TM 5-814-8
only sparingly water soluble. The typical precipi-
(a) Acid waste neutralization. Neutraliza-
tation process using sodium hydroxide or lime as
tion of an acid waste (low pH) can be accom-
a reactant is generally applicable to copper, zinc,
plished by adding alkaline materials such as
iron or nickel removal with no special modifica-
crushed limestone, lime, soda ash or sodium
tions.
neutralization of a waste containing sulfuric acid
Chromium exists in wastewater in both
the highly toxic hexavalent and the
forms a salt of limited volubility (CaS04) which cn
less toxic trivalent forms. To precipi-
cause adherent deposits on equipment surfaces
and piping. Hydrated lime (Ca(OH)2) or quicklime
tate chromium, the hexavalent form
must first be reduced to the trivalent
(CaO) are more commonly used, since these mate-
rials have more neutralizing capacity per pound
form using reducing agents such as
than limestone. However, lime may also form
sulfur dioxide, ferrous sulfate, metallic
iron, or sodium bisulfite. The reaction
calcium sulfate sludges. Strong bases such as
is best performed in an acidic solution
soda ash (Na2C 03) or sodium hydroxide (NaOH)
quickly neutralize strong acids, forming soluble
with a pH of 2.0 to 3.0. The trivalent
salts and virtually eliminating the sludge prob-
chromium is precipitated as chromium
lem, although increasing the dissolved solids
hydroxide by raising the pH with lime
or sodium hydroxide (34)(39)(86).
content of the water. Strong bases require special
Cadmium hydroxide precipitation by
equipment and handling and are four to eight
lime occurs at high pH. If cyanide is
times as expensive as lime or limestone.
also present (as inplating waste), it
ization of an alkaline or basic wastewater (high
must be eliminated first by adding
sodium sulfide. The proprietary
pH) can be accomplished by adding acidic materi-
als such as carbon dioxide (CO2) or sulfuric acid
"Kastone" process is a hydrogen perox-
(H2S04). Carbon dioxide may be added by passing
i d e oxidation-precipitation system
boiler flue gas or bottled CO2 gas through the
which simultaneously oxidizes and pre-
alkaline waste, forming carbonic acid (H 2C O3)
cipitates cadmium as cadmium oxide
which then neutralizes the base. Sulfuric acid
which can be recycled to some process
readily neutralizes bases, although it is highly
solutions (130).
Lead may be precipitated by substitut-
corrosive and requires special equipment and
handling. Other strong acids, such as hydrochlo-
ing soda ash for lime in the conven-
ric acid (HC1), can be used depending on acid
tional lime precipitation scheme. Both
mercury and silver as well as lead may
costs.
( 2 ) Heavy metal removal and recovery.
be precipitated as sulfides with the
addition of combinations of sodium sul-
Heavy metals which are of most concern are
fide, sodium thiosulfate or sodium hy-
silver (Ag), cadmium (Cd), chromium (Cr), copper
droxide (21)(86). The precipitated sul-
(Cu), iron (Fe), mercury (Hg), lead (Pb), nickel (Ni),
fide sludge may be sold to a refinery
tin (Sri), and zinc (Zn) because of their toxicity
for recovery (130).
and/or high market value (86). Military sources of
(b) Metallic replacement. The metallic re-
heavy metals include munitions production, metal
placement or displacement process is used when
plating, aircraft and motor vehicle washing, paint-
metal recovery is desirable, such as silver recov-
stripping and metal-working, photographic pro-
ery from photographic wastes and copper recov-
cessing and cooling water system blowdown. The
ery from brass-working wastes. In this process, a
most commonly used heavy metal removal tech-
metal which is more active than the metal to be
niques are chemical precipitation, metallic replace-
recovered is placed into the waste solution. The
ment, electrodeposition, ion exchange, evapora-
more active metal goes into solution, replacing
tion, and reverse osmosis, although solvent
extraction, activated carbon adsorption and ion
the less active metal which precipitates (or plates)
flotation are being developed and are applicable in
out and is recovered. Zinc or iron, in the form of
some situations (32)(33)(39)(86).
either dust or finely-spun wool, is often used to
recover silver or copper (30)(86). A proprietary
monly used removal method, particularly when
spun-iron cartridge is used to recover silver from
metal recovery is not a consideration, is precipita-
waste photographic fixing solutions in normally a
tion. This process is based on the fact that most
c o n t i n u o u s operation (111). The treated fixing
metal hydroxides are only slightly soluble and
solution may still contain at least 1,000 mg/L of
that some metal carbonates and sulfides are also
silver as well as the ionized iron and cannot be
6-28
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