WO2008006946A1

WO2008006946A1 - Process for the treatment of a filtrate with oxygen and use of oxygen for reducing harmful components in a filtrate

Info

Publication number: WO2008006946A1
Application number: PCT/FI2007/050428
Authority: WO
Inventors: Hannu Leino; Ragnar SJÖDAHL
Original assignee: Linde Aktiengesellschaft
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2008-01-17
Also published as: FI20065494A0

Abstract

The invention relates to a process for the treatment of a filtrate with oxygen and to the use of oxygen for reducing harmful components in a filtrate. The filtrate is produced in the early stages of a mechanical pulping process in a dewatering device located upstream of any bleaching stage of said process, At least part of the filtrate is treated with oxygen to eliminate harmful components from the filtrate. The filtrate typically circulates in a water circulation or loop and is returned to the pulping process; the filtrate or part thereof may be discharged from the process.

Description

Process for the treatment of a filtrate with oxygen and use of oxygen for reducing harmful components in a filtrate

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a process for the treatment of a filtrate from a dewatering device located upstream of any bleaching stage in a mechanical pulping process, more particularly located in a water circulation or loop before any bleaching stage. The present invention further concerns the use of oxygen for reducing the amount of organic oxidizable material and metals in a filtrate stream from a dewatering device.

BACKGROUND OF THE INVENTION

In a pulp mill it is desirable for environmental reasons to minimize the discharge of washing filtrates. This can be done by "closing" the process of a pulping facility so that there is no or very little discharge of washing filtrates into the environment or any filtrates are effectively treated so that there is essentially zero pollution associated with the discharge thereof. The organic material in the filtrate increases the chemical oxygen demand (COD), which is a not wanted property of a wastewater. Therefore, the circulated water as well as any discharged water should have as low chemical oxygen demand as possible.

In mechanical pulping, water and wood is charged to a grinder or refiner and the ground material defibered in the grinder/refiner is then passed forward in the process. The stream containing water and pulp is passed through a dewatering device, in which the pulp is dewatered and passed for further treatment. The filtrate obtained in the dewatering device is collected in a tank and passed from the tank back to the grinder/refiner, in this process most of the material released from the wood feed stays in the circulating water (filtrate). However, some of this water passes away from the circulation with the pulp stream to further treatment. Therefore, there has to be also addition of clear water filtrate or the like water to the system. This has traditionally been considered to be acceptable, as the accumulation of impurities in the circulating water could be controlled in this way. By doing so, a substantial amount of organic material and metals have been passed to the bleaching stage with the pulp. The problem with the impurities in the water has been so far taken care of in the washing stages during bleaching.

Mechanical cellulose pulp is typically bleached in one or several successive bleaching steps by the supply of an oxidizing bleaching agent, such as hydrogen peroxide, or a reducing bleaching agent i.e. dnhionite, to the pulp. Liquid is removed from the pulp after each bleaching step in such a manner that a residue liquid or filtrate is obtained. Since this residue liquid still contains a certain amount of the bleaching agent, it is common practice during bleaching of mechanical pulp to convey the residue liquid back to a preceding washing step in a counter flow in order to utilize available peroxide or other oxidizing bleaching agents instead of conveying it directly to the effluent cleaning plant. In this manner the degree of utilization of the oxidizing bleaching agent may be improved.

The residue liquid obtained from the bleaching forme, after having been conveyed in a counter flow to the pulp flow, an effluent flow, which is typically conveyed to an effluent plant. The effluent flow contains oxygen consuming organic substances, so called COD (chemical oxygen demand), which must only be contained in a limited quantity of tons per day in the effluent liquid leaving a mill. Nowadays, it is possible to decompose such organic substances in the effluent plants available. Nevertheless, problems frequently occur if the amount of these substances suddenly for any reason is greater than calculated, since the existing effluent plant is dimensioned to manage a certain quantity thereof. Likewise, heavy investment expenses are involved when the requirements from the authorities are made more stringent and the maximal permissible level is decreased.

it is to be noted that it is generally considered to be a problem, in connection with the production of pulp, to utilize the residue liquid, or the filtrate, in the process, since it is considered to contain organic substances consuming bleaching agents and metal ions being harmful to the bleaching agent. If organic substances or the amount of harmful metal ions is high, the bleaching result by the use of such a liquid has been unsatisfactory. A recirculation of residue liquid in connection with pulp bleaching results in an increased need of peroxide or other oxidizing bleaching agents in the bleaching reactor for obtaining the same bleaching result.

