FI107743B - Procedure for reducing the silica content of the green liquor - Google Patents

Description

1 107743

METHOD FOR REDUCING THE PIPE CONTENT OF GREEN PIPE

The present invention relates to a process for lowering the green liquor content of a pulp mill by means of a 2- or 3-value cationic compound, comprising concentrating the cooking chemicals and the siliceous liquor containing the alkali pulping process to increase the solids solids content,

In conventional pulp cooking processes such as sulfate and soda processes, Kemi cabbages are recovered by evaporating water from the waste liquor (black liquor) and then burning the organic matter contained in the liquor in a recovery boiler to liberate the chemicals. From the recovery boiler, the chemicals are removed in a melt which is dissolved in water or weak liquor to form a green liquor. Green liquor contains insoluble or poorly soluble impurities in the form of solid particles which are removed from the green liquor by settling in a clarifier or by filtration. The sodium carbonate contained in the purified green liquor 15 is causticized with lime (CaO), which is burned into sodium hydroxide for pulping. First the lime is extinguished:

CaO + H20 -> Ca (OH) 2 (1) • · • · ·. This is followed by the actual causticization reaction: 20 Ca (OH) 2 + Na 2 CO 3 -> 2NaOH + CaCO 3 (2) • The white liquor and calcium carbonate thus obtained containing sodium hydroxide and in the sulphate process (lime) is separated and the white liquor is recycled into pulp: T: soup. The separation of white liquor and lime sludge can be done in either the clarifier or: 4: filter. The honey is regenerated into calcium oxide in a calciner, such as a 25 lime kiln, to be returned to causticization.

• · · • · • · '*. * Some of the raw materials used to make pulp contain cellulose • · *. *: High in silica (SiO2). The most common of these raw materials are annuals: plants such as bamboo, sugar cane, rice and wheat. But it has also been found that some tropical wood species also contain silicon to the extent of 2,107,743 to the detriment of the pulp production process. In the production of pulp from parts of such plants, the silica in particulate form dissolves in the basic broth as silicate ions. There may also be other hidden sources. For example, silicon can come from lime, wood surfaces and bleaching chemicals when bleaching filtrates are recycled in the process.

The dissolved silica in the process causes problems during the various stages of the chemical cycle described above. At the evaporation plant, the silicates of the waste liquor reach their solubility limits and precipitate as various compounds on the heat surfaces of the evaporators, thus impairing the operation of the equipment. Also in the recovery boiler, silicates form deposits on thermal surfaces and increase the viscosity of the chemical melt. In causticization, the silicates 10 precipitate as calcium hydrosilicates among the lime. It has been found that a significant amount of silicate-containing lime mud interferes with the operation of the lime kiln. Often such lime is not worth burning at all, but must be landfilled, or only part of the mill's lime is burned and a significant amount of lime needs to be satisfied with new lime.

Instead of destroying the impure lime, it has been proposed to separate the silica from the black liquor or the green liquor by lowering the pH of the liquor, for example by means of carbon dioxide, to about 9.1-10.2. In the alkaline ionic form, mostly as ions HS1O43 'and S1O32', the solubility of the dissolved silica is reduced and precipitated as a colloidal silica gel. In practice, such methods have not worked as desired. The problem is the apparent filterability of the gel-like precipitate.

«·· ·· ♦ ·; ***; A chemical can be added to the waste or green liquor with which the silicon precipitates, and ···: ***: can then be separated from the lye. Usually the chemical contains a divalent or trivalent: T: cation by which the silicon is precipitated.

From Japanese Patent Application 60-45692, a method is known whereby an aqueous solution of a magnesium salt such as MgSO 4 or an aqueous solution of an aluminum salt such as A 12 (SO 4> 3) is added to black liquor or green liquor. for precipitating silicon from liquids.

r- · *:: The problem with using soluble salts for silicon removal is their high cost. In addition, • · ·: * the chemical dose may be disadvantageous in relation to the amount of silicon to be removed.

The lower the silicon precipitation is, the higher the molar ratio of precipitating *: * *: cation to silicon requires from the solution to precipitate the silicate compound. Thus, especially when tending to silicon contents below 1 g / l, the molar ratio of precipitating cation to precipitated silicon becomes very disadvantageous. In addition, when using aluminum compound 3 107743, the soluble aluminum content of the lye increases, which may be detrimental, for example, to the evaporation of the waste liquor. Calcium and magnesium are usually at least within the saturation limit at least in the green liquor, so that there is no change in their concentration.

