SE435304B - Utilization of coke liquor preparation prepared from the sulfur whites in the preparation of chlorine dioxide for bleaching - Google Patents

Description

the wires? The green liquor is then causticized, usually by treatment with CaO to transfer sodium carbonate to sodium hydroxide. The resulting liquid is white liquor and is useful in the first boiling step to form at least a portion of the boiling liquid.

The sequence or cycle described above is well known and is hereinafter referred to as a cooking liquor cycle. Bleaching operations usually involve a series of bleaching and purification steps, which may be combined with washing steps.

The bleaching 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, lute extraction.

A particular bleaching sequence used in one embodiment of the invention comprises a first bleaching of the pulp with an aqueous solution containing chlorine dioxide and chlorine, an intermediate wash, a lye extraction using aqueous sodium hydroxide solution, a further wash, a bleaching with an aqueous solution of chlorine dioxide, another wash, a further lute extraction using aqueous sodium hydroxide, a further wash, a final bleaching with chlorine dioxide solution and a final wash. These constitute the so-called DCEDED or D / C EDED sequences.

A common source of chlorine dioxide for bleaching work is a chlorine dioxide generating plant, which forms chlorine dioxide, usually an aqueous solution of chlorine dioxide and chlorine, by reduction of a chlorate salt, eg sodium chlorate. Such chlorine dioxide generating plants utilize a feedstock of H 2 SO 4, which is reacted with a. mixture of sodium chlorate and any sodium fl orid to produce a bleaching product of chlorine dioxide and a by-product of sodium sulphate: (carpenter).

In the present invention, the boiling liquid cycle is used, as described above, in combination with a sulfur cycle. The sulfur cycle uses part of the green liquor from the boiling cycle and has a common burning and melt dissolution step with the boiling liquid cycle. The green liquor used in the sulfur cycle is carbonated, using flue gas, forming three phases. One is a gasase of substantially pure hydrogen sulfide. One consists of a solid phase of sodium bicarbonate. The remaining phase is an aqueous liquid phase containing soluble impurities from the green liquor, mainly chlorides. The hydrogen sulfide phase is converted to H 2 SO 4 by oxidation and the product is used as a feedstock in a chlorine dioxide generating device. The solid phase of NaHCO 3 can be transferred by causticification to produce relatively pure feedstock for the lye extraction steps in a bleaching sequence or can be burned to produce additional soda melt. The aqueous phase, which has a high chloride content, is suitably used as a feed material in a chlorine dioxide generating device and is a source of chlorides or can be fed to a salt extracting device, whereby chlorides are removed from the operation in both cases.

The sodium sulphate by-product from the chlorine dioxide generating device is burned together with evaporated or concentrated black liquor to form a common soda melt and then further green liquor, which ends the sulfur cycle.

It is probable that the carbonation reactions according to the present invention take place as follows; however, it is understood that the reactions are shown only to increase understanding and are not limiting of the invention, as it must be taken into account that the green liquor and flue gas reactants are extremely impure products Na S + H 2 20 + coz _) Nans + Nancoål 'Na fi s + znzo + The present invention also relates to a balanced pulp mill work, in which the cooking liquid cycle and the sulfur cycle are used in combination. ../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../ ..

The present invention is further described with reference to and discussion of the accompanying drawings. In this case, Fig. 1 shows a schematic flow diagram illustrating a typical operation of a pulp mill, in which a boiling liquid cycle, a bleaching plant and a chlorine dioxide generating device for the preparation of the bleaching solutions are used. Fig. 2 shows a schematic flow diagram illustrating the operation of a pulp mill, in which a boiling liquid cycle is used in combination with a sulfur cycle and a bleaching plant.

In this embodiment, a portion of the sodium bicarbonate produced is causticized and used in the bleaching work, and the effluent from the carbonation tower is used as a feed material to the chlorine dioxide generating device. 7714so1l7 The chemical consumption quantities and quantities required in the following description are given in kg required for the production of 1 tonne of air-dried pulp.

Fig. 1 shows the basic cooking liquid cycle in combination with a chlorine dioxide generating device. Thus, cellulosic fibrous material, such as chips, is fed through line 10 into the digester 12, wherein the chips are boiled with a cooking liquid, which is supplied through line 14 and which contains NaOH and Na 2 S as the main cooking chemicals. Various losses in the boiler amount to about 11.1 kg Na 2 O equivalents, about 2.04 kg sulfur and about 0.27 kg sodium hydroxide, calculated on 1 ton of air-dried pulp produced from the plant. Cooking liquid supplied through line lä contains about 129.7 kg of sodium sulphide, about 310.3 kg of sodium hydroxide, about 3.2 kg of sodium sulfate, about 31.8 kg of sodium carbonate, about 0.45 kg of sodium chloride and about 2,586 kg of water.

