Method for electrolytic deposition of manganese
The present invention is directed to the electrolytic deposition of manganese. More, particularly the present invention is directed to the electrodeposition of manganese metal from an electrolyte containing additions of sulfur dioxide, selenium and a polyacrylamide compound. 
     The electrodeposition of manganese is well known and it is also known to introduce sulfur dioxide and selenium compounds into the manganese metal electrolyte in an effort to increase the current efficiency of the electrolytic cell as disclosed in U.S. Pat. No. 3,696,011--Lai. However, as disclosed in the later U.S. Pat. No. 3,821,096--Lai, the practice of U.S. Patent 3,696,011 results in a disadvantageous precipitation of amorphous selenium, which requires the replenishment of relatively expensive selenium, and the relatively high concentrations of selenium required results in selenium contamination of the manganese product. U.S. Pat. No. 3,821,096 attempts to overcome the above-noted disadvantages by using zinc together with lesser amounts of selenium and decreased manganese concentration in the electrolyte. 
     It is an object of the present invention to provide a method for electrodepositing manganese metal from conventional manganese metal electrolytes at high current efficiency, the manganese metal deposit obtained being sound and generally smooth and free of excessive treeing i.e. dendritic growth. 
    
    
     Other objects will be apparent from the following description and claims taken in conjunction with the drawing wherein FIGS. 1(a) and 1 show photographs at a magnification of 10X of a top surface and side view respectively of manganese metal product made in accordance with the present invention, and FIGS. 2(a) and 2 show similar photographs at the same magnification of manganese metal product made by prior art techniques. 
    
    
     A method in accordance with the present invention is an improvement in electrodepositing manganese metal from an electrolyte containing a source of manganese and comprises introducing into the electrolyte a selenium compound in an amount sufficient to provide from about 0.002 to 0.02 gram per liter of selenium and a polyacrylamide polyelectrolyte in an amount sufficient to provide about 0.1 to 2 mg per liter, and effecting deposition of manganese metal in the presence of sulfur dioxide in an amount of from about 0.1 to 1. grams per liter. 
     In the practice of a particular embodiment of the present invention, a conventional manganese electrolyte feed solution containing ammonium sulfate and manganese sulfate, with additions of sulfur dioxide, selenium dioxide, and a water soluble polyacrylamide polyelectrolyte in predetermined proportions, is added continuously to the catholyte solution in a conventional electrolytic diaphragm cell, e.g. of the type described in U.S. Pat. No. 2,739,116. The feed solution flow rate is chosen following techniques known to the art to give a desired amount of stripping, i.e. manganese depletion from the electrolyte. The manganese depleted solution passes from the cathode compartment through a diaphragm into the anode compartment, and ultimately exits the cell. The cathodes and anodes may be of any suitable materials, e.g., titanium or stainless steel for cathodes, and lead--1% silver for anodes. Normally because of solubility limits, the feed solution contains about 30-35 g. Mn/l., and this may be stripped, i.e. depleted during electrodeposition to, for example, 10-15 g./l. The ammonium sulfate is used to maintain manganese solubility and can be varied within fairly wide limits, but too little, e.g. less than about 100 g./l. in the feed will cause manganese hydroxide precipitation in the catholyte because of insufficient buffering action, and too much e.g. more than about 150 g./l. in the feed causes a decrease in current efficiency. The preferred amount for manganese concentration of 30-35 g. Mn/l. is about  110-150 g. of (NH 4 ) 2  SO 4  /l. The amount of sulfur dioxide in the cell feed is 0.1-1.0 g./l., preferably 0.3-1.0 g./l. This can be added conventionally as SO 2  gas or as sulfite salts such as Na 2  SO 3 . The selenium addition should be at least 0.002 g./l., and preferably at least 0.005 g./l. The higher selenium additions, e.g., 0.1/g.l, are disadvantageous since selenium is an expensive additive and a relatively high proportion of the selenium addition is precipitated as metal during electrolysis, and cannot be readily recycled to the system. Also, a significant proportion of the selenium codeposits with the manganese, leading to an undesirably impure product with high selenium additions since codeposition of selenium increases in proportion to its concentration in the electrolyte. Consequently, the selenium should be present in the feed solution in an amount from about 0.002 g./l. to about 0.02 g./l. At the upper level of selenium, the manganese metal product contains no more than about 0.10-0.13% Se. The selenium is conveniently added as SeO 2 , but other selenium compounds such as SeO 3 , H 2  SeO 4 , H 2  SeO 3 , and selenite or selenate salts can be used. The amount of water-soluble polyacrylamide polyelectrolyte to be added should fall within the range of 0.1-2.0 mg./l., with the preferred range about 0.15-1.0 mg./l. Higher quantities of polyelectrolyte are detrimental to the plating, as the manganese becomes highly stressed under such circumstances and can separate prematurely from the cathode during electrolysis. 
     The polyacrylamide polyelectrolyte compounds referred to herein are water soluble acrylamide homopolymers with the structure ##STR1## or water soluble copolymers of acrylamide with not more than 25 mole % of other suitable monomers, e.g. acrylic acid, vinyl chloride, and the like. The polymers in water solution may be nonionic, or slighty anionic, e.g. from the hydrolysis of some of the amide groups to carboxyl groups. Typical examples of the polyacrylamides are manufactured by Dow Chemical Company, e.g. Separan NP-10, Separan NP-20, Separan MG-250 (all slightly anionic) and Separan MGL (Nonionic). 
     The following example will further illustrate the present invention. 
     EXAMPLE 
     A small diaphragm cell containing one titanium alloy cathode and two lead-silver anodes, one on each side of the cathode, was operated 48.0 hr. at 18.0A (36A/ft. 2  initial cathode current density) at 35° C. The feed to the cell contained 32-34 g. Mn/l. and approximately 130 g. (NH 4 ) 2  SO 4  /l. The pH was 7.15. Selenium as SeO 2 , sulfur dioxide as Na 2  SO 3 , and polyacrylamide polyelectrolyte as Dow Chemical Company&#39;s Separan NP-10, were added in the amounts recorded in Table I. Feed rates were adjusted as necessary to give a catholyte of approximately 11-14 g. Mn/l. The catholyte pH was about 8.8-9.0. 
     
