Note: Descriptions are shown in the official language in which they were submitted.
%2
When, e.g., according to the process of Canadian Patent No.
1,077,240, issued May 13, 1980 a~ SO2-containing waste gas is scrubbed with
circulating calcium-ion-containing clear wash liquid having a maximum initial
pH of 12 and a minimum subsequent pH in the weakly-acid ~one, b) acid or
salt ingredient means are then added to the gas-contacted wash liquid to in-
crease the degree of calcium-ion dissociation therein, c) calciUrQ bisulfite
in the ingredient-means-containing wash liquid is subsequently oxidi~ed to
calcium sulfate at a pH of at least 4.5, d~ one or more calcium-ion-providing
compounds, e.g. calcium carbonate, calcium oxide and calcium hydroxide, are
thereafter added to the wash water and e) the resulting calcium-ion-contain-
ing wash liquid is clarified before scrubbing SO2-containing gas therewith,
comparatively large quantities of wash liquid and correspondingly large
equipment are required to effect any particular degxee of SO2 removal from
the S02-contaminated gas being scrubbed.
,
12:~
The so-called "wa~er factor" is the volume o water
(in liters) which is re~uired in the scrubber (washer) to remove
~0% (by volume) o~ the S02 from 1 Nm3 ~one cubic m~ter at
atmospheric pressure and 20 degrees centigrade) of gas, e.g.
waste gas or flue gas, containing 2.5 grams (y) per Nm3 of S02.
Even when ~he mèans to increase the calcium-ion~aissoci2tion
includes both strong inorganic, e.g. hydrochloric, acid or salt
thereof and monobasic organic acid, such a~ formic acid~ or a
salt thereof, the water factor is 2.8 liters per Nm3 of moist
~ 10 flue gas. Ever-increasing requirements for purifying gases,
; particularly for removing harmful or noxious substances from
~lue gas, rendex extraordinarily significant any material
reduction in the water factor and concomitant reduction in
operation and e~uipment costs.
Summary of t~e Inv~ntion
h When the ingredlent means to increase the degree o~
; calcium-ion dissociation in contaminated-gas-con-~acted wash
li~uid (circulated in the scrubbin~ process of parent applica-~
~ ~ tion SN 606,785) contains dibasic carboxylic, e.g. tartaric,
;' 20 or other polybasic carboxylic, e.g. citric, acid or a salt
o~ either in addition to or in lieu of monobasic carboxylic,
;~ e.g~ formic, acid or a salt thereof, the noxious-contaminant,
e.gO S02, absorption rate and wash-liquid capacity are signi
ficantly increased over those obtainable without such a multi-
basic carboxylic acid or salt thereof.
An object of this in~ention is to improve the
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ef~ectivenes~ of wash liquid in scrubbing noxious components, e.g. S02, HF,
NOX (NO, NO2, N2O5, etc.) from gas contaminated thereby. A further object ia
to increase the capacity and/or efficiency of existing apparatus for scrub-
bing such contaminated gas with circulating wash liquid or to reduce the size
of apparatus required to achieve the same degree and/or rate of purification
of such contaminated gas.
In one aspect the present invention provides in a process for re-
moving sulfur dioxide from gas contaminated therewith and comprising: a)
contacting the gas with circulating calcium-ion-containing wash liquid having
a pH from a maximum initial pH of 12 to a minimum subsequent pH of 3; b) add-
ing to the gas~contacted circulating wash liquid acid or salt ingredient means
to increase the degree of calcium-ion dissociation therein; c) oxidizing the
ingredient-means-containing circulating wash liquid to produce calcium sulfate
d) adding to the circulating wa~h liquid a sufficient amount of a substance
selected from the group consisting of calcium carbonate, calcium oxide and
calcium hydroxide to form calcium-sulfur compounds from components in the
circulating wash liquid and to pro~ide said circulating wash liquid with cal-
cium ions for step (a); and e) clarifying the circulating wash liquid before
contacting the sulfur-dioxide-containing waste gas therewith; the improvement
wherein the wash liquid of step (a) contains, in com~ination, anions of a
strong inorganic acid and anions of a polybasic carboxylic acid.
