Lawrence K Wang Veysel Eroglu and Ferruh Erturk

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28.1 Introduction 1191

28.1.1 Metal Finishing Industry 1191

28.1.2 Acid Pickling and Acid Cleaning of Metal Surface 1192

28.1.3 Pickling Liquor and Waste Pickling Liquor 1192

28.1.4 Acid Pickling Operation 1192

28.1.5 WPL Treatment and Recycle 1192

28.2 Pickling Process Reactions and WPL Characteristics 1193

28.2.1 WPL Generation 1193

28.2.2 Sulfuric Acid Pickling Reaction 1193

28.2.3 Hydrochloric Acid Pickling Reactions 1193

28.3 Treatment of WPLs and Cleaning Wastes 1194

28.3.1 Treatment, Disposal, or Recycle 1194

28.3.2 Neutralization and Clarification (Sedimentation or DAF) 1194

28.3.3 Crystallization and Regeneration 1195

28.4 Treatment of Wastewater from Acid Pickling Tanks in a Galvanized Pipe Manufacturing Factory Using Sulfuric Acid 1197

28.4.1 General Description 1197

28.4.2 Characteristics of Wastewaters 1197

28.4.3 Treatment Methods 1201

28.5 Management and Treatment of Wastewater and Air Emissions from

Acid Pickling Tanks Using Hydrochloric Acid 1202

28.5.1 Environmental Management at Steel/Iron Hydrochloric

Acid Pickling Plants 1202

28.5.2 Manufacturing Plant of Stainless Steel Pipes and Fittings:

A Case History 1205

28.6 Summary 1210

References 1210

28.1 I NTRODUCTION 28.1.1 Metal Finishing Industry

Metal industries use substantial quantities of water in processes such as metal finishing and galvanized pipe manufacturing in order to produce corrosion-resistant products. Effluent wastewaters

CONTENTS

from such processes contain toxic substances, metal acids, alkalis, and other substances that must be treated, such as detergents, oil, and grease. These effluents may cause interference with biological treatment processes at sewage treatment plants. In the case when the effluents are to be discharged directly to a watercourse, treatment requirements will be more stringent and costly.1-8

28.1.2 Acid Pickling and Acid Cleaning of Metal Surface

Laser cutting, welding, and hot working leave a discolored oxidized layer or scale on the surface of the worked steel. This must be removed in order to perform many of the surface finishing processes. The acid pickling process is used to remove the oxide or scale of metals and corrosion products, in which acids or acid mixtures are used.

Acid cleaning is also used for removing inorganic contaminant not removable by other primary cleaning solutions. Acid cleaning has its limitations in that it is difficult to handle because of its cor-rosiveness and is not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels. The hydrogen from the acid reacts with the surface and makes it brittle and crack. Because of its high reactivity to treatable steels, acid concentrations and solution temperatures must be kept under control to ensure desired pickling rates.

Technically speaking, acid pickling is a treatment of metallic surfaces that is done to remove impurities, stains, rust, or scale with a solution called pickle liquor, containing strong mineral acids, before subsequent processing, such as extrusion, rolling, painting, galvanizing, or plating with tin or chromium. The two acids commonly used are hydrochloric acid and sulfuric acid.

28.1.3 Pickling Liquor and Waste Pickling Liquor

The most common acid used for pickling is sulfuric acid. Other acids such as hydrochloric, phosphoric, hydrofluoric, or nitric acids are also used individually or as mixtures. Sulfuric or hydrochloric acids are used for pickling carbon steels, and phosphoric, nitric, and hydrofluoric acids are used together with sulfuric acid for stainless steel. Water is used in pickling and rinsing. The quantity of water used can vary from <100 to 3000L/ton, depending on whether once-through or recycle systems are used.1,2

Carbon steel is pickled usually by either sulfuric acid or hydrochloric acid. At one time, sulfuric acid was the pickling agent of choice for picklers running integrated steel works.1 Hydrochloric acid is chosen in more modern lines when bright surfaces, low energy consumption, reduced overpick-ling, and the total recovery of the pickling agent from the waste pickle liquor are desired.

