Abstract:
The specification discloses an apparatus and method for treating a slurry containing sodium sesquisulfate to recover sulfate and acid constituents therefrom. The apparatus includes a treatment vessel having a separation wall delineating a clarifying zone and a mixing zone. Slurry containing sesquisulfate crystals is introduced into the mixing zone along with water and the material is mixed to promote dissolution of the crystals and formation of sodium sulfate solids. Sodium sulfate solids are collected in a lower portion of the treatment vessel and conveyed out of the vessel, and liquid from the clarifying zone is conducted from an upper end of the treatment vessel to a conventional liquid processing unit. Treatment of a sesquisulfate-containing slurry in accordance with the invention provides sodium sulfate containing little or no sesquisulfate crystals thereby reducing the need for vacuum filtration or other expensive separation techniques to recover sulfuric acid and sodium sulfate solids from the slurry.

Description:
This application is a divisional of application Ser. No. 09/036,820 filed Mar. 9, 1998 now U.S. Pat. No. 6,126,702. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the treatment of sesquisulfate waste streams to recover acid and sulfate values therefrom. More particularly, the invention relates to an apparatus and method for treating a sodium sesquisulfate (Na 3 H(SO 4 ) 2 )-containing waste stream from a chlorine dioxide production process to recover a sulfuric acid (H 2 SO 4 ) solution and sodium sulfate solids (Na 2 SO 4 ). 
     BACKGROUND OF THE INVENTION 
     Bleaching or whitening of pulp is typically accomplished by chemically altering and/or removing colored matter in the pulp and imparting a higher brightness thereto. Chlorine-based chemicals such as chlorine, chlorine dioxide and hypochlorite have been used in pulp bleaching for many years and continue to be used for removing lignin and bleaching the pulp to high brightness. Growing environmental concerns have led to an increase in the use of chlorine dioxide and nonelemental chlorine agents such as oxygen, peroxide and/or ozone in bleaching processes. 
     Waste streams containing sodium sesquisulfate result from the production of chlorine dioxide for use in bleaching pulp. The increasing use of chlorine dioxide in pulp mills thus makes recovery of the acid and sulfate values of such waste streams of considerable economic importance since the components recoverable from the waste streams may be recycled for a variety of uses. 
     One process for recovering sodium sulfate and sulfuric acid from a stream containing sodium sesquisulfate is described in U.S. Pat. No. 5,116,595 and is commonly referred to as the “R10” process. The &#39;595 patent is specifically directed to treatment of a waste stream obtained from a process for generating chlorine dioxide for use in the pulp mill commonly known in the art as the “R8” process, and described in U.S. Pat. No. 4,081,520. 
     In accordance with the method described in the &#39;595 patent, a slurry containing sulfuric acid and sodium sesquisulfate is filtered, contacted with water, and thereafter filtered again to separate the resulting sodium sulfate precipitate from the sulfuric acid solution. One significant disadvantage of recovery processes of this type is that they require filtration techniques, typically vacuum filtration techniques, and the concomitant equipment and operational costs associated with these filtration steps. However, such filtration techniques have been considered vital to achieving sulfate solids of satisfactory yield. 
     Accordingly, it is an object of the present invention to provide a method and apparatus for treating a sesquisulfate-containing stream to recover valuable constituents thereof. 
     It is an additional object of the invention to provide an apparatus and method for treating a sesquisulfate stream to recover acid and sulfate values of the stream. 
     Another object of the invention is to provide an apparatus and method for treating a sodium sesquisulfate-containing waste stream to recover a sulfuric acid solution and a sodium sulfate solids. 
     A further object of the invention is to provide an apparatus and method of the character described which avoids or limits the need for mechanical filtration such as vacuum filtration. 
     Still another object of the invention is to provide an apparatus and method of the character described which is uncomplicated and economical as compared to conventional methods and which does not compromise the quality or quantity of the yield. 
     SUMMARY OF THE INVENTION 
     With regard to the foregoing and other objects, the invention provides an apparatus for treating a slurry containing sodium sesquisulfate crystals to recover sulfate and acid constituents therefrom. In general, the apparatus includes an upright treatment vessel having an upper end and a lower end and a separation wall defining on one side a mixing zone and on another side a clarifying zone. The separation wall includes a lower edge spaced above the lower end of the vessel to enable fluid flow communication between the mixing zone and the clarifying zone by flow of liquid under the lower edge between the zones. 
