Process for removing iron oxide deposits

Abstract

The invention relates to a process for removing iron oxide deposits such as rouging, golding and blacking of stainless steel, in which a solution of alkali formate and alkali sulfite is brought into contact with the stainless steel. According to the invention, the alkali is at least predominantly potassium and the alkali sulfite is an alkali hydrogen sulfite, as a result of which virtually neutral pH values of 5-7 are nevertheless possible with good action. A hydroxycarboxylic acid, in particular lactic acid or glycolic acid, is preferably additionally present in the solution.

Description

METHOD FOR REMOVAL OF IRON OXIDE DEPOSITS Technical area

[0001] The present invention relates to a process for removing iron oxide deposits such as rouging, golding and blacking from stainless steel, wherein a solution of alkali formate and alkali sulfite is brought into contact with the stainless steel. State of the art

[0002] The most common form of corrosion is iron rusting. To prevent this reaction, steels are alloyed with a chromium content of at least approximately 10%. These stainless steels can also corrode, and their corrosion products are referred to as "golding," "rouging," or "blacking." These stainless steels, beginning with the numbers 1.40, 1.41, 1.42, 1.43, 1.44, 1.45, and 1.46, contain additional alloying components for their specific applications. For example, molybdenum additions of > 2% lead to increased corrosion resistance to chloride. A Ni-Cr content of 30% in the material is characterized by high corrosion resistance to strong acids such as sulfuric acid.

[0003] Even if these stainless steels are corrosion-resistant, corrosion by the medium with cumulative effects cannot be ruled out and is only a matter of time. For example, hot, deionized steam is used to sterilize production equipment in the pharmaceutical industry, but also in medical technology to sterilize surgical instruments. Surface corrosion occurs at the corresponding inlets of the containers, or iridescent tarnish forms across the entire surface of the sterile metal objects. Water for injection is disinfected at 80°C with ozone, among other processes, filtered, and used as the basis for all aqueous medicines. The European Medicines Agency (EMA) regulations apply in Europe for maintaining water quality in sterile medical applications, requiring only "purified water" or WFI to be used in all production steps in the pharmaceutical manufacturing industry.Further cumulative effects lead to other types of corrosion (e.g. contact corrosion, stress corrosion cracking) when hot, ultra-pure water is used in stainless steel systems. In biotechnology, chlorides are also used extensively at temperatures of 30–60°C, which are a source of corrosion. For example, guanidine chloride is used to denature or unfold proteins. Halides lead to increased pitting corrosion on rough stainless steel surfaces. Furthermore, crevice corrosion occurs in fine-mesh filters or chromatography columns due to poor rinsing properties. Biocorrosion is becoming increasingly common in bioreactors and the food industry (e.g. beer). In both the food industry and in restaurant kitchens, salty mixtures, temperatures up to boiling point, organic acids and microorganisms, among other things, lead to almost all possible types of corrosion in the stainless steel materials used.

[0004] Generally, the corrosion products of stainless steel, known as "blacking," are observed in the vapor phase above 100°C, either over the entire surface of the affected metal or directly at the liquid level in the form of a blue-burgundy-black ring on a container wall. This primarily crystallizes the spinel magnetite FeFe2O4, and to a lesser extent the spinel chromite FeCr2O4 at the weld seams. "Rouging" is generally observed in all hot aqueous liquid systems from 50°C to 100°C, but also as an oxidation product of "blacking" according to the reaction 2 Fe II Fe III 2O4 + ½ O2 → 3 Fe III2O3 above 100°C. The main component is hematite Fe2O3, which grows in trigonal crystal form and is found in all shades of red. "Golding" is found predominantly in all cold aqueous systems between 20°C and 50°C in the form of various iron oxide hydrates FeO(OH) in golden-yellow-brown hues. "Golding" is the hydration product of rouging according to the reaction: Fe2O3 + H2O → 2FeO(OH) and can therefore only occur in aqueous systems. The main components of "golding" are the minerals goethite, limonite, lepidocrocite, or, with chloride inclusions, akaganeite FeO(OH, Cl).

[0005] Regardless of the specific application, corrosion residues are undesirable and even prohibited. Chemical, pharmaceutical, or food processing industrial plants can be restored to their original condition with the help of a wet-chemical process by stopping corrosion before the corrosion products spread and eliminating it from the system. Ideally, this occurs at a time when no significant material attack has occurred and the electropolishing of the base material is still intact. This process selectively dissolves a wide variety of corrosion products using wet chemicals and then professionally disposes of them.

