FI120504B - Electrochemical purification method and purification unit - Google Patents

Description

Electrochemical purification method and purification unit

The present invention relates to a process for cleaning contaminated electrically conductive surfaces or electrochemically retaining electrically conductive surfaces wherein the structure comprising the surfaces to be cleaned is immersed in a cleaning solution comprising an aqueous solution of an inorganic acid, an inorganic base or an inorganic salt.

10 Contamination of process heat exchangers, such as the formation of precipitates on their surfaces, can be a major problem, e.g. due to the difficulty of removing dirt and the cost of removal. In some cases, the dirty heat exchanger surfaces of a plate heat exchanger may not be cleaned sufficiently and therefore, some plates will have to be scrapped and decommissioned.

15

There are several different solutions to the precipitation problems of surfaces such as heat transfer surfaces of heat exchangers. By far the most widely used solution is to clean heat exchangers. In washing, the changers are opened and typically washed with water containing detergent at very high pressure. Washing can also be carried out without pressure, provided that the precipitate can be easily dissolved.

Recently, the use of electrochemical methods for cleaning heat exchanger surfaces has also been investigated, but these methods have been simple and optimal efficiency has not been achieved.

RU publication 2152575 discloses a tube cleaning apparatus operating on a pulse current.

U.S. Pat. No. 4,396,434 discloses a method for cleaning heat exchangers wherein the purification process is carried out by injecting an aqueous solution of ammonium nitrate and potassium nitrate into the apparatus and removing the precipitate by means of a sound source.

JP 2004176088 describes a plate-type heat exchanger for removing or preventing the formation of deposits. Anode and cathode plates are provided with direct current from a power source.

2 JP 09041200 discloses enhancing the cleaning of iron plates. The purification is carried out by electrolysis using an aqueous solution of sodium sulfate and an organic acid as the electrolyte, whereby hydrogen gas is formed on the cathodic side and sulfuric acid on the anodic side.

US publication 2004025005 discloses protection of a plate heat exchanger against sulfuric acid corrosion. Shielding is achieved by providing anodic shielding by providing a DC power supply to the equipment.

10

One problem with high-pressure cleaning of heat exchangers is that the high pressure washer increases surface roughness, which in turn increases the fouling rate.

Often the surfaces become so dirty and the deposits adhere to the surface so tightly that the changer-15 slots must be opened and the plates removed from the factory to a separate service center where cleaning is carried out. Cleaning at the service center may take more than a week. In some cases, the surfaces remain so dirty that even with this treatment they are not clean enough and heat exchangers must be rejected.

Precipitates which are difficult to purify are generally those which are insoluble in aqueous solutions used in processes or separate purifications. Common examples of such insoluble precipitates are titanium oxides.

The problem with prior art electrolysis purification methods is 25 mm. limitations on their applicability. Most are designed specifically for cleaning plate heat exchangers. Cleaning methods for tube heat exchangers, on the other hand, are not suitable for plate-shaped rigid systems. In addition, in most prior art processes, the processes must be terminated and the heat exchangers opened prior to the start of purification. Often cleaning is even carried out elsewhere, whereby the heat exchangers have to be transported to the cleaning site.

The object of the present invention is to eliminate at least some of the problems associated with the prior art and to provide a solution which allows relatively easy, fast and efficient cleaning of heat exchangers and similar structures to be cleaned, while keeping the cost low and maintaining the structures to be cleaned.

The present invention relates to a process for cleaning contaminated electrically conductive surfaces or electrochemically retaining electrically conductive surfaces wherein the structure comprising the surfaces to be cleaned is immersed in a cleaning solution of an inorganic acid, an inorganic base or an inorganic salt, an aqueous solution, that - at least two electrodes are placed in the cleaning solution, each of which may consist of either a part of the structure to be cleaned or a counter electrode, and - a pulse current is applied to the cleaning solution via the electrodes.

More specifically, the method of the present invention is characterized by what is set forth in the characterizing part of claim 1.

15

The cleaning unit according to the present invention, in turn, is characterized by what is stated in the characterizing part of claim 26.

Advantages of the process include its speed and its adaptability to various precipitates and surfaces to be cleaned. The method can quickly and efficiently clean metallic or, in principle, other electrically conductive surfaces contaminated by electrochemical processes either in a separate cleaning unit or, in some cases, in the process solution in which they become soiled. Suitable surfaces include, for example, heat transfer surfaces of heat exchangers. The invention can also, if desired, prevent harmful precipitation of heat exchangers or similar metallic or otherwise electrically conductive surfaces in solution under process conditions. This cleaning or prevention of precipitation can, if desired, be carried out even without opening the heat exchanger. The process can purify a wide variety of precipitates, but the purification process itself in solution typically takes up to a few tens of minutes. In addition, the surfaces are completely bleached without increasing their roughness.

The above advantages are readily achieved by selecting the correct one for the removal of the particular precipitate from the cleaning solution and electrode options mentioned below.

