FIELD OF THE INVENTION

The invention relates to a method for reduc ^¬ ing the phosphorus load in effluent from a pulp pro ^¬ duction process. The invention further relates to uses of at least one chelating agent. BACKGROUND OF THE INVENTION

The content of compounds harmful for the en ^¬ vironment in effluent or waste water coming from a pulp production process is nowadays rather strictly controlled for environmental reasons in order to pro- vide a sustainable production process. Plants have different kinds of solutions that are used for waste water treatment in order to ensure that environmental regulations are fulfilled for the plant.

Traditional solutions for phosphorus removal are mainly for the effluent treatment plant, including tertiary treatment, use of phosphorus concentrating (P-concentrating) microorganisms, precipitation with different chemicals, etc. A drawback of tertiary treatment is a high investment and operating cost. P- concentrating microorganisms work in anaerobic media, which complicates the needed operations and requires specific treatments for the effluent to be used. Pre ^¬ cipitation with chemicals involves a high operating cost and depending on the precipitation chemical used can pose challenges for non-process elements accumula ^¬ tion in the recovery cycle when biosludge is burnt in a recovery boiler.

The inventors have recognized the need for a method to reduce the phosphorus load in the effluent coming from the pulp production process so that less phosphorus needs to be dealt with in e.g. a waste wa ^¬ ter treatment plant.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide a new type of method for reducing the phosphorus load in effluent from a pulp production process. Further, the purpose of the invention is to provide new uses of at least one chelating agent.

SUMMARY

The method according to the present invention is characterized by what is presented in claim 1.

The uses according to the present invention are characterized by what is presented in claims 7 and 8.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is included to provide a further understanding of the invention and constitutes a part of this specification, illus ^¬ trates an embodiment of the invention and together with the description helps to explain the principles of the invention. In the drawing:

Fig. 1 is a flow chart illustration of a method for reducing the phosphorus load in effluent from a pulp production process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for reducing the phosphorus load in effluent from a pulp production process, which process comprises the steps of a) digesting lignocellulosic material for forming cooked pulp, b) washing the pulp, and c) bleaching the pulp, wherein digesting the lignocellulosic material in step a) is carried out in the presence of at least one chelating agent for preventing the formation of insoluble salt containing calcium and/or magnesium and phosphorus .

The expression "pulp production process" should be understood in this specification, unless otherwise stated, as referring to a manufacturing pro ^¬ cess that converts lignocellulosic material into a fi ^¬ brous material, which is also referred to as pulp. In step a) , the lignocellulosic material is cooked, whereby heat and chemicals break down preferably lig- nin. In step b) the pulp is washed, whereby breakdown products are washed out from fibrous material. In step c) , i.e. bleaching, the brightness of fibrous material is increased. The fibrous material produced can be e.g. further processed into paper.

The expression "pulp" should be understood in this specification, unless otherwise stated, as a lig ^¬ nocellulosic fibrous material prepared by chemically and/or mechanically separating cellulose fibres from a lignocellulosic material. So-called kraft or sulphate processes can be used for producing pulp.

In this specification, unless otherwise stat ^¬ ed, the expression "lignocellulosic material" should be understood in as any plant derived organic matter that includes cellulose, hemicellulose, and lignin. Lignocellulosic material can originate from virgin bi- omass from trees, bushes and grass or waste biomass from various sectors like straw, sugarcane bagasse, corn stover etc. from the agricultural sector as well as saw mill and paper mill discards from the forestry sector .

In one embodiment of the present invention the lignocellulosic material originates from at least one wood species. In one embodiment of the present in ^¬ vention the lignocellulosic material originates from at least one hardwood species. In one embodiment of the present invention, the lignocellulosic material originates from at least one eucalyptus species.

In one embodiment of the present invention the lignocellulosic material comprises wood chips. In one embodiment of the present invention the wood chips comprise chips of at least one hardwood species. In one embodiment of the present invention the wood chips comprise chips of at least one eucalyptus species.

The present invention further relates to the use of at least one chelating agent in a pulp produc ^¬ tion process, which process comprises the steps of a) digesting lignocellulosic material for forming cooked pulp, b) washing the pulp, and c) bleaching the pulp, for preventing the formation of insoluble salt con- taining calcium and/or magnesium and phosphorus.

The present invention further relates to the use of at least one chelating agent in a pulp produc ^¬ tion process, which process comprises the steps of a) digesting lignocellulosic material for forming cooked pulp, b) washing the pulp, and c) bleaching the pulp, for reducing the phosphorus load in effluent from the pulp production process.

In one embodiment of the present invention the step c) of bleaching the pulp comprises at least one step of bleaching the pulp in acidic conditions. In one embodiment of the present invention the step c) of bleaching the pulp comprises at least one step of bleaching the pulp in conditions with neutral pH .