The spent pickling liquor is called waste pickling liquor (WPL), which must be properly treated for disposal or reuse. Wastewaters from pickling include acidic rinse waters, metallic salts, and waste acid. WPL is considered a hazardous waste by the U.S. Environmental Protection Agency (U.S. EPA).

28.1.4 Acid Pickling Operation

Pickle solutions that are used in the removal of metal oxides or scales and corrosion products are acids or acid mixtures.

Depending on the product being pickled, the acid pickling operation can be a batch or continuous process. In continuous strip pickling, more water is required for several operations such as the uncoilers, looping pit, and coilers. In the case of pickling hot rolled coils, the coils are transported to the pickling line. In the uncoiler section, the coil is fed through a pit containing water for washing off the surface dirt and then fed through the pickling line.

28.1.5 WPL Treatment and Recycle

Lime or alkaline substances are used to neutralize the waste pickle liquor. In addition, 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), oil and grease (O&G), ammonium nitrogen (NH4+ -N), pH, cyanides, fish toxicity, and several relevant metal ions such as cadmium (Cd2+), iron (Fe2+), zinc (Zn2+), nickel (Ni2+), copper (Cu2+), and chromate (Cr6+) have to be reduced below the maximum allowable limits.

Some acid pickling plants, particularly those using hydrochloric acid, operate acid recovery plants where the mineral acid is boiled away from the iron salts, but there still remains a large volume of highly acid ferrous sulfate or ferrous chloride to be disposed of. Since the 1960s, total hydrochloric acid regeneration processes have reached widespread acceptance.5 The by-product of nitric acid pickling is marketable to a couple of secondary industries including fertilizers.

28.2 PICKLING PROCESS REACTIONS AND WPL CHARACTERISTICS

28.2.1 WPL Generation

During the application of the pickling process in the finishing of steel, in which steel sheets are immersed in a heated bath of acid (sulfuric, hydrochloric, phosphoric, etc.), scales (metallic oxides) are chemically removed from the metal surface. The pickling process can be a batch or continuous process. In these processes, water is used in pickling and rinsing operations. In continuous pickling, wet fume scrubbing systems are also used. The effluent water from the pickling tanks, which is called the waste pickle liquor (WPL), consists of spent acid and iron salts. Waste hydrochloric liquor contains 0.5-1% free hydrochloric acid and 10% dissolved iron, and the production of WPL is approximately 1 kg free hydrochloric acid and 10 kg dissolved iron per ton of steel pickled.2 In waste sulfuric acid pickle liquor, the free acid and dissolved iron content are approximately 8% each, resulting in 10 kg each of free sulfuric and dissolved iron per ton of steel pickled. WPL may also contain other metal ions, sulfates, chlorides, lubricants, and hydrocarbons. Rinse water, which contains smaller concentrations of the above contaminants, ranges in quantities from 200 to 2000 L/ton. Fume scrubber water requirements range from 10 to 200L/ton.2

In hot rolling processes, pickling is used for further processing to obtain the surface finish and proper mechanical properties of a product. In the case of pickling hot rolled coils, the coil is fed through a pit containing water for washing off surface dirt and then fed through the pickling line. In the pickling section, the coil strip comes in contact with the pickle liquor (sulfuric or hydrochloric acid). Wastewater sources are processor water, waste pickle liquor, and rinse water.

In the case of batch pickling, the product is dipped into a pickling tank and then rinsed in a series of tanks. The quantity of wastewater discharged from a batch process is less than that from continuous operation. The wastewater is usually treated by neutralization and sedimentation.

28.2.2 Sulfuric Acid Pickling Reaction

In sulfuric acid pickling, ferrous sulfate is formed from the reaction of iron oxides with sulfuric acid:

The ferrous sulfate that is formed in the above reaction is either monohydrate or heptahydrate (FeSO4 ■ 7H2O).

28.2.3 Hydrochloric Acid Pickling Reactions

During the hot forming or heat treating of steel, oxygen from the air reacts with the iron to form iron oxides or scale on the surface of the steel. This scale must be removed before the iron is subsequently shaped or coated. One method of removing this scale is pickling with hydrochloric acid.5

Pickling is conducted by continuous, semicontinuous, or batch modes depending on the form of metal processed. In developing a National Emission Standard for the steel pickling industry, U.S.

