CN1335905A - Method of reducing the silicon content of green liquor - Google Patents
Method of reducing the silicon content of green liquor Download PDFInfo
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- CN1335905A CN1335905A CN00802578A CN00802578A CN1335905A CN 1335905 A CN1335905 A CN 1335905A CN 00802578 A CN00802578 A CN 00802578A CN 00802578 A CN00802578 A CN 00802578A CN 1335905 A CN1335905 A CN 1335905A
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- Prior art keywords
- silicon
- liquor
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- compound
- green liquor
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- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
- D21C11/0092—Substances modifying the evaporation, combustion, or thermal decomposition processes of black liquor
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- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/10—Concentrating spent liquor by evaporation
- D21C11/106—Prevention of incrustations on heating surfaces during the concentration, e.g. by elimination of the scale-forming substances contained in the liquors
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P40/00—Technologies relating to the processing of minerals
- Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills
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- Paper (AREA)
- Compositions Of Oxide Ceramics (AREA)
- Separation Of Suspended Particles By Flocculating Agents (AREA)
Abstract
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. An essential characteristic of the invention is 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 a cation and silicon, which compound is separated from the green liquor.
Description
The invention relates to a method for reducing the silicon content of green liquor (green liquor) in a pulp mill by means of a compound containing divalent or trivalent cations, by concentrating spent liquor containing cooking chemicals and silicon in alkaline pulping to increase the solids dry content of the liquor, and burning the concentrated spent liquor to form clinker, and dissolving the clinker to form green liquor.
In alkaline pulping, such as sulfate and soda processes, chemicals are recovered by evaporating water from the spent liquor (black liquor) and then combusting the organic matter in the solution in a waste heat boiler to precipitate the chemicals. These chemicals are discharged from the waste heat boiler as a clinker which is soluble in water or weak lye to produce green liquor. The green liquor contains insoluble or poorly soluble impurities in the form of solid particles, which can be removed from the green liquor by sedimentation in a clarifier or by filtration. Sodium carbonate in the refined green liquor is causticised to form the sodium hydroxide required for digestion of the pulp with quicklime (CaO) in the following manner. First, the lime is slaked:
the white liquor of calcium carbonate (lime sludge) and sodium hydroxide formed therefrom, and sodium sulfide in the case of kraft pulping, is separated from one another and recycled to the pulp digestion. The separation of the white liquor and lime slime can be completed by a clarifying tank or a filter. The lime sludge can be regenerated in a calcination apparatus, such as a lime sludge reburning furnace, to become calcium oxide, which is returned to causticization.
Some raw materials used in the manufacture of pulp contain large amounts of silicon dioxide (SiO) in their meristems2). The most common of these raw materials are annual plants such as bamboo, sugar cane, rice and wheat. However, it has been noted that also some tropical wood species contain silicon to the extent that it is detrimental to the pulping process. When pulp is produced from these kinds of plant branches, particulate typeSilicic acid is dissolved in the alkali-digested solution to form silicate ions. Other sources of silicon are also possible. Silicon may be fed into the process together with lime, wood-independent silicic acid substances and bleaching chemicals when the bleaching filtrate is recycled in the process.
The dissolved silica at the end of the process can cause problems at various stages of the chemical cycle as described above. Some of the silicates in the waste liquid in the evaporator unit reach their solubility limit and precipitate as various compounds on the heat transfer surface of the evaporator, impairing the operation of the equipment. These silicates also form scale on the heat transfer surfaces of the waste heat boiler and increase the viscosity of the chemical clinker. During causticisation, these silicates precipitate as hydrated calcium silicate salts in lime sludge. Lime sludge having a high silicate content has been shown to interfere with the operation of lime sludge reburning furnaces. Burning such lime sludge is often not advantageous but has to be sent to a landfill or only a part of the mill lime sludge is burned, and a large part of the demand for lime has to be met by supplying fresh lime.
It has been proposed to separate silica from black or green liquor without discarding this impure lime, by lowering the pH of the solution with carbon dioxide to a range of about 9.1-10.2. In the ionic form (mainly HSiO)4 3-And SiO3 2-) The solubility of silica dissolved in the solution decreases, and the silica precipitates as a colloidal silica gel. In practice these methods do not work as well. Apparently, poor filterability of gel-type precipitates has been problematic.
This waste liquor and green liquor may be supplemented with chemicals by means of which silicon is precipitated as silicates and then removed from the liquor. Typically such chemicals contain divalent or trivalent cations that precipitate the silicon.
