Membrane Electrolysis of Borax Pentahydrate (Na₂B₄O₇.5H₂O)

Abstract

In the present work, membrane electrolysis of nearly saturated borax solution prepared from borax pentahydrate (Na 2 B 4 O 7 .5H 2 O) was studied. Mono layer cation-exchange membrane (Nafion 551) was used as a separator in a two-compartment electrolysis cell for the production of sodium hydroxide and boric acid. The anolyte concentration was adjusted in order to obtain a Na 2 O/B 2 O 3 mol ratio of 0.1 by dissolving boric acid and borax pentahydrate together. Experiments were performed in continuous operation mode by feeding nearly saturated borax solution. The effects of different catholyte concentrations (10, 20, and 30% NaOH) on the current efficiency and the specific energy consumption were calculated and the quality of boric acid products was determined.Membrane electrolysis is one of the processes used in the chloralkali industry. The industry is one of the largest industries producing annually more than 48 million tons of sodium hydroxide (NaOH) and more than 42 million tons of chlorine (Cl 2 ). 1 The raw material is sodium chloride (NaCl) and the reaction proceeds according to:Chlorine gas, caustic soda, and hydrogen gas are produced simultaneously in the chlor-alkali process. Approximately 0.9 tons of chlorine gas is produced together with a ton of sodium hydroxide. Generally, sodium hydroxide production cannot be increased to meet the demand, because a market cannot be found for all the chlorine gas produced. Therefore, it is thought that sodium hydroxide could be produced from other sodium salts, i.e. sodium borates. 2 On the other hand, current boric acid production processes have some disadvantages such as high consumption of sulphuric acid and accumulation of a large amount of gypsum wastes. 3 In conventional boric acid production process, colemanite mineral (Ca 2 B 6 O 11 .5H 2 O) is reacted with sulfuric acid solution at 88-92 • C (Eq. 2).[2] Precipitated gypsum is removed by filtration and boric acid is crystallized by adiabatic evaporation of the filtrate. 4 Approximately, 400.000 tons of gypsum waste (on dry basis) is generated in Turkey every year by boric acid production. 5 To overcome the environmental problems existing in the conventional production methods of boric acid and sodium hydroxide and to meet the demand of sodium hydroxide in Turkey, membrane electrolysis process was applied to sodium borate solution. In the proposed process, boric acid and oxygen are produced in the anodic compartment, sodium hydroxide and hydrogen are produced in cathodic compartment. The anode compartment provides Na + ions for the reaction and Na + ions penetrate through the membrane from the anode to the cathode compartment. The reactions are given as follows: 2,8 Dissolution and Hydrolysis:Anodic Reaction:Cathodic Reaction:] Total Reaction: Overall : Na 2 B 4 O 7 · 5H 2 O + 3H 2 O → 4H 3 BO 3 + 2NaOH + 1/2O 2(g) + H 2(g) [6] z E-mail: iffet.elbeyli@tubitak.gov.trAccording to the stoichiometry, 3.5 tons of boric acid is produced together with 1 ton of sodium hydroxide. 2 The ion exchange membrane is one of t...

The electrochemical production of boric acid

“…In previous studies different Nafion membranes were tested for the electrochemical production of boric acid. In these studies, the specific energy consumption of the cell was calculated as 2535 kWh ton −1 (6.2 V, 1.2 kA m −2 ) and 2630 kWh ton −1 (6 V, 1.2 kA m −2 ), for Nafion®424 and Nafion®551, respectively …”

“…In these studies, the specific energy consumption of the cell was calculated as 2535 kWh ton −1 (6.2 V, 1.2 kA m −2 ) and 2630 kWh ton −1 (6 V, 1.2 kA m −2 ), for Nafion ® 424 and Nafion ® 551, respectively. 15,16 The process flow diagram for the electrochemical process of boric acid The process flow diagram and the required equipments for the electrochemical process of borax pentahydrate are given in (Fig. 4).…”