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Title:
A METHOD FOR THE TREATMENT OF CALCIUM-CONTAINING SOLID SIDE STREAM MATERIAL
Document Type and Number:
WIPO Patent Application WO/2019/158814
Kind Code:
A1
Abstract:
A method for the treatment of calcium-containing solid side stream material (1) obtainable from a chemical recovery process (2) of a chemical pulping process (3) is disclosed. The method comprises treating the calcium-containing solid side stream material with an acid (4), thereby obtaining a solution (5) comprising a calcium and/or magnesium salt of the acid. A system (14) for the treatment of the calcium-containing solid side stream material and products (5, 13) obtainable by the method are also disclosed.

Inventors:
SAHLBERG TEPPO (FI)
VIRTANEN PASI (FI)
HÄKKINEN KAISA (FI)
Application Number:
PCT/FI2019/050101
Publication Date:
August 22, 2019
Filing Date:
February 11, 2019
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
UPM KYMMENE CORP (FI)
International Classes:
D21C11/00; B09B3/70; C02F11/02
Domestic Patent References:
WO2015150631A12015-10-08
WO2008079072A12008-07-03
WO2013072559A12013-05-23
Foreign References:
US5961941A1999-10-05
US20170292133A12017-10-12
DE102009037946A12011-02-24
EP2853583A12015-04-01
US5628875A1997-05-13
FI105112B2000-06-15
US3165299A1965-01-12
US3589363A1971-06-29
Attorney, Agent or Firm:
PAPULA OY (FI)
Download PDF:
Claims:
CLAIMS

1. A method for the treatment of calcium- containing solid side stream material (1) obtainable from a chemical recovery process (2) of a chemical pulping process (3) , wherein the calcium-containing solid side stream material optionally further contains magnesium; wherein the method comprises

treating the calcium-containing solid side stream material with an acid (4) , thereby obtaining a solution (5) comprising a calcium and/or magnesium salt of the acid,

wherein the acid comprises at least one of nitric acid, an organic acid, or any combination or mixture thereof.

2. The method according to claim 1, wherein the calcium-containing solid side stream material comprises green liquor dregs, calcium carbonate containing solid side stream material, calcium oxide containing solid side stream material, fly ash, or any mixture or combination thereof.

3. The method according to claim 1 or 2, wherein the calcium-containing solid side stream material comprises green liquor dregs, and the green liquor dregs are pressure filtered and optionally washed prior to treating them with the acid.

4. The method according to claim 3, wherein the green liquor dregs are pressure filtered using a vertical pressure filtration unit (6) prior to treating them with the acid.

5. The method according to any one of claims 1 - 4, wherein the method comprises mixing the calcium-containing solid side stream material in a dry or slurried form with the acid and optionally with water to obtain a slurry (7) .

6. The method according to any one of claims 1 - 5, wherein the calcium-containing solid side stream material in a dry or slurried form is mixed with the acid by feeding them to a zone of high shear forces (8) within a high shear mixer (9) and subjected simultaneously to the zone of high shear forces.

7. The method according to claim 5 or 6, wherein the pH of the slurry (7) is adjusted to a pH of 5 or lower, or to a pH in the range of about 2 to about 5, or to a pH in the range of about 4 to about 5.

8. The method according to any one of claims 1 - 7, wherein the method further comprises separating the solution (5) obtained from residual solids (10), optionally by filtering.

9. The method according to any one of claims 1 - 8, wherein the method further comprises using the solution (5) as a nutrient solution in biological waste water treatment.

10. The method according to any one of claims 1 - 9, wherein the method further comprises treating the solution by electrolytical water treatment (12) , thereby removing at least a portion of harmful substances, such as heavy metals, contained in the solution .

11. The method according to any one of claims 1 - 10, wherein the calcium and/or magnesium salt of the acid contained in the solution is precipitated and/or crystallized from the solution.

12. A solution (5) comprising a calcium and/or magnesium salt, wherein the solution is obtainable by the method according to any one of claims 1 - 11.

13. A product (13) comprising the calcium and/or magnesium salt of the acid precipitated and/or crystallized from the solution obtainable by the method according to any one of claims 1 - 11, wherein the product is optionally a fertilizer product.

14. A system (14) for the treatment of calcium-containing solid side stream material (1) obtainable from a chemical recovery process (2) of a chemical pulping process (3) , wherein the system comprises

an apparatus (9) for mixing the calcium- containing solid side stream material with an acid (4) to form a solution (5) comprising a calcium and/or magnesium salt of the acid; and

a separation apparatus (15) for separating the solution comprising the calcium and/or magnesium salt of the acid from residual solids (10) .

15. The system according to claim 14, wherein the system further comprises a pressure filtration unit (6), such as a vertical pressure filtration unit, for filtering the calcium-containing solid side stream material, such as green liquor dregs, prior to mixing it with the acid.

16. The system according to claim 14 or 15, wherein the system further comprises a flow connection (16) for conveying the solution comprising the calcium and/or magnesium salt of the acid to a waste water treatment plant (17) for use as a nutrient solution in biological waste water treatment (11) within the waste water treatment plant.

Description:
A METHOD FOR THE TREATMENT OF CALCIUM-CONTAINING SOLID SIDE STREAM MATERIAL

TECHNICAL FIELD

The present disclosure relates to a method and system for the treatment of calcium-containing solid side stream material obtainable from a chemical recovery process of a chemical pulping process and to products obtainable therefrom.

BACKGROUND

Chemical pulping processes and the chemical recovery processes involved produce several waste or side stream material byproducts that need to be dis- posed of. Traditionally, solid side stream materials such as green liquor dregs produced in the chemical recovery processes have been landfilled. However, as environmental regulations restricting waste disposal tend to become stricter, it may be desirable to reduce the total amount of the side stream material or waste and recycle or reuse at least a part of the side stream material.

SUMMARY

A method for the treatment of calcium- containing solid side stream material obtainable from a chemical recovery process of a chemical pulping process is disclosed. The method may comprise treating the calcium-containing solid side stream material with an acid, thereby obtaining a solution comprising a calcium and/or magnesium salt of the acid.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments. In the drawings:

Fig. 1 illustrates a method and a system for the treatment of calcium-containing solid side stream material ;

Fig. 2A illustrates a cross-sectional side view of an embodiment of a high shear mixer;

Fig. 2B shows the same embodiment of the high shear mixer as a cross-sectional top view;

Fig. 3 shows EDS results of a dried liquid fraction (salt product) ;

Fig. 4 shows sulphate and nitrate contents of samples of the dried liquid fraction (salt product) ;

Fig. 5 shows ICP results from the same samples ;

Fig. 6 illustrates EDS results of salt products ;

Fig. 7 shows ICP results of salt products;

Fig. 8 shows EDS results of samples of salt products ;

Fig. 9 illustrates ICP results of samples of salt products;

Fig. 10 shows EDS results of samples of salt products ;

Fig. 11 shows ICP results of salt liquids;

Fig. 12 shows IC results of salt liquids;

Fig. 13 shows ICP results of salt liquids;

Fig. 14A and 14B illustrate EDS results of two trial points; and

Fig. 15 shows acetate, sulphate and nitrate contents of the formed salt liquids analysed by IC.

