Abstract:
Processes and systems for the treatment of wood chips prior to cooking remove substantially (e.g., at least about 30%) water-soluble compounds therein, especially those compounds which disassociate in water to form potassium and/or chloride ions and/or water-soluble metal compounds such as manganese, calcium and potassium. In preferred embodiments, wood chips entering the cooking system containing water-soluble compounds, such as those compounds which disassociate in water to form potassium and/or chloride ions and/or water-soluble metal compounds such as manganese, calcium and potassium, may be treated prior to cooking by being brought into contact with an aqueous treatment stream (e.g., water or steam condensate) at a temperature (e.g., between about 20° C. to about 120° C.) and for a time (e.g., between about 2 to 200 minutes) so as to reduce substantially (e.g., at least about 30%) such water-soluble compounds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is based on, and claims domestic priority benefits under 35 USC §119(e) from, U.S. Provisional Patent Application Serial No. 60/299,770 filed on Jun. 22, 2001, the entire content of which is expressly incorporated hereinto by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to the field of wood chip treatments. In especially preferred embodiments, the present invention relates to treatment of wood chips prior to cooking in a pulp manufacturing process.  
         BACKGROUND AND SUMMARY OF THE INVENTION  
         [0003]    The presence of some water-soluble compounds can have deleterious effects in pulping processes. For example, compounds which disassociate in water to form potassium and/or chloride ions, lower the melting point of fly ash in the recovery boiler of a wood pulp mill. This reduced melting point of the fly ash in turn requires that the recovery boiler be operated at a lower temperature, thereby reducing the efficiency of the boiler. To overcome this problem, it is highly desirable that water-soluble compounds be removed from wood chips prior to being cooked.  
           [0004]    Similarly, water soluble metal compounds such as manganese, calcium, potassium and the like should be removed prior to cooking of wood chips so as to ensure that the resulting pulp may be more easily bleached. In this regard, manganese is especially harmful to the bleaching process and is known to cause poor bleaching results. Moreover, the removal of calcium results in reduced scaling in the digester and evaporators.  
           [0005]    Broadly, the present invention is embodied in processes whereby wood chips are treated prior to cooking so as to remove substantially (e.g., at least about 30 wt. %, more preferably at least about 50 wt. % and most preferably at least 70 wt. %) water-soluble compounds, especially those compounds which disassociate in water to form potassium and/or chloride ions.  
           [0006]    More specifically, as described briefly above, wood chips entering the cooking system contain water-soluble compounds, such as those compounds which disassociate in water to form potassium and/or chloride ions. According to the present invention, wood chips are treated prior to cooking by being brought into contact with an aqueous treatment stream (e.g., water or steam condensate) so as to reduce substantially (e.g., at least about 30 wt. %) such potassium- or chloride-containing water-soluble compounds. By treatment of the wood chips in accordance with the present invention, water-soluble compounds, specifically potassium- or chloride-containing water-soluble compounds, present in the wood chips are displaced from the interior of the chips and accumulate in the aqueous treatment liquid present. The aqueous treatment liquid containing the water-soluble compounds may then be drained from the chips prior to the chips entering the cooking system. These wood chips can be treated (washed) multiple times so as to sequentially increase the amount of water-soluble compounds, specifically potassium- and/or chloride-containing compounds, removed therefrom.  
