Treatment of effluents

Soluble thiosulphates present in aqueous redox systems comprising at least one salt of an anthraquinone di-sulphonic acid and at least one water soluble compound of metal having at least two stable oxidation states, for example a vanadate, are converted to sulphates by introducing oleum or sulphuric acid into the lower half of a vessel containing said thiosulphate containing redox solution, wherein the sulphuric acid has a concentration of from 10-98% w/w H.sub.2 SO.sub.4 and the amount of said sulphuric acid is such that the gram molecular weight ratio of sulphuric acid to thiosulphate is from 1:2 to 1:3, and thereafter separating the sulphate formed for example by using a solubility lowering technique. The process is carried out at ambient temperature and at a temperature of from 70.degree. C. up to the boiling temperature of the acidic mixture.

This invention relates to the treatment of aqueous effluents derived from 
wash liquors employed for the purification of gases and hydrocarbon 
liquids, by the removal of undesirable contaminants from such effluents. 
More particularly the invention relates to the removal of solubilised 
sulphur compounds from wash liquors employed for the removal of hydrogen 
sulphide from gases and hydrocarbon liquids. 
At present there are available several processes for removing H.sub.2 S 
from gases by scrubbing or washing with aqueous reagents and typically the 
reagents may be redox systems whereby the hydrogen sulphide is solvated 
and oxidised to elemental sulphur with the concomitant reduction of the 
reagent. The reagent may be reoxidised and thus regenerated for further 
use. One such process for H.sub.2 S removal using a redox system is known 
as the Stretford Process and is described in our U.K. Patent Specification 
No. 948270 wherein the redox system is typically a mixture of 
anthraquinone disulphonic acid salts and alkali metal vanadates. The 
reagent system of Stretford Process is reoxidised and regenerated by 
air-blowing through the reduced liquor. The air-blowing is also employed 
to remove elemental sulphur from the liquor which is forced during 
reduction of the reagent. The presence of air and elemental sulphur causes 
some of the elemental sulphur to be solubilised in the form of sulphoxy 
compounds such as sulphate and thiosulphate. Solubilisation is also 
believed to occur at other times during the progress of the Stretford 
Process cycle. The build-up of these sulphoxy compounds may cause the 
redox reagents to come out of the solution necessitating the discharge of 
at least a portion of the liquor to be wasted and, in which case the 
sulphur compounds, particularly thiosulphates have an undesirable and 
pollutant effect on the environment owing to their high biological oxygen 
demand. Current legislation, throughout the world, makes the safe disposal 
of these liquors a difficult and costly task. 
Processes are known whereby thiosulphates may be destroyed by pyrolysis, 
reduction or hydrolysis. However, the temperatures employed for these 
processes will also destroy the organic constituents, e.g. anthraquinone 
disulphonic acid salts and organic sequestrants such as citrates. Thus 
such processes add a burden to the process economics of the main H.sub.2 S 
removal process in that valuable reagents are lost which have to be 
replaced. 
It has been recognised that thiosulphates may be destroyed by reaction with 
sulphuric acid. For example, this technique has been described in `Coke 
and Chemistry (USSR)` 1963, Volume 8, Pages 42-45 and 1975, Volume 3, 
Pages 38-40 as well as in Japanese Patent Specification No. 52-1392. 
However, in these processes sulphur dioxide is produced in significant 
quantities and this gas is as much a pollutant as the original 
thiosulphate. The 1975 `Coke and Chemistry` reference discloses a method 
for reducing sulphur dioxide production by carrying out the thiosulphate 
acidification reaction at elevated temperature and pressure, e.g. at 
165.degree. C. and at a pressure of 6 atmospheres. 
The present invention proposes a process for destroying thiosulphates by 
reaction with sulphuric acid with substantially no sulphur dioxide 
production and under conditions which avoid the economical burdens 
proposed by the prior art processes. In accordance with the present 
invention there is provided a process for the conversion of water soluble 
thiosulphates to the corresponding sulphates wherein an aqueous solution 
contaning said thiosulphates is fed to a reaction vessel, reacted with 
concentrated sulphuric acid and the reaction product comprising a sulphate 
is removed from the reaction vessel, the improvement consisting in 
effecting said reaction at about ambient pressure and at a temperature of 
from about 70.degree. C. to about 100.degree. C. and wherein the sulphuric 
acid is sparged into the lower and of the vessel containing said aqueous 
solution. 
