FI67243C - KOKNINGS- OCH BLEKNINGSFOERFARANDE FOER CELLULOSA - Google Patents

Description

ΚΖ5 ^ Ϊ1 Γ »Ί« «KWULUTUSJULKAISU rn η Λ Ύ Μ α ^ ^ ^ utlAggningsskrift 6 7243 C Fa t? N 11 nySnnctty 11 02 1935 * · Γ 1¾ Patont cieddelat (51) c * .Va.3 D 21 C 11 / 00 FINLAND — FINLAND (M) - **, 773860 (22) HsInwNpiM — AF.Btailt ^ f 20.12.77 (23) AtoplM — GHUclMttdac 20.12.77 (41) TaftK | «HUMks <- BttvN affMOf 23.06.78

Pttntti · is registry administration ...________. . _ _ '(44) tWhtntfcUpnow μ lumUufcil — iy »m.- PUwt- och r ^ httfUyrthW' AmMum uthgd odk utLskrAan (wbNcsnrf 31.1Q.84 (32) (33) (31)« CMolkain — B «(ird prtortt * 22.12.76 USA (US) 753656 (71) KemaNord Ab, Box 11020, S-100 61 Stockholm, Sweden-Sweden (SE) (72) Wi Hard A. Fuller, Grand Island, New York, USA (US) ( (74) Berggren Oy Ab (54) Method for cooking and bleaching cellulose -

This invention relates to the production of bleached cellulosic pulp. More specifically, the present invention relates to a process for producing bleached fibrous cellulosic pulp, in which the chemical compounds used and formed in the various steps and parts are utilized as efficiently as possible with minimal losses and resulting contamination by recovering and reusing products previously waste from pulp bleaching processes.

In a conventional sulphate process, a raw cellulosic fibrous material, usually wood chips, is boiled by heating in a cooking liquor (white liquor) containing sodium sulphide and sodium hydroxide as active cooking chemicals. The soup produces pulp and spent cooking liquor (black liquor). The black liquor is separated from the pulp by washing in a brown pulp scrubber and the pulp is used as a feedstock in a bleaching plant for clarification and cleaning operations.

The black liquor is then concentrated, usually by evaporation, and the concentrated black liquor is burned in a reduction furnace to give a melt containing 2,6473 of sodium carbonate and sodium sulfide.

The melt is then dissolved in water to give a crude soda liquor which can be clarified to remove insoluble solids. The soda liquor is then causticized, usually with CaOr, to convert the sodium carbonate to sodium hydroxide. The resulting liquid is a white liquor and is useful in the first cooking step to provide at least a portion of the cooking liquor.

The above procedure or circulation process is well known and is referred to herein as boiler liquor circulation.

The functions of a bleaching plant usually involve a series of clarification and cleaning steps, which may be combined with washing steps. Clarification steps usually involve the use of bleaching agents such as chlorine or chlorine dioxide. The purification steps include washing and treatment with sodium hydroxide solution, base extraction.

A particular set of bleaching applications for use in one aspect of this invention comprises initial bleaching of the pulp with an aqueous solution containing chlorine dioxide and chlorine, intermediate washing, alkaline extraction using aqueous sodium hydroxide solution, further washing, bleaching with aqueous chlorine dioxide solution, second washing with chlorine dioxide solution and final wash. These are the so-called D EDED or D / C EDED sequences.

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A common source of chlorine dioxide for the bleaching operation is a chlorine dioxide generator which produces chlorine dioxide, usually as an aqueous solution of chlorine dioxide and chlorine, by reducing a chlorate salt, e.g. sodium chlorate. Such chlorine dioxide generators use a H 2 SO 4 feed which reacts with a mixture of sodium chlorate and some sodium chloride to form a chlorine dioxide bleach product and a sodium sulfate by-product (salt cake).

