From Roast to Paints
How to Use Pyrohydrolysis for Inorganic Oxide Prime Material for Ceramics
Alumina (Al2O3) from pyrohydrolytic processes are uncommon and only made when aluminium stripes are being pickled with hydrochloric acid. Starting with fresh hydrochloric acid of 20 wt %, the aluminium is being pickled to produce (AlCl3)n solutions of a certain polymeric structure which allow for a high concentrations of aluminium-Ions in the solution.
The pyrohydrolysis process is run at temperatures of around 750 °C, resulting in a pure white powder of Al2O3 with fluffy consistence. Further processing to produce ceramics or refractories require milling in ball or attritor mills. The final product can easily be pressed into the required forms by isostatic presses.
A co-spraying process can be performed to produce refractory material to make for example Al2O3.MgO and will be detailed in the next chapter. Important during the pressing process is the avoidance of hollow spheres or voids in the final ceramic. Unmilled spray roasted powder has an enormous shrinkage, thus proper milling is a must.
Producing Magnesia for Pharma and Liefstyle
The production of spray roasted magnesia (MgO) usually starts with natural carnallitic brines or Dead Sea water, as well as from hydrochloric solutions deprived from leachings of serpentines, or natural magnesites. From these raw materials, pure solutions are produced, by special cleaning processes, like repeated solvation , precipitation and washing processes, to extract impurities like Fe, Na, K, Ca, B which are unwanted for example for refractory use or the production of ultra-pure magnesia required in pharmaceutical or cosmetic industry.
Solutions for spray roasting in thermally heated reactors contain a higher concetration of MgCl2, nearly twice as such during the production of iron oxide. The temperature requirement is around 100°C higher than with other processes. Specific energy consumption to make MgO amounts 8000 – 8500 kcal/kmol. A pure white powder of hollow spheres is produced by this reaction which requires further milling to be processed in refractories.
An other refractory material such as Al2O3.MgO can be made by a co-spray-roasting process, starting from solutions of MgCl2 and AlCl3 . Also the production of Cr2O3.MgO, an equal important material starts from MgCl2 and CrCl3 solutions. Production temperatures in both cases amount around 850 – 900°C.