The present disclosure relates to recovery of lignin from a black liquor, particularly from Kraft pulp production, and use said recovery in conjunction with inorganic materials removal for increasing pulp production and in a Kraft process.

BACKGROUND ART

The recovery system of a Kraft pulp production constitutes an important aspect for the financial profitability of a Kraft Pulp mill. The recovery system fulfills two major functions: a) recovery and regeneration of chemicals produced by cooking wood chips and b) energy production such as steam and electricity. First, the recovery system was designed to recovery and regenerate chemicals contained in the used cooking liquor (black liquor), either caustic soda (NaOH) and sodium sulfide (Na ₂ S), which are the major components of the cooking liquor used for pulping (white liquor). Secondly, the recovery boiler, a key element of a conventional recovery system, produces steam by burning dissolved organic materials from the cooking of wood chips.

The capacity of the recovery boiler is often limited by the heat load and/or the solid load from the black liquor, limiting therefore pulp production. The recovery boiler usually operates at its maximum steam production capacity (thermal load) which was established in the original design.

Reducing the heat load of a recovery boiler may be done by precipitating a fraction of the lignin contained in the black liquor solids. It is therefore clear that partial extraction of the lignin contained in the black liquor induces an effective decrease in the heat load as well as in the solid load fed into the boiler.

Several approaches have been proposed to extract lignin from the black liquor for the purpose of reducing the load on the boiler. Chemical acidification consists of neutralizing the alkalinity of black liquor by adding acid agents such as sulfuric acid and/or carbon dioxide to reduce the pH of black liquor to the point of precipitation of the lignin. However, precipitation of lignin by chemical acidification has the effect of increasing the load of inorganic materials in the recovery boiler, therefore contributing to reduce calorific value of the solids fed into the recovery boiler.

Electrochemical acidification consists in neutralizing the alkalinity of black liquor through electrochemical reactions such as electrolysis or by extracting the alkaline salts by electrodialysis.

SUMMARY OF THE DISCLOSURE

In an aspect, the disclosure relates to a process for the recovery of lignin from black liquor, the process comprising: electrochemically treating the black liquor to reduce the pH thereof below about 11 ; precipitating the lignin from the electrochemically treated black liquor; and recovering the solid lignin.

In another aspect, there is provided a process for increasing production of pulp in a Kraft process limited by the capacity of a recovery boiler, the process comprising: electrochemically treating at least one portion of a black liquor produced by said process before entry into said recovery boiler to reduce the pH thereof below 11 and optionally extracting at least a portion of inorganic materials therefrom; precipitating and recovering lignin from the electrochemically treated black liquor; and feeding the resulting liquor to the pulp production process before the evaporation train of the Kraft process (i.e. the recovery system).

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures, in which;

Fig. 1 is flow diagram of a process for increasing pulp production in a Kraft pulping process limited by the capacity of a recovery boiler in accordance with an embodiment of the present disclosure;

Fig. 2 illustrates the effect of the extraction of at least a portion of the inorganic materials by direct electrochemical treatment of the black liquor with respect to the pH in accordance with an embodiment of the present disclosure;

Figs. 3a to d illustrate the effect of the different extraction rates: 0%, 25%, 50%, and 75% of inorganic materials on the load of material fed into the boiler, and on the calorific value of this load of materials in accordance with an embodiment of the present disclosure;

Fig. 4 shows typical variation of specific calorific value and relative flow of solids fed to a recovery furnace, as function of the rate of increase in production in accordance with methods in the art for extracting lignin from the black liquor based on the addition of chemicals reactants;

Fig. 5 illustrates the comparison of increased pulp production between the process for recovery of lignin based on chemical precipitation and the electrochemical processes in accordance with embodiments of the present disclosure;

Fig. 6 illustrates a mass balance of an electrolysis treatment of black liquor having a concentration of 30% by weight to obtain a final pH of 9.0 in accordance with an embodiment of the present disclosure;

Fig. 7 illustrates sodium sulfate electrolysis combined with a separation system for separating the acid produced from the unconverted sulfate in accordance with an embodiment of the present disclosure;

Fig. 8 shows a process for recovery of lignin wherein black liquor is electrochemically treated by electrolysis in accordance with an embodiment of the present disclosure;

Fig. 9 shows a process for recovery of lignin wherein black liquor is electrochemically treated by electrodialysis in accordance with an

embodiment of the present disclosure;

Fig. 10 illustrates a process for recovering lignin wherein the black liquor is electrochemically treated by a first treatment with electrodialysis followed by a second treatment with electrolysis in accordance with an embodiment of the present disclosure;

Fig. 11 shows a process for recovery of lignin wherein black liquor is electrochemically treated by electrolysis in accordance with an embodiment of the present disclosure; and

Fig. 12 shows a process for recovery of lignin wherein black liquor is mixed with an acid generated electrochemically in accordance with an embodiment of the present disclosure. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

It was found that with the processes in accordance with the disclosure, the pulp production of a Kraft mill may be substantially increased by complying with the maximum heat load of the recovery boiler and without significantly reducing the calorific value of the black liquor fed into the boiler.

Black liquor refers to the by-product liquor from the cooking step of the Kraft pulping process that comprises: water; organic material such as lignin, hemi- celluloses and other organic molecules; and inorganic material such as sodium salts and NaOH. The inorganic and organic materials are also known as Black Liquor Solids (BLS). The Gross Calorific Value of black liquor solids is about 14.0 MJ/kg or 3,900 kWh/t of which about 70 percent comes from the lignin which constitutes about 40 per cent of the solids with a calorific value of 26.9 MJ/kg or 7,470 kWh/t.

In accordance with one embodiment, Fig. 1 schematically illustrates a process for increasing production of pulp in a Kraft process limited by the capacity of a recovery boiler. The process comprises electrochemically treating at least one portion of a black liquor produced by said process, before entry into said recovery boiler, to reduce the pH thereof below 1 1 and optionally extracting at least a portion of inorganic materials therefrom, precipitating and recovering lignin from the electrochemically treated black liquor, and feeding back the resulting liquor to the pulp production process before the evaporation train of the Kraft process (i.e. the recovery system).

