Chemical Engineering June 2020 - 39

include oxidation reactors, precipitation,
filtration, distillation and rectification,
as well as absorption and
stripping equipment.
Composition of spent acids
spent acids result from a variety of
processes in which nitric or sulfuric
acids are used as reactants, leaching
solutions or as a catalyst. depending
on the production process, the composition
of the sa varies considerably
in terms of acid concentration and impurities.
figure 1 illustrates the classification
of sa resulting from nitration
or sulfonation reactions (figure 2) and
acid leaching based on the containing
acid (nitric or sulfuric acid or both)
and the type of contaminants (organic
or inorganic). While hydrochloric acid
is also used in various industrial processes,
chemical or thermal recycling
of spent hydrochloric acid is applied
less often as compared to spent nitric
and sulfuric acids and therefore will
not be discussed further here.
in industrial production processes
involving nitration and sulfonation
reactions (for example, in the production
of polymers, drugs, explosives
or pigments), the sa is contaminated
with organic compounds.
these contaminants include at least
traces of the reactants and product
(for instance, nitrocellulose, nitrobenzenes,
benzenesulfonic acid, glycolnitrates,
picric acid, nitrotoluenes)
but also byproducts. for recycling
of these sa, in some cases, a complete
removal of certain contaminants
is required for safety reasons
(such as for explosive compounds)
while some contaminants can be
recycled to the originating process.
depending on the acid used for the
nitration, the sa is either a nitric acid
or a mixture of nitric and sulfuric acid
with organic impurities.
spent acids from inorganic industries
like the production of inorganic
pigments or battery chemicals (for
example, titanium dioxide via the sulfate
route or lithium refining) or metallurgical
industries (such as copper
electrolysis, or demister acid from
smelter sulfuric acid plants) contain
inorganic impurities that could precipitate
in the production process
and/or affect the product quality.
these inorganic impurities must be
depleted or removed entirely.
the concentration levels of the
acid in the sa range from diluted
solutions (below 10 wt.% acid) to
high concentrations with partly anhydrous
acids.
Process modules
the treatment of sa applying process
modules consists of unit operations
for mechanical separation (filtration),
thermophysical separation
(evaporation, distillation/rectification)
as well as chemical (oxidation)
and physicochemical operations
(precipitation, stripping and absorption).
together with the auxiliary machines
(pumps, vacuum pumps and
compressors) and static equipment
(heat exchangers, evaporators, tray
columns, packed-bed columns, reFor
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