Page 170 - Materials Chemistry, Second Edition

CAT3525_C06.qxd 1/29/2005 9:56 AM Page 141
Recycling Solid Wastes 141

TABLE 6.4
Other Secondary Uses for Cullet

Abrasives Finely ground container and noncontainer glass used in sand blasting. Such abrasives
contain no silica, which is the causative agent of silicosis.
Aggregate substitute Container and noncontainer glass utilized as drainage medium, backfill or for landscaping
purposes
Bead manufacturing Container and noncontainer glass is melted into rounded glass pellets or beads and used in
reflective paint for highways
Decorative applications Ceramic tiles, picture frames, costume jewelry, and some household items may include
recycled container and noncontainer glass
Frictionators Recycled glass is used to make frictionators needed for firing ammunition and lighting
matches
Fluxex or other additives Glass powders used as lubricants, core additives and fluxes in metal foundry work and
fabrication, as well as flux/finders in the ceramics industry
Source: Glass Packaging Institute, n.d. Reproduced with kind permission of the Glass Packaging Institute

energy to manufacture glass products compared with raw materials will hold down manufacturing
costs. There are also fewer gaseous emissions when working with cullet.
New glass containers manufactured from cullet possess the same quality and structural integrity
as do containers made with raw materials only. Using recycled glass saves wear on furnaces, result-
ing in extended furnace life and savings on maintenance. Recycling glass also reduces the amount
of solid waste brought to landfills. Lower volumes of solid waste lessen the demand for landfill
space and reduce disposal costs. Finally, recycled glass is usually closer to the bottle plants than the
sources of the raw materials (CMI, 2002).

6.9 ALUMINUM
6.9.1 ALUMINUM MANUFACTURING

The starting material for primary aluminum manufacture is bauxite ore, a mined mineral. Dissolving
powered bauxite in sodium hydroxide produces alumina, which serves as the raw material for pri-
mary aluminum production. The aluminum industry utilizes the Bayer process to produce alumina
from bauxite. The three major stages in the Bayer process are extraction, decomposition, and calci-
nation. During extraction, hydrated alumina is selectively removed from other insoluble oxides by
transferring it into a solution of sodium hydroxide (caustic soda) (World-Aluminum, 2000):

Al O ⋅xH O 2NaOH → 2NaAlO (x 1)H O (6.3)
2
3
2
2
2
This product is transferred to a heated pressure digester. Conditions within the digester (concen-
tration, temperature, and pressure) vary according to the properties of the bauxite ore being used.
Modern plants typically operate between 200 and 240°C and involve pressures of approximately
30 atm.
After the extraction stage the liquor, containing the dissolved Al O , is separated from the insol-
2
3
uble bauxite residue, purified and filtered before it is delivered to the decomposer. The mud is thick-
ened and washed so that the caustic soda can be removed and recycled. During the decomposition
phase, crystalline alumina trihydrate is extracted from the digestion liquor by hydrolysis (World-
Aluminum, 2000):
2NaAlO 4H O → Al O ·3H O 2NaOH (6.4)
2
2
2
3
2