ITMI982612A1 - APPARATUS AND METHOD TO CONTROL PERACCIAIO PICKLING PROCESSES. - Google Patents

Abstract

A device and a method to control pickling processes are described, where the control device comprises means (C) to take a sample of the bath to be analysed; means (CA, D, EM) to analyse said sample in order to measure a number of parameters according to specific conductivity and potentiometric methodologies as well as the redox potential value of said samples and its temperature; restoring means, apt to calculate, according to the above measured values, the quantity of corrective chemicals to be added to the pickling bath in order to restore at the desired level the value of said parameters and to actuate at least a device to add into said pickling bath necessary quantities of correction chemicals. The parameters measured according to conductivity methodologies are the concentrations of sulphuric acid, of hydrofluoric acid, or of another inorganic acid; the parameters measured according to potentiometric methodologies are concentrations of bivalent and trivalent iron ions and of hydrogen peroxide and the corrective chemicals are sulphuric acid, hydrofluoric agent and an oxidising agent.

Description

Field of the invention

The invention consists of an apparatus and a method for controlling pickling processes for steel, austenitic, ferritic and martensitic stainless steels, duplex steels and special alloys, in which this equipment automatically manages the sampling of pickling bath samples, the analysis of the samples themselves in order to determine (according to specific conductometric and potentiometric methods) critical process parameters and the restoration of the predetermined concentrations in the pickling tanks through the dosage of the necessary chemical products.

The invention also makes it possible to manage specific pickling conditions for the type of steel treated by defining operating procedures that can be activated remotely with which the most suitable operating conditions for pickling the specific type of material processed are automatically recalled and created.

Prior art.

During the manufacturing processes of steel products (such as sheets, metal strips, pipes, wire rod) which are subjected to rolling, drawing, drawing, extrusion and heat treatments, oxide layers are formed on the surface of these materials which must necessarily be removed both to ensure a final appearance and suitable passive and anticorrosive properties of the finished product and to allow a further lamination process of the material.

These surface layers of oxides are generally removed by a chemical process (pickling) which consists in subjecting the metal material to the action of one or more acid baths consisting of inorganic mineral acids (sulfuric, hydrochloric, nitric, hydrofluoric), alone or in various ratios, suitably diluted and heated, followed by at least one final rinsing phase with water.

For stainless steels, the traditionally used pickling processes (which can be defined as "immersion", "spray" or "turbulence") involve the use of a mixture of nitric acid and hydrofluoric acid: these processes involve very serious problems ecological due to the emission of reaction by-products into the atmosphere (extremely toxic nitrogen oxides) as well as to the presence of large quantities of nitrates in the waste water.

For these reasons, in recent years alternative processes defined as "ecological" have been developed, characterized by the fact that they operate in the absence of nitric acid.

Among these, processes that use mixtures of sulfuric or hydrochloric acid, hydrofluoric acid and ferric ions, in which the correct concentration of said ions in the pickling bath is maintained through the addition of hydrogen peroxide, have proved to be particularly effective at an industrial level. Examples of such processes are reported in Italian patents n ° 1245594 and n ° 1255655.

In traditional pickling technology, the management of the process normally involves an occasional control of the pickling bath through manual titrations of the acidity or conductivity of the solution and of the iron content (or total metals, by measuring the density of the solution) present in the bath; analytically it is also possible to determine the content of hydrofluoric acid by means of a specific selective ion electrode.

Some of these techniques have also been used to automate individual operations in nitric acid-based pickling processes for stainless steel.

US patent 4060717 (LECO Corp.) claims the use of selective ion electrodes for hydrogen ion and fluoride ion for the determination of nitric acid (or other strong acid) and hydrofluoric acid in pickling baths based on nitric acid and hydrofluoric acid; the electrical voltage data acquired by a control circuit are processed by a microprocessor to calculate the concentrations of the two acids and regulate their concentration.

Patent JP 55040908 (Nippon Steel Corp.) claims the determination of hydrofluoric acid and another strong acid (nitric, hydrochloric, sulfuric) through the determination with selective ion electrodes of the respective anions after passage of the solution through anionic exchange membranes for adjust the dosage of acids.

US patent 5286358 (FOXBORO Corp.) determines the concentration of hydrofluoric acid in a nitric acid / hydrofluoric acid mixture through the complexing capacity of the trivalent iron ion for the fluoride ion, allowing the dosage of the acids in the mixture.

The continuous automatic management of these nitric acid-based pickling processes, even if it is an improvement compared to an occasional check (manual or automatic) carried out (for example) a few times a day, is not essential for the management of the process in terms of quality. of the processed material due to the functional characteristics of such baths; in particular, in the case of stainless steel pickling, these baths normally contain high concentrations of nitric acid (normally 12-15%) and hydrofluoric acid contents of the order of 2-5%. The high concentration of nitric acid in the solution guarantees at the same time high acidity and almost constant oxidizing power, making it possible to manage the process through occasional additions of replenishing chemicals. Furthermore, the determination of the acid concentration is sufficient to have an adequate control of the pickling capacity of the bath.

On the contrary, nitric acid-free pickling systems such as those previously mentioned base the oxidizing properties of the system on the control of the quantity of ferric ions (Fe <3 *>) present or better on the control of the Fe <3> 7Fe <2> ratio. \

In this case, due to the pickling reaction (1)

2 Fe <3+> + FeO - »3 Fe <2+> (1)

in a continuous process for the production of stainless steel strips or in high-productivity automatic systems for pickling the wire rod, the concentration of trivalent iron, the ratio Fe <3> 7Fe <2+> and therefore the oxidizing capacity of the solution tend to decrease rapidly, by continuously and drastically changing the behavior of the pickling bath.

The optimal conditions must therefore be continuously restored through the use of oxidizing agents such as hydrogen peroxide.

Furthermore, the variation in the concentration of trivalent iron also indirectly affects the concentration of free acids present in the bath.

For example, in a pickling system based on mixtures of sulfuric acid, hydrofluoric acid and ferric salts, this influence is linked to the following two preferential balances:

Fe <3+> + n F- → Fe Ffl <(3> - <n> +>

Fe <2 *> + S04 <2 '> → FeS04

This means that during the oxidation / reduction reactions of the Fe <3> 7Fe <2+> couple, sulfuric acid and hydrofluoric acid will be released from the corresponding complex salts, with consequent modification of the composition of the bath.