[0006] In general, all iron oxides containing trivalent iron (i.e., rouging and golding) can be completely dissolved with derouging media. However, blacking consisting of divalent and trivalent iron can only be partially removed with these media. This usually requires an additional process step, but in the case of the sole presence or precise characterization of blacking, a single process step may suffice.

[0007] A process of the type mentioned above is known from WOWO 2017 / 029405A. According to this document, in Examples 1 and 3, a solution is prepared as follows: 8 kg of 50% NaOH and 7 kg of 85% formic acid are added to 79 kg of water. Then, 5 kg of solid sodium sulfite are added. The pH is then approximately 6. Finally, immediately before use, another 1 to 2 kg of 85% formic acid are added until a pH of 4.5 or 4.1 is reached.

[0008] According to Example 4, a solution with a pH of 4.5 is immersed at 70°C for 2 hours or left to act at 70°C for 1 hour. In the spraying process, a solution with a pH of 4.1 is left to act at 80°C for 4 hours or left to act at 80°C for 2 hours.

[0009] The idea of ​​adding formic acid immediately before use is likely based on the fact that this releases sulfur dioxide SO2 (see formulas 3 and 4 on page 5), similar to how carbon dioxide CO2 is released when carbonic acid is added. The iron compounds to be removed are then reduced by sulfur dioxide (formula 5 on page 5) and subsequently dissolved by formic acid. The fact that sulfur dioxide is only released from sulfite through dehydration at a pH below 4.8 explains why pH values ​​of up to 4.8 are claimed in the patent claim of this document.

[0010] The chemical formulas on page 5 indicate that "alkali sulfite" refers to the normal, neutral, or secondary sulfite: Na2SO3 (Formula 2 on page 5). NaOH and formic acid react to form sodium formate (Formula 1 on page 5). Thus, the use of alkali formate and alkali sulfite is known, as expressed by the generic term of patent claim 1.

[0011] A disadvantage of this derouging solution is that it is relatively acidic, which poses problems both in terms of occupational safety and environmental friendliness during disposal. The relatively low pH value is caused by an excess of formic acid. This excess formic acid partially evaporates during use due to the high working temperature of 70°C or 80°C, which also poses a problem in terms of occupational safety. Therefore, closed systems and a protective atmosphere are required, which makes visual in-process control impossible. Complex accompanying analysis is therefore necessary. Description of the invention

[0012] It is an object of the present invention to eliminate these disadvantages.

[0013] This object is achieved according to the invention by a process of the type mentioned at the outset in that the alkali is at least predominantly potassium, that the alkali sulfite is an alkali hydrogen sulfite and that the pH is 5-7.

[0014] According to the invention, the sulfite used is a hydrogen sulfite, also called primary or acid sulfite or bisulfite, and not, as in the known process, a normal sulfite (also called neutral or secondary sulfite), i.e. AHSO3 instead of A2SO3 (where A stands for alkali), in particular KHSO3 instead of K2SO3 (when potassium is used as alkali).

[0015] Although the alkali cations do not participate in the reactions, the inventors have found in numerous experiments that using only sodium as the alkali does not produce satisfactory results at pH values ​​above 5. However, if potassium is used as the alkali, the result is acceptable. It is crucial that at least the majority of the alkali, i.e., at least 50 mol%, is potassium. However, it is possible that some of it is sodium, which clearly provides a cost advantage. The reason for this could be that potassium salts are significantly more water-soluble than sodium salts.

[0016] In short: By using potassium as an alkali, it becomes possible to carry out the known process at higher pH values.

[0017] The ratio of K and Na is adjusted so that the solubility of the alkali bicarbonate is not exceeded in the range of 25 to 1.8 wt.%.

[0018] Due to the relatively high pH range of 5-7, which often increases during the reaction, to approximately 6-7, the possibility of visual process control without danger to the environment and the process becomes possible, especially in the immersion process.

[0019] Surprisingly, it was found that even at these higher pH values, sulfur dioxide is formed, which is essential for derouging.

[0020] Without being bound to these theories, the inventors suspect the following two mechanisms: A) Mechanism 1

[0021] In a pH range of 5 to 7, the oxidizing agent hydrogen sulfite is reduced to hydrogen hyposulfite, i.e. to AHSO2 (in the example where alkali stands for potassium, i.e. to KHSO2): HCOOA + AHSO3 → AHSO2 + AHCO3(I)

[0022] Hydrogen hyposulfite is a highly reactive reducing agent, which greatly increases reactivity, meaning that less concentrated solutions or shorter exposure times are required to remove the deposits.