4

Other details and advantages of the invention will be apparent from the following detailed description, which includes a number of Examples of Use.

Figure 1 shows the current density of sulfuric acid purification as a function of time with anodic 5 voltage of + 3 V vs. SCE (duration 30 s) and cathodic voltage of -0.8 V vs. SCE (duration 30 s) (10 wt% H 2 SO 4, 60 ° C).

Figure 2 shows the current density of purification in sulfuric / citric acid solution over time with anodic voltage of +3 V vs. SCE (duration 30 s) and cathodic voltage of -0.8 10 V vs. SCE (duration 30 s) % H2SO4 + citric acid, 60 ° C).

Figure 3 shows the current density of purification in the process solution as a function of time when the anode voltage is +3 Y vs. SCE (duration 45 s) and the cathode voltage is -0.8 V vs. SCE (duration 15 s) (60 ° C).

15

Figure 4 shows the current density of purification of sulfuric / citric acid in solution as a function of time after determining the minimum current density at which the precipitate is released (10% by weight EESOV citric acid, anode voltage +4 V vs. SCE for 30 s and cathodic voltage -0.5 V vs. SCE 30 s, 25 ° C) 20

Figure 5 shows the purification potential of sulfuric / citric acid in solution as a function of time after determining the minimum potential to precipitate out (10% w / w EESOV citric acid, current density ± 1 mA / cm 2 for 60 s / 60 s, 25 ° C).

Preferably, the surface to be cleaned by the process of the present invention is a heat exchanger, such as a plate or tubular heat exchanger, or a part thereof.

'' Heat exchanger '' is a device used in industry for example. for varying cooling and heating processes and pasteurization. Heat exchangers are common e.g. in breweries 30 and in dairies for pasteurization of liquids for both cooling and heating. They are also used for food and pharmaceutical products. The operation of air coolers and fans may also be based on heat exchangers. Other applications include e.g. condensers and evaporators. Common heat exchanger models include plate heat exchangers and tube heat exchangers. They can be made from almost any electrically conductive material.

In plate heat exchangers, the heat transfer surface is formed by profiled duct plates. The edges and corners of each duct plate are rotated by a seal made of a suitable material. The various plate designs adapt the heat exchanger to suit each task. In the plate heat exchanger, the substances flow in the flow soles formed by the duct plates. The first substance is conducted to every other passage and the second (substance) is passed between the preceding passages.

10 The advantage of tube heat exchangers over plate heat exchangers is that they require less coolant eg due to their compact external dimensions.

'' Electrolysis '' is the production of a chemical reaction by passing an electric current through an electrolyte. The reaction is an oxidation-reduction reaction in which a substance, for example an electrolyte, decomposes into positively dense and negative ions. The reaction takes place in an electrochemical cell at the interface between electrodes and solutions. The electrode undergoing oxidation is called the "" anode "and the electrode undergoing reduction is called the" "cathode. In electrolysis, the positive ions are first transported to the cathode where they are reduced, while the negative ions are transported to the anode where they are oxidized.

20

If there is no external voltage source between the electrodes, it is a "galvanic cell" where the oxidant (or cathode) draws electrons through a conductor from the reductor (anode). Electrons cause a reaction on the cathode, whereby negative ions are formed on the cathode. At the same anode, positive ions are formed due to electron deficiency. For example, the reaction may be as follows: 2H + (aq) + Zn (s) -> Zn2 + (aq) + H2 (g)

The force that pulls electrons in a conductor is called the potential E of the cell, or electromotive force (SMV). The galvanic cell has a positive potential:

AG = -nFE

6

In order to utilize the energy released in the oxidation-reduction reactions, the reagents must be separated. This is generally done either by a salt bridge or a membrane. Thus, electrochemical cells which serve to generate electrical energy or certain ions have two sides, i.e., the '' anodic side '' and '' the cathodic side.

5

Common electrode materials include metals such as platinum, titanium, lead stainless steel. Examples include inert electrodes, such as platinum wires, and metals coated with their sparingly soluble salt. In order for the reaction to occur, the electrodes must be placed in the solution, i.e. the '' electrolyte ', where the reaction is to take place.

10

Corrosion is the corrosion of objects, usually metal objects. In chemical corrosion, corrosion is caused by oxidation-reduction reactions. The compounds reacting in electrochemical corrosion are not in direct contact with each other, i.e. they are separated by, for example, the salt bridge or membrane mentioned above. The pure form of the metals is mod-15 thermodynamically unstable, ie corrosion occurs.

