Abstract:
A process for regeneration of electrolytes, in particular Na 2 SO 4 , from waste pickles created during picking of stainless steel, in particular stainless steel strips. The electrolytes are regenerated by adding Na x H y S z O v  in an acid solution whereby Cr 6+  is reduced to Cr 3+ . Variables x, y, z and v having the following values: x=0 to 2, y=0 to 2, z=1 to 6, v=2 to 6.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a process for regeneration of electrolytes, in particular of Na 2 SO 4  from spent pickling liquid, or pickle liquors, for stainless steel, in particular, stainless steel strips. 
     The “neolyte” process using a neutral electrolyte has become widely accepted all over the world for electrolytically pickling stainless steel strip, in particular cold strip. This process provides for the strip to be de-scaled in a sodium sulphate solution by means of electric current. In the process, metals build up in the electrolyte—i.e., the sodium sulphate solution in this case. Chromium is dissolved in the form of chromate (Cr 6+ ), while the other metals such as iron and nickel are present in the form of hydroxides. 
     The metal content of the pickling acid is kept constant by continuous extraction of electrolyte from the pickling bath and its substitution with fresh sodium sulphate solution. This causes considerable operating expenses for sodium sulphate and for the disposal of the spent electrolyte solution. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention in a preferred form is a process for regenerating the electrolyte present in the pickle liquors which reduces both the operating cost and the environmental impact of so-called neolyte pickling plants. 
     In the process according to the invention, this objective is achieved by adding Na x H y S z O v  in an acid solution, which causes Cr 6+  to be reduced to Cr 3+ , with the suffixes being x=0 to 2, y=0 to 2, z=1 to 6, v=2 to 6. By adding this substance, Na 2 SO 4  is produced in any of the reactions and can be easily recycled to the process. 
     An advantageous design of the invention is characterized by the fact that no foreign atoms are introduced to the solution. This avoids formation of other, undesirable reaction products apart from Na 2 SO 4 . 
     An advancement of the invention is characterized by the fact that the reaction takes place at a pH in the range of approx. 2-3. The decrease of the pH is favorably effected with H 2 SO 4 , which also means that no foreign atoms are introduced to the solution. Having the reduction take place in the acid range speeds up the reaction in an advantageous manner. 
     An advantageous design of the invention is characterized by the fact that the acid solution produced is neutralized by adding NaOH. This neutralization with NaOH causes Na 2 SO 4 , which can be easily recycled to the process, to be formed also in this step. 
     An advantageous advancement of the invention is characterized by the fact that the metal hydroxides produced by neutralization are separated from the pickling acid (Na 2 SO 4 ) and the solution is recycled to the pickling process. By recycling the pickling acid, the high cost of fresh pickling acid is avoided, and by separation of the metal hydroxides from the pickling acid these can be deposited at a higher concentration, i.e., with a lower content of residual acid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawing in which: 
     FIG. 1 is a schematic diagram of a pickling system with neolyte recovery in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an electrolytic pickling system  1  with electrode pairs  2 , through which a metal strip  3  is routed, and more particularly through section  4  which is filled with pickling acid. As pickling acid, a neutral electrolyte like Na 2 SO 4  is used. The pickling system is followed by a cleaning system  5 , which contains cleaning brushes  6 . 
     The neutral electrolyte is introduced to the pickling plant  1  from the circulation tank  7  of the pickling plant  1  and via a pump  8  that is advantageously adjustable, a heat exchanger  9  whose temperature can be adjusted and a pipe  10 . This can be done as required, preferably at several points. The waste pickle acid, or pickle liquor, enriched with the scale from the metal strip  3  as well as a large portion of Cr 6+  ions, apart from iron and nickel, is piped to the circulation tank  7  via a pipe  11 . The electrolyte enriched with metal ions is piped to the circulation tank  7  and further on to a reaction tank  12 . In one embodiment, fresh sulfuric acid (H 2 SO 4 ) is added via a pipe  13  to decrease the pH, and a reduction agent such as Na 2 S 2 O 5  for reducing Cr 6+  is added via pipe  14 . The pH in the reaction tank  12  is in the acid range, and is, for instance, pH=2. As a result of the reaction, Na 2 SO 4  is produced in accordance with the following representative formulae. 
     
