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
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/16—Bleaching ; Apparatus therefor with per 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
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents

Definitions

• the invention relates to a process for the bleaching of a high yield pulp, wherein, before peroxide bleaching, the pulp is pretreated with a chelating agent in order to eliminate the adverse effects of any heavy metals present in the pulp.
• high yield pulps are meant above all mechanical pulps the bleaching of which differs from the bleaching of chemical pulps in that the aim is to avoid delignification.
• the yield of pulp is maintained at a high level.
• the aim in bleaching is to ren ⁇ der colorless the color-producing components (chromophoric groups) present in lignin.
• high yield pulps are considered as also including recycled fibers in the bleach ⁇ ing of which the aim is to remove printing ink, not lignin.
• the chelating carried out in connection with the bleaching of mechanical pulps differs from the pretreatment carried out in connection with the bleaching of chemical pulps. In the latter case the complexes formed in the chelating are removed by a wash. In the case of mechanical pulps, no wash is carried out in the pretreatment; instead, the pretreated pulp is compressed to the consistency used in bleaching, in which case only a small proportion of the metal complexes are removed from the pulp.
• Mechanical pulps can be divided into two principal categories, pure mechanical pulps and chemi-mechanical pulps. These pulps can further be divided into subcategories so that mechanical pulps include stone groundwood (SGW), pressure groundwood (PGW), refiner mechanical pulp (RMP) , thermomechanical pulp (TMP) and others such as TRMP and PRMP. Respectively, the chemi-mechanical pulps include low-sulfonated pulps (chemi- thermomechanical pulp CTMP amd BCTMP), chemically modified pulps (OPCO) and high-sulfonated pulps (CMP, UHYS). Mechanical pulps are used in the making of, for example, news ⁇ print, magazine paper and porous paper grades (tissue papers). Certain highly bleached chemi-mechanical pulps (BCTMP grades) are also used in bleached printing papers to replace chemical pulp.
• SGW stone groundwood
• PGW pressure groundwood
• RMP refiner mechanical pulp
• TMP thermomechanical pulp
• CMP high-sulfonated pulps
• Mechanical pulps
• Hydrogen peroxide is used for the bleaching of mechanical pulps and of recycled fiber which contains paper fiber made from mechanical pulps.
• the bleaching conditions must be rendered such that the hydro ⁇ gen peroxide will not dissociate.
• Heavy metals catalyze the dissociation of hydrogen peroxide and peroxy compounds.
• the ions the most detrimental in terms of bleaching are manganese (Mn), iron (Fe) and copper (Cu).
• other heavy metals such as chromium ions (Cr), etc., have a detrimental effect on the consumption of peroxy compounds.
• Detrimental heavy metals originate in the pulp, the treatment waters and the pulp-treatment apparatus.
• heavy metals are bound by using agents which chelate metal ions, for example poly- aminocarboxylic acids.
• agents which chelate metal ions for example poly- aminocarboxylic acids.
• EDTA ethylene diamine tetra-acetic acid and its salts
• DTPA diethylene triamine penta-acetic acid and its salts
• DTMPA diethylene triamine pentamethylenephosphonic acid
• the bleaching of mechanical pulps and recycled fiber is common ⁇ ly carried out with hydrogen peroxide at a high pH.
• Magnesium sulfate and waterglass are added during the bleaching step in order to stabilize the peroxide.
• Many hypotheses have been presented regarding the stabilizing effect of the above-men ⁇ tioned compounds.
• the stabilizing effect of waterglass is prob ⁇ ably based on waterglass surrounding the hydroxides, oxyhydrox- ides and oxides formed under alkaline conditions when iron precipitates, thus inactivating the catalytic surfaces of these solids.
• EDTA and DTMPA are regarded as non-biodegradable.
• DTPA is poor ⁇ ly biodegradable.
• the bleaching of recycled fibers can often also be carried out using reducing agents such as dithionite, and peroxide bleach ⁇ ing is not always necessary. Hydrogen peroxide is used more in the slushing of recycled pulp than in actual bleaching. For this reason the treatment of recycled fiber involves the same problems as regards the stability of hydrogen peroxide.
