CN105074056A - Techniques and prefabricated cathode structures for the manufacture of chlorinated products - Google Patents

Abstract

Techniques for producing chlorinated products include an electrochemical process that includes the steps of providing an anode and a cathode in an electrolyte comprising impurities such as calcium ions, applying a voltage between the anode and the cathode under conditions to form an electrolysis product such as sodium chlorate in the electrolyte, and providing sufficient phosphate ions to form with at least a portion of the calcium ions a protective external layer including a calcium phosphate compound such as hydroxyapatite on the cathode, while preferably avoiding other phosphate precipitations. A pre-determined amount of phosphate ions may be added, for example, based on the surface area of the cathode in order to form the protective layer. Related uses and systems are also described. Prefabricated cathodes may include a substrate, a catalytic intermediate layer and a calcium phosphate protective layer.

Description

Techniques and prefabricated cathode structures for the manufacture of chlorinated products

technical field

The present invention relates generally to the field of manufacture of chlorination products, and more particularly to cathodic protection techniques and prefabricated cathodic structures in the manufacture of sodium chlorate.

Background technique

Sodium chlorate (NaClO ₃ ) is produced commercially by an electrochemical process according to the following overall reaction:

NaCl+3H ₂ O=>NaClO ₃ +3H ₂ (1)

Hydrogen evolution (H ₂ ) occurs on the cathode side as follows:

2H ₂ O+2e ^- ＝>2OH ^- +H ₂ (2)

And on the anode side chlorate (ClO ₃ ^- ) is formed through a series of reactions:

2Cl ^- ＝>Cl ₂ +2e ^-

Cl ₂ +H ₂ O=>HClO+Cl ^- +H ⁺

HClO＝>ClO ^- +H ⁺

2HClO+ClO ^- ＝>ClO ₃ ^- +2Cl ^- +2H ⁺ (3)

On the cathode side, the hydrogen current efficiency (CE) is defined as the ratio between the hydrogen flow rate (J _H2 ) and the total current applied to the electrochemical cell (cell):

CE＝J _H2 /(I/2F)(4)

where I is the applied current and F is Faraday's constant.

For the cathode, mild steel with a low carbon content is generally used, and a dimensionally stable anode (DSA) is used as the anode. In practice, electrochemical reactions often take place in undivided cells using monopolar or bipolar configurations. In a bipolar configuration, the anode portion is in physical and electrical contact with the cathode portion and as a result, severe galvanic corrosion problems can occur during shutdown when cathodic protection is no longer in place.

A typical electrolyte solution comprises about 550 g/l NaClO ₃ , 110 g/l NaCl and 1-3 g/l NaClO. The process typically takes place at a pH of about 6.5, a temperature of 60°C-85°C, and a current density of 2-4 kA/m ² .

Several side reactions can lead to reduced CE, such as the reduction of hypochlorite and chlorate on the cathode side:

ClO ^- +H ₂ O+2e ^- ＝>Cl ^- +2OH ^- (5)

ClO ₃ ^- +3H ₂ O+6e ^- ＝>Cl ^- +6OH ^- (6)

Another parasitic reaction that can be important (especially during start-up) is the reduction of iron oxides on the steel cathode, which leads to an explosive release of oxygen from the cell:

MO+2e ^- ＝>M+O ^2- (cathode)(7)

O ^2- ＝>1/2O+2e ^- (anode)

Oxygen can also be generated by anodic oxidation of hypochlorite according to:

OCl ^- +H ₂ O＝>O ₂ +2H++Cl ^- +2e ^- (8)

And it can also be formed from the decomposition of hypochlorite as follows:

2OCl ^- ＝>2Cl ^- +O ₂ (9)

2HClO＝>O ₂ +2HCl(10)

The presence of Ni, Co, or Cu ionic impurities in the electrolyte can accelerate the decomposition of hypochlorite. At 1ppm level content, Ni, Co, and Cu can lead to an increase of 2.0%, 1.0%, and 0.7% _O2 in the electrolyzer gas at 70°C and 3kA/ ^m2 current density, respectively. Therefore, among the various elements that may be part of an electrochemical cell or electrode, Ni is often particularly avoided.

In order to reduce the aforementioned cathode parasitic reactions and increase the hydrogen CE, the industry typically adds dichromate (Na ₂ Cr ₂ O ₇ ) to the electrolyte at a concentration of 3-8 g/l. During cathodic polarization, chromium (VI) is reduced to chromium (III) and a thin film of chromium (III) hydroxide forms on the cathode surface. The porous membrane helps protect the cathode from corrosion, hinders the migration of anions to the electrode surface and suppresses unwanted side reactions, while still allowing the hydrogen evolution reaction to occur.

Cr ₂ O ₇ ^-2 +8H ⁺ +6e ^- ＝>2Cr(OH) ₃ +H ₂ O(11)

Chromate also acts as a pH buffer and it reduces the production of oxygen by-products. Due to the toxicity of hexavalent chromium, which adds to the processing costs, it is desirable to find a replacement or replacement for this practice. Therefore, it is desirable to find non-toxic coating materials that, once deposited on the surface of the cathode, inhibit the reduction of hypochlorite and chlorate and improve CE.

The electrolyte is highly corrosive and iron cathodes are susceptible to corrosion in such environments in the absence of cathodic protection. Corrosion shortens the lifetime of the cathode and also contaminates the electrolyte with iron impurities. Depending on operating conditions and shutdown frequency, cathode life can be as short as five years. For this reason, some industries use thicker cathodes and operate with larger inter-electrode separations to avoid short circuits caused by corrosion products. Furthermore, iron oxides have been found to catalyze chlorate reduction and lower CE. However, iron cathodes are cheap, they have a relatively low cathodic overpotential and their surface is renewed as the corrosion layer is removed each time a prolonged power interruption occurs. Therefore, after a shutdown event, the cell voltage is usually lower, but CE is still active until the chromium hydroxide film forms again on the surface.

Another challenge in the field of sodium chlorate manufacture is corrosion on the cathode, especially when iron cathodes are used.

To minimize corrosive effects on the cathode, stainless steel cathodes can be considered instead of iron, but stainless steel generally has a much higher cathodic overpotential due to the chromium in the alloy. The overpotential of pure chromium is at least 100 mV higher than that of pure iron. In addition, the 300 series austenitic stainless steels also contain Ni, which can affect the production of oxygen by-products as mentioned previously. In the absence of dichromate, the rate and CE of chlorate reduction strongly depend on the nature and properties of the electrode material. For the last twenty to thirty years, research to find a replacement for iron cathodes has been ongoing, but iron cathodes are still in use. Therefore, new cathode materials are desired that are electroactive, highly corrosion resistant and almost as noble as DSA cathodes to minimize galvanic corrosion in bipolar configurations.

Another challenge in the field of sodium chlorate manufacturing involves electrolyte impurities. During electrolysis, a gradual increase in cell voltage is observed when electrolyte impurities plate or deposit on the electrodes and cloud or poison the electrocatalytic activity. Under the action of an electric field, positively charged impurities such as Ca ²⁺ and Mg ²⁺ are attracted towards the cathode, while negatively charged impurities such as sulfate ions move towards the anode. The paper entitled "Electrolyticsodium chloride technology: current status" published in Electrochemical Society Proceedings, vol. 99-21 by BV Tilak et al. in 1999 mentioned that the deposits usually found on the cathode are hydroxides of calcium and magnesium. BV Tilak et al. showed that Ca ⁺² impurity at a level of 1 ppm can lead to a voltage increase of about 50-75 mV per month, and at a level of 1.5 ppm can cause a voltage increase of about 100 mV per month. At the 20ppb level, the voltage rise was only 50mV over two years of operation. Since most manufacturing plants operate at ppm calcium levels, they typically clean their electrolytic cells several times a year using acid washes to remove these masking deposits.

Recently, novel cathodes exhibiting an overpotential approximately 200 mV lower than that of iron have been reported. Canadian Patent No. 2687129 and Canadian Patent Application No. 2778865 to Schulz et al. describe new cathode materials _of the type Fe3Al(Ru) and _Fe3AlTa (Ru) which can be used to make chlorine with improved conditions relative to iron Sodium acid. These materials have catalytic species (Ru) within an iron aluminide metal matrix. Despite their efficiency for the hydrogen evolution reaction, unfortunately, these new cathode materials are also affected by calcium impurities. In some cases, calcium impurities above ppm fractions can have a negative impact on electrode performance.

Calcium impurities can also have a negative impact in the case of chlor-alkali technology where a highly porous membrane or membrane is used to separate the anolyte and catholyte compartments. Calcium impurities at ppm levels can clog these membranes fairly easily and for this reason, methods have been developed to reduce calcium impurities to ppb levels. Such a method is described in US Patent No. 4,176,022 to Darlington in 1979. After removal of particulates, it typically begins by treating the brine with soda ash to precipitate most of the calcium as calcium carbonate, which is separated from the electrolyte by filtration or other physical separation methods. This usually brings calcium to a level of 2-3ppm. Thereafter, the electrolyte can be passed through an ion exchange column to obtain a brine containing less than about 0.5 ppm (500 ppb) calcium. Finally, US Patent No. 4,176,022 proposes adding phosphate groups to alkaline brines to form calcium phosphate compounds believed to be calcium apatite which are substantially insoluble in brines and then separating the compounds from the electrolyte. The pH of the brine was kept above 10 and the temperature above 40°C during the formation of the compound to further reduce its solubility in the brine. They also proposed adding seeds such as calcium phosphate (Ca ₃ (PO ₄ ) ₂ ) or calcium hydroxide to the electrolyte to facilitate the precipitation reaction. The final calcium impurity level was about 20 parts per billion. Typically, the concentration of phosphate added to the brine is from about 0.1 to about 1% by weight. As an example, they added 0.44g and 2.24g phosphoric _acid (85% H3PO4 ₎ to a liter of brine to reduce calcium to 200ppb and 20ppb levels, respectively. More recently, in 2008, Canadian Patent Application No. 2655726 proposed a similar method to remove calcium from brine. This application teaches adding _2g of _Na2HPO4 to 60ml of electrolyte to reduce calcium impurity levels to below ppm levels. Such references disclose the addition of relatively high amounts of phosphate for the purpose of precipitating calcium ions from the bulk solution to achieve a targeted final calcium impurity level.