The inorganic materials, preferably in the form of caustic soda or a mixture of caustic soda and sodium salts are extracted from the black liquor, which makes it possible to avoid feeding them into the recovery boiler. Fig. 2 illustrates a typical tendency of the sodium recovery rate by direct electrochemical treatment of the black liquor with respect to the pH of the black liquor treated. As shown in Fig. 2, at a pH of 9, the sodium recovery rate may be about 25 to 30 percent. At a pH of 7, the sodium recovery rate may be increase to about 60%.

Figs. 3a to d illustrate the effect of the increase in the pulp production capacity, following extraction of inorganic materials and lignin precipitation by electrochemical treatment of a black liquor, on the load of material fed into the recovery boiler, and on the calorific value of this load of materials, while maintaining heat load of the recovery boiler.

As shown in Figs. 3a to d, a portion of the black liquor solids, namely BLS, is processed in accordance with an embodiment to increase the pulp production by precipitating lignin extracting at least a portion of the inorganic materials such as sodium. The BLS may be but not limited to in the form of pre- concentrated black liquor from an evaporation step of the Kraft process. To calculate the results presented in Figs. 3a to d, it is assumed that the sodium is extracted in the form of caustic soda (NaOH). The results presented in Figs. 3a to d were obtained using mass and energy balance based on the hypotheses shown in Table 1. For the purpose of calculation, it was assumed that the lignin produced does not contain inert material, therefore, no sodium. The parameter values as shown in Table 1 were used. For calculation purposes, it is considered that the equivalent of 70 percent of the lignin comprised in the black liquor from the Kraft pulp mill (BLS), is recovered and separated.

Figs. 3a to d illustrate the effect of the increase in pulp production, while maintaining heat load of the recovery boiler, on the following variables:

The fraction of black liquor solids (BLS) that must be rerouted or deviated towards the process for increasing pulp production in accordance with the present disclosure to separate the lignin, while maintaining the heat value of the boiler. This variable is indicated by the rate of rerouting to the process.

The ratio of the load (or the mass flow) of solids at the inlet of the recovery boiler on the value of this load of solids before increasing pulp production. This ratio, identified by: Flow-solids/Flow-initial BLS, is the maximum relative amount of total solids capable to be fed into the recovery boiler in order to be able to maintain the heat load of the recovery boiler.

The ratio of the Gross Calorific Value (GCV) of the liquor at the inlet of the recovery boiler, on the Gross Calorific Value of this liquor before increasing production. This ratio, identified by: GCV-solids/GCV-BLS is a relative measure of the calorific value of all the solids fed into the recovery boiler. For each of Figs. 3a to d, the abscissa refers to the rate of increase (expressed in percentage) of the pulp production capacity of a Kraft mill. This rate of increase is assumed to correspond to the rate of increase in the flow of black liquor solids (BLS) going towards the Kraft mill evaporation system (see Fig. 1).

Figs. 3a to d respectively refer to a given inorganic material extraction rate: 0% (3a), 25% (3b), 50% (3c), and 75% (3d). These rates represent the fraction of inorganic material extracted (or purged) from the black liquor during the electrochemical treatment of the black liquor. This represents the ratio between the sodium mass flow extracted in the form of NaOH and the mass flow of inert material contained in the black liquor (BLS) at the inlet (see Fig. 1). For a given increase in the rate of production, the maximum fraction of the black liquor solids (BLS) that may be rerouted to the process in accordance with the present disclosure without increasing the heat load of the boiler may be calculated. This maximum fraction is called: the rerouting rate of BLS to the process. Once the latter is calculated, the other two above-mentioned variables, namely "Flow-solids/Flow-initial-BLS" and "GCV-solids/GCV-BLS" may also be calculated.

These calculations were obtained using mass and energy flow balance based on the hypotheses shown in Table 1. These calculations are based on constant heat load to the recovery boiler. Heat load of the recovery boiler may be considered as being the foregoing:

Heat load = (mass flow of lignin) x (GCV-lignin)

+ (Mass flow of non-lignin organic material) x (GCV-non-lignin organic material) (1)

where GCV indicates the gross calorific value for a given component category.

It is possible to see the advantage of the process in accordance with the present disclosure that allows extraction of sodium from the black liquor circuit, as is shown by Fig. 1 and by the results shown in Figs. 3a-d. In Fig. 3a for an increase of 25% in the rate of pulp production, if no sodium is extracted while the lignin is precipitated, the gross calorific value of the solids at the input of the boiler would be reduced to 90% of the initial value. In the case where the extraction rate is equal to 50%, the calorific value is higher or 96% of the initial value (Fig. 3c). Fig. 3d illustrates an extraction rate of 75% with a calorific value that remains almost constant regardless of the increase of production. As shown in Figs. 3a to d, the higher the inorganic materials extraction rate, the lower the reduction in the calorific value of the solids fed into the boiler and the lower the increase in the proportion of inorganic solids fed into the recovery boiler.

Table 1 - Parameters for mass and heat balance used to calculate the curves in Figs. 3a to d.

According to the results of mass and energy balance shown above, since the calorific value may be maintained to the initial level, It would be theoretically possible to treat the entire amount of the total black liquor flow, extracting 70% of the lignin and thereby significantly increasing the production capacity (even as much as 80%) assuming that recovery system units other than the causticization system, are not overloaded.

It was found that with the process in accordance with the disclosure, the pulp production of a Kraft mill may be substantially increased by complying with the maximum heat load of the recovery boiler and without significantly reducing the calorific value of the black liquor fed into the boiler. Methods known in the art for extracting lignin from the black liquor based on the addition of chemicals reactants reduce the calorific value of the solids fed to the recovery boiler, which may thus limit additional production of pulp. Fig. 4 shows the variation of the GCV-solids/GCV-BLS ratio, as well as the flow-solids/lnitial BLS flow ratio, in terms of the rate of increase in production.

It is assumed that a process for recovery of lignin from black liquor based on the chemical precipitation of lignin implies feeding at least about 0.5 tons of inorganic material into the black liquor circuit per ton of lignin precipitated. For an increase in production of 25%, the calorific value of the solids fed into the recovery boiler corresponds to 85% of the calorific value of the black liquor solids. Fig. 4 illustrates the substantial increase in the quantity of solids. To maintain a calorific value of the solids of at least 95% of the calorific value of the black liquor solids, the increase in production capacity should not exceed 7%.

