FI20176189A1

FI20176189A1 - A method and a system for adjusting S/Na -balance of a pulp mill

Info

Publication number: FI20176189A1
Application number: FI20176189A
Authority: FI
Inventors: Risto Hämäläinen; Seppo Tuominiemi
Original assignee: Valmet Technologies Oy
Priority date: 2017-12-29
Filing date: 2017-12-29
Publication date: 2019-06-30
Also published as: FI129615B

Description

A method and a system for adjusting S/Na -balance of a pulp mill

Technical field

The invention relates to a method and a system for adjusting S/Na -balance of a pulp mill. Some aspects of the invention relate to a method and a system for separating sulphur from pulp mill liquor. Some aspects of the invention relate to a method and a system for biological oxidation of sulphur compounds of pulp mill liquor inside a pulp mill.

Background

Industrial pulping processes, chemical pulping processes in particular, are utilized to remove hemicelluloses and lignin from the wood-based raw 15 material in order to provide cellulose fibres. The chemical cooking process, sulphate cooking in particular, uses a combination of high temperature and pulping chemicals to break the chemical bonds of lignin, which is a natural biopolymer in the wood that binds the cellulose fibres together. In a sulphate cooking process, wood-based material is mixed in a digester with an 20 aqueous solution of pulping chemicals, and then heated with steam. An example of a sulphate process is the Kraft process, wherein the main pulping chemicals are sodium hydroxide (NaOH) and sodium sulphide (Na2S). The chemical cooking process separates cellulose fibres from the lignin and hemicellulose components, and produces spent cooking liquor, referred to as 25 black liquor. This liquor containing the spent cooking chemicals and byproducts is then concentrated and typically burned to recirculate the cooking chemicals. Recirculation of the cooking chemicals is typically referred to as the liquor cycle or the chemical recovery cycle of a pulp mill.

Due to tightened legislation relating to environmental protection, modern pulp mills need to circulate chemicals more carefully as well as try to diminish the accumulation of sulphur compounds in the environment. Conventional means for dealing with sulphur containing side streams formed at the pulp mill processes have been to dump the side streams as a fly ash or to recirculate 35 the sulphur containing side streams to other processes for manufacture of industrial chemicals. One example for sulphur recovery is the combustion of

20176189 prh 29 -12- 2017 malodorous gases, which are formed as a by-product of the pulp manufacturing process. The combustion of the malodorous gases produces flue gas containing sulphur oxides, which may be recovered and further used to manufacture for example sulphuric acid. Sodium bisulphite, dithionite and 5 gypsum are other examples of possible products which may be manufactured from the sulphur containing side streams of a pulp mill. However, the refining of pulp mill flue gas or sulphur containing side streams to more valuable chemicals requires massive capital investments and separate chemical plants. The refining may further be problematic from the 10 environmental perspective. Furthermore, such investments are time consuming and may be difficult to retrofit to already existing processes at conventional pulp mills.

Sulphur is a critical chemical in the chemical cooking process of a sulphate 15 pulp mill and needs to be removed from and replenished to the chemical recovery cycle on a continuous basis. A particular downside related to the conventional ways for recovering sulphur from the pulp mill is the concomitant loss of sodium from the chemical cooking process, which is typically recovered together with the sulphur. This leads to loss of two critical 20 elements in the cooking chemicals, which is undesirable for the S/Na balance of the pulp mill. It is therefore a constant dilemma how the total sulphur content of the chemical recovery cycle could be reduced and how the S/Na -balance of the pulp mill could be improved in view of stricter legislation. The accumulation of sulphur into the chemical recovery cycle is a continuous 25 challenge for the efficient operation of the pulp mill. Thus, there is a need for a cost-effective and environmentally friendly method and system for controlling the S/Na -balance of a pulp mill that are easier to implement on an already existing process of a conventional pulp mill.

Summary

The above disclosed problems may be addressed by providing a method and a system which enables adjustment of S/Na -balance of a pulp mill by separation of sulphur compounds from pulp mill liquors, such as green or 35 white liquors, which comprise sulphides, and oxidation of sulphides into elemental sulphur with microbes. An advantage is that the total sulphur

20176189 prh 29 -12- 2017 content of the pulp mill processes may be reduced, since the circulation of sulphur in the pulp mill processes is shorter, when the excessive sulphur is recovered from the liquor cycle, instead of later phases of the process, such as the gases or fly ash formed in the pulp mill processes. A further advantage 5 is, that adjusting the S/Na -balance of the pulp mill may be implemented in a simpler and faster manner. Moreover, sulphur may be recovered in its elemental form without losing sodium at the same time. This reduces the need for adding make-up NaOH in order to adjust the sulphidity of the pulp mill, thereby lowering the costs and enabling avoidance of unnecessary use 10 of chemicals. Thus, adjusting S/Na -balance of the pulp mill in a cost-efficient and environmentally friendly manner is enabled.