In U.S. Patent No. 4,004,988, Mollard et al. propose the use of phosphoric acid or alkali metal salts of these acids to the electrolyte for complexing calcium in a diaphragmless electrolyzer for the manufacture of sodium chlorate. Mollard et al. disclose that complexing agents remove most of the calcium during the electrolysis process in the form of easily filterable precipitates. In an example, Mollard et al. added 0.5-2 g sodium tripolyphosphate (Na ₅ P ₃ O ₁₀ ) to 1 kg brine containing 60 ppm-100 ppm calcium, which corresponds to about 0.7 g-2.6 g/liter. After passing through the electrolysis cell, the solution was filtered and found to contain 5 ppm-10 ppm calcium. As in the previously described references, the concentration of the phosphate additive was in the range of grams per liter of brine, which means in the range of 1000 ppm.

UK Patent Application No. 2039959 discloses a similar approach, but instead of adding phosphoric acid directly to the brine, the phosphoric acid is mixed with hypochlorous acid which is periodically added to the brine for pH balancing. In the examples, for a brine containing about 35ppm calcium and 2ppm Fe, in one case they added 1g-2g phosphoric acid (85% _of H3PO4 ₎ /kg of sodium chlorate produced, and in the second In this case 0.15 g acid/liter electrolyte was added. These amounts are relatively high and in ranges similar to those previously disclosed in other references.

US Patent Application No. 2008/0230381A1 by N. Krstajic et al also discloses the addition of at least 1 g/l of phosphate ions to sodium chloride brine to act as a buffer. As a cathode, Krstajic et al. proposed a low carbon steel substrate coated with an electrodeposited Fe-Mo alloy layer with a thickness of 10 μm-50 μm. With the addition of phosphate, Krstajic et al. mention that the voltage drop observed with this type of activated coating can reach values as high as 500 mV at the usual current densities of 2.5-3 kA/m ² , while using known In saline, the drop is limited to 100-150 mV. Krstajic et al. also proposed that the concentration of dichromate in the electrolyte could be reduced to 0.1 g/l or eliminated completely without greatly affecting the CE.

The problem with known methods relates to the fact that increasing amounts of phosphate additives are added with the aim of removing significant amounts of calcium impurities in the electrolyte and in order to bring the calcium impurities to a level where they no longer affect the cathode. The addition of large amounts of anions such as PO ₄ ^-3 can cause additional problems when iron impurities are also present in the electrolyte. In a paper titled "Effects of Electrolyte in Chlorate Cells," RAKus mentions that when iron and phosphate impurities are present together at significant concentration levels, a voltage increase is observed due to a synergistic effect between these impurities that affects anode performance. For this reason, the industry currently tends not to use phosphoric acid additives, since they almost all use iron cathodes and thus have iron impurities in their electrolytes in addition to calcium impurities.

Given the various challenges in the field of sodium chlorate manufacturing, there is a need for technologies that provide at least some solutions.

Contents of the invention

The present invention responds to the above needs by providing enhanced technology for the manufacture of chlorinated products such as sodium chlorate.

In some implementations, an electrochemical process is provided comprising: disposing an anode and a cathode in an electrolyte including impurities comprising calcium ions; between the anode and the cathode under conditions in which electrolysis products are formed in the electrolyte applying a voltage; and providing phosphate ions in the electrolyte in an amount sufficient to form a protective outer layer on the cathode with at least a portion of the calcium ions, the protective outer layer comprising a calcium phosphate compound, and The amount is sufficient to substantially avoid precipitation of calcium phosphate compounds in the electrolyte. It should be noted that the step of providing phosphate ions may be performed before, after or during the step of applying a voltage.

In some implementations, the phosphate ions are added in an amount based on the surface area of the cathode in contact with the electrolyte.

In some implementations, the process further includes applying the protective outer layer over the catalytic intermediate layer prior to immersing the cathode in the electrolyte.

In some implementations, applying the protective outer layer comprises sputtering, dipping, sol-gel, electrochemical deposition, biomimetic coating, hot isobaric coating, or plasma spraying.

In some implementations, the process further includes forming the protective outer layer on the catalytic intermediate layer after dipping the cathode in the electrolyte.

In some implementations, the step of forming the protective outer layer includes: providing phosphate ions in the electrolyte; ensuring sufficient calcium ions are present in the electrolyte; and providing sufficient calcium ions to cause formation of the protective outer layer. conditions of electrolysis.

In some implementations, the formation of the protective outer layer includes: reacting Ca(OH) ₂ with H ₃ PO ₄ to produce Ca ₃ (PO ₄ ) ₂ and water; and reacting Ca(OH) ₂ with Ca ₃ ( PO ₄ ) ₂ reacts to produce Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ , wherein Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ forms at least part of the protective outer layer.

In some implementations, phosphate ions are provided in the electrolyte at a phosphate concentration of up to about 75 ppm, or about 5 ppm to about 50 ppm.

In some implementations, the phosphate concentration is low enough to prevent the formation of iron phosphate compounds or deposits on the anode, an increase in _O2 levels in the electrolyte, and/or an increase in voltage requirements.

In some implementations, the phosphate ions are provided at least in part by the addition of H ₃ PO ₄ . Phosphate ions may also be provided, at least in part, by their inherent presence in the electrolyte. Calcium ions may also be provided, at least in part, by their inherent presence in the electrolyte.

In some implementations, the electrolyte includes a first portion of calcium ions for reacting with phosphate ions to form a calcium phosphate compound on the cathode, and a second portion of calcium that remains unreacted in the electrolyte.

In some implementations, the process is used to produce chlorinated products. The chlorination products may include sodium chlorate and/or sodium hypochlorite.

In some implementations, there is provided the use of phosphate ions in an electrochemical process for producing chloride products in an electrolyte comprising calcium ions, wherein the phosphate ions are present in an amount sufficient to form with at least a portion of the calcium ions at the cathode comprising calcium phosphate A protective outer layer of salt compounds is provided in an amount sufficient to avoid precipitation of calcium phosphate compounds in the electrolyte.

In some implementations, an electrochemical system is provided comprising: an electrolysis chamber for containing an electrolyte, wherein the electrolyte includes calcium ions and phosphate ions; an anode located in the electrolysis chamber; a cathode located in the electrolysis chamber and an ion regulator configured to regulate the level of ions in the electrolyte such that the electrolyte comprises sufficient calcium ions to form a protective outer layer comprising calcium phosphate compounds on the cathode and sufficient to avoid calcium phosphate The amount of phosphate ions in which the salt compound precipitates in the electrolyte.

In some implementations, the ion regulator includes an inlet in fluid communication with the electrolysis chamber for providing the amount of phosphate ions into the electrolysis chamber. The ion regulator may further comprise at least one measuring device for measuring the concentration of phosphate ions, iron ions and/or calcium ions in the electrolyte. The ion regulator may also include a controller connected to the measuring device and the inlet for controlling the input amount of phosphate ions in response to a reading from the measuring device. The system may be configured for the production of chlorinated products such as sodium chlorate and/or sodium hypochlorite.

In some implementations, an electrochemical process is provided comprising: disposing an anode and a cathode in an electrolyte comprising impurities including calcium ions and iron ions; applying a voltage across the cathodes; and providing phosphate ions in the electrolyte in an amount sufficient to form a protective outer layer on the cathode with at least a portion of the calcium ions, the protective outer layer comprising calcium phosphate compound, and the amount is sufficient to substantially avoid precipitation of iron phosphate compounds in the electrolyte.

The above process may also have one or more features as described in other implementations described herein.

In some implementations, there is provided the use of phosphate ions in an electrochemical process for producing chloride products in an electrolyte comprising calcium ions and iron ions, wherein the phosphate ions are formed in an amount sufficient to form with at least a portion of the calcium ions at the cathode comprising a protective outer layer of calcium phosphate compound and provided in an amount sufficient to prevent precipitation of iron phosphate compound in said electrolyte.

In some implementations, an electrochemical system is provided comprising: an electrolysis chamber for containing an electrolyte, wherein the electrolyte includes calcium ions, iron ions, and phosphate ions; an anode located in the electrolysis chamber; an anode located in the electrolysis chamber; and an ion regulator configured to regulate the level of ions in the electrolyte such that the electrolyte comprises sufficient calcium ions to form a protective outer layer comprising a calcium phosphate compound on the cathode and sufficient Amounts of phosphate ions in which iron phosphate compounds precipitate in the electrolyte are avoided.

In some implementations, an electrochemical process is provided comprising: disposing an anode and a cathode in an electrolyte including impurities comprising calcium ions, the cathode having a surface area in contact with the electrolyte; forming an electrolysis product in the electrolyte A voltage is applied between the anode and the cathode under conditions of ; and phosphate ions are provided in the electrolyte in a predetermined amount based on the surface area of the cathode: the predetermined amount is such that the phosphate ions and At least a portion of the calcium ions are consumed in the formation of a protective outer layer covering the surface area of the cathode in contact with the electrolyte.