       
         3 Na 2 SO 3 +3 H 2 SO 4 +2 H 2 CrO 4 →Cr 2 (SO 4 ) 3 +3 Na 2 SO 4 +5 H 2 O  1) 
       
     
     
       
         6 Na 2 S 2 O 3 +6 H 2 SO 4 +2 H 2 CrO 4 →Cr 2 (SO 4 ) 3 +3 Na 2 SO 4 +3 Na 2 S 4 O 6 +8 H 2 O  2) 
       
     
     
       
         3 Na 2 S 2 O 5 +3 H 2 O&lt;==&gt;6 NaHSO 3  6 NaHSO 3 +2 H 2 CrO 4 →Cr 2 (SO 3 ) 3 +3 Na 2 SO 4 +5 H 2 O  3) 
       
     
     leads to 
     
       
         3 Na 2 S 2 O 5 +2 H 2 CrO 4 →Cr 2 (SO 3 ) 3 +3 Na 2 SO 4 3+2 H 2 O 
       
     
     
       
         3 Na 2 S 2 O 4 +3 H 2 SO 4 +4 H 2 CrO 4 →Cr 2 (SO 4 ) 3   +Cr   2 (SO 3 ) 3 +3 Na 2 SO 4 +7 H 2 O  4) 
       
     
     
       
         3 Na 2 S 2 O 6 +2 H 2 CrO 4 →Cr 2 (SO 4 ) 3 +3 Na 2 SO 4 +2 H 2 O  5) 
       
     
     The above formulae show possible reactions in an exemplary manner, with Cr 6+  being reduced to Cr 3+  and Na 2 SO 4  being formed again as neutral electrolyte. 
     Preferably, the acid and reduction agent that are added to the waste pickle acid contain only chemical elements that are already present in the waste pickle acid. That is, the acid and reduction agent contain no foreign chemical elements (chemical elements that are foreign to the waste pickle acid). This ensures that the chemical reaction produces Na 2 SO 4  and prevents formation of other, undesirable reaction products. 
     The reaction products such as iron and nickel hydroxide are routed to a neutralization stage  15 . For neutralization, NaOH is added via a pipe  16 . The pH in the neutralization tank  15  is in the neutral range, i.e., pH=8-9. The sludge (the mother liquor) in neutralization stage  15  is then pumped to a filter  18  via a pump  17 , and from there, the electrolyte, i.e., the pure Na 2 SO 4  solution, is led to circulation tank  7  via pipe  19  and thus recycled to the process. The phase enriched with metal oxides is extracted from filter  18  via pipe  20  as sludge, and can then be deposited. 
     In the following example, the process according to the invention is compared to the hitherto known neolyte process. Cold strip sized 1250 mm×0.8 mm and made of quality AISI 304 steel was pickled at a strip speed of 85 m/min. The average pickling loss amounted to 3 g/m 2 . The Na 2 SO 4  concentration was 150 g/l and the Cr 6+  concentration 5 g/l. 
     This yielded the following quantities: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Process as 
                 Difference 
                   
               
               
                   
                 Neolyte 
                 per the 
                 between the 
                   
               
               
                   
                 process 
                 invention 
                 invention and 
                   
               
               
                   
                 with Cr 6+   
                 with recovery 
                 state of the 
                   
               
               
                   
                 reduction 
                 of neolyte 
                 art 
                 Unit 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Na 2 SO 4   
                 206.6 
                 23.5 
                 −183.1 
                 kg/h 
               
               
                 H 2 SO 4   
                 33.9 
                 35.4 
                 1.5 
                 kg/h 
               
               
                 Na 2 S 2 O 5   
                 18.7 
                 18.9 
                 0.2 
                 kg/h 
               
               
                 NaOH 
                 0.0 
                 36.1 
                 36.1 
                 kg/h 
               
               
                 Water for 
                 1775.6 
                 321.3 
                 −1454.3 
                 l/h 
               
               
                 neolyte 
               
               
                 Rinse water 
                 1500.0 
                 1500.0 
                 0.0 
                 l/h 
               
               
                 Waste pickle to 
                 1458.4 
                 792.7 
                 −665.7 
                 l/h 
               
               
                 neutralization 
               
               
                 pH of sludge 
                 pH = 2 
                 pH = 8 
               
               
                   
               
             
          
         
       
     
     The above chart shows that the consumption of Na 2 SO 4  for the pickle was substantially reduced. Also, the consumption of water was reduced correspondingly. The waste sludge amount was about halved, with the pH now being in the neutral range. The savings of lime for neutralization were not taken into account in the above overview. 
     Rinsing water is added in the cleaning stage  5  which follows the pickling plant  1  by a pump  21  and a pipe  22  and sprayed onto the strip  3  for instance at the entrance of the metal strip  3  into the cleaning stage  5  or near the brush rolls  6 . The sludge is routed to a settling tank  24  via a pipe  23  and then extracted from there through a pipe  25  and deposited. Wash water is led to the reaction tank  12  via pipe  26 . 
     The invention is not limited to the design shown in FIG. 1, it may well be laid out without a cleaning system or with a cleaning system designed in a different manner. Also, several pickling systems may be arranged one after the other.