In the present invention, a boiler liquor cycle as described above is used sequentially with a sulfur cycle. The sulfur cycle uses a portion of the soda liquor from the boiler cycle and has in common the furnishing and melt dissolution steps of the boiler liquor cycle. The soda-liquor used in the sulfur cycle is saturated with carbon dioxide using flue gas to form three phases. One is the gas phase formed by substantially pure hydrogen sulfide. The other is the solid phase formed by sodium bicarbonate. The remaining phase is an aqueous liquid phase containing soluble impurities in the soda liquor, mainly chlorides. The hydrogen sulfide phase is converted to I 2 SO 4 by oxidation and the product is used as a feedstock to the chlorine dioxide generator. The solid phase formed by NaHCO 3 can be converted by causticization to give a relatively pure feedstock for the base extraction steps in a bleaching sequence, or it can be kilned to obtain additional melt. The aqueous chloride-rich phase is suitably used as a feed to a chlorine dioxide generator as a source of chlorides or may be fed to a salt extraction apparatus, in both cases to remove chlorides from the process.

The sodium sulfate by-product of the chlorine dioxide generator is co-fired with the evaporated or concentrated black liquor to form a co-melt and then a new soda liquor, which completes the sulfur cycle.

It is noted that the carbonation reactions of this invention proceed as follows; however, it is to be understood that the reactions are presented for clarity of understanding and are not intended to limit the invention, as the soda-liquor and fuel gas reagents are to be considered as highly impure products.

Na 2 S + H 2 O + CO 2 -> NaHS + NaHCO 3

Na2S + 2H2O + CO2-> 2NaHCO3i + H2S

Na2S + H2S -> · 2NaHS

Na 2 CO 3 + H 2 O + CO 2 -> 2 NaHCO 3 The present invention also contemplates a balanced paper mill process using successive boiler and sulfur circuits.

The present invention will be described in more detail with reference to the drawings and the invention.

67243

Figure 1 is a schematic flow diagram showing the typical operation of a cellulose mill using a cooking liquor cycle, a bleaching plant and a chlorine dioxide generator to produce bleaching solutions.

Fig. 2 is a schematic flow diagram showing the operation of a pulp mill using a boiler liquid circuit together with a sulfur circuit and a bleaching plant. In this form, a portion of the sodium bicarbonate produced is causticized and used in the bleaching process and the effluent from the carbonation tower is used as feed to the chlorine dioxide generator.

The chemical consumption and requirements in the following descriptions are given in kilograms required to produce one tonne of air-dried cellulose.

To now consider Figure 1, the basic cycle of boiler liquor is shown together with the chlorine dioxide generator. Thus, a fibrous cellulosic material, i.e. the wood chips are fed along a pipe 10 to a cooking pot 12, where the wood chips are cooked with cooking liquor fed by a pipe 14 and containing NaOH and Na 2 S as the main cooking chemicals. The miscellaneous losses in the boiler increase to about 11.1 kg of Na 2 O equivalent, to about 2.0 kg of sulfur and to about 1.0 kg of sodium hydroxide based on one ton of air-dried cellulose product from the plant. The cooking liquor fed through tube 14 contains about 130 kg of sodium sulfide, about 310 kg of sodium hydroxide, about 3.2 kg of sodium sulfate, about 32 kg of sodium carbonate, about 0.05 kg of sodium chloride and about 2590 kg of water.

The resulting pulp and spent cooking liquor are fed through line 15 and the pulp is washed and separated from the waste cooking liquor in a brown pulp washer 16. The washed but unbleached pulp is fed through line 20 to a bleaching plant 22 where the pulp undergoes a series of bleaching and purification operations. These operations may include additional bleaching or purification steps using base extraction to which aqueous sodium hydroxide solutions are fed through line 26. Generally, the pulp is washed during operation of the bleaching plant, typically after each bleaching or base extraction step, with water fed through the tube 28. The spent wash water from the bleaching plant washing operations 67243, together with the used chemicals from the bleaching and base extraction steps, form the bleach plant effluent in the tube 18.

The bleach plant effluent in line 18, which is a potential loss and contaminant, could be considered for addition to the spent cooking liquor in line 36; however, this procedure is not possible because significant amounts of chlorides would be added to the pulp cooking system, causing wear on the equipment, and because the chlorides do not react in this cycle, they accumulate in large amounts of unusable materials in the cycle. As shown in the remaining figures, the present invention alleviates the problem of accumulation of unusable chlorides in the cooking liquor circuit and further provides a sodium bicarbonate product that can be easily converted to sodium hydroxide and returned as a useful component to either the cooking liquor circuit or bleach.