EPA recently surveyed the industry and produced a background information document containing detailed information concerning the various processes in the industry, pollution control devices, and emissions.5

When iron oxides dissolve in hydrochloric acid, ferrous chloride is formed according to the following reactions:

Since Fe3O4 is Fe2O3FeO, the reaction for Fe3O4 is the sum of the two reactions. Some of the base metal is consumed in the above reaction as well as in the following reaction:

An inhibitor is usually added to lessen the acid's attack on the base metal while permitting it to act on the iron oxides. The rate of pickling increases with the temperature and concentration of HCl. As pickling continues, HCl is depleted and ferrous chloride builds up in the pickling liquid to a point where pickling is no longer effective. At this point, the old liquid is discharged and the pickling tank is replenished with fresh acid. Typical HCl concentrations in a batch pickling process are 12 wt% for a fresh solution and 4 wt% before acid replenishment. At these concentrations, the concentration of HCl in the vapor phase increases rapidly with temperature.5

28.3 TREATMENT OF WPLs AND CLEANING WASTES

28.3.1 Treatment, Disposal, or Recycle

Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralization. The lime neutralization process raises the pH of the spent acid and makes heavy metals in the sludge less likely to leach into the environment. Today, however, some of the spent pickle liquor can be recycled or regenerated on-site by steel manufacturers.5

The treated wastewater effluents, in general, can be either discharged to a watercourse or a public sewer system. In the former case, the treatment requirements will be more stringent.

The waste pickle liquor, rinse water discharges, and fume scrubber effluent can be combined in an equalization tank for subsequent treatment. Basically, three methods are used to treat the WPL:

1. Neutralization and clarification [sedimentation or dissolved air flotation (DAF)]

2. Crystallization of ferric sulfates and regeneration of the acid

3. Deep-well disposal.

The most commonly used methods are the first two.

28.3.2 Neutralization and Clarification (Sedimentation or DAF)

In old plants, neutralization and sedimentation are applied to the treatment of wastewaters in general, including WPL. A typical treatment system for continuous pickling water is shown in Figure 28.1.1,3 In an integrated steel mill, a central wastewater treatment system is used to treat wastewater from pickling lines, cold rolling mills, and coating lines.

The pickling wastewater has a low pH and contains dissolved iron and other metals. The blow-down and dumps from the cold rolling mill solutions, which may contain up to 8% oil, are collected in emulsion-breaking tanks in which the emulsions are broken by heat and acid. The oil is then

FIGURE 28.1 Typical treatment system for pickling. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

FIGURE 28.1 Typical treatment system for pickling. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

skimmed, and the water phase containing 200-300 mg/L of oils is treated together with the wastewaters from pickling, cold rolling, and coating lines. The combined wastewater flows to a settling and skimming tank where solids and oil are removed. The effluent from the settling/skimming tank is then treated in a series of settling tanks where chemicals (coagulants and/or lime) and air are added to oxidize the remaining iron to ferric ions (Fe3+), to further break the oil emulsions and neutralize the excess acid in the wastewater. The effluent from the mixing tanks then enters a flocculator/clarifier system, the overflow from the clarifier is discharged, and the settled sludge is pumped to a dewatering system consisting of centrifuges, belt, or vacuum filters. The dewatered sludge is disposed and the water phase returned to the clarifier effluent.

The clarifier shown in Figure 28.1 can be either a sedimentation clarifier, a DAF clarifier, or a dissolved air flotation-filtration (DAFF) clarifier, depending on the space availability, pretreatment requirements, effluent limitations, and costs.6-11 Modern pickling plants use DAF or DAFF for more cost-effective clarification or more efficient clarification, respectively.

28.3.3 Crystallization and Regeneration

The use of lime or other alkaline substances to neutralize acid is quite costly, especially when large capacities are involved. Also there are potential values in the acids and ferrous ion, and therefore, recovery of these substances will not only reduce the pollution load, but their sale or reuse will represent a profit to the industry.

