CS210643B2 - Method of recovery of magnesium sulphite coocing acid and apparatus for making the same - Google Patents

Abstract

Recovery of MgSO3 from MgSO3-contg. digester liquors (from cellulose mfr.) is carried out by (a) evaporating and roasting the liquors to produce a SO2-contg. gas, (b) mechanically separating solids from the gas, (c) scrubbing SO2 from the gas with a circulating wash liq. maintained at 63-65 deg.C and pH 3.5-4.5, (d) withdrawing part of the wash liq. and adjusting it to a temp. corresp. to a MgSO3 solubility minimum (between ambient temp. and 40 deg.C or between 75 and 80 deg.C), (e) adjusting the pH to 6.8- 7.8 (pref. 7.0-7.2) by adding an aq. Mg(OH)2 slurry, (f) separating the ppted. MgSO3, and (g) purging part of the residual liq. from the system and recycling the rest to the scrubbing circuit. The process gives high yields of MgSO3 with a low chloride content and eliminates the need for filtration in step (f) (cf. US3844879).

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for the recovery of magnesium sulphite boiling acid containing corrosive substances, in particular chlorides, and at high yields of recovered useful chemicals, in particular magnesium oxide, by reacting sulfur-containing gases obtained by burning magnesium sulphite liquors with an aqueous hydroxide suspension. and then the sulfur-containing gases, which are freed from the bulk solids, are then washed with a recirculated washing liquid. SUMMARY OF THE INVENTION is to separate a portion of the wash liquid from a recirculated wash liquid having a temperature of 63 to 65 ° C and a pH of 3.5 to 4.5, and then cool the portion of the wash liquid to a temperature within the temperature range room temperature and 40 ° C or heated to 75 to 80 ° C and adjust the pH of a portion of the wash liquid with an aqueous magnesium hydroxide suspension.

6.8 to 7.8, whereupon the reaction product is allowed to settle and one part of the supernatant liquid is discharged from the process and the other part of the chloride-containing liquid is returned to the wash liquid circulation.

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BACKGROUND OF THE INVENTION The present invention relates to a process for recovering magnesium sulphite boiling acid with a low content of corrosive substances, in particular chlorides, and to a high yield of recoverable usable chemicals, in particular magnesium oxide. The process is based on the reaction of sulfur dioxide-containing gases obtained by combustion of magnesium sulphite liquors with an aqueous suspension of magnesium hydroxide, whereupon the gases containing sulfur dioxide and free of the bulk solids are then washed with recycled washing liquid.

The process for recovering the sulphite boiling acids used for pulp production is based on the requirement of high yields of recoverable usable chemicals and on the requirement of as little environmental contamination as possible. It is also a particular object of the present invention to prevent the accumulation of corrosive substances in the recovery stage of sulfite boiling acid and thereby to ensure a longer service life of the device.

When recovering magnesium sulphite boiling acids from magnesium sulphite leaches, the problem is that the sulfur-containing gases obtained by burning the sulphite liquor carry fine particles of magnesium oxide dust which cannot be completely removed from the flue gas during dry cyclone separation. Electrostatic filters then make the whole plant more expensive.

The problem with wet separation is that corrosive substances, especially chlorides, remain in the washing liquid and accumulate there. To overcome these difficulties, it has been proposed in U.S. Patent No. 3,844,879 to withdraw a portion of the recycled liquid, to react with a magnesium hydroxide slurry, to filter out the magnesium sulphite formed and to add it to the sulphite boiling acid while the chloride-containing filtrate is drained.

However, this known process has various disadvantages, namely that the filtrate containing chloride still contains a considerable amount of magnesium compounds, mainly magnesium sulphite, which are lost from the recovery stage of the sulphite boiling acids and thus the recovery of these chemicals is reduced. In addition, filtration is an expensive process that is very susceptible to failure.

SUMMARY OF THE INVENTION It is an object of the present invention to provide the possibility of reacting a partial stream from a recycled wash liquid stream with magnesium hydroxide while maintaining optimal conditions determined by the minimum solubility of the magnesium sulphite formed.

