Abstract
The invention concerns a continuous pickling plant (101) for the treatment of metal products (109) in the form of a strip or tube which includes a plurality of acid solution process tanks (102) each divided into a plurality of turbulence cells (114) and a recirculation system of the acid solution. The plant also includes, upstream of the plurality of process tanks (102), a device for heating (124; 126) the metal product (109), namely an induction heating (124) system and/or a tunnel with water sprays (126). A solution with only two pickling tanks (102) is also presented. Devices are described (135; 136; 138, 138a) to increase turbulence in the tanks and to accelerate pickling, e.g. by adding fresh acid (146) in the first tank. The invention also concerns a related pickling process. The invention optimizes the effect of the acid and reduces the residence time of the strip (109) in the plant.
Claims
1. A continuous pickling plant for the treatment of metal products in the form of a strip or tube comprising (a) a plurality of acid solution process tanks each divided into a plurality of turbulence cells; (b) a recirculation system of the acid solution; and (c) a heating device of the metal product upstream of the plurality of process tanks, wherein said heating device is an induction heating system and/or a water spray tank.
2. The continuous pickling plant according to claim 1, wherein it comprises one or more devices to increase the turbulence in the acid solution selected from the group consisting of (i) upper and lower turbulence sprays placed at the entrance and exit of the first and last turbulence cell of each tank; (ii) turbulence fins arranged inside each turbulence cell; and (iii) in the last pickling tank and precisely in the first turbulence cell a pickling accelerator consisting of a series of upper and lower acid spray bars.
3. The continuous pickling plant according to claim 1, wherein it further comprises: (d) at the entrance and exit of each tank immersion rollers dimensioned such that the level of overflow of the free surface of the acid solution is lower than the central axis of the roller, wherein these rollers define the passage of the metal product in the turbulence cells dividing them into an upper turbulence cell and a lower turbulence cell.
4. The continuous pickling plant according to claim 1, wherein it further comprises: (e) upstream of the pump of the first process tank, a pipe with a suitable regulation valve for adding fresh acid.
5. The continuous pickling plant according to claim 1, wherein it further comprises (f) downstream of the heating device a temperature measuring device wherein the signal generated by the latter is sent to a closed loop control unit in order to maintain the temperature deviation of the metal product within a tolerance range, of ±2-5° C.
6. The continuous pickling plant according to claim 1, wherein the last tank of the plurality of process tanks, on the bottom thereof, comprises a first overflow drain with a low iron concentration and a second overflow drain at increased iron concentration, wherein the two drains feed the respective recirculation tank that for this purpose contains a first chamber for the low concentration of iron and a second chamber for the increased concentration of iron, separated by a wall of weir, and wherein the plant provides for the presence of only two process tanks.
7. A pickling process, performed with a plant according to claim 1, comprising the following steps: a) a pre-heating of a strip-shaped metal product by means of a heating device, wherein said heating device is an induction heating system and/or a water spray tank; b) pickling of the strip with removal of the scale in a plurality of process tanks with acid solution.
8. The pickling process according to claim 7, wherein the descaling capacity of the first process tank is increased by introducing fresh acid into the first tank.
9. The pickling process according to claim 7, wherein the turbulence inside the tanks is increased by inserting into the tanks one or more of the devices from (i) to (iii) of claim 2, in particular the devices with acceleration capacity.
10. The pickling process according to claim 7, comprising the following steps: a) a pre-heating of a strip-shaped metal product by means of a heating device, wherein said heating device is an induction heating system and/or a water spray tank; b) pickling of the strip with removal of the scale in a plurality of process tanks with acid solution.
11. The pickling process according to claim 10, wherein the strip-shaped metal product is a hot-rolled strip.
12. continuous pickling plant according to claim 1, wherein a solution discharged from the overflow of each process tank feeds through a discharge pipe and an external recirculation tank from which the acid solution is returned by a pump through an intake pipe to the process tank by optionally passing through a heat exchanger to be heated
Description
DESCRIPTION OF PREFERRED EMBODIMENTS
[0073] FIG. 1 shows the diagram of a continuous pickling plant including the acid recirculation system according to the state of the art.
[0074] FIG. 2 schematically shows in lateral section the realization of the process tanks including the turbulence cells of the plant of FIG. 1 according to the state of the art.
[0075] FIG. 3 schematically shows in lateral section an embodiment of a pickling plant according to the invention equipped with elements for pre-heating the strip entering the pickling tanks.
[0076] FIG. 4 schematically shows in lateral section an embodiment of a pickling tank according to the plant of the invention equipped with elements for increasing the turbulence in the plant.
[0077] FIG. 5 schematically shows in lateral section another embodiment of a pickling plant according to the invention equipped with a variant for introducing acid into the first tank of the plant.
[0078] FIG. 6 shows a schematic view of a plant with only two continuous pickling tanks preceded by a strip pre-heating unit.
[0079] FIG. 1 shows the diagram of a continuous pickling plant 1 including the acid recirculation system according to the state of the art. A succession of three pickling tanks 2 is noted. Systems of discharge pipes 3 remove the liquid contained in the tanks 2 and transport it to the respective drain tanks 4. From each tank 4 the liquid is guided by a delivery pump 5 along a recirculation pipe 6 equipped with a heat exchanger 7 through relative systems of intake pipes 8 back into the corresponding pickling tank. A metal strip 9 passes through the tanks 2 in the direction of the arrow a meeting at the beginning of the line, at the end of the same and between the tanks 2 of the squeezer rollers 10. The drain tanks 4 connected together in cascade with the opposite travel direction of the strip (arrow a) through pipes 11.
