Page 878 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Appendix F: Alum Data and Conversions

Aluminum ion, Al , and the ferric ion, Fe , are the com- The product formed called ‘‘dry’’ alum is a crystal, i.e.,
3þ
3þ
mon metal coagulants used in water treatment, with Al 3þ Al 2 (SO 4 ) 3 14H 2 O, and must be ground to be used in water
being used most frequently. The Al 3þ ion is provided com- treatment. As seen in Figure F.1, grinding and screening
mercially in the form of hydrated aluminum sulfate, produces powdered, ground, and granular, which are the
Al 2 (SO 4 ) 3 14H 2 O, called alum, and Fe 3þ is provided as common commercial sizes, with lump being available in
either Fe 2 (SO 4 ) 3 7H 2 Oor Fe 2 Cl 3 6H 2 O. some cases in other countries. The dry alum may be shipped
In using alum, the principles are straightforward. A given in bulk and is also packaged in 50 or 100 lb multiwalled paper
mass of alum added to a given volume of water results in a bags and, in some cases, plastic bags.
known concentration. To implement this ostensibly simple
idea, however, requires knowledge of the manufacture of
alum, and its resulting characteristics and conventions in F.2 SOLID ALUM
expressing concentration. Both solid and liquid forms are
The alum product manufactured is chemically,
considered here.
Al 2 (SO 4 ) 3 14H 2 O, which has a solid crystal structure and
is called ‘‘dry alum.’’ Since this hydrated form is the ‘‘added
F.1 MANUFACTURER OF ALUM mass’’ in water treatment, concentrations are expressed as mg
Al 2 (SO 4 ) 3 14H 2 O=L solution and mass flows are expressed
The principal aluminum ore is bauxite, a mixture of
as mass of Al 2 (SO 4 ) 3 14H 2 O per unit time.
hydrous aluminum oxides, varying physically according to
their deposits. In general, commercial deposits have about
52% hydrated aluminum oxide. Most of the bauxite mined F.2.1 DESCRIPTION
is refined into alumina, which has several forms of which
Table F.1 describes some of the properties of the ‘‘standard
aluminum oxide is the principal component (Britannica,
ground’’ dry alum. The data may be useful in designing
1974). The commercial chemical grade bauxite, from
storage for bulk alum, in sizing a reactor for dissolution (for
which alum is produced, is 59% Al, expressed as Al 2 O 3 .
standard ground alum), and in providing a general back-
Metallurgical grade bauxite is lower assay and higher in
ground concerning alum characteristics.
contamination.
Aluminum sulfate is produced by the reaction between
sulfuric acid and hydrated aluminum oxide with product
F.2.2 MASS FLOW OF ALUM
Al 2 (SO 4 ) 3 14H 2 O. The reaction, sans the waters of hydration is
The dry chemical is free flowing and feeds well from bulk
Al 2 O 3 þ 3H 2 SO 4 ! Al 2 (SO 4 ) 3 þ 3H 2 O (F:1) storage hoppers (Harringer, 1984). Feeders are of two types:
volumetric and gravimetric. The accuracy of the volumetric
According to Harringer (1984), alum is manufactured by feeder is within 1%–2% of the amount set and is common for
‘‘digesting’’ an aluminum-bearing ore with sulfuric acid. smaller plants. The gravimetric feeders are more accurate and
Figure F.1 depicts the manufacture, showing the sequence of are favored by the larger plants. The designated rate of alum
processes and operations. The waters of hydration, i.e., n in feed is dropped into a dissolving chamber which is agitated by
Al 2 (SO 4 ) 3 nH 2 O is determined by the point at which the a mixer and from here fed to the rapid mix.
crystallization is arrested in the evaporation process with Figure F.2 shows schematically the feed of granular alum
n 18. The 14 waters of hydration is a stable form, losing from palettes to a hopper for storage and metering. The
water slowly. Also, with slightly lower mass than for n ¼ 18, metered granular alum drops to a belt and then drops into a
the cost to transport is less (the foregoing statements were dissolution reactor. The dissolved alum concentrate is then
adapted from Harringer, 1984). The hydrated state of com- metered by a positive displacement pump into the rapid mix
mercial alum is approximately 14H 2 O as a mixed hydrate, and (or into the flow to the rapid mix).
is expressed normally as 14H 2 O. For reference, 14.3H 2 Ois The mass flow of granular alum has three stages: (1) from
used commonly in the literature, but 14 was suggested by the alum hopper to the dissolution reactor, (2) from the dis-
General Chemical, a major manufacturer of alum in the solution reactor to the rapid mix as a concentrated dissolved
United States. alum solution, and (3) as dissolved alum in the main

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