 and water in the solution are removed from the system. Subsequently, the column is washed with water and HCl retained in the resin is recovered. [10] Studies in the literature have shown that with this method approximately 80-90% of the acid in the waste solution is recovered, while 45-60% of FeCl2 can be removed from the waste solution. Although the acid recovery rate is high, the acid concentration obtained contains approximately 8% HCl, which is lower compared to other methods. At the same time, the separation of 45-60% of FeCl2 from the solution is also a much lower rate compared to other methods. [4, 10]

4. Diffusion Dialysis

In the diffusion dialysis method, a selective permeable membrane is used. As seen in Figure 3, solution is fed from one side of the membrane while deionized water is fed from the other side. Due to the difference in acid concentration in the solutions on both sides of the membrane, acid is transferred through the selective permeable membrane to the deionized water side. Metal-containing solution with acid removed is withdrawn from one side of the membrane, while HCl-containing solution is obtained from the other side. As a result of the process, acid recovery is achieved at 80-90% level, while FeCl2 recovery can be achieved at 75% level. HCl solution is obtained at approximately 6% level. This concentration is lower compared to other methods. [4, 11]   [caption id="attachment_123241" align="aligncenter"] Figure 3. HCl Recovery by Diffusion Dialysis Method [11][/caption]

5- Recovery of Iron Sulfate Heptahydrate from Waste Solution and Hydrochloric Acid Recovery

In this method, sulfuric acid is added to the waste solution and through the following reaction, iron (II) sulfate and hydrochloric acid are formed. FeCl2 + H2SO4 → FeSO4 + 2HCl The solution obtained as a result of the reaction is cooled and crystallized to obtain the commercially valuable FeSO4.7H2O by-product (see Figure 4). As a result of the reaction, 30% HCl solution is obtained by weight. With this method, 90% of the iron in the waste solution can be recovered as iron sulfate. [7, 12, 13] [caption id="attachment_123242" align="aligncenter"] Figure 4. Recovery of Iron Sulfate Heptahydrate from Waste Solution and Hydrochloric Acid Recovery [7, 13] . [/caption]  Research conducted in scientific and technological fields has revealed numerous studies on the recovery of waste acid solutions generated as a result of metal surface treatment. Pyrometallurgical methods are among the most commonly used methods in industrial facilities. However, since the investment and operating costs of pyrometallurgical methods are higher compared to other systems, the payback period for investment in low-capacity systems is long and for this reason they cannot be applied in low-capacity systems. In ion exchange and membrane separation methods, higher amounts of FeCl2-containing products are obtained compared to other methods. Although the FeCl2 by-product has commercial value, its conversion to FeCl3, which has more widespread commercial use, would add greater value to the system. From the perspective of removing iron ions from waste solutions, ion exchange and membrane separation methods provide lower separation efficiency compared to other methods. As mentioned above, the low iron ratio in the recovered solution affects the efficiency of the surface cleaning process.

References

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Nurcan Demirçivi Chemical Engineer Sistemas