The comparison of increased pulp production between the process for recovery of lignin based on chemical precipitation and electrochemical processes in accordance with embodiments of the present disclosure is shown in Fig. 5. According to Fig.5, the electrochemical processes of the present disclosure offers a potential for increasing production that is substantially higher compared with the chemical option (C0 ₂ ). Assuming that it is acceptable to reduce at 90% the GCV of the black liquor fed into the recovery boiler, the electrochemical processes in accordance with embodiment of the present disclosure allow increasing the pulp production limited by the heat load in the recovery boiler by more than 50%, while the chemical precipitation of the lignin (by C0 ₂ for example) is limited to about 15% or less.

It is understood that for the purpose of the present disclosure, the black liquor can be pre-concentrated (using evaporation or other known techniques) to contain from about 15 to about 50 wt% solids; alternatively between about 15 to about 35 wt% solids before being electrochemically treated. The black liquor may also be directly electrochemically treated without being evaporated, the concentration of the black liquor then being at least about 15 wt%.

The step of electrochemically treating the black liquor can be made by electrolysis. The black liquor may be fed into the anode compartment of electrolysis cell equipped with a cation exchange membrane. In a preferred embodiment, the black liquor may be weak black liquor (15%), pre- concentrated black liquor (15 - 35% or more). The unit cell consists of an anode and a cathode separated by a membrane that is selective to monovalent Na ⁺ cations, thus defining two types of compartments,: an anode and cathode compartment. In the processes of electrolysis of black liquor, oxygen is produced with the formation of water in accordance with equation (2):

2 OH-(aq) -> H ₂ 0 + ¹ / ₂ 0 ₂ (g) + 2 e- (2)

Under the influence of the electrical field crossing the cell, the Na ⁺ ions migrate from the anode compartment to the cathode compartment by passing through the cation membrane. In the cathode compartment, the Na ⁺ ions may combine with OH ^" ions formed at the cathode, producing caustic soda and hydrogen as shown by equation (3):

2 H ₂ 0 + 2e ^" -» 2 OH ^" (aq) + H ₂ (g) (3)

In an embodiment, the inorganic materials extracted comprises sodium hydroxyde in the form of a caustic soda solution (5 - 10% by weight). The caustic solution may be recycled to but not limited to the preparation of white liquor. During electrolysis, the alkalinity (hence the level of pH) of the black liquor is substantially reduced below 1 1 , in preparation for the lignin precipitation step. In an embodiment, the pH is reduced between 4.0 and 9.5; preferably the pH is reduced between 7.5 and 9.0.

Fig. 6 shows the mass balance of an electrolysis test of black liquor having an initial concentration of 30% by weight and a final pH of 9.0. Fig. 6 shows that 26.1 % of the sodium content in the black liquor fed into the cell has been recovered in the form of caustic soda (0.261 = 33.81 · 23/40/74.5), wherein 33.81 is the production (g/h) of caustic soda, 23 and 40 are the respective molecular weight (g/mol) of sodium and caustic soda, and 74.5 is the sodium fed (g/h) into the electrolysis process in accordance with Fig. 6. From the production rate of the caustic soda, the time of operation, and the current used, it was calculated that the Faradic efficiency was more than 80%. An average anode-cathode voltage of 4.18 VDC was measured. Energy consumption corresponds to 3,500 kWh per t of NaOH produced or 870 kWh per t of lignin recovered at a rate of 66% (870 = 3500 · 33.81 / 206 / 0.66 wherein 206 is the flow of lignin (g/h) in the black liquor).

The energy used in the electrolytic treatment represents less than 18% of the gross calorific value of the lignin (considered to have a calorific value greater than or equal to 7,470 kWh per t of lignin). By assuming that the lignin is used as biomass in a cogeneration system with a rate of energy conversion of 25%, the electrolytic treatment process of the black liquor in order to precipitate the lignin presents a net gain in electricity of 1 ,000 kWh/t of lignin (7470 · 0.25 - 870).

The step of electrochemically treating the black liquor can be by electrodialysis. Electrodialysis technology is a separation technique applying an electrical field to force the migration of ions through selectively charged membranes permitting selective separation. Sodium salts as well as caustic soda are ionic substances that dissociate when dissolved in water. Thus, an aqueous solution such as black liquor containing NaOH, Na ₂ S (in the form of NaHS), Na ₂ C03, Na ₂ S0 ₄ , may be considered as an ionic solution containing cations, mostly positive Na ⁺ ions, and anions i.e. negative ions such as OH ^" , HS ^" , HC0 ₃ ^" , S0 ₄ ⁼ , and other anion species. The application of an electrical field through an electrodialytic cell forces the migration of ions to the electrodes having opposing charges modifying the ionic composition of the treated solution and leading to the extraction of the sodium salts.

The electrodialytic system includes several unit cells, each cell comprising a cation membrane, a concentration compartment (concentrate), an anion membrane, and a dilution compartment (diluate). In an embodiment, neutral polymer membranes may replace anion membranes. In a preferred embodiment, anion membranes may be used, given their selectivity for monovalent anions. In an embodiment, the concentration of the solids in black liquor from the evaporator train of the Kraft mill may vary between 15 to 35%, and may be fed into the dilution compartments of the electrodialytic cell.

The Na ⁺ ions migrate through the cation membranes and the anions (OH ^" , HS ^" , HC0 ₃ ^" ,S0 ₄ ⁼ ) migrate through the anion membranes. The extraction of the sodium salt reduces the alkalinity of the treated black liquor in order to precipitate the lignin. In an embodiment, the anion membrane is impermeable to S0 ₄ ⁼ anions.

In an alternate embodiment, the concentration compartment may be fed by but not limited to briny water, preferably comprising a certain quantity of sodium sulfate. Alternatively the concentrate compartment may also be fed by different types of aqueous solutions provided that their respective conductivity is sufficiently high. Sodium sulfate brine or weak black liquor (containing around 15% by weight of solids) may also be used.

The inorganic material extracted from the black liquor during the electrodialytic treatment is transferred in the concentration compartment (concentrate) where the concentrate comprises NaOH and sodium salts: Na ₂ S and a ₂ C0 ₃ and Na ₂ S0 ₄ Thereafter, this solution may be recycled for the preparation of the cooking liquor or returned to the black liquor circuit to the inlet of the evaporators. In the latter case, the alkalinity of this waste liquor may also serve to adjust the pH of the water from the acid washing stage of the lignin.

In an embodiment, the pH is reduced between 9 and 1 1. Preferably, the pH is reduced between 9.5 and 10.5.

