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
  • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
  • D21C11/0028—Effluents derived from the washing or bleaching plants
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills

Definitions

• Wood is composed of two main partsa fibrous carbohydrates or cellulosic portion and a non-fibrous portion comprising phenolic-type compounds, this latter portion being commonly referred to as lignin.
• the weak black liquor (at approximately 15-20% solids) is treated in a series of evaporators, commonly referred to as a multiple effect evaporator, and is discharged to a direct contact evaporator where further water is evaporated until the solids content is approximately 65-70%.
• the evaporator liquor is then screened and passed through a recovery unit where (a) make-up chemical or salt cake (sodium sulfate) is added; (b) the remaining water is removed, (c) sodium salts are recovered mostly as sodium carbonate and sodium sulfide; and ((1) heat is obtained by burning the organic compounds contained in the liquor.
• the molten ash, or smelt, from the recovery unit which consists of sodium carbonate and sodium sulfide, is dissolved in water to form green liquor.
• the green color is dueto iron impurities which, along with silica and other suspended solids, are removed by treatment .with
• the clarification step is followed by a causticizing step in which the green liquor is treated with lime (calcium oxide) to convert the sodium carbonate to sodium hydroxide, the sodium sulfide remaining unchanged.
• lime calcium oxide
• the sludge from the lime treatment is removed, and the resulting solution is referred to as white liquor which can be reused as cooking liquor in the digester.
• the sludge is washed in a lime mud washer and the filtrate used to dissolve additional smelt from the recovery unit.
• the lime mud is treated in a kiln to yield reburned lime. During the burning operation some calcium oxide and sulfur compounds are collected in the kiln stack and removed by a scrubber in said stack.
• Unbleached kraft, or sulfate, pu'lps are generally bleached to an acceptable brightness by a multistage process employing chlorine or chlorine-containing compounds such as calcium or sodium hypochlorite and chlorine dioxide.
• chlorine and chlorine-containing compounds have proven to be effective bleaching agents, these compounds are difiicult to handle, introduce the problem of corrosion to the paper-making equipment, and render the efiluents from the bleach plant incapable of being recovered and recycled.
• waste liquors and wash water from the bleach plant incorporating such compounds can cause a serious pollution problem.
• Pre-treatments of the wood pulp with acids such 'as sulfuric acid and sulfur dioxide prior tothe oxygen bleaching step have also been suggested as a means for decreasing the degradation of the wood pulp which might otherwise occur during the bleaching operation.
• acids such 'as sulfuric acid and sulfur dioxide prior tothe oxygen bleaching step
• these acid pre-treatments were always followed by a washing step prior to the bleaching operation since it appeared that to introduce acid into the bleaching opera tion would result in a decrease in the efliciency of said operation due to a neutralization of some of the alkali required for bleaching.
• kraft pulp is bleached in an improved oxygen-based bleaching process wherein the improvements comprise (a) employing untreated white liquor from a kraft recovery system as a source of alkali in the oxygen-based bleaching process and (b) recycling the eflluent from the bleaching process directly to the kraft recovery system.
• the present invention involves the combination of a kraft recovery system and an oxygenbased bleaching process.
• Pulp to be treated in accordance with the present invention is prepared in a conventional kraft pulping operation as is well-known to those skilled in the art.
• the effluent from this pulping operation is then introduced into a kraft recovery system, such as that described above.
• the resulting pulp is finally bleached to a high brightness by an oxygen-based bleaching process as is also well-known to those skilled in the art.
• What is done in accordance with the present invention is to (la) employ untreated white liquor from the kraft recovery system as a source of alkali in the oxygen-based bleaching process and (b) to introduce the efiluent from the oxygen-based bleaching process directly into the kraft recovery system without any pre-treatment of said effluent.
• untreated white liquor from the kraft recovery system can be employed as a source of alkali in the oxygen bleaching process without having any detrimental eifect on the brightness or other properties of the resulting pulps.
