FI122837B - Method for recovering chemicals from a pulp mill - Google Patents

Description

METHOD FOR THE RECOVERY OF CHEMICALS IN A PULP MILL

The present invention relates to a process and arrangement for recovering chemicals 5 from pulp mills, which include lime slaking for white liquor production, in particular lime slaking in a manner that can improve the treatment properties of causticizing lime slurry and efficiently recovering slugging energy.

1 o In pulp cooking processes such as sulphate and soda processes, chemicals are recovered by evaporation of water from the digestion liquor. black liquor, and then burning the organic matter contained in the broth in a chemical recovery boiler, a recovery boiler, to release the chemicals. From the recovery boiler, chemicals run molten from the bottom of the furnace into a separate leaching tank. The melt is dissolved in water or lean white liquor in a tank. The resulting solution is called green liquor. Insoluble matter is removed from it, for example by clarification or filtration before causticization. The sodium carbonate contained in the green liquor is causticized with lime (CaO), which is burned into sodium hydroxide for pulping. The causticization occurs in two reaction steps. The calcium oxide of the burnt lime first reacts with the water contained in the green liquor to form calcium hydroxide (Ca (OH) 2) in an exothermic lime quenching reaction. The extinguishing reaction is of the form:

CaO + H 2 O -> Ca (OH) 2 + 1130 kJ / kg CaO (1)

The heat released in the reaction is distributed in the green liquor with suitable mixing to prevent local overheating.

The resulting calcium hydroxide then reacts immediately with the green liquor sodium carbonate to give o Y sodium hydroxide. This reaction, or causticization reaction, is written as: o ^ 30 Ca (OH) 2 + Na 2 CO 3 -> 2 NaOH + Ca CO 3 (2) □ _

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The white liquor containing sodium hydroxide thus obtained is separated from the calcium carbonate (lime mash) and the white liquor is recycled for pulping. The difference in cm between white liquor and lime mud can be made either in the clarifier or by filtration. White liquor filters are usually candle filters or drum or disk filters which operate under pressure or vacuum. The honey is washed to reduce sodium losses and thickened with a honey filter. All washing liquids are collected in a common, so-called. laihavalkolipeäsäiliöön.

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Lean white liquor is used to dissolve the molten material. The lime is regenerated by burning the honey at high temperature in a calciner such as a lime kiln. Calcium carbonate is then decomposed into calcium oxide and carbon dioxide.

5 Honey ovens, which are typically drum ovens, use liquid, gaseous and solid fuels such as oil, natural gas and coal dust as heat sources. The burner is attached to the hot end of the oven. The hot product formed during the combustion, calcium oxide, is recovered by transferring the heat contained therein to the combustion air required for combustion of the fuel used in the process. In this case, this air (so-called secondary air) is generally introduced into the furnace via a condenser past the burner and only so-called through the burner. primary air. The proportion of primary air varies by burner and application,

As mentioned above, the quenching reaction generates a great deal of heat. When "unlimited" water is available at the time of quenching, as in the case of quenching the addition of lime to the green liquor as described above, the reaction occurs rapidly and so does the temperature of the mixture rise rapidly. However, this is followed by rapid cooling by the action of water, since liquid water also acts as a coolant. In quenching, the volume of the solid increases as the calcium oxide reacts to form calcium hydroxide. 20 Looking at the situation from the point of view of a single lime particle, as the solid mass increases, the lime particle crumbles into small parts which are separated (separated) by the liquid. As a result, the lime is chopped finely. The causticization reaction takes place with these small cleaved Ca (OH) 2 particles and results in fine lime slurry (CaCO 3 particles).

Several studies have found that leachate leachability is essentially dependent on the grain size of the honey (as is usually the case with solids separation). Generally, Y, the larger the particles, the better their leachability. In well-known § processes, lime slurry is relatively finely divided.

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Separation of the solid by filtration and washing of the resulting filter cake are important sub-processes in causticization. It is an object of the invention to eliminate the disadvantages of the bulk causticisation process and to provide a lime slaking process whereby the resulting lime is causticized into a lime slurry which has better drainage properties than hitherto. If the lime sludge separation processes can be enhanced by improving the leaching leaching properties, significant operational benefits will be obtained throughout the causticization process. The white liquor filter and the lime filter can be loaded 3 times more. The honey filter achieves a higher dry matter content and a lower residual alkali content.

Current practice is to recover the quenching energy typically by cooling the green liquor prior to quenching to obtain about 75 liters of hot water. The pulp mill has many sources of this warm water, so the value of the heat energy thus recovered remains low.

