Title of Invention: Electrode for Electrolysis
technical field
[One]
Cross-Citation with Related Applications
[2]
This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0021361 dated February 22, 2019, and all contents disclosed in the literature of the Korean patent application are incorporated as a part of this specification.
[3]
technical field
[4]
The present invention relates to an electrode for electrolysis and a method for manufacturing the same, and to an electrode for electrolysis in which a metal substrate layer of the electrode is planarized and a method for manufacturing the same.
background
[5]
A technique for producing hydroxide, hydrogen and chlorine by electrolyzing inexpensive brine such as seawater is widely known. This electrolysis process is usually called a chlor-alkali process, and it can be said that it is a process whose performance and reliability of technology have been proven through commercial operation for several decades.
[6]
In the electrolysis of brine, an ion exchange membrane is installed inside the electrolyzer to divide the electrolyzer into a cation chamber and an anion chamber, and the ion exchange membrane method, which uses brine as an electrolyte to obtain chlorine gas from the anode and hydrogen and caustic soda from the cathode, is currently the most widely used. method being used.
[7]
On the other hand, the electrolysis process of brine is made through a reaction as shown in the following electrochemical reaction formula.
[8]
Anode reaction: 2Cl - → Cl 2 + 2e - (E 0 = +1.36 V)
[9]
Cathode reaction: 2H 2 O + 2e - → 2OH - + H 2 (E 0 = -0.83 V)
[10]
Overall reaction: 2Cl - + 2H 2 O → 2OH - + Cl 2 + H 2 (E 0 = -2.19 V)
[11]
[12]
In carrying out the electrolysis of brine, the electrolysis voltage must consider all of the voltage required for the electrolysis of the brine in theory, the overvoltage of the anode, the overvoltage of the cathode, the voltage due to the resistance of the ion exchange membrane, and the voltage due to the distance between the anode and the cathode. Among these voltages, the overvoltage by the electrode is acting as an important variable.
[13]
Accordingly, a method for reducing the overvoltage of the electrode is being studied. For example, a noble metal-based electrode called DSA (Dimensionally Stable Anode) has been developed and used as an anode, and an excellent material with low overvoltage and durability for the anode has also been developed. this is being requested
[14]
Stainless steel or nickel has been mainly used as such anode. Recently, in order to reduce overvoltage, the surface of stainless steel or nickel is coated with nickel oxide, an alloy of nickel and tin, a combination of activated carbon and oxide, ruthenium oxide, platinum, etc. How to use it is being studied.
[15]
In addition, in order to increase the activity of the anode by controlling the composition of the active material, a method of controlling the composition using a platinum group element such as ruthenium and a lanthanide element such as cerium is being studied. However, an overvoltage phenomenon occurred and deterioration due to reverse current occurred.
[16]
Prior art literature
[17]
(Patent Document 1) JP2003-2977967A
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[18]
It is an object of the present invention to provide an electrode for electrolysis in which overvoltage is improved by increasing adhesion to a membrane and reducing gas traps.
means of solving the problem
[19]
In order to solve the above problems, the present invention is a metal base layer having a mesh structure; and a coating layer comprising a ruthenium-based oxide, a cerium-based oxide, a platinum-based oxide, and an amine-based compound, wherein the coating layer is formed on the surface of the wire constituting the mesh structure, and the aspect ratio of each wire section is 120% It provides an electrode for electrolysis that is the above.
[20]
In addition, the present invention comprises the steps of flattening the metal substrate having the mesh structure so that the aspect ratio of the individual cross-sections constituting the mesh structure is 120% or more; applying a coating composition on the wire surface of the flattened metal substrate; and drying and baking the metal substrate to which the coating composition is applied, and coating the coating composition, wherein the coating composition comprises a ruthenium-based precursor, a cerium-based precursor, a platinum-based precursor, and an amine-based compound. provide a way
Effects of the Invention
[21]
In the electrode for electrolysis according to the present invention, the metal substrate is planarized to have high adhesion to the membrane, and accordingly, the gas trap is reduced, and thus the overvoltage can be improved as the gas can be effectively desorbed.
Brief description of the drawing
[22]
1 is a diagram schematically illustrating an effect that may appear when a metal substrate is planarized in the present invention.