The aforementioned drawbacks have no method of recovering magnesium sulphite boiling acid containing corrosive substances, in particular chlorides, of not more than 200 mg / l and yielding useful chemicals, in particular magnesium oxide, of at least 80 percent, by reacting sulfur dioxide containing gases obtained by burning magnesium sulphite. extracts, with an aqueous suspension of magnesium hydroxide, after which the sulfur dioxide-containing gases, which have been removed from the bulk of the solids, are then washed with a recirculated washing liquid which consists of recirculating washing liquid having a temperature of 63 to 65 ° C and pH 3.5 to 4.5, separating a portion of the washing liquid, after which the washing liquid is cooled to a temperature between room temperature and 40 ° C, or heated to 75 to 80 ° C, and the aqueous magnesium hydroxide slurry is adjusted to a high degree The reaction product is allowed to settle and one portion of the supernatant liquid is removed from the process and the other portion of the chloride-containing liquid is returned to the wash liquid circulation.

The part of the washing liquid which is separated from the circulating washing liquid is cooled by adding fresh cold water.

The invention also includes an apparatus for carrying out the aforesaid method, comprising a vaporizer and a magnesium sulphite liquor combustion apparatus, a dry separator, preferably a cyclone, for separating solids from the flue gas, and an absorber for absorbing a major proportion of sulfur dioxide. a gas scrubbing liquid circuit and a reactor for contacting the scrubbing liquid stream with magnesium hydroxide, the reactor having a cooling and heating device, a stirrer and a second water supply line, the reactor being connected to a a refining tank to indent and separate the reaction product.

An advantage of the invention is that it is as described above. it not only removes corrosive substances, especially chlorides, but also provides high recovery rates for the recovery of magnesium oxide and sulfur dioxide on a simple plant. The minimum solubility of magnesium sulphite is maintained by limiting the temperature and pH within the limits set above.

The reaction of the recycle wash liquid stream containing magnesium bisulfite and magnesium chloride with the magnesium hydroxide slurry requires a reaction time of at least 30 minutes to obtain a crystalline precipitate. This precipitate consists essentially of magnesium bisulfite. The hydrogen sulphite present in the reactor is converted to normal magnesium sulphite by the pH value.

The method according to the invention and the apparatus for carrying it out are illustrated schematically in the attached figure.

The apparatus comprises a washer

1, preferably a Venturi scrubber, into which flue gas, free of the major part of the solids, is fed through the inlet line 2. However, these flue gases still contain to a considerable extent fine particles of magnesium oxide as well as corrosive substances such as chlorides. These substances are eluted by a washing liquid which is circulated between the washer 1, the liquid separator 3, the pump 4, the cooler 5 and the washer 1 through the circulation line 6. Fresh water can be supplied to this circulation through the supply line 7. The circulation line 6 is closable with shut-off valve 8.

From the liquid separation vessel 3, the washed and dried sulfur dioxide and chloride-free sulfur dioxide are discharged through the outlet conduit 9 to an absorption device (not shown). In the process of the invention, the wash liquid temperature is 63-65 ° C and the pH is in the range of 3.5-4. The wash liquid contains normal magnesium sulfite, magnesium bisulfite and magnesium chloride in addition to a small amount of magnesium sulfate. From the circulation of the washing liquid, a branch line 12 on which a blocking valve 11 is located, is branched in the branch 10 · 10. via a line 16, the first supply line 15 supplying the magnesium hydroxide slurry and the second supply line 16 supplying fresh water, namely cold water. Branched line 12. and the reactor 13 may be additionally provided with cooling and heating devices to set the desired temperatures. The desired temperature lies in one embodiment of the process according to the invention with respect to the first solubility minimum of magnesium sulphite between room temperature and 40 ° C, and with respect to the second solubility minimum of magnesium sulphite lies in a temperature range of 75 to 80 ° C. Depending on whether one or the other solubility minimum is used, the temperatures in the reactor 13 are selected. The pH of the reaction mixture which is in the reactor 13 is adjusted to a value in the range of 6.8 to 7 by adding an appropriate amount of magnesium hydroxide ^1. , 8. The reaction time required for the reaction is at least 30 minutes and the preferred time is about one hour.