A problem associated with the use of washing filtrates (or effluents) from a bleach plant in a closed or partly closed system is that impurities accumulate into the water circulation and harm the pulping process. As said above, the consumption of chemicals during the bleaching may be unreasonable high if there arc too many potentially harmful metallic ions. Therefore it is desirable to remove metals, such as Mn, Fe and Cu, to prevent buildup of those metals to a level adversely affecting the bleaching reactions. The bleaching chemicals also react with organic material in the liquid phase of the pulp rather than reacting with the pulp fibers, it is highly desirable to have the bleaching chemicals react only with fibers so that the consumption of chemicals is minimized. Therefore, it is also desirable to remove or at least reduce the amount of organic material in the liquid before bleaching. In mechanical pulping, the metal impurities are traditionally removed either by acidification of the pulp or by treating the pulp with chelating agents to release the metal ions from the pulp. The so treated pulp is then washed or dewatered to remove the freed ions.

There are many known chemical pulping processes where the filtrate is circulated in the bleaching stage. In these processes the streams from washers are treated physically and/or chemically in order to eliminate harmful components from said filtrates. However, these processes and their applications are only located in the bleaching zone, not before any bleaching stage. One such a process is disclosed in CA 2,170,553, wherein a kraft cellulose pulp is bleached in several steps and a residue liquid is separated from the pulp and returned to the pulp stream. Before being retumed, the residue liquid is brought into contact with an oxidation gas in an oxidation reactor. According to this document, the preferred oxidation gas is ozone.

WO 94/21857 discloses a method for removing metals from a filtrate obtained in a dewatering device in a kraft pulp stream and using the treated filtrate in the bleach plant of the pulp mill. This document discloses the supply of an oxidizing agent, for instance oxygen gas, and alkali to the filtrate in a reactor, in such a manner, the metals dissolved in the filtrate may be precipitated as solid particles and removed there from. This publication does not teach the removal of organic material from the filtrate.

EP 564443 discloses a method for the treatment of kraft cellulose pulp along a pulp stream comprising bleaching in several steps. A residue liquid is obtained after a bleaching step and it is returned to a prior washing step. The residue liquid obtained is heated and conveyed to a reactor to react with oxygen gas. Thereafter, the residue liquid Is conveyed to a heat exchanger and from there back to the pulp stream.

Furthermore, EP 950136 discloses a process, which is applicable in the bleaching of chemical as well as mechanical pulps. In connection with the pulp bleaching, a residue liquid is provided, which is less harmful from an environmental point of view and is more appropriate for being utilized during the treatment of cellulose fibres. The method comprises treatment of the residue liquid flow from a bleaching stage by oxygen by injecting oxygen into said flow.

The above disclosed methods are ail used in bleaching stages using controlled combinations of chemicals. Such methods are not suggested for the early stages of mechanical pulping processes i.e. after (he refiner or grinder and before any bleaching stage. At this early stage the process stream and the filtrates are very dirty. Also the temperature of the stream is typically high, about 90 to 95 °C, which makes the treatment of the filtrate liquids more difficult. Especially, the use of gaseous chemicals, such as oxygen, is not considered meaningful as the solubility of gases into liquids decreases when the temperature rises if the pressure is constant Therefore most of the effect of oxygen would be lost, because the oxygen can oxidize the contaminants in the liquid only if it is dissolved in it

EP0683830 discloses a method of producing chemi-mechanical pulp, wherein the properties of pulp produced in a pressurized grinding mill at elevated temperatures are improved by supplying oxygen via the spray water to the grinding chamber. This method comprises the steps of grinding wood in a grinding mill housed in a grinding chamber, maintaining the chamber ai an overpressure, spraying water onto the wood, and supplying oxygen and hydrogen peroxide via the spray water to the grinding chamber. This publication is focused on properties of the pulp and the does not relate to the problems associated with the filtrate, such COD content and dissolved metals.

