FI98225C - Procedure for Removing Harmful Impurities in Alkaline Delignification of Cellulosic Material - Google Patents

Description

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The present invention relates to a process for the removal of impurities in the chemical treatment of alkali in the production of chemical chemicals such as silicon and phosphorus.

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In countries with scarce wood resources, annual crops such as bamboo, sugar cane, rice and wheat are used as raw material for pulp production. There has also been a lot of talk in Finland recently about the so-called 15 from the manufacture of agrofuels. Manufacturers of such pulp have completely different problems than manufacturers of wood pulp. The problems are due to both the properties of the raw material and the generally small size of the pulp mills. A major problem is the recovery of cooking chemicals, which is hampered by certain properties of the waste liquors. The economics of recovery processes are usually very low and often small plants do not recover chemicals at all. Due to changed economic circumstances and environmental damage, the development of chemical recovery 25 has recently become increasingly important.

The cell of annual plants is rich in silica (SiO 2). In the production of pulp from parts of such plants, the silicic acid in particulate form dissolves in the basic broth as silicate ions. Recovery of chemicals from broth effluent has proven to be very difficult precisely because of the dissolved silica. In time-consuming cooking processes, such as sulfate and soda processes, chemicals are recovered by evaporating the waste liquor, black liquor, water, and then burning this organic matter inside the broth in a recovery boiler to release the chemicals. The chemicals are removed from the recovery boiler as a 2 O "O O Γ yo z Z o melt, which is dissolved in water to green liquor, the sodium carbonate in which is causticized to the sodium hydroxide needed for pulp cooking with burnt lime.

Silica dissolved in alkaline black liquor causes problems at all stages of the chemical cycle. Evaporators) in black liquor silicates reach their solubility limits and precipitate as various compounds on the thermal surfaces of the evaporators, thus impairing the use of the equipment. Also in the recovery boiler, silicates form deposits on the heating surfaces and increase the viscosity of the chemical melt vis-15. In causticization, silicates precipitate as calcium hydrosilicates among the lime used in causticization. Calcium carbonate, which is rich in silicate, is poorly filtered, leaving moisture in the lime, with which a lot of alkali enters the lime kiln, which in turn impairs the operation of the lime kiln. Often such lime is not worth burning at all, but has to be taken to a landfill. In addition, the amount of white liquor thus obtained is reduced, and valuable cooking chemicals also go to landfill with the lime.

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Instead of disposing of impure lime, it has been proposed to separate silica from black liquor by lowering the pH of the liquor to about 9.Ι-ΙΟ with carbon dioxide. 2. The solubility of silica dissolved in the ionic form of black liquor, mostly HSiOA3 and SiO32, decreases and precipitates as a colloidal silica gel. Developed by the United Nations Industrial Development Organization (UNIDO) and the Swedish International Development Authority (SIDA), 35% of the silage liquor (6 g SiO2 / 1) has been reported to be separated from silicate.

In this case, carbon dioxide is bubbled in the bubbling reactor to black liquor, after which the precipitated silica is separated by filtration. 3 3 Γ · ^ Ο Γ- ο. Similar solutions have been developed by CPPRI and Lurgi. What these solutions have in common is that they have not worked in the desired way in practice. The problems have apparently been the poor filterability of the gel-like precipitate, the co-precipitation of the lignin in the black liquor with the silicic acid, the chemicals lost with the organic matter, and the deterioration of the calorific value of the black liquor.

10 In the same way as black liquor, silica can be separated from green liquor by means of carbon dioxide. The green liquor may contain about 10-20 g of S102 / l silicate, depending on the raw material used - sometimes even more - which is considerably more than in the corresponding low-black-15 head, which can be expected to lead to a better yield. Separation of the silica precipitate from green liquor is also not as problematic as black liquor due to the lack of organic matter in green liquor.

However, as the pH decreases under the influence of carbon dioxide, the free alkali contained in the green liquor disappears at the same time as it is replaced by carbonates and bicarbonates which must be removed from the green liquor during causticization. This considerably increases the need for causticization and thus the sale of quicklime.