It was found that, by using electrodialysis, the pH of the treated black liquor may be reduced to about 10.0 while extracting 20% or more of sodium of the inorganic materials. In an embodiment, the composition of the black liquor may be the following: Approximately 15% by weight of total solids, these solids containing 35.6% lignin and 49.3% ash (or inorganic materials) and 19.5% sodium by weight. The sodium/lignin ratio of the initial black liquor was 0.55. The inference is that the process may extract 0.1 1 tons of sodium (Na) per ton of lignin. The typical voltage for the operation of an electrodialysis cell unit was about 1 Volt DC. By assuming a current efficiency of 80%, it is estimated that the energy linked to the transport of sodium ions corresponds to 1 ,457 kWh/t-Na or 838 KWH for tons of sodium in the form of NaOH. This corresponds to 160 kWh per tons of lignin in solution. If a lignin recovery rate of 66% is considered, one obtains an energy consumption of 242 kWh per tons of pure lignin.

In an embodiment, the step of electrochemically treating the black liquor is made first by an electrodialysis followed by electrolysis as defined above.

In an embodiment, the first treatment with electrodialysis reduces the pH of the black liquor between 9 and 1 1. Preferably, the first treatment with electrodialysis reduces the pH of the black liquor between 9.5 and 10.5.

In an embodiment, the second treatment with electrolysis reduces the pH of the black liquor between 4.0 and 9.5. Preferably, the second treatment with electrolysis reduces the pH of the black liquor between 7.5 and 9.0.

In an embodiment, the at least a portion of inorganic materials extracted from the first treatment with electrodialysis comprises sodium hydroxyde and sodium salts. In an embodiment, the sodium salts are sodium sulfide and sodium carbonate.

In an embodiment, the sodium hydroxyde and the sodium salts may be recycled in the Kraft pulping process, preferably in the preparation of the cooking liquor (white liquor).

In an embodiment, at least a portion of inorganic materials extracted from the second treatment with electrolysis comprises sodium hydroxyl. The sodium hydroxyl may be recycled in the Kraft pulping process.

In an embodiment, the lignin may be precipitated from the electrochemically treated black liquor. In an embodiment, the precipitation may be made by coagulation. The variables of coagulation are the temperature, ionic strength of the solution, and the pH.

In an embodiment, the step of coagulating the electrochemically treated black liquor comprises heating the electrochemically treated black liquor between 80 to 100°C. The step of coagulating the electrochemically treated black liquor may further comprises adding sodium sulfate or another salt thereto, in order to adjust its ionic strength to the equivalent of at least 0.1 moles of sodium per kg of water, preferably between 0.5 - 2 mole/kg of water. In an embodiment, the step of coagulating the electrochemically treated black liquor may further comprises adjusting the pH below 9 by adding sulfuric acid or another acid.

In an embodiment, the step of coagulating the electrochemically treated black liquor comprises: heating the electrochemically treated black liquor between 80 to 95°C; adjusting the ionic strength of the electrochemically treated black liquor with a sodium salt to obtain a sodium molality greater than 0.1 mole/kg per kg of water, preferably between 0.5 - 2 mole/kg of water.; adjusting the pH of the electrochemically treated black liquor below 9 with an acid; or a combination thereof.

In one embodiment, the precipitated lignin may be separated mechanically, such as via centrifuge or even vacuum filtration or filtration under pressure. In an embodiment, the lignin recovered may be further submitted to washing stages. In an embodiment, at least one washing may be performed with an acid solution preferably based on sulfuric acid having a concentration of between 5 to 20% by weight, according to a quantity equivalent to between 1 to 10, preferably 3 to 4 times the mass of the solid lignin depending on the purity sought.

In an embodiment, the acid required for the washings is sulfuric acid and may be produced by electrochemical treatment of sodium sulfate. The sodium sulfate is treated by electrolysis or electrodialysis.

In at least an embodiment, lignin may be precipitated by mixing black liquor (not electrochemically treated) with the acid generated by electrochemical treatment of the sodium sulfate. In a preferred embodiment, the black liquor may be concentrated black liquor with total solid content of 50 % or above. Mixing of the black liquor with the acid generated electrochemically lower the pH of the black liquor below 11. In an embodiment, the pH is reduced between 4.0 and 9.5; preferably between 7.5 and 9.

In an embodiment, the sodium sulfate may be converted to sulfuric acid and caustic soda (salt split). The sulfuric acid solution may also comprises sodium salt (i.e. unconverted sodium sulfate). In an embodiment, the sodium sulfate is converted to sulfuric acid and caustic soda by electrolysis in a two compartment electrolyser. The overall reaction may be shown as follows:

Na ₂ S0 ₄ (aq) + 3 H ₂ 0 ^ 2NaOH(aq) + H ₂ S0 ₄ (aq)+ H ₂ (g) + ¹ / ₂ 0 ₂ (g) (4)

In an embodiment, the unit cell of the electrolyser consists of an anode and a cathode separated by a membrane that is selective to Na ⁺ ions defining two types of compartments, which are the anode compartment and the cathode compartment. A concentrated, preferably saturated, sodium sulfate solution is fed into the anode compartment. In a first step, when the solution is alkaline, oxygen will be produced with consumption of OH ^" ions according to the equation (2). Consequently, during electrolysis, the pH of the solution is reduced gradually as the OH ^" ions are neutralized. In a second step, when the solution becomes acid, H ⁺ ions are produced and they combine with the sulfate ions (S0 ₄ ^" ) present in the solution to form sulfuric acid (H ₂ S0 ₄ ). Under the influence of the electrical field in the cell, the Na ⁺ ions migrate from the anode compartment through the cation membrane to the cathode compartment. In the cathode compartment, the Na ⁺ ions combine with the OH ^" ions formed at the cathode according to the equation (3). In the cathode compartment, caustic soda is produced with release of hydrogen.

It was observed that a conversion rate near 100% of sodium sulfate to sulfuric acid may be difficult to obtain due to excessive consumption of electricity. It was observed that, a portion of the sodium sulfate, preferably 50 - 60% may be converted to sulfuric acid and caustic soda with an acceptable consumption of electricity. The sulfuric acid solution exiting from the anode cell compartments therefore contains unconverted residual sodium sulfate. In an embodiment, the sulfuric acid solution may be used for the precipitation steps and washing of the lignin.

