Brine supply

The brine used in the mercury cell and membrane cell processes is normally saturated with solid salt although there are some installations that use solution-mined brine on a once-through basis. The brine supply for diaphragm cells is always used on a once-through basis, although the salt recovered from caustic soda evaporators may be recycled into the brine supply. 

The basic raw material for the mercury cell and membrane cell processes is usually solid salt. This may be obtained from three sources rock salt, solar salt, or vacuum-evaporated salt from purifying and evaporating solution-mined brine. 

In the United States and Europe, rock salt is most commonly used. The most important impurities are shown in Table 3. The concentrations of these impurities depend on the method of production and on the different grades crude rock salt, prepared rock salt, and evaporated salt. Solar salt is used in Japan and many other parts of the world, the most important sources being Australia, Mexico, China, Chile, India, and Pakistan. The salt produced by solar evaporation is usually much less pure than rock salt. In a few cases the salt may be obtained from other processes, such as caustic soda evaporation in the diaphragm process. 

A new upgrading process (Salex) has been developed by Krebs Swiss [48]. It removes the impurities by selective cracking of the salt crystals and a washing process. Salt losses are minimized, and the purity exceeds 99.95 % NaCl. 

Brine Resaturation. In older plants, the open vessels or pits used for storing the salt are also used as resaturators. The depleted brine from the cells is sprayed onto the salt and is saturated, the NaCl concentration reaching 310 - 315 g/L. Modern resaturators are closed vessels, to reduce environmental pollution [49], which could otherwise occur by the emission of a salt spray or mist. The weak brine is fed in at the base of the resaturator, and the saturated brine is drawn off at the top. If the flow rates of the brine and the continously added salt are chosen carefully, the differing dissolution rates of NaCl and CaS04 result in little calcium sulfate dissolving within the saturator [50]. Organic additives also reduce the dissolution rate of calcium sulfate [51]. The solubility (g per 100 g of H2O) of NaCl in water does not increase much with temperature (t, °C), whereas the solubility of KCl does  

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Failure of chilled brine supply to primary reactor. 

The important point to be taken from this discussion is that the plant designer needs access to the full analytical history of the salt or brine supply. If data are insufficient, collection of new data should begin before or early in the process of plant design. 

T.E O Brien, Brine Supply Requirements in Plant Conversions, 46th Chlorine Institute Plant Operations Seminar, Chicago, IL (2003). 

In hybrid plants that operate diaphragm cells in combination with other types, there are other options for use of the evaporator salt. One of the advantages of diaphragm cells is their ability to operate without penalty on salt supplied in the form of brine. With the other types, a brine supply presents a problem with the water balance. With mercury cells, for example, solid salt is needed to resaturate the depleted brine for recycle. Evaporator salt can fill this need, and, with the right division of production between the two types of cell, it is possible to run both types without a supply of solid salt. Several plants operate this way. With membrane cells, there are other ways to integrate the two types of cell (Section 9.4.1), but the basic idea is that the availability of salt from the evaporators again allows the combination to operate from an all-brine supply. 

Figure 24. Schematic diagram of a brine circulation system in the mercury cell process a) Electrolysis cell b) Anolyte tank c) Vacuum column dechlorinator d) Cooler e) Demister f) Vacuum pump g) Seal tank h) Final dechlorination i) Saturator k) Sodium carbonate tank I) Barium chloride tank m) Brine reactor n) Brine filter o) Slurry agitation tank p) Rotary vacuum filter q) Vacuum pump r) Brine storage tank s) Brine supply tank...

The potassium sulfate and boric acid plants (Fig. 1.66 with capacities of 250,000 and 16,000 mt/yr, respectively) started production in 1998 using a separate brine supply and solar evaporation system. After the brine had left the initial halite ponds all of the potassium and sulfate salts were allowed to crystallize and be harvested together from one set of ponds. In 2002 the first processing step was to leach its halite content, and then convert the residue to schoenite with return liquor from... 

A large reserve of caUche ore bearing iodine is being processed in the Atacama Desert. Production of iodine there is relatively inexpensive. About 40% of the world supply of iodine is made from these Chilean deposits. The process consists of leaching the caUche with water. Brine is stripped of iodine using an organic solvent. The iodine is then removed from the solvent to form a slurry. SoHd-phase iodine is separated from the slurry in conventional flotation cells, dried, and packaged. Details of the process are proprietary. 

Sodium, 22 700 ppm (2.27%) is the seventh most abundant element in crustal rocks and the fifth most abundant metal, after Al, Fe, Ca and Mg. Potassium (18 400 ppm) is the next most abundant element after sodium. Vast deposits of both Na and K salts occur in relatively pure form on all continents as a result of evaporation of ancient seas, and this process still continues today in the Great Salt Lake (Utah), the Dead Sea and elsewhere. Sodium occurs as rock-salt (NaCl) and as the carbonate (trona), nitrate (saltpetre), sulfate (mirabilite), borate (borax, kemite), etc. Potassium occurs principally as the simple chloride (sylvite), as the double chloride KCl.MgCl2.6H2O (camallite) and the anhydrous sulfate K2Mg2(S04)3 (langbeinite). There are also unlimited supplies of NaCl in natural brines and oceanic waters ( 30kgm ). Thus, it has been calculated that rock-salt equivalent to the NaCl in the oceans of the world would occupy... 

The salts content of soils may be markedly altered by man s activities. The effect of cathodic protection will be discussed later in this section. Fertiliser use, particularly the heavy doses used in lawn care, introduces many chemicals into the soil. Industrial wastes, salt brines from petroleum production, thawing salts on walks and roads, weed-killing salts at the base of metal structures, and many other situations could be cited as examples of alteration of the soil solution. In tidal areas or in soils near extensive salt deposits, depletion of fresh ground-water supplies has resulted in a flow of brackish or salty sea water into these soils, causing increased corrosion. 

An American Salt Company plant and the Dow Chemical Company s Midland plant also benefit directly from each other s presence. Dow found that after recovering bromine from brine it had more salt left than it desired. American Salt needed salt. By locating next to Dow s plant it was able to buy this salt stream for less than it would cost to mine it or pump it from natural underground reservoirs. In turn, Dow was able to sell an unwanted stream that it would otherwise have had to pump back into the ground. The American Salt plant is typical of many satellite plants. These are plants that either use a by-product or a waste stream from another plant or are built mainly to supply a needed chemical to an adjacent plant. The nearby presence of another plant determines their location. 

When ion-exchange resin containing zirconium hydroxide comes into contact with acidic brine in the RNDS , zirconium hydroxide adsorbs bisulphate ions thus sulphate is removed from brine. For regeneration of the ion-exchange resin, a basic solution is supplied and when it comes into contact with the resin, sulphate desorption starts. 

In the operating commercial plant, columns are filled with ion-exchange resin containing zirconium hydroxide. Acidic brine is supplied to the column from the bottom, where bisulphate ions are adsorbed, by way of a fluidised bed arrangement, based on the equations ... 

Upon completion of adsorption over a pre-set time, brine drains from the column. To recover the salt adhering to resin, water is supplied to the resin column from the top. 

The sulphate adsorption and desorption processes run cyclically by supplying brine and water to the resin column under automatic control. A number of optional operation parameters are available. 

Ion-exchange resin, containing zirconium hydroxide, is packed into the column to which brine containing iodide is supplied from the top. The observed sequence by rate of removal is iodide ion [Pg.171]

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