Title of Invention

A METHOD OF PRODUCING A COOKING LIQUOR WITH DECREASED SILICON, PHOSPHOR AND A ALUMINIUM CONTENTS

Abstract

ABSTRACT (1295/MAS/95) The present invention relates to a method of producing a cooking liquor with decreased silicon, phosphor and/or aluminium contents, said process comprising the steps of a) delignifying cellulose-containing material with alkaline cooking liquor to obtain a pulp; b) separating the resulting black liquor from the pulp; c) evaporating and combusting the black liquor to produce a melt containing sodium carbonate, silicon, phosphor and/or aluminium; d) dissolving the melt obtained from black liquor combustion to obtain a solution containing dissolved sodium silicates, sodium phosphates and/or sodium aluminates and recovering the sodium carbonate in solid form or as a solution; e) dissolving the sodium carbonate for forming a solution, which has a low silicon, phosphor and/or aluminium content.

Full Text

The present invention relates to a method of producing a cooking liquor with decreased silicon, phosphor and aluminium contents in connection with the recovery of chemicals in an alkali delignification process of cellulose-containing material. In countries of scanty wood resources, annual plants, such as bamboo, sugar cane, rice, and wheat, are used as raw materials for manufacturing chemical pulp. Manufac¬turers of this kind of pulp have quite different problems than manufactures of wood pulp. The problems are attribu¬table to properties of the raw material as well as the small size of pulp mills in general. An essential problem is the recovery of cooking chemicals, aggravated by cer¬tain properties of waste liquors. Generally, the recovery processes are not economical, and small pulp mills often do not recover chemicals at all. Due to economical changes and environmental aspects, development in the recovery of chemicals has been found more and more im¬portant lately. Meristematic tissues of annual plants contain plenty of silica (Si02) - When pulp is manufactured from limbs of this kind of plants, silicon acid, which has been in particulate form, dissolves in alkaline cooking liquor as silicate ions. Recovery of chemicals from the cooking waste liquors has proved to be highly difficult just because of the dissolved silica. In alkaline cooking methods, such as sulphate and soda processes, chemicals are recovered by evaporating water from the waste liquor, i.e., black liquor, and then by combusting the organic substance contained in the liquor in a recovery boiler for releasing the chemicals. From the recovery boiler, chemicals are discharged as melt, which is dissolved in water to form green liquor. The green liquor contains sodium carbonate which is causticized, by means of cal- cium oxide, to sodium hydroxide needed in pulp produc¬tion. The white liquor thereby obtained is circulated to the pulp production system, and the calcium carbonate (lime mud) formed is regenerated in the lime kiln. The silica dissolved in alkaline black liquor causes problems in all stages of the chemical circulation. In the evaporation plant, the silicates of black liquor reach their solubility limit and precipitate as different compounds onto the heat transfer surfaces of the evapora¬tors, thereby impairing the operation of the equipment. Also in the recovery boiler, silicates form deposits on the heat transfer surfaces and increase the viscosity of the chemical melt. In causticizing, silicates precipi¬tate as calcium-hydrosilicates among the lime mud. The lime mud rich in silicate infiltrates poorly, leaving moisture in the lime mud. Plenty of alkali is carried into the lime kiln along with the moisture, which hampers the operation of the lime kiln. Often, this kind of lime mud is not worth combusting, but it has to be taken to a landfill area instead. Furthermore, the amount of white liquor thereby obtained decreases, and also valuable cooking chemicals end up in the landfill area, along with the lime mud. Instead of discharging impure lime, it has been suggested to separate silica from the black liquor by lowering the pH of black liquor with carbon dioxide to the range of about 9.1 to 10.2. In the ionic form of black liquor, i.e. mostly HSiOj'*" and SiOj^'the solubility of dissolved silica decreases and the silica precipitates as colloidal silica gel. It has been reported that, in the method developed by United Nations Industrial Development Organ¬ization (UNIDO) and Swedish International Development Authority (SIDA), even 90% of the silicate has been suc-™ ,. _ cessfully separated from weak black liquor (6 g Si02/1) . In this case, carbon dioxide is caused to bubble into the black liquor in a bubbling reactor, whereafter the pre¬cipitated silica is separated by filtering. Similar ar¬rangements have been developed by CPPRI and Lurgi. It is common to these arrangements that, in practice, they have not operated as desired. The problems have obviously been poor filterability of the gel-like precipitate, precipi¬tation of lignin contained in the black liquor simulta¬neously with silica, chemicals being lost along with organic substance as well as a decline of the caloric value of black liquor. In the same way as from black liquor, silica may be sep¬arated from green liquor by means of carbon dioxide. Depending on the raw material used, the silicate content of green liquor may be about 10 to 20 g Si02/1, sometimes