In some implementations, the predetermined amount of phosphate ions is about 0.025 mg/cm ² cathode to about 0.2 mg/cm ² cathode, or about 0.05 mg/cm ² cathode to about 0.15 mg/cm ² cathode.

In some implementations, the electrolyte further includes iron ions, and phosphate ions are further added in an amount to avoid precipitation of iron phosphate compounds in the electrolyte. Phosphate ions may further be added in an amount to avoid precipitation of additional calcium phosphate compounds in the electrolyte.

In some implementations, the predetermined amount of phosphate ions is calculated based on a target thickness of the protective outer layer to be obtained.

The above process may also have one or more features as described in other implementations described herein.

In some implementations, there is provided the use of phosphate ions in an electrochemical process for producing chloride products in an electrolyte comprising calcium ions, wherein the phosphate ions are provided in a predetermined amount based on the surface area of the cathode as follows : said predetermined amount is such that phosphate ions and at least a portion of calcium ions are consumed in the formation of a protective outer layer covering the surface area of said cathode in contact with said electrolyte.

In some implementations, an electrochemical system is provided comprising: an electrolysis chamber for containing an electrolyte, wherein the electrolyte includes calcium ions and phosphate ions; an anode located in the electrolysis chamber; a cathode located in the electrolysis chamber and an ion regulator configured to adjust the level of ions in the electrolyte such that the electrolyte includes a predetermined amount of phosphate based on the surface area of the cathode: the predetermined amount is such that phosphate ions and at least a portion of the calcium Ions are consumed in the formation of a protective outer layer covering the surface area of the cathode in contact with the electrolyte.

In some implementations, a prefabricated cathode is provided comprising: a substrate; a catalytic intermediate layer; and a protective outer layer comprising a calcium phosphate compound.

In some implementations, the substrate includes stainless steel. The stainless steel may be 400 series stainless steel. The substrate may comprise a material having sufficient corrosion resistance to prevent ferric ions from entering the electrolyte during periods of shutdown of the electrolytic cell.

In some implementations, the catalytic intermediate layer is adjacent to the outer surface of the substrate.

In some implementations, the catalytic interlayer includes a metal matrix doped with a catalytic compound. The metal matrix can be iron aluminide. The catalytic compound includes Ru.

In some implementations, the calcium phosphate compound comprises a hydroxy calcium phosphate compound, such as hydroxyapatite. The protective outer layer may consist essentially of hydroxyapatite.

In some implementations, the protective outer layer has a thickness of about 0.25 microns to about 1.5 microns. The protective outer layer may have a thickness of from about 0.5 micron to about 1 micron.

In some implementations, the protective outer layer is configured to cover the entire outer surface of the catalytic intermediate layer to prevent direct contact of the catalytic intermediate layer with calcium impurities in the electrolyte. The protective outer layer may be sputter coated, dip coated, sol-gel applied, electrochemically deposited, biomimetic coated, hot isobaric coated, or plasma sprayed onto the catalytic intermediate layer.

In some implementations, the protective outer layer has a network structure. The protective outer layer may have a honeycomb structure.

In some implementations, the protective outer layer has a structure that enables the hydrogen evolution reaction to occur thereunder while preventing calcium impurities from poisoning the intermediate catalytic layer (catalytic layer).

In some implementations, the protective outer layer has a structure that prevents chlorate and hypochlorite ions from reaching the surface of the intermediate catalytic layer to reduce or avoid the following reaction: ClO ⁻ +H ₂ O + 2e ⁻ = >Cl ⁻ +2OH ⁻ ; and/or ClO ₃ ⁻ +3H ₂ O+6e ⁻ =>Cl ⁻ +6OH ⁻ .

In some implementations there is provided the use of a prefabricated cathode as defined herein in an electrolysis cell for the manufacture of chlorinated products such as sodium chlorate and/or sodium hypochlorite.

In some implementations, an electrochemical process is provided comprising: disposing an anode and a prefabricated cathode as defined herein in an electrolyte comprising impurities comprising calcium ions; A voltage is applied between the anode and the prefabricated cathode. The process may also include one or more of the features as described herein.

In some implementations, there is provided a method for making a prefabricated cathode for use in the manufacture of chlorinated products, comprising: providing a substrate; providing a catalytic intermediate layer atop the substrate; and applying a protective outer layer to the on the catalytic intermediate layer, wherein the protective outer layer comprises a calcium phosphate compound.

The method can be performed to produce a prefabricated cathode having one or more features as described herein.

In some implementations, the step of applying the protective outer layer comprises sputtering, dipping, sol-gel methods, electrochemical deposition, biomimetic coating methods, hot isobaric coating, and/or plasma spraying.

In some implementations, an electrochemical process is provided comprising disposing an anode and a cathode in an electrolyte comprising impurities comprising calcium ions, wherein the cathode comprises a substrate composed of a corrosion-resistant material, and a catalytic intermediate layer; for an electrolysis period, performing electrolysis comprising applying a voltage between the anode and the cathode under conditions in which electrolysis products are formed in the electrolyte; periodically shutting down the electrolysis for a shutdown period comprising terminating said voltage, wherein the corrosion-resistant material of said substrate prevents iron ions from being released into said electrolyte during each of said shutdown periods; and providing phosphate ions in said electrolyte in an amount sufficient Such that during each electrolysis phase, the phosphate ions and at least a portion of the calcium ions form or reform a protective outer layer comprising a calcium phosphate compound on the catalytic intermediate layer of the cathode.

The above process may also have one or more features as described in other implementations described herein.

In some implementations, an electrochemical process is provided comprising: disposing an anode and a cathode in an electrolyte that includes impurities comprising alkaline earth metal ions; and providing phosphate ions in the electrolyte in an amount sufficient to form a protective outer layer on the cathode with at least a portion of the alkaline earth metal ions, the protective outer layer comprising an alkaline earth metal phosphate compound, and the amount is sufficient to substantially avoid precipitation of the alkaline earth metal phosphate compound in the electrolyte. The alkaline earth metal may be calcium. Note also that various implementations of the processes described herein can be performed with general alkaline earth metals other than specifically calcium.

In some implementations, providing an electrochemical process comprising: disposing an anode and a cathode in an electrolyte that includes an impurity; applying a voltage between the anode and the cathode under conditions such that an electrolysis product is formed in the electrolyte; and ensuring that sufficient phosphate ions and calcium ions are present in the electrolyte such that the phosphate ions and at least a portion of the calcium ions form a protective outer layer on the cathode, the protective outer layer comprising a calcium phosphate compound , and it substantially avoids the precipitation of calcium phosphate compounds in the electrolyte, substantially avoids the precipitation of iron phosphate compounds in the electrolyte when iron ions are present in the electrolyte, and/or in the protective outer layer Essentially all the phosphate ions are consumed in the formation of .

Note also that one or more further optional features, aspects and implementations of the processes, systems, uses, cathodes and methods of making cathodes which will be described in more detail in the following description may be combined with the various implementations described above.

Description of drawings

Figure 1 is a schematic diagram of an electrolysis system for the manufacture of sodium chlorate.

Figure 2 is a graph of _O2 content, voltage and efficiency versus time. The efficiency curve is the top curve; the voltage curve is the middle curve; and the _O2 content is the bottom curve.

Figure 3 is a plot of counts (arbitrary units) versus 2Θ (degrees) from an x-ray scan of the cathode surface.

Figure 4 is a graph of voltage versus time for a chlorate electrolyzer with calcium addition.

Figure 5 is a graph of counts (arbitrary units) versus 2Θ (degrees) from an x-ray diffraction spectrum of the cathode surface after electrolysis and addition of calcium and phosphoric acid to the electrolyte, inset is an EDX spectrum showing the elements present on the cathode surface.

Figure 6 is another graph of _O2 content, voltage and efficiency versus time. The efficiency curve is the top curve; the voltage curve is the middle curve; and the _O2 content is the bottom curve.

7a and 7b are scanning electron micrographs of a cross-section of a cathode structure.

FIG. 8 is a pair of scanning electron micrographs of a cross section of a cathode structure.

Figure 9 is a pair of photomicrographic chemical maps (maps) of the protective outer layer.

Figure 10 is a graph of voltage versus time with different amounts of calcium added to the electrolyte.

Figure 11 is a graph of voltage versus time for the addition of calcium ions followed by phosphoric acid.

Figure 12 is a graph of voltage versus time with the addition of phosphoric acid followed by the addition of calcium ions.

Figure 13 is a graph of voltage versus time with the addition of phosphoric acid.

Figure 14 is a block diagram.

Detailed ways

The various techniques described herein for electrolysis support the use of relatively small amounts of phosphate to form a protective layer on the cathode in an electrolysis system. A further technology provides for the electrolytic production of chlorinated products such as sodium chlorate (NaClO or ClO _{3 −} ) ^, which is a compound used in paper bleaching applications, or sodium hypochlorite (NaClO or ClO ⁻ ), which is a water treatment agent. A prefabricated cathode with a protective layer comprising a calcium phosphate compound in . It should be understood that while various implementations and aspects will be discussed herein with respect to the manufacture of sodium chlorate, such techniques may be adapted for the manufacture of other chlorination products such as sodium hypochlorite and the like.

In some implementations, the production of sodium chlorate by electrolysis is enhanced by providing the cathode with a protective outer layer comprising a calcium phosphate compound formed by adding phosphate ions in an amount that results in Calcium phosphate compounds are formed at the surface of the cathode while preventing precipitation of iron phosphate compounds and/or calcium phosphate compounds which could have a detrimental effect on electrolysis. To form the protective outer layer, phosphate may be provided in an amount based on the surface area of the cathode.

