EP1234897B1 - Procédé de dézingage de l'acier galvanisé - Google Patents

Procédé de dézingage de l'acier galvanisé Download PDF

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Publication number
EP1234897B1
EP1234897B1 EP02075408A EP02075408A EP1234897B1 EP 1234897 B1 EP1234897 B1 EP 1234897B1 EP 02075408 A EP02075408 A EP 02075408A EP 02075408 A EP02075408 A EP 02075408A EP 1234897 B1 EP1234897 B1 EP 1234897B1
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EP
European Patent Office
Prior art keywords
zinc
galvanized steel
process according
steel
scrap
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP02075408A
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German (de)
English (en)
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EP1234897A1 (fr
Inventor
William A. Morgan
Frederick J. Dudek
Edward J. Daniels
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Metals Investment Trust Ltd
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Metals Investment Trust Ltd
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Publication date
Priority claimed from US08/680,344 external-priority patent/US5779878A/en
Application filed by Metals Investment Trust Ltd filed Critical Metals Investment Trust Ltd
Priority to DE69825799T priority Critical patent/DE69825799D1/de
Publication of EP1234897A1 publication Critical patent/EP1234897A1/fr
Application granted granted Critical
Publication of EP1234897B1 publication Critical patent/EP1234897B1/fr
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F5/00—Electrolytic stripping of metallic layers or coatings

