Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1999-08-23
2002-07-30
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S681000, C210S687000, C210S688000
Reexamination Certificate
active
06426008
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for reducing the concentration of multivalent metal cations in a brine solution containing a metal chelating agent. In particular, this invention relates to a method for reducing the concentration of multivalent metal cations in a brine solution involving the use of a membrane electrolyzer. The brine solution is a product of a condensation polymer manufacturing process and contains a water-soluble chelating agent, such as sodium gluconate.
BACKGROUND OF THE INVENTION
The manufacture of condensation polymers often produces a brine solution as a by-product. For example, a brine solution is produced in the manufacture of polycarbonate resins through the reaction of phosgene with at least one bisphenol compound in an organic solvent in the presence of aqueous sodium hydroxide. A common example is the reaction of bisphenol A with phosgene in dichloromethane in the presence of aqueous sodium hydroxide to produce bisphenol A polycarbonate and sodium chloride solution.
To reduce production costs and avoid environmental pollution, such brine solutions are often recycled to a chlor-alkali plant for electrolysis to produce chlorine gas, sodium hydroxide solution, and hydrogen gas. The electrolysis cells in such chlor-alkali plants frequently comprise an anode compartment and a cathode compartment with an appropriate separator in between the two compartments. The purpose of the separator is to separate the anolyte solution and the chlorine gas evolved at the anode from the catholyte solution and the hydrogen gas evolved at the cathode, within the electrolysis cell. The separator may be at least partially porous to water. The types of separators used in electrolysis cells include diaphragms and membranes.
During membrane electrolysis cell operation, the ion exchange membrane separator may gradually become plugged by the accumulation of solid material, retarding the passage of water and dissolved species from anolyte solution to catholyte solution. Separator plugging decreases the efficiency of cell operation and lowers the production rate of products arising from electrolysis. When plugging reaches a critical point, the separator must be replaced, often before its expected lifetime is reached. To achieve most economical electrolysis cell operation, it is necessary that the cell separator have as long a lifetime as possible.
Brine solutions arising as by-products from condensation polymer manufacture often contain both organic and inorganic contaminants. Organic contaminants may include residual solvent, catalyst, and aqueous-soluble organic species such as monomer and low molecular weight oligomer. Inorganic contaminants may include multivalent alkaline earth and transition metal cations, particularly iron, calcium, and magnesium. When brine solution containing one or more such contaminants is electrolyzed, both organic species and metal species may precipitate on the surface of and within an electrolysis cell separator to cause plugging. To achieve maximum lifetime of a separator in an electrolysis cell, the concentration of contaminating organic species and multivalent metal cations must be reduced to as low a level as economically possible in the feed-brine solution.
In order to lower the concentrations of organic and inorganic contaminants to levels suitable for feeding the brine to membrane electrolytic cells, primary and secondary brine treatment are often employed. In primary brine treatment, the brine pH is elevated to above about 10 in the presence of a molar excess of carbonate ion in order to precipitate alkaline earth and transition metals as their carbonates and/or hydroxides, followed by a filtering or settling process such as clarification. This is followed by acidification and stripping of the brine to remove carbonate ion as well as organic contaminants such as organic solvents and dissolved catalysts. Additional treatment such as adsorption may be utilized as necessary to remove organic species such as monomer and low molecular weight oligomer from the brine.
In secondary brine treatment, the brine pH is adjusted to about 8-11 and the brine is treated in a chelating ion exchange resin such as aminomethylphosphonic acid-functionalized polystyrene resin (AMP resin) or iminodiacetic acid-functionalized polystyrene resin (IDA resin). These resins are both chelating cation exchange resins and are commonly used in the chlor-alkali industry for secondary brine treatment, particularly AMP resin. This treatment normally reduces the concentration of alkaline earth metal ions such as calcium and magnesium to levels that are acceptable for use in membrane electrolyzers. Typical membrane electrolyzers require that the combined calcium plus magnesium concentration in the brine be less than 20 ppb.
This combined primary and secondary brine treatment procedure may be effective for reducing impurity concentrations in brine solutions to levels specified for membrane electrolyzers. The concentration of alkaline earth metals is particularly important for membrane electrolyzer operation (20 ppb combined calcium and magnesium). However, it has been found that when a brine solution which results from a condensation polymer manufacturing process, such as a polycarbonate manufacturing process, is treated by primary and secondary brine treatment, the concentration of alkaline earth metal cations in the treated brine exceeds the tolerable level and the membrane electrolyzer separator becomes plugged at an unexpectedly rapid rate, resulting in premature failure.
After careful experimentation it has been discovered that the cause of rapid membrane separator plugging during electrolysis of such brine solution is the precipitation of alkaline earth metal hydroxide species, primarily derived from residual calcium and magnesium in the feed-brine, on the surface of and within the electrolysis cell membrane separator. Analysis has revealed that there is still a very low concentration of alkaline earth metal cations present in electrolyzer feed brine even after primary and secondary brine treatment. Without being bound by any theory, the cause of this problem is believed to be the presence of a water-soluble chelating agent in the brine solution. The chelating agent apparently retains some fraction of the transition metal cations as water-soluble complexes so that these complexed cations are not precipitated as salts during primary brine treatment. These complexed transition metal cations are more strongly bound to the ion exchange resin than alkaline earth metal cations in secondary brine treatment. Therefore, during ion exchange treatment (secondary brine treatment) they displace alkaline earth metal cations from the ion exchange resin into the brine solution. These displaced alkaline earth metal cations then exit the ion exchange column with the brine and cause precipitation on an inside the membrane separator in the electrolytic cell. The chelating agent is typically a sugar acid such as gluconate anion.
Gluconate anion is often added in the form of sodium gluconate in condensation polymer manufacturing processes to form water-soluble complexes with a fraction of the multivalent transition metal cations such as iron (III), nickel (II), and chromium (III). Complexation beneficially hinders transition metal salts from precipitating in the manufacturing equipment and from contaminating the polymer product. With iron (III), for example, gluconate anion forms an iron-gluconate complex, thereby solubilizing iron in the brine solution so that the polymer product is produced substantially free of iron contamination. However, when the brine solution undergoes primary brine treatment, the fraction of a transition metal species such as iron (III) that exists as a gluconate complex remains strongly chelated and thus remains in solution through the end of primary brine treatment. These transition metal gluconate complexes such as iron (III) gluconate are much more strongly bound to both AMP and IDA resins than are alkaline earth metal cations. Therefore, whe
Foust Donald Franklin
Fyvie Thomas Joseph
Silva James Manio
Caruso Andrew J.
Cintins Ivars
Johnson Noreen C.
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