Sulfate control purge

Selection of salt chemical treatment ion exchange brine purge choice of alternative process for sulfate control... 

The major anionic impurity in most brine systems is sulfate. Control of its concentration is an issue mostly in membrane cells. In the diaphragm-cell process, sulfate passes with the rest of the anolyte into the cathode side of the cells. It can be separated from caustic soda in the evaporators and purged from the system as Glauber s salt This is covered in Section 9.4.2.1. Mercury cells are least sensitive to sulfate. Its concentration is frequently allowed to build to the point where dissolution of calcium sulfate from the salt is inhibited. The greatest problem then caused by the sulfate is a reduction in the solubility of NaCl or KCl. 

Concentrated Purge. New methods of sulfate control have been developed in response to the membrane-cell brine problem. These include a novel application of the familiar ion-exchange technique and a process based on the relatively new technique of nanofiltration. The processes use physical or chemical means to make a partial separation between chloride and sulfate the problem of disposal of the sulfate remains. By concentrating the sulfate and removing most of the chloride, they may allow safe, legal, and economic disposal of the sulfate by a simple purge. In other cases, their value lies in providing a much smaller stream to be treated for the ultimate disposal of the sulfate (e.g., by precipitation). 

As the solution contacts the gases in the absorber, a moderate amount of the circulating solution is oxidized to non-regenerable sulfate by the oxygen or sulfur trioxide in the gas. To control the sulfate level, a small stream of the solution is purged from the system. This purge stream... 

Control Method—Commercial Experience Sodium sulfate concentration in the solution-mined brine is suppressed by additives that reduce the solubility of anhydrite. Sodium sulfate can be precipitated from brine with barium or calcium ions, and its concentration can be controlled by purging the brine system (see Chapter 7.5.7.2A for additional details). 

The quantities of chlorate produced are not great. The rate of formation of sodium chlorate in most membrane electrolyzers is of the order of 1 or 2 kg of NaClOs per ton of chlorine produced. Allowable concentrations are above 10 gpl NaClOa. The specification then usually can be met by purging a small part of the total brine flow, and this is a popular method. Most plants that purge brine in order to control the sulfate concentration have incidentally avoided a chlorate-accumulation problem. 

IE. Effects of Sulfate. Excessive amounts of sulfate in the brine can lead to the precipitation of certain sulfate compounds in the carboxylic layer of the membrane. This reduces the current efficiency. Therefore, a maximum sulfate level of 8gplNa2S04 in the feed brine is recommended. The sulfate concentration can accumulate to this level unless it is controlled by purging or by some removal process. Section 7.5.7 discusses a number of candidate processes. 

Finally, too high a concentration of SO2 has an effect somewhat similar to that of HCN, and leads to excessive base consumption and the formation of sulfite, sulfate, or thiosulfate salts. In most cases, a normal solution purge, coupled with good pH control, is sufficient to prevent excessive sulfur salt buildup. 

---