even more, which is much more than in corresponding weak black liquor, whereby a better yield may be expected. Separation of silica precipitate from green liquor is not so problematic than separation from black liquor, because green liquor lacks organic substance. As the pH lowers due to the effect of the carbon dioxide, the free alkali contained in the green liquor, however, disappears, being at the same time replaced with carbon¬ates and bicarbonates, which have to be removed from the green liquor in causticizing. This increases the need for causticizing considerably, which again adds to the con¬sumption of calcium oxide. Other harmful impurities contained in the green liquor are phosphor and aluminium. Like silicon, they tend to build up in the lime cycle and form an inert load there¬in. For this reason, e.g., phosphor has been removed by taking away part of the lime from the circulation and by replacing it with new lime. Phosphor tends to become concentrated in the fine dust of the lime kiln and, if possible, it has usually been removed in this very form. In some places, fine dust has been sold as phosphor-con¬taining material for soil improvement purposes. An object of the present invention is to provide a method which is simpler and more efficient in comparison with prior art methods for essential reducing the silicon, phosphor, and aluminium contents of cooking liquors. Special attention has been paid to making the above men¬tioned impurities into a readily separable form. In order to accomplish the above identified goals, it is characteristic to the present invention that - the melt obtained from black liquor combustion and containing sodium carbonate is so treated that the sodium carbonate is recovered in solid form, whereas silicon, phosphor and/or aluminium is separated as a solution con¬ taining dissolved sodium silicates, sodium phosphates and/or sodium aluminates; and that - the solid sodium carbonate is dissolved, for forming a solution which has a low silicon, phosphor, and/or alu¬ minium content. Sodium silicates, sodium phosphates and sodium aluminates dissolve almost limitless in water, whereas solubility of sodium carbonate in water is limited. As sodium carbonate is, according to the present invention, brought into a crystal state, it may be separated as pure sodium carbon¬ate from the mother liquor rich in sodium silicate, sodium phosphate, and/or sodium aluminate. After separ¬ation, crystal sodium carbonate is dissolved, and the solution thereby obtained may be treated in a conven¬tional manner in the causticizing plant, for producing sodium hydroxide solution for pulping and other needs. In a conventional chemical regeneration process, melt containing sodium carbonate is dissolved either in water washing of lime mud. The sodium carbonate may be separated from this kind of liquor by evaporating liquor as long as crystallization takes place. Evaporation may be a conventional multi-effect process. After evaporation, crystals are separated by processes known per se, such as filtering, from the mother liquor, which contains soluble sodium silicates, sodium phosphates and sodium aluminates. Sodium carbonate may be separated from readily soluble silicates, phosphates and aluminates also so that readily soluble compounds are leached from the recovery boiler melt to a small amount of liquid, whereby the sodium carbonate, which does not dissolve so easily, stays main¬ly solid. Leaching may be effected, for example, by applying the melt dissolving method developed by Ebara, in connection with a NSSC recovery process (US patents 4,212,702 and 4,141,785; Teder, A., Nordisck Cellulosa, 1984, No. 2, pp. 12-14). In accordance with another method, soda melt may be cooled, if necessary, and grind it thereafter to a desired particle size for leaching. As mentioned earlier, silicate constitutes an especially difficult problem in those pulp mills where pulp is pro¬duced from annual plants. In these cases, the deligni-fication process is typically a soda process, using sodium hydroxide as a cooking chemical. However, the present invention is not limited to a soda process, but it may be correspondingly applied also to processes util¬izing sulphur containing cooking liquors, such as a sul¬phate process. However, in that case the sodium sulphide, which dissolves more readily than the sodium carbonate, ends up in the mother liquor containing sodium silicates. The silicates, phosphates and/or aluminates may, however, be separated from the mother liquor by precipitating them, e.g., with lime or carbon dioxide, thereby obtain¬ing a solution containing sodium sulphide, to be returned to the chemical circulation of the mill. The advantageousness of the method of the present inven¬tion is still more prominent because its "waste", i.e. sodium silicate solution has commercial value. In some cases, the value may be even higher than that of the replacement chemical of the alkali dispharged in separat¬ing the silicon (all sodium carbonate of the melt is not obtained in solid form; but part of it is wasted in solute form). The situation may be advantageous to those mills which, besides soda cooking, produce high pulps or waste paper pulps. Peroxide bleaching of waste paper pulps, namely, utilizes large amounts of sodium silicate as a stabilizer to prevent decomposition of peroxide. FI patent 84190 further discloses a method of recovering alkalis from a