US20020005210A1 - Regenerating spent pickling liquor - Google Patents

Regenerating spent pickling liquor Download PDF

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US20020005210A1
US20020005210A1 US09/740,465 US74046500A US2002005210A1 US 20020005210 A1 US20020005210 A1 US 20020005210A1 US 74046500 A US74046500 A US 74046500A US 2002005210 A1 US2002005210 A1 US 2002005210A1
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acid
pickling
metal
metal salt
hydrochloric acid
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Douglas Olsen
Charles Blumenschein
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Priority to US09/740,465 priority Critical patent/US20020005210A1/en
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Priority to US10/245,418 priority patent/US20030026746A1/en
Priority to US11/129,639 priority patent/US7351391B1/en
Abandoned legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/36—Regeneration of waste pickling liquors
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B7/00—Halogens; Halogen acids
    • C01B7/01—Chlorine; Hydrogen chloride
    • C01B7/03—Preparation from chlorides
    • C01B7/035—Preparation of hydrogen chloride from chlorides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B7/00—Halogens; Halogen acids
    • C01B7/01—Chlorine; Hydrogen chloride
    • C01B7/07—Purification ; Separation
    • C01B7/0706—Purification ; Separation of hydrogen chloride
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/14—Sulfates

Definitions

  • This invention relates to an apparatus and method for regenerating spent pickling liquor.
  • this invention relates to an apparatus and method for regenerating spent pickling liquor from the acid pickling of a metal.
  • Pickling is a process for the removal of a scale, oxides, or other impurities from a metal surface by immersion in an inorganic acid, usually sulfuric acid, hydrochloric acid, nitric, hydrofluoric, or phosphoric acid.
  • Pickling processes are used to clean the metal surface, e.g., steel.
  • the pickling process removes thin layers of the scale and oxides formed on the metal surface during process operations such as rolling and annealing, and also from exposure to water and the environment.
  • Hot rolled steel for example, has very thin layers of scale, e.g., at depths of from about 0.000228 inch to about 0.000380 inch thick.
  • the rate of scale removal varies inversely with concentration and temperature.
  • the usual concentration is 15% at or above 100° C.
  • the rate also is increased by electrolysis.
  • Operating conditions for batch pickling in hydrochloric acid solutions involve acid concentrations in the range from about 8 to about 12 grams per 100 milliliters, temperatures in the range from about 100° F. to about 110° F., and immersion times in the range from about 5 minutes to about 15 minutes.
  • Operating conditions for continuous pickling in hydrochloric acid solution involve acid concentrations in the range of about 2 grams to about 20 grams per 100 milliliters, temperatures in the range from about 150° F. to about 200° F., and immersion times in the range from about 1 minute to about 2 minutes.
  • Sulfuric acid and hydrochloric acid are agents to pickle steel.
  • Pickling is meant principally, a process to clean the steel before subsequent operations, principally by removing scale and oxides from previous operations such as rolling and annealing and also from exposure to water and the environment. Large scale pickling operations are conducted at temperatures up to 200° F.
  • Sulfuric acid dissolves principally the ferrous oxide (FeO) fractions of the scale. Sulfuric acid penetrates the scale by acting on the base metal, generating hydrogen which acts to pop off the unreacted scale of miscellaneous oxides into the acid. The carbon contained in the steel, exposed by the action of the acid on the metal, and other fine particles remain on the surface of the metal in a form called smut. The action of sulfuric acid is inhibited by the presence of increasing concentrations of iron salts in the pickling bath. Iron concentrations typically are maintained below 8 gms/100 ml. Sulfuric acid concentration is not practical for pickling when the acid concentration has dropped to less than 4 gms/100 ml.
  • FeO ferrous oxide
  • Hydrochloric acid dissolves all of the scale and consistently leaves a uniform light gray finish on both low and high carbon steel. It does not leave smut behind on the surface of the metal. Rinsing is facilitated because of the high solubility of the chlorides compared to the sulfates. Overpickling is less likely with hydrochloric acid than with other acids. The amount of iron contained in the solution is as high as 13 g/100 ml without deleterious effect on pickling. Hydrochloric acid has been utilized in pickling baths until its concentration has reached as low as 1.5-2 gm/100 ml.
  • the spent pickle liquor (SPL) has been sent to a water treatment facility, a wastewater treatment facility, or a combination of water treatment facilities and wastewater treatment facilities for recovering a ferrous chloride value.
  • the spent pickle liquor (SPL) can be treated in a wastewater treatment plant, injected into deep well, or regenerated by a roosting process.
  • High temperature systems for the regeneration of spent hydrochloric acid pickle liquors are necessary.
  • ferrous chloride in the spent pickle liquor is hydrolyzed in a reaction at approximately +840 degrees Fahrenheit to produce iron oxide and hydrogen chloride gas.
  • the hydrogen chloride gas then is absorbed into water to form a suitable concentration of aqueous hydrochloric acid to be returned to the pickling line as a regenerated pickling liquor.
  • the high temperatures involved in the roasting processes are disadvantageous for a number of reasons, including the relatively high maintenance and operating costs attendant high temperatures.
