WCO Explanatory Notes reproduced for reference. © World Customs Organization. Verify against official WCO publication.
28.39 - Silicates; commercial alkali metal silicates. - Of sodium : 2839.1 1 - - Sodium metasilicates 2839.19 - - Other 2839.90 - Other Subject to the exclusions mentioned in the introduction to this sub-chapter, this heading covers silicates, metal salts of the various silicic acids, non-isolated in the free state and derived from silicon dioxide (heading 28.1 1). (1) Sodium silicates. Obtained by melting sand and sodium carbonate or sulphate. Their composition is ve diverse (monosilicate, metasilicate, polysilicate, etc.), and their degree of hydration and xeir solubi,, vary with the method of manufacture and the de ee of purity. Occur in colourless crystals or in powder, in vitreous masses water-glasror in more or less viscous aqueous solutions. The deflocculate the gangues o ores and are used as flotation regulators. Also employed as fil ers for manufacturing silicate soaps; as binders or adhesives in the manufacture of paperboard or agglomerated coal; as fire-proofing materials; for preserving eggs; in the manufacture of non-putrefymg adhesives; as hardening agents in the preparation of corrosion-resistant cements, of lutings or of artificial stones; for the manufacture of washing pre arations; for pickling metals; as anti-scaling products (see Explanatory Note to head~ng3f.24). r I (2) Potassium silicates. Used for similar purposes to sodium silicates. (3) Manganese silicate (MnSi03). Orange-coloured powder, insoluble in water. Used as a ceramic colour and as a drier for paints or varnishes. (4) Precipitated calcium silicates. White powders obtained fiom sodium or potassium silicates. Used for manufacturing fire-proofpisis, and dental cements. (5) Barium silicates.White powders used for manufacturing barium oxide and optical glass. (6) Lead silicates. Occur as powder or in vitreous white masses; used as a glaze in ceramics. (7) Other silicates, including commercial alkali metal silicates other than those mentioned above. These include caesium silicate (yellow powder, used in ceramics), zinc silicate (coating of fluorescent tubes), aluminium silicate (manufacture of porcelain and refractory products). Natural silicates are excluded from the heading, e.g. (a) Wolastonite (calcium silicate), rhodonite (manganese silicate), phenacite (or phenakite) (beryllium silicate), and t~tanite(titanium silicate) (heading 25.30). (b) Ores such as co per silicates (chrysocolla, dioptase), zinc hydrosilicate (hemimorphite), and nrconium silicate fzircon) (headings 26.03,26.08 and 16.15). (c) The precious stones of Chapter 71. -
1.- Except where the context otherwise requires, the headings of this Chapter apply only to : (a) Separate chemical elements and separate chemically defined compounds, whether or not containing impurities; (b) The products mentioned in (a) above dissolved in water; (c) The products mentioned in (a) above dissolved in other solvents provided that the solution constitutes a normal and necessary method of putting up these products adopted solely for reasons of safety or for transport and that the solvent does not render the product particularly suitable for specific use rather than for general use; (d) The products mentioned in (a), (b) or (c) above with an added stabiliser (including an anti-caking agent) necessary for their preservation or transport; (e) The products mentioned in (a), (b), (c) or (d) above with an added anti-dusting agent or a colouring substance added to facilitate their identification or for safety reasons, provided that the additions do not render the product particularly suitable for specific use rather than for general use.