           [0007]    These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0008]    Reference will hereinafter be made to the accompanying drawings, wherein like reference numerals throughout the various FIGURES denote like structural elements, and wherein;  
         [0009]    FIGS.  1 - 3  each represent a schematic view of a possible wood chip treatment technique in accordance with the present invention;  
         [0010]    [0010]FIGS. 4A and 4B are graphical plots in accordance with the Examples below of potassium and chloride ion concentrations (mg/L) in chip filtrate versus leach time for 2-3 mm unsteamed and steamed wood chips, respectively;  
         [0011]    [0011]FIGS. 5A and 5B are graphical plots in accordance with the Examples below of potassium and chloride ion concentrations (mg/L) in chip filtrate versus leach time for 4-6 mm unsteamed and steamed wood chips, respectively;  
         [0012]    [0012]FIGS. 6A and 6B are graphical plots in accordance with the Examples below of potassium and chloride ion concentrations (mg/L) in chip filtrate versus leach time for 8-10 mm unsteamed and steamed wood chips, respectively; and  
         [0013]    [0013]FIG. 7 is plot of the percentage of total potassium removed from three different wood chip size categories in accordance with the Examples below.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    The aqueous wash stream that may be employed in the practice of the present invention can be virtually any stream which is capable of being processed in the pulp mill&#39;s existing wastewater treatment system. Such streams include, but are not limited to, water, evaporator (and/or other mill) condensates, collected rainwater, and bleach plant effluent. The wash may advantageously be carried out at temperatures between about 20° C. to about 120° C. (e.g., advantageously between about 30° C. to about 90° C.), at pressures up to the operating pressure of the downstream digester. The residence time in which the wood chips may be in contact with the aqueous wash liquid can be from about 2 to about 200 minutes. The wash is most preferably conducted at a pH of between about 2 to about 12, and more preferably between about 4 to about 10. It is especially preferred that the wash be conducted at elevated pressures which, as noted above, can be up to the pressure condition of the downstream digester.  
         [0015]    Various techniques may be employed in accordance with the present invention. For example, as shown in accompanying FIG. 1, one system  10  includes a chip bin  12 , such as disclosed in U.S. Pat. No. 5,500,083 (the entire content of which is expressly incorporated hereinto by reference) and available commercially from Andritz Inc. of Glens Falls, N.Y. as Diamondback® Steaming Vessel. Steam may be injected into the wood chips contained within the chip bin  12  via line  12 - 1 . The chip bin  12  is provided with a meter screw  14 , such that condensate is capable of being drained therefrom via line  16 . Such drained condensate may then be sent to the pulp mill&#39;s existing wastewater treatment system (not shown). The chip bin  12  shown in FIG. 1 could, if desired, be provided without steaming, but with a meter screw and a horizontally or vertically disposed wash vessel (not shown) downstream of the meter screw where wash liquid may be introduced. The wash effluent may thus be drained from the wash vessel and processed in the pulp mill&#39;s existing wastewater treatment system.  
         [0016]    The system  20  shown in FIG. 2 includes a similar chip bin  12  as depicted in FIG. 1, in that it is steam may be injected into the wood chips contained therein by line  22 - 1  and that it is provided with a meter screw  24 . In addition, the chip bin  22  is provided with a plurality of serially arranged horizontally or vertically disposed wash vessels  25 ,  26  downstream thereof where wash liquid may be introduced. The chips may be washed with water or other suitable aqueous stream via line  27 . The liquid may be drained from an initial one of the wash vessels  25  via line  25 - 1  and then directed to a second downstream wash vessel  26  where the chips are washed a second time. Liquid drained from the second wash vessel may be divided into respective portions, with one portion thereof being employed as wastewater treatment via line  26 - 1 , and another portion thereof used to flow counter-current via line  26 - 2  with the wood chips in the first wash vessel  25 . Thus, with the series of wash vessels  25 ,  26  as depicted in FIG. 2, each one has the ability to gather the drained wash liquid and return it to the preceding wash vessel or be sent to the pulp mill&#39;s existing wastewater treatment system.  
         [0017]    The last wash vessel in the series could be, for example, an in-line separator, while an intermediate wash vessel could be a retention vessel to increase the retention time of wash material in contact with wood chips. Such a system  30  is depicted in the embodiment depicted in accompanying FIG. 3. Alternatively, instead of a dedicated wash vessel, the final chip wash could occur physically in the top-separator of the digester.  
