Chemical Engineering June 2020 - 44

Feature Report
acid recovery and
recycle technologies
This overview of a number of different methods for handling spent acids will help
operators select the technology most suited for a given application
Bryan and Jared
Cullivan
Beta control Systems
In BrIef
Sulfuric acid
recovery:
cryStallization
hydrochloric
acid recycle -
PyrohydrolySiS
hydrochloric acid
recovery
Mixed acid (hf/hno3)
recovery
t
he most
common
pickling,
leaching
and passivation
acids are the mineral
acids: sulfuric,
hydrochloric and
nitric. Mixtures of
hydrofluoric acid
and nitric acid are
also used in cleaning,
etching, passivating
and chemical
milling. although
the organic acids
like acetic, phosphoric,
citric, oxalic
and tartaric have
Part 2
Figure 1. Crystallization is one way to recycle spent sulfuric acid and recover iron
been used in small quantities and in limited
applications, the mineral acids continue
to dominate. in production uses of highly
corrosive mineral acid, the rate of reaction
slows as metal oxides dissolve into soluble
salt. When the acids wane and reach
the end of their effective life, they are either:
1) discarded as a hazardous waste;
2) neutralized into a solid waste sludge; or
3) processed through a recovery system to
reclaim the valuable remaining acid. focusing
on the latter, this article describes the
operation of the most prevalent acid-recovery
and recycle technologies in use today.
these include the following:
* Sulfuric acid recovery - low temperature
crystallization (figure 1)
* hydrochloric acid recycle - pyrohydrolysis
(spray roaster/fluidized bed)
* hydrochloric acid recovery - vacuum
evaporation, acid sorption, diffusion dialysis,
chemical replacement
* Mixed acid recovery (hf/hno3) - acid
sorption, vacuum evaporation, pyrohydrolysis,
nanofiltration
44
H2SO4 recovery: Crystallization
Pickling and etching of steel and copper is
usually performed in a 12-24% acid solution
at bath temperatures between 55°c
(130°f) and 75°c (167°f). Sulfuric solutions
for electrocleaning and ore leaching are usually
kept close to 40°c. the higher solubility
of many metal salts in a heated solution allows
a saturated solution to be chilled, creating
hydrated crystals in the chilled solution.
the crystals are commonly separated and
collected from the chilled solution through
centrifugation or gravity filtration.
figure 2 shows a solubility curve for iron
in various concentrations of sulfuric acid as
a function of temperature. Such curves are
needed for the design of a crystallizer plant.
the process flow diagram shown in figure
3 depicts a typical sulfuric acid crystallizer
used in a steel pickling plant. for the example
shown, the spent acid contains a high
concentration of iron. Precipitating about
half of this iron as ferrous sulfate heptahydrate
(feSo4
.7h2o) produces a recovered
acid that can be reused in the pickling plant.
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