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= Proceso de Cloroálcali =

El proceso de Cloroálcali (también llamado clor-alcali y clor alcali) es un proceso industrial para la electrólisis de soluciones de Cloruro de Sodio NaCl. Esta teconología es usada para la producción de Cloro e Hidróxido de Sodio (lejía/soda cáustica), ambos productos químicos requeridos por la industria. En 1987, fueron producidos 35 millones de toneladas de cloro y fueron preparadas por este proceso. La producción a escala industrial comenzó en 1892.

Usually the process is conducted on a brine (an aqueous solution of NaCl), in which case NaOH, hydrogen, and chlorine result. When using calcium chloride or potassium chloride, the products contain calcium or potassium instead of sodium. Related processes are known that use molten NaCl to give chlorine and sodium metal or condensed hydrogen chloride to give hydrogen and chlorine.

The process has a high energy consumption, for example over 4 billion kWh per year in West Germany in 1985. Because the process gives equivalent amounts of chlorine and sodium hydroxide (two moles of sodium hydroxide per mole of chlorine), it is necessary to find a use for these products in the same proportion. For every mole of chlorine produced, one mole of hydrogen is produced. Much of this hydrogen is used to produce hydrochloric acid or ammonia, or is used in the hydrogenation of organic compounds.



Sistemas de proceso
Three production methods are in use. While the mercury cell method produces chlorine-free sodium hydroxide, the use of several tonnes of mercury leads to serious environmental problems. In a normal production cycle a few hundred pounds of mercury per year are emitted, which accumulate in the environment. Additionally, the chlorine and sodium hydroxide produced via the mercury-cell chloralkali process are themselves contaminated with trace amounts of mercury. The membrane and diaphragm method use no mercury, but the sodium hydroxide contains chlorine, which must be removed.

The performance of these devices is governed by the considerations of electrochemical engineering.

Celda Membrana


El proceso más común es la electrólisis de una disolución acuosa de cloruro de sodio (una salmuera) in a membrane cell. A membrane, such as one made from Nafion, is used to prevent the reaction between the chlorine and hydroxide ions.

Saturated brine is passed into the first chamber of the cell where the chloride ions are oxidised at the anode, losing electrons to become chlorine gas (A in figure):
 * 2Cl− → + 2e−

At the cathode, positive hydrogen ions pulled from water molecules are reduced by the electrons provided by the electrolytic current, to hydrogen gas, releasing hydroxide ions into the solution (C in figure):
 * 2 + 2e− → H2 + 2OH−

The ion-permeable ion exchange membrane at the center of the cell allows the sodium ions (Na+) to pass to the second chamber where they react with the hydroxide ions to produce caustic soda (NaOH) (B in figure). The overall reaction for the electrolysis of brine is thus:
 * 2NaCl + 2 → +  + 2NaOH

Celda de diagrama
In the diaphragm cell process, there are two compartments separated by a permeable diaphragm, often made of asbestos fibers. Brine is introduced into the anode compartment and flows into the cathode compartment. Similarly to the Membrane Cell, chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. The diaphragm prevents the reaction of the caustic soda with the chlorine. A diluted caustic brine leaves the cell. The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of caustic soda. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.

Celda de Mercurio


In the mercury-cell process, also known as the Castner–Kellner process, a saturated brine solution floats on top of a thin layer of mercury. The mercury is the cathode, where sodium is produced and forms a sodium-mercury amalgam with the mercury. The amalgam is continuously drawn out of the cell and reacted with water which decomposes the amalgam into sodium hydroxide, hydrogen and mercury. The mercury is recycled into the electrolytic cell. Chlorine is produced at the anode and bubbles out of the cell. Mercury cells are being phased out due to concerns about mercury poisoning from mercury cell pollution such as occurred in Canada (see Ontario Minamata disease) and Japan (see Minamata disease).

Unpartitioned cell
Without a membrane, the chlorine and hydroxide ions react, and the chlorine disproportionates to form chloride and hypochlorite ions:
 * + 2OH− → Cl− + ClO− +

Above about 60 °C, chlorate can be formed:
 * 3 + 6OH− → 5Cl− + − + 3

If current is interrupted while the cathode is submerged, cathodes that are attacked by hypochlorites, such as those made from stainless steel, will dissolve in unpartitioned cells.

If producing hydrogen and oxygen gases is not a priority, the addition of 0.18% sodium or potassium chromate to the electrolyte will improve the efficiency of producing the other products.

Electrodes
Due to the corrosive nature of chlorine production, the anode (where the chlorine is formed) must be non-reactive and has been made from platinum metal, graphite (called plumbago in Faraday's time), platinized titanium, or a mixed metal oxide clad titanium anode (also called a dimensionally stable anode). Historically, platinized platinum, magnetite, lead dioxide, manganese dioxide, and ferrosilicon (13-15% silicon ) have also been used as anodes. Platinum alloyed with iridium is more resistant to corrosion from chlorine than pure platinum. Unclad titanium cannot be used as an anode because it anodizes, forming a non-conductive oxide and passivates. Graphite will slowly disintegrate due to internal electrolytic gas production from the porous nature of the material and carbon dioxide forming due to carbon oxidation, causing fine particles of graphite to be suspended in the electrolyte that can be removed by filtration. The cathode (where hydroxide forms) can be made from unalloyed titanium, graphite, or a more easily oxidized metal such as stainless steel or nickel.

Laboratory procedure
Electrolysis can be done with beakers, one containing a brine solution (salt water) and one containing pure water connected by a salt bridge. Anodes are made ideally from platinum group metals, which resist corrosion. Since corrosion is less severe at the cathode, it can be stainless steel or silver.

Manufacturer associations
The interests of chloralkali product manufacturers are represented at regional, national and international levels by associations such as Euro Chlor and The World Chlorine Council.

Lecturas sugeridas

 * Bommaraju, Tilak V.; Orosz, Paul J.; Sokol, Elizabeth A.(2007). "Brine Electrolysis." Electrochemistry Encyclopedia. Cleveland: Case Western Rsserve University.

Category:Chemical processes Category:Electrolysis Category:Industrial gases