                                           TABLE I                                 
__________________________________________________________________________
Manganese Electrodeposition in 48.0 Hr. at 36A/Ft..sup.2, 35° C.   
Cell Feed Composition                                                     
              mg Separan                                                  
                    Current                                               
Test                                                                      
   q. SO.sub.2 /L.                                                        
         q. Se/L                                                          
              NP-10/L                                                     
                    Eff. (%)                                              
                         Metal Characteristics                            
__________________________________________________________________________
1  0.60  0    0     65.0 good base, small trees                           
2  0.60  0    0.88  65.4 thick base, smoother than                        
                         that of Test 1                                   
3  0.60  0.0080                                                           
              0     72.2 thin base, highly treed                          
4  0.60  0.0080                                                           
              0.88  72.8 good base, less treed                            
                         than in Test 3                                   
5  0.60  0.0080                                                           
              0.88  72.2   &#34;                                              
6  0.40  0    0     66.6 good base, small trees                           
7  0.40  0    0.88  67.8 thick base                                       
8  0.40  0.0050                                                           
              0     68.4 thin base                                        
9  0.40  0.0050                                                           
              0     70.3 thin base                                        
10 0.40  0.0050                                                           
              0.88  69.5 thicker base than that                           
                         of Tests 8-9                                     
__________________________________________________________________________
 
    
     The metal produced with the selenium and polyacrylamide additions in accordance with the present invention, Tests 4, 5 and 10, was significantly less treed than that produced with only selenium and SO 2  additions and high current efficiencies were achieved as compared to the other tests. The thin based metal from the selenium-only Tests 3, 8 and 9, was substantially all trees. This condition is very detrimental in large scale commercial practice; often the treeing is even more intense because of generally unequal current distribution to the cathodes and the trees tend to fall off and redissolve in the electrolyte, frequently when the cathode is extracted from the cell. Also, large trees tend to redissolve at their base while still attached to the cathode. These phenomena can result in a net decrease in current efficiency, which, in turn, translates to increased power costs per pound of metal produced. FIGS. 1 and 1(a) showing photographs of the manganese metal product obtained in Test 5 in accordance with the present invention (SO 2 , Se, polyacrylamide additions) exhibit the minimal &#34;treeing&#34; and thick, sound metal base achieved in the practice of the present invention. FIGS. 2 and 2(a) show the metal product of Test 3 (SO 2 , Se additions) which exhibits gross &#34;treeing&#34;, cracking and a thin base.