The obtained pulp and liquor are fed through line 15 and the pulp is washed and separated from the liquor in the brown mill laundry 16. The washed mixed bleach is fed through line 20 to the bleaching plant 22, wherein the pulp is subjected to a series of bleaching and purification processes, comprising in at least one step use of chlorine dioxide. These processes may include additional bleaching or purification steps, using a lute extraction, feeding aqueous solutions of sodium hydroxide through line 26. In general, the pulp is washed while operating the bleaching plant, typically after each bleaching or lye extraction step by water supply through line 28. The used wash water from the bleaching plant washing step together with the chemicals used from the bleaching and lye extraction steps, forms the bleaching plant effluent in line 18. - The bleaching plant effluent in line 18, a potential loss and contamination, can be considered to be trapped in line 36; however, this process is not possible because significant amounts of chlorides would be added to the pulp cooking system, which causes erosion of the equipment, and since the chlorides do not react in this cycle, they accumulate in large amounts as unusable material in the cycle. As shown in the remaining figures, the present invention avoids this problem of collecting unusable chlorides in the boiling liquid cycle and according to the invention further produces a sodium bicarbonate product which can be easily converted to sodium hydroxide and recycled as a useful component to either the boiling liquid cycle or to the bleaching plant. A chlorine dioxide generating device 24 uses H 2 SO 4, NaClO 3 and NaCl as feedstock and forms chlorine dioxide, which is supplied through line 30 to the bleaching plant 22. Typically, about 10.9 kg of NaCl, about 18.46 kg of NaClO 3 and approx. 43.1 kg HZSO4 for the production of approx. 11.11 kg chlorine dioxide and approx. 6.89 kg chlorine. About 25.54 kg of salt cake, Na2SO4. is formed and fed through line 32 to furnace 34. Although furnace 34 is equipped with dust separator 36, a smoke loss of about 1.315 kg Na 2 O equivalents, 1,678 kg sulfur and about 0.14 kg NaCl is obtained.

The effluent is fed through line 36 to the evaporator 38 and evaporated materials are fed through line 40 to the furnace 34. The soda melt produced by the furnace 34 is fed to a melt solvent 42 for the production of green liquor, which is pre-causticated by the addition of calcined soda, in an amount of about 52.6 kg, and is re-causticated in the re-caustic 48.

The re-caustic solution is fed through line 46 to the digester 12, terminating the boiling liquid cycle.

Fig. 2 shows the boiling liquid cycle from Fig. 1 in combination with a sulfur cycle. An amount of green liquor containing Na 2 S corresponding to about 42.6 kg of H 2 SO 4 (the need for the chlorine dioxide generating plant 24 based on 1 ton of air-dried pulp product) is taken from the melt dissolver 42 through line 50 and fed to the pre-carbonation vessel 52. The pre-carbonation vessel receives CO 2 gas feedstock 54 from the bicarbonate tower 56. The precarbonated material is fed through line 58 to the H28 evaporation tower 60, where additional bicarbonate solution or slurry is fed through line 62 and depleted solution is returned through line 64 to the bicarbonate tower 56. substantially pure H28 in gaseous form is fed through line 66 to the oxidation plant 68 for the production of HZSO4 or HZSO3.

Sulfuric acid produced in this way is fed through line 70 as feed material to the chlorine dioxide generating plant 24, whereby the sulfur cycle ends.

In the following, the bicarbonate tower 56, which forms an integral part of the present invention, is discussed in more detail. This tower receives as a CO2 source flue gas through line 72, although it is understood that CO2 from any source can be used. The carbonation reaction precipitates a relatively pure NaHCO 3 product, which can be easily separated by elutriation and re-causticized by treatment with CaO to give a substantially pure NaOH product for use in the bleaching plant 22 or for pre-caustic ("butt") or for preparation of additional cooking liquid. Some of the carbonated product was used in the H28 stripper 60 to release H28. 1v14so1-7 The mother liquor from the elutriation, mainly chlorides, was used at least in part as a source of chlorides for the chlorine dioxide generating plant 216. It should be pointed out that the bleaching plant effluent, especially extraction effluents containing large amounts of chlorides, can now be recycled to boiling water because the chlorides are removed in the sulfur cycle and used as feed material for the chlorine dioxide generating plant. Thus, the extraction effluent from the bleaching plant 22 is returned through the line 19 to the brown malt laundromat 16 and is used as a washing supplement.

The flue gases fed to the bicarbonation tower 56 through line 72 from the dust trap 36 contain about 15% by weight of CO2. The CO 2 gas reacted with Na 2 S and NaCO 3 to produce NaHCO 3. Although the bicarbonation tower 56 can be operated at pressures from atmospheric pressure to about 0.69 MPa gauge and at a temperature of from about 25 to about 100 ° C, a more practical range is atmospheric pressure to a gauge pressure of about 0.21 MPa and about 50-80 ° C. The main products from the bicarbonation tower 56 are NaHCO 3 and a chloride feed material for the chlorine dioxide generating device 2ü. In the pre-carbonation vessel 52, green liquor from line 50 is treated with a solution of NaHCO 3 containing a certain amount of CO 2 'from the bicarbonation tower 56. The desired reaction is the transfer of Na 2 S to NaHS without releasing H 28. This treatment is preferably performed in a closed vessel at atmospheric pressure to minimize the formation and loss of H25. The treated solution from the pre-carbonation vessel 52 is fed through line 56 to the evaporation column 60, where the mixture is evaporated, preferably using steam to release H23 in concentrated and substantially pure form. H23 is preferably removed from the solution by expansion evaporation or vacuum evaporation, in which solution containing NaHCO 3 is fed through line 62 and depleted solution containing Na 2 CO 3 is returned to the bicarbonation tower 56 through line 64. is easily performed by condensing the steam from the mixture of steam and H28.

