Generally speaking what proportion of NaOH would be typically required to dissolve one tonne of crushed bauxite of average purity?
Also are varying quantities required for gibbsite, böhmite and diaspore respectively and how do they differ?
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Bauxite contains usually 45 - 60 % Al2O3. To get an order of magnitude, one ton bauxite contains about 500 kg Al2O3, or about 5000 mole Al2O3. If 2 NaOH are needed to dissolve 1 Al2O3, it means that a minimum of 10'000 mol NaOH is needed to dissolve Al2O3, which weighs 400 kg. But more NaOH is necessary, because the bauxite contains also some silica, which is also dissolved in NaOH. So the amount of NaOH must be higher than 400 kg.
My advice is start with the best theoretical amount (per above) and apply it with varying deviations of concentration to some test samples. Heat or stir as you may be able to perform when working with larger quantities in bulk.
My understanding, even after your sample testing, you will find that working in larger quantities is more likely difficult to get the best results than you did experience in your small test sampling efforts. Here is a source noting an issue concerning surface charging properties of metal oxides (including Al2O3) in bulk.
Perhaps, applying some sonolysis (which could introduce powerful radicals from the water itself with sound frequencies) to your bulk preparations, which are harder to evenly heat, may prove fruitful.
There are also an mechanochemical option.