The present invention relates to a method of regenerating by crys¬ tallization 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 in¬ dustries and the workshop industry for surface treating articles made of iron, steel and other iron alloys. In the steel industry, 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 manu¬ facture. These layers are mainly the result of the thermal treat¬ ment processes to which the workpieces are subjected, e.g. to annealing processes and subsequent rolling operations.

In the workshop industry, workpieces are picκled in acid baths in order to remove rust and other contaminants from the surfaces of the workpieces. The 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.

In the steel industry, sulphuric acid and hydrochloric acid pickling solutions are primarily used to pickle non-alloyed steel or low- alloyed steel (commercial steel). Those pickling baths used in the workshop industry are mainly sulphuric-acid solutions.

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When scale-coated articles made of non-alloyed or low-alloyed steels are pickled in acid pickling baths, the scale is first partially dis¬ solved 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.

In normal cases the concentration of the solution and the temperature at which the pickling process is carried out are those given in the following table:

Concentration Temperature Pickling acid mol/1 ^C

Hydrochloric acid . 2-5 20-60

Sulphuric acid 1-2.5 60-90

The pickling time for the complete removal of scale in a rolling mill is about 1 minute.

When pickling workpieces to remove rust therefrom, rust and other contaminants on the surfaces of the workpiece are progressively dissolved, and the conditions are somewhat different to those when pickling in order to remove scale. Normal concentrations and tem¬ peratures are given in the following table:

Concentration Temperature Pickling acid . mol/1 ^C Hydrochloric acid 1-4 20-60

Sulphuric acid 0.5-1 20-80

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, re¬ sulting 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 pro¬ tect 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 simul¬ taneously inhibiting the exposed metal surfaces against dissolution by acid attack. According to this method 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.

Irrespective of the pickling acid used, 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.

Various methods of disposing of the consumed pickling solution are known to the art. These methods are applied technically with dif¬ fering degrees of success. The methods are either concerned with the destruction of the solution or its regeneration for re-use.

In those methods pertaining to the destruction of the solution, said solution is neutralized by adding lime, sodium hydroxide or some other alkali thereto. At the same time herewith, iron and other metals are precipitated in hydroxide form. Subsequent to separating

the precipitate, the water phase can be discharged to a recipient, while observing those requirements appertaining to the care and protection of the environment. Because of the low economic value of the precipitate, its metal content cannot be utilized. It must there- fore be dumped. The disadvantages with the destruction of pickling bath solutions are many, and in a majority of cases obvious, although the costs of the lime and the low economic value of the precipitate are among the most serious of these disadvantages. Hydroxide precip¬ itates are also difficult to handle and are thus particularly dif- ficult to separate from the water phase. Since the acid is neutral¬ ized, it cannot be re-used or worked up. Thus, the regeneration of pickling bath solutions is of much more interest in the present context. The different regenerating methods which have been devel¬ oped 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. In one of the previously known methods according to Keramche ie, the crys¬ tallization process is carried out in a vacuum crystal!izer at a temperature of about 10°C. In another known crystallization method (The Otto-method) the crystallization of iron sulphate is effected by strong indirect cooling without vacuum in one or more intercon¬ nected crystal!izers. In both of these methods iron sulphate -heptahydrate is separated from the water phase by centrifugation. The mother liquor is returned to the pickling bath, to which con- centrated sulphuric acid is added so as to again obtain suitable pickling conditions.

It is also since long known to regenerate sulphuric acid pickling solutions by first increasing the sulphuric acid content of the pickling solution by supplying concentrated sulphuric acid prior

to a final cooling of the solution in order to crystallize iron sul¬ phate therefrom. Such processes, which -for example are disclosed in DE-C-561514 (Sierp and Fransemeier), DE-B-1124781 (Niedner) and US-A-1589610 (Marsh et al), show much better regenerating efficiency than those using solely the cooling.

However, a common feature of the known crystallization methods is that large quantities of energy are required, since both evaporation and the strong cooling of the bath are unit processes requiring large quantities of heat.