[23]
2 is a graph showing the performance change with time of the electrodes of Examples 1 and 2 and Comparative Examples 1 and 2 of the present invention.
[24]
3 is a view of the electrode surface of Example 1 and Comparative Example 2 of the present invention observed.
Best mode for carrying out the invention
[25]
Hereinafter, examples and experimental examples will be described in more detail to describe the present invention in detail, but the present invention is not limited by these examples and experimental examples. Embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.
[26]
[27]
ingredient
[28]
In this embodiment, a nickel substrate (Ni purity of 99% or more, diameter 200㎛, thickness 370㎛) manufactured by Ildong Gold Mesh Co. was used as the metal substrate. As the ruthenium-based precursor, Heraeus' ruthenium chloride hydrate, and the platinum-based precursor were used. Alfa Aesar's platinum chloride (platinum (IV) chloride, 99.9%), Sigma-Aldrich's cerium nitrate hexahydrate was used as a cerium-based precursor, and urea from Daejeong Hwa-Geum was used as an amine-based compound. In addition, as solvents, isopropyl alcohol and 2-butoxy ethanol from Daejeonghwageum Co., Ltd. were used.
[29]
[30]
Preparation of coating composition
[31]
Metal precursor RuCl 3 ·nH 2 O, Ce(NO 3 ) 3 ·6H 2 O and PtCl 4 were mixed in a molar ratio of 5::1:0.4, and isopropyl alcohol and 2-butoxyethanol were mixed in a volume ratio of 1:1. It was dissolved in the mixed solvent. After that, when the metal precursor was dissolved, urea, an amine compound, was added at a molar ratio of 3.13, and stirred at 50° C. overnight to prepare a coating composition solution having a concentration of 100 g/L based on ruthenium.
[32]
[33]
Example
[34]
Example 1. Preparation of an electrode for electrolysis using a nickel base flattened by rolling
[35]
The nickel substrate of the material was rolled to prepare a flattened nickel substrate having a thickness of 170 μm. The aspect ratio of the individual wire cross-sections of the planarized nickel substrate was measured, and the value was 120 to 169%. Thereafter, the surface of the substrate was sandblasted with aluminum oxide (120 mesh) under 0.4 MPa conditions to form a structure with concavities and convexities. Thereafter , the processed nickel substrate was put into a 5M H 2 SO 4 aqueous solution at 80° C. and treated for 3 minutes to complete the pretreatment process. Then, the coating composition solution prepared in advance on the pre-treated nickel substrate was coated by a brush method, dried in a convection drying oven at 180° C. for 10 minutes, and then put into an electric heating furnace at 500° C. and fired for 10 minutes. After performing the coating, drying and firing process 9 times, the electrode for electrolysis was finally fired in an electric heating furnace heated to 500° C. for 1 hour.
[36]
[37]
Example 2. Preparation of an electrode for electrolysis using a nickel substrate planarized by chemical etching
[38]
An electrode for electrolysis was prepared in the same manner as in Example 1, except that chemical etching was performed instead of rolling. The aspect ratio of the individual wire cross-sections of the planarized nickel substrate was measured, and the value was 132 to 155%.
[39]
[40]
Example 3. Preparation of an electrode for electrolysis using a nickel substrate flattened by a press
[41]
An electrode for electrolysis was prepared in the same manner as in Example 1, except that it was pressed instead of rolling. However, in the case of press treatment, flattening was not uniformly performed compared to the case of flattening by other methods, but it was confirmed that the value of the aspect ratio was 120 to 180%, which was larger than the case of flattening by other methods.
[42]
[43]
Comparative Example 1. Commercial electrode
[44]
After voltage stabilization was completed through an electrolysis process, the fresh electrode was activated and used as a commercial electrode of Comparative Example 1.
[45]
[46]
Comparative Example 2. Preparation of an electrode for electrolysis using a non-planarized nickel substrate
[47]
An electrode for electrolysis was prepared in the same manner as in Example 1, except that it was not planarized. The aspect ratio of the individual wire cross-sections based on unplanarized nickel was 100%.