A process control through sporadic analytical determinations followed by massive additions of chemical products to restore the optimal pickling conditions therefore determines too wide variations of the bath parameters with negative consequences on the quality of the material and on the process costs.

On the other hand, carrying out frequent checks manually and subsequent corrective actions is time-consuming and expensive as it requires the use of numerous staff to ensure a control frequency of no less than a predetermined minimum (eg. One check / hour).

The criticality of nitric acid-free pickling processes is obviously a function of the total dissolved iron concentration in the unit of time, the number of solution tanks to be controlled, the number of materials that require different operating conditions and the practical possibility of being able to control manually frequent additions of acids to the tanks.

The management of stainless steel pickling processes such as those previously mentioned on systems for the continuous pickling of stainless steel strips or on high productivity automatic systems for the treatment of wire rod has proved to be critical for the quality of the material or is uneconomical if carried out without the aid of an automatic system for taking, checking and dosing the feed products.

The equipment and the control method object of this invention provide for the adoption of specific measures and analytical methodologies that make them suitable for the correct management of the aforementioned processes.

Summary of the invention.

The present invention relates to an apparatus and a control method for nitric acid-free pickling baths which include means for acquiring a sample of the bath to be analyzed; means suitable for analyzing the aforementioned sample to determine a plurality of parameters according to specific conductometric methods (to determine the concentration of hydrofluoric acid, sulfuric acid or another strong inorganic acid) and potentiometric (to determine the concentration of divalent iron and of trivalent iron) as well as the redox potential values of the aforementioned diluted sample and its temperature; restoration means, suitable for calculating, in response to the detected values, the quantities of correction elements (preferably hydrofluoric acid, sulfuric acid and an oxidizing agent) to be added to the pickling bath to restore the preset values of the aforementioned parameters and to control at least an actuator for introducing the aforesaid quantities of the correction elements into the pickling bath.

List of figures

The invention will now be better described with reference to a non-limiting embodiment illustrated in the attached figures, where:

- Figure 1 schematically shows a system comprising an analysis apparatus according to the invention;

- Figure 2 shows a simplified block diagram of an analysis equipment according to the invention;

- figure 3 schematically shows the analysis vessel CA of figure 2, comprising a conductometric type measuring system and a preferred embodiment of the washing means of the vessel itself and of the measuring electrode;

- figure 4 schematically shows the analysis vessel CA of figure 2, comprising a measuring system of the potentiometric type and a preferred embodiment of the washing means of the vessel itself and of the measuring electrodes.

In the attached figures, the corresponding elements will be identified by the same references.

Detailed description of the invention

Figure 1 schematically shows a plant, comprising an analysis apparatus according to the invention, consisting of:

- a plurality of pickling tanks V (V1,. Vn);

- an analysis apparatus A (which will be described with reference to the simplified block diagram of figure 2) which, in the example of embodiment described here, consists of a pair of analysis apparatus (A1, A2) operating simultaneously on parameters different; - a plurality of reservoirs S (S1, S2, S3), each of which contains a solution of a predetermined concentration of one of the correction elements [a strong mineral acid (preferably sulfuric acid), hydrofluoric acid and an oxidizing product consisting preferably, but not necessarily from hydrogen peroxide] to be introduced into the pickling bath contained in one of the tanks V;

- a plurality of permanent recirculation pipes, designed to ensure the representativeness of the solution to be analyzed, which neatly connect the tanks V to the sampling inlets l (Figure 2) of the analysis equipment A;

- a plurality of pipes for dosing the correction elements, which connect the tanks S to the tanks V;

- control means that allow the analysis equipment A to control the delivery of the correction elements contained in the tanks S.

In figure 1, for the sake of simplicity of graphic realization (electro) valves, pumps, actuators, filtering and washing means (known per se) activated at a predetermined rate and other (possible) circuit components not relevant for the purpose of the present description have been omitted.

The analysis apparatus A comprises (figure 2) means suitable for acquiring a sample of the pickling bath to be analyzed from a tank V, for analyzing it in order to determine the predetermined parameters (the concentration of the strong mineral acid, for example of sulfuric acid, hydrofluoric acid, divalent iron and trivalent iron) as well as the redox potential of the aforementioned diluted sample and its temperature, to calculate the quantities of the correction elements to be introduced from the tanks S into the tank V for correcting the predetermined parameters and controlling the actuators located at the outlet of the tanks S to introduce the calculated quantities of these correction elements into the pickling bath.

In the present description, the term "sulfuric acid" indicates any strong mineral acid.

Since the time for the analytical determination of the concentration of sulfuric acid and hydrofluoric acid is much shorter than that required to determine the concentration of iron ions (some minutes versus 30 minutes approximately), preferably the analytical equipment (A1, A2) they are separate devices, each of which is specialized in one of the above analyzes (determination of the concentrations of sulfuric acid and hydrofluoric acid, respectively determination of the concentration of iron ions).

The analysis equipment (A1, A2) can be managed by a higher level logic unit, not shown in the figures for simplicity of graphic representation, which can be provided "on site" or allocated in a remote location and connected to the analysis equipment (A1, A2) by bidirectional transmission means known per se.

Alternatively, the analysis equipment (A1, A2) can be identical to each other and comprising the analysis means suitable for determining the concentrations of both the acids (sulfuric acid and hydrofluoric acid) and the iron ions.

In this case, the plant according to the invention could also operate in the event of a malfunction of one of the analysis apparatuses (A1, A2).

Figure 2 shows a simplified block diagram of an analysis equipment A (A1, A2) of Figure 1, which includes in combination with each other:

- a sampling module C, whose sampling inputs I (11,. In) are neatly connected to the permanent recirculation pipes between the pickling tanks V (V1. Vn; Figure 1) and the analysis equipment A; inside the sampling module C there is at least one tank (not shown in the figure) in which the pickling bath sample to be analyzed is loaded;

- a DR reagent depot, containing at least the containers for the reagents needed for the analyzes;

- dosage means D (D1, D2) suitable for withdrawing predetermined quantities of the reagents necessary for the analysis from the aforementioned containers and for transferring them into the analysis vessel CA, where part of the dosage means (D) is suitable for withdrawing large quantities of reagent with a low precision (from about 2 to 5%), while the remainder are suitable for withdrawing small quantities of reagent with a high precision (about 0.1%); in figure 2 the low-precision and high-precision metering means D have been grouped into two distinct functional units (D1.D2);

- the analysis vessel CA, in which the measuring electrodes (indicated schematically in figure 2 with EM) used for the analyzes are allocated, which receives the bath sample to be analyzed from the sampling module C, from the dosing means D i reagents required for the analysis and from a tank W (not shown in the figure) the water (preferably having a conductivity lower than 100 microsiemens) necessary to dilute the above sample according to a predetermined dilution ratio; in figure 2 further elements (for example: agitators) possibly present in the analysis vessel CA but extraneous to the present invention have been omitted for simplicity of graphic representation.

