Patent Application
20150354073
The present invention generally relates to the field of producing chlorinated products and, more particularly, to techniques for cathode protection and prefabricated cathode structures in the production of sodium chlorate.
Sodium chlorate (NaClO3) can be commercially produced by an electrochemical process according to the following overall reaction:
NaCl+3H2O=>NaClO3+3H2 (1)
Hydrogen discharge (H2) takes place on the cathodic side as:
2H2O+2e−=>2OH−+H2 (2)
and chlorate (ClO3−) is formed on the anodic side through a series of reactions:
2Cl−=>Cl2+2e−
Cl2+H2O=>HClO+Cl−+H+
HClO=>ClO−+H+
2HClO+ClO−=>ClO3−+2Cl−+2H+ (3)
On the cathodic side, the hydrogen current efficiency (CE) is defined as the ratio between the hydrogen flow rate (JH2) and the total applied current to the electrochemical cell:
CE=JH2/(I/2F) (4)
where I is the applied current and F is the Faraday number.
Mild steels with low carbon content are usually used for the cathode and dimensionally stable anodes (DSA) are used as the anode. In practice, the electrochemical reaction often takes place in undivided cells using mono-polar or bi-polar configurations. In the bi-polar configuration, the anodic part is in physical and electrical contact with the cathodic part and, as a result, severe galvanic corrosion problems can occur during shutdowns when cathodic protection is no longer in place.
A typical electrolytic solution includes about 550 g/l of NaClO3, 110 g/l of NaCl and 1-3 g/l of NaClO. The process typically takes place at pH around 6.5, temperatures between 60° C. and 85° C. and current densities between 2 and 4 kA/m2.
Several side reactions can lead to a reduced CE, such as the reduction of hypochlorite and chlorate on the cathodic side:
ClO−+H2O+2e−=>Cl−+2OH− (5)
ClO3−+3H2O+6e−=>Cl−+6OH− (6)
Another parasitic reaction that may be important, especially during start-up, is the reduction of iron oxides on steel cathodes, which leads to an oxygen burst release from the cell:
MO+2e−=>M+O2−(cathode)
O2−=>½O+2e−(anode) (7)
Oxygen can also be produced through the anodic oxidation of hypochlorite according to:
OCl−+H2O=>O2+2H++Cl−+2e− (8)
and it can also form from the decomposition of hypochlorite as:
2OCl−=>2Cl−+O2 (9)
2HClO=>O2+2HCl (10)
The decomposition of hypochlorite can be accelerated by the presence of Ni, Co or Cu ion impurities in the electrolyte. At one ppm level content, Ni, Co, and Cu can lead to an increase of O2 in the cell gas by 2.0%, 1.0%, and 0.7% respectively at 70° C. and 3 kA/m2 current density. Therefore, amongst the various elements which can be part of an electrochemical cell or an electrode, Ni is often particularly avoided.
To reduce the cathodic parasitic reactions mentioned previously and increase hydrogen CE, industries add dichromate (Na2Cr2O7) to the electrolyte generally at a concentration of 3-8 g/l. During cathodic polarisation, chromium (VI) is reduced to chromium (III) and a thin film of chromium (III) hydroxide forms on the cathode surface. This porous film helps to protect the cathode against corrosion, impede the transport of the anions to the electrode surface and hinder the unwanted side reactions while still allowing the hydrogen evolution reaction to take place.
Cr2O7−2+8H++6e−=>2Cr(OH)3+H2O (11)
Chromate also acts as pH buffer and it reduces the production of the oxygen by-product. Due to the toxicity of hexavalent chromium which increases process treatment costs, one would wish to find a substitute or an alternative to this practice. Therefore, it would be desirable to find a non-toxic coating material which, once deposited on the surface of the cathode, would inhibit the reduction of hypochlorite and chlorate and improve CE.
The electrolyte is highly corrosive and iron cathodes corrode readily in such environment when there is no cathodic protection. The corrosion reduces the lifetime of cathodes and also contaminates the electrolyte with iron impurities. Depending on the operating conditions and the frequency of shutdowns, cathode lifetime can be as short as five years. For this reason, some industries use thicker cathodes and operate at larger inter-electrode spacing to avoid short circuits by corrosion products. Moreover, iron oxides have been found to catalyze chlorate reduction and reduce CE. Nevertheless, iron cathodes are cheap, they have relatively low cathodic overpotential and their surfaces are renewed with the removal of the corrosion layer each time power interruption for an extended period of time occurs. Therefore, after a shutdown event, cell voltages are usually lower but CE efficient also until the chromium hydroxide film forms again on the surface.
Another challenge in the field of sodium chlorate production is corrosion on cathodes particularly when iron cathodes are used.
To minimize the effect of corrosion on cathodes, one may think of using stainless steel cathodes instead of iron, but stainless steels usually have much higher cathodic overpotentials because of chromium in the alloys. The overpotential of pure chromium is at least 100 mV higher than that of pure iron. Moreover, the austenitic stainless steel of the 300 series also contains Ni, which may affect the production of the oxygen by-product as mentioned previously. In the absence of dichromate, the CE and the rate of chlorate reduction depends strongly on the nature and properties of the electrode material. Research to find a substitute to iron cathodes has been ongoing for the last twenty to thirty years, but iron cathodes are still being used. Therefore, a new cathodic material which would be electro-active, highly resistant to corrosion and almost as noble as the DSA cathodes to minimize galvanic corrosion in bipolar configuration would be desirable.
