Note: Descriptions are shown in the official language in which they were submitted.
106358~
The present invention relates to effluent treatment
and more particularly to the treatment of aqueous alkali
effluent containing sulphur compounds.
In one known process for the removal of hydrogen
sulphide from fuel gas such as manufactured coal gas, the - -
gas is washed in an aqueous alkali wash liquor and the
resulting hydrosulphide is oxidised to elemental sulphur ;
which is removed from the wash liquor by froth flotation.
The hydrosulphide is oxidised to sulphur by means of a salt
of a metal in a high oxidation state, a typical metal being
pentavalent vanadium which during oxidation of the hydro- ~-
sulphide is itself reduced to quadrivalent vanadium. The
reduced vanadium is reoxidised to pentavalent vanadium by
aerial oxygen in the presence of anthraquinone disulphonate.
The net result is the oxidation of hydrogen sulphide to
elemental sulphur by aerial oxygen.
Whilst this process is extremely efficient and is
widely used, there are undesirable side reactions which
cause contamination of the alkali wash liquor so that the
wash liquor must be discarded and replaced. The major
undesirable by-product is thiosulphate which arises from
direct aerial oxidation of hydrosulphide in solution or of
elemental sulphur in suspension in the wash liquor. Certain
of the thiosulphate may be further oxidised to sulphate by
the anthraquinone in the wash liquor. Thiosulphate and
sulphate are undesirable in that their formation involves
the fixation of alkali and sulphur so that wash liquor is
consumed and sulphur is lost. Furthermore, very high
thiosulphate concentrations may cause the precipitation
from solution of expensive vanadium salts.
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Thiosulphate concentration is controlled by continu-
ously bleeding off contaminated wash liquor and making up
with fresh liquor. Disposal of the contaminated effluent
wash liquor is generally by dumping after evaporation.
The problem of wash liquor contamination is very
much accentuated by the presence of cyanide in the fuel gas
under treatment. Hydrogen cyanide is absorbed with extremely -
high efficiency by the alkali wash liquor and forms thiocyanate
by reaction with elemental sulphur in suspension. Formation of
thiocyanate, like the formation of thiosulphate, is undesirable
in that it involves the fixation of alkali and sulphur, and
presents an even more serious disposal problem as thiocyanate
is biologically offensive and cannot be dumped without prior
treatment such as oxidative combustion or very considerable
dilution.
In a development of the above process, gases
containing hydrogen sulphide are pretreated by washing with
an aqueous alkali solution of polysulphide. The polysulphide
reacts with the cyanide to form thiocyanate whlch dissolves
in the wash liquor. Whilst this avoids contamination by
cyanide of the wash liquor used in the hydrogen sulphide
washing plant, the formation of thiocyanate in the wash
liquor in the hydrogen cyanide washing plant again involves
the fixation of alkali and sulphur and presents the same
problem of contaminated liquor disposal.
The present invention seeks to provide a process
for treating alkali effluent liquors containing sulphur
compounds so as to recover alkali and sulphur.
According to the present invention, there is provided
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a proce~s fox ~ncineXRtin~ ~UeoUs llquox containin~ alkali ;
or metal s~lt which ls preconcentrated by heat exchange ~rom
incineration gases, comprising atomising said liquor into droplet
form, bringing said liquor in droplet form into direct contact with
hot incineration gases to effect partial evaporation of liquor from ~ :
the surface of the droplets and cooling of said gases, and
passing the partially evaporated liquor in the vapor and/or
particulate phase to an incineration chamber for incineration
of the partially evaporated liquor and the production of hot
incineration gases containing substantially all the solid
particulate product of the incineration for preconcentration ~ :
of subsequently atomized liquor.
According to a second aspect of the present invention,
in a process for washing hydrogen sulphide and hydrogen cyanide
from a gaseous medium, in which the hydrogen sulphide is oxidised
in an aqueous alkali liquor to elemental sulphur and the hydrogen
cyanide is removed by polysulphide in an aqueous alkali liquor,
: liquor contaminated by hydrogen cyanide removal is thermally
reconstituted in a combustion chamber under a reducing atmosphere
to produce a gaseous combustion product having finely divided
particulate matter entrained therein, said combustion product
then being conducted from the combustion chamber through a
scrubber wherein said combustion product is quenched and said
particulate matter passes into aqueous solution. : .
Preferably, the scrubber operates by introducing re-
constituted aqueous liquor into a stream of combustion product -
flowing through the scrubber. In a preferred embodiment the
scrubber is provided with a spray device through which quench
liquor is introduced into the stream of combustion product, the : :
- 30 stream of combustion product preferably being accelerated upon :
introduction of the quench liquor and the resulting aqueous solution
passing from the scrubber in droplet form.