- the UL logic unit, which controls and manages the analysis procedures, acquires and processes the information provided by the EM measuring electrodes and controls the actuators to introduce the solutions of the correction elements contained in the tanks S into the pickling bath (figure 1 ). In a preferred but non-limiting embodiment, the metering means D belonging to the functional unit D1 consist of peristaltic pumps with constant flow rate, while those belonging to the functional unit D2 consist of syringes made of antacid material (for example PES), driven by an electric stepping motor.

Still in a preferred embodiment, the analysis apparatus A further comprises means (which will be described with reference to Figures 3 and 4) which allow to wash the analysis vessel CA and the EM measuring electrodes with water after each measurement and with a chemical solution (preferably but not necessarily 10-20% hydrochloric acid) after a predetermined number of analyzes: this allows to keep the EM measuring electrodes in optimal conditions, to obtain reliable analytical results, to minimize the interventions of maintenance and greatly extend the life of the EM electrodes.

To guarantee a constant quality of the finished product, each type or family of materials to be pickled must be treated according to standard and characteristic working parameters (concentration of hydrofluoric and sulfuric acid, concentration of divalent and trivalent iron, ratio between trivalent iron ions and ions bivalent iron, concentration of hydrogen peroxide, temperature of the sample to be analyzed, etc.): in a preferred embodiment of the invention the parameters that characterize each process and those of operation of the analysis apparatus A which allow to carry out the different types of analyzes on the pickling baths associated with the specific process are grouped into operating procedures, bi-uniquely correlated with the material itself and stored in the UL logic unit, which are recalled from time to time according to the materials to be pickled.

Preferably but not necessarily, an operating procedure comprises at least the following information:

- the sequence and type of analysis to be carried out;

- the pre-set values of the pre-set parameters in the pickling bath;

- the extent of the allowable rejects with respect to the predetermined values, after which the logic unit UL commands the actuators to enter the solutions of the correction elements contained in the tanks S into the pickling bath;

- the dilution ratios with water in the CA vessel of the pickling bath sample to be analyzed.

It is advantageous to check the correct functioning of the analysis apparatus A cyclically and automatically: for this purpose, in a preferred embodiment of the invention, a further self-calibration operating procedure is stored in the logic unit UL and is activated after a predetermined number. of analysis and which includes the functional steps of taking from a container (preferably but not necessarily allocated in the reagent deposit DR) a predetermined quantity of a standard solution with a known composition, to transfer it to the analysis vessel CA, to analyze it, to compare the values resulting from the analysis with those expected and to activate any alarm signals if the differences between the values resulting from the analysis and those expected exceed predetermined values.

According to a possible embodiment of the invention, not shown in the figure, the UL logic unit can be connected to a central operator post and / or to a higher level logic unit, from which it can be controlled and managed; as previously mentioned, this higher level logical unit can be allocated "on site" or be remote.

In particular, at each change of processing the operator of the central station can modify the operating procedure performed by one or more logic units UL by activating the one relating to the processing in progress from time to time; the operator can also recall an operating procedure from one or more UL logic units, modify it and have it executed by the UL logic units and / or enter a new operating procedure by storing it in the UL logic units.

The analytical methodologies used for the analysis of the pickling baths are now described, in order to better understand the details described as part of the present invention,

a) Conductometric determination of sulfuric acid and hydrofluoric acid (or another strong acid compared to hydrofluoric acid).

This determination is based on the principle that, in an aqueous solution consisting of a mixture of a weak acid such as hydrofluoric acid and an acid stronger than it (such as sulfuric acid), the conductivity of the solution is practically equivalent to that deriving from strong acid alone at the same concentration; the method also exploits (in a stage subsequent to a first conductivity reading made on a suitably diluted pickling bath sample to determine the concentration of sulfuric acid) the high affinity of hydrofluoric acid for a metal cation dosed in the solution below form of salt. The anion of the salt should preferably derive from a strong acid (for example, nitric or hydrochloric acid) in such a way that the reaction of formation of fluoro complexes between the metal cation and the hydrofluoric acid generates a significant increase in conductivity due to the release of an equivalent quantity of completely dissociated strong acid, detected through a second conductometric measurement.

For example:

n HF Fe (N03) 3 → FeFn <(3'n) *> + n HN03

This increase in conductivity is therefore proportional to the concentration of hydrofluoric acid which, after appropriate calibration, can be quantitatively determined.

Examples of such salts are ferric nitrate, ferric chloride, aluminum nitrate and aluminum chloride; in the preferred embodiment of the invention a solution of ferric nitrate nonohydrate at a concentration of 750 g / l is used.

To ensure a sufficiently linear response of the conductivity to the variation of the acid concentration, the dilution of the sample must be carefully evaluated according to the concentrations of the acids present in the bath to be analyzed; by way of non-limiting example, dilution ratios from 1: 100 to 5: 100 and preferably 4: 100 are suitable for concentrations of sulfuric acid lower than 200 g / l and hydrofluoric acid lower than 60 g / l.

Another fundamental variable to obtain reliable results and which must be managed by the UL logic unit of the analysis equipment A is the temperature of the sample after dilution with water; in fact, in an industrial environment the water temperature can have significant fluctuations (generally from 5 ° C to 40 ° C) depending on the season, the origin of the water and the storage time in the tank W. It is evident that a determination conductometric is strongly influenced by temperature and normally this problem is overcome through the automatic compensation system incorporated in the measuring instrument; in the case in question, the automatic compensation is able to correctly compensate only the effect on the first conductometric reading (determination of the concentration of sulfuric acid), but not on the second (determination of the concentration of hydrofluoric acid) carried out after the addition of ferric nitrate since, having changed the composition of the solution, the dependence of the latter on the temperature is different from that which occurs before the addition of the ferric nitrate.

