The present invention relates to a method of controlling or regu- lating the composition of pickling bath solutions for acid pickling metallic materials.

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 manufac¬ ture. 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.

In the workshop industry, workpieces are pickled 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 com¬ prising, for example, finished goods from the steel industry or inter¬ mediate 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 surface of the workpieces some protection against corrosion.

For similar reasons, magnesium, copper, brass and other copper alloys are also subjected to acid pickling processes in other metal-working industries.

OMPI

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

When pickling stainless material and other not-readily workable grades, the baths mainly comprise nitric acid or hydrofluoric acid or mixtures thereof.

When pickling magnesium, chromic acid or citric acid is often used, while when pickling copper and alloys thereof a sulphuric acid bath containing chromic acid is used.

Although in principle phosphoric acid can be equated with, hydrochloric acid and sulphuric acid when pickling steel, its use is limited be¬ cause of the longer pickling times required and the higher costs in¬ volved.

When scale-coated articles made of non-alloyed or low-alloyed steels are pickled in acid pickling baths, 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.

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:

Pickl ing acid Concentration Temperature mol /kg solution °C

Sulphuric acid 2-5 20-60 ^' Hydrochloric 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 dis¬ solved, and the conditions are somewhat different to those when pickling in order to remove scale. Normal concentrations and temper¬ atures are given in the following table:

Pickling acid Concentration Temperature mo!/kg solution o,

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, sometimes 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. The use of a- restrainer, how¬ ever, is made difficult by the fact that the substance is readily adsorbed on the exposed metal surfaces in the pickling tank or on the surfaces of the scale particles collected in the bath solution. With present day techniques it is practically impossible to deter- mine the concentration of inhibitor in the bath.

Described in Swedish Patent application 7909187-2 is a method for acid-pickling workpieces while simultaneously inhibiting the exposed metal surfaces against attack by the acid solution. According to this method the workpieces are brought into contact with a solution con- taining at least tv., mineral acids, of which one is phosphoric acid

Is.

in an amount sufficient to provide said inhibiting effect without appreciably lowering the pickling effect, which effect is afforded by the remaining components in the solution, .for a period of time sufficient to remove oxide scale and other undesirable coatings _, on the surfaces of the workpieces.

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 off 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 out in hydroxide form. Subsequent to separating the precipitates, 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 precip tate, the metal content thereof cannot be util¬ ized. It must therefore be dumped.

The disadvantages with the destruction of pickling bath solutions are many, and in a majority of cases obvious, whereby the costs of the lime and the low economic value of the precipitate are among the most serious of these disadvantages. Hydroxide precipitates are also difficult to handle and are thus particularly difficult to separate out.

Since the acid is neutralized, 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 developed thus pertain to the re-use of either, re- sidual acid or dissolved metals or, optionally, a combination thereof.

In Swedish Patent Application 8000444-3 there is described a method for regenerating by crystallization bath solutions based on sulphuric acid or hydrochloric acid and used for pickling iron, steel or other iron alloys. This method enables pickling acid to be re-used by re¬ turning the acid to the bath for re-use subsequent to regeneration.

In order to carry out an acid pickling operation in an advantageous manner, it is necessary to know how composition variables and other physical magnitudes affect the pickling process. Pickling can then be effected in an economic manner, to achieve low acid consumption, low energy and water consumption and good surface properties of the pickled material.

It is of particula importance that when the pickling acid is to be regenerated the pickling process can be controlled or regulated, since particularly advantageous conditions can then be achieved, such as pickling at constant pickling parameters.

The following description will mainly deal with controlling the pickling sequence when pickling non-alloyed steel. _. This is in no way restrictive of the invention, since the method according to the invention can also be applied when pickling other metallic materials.

Those quantities which most affect he pickling, result- re ^* .acid- con¬ tent, iron(II)concentration, temperature and pickling time. The pickling result is also influenced by the concentration of inhibitor, when inhibitors are used.

Of these quantities no problems are encountered when measuring and controlling the temperature and pickling time under normal condi¬ tions. In the event of a breakdown in operations, however, the pickling time may be difficult to control, for example when the object being pickled is a continuous strip of material and the strip stops. On the other hand, serious problems are encountered when measuring the acid content and iron concentration of the bath, par¬ ticularly if it is desired to monitor the pickling sequence con- tinuously.