The early stages of mechanical pulping produce a vast amount of COD and metals dissolved in water. A part of these impurities follow the pulp into the bleaching stages and are handled there, consuming bleaching chemicals. A part of the impurities are typically removed with the filtrate streams from the initial loops and have to be destroyed before the effluent is discharged from the plant. Thus, the problem with the known processes is that the amount of COD and dissolved metals are still too high in the filtrate streams in the early stage of a pulping process.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a method so as to alleviate the above disadvantages. One of the primary object of the present invention is to facilitate the effective removal of metals, which adversely affect the bleaching reactions, from filtrates in cellulose pulp mills, while at the same time producing a treated filtrate that can be used in the pulp mill. The objects of the invention are achieved by a process, which is characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

Accordingly the present invention provides as a first aspect a method for the treatment of an aqueous filtrate from a dewatering device located upstream of any bleaching stage in a mechanical pulping process, wherein at least part of said filtrate is treated with oxygen. The filtrate is returned to the pulping process or discharged from the plant. The oxygen is added to the fihrate in order to eliminate harmful components from said filtrate.

In a second aspect the invention provides the use of oxygen for reducing the amount of oxidizable organic material and metals in a filtrate from a dewatering device located upstream of any bleaching stage in a mechanical or recycled paper pulping process. The filtrate is returned to the pulping process or discharged.

The invention is based on the realization that a major part of the problematic/dissolved COD in a mechanical pulping process is in the filtrate from a dewatering device before any bleaching stage. The filtrate comprises an aqueous liquid separated from an aqueous pulp stream in a concentration (dewatering) step. Any components which are dissolved in the aqueous phase mainly follow the filtrate while most of the solids follow the pulp. However, the filtrate typically also contains a minor portion of the pulp fibres. The filtrate circulates in one or more loops at this stage of the process and huge amounts of impurities accumulate in the water. Thus, the overall COD of wastewaters of a mechanical pulp mill can be reduced significantly if even a part of the COD from this early stage can be eliminated. Treating a part only of the huge streams of filtrate enables applying optimum oxidation conditions as regards pressure, temperature, residence time etc., and to obtain a significant reduction of the impurities with a cleaning process, which might be considered industrially too complicated and/or too costly if applied to the whole of the filtrate stream.

It has been noted that the use of oxygen in this stage also precipitates metal ions in the filtrate. The oxidized (no longer harmful) metals can be filtrated from the filtrate stream or they can be conveyed back to or be precipitated in the pulp stream, wherein they attach to the pulp and are carried away from the water circulation with the pulp stream.

BRIEF DESCRIFΠON OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figure 1 is a block diagram showing the use of oxygen in the hot loop of a PGW process. Figure 2 is a block diagram showing the use of oxygen in a TMP process.

SUMMARY OF THE INVENTION

According to the first embodiment of the invention at least part of a filtrate from a dewatering device located upstream of any bleaching stage in a mechanical pulping process is treated with oxygen. The filtrate or a part of the filtrate is returned to the pulping process or is discharged from the process. Oxygen is added to the filtrate in order to eliminate harmful components from the filtrate. When the filtrate is returned to the pulping process, the oxygen is preferably added before the filtrate is returned to the process. It is also possible that all of the filtrate is treated with oxygen before being returned to the pulping process or before discharging. Preferably, at least a part of the oxygen treated filtrate is returned to the pulp stream at a position upstream of the dewatering device.

In one embodiment of the invention the oxygen is contained in an oxygen containing gas, such as air or substantially pure oxygen, such as industrially supplied under the term oxygen. Typically the harmful components mat are eliminated from the filtrate are selected from a group comprising metals and oxidizabte organic compounds. Preferably, the elimination of organic material reduces the amount of chemical oxygen demand (COD) of the filtrate. Typically the harmful metals include manganese (Mn), copper (Cu) and iron (Fe).

The oxygen may be supplied to the filtrate under very simple conditions, for instance by injecting oxygen gas to the transport conduit normally existing in a pulp plant for conveying the filtrate directly back to the pulp stream or to the grinder or refiner. Preferably, the gas is injected into the filtrate on the pressure side of a pump used to pump the filtrate. The oxygen should be supplied in very fine bubbles so as to be effectively dissolved in the filtrate. By such a treatment of the filtrate, possibilities are provided for changing or rendering harmless a significant percentage of the undesired oxygen consuming organic substances and of the undesired harmful metal ions, which are present in the filtrate when it leaves the dewatering device.

The oxygen may also be supplied to the filtrate after it has been relumed in the loop to the pulping process. The filtrate is at that stage mixed with the pulp stream and most metals and/or organic compounds, which are precipitated by the oxygen, will attach to the pulp fibres and will follow the pulp out of the initial loop or loops. This will reduce the amount of impurities in the circulating filtrate. However, in some cases the precipitated metals and organics may cause harm in the later stages of the pulping and/or papermaking process, such as in the bleach plant, Ii is therefore preferred to remove the harmful impurities from the filtrate when not in contact with the pulp.