In practice, cheaper sources of magnesium or aluminum would be solid compounds of these materials, such as magnesium oxide, dolomite lime or bauxite.

Addition of the solid compound as such to precipitate the waste or green liquor will in practice result in an even more unfavorable molar ratio than the use of soluble salts or excessively long reaction times, since the precipitating cation must first dissolve from the solid to reach the silicon.

U.S. Patent No. 4,331,507 describes a process for adding bauxite ore, which consists primarily of hydrated alumina, to black liquor. In this case, the soluble silica in the lye precipitates as wariteite, which is separated from the black liquor. Addition of the aluminum compound to the black liquor is carried out before or during the intermediate evaporation. The process requires connecting the reactor and the separator to the evaporator line 15 for the reaction between the aluminum compound and the silicon and to separate the silicon-containing precipitate generated in the reactor.

The problem is that the silicon separation methods known to date have not, economically, been profitable to remove silicon from the alkali so that it is virtually non-existent. * precipitated in causticization. When tropical mixed trees are cooked, the green liquor has a silicon content ··· '"I> 20 of about 1.0 to 2.5 g SiO 2/1. In normal white liquor, silica may be · · causticized after ca. 0.7-0.9 g / 1. Lime sludge precipitated during ·. * · * The spent lime sludge is regenerated to lime, which means that the silicon precipitated with the sludge along with the lime sludge also returns to the following lime sludges for recycling.

25 • · · *. *: *: It is an object of the present invention to provide a more usable '. a process in which solid, soluble, or pre-solubilized 2- or 3-value cations • ·, ···. containing compound can be used to precipitate silicon to reduce the • content of green liquor. In particular, it is intended to provide a process wherein the molar ratio of precipitating cationic compound to silicon in the precipitating compound is • · lower than in known methods when the final concentration of silicon in the 4,107,773 green liquor is the same. A further object is to provide a process for obtaining as little clear solids as possible inside green liquor.

To accomplish these purposes, the present invention is characterized in that a cation containing a 2- or 3-value cation is added to the spent liquor so that the cation containing compound reacts with the silicon in the spent liquor incinerator to form a cation and silicon containing compound which is separated from the green liquor.

According to the present invention, low-silicon green liquor can be produced at a lower amount of the precipitating compound than in the known processes. The effectiveness of the invention is based on the reaction of the 2- or 3-membered cation containing compound with silicon soluble in the waste liquor at high temperature in the waste liquor incinerator. According to a new method, a solid or soluble cation-containing compound is added to the waste liquor immediately before or during incineration. The cation compound added in the melt in the incinerator reacts with the silicon in the melt 15 to form the added cation-containing silicate. The melt is removed from the incinerator and dissolved in, for example, water or weak liquor as green liquor. Normally, the total alkali of the green liquor (TTA) is greater than 115 g / l Na 2 O and the degree of causticization is less than 25%. The insoluble silicate is removed from the green liquor by a separation process known per se, to obtain pure, low silicon lye. By the method of the invention 20, the silicon content of the green liquor can be significantly reduced.

• ·. The silicon content of normal green liquor can be obtained to a level below 2.5, most preferably ····: * * *: less than 0.9 g SiO2 / 1. These concentrations are based on silicon made in the pulp mill environment ···; * * *: for solubility measurements. There is no real obstacle to the operation of the method even in dilute liquids. With an alkaline dilute lye, the most preferred concentration of T: 25 mentioned above would be lower, but on the other hand, this concentration would be achieved with lower cation consumption than with normal lye. However, for practical reasons, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·

λ · • *

The process of the invention can significantly reduce the amount of chemical silicon that is • · - '· * 30 circulated. When causticizing green liquor has a content of less than 0.9 g • · · * · * j S1O2 / I, causticization no longer accumulates in the lime. Also, the white liquor formed during causticization contains a small amount of silicon, thus reducing the silicon load of the waste liquor produced during cooking.