This critical problem is solved by an analysis equipment A made according to the invention, whose logic unit UL takes into account the variation in conductivity due to the addition of the volume v3 of the ferric nitrate solution as a function of the temperature of the sample. The amount of ferric nitrate during the titration must be such as to ensure the total complexation of the hydrofluoric acid; in the system under examination, for hydrofluoric acid concentrations lower than 60 g / l the ratio between the volume v1 of the bath sample taken and the volume v3 of a 750 g / l solution of ferric nitrate nonohydrate must be greater than 0.5 and preferably equal to 1.

By way of non-limiting example, the operating procedure and the relative calculations are shown for a relative dilution of the sample of 4: 100 by volume:

- filling of the analysis vessel CA, by means of the dosing means D2, with a predetermined volume v2 of water with conductivity lower than 100 microsiemens to obtain a dilution ratio 4: 100;

- taking from the sampling module C (by means of the dosing means D2) of a predetermined volume vt of the pickling bath sample to be analyzed;

- start of agitation of the solution;

- first conductometric reading (L,);

- addition of a predetermined volume v3 = v, of a solution at 750 g / l of ferric nitrate nonohydrate;

- agitation of the solution and detection of the temperature T;

- second conductometric reading (L2).

The logic unit UL acquires the data Lu L2, T and automatically determines the concentrations of acids through the following calculations:

- concentration of sulfuric acid

- concentration of hydrofluoric acid

where: a, b, c, alP b1t c, = coefficients of the quadratic equations; δ = L2 -

= constants dependent on the amount of ferric nitrate added to the diluted sample before the second conductometric reading.

In the example in question:

a = 0.0066; b = 5.015; c = 6.98;

a, = 0.0120; b, = 2.881; c ^ .3.81;

3⁄4 = 9.632; .C3 = 0.297.

Figure 3 shows the characteristics of the CC conductivity cell, whose particular shape allows to minimize the negative effects due to the high viscosity of the solution, as well as to facilitate the washing of the platinized reading plates.

It consists of a hollow body B, made of glass and of a substantially cylindrical shape, inside which the pair of blackened platinum plates EL is allocated; in the lower and upper part of the hollow body B there are two holes (F1, F2) to circulate the sample to be analyzed inside the hollow body B.

Preferably, the hollow body B has a diameter of about 20 mm (and in any case between about 17 and 23 mm) and a height of about 40 mm (and in any case between about 35 and 45 mm); the dimensions of the EL plates are approximately 10 x 5 mm (and in any case between approximately 8 x 12 mm and approximately 3 x 7 mm) and their distance is approximately 15 mm (and in any case between approximately 12 and 18 mm).

To avoid polarization of the EL electrodes, the electrical measuring circuit connected to the DC conductivity cell (not shown in the figure for simplicity of graphic representation) must work at high frequency (from 25 to 40 kHz).

b) Determination of divalent iron

The determination of divalent iron is performed through potentiometric analysis by titration with potassium permanganate according to the classical method.

The operational sequence includes:

- filling of! analysis vessel CA with a volume v2 of water, predetermined through the overflow pipe TP and such as to obtain a dilution ratio> 1: 50;

- taking from the sampling module C (by means of the dosing means D2) of a predetermined volume v, of the pickling bath sample to be analyzed and its introduction into the analysis vessel CA;

- acidification of the sample of the diluted pickling bath by introducing into the analysis vessel CA (by means of dosing means D1) a known but not critical volume of a solution of a strong acid, for example of a solution of sulfuric acid 1: 1 by weight ;

- potentiometric titration with fixed end point or with automatic search of the end point with a normal 0.1 potassium permanganate solution added to the analysis vessel CA by means of dosing means D2;

- empty and wash the CA analysis vessel with water.

c) Determination of trivalent iron

The determination of trivalent iron exploits the iodometric titration technique, but employs some specific measures so that it can be applied through automatic equipment with reliable and reproducible results.

It provides for the following operational sequence:

- filling of the analysis vessel CA with a volume v2 of water, predetermined through the overflow pipe TP and such as to obtain a dilution ratio> 1: 50;

- taking from the sampling module C (by means of the dosing means D2) of a predetermined volume v, of the pickling bath sample to be analyzed and its introduction into the analysis vessel CA;

- start of agitation of the solution;

- addition in the analysis vessel CA (by means of the dosage means D1) of a predetermined but not critical volume of a solution of lanthanum nitrate at a known concentration;

- wait for 30 seconds in quiet conditions;

- addition in the analysis vessel CA (by means of the dosing means D1) of a predetermined but not critical volume of a hydrochloric acid solution 1: 1 by volume;

- addition in the analysis vessel CA (by means of the dosage means D1) of a predetermined but not critical volume of a solution of potassium iodide, for example with a concentration of 1 kg / l;

- wait for 5 minutes in quiet conditions;

- start of stirring of the solution;

- titration by potentiometric way with normal sodium thiosulfate 0.1 (added by means of dosage D2) of the iodine released by the reaction of trivalent iron with potassium iodide;

- emptying and washing the CA analysis vessel with water.

An important aspect for the automatic application of this analysis is the use of lanthanum nitrate; the addition of a salt containing a cation capable of complexing the fluoride ion bound to the iron ion is essential to make the determination of the ferric ion itself quantitative by iodometric analysis.

By operating manually, this result can be achieved using a calcium chloride solution, but it has been verified that the titration of trivalent iron using calcium chloride cannot be managed by automatic equipment, as over time the precipitation of calcium fluoride and calcium sulphate which as a result, it continuously dirties the electrodes contained in the analysis vessel CA causing significant errors and heavy maintenance interventions; it has been found that the use of lanthanum salts is able to quantitatively release the ferric ion by generating lanthanum fluoride precipitates with a powdery and non-adherent consistency, thus allowing the process to be managed automatically with high reliability and negligible maintenance.

Alternatively, the same result can be obtained by adding to the system a complexing agent for the iron ion which then makes it quantitatively available for the subsequent reaction with potassium iodide; it has been verified that the use of complexing agents such as EDTA may be suitable.

The potentiometric system, schematically illustrated in figure 4, comprises a measuring electrode E (in inert material in the working environment) immersed in the analysis vessel CA and a reference electrode R (preferably in glass of the Ag / AgCI type) placed outside the analysis vessel CA and placed in contact with the solution subjected to measurement by means of a saline bridge, consisting of an electrolyte (contained in an SR tank) which flows continuously through a porous septum SP placed at the end of a tube T in plastic material.