DETAILED DESCRIPTION

Recycling, reuse or disposal of calcium- containing side stream materials from chemical pulping processes and the chemical recovery processes involved may be somewhat challenging in view of environmental regulations and other factors, their composition and consistency. For example, calcium-containing solid side stream materials such as green liquor dregs typically have a relatively high water content and may have a relatively high content of certain components, for example impurities such as heavy metals.

Such calcium-containing solid side stream materials may, however, be treated with an acid to obtain a solution containing a calcium and/or magnesium salt of the acid. The solution may be used for various purposes, or the calcium and/or magnesium salt may be precipitated or crystallized therefrom and recycled or reused for various purposes. This can significantly reduce the amount of solid waste to be disposed of e.g. to landfills and lower the environmental impact of the calcium-containing solid side stream material. Furthermore, the method may also provide a process to separate a useful fraction from a less attractive or useful fraction of the calcium- containing solid side stream material.

Further, in many processes calcium-containing solid side stream materials such as green liquor dregs are filtered using lime mud. For example, green liquor dregs may be filtered using a rotary vacuum drum filter using lime mud as a precoat, such that a desired dry solids content can be obtained for the resulting cake. But on the other hand a significant amount of the precoat lime mud may then end up in waste instead of being used or recycled for other purposes. Lime mud typically has a composition that makes it relatively well suitable for recycling and reuse. According to some embodiments of the method, it may be possible to reduce the use of lime mud, which may then be recycled in other ways .

A method for the treatment of calcium- containing solid side stream material obtainable from a chemical recovery process of a chemical pulping process is disclosed.

In the context of this specification, the term "calcium-containing solid side stream material" may be understood as referring to a calcium-containing solid side stream material obtainable from a chemical recovery process of a chemical pulping process. The calcium-containing solid side stream material may be any solid side stream material obtainable from a chemical recovery process of a chemical pulping process, for instance a causticizing process, provided it contains calcium. The calcium contained in the solid side stream material may be e.g. in the form of calcium carbonate (CaCO 3 ) , calcium oxide (CaO) , calcium hydroxide (Ca(OH) 2 ), and/or any mixture or combination thereof. Suitable calcium-containing solid side stream materials may comprise green liquor dregs, calcium carbonate containing solid side stream material, calcium oxide containing solid side stream material, fly ash, or any mixture or combination thereof. Other suitable calcium-containing solid side stream materials may comprise or be e.g. lime mud, lime slaker grits, solid side stream material obtainable from purification of flue gas from lime sludge reburning, burnt lime and any mixtures thereof.

The calcium-containing solid side stream material may further contain magnesium. The calcium- containing solid side stream material may comprise various other components derived from the raw material for pulping, such as wood; for instance magnesium in the form of carbonate, oxide or hydroxide, aluminium, phosphorus (e.g. in the form of phosphates), manganese, sodium, sulphur, silicon, iron, zinc, various heavy metals, incombustible material, such as material derived from lignin, and/or any mixtures or combinations thereof. The calcium-containing solid side stream material may be a fraction that is (without the treatment with the acid) be considered a calcium- containing solid waste.

In an embodiment, the calcium-containing solid side stream material comprises or is green liquor dregs. The method may be particularly useful for the treatment of green liquor dregs, which are a complex side stream material to dispose and/or to treat further .

The term "green liquor dregs" may, in principle, refer to any solid matter separated from green liquor. Green liquor dregs may be formed when the molten smelt from a recovery boiler is dissolved in weak liquor (weak wash) to produce green liquor. Suspended particles, i.e. the solid matter, in the green liquor are referred to as dregs and are typically removed in a clarifier. Such solid matter may thus be separated from green liquor using a clarifier. For example, green liquor dregs removed in a sedimentation clarifier or another type of clarifier may be filtered and washed to increase the solid content of the cake containing the green liquor dregs.

Solid matter separated from green liquor may be obtainable by filtering using a filter, for example a drum filter, pre-coated with lime mud, or a filter belt press. Green liquor dregs separated using a clarifier may further be filtered. Various other methods and apparatuses for separating the solid matter may also be contemplated.

Green liquor dregs (GLD) may therefore refer, additionally or alternatively, to solid matter separated by filtering green liquor using a filter, for example a drum filter, precoated with lime mud. GLD may therefore optionally comprise lime mud. A calcium-containing solid side stream material comprising green liquor dregs and lime mud may also be referred to as "GLD to landfill". The GLD to landfill may, in an embodiment, be obtainable by filtering using a filter pre-coated with lime mud.

Green liquor dregs may, additionally or alternatively, be obtained by filtering green liquor and/or green liquor dregs separated using a clarifier, using a suitable filter without the use of lime mud. For example, they can be separated by pressure filtration or using a decanter centrifuge. Green liquor dregs separated without the use of lime mud may be referred to as "green liquor dregs to filter". With pressure filtration, it may be possible to reach a relatively high dry solids content. With pressure filtration, it may also be possible to return more sodium back to the circulation of chemicals of the chemical pulping process. In an embodiment, the calcium-containing solid side stream material comprises green liquor dregs, calcium carbonate containing solid side stream material, calcium oxide containing solid side stream material, fly ash, and/or any mixture or combination thereof.

The green liquor dregs, or any other calcium- containing solid side stream material, may be pressure filtered and optionally washed prior to treating them with the acid. For example, they may be pressure filtered using a vertical pressure filtration unit and optionally washed prior to treating them with the acid. Green liquor dregs with a relatively high solids content may thereby be obtained, for example a solids content of at least 60 % (w/w) . It may also allow returning more sodium to the circulation of chemicals of the chemical pulping process. The pressure filtering and the washing after the filtering may significantly decrease the amount of sodium ending up in the GLD cake after filtering. Further, a smaller amount of acid may be required.

The method may comprise treating the calcium- containing solid side stream material with an acid, thereby obtaining a solution comprising a calcium and/or magnesium salt of the acid. When treating the calcium-containing solid side stream material, the calcium-containing solid side stream material and the acid are brought into contact. They may thus react, and the acid may dissolve at least a part of the calcium-containing solid side stream material, in particular at least a part of the calcium carbonate (CaCO 3 ) , magnesium carbonate (MgCO 3 ) , calcium oxide (CaO) , magnesium oxide (MgO) , calcium hydroxide (Ca(OH) 2 ), magnesium hydroxide (Mg(OH) 2 ), or any mixture or combination thereof, contained in the calcium-containing solid side stream material. The calcium and/or magnesium salt of the acid may be soluble in aqueous solutions, so that when the acid is added e.g. as an aqueous solution and/or when the calcium-containing solid side stream material in a slurried form is contacted with the acid, the solution is formed. It may therefore be understood that as the acid and the calcium-containing solid side stream material are mixed or slurried, the mixture or slurry may contain an amount of the resulting solution. Depending on the solubility of the calcium and/or magnesium salt of the acid, at least a part of the calcium and/or magnesium salt of the acid or possibly even the entire calcium and/or magnesium salt of the acid may be dissolved in the solution. Some calcium and/or magnesium salts, such as calcium sulphate, may be less readily soluble. Parts of the calcium- containing solid side stream material which do not dissolve and/or react may remain as residual solids. In other words, both the solution and residual solids may be obtainable by the method.