Another challenge in the field of sodium chlorate production relates to electrolyte impurities. During electrolysis, one observes a gradual increase of the cell voltage when electrolyte impurities are plated or deposited on electrodes and blind or poison the electrocatalytic activity. Positively charged impurities such as Ca2+ and Mg2+ are attracted toward the cathode while negatively charge impurities such as sulphate ions move toward the anode under the effect of the electric field. The paper entitled “Electrolytic sodium chlorate technology: current status” by B. V. Tilak et al. published in the Electrochemical Society Proceedings, vol. 99-21 in 1999 mentions that the deposits usually found on cathodes are calcium and magnesium hydroxides. B. V. Tilak et al. indicate that Ca+2 impurities at the level of 1 ppm can lead to voltage increases of about 50 to 75 mV per month and about 100 mV per month at 1.5 ppm level. At the 20 ppb level, the voltage increase is of only 50 mV in two years of operation. Since most production plants operate at the ppm calcium level, they usually clean their cells several times a year using acid wash to remove these blind deposits.
Recently, novel cathodes presenting overpotentials about 200 mV lower than that of iron have been reported. Canadian patent No. 2687129 and Canadian patent application No. 2778865 of Schulz et al. describe new cathodic materials of the type Fe3Al(Ru) and Fe3AlTa(Ru), which can be used for producing sodium chlorate with improvements over iron. These materials have a catalytic species (Ru) within an iron aluminide metallic matrix. In spite of their efficiency for the hydrogen evolution reaction, these new cathodic materials are, unfortunately, also affected by calcium impurities. In some cases, calcium impurities above a fraction of ppm can have a negative impact on electrode performance.
Calcium impurities can also have a negative impact in the case of the chlor-alkali technology where highly porous membranes or diaphragms are used to separate the anolyte and catholyte compartments. Calcium impurities at the ppm level can block these membranes quite readily and for this reason, methods have been developed to reduce calcium impurities to the ppb level. U.S. Pat. No. 4,176,022 of Darlington described in 1979 such a method. After removing the particulates, one usually starts by treating the brine with soda ash to precipitate a major portion of the calcium in the form of calcium carbonate which is separated from the electrolyte by filtration or other physical separation methods. This usually brings the calcium to 2-3 ppm levels. Thereafter, one may pass the electrolyte through an ion exchange column to obtain brine containing less than about 0.5 ppm (500 ppb) of calcium. Finally, U.S. Pat. No. 4,176,022 proposes the addition a phosphate to the alkaline brine to form a calcium phosphate compound believed to be calcium apatites substantially insoluble in brine and thereafter, separating the compound from the electrolyte. The pH of the brine is maintained above 10 and the temperature above 40° C. during the formation of the compound to further decrease its solubility in the brine. They also propose to add seeds to the electrolyte such as calcium phosphate (Ca3(PO4)2) or calcium hydroxide to ease the precipitation reaction. The final calcium impurity level is around 20 parts per billion. Typically, the concentration of phosphate added to the brine is from about 0.1 to about 1 wt %. As an example, they add 0.44 g and 2.24 g of phosphoric acid (85 percent H3PO4) to one liter of brine to reduce the calcium to 200 ppb and 20 ppb levels respectively. More recently, in 2008, Canadian patent application No. 2 655 726 proposed a similar method to remove calcium from brine. This application teaches the addition of 2 g of Na2HPO4 to 60 ml of electrolyte to reduce the calcium impurity level to below the ppm level. Such references disclose the addition of phosphate in relatively high amounts with the purpose of precipitating calcium ions out of the bulk solution in order to obtain a targeted final calcium impurity level.
In U.S. Pat. No. 4,004,988, Mollard et al. propose to use a method of adding phosphoric acids or an alkali-metal salt of these acids to the electrolyte for complexing calcium in undivided cells used for the production of sodium chlorate. Mollard et al. disclose that the complexing agent removes a large fraction of the calcium during the course of electrolysis in the form of an easily filterable precipitate. In an example, Mollard et al. add 0.5 to 2 g of sodium tripolyphosphate (Na5P3O10) to one kg of brine containing 60 ppm to 100 ppm of calcium which is equivalent to about 0.7 g to 2.6 g per liter. After passage through an electrolytic cell, the solution is filtered and found to contain 5 ppm to 10 ppm of calcium. As in the previously described references, the concentration of phosphate additive is in the range of gram per liter of brine which means in the 1000 ppm range.
UK patent application No. 2039959 discloses a similar method but instead of adding the phosphoric acid directly to the brine, the phosphoric acid is mixed with the hydrochloric acid that is regularly added to the brine for balancing the pH. In an example, for brine containing about 35 ppm of calcium and 2 ppm of Fe, they add 1 g to 2 g of phosphoric acid (85 percent H3PO4) per kg of sodium chlorate produced in one case and 0.15 g of acid per liter of electrolyte in a second case. These quantities are relatively high and are in a similar range as the ones previously disclosed in other references.
