Method for purifying pickling solution

Abstract

The invention relates to a pickling plant for pickling a pickling solution for electrical steel, comprising:-a pickling bath,-a concentration bath, where the concentration bath is connected to the pickling bath,-a cross-flow filtration device capable of generating a permeate and a trapped fluid, comprising an inlet side, a permeate outlet side and a trapped fluid outlet side, where the inlet side is connected to the concentration bath,-the cross-flow filtration device is capable of generating the permeate and the trapped fluid, and-the cross-flow filtration device is capable of generating the permeate and the trapped fluid. The trapped fluid outlet side is connected to the inlet side and to the concentration tank.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method and a plant for purifying pickling liquor. The method is particularly suitable for pickling liquor used for pickling steel, in particular electrical steel. BACKGROUND

[0002] The development of new steel grades leads to higher alloy element contents, in particular silicon contents for steels used in electrical applications. Unfortunately, the alloy elements contaminate the pickling bath by dissolution during the pickling process.

[0003] For example, silicon dissolved in the pickling liquor can lead to the formation of monosilicic acid Si(OH)4, which subsequently condenses to form colloidal silicon dioxide and eventually forms a suspension and / or a co-precipitate with other substances. Silicon dissolves in the pickling tank and more globally in the pickling plant. Eventually, this silicon precipitates and reduces the plant lifetime of the pickling plant. To remove the precipitates and to eliminate the blockages, the pickling process needs to be shut down during a cleaning process.

[0004] Patent WO 201436575 describes a method for purification and desiliconization of spent pickling bath. In this method, the spent pickling liquor from the pickling plant is supplied to a collection tank. The spent pickling liquor is then supplied to a microfiltration unit, which produces a first permeate and a first retentate. The first permeate is supplied to an ultrafiltration unit, which produces a second permeate and a second retentate. The second permeate is then sent to a thermal regeneration device. The first and second retentates eventually enter a sludge container to be treated in a sewage treatment plant. However, this method requires two sets of filtration equipment, leading to high maintenance and more frequent production line shutdowns, and does not allow the recirculation of the filtered pickling liquor to the pickling plant.

[0005] Patent EP 3 728 696 B1 describes a method for treating spent acid. In this method, the spent pickling liquor from the pickling plant is supplied to a recirculation tank. The spent pickling liquor is then supplied to an ultrafiltration device, which produces a permeate and a retentate. The permeate is supplied to the recirculation tank.

[0006] However, this method is mainly suitable for a turbulent pickling plant with an available recirculation tank and mainly producing low silicon steel grades, i.e. non-electrical steel grades.

[0007] Patent AT 411 575 relates to a method for purifying contaminated acidic pickling wastewater by means of cross-flow microfiltration. To do so, the produced pickling liquor is left to stand in a settling container and then purified in a cross-flow microfilter in a temperature range of 10°C to 55°C. The purified pickling liquor is then treated in an acid regeneration device according to the principle of spray roasting. This method aims at preventing device malfunctions due to pipe fouling, filter and nozzle clogging.

[0008] However, this method is not energy efficient, since the pickling liquor has to be cooled to be treated and re-heated to be used again in the pickling plant, and does not allow the recirculation of the filtered pickling liquor to the pickling plant. SUMMARY

[0009] The object of the present invention is to provide a method and a plant able to purify the pickling liquor from the pickling plant and to re-use the purified pickling liquor in said pickling plant. BRIEF DESCRIPTION OF DRAWINGS

[0010] In order to illustrate the present invention, a number of embodiments and a number of non-limiting examples will be described, with particular reference to the following drawings:

[0011] - Figure 1 is a schematic representation of an embodiment of the present invention,

[0012] - Figure 2 is a schematic representation of a second embodiment of the present invention. DETAILED DESCRIPTION

[0013] As Figure 1 The present invention also relates to a plant for pickling liquor for pickling electrical steel, comprising, as shown in

[0014] - a pickling tank 1,

[0015] - a concentration tank 3, wherein said concentration tank is connected to said pickling tank 1,

[0016] - a cross-flow filtration device 5, able to produce a permeate and a retentate, comprising an inlet side 5e, a permeate outlet side 5p and a retentate outlet side 5r, wherein

[0017] - said inlet side 5e is connected to said concentration tank 3,

[0018] - said permeate outlet side 5p is connected to pickling tank 1,

[0019] - said retentate outlet side 5r is connected to said inlet side 5e and to said concentration tank 3.

[0020] The pickling tank 1 can comprise a pickling liquor 2, forming a pickling bath inside said pickling tank 1.