Another interfering impurity contained in green liquor may be phosphorus, which, like silicon, tends to enrich the lime cycle and form an inert rotational load. For this reason, phosphorus has been removed by disposing of part of the lime from the chemical cycle and replacing it with a new one. Phosphorus tends to be enriched in the air dust of the lime kiln and has been preferably removed in this particular form. Air dust has been sold in some places as a phosphorus-35-containing soil improver.

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It is an object of the present invention to provide a simpler and more efficient method than known methods for substantially reducing the silicon and phosphorus content of cooking chemical liquors. In particular, attention is paid to the fact that the above-mentioned impurities are in an easily separable form.

To achieve these objectives, it is essential to the process of the present invention that the melt containing sodium carbonate from the combustion of 10-black liquor is treated to recover sodium carbonate as a solid, while separating silicon and / or phosphorus as dissolved sodium silicates / sodium phosphate solutions; and that the 15-solid sodium carbonate is dissolved to form a solution having a low silicon and / or phosphorus content.

Sodium silicates and phosphates are almost infinitely soluble in water, while the solubility of sodium carbonate in water is only limited. When the sodium carbonate is brought to a solid state according to the present invention, it can be separated neat from the sodium silicate / phosphate-rich mother liquor. After separation, the solid sodium carbonate is dissolved, and the solution thus obtained can be treated in a conventional manner in a causticizing plant into a sodium hydroxide solution for pulp cooking, bleaching and other purposes.

In a conventional chemical regeneration process, a melt containing sodium carbonate is dissolved in water or a low white liquor from a lime wash. From such a lye, sodium carbonate can be separated by evaporating the lye until crystallization occurs. Evaporation can be done by conventional multi-stage evaporation. After evaporation, the crystals are separated by a separation process known per se, such as filtration, from a mother liquor containing soluble sodium silicates and phosphates.

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Sodium carbonate can also be separated from readily soluble silicates and phosphates by extracting the readily soluble compounds from the recovery boiler melt into a small amount of liquid, leaving the sodium carbonate more difficult to dissolve for the most part. The extraction can be carried out, for example, by applying the melt leaching developed by Ebara in connection with the NSSC recovery process (U.S. Patents 4,212,702 and 4,141,785; Teder, A., Nordisk Cellulosa, 1984, No. 2, pp. 12-14). According to another method, if necessary, the melt 10 can be cooled and then ground to the desired extraction roughness.

As mentioned earlier, silicate is a particularly difficult problem in pulp mills where pulp is produced from 15 annual plants. The delignification process is then typically a soda process in which the cooking chemical is sodium hydroxide. However, the present invention is not limited to the soda process, but can accordingly be applied to processes utilizing sulfur-containing cooking liquors, such as the sulfate process. In this case, however, the sodium sulphide, which is more soluble in sodium carbonate, enters the mother liquor solution containing sodium silicates. However, the silicates / phosphates can be separated from the mother liquor by mixing, e.g., with quicklime or carbon dioxide, to obtain a sodium sulfide-containing solution for return to the plant chemical cycle.

The advantage of the method according to the present invention is emphasized by the fact that the sodium silicate or water glass solution generated as a "waste" has commercial value. In some cases, the value may even be higher than the value of the replacement chemical for the alkali removed in the silicon separation (not all of the molten sodium carbonate is obtained as a solid, but some ver-35 is lost in soluble form). An advantageous situation may arise in factories which, in addition to soda soup, produce high-yield or. · 'R ··. r \ f. A: y / ‘•’ i 6 waste paper pulps. Namely, in the peroxide bleaching of the latter, large amounts of water glass are used as a protective chemical to prevent the decomposition of peroxide. F1 patent 84190 further discloses a method for recovering alkalis 5 from a used washing solution. Therefore, the replacement of the alkali losses previously reported could be handled with the alkali obtained from this recovery. Silicon can be removed from a low carbonate silicate-containing solution very efficiently, for example by means of burnt skull, because a causticization reaction which otherwise impairs the use of lime to remove silicon from green liquor does not take place. Thus, the advantage of the invention is not limited to the continued use of sodium silicate.

