CS237301B2 - Method of regaining of hydrogen magnesium sulphite from waste gas in combustion of sulphite waste liquor - Google Patents

Abstract

The absorption of SO2 in the gas occurs in at least one magnesium sulphite (MS) stage and at least one magnesium bisulphite (MBS) stage. More MS is made than is required for the prodn. of bisulphite, and the excess (E) is removed as MS from the MBS system. Excess (E) corresponds with the difference of the amt. of S in the thick lye and the total bonded S in the tower acid. The MS removed is pref. employed for mfg. H2SO4; or is cracked by heating to obtain MgO recycled to the MBS stage and SO2 fed into the purified bisulphite soln. The MS is pref. cracked at 500-900 degrees C., possibly using some heat from the lye boiler. The MS is pref. dried before craking, which pref. occurs in a stream of SO2, and the resulting SO2 is pref. liquefied. Used in the prodn. of cellulose, using a completely closed circuit for the chemicals, and to produce a digestion acid contg. any desired amt. of SO2 in soln.

Description

The sum of the amount of chemicals shows that the circulation of chemicals corresponding to the tower acid content is, on average, greater in the Magnefite process than in the acid process; when acidic (-100%) is used as the basis, this circulation is greater by about 100% for magnesium oxide and about 40% for sulfur.

For both of these processes to achieve the same raw material consumption at the same pulp quality, this means that the loss quota of the Magnefite process must be smaller in proportion to the higher chemical circulation. This circumstance represents an advantage for the acid process. For example: - for magnesium oxide:

a recovery rate of 90% for the acid process requires a recovery of 95% for the Magnefite process;

- for sulfur:

a recovery rate of 90% for the acid process requires a recovery of 93% for the Magnefite process.

By means of known methods using magnesium bisulphite with chemical recovery, it is possible at best to produce almost pure stoichiometric bisulfite acid, but not free sulfuric acid dissolved as sulfur dioxide, from sulfur recovered exclusively through the concentrated liquor.

Crude acids can be produced by known methods in which, based on stoichiometric bonded bisulfite sulfuric acid, the following percentage of sulfur is missing:

- for the Magnefite method:

about 4 to 5%, for an acidic process: about 3 to 4%.

The formation of 100% bisulfite acid or free sulfur in bisulfite acid to form the tower acid takes place in a so-called cross-linking tower using a gas with a high content (at least 15% by volume) of sulfur dioxide supplied from the sulfur combustion plant. The amount of sulfur that corresponds to the total losses in the system is fed to the plant. This crosslinking can also be carried out with liquid sulfur dioxide.

These acid production conditions, due to the production technology, make minimum sulfur losses necessary for the production of the desired acid in each of the two processes. These minimum losses are - for the Magnefite method:

3,5 to 4,5% of the total sulfur content of the tower acid, - for the acid process:

up to 30% of the total sulfur content of the tower acid (at 100% degree of crosslinking efficiency).

Under current and future regulations envisaged, these environmental contaminations are unacceptable, especially in the acid process, especially when about 30% of these losses are exited from the gaseous sulfur dioxide plant.

SUMMARY OF THE INVENTION The object of the present invention is to provide boiling acid in a completely closed chemical cycle, in which the proportion of physically dissolved sulfur dioxide is freely selectable.

The process according to the invention is characterized in that the absorption produces more normal magnesium sulphite than that required for the production of bisulphite, and moreover a quantity corresponding to the difference between the amount of sulfur in the concentrated liquor and the amount of total sulphite and sulphate-bound sulfur in the tower. acid, is discharged from the bisulfite circuit in the form of normal magnesium sulfite.

In particular, the normal magnesium sulphite thus obtained is used as a basic raw material in the production of sulfuric acid. Preferably, the discharged normal magnesium sulphite is decomposed by the action of heat into sulfur dioxide and magnesium oxide, and the resulting sulfur dioxide is dissolved in the purified bisulfite solution, while the magnesium oxide is returned to the monosulfite step.

In particular, the discharged normal magnesium sulphite decomposes by mere thermal cleavage without additives in the temperature range of 500 to 900 ° C.

This decomposition of normal magnesium sulphite is preferably carried out by part of the heat released in the leachate boiler.

According to another aspect of the invention, normal magnesium sulfite is decomposed into magnesium oxide and sulfur dioxide, and the resulting sulfur dioxide is liquefied.

The normal magnesium sulphite is dried prior to decomposition and is decomposed by indirect heating, preferably in a gas stream consisting essentially of sulfur dioxide.

According to a further feature of the invention, part of the produced sulfur dioxide is in the form of fine bubbles in contact with the magnesium bisulfite solution produced in the absorption stage in a venturi tube system.

The sulfur dioxide stream contacted with the magnesium bisulfite solution is preferably divided into a plurality of partial streams. These partial streams are fed to the point of the narrowest cross-section of the downflow channel operating on the venturi principle.

The invention is illustrated schematically and by way of example in FIGS. 1 and 2. FIG. 1 is a flow chart of a normal sulfite decomposition apparatus, and FIG. 2 is a heat mode diagram of a similar apparatus.

According to FIG. 1, the aqueous magnesium sulphite suspension is conveyed from the magnesium bisulphite recovery plant to a centrifuge 1, in which it is concentrated to a dry matter content of greater than 70% and conveyed to a jet dryer by a conveying screw 2.

