Because of the increasing interest in environmental issues, there is a great wish to decrease the emission of pollutants from human activities. The pulp and paper pro- ducers are considered to cause environmental problems.

However, in the last few years considerable resources have been spent on decreasing the emission from pulp and paper mills and much progress has also been made.

The major aim has been to provide a closed pulp mill, that is a pulp mill which minimises emissions by, as far as possible, regenerating chemicals that are available in the process and reusing the spent liquors which have formed. One part of this is to try to recircu- late spent liquors countercurrently to the pulp in the process. A problem which arises in this context is the risk that certain process-foreign matters, such as ions of transition metals and/or alkaline earth metals which are added to the process by, for example, the raw mate- rial of wood, are concentrated in the system when the spent liquors are recirculated.

Alkaline solutions may be applied in many positions in pulp and paper mills. In order to obtain a process which is as non-expensive and environment-friendly as possible, attempts have been made to use as far as possi- ble, the alkaline spent liquors which are generated in the pulp and paper mills, instead of adding new sodium hydroxide in these positions of application. One example of such a position for using white liquor is in the bleaching stage.

White liquor is an important source of alkali in a large number of the process stages in a pulp mill. How-

ever, white liquor contains sodium sulphide which is un- desirable in most applications where alkalis are needed.

A more and more common way of making the white liquor useful for these applications is to oxidise the white liquor by air or oxygen in order to remove most of the sodium sulphide by transforming it to oxidised sulphur compounds.

White liquor contains, apart from mainly sodium sul- phide, sodium hydroxide and water, also a large number of ions of transition metals and/or alkaline earth metals, which each one, or in combination with each other, can give rise to precipitates on the pulp.

An increasing part of the papermaking pulp is nowa- days bleached by means of hydrogen peroxide, peracetic acid or ozone. These bleaching processes are disturbed in different ways by ions of transition metals and/or alka- line earth metals. Thus, it is necessary for the amount of ions of transition metals and/or alkaline earth metals to be minimised in the spent liquors which are re-used in different positions of application of the process.

From EP 0 726 834 it is known to reduce the amount of complexed ions of manganese, copper or iron in liquid which is obtained when producing pulp containing ligno- cellulose, and more particularly white water, by adding a compound containing alkaline earth metals to form a pre- cipitate which contains said metals.

US 4 053 352 discloses a process for producing oxi- dised white liquor, in which the sodium sulphide of the white liquor is oxidised to sodium thiosulphate. The oxi- dised white liquor can be treated by adding a magnesium, calcium or zinc compound for precipitating foreign, dis- solved substances.

An object of the present invention is to provide a process for minimising the amount of ions of transition metals and/or alkaline earth metals in white liquor.

Other objects, features and advantages of the pres- ent invention will be evident from the following descrip- tion.

The objects are achieved by a process according to claim 1. Especially preferred embodiments are defined in the dependent claims.

Briefly, the present invention relates to a process for treating white liquor, which apart from mainly sodium sulphide, sodium hydroxide and water, also contains ions of transition metals and/or alkaline earth metals, in pulp and paper mills. According to the invention, the white liquor is treated on the one hand in a stage a), in which the white liquor is oxidised by first being con- tacted with an oxygen-containing gas for oxidising sodium sulphide to sodium thiosulphate, after which the white liquor is contacted with hydrogen peroxide for oxidising sodium thiosulphate to sodium sulphate, and, on the other hand, in a stage b), in which a magnesium salt is added to the white liquor and the formed precipitate is sepa- rated from the white liquor, the stages a) and b) being carried out in an optional order.

An advantage of the present invention is that the amount of ions of transition metals and/or alkaline earth metals is minimised in the regenerated source of alkali.

Thus, the risks of concentrating these metals are de- creased in the pulp process and pure sodium hydroxide added in the bleaching stage can be replaced by oxidised white liquor, which has been treated according to the present invention.

By the expression ions of transition metals and/or alkaline earth metals is meant ions of metals selected from the group consisting of Al, Ca, Fe, Mn, Ni, Zn, etc.

In a preferred embodiment of the present invention, also a polymer, preferably polyacrylamide, is added to the white liquor in order to, if necessary, facilitate the precipitation of the metal compounds.

In another preferred embodiment, magnesium sulphate is added to the white liquor to precipitate ions of the transition metals and/or alkaline earth metals. Other ex- amples of suitable magnesium salts comprise all soluble salts of magnesium.

In a further preferred embodiment, the formed pre- cipitate is separated by flotation, filtration or sedi- mentation.

In yet another preferred embodiment, the separated precipitate is added to the process water of a bleaching stage in the separation of precipitated, flocculated metal compounds, the separation being made by flotation.

In the following, further preferred embodiments of the present invention will be described by means of a few Examples. They must not be considered to limit the scope of the present invention.

The unit ppm which is used in the Examples below is based on weight, i. e. 1 g/tonne = 1 ppm.

