FR2521874A1 - Macroporous alkylamino:phosphonic chelating resin beads - useful for removing impurities from electrolytic brines - Google Patents

Abstract

Macroporous alkylaminophosphonic chelating resin with an apparent density of 0.35-0.425 g/ml, a particle size of less than 0.8 mm, a water retention of 50-60% in the acid form, a porosity of 800-1100 mm3/g, a total theoretical capacity of the fixation of calcium ions of not greater than 31 g/l of resin and the Na form and an osmotic resistance such that more than 90% of the beads are intact after 30 shocks. The resins are made by (a) selecting 0.3-0.5 mm beads of a macroporous reticulated vinyl-aromatic copolymer having a porosity of 700-1000 mm3/g and a vol. swelling factor in toluene of 1.65-1.9; (b) (i) chloromethylating the heads to a desired Cl content, (ii) aminating the chlorinated beads to form chloraminated beads, (iii) hydrolysing the chloroaminated heads under moderate conditions with dil. acid and (iv) alkylphosphonating the hydrolysed beads so as not to produce sec. crosslinking in the copolymer, (c) the reaction conditions in (b) being such as to produce a prod. with the stated properties and esp. an apparent density of 0.4-0.425 g/ml, a water retention of 53-55% in acid form, a porosity of 850-950 mm3/g and a calcium fixation capacity of not higher than 29 g/l of resin in the Na form. The prods. have a high resistance to osmotic shock and are esp. useful for removing (in the form of complexed cations) the impurities contained in concn. electrolytic brines as used in the electrolytic prodn. of chlorine or alkali metals in membrane cells or diaphragm cells and for the electrolytic prodn. of chlorates.

Description

 The present invention relates to a process for the purification of concentrated brines used for the electrolytic production of chlorine, by means of an ion exchange resin having chelating functional groups of the alkylaminophosphonic type.

 Each of the three current industrial processes used for the manufacture of chlorine by electrolysis of concentrated brines of alkali chlorides require an adequate control of the quantities of calcium and magnesium present as impurities in these brine solutions.

 The concentration of calcium is usually maintained around 10 mg / l in mercury cells and around 3 to 5 mg / l for efficient functioning of diaphragm cells. It must be reduced below 0.05 mg / l for efficient functioning of membrane cells.

 In fact, membrane cells require the use of high purity brines and any excess of calcium above the indicated limit reduces the performance and the lifespan of the membranes.

 The amount of calcium in brines can be reduced to 2 to 10 mg / l by adequate chemical pretreatments, decanting and filtration.

 Further reduction of the calcium content could only be achieved practically and economically by the use of chelating ion exchange resins.

 The aim of the present invention is to provide a process for the elimination of calcium and magnesium ions present in saline electrolysis brines up to a high degree of purification, (calcium below 0.05 mg / l) using ion exchange resins with chelating properties used in simple and conventional equipment.

 In the practice of ion exchange, it is well known that if a solution containing monovalent ions (for example sodium) and divalent ions (for example calcium) is passed through, which is the case with electrolytic brines , on an ion exchange resin in an appropriate ionic form and completely free of divalent ions, said solution is obtained at the outlet of the ion exchange column which no longer contains divalent ions. technically and economically viable, the ion exchanger under consideration must have a sufficiently high useful capacity, that is to say that the actual quantity of divalent ions fixed under the practical operating conditions is sufficiently high, when it is in equilibrium with the influent solution. If this useful capacity is low, the volume of solution free of calcium ions 1 for example, obtained by exchange cycle will be reduced and the process will not be technically and economically beneficial health. This useful capacity is strongly influenced by various parameters such as the exchange kinetics, the theoretical total capacity of the ion exchange resin, the concentration of ions in solution and by the selectivity of the ion exchanger considered with respect to -vis ions to eliminate.

 We contact a recent process for the elimination of calcium and magnesium ions in alkaline electrolysis brines in which the use of chelating resins having aminoacetic groups fixed on styrene-butadiene, styrene-divinylbenzene, N-glycine- copolymers is proposed. glycidylmethacrylate, on polymers of epichlorohydrin or preferably of chelating compounds, derived from aminoacetic acids, adsorbed on inert supports such as activated carbon, silica gel, alumina, zeolite or synthetic adsorbent polymers. These chelating compounds adsorbed on an inert support would have the following advantages: mechanical resistance and higher chemical stability, lower cost price and improved performance. However, the use of the products mentioned makes it difficult to lower the content of calcium and magnesium ions in brine below 10 mg / l.

 Chelating resins of the alkylaminophosphonic type are known.