The continuous flow of the electrolyte through the porous septum SP has the purpose of allowing electrical contact, to prevent the porous septum SP from coming into direct contact with the hydrofluoric acid contained in the pickling bath and to regenerate the electrolyte that is renewed continuously. In one of its preferred embodiments, the measuring electrode E consists of a body made of antacid material which carries at the end a platinum plate P whose surface, mirror polished, faces downwards: this arrangement prevents salts deriving from the reaction products to deposit on the plate P of the measuring electrode E and to soil it.

Advantageously, the electrolyte, generally consisting of 3 molar potassium chloride, is added with 10% glycerin (or with another product having a viscosity at 20 ° C between 1.15 and 1.45 centipoise, inert to the working system and functionally equivalent) to increase its viscosity and reduce the flow rate, thus increasing the autonomy of the potentiometric system for the same volume of the SR tank.

d) Determination of hydrogen peroxide.

The determination - of the free hydrogen peroxide - in the nitric acid-free pickling processes such as those described is necessary in the case of the control of the finishing / passivation baths generally used as the last stage before the final rinsing in the treatment of ferritic and martensitic steels: normally these baths consist of sulfuric acid (20-60 g / l), oxygenated water (3-10 g / l) and possibly hydrofluoric acid.

The analytical methodology and the operational sequence used for the determination of water. oxygenated are the same used for the determination of divalent iron in pickling baths,

e) Determination of the redox potential.

The apparatus in question, before carrying out the determination of the divalent iron, determines on the diluted sample of the pickling bath the value of the redox potential of the solution by means of a measurement with the previously described potentiometric system; the value thus determined is very close (± 20 mV) to the value of the redox potential measured in the bath before dilution of the bath itself. The determined value is compared with a "range" of values (normally from 200 to 550 mV) set as desired in the UL logic unit and used as a first index of the correct functioning of the system: if the measured value "goes out" from the aforementioned " range "the UL logic unit of analysis equipment A blocks the analysis procedure and generates an alarm signal. Furthermore, the determination of the redox potential is carried out with a predetermined frequency (usually once a week) on a standard solution with a known potential (normally 468 mV) for the calibration of the potentiometric system.

As previously mentioned, the logic unit UL of an analysis apparatus A made according to the invention (after having determined the predetermined parameters in the sample of the pickling bath analyzed) calculates the quantity of each of the solutions with a predetermined concentration of the correction elements (sulfuric acid, hydrofluoric acid and oxidizing product) contained in the tanks S to be added to the pickling bath to restore the predetermined values of the predetermined parameters and controls the actuators (consisting for example of dosing pumps or solenoid valves) placed at the outlet of the tanks S to introduce the calculated quantities of the solutions of the correction elements into the pickling bath.

Since the characteristics of the system are known (volume of the tank V, flow rate of each actuator, predetermined values of the predetermined elements, concentration of the correction elements, etc.), to calculate the quantities of the solutions of the correction elements to be added to the pickling bath it is it is sufficient for the logic unit UL to determine how long to activate the corresponding actuators.

Studies and verifications carried out by the Applicant have shown that, to bring the concentrations in the pickling bath of sulfuric acid, hydrofluoric acid, trivalent iron ion and oxidizing product back to the predetermined values, the logic unit UL must activate each of the actuators which regulate the introduction into the pickling bath of the solutions of sulfuric acid, hydrofluoric acid and the oxidizing product for a time s (in seconds) expressed by the following formula:

where is it:

s = actuator activation time (sec);

K = product factor inversely proportional to the concentration of the correction element (l / g);

v0 = predetermined concentration of the predetermined element (g / l) vm = concentration of the predetermined element resulting from the analysis (g / l); vb = volume of the tank V

p = actuator flow rate (l / sec)

To restore the ratio R between the concentration of the trivalent iron ion and that of the divalent iron ion to the predetermined value, the logic unit UL determines the activation time s1 (in seconds) of the actuator that regulates the introduction into the bath as follows pickling of the oxidizing product solution:

1) calculate: B, = A * R

where A is the concentration of the divalent iron ion (g / l) resulting from the permanganometric analysis, R is the predetermined ratio between the concentration of the trivalent iron ion and that of the divalent iron ion and B, is the theoretical concentration of the trivalent iron ion, 2) compare BT with B (concentration of the trivalent iron ion resulting from the iodometric analysis; g / l);

3) if B ≥ BT (the measured trivalent iron ion concentration is greater than the theoretical one) the UL logic unit does not perform any intervention;

4) if B <B, (the concentration of the trivalent iron ion is lower than the theoretical one), the logic unit UL determines the activation time s1 (in seconds) of the actuator that regulates the introduction of the solution into the pickling bath of the oxidizing product using the following formula:

where is it :

s1 = actuator activation time (sec)

K = product factor, inversely proportional to the concentration of the correction element (l / g);

K, = factor proportional to the volume of the tank V (I);

C = (BrB) / R = quantity of divalent iron ion to be oxidized to restore the predetermined iron value (g / l);

p = actuator flow rate (l / sec)

Figure 3 schematically shows an exploded view of the analysis vessel CA of figure 2, comprising a conductometric type measuring system and a preferred embodiment of the washing means of the analysis vessel CA and the measuring cell CC.

Figure 3 shows:

- the CC conductivity measuring cell, used for conductometric measurement;

- the CA analysis vessel;

- the overflow drain TP, mobile, whose position (controlled by the logic unit UL) allows to determine the level of the liquid in the analysis vessel CA, respectively to empty the vessel itself;

- the washing means (F, U), controlled by the UL logic unit, which allow washing the CA analysis vessel and the CC conductivity measuring cell.

Figure 4 schematically shows an exploded view of the analysis vessel CA of figure 2, comprising a measuring system of the potentiometric type and a preferred embodiment, similar to that of figure 3, of the washing means of the analysis vessel CA and of the measuring electrodes.

