This invention relates to a method of treating process water containing metal ions, in connection with the bleaching of pulp of lignocellulose-containing material.

In order to minimize the effect by various emissions, such as BOD, COD,colour etc. on the environment, there is a strong interest to substantially restrict the emissions by closing the wastewater system of the bleach plants. This implies, that the wastewater should be recycled to the process instead of being emitted. As a result of closing the system, however, the content of metal ions, such as manganese, copper, iron etc. in the system can increase to a level giving rise to problems at bleaching with hydroperoxide and, respectively, ozone.

The metal ions enter the system via the wood raw material and, therefore, the affluent of metal ions cannot be eliminated as would be desirable for process reasons. The content of these metal ions in the pulp, however, is very low and, therefore, they do not cause any environmental problem, not in the local working environment and not in the recipient, either.

In future, the bleaching of papermaking pulp will be based to a high degree on hydroperoxide bleaching and ozone bleaching. For achieving high ISO-brightness, this type of bleaching requires the unbleached pulp and process water to be free, or substantially free, of certain metal ions, because otherwise a considerable decomposition of charged hydroperoxide to water and oxygen and, respectively, of charged ozone to oxygen will take place, without any simultaneous bleaching effect .

In order to remove the metal ions prior to bleaching, the pulp must be pretreated with chemicals, for example chelating agents such as EDTA or DTPA , which selectively bind the undesired metal ions and thereby render it possible to wash them out prior to the bleaching. For process reasons, the resulting washing water must be eliminated from the system, and at the same time environmental reasons require that the washing water is not emitted.

The present invention offers a solution for the aforesaid problems. The invention, thus, has the object to remove these metal ions from the process water in the bleach plant and thereby render it possible to close the wastewater system of the bleach plant without causing problems at the bleaching.

The invention is described in greater detail in the following, with reference to the accompanying Figure 1 showing by way of a flow sheet an application of the method according to the invention.

Fig. 2 is a diagram showing the manganese content as a function of the reaction time at one embodiment.

Unbleached pulp is supplied through a duct 1 to a pretreatment step 2 where the pulp is treated with chelating agents of the type EDTA or DTPA. Metals such as manganese are here solved out of the fiber material. The pulp is directed from here to a first washing step 3, for example in the form of a washing press or washing filter where wastewater containing the critical metal ions is removed through a duct 4.The pulp free of critical metal ions is directed to a bleaching step 5 for being bleached with hydroperoxide or ozone.

After the bleaching step 5 the pulp is directed to a second washing step 6, from where the washed pulp is removed through a duct 7. The wastewater removed through the duct 4 is directed to a vessel 8 where the wastewater is brought into contact with an alkaline liquid, which preferably also contains sulphide, for example green liquor or white liquor from a sulphate pulp mill. In this vessel 8 the metal ions are precipitated mainly as compounds difficult to solve in the form of hydroxides and sulphides. These difficultly soluble compounds thereafter can be separated from the liquid phase in a suitable way, for example by sedimentation, filtering or the like. The wastewater free of metal ions thereby can be used as washing water in positions where a low content of metals is of importance, for example in a bleach plant entirely free of chlorine. The waste¬ water from the vessel 8 is removed through a duct 9 and can be used again as washing water in other process steps .

The precipitated metal compounds can be recycled through a duct 10 to the chemical recovery plant of the pulp mill, where they gradually will be ejected, for example into the green liquor slime. This eject passage exists already previously and, therefore, does not additionally harm the environment. As an alternative ^' method, the precipitated metal compounds are ejected directly out of the system without passing through the chemical recovery plant. The undesired metals, thus, are i-n both cases transported out of the bleach plant, and the accumulation effect in the bleach plant thereby can be avoided when the drain system is closed. As the method does not include an evaporation process, the energy balance of the mill is not deteriorated, either.

It was found that suitable conditions for achieving an effective elimination of the metal ions in the reaction vessel 8 shall be an alkaline pH, preferably higher than pH 11 at room or higher temperature .The reaction time can be very short, less than 1 minute. A longer reaction time, 1--60 minutes, however, has no detrimental effect on the precipitation reaction. The amount of charged hydroxide and sulphide shall exceed the stoichiometric demand in order to bind the undesired metals.

Example

50 ml wastewater from a complex former step was treated with 25 and, respectively, 50 ml green liquor from a pulp mill. Final pH was 12. -12.9.The result is shown in Fig. 1 where the manganese content is shown as a function of the reaction time; As appears from the diagram, the manganese content decreases rapidly, in less than 1 minute, at both green liquor charges from 1.4 mg/1 to below 0.2 mg/1. Thereafter an insignificant additional decrease of the manganese content at longer reaction times is observed.

The invention, of course, is not restricted to the embodiment described above, but can be varied within the scope of the invention idea.