When the potential of the electrochemical cell is deviated from its equilibrium value (its thermodynamic value), the "net current" flows in the cell. The change in the potential of a cell from its equilibrium value due to current travel is called polarization. The magnitude of polarization 20 is represented by the over-potential, which is the deviation of the potential from its equilibrium value: η = E - Etp

In an embodiment of the invention, precipitation is removed, or precipitation is prevented, e.g., by "anode-polarization" or "" cathodic polarization ", which terms mean to change the charge of the surfaces to the same sign as the charge of the particles of the precipitates. For example, anodic polarization, which is achieved by an '' anodic current '', that is, a positive current, causes the precipitates to oxidize (the pH drops), whereupon carbonates are dissolved, for example. The '' cathodic current '' causes the opposite reaction. The polarization can be either long-lasting 30 or can be caused by generating a '' pulse current ''.

In the '' Galvanostatic '' method the current is kept constant, whereas in the '' Potentiostatic '' or '' Potential '' the voltage is kept constant.

7

In the embodiment of the invention, the cleaning solution used for electrochemical cleaning is selected based on both the corrosion resistance of the material to be cleaned, in this case the heat transfer surface, and the chemical composition of the precipitate on the surface. In principle, any solution having a conductivity sufficient to effect the required electrochemical reaction is suitable for the process. If the conductivity of the solution is poor, the method may possibly still be applied by placing the electrodes closer together, if possible. In this case, the working electrodes and counter electrodes must also have substantially the same surface area. Otherwise, the current distribution will be uneven.

10

The choice of solutions and their concentrations takes into account not only the cleaning performance, but also any occupational safety factors as well as the corrosion resistance of the material to be cleaned at the concentration used in that solution. For occupational safety, less than 5% sulfuric acid can be treated as an aqueous solution and not as an acid solution. Often, however, more concentrated solutions are required. On average, acid solutions of the order of 10% are sufficient for most of the precipitates to be used in electrochemical purification. Again, the material to be cleaned must have sufficient corrosion resistance in the solution to be cleaned to prevent the surfaces from being damaged by corrosion during electrochemical cleaning.

Based on the electrochemical properties, in the embodiment of the invention, the current density and / or potential used in material purification is selected. Stainless steels and titanium have different currents used for electrochemical cleaning. In the purification of titanium, the cathodic current is of great importance as compared to the anodic current, and accordingly, in the cleaning of stainless steels, the anodic current is of greater importance than the cathodic current. The key to this pulsing, however, is that it causes the current densities to rise significantly (potentiostatic method) or potentials (galvanostatic method). This in turn promotes the formation of gases (such as H2 and O2).

In the embodiment of the invention, factors affecting the method of preventing electrochemical precipitation under process conditions are evaluated by the same methods as cathodic and anodic shields. The implementation of the method, for example in heat exchangers, is influenced by several factors, the most important of which are related to the type and structure of the heat exchanger and the solution environment. Absolute prerequisites are to have both electrodes 8 in the same solution close enough to one another and an electrical conductor between the anodes and cathodes. Other critical factors include sufficient conductivity of the solution, as well as the geometry of the heat exchanger and the area at which it is desired to prevent precipitation.

5

According to a preferred embodiment of the invention, cleaning of surfaces such as heat exchanger plates is carried out in a separate cleaning unit by placing the plates in a warm acid solution, which preferably comprises an inorganic acid with the addition of a suitable complexing agent (e.g. citric acid). The plates are placed in a clean 10 sink so that they are not in contact with each other. Every other plate is connected to the anode and every other plate to the cathode. In addition, one or more reference electrodes may be applied to the bath solution as required. A pulse current is connected between the anodes and the cathodes so that the different electrodes are alternately anodes and alternately cathodes, whereby purification at each electrode is effected alternately by the anodic current and the cathodic current.

The temperature during the purification process is maintained below the boiling point of the solution used, preferably between 40 and 60 ° C. Preferably, the temperature during the process is 60 ° C. High temperatures are effective for cleaning efficiency, but if the temperature is raised too close to the boiling point of the solution, evaporation will begin to interfere with the process.

The method is based on the pulsed alternation of the electrical potential between the anodic and cathodic potentials. Preferably, this polarity of the electrodes is changed at intervals of about 10 to 50 seconds, more preferably at intervals of about 15 to 45 seconds, most preferably about 30 seconds. Deposits that do not reduce or dissolve during polarization are also efficiently and rapidly removed from the surface. Either hydrogen evolution in cathodic polarization or oxygen evolution in anodic polarization mechanically removes these impurities from the metal surface. In addition, rapid reversal of polarization direction contributes to the release. The evolution of gases, especially BLm and C > 2, is essential for the purification. It is precisely by pulsing that it can be enhanced. If it is desired to prevent the formation of precipitates instead of removing the formed precipitates, the pulse times can be extended up to several hours, but in this case the current densities are often reduced.

For example, if the cathodic potential corrodes the material to be cleaned more efficiently than the anodic potential, the duration of the cathodic current can be reduced. To provide a cathodic 9 current, the potential is lowered below the hydrogen generation potential. Depending on the material, the potential can be raised to higher potentials than oxygen development to provide an anode current. One important parameter in the purification process is usually a sufficiently high cathodic current.

