PROCESS FOR THE RECOVERY AND UTILIZATION OF SULPHUR DIOXIDE CONTAINED IN SULPHUROUS GASES, SUCH AS FLUE GASES

Abstract: The sulphur dioxide contained in the gas is absorbed in a saturated or almost saturated aqueous potassium sulfite (K2SO3) solution, and the absorption is continued until the solution is saturated with potassium bisulfite. The crystallized potassium bisulfite is separated from the solution and subjected to heat treatment at about 200<o>C. The potassium sulfate obtained may be widely used, e.g. for the preparation of fertilizers.

Description: Process for the recovery and utilization of sulphur dioxide contained in sulphurous gases, such as flue gases The present invention concerns a process in accordance with the preamble of claim 1 for recovering and utilizing sulphu dioxide contained in sulphurous gases, such as flue gases. According to the present process, sulphur dioxide is first absorbed in a solution containing sulphite iones, whereupon the sulphur-enriched solution is treated in a suitable man¬ ner to produce the end product. The processes for recovering sulphur dioxide are in many cases based upon neutralizing sulphur dioxide by means of calcium carbonate or calcium hydroxide. The neutralization process may be carried out either before the stream of sulphur dioxide leaves the combustion process or an equivalent process or in connection with the sulphur dioxid recovery process. Depending on the process used, calcium sulphite or calcium sulphate are obtained. As an example of an industrial process, the Pyroflow -technique developed by A. Ahlstrδm Oy may be mentioned. The products obtained have no reuse value, and they may eve be detrimental in, for instance, the ash which can be used for different purposes. The sulphur dioxide in flue gases may also be neutralized by means of sodium, potassium, and ammonium bases, e.g. in a McGill process, whereby sodium sulphate is obtained, or i the alther GmbH-process, whereby ammonium sulphate may be produced. In the so called ellman-Lord-process the sulphur dioxide is sorbed in a solution of Na2S0 3 , from which it is trans¬ ferred in form of an enriched stream to the required pro¬ cess. By using potassium bases potassium salts may be obtained. There are several disadvantages related to the above- mentioned solutions: - the bases are comparatively expensive to use, since the products obtained are all bulk-products, - except for potassium sulphate, the use value for all of the products obtained is small, - during the preparation of the base, chlorine might be formed reducing to some extent the total environmental benefits of the air-cleaning process. It is known per se to use aqueous mixtures of sulphur dioxi¬ de and acetone, the sulphur dioxide being converted to an acid the properties of which are comparable to those of strong acids. The use value of the sulphur dioxide is thus substantially increased. Potassium chloride (KC1), a mineral which is obtained from nature, is in this case used for preparing potassium sulphate. As an example of such a method, the solution disclosed in the Canadian Patent Specification No. 832,931 (Wilson and Bejoy) may be mentioned. Said methods are satisfactory and useful per se, but they may only be employed for utilizing sulphur dioxide which has already been recovered. Moreover, a potassium sulphite solution is obtained, which must be further evaporated before a crystalline product may obtained, which in its turn is processed further to give potassium sulphate. Evaporation is a unit process requiring extensive amounts of energy. The object of the present application is to eliminate the drawbacks of the known solutions and provide a novel process for recovering and utilizing sulphur dioxide. The invention is based upon using a solution of K2SO3 as an absorbent in a process for recovering sulphur dioxide, the absorbed sulphur dioxide forming potassium bisulphate by reaction with the potassium sulphite and water. The potassiu bisulphate is crystallized and it may be recovered and pro¬ cessed to potassium sulphate. The term "to absorb" is in this connection consequently used for expressing the operation of sorbing sulphur dioxide in a solution. This is so in spite of the fact that said event is only partially chemical, comprising as a first step a substantially physical adsorption. The product crystallizing from the solution of potassium sulphite is more specifically the anhydride of potassium bi¬ sulphite, i.e. potassium ( eta)bisulphite K2S2O5. Within the scope of this application the term "potassium bisulphite" (KHSO3) ill De used for said product for reasons of simpli¬ city. More specifically, the process in accordance with the invention is characterized in what is stated in the charac- terizing part of claim 1. By means of the invention, significant advantages are achieved. Thus, the total economy of the process is exten¬ sively advantegous . The energy costs are considerably less than those of the known processes, since the amount of energ used for the crystallization and the treatment of the crys¬ tallized material is smaller than that of crystallization. The potassium sulphate obtained may be widely used, e.g. for the preparation of fertilizers. The invention will be examined in more detail by means of the following examples with reference to the attached drawings . Figure 1 depicts a simplified flow sheet diagram for an embodiment of the invention. Figure 2 depicts the flow sheet diagram of another, prefer¬ red embodiment of the invention. Figure 3 depicts the flow sheet diagram of an alternative