Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
  • D21C11/0078—Treatment of green or white liquors with other means or other compounds than gases, e.g. in order to separate solid compounds such as sodium chloride and carbonate from these liquors; Further treatment of these compounds
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
  • D21C11/0071—Treatment of green or white liquors with gases, e.g. with carbon dioxide for carbonation; Expulsion of gaseous compounds, e.g. hydrogen sulfide, from these liquors by this treatment (stripping); Optional separation of solid compounds formed in the liquors by this treatment

Definitions

• the present invention relates to a process for treating white liquor in pulp and paper mills. More particularly, the present invention relates to a process for oxidising white liquor. 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 producers 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 recirculate 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 material 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.
• attempts have been made to use as far as possible, 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 spent 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 undesirable 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 .
• US-A-4 053 352 discloses a process where white liquor is oxidised by air at an elevated temperature.
• US-A-5 382 322 discloses a process where white liquor is oxidised by a gas rich in oxygen which preferably contains at least 80% oxygen.
• WO 94/01616 discloses a process where white liquor is oxidised by oxygen.
• White liquor mainly contains a mixture of sodium sulphide, sodium hydroxide and water.
• sodium sulphide is oxidised to sodium thiosulphate.
• sodium thiosulphate is undesirable in a plurality of positions of application where alkalis are needed.
• white liquor containing sodium thiosulphate cannot be added to a bleaching stage where hydrogen 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 .
• An object of the present invention is to provide a process which results in an essentially total oxidation of white liquor (i.e. the sodium sulphide is oxidised via sodium thiosulphate to sodium sulphate) at reasonable investment and operational costs.
• the present invention relates to a process for treating white liquor in pulp and paper mills, the process comprising oxidation of white liquor.
• white liquor which mainly contains sodium sulphide, sodium hydroxide and water, is first contacted 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.
• the oxygen-containing gas comprises air.
• Example 1 White liquor, which mainly contained sodium sulphide, sodium hydroxide and water, with a temperature of 80-90°C and a pH of 12-13 was continuously pumped into an oxidation reactor at a flow rate of 14 p /h.
• the white liquor was oxidised by means of an incoming air current. The air was supplied at a flow rate of about 2000 m 3 /h.
• a sample was taken of the outgoing current from the oxidation reactor and the amount of sodium sulphide, sodium thiosulphate and sodium sulphate per litre white liquor was measured in the sample. The results are shown in Table 1 below.
• Example 2 Table 1 below.
• Oxidised white liquor according to Example 1 was continuously pumped further into a second oxidation reactor where the white liquor was mixed with an incoming current of hydrogen peroxide, 35 g of hydrogen peroxide being added per litre oxidised white liquor.
• a sample was taken of the outgoing current from the second oxidation reactor and the amount of sodium sulphide, sodium sulphite, sodium thiosulphate and sodium sulphate per litre white liquor was measured. The results are shown in Table 1 below.
• Example 4 The process according to Example 2 was carried out except that 47 g hydrogen peroxide was added per litre oxidised white liquor. The results are shown in Table 1 below.
• Example 4 The process according to Example 2 was carried out except that 47 g hydrogen peroxide was added per litre oxidised white liquor. The results are shown in Table 1 below.
• Example 3 of the examples above corresponds to a 100 % addition of hydrogen peroxide on the basis of the theoretically calculated amount of hydrogen peroxide, which is needed for a total oxidation of the white liquor.
• Example 5 The white liquors obtained according to Examples 1- 4, which have been oxidised to a larger or smaller extent, are used as a source of alkali when bleaching pa- permaking pulp. NaOH of PA quality is used as a reference (R) . The results of the bleaching by means of the differ- ent sources of alkali are shown in Table 2 below. Table 2 :
• the concentration of certain process-foreign matters also constitutes a problem when trying to make a closed pulp mill.
• White liquor also contains, apart from sodium sulphide, sodium hydroxide and water, ions of transition metals and/or alkaline earth metals, which each one or in combination with each other, can give rise to precipi- tates on the pulp.
• 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.
• magnesium sulphate and optionally polyacrylamide are added to the white liquor for precipitation of the ions of the transition metals and/or alkaline earth metals.
• suitable magnesium salts comprise all soluble salts of magnesium.
• the formed precipitate is separated by flotation, filtration or sedimenta- tion.
• 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.

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Citations (3)

• 1999
  • 1999-01-18 SE SE9900143A patent/SE513403C2/sv not_active IP Right Cessation
• 2000
  • 2000-01-04 WO PCT/SE2000/000007 patent/WO2000044978A1/en active Application Filing
  • 2000-01-04 AU AU23350/00A patent/AU2335000A/en not_active Abandoned
• 2001
  • 2001-07-17 FI FI20011551A patent/FI20011551A/sv not_active IP Right Cessation

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