5

The current densities in the process are maintained between 1 and 15 mA / cm 2, preferably between 2 and 7 mA / cm 2. The potentials, in turn, are held between -0.8 and -0.5 V vs. SCE (Saturated Calomel Electrode) at the cathode and between 2 and 10 V vs. SCE at the anode. When using a separate counter electrode, such as a lead electrode, current densities increase.

10 The cleaning unit used in this embodiment comprises: 1 a basin 2 a suspension system for the electrodes 3 a pulse current generating power supply 15 4 a harmful gas removal system

The purification unit may also comprise one or more reference electrodes and a solution recycling and purification system.

According to another preferred embodiment of the invention, the surfaces, for example in a heat exchanger, are cleaned in a process. This can be done when the process solution is suitable for purification, e.g. sulfuric acid solutions. In plate heat exchangers, the plates are in contact with one another, which requires separate counter electrodes to be used for cleaning in this embodiment, for example, placed in the channels of the plate heat exchangers where the process solution flows. Optional reference electrodes can also be placed in these channels. Counter-electrodes can also be installed between plates if the conductivity of the solution is poor.

A similar installation method is used to protect plate heat exchangers with either anodic or cathodic protection.

If gases (such as hydrogen) or precipitates from the surface of the plates cannot be allowed to enter the process solution or if the process solution is not suitable for cleaning, the heat exchanger must be cleaned by removing it from the process circuit. If a large amount of sediment comes off the surface, the solution should be filtered, for example. Also, large amounts of hydrogen that may be formed must be properly treated.

The cleaning unit used in this second embodiment comprises: 1 counter electrodes of a pool 2 for the electrodes of the hanging system 3 counter-electrodes of a power supply generating a pulse current 4 5

The purification unit may also comprise one or more reference electrodes and a solution recycling and purification system.

According to a third preferred embodiment, the formation of deposits is prevented by the same method as the deposits according to the above-mentioned embodiments are removed from the surfaces. However, lower current densities during polarization are sufficient to prevent precipitation. The temperature during this inhibition is maintained at the same as under normal process conditions or may be increased slightly if possible.

Current densities for inhibiting precipitation are of the order of -5 to +5 mA / cm 2, preferably about -0.5 to ry +0.5 mA / cm and most preferably about -0.1 to + 0.1 mA / cm, while the potentials for precipitation are are of the same class as the precipitate removal.

25

For example, the formation of precipitates on the titanium surface can be prevented by a pulsed current of TiCl 2 containing sulfuric acid having a potentiostatic anodic polarization (2 V vs. SCE) duration of 24 hours and a cathodic polarization duration of 2-3 minutes (-0.7 to -0.8 V vs. .SCE). The surfaces are kept clean at 90 ° C for at least 11 days. 30 Anodic polarization alone slows down but not completely prevents precipitation. When cathodic polarization is used, there is a risk of titanium corrosion. If the cathodic polarization is enhanced or the polarization time is extended, titanium corrosion can occur.

11

In the embodiment of the invention, the formation of precipitates is also prevented in laboratory scale tube heat exchangers. A wire-like counter electrode is placed inside each tube. Titanium or stainless steel, preferably mixed oxide coated titanium, are suitable as the counter electrode material. Precipitate formation can be prevented by providing pulse current to only a portion of the tubes, but the power supply must be high. There are no other obstacles to its use. For example, the formation of calcium sulfate-based precipitates can be prevented by using galvanostatic polarization for 10 minutes (± 5 mA / cm 2, 30 s / 30 s), after which the tube is kept unpolarized for 50 minutes.

According to a preferred aspect, prior to electrochemical cleaning, the surfaces are pretreated by heating them in an oven. This method is particularly applicable in cases where the dirt layers consist of a dense relatively thick carbonate layer. By this the purification is considerably enhanced, after which the precipitates are removed in both alkaline and acidic solutions. In practice, the pretreatment is carried out in an oven at a temperature of about 650 to 700 ° C, preferably close to 700 ° C, and the treatment time can be up to 10 minutes.

The process according to an embodiment of the invention can be carried out in a cleaning solution which is acidic, e.g. in aqueous acidic solution, preferably sulfuric or phosphoric acid, the effectiveness of which may be increased by adding a complexing agent, e.g. citric acid, to the solution. The acid present in the solution must be such that the metal to be purified has a low corrosion rate and, if corrosion occurs, is uniform and not local. Sulfuric and phosphoric acids are such acids in which the most common austenitic stainless or duplex steels and titanium have low corrosion rates and the corrosion occurs as a continuous corrosion. The concentration of the aqueous solution of the inorganic acid is preferably about 5 to 30% by weight, more preferably about 5 to 25% by weight and most preferably about 5 to 10% by weight. Complexing agents (eg, citric acid, EDTA, sulphonic acid or the like complexing agent or a mixture of two or more complexing agents) increase the efficiency of the acid by binding surface dissolved compounds or metals to form complexes therewith. These are preferably added to the solution at about 0.5 to 5 g / L, more preferably about 0.7 to 2 g / L, most preferably about 1 g / L.