  • Organic acid inhibitors used in acid pickle liquors are destroyed in the high temperatures.
  • a distillation process has disadvantages of high distillation temperatures, the attendant high maintenance and operating costs, and the destruction of organic acid inhibitors.
  • the spent pickle liquor (SPL) is regenerated to iron oxide and about 18% hydrochloric acid by a spray roasting process.
  • the principal hydrochloric acid spent pickle liquor (SPL) regeneration process is the acid spray roasting process, which converts the spent pickle liquor (SPL) to between about 18% to about 20% hydrochloric acid solution containing approximately 0.25% iron and a fine iron oxide dust.
  • the iron oxide is used for manufacturing magnetic products or discarded.
  • the spray roasting process has many disadvantages, including large equipment sizes, high capital costs, and high operating costs.
  • Various methods have been introduced to treat the pickling waste liquor. As described in Beecher U.S. Pat. Nos.
  • a roasting process described in Barczak U.S. Pat. No. 4,436,681 recovers HCl from pickle liquor by injecting the pickle liquor into a roasting chamber at very high temperatures (1,600° F.) using a very high pressure to obtain a fine spray.
  • the ferrous chloride is converted to Fe 2 O 3 and HCl.
  • the roasting process uses a great deal of energy and requires high maintenance costs.
  • the Fe 2 O 3 produced is of low marginal quality and value.
  • ferric oxide, ferrous sulfate, and HCl are of relatively low marginal value.
  • the prior art processes are expensive to setup and operate and difficult to be justified economically.
  • Hydrochloric acid-based metal pickling produces a spent pickle liquor (SPL) having low acid concentration and containing soluble ferrous chloride.
  • Hydrochloric acid reduces harmful over-pickling because the hydrochloric acid reacts mainly with the scale. Sulfuric acid, on the other hand, reacts directly with the steel itself.
  • the amount of iron present in the hydrochloric acid pickling solution is as high as about 13 grams per 100 milliliters without having a deleterious effect on pickling.
  • Hydrochloric acid has been utilized in pickling baths until its concentration has reached as low as about 1.5 grams to about 2 grams per 100 milliliters, or lower in the galvanizing industry where it is used to strip zinc from off-specification product.
  • the apparatus and process of the present invention provide means and method for regenerating a metal pickling process solution containing a metal salt of a first acid.
  • a second acid is added to the metal pickling process solution to produce a regenerated first acid and a metal salt of the second acid.
  • the metal salt of the second acid is crystallized, and removed from the solution.
  • the apparatus and method of the present invention provide means and method for regenerating a ferrous metal pickling process solution containing ferrous chloride.
  • Sulfuric acid is added to the solution to produce regenerated hydrochloric acid and ferrous sulfate.
  • the ferrous sulfate is crystallized, and removed from the solution.
  • the apparatus and method of the present invention provide for pickling a metal.
  • a solution including a first acid contacts the metal for a time sufficient to provide pickling.
  • the metal is removed from the solution, a second acid is added to the solution to produce a regenerated first acid and a metal salt of the second acid, and the metal salt of the second acid is crystallized, and removed from the solution.
  • a second acid is added to the solution to produce a regenerated first acid and a metal salt of the second acid at a specified temperature.
  • a mixture of first and second acid from regeneration can contact the metal with effective results.
  • the present invention provides novel process and apparatus for the closed-loop regeneration of spent hydrochloric acid pickle liquors used to pickle ferrous metals by recovering ferrous chloride from the spent pickle liquors at low temperatures. Ferrous sulfate heptahydrate crystals formed at low temperatures are removed from the pickle liquor, which then permits reuse of the free hydrochloric acid remaining within the regenerated pickle liquor in normal pickling operations.
  • the low temperature, low energy requirements of the apparatus and processes of the invention make for economic superiority to high temperature closed-loop regeneration processes for spent hydrochloric acid pickle liquors, and allow the recycling of a much larger percentage of organic acid inhibitors present in spent pickle liquor.
  • the present invention provides apparatus and process for recovering spent hydrochloric acid pickling liquors used to pickle ferrous metals through a spent hydrochloric acid pickling liquor having sufficient amounts of free hydrochloric acid and iron to allow subsequent precipitation of ferrous sulfate heptahydrate crystals at low temperatures without freezing the pickling liquor; cooling the liquor to a temperature sufficiently low to cause ferrous sulfate heptahydrate crystals to form; and separating the precipitated ferrous sulfate heptahydrate crystals from the resultant supernatant.
  • the present invention provides apparatus and process for recovering a spent hydrochloric acid pickling liquor used to pickle ferrous metals by a first supply of spent pickle liquor, the first supply containing spent hydrochloric acid pickling liquor; a first container suitable for holding the spent pickling liquor; a valved conduit connecting the first container with the first supply for delivering spent pickling liquor from the first supply to the first container; refrigerating means including a refrigerating heat exchanger positioned in association with the first container for actively cooling spent pickling liquor within the first container to a temperature sufficiently low to cause ferrous sulfate heptahydrate crystals to form; agitating means to cause the spent pickling liquor to flow within the first container while being actively cooled; and separating means for separating precipitated ferrous sulfate heptahydrate crystals from the resultant supernatant.
