Process control brine purge

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

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. 

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). 

Depleted brine will be physically saturated with chlorine, and some chlorine wUl react to form hypochlorite (Section 7.5.9.1). This chlorine value represents an economic asset to be recovered and, particularly in the case of membrane cells, an intolerable contaminant in the brine treatment system. There are several approaches to this problem [208], and we cover these below. We divide them into methods aimed at recovery of the bulk of the chlorine in a useful form (primary dechlorination Section 7.5.9.2) and those whose purpose is to reduce the active chlorine to chloride and safeguard the environment or other parts of the process (secondary dechlorination Section 7.5.9.3). Some of the hypochlorite that forms in the anolyte will continue to react to form chlorate. This is a much less harmful impurity in the cells, and higher concentrations are tolerable. Many plants keep the chlorate concentration under control by natural or deliberate purges from the brine system (Section 7.5.7.2A). In others, it is necessary to reduce some of the chlorate ion to chloride in order to maintain control (Section 7.5.9.4). 

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