SE patent 460486 proposes that the lime is quenched mainly by greening at atmospheric pressure or elevated pressure, and the heat generated by the quenching is recovered as steam. Steam is shown for use in a pulp mill, such as white liquor heating, cooking, black liquor evaporation, pulp drying, electrical power generation.

WO 02/096820 discloses quenching of burnt lime with water vapor. This allows heat to be recovered at a higher temperature than liquid quenching. Heat is shown to be used to produce steam for the quenching reaction, to produce hot water, or to dry the lime raw material prior to combustion. In addition, it is suggested that heat be used in the turbine, and turbine exhaust steam for quenching. The lime from Sam-20 is cooled and the steam generated is also used in the fire extinguisher. For the purposes of this publication, the lime-containing raw material may be lime mud, limestone, dolomite, lime-containing sludge, which is incinerated and quenched. This publication does not directly seek to influence the energy economy of the lime burner itself nor the causticizing lime mud properties.

25 ^ The combustion of honey in a lime kiln is one of the most energy consuming processes in the pulp mill for the recovery of primary energy such as oil. Therefore, there is a need for processes ό γ to improve the energy economy of lime sludge combustion. The pulp mill also has other 0 incinerators, such as a recovery boiler, for which there is a similar need.

^ 30 A preferred energy-efficient way of lime slaking and lime has not been described previously

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for lime sludge incineration or other pulp mill incineration.

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It is also an object of the present invention to improve the energy cm of a pulp mill by utilizing the heat generated in the lime slaking process in an advantageous manner.

The present invention relates to a method for treating lime sludge in the recovery of chemicals in a pulp mill according to the preamble of claim 1. The lime is then burned in a lime kiln to produce lime which is further processed to produce slaked lime, the slaked lime is contacted with green liquor 5 to produce lime and white liquor, and the lime is separated from the white liquor. It is essential to the invention that the lime burned is brought into contact with water or water vapor, whereby the lime is extinguished substantially with water in the gas phase, and the heat released in the quenching reaction is used to heat the combustion air of the pulp mill combustor.

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The invention also relates to an arrangement for recovering chemicals in a pulp mill, which comprises at least a lime kiln, a fire extinguisher and causticizing vessels. The essential features of the arrangement are set forth in the appended claims.

15 Steam lime stripping is known per se and is used in the production of slaked lime, for example, in agriculture and in the construction industry. Thereby, it has been the objective to obtain a dry final product as also disclosed in WO 02/096820, mentioned above. It has never been shown for use in a pulp mill caustic mill in accordance with the present invention. In fact, the steam quenching method is more complicated in terms of the known needs of causticization compared to the method used (direct green liquor quenching).

However, it has now been found in experiments carried out that by performing steam quenching of lime lime, the filtration and dewatering properties of lime mud can be clearly improved. It was found in the experiments that the particle size of the slaked lime increases with increasing temperature, which results in better leachability and separation of the white liquor.

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of bodyweight. In the experiments, the temperature was 100-300 ° C and based on them the optimum range is about 200-300 ° C, most preferably 200-250 ° C. In experiments it was found that the quenching reaction slows down exponentially as the temperature rises linearly. Raising the temperature slows down ^ 30 quenching reactions, but at the same time leaching filtration resistance decreases and

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the substance content rises as the temperature rises and heat energy is also better obtained

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recovered. Figure 3 shows the properties of lime sludge taken from two different mills when treated in accordance with the present invention. The extinguishing was done with steam. The figure shows the average filtration resistance of the lambs as a function of the quench temperature 35. From this it can be stated that the filtering resistance drops clearly in this case. as the temperature rises. As is known in the art, the filtration resistance of reference sacks quenched with green liquor below 100 ° C (eg 97 ° C) is greater than that of lime sludge from the same source treated according to the invention. The improved filtration properties of the honey appeared to be due to the fact that quenching at temperatures higher than those known in the art produces slaked lime, which further produces causticizing larger lime particles with a lower specific surface area. Higher drainage of honey also results in a higher dry matter content by removing white liquor and washing it before feeding it into the oven. In terms of the properties of the honey, the optimum conditions for quenching are influenced by the quality of the source honey to be burned.

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The raw material used in the tests was burnt lime, which was in the form of small round granules. During quenching, the granules disintegrated into smaller particles and eventually became flour. This pulverization offers some advantages over the known lime form at the pulp mill. Due to the powdery shape, pneumatic conveying of lime can be used instead of belt and bump conveyors. In addition, impurities such as sand are easier to remove from the lime based on gravity before causticizing the green liquor. At present, the use of pneumatic transport is prevented by the fact that some of the slaked lime is in large particles. Similarly, sand removal from dry lime is prevented because the lime to be recovered is in chunks.