  • FIG. 1 is a flow diagram of the present invention.
  • FIG. 2 is a schematic flow diagram of apparatus and process for regenerating spent hydrochloric acid pickle liquor according to the present invention for treating steel from a continuous steel strip mill.
  • FIG. 3 is a schematic flow diagram of apparatus and process for regenerating spent hydrochloric acid pickle liquor and the crystallization of ferrous sulfate in a separate crystallizer according to the present invention.
  • FIG. 4 is a graph of the solubility of ferrous sulfate in hydrochloric acid as a function of temperature.
  • the present invention provides novel apparatus and method for pickling iron and steel in combination with the on-site regeneration of the pickling acid from the metal salt resulting from the pickling.
  • the present invention provides apparatus and process for pickling the steel in a solution of hydrochloric acid with some ferrous sulfate.
  • the solution at the completion of pickling contains ferrous chloride with some ferrous sulfate and residual hydrochloric acid.
  • the solution at the completion of pickling is fed to a chilled reactor where concentrated sulfuric acid is added, sufficient to react with the ferrous chloride.
  • the resultant solution is chilled to 0-35° F., facilitating the crystallization of ferrous sulfate heptahydrate.
  • the ferrous sulfate heptahydrate crystals are removed from the solution.
  • the remaining solution is heated, its concentration adjusted, with water, and recycled to the pickling tank as fresh pickling acid.
  • the ferrous sulfate heptahydrate is dewatered and sold as a by-product.
  • the present invention produces ferrous sulfate heptahydrate and hydrochloric acid from ferrous chloride or liquors containing ferrous chloride.
  • the present invention produces ferrous sulfate heptahydrate and hydrochloric acid from ferrous chloride or liquors containing ferrous chloride produced in the production of steel or from other industries.
  • the present invention regenerates spent pickle liquors.
  • the present invention regenerates spent hydrochloric acid pickle liquors used for ferrous metal pickling by recovering ferrous chloride from the spent pickle liquors at low temperatures.
  • the apparatus and process of the present invention operate through the steps of (1) providing a waste acid of HCl and adding a second acid of H 2 SO 4 ; (2) providing a preferred critical temperature; (3) chilling to form ferrous sulfate heptahydrate; (4) removing crystals; and (5) obtaining HCl.
  • the apparatus and process of the present invention add sulfuric acid to cover iron sulfate heptahydrate.
  • the apparatus and process of the present invention operate to save energy, use less equipment, require less capital, and operate with low flows.
  • the apparatus and process of the present invention operate to tolerate some sulfuric, even providing a benefit because the sulfuric reduces HCl loss from vaporization.
  • the apparatus and process of the present invention operate to include iron in the pickling feed also. This inclusion of a small amount of iron activates the pickling of the initial acid.
  • the apparatus and process of the present invention operate to regenerate HCl by adding H 2 SO 4 and crystallizing.
  • the present invention operates at a preferred critical temperature. If the temperatures are too high or too low, the process does not operate to specification. At the preferred temperature, the kinetics of the process operate to specification. If too hot, the crystallize step will not operate to specification, and too much iron will be left in solution. If too cold, ice crystals will form and dilute the ferrous sulfate heptahydrate product.
  • the present invention operates at a preferred critical temperature, including a controlled bath of 30-45° F. for adding H 2 SO 4 , preferably at different times.
  • the present invention operates in mixing sulfuric acid with spent hydrochloric acid at a certain temperature and forming FeSO 4 capable of being crystallized by temperature reduction at a specific temperature to 0-35 degrees F.
  • the amount of sulfuric acid also is important to balance the sulfate with the iron, to manage the hydrogen ions, to add enough sulfate, and to form and remove FeSO 4 . Adding excess SO 4 ions reduces Fe solubility and HCl vaporization.
  • the process of the present invention allows the reduction of the Fe to 4 grams/100 ml solubility.
  • the process preferably does not remove too much sulfate or allow too much sulfate in the recovered acid, but some sulfate in the recovered acid is not a problem in recycling operations, e.g., 13% HCl and 1% H 2 SO 4 , where the H + ions stay in solution to provide HCl, and the Fe goes with SO 4 as FeSO 4 and is removed.
  • a metal pickling process solution 10 contains a metal salt of a first acid.
  • the first acid preferably is hydrochloric acid.
  • the first acid can be nitric acid, hydrofluoric acid, or a halogen acid.
  • the metal pickling process solution 10 is referred to as spent pickle liquor (SPL).
  • SPL spent pickle liquor
  • the metal pickling process solution 10 is regenerated with a second acid.
  • the second acid is added to the spent pickle liquor (SPL) 12 , producing a regenerated first acid and a metal salt of the second acid 14 .
  • the metal salt of the second acid is crystallized 16 and removed from the solution 18 .
  • the second acid of the present invention reacts with the metal salt of the first acid to form a metal salt of the second acid.
  • the metal salt of the second acid then is crystallized.