Crystallization is one of the treatment methods for sulfuric acid waste pickle liquor. Thus, it is possible to decrease the pollution load and at the same time recover various hydrates of FeSO4. The crystallization of FeSO4 depends on the characteristics of the water and acid, and solubility of FeSO4. The solubility of ferrous sulfate as a function of temperature and sulfuric acid concentration is shown in Figure 28.2.4 In this figure, FeSO4 ■ 7H2O is dominant in region A, FeSO4 ■ 4H2O in region B, and FeSO4 ■ H2O in region C.

The crystallization of ferrous sulfate as heptahydrate is commonly used today. The concentration of iron in the acid bath is approximately 80 g/L as Fe3+. The crystallization of FeSO4 ■ 7H2O is achieved by cooling the acid waters in heat exchangers or evaporation under vacuum after pickling.

g 20

g 20

% H2SO4

% H2SO4

10__

25_

Temperature, °C

40 60

Temperature, °C

FIGURE 28.2 Solubility of ferrous sulfate FeSO4 as a function of temperature and sulfuric acid concentration. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

Make-up acid must be added to the bath. During countercurrent cooling, the acid bath waste passes through two to three crystallization tanks and is cooled down between 0°C and 5°C. The crystallized ferric sulfates are recovered by centrifuging. A typical flow diagram of FeSO4 ■ 7H2O crystallization is shown in Figure 28.3.

The WPL is sprayed above a cyclone crystallizer, and air is blown from the bottom countercurrent to the liquid. A packing material is also present in order to increase the area of contact between the air and the liquid. The acid wastewaters are then cooled, and the FeSO4 ■ 7H2O crystals are recovered by centrifuging.

In the Ruthner process,1 the WPL is first concentrated in an evaporator. The concentrate is then pumped to a reactor where it is combined with hydrochloric acid gas, in which ferrous chloride and sulfuric acid are formed. The sulfuric acid is then separated by centrifuging. The ferrous chloride

FIGURE 28.3 Flow diagram of ferrous sulfate FeSO4 • 7H2O crystallization. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

FIGURE 28.3 Flow diagram of ferrous sulfate FeSO4 • 7H2O crystallization. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

goes to a roaster in which it is converted to ferric oxide. The gases liberated from the roaster and the acid from the centrifuge go to a degassing chamber, and the sulfuric acid is removed and returned to the pickling process, or can be sold. The remaining gases from the degasser are passed through an absorption system and then reused in the reaction chamber.

In the Lurgi process1 that was developed in Germany, hydrochloric acid is recovered from the WPL. The acid is regenerated in a fluidized bed. During pickling with HCl, the acid circulates between a pickling tank and a storage tank and the acid reacts with the iron oxide scale from the steel producing ferric chloride, resulting in increasing concentration of dissolved iron and decreasing concentration of acid.

In the Lurgi system,1 the acid level in the pickling liquor stays constant at about 10%. A continuous bleed stream is removed from the system at the same rate as it is pickled. The bleed stream, or spent pickle, is fed to a pre-evaporator and heated with gases from the regeneration reactor. Concentrated liquor from the pre-evaporator then enters the lower part of the reactor containing 13% acid and 20% ferrous chloride. The reactor contains a fluidized bed of sand and is fired by oil or gas to maintain an operating temperature of about 800°C. The reaction products leave from the top of the reactor. The ferric oxide is removed by a cyclone, and the hot gases enter the pre-evaporator. The overhead from the evaporator, which is at a temperature of about 120°C, contains water vapor, HCl, combustion products, and also some HCl that vaporizes directly from the plant liquor that enters the system. The gas mixture from the pre-evaporator enters the bottom of the adiabatic absorption tower, where HCl is absorbed by another bleed stream of the pickle liquor, and thus the regenerated acid is placed back in the pickle liquor circuit. The regenerated acid contains 12% acid and about 70 g/L of iron. The unabsorbed gases move to a condenser.

