FI96878C - Procedure for handling fly ash from recovery boiler for disposal - Google Patents

Description

96878 METHOD FOR TREATMENT ASH FOR WASTE LEAF RECOVERY BOILER 5

The present invention relates to the treatment of fly ash from the combustion of waste liquor and the recovery of its chemicals for the production of cooking liquors.

10 Waste liquor from the pulp and paper industry in the pulp and paper industry, the so-called black liquor, usually burned in a chemical and heat recovery boiler. In a conventional recovery boiler, the process chemical is recovered by injecting the liquor into an oven, whereupon it dries rapidly and burns under reducing conditions above the bottom, creating a melt. This melt contains mainly sodium carbonate, sodium sulfide and sodium sulfate. Other compounds are also abundant, but the main components are essential for the production of cellulosic pulp.

25 The flue gases of a soda boiler are rich in ash, which *. separated in the ash hopper and electrostatic precipitators before removing the gas from the boiler. This so-called fly ash is mainly sodium sulfate. As boiler emissions decrease, the amount of ash has increased sharply. 30 Usually, this ash is recycled back to the soda space by mixing it with a strong black head fed to the boiler. As a result, the dead load of the recovery boiler increases as the amount of the component to be reduced in the recovery boiler, Na 2 SO 4, increases, i. the efficiency of the reduction zone decreases with the increase of the aerobatic 35. In addition, the sulfidity of the boiler increases as the amount of ash increases, as the S / Na ratio of the ash is higher than that of black ash normally. Ts. Increasing recycling of sodium sulphate to black liquor increases the internal sulphidity of the boiler 2 96878, which increases the corrosion of the boiler.

Finnish patent publication 27748 discloses a method for compensating for alkali and sulfur losses in the sulphate cellulose process. In this process, make-up Na2SC> 4 and possibly also Na2SC> 4 from electrostatic precipitators are reduced to Na2S in a separate device and not in a recovery boiler. However, the publication does not disclose any detailed, technically feasible method for performing this reduction.

It is an object of the present invention to provide a simple and efficient method for treating fly ash without having to return it to a recovery boiler, thereby preventing a reduction in the power of the reduction zone and minimizing corrosion.

It is a further object of the present invention to provide a process in which the cooking chemicals contained in the ash can be utilized in a more versatile and freer manner than before in the production of cellulose pulp.

The central idea of the present invention is that fly ash with a high S / Na2 ratio (about 1) is utilized in the pulp production process. A near-pure sodium sulfate feedstock is a prerequisite for obtaining a near-pure sodium sulfide product that can be used to increase the sulfidity of the pulp mill lye streams in an optimal manner.

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The present invention relates to a process for treating fly ash from flue gases from waste liquor combustion in a separate process, characterized in that the fly ash separated from the flue gases is contacted with carbon to reduce 35% sodium sulphate to predominantly sodium sulfide to dissolve the 3 96878 salts it contains, and the carbon is separated from the resulting solution, which is recovered for use in the manufacture of pulp.

Sodium sulfate is reduced to sodium sulfide according to the following reactions: (1) Na 2 SO 4 + 4C -> Na 2 S + 4 CO 2 (2) Na 2 SO 4 + 2 C -> Na 2 S + 2 CO 2 In the process of the present invention, . The treatment is carried out in a reactor with a temperature above 600 ° C but below the inorganic Na salts, i.e. Melting point of Na 2 SO 4, Na 2 S and Na 2 CO 3. Preferably the temperature is 650-750 ° C, most preferably 690-740 ° C. The reactor atmosphere may be inert (e.g. nitrogen) or reducing. The odorous gases from the pulp mill can be used to promote reducing conditions in the reactor. The sulfur compounds of the odorous gases 20 bind as active reactive components to the reaction vessel, which in turn increases the sulfidity.

The carbon required for the reduction may be derived from the carbonaceous materials used in the pulp mill. Carbon can be produced by charring bark, drying or pyrolyzing a black liquor. It can also be low ash coke.

In the heat treatment in the reactor, sodium sulfate is reduced to sodium sulfide according to reactions (1) and (2). Some of the sodium sulfate can be converted to sodium carbonate.