In an embodiment, the sulfuric acid solution may be purified in a separation unit in which a first acid solution and second solution containing unconverted residual salt are obtained. Figure 7 illustrates an embodiment in which the sodium sulfate electrolysis process is combined with a separation system allowing a separation of the acid produced and the unconverted sulfate. The unconverted sulfate solution may be recycled and converted to sulfuric acid with another electrolysis treatment. It is of note that recycling the unconverted sodium sulfate solution permits to increase the overall conversion rate of the salt to the acid and the base, especially when the quantity of sodium available is limited. In an embodiment, the acid solution may be used in the coagulation step and/or for washing the filtered lignin.

The electromotive force of the sulfate electrolysis reaction as described by the equation (4) is 2.06V, under standard conditions (25°C, 1 atm). A voltage between 4 and 5 volts was measured with a current efficiency of 75 - 85 % with a sulfate conversion rate of 60%. In the case of treating a sodium sulfate saturated solution in an electrolyzer at an operating temperature of about 80°C, the acid and caustic soda concentrations that are achieved may be up to 20% by weight. With a typical voltage of 4V DC and a current efficiency of 80%, the energy consumption of the sulfate electrolysis may be 3,350 kW per ton of NaOH. According to the equation (4), for one mole (i.e., 142 g) of Na ₂ S0 ₄ , 1 mole (98 g) of H ₂ S0 ₄ and 2 moles of NaOH (80 g) may be produced. The specific energy consumption may be around 2,735 kWh per ton of acid produced.

The sodium sulfate solution may contain salts other than sodium sulfate. For example, the solids collected by the electrostatic precipitator for treating the flue gas of the recovery boiler may contain sodium sulfate but also a significant fraction of sodium carbonate, typically between 10 and 20% by weight. In such case, sodium carbonate may be converted into C0 ₂ in the anode compartment of the cell and into NaOH in the cathode compartment. This process is described by equation (5):

Na ₂ C0 ₃ (aq) + 2 H ₂ 0 -» 2 NaOH(aq) + C0 ₂ (g) + H ₂ (g) + ½ 0 ₂ (g) (5)

This reaction may be beneficial as it implies an extraction of the fraction of sodium present for example in the solid of the precipitator and the release of carbon dioxide. Therefore, the sodium sulfate and the sodium carbonate may be deviated from the recovery boiler partially reducing first the inorganic load on the recovery boiler and secondly the sodium carbonates load in the recausticizing system. It is to be noted that C0 ₂ may also be used to acidify the black liquor.

The sodium sulfate may be converted in sulfuric acid and caustic soda by electrolysis in a three compartment electrolyser. In an embodiment, the cell unit of the electrolyser includes an anode and a cathode between which there is a cation selective membrane for the passage of Na ⁺ ions and an anion selective membrane for the passage of S0 ₄ ⁼ ions, defining three types of compartments, an anode compartment, a dilution compartment, and a cathode compartment.

The electrolyser has then three circuits linked respectively to each of the compartment types described above and are isolated from each other, i.e., that they are different. The circuit linked to the dilution compartment may be fed by saturated sulfate solutions and in this way a substantially higher sodium sulfate conversion rate may be obtained. The sulfate is then separated from the acid formed without the need of a sulfate/acid separation step. The anode compartment may be fed by water or preferably by a sulfuric acid solution while the cathode compartment may be fed by water, or preferably by a caustic soda solution. Under the influence of the electrical field crossing the cell, the Na ⁺ ions migrate from the dilution compartment through the cation membrane to the cathode compartment where the Na ⁺ ions combined with the OH ^" ions produced at the cathode. At the same time, the S0 ₄ ⁼ ions migrate from the dilution compartment through the anion membrane to the anode compartment of the cell, where the S0 ⁼ ions combined with the H ⁺ ions produced at the anode. A reduction of sulfates concentration may be obtained in the dilution compartment and, at the same time, production of acid with the release of oxygen in the anode compartment; and production of a base with a release of hydrogen in the cathode compartment.

The sodium sulfate may be converted to sulfuric acid and caustic soda (salt split) by bipolar membrane electrodialysis according to equation (6):

Na ₂ S0 ₄ (aq) + 2 H ₂ 0 -» 2NaOH(aq) + H ₂ S0 ₄ (aq) (6)

In an embodiment a three compartment bipolar electrodialyser can be used. The elecrodialyser cell unit includes one bipolar membrane, a cation selective membrane (for the passage of Na ⁺ ions) and an anion selective membrane (for the passage of S0 ₄ ⁼ ions) all of which define three types of compartments: a compartment for the formation of a base, a dilution compartment containing a sodium sulfate solution and a compartment for the formation of an acid. In use, a saturated sodium sulfate solution may be fed into the dilution compartment, or the central compartment. The acid compartment may be fed by water or preferably by a sulfuric acid solution while the base compartment may be fed by water, or preferably by a caustic soda solution. Under the influence of the electrical field crossing the cell, the Na ⁺ ions migrate from the dilution compartment through the cation membrane to the base compartment where the Na ⁺ ions combine with the OH ^" ions produced on the anion side of a bipolar membrane. At the same time, the S0 ₄ ⁼ ions migrate from the dilution compartment through the anion membrane to the acid compartment of the cell, where the S0 ₄ ⁼ ions combine with the H ⁺ ions produced on the cation side of a bipolar membrane. This gives a reduction of sulfates concentration in the dilution compartment; production of acid in the acid compartment; and production of NaOH in the base compartment.

In an embodiment, the three compartment electrodialyser has three circuits linked respectively to each of the compartment described above and are isolated from each other, i.e., that they are different. The circuit linked to the dilution compartment may be fed by saturated sulfate solutions and in this way a substantially higher sodium sulfate conversion rate may be obtained. Preferably, a conversion rate substantially closer to 100% is obtained.

In an embodiment, the cell unit may be stacked in series to form a full stack with at each ends of the stack, an anode and a cathode for the generation of the electric field required. A circuit must be provided for the movement of an electrolyte within the electrode compartments, but this circuit is not predominent if the number of unit cells, each comprising a bipolar membrane, an anion membrane, and a cation membrane, is high in the stack.

