Brine purification calcium

Influence of Electrolysis Conditions. Among the various electrolysis conditions, brine purity has the most significant effect on the life of the membranes. The presence of a small amount of multivalent cations leads to formation of metal hydroxide deposits in the membrane, and thus causes a decrease in current efficiency, an increase in cell voltage, and damage to the polymer structure of the membrane. With perfluorocarboxylic acid membrane, the presence of more than 1 ppm of calcium ion will begin to cause these problems in a very short period (1 - 8). To obtain stable current efficiency and cell voltage, it is therefore essential to establish effective brine purification methods. 

With this wide variety of sources, one would expect to find great variations in the type and abundance of impurities in salt. Still, there are a number of useful generalizations. Table 7.6 shows that calcium, magnesium, and sulfate ions are the other major components of seawater and therefore the major impurities in most salts [63], and the removal or eontrol of these constituents is a primary objective of the brine purification process. The most widespread impurity in NaCl deposits is CaS04. Magnesium compounds, and to a lesser extent iron compounds, also are present in most natural salts. Oxides of silicon and aluminum are also found, as well as traces of other metals and sometimes anions such as iodide. The importance of these impurities depends greatly on the type of cell in use, and the methods used for purification of brine reflect all the above factors. 

The first step in brine purification is chemical treatment to remove certain impurities. The elements of hardness (calcium and magnesium) must be removed, along with iron and heavy metals. This is done by precipitation, adding a source of carbonate ion to remove calcium and a source of hydroxide ion to remove the other metals. Sulfate ion also can be removed by precipitation, using either calcium or barium ion. Precipitation processes cannot usefully be considered in isolation. The nature of the solids formed in this way determines how they will behave in the later processes that are designed for their physical removal. The subsections that follow therefore describe in a general way the flow behavior and settling rates of precipitated particles. Later sections of the chapter cover the details of sedimentation and filtration of the solids. 

Due to the higher concentrations of Ca and Mg in those solar pond brines, brine purification (easing the task to achieve product purity) cannot be considered. The high consumptions of soda ash, calcined soda, and calcium chloride (or lime milk and hydrochloric acid), and the disposal costs for the filter cakes including the additional investment costs caimot reach feasibility. In this case and for this concept. 

Brine Purification. In mercury cells, traces of heavy metals in the brine give rise to dangerous operating conditions (see p. 32), as does the presence of magnesium and to a lesser extent calcium (521. In membrane cells, divalent ions such as Ca or Mg are harmful to the membrane. The circulating brine must be rigorously purified to avoid any buildup of these substances to undesirable levels [7]. Calcium is usually precipitated as the carbonate with sodium carbonate magnesium and iron, as hydroxides with sodium hydroxide and sulfate, as barium sulfate. 

It is important to consider not only the impurities themselves but also their interaction. The presence of one impurity may not be harmful, but its synergistic combination with others may be [144]. For example, silica itself is not harmful for membranes. Only in the presence of calcium and aluminum do precipitates form and damage the membrane irreversibly. The concentration of silica and/or the concentration of aluminum plus calcium can be adjusted to give the optimum operating conditions. For example, with an effective secondary brine purification, higher levels of silica can be tolerated. Similarly, if aluminum concentration is high, calcium or silica concentration must be reduced to maintain acceptable membrane performance. 

Novel ion-exchange technology has recently been developed for purification of brine. Different resins have been developed for removal of sulphate impurities as well as calcium and magnesium hardness. The process is very simple and since only water is consumed to regenerate the ion-exchange resins, the operating costs are extremely low. The equipment, which is similar to that currently widely utilised for purification of waste acid, is very compact. It is expected that commercial-scale systems of both types will be installed later in 2000. 

Rock salt is generally contaminated with calcium and magnesium sulfates and carbonates and with polyhalite (K2SO4 2CaS04 MgS04 2H2O). Crude brine, therefore, contains calcium, magnesium and sulfate ions. Purification of the brine is necessary before it is used in many processes, for example, the ammonia soda process (section 31.18), the electrolytic production of chlorine/caustic soda, and the production of purified salt. 

The production cycle starts with the extraction of sodium chloride. About 20% of the world s salt consumption goes into soda ash production [24]. The next step after rock salt mining is the production and purification of brine yielding a concentrated aqueous sodium chloride solution [8,25-27]. A parallel step is the production of carbon dioxide gas by calcination of limestone. The brine is treated with ammonia and carbon dioxide under precipitation of the less-soluble sodium hydrogen-carbonate. Ammonia is recovered by mixing the mother liquor with calcium hydroxide and stripping off the ammonia with steam. Thermal decomposition of sodium hydrogencarbonate yields synthetic soda ash [8,20,22,23,28-38]. The output of soda ash produced by the ammonia-soda process amounts to about two-thirds of the world production [22,23]. 

The preparation and purification of brine. The brine prepared for the electrolytic process for CI2 and NaOH contains calcium and magnesium ions which will be precipitated during the manufacturing process, in pipes in particular, blocking them up. This is prevented by deposition of the ions by sodium carbonate and sodium hydroxide before commencing the process. 

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