While other approaches have emphasized the addition of higher levels of phosphate ions for the purpose of precipitating calcium ions from the electrolyte, the various techniques described herein provide reduced amounts of phosphoric acid sufficient to form a protective layer on the cathode surface Favorable addition of root ions. In some cases, phosphate ions can be provided at significantly low levels to form a protective layer, which not only enables improved operation of the electrolysis despite residual calcium ions in the electrolyte, but also reduces or prevents iron phosphate, which can have a negative impact on electrolysis. compound formation. Thus, although the electrolyte may have various impurities such as calcium ions and iron ions, the protective outer layer may provide protection from electrolyte impurities and maintain efficient electrolytic operation.

In some cases, the beneficial aspects of phosphate additives can be achieved at phosphate concentrations that are one or more orders of magnitude lower than previously used. Instead of adding phosphate according to the concentration of calcium ions in the electrolyte with the aim of precipitating calcium impurities, phosphate can be provided with the aim of forming a calcium phosphate protective layer to counteract the effect of calcium on the electrolyte in sodium chlorate cells without a diaphragm. negative impact on the cathode surface.

Providing such a protective layer may be particularly advantageous when used in combination with a cathode comprising a substrate, such as stainless steel, and a catalytically active coating, such as a metal matrix doped with a catalytic species such as Ru. Calcium impurities can interfere with such a catalytically active coating and form calcium hydroxide during the hydrogen evolution reaction (Eq. 2), causing a gradual increase in cell voltage as this masking deposit builds up. Conditions that result in the deposition of an insoluble calcium phosphate compound such as hydroxyapatite may be provided close to the cathode surface. Even though the average pH in the bulk electrolyte remains around 6.5, the pH near the cathode surface is much higher, eg, well above 10, due to the formation of hydroxyl groups according to Equation 2. A locally elevated pH reduces the local solubility of calcium phosphate compounds and leads to the formation of deposited layers. In addition, calcium hydroxide naturally forms on the surface of the cathode and these clusters of calcium hydroxide can act as seeds for the formation of calcium phosphate (Ca ₃ (PO ₄ ) ₂ ). When a small amount of phosphoric acid is present near the cathode surface, the following reaction occurs:

3Ca(OH) ₂ +2H ₃ PO ₄ ＝>Ca ₃ (PO ₄ ) ₂ +6H ₂ O(12)

These calcium phosphate molecules further react with calcium hydroxide to form hydroxyapatite on the cathode surface according to the following:

Ca(OH) ₂ +3Ca ₃ (PO ₄ ) ₂ ＝>Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ (13)

The high temperature (about 60°C to about 85°C) that is characteristic of chlorate operation also favors this deposition, since the higher the temperature, the lower the solubility of the calcium phosphate compound. Indeed, conditions are provided for the heterogeneous formation of hydroxyapatite on the cathode surface, even though they may not be suitable for the precipitation of phosphate compounds in the bulk of the electrolyte. Various techniques described herein support the use of conditions for the intended addition of phosphate to be substantially completely consumed in the formation of a calcium phosphate protective layer on the cathode surface. Note also that one or more other calcium phosphate compounds such as Ca ₃ (PO ₄ ) ₂ may also be present and have beneficial protective properties.

Various aspects of the invention are described below, including systems and processes for making sodium chlorate, prefabricated cathodes and uses, and methods of making prefabricated cathodes.

Electrolysis system for sodium chlorate manufacture

Figure 1 schematically illustrates a system 10 for producing sodium chlorate. The system 10 includes an electrolysis cell 12 having an electrolysis chamber 14 filled with an electrolytic solution 16 (also referred to herein as an electrolyte). System 10 also includes an anode 18 and a cathode 20, the possible structure and composition of which will be discussed further below. Cathode 20 may have a structure including substrate 22 , intermediate catalytic layer 24 and protective outer layer 26 .

Still referring to FIG. 1 , the electrolysis cell 12 produces a sodium chlorate-rich solution 28 that is withdrawn from the electrolysis chamber 14 and may be supplied to downstream units 30 such as settlers, filters, evaporators, crystallizers, and dryers for further Processing to produce sodium chlorate 32 in solid and/or concentrated form.

Cathode 20 (which may also be referred to as a cathode structure) may include three or more layers. In some cases, the cathode structure has three layers comprising: substrate 22 , intermediate catalytic layer 24 disposed directly on substrate 22 , and protective outer layer 26 disposed directly on intermediate catalytic layer 24 . It is also conceivable to provide one or more further layers between different layers.

In some implementations, substrate 22 may be composed of a corrosion resistant material such as stainless steel, which may be from the 400 series. More about the substrate will be described further below.

In some implementations, the intermediate catalytic layer 24 may be composed of a highly porous and highly active catalytic material such as Fe3Al(Ru ₎ and _Fe3AlTa (Ru). More about the intermediate catalytic layer will be described further below.

In some implementations, protective outer layer 26 includes a calcium phosphate compound. More on the formation and properties of the protective outer layer will be described further below.

Phosphate dosing and protective layer formation

The protective outer layer can be formed in a number of ways. In one instance, the protective outer layer is formed in situ within the electrolytic cell. In other cases, the protective outer layer can be formed exsitu to make a prefabricated cathode that can be used in an electrolysis system. More on the prefabricated cathode and the ex-situ method will be described further below.

As indicated above, the protective outer layer can be formed in situ within the electrolytic cell by adding phosphate ions and in some cases calcium ions.

Phosphate ions may be added in relatively low amounts sufficient to form the protective outer layer and avoid one or more other reactions such as precipitation of calcium or iron ions from the bulk electrolyte solution and/or deposition of iron phosphate compounds at the anode.

In terms of the amount of phosphate ions added to the electrolyte, this can depend on a number of factors including: the surface area (surface area) of the cathode to be protected, the thickness of the protective layer to be formed, the composition and structure of the electrodes, the electrolyte composition and properties, etc.

In some cases, the phosphate is added in an amount sufficient to form a protective layer having a thickness of about 0.25 microns to about 1.5 microns, about 0.5 microns to about 1 micron, or about 0.6 microns to about 0.9 microns. Figure 8 illustrates a protective layer formed on the catalytic interlayer and having a thickness generally in the range of about 0.5 micron to about 1 micron.

Additionally, a general guideline can be followed whereby the amount of phosphate added to the electrolyte is at most sufficient to form a protective layer having a thickness of about 1 micron. For example, in one case discussed in the Examples section below, about 0.1 mg phosphate (PO ₄ )/cm ² cathode may be added. It will therefore be understood that the amount of phosphate to be added may be determined based on the surface area of the cathode rather than the volume of the electrolyte or the amount of calcium ions present in the electrolyte. For example, the amount of phosphate to be added may be from about 0.025 mg/cm ² cathode to about 0.2 mg/cm ² cathode, or, for example, from 0.05 mg/cm ² cathode to about 0.15 mg/cm ² cathode.

Furthermore, the amount of phosphate to be added may be predetermined by calculation and/or empirical testing, as will be appreciated from the examples. In one example, no more than 75 ppm, 50 ppm, 30 ppm, 20 ppm, or 15 ppm of phosphate ions are added to the electrolyte in order to form a protective layer on the cathode.

In some cases, the electrolyte may initially not contain enough calcium ions to form a protective layer with the phosphate ions. The addition of calcium ions can be done before, during or after the addition of phosphate ions in an amount sufficient to allow the formation of a protective layer. Figures 11 and 12 illustrate that calcium and phosphate ions can be added in various orders in order to stop the voltage rise.

It should also be noted that the phosphate may be added as a single dose or may be added periodically in increasing doses. Figure 13 illustrates the incremental addition of multiple phosphate doses to the system over a period of time.

Referring back to Figure 8, it can be seen that the protective layer is porous, having a reticular or honeycomb structure with a network of structural elements and dispersed void spaces. In some cases, the porosity and permeability of the protective layer are low enough to protect the underlying catalytic layer from calcium poisoning and high enough to avoid hindering the hydrogen evolution reaction from occurring.

Referring back briefly to FIG. 1 , phosphate may be added to electrolyte 16 via one or more phosphate addition lines 34, which may be separate lines or may be configured to add phosphate to the electrolytic cell Another inlet at 12, for example, a dilute HCL inlet as shown. Phosphate can be added manually or automatically. It may be added in response to measurements or readings taken about the electrolysis system. In this sense, the phosphate line 34 may be part of an ion regulator that regulates the amount of phosphate ions and possibly calcium ions in the electrolyte to ensure that a small amount of phosphoric acid sufficient to form a protective layer is provided. root. The ion conditioner may have various other components such as measurement devices and controllers.

_In some implementations, the phosphate is added in the form _of phosphate H3PO4. It should be noted, however, that the phosphate groups may be added in other forms such as acids or salts, as monophosphate or polyphosphate compounds, or otherwise.

In addition, the addition of phosphate may be performed to facilitate substantially complete consumption of phosphate in the formation of the protective layer. For example, controlling the conditions of the electrolysis system (eg, temperature, pH, composition, etc. of the electrolyte) to support the formation of calcium phosphate compounds on the cathode surface can limit or prevent any other reactions involving phosphate. This control step can be performed at or near the start of the electrolytic operation so that the protective layer can be formed as quickly as possible. As will be described further below, the cathode can also be coated ex-situ with a protective layer using a number of methods so that the cathode has a protective layer at the start of the electrolytic operation.