[0021] The concentration tank 3 can comprise a concentration liquor 4, so that the pickling liquor 2 from the pickling tank 1 can be supplied to said concentration tank 3.

[0022] The inlet side 5e is connected to said concentration tank 3, so that the concentration liquor 4 can be supplied to said inlet side 5e of said cross-flow filtration device 5.

[0023] The permeate outlet side 5p is connected to the acid washing tank 1, so that the permeate can be supplied to said acid washing tank 1.

[0024] The retentate outlet side 5r is connected to said inlet side 5e and to said concentration tank 3, so that the retentate can be supplied to the filtration device 5 and to the concentration tank 3.

[0025] A pre-filter able to remove coarse particles from 2.5 pm to 5 mm, preferably from 2.5 pm to 10 pm, can be placed between the concentration tank 3 and the cross-flow filtration device 5, allowing the filtration of the concentrated liquid flowing from the concentration tank 3 to the cross-flow filtration device 5.

[0026] The cross-flow filtration device 5 is made of a material resistant to acidic or basic media. For example, the cross-flow filtration device can be made of a polymer, such as polypropylene and / or polyethylene and / or polysulfone and / or polyvinylidene fluoride, or of a metallic material, such as titanium or stainless steel and / or other metals with a coating.

[0027] The cross-flow filtration device is preferably a ceramic membrane cross-flow filtration device.

[0028] The cross-flow filtration device 5 can be an ultrafiltration device. The membrane pore size of the ultrafiltration device is preferably from 5 kDa to 500 kDa and even more preferably from 15 kDa to 300 kDa. The total filtration area of the ultrafiltration device is preferably from 10 m 2 to 600 m 2 and even more preferably from 20 m 2 to 450 m 2 .

[0029] Preferably, the ultrafiltration device comprises a tubular membrane consisting of an asymmetric ceramic porous structure support on which a filtration active layer is deposited. For example, the ceramic porous structure support can be made of a metal oxide, such as aluminum oxide (AI2O3) or titanium dioxide (TiO2). The filtration active layer can comprise at least three layers having a thickness of 3 pm to 80 pm. These active layers can also be made of a metal oxide, typically AI2O3, TiO2or ZrO2. The membrane is assembled in a metallic casing (made of titanium or stainless steel or coated metal), which can comprise several membranes.

[0030] As shown in Figure 2 , the pickling plant can comprise a feed pump 8 able to regulate the flow of concentrated liquid 4 from the concentration tank 3 to the filtration device 5.

[0031] As shown in Figure 2 , the pickling plant preferably comprises a recirculation pump 16 upstream of the filtration device 5. This pump allows the preferred conditions for the cross-flow filtration process to be achieved.

[0032] The permeate outlet 5p is also preferably connected to an intermediate collection tank and / or an acid regeneration unit (ARP).

[0033] like Figure 2 As shown, the pickling equipment preferably includes a separation device 9 connected to the concentration tank 3, allowing the concentrate 4 to be supplied to the separation device 9. The separation device 9 is preferably a decanter. Figure 2 As shown, the pickling equipment preferably includes a discharge pump 10 capable of regulating the flow rate of the concentrate 4 from the concentration tank 3 to the separation device 9.

[0034] like Figure 2 As shown, in addition to the separation device 9, the pickling equipment may also include a dehydration system 13, which is connected to the separation device 9 and to the concentration tank 3 and / or to the separation device 9 and / or to any other collection device.

[0035] The present invention also relates to the following methods for purifying pickling solutions.

[0036] A portion of the pickling solution 2 from pickling tank 1 is supplied to concentration tank 3.

[0037] Pickling solution 2 can be an acidic solution, an alkaline solution, or a mixture of both. The pickling solution preferably comprises 0.5 to 30% by weight of hydrochloric acid and / or sulfuric acid and / or nitric acid. Preferably, the pickling solution does not include hydrofluoric acid, as hydrofluoric acid may damage the ceramic membrane of the filtration system.

[0038] Pickling solution 2 may include any alloying element of the steel product, such as Fe, Mn, Zn, Cr, Sn, SiOx, or silicon-based compounds. For example, the pickling solution may include iron in the form of iron salts, such as iron chlorides and / or iron oxides. For example, the pickling solution may include 60 to 350 grams of iron per liter and 0.001 to 20 grams of manganese per liter.

[0039] Pickling solution 2 may also include additives such as inhibitors to prevent over-pickling, such as organic and / or inorganic compounds.

[0040] Pickling solution 2 may also include other elements generated from the pickling of steel products and from upstream processing steps of the pickling of steel products.