28.4 TREATMENT OF WASTEWATER FROM ACID PICKLING TANKS IN A GALVANIZED PIPE MANUFACTURING FACTORY USING SULFURIC ACID

28.4.1 General Description

This study was conducted at Cayirova Boru Sanayii AS (a galvanized pipe manufacturing factory) in Gebze, Kocaeli, Turkey.1,3 At this plant, batch pickling is applied. During the manufacturing process, the pipes are immersed in an acid bath that contains 25% sulfuric acid at 80°C and then prepared for the galvanization process by passing through cold water, hot water, and flux baths. The purpose of a cold water bath is to clean the acid from the surface of the pipes following pickling. A hot bath is applied in order to dry and prevent water and acid from entering a flux bath. The purpose of the flux bath, in which ammonium zinc chloride (NH4ZnCl3) is used, is to prepare a suitable surface for galvanization and prevent oxidation of the pipe. The flow diagram of the baths is shown in Figure 28.4.

Acid bath wastewaters are usually discharged once a week. The average flow rate of these wastewaters is 4 m3/h, with a maximum of 8 m3/h. The hot and cold water baths are discharged once every 15 days. The quantities and flow rates of these wastewaters are shown in Table 28.1.1,3

28.4.2 Characteristics of Wastewaters

Wastewater characteristics must be known in order to select a suitable treatment system. For this purpose, the wastewater samples taken from the sources were analyzed to determine various parameters. Also, the quantities of chemicals (NaOH) required for neutralization and settling characteristics were determined. These were made separately for continuous and batch discharges. Since the system is to be designed according to the continuous discharge of wastewaters from the batch system to the treatment plant, "mixed wastewater" was prepared in quantities proportional to the flow rates. The quantity of NaOH required for 1000 mL of mixed wastewater is shown in Table 28.2.1,3

Clean water Water

FIGURE 28.4 Flow diagram showing sources of wastewaters in the galvanized pipe manufacturing process. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

Clean water Water

FIGURE 28.4 Flow diagram showing sources of wastewaters in the galvanized pipe manufacturing process. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

TABLE 28.1

Types and Quantities of Wastewaters in Acid and Flux Baths

TABLE 28.1

Wastewater Source

Average

Maximum

Continuous discharge from hot and cold water baths

4m3/h

6m3/h

Intermittent discharge (once every 7 days)

15 m3

15 m3

Cold water bath (once every 15 days)

15 m3

15 m3

Hot water bath (every 15 days)

15 m3

15 m3

Flux bath

5 m3

5 m3

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

TABLE 28.2

Quantities of Wastewater Required for 1000 mL "Mixed Wastewater"

Units Flow Rate (m3/2 months) Quantity of NaOH Required for 1000 mL

Continuous discharge 8640 971

Acid bath 129 14.5

Cold water bath 60 6.8

Hot water bath 60 6.8

Flux bath 5 0.56

Total 8894 1000

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

TABLE 28.3

Experimental Results for Continuous Discharge

TABLE 28.3

Experimental Results for Continuous Discharge

Parameter

Unit

Original Sample

After Neutralization and !

Total iron

mg/L

598G

350

Chromate

mg/L

G

0

Lead

mg/L

G

0

COD

mg/L

35G

20

Zinc

mg/L

G

0

pH

1.6

8.0

Color

Greenish

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

Since the continuous discharge quantities are much larger compared to batch discharges, they were analyzed separately. The wastewaters from continuous discharge were neutralized with 2NNaOH. The results are given in Table 28.3.13 The settling characteristics of the continuous discharge wastewaters are shown in Table 28.4.13 The experimental results from the "mixed wastewaters," the quantities of which were shown in Table 28.2, are given in Table 28.5.13 The settling characteristics of the mixed wastewaters are shown in Table 28.6.3

Neutralization can also be carried out by a combination of NaOH and lime. Experiments were conducted in order to determine the optimum combination of NaOH and lime. For this purpose, various quantities of lime were added to 1L of mixed wastewater, and then the amount of NaOH required was determined to obtain a pH of 8.5. The results are shown in Table 28.7.3