In another aspec~ the invention provides in a process for removing
sulfur dioxide from gascont~min:~ed therewith hy washing the contaminated gas
with calcium-ion- and strong-inorganic-acid-anion-containing wash liquid, and
subsequently separating a calc1um-sulfur compound the improvement wherein the
wash liquid comprises a water-~actor-reducing amount of polybasic-carboxylic-
acid anions.
me sole figure is a schematic diagram illustra-ting an arrangement of
apparatus suitable for conducting the process aspects of this invention.
Noxious, generally acidic, components, such as SO2, HF and NO (N0
N02, N2O5, etc.) often contaminate gas designated, e.g., as crude gas, waste
gas, exhaust gas or byproduct gas. me utility of such gas is often material-
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ly increased by removing such noxious conta~linan-ts, particularly S02.
relatively-simple way has now heen found for increasing the efficiency of
such noxious-component removal ana of the capacity of existing equipmen-t to
effect such removal.
In a proCeSs which comprises: I) Scrubbing or washing contaminated
gas by contacting it with recycling clarified aqueous wash liquid, con-taining
:~ - 4a -
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a) strong inorganic, e.g. hydrochloric, acid or a
salt thereof to increase the degree of calcium-compound
dissociation and, optionally, b) monocarboxylic oryanic,
e.g. formic, acid or a salt therPof;
II) oxidizing calcium--sulfur compounds, e.g. calcium bisul-;
fite, in the resulting wash liquid into calcium sulfate;
III) increasing the caleium-ion concen~ration o~ the wash
liquid by dissolving therein one or more calcium compounds
such as calci~n carbonate, calcium oxide or calcium
hydroxide, to bind, e.g., sul~ur dioxide during the
scrubbing or washing;
IV) separating precipitated calcium sulfate from and clari-
fying the wash liquid; and
v) scrubbing or washing further and similarly-contaminated
gas by contacting it with thus-clariied wash liquid;
the absorption capacity of the wash li~uid for holding and
its efficiency ~or removing the noxious contaminants are
materially increased by incorpora-ting multi-basic carboxylic
acid in the recycliny wash li~uid.
To increase the intensity of the washing process in
connection with, e.g., flue-gas washinq, a strong inorganic
acid, especially hydrochloric acid, is added to the wash
liquid to increase the degree of calcium dissociation. This
results in a considerably~increased number of dissolved calcium
~5 ions, which combine with and thus bind sulfur dioxide from the
flue gas considerably faster and more intensively than is
possible with/ e.g., suspended li~e or other calcium-compound
1~ P articles During the oi~ding of sulur dioxide to dissolved
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111 OQ12Z
calcium ions, the added stxong acid is released to dissolve
any lime or other calcium-compound particles, which may still be
in suspension or developing incrustation. The s~rong inorganic
acid both increases t~e ef~iciency of the system and prevents
incrustation development.
Notwithstanding the foregoing, continued aifficulty is
unfortunately, encoun~ered with incrustation. By virtue of the
surplus supply o~ lime or other calcium-containing compound
present in the wash li~uidl ~he amount o~ lime or other calcium~
containing compound in the wash liquid that is dissolved and
the amount of developing strong acid that is availa~le (depend- .
ing on the amount of sulfur dioxide in the flue gas) for
. incrustation dissolution are stoich.iometrically indeterminable.
The resulting tendency to add a relatively large excess o ~:
~trong acid to the wash li~uid must be resisted because such ~
addition also constitutes a stxess on system components~ The :
.~ apparent dilemma is resolved by adding a mixture of strong
inorganic acid and weak organic acid, especially formic acid,
. to the wash liquid. By virtue of ~he formic or other weak
organic acid, the efect of the strong inorganic, e.g. hydro-
: . chloric, acld is buffered. Sulfur dioxide from the flue gas :
. first releases and then combines with calcium ions from the
produced calcium saIt of the weak organic acid,. e.g. calcium
foxmate/ thus slowly reducing the pI~ of the waqh liquid so ..
2S . that sul~ur dioxide wash-out takes place with good ~fficiency
and with intensive ~alciwn containing compound u~ilization.
~: only when calcil~m ions of the salt of the weak organic acid
have been s d up are c~lcium ion, of the salt of the ~trong
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110~1~2
inoryanic acid released, resulting in a fast2r decrease in p~
to that ~alue required for reliable disso1lltion of incrustation.
