Title of Invention

A METHOD OF REDUCING THE SILICON CONTENT OF GREEN LIQUOR IN A PULP MILL

Abstract

A method of reducing the silicon content of green liquor in a pulp mill by means of a compound containing a divalent or trivalent cation, in which method waste liquor containing cooking chemicals of an alkaline pulping process and silicon is concentrated in order to increase the dry solids content of the liquor, and the concentrated waste liquor is combusted in order to produce smelt and the smelt is dissolved in order to form green liquor, characterized in that a compound containing a divalent or trivalent cation is added into the waste liquor so that the cation-containing compound reacts in the smelt with silicon in order to form a compound containing said cation and silicon, which compound is separated from the green liquor.

Full Text

METHOD OF REDUCING THE SILICON CONTENT OF GREEN LIQUOR The present invention relates to a method of reducing the silicon content of green liquor in a pulp mill by means of a compound containing a divalent or trivalent cation, in which method waste liquor containing cooking chemicals of an alkaline pulping process and silicon is concentrated in order to increase the dry solids content of the liquor, and the concentrated waste liquor is combusted in order to produce smelt and the smelt is dissolved in order to form green liquor. In alkaline pulping processes such as sulfate and soda processes chemicals are recovered by evaporating water from waste liquor (black liquor) and then combusting the organic matter of the liquor in a recovery boiler in order to release chemicals. The chemicals are discharged from the recovery boiler in the form of smelt which is dissolved in water or weak liquor to produce green liquor. The green liquor contains insoluble or poorly soluble impurities in the form of solid particles which are removed from the green liquor by sedimenting in a clarifier or by filtering. The sodium carbonate of the purified green liquor is causticized to form sodium hydroxide needed for the digestion of pulp by burnt lime (CaO) in the following way. First, the lime is slaked: CaO+H20-»Ca(OH)2 (1) after which the actual causticizing reaction takes place: Ca(OH)2 + Na2C03 -» 2NaOH + CaCCb (2) Calcium carbonate (lime sludge) and the white liquor thus produced containing sodium hydroxide - and sodium sulfide in case of the sulfate process - are separated from each other and the white liquor is circulated to the digestion of pulp. The separation of the white liquor and the lime sludge can be performed in a clarifier or a filter. The lime sludge is regenerated to become calcium oxide in a calcinating device, e.g. a lime sludge rebuming kiln, in order to be returned back to the causticizing. Certain raw materials used in the manufacture of pulp contain great amounts of silica (Si02) in their meristematic tissues. The most common of these raw materials are annual plants such as bamboo, sugar cane, rice and wheat. However, it has been noticed that also certain tropical wood species contain silicon to a harmful extent as regards the pulp manufacturing process. When manufacturing pulp of limbs of these kinds of plants, the silicon acid in a particle form is dissolved in the alkaline cooking liquor and silicate ions are formed. There may also be other sources of silicon. Silicon may enter the process with lime, extraneous siliceous substance on wood material and bleaching chemicals when circulating the bleaching filtrates in the process. The dissolved silica that has ended up in the process causes problems at various stages of the chemical circulation described above. In the evaporation plant the silicates of the waste liquor reach their solubility limit and precipitate as various compounds on the heat transfer surfaces of evaporators and thus, impair the operation of the devices. The silicates form scaling also on the heat transfer surfaces of the recovery boiler and increase the viscosity of the chemical smelt. In the causticizing the silicates precipitate as calcium hydrosilicates in the lime sludge. It has been established that lime sludge that contains considerable amounts of silicate impairs the operation of the lime sludge reburning kiln. Often it is not profitable to combust this kind of lime sludge, but it has to be taken to a landfill site, or only a part of the lime sludge of the mill is combusted and a significant part of the need for lime has to be met by providing fresh lime. Instead of disposing of the impure lime it has been suggested that the silica be separated from black liquor or green liquor by lowering the pH of the liquor to a range of approx. 9.1 - 10.2, e.g. by means of carbon dioxide. The solubility of the silica dissolved in the liquor in an ionic form (mainly as HSiO42- and Si032~) decreases, and the silica precipitates as colloidal silica gel. In practice these kinds of methods have not functioned in a desirable manner. Apparently, the problem has been the poor filterability of the precipitates in a gel form. A chemical can be added to the waste liquor and the green liquor and by help of said chemical the silicon will precipitate as silicates and then be removed from the liquor. Usually the chemical contains a divalent or trivalent cation, by means of which the silicon is precipitated. Japanese patent application no. 60-45692 discloses a method whereby an aqueous solution of a magnesium salt such as MgS04 or an aluminum salt such as A12(S04)3 is added to black liquor or green liquor in order to precipitate silicon from the liquors. A problem with the use of soluble salts in connection with the removal of silicon is the high price of the salts. Furthermore, the dosage of chemicals in relation to the amount of silicon