~dvantageously, the scrubber is a venturi scrubber
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i~363S84 " t
having a reduced throat and a liquor spray in or upstream of
the throat.
According to an advantageous feature of the present
invention, the liquor is concentrated by evaporation before
thermal reconstitution in the combustion chamber. The arrange-
ment is preferably such that a proportion of the water in the
liquor is separated from the liquor by evaporation and by-passes
the combustion chamber, only the concentrated liquor being
reconstituted in the combustion chamber. The two fractions
- 10 are then combined in a later stage in the system, after
reconstitution.
The evaporator is preferably a direct contact
evaporator in which the liquor is heated and concentrated by
hot products of combustion from the combustion chamber.
Conveniently, the evaporator and combustion chamber are
combined into one vessel.
According to a further advantageous feature of the
present invention, said reducing atmosphere is generated at
an elevated temperature in the absence of the liquor to be
reconstituted and said liquor is thereupon introduced into
the hot reducing atmosphere whereupon said liquor is thermally
reconstituted.
Generation of the hot reducing atmosphere and thermal
reconstitution of said liquor-preferably take place in res-
pective separate zones of a chamber. Whilst this chamber is
referred to above as a combustion chamber, combustion, which
is generally understood to be an oxidative process, takes
place only in the first zone in which, for example, fuel
gas such as coke oven gas is combusted in an amount of air
which results in incomplete combustion of the fuel gas and
thereby gives rise to a hot reducing atmosphere. The
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thermal reconstitution of the liquor is a reductive rather
than an oxidative process.
When the liquor is to be concentrated by evapora-
tion in the chamber in which the thermal reconstitution occurs, -
the chamber will have three more or less distinct zones,
namely an upstream combustion zone in which the hot reducing
atmosphere is generated, an intermediate reaction zone in
which the liquor is thermally reconstituted, and a downstream
cooling and evaporating zone in which heat is transferred
from the hot gases to the incoming liquor to concentrate the
liquor by evaporation and to cool the gases.
The invention is further described, by way of
example, with reference to the accompanying drawings, in -
which:-
Fig. 1 is a diagrammatic representation of a plant
which operates in accordance with a first embodiment of the
invention, shown in combination with a cyanide washing plant; -
Fig. 2 is a diagrammatic representation of a plant
which operates in accordance with a second embodiment of the
invention, also shown in combination with a cyanide washing
plant;
Fig. 3 is a diagrammatic representation of a plant
which operates in accordance with a third embodiment of the
invention, showing three distinct zones in a heat treatment
chamber; and
Fig. 4 shows a modified version of the plant of
Fig. 3.
The cyanide washing plant shown in Fig. l comprises
a scrubbing tower 10 and a sodium polysulphide generator 12.
At the base of the tower 10 is a wash liquor tank 14 from
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` 1063584
which liquor is transferred through a line 16 by a pump 18
to the top of the tower, the wash liquor then permeating ~ -
down through the tower to return to the tank 14. Sodium
polysulphide from the generator 12 is mixed with the liquor ' '
in the tank 14 and reacts, in the tower 10, with hydrogen
cyanide contained in the gas entering the tower at 20 to
form sodium thiocyanate. Washed gas passes out of the
tower through a line 22.
.~ .
To control thiocyanate concentration in the liquor,
10 it is necessary to bleed off contaminat'ed liquor by way of
a line 24 and to introduce fresh liquor by way of the poly-
sulphide generator 12. Hitherto, contaminated liquor removed
through the line 24 has been treated as a waste product.
Disposal of the waste product has caused problems especially
; as it contains sodium thiocyanate which is biologically
objectionable. Whilst thiocyanate is bio-degradable in the
presence of certain organisms at concentrations in the order
of 1000 ppm, dumping of contaminated liquor would involve a
very heavy dilution water requirement and also, of course,
20 result in loss of reagents. Alternatively, the contaminated
liquor may be treated, by for example oxidative'combustion,
before dumping.