In general, ~he p~-value of the wash liquid is adjusted
prior to contact with flue or other contaminated gas to a value
which is not in excess of 12 or below 6.8. At a p~ in e~cess of
12 the desired sulfur dio~i~e wash-out, incrustation pr~vention
and calcium bisul~i~e o~na~ion are ad~rsely affected. The pH
value of 12 thereore represents an upper limit for effective
wash li~uid. Care must also be taken that the p~ value of the
wash liquid, upon contact with the flue gag, will reach the
weak-acid range but will not drop below a pH of 3, ad~antageous-
ly not below a pH of 4~ and preferably not below a pH of 4.5. In
the basic to slightl~-acid range, extraction of sulfur dioxide
from flue gas takes place with particular intensity. Moreover,
calcium bisul~ite, rather than calcium sul~ite, is pre~er~n-
tially formed in the weak-acid range. Calcium bisulite is
readily oxidized to calcium sulfate, a product which is very
~ifficult to produce by oxidizing calcium sulfite. Calcium
sulfate is practically insoluble in water and is readily pre-
cipitated and separated therefrom ~ithout any precautions.
Even with a monobasic carboxylic acid, such as formic
acid, in the wash liquid, the watex fac~or obtained by fo~low-
ing this procedure for scrubbing contaminated gas is at 2.~
liters per standard cubic meter of ~oist flue gas. Surpris-
ingly, the water factor i~ significantly reduced when a multi-
basic car~oxylic acid is used in place of or in addition to
the noted weak organic (monobasic) acid~
Although dibasic carboxylic acids, such as tartaric
acid, mal c acid, malic acid, maloli.c acid ~Dd
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110~122
succinic acid, are suitable for this purpose, a tribasic
carboxylic acid, especially citric acid, is preferred. ~ith
such polybasic carboxylic acids in ~he wash li~uid, the sulfur-
dioxide absorption rate is considerably higher and the absorp-
tion capacity o~ the wash liquid is greater than when a mono-
basic carboxylic acid, such as ormic acid, is the sole
carboxylic acid in the wash liquid. Wh~n clarified wash liquid
contains a polybasic carboxylic acid, a specified volume of
gas is relieved o~ its noxious impurities with a considerably
smaller volwne of wash liquid and without reducing or impairing
the degree of noxious-component removal.
The presence of polybasic carboxylic acid in clari-
fied wash liquid used to scrub, e.g.,flue gas makes it possible
to work with a considerabl~-lower wash-lic~uid factor (liters
of wash li~uid per cubic meter o~ flue gas). Accordingly,
less liquid has to be pumped into the system, thus reducing
the amount of work required by and/or the size ~f -the pump
and other components of the system. Wash-liquid containers,
oxidation vessels and clarification units are commensurately
scaled down in size.
As noxious acidic contaminants are absorbed ~y the wash
liq~id from contaminated gas during scrubbing, the pH of the wash
liquid lowers at least to a p~l of 6,6.This lowering pH does not
result in any precipitation; there is no formation of calcium sul
fite and/or of calci~n carbvnate. Xn the p~ range from 6 to 6~B
calcium bisulfite is fairly readily oxidi~ed to calcium sulfa~e~
Tribasic carboxylic acid, such as citric acid, acts
as a buffer both in the alkaline and in the acid range. The
strong buffer action of pol~basic carboxylic acid in the wash
liquid in the pH xange of, e.g. 4 ~o 7 also assures extensiVe
corrosion protection for system components.
Another advantage of wash li~uid con~aining polybasic
~arboxyli~ acid is that considerably-larger crystals of calcium
sulfate (gypsum) are formed during oxidation than are formed
when other wash liquids are used for scrubbing. This assures
considerably-better reduction of the water content in the
calcium sulfate residue, which must be filtered off, e.g., by
means of drum vacuum filters or centrifuges.
When, e.g., citric acid is the sole organic acid in
the wash li~uid and the pH of the wash liquid is permitted to
exceed 10 or to reach a value of less than ~, comparatively-
respectively precipitated
large quantities of added citric acid are decomposed (used up)
and must be replaced. ~he involved expense is to be avoided~
This limitation of the process is readily overcome by including
in the wash li~uid both a monobasic carboxylic acid, such as
formic acid, and a multibasic carboxylic acid, such as citric
ac id .
By including both mo~obasic and polybasic carboxylic
ids in the wash liquid, decomposi~ion of the multlbasic car-
boxylic acid is precluded in the pH range above 10 and below 4.
