(H2SO4) and hydrochloric acid (HCl) are commonly used for pickling steel.

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1 ACID RECYCLING BY KURT ETTER AND THOMAS LANGILL, PH. D ACID IN THE GALVANIZING PROCESS One of the most important operations in hot-dip galvanizing is the surface preparation of the steel. The quality of the zinc coating depends on the thoroughness of the metal surface preparation prior to dipping the work into the zinc bath. The metal surface should be entirely free from any oil or grease films and from any adhering metal oxides, mill scale, and rust. Since the galvanizing process is a metallurgical reaction between clean steel and molten zinc, it is critical to prepare the surface so that iron and zinc can easily interdiffuse. The most critical surface preparation step is the pickling of the work in an acid bath. The main purpose of acid pickling is to remove iron oxides. Any traces of iron oxides or scale will prevent iron and zinc interdiffusion, leaving an uncoated area on the steel. On the other hand, over-pickling (acid attacking the base steel) can result in pitting of metal which leads to a rough, blistered coating. Normally, all ferrous metals have a surface oxide or scale as a result of high temperature rolling and annealing and this layer must be removed prior to hot-dip galvanizing. Pickling is the chemical removal of these oxides from the surface by immersion in an acid solution. Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are commonly used for pickling steel. HYDROCHLORIC ACID Hydrochloric acid pickles rapidly at room temperature and is less corrosive to handling equipment than sulfuric acid. However, the fumes released are a greater problem with hydrochloric acid but they can be controlled with modern hoods and scrubbers. Most galvanizers find working with hoods to be very difficult so they are not often found in galvanizing plants. The hydrochloric acid fumes condense on cold surfaces around the plant and are very destructive to any steel parts. The origin of hydrochloric acid being used with iron and steel comes from historical chemical processing. In the Leblanc process, which was created around 1790, salt is converted to soda ash using sulfuric acid, limestone, and coal. This process created sodium carbonate and released hydrogen chloride as a by-product. Until the Alkali Act of 1863 prohibited it, excess hydrochloride gas was vented to the air. After the passage of the act, soda ash producers were obliged to absorb the waste gas in water, producing hydrochloric acid on an industrial scale. The 3D model of a hydrochloric molecule can be Copyright 2005 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use. 1

2 seen in Figure 1 and the chemical properties for hydrochloric acid can be seen in Table 1 in Appendix 2. Figure 2: 3D Model of Sulfuric Acid Figure 1: 3D Model of Hydrochloric Acid SULFURIC ACID Sulfuric acid is a less costly material to purchase than hydrochloric acid, but must be operated at higher temperatures and requires periodic removal of less soluble salts. In addition, sulfuric acid is corrosive to handling equipment and can produce highly corrosive mists and fumes. Concentrated sulfuric acid may also react explosively with water addition. Sulfuric acid was first discovered around the year 1600 by Johann Van Helmont who used the combination of destructive distillation of green vitriol or ferrous sulfate and burned sulfur. The first major industrial demand for sulfuric acid was created by the Leblanc process of producing soda ash. Sulfuric acid was first produced in bulk at Nordhausen, Germany, from the Van Helmont process but was expensive. The contact process was originally developed around the year 1830 by Peregrine Phillips in England, but was not used much until a need for concentrated acid arose, particularly for the manufacturing of synthetic organic dyes. The 3D model of a sulfuric molecule can be seen in Figure 2 and the chemical properties for sulfuric acid can be seen in Appendix 2. RECYCLED ACIDS Recycled hydrochloric acid is very clean and returns a contaminant-free acid to the pickling tank. The ferrous chloride by-product of the recycling process includes any of the contaminants that may have made their way into the pickling tank. However, the opposite is true with recycled sulfuric acid. A very clean zinc-iron crystal byproduct results but the recovered acid stream returned to the pickle tank may include other contaminants. Acid recycling can be done using either an in-line or off-line process. The in-line process is completed by pumping the pickling solution directly from the pickling tank, recycling it, and then returning it to the tank for service. The off-line process is completed by pumping the pickling solution to a separate tank where it is batch treated. EVAPORATIVE RECYCLING OF HYDROCHLORIC ACID The evaporative recycling process returns a concentrated acid with a very low level of metallic contaminants. In addition, it also produces a concentrated by-product, which can be further processed into a crystalline form if desired. The cost of building an evaporative recycling process system to handle smaller industrial volumes of hydrochloric acid waste is now much more affordable because of the advances in superplastics, automated controls, and new fabrication techniques. 2

