WO2025178544A1

WO2025178544A1 - Sodium recovery process and purge purification

Info

Publication number: WO2025178544A1
Application number: PCT/SE2025/050151
Authority: WO
Inventors: Riccardo SUBIACO; Sami ESKELINEN
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2024-02-19
Filing date: 2025-02-19
Publication date: 2025-08-28
Anticipated expiration: 2026-08-19

Abstract

A sodium recovery process (1) for recovering sodium salts from recovering sodium salts from ash in an ash treatment plant in a pulping mill, said sodium recovery process comprising the feature of mixing ash from burning of black liquor with a liquid to form an ash suspension by mixing ash and liquid in a ratio between 1:1,5 and 1:4 by weight, said ash suspension subjected to temperature between 40 and 100 degrees °C whereby sodium salts are precipitated and forming a stream of solids separated from the liquid phase, said liquid phase forming a purge (2), the sodium recovery process (1) further includes the steps of: - diluting the purge (2) by addition of 2 % by weight to 60 % by weight of water (8); cooling the purge (2) to a temperature of below 15 °C; - allowing sodium salts to precipitate from the cooled and diluted purge (2); - separating a liquid phase (12) from the precipitated sodium salts (11); - withdrawing the liquid phase (12) from a slurry (16) of the precipitated sodium salts (11).

Description

SODIUM RECOVERY PROCESS AND PURGE PURIFICATION

TECHNICAL FIELD

The disclosure pertains to a process for recovering chemicals, such as sodium salts, from a liquid stream. The process for recovering chemicals such as sodium salts is particularly suitable for recovering chemicals from a purge of an ash treatment plant in a Kraft pulping mill.

BACKGROUND

In the operation of pulp mills, there is an ongoing effort to become more environmentally friendly and sustainable. One way of improving sustainability is to minimize the amount of make-up chemicals which are added to the process by reducing the chemical loss to the effluent.

In a Kraft pulping mill, black liquor is burnt in a recovery boiler to produce energy and recover important chemicals. Ash is separated from the recovery boiler flue gases, e.g. by the use of Electrostatic precipitators (ESP:s). Recovery boiler flue gas ash contains a large amount of cooking chemicals (also commonly referred to as Process Elements, PE:s) in the form of sodium salts (mostly sodium sulphate Na2SO4). However, recovery boiler flue gas ash also contains chlorine (Cl) and potassium (K) compounds, which are unwanted Non Process Elements (NPE:s) in the pulp mill liquor cycle. Chlorine and potassium are removed from the pulping process by discharging recovery boiler flue gas ash directly or by sending recovery boiler flue gas ash to an ash treatment plant (also commonly referred to as ash treatment system or ash treatment process).

An ash treatment plant is a plant in a pulp mill responsible for: receiving all or a part of the ash from the recovery boiler; treating the ash by using a specific separation technology (leaching, crystallization and ion exchange are the most known); returning the treated ash to the black liquor cycle with a reduced concentration of undesired components such as chlorine and potassium. The separation products of an ash treatment plant are usually a recovered solid stream containing sodium and sulphate salts and a liquid stream enriched with potassium and chloride, called purge. Purge is also commonly referred to as bleed.

The purge still contains cooking chemicals such as sodium and sulphur which are desired to reuse and result in an undesired loss of cooking chemicals from the pulping process unless recovered.

A method for purifying ash, mainly comprising sodium sulphate, from a recovery boiler is disclosed in US 6,162,408. Ash is purified by leaching or evaporation-crystallizing contaminants such as chlorides and potassium salts. The leaching or the evaporationcrystallization takes place in an aqueous solution at a temperature above 32 DEG C. The leached or evaporation-crystallized ash is returned to the black liquor or to the recovery boiler while the leaching or evaporation-crystallization water is cooled to less than 32 DEG C, preferably to approximately 10-15 DEG C. Sodium sulphate containing water of crystallization will be crystallized and separated in order to be reintroduced into the black liquor, or the recovery boiler or into the leaching solution or evaporation-crystallization solution.