As can be seen from Table 28.7, the required dosage of NaOH does not increase significantly when the limb dosage is more than 20g/1000mL. The mixed wastewater, which was treated with the dosages of lime and NaOH shown in Table 28.7, was then aerated for 15min after the pH reached 8.5. After aeration, it was allowed to settle for a period of 30-120 min. An analysis of the clear phase after settling is shown in Table 28.8.3 The wastewater was treated with 15g/L of lime

TABLE 28.4

Settling Characteristics of Continuous Discharge Wastewaters

Time Volume of Clear Phase (mL/L)

15 min 20

30 min 50

3.5h 400

20 h 720

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/ hcl_section_3_1_4_steel_pickling.htm

TABLE 28.5

Experimental Results for Mixed Wastewater Samples

After Neutralization and Separation

TABLE 28.5

After Neutralization and Separation

Parameter

Original Sample

in Clear Phase

Total iron, mg/L

6100

300

Sulfate, mg/L

19,000

16,000

Chromate, mg/L

0

0

Lead, mg/L

0

0

Zinc, mg/L

15

0

COD, mg/L

360

15

pH

0.7

8.5

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

TABLE 28.6

Settling Characteristics of Mixed Wastewaters

Time Volume of Clear Phase (mL/L)

30 min 40

1h 100

3.5h 350

4.5h 410

53 h 460

20 h 700

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http:// www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_ 4_steel_pickling.htm

TABLE 28.7

Quantities of Sodium Hydroxide Required for Obtain a pH of 8.5

I 10g Lime II 20g Lime pH NaOH Added (mL) pH NaOH Added (mL)

Different Quantities of Lime to

III 26g Lime IV 32g Lime_

pH NaOH Added (mL) pH NaOH Added (mL)

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http:// www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

TABLE 28.8

Analysis of Mixed Wastewater after Neutralization, Aeration, and Clarification

TABLE 28.8

Analysis of Mixed Wastewater after Neutralization, Aeration, and Clarification

Parameter (mg/L)

Settling Time (min)

10

20

26

32

Iron

30

125

30

5

0

120

0

0

0

0

Sulfate

30

5750

5759

5000

3000

120

5750

5750

5000

2750

Settlable matter

30

120

280

320

440

120

400

520

410

480

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm

TABLE 28.9

Analysis of Wastewater after Neutralization, Aeration, and Clarification

Parameter Concentration (mg/L)

COD 0

Total iron 0

Zinc 0

Sulfate 2100

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_ section_3_1_4_steel_pickling.htm and NaOH to attain a pH of 8.5, aerated for 1 h, mixed for 23 h, and an additional hour was allowed for clarification. The analysis of the clear clarifier effluent is shown in Table 28.9.1,3

28.4.3 Treatment Methods

As was indicated in the previous section, the concentration of iron in the mixed wastewaters ranged from 5980 to 6100 mg/L; its pH was 0.7 and zinc concentration was 15 mg/L. Since these wastewaters come only from acid baths and not from other processes of the plant, parameters such as cadmium and fluoride are not encountered. The discharge standards for the metal industry effluents set by the Turkish Water Pollution Control Regulation (Official Gazette, Table 15.7, September 4, 1988) are shown in Table 28.10.1

The experiments conducted on wastewaters, the results of which were shown in the previous section, indicated that neutralization/aeration/settling gave satisfactory results. The sludge formed must be disposed after dewatering in a filter press, a horizontal belt filter, or a centrifuge. An equalization tank is required in order to compensate for the effects of intermittent discharges. The treated wastewater can then be recycled to be used in the process or discharged to the river. The flow diagram of the selected system is shown in Figure 28.5.

TABLE 28.10

Effluent Standards for Metal Industry Wastewaters in Turkey

Parameter 2-h Composite Sample (mg/L, except pH)

TABLE 28.10

Effluent Standards for Metal Industry Wastewaters in Turkey

Parameter 2-h Composite Sample (mg/L, except pH)

COD

200

Suspended solids

125

Oil and grease

20

Ammonium nitrogen

400

Cd

0.1

Fe

3

Flouride

50

Zn

5

Fish toxicity

10

pH, units

6-9

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/ WebHelpZhcl_section_3_1_4_steel_pickling.htm