may turn out to-be unprofitable. The lower the silicon content of the substance in which the precipitation takes place, the greater the molar ratio of the precipitating cation and silicon that is required for the precipitation of the silicate compound from the solution. Thus the molar ratio of the precipitating cation to the silicon to be precipitated will be very unprofitable, particularly when attempting to reach the silicon content of less than 1 g/L In connection with the use of an aluminum compound also the content of the dissolved aluminum in the liquor increases, which may cause difficulties e.g. in the evaporation of the waste liquor. Usually at least in green liquor calcium and magnesium have almost or completely reached their solubility limits and thus, there will be no changes in the content of calcium and magnesium. Inexpensive sources of magnesium and aluminum would in practice be the solid compounds of these substances, such as magnesium oxide, dolomitic lime or bauxite. Adding a solid compound as such into waste liquor or green liquor in order to precipitate silicon leads in practice to an even more undesirable molar ratio than the use of soluble salts, or it may lead to unreasonably long reaction times, because in order to come into contact with the silicon, the precipitating cation must first be dissolved from a solid compound. US patent no. 4,331,507 discloses a method whereby bauxite ore is added into black liquor, said bauxite ore being comprised mainly of hydrated aluminum oxide. Thus the soluble silica present in the liquor precipitates as sodalite which is separated from the black liquor. The addition of the aluminum compound is performed before the evaporation or at an intermediate stage of evaporation. The method requires that a reactor and a separator be connected to the evaporation line for the reaction between the aluminum compound and silicon and for the separation of the silicon-containing precipitates formed in the reactor. The problem is that so far, by means of known methods of separating silicon, it has not been economically profitable to remove silicon from liquor so that it would not precipitate at all during the causticizing. When digesting mixed tropical woods the silicon content of the green liquor has been observed to range from 1.0-2.5 g S1O2 /I. Normal white liquor may contain dissolved silica approx. 0.7-0.9 g/I after the causticizing. The rest of the silicon has precipitated in the lime sludge during the causticizing. The used lime sludge is regenerated to form lime and therefore, also the silicon precipitated during the causticizing will return to the subsequent causticizing cycles where the lime is circulated A purpose of the*present invention is to provide a more applicable method in comparison with previous ones, in which method a compound containing a solid, soluble or dissolved divalent or trivalent cation may be used to precipitate silicon in order to reduce the silicon content of the green liquor. Specifically, another purpose is to provide a method, whereby the molar ratio between the precipitating cation compound and silicon in the compound to be precipitated can be brought lower than in prior art methods, while the final content of silicon in the green liquor is the same. Further, yet another purpose is to provide a method of producing green liquor that is as clear as possible and contains as little solid matter as possible. RECTIFIED SHEET (RULE 91) In order to achieve these objectives, it is characteristic of the present invention that a compound containing a divalent or trivalent cation is added into waste liquor so that the cation-containing compound reacts in the smelt with silicon in order to form a compound containing said cation and silicon, which compound is separated from the green liquor. By means of the invention it is possible to produce green liquor having a lower silicon content by using a smaller amount of the precipitating compound than in prior art methods. The effectiveness of the invention is based on the reaction of a compound containing a divalent or trivalent cation with the silicon dissolved in the waste liquor at a high temperature in a combustion device for waste liquor. By utilizing the new method the solid or soluble compound containing said cation is added into the waste liquor just before or during the combustion. The added cation compound reacts in the smelt in the combustion device with the silicon in the smelt and becomes silicate containing the added cation. The smelt is removed from the combustion device and dissolved in e.g. water or weak liquor to form green liquor. Normally the total alkali (TTA) of the green liquor is greater than 115 g/1 as Na20 and the causticizing degree is below 25 %. Insoluble silicate is removed from the green liquor in a method of separation known per se, whereby clean green liquor containing very little silicon is obtained. The silicon content of the green liquor can be reduced considerably by using the method according to the invention. The silicon content of normal green liquor can be brought to a level below 2.5, preferably below 0.9 g Si02/l. These figures are based on measurements of solubility of silicon made in a pulp mill environment. There is no actual limitation to the applicability of the method in connection with diluted liquors. As regards green liquor with a lower alkali content, the most preferable silicon content would be lower, but on the other hand, this content could be reached by means of a lower consumption of the cation than with normal green liquor. For practical reasons, it is usually not worthwhile to dilute green liquor having a normal alkali level for the removal of the silicon. By means of the method according to the invention