Acid regeneration process for pickling process

Abstract

The invention relates to a method for treating a spent acid, for steel pickling, wherein the following steps are performed: i. mixing said spent acid with a solution of sulphuric acid, to effect FeCl2 + H2SO4 + n H2O ⇆ FeSO4.nH2O + 2 HCl, so as to obtain a reaction mixture comprising hydrochloric acid and hydrated forms of iron sulphate, wherein said reactants comprises from 5 to 40 weight percent of sulphuric acid and ii. distilling said reaction mixture to obtain a distillate and a concentrate, iii. mixing said concentrate with water such that the weight ratio between the concentrate and the water is from 0.1 to 10, to obtain an aqueous solution, iv. setting said aqueous solution to a temperature from -10 to 50°C to obtain a liquid phase and a solid phase, v. separating said liquid phase and said solid phase obtained in step iv.

Description

[0001] ACID REGENERATION PROCESS FOR PICKLING PROCESS

[0002] The invention relates to a method to treat a spent acid so as to regenerate a fresh acid and produce hydrated forms of ferrous sulphate. The invention is particularly intended to treat a spent acid resulting from the pickling of steel products with a solution comprising hydrochloric acid and to produce ferrous sulphate heptahydrate and fresh acid.

[0003] The pickling process is a surface treatment used to remove impurities such as stains, inorganic contaminants, rust or scale from steel. A pickling bath, also called pickle liquor, containing strong acids is used to remove the surface impurities.

[0004] The strong acid reacts with the oxides and base metal to produce dissolved metal salts which neutralizes the acid.

[0005] The pickling of steel sheets produces a large quantity of spent acid, i.e. spent pickle liquor, containing dissolved metals salts, and especially iron salt, as well as residual free acid. This spent acid poses a hazard to the environment and is difficult to dispose of in an economical manner. It can be disposed of by chemical neutralization with alkali or regenerated.

[0006] Some methods have been developed to regenerates the acid such as pyro-hydrolysis or such as roasting. Even though those techniques convert the dissolved iron into iron oxide powder, they are very energy intensive.

[0007] More recent processes have been developed to treat the pickling liquor so as to obtain a useful iron product such as ferrous sulphate heptahydrate in a less energy intensive manner.

[0008] For example, WO 2021 074 904 Al discloses a method wherein a pickling bath comprising hydrochloric acid and ferric chloride is mixed with a sulphuric acid so as to obtain a solution comprising hydrochloric acid containing ferrous sulphate. This solution is then directly cooled below 0°C to obtain a ferrous sulphate precipitate which is then separated into a ferrous sulphate product and a regenerated hydrochloric acid solution

[0009] However, this method consumes a lot of energy as the hydrochloric acid needs to be heated before being used in the pickling process and results in a ferrous sulphate heptahydrate having a low purity, e.g. comprising other metals than iron that were included in the steel, such as manganese and tin. The aim of the present invention is to provide a method for treating a spent hydrochloric acid permitting to increase the recovery of the hydrochloric acid and iron sulphate heptahydrate.

[0010] Other characteristics and advantages will become apparent from the following detailed description of the invention.

[0011] The invention will be described, in a non-limitative way, in reference to the following drawing: figure 1 is a flow diagram of an embodiment of a process according to the present invention.

[0012] Table 1 presents the concentration of iron, chlorides and sulphates depending on the concentration of sulphuric acid in the reactants mix in step i for an embodiment of the present invention.

[0013] The invention relates to a method for treating a spent acid comprising 0.5 to 18 weight percent of hydrochloric acid and 60 to 350g.L4of iron, wherein the following steps are performed: i. mixing said spent acid with an aqueous solution of sulphuric acid, to effect Reaction (1),

[0014] Reaction (1) FeCl2+ H2SO4+ n H2O FeSO4mH2O + 2 HC1, so as to obtain a reaction mixture comprising from 5 to 40 weight percent of sulphuric acid, ii. distilling said reaction mixture so as to obtain a distillate comprising HC1 with a weight percent from 10 to 40, and a concentrate comprising essentially hydrated forms of ferrous sulphate, iii. mixing said concentrate with water such that the weight ratio between the concentrate and the water is from 0.1 to 10, so as to obtain an aqueous solution comprising hydrated forms of iron sulphates, iv. setting said aqueous solution to a temperature from -10 to 50°C so as to obtain a liquid phase comprising metal chlorides and metal sulphates metal salts and acids and a solid phase comprising mainly hydrated forms of iron sulphate, v. separating said liquid phase and said solid phase obtained in step iv.