c In the plant illustrated in Fig. 1, the contaminated
liquor removed through the line 24 is introduced by way of
a spraying device 26 into a combustion chamber 28 in which
the liquor is reconstituted at high temperature under a
reducing atmosphere. The temperature in the combustion
chamber 28 is preferably maintained at 700 to 1100C by
means of a burner 30 supplied with closely controlled quantities
3Q 'of fuel, gas and air. The ratio of gas to air introduced into
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~(~63S~4
the combustion chamber is typically such as to produce an
atmosphere in the chamber comprising 1 to 4 volume percent
carbon monoxide, 2 to 10 volume percent hydrogen and a
balance of water vapour, carbon dioxide, nitrogen and hydro-
carbons. The spraying device 26 through which the contamina-
ted liquor is introduced into this reducing atmosphere is
preferably formed of steam atomisation sprays to ensure
the formation of fine droplets of liquor whilst avoiding
the introduction of air into the chamber. As is clear from
10 Fig. 1, the spraying device 26 enters the chamber 28 ~ -
appreciably downstream of the burner 30 so that the hot
reducing atmosphere is generated in the absence of the
contaminated liquor and the contaminated liquor is sub-
sequently sprayed into the hot reduclng atmosphere for
thermal reconstitution. Combustion of the liquor in the
chamber 28 liberates the majority of the sulphur in the
thiocyanate as hydrogen sulphide which is conducted,
together with such other gaseous materials as may be present
and finely divided particulate matter containing sodium,
thraugh a line 32 to a venturi scrubber 34 in which vapours
are condensed and the particulate matter is taken into
aqueous solution to form reconstituted alkali wash liquor
in fine droplet form. The venturi scrubber 34 comprises a
narrow throat 36 and frustoconical lead-in and lead-out
portions 38 and 40. Liquor is introduced by way of a line
42 through a spray (not shown) situated within the venturi
scrubber 34 upstream of the narrow throat 36.
Dissolved particulate matter, condensed vapours, ~ ;
uncondensed vapours and gases pass from the venturi scrubber
34 by way of a line 44 to a cyclone separator 46 in which
the liquid and gaseous materials are separated. Liquid,
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~ 10635~4which will basically be sodium carbonate solution, collects in
- a tank 48 and the gaseous materials pass through a condenser -
50, condensed liquid from the condenser 50 being passed to
the tank 48 and uncondensed gases, which are rich in hydrogen
sulphide, being conducted away through a line 52 which joins
the gas outlet line 22 from the tower 10.
A pump 54 conducts liquor from the tank 48 through
an outlet line 56 from which branch the line 42 to the
venturi scrubber 34 and a line 58. The line 58 supplies
liquor to the polysulphide generator 12 which is also
supplied through a line 60 with elemental sulphur slurried
with alkali liquor. The amount of liquor entering the poly-
sulphide generator 12 with the sulphur slurry is substantially
equal, at least in terms of dissolved alkali, to the amount
of liquor passing from the plant through the line 56.
If it were desired merely to remove hydrogen
cyanide from the gas under treatment to produce gas having
a greater hydrogen sulphide content than it may have had
initially, the plant illustrated in Fig. 1 could be made
complete and self-contained by slurrying elemental sulphur
with the wash liquor issuing from the line 56 and supplying
the sulphur slurry to the polysulphide generator through
the line 60. The added sulphur would then be consumed and
would escape as hydrogen sulphide through the line 22.
However, the plant shown in Fig. 1 is particularly suitable
for use with a hydrogen sulphide washing plant in which
elemental sulphur is recovered from the gas under treatment.
.In such a plant, gas from the line 22 is washed in an aqueous
alkali wash liquor and the resulting hydrosulphide is oxidised
to elemental sulphur which is removed from the wash liquor
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i(~6;~51~
by froth flotation. The hydrosulphide is oxidised to sulphur
by pentavalent vanadium which is complexed with citrate and
which during oxidation of the hydrosulphide is itself reduced
to quadrivalent vanadium. The reduced vanadium is reoxidised
to pentavalent vanadium by aerial oxygen in the presence of
anthraquinone disulphonate. The net result is-the oxidation
- of hydroqen sulphide to elemental sulphur by aerial oxygen.
When the gas entering the cyanide scrubbing tower at 20 already
contains hydrogen sulphide, only a portion of the recovered
10 sulphur need be slurried with wash liquor and returned to the
polysulphide generator 12.
Whilst the invention has been described in connection
with a cyanide washing plant, it is also applicable, inter
alia, to a hydrogen sulphide washing plant of the type des-
cribed above. During aerial oxidation of quadrivalent vanadium
to pentavalent vanadium, any hydrosulphide remaining in
solution will be liable to oxidise to thiosulphate and some
sulphur in suspension will also oxidise to thiosulphate. The
anthraquinone will also oxidise thiosulphate to sulphate.
20 Therefore, as in the above described cyanide washing plant,
the alkali wash liquor must be discarded and replaced, disposal
of the discarded liquor resulting in loss of alkali and sulphur.