However, the use of such resins (as described in French patent application No. 74/25610) is expensive because their mechanical resistance to osmotic shocks due to repeated passage from the regenerated form to the saturated form is insufficient. . The large percentage of broken and cracked beads of such resins prevents their economical use on an industrial scale for the treatment of brines in the chlorine industry.

The subject of the present invention is a process for the purification of electrolytic brines using a new alkylaminophosphonic resin having a high resistance to osmotic shocks and a satisfactory capacity for retaining calcium, making it possible to achieve in a simple and economical manner. ion exchange up to final calcium concentrations of less than 0.05 mg / l brine. The manufacture of this resin is described in French patent application n 82 03 114 and its main physical characteristics are - apparent density 0.400 - 0.425 g / ml - particle size ..................... <0.8 mm - water retention ................. 53 - 55% in hydrogen form - osmotic resistance ..............> 90% intact beads after 30 shocks - porosity (BJ.I <method) 850 - 950 mm3 / g
The method according to the invention comprises the following five steps
(a) Separation by ion exchange
(b) Travel
(c) Regeneration
(d) Washing
(e) Conversion to the Na + form
During the separation step (a), the brine to be purified is passed through the resin bed and the calcium ions are replaced by sodium ions, until the calcium concentration in the effluent brine is reduced to the limit required.

 In general, the total capacity of the resin is not used, 15 to 20% of the capacity of the resin bed being still unused at the end of the operation (a).

 In order to make the best use of the capacity of the resin without the concentration of calcium in the effluent exceeding the accepted level, two beds of said resin can be used in series.

 The displacement (b) of the brine containing chlorates must be used before the hydrochloric acid regeneration cycle, since in the eventual contact with this acid, the chlorates usually found in recycled brines decompose forming chlorine free.

 This displacement can be carried out by washing the resin with brine which does not contain chlorates, or with water.

 The regeneration (c) is carried out with 2N to 4N hydrochloric acid, in order to remove the cations complexed by the resin, and bring it to its acid form.

 A contact time of the resin with the acid of approximately 30 minutes is necessary for the complete elimination of the complexed cations and flow rates of 2 to 6 BV / hour are considered satisfactory.

 The washing (d) of the regenerated resin is carried out with softened and deionized water, at a rate allowing the expansion of the resin bed from 50% to 75% in order to remove fine particles and solids insoluble which can be found in the resin bed.

 After washing, the resin bed is allowed to settle. The washing operation could also be carried out before the regeneration, but carrying it out after the regeneration contributes to minimizing the osmotic shocks that the resin undergoes.

 The conversion (e) to the sodium form can be carried out by passing a 2N to 4N solution of sodium hydroxide, through the resin bed.

 Introducing twice the volume of the sodium hydroxide resin bed (2bu) over a period of about 30 minutes is sufficient for the resin to be ready to be put back into operation.

The following operating conditions lend themselves to better use of the resin in the process according to the invention
1 - Use of a chemically pretreated brine preferably having a calcium concentration of less than 10 mg / l, in order to reduce this concentration below 0.05 mg / l in the most effective manner.

brine
2 - A flow rate / of 10 BV / h is recommended as a compromise between an economic size of the equipment and the duration of the cycles for the resin used during the implementation of the invention.

 3 - Effective removal of calcium can be carried out at a pH between 8 and 11.

 4 - As the exchange capacity of the resin increases with the temperature of the brines, it is preferable to carry out the operating cycles with brines at 60 "Cl but not to exceed 80" C.

 5 - The alkylaminophosphonic resin which is the subject of the present invention is sensitive to the action of chlorine or other oxidizing agents, hence the need to keep the amounts of chlorine present at an absolute minimum. Thus, the free chlorine or chlorates generally present in brines and coming from the anode compartment of the electrolysers must be eliminated before the contact of these brines with the resin. Likewise, the formation of free chlorine from chlorates must be avoided during possible contact with hydrochloric acid during regeneration, by separating these chlorates. The usual means for retaining free chlorine in solutions is to pass them through columns of activated carbon or to subject them to an appropriate chemical treatment.

6 - The accumulation of suspended solids in the resin bed determines pressure drops through the columns and the formation of channels, hence the need to filter out these suspended solids, if they are present. -
The following examples illustrate the implementation of the present invention.

Example 1
An installation is used comprising two steel columns coated with rubber and each having a useful volume of 150 l.

 The first column is filled with activated carbon and is used as a filter, and the second, as an exchange column, filled with the resin which is the subject of the present invention. The exchange column was supplied with an industrial brine used for electrolysis in a diaphragm cell whose concentration varies between 2 and 5 mg / l in calcium and 0.15 to 0.20 mg / l in magnesium, and whose flow rate varies between 1.2 and 1.6 m3 / h. The time between regenerations was more than 15 days.