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Phosphorus compounds can also be removed from the solution by means of quicklime, resulting in a small amount of a particularly high phosphorus-containing precipitate. In this way, the amount of lime to be removed from the cycle can be reduced and more valuable lime can be obtained as 20 products for soil improvement purposes.

The main advantage of the silicon-phosphorus separation process according to the invention is its low operating costs. The silicon / phosphorus separation by extraction is likely to be a more cost-effective alternative to evaporative crystallization in terms of both operating and investment costs.

The invention will be described in more detail with reference to the accompanying figures, in which Figure 1 shows the silica content of pure green liquor in an experiment 30 depending on the degree of evaporation, Figure 2 shows a preferred embodiment for carrying out the process according to the invention, Figure 3 shows another preferred embodiment for carrying out the process according to the invention. a preferred embodiment for carrying out the method according to the invention.

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Example:

According to laboratory experiments, the silica content of the synthetic soda liquor containing 48 g of SiO 2/1 decreased with the evaporation crystallization proceeding as shown in Fig. 1, which shows the dependence of the SiO 2 content of the solution obtained by dissolving the separated sodium carbonate crystals on the evaporation concentration.

10 The result is obtained by analyzing the sodium and silicon contents of the separated crystals and calculating the silica content which would remain in the purified green liquor otherwise corresponding to the initial situation if all the sodium carbonate could be separated. It can be seen from the results that by evaporating the green liquor to a volume of about one-seventh of the original, the final product is sodium carbonate liquor with a silica content of about 5 g / l. This value can also be improved by more efficient crystal washing.

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From the ground sodium silicate / sodium urea carbonate melt, silica was extracted in the laboratory into a small amount of room temperature water. In this case, it was found that by optimizing the extraction temperature and processing time, it is possible to reach as low silicon concentrations as by evaporative crystallization.

Figure 2 shows a process in which sodium carbonate is separated by evaporative crystallization. The black liquor is fed through nozzles 2 to a waste liquor boiler 1, where it is burned by means of air 3. In this case, the inorganic substance contained in the black liquor remains mainly at the bottom of the boiler as a melt containing sodium carbonate or sodium carbonate and sodium sulphide - depending on the cooking process. Siel-35 is melted in a manner known per se to a solvent, where it is dissolved in water or lean liquor together to form a green liquor. Because green liquor contains 8 o ^ n r \ f ^ zz insoluble impurities, it is usually clarified or filtered to remove this green liquor precipitate. Preferably, the filter described in patent application PCT / FI94 / 00485 can be used as the filter 6. The filtered 5 green liquors are passed together through 7 to the evaporator 8.

The evaporation is most preferably carried out in several stages and using descending membrane evaporators in which a green liquor flows on the outer surface of the evaporating element, tube or lamella. Upon evaporation, the green liquor is concentrated so that sodium carbonate crystallizes. By controlling the amount of water leaving the evaporation, the desired proportion of sodium carbonate is crystallized.

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The crystal slurry is removed from the lye circuit of the evaporator via one 9 to separate the crystals, which in this case is done as a filter 10. The silicate / phosphate-containing filtrate is passed through one 11 for possible further use. The sodium carbonate crystals 12, in turn, are dissolved in the liquid 15 in the mixing tank 14. This gives a silicon / phosphorus-pure lye 17 of the original green liquor of the solvent 4, which can be causticized in a manner known per se for use in delignification.

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Along with the filtrate 11, alkali may be wasted and replaced by the addition of alkali 13 to the mixing tank 14.

Figure 3 shows another way to separate solid sodium carbonate-30, which is based more easily on Liu lubricating compounds, i. extraction of sodium silicates and phosphates with a small amount of water or lean, so that the sodium carbonate remains less soluble when less soluble.