Japanese patent application No.60-45692 discloses the addition of a magnesium salt such as MgSO4Or aluminium salts such as Al2(SO4)3To black liquor or green liquor, precipitating silicon therein.
One problem with using soluble salts associated with the removal of silicon is that these salts are expensive. In addition, the amount of chemical reagent used, which is related to the silicon content, may also be turned to disadvantageous. The lower the silicon content of the material undergoing precipitation, the greater the molar ratio of precipitating cations to silicon required to precipitate the silicate compound from solution. Thus, the molar ratio of the precipitating cations to the silicon to be precipitated can be very disadvantageous, especially when trying to achieve silicon contents below 1 g/l. In connection with the use of aluminum compounds, the content of dissolved aluminum in the solution also increases, which may lead to difficulties such as evaporation of the waste liquid. Generally, calcium and magnesium almost or already reach their solubility limits at least in green liquor, so that the calcium and magnesium content does not change.
Inexpensive sources of magnesium and aluminum are virtually always solid compounds of these materials, such as magnesia, high-magnesium lime or bauxite.
The addition of solid compounds to the waste liquor or green liquor to precipitate the silicon in this way leads in practice to less than optimal molar ratios than with soluble salts or to unreasonably long reaction times, since the precipitating cations must first be dissolved from the solid compounds in order to be brought into contact with the silicon.
US 4,331,507 discloses a method of adding bauxite ore, which consists essentially of hydrated alumina, to black liquor. Thus, the soluble silica present in the solution precipitates as sodalite which can be separated from the black liquor. The addition of the aluminium compound is done before or at an intermediate stage of evaporation. This process requires that the reactor and the separator are in communication with the evaporation line for the reaction between the aluminium compound and the silicon and for separating the precipitate containing silicon formed in the reactor.
The problem is that hitherto it has been economically disadvantageous to remove silicon from solution by means of known methods for separating silicon, so that it should not precipitate at all during causticisation. In digesting mixed tropical wood, it has been observed that the silicon content of the green liquor is between 1.0 and 2.5 grams SiO2In the range of/liter. The causticized normal white liquor may contain about 0.7-0.9 g/l of dissolved silica. The rest of the silicon is precipitated in the causticizing periodLime is used as the clay. The spent lime sludge is regenerated to produce lime, and thus the silicon precipitated during causticisation is also returned to the subsequent causticisation cycle of recycled lime.
The object of the present invention is to provide a process which is more suitable than the known processes and in which the silicon content of the green liquor is reduced by precipitating silicon by means of a compound containing solid, soluble or dissolved divalent or trivalent cations. In particular, another object of the invention is to provide a process which allows a lower molar ratio between the precipitating cationic compound and the silicon in the precipitated compound than in the prior art, while the final silicon content in the green liquor is the same. Furthermore, it is a further object to provide a process for producing a green liquor which is as transparent as possible and which contains as little solid matter as possible.
To achieve these objects, the invention is characterized in that a compound containing a divalent or trivalent cation is added to the waste liquor, and the cation-containing compound is reacted with silicon in the clinker to form a compound containing said cation and silicon which is separable from the green liquor.
With the process of the invention it is possible to produce green liquor with a lower silicon content than the prior art processes with a smaller amount of precipitating compounds. The effectiveness of the invention is based on the high temperature reaction of a compound containing divalent or trivalent cations with silicon dissolved in the spent liquor in a spent liquor combustion plant. With this new process, the solid or soluble compound containing the cation is added to the waste stream before or during combustion. The added cationic compound reacts with the silica in the clinker in the combustion device to become silicate containing the added cations. The clinker is removed from the combustion apparatus and dissolved, for example, in water or weak lye to form a green liquor. Typically, the total alkali metal content (TTA) of the green liquor is as Na2The O content is more than 115 g/L, and the causticity degree is less than25%. The insoluble silicate is removed from the green liquor using separation methods known per se to give a clean green liquor containing little silicon. The method of the invention can obviously reduce the silicon content of the green liquor. The silicon content of the normal green liquor can reach 2.5 g Si2O/l or less, preferably 0.9 g Si2O/liter or less. These numbers areBased on the silicon solubility as determined in the pulp mill environment. The method is relevant to the solution to be diluted, and the applicability thereof is practically unlimited. For green liquors with low alkali levels, the most preferred silicon level will be lower, but on the other hand such a silicon level can be achieved with less cation consumption than normal green liquors. For practical reasons, it is generally not worth diluting the green liquor at normal alkaline levels for silicon removal.
