WCO Explanatory Notes reproduced for reference. © World Customs Organization. Verify against official WCO publication.
28.50 - H drides, nitrides, azides, silicides and borides, whether or not chemically defined, ot ex than compounds which are also carbides of heading 28.49. K The four groups of compounds covered by this heading each contain two or more elements, one of which is described by the term used (hydrogen, nitrogen, silicon or boron), the others being non-metals or metals. (A) HYDRIDES The most im ortant hydride is calcium h dride (Call2) (hydrolith) obtained by direct combination o its elements; white masses wit a crystalline fracture, decomposing in the cold on contact with water and givin off hydrogen. It is a reducing agent used for producing sintered chromium fiom chromic cflloride. i' h There are also hydrides of arsenic, silicon, boron (including sodium borohydride), lithium (and aluminium-lithium), sodium, potassium, strontium, antimony, nickel, titanium, zirconium, tin, lead, etc. This heading does not include compounds of hydrogen with the followin elements : oxygen headings 22.01, 28-45, 28.47 and 28.531, nitrogen (headings 18.11, 28.14 an% 28-25), phos horn beading 28-48], carbon (heading 29.011, and certain other non-metals (headings 28.06 and i8.11). Palladium hydrides and other precious metal hydrides fall in heading 28.43. (B) NITRIDES (1) Non-metal nitrides. Boron nitride (BN) is a light white powder, high1 refractory. A heat and electricity insulator; used for lining electric ovens or for the manu acture of crucibles. Silicon nitride (Si3N4)is a greyish-white powder. B (2) Metal nitrides. Aluminium, titanium, zirconium, hafnium, vanadium, tantalum or niobium nitrides are obtained either by heating the pure metal in nitrogen at a temperature of 1,100 "C or 1,200 "C, or by heating at a higher temperature a mixture of oxide and carbon in a current of nitrogen or ammonia gas. This heading does not cover combinations of nitro~en with the following elements: oxy en heading 2 8 . 1 , halogens (heading 28.12), sulphur (head~ng28.13), hydrogen (heading 28.14), rar%on [heading 28.5 ). Silver nitrides and other precious metal n~tridesfall in beading 28.43, thorium and uranium nitrides in heading 28.44. (C) AZIDES Metal azides can be regarded as salts of hydrazoic acid (HN3). (1) Sodium azide (NaN3). Obtained by action of nitrous oxide on sodium amide, or fiom hydrazine, ethyl nitrite and sodium hydroxide; colourless crystalline flakes. Soluble in water, deteriorates slightly in humid atmosphere. Strongly affected by the carbon dioxide in the air. It is sensitive to shock, like mercury fulminate, but less sensitive to heat than the latter. Used for preparing primer explosives for detonators. (2) Lead azide (PbN6). Obtained from sodium azide and lead acetate. White crystalline powder, very sensitive to shock, preserved under water. Used instead of mercury fulminate as an explosive. @) SILICIDES (1) Calcium silicide. Very hard, ey, crystalline masses. Used in metallurgy, for local production of hydrogen, and in e manufacture of smoke bombs. (2) Chromium silicides. There are several chromium silicides; these are very hard substances used as abrasives. other than copper silicon Re ucing agent for refinin copper, resistance to ru ture; it ecreases the Also used in the manufacture o silicon bronze or of B P (4) Magnesium or manganese silicides. This headin does not cover combinations of silicon with the following elements : oxy en (heading 28,fl), halogens (headin 28.121, sulphur (heading 28.13), phosphorus (headin 1.88). Carbon silicide (silicon carbide) fa1 s in beading 28.49, platinum and other precious metal sificides in heading 28.43, ferro-alloys and master alloys contairun silicon in heading 72.02 or 74.05, and alumin~um-siliconalloys m Chapter 76. See paragraph $A) above for eomb~nstionsof silicon and hydrogen. f Q BORIDES (1) Calcium boride (Ca6). Obtained by electrolysing a mixture of a borate and calcium chloride; dark crystalline powder. A powerful reducing agent used in metallurgy. (2) Aluminium boride. Obtained in an electric furnace; crystalline masses. Used in the manufacture of glass. (3) Titanium, zirconium, vanadium, niobium, tantalum, molybdenum and tungsten borides are obtained by heating mixtures of metal owder and of pure boron powder in a vacuum at a tern erature of 1,800 OC to 2,200 or by treating vaporised metal with boron. These pro ucts are very hard and are good conductors of electricity. They are used in hard sintered compositions. i (4) Magnesium, antimony, manganese, and iron borides, etc. This headin does not include corn ounds of boron with the following elements: oxygen (heading t8.f0), halogens (heading 28.1 ), sulphur (heading 18.13), recious metals headin 28.43), pbos horn (heading 28-48), carbon (heading 28.4%. See paragrapRs (A), (B) an (D) a ove far corngnations with hydrogen, nitrogen or silicon. !i d The heading excludes copper-boron master alloys (see the Explanatory Note to heading 74.05). -
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.