Acid Recovery from Waste Surface Cleaning Solutions

Acid pickling is the process of cleaning metal surfaces using strong mineral acids known as surface cleaning solutions to remove impurities such as rust and metal oxides from metal surfaces. Hydrochloric acid and sulfuric acid are commonly used acids. Phosphoric, hydrofluoric, or nitric acid are also used. Sulfuric or hydrochloric acid is used for surface cleaning of carbon steel; phosphoric, nitric, and hydrofluoric acid, together with sulfuric acid, are used for surface cleaning of stainless steel. Acid pickling operations can be batch or continuous depending on the product being processed. This article examines the recovery of sulfuric acid or hydrochloric acid from waste surface cleaning solutions.

Chemical reactions:

Impurities on the steel surface may be metal oxides such as FeO, Fe2O3, and Fe3O4. The main reactions occurring during the process are as follows.

In surface cleaning with sulfuric acid:

  The resulting ferrous sulfate can be in monohydrate (1 mole hydrated) or heptahydrate (7 moles hydrated) form depending on crystallization temperature. In surface cleaning with hydrochloric acid: Inhibitors are generally added to the solution to reduce the acid's effect on the base metal. The added inhibitor does not change the acid's effect on the metal oxide layer. As shown in Figure 1 and Figure 2, an increase in temperature and acid concentration increases the surface cleaning rate. As shown in Figure 3, an increase in FeCl2 concentration up to a certain level increases the surface cleaning rate. As surface cleaning continues, the acid in the solution becomes depleted and iron compounds increase, at which point the surface cleaning operation becomes inefficient. At this point, the spent solution is discharged and the tanks are refilled with fresh acid. The following processes can be applied to these solutions: 1. Off-site disposal, 2. Neutralization and settling in-house followed by treatment in wastewater treatment facilities, 3. Acid recovery. The disadvantages of operations 1 and 2 are listed below: • High cost of off-site disposal or neutralization/settling operations, • Loss of acid through disposal. Therefore, increased need for fresh acid, • Time and production loss caused by emptying and refilling acid tanks, • Adverse effects of variable acid and iron concentrations in cleaning tanks on cleaning time and quality, • Environmental unfriendliness. Figure 1. Surface cleaning time for sulfuric acid [2] Figure 2. Surface cleaning time for hydrochloric acid [2]

Figure 3. Effect of FeCl2 concentration on surface cleaning time (20°C). [2]

Advantages of acid recovery:

• Environmental friendliness, • Absence of disposal or neutralization/settling costs, • No acid loss. Therefore, reduced need for fresh acid, • Increased production efficiency since it is a closed-loop system, • Increased cleaning rate and quality due to ideal and stable acid/iron concentrations in surface cleaning tanks, • Reuse of water obtained in HCl recovery system within the operation, • Production of commercially valuable by-products such as ferrous sulfate heptahydrate and iron(II) chloride, • Ease of installation and operation of modular recovery systems.

Figure 4. Acid Recovery Systems

The acid solution from cleaning tanks is fed to the recovery system. Through the recovery system, by-products such as ferrous sulfate heptahydrate and iron(II) chloride are obtained. The recovered acid is sent back to the cleaning tanks. Acid is added to the system in accordance with the chemical reactions mentioned above. In the sulfuric acid recovery system, ferrous sulfate along with water exits the system in heptahydrate form. According to total mass balance, water must be added to the system. Rinse water can be used for this purpose. In the hydrochloric acid recovery system, the water obtained from the condenser can be reused in the operation as rinse water or for other purposes.