Recycling of the spent cooking chemicals in a pulp mill is denoted as a liquor cycle or chemical recovery cycle of the pulp mill. The used cooking chemicals 15 may be burnt in a recovery boiler thus forming a molten ‘smelt’ that may be dissolved into a liquid. Thus formed liquid may be denoted as green liquor due to a characteristic green color. Green liquor may be used to prepare white liquor for the pulping process. The liquor cycle is designed to recover the chemicals used in the pulping.

Sulphur balance control is important in a pulp mill. As sulphur is introduced to the cooking process, typically as sodium sulphide (Na2S), sulphur also has to be removed from the chemical recovery cycle in some form in order to avoid excessive sulphur content in the cycle. Excessive sulphur content as well as 25 unnecessary low sulphur content in the chemical recovery cycle may cause operational problems resulting for example in poor pulping liquor quality, increased mill energy consumption, and decreased mill production capacity. S/Na -balance of a pulp mill is related to sulphidity. Sulphidity is a percentage value of a ratio between amounts of Na2S and active alkali in the pulp mill 30 white liquor. Active alkali refers to NaOH and Na2S. The optimum sulphidity depends on several factors, such as wood species, alkali charge, cooking temperature and properties desired in the final product. Typically the sulphidity may vary between 20-50 %.

Green liquor containing Na2S and NaHS is an essential part of the liquor cycle taking care of the recovery of chemicals used in the pulping. White

20176189 prh 29 -12- 2017 liquor, which is formed of green liquor also contains sulphides as disclosed above. Thus, a green liquor stream diverted from a recovery boiler or a green or white liquor stream diverted later from the process represent convenient sources of material for adjustment of S/Na -balance of a pulp mill by 5 removing sulphur from the chemical recovery cycle.

According to an aspect of the invention, at least part of a pulp mill liquor stream, such as green or white liquor stream, containing sulphides is diverted into a bioreactor. The liquor containing sulphides may then be oxidized 10 biologically in the bioreactor by means of sulphur-oxidizing microbes, thus forming elemental sulphur. The elemental sulphur may then be recovered.

According to another aspect of the invention, at least part of a pulp mill liquor stream, such as green or white liquor stream, containing sulphides may be 15 diverted into a stripper. The pulp mill liquor containing sulphides may be stripped in the stripper with an acidic agent. The acidic agent lowers the pH of the pulp mill liquor. By this way, sulphides of the pulp mill liquor may be transformed into gaseous H2S. Thus, a gas stream containing H2S and a residual pulp mill liquor stream may be obtained. The gas stream containing 20 H2S is then scrubbed in a scrubber with an aqueous scrubbing solution containing an alkaline agent, such as NaOH. When contacted, H2S reacts with the alkaline agent, thereby producing an aqueous spent scrubbing solution containing sulphides, such as Na2S and NaHS, which sulphides, when reacted, transfer themselves from the gaseous phase into the liquid 25 phase, such that a selective sulphide conversion may be obtained. The aqueous spent scrubbing solution containing sulphides is then oxidized biologically in a bioreactor by means of sulphur-oxidizing microbes, thereby forming elemental sulphur. The elemental sulphur may then be recovered.

Therefore, there is provided a method for adjusting S/Na -balance of a pulp mill, which method comprises

- diverting an aqueous pulp mill liquor containing sulphides into a bioreactor,

- oxidizing the aqueous pulp mill liquor containing sulphides in the bioreactor biologically in an oxidizing reaction by means of sulphur

20176189 prh 29 -12- 2017 oxidizing microbes, thereby producing an aqueous suspension containing elemental sulphur, and

- separating the elemental sulphur from the aqueous suspension in a sulphur separation unit located downstream of the bioreactor, thereby obtaining a residual solution and a precipitate containing the elemental sulphur.

Optionally, a method for adjusting S/Na -balance of a pulp mill may comprise

- diverting an aqueous pulp mill liquor containing sulphides into a stripper,

- stripping the aqueous pulp mill liquor containing sulphides in the stripper with an acidic agent, thereby obtaining a gas stream containing H2S and a residual pulp mill liquor stream,

- scrubbing the gas stream containing H2S in a scrubber located downstream of the stripper with an aqueous scrubbing solution containing an alkaline agent, whereby at least some of the H2S reacts with the alkaline agent, thereby producing a residual gas stream and an aqueous spent scrubbing solution containing sulphides,

- conducting the aqueous spent scrubbing solution into a bioreactor,

- oxidizing the aqueous spent scrubbing solution containing sulphides in the bioreactor biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby producing an aqueous suspension containing elemental sulphur, and

Objects according to the invention are further described in the appended 30 claims.