[48]
[49]
Comparative Example 3. Preparation of an electrode for electrolysis using a thin nickel substrate without planarization
[50]
An electrode for electrolysis was prepared in the same manner as in Example 1, except that the thickness was 170 μm and a non-planarized nickel substrate was used. The aspect ratio of the individual wire cross-sections based on unplanarized nickel was 100%.
[51]
[52]
Information on the electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 is summarized in Table 1 below.
[53]
[Table 1]
division Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3
planarization method rolled chemical etching Press - - -
aspect ratio 120-169% 132-155% 120-180% 100% 100% 100%
Thickness of the substrate (μm) 170 180 160 330 380 170
Composition ratio of coating layer (Ru:Ce:Pt:urea) 5:1:0.4:3.13 5:1:0.4:3.13 5:1:0.4:3.13 5:1:0.4:3.13 5:1:0.4:3.13 5:1:0.4:3.13
[54]
[55]
Experimental Example 1. Confirmation of the performance of the prepared electrode for electrolysis
[56]
In order to check the performance of the electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 3, a single cell device for measuring overvoltage at a constant current was used. As the anode, an oxidized electrode of AKC, the used electrode, was used, and as the cathode, a 5X5cm 2 cell capable of implementing a zero-gap cell in the form of placing a nickel mattress on the current meter and raising the electrode was used. did. Aciplex's F6808 was used as the membrane, and the experiment was performed under the condition of a current density of 6.2 kA/m 2 as a constant current . A cathodic voltage measurement experiment using a half cell in Chlor-Alkali Electrolysis was performed. The electrolyte on the oxidation electrode side was 305 g/L sodium chloride, and the electrolyte on the reduction electrode side used 30.6% sodium hydroxide. The reaction temperature was set to 90° C., and the experiment was performed while flowing a flow rate of 15 ml/min to both the oxidation electrode and the reduction electrode. The results are summarized in Table 2 below, and the change over time is shown in FIG. 2 .
[57]
[Table 2]
Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3
Performance (V) 3.040 3.035 3.043 3.086 3.045 3.126
[58]
As can be seen in Table 2, it was confirmed that the electrode of the example having an aspect ratio of 120% or more exhibited a lower overvoltage than the electrode of the comparative example that was not subjected to the planarization treatment. In addition, as confirmed in FIG. 2 , the electrode of Comparative Example 1 exhibited a high overvoltage from the beginning, and the electrodes of Examples 1 and 2 showed a lower value compared to the electrode of Comparative Example 2 at the overvoltage values ​​converging after a certain period of time. was confirmed.
[59]
[60]
Experimental Example 2. Observation of the surface of the electrode
[61]
The surfaces of the electrodes prepared in Example 1 and Comparative Example 2 were observed, and this is shown in FIG. 3 . Observation was performed through a scanning electron microscope (SEM). It was confirmed from FIG. 3 that when the rolling process was performed, the wires intersecting in the mesh structure were in contact in a wide area, and thus a coating layer having a larger area could be secured. That is, it was confirmed that the electrode of Example 1 can perform the electrolysis reaction smoothly compared to Comparative Example 2.
Modes for carrying out the invention
[62]
Hereinafter, the present invention will be described in more detail.
[63]
The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. Based on the principle that there is, it should be interpreted as meaning and concept consistent with the technical idea of ​​the present invention.
[64]
[65]
Definition of Terms
[66]
As used herein, “aspect ratio” refers to the ratio of width to height (width/height).
[67]
As used herein, "mesh structure" refers to a mesh structure formed by intertwining wires.
[68]
[69]
Electrode for electrolysis
[70]
The present invention is a metal base layer having a mesh structure; and a coating layer comprising a ruthenium-based oxide, a cerium-based oxide, a platinum-based oxide, and an amine-based compound, wherein the coating layer is formed on the surface of the wire constituting the mesh structure, and the aspect ratio of each wire section is 120% It provides an electrode for electrolysis that is the above.
[71]
[72]
The metal substrate may be nickel, titanium, tantalum, aluminum, hafnium, zirconium, molybdenum, tungsten, stainless steel, or an alloy thereof, of which nickel is preferable. When nickel is used as the metal substrate, durability and electrode performance may be excellent.