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://www.epa.gov/tri/TWebHelp/ WebHelpZhcl_section_3_1_4_steel_pickling.htm

discharge

FIGURE 28.5 Flow diagram of the selected treatment system. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

discharge

FIGURE 28.5 Flow diagram of the selected treatment system. (From Eroglu, V. and Erturk, F., in Handbook of Industrial Waste Treatment, Wang, L.K. and Wang, M.H.S., Eds, Marcel Dekker, New York, 1991, pp. 293-306; Eroglu, V., Topacik, D., and Ozturk, I., Wastewater Treatment Plant for Cayirova Pipe Factory, Environmental Engineering Department, Istanbul Technical University, Turkey, 1989. With permission.)

28.5 MANAGEMENT AND TREATMENT OF WASTEWATER AND AIR EMISSIONS FROM ACID PICKLING TANKS USING HYDROCHLORIC ACID

28.5.1 Environmental Management at Steel/Iron Hydrochloric Acid Pickling Plants

Hydrochloric acid aerosols are produced and released into the air during the pickling process as HCl volatilizes, and steam and hydrogen gas with entrained acid fumes rise from the surface of the pickling tank and from the pickled material as it is transferred from the pickling tank to the rinse tank. Pickling and rinse tanks are covered and the acid fumes are generally collected and treated by control devices (e.g., packed tower scrubbers) to remove HCl. Emissions from many batch operations are uncontrolled. Pickling is sometimes accomplished in vertical spray towers. In this process, all the HCl in the pickling solution produces hydrochloric acid aerosols that are also used. Acid storage tanks and loading and unloading operations are also potential sources of HCl emissions. Uncontrolled HCl emissions from a storage tank may be of the order of 0.07-0.4 tons per year (tpy) of HCl per tank, depending on the tank size and usage. For each million ton of steel processed at continuous coil or push-pull coil model facilities, storage tank losses are estimated to amount to 0.39 tpy. For other types of pickling facilities, storage tank losses are estimated to be about 11.19 tpy of HCl per million ton of steel processed.

The U.S. EPA guidance for acid storage tanks can be applicable to storage tanks used in conjunction with the pickling process and may be extended to apply to the pickling process itself.6 For storage tanks, one applies the amount of hydrochloric acid aerosol generated from a tank under average 19 capacity and other conditions to the manufacturing threshold and multiplies that by the number of times the tank has been drawn down and refilled. The amount of acid aerosol manufactured during the pickling process can be similarly determined by the amount of HCl generated from the pickling tanks during the processing of a certain amount of material and scaling up that figure to apply to all the material processed by the same process and under the same conditions. The amount of hydrochloric acid aerosols lost from the pickling tanks counts toward the material released to air unless the aerosol is collected and removed before exiting the stack. The hydrochloric acid aerosol collected in a scrubber is converted to the nonaerosol form, not reportable; the hydrochloric acid aerosol removed by the scrubber is considered to have been treated for destruction.

Hydrochloric acid may be recovered from the WPL in an acid regeneration process. This process has the potential for emitting significant amounts of hydrochloric acid aerosols. The annual capacities of 10 acid regeneration plants surveyed by the U.S. EPA5 ranged from 3.2 to 39.8 million gallons (MG) per year for a single facility. The spray roasting acid regeneration process is the dominant one presently employed. One older facility used a fluidized bed roasting process.

In the spray roasting acid regeneration process, WPL at 2-4% HCl comes into contact with hot flue gas from the spray roaster that vaporizes some of the water in the WPL. The WPL then becomes concentrated pickling liquor (CPL). The CPL is then sprayed on the spray roaster where ferrous chloride in the droplets falling through the rising hot gases reacts with oxygen and water to form ferric oxide and HCl,

Flue gas containing HCl goes to a venturi preconcentrator and an absorption column. There, the generated acid contains approximately 18% HCl by weight. Emissions from acid regeneration plants range from about 1 to more than 10 tpy from existing facilities with and without pollution control devices (controlled and uncontrolled facilities).

