Process for the removal of sulfur oxide constituents from a waste gas

A waste gas from a chemical process is treated to remove undesirable substances, especially sulphur dioxide, by contacting said waste gas with acidic solution, e.g. industrial waste solution, at pH less than 2 to effect pre-cleaning, and then contacting said waste gas with alkaline solution e.g. seawater.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method and apparatus for the treatment of waste 
gases produced in chemical processes. In particular the invention relates 
to the treatment of waste gases to remove undesirable constituents, such 
as sulphur dioxide. 
2. Description of the Prior Art 
It is known to treat waste gases with an alkaline solution, e.g. 
environmental or surface water such as seawater, in order to remove 
harmful substances. For example, a method is known in which waste gase is 
scrubbed with seawater. Seawater is useful in this context because of its 
natural alkalinity. 
In this gas scrubbing method, seawater absorbs sulphur dioxide, and then 
the sulphite formed is converted by oxidation to sulphate. This process 
can be carried out in an absorber and a downstream aerator. The used 
seawater laden with sulphate is then returned into environmental surface 
water. Examples of such processes using seawater are given in U.S. Pat. 
No. 4,152,218, GB-A-1356972 and GB-A-1410884. A drawback of this known 
method is that the returned seawater is also laden with other harmful 
constituents of the waste gas. In many chemical processes these 
constituents are produced and load the seawater in such a high 
concentration that the used seawater cannot be returned to the surface 
water. As a result of this, the known method has only limited application. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of the kind 
described above in which the loading and contamination of the alkaline 
solution, e.g. seawater is reduced. 
Another object of the invention also is to provide a method of the kind 
described above which is applicable in a broad range of chemical 
processes. 
The method in accordance with the invention is characterised in that, prior 
to the treatment with an alkaline solution, the waste gas is precleaned by 
contacting it with an acidic solution, e.g. acidic waste water This has 
the advantage that components from the waste gas which are not desired in 
the alkaline solution, are removed from the waste gas by the pre-cleaning. 
For example, acid pre-scrubbing removes gaseous and dusty components and 
polycyclic aromatic hydrocarbons (PAHs) to a large extent. 
A further advantage is that the amount of alkaline solution required for 
removing constituents from waste gas is less in the method according to 
the invention because some of the constituents are absorbed by the acidic 
waste water. 
Yet another advantage is the prevention of a sudden large loading of the 
alkaline solution, e.g. seawater, with a heavy contamination. An example 
of this could be a sudden loading as a result of a defect in an 
installation part such as a dust filter. With the method in accordance 
with the invention the loading is considerably reduced because a large 
part of it is taken up in the acidic water. 
Another advantage is that the waste gas already gives off part of its heat 
during the pre-cleaning so that, when hot waste gas is being treated, the 
temperature rise of the alkaline solution such as seawater is less than 
with the known process. There follows the important advantage that the 
oxygen concentration in the seawater is also higher and this is favorable 
from the biological point of view. For example, any living organisms 
present in the seawater are better able to survive if the oxygen content 
is maintained and temperature fluctuations avoided. 
In one preferred form of the method according to the invention, the acidic 
solution comprises residual water from the chemical process producing the 
waste gas or from an auxiliary process related to this chemical process. 
In this embodiment the acidic waste water is chemically cleaned in a 
further cleaning process. 
In many chemical processes, water is used (in the process itself or an 
auxiliary process) and ends up as residual waste water which is taken 
straight to a chemical water cleaning station. By using this residual 
water as the acidic solution in pre-cleaning, a need for an additional 
flow of water may be avoided. 
Another advantage is that the residual water may contain elements or 
compounds which react chemically with elements or compounds from the waste 
gas so that a very high degree of waste gas cleaning can be achieved. 
At the same time this achieves the advantage that the acidic solution may 
be cleaned simultaneously with other residual water, by chemical water 
purification, and then released as clean water after cleaning. 
In a preferred embodiment of the method according to the invention, acid is 
added to the acidic solution depending on the concentration of metal in 
the waste gas and/or the amount of sulphur dioxide in the waste gas and/or 
the amount of chloride in the acidic solution. Because pre-cleaning takes 
place with an acidic solution and sulphur dioxide is removed with the 
alkaline solution, it becomes possible to adjust the pH of both kinds of 
solution relative to one another to an optimum value. 
Particular advantages are achieved if the pH of the acidic solution is low. 
In a number of chemical processes the waste gas still contains oxygen in 
molecular form. This is the case, for example, if the waste gas is a flue 
gas from a combustion installation or if a waste gas originates from an 
extraction unit in which air is used as carrier gas for discharging 
undesired gaseous components from the chemical process. 
When seawater is used as the alkaline solution, typically the pH before 
contact with the waste gas is 7.7 to 8.6. 