Even though US 6,162,408 discloses a method for purifying ash and recovering sodium, there is a desire to further improve the purification of ash and recovering of cooking chemicals comprised in the purges.

SUMMARY

An object of the present invention is to offer a way of reducing the loss of sodium from the ash treatment plant of a pulp mill, e.g. a Kraft pulp mill.

The above and further objects may be achieved with a crystallization process for recovering sodium salts from a purge according to claim 1. Variations of the disclosure are set out in the dependent claims.

During the years, there has been a development of processes and recovery boilers used in a Kraft pulp mill. Modern recovery boilers’ flue gas ash presents in particular higher carbonate content and lower sulphate content compared to old ones, resulting in an increased concentration of sodium carbonate (Na2CO3) salt. Due to this technology development of the recovery boilers, it becomes increasingly important for ash treatment plants to enhance sodium recovery by precipitating sulphate and carbonate compounds from the liquid solution directed to the effluent, called purge, containing dissolved chlorine and potassium.

By the expression purge is meant a liquid stream from an ash treatment plant containing Na, K, Cl, SO4, CO3 with or without a fraction of solids. The purge purification process disclosed herein is used for a purge from an ash treatment plant in a pulping mill.

The process described herein aims at recovering process elements (PE) used as cooking chemicals such as sodium salts. It is a desire to separate these PE from non-process elements (NPE) which are for example K and Cl. In the process described herein, sodium is mainly precipitated as salt while K and Cl will be maintained in the liquid phase.

The sodium recovery process disclosed herein is directed to recover sodium comprised in ash from a recovery boiler in a pulping mill. The ash is collected to be treated in an ash treatment plant of the pulping mill. The sodium recovery process comprising the feature of mixing the ash originating from burning of black liquor with a liquid to form an ash suspension. The ash suspension is formed by mixing ash and liquid in a ratio between 1 :1 ,5 and 1 :4 by weight. Hence, the mixture is formed by mixing 20 to 40 % w/w ash with 40 to 60 % w/w liquid. The liquid normally used is water. The temperature of the ash suspension is controlled to be between 30 and 100 degrees °C whereby sodium salts are precipitated, mainly as sodium sulphate but also other salts such as sodium carbonate, and forming a stream of solids separated from the liquid phase. The liquid phase is separated to form a liquid stream which is commonly referred to as a purge. The purge will be subjected to further treatment in order to recover more of the sodium comprised in the saturated purge.

The purge purification process for recovering sodium salts from a purge comprises:

- diluting the purge by addition of between 2 and 30 % by weight of water, more preferably between 2 and 25 % by weight of water, even more preferably between 5 and 20 % by weight of water and most preferably between 5 and 15 % by weight of water;

- cooling the diluted purge to a temperature of 15 °C or below, such as to a temperature between 2 and 15 °C, more preferably to a temperature between 2 and 10 °C and most preferably to a temperature between 5 and 10 °C;

- precipitating sodium salts from the cooled and diluted purge;

- separating a liquid phase from the precipitated sodium salts; - withdrawing the liquid phase from a slurry of the precipitated sodium salts.

Dilution of the purge by liquid such as water can be made in one single step. The dilution with water can also be carried out in two or more steps and can be made at different stages of the process. Likewise, cooling of the purge can be made in one step or carried out in two or more steps and can be made at different stages of the process. Dilution and cooling may be at least partly performed at a same stage in the process. Accordingly, the dilution and cooling steps may be carried out in different ways and at different stages in the process. It may be beneficial for the process if the dilution is performed in several steps while the purge is cooled in order to provide an improved precipitation of hydrated salts. The dilution and cooling steps may for example be carried out at separate stages of the process and may be actions which are at least partly overlapping in time and/or location in the ash treatment process. Hence, the cooling and dilution may be performed by turns one after another or simultaneously. In particular, it may be beneficial to add water at least once at a temperature above 25 degrees °C and at least once below 25 degrees °C in order to have sufficient water for precipitation of hydrated salts while also keeping the saturation level high during the precipitation. There can for example be a multistage or continuous addition of water during cooling of the purge from about 30 degrees to its final temperature below 15 degrees, more preferably below 10 degrees. To control the cooling and addition of water properly may thus improve the process. After dilution and cooling, the precipitated salts are separated from the liquid phase.

