FI105532B - Double calculated desulphurisation - Google Patents

Description

105532

Double - alkali desulphurisation

The present invention relates to a process for the recovery and treatment of sulfur dioxide contained in flue gases, comprising is oxidized to form alkali sulfate in the first container.

a 'Purification of sulfur dioxide from gases takes place in several ways. The washing process is called so-called.

wet scrubbing, whereby the sulfur dioxide-containing gas is contacted as effectively as possible with the washing liquid. At the same time, the gas cools and exits the dewatering process. The solubility of sulfur dioxide in the scrubbing liquid depends essentially on several factors in the scrubbing liquid, the most important of which, however, are the pH and the sulfur dioxide contained in the scrubbing liquid. If the pH is high, it is clear that no soluble sulfur dioxide is formed, but sulphite salts are formed. If, on the other hand, the amount of the washing liquid 25 is high in relation to the sulfur dioxide contained in the gas and the saturation limit of solubility is not reached, it is clear that the scrubbing of the gas with sulfur dioxide is successful.

m% <* · -

There are several neutralizing agents. Smaller and more simple 3 0 neutralization plants use either carbonates or hydroxides of the alkali metals (Na and K) as neutralizing agents. By doing so, no harmful crystallization occurs at any part or step of the process. Neutralization also works well, usually over 105532 2 90%. The worst drawback is the high cost of the neutralizing agent, which in practice is up to ten times that of lime-based neutralization used in large plants. Thus, alkali metal-based neutralization is used only for highly desulphurization purposes and in small plants where automation is significant. The washing water produced by alkaline neutra-5 sweeping wash contains sodium / potassium sulphite, which is easily oxidized to sulphates. Oxidation can take place either by direct oxidation of circulating wash water or in a separate oxidation reactor. The oxidation is not particularly accurate at the prevailing pH and thus the ease of oxidation favors alkaline neutralization.

Another and economically significant method of neutralization is to use lime either as carbonate or lime milk from calcined lime, which, in addition to dissolved calcium hydroxide, is insoluble. In practice, the washing process is difficult and precise to run because, at pH 6 and above, calcium forms sulfuric acid to neutralize calcium sulphite, which has a very low solubility. Thus, 15 is rapidly formed in the piping, pumps or other parts of the washing process; unwanted crystallizations. These often lead to clogging and even to the end of the entire washing process. Thus, in the presence of calcium, the pH of the sulfur dioxide scrubbing process should be below 6, even below 5.5. In this case, the calcium as a soluble compound is so-called. as calcium bisulfite (Ca (HS03) 2). In this form, the dissolved Ca salt is easily oxidized to give Ca sulfate 20, i.e. gypsum.

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As mentioned above, even with calcium rising to pH above 5.5, even nasty crystals are produced.

For this reason, the wash water cycle is intended to keep calcium out. Many different methods and solutions have been developed and invented for this purpose.

25

One is the so-called. double alkaline alloy, in which magnesium (Mg) is used in addition to calcium (Ca) * as neutralizing agents. Magnesium salts, e.g. Mg sulfate and sulfite, are much more soluble than the corresponding calcium salts. This often eliminates the process-blocking crystallization and the process is not as sensitive to pH as 30 above.

105532 3

In double alkali, first, the washing process is neutralized with Na or K hydroxide or carbonate. Thereafter, the washing solution is oxidized to sulfate. The sulphates formed are most preferably treated with calcium hydroxide back to alkali hydroxides and calcium forms gypsum. However, the gypsum is too soluble to avoid the above calcium calcium deposits without the need for special operations and treatments. Thus, it is sought to remove the Ca ion from the solution as accurately as possible, or at least below the solubility limit of Ca sulfite, which is about 1 / 50th of the solubility of gypsum. These include: additions of chemical lithium in Ca ion exchange are added to water eg Na carbonate. The Ca ion content then falls below the solubility limit of said Ca sulfite. This method is complex and well managed.