US patent application No. 2008/0230381A1 of N. Krstajic et al. also discloses adding at least 1 g/l of phosphate ions to sodium chloride brines to act as a buffering agent. As a cathode, Krstajic et al. propose a low carbon steel substrate coated with an electrodeposited layer of a Fe—Mo alloy whose thickness is between 10 μm and 50 μm. With the addition of phosphate, Krstajic et al. mention that the observed voltage decrease with this type of activated coating can reach values as high as 500 mV at the usual current densities of 2.5-3 kA/m2 while the decrease is limited to 100-150 mV with known brines. Krstajic et al. also suggest that the dichromate concentration in the electrolyte can be reduced to 0.1 g/l or even be eliminated completely without greatly affecting the CE.
A problem with known methods concerns the fact that elevated amounts of phosphate additive are added with the goal of removing a significant amount of calcium impurities in the electrolyte and to bring the calcium impurities to levels where they are no longer affecting the cathodes. The addition of large amounts of anions such as PO4−3 can lead to additional problems when iron impurities are also present in the electrolyte. In the paper entitled “Effects of Electrolyte Impurities in Chlorate Cells” R. A. Kus mentions that voltage increases are observed when iron and phosphate impurities are present together at a significant concentration level due to a synergetic effect between these impurities which affects anode performance. For this reason, industry currently tends not to use phosphoric acid additives since almost all of them uses iron cathodes and therefore have iron impurities in their electrolyte in addition to calcium impurities.
In view of the various challenges in the field of sodium chlorate production, there is a need for a technology that provides at least some solutions.
The present invention responds to above mentioned need by providing enhanced techniques for the production of chlorinated products, such as sodium chlorate.
In some implementations, there is provided an electrochemical process including providing an anode and a cathode in an electrolyte comprising impurities comprising calcium ions; applying a voltage between the anode and the cathode under conditions to form an electrolysis product in the electrolyte; and providing phosphate ions in the electrolyte in an amount sufficient to form with at least a portion of the calcium ions a protective external layer on the cathode, the protective external layer comprising a calcium phosphate compound, and to substantially avoid precipitation of calcium phosphate compounds in the electrolyte. It should be noted that the step of providing phosphate ions may be done before, after or during the step of applying the voltage.
In some implementations, the phosphate ions are added in an amount based on a surface area of the cathode that is in contact with the electrolyte.
In some implementations, the process further includes applying the protective external layer on the catalytic intermediate layer prior to immersing the cathode into the electrolyte.
In some implementations, the step of applying the protective external layer comprises sputter coating, dip coating, sol-gel, electrochemical deposition, biomimetic coating, hot isostatic coating, or plasma spraying.
In some implementations, the process further includes forming the protective external layer on the catalytic intermediate layer after immersing the cathode into the electrolyte.
In some implementations, the step of forming the protective external layer comprises providing phosphate ions in the electrolyte; ensuring sufficient calcium ions are present in the electrolyte; and providing electrolytic conditions sufficient to induce formation of the protective external layer.
In some implementations, formation of the protective external layer comprises reacting Ca(OH)2 with H3PO4 to produce Ca3(PO4)2 and water; and reacting Ca(OH)2 with Ca3(PO4)2 to produce Ca10(PO4)6(OH)2, wherein the Ca10(PO4)6(OH)2 forms at least part of the protective external layer.
In some implementations, the phosphate ions are provided in the electrolyte in a phosphate concentration up to about 75 ppm, or between about 5 ppm and about 50 ppm.
In some implementations, the phosphate concentration is sufficiently low to prevent formation of an iron phosphate compound or deposit 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 at least partially provided by addition of H3PO4. The phosphate ions may also be at least partially provided by inherent presence in the electrolyte. The calcium ions may also be at least partially provided by inherent presence in the electrolyte.
In some implementations, the electrolyte comprises a first portion of calcium ions for reacting with the phosphate ions to form the calcium phosphate compound on the cathode, and a second portion of calcium remaining unreacted in the electrolyte.
In some implementations, the process is for producing chlorinated products. The chlorinated products may include sodium chlorate and/or sodium hypochlorite.
In some implementations, there is provided a use of phosphate ions in an electrochemical process for producing chlorinated products in an electrolyte comprising calcium ions, wherein the phosphate ions are provided in an amount sufficient to form with at least a portion of the calcium ions a protective external layer comprising a calcium phosphate compound on a cathode and to avoid precipitation of calcium phosphate compounds in the electrolyte.
In some implementations, there is provided an electrochemical system comprising an electrolysis chamber for containing an electrolyte, wherein the electrolyte comprises calcium ions and phosphate ions; an anode located in the electrolysis chamber; a cathode located in the electrolysis chamber; and an ion adjuster configured to adjust ion levels in the electrolyte such that the electrolyte comprises an amount of phosphate ions sufficient to form with at least at least a portion of the calcium ions a protective external layer comprising a calcium phosphate compound on the cathode and to avoid precipitation of calcium phosphate compounds in the electrolyte.
In some implementations, the ion adjuster comprises an inlet in fluid communication with the electrolysis chamber for providing the amount of phosphate ions into the electrolysis chamber. The ion adjuster may further include at least one measurement device for measuring the concentration of phosphate ions, ferric ions and/or calcium ions in the electrolyte. The ion adjuster may also include a controller coupled to the measurement device and the inlet for controlling an input amount of phosphate ions in response to readings from the measurement device. The system may be configured for producing chlorinated products, such as sodium chlorate and/or sodium hypochlorite.