process for preparing the potassium sulphite used in the invention. In the embodiment in accordance with figure 1, sulphur di¬ oxide containing pressurized gas, that is the air to be purified, which may derive from, e.g. a combustion process for fossil fuels, is conducted through a blower 1 to a gas washer 2, i.e. scrubber, the SO2 being absorbed in an aqueous solution of potassium sulphite. The gas washer 2 used in the process may be of any conventional type, e.g. an empty tower or a tower packed with packings, provided with spray nozzles at the top. The gas washer may also be a deflector washer provided with baffle plates, which cause sudden changes in direction of the gas enhancing the mass transfer. The potassium sulphite solution entering at- the top of the washer 2 is atomized or sprayed into the ' gas flowing upward from the bottom of the washer, most of the sulphur dioxide being absorbed by the drops. [When a co-currently operating washer (e.g. the Venturi scrubber) is used, the gas and the potassium sulphite solution flow, of course, in the same direction, that is from top to bottom]. The sulphur dioxide forms potassium bisulphite by reaction with the potassium- sulphite-water-system. The sulphur dioxide-free gas leaves the top of the washer tower, and it may either be released to the ambient air or used as process air. The operational parameters of the washer, i.e. the prevailing pressure and temperature as well as the liquid and gas flow rates, are selected such that the K2SO3 solution withdrawn from the bottom of the tower is enriched in bisulphite, however without having reached the state of saturation. The potassium sulphite content of the solution used as absorbent is preferably as high as possible. It is therefore preferable to use a saturated or almost saturated solution. The liquid stream leaving the washer 2 is passed to a absor¬ ber 3, where more sulphur dioxide is introduced until the state of saturation of potassium bisulphite is reached. The KHSO3 f° rme is crystallized. The absorber 3 used may be of any conventially used type,, such as a packed tower or a sieve-plate tower. To eliminate the cleaning problems caused by crystallization, an absorber in which the gas is allowed to flow in bubbles through the liquid layer may also be used. It is also possible to carry out crystallization first after the absorber by reducing the temperature. Saturation and crystallization may also be conducted in a crystallizer fed with a surplus of sulphur dioxide. The outlet stream is divided into two parts by separating the crystallized KHSO3 from the liquid stream by filtering. One outlet stream (the crystallized potassium bisulphite) is transferred to calcination 4 and the other (the aqueous potassium sulphite solution) to the stripper 5. During calcination 4 the potassium bisulphite crystals are heated to about 200°C. The bisulphite molecules decompose at said temperature forming solid potassium sulphate and gaseous sulphur dioxide and steam. Potassium sulphate obtained from calcination is recovered, and the gaseous mixture of sulphur dioxide and steam is divided into two parts, one of which is recycled to the absorber 3, the other being transferred to the production of potassium sulphite. Since the temperature needed in calcination is as low as 200°C, the equipment use for calcination may comprise, e.g. some conventional (continuous) dryers for solid substances, such as screw- conveyor dryers. Various rotary calcination ovens or equiva lent equipment may of course be used, if desired. Adsorbed, but unreacted sulphur dioxide is removed from the aqueous potassium sulphite solution transferred to the stripper 5 by heating said solution to a temperature in the range from 80 to 100°C. The outlet streams of the stripper 5 are recycled to the gas washer 2 by conducting the sulphu dioxide directly back to the gas washer 2, and by combining potassium sulphite to the fresh feed. The potassium sulphit may in spite of the heat treatment still contain minor amounts of potassium bisulphite, which thus are introduced into the fresh feed through the recycling operation. Small amounts of potassium bisulphite do not, however, have a detrimental effect on the absorption process in the gas washer. Sulphur dioxide may also be separated from the potassium sulphite obtained from the absorber at reduced pressure. However, it is preferred in practice to increase the tempe¬ rature. In the embodiment depicted in figure 2 sulphur dioxide con¬ taining gas is fed under pressure through a blower 11 to a gas washer 12, as described above. The SO2 is absorbed in the aqueous potassium sulphite solution in the gas washer. In the embodiment in accordance with said figure, absorptio is continued until the potassium sulphite solution is satu¬ rated in potassium (meta)bisulphite and crystallization is initiated. The crystals formed are withdrawn from the botto of the absorber, whereupon they as well as the mother liquo are treated as mentioned above; the calcination being per- formed in calcination equipment 13, and sulpur dioxide bei-n removed from the aqueous potassium sulphite solution in -a stripper 14. No separate adsorber is thus used in this embodiment, resul ting in a decrease in equipment costs. The washer 12 used may be of any above-mentioned type, however, preferably not a packed tower. To improve the absorption in the washer 2 or 12, the gas ma be recycled until the air has reached the required low sulphur dioxide content. The absorption time depends on the sulphur content of the gas, the concentration of the potas¬ sium sulphite solution and the recycling ratio. It may, e.g lie in the range from a few minutes to one day or more. The potassium sulphite used in the process may as known per se be prepared, e.g. by passing sulphur dioxide gas into a potassium hydroxide or potassium carbonate solution. One alternative preparation process is depicted in figure 3 (Wilson, W.J., Sulphur dioxide and acetone-water in the pre¬ paration of fertilizers and potassium chemicals, DRIE Report No. 3124, May 1984). The enriched SO2 stream derived from calcination is trans- ferred to regeneration, where it is absorbed in water. The aqueous solution enriched in SO2 is pumped to a regenerator where acetone is added. The strong hydroxy sulphonic acid solution thus obtained is pumped to an ion exchanger in K + form. Potassium hydroxy sulphonate thus obtained is pumped to distillation, where acetone and the SO2 surplus is re¬ moved and the solution is returned to circulation. The invention will be described in detail by means of the following non-limiting examples. Example 1 Sulphur dioxide containing gas was passed into a solution, the K2SO3 concentration of which is 780 g/1. A countercur- rently operating Lisop scrubber was employed as gas washer. The initial sulphur dioxide concentration was 4800 mg SO2 per m^, and the temperature of the process was maintained a about 20°C. When the SO2 concentration of the solution had increased to 105,5 grams per liter (the sulphur dioxide being calculated as bisulphite), the solution was removed and transferred to crystallization. A surplus of sulphur dioxide was introduced into it until the anhydrite of potas sium bisulphite was crystallized. The amount of bisulphite crystallized per liter of solution was 120 g. The mother liquor was passed after filterization to a SO2 stripper, where the additional sulphur dioxide was removed. The mainly potassium sulphite containing solution subsequently obtained was recycled to the gas washer combined with the solution obtained from the preparation of potassium sulphite. Potassium (meta)bisulphite obtained from crystallization was calcined at 200°C and the sulphur dioxide formed retur¬ ned to the KHSO3 solution. Potassium sulphate K2SO4) obtai¬ ned from calcination was suitable for preparing fertilizers Example 2 Sulphur dioxide containing gas, the SO2 concentration of which was about 5000 ppm (i.e. about 13 grams of SO2 per nm- 1 ), was recirculated into a solution containing 48 % per weight of K 2 S0 3 . The- air flow was 1200 nm^/h and the temperature about 80°C. The flow of potassium sulphite in the scrubber was 900 kg/h, The SO2 concentration of the outlet air from the scrubber was about 90 ppm SO2 (235 mg S0 2 /nm3) . After about 17 hours of operation saturation degree of potassium bisulphite was achieved and K2S2O5 was beginning to crystallize. The crystals formed were removed from the bottom of the scrubber and the mass containing in addition to potassium (meta)bisulphite crystals about 50 % mother liquor containing K2SO3-KHSO3, was dryed and calcined at K2SO3 solution, the concentration of which was 48 per cent, was fed to scrubber at a rate of 120 kg/h after the initia¬ tion of crystallization. Example 3 To a K2SO3 solution, the concentration and flow rate of which are similar to those mentioned in the above examples, SO2 gas was fed. Said gas contained 460 g Sθ2/nm 3 ' tne flow rate of the gas was 1200 nm^/h. Crystallization of potassium metabisulphite was initiated after about 5 minutes in the scrubber. The metabisulphite was separated and further converted to K2SO4.

Claims: Claims : 1. A process for the recovery and utilization of sulphur dioxide contained in sulphurous gases, e.g. flue gases, comprising absorbing the sulphur dioxide in a liquid con¬ taining sulfite ions, and subsequently treating the liquid enriched with sulphur dioxide in a suitable manner to pre¬ pare the desired end product, c h a r a c t e r i z e d by the steps of a) absorbing the sulphur dioxide in an aqueous potassium sulfite solution containing not more than a trace of potassium bisulfite, b) continuing the absorption until the solution is saturate with potassium bisulfite, c) separating the crystallized potassium bisulfite from the solution, d) subjecting it to heat treatment to produce potassium sulfate, and e) circulating at least a part of the by-product obtained in steps c) and d), possibly after further processing. 2. A process as claimed in claim 1, c h a r a c ¬ t e r i z e d by using as absorbent a saturated or almost saturated potassium sulfite solution. 3. A process as claimed in claim 2, c h a r a c ¬ t e r i z e d in that the step a) is performed in a scrubber (2) and the step b) in an absorber (3), the pressu and the temperature in the scrubber, as well as the flow rate of the solution and the gas containing sulphur 'dioxide being selected such that the K2SO3 solution, which contains potassium bisulfite, is not saturated with potassium bi¬ sulfite. 4. A process as claimed in claim 2, c h a r a c ¬ t e r i z e d in that the steps a) and b) are performed in the gas washer (12). 5. A process as claimed in claims 1 - 4, c h a r a c ¬ t e r i z e d by heating the potassium bisulfite to about 200°C in step d) . 6. A process as claimed in any of the previous claims, c h a r a c t e r i z e d by heating the solution obtained after step σ) to a temperature of between 80°C and 100°C, whereby it may be used in process step a).

Inventors: SARKKINEN VELI, SARKKINEN, VELI
Applicants: CATENA OY
Application year: 1986
Main classification: B01D53
IPC codes: B01D53, C01B17