5, 107743

When the waste liquor is fed to the incinerator, usually a recovery boiler, it dries quickly and returns under reducing conditions to produce a melt. At the bottom of the recovery boiler, the temperature of the fusion furnace is typically 1000 to 1100 ° C. At such a high temperature, the waste liquor silica and the added cation compound can form a compound,

Wherein the molar ratio of cation to silica is lower than that of compounds formed at lower temperatures. Thus, the process of the invention provides a compound having a stoichiometry such that the chemical consumption is reduced as compared to the previous methods wherein the silicon precipitating compound is added to the black or green liquor and the silicates precipitate at about 80-200 ° C.

10 temperature. Low temperature precipitating compounds are most likely a mixture of hydrosilicates and hydroxides containing a high precipitating cation, whereas at 1000 ° C, the cation reacts with the molten SiCl ^ to form a less cationic silicate such as MgSiC? 3, Al2C? silicate compounds.

The process according to the invention is very advantageous because, due to the conditions therein, a solid cationic compound can be effectively used. Preferably, the 2 or 3 value cation is magnesium or aluminum. The most preferred sources of these cations are their solid compounds such as magnesium oxide, dolomite products (CaO-MgO, CaCO3-MgO) or bauxite (major component Al2O3 3H2O). Cation-soluble cationic compounds readily soluble in water can also be used, such as MgSO 4, Al 2 (SO 4> 3, etc.), although the disadvantages of these compounds are their high cost. 10 moles, preferably 2.5 to 5 moles, per mole of precipitated SiO2> The cation is preferably · · · · capable of lowering normal green liquor (TTA> 115 g / l in Na NaO, · · ·, causticization degree less than 25%). ) with a silicon content of less than 2.5 g S1O2 / I, for example, with calcium, it is not possible to achieve this, but at slightly higher silicon concentrations, the use of calcium chemicals may be advantageous.

Most preferably, the cation compound to be added is included in the molten liquor from the waste liquor by adding it to the waste liquor immediately before the liquor is fed to the incinerator.

Preferably, the cationic compound is added together with the ash to the strong waste liquor.

• *: 30 The cationic compound can also be added during combustion, eg with combustion air or other • · •: · *: direct injection into the incinerator.

g, 107743

In the process of the invention, the molten silicon and cation containing compound tends to partially dissolve in the downstream liquor solvent to achieve a thermodynamic equilibrium, but due to the low temperature and low dissolution force, it is virtually impossible to dissolve the molten liquor and lye.

The same thermodynamic equilibrium is also sought in the known methods of adding a 2- or 3-value cation-containing compound, such as MgSO 4 or MgO, to green liquor to precipitate SiO 2 as magnesium silicate. In these known cases, the equilibrium state is only approached from the opposite direction. Here too, achieving the equilibrium state 10 is due to the low temperature and the weakness of the driving force, and the retarder equilibrium cannot be fully achieved. When using MgO, the process is further slowed down by the solid state of the chemical.

The green liquor is removed by filtration, preferably with filtration, of the precipitated silicon and cation compound and the unreacted cation compound together with other green liquor impurities. Preferably, the filter is a filter described in FI patent application 941194, sold by Ahlström Machinery Oy as X-Filter ™, or a filter press type filter used in the same company's DreX ™ process. Pressurized disk or drum filters known per se may also be used. Green liquor; e.g., magnesium oxide is a compound which is difficult to clarify. Finely divided, it • · ·· • · 20 easily passes over the clarifier. Thus, according to the present invention, the unreacted compound entrained in the molten green liquor and · · ·. ***. the silicate compound formed is most preferably separated from the lye by filtration.

··· ·· · • · «

Example: • · · * · *

In the laboratory, three experiments were performed with synthetic green liquors containing 175 g / l ··· 1 25 Na 2 CO 3 and 23 g / 1 NaOH and minor amounts of dissolved silicon added as • ·· * Na 2 SiO 3, 5H 2 O. Of these Spectacular silicon was chelated with magnesium to reduce the Spectacular *: **: silicon content. Mg addition was tested at various molar ratios of silicon to be sheared.