[73]
In the metal substrate included in the electrode for electrolysis, the individual wires constituting the mesh structure thereof are planarized, so that the aspect ratio of the cross section of each wire is 120% or more. Preferably, the lower limit of the aspect ratio may be 120%, 125% or 130%, and the upper limit of the aspect ratio may be 180%, 170%, 160% or 150%. As can be seen from FIG. 1 , when the aspect ratio of the cross section of the individual wires constituting the mesh structure is 120% or more by flattening the metal substrate having the mesh structure, the adhesion with the membrane is increased to reduce the gas trap, and consequently the overvoltage is reduced. can be improved, and a smooth electrolysis reaction can proceed. On the other hand, if the aspect ratio is too large, the durability of the metal substrate itself may be weakened. The planarization can be performed without any limitation as long as the aspect ratio of the cross section of the individual wires constituting the mesh structure can be 120% or more without affecting the durability of the metal substrate, and preferably through pressing, rolling or chemical etching. can be performed.
[74]
The ruthenium-based oxide, cerium-based oxide, and platinum-based oxide of the coating layer serve to lower the overvoltage of the electrode, and in particular, the platinum-based oxide can improve the stability of the additional overvoltage-improving catalyst layer, and the cerium-based oxide can improve durability and of the catalyst layer Stability can be improved.
[75]
The thickness of the metal substrate may be 100 to 300 μm, preferably 120 to 280 μm, and more preferably 150 to 250 μm. If the metal substrate is too thin, for example, thinner than 100 μm, there may be problems in use due to the weak durability of the electrode, and if the metal substrate is too thick, for example, if it exceeds 300 μm, a lot of cost is consumed in manufacturing the electrode, When a metal substrate having a thick mesh structure is used, the hardness of the substrate is high, so that the adhesion between the electrode and the membrane in the zero-gap cell is reduced, and thus the electrolysis reaction may not smoothly occur.
[76]
[77]
Manufacturing method of electrode for electrolysis
[78]
The present invention comprises the steps of flattening the metal substrate having the mesh structure so that the aspect ratio of each cross section of the wire constituting the mesh structure is 120% or more; applying a coating composition on the wire surface of the flattened metal substrate; and drying and baking the metal substrate to which the coating composition is applied, and coating the coating composition, wherein the coating composition comprises a ruthenium-based precursor, a cerium-based precursor, a platinum-based precursor, and an amine-based compound. provide a way
[79]
The planarization may be performed through the same method as described above, and is preferably performed through rolling or chemical etching.
[80]
The ruthenium-based precursor is a material that provides ruthenium as an active material to the catalyst layer of the anode for electrolysis. The ruthenium-based precursor is ruthenium hexafluoride (RuF 6 ), ruthenium (III) chloride (RuCl 3 ), ruthenium (III) chloride hydrate (RuCl 3 ·xH 2 O), ruthenium (III) bromide (RuBr 3 ), ruthenium (III) may be at least one selected from the group consisting of bromide hydrate (RuBr 3 ·xH 2 O), ruthenium iodide (RuI 3 ) and ruthenium acetate salt, among which ruthenium (III) chloride hydrate is preferable.
[81]
The cerium-based precursor is a material that provides a cerium element to the catalyst layer of the cathode for electrolysis. The cerium element can minimize the loss of ruthenium in the catalyst layer of the electrode for electrolysis during activation or electrolysis by improving the durability of the cathode for electrolysis. Specifically, during the activation or electrolysis of the anode for electrolysis, the particles containing ruthenium in the catalyst layer become metallic Ru (metallic Ru) or partially hydrated to reduce the structure to an active species without changing the structure. do. In addition, the structure of the particles containing the element cerium in the catalyst layer is changed to form a network with the particles containing ruthenium in the catalyst layer, and as a result, it is possible to prevent the loss of ruthenium in the catalyst layer by improving the durability of the cathode for electrolysis. .
[82]
The cerium-based precursor is cerium (III) nitrate hexahydrate (Ce(NO 3 ) 3 .6H 2 O), cerium (IV) sulfate tetrahydrate (Ce(SO 4 ) 2 .4H 2 O) and cerium (III) It is at least one selected from the group consisting of chloride heptahydrate (CeCl 3 ·7H 2 O), and among them, cerium (III) nitrate hexahydrate is preferable.