The purpose of the provision of a purge purification system for an ash treatment plant as disclosed herein is to reduce the amount of sodium salts in the purge, thereby improving recovery of cooking chemicals. By mixing the purge in water and cooling it to a temperature of below 15 °C, such as to a temperature of 2-15 °C, hydrated sodium salts such as hydrated Na2SO4 and Na2COs (Na2SO4*10H2O, etc.) will precipitate out of the liquid phase. The hydration degree of the salts may vary. The solubility of Cl and K is relatively high at 0-15 °C, such that these unwanted NPE will remain in the liquid phase. The liquid phase which is separated from the precipitated sodium salts may then be removed from the system and sent to further treatment, e.g. wastewater treatment or a next recovery step. The solid phase containing the precipitated sodium salts together with some liquid can either be further separated in a separator or returned to the process. Cooling to low temperatures, such as cooling below 10 °C (e.g. between 2 and 10 °C or 5 and 10 °C) causes a greater amount of sodium salts, in particular Na2CO3 (hydrated), to precipitate from the purge than when cooling to 15 °C. Although cooling to temperatures below 5 °C may be beneficial in terms of improved precipitation, cooling to a temperature in the range of from 10 °C to 5 °C may be preferred in view of process efficiency and economy.

It is not desirable to precipitate potassium salts from the purge. Cooling and water addition should therefore be adapted such that potassium salts remain in the solution and are not precipitated. This generally means that cooling to lower temperatures requires a larger addition of water.

Water addition is desired not only for keeping K and Cl in the solution but also for the formation of the hydrated Na salts Na2SO4 + 10H2O, Na2SO4 + 7H2O, Na2CO3 + 10H2O and Na2CO3 + 7H2O. Typically, dilution of a solution would lead to a larger volume of solvent present, meaning higher losses of the elements of interest, as there is more solvent to stay dissolved in. However, in this process water is consumed for the formation of the hydrated solids, which would not precipitate otherwise. Hence, water is not added in the process to dissolve initial solid from the purge but primarily to provide water for hydration of salts.

In the process as disclosed herein, the precipitated sodium salts may be allowed to separate from the liquid phase by, for example, means of gravity. Forced separation means may also be used, alone or in combination with gravitational separation, although gravitation-based methods may be preferred due to economical and energetic advantages. Gravitational separation may be sufficient, which means that it is generally not necessary to use centrifugation or other forced separation means.

The slurry of precipitated sodium salts may be at least partly transferred back to the ash treatment plant. Another part of the slurry of precipitated sodium salts may be subjected to a further separation step for recovery of solids from the slurry of precipitated sodium salts.

The purge from the ash treatment process may typically have an initial temperature in the range of from 70-100 °C but can also be lower, e.g. even as low as 30 degrees. The purge may be diluted by mixing with water having a lower temperature than the purge.

The purge may be diluted by mixing with water prior to cooling the diluted purge to a final temperature in a precipitation vessel, also commonly referred to as crystallizer, in which precipitation of the sodium salts takes place.

The diluted purge may be subjected to pre-cooling before transfer of the diluted purge to the precipitation tank where a final cooling step is carried out.

The purge may be a purge which is derived from a crystallization process in the ash treatment plant. Alternatively, the purge may be a purge which is derived from a leaching process in the ash treatment plant.