In some implementations, there is provided an electrochemical process comprising providing an anode and a cathode in an electrolyte impurities comprising calcium ions and ferric ions; applying a voltage between the anode and the cathode under conditions to form electrolysis product in the electrolyte; and providing phosphate ions in the electrolyte in an amount sufficient to form with at least a portion of the calcium ions a protective external layer on the cathode, the protective external layer comprising a calcium phosphate compound, and 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 a use of phosphate ions in an electrochemical process for producing chlorinated products in an electrolyte comprising calcium ions and ferric ions, wherein the phosphate ions are provided in an amount sufficient to form with at least a portion of the calcium ions a protective external layer comprising a calcium phosphate compound on a cathode and to avoid precipitation of iron phosphate compounds in the electrolyte.
In some implementations, there is provided an electrochemical system comprising an electrolysis chamber for containing an electrolyte, wherein the electrolyte comprises calcium ions, ferric ions and phosphate ions; an anode located in the electrolysis chamber; a cathode located in the electrolysis chamber; and an ion adjuster configured to adjust ion levels in the electrolyte such that the electrolyte comprises an amount of phosphate ions sufficient to form with at least a portion of the calcium ions a protective external layer comprising a calcium phosphate compound on the cathode and to avoid precipitation of iron phosphate compounds in the electrolyte.
In some implementations, there is provided an electrochemical process comprising providing an anode and a cathode in an electrolyte comprising impurities comprising calcium ions, the cathode having a surface area in contact with the electrolyte; applying a voltage between the anode and the cathode under conditions to form an electrolysis product in the electrolyte; and providing phosphate ions in the electrolyte in a pre-determined amount based on the surface area of the cathode such that the phosphate ions and at least a portion of the calcium ions are consumed in the formation of a protective external layer covering the surface area of the cathode in contact with the electrolyte.
In some implementations, the pre-determined amount of phosphate ions is between about 0.025 mg per cm2 of the cathode and about 0.2 mg per cm2 of the cathode, or between about 0.05 mg per cm2 of the cathode and about 0.15 mg per cm2 of the cathode.
In some implementations, the electrolyte further comprises ferric ions and the phosphate ions are further added in an amount to avoid precipitation of iron phosphate compounds in the electrolyte. The phosphate ions may be further added in an amount to avoid precipitation of additional calcium phosphate compounds in the electrolyte.
In some implementations, the pre-determined amount of phosphate ions is calculated based on a target thickness of the protective external layer to obtain.
The above process may also have one or more features as described in other implementations described herein.
In some implementations, there is provided a use of phosphate ions in an electrochemical process for producing chlorinated products in an electrolyte comprising calcium ions, wherein the phosphate ions are provided in a pre-determined amount based on the surface area of the cathode such that the phosphate ions and at least a portion of the calcium ions are consumed in the formation of a protective external layer covering the surface area of the cathode in contact with the electrolyte.
In some implementations, there is provided an electrochemical system comprising an electrolysis chamber for containing an electrolyte, wherein the electrolyte comprises calcium ions and phosphate ions; an anode located in the electrolysis chamber; a cathode located in the electrolysis chamber; and an ion adjuster configured to adjust ion levels in the electrolyte such that the electrolyte comprises a pre-determined amount of phosphate based on the surface area of the cathode such that the phosphate ions and at least a portion of the calcium ions are consumed in the formation of a protective external layer covering the surface area of the cathode in contact with the electrolyte.
In some implementations, there is provided a prefabricated cathode comprising a substrate; a catalytic intermediate layer; and an protective external layer comprising a calcium phosphate compound.
In some implementations, the substrate comprises stainless steel. The stainless steel may be a 400 series stainless steel. The substrate may include a material with sufficient corrosion resistance to prevent ferric ions from entering an electrolyte during shutdown periods of an electrolytic cell.
In some implementations, the catalytic intermediate layer is contiguous with an outer surface of the substrate.
In some implementations, the catalytic intermediate layer comprises a metal matrix doped with a catalytic compound. The metal matrix may be an iron aluminide. The catalytic compound comprises Ru.
In some implementations, the calcium phosphate compound comprises a hydroxy calcium phosphate compound, such as hydroxyapatite. The protective external layer may consist essentially of hydroxyapatite.
In some implementations, the protective external layer has a thickness between about 0.25 micron and about 1.5 microns. The protective external layer may have a thickness between about 0.5 micron and about 1 micron.
In some implementations, the protective external layer is provided in order to cover an 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 external layer may be sputter coated, dip coated, sol-gel applied, electrochemically deposited, biomimetically coated, hot isostatically coated, or plasma sprayed onto the catalytic intermediate layer.
In some implementations, the protective external layer has a reticulum structure. The protective external layer may have a honeycomb structure.
In some implementations, the protective external layer has a structure enabling the hydrogen evolution reaction to take place there-under while preventing calcium impurities from poisoning the intermediate catalytic layer.
In some implementations, the protective external layer has a structure enabling blocking of chlorate and hypochlorite ions from reaching a surface of the intermediate catalytic layer to reduce or avoid the following reactions: ClO−+H2O+2e−=>Cl−+2OH−; and/or ClO3+3H2O+6e−=>Cl−+6 OH−.