Prefabricated cathodes and ex situ fabrication

The prefabricated cathode can include a substrate, a catalytic layer, and a protective layer. The prefabricated cathodes described here can be used to replace iron cathodes currently used in industry to provide corrosion resistance, high activity for hydrogen evolution reaction, good hydrogen current efficiency, and low hydrogen overpotential. In some cases, the prefabricated cathode does not catalyze the reduction of hypochlorite and chlorate and does not produce significant oxygen by-products.

The prefabricated cathode can provide several advantages such as providing a protective coating of the catalytic layer at the start of the electrolysis operation and providing tolerance to electrolyte impurities and contributing to counteract the negative effects of calcium impurities on the cathode surface.

The method of making said prefabricated cathode may comprise the step of applying said protective outer layer by sputtering, dipping, sol-gel methods, electrochemical deposition, biomimetic coating, hot isobaric coating, or plasma spraying . The method of applying the protective layer can be selected and implemented to achieve certain properties of the protective layer, such as layer thickness, permeability and porosity within certain ranges.

Referring to Figure 14, the cathode structure 36, which may include a substrate and a catalytic layer, may be supplied to an ex situ pretreatment unit 38 for applying a protective layer. Preprocessing unit 38 may be configured to perform one or more of the methods described above for applying the protective layer to produce prefabricated cathode 20 . The prefabricated cathode 20 is then supplied to the electrolytic cell 12 for the production of sodium chlorate.

In some cases, the prefabricated cathodes may remain in the electrolysis cell until the end of their working life. If a pre-applied protective coating is damaged or partially removed, in situ methods of regenerating the protective layer may be used, such as adding a predetermined amount of phosphate as described herein. Additionally, in some cases, after a certain period of operation, the electrolyte may be removed from the cell 12 and the cell may be cleaned by introducing a mild acid solution, which is typically provided in the cell for about 1 Hour. If such a cleaning or other maintenance operation is performed to remove the protective outer layer from the cathode, the protective outer layer can be reapplied in situ to enable continued protection during the next stage of electrolysis.

Alternatively, after a certain period of operation, the spent cathode 40 may be removed from the electrolysis cell 12 for cleaning and/or maintenance. The spent cathodes can be removed, for example, during shutdown operation of the electrolysis cell 12 . Spent cathodes 40 may be provided to cleaning unit 42 for inspection and cleaning of the cathodes to produce cleaned cathodes 44 . The cleaning process may remove the protective layer from the cathode structure and thus the cleaned cathode 44 may be supplied to the pretreatment unit 38 to reapply the protective layer so that the cathode may be reused in the electrolytic cell middle.

Cathode substrate and catalytic layer

In some implementations, as previously mentioned, the cathode has a structure that includes a substrate and a catalytic layer.

The substrate may comprise a nickel-free stable corrosion-resistant substrate. The substrate may be 400 series stainless steel or another material with corrosion resistant properties. 400 series stainless steels can be used, albeit with a higher cathodic overpotential. As shown in Table 1 below, which lists commercial stainless steels available from AK Steel Corporation, these alloys have very low carbon contents similar to those of mild steels and are nickel free. They are highly resistant to corrosion in media containing chloride ions. In electrolytic operations, corrosion problems can arise during shutdown when cathodic protection is not present, and mild steel cathodes can suffer from degradation. Corrosion products on steel remove the surface layer every time power is interrupted. While calcium and phosphate still present in the electrolyte can lead to regeneration of the protective layer, another advantageous feature is the provision of an improved substrate compared to mild steel.

Table 1: Commercial stainless steels available from AKSteel

The catalytic layer may comprise a metal matrix doped with a catalytically active compound. For example, the catalytic layer may include Fe3Al(Ru ₎ and _Fe3AlTa (Ru), which are useful in making sodium chlorate and are improved over iron. These materials have catalytic species (Ru) within an iron aluminide metal matrix. Despite their efficiency for the hydrogen evolution reaction, unfortunately, these new cathode materials are also affected by calcium impurities. As shown in Figure 4, in electrolytic cells containing these alloys, a voltage rise of 60 mV was observed after the addition of 2 ppm calcium impurity to the electrolyte. Calcium deposits on the cathode tend to poison the electrode and reduce the activity of catalytic species. This voltage increase occurs on a time scale of one hour rather than months, which is faster than on conventional cathodes. The reason for this comes from the fact that these cathodes are highly porous. They typically have an effective surface about 100 times higher than that of a steel cathode. Thus, masking deposits on the surface can fairly quickly block the large number of catalytic sites located within the pores of the catalytic structure. However, the use of a protective layer comprising a calcium phosphate compound enables the catalytic layer to provide beneficial operation without being poisoned by calcium impurities.

In order to have sufficient lifetime, the cathode should be able to tolerate power interruptions. The most severe corrosion conditions often occur during power interruptions when cathodic current protection is not present.

In some cases, the prefabricated cathode has a structure that can withstand multiple current interruptions.

Various aspects and implementations described herein provide advantages such as: reduced phosphate demand, extended operating time between electrolyser cleaning, maintenance of low voltage levels, enhanced protection of the cathode, reduced negative impact on the anode wait.

Examples and experiments

Various examples and experimental results will now be described.

Figure ² shows a series of shutdowns in a chlorate electrolyzer operating at 70°C and 2.5kA/m2 with an iron cathode and a DSA anode. The electrolyte contained 550 g/l NaClO ₃ , 110 g/l NaCl and 3 g/l dichromate. Observations were made for current interruptions in the form of open circuits (OC) for 2, 5, 10 and 15 minutes followed by interruptions in the form of short circuits (CC) for 15 minutes. In an open circuit, the voltage of the cathode relative to the DSA reaches its corrosion potential at 1.08V rather quickly. The longer the interruption, the greater the oxygen burst and the lower the CE after shutdown. When the interruption occurs under CC conditions (which correspond to a bipolar configuration), the corrosion is so severe that a large shell of corrosion product falls to the bottom of the cell and, as a result, the oxygen release after the event is relatively small.

Figure 3 shows an x-ray scan taken from the surface of the cathode after several months of electrolysis. In addition to sodium chlorate, deposits of calcium hydroxide (hydroxylite) and some calcium sulphate hydroxide (waltonite) were also observed.

These masking deposits are responsible for the increase in cell voltage observed after long-term operation.

Example 1

The beneficial effect of the addition of very small amounts of phosphate additives on the cathode was observed in the following experiments.

A bath of 350 liters of sodium chlorate electrolyte containing 550 g/l of NaClO ₃ , 110 g/l of NaCl and 3 g/l of dichromate was used for electrolysis. The pH and temperature were maintained at 6.5 and 70°C, respectively. Steel cathodes and DSA anodes were used in the experiments. After 74 hours of continuous electrolysis at 2.5 kA/m ² , the cell voltage was 3.056 volts. Then 4ppm calcium impurity was added to the electrolyte in the form of _CaCl2 and in only 1 hour, the voltage rose to 3.074, which means a rise of 18mV. A 15 minute shutdown event without cathodic protection (ie, open circuit - OC) was performed to cause some corrosion of the cathode and generation of iron impurities in the electrolyte and after 22 hours of continuous electrolysis, the voltage reached 3.060 volts. This voltage is close to the original value, which indicates that part of the surface impurities and corrosion products on the cathode surface are removed by this sequence of events (shutdown with OC followed by hydrogen evolution).

Afterwards, a second shutdown in open circuit was performed for 15 minutes and during this event 10 ml of phosphoric acid (H ₃ PO ₄ -85%) was added to the 350 liters of electrolyte. Taking into account the density of the electrolyte 1.3 g/l and phosphoric acid 1.7 g/l, this addition corresponds to 0.047 g/l or 36 ppm of phosphate ions (PO ₄ ⁻³ ) to the electrolyte. This also corresponds to 15.3 mg/l of phosphorus in the electrolyte. This very small addition of phosphate compound resulted in a large voltage drop of 62 mV after 22 hours of constant polarization, as shown in Table 2 summarizing this series of experiments. Finally, a second addition of 10 ml phosphoric acid was made during another shutdown event of 15 minutes, and an additional voltage drop of 71 mV was observed after 16 hours of electrolysis. The total voltage drop (3.074 to 2.927 volts) from the value observed after calcium impurity addition was 147 mV. This is a large drop for a very small addition of phosphate ions corresponding to less than 0.1 g/l or 75 ppm. At these very low levels of phosphate in the electrolyte, there is no precipitation of iron phosphate compounds on the anode (which, as mentioned before, can be detrimental to the overall operation of the electrolysis cell).

Table 2: Series of events performed as part of electrolysis experiments

After the above experiment, the electrolyte was analyzed for impurity content and the results are shown in Table 3.

Table 3: Impurity content in the bath after the experiment

Ca P Fe Impurity content (mg/l) 2.9 19 4.3

Due to the addition of 4 ppm calcium and a total of 30.6 mg/l (twice the amount of 15.3 mg/l) of phosphorus to the electrolyte, Table 3 shows that there were still significant amounts of impurities in the electrolyte at the end of the experiment. Due to the protective coating on the cathode surface, residual calcium impurities in the bath no longer affect the cell voltage.

Figure 5 shows the x-ray diffraction spectrum of the surface of the electrode after the cathode was removed from the bath at the end of the experiment. The surface was air dried prior to analysis. The spectrum clearly reveals the presence of calcium oxyphosphate Ca ₁₀ (PO ₄ ) ₆ O from the dehydration of hydroxyapatite Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ . This demonstrates the presence of a thin layer of hydroxyapatite on the surface of the cathode in a chlorate electrolysis cell when both calcium impurities and very small amounts of phosphate are present in the electrolyte. This thin layer of hydroxyapatite protects the cathode surface from the deleterious effects of calcium impurities and leads to a significant drop in the cell voltage. At phosphate concentrations below 0.1 g/l or 75 ppm, conditions exist to promote the formation of hydroxyapatite at the cathode surface (including high pH, high temperature, and the presence of _Ca (OH) seeds) , even if the conditions for the precipitation of calcium phosphate compounds in the electrolyte bulk may be insufficient. Thus, calcium ions can still be present in the electrolyte in fairly high concentrations, but their negative impact on the cathode is reduced due to the protective layer of hydroxyapatite. Furthermore, the concentration of phosphate ions is low enough to avoid the formation of iron phosphate compounds on the anode surface.

