Note: Descriptions are shown in the official language in which they were submitted.
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PR~CESS FOR PURIFYING FLUE GA~ HAVTN~ HI~H CHLORIDE CONTENT
The present invention relates to a process for
purifying flue gas having high chloride content, and more
specifically of flue gas from waste and special waste
incineration plants by a multi-stage absorption of con-
taminats, wherein initially, strongly acidic components of
the flue gas such as HCl and HF are removed by washing with
water and/or alkaline solutions, and in subsequent stages the
less acidic flue gas components such as SO2 and NOx are
removed.
In the purification of flue gases, HCl and HF play a
big part, besides SO2 and NOx, and especially so if the flue
gas is derived from a waste incineration plant. Depending on
composition of the waste subjected to incineration, the HCl
content of the flue gas may amount to even more than 10 g/m3.
As a rule, such high ~Cl contents require particular treat-
ment in flue gas purification plants. Today, this is effected
primarily by introducing a stage of HCl removal preceding the
stage of SO2 removal. Here, HCl removal frequently is
effected with simultaneous quenching of the flue gas, with
alkaline chemicals being already added in most of the casesO
Furthermore, in such quenching, a ma~or part of residual dust
remaining in the flue gas is deposited.
The solution or suspension obtained during the
deposition process and mostly still being acidic due to
hydrochloric acid generally contains the major part of heavy
metals contained in the flue gas. This hydrochloric acid
solution or suspension mostly is subjected to waste water
treatment. Upon addition of alkali or alkaline earth car-
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bonates or hydroxides, neutralization along with precipita-
tion of heav~v metals occurs. Subsequent to filtrating the
suspension, the filtrate usually is fed into a preclarifier
or is evaporated, with the residue being conveyed to a
disposal site. When entering the preclarifier, the waste
water in most of the cases still contains considerable
amounts of soluble salts as well as certain residual contents
of heavy metal.
If it is intended to recover utilizable salts such as
common salt or calcium chloride from these residues, an
efficient and expensive procedure of waste water re-pxocess-
ing must generally be carried out prior to evaporation and,
depending on the circumstances, the crystal mass precipitated
during evaporation must be subjected to one or more recrys-
tallization steps which involves a high apparative and
energetic input while, nevertheless, degrees of purity for
the salts as demanded in practice cannot be guaranteed.
It is an object of the present invention to further
improve the process for purifying flue gas having a high
chloride content, in particular, flue gas from a waste and
special waste incineration plant, and render it simpler and
safer, with maximum possible amounts of re-usable products
and only small amounts of waste material loading waste water
or disposal site being formed. More particularly, it is an
object of the invention to conduct the removal of the
strongly acidic components such as HCl and HF ~rom the flue
gas in a manner such that re-usable substances are recoverPd
thexefrom with low costs and in reliable fashion. The process
is based on such processes where in a pre-stage, strongly
acidic components of the flue gas such as HCl and HF are
washed out with water initially containing up to 20 g/l of
HCl, and in subsequent stages the less acidic components of
the flue gas such as SO2 and NOX are removed.
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Said object can be attained in a surprisingly simple
way by pre-washing with water in a quencher pre-stage, then
washing once more with water in at least one subsequent
washing stage, and then washing once more with water in at
least one further subsequent washing stage at pH values of
< 3, preferably at pH values of < 1 with neutralization using
alkaline solutions, preferably sodium hydroxide solution or
calcium hydroxide, and only then desulfurizing and denitriz-
ing in the usual fashion, with the washing solution from the
first washing stage(s) being re-fed into the quencher, and
the washing solution neutralized with alkaline solution being
eliminated from the process on exceeding the desired maximum
concentrations o~ salts.
Surprisingly, it has turned out that according to the
invention, in the quencher stage and in at least one of the
su~sequen~ washing stages using water, virtually the entire
amount of heavy metals and fluoride - except for r~sidual
flue ash - is deposited so that in the subsequent washing
stage(s) using sodium hydroxide solution or calcium hydrox-
ide, sodium chloride or calcium chloride is obtained with
high purity. The quencher bottom, in addition to heavy metals
and possibly fluoride, consists of hydrochloric acid cor-
responding to the equilibrium partial pressure of hydrogen
chloride in the flue gas. Once this equilibrium concentration
is reached, the flue gas again contains the original HCl
concentration after the quencher stage.
By adding aluminum chloride to the first washing
solution after the quencher stage, it is possible to deposit
sparingly soluble aluminum fluoride which may be utilized
separately.