In some implementations, there is provided a use of the prefabricated cathode as define herein, in an electrolytic cell for producing a chlorinated product, such as sodium chlorate and/or sodium hypochlorite.
In some implementations, there is provided an electrochemical process comprising providing an anode and the prefabricated cathode as defined herein in an electrolyte comprising impurities comprising calcium ions; and applying a voltage between the anode and the prefabricated cathode under conditions to form an electrolysis product in the electrolyte. This 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 production of a chlorinated product, including providing a substrate; providing a catalytic intermediate layer on top of the substrate; and applying an protective external layer onto the catalytic intermediate layer, wherein protective external layer comprises a calcium phosphate compound.
The method may be performed to produce the prefabricated cathode having one or more features as described herein.
In some implementations, the step of applying the protective external layer comprises sputter coating, dip coating, sol-gel methods, electrochemical deposition, biomimetic coating methods, hot isostatic coating and/or plasma spraying.
In some implementations, there is provided an electrochemical process comprising providing 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; performing electrolysis for electrolysis periods, comprising applying a voltage between the anode and the cathode under conditions to form an electrolysis product in the electrolyte; periodically shutting down the electrolysis for shutdown periods, comprising terminating the voltage, wherein the corrosion resistant material of the substrate prevents release of ferric ions into the electrolyte during each of the shutdown periods; and providing phosphate ions in the electrolyte in an amount sufficient such that, during each electrolysis period, the phosphate ions form or re-form with at least a portion of the calcium ions a protective external layer on the catalytic intermediate layer of the cathode, the protective external layer comprising a calcium phosphate compound.
The above process may also have one or more features as described in other implementations described herein.
In some implementations, there is provided an electrochemical process comprising providing an anode and a cathode in an electrolyte comprising impurities comprising alkaline earth metal ions; applying a voltage between the anode and the cathode under conditions to form an electrolysis product in the electrolyte; and providing phosphate ions in the electrolyte in an amount sufficient to form with at least a portion of the alkaline earth metal ions a protective external layer on the cathode, the protective external layer comprising an alkaline earth metal phosphate compound, and to substantially avoid precipitation of alkaline earth metal phosphate compounds in the electrolyte. The alkaline earth metal may be calcium. It is also noted that various implementations of the processes described herein may be performed in relation to an alkaline earth metal in general rather than calcium in particular.
In some implementations, there is provided an electrochemical process comprising: providing an anode and a cathode in an electrolyte comprising impurities; applying a voltage between the anode and the cathode under conditions to form an electrolysis product in the electrolyte; and ensuring that sufficient phosphate ions and calcium ions are present in the electrolyte such that the phosphate ions form with at least a portion of the calcium ions a protective external layer on the cathode, the protective external layer comprising a calcium phosphate compound, and to substantially avoid precipitation of calcium phosphate compounds in the electrolyte, to substantially avoid precipitation of iron phosphate compounds in the electrolyte when ferric ions are present in the electrolyte, and/or to consume substantially all of the phosphate ions in the formation of the protective external layer.
It is also noted that one or more further optional features, aspects and implementations of processes, systems, uses, cathodes and methods of making cathodes, which will be described in further details in the below description, may be combined with various implementations described above.
a and 7b are scanning electron microscope pictures of the cross-sections of cathodic electrode structures.
Various techniques that are described herein for electrolysis leverage the use of a relatively small amount of phosphate in an electrolytic system to form a protective layer on the cathode. Further techniques provide a prefabricated cathode that has a protective layer including a calcium phosphate compound for use in the electrolytic production of chlorinated products, such as sodium chlorate (NaClO3 or ClO3) that is a compound used in paper bleaching applications or sodium hypochlorite (NaClO or ClO−) that is a water treatment agent. It should be understood that while various implementations and aspects will be discussed herein in relation to producing sodium chlorate, such techniques may be adapted to the production of other chlorinated 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 an protective external layer including a calcium phosphate compound, which is formed by the addition of phosphate ions in an amount to induce formation of the calcium phosphate compound at the surface of the cathode while preventing precipitation of calcium phosphate compounds and/or iron phosphate compounds that can have deleterious effects on the electrolysis. The phosphate may be provided in an amount based on the surface area of the cathode in order to form the protective external layer.
While other methods have emphasized the addition of higher levels of phosphate ions with the aim of precipitating calcium ions out of the electrolyte, various techniques described herein provide for the advantageous addition of a reduced amount of phosphate ions sufficient to form the protective layer on the surface of the cathode. In some scenarios, the phosphate ions can be provided in notably low levels to form the protective layer, which not only enables improved functioning of the electrolysis despite remaining calcium ions in the electrolyte, but also reduces or prevents the formation of iron phosphate compounds that can have a negative impact on the electrolysis. Thus, while the electrolyte may have various impurities, such as calcium ions and ferric ions, the protective external layer can provide protection against electrolyte impurities and maintain efficient electrolytic operations.
In some instances, beneficial aspects of phosphate additives can be achieved at phosphate concentrations that are one or more orders of magnitude lower than the concentrations previously used. Instead of adding phosphates in accordance with the calcium ion concentration of the electrolyte with the goal of precipitating calcium impurities, the phosphates can be provided with the purpose of forming the calcium phosphate protective layer in order to counteract negative effects of calcium on cathode surfaces in undivided sodium chlorate cells.