the amount of the silicon that ends up in the chemical circulation can be reduced considerably. When the silicon content of the green liquor to be causticized is below 0.9 g Si02/1, no silicon accumulates in the lime any more in the causticizing. Also the white liquor formed in the causticizing contains very little silicon and thus, the amount of silicon in the waste liquor formed in the digestion is reduced. When feeding waste liquor into a combustion device, usually to a recovery boiler, it dries quickly and bums in reducing conditions, whereby smelt is formed. The temperature on top of the char bed at the bottom of the recovery boiler is typically 1000-1100 °C. At such a high temperature the silica in the waste liquor and the added cation compound may form a compound where the molar ratio of the cation to the silica is smaller than in compounds formed at lower temperatures. In the method according to the invention a compound is produced having such a stoichiometric property that the consumption of the chemical will decrease in comparison with I previous methods where the compound precipitating silicon is added into the black * liquor or the green liquor and the precipitation of silicates takes place at a temperature of about 80-200 °C. The compounds precipitated at low temperatures are most likely mixtures of hydroxides and hydrosilicates containing great amounts of the precipitating cation, whereas at the temperature of 1000 °C, the cation reacts with the Si02 in the smelt and forms a silicate in which there is less said cation, such silicates being e.g MgSi03, Al203*2Si02 or other corresponding silicate compounds. The method according to the invention is economical because the prevailing conditions in the method allow effective use of a solid cation compound. Preferably the divalent or trivalent cation is magnesium or aluminunL The most preferable sources of these cations are the solid compounds thereof, such as magnesium oxide, dolomite products (CaO*Mg0, CaCCVMgO) or bauxite (main component being A1203«3H20). It is also possible to use cation compounds that dissolve easily in water, such as MgS04, Al2(S04)3 etc. even though the high price of these compounds is a disadvantage. The amount of magnesium used is 2-10 mol, preferably 2.5-5 mol RECTIFIED SHEET (RULE 91) per a precipitated SiCVmol. Preferably the cation is capable of lowering the silicon content of normal green liquor (TTA>115 g/1 as NaO, the causticizing degree below 25 %) to a level below 2.5 g S1O2/I. For instance, with calcium it is not possible to reach such a low value but with a slightly higher silicon content, the use of calcium chemicals may be preferable. The cation compound to be added is brought into the smelt formed of waste liquor preferably by adding it into the waste liquor just before feeding the liquor to the combustion. Preferably the cation compound is added along with ash into the strong waste liquor. The cation compound may be added also during combustion e.g. with the combustion air, or by some other manner of injecting directly into the combustion device. In the method according to the invention the compound formed in the smelt and containing silicon and cation will attempt to dissolve partly in the dissolving tank for green liquor after the combustion device in order to reach a thermodynamic equilibrium but due to the low temperature and the low intensity of the force leading to the dissolution, the compound will not in practice have enough time to dissolve during the time reserved for the dissolution of the smelt and the green liquor purification. The same thermodynamic equilibrium is aimed at also in prior art methods whereby a compound with a divalent or trivalent cation' is added into green liquor (such as MgS(>4, MgO) in order to precipitate Si02 as magnesium silicate. In these known methods the balance is approached in the opposite direction. But then too this equilibrium is slow to achieve and due to the low temperature and the low intensity of the force the equilibrium cannot be folly reached. When using MgO, the process is slowed down also by the solid form of the chemical. The precipitated compounds of silicon and cation and the non-reacted cation compound are removed from the green liquor along with other impurities present in the green liquor preferably by filtering. The filter is preferably the one disclosed in WO 95/12446, marketed by Ahlstrom Machinery Oy under the trademark X-Filter™ or a filter of filter press type utilized in the DreX™ -method, also by Ahlstrom Machinery Oy. Pressurized disc or drum filters known per se may also be used. In the green liquor e.g. magnesium oxide is a difficult compound as regards clarification. It is very fine and therefore, it easily moves to the overflow of the sedimentation in a clarifier. Therefore, according to the present invention the non-reacted compound ending up in the green liquor with the smelt and the silicate compound formed in the smelt are preferably to be separated from the liquor by filtering. Example: Three laboratory tests were performed by using synthetic green liquors containing 175 g/1 of Na2C03 and 23 g/1 of NaOH and small amounts of dissolved silicon that was added as Na2SK)3*5H20. The silicon was precipitated from these liquors by help of magnesium in order to lower the silicon content of the liquors. The amount of the added Mg was adjusted by using different molar ratios in relation to the silicon to be precipitated. In the first tests series the source of the added Mg was magnesium sulfate, MgS04«7H20 that was added into the green liquor as a solution, and in two other series the source was magnesium oxide MgO (pro analysis). At first, smelts were made of the first series of MgO in steel crucibles at