~s an added bonus, a furth*r increase in the sulur-dioxide
~bsorption rate is surprisingly attained, thus leading to a
Eurther reduction in ~he wash-water iactcJr. When a strong
inorganic acid, such as hydrochloric acid, is the sole acid
in the wash liquid or is in admixture with a monobasic carboxy-
lic acid, such as formic acid, the wash-water factor is in the
110~1ZZ
range o~ about 2.8. The combination of such strong inorganic
acid with a polybasic acid, such as citric acid, in the wash
liquid yields a wash-water factor in the range of about 1.96.
The combination of a strong inorganic acia wi-th both a mono-
basic carboxylic acid and a po3~basic carboxylic acid in the
wash liquid reduces the wash-water factor to abouk 1.4~ This
means that only 1.4 liters o~ wash liquid are required per
standard cubic meter of moist fl~e gas. In the latter case the
produced cryst~lline calcium sulfate (gypsum) is readily suit-
able for fur~her processin~, no corrosi,on of system components
is observed and loss oE added ac~d is minor.
When both a monobasic carboxylic acid and a m~ltibasic
carboxylic acid are employed in the wash liquidl the preferred
ratio between the two depends on the sulfur-dioxide content
of the eontaminated gas and the volume of wash liquid employed.
At a sulfur-dioxide content of 2.5 gra~s per standard cubic
meter of, e.g., crude gas a convenient ratio of monobasic
carboxylic acid to multibasic carboxylic acid is about 3:1. A
particularly advantageous volume ratio is about 10:1. The use-
ful range of such volume ratio ext~nds to 100:1.
Carboxylic acid is continuously added to circulating
wash liquid to replenish that which is constantly removed from
the cyele along with moist calcium sulfate or along with
removed water~ 'Strong inorganlc acid, such as hydrochloric
acid, is added to circulating wash liquid only when needed.
Such addition is not required when contaminated gas (being
scrubbed~ contains an adequate concentration o~ chlorine ions.
Under sueh circumstanees the chlorine-ion concen-tra~ion need
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not be implemented. When the contaminated gas contains such a
high concentra-tion of, e.g., chlorine ions that the concentra-
tion desired in the wash liquid is actually exceeded, a cor-
respond:ing volume of wash liqui.d is removed from -the wash-
liquid cycle to re~uce the ch~.vrine-ion concentration.
Although previously referred to primarily in terms
of acid additives to the wash liquid, at least the monobasic
carboxylic acid is best incorpora-ted in the wash liquid in the
form of~water~soluble alkaline (alkali-metal, SUCll as sodium
or potassium; alkaline earth me-tal, such as calcium or
magnesium; or ammonium) salt. Correspondingl~, polybasic
carboxylic acid is optionaliy introduced in-to the wash liquid
in the form of, e.g., an alkaline citrate. Under such circum-
stances it is also advantageous to incorporate the strong
inorganic acid, such as hydrochloric acid, in the wash liquid
in the form of an alkaline chloride salt hav:i.ng a common cation
with the alkaline salt of the polycarboxylic acid .
The concentration of polycarboxylic acid, e.g.
. citric acid or alkaline salt thereof, e.~. sodium citrate,
in the wash liquid depends on the concentration of noxious
components in contaminated gas being scrubhed. If alkaline salts
are used a concentration by weight of 5 percent or even more of
e.g. sodium citrate in the wash liquid is suitable.
The capaci~y of wash liquid -to absorb sulfur
dioxide when the wash li.quid contains the anion of a polybasic
carboxylic acid is greater by at least one order oE magnitude
than that wherein the only anion of a carbox~lic acid with
which the ~ sh liquld is enr ched is that of a monobasic
l1~a~
carboxylic acid, such as formic acid. For a flue gas from
a coal-fired power plant and wi-th a sulfur-dioxide content of
2,000 milligrams, e.g., about 2Q times more sulfur-dioxide
ions are absorbed by wash liquid containing the anion of a
polycarboxylic acid than with a comparable wash liquid wherein
the sole anion of a carboxylic acid is that of a monobasic
carboxylic acid. The presence in the wash liquid of a poly-
basic carboxylic acid or a salt thereof results in a lower
wash-liquid factor with corresponclingly lower requirements
for wash li~uid, pump outpu~ and size of system components.
In addition to previously-noted enlargement of
developing-gypsum crystals and ~he resultant possible improve-
ment during drainage of calcium sulfate residue, a further
. advantage of the subject process is that iron is kept in . .