In another embodiment, a two compartments bipolar electrodialyser may also be used to convert sodium sulphate to sulphuric acid and caustic soda. The elecrodialyser cell unit includes one bipolar membrane and a cation selective membrane (for the passage of Na+ ions) all of which define two types of compartments: a compartment for the formation of a base and a compartment for the formation of an acid. The acid compartment may be fed by sodium sulfate aqueous solution while the base compartment may be fed by water or preferably by a caustic soda solution. Under the influence of the electrical field crossing the cell, the Na+ ions migrate from the acid compartment through the cation membrane to the base compartment where the Na+ ions combine with the OH- ions produced on the anion side of a bipolar membrane. At the same time, there is production of protons H+ at the cation side of an adjacent bipolar membrane. This allows production of acid in the acid compartment and production of NaOH in the base compartment. It is said that solution at the exit of the acid compartment will contain sulfuric acid and non converted sodium sulfate. The two compartment bipolar electrodialyser has then two circuits linked respectively to each of the compartment described above and are isolated from each other, i.e., that they are different.

In an embodiment, the disclosure also provides a process for the recovery of lignin from black liquor. The process comprises:, a) electrochemically treating a sodium sulphate by-product recovered from a Kraft pulping process to produce an aqueous solution comprising sulphuric acid; b) electrochemically treating the black liquor to reduce the pH thereof below 1 1 to produce an electrochemically treated black liquor; c) Precipitating the lignin from the electrochemically treated black liquor to produce a liquor comprising solid lignin; d) Filtrating and washing the solid lignin with the aqueous solution comprising aqueous sulphuric acid obtained in the first process; e) recovering the solid lignin substantially free of inorganic materials, f) recovering waste liquor .

Waste liquors (i.e. filtrate) from the lignin recovery process may comprise organic materials other than lignin, the remainder of the lignin not recovered and inorganic material not extracted.

In an embodiment the black liquor is directly electrochemically treated or may be pre-concentrated between 15-35 wt% before being electrochemically treated. Fig. 8 shows an embodiment of the process for recovery of lignin wherein the black liquor is electrochemically treated by electrolysis as defined above. In an embodiment the pH is reduced between 4.0 and 9.5, preferably between 7.5 and 9.0. In an embodiment, inorganic materials are extracted during the electrodialysis of the black liquor. The inorganic materials extracted comprise sodium hydroxyl. In an embodiment, the sodium hydroxyl may be recycled in the Kraft pulping process.

Fig. 9 shows an alternative embodiment where the black liquor is electrochemically treated by electrodialysis as defined above. In an embodiment, inorganic materials are extracted during the electrodyalysis of the black liquor. In an embodiment, the inorganic materials extracted comprises sodium hydroxyde and sodium salts. Preferably, the sodium salts include sodium sulfide and sodium carbonate. In an embodiment, the sodium hydroxyl and the sodium salts may be recycled in the Kraft pulping process.

Fig. 10 illustrates an embodiment of the process for recovering lignin wherein the black liquor may be treated first by electrodialysis in order to lower the pH below 1 1 , preferably between 9 and 1 1 , and most preferably between 9.5 and 10.5 to produce an electrodialysed treated black liquor. Afterwards, the electrodiaiyzed black liquor may be treated by electrolysis to further reduce the pH below 9.0. to produce electrochemically treated black liquor. In an embodiment, in the second treatment with electrolysis the pH is reduced between 4.0 and 9.5, preferably between 7.5 and 9.0.

In an embodiment, inorganic materials may be extracted during the electrodialysis of the black liquor. The inorganic materials extracted comprises sodium hydroxyl and sodium salts. The sodium salts include sodium sulphide and sodium carbonate.

In an embodiment, inorganic materials may also be extracted during the electrolysis of the black liquor. The inorganic materials extracted comprise sodium hydroxide.

In an embodiment, the inorganic materials may be recycled in the Kraft pulping process as mentioned previously. In an embodiment, the electrochemically treated black liquor may be treated under condition of heat and/or chemical coagulation in order to precipitate the lignin. In the coagulation step, the ionic strength of the treated solution may be increased by adding salts coming from the electrostatic precipitator and/or from the chlorine dioxide generation system of the Kraft pulp process. In an embodiment, the pH of the treated solution may be adjusted below 9 if needed, by adding a sulfuric acid solution made from the electrochemical treatment of sodium sulfate.

In an embodiment, the coagulation may be performed by simple heating, without recourse to adding salts or acid, if the pH level of the treated solution is sufficiently low (preferably lower than or equal to 9.0).

The solution stemming from the coagulation may be filtered and the solid lignin recovered may be washed with a dilute sulfuric acid solution which may contain sodium sulfate made from the electrochemical treatment of sodium sulfate, before being filtered again and washed again with water and thereafter being submitted to a last step of filtration. The purified lignin is then recovered in the form of a wet solid.

In an embodiment, the step of electrochemically treating the sodium sulphate may be made by electrolysis or by electrodialysis as defined above.

In an embodiment, the step of electrochemically treating sodium sulphate further comprises producing an aqueous solution comprising sodium hydroxyl. The aqueous solution comprising sodium hydroxyl may be recycled in the Kraft pulping process.

In an embodiment, the step of precipitating the lignin comprises heating the electrochemically treated black liquor between 80 to 95°C.

In an embodiment, the step of precipitating the lignin comprises adjusting the ionic strength of the electrochemically treated black liquor with a sodium salt to obtain a sodium molality greater than 0.1 mole/kg per kg of water, preferably more than 0.5 - 2 mole/kg of water.

In an embodiment, the step of precipitating the lignin comprises adjusting the pH of the electrochemically treated black liquor below 9 with the sulphuric acid obtained by electrochemical treatment of sodium sulphate . Fig. 12 illustrates an embodiment of the process for recovering lignin wherein the black liquor is acidified by mixing with an sulfuric acid solution made by electrochemical treatment of sodium sulfate. The acidification lower the pH of the black liquor below 11 , most preferably below 9.

In an embodiment, the step of precipitating the lignin comprises at least one of the following: heating the electrochemically treated black liquor between 80 to 95°C; adjusting the ionic strength of the electrochemically treated black liquor with a sodium salt to obtain a sodium molality greater than 0.1 mole/ kg per kg of water, preferably more than 0.5 - 2 mole/kg of water; adjusting the pH of the electrochemically treated black liquor below 9 with sulphuric acid.

In an embodiment, for the step of filtrating and washing the solid lignin, the aqueous sulphuric acid has a concentration between 5 to 20% by weight. The amount of sulfuric acid solution used is equivalent to between 1 and 10 times, preferably 3 to 4 times the mass of the solid lignin depending on the purity desired.