  • the second acid has a higher affinity for the metal than the first acid, and the reaction equilibrium favors the formation of the metal salt of the second acid.
  • the metal salt of the second acid is less soluble than the metal salt of the first acid for crystallization. The second acid does not produce by-products detrimental to the pickling process.
  • the first acid is hydrochloric acid and the metal is iron or steel.
  • the acid reacts with the scale on the metal as shown in Equations 1-3.
  • the spent pickle liquor (SPL) from a hydrochloric acid pickling process of iron or steel contains ferrous chloride as the metal salt of the first acid, though the iron stays in solution in the hydrochloric acid as ferrous ion (Fe 2+ ).
  • the second acid is sulfuric acid
  • the metal salt of the second acid is ferrous sulfate.
  • ferrous sulfate forms ferrous sulfate heptahydrate.
  • Sulfuric acid has a higher affinity for iron than hydrochloric acid. The acid reacts with ferrous chloride as shown in Equation 4.
  • the solubility of ferrous sulfate heptahydrate in hydrochloric acid is lower than the solubility of ferrous chloride.
  • the lower solubility provides the ferrous sulfate to be crystallized more readily than ferrous chloride.
  • the second acid is added to the spent pickle liquor (SPL) in a manner providing adequate mixing and other process conditions to react the metal salt of the first acid and the second acid to form the metal salt of the second acid.
  • the second acid and the spent pickle liquor (SPL) are reacted in an apparatus of the present invention, including a reactor vessel or inline mixing apparatus.
  • the reaction of the second acid and the spent pickle liquor (SPL) are reacted in a reactor vessel.
  • the reactor vessel is of a type or configuration facilitating the reaction between the metal salt of the first acid and the second acid.
  • the reactor vessel includes means and method for providing agitation or static mixing.
  • the reactor vessel is run under process conditions to allow the reaction between the metal salt of the first acid and the second acid.
  • the process conditions are controlled to drive the reaction toward completion. For example, the production of ferrous sulfate from sulfuric acid and ferrous chloride is promoted by cooling.
  • the sulfuric acid is added to the spent pickle liquor (SPL) containing the ferrous chloride in a chilled reactor vessel.
  • the reactor vessel is cooled down to temperatures to promote the reaction and to prevent the formation of unwanted byproducts.
  • the preferred temperature of the reactor vessel is determined by the required degree of regeneration and the cost of power and capital for refrigeration.
  • the reactor is cooled down to minimize the oxidation of Fe 2+ to Fe 3 ⁇ .
  • Fe 3+ forms ferric sulfate with sulfuric acid rather than ferrous sulfate.
  • Ferric sulfate is more soluble that ferrous sulfate, and more difficult to remove from the regenerated first acid.
  • a temperature between about 0° F. and about 65° F. is used.
  • the metal salt of the second acid is removed from the regenerated first acid.
  • the metal salt of the second acid is crystallized to provide separation of the metal salt of the second acid from the regenerated first acid.
  • the separation step is performed under process conditions favoring the crystallization of the metal salt of the second acid. Where cooler conditions promote the crystallization of the metal salt of the second acid, the step is carried out under cooled temperatures to promote the crystallization.
  • the crystallization temperature is determined based on practical and economic factors, such as the maximum allowable iron concentration in the regenerated first acid and the cost of power and capital for refrigeration.
  • the crystallization is carried out at cooled temperatures to promote crystallization and to lower the iron content of the regenerated first acid.
  • water forms ice crystals that are removed along with the ferrous sulfate heptahydrate crystals causing a high moisture content in the crystals and handling problems.
  • the level of iron removal from the regenerated first acid and the presence of water in the crystals are balanced for the specific process.
  • crystallization is improved by the addition of other substances which lower its solubility.
  • the addition of sulfate ions decreases the solubility of ferrous sulfate.
  • sulfate ions for example, in the form of sodium sulfate, are added to the solution to improve crystallization.
  • Sulfate ions also are introduced by adding greater than the stoichiometric amount of sulfuric acid, or by maintaining a residual level of sulfate ions in the pickling liquor.
  • Crystallization is carried out as the metal salt of the second acid is present.
  • crystallization occurs in the reactor vessel as the metal salt of the second acid is produced, or it occurs separately in a crystallizer.
  • the use of a separate crystallizer is preferred where the reaction conditions are not the same as the crystallization conditions.
  • the crystallized metal salt of the second acid is removed from the regenerated acid by a method providing the regenerated acid in a form usable for its intended application. For example, removal of the metal salt of the second acid from the regenerated first acid is performed in a unit such as a filter or a particle separator. Following separation, the regenerated acid is treated further, if necessary, for its desired use. For example, the acid is heated, or the acid concentration is adjusted, or the acid is heated and its concentration adjusted. After separation and treatment, the regenerated acid is recycled to the metal pickling line as fresh pickling acid.
  • the metal salt of the second acid also is treated following separation. For example, residual water on crystals of the metal salt of the second acid are removed. The removal is accomplished, for example, in a dewatering process. Following treatment, the crystals are used or disposed.
  • heat is exchanged between flow streams.