[0015] Step i

[0016] The spent acid comprises from 0.5 to 18 weight percent of hydrochloric acid and from 60 to 350 grams per litre of iron. The spent acid can comprise from 1.5 to 18 weight percent of hydrochloric acid. This iron is mainly present in the form of FeClz and result from the reaction between the iron oxide present on the steel product and the sulphuric acid according to the following reactions: FeO + 2HC1 — > FeCl2+ H2O, Fe3O4+ 8 HC1 — > FeCl2+ 2 FeClj +4 H2O and from the reaction between the iron of the steel product and the sulphuric acid according to the following reaction: Fe + 2 HO —> FeCl2+ H2. Iron is also present in the form of FeC13 due to the previously mentioned reaction and also due to the following reaction: Fe2O3+ 6 HCL — > 2 FeCl3+ 3 H2O.

[0017] The spent acid can also comprise other pickling acids such as hydrofluoric acid, sulphuric acid and / or nitric acid. The solution can then also comprise iron in the form of FeF2, FeF3, FeSO4, Fe2(SO4)3, Fe(NO3)2and / or Fe(NO3)3.

[0018] The spent acid may comprise any alloying elements of steel products, such as Mn, Zn, Cr, Sn, SiO2or silica-based compounds. The alloying elements can form various compounds such as metallic salts and / or metallic oxides which can be present in the spent acid.

[0019] The spent acid may also comprise additives such as inhibitors for over-pickling avoidance such as organic and / or inorganic compounds.

[0020] The spent acid may also comprise impurities due to the pickling of a steel product.

[0021] As illustrated in Figure 1, the spent acid is mixed with an aqueous solution of sulphuric acid so as to effect Reaction (1): FeSCfonHzO + 2 HC1

[0022] The acid mixture, i.e. the reactants, comprises from 5 to 40 weight percent of sulphuric acid.

[0023] Apparently, below 5 weight percent of sulphuric acid, it is not possible to recover at least 50 percent of the chlorides, which are present essentially in the form of metal chlondes, present in the spent acid. Apparently, if it is greater than 40 weight percent, the yield is not improved.

[0024] Preferably, the acid mixture comprises at least 10 weight percent of sulphuric acid. Preferably, the acid mixture comprises at most 24 weight percent of sulphuric acid.

[0025] However, due to the possible presence of other compounds, other reactions can take place such as the reactions (2) and (3):

[0026] (2) 2 FeCl3+ 3 H2SO4+ nH2O Fe2(SO4)3mH2O + 6 HC1

[0027] (3) 2 MClx + x H2SO4+ nH2O ±5 M2(SO4)x + 2x HC1 In addition to ferrous chloride, sulphuric acid, water, hydrochloric acid and hydrated forms of iron sulphate FeSO^nHzO and HC1, the reaction mixture may comprise any compound or acid present in the spent acid and / or any product of the reactions (2) and (3) and / or resulting from other reactions. For example, the reaction mixture can comprise ferric or ferrous chloride (such as FeCls, FeCh,) and / or hydrated forms of ferrous or ferric sulphate, and / or other metal salts or metal oxides.

[0028] Preferably, the aqueous solution of H2SO4 is prepared by diluting a solution of HZSO4, having a concentration from 90 to 99 weight percent with an aqueous solution. The aqueous solution can be clean water and / or rinse water from the pickling line and / or other effluent from the process. It improves the yield of the process and thus the recovery of iron and chlorides.

[0029] This mix can be done prior to or during the mix of the sulphuric acid with the spent acid or at the same time than the distillation.

[0030] The dilution of sulphuric acid with an aqueous solution is a highly exothermic process.