The iron content of hydrochloric-acid pickling bath solutions can be recovered therefrom in the form of iron chlorides or iron oxide.

In the so-called Ruthner-process, described in A Hake, Dsterr.

Chemiker Zeitung 68, 180/85 (1967), for example, a roasting process is applied, in which the spent pickling bath solution is sprayed into a heated reactor space. The water in the solution evaporates and hydrogen chlorine and iron oxide are formed from the metal chlorides and water vapour present, said hydrogen chlorine and iron oxide being removed from the reaction gases by cyclone separation. The hydrogen chlorine gas is absorbed in the water in a column and the obtained hydrochloric acid can be re-used for pickling purposes.

A similar method is described in DE-A-1546164 and 1621615 (Metal1- gesellschaft AG, Frankfurt), in which the pickling bath solution, subsequent to a certain degree of pre-vaporization, is charged to an oil-heated or gas-heated fluidized bed furnace and there split up while recovering metal oxide and hydrogen chloride from the fumes, by oxidation with an oxygen - containing fluidizing gas.

According to the Pori-process, described in detail in Burtch I W, Iron and Steel Engineer, 50 (1973) :4, 40-42 ferrous chloride in the pickling bath solution is oxidized to ferric chloride, which is then hydrolyzed at high temperatures to iron oxide and hydrochloric acid.

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Another known method described in NO-A-7302621 concerns the produc¬ tion of iron sulphate from the spent pickling bath by adding sul¬ phuric acid to the bath and electrode-heating said bath to a tem¬ perature of 120°C.

It will be evident from the aforedescribed descriptions of the known processes for regenerating pickling bath solutions based on sulphu¬ ric acid and hydrochloric acid that present ^' day techniques require er high energy inputs.

Thus, when crystallizing iron sulphate for the treatment of sul¬ phuric acid baths in which the iron is a high concentration and the acid in low concentration, it is necessary to subject the solution to a vacuum or to strong cooling (from about 60°C to about 10 C) or a combination thereof. This requires complicated apparatus, and the installation and operational costs are so high as to render the method unacceptable, at least in the case of pickling installations of average size within the workshop industry. In practice, it is only possible to recover sulphuric acid in those cases when the consumed pickling bath solution is subjected to evaporation pro¬ cesses. In other cases the acid concentration of the residual solu¬ tion is too low.

The previously described methods available for recovering hydro- chloric acid from consumed hydrochloric acid bath solutions re¬ quire even more energy, even though it is possible by these methods to recover hydrochloric acid with a relatively high degree of effi¬ ciency. The practical operation of the kind of apparatus required herefore, is however, extremely complicated.

The object of the present invention is to eliminate the aforemen¬ tioned 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. Thus, 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 elimi¬ nate 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.

It is also an object of the present invention to permit operation of a continuous picκ ^' ling operation, in which substantially constant pickling conditions can be maintained. According to another object of the invention it is also possible to utilize the iron salts which are precipitated from the pickling solutions, for example as water purifying agents.

It has now surprisingly been found that these objects easily can be realized in a method where the solubility properties of the iron salts in question are utilized in a suitably manner. The method is characterized in that sulphuric acid or hydrochloric acid is added to the solution in a concentration higher than that of said solu¬ tion in a quantity sufficient to substantially decrease the solu- bility of the iron salt corresponding to said acid at the temper¬ ature 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 solu- bility 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.

In those acids or acid mixtures used when pickling iron, steel ard other iron alloys the solubility of corresponding iron salts is de¬ pendent on the acid concentration and on the temperature. It has now further been found that presence of phosphoric acid has an in¬ fluence on the solubility of the iron salts in a favourable direc¬ tion in those concentration ranges being actual when regenerating the pickling solutions. By adjusting the acid concentration and tem¬ peratures to suitable values it is thus possible to precipitate iron salts from pickling solutions containing phosphoric acid in a manner that is favourable from an economic point of view and also from an energy point of view.

The reason why a phosphoric acid content of the magnitude set forth in the claims, that is up to 1 mole per litre, in this connection has such a favourable influence on the solubility conditions for iron is not quite made clear, but probably a displacement of the equilibria of the pickling solution will occur.