The chlorine dioxide generating unit 24 uses I 2 SO 4, NaClO 2 and NaCl as feedstock and produces chlorine dioxide which is fed through line 30 to bleaching plant 22. Typically n.

11 kg of NaCl, about 18.5 kg of NaClO 2 and about 43 kg of H 2 SO 4 are used to produce about 11.1 kg of chlorine dioxide and about 6.89 kg of chlorine. About 25.5 kg of salt cake, Na 2 SO 4, is formed and fed through line 32 to furnace 34. Although furnace 34 is equipped with precipitator 36, there is a removal loss of about 1.32 kg of Na 2 O equivalent, 1.68 kg of sulfur and about 0.14 kg of kg NaCl: a.

The waste cooking liquor is fed via line 36 to evaporator 38 and evaporator materials are fed through line 40 to furnace 34. The melt produced by furnace 34 is fed to solvent device 42 to produce soda liquor, supplemented by adding about 52.6 kg of soda ash and re-causticized in re-causticizer 48. 46 to the boiler 12, which completes the circulation of the boiler liquor.

Referring now to Figure 2, this figure shows the rotation of the cooking liquor of Figure 1 in succession with the sulfur cycle. The amount of soda liquor containing Na2S containing about 43 kg of H 2 SO 4 (requirement for a chlorine dioxide generator 24 based on one 67243 ton of air-dried pulp mill product) is removed from the melt dissolution device 42 via line 50 and fed to a precarbonation vessel 52. via line 54 from bicarbonate tower 56. Precarbonated material is fed through line 58 to H2S stripping tower 60, to which additional bicarbonate solution or slurry is fed through line 62 and the removed solution is returned via line 64 to bicarbonate tower 56. Substantially pure H2S is fed in gaseous form via line 66 oxidant 68 to produce H 2 SO 4 or H 2 SO 4. The sulfuric acid thus prepared is fed through line 70 as a feedstock to the chlorine dioxide generator 24, which completes the sulfur cycle.

To now consider in more detail the bicarbonation tower 56, which is an integral part of the present invention, this tower enters (Χ> 2 ~ a source of flue gas through line 72, although it will be appreciated that CO 2 from any other source may be used. The carbonation reaction precipitates relatively pure NaHCO ^ product, which can be easily separated by decantation and causticization by treatment with CaOt to give a substantially pure NaOH product for use in bleaching plant 22 or to supplement or produce additional cooking liquor.A portion of the carbonated product is used in an H2S stripper 60 to release H2S. chlorides are used at least in part as a source of chlorides for the chlorine dioxide generator 24.

At this stage, it can be noted that the blast furnace effluent, in particular the extraction effluent, which contains large amounts of chlorides, can now be returned to the boiler liquor circuit, while the chlorides are removed in the sulfur cycle and used as feed to the chlorine dioxide generator. Thus, the extraction stream is returned from the bleaching plant 22 through line 19 to the brown pulp scrubber 16 and used as a washing supplement.

The flue gases fed to the bicarbonation tower 56 via line 72 from precipitator 36 contain about 15 weight percent CO 2. The incoming CO2 reacts with Na 2 S and NaCO 3 to produce NaHCO 3. Although the bicarbonation tower 56 can operate at pressures ranging from normal pressure to about 7 ata and temperatures between about 25-100 ° C, a more practical range is from normal pressure to about 2 ata and about 50-80 ° C. The main products of the Bikai® surface tower 56 are NaHCO 3 and the chloride feedstock to the chlorine dioxide generator 24. In the precarbonation vessel 52, the soda liquor from tube 50 is treated with a solution of NaHCO 3 containing some CO 2 from bicarbonate tower 56. The desired reaction is to convert Na2S to NaHS by releasing H2S. It is recommended that this treatment be performed in a closed vessel at normal pressure to minimize H2S formation and losses. The treated solution from the precarbonation vessel 52 is fed through line 58 to a stripping column 60 where the mixture is stripped, preferably using steam, to release H 2 S in a concentrated and substantially pure form. H2S is removed from the solution preferably to equilibrate! by vacuum stripping, to which a solution containing NaHCO 3 is fed through line 62 and a solution containing Na 2 O 3 is removed, which is returned to bicarbonation tower 56 via line 64. It is recommended to use steam as a stripping agent, because the recovery of concentrated H2S gas can be performed simply by condensing the steam from a mixture of steam and H2S.

