PROCESS FOR STEEL BAND SPRAY PICKLING AND APPARATUS FOR CARRYING OUT SAID PROCESS

Abstract: The following description sets forth a process for steel band spray pickling and an apparatus suitable for carrying out said pickling process, wherein the steel band (7) to be pickled slides between two sets of pipes (6, 8) (each bearing a series of nozzles spraying the pickling solution) positioned crosswise in respect of the band feed and facing the opposite sides of the steel band (7) to be pickled.

Description: Process for steel band spray pickling and apparatus for carrying out said process Field of the invention The present invention relates to a steel band spray pickling process and to an apparatus suitable for carrying out said pickling process, wherein the steel band to be pickled slides between two sets of pipes (each bearing a series of nozzles spraying the pickling solution) positioned crosswise in respect of the band feed and facing the opposite sides of the steel band to the pickled. Prior art During the manufacturing process, steel products (e.g. metal sheets and/or bands), are rolled, in particular hot-rolled, and are inevitably subject to the formation of oxide layers, which must be removed before cold-rolling. Said surface oxide layers must be removed also before applying one or more protective coats to the metal. The surface layers of oxides are generally removed through a chemical process, called "pickling", usually providing for the immersion of the metal body to be treated (sheet or band) into an acid bath generally consisting of mineral inorganic acids (sulphuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid), either alone or as mixtures thereof in various ratios, suitably diluted and heated, followed by a rinsing step in running water. Stainless steel pickling is almost exclusively based on the use of a nitric- hydrofluoric acid mixture. Said pickling processes lead to good results and would be per se inexpensive, should they not involve extremely serious problems for the environment and for the health of the persons in charge of the plant, brought about by the formation of highly polluting and aggressive nitrogen oxide vapours and by the presence of nitric acid and/or nitrates in wastewater. Therefore, the use of pickling baths containing nitric acid requires purification and flue gases abatement plants, which involve high implementation and operating costs. To obviate said serious drawbacks, "ecological" pickling baths, i.e. free from nitric acid, have been studied and are successfully replacing the traditional pickling baths based on the use of a nitric-hydrofluoric acid mixture. Said "ecological" baths are described, e.g. in Italian Patents No. 1 ,245,594, filed on March 29, 1991 , and No. 1 ,255,655, filed on August 6, 1992. Pickling processes "by immersion", wherein the metal body to be treated is immersed into a pickling bath, require a non-negligible bath volume. Said baths may operate under static conditions or with a vigorously agitated pickling solution (obtained by recycle pumps and/or air flow) securing an effective change of the pickling solution on the body surface to be treated and, consequently, a high efficiency of the pickling bath resulting in increased pickling rate and plant production. Processes and apparatus for spray pickling (wherein the metal body to be treated is sprayed with a pickling solution without being immersed into said solution) are known and used to treat sheets or the like. Conversely, processes and apparatus for continuous steel band spray pickling have never been developed. It is an object of the present invention to provide a "spray" type process and apparatus for steel band pickling, which at least partially overcome drawbacks and/or limitations of "immersion" pickling processes and apparatus known in the art and used for continuous steel band pickling. Summary The present invention provides for a steel band pickling process consisting in the application "by spraying" of the pickling liquid to a steel band as well as a pickling apparatus suitable for carrying out said process, wherein the steel band to be pickled slides between two sets of pipes positioned crosswise in respect of the band feed and facing the steel band opposite sides, each pipe bearing a series of nozzles spraying the pickling solution on one side of the steel band. Brief description of the drawings Said