[0023] Just as sulfite releases sulfur dioxide upon acidification to a pH below 4.8, sulfur monoxide is released from hyposulfite upon acidification, but already at a pH of 7:AHSO2 → SO + AOH (II)

[0024] And sulfur monoxide reacts immediately to form sulfur dioxide, e.g. with atmospheric oxygen: SO + ½ O2 → SO2(III)

[0025] This also produces sulfur dioxide, which is also a highly reactive reducing agent.

[0026] The formation of alkali bicarbonate AHCO3 according to formula (I) is favorable. Alkali bicarbonate AHCO3 forms in the pH range of 5-7 from alkali formate, and also by reaction with oxygen in water, but also with atmospheric oxygen in partially flooded containers: HCOOA + 1 / 2 O2 → AHCO3(IV)

[0027] By using the reducing agent alkali formate, its oxidation is forced to convert it into an alkali bicarbonate.

[0028] Alkali bicarbonate is advantageous because it is further converted into carbon dioxide: AHCO3 ↔ CO2 + AOH (V)

[0029] This creates a protective gas atmosphere during atomization, making the derouging medium insensitive to oxygen during the spraying process. Therefore, not only is oxygen consumed, but CO2 is also formed.

[0030] As a result of the consumption of oxygen, oxidation of (hydrogen)hyposulfite to (hydrogen)sulfite and further to (hydrogen)sulfate is largely prevented; the equilibrium lies on the side of (hydrogen)hyposulfite. B) Mechanism 2

[0031] Another mechanism that is also suspected is that the formation of slightly water-soluble SO2 in the pH range of 5-7 in the presence of an alkali formate can also occur catalytically: (VI)

[0032] It is advantageous if the solution also contains a hydroxycarboxylic acid or amino acid, for example, in an amount of 0.2 to 3 wt.%. This removes the iron ions from the reaction process. This hydroxycarboxylic acid or amino acid can be present in the solution from the beginning, making the process user-friendly. Lactic acid and glycolic acid are preferred, but citric acid, ascorbic acid, and glycine are also possible.

[0033] Suitable concentrations are, for example: 9.35 mol% potassium formate 2.35 mol% potassium hydrogen sulfite 1.66 mol% lactic acid

[0034] Or:0.19 mol% potassium formate0.04 mol% potassium hydrogen sulfite0.034 mol% lactic acid

[0035] High concentrations are used in the spraying process, low concentrations in the dipping process.

[0036] The following reactions take place according to the invention (A stands for alkali, X for the salt of the hydroxycarboxylic acid, e.g. lactate): Degolding reaction I with Fe(III) reduction, oxidation of hyposulfite to hydrogen sulfite

[0037] 2 Fe III O(OH) (s) + AHS II O 2(aq) + 2 S IV O 2(aq) + 2 AX (aq) + 2 HX (aq) →2 Fe II X 2(aq) + 3 AHS IV O 3(aq) + H2O (VII) Derouging reaction I with Fe(III) reduction, oxidation of hyposulfite to hydrogen sulfite

[0038] Fe2 III O3(s) + AHS II O 2(aq) + 2 S IV O 2(aq) + 2 AX (aq) + 2 HX (aq) →2 Fe II X 2(aq) + 3 AHS IV O 3(aq) (VIII) Degolding reaction II with Fe(III) reduction, oxidation of sulfur dioxide to hydrogen sulfate

[0039] 2 Fe III O(OH) (s) + 2 S IV O 2(aq) + 2 AX (aq) + 2 HX (aq) →2 Fe II X 2(aq) + AHS VI O 4(aq) + AHS IV O 3(aq) + H2O (IX) Derouging reaction II with Fe(III) reduction Oxidation of sulfur dioxide to hydrogen sulfate

[0040] Fe2 III O 3(s) + 2 S IV O 2(aq) + 2 AX (aq) + 2 HX (aq) →2 Fe II X 2(aq) + AHS VI O 4(aq) + AHS IV O 3(aq) (XIV)

[0041] After these reactions, the reactant, hydrogen sulfite, is partially regenerated. This represents a significant economic advantage over previous derouging media.

