PRESSURIZED PULP CHLORINATION, AND CHLORINE INTRODUCTION IN MINUTE BUBBLES

Abstract: A method and apparatus are provided for increasing the efficiency of treatment of a paper pulp slurry with chlorine gas, such as in the bleaching or delignification of deciduous wood chemical pulps, coniferous wood chemical pulps, and non-wood fibrous chemical pulps. Chlorine gas is intimately mixed with the pulp in a fluidizing mixer (14), the pulp preferably having a consistency of about 7-12%, and then is passed to a retention vessel (20). The discharge of pulp from the mixer to the retention vessel, and from the retention vessel to other treatment structures, is throttled by valves (30, 31) so as to increase the pressure in both the mixer and the retention vessel to above atmospheric, and preferably in the range of about 15-150 psig (1 to 10 bar above atmospheric pressure). The chlorine gas is added to the mixer through one more discs (53) of porous materials having pore sizes in the range of about 5-175 microns, such as porous ceramic or sintered metal discs. The increased pressure during the chlorine treatment, and the introduction of the chlorine gas in the form of minute bubbles, improve the solubility of the chlorine in the pulp slurry and thereby increase the efficiency of treatment. (Figure 1).

Description: BACKGROUND AND SUMMARY OF THE INVENTION In typical medium consistency chlorination facilities, such as in the bleaching or delignification of chemical pulps, chlorine is injected through nozzles into a mixer, and then is passed to a retention vessel which is at approximately atmospheric pressure. During the residence of the pulp in the mixer and the retention vessel the chlorine reacts with impurities and encrustants (such as lignin and bark) in the pulp slurry, to effect desired bleaching or delignification. According to the present invention, a method and apparatus are provided for significantly enhancing the efficiency and efficacy of the treatment of comminuted cellulosic fibrous material slurries with chlorine gas, particularly for the bleaching or delignification of chemical pulps. According to the present invention it has been found that if the pressure within the mixer and/or the retention vessel is increased, such as to the range of about 15-150 psig, the solubility of the chlorine gas increases. Also at increased pressure the chlorine gas has a tendency to take the form of minute bubbles thus increasing the overall reactivity area, and the network of interlocked fibers at this pressure inhibits the coalescence of gas bubbles. Also, according to the present invention, the chlorine gas is introduced into the mixer through porous material discs, such as porous ceramic discs or porous sintered metal discs having a pore size of about 5-175 microns, so that from the time of introduction the chlorine is in the form of minute bubbles. The improvements in solubility of the chlorine gas in the pulp slurry that are achieved according to the present invention result in a number of beneficial effects, including decreased overall chemical consumption, use of more chlorine relative to chlorine dioxide (which is also added during treatment, and which is more costly than chlorine gas), decreased volume of slurry in the retention vessel, treatment to lower Kappa numbers, less chemical wastage, and decreased consumption of bleaching agents in subsequent stages. According to the present invention it is preferred to intimately mix the chlorine gas with the pulp slurry in a fluidizing mixer, the pulp having a consistency of about 2.5-16%, and preferably about 7-12%, and then pass the slurry through a discharge line into a retention vessel. At the discharge line from the mixer a throttling valve is utilized to throttle the discharge and thereby increase the pressure within the mixer and maintain it at the range of about 15-150 psig (1 to 10 bar above atmospheric pressure). A similar throttling valve is placed at the discharge of the retention vessel to also maintain the pressure therewithin within the range of 15-150 psig (1 to 10 bar above atmospheric pressure). The chlorine gas when introduced into the mixer is introduced through porous material discs so as to be in the form of minute bubbles. It is the primary object of the present invention to provide for the more efficient treatment of comminuted cellulosic fibrous material slurries with chlorine gas. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic view of exemplary apparatus according to the present invention, for the bleaching of chemical pulp according to the present invention; FIGURE 2 is a detail end view of the front of the mixer of the apparatus of FIGURE 1, showing the interconnection thereof to chlorine gas and chlorine dioxide liquid sources; FIGURE 3 is a side cross-sectional view of the structure 44 of FIGURE 2; FIGURE 4 is a side cross-sectional view of an introduction nozzle 47 of FIGURE 2; and FIGURE 5 is an end view of the nozzle of FIGURE 4. DETAILED DESCRIPTION The invention is applicable to a wide variety of procedures for the treatment of comminuted cellulosic fibrous material slurries with chlorine gas. In particular the invention is useful as the first stage of a multistate bleach feed in the bleaching of deciduous wood chemical pulps, coniferous wood chemical pulps, mixtures of chemical pulps, and non-wood fibrous chemical pulps; as the first stage of a delignification procedure as a stage subsequent to an initial delignification using oxygen; and in like bleaching, delignification, and related procedures. For the purposes of description, the method according to the invention will be described with respect to the chlorine bleaching, along with chlorine dioxide, of chemical pulp, but it is to be understood that the invention has wider applicability. Chemical pulp is provided in a conventional bleach feed storage tank 10, the pulp being introduced therein through an inlet 11, and being pumped from the bottom of the tank 10 utilizing a conventional fluidizing centrifugal pump 12, such as the type shown in U.S. Patent 4,435,193. The pulp has a consistency of about 2.5-16%, preferable 3.5-13%, and most preferably about 7-12%. The temperature of the pulp slurry typically will be from 50°F-160°F (10 to 71°C), more preferably 80-150°F (27 to 66°C). The pulp is pumped in line 13 to conventional fluidizing centrifugal mixer 14. The mixer 14 preferably is capable of fluidizing pulp even if it has a consistency up to about 16%, to effectively intimately mix the pulp with chemicals that are added