12

The choice of acid is also affected by the chemical composition of the precipitates. The precipitates need not be dissolved in the cleaning solution. All they have to do is remove them from the surface and then remove them by filtration.

Oxidic precipitates are best removed in these acidic solutions.

In an embodiment of the invention, a basic solution, e.g. an inorganic basic aqueous solution, preferably an aqueous solution of sodium hydroxide (NaOH), may also be used for the purification. The concentration of the aqueous solution of the inorganic base is preferably about 5 to 50% by weight, more preferably about 5 to 25% by weight and most preferably about 5 to 10% by weight. With alkaline solutions, for example, sulphate and carbonate based dirt layers are best removed. However, the chemical stability of the precipitates does not directly assess the effectiveness of the cleaning solution in electrochemical cleaning. The structure and, for example, the density and thickness of the precipitating layer significantly influence the cleaning properties. In practice, it may be very difficult to purify thick 15 and, in principle, relatively easily soluble compounds, such as Ca-CC 3, due to the dense and thick structure of the precipitate. Crystalline precipitates have been found to be more difficult to purify than amorphous ones.

An aqueous solution containing an inorganic salt is also a cleaning solution. The saline solution may advantageously be sea water or a saline solution chemically equivalent thereto, wherein the solution contains one or more inorganic salts. The salts used in aqueous solutions are preferably substantially water soluble salts of alkali metals or alkaline earth metals. Preferably the salts are halides, nitrates, sulfates, carbonates, borates or phosphates or combinations thereof, more preferably halides or combinations thereof, most preferably chlorides.

Organic salts would also be suitable for use in the process of the present invention, but due to their adverse environmental effects, organic salts are avoided. The salt content in the purification process is 5 to 50% by weight, preferably 5 to 25% by weight, most preferably 5 to 10% by weight.

The purification process can also be carried out without changing the solution, that is, as the electrolyte in the electrochemical purification, a process solution can be used which may be the same concentration as the one used in the conventional process of the apparatus. If this solution is too concentrated (for example, would cause corrosion under the conditions available for cleaning), it can be diluted. A suitable concentration of the dilution may be, for example, 10-80% by weight of the stock solution, preferably 20-70%, most preferably 40-60% by weight. This embodiment is suitable for use, for example, if the process solution comprises the aforementioned acids, bases or salts. Generally, process solutions also include solutes therein or solids at the base, such as carbonates, for example derived from liberated precipitation. Up to 80% by weight of the process solution may consist of solids. In this case, the water solution is 20% by weight or more.

In the implementation of electrochemical purification, two electrode options are used in the embodiment of the invention. According to one aspect of the invention, the material to be purified is used as both anode and cathode. In another aspect, the inert electrode material is selected as the second electrode material. In this case, the same materials as those used, for example, in cathodic shielding as an anode can be used as the counter electrode material. The electrode material used must not corrode in the cleaning solution and must withstand pulsing conditions. Such materials include, for example, various metals and alloys such as platinum, titanium, coated titanium, lead, stainless steel, graphite, and nickel alloys, preferably mixed oxide coated titanium and platinum titanium.

The invention can in principle be applied to all electrically conductive surfaces with deposits. It is a prerequisite that the surfaces to be cleaned must be immersed in a suitable Liu-20 and that sufficient electricity must be applied to the surfaces so that the electrochemical potential of the surface can be changed sufficiently.

Depositioning in an embodiment of the present invention generally takes less than 5 minutes to half an hour. When sulfuric acid or phosphoric acid solution with the addition of a complexing agent is used as the electrolyte solution, the time to remove the precipitates is generally as short as a few minutes, for example 1-5 minutes. When the electrolyte stock solution is sulfuric or phosphoric acid or an inorganic base such as sodium hydroxide, it usually takes slightly more time to remove the precipitate than the above electrolyte, for example about 10-15 minutes. When the purification process is carried out in the process solution of the purifying device, it will take even more time, such as 20-25 minutes, but it may be more advantageous to carry out the inhibition of precipitation in the process solution without time limitation instead of starting the precipitates remove. The purification times depend on many factors such as the composition of the precipitate, the thickness, the surface to be cleaned, etc. 14, so the above times are only indicative and are intended only to indicate differences in the efficiency of the various electrolyte solutions.

The following non-limiting examples illustrate the invention and its advantages.

5

Examples Example 1 Electrochemical purification 10

Comparison of the electrochemical purification of the precipitates from the plate heat exchangers in two different cases, i.e., the formation of TiO 2 precipitates from the sulfuric acid solution and the calcium carbonate precipitates from the process solution.

