Sodium sulfate electrolysis

But the conventional membrane setup described is only one possibility of a sodium sulfate electrolysis. Other proposals include the use of an ODC or even an HDA (Hydrogen Depolarized Anode) for cell voltage reduction or the use of anion exchange membranes instead of the cation variety [7]. Last but not least electrodialysis may be used in a three compartment unit the salt is converted into free acid and the free base [40]. 

Substantial savings of money, which otherwise would have been required for disposal of by-product salts. In the specific case of sodium sulfate, electrolysis becomes economically interesting when the disposal cost of the salt is in the order of 100 per tonne, with a caustic soda price in the range of 200 per tonne. 

Figure 101. Flow diagram of sodium sulfate electrolysis in two-compartment electrolyzers (production of acid sodium sulfate, oxygen-evolving anodes)...

Figure 103. Flow diagram of sodium. sulfate electrolysis in three-compartment electrolyzers (bufler compartment) MC = Cation-exchange membrane...

Voltage (V)-current density (CD) relationships for three internal structures of the elementary cell for sodium sulfate electrolysis ... 

With a porous diaphragm, it is difficult to achieve efficient separation of the acidic anolyte from the alkaline catholyte. The result is a significant loss of current efficiency. Also, the choice of a diaphragm resistant to both sulfuric acid and caustic soda and the selection of an anode stable in sulfuric acid solution are limited. These difficulties have retarded the development of sodium sulfate electrolysis. 

These anodes are used extensively in electrogalvanizing, tin electroplating, electrochemical production of copper foil for printed circuit boards, and electrowinning of copper and zinc [91-95]. The application of oxide-coated anodes to sodium sulfate electrolysis so far is small and is not a major driver of electrode development programs. However, environmental concerns associated with byproduct or waste sodium sulfate, along with possible imbalances in the demand for chlorine and caustic soda are enough to maintain interest in the technique. 

Magnesium is reduced from a mixture of magnesium, calcium, and sodium chlorides. Electrolysis from aqueous solution is also possible zinc, copper, and manganese dissolved as sulfates in water can be reduced electrolytically from aqueous solution. 

Electrolysis of aqueous solutions of the following using inert electrodes sodium chloride, copper(n) sulfate, sodium sulfate and sodium hydroxide. 

Calculate the time required to produce 2.50 g of hydrogen by the electrolysis of water (containing sodium sulfate to carry the charge) with a current of 5.00 A. 

Hydrochloric acid is a convenient acid for laboratory-scale electrolysis it is easy to remove during the workup by evaporation. The oxygen-containing acids, such as sulfuric or perchloric acid, must be neutralized before basic products can be extracted. Sulfuric acid is most conveniently neutralized by concentrated ammonia rather than by sodium hydroxide, whereby the precipitation of sodium sulfate is avoided. 

The electrolysis of water, shown in Figure 16, leads to the overall reaction in which H2O is broken down into its elements, H2 and O2. Pure water does not have enough ions in it and is not conductive enough for electrolysis. An electrolyte, such as sodium sulfate, must be added. The Na (a ) and SO aq) ions play no part in the electrode reactions, which are as follows ... 

Into the cell is then placed a solution of 15 g. sodium sulfate. 15. g. boric acid and 14 g. of salicylic acid (0.1 mole), just neutralized with the calculated amount of sodium hydroxide. The solution is then diluted to, 175 cc. All the boric acid not dissolve in this quantity of solution, but is kept in suspension by means of rapid mechanical stirring. The cell is placed in a cooling mixture, and when the temperature reaches 15° to 18° the current is turned on. A temperature of 15° to 18° is maintained throughout the experiment. A current of 3 amp. (6 amp. per sq. dm.) is then passed through the solution for a period of 1 hr. 55 min., which is slightly more than the calculated amount (5.4 amp.-hr.) necessary to reduce the salicylic acid to salicylic aldehyde. During the electrolysis 20 g. sodium bisulfite are added at the rate of about 1.5 g. every 10 min. It has been found best not to begin the addition of the sodium bisulfite until the electrolysis has been started about 5 min, since the bisulfite reduced to sulfur when added to soon, or too rapidly thereafter. 

In addition to the reduction of sodium sulfate, an electrolysis process is operated in which a sodium polysulfide solution (from sodium sulfide solution and... 

In the electrolysis of aqueous sodium sulfate using inert electrodes, we observe the following. 