         [0018]    More specifically, as shown in FIG. 3, the chip bin  32  includes a meter screw  34  and is provided with a line  32 - 2  to allow steam injection into the wood chips contained therein. A series of wash vessels  35 ,  36  and  37  are provided to allow the wood chips to be sequentially washed in countercurrent fashion via fresh wash water supplied via lines  39 - 1 ,  39 - 2  and  39 - 3 , respectively. As noted briefly above, the last wash vessel  37  in the series could be, for example, an in-line separator. A retention vessel  40  is preferably interposed in the chip flow between the initial was vessel  35  and the intermediate wash vessel  36 . The retention vessel serves to increase the retention time of wash material in contact with wood chips.  
         [0019]    Liquid drained from the intermediate and final wash vessels  36 ,  37  may be divided into respective portions, with one portion thereof being employed as wastewater treatment via lines  36 - 1  and  37 - 1 , respectively, and another portion thereof used to flow counter-current via lines  36 - 2  and  37 - 2  with the wood chips in the first and intermediate wash vessels  35  and  36 , respectively. Thus, as was the case with the series of wash vessels  25 ,  26  as depicted in FIG. 2, the wash vessels  36 ,  37  in the system  30  shown in FIG. 3 has the ability to gather the drained wash liquid and return it to the preceding wash vessel or be sent to the pulp mill&#39;s existing wastewater treatment system.  
         [0020]    It has been found that the removal of water-soluble compounds can be accomplished. In one specific embodiment of the invention, it has been found that water-soluble potassium can be removed from wood chips. More specifically, in laboratory experiments carried out at atmospheric pressure and at temperatures ranging from 30° C. to 90° C. with wash liquid contact time of about 30 minutes, potassium-containing compounds may be reduced substantially in an amount of at least about 30 wt. % (e.g., between about 30 wt. % to about 40 wt. %). The operational conditions used in the laboratory experiments were selected due to the practical limitations existing in the laboratory and are thus non-limiting to the present invention. That is, other operational conditions may be employed within the context of the present invention. Based on the results of the laboratory experiments, it is expected that potassium removal will increase at higher temperatures and/or operating pressures. It is also expected that the commercial operating conditions will be different from those used in the laboratory, specifically higher temperature, higher pressure and longer retention time.  
         [0021]    The present invention will be further understood from the following non-limiting Examples.  
       EXAMPLES  
       [0022]    Wood chips were tested for moisture content before any treatment was performed. Chips were separated into two portions—that is, one portion for those chips to be pre-treated with steam prior to the leaching treatment and another portion for those chips to be treated only by the leaching treatment (i.e., without any steam pre-treatment). The condensate formed during the steaming treatment was drained and tested for potassium content. Chips to be pre-treated with steam were segregated into three different chip size categories—that is, chips having a size, on average, of 2-3 mm, 4-6 mm and 8-10 mm, respectively. Numerous bags containing 50 g of wood chips for each chip size category were prepared. Respective bags of wood chips were then subjected to each of three different temperature regimes (i.e., 30° C., 60° C. and 90° C.) and five different retention time regimes (i.e., 1 min., 2 min., 5 min., 10 min. and 30 min.). A total of 45 bags of chips was prepared for the steamed wood chips. The same procedures as described above were also followed for preparation of wood chip-containing bags for the non-steam pre-treated chips.  
         [0023]    A container of water was heated to the desired temperature (i.e., 30° C., 60° C. and 90° C.). Once at temperature, a bag of chips was introduced and held at temperature for the desired retention time. After reaching the desired retention time, the heating was terminated and the filtrate tested for chloride and potassium content. Standard Method 4500 (Standard Methods, 18 th  Ed., Meruric Nitrate Method 4500-Cl, Section C, 1992) was used for chlorides and Standard Method 3111 was used for potassium (Standard Methods, 18 th  Ed., Metals by Flame Atomic Absorption Spectrometry 3111 A &amp; B, 1992)  
         [0024]    The concentrations of potassium and chloride present in the filtrate versus the leach time were plotted for each of the unsteamed and steamed wood chips size categories and appears as FIGS.  4 A- 4 B,  5 A- 5 B and  6 A- 6 B, respectively.  
         [0025]    The percentage of potassium removed by steaming and leaching was also calculated and such data appear in FIG. 7.  
         [0026]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.