The reduction of sodium sulphate can be carried out in a reasonably short time when carried out according to the invention with solid ash having a very high specific surface area. In this case, however, sintering of the salt must be prevented in order to maintain a sufficient surface area. The ash-carbon mixture remains a solid mixture during the heat treatment without sintering when the particle size and amount of carbon powder 4 96878 are proportional to the particle size and amount of fly ash. Of course, the carbon used also increases the surface area required for the reactions. In experiments at about 690 ° C, it was found that a suitable carbon powder / ash powder ratio of 5 is about 1: 2 (w / w).

Reduction below the melting point also reduces the amount of heat required because no heat is used to melt the ash salt.

After the heat treatment, the reaction mixture is cooled and slurried in water or a lean liquor to form a solution containing salts from which the carbon can be removed, e.g. by filtration. The result is green liquor with a sulphidity of 55-15 100 mol% (sulphidity should be understood in this context as the sulphidity that would be obtained if the liquor were causticised 100%) and the proportion of sodium carbonate may be considerably lower than that of green liquor from melting boiler melt, usually with 60-70 mol-20% Na 2 CO 3 and 30-40 mol% Na 2 S. Alternatively, the reaction mixture may be slurried in a basic solution such as white liquor, low white liquor or green liquor.

The removed carbon can be reused in the process after 25 washes and drying. If reuse is not necessary, then the coal can be burned in a recovery boiler or shell boiler, for example.

The reactor may be a simple drum furnace or a through-furnace, into which, for example, the ash-carbon mixture in iron vessels is introduced for sufficient time to carry out the reactions. The heating of the furnace is preferably carried out indirectly, so that the required heat can be obtained by burning the carbon monoxide produced by the reduction of sodium sulphate.

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E.g.

The soda boiler electrostatic precipitator and activated carbon were mixed (1/3: 2/3 w / w) in a ball mixer. The granite crucible 5 96878 was filled with the mixture to give a reduction mass of about 75 g. The crucible was placed in an oven and maintained at the reaction temperature (640, 690 or 740 ° C) for 5 or 24 hours. It was then cooled. The furnace had a nitrogen purge of 5 lu throughout the experiment. Immediately after removal from the oven, the reaction mixture was slurried in cold water and filtered. The filtrate (green liquor) was analyzed for SO 2, SO 2 and CO 3. Tables 1, 2 and 3 show the concentrations in molar percentages.

The results clearly show that by the simple process of the present invention 10, fly ash sodium sulfate can be reduced to useful sodium sulfide at a reasonable reaction rate.

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Table 1. Anion composition of the salt to be reduced before the reduction experiment.

T / ° C S ~ CO3- SQ4- 20 I 25 1 0 <2 I 98

Table 2. Anion composition of the salt to be reduced at three temperatures after a reaction time of 5 hours.

T / ° C S ~ CQ3— SQ4- 640__0__4__96 25 690__31__41 28 740 75 22 3 ~

Table 3. Anion composition of the salt to be reduced at three temperatures after a reaction time of 24 hours 30 ΤΛΟ S-CO3- SQ4- 640__0 23 77: 690__63___30__7 740 I 96 ~ 3 1 35 6 96878

The sulfidity of the solution obtained in the process of the present invention depends, as in eo. an example can also be seen from the temperature used in the heat treatment. Thus, an additional advantage of the invention is the preparation of a lye, the sulphidity of which can be controlled by means of a reduction temperature. This is very advantageous because nowadays the aim is to adjust the sulphidity during cooking so that the sulphidity is high at the beginning of the cooking.

10 The yield and quality of pulp can be improved if cooking liquors with different sulphidities can be used in their production at different stages of cooking. The sulphidity of white liquor traditionally used in cooking is 30-40%. In an improved manner, lyes with a high sulfide-15 ratio (e.g., 60%) are used in the early stages of cooking, and lyes with low sulfidity (e.g., 20%) are used in the final stages of cooking. In the embodiment described above, in which the reaction mixture is slurried in water, a green liquor made from the ash of a recovery boiler is obtained, which has a high sulfide-theta and can thus be used directly in the early stages of cooking to improve the cooking result. Alternatively, the above green liquor can be mixed with a portion of a conventionally prepared white liquor, the sulfidity of which is thus increased. Alternatively, the reduced ash mixture 25 may be slurried in conventionally prepared green liquor, white liquor, or low white liquor to adjust their sulfidity.

