mx5kevin Posted August 18 Posted August 18 Slow cell (production) problem fixing. Before we start, we have to decide that we want a small electrode for a small cell with a cheap assembly. Or a large electrode with a much more expensive and complicated assembly. In terms of value for money, if someone uses 1-2 kg of chlorate or perchlorate per year, it is worth thinking about a 2 liter cell. The surface of the electrode is the first thing that decides how quickly it can be produced. The minimum anode parameter in platinum are 3x70mm silver rod with minimum 50 micron thick pure platinum. Small platinum 10x10 tiles and small wires are not good here. And below with a adapter 6V 4A, it is not possible to produce normally under this parameter. The amount of solution that is worth working with minimum 400ml using a 700ml round jar in a single cell. But it is not worth processing below 1200 ml of solution (3x400ml solution). This provides around 500g of potassium chlorate or perchlorate each time. For these cells, the amperage drops as the process progresses, so that it drops to at least half by the end of the process, which is completely natural. Electrolysis has a constant rate, it cannot happen too slowly, but it is not a fast process. When we start from the chlorides, the process is too fast at the beginning, if we have a precisely adjustable adapter, this must be slowed down. The wire oxidizes at the connection points, which must be cleaned and replaced at certain intervals. Conductivity can be significantly improved at the connection points by treating them with hydrochloric acid. If there is tin, it is worth soldering on the connection points and insulating them with plastic so that no air can reach them. We use few volts, so thick wires are needed to transfer the amperes. The cable between the adapter and the cell should always be replaceable! There are points on the cell where the contacts oxidize, the conductivity deteriorates, and here the wires must be regularly replaced or cleaning is required. In a pH controlled NaCl or KCl cell always use Na or K persulfate additive. If PbO2 anode are used this is are not compatible with potassium dichromate! For platinum use potassium dichromate. Persulfate additive to some extent reduce the chlorine escaping and increase the oxygen evolution. Potassium dichromate on the cathode reduce the reduction. If having a precise lab power supply when the chloride started to electrolysis start it to 3V later vill be slowly (6-7V) 7V maximum will be necessary. The cell should have a stable constant temperature, do not overheat. If the cell is very hot and later cools down, much more deposits will form on the cathode. The cathode must be cleaned regularly. First, it must be cleaned with a kitchen towel, then put in hot water, which will remove all the deposits in a few minutes. Without it, the pH control by itself is not effective enough. How many amperes pass through the cell, the larger the surface of the electrode, the more. Each power supply unit has a limit that it can sustain in the long term stably. Weak power supplies cannot be used as electrolysis requires amperes. Even the Volt it's shorter, it won't tolerate too many amperes the supply anyway. This can be a problem at the beginning of the process. It can be regulated with a precise laboratory power supply to avoid overloading. If the power supply is very powerful, the other problem is that the cell can be overheated. The two problems can be the adapter being overloaded or the cell overheating. The current should be kept more or less constant at a value determined by the cell design. As a rule of thumb, supply, keep under 2 amperes of current per every 100 ml of electrolyte. If graphite anodes are used it is better to supply less current since that will increase anode life (30 mA per square centimetre of anode surface area is typical). A constant current supply is of course the most convenient to use for regulating current, but manually adjusting the voltage from time to time also works well. If you have a 10 amp adjustable power supply, it is recommended to use only 8 amps of it. If the power supply gets hot, there is a problem, if the cell is too hot, there is also a problem. In non-adjustable power supplies have a safety fuse, these should be used in such a way that the amperes taken by the cell are below this. The amperes that an adapter can stably deliver in the long term, and the maximum that the latter value can deliver, are significantly below the first value. A cooler 45°C chlorate cell maybe slower but much more ideal. The cell solution and the atmosphere are also corrosive, all unnecessary metal components that could contaminate the solution must be excluded. If the solution later becomes discolored or dirty, something that is not need there is dissolved in it. In severe cases, this could be the anode. To prevent the adapter itself and many other things from being damaged by chlorine, we can avoid this by running the cell outdoors. It is impossible to ensure a constant temperature in winter and summer. The anode and cathode should be close to each other for fast production. An anode between two cathodes is the most ideal for chlorates. This significantly reduces the acidic medium on the anode and the escape of chlorine from the cell. The gases formed at the anode should come into contact with the alkaline solution formed at the cathode as directly as possible, and should travel slower and longer through the solution. At the very beginning of the process, if the adapter can be regulated (and after pH control keep is slow), do not use more than 3 Volts at the beginning. Some overdosing in pH controlling are natural. If the cell is properly maintained, at regular intervals, the same product is produced in the same amount of time with minor differences. Once you know how much running time a specific product needs, it will always be that much. Periodic maintenance is required. My advice is that the chlorate or perchlorate what needed for a longer period, it should be prepared with continuous production, so that the cell can run for several months. This is beneficial because it reduces the rate of equipment failure. For the beaker always use a heat fence Wire Gauze with Ceramic Center. Protect the Hot plate with aluminum foil, and never heat chlorides or the cell solution in stainless steel vessel. Run the cell outdoor, and work the solution outdoor too.