     A collector is located within the treatment vessel generally below the mixing zone and includes an upwardly opening reservoir for collecting solid particles descending by gravity from the mixing zone. A diffuser in the reservoir of the collector is connected in flow communication with a source of pressurized gas for releasing a flow of gas bubbles through solid particles collected in the reservoir to agitate particles contained therein, and to provide a flow of gas bubbles up through the mixing zone to promote a turbulent mixing flow regime within the mixing zone for dissolution of sodium sesquisulfate crystals and formation of sodium sulfate solids. 
     An inlet conduit is connected in flow communication with the mixing zone for delivering the slurry into the mixing zone. A water inlet is provided for introducing water into the mixing zone for mixing with the slurry. 
     An outlet conduit adjacent the upper end of the treatment vessel is provided for directing liquid from the clarifying zone to a conventional liquid processing unit. A solids outlet is provided adjacent the lower end of the treatment vessel for directing sodium sulfate solids out of the vessel. 
     In a preferred embodiment, the separation wall is provided by an upright elongate cylindrical conduit generally centrally located within the treatment vessel. The mixing zone is defined within the conduit as an elongate upright cylindrical columnar space and the clarifying zone is defined outside of the conduit as an elongate upright annular space between the conduit and the inner wall of the vessel. The lower edge of the wall is defined as the lower continuous edge of a generally conical, downwardly opening flow expansion member on the conduit. 
     The invention also provides a method for treating a slurry containing sodium sesquisulfate crystals to recover a sodium sulfate solids therefrom. In accordance with the method, the slurry is conducted through an elongate generally upright mixing zone in a treatment vessel and is mixed therein to promote dissolution of the crystals and formation of sodium sulfate solids. The slurry is then conducted to a clarifying zone. 
     Sesquisulfate crystals descending from the mixing zone are collected in the reservoir of a collector located beneath the mixing zone. Gas is bubbled through the crystals in the reservoir to promote dissolution thereof and to provide a flow of gas bubbles ascending through the mixing zone to promote a turbulent flow regime therein and consequent mixing of the slurry. Sodium sulfate solids descending from the slurry in the clarifying zone are collected and conducted out of the vessel along with entrained liquid consisting principally of sulfuric acid and water. The balance of the liquid from the clarifying zone is conducted out of the vessel along with a small amount of entrained solids consisting principally of sodium sulfate. 
     Treatment of a sesquisulfate-containing stream in accordance with the invention yields an effluent containing sulfuric acid, water and a small amount of dissolved sodium sulfate and a sodium sulfate solids product with entrained sulfuric acid containing little or no sesquisulfate crystals, limiting the need for vacuum filtration and other expensive, complicated separation techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and aspects of the present invention will become further known from the following detailed description considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a preferred embodiment of an apparatus for treating sodium sesquisulfate in accordance with the present invention; 
     FIG. 2 is a perspective view of a preferred embodiment of a mixing conduit and collector for use in practicing the invention; and 
     FIG. 3 is a perspective view of another embodiment of a mixing conduit and collector for use in practicing the invention. 
    
    
     DETAILED DESCRIPTION 
     With initial reference to FIG. 1, there is shown a preferred embodiment of a separator  10  in accordance with the invention for continuously treating waste streams containing sodium sesquisulfate crystals (Na 3 H(SO 4 ) 2 ) to recover acid and sulfate constituents thereof, namely, a sulfuric acid (H 2 SO 4 ) solution and sodium sulfate solids (Na 2 SO 4 ) 
     In general, a waste stream containing sesquisulfate crystals is introduced into the separator  10  and treated to yield a sulfuric acid solution effluent L and sodium sulfate solids S. The effluent L will generally contain water, sulfuric acid and a small amount of sodium sulfate solids. the solids S will generally contain principally sodium sulfate solids (i.e., crystalline anhydrous neutral sodium sulfate), along with from about 2 to about 10 wt. % sulfuric acid at a normality of from about 0.2 N to about 1.5 N and it may contain a small amount of sodium sesquisul fate crystals. 