[83]
The catalyst composition may include 0.01 to 0.5 mol or 0.05 to 0.35 mol of the cerium-based precursor, preferably 0.05 to 0.35 mol, of the cerium-based precursor, based on 1 mol of the ruthenium-based precursor.
[84]
If the above-described range is satisfied, the durability of the prepared electrode may be improved to minimize the loss of ruthenium in the catalyst layer during activation or electrolysis.
[85]
[86]
The platinum-based precursor is a material that provides platinum to the catalyst layer of the cathode for electrolysis. The platinum may improve an overvoltage phenomenon of the electrode. In addition, the platinum may minimize the deviation between the initial performance of the electrode and the performance after a certain time has elapsed, and as a result, the electrode may not perform or minimize a separate activation process.
[87]
The platinum-based precursor is chloroplatinic acid hexahydrate (H 2 PtCl 6 .6H 2 O), diamine dinitro platinum (Pt(NH 3 ) 2 (NO) 2 ) and platinum (IV) chloride (PtCl 4 ), platinum ( II) chloride (PtCl 2 ), potassium tetrachloroplatinate (K 2 PtCl 4 ), potassium hexachloroplatinate (K 2 PtCl 6 ) may be at least one selected from the group consisting of, of which platinum (IV) chloride is preferable
[88]
[89]
The catalyst composition may include 0.01 to 0.7 moles or 0.02 to 0.5 moles of the platinum-based precursor based on 1 mole of the ruthenium-based precursor, and preferably contains 0.02 to 0.5 moles of the platinum-based precursor.
[90]
When the above-described range is satisfied, the overvoltage phenomenon of the electrode can be remarkably improved. In addition, since the initial performance of the electrode and the performance after a certain period of time can be maintained constant, an activation process of the electrode is unnecessary. Accordingly, it is possible to reduce the time and cost required for the electrode activation process.
[91]
[92]
The amine-based compound is known to play a role in reducing the particle phase by adding it as an additive when manufacturing nanoparticles, etc., and has an effect of making the ruthenium oxide crystal phase small in electrode coating. In addition, since the catalyst composition contains the amine compound, the cerium network structure formed by increasing the size of the needle-like structure of cerium serves to more firmly fix the ruthenium particles, thereby improving the durability of the electrode. And, as a result, even when the electrode is operated for a long time, there is an effect that the peeling phenomenon of the electrode can be significantly reduced.
[93]
[94]
The catalyst composition may contain 0.5 to 1 mol or 0.6 to 0.9 mol of the amine-based compound with respect to 1 mol of the ruthenium-based precursor, and preferably includes 0.6 to 0.9 mol of the amine-based compound.
[95]
If the above content is satisfied, the amine-based compound can form a network in the catalyst layer by rapidly changing the structure of the particles containing the cerium element after activation of the electrode or during electrolysis, compared to when the amine-based compound is not used. and, as a result, the durability of the electrode may be improved. Specifically, the amine-based compound may improve the durability of the electrode by increasing the needle-like structure of particles containing cerium.
[96]
[97]
The amine-based compound is preferably urea. When urea is used, the stability and safety of the coating solution are superior compared to those used with other amine-based compounds, and there are advantages in that harmful substances and odors are less generated even when manufacturing an electrode over a large area.
[98]
[99]
In the manufacturing method of the present invention, it may include a step of pre-treating the metal substrate before performing the coating step.
[100]
The pretreatment may be to form irregularities on the surface of the metal substrate by chemical etching, blasting, or thermal spraying of the metal substrate.
[101]
The pretreatment may be performed by sandblasting the surface of the metal substrate to form fine irregularities, and performing salt treatment or acid treatment. For example, the surface of the metal substrate may be sandblasted with alumina to form unevenness, immersed in an aqueous sulfuric acid solution, washed and dried, and thus pre-treated to form fine unevenness on the surface of the metal substrate.
[102]
[103]
The application is not particularly limited as long as the catalyst composition can be evenly applied on the metal substrate and may be performed by a method known in the art.
