Method for recovery of reusable chemicals from C102 generator effluent
BACKGROUND OF THE INVENTION 
     Process for the recovery of spent acid from a Mathieson or Solvay type chlorine dioxide generator and more particularly a process for the recovery of salt cake and reusable sulfuric acid from the effluent of a chlorine dioxide generator. 
     Those skilled in the art are aware that chlorine dioxide is used in the bleaching of cellulosic materials such as wood pulp produced by the Kraft or sulfite processes. It is also well known in the pulp and paper industry that the Hooker SVP and Erco R-3 systems offer an approach to the recovery of effluent from the chlorine dioxide generator by operating the generator under chemical conditions which favor precipitation of salt cake. The resystems have the disadvantage of generating significant quantities of chlorine gas which is dissolved in the chlorine dioxide and is harmful and damaging to the pulp. 
     On the other hand, the Mathieson or Solvay process does not produce chlorine gas but does yield an acid effluent which is difficult to dispose of since it cannot be dumped and to date, at least commercially, the acid effluent has not been successfully treated. Accordingly, the Mathieson or Solvay process for generation of chlorine dioxide is not used to any significant extent in the United States and Canada because of the environmental protection laws regulating the dumping of its effluent. As is also well known, recycling the C10 2  generator effluent with the black liquor upsets chemical balances and values so that the Mathieson or Solvay system has fallen out of general use and favor worldwide. 
     It has been recognized in the prior art that in the evaporation and crystallization of the residual acid it is not only difficult but expensive to remove the sodium sulfate. However, while the industry is aware of the general chemistry involved, it has not developed a system for the purpose of disposing of the acid effluent in a closed system. 
     Among the prior art references known to applicants, only three would appear to be of interest. U.S. Pat. No. 4,329,199 is concerned with all of the environmental pollution substances for a pulp mill including black liquor, effluent from the bleaching operation, flue gases, condensate from the digester, and the like. This particular reference, however, does not direct itself to the process steps of this invention but simply mentions evaporation/crystallization steps and no more. The reference is considered to be of interest only for its recognition of the general chemistry. 
     U.S. Pat. No. 3,789,108 owned by Erco is directed to the production of chlorine dioxide. In this particular patent, a three-stage reaction zone for the production of chlorine dioxide involves the reacting of an alkali metal chlorate with alkali metal chloride and sulfuric acid to which in each of the generators air is introduced for removing chlorine. While the method of the Erco patent involves a three-stage operation for the production of sodium sulfate, its purpose is not to recover the acid effluent but to generate chlorine dioxide in each of the three stages. Accordingly, it adds sulfate ion-containing acidic material to all three of the stages. 
     Finally, U.S. Pat. No. 3,341,288 assigned to Hooker Chemical Corporation also is directed to the production of chlorine dioxide but does not concern itself with the acid effluent resulting from an Olin-Mathieson or Solvay type chlorine dioxide generating process. 
     Applicants are not aware of any other references at the time of filing this application that have any bearing on the steps of the instant application for the recovery of spent acid effluent in the form of salt cake and sulfuric acid. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     As will be observed from the drawing, the chlorine dioxide generator utilizes the conventional Mathieson system constituents of sodium chlorate, sodium chloride, sulfuric acid and sulfur dioxide in the reaction zone or generator 10. In the Solvay system, methanol is used in place of sulfur dioxide. Spent acid effluent from the generator is directed to the evaporator/crystallizer 12. It will be appreciated that the spent acid recovery system of this invention is based upon the solubility system of sulfuric acid-sodium sulfate-water. Depending upon the concentration of three inorganic chemicals, the excess salts present are precipitated out in the first stage as sodium bisulfate, in the second stage as sodium sesquisulfate and in the third stage as sodium sulfate. Water is internally evaporated, condensed and recycled within the system while the excess is either discharged or reused to dissolve incoming sodium chlorate as is shown in the flow chart. 
     In accordance with the present invention, three evaporator/crystallizers are operated in sequence, the conditions being somewhat different for the first stage as opposed to the second and third stages. In the first stage, effluent from the chlorine dioxide generator is directed via line 12 to the heater evaporator 14 where the spend acid is boiled at or above atmospheric pressure at approximately 128 C. in order to obtain first stage crystals. The spent acid solution is boiled to yield a weight reduction of approximately 40%. The solution is then transferred to the crystallizer tank 16 where it is cooled. Crystallization commences between 98 degrees to 103 degrees C. The slurry is cooled to 60 degrees C. and held at that temperature for approximately 3 hours in order for the solution to reach equilibrium. The crystals were analyzed and found to have a Na/SO 4  ratio of 0.26 which is indicative of NaHSO 4 . X-ray diffraction analysis performed on a sample of the crystals confirmed the presence of NaHSO 4  and NaHSO 4  :H 2  O, the latter of which may have formed as a result of subsequent hydration of the sodium bisulfate. The crystals were also observed by a scanning electron microscope. 
     The crystals and filtrate are separated with the filtrate going via lines 18 and 20 back to the ClO 2  generator. The crystals were transferred to the holding tank 24 and consisted of 59.2% sodium sulfate and approximately 40.8% sulfuric acid. 
     At holding tank 24 the crystals are redissolved and transferred to the second stage heater evaporator 26 with the solution containing 39.2% sodium sulfate, 26% sulfuric acid and 34.8% water. The solution in the stage 2 evaporator is heated to 63 degrees C. under a partial vacuum of 673 mm Hg to induce evaporation. The crystals formed were found to have a Na/SO 4  ratio comparable to Na 3  H(SO4) 2 . The stage 2 crystals transferred to the holding tank 34 via line 30 were approximately 81.3% sodium sulfate and 18.7% sulfuric acid. 
     The stage 2 crystals transferred to holding tank 34 are then redissolved and heated to 85 degrees C. under 673 mm Hg vacuum. It was found that a lower temperature at this stage resulted in too slow a reduction of volume. 
     After cooling, the crystals were analyzed to have a Na/SO 4  ratio comparable to sodium sulfate. The salt-cake is then transferred to its point of use while the filtrate is transferred via line 42 to the holding tank 24 at stage 2. The filtrate from stage 2 via line 32 is transferred to the stage 1 heater evaporator 14 along with original effluent from the generator 10 via lines 12 and 15. 
     Reusable salt cake was obtained from the spent acid effluent as set forth. The crystals formed in all three stages contained no significant traces of iron or chromium which indicates the recycled acid will contain any impurities introduced in the system.