The bicarbonation and stripping operations can be performed with any type of equipment conventionally used for gas absorption or stripping processes. Thus, packed columns, bottom columns, spray columns and continuous liquid phase columns are very suitable for use. The mixed gas dispersing equipment can be advantageously used in a bicarbonation tower.

The released H 2 S is fed through line 66 to a sulfuric acid plant 68 where H 2 S is oxidized to produce SO 2, which is dissolved in aqueous solution to give the H 2 SO 2 product.

2 4

The H 2 SO 4 produced is fed to the chlorine dioxide generator 24 as feedstock. The salt cake produced as a by-product from the chlorine dioxide generator 24 is fed through a tube 32 to an oven 34 where it is co-fired with the evaporated waste cooking liquor obtained from the evaporator 38.

In this form of the invention, excess NaHCO 3, about 94.3 kg, is fed to a re-causticizer 76 which converts Na 2 O 2.

To 43 kg of NaOH, which is suitably fed via line 78 to the bleaching plant 22 and used in the pulp bleaching process, although if required it can be used to supplement or produce cooking liquor.

8V? R f 67243

Due to losses at different stages of the operation of the sulphate pulp mill, more lye (NaOH) and in some cases more sulfur replenishment are required. This is done as much as possible using salted cake, but due to its sulfur content, salted cake cannot usually be fully utilized and more lye needs to be obtained. It should be noted that the process of Figure 2 produces excess lye from the salt cake without adding excessive amounts of sulfur, which is added to the cooking pot liquor.

It is to be understood that many modifications may be made to this invention, for example, different bleaching arrangements will produce effluents of varying composition, or other papermaking operations having chemical requirements or by-products may be taken care of by altering the process in question.

Economic or environmental factors can also be taken care of by modifications, for example, in one mode of operation of the present invention, the bleaching plant 22 uses a DCEDED pulp bleaching sequence. When this order is used, the entire effluent from the bleaching plant can be returned to the brown pulp scrubber 16, thus utilizing and eliminating all effluent from the bleaching plant process. If desired, the salt crystallizer can be installed in connection with the bicarbonation tower 56 and recover sodium chloride in substantially pure form.

The following table provides comparable chemical requirements and credit residues for the process of this invention when operating under the conditions of Figure 2 (column A) and the total recycle stream of the bleaching plant (column B) compared to a plant operating at normal boiler liquor circulation (column C). Claims and credits are given in kg of chemicals needed to produce one tonne of air-dried cellulose.

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table 1

Requirements A B C

H2SO4 - - 17.6

NaClO3 43.9 43.9 18.5

NaCl - - 10.9

Na 2 CO 3 42.7 52.6 18.5

NaOH - - 43.0

Na 2 SO 4 21.2 19.9

Refunds

Na 2 SO 4 - - 2.9

NaCl - 17.7

Claims (12)

1. An integrated process for boiling and bleaching cellulose as well as for the recovery of sulfur, characterized in that it comprises in combination the following steps: a) a fibrous cellulose material is boiled; in a furnace to form a melt and flue gas, d) the melt is dissolved to form green liquor and a portion of the green liquor is causticized for use in step a); solution for preparing a precarbonized solution containing NaHS; a NaHCO 3 solution and a chloride stream, and a portion of the NaHCO 3 solution is used for entry into step e) and another portion for entry into step f), h) at least a portion of the hydrogen sulfide is oxidized to i) at least a portion of the obtained H in step c). Process according to claim 1, characterized in that the flue gas coming from step c) is used as CO 2 source in step g). Process according to claim 1, characterized in that at least part of the chloride flow obtained in step g) is utilized in the chlorine dioxide generator in step i). Process according to claim 1, characterized in that the fibrous cellulose material is wood.