It is known that the total content of ionic compounds in diluted solutions, such as rinsing baths (SE,B,346339, SE,B,362447), and in electrolysis baths (US,A,3019799, 0S,A,3490467) can be controlled by measuring the electrical conductivity of the baths.

A process for a continuous regeneration of chemical nickeling solutions is known, which solutions contain nickel ions and hypophosphite aπions . and whereby nickel ions and hypophosphite anions are consumed, while causing a decrease in pH. One can thereby by measuring the pH calculate the nickel ion consumption and add an equal amount of nickel ions and hypophosphite consumed to the nickeling bath. This is a pure chemical system where a consumed ion is replaced, (DE,A, 1,107,045). A similar system for adding one or more compounds or solutions of compounds to metallizing baths, either electrolytic or chemical, is known (DE,A, 1,957,087).

There is no knowledge about pickling bath solutions and their use using a single physical quantity for their control.

It is also known when measuring the electrical conductivity of a bath to control the amount of acid or alkali metered to baths of high acidity or alkalinity (DE,A,203991).

For the purpose of determining the metal salt content of a pickling bath it is known to measure the turbidity of a suspension subsequent

to precipitating a metal salt in a sample taken from the bath (DE,A,2828547).

For the purpose of simultaneously controlling two concentrations, e.g. of acid and metal salt, it is known to measure the density of ^* the solution and, at the same time, e.g. by titration to determine the concentration of one metal ion (DE,B,2041815).

Al ernatively, it is possible to measure the density of the solution and to determine the iron-salt concentration indirectly, by measuring the absorbance of the solution in a selected wave-length field by colorimetry) or at a given wavelength (by photometry) (US,A,3074277, US,A,3427198, US,A,3433670). It is also possible to measure the acid content directly and to determine the metal-ion concentration (AT,B,346150).

As will be gathered from the aforegoing, it is important when pickling non-alloyed steel that both the iron(II)concentration and the acid concentration can be controlled or regulated in a simple fashion.

Hitherto, this has been achieved by measuring two physical quantities, or one physical variable and one composition variable in a manner previously applied.

By way of a further example it can be mentioned that the concentration* of iron(II) and the acid concentration can be clearly determined by measuring the electrical conductivity and the density of a sample taken from the bath, or by measuring said conductivity and said density directly in the bath solution (US,A,2927871 , US,A,3062223). This is made possible by the fact that both the electrical conduc¬ tivity and the density of the bath solution varies with the content of iron(II) and the acid concentration, although in different as¬ pects.

It has now been surprisingly found, however, that the same clear de¬ terminations can be made by measuring one physical quantity only. It is an object of the present invention to provide a method whereby a pickling bath can be controlled in a simpler and less expensive manner by utilizing firstly the relationship between said quantity and the two concentration quantities (iron(II) and acid), and secondly the relationship between said two quantities. The characterizing features of the invention are set forth in the accompanying claims.

By operational line is meant here and in the following that line which is drawn when pickling in accordance with a two-phase diagram where the concentration of iron(II) is plotted on one axis and the acid concentration is plotted on the other axis. The operational line provides the stoichiometric relationship between the dissolution of iron*and the consumption of acid in the pickling bath solution. This can be calculated theoretically with knowledge of the nature of the material being pickled and the manner in which the pickling process is carried out, or in practice by testing the material to be pickled. It is the operational line which is utilized as one of the relationships in the controlling operation.

The method can be applied to particular advantage when the physical quantity to be measured is the electrical conductivity of the bath solution, since the electrical conductivity varies within a wide area and is influenced to a large extent by the amount of hydrogen ions in the solution.

The method is particularly advantageous when pickling in mixtures comprising at least two mineral acids, of which one is phosphoric acid, as described in the previously mentioned SE,A,7909187-2. In this case, dissolution of the basic metal is strongly inhibited as a result of the phosphoric acid. The stoichiometric relationship bet¬ ween iron(II) and the acid is thus simplified, since the pickling reaction concerns practically solely the dissolution of scale and/or rust.

At the present time it is not possible to control the composition of the pickling bath solution, since it is practically impossible to carry out the aforementioned determination, for reasons which should be apparent from the aforegoing.