[0041] The present invention is particularly suitable for treating pickling solutions of electrical steel grades, which can contain up to 6.5 weight percent of silicon. The pickling solution 2 in the pickling tank 1 preferably has a silicon content of 30 mg to 10000 mg per liter of pickling solution. Even more preferably, the pickling solution 2 in the pickling tank 1 preferably has a silicon content of 500 mg to 10000 mg per liter of pickling solution. Silicon compounds can be found in dissolved, colloidal or suspended particulate form. More than 50% of the total silicon dioxide can be in colloidal or suspended form.

[0042] A portion of the pickling solution 2 can be supplied to the concentration tank 3 by gravity, or can be pumped to any other suitable device.

[0043] The steel pickling plant can comprise more than one pickling tank. A portion of the pickling solution 2 from more than one pickling tank can be supplied to the concentration tank 3.

[0044] The volume loss due to the supply of pickling solution 2 from the pickling tank 1 to the concentration tank 3 can be compensated or reduced by means of the feed of pickling solution.

[0045] A portion of the concentrated solution 4 from the concentration tank 3 is supplied to the cross-flow filtration device 5. In cross-flow filtration, the filtration effect is achieved by means of a dynamic shear stress gradient established in the direction of filtration. In comparison with static filtration, cross-flow filtration allows a sustained filtration performance to be achieved at high filtrate flow rates. The accumulation of filter cake, for example impurities, on the membrane pores can be removed by means of regular counter-flow, i.e. backwash, in order to clean the filtration device. This can be done by means of a Clean-in-place (CIP) method well known to the person skilled in the art. The devices and pipes involved in this method are not shown in Figure 1 .

[0046] The purpose of the ultrafiltration device 5 is to separate the acid or base of the pickling solution from the colloidal and suspended impurities, for example alloying elements, and in particular from silicon-based compounds, for example colloidal silicon dioxide.

[0047] The ultrafiltration device 5 is then supplied with the concentrated solution 4 from the concentration tank 3, which produces a retentate 6 and a permeate 7.

[0048] The retentate has a higher concentration of impurities than the concentrated solution 4 supplied to the ultrafiltration device 5. The permeate, also known as filtrate, has a smaller concentration of impurities than the concentrated solution 4 supplied to the ultrafiltration device 5.

[0049] In the case of the use of a recirculation pump upstream of the ultrafiltration device 5, the pump allows a transmembrane overpressure of 0.5 bar to 6 bar to be obtained.

[0050] Preferably, the volume of permeate is greater than 70% of the concentrated liquor 3 entering the filtration device, and preferably greater than 90%. Preferably, the rejection rate is less than 30%, and more preferably less than 10%.

[0051] Preferably, the concentrated liquor can comprise a small portion of the spent liquor from the pickling liquor 2, a small portion of the retentate from the filtration device 5 and a small portion of the clarified concentrated liquor 1 1. This mixed stream or concentrated liquor can be fed to the filtration device 5 or the separation device 9. The silicon concentration in this concentrated liquor can range from 0.5 g / L to 50 g / L, and the temperature can be between 60°C and 85°C.

[0052] The permeate 7 can be supplied to the pickling tank 1. The permeate can also be supplied to an intermediate collection tank and / or an acid regeneration plant (ARP). The retentate 6 is recirculated through a recirculation loop. Preferably, the flow rate recirculated through the recirculation loop is significantly higher, 5 to 60 times greater than the flow rate provided by the feed pump 8, using a pump 16. This allows the preferred conditions for a cross-flow filtration process to be achieved. Preferably, the transmembrane pressure is from 0.5 bar to 6 bar, and the cross-flow velocity is from 2.5 m / s to 5 m / s. These conditions allow a turbulent regime to be created, reducing the fouling of the membrane.

[0053] Furthermore, the increase in concentration in the recirculation loop allows the fouling of the filtration device to be limited, in particular for filtration devices with membranes, as the largest colloidal particles are less likely to foul the filtration device. A small portion of the retentate can be recirculated from the filtration device 5 to the concentrated tank 3.

[0054] The apparatus allows a recirculation loop to be formed between the concentrated tank 3 and the ultrafiltration device 5. Furthermore, as the permeate leaves the recirculation loop, the concentrated liquor 4 increases the concentration of the concentrated liquor 4 in terms of silicon content.

[0055] The method can also comprise a step in which a portion of the concentrated liquor 4 from the concentrated tank 3 is supplied to the separation device 9. Optionally, the method can comprise an operating mode in which a small portion of the retentate 6 leaving the filtration device 5 is supplied directly to the separation device 9, bypassing the concentrated tank 3, in a continuous feed-and-draw operating mode, supplied via a pump or by gravity.

