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25.19 ‑ Natural magnesium carbonate (magnesite); fused magnesia; dead‑burned (sintered) magnesia, whether or not containing small quantities of other oxides added before sintering; other magnesium oxide, whether or not pure.
This heading covers magnesite (or giobertite) which is a naturally occurring magnesium carbonate with impurities in various proportions.
The heading also covers various types of magnesia (magnesium oxide) obtained from natural magnesium carbonate, basic magnesium carbonate, magnesium hydroxide precipitated from sea water, etc. The main types are :(1) Fused magnesia, obtained by fusion. It is usually colourless but may be slightly yellowish or greenish. It is less soluble than other types of magnesia and is used, for example, in the manufacture of crucibles or heating elements for electric ovens. (2) Dead‑burned (sintered) magnesia, obtained by high temperature (about 1400‑1800 °C) calcination. Sintered magnesia may contain small quantities of other oxides (e.g., iron oxide or chromium oxide), added before sintering in order to lower the sintering temperature. It is used in the manufacture of refractory bricks. (3) Caustic‑burned magnesia, usually obtained from magnesite by relatively low temperature (lower than 900 °C) calcination. It is more chemically reactive than fused or sintered magnesia and is used, for example, in the production of magnesium compounds, decolouring agents or oxychloride cement.
Light and heavy magnesium oxides are usually obtained by calcination of pure precipitated magnesium hydroxide or basic carbonate at temperatures from 600 °C to 900 °C. These magnesium oxides are practically insoluble in water but are readily soluble in dilute acids and are more chemically reactive than other types of magnesia (i.e., sintered magnesia and fused magnesia). They are used in the manufacture of medicaments, cosmetics, etc.
The heading does not cover :(a) Hydrated basic magnesium carbonate, sometimes known as "pharmacist's white magnesia" (heading 28.36). (b) Cultured crystals (other than optical elements), of magnesium oxide, weighing not less than 2.5 g each (heading 38.24); optical elements of magnesium oxide (heading 90.01).
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