Cell Brine Systems

Evaporator salt, if not otherwise contaminated during its processing, could be used as a low-sulfate feed to membrane cells. However, there is a more effective method of integration in which the diaphragm cells serve as a purge for the membrane-cell brine system. Section 9.4.1 also covers this topic. 

Removal of Sulfate from Brine. It is the recycle of brine around a continuous system that causes the sulfate problem. With no escape from the brine loop, any amount of sulfate in the incoming salt or brine will eventually build up to the point where the concentration specification is violated. What sulfate enters a membrane-cell brine system must therefore be removed. This means that a purge from the brine system is necessary, and the techniques used range from a simple purge of the brine itself to more elaborate schemes of precipitation or crystallization. Between these extremes are new techniques developed specifically as a response to the membrane-cell sulfate problem. 

T.F. O Brien, Control of Sulfates in Membrane-Cell Brine Systems. In K. Wall (ed.). Modem Chlor-Alkali Technology, vol, 3, Ellis Horwood, Chichester (1986), p. 326. 

When some of the salt is normally transferred to membrane or mercury cells, prudent design will also allow its occasional use in the diaphragm circuit. In the membranecell version, this facility can be a simple branch in the slurry-transfer system. In the mercuty-cell case, the use of separate tanks and transfer pumps and lines will prevent contamination of the diaphragm-cell brine system with mercury. 

This chapter concentrates on the mercury technology with waste brine system, as used at Runcorn and in particular a mathematical model of a mercury cell which is being used to improve the operability and efficiency of chlorine production. In general, however, the use of the techniques described here is applicable to all cell technologies. 

Typical areas where titanium has found widespread industrial use in membrane technology are cells, anodes, anolyte headers, anolyte containers, filters, heat exchangers, chlorate removal systems and various parts of the brine system. 

After listening intently, the operating foreman explained that mere traces of chrome salts in the brine system could create an explosive situation within the electrolytic chlorine cells. Traces of chrome salts in the feed brine to the chlorine cells liberate hydrogen gas in the chlorine cell gas. Hydrogen in the chlorine cell gas has a very wide explosive range. Installation of stainless steel equipment in sodium chloride brine systems has devastated chlorine processing equipment within other similar chlorine manufacturing plants. The maintenance foreman had the improper pump impeller removed immediately before any problems occurred. 

A more permanent and less troublesome alternative was simply to separate the diaphragm and mercury cell resaturation systems by installing an additional dissolver or by drilling additional brine wells. 

A block diagram of the apparatus is shown in Figure 1. The system is constructed to use three sodium chloride anolyte and four sodium hydroxide catholyte concentrations. The starred anolyte compartments refer to separate solutions which have been doped with radiotracer. These solutions are used only for determinations of transport number the nonradioactive brine solutions are used for system flushing and membrane equilibrations. Solutions are selected and pumped into the cell, under computer control, through an all-Teflon pump-valve system. The solutions are heated during these transfers to ensure rapid attainment of experimental temperature in the cell. The brine system is designed to enable the return of radiotracer solutions to their storage vessels after each use. This serves to reduce consumption of radioactive solutions. 

Impurities in brine affect electrode reaction kinetics, cell performance, the condition of some cell components, and product quality. Treatment of brine to remove these impurities has always been an essential and economically significant part of chlor-alkali technology. The brine system typically has accounted for 15% or more of the total capital cost of a plant and 5-7% of its operating cost. The adoption of membrane cells has made brine specifications more stringent and increased the complexity and eost of the treatment process. Brine purification therefore is vital to good electrolyzer performance. This section considers the effects of various impurities in all types of electrolyzer and the fundamentals of the techniques used for their control. Section 7.5 covers the practical details of the various brine purification operations. 

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. 

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). 

In the diaphragm-cell process, the treated brine is less than saturated with NaCl. The recovered salt is used to saturate this stream before it flows to the cells. This consumes only a portion of the available salt. The rest also is dissolved for feed to the cells. This requires the addition of water to the process. Recovered evaporator condensate, like the salt, is soft and will add few impurities to the brine system. Since minimal amounts of hardness enter with the recovered salt and the condensate, many plants use this brine without further chemical treatment. In this case, the two functions of dissolving all the salt while resaturating the treated brine obviously can be done in a single step. 

In any event, there is a need to bring together the salt produced and the stream requiring saturation. The usual approach is to transport the salt as a slurry to the appropriate brine system. In an all-diaphragm plant, the carrier fluid may be treated brine, evaporator condensate, or a mixture of the two. Condensate dissolves much of the salt and therefore reduces the size of the stream to be transferred. In a combined plant, the transfer fluid is the depleted and dechloiinated brine from the non-diaphragm cells. 

If capacities are imbalanced toward the diaphragm side, some of the brine from the primary treatment must flow to the resaturator. If the membrane-cell capacity is high, some of the resaturated brine can be sent to those cells. This re-establishes the recycle loop, but the diaphragm cells remain as a gigantic purge from the brine system, and recycle accumulation of impurities still is strongly limited. 

The brine system should be fully operational, on recycle and bypassing saturation and the electrolyzers, at a rate equivalent to that required for operation at 2-3 kA m. The brine temperature should be about 50°C at the cell room. Concentration should be as specified for normal operation (typically 300 gpl), and quality should meet all specifications. Analytical routines for the brine system should be in full operation by this time. 

The mercury removed from the wastewater is dissolved in the regenerating acid. It can return to the cells by way of the brine system. 

Current is fed into the electrolyzer by means of anodic and cathodic end elements. The anodic compartment of each cell is joined to an independent brine feed tank by means of flanged connections. Chlorine gas leaves each cell from the top, passing through the brine feed tank and then to the cell room collection system. Hydrogen leaves from the top of the cathodic compartment of each cell the cell Hquor leaves the cathodic compartment from the bottom through an adjustable level connection. 

Fig. 12. OxyTech Systems MDC cell a, brine feed rotometer b, head sight glass c, cell head d, cathode assembly e, tube sheet f, grid plate g, cathode...

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