15 a) TiO 2 precipitates

Precipitation is removed by the electrochemical method both from the surfaces of the titanium heat exchanger plates which have been rejected due to contamination in the process and from specimens that have undergone, under laboratory conditions, a TiCl 2 precipitate from the process solution. Potential and galvanostatic methods as well as pulse current are used for purification.

The tests are carried out with a direct current source and using two contaminated Ti specimens as electrodes, which have been cut from the heat exchanger plate rejected for contamination. Experiments are performed on pure sulfuric acid (10%), a mixture of phosphoric acid and sulfuric acid, ibid.

25 in citric acid (10%) and in process solution. The temperature of the solution is 60 ° C. The test pieces act alternately as an anode and cathode in the tests. Some experiments are also carried out using a lead electrode as a counter electrode. The polarity of the electrodes is changed every 30 s and the current density used is 5-12 A / dm2.

30 Using two titanium electrodes, the samples are cleared in minutes. The use of citric acid clearly enhances purification. When using two titanium electrodes, the current limiting factor is the passivation of titanium during anodic polarization. Purification is further enhanced by the use of lead, graphite or mixed oxide coated titanium electrodes as counter electrodes, since the current densities of the cathodic phase are thereby increased. Titanium has a uniform dissolution and, in the cleaning times used, its weight loss has not been significant. In pure sulfuric acid solution, titanium is purified in 10-15 minutes, in sulfuric acid / citric acid solution for a few minutes and in process solution for 20-25 minutes.

5 At 60 ° C, the anode current alone does not allow the samples to be purified even at very high potential (10 V vs. SCE). With cathodic current, the sample is purified at a potential of <-0.6 V vs. SCE. If cathodic polarization is continued after purification, there is a risk of titanium corrosion. By using a pulse current, the cleaning efficiency can be clearly improved. In the experiments, the lengths of the current cycles vary between 10 and 50 s. Purification in sulfuric acid solution takes place efficiently and without the addition of citric acid. Cleaning time is less than 5 min. With the process solution, the purification is slightly slower, but the purification time is less than 10 minutes if the cathodic potential is <-0.8 V vs. SCE.

Figure 1 shows the current density of purification with 10% sulfuric acid as a function of time at anodic voltage of +3 V vs. SCE for 20 s and cathodic voltage of -0.8 V vs. SCE for 30 s. The surface is shown to be pure when the cathodic current reaches its maximum value. or just after. In Figure 2, purification is done in 10% ELSO4 citric acid solution at anodic voltage of +3 V vs. SCE for 30 s and cathodic voltage of -0.8 V vs. SCE for 30 s.

20 Citric acid significantly reduces the time required for purification. In the process solution at 60 ° C, titanium purification (anode voltage +3 V vs. SCE for 45 s and cathode voltage -0.8 V vs. SCE 15 s) takes considerably longer (Fig. 3). From the graph, it is difficult to evaluate the efficiency of purification in the process solution.

25 i) Minimum current densities / potentials

Because the current densities are relatively high in the experiments performed, the minimum current densities / potentials in which test specimens can be purified in 10 wt% sulfuric acid supplemented with citric acid are investigated. The cleaning is then carried out at room temperature.

The cathodic voltage is -600 mV and the current density is 2 mA / cm2. In potentiostatic tests, the voltages are -500 and -550 mV vs. SCE as the cathodic pulse and 4 V vs. SCE as the anodic voltage. The duration of both pulses is 30 s. Some precipitation is observed, but after one hour, there is still some precipitate remaining (Fig. 4). Current densities in galvanostatic tests are ± 1 mA / cm2. The pulse durations are 30 and 30 s. The cleaning result is clearly better than the potentiostatic tests, but the samples are not completely pure in the one hour tests (Figure 5).

b) CaCO 3 based precipitates 5

In the process where the solution contains large amounts of CaCC> 3, some of the plate heat exchangers become soiled and a hardly cleanable precipitate forms on the heat transfer surfaces. The precipitate is a CaCO3 precipitate. The plate heat exchangers are made of stainless steel.

10 The test solution used is a process solution (70% CaCC3 solution) or dilution thereof (20, 40, 50, 60%) and a synthetic 40% CaCC3 solution. The purification of precipitates formed from these solutions under laboratory conditions is investigated by electrochemical methods. The cleaning of the heat transfer surfaces of heat exchangers contaminated under process conditions is likewise studied.

15 For electrochemical cleaning, stainless steel and sulfuric and phosphoric acid solutions with citric acid are used as test materials. In addition, experiments are performed in NaOH solution.

20 The surface of the specimens in the tests is mainly cleaned during anodic polarization. The use of pulse current increases the cleaning efficiency slightly. Most of the carbonate precipitate is soluble in acid, but when using 10% NaOH as a cleaning solution, the cleaning result is slightly better. However, in both cases a dark precipitate remains on the metal surface.