Example 2

Figure 6 shows an electrochemical experiment similar to that shown in Figure 2 except using a stainless steel cathode. Specifically, Figure 6 shows a series of shutdowns in a chlorate electrolysis cell operated at 70°C and ^2.5kA /m2 with 400 series stainless steel cathodes and DSA anodes. Observations were made for current interruptions in the form of open circuit (OC) for 2, 5, 10 and 15 minutes followed by interruptions in the form of short circuit (CC) for 15 minutes. As can be seen by comparing Figure 6 with Figure 2, the cell with the stainless steel cathode had a potential 190 mV higher than that of the iron cell (4.42 - 4.23 volts). As mentioned before, this is due to the presence of chromium in the stainless steel alloy. Background _O2 release was also slightly higher by 0.4% (3.1% vs. 2.7%). Surprisingly, however, the cathodic current efficiency of the stainless steel cathode was much higher and remained high even during current interruptions.

This is due to the stainless steel surface which is stable even under corrosive conditions. In an open circuit, a very stable passivating surface layer forms on stainless steel. This passivating surface layer is similar to the chromium hydroxide layer formed by adding dichromate to the electrolyte and provides high cathodic current efficiency. The presence of chromium in stainless steel alloys therefore always provides a high current efficiency, whereas in the case of mild steel cathodes, the iron oxide scale reduces the current efficiency considerably every time there is a shutdown. Also noticeable on Figure 6 is that the open circuit voltage of the stainless steel electrolyzer is much lower than that of the iron electrolyzer (0.35 vs. 1.08 volts), which indicates that stainless steel is more noble than mild steel and as a result, under bipolar or short circuit conditions Under the galvanic corrosion will be significantly reduced. Furthermore, the high corrosion resistance of stainless steel reduces the total amount of iron impurities in the chlorate electrolyte and, as a result, reduces the possibility of iron phosphate formation on the anode.

To overcome the high overpotential of stainless steel, the stainless steel substrate can be coated with a thin catalytic layer that facilitates the hydrogen evolution reaction described in Equation 2. Examples of such catalytic layers are Fe3Al(Ru ₎ and _Fe3AlTa (Ru) alloys.

Figures 7a and 7b show scanning electron micrographs taken from a cross section of such a cathode structure comprising a 400 series stainless steel substrate and a thin catalytic top layer of the type previously described.

To complete the cathode structure according to one embodiment of the invention, a coating is added on top of the bilayer to protect the electrode from impurities in the electrolyte. The top surface layer may be the hydroxyapatite protective layer previously described.

Examples of overall cathode structures include: (i) a 400 series stainless steel substrate containing a small amount of carbon and containing no nickel; (ii) a catalytic interlayer such as based on an iron-aluminide metal matrix doped with a catalytic element such as Ru and A catalytic interlayer as described in Canadian Patent Document Nos. 2687129 and/or 2778865; and (iii) comprising or consisting essentially of hydroxyapatite on the surface of said catalytic interlayer and The top layer that protects the cathode from impurities present in the electrolyte.

As previously discussed, the top layer of hydroxyapatite on the cathode surface can be obtained by introducing small amounts of phosphate-containing compounds in the electrolyte (e.g. at less than 0.1 g/l or 75 ppm phosphate ion (PO ₄ ^-3 ) amount) and formed in situ. This surface layer of hydroxyapatite can also be prepared ex situ by depositing the coating prior to use of the electrode assembly in an electrochemical chlorate electrolysis cell. Indeed, coatings of hydroxyapatite can be deposited by several methods including thermal spraying, sputtering, dipping, sol-gel, electrochemical and electrophoretic deposition, biomimetic coating and hot isobaric coating. The most common method of manufacturing hydroxyapatite coatings is by plasma spraying, which is classified as a thermal spray technique.

Example 3

Figure 8 shows two scanning electron micrographs taken from a cross section of a cathode structure comprising a 400 series stainless steel substrate, a catalytic intermediate layer and a thin hydroxyapatite top layer. Note that a tungsten coating was provided on top of the sample prior to electron microscopy to protect the sample during processing.

Figure 9 shows a chemical map of a hydroxyapatite layer showing the presence of calcium and phosphorus elements.

Example 4

Electrolyzers comprising cathodes _of the Fe3Al(Ru) type were tested at room temperature with different amounts of calcium impurities added to the electrolyte. It can be noted that a 100-150 mV rise in only 1 or 2 hours when 1-2 ppm calcium is added indicates the negative effect of calcium on such electrodes. Figure 10 shows the results of these experiments.

Example 5

Figure 11 shows the voltage over time at a temperature of 68°C for an electrolysis cell containing such catalytically enhanced electrodes. After about 4 minutes of electrolysis, 2 ppm of Ca ⁺² ions were added to the electrolyte. The voltage is then systematically increased. After about 45 minutes of electrolysis, the phosphate ion PO4 ⁻³ was added in the form of phosphoric acid. The effect is to immediately stop the increase in cell voltage.

Example 6

In another experiment as reported in Figure 2, the voltage evolution was shown for an electrolyser similar to that in Example 5, except that phosphoric acid was added to the electrolyte prior to the addition of calcium ions. After about 2 hours of electrolysis, 2 ppm Ca ⁺² was added. No significant increase in voltage was observed after this addition of calcium.

Example 7

Figure 13 illustrates that low levels of phosphate prevent negative (polar) voltage rises caused by calcium impurities. Calcium is present in an amount of about 2 ppm.

Example 8: Calculation estimates for phosphate addition

As discussed above, phosphate may be added to or otherwise provided to the electrolyte based on the surface area of the cathode to be covered by the protective layer.

In cases where phosphate is added to the electrolyte to form a protective layer, the amount of phosphate can be determined in a number of ways. In one example, phosphate addition is predetermined based on a computational methodology, such as the computational methodology described below.

Various properties of calcium phosphate compounds can be measured or obtained from the literature, such as:

The molecular formula of hydroxyapatite: Ca ₅ (PO ₄ ) ₃ OH;

The molecular weight of hydroxyapatite: 502.31g;

Mass fraction of phosphate (PO ₄ ) in hydroxyapatite (284.91/502.31): 56.7%;

Calculated density of hydroxyapatite (100% dense): 3.156 g/cc; and

Porous hydroxyapatite can have a porosity of up to 90% (0.35 g/cc => 89%, from literature).

The maximum phosphate demand on the cathode can then be calculated as follows:

It is assumed that a protective layer of about 1 micron thickness is to be obtained, as this thickness has been found to be sufficient to provide protective properties;

For a surface of 1 cm ² => volume of the layer: 10 ⁻⁴ cm ³ ;

Phosphate root mass (100% compact) per cm ² cathode surface => 3.156g/ccx10-4ccx56.7% = 0.179mg;

Thus, less than 0.18 mg phosphate (PO4)/cm ² cathode surface can be provided; and

If a 1 micron thick surface layer of hydroxyapatite with 45% porosity is sufficient to prevent calcium from poisoning the catalytic interlayer, then 0.18 mg x (1-0.45) = 0.1 mg of phosphate (PO ₄ )/cm ² cathode would be required .

Claims (114)

1. electrochemical process, comprising:

Anode and negative electrode are arranged in the ionogen comprising the impurity comprising calcium ion;

Between described anode and described negative electrode, voltage is applied form the condition of electrolysate in described ionogen under; With

Phosphate anion is provided to measure as follows: described amount is enough to form protectiveness skin on the cathode with calcium ion at least partially in described ionogen; described protectiveness skin comprises calcium phosphate compound, and described amount avoids calcium phosphate compound to precipitate in described ionogen substantially.

2. the electrochemical process of claim 1, wherein phosphate anion adds with the amount of the surf zone contacted with described ionogen based on described negative electrode.

3. the electrochemical process of claim 1 or 2, is included in further before being immersed in described ionogen by described negative electrode on catalytic middle layer, applies described protectiveness skin.

4. the electrochemical process of claim 3, wherein applies the outer field step of described protectiveness and comprises dash coat, dip-coating, sol-gel, electrochemical deposition, bionical coating, the isobaric coating of heat or plasma spraying.

5. the electrochemical process of claim 1 or 2, is included in further after being immersed in described ionogen by described negative electrode on catalytic middle layer, forms described protectiveness skin.

6. the electrochemical process of claim 5, wherein forms the outer field step of described protectiveness and comprises:

Phosphate anion is provided in described ionogen;

Ensure to there is enough calcium ions in described ionogen; With

The electrolytic condition being enough to cause the outer field formation of described protectiveness is provided.

7. the electrochemical process of any one of claim 1-6, the outer field formation of wherein said protectiveness comprises:

Make Ca (OH) ₂with H ₃pO ₄reaction is to produce Ca ₃(PO ₄) ₂and water; With

Make Ca (OH) ₂with Ca ₃(PO ₄) ₂reaction is to produce Ca ₁₀(PO ₄) ₆(OH) ₂, wherein Ca ₁₀(PO ₄) ₆(OH) ₂form described protectiveness at least partially outer field.

8. the electrochemical process of any one of claim 1-7, wherein provides phosphate anion with the phosphate concentration being up to about 75ppm in described ionogen.

