Process for improving the reactivity of zinc particles in producing sodium dithionite from zinc dithionite

Abstract

The present invention relates to a process for preparing a zinc dithionite solution wherein the process comprises reacting an aqueous slurry of zinc particles with sulfur dioxide in presence of a promoter compound. The zinc particles can either be a commercial particulate zinc material or a particulate zinc material prepared by electrolyzing an aqueous alkaline suspension or slurry containing zinc based species. The addition of the promoter compound significantly increases the conversion of zinc to zinc dithionite and significantly reduces the cycle time of the reaction. The promoter compound is selected from the group consisting of sodium hydroxide and metal compounds. The promoter metal compounds are selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal zincate, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.

I claim:  
     
         1 . A process for preparing a zinc dithionite solution which comprises reacting an aqueous slurry of zinc metal with sulfur dioxide in presence of an effective amount of a promoter compound.  
     
     
         2 . The process of  claim 1  wherein the reacting step is maintained at a pH above about 3.3 and a reaction temperature below about 54° C.  
     
     
         3 . The process of  claim 1  wherein the promoter compound comprises an alkali metal hydroxide and the reacting step is maintained at a pH above about 4.0.  
     
     
         4 . The process of  claim 1  wherein the sulfur dioxide is introduced to the aqueous slurry of zinc metal at a constant rate and the promoter compound is admixed with the aqueous slurry of zinc metal either before or after the sulfur dioxide is introduced.  
     
     
         5 . The process of  claim 3  wherein the constant rate of introducing sulfur dioxide is adjusted in a series of constant rate increments to maintain pH above about 3.2.  
     
     
         6 . The process of  claim 1  wherein the zinc metal comprises particulate zinc material and/or zinc powder.  
     
     
         7 . The process of  claim 1  wherein the promoter compound comprises at least one compound selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc hydroxide, an alkali metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.  
     
     
         8 . The process of  claim 1  further comprising electrolyzing an alkaline suspension or slurry of a zinc mud comprising zinc hydroxide in an aqueous alkaline suspension to provide the aqueous slurry of zinc metal.  
     
     
         9 . The process of  claim 8 , wherein the electrolyzing step takes place in an electrochemical cell having cathodes based on metals selected from the group consisting of copper, magnesium, stainless steel, and nickel.  
     
     
         10 . The process of  claim 9  wherein the electrochemical cell comprises an undivided electrochemical cell having a copper cathode and a nickel anode.  
     
     
         11 . The process of  claim 9  wherein the electrochemical cell comprises an undivided electrochemical cell having a copper cathode and a stainless steel anode.  
     
     
         12 . The process of  claim 8 , wherein the electrolyzing step is carried out at a temperature of about 20 to about 65° C.  
     
     
         13 . The process of  claim 1 , wherein the effective amount of the promoter compound comprises 2% to 15% by weight of the zinc metal.  
     
     
         14 . The process of  claim 1 , wherein the effective amount of the promoter compound comprises 3% to 9% by weight of the zinc metal.  
     
     
         15 . The process of  claim 1 , wherein the effective amount of the promoter compound comprises 4% to 6% by weight of the zinc metal.  
     
     
         16 . A process for the production zinc dithionite solution, said process comprising: 
 a) introducing a sulfur dioxide stream to a reaction zone containing an aqueous slurry of zinc metal in presence of a promoter compound comprising a compound selected from the group selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc hydroxide, an alkali metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof;    b) reacting the zinc metal with the sulfur dioxide to produce a zinc dithionite effluent comprising a zinc dithionite solution and insolubles; and    c) recovering the zinc dithionite solution.    
     
     
         17 . The process of  claim 16  wherein the promoter compound is introduced to the reaction zone prior to or simultaneously with the passing sulfur dioxide stream.  
     
     
         18 . The process of  claim 16  further comprising recovering and returning at least a portion of the insolubles to the reaction zone.  
     
     
         19 . The process of  claim 16  further comprising passing at least a portion of the zinc dithionite reactor effluent at effective sodium dithionite reaction conditions to a sodium dithionite reaction zone and therein contacting a dilute caustic solution to provide a sodium dithionite product solution.  
     
     
         20 . The process of  claim 16  further comprising removing at least a portion of the insolubles from the zinc dithionite effluent and returning at least a portion of the insolubles to an electrochemical cell for producing the aqueous slurry of zinc metal.  
     
     
         21 . A process for preparing a sodium dithionite product solution comprising: 
 a) admixing a zinc mud stream comprising zinc oxide with an electrolyte stream comprising water and caustic in an undivided electrochemical reaction zone and electrolyzing said reaction zone to provide a zinc metal suspension;    b) recovering a zinc metal stream from the zinc metal suspension;    c) washing the zinc metal stream to provide an aqueous zinc metal suspension;    d) admixing a promoter compound with water in a zinc dithionite reaction zone to provide an aqueous zinc compound suspension;    e) admixing the aqueous zinc metal suspension with the aqueous zinc compound suspension to provide a mixed zinc suspension;    f) contacting the mixed zinc suspension with a sulfur dioxide stream in a zinc dithionite reaction zone and reacting the mixed zinc suspension with the sulfur dioxide to provide a zinc dithionite solution and insolubles;    g) contacting the zinc dithionite solution with a dilute caustic solution in a sodium dithionite reaction zone to provide a sodium dithionite effluent stream comprising sodium dithionite solution and a solid zinc hydroxide material;    h) separating the solid zinc hydroxide material from the sodium dithionite effluent stream to provide a recovered zinc oxide stream and the sodium dithionite product solution;    i) washing the recovered zinc oxide stream to provide a washed zinc oxide stream;    j) combining at least a portion of the washed zinc oxide stream with the zinc oxide feed stream to provide the zinc mud stream;    k) combining at least a portion of the washed zinc oxide stream with water in step (d), and    l) recovering the sodium dithionite product solution.    
     
     
         22 . The process of  claim 21  wherein the promoter compound is selected from the group consisting of an alkali metal hydroxide, zinc mud, zinc hydroxide, an alkali metal zincate, zinc oxide, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.  
     
     
         23 . The process of  claim 21 , further comprising recovering at least a portion of the insolubles and returning at least a portion of said insolubles to the undivided electrochemical reaction zone to improve the electrolyzing step therein.  
     
     
         24 . The process of  claim 21 , further comprising recovering at least a portion of the insolubles and returning said insolubles to the zinc dithionite reaction zone.  
     
     
         25 . The process of  claim 21 , further comprising at least partially converting the mixed zinc suspension with the sulfur dioxide in step (f), recovering at least a portion of the zinc dithionite solution and returning unreacted zinc suspension and insolubles to the zinc dithionite reaction zone.  
     
     
         26 . The process of  claim 7 , wherein the alkali metal comprises sodium.  
     
     
         27 . The process of  claim 16 , wherein the alkali metal comprises sodium.  
     
     
         28 . The process of  claim 22 , wherein the alkali metal comprises sodium.