Page 326 - Chemical engineering design

301
MATERIALS OF CONSTRUCTION
7.8.13. Silver
Silver linings are used for vessels and equipment handling hydrofluoric acid. It is also
used for special applications in the food and pharmaceutical industries where it is vital
to avoid contamination of the product.
7.8.14. Gold
Because of its high cost gold is rarely used as a material of construction. It is highly
resistant to attack by dilute nitric acid and hot concentrated sulphuric acid, but is dissolved
by aqua regia (a mixture of concentrated nitric and sulphuric acids). It is attacked by
chlorine and bromine, and forms an amalgam with mercury.
It has been used as thin plating on condenser tubes and other surfaces.

7.8.15. Platinum
Platinum has a high resistance to oxidation at high temperature. One of its main uses has
been, in the form of an alloy with copper, in the manufacture of the spinnerets used in
synthetic textile spinning processes.

7.9. PLASTICS AS MATERIALS OF CONSTRUCTION
FOR CHEMICAL PLANT

Plastics are being increasingly used as corrosion-resistant materials for chemical plant
construction. They can be divided into two broad classes:

1. Thermoplastic materials, which soften with increasing temperature; for example,
polyvinyl chloride (PVC) and polyethylene.
2. Thermosetting materials, which have a rigid, cross-linked structure; for example, the
polyester and epoxy resins.
Details of the chemical composition and properties of the wide range of plastics used as
engineering material can be found in the books by Butt and Wright (1980) and Evans (1974).
The biggest use of plastics is for piping; sheets are also used for lining vessels and for
fabricated ducting and fan casings. Mouldings are used for small items; such as, pump
impellers, valve parts and pipe fittings.
The mechanical strength and operating temperature of plastics are low compared with that
of metals. The mechanical strength, and other properties, can be modified by the addition of
fillers and plasticisers. When reinforced with glass or carbon fibres thermosetting plastics
can have a strength equivalent to mild steel, and are used for pressure vessels and pressure
piping. Unlike metals, plastics are flammable. Plastics can be considered to complement
metals as corrosion-resistant materials of construction. They generally have good resistance
to dilute acids and inorganic salts, but suffer degradation in organic solvents that would not
attack metals. Unlike metals, plastics can absorb solvents, causing swelling and softening.
The properties and typical areas of use of the main plastics used for chemical plant are
reviewed briefly in the following sections. A comprehensive discussion of the use of plastics
as corrosion-resistant materials is given in a book by Fontana (1986). The mechanical
properties and relative cost of plastics are given in Table 7.9.