As a result of cleaning the waste gas in two stages, the final pH in the 
stage in which an alkaline solution is used may be set higher than in a 
method in which only a single stage is used. As an example of the 
usefulness of this, oxidation of sulphur dioxide from sulphite t sulphate 
works better at this higher pH. Moreover, under those circumstances less 
release of sulphur dioxide to the atmosphere takes place. Indeed, 
oxidation is additionally encouraged by longer-lasting contact of the 
waste gas with the two kinds of solution compared with contact with a 
single kind of solution in the known method. 
For environmental reasons, after it is used to clean the waste gas, the 
alkaline solution is aerated, using, for example, an aerator. One 
advantage of this is that it converts sulphite into sulphate and thereby 
reduces consumption of the chemical oxygen. The pH of the alkaline 
solution after the waste gas is cleaned is higher than with the known 
method which means that, for example, release of sulphur dioxide in the 
aerator is reduced. 
Preferably the acidic solution is at a pH under 2, e.g. 1.5. This gives the 
advantage that due to a low pH during pre-cleaning, heavy metals such as 
nickel, chromium and calcium may be removed from the waste gas 
considerably better than with a high pH. If hydrochloric acid is used as 
acid the metals then combine as metal chloride that is easily removed in a 
downstream chemical water purification stage, for example, as metal 
hydroxide. 
When the method of the invention is performed in this way the alkaline 
solution to be returned to surface water is considerably less laden with 
heavy metals. 
Preferably, the waste gas is saturated with water vapor from the acidic 
solution. In many chemical processes the waste gas can still take up 
substantial amounts of water vapor. This means that the amount of used 
residual water which has to be cleaned in the chemical water purification 
stage reduces by the same amount. Because, within broad limits the 
investment and operating costs of chemical water purification are in 
proportion with the amount of water to be purified, the costs of chemical 
water purification also reduce substantially. At the same time the amount 
of water to be released becomes less which results in less imposition o 
and pollution of the environment. 
When the proposed method is used for the removal of sulphur dioxide from 
waste gas, using seawater as alkaline solution, it gives a lower 
germicidal effect for the seawater released than the known method in which 
only one stage is used for scrubbing. This lower germicidal effect arises, 
among other reasons, from the smaller temperature rise of the seawater, 
the lower concentration of heavy metals, the lower dust loading, the 
higher oxygen content and, in a chemical process where fluorides are 
released, a smaller fluoride concentration. 
Research has shown that a method according to the invention may be carried 
out economically when treating waste gases that have a sulphur dioxide 
concentration of up to approximately 1000 mg sulphur dioxide per normal 
cubic meter of waste gas. 
In another aspect, the invention provides an apparatus for removing 
substances from a waste gas of a chemical process by gas-scrubbing the 
waste gas with alkaline solution which has two chambers, one for 
contacting the waste gas with an acidic solution, and another for 
contacting the waste gas with an alkaline solution. As an example, the 
chamber for contacting the waste gas, with an acidic solution can be a 
pre-absorber. The pre-absorber may be provided with a fluid supply for the 
acidic solution, and a fluid discharge unit linked to a water cleaning 
unit. In the pre-absorber the waste gas is relieved of a number of 
undesirable components such as PAHs, dust and heavy metals. At the same 
time the waste gas takes up water vapor so that delivery to the waste 
water purification unit is reduced. Removal of heavy metals may then be 
further improved in an apparatus which is provided with means for 
supplying an acidic component to the acidic solution. 
Preferably according to the invention the apparatus is provided with means 
for analysing the waste gas, which are control-linked to means for 
metering an acidic component. In another embodiment the apparatus is 
provided with means for analysing the composition of the acidic solution 
or residual waste water which is to act as the acidic solution, and these 
means are control-linked to a means for supply of an acidic component. 
In an apparatus according to the invention the pH of the acidic solution 
during removal of (for example) sulphur dioxide is held at a desired value 
by analysis of the waste gas and/or the acidic solution and, based on the 
measurement results, by conveying an acidic component to the acidic 
solution until a desired pH is reached. 
Using the apparatus according to the invention it is possible to clean the 
waste gas in two stages by scrubbing with water, and the pH of the used 
water may be set in each stage. This achieves several advantages: heavy 
metals from the waste gas are removed to a large extent, the oxidation 
effect of sulphur dioxide is improved, the risk of release of sulphur 
dioxide in a downstream oxidation stage is reduced, the risk of sudden 
loading of seawater is substantially eliminated, and the germicidal effect 
of the released seawater is less, among other reasons, through the lower 
temperature rise of the released seawater.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a diagram in which sulphur dioxide is being removed from two 
flows of waste gas by applying the method in accordance with the invention 
twice, in parallel. In FIG. 1 elements with corresponding functions in 
each of the two flows are indicated by the same reference number, with or 
without the additional reference letter "a". 
