The issue of rising voltage in ion membrane cells
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This post was last edited by sunjl1981 on 2013-1-7 at 00:27. Our company uses Asahi Kasei ion-exchange membrane electrolyzers; the current operating current is 12 KA. Over the past 30 months, there has been an increase in the cell voltage. The electrolyzers have been in operation for 6 months without any shutdowns. The calcium and magnesium levels in the brine are less than 10 PPb, but the problem now is that the sulfate level in the brine is somewhat high, at 7 g/l. The cell voltage has increased by 1.5 V within 4 days. The parameters of pure water and high-purity acid are all within acceptable ranges. Please help analyze the reasons for this issue. # hcbbs
Based on the data provided by the poster, the sulfate content is not very high; it would be best to analyze whether the concentration of alkalis in your sample is too high, as well as the chloride content.
There are many factors that can affect the increase in cell voltage. Based on experience, the key things to consider are whether there are any changes in the flow rates at the anode and cathode, whether there are changes in the pH values at the inlet and outlet of the cell, the concentrations at the anode and cathode, the quality and concentration of the saline solution, as well as the pressure in the anode and cathode chambers. It’s necessary to first check for any changes in these aspects. Last edited by yzhms on 2009-4-11 08:16.]
There are many factors that can cause the cell voltage; a careful analysis is necessary: saltwater composition, caustic soda concentration, cell temperature...
Iron ions in saltwater may be high, resulting in a high cell voltage.
There are many reasons for an increase in cell voltage. Judging from the quality of your saltwater, there seems to be no problem; what about the concentration of the alkali fed into the cell? Pressure difference? Trough temperature? If the pH values at the inlet and outlet of the tank don’t change, then it’s necessary to check whether the coating on the electrode plates has been damaged! The coating on the electrode sheets has peeled off, and the slot voltage has risen very quickly! This post was last edited by yzhms on 2009-4-11 14:46]
The sulfate content is indeed not very high; analyze the contents of iron and other metal elements first.
Analyze whether the iron ion content is too high, and then examine the iodine content in the saltwater; high iodine levels can reduce current efficiency.
The sum of calcium and magnesium levels in the brine is less than 10 PPb; the quality of the brine in terms of calcium and magnesium is satisfactory. However, it is also necessary to analyze the levels of other heavy metals in the brine, especially iron. In addition, it is important to check whether there are any changes in the pH values at the inlet and outlet of the brine tank, as well as the concentrations of the anode and cathode, and the pressures in the anode and cathode chambers. This post was last edited by yzhms on 2009-4-11 08:16]
On the first floor, OP, what are the requirements regarding the sulfate content in the brine you use?
Due to a sudden and significant increase in sulfate levels in the raw salt, the sulfate concentration in the brine exceeded the allowable limits. Our ICP analysis results show that the combined amount of calcium and magnesium in the secondary brine is less than 10 PPb, while the iron level is very low, essentially indicating that it is not detectable； High-purity acid is analyzed once a week; the combined level of calcium and magnesium is 0.5 PPm, while the iron level is around 0.3–0.5 PPM, with the quality not being very stable. The last time we stopped was in September 2008. We are now also suspecting high-purity acid and are conducting an investigation. This post was last edited by yzhms on 2009-4-13 15:40]
Reason: 1. The electrolyte temperature drops below 85°C or even lower. 2 The concentration in the anode solution increases. 3 Overcurrent caused by rectifier failure. 4 Increase in catholyte concentration. 5 The membrane is contaminated by metal precipitates.
A comprehensive analysis is conducted on the quality of brine and raw salt. Including TOC. A continuous increase in slot voltage must be due to the influence of ions. What do you use to remove sulfate ions?
If that’s the case, did sodium sulfate affect the saturation of sodium chloride?
The operator measures the alkali specific gravity; if it is too high, pure water is added to prevent an increase in voltage! This post was last edited by yzhms on 2009-4-13 18:19]
Could it be that the anode or cathode has failed?
The people upstairs have provided a fairly comprehensive analysis; I’d like to add some insights regarding the electrodes. The issue with the anode can basically be ruled out, unless there is a sudden change in the voltage of certain cells (an increase of over 100 mv in a short period of time). May I ask what type of cathode is used in the owner’s electrochemical cells? In previous years, Asahi Kasei generally used active nickel cathodes, while more recent models employ ruthenium-coated cathodes. Ruthenium-coated cathodes remain relatively stable unless there is a shutdown – in that case, polarized rectification protection is available – unless there is an abnormally high level of iron in the alkali solution. During long-term operation, active nickel cathodes experience periodic declines in performance, manifested as an increase in voltage. This is because the composition of active nickel cathodes is porous nickel oxide. The degradation of these cathodes occurs in two stages: first, the nickel oxide is reduced to porous nickel, and then the porous nickel detaches and is lost. This change is not linear throughout the cathode lifetime. Additionally, if the salt types are mixed, perform ICP analysis to check for aluminum and nickel.
Consider that the concentration of saltwater is too low and the concentration of alkali is too high
Thank you all for your responses. After examining each of the issues raised, we have essentially identified the cause: it is due to iron ions, which originate from high-purity hydrochloric acid. To protect the ICP, samples of this high-purity hydrochloric acid are tested on a weekly basis, and quality problems arose during that process.
As a lesson to take to heart, our high-purity acids are analyzed on a daily basis; it is only when they contain iron that the color of the acid changes to a darker shade. Hehe.
After ruling out the reasons for the operation, check for heavy metals. Generally, such significant increases are related to membrane contamination