In carrying out the invention, it has been found essential to contact the 
sulphuric acid with the liquor at the base of the reaction vessel. This 
may be achieved by either pumping the acid directly into the bottom of the 
tank or by introducing the acid from the top directly through a dip tube 
terminating near the bottom of the vessel. Preferably the contents of the 
reaction vessel are continuously agitated, e.g. by stirring. 
Under the conditions of operation, according to the invention following 
reactions are believed to take place. 
EQU Na.sub.2 S.sub.2 O.sub.3 +H.sub.2 SO.sub.4 =Na.sub.2 SO.sub.4 +H.sub.2 
O+SO.sub.2 +S. I 
Since the acid is introduced below the main mass of thiosulphate liquor in 
the reaction vessel, the sulphur dioxide produced in reaction I reacts 
with thiosulphate Viz 
EQU 2Na.sub.2 S.sub.2 O.sub.3 +SO.sub.2 =2Na.SO.sub.4 +3S. II 
thereby giving an overall reaction. 
EQU 2Na.sub.2 S.sub.2 O.sub.3 +H.sub.2 SO.sub.4 =3Na.sub.2 SO.sub.4 +4S+H.sub.2 
O. III 
The process of the invention is particularly advantageous for destroying 
thiosulphates present in Stretford Liquors. In addition to the benefits 
afforded by the invention as regards minimizing sulphur dioxide 
production, the presence of vanadium, inherently present in Stretford 
Liquors, is also believed to have a catalytic effect in reducing sulphur 
dioxide formation. 
Our investigations have shown that there is some evidence that the vanadium 
component of the redox liquor will oxidise the sulphur dioxide by 
promoting the reaction: 
EQU SO.sub.3.sup.2- =SO.sub.4.sup.2- IV 
Thus, if the sulphuric acid is contacted with the liquor at the bottom of 
the reaction vessel, the rising sulphur dioxide reacts with both the 
vanadium and thiosulphate present in the liqor and no free SO.sub.2 is 
detectable at the top of the vessel. At temperatures below 70.degree. C. 
little reaction between sulphur dioxide and thiosulphate is oberved even 
in the case where the gram molecular weight ratio of thiosulphate to 
sulphuric acid is 3:1. 
The concentration of sulphuric acid in total liquor will vary depending 
upon the temperature at which it is desired to operate the process of the 
invention. The acid concentration will therefore range from 0.3 to 1.0 gm 
mole for every gram mole of thiosulphate. However, for reactions carried 
out at high temperatures it is preferred to provide a slight 
stoichiometric excess, e.g. 1 gm mole H.sub.2 SO.sub.4 /2 gm mole S.sub.2 
O.sub.3 2. 
The temperature at which the process is carried out will vary from 
70.degree. C. to 100.degree. C., and preferably will be from about 
85.degree. C. to 95.degree. C. 
The acid strength may vary from above 50% w/w to the most concentrated form 
available including oleum. However, it is preferred to employ from above 
50% to 98% w/w H.sub.2 SO.sub.4. 
The process of the present invention may be carried out at any thiosulphate 
loading up to the solubility point of the thiosulphate, the solution 
undergoing reaction having regard to the temperature at which redox wash 
liquor normally works. For example, a typical Stretford liquor works at 
about 40.degree. C., at which temperature the solubility of sodium 
thiosulphate is about 300 gm/liter. However, the Stretford effluent may 
contain other solubilised sulphoxy compounds. Thus, it is not possible to 
achieve the theoretical 100% saturation loading of thiosulphate. A 
Stretford Liquor at 40.degree. C. may be regarded as nearly saturated with 
sulphoxy compounds when it contains 120 gm/liter Na.sub.2 S.sub.2 O.sub.3 
and 100 gm/liter Na.sub.2 SO.sub.4. Preferably the invention is carried 
out at the highest loading consistent with maintaining solubility of all 
the solubilised sulphoxy compounds. It should be emphasised that the 
relationship of sulphoxy compound loading with temperature is considered 
for the working temperature of the wash liquor and not necessarily the 
temperature at which the acidification is effected. Thus for acidification 
reaction which is carried out at temperatures above 70.degree. C. one 
still has to consider the sulphoxy compound loading at, say, 40.degree. C. 
Upon conversion of the thiosulphate to sulphate in accordance with the 
process of the invention, the sulphate present in the reaction product may 
be removed by conventional techniques for lowering its solubility in 
solutions. For example, the sulphate-containing solution may be cooled to 
induce crystallisation or the liquor may be heated to evaporate water. The 
latter technique may be especially useful since the process system already 
contains appreciable amounts of useful heat. However, since the liquor 
contains other compounds, e.g. the redox components, the water should not 
be evaporated off to such a degree whereby the other solution components 
also come out of solution. Alternatively the sulphate may be rendered 
insoluble by a precipitation technique such as by the addition of a barium 
salt or by addition of an organic precipitant such as a lower alkanol, 
e.g. methanol. 