  • the spent pickle liquor (SPL) from the pickling process and the regenerated acid fed to the pickling process exchange heat.
  • the heat of the spent pickle liquor (SPL) is harnessed to reheat the regenerated acid prior to recycling.
  • the spent pickle liquor (SPL) is cooled prior to the addition of the second acid or crystallization.
  • the heat exchange is performed by a cross-flow heat exchanger such that the spent pickle liquor (SPL) exchanges heat with the regenerated first acid returning to the pickling process.
  • the cross-flow heat exchange device or method, or other heat exchange device or method uses a cooling or warming media to adjust further the temperature of the spent pickle liquor (SPL) or the regenerated acid.
  • SPL spent pickle liquor
  • a complete spent pickle liquor (SPL) regeneration apparatus and process 2 of the present invention are illustrated in a schematic process flow diagram FIG. 2.
  • Steel sheet 20 was fed continuously to a series of hydrochloric acid-based pickling baths 22 .
  • spent pickle liquor (SPL) containing about 0.15 pounds per gallon hydrochloric acid, about 0.89 pounds per gallon ferrous sulfate, and about 2.32 pounds per gallon ferrous chloride at about 170° F. flowed at a rate of about 100 gallons per minute into an spent pickle liquor (SPL) line 24 .
  • the spent pickle liquor (SPL) in line 24 flowed through a heat exchanger 26 where it was cooled by regenerated hydrochloric acid flowing at about 32° F. in line 46 and water flowing at about 50° F.
  • Reactor vessel 28 included a heat exchanger which received coolant from a chiller 24 via a cooling line 36 . Reactor vessel 28 and its contents were maintained at about 32° F.
  • the ferrous chloride in the spent pickle liquor (SPL) reacted with the sulfuric acid, producing ferrous sulfate and regenerated hydrochloric acid.
  • the ferrous sulfate was crystallized in the form of ferrous sulfate heptahydrate crystals in reactor 28 .
  • An outlet line 38 carried a solution containing about 1.49 pounds per gallon hydrochloric acid, about 0.89 pounds per gallon ferrous sulfate in solution, and 5.08 lb/gallon of ferrous sulfate heptahydrate crystals from reactor 28 at a flow rate of 115 gallon per minute.
  • Outlet line 38 fed the regenerated hydrochloric acid, ferrous sulfate heptahydrate crystals, and ferrous sulfate into a dewatering unit 40 .
  • dewatering unit 40 ferrous sulfate heptahydrate was separated from the regenerated hydrochloric acid.
  • the ferrous sulfate heptahydrate was sent to a ferrous sulfate heptahydrate line 42 at a rate of about 508 pounds per minute.
  • the ferrous sulfate heptahydrate in line 42 passed to shipping 44 where it was packaged.
  • the regenerated hydrochloric acid solution containing about 1.49 pounds per gallon hydrochloric acid and about 0.89 pounds per gallon ferrous sulfate, flowed into line 46 at a rate of about 83 gallons per minute.
  • the regenerated hydrochloric acid solution flowed through line 46 to a mixing point where water at about 50° F. flowed from a water line 48 was mixed with the acid.
  • the water brought the flow rate of the regenerated hydrochloric acid solution to about 100 gpm.
  • the regenerated acid in line 46 then passed through heat exchanger 26 where the regenerated hydrochloric acid solution exchanged heat with the spent pickle liquor (SPL) in line 24 , increasing its temperature to about 140° F.
  • SPL spent pickle liquor
  • Line 46 then passed through a second heat exchanger 54 where the regenerated hydrochloric acid solution exchanged heat with steam in line 56 , increasing its temperature to about 170° F. or higher.
  • the regenerated acid solution was returned to pickling baths 22 via line 46 .
  • FIG. 3 illustrates apparatus and process 3 of the present invention wherein the crystallization of the ferrous sulfate occurred in a separate crystallizer 50 .
  • a novel apparatus and process of the present invention provide a zero-discharge process for regenerating the hydrochloric acid in spent pickle liquor (SPL.)
  • the novel apparatus and process of the present invention regenerate hydrochloric acid spent pickle liquor (SPL).
  • the process uses sulfuric acid to produce ferrous sulfate heptahydrate from the ferrous chloride in the SPL, leaving a solution with a restored concentration of the hydrochloric acid suitable for recycling to the pickle tub.
  • the ferrous sulfate crystals are a product in demand, resulting in zero discharge. As a byproduct of the process, ferrous sulfate crystals are sold.
  • the process generates no wastewater or residual waste requiring disposal.
  • the novel apparatus and process of the present invention provide significant operating and capital cost savings.
  • Spent hydrochloric acid pickling liquor (SPL) in the steel rolling and finishing industry contains 25 to 30 wt. % ferrous chloride and 0.5 to 2 wt. % hydrochloric acid. Approximately 1.5 billion gallons of spent pickle liquor are produced annually in U.S. integrated mills and by outside processors. Over 90% is from HCl pickling lines.
  • the principal acid regeneration process is the acid spray roasting process, which converts the SPL to a distilled hydrochloric acid of approximately 18%, plus a fine iron oxide dust. The iron oxide is used for manufacturing magnetic products, and otherwise, it is discarded.