Definitions

  • the present invention relates, in general, to a process for dezincing steel scrap and, in particular, to a galvanic dezincing process in which the cathode is steel or another metal or alloy which does not have a low hydrogen overvoltage.
  • Zinc coated (galvanized) steel is widely used in automotive, construction, and agricultural equipment and other industries. These industries and the mills producing galvanized sheet generate a considerable quantity of fresh steel scrap, at least some of which is galvanized, which can be recycled and reused as a starting material in steel and iron-making processes.
  • the presence of zinc in the steel scrap used in steel and iron-making processes increases the cost of compliance with environmental regulations due to costs associated with dust disposal and possible pretreatment of dust as a hazardous waste, treatment of waste water for removal of zinc and collection of fumes to maintain the shop floor environment and to restrict roof-vent emissions. As a result, there is great interest in development of an economical method of removing zinc from steel scrap.
  • the steel scrap is immersed in an acid such as hydrochloric acid or sulfuric acid.
  • an acid such as hydrochloric acid or sulfuric acid.
  • Iron however, is co-dissolved with the zinc in the acid solution and the separation of the iron from the zinc has not been found to be economically feasible.
  • caustic soda solution to dissolve zinc from galvanized steel scrap has also been proposed.
  • An inherent advantage of this method is that iron is stable in caustic and thus, separation of iron from zinc in solution is not a significant problem.
  • a disadvantage of this method is the relatively slow rate at which zinc is removed from the galvanized surface which leads to low productivity or inadequate zinc removal.
  • LeRoy et al. disclose other methods for accelerating the dissolution of zinc from galvanized steel in caustic electrolyte in U.S. Patent Nos. 5,302,260 and 5,302,261.
  • LeRoy et al. suggest that the galvanized steel be immersed in a caustic electrolyte and electrically connected to a cathodic material which is stable in the electrolyte and which has a low hydrogen overvoltage.
  • such cathodes include high-surface-area nickel-based and cobalt-based materials such as Raney nickel type and Raney Cobalt type, nickel molybdates, nickel sulfides, nickel-cobalt thiospinels and mixed sulphides, nickel aluminum alloys, and electroplated active cobalt compositions. If the scrap is clean, unpainted, or shredded, no external source of voltage is applied to the cathode material.
  • LeRoy et al. U.S. Patent No. 5,302,261 at col. 2, lines 37-47. If bundles of scrap are to be dezinced, however, they suggest applying an external source of voltage to the cathode to increase the rate of zinc stripping.
  • the provision of a process for dezincing steel scrap in a caustic electrolyte is the provision of a process for dezincing steel scrap in a caustic electrolyte; the provision of such a process in which the cathode is steel or another metal having a relatively high hydrogen overvoltage; the provision of such a process in which an external source of voltage need not be applied to the cathode material to increase the dissolution rate; and the provision of such a process in which the zinc removal rate is accelerated relative to the rate at which zinc would be removed from scrap which is simply immersed in caustic electrolyte.
  • Fig. 1 is a schematic view illustrating steel scrap movement and caustic electrolyte circulation through a dezincing process of the present invention.
  • the process of the present invention is carried out in a system in which the steel scrap is immersed in a caustic electrolyte such as caustic soda (sodium hydroxide) or caustic potash (potassium hydroxide).
  • a caustic electrolyte such as caustic soda (sodium hydroxide) or caustic potash (potassium hydroxide).
  • caustic soda is preferred over potassium hydroxide, however, due to its relative cost advantage.
  • the zinc-coated steel is galvanically corroded with the zinc-coated surface of the scrap serving as the anodic material and an exposed steel surface or another metal having a relatively high hydrogen overvoltage serving as the cathodic material.
  • the scrap is treated in a manner to increase the surface area of the cathodic material relative to the surface area of the anodic material.
  • the rate of dissolution of the zinc increases with increasing concentration of the caustic soda in and the temperature of the electrolyte.
  • the electrolyte is an aqueous solution comprising caustic soda in a concentration of at least 15% by weight. More preferably, the concentration of caustic soda in the electrolyte is between 25% and 50% by weight and most preferably it is maintained within the range of 30% to 40% by weight. At these concentrations, the electrolyte can be relatively viscous depending upon the temperature. Accordingly, the temperature of the electrolyte is preferably at least 75° C but less than the temperature at which the electrolyte boils, more preferably between 85° C and 95° C, and most preferably between 90 °C and 95°C.
  • the cathodic material may be any metal or alloy which is more noble than zinc in the galvanic series of metals and alloys.
  • High-surface-area nickel-based or cobalt-based materials, nickel molybdates, nickel sulfides, nickel-cobalt thiospinels and m ⁇ xed sulphides, nickel aluminum alloys, and electroplated active cobalt compositions and any other such low-hydrogen overvoltage materials are too expensive and thus are preferably not used as the cathodic material.
  • the cathodic material is principally iron, an alloy of steel, or another alloy or metal having a standard electrode potential (reduction potential) intermediate that of the standard electrode potential of zinc (-0.76 V) and cadmium (-0.4 V) in the electrochemical series which is relatively inexpensive.
  • the cathodic material pieces of galvanized scrap or regions thereof from which the zinc coating has been removed serve as the cathodic material.
  • the size of the cathodic area relative to the size of the anodic area of the steel scrap may be increased by a variety of methods.
  • the steel scrap may be heated or mechanically abraded or deformed to increase the number density and total surface area of cathodic areas in the scrap, or (ii) it may be intimately mixed with a cathodic material.
  • these methods may be carried out before the scrap is immersed in the electrolyte or while it is immersed in the electrolyte.
  • the galvanized scrap is heated to a temperature in excess of the melting point of zinc in order for this transformation to occur in a commercially acceptable time period. More preferably, the galvanized scrap is heated to a temperature of at least 470° C, still more preferably at least 500 °C, and most preferably at least 600° C.
  • the period of time at which the galvanized scrap is held at these temperatures to achieve the desired effect will be a function of temperature. In general, however, it is preferred that the holding period be between 5 and 20 minutes, with time periods of 10 to 15 minutes being particularly preferred.