The illustrated plant, with the omission of the parts 10 to 22
and 58 and 60 may be used to reconstitute effluent wash liquor
contaminated with thiosulphate, sulphate and, if the incoming
gas contains cyanide, thiocyanate. Wash liquor is bled off
through the line 24 and reconstituted alkali liquor is
returned through the line 56.
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~ i~6358~
The embodiment of Fig. 2 is similar to that shown
in Fig. 1 and like reference numerals have been used to
denote like parts. Fig. 2 differs from Fig. 1 in including
an evaporator 25 through which the contaminated liquid is
passed before thermal reconstitution.
The evaporator 25 serves to concentrate and pre-
heat the contaminated liquor drawn off through the line 24,
concentrated liquor passing from the evaporator by way of
a pump 64 and line 66 to the spraying device 26, and water
vapour being conducted from the evaporator through the line
32. A proportion of the water in the liquor drawn off through
the line 24 therefore by-passes the combustion chamber 28,
that is, passes directly from the line 24 to the line 32
without being sprayed into the combustion chamber by the
device 26. This by-passing of water past the combustion
chamber permits a very significant reduction in size of
several of the plant items and a reduction in the amount of
fuel required to combust the liquor. Reduced fuel consumption
results in a reduction in the amount of water vapour
introduced into the system by the burner 30 so that the
amount of water which needs to be evaporated at a later
stage to maintain a water balance is also reduced.
In Fig. 2, the combustion chamber 28 and evaporator
25 are combined into one vessel for direct heat transfer from
the products of combustion to the liquor to be concentrated.
The liquor is thereby concentrated and preheated and the
products of combustion are cooled and mixed with water vapour
from the liquor. The venturi scrubber 34 can thus be a low
temperature device which may readily be designed to include an
adjustable throat.
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- ` lt:~635~34
As the liquor is very corrosive, a conventional
tubed evaporator is not appropriate, the evaporator 25
preferably being a direct contact heat exchanger in the ' . ~
form of a spray tower which is so designed that water is
evaporated from the liquor by the hot products of combustion,
but regenerated solids in the products of combustion are not -
removed by the liquor. However, the liquor may advantageously
remove from .the vessel 25, 28 such heavy solids as may be
~,
' formed in the combustion chamber.
It is found that the heat content in the products
of combustion may be such that the liquor is overconcentrated'
in the evaporator and becomes too viscous to be sprayed by
, the device 26. There is accordingly shown in Fig. 2 a .. ~,~
line 68 through which.a proportion of the condensate from
~ the condenser 50 may be mixed with the liquor in the line
24. . ~'
Referring to Fig. 3, a heat.treatment vessel 123
encloses a chamber having.three separate zones, namely an '- '
' upstream combustion zone 127 in which an exces,s of fuel ;
; 20 gas is burned in air to generate a hot reducing atmosphere;
-an intermediate reaction zone 128 into which the hot reducing
atmosphere passes to thermally reconstitute concentrated
liquor which.is introduced at 126, downstream of the .
combustion zone 127; and a downstream evaporator zone 125 ,: ~.
in which liquor is concentrated by evaporation.
Contaminated liquor for reconstitution is intro-
duced by way of the line 124 into the evaporator zone 125,
, is concentrated by direct contact with hot gases in the
vessel 123 and is passed by a pump 164 through a line 166
into the reaction zone 128 at the point 126. Reaction
products which have been cooled by heat transfer to incoming
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i~;3S84
liquor in the evaporator 125 are conducted through~a line132 to a venturi scrubber 134 in which vapours are condensed
- and particulate matter is taken into aqueous solution to
. form reconstituted alkali wash liquor. The venturi scrubber
134 comprises a narrow throat L36 and frustroconical lead-
in and lead-out portions 138 and 140. Liquor is introduced
by way of a line 142 through a spray (not shown) situated
within the venturi scrubber 134.
Dissolved particulate matter, condensed vapours,
uncondensed vapours and gases pass from the venturi scrubber
; 134 by way of a line 144 to a cyclone separator 146 in
which the liquid and gaseous materials are separated. Liquid,
which will basically be sodium carbonate solution, collects
in a tank 148 and the gaseous materials pass through.a
condenser 150, condensed liquid from the condenser being
passed to the tank 148 and uncondensed gases, which are
rich in hydrogen sulphide, being conducted away through a
line 152.
A pump 154 conducts liquor from the tank 148
through an outlet line 156 and through the line 142 to
the venturi scrubber 134.
The heat content in the reaction products may be
such that the liquor is overconcentrated in the evaporator
zone 125 and becomes too viscous to be sprayed.into the
reaction zone 128. There is accordingly shown a by-pass
line 168 through which a proportion of the condensate from
the condenser 150 may be mixed with the liquor in the line
124.
