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Zinc Borate in Treatment of Wastewater

Can you please take a look at the attached and give me some feedback. I want to understand why specifically pH is driven way low and then way high to promote precipitation of the zinc solids.

Please take a look and comment.

In our process, zinc borate (2ZnO-3H3BO3-3H2O) ultimately splits and dissociates to zinc oxide and boric acid.

In the first equation (because I'm only concerned about the zinc), the zinc oxide combines with water in the wastewater plant to form zinc hydroxide.

Then, the current treatment scheme doses the zinc hydroxide with phosphoric acid and drives pH down to approx. 2.5 ---- it would appear that zinc phosphate and water are by-products

Then, the wastewater is dosed with sodium hydroxide to raise pH to approx. 8.5 to 9.0

This is the current chemistry that allows the zinc to be precipitated.

Does this make sense to you and can you explain a little bit better why we have to get back to a hydroxide of zinc to allow precipitation.....why can't the zinc phosphate be precipitated directly???

Also...are my balanced equations correct?

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Solution Summary

The precipitation processes of zinc borate in wastewater are discussed