I have a solid residue which is supposed to be mainly composed by alumina. My goal is to dissolve it or at least weaken it. I have tried concentrated inorganic acids such as HF and HCl, organic acids such as glycolic and citric, bases such as NaOH, Na2CO3... All at 60-90ºC. Nothing seems to attack it. I appreciate any advice or recommendation
Compact alumina can only be dissolved in a melt. If you find a platinum crucible, you can dissolve Al2O3 for example in a melt of NaOH, Na2CO3 or Na2B4O7, or NaF. Such cooled melts with Al3+ ionen are water-soluble.
From my experience, we needed to dissolve etched alumina on top of aluminum. So we mostly used a mixture of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 50 oC. The time depended on the alumina layer (for example 5 μm needed around 3 hours)
Molten salts certainly can dissolve it, such as NaF-AlF3 melts at 800-1000 degrees C. I would think that fuming HF also will do the job.
conc. HF at relatively higher temperature can dissolve alumina after soaking for 2-3 days.
Look up the Bauxite or Bayer-process.
Those are the industrial processes for dissolving alumina.
Another possibility: solid-state reaction at 900 C Al2O3 + NaHSO4 (or Na2S2O7)
For a somewhat related discussion, you may check: https://www.researchgate.net/post/What_acid_usable_of_alumina_leaching_processing
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