Other chemical additions have also been used to eliminate the Ca ion, either to precipitate Ca or to make it into its own complex soluble compound from crystallization. There are acetic acid supplements, oxalic acid, tartaric acid and the like. Few of these have evolved into commercial solutions, at least in the broader sense. The most common; The method is a lime (CaCO3) based wash water cycle that is kept below pH 5.5 at all times. Such a wash water circuit has a separate oxidation reactor step to produce gypsum. Gypsum is removed from the washing circuit, washed and generally used in industry for the production of building elements. Such sulfur dioxide scrubbers are typically large in size and highly auto-controlled. The amount of waste water they produce is also large and difficult to handle. Thus, for them too, these wet scrubbers are expensive investments that are often too expensive for medium-sized plants to try and find other economical and viable solutions.

It is an object of the present invention to provide a process by which a neutralization scrubbing of flue gases containing sulfur dioxide can be carried out by double alkali treatment without wastewater, whereby the alkali metal, preferably e.g. sodium Na, is recovered at 70-95% without the risk of Ca ionization.

30 4 105532

The method of the invention is characterized in that d) measuring the concentration of alkali sulphate in the circulating liquid in the first container, and 5 e) passing the circulating liquid from the first container to a second container at a preselected concentration where said circulating liquid is partially ion-exchanged gypsum (CaSO4) and alkaline hydroxide are formed in the tank.

The method of treating the scrubbing liquid of the invention simplifies the double alkaline process for scrubbing substantially sulfur dioxide-containing gases. This makes the automation and, above all, the control of the acidity easier and simpler to operate. Wastewater treatment is very expensive, not only in terms of investment, but above all, it costs a lot to clean up to the standards set, not to mention that their automatic purification control is a continuous process in which the authorities also have an active role. Even financiers are calling for effluent treatment to comply with standards 20.

The invention thus achieves two main objectives, both of which have meaning, i.e. the water discharges completely and the expensive neutralizer is recycled. This saves at least 70-80% in the use of alkali metals. This saving 25 is very significant, not only in terms of cost but also in terms of nature. 1

A preferred embodiment of the method is based on monitoring / measuring the specific weight of oxidized washing water at the desired concentration of sodium sulfate, whereby a measured quantity of known concentrated solution is taken and treated with 3 0 of known concentrated S = 0.5-0.9), whereupon a gypsum precipitate is formed in the solution which renders the solubility of calcium non-existent due to the Na-sulfate remaining in the solution. The solution is filtered out of the gypsum, the precipitate possibly washed, and the resulting alkali hydroxide solution is returned to neutralization and evaporated with the moisture of the gas at the dew point, thereby avoiding wastewater.

5

The invention will be described in detail with reference to a preferred embodiment of the method of the invention shown in the figures of the accompanying drawings, but which is not intended to be limited thereto.

Fig. 1 is a schematic side view of a preferred embodiment of the device solution used in the method of the invention.

Figure 2 is a schematic side view of a preferred embodiment of the specific gravity measurement used in the method of the invention.

15; As shown in Figure 1, the gas flow A0 containing sulfur dioxide SO2 flows into the scrubber 1 which receives the wash water flow B from the pump 3. The pump 3 draws the wash water from the recirculating water tank 2 to the scrubber 1 after washing, containing impurities contained in the gas stream A0. The flow B exiting the pump 3 has a branch flow D which is passed through a specific gravity meter / concentration meter 4. The specific gravity meter 4, an embodiment of which is shown in Fig. 2 and its description, continuously measures the specific gravity of branch flow D of the same composition as flow B, which is measured by a change in specific gravity mainly due to Na2SO4 or at least directly correlated to its concentration. is directly related to the concentration of sodium sulfate. This Na2SO4 concentration dependence is. can be tested on a few different specific gravity samples.

