Chemical Engineering June 2020 - 51

leaving a final stream of water condensate.
the recovered metal concentrate
can be crystallized or left in
liquid form.
Pyrohydrolysis. Spray roasting
technology also works on mixed
acid. the capital cost is quite high,
but the high cost of the acid can offset
it in large stainless-steel production
plants. Plants producing more
than forty tons of spent acid each
day might consider applying pyrohydrolysis.
as with pyrolytic recycle
of hydrochloric acid, the location of
the recycling plant and air-quality restrictions
should be studied.
Nanofiltration
and
crystallization
(experimental). acid recovery
using nanofiltration membranes has
been attempted many times over
the past several decades. the three
problems preventing the technology
from advancing as a cost-effective
solution have been the concentration
limitation, flux rate, and the lack
of corrosion-resistant high-pressure
pumps and robust membranes. the
ability to pass mixed acid through
the membrane while sequestering
the metal ions in a concentrate
stream has been proven. however,
as the metal ion concentration rises,
the flux rate (throughput of recovered
acid) diminishes. in tests at
pressures up to 30 bars, the practical
limit of metal ions in the spent
acid was approximately 30 g/l.
the same acid-resistant nanofilter
membranes pilot-tested for mixed
acid recovery are used in copper recovery
in sulfate leach solutions. the
copper-sulfate-concentrate stream
from nanofiltration is further processed
in a crystallizer or electrowinning
cell. trial studies on mixed acid
waste from stainless-steel cleaning
lines in Sweden followed a similar
approach - crystallizing the concentrate
from the nanofiltration step
and reprocessing the post-crystallization
filtrate. crystallization of the
iron fluoride hydrate (fef3
.3h2o)
proved difficult in the test trials, with
the nickel and chrome hydrates
even more so. the copper sulfate
recovery from the concentrate of the
membrane process was much simpler
by comparison.
Microfiltration of the spent acid in
the first step of recovering the acid
was found to be imperative. ion-selective
membranes and sorption resins
require protection against incursion
from solids and oil that cause
blinding and fouling without excellent
precleaning steps. in the case
of filtration of the hydrofluoric/nitric
mixed acid feed, some installations
employ silicon carbide microfilters
and corrosion-resistant components
to extract the larger molecular organics
like oil, as well as any microsolids
that blind surfaces and impact
the hydraulic process.
Final remarks
environmental protection agencies
have set goals and standards that
guide us toward what we now call
zld (zero liquid discharge). Meeting
the environmental standards
with minimum impact on the cost
of production has become the challenge
to industry. over the past fifty
years, many recovery technologies
have been applied to the mining,
milling and metal-finishing industries
to cost-effectively recover or
recycle their most expensive and
dangerous waste - acid. advancements
in materials and automation
over that half-century have greatly
reduced the capital cost of recovery
and recycle, while also bringing
operating costs into line. this challenge
will continue.
Scientists and engineers work
every day to create more costefficient,
user-friendly
processes.
as
we advance toward zld, innovations
in recovery technologies
create co-products rather than
waste products. in many cases, the
co-product revenue exceeds the
operating cost. the goal has not
changed, but the tools to reach it
have advanced. as famed american-football
coach vince lombardi
once surmised, " Perfection is not
attainable, but if we chase perfection,
we can catch excellence. " n
Edited by Gerald Ondrey
References
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ChemiCal engineering www.Chemengonline.Com June 2020
51
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Authors
Bryan Cullivan is the founder
(1980), president and CEO of
Beta Control Systems, Inc. (6950
SW 111th Ave., Beaverton, OR
97008; Phone: 503-646-3399;
Email: bryanc@betacontrol.com;
Website: www.betacontrol.com),
which has designed and installed
over 120 environmental recovery
facilities worldwide. He holds a
B.S.Ch.E. degree from Oregon State University. Prior to
founding Beta, he managed one of the largest integrated,
industrial-wastewater-treatment plants in the
world at a Fortune 500 company.
Jared Cullivan is a process engineer
and project manager at Beta
Control Systems, Inc. (same address
as above), where for the last
15 years, he has assisted with the
design, automation and commissioning
of hydrochloric-, sulfuric-,
and
hydrofluoric-acid-recovery
systems. He holds a B.S.M.E. degree
from Santa Clara University.

http://www.icis.com/chemi http://premium http://www.docstoc.com http://www.lime.org/docu http://www.water http://www.info.org/resources/water-facts http://www.eia http://www.eia.gov/ http://www.betacontrol.com http://www.nmfrc.org http://www.Chemengonline.Com

Chemical Engineering June 2020

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