Ferric Chloride vs Ferric Sulfate Coagulants

[ScottATL]

What is the difference between ferric chloride and ferric sulfate with respect TOC removal, handling, sludge production, effective pH range,...? I have a client who is looking into switching coagulants from aluminum sulfate to either of the ferrics for increase TOC removal.

Any lessons learned from switching from alum to ferric?

Thank you for your help.

 

[cub3bead]

With either ferric salt you precipitate Fe(OH)3. I have used both but not specifically for organics removal.

Both are available as a liquid in tote bins which I prefer from a handling stand point. Ferrifloc is dry ferric sulfate but I have had difficulty making it dissolve.

From a corrosion view ferric sulfate wins because it not as aggressive ferric chloride.

 

[kenvlach]

Ferric has wider effective pH range than alum, whose effectiveness decreases rapidly above pH 8. Important for precipitating metals at pH 9 or 10, maybe not relevant for TOC.

Consider using ferrous sulfate (aka copperas). It is very cheap, (probably) more soluble than ferric, and you can aerate to ferric. The ferrous may also catalyze oxidation reactions, as with Fenton's reagent, and the process may already include aeration steps for oxidizing organics.

 

[countryham]

I have used all three over the past thirty five years.. I have done about everything that can be done (wrong) while by making poorly assumed "educated guesses"....I have found nothing that compares to alum as a surface water coagulant but alum has at least two shortcomings.-1.It forms three separate concurent hydroxides that shift in concentration,and solubility, dependent upon pH... The most cationic is at the lower ph.The least soluble and the most cationic concentrated form is at pH 5.7.....These variables are reasons that alum has been identified as the culprit behind fouled heat exchangers..Since the pH falls by increasing concentrations and the optium pH may easily be passed with increased demand,, Many times sodium aluminate is added in conjunction with aluminum sulfate (or chloride) to keep the pH at a given "fixed" level. 2.Alum dewaters poorly in comparison to ferric BUT for any given amount of cation demand (chemical usage),-The ferric ion Fe+3depresses the pH far greater than an equavelent alum ion Al+3 requirement- assuming that all FMA (free mineral acid has been removed from both samples).......You can prove it by titrating a given quantity of ferric ion and the same quantity of aluminum ion against a standardized caustic solution to pH 7.0......Also, ferric chloride is much more soluble than ferric sulfate (copperous) but it is extremely corrosive, even on stainless steel at levels approaching 1500ppm...Also, it is very expensive to form ferric hydroxide with caustic soda and it is also a much less effective flocculant than ferric and lime from a number of standpoints....BUT ....FERRIC CAN LOWER HARDNESS in surface water clarification whereas alum will not..Good luck!