Process for recovering compounds from kraft boiler ash

Abstract

The present invention relates to a process for recovering compounds from Kraft boiler ash. The process recovers valuable elements from ash, dissolved ash purge or ash leaching purge; from kraft recovery boiler.

Description

[0001] PROCESS FOR RECOVERING COMPOUNDS FROM KRAFT BOILER ASH DESCRIPTION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a process for recovering compounds from Kraft boiler ash. The process recovers valuable elements from ash, dissolved ash purge or ash leaching purge; from kraft recovery boiler.

[0004] BACKGROUND OF THE INVENTION

[0005] Kraft boiler is one of the most important equipment in Kraft pulp mills. The main function of this equipment is to recover the chemicals used in the pulping process. These chemicals are present in the black liquor (by-product of the pulping process) which is burned under reducing conditions in the boiler to obtain sodium sulfide, one of the pulping chemicals. The combustion of the black liquor is also important due to two other factors: it produces steam and power for the mill and provides a means for disposal of lignin and other waste residues from the pulping process. Specific components of the black liquor can adversely affect the boiler's performance, therefore there is some process to eliminate some of these components.

[0006] The international patent application WO2013036190 A1 refers to a process for treating a hardwood black liquor from the manufacturing of hardwood pulp in order to precipitate calcium compounds present in the liquor wherein the process comprises the steps of; providing a hardwood black liquor from the manufacturing of hardwood pulp and treating at least one part of said liquor at a temperature of above 60°C wherein the calcium compounds present in the black liquor will precipitate.

[0007] The international patent application WO2014976361 A1 refers to method deals with the leaching of ash originating from the recovery boiler of a pulp mill, particularly when the ash contains a significant amount of carbonate. The objective of the ash-leaching process is to purge chlorine and potassium, two of the so-called non-process elements, from the chemical recovery cycle of the pulp mill. The international patent application WO 2018 / 197753 describes a method of treating fly ash containing sodium sulfate from a recovery boiler of a chemical pulp mill. In this document the pH is adjusted with alkali and the calcium is precipitated after obtaining the sulphate crystals.

[0008] In kraft recovery boiler operation, fly ash generated from black liquor combustion is mixed with the virgin black liquor in a mix tank and returned to the boiler with the as-fired black liquor. The selective crystallization of Kraft boiler ashes has proven to be the most efficient ash treatment system. Nevertheless, it is affected by the variability in ash composition and faces several issues, primarily high energy costs, efficiency loss based on ash composition, and the loss of valuable chemicals. Therefore, there is significant interest in developing new procedures to address these challenges.

[0009] SUMMARY OF THE INVENTION

[0010] Kraft pulping is the most common form of chemical pulping. Kraft pulping involves the digesting of wood chips at elevated temperature and pressure in “white liquor”. White liquor is a water solution of sodium sulphide and sodium hydroxide. The lignin that binds the cellulose fibres together in the wood is chemically dissolved by this white liquor and it is obtained fibre and a black liquor. This black liquor is concentrated in an evaporator train until a point where it can be effectively burned in a kraft recovery boiler.

[0011] In a kraft recovery boiler, concentrated black liquor is burned in the furnace. In kraft recovery boilers the ash is formed by several mechanisms, alkali metals in burning black liquor droplets vaporize. They subsequently react with furnace gases and condense to small particles. This process is called fume formation. Most of the ash in recovery boiler flue gases is fume.

[0012] In the ashes, valuable components that are desired to be recovered include sodium, carbonates, and sulphates, along with impurities such as chlorides and potassium, which can be detrimental to the operation of the boiler. The aim is to reduce these impurities; hence, an object of the invention's process is to eliminate the majority of potassium and chlorides with minimal loss of potentially valuable components and their subsequent recovery. Chloride and potassium accumulation often leads to significant increases in the frequency of shutdowns of the chemical recovery boiler due to plugging of the heat exchanger passages (sticky dust). Frequent recovery boiler shutdowns and washes not only cause loss of production but can contribute significantly to recovery boiler corrosion. Both chloride and potassium contribute to a lowering of the melting point of the fume particles produced in the recovery boiler. This causes particles to be softened and sticky at temperatures where they would normally be hard. Sticky particles can attach themselves to surfaces such as the heat exchange surfaces in the upper part of the recovery boiler. Eventually the gas passages may become partially or almost entirely plugged and this will require a shutdown and water wash.

[0013] The present description describes a process for recover compounds of interest such as sodium sulfate, sodium carbonate, potassium sulfate, glaserite, leaving as residue an effluent with chloride ions (Cl brine).