apparatus is shown in detail in the attached figures, intended as illustrative of a particular embodiment of the present invention, but not limiting: Figure 1 is a schematic, simplified, cross-sectional view of a pickling tank belonging to a pickling apparatus of the prior art; Figure 2 is a schematic, simplified, cross-sectional view of a pickling tank belonging to a pickling apparatus according to the invention; Figure 3 is a schematic, longitudinal-sectional view of a pickling tank belonging to a pickling apparatus according to the invention; Figure 4 is a schematic, simplified, cross-sectional view of the pickling tank of Figure 2, showing the support elements applied to the tank edges. Description of the invention The liquid used for steel band spray pickling according to the present invention consists of mineral acids solutions, such as HNO 3 , HF, H 2 S0 4 , either alone or as mixture thereof. Preferably, the pickling liquid is a mineral acids solution consisting of sulphuric acid and hydrofluoric acid, with pH <2.5, and containing Fe 3+ and/or Fe 2+ ions, wherein Fe 3+ ions + Fe 2+ ions are no more than 100 g/l, the Fe 2+ ions concentration may range from 0 to 100 g/l and the of Fe 3+ ions concentration is no more than 70 g/i- The pickling solution temperature ranges from room temperature to about 80°C. The following example is given further to illustrate the present invention. The scope of this invention is not, however, limited to the specific example reported below. EXAMPLE A shotblasted steel band (AISI 300), approx. 3.5 mm thick, was pickled in a spray pickling plant according to the present invention. The plant consisted in two spray treatment sections, each being 10 m long. Two washing areas with water and cleaning brushes were located between the two pickling sections and, respectively, at the outlet of the second pickling section. Wring rolls limiting the solution flow from one section to the other were also located between the two sections. The pickling tanks capacity was 25 nrVtank with volume of liquid of 15 rrrVtank. The solution was recycled to the sprayer nozzles through a pump having 90 m 3 /h flow. Several tests were made on embodiments of the claimed process. The composition of the pickling solutions used was within the limits defined below: sulphuric acid 90 to 130 g/l hydrofluoric acid 20 to 35 g/l Fe 3+ ≥35 g/l Fe 3 7Fe 2+ 1 to 2 Fe 3+ + Fe 2+ 90 g/l The pickling solution temperature ranged from 55°C to 65°C. In all tests the steel band was fed at a rate of 14 to 18 m/min., depending on its thickness. In all tests, pickling of steel (AISI) band was absolutely satisfactory. The pickling apparatus for carrying out the aforesaid pickling process included at least a tank suitable for collecting and/or containing the pickling solution, recycle and filtering means of the pickling solution, steel band guide rolls in the tank, a cover covering at least the tank central section, and two sets of sprayer pipes (positioned crosswise in respect of the steel band feed and facing the upper side and, respectively, the lower side of the steel band), each bearing a series of nozzles spraying the pickling solution on one side of the steel band, which slides between the two sets of sprayer pipes without being immersed into the pickling solution. In the attached figures, corresponding elements are identified by the same reference numbers. As already mentioned, Figure 1 is a schematic, simplified, cross-sectional view of a pickling tank belonging to a pickling apparatus of the prior art. The Figure shows a tank (1 ) covered with mobile cover (2) (represented, when open, with a broken line) supported by a plurality of arms (3) (only one is visible) actuated by hydraulic pistons (4) (only one is visible) and/or by other means already known and performing an equivalent function. For the sake of simplicity, the following elements have been omitted in Figure 1 : the tank (1 ) coating (in acid-proof rubber and/or inert material), the steel band to be pickled, the pickling solution contained in tank (1 ) as well as other already known elements belonging to the pickling apparatus, such as pickling solution recycle means, filtering means, if any, steel band guide rolls in tank (1 ) and manual or automatic control systems. As already mentioned, Figure 2 is a schematic, simplified, cross-sectional view of a pickling tank belonging to a pickling