to it within the mixer. A form such a mixer 16 might take is shown in Canadian Patent 1102604, and such a mixer also is commercially available from Kamyr, Inc. of Glens Falls, New York and sold under the trademark "MC". Chlorine gas from source 15 passes through barometric loop 16' in line 16 and is added to the mixer 14, as is chlorine dioxide from source 17 through line 18. Typically the chlorine gas will have a temperature of about 75°F-140°F (24 to 60°C), and more commonly in the range of about 90°F-125°F (32 to 52°C). The pressure at source 15 and in line 16 typically will be in the range of about 60-125 psig (4,1 to 8,6 bar above atmospheric pressure) conventional safety devices, valves, and interlocks will be used with the chlorine gas system. Chlorine dioxide from source 17 will typically have a temperature of 35°F-100°F(1,6 to 38°C), more commonly 40°F-55°F (4,4 to 13°C). The chlorine dioxide will be added in liquid form, with concentrations ranging from 4-14 gpl, preferably 6-10 gpl. Total retention time in the pressurized zone (mixer 14 and/or vessel 20) is about 0.01-60 minutes, preferably about 0.1-20 minutes. The invention is particularly applicable to the bleaching of deciduous wood chemical pulps with Kappa numbers in the range of 10-25 (more commonly 15-20), bleaching of coniferous wood chemical pulps with Kappa numbers in the range of 25-150 (more commonly 28-38), bleaching of mixtures of chemical pulps, and bleaching of non-wood fibrous chemical pulps with Kappa numbers in the range of 14-150. From mixer 14 the pulp is passed to retention vessel 20 and flows upwardly therein with dilution liquid added through introduction structure 21. The pulp is discharged by a scraper into the discharge line 23 at the top of the retention vessel, and then passes into a stock line and from that preferably into a pressure diffuser or like vessel at the same pressure, for further retention or washing under pressure. According to the present invention it has been found that by pressurizing the mixure 14 and/or the vessel 20 the solubility of chlorine in the pulp is significantly increased thus causing more efficient reaction with the impurities in encrustants (such as lignin and bark) on, with, and in the pulp fibers. Also the application of pressure favors the production of minute bubbles of chlorine gas, and the network of interlocked fibers coupled with the increased pressure inhibits the coalescence of gas bubbles. The end result is decreased overall chemical consumption, decreased degree of substitution of chlorine dioxide for chlorine (chlorine dioxide being more expensive), a decreased volume of slurry in the vessel 20, lower extracted Kappa numbers for the pulp, less chemical wastage, and decreased consumption of bleaching agents in subsequent stages. Pressurization of the mixer 14 and vessel 20 is preferably effected utilizing a valve 30 at the discharge from the mixer 14, and a valve 31 at the discharge 23 from the tank 20. The valves 30, 31 may be selected from a wide variety of available throttling valves, such as valves having all wetted parts of materials resistant to the corrosive-aggressive conditions that are present. For instance the valve 30 may comprise a four inch (100 mm) diameter ball valve, and the valve 31 may comprise an eight inch (200 mm) diameter ball valve. Typical throttling ball valves that can be utilized in this regard are available from WKM of Houston, Texas, and are sold under the tradename DynaSeal 350. Of course a wide variety of other throttling valves that are commercially available may be utilized. The valves 30, 31 are operated, either manually or automatically, to control the pressure within the mixer 14 and/or the tank 20 so that it is within the range of about 15-150 psig (1 to 10 bar above atmospheric pressure) (preferably about 30-120 psig) (2 to 8,3 bar above atmospheric pressure). In order to ensure that the chlorine gas, from the time of its introduction, is in the form of minute bubbles, the structures illustrated in FIGURES 2 through 5 preferably are utilized. FIGURE 2 shows the chlorine source 15 connected through line 16 to the mixer 14. The particular interconnection structures illustrated in FIGURE 2 include the branched connection 40 which has branch conduits 41,42 extending therefrom. In each branch conduit is disposed a filter 44 and one or more valves 45, and each branch conduit terminates in a chlorine gas injecting nozzle 47. Two such structures are utilized in case one malfunctions (in which case it can be closed off and the other utilized until repair is effected), and also to evenly introduce the chlorine gas into the mixer. The injection nozzles 47 (any reasonable number of nozzles may be provided) in the embodiment illustrated in FIGURE 2 are connected to the mixer 14 at opposite sides of the circular cross-section pulp inlet portion 50 of the mixer. A typical filter 44 is illustrated in FIGURE 3, and includes the solid annular metal body 52 which mounts a circular disc 53 of porous material. Typical porous material for the disc 53 include porous ceramics and sintered metals. A typical chlorine gas injection nozzle 47 is illustrated in FIGURES 4 and 5 and includes a metal housing including annular flange 55 and annular body 56, with a conical surface 57 formed at the innermost end of the body 56 (the end closest to, or extending into, the mixer 14). Disposed within the body 56 and having a conical surface abutting the conical surface 57 is disc 58. The disc 58 comprises means for introducing the chlorine gas in the form of minute bubbles, and comprises a porous material having pore sizes ranging from about 5-175 microns. Typical porous materials utilizable for the formation of the disc 50 are conventionally available porous ceramics and porous sintered metals. The disc 58 is held in place against the conical surface 57 by the gasket 59, locking ring 60, and locking clip 61. The locking ring 60 can be screw-threaded into engagement with the body portion 56 (which has internal threads therein), with the clip 61 engaging the body 56 and the ring 60 to hold them in the position illustrated in FIGURE 4 wherein the ring 60 presses against the gasket 59 and the disc 58. It will thus be seen that according to the present invention a method and apparatus are provided for increasing the efficiency and efficacy of the treatment of comminuted cellulosic fibrous material slurries with chlorine gas. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to emcompass all equivalent methods and apparatus.