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Computationally, these results obtained in steam quenching would result in smaller white liquor and lime sludge filters than the prior art devices, so that the invention also has considerable economic potential, although quenching the lime lime itself is thus more complicated and apparently more expensive. Higher drainage of honeys 25 also leads to better washability, whereby emissions from lime sludge combustion can be reduced and furnace operation facilitated. In addition, higher lime dry matter saves energy in the lime kiln.

The invention is thus based on the idea that the burnt lime 30 obtained from the lime kiln is quenched with steam, whereby the lime retains its grain size and causticization better

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the resulting lime has better drainage properties than before. Lisäk-

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The heat generated by the quenching is advantageously utilized in the heating of the combustion air.

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Steam lime quenching can be accomplished, for example, by mixing lime in a screw-type mixer and introducing water vapor or by spraying water or by introducing a mixture of water and steam into the lime by adjusting the temperature so that lime 6 remains dry (lime temperature above 100 ° C). The water sprayed into the hot lime evaporates and quenches the lime as steam. The energy required for evaporation is obtained from the heat released in the reaction. There is still excess energy so that it is advantageous to remove heat from the reaction space. Unless excess heat is removed from the reaction space, the temperature of the lime 5 may rise so high that the quenching reaction is prevented (at atmospheric pressure above 500 ° C). The lime can be mechanically mixed instead of the screw, for example with stirrers, hoists or a rotating drum.

The lime and steam can also be brought together by stirring the lime with steam. The water used in the extinguishing can also be indirectly vaporized by the reaction heat, after which the steam and lime thus obtained are brought into contact with each other. The lime can be blended with a gas stream (steam), for example, steam jet jets, bubbling beds, fluidized bed or circulating beds.

15 Extinguishing can be done with or without pressure.

The amount of water used to quench the lime preferably corresponds substantially to the amount of water required for the reaction. It is also evident that the water used for the extinguishing need not be pure, but also water containing some impurities, green liquor, lean white liquor or condensate or mixtures thereof may be used.

All of the above lime quenching methods involve essentially removing excess heat from the process, either directly from the lime or, for example, by cooling the steam in the process. According to the invention, the heat is recovered in the combustion air supplied to the pulp mill's combustor. The heat energy can be transferred away as water vapor or as a mixture of steam and lime, the heat of which can be recovered elsewhere in the air in the heat exchanger. Heat can be removed from the sides of the reaction vessel indirectly to the combustion air, and optionally using additional heat transfer ribs or other heat transfer device. Heat can also be transferred directly to the reaction air circulating in the reaction space, especially when quenching is done with lime in the bed

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dossa (fluidized bed, bubbling bed, twisting bed). Heat transfer to the combustion air can be done

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^ also outside the extinguisher by direct contact with the steam-g lime mixture. Of course, several processes and combinations thereof may be used simultaneously in the process to recover thermal energy. If not all of the heat can be utilized for heating the combustion air, then the steam generated in addition to the heating can be utilized by known methods such as direct steam utilization, for example, for heating green liquor or for producing hot water or other pulp mill steam-intensive processes. Further, the quench reaction temperature can be adjusted by adding water and removing excess steam from the quencher.

It is known that the combustion air of the lime kiln is heated in a lime cooler which is integrated into the lime kiln or is a separate device. By taking the heat of the quenching into the combustion air according to the invention, the air at 0 ° C, for example, can be heated to 350 ° C, for example. The air is further heated by passing it to the lime cooler from the furnace connected to the lime kiln before being fed into the furnace itself. It is also advantageous to cool the burnt lime from the furnace before feeding it into the fire extinguisher, since the temperature of the burnt lime is high, e.g. about 1000 ° C, whereby it could raise the temperature in the fire extinguisher to an unfavorable high for the quench reaction.

Heated air can also be used in other pulp mill combustion plants, such as a recovery boiler. However, it is most advantageous to use the air in the lime kiln as it is technically located closer to the extinguisher.

Steam quenching with steam can also be combined with additional features that improve the quality of the product, such as the separation of unreacted lime and excess material (sand). Especially for solutions in which lime is moved by a gas stream, sand separation can be combined.

After quenching with steam, lime is added to the green liquor to causticize it. Because the lime is quenched "dry", the lime better retains its grain size and forms larger lime granules when reacting with the green liquor sodium carbonate. The invention will now be described in more detail by way of example and with reference to the accompanying drawings, in which

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Figure 1 shows a preferred embodiment of the invention for treating lime.

Figure 2 shows another preferred embodiment of the invention for treating lime.