AustralianPyromaniac Posted August 18 Author Posted August 18 (edited) 6 hours ago, mx5kevin said: Persulfate additive to some extent reduce the chlorine escaping and increase the oxygen evolution Persulphate does not decrease the production of chlorine more is made with persulphate than a totally pH-uncontrolled cell, where nearly no chlorine is produced. Regarding oxygen evolution, I am not sure - I have not heard anything about how it affects oxygen evolution. But oxygen evolution is bad - it is directly a current loss in the system. The mechanism for persulphate pH control does not seem to be fully understood, I have some theories myself, but none great. 6 hours ago, mx5kevin said: A cooler 45°C chlorate cell maybe slower but much more ideal. Electrode decay is about the same at 40c and 60c. The improvement in chlorate formation is much more important. Hypochlorite disproportionation and bulk chlorate formation will occur MUCH slower at cooler temps. If we look at industrial cells, they run between 65c and 90c. Around 80 is normal. A 10c in cell temp results in about a 6% increase in C.E. In pH uncontrolled cells yes maybe 45c is ok, but still not ideal. I think 50-60 is a good target for us. 6 hours ago, mx5kevin said: The cathode must be cleaned regularly. First, it must be cleaned with a kitchen towel, then put in hot water, which will remove all the deposits in a few minutes. What is the deposit made out of? Are you saying chlorate crystals form on your cathode? Or carbonate/ hydroxide deposits? None of these should form if your system is running correctly. Carbonate really cannot be washed away with water, but hydroxides sort of can, so it may be Ca/Mg hydroxide. 6 hours ago, mx5kevin said: The anode and cathode should be close to each other for fast production This is wrong. The resistivity of the solution is so small that 0.5cm or 5cm - the difference is hardly felt in cell current. The electrical load that the PSU sees is caused by the reduction potential of chlorine (plus overvoltage), and very little by the ohmic load of the cell. More heat is generated, which could be an issue, but almost the same amount of current flows and so the rate of formation is the same. 6 hours ago, mx5kevin said: An anode between two cathodes is the most ideal for chlorates. This significantly reduces the acidic medium on the anode and the escape of chlorine from the cell. The gases formed at the anode should come into contact with the alkaline solution formed at the cathode as directly as possible, and should travel slower and longer through the solution. I am not 100% sure what this section means. The anodic boundary layer is not really that acidic, yes some HOCl is made there, which is an acid, but the high concentration of chloride mostly prevents this, chlorine would rather dissolve than hydrolise directly adjacent to the anode. The two cathode thing is to ensure the current density is equal on both sides of the anode, and I believe it also prevents some side reactions from happening when you have a higher cathodic surface area and, therefore, lower cathodic current density. Alkali is formed at the cathode, but this should really be considered a side reaction. If we could stop it from happening we would. I think maybe you are mistaken how the hypochlorite group forms - because there are two ways. The first way is when the chlorine gas directly reacts with OH- forming OCl- (1), but the more common way is when chlorine reacts with water forming HOCl and HCl (2). Again unsure what you mean by the "slower and longer." Once HOCl forms it diffuses away from the cathode and then dissociates once H+ concentration drops enough, but again this is not a critical part of the process, except to say that it is required for bulk chlorate formation. In industrial setups, you ideally want to have the electrolyte away from the electrodes to encourage the formation of chlorate by reaction between OCl- and HOCl (3) not by annodic oxidation. (1) Cl2 + 2 OH– → Cl– + ClO– + H2O (2) Cl2 + H2O ⇋ HClO + H+ + Cl– (3) HClO + ClO– → ClO3– + 2Cl– + 2 H+ Edited August 18 by AustralianPyromaniac