In an embodiment, the step of filtrating and washing the solid lignin further comprises washing the solid with water. The amount of water used is equivalent to between 1 and 10 times (preferably 3 to 4) times the mass of the solid lignin depending on the purity desired. In an embodiment, the acid and water washing steps may be integrated into the mechanical filtration process for the lignin.

The waste liquor (filtrate) from the lignin separation step comprises: organic materials other than lignin, the remainder of the lignin not recovered, and remainder of inorganic material not extracted. In an embodiment, the process for recovery of the lignin from black liquor further comprises: combining the waste liquor and the washing solution from the separation step; and feeding the combined waste liquor and washing solution to the evaporation system of the Kraft pulping process.

In an embodiment, the electrical consumption is to be limited as possible and represent a fraction, preferably less than 25% of the calorific value of the pure lignin being produced by a process in accordance with the present disclosure. Lignin recovery rate during process in accordance with an embodiment may be about 50 % or more, preferably greater than 60 % and the following electrical conditions prevail regarding concerns with limitation of the electrical consumption of the complete lignin recovery process:

Voltages of the unit cell: for:

a) Black liquor electrolysis: V < 5 VDC

b) Black liquor electrodialysis: V < =1.0 VDC

c) Electrolysis/electrodialysis of salts: V < 5 VDC

d) Current efficiency: >80%

e) Treatment sequence electrical consumption: < 1 ,875 kWh/TM lignin

EXAMPLE 1 Process for recovery lignin by Electrolysis.

Tables 2a and 2b summarize mass balances of a process for recovery of lignin in accordance with the present disclosure, as schematically illustrated in Fig. 1 1 , for producing 1 ,000 kg/h of pure lignin, based on the results from a pilot testing. Upon entering into the treatment sequence, 11 ,220 L/h of black liquor flow containing 379.5 g/L of total solids (which gives us a concentration of about 30%), 135 g/L of lignin pure and 74.06 g/L of sodium (Na) is fed. In this stream, there is thus 1 ,515 kg/h of lignin accompanied by 831 kg/h of sodium. The black liquor is treated first with Electrolysis to extract 160 kg/h of NaOH in an aqueous solution at 5.3% by weight of NaOH (stream 2). The electrolyzed black liquor was heated at 80 - 85°C for a period of 1 to 3 hours. Afterwards, the solution is filtered and the cake is submitted to washing and filtration cycles involving successively: 4,808 L/h of a sodium sulfate solution at 50 g/L, which represents feeding 240 kg/h of sulfate or 78 kg/h of sodium in the form of Na (stream 5), 4,808 L/h of sulfuric acid solution at 49 g/L, which represents feeding 236 kg/h of H ₂ S0 ₄ (stream 6), and finally, 4,808 L/h of water for a final rinse of the lignin produced (stream 7). 1 ,021 kg/h of lignin is produced substantially free of inorganic materials such that 1 ,000 kg/h of pure lignin is obtained (stream 8).

The ratio between the sodium flow in stream 2 and that of stream 1 gives us the sodium extraction rate, which is 19.3%. This sodium is extracted in the form of caustic soda (NaOH). Finally, the lignin recovery rate is 66%. As already mentioned, 236 kg/h of sulfuric acid is needed. This acid is made by electrolysis or electrodialysis of sodium sulfate. To produce 236 kg of sulfuric acid, 570 kg/h of sodium sulfate is needed assuming a conversion rate of 60%.

Table 2-a Composition and characteristics of the streams of Figure 11

Table 2-b Composition and characteristics of the streams in Figure 11 (continued)

The electrical energy may be calculated for the entire treatment sequence. To do this, it is necessary to calculate the electrical energy required for electrolysis and that required for the electrochemical conversion of sodium sulfate to sulfuric acid. Assuming a cell voltage of 4.18 VDC and a current efficiency of 80%, a specific consumption of 3500 kWh per t of NaOH produced is obtained and an electrical consumption of 954 kWh per ton of lignin recovered is calculated.

The amount of energy required for the electrochemical conversion of sodium sulfate to sulfuric acid needed to wash the lignin may be estimated. Assuming a voltage of 4 VDC and a current efficiency of 80% for electrolysis conversion of sodium sulfate to sulfuric acid, an electrical consumption of 632 kWh per ton of recovered lignin is obtained. In total, for the entire process, 1 ,586 kWh was consumed per ton of lignin recovered (954 + 632). This represents a quantity of electricity equivalent to less then 25% of the calorific value of lignin (which represents 1 ,870 kWh per ton of lignin).

EXAMPLE 2 Electrodialysis of weak black liquor.

Table 3 summarizes the results obtained by the electrodialysis of weak black liquor in accordance with the present disclosure, as schematically illustrated in Fig. 9.

2.5 L of weak black liquor containing at least about 15wt% solids is treated by electrodialysis in an electrodialysis system including several cell units in accordance with an aspect of the invention. The electrodialysis cell used includes an arrangement of 4 anionic membranes and 4 cationic membranes, each being 180 cm ² , providing an overall efficient surface of 720 cm ² . A current density of 22,2 mA/cm ² at a temperature of 60°C is applied to the electrodialysis system. During the electrodialysis treatment, which lasted 4.8 h, the volume of weak black liquor was reduced from 2.5 L to 1.6 L, due to the transfer of water accompanying the ions. The electrodialysis treatment consumed 86.35 Wh. The quantity of lignin present in the final diluate is 130.7 g. Consequently, the specific consumption of energy of electrodialysis corresponds to 661 kWh per ton of total lignin present in the solution to be treated.

During the electrodialysis treatment, the solution to be treated is recirculated in the dilution circuit. Weak black liquor containing at least about 15wt% solids is recirculated in the concentrate circuit. The inorganic material is extracted from the diluate in the dilution compartment. The anions (OH ^" , HS ^" , C0 ₃ ⁼ , S ₂ 0 ₃ ⁼ , C0 ₃ ^" , S0 ₄ ⁼ ) migrate through the anionic membrane to the anode and the Na+ cations migrate through the cationic membrane to the cathode.