 After 8 months of operation, no deterioration of the resin was detected. Current yields did not decrease significantly during this period in the cell operating with purified brine and the percentages of chlorine and oxygen were almost constant during this period.

In contrast, other cells, operating with unpurified brine, experienced decreases in current efficiency of 5-10%. This results in a marked improvement in the performance of the diaphragm cell using the method which is the subject of the invention.

Example 2
The goal is to lower the calcium concentration of concentrated industrial brines used for electrolysis in membrane cells to less than 0.05 mg / l.

 An installation of three columns with a capacity of 0.85 m3 was used, each operating in series.

 After filling with the resin which is the subject of the invention, the columns were supplied with an industrial brine containing 3 - 4 mg / l of calcium having a temperature of 60 "C, a pH of 9 to 10 and a flow rate of 5 to 6 volumes / hour.

 The brine was dechlorinated before entering the first column.

 The overhead column was taken out of service and regenerated when the effluent brine reached a calcium concentration of 0.5 mg / l. The final calcium concentration in the purified brine was less than 0.05 mg / l.

 It should also be noted that the flow rates of the brine on the resin have been varied. Thanks to the capacity and high kinetics of the resin, it was possible to maintain a calcium concentration of less than 0.10 mg / l in the effluent while varying the specific charges from 4 to 11 volumes of brine per volume of resin and per hour.

Industrial applications
The process which is the subject of this invention can be used for the efficient and economical removal of calcium and magnesium ions from concentrated brine solutions supplying the electrolysis cells for the production of chlorine or alkali metals and especially of the cells to membrane.

 The application of this purification process makes it possible to greatly increase the life span of the electrolytic membranes and the current yield of the electrolysis cells.

 Its efficiency and ease of implementation also make it advantageous from an economic point of view for the treatment of brine purification of diaphragm cells.

The physical characteristics of the chelating resins of the aminoalkylphosphonic type used in the process according to the invention are determined by means of the following methods - Bulk density: volume / weight ratio of the dried resin at 600C for
12 hours.

- Granulometry: sieving under water of the resin on a series of sieves. The
particle size fractions collected in test tubes covered with water,
are packed to constant volume.

- Water retention: the resin in determined ionic form is rinsed. Esso
controlled rabies is carried out in a BUCHNER filter in a damp cloth. Resin
is dried at 1000C overnight.

- Osmotic resistance: optical determination of the percentage of re beads
sine broken and cracked, after successive cycles of contraction and ex
expansion of the beads due to their subsequent change in ionic shape
the passage of solutions of chemical reagents (acid-base) on the resin. A
contraction-expansion cycle represents an osmotic shock.

- Porosity: determined by the nitrogen adsorption and desorption isotherms
resin, BJH method (Journal American Chemical Society, 73, 373-380,
1951).

Claims (8)

1 - Process for purifying concentrated brines, intended for electrolysis by means of a chelating ion-exchange resin, characterized in that the brine to be purified is passed through a bed of resin beads chelating agent of the alkylaminophosphonic type, which has the following properties: a ball size of less than 0.8 mm, a porosity of between 850 and 950 mm 3 / g, and a total capacity for fixing calcium ions at most equal to 31 g / l of resin in the sodium form. 2 - Method according to claim 1, characterized in that said resin comprises a macroporous matrix based on crosslinked polystyrene, having a particle size between 0.3 and 0.5 mm, a porosity between 800 and 900 mm3 / g, a volume of swelling in toluene of between 1.7 and 1.8. 3 - Method according to claim 2, characterized in that said chelating resin is obtained by fixing, on said matrix, alkylaminophosphonic groups so that its calcium fixing capacity is limited to 29 g / l of the resin in the form sodium. 4 - Process according to claim 1, characterized in that the brine is purified at a temperature between 50-900C. 5 - Process according to claim 3, characterized in that the temperature of the brine to be purified is approximately 60 "C. 6 - Process according to claim 3, characterized by the fact that the calcium content of the purified brine is kept at a reduced value of less than 0.05 mg / l of brine by periodically carrying out a regeneration step by passage, at through said bed, a volume of HC1 2N corresponding to approximately 1-3 times the volume of said bed. 7 - Process according to claim 6, characterized in that one carries out the conversion of said resin from its acid form to its sodium form, by passing, through the regenerated bed, a volume of NaOH 2N corresponding to 1 - 3 times the volume of said bed. 8 - Process according to claims 6 and 7, characterized in that the purification of the brine is carried out at specific charges of between 4 and 11 volumes of brine per volume of resin and per hour for a calcium leak of less than 0, 10 mg / l.