Providing such a protective layer may be particularly advantageous when used in combination with cathodes that include a substrate, such as stainless steel, and a catalytically active coating, such as a metallic matrix doped with a catalytic species like Ru. Calcium impurities can interfere with such catalytically active coatings in the process of the hydrogen evolution reaction (Equation 2) and form calcium hydroxides, resulting in cell voltage increases gradually with the built-up of this blinding deposit. The conditions to induce deposition of insoluble calcium phosphate compounds, such as hydroxyapatite, can be provided close to the surface of cathodes. Even if the average pH in the bulk electrolyte is maintained around 6.5, the pH near the surface of the cathode is much higher, for instance well over 10, because of the formation of hydroxyl groups according to Equation 2. The local elevated pH decreases the local solubility of calcium phosphate compounds and results in the formation of a deposited layer. In addition, calcium hydroxide forms naturally on the surface of cathodes and these clusters of calcium hydroxide can act as seeds for the formation of calcium phosphate (Ca3(PO4)2). When small amounts of phosphoric acid are present near the surface of the cathode, the following reaction takes place:
3Ca(OH)2+2H3PO4=>Ca3(PO4)2+6H2O (12)
These calcium phosphate molecules further react with calcium hydroxide to form hydroxyapatite on the surface of the cathode according to:
Ca(OH)2+3Ca3(PO4)2=>Ca10(PO4)6(OH)2 (13)
The high temperatures typical of industrial chlorate operations (about 60° C. to about 85° C.) are also beneficial for the deposition because the higher the temperature, the lower is the solubility of the calcium phosphate compounds. Indeed, the conditions for the heterogeneous formation of hydroxyapatite on the surface of the cathodes are provided even though they may not be adequate for the precipitation of phosphate compounds in the bulk of the electrolyte. Various techniques described herein leverage such conditions for the pre-determined addition of phosphate to be substantially fully consumed in the formation of the calcium phosphate protective layer on the cathode surface. It is also noted that one or more other calcium phosphate compounds, such as Ca3(PO4)2, may also be present and have beneficial protective properties.
Various aspects of the invention will be described below, including systems and processes for the production of sodium chlorate, prefabricated cathodes as well as uses and methods of making prefabricated cathodes.
Electrolytic Systems for Sodium Chlorate Production
Still referring to
The cathode 20, which may also be referred to as the cathodic electrode structure, may include three or more layers. In some scenarios, the cathodic electrode structure has three layers including the substrate 22, the intermediate catalytic layer 24 that is provided directly onto the substrate 22, and the protective external layer 26 that is provided directly onto the intermediate catalytic layer 24. It may also be envisioned to provide one or more additional layers in between the other layers.
In some implementations, the substrate 22 may be composed of a corrosion resistant material, such as stainless steel that may be from the 400 series. More regarding the substrate will be described further below.
In some implementations, the intermediate catalytic layer 24 may be composed of a high porosity highly active catalytic material, such as Fe3Al(Ru) and Fe3AlTa(Ru). More regarding the intermediate catalytic layer will be described further below.
In some implementations, the protective external layer 26 includes a calcium phosphate compound. More regarding the formation and properties of the protective external layer will be described further below.
Phosphate Dosing and Formation of Protective Layer
The protective external layer may be formed in a number of ways. In one scenario, the protective external layer is formed in situ within the electrolytic cell. In other scenarios, the protective external layer may be formed ex situ in order to make a prefabricated cathode that can be used in the electrolysis system. More regarding the prefabricated cathode and ex situ methods will be described further below.
As noted above, the protective external layer may be formed in situ within the electrolytic cell by the addition of phosphate ions and, in some cases, calcium ions.
The phosphate ions may be added in a relatively low amount that is sufficient to form the protective external layer and to avoid one or more other reactions such as the precipitation of calcium or ferric ions from the bulk electrolyte solution and/or deposition of iron phosphate compounds at the anode.
In terms of the quantity of phosphate ions added to the electrolyte, this may depend on a number of factors including the 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 composition and properties of the electrolyte, and so on.
In some scenarios, the phosphate is added in an amount sufficient to form the protective layer with a thickness between about 0.25 micron and about 1.5 microns, between about 0.5 micron and about 1 micron, or between about 0.6 micron and about 0.9 micron.
In addition, a general guideline may 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 instance, in one scenario that is discussed in the Examples section below, about 0.1 mg of phosphate (PO4) per cm2 of cathode may be added. It should therefore be understood that the amount of phosphate to add may be determined based on the surface area of the cathode rather than the volume of the electrolyte or the quantity of calcium ions present in the electrolyte. For instance, the amount of phosphate to add may be between about 0.025 mg per cm2 of cathode and about 0.2 mg per cm2 of cathode, or 0.05 mg per cm2 of cathode and about 0.15 mg per cm2 of cathode, for example.
Furthermore, the amount of phosphate to add may be pre-determined by calculations and/or empirical tests, 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 is added to the electrolyte in order to form the protective layer on the cathode.
In some scenarios, the electrolyte may not initially contain sufficient calcium ions to form the protective layer with the phosphate ions. Addition of calcium ions may be performed before, during or after the addition of phosphate ions, in a sufficient amount to allow formation of the protective layer.
It should also be noted that the phosphate may be added as a one-time dose or may be added periodically in incremental doses.