Hydrochloric acid is a waste by-product from the chlorination of many organic compounds. In certain cases, it would be desirable to recover the chlorine from HC1 by electrolysis of the acid. Sodium sulfate is also a common by-product of many commercial processes. This material can be converted to caustic soda and sulfuric acid by electrolysis of Na2S0i, as shown in Figure 4. 

The following example deals with the electrolysis of an aqueous solution of sodium sulfate (Na2S04). 

Moissan and Lebeau (1901) produced sulfuryl fluoride by the combination of sulfur dioxide with fluorine (217). Other processes which have been used to produce the gas are (a) the thermal decomposition of barium fluorosulfonate or certain other fluorosulfonates (188, 221, 808), (b) the reaction of sulfur dioxide with chlorine and hydrogen fluoride in the presence of activated charcoal at 400° (11), (c) the reaction of sulfur dioxide and chlorine with potassium or sodium fluoride at 400° (328), (d) the disproportionation of sulfuryl chlorofluoride at 300-400° (328), (e) the reaction of sulfuryl chloride with a mixture of antimony trifluoride and antimony pentachloride at about 250° (86), (f) the reaction of sulfur dioxide with silver difluoride (86), (g) the reaction of thionyl fluoride with oxygen in an electrical discharge (314), (h) electrolysis of a solution of fluorosulfonic acid in hydrogen fluoride (264), ( ) the reaction of fluorine with sodium sulfate, sodium sulfite or sodium thiosulfate (229, 239), (j) the reaction of hydrogen fluoride with sulfuryl chloride (820). 

However, disposal will become increasingly difficult, if not impossible, in the near future due to tighter environmental regulations. In any case, it will certainly become increasingly expensive and for this reason, at least, the recovery by electrolysis of sodium sulfate—and in general of other neutral salts, such as sodium nitrate and sodium chloride, which are also generated in some chemical processes—is expected to have a bright future. 

Structure of the Electrolyzers. The following discussion is based on sodium sulfate, which is the by-product salt of greatest concern [228], [229]. However, many of the conclusions also apply to the electrolysis of other salts such as potassium sulfate and sodium and potassium nitrate. 

With two-compartment cells the cathodic efficiency also represents the total current efficiency of the process. In the three-compartment cells, the current efficiency for sulfuric acid production (anodic efficiency) must also be taken into consideration. The overall current efficiency of electrolysis is represented by the lower of the two efficiencies. If, for example, the anodic efficiency is lower than the cathodic, acidity builds up in the sodium sulfate compartment during operation so that the cathodic current efficiency progressively decreases when it reaches the same value as the anodic, stationary process conditions are maintained. Therefore, the process is provided with a self-regulating mechanism. Alternatively, if the accumulation of acidity in the sodium sulfate compartment cannot be allowed, then a portion of the caustic soda produced can be fed into the sodium sulfate loop. The quantity of caustic soda available for use outside the electrolysis plant again represents the overall current efficiency, which still remains a function of the anodic efficiency. The anodic and cathodic current efficiencies can be made independent when the control of acidity in the sodium sulfate compartment is performed by means of sodium carbonate addition. 

Auxiliary Sections. The electrolysis plant may contain auxiliary units necessary for functional integration with the upstream plants where sodium sulfate is generated and the electrolysis products are fed back. 

As a purely indicative example, an electrolysis plant based on three-compartment cells is considered, having a capacity of3000 t/a of sodium sulfate (100 % basis) fed to the cells as a 20 % solution The investment for a doubleeffect evaporation unit represents about 10-15% of the total investment for the complete plant, comprising cells, rectifier, pretreatment unit, evaporation, and construction. The cost of steam amounts to about 50% of the total electric energy cost. 

Before beginning an economic analysis to define the most convenient trade-oflF between evaporation and electrolysis cost, the upstream plants where sodium sulfate is generated should be inspected carefully to determine whether the dilute solutions are formed as such or are the result of mixing with other process wastewaters. Internal dilution is often a consequence of the old attitude toward by-product salt solutions, which were diluted as much as possible within the battery limits of the plants to reach the limit allowed for discharge. 

Recovery of Caustic Value. Byproduct sodium sulfate is not a readily marketable commodity. The United States alone generates more than 1.5 million tons per year, and those chlor-alkali plants which sell the material may not recover even the cost of crystallization [166]. Modified membrane cells that regenerate caustic soda and sulfuric acid by electrolysis of Na2S04 solution therefore have been studied. These reverse the neutralization reaction and regenerate the acid and base ... 

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