Classifications

• • 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

Definitions

• the present invention relates to a method of regenerating by crystallization on cooling pickling bath solutions based on sulphuric acid and/or hydrochloric acid and used for pickling iron, steel and other iron alloys.
• Acid pickling processes are applied within the iron and steel industries and the workshop industry for surface treating articles made of iron, steel and other iron alloys.
• acid pickling processes are primarily applied in order to dissolve the scale and other oxide and hydroxide layers formed on the metal surfaces of % workpieces during the various processes of their manufacture. These layers are mainly the result of the thermal treatment processes to which the workpieces are subjected, e.g. to annealing processes and subsequent rolling operations.
• rust is formed during the transportation, storage and handling of the workshop raw materials, these raw materials comprising, for example, finished goods from the steel industry or intermediate products from other metal -processing industries, e.g. products obtained from different subcontractors.
• the contaminants originate from the same sources, and may comprise welding, soldering or brazing residues and handprints. They may also originate from oil coatings and other coatings resulting from processing operations and from surface treatment processes carried out in order to afford the steel surfaces of the workpieces some protection against corrosion.
• sulphuric acid and hydrochloric acid pickling solutions are primarily used to pickle non-alloyed steel or lowalloyed steel (commercial steel).
• Those pickling baths used in the workshop industry are mainly sulphuric-acid solutions.
• the scale is first partially dissolved at the same time as the major part of said scale is removed mechanically by the development of hydrogen gas in pores and cavities in the scale. The thus exposed surfaces of the basic material are then liable to attack by the acid, and if the pickling process is not stopped at the correct point of time an unnecessarily large quantity of material is consumed, without any advantage being gained thereby.
• the pickling time for the complete removal of scale in a rolling mill is about 1 minute.
• the time required when pickling to remove rust is normally between 5 and 20 minutes.
• One common feature of all pickling processes is that when the raw metal surface is exposed it is readily attacked by the acid, resulting in an unnecessary loss of material by dissolution of the metal. To avoid this it is normal to add an organic inhibitor, scme- times referred to as a restrainer, whose purpose is to block the exposed, free metal surface by adsorption thereon, thereby to protect said surface from acid attack.
• Described in European Patent Application No. 80850166.2 is a method for acid-pickling metallic material having > 80 % Fe while simultaneously inhibiting the exposed metal surfaces against dissolution by acid attack.
• the material are brought into contact with an aqueous solution containing phosphoric acid and one or both of the mineral acids hydrochloric acid and sulphuric acid.
• the phosphoric acid concentration is adjusted to at least about 0.01 M (i.e. mol/1) and at most about 1 M, while the total acid concentration is adjusted to the range between about 0.5 and 4 mole per litre.
• the concentration of acid will decrease as the process proceeds, while the concentration of dissolved iron (and possibly other alloying metals) will increase at the same time. Ultimately the pickling effect will be non-existent or will have decreased to such an extent as to render it no longer possible to utilize the solution for pickling purposes.
• the different regenerating methods which have been developed thus pertain to the re-use of either residual acid or dissolved metals or, optionally, a combination thereof.
• the iron content of sulphuric - acid pickling bath solutions can be recovered therefrom in the form of iron sulphates by crystallization. This method, which is the one most used today for working-up pickling bath .solutions, requires the crystallization process to be effected either by strong cooling of the solution or by evaporating the solution under a vacuum. Equipment for such methods is commercially available.
• the crystallization process is carried out in a vacuum crystal! izer at a temperature of about 10°C.
• the crystallization of iron sulphate is effected by strong indirect cooling without vacuum in one or more interconnected crystal! izers.
• iron sulphate -heptahydrate is separated from the water phase by centrifugation. The mother liquor is returned to the pickling bath, to which concentrated sulphuric acid is added so as to again obtain suitable pickling conditions.
• the iron content of hydrochloric-acid pickling bath solutions can be recovered therefrom in the form of iron chlorides or iron oxide.
• the object of the present invention is to eliminate the aforementioned disadvantages encountered when regenerating pickling bath solutions based on sulphuric acid and hydrochloric acid, and to provide a technically simple, energy-saving solution to the problems encountered when working-up the pickling bath solution for re-use by the normal user, for example within the workshop industry.