The method may be a continuous and/or an online method. It may be performed at an industrial scale . The acid may comprise or be nitric acid (HNO 3 ) ; sulphuric acid (H 2 SO 4 ) ; phosphoric acid (H 3 PO 4 ) ; an organic acid; or any combination or mixture thereof. Examples of possible organic acids include carboxylic acids, such as acetic acid (CH 3 COOH) , citric acid, formic acid (HCOOH) , gluconic acid, lactic acid, oxalic acid, tartaric acid, or any mixtures or combinations thereof. The acid may be an aqueous solution of the acid, including any acids described above. The concentration of the aqueous solution of the acid may be selected such that a desired pH and/or volume of the slurry and/or solution is achieved. A concentrated acid solution, for example a concentrated sulphuric acid or nitric acid, may be used so as to avoid introducing large volumes of water into the solution (or slurry, as described below) .

In an embodiment, the acid is an acid other than hydrochloric acid (HC1) , or an acid other than a hydrogen halide. In an embodiment, the acid comprises at least one of nitric acid, an organic acid, or any combination or mixture thereof. In an embodiment, the acid comprises nitric acid (HNO 3 ) , an organic acid or any mixture or combination thereof. In an embodiment, the acid comprises nitric acid (HNO 3 ) , acetic acid (CH 3 COOH) or any mixture or combination thereof.

The calcium salt of the acid may comprise one or more calcium salts of the acid(s), depending on the acid or acids used. For example, the calcium salt of the acid may comprise calcium nitrate (Ca(NO 3 ) 2 ); calcium sulphate (CaSO 4 , which may be anhydrous or a hydrate in solid form) ; calcium phosphate (Ca 2+ combined with PO 4 3- , HPO 4 2- , and/or H 2 PO 4 -) ; a calcium salt of an organic acid; or any combination or mixture thereof. Examples of possible salts of organic acids include calcium salts of carboxylic acids, such as calcium acetate (Ca (CH 3 COO) 2 , which may be anhydrous or monohydrate in solid form) ; calcium citrate, calcium formate (Ca(HCOO) 2 ), calcium gluconate, calcium lactate, calcium oxalate, calcium tartrate, or any mixtures or combinations thereof.

The calcium-containing solid side stream material may further comprise other components than calcium carbonate, for example carbonates of magnesium and/or sodium. The solution may therefore also comprise magnesium (Mg) salts of the acid and/or sodium (Na) salts of the acid. The corresponding Mg and/or Na salts may be salts of any of the acids described above. The term "calcium and/or magnesium salt" may therefore refer to a calcium salt; to a magnesium salt; or to a calcium salt and optionally a magnesium salt.

The magnesium salt of the acid may, likewise, comprise one or more magnesium salts of the acid(s), depending on the acid or acids used. For example, the magnesium salt of the acid may comprise magnesium nitrate (Mg(NO 3 ) 2 ); magnesium sulphate (MgSO 4 , which may be anhydrous or a hydrate in solid form) ; magnesium phosphate (Mg 2+ combined with PO 4 3- , HPO 4 2- , and/or H 2 PO 4 -) ; a magnesium salt of an organic acid; or any combination or mixture thereof. Examples of possible salts of organic acids include magnesium salts of carboxylic acids, such as magnesium acetate (Mg (CH 3 COO) 2 , which may be anhydrous or monohydrate in solid form) ; magnesium citrate, magnesium formate (Mg(HCOO) 2 ), magnesium gluconate, magnesium lactate, magnesium oxalate, magnesium tartrate, or any mixtures or combinations thereof.

As the calcium-containing solid side stream material is treated with the acid, the calcium- containing solid side stream material is contacted with the acid so that the acid and one or more components of the calcium-containing solid side stream material may react. They may be allowed to react for a certain reaction time. For example, they may, in some embodiments, be allowed to react for up to about 60 minutes, or for at least about 30 minutes, or for about 30 to about 60 minutes. However, the reaction time may depend e.g. on the mixing. When using a high shear mixer, as described below, the reaction time may be much shorter, for example about 3 to 5 seconds. At least a part of the calcium-containing solid side stream material may then dissolve into the solution, such that the solution formed comprises a calcium and/or magnesium salt of the acid (the calcium of the salt being derived from the calcium-containing solid side stream material) . Examples of the reactions may be presented according to the following formulas, in which the acid is either nitric acid or acetic acid and the calcium-containing solid side stream material comprises calcium carbonate:

The method may comprise mixing the calcium- containing solid side stream material in a dry or slurried form with the acid and optionally with water to obtain a slurry. The volume and/or consistency of the slurry may be adjusted. The concentration of the aqueous solution of the acid may be selected such that a desired consistency of the slurry is achieved. Alternatively or additionally, water may be added to the slurry and/or to the calcium-containing solid side stream material to adjust its volume and/or consistency. Adding water to adjust the volume may improve dissolution of the calcium-containing solid side stream material, in particular in embodiments in which a high shear mixer, such as an Atrex-type mixer, is used. The slurry may also comprise the resulting solution. As the reaction between the calcium- containing solid side stream material and the acid progresses, the (relative) amount of the solution obtained may increase in the slurry, and the (relative) amount of the calcium-containing solid side stream material and the acid may decrease. The slurry may also comprise residual solids that are not dissolved during the treatment.

The surface area of the calcium-containing solid side stream material may be increased before or during mixing it with the acid. For example, the calcium-containing solid side stream material may be preground prior to treating it with the acid. The pregrinding may increase the rate and/or extent the reaction between the calcium-containing solid side stream material and the acid.

The pH of the slurry may be adjusted to a pH of 5 or lower, or to a pH in the range of about 2 to about 5. The pH may be such that the calcium- containing solid side stream material dissolves and/or reacts to a sufficient extent and/or at a sufficient rate. The pH may further be such that the solubility of certain components, for example heavy metals such as Cd, is minimized. For example, a lower pH may be associated with improved removal of heavy metals, such as Cd. For example, the pH of the slurry may be adjusted to a pH value in the range of about 2.5 to about 5, or about 3.5 to about 5, or of about 2.5 to about 3.5. At such a pH, at least a portion of the heavy metals, such as Cd, contained in the calcium- containing solid side stream material will not dissolve into the solution but will remain in the residual solids. Thus the amount of potentially harmful contaminants, such as heavy metals, in the solution and any resulting products obtainable from the solution may be kept low. For example, at a pH in the range of about 2.5 to 3.5, the calcium-containing solid side stream material may dissolve relatively efficiently, but the amount of potentially harmful contaminants, such as heavy metals, remains relatively low. The pH may however also be optimized such that the consumption of the acid remains economical.