1.0 Recovery of Sulfuric Acid by Cooling Crystallization Method

Surface cleaning time for sulfuric acid decreases as temperature increases (Figure 1). Many surface cleaning tanks operate at 15-20% sulfuric acid concentration and in the range of 45°C to 60°C. When this solution is cooled to around 10-15°C, a large portion of the ferrous sulfate in the solution crystallizes in ferrous sulfate heptahydrate form and separates from the solution. The solubility of ferrous sulfate in sulfuric acid can be seen in Figure 5. Iron concentration in cleaning tanks is maintained around 55-65 g/L. The process can be described as follows (Figure 6): 1. The acid solution is passed through a filter, then cooled through an economizer before crystallization. This solution is cooled with the cold solution exiting the crystallization system. In the economizer, the temperature of the recovered acid is increased while the temperature of the solution fed to the crystallization system is decreased. This way, energy required for heating and cooling is kept to a minimum. 2. The cooled acid solution is fed to the crystallization system. In the crystallizer, ferrous sulfate heptahydrate (FeSO4.7H2O) crystals form due to supersaturation created in the further cooled solution. Care should be taken in the design to prevent scale formation in the heat exchanger. 3. Since acid recovery processes are closed-loop continuous systems, ideal and stable acid/iron concentrations can be maintained in the surface cleaning tanks. 4. Ferrous sulfate heptahydrate (FeSO4.7H2O) is a commercially valuable by-product. It can be used in the agricultural sector and in wastewater treatment facilities.

Figure 5. Solubility of Ferrous Sulfate in Sulfuric Acid [3]

As mentioned previously, water separates from the system in ferrous sulfate heptahydrate form. According to total mass balance, water must be added to the system. Another important issue is that the working environment is highly corrosive due to high temperature and acid concentration. Equipment materials must be selected carefully.

Figure 6. Recovery of Sulfuric Acid by Cooling Crystallization Method

Figure 7. Modular Design of Sulfuric Acid Recovery Facility

2.0 Recovery of Hydrochloric Acid with Evaporation Technology

Recovery of hydrochloric acid from surface cleaning solutions differs from the sulfuric acid recovery method. The process can be described as follows (Figure 8): 1. After filtration, the acid solution is fed to the evaporator. With the evaporation of acid and water, the concentration of iron(II) chloride in the liquid phase increases. If further evaporation is performed, iron(II) chloride dihydrate (FeCl2.2H2O) crystals are obtained. 2. The acid and water vapor exiting the evaporator enter the enrichment column. This is a type of distillation column. Water vapor is enriched in the column. Hydrochloric acid is taken as the bottom product and water as the top product. By changing the reflux ratio, product purity can be adjusted. Due to the azeotrope between HCl and water, acid concentration can reach up to 18%. 3. Water vapor condenses in the condenser and is sent back to the process for reuse. 4. The recovered acid is sent to the surface cleaning tanks. 5. Iron(II) chloride, which has commercial value, can be used in wastewater treatment facilities. It is used to eliminate odor problems in sewer systems. It reduces odors from hydrogen sulfide (H2S) by forming harmless iron compounds. It also reduces corrosion problems.

3.0 Economic Evaluation of the Acid Recovery System

Case Study: A facility with a steel processing capacity of 150 tons/day was analyzed. The off-site acid disposal method and the evaporation acid recovery method were compared. Assumed Values: Off-site acid disposal cost: USD 50.42/ton Fresh acid cost - HCl (32%): USD 102.5/ton Electricity unit price: USD 0.083/kWh Natural gas unit price: USD 0.3227/Nm³ Operating hours: 7,035 hours/year FeCl2.2H2O sales price: USD 100/ton Auxiliary Facilities: Electricity consumption: 15 kW Natural gas consumption: 32.25 Nm³/hour Table 1. Cost comparison of the two methods If an acid recovery system is used: Annual savings: 307,936 – 169,073 = USD 138,863/year Income from by-product sales: USD 61,200/year Total annual gain: 138,863 + 61,200 = USD 200,063/year Investment cost: USD 500,000 Payback period of investment: (USD 500,000)/(USD 200,063)≈2.5 years

References [1] Lawrence K. Wang, Yung-Tse Hung, Nazih K. Shammas, Handbook of Advanced Industrial and Hazardous Wastes Treatment, CRC Press 2010. [2] Mika Maanonen, Steel Pickling in Challenging Conditions, Thesis 2014. [3] J.K. Seyler, W.E. Thornton, M.K. Householder, Sulfuric Acid and Ferrous Sulfate Recovery from Waste Pickle Liquor, EPA Jan 1974. [4] T. Özdemir, C. Öztin, N. S. Kıncal, Treatment of Waste Pickling Liquors: Process Synthesis and Economic Analysis [5] J. Cullivan, B. Cullivan, Economic and Chemical Comparisons of Hydrochloric Acid Recovery Technologies for Iron Pickling Operations, Wire Journal International, March 2016.

Şaban Kantaşlı Chemical Engineer Project Development Manager Sistemas Teknoloji A.Ş.