     The fluid-contacting surfaces of separator  10  are preferably made of conventional materials such as stainless steel or a polymer-lined or polymeric material suitable for the processing materials such as sodium sesquisulfate and sulfuric acid. As will be explained below, the separator is configured to provide several zones which cooperate to accomplish the desired treatment of the sesquisulfate stream. 
     In a preferred embodiment, separator  10  includes an upright vessel  11  having a generally cylindrical upper portion  12 , a generally funnel or inverted cone-shaped middle portion  14  and a generally cylindrical lower neck portion  16 . The separator  10  also includes an upright elongate generally cylindrical conduit  18  defining a mixing zone  19  for treating a sodium sesquisulfate slurry  20  provided through conduit  22  from a waste source  24  such as a waste stream in a pulp mill. Conduit  18  provides a separation wall for separating mixing zone  19  from a clarifying zone  21 . 
     Water  26  for a dilution of the slurry  20  is provided through conduit  28  from a water source  30 . The slurry conduit  22  and water conduit  28  are provided with suitable flow control valves and pumps of a type known in the art for controlling the flow of slurry  20  and water  26  to the mixing zone  19 . 
     Conduit  18  includes an inlet opening  32  adjacent an upper portion  34  thereof for receiving the slurry  20  and water  26  into the mixing zone  19 . The conduit  18  also contains a lower edge  36  spaced above the lower neck portion  16  of the vessel  11  to enable fluid flow communication between the mixing zone  19  and clarifying zone  21 . In the illustrated embodiment, liquid from the mixing zone  19  together with entrained solids flows under the lower edge  36  of the conduit  18  into the clarifying zone  21 . 
     Conduit  18  preferably has an elongate central cylindrical portion  40  and a lower conical portion  41 , and is generally centrally located within the treatment vessel  11 . By virtue of this arrangement, clarifying zone  21  is defined as a generally annular space between the outer surface of conduit  18  and the inner surface of the upper portion  12  of vessel  11  and, moving downwardly from upper portion  12 , converges or is narrowed according to the shape of middle position  14  to neck portion  16  so as to funnel solid particles descending through zone  21  to the bottom-most portion of vessel  11 . Conical portion  41  of conduit  18  expands the flow area of material moving down through the conduit thereby decelerating the material and providing a transition zone  42  before the slurry is released into clarifying zone  21 . 
     Conduit  18  may be supported within vessel  11  by any suitable means such as a plurality of horizontally spaced apart radially extending bars or rods  43  connected in supporting relation between the outer surface of conduit  18  and the inner surface of the upper portion  12  of vessel  11 . 
     As will be explained more fully below, the sesquisulfate stream is preferably contacted with water in the mixing zone  19  under conditions which promote dissolution of sodium sesquisulfate crystals to sodium sulfate solids. In this regard, and without being bound by theory, it is believed that the sesquisulfate crystals dissolve in the water with the sulfate saltcake forming as a precipitate. 
     Water  26  is preferably introduced into the zone  19  with the sesquisulfate slurry  20  as previously described and/or water is flowed upwardly into the lower portion  41  of the mixing zone  19  to promote substantially turbulent conditions in the mixing zone  19  along with dilution of the slurry. 
     As used herein, the term “turbulent” is a relative term and will be understood to refer generally to non-stagnant fluid conditions so as to promote dissolution of the sesquisulfate crystals. Those of ordinary skill will recognize that consistent with the aim of the invention to promote dissolution of the sesquisulfate crystals in the slurry fed to the separator  10 , a sufficient turbulence is applied in the mixing zone  19  to substantially accomplish this objective. The degree of turbulence will vary depending on the circumstances of feed concentration, temperature, vessel dimensions and the like and may be determined in the exercise of ordinary skill as appropriate to accomplish the invention objectives on a case-by-case basis. 
