generally mixtures of HCl and HNO3 (20–50%) is recommended for washing the alumina but using that small cracks form on surface of tube which can be opened later. can anyone suggest the washing chemical for alumina so that no cracks forms. Thanks in advance
As a generic method for acid cleaning alumina tubes, you may first try HCl (≥25 wt%). Only if that is found ineffective, it may be worth to try HCl + HNO3 (1:1 v/v; ≥25 wt%), with the safety concerns that apply to aqua regia. Both the single acid and the mixture may be found somewhat more effective if heated, but that could only be practically considered for rather small tubes and at the fume hood. Other acids or acids mixtures, and even salt aqueous solutions, may be possibly considered while dealing with specified contaminants. After the acid cleaning the tube should be rinsed with tap water, then with distilled or deionized water, and should not be taken to high temperatures at high heating rates in the first after-cleaning run. Note that it may be rather difficult, being often impossible in practice, to completely clean the tube from contamination due to high temperature exposure to the contaminant, due to possible high temperature diffusion beneath the exposed alumina surface. Organics may be usually burned off by heating above 900 ºC in air, or at a somewhat lower temperature in oxygen enriched atmosphere, but possible tar formation should be avoided.
What kind of material do you want to remove from the alumina tube ? If these are organic compounds, tars or pyrolytic carbon, the safest way is just heating slowly the tube up to 1000°C in air and let the organic matter burn completely until you recover a nice, clean, white tube. When doing this, carefully open windows and/or switch fume hoods on, as CO is emitted during the process.
Alain
Thanks Alain Celzard for your reply. its just routine base washing as Alumina tube needs washing on regular basis therefore we used mixtures of HCl and HNO3 (20–50%) as recommended but it creates microcracks therefore i asked any suggestion ...
As a generic method for acid cleaning alumina tubes, you may first try HCl (≥25 wt%). Only if that is found ineffective, it may be worth to try HCl + HNO3 (1:1 v/v; ≥25 wt%), with the safety concerns that apply to aqua regia. Both the single acid and the mixture may be found somewhat more effective if heated, but that could only be practically considered for rather small tubes and at the fume hood. Other acids or acids mixtures, and even salt aqueous solutions, may be possibly considered while dealing with specified contaminants. After the acid cleaning the tube should be rinsed with tap water, then with distilled or deionized water, and should not be taken to high temperatures at high heating rates in the first after-cleaning run. Note that it may be rather difficult, being often impossible in practice, to completely clean the tube from contamination due to high temperature exposure to the contaminant, due to possible high temperature diffusion beneath the exposed alumina surface. Organics may be usually burned off by heating above 900 ºC in air, or at a somewhat lower temperature in oxygen enriched atmosphere, but possible tar formation should be avoided.
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