Figure 4 shows:

- the potentiometric system, which includes the measuring electrode E, the reference electrode R placed outside the analysis vessel CA and the salt bridge, consisting of an electrolyte contained in the SR tank that flows continuously through the porous septum SP placed at the end of the plastic tube T;

- the CA analysis vessel;

- the overflow drain TP, mobile, whose position (controlled by the logic unit UL) allows you to determine the level of the liquid in the analysis vessel CA, respectively to empty the vessel itself;

- the washing means (F, U), controlled by the UL logic unit, which allow you to wash the analysis vessel CA, the end of the electrode E and the porous septum SP.

In the preferred embodiment illustrated in Figures 3 and 4, these washing means comprise a plurality of slots F arranged along the upper edge of the analysis vessel CA and a nozzle U adapted to spray the end of the the measuring electrode E and the porous septum SP, respectively the conductivity measuring cell CC; Figures 3 and 4 also show the lid CP of the analysis vessel CA and means MS designed to support the electrode E and the tube T of the potentiometric system, respectively the measuring cell of the conductivity CC, and the tubes (not explicitly indicated in figure 3 and / or 4) which connect the metering means D (D1, D2) to the analysis vessel CA; the lid CP and the support means MS will not be described because they are known per se and in any case unrelated to the present invention.

Preferably the analysis vessel CA, the measuring electrode E and the porous septum SP (respectively the analysis vessel CA and the conductivity measuring cell CC) are washed with water after each analysis, while they are washed with a solution chemistry after a predetermined number of analyzes.

To wash the above elements with water after each analysis, the logic unit UL carries out the following functional steps in an orderly fashion:

- completely empty the CA analysis vessel;

- let an abundant flow of water flow through the slits F;

- fill the analysis vessel CA with water up to immerse the end of the electrode E and the porous septum SP, respectively the CC conductivity measuring cell;

- empty the CA analysis vessel;

- further clean the end of the electrode E and the porous septum SP, respectively the CC conductivity measuring cell with a jet of water sprayed directly through the nozzle U;

- empty the water from the CA analysis vessel and prepare it for the next analysis.

To wash the analysis vessel CA, the end of the electrode E and the porous septum SP (respectively the analysis vessel CA and the conductivity measuring cell CC), the logic unit UL fills the analysis vessel CA with water through the slits F until the end of the electrode E and the porous septum SP are immersed, respectively the measuring cell of the conductivity CC, takes from a tank (preferably but not necessarily placed in the reagent deposit DR) the quantity of product (preferably hydrochloric acid) necessary to make the aforementioned washing chemical solution and pours it into the analysis vessel CA; after a predetermined time, the logic unit (JL empties the CA analysis vessel and removes all traces of the chemical solution by washing with water.

Furthermore, in rest conditions the analysis vessel CA is completely filled with water through the slits F and the nozzle U to prevent the end of the electrode E and the porous septum SP, respectively the measuring cell of the conductivity CC, from being able to be soiled and / or damaged.

Without departing from the scope of the invention, it is possible for a technician to make all the modifications and improvements suggested by normal experience and the natural evolution of the technique to the control equipment for the pickling bath object of this description.

Claims (48)