     The treated slurry emerging from the mixing zone  19  containing sodium sulfate solids, sulfuric acid and/or water discharges into the less turbulent clarifying zone  21  thereby promoting descension of sodium sulfate solids which settle in the lower neck portion  16  of the vessel  11  and accumulate as solids S which are discharged from the vessel  11 . The liquid effluent L in clarifying zone  21  is discharged from the separator  10  through conduit  44  in the upper cylindrical portion  12  of the vessel  11 . A control valve  46  controls the flow of effluent L from the separator  10  so that a substantially constant flow is maintained. The effluent L from the separator  10  is conducted to a conventional treatment system  48  for concentration of sulfuric acid, removal of any entrained solids and recycle to the mill. 
     Additional water from a source  50  may be introduced into the lower portion  52  of the vessel  11  via a conduit  54  and associated valve  56  for further dilution of slurry  20 . A flow distributor  58  is preferably provided on the end of the conduit  54  to direct a diffused flow of water in a generally upwardly and radially outward direction into the clarifying zone  21 . 
     With additional reference to FIG. 2, a generally inverted cone-shaped collector  60  is preferably provided in the separator  10  positioned below mixing zone  19  adjacent the middle portion  14  of the separator in substantial vertical alignment with and spaced below lower conical portion  41  of the conduit  18 . Collector  60  defines an upwardly opening reservoir  61  therein for intercepting and collecting undissolved sodium sesquisulfate crystals  78  which descend from transition zone  42 . The diameter and configuration of the opening of reservoir  61  defined by collector  60  preferably corresponds substantially to that of the central cylindrical portion  40  of conduit  18  to catch a substantial portion of the relatively large sodium sesquisulfate crystals  78  descending from the conduit. 
     To further promote mixing and dissolution of sesquisulfate crystals within mixing zone  19 , a flow of gas such as air from a source of pressurized gas  62  is preferably emitted from collector  60  at a rate of from about  10  to about  80  cubic meters per ton of sesquisulfate via a gas inlet conduit  64  which doubles as a support for collector  60 . Gas flow may be regulated as by flow control valve  66 . 
     Gas inlet conduit  64  is preferably connected to the collector  60  at its base  68  in order to introduce a flow of gas bubbles  69  upwardly through the reservoir  61 . As will be appreciated, a flow of gas bubbles from collector  60  moves generally in the direction indicated by arrows C up through the mixing zone  19  to promote turbulence which aids in dissolution of sesquisulfate crystals therein. In this connection, it is further noted that expanded conical portion  41  of conduit  18  serves to collect gas bubbles  69  ascending from collector  60  into the mixing zone  19 . Accordingly, the lower edge  36  of conical portion  41  is preferably dimensioned and positioned so that the conical portion  41  in the manner of a hood substantially envelopes and thus captures bubbles ascending from collector  60 , while decelerating the downward flow of material from central portion  40  of the mixing zone  19  and also accelerating the flow of bubbles  69  through the transition area for an enhanced mixing effect. 
     A screen  70  is preferably provided at the base  68  of the collector  60  in reservoir  61  to diffuse the flow of gas into the small bubbles  69  and to limit entry of solids into the conduit  64 . Conduit  64  also preferably includes a U-shaped bend or trap  72  to collect solids which may enter the conduit  64  and to limit further migration of solids in the conduit  64 . 
     Thus, it will be appreciated that in operation sesquisulfate and water introduced into upper portion  34  of the conduit  18  flows downwardly through mixing zone  19  toward the lower portion  41  countercurrently with the upward flow of bubbles  69 . In this connection, it is noted that upper portion  34  of conduit  18  is preferably flared outwardly at the inlet opening  32  thereof so as to funnel the entering water and sodium sesquisulfate slurry toward the central portion  40  of the conduit  18  to promote integration and mixing of the water and sesquisulfate therein. Additional mixing in zone  19  may be provided by flights projecting into the zone and/or one or more motordriven impellers. 
     It has been found that the substantially turbulent mixing of sesquisulfate stream and water in accordance with the invention promotes a dissolution of the crystals in the water. However, some portion of the sesquisulfate crystals, particularly the larger crystals, may not fully dissolute prior to exiting the mixing zone  19 . Accordingly, collector  60  is positioned below the lower portion  41  of the conduit  18  to intercept and collect a substantial portion of such undissoluted crystals  78 . It is noted that the larger of these crystals  78  may tend to stay within the reservoir  61  of collector  60  and undergo repeated rising and falling, colliding with other particles, and that as these crystals grow smaller, they may become entrained and carried back into the mixing zone  19  by the bubbles  69  eventually dissoluting and settling as sodium sulfate solids S. 
