[soosenbinder99]

I overheard people at my university talking about an experiment the did, and it stuck on my mind. I'm just starting out studying and I'm also not a native English speaker, so excuse the language barrier.

They had waster water from recycling plant, and they wanted to precipitate Lithiumphosphate from it. They first used phosphoric acid and later put in NaOH to reach a pH value of around 11.

I was wondering if it wouldn't be smarter to just use something like Sodiumphosphate as a solid to precipitate Lithiumphosphate. Why would they choose H3PO4 for it and what actually happened here?

I know it would be smarter to speak to them directly but I never saw them afterwards and don't know their supervisor. As it just a question I ask myself I didn't want to create that big a fuzz about it

Any help would be greatly appreciated!

[Hunter2]

Cost factor. Phosphoric acid and sodium hydroxid solution is cheaper as the sodium hydrogen phosphate. Also need to dissolve. Acid and caustic Soda is available as liquid. The reaction is precipitation, because lithiumphosphate solubility is low.

[Enthalpy]

The waste water may very well contain more compounds and ions. Could H₃PO₄ under acidic conditions serve for a first separation? If lithium extraction is the goal, two steps would obtain it more concentrated.

Take with mistrust, I'm outside my knowledge here.

[soosenbinder99]

That makes a lot of sense. Thanks for your ideas

[Enthalpy]

Maybe sodium phosphate dissolves more slowly in the wastewater. Phosphoric acid then sodium hydroxide would save time.

Separate additions let also obtain the desired pH from an initial wastewater pH very different or badly reproducible. pH=11 from only sodium phosphate may be out of reach.