[0014] Therefore, the aspect of the present invention refers to a process for recovering compounds from an ash product selected from: kraft recovery boiler dissolved ash purge and / or other purge from a kraft recovery boiler ash treatment process, that comprises Cl K+, Na+, CO32-and SO?’ comprising the steps of: a) purifying the ash product by controlled precipitation of calcium compounds and filtration of calcium compounds precipitated and insoluble matter; b) evaporating the ash product purified from step a) in a crystallizer to obtain K+, Na+, CO32’ and SO?’ crystals, concentrated Cl’ brine and step b) water / condensate; c) separating the K+, Na+, CO32’ and SO ’ crystals obtained in b) and re-slurring them with brine from f) and water / condensate from step b) to obtain CO32’ brine and K+, Na+and SO ’ crystals; d) separating the K+, Na+and SO?’ crystals obtained in c) and re-slurring them with brine from I) to obtain glaserite crystals and K+, Na+and SO?’ brine; e) evaporating CO32’ brine from c) in a crystallizer to obtain Na+, CO32’ and SO?’ crystals, a step e) brine and step e) water / condensate; f) separating the crystals from e) and recycling them to the Kraft process and recycling step e) brine to c); g) separating the glaserite crystals from d) and re-slurring them with step e) water / condensate to obtain K2SO4 crystals; h) evaporating d) brine in a crystallizer to obtain glaserite crystals; i) separating the glaserite crystals from h) and re-slurring them with step e) and / or step b) water / condensate to obtain K2SO4 crystals and a step i) brine; j) cooling brine from i) to obtain glauber salt (Na2SO4.10 H2O) crystals; k) separating the glauber salt crystals from j) t; l) separating the K2SO4 crystals from g) and i) and recycling brine to d) ; wherein the energy source for the steps b), e) and h) is the residual steam from the black liquor evaporation train going to the Kraft boiler.

[0015] The term “Kraft boiler dissolved ash purge" refers to the ash that results from the combustion of black liquor in a kraft recovery boiler dissolved in water. The black liquor, a byproduct of this process, is burned in a recovery boiler to generate energy and recover chemicals. The dissolved ash refers to the inorganic components that remain in solution in the black liquor and become ash when the liquor is burned. These components typically include various chemicals and minerals, and the composition of kraft boiler dissolved ash can vary depending on the wood source and the specific processes used in pulp production.

[0016] The "kraft boiler ash treatment process" refers to a series of procedures and methods designed to manage and treat the ash generated in a kraft recovery boiler. The treatment process may include various steps such as separation, purification, and recovery of valuable chemicals from the ash by for example leaching, evaporation-crystallization, freezing-crystallization and ion exchange process.

[0017] "Controlled precipitation" refers to a process in which the formation of solid particles or precipitates from a solution is intentionally regulated or manipulated under specific conditions of retention time, temperature and seeding. The ash product has pH values from 5 to 13, temperature from 50°C to 90°C and carbonate ion concentration from 5 g / l to 50 g / l. "The residual steam from the black liquor evaporation train going to the Kraft boiler" refers to the steam going to the condenser from black liquor evaporation train.

[0018] DETAILED DESCRIPTION OF THE INVENTION

[0019] Therefore, as it is said above the aspect of the invention refers to a process for recovering compounds from an ash product selected from: kraft recovery boiler dissolved ash purge and / or other purge from a kraft recovery boiler ash treatment process, that comprises Cl K+, Na+, CO32' and SC>42' comprising the steps of: a) purifying the ash product by controlled precipitation of calcium compounds and filtration of calcium compounds precipitated and insoluble matter; b) evaporating the ash product purified from step a) in a crystallizer to obtain K+, Na+, CO32' and SO ' crystals, concentrated Cl' brine to be discarded and step b) water / condensate; c) separating the K+, Na+, CO32' and SO ' crystals obtained in b) and re-slurring them with brine from f) and water / condensate from step b) to obtain CO32' brine and K+, Na+and SO?' crystals; d) separating the K+, Na+and SO?' crystals obtained in c) and re-slurring them with brine from I) to obtain glaserite crystals and K+, Na+and SO?' brine; e) evaporating CO32' brine from c) in a crystallizer to obtain Na+, CO32' and SO?' crystals, a step e) brine and step e) water / condensate; f) separating the crystals from e) and recycling them to the Kraft process and recycling step e) brine to c); g) separating the glaserite crystals from d) and re-slurring them with step e) water / condensate to obtain K2SO4 crystals; h) evaporating d) brine in a crystallizer to obtain glaserite crystals; i) separating the glaserite crystals from h) and re-slurring them with step e) and / or step b) water / condensate to obtain K2SO4 crystals and a step i) brine; j) cooling brine from i) to obtain glauber salt (Na2SO4.10 H2O) crystals; k) separating the glauber salt crystals from j); l) separating the K2SO4 crystals from g) and i) and recycling brine to d) ; wherein the energy source for the steps b), e) and h) is the residual steam from the black liquor evaporation train going to the Kraft boiler.

[0020] Preferably the controlled precipitation of calcium compounds is by controlling temperature, stirring or seeding. More preferably, the temperature is controlled in the step a), from 50°C to 90°C. Preferably the crystals from e) are recycled to the Kraft process, particularly are mixed with the black liquor.