Brief description of the drawings

Figure 1 illustrates, by way of an example, a process diagram of a system configured to adjust S/Na -balance of a pulp mill,

Figure 2a

Figure 2b

Figure 3

Figure 4 illustrates, by way of an example, a variation of a process diagram of a system configured to adjust S/Na -balance of a pulp mill, illustrates, by way of an example, another variation of a process diagram of a system configured to adjust S/Na balance of a pulp mill, illustrates, by way of an example, a stripper configured to separate sulphur from a pulp mill liquor stream, illustrates, by way of an example, a scrubber configured to separate sulphur from a pulp mill liquor stream, and

Figure 5 illustrates, by way of an example, a bioreactor configured to separate sulphur from a pulp mill liquor stream.

The figures are schematic. The figures are not in any particular scale.

Detailed description

20176189 prh 29 -12- 2017

The term “scrubber” refers to an air pollution control device which is used to remove particulates or compounds from a pulp mill exhaust gas stream. An aqueous solution may be introduced into the scrubber to collect unwanted pollutants from a gas stream into an aqueous spent scrubbing solution.

The term “efficiency” refers to a quantitative ratio of output to the total input. Unless otherwise stated, efficiency in this context is calculated as a percentage of the theoretical maximum, which the given total input quantities could yield. In other words, efficiency is expressed as a percentage of the result that could ideally be expected.

The term “weak malodorous gas” typically refers to a gas having a sulphur concentration of less than 0,5 g/m³ Weak malodorous gas may also be called a diluted malodorous gas. The weak malodorous gases may in a pulp

20176189 prh 29 -12- 2017 mill environment originate for example from chip-pre-steaming, screening, pulp washing, smelt dissolver and ventilation of various tanks.

The term “strong malodorous gas” typically refers to a gas having a sulphur concentration above 5 g/m³ The strong malodorous gases may in a pulp mill environment originate for example from digester, evaporation plant and condensate stripper.

The term “volumetric flow rate” refers to a volume of a fluid passing per unit of time.

The term “mass flow rate” refers to a mass of a substance passing per unit of time.

Within the context of this specification, the term “sulphides” refers to compounds or substances comprising HS or S² entities. Those compounds or substances include, for example, NaHS and Na2S, as well as their hydrates.

The term “clarifying” refers to a process in which a fluid, usually a liquid, is made clear by removing impurities or solid matter.

The term “aerating” refers to supplying oxygen or air. Aeration is a process by which air is circulated through, mixed with or dissolved in a liquid, thereby allowing oxygen to be transferred into the liquid, such as an aqueous solution.

In a chemical pulp production cooking is used for recovering fibres from chips in a digester by using chemicals and heat in order to remove fibre binding lignin and, in addition, to remove wood extractives which may later cause foaming and precipitants in the process. Therefore, chemicals which dissolve as much lignin and as little cellulose as possible are typically used in the pulping process. Typically, the process for manufacturing bleached chemical pulp comprises pulping, washing, screening, bleaching, and cleaning stages. Nowadays sulphate cooking, also called as Kraft cooking or pulping, which uses a mixture of sodium hydroxide (NaOH) and sodium sulphide (Na2S), is

20176189 prh 29 -12- 2017 the most commonly used pulp production method. The cooking process may be based on batch cooking or continuous cooking comprising a digester or several digesters. The chemicals required for this process are used in a mixture denoted as white liquor.

In pulping, sodium sulphide (Na2S) and sodium hydroxide (NaOH) of white liquor react with water forming hydrosulphide (HS ) and hydroxyl (OH ) groups according to equations 1 and 2.

Na₂S + H₂O 2Na⁺ + HS~ + OH~ (Equation 1)

NaOH -► Na⁺ + OH~ (Equation 2)

As a result of the pulping process, black liquor is formed. The pulp coming 15 from the digester contains both fibres and spent cooking liquor (black liquor).

A large amount of chemicals is used in a chemical pulp production, and recovery and re-use of these chemicals is required. The main process units in the chemical recovery system of a pulp mill are the evaporation of the black liquor, burning of the evaporated liquors in a recovery boiler and 20 causticizing, including lime generation.