[0056] The apparatus can comprise several filtration stages (e.g. 2 to 7) operating in series or in parallel, to increase the efficiency of the process, reducing the total membrane area required. For example, the apparatus can comprise 2 to 7 filtration stages in series or in parallel.

[0057] The separation device 9 is supplied periodically or continuously. The term periodically is to be understood in the sense that the supply can be "turned on" and "turned off", but it does not necessarily take place at regular time intervals. The supply can be controlled by a discharge pump 10 or by gravity.

[0058] The separation device favors the sedimentation of the supplied concentrate 4, which will result in the presence of a multi-phase solution comprising at least a clear concentrate and a darker concentrate.

[0059] The clear concentrate 11 has a lower silicon content than the supplied concentrate 4 or the reject 6 from the concentration tank 3. The clear concentrate should be on top of the separation device.

[0060] The darker concentrate 12 has a greater silicon content than the supplied concentrate 4 or the reject 6 from the concentration tank 3. The darker concentrate should be at the bottom of the separation device.

[0061] Preferably, less than 30% of the flow entering the filtration device 5e is supplied to the separation device 9.

[0062] The clear concentrate 11 can be supplied to the pickling tank 1 and / or to the concentration tank 3 and / or to the ultrafiltration device 5 and / or to the tank.

[0063] The darker concentrate can be supplied to the dewatering system 13.

[0064] The dewatering system 13 can be any system or device capable of concentrating the solid fraction and separating the liquid phase. For example, the dewatering system 13 can be a filter press, a belt filter or a centrifuge. The dewatering system 13 can also comprise more than one equipment capable of concentrating the solid fraction and separating the liquid phase. For example, the dewatering system 13 can comprise a filter press and a centrifuge.

[0065] The dewatering system produces a solid fraction 14 and a liquid fraction 15. The liquid fraction can be supplied to the separation device 9 and / or to the concentration tank 3 and / or to the pickling tank 1 and / or to the ultrafiltration system 5 and / or to the collection tank.

Claims

1. A pickling apparatus for pickling electrical steel with a pickling solution, comprising: - Pickling tank (1) - Concentration tank (3), wherein the concentration tank is connected to the pickling tank (1). - Cross-flow filtration device (5), capable of generating permeate and retentate, including an inlet side (5e), ​​a permeate outlet side (5p), and a retentate outlet side (5r), wherein - The inlet side 5e is connected to the concentration tank (3). - The retentate outlet side (5r) is connected to the inlet side (5e) and to the concentration tank (3).

2. The pickling equipment according to claim 1, wherein the cross-flow filtration device (5) is an ultrafiltration device.

3. The pickling equipment according to any one of claims 1 or 2, wherein the cross-flow filtration device (5) is made of a polymer selected from polypropylene, polyethylene, polysulfone, polyvinylidene fluoride or combinations thereof, or of a metallic material selected from titanium or stainless steel and / or other coated metals.

4. The pickling equipment according to claim 2, wherein the membrane pore size of the ultrafiltration device is from 5 kDa to 500 kDa.

5. The pickling equipment according to any one of claims 1 to 4, wherein the pickling equipment includes a separation device (9) connected to the concentration tank (3).

6. The pickling equipment according to claim 5, wherein the pickling equipment includes a dehydration system (13) connected to the separation device (9) and connected to the concentration tank (3).

7. The pickling apparatus according to any one of claims 1 to 6, wherein the permeate outlet side (5p) is connected to the pickling tank (1).

8. A method for purifying pickling solution in a pickling apparatus according to any one of claims 1 to 7, wherein the pickling tank (1) comprises pickling solution (2) and the concentration tank (3) comprises concentrate (4), the method comprising the following steps: i. A portion of the pickling solution (2) is supplied to the concentration tank (3). ii. A portion of the concentrate (4) is supplied from the concentration tank (3) to the cross-flow filter device (5). iii. Using the cross-flow filtration device (5), retentate (6) and permeate (7) are generated. iv. The retentate (6) is supplied to the concentration tank (3) and to the filtration device (5).

9. The method according to claim 8 is carried out in the pickling apparatus according to claim 6, wherein the permeate (7) is supplied to the pickling tank (1).

10. The method according to claim 8 or 9, wherein the pickling solution (2) in the pickling tank (1) has a silicon content of 30 mg to 10,000 mg per liter of pickling solution.

11. The method according to any one of claims 8 to 10 is carried out in the pickling apparatus according to claim 4, wherein a portion of the concentrate (3) from the concentration tank (4) is supplied to the separation device (9).

12. The method according to claim 11, wherein less than 30% of the flow rate entering the filter device 5e is supplied to the separation device (9).

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