mx5kevin Posted August 18 Posted August 18 5 hours ago, AustralianPyromaniac said: Persulphate does not decrease the production of chlorine more is made with persulphate than a totally pH-uncontrolled cell, where nearly no chlorine is produced. Regarding oxygen evolution, I am not sure - I have not heard anything about how it affects oxygen evolution. But oxygen evolution is bad - it is directly a current loss in the system. The mechanism for persulphate pH control does not seem to be fully understood, I have some theories myself, but none great. Electrode decay is about the same at 40c and 60c. The improvement in chlorate formation is much more important. Hypochlorite disproportionation and bulk chlorate formation will occur MUCH slower at cooler temps. If we look at industrial cells, they run between 65c and 90c. Around 80 is normal. A 10c in cell temp results in about a 6% increase in C.E. In pH uncontrolled cells yes maybe 45c is ok, but still not ideal. I think 50-60 is a good target for us. What is the deposit made out of? Are you saying chlorate crystals form on your cathode? Or carbonate/ hydroxide deposits? None of these should form if your system is running correctly. Carbonate really cannot be washed away with water, but hydroxides sort of can, so it may be Ca/Mg hydroxide. This is wrong. The resistivity of the solution is so small that 0.5cm or 5cm - the difference is hardly felt in cell current. The electrical load that the PSU sees is caused by the reduction potential of chlorine (plus overvoltage), and very little by the ohmic load of the cell. More heat is generated, which could be an issue, but almost the same amount of current flows and so the rate of formation is the same. I am not 100% sure what this section means. The anodic boundary layer is not really that acidic, yes some HOCl is made there, which is an acid, but the high concentration of chloride mostly prevents this, chlorine would rather dissolve than hydrolise directly adjacent to the anode. The two cathode thing is to ensure the current density is equal on both sides of the anode, and I believe it also prevents some side reactions from happening when you have a higher cathodic surface area and, therefore, lower cathodic current density. Alkali is formed at the cathode, but this should really be considered a side reaction. If we could stop it from happening we would. I think maybe you are mistaken how the hypochlorite group forms - because there are two ways. The first way is when the chlorine gas directly reacts with OH- forming OCl- (1), but the more common way is when chlorine reacts with water forming HOCl and HCl (2). Again unsure what you mean by the "slower and longer." Once HOCl forms it diffuses away from the cathode and then dissociates once H+ concentration drops enough, but again this is not a critical part of the process, except to say that it is required for bulk chlorate formation. In industrial setups, you ideally want to have the electrolyte away from the electrodes to encourage the formation of chlorate by reaction between OCl- and HOCl (3) not by annodic oxidation. (1) Cl2 + 2 OH– → Cl– + ClO– + H2O (2) Cl2 + H2O ⇋ HClO + H+ + Cl– (3) HClO + ClO– → ClO3– + 2Cl– + 2 H+ Na2S2O8 especially in NaCl electrolysis much less chlorine is released into the environment. But with KCl are more effective. From the point of view of industrial production, the highter temperature is good, but it cannot be maintained in winter and summer using a homemade setup. A cooler cell is much more gentle on the cell and the circuit, and during perchlorate production it must be kept below 45°C without external cooling, otherwise the anode will be damaged. With a lead dioxide anode in a warm perchlorate cell, I have seen how quickly the electrode falls apart in an overheated cell. Chlorine is better absorbed in the alkaline medium between two large-surface cathodes, which covers two sides the anode. The cathode is always alkaline and the anode is always acidic, the acid-base reaction between the two cathodes is more efficient. Hydrogen leaves more gently at the cathodes. And if a deposit forms on it, the electrolysis is less affected. The deposit is usually a mixture of various hydroxides, chloride and chlorate. Electrodes far apart are not ideal. Although chlorine is not produced at the cathode, and alkali is produced there, but chlorine still actively leave there. With the sum of several small settings, significant improvements can be achieved in several areas. I have tried all of the ones I recommended, I not only think, but I have experienced that these solutions are beneficial. As a cathode, I use graphite, not titanium for chlorates and perchlorates, a deposit always slowly forms over this by time. I think the cause of your problem is that the circuit does not conduct the current properly at some point or points (wear oxidation or contamination).
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