3. The water separated in the centrifuge 1 flows through line 13 back to the magnesium bisulfite recovery plant. The dried crystalline normal magnesium sulphite is separated from the drying air in cyclone 4 downstream of the jet dryer 3 and falls into the container 5. The dryer air is led from the cyclone separator to a magnesium bisulphite plant. Normal magnesium sulphite is conveyed via a rotary dispenser 6 via a screw conveyor 7 to a sulphite thermal cracking device 8 operating in a temperature range, preferably 500 to 900 ° C. At these temperatures, normal magnesium sulfite decomposes into magnesium oxide and sulfur dioxide. Sulfur dioxide with blower 9 blows into the. of the gas cooler 10, while cooling to about 65 ° C and fed to a booster tower to form free dissolved sulfur in the form of sulfur dioxide in the magnesium bisulfite solution. However, sulfur dioxide can also be liquefied. The magnesium oxide leaves the thermal cracking device 8 via a hopper and a pocket rotary dispenser 11 and enters a hydration vessel 12 where it is mixed with water and pumped in the form of dissolved magnesium hydroxide to the magnesium bisulfite recovery device as a sulfur dioxide absorber.

Fig. 2 shows thermal decomposition in apparatus 8 (thermal cracking reactor J (hereinafter also referred to as decomposition reactor)), wherein the decomposition reactor is indirectly heated by heating gas from combustion chamber 14 heated to approximately 900 degrees Celsius. leaving the decomposition reactor 8 and cooled to about 800 ° C, cooled to about 600 ° C in the heat exchanger 15, and then passed to a normal magnesium sulfite dryer 16. From this dryer, a gas of about 400 ° C exits and passes The heat exchanger 17 is then fed to a further heat exchanger 17 in which it is cooled to a temperature of approximately 300 [deg.] C., which is approximately the temperature of the waste gases from the leach boiler. from the combustion boiler.

From the absorber 27, a portion of the wet normal magnesium sulphite produced is fed to a dryer 16. The dried normal magnesium sulphite is introduced through the breech 19 into a sulphite cracking apparatus 8, in which a decomposition temperature of approximately 800 ° C is maintained. A hot sulfur dioxide-containing gas mixture flowing through the sulfite splitting device 8, which at its outlet contains water vapor and magnesium fractions and which upon cooling from the sulfite splitting device 8 is cooled in a regenerative heat exchanger 20, in a heat exchange cyclone 21 as well as in a heat exchanger 23 to about 200 ° C before part of the mixture is returned via a condenser 25 to the amplifier 24 via the condenser 25. In the heat exchange cyclone 21, the dust fractions of magnesium oxide formed during decomposition are separated and returned After the blower 22, the majority of the sulfur dioxide gas mixture is passed back to the sulphite cleavage device via the heat exchangers 20, 15, and is again heated up to almost at the heat exchangers 20, 15. decomposition temperature.

To improve heat utilization, fuel and / or combustion air is preheated in a heat exchange cyclone 21 or heat exchanger 17. The amplifier 24 consists essentially of a venturi arranged in a closed vessel, at the narrowest point of which sulfur dioxide is supplied, which dissolves in the crude, circulating acid.

Within the scope of the invention, it is possible to heat the decomposition apparatus directly or indirectly by the flue gas stream from the leachate combustion boiler 26. Furthermore, if only a lower need for acid in the bisulfite solution is necessary, it is possible to remove a portion of the magnesium sulfite produced in the absorber from the chemical circulation and to process, for example, sulfuric acid.

Claims (10)

SUBJECT A process for the recovery of magnesium bisulfite from waste gases from a leachate combustion boiler, wherein the absorption of sulfur dioxide is carried out in at least one stage working with normal magnesium sulfite and in at least one stage working with magnesium bisulfite, characterized in that produces a larger quantity of normal magnesium sulphite than is required for the production of hydrogen sulphite, and moreover the quantity produced corresponding to the difference between the amount of sulfur in the concentrated liquor and the amount of total sulphite and sulphate-bound sulfur in the tower acid is removed from the magnesium sulphite. 2. The process of claim 1 wherein the normal magnesium sulfite discharged from the magnesium bisulfite circuit is decomposed by heat treatment to sulfur dioxide and magnesium oxide, and the produced sulfur dioxide dissolves in the purified hydrogen sulfide solution while the magnesium oxide is returned to the working with normal sulphite. 3. A process according to claim 2, characterized in that the normal magnesium sulphite discharged from the magnesium bisulphite circuit is decomposed by mere thermal cleavage without additives in the temperature range of 500 to 900 [deg.] C. 4. A process according to claim 3, characterized in that a portion of the heat released by the leachate boiler is used for the cleavage of normal magnesium sulfite. 5. The process of claim 2 wherein the normal magnesium sulfite is cleaved into magnesium oxide, and the sulfur dioxide and sulfur dioxide are liquefied. 6. The process of claim 3 wherein the normal magnesium sulfite is dried prior to its cleavage. 7. The process of claim 3, wherein the cleavage of normal magnesium sulfite is by indirect heating. 8. The process of claim 3, wherein the cleavage of normal magnesium sulfite is carried out in a gas stream consisting essentially of sulfur dioxide. 9. The method of claim 2, wherein a portion of the produced sulfur dioxide is in the form of fine bubbles in contact with the magnesium bisulfite solution produced in the absorption stage in a venturi tube system. 10. The method of claim 9, wherein the sulfur dioxide stream contacted with the magnesium bisulfate solution is divided into a plurality of partial streams.