The temperature of the slurry may be adjusted to improve the rate (reaction kinetics) and/or extent of the reaction of the calcium-containing solid side stream material with the acid. For example, the temperature of the slurry may be adjusted to at least 50 °C. At a temperature of at least 80 °C, the rate of the reaction may be particularly increased.

The slurry may be subjected to mixing in order to improve the rate (reaction kinetics) and/or extent of the reaction of the calcium-containing solid side stream material with the acid. This may be done e.g. in a suitable mixing apparatus, for example in a mixing tank. Suitable mixers for this purpose may be e.g. a blade mixer comprising a dispersion blade, a high shear impeller, or a rotor-stator mixer.

The mixing and the formation of the solution may be particularly efficient, if a high shear mixer is used. In an embodiment, the calcium-containing solid side stream material in a dry or slurried form is mixed with the acid by feeding them (i.e. both the calcium-containing solid side stream material and the acid) to a zone of high shear forces within a high shear mixer and subjected simultaneously to the zone of high shear forces. Thereby the calcium-containing solid side stream material is treated with the acid and the solution comprising the calcium and/or magnesium salt of the acid is formed at least partially in the high shear mixer. Since the calcium- containing solid side stream material and the acid, or the slurry, are fed to and subjected to the zone of the high shear forces, these forces may be exerted to substantially the entire volume of the calcium- containing solid side stream material and acid. For example, blade mixers may produce high shear forces at the rim of the blade, but they do not form a zone of high shear forces, through which all or essentially all of the calcium-containing solid side stream material and acid would be forced. On the other hand, high shear mixers such as impact mixers (e.g. Atrex- type mixers) may produce high shear forces and all or essentially all of the calcium-containing solid side stream material and the acid, due to the geometry of the mixer, is forced through the zone of high shear forces formed by the rotors. For example, at least about 90 w-%, or at least about 95 w-%, or at least about 99 w-% of the calcium-containing solid side stream material and acid or the slurry may pass through the zone of high shear forces.

The calcium-containing solid side stream material and the acid are therefore efficiently mixed, and the mixing may be faster than e.g. using a conventional mixer, i.e. the residence time through the high shear mixer may be reduced. Energy consumption may also be reduced. Furthermore, it may be possible to use an acid of a higher concentration and/or a slurry with a higher solids content than when using another type of mixer, such as a conventional blade mixer.

The energy intensity of the high shear forces to which the calcium-containing solid side stream material and the acid, or the slurry, are subjected in the high shear mixer may be selected depending on various factors. The energy intensity may be such that the calcium-containing solid side stream material is efficiently slurried with the acid. The energy intensity may therefore be, for example, at least 400 kWh/m 3 .

Any energy intensities described in this specification may be calculated on the basis of the volume occupied by the calcium-containing solid side stream material and the acid and/or the slurry fed into the high shear zone. The residence time of the calcium-containing solid side stream material and the acid and/or of the slurry in the zone of high shear forces may be about 0.01 to 60 seconds, or longer, if desired.

The high shear mixer may be capable of operating continuously.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least one rotating rotor element.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least one static stator element and at least one rotating rotor element.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least two counter-rotating rotors.

An example of a high shear mixer may be an impact mixer, for example an impact mixer sold under the trade name Atrex® (Megatrex Oy) . Such a high shear mixer may comprise a first rotor provided with blades and a second rotor provided with blades, wherein the first and second rotor are arranged concentrically with each other and configured to rotate to opposite directions in relation to each other, and the calcium- containing solid side stream material and the acid and/or the slurry are supplied through the rotors such that they are repeatedly subjected to shear forces by the effect of the blades, the effect of the blades thereby forming the zone of high shear forces and mixing the calcium-containing solid side stream material and the acid. The rotor elements may be capable of rotating at a speed of about 500 to 5000 rpm. Examples of such a high shear mixer are described e.g. in WO 2013/072559 (page 7, line 1 - page 11, line 17 and Figures 1-4) or in FI 105112 B (e.g. the apparatus described in Figs. 1 - 5 and associated paragraphs in the text, e.g. p. 5, 1. 30 to p. 8, 1. 31) , which are herein incorporated by reference in their entirety.

Another type of high shear mixer is the mixer sold under the trade name Cavitron® (Hagen & Funke GmbH) . Such a Cavitron-type high-shear mixer may comprise a dispersing unit or shock-wave reactor. In the dispersing unit or shock-wave reactor, a zone of high shear forces is induced by a rotor/stator system having passage gaps at the rotor and stator. The Cavitron-type high-shear mixer may be configured to fill the gaps arranged in a row with the calcium- containing solid side stream material and the acid or the slurry, such that they are/it is centrifugally accelerated by the rotor to gaps in an adjacent row of gaps, thereby generating alternating pressure fields. Examples of possible high shear mixers may be described e.g. in US3165299A and US3589363.

The method may further comprise separating the solution obtained from residual solids. This can be done, for example, by filtering or using centrifugal forces. In an embodiment, the residual solids are separated using at least one of a decanter centrifuge or a pressure filter. The pressure filter may be a vertical pressure filter. A vertical pressure filter may have a relatively good performance for separating the residual solids. However, the pressure filter may also be a horizontal pressure filter; other filter types may also be contemplated. The residual solids may further be treated by washing them with acid. The acid used in the washing may be the same acid used in the treatment, or it may be selected independently. The acid washing may further dissolve the residual solids.

The solution and optionally also the residual solids may then be recovered.

The solution may be utilized for various purposes, either as such or after further treatment. The method may further comprise concentrating the solution prior to use and/or further treatment.

The method may further comprise using the solution as a nutrient solution in biological waste water treatment. The solution comprising the calcium and/or magnesium salt of the acid may be conveyed to a waste water treatment plant for use as a nutrient solution in biological waste water treatment within the waste water treatment plant. This may allow nutrients obtainable from the green liquor dregs of a chemical pulping plant to be reused at the waste water treatment plant of the same chemical pulping plant and may thus be logistically beneficial. As the nutrient solution can be conveyed and used as such, or after being concentrated, there may not be any need to further treat the solution prior to using it in the biological waste water treatment. Microorganisms, such as bacteria, used in the biological waste water treatment can utilize components of the solution as nutrients. The pH of the solution may be adjusted prior to using it as the nutrient solution, if desired. It may also be possible to purify the solution from certain harmful components prior to using it as the nutrient solution, if desired.

The method may further comprise treating the solution by electrolytical water treatment, thereby removing at least a portion of harmful substances, such as heavy metals, contained in the solution.