The bicarbonate and stripping operations can be performed in any type of equipment conventionally used for gas absorption or for stripping operations. Thus, packed columns, columns with bottoms, spray columns and columns with a continuous liquid phase can be suitably used. Equipment with stirred gas dispersion can be advantageously used in the bicarbonation network. D 221714601-1 Released H28 is fed through line 66 to the sulfuric acid plant 68, where H28 is oxidized to produce SO3, which is dissolved in an H2 O solution to form an HZSOM product.

Produced HZSOH is fed to the carbon dioxide generating device 24 as feedstock. The salt cake formed as a by-product in the chlorine dioxide generating device 2N is fed through line 32 to the furnace BN, where it is burned together with evaporated effluent from the evaporator 38.

In this embodiment of the invention, excess NaHCO 5 is fed to about 90.7 kg, to a re-causticizer 76, which transfers Na 2 CO 3 to about 44.1 kg of NaOH, which is conveniently fed through line 78 to bleaching plant 22 and used in the pulp bleaching operation, although it if required can be used for pre-causticization or for the production of cooking liquid.

Due to losses in the various steps in the pulp production, additional caustic soda (NaOH) and in some cases additional sulfur are required as a supplement. This is done as much as possible by using a salt cake, but due to its sulfur content, the cake can usually not be fully utilized and additional sodium hydroxide must be acquired. It should be noted that in the process of Fig. 2, sodium hydroxide was produced in excess from brine without passing the addition of excessive amounts of sulfur to the cooking liquid.

It is understood that many modifications may be made to the present invention, for example, different bleaching sequences result in varying compositions or other papermaking processes which may have chemical requirements or by-products which may be met by modifying the present process.

Economic or environmental factors can also be met by modifications, for example in an embodiment of the present invention a mass bleaching sequence DCEDED is used in the bleaching plant 22.

When this sequence is used, the entire effluent from the bleaching plant can be returned to the brown malt laundry 16, whereby all effluent from the bleaching plant is thus used and eliminated. If desired, a salt crystallizer can be installed in conjunction with the bicarbonate tower 56 and sodium chloride in substantially pure form is recovered.

The following table compares the chemically required amounts and amounts recovered for the present process when operating under the conditions of Fig. 2 (column A) and during a total reversal of the bleaching plant effluent (column B) in comparison with a plant operated under normal boiling liquid cycle (column C). Required quantities and quantities recovered are expressed in kg of chemicals required for the production of one tonne of air-dried pulp.

Table 1 Required Amounts ABCH¿sou - - 17, 6 NJ 103 143.9 143.9 13.5 NaCl - - 10.9 NaZCOB 142.7 52.6 18.5 NaOI-I »- - 143, 1 Amounts Recovered NaaSün - - 2.9 NaCl ~ 17.7 '

Claims (1)

A process for the boiling and bleaching of cellulosic pulp recovery of sulfur, in which a fibrous cellulosic material is boiled, the fibrous pulp and the liquor produced by boiling. the step is separated, the liquor is evaporated and burned in an oven to form a soda melt and a flue gas, the melt is dissolved to form a green liquor and a part of the green liquor is causticized for use in the cooking step a), characterized by treating a part of the green liquor with NaHCO 3 containing CO 2 in a closed vessel to prepare a precarbonated solution containing NaHS, evaporation of the precarbonated solution to form hydrogen sulfide and a solution of Na 2 CO 3, use of the Na 2 CO 3 solution from f) and CO 2 to prepare a solution of NaHCO 3 and a chloride stream and using a part of the NaHCO 3 solution as feedstock ie) and a further part of the NaHCO 3 solution as feedstock if), oxidizing at least one d electricity of the hydrogen sulphide to form H 2 SO 4 and using at least a portion of the formed H 2 SO 4 as feedstock to a chlorine dioxide generating plant to react with an alkali metal chlorate and an alkali metal chloride to produce C102 and an alkali metal sulphate. A method according to claim 1, characterized in that the fibrous cellulosic material consists of wood. Process according to Claim 1, characterized in that the alkali metal is sodium. Process according to claim 1, characterized in that flue gas from the furnace in c) is used in e) for the supply of CO 2 for the treatment of the green liquor for the production of NaHCO 3. 5. A process according to claim 1, characterized in that the C102 product from h) is used for bleaching pulp.