Referring back to
Referring briefly back to
In some implementations, the phosphate is added in the form of phosphoric acid H3PO4. However, it should be note that the phosphate may be added in other forms, such as acids or salts, as monophosphate or polyphosphate compounds, or in other forms.
Furthermore, addition of the phosphate may be performed in order to promote substantially total consumption of the phosphate in the formation of the protective layer. For example, controlling the conditions of the electrolytic system (e.g. temperature, pH, composition of electrolyte, etc.) so as to favor the formation of the calcium phosphate compounds on the surface of the cathode may limit or prevent any other reactions involving phosphate. This controlling step may be done at or near the start of the electrolytic operations so that the protective layer can form as quickly as possible. As will be described further below, the cathode may also be coated ex situ with the protective layer using a number of methods, such that the cathode has the protective layer upon commencing the electrolytic operations.
Prefabricated Cathodes and Ex Situ Manufacturing
The prefabricated cathode may include the substrate, the catalytic layer and the protective layer. The prefabricated cathodes described herein may be used to replace iron cathodes currently used in the industry, in order to provide corrosion resistance, high activity toward the hydrogen evolution reaction, a good hydrogen current efficiency and low hydrogen overpotential. In some cases, the prefabricated cathodes do not catalyze the reduction of hypochlorite and chlorate and do not produce significant oxygen by-product.
The prefabricated cathodes can provide several advantages, such as providing a protective coating of the catalytic layer at the beginning of the electrolytic operations and also providing tolerance to electrolyte impurities and contribute to counteracting negative effects of calcium impurities on cathode surfaces.
The method of manufacturing the prefabricated cathode may include the step of applying the protective external layer by sputter coating, dip coating, sol-gel methods, electrochemical deposition, biomimetic coating, hot isostatic coating, or plasma spraying. The method of applying the protective layer may be chosen and carried out in order to obtain certain properties of the protective layer, such as a layer thickness, permeability and porosity within certain ranges.
Referring to
In some scenarios, the prefabricated cathodes may remain in the electrolytic cell until the end of their working life. If the pre-applied protective coating is damaged or partially removed, an in situ method of regenerating the protective layer may be used, such as adding a pre-determined amount of phosphate as described herein. In addition, in some situations, after a certain period of operation the electrolyte may be removed from the electrolytic cell 12 and the cell may be cleaned by introducing a mild acid liquor that is usually provided in the cell for about one hour. If such cleaning or other maintenance operations are conducted which remove the protective external layer from the cathodes, one may reapply the protective external layer in situ to enable continued protection during the next phase of electrolysis.
Alternatively, after a certain period of operation, the used cathodes 40 may be removed from the electrolytic cell 12 for cleaning and/or maintenance. The used cathodes may be removed, for example, during shutdown operations of the electrolytic cell 12. The used cathode 40 may be provided to a cleaning unit 42 in order to inspect and clean the cathode to produce a cleaned cathode 44. The cleaning process may remove the protective layer from the cathode structure and thus the cleaned cathode 44 may be supplied to the pre-treatment unit 38 in order to reapply the protective layer so that the cathode can be reused in the electrolytic cell.
Cathode Substrates and Catalytic Layers
In some implementations, as mentioned previously, the cathode has a structure including a substrate and a catalytic layer.
The substrate may include a stable corrosion resistant substrate which does not contain nickel. The substrate may be stainless steel of the 400 series or another material having anti-corrosion properties. Stainless steel of the 400 series may be used, in spite of having higher cathodic overpotentials. As shown in the below Table 1 listing commercial stainless steel available from the AK Steel company, these alloys have very low carbon contents similar to those of mild steels and no nickel. They are highly corrosion resistant in media containing chloride ions. In electrolytic operations, corrosion problems can occur during shutdowns when there is no cathodic protection and mild steel cathodes can suffer from degradations. The corrosion product on the steel can remove the surface layers at each power interruption. While calcium and phosphate still present in the electrolyte can lead to the regeneration of the protective layer, another advantageous feature is to provide an improved substrate compared to mild steel.
The catalytic layer may include a metallic matrix doped with a catalytically active compound. For instance, the catalytic layer may include Fe3Al(Ru) and Fe3AlTa(Ru), which can be used for producing sodium chlorate with improvements over iron. These materials have a catalytic species (Ru) within an iron aluminide metallic matrix. In spite of their efficiency for the hydrogen evolution reaction, these new cathodic materials are, unfortunately, also affected by calcium impurities. As shown in
In order to have a sufficient lifetime, cathodes should be able to sustain power interruptions. The most severe corrosion conditions often occur during a power interruption when there is no cathodic current protection.
In some scenarios, the prefabricated cathodes have a structure that can sustain multiple current interruptions.
Various aspects and implementations described herein provide advantages such as reduced phosphate demand, prolonged operating times between cell cleaning, maintenance of low voltage levels, enhanced protection of cathodes, reduced negative impact on anodes, and so on.
Various examples and experimental results will now be described.
These blind deposits are responsible for cell voltage increases observed after a long period of operation.
Beneficial effects of the addition of very small amounts of phosphate additives on the performance of cathodes were observed in the following experiment.