Sodium sulphate is a marketable commodity provided it can be produced in 
pure form. The glass and detergent industries, for example, utilize pure 
sodium sulphate in their technologies. One of the disadvantages of 
removing sulphate directly from the acidification liquor is that the 
sulphate crystals are likely to be contaminated with elemental sulphur, 
which is also produced in the acidification process of this invention. 
There is a problem of the disposal of the acidification liquor, even if 
the sulphate components are recovered, since the liquor will also contain 
polythionates which are produced as a side reaction during the 
acidification step. 
Thus, for the removal of sulphate, solubility lowering techniques may be 
applied to the acidification effluent, or preferably, to a separated side 
stream. 
We therefore propose a process whereby gases or non-polar liquids 
containing hydrogen sulphide may be purified, whereby sodium sulphate of 
high quality may be recovered and whereby other sulphoxy compounds which 
are produced may be destroyed. 
This process comprises contacting said gas or non-polar liquid with an 
aqueous alkaline wash liquor comprising a salt of an anthraquinone 
disulphuric acid or derivative thereof, and a compound of vanadium, 
contacting said liquor, after contact with said gas or liquid, with an 
oxygen containing gas, taking a first portion of said oxygenated liquor, 
acidifying said portion as described aforesaid, returning said acidified 
liquor to the remainder of the oxygenated liquor, taking a second portion 
of said mixed oxygenated and acidified liquor, removing elemental sulphur 
from said second portion and subjecting said elemental sulphur fee second 
portion to a solubility lowering technique to induce crystallisation of 
sodium sulphate, removing said sodium sulphate and returning said second 
portion, now free of elemental sulphur and sodium sulphate, to the 
remainder of said oxygenated liquor. 
As described aforesaid, the process of the present invention may be 
employed to increase the overall efficiency of the Stretford Process. 
Thus, the main purification process may be carried out as described in our 
U.K. Patent Specification No. 948270 as well as according to our earlier 
U.K. Patent Specification Nos. 971233 and 878251 and U.S. Pat. Nos. 
2,997,439 and 3,035,889, all of which are incorporaated herein by 
reference. 
The aqueous alkaline solution may contain approximately 0.5% by weight of 
any of the isomeric anthraquinone disulphonic acids (which of course will 
be present in the form of their salts), and may be initially made alkaline 
by adding ammonia or an alkali metal carbonate or bicarbonate or other 
base. It has a pH above 7, the preferred value being from 8.5 to 9.5. 
The compound of a metal having at least two valency states may be an 
ortho-, meta-, or pyrovanadate of ammonia vanadate or sodium 
orthovanadate. Which ever salt is initially added, it would appear that a 
meta-vanadate is formed in a solution having a pH of about 9. It is 
preferably added in such quantity as to give a solution of concentration 
M/1000 to M/20, although concentrations outside this range may be used. 
Other metal compounds which may be used in addition to the vanadates are 
salts or iron, copper maganese, chromium, nickel and cobalt, for example 
ferrous sulphate or ferric chloride. Such salts may be used in 
concentrations of M/1000 to M/100. 
Good results are obtainable also by using vanadates together with salts of 
iron. 
It is preferable to add a chelating or sequestering agent for the vanadate. 
Examples of such agents include soluble tartrate such as sodium potassium 
tartrate or tartaric acid or ethylene diamine tetracetic acid (referred to 
hereinafter as EDTA), or citric acid or soluble citrates, present in 
sufficient quantity to complex at least a portion of the vanadate, in 
order to maintain the solubility of the vanadate in the presence of 
hydrosulphide. 
In carrying out the process of removal of hydrogen sulphide it appears that 
absorption of the hydrogen sulphide in alkaline solution occurs with 
formation of hydrosulphide which is then oxidised. This procedure results 
in the acceleration of the oxidation process as shown by the following 
figures relating to the time taken to oxidise 50% of the hydrosulphide in 
a solution initially containing 340 parts per million of hydrosulphide, 
using different concentrations of vanadate. 