[0042] sustainability

[0043] With a single application of the derouging medium at concentrations of 2 wt.% using the flooding method or ten applications at concentrations of 20 wt.% using the spraying method, this process, with the food additives used, is both economical and ecological. The product is a mixture of pH-neutral, harmless salts (alkali bicarbonates, excess alkali formate, alkali bisulfates, Fe(II)X2, e.g., iron(II) dilactate, iron(II) diformate, CO2). The non-toxic iron compounds, for example, increase the productivity of the upper layers of the oceans. Potassium compounds and CO2 enhance the photosynthetic performance of plants. Bicarbonates are a starting material for mussels and snails. In general, the use of these food additives and the resulting products can be expected to offer significant benefits for health, safety, and the environment. Best way of carrying out the invention

[0044] The present invention is explained in more detail using the following comparative experiments.

[0045] A derouging solution according to Examples 1, 3 and 4 of WO 2017 / 029405 A1 ("Solution B") and a derouging solution according to the invention ("Solution A") are compared.

[0046] The "Solution A" is made from two components, "Component A" and "Component B".

[0047] Production of 100 kg of "Component A" (density = 1.535 g / cm 3 ):93.8 kg of a 75% potassium formate solution are mixed with 6.2 kg of an 80% lactic acid solution.

[0048] Production of 100 kg of "component B" (density = 1.305 g / cm 3 ):11.8 kg of an 80% lactic acid solution is diluted with 13.1 kg of water. This dilution is mixed with 34 kg of a 45% potassium sulfite solution, and this mixture is mixed with 41.1 kg of a 38% sodium hydrogen sulfite solution.

[0049] The composition of the resulting solutions is now as follows (all proportions rounded to tenths of a percent):

[0050] Component A: Potassium formate 70.3% Lactic acid 5.0% Water 24.7%

[0051] Component B: Sodium hydrogen sulfite 15.6% (equivalent to 12.0% sulfite) Potassium sulfite 15.3% (equivalent to 7.8% sulfite) Lactic acid 9.4% Water 59.7%

[0052] It contains 19.8% sulfite.

[0053] To prepare "Solution A" we now work by mixing by volume.

[0054] 5.3 liters of water (equivalent to 5.3 kg) are added. Then, 1 liter of component A (equivalent to 1.535 kg) is added and mixed. Next, 1 liter of component B (equivalent to 1.305 kg) is added and mixed. This results in 8.14 kg.

[0055] This contains 1.305 kg x 19.8% = 0.258 kg of sulfite. This corresponds to 3.2%. The solution according to WO 2017 / 029405 A1 ("Solution B") also has a sulfite content of 3.2%.

[0056] The tests with "Solution A" and "Solution B" were conducted at the same process temperature (70°C and 40°C) and on rouged material samples in the same location. The minimum exposure time for successful rouging was determined on three different material samples in order to compare the effectiveness. The pH of Solution B (pH = 4.5 in the immersion process) was then adjusted to that of the inventive Solution A (pH = 5.4) (referred to as "Solution C"), and the procedure was repeated as described above.

[0057] If no effect is achieved after 300 minutes of exposure, this is considered a failure ("not good" in the following table), if no effect is achieved at all, this is referred to as "not good".

[0058] Material sample: W1: Material: 1.4571, vessel wall [BW], non-wipeable rouging) W2: Material: 1.4435, secondary side steam line [SDL], non-wipeable rouging) W3: Material: 1.4404, plate heat exchanger [PWT], wipeable rouging)

[0059] Measured values Table 1 123456SolutionABCABCTemperature70°C70°C70°C45°C45°C45°CpH5,44,55,45,44,55,4W1t [min]10545240n.iOkRW2t [min]16740n.iOniOkRW3t [min]6630135135k.R.

[0060] At 70°C, a certain superiority of solution B over solution A is evident, but this comes at the cost of a lower pH value. If the pH value of solution B is raised to that of the inventive solution A, the inventive solution A is clearly superior. This becomes even more evident at 45°C (which, however, is not an optimal temperature for either solution), where solution A performs better than solution B for material W1 and equally well for material W3. Material W2 cannot be cleaned with any of the three solutions at 45°C.

Claims

Method for removing iron oxide deposits such as rouging, golding and blacking from stainless steel, wherein a solution of alkali formate and alkali sulfite is brought into contact with the stainless steel, characterized in that the alkali is at least predominantly potassium, that the alkali sulfite is an alkali hydrogen sulfite and that the pH value is 5-7. The method according to claim 1, characterized in that the solution additionally contains a hydroxycarboxylic acid. The method according to claim 2, characterized in that the hydroxycarboxylic acid is selected from lactic acid and glycolic acid. Method according to claim 1, characterized in that the solution additionally contains an amino acid, in particular glycine.

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