The filtrate circulating in the present early stages of mechanical pulping processes typically has a high temperature and care must be taken to ensure a sufficient dissolution of the oxygen in the fluid and a sufficient contact time between the hot filtrate and the gas. The filtrate typically has a temperature between 50 and 95 °C depending on the mechanical pulping process in question. When being treated with oxygen, the filtrate preferably has a temperature between 60 and 160 X and more preferably between 70 and 120 °C. Most preferably, the oxidation is performed at a temperature between 95 and 1 IO °C. Since the invention has shown that a high temperature increases the reaction rate, the filtrate or the part of the filtrate, which is to be treated, is preferable at said elevated temperature or is heated to a desired elevated temperature.

In order to efficiently introduce the oxygen into the filtrate and into contact with the impurities in the filtrate, the oxidation is performed at super-atmospheric pressure. The filtrate may be pressurized before the addition of the oxygen, such as at the pressure side of a pump. If a reactor is used, the pressure may be provided by the oxygen containing gas itself. The pressure at the oxidation reaction should exceed 200 kPa (2 bar) and it is preferably between 500 and 1500 kPa (5 to 15 bar) more preferably between 700 and 1200 kPa (7 to 12 bar). A thorough mixing of the reaction mixture also improves the reaction and reduces the oxidation time.

Using oxygen in this manner and at this location in a mechanical pulping process has not previously been suggested. In fact, the art seems hardly to have realized that the filtrate could be cleaned. A skilled person would have considered that the amount of filtrate circulating in the early loops was far too huge to warrant the use of any sophisticated cleaning methods and he would have thought that oxygen should not be added to such a hot and dirty filirate where it would not have any meaningful effect. Therefore, the prior art has mainly reduced the COD content and the amount of metal ions in the filtrate streams by discharging a part of the filtrate as effluent and/or making up for lost filtrate by adding cleaner waters from the downstream part of the process. The actual cleaning processes have mostly taken place in the bleaching zone where the filtrates are clearer and temperatures are not that high.

In another embodiment of the invention at least pan of the filtrate stream is conveyed to a separate reactor for the oxygen treatment. Preferably the reactor comprises one or several pressure chambers, wherein the filtrate is mixed under pressure with oxygen. The oxygen is added to the filtrate in connection with the reactor. The oxygen can be injected to the filtrate stream just before it enters the reactor or it can be injected directly into the reactor. The reactor should have means for dissolving the gaseous oxygen into the filtrate. If the oxygen is added to the filtrate stream in the separate reactor and not just before it, then care should be taken that oxygen has time io dissolve into the filtrate in the reactor. In accordance with one embodiment of the invention oxygen or air is added under pressure to the filtrate (both the filtrate and the air/oxygen has an increased pressure) with a subsequent forceful mixing. Advantageously the mixing is carried out by means of a cyclone, it is an advantage of this embodiment of the invention that the oxygen quickly and efficiently is distributed in the filtrate.

The high pressure will cause more oxygen to be dissolved in the filtrate. The increased concentration of oxygen results in correspondingly faster reactions with the metal ions in the filtrate that therefore precipitate faster and provide a fast treatment of the filtrate. Also the reactions with organic substances are faster. Through the use of cyclones for the mixing of oxygen and filtrate, a very fast and extremely good atomizing of the oxygen bubbles in the filtrate take place. This provides a large contact area between oxygen and liquid and thus fast dissolving of the oxygen in the filtrate. The filtrate circulates evenly and uniformly in the cyclone, which makes it also possible to feed the oxygen to the passing filtrate constantly so that the oxygen distribution in the filtrate will be homogenous. In addition to the above phenomena one can conceive that the further increased pressures that arise in cyclones contribute to the dissolving ability and reaction rate. Oxygen (calculated as pure O₂) is preferably added in an amount of 1 to 40 kg O₂ per ADT (Air Dry Ton) pulp passing the dewatering device. More preferably the amount of oxygen is from 5 to 30 kg, most preferably from 8 to 20 kg, per ADT pulp passing the dewatering device. In the ideal situation all of the oxygen reacts with the organic compounds and/or metal ions. However, in practice this is not necessarily the case and some un-reacted oxygen passes out from the filtrate and is lost

In another embodiment of the invention the process comprises a further a step, wherein solids are removed e.g. by filtration from the liquid after the addition of oxygen and before returning it to the pulping process or discharging it If a separate reactor is used for dissolving oxygen into the filtrate, then it is preferred that this filtration step is after the reactor. The filtration is preferably used for removing oxidized metals from the filtrate stream. The precipitated oxidation products can advantageously be filtered out by means of cyclones since for instance metal oxides are considerably heavier than water.