: 15 solution in the form of a complex, thus precluding dehydra-
tion inhibiting iron-hydroxide formation, which would impede
expulsion of water from the calcium-sulfate residue. In the
event of any iron-hydroxide formation, such iron hydrox.ide is
advantageously precipitated at intervals (through hiyher
alkaline adjustment of the washing liquid to p~ values oE more
than 8) and withdrawn from the cycle. ....
The use of an alkaline chloride (in place of hydro-
chloric acid) together with an alkaline citrate (in place of
citric acid) and, possibly, an alkaline formate (i.n place o:E
~ormic acid), in case of SO~-ion exchancJe, via Ca(O~)2, leads to
more-quantitative precipitation of calcium sulfate (gyps~n).
Quantitati~e values are providea by the following
table, which reflects results of three experimen-ts with wash
Z
liquids of differing compositlons. The addi~iv~s, i.e. hydro-
chloric acid, formic acid and ci~ric acid, were addéd to the
wash liquid in each case in the form of an aqueous solution of
the corxesponding calci~n sal-t.
~ABLE OF EXAMPLES
Experiment
1 2 3
Gas (~oist) Volume 100,000 100,000 100,000
~Nm /h)
Wate~ Clrculati~n Volume 280 196 140
(m /h)
Additives:
Hydrochloric Acid (mg/l~ 3~500 3,500 3,500
Formic Acid (mg/l) l,S00 -_ ~oo
Citric Acid (mg/l) -- 700 80
Water Factor (l/Nm3 - moist) 2.8 1.96 1.4
Desulfurization Degree (%) 90 90 90
p}~ Value of Washing Liq~lid ,
- upon entry into gas washer 10.8 10.8 10.8
- upon exit from gas w~sher 4 6.5 4
The pH va.lue of the wash liquid, upon entry into the
gas washer, is, optionally, varied within a broad range between
pH 6.8 and pH 12 for the purpose of attaining the desired clegree
o~ desulfurization and/or adapting to ~che sulfur dioxide content
of the crude gas, Similar results are attained by alte.ring the
amount of added calciwn hydroxide, calcium oxide, calci~n
carbonate, or other calcium-con-taining compound wi-thout essen-
tially changing the pH value of the wash liquid upon departure
from the gas washer.
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The preceding table clearly confirms that substituting
a tribasic carboxylic acid, such as ci-tric acid, for a monobasic
carboxylic acid, such as formic acid, results in a considerable
reduction in the water factor, i.e., from 2.8 down to 1.96. The
pH of the wash liquid upon exit from the yas washer is naturally
higher when ci-tric acid is the sole organic acid than when formic
acid is employed.
The combination of a monobasic carboxylic acid, such as
ormic acid, with a multibasic carboxylic acid, such as citric
acid, as additives to the wash liquid, lowers the wa-ter factor
from 2.8 to 1.4, thus cutting it in half. In practical opera-
tion, this means that gas-purification operating costs are effec-
tively cut in half. When the pH of the wash liquid coming out of
the gas washer is 4, the formed gypsum is separaked with particu-
lar ease and has expecially-good utilization properties.
The clear or clarified wash liquid when enteri~ the
washer has a pH of 10 or more and contains between 0,5 and 1,5
grams per liter of calcium hydroxide and the calcium salts oE
hydrochloric,citric and optionally formic acid. These salts are
present in quantities which correspond to the following
quantities of acids:
hydrochloric acid: 0,2 to 150, preferably to 3,5 grams per liter,
citric acid: 0,005 to 0j8 grams per liter,
formic acid: 0,4 to 5, preferably to 0,9 grams per liter.
If howe~er alkaline salts are addedlthe clear or
clarified wash liquid entering the washer has a pll of 10 or more
and contains between 0,5 and 1,5 grams per llter of sodium
; hydroxide and the alcaline salts of hydrochloric~ ci-tric and
optionally formic acid These salts are present in quantities
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which correspond to the followiny quant:ities of acids:
hydrochloric acid: 0,2 to 150, preferably to 7 ~rams per liter,
citric acid: 0,2 to 50, perferably to ~5 grams per liter,
formic acid: o to 50, preferably 5 grams per liter.