In the subsequent washing stages with neutralization
using alkaline solutions such as sodium hydroxidP solution or
calcium hydroxide, the HCl gas is removed from the flue gas
almost completely, without substantial removal of SO2 and NOx
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occurring. Decisively important is the pH value which is
maintained in all the washing stages at < 3, preferabl~ < 1.
Addition of alkaline solutions such as sodium hydroxide
solution or of calcium hydroxide is always done in only such
extent as is necessary to neutralize absorbed HCl. Thus,
conveniently, controlling the addition of each alkaline
solution is effected via pH values of the washing solutions.
The absorption capacity of the washing solutions for
HCl is diminished with increasing concentrations or dissolved
salts such as sodium chloride or calcium chloride. Thus,
according to the invention, 2 washing stages with neutraliza-
tion are provided. While in the first washing stage con-
centrating the salt solutions up to a desired maximum value
is effected, the second and optionally further washing stages
serve to more completely remove HCl from the flue gas.
Conveniently, eliminating the washing solutions from each
washing stage is done according to thP concentrations of
sal~s. Most simply, these can be measured via conductivity or
density. In principle, it would also be possible to determine
the overall chloride content, but with hitherto known
methods, this is much more costly.
At last, an amazingly pure and almost neutralized
sodium chloride solution is eliminated from the first washing
stage with neutralization using sodium hydroxide solution.
Purity and concentration of these sodium chloride solutions
are so high that it is possible to employ them in the
electrolysis of alkaline chloride salts with relatively lo~
effort. ~iXewise, the removed calcium chloride is amazingly
pure and may be used like any other pure calcium chloride~ If
desired, the salt solutions may also be evaporated to dryness
and thus, solid salts may be recovered. Production of pure
solid sodium chloride has proven especially advantageous.
In particular, having relatively high contents of HF,
it has proven convenient to add a water soluble fluoride-
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binding substance to the first washing stage subsequent tothe quencher stage. As the fluoride-binding substance,
aluminum chloride but optionally also calcium chloride or
boric acid are possible above all. Thus, residual entrained
flue ash, heavy metals and above all, precipitated fluorides
are eliminated from the first washing solution. If desired,
the quencher bottom may also be subjected to separation and
processing in order to recover further utilizable products.
Most simply, this is done by fractionated precipitation using
calcium hydroxide or sodium hydroxide solution, for instance.
If high fluoride concentrations are present, the
precipitated fluoride salts and fluoride compounds may also
be removed and recycled.
- It has turned out that in spite of the expense ofhaving several separate washing stages and circulations of
washing solutions and seeming greater initially, it is much
more lower in cost and simpler to remove the contaminants
that are removable beforehand such as flue ash, heavy metals,
HF, and HCl and convert them to utilizable products instead
of removing them together and attempting later to re&over
useful fractions again.
The process of the invention operates especially
efficient when entrainment between washing stages is prevent-
ed by mist eliminators. By installing mist eliminators it is
possible to reduce entrainment from one washing stage to the
next from about 10% to about 2-3%. Instead of having several
washing stages and the expense associated therewith, mist
eliminators permit to have fewer washing stages and never-
theless, to achieve the same effects.
According to another variant of khe process of the
invention, hydrochloric acid is removed by distillation from
one of the acid washing stages following the quencher, the
condensate i~; purified over active charcoal and neutralized
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using sodium hydroxide solution with cooling to give a sodium
chloride solution of at least 100 g, preferably 150 to 250 g
NaCl/l. It has turned out that with this variant of the
process as well, residual dust, heavy metals and organic
contaminants are predominantly concentrated in the bottom of
the distillation, and that residues of organic compounds
passing over with the hydrochloric acid vapors may be removed
by an active charcoal adsorber. On neutrali~ing the thus
obtained hydrochloric acid with sodium hydroxide solution
with cooling, sodium chloride solutions or - on evaporation
to giva the salt - solid salts are obtained with such purity
that it is possible to further use them as high grade saline
or pure salts. According to the invention, the expense for
pur~fying the salts is less than in conventional purification
of the salts, in particular of sodium chloride by stepwise
evaporation and precipitation of the crystal mass.
At the same time, purification of the waste gases can
be achieved better, safer and more economical in this
~ashion, with less formation of waste water and waste
material which loads the disposal sites.
For example, the process of the invention may be
employed in close vicinity to a plant for electrolysis of
alkaline chloride salts where most frequently, chlorinated
waste material is formed as well which is intended to be
disposed in the special waste incineration. ~ikewise/ sodium
hydroxide solution needed to neutralize the hydrogen chloride
is directly available. Without major expense, the recovered
sodium chloride solution may be re-fed to the electrolysis of
alkaline chloride salts so that virtually, all the by
products of such a plant may be recovered and recycled.
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