______________________________________ 
Anthraquinone Time for 50% 
Disulphonic Acid 
Vanadate Conversion 
______________________________________ 
M/100 Nil 60 minutes 
M/100 M/1000 18 minutes 
M/100 M/500 8 minutes 
M/100 M/200 2 minutes 
M/100 M/100 1 minute 
______________________________________ 
For best results the higher vanadate and/or metal salt concentrations 
should be used where the hydrosulphide concentration in the alkaline 
solutions due to the absorption of hydrogen sulphide is higher. 
The precipitated sulphur may be removed, e.g. by filtration, either before 
or after the regeneration of the solution. 
After acidification of the portion of oxidised liquor, the portion is 
returned to the main liquor stream. The portion will contain all the 
original components of the Stretford liquor, elemental sulphur, sulphuric 
acid, sulphate and thionates. Although the pH of the portion is acid, when 
admixed back into the main stream, the overall pH will not drop. 
A further portion of this main stream is now taken and subjected to a 
sulphur removal step. The sulphur present in the stream is solid elemental 
sulphur suspended in the liquid phase. This sulphur may be removed either 
by a physical removal technique such as filtration or by chemical 
techniques to solubilise the sulphur in the aqueous phase of the liquor. 
The sulphur may be solubilised for example by heating the oxide stream to 
between 85.degree. C. and boiling. Preferably the solubilising reaction is 
effected in a delay tank and the residence time may be from 2 to 5 hours. 
At the end of the solubilising step, all the liquor components are in the 
aqueous phase. This aqueous phase may then be subjected to a solubility 
lowering technique, for example as described above. The precipitated 
sulphate may then be removed for example by filtration and the mother 
liquor returned to the main process stream. The thionates present in the 
liquor may be destroyed by contacting the liquor with hydrogen sulphide, 
i.e. when the liquor is reused for the hydrogen sulphide purification 
step. 
Where the gases or liquid undergoing purification, according to the 
invention, also contain hydrogen cyanide, we have found that the build up 
to thiocyanates in the liquor can be controlled by the acidification 
treatment of the invention in an analogous manner to that for 
thiosulphates.

By this process, coal and other fuel gases, effluent air streams, liquid 
hydrocarbons and other materials can be purified so as to be free from 
hydrogen sulphide, as shown by the following examples. 
EXAMPLE I 
Two 500 ml aliquots of a Stretford Liquor having a pH of 9 and containing 4 
gm/liter of sodium anthraquinone sulphonate, M/32 (as sodium meta 
vanadate) and 81.9 gm/liter of sodium thiosulphate were each admixed with 
8.2 cc of a 25% (w/w) solution of sulphuric acid. The Liquor aliquots and 
each of the sulphuric acid solutions were preheated to 70.degree. C. and 
80.degree. C. respectively prior to admixture. 
Each of the acidified solutions were allowed to stand and the thiosulphate 
contents measured after predetermined periods of time. The results of 
thiosulphate content reduction with time are shown in Table 1. 
TABLE 1 
______________________________________ 
70.degree. C. 80.degree. C. 
Time Time 
(minutes) g/l S.sub.2 O.sub.3 
(minutes) g/l S.sub.2 O.sub.3 
______________________________________ 
0 81.9 0 81.9 
2 79.7 3 68.3 
8 66.7 10 54.4 
15 59.4 18 45.8 
23 53.1 26 41.7 
31 48.1 33 40.5 
60 37.5 60 39.2 
90 38.9 75 37.0 
135 38.3 
______________________________________ 
EXAMPLE II 
To a 500 ml sample of Stretford Liquor having the same composition as that 
described in Example I, except that the thiosulphate concentration was 
248.7 gm/l, was added 55.3 ml of 25% (w/w) sulphuric acid to give a molar 
ratio of Na.sub.2 S.sub.2 O.sub.3 :H.sub.2 SO.sub.4 of 2:1. Both the 
liquor sample and the acid solution was preheated to 93.degree. C. prior 
to admixture and maintained at 93.degree. C. after admixture. The reaction 
mixture was analysed for thiosulphate content at predetermined periods of 
time and the results are shown in Table 2. 
TABLE 2 
______________________________________ 
Time 
(minutes) 
S.sub.2 O.sub.3 g/l 
______________________________________ 
0 248.7 
2 139.1 
10 59.4 
18 59.1 
27 57.0 
35 53.8 
60 45.2 
90 34.8 
135 23.4 
180 16.0 
210 14.2 
______________________________________ 
At the end of the 31/2 hour period the solution was cooled to 1.degree. C. 
where upon sodium sulphate was observed to crystallise out. The solution 
was maintained at this temperature until no further crystallisation was 
observed. The mother liquor was decanted from the solid sulphate residue 
and filtered to remove suspended sulphate and sulphur. A 25% solution of 
sodium carbonate solution was added to the filtrate until its pH reached 9 
and the pH adjusted solution was qualitatively and quantitatively analysed 
for anthraquinone disulphonic acid salt and vanadium. The analysis showed 
that both components were present in the same form and substantially the 
same amounts at the end of the acidification process as they were at the 
beginning. 
