NO344281B1 - Process for the preparation of chlorine-containing compounds - Google Patents

Description

The present invention relates to a method for producing chlorine or sodium chlorate by starting from an aqueous salt solution containing at least 100 g/l sodium chloride and a contaminating amount of polyvalent cations. The procedure includes the steps that proceed from claim 1.

In aqueous salt solutions containing at least 100 g/l sodium chloride from a natural source, a majority of anions and cations, such as magnesium, calcium, strontium, iron and sulfate, are present. In the manufacturing process of salt and the manufacturing processes of products in which salt is a raw material, most of these anions and cations must be removed. Hitherto, it has generally been impossible to remove polyvalent ions such as calcium, magnesium, strontium, iron, and/or sulfate from an aqueous salt solution to a significant extent other than by means of a process involving the addition of a large amount of soda ash and concentrated NaOH solution to the aqueous salt solution and subjecting the solution to several deposition steps (sedimentation steps) after which the aqueous salt solution is subjected to a multi-stage evaporation process. The relatively pure salt produced in this way can be sold as is and constitutes a suitable raw material for other processes, the most important of which is the production of chlorine-containing compounds, such as e.g. chlorine and sodium chlorate. For these production processes of chlorine-containing compounds, the salt is re-dissolved in water and the chloride ions are converted to chlorine-containing compounds by means of an oxidation step, e.g. using an electrolysis process.

The above prior process for the production of chlorine-containing compounds includes many steps that require large reactors and deposition vessels and therefore lower the desired profitability. It also requires a large amount of chemicals, such as soda ash and caustic alkali, which are undesirable. Furthermore, the prior art process requires a not too contaminated aqueous salt solution as a starting material, as it has become clear that aqueous salt solutions comprising a large amount of impurities are impossible to purify to a sufficient extent using the prior process.

Furthermore, if it would be possible to substantially remove polyvalent ions from an aqueous salt solution containing at least 100 g/l sodium chloride, it would no longer be necessary to first prepare a dried salt, only for it to be redissolved in water for further processing in a subsequent step. Also, if it were possible to remove polyvalent ions from an aqueous solution containing at least 100 g/l sodium chloride to a greater extent, this aqueous solution could have a lower quality, i.e. could contain a higher amount of unwanted polyvalent ions, but it would remain suitable for the production of chlorine-containing compounds.

Accordingly, there is a need in the market for a process for the production of chlorine-containing compounds in which an aqueous salt solution comprising at least 100 g/l sodium chloride, more specifically a concentrated salt solution, is purified to mainly remove polyvalent ions therefrom, the purified solution then being further reacted to give the chlorine-containing compounds.

US 5,858,240 describes a chloralkali process in which salt solutions comprising at least 50 g/l sodium chloride are purified by means of a nanofiltration step to remove unwanted ions therefrom and then converted to chlorine or sodium chlorate by means of an electrolysis step. However, the retention of calcium and magnesium as discussed in this document is still in need of improvement. In particular, as shown by the examples in this reference, when the amount of sodium chloride in the aqueous salt solution is increased, the retention of calcium decreases greatly, the calcium retention being 56.3% when the sodium chloride concentration is 139.6 g/l and only 12.3% when the sodium chloride concentration has increased to 288.7 g/l. Accordingly, the purification step in which these unwanted ions are removed from the aqueous salt solution becomes more cumbersome as the salt concentration increases, as the (unwanted) passage of calcium ions through the nanofiltration membrane along with the purified salt solution increases considerably at higher salt concentrations.