used wash fluid. For this reason, earlier described alkali losses could be replaced with alkali received from the recovery process. Silicon may be effi¬ciently removed from a silicate-containing solution poor in carbonate, for example, with calcium oxide, because a causticizing reaction, otherwise obstructing use of cal¬cium oxide for removing silicon from green liquor, does not take place. Thus, the advantageousness of the inven¬tion is not solely bound up with further use of sodium silicate. Also other inorganic impurities may be removed from the mother liquor preferably by "causticizing" with calcium oxide. As a result, a small amount of lime mud is obtained, in which the content of silicon, phosphor, and aluminium is very high. In this way, the amount of lime to be discharged from the cycle may be decreased, and it is possible to produce more valuable lime mud for soil improvement. The greatest advantage of the method of separating sili¬con, phosphate and aluminium in accordance with the in¬vention is the inexpensive operating costs. Separation of silicon by leaching is probably more advantageous, both in terms of operating and investment costs, when compared with crystallizing evaporation. Accordingly, the present invention provides a method of producing a cooking liquor with decreased silicon, phosphor and/or aluminium contents, said process comprising the steps of a) delignifying cellulose-containing material with alkaline cooking liquor to obtain a pulp; b) separating the resulting black liquor from the pulp; c) evaporating and combusting the black liquor to produce a melt containing sodium carbonate, silicon, phosphor and/or aluminium; d) dissolving the melt obtained from black liquor combustion to obtain a solution containing dissolved sodium silicates, sodium phosphates and/or sodium aluminates and recovering the sodium carbonate in solid form or as a solution; e) dissolving the sodium carbonate for forming a solution, which has a low silicon, phosphor and/or aluminium content. The invention will be described in further detail below, by way of example, with reference t6 the accompanying drawings, in which Fig. 1 shows a test result, representing the silica content of clean green liquor, depending of the evaporation degree. Fig. 2 illustrates a preferred embodiment for imple¬menting the method of the invention. Fig. 3 illustrates a second preferred embodiment for implementing the method of the invention, and Fig. 4 illustrates a third preferred embodiment for implementing the method of the invention. Example: In accordance with laboratory tests, the silica content of synthetic green liquor, which contained 48 g SiOj/l, decreased during the advance of crystallizing evaporation in accordance with Fig. 1, which shows the dependency of the Si02 content of the obtained solution on the concen¬tration degree of evaporation, in a solution of separated sodium carbonate crystals. The result was received by analyzing the sodium and sili¬con contents of the crystals separated in the laboratory, and by calculating, on the basis of these, what the sil¬ica content would be in the purified green liquor other¬wise corresponding to that of the initial situation, if all sodium carbonate could be separated. The results indicate that, by evaporating the green liquor to a vol¬ume of about one seventh of the original, an end product is received, which is green liquor having a silica con- tent of about 5 g/1. This value may be still further improved by a more efficient crystals washing. In a laboratory, silica was leached out of a ground melt of sodium silicate and sodium carbonate to a small amount of water having a room temperature. It was found that optimization of the leaching temperature and treating time, it was possible to achieve as low silicon contents as with crystallizing evaporation. Fig. 2 illustrates a process where sodium carbonate is separated from green liquor by crystallizing evaporation. Black liquor is introduced via nozzles 2 into a waste liquor boiler 1, where it is combusted with air 3. Thus, the inorganic substance contained by black liquor, which inorganic substance is mainly sodium carbonate, or sodium carbonate and sodium sulphide, depending on the cooking process, remains as melt at the bottom of the boiler. Therefrom, the melt is fed, in a manner known per se, into a dissolving means where it is dissolved in water or weak liquor from conduit 5 in order to form green liquor. As green liquor contains insoluble impurities, it is normally clarified or filtered in order to remove the green liquor dregs. Filter 6 may preferably be a filter disclosed in international patent application PCT/FI94/-00485. The filtered green liquor is conveyed via conduit 7 to an evaporating means 8. Evaporation is most preferably accomplished in several effects and by using falling film evaporators, in which green liquor flows along the outer surface of the evapor¬ating means, i.e., tube or plate. When evaporating green liquor, it concentrates so that sodium carbonate crystal¬lizes. By regulating the amount of water being removed in evaporation, it is possible to make a desired part of sodium carbonate crystallize. The green liquor containing crystals is discharged from the liquor circulation system of the evaporators via conduit 9 to the crystals separation process, which is in this case effected by filters 10. The filtrate containing silicate, phosphate