The graphite crucible is resistant to acid and alkali corrosion, high oxidation resistance and fast heat transfer; however, when it melts, impurities in the metal solution and additives form slag and adhere to the crucible wall. Since the slag contains additives, it corrodes the crucible wall. Therefore, we need to remove the slag once a day.

The method of removing slag from graphite crucible is as follows.

I. Special tools

When the slag has not solidified, we should use special tools to remove it in time. This is because the slag will be harder to remove after it has solidified. We should be careful when cleaning so as not to scratch the crucible wall.

II. Hydrochloric acid

When we encounter many layers of slag solidified on the crucible wall, it will erode the graphite nest and also affect its thermal conductivity, and even cause the graphite crucible to crack when heated and expanded, shortening the service life of the crucible.

For the above mentioned problems, we can use hydrochloric acid for cleaning. Most of them can be dissolved. If some residues containing carbon cannot be dissolved by hydrochloric acid. We can use nitric acid for cleaning. Since nitric acid is a strong oxidizing agent, it can also be heated appropriately during cleaning.

Third, dilute hydrochloric acid or dilute nitric acid

If those substances that are still difficult to clean, we can boil in dilute nitric acid or dilute hydrochloric acid. But we need to pay attention to: the use of hydrochloric acid is not allowed to contain halogens, nitrates, nitric acid and other oxidizing agents. If dilute nitric acid cannot be cleaned, it can be melted and cleaned with sodium carbonate, potassium metabisulfate or borax.

The graphite crucible should be kept clean and shiny inside and out. The outside of the crucible may become dull after a long period of combustion, and over time, unclean material may enter the interior and cause the graphite nest to break fragile, so we need to remove unclean material. If the surface is still opaque or stained, we need to use a sifted fine sand without sharp edges and rub it gently with water to restore the surface luster.