Electrolytic Production of Bromates

An electrolytic process for the product ion of potassium or sodium bromate using a pure lead peroxide anode has been operated successfully. A concent ra ted bromate solution was electrolyzed continuously at a cathode of stainless steel wi th an anodic current density of 20 amp/d in 2 at 70~ current efficiency, about 90%; anode consumption about 50-60 m g / K amp-hr. Pure crystal l ine bromate could be obtained by cooling the cell effluent at room temperature . A m o n g a lka l i m e t a l ha logena tes , only ch lora tes h a v e been p roduced c o m m e r c i a l l y in J a p a n by the e l ec t ro ly t i c process. N e w uses for b r o m a t e b r o u g h t about its e l ec t ro ly t i c p roduc t ion , a l t h o u g h in m u c h sma l l e r amoun t s t han chlora te . A n e w e lec t ro ly t i c process us ing a lead p e r o x i d e anode is desc r ibed here . Anode for Electrolytic Production of Bromate Elec t ro ly t i c p r o d u c t i o n of b r o m a t e is usua l ly ca r r i ed out a t a g r a p h i t e anode. H o w e v e r , the use of g raph i t e has u n f a v o r a b l e effects. I t spal ls d u r i n g electrolysis , f o r m i n g a m u d w h i c h m a k e s con t inu ous ope ra t ion difficult. Also, the final p r o d u c t be comes s l igh t ly y e l l o w 1 and can be decolor ized only w i t h difficulty. These two facts a re the m a i n r e a sons for the sea rch for a be t t e r ma te r i a l . A b r i e f i nves t i ga t i on showed tha t the l ead p e r ox ide e lec t rode m a n u f a c t u r e d by a process r e p o r t e d ea r l i e r (2) was the most p rac t i ca l choice for an anode ma te r i a l . A compac t lead p e r o x i d e l a y e r is depos i ted e lect r o ly t i c a l l y f r o m n e u t r a l lead n i t r a t e so lu t ion upon the i nne r su r face of an i ron cy l inde r w h i c h acts as an anode. An e x a m p l e of the ope ra t i ng condi t ions is shown in T a b l e I. Appara tus , used is shown in Fig. 1. D u r i n g e lect rolysis , the ac id i ty of the e l ec t ro ly t e Table I. Operating conditions Electrolyte: 21-22% lead ni t ra te solution. Anode: iron cyl inder wi th surface polished inside; length 500 mm, ID 204 mm, thickness 8 ram, weight 20.6 kg, avai lable surface area 32 dm ~. Cathode: copper rod (diameter 25 mm) Current : 172 amp Current density, anodic: 5.4 amp/dm" Voltage: 7.4 v Temp: 50~ Flow rate of electrolyte: 4 to 5 l / ra in Time: 66 hr 1 This coloration has been considered hitherto to be due to the presence of chromate added to the cell (I). By the use of a lead peroxide anode instead of graphite, colorless crystals were obtained as shown here. 448 g r a d u a l l y inc reased and the concen t r a t i on of Pb § dec reased accord ing to the fo l lowing react ion . 2Pb(NO~)~ + 2H~O-~ PbO~ + Pb + 4HNO~ The change of these two was found to g ive a b r i t t le depos i t of lead p e r o x i d e wh ich could not be cut for f in ishing or not used for an anode of t h e elect rolysis . Moreove r , sudden c h a n g e of pH due to d i rec t n e u t r a l i z a t i o n of acid in t he e l ec t ro ly t i c b a t h g a v e a deposi t consis t ing of he t e rogeneous l ayers wh ich was also br i t t le . There fo re , the pH and the concen t r a t i on of Pb § should be m a i n t a i n e d as cons tan t as poss ib le d u r i n g e lect rolysis . This was accompl i shed by f lowing the e l e c t r o l y t e at cons tan t r a t e and neu t r a l i z i ng the inc reas ing ac id i ty wi th P b ( O H ) ~ by us ing the a p p a r a t u s shown in Fig. 1. The e lec t ro lys is was conduc ted un t i l t he th ickness of t he deposi t r e ached about 8 ram. Each end of the m o t h e r cy l inde r 75 m m in l eng th was cut off; t hen by cu t t ing the i ron pa r t of t he cy l inde r l eng thwise , a lead pe rox ide cy l inde r of su i t ab le s t r eng th and compac tness as we l l as of a smoo th ou te r su r face was