CLAIMS 1) Control apparatus for nitric acid-free pickling baths, characterized in that it comprises means suitable for acquiring a sample of the bath to be analyzed; means for analyzing the aforesaid sample to determine a plurality of parameters therein according to specific conductometric and potentiometric methods as well as the redox potential values of the aforesaid diluted sample and its temperature; reset means able to calculate, in response to the detected values, the quantities of correction elements to be added to the pickling bath to restore the predetermined values of the aforementioned parameters and to control at least one actuator to introduce the aforementioned quantities of the elements into the pickling bath correction. 2) Equipment as in claim 1, characterized in that the aforementioned parameters are the concentration of sulfuric acid, hydrofluoric acid and divalent and trivalent iron ions. 3) Apparatus as in claim 1, characterized in that the restoring means introduce calculated quantities of solutions of predetermined concentration of the correction elements into the pickling bath. 4) Equipment as in claim 3, characterized in that the correction elements are sulfuric acid, hydrofluoric acid and an oxidizing product. 5) Apparatus as in claim 4, characterized in that the oxidizing product consists of hydrogen peroxide. 6) Equipment as in claim 1, characterized in that it comprises at least one analysis equipment (A). 7) Apparatus as in claim 6, characterized in that it comprises two analysis apparatuses (A1, A2) operating simultaneously on different parameters. 8) Apparatus as per claims 2, 4 and 7, characterized in that one of the analysis apparatuses (A1, A2 respectively) determines the concentrations in the pickling bath of sulfuric acid and hydrofluoric acid and introduces sulfuric acid and hydrofluoric acid to restore the predetermined concentration values and from the fact that the other analysis equipment (A2, respectively A1) determines the concentrations in the pickling bath of the iron ions and introduces the oxidizing product into it to restore the concentration of the trivalent iron ion and / or the ratio of trivalent iron ion to divalent iron ion. 9) Apparatus as per claim 6, characterized in that the analysis apparatus (A) comprises, in combination with each other: - a sampling module (C), comprising sampling inputs (I) neatly connected to the pickling tanks ( V) to load the pickling bath sample to be analyzed into at least one tank placed inside the sampling module (C); - a reagent deposit (DR), comprising at least the containers for the reagents used to analyze the pickling bath sample; - dosing means (D), suitable for withdrawing predetermined quantities of reagents from containers allocated in the reagent deposit (DR) and transferring them into an analysis vessel (CA); - the analysis vessel (CA), in which the measuring electrodes (EM) used for the analysis of the pickling bath sample are allocated and which receives the pickling bath sample from the sampling module (C) - from analyzing and from the assay media (D) the reagents required for the analysis; - a logic unit (UL), able to control and manage the analysis procedures, to acquire and process the information provided by the measuring electrodes (EM) and to drive the at least one actuator to introduce the elements into the pickling bath correction. 10) Apparatus as in claim 9, characterized in that part of the metering means (D) is suitable for withdrawing large quantities of reagent with low precision (from about 2 to 5%) and that the remaining metering means (D) they are suitable for withdrawing small quantities of reagent with high precision (about 0.1%). 11) Apparatus as in claim 10, characterized in that the low-precision and high-precision metering means (D) are grouped into two distinct functional units (D1, D2). 12) Apparatus as in claim 9, characterized in that it further comprises means for supplying the analysis apparatus (A) with the water for washing the analysis vessel (CA) and the measuring electrodes (EM) and for dilute the pickling bath sample contained in the analysis vessel (CA) according to a predetermined dilution ratio. 13) Equipment as per claim 12, characterized by the fact that the washing and dilution water has a conductivity of less than 100 microsiemens. 14) Equipment as in claim 9, characterized by the fact that each logic unit (UL) is connected to a central operator post and / or to a higher level logic unit, from which it is controlled and managed. 15) Apparatus as in claim 1, characterized in that the means for carrying out conductometric measurements comprise a conductivity measuring cell (CC) having at one end a hollow glass body (B), of substantially cylindrical shape, inside which a pair of blackened platinum plates (EL) is allocated, in the lower and upper part of the hollow body (B) there are holes (F1, F2) to circulate the bath sample inside the hollow body (B) of pickling contained in the analysis vessel (CA). 16) Apparatus as per claim 15, characterized by the fact that the hollow body (B) has a diameter between about 17 and 23 mm and a height between about 35 and 45 mm, by the fact that the dimensions of the plates (EL) they are comprised between about 8 x 12 mm and about 3 x 7 mm and by the fact that their distance is between about 12 and 18 mm. 17) Apparatus as in claim 16, characterized by the fact that the hollow body (B) has a diameter of about 20 mm and a height of about 40 mm, by the fact that the dimensions of the plates (EL) are 10 x 5 mm and from the fact that their distance of about 15 mm. 18) Apparatus as in claim 1, characterized in that the means for carrying out potentiometric measurements comprise a measurement electrode (E) immersed in the analysis vessel (CA) and a reference electrode (R) placed outside the vessel d analysis (CA) and connected to the solution subjected to measurement by means of a saline bridge consisting of an electrolyte that flows continuously through a porous septum (SP) placed at the end of a plastic tube (T). 19) Equipment as per claim 18, characterized in that the electrolyte is added with a product having a viscosity at 20 ° C between 1.15 and 1.45 centipoise. 20) Equipment as per claim 19, characterized in that the electrolyte is added with 10% glycerin. 21) Apparatus as per claim 18, characterized by the fact that the measuring electrode (E) is made up of a body in antacid material which has at the end a platinum plate (P) whose surface, mirror polished, faces downward. 22) Apparatus as per claims 9, 15 and 18, characterized in that the analysis apparatus (A) further comprises means for chemical washing and for washing with water of the analysis vessel (CA), of the electrode (E) and of the porous septum (SP) belonging to the saline bridge, respectively of the analysis vessel (CA) and of the conductivity measuring cell (CC), said washing means comprising at least slots (F) arranged along the upper edge of the analysis vessel (CA) and a nozzle (U) designed to spray the end of the measuring electrode (E) <1> and the porous septum (SP), respectively the cell, with a jet of water conductivity measurement (CC). 23) Method for controlling nitric acid-free pickling baths, characterized by the fact of including at least the operations of: a) determining the concentration in the acid pickling bath; b) determining the concentration of the divalent iron ion in the pickling bath; c) determining the concentration of the trivalent iron ion in the pickling bath; d) determine the redox potential of the pickling bath. 24) Method as in claim 23, characterized by the fact that it also includes the operation of determining the concentration of free hydrogen peroxide in the finishing / passivation baths used as the last stage before the final rinse! treatment of ferritic and martensitic steels. 25) Method as in claim 23, characterized in that the determination of the concentration in the acid pickling bath comprises at least the following operations: - filling the analysis vessel, by means of high precision dosing means, with a predetermined volume of water with a conductivity lower than 100 microsiemens to obtain a predetermined dilution ratio; - taking from a sampling module, by means of the high precision dosing means, a predetermined volume of the pickling bath sample to be analyzed and introducing it into the analysis vessel; - shake the solution; - make a first conductometric reading (L1); - add a predetermined volume of a solution of ferric nitrate nonohydrate to the analysis vessel; - shake the solution and measure its temperature (T); - carry out a second conductometric reading (L2); - empty the analysis vessel. 26) Method as in claim 25, characterized in that a volume of a 750 g / l solution of ferric nitrate nonohydrate equal to that of the pickling bath sample to be analyzed is added to the analysis vessel. 27) Method as in claim 25, characterized in that the concentration (as) in the pickling bath of sulfuric acid is determined by the following relationship: where a, b and c = coefficients of the quadratic equation. 28) Method as in claim 25, characterized in that the concentration (af) in the hydrofluoric acid pickling bath is determined by the following relationship: where a „b, and c, = coefficients of the quadratic equation; = constants depending on the quantity of ferric nitrate nonohydrate added to the analysis vessel. 29) Method as in claim 23, characterized in that the determination of the concentration of the divalent iron ion in the pickling bath is carried out by permanganometry. 