The present invention provides a useful method 30 for use in a pulp mill to reduce the sulfur load of a recovery boiler and to produce cooking liquor having the desired sulfidity. The figure below shows how the process according to the invention can be integrated into a pulping process.

Is the Jentot ash 1 from the boiler plant mixed in a container? or the like h i. i 1 ipitoiser. with the powder from the plant or elsewhere, such as chalite crushed stone.

li 35 7 96878

The vessel containing the powder mixture is introduced into a suitable oven 3 or the like, where the mixture is preferably kept at 650-760 ° C for the time required for the desired sulphidity. The oven can advantageously be a through oven, into which the containers containing the powder mixture are introduced by a conveyor and removed in the same way. The treated ash from the soda boiler can be temporarily stored at the rate required to produce the reduction product.

The mixture in the oven is cooled, slurried in water in step 4 and decarbonized in step 5, e.g. by filtration. The spent carbon powder can be washed and dried and reused for a new batch of fly ash. The filtrate obtained is green liquor 6, which can be used as desired in the production of pulp 15. The sulphidity of green liquor can be made very high (e.g. 75-100%), so that it can be used as such in a digester. Alternatively, the reduced ash may be slurried in pulp mill green liquor, low white liquor, or white liquor to control their sulfidity.

The present invention provides a preferred way to perform sodium sulfate reduction instead of recovery boiler in a separate process. It has now been realized that nearly 25 pure sodium sulfates, fly ash, can be utilized alone to provide cooking liquors with high sulfidity, and an effective method has been invented for how this utilization can be accomplished. This also avoids the problems caused by the high sodium sulfate recovery boiler 30 that would occur if fly ash had to be treated there. No simple embodiment of the present invention has been previously realized to achieve the above objects.

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Claims (12)

96878 S A process for treating waste fly ash from flue gases in a separate process in which the sodium sulphate contained in the ash is reduced in the presence of a carbonaceous substance, characterized in that the fly ash separated from the flue gases is contacted with pulverized carbon at 600 ° C and below the melting point of said salts, the reduction product is slurried in water or an alkaline solution to dissolve the salts it contains, and the carbon is separated from the resulting solution, which is recovered for use in the production of pulp. 15 Process according to Claim 1, characterized in that the temperature is 650 to 750 ° C, preferably 690 to 740 ° C. Process according to Claim 1, characterized in that the carbon is derived from black liquor. A method according to claim 1, characterized in that the carbon is recycled back to the process. ] A method according to claim 1, characterized in that the treatment is carried out in a reducing or inert atmosphere. Process according to Claim 5, characterized in that a reducing atmosphere is obtained using the odorous gases of the pulp mill. Process according to Claim 1, characterized in that the basic solution is a white liquor, a weak white liquor or a green liquor. Process according to Claim 1 or 7, characterized in that the sulphidity of the solution obtained is determined by adjusting the reduction temperature. Process according to Claim 8, characterized in that the solution obtained has a higher sulphidity than usual and is used to provide a lye stream with a high sulphidity in the cellulose soup. 10. A method for producing a high sulphidity cooking liquor for sulphate cooking, wherein the fly ash from the combustion of waste liquor is treated in a separate process in which the sodium sulphate in the ash is reduced in the presence of a carbonaceous substance, characterized in that the essentially to sodium sulfide in a solid state at a temperature that provides the necessary effective reduction, the reduction product is slurried in water or an alkaline solution to dissolve the salts it contains, and the carbon is separated from the solution recovered as a high sulfide liquor for use in making cellulose pulp. Method according to Claim 10, characterized in that! 25 that the temperature is 650-750 ° C, preferably 690-740 ° C. Process according to Claim 10, characterized in that the basic solution is white liquor, green liquor or low-white liquor. 30 96878