The precipitated sodium salts comprise one or more of Na2SO4*7H2O, Na2SO4*10H2O, Na2CO3*7H2O and Na2CO3*10H2O. The precipitated sodium salts contain sodium, sulphate and carbonate are recycled. As disclosed above, the content of carbonates relative the content of sulphates in the ash has been increased as a consequence of development of modern recovery boilers. The purge purification process disclosed herein is adapted to work well for ashes from these modern recovery boilers rendering the diluted purge having a relative content of carbonate to sulphate above 1 :35 and even higher such as 1 :25, 1 :20 or 1 :17.

The liquid phase of the purified purge, containing the unwanted non-process elements (NPE) chloride and potassium, which is separated from the precipitated sodium salt can be withdrawn via the ash-treatment plant and further processed or withdrawn from the Kraft pulping process.

BRIEF DESCRIPTION OF DRAWINGS

The process as disclosed herein will be further explained hereinafter with reference to the appended drawing wherein:

Figure 1 shows a schematic illustration of a sodium recovery process. DETAILED DESCRIPTION

Different aspects of the present disclosure will be described more fully hereinafter with reference to the enclosed schematic drawing.

In a Kraft pulping process, also referred to as a sulphate process, wood chips are treated in a digestor with an aqueous cooking liquor, commonly referred to as “white liquor” and containing sodium hydroxide (NaOH) and sodium sulphide (Na2S). The cooking process breaks the bonds that link lignin, hemicellulose, and cellulose. After cooking, the cellulose fibres are further treated in a pulp production process involving washing, bleaching, and drying to produce cellulose pulp.

Process liquid, commonly referred to as “black liquor”, is derived from the pulp production process. The black liquor is subjected to evaporation in an evaporation plant where the black liquor from the pulping process is concentrated. The concentrated black liquor is then transferred to a recovery boiler.

The recovery boiler is part of the recovery system. In the recovery boiler, black liquor from the pulping process is burnt to recover chemicals and thermal energy generated by the combustion process.

An ash separator, e.g. an electrostatic precipitator (ESP), is used to remove ashes from the flue gas from the recovery boiler. As disclosed herein, the ashes are further treated in order to capture and reuse cooking chemicals such as sodium, sulphate and carbonate.

The liquor flows in the Kraft pulp mill may be referred to as a “cyclic liquor flow system” comprising a main liquor cycle where at least a part of liquor which is used in the digester in the pulp mill is reconditioned and recycled to the digester. The cyclic liquor flow system comprises sub-cycles of liquor flow from different process equipment in the pulp mill. It is also to be understood that water and chemicals are added and withdrawn from the cyclic liquor flow system as part of a chemical recovery system in the pulp mill.

Ash treatment plants in a Kraft pulping mill are commonly based on crystallization or leaching technology, with a minor part being based on ion exchange technology. The common purpose of these systems is to remove chloride (Cl) and potassium (K) from ash, while recovering sodium salts (Na2SO4 and Na2COs). A multiple stage system for removal of chloride and potassium is disclosed in US 8,721,837 B2. The purge treatment system in US 8,721 ,837 B2 takes advantage of salt solubility differences at different temperatures. The multiple stage crystallizers described in US 8,721,837 B2 have a first stage operating as a conventional evaporative crystallizer at 90-100 °C. A second stage is operated at 35-50 °C for precipitation of the potassium salt Glaserite. A single stage cold leaching system is disclosed in US 5,840,085 A. The system in US 5,840,085 A involves mixing ash and water to create a slurry which is cooled with ice to 10-15 °C and then separated in centrifuge.

The purge flow from existing systems is saturated on sodium sulphate/sodium carbonate which leads to a loss of cooking chemicals in the Kraft pulping mill. Typically, 10-30 % of incoming sodium (Na), sulphate (SO4) and carbonate (CO3) are lost in the process and need to be replaced by make-up chemicals.