[0021] Preferably the glauber salt crystals are recycled to the Kraft process, particularly mixed with the black liquor or further processed to obtain Na2SC>4 crystals. Preferably the K2SO4 crystals are used as fertilizer.

[0022] Preferably the dissolved ash purge is obtained by dissolved the ash in water and filtering to separate the insoluble matter.

[0023] Preferably the process further comprises the steps of: n) cooling the concentrated Cl' brine released in b) to obtain Na+, COa2' and SO?' crystals and enriched Cl' brine; o) separating the Na+, COa2' and SO?' crystals and recycling them to c); p) evaporating enriched Cl' brine to obtain NaCI crystals and high Cl' brine; q) separating the NaCI crystals and recycling high Cl' brine from p) to b), wherein the steps n) to q) is carried out in any time after the step c).

[0024] Preferably the invention refers to the process for recovering compounds from kraft recovery boiler dissolved ash purge, that comprises Cl Na+, K+, COa2' and SO?' consisting in the steps of: r) purifying the kraft recovery boiler dissolved ash purge by controlled precipitation of calcium compounds and filtration of calcium compounds precipitated and insoluble matter; s) evaporating the kraft recovery boiler dissolved ash purge purified from step r) in a crystallizer to obtain Na+, COa2' and SO ' crystals and a brine comprising Cl' and K+wherein the energy source is the residual steam from the black liquor evaporation train going to the Kraft boiler; t) separating the Na+, COa2' and SO?' crystals obtained in s) to be recycled to the Kraft process u)purging the brine obtained in s) and so discarding the K+and Cl'.

[0025] Preferably the K+recovery as by-product from 85% wt to 95% wt respect the initial weight of K+.

Claims

CLAIMS1. A process for recovering compounds from an ash product selected from: kraft recovery boiler dissolved ash purge and / or other purge from a kraft recovery boiler ash treatment process, that comprises Cl K+, Na+, CO32' and SC>42' characterized by comprising the steps of: a) purifying the ash product by controlled precipitation of calcium compounds and filtration of calcium compounds precipitated and insoluble matter; b) evaporating the ash product purified from step a) in a crystallizer to obtain K+, Na+, CO32' and SO ' crystals, concentrated Cl' brine to be discarded and step b) water / condensate; c) separating the K+, Na+, CO32' and SO ' crystals obtained in b) and re-slurring them with brine from f) and water / condensate from step b) to obtain CO32' brine and K+, Na+and SO?' crystals; d) separating the K+, Na+and SO?' crystals obtained in c) and re-slurring them with brine from I) to obtain glaserite crystals and K+, Na+and SO?' brine; e) evaporating CO32' brine from c) in a crystallizer to obtain Na+, CO32' and SO?' crystals, a step e) brine and step e) water / condensate; f) separating the crystals from e) and recycling them to the Kraft process and recycling step e) brine to c); g) separating the glaserite crystals from d) and re-slurring them with step e) water / condensate to obtain K2SO4 crystals; h) evaporating d) brine in a crystallizer to obtain glaserite crystals; i) separating the glaserite crystals from h) and re-slurring them with step e) and / or step b) water / condensate to obtain K2SO4 crystals and a step i) brine; j) cooling brine from i) to obtain glauber salt (Na2SO4.10 H2O) crystals; k) separating the glauber salt crystals from j); l) separating the K2SO4 crystals from g) and i) and recycling brine to d) ; wherein the energy source for the steps b), e) and h) is the residual steam from the black liquor evaporation train going to the Kraft boiler.

2. The process according to claim 1 characterized by comprising the steps of: n) cooling the concentrated Cl' brine released in b) to obtain Na, CO3and SO4 crystals and enriched Cl' brine;o) separating the Na, COa2’ and SO4 crystals and recycling them to c); p) evaporating n) brine to obtain NaCI crystals and high Cl’ brine; q) separating the NaCI crystals to be released as a by-product and recycling high Cl’ brine from p) to b).

3. The process for recovering compounds from kraft recovery boiler dissolved ash purge, that comprises Cl’, K+, Na+, COa2’ and SO?’ consisting in the steps of: r) purifying the kraft recovery boiler dissolved ash purge by controlled precipitation of calcium compounds and filtration of calcium compounds precipitated and insoluble matter; s) evaporating the kraft recovery boiler dissolved ash purge purified from step r) in a crystallizer to obtain Na+, COa2’ and SO ’ crystals and a brine comprising O’ and K+wherein the energy source is the residual steam from the black liquor evaporation train going to the Kraft boiler; t) separating the Na+, COa2’ and SO ’ crystals obtained in s) to be recycled to the Kraft process u)purging the brine obtained in s) and so discarding the K+and Cl.

4. The process according to the claims 1 and 2 characterized by the K recovery as byproduct from 85% wt to 95% wt respect the initial weight of K.