• a further object of the present invention is to utilize the solubility of the iron salts at differing acid concentrations and temperatures and in the presence of phosphoric acid in a manner such as to eliminate such energy-consuming operations as high-temperature splitting, or cooling to low temperatures, when regenerating spent pickling bath solutions.
• Another object of the invention is to eliminate or reduce the necessity of destroying spent or consumed pickling bath solutions, by using regenerating agents of suitable composition, which, in comparison with previously known methods, provides a method which has a less negative effect on the environment and which requires less energy.
• Still another object of the invention is to eliminate the need of complicated and expensive apparatus and to reduce the operational costs of regenerating pickling bath solutions.
• the method is characterized in that sulphuric acid or hydrochloric acid is added to the solution in a concentration higher than that of said solution in a quantity sufficient to substantially decrease the solu- bility of the iron salt corresponding to said acid at the temperature in question, in that a content of about 0.01 - 1 mole per litre of phosphoric acid is maintained in the solution, in that the solution is then cooled to a given temperature to crystallize out the iron salt, said temperature being so selected that the solubility of said iron salt at said temperature is sufficiently low for the solution to again be used for pickling purposes and in that the crystallized iron salt is removed from the solution.
• phosphoric acid containing pickling bath solutions are thus treated in three stages, whereby excess of dissolved iron is removed from the solution and the contents of acid and iron are re-set to values which are suitable for re-use as a pickling solution.
• a concentrated acid or an acid mixture containing one or both of these acids whereat the composition of the solution is displaced tov/ards the saturation value of corresponding iron salts at the temperature prevailing in the bath, which temperature, of course, can be slightly increased by the heat of solution as the acid is added.
• the spent pickling bath solution does not contain any phosphoric acid also such acid is supplied to the solution in this stage, suitably in the form of a mixture with sulphuric acid or hydrochloric acid. If phosphoric acid already is present in the spent pickling bath solution a minor amount phosphoric acid may be added to compensate for losses during the pickling.
• the saturation value is exceeded after only a relatively small amount of acid is added, whereat precipitation of an anhydrous or hydrous iron salt commences.
• the saturation value is not normally exceeded in this process stage.
• the amount of acid added in this stage is suitably adapted so as to at least substantially obtain the desired acid concentration in the pickling bath, and the desired maximum iron content can be adjusted with the cooling capacity available.
• the pickling bath solution with the acid added thereto is cooled to a given temperature above or in the proximity of the ambient temperature, using some conventional method herefore. These methods, for example, may comprise conventional air cooling, water cooling or self-cooling processes.
• the iron salts begin to precipitate when the saturation value is exceeded in said cooling stage.
• the iron content of the pickling bath decreases continuously with decreasing temperature. Since iron is precipitated as hydrous salts, the acid content of the solution increases indirectly during the cooling and iron-sal t preci pi tati on processes . Cool i ng i n the second stage is continued to a temperature at which acid and iron contents of the pickling bath solution are suitable for re-use in pickling processes.
• the crystallized iron salt is separated from the bath solution.
• This separation can be effected by conventional methods, e.g. filtration, sedimentation and centrifugation.
• the residual solution freed of crystals is returned to the pickling operation.
• the acid in the solution is again con sumed and iron is dissolved in said solution until said solution loses its pickling ability, v/hereupon it is again considered a consumed or spent solution and the aforedescribed regeneration stages are repeated.
• the regeneration method can be carried out batchwise, whereat a major portion of a consumed pickling bath is removed and subjected to the aforedescribed stepwise treatment process.
• the regneration process can also be carried out semi-continuously or continuously, whereat minor portions of the bath or a continuous flow of bath solution are withdrawn and processed in accordance with the above. In this way variations in concentration during the pickling process are limited or completely eliminated.
• the described treatment cycle can be repeated without changing the quantity of bath solution.
• the amount of solution removed by crystallization can be compensated by the quantity of acid added.
• liquid is also lost by adhesion, vaporization and spillage. This loss must also be replaced by suitable additions to the bath.
• the crystallization process is carried outwhile increasing the concentration in the bath, and displacing the equilibria by the presence of the phosphoric acid the temperature difference required for the desired degree of crystallization is lower than that required in known methods based on crystallization, and therefore the bath need not be cooled to temperatures beneath ambient temperature or exposed to a vacuum.