apparatus according to the invention, wherein pickling is carried out by steel band spraying without immersion of same into the pickling solution contained in tank (1). Figure 2 shows: - the tank (1 ) covered with a mobile cover (5) (represented, when open, with a broken line) supported by a plurality of arms (3) (only one is visible in Fig. 2) actuated by hydraulic pistons (4) (only one is visible in Fig. 2) and/or by other means, already known and performing an equivalent function; - a first sprayer pipe (6), integral with the inner side of cover (5) and bearing a plurality of nozzles spraying the pickling solution on the upper side of steel band (7); - a second sprayer pipe (8), fastened (by conventional means, not shown) to support elements (12 and 15) (better visible in Fig. 4) at the height of the tank (1) edges and bearing a plurality of nozzles spraying the pickling solution on the lower side of steel band (7); - the steel band (7), positioned between the first sprayer pipe (6) and the second sprayer pipe (8); - one of the slide rolls (9) of steel band (7), better visible in Fig. 3; - the aforesaid support elements (12, 15), applied to the longer sides of the tank (1 ), at least where the tank (1 ) is covered with the mobile cover (5), bearing second sprayer pipes (8), and mobile cover (5) when closed. At the upper end of support element (12) there is U-shaped groove (13), which (when mobile cover (5) is closed) accommodates an appendix (14) applied to a longer side of mobile cover (5). At the upper end of support element (15) there is obtuse-angle shaped seat (16), which (when mobile cover (5) is closed) accommodates an inclined appendix (17) which is applied to the other longer side of mobile cover (5) and acts as a hinge for the inclined appendix (17), at least when the mobile cover (5) starts opening. Support elements (12, 15) may be applied to the longer sides of a pickling tank of the prior art, without needing structural modifications and/or adjustments. It follows that a pickling apparatus "by immersion" may be converted to a pickling apparatus "by spraying" according to the present invention, in a relatively simple and inexpensive way. Without departing from the scope of the invention, support elements (12, 15) may be incorporated in tank (1 ) longer sides, which must be suitable for bearing second sprayer pipes (8) and at whose upper ends groove (13) and, respectively, seat (16), which are positioned at their upper ends and are suitable to accommodating the corresponding appendices (14 and, respectively, 17) applied to the longer sides of mobile cover (5), must be provided. For the sake of simplicity, the following elements have been omitted in Figure 2: the tank (1 ) coating (in acid-proof rubber and/or inert material), the pickling solution contained in tank (1 ) as well as other already known constituent elements of the pickling apparatus, such as pickling solution recycle means, filtering means, if any, pickling bath heating systems, manual or automatic control systems, etc. Spray pickling by the apparatus according to the present invention offers several advantages over the traditional pickling "by immersion", i.e.: - pickling solution spraying through the nozzles of sprayer pipes (6) and (8) secures the delivery of the pickling solution in a high volume per steel band (7) unit area, which results in a more effective pickling and, consequently, in a higher line rate (increased production) or, at the same line rate, in a lower consumption of pickling products. It is also advantageous to use a pickling solution recycle system providing for high spraying rates at low pressures; to this end sprayer pipes (6) and (8) are provided with nozzles suitable for high spraying rates; - the pickling solution sprayed on steel band (7) is continuously changed. Conversely, in pickling "by immersion" the moving steel band entrains a thinner or thicker layer of exhaust pickling solution. That is one of the reasons why spray pickling exhibits an improved efficiency over immersion pickling; - the pressure and/or rate of the solution sprayed by each pair of sprayer pipes (6, 8) [and/or by adjacent groups of sprayer pipes (6, 8)] can be so adjusted as to spray each steel band (7) section with an optimal quantity of pickling solution, with the result that band pickling is faster and/or more effective. Further advantages will be manifest