The recovery boiler is used to recover the cooking chemicals. When burnt, the cooking chemicals form a molten ‘smelt’ at the bottom of the recovery boiler. The smelt may be dissolved into a liquid. Thus formed liquid may be 25 denoted as green liquor due to a characteristic green color. Green liquor may be used to prepare white liquor for the pulping process. The recycling of these spent cooking chemicals is denoted as a liquor cycle. The liquor cycle is designed to recover the chemicals used in the pulping. In particular, the recovery boiler aims to recover sodium carbonate (Na2COs) and sodium 30 sulphide (Na₂S). The green liquor is clarified and causticized with lime, in which process Na2COs is converted to NaOH. Besides NaOH and Na2S, white liquor also comprises other sodium salts, such as sodium sulphate (Na2SO4), and small amounts of sulphites and chlorides.

Sulphur balance control is important in a pulp mill. As sulphur is introduced to the cooking process, sulphur also has to be removed from chemical recovery cycle in order to avoid excessive sulphur content in the cycle. S/Na -balance of a pulp mill is related to sulphidity. Sulphidity is a percentage value of a ratio between amounts of Na2S and active alkali in the pulp mill white liquor. Active alkali refers to NaOH and Na2S. Sulphidity may typically vary between 5 20-50 %. Equation 3 may be used to express sulphidity. The amounts of

Na2S and NaOH may be expressed in grams of NaOH equivalents, or in percentages of dry wood. Sulphidity of a pulp mill may be determined using standards NaOH SCAN-N 30:85 and Na₂S SCAN-N 31:94. Sulphidity of the pulp mill may be maintained at a desired level by adding make-up NaOH to 10 the chemical recovery cycle. This, however, causes extra costs and requires unnecessary use of chemicals.

Na₂S

NaOH+Na₂S

100 (Equation 3)

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The current specification discloses a method and a system for adjusting S/Na -balance of a pulp mill by removing sulphur compounds from the chemical recovery cycle in a pulp mill, as well as for processing of the sulphur compounds into elemental sulphur, which is of high intrinsic value. Chemically, sulphur reacts with almost all elements except for some noble 20 metals and the noble gases. Elemental sulphur may be used as a precursor to other chemicals, such as sulphuric acid. Further, the disclosed method and system enable recovery of sulphur without losing sodium at the same time. The recovery of sulphur without sodium may be used to adjust the S/Na balance of the pulp mill.

Figure 1 illustrates, by way of an example, a system 100 for adjusting S/Na balance of a sulphate pulp mill. The system 100 comprises a bioreactor 102 and a sulphur separation unit 106 located downstream of the bioreactor 102.

In a method implementable by the system 100, an aqueous pulp mill liquor 109 containing sulphides is collected. The pH of the aqueous pulp mill liquor 109 is alkaline. The pH of the aqueous pulp mill liquor 109 containing sulphides may be about 14. The aqueous pulp mill liquor 109 may comprise for example a pulp mill green liquor stream or a pulp mill white liquor stream.

20176189 prh 29 -12- 2017

The pulp mill green liquor stream may originate from a recovery boiler, in which the concentrated black liquor is combusted. The combustion forms a molten ‘smelt’ at the bottom of the recovery boiler. The smelt contains for example Na2COs and Na2S. The smelt may be dissolved into a liquid, which 5 may be for example water or weak white liquor. A liquid thus formed is denoted as green liquor due to a characteristic green color. The green liquor contains sulphides, such as Na2S and NaHS. The pulp mill green liquor stream may be clarified at a clarifier unit in order to provide the aqueous pulp mill liquor 109, or the pulp mill green liquor stream may be used as such in 10 the method according to the invention. In the latter case, the pulp mill green liquor stream corresponds to the aqueous pulp mill liquor 109.

The aqueous pulp mill liquor 109 is diverted into a bioreactor 102. Figure 5 illustrates, by way of an example, the bioreactor 102, 202 with reference to 15 figures 1, 2a and 2b. The temperature of the aqueous pulp mill liquor 109 is above room temperature prior to entering the bioreactor 102. Preferably, the temperature of the aqueous pulp mill liquor 109 is in the range of 40 to 60 °C prior to entering the bioreactor 102. When necessary, the temperature of the aqueous pulp mill liquor 109 may be lowered by a heat exchanger arranged 20 upstream of the bioreactor 102. In the bioreactor 102 the aqueous pulp mill liquor 109 containing sulphides is oxidized biologically in an oxidizing reaction. The oxidizing takes place by means of sulphur-oxidizing microbes. In an exemplary pulp mill that produces one million air-dry tons of pulp per year, the volumetric flow rate of the aqueous pulp mill liquor 109 diverted into 25 the bioreactor 102 may be 6,9 m³ per hour. Na2S concentration of the aqueous pulp mill liquor 109 diverted into the bioreactor 102 may be 46,8 g/l.