[0031] The acid recovery installation can comprise a heat exchanger able to transfer the heat resulting from the dilution of the sulphuric acid with the aqueous solution to the distillatory system or to the distillate resulting from the distillation of step ii., i.e. essentially hydrochloric acid, before reusing the distillate in the pickling line.

[0032] Step ii

[0033] As illustrated in Figure 1, the reaction mixture is then distilled so as to obtain a distillate essentially comprising hydrochloric acid and a concentrate comprising essentially hydrated forms of ferrous sulphate, i.e. FeSCbmHzO wherein n is from 0 to 7.

[0034] The distillation is preferably performed at a pressure from 0.01 to 500 mbar, even more preferably at a pressure from 30 to 200 mbar.

[0035] The distillation is preferably performed at a temperature from 60 to 95°C, even more preferably at a temperature from 75 to 85°C.

[0036] The distillation can be performed using membranes distillation or column distillation.

[0037] Alternatively, the distillation is performed using a rotary evaporator.

[0038] The duration of the distillation depends on the distillation condition such as the temperature and the pressure. The distillation can for example lasts from 5 to 90 minutes, depending on the distillation conditions, such a pressure and temperature. In another alternative, the distillation can be performed by flash distillation.

[0039] Without to be bound by any theory, distilling the reaction mixture of Reaction (1) is advantageous as during the distillation, the iron stays with the sulphate allowing the Cl' to be freely distilled. Consequently, adding an aqueous solution of sulfuric acid and then distilling leads to a greater yield, in terms of recovery of hydrochloric acid, than distilling the spent acid.

[0040] The distillate contains hydrochloric acid in a concentration from 10 to 40 weight percent, preferably from 15 to 20 weight percent. The concentration of hydrochloric acid in the distillate depends on the water concentration in the reactants. Apparently, a greater concentration of water leads to a lower concentration of hydrochloric acid while a lower concentration of water leads to a greater concentration of hydrochloric acid but with a maximum limit of 40 weight percent of hydrochloric acid for the conditions described above.

[0041] The distillate may comprise sulphuric acid, sulphates and / or impurities such as metal chlorides and / or other metal salts and / or metal oxides. Preferably, the distillate comprises less than 10000 ppm of sulphates, more preferably less than 1000 ppm and even more preferably less than 100 ppm.

[0042] This distillate may then be reused in a pickling line as fresh liquor. For example, the distillate can be sent to a pickling tank so as to replenish a pickling bath.

[0043] The concentrate comprises essentially hydrated forms of ferrous sulphate. The concentrate essentially comprises FeSCfoHzO, FeSCfoSHzO, FeSO4’4H2O and FeSCfoVHaO. Even more preferably, the concentrate comprises mainly iron sulphate heptahydrate FeSCfoVHzO.

[0044] The concentrate may comprise impurities such as metal sulphate including ferric sulphate, other metal sulphates, hydrochloric acid, sulphuric acid, metal chlorides, metal oxides, metal salts and / or other hydrated forms of ferrous sulphate.

[0045] Table 1 illustrates the concentration of iron, chlorides and sulphates in the concentrate resulting from step 2 for various concentration of sulphuric acid in the reaction mixture used in the reactants of step 1. The yield of recovered chlorides as HC1 is the amount of Cl present in the distilled acid, in the form of HC1, divided per the total amount of chlorides present in the spent acid, in the form of HC1 + combined chlorides, such as FeQz and FeClj or metal chlorides MCL

[0046] The amount of chlorides in the form of iron chlorides present in the spent acid can be estimated based on the acidity of the spent acid and the total amount of chlorides. A first titration has been performed to estimate the total amount of chlorides and a second titration has been performed to estimate amount of free chlorides as HC1. Then the quantity of total chlorides can be estimated based on the following formula: Combined Cl' |g / L] = Total Cl' [g / 1] — Free chlorides as HC1 [g / L].

[0047] The amount of chlorides in the form of HO in the distilled acid can be estimated based on the acidity and the total amount of chlorides.

[0048] Table 1

[0049] The elements of Table 1 result of the process described wherein the spent acid comprises 6 weight percent of hydrochloric acid and 119 grams per litre of iron.