30) Method as in claim 29, characterized in that the determination of the concentration of the divalent iron ion in the pickling bath comprises at least the following operations: - fill the analysis vessel with a predetermined volume of water, to obtain a predetermined dilution ratio; - taking from the sampling module, by means of the high precision dosing means, a predetermined volume of the pickling bath sample to be analyzed and introducing it into the analysis vessel; - acidifying the sample of the diluted pickling bath by introducing into the analysis vessel, by means of low precision dosing means, a known but not critical volume of a solution of a known concentration of a strong acid; - carry out a potentiometric titration with a fixed end point or with automatic search of the end point with a potassium permanganate solution of known concentration added to the analysis vessel by means of high precision dosing means; - empty the analysis vessel. 31) Method as in claim 23, characterized in that the determination of the concentration of the trivalent iron ion in the pickling bath is carried out by iodometric titration. 32) Method as in claim 31, characterized in that the determination of the concentration of the trivalent iron ion in the pickling bath comprises at least the following operations: - fill the analysis vessel with a predetermined volume of water, to obtain a predetermined dilution ratio; - taking from the sampling module, by means of the high precision dosing means, a predetermined volume of the pickling bath sample to be analyzed and introducing it into the analysis vessel; - shake the solution; - add in the analysis vessel, by means of low-precision dosing means, a predetermined but non-critical volume of a solution of a known concentration of a salt of an element which, reacting with sulfuric and hydrofluoric acids, forms soluble or easily precipitated salts removable; - wait for a predetermined time in quiet conditions; - adding a predetermined but non-critical volume of a hydrochloric acid solution of known concentration to the analysis vessel by means of the low-precision dosing means; - adding a predetermined but non-critical volume of a potassium iodide solution of known concentration to the analysis vessel by means of the low-precision dosing means; - wait for a predetermined time in quiet conditions; - shake the solution; - carry out a titration by potentiometric way with sodium thiosulfate, added by means of high precision dosing means, of the iodine released by the reaction of trivalent iron with potassium iodide; - empty the analysis vessel. 33) Method as in claim 32, characterized in that the salt of an element which, reacting with sulfuric and hydrofluoric acids, forms soluble salts or easily removable precipitates is lanthanum nitrate. 34) Method as claimed in claim 30 or 32, characterized in that the volume of water in the analysis vessel is prefixed through an overflow pipe belonging to the analysis vessel. 35) Method as in claim 23, characterized by the fact that the determination of the redox potential of the pickling bath is carried out before determining the concentration of the divalent iron, by the fact that the value of the redox potential thus determined is compared with a "range" of preset values and by the fact that, if the determined value does not belong to the aforementioned "range", the analysis procedure is interrupted and an alarm signal is generated. 36) Method as per claim 24, characterized in that the determination of free hydrogen peroxide includes at least the following operations: - fill the analysis vessel with a predetermined volume of water, such as to obtain a predetermined dilution ratio; - taking from the sampling module, by means of the high precision dosing means, a predetermined volume of the pickling bath sample to be analyzed and introducing it into the analysis vessel; - acidifying the sample of the diluted pickling bath by introducing into the analysis vessel, by means of the low precision dosing means, a known but not critical volume of a solution of a known concentration of a strong acid; - carry out a potentiometric titration with a fixed end point or with automatic search of the end point with a potassium permanganate solution of known concentration added to the analysis vessel by means of high precision dosing means; - empty the analysis vessel. 37) Method as in claim 23, characterized in that it further comprises a washing operation with water, carried out after each analysis, of the analysis vessel and of the means for carrying out potentiometric measurements, respectively of the analysis vessel and of the conductivity measurement and a chemical washing operation, carried out after a predetermined number of analyzes, of the analysis vessel and the means for carrying out potentiometric measurements, respectively of the analysis vessel and of the conductivity measuring cell. 38) Method as in claim 37, characterized in that the washing operation with water includes at least the following operations: - completely empty the analysis vessel; - let an abundant flow of water flow through slits along the upper edge of the analysis vessel; - fill the analysis vessel with water until the end of the means to carry out potentiometric measurements, respectively the conductivity measuring cell, is immersed in it; - empty the analysis vessel; - spray the end of the means for carrying out potentiometric measurements, respectively the conductivity measuring cell, with a jet of water coming out of a nozzle placed in the analysis vessel; - empty the analysis vessel from the water and prepare it for the subsequent analysis. 39) Method according to claims 37 and 38, characterized in that the chemical washing operation comprises at least the following operations: - fill the analysis vessel with water through slits arranged along the upper edge of the analysis vessel until the end of the means for carrying out potentiometric measurements, respectively the conductivity measuring cell, is immersed; - take the amount of product needed to make the chemical washing solution from a tank and put it in the analysis vessel; - after a set time, empty the analysis vessel and remove all traces of the chemical solution by washing with water. 40) Method as in claim 39, characterized in that the chemical washing is carried out by means of hydrochloric acid at 10-20%. 41) Method as in claim 39, characterized in that the quantity of product necessary to make the chemical washing solution is taken from a tank placed in the reagent deposit. 42) Method as in claim 23, characterized by the fact that, in rest conditions, the analysis vessel is completely filled with water through slits arranged along the upper edge of the analysis vessel and a nozzle placed in the analysis vessel. 43) Method as in claim 23, characterized by the fact that the concentrations in the pickling bath of sulfuric acid, hydrofluoric acid, trivalent iron ion and oxidizing product are brought back to the predetermined values by activating each of the actuators that regulate the input in the pickling bath of the corresponding correction element - for a time (s) expressed by the following formula: where is it s = activation time of the actuator; Κ = product factor, inversely proportional to the concentration of the correction element; v0 = predetermined concentration of the predetermined element; vm = concentration of the predetermined element resulting from the analysis; vb = volume of the tank; p = actuator capacity. 44) Method as in claim 23, characterized by the fact that the ratio R between the concentration of the trivalent iron ion in the pickling bath and that of the divalent iron ion is brought back to the predetermined value by carrying out the following sequence of operations: - calculate B, = A * R, where A is the concentration of the divalent iron ion resulting from the permanganometric analysis, R is the predetermined ratio between the concentration of the trivalent iron ion and that of the divalent iron ion and B, is the theoretical concentration of the trivalent iron ion; - compare B with B, where B is the concentration of the trivalent iron ion resulting from the iodometric analysis; - if B ≥ BL do not activate the actuator that regulates the introduction of an oxidizing product into the pickling bath; - if B,> B, activate the actuator that regulates the entry into the pickling bath of an oxidizing product for a time (s,) expressed by the following formula: where is it ST = actuator activation time; K = product factor, inversely proportional to the concentration of the correction element; K., = factor proportional to the volume of the tank; C = (BrB) / R = quantity of divalent iron ion to be oxidized to restore the predetermined R ratio; p = actuator capacity. 45) Method as in claim 23, characterized by the fact that the logic unit controls the pickling bath by executing one of the operating procedures loaded into its memory and comprising a plurality of parameters characterizing a specific process and the operating parameters of the analysis to analyze the pickling bath associated with the aforementioned specific process. 46) Method as in claim 45, characterized by the fact that each operating procedure comprises at least the following information: - the sequence and type of analysis to be carried out; - the pre-set values of the pre-set parameters in the pickling bath; - the extent of the allowable deviations with respect to the aforementioned predetermined values, after which the logic unit drives the actuators to enter the correction elements in the pickling bath; - the dilution ratios with water of the pickling bath sample to be analyzed in the analysis vessel. 47) Method as in claim 45, characterized by the fact that the logic unit also performs an operating procedure of self-calibration, activated after a predetermined number of analyzes, which includes the operations of: - withdrawing from a container a predetermined quantity of a solution of known composition; - transfer the solution of known composition into the analysis vessel and analyze it; - compare the values resulting from the analysis with the expected ones; - activate alarm signals if the differences between the values resulting from the analysis and those expected exceed predetermined values. 48) Method as in claim 47, characterized in that the solution of known composition is taken from a container located in a reagent deposit.