The sulphur-oxidizing microbes may be autotrophic, heterotrophic or mixotrophic aerobic bacteria. The sulphur-oxidizing microbes may be 30 alkaliphilic. The sulphur-oxidizing microbes may include for example the bacteria of the genera Thiobacillus and Thiomicrospora. The bacteria capable of oxidizing sulphide to elemental sulphur may be obtained for example from geothermal springs, oceanic geothermal vents, sulphidic cave systems, sulphide-rich industrial sites, sewage sludge, soil, salt marshes, 35 soda lakes and cold springs. Alkaliphilic sulphur-oxidizing bacteria such as Thioalkalimicrobium, Thioalkalivibrio and Thioalkalispira may be isolated from

20176189 prh 29 -12- 2017 soda lakes. They may be halophilic or halotolerant to varying degrees. The sulphur-oxidizing microbes may have at least one of the following properties: pH optimum above 9, usually below 10,5, in particular around 9,5; capability of oxidizing at least H2S/HS ; growth over a temperature range of 10-65 °C;

tolerance for NaCI and sodium carbonates.

The bioreactor 102 may be aerated with a gas 105 comprising air and/or weak malodorous gas from the pulp mill. In the oxidizing reaction most of the sulphides of the aqueous pulp mill liquor 109 get oxidized into elemental 10 sulphur. The efficiency of the oxidizing reaction may be equal to or more than

%. As the chemical stability of the elemental sulphur produced decreases with increasing pH and temperature, the temperature inside the bioreactor should not exceed 65 °C. The pH of a reaction medium inside the bioreactor 102 may be between 8-11. By aerating the bioreactor 102 with weak 15 malodorous gas the pH of the reaction medium may be lowered. The bioreactor 102 may be a mixing reactor. The system 100 may contain more than one bioreactor. The bioreactors may be arranged in parallel.

The oxidizing reaction yields an aqueous suspension 103 containing 20 elemental sulphur. The oxidizing reaction also yields a gas stream 104. The gas stream 104 may be forwarded from the bioreactor 102 to a processing of weak malodorous gases of the pulp mill. The processing of weak malodorous gases may be performed in the recovery boiler, in such a way that the weak malodorous gases are fed into the combustion air of the recovery boiler.

The aqueous suspension 103 containing elemental sulphur from the bioreactor 102 is conducted to a sulphur separation unit 106. In the sulphur separation unit 106 the elemental sulphur is separated from the aqueous suspension 103. A residual solution 108 and a precipitate 107 containing the 30 elemental sulphur are thereby obtained. The sulphur separation unit 106 may be a conical separator. The separation may be performed for example by filtration, settling or flocculation. In an exemplary pulp mill that produces one million air-dry tons of pulp per year, the amount of elemental sulphur produced may be 128 kg per hour. From the sulphur separation unit 106, the 35 residual solution 108, from which the precipitate 107 has been separated, may be directed to causticizing.

20176189 prh 29 -12- 2017

Figures 2a and 2b illustrate, by way of an example, a further system for separating sulphur from a pulp mill liquor stream. The system 200 comprises a stripper 210, a scrubber 214 located downstream of the stripper 210, a 5 bioreactor 202 located downstream of the scrubber 214 and a sulphur separation unit 206 located downstream of the bioreactor 202.

In a method implementable by the system 200, an aqueous pulp mill liquor 109 containing sulphides is collected. The pH of the aqueous pulp mill liquor 10 109 is alkaline. The pH of the aqueous pulp mill liquor 109 containing sulphides may be about 14. The aqueous pulp mill liquor 109 may comprise for example a pulp mill green liquor stream or a pulp mill white liquor stream.

The aqueous pulp mill liquor 109 is diverted into the stripper 210. In an exemplary pulp mill that produces one million air-dry tons of pulp per year, a 15 volumetric flow rate of the aqueous pulp mill liquor 109 diverted into the stripper 210 may be 54,2 m³ per hour. Na2S concentration of the aqueous pulp mill liquor 109 diverted into the stripper 210 may be 46,8 g/l.

The aqueous pulp mill liquor 109 containing sulphides is stripped in the 20 stripper 210 with an acidic agent. The acidic agent may be for example carbon dioxide (CO2) or an acidic solution. Into the stripper 210, a stripping fluid stream 213 comprising the acidic agent is fed. The stripping fluid stream 213 may comprise for example pure carbon dioxide or flue gas. In the stripper 210, the stripping fluid stream 213 lowers the pH of the aqueous pulp 25 mill liquor 109, thereby causing formation of H2S from the sulphides of the aqueous pulp mill liquor 109. A pH of the aqueous pulp mill liquor 109 while stripping may be 7 or less.