TABLE 3 
______________________________________ 
From From 
Filtrate* 
Residue** 
Time (minutes) 
0 210 210 % Recovery 
______________________________________ 
Vanadium (gm/l) 
1.57 1.33 0.20 97.2 
ADA (gm/l) 3.89 3.14 0.62 96.6 
______________________________________ 
*After pH adjustment 
**After the residue had been washed with 0% of the recovered pH adjusted 
filtrate. The filtrate was analysed again and the Vanadium and ADA conten 
from the residue determined by difference. 
After the removal of sulphate, the pH of the filtrate was brought back to 
pH 9 by the addition of 5 mg/l Na.sub.2 CO.sub.3 and 25 gm/l NaHCO.sub.3. 
The liquor was then loaded with hydrogen sulphide gas to the level of 0.21 
gm/l and stood for 25 minutes. The concentrations of sodium tetrathionate 
present in the liquor prior to H.sub.2 S introduction and after the 25 
minutes standing period were determined. It was found that the thionate 
level before H.sub.2 S addition was 1.48 gm/l and that the end of the 
standing period the level had dropped to 0.30 gm/l. 
EXAMPLE III 
The procedure of Example II was repeated except that after the expiration 
of the 210 minute acidification step, the liquor was added to about 5 
volumes of a Stretford Liquor which had not undergone acidification. The 
ADA concentration of the mixed solution was 3 gm/l and the suspended 
elemental sulphur amounted to 1.42 gm/l. 
200 ml aliquots of this solution were placed in 1.5.times.18" test tubes 
and heated on a water bath. Air was passed into each solution via a 
sintered bubbler and when it was not passed into the solution, the sulphur 
was maintained in suspension by an electrically drives glass stirrer. 
The results of these experiments are reported in Table 4. In experiments 
1-6, air was passed in throughout the experiment (i.e. heating time) at a 
rate of 50 ml/min. In experiments 7-9 air was blown in for 15 minutes 
after the expiration of the heating period. 
The amount of sulphur solubilised was determined by difference from the 
original suspended sulphur, by filtering and weighing. 
TABLE 4 
______________________________________ 
Sulphur 
Experiment 
Heating Dissolved ADA loss 
No. Temp. .degree.C. 
Time-Hours gm/l % 
______________________________________ 
1 60 2 0.057 2.3 
2 60 4 0.084 2.8 
3 80 2 0.33 0.9 
4 80 4 0.34 7.1 
5 95 1 1.27 0.9 
6 95 2 1.32 1.8 
7 80 2 0.73 Nil 
8 80 4 1.17 Nil 
9 95 0.8 1.32 Nil 
______________________________________ 
EXAMPLE IV 
This example is presented to demonstrate the effect of vanadium in 
minimising the evolution of sulphur dioxide under acid conditions. 
500 cc of a solution containing 100 gm/l Na.sub.2 S.sub.2 O.sub.3, 4 gm/l 
ADA and 2 gm/l vanadium was acidified with 16.5 mls. 98% H.sub.2 SO.sub.4 
at 20.degree. C. The molar ratio of sulphuric acid to sodium thiosulphate 
was 1:1. 
The acid was added via a funnel to the bottom of the reaction vessel. The 
quantity of SO.sub.2 evolved with time was estimated by arranging that the 
gaseous effluate should pass through a wash bottle wherein any SO.sub.2 
was oxidised to the equivalent amount of sulphuric acid by hydrogen 
peroxide. The sulphuric acid so produced being continuously titrated (to 
neutrality) with caustics soda. The quantities of caustic soda used were 
proportioned to the amounts of SO.sub.2 evolved, these quantities 
expressed as gas evolved per mole of thiosulphate are shown in curve I of 
the accompanying drawing. 
The experiment was repeated at 95.degree. C. using 5.5 mls of 98% sulphuric 
acid to give a molar ratio of sulphuric acid to sodium thiosulphate of 
1:3. 
First in the absence of ADA and Vanadium (curve II) and secondly in their 
presence (curve III). In the third experiment ADA and vanadium were 
present in the respective molar concentrations M/80 and M/32.