A schematic purge purification process 1 as disclosed herein is shown in Fig.1. The sodium recovery process 1 is configured for treatment of a purge 2 from an ash treatment plant 3. In the ash treatment plant 3, ash is added to a liquid such as water and a portion of the sodium comprised in the ash will be precipitated and removed as solids from the ash treatment plant while the saturated liquid phase will form the purge 2 to be further treated. The ash treatment plant 3 may be based on crystallization technology or leaching technology, as disclosed herein. The purge purification process 1 which is shown in Fig. 1 and is performed on the purge 2 leaving the ash treatment plant comprises a mixing step 5 in which water is added to the purge 2, a cooling step 6, a precipitation step 7, and a separation step 17.

It is to be understood that the process steps disclosed herein can be combined or carried out in one or several devices and/or in a different order. For instance, mixing and cooling may be performed simultaneously, and/or cooling may be performed before mixing. Furthermore, cooling may be performed together with precipitation. The process steps may be carried out in one or more sub-steps. By way of example, the cooling step may be carried out in sub-steps at different stages of the purification process 1. The purge 2 from the ash treatment plant 3 is mixed with water 8. The purge 2 is diluted by addition between 2 and 60 % by weight of water 8, more preferably between 2 and 50 % and most preferably between 5 and 40 %. The optimal dilution rate is a function of the target cooling temperature and purge initial composition, the latter a function of the composition of the ESP ash treated by the ash treatment plant.

When leaving the ash treatment plant 3, the purge 2 normally has a temperature in the order of 70-100 °C, but the temperature may be lower, depending on the ash treatment plant being used.

The water 8 which is added in the mixing step 5 preferably has a temperature which is lower than the temperature of the purge 2, to contribute to cooling of the purge 2. In the purge purification process 1 which is shown in Fig. 1 , the diluted purge 9 from the mixing step 5 undergoes a cooling step 6. Further water 8 may be added to the diluted purge 9 during the cooling step 6, if desired. The cooled and diluted purge 10 is then subjected to a precipitation step 7 in which the temperature of the cooled and diluted purge 10 is below 20 °C, such as between 0 and 20 °C, more preferably between 0 and 15 °C and even more preferably between 0 and 10 °C. The diluted purge 9 can for example be cooled to a temperature preferably to below 8 °C, such as between 0 to 8 °C or between 2 to 8 °C.-

It is further to be understood that water 8 may be added only in a single mixing step 5 or that water may be added during the cooling step 6, during the precipitation step 7 or in a combination of two or all three of these steps. It is of course also possible that the cooling and dilution starts simultaneously, and the purge is cooled while being diluted. The specific order in which these operations, i.e. cooling and dilution, are performed may shift and if they are performed in a single step or divided in two or several sub steps may also change. However, dilution must have started before the purge has reached its desired cooling temperature, otherwise there may be an undesired precipitation of potassium and chloride which will be mixed with precipitated sodium salts. As a general rule, the purge shall be diluted by at least the indicated lower limit for dilution before the purge has reached the indicated final temperature. As disclosed herein, the purge 2 may be diluted by adding water 8 at any suitable stage of the purge purification process 1. However, it may be preferred that all, or at least a major part of the water 8 is added early in the purge purification process 1 in order to avoid precipitation of potassium and chloride. The mixing step 5, the cooling step 6 and the precipitation step 7 can all be performed in the same vessel or there may be two or more vessels where one or several of these steps are performed.

By mixing the purge 2 from the ash treatment plant 3 with water to form a diluted purge 9 and cooling it to a temperature of between 0 and 15 °C, or within any of the other temperature intervals disclosed above, hydrated sodium salts such as hydrated Na2SO4 and Na2CC>3 (Na2SO4*10H2O, etc.) will precipitate out of the liquid phase 12 and form a sediment of precipitated sodium salts 11 in the precipitation stage 7. The solubility of K and especially of Cl is relatively high at 0-15 °C, such that these unwanted NPE will remain in liquid phase 12. The liquid phase 12 which is separated from the precipitated sodium salts 11 is then removed from the purge purification process 1 and sent to wastewater treatment 15.