[0080] FIG. 2 schematically shows in lateral section the embodiment detail of a single process tank including turbulence cells also present in the plant of FIG. 1, as known in the state of the art. At the entrance and exit of the tank 2, note two pairs of squeezer rollers 10, useful for dragging the product, preventing it from retaining excessive acid. At the entrance and exit of each tank 2 are positioned respectively inlet hydraulic seal sprays 12 and outlet hydraulic seal sprays 13 adapted to limit the leakage of acid from the tank. The tank 2 is divided into a series of turbulence cells 14 each composed of an upper turbulence and recirculation cell section 15 and a lower turbulence cell section 16, separated from each other by the strip 9 being processed. The individual cells 14 are separated from each other at the lower part by stone blocks 17 and at the upper part by rounded blocks 18. Note also inclined panels 15 or ramps that serve, together with the elements 17, 18 to create turbulence by favouring the recirculation of the liquid inside the cell; this is well known in the art and is described, for example, in the document EP 1 054 079 B1. The discharge pipe system 3 is also depicted. The cells are enclosed below by a tank bottom 20 and above by a cover 21. To further increase turbulence within the tank, a number of lateral auxiliary sprayers 22 can be provided.
[0081] FIG. 3 schematically shows in lateral section an embodiment relating to the continuous pickling plant 101 according to the invention, provided with elements for pre-heating the strip 109 entering the pickling line. Only the first tank 102 of the plant is depicted. At the entrance and exit of the tank 102 there are pairs of squeezer rollers 110. Upstream of the first tank 102 relative to the feed direction (arrow a) of the strip 109 a heating unit is provided: for example, an induction or gas or resistance furnace 124 or the like, adapted to heat the strip before it enters the first tank 102. The depicted tank 102 is a heating tank which uses water and has a series of water sprayers 126, preferably arranged in spray tunnels that allow pre-heating the strip 109 to a temperature between 65 and 75° C. In fact, through the pipe 128 and passing through a self-cleaning filter 129 the discharge reaches the discharge tank 104 which can also be fed by the discharge of rinsing water 130. A delivery pump 105 forces the liquid to pass through a heat exchanger 107 fed by, for example, water vapour, which heats the passing liquid while it is again sent through the intake pipe 108 in the spray system 126 and thus heats the strip 109 passing through the tank 102. Suitable temperature measuring devices 132, e.g. infrared, are arranged upstream and downstream of the tank 102 and transmit temperature values to a temperature control unit 134 capable of controlling and regulating the temperature of the water fed to the water sprayers 126 and the induction heating system 124. In any case, said heating unit 124 and the water heating system 126 can be realized as alternatives or in combination.
[0082] FIG. 4 schematically shows in lateral section an embodiment of a pickling tank 102 according to the plant of the invention equipped, in particular, with elements for further increasing the turbulence in the plant. The elements depicted help increase the turbulence in the system and thus accelerate the pickling and can be used as alternatives or in various combinations therebetween. As already illustrated above with reference to the state of the art, for example, three turbulence cells 114 can be identified inside the tank 102. Each cell 114 is divided by the strip 109 into an upper turbulence cell 115 and a lower turbulence cell 116. The individual cells 114 are separated therefrom by lower blocks 117 and upper blocks 118, similar to that existing in the prior art. The tank 102 has a bottom 120 and a cover 121, as well as the inclined panels or ramps 119 as already described. Side sprayers 122 increase turbulence in a known manner. Lower and upper turbulence sprayers at the entrance and exit 135 further contribute to the increase in turbulence. Additional fins 136 that help increase turbulence are applied to the bottom 120 and/or the lower surface of the inclined panels 119. As additional pickling effect accelerators, particularly in the last tank, upper and lower sprayers 138 can be arranged applied to the bottom 120 of the tank 102 and/or to the lower surface of the inclined panels 119. This new solution also includes immersion rollers 140 of large diameter, which press the strip under the free surface 142 of the acid (dotted line). The large diameter ensures that the roller axis is always above the free overflow discharge surface 142 of the tank 102, so that the bearings are never in contact with the acid solution and last longer. The immersion rollers 140 thus render the hydraulic sealing sprays at the entrance and exit of the tank 102 superfluous, simplifying the layout.
[0083] FIG. 5 schematically shows in lateral section another embodiment of a pickling plant according to the invention equipped with a variant for introducing acid into the first tank 102 of the plant. For simplicity a diagram similar to that in FIG. 1 is illustrated, but it is understood that the plant can be further provided with one or more of the heating and/or turbulence increasing elements according to the invention and as depicted in FIGS. 3 and 4. Referring to FIG. 1, note a pipe 144 for the addition of fresh acid from a further tank not shown, arranged between the discharge tank 104 and the delivery pump 105. The amount of fresh incoming acid 146 is regulated by an automatic valve 148, and is arranged in case the solution is excessively diluted.
[0084] FIG. 6 shows a schematic view of a plant with only two continuous pickling tanks preceded by a strip pre-heating unit. The strip pre-heating unit was first described with reference to FIG. 3. The strip 109 exiting the pre-heating unit passes through a first pickling tank 104 equipped with a “traditional” acid recirculation system and subsequently through a second pickling tank 102 in which the acid recirculation system provides for an important difference with respect to the “traditional” one of the state of the art (see FIG. 1), namely the innovation that the discharge tank is fed by an overflow drain at a low concentration of iron 152 and an increased concentration drain 150 realized directly on the bottom of the pickling tank 102 placed as the second and last tank in the plant that is connected to the previous tank through the cascade pipe 111. The two drains 150 and 152 pour the solution into the recirculation tank 104 in which two chambers 156 and 158 are realized, separated by a wall of weir 154, thereby obtaining correct use of the acid solution.
[0085] During operation, further embodiment modifications or variants of the pickling plant and pickling process of the invention, not described herein, may be implemented. If such modifications or such variants should fall within the scope of the following claims, they should all be considered protected by the present patent.