Figure 3 shows the effect of quenching temperature on the leachability of lime sludge produced during causticization

Figure 1 illustrates a process according to the invention in which quench heat is indirectly recovered in the combustion air.

In Figure 1, the lime slurry 20 is burned in a lime kiln 1, from which the lime 3 is discharged through a condenser 2. In the figure, the cooler is integrated into a lime kiln, e.g. a sector or satellite cooler known per se, but it can also be a separate cooler. The lime is led to quench 4, where water, steam or vapor-gas mixture is also introduced through line 5 for the quench reaction. The quenching reactor 4 may be, for example, a vessel equipped with a screw or other agitator to improve contact between the lime material and the steam and thereby promote the reaction to produce slaked lime. In an exothermic reaction, the fire extinguisher generates water vapor, which is conveyed via one 7 to an indirect heat exchanger 9, where the steam releases heat to the air introduced along the compound 11. The heated air 10 is supplied to the cooler 2 of the lime kiln furnace 15 where it meets the burnt lime and is heated further before entering the furnace combustion chamber. Steam delivered from the extinguisher to the heat exchanger typically also contains lime, and the cooled steam-lime mixture is returned to the quenching time, where the steam participates in quenching due to the solids contained in the vapor, e.g. a lamellar type heat exchanger.

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Steam 22 can also be removed from the quench, which can be utilized in a manner known per se at the factory, e.g. for hot water production.

The slaked lime from reaction vessel 6 is introduced into causticization 12, where it reacts with green liquor 13 to form lime (calcium carbonate) and white liquor. Because of the fines of the slaked lime, it can be pneumatically transferred to a causticizing plant, where the lime tank is typically first located before the causticizing vessels.

Y The white liquor is separated from the lime 17 and drained together through 16. The honey is washed o with liquid 21, and the lean white liquor thus formed is fed through 18 through the extension 30 for use, and the washed lime into one 20 for burning. The difference between white liquor and lime mixture 15

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e.g., filter 14 and washing of lime with filter 19 are now more effective due to better drainage properties of leachate g than with conventional known process processes.

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The structure of Figure 1 is not limited to that shown herein, but may also be of a different shape. In extinguishing, the lime may be mechanically mixed in addition to the aforementioned screw or 9 stirrers, e.g. Lime can be blended with a gas stream (such as steam) for example: - steam jets - bubbling bed 5 - fluidized bed - circulating bed.

A separate indirect heat exchanger may also be located inside the quench vessel in the reaction space. It may also be replaced by a solution having a double jacket formed by the inner wall and the outer wall in the fire extinguishing vessel and the air flowing in the space between the walls whereby it is heated by indirect heat transfer due to heat from the inside of the extinguisher.

Additional features such as sand extraction can be added to the process (especially for solutions transported by 15 gas streams).

In the embodiment of Figure 1, the quencher may be a fluidized bed reactor to which burnt lime is fed from the lime kiln cooler and the lime floats with steam supplied to the reactor. In a fluidized bed, sand etc. can be separated from the lime, which can be removed from the bottom of the reactor. The slaked lime and steam mixture generated in the reactor is introduced into an indirect heat exchanger to heat the combustion air. From the heat exchanger, said mixture is introduced into a separator, e.g. a cyclone, on the basis of which some of the lime is removed for causticization, while some of the lime circulates with the steam in the extinguisher. The pipeline connecting the separator to the extinguisher is provided with a blower or equivalent to supply the mixture to the extinguisher. Water or vapor may be added to the mixture after the blower before the extinguisher to provide sufficient water for the quench reaction, § § The embodiment of Figure 2 differs from the solution of the figure in that heat is transferred from the quench reaction directly to heated combustion air, thus indirect heat transfer.

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breast 9 is not needed. The air can be introduced into direct contact with the co g hot lime and steam in the extinguisher. The heated air is led to the lime kiln. If necessary, descaler can be removed from the separator and returned to a fire extinguisher, for example.

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According to one embodiment, the slaked lime can be led from the extinguisher, for example, by means of a screw to a riser where the air to be heated flows upwards and where the air now warms as hot lime mixes with it. Preferably, impurities such as sand, which are removed from the lower portion of the riser, may also be leached from the lime in the riser. The lime-air mixture is led to a separator, from which the air is fed to the combustion plant and the lime is causticized.

It is also possible that the fire extinguisher comprises more than one step whereby the air to be heated has an indirect contact with the hot lime / steam and partly a direct contact with the lime / steam in e.g. a cyclone heat exchanger.