When the specific gravity meter 4 provides information on the specific gravity concentration of Na2SO4 for valve 5, preferably 30 solenoid valves, according to branch flow D, this preferably allows the flow rate from the pump 3 to flow from 6 to 10 or 6 the volume of which is monitored by volume measurement or e.g. by the known horizontal technique 7. When the measured / desired amount of the known concentrated Na2SO4 solution / washing liquid measured by the circulating tank specific gravity meter 4 is obtained, the CaO / Ca (OH) 2 / Ca (OH) 2 powder / liquid calculated as Ca-vir-5 as F to effect ion exchange of Na - * Ca to the desired extent. The Ca content of the flow F should be calculated in sub-cymmetry (Ca: S = <1), preferably 10% below stoichiometry = 1.

In the reaction tank 6, the Na2SO4-containing liquid in the ion exchange stream E reacts with the amount of Ca ion contained in the Ca flow F, whereupon Na replaces the Ca ion to form sodium hydroxide (NaOH) and the sulfate forms gypsum (CaSO4) with Ca containing crystalline water (CaSO4 + 2H2O). Sodium hydroxide is dissolved in washing water. Due to the ion-exchange reaction, the pH of the reaction tank 6 rises up to 14. The ion exchange can be more efficiently enhanced, e.g. by agitator 15. After a suitable agitation 15 and ion exchange reaction time, the reaction tank 6 is emptied as a stream G by sedimentation. through filtration / filtration 9, where the gypsum and any other solids in the washing liquid are removed as dry as possible through H and the filtered liquid is discharged as flow I into the neutralization tank 10. Flow H can be washed / rinsed with fresh water / sedimentation / filtration 9 / with a liquid that is preferably within 2 o of the flow I.

The pH meter 12 in the circulation tank 2 controls the dosing pump 11 of the neutralizer tank 10 to neutralize (flow J) the sulfur dioxide dissolved in the scrubber 1 from the gas stream Aq to the scrubbing liquid stream C used. Dissolved sulfur dioxide forms sulfuric acid, which is then oxidized by oxidant 14, preferably in a circulating water tank 2.

I * * w

Because the Aq of the scrubbing gas stream is generally low in humidity, the dew point is 20-30 ° C, its water content is low. The gas A 1 after the scrubber 1, or outgoing from the scrubber 1, is at 30 dew points and the exit temperature is about 50 ° C. Its water content is up to three times that of the previous one. As a result, 105532 7 water is continuously discharged from the washing liquid circuit. This discharge is partially offset by flow J, but is largely replaced by a new water / liquid flow K through the solenoid valve 13, preferably controlled by the surface boundary monitor 16, preferably into the circulation tank 2.

Figure 2 shows, as a solution, but not the only one, a specific gravity meter substantially included in a preferred embodiment of the invention for easy measurement of the concentration of Na 2 SO 4 in the washing liquid. The gauge 20 consists of a balance 21 with a tube clamp 22 containing a tube loop of the desired length and diameter, however suitably such that the volume of fluid flowing inside it is desired, e.g., about one liter. In practice, flow D from flow B to scrubber 1 is taken through valve 23, in this case e.g. solenoid valve, to pass through tube winder 22, whereby balance 21 weighs total weight and preferably transmits data to central control device 30. Subsequently, for comparison, feeds 24 and 25 alternately The solutions / media b and c of known specific gravity are known by the central control device 30. with these weights, the specific gravity of the flow D. When still testing for flow <D, e.g., two different specific gravity concentrations for Na2SO4, it allows for a very accurate measure of the amount of Na2SO4 contained in the ion exchange solution. By this solution, which is by no means the only way to measure the amount of sodium sulfate to be processed, the operation and object of the invention are easy to accomplish.

The removal of Ca ion from water in stream I, i.e. the solubility of gypsum, is prevented through chemical reaction and solubility equilibrium constants. Reactions 25 and solubilities in chemistry are represented by the formulas: »··

• I

1) Gypsum dissolves into CaSO4 - * Ca ++ - + S04 'ions in water, of which 30 8 105532 CCa x CS04 2) Equilibrium constant = Kp -------------------- constant CCaS04 5

So if we have gypsum in solution alone, it will dissolve as shown. In practice, the solubility of gypsum is about 400 mg Ca / l.