When carrying out the method of the invention in practice, samples of the bath are taken at regular intervals for measuring, for example, the electrical conductivity of the solution. When the operational line is known, concentration variables can be readily determined, thereby enabling it to be determined whether the bath can be used for a further period of time or whether the acid shall be replaced or the bath dumped.

When continuously measuring a variable in the bath solution, a measuring instrument is immersed in said solution or in a flow of solution taken from the bath, the measuring data obtained being utilized in the same manner as that when measuripg said variable from a sample taken from the bath.

in a particularly advantageous embodiment of the invention, input signals are processed in an instrument, and control signals are then transmitted from said instrument to bath-controll ng devices. These signals may be operative to cause acid or water to be metered to the bath, or to cause a given quantity of solution to be removed from the bath, or to cause these physical quantities to be controlled in a timed pattern.

Present day microcomputer techniques enable such an instrument to be readily constructed. It is then relatively easy to program the operational line, control ranges etc.

The invention will now be illustrated more clearly with reference to two non-limitive examples relating to the application of the inven¬ tion when pickling steel, and to three figures of which Figure 1 illustrates the system FeSO^-HpSO.-HpO with operational lines from

the example plotted therein; Figure 2 is a diagram illustrating electrical conductivity in S/m as a function of the acid content of the bath; and Figure 3 is a diagram illustrating the density as a function of acid content of the sulphuric acid pickling bath accord- ng to the examples.

EXAMPLE 1

In one test, sheet-metal covered with scale was pickled by sub¬ merging the sheet metal in an aqueous solution which contained ini- tially 1.5 mole sulphuric acid per liter. During the pickling pro¬ cess, the electrical conductivity was reduced from an initial value of 34 S/m to 12 S/m. At the same time, the sulphuric acid content was reduced to about 0.5 mole sulphuric acid per liter and the iron content was increased from about 0.8 mole per liter to about 1.74 mole per liter, i.e. from A to B in Figure 2.

The pickling process was then interrupted and the bath was regen¬ erated by adding thereto 98% H^SO, and by cooling the bath to ambient temperature, i.e. from point B to point C, and then from point C to point A in Figure 1. The acid added to the bath increased the electrical conductivity to 18.4 S/m and when iron sulphate crys¬ tallized out as the solution cooled, the electrical conductivity was again increased to 35.6 S/m, i.e. approximately to the starting value (point A' in Figure 2). By regulating the pickling process with the electrical conductivity of the bath being used as a control variable, the acid concentration and iron concentration were con¬ stantly known and acid could be etered to the bath in quantities corresponding to the measurement values.

Figure 1 illustrates the operational line for pickling and regen¬ erating the bath of Example 1. Figure 2 illustrates how the electri¬ cal conductivity varied as a function of the acid concentration. The composition variables are clearly determined in this case by the two graphs.

^■

EXAMPLE 2

In a similar test, metal-sheet coated with scale was pickled in a pickling bath under the same conditions as those recited in Example 1 The density of the bath was then increased from an initial value of

3 3 1218 kg/m to 1317 kg/m , while the acid content was decreased and the iron content increased in the same manner as that recited in Example 1, i.e. from point A to point B in Figure 3.

The pickling process was then interrupted and the bath regenerated by adding 98% HgSO _* and by cooling the bath to ambient temperature.

The acid addition caused the density to increase to 1334 kg/m

(from point B to point C in Figure 3) and when cooling the bath the density increased to 1360 kg/m at 40°C (from point C to point C in

Figure 3). When crystallisation had taken place and the temperature had, at the same time, decreased to ambient temperature, the density

3 decreased to 1270 kg/m (from point C to point C" in Figure 3).

When the solution was re-heated to 70°C, the density was approx¬ imately the same as that at the beginning of the test (point A' in Figure 3).

By controlling the bath using the density as a control variable, the acid and iron concentration was constantly known and additions to the bath could be varied in accordance with the measurement values.

Figure 1 illustrates the operational line for pickling and regen¬ erating according to the example. Figure 3 illustrates how the den¬ sity varied as a function of the acid concentration. The composition variables could be clearly determined in this case by the two graphs.

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