Disposition Of Spent Liquor

Spent pickle liquor disposal methods include discharge to a waterway, hauling by a contractor, deep well disposal, recovery of acid values, neutralization, and regeneration of both acid and iron values.

and can be separated from the additional gypsum produced by a hydrocyclone or differential settling. This finer crystal can then be used to seed first stage crystallization. Titration curves (Figure 8.4.2) illustrate the neutralization characteristics of iron in its two oxidation states.

A substantially more compact iron precipitate is produced from ferric hydroxide. Figure 8.4.3 shows the effect of iron oxidation state on chemical sludge settling rate. The solubility product of Fe(OH)2 is 1.64 X 10—14, whereas that of Fe(OH)3 is 1.1 X 10—36.

Ferrous iron is oxidized to ferric iron by blowing air, oxygen, and nitrous oxide (prepared from the catalytic oxidation of ammonia) through it. This oxidation can be carried out on the hot spent liquor or the cooled colloidal suspension of ferrous hydroxide, resulting in black magnetic iron oxide (Fe3O4), ferric oxide (Fe2O3), or ferrifer-rous oxide.

Recovery of Acid Values

Recovery of acid values refers to the removal of iron salts from the spent liquor by concentrating then cooling the liquor, which crystallizes the ferrous salts. The resulting mother liquor contains less iron, and its acid strength is returned to operating levels by the addition of concentrated acid.

Ferrous sulfate crystallizes as the heptahydrate. If the crystal is collected, dehydrated to the monohydrate, and added in excess to the sulfuric acid spent liquor, the liquor dehydrates and the heptahydrate forms and can be crystallized out of the cooled liquor.

Gaseous chlorine can be fed into the spent hydrochloric acid liquor to oxidize ferrous iron to the ferric state and crystallize the ferric chloride.

Recovered iron salts can be used as soil stabilizers and in water and wastewater treatment plants.

FIG. 8.4.2 Effect of oxidation state of iron on neutralization characteristics of waste pickle liquor as exhibited by titration with sodium hydroxide. Key: A = curve for mixture of ferrous sulfate and sulfuric acid; B = curve for mixture of ferrous sulfate plus ferric sulfate plus sulfuric acid; C = curve for mixture of ferric sulfate and sulfuric acid; D = curve for pure sulfuric acid

FIG. 8.4.2 Effect of oxidation state of iron on neutralization characteristics of waste pickle liquor as exhibited by titration with sodium hydroxide. Key: A = curve for mixture of ferrous sulfate and sulfuric acid; B = curve for mixture of ferrous sulfate plus ferric sulfate plus sulfuric acid; C = curve for mixture of ferric sulfate and sulfuric acid; D = curve for pure sulfuric acid

Neutralization

Neutralization is performed in an aqueous solution, forming an iron hydroxide that may be recovered for return to steel melting furnaces. With neutralization, iron values are recovered as iron oxide.

Lime, as ground limestone, calcium carbonate, or cement clinker flue dust; lime slurry from acetylene manufacture; and powdered slag from electric furnaces have been used to treat sulfuric acid liquors. If the liquor is neutralized, a mixture of gypsum (CaSO4 • 2 H2O) and iron oxide is formed, which can be separated if the pH is controlled. Neutralization to a pH of 0.6 to 2.0 results in the production of gypsum (hydrated calcium sulfate) which precipitates out or is centrifuged from the liquor containing most of the iron. When the pH is increased to a range of 6 to 10, the iron is precipitated as ferrous hydroxide

FIG. 8.4.3 Effect of oxidation state of iron on sludge volume produced upon neutralization of waste pickle liquor by sodium hydroxide. Key: A = curve for 100 percent ferric iron, 0 percent ferrous iron; B = curve for 0 percent ferric iron, 100 percent ferrous iron; C = curve for 65 percent ferric iron, 35 percent ferrous iron

FIG. 8.4.3 Effect of oxidation state of iron on sludge volume produced upon neutralization of waste pickle liquor by sodium hydroxide. Key: A = curve for 100 percent ferric iron, 0 percent ferrous iron; B = curve for 0 percent ferric iron, 100 percent ferrous iron; C = curve for 65 percent ferric iron, 35 percent ferrous iron

Ammonia neutralization involves a single step process wherein the sulfate is recovered as ammonium sulfate crystal, which can be used as fertilizer.