[0050] In this experiments, various concentration of sulphuric acid in the reactants mixture in step i. have been tested.

[0051] Then in step ii., the distillation has been performed in a rotatory evaporator at a temperature of 75°C and a pressure of 150 mbar. Then the composition of the concentrate has been studied to identify the concentration of iron, chlorides and sulphates depending on the concentration of sulphuric acid in the reactants mix in step i.

[0052] It can be seen that for a concentration of sulphuric acid below 10 weight percent, for example 4 weight percent, the yield of recovered chlorides is below 60 percent. Moreover, below 15 weight percent of sulphuric acid, which is the lower value of the preferred range, the yield of recovered chlorides is below 85%.

[0053] Apparently, above 40 weight percent of sulphuric acid, the Reaction 1 cannot take place.

[0054] Step iii

[0055] As illustrated in Figure 1, the concentrate is then mixed with water such that the weight ratio between the concentrate and the water is from 0.1 to 10 so as to obtain an aqueous solution comprising hydrated forms of iron sulphates. This aqueous solution may also comprise chlorides and other metal salts and / or metal oxides.

[0056] Apparently, if the ratio is lower than 0.1, the concentrate cannot be efficiently dissolved resulting in a greater concentration of chlorides in the solid phase in the next step. Apparently, if the ratio is greater than 10, the yield of the precipitation is severely reduced.

[0057] Preferably, the concentrate is then mixed with water such that the weight ratio between the concentrate and the water is from 1 to 3 so as to obtain an aqueous solution comprising hydrated forms of iron sulphates.

[0058] The water is preferably clean water but can be rinse water from a pickling process or an aqueous solution from step v.

[0059] Step iv

[0060] As illustrated in Figure 1, the aqueous solution is then set to a temperature from -10 to + 50°C so as to obtain a liquid phase comprising metal chlorides and metal sulphates and a solid phase comprising mainly hydrated forms of iron sulphate. This step permits to precipitate the ferrous sulphate heptahydrate. The aqueous solution is preferably set to a temperature from -5°C to 20°C and even more preferably from -5°C to 5°C. The solubility in water of iron sulphate is lower for lower temperature.

[0061] The liquid phase may also comprise metal acids, acids, such as H2SO4 and HC1, sulphates such as FeSCh and M^SC^x and metal salts such as FeCh, MC1Xwhere M is any metal present in the pickled steel and x its oxidation state. The liquid phase comprises water. The liquid phase may also comprise free hydrochloric and sulphuric acids

[0062] The solid phase comprises mainly iron sulphate heptahydrate. The solid phase may also comprise water, H2SO4 and HC1, other hydrated forms of FeSCh and M2(SC>4)x and metal salts such as FeCh and MClx where M is any metal present in the pickled steel and x its oxidation state.

[0063] Step v

[0064] As illustrated in Figure 1, the liquid phase and the solid phase are then separated. This separation can be performed by any means such as centrifugation, filtering or gravity separation.

[0065] The liquid phase essentially comprises water, hydrochloric acid, sulphuric acid, iron sulphate, iron chloride and / or other metal salts and / or metal oxides.

[0066] The solid phase comprises mainly iron sulphate heptahydrate. The solid phase may also comprise water, H2SO4 and HC1, other hydrated forms of FeSCh and Jv^SCh)* and metal salts such as FeCl2and MC1Xwhere M is any metal present in the pickled steel and x its oxidation state.

[0067] The claimed method permits to obtain a regenerated acid quasi without metal or sulphates. On the contrary, with the method from the patent WO 2017 017 494 Al, the regenerated acid of lower purity, i.e. comprising metals and sulphates.

[0068] Moreover, the claimed method permits to increase the purity of heptahydrate iron sulphate, by lowering the amount of chlorides, compared to existing process.

[0069] The method can comprise the additional step of mixing the solid phase obtained in step v with water such that the weight ratio between the solid phase and the water is from 0.1 to 10 so as to obtain a second aqueous solution comprising hydrated forms of iron sulphates. This second aqueous solution may also comprise metal chlorides, other metal salts and acids.