As illustrated by Figure 3, the stripping in the stripper 210 is performed in a 30 counter current manner. The aqueous pulp mill liquor 109 containing sulphides is fed into the stripper 210 at the upper part of the stripper 210 and is arranged to flow downwards towards the lower part of the stripper 210. The stripping fluid stream 213 is fed into the stripper 210 at the lower part of the stripper 210 and is arranged to flow upwards towards the upper part of the 35 stripper 210. The stripper 210 may be a plate column or a packed bed column.

20176189 prh 29 -12- 2017

The stripping yields a gas stream 211 containing H2S and a residual pulp mill liquor stream 212. The H2S concentration of the gas stream 211 may be 99 vol-%. The residual pulp mill liquor stream 212 may be fed back to the 5 chemical recovery cycle of the pulp mill. In an exemplary pulp mill that produces one million air-dry tons of pulp per year, the mass flow rate of the gas stream 211 containing H2S may be 553 kg per hour. The volumetric flow rate of the residual pulp mill liquor stream 212 may be 54,2 m³ per hour.

Na2S concentration of the residual pulp mill liquor stream 212 may be 23,4 10 g/l.

Figure 4 illustrates, by way of an example, the scrubber 214 with reference to figures 2a and 2b. The gas stream 211 containing H2S is fed into the scrubber 214. In the scrubber 214 the gas stream 211 containing H2S is 15 scrubbed with an aqueous scrubbing solution 215. The pH of the aqueous scrubbing solution 215 may be adjusted with an alkaline agent. A stream 216 comprising the alkaline agent may be configured to feed the alkaline agent to the aqueous scrubbing solution 215. The alkaline agent may be for example NaOH solution or oxidized white liquor. The pH of the aqueous scrubbing 20 solution 215 may be above 8. Preferably, the pH of the aqueous scrubbing solution 215 is above 11.5. The pH of the aqueous scrubbing solution 215 may be in the range of 12 to 14. The efficiency of scrubbing improves with higher pH. When NaOH is utilized as the alkaline agent, the mass flow rate of NaOH fed into the aqueous scrubbing solution 215 may be 25 kg per hour in 25 an exemplary pulp mill that produces one million air-dry tons of pulp per year.

In the scrubber 214, intensive contact between the gas stream 211 containing H2S and the aqueous scrubbing solution 215 is enabled. At least some of the H2S of the gas stream 211 reacts with the alkaline agent of the 30 aqueous scrubbing solution 215, thereby forming sulphides, such as Na2S and NaHS. A residual gas stream 217 and an aqueous spent scrubbing solution 201 containing sulphides are produced in the scrubber 214.

Na2S/NaHS mixture ratio of the aqueous spent scrubbing solution 201 is dependent on the pH of the aqueous spent scrubbing solution 201. The 35 residual gas stream 217 may be forwarded from the scrubber 214 to a processing of strong malodorous gases of the pulp mill. The processing of

20176189 prh 29 -12- 2017 strong malodorous gases may comprise burning of the gases for example in a recovery boiler.

The scrubber 214 may be an absorption tower of a packed bed column type.

The scrubber 214 provides a straight contact area between a gas and a liquid.

The aqueous spent scrubbing solution 201, 201a containing sulphides is conducted into the bioreactor 202 (Fig. 5). The temperature of the aqueous 10 spent scrubbing solution 201, 201a prior to entering the bioreactor 202 is above room temperature. Preferably, the temperature of the aqueous spent scrubbing solution 201, 201a is in the range of 40 to 60 °C prior to entering the bioreactor 202. In the bioreactor 202 the aqueous spent scrubbing solution 201,201a containing sulphides is oxidized biologically in an oxidizing 15 reaction. The oxidizing takes place by means of sulphur-oxidizing microbes.

According to an embodiment illustrated in Figure 2b, at least some of the aqueous spent scrubbing solution 201b is recirculated by a pump 218 back to the scrubber 214. Thus, the aqueous spent scrubbing solution 201 is divided 20 into two portions 201a and 201b. By this arrangement, the sulphur compounds of the gas stream 211 may be more efficiently converted into sulphides.