The solution of the present invention achieves many significant advantages in the operation of the entire causticization process. Honey separation processes become more efficient. The white liquor filter can be loaded more heavily, allowing a smaller filter to be added to new equipment deliveries or a higher capacity of existing equipment. The same goes for the lime filter. In addition, the lime sludge filter achieves higher dry matter, which in turn means savings in the lime kiln fuel. 15 By lowering the residual alkali content of the lime, the lime kiln has less flue gas emissions and improves the runnability of the furnace. In addition, the energy economy of pulp mill combustion equipment, such as a lime kiln, can be substantially improved by heating the combustion air with the heat generated during the quenching process, whereby the value of the heat energy recovered during quenching is high.

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It should be noted that the preferred embodiments described herein are exemplary only and are not to be construed as limiting the scope of the present invention as defined in the appended claims. For example, a fire extinguisher may comprise more than one reactor.

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Claims (24)

A process for recovering pulp mill chemicals, wherein the lime slurry 5 is incinerated to produce lime burned in a lime kiln, further processed to form slaked lime which is contacted with green liquor to produce lime slurry and white liquor, and the lime slurry is separated by filtration, water vapor to form slaked lime, wherein the lime is extinguished substantially with water in the gas phase, and the heat released in the quenching reaction is used to heat the combustion air of the pulp mill combustor. Method according to Claim 1, characterized in that the heat generated during the extinction is transferred indirectly to the combustion air using a heat exchanger. 15 Method according to Claim 1, characterized in that the heat generated during the quenching is transferred to the combustion air by directing the air in contact with the stream of material formed from the quenching process. Method according to Claim 3, characterized in that the combustion air is supplied to the extinguisher directly in contact with the lime and steam for heating the air. 5. A method according to claim 3, characterized in that the combustion air is led directly into contact with the slaked lime removed from the fire extinguisher in a direct heat exchanger to heat the air, o (Mo Process according to Claim 1, characterized in that the amount of water used for quenching the lime corresponds substantially to the amount of water required for the reaction. jr 30 CL Process according to Claim 2, characterized in that the steam involved in the reaction is circulated through a heat exchanger to transfer heat from the quenching process to the combustion air. o OJ 8. A method according to claim 7, characterized in that the steam recycled in the heat exchanger is accompanied by lime. Method according to claim 8, characterized in that the lime is separated from the steam before returning the steam to the extinguisher. Method according to one of the preceding claims, characterized in that the heat released from the quenching is used in part to generate steam or hot water. Method according to one of the preceding claims, characterized in that the sand is separated from dry slaked lime. 10 A method according to any one of the preceding claims, characterized in that the transfer of the dry slaked lime to the process step after slaking is carried out pneumatically. A method according to any one of the preceding claims, characterized in that the incinerator is a lime kiln. Method according to one of the preceding claims 1 to 13, characterized in that the combustion apparatus is a chemical recovery boiler. 20 Method according to any one of the preceding claims, characterized in that the burnt lime removed from the incineration is cooled by air prior to quenching. Method according to one of the preceding claims, characterized in that the extinguishing is pressurized. δ Method according to one of the preceding claims 1 to 15, characterized in that the quenching is carried out at atmospheric pressure. <£> ox 30 18. Arrangement for pulp mill chemical recovery, comprising at least a CL lime kiln, a fire extinguisher and causticizing vessels, characterized in that the lime burned in the lime kiln is arranged in contact with water or water vapor in the extinguishing reactor such that water in the gas phase, and cm that an indirect heat exchanger is provided in connection with the fire extinguisher, which is connected to a pulp mill incinerator to heat the combustion air supplied therein from the extinguisher. Arrangement according to Claim 18, characterized in that the indirect heat exchanger is connected to the fire extinguisher by circulating steam and / or lime from the fire extinguisher through the heat exchanger to transfer the heat energy to the combustion air. Arrangement according to Claim 19, characterized in that a separator is provided between the heat exchanger and the extinguisher after the heat exchanger for separating at least part of the lime from the steam. An arrangement for recovering chemicals in a pulp mill, comprising at least 10 lime kilns, a fire extinguisher and causticizing vessels, characterized in that the lime burned in the lime kiln is arranged in contact with water or steam to extinguish substantially with water in the gas phase; to heat the incoming combustion air so that the air is heated by direct contact with the slaked lime in the quench reaction space or outside the quencher in a direct heat exchanger. Arrangement according to Claim 18 or 21, characterized in that the incinerator is a lime kiln. 20 Arrangement according to Claim 18 or 21, characterized in that the incinerator is a recovery boiler. Arrangement according to Claim 18 or 21, characterized in that the slaked lime 25 is transferred to the causticization by means of a pneumatic device. o {M i o CO 0 X IX CL CD 01 O) o CO o o (M