In one embodiment of the invention also other chemicals than oxygen are added to the filtrate before returning said filtrate to the pulping process or discharging it Such other chemicals can be selected from flocculating agents, peroxides, and alkaline and acidic substances, the latter two being used for controlling pH of the filtrate. In an embodiment of the invention floccutation agents may be added to the filtrate in order to get some impurities to be precipitated as floccules. Dosage pumps may be arranged for the adding of chemicals, for instance flocculation agent that is distributed efficiently by the mixing cyclone.

In one embodiment of the invention, effluents from another part of the pulping process downstream of the dewaiering device are added to the filtrate. It should be noted the mention of the pulping process here is meant to include also a possible papeπnaking process in an integrated mill. Such effluents are typically used as make-up water for the grinding or refining and they may in their own way influence the circulating filtrate. According to the present invention the mechanical pulping process is preferably a process for producing a pulp selected from a group consisting of pressure groundwood (PGW), thermomechanical pulp (TMF), chemhhermomechanicai pulp (CTMP), stone groundwood (SGW) and refiner mechanical pulp (RMP). The use of the process in a PGW process is especially preferred.

Figure 1 illustrates one embodiment of the present invention, wherein filtrate in the hot loop of a PGW process is treated with oxygen. The wood chips are introduced at reference number 1 to the grinder/refiner 2, in which the fibres are mechanically liberated from each other. The fibre concentration is decreased by addition of filtrate from a shower water tank 5. The resulting suspension is dewatered in the thickener 3. The dewatered pulp is conveyed to the bleaching plant 8, whereas the filtrate from the dewatering is circulated back to the shower water tank S. The filtrate or a stream of the filtrate, going from the thickener 3 to the shower water tank S is treated with oxygen in a reactor 4 for oxidation of organic material and metal ions.

Some of lhe filtrate is discharged from the process as effluent 7 and a corresponding amount of make-up water is introduced from a clear water rank 6 belonging to the downstream pulping process.

. The filtrate stream, which is to be treated with oxygen is passed via a conduh to a cyclone of the reactor 4 and oxygen gas is added to the filtrate stream before H enters the cyclone. Oxygen is added in an amount of from S to 10 kg per ADT of pulp passing the thickener 3. The oxygen treated filtrate is then passed from the reactor 4 back to the conduit leading from the thickener 3 to shower water tank 5. Before the filtrate is rccirculated to the grinder/refiner 2 or is discharged at 7, it is preferably filtrated by means of a cyclone (not shown) in order to remove the precipitated material and especially the metals.

Figure 2 illustrates another embodiment of the present invention, wherein filtrate in the loop of a TMP process is treated with oxygen. The wood chips 1 enter a refiner 9 and the resulting liberated fibres mixed with water from clear water tank 11 are conveyed to a dewatering disc filter 10. The dewatered pulp is conveyed to the bleaching plant 8, while the filtrate from the filter 20 is directed to the clear water tank 1 i . Make-up water 12 from a downstream paper process is also introduced into the tank I i to make up for water, which has passed out of the loop with the pulp or is possibly discharged as effluent 7.

A reactor 4 with an oxygen cyclone is connected to the clear water tank 11 and a part of the filtrate is continuously drawn from the tank and treated with oxygen in the reactor as discussed above in connection with the PGW process. The oxygen treated filtrate is preferably filtered before being conveyed back to the tank 1 1. In accordance with the preferred embodiment of the invention, the oxygen treated filtrate or a significant part thereof ends up in the pulp stream going to the bleach plant or to another treatment stage of the mill, which may operate in any conventional manner known to those skilled in the art. The pulp is eventually processed into an end product, which typically comprises paper, board or dried pulp. Such processing may be performed according to conventional procedures.

As disclosed above, oxygen is used for reducing the amount of organic material and metals in an filtrate from a dewatering device located upstream of any bleaching stage in a pulping process, and the filtrate is returned to the pulping process or discharged.