The expression "effluent" should be under- stood in this specification, unless otherwise stated, as referring to waste water discharged from a pulp production process. The waste water discharged from a pulp production process is usually purified at a waste water treatment plant before being conveyed further to e.g. a river, a lake or the sea. The expression "phosphorus load in effluent" should be understood as referring to the amount of phosphorus in the effluent.

Especially a phosphorus load in the effluent reaching territorial waters may be harmful to the sur ^¬ rounding environment. Thus regulatory agreements in certain countries are becoming increasingly strict. One of the main sources for the phosphorus load coming from a pulp production process or plant is the bleach- ing stage. The phosphorus of the pulp production pro ^¬ cess mainly originates from the lignocellulosic mate ^¬ rial, e.g. wood, used in the process. The inventors of the present invention found out that the phosphorus content together with the calcium content of the used lignocellulosic material affects the amount of phos ^¬ phorus that will pass through the pulp production pro ^¬ cess and end up in the effluent thereof and especially in the effluent from the bleaching step. Magnesium, and possibly also other elements, may also precipitate in the same manner as calcium. It was found out that calcium and possibly also magnesium present in the step of digesting the lignocellulosic material is able to precipitate phosphates from the lignocellulosic ma ^¬ terial. This formed precipitate is then carried fur- ther through the fiberline process until it is dis ^¬ solved under the acidic or neutral conditions used e.g. in the step of bleaching the pulp. As the phos ^¬ phorus is being dissolved during the bleaching step, it will end up in the effluent received from this step.

The inventors of the present invention sur ^¬ prisingly found out that the use of at least one che ^¬ lating agent in the step of digesting the used ligno ^¬ cellulosic material is able to bind the calcium and/or magnesium present in a manner that the formation of insoluble salt containing calcium and/or magnesium and phosphorus is decreased or prevented. In one embodiment of the present invention the insoluble salt containing calcium and/or magnesium and phosphorus comprises calcium phosphate precipi ^¬ tate. In one embodiment of the present invention the insoluble salt containing calcium and phosphorus con ^¬ sists of calcium phosphate precipitate. The insoluble salt may also contain other components, such as magne ^¬ sium phosphate and other calcium and/or magnesium salts .

The inventors of the present invention found out that the use of the method according to the pre ^¬ sent invention can result in effluent, coming from the pulp production process and especially from the bleaching step thereof, with a reduced phosphorus load compared to a corresponding process, where no chelat ^¬ ing agent is used in step a) of digesting the ligno- cellulosic material.

A chelating agent or a complexing agent is a substance capable of forming a complex compound with another material in solution. It was found out that when using at least one chelating agent in the step of digesting the lignocellulosic material the calcium and/or magnesium will be part of the complexes formed with the chelating agent instead of forming calcium and/or magnesium phosphates.

The amount of the at least one chelating agent in step a) may be selected e.g. depending on the amount of calcium present in the wood raw material species. For example, in Eucalyptus species with low calcium contents of up to 500mg/kg od wood, the effec ^¬ tive dosage of a commercial industrial chelating agent could range from 0,5 to 8 kg/ADt depending on the de ^¬ sired effect (e.g. how large a percentage of the phos ^¬ phorus in effluent should be removed) . For commonly occurring calcium contents of about lOOOmg/kg od wood dose, the amount may be e.g. about 15kg/ADt. For high calcium containing species, the amount may be even higher. Further, a stoichiometric excess of chelating agent may be added with respect to the stoichiometric amount of calcium and/or magnesium.

In one embodiment, the amount of the at least one chelating agent in step a) is at least 0,5 kg/ADt. kg/ADt should be understood as referring to kilograms of the chelating agent per air dry tonne of pulp. Thus 0,5 kg/ADt corresponds to 0,05 % (w/w) of the at least one chelating agent based on the weight of air dry pulp.

In one embodiment of the present invention the amount of the at least one chelating agent in step a) is 0,5 - 15 kg/ADt.

In one embodiment, the amount of the at least one chelating agent in step a) is 0,5 - 8 kg/ADt.

In one embodiment, the amount of the at least one che ^¬ lating agent in step a) is 8 - 15 kg/ADt. In one em ^¬ bodiment, the amount of the at least one chelating agent in step a) is at least 15 kg/ADt.

In one embodiment, the at least one chelating agent is present in stoichiometric excess in compari ^¬ son to the amount of calcium and/or magnesium in step a) . In one embodiment, the at least one chelating agent is present in frank or large stoichiometric ex- cess relative to calcium and/or magnesium in step a) . In one embodiment, the molar ratio of the at least one chelating agent to calcium and/or magnesium in step a) is in the range of 1:1 to 10:1, or in the range of 2:1 to 6:1, or in the range of 3:1 to 5:1.