  • the spray roasting process has many disadvantages, including the large size, high capital cost, and high operating cost required for regenerating the acid.
  • the novel apparatus and process of the present invention use a metal salt of a second acid having a solubility in a first acid, e.g., of hydrochloric acid lower than ferrous chloride.
  • a first acid e.g., of hydrochloric acid lower than ferrous chloride.
  • ferrous sulfate heptahydrate is crystallized, removing iron from the solution together with sulfate introduced with the sulfuric acid.
  • the hydrochloric acid thereby is regenerated and is recycled to the pickling line to form a continuous processing loop.
  • the by-product ferrous sulfate heptahydrate is a valuable commodity in short supply in the United States and is sold.
  • the novel apparatus and process of the present invention does not generate any wastewaters or other residual wastes requiring additional treatment or disposal.
  • the novel apparatus and process of the present invention provide a zero discharge process having significant environmental advantages for facilities currently sending SPL off site for disposal.
  • Hydrochloric acid is used to pickle hot rolled or heat-treated carbon steel sheet, rod, and wire. Many steel producers prefer hydrochloric acid because of the speed of pickling and the uniform surface characteristics in continuous pickling operations.
  • Operating conditions for batch pickling in hydrochloric acid involve acid concentrations from 8-12 g/100 ml, temperatures of 100 to 110° F., and immersion times from 5 to 15 minutes with a maximum allowable iron concentration of 13 g/100 ml.
  • Operating conditions for continuous pickling in hydrochloric acid solution typically involve acid concentrations of 2-20 g/100 ml, temperatures of 150-200° F., and immersion times of 1 to 20 minutes.
  • Hydrochloric acid offers a number of advantages when compared with sulfuric or other acids. Effective pickling is obtained with iron concentrations as high as 13 g/100 ml. Acid is used until the free acid concentration is as low as 1-2 wt. %, or lower in the galvanizing industry where it is used to strip zinc.
  • the exiting acid exchanges heat with the acid returning to the system.
  • the acid to be regenerated then flows into a reactor where it is contacted with 66°Be (93 wt. %) sulfuric acid.
  • the reaction mix is chilled to temperatures of 0-35° F. in a crystallizer. Because of the limited solubility of ferrous sulfate heptahydrate in acid solutions, it precipitates, crystallizes, effectively removes the majority of the iron and sulfate, leaving behind hydronium ions from the sulfuric acid and the chloride ions from the ferrous chloride. Depending on the crystallization conditions, more or less Fe ++ and SO 4 ++ ions return with the recycling acid. The regenerated acid is recycled as essentially hydrochloric acid.
  • the by-product of the process is marketable ferrous sulfate heptahydrate, which is separated from the regenerated acid by a suitable filter or centrifuge.
  • the process and apparatus of the present invention operate on a difference in the solubility of ferrous sulfate and ferrous chloride in acid to provide the preferred separation of the present invention possible.
  • the solubility of these salts in their respective acids As the sulfuric acid regenerates hydrochloric acid, the solubility of the iron is repressed, and the iron is driven out of solution as ferrous sulfate. Significantly low residual concentrations of iron and residual sulfate are possible.
  • a residual amount of iron in the returning pickling bath is not a disadvantage. Many pickling operations prefer some residual metal to totally fresh acid.
  • the level at which to maintain iron, hydrochloric acid, and sulfate ions in the regenerated hydrochloric acid relates to the metal being processed and the results preferred from the pickling operation.
  • the comparative economics of pickling operations using fresh hydrochloric acid or hydrochloric acid regenerated with sulfuric acid are a function of a comparative cost of acid to remove scale, net savings of the recovery process, cost of disposal of SPL vs. income from sales of ferrous sulfate crystals, operating costs, including power, labor, and maintenance, and capital cost Comparative cost of acid to remove scale.
  • Hydrochloric acid purchased in bulk at 32 wt. % acid has a delivered price of $145-180 per ton, depending on the area of delivery.
  • the equivalent 100% (anhydrous) price for hydrochloric acid is $453-563 per ton.
  • Sulfuric acid prices have a wider variation.
  • the bulk delivered price of sulfuric acid is $48-110 per ton.
  • Hot rolled steel has a scale of 0.000228-0.000380 inch thickness.
  • the theoretical dosage of hydrochloric acid to remove the scale is 9.03 lb of anhydrous HCl or 13.9 lb of 66°Be (93.5 wt. %) sulfuric acid per 1,000 square feet of surface.
  • Strip 0.09 inch thick has 1,086 sq.ft./ton.
  • Heavier gauge sheet has proportionally less surface area and requires proportionally less acid per ton.
  • Table 1 shows the comparative acid cost for pickling with fresh hydrochloric acid as compared to pickling with hydrochloric acid regenerated from sulfuric acid. TABLE 1 Comparative Raw Material Cost for Pickling Hot Rolled Strip $/M sq. ft.
  • the cost for hauling away spent pickle liquor is $0.60/gal, and is in the range of $0.20 to $1.00/gal in some locations. Acid removal costs $3.36/Mft 2 pickled, with a range from $1.162-5.60/Mft 2 .