The melt is led from the bottom of the recovery boiler 1 to a hopper 21, where it comes into contact with a slurry 22 which is 35 ° C; f 'r-' y C / 1 Z. 0 9 contains solid sodium carbonate in sodium silicate and phosphate solution. This causes the melt to decompose and solidify into small particles. The smallest particles pass through a screen 23 into a slurry tank 24, where sodium silicates and -5 phosphates dissolve from the particles. The coarsest particles enter the second dissolution tank 25, where they dissolve and to which water 29 is added for dissolution. The solution floods from this tank 25 to the first solution tank 24, from which the slurry 10 containing solid sodium crystals is passed together 27 through a filter 28 to separate the crystals from the mother liquor.

The sodium carbonate crystals are dissolved in water in a tank 30, from which the sodium carbonate solution 31 is subjected to causticization 15 in a manner known per se. The filtrate 32 from the filter 28 can be treated to remove silicates and phosphates. Burnt lime or lime milk from a total of 33 is added to it to precipitate silicon and phosphate in a clarifier 34. The clarified silicon / phosphorus-free alkaline solution 35 pa-20 is recycled to a chemical circuit 31, and the silicon / phosphorus-containing precipitate 36 is recycled.

In connection with Figure 4, the extraction of the recovery boiler melt has been done in an alternative way. The melt is decomposed and cooled by air jets 40 in a condenser 41.

The hot air generated in this case is used as combustion air 42 in the recovery boiler 1.

The solidified salt is decomposed by a crusher 43. It is extracted with an amount of liquid in the solvent 44 at which the sodium silicates and phosphates dissolve while the sodium carbonate remains substantially solid. The sodium carbonate is separated as a filter 45. The silicate and phosphate-containing filtrate 46 is discarded for further use, while the sodium carbonate 35 is dissolved in water 48 in a solvent.

The alkali lost with the filtrate is replaced by alkali addition, C. 'r; o r.

"V ... · 10 with 49. The sodium carbonate solution 50 is subjected to causticization as previously described.

The various details of the invention may vary and differ from those exemplified above only within the scope of the inventive idea defined by the appended claims.

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Claims (11)

A process for reducing the silicon and / or phosphorus-5 contents of cooking chemical liquors in the recovery of chemicals in an alkaline delignification process of a cellulosic material, characterized in that - sodium carbonate-containing melt from black liquor combustion is treated to recover sodium carbonate as a solution containing dissolved sodium silicates / sodium phosphates; and that - the solid sodium carbonate is dissolved to form a solution with a low silicon and / or phosphorus content. Process according to Claim 1, characterized in that the melt obtained from the combustion of the black liquor is dissolved and the solution thus obtained is evaporated to crystallize sodium carbonate. Process according to Claim 1, characterized in that the molten black liquor is extracted from the combustion with an aqueous solution so that the sodium carbonate is recovered as a solid. Process according to Claim 3, characterized in that the molten black liquor is cooled with air before extraction. 30 A method according to claim 4, characterized in that the air heated in the cooling of the melt is used as combustion air in the combustion of black liquor. Process according to Claim 1, characterized in that the sodium silicon f r ο λ separated from the sodium carbonate. The r-12-containing solution is used for pulp bleaching or other utilization. Process according to Claim 1, characterized in that a silica dopant, preferably quicklime, is added to the sodium silicate-containing solution separated from the sodium carbonate in order to separate the silicon. Process according to Claim 1, characterized in that the sodium silicate-containing solution separated from the sodium carbonate is treated with a carbon dioxide-containing gas to precipitate and separate the silicon as a silica gel. 15 Process according to Claim 1, characterized in that the sodium phosphate-containing solution separated from the sodium carbonate is treated with quicklime to separate the phosphate. 20 Process according to one of the preceding claims, characterized in that the alkaline delignification takes place by the soda process. Process according to one of Claims 1 to 9, characterized in that the alkaline delignification takes place by the sulphate process. No; oop L / £; j 13