What is the red precipitate in saltwater?

Main components of clay: silica 65.18–71.86%, alumina 15.02–17.99%, iron oxide 3.27–6.61%, calcium oxide 0.75–1.68%, magnesium oxide 0.89–2.07%, loss on ignition 4.19–6.20%. I believe that the main reason for this phenomenon is that ferric chloride was added to the brine, existing in the form of iron hydroxide in the alkaline brine. We have this problem here as well; that’s my personal opinion

　　Regarding this question, let me add more: under alkaline conditions, saltwater is colorless and transparent; upon the addition of high-purity acid, a red precipitate forms. At this point, the pH of the saltwater is between 7 and 8. Will iron hydroxide be formed under these conditions?

Iron hydroxide is formed in a pH range of 3-4

What impact do such precipitates have on the diaphragm system? Our current situation is that the cell voltage is rising, but the concentration in the cell is decreasing. Puzzling. :(

Isn’t iron hydroxide formed under alkaline conditions? Please explain!!

The conditions for the formation of iron hydroxide are within this range. Regarding the issue of the increase in cell voltage, it is because iron hydroxide adheres to the surface of the membrane on the anode side, causing an increase in membrane pressure drop; this is why, in the production of ion exchange membranes, it is necessary to maintain the pH at the anode outlet at 2–2.5 (Asahi Kasei)

　　For ion-exchange membrane systems, the impact of iron may be smaller, as iron is formed on the anode side and does not enter the ions; it only deposits on the anode side of the membrane, so its effect on the cell voltage is likely to be modest. 　　For diaphragm systems, is it a different situation: something enters inside the diaphragm, causing the cell voltage to rise, but in that case, the concentration in the cell should also increase accordingly. Is such an analysis correct?

1. Will iron hydroxide precipitate still form under alkaline conditions? 2. Does the precipitate of iron hydroxide still dissolve under acidic conditions with a pH lower than 3? 3. Are the forms and states of iron hydroxide as a colloid and as a precipitate different? It seems I still need to first learn some knowledge about iron hydroxides. :Q

Solid impurities formed when there are fluctuations in the mass of FeCl3 or saline added end up being carried into the system :) This post was last edited by yzhms on 2008-3-2 00:12.]

A qualitative analysis was conducted on the resulting solid: its main components are sodium, sulfur, silicon, etc. Therefore, I personally conclude that it is a complex salt formed from silicates and sodium sulfate and similar substances. I’m not sure if this is correct; I would appreciate your guidance.

1. Under alkaline conditions, it exists in the form of an iron hydroxide precipitate, of course； 2. When pH is less than 2 (it was originally said to be 3; this is a correction), it exists in the form of iron ions and does not precipitate ； 3. The effect of iron hydroxide on the voltage of ion-exchange membrane cells is significant; in fact, it is quite substantial. However, its impact on current efficiency is minimal ； 4. As for my brother’s qualitative analysis of this substance, I dare not draw any conclusions; I still think it is related to iron ；

1. Have you come across any cases where TOC has an impact on ion exchange membranes? I heard that Changhua had to replace all its membranes for this reason. 2. Regarding the red precipitate in saline solution as mentioned, I will conduct further follow-up on this ； 3. Regarding ferric trichloride, I conducted experiments; under slightly altered acidic conditions, iron hydroxide is formed, which is different from what I described above ； 4. Under alkaline conditions, the saltwater is colorless and free of suspended particles; color appears when it is adjusted to neutral pH.

1. I have not encountered any issues with TOC, as sea salt is used in my applications. In some cases, problems may arise in conventional chemical processes due to the high proportion of salts present; however, it is difficult to quantify TOC levels at present due to limitations in the instruments used. As far as I know, when sea salt is used, the TOC level is generally below 10 ppm. There are excellent discussions on the impact of organic substances in related literature, which can be found by searching for them； 2. The issue of the form in which iron exists under alkaline conditions: The so-called 2-4 refers to the period during which precipitation begins at level 2 and continues until complete precipitation occurs at level 4. Therefore, I still believe that iron hydroxide must be present under alkaline conditions. The reason it cannot be observed is that its amount is too small to be detected by the naked eye; however, over time, it accumulates in pipes and equipment and can then be noticed.

An anti-caking agent, potassium ferrocyanide, is used in refined salt; it cannot be removed during the treatment of the secondary brine, and can only be oxidized under the operating conditions of the electrolytic cell to form iron oxide.

Regarding this question, let me add more: under alkaline conditions, saltwater is colorless and transparent; upon the addition of high-purity acid, a red precipitate forms. At this point, the pH of the saltwater is between 7 and 8. Will iron hydroxide be formed under these conditions?

It is precisely for this reason that it is problematic: iron hydroxide is not formed under alkaline conditions, but rather under neutral conditions; it should instead be a coprecipitate formed by substances such as aluminum and silicates. :L

I’m very grateful for this friend’s question; it reminded me of the discussion I had with Japanese experts before the New Year. Back then, those Japanese experts were absolutely certain that it was clay! I’m not really sure about the specific reasons either!

It seems to be the correct solution, as red substances appear in the anode solution outlet hose after prolonged operation

Could we discuss in detail the issues related to Japanese experts? Thank you. What did the Japanese experts say at that time? What is clay then? This post was last edited by yzhms on 2008-3-2 22:03]

It should be an iron salt; did you add too much FeCl3? Under alkaline conditions, there shouldn’t be such a red precipitate