In accordance with the method of present invention, 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.

In a first stage there is added to the consumed pickling bath solu- tio ^' ns 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 tem¬ perature prevailing in the bath, which temperature, of course, can be slightly increased by the heat of solution as the acid is added. If the spent pickling bath solution does not contain any phosphoric acid also such acid is supplied to the solution in this stage,

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suitably in the form of a mixture with sulphuric acid or hydro¬ chloric 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. If the solution has a high iron content, 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. When the iron- content of the solution is low, however, 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 substan¬ tially obtain the desired acid concentration in the pickling bath, and the desired maximum iron content can be adjusted with the cooling capacity available. Thus, in the next stage 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. If precipitation has already commenced in the first pro¬ cess stage, it will continue during the cooling stage. Otherwise, the iron salts begin to precipitate when the saturation value is exceeded in said cooling stage. During the 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-salt precipitation processes. Cooling in 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.

In a third and last stage of the method, the crystallized iron salt is separated from the bath solution. This separation can be ef¬ fected 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 su ed and iron is dissolved in said solution until said solution loses its pickling ability, v/hereupon it is again considered a con¬ sumed 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 pro¬ cess are limited or completely eliminated.

By suitable selection of acid concentration and temperature, the described treatment cycle can be repeated without changing the quantity of bath solution. The amount of solution removed by crys¬ tallization, can be compensated by the quantity of acid added. In practice, liquid is also lost by adhesion, vaporization and spillage. This loss must also be replaced by suitable additions to the bath.

Since the crystallization process is carried out ^' while 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 ex¬ posed to a vacuum. The crystallization process can thus be carried out in a comparatively simpler apparatus with less cooling require¬ ments 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.

By suitable selection of operating conditions, it is possible to control the actual pickling operation in an optimal manner, since both the concentration of acid and of iron in the pickling solution can be selected within reasonable limits already during the re- generation process. Thus, the acid content can be set at a value preferably within the range of 0.5-4 mol/1, whereat in the case

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of sulphuric-acid based solutions there is suitably selected a con¬ tent of 0.5-2.5 mol/1. The iron content, which in the consumed solu¬ tion can be permitted, to reach to 100 g/1 or thereabove, can thus be lowered to a value which normally lies beneath 50 g/1. As will be understood, even lower contents can be obtained if desired, by a suitable combination of acid addition and cooling.

By the aforementioned selection of iron content it is possible in practice to utilize the well known phenomenon! that a given content of iron in the pickling bath solution provides better pickling con¬ ditions than the normal starting position for pickling, when be¬ ginning with a freshly prepared acid solution which does not contain iron.

In practice suitable operating conditions are selected, such that both the iron concentration and the acid concentration are allowed to vary within the optimum range. In a continuous method, the con¬ centrations of iron and acid may be selected so as to be practically constant at suitably selected operational conditions.

During a pickling operation components other than iron are also taken up in the pickling solution, e.g. alloying metals and oil. When prac¬ tising the regenerating method according to the present invention, 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 re¬ placed or subjected to special purifying operations for removing these components.

The ⁿ icklinQ and rsgeneratini procedures are best illustrated on phase diagrams of actual systems, such as FeSO, - H^SO. - H _^ O or FeCl ₂ - HC1 - H ₂ 0. One example of such a diagram is given in figure 1, in which the contents of FeSO, and H^SO, present in aqueous solution in equil brium with solid FeSO, are shown at different temperatures. Figure 2 shows a corresponding phase diagram for the system FeCl^ -

HC1 - H ₂ 0. Figure 3 illustrates on a phase diagram over the system FeSO. - HpSO, - H ₂ 0 a regenerating procedure both with and without presence of phosphoric acid for a sulphuric acid based pickling solu¬ t ⁱ on. Figure 4 illustrates in connection with a flow sheet showing the process of the present invention a mass and energy balance ac¬ cording to Example 3.

The invention will now further be illustrated by means of a number of non-limiting examples, in which "%" means "% by weight".