the oven temperature of 900 °C. The smelts included all the salts of green liquor and the added MgO. After the melting, the smelt was dissolved in water in order to form green liquor. In another series with MgO, MgO was added in a powder form into the green liquor containing the dissolved salts. In all cases the liquors were allowed to remain at the temperature of 80 °C for 16 hours after which the solid matter in the liquor was removed by filtering. The composition of the silicon-containing precipitate was defined by calculating it from the differences in the solubility of silicon in the individual tests of each test series, when the amount of the addition of magnesium used in each test was known. The results of the tests are presented in the appended figure. It illustrates the ratio of the precipitating cation and the silicon in the resulted precipitate with different silicon contents of the green liquor. A conclusion that may be drawn from the results is that in the method (MgO-smelt) according to the invention, the molar ratio of the precipitating compound and silicon can be brought lower in a precipitating compound with the same final content of silicon in the green liquor than in other known methods (i.e. MgSC>4 and solid MgO), which equals to a smaller amount of the precipitating chemical. Judging from the results, one can also notice that when adding solid fine MgO into the green liquor containing 2 g/I dissolved Si02, MgO is needed in the amount of 11.8 mol/mol of Si02 to be precipitated for lowering the silicon content of the liquor to a level below 0.9 g/l. When using MgS04 the required amount is 4.8 mol/mol. By adding the solid fine MgO into the waste liquor to be fed in the recovery boiler, the required amount is 2.5 mol/mol, which is considerably less than in the known methods (11.8 or 4.8 mol/mol). The result is significant because now, for the first time, one has succeeded in lowering the silicon content of the liquor with a reasonable consumption of a chemical to a level where silicon can no longer accumulate in the lime cycle of the causticizing. What is also verj; significant is that the result is achieved by utilizing a solid cation compound, such as magnesium oxide, whereby it is possible to use inexpensive magnesium-containing lime, dolomite or some other inexpensive source of magnesium for the removal of silicon. When analyzing the green liquor precipitates of a pulp mill using mixed tropical wood as raw material, it was noticed that the ratio of MgO/Si02 was in average 0.93 mol/mol while the S1O2 content of the green liquor was in average 1.3 g/l at the time RECTIFIED SHEET (RULE 91) of the measurement The ratio is very close to what was simulated in the laboratory environment by using synthetic chemicals. In addition to magnesium, also aluminum was taken under examination, aluminum as such dissolving easily into green liquor. Such a great amount of aluminum was detected in the precipitates that the ratio of (AI2O3+MgO) to SiO2 rose to 1.11 mol/mol. Thus it is likely that in addition to magnesium silicates, also aluminum or magnesium silicates have been formed in the smelt. According to the present knowledge calcium cannot precipitate calcium hydrosilicates from normal green liquor when the silicon content of the liquor is this low, i.e. 1.3 g S1Q2/I. The precipitates did not contain considerable amounts of other substances that could possibly precipitate silicate, and therefore, the presence of the silicon in the precipitates can be explained only by the high content of magnesium and aluminum in the precipitates. CLAIMS 1. Method of reducing the silicon content of green liquor in a pulp mill by means of a compound containing a divalent or trivalent cation, in which method waste liquor containing cooking chemicals of an alkaline pulping process and silicon is concentrated in order to increase the dry solids content of the liquor, and the concentrated waste liquor is combusted in order to produce smelt and the smelt is dissolved in order to form green liquor, characterized in that a compound containing a divalent or trivalent cation is added into the waste liquor so that the cation-containing compound reacts in the smelt with silicon in order to form a compound containing said cation and silicon, which compound is separated from the green liquor. 2. Method according to claim 1, characterized in that the compound containing a cation is added into the waste liquor just before feeding the liquor to the combustion. 3. Method according to claim 2, characterized in that the compound containing a cation is added into the waste liquor along with ash. 4. Method according to claim 1, characterized in that the compound containing a cation is added during the combustion. 5. Method according to claim 1, characterized in that the compound containing a cation is a magnesium compound. 6. Method according to claim 1, characterized in that the compound containing a cation is substantially non-water-soluble. 7. Method according to claim 1, characterized in that the compound containing silicon and the cation is separated from the green liquor by filtering. 8. Method according to claim 1, characterized in that the silicon content of the green liquor produced is less than 2.5, preferably less than 0.9 g S1O2/I. 9. Method according to claim 5, characterized in that a magnesium compound is used in the amount of 2-10, preferably 2.5-5 mol Mg/mol of precipitated SiO2 10. Method according to claim 1, characterized in that said cation is some other than a calcium ion. 11. Method of reducing the silicon content of green liquor substantially as hereinbefore described with reference to the accompanying drawings.

Documents:

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