3 One evaporative recycling system that is available starts with a pump that forces the spent hydrochloric acid through a pre-filter and into the evaporator loop. In the evaporator loop, the spent acid begins to vaporize at approximately 216 F (102 C) and the solution increases in concentration as the temperature increases in the loop. When the proper solution temperature and concentration is reached, a ferrous chloride concentrate is slowly extracted from the loop and transferred to a storage tank. The acid and water vapors are forced by expansion out of the separator and into the rectifier. The concentration of acid is also controlled in this step in order to return the best quality hydrochloric acid possible. The remaining water vapor is stripped of any acid and is sub-cooled and condensed to nearly pure water in the condenser. This water can then be reused in the pickling rinse process or can be returned to the pickling tank with the concentrated acid. A photo of this system is shown in Figure 3, and the flow diagram is shown in Figure 5 in Appendix 1. Figure 3: Evaporative Recycling Process MOLECULAR DISPLACEMENT OF HYDROCHLORIC ACID One molecular displacement system that is available uses a patented, closed-loop, molecular displacement technology that regenerates a noncontaminated hydrochloric acid and a virtually chloride-free by-product without any waste. It is a low maintenance, computer operated system that requires only 600 square feet of space. The system also has a low energy requirement and provides a savings of approximately 50 percent on combined acid and disposal costs. FREEZE CRYSTALLIZATION OF SULFURIC ACID The freeze crystallization recycling process will efficiently chill the acid and significantly reduce the contaminating iron concentration. The iron is removed in the form of crystalline ferrous sulfate heptahydrate. The recovered acid is then preheated and returned to the pickling tank for further use. The by-product, which has numerous agricultural and commercial uses, can then be sold and may produce enough revenue to offset the cost of operating the equipment. One freeze crystallization system that is available starts with a chemical pump drawing the pickling acid from the tank and forcing it through a prefilter to remove any gross particles. Then, once the acid solution has been filtered, the fluid is passed through a valve and flow meter, which allows the operator to adjust the flow rate and view the clarity of the solution. The acid solution then enters the primary exchanger where the temperature is reduced. Then, in the reactor, the acid is chilled to a level where the iron begins to form an iron sulfate crystal or crystalline ferrous sulfate heptahydrate. This is done using special thermoplastic heat exchangers in the solution to pass a refrigerant through the inner tubes that absorb heat from the solution and displaces it in the chiller condenser loop. In addition, a specially designed mixer in the reactor keeps the crystals from forming a cake of solid crystal on the exchanger by agitating the solution across the exchange tubes. Then, the solution is fed through the crystal settler that is a conical-bottomed, fiberglass tank with an overflow port that allows the clarate, which is a solution without iron crystals, to return to one-half of the two-stage pump back station. Then, this chilled acid is pumped through the primary exchanger and is heated by the hot acid coming in from the process tank. 3

4 The iron crystals are gravity fed to the crystal settler and then pumped to the centrifuge, where they are separated from the remaining solution. The centrate, which is the solution with iron crystals that has been discharged by the centrifuge, is then returned to the other half of the two-stage pump back station. Then the solution is pumped back to the reactor where the small crystals that remain are reseeded and grown. Then, the acid is drawn from the tank by means of a centrifugal, chemical pump and fed through the pre-filter to remove any remaining gross particles. It is passed through a valve and flow meter, which again allows the operator to adjust the flow rate and view the clarity of the recycled solution. A microprocessor is located in the control panel and creates a highly automated system to minimize the need for operator intervention. The only routine maintenance needed is the removal of the dry crystals from the crystal collector and changing of the filters. A photo of this system is shown in Figure 4, and the flow diagram is shown in Figure 6 in Appendix 1. Figure 4: Freeze Crystallization Process Another freeze crystallization system that is available starts with a chemical pump drawing the 150 F pickling acid from the tank. Then, it is forced through a Teflon impregnated heat exchanger in the plate & frame heat exchanger, which cools it to 110 F. The pickling acid is then pumped through the chiller, which cools it to 55 F. This cooling starts the formation of the ferrous sulfate heptahydrate crystals. The cooled acid then enters a specially designed conical tank where the crystals precipitate and settle to the bottom of the tank. The precipitated crystals are then pumped from the bottom of the tank into the centrifuge that separates the remaining acid from the crystal slurry to create a dry ferrous sulfate heptahydrate crystal, which falls from the centrifuge into a receptacle. The remaining clean acid is reheated, then goes through the pump back station, and finally is sent to the plate & frame heat exchanger, where it is reheated to 110 F and returned to the pickling tanks for reuse. The flow diagram for this system is shown in Figure 7 in Appendix 1. OTHER SYSTEMS AVAILABLE There are a few other systems that are available which use different recycling processes for hydrochloric and sulfuric acid. The systems for hydrochloric acid recycling use a distillation process and a reaction plant. The systems for sulfuric acid recycling include a stripping process and a SATCO process. These processes are similar to the other processes described above and the flow diagrams for these systems are shown in Figure 8 - Figure 12 in Appendix 1. TYPICAL COST OF A SYSTEM A small to medium-size galvanizing or steel manufacturing plant spends approximately $14 per ton to clean its products. With any of these recycling systems, the plant can slash that cost to approximately $3.40 per ton. A current statement from one of the suppliers of these recycling systems estimated the price of them at about $250,000 with around a 73 percent internal rate of return, and a payback of 1.4 years for a onegallon-per-minute system. The recycling systems also save energy. The steam consumed by a recycling system costs approximately $0.03 of energy per gallon of acid processed compared with about $3 per gallon to neutralize acid, which is the process currently used to ready acid for disposal. An HCl acid recycling system operating at full capacity, which is approximately 6,600 gallons or 25,000 liters processed per day, is estimated to save about 24 4