The bioreactor 202 may be aerated with a gas 205 comprising air and/or 25 weak malodorous gas from the pulp mill. In the oxidizing reaction most of the sulphides of the aqueous spent scrubbing solution 201, 201a get oxidized into elemental sulphur. The efficiency of the oxidizing reaction may be equal to or more than 95 %. As the chemical stability of the elemental sulphur produced decreases with increasing pH and temperature, the temperature 30 inside the bioreactor should not exceed 65 °C. The pH of the reaction medium inside the bioreactor 202 may be between 8-11. By aerating the bioreactor 202 with weak malodorous gas the pH of the reaction medium may be lowered. By this way, use of somewhat higher pH than what is optimal for the bioreactor 202, in the scrubber 214, may be compensated by 35 aerating the bioreactor 202 with weak malodorous gas capable of lowering the pH of the reaction medium. The bioreactor 202 may be a mixing reactor.

20176189 prh 29 -12- 2017

The system 200 may contain more than one bioreactor. The bioreactors may be arranged in parallel.

The oxidizing reaction yields an aqueous suspension 203 containing elemental sulphur. The oxidizing reaction also yields a gas stream 204. The gas stream 204 may be forwarded from the bioreactor 202 to a processing of weak malodorous gases of the pulp mill. The processing of weak malodorous gases may be performed in the recovery boiler, in such a way that the weak malodorous gases are fed into the combustion air of the recovery boiler.

The aqueous suspension 203 containing elemental sulphur from the bioreactor is conducted to a sulphur separation unit 206. In the sulphur separation unit 206 elemental sulphur is separated from the aqueous suspension 203. A residual solution 208a, 208b and a precipitate 207 15 containing the elemental sulphur are thus obtained. The sulphur separation unit 206 may be a conical separator. The separation may be performed for example by filtration, settling or flocculation. In an exemplary pulp mill that produces one million air-dry tons of pulp per year, the amount of elemental sulphur produced may be 500 kg per hour. The mass flow rate of the residual 20 solution 208a, 208b with respect to sulphur may be 10 kg per hour.

The embodiment illustrated in Figure 2b, in which at least some of the aqueous spent scrubbing solution 201b is recirculated by a pump 218 back to the scrubber 214, enables use of a smaller sulphur separation unit 206 25 compared to the system disclosed in Figure 2a. As the sulphur compounds of the gas stream 211 are more efficiently converted into sulphides, the volume of the aqueous suspension 203 containing elemental sulphur may be smaller, and thus a smaller unit is needed for separation of the residual solution 208 and the precipitate 207 containing the elemental sulphur.

From the sulphur separation unit 206, at least some of the residual solution 208a, from which the precipitate 207 has been separated, may be directed back into the scrubber 214 to replenish the aqueous scrubbing solution 215. Thus, the possible un-oxidized sulphur compounds of the residual solution 35 208a may be directed back to the bioreactor 202 for oxidizing. Further, recirculating the liquid diminishes the need for fresh water and reduces the unnecessary use of the valuable natural resources. The residual solution 208b may be fed back to the chemical recovery cycle of the pulp mill.

Many variations of the method and system will suggest themselves to those 5 skilled in the art in light of the description above. Such obvious variations are within the full intended scope of the appended claims.

Claims

Claims:

1. A method for adjusting S/Na -balance of a pulp mill, the method comprising:

5 - diverting an aqueous pulp mill liquor (109) containing sulphides into a bioreactor (102),

- oxidizing the aqueous pulp mill liquor (109) containing sulphides in the bioreactor (102) biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby producing an aqueous suspension

10 (103) containing elemental sulphur, and

- separating the elemental sulphur from the aqueous suspension (103) in a sulphur separation unit (106) located downstream of the bioreactor (102), thereby obtaining a residual solution (108) and a precipitate (107) containing the elemental sulphur.
2. A method for adjusting S/Na -balance of a pulp mill, the method comprising:

- diverting an aqueous pulp mill liquor (109) containing sulphides into a stripper (210),

20 - stripping the aqueous pulp mill liquor (109) containing sulphides in the stripper (210) with an acidic agent, thereby obtaining a gas stream (211) containing H2S and a residual pulp mill liquor stream (212),

- scrubbing the gas stream (211) containing H2S in a scrubber (214) located downstream of the stripper (210) with an aqueous scrubbing

25 solution (215) containing an alkaline agent, whereby at least some of the H2S reacts with the alkaline agent, thereby producing a residual gas stream (217) and an aqueous spent scrubbing solution (201, 201a) containing sulphides,

- oxidizing the aqueous spent scrubbing solution (201, 201a) containing

30 sulphides in a bioreactor (202) biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby producing an aqueous suspension (203) containing elemental sulphur, and

- separating the elemental sulphur from the aqueous suspension (203) in a sulphur separation unit (206) located downstream of the

35 bioreactor (202), thereby obtaining a residual solution (208a, 208b) and a precipitate (207) containing the elemental sulphur.