[0070] Apparently, if the ratio is lower than 0.1 the crystals concentrate cannot be dissolved. Apparently, if the ratio is greater than 10, the yield is severely reduced. In this additional step, the second aqueous solution is then set to a temperature from -10 to 40°C so as to obtain a second liquid phase comprising chlorides and a second solid phase comprising hydrated forms of iron sulphate. The second aqueous solution is preferably set to a temperature from -5 to 10°C.

[0071] The second liquid phase also comprises part of the remaining acids and / or metal salts in the solid with iron sulphate.

[0072] The second solid phase also comprises the rest of the remaining acids, chlorides and / or metals salts.

[0073] In this additional step, the second liquid phase and the second solid phase are separated. This separation can be performed by any means such as centrifugation, filtering or gravity separation.

[0074] The second liquid phase essentially comprises water, hydrochloric acid, sulphuric acid, iron sulphate, iron chloride and / or other metal salts and / or metal oxides.

[0075] The second solid phase mainly comprises hydrated form of iron sulphates.

[0076] This optional step permits to increase the weight percent of ferrous sulphate heptahydrate in the second solid phase obtained in step vi compared to the solid phase obtain in step v.

Claims

CLAIMS1. A method for treating a spent acid comprising 0.5 to 18 weight percent of hydrochloric acid and 60 to 350g.L4of iron, wherein the following steps are performed: i. mixing said spent acid with an aqueous solution of sulphuric acid, to effect Reaction (1),Reaction (1) FeCl2+ H2SO4+ n H2O ±5 FeSO4.nH2O + 2 HC1, so as to obtain a reaction mixture comprising from 5 to 40 weight percent of sulphuric acid, ii. distilling said reaction mixture so as to obtain a distillate comprising HC1 with a weight percent from 10 to 40, and a concentrate comprising essentially hydrated forms of ferrous sulphate, iii. mixing said concentrate with water such that the weight ratio between the concentrate and the water is from 0.1 to 10, so as to obtain an aqueous solution comprising hydrated forms of iron sulphates, iv. setting said aqueous solution to a temperature from -10 to 50°C so as to obtain a liquid phase comprising metal chlorides and metal sulphates metal salts and acids and a solid phase comprising mainly hydrated forms of iron sulphate, v. separating said liquid phase and said solid phase obtained in step iv.

2. The method according to claim 1, wherein the spent acid comprises from 14 to 22 weight percent of hydrochloric acid.

3. The method according to claim 1 or 2, wherein the distillation of step ii. is performed using a membranes distillation.

4. The method according to claim 1 or 2, wherein the distillation of step ii. is performed using a column distillation.

5. The method according to claim 1 or 2, wherein the distillation of step ii. is performed using a rotary evaporator.

6. The method according to any one of the claims 1 to 5, wherein the distillation of step ii. is performed at a temperature from 60 to 95°C.

7. The method according to claim 6, wherein the distillation of step ii. is performed at a temperature from 75 to 85°C.

8. The method according to any one of the claims 1 to 7, wherein the distillation of step ii. is performed at a pressure from 0.01 to 500 mbar.

9. The method according to claim 8, wherein the distillation of step ii. is performed at a pressure from 30 to 200 mbar.

10. The method according to any one of the claims 1 to 9, wherein the distillation is performed to obtain a distillate comprising essentially HC1 with a weight percent from 15 to 20.

11. The method according to any one of the claims 1 to 10, wherein in step iv., said aqueous solution is set to a temperature from -5 to 10°C.

12. The method according to any one of the claims 1 to 11, wherein in step v. the separation is performed by means of centrifugation.

13. The method according to any one of the claims 1 to 11, wherein in step v. the separation is performed by means of filtering.

14. The method according to any one of the claims 1 to 13, wherein the method comprises the following steps vi. mixing the solid phase with water such that the weight ratio between the solid phase and the water is from 0.1 to 10 so as to obtain a second aqueous solution comprising hydrated forms of iron sulphates. vii. setting said aqueous solution to a temperature from -10 to 40°C so as to obtain a liquid phase comprising chlorides and a solid phase comprising hydrated forms of iron sulphate, viii. separating said liquid phase and said solid phase obtained in step iv.

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