The precipitated sodium salts 11 are separated by gravity and removed as a slurry 16. The slurry 16 can either be returned to the ash treatment plant 3 for recovery in the form of a slurry or a diluted liquid or can be further separated in a separation step 17, such as centrifugation. In the separation step 17, the remaining liquid 18 is removed from the solids 20 and sent to effluent or to further treatment 15. The solids 20 are recovered and returned to the Kraft pulping process.

The purpose of the sodium recovery process 1 for the ash treatment plant 3 as disclosed herein is to reduce the amount of sodium salts in the purge, thereby increasing the cooking chemical recovery of the system and reducing the negative impact of sodium salts on the environment.

Claims

1. A sodium recovery process (1) for recovering sodium salts from ash in an ash treatment plant in a pulping mill, said sodium recovery process comprising the feature of mixing ash from burning of black liquor with a liquid to form an ash suspension by mixing ash and liquid in a ratio between 1 :1 ,5 and 1 :4 by weight, said ash suspension subjected to temperature between 40 and 100 degrees °C whereby sodium salts are precipitated and forming a stream of solids separated from the liquid phase, said liquid phase forming a purge (2), characterized by

- diluting the purge (2) by addition of between 2 and 30 % by weight of water (8), more preferably between 2 and 25 %, even more preferably between 5-20 % and most preferably between 5 and 15 % to form a diluted purge (9)

- cooling the diluted purge (9) to a temperature of 15 °C or below, such as to a temperature between 2 and 15 °C, more preferably between 2 and 10 °C and most preferably between 5 and 10 °C

- precipitating sodium salts from the cooled and diluted purge (10);

- separating a liquid phase (12) from the precipitated sodium salts (11);

- withdrawing the liquid phase (12) from a slurry (16) of the precipitated sodium salts (11).

2. Process according to claim 1 , wherein dilution of the purge with water (8) is carried out in two or more steps.

3. Process according to claim 2, wherein water is added at least once at a temperature above 25 degrees °C and at least once below 25 degrees °C.

4. Process according to claim 1 or 2, wherein cooling of the purge is carried out in two or more steps.

5. Process according to any one of the preceding claims, wherein the precipitated sodium salts (11) are separated from the liquid phase (12) by means of gravity.

6. Process according to any one of the preceding claims, wherein at least a part of the slurry (16) of precipitated sodium salts (11) is transferred back to the ash treatment plant (3).

7. Process according to any one of the preceding claims, wherein at least a part of the slurry (16) of precipitated sodium salts (11) is subjected to a further separation step for recovery of solids from the slurry (16) of precipitated sodium salts (11).

8. Process according to any one of the preceding claims, wherein the purge (2) is diluted with water (8) having a lower temperature than the purge (2).

9. Process according to any one of the preceding claims, wherein the purge (2) is diluted by mixing with water (8) prior to cooling the diluted purge (9).

10. Process according to any one of the preceding claims, wherein the purge (2) is subjected to cooling before subjecting the cooled and diluted purge (10) to precipitation in a precipitation step (7).

11. Process according to any one of the preceding claims, wherein the purge (2) is a purge from a crystallization process or a leaching process in the ash treatment plant (3).

12. Process according to any one of the preceding claims, wherein the precipitated sodium salts (11) comprise one or more of IX^SC ^FW, IS^SO lO W, IS^COs^bW and Na2CO3*10H2O.

13. Process according to any one of the preceding claims, wherein the liquid phase (12) which is separated from the precipitated sodium salts (11) contains chloride and potassium.

14. Process according to any one of the preceding claims, wherein the relative content of carbonate to sulphate in the diluted purge (9) is above 1 :35 (or 1:25, or 1:20, preferably 1 :17).