20176189 prh 29 -12- 2017
3. The method according to claim 2, further comprising

- directing at least some of the residual solution (208a), from which the precipitate (207) has been separated, back into the scrubber (214) to

5 replenish the aqueous scrubbing solution (215).
4. The method according to claim 2 or 3, further comprising

- directing at least some of the aqueous spent scrubbing solution (201 b) by a pump (218) back into the scrubber (214) for re-scrubbing.
5. The method according to any of the previous claims, further comprising

- clarifying pulp mill liquor stream at a clarifier unit, thereby providing the aqueous pulp mill liquor (109).
6. The method according to any of the previous claims, wherein

- the aqueous pulp mill liquor (109) or

- the aqueous spent scrubbing solution (201,201 a) has a temperature above room temperature, preferably in the range of 40 20 to 60°C prior to entering the bioreactor (102, 202).
7. The method according to any of the claims 2-6, further comprising

- adjusting the pH of the aqueous scrubbing solution (215) with the alkaline agent, such that the pH of the aqueous scrubbing solution

25 (215) is above 8, preferably above 11.5, such as in the range of 12 to

14.
8. The method according to any of the previous claims, wherein the method further comprises aerating the bioreactor (102, 202) with a gas (105, 205) comprising air and/or weak malodorous gas from the pulp mill.

30
9. Use of a bioreactor (102, 202) containing sulphur-oxidizing microbes to separate sulphur from a pulp mill liquor stream.
10. Use of a bioreactor (102, 202) containing sulphur-oxidizing microbes to adjust the S/Na -balance of a pulp mill.

20176189 prh 29 -12- 2017
11. Use of a bioreactor (102, 202) containing sulphur-oxidizing microbes to produce elemental sulphur from a pulp mill liquor stream.
12. A system (100) arranged to adjust S/Na -balance of a pulp mill, the 5 system (100) comprising

- a means configured to collect an aqueous pulp mill liquor (109) containing sulphides,

- one or more conducts configured to conduct the aqueous pulp mill liquor (109) into a bioreactor (102),

10 - the bioreactor (102) configured to oxidize the aqueous pulp mill liquor (109) with sulphur-oxidizing microbes, the bioreactor (102) thereby configured to produce an aqueous suspension (103) containing elemental sulphur, and

- a sulphur separation unit (106) located downstream of the bioreactor

15 (102), the sulphur separation unit (106) configured to produce a residual solution (108) and a precipitate (107) containing the elemental sulphur.
13. A system (200) arranged to adjust the S/Na -balance of a pulp mill, the 20 system (200) comprising

- a means configured to collect an aqueous pulp mill liquor (109) containing sulphides,

- one or more conducts configured to conduct the aqueous pulp mill liquor (109) into a stripper (210),

25 - the stripper (210) configured to strip the aqueous pulp mill liquor (109) with an acidic agent, the stripper (210) thereby configured to produce a gas stream (211) containing H2S and a residual pulp mill liquor stream (212),

- a scrubber (214) located downstream of the stripper (210), the

30 scrubber (214) configured to scrub the gas stream (211) containing

H2S with an aqueous scrubbing solution (215) containing an alkaline agent, the scrubber (214) thereby configured to produce a residual gas stream (217) and an aqueous spent scrubbing solution (201, 201a) containing sulphides,

- one or more conducts configured to conduct the aqueous spent scrubbing solution (201, 201a) containing sulphides into a bioreactor (202),

- the bioreactor (202) located downstream of the scrubber (214), the

5 bioreactor (202) configured to oxidize the aqueous spent scrubbing solution (201, 201a) containing sulphides with sulphur-oxidizing microbes, the bioreactor (202) thereby configured to produce an aqueous suspension (203) containing elemental sulphur, and

- a sulphur separation unit (206) located downstream of the bioreactor

10 (202), the sulphur separation unit (206) configured to produce a residual solution (208a, 208b) and a precipitate (207) containing the elemental sulphur.
14. The system (200) according to claim 13, the system (200) further
15 comprising a pump (218) and a conduct configured to direct at least some of the aqueous spent scrubbing solution (201b) back into the scrubber (214) for re-scrubbing.

15. The system (100, 200) according to claims 12-14, the system (100,

20 200) comprising more than one bioreactor (102, 202).
16. The method according to any of the claims 1-8 or the use according to any of the claims 9-11 or the system according to any of the claims 12-15, wherein the aqueous pulp mill liquor (109) is green liquor or white liquor.