• the crystallization process can thus be carried out in a comparatively simpler apparatus with less cooling requirements than for prior art crystallization processes.
• the cooling therefore can readily be effected with air or available cooling water.
• the method is energy saving, and incurs lower investment costs and operational costs than hitherto known techniques.
• the acid content can be set at a value preferably within the range of 0.5-4 mol/1, whereat in the case of sulphuric-acid based solutions there is suitably selected a content of 0.5-2.5 mol/1.
• even lower contents can be obtained if desired, by a suitable combination of acid addition and cooling.
• the concentrations of iron and acid may be selected so as to be practically constant at suitably selected operational conditions.
• components other than iron are also taken up in the pickling solution, e.g. alloying metals and oil.
• these components will be accumulated. in the bath solution and may gradually build up to such concentrations as to disturb the pickling operation. In such a case, the pickling bath solution must be replaced or subjected to special purifying operations for removing these components.
• phase diagrams of actual systems such as FeSO 4 - H 2 SO 4 - H 2 O or FeCl 2 - HCl - H 2 O.
• Figure 1 shows a corresponding phase diagram for the system FeCl 2 HC1 - H 2 O.
• Figure 3 illustrates on a phase diagram over the system FeSO 4 - H 2 SO, - H 2 O a regenerating procedure both with and without presence of phosphoric acid for a sulphuric acid based pickling solution.
• Figure 4 illustrates in connection with a flow sheet showing the process of the present invention a mass and energy balance according to Example 3.
• the acid concentration is supposed permitted to vary between about 16 and about 10 H 2 SO 4 .
• the pickling and regenerating procedure is illustrated by the diagram in Figure 3.
• the pickling starts at the point A in Figure 3, where the pickling solution contains 16 % H 2 SO, and about 13 % FeSO 4 .
• the pickling acid is consumed and iron sulphate simultaneously formed.
• the pickling is broken off.
• the iron sulphate content is now 21 %, which can be seen from point B in the diagram.
• the regenerating process is commenced by an addition of a 95 % sulphuric acid, whereby the H 2 SO 4 content of the pickling solution increases, whereas the iron sulphate content decreases due to dilution.
• the acid supply is interrupted at point C 1 when the H 2 SO 4 content has increased to a magnitude which is determined by the desired composition of the pickling solution being regenerated.
• the solution contains then about 13 % H 2 SO. and 20 % FeSO 4 .
• Pickling with sulphuric acid is generally carried out at elevated temperatures, such as about 70°C.
• elevated temperatures such as about 70°C.
• sulphuric acid is added to the spent pickling solution a further temperature increase will occur.
• the solution in the example has a temperature of about 77o C.
• iron sulphate precipitation will commence when the temperature reaches a value corresponding to the solubility of the sulphate. In the present case sulphate will begin to precipitate in solid form at about 37°C.
• the content of FeSO 4 in the solution will decrease whereas the content of H 2 SO 4 will decrease due to the fact that FeSO 4 XH 2 O is removed from the solution.
• the point C 1 has been suitable chosen the crystallization will be completed at the point A 1 , at which point the solution again has the composition which is suitabl e for pickl ing.
• the ssoolluuttion must be cooled to about 21°C to reach the desired composi tion.
• the pickling procedure is performed following the same conditions as in Example 1, but the pickling solution is supplied with phosphoric acid so as to reach a content of 0.5 mol/1 prior to the regenerating operation.
• the solubility of iron sulphate is hereby lowered, so that the 21o C equilibrium curve (corresponding to the final temperature in Example 1) lies about 5o C lower than in the system free from phosphoric acid. See Figure 3.
• Precipitation of iron phosphate will not occur at prevailing conditions.
• the pickling operation starts with a solution containing 17 % H 2 SO 4 and 11.5 % FeSO 4 corresponding to point A 11 .
• the acid content decreases to about 10 %, while the iron sulphate content will increase to about 21 %, point B 11 .
• sulphuric acid is supplied until point C 11 is reached.
• the acid content is about 13.5 % and the iron sulphate content about 20 %.
• EXAMPLE 3 Energy balances.
• the benefit of utilizing a phosphoric acid containing pickling solution for the regenerating operation is set forth of the energy balances composing the Examples 1 and 2, respectively.
• 1000 m 2 steel sheet is pickled and the FeO scale removed corresponds to 40 g/m 2 .

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• 1981
  • 1981-01-16 EP EP81850009A patent/EP0032886B1/en not_active Expired
  • 1981-01-16 DE DE8181850009T patent/DE3163015D1/de not_active Expired
  • 1981-01-16 JP JP50046081A patent/JPS56501886A/ja active Pending
  • 1981-01-16 WO PCT/SE1981/000007 patent/WO1981002026A1/en active IP Right Grant
  • 1981-01-16 AT AT81850009T patent/ATE7050T1/de not_active IP Right Cessation
  • 1981-09-10 FI FI812817A patent/FI68668C/fi not_active IP Right Cessation

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