to those expert in the art. Figure 3 is a schematic, longitudinal-sectional view of a pickling tank belonging to a pickling apparatus implemented according to the invention. As may be inferred from said Figure, tank (1 ) central section is covered with the cover (5) (with sprayer pipes (6) fastened to the inner side thereof), while the two lateral areas accommodating rolls (11), which control the height of steel band (7) in respect of the pickling solution level in tank (1), are covered with other covers. Rolls (11 ) and relevant covers will not be described herein being already known. Without departing from the scope of the invention, tank (1 ) may be covered with a single cover, at whose inner side first sprayer pipes (6) are fastened at least over tank (1 ) central section. Steel band (7) (coming e.g. from a decoiler reel or a roll train) is caused to pass under the first roll (11 ), to slide between a plurality of sprayer pipes (6, 8) pairs bearing the nozzles spraying the pickling solution on its two sides, and to pass under the second roll (11) before being taken up on a coiler reel or sent to a further roll train or subjected to further treatments, if any (washing, etc.). Due to the applied stress, steel band (7) follows a substantially horizontal and rectilinear path of travel in tank (1 ), where it is supported by slide rolls (9). In view of the high efficiency of a pickling process carried out by an apparatus according to the invention and of the specific applications thereof, in an apparatus according to the present invention the pickling solution temperature is kept in a range between room temperature and about 80°C. For the sake of simplicity, Figure 3 shows - with the relevant reference numbers - only one pair of sprayer pipes (6, 8) and one slide roll (9). Finally, as may be inferred from Figure 3, the bottom of the tank (1 ) consists of two flat slightly inclined surfaces, converging on the tank central area, where oxide scales removed from the steel band surface and/or the mud, if any, derived from chemical pickling processes are gathered (or tend to gather). If the pickling solution to be sent to sprayer pipes (6) and (8) is taken from tank (1 ) areas sufficiently far from the tank central area, the pickling solution sent to sprayer pipes is (practically) free from mud and/or foreign matters. Figure 4 is a schematic, simplified, cross-sectional view of pickling tank (1 ) of Figure 2, showing support elements (12) and (15) applied to tank (1 ) longer sides, at least where the tank (1 ) is covered with mobile cover (5). Figure 4 clearly shows that at the upper end of support element (12) there is U- shaped groove (13), which accommodates the corresponding appendix (14) (Figure 2) applied to one of the longer sides of mobile cover (5). At the upper end of support element (15) there is obtuse-angle shaped seat (16), which accommodates the corresponding appendix (17) (Figure 2) applied to the other longer side of mobile cover (5): the obtuse-angle shape of seat (16) allows its acting as a hinge for appendix (17), at least when mobile cover (5) starts opening. In the embodiment shown in Figure 4, support element (15) is also provided with splash guard element (18) located below seat (16). Support elements (12) and (15) [bearing the weight of second sprayer pipes, (8) and of mobile cover (5)] and at least appendices (14) and (17) applied to the longer sides of mobile cover (5), which transfer the weight of mobile cover (5) (including the weight of first sprayer pipes (6) fastened to the inner surface thereof) to support elements (12, 15), are made by a material having such a mechanical resistance that they can bear the aforesaid weights as well as a good resistance to the pickling bath chemical agents. In a possible embodiment of the pickling apparatus according to the present invention, the whole mobile cover (5) is made in a material suitable to resist to the action of the pickling bath chemical agents. Figure 4 schematically shows steel band (7) positioned between a pair of sprayer pipes (6, 8) bearing a plurality of pickling solution sprayer nozzles. Without departing from the scope of the present invention, numerous variations and modifications to the pickling process and/or apparatus object of the invention may be made by those expert in the art, as suggested by experience and by the normal evolution of the technique.