Although the composition of the solution and also of the salt(s) of the acid may, at least to some extent, depend on the composition of the calcium- containing solid side stream material, it may be possible to separate the calcium and/or magnesium salt from the solution either together or separately.

The method may comprise precipitating and/or crystallizing the calcium and/or magnesium salt of the acid contained in the solution from the solution. The calcium and/or magnesium salt thus obtainable may be used for various purposes, for example for fertilizing and/or soil conditioning.

The precipitating and/or crystallizing may be done by evaporating the water in the solution. The method may further comprise drying and/or flaking the product thereby obtainable comprising the calcium and/or magnesium salt of the acid precipitated and/or crystallized from the solution.

The yield of the calcium and/or magnesium salt of the acid may be e.g. at least about 70 % (w/w) , or at least about 80 % (w/w) , or at least about 85 % (w/w), of the calcium and/or magnesium contained in the calcium-containing solid side stream material, but may depend e.g. on the type of the calcium- containing solid side stream material, the amount of the acid and other factors.

The residual solids may be discarded, for example by disposing of them in a landfill. It may also be possible to treat them further to reduce the amount of harmful substances in them. It may also be possible to utilize or reuse them further.

Alternatively or additionally, the treatment with an acid may be repeated after the first treatment, i.e. treating the calcium-containing solid side stream material with the acid. In other words, in a further treatment, the residual solids may be further treated with an acid (the same or a different acid, which may be selected independently from the acid used to treat the calcium-containing solid side stream material in the first treatment) , thereby obtaining a second solution comprising a calcium and/or magnesium salt of the acid. The second solution may be combined with the solution obtained in the first treatment, or the solutions may be used and/or treated further separately. A solution comprising a calcium and/or magnesium salt is also disclosed. The solution may obtainable by the method according to one or more embodiments described in this specification. The calcium and/or magnesium salt may be any calcium and/or magnesium salt of the acid described in this specification. The solution may be used for various purposes, such as for fertilizing or soil conditioning. For example, nitrate salts of calcium and optionally of magnesium and/or sodium may be useful and well suited for fertilizing purposes.

A product comprising the calcium and/or magnesium salt of the acid precipitated and/or crystallized from the solution obtainable by the method according to one or more embodiments described in this specification is also disclosed. The product may be used for various purposes, such as for fertilizing or soil conditioning. The product may be, for example, a fertilizer product or a soil conditioner. The product may be or comprise the dried calcium and/or magnesium salt of the acid. The product may further comprise other salts, for example sodium (Na) salts of the acid, and possibly also other calcium, magnesium and/or sodium salts. The product may be a solid product.

The cadmium (Cd) content of the product and/or of the solution may be less than or equal to 3 mg/kg, or less than or equal to 2.5 mg/kg, or less than or equal to 2 mg/kg, or less than or equal to 1.5 mg/kg, based on the total dry weight of the product or solution .

Use of the product comprising the calcium and/or magnesium salt of the acid precipitated and/or crystallized from the solution obtainable by the method according to one or more embodiments described in this specification for fertilizing or soil conditioning is also disclosed. A system for the treatment of calcium- containing solid side stream material obtainable from a chemical recovery process of a chemical pulping process is disclosed. The system may comprise

an apparatus for mixing the calcium- containing solid side stream material with an acid to obtain a solution comprising a calcium and/or magnesium salt of the acid; and

a separation apparatus for separating the solution comprising the calcium and/or magnesium salt of the acid from residual solids.

The apparatus for mixing the calcium- containing solid side stream material with an acid to form a solution comprising a calcium and/or magnesium salt of the acid, i.e. the apparatus configured to mix the calcium-containing solid side stream material with an acid, may comprise or be a high shear mixer.

In an embodiment, the high shear mixer comprises a first rotor provided with blades and a second rotor provided with blades, wherein the first and second rotor are arranged concentrically with each other and configured to rotate to opposite directions in relation to each other, so that the calcium- containing solid side stream material and the acid or the slurry thereof are repeatedly subjected to shear forces by the effect of the blades, the effect of the blades thereby mixing the calcium-containing solid side stream material and the acid. Such a high shear mixer is thus configured to form the zone of high shear forces by the effect of the blades.

In an embodiment, the high shear mixer is a Cavitron-type mixer.

The system may further comprise a pressure filtration unit for filtering the calcium-containing solid side stream material prior to mixing it with the acid, i.e. a pressure filtration unit configured to filter the calcium-containing solid side stream material prior to mixing it with the acid. The calcium-containing solid side stream material may be any calcium-containing solid side stream material described in this specification, for example green liquor dregs. The pressure filtration unit may be a vertical pressure filtration unit.

The separation apparatus, i.e. the separation apparatus configured to separate the solution comprising the calcium and/or magnesium salt of the acid from residual solids, may be, for example, a filtration apparatus, a centrifugation apparatus, a decantation apparatus, a clarification apparatus, a flotation apparatus or a sedimentation apparatus. In an embodiment, the separation apparatus is a pressure filter, such as a vertical pressure filter or a horizontal pressure filter.

The system may also comprise e.g. an evaporation apparatus for evaporating water from the solution comprising the calcium and/or magnesium salt of the acid, i.e. configured to evaporate water from the solution comprising the calcium and/or magnesium salt of the acid. The system may further comprise an apparatus for drying, i.e. configured to dry; an apparatus for crystallizing, i.e. configured to crystallize; and/or an apparatus for flaking, i.e. configured to flake, the product comprising the calcium and/or magnesium salt of the acid.

The system may further comprise a waste water treatment plant.

The system may further comprise a chemical pulping plant and a waste water treatment plant for treating waste water obtainable from the chemical pulping plant, i.e. configured to treat waste water obtainable from the chemical pulping plant.

The system may further comprise a flow connection, e.g. a conduit or piping, for conveying the solution comprising the calcium and/or magnesium salt of the acid to the waste water treatment plant for use as a nutrient solution in biological waste water treatment within the waste water treatment plant (i.e. configured to convey the solution to the waste water treatment plant) .

EXAMPLES

Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawing.

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components.

Fig. 1 illustrates a method and a system 14 for the treatment of a calcium-containing solid side stream material 1. The calcium-containing solid side stream material 1 may be obtainable from a chemical pulping process 3, and specifically from the chemical recovery process 2 of the chemical pulping process 3 of a chemical pulping plant. For example, the calcium- containing solid side stream material 1 may be green liquor dregs obtainable by clarifying green liquor, e.g. using a suitable clarifier. Although green liquor dregs are depicted in this exemplary method and system, other calcium-containing solid side stream materials may also be obtainable from the chemical recovery process 2 of the chemical pulping process 3. The system 14 may therefore comprise e.g. a digestion apparatus for cooking pulp; a washing apparatus for washing pulp; and a bleaching apparatus for bleaching pulp (not shown for simplicity) . In an embodiment, the system comprises a chemical recovery system. The chemical recovery system may comprise e.g. a soda recovery unit, a lime slaking unit, a lime sludge reburning kiln, and/or an apparatus purification of flue gas from lime sludge reburning, such as an electrostatic precipitator. The chemical recovery system may further comprise a clarifier for clarifying green liquor; green liquor dregs 1 may be obtainable from the clarifier.