  • the steel scrap may be mechanically abraded or deformed to increase the galvanic corrosion rate. Abrading the steel scrap will remove the zinc from local areas. Deforming the steel scrap may crack or otherwise stress the zinc coating. Because these exposed and deformed areas are generally surrounded by zinc-coated regions, the number density and total surface area of cathodic areas in the scrap is increased at the surface of the steel scrap thus increasing the galvanic corrosion rate of the scrap when it is immersed in the electrolyte.
  • the steel scrap may be mechanically abraded or deformed, for example, by shredding the scrap, by relative movement of the scrap against itself or another abrasive surface, or by hammer-milling it. Steel scrap is typically available in pieces ranging in size from 2.5 to 120 cm.
  • the shredded pieces preferably have a size distribution of 10 to 20 cm., with the majority of shredded pieces having a size distribution of about 10 to about 15 cm. wherein size is determined by reference to the dimensions of square openings in a grate through which the pieces are passed. If the pieces of steel scrap are mechanically deformed, e.g., bent or scraped, it is preferred that the deformation sites be uniformly distributed over the galvanized surface and that, on average, the deformed surface area exceed 10%, more preferably 15%, and most preferably at least 20% of the surface area of steel scrap.
  • the size of the cathodic area may be increased relative to the size of the anodic area of the galvanized steel scrap by forming a mixture of galvanized steel scrap and uncoated material, i.e., a metal or alloy-which is more noble than zinc in the galvanic series and which lacks a zinc coating.
  • the mixture of uncoated material and galvanized steel scrap comprises at least 5% by weight uncoated material, preferably at least 10% uncoated material, more preferably at least 20% uncoated material, and optimally at least 30% uncoated material.
  • Such mixtures may be available directly from some scrap producers or may be formed by mixing the galvanized steel scrap with uncoated material.
  • the uncoated material is steel scrap from which the zinc coating has at least been partially removed.
  • the steel scrap is immersed in and/or carried through the electrolyte by a conveyor consisting essentially of a cathodic material which is more noble than zinc, such as a steel alloy.
  • the conveyor may be, for example, an endless moving steel belt or a track with a carriage for holding the steel scrap suspended from the track.
  • the carriage is a rotating drum having openings in the wall thereof through which electrolyte can pass when it is immersed in the electrolyte.
  • Rotation of the drum in the electrolyte causes movement of the steel scrap relative to itself and to drum which causes mechanical abrasion of the galvanized steel and acceleration of the galvanic corrosion rate.
  • rotation of the drum causes the steel scrap to move relative to the electrolyte, thereby decreasing the thickness of the boundary layer and further accelerating the galvanic corrosion rate.
  • reference numeral 10 generally illustrates a preferred embodiment of an apparatus for carrying out the process of the present invention.
  • Dezincing apparatus 10 comprises dezincing tank 12, rinse tanks 14, 16 and a series of endless moving belts 18, 22, 24 and 26.
  • Steel scrap such as shredded loose clippings is fed to conveyor 18 which delivers the steel scrap to dezincing tank 12 which contains an aqueous sodium hydroxide solution containing from 150 grams/liter to 500 grams/liter NaOH at temperatures ranging from 50° C to 100° C.
  • moving belt 20 is supported by pads 21 which, in addition, electrically isolate moving belt 20 from dezincing tank 12 and from ground.
  • Moving belt 20 delivers the black scrap to moving belt 22 which carries the black scrap up and out of dezincing tank 12 and delivers it onto moving belt 24.
  • Moving belt 24 carries the scrap through rinse tank 14 and delivers the rinsed scrap onto moving belt 26 which carries the scrap through rinse tank 16 for a second rinsing. The rinsed, black scrap is then transferred to a storage bin or directly to a customer.
  • Electrolyte containing dissolved zinc is continuously withdrawn from dezincing tank 12 via line 28, purified to remove aluminum, lead, copper, bismuth and iron in a tank 30, pumped by slurry pump 32, filtered in a vacuum drum or other suitable filter 34 and delivered to electrolytic zinc recovery cell 36 connected to a transformer rectifier 38.
  • electrolytic zinc recovery cell 36 the zinc metal is deposited on the cathode (e.g., a magnesium cathode) as a powder and/or in dendritic form and is continuously caused to be removed from the cathode to settle to the bottom of the electrolysis cell.
  • zinc metal powder slurry is withdrawn and pumped via line 40 and slurry pump 42 to filter 44 (or centrifuge).
  • Damp zinc cake discharged from horizontal tank filter 44 is transferred by line 46 to a briquetting unit 48 which produces zinc powder briquettes 50 which are ready for storage or sale to a customer.
  • the electrolytic process regenerates caustic soda which is returned to the dezincing tank; the spent electrolyte with a reduced zinc content (i.e., less than 20 gm./l of zinc) is returned to the dezincing tank for further use.
  • Preferred operating temperatures for the electrolysis solutions are 30 to 45 °C and an input range of 25 to 40 grams/liter of zinc with a free caustic level of 150 to 300 grams/liter of NaOH.
  • the removal rate of zinc can be increased by deforming the surface of the scrap prior to immersion in the tank of sodium hydroxide solution with dezincing times being reduced from 80 minutes to less than 20 minutes.
  • the dezincing effect starts at the deformed site on the steel, e.g. a bend or scratch and proceeds across the surface of the steel. It has been demonstrated that the greater the number of these deformed sites the greater the improvement in rate of effectiveness of the process, e.g., if the steel is shredded into smaller pieces in a hammer mill. This creates sites of high energy (deformation) and areas where zinc has been mechanically removed in close proximity to coated areas. In all of the above cases the galvanic dezincing effect is enhanced. No external current or oxidant need to be used.
  • the process can be performed by charging the materials to be dezinced to a shredder such as a hammer mill which is operated to deform the steel, mechanically remove zinc from part of the surface and concurrently heat the steel.
  • a shredder such as a hammer mill which is operated to deform the steel, mechanically remove zinc from part of the surface and concurrently heat the steel.
  • Example 2 The test of Example 2 was repeated except that some of the samples were heated to a temperature of 750 °C prior to being immersed in the NaOH solution. The results are presented in Table 2.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacture And Refinement Of Metals (AREA)

