Although graphite crucibles possess characteristics such as high bulk density, high temperature resistance, rapid heat transfer, resistance to acid and alkali corrosion, high strength at high temperatures, and excellent oxidation resistance, cleaning them after use can be troublesome, especially when processing certain raw materials.
The first step in cleaning a graphite crucible is to remove the chemical substances inside, followed by cleaning. The cleaning method depends on the type of residue remaining in the crucible. Generally, the deposits on graphite crucibles are inorganic combustion residues, which can be cleaned with hydrochloric acid, as most of them are soluble. However, some residues containing carbon components are not soluble in hydrochloric acid. In such cases, nitric acid can be used for cleaning, as it has strong oxidizing properties. Heating can also be applied during the cleaning process.
For substances that are still difficult to clean, they can be boiled in dilute hydrochloric acid or dilute nitric acid. Note that the hydrochloric acid used must not contain oxidizing agents such as nitric acid, nitrates, or halogens. If dilute nitric acid is still insufficient for cleaning, then molten potassium pyrosulfate, sodium carbonate, or borax can be used for cleaning.
Graphite crucibles need to be kept clean, with both the inside and outside surfaces being bright and shiny. After prolonged heating, the outer surface of the crucible may become dull, and over time, this dullness will penetrate the interior, causing the crucible to become brittle and crack. Therefore, any unclean substances must be removed. If there are still stains or the surface is dull, gently rub the surface with fine sand that has passed through a 100-mesh sieve and has no sharp edges, using water as a lubricant, to restore the surface's luster.