25 i) Surface preparation The effect of the heat treatment is investigated by heating the samples (5-10 min) in an oven at different temperatures. A significant part of the precipitate is already released into the water after quenching after heat treatment. When the temperature is raised to 700 ° C, the precipitate can be removed relatively quickly by an electrochemical method. However, at temperatures higher than this, cleaning cannot be done as the processing will adversely affect the properties of the steel. The pure passive metal surface is achieved by first heating the samples in an oven at 700 ° C for 10 minutes, after which the sample is cooled in water. The electrochemical purification 17 is done in two steps. First, the test piece is immersed in NaOH solution (50%) for 5 min and then in LUSCO 2 solution (25%) for 5 min with addition of 1 g / l citric acid. The anodic and cathodic phases of the pulse current used each last for 10 s. The current densities are between 3.5 and 7 A / dm2. The efficiency of the treatment is improved by raising the temperature of the solution to 40-60 ° C.

EXAMPLE 2 Electrochemical Inhibition of Precipitation Formation 10 Electrochemical inhibition of precipitation formation on plate heat exchangers and tubular heat exchangers is investigated.

a) TK Deposits 15 Precipitation formation on titanium plate heat exchanger surfaces is performed with cathodic and anodic current and pulse current. Anodic polarization alone slows down but not completely prevents precipitation. When cathodic polarization is used, there is a risk of titanium corrosion. At a current density of ± 0.1 mA / cm2, the formation of precipitates cannot be prevented. Using a pulse current with potentiostatic anode-20 polarization (2 V vs. SCE) duration of 24 hours and cathodic polarization duration of 2-3 minutes (-0.7 to -0.8 V vs. SCE), the surfaces are kept clean . The temperature of the solution is 90 ° C. After the test, no significant fouling is observed on the surface of the sample and no weight loss is observed in the sample. In the prevention of precipitation, pulsation can be done in this way with long polarization durations. More often, pulsation could occur, further slowing down the formation of precipitates, but with the risk of titanium corrosion.

(b) Calcium sulphate based precipitates

The inhibition of precipitate formation is investigated with a laboratory tube heat exchanger. In the apparatus 30, the process solution is circulated and the tube is immersed in a hot oil bath, bringing the temperature conditions to match those of the actual heat exchanger so that the surface temperature of the tube is higher than the temperature of the solution. The temperature of the oil is 120 ° C and the temperature of the process solution inside the tube is 87 ° C. However, the flow rates are considerably lower than the flow rates used in the process. A counter electrode wire is placed in the center of the tube near the outlet for polarization of the counter electrode tube. With the equipment, a precipitate is formed within a few hours, and as the test continues, the precipitate intensifies strongly. Thereafter, inhibition of precipitation is tested by pulse current. The assay is performed galvanostatically (± 0.5 mA / cm 2, 1 min / 1 min). No precipitate is formed. A small amount of iron is observed on the surface of the tube.

Preventing tube fouling is also accomplished by periodic polarization. The tube is galvanostatically polarized for 10 min (± 5 mA / cm 2, 30 s / 30 s) and then kept for 10 min without polarization. After 24 hours, the tube has remained clean. Thus, it may be possible to keep the heat exchanger tubes clean by polarizing only a portion of the tubes at a time, thereby reducing the total amount of electricity used.

(c) Calcium carbonate - based precipitates

The formation of precipitates is investigated in a laboratory tube heat exchanger. In tests, the oil temperature ranges from 100 to 130 ° C and the process solution from 70 to 90 ° C.

In the experiments performed, the flow rate of the process solution is low compared to the actual process conditions, and calcium carbonate precipitates rapidly on the surface of the tube as a particle precipitate. In the process, a siliceous precipitate could first form on the surface of the tube, after which the carbonate would also begin to adhere to it.

DC and pulse current (-0.5 to 5 mA / cm 2, 30 sec / 30 sec) do not prevent precipitation in 70% process solution. The purification cell voltage in the carbonate solution is quite high due to the low conductivity of the solution and especially of the resulting precipitate. When using a diluted (20%) process solution in the tests, no mass precipitation occurs, but a thin oxide layer forms on the surface of the tube, from which the carbonate is much more easily removed than the pure metal surface.

The use of alternating current to prevent the formation of precipitates is also being studied in the carbonate solution. The current density used is 1 mA / cm2, which is the maximum allowed current density of the equipment when using AC power. The AC frequencies used are 5, 50, 1000 30 19 and 10000 Hz. However, the use of alternating current is not observed to inhibit or slow down the formation of a precipitate in 70% process solution or 40% dilution.

