|Slight arsenic additives 0.2-1.0% increase surface-tension of molten lead in gun shot production because it makes the shape of pellets spherical. Arsenic sometimes is used also as antimony substitute in some babbitts and type metals.|
Pure arsenic is not toxic; however its water soluble compounds as well as those which dissolve in gastric juice, especially arsenic hydride and arsenous anhydride, are highly poisonous. Almost all sulphide ores of non-ferrous metals as well as sulphur ore contain arsenic, so during the process of oxidizing roasting along with sulphurous anhydride SO2 also As2O3 is always evolved. Most of it is concentrated in chimney flue, however, in the case of their low efficiency or dirtiness flue gases from ore-roasting furnace may transport significant amounts of As2O3. Non-toxic pure arsenic exposed to air is also coated by poisonous As2O3. Proper ventilation is crucial during metals etching treatment by sulphuric or chlorohydric acid which may contain arsenic impurities.
Uses of Arsenic
|manufacture of arsenic compounds, and it is a useful constituent of certain alloys. Small quantities, 0.1 to 0.2 per cent., are added to lead in the production of lead shot, which is obtained by allowing drops of the molten metal to fall into water. Pure melted lead, when dropped from a height, forms tailed drops, but the addition of arsenic leads to the formation of nearly spherical pellets. The effect appears to be due to the fluxing of the film of lead oxide which forms and surrounds the liquid particle as it falls. Alloys of lead containing small amounts of arsenic are used for making bearings, sheaths for electric cables and battery grids. Examples of such alloys are (1) lead with not more than 4 per cent. Sb and not more than 1 per cent. As, (2) lead with Sn 9 to 11, Sb 9 to 11, Cd 1.4 to 1.8, Cu 1.2 to 1.6, As 0.9 to 1.7. The addition of more than 1 per cent, of arsenic to alloys of lead and tin causes increased hardness and compressive strength but the toughness is diminished; such alloys are useful for bearings to withstand high bearing pressure free from impact. Lead anodes containing arsenic (less than 0.5 per cent.) are used in the electrolytic preparation of zinc. |
Small amounts of arsenic in iron permit the latter to take a brilliant polish and such metal is used for making chains and ornaments. A film of arsenic electrolytically deposited on iron provides a resistant protective coating against subaqueous corrosion. Alloys of arsenic with iron, manganese, molybdenum or tungsten, separately or mixed, are resistant to acids, particularly hydrochloric acid. The presence of arsenic in steels is not desirable. Small quantities (up to 0.05 per cent.) have no appreciable effect on the mechanical properties, even on impact, but larger quantities (up to 0.46 per cent.), although slightly increasing the elastic limits and ultimate strength, decrease ductility, elongation and impact resistance, especially for medium-carbon steels. There is a slight embrittling effect, and while the weakening is less than that caused by phosphorus or sulphur, it intensifies the effects of these elements if present. Arsenic appears to promote in steel the development of a dendritic heterogeneous structure which easily cracks under dynamic tests. It adversely affects hot malleability and tool life of high-speed tool steel, although the hardness is not appreciably affected.
Copper containing small quantities of arsenic (up to 1 per cent.) in the absence of other impurities shows remarkable malleability and ductility and can be worked hot or cold to any desired extent. The effect is enhanced if antimony is also added (up to 5 per cent, of each element). The electrical conductivity of copper is reduced by the addition of arsenic. The presence of arsenic is useful in copper which is to be subject to reducing conditions at high temperature. Boiler tubes and plates for locomotive fireboxes are generally made of such copper. Arsenical copper is frequently brittle, but the brittleness develops only after heat treatment below 650° C.; the critical temperature for its development appears to be 500° C. Up to 2 per cent, of arsenic is beneficial in certain bronzes and speculum metals; Britannia metal also, and brasses for high temperature work, may advantageously contain arsenic. In ordinary brass (60:40), however, as little as 0.12 per cent, of arsenic halves the ductility. The element is also contained in certain corrosion-resisting alloys of aluminium and of silver.
Arsenic in the form of an amalgam is used to a small extent in medicine. It also finds application as a poison in fly-traps.