Fig. 4 shows a modified system in which a vessel
223 defines a lateral chamber constituting a combustion zone
,
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35~4
227 in which a hot reducing atmosphere is generated. Co~cen-
trated liquor passed through a line 266 by a pump 264 from
a lower evaporator zone 225 is introduced through a spray
226 disposed vertically above a reaction zone 228. Contamina-
ted liquor for concentration enters along a line 224 and
passes into the evaporator zone 225 through a spray 229.
Reaction products pass directly into a venturi scrubber 234
and then to a cyclone separator 246. Reconstituted liquor
from the cyclone separator 246 is passed by a pump 254 along
a line 255 which joins an outlet line 256 and a line 242
which supplies a proportion of the liquor to the venturi
scrubber 234. Vapours from the cyclone separator 246 pass
to a condenser 250 from which gaseous materials are conducted
away along a line 252 and from which condensate is conducted
directly to the input side of the pump 254. A branch line 259
is used to introduce a proportion of the condensate into the
evaporator zone 225 by way of a spray 261 to control the
consistency of the concentrated liquor which is introduced
into the reaction zone 228.
In the embodiments of Figs. 2, 3 and 4, the
contaminated liquor is concentrated by spraylng the liquor
in course droplet form into incineration gases issuing from
the reaction zone where previously concentrated liquor is
; undergoing thermal reconstitution. The course liquid
droplets present a sufficient surface area to give rise to
efficient evaporation from the surface but the droplets
are too coarse to collect any appreciable amount of finely
divided solid product e~isting in the incineration gas.
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3584
his enables the incineration gas to be cooled but to retain
~ its solids content until it passes from the evaporator into
the venturi scrubber or other suitable dust collection
device. The liquor thus concentrated is collected from
the base of the evaporator zone and recycled to the high-
temperature reaction zone where it is very finely atomised
so that solid products of incineration exist as a fine fume
which is able to pass through the evaporator zone without
being collected by the co~rse droplets undergoing concentra-
10 tion.
Evaporation by direct contact between coarse
droplets and hot gases avoids difficulties which are
encountered with other evaporation techniques. Indirect
heat exchangers such as shell and tube or plate-to-plate
evaporators are expensive, difficult to clean and prone
to problems of deposition and corrosion. Another prior
art technique is to hold the liquor in bulk in a tank and
bubble hot incineration gases through the bulk liquor. In
this technique, it is difficult to achieve stable gas
20 pressure and there are problems arising from foaming. Also,
it is impossible to avoid collecting substantial quantities
; of solids in the bulk liquor.
Example
The incineration of a liquor containing 200 gramme
per litre sodium thiocyanate was achieved by subjecting the
liquor to a reducing atmosphere at an elevated temperature
over 600C. If this liquor had been injected directly into
the high temperature section of the incinerator, the -
ancillary fuel requirement would have been in the order
A 30 of 3000 Btu/lb of effluent feed. However, by concentrating
the raw effluent, the net auxiliary fuel requirement was
- 15 - ',' ~
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reduced to below 2000 Btu/lb. Concentration was achieved by
, - spraying the raw feed into the evaporator section of the
incinerator so as to produce droplets substantially above
1000 micron diameter. The partially evaporated liquor was '
' collected in the base of the incinerator at a solids concentra-
` tion of up to 450 gramme per litre and was then recycled to ,~
the incinerator high temperature sprays where steam atom~
isation produced droplets below 100 micron, mainly around
10 micron. Reduction of this liquor by the high temperature ~
10 gases led to the production of`a sub-micron fume which was , '~ ,
able to pass right through the pre-concentrate sprays, even
though its carrier gas was cooled to 120C. The cooled
product was then passed to a high pressure venturi scrubber
where the solid sodium carbonate product was collected in
solution. In a second trial, aqueous liquor containing
sodium thiosulphate was subjected to oxidative incineration
at an elevated temperature of above 700C to produce sodium
carbonate and sulphur dioxide. The moist acidic gas thus
produced is highly corrosive and would damage heat exchange
surfaces in conventional indirect heat exchanges and give
rise to solids deposition. To avoid t,hese problems,
, concentration of raw liquor was effected by passing the
incineration gases through a coarse spray of raw liquor,
using droplets in the order of 1000 micron and a residence
time of about one second. The concentration of solute in
the liquor was thereby raised from 150 to 300 grammes per
litre and the incineration gases were cooled to about 80C.
It was found that the coarse droplets had collected only
about 15% of the sodium carbonate and very little of the '
30 sulphur dioxide. ! ~
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