[104]
The application may be performed by any one method selected from the group consisting of doctor blade, die casting, comma coating, screen printing, spray spraying, electrospinning, roll coating, and brushing.
[105]
[106]
The drying may be performed at 50 to 300° C. for 5 to 60 minutes, and preferably at 50 to 200° C. for 5 to 20 minutes.
[107]
When the above conditions are satisfied, the solvent can be sufficiently removed and energy consumption can be minimized.
[108]
[109]
The calcination may be performed at 400 to 600° C. for 1 hour or less, and is preferably performed at 450 to 550° C. for 5 to 30 minutes.
[110]
The firing serves to convert the metal precursor into an oxide. When the sintering satisfies the above-described conditions, impurities in the catalyst layer may be easily removed and the strength of the metal substrate may not be affected.
[111]
[112]
On the other hand, the coating may be performed by sequentially repeating application, drying and firing so as to be 10 g or more based on ruthenium per unit area (m2) of the metal substrate. That is, in the manufacturing method according to another embodiment of the present invention, the catalyst composition is applied on at least one surface of the metal substrate, dried and fired, and then applied again on one surface of the metal substrate to which the first catalyst composition is applied, and dried. and coating to be fired may be repeatedly performed. The repetition may be performed 5 to 20 times.
Claims
[Claim 1]
a metal base layer having a mesh structure; and a coating layer comprising a ruthenium-based oxide, a cerium-based oxide, a platinum-based oxide, and an amine-based compound, wherein the coating layer is formed on the surface of the wire constituting the mesh structure, and the aspect ratio of each wire section is 120% An electrode for electrolysis that is more than one.
[Claim 2]
The electrode for electrolysis according to claim 1, wherein the aspect ratio is 120 to 180%.
[Claim 3]
The electrode for electrolysis according to claim 1, wherein the metal is nickel, titanium, tantalum, aluminum, hafnium, zirconium, molybdenum, tungsten, stainless steel, or an alloy thereof.
[Claim 4]
The electrode for electrolysis according to claim 1, wherein the metal base layer has a thickness of 100 to 300 μm.
[Claim 5]
flattening the metal substrate having the mesh structure so that the aspect ratio of each cross section of the wire constituting the mesh structure is 120% or more; applying a coating composition on the wire surface of the flattened metal substrate; and drying and baking the metal substrate to which the coating composition is applied, and coating the coating composition, wherein the coating composition comprises a ruthenium-based precursor, a cerium-based precursor, a platinum-based precursor, and an amine-based compound. Way.
[Claim 6]
The method of claim 5, wherein the planarization is performed through rolling or chemical etching.
[Claim 7]
The method of claim 5, wherein the metal substrate is nickel.
[Claim 8]
According to claim 5, wherein the ruthenium-based precursor is ruthenium hexafluoride (RuF 6 ), ruthenium (III) chloride (RuCl 3 ), ruthenium (III) chloride hydrate (RuCl 3 ·xH 2 O), ruthenium (III) bromide (RuBr 3 ), ruthenium (III) bromide hydrate (RuBr 3 ·xH 2 O), ruthenium iodide (RuI 3 ), and at least one selected from the group consisting of ruthenium acetate salt, the cerium-based precursor is cerium (III) ) nitrate hexahydrate (Ce(NO 3 ) 3 .6H 2 O), cerium(IV) sulfate tetrahydrate (Ce(SO 4 ) 2 .4H 2 O) and cerium(III) chloride heptahydrate (CeCl 3 .7H) 2O) at least one selected from the group consisting of, the platinum-based precursor is chloroplatinic acid hexahydrate (H 2 PtCl 6 .6H 2 O), diamine dinitro platinum (Pt(NH 3 ) 2 (NO) 2 ) and platinum(IV) chloride (PtCl 4 ), platinum(II) chloride (PtCl 2 ), potassium tetrachloroplatinate (K 2 PtCl 4 ), potassium hexachloroplatinate (K 2 PtCl 6 ) A method for producing an electrode for electrolysis that is one or more kinds.
[Claim 9]
The method of claim 5, wherein the amine-based compound is urea.
[Claim 10]
[Claim 6] The method of claim 5, wherein the coating, drying and firing are each repeated 5 to 20 times.