  • the revenue from the sale of most by-product ferrous sulfate heptahydrate ranges from $15-65/ton, providing a revenue of $0.33- $1.41/Mft 2 .
  • Table 2 summarizes the potential savings using regenerated acid. TABLE 2 Savings from Raw Material Purchases and By-Product Sales $/M sq. ft. Pickled Budget Min. Max. Acid purchases Sulfuric vs. 1.56 1.55 1.57 Hydrochloric Acid disposal/hauling cost 3.32 1.11 5.53 Crystal sales revenue 0.43 0.33 1.41 Power cost .24 Labor cost .19 Total Savings and Revenue $4.71 .$2.39 $8.08
  • the operating cost for the apparatus and process of the present invention includes power, operators, and maintenance.
  • the power cost is linear and is $0.24/ft 2 pickled.
  • Operator labor and maintenance costs are $0.19/Mft 2 .
  • C 1 Cost of base facility rated for R 1 tons Fe removal/day
  • C 2 Cost of base facility rated for R 2 tons Fe removal/day
  • the apparatus and process of the present invention provide attractive paybacks for even the smallest mills. In some cases, paybacks occur at less than one year.
  • the apparatus and process of the present invention provide substantial positive energy and environmental effects. Delivering sulfuric acid instead of hydrochloric acid requires fewer trucks and saves fuel, reduces the generation of greenhouse gas, and minimizes exposure of the environment to potential accidental spills. The energy cost for manufacturing sulfuric acid is less than that for hydrochloric acid, saving both energy and green-house gases.
  • the apparatus and process of the present invention replace the practice of regenerating SPL at an off-site roasting facility.
  • the on-site chemical regeneration saves fuel, reduces the generation of greenhouse gasses, and minimizes incidents of exposure of the environment to potential accidental spills.
  • the energy cost of regenerating hydrochloric acid with sulfuric acid is less than that of regenerating hydrochloric acid by the roasting process, saving energy and reducing greenhouse gases.
  • the apparatus and process of the present invention replace deep well injection and replace the treatment of acid and subsequent landfill of treated waste sludge, for a significant environmental advantage because it produces marketable ferrous sulfate heptahydrate, rather than a waste.
  • Ferrous sulfate crystals are used in several industries and are imported in large quantities from Europe.
  • the present invention provides novel apparatus and method for pickling iron and steel in combination with the on-site regeneration of the pickling acid from the metal salt resulting from pickling.
  • the present invention provides apparatus and process for pickling the steel in a solution hydrochloric acid with some ferrous sulfate.
  • the solution at the completion of pickling contains ferrous chloride with some ferrous sulfate and residual hydrochloric acid.
  • the solution at the completion of pickling is fed to a chilled reactor where concentrated sulfuric acid is added, sufficient to react with the ferrous chloride.
  • the resultant solution is chilled to 0-35° F., facilitating the crystallization of ferrous sulfate heptahydrate.
  • the ferrous sulfate heptahydrate crystals are removed from the solution.
  • the remaining solution is heated, its concentration adjusted, with water, and recycled to the pickling tank as fresh pickling acid.
  • the ferrous sulfate heptahydrate is dewatered and sold as a by-product.
  • the present invention regenerates hydrochloric acid from the spent pickle liquor resulting from steel pickling.
  • the present invention provides many benefits, including significant energy savings, the ability to perform on-site closed-loop pickling and regeneration, the elimination of transport and off-site processing of hazardous spent pickle liquor, the availability of a cost-effective regeneration system for very small mills, and many cost and resource savings associated with all of these improvements.
  • the present invention provides a profound impact on energy use, pickling costs, and pollution generation throughout the steel industry.
  • the present invention regenerates spent hydrochloric acid from steel pickling, that results in approximately 95% energy savings, 52% cost savings, and 91% reduction in CO 2 over conventional technologies.
  • the process generates no wastewater or residual waste requiring disposal and results in significant operating and capital cost savings in addition to major energy savings.
  • the technology uses sulfuric acid to produce ferrous sulfate heptahydrate from the ferrous chloride in spent pickle liquor, leaving a solution with a restored concentration of the hydrochloric acid suitable for recycling to the pickle tub.
  • Ferrous sulfate crystals, a by-product, is sold for reuse.
  • Spent pickle liquor is an EPA listed hazardous waste (waste category K062) requiring costly and energy intensive handling, treatment, and disposal. Eighty per cent (80%) of SPL is sent to publicly owned treatment works (POTWs) or regenerated; approximately 20% is neutralized and landfilled or disposed by deep well injection. The SPL sent to POTWs and treatment plants contains acid that must be neutralized with lime or caustic, requiring the use of energy and raw materials.
  • the most common method of regeneration is an acid spray roasting process that requires high investment, significant energy use, and produces a waste ferric oxide dust.
  • a 40,000-gallon/day SPL roasting process plant costs approximately $7 million and uses 1.8 ⁇ 10 11 BTU/year (See Table 3).
  • the roasting process is uneconomical for small quantities of SPL.