The invention also includes apparatus -for implementing
the me-thod wi-th a known gas washer and a known oxidizer. Oxida-
tion oE used wash liquid is simple with a mechanism characterized
by an oxidizer consisting of a vessel with several interconnected
chambers and wherein at least one chamber is designed as an oxi-
dation chamber with an air inlet, another chamber is designed
as a reaction chamber wi-th a calcium-containing compound, e.g.lime
or lime mil , supply, and a third chnmber serves as ~ settling
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chamber. The reaction chamber optionally has an air inlet.
To remove precipitated components, a drag belt is pro--
vide~ at least at the bottom of the settling chamber. This
drag belt, however, optionally extends throughout all of the
Ivessel's chambers.
Both process and apparatus aspects of the invention
are illustrated in the drawing; the sole figure provides a dia-
gram of a mechanism for purifying crude or other gas conkaining
sulfur dioxide as a contaminant.
la Crude gas (which contains SO~ and, possibly, additional
harmful substances, such as HF and NOX) is suppli~, in the di-
rection of arrow 1, to a conventional,gas scrubber or washer 2,
from which it is discharged in the direction of arrow 3. Wash
liquid (having the composition of that o Experiment 3 in the
Table) is supplied to the gas washer 2 via pipeline 4. The wash
liquid is moved by pump 5 from tank or vessel 6 into pipeline 4
and the gas washer 2, from which it is again returned to tank 6
via pipeline 7.
Tank 6 is designed as an oxidizer for the wash liquid
and~or components therein. It has several series-connect~d cham-
bers, that is, an oxidation chamber 8, a reaction chamber 9, and
a settling basin 10. Air pipeline 11, supplied at one end by
fan 13 via collecting line 14, has several outlet openings 12
arranged at its other end in oxidation chamber ~. Adjoining the
collectin~ line 14 is a branch line 15 which ends in reaction
chamber 9, in which it has several discharge openings 16. Air
pipeline 11 and branch line 15 are e~uipped with respective
adjusting valves 17 and 18 for selectively adjusting the air
¦volume introduced into each such chamber.
3Q Reaction chamber 9 is provided with conventional
chargipg means 21 for introducing calcium-containing material,
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such as lime, into the re~ction chamber. The three chambers,
8, 9, ana 10, are separated from eac~ other by walls 19 and 2Q.
In the area adjacent the bottom, wa~ls 19 and 20 have recesses
(not illustrated) through which a scraper conveyor 22 extends.
In chambers 8 and 9 and in a portion of chamber 10 the scraper
conveyor is conducted direc~l~ over the tank bottom, which is
therein horizontall~v disposed. In the settling chamber 10 the
scraper conveyor rises to a discharge opening 23, ~rom which
precipitated calcium sulfate, picked up by the ~craper conveyor,
is moved out in the direction of arrow 24.
In detail, the illustrated mechanism works as follows:
wash liquid, flowiny out of the gas washer 2, is first collected
in oxidation chamber 8, in which the calcium-sulfur compounds,
formed during or subsequent to scrubbing, are oxidized (with
air oxygen) into calcium sulfate. Then the wash liquid flows
into reaction chamber 9 where, e.g., fresh lime is added. In
the downstream-connected se~tliny chamber the formed calcium
sulfate sinks to the scraper conveyor 22 so that the pump 5
sucks in substantially clear wash li~uid from tank 6.
Washing or scrubbing the SO2-contaminated gas with
clarified wash liquid from settling chamber 10 produces calcium
bisulfite in the gas-scrubbed wash liquid transmitted to vessel
6. This calcium bisùlfite is readily oxidized to calcium
; sulfa~e.
The calcium sulfate~ discharyed at 23, is then
conventiona]ly dehydrated in a centrifuye or in a drum vacul~
filter to a moisture content of less than 10%. Fur-ther drying
of the calcium sulfate residue is then op-tionally effected in a
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llO~lZZ
spray dxyer, which requires comparatively-little heating
energy. The heat energy needed for the spray dryer, at any
rate, is mlnimal, i.e. compared to the heat energy needed to
dry calcium sulfate residue from sulfur-dioxide washers
operated with lime sludge, such residue having been obtained
according to previously-known methods.
The invention and its advantages are readily under-
stood from the Eoregoing description. Various changes may be
made in the composition of the wash liquid/ in the process
and in the arrangement and construction of the apparatus
without departing from the spirit and scope of the invention
or sacrificing its material advantages. The compositions,
processes and apparatus hereinbefore described are merely
illustrative of preferred embodiments.