and/or aluminate iB taken via conduit 11 for potential further use. The sodium carbonate crys¬tals 12 are dissolved in a mixing tank 14 into a liquid, which is in this case condensate 15 taken from condenser 16 of the evaporator. Thus, green liquor 17 is received, which is purer with regard to silicon, phosphor and/or aluminium than the original green liquor from the dis¬solving means 4. This purer green liquor may be caustici-zed in a manner known per se for use in delignification. Alkali may be wasted along with filtrate 11. This alkali is replaced with an alkali addition 13 into the mixing tank 14. Fig. 3 illustrates a second manner of separating solid sodium carbonate. This manner is based on leaching of more easily dissolving compounds, i.e., sodium silicates, sodium phosphates and sodium alurainates into a minor amount of water or into a weak white liquor so that the sodium carbonate, which is the hardest to dissolve, remains solid. The melt is conveyed from the bottom of recovery boiler 1 into a hopper 21, where is becomes into contact with slurry 22, which contains solid sodium carbonate in a solution of sodium silicate, sodium phosphate and sodium aluminate. Thereby, the melt dissolves and solidifies into small particles. The smallest particles pass through a screen 23 and enter a dissolving tank 24, where sodium silicates, sodium phosphates and sodium aluminates are dissolved from particles. The coarsest particles pass on to a second dissolving tank 25, where they are dissolved and whereto water 29 is added for dissolving. The sol- ution overflows from this tank 25 into the first dissolv¬ing tank 24, wherefrom the lime mud containing solid sodium carbonates is taken via conduit 27 into a filter 28 for separating solid crystals from the mother liquor. The sodium carbonate crystals are dissolved into water in tank 30, wherefrom the sodium carbonate solution 31 is taken to be causticized in a manner known per se. The filtrate in conduit 32 from filter 28 may be treated in order to remove silicates, phosphates and aluminates. Calcium oxide or lime milk is added to it from conduit 3 3 in order to precipitate silicates, phosphates, and alum¬inates in a clarifier 34. The clarified silicon, phosphor and/or aluminium free alkali solution 3 5 is returned to the chemical circulation via a conduit 31, and the dregs in conduit 36 containing silicon, phosphor and/or alumin¬ium is taken to further use. In Fig. 4, leaching of melt from the recovery boiler is effected in an alternative manner. The melt is dissolved and cooled with air jets 40 in a cooler 41. The hot air thereby produced is used as combustion air 4 2 in recovery boiler 1. The solidified salt is disintegrated with a crusher 43. It is leached in such an amount of water in a dissolving means 44, that sodium silicates, sodium phosphates and sodium aluminates are dissolved whereas the sodium car¬bonate remains mainly solid. The sodium carbonate is separated with a filter 45. The filtrate in conduit 46, containing silicates, phosphates, and aluminates, is removed for further use, whereas the sodium carbonate is dissolved into water from conduit 48 in the dissolving means. The alkali lost with the filtrate is replaced with an alkali addition 49. The sodium carbonate solution is conveyed via conduit 50 into a causticizing process, as. described earlier. The details of the invention may vary and deviate from the exemplary details described above, within the scope of invention defined by the accompanying claims. WE CLAIM; 1. A method of producing a cooking liquor with decreased silicon, phosphor and/or aluminium contents, said process comprising the steps of a) delignifying cellulose-containing material with alkaline cooking liquor to obtain a pulp b) separating the resulting black liquor from the pulp; c) evaporating and combusting the black liquor to produce a melt containing sodium carbonate, silicon, phosphor and/or aluminium; d) dissolving the melt obtained from black liquor combustion to obtain a solution containing dissolved sodium silicates, sodium phosphates and/or sodium aluminates and recovering the sodium carbonate in solid form or as a solution; e) dissolving the sodium carbonate for forming a solution, which has a low silicon, phosphor and/or aluminium content. 2. The method as claimed in claim 1, wherein the step of dissolving is selected from dissolving in water, dissolving in white liquor obtained from washing of lime mud, leaching with water and leaching with a known aqueous solution. 3. The method as claimed in claims 1 or 2 wherein the solution containing sodium carbonate is evaporated for crystalli2dng sodium carbonate. 4. The method as claimed in claim 3, wherein the water separated in the evaporation is used for dissolving sodium carbonate crystals. 5. A method of producing a cooking liquor with decreased silicon, phosphor and/or aluminium contents substantially as herein described and exemplified with reference to the accompanying drawings.

Documents:

1295-mas-1995 abstract.pdf

1295-mas-1995 claims.pdf

1295-mas-1995 correspondence-others.pdf

1295-mas-1995 correspondence-po.pdf

1295-mas-1995 description(complete).pdf

1295-mas-1995 drawings.pdf

1295-mas-1995 form-1.pdf

1295-mas-1995 form-13.pdf

1295-mas-1995 form-26.pdf

1295-mas-1995 form-4.pdf

1295-mas-1995 form-6.pdf

1295-mas-1995 form-9.pdf

1295-mas-1995 others.pdf

1295-mas-1995 petition.pdf