According to the method of the invention, the amount of silicon at the end of the chemical cycle can be significantly reduced. When the silicon content of the causticized green liquor is 0.9 g Si2Below O/liter, silicon in lime does not accumulate during causticization. Furthermore, the white liquor formed in causticization contains very little silicon, thus reducing the amount of silicon in the digestion-forming effluent.
When the spent liquor is fed to a combustion plant, typically a waste heat boiler, the spent liquor is rapidly dried and combusted under reducing conditions, thereby forming a clinker. The top surface temperature of the char bed at the bottom of the waste heat boiler is typically 1000-1100 ℃. In such high temperature waste streams, the silica and added cationic compound may form a compound having a lower cation to silica molar ratio than the compound formed at the lower temperature. According to the process of the invention, a compound is produced which acts as a precipitate of silicon, which compound has the stoichiometric characteristics that, when added to black or green liquor, it reduces the consumption of chemicals compared to the prior art processes and allows the precipitation of silicates at temperatures of about 80-200 ℃. The compound precipitated at low temperature is most likely a mixture of hydroxide and hydrated silicate containing a large amount of precipitating cations which are bound to the SiO in the frit at a temperature of 1000 deg.C2Reacting to form a silicate in which the cations are less abundant, such silicate being e.g. MgSiO3、Al2O3·2SiO2Or other corresponding silicate compounds.
The process according to the invention is economical, since the main conditions of the process allow an efficient use of the solid cationic compound. Superior foodOptionally, the divalent or trivalent cation is magnesium or aluminum. The most preferred sources of these cations are solid compounds thereof, such as magnesium oxide, dolomite products (CaOMgO, CaCO)3MgO) or bauxite (Al as the main component)2O3·3H2O). In addition, it is also possible to use cationic compounds which are readily soluble in water, such as MgSO4、Al2(SO4)3Etc., although these compounds are expensive and disadvantageous. The amount of magnesium used is 2 to 10 moles, preferably 2.5 to 5 moles, per mole of precipitated silica. Preferably, the cation is capable of reducing the silicon content of normal green liquor (TTA>115 g/l, causticity below 25% in terms of NaO) to 2.5 g SiO2At levels below the liter. For example, such low silicon content cannot be achieved with calcium, but will be slightly higher, and use of calcium chemistry may be preferred.
The added cationic compound causes the spent liquor to form a clinker, which is preferably added to the spent liquor immediately beforethe liquor is sent to combustion. Preferably, the cationic compound is added to the concentrated waste stream together with the ash. The cationic compound may also be added during combustion, such as with combustion air, or injected directly into the combustion apparatus by other means.
According to the method of the invention, the compound formed in the clinker and containing silicon and cations is dissolved as partially as possible in the green liquor dissolution tank after the combustion plant in order to achieve a thermodynamic equilibrium, but the low temperature results in a low dissolution driving force, so that the compound does not actually have sufficient time to dissolve during the predetermined time of dissolution of the clinker and purification of the green liquor.
The prior art has resorted to the addition of compounds having divalent or trivalent cations to green liquor (such as MgSO)4MgO) to precipitate SiO2The process for magnesium silicate is also directed to the same thermodynamic equilibrium. In these known methods, the equilibrium is approached in the opposite direction. However, this equilibrium is only achieved slowly and not sufficiently due to the low temperature and low driving force. When MgO is used, this process is also due to the chemical agent being in the solid stateBut is slowed down.
Both the precipitated silicon and cationic compounds and unreacted cationic compounds are removed from the green liquor, preferably by filtration, together with other impurities present in the green liquor. The Filter is preferably one of the filters disclosed in WO 95/12446 and is available under the registered trade name X-Filter from Ahlstrom mechanical oxygenTMSold or a type in DreXTMFilter press type filters used in the process, also sold by Ahlstrom Machinery Oy. The pressure disk or drum vacuum filters known per se can also be used. In green liquor,magnesium oxide, for example, is a poorly clarified compound. It is a very fine powder and therefore tends to flow out with the settled supernatant in the clarifier. Thus, according to the invention, the last unreacted compounds in the green liquor are preferably separated from the solution by filtration together with the clinker and the silicate compounds formed in the clinker.
Examples
Utilizes 175 g/l of Na2CO323 g/l NaOH and a small amount of Na2SiO3·5H2O added synthetic green liquor of dissolved silicon, three laboratory tests were completed. Magnesium is used to precipitate silicon from these solutions to reduce the silicon content of the solution. The amount of Mg added is adjusted by different molar ratios of precipitated silicon.