EXAMPLES

Example 1

Laboratory trials were performed on filtrate from a PGW shower water tank of an industrial process. All filtrates used for the laboratory trails (Table 1) were taken at the same time from the shower water tank. The filtrate samples (250 ml/sample) were heated in autoclaves (i.S L) rotated in a polyglycol bath. Air or oxygen was added to the autoclaves with a pressure of 700 kPa (7 bars). The oxygen was 99 % pure oxygen commercially supplied by AGA AB. Sweden. Reaction time and temperature were varied according to Table I . Table I. influence of an oxygen containing gas on the COD content of a PGW filtrate

As can be seen in Table 1, it is possible to cause degradation of COD by introducing an oxygen containing gas into the filtrate. Table 1 also shows that when the temperature is increased from about 60 to about 100°C and pure oxygen gas is used, the reaction time is dramatically reduced.

Example 2

An experiment was conducted for ascertaining the influence of oxygen on metal ions in an aqueous liquid at an elevated temperature. Fe²⁺ ions were provided in water at the concentrations indicated in Table 2. The water was heated to 90 °C and oxygen was introduced, whereafter the concentration was again measured. Oxygen gas was supplied to the water lor 30 minutes in the same manner as in Example 1. Table 2, Fe ²⁺ -oxidation in water with addition of oxygen at 90 °C and a reaction time of 30 min.

As can be seen in Table 2, it is possible to eliminate metal ions from an aqueous liquid by the use of an oxygen containing gas. The metal ton concentration in the treated liquid was significantly reduced.

Claims

1. A process for the treatment of a filtrate from a dewatering device located upstream of any bleaching stage in a mechanical pulping process, characterized in that at least part of said filtrate Is treated with oxygen and is returned to the pulping process or discharged from the process, and wherein oxygen is added to the filtrate in order to eliminate harmful components from said filtrate.

2. A process according to claim I, wherein said oxygen is contained in an oxygen containing gas.

3. A process according to claim 2, wherein oxygen containing gas is air or oxygen gas.

4. A process according to claim I , wherein said filtrate when treated with oxygen has a temperature between 60 and 160 °C, preferably between 70 and 120 °C.

5. A process according to claim 1 , wherein said treatment with oxygen is performed at a pressure between 500 and 1500 kPa.

6. A process according to any of the claims, wherein the harmful components are selected from a group comprising metals and organic oxidizablc compounds.

7. A process according to claim 6, wherein the metals comprise Mn, Cu and Fe.

8. A process according to claim 6, wherein the elimination of organic compounds reduces the COD content of the filtrate.

9. A process according to claim 1 , wherein said oxygen is injected into said filtrate.

10. A process according to claim 1, wherein said oxygen is injected into said filtrate after it has been returned to the pulping process.

11. A process according to any of claims 1 to 8, wherein said al least part of the filtrate stream is conveyed in a separate reactor before being returned to the pulping process.

12. A process according to claim 111 wherein oxygen is added to the filtrate stream in connection with the separate reactor.

13. A process according to claim 11 or 12, wherein said reactor comprises one or several cyclones, wherein the filtrate is mixed under pressure with oxygen.

14. A process according to any of the preceding claims, wherein at least a part of the oxygen treated filtrate is returned to the pulping process upstream of said dewateriπg device.

15. A process according to any of the preceding claims, wherein all of said filtrate is treated with oxygen before being returned to the pulping process or before being discharged.

16. A process according to any of the preceding claims, wherein other chemicals than oxygen and/or other effluents from the downstream pulping process are added to the nitrate before returning said filtrate to the pulping process or discharging it.

17. A process according to any of the preceding claims, wherein the mechanical pulping process is for producing a pulp selected from a group comprising pressure groundwood (PGW), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), stone groundwood (SOW) and refiner mechanical pulp (RMP).

18. A process according to any of the preceding claims, wherein oxygen is added in an amount of 1 to 40 kg O₂ per air dry ton (ADT) of pulp passing the dewatering device.

19. A process according io claim 17, wherein oxygen is added in an amount of 5 to 30 kg O₂, per ADT of pulp passing the dewatering device.

20. A process according to any of the preceding claims, wherein solids are removed from the filtrate after the addition of oxygen and before returning it to the pulping process or discharging it.

21. A process according to any one of the preceding claims, wherein said oxygen treated filtrate or a part thereof is included in the pulp stream and the resulting pulp is processed into paper, board or pulp.

22. Use of oxygen for reducing the amount of oxidizable organic compounds and metals in a filtrate from a dewatering device located upstream of any bleaching stage in a mechanical pulping process, wherein said filtrate is returned to the pulping process or discharged.