EXAMPLE 1

When pickling iron in a sulphuric acid solution, the acid concentra¬ tion is supposed permitted to vary between about 16 and about 10 H SO.. The pickling and regenerating procedure is illustrated by the diagram in Figure 3. Thus, the pickling starts at the point A in Figure 3, where the pickling solution contains 16 % H^SO, and about 13 % FeSO.. During the pickling process acid is consumed and iron sulphate simultaneously formed. When the acid concentration has fallen to about 10 * the pickling is broken off. The iron sul- phate 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^SO- content of the pickling solution increases, whereas the iron sulphate content decreases due to dilu¬ tion. The acid supply is interrupted at point C when the H ₂ S0 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^SO. and 20 % FeSO _* .

Pickling with sulphuric acid 1? generally carried out at elevated temperatures, such as about 70°C. When sulphuric acid is added to the spent pickling solution a further temperature increase will occur. At the point C the solution in the example has a temper- ^" ature of about 77 C.

When the sulphuric acid supplied pickling solution is cooled, 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.

During the crystallization process the content of FeSO. in the solu¬ tion will decrease whereas the content of H^SO, will decrease due to the fact that FeSO. • XH ₂ 0 is removed from the solution. Provided that the point C has been suitable chosen the crystallization will be completed at the point A , at which point the solution again has the composition which is suitabl e for pickl ing. In this exampl e the ssoolluuttion must be cooled to about 21°C to reach the desired composi tion.

EXAMPLE 2

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 21 C equilibrium curve (corresponding to the final tempera¬ ture in Example 1) lies about 5 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-SO, and 11.5 % FeSO, corresponding to point A . During the pickling operation the acid content de¬ creases to about 10 %, while the iron sulphate content will increase to about 21 %, point B . Then sulphuric acid is suppl ed until

11 11 point C is reached. At C the acid content is about 13.5 % and the ^' iron sulphate content about 20 %.

During the cooling operation iron sulphate is precipitated from the

11 11 solution, which will move the operation point from C back to A , which point corresponds to the initial composition of the pickling solution. Theoretically no phosphoric acid will be ^' consumed during

14

the pickling-regeneration procedure irrespective of minor amounts which will adsorb on the pickling goods and also of spill and other inevitable losses. The pickling bath may thus theoretically be re¬ generated by addition of solely sulphuric acid, but, in practice, the solution must be replenished with sulphuric acid containing a phosphoric acid content adjusted to the circumstances.

EXAMPLE 3 Energy balances. The benefit of utilizing a phosphoric acid containing pickling solu¬ tion for the regenerating operation is set forth of the energy bal¬ ances composing the Examples 1 and 2, respectively. In both cases 1000 m steel sheet is pickled and the FeO scale removed corresponds to 40 g/m ² .

Provided that the process is performed using a heat exchange appara¬ tus as indicated of the flow sheet in Figure 4 mass and enerby bal¬ ances can be calculated as shown by the following table.

Exampl e 1 Exampl e 2

Spent pickl i ng sol ution amount i n kg 1030 842 temperature C 70 70

Subsequent to acid supply • amount i n kg 1070 882 temperature C 77 77

Iron sulphate, kg 155 155

Regenerated pickl ing sol ution amount in kg 915 727 temperature C 21 21

Ftisyπv rl αma nf. M.I final cool i ng-crystal l i zation 89 75 heati ng to operation temp. 26 18

115 93

Note: In practice it will be water losses in both cases by evapora¬ tion and so, which losses have been supposed to influence the mass and energy balance equally.

The presence of phosphoric acid will consequently result in a lowering of the amount pickling acid in operation and thereby also a reduction of the energy demand with almost 20 , which is clearly shown of the figures corresponding to Example 2.

It will be evident that similar results as those obtained in the above examples may also be achieved also for hydrochloric acid pickling baths. The lowering of the solubility of FeCl ₂ is here even more evident in this case and is furthermore dependent on the HC1 concentration as is shown in the diagram of Figure 2 by the dotted line corresponding to a temperature of 20 C.

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