• Biodegradable co plexing agents have been developed for deter ⁇ gent builders. They must at the same time have softening action on water, i.e. they must bind calcium ions and magnesium ions.
• One such sequestering agent is ethylenediamine disuccinic acid (EDDS). This compound has three stereoisomers.
• EP patent application 556 782 discloses the use of EDDS as an iron complexer in photography chemicals.
• Example 9 of the pub ⁇ lication mentions that the ferric salts of EDTA, DTPA and HEDTA are not biodegradable.
• the Fe 2+ salt of EDDS is biodegradable. The biodegradabilities of the compounds were tested in the example by a generally approved testing method
• Another known biodegradable complexing agent is 2,2'-imino- disuccinic acid (ISA).
• ISA 2,2'-imino- disuccinic acid
• the use of this compound in alkaline detergents is disclosed in EP patent application 509 382.
• the patent application mentions the use of 2,2' -iminodisuccinic acid as a stabilizer of peroxide compounds, in particular in alkaline detergent compositions which contain hydrogen peroxide and its derivatives.
• only perborate is used. Since perborate releases hydrogen peroxide only slowly, no far-reaching conclusions can be drawn from the examples of the patent regarding the stabilization of hydrogen peroxide in similar detergent compositions.
• DE patent application 4 216 363 discloses the use of ISA as a stabilizer of tensides. There is no mention of the use of a peroxide compound.
• EP patent application 513 948 mentions the use of the substance in detergents which are intended for hard surfaces and contain an organic solvent boiling at approx. 90 °C. There is no mention of the use of a peroxide compound.
• DE patent application 4 340 043 discloses the use of ISA as a bleaching agent in the bleaching of groundwood.
• the purpose of ISA is the stabilization of hydrogen peroxide, and the examples show that at a pH of 10 it is a better stabilizer than DTPA.
• It is an object of the present invention is to eliminate the adverse effects of heavy metals in the bleaching of high yield pulps.
• the aim is to provide for use a biodegradable chelating agent which yields a good bleaching result.
• n 1-3
• m 0-3
• p 1-3
• R , R3 and R4 are H, Na, K, Ca or Mg
• R 5 and R 6 are H, CH 2 OH, CH 2 CH 2 OH or
• DTPA which is commonly used in the bleaching of pulp, chelates heavy metals best at a pH of approx. 5.
• the process according to the invention can be used in the pre ⁇ treatment of mechanical pulps, and possibly also in the actual bleaching following it.
• the process is suitable for the treat ⁇ ment of all mechanical pulps, such as stone groundwood, pres ⁇ sure groundwood, refiner mechanical pulp, thermomechanical pulp, chemi-mechanical refiner pulp and chemi-mechanical pulp.
• the process is also suitable for the bleaching of recycled fiber and textile fiber.
• the process can be used both in single-step and in two-step peroxide bleaching of mechanical pulp.
• the treatment is also suitable for mechanical pulp bleaching in which dithionite or formadine sulfinic acid bleaching is used instead of peroxide in some step.
• the compounds according to Formula I may be added already to the impregnation liquor, whereby the efficacy of the bleaching can be improved.
• the treatment may be performed on pulps obtained from various fiber raw materials, such as softwood or hardwood.
• the pH control of the acid chelating step can be carried out using conventional mineral acids, such as sulfuric acid, sulfur dioxide or an aqueous solution thereof, carbon dioxide, or organic acids such as formic acid and acetic acid.
• mineral acids such as sulfuric acid, sulfur dioxide or an aqueous solution thereof, carbon dioxide, or organic acids such as formic acid and acetic acid.
• an especially suit ⁇ able chelating agent according to Formula I is ethylenediamine- N,N'-disuccinic acid, its various isomers or its alkali metal salts, such as sodium and potassium salts, or its earth-alkali metal salts, such as calcium and magnesium salts. It is also possible to use ethylenediamine-N,N'-disuccinic acid together with calcium sulfate and/or magnesium sulfate.
• Another particu ⁇ larly suitable chelating agent is 2,2'-iminodisuccinic acid, its various isomers and its alkali metal salts, such as sodium and potassium salts, or its earth-alkali metal salts, such as calcium and magnesium salts. It is also possible to use 2,2'- iminodisuccinic acid together with calcium sulfate and/or mag ⁇ nesium sulfate.