The reactor 13 is connected by a connecting line 17 and a clarification tank 18, which is also provided with a cooling and possibly a heating device. The temperature of the clarified mixture must be in the same range as the reactor temperature. Crystalline magnesium sulphite 19 settles at the bottom of the clarification tank 18. The liquid 20 above the sediment. . It contains magnesium chloride, the liquid being withdrawn through the overflow 21 via the discharge conduit 22. One part of the magnesium chloride solution is discharged from the process through the waste conduit 23, the other part of the solution is returned to the washing circuit via the return conduit 24. liquid. In this way a constant magnesium chloride content in the wash liquid circuit and thus constant circulation conditions is achieved.

Example

When burning the sulphite liquor and washing the resulting sulfur dioxide gas, the washing liquid results in the following composition:

chloride content: 180 · mg / l, magnesium bisulfite: 34 g / l, unreacted magnesium oxide: 6 g / l.

8 m3 of washing liquid per hour are withdrawn from this washing liquid; this liquid is mixed with an aqueous suspension of magnesium hydroxide to achieve a pH of 7.5. In doing so, all the magnesium bisulfite is converted to magnesium sulfite. Because of the low solubility, only 6 kg of magnesium sulphite remains dissolved in 1 m 3 of the treated wash liquid.

The withdrawn washing liquid is then collected in a settling tank from which 7 m 3 of clear chloride-containing supernatant is collected. In this way 10.8 kg of chloride per hour are removed from the circulating wash liquid. The supernatant obtained is centrifuged. 36 kg of magnesium sulphite is lost in the supernatant. Regeneration is 275.2 kg, which corresponds to a degree of regeneration of 88.49 · ° / o. In this way, the chloride content of the wash liquid is reduced to about 180 mg per liter.

Claims (3)

SUBJECT A process for the recovery of magnesium sulphite boiling acid containing corrosive substances, in particular chlorides, of not more than 200 mg / l and yielding recovered useful chemicals, in particular magnesium oxide, of at least 80% by reaction of sulfur-containing gases obtained by combustion magnesium sulphite liquor, with an aqueous magnesium hydroxide suspension, after which the sulfur dioxide-containing gases are stripped of the bulk solids, then washed with a recirculated washing liquid, characterized in that they are recycled from a washing liquid having a temperature of 63 to 65 ° C; pH 3.5 to 4.5 separate a portion of the wash liquid, then cool the wash liquid to a temperature between room temperature and 40 ° C, or heat to 75 to 80 ° C and adjust the aqueous magnesium hydroxide slurry the pH of a portion of the washing liquid to 6.8-7.8, whereupon the reaction product is allowed to settle and one part of the liquid above the crystalline grounds is removed from the process and the other part of this BACKGROUND OF THE INVENTION Chlorine-containing palpes are recirculated to the wash liquid circulation. 2. A method according to claim 1, wherein the cooling of the part of the washing liquid which is separated from the circulated washing liquid is carried out by adding fresh cold water. 3. Apparatus for carrying out the process according to Claims 1 and 2, comprising a vaporizer and a combustion apparatus of a magnesium sulphite liquor, a dry separator, preferably a cyclone, for separating solid particles from combustion gases, and an absorber for absorbing a major proportion of sulfur dioxide. wherein between the separator and the absorber there is arranged a circulation of the scrubbing liquid for sulfur-containing gases and a reactor for contacting the sub-stream of the scrubbing liquid with magnesium hydroxide, characterized in that the reactor (13) is equipped with a cooling and heating device, a stirrer ( 14) and a second supply line (16) for supplying water, the reactor (13) being connected to a clarification tank (18) to separate and separate the reaction product.