9. the electrochemical process of any one of claim 1-7, wherein phosphate concentration is about 50ppm for about 5ppm-.

10. the electrochemical process of any one of claim 1-9, wherein phosphate concentration is low to being enough to prevent from forming iron phosphate compounds or settling on described anode.

The electrochemical process of 11. any one of claim 1-10, wherein phosphate concentration is low to being enough to prevent the O in described ionogen ₂the rising of level.

The electrochemical process of 12. any one of claim 1-11, wherein phosphate concentration is low to being enough to the rising preventing voltage request.

The electrochemical process of 13. any one of claim 1-12, wherein phosphate anion is at least in part by H ₃pO ₄interpolation provided.

The electrochemical process of 14. any one of claim 1-13, wherein phosphate anion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 15. any one of claim 1-14, wherein calcium ion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 16. any one of claim 1-15, wherein said ionogen comprises for reacting first part's calcium ion to form calcium phosphate compound on the cathode with phosphate anion, and in described ionogen, keep unreacted second section calcium.

The electrochemical process of 17. any one of claim 1-16, wherein said technique is for the manufacture of chlorizate.

The electrochemical process of 18. claims 17, wherein said chlorizate comprises sodium chlorate and/or clorox.

The electrochemical process of 19. claims 17, wherein said chlorizate is sodium chlorate.

20. phosphate anions are in the purposes for manufacturing in the ionogen comprising calcium ion in the electrochemical process of chlorizate, and wherein phosphate anion is that to be enough to be formed on negative electrode with calcium ion at least partially the protectiveness comprising calcium phosphate compound outer and be enough to avoid the amount that calcium phosphate compound precipitates in described ionogen to provide.

The purposes of 21. claims 20, wherein said chlorizate comprises sodium chlorate and/or clorox.

22. electro-chemical systems, comprising:

For comprising electrolytical tank room, wherein said ionogen comprises calcium ion and phosphate anion;

Be arranged in the anode of described tank room;

Be arranged in the negative electrode of described tank room; With

Ion-select electrode device; it is configured to regulate the ion concentration in described ionogen, described ionogen is comprised be enough to be formed on the cathode with calcium ion at least partially the protectiveness comprising calcium phosphate compound outer and be enough to the phosphate anion of the amount avoiding calcium phosphate compound to precipitate in described ionogen.

The electro-chemical systems of 23. claims 22, wherein said ion-select electrode device comprises the entrance be communicated with described tank room fluid for being provided to by the phosphate anion of described amount in described tank room.

The electro-chemical systems of 24. claims 23, wherein said ion-select electrode device comprises at least one measuring apparatus of the concentration for measuring phosphate anion in described ionogen and/or calcium ion further.

The electro-chemical systems of 25. claims 24, wherein said ion-select electrode device comprises the controller being connected to described measuring apparatus and described entrance for controlling the input of phosphate anion in response to the reading from described measuring apparatus further.

The electro-chemical systems of 26. any one of claim 22-25, it is configured to for the manufacture of chlorizate.

The electro-chemical systems of 27. claims 26, wherein said chlorizate comprises sodium chlorate and/or clorox.

28. electrochemical process, comprising:

Anode and negative electrode are arranged in the ionogen comprising the impurity comprising calcium ion and iron ion;

Between described anode and described negative electrode, voltage is applied form the condition of electrolysate in described ionogen under; With

Phosphate anion is provided to measure as follows: described amount is enough to form protectiveness skin on the cathode with calcium ion at least partially in described ionogen; described protectiveness skin comprises calcium phosphate compound, and described amount is enough to substantially avoid iron phosphate compounds to precipitate in described ionogen.

The electrochemical process of 29. claims 28, wherein phosphate anion adds with the amount of the surf zone contacted with described ionogen based on described negative electrode.

The electrochemical process of 30. claims 28 or 29, is included in further before being immersed in described ionogen by described negative electrode on catalytic middle layer, applies described protectiveness skin.

The electrochemical process of 31. claims 30, wherein applies the outer field step of described protectiveness and comprises dash coat, dip-coating, sol-gel, electrochemical deposition, bionical coating, the isobaric coating of heat or plasma spraying.

The electrochemical process of 32. claims 28 or 29, is included in further after being immersed in described ionogen by described negative electrode on catalytic middle layer, forms described protectiveness skin.

The electrochemical process of 33. claims 32, wherein forms the outer field step of described protectiveness and comprises:

Phosphate anion is provided in described ionogen;

Ensure to there is enough calcium ions in described ionogen; With

The electrolytic condition being enough to cause the outer field formation of described protectiveness is provided.

The electrochemical process of 34. any one of claim 28-33, the outer field formation of wherein said protectiveness comprises:

Make Ca (OH) ₂with H ₃pO ₄reaction is to produce Ca ₃(PO ₄) ₂and water; With

Make Ca (OH) ₂with Ca ₃(PO ₄) ₂reaction is to produce Ca ₁₀(PO ₄) ₆(OH) ₂, wherein Ca ₁₀(PO ₄) ₆(OH) ₂form described protectiveness at least partially outer field.

The electrochemical process of 35. any one of claim 28-34, wherein provides phosphate anion with the phosphate concentration being up to about 75ppm in described ionogen.

The electrochemical process of 36. any one of claim 28-35, wherein phosphate concentration is about 50ppm for about 5ppm-.

The electrochemical process of 37. any one of claim 28-36, wherein phosphate concentration is low to being enough to prevent from forming iron phosphate compounds or settling on described anode.

The electrochemical process of 38. any one of claim 28-37, wherein phosphate concentration is low to being enough to prevent the O in described ionogen ₂the rising of level.

The electrochemical process of 39. any one of claim 28-38, wherein phosphate concentration is low to being enough to the rising preventing voltage request.

The electrochemical process of 40. any one of claim 28-39, wherein phosphate anion is at least in part by H ₃pO ₄interpolation provided.

The electrochemical process of 41. any one of claim 28-40, wherein phosphate anion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 42. any one of claim 28-41, wherein calcium ion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 43. any one of claim 28-42, wherein said ionogen comprises for reacting first part's calcium ion to form calcium phosphate compound on the cathode with phosphate anion, and in described ionogen, keep unreacted second section calcium.

The electrochemical process of 44. any one of claim 28-43, wherein said technique is for the manufacture of chlorizate.

The electrochemical process of 45. claims 44, wherein said chlorizate comprises sodium chlorate and/or clorox.

The electrochemical process of 46. claims 44, wherein said chlorizate is sodium chlorate.

47. phosphate anions are in the purposes for manufacturing in the ionogen comprising calcium ion and iron ion in the electrochemical process of chlorizate, and wherein said phosphate anion is that to be enough to be formed on negative electrode with calcium ion at least partially the protectiveness comprising calcium phosphate compound outer and be enough to avoid the amount that iron phosphate compounds precipitates in described ionogen to provide.

The purposes of 48. claims 47, wherein said chlorizate comprises sodium chlorate and/or clorox.

49. electro-chemical systems, comprising:

For comprising electrolytical tank room, wherein said ionogen comprises calcium ion, iron ion and phosphate anion;

Be arranged in the anode of described tank room;

Be arranged in the negative electrode of described tank room; With

Ion-select electrode device; it is configured to regulate the ion concentration in described ionogen, described ionogen is comprised be enough to be formed on the cathode with calcium ion at least partially the protectiveness comprising calcium phosphate compound outer and be enough to the phosphate anion of the amount avoiding iron phosphate compounds to precipitate in described ionogen.

The electro-chemical systems of 50. claims 49, wherein said ion-select electrode device comprises the entrance be communicated with described tank room fluid for being provided to by the phosphate anion of described amount in described tank room.

The electro-chemical systems of 51. claims 50, wherein said ion-select electrode device comprises at least one measuring apparatus of the concentration for measuring phosphate anion, iron ion and/or calcium ion in described ionogen further.

The electro-chemical systems of 52. claims 51, wherein said ion-select electrode device comprises the controller being connected to described measuring apparatus and described entrance for controlling the input of phosphate anion in response to the reading from described measuring apparatus further.

53. electrochemical process, comprising:

Anode and negative electrode are arranged in the ionogen comprising the impurity comprising calcium ion, described negative electrode has the surf zone contacted with described ionogen;

Between described anode and described negative electrode, voltage is applied form the condition of electrolysate in described ionogen under; With

Phosphate anion is provided with the following predetermined amount of the described surf zone based on described negative electrode: described predetermined amount makes phosphate anion be consumed in the outer field formation of protectiveness of the surf zone contacted with described ionogen covering described negative electrode with calcium ion at least partially in described ionogen.

The electrochemical process of 54. claims 53, the phosphate anion of wherein said predetermined amount is about 0.025mg/cm ²negative electrode-Yue 0.2mg/cm ²negative electrode.

The electrochemical process of 55. claims 53, the phosphate anion of wherein said predetermined amount is about 0.05mg/cm ²negative electrode-Yue 0.15mg/cm ²negative electrode.

The electrochemical process of 56. any one of claim 53-55, wherein said ionogen comprises iron ion further, and phosphate anion is the amount interpolation avoiding iron phosphate compounds to precipitate in described ionogen further.

The electrochemical process of 57. any one of claim 53-56, wherein phosphate anion is that the amount avoiding extra calcium phosphate compound to precipitate in described ionogen is added further.

The electrochemical process of 58. any one of claim 53-57, is included in further before being immersed in described ionogen by described negative electrode on catalytic middle layer, applies described protectiveness skin.