** In the first set of experiments, the Mg source added was magnesium sulfate, MgSO 4 * 7H 2 O, which was added as a solution in green liquor and two other hundred magnesium oxide, MgO (pro analysis). The first MgO-Saija was first melted in steel cups 7107743 at 900 ° C. The digests contained all the salts of the lye and added MgO. After thawing, the melt was dissolved in water to form a green liquor. In another series of MgO, MgO was added as a powder to the green liquor containing the dissolved salts. In all cases, the lye was allowed to stand at 80 ° C for 16 h. After which time, the solids contained in the lye were filtered off. The composition of the silicon-containing precipitate was determined by calculating the differences in the solubility of silicon between the individual experiments of each testqa, as the magnesium addition used in each experiment was known. The results of the experiments are shown in the figure below. It shows the ratio of precipitating cation to silicon in the resulting precipitate at different concentrations of green liquor. From the results, it can be seen that the molar ratio of precipitating compound to silicon in the process of the invention (MgO molten) is lowered in the precipitating compound than in known methods (MgSO 4 and MgO solid) with the same concentration of silicon in green liquor.

From the results, it can be further determined that when soluble fine MgO dissolved in green liquor containing 15 g of SiO 2 is added, 11.8 mol / mol precipitated S1O2 is required to lower the alkali silicon content to less than 0.9 g / l. For MgSCV this amount is 4.8 mol / mol. By adding a solid finely divided MgO to the waste liquor fed into the recovery boiler, this amount is 2.5 mol / mol, which is considerably less than in known methods (11.8 or 4.8 mol / mol).

• ♦ :. *: 20 The result is significant as it is the first time that the silicon content of the lye has been obtained,. '*' Reduced with moderate chemical consumption to a level where silicon no longer • · · • ... · accumulates in the causticization lime cycle.

• · · • · ♦ · • · ·. * I *; It is also very significant that the result has been achieved using a solid cationic compound such as magnesium oxide, whereby it is possible to use 25 cheap magnesium-containing lime, dolomite or other inexpensive source of magnesium for the silicon removal.

··· • · · · · «

Analyzing the green liquor precipitates of a pulp mill using an Indonesian tropical mixed wood as a raw material, the average Mg0 / SiO2 ratio of the precipitate was found to be 0.93 mol / mol with a SiC> 2 concentration of green liquor at the time of sampling. ·: 30 1.3 g / l. The ratio is very close to what was achieved under the laboratory conditions by synthetic chemicals.

8 107743

When considering not only magnesium but also aluminum, which as such is highly soluble in green liquor, but found in such significant amounts in the fines, the (Al 2 O 3 + MgO) / SiO 2 ratio increased to 1.11 mol / mol. Therefore, it is likely that not only magnesium silicates but also aluminum or magnesium aluminum silicates are formed in the salt melt.

To our present knowledge, calcium is not capable of precipitating calcium hydrosilicates from normal green liquor at such low levels (1.3 g SiO 2/1). The precipitates did not contain significant amounts of other silicate precipitating agents, so that the presence of silicon in the fines was only explained by the high magnesium and aluminum content of the fines.

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Claims (10)

A process for lowering the silicon content of the pulp mill's green liquor by means of a compound containing a 2- or 3-host cation, in which a liquor containing boiling chemical and silicon process from an alkaline pulp preparation process is concentrated to increase its dry matter, the concentrated effluent is combusted to produce a melt and the melt dissolved to form green liquor, characterized in that the compound containing a 2- or 3-host cation is added to the effluent so that the cation containing the compound reacts with silicon in the furnace's melt. to form a compound containing said cation and silicon, which compound is separated from the green liquor. Process according to Claim 1, characterized in that the cation-containing compound is added to the effluent just before the liquor is measured for combustion. 15 Process according to Claim 2, characterized in that the cation-containing compound is added to the effluent together with ash. 4. A process according to claim 1, characterized in that the cation-containing compound is added to the effluent during combustion. 5. A process according to claim 1, characterized in that the cation containing: * ·: the compound is a magnesium compound. • · • · · «· · · • · · 25 6. A process according to claim 1, characterized in that the cation-containing compound is essentially insoluble in water. • # ♦ • · · • c 7. A method according to claim 1, characterized in that the compound containing silicon and cation is separated from the green liquor by filtration. • · • · · ** 30 8. A method according to claim 1, characterized in that the silicon content of the green liquor produced in the ·· · · · · · · · is below 2.5, preferably below 0.9 g SiO2 / 1. · · · · · · · · ♦ ♦ ♦ ♦ ♦ ♦ ♦ • · «·· • ·« · ··· 12 107743 9. A process according to claim 5, characterized in that the amount of magnesium compound used is 2-10, preferably in 2.5-5 mol Mg / mol precipitated SiO 2. 5 10. A process according to claim 1, characterized in that the active cation is other than a calcium ion. • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · Il • · «· • Il