  • Smaller pickling mills e.g., 80% of U.S. pickling mills, either ship their SPL by tank trucks to central processing plants at distances of up to several hundred miles, and then the regenerated acid is shipped back, or they ship to a POTW and receive shipments of fresh acid.
  • the transport involves significant cost, fuel consumption, and exposure of the public and the environment from the transport of hazardous materials.
  • the present invention (1) saves 230,000 BTU per ton of steel pickled compared to the conventional roasting process; (2) enables on-site closed loop pickling and regeneration, eliminating the need for transport of SPL and regenerated acid and off-site processing, saving 179,000 BTU/ton of steel; (3) is cost effective for very small mills for which regeneration has been prohibitively expensive in the past; (4) produces a marketable by-product (ferrous sulfate heptahydrate) in short supply in the U.S. rather than a waste or unmarketable substance; (5) uses the preferred hydrochloric acid in the pickling process, and (6) uses less costly concentrated sulfuric acid at approximately 1 ⁇ 3 of the volume of HCl required from the conventional roasting process
  • the present invention provides energy savings for a 40,000-gallon/day SPL plant of 2.96 ⁇ 10 11 BTU/year, which includes savings from both processing and transport energy. This equates to an estimated 91% savings in energy use over the existing roasting technology.
  • the present invention provides environmental (waste) savings.
  • the projected annual energy savings for a 40,000-gallon/day SPL plant are 10,621 tons of CO 2 based on energy production and transportation emissions. This equates to an estimated 91% savings in waste generation over the existing roasting technology. See Table 3 below for a summary of cost, energy, and environmental savings.
  • the present invention provides economic benefits for pickling with regenerated hydrochloric acid in a 52% cost saving over pickling using fresh hydrochloric acid, as summarized in Table 3. TABLE 3 Annual Energy, Cost, and Waste Savings for 40,000 gallon/day SPL HC1 Recovery Plant 730,000 TPY steel.

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US11/129,639 US7351391B1 (en) 2000-12-19 2005-05-16 System and method for converting the spent remnants of a first pickling acid solution into a usable second pickling acid solution

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ES2239900A1 (es) * 2004-03-22 2005-10-01 Condorchem Iberica, S.L. Proceso de recuperacion de acido clorhidrico de baños agotados de decapado de metales mediante la sustitucion por desplazamiento con acido sulfurico produciendo sulfatos de hierro y de cinc.
US20050281732A1 (en) * 2004-06-22 2005-12-22 Alexander Kehrmann Method of producing ferrous sulfate heptahydrate
ITVI20090126A1 (it) * 2009-06-03 2010-12-04 Ekipo S R L Processo di rigenerazione di bagni di strippaggio di zinco e decapaggio di ferro con recupero di acido cloridrico e produzione di solfato di zinco e solfato di ferro
CN110065930A (zh) * 2019-05-30 2019-07-30 李永飞 钢材酸洗废液的回收再利用方法及回收再利用装置
CN115140711A (zh) * 2022-07-06 2022-10-04 北京水木方科技有限公司 一种含铁废盐酸用硫酸再生盐酸的方法
CN115304267A (zh) * 2022-08-19 2022-11-08 陕西合兴硅砂有限公司 一种光伏玻璃用石英砂酸洗液的回收方法

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FI120682B (fi) * 2006-11-30 2010-01-29 Kemira Oyj Menetelmä suolahapon talteenottamiseksi rautakloridiliuoksesta
IL200860A (en) 2009-09-10 2015-07-30 Green Future Ltd Galvanic wastewater treatment and the production of iron oxide sized nanoparticles
EP3134559A4 (en) 2014-04-22 2018-04-04 Green Future Ltd. Method and formulations for removing rust and scale from steel and for regenerating pickling liquor in hot-dip galvanization process
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IT202000005848A1 (it) * 2020-03-19 2021-09-19 Tenova Spa Processo per decapare e/o passivare un acciaio inossidabile.
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ES2239900A1 (es) * 2004-03-22 2005-10-01 Condorchem Iberica, S.L. Proceso de recuperacion de acido clorhidrico de baños agotados de decapado de metales mediante la sustitucion por desplazamiento con acido sulfurico produciendo sulfatos de hierro y de cinc.
US20050281732A1 (en) * 2004-06-22 2005-12-22 Alexander Kehrmann Method of producing ferrous sulfate heptahydrate
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ITVI20090126A1 (it) * 2009-06-03 2010-12-04 Ekipo S R L Processo di rigenerazione di bagni di strippaggio di zinco e decapaggio di ferro con recupero di acido cloridrico e produzione di solfato di zinco e solfato di ferro
CN110065930A (zh) * 2019-05-30 2019-07-30 李永飞 钢材酸洗废液的回收再利用方法及回收再利用装置
CN115140711A (zh) * 2022-07-06 2022-10-04 北京水木方科技有限公司 一种含铁废盐酸用硫酸再生盐酸的方法
CN115304267A (zh) * 2022-08-19 2022-11-08 陕西合兴硅砂有限公司 一种光伏玻璃用石英砂酸洗液的回收方法

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