• the white liquor contains, in addition to the desired sodium hydroxide, a significant amount of sodium sulfide which is known to be a strong reducing agent. It had been expected that the introduction of a reducing agent into the bleaching process would decrease the efficienc of the oxidation reaction occurring therein. However, this has not been found to be the case. To the contrary, it has been found that white liquor having a sulfidity of from about 20% to about 40% may be employed in the bleaching operation without, in 'any way, degrading the properties of the resulting pulp.
• a conventional oxygen-based bleaching process is employed.
• this process utilizing pulp at a consistency of from about 20% to about 40%, an oxygen pressure of from about 80 to about 150 p.s.i., and 'an elevated temperature of from about 80 C. to about 160 C. fora time of from about 20 to about 90 minutes.
• Optimum results have been achieved when the bleaching operation is carried out at an alkaline pH of from about 8 to 13.
• the required alkaline pH may be achieved by treating the pulp with the white liquor obtained from a kraft recovery system without any pre-treatment of the white liquor to remove the sulfur contained therein.
• the sulfidity of white liquor is generally in the range of from about 20 to about 25%, it has been found that satisfactory results can be achieved when a white liquor having a sulfidity of up to about 40%, such as those commonly encountered in commercial operations,
• the amount of White liquor employed in treating the pulp prior to its treatment with oxygen in the bleaching process can be varied over a wide range depending upon the type of pulp employed and the properties desired in the final pulp. For a conventional hardwood pulp satisfactory results have been achieved when the amount of white liquor employed is sufiicient to add from about 1% to about 8% active alkali based on the dry weight of the pulp. To produce a pulp useful in the preparation of sheets having optimum physical prop erties, it is especially preferred to employ an amount of white liquor such that from about 1% to about 4% active alkali is "added to the pulp.
• the oxygen-based bleaching process may be utilized either alone or in combination with other treatments conventionally employed in the bleaching of wood pulp. It has also been found, in accordance with the present invention, that efiiuent from the above-described, oxygenbased bleaching operation can be introduced directly into the kraft recovery system. Thus, no pre-treatment is required to remove excess alkali from this efiluent prior to its recycling. It is especially preferred to introduce the effluent into the recovery system by utilizing it in the lime kiln scrubber or brown stock washer so that the solids contained therein are removed, incinerated and causticized in the normal operation of that system.
• One advantage of the present process is that it results in a minimum amount of fresh water being required and in a significant reduction in the stream and air pollution associated with the previously employed pulping and bleaching operations.
• Handsheets were prepared by the procedure described in TAPPI Standard T-28 lm for optical tests and in accordance with TAPPI Standard T-205m for physical tests.
• Brightness of the sheets was measured on a General 'Electric photometer in accordance with TAPPI Standard T-217m and is expressed in terms of GE. units.
• Tensile strength of the sheets was measured in accord ance with TAPPI Standard T-220m60 and is expressed in terms of meters.
• the tear value of the handsheets was also measured in accordance with TAPPI Standard T-220 using a junior Elmendorf tear tester.
• the viscosity of the pulp was measured in accordance with TAPPI Standard T-230 and is reported in terms of centipoise.
• EXAMPLE I Poplar hardwood chips were reduced to a pulp by the kraft process. Handsheets prepared from a sample of the pulp had a brightness of 36.0.
• the fibers were then treated with white liquor from a kraft recovery porcess so that 4.5% by weight, based on the weight of the fiber, of the white liquor was added.
• the treated sample was placed in a Parr bomb which was charged with oxygen until the pressure rose to approximately p.s.i. The Parr bomb was maintained at this pressure and heated to approximately 100 C. for a period of about 30 minutes.
• the pulp was then removed from the bomb and washed thoroughly with water and its pH adjusted to about 4.0
• the effluent from the bleaching step is collected and introduced directly into the chemical recovery system of the kraft pulping process.
• a second sample of the pulp was treated as above except that the white liquor was replaced by an equivalent amount of an aqueous solution of sodium hydroxide.
• Handsheets prepared from pulp bleached in this manner had a brightness of about 52.3.

Landscapes

• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=25766824

Family Applications (1)

Country Status (3)

Cited By (4)

• 1972
  • 1972-04-24 US US00247119A patent/US3798119A/en not_active Expired - Lifetime
• 1974
  • 1974-03-15 DE DE19742413123 patent/DE2413123B2/de not_active Withdrawn
  • 1974-06-26 BE BE1006046A patent/BE816863Q/xx active

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