The inorganic material is concentrated in the concentration compartment, producing an enriched solution circulating in the concentrate circuit. About 72% of the sodium was removed from the treated weak black liquor (Table 3). The electrodialysis treatment of black liquor has reduced the alkalinity of the weak black liquor by extracting inorganic materials such as caustic soda (NaOH), sodium sulfide (Na ₂ S) and sodium carbonate (Na2CC>3), lowering the pH of the weak black liquor from 11.5 to 10.4 before precipitation of lignin.

The concentrate initially comprising weak black liquor now comprises at least 62% of the inorganic material stemming from the fact that inorganics have been extracted from the diluate in the dilution compartment, to the concentrate in the concentration compartment. During the electrodialysis treatment, the volume of diluate in the dilution compartment was reduced from 2.5 L to 1.6 L while the volume of the concentrate in the concentration compartment was increased from 2.5L to 3.0L due to the transfer of water accompanying the ions.

A 300 ml sample was taken from the final diluate to test the coagulation of lignin. The lignin was precipitated by heat at 80 °C and by adding 6 ml of sulphuric acid 45wt% solution. The pH was lowered from 10.15 to 8.61. The precipitated lignin was isolated by filtration. 23.12g of lignin having a moisture content of 60% and an ash content of 11.77% was recovered. The rate of lignin recovery in this acidification step is 33 %, meaning that 33 % of the lignin present in the 300 ml sample, was recovered. The quantity of sulfuric acid used corresponds to 0.15g of H ₂ S0 ₄ per g of lignin present in the sample.

The filtrate liquor resulting from the filtration of lignin was recovered. Lignin was precipitated by heat at 80 °C and by adding 4.5 ml of sulphuric acid 45wt% solution. The pH was further lowered to 4.11. The precipitated lignin was isolated by filtration on filter paper of 250 microns (60 mesh). 13g of lignin with a moisture content of 60% and an ash content of 12.35% was recovered. The rate of dry lignin recovery for this acidification step is 16% (i.e. 16% of the lignin initially contained in the sample of 300 ml, was recovered in the second step of coagulation). The quantity of acid used corresponds to 0.11g of H ₂ S0 ₄ per g of lignin initially present in the sample of 300 ml.

If the results of the two steps of coagulation are combined, the total recovery rate of lignin is 49% (33% + 16%). The total quantity of acid used to recover lignin corresponds to 0.26g of H ₂ S0 ₄ per g of lignin present in the sample (0.15 + 0.11).

Table 3 Mass balance - batch electrodialysis of black liquor 15% , 4 membrane pairs of 180 cm ² , total of 720 cm ² ), current density of 22.2 mA/cm ² or current of 4 A, for 4.8 hrs.

Initial concentrate Final concentrate

Volume (L) 2.5 3.0 Transfert Rate g/L g/h g/L g % g/h kg/h/m*

Density 1.077 1.103

Solids 161.6 403.9 184.2 552.7 148.7 36.82 31.0 -0.431

Inorg 82.08 205.2 110.7 332.1 127.0 61.87 26.5 -0.368

Org 79.49 198.7 73.50 220.5 21.78 10.96 4.5 -0.063

Na ⁺ 33.12 82.80 52.78 158.4 75.55 91.23 15.8 -0.219

Na ₂ 0 7.868 19.67 11.48 34.43 14.76 75.03 3.1 -0.043

NaOH 10.15 25.382 14.81 44.43 19.04 75.0 4.0 -0.055

STOT 7.410 18.52 10.20 30.61 12.09 65.25 2.5 -0.035

Lignin 62.79 157.0 52.78 158.4 1.383 0.881 0.3 -0.004

C0 ₃ ⁼ 8.935 22.34 14.65 43.94 21.60 96.70 4.5 -0.063

S ₂ 0 ₃ ⁼ 3.377 8.442 5.140 15.42 6.977 82.65 1.5 -0.020

S0 ₄ ⁼ 2.359 5.897 3.703 11.11 5.21 88.36 1.1 -0.015

S= 6.220 15.55 7.792 23.38 7.826 50.33 1.6 -0.023

EXAMPLE 3: Process for recovery of lignin wherein black liquor is mixed with a sulphuric acid solution that is generated electrochemically in accordance with an embodiment of the present disclosure.

150 ml (or 215 g) of concentrated black liquor from a typical Kraft mill, having a solid concentration of 70.1 % including 411 g/L of lignin, was acidified to pH 9 by mixing the black liquor sample with a 10% wt sulfuric acid solution (or 106.6 g/l or 2.28 N acid). Sample was fed into an Erlenmeyer at 75 °C and the mixing was made by progressively adding sulfuric acid solution at a rate of 4.8 ml/min. The mixture was continuously agitated with an impeller. It was determined that 126 ml of acid solution was required to reach pH of 9.0. The lignin was precipitated by heating the mixture at 75 °C for 1 hour while maintaining a gentle agitation (impeller at 100 rpm). The mixture was filtered (first filtration) under a vacuum of 5 to 10 inch Hg in a stainless steel 150 micrometers wire mesh disc of 11 cm diameter, until there was no significant flow of filtrate. Duration of this first filtration was 35 seconds. The wet cake was recovered and mixed with 300 ml of sulfuric acid 1 N (49 g/L of acid) in order to form a slurry. A second filtration under conditions identical to the first filtration was made. The duration of this second filtration was 16 seconds. The cake was washed with 300 ml of water slowly added to the cake. The cake was dried under vacuum. Duration of the filtration after the addition of water, was 60 sec.

The filter cake was weight was 90.52 g. The filter cake was dried in an oven for 24 hours. The weight of the solid was 35.27 g. It was determined that the ash concentration of this solid lignin is 4.08 %. It was considered that 35.27x(1-4.08/100) or 33.83 g of pure lignin was recovered. From this example, the ratio of the recovered lignin over the initial pure lignin content of the sample was 33.83/(150/1000 x 41 1 ) or 54.9 %.

The quantity of acid required for the acidification was 126/1000x106.6 or 13.43 g, this was for the recovery of 33.83 g of pure solid lignin. This means 0.40 g of H ₂ S0 ₄ per g of product. The quantity of acid required for the acid wash was 300/1000x49=14.7 g of H ₂ S0 ₄ . This was for the recovery of 33.83 g of lignin. This represent an additional quantity of 0.43 g of H ₂ S0 ₄ per g of recovered pure solid lignin. Globally, 0.83 g of H ₂ S0 ₄ was required per gram of pure recovered lignin for this run.