The green liquor dregs 1 may be filtered using a suitable filter, for example a pressure filter 6. The pressure filter 6 may be a vertical pressure filter, but could also be a horizontal pressure filter. The filtering concentrates the green liquor dregs 1. It may also allow returning more sodium back to the circulation of chemicals of the process.

The green liquor dregs 1 may also be filtered using a filter, for example a rotary drum vacuum filter 18. The filter is precoated with lime mud. The green liquor dregs cake thereby obtained will then comprise both the green liquor dregs and lime mud, for example at a ratio of 1:1. The lime mud may typically comprise mainly CaCO 3 . This type of green liquor dregs 1 may be referred to as GLD to landfill.

Either type of green liquor dregs 1 obtained as a cake may be preground, i.e. comminuted, so that its surface area increases. This can be done using a suitable grinding apparatus 19, such as a mill. However, in many embodiments, pregrinding may not be necessary. The grinding apparatus 19 and the pregrinding are therefore fully optional.

The green liquor dregs 1, or any other calcium-containing solid side stream material, either dry or in a slurried form, may be premixed with an acid 4 and optionally with water using a suitable mixing apparatus 20, for example a mixer or a mixing tank. Such a mixer may be e.g. a simple blade mixer. A slurry 7 may thus be obtained. However, in many embodiments the premixing is not necessary, but instead the green liquor dregs 1 and the acid 4 may be fed directly and separately to an apparatus 9 for mixing the calcium-containing solid side stream material (in this embodiment, green liquor dregs) with the acid 4 to form a solution 5 comprising a calcium and/or magnesium salt of the acid. In the apparatus 9 they are then mixed, thereby forming a slurry 7. In particular, when the apparatus 9 is a high shear mixer, there may be no need for the premixing.

As soon as the calcium-containing solid side stream material, e.g. green liquor dregs 1, is brought in contact with the acid 4, they will react, and a solution 5 comprising the calcium and/or magnesium salt of the acid will be formed. The required reaction time may depend on various factors, for example the composition of the calcium-containing solid side stream material 1, the concentration and amount of the acid 4, the intensity of the mixing, and/or the pH of the slurry 7. The pH of the slurry 7 may be adjusted to a desired value.

In this exemplary embodiment, the apparatus 9 for mixing the calcium-containing solid side stream material 1 with the acid 4 to form the solution 5 comprising a calcium and/or magnesium salt of the acid is a high shear mixer. The high shear mixer 9 may be configured to form a zone 8 of high shear forces within the high shear mixer and to subject the calcium-containing solid side stream material 1 and the acid 4, or the slurry 7, simultaneously to the zone 8 of high shear forces, thereby mixing them. An embodiment of such a high shear mixer is described in detail in Figs. 2A and 2B. However, any mixer, in particular any high shear mixer (for example, a Cavitron-type mixer) , described in this specification could be contemplated instead. All or essentially all of the calcium-containing solid side stream material 1 and the acid 4 and/or the slurry 7 may pass through the zone 8 of high shear forces, such that all or essentially all of them are subjected to the high shear forces.

After the reaction between the calcium- containing solid side stream material 1 and the acid 4 has progressed sufficiently or completed, the resulting solution 5 and residual solids 10 (if any) may be conveyed to and separated using a separation apparatus 15 for separating the solution 5 comprising the calcium and/or magnesium salt of the acid from the residual solids 10. The separation apparatus 15 may comprise or be e.g. a filtration apparatus, a centrifugation apparatus, a decantation apparatus, a clarification apparatus, a flotation apparatus or a sedimentation apparatus. The filtering apparatus may, in an embodiment, be a pressure filter, e.g. a vertical pressure filter or a horizontal pressure filter.

The solution 5, or at least a portion thereof, obtained may be treated further.

The system 14 may comprise an electrolytical water treatment apparatus 12 for removing at least a portion of harmful substances, such as heavy metals, contained in the solution. However, the electrolytical water treatment is, in many embodiments, not necessary. Other suitable apparatuses for removing at least a portion of harmful substances may also be contemplated, for example an ion exchange apparatus.

The system 14 may also comprise e.g. an evaporation apparatus 21 for evaporating water from the solution comprising the calcium and/or magnesium salt of the acid. The evaporation apparatus 21 may be used for precipitating and/or crystallizing the calcium and/or magnesium salt of the acid and any other salts present in the solution 5; other crystallizing appa- ratuses may also be contemplated. The system 14 may further comprise an apparatus 22 for drying; and/or an apparatus 23 for flaking the calcium and/or magnesium salt of the acid. A product 13 comprising the calcium and/or magnesium salt of the acid precipitated and/or crystallized from the solution 5 may thus be obtained.

Additives or other substances may be added to the product 13, if desired. Examples of such additives may include e.g. additives for assisting with forming granules of the product, coatings, or components for improving the nutritional content of the product. For example, nitrogen-containing compounds such as nitrogen salts or urea may be added to increase the nitrogen content of the product; ash; one or more of other nutrients, such as one or more of phosphorus, potassium, calcium, magnesium, sulphur, boron, chlorine, manganese, iron, zinc, copper, cobalt, molybdenum, nickel, silicon, selenium or sodium, or other components of a fertilizer or soil conditioner. The product may thus comprise other components, for example any of the components described above.

The system 14 may comprise a waste water treatment plant 17 and a biological waste water treatment system 11 therein. The solution 5, or at least a portion of the solution 5, may be conveyed to a waste water treatment plant 17. The system 14 may therefore comprise a flow connection 16, for example a suitable conduit or piping, for conveying the solution comprising the calcium and/or magnesium salt of the acid to the waste water treatment plant 17 for use as a nutrient solution in biological waste water treatment 11 within the waste water treatment plant. The solution 5 may be added to the biological waste water treatment process, for example to a biological purification process, in which the microbes, e.g. bacteria, may utilize the calcium and/or magnesium salt of the acid and optionally any other components in the solution 5 as nutrients. The solution 5 obtainable from the electrolytical water treatment apparatus 12, i.e. the solution 5 treated by electrolytical water treatment, may also be conveyed to the waste water treatment plant 17.

The system 14 may further comprise conduits or flow connections, for example pipings, between the parts of the system for conveying the calcium- containing solid side stream material 1; the acid 4; the slurry 7; the solution 5; the residual solids 10; and/or the product 13.

Fig. 2A illustrates a cross-sectional side view of an exemplary high shear mixer 9 which can be used for mixing the calcium-containing solid side stream material 1 and the acid 4. Fig. 2B shows the same exemplary high shear mixer 9 as a cross-sectional top view. This embodiment of the high shear mixer 9 is merely an example, and it is clear to a skilled person that various other high shear mixers or other mixers may be utilized for the same purpose and that their structures and operations may differ from the one described herein. The high shear mixer 9 described in these Figs, is similar e.g to Atrex CD 500 G45 which is suitable for the method.

