Electrochemistry allows the decomposition of salts into their acid and base components. In the case of this process using a cation exchange membrane, the cation (such as Na+ or K+) of the salt dissolved in the anode electrolyte is forced into the cathode electrolyte by an electric current through the membrane. The cation in the anode electrolyte is replaced by hydrogen ions produced by anodic oxidation of water. Electrode products hydrogen and oxygen are unintended byproducts of this process. The main advantage of the electrochemical process over conventional acidification using sulfuric acid is the elimination of sodium sulfate as a by-product. As a result, a higher purity acid is obtained and the yield loss involved in separating sodium sulfate from the product acid is avoided. The method works with any acid, especially weak acids, which are made from their salts. Examples are chromic, boric, and salicylic acids. See also section 5.3.
The process is used on a commercial scale to produce chromic acid from sodium dichromate. Interestingly, the main driver of new technology adoption is environmental protection. Even chromic acid contaminated sodium sulfate is difficult to dispose of. It can be said that the purity and yield difference of main products is not as serious as the purity difference of waste products.
The process of making chromic acid begins with the alkaline roasting of chromium ore in a rotary kiln. Air is used to oxidize the trivalent chromium in the ore to a hexavalent state in the form of sodium chromate. It is leached with water to produce a concentrated solution of sodium chromate. Half of its sodium content can be cheaply removed by acidification with carbon dioxide under pressure. Although carbon dioxide is a fairly weak acid, the process is driven by the low solubility of the sodium bicarbonate formed. Filtration removes sodium bicarbonate and heat converts it to sodium carbonate. The sodium carbonate is sent back to the rotary kiln, closing half the alkalinity loop.
In this respect, the electrochemical route differs from the older technique of sulfuric acid acidification. The sodium dichromate solution is fed into the anode chamber of the electrolytic cell. The current forces sodium ions through the membrane into the cathode chamber, where they can form sodium hydroxide, as in the electrolysis of NaCl. However, the low pH of the anode electrolyte precludes the use of carboxylic acid/sulfonic acid double membrane, which allows the production of 30-35% NaOH at high CE. In contrast, sulfonic acid membranes produce 15% NaOH at about 85% CE. To avoid the expensive evaporation required to return this dilute solution to the alkalinity loop, it is preferred to use sodium dichromate solution as the cathode electrolyte (i.e., sodium dichromate solution as the anode electrolyte and cathode electrolyte feed). As sodium ions pass through the membrane and enter the cathode chamber, the cathode electrolyte is gradually converted to sodium chromate and the anode electrolyte to chromate. The sodium chromate formed in the cathode electrolyte is then recycled back to the carbon dioxide acidification process, resulting in a solid form of alkalinity. This closes the second half of the alkalinity circuit.