If the solution contains sodium sulfate Na 2 SO 4, it will dissolve at 50 ° C at least 10 350 g / l of washing solution, i.e. more than 800 times the solubility of gypsum. If, according to the invention, for example, 10% of sodium sulfate is left free, i.e. without ion-exchange with Ca, so much SO4 radicals remain in the solution, exceeding the solubility constant of gypsum, that is, mathematically, about 87.5 times. The Kp formula CCa x CSO4 then changes so that the CCa concentration drops to zero. This results in a soluble Ca content of gypsum that is well below 1 mg / L. This prevents the known formation of CaSO 3 in the washing process, since the soluble Ca content thereof exceeds the solubility of the gypsum stated above when it already has a 10% Na 2 SO 4 solution.

Of course, the solution of the present invention can be applied to other *: ion exchange processes as required. In the embodiment according to the invention, only the dust possibly present in the gas flow A0 is a disturbing factor, which can give its own disturbance factor to the specific gravity measurement, if its proportion is significant. Therefore, besides medium b and c, it is necessary to analyze the specific gravity of at least two different compositions of stream D in order to obtain a "refined concentration of sodium sulfate in stream D."

The batch processing solution disclosed in the specification is not strictly necessary, since it is possible to carry out the ion exchange with a constant concentration and flow rate of 30 controlled / controlled flows E and F such that said stoichiometry can be realized. The mixing tank 6 with the mixer 15 is then only without measuring devices.

Claims (9)

A process for the collection and treatment of sulfur dioxide (SO2) contained in flue gases, wherein process a) the flue gases are conducted as a flow (A0) to a wash part (1), b) to the wash part (1) is led from a first container (2) along a flow line (B) a circulation washing liquid, which is neutralized by alkali metal neutralization, the sulfur dioxide (SO2) entering the flue gases dissolved in the circulation washing liquid, c) the circulating washing liquid is oxidized, Characterized in that d) the alkali sulfate concentration of the circulating washing liquid is measured in the first container (2), and 20, e) the circulating washing liquid is conducted from the first container (2) as a flow (E) when said circulating washing liquid reaches a preselected concentration. to a second container (6) in which the circulating wash liquid is partially ion exchanged with a calcium (Ca) gases (CaSO 4) and alkali hydroxide are formed in the second container (6). Process according to Claim 1, characterized in that alkali sulphate concentration. the circulation washing liquid in the first container (2) is measured as a measurement of the specific weight. 30 Method according to claim 1 or 2, characterized in that the alkali hydroxide is returned as neutralizing agent from the second container to a third container (10) as a flow (G, I). 12 105532 Method according to claim 3, characterized in that the second container is discharged as a flow (G) into a sedimentation / filtration container (9), from which the gypsum (CaSO 4) and possible other solids in the washing liquid discharge as a flow (H). and the filtered alkali hydroxide is conducted from the sedimentation / filtration vessel (9) as a flow (I) to the third vessel (10). Process according to any of claims 1-4, characterized in that the alkali hydroxide is led from the third container (10) to the first container (2) as a flow (J). 10 Process according to any of claims 1-5, characterized in that replacement washing liquid is fed to the first container (2) as a flow (K). Process according to any of claims 1-6, characterized in that the acidity of the circulating washing liquid in the first container (2) is measured with a pH meter (12). Method according to any one of claims 1-7, characterized in that the amount of the circulating washing liquid led to the second container (6) is weighed with a path (7) and to the second container (6) is conducted from a container (8). predetermined 2. amount of calcium (Ca) containing powder / liquid as a flow (F). * Method according to any one of claims 1-8, characterized in that the amount of circulating washing liquid in the first container is monitored by means of a surface boundary monitor (16). 25 «V