In the first test series, the Mg source added was magnesium sulfate, MgSO4*7H2O, was added as a solution to the green liquor, while in the other two series of tests the Mg source added was magnesium oxide MgO (according to analysis). First, a frit was made from a first series of MgO at an oven temperature 900 in a steel crucible. The frit comprises all salts in green liquor and added MgO. After the fusion cake is melted, the fusion cake is preparedThe frit was dissolved in water to form green liquor. In another series of tests using MgO, MgO was added in powder form to green liquor containing the dissolved salts. In all cases, the solution was kept at a temperature of 80 ℃ for 16 hours, after which the solid material in the solution was filtered off. The silicon-containing precipitate is composed ofDetermined by calculation from the difference in solubility of silicon in each test series when the amount of magnesium added is known in each test. The test results are shown in the attached figures. The figure illustrates the ratio of cations to silicon that precipitated in the resulting precipitate at different silicon contents in the green liquor. From these results, it can be concluded that, according to the process of the invention (MgO-frit), in the precipitating compound of green liquor of the same final silicon content, the molar ratio of precipitating compound to silicon can be made lower than in the other processes known (i.e. MgSO. RTM.)4And solid MgO), i.e., less chemical agent in an amount equivalent to precipitation.
From these results, it can also be seen that when SiO is dissolved in a solution containing 2 g/l2When MgO, which is a solid powder, is added to the green liquor of (1), SiO precipitated from MgO is required2The amount was 11.8 moles/mole to reduce the silicon content of the solution to a level below 0.9 grams/liter. When using MgSO4In this case, the amount of the precipitate required was 4.8 mol/mol. The addition of solid powdered MgO to the waste stream fed to the waste heat boiler required a precipitation level of 2.5 mol/mol, which is much lower than (11.8 or 4.8 mol/mol) of the known process.
This result is significant because the silicon content of such solutions is now successfully reduced for the first time and the chemical reagents are consumed reasonably enough that silicon can no longer accumulate during the lime causticization cycle.
It is also important to achieve this result by using a solid cationic compound such as magnesium oxide, thereby making it possible to remove silicon using inexpensive magnesium-containing lime, dolomite or other inexpensive magnesium source.
When analyzing pulp mill green liquor deposits using mixed tropical wood as a feedstock, it has been found that MgO/SiO2The ratio of (A) was 0.93 mol/mol on average, while green liquor SiO was measured2The content was 1.3 g/l on average. Thisratio is very close to the ratio simulated in the laboratory with synthetic chemicals.
In addition to magnesium, aluminum was also used in the investigation, which also dissolves readily in this green liquor. A large amount of aluminum (Al) was detected in the precipitate2O3+ MgO) to SiO2The ratio of (A) to (B) is as high as 1.11 mol/mol. It is therefore likely that, in addition to magnesium silicate, aluminium or magnesium silicate is formed in the frit.
According to the prior knowledge, when the silicon content of the solution is so low, i.e. 1.3 g SiO2Calcium in each literThe calcium silicate hydrate cannot be precipitated from the normal green liquor. These precipitates do not contain significant amounts of other substances which can precipitate silicates, and therefore the presence of silicon in the precipitates can only be explained by the high magnesium and aluminium content of the precipitates.
Claims (10)
1. A method for reducing the silicon content of green liquor in a pulp mill by means of a compound containing divalent or trivalent cations, in which method spent liquor containing cooking chemicals and silicon in alkaline pulping is concentrated to increase the dry solids content of the liquor, and the concentrated spent liquor is combusted to form clinker, which clinker is then dissolved to form green liquor, characterized in that a compound containing divalent or trivalent cations is added to the spent liquor, and the cation-containing compound reacts with silicon in the clinker to form a compound containing said cations and silicon, which compound can be separated from the green liquor.
2. A method according to claim 1, characterized in that said compound containing a cation is added to the waste liquor immediately before the liquor is sent to combustion.
3. A method according to claim 2, characterized in that said compound containing a cation is added to the waste liquid together with the ashes.
4. A method according to claim 1, characterized in that said compound containing a cation is added during the combustion.
5. A process according to claim 1, characterized in that said compound containing a cation is a magnesium compound.
6. A method according to claim 1, characterised in that said compound containing a cation is substantially water-insoluble.
7. A process according to claim 1, characterized in that said compounds containing silicon and said cations are separated from the green liquor by filtration.