In one embodiment of the present invention the chelating agent is selected from a group consist ^¬ ing of ethylenediaminetetraacetic acid (EDTA) , 2,3- dimercaptopropanesulfonic acid (DMPS), thiamine tetra- hydrofurfuryl disulfide (TTFD) , diethylene triamine pentaacetic acid (DTPA) , tri ( carboxymethyl ) amine (NT), and 2 , 3-dimercaptosuccinic acid (DMSA) . In one embodiment of the present invention a further amount of at least one chelating agent is add ^¬ ed before step c) , and preferably in step b) .

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to ^¬ gether to form a further embodiment of the invention. A method, or a use, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore.

An advantage of the present invention is that the content of phosphorus in effluent coming from a pulp production process can be reduced by the method according to the present invention. This results in less phosphorus being needed to be treated in e.g. a waste water treatment plant, which may have a benefi ^¬ cial effect also for the waste treatment plant.

An advantage of the present invention is that the method enables the use of different kinds of lig- nocellulosic material in the pulp production process as it diminishes the effect of different elements con ^¬ tents, such as the phosphorus and calcium and/or mag ^¬ nesium content thereof, of the source material.

An advantage of the method according to the present invention is that it does not require signifi ^¬ cant investments for equipment of the pulp production plant .

EXAMPLES

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

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

Figure 1 illustrates a method according to one embodiment of the present invention for reducing the phosphorus load in effluent from a pulp production process. The pulp production process comprises, in step a) , digesting or cooking lignocellulosic material for forming cooked pulp material. Step a) of digesting the used lignocellulosic material is carried out in the presence of at least one chelating agent. The use of the chelating agent in step a) prevents the for ^¬ mation of insoluble salt containing calcium and/or magnesium and phosphorus, such as calcium phosphate precipitate, during the cooking step. The at least one chelating agent may be used e.g. in an amount of 0,5 - 15 kg/ADt.

After step a) of forming cooked pulp, the pulp is washed in step b) followed by step c) of bleaching the pulp in e.g. in acidic conditions. The effluent coming from the pulp production process and especially from the bleaching step thereof has a re ^¬ duced phosphorus load compared to a corresponding pro ^¬ cess, where no chelating agent has been used in step a) of digesting the lignocellulosic material.

EXAMPLE 1 - Reducing phosphorus load in effluent coming from a pulp production process; laboratory simulation

Eucalyptus wood chips were laboratory cooked using an airbath digester with 1L autoclaves, using an alkali dose of 20% EA as NaOH and H factor to reach kappa number in the range of 16 to 18. In the blank no chelating agent was added, while in the different ex ^¬ periments a chelating agent was added to the cooking liquor before the beginning of the cook. Cooked chips were disintegrated, screened and thoroughly washed with water. After that an acidic extraction with sul- furic acid simulating a bleaching A stage was performed, using following conditions: temperature 85°C, final pH 3, consistency 5% and retention time 180 min. In one example, wood chips were clonal Eucalyptus dun- nil chips with a high calcium content of 9,4g/kg od wood and a phosphorous content of 110 mg/kg od wood. Wood chips were cooked using H factor 600. Chelating agent was EDTA disodium salt ppa added in frank excess with respect to calcium content (5mol EDTA/ mol calci- urn) to prove the concept. Phosphorous extracted from A stage (simulating P carried to effluent) decreased from 34mgP/kg od wood to non-detectable (<0,5mg P/kg od wood) . EXAMPLE 2

Wood chips were mill chips taken from UPM Fray Bentos Mill production, consisting of a mix of E. grandis 46%, E. benthamll 20%, E. dunnll 16%, E. glob- ulus 11% and E. maldenll 7%. Phosphorous content in chips was lOOmg/kg od wood and calcium content l,0g/kg od wood. Mill chips were cooked to kappa 17, with H factor 450. The chelating agent was EDTA disodium salt ppa added in a dose of 1,0 kg/ADt. Phosphorous ex- tracted from A stage decreased from 13mg P/kg od wood to 8mg P/kg od wood (38% phosphorous reduction to ef ^¬ fluent ) .

EXAMPLE 3

The same mill chips as in Example 2 were cooked to kappa 15 (H factor 600) and treated with commercial chelating agents: EDTA with a chelating power of lOOmg Ca/kg commercial product and DTPA with a chelating power of 80mg Ca/kg commercial product, both in a dose of 2kg/ADt pulp. Commercial EDTA re ^¬ duced phosphorous extracted in A stage from 21,4mg P/kg od wood to 17,7mg P/kg od wood (17% phosphorous reduction to effluent) and DTPA to 15,3mg P/kg od wood (29% phosphorous reduction to effluent). It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.