The high shear mixer 9 comprises a first rotor 24 and a second rotor 25 arranged concentrically within each other such that they are configured to rotate around a common rotation axis 26. The first rotor 24 and the second rotor 25 are configured to rotate to opposite directions in relation to each other. The first and second rotor 24, 25 thus form a pair of counter-rotating rotors. However, the high shear mixer 9 may comprise two, three or more rotors. In other embodiments, one of the first or second rotors 24, 25 could be replaced by a stator. However, solutions comprising at least two counter-rotating rotors may be more efficient at causing high shear forces .

The rotors 24, 25 are provided with blades or ribs 33. The blades 33 are arranged in at least two concentric rims 27, 28, 29, 30, 31, 32. The blades 33, connected to the rotors 24, 25, are thus also configured to rotate around the common rotation axis 26. The at least two concentric rims are configured to rotate to opposite directions in relation to each other. The exemplary embodiment of the high shear mixer 9 shown in Figs. 2A and 2B comprises blades arranged in a plurality of rims 27, 28, 29, 30, 31, 32 - specifically six in this embodiment. The blades 33 of three of the rims 27, 29, 31 are configured to rotate in the same direction. The blades of the other three rims 28, 30, 32 are configured to rotate in the opposite direction. The rims 27, 28, 29, 30, 31, 32 are arranged pairwise such that one rim is always followed and/or preceded in the radial direction by a rim of counter-rotating blades. The rotors may be capable of rotating at a speed of about 500 to 5000 rpm, but speeds lower than about 500 or greater than about 5000 may also be contemplated.

The blades 33 may be elongated pieces, the height of which may be greater than their width (i.e. their dimension in the radial direction of the rotors) . Figs. 2A and 2B show measurements of certain dimensions of the exemplary high shear mixer 9 in millimetres, but various geometries and dimensions for the rotors and the blades may be contemplated. For example, in Fig. 2B the blades 33 are parallel to the radial direction, but at least some of the blades 33, for example those arranged in the outermost rim 32 or in the two outermost rims 31, 32, may be arranged at an angle to the radial direction. This may increase the residence time in the high shear mixer 9 and thereby possibly also the dissolution of the calcium- containing solid side stream material 1. Further, the dimensions of the high shear mixer 9 may be selected e.g. on the basis of the amount of the calcium- containing solid side stream material 1 to be treated in a given time period. The blades 33 and optionally other parts of the rims and rotors may be formed of an acid compatible alloy.

The high shear mixer 9 comprises a housing 34 within which the rotors 24, 25 may be arranged. The high shear mixer 9 further comprises an inlet 35 for feeding in the calcium-containing solid side stream material 1 and the acid and/or the slurry 7. The inlet 35 opens to the innermost rim 27. The high shear mixer 9 further comprises an outlet 36 for removing the slurry 7. The outlet 36 opens to the outermost rim 32 and extends through the housing 34. As the calcium- containing solid side stream material 1 and the acid and/or the slurry 7 are fed into the high shear mixer 9 via the inlet 35, it is subsequently fed through the zone 8 of high shearing forces towards the outlet 36. The rotors 24, 25 can be considered to be flow-through rotors, such that the calcium-containing solid side stream material 1 and the acid and/or the slurry 7 may pass through the rotors via gaps between the blades 33 extending in the direction of the rotation axis 26 and therefore also through the zone 8 of the high shear forces. The calcium-containing solid side stream material 1 and the acid and/or the slurry 7 may pass through the zone 8 of high shear forces in a given residence time.

The calcium-containing solid side stream material 1 and the acid 4 and/or the slurry 7 may thus be supplied outwards in the radial direction with respect to the rotation axis 26 of the rotors 24, 25 such that the calcium-containing solid side stream material 1 and the acid and/or the slurry 7 are repeatedly subjected to shearing and impacting forces by the effect of the blades 33 of the counter-rotating rotors. The blades 33 provide collision surfaces for impacting. The zone 8 of high shearing forces is thus generated in the space along which the blades move upon the rotation of the rotors. The extent of the zone 8 of high shearing forces is shown in Fig. 2A in the direction of the rotation axis 26 and in Fig. 2B in the radial direction. The calcium-containing solid side stream material 1 and the acid or the slurry 7 may thus be supplied through the rotors 24, 25 such that they are repeatedly subjected to shear forces by the effect of the blades 33, the effect of the blades 33 thereby forming the zone 8 of high shear forces and mixing the calcium-containing solid side stream material 1 with the acid 4 intimately.

Further, the rotary movement of the blades 33 produce narrow gaps 37 between the blades 33, in which the calcium-containing solid side stream material 1 and the acid and/or the slurry 7 are subjected to shear forces. As each pair of counter-rotating rims, i.e. 27 and 28; 29 and 30; and 31 and 32, of blades generates a number of the narrow gaps 37 and correspondingly reversals of impact directions during each complete rotation of a pair of rims, the direction of the impacts caused by the blades 33 changes at a high frequency.

The calcium-containing solid side stream material and the acid are therefore repeatedly impacted by and subjected to the shearing forces caused by the blades 33. These impacting forces and shearing forces therefore cause the mixing of the calcium-containing solid side stream material and the acid, and may also cause the calcium-containing solid side stream material to be ground to smaller particles. The calcium-containing solid side stream material 1 and the acid 4 and/or the slurry 7 fed into the zone 8 of high shear forces thus can occupy a volume within the rims, i.e. the space along which the blades 33 move upon the rotation of the rotors and which is not occupied by the blades or other parts of the rotors.

Various parameters such as the number of the rotors and rims, the number and/or density of the blades in each rim and/or rotor, the geometry (e.g. angles) of the blades and/or the rotation speeds of the rotors may be used to affect the shear forces and the energy intensity to which the calcium-containing solid side stream material and the acid and/or the slurry are subjected.

The energy intensity in the zone 8 of high shear forces may be calculated or estimated by dividing the input power of the high shear mixer by the volume of the calcium-containing solid side stream material 1 and the acid 4 and/or the slurry 7 fed into the zone 8 of high shear forces.

The residence time of the calcium-containing solid side stream material and the acid and/or the slurry in the zone 8 of high shear forces may be about 0.01 to 60 seconds, or longer, if desired. The type of high shear mixer described in these Figs, may however be quite efficient, and residence times of a few seconds may in many cases suffice.

EXAMPLE 1

Green liquor dregs were obtained from a pulp mill and treated with a vacuum drum filter. Lime mud was used as a precoat. This treated fraction was referred to as green liquor dregs to landfill. Based on four separate measurements, the calcium content of the lime mud had been 163-430 g/kg ds measured by ICP-OES. Green liquor dregs to filter was the fraction entering the vacuum filter without any lime mud. The green liquor dregs, water and acid were mixed. After a reasonable mixing time, the solution was filtrated with a Buchner filter, as the soluble salts remained in the filtrate. The cake and the fil- trate were separated and dried overnight at 105 °C.