The two flows of waste gas may originate, for example, from two 
installations for the manufacture of aluminium from aluminium oxide by 
means of electrolysis. The electrolysis takes place in electrolytic cells 
into which aluminium oxide, among other things, is metered, and from which 
molten aluminium is tapped off. Gases produced during the process are 
extracted above each electrolytic cell as waste gas. In addition to oxygen 
and nitrogen air, the waste gas also contains sulphur dioxide, carbon 
dioxide and carbon monoxide, fluoride, heavy metals, PAHs and other 
hydrocarbons and large amounts of dust. The sulphur dioxide content is in 
the range approximately 100-400 mg per normal cubic meter, with a typical 
value of 250 mg/Nm.sup.3. 
In FIG. 1 line 1 is the supply of the waste gas to be cleaned from which 
fluorine is being removed in a dry flue cleaning stage 2. The waste gas 
flows through a pipe 3 to a first cleaning stage 4 in which the acidic 
scrubbing takes place with aqueous acidic waste water. The acidic waste 
water is conveyed through pipe 5 as used residual water. 
In the manufacture of aluminium from aluminium oxide the residual water 
obtained contains chlorinated waste water from the foundry of aluminium 
ingots, drain water from cooling towers, leakage water from the pit floors 
beneath the electrolysis cells and possibly rainwater which contains 
metal. This residual water is used for the acidic solution for 
gas-scrubbing, in this embodiment of the invention. 
The used residual water is mixed in a mixing vessel 6 with an acid, e.g. 
HCl, originating from a storage vessel 7. The addition of acid may take 
place on the basis of the measured or otherwise known composition of the 
used residual water and/or according to the composition of the waste gas. 
The pH aimed at is less than 2, e.g. 1.5. 
From the first cleaning stage 4 the used residual water flows through a 
pipe 8 to a buffer vessel 9 from which the two pipes 8 and 8a emerge. The 
buffer vessel 9 is linked to a chemical water purification unit 10. 
The pre-cleaned waste gases from the first cleaning stage 4 flow from a 
pipe 11 to the sulphur dioxide scrubber 12. Seawater, e.g. of initial pH 
8.0, is added to the sulphur dioxide scrubber 12 through a pipe 13 by 
means of an inlet system 14. The cleaned waste gas leaves the sulphur 
dioxide scrubber 12 via a pipe 15 which is linked to a chimney stack 16. 
The target value for the sulphur dioxide content in the exhaust gas after 
cleaning is 25 mg/Nm.sup.3, which is achievable. 
The seawater from the sulphur dioxide scrubber having for example a pH of 
6.2 (from an initial pH of 8.0) is conveyed along a pipe 17 to a seawater 
oxidation stage (aerator) in which sulphite dioxide in the seawater is 
oxidized by means of air supplied through a pipe 18. The seawater thus 
processed is drained off through pipe 20. 
It has been found that the reaction of bicarbonate in the seawater with 
absorbed sulphur dioxide results in the formation of carbon dioxide that 
leaves the seawater, thereby raising the pH towards its original value. 
FIG. 2 shows a single-flow apparatus in accordance with the invention. A 
twin-flow embodiment, corresponding to FIG. 1, is within the scope of the 
expert. 
FIG. 2 shows a two-stage scrubbing tower 31 provided with an inlet 32 for 
the waste gas to be cleaned. A pipe 33 is connected to a storage vessel 
for acid. The pipe 33 has a control valve 34 which allows passage of the 
permitted amount of acid depending on a control signal originating from 
measurement means not shown in the drawing which analyse the used residual 
water and/or the waste gas to be cleaned. 
The acid from the pipe 33 is mixed in a pipe 35 with the flow of used 
residual water which passes along the pipe 35. The pipe 35 emerges into a 
spray system 36 of the first, and bottom stage of the scrubber tower. If 
desired, a part of the used residual water may be recirculated by means of 
a pump 37. Used residual water leaves the scrubber tower through a pipe 38 
to go to a chemical water cleaning unit not shown in the drawing. 
Seawater, which is alkaline, is conveyed along a pipe 40 and sprayed 
through sprayers 41 into one or more spray layers at the top of the 
scrubber tower. Seawater dropping down is fed out via a gutter 42, through 
a pipe 43 to the oxidation tank 44. The oxidation tank 44 has a coupling 
pipe 45 for air which is bubbled into the oxidation tank by a diffuser. 
Used seawater is drained out of the oxidation tank 44 by means of a pipe 
47 and a pump 48. 
Although described here for a use in a process of winning aluminium by 
electrolysis, the invention is applicable in a similar manner to other 
processes of electrolysis of metal oxides to gain the metal. Another 
possible use of the invention is in cleaning exhaust gases from a 
sintering plant for preparing iron ore as a feedstock for a blast furnace. 
The acid water, or a water which is to be used as the acid water, can be 
chosen from a variety of supplies e.g. drinking water, river-water, spring 
water or a waste water. This waste water is present in industry again in a 
great variety: cooling tower blow-off water, cooling water from a casting 
process, collected rain water. The desired pH of the water in the first 
washing step is 1.5. This is reached due to the acid contaminants in the 
exhaust gas or by extra addition of acid. The value of 1.5 will result in 
a high solubility of metallic contaminants.