Claims: 1. A method of treating a slurry of comminuted fibrous cellulosic material with chlorine gas, comprising the steps of: (a) intimately mixing chlorine gas with the slurry; and (b) feeding the slurry to a retention vessel (20); and characterized by (c) maintaining the pressure in the retention vessel between about 15 psig -150 psig (1 to 10 bar above atmospheric pressure) to enhance the solubility of chlorine gas in the slurry and thereby enhance the efficiency and/or efficacy of the chlorine treatment. 2. A method as recited in claim 1 wherein step (a) is practiced in a mixer (14), and further characterized by the step of maintaining the pressure in the mixer between about 15 psig -150 psig (1 to 10 bar above atmospheric pressure). 3. A method as recited in claim 2 further characterized in that the slurry is retained at the pressure of between about 15-150 psig (1 to 10 bar above atmospheric pressure) for a time within the range of about 0.01-60 minutes. 4. A method as recited in claim 2 further characterized in that the consistency of the slurry is within the range of about 7-12% during practice of steps (a)-(c). 5. A method as recited in claim 1 further characterized in that the clorine is introduced into the mixture in the form of minute bubbles, so as to improve solubility in the slurry and thereby increase efficiency of treatment, by passing the chlorine gas through a porous disc (53) having pore sizes within the range of about 5-175 microns. 6. A method as recited in claim 5 further characterized in that the slurry has a consistency during the practice of steps (a)-(c) of between about 2.5-16% and is retained at a pressure of about 30-120 psig (2 to 8,3 bar above atmospheric pressure) during practice of steps (a)-(c). 7. A method as recited in claim 2 further characterized in that the pressure in the retention vessel and the mixer is retained by controlling the discharge of slurry from the retention vessel with a first throttling valve (30), and controlling the discharge of slurry from the mixer with a second throttling valve (31). 8. Apparatus for treating a slurry with chlorine gas comprising: a fluidizing mixer (14) for intimately mixing chlorine gas with a slurry, said mixer including at least one chlorine gas introduction nozzle (47), and a discharge line extending therefrom; and characterized by a first throttling valve (30) disposed in the discharge line for throttling the slurry discharged from the mixer to thereby control the pressure within the mixer; a retention vessel (20) operatively connected to said discharge line on the opposite side of said first throttling valve, from said mixer, said retention vessel including an inlet line connected to the mixer discharge line (23), and a retention vessel discharge line; and a second throttling valve (31) disposed in the retention vessel discharge line for controlling the pressure within the retention vessel. 9. Apparatus as recited in claim 8 further characterized in that said at least one chlorine gas introduction nozzle comprises means for introducing chlorine gas into the mixer in the form of minute bubbles, comprising a disc (53) of porous material disposed in said nozzle and through which the chlorine gas must pass into said mixer, said material having pore sizes in the range of about 5-175 microns. 10 Apparatus as recited in claim 9 further characterized in that said at least one chlorine gas introduction nozzle comprises a pair of chlorine gas introduction nozzles (47) disposed approximately opposite each other in operative association with said mixer, said nozzles connected through branch conduits (41, 42) to a common line (16) containing chlorine gas under pressure, and each of said chlorine gas introduction nozzles including a first porous material disc; and further comprising a second porous material disc disposed in each of said branch conduits acting as a filtering means (44) for filtering particles out of the stream of chlorine gas.

Inventors: PHILLIPS, JOSEPH R., ROWLANDSON, GORDON, SHERMAN, MICHAEL I.
Applicants: KAMYR, INC.
Application year: 1986
Main classification: D21C9
IPC codes: D21C9