Ion chromatography (or ion-exchange chromatography) , inductively coupled plasma optical emission spectrometry (ICP-OES) , Energy Dispersive X- ray Spectroscopy (EDS) and FTIR (Fourier Transform Infrared Spectrometer) were used in the analyses.

Green liquor dregs (GLD to landfill) from a pulp mill were mixed with distilled water and nitric acid by a magnetic mixer. The trial points are presented in Table 1. Nitric acid with different concen- trations, elevated temperature and different mixing times were tested.

It appeared that the lower the pH, the bigger the dregs reduction. Some of the salts obtained were dried at 105 °C overnight.

EDS was performed for some of the samples and the results are presented in Fig. 3. The EDS results showed that the formed salts are likely calcium, magnesium and sodium nitrates.

The sulphate and nitrate contents of the three samples were defined by IC, and the results are presented in Fig. 4. The calcium, magnesium and sodium contents of the few samples were measured by ICP. The results are presented in Fig. 5. Based on IR spectra obtained (not shown) , the composition of formed salts are likely calcium nitrate, but also magnesium nitrate is present.

Based on the results of IC, ICP, EDS and FTIR it was concluded that the major part of the formed salts were calcium nitrate, but also magnesium and so- dium nitrate were present. Some additional impurities such as sulphur were present. EXAMPLE 2

A mechanical mixer was used in the present example. The trial points are presented in Table 2. Nitric acid was used and the pH target was near neutral .

The EDS results of the salts are presented in Fig. 6. The formed salts contained mainly calcium. Also magnesium, sodium and sulphur were present.

The contents of calcium, magnesium, sodium and sulphur were also measured from a few test points by ICP. The formed salt was quite pure calcium nitrate, because the contents of magnesium, sodium and sulphur were quite low. The other samples also contained mainly calcium as seen in Fig. 7.

Based on FTIR analyses (not shown) , the formed salts were nitrates, mainly calcium nitrates but also magnesium nitrates were found to be present.

The amount of cadmium was measured from some of the samples. The results of ICP measurements are presented in Table 3.

Table 1. Cadmium content of the cake and filtrate measured by ICP

EXAMPLE 3

Acetic acid was used to form calcium acetate. The raw material of these experiments was green liquor dregs to landfill. The trial record is presented in Table 4.

The EDS results of these samples are presented in Fig. 8 and ICP results in Fig. 9.

Calcium, magnesium and sodium acetates were formed.

FTIR analyses (not shown) showed that the formed salts are likely to be calcium acetates. There might also be some magnesium acetate.

The cadmium contents of the cake and filtrate were analysed. The results are presented in Table 5.

Table 5. Cadmium content of the cake and filtrate

EXAMPLE 4

Green liquor dregs to filter was selected as the raw material for these tests, because the lime mud used as a pre-coat when green liquor dregs to landfill is formed, could be utilized elsewhere due to its heavy metal free composition.

Nitric acid, phosphoric acid and acetic acid were used in these trials. The trial record of these tests is presented in Table 6.

The EDS results of these test points are presented in Fig. 10.

Based on the EDS and FTIR spectra (not shown) it seemed that the formed salts were mainly sodium salts. Nitric acid formed sodium nitrate, phosphoric acid formed sodium phosphate and acetic acid formed sodium acetate.

The measured cadmium contents of the cake and filtrate of these samples are presented in Table 7.

Table 7. Cadmium amounts measured by ICP

EXAMPLE 5

Two Larox pressure filters, vertical and horizontal, were tested.

The vertical Larox pressure filter was tested in order to see if bigger cakes could be obtained. The raw material was green liquor dregs to filter from a pulp mill. Three chambers were used in the filter.

ICP analyses were done from the initial green liquor dregs to filter and from the filtrated and fil- trated+washed green liquor dregs. The results are presented in Table 8.

Table 8. ICP measurements of green liquor dregs before and after Larox pressure filter

EXAMPLE 6

The Larox filtrated green liquor dregs to filter were acidified with nitric acid to form calcium nitrate and with acetic acid to form calcium acetates. The performed trials and results are presented below.

Nitric acid trial points

The Larox filtered green liquor dregs to filter was treated with nitric acid to form calcium nitrate .

All the cakes were combined and dried at 105 °C overnight before the analysis. Filtrates were com- bined and analyzed.

The other trial points were performed in the same way, only the amount of acid was changed. The amount of acids and the trial record are presented in Table 9.

The filtrate was analysed as such without any drying procedures. The ICP measurements of the salt liquids are presented in Fig. 11. The amount of calcium was similar in all test points. Quite some sodium, magnesium and sulphur were also present.

The amount of sulphates and nitrates were measured by IC. The results are presented in Fig. 12.

Based on FTIR analyses (not shown) the salts were likely to be nitrate salts containing calcium ni- trate but possibly also sodium and magnesium nitrates.

The amounts of nitrates calculated based on the ICP analysis are presented in Table 10.

Approximately half of the formed salts were calcium nitrates, one fourth magnesium nitrates and one fourth sodium nitrate.

The cadmium contents of the remained dregs and filtrate (salt) were measured by ICP. The results are presented in Table 11.

Table 11. Cadmium content of the rema dregs and filtrate (salt)

To summarize, the formed salts were nitrates, mainly calcium nitrates but also magnesium nitrates. Based on the IC analysis, the major part of the ions are nitrates and there was only very little of sulphates. Based on the ICP measurements, approximately half of the analysed elements were calcium, but also quite some magnesium and sodium were present.

Acetic acid trial points The Larox filtered green liquor dregs to filter was treated with acetic acid to form calcium acetate. The trial procedure was similar to the nitric acid experiments, only the acid grade and the amount of acid were changed. The trial record is presented in Table 12.

Two trial points were performed. The raw material of the trial point 11 was Larox filtrated green liquor dregs to filter without wash stage and the raw material of the trial point 14 was Larox filtrated and washed green liquor dregs to filter. The results of the ICP analysis are presented in Fig. 13. Most of the salts were made of calcium, but there was also quite a lot sodium, magnesium and sulphur in the filtrate. The results of the EDS analyses are presented in Figs. 14A and 14B. IC results are presented in Fig. 15. A major part of the ions were acetate ions and there were also very small amounts of sulphates and nitrates.

Based on FTIR analyses (not shown) it was concluded that the formed salts were acetate salts, mainly calcium acetates, but also magnesium and sodium acetates were present.

The cadmium contents of the remained dregs and filtrate (salt) were measured by ICP. The results are presented in Table 13.

Table 13. Cadmium content of the remaining dregs and filtrate (salt)

It is obvious to a person skilled in the art that with the advancement of technology that the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, a product, a system, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.