Spent hydrochloric acid liquors can be neutralized with calcium compounds or ammonia. Sodium hypochloride is also used as an oxidizing agent and to raise the pH to precipitate a ferric oxide.

The small pickler (using a batch, not a continuous, pickling process) will first concentrate spent liquor by a factor of 10; after adding lime, the heat of neutralization vaporizes the water, drying the mass into a pourable solid. The steam from the initial concentration should be condensed and reused for pickling, because it can pollute if released to the atmosphere.

Problems associated with neutralization include oxidation rates from ferrous to ferric iron, settling kinetics of the two hydroxides and their mixtures, separation of neutralization products from liquors, and final disposition of residues obtained. Synthetic flocculants may be of value in settling the hydroxide and in vacuum filtration of the settled sludges. Figure 8.4.4 shows the treatment of waste-water from a chemical rinse treatment plant (Liptak 1973).

Regeneration of Acid and Iron Values

The regeneration of acid and iron values from hydrochloric acid spent liquors requires either a wet process or an elevated temperature process.

In the wet process, the liquor is treated with lime, with or without oxidation, to precipitate the iron. The resulting calcium chloride solution is treated with sulfuric acid to precipitate gypsum and to produce hydrochloric acid for recycle to the pickle line.

The thermal processes are usually two-step. The first step involves drying the ferrous chloride by evaporation of free water and hydrogen chloride; it is then oxidatively decomposed to Fe2O3 or Fe3O4 with further release of the chloride (as hydrogen chloride) by the following en-dothermic reaction:

In some thermal units, the two processes occur simultaneously, while in others the processes occur in two different areas, which are heated to different temperatures. Evaporation can be performed at 500° to 600°C. Roasters, fluidized beds, and multiple-hearth incinerators are used, producing an iron oxide that is removed as an ash from the vapor stream by cyclones. The water and hydrogen chloride in the vapor are condensed to produce 20% hydrochloric acid for recycling to the pickling line. Various economizers are used to cool the leaving gases and to preheat the air and spent liquor feed.

Miscellaneous Treatment Methods

Ion exchange processes exchange ferrous iron for hydrogen ions. Regeneration results in a concentrated iron salt that can be processed by one of the several methods described earlier. Electrodialysis segregates the returnable liquor from a concentrated iron salt. Ion exchange membranes separate the various compartments in this process.

Spent nitric, phosphoric, and hydrofluoric acid liquors and their various mixtures can be treated by one of the several processes already described. Lime will remove fluorides, but hydrogen fluoride can be distilled from an acidic solution at about 200°C. If silicon enters the liquor, fluorosilicic acid will also distill off with the hydrogen fluoride.

FIG. 8.4.4 Treatment of wastewater from a chemical rinse treatment plant. Key: pHRC = pH recording controller; pHIT = pH indicating transmitter; FRC = flow recording controller; FE = flow element; LRC = level recorder controller; LT = level transmit-

© 1999 by CRC Press LLC

References

American Petroleum Institute (API). 1969. Manual on disposal of refinery wastes. Volume on liquid wastes. Chapter 11, p. 6.

Beychok, M.R. 1967. Aqueous wastes from petroleum and petrochemical plants. London: Wiley.

De Lorenzi, O. (ed.). 1951. Combustion engineering (1st ed). New York: Superheater, Inc.

Hamer, P., J. Jackson, and E.F. Thurston. 1961. Industrial waste treatment practice. London: Butterworths.

International Union of Pure and Applied Chemistry (IUPAC). 1963. Reuse of water in industry. A Contribution to the Solution. Liptâk, B.G. (ed.). 1973. Instrumentation in the Processing Industries.

Chapter VI. Philadelphia, Pa.: Chilton. Parsons, W.A. 1965. Chemical treatment of sewage and industrial wastes.

National Lime Association. Price, A.R. 1967. Turn treating costs into profit. Hydrocarbon Processing 46(9): 149.

U.S. Environment Protection Agency (EPA). Fluid Bed Incineration of Petroleum Refinery Wastes. Project No. 12050EKT. Washington, D.C.: U.S. Government Printing Office.

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