Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
1999-07-27
2004-06-08
Cintins, Ivars C. (Department: 1724)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S536000, C210S712000, C210S726000, C423S182000
Reexamination Certificate
active
06746592
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to chlor-alkali membrane electrolytic cells particularly to brine as a feedstock for said cells and, more particularly, to removal of aluminum species from said feedstock.
BACKGROUND OF THE INVENTION
The quality of feed brine solution required for a modern chlor-alkali plant equipped with membrane cells is much more stringent than conventional diaphragm and mercury operations. The electrical efficiency of the membrane cells is easily compromised by the presence of various cationic and anionic impurities commonly found in the metal halide brine feedstock. A concentration of greater than 20 ppb of calcium or magnesium in the feed brine results in physical disruption of the sulfonic/carboxyl layers membrane interface through deposition of insoluble precipitates of these metals. A feed brine solution containing more than 100 ppb aluminum and greater than 10 ppm silica leads to precipitation of aluminosilicate near the cathode surface which ultimately damages the membrane separator and significantly affects its sodium and water transport properties. Hence, the control of the concentration of these impurities to their respective acceptable maximum concentrations is crucial to prevent membrane degradation and anode blinding.
In order to achieve a high purity feed brine using solar salt or rock salt as a raw material, both primary and secondary brine treatment processes are employed. During the primary treatment operation, caustic and soda ash are added either together or in series to a treatment tank to initiate primary precipitation of calcium as calcium carbonate and magnesium as magnesium hydroxide, the two main cationic impurities present in solar and rock salt. This is followed by secondary precipitation or co-precipitation of other cationic impurities such as aluminum, iron, barium, manganese, strontium, cobalt, nickel and like heavy metals, which are also commonly found in the feedstock salt, but in much lower concentrations than the alkali earth metals. The secondary co-precipitation process usually involves physical occlusion and/or adsorption onto primary precipitates, and is a much slower reaction than the primary precipitation. Hence, related kinetic factors such as residence time, temperature and reactant concentrations must be optimized to ensure the effective removal of these impurities. This is normally achieved by use of a large capacity clarifier operated with sludge recirculation, consisting mainly of the primary precipitates, calcium carbonate and magnesium hydroxide. The subsequent clarified solution is then filtered through conventional press or leaf filters, and the resulting filtered solution, usually Containing less than 10 ppm of calcium and magnesium cations, is then introduced to a secondary treatment system equipped with cation-exchange resins. All the major cationic impurities are reduced through chelation to the trace level of less than 50 ppb, a standing maximum concentration which membrane manufacturers accept for use in chlor-alkali membrane electrolysis. However, the cationic exchange activities of most of these impurities, particularly calcium and magnesium, are best effected under alkaline conditions, such as, pH 9 to 11, when using iminodiacetic or aminophosphlonic functionalized chelating resins. Techniques for the primary and secondary brine treatment processes are well-known in the art.
Thus, the primary and secondary treatments are effective in removing most major cationic impurities under alkaline conditions, except aluminum and silica under alkaline pH 9 to 11. At this pH, the anionic complexes, aluminate (AlO
2
) and metamonosilicate (HSiO
3
) predominate, which renders their removal by primary and secondary treatment processes to be limited.
A number of different processes have been used to attempt to remove and control the aluminum species concentration in alkali brine. One method is described in U.S. Pat. No. 4,450,057, issued May 22, 1984 to Olin Corporation, which discloses the acidification of saturated alkali metal halide brine to a pH of between 2.0 to 3.0 to convert the aluminum species present to the soluble cationic form Al
3+
, followed by contact of the acidic brine with a strong macroreticular cationic chelating resin to remove the dissociated aluminum cations at the negative hydroxyl sites on the resin.
However, in the cationic exchange process, the highly mobile hydrogen ions from the acidic brine compete with the less mobile trivalent aluminum cations for the chelating sites on the resin, and as a result, the neutralization of these negative hydroxyl sites with the hydrogen ions significantly lowers the dynamic loading capacity of the clielating resin by making it less effective for aluminum removal. Moreover, tile required frequent regeneration of the resin bed leads to additional capital and operating costs.
U.S. Pat. No. 4,966,764, issued on Oct. 30, 1990 to Olin Corporation describes the removal of aluminum in brine in the calcium chloride feed stream by recycling brine from the calcium carbonate settler at a lower pH to solubilize the aluminum, followed by raising the pH to reprecipitate the aluminum onto carrier particles with high surface area to facilitate removal. Although this technique addresses the gradual increase in aluminum concentration within a closed-loop chlor-alkali brine circuit, it does require a large standing inventory of calcium carbonate settler solids along with additional large holding vessels and associated equipment. Moreover, during the acidification of these solids to solubilize the aluminum much of the other precipitated cationic impurities also dissolve. This not only results in greater consumption of hydrochloric acid, but also in caustic usage when these dissociated impurities are reprecipitated through recausticization for subsequent separation. In addition, with the ever increasing awareness on environmental protection pressure has been mounting on chemical manufacturers such as chlor-alkali producers to limit their plant outflows, both liquids and solids, and hence the need to continually dispose large quantity of sludge materials containing heavy metals clearly poses an environmental concern.
Two other references, one entitled “The Removal of Aluminum From The Recovery System Of A Closed Kraft Pulp Mill” by Per Ulmgren of Swedish Forest Products Research Laboratory, and the other entitled “The Solubility Of Aluminosilicates In Kraft Green And White Liquors” by P. N. Wannenmacher of Oregon State University, W. J. Frederick of The Institute of Paper Science and Technology, and K. A. Hendrickson and K. L. Holman both of Weyerhaeuser Company, have also described the effective removal of aluminum and silica from green and white liquors by precipitation with magnesium salts and efficient dregs removal. However, these techniques are developed to remove high levels of aluminum and silica concentrations present in highly alkaline solutions such as pulping liquors where titratable alkali content normally exceeds 160 g/L NaOH, and the residual aluminum content after such treatment still remains at the 10 ppm region despite using an optimum Mg/Al molar ratio and 24 hours of residence time. In addition, it is well-known that aluminum and its dissociated complexes are amphoteric in nature and since the precipitation reactions are carried out under strongly alkaline conditions the resulting equilibrium concentration of aluminum complexes is significantly affected.
In consequence of increasing environmental consciousness coupled with highly competitive markets, modern chlor-alkali producers are forced to look to alternate ways to not only reduce operating and capital costs, but to also minimize the amount of solids and liquids effluents. The current solution to these problems is to replace the solar or rock salt raw materials with evaporated salt which is a much purer and cleaner salt having amounts of alkali earth metals and other heavy metals orders of magnitude lower in concentration. Upon dissolution of the purer salt, the resulting brine solut
Mok Felix
Subramanian R. Ganapathy
Cintins Ivars C.
Kvaerner Canada Inc.
Morgan & Lewis & Bockius, LLP
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