Brine recycling

Horton, B.S. 1997. Water, chemical and brine recycle or reuse applying membrane processes. Aust. J. Dairy Technol. 52, 68-70. 

As previously mentioned, the mercury cellroom at Runcorn is operated as a waste brine process with some of the brine recycled through the cells. This process involves a trade-off between electricity and brine costs since with increased brine flow the cells operate at a higher average concentration, which saves power. 

Figure 4 is a diagram of the experimental unit. In its operation the two-phase feed of brine and methylene chloride enters the flash column. Ice slurry forms in the upper part of the column as the methylene chloride vaporizes, and the slurry is transferred by the slurry pump to the glass tee at the bottom of the separation column. In the glass tee, the ice floats into the separation column, where it is washed and removed. The brine recycles from the bottom of the separation unit to the ice generator by joining a stream of fresh feed and liquid refrigerant. 

Some of the limitations to the use of ion exchange include the production of a highly concentrated waste by-product stream that poses a disposal problem (this problem can be reduced by brine recycling). Run length is affected by sulfate level. The technology is only recommended primarily for small groundwater systems with low sulfate and low TDS. Another limitation to its use is that it requires a high level of operator skill and therefore not popular with many end-users. 

Other contaminants found in brine as produced include sulfate and a group of less common impurities that require treatment in some plants. Finally, brine treatment must deal with certain byproducts of cell operation that must be removed from brine recycle systems. These include dissolved chlorine (hypochlorite) and chlorate. The latter forms... 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

The discussion of brine purification so far has dealt with the removal of impiuities that enter the plant accidentally or with the salt, the process water, or auxiliary materials. In mercury- and membrane-cell plants, the partly exhausted brine, or depleted brine, that leaves the cells must be recovered and resaturated for recycle to the cells. With those technologies, therefore, impurities that form or accumulate in the cells or the brine recycle loop are also important. 

Reuse of existing facilities requires removal of contained mercury but not dismantling and detoxification of the debris. Demercurization of brine treatment systems, for example, is straightforward and sometimes quite easy to apply. TTiis is particularly so when the brine recycle system has been operated with a low level of dissolved chlorine to prevent mercury precipitation. Lott [19] reports that such a system was drained, washed with dilute acid to remove deposits and then with dilute hypochlorite solution to solubilize mercury, and finally flushed with hot water. The residual mercury concentration was well within the specification for the new membrane cells. 

The reaction is initiated with nickel carbonyl. The feeds are adjusted to give the bulk of the carbonyl from carbon monoxide. The reaction takes place continuously in an agitated reactor with a Hquid recirculation loop. The reaction is mn at about atmospheric pressure and at about 40°C with an acetylene carbon monoxide mole ratio of 1.1 1 in the presence of 20% excess alcohol. The reactor effluent is washed with nickel chloride brine to remove excess alcohol and nickel salts and the brine—alcohol mixture is stripped to recover alcohol for recycle. The stripped brine is again used as extractant, but with a bleed stream returned to the nickel carbonyl conversion unit. The neutralized cmde monomer is purified by a series of continuous, low pressure distillations. 

Mercury cells are operated to maintain a 21—22 wt % NaCl concentration in the depleted brine and thus preserve good electrical conductivity. The depleted brine is dechlorinated and then resaturated with soHd salt prior to recycling back to the electroly2er. 

Eig. 19. CME monopolar electrolyzer a, membrane b, cathode element c, half-cathode element d, current distributor e. Teflon tube f, CI2 + depleted brine manifold g, conductor rod h, CI2 + depleted brine outlet nozzle i, base frame j, recycled NaOH manifold k, recycled NaOH inlet nozzle 1, gasket (the gasket-to-element ratio is quite small) m, tie rod n, anode element o, H2 + NaOH manifold p, end plate, q, under cell bus bar (simplifies piping... 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

The commercial recovery of iodine on an industrial scale depends on the particular source of the element.Erom natural brines, such as those at Midland (Michigan) or in Russia or Japan, chlorine oxidation followed by air blowout as for bromine (above) is much used, the final purification being by resublimation. Alternatively the brine, after clarification, can be treated with just sufficient AgNOs to precipitate the Agl which is then treated with clean scrap iron or steel to form metallic Ag and a solution of EeU the Ag is redissolved in HNO3 for recycling and the solution is treated with CI2 to liberate the h ... 

Concentrate recycle RO plants allow some of the brine reject water to recycle back through the plant, which improves the permeate recovery rate. (The reduced flow of brine reject water does of course have a proportionally higher TDS level.) Various types of high pressure, corrosion-resistant pumps are used, including multistage, centrifugal and plunger pumps, each with their own benefits and area of application. 

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