8. A process according to claim 1, characterized in that the green liquor produced has a silicon content of less than 2.5 g SiO2Per liter, preferably less than 0.9 g SiO2And/liter.
9. A process according to claim 5, characterized in that the magnesium compound is used in an amount of 2-10 moles Mg per mole of precipitated SiO2Preferably 2.5-5 moles Mg per mole precipitated SiO2。
10. A method according to claim 1, characterized in that said cation is a cation other than calcium.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI990042 | 1999-01-12 | ||
| FI990042A FI107743B (en) | 1999-01-12 | 1999-01-12 | Procedure for reducing the silica content of the green liquor |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN1335905A true CN1335905A (en) | 2002-02-13 |
| CN1143024C CN1143024C (en) | 2004-03-24 |
Family
ID=8553314
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CNB008025789A Expired - Fee Related CN1143024C (en) | 1999-01-12 | 2000-01-07 | Method of reducing the silicon content of green liquor |
Country Status (4)
| Country | Link |
|---|---|
| CN (1) | CN1143024C (en) |
| FI (1) | FI107743B (en) |
| ID (1) | ID29518A (en) |
| WO (1) | WO2000042251A1 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102041703A (en) * | 2010-12-02 | 2011-05-04 | 陕西科技大学 | Process for removing silicon in green liquor of papermaking alkali recovery section by seeding |
| CN102094346A (en) * | 2010-12-02 | 2011-06-15 | 陕西科技大学 | Acid method silica removal process for green liquor in alkali recovery section of papermaking |
| CN102619123A (en) * | 2012-03-16 | 2012-08-01 | 陕西科技大学 | Method for removing silicon from green liquor through synergism of calcium oxide, carbon dioxide and aluminum-containing compound |
| CN103469664B (en) * | 2013-08-26 | 2015-09-16 | 陕西科技大学 | A kind of alkali collection workshop section green liquor flocculation silicon removing method |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115652676B (en) * | 2022-11-17 | 2023-09-26 | 陕西科技大学 | Green liquid silicon removal process for pre-stage silicon removal process |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4331507A (en) * | 1979-12-10 | 1982-05-25 | Dorr-Oliver Incorporated | Desilication in alkaline pulp processes |
| US4347102A (en) * | 1981-04-02 | 1982-08-31 | Combustion Engineering, Inc. | Elimination of potassium compounds from sodium-based pulped cycles |
| JPS6045692A (en) * | 1983-08-18 | 1985-03-12 | 石川島播磨重工業株式会社 | Removal of silica from pulp black liquor and green liquor |
| DK154659C (en) * | 1986-11-03 | 1989-05-08 | Fredericia Cellulosefabrik Akt | PROCEDURE FOR REDUCING SILICON CONTENT IN BLACK CELLULOSE MANUFACTURING |
| SE9401663L (en) * | 1994-05-13 | 1995-06-12 | Kvaerner Pulping Tech | Procedure for precipitating magnesium with aluminum during green clearing |
-
1999
- 1999-01-12 FI FI990042A patent/FI107743B/en active
-
2000
- 2000-01-07 WO PCT/FI2000/000009 patent/WO2000042251A1/en active Application Filing
- 2000-01-07 ID IDW00200101719A patent/ID29518A/en unknown
- 2000-01-07 CN CNB008025789A patent/CN1143024C/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102041703A (en) * | 2010-12-02 | 2011-05-04 | 陕西科技大学 | Process for removing silicon in green liquor of papermaking alkali recovery section by seeding |
| CN102094346A (en) * | 2010-12-02 | 2011-06-15 | 陕西科技大学 | Acid method silica removal process for green liquor in alkali recovery section of papermaking |
| CN102619123A (en) * | 2012-03-16 | 2012-08-01 | 陕西科技大学 | Method for removing silicon from green liquor through synergism of calcium oxide, carbon dioxide and aluminum-containing compound |
| CN103469664B (en) * | 2013-08-26 | 2015-09-16 | 陕西科技大学 | A kind of alkali collection workshop section green liquor flocculation silicon removing method |
Also Published As
| Publication number | Publication date |
|---|---|
| WO2000042251A1 (en) | 2000-07-20 |
| ID29518A (en) | 2001-09-06 |
| FI990042A (en) | 2000-07-13 |
| FI107743B (en) | 2001-09-28 |
| FI990042A0 (en) | 1999-01-12 |
| CN1143024C (en) | 2004-03-24 |
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