The need for white liquor which is purified of met- als, totally oxidised or not oxidised depends on the size of the mill and what bleaching stages are used in the bleach plant. A so-called normal-sized pulp mill may use about 5-10 m3 white liquor per hour in the bleach plant.

Example 1 White liquor with a temperature of about 70°C from a causticising stage was pumped through a pipe to a set- tling cone. In the pipe before the settling cone, 200 ppm MgSO4 (calculated as Mg2+) was added. Directly after the point of addition of the magnesium salt, a static mixer was installed to perform satisfactory addition of the magnesium salt. Directly before the settling cone, 10 ppm polyacrylamide was subsequently added to accelerate the sedimentation of the formed metal floccules. The formed metal floccules were settled in the settling cone and the purified, clear white liquor was pumped to a storage tank to be used later in the bleach plant or in another posi-

tion in the process where needed. The results of the metal purification are shown in Table 1 below.

Example 2 The experiment was carried out as described in Exam- ple 1 except that 400 ppm MgSO4 (calculated as Mg2+) and 15 ppm polyacrylamide were added to the white liquor in- stead of 200 ppm MgSO4 and 10 ppm polyacrylamide, respec- tively. The results of the metal purification are shown in Table 1 below.

Table 1 Metals White White Reduc-White Reduc- (mg/1) liquor liquor tion (%) liquor tion (%) (in) (out) Ex. 1 (out) Ex. 2 Ex. 1 Ex. 2 Al 3. 3 2 39. 4 1. 2 63.6 Ca 13 8. 1 37. 7 5. 3 59.2 <BR> <BR> <BR> Cu <0. 1 <0. 1 0. 0 <0. 1 0.0<BR> <BR> <BR> Fe 3 0. 9 70. 0 0. 3 90.0 Mn 3. 2 0. 4 87. 5 0. 1 96.9 <BR> <BR> Ni 0. 5 0. 4 20. 0 0. 4 20.0<BR> <BR> <BR> Zn 0. 7 0. 5 28. 6 0. 5 28.6 Example 3 The experiment was carried out as described in Exam- ple 1 except that it was based on oxidised white liquor.

The results of metal purification are shown in Table 2 below.

Example 4 The experiment was carried out as described in Exam- ple 1 except that 400 pmm MgSO4 (calculated as Mg2+) and 15 ppm polyacrylamide were added to the oxidised white liquor. The results of the metal purification are shown in Table 2 below.

Table 2 Metals Ox white Ox white Reduc-Ox white Reduc- (mg/1) liquor liquor tion (%) liquor tion (%) (in) (out) Ex. 3 (out) Ex. 4 Ex. 3 Ex. 4 Al 3. 8 2. 1 44. 7 2. 1 44.7 Ca 15 10 33. 3 7. 3 51.3 <BR> <BR> <BR> Cu <0. 1 <0. 1 0. 0 <0. 1 0.0<BR> <BR> <BR> Fe 2. 6 1 61. 5 0. 5 80.8 Mn 4. 9 0. 7 85. 7 0. 1 98.0 <BR> <BR> Ni 0. 5 0. 4 20. 0 0. 4 20.0<BR> <BR> <BR> Zn 0. 5 0. 5 0. 0 0. 5 0.0

As is evident from Table 1 and Table 2 above, a con- siderable reduction of the amount of remaining ions of transition metals and/or alkaline earth metals was ob- tained by the process according to the present invention.

In the process according to the present invention, addition is made of at least 20 ppm, preferably at least 200 ppm, and at the most 1000 ppm, preferably at the most 600 ppm MgSO4 (calculated as Mg2+). Furthermore, 0-25 ppm, preferably 10-15 ppm, polymer is added.

As mentioned above, the white liquor which contains sodium sulphide is subjected to a stage a) involving oxi- dation either before or after the stage b) involving ad- dition of a magnesium salt.

According to prior art, the white liquor is oxidised by air or oxygen in order to remove most of the sodium sulphide by converting it to oxidised sulphur compounds.

When white liquor is contacted with air, sodium sul- phide is oxidised to sodium thiosulphate. However, sodium thiosulphate is also undesirable in a plurality of posi- tions of application where alkalis are needed.

For instance, white liquor containing sodium thio- sulphate cannot be added to a bleaching stage where hy- drogen peroxide is used as a bleaching chemical. If this is the case, the hydrogen peroxide first reacts with the

sodium thiosulphate and forms radicals which in various ways have a detrimental effect on the quality of the pa- permaking pulp.

The use of oxygen for oxidising white liquor is not preferred since the costs which arise on the one hand when an existing plant is rebuilt and, on the other hand, when in operation, are considerable and, thus, oxidation by oxygen, is not economically defensible.

According to the present invention, oxidation is in- stead carried out by the white liquor first being con- tacted with an oxygen-containing gas for oxidising sodium sulphide to sodium thiosulphate, after which the white liquor is contacted with hydrogen peroxide for oxidising sodium thiosulphate to sodium sulphate. According to a preferred embodiment the oxygen-containing gas is air.