5 billion Btu per year over conventional transportation and disposal energy use. In addition to the energy that is saved by eliminating the acid-neutralization step, the cost of transporting and disposing of the waste is also eliminated and the ferrous chloride by-product of the recycling process can be sold for up to $100 per ton. SELLING OF BY-PRODUCTS PRODUCED The process of selling the by-product from recycled hydrochloric acid has been somewhat problematic. The only industry that uses the ferrous chloride is the wastewater treatment industry and zinc in the ferrous chloride cannot be tolerated because it kills the microbes used for organic treatment of wastewater. This means that unless the galvanizer can guarantee zinc is isolated from the pickle solution the ferrous-zinc chloride that is created will not be appropriate for wastewater and will require disposal. However, the process of selling the by-product from recycled sulfuric acid, even when containing zinc, is usually not a problem. The three industries that use these crystals are the ph adjustment industry, the micronutrients industry, which uses it for both fertilizer and animal feed, and certain chemical processing industries (zinc production and cyanide reduction) where it is used in gold mining operations. SHOULD YOU CONSIDER A RECYCLING SYSTEM The disposal of either hydrochloric or sulfuric acid is definitely an easier process than the recycling process because the disposal process requires no maintenance and demands minimal operator attention. The main problem with the disposal process is that the pickle tank can be out of service for a certain period and if sulfuric acid is being used, a reheating period is required. In addition, the escalating cost of the disposal process as well as changing community standards can make acid disposal an even less attractive option. When considering the recycling process versus the disposal process, the long-term condition of the pickling tank should also be considered. The pickling tank is not regularly emptied and cleaned, and this creates the possibility that over time a build-up of chemicals can adversely affect either the recycling process or the equipment being used. In addition, the environmental restrictions on the disposal of hazardous wastes in North America have increased the cost and liability of handling spent acid to such a high level that recycling of the acid at the source is both environmentally and economically preferred. In addition, as the cost of the technology required for recycling of either hydrochloric or sulfuric acid decreases, there is an expected increase in the number of recycling systems that can be used economically. 5

6 APPENDIX 1: FLOW DIAGRAMS Figure 5: Evaporative Recycling Process for Hydrochloric Acid Recycling Figure 6: Freeze Crystallization Process for Sulfuric Acid Recycling 6

7 Figure 7: Freeze Crystallization Process for Sulfuric Acid Recycling Figure 8: Distillation Process for Hydrochloric Acid Recycling 7

8 Figure 9: Spray Roast Process for Hydrochloric Acid Recycling Figure 10: Reaction Process for Hydrochloric Acid Recycling 8

9 Figure 11: Stripping Process for Sulfuric Acid Recycling Figure 12: SATCO Process for Sulfuric Acid Recycling 9

10 APPENDIX 2: PROPERTY TABLES Table 1: Chemical Properties of Hydrochloric Acid General Systematic name Hydrochloric acid Other names Muriatic acid Molecular formula HCl in water (H 2 O) Molar mass g/mol (HCl) Appearance Clear liquid, colorless in pure form CAS number [ ] (HCl) Properties Density, phase 1.18 g/cm 3, Solubility in water fully miscible Melting point -26 C (247 K) Boiling point 48 C (321 K) Viscosity 28.7 cp at 20 C Hazards MSDS External MSDS Table 2: Chemical Properties of Sulfuric Acid General Systematic name Sulfuric Acid Other names Oil of Vitriol Molecular formula H 2 SO 4 Molar mass g/mol Appearance Clear, colorless, odorless oil CAS number [ ] Properties Density and phase 1.84 g/cm 3, liquid Solubility in water fully miscible Melting point 10 C (283 K) Boiling point 337 C (610 K) Viscosity 26.7 cp at 20 C Hazards MSDS External MSDS NFPA 704 NFPA 704 R-phrases R34, R37 R-phrases R35 S-phrases S26, S36, S45 S-phrases S1/2, S26, S30, S45 RTECS number MW RTECS number WS

11 APPENDIX 3: REFERENCES American Galvanizers Association, < (Accessed ). ASTEC Engineering < (Accessed ). Beta Control Systems, Inc., An Ecologically Sound Alternative to Dumping Waste Pickling Acid, November 2005, USA. Beta Control Systems, Inc. < (Accessed ). Depend EQ < (Accessed ). Greener Industry < (Accessed ). New Zealand Steel < (Accessed ). Phoenix Systems, Inc. < (Accessed ). Wikipedia < (Accessed ). Photos and flow diagrams courtesy of: o ASTEC Engineering o Beta Control Systems, Inc. o Corradina Enterprises, LLC o DependEQ o Greener Industry o New Zealand Steel o Phoenix Systems, Inc. 11