• Usable chelating agents also include N-(l,2-dicarboxyethyl)-N- (2-hydroxyethyl) aspartamic acid, its various isomers and its alkali metal salts and earth-alkali metal salts.
• the acid may also be used together with potassium sulfate or magnesium sulfate.
• the chelating agent may be added in an amount of 0.1-5 kg, preferably 0.5-2 kg, per metric ton of dry pulp.
• EDDS and ISA can be used to ⁇ gether with hydroxycarboxylic acids without the bleaching result being worsened. It is advantageous to use EDDS or ISA together with chelating agents which do not contain nitrogen. These include hydroxycarboxylic acids having the general for ⁇ mula II
• n 1-8, m is 0-2n, p is 0-n, q is 0-2,
• R j is COOH
• R 2 is H, CH 2 OH or COOH.
• carboxyiic acids, hydroxycarboxylic acids, polyhydroxycarboxylic acids and hydroxypolycarboxylic acids according to Formula II such as citric acid, tartaric acid, lactic acid, pimelic acid, glutamic acid, glu ⁇ oheptonic acid, ascorbic acid, glycolic acid, glutaric acid, adipic acid, succinic acid or malonic acid, can be used as replacement che ⁇ lating agents.
• Citric acid has been used as a replacement for phosphates in phosphate-free detergents and cleansing agents, in which the substances are required to bind calcium and magnesium. Espe ⁇ cially the binding of magnesium should be disadvantageous in terms of bleaching.
• the best pH range for metal removal can be sought by means of washing experiments. With the help of washing experiments car ⁇ ried out on the same pulp at the same pH it is possible to compare the efficacies in the chelating of metal ions by dif ⁇ ferent chelating agents.
• Na 5 DTPA stands for the pentasodium salt of DTPA
• Na 4 EDTA stands for the tetrasodium salt of EDTA
• H 4 EDDS stands for the acid form of EDDS.
• the pH used will determine how the chelating agents are dissociated, i.e. in which form they actually appear in the treatment.
• the H ⁇ EDDS (reaction mixture) mentioned in the table refers to experiments in which the chelating agent used was an unpurified reaction product directly from the process for the preparation of EDDS.
• Table 2 shows the results of washing experiments similar to those described in Example 1, when EDDS was diluted with cer ⁇ tain hydroxy acids.
• DTPA is usually dosed into a softwood pulp at a rate of approx. 2 kg/tp.
• Table 2 the effect of the DTPA dose on the chelating of metals was first investigated. Chelating was clearly less when the dose of EDDS was reduced from a rate of 2.0 kg/tp to a rate of 1.0 kg/tp or 0.5 kg/tp. In the previous series of experiments (Table 1) it was observed that when Na ⁇ EDDS was used at a rate of 1.5 kg/tp, the chelating was as complete as when Na ⁇ DTPA was used at a rate of 2.0 kg/tp.
• thermomechanical pulp was treated with a chelating agent and was bleached with hydrogen peroxide in the conditions described in Table 3.
• the chelating agent was added in the pretreatment step at a rate of 1.5 kg/metric ton of pulp (kg/tp).
• each chelating agent was added at a rate of 1.5 kg/tp.
• a refiner mechanical pulp was treated with chelating agents and was bleached with hydrogen peroxide in the conditions described in Table 4.
• the chelating agent was added at a rate of 2.0 kg/metric ton of pulp in the pretreatment step. No chelating agents were used in the bleaching step.
• Refiner mechanical pulp was treated with chelating agents and was bleached with hydrogen peroxide in the conditions described in Table 5.
• the chelating agent was added at a rate of 2.0 kg/metric ton of pulp in the pretreatment step.
• each chelating agent was added at a rate of 2.0 kg/tp. The bleaching result is described relatively well by the residual peroxide content. Therefore the brightness of the pulp was not determined in all of the experi ⁇ ments.

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Citations (4)

• 1996
  • 1996-02-19 FI FI960757A patent/FI115642B/fi not_active IP Right Cessation
• 1997
  • 1997-02-19 EP EP97903409A patent/EP0882152B1/de not_active Revoked
  • 1997-02-19 AU AU17974/97A patent/AU1797497A/en not_active Abandoned
  • 1997-02-19 WO PCT/FI1997/000107 patent/WO1997030209A1/en active IP Right Grant
  • 1997-02-19 DE DE69731617T patent/DE69731617T2/de not_active Revoked

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