The electrochemical process of 59. claims 58, wherein applies the outer field step of described protectiveness and comprises dash coat, dip-coating, sol-gel, electrochemical deposition, bionical coating, the isobaric coating of heat or plasma spraying.

The electrochemical process of 60. any one of claim 53-57, is included in further after being immersed in described ionogen by described negative electrode on catalytic middle layer, forms described protectiveness skin.

The electrochemical process of 61. claims 60, wherein forms the outer field step of described protectiveness and comprises:

Phosphate anion is provided in described ionogen;

Ensure to there is enough calcium ions in described ionogen; With

The electrolytic condition being enough to cause the outer field formation of described protectiveness is provided.

The electrochemical process of 62. any one of claim 53-61, the outer field formation of wherein said protectiveness comprises:

Make Ca (OH) ₂with H ₃pO ₄reaction is to produce Ca ₃(PO ₄) ₂and water; With

Make Ca (OH) ₂with Ca ₃(PO ₄) ₂reaction is to produce Ca ₁₀(PO ₄) ₆(OH) ₂, wherein Ca ₁₀(PO ₄) ₆(OH) ₂form described protectiveness at least partially outer field.

The electrochemical process of 63. any one of claim 53-62, wherein provides phosphate anion with the phosphate concentration being up to about 75ppm in described ionogen.

The electrochemical process of 64. any one of claim 53-63, wherein phosphate concentration is about 50ppm for about 5ppm-.

The electrochemical process of 65. any one of claim 53-64, wherein phosphate concentration is low to being enough to prevent the O in described ionogen ₂the rising of level.

The electrochemical process of 66. any one of claim 53-65, wherein phosphate concentration is low to being enough to the rising preventing voltage request.

The electrochemical process of 67. any one of claim 53-66, wherein phosphate anion is at least in part by H ₃pO ₄interpolation provided.

The electrochemical process of 68. any one of claim 53-67, wherein phosphate anion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 69. any one of claim 53-68, wherein calcium ion provided by the intrinsic existence in described ionogen at least in part.

The electrochemical process of 70. any one of claim 53-69, wherein said ionogen comprises for reacting first part's calcium ion to form calcium phosphate compound on the cathode with phosphate anion, and in described ionogen, keep unreacted second section calcium.

The electrochemical process of 71. any one of claim 53-70, wherein said technique is for the manufacture of chlorizate.

The electrochemical process of 72. claims 71, wherein said chlorizate comprises sodium chlorate and/or clorox.

The electrochemical process of 73. claims 71, wherein said chlorizate is sodium chlorate.

The electrochemical process of 74. any one of claim 53-73, the phosphate anion of wherein said predetermined amount calculates based on the outer field target thickness of protectiveness to be obtained.

75. phosphate anions are in the purposes for manufacturing in the ionogen comprising calcium ion in the electrochemical process of chlorizate, and wherein phosphate anion provides with the following predetermined amount of the surf zone based on negative electrode: described predetermined amount makes phosphate anion be consumed in the outer field formation of protectiveness of the surf zone contacted with described ionogen covering described negative electrode with calcium ion at least partially.

The purposes of 76. claims 75, wherein said chlorizate comprises sodium chlorate and/or clorox.

77. electro-chemical systems, comprising:

For comprising electrolytical tank room, wherein said ionogen comprises calcium ion and phosphate anion;

Be arranged in the anode of described tank room;

Be arranged in the negative electrode of described tank room; With

Ion-select electrode device; it is configured to regulate the ion concentration in described ionogen, makes described ionogen comprise the phosphate radical of the following predetermined amount of the surf zone based on described negative electrode: described predetermined amount makes phosphate anion be consumed in the outer field formation of protectiveness of the surf zone contacted with described ionogen covering described negative electrode with calcium ion at least partially.

The electro-chemical systems of 78. claims 77, wherein said ion-select electrode device comprises the entrance be communicated with described tank room fluid for being provided to by the phosphate anion of described amount in described tank room.

The electro-chemical systems of 79. claims 78, wherein said ion-select electrode device comprises at least one measuring apparatus of the concentration for measuring phosphate anion in described ionogen and/or calcium ion further.

The electro-chemical systems of 80. claims 79, wherein said ion-select electrode device comprises the controller being connected to described measuring apparatus and described entrance for controlling the input of phosphate anion in response to the reading from described measuring apparatus further.

81. prefabricated negative electrodes, comprising:

Substrate;

Catalytic middle layer; With

The protectiveness comprising calcium phosphate compound is outer.

The prefabricated negative electrode of 82. claims 81, wherein said substrate comprises stainless steel.

The prefabricated negative electrode of 83. claims 82, wherein said stainless steel is 400 series stainless steels.

The prefabricated negative electrode of 84. any one of claim 81-83, wherein said substrate comprise have be enough to prevent electrolyzer close down the phase during iron ion enter the material of electrolytical erosion resistance.

The prefabricated negative electrode of 85. any one of claim 81-84, wherein said catalytic middle layer adjoins with the outside surface of described substrate.

The prefabricated negative electrode of 86. any one of claim 81-85, wherein said catalytic middle layer comprises the metallic matrix doped with catalytic compound.

The prefabricated negative electrode of 87. claims 86, wherein said metallic matrix is iron aluminide.

The prefabricated negative electrode of 88. claims 86 or 87, wherein said catalytic compound comprises Ru.

The prefabricated negative electrode of 89. any one of claim 81-88, wherein said calcium phosphate compound comprises hydroxyl calcium phosphate compound.

The prefabricated negative electrode of 90. any one of claim 81-89, wherein said protectiveness skin comprises hydroxyapatite.

The prefabricated negative electrode of 91. any one of claim 81-90, wherein said protectiveness skin is made up of hydroxyapatite substantially.

The prefabricated negative electrode of 92. any one of claim 81-91, wherein said protectiveness skin has the thickness of about 0.25 micron of-Yue 1.5 microns.

The prefabricated negative electrode of 93. any one of claim 81-91, wherein said protectiveness skin has the thickness of about 0.5 micron of-Yue 1 micron.

The prefabricated negative electrode of 94. any one of claim 81-93, the whole outside surface that wherein said protectiveness skin covers described catalytic middle layer directly contacts with the calcium impurities in described ionogen to prevent described catalytic middle layer.

The prefabricated negative electrode of 95. any one of claim 81-94, wherein said protectiveness skin is that dash coat, dip-coating, sol-gel applying, electrochemical deposition, bionical coating, the isobaric coating of heat or plasma spraying are on described catalytic middle layer.

The prefabricated negative electrode of 96. any one of claim 81-95, wherein said protectiveness skin has network structure.

The prefabricated negative electrode of 97. any one of claim 81-96, wherein said protectiveness skin has honeycomb structure.

The prefabricated negative electrode of 98. any one of claim 81-97, wherein said protectiveness skin has evolving hydrogen reaction lower can be occurred at it, prevent calcium impurities from poisoning the structure of described intermediate catalyst layer simultaneously.

The prefabricated negative electrode of 99. any one of claim 81-98, wherein said protectiveness skin has and makes it possible to stop chlorate anions and hypochlorite ion to arrive the surface of described intermediate catalyst layer to reduce or to avoid the structure of following reaction:

ClO ^-+ H ₂o+2e ^-=>Cl ^-+ 2OH ^-; And/or

ClO ₃ ^-+3H ₂O+6e ^-＝>Cl ^-+6OH ^-。

100. if the prefabricated negative electrode that limits in any one of claim 81-98 is for the manufacture of the purposes in the electrolyzer of chlorizate.

The purposes of 101. claims 100, wherein said chlorizate comprises sodium chlorate and/or clorox.

102. electrochemical process, comprising:

Anode and the prefabricated negative electrode as limited in any one of claim 81-98 are arranged in the ionogen comprising the impurity comprising calcium ion;

Between described anode and described prefabricated negative electrode, voltage is applied form the condition of electrolysate in described ionogen under.

The technique of 103. claims 102, comprises one or more features of claim 1-19, any one of 28-46 and 53-74.

104. for the manufacture of the method for the prefabricated negative electrode be used in the manufacture of chlorizate, and described method comprises:

Substrate is provided;

The top of described substrate provides catalytic middle layer; With

Be applied on described catalytic middle layer by protectiveness skin, wherein protectiveness skin comprises calcium phosphate compound.

The method of 105. claims 104, wherein said prefabricated negative electrode is as limited in any one of claim 81-98.

The method of 106. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises dash coat.

The method of 107. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises dip-coating.

The method of 108. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises sol-gel process.

The method of 109. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises electrochemical deposition.

The method of 110. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises bionical coating method.

The method of 111. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises the isobaric coating of heat.

The method of 112. claims 104 or 105, wherein applies the outer field step of described protectiveness and comprises plasma spraying.

113. electrochemical process, comprising:

Anode and negative electrode are arranged in the ionogen comprising the impurity comprising calcium ion, wherein said negative electrode comprises:

The substrate be made up of corrosion resistant material; With

Catalytic middle layer;

For the electrolysis phase, carry out electrolysis, it applies voltage under being included in the condition forming electrolysate in described ionogen between described anode and described negative electrode;

Periodically described electrolysis closed down for the phase of closing down, it comprises and stops described voltage, the described corrosion resistant material of wherein said substrate prevent described close down each of phase during iron ion be discharged in described ionogen; With

Phosphate anion is provided to measure as follows: described amount enough makes during each electrolysis phase in described ionogen; it is outer that phosphate anion and at least partially calcium ion form or again formed protectiveness on the catalytic middle layer of described negative electrode, and described protectiveness skin comprises calcium phosphate compound.

The technique of 114. claims 113, comprises one or more features of claim 1-19,28-46,53-74,102 and 103 any one.