     Material exiting the mixing zone  19  (excepting solids setting into collector  60 ) enters the clarifying zone  21  as indicated by arrows R. As the material flows out of the mixing zone  19  through transition zone  42  it expands radially and its velocity decreases. Sodium sulfate solids descend within the relative calm of the clarifying zone  21  under the influence of gravity and collect as solids S in the lower neck portion  16  of vessel  11 . Substantially solids-free liquid effluent rises through the clarifying zone  21  from the lower edge  36  of the conduit  18  and is directed out of vessel in conduit  44 . Thus, an increasing solids gradient will generally be observed moving downwardly through clarifying zone  21 . 
     Solids S may be removed from separator  10  via a solids effluent outlet delivery conduit  80  and associated control valve  82  located adjacent the lower neck portion  16  of the separator  10  and thereafter further processed using conventional recovery techniques in a recovery unit indicated generally at  84  to recover sodium sulfate substantially free of entrained sulfuric acid. Outlet delivery conduit  80  may be any conventional wet solids conveyance apparatus such as a screw conveyor or the like. 
     It is further noted that separator  10  may be provided in various geometrical configurations without deviating from the spirit of the invention. For example, FIG. 3 shows an alternate configuration for conduit  18 , designated  18 ′, and for the collector  60 , designated  60 ′. As can be seen, conduit  18 ′ includes upper and lower portions  34 ′ and  41 ′ provided by spaced-apart elongate panel members  90  which angle outwardly from a pair of uniformly spaced apart upright panel members  86  and  88  which define between them mixing zone  19 ′. Likewise, the collector  60 ′ is provided by a pair of side panel members  92  and  94  which angle outwardly from a slotted tube or screen  70 ′ in flow communication with a conduit  64 ′ for delivering air bubbles from the collector  60 ′ up into the mixing zone  19 ′ in a manner substantially equivalent to that described previously. 
     With respect to the operation of the separator  10  according to the invention, the sodium sesquisulfate slurry treated in the separator  10  may be a waste stream from a pulp mill which contains sodium sesquisulfate crystals, e.g., such as may be produced during the generation of chlorine dioxide for use in the mill. Such waste streams typically contain from about 30 to about 80 percent by weight solids, most typically about 80 weight percent solids, which solids consist primarily of sodium sesquisulfate crystals and sodium sulfate solids. By volume, the slurry typically contains from about 2 to 3 parts sodium sesquisulfate crystals per 1 part sulfuric acid and such feed typically has an acid normality of from about 0.5 to about 4.8 N. 
     The sesquisulfate slurry introduced into the mixing zone  19  via the conduit  20  preferably has a temperature of from about 20° C. to about 60° C. and water introduced into the mixing zone  19  via conduits  28  and  54  preferably has a temperature of from about 20° C. to about 100° C. If necessary, the incoming streams may be passed through one or more heat exchangers to obtain the desired inlet temperatures. 
     In an exemplary embodiment water is conducted to the mixing zone  19  via the conduit  28  at a rate of from about 0 liter/Kg sesquisulfate to about 0.5 liter/Kg sesquisulfate and via the conduit  54  at a rate of from about 0.1 liter/Kg sesquisulfate to about 1 liter/Kg sesquisulfate. It is preferred that the amount of water and sesquisulfate introduced be controlled such that the ratio of the total weight of water to the total weight of sesquisulfate is from about 0.6 to about 1.0, preferably from about 0.7 to about 0.9 and the slurry diluted by a factor of from about 0.2 to about 0.5. 
     As will be appreciated, the invention offers economic advantages over conventional processes and apparatus which involve the use of expensive vacuum filtration equipment. By avoiding or limiting the use of such equipment, considerable savings may be achieved without sacrificing the quality or quantity of the yield. 
     The foregoing description of certain embodiments of the present invention has been provided for purposes of illustration only, and it is understood that numerous modifications or alterations may be made without departing from the spirit and scope of the invention as defined in the following claims.