A bath of 350 litres of sodium chlorate electrolyte containing 550 g/l of NaClO3, 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. A steel cathode and a DSA anode were used in the experiment. After 74 hours of continuous electrolysis at 2.51 kA/m2, the cell voltage was 3,056 volt. Then 4 ppm of calcium impurities was added to the electrolyte in the form of CaCl2 and in only one hour, the voltage raised to 3,074, which means an increase of 18 mV. A shutdown event of 15 min without cathodic protection (i.e. open circuit—OC) to induce some corrosion of the cathode and generate iron impurities in the electrolyte was performed and, after 22 hours of continuous electrolysis, the voltage reached 3,060 volt. This voltage is near the original value which suggests that part of the surface impurities and corrosion product on the cathode surface was removed by this sequence of events (shutdown in OC followed by hydrogen discharge).
Thereafter, a second shutdown in open circuit of 15 min was conducted and during this event, 10 ml of phosphoric acid (H3PO4 —85%) was added to the 350 litres of electrolyte. Considering a density of electrolyte of 1.3 g/l and the density of phosphoric acid of 1.7 g/l, this addition corresponds to 0.047 g/l or 36 ppm of phosphate ions (PO4−3) to the electrolyte. This is also equivalent to 15.3 mg/l of phosphorus in the electrolyte. This very small addition of phosphate compound lead to a large voltage drop of 62 mV after 22 hours of constant polarization as indicated in the Table 2, which summarizes the series of experiments. Finally, a second addition of 10 ml of phosphoric acid was performed during another shutdown event of 15 min and an additional voltage drop of 71 mV was observed after 16 hours of electrolysis. The total voltage reduction from the value observed after adding the calcium impurities (3.074 to 2.927 volt) was 147 mV. This is a large decrease for a very small addition of phosphate ions which corresponds 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 previously, could be detrimental to the overall operation of the cell.
After the experiment described above, the electrolyte was analysed for impurity content and the results are shown in Table 3.
Since 4 ppm of calcium and a total of 30.6 mg/l (twice the amount of 15.3 mg/l) of phosphorous were added to the electrolyte, Table 3 shows that a significant amount of impurities is still present in the electrolyte at the end of the experiment. The residual calcium impurities in the bath no longer affect the cell voltage because of a protective coating on the surface of the cathode.
This is due to the surface of stainless steel which is stable even under corrosion conditions. In open circuit, a very stable passive surface layer forms on the stainless steel. This passive surface layer is similar to the chromium hydroxide layer which forms by the addition of dichromate in the electrolyte and which provides high cathodic current efficiency. The presence of chromium in the stainless steel alloys thus provide high current efficiency at all times while in the case of the mild steel cathode, the iron oxide scale lowers current efficiency significantly each time there is a shutdown. Also notable on
To overcome the drawback of the higher overpotential of the stainless steels, the stainless steel substrates may be coated with thin catalytic layers which ease the hydrogen evolution reaction described in Equation 2. Examples of such catalytic layers are the Fe3Al(Ru) and Fe3AlTa(Ru) alloys.
a and 7b show scanning electron microscope pictures taken from the cross-sections of such cathodic electrode structures containing a stainless steel substrate of the 400 series and a thin catalytic top-layer of the type described previously.
To complete the cathodic electrode structure according to one embodiment of the present invention, on top of this bi-layer, a coating is added which protects this electrode from impurities in the electrolyte. This top surface layer may be the hydroxyapatite protective layer described previously.
An example of the overall cathodic electrode structure includes (i) a stainless-steel substrate of the 400 series containing a small amount of carbon and no nickel; (ii) catalytic mid-layer such as the ones based on an iron-aluminide metal matrix doped with a catalytic element such as Ru and as described in Canadian patent documents Nos. 2687129 and/or 2778865; and (iii) a top-layer including or consisting essentially of hydroxyapatite on the surface of the catalytic mid-layer to protect the cathode from the impurities present in the electrolyte.
As discussed previously, the top-layer of hydroxyapatite on the surface of the cathode can be formed in-situ by introducing in the electrolyte a small amount of phosphate containing compounds, for example in quantities less than 0.1 g/l or 75 ppm of phosphate ions (PO4−3). This surface layer of hydroxyapatite can also be prepared ex-situ by depositing the coating prior to use the electrode assembly in an electrochemical chlorate cell. Indeed, coatings of hydroxyapatite can be deposited by several methods including thermal spray, sputter coating, dip coating, sol-gel, electrochemical and electrophoretic deposition, biomimetic coating and hot isostatic coating. The most commonly used method to fabricate a hydroxyapatite coating is by plasma spray which is classified as a thermal spray technique.
At room temperature, a cell containing a Fe3Al(Ru) type cathode was tested with different quantities of calcium impurities added to the electrolyte. One can note an increase of 100 to 150 mV in only one or two hours when 1 to 2 ppm of calcium is added, showing the negative impact of calcium on such electrodes.
In another trial as reported in
As discussed above, the phosphate may be added to the electrolyte or otherwise provided based on the surface area of the cathode to be covered by the protective layer.
In a case where phosphate is added to the electrolyte in order to form the protective layer, the amount of phosphate may be determined in a number of ways. In one example, the phosphate addition is pre-determined based on a calculation methodology such as the one described below.
Various properties of the calcium phosphate compound may be measured or taken from literature, for example:
The maximum phosphate requirement on cathodes may then be calculated as follows:
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2013/050136 | 2/22/2013 | WO | 00 |