Claims (14)

  1. Procédé de retrait du zinc de l'acier galvanisé comprenant l'immersion de l'acier galvanisé dans un électrolyte aqueux contenant de l'hydroxyde de sodium ou de potassium,
    la corrosion galvanique du zinc depuis la surface de l'acier galvanisé dans une réaction où il y a une anode et une cathode, où le zinc sert d'anode et le matériau servant de cathode est principalement un matériau ayant un potentiel d'électrode standard qui est intermédiaire entre ceux du zinc et du cadmium dans la série électrochimique, et
    le traitement de l'acier galvanisé pour accélérer la vitesse de corrosion du zinc depuis l'acier galvanisé, ledit traitement comprenant le chauffage de l'acier galvanisé pour former un alliage de zinc sur la surface de l'acier galvanisé.
  2. Procédé selon la revendication 1 où la vitesse de corrosion galvanique est augmentée par chauffage de la surface de l'acier galvanisé à une température qui amène le zinc provenant du revêtement de zinc à diffuser dans l'acier et le fer provenant de l'acier à diffuser dans le revêtement de zinc.
  3. Procédé selon la revendication 2 où la surface de l'acier galvanisé est chauffée à une température d'au moins environ 470°C.
  4. Procédé selon la revendication 3 où la surface de l'acier galvanisé est chauffée à une température d'au moins 600°C.
  5. Procédé selon l'une quelconque des revendications 1 à 4 où la vitesse de corrosion galvanique est augmentée par abrasion ou déformation mécanique de l'acier galvanisé.
  6. Procédé selon la revendication 5 où l'aire de surface abrasée ou déformée mécaniquement dépasse environ 10 % de l'aire de surface de l'acier galvanisé.
  7. Procédé selon la revendication 6 où l'aire de surface abrasée ou déformée mécaniquement dépasse environ 15 % de l'aire de surface de l'acier galvanisé.
  8. Procédé selon l'une quelconque des revendications 1 à 7 où la vitesse de corrosion galvanique est accélérée par mélange de l'acier galvanisé avec un matériau ayant un potentiel d'électrode standard intermédiaire entre ceux du zinc et du cadmium dans la série électrochimique, la proportion dudit matériau étant au moins 5 % en masse du mélange.
  9. Procédé selon l'une quelconque des revendications 1 à 8 où la vitesse de corrosion galvanique est accélérée par mélange de l'acier galvanisé avec un matériau ayant un potentiel d'électrode standard intermédiaire entre ceux du zinc et du cadmium dans la série électrochimique, la proportion dudit matériau étant au moins 10 % en masse du mélange.
  10. Procédé selon l'une quelconque des revendications 1 à 9 où l'acier galvanisé est dans un chariot qui tourne tandis que l'acier galvanisé est immergé dans l'électrolyte.
  11. Procédé selon l'une quelconque des revendications 1 à 10 où l'acier galvanisé est déchiqueté en fragments ayant une taille entre 10 cm et 20 cm pour accélérer la vitesse de corrosion.
  12. Procédé selon l'une quelconque des revendications 1 à 11 où l'acier galvanisé est déchiqueté en fragments dont la majorité ont une taille entre 10 cm et 15 cm pour accélérer la vitesse de corrosion.
  13. Procédé selon l'une quelconque des revendications 1 à 12 où l'électrolyte contient au moins 15 % en masse d'hydroxyde de sodium ou de potassium et a une température d'au moins 75°C.
  14. Procédé selon la revendication 13 où l'électrolyte contient au moins 30 % en masse d'hydroxyde de sodium et a une température d'au moins 85°C.
EP02075408A 1996-07-17 1998-04-24 Procédé de dézingage de l'acier galvanisé Expired - Lifetime EP1234897B1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
DE69825799T DE69825799D1 (de) 1998-04-24 1998-04-24 Verfahren zum Entzinken galvanisierten Stahls

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US08/680,344 US5779878A (en) 1996-07-17 1996-07-17 Process for dezincing galvanized steel
EP98918703A EP0996775B1 (fr) 1996-07-17 1998-04-24 Procede de dezingage de l'acier galvanise

Related Parent Applications (1)

Application Number Title Priority Date Filing Date
EP98918703A Division EP0996775B1 (fr) 1996-07-17 1998-04-24 Procede de dezingage de l'acier galvanise

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Publication Number Publication Date
EP1234897A1 EP1234897A1 (fr) 2002-08-28
EP1234897B1 true EP1234897B1 (fr) 2004-08-18

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CN114107679B (zh) * 2021-12-06 2024-03-19 昆明同越科技开发有限公司 防止镀锌钢板回收钢板生锈的方法

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AU3115295A (en) * 1994-07-19 1996-02-16 Hoogovens Staal Bv Process for electrochemically dissolving a metal such as zinc or tin
US5779878A (en) * 1996-07-17 1998-07-14 Metal Recovery Industries (Us) Inc. Process for dezincing galvanized steel

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