Acid regeneration plants have storage tanks for spent and regenerated acid and these tanks are potential sources of HCl emissions. Emission estimates for uncontrolled and controlled storage tanks at acid regeneration facilities are 0.0126 and 0.008 tpy per 1000 gallons of storage capacity, respectively.

Acid recovery systems are used to recover the free acid in the WPL. They are not employed in larger facilities because they recover only 2-4% free HCl from the spent acid, but leave the FeCl2 in the solution that must be processed or disposed of separately. These acid recovery systems are generally closed-loop processes that do not emit HCl. In their survey, U.S. EPA compiled data from different types of pickling operations and their estimated emissions.5 This information is reproduced in Table 28.11.

In order to estimate emissions from pickling facilities, U.S. EPA developed 17 model plants to represent five types of pickling operations and one acid regeneration process.12 The model plants include one or more size variation for each process model. The model plants were developed from information obtained from a survey of steel pickling operations and control technologies. U.S. EPA estimated emission rates for model facilities. Using these emission rates and the production and hours of operation for the model pickling plants, emission factors were calculated. These appear in Table 28.12.

TABLE 28.11

Annual Emission Estimates from Steel Pickling Operations

TABLE 28.11

Number of

Number of

Uncontrolled Emissions

Controlled Emissions

Type of Facility

Facilities

Operations

(Mg/yr)

(Mg/yr)

Continuous coil

36

64

22,820

2640

Push-pull coil

19

22

815

29

Continuous tube

20

55

6524

4252

Batch

4

11

100

52

Acid regeneration

26

59

2632

1943

Storage tanks

10

13

5662

393

99

369 (estimated)

41

24

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at

http://www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm Note: Mg = million grams.

TABLE 28.12

Air Emissions and Emission Factors for Model Pickling Plants

TABLE 28.12

Type of Facility

Production (tpy)a

Hours of Operation (h)

Uncontrolled HCI Emissions (lb/h)

Control Efficiency

(%)

Emission Factor (lb HCI/tons Processed)b

(U) (C)

Continuous coil (S)

450,000

6300

111

93

1.6

0.1

Continuous coil (M)

1,000,000

6300

179

92

1.1

0.1

Continuous coil (L)

2,700,000

7000

347

92

0.9

0.1

Push-pull coil (S)

300,000

5000

12

98

0.2

0.0

Push-pull coil (M)

550,000

4400

27

98

0.2

0.0

Push-pull coil (L)

1,300,000

8760

42

95

0.3

0.0

Continuous rod/wire (S)

10,000

5100

46

98

23.5

0.5

Continuous rod/wire (M)

55,000

7800

119

84

16.9

2.7

Continuous rod/wire (L)

215,000

7200

413

13.8

Continuous tube (S)

80,000

6400

73

95

5.8

0.3

Continuous tube (L)

420,000

6700

312

95

5.0

0.2

Batch (S)

15,000

4400

16

94

4.7

0.3

Batch (M)

75,000

4600

65

90

4.0

0.4

Batch (L)

170,000

5700

147

81

4.9

0.9

Acid regeneration (S)

4

8200

7

98

14,350.0

287.0

Acid regeneration (M)

13.5

7700

28

98

15,970.4

319.4

Acid regeneration (L)

30

8760

1064

98.5

310,688.0

4660.3

Source: U.S. EPA, Steel Pickling, U.S. Environmental Protection Agency, Washington, DC, June 2008. Available at http://

www.epa.gov/tri/TWebHelp/WebHelp/hcl_section_3_1_4_steel_pickling.htm a The production for acid regeneration facilities is in units of million gallons per year.

b The emission factor units for acid regeneration facilities is in units of lb of HCl per million gallons of HCl produced. S: small; M: medium; L: large; U: uncontrolled; C: controlled.

A National Emission Standard for Hazardous Air Pollutants (NESHAP) for new and existing hydrochloric acid process steel pickling lines and HCl regeneration plants pursuant to Section 112 of the Clean Air Act as amended in November 1990 has been proposed (62 FR 49051, September 18, 1997). The purpose of this rulemaking is to reduce emissions of HCl by about 8360 megagrams per year.

28.5.2 Manufacturing Plant of Stainless Steel Pipes and Fittings: A Case History

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