Claims: Claims 1. Process for steel band pickling, wherein the pickling solution is applied by spraying. 2. A process as claimed in claim 1 , wherein the pickling liquid is a solution of mineral acids or mixtures thereof. 3. A process as claimed in claim 1 , wherein the pickling liquid used is a mineral acids solution consisting of sulphuric acid and hydrofluoric acid, with pH <2.5, and containing Fe 3+ and/or Fe + ions. 4. A process as claimed in claim 3, wherein Fe 3+ ions + Fe 2+ ions are no more than 100 g/l, the Fe 2+ ions concentration may range from 0 to 100 g/l and the Fe 3+ ions concentration is no more than 70 g/l. 5. A process as claimed in any of claims 2 to 4, wherein the pickling solution temperature ranges from room temperature to about 80°C. 6. Apparatus for steel band pickling as claimed in claim 1 , including at least one tank (1 ) suitable for collecting and/or containing the pickling solution, means for recycling and filtering the pickling solution and guide rolls (9) of steel band (7) in the tank (1 ), where at least its central section is covered with a cover (5) and in which steel band (7) slides between two sets of sprayer pipes (6, 8) positioned crosswise in respect of the steel band (7) feed and facing the upper side and, respectively, the lower side of steel band (7), without immersing said steel band into the pickling solution, each sprayer pipe (6, 8) bearing a series of nozzles spraying the pickling solution on one side of the steel band (7). 7. An apparatus as claimed in claim 6, wherein sprayer pipes (6) bearing the nozzles meant for spraying the pickling solution on the upper side of steel band (7) are integral with the inner side of cover (5) covering at least the central section of the tank (1 ). 8. An apparatus as claimed in claim 6, wherein the cover (5) is mobile. 9. An apparatus as claimed in claim 6, wherein a first and a second support element (12, respectively 15) are provided which are suitable for bearing the cover (5), when closed, each support element (12, 15) being applied to one of the longer sides of the tank (1 ), at least where the tank (1 ) is covered with cover (5). 10. An apparatus as claimed in claim 9, wherein support elements (12, 15) bear at least sprayer pipes (8) bearing the nozzles for spraying the pickling solution on steel band (7) lower side. 11. An apparatus as claimed in claim 9, wherein at the upper end of the first support element (12) there is a U-shaped groove (13), suitable to accommodate the first appendix (14) applied to one of the longer sides of cover (5). 12. An apparatus as claimed in claim 9, wherein at the upper end of the second support element (15) there is seat (16), suitable to accommodate the second appendix (17) applied to the other longer side of cover (5), said seat being obtuse- angle shaped and acting as a hinge for the second appendix (17), at least when the cover (5) starts opening. 13. An apparatus as claimed in claim 9, wherein the second support element (15) is also provided with splash guard element (18) located below seat (16). 14. An apparatus as claimed in claim 9, wherein the first and the second support elements (12, 15) are made by a material having a good resistance to the pickling bath chemical agents and such a mechanical resistance that they can bear the weight of cover (5). 15. An apparatus as claimed in claim 10, wherein the first and the second support elements (12, 15) are made of a material having a good resistance to the pickling bath chemical agents and such a mechanical resistance that they can bear the weight of the cover (5) and of sprayer pipes (8). 16. An apparatus as claimed in claim 11 or 12, wherein at least the appendices (14, 17) applied to the longer sides of the cover (5) are made by a material having a good resistance to the pickling bath chemical agents and such a mechanical resistance that they can bear the weight of cover (5). 17. An apparatus as claimed in claim 6, wherein the sets of sprayer pipes (6, 8) facing the upper and lower sides of steel band (7) bear series of nozzles suitable for spraying the pickling solution at high rates. 18. A pickling apparatus as claimed in claim 6, wherein the cover (5) is entirely built in a material exhibiting a good resistance to the pickling bath chemical agents.

Inventors: DEMERTZIS IOANNIS, FORTUNATI SANDRO, GIORDANI PAOLO, DEMERTZIS, IOANNIS, FORTUNATI, SANDRO, GIORDANI, PAOLO
Applicants: ACCIAI SPECIALI TERNI SPA, DEMERTZIS IOANNIS, FORTUNATI SANDRO, GIORDANI PAOLO, HENKEL KGAA, ACCIAI SPECIALI TERNI S.P.A., DEMERTZIS, IOANNIS, FORTUNATI, SANDRO, GIORDANI, PAOLO
Application year: 1998
Main classification: C23F
IPC codes: C23G1, C23G3