The Brine Treatment Process

This route, properly controlled, is used in the commercial production of sodium bicarbonate. When carbonate is the desired product, as in our application, bicarbonate should be avoided for two reasons. First, it reacts with caustic added later in the brine treatment process, leading to the back-formation of carbonate ... 

This complicates the control of the brine treatment process. Second, NaHC03 is less soluble than the carbonate, and its formation can allow solids to deposit. The problems of bicarbonate formation disappear when the ratio of caustic soda to CO2 in the combined feed to the process is correctly maintained. 

Nearly all evaporator salt is redissolved for use in chlor-alkali manufacture. Its designation as CP or chemically pure salt reflects its high quality. Hardness elements are largely absent, having been removed in the brine treatment process. The sulfate content may be reduced by treatment described below. On the other hand, chlorate formed in the cells and small amounts of corrosion products will have appeared. 

The sulfate accompanying NaCl brine is sometimes recovered as a by-product. In the diaphragm-cell process, sulfate is available as a concentrated purge from the evaporators. In the membrane-cell process, there is no natural point of high sulfate concentration, but it is possible to isolate sodium sulfate in the brine treatment process (Section 7.5.7.2B). 

Process water for brine treatment and catholyte adjustment may also contain impurities. On the catholyte side, the likely impurities are silica from the water supply and iron and nickel from metallic corrosion. However, the catholyte is removed continuously from the system, and while the presence of impurities may be of concern, they are not permitted to accumulate to high concentrations. On the other side, the anolyte is recirculated between the brine-treatment plant and the electrolyzers, allowing the accumulation of trace impurities to undesirable levels. 

Finally, the backwash containing the solids must be dealt with. After collection in a tank, the fluid can be returned to the brine treatment system. To avoid disturbing the process with sudden large flows, the collection tank should be used as a surge vessel and the fluid returned to the process at a more nearly uniform rate. It may go to the clarifier or, preferably, to a treat tank. The collecting tank often has a conical bottom to allow some of the solids to settle out. Some solid material will remain in the supernatant liquid, but continued particle growth will allow it ultimately to settle in the clarifier. 

Precipitation. Sulfate can be precipitated by either the barium or calcium ion, and both are used commercially. Turning first to barium, we note that the solubility of BaS04 is so low that its precipitation is the standard gravimetric technique for analysis for the sulfate ion. In a brine loop, the precipitation step could be located anywhere, but it is most convenient to combine it with the precipitation of metals in the brine treatment tanks (Section 7.S.2.2) and not to add another step to the process. 

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). 

Brine Preparation. Sodium chloride solutions are occasionally available naturally but they are more often obtained by solution mining of salt deposits. Raw, near-saturated brines containing low concentrations of impurities such as magnesium and calcium salts, are purified to prevent scaling of processing equipment and contamination of the product. Some brines also contain significant amounts of sulfates (see Chemicals FROMBRINe). Brine is usually purified by a lime—soda treatment where the magnesium is precipitated with milk of lime (Ca(OH)2) and the calcium precipitated with soda ash. After separation from the precipitated impurities, the brine is sent to the ammonia absorbers. 

In the recovery of cadmium from fumes evolved in the Imperial Smelting process for the treatment of lead—zinc concentrates, cadmium is separated from arsenic using a cation-exchange resin such as Zeocarb 225 or Ambedite 120 (14,15). Cadmium is absorbed on the resin and eluted with a brine solution. The cadmium may then be recovered direcdy by galvanic precipitation. 

The sodium chlorate manufacturing process can be divided into six steps (/) brine treatment 2 electrolysis (J) crystallisation and salt recovery (4) chromium removal (5) hydrogen purification and collection and (6) electrical distribution. These steps are outlined in Figure 3. 

Texture In the figure 2 the values of firmness along the process for two consecutive seasons is shown, very similar results were found for both seasons. The most important decreases took place during the lye treatment and subsequent wash step, reaching values of 50% of the initial. When the fruits were placed in the brine solution the firmnes recovered to 80% of the initial and, finally, during fermentation there was a new decrease to 60% of the initial. 

Abstract This chapter discusses the characteristics of membrane concentrate, and the relevance that the concentrate has on the method of disposal. Membrane concentrate from a desalination plant can be regarded as a waste stream, as it is of little or no commercial benefit, and it must be managed and disposed of in an appropriate way. It is largely free from toxic components, and its composition is almost identical to that of the feed water but in a concentrated form. The concentration will depend on the type of desahnation technology that is used, and the extent to which fresh water is extracted from the brine. Based on the treatment processes that are used, a number of chemicals may also be present in the concentrate, albeit in relatively small quantities. 

In most commercial processes, the compound is either derived from the sea water or from the natural brines, both of which are rich sources of magnesium chloride. In the sea water process, the water is treated with lime or calcined dolomite (dolime), CaO MgO or caustic soda to precipitate magnesium hydroxide. The latter is then neutralized with hydrochloric acid. Excess calcium is separated by treatment with sulfuric acid to yield insoluble calcium sulfate. When produced from underground brine, brine is first filtered to remove insoluble materials. The filtrate is then partially evaporated by solar radiation to enhance the concentration of MgCb. Sodium chloride and other salts in the brine concentrate are removed by fractional crystallization. 

The brine from the electrolytic cells is often contaminated with mercuric chloride due to the oxidation of the mercury cathode. As a waste treatment process, in 1963, G.E. Edwards passed the contaminated brine through a cell consisting of a graphite anode and a steel wool cathode. The mercury metal deposited on the steel wool. 

Iodine can also be produced from brine. This process (Fig. 2) consists of cleaning the solution (of clays and other materials), adding sulfuric acid to a pH <2.5 followed by treatment with gaseous chlorine ... 

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