Claims (29)

A process for cleaning contaminated electrically conductive surfaces or electrochemically maintaining electrically conductive surfaces wherein the structure comprising the surfaces to be cleaned is immersed in a cleaning solution comprising an inorganic acid, inorganic base or inorganic salt aqueous solution or process solution, and at least two may consist of either a part of the structure to be cleaned or a counter electrode or reference electrode, 10 characterized in that: - a pulsed current is applied to the cleaning solution via electrodes, and - current densities used for surface cleaning are maintained between 1 and 5 mA / cm2; mA / cm 2, and the potentials used are kept between -0.8 and -0.5 V vs. SCE with cathodes and 15 between 2 and 10 V vs. SCE with anodes. Method according to Claim 1, characterized in that the surface to be cleaned consists of an electrically conductive material. Method according to claim 1 or 2, characterized in that the electrodes consist of a metal or an alloy such as platinum, titanium, coated titanium, lead, stainless steel, graphite or nickel alloy, preferably mixed oxide coated titanium or platinum plated titanium. Method according to one of the preceding claims, characterized in that the pulse current is generated alternately by switching every other electrode to an anode and every other electrode into a cathode, whereby purification at each electrode is carried out alternately with the anode current and the cathodic current. The method according to claim 4, characterized in that the polarity of the electrodes is changed at intervals of about 10 to 50 seconds, preferably at intervals of about 15 to 45 seconds, preferably at intervals of about 30 seconds. Method according to one of the preceding claims, characterized in that the surface to be cleaned is a plate or tube heat exchanger or a part thereof. Method according to claim 6, characterized in that the reference or counter electrodes, or each of them, are placed in or between the channels in the plate heat exchanger. Method according to one of the preceding claims, characterized in that an electrical conductor is connected between the anodes and the cathodes. 10 Method according to one of the preceding claims, characterized in that it is carried out either potentiostatically or galvanostatically. Method according to one of Claims 1 to 9, characterized in that the current densities used for cleaning the surfaces 15 are between 1 and 15 mA / cm 2, preferably between 2 and 7 mA / cm 2. Method according to one of Claims 1 to 9, characterized in that the current densities used to keep the surfaces clean are between -5 and +5 mA / cm2, preferably between -0.5 and +0.5 mA / cm2, preferably between -0 , 1 to + 0.1 mA / cm 2. Process according to one of Claims 1 to 11, characterized in that the cleaning solution comprises an acid, such as an inorganic acid, which is preferably sulfuric acid or phosphoric acid. 25 Process according to Claim 12, characterized in that the concentration of the acid in the purification is 5 to 30% by weight, preferably 5 to 25% by weight, preferably 5 to 10% by weight. Process according to one of Claims 1 to 11, characterized in that the cleaning solution comprises a base, such as an inorganic base, which is preferably sodium hydroxide. Process according to Claim 14, characterized in that the concentration of the base in the purification is 5 to 50% by weight, preferably 5 to 25% by weight, preferably 5 to 10% by weight. Process according to one of Claims 1 to 11, characterized in that the cleaning solution is an aqueous solution of an inorganic salt comprising one or more salts of alkali metals or alkaline earth metals. 5 Process according to Claim 16, characterized in that the salt content in the purification is 5 to 50% by weight, preferably 5 to 25% by weight, preferably 5 to 10% by weight. Method according to one of Claims 1 to 11, characterized in that the cleaning solution 10 comprises a process solution in which the surface to be cleaned has been submerged before cleaning. Method according to one of the preceding claims, characterized in that the cleaning solution further comprises a complexing agent. 15 Process according to claim 19, characterized in that the complexing agent is citric acid, EDTA or sulfamic acid or a mixture thereof. Process according to Claim 19 or 20, characterized in that the content of the complexing agent is from 0.5 to 5 g / l, preferably from 0.7 to 2 g / l, most preferably from 1 g / l. Method according to one of Claims 1 to 21, characterized in that the temperature of the solution during surface cleaning is maintained below the boiling point of the solution, preferably between 40 and 60 ° C, more preferably at 60 ° C. 25 A method according to any one of the preceding claims, characterized in that, prior to cleaning the surfaces, a pre-treatment is carried out in which the plates to be cleaned are heated in a furnace to a temperature of 650-700 ° C, preferably 700 ° C, A process according to claim 23, characterized in that the cleaning solution used in the pretreatment is a basic solution, such as a solution of an inorganic base, which preferably is sodium hydroxide. Method according to one of the preceding claims, characterized in that the cleaning solution is circulated through the filters. 26. A cleaning unit that can be used to clean contaminated electrically conductive surfaces 5 or electrochemically to keep electrically conductive surfaces clean, comprising a basin, - a suspension system for electrodes, and - a pulse current generating power supply for adjusting the rate at which for surface cleaning, current densities are maintained between 1 and 15 mA / cm2, for maintaining surfaces clean, current densities are between -5 and +5 mA / cm2, and potentials are maintained between -0.8 and -0.5 Y vs. SCE with cathodes and between 2 and 10 Y vs. SCE with anodes, and that the unit further comprises a system for removing harmful substances. 15 A cleaning unit according to claim 26, characterized in that it further comprises counter electrodes. A cleaning unit according to claim 26 or 27, characterized in that it further comprises reference electrodes. A purification unit according to claim 27 or 28, characterized in that it further comprises a solution recycling and purification system. 25