GB 2 395 946 relates to a process in which seawater is purified to subject the solution to a nanofiltration step. The seawater is sent to a nanofiltration process with a higher repulsion of sulphate ions compared to sodium ions or chloride ions. The permeate from the nanofiltration process is then sent to a thermal desalination process to increase the sodium chloride concentration in the water. Finally, the sodium chloride is precipitated in a crystallizer. The retentate obtained after the nanofiltration step can be discharged to a discharge, or it can be sent to a process for mineral extraction of components such as magnesium, sulphate or calcium. It is mentioned that in the nanofiltration process it must be ensured that the salt concentration is sufficiently low to prevent the precipitation of calcium carbonate. GB 2 395 946 does not show nanofiltration of concentrated sodium chloride streams, i.e. aqueous salt solutions comprising at least 100 g/l sodium chloride. US 4 176 022 describes a method for the electrolysis of an alkali metal chloride salt solution in an electrolysis cell with an anolyte compartment separated from a catholyte compartment by means of a semipermeable barrier, the method comprising (i) a calcium ion-containing salt solution being maintained alkaline, (ii) a phosphate selected from the group consisting of of phosphoric acid, sodium orthophosphate, sodium metaphosphate, sodium polyphosphate, potassium polyphosphate, and mixtures thereof are added to the aforementioned alkaline salt solution and form an insoluble precipitate with the stoichiometric formula (CaX2) � (Ca(PO4)2)3 in which X is selected from the group consisting of F , Cl and OH, (iii) the precipitate is separated from the salt solution, (iv) the salt solution, containing less than 20 parts per billion calcium ions is fed to the anolyte compartment of the cell, and (v) an electric current is passed through the cell. In summary, this reference describes a method for removing calcium from a salt solution in which an additive is added to the salt solution which will form an insoluble stoichiometric complex with the calcium ions present therein and which must then be removed by filtration and discarded. As a consequence, this reference does not relate to a process in which the additive can be recycled, which makes the process less economically attractive. Furthermore, it was found that the feed as described in US 4,176,022 is not suitable to be subjected to a nanofiltration step, as nanofiltration membranes are sensitive to mechanical damage in the top layer due to any solids present in the feed.

A process has now been found for the production of chlorine-containing compounds, in particular chlorine and sodium chlorate, in which the retention of polyvalent ions, such as calcium, in aqueous solutions containing at least 100 g/l sodium chloride can be significantly increased in one step, so that in a subsequent step the chloride anion can be directly reacted to give the desired chlorine-containing compound.

In more detail, the invention provides a process for the production of chlorine or sodium chlorate using an aqueous salt solution containing at least 100 g/l sodium chloride and a contaminating amount such as at least 0.01 ppm (parts per million) of polyvalent cations comprising the steps:

(i) producing an aqueous salt solution containing at least 100 g/l sodium chloride and at least 0.01 ppm polyvalent cations by dissolving a sodium chloride source in water, where the polyvalent cations are magnesium, strontium, iron, barium, aluminum or a mixture thereof,

(ii) adding an effective amount of at least one positive retention-enhancing component selected from the group consisting of (poly)carboxylic acids, phosphinocarboxylic acids, polyacrylic acids, polymaleic acids, glucose, sucrose, sucrose and sodium gluconate to the aqueous solution wherein the total amount of positive retention-enhancing components as added is at least 15 ppm,

(iii) the solution is then subjected to a nanofiltration step, with a molecular weight threshold of between 100 Da and 10,000 Daltons, so that the solution is separated into a retentate enriched in polyvalent cations and a permeate which is the purified aqueous salt solution,

(iv) the chloride anions in the permeate are converted to chlorine or sodium chlorate by means of an electrolysis step, and

(v) at least a portion of the retentate is recycled to the dissolution step (i).

By the term "positive retention improving component" is meant any additive which, when added to an aqueous solution comprising calcium and other polyvalent cations such as magnesium, strontium, iron barium and/or aluminium, will lead to an increase in retention of calcium and preferably also in the retention of one or more of the polyvalent cations selected from the group consisting of magnesium, strontium, iron, barium and aluminium, when this solution is subjected to a nanofiltration step. In order to determine whether an additive is suitable for use as a positive retention enhancing component PREC ("positive retention enhanding component") or not in the method according to the present invention, the following test can be used.

A synthetic salt solution is prepared by dissolving 1120 g of ultrapure sodium chloride from Merck in 3600 g of water. Then 17.04 g Na2SO4 and 13.2 CaCl2 are dissolved in the synthetic salt solution. The resulting saline, designated synthetic saline, is fed to a DSS lab stack M20 unit containing 0.036 m 2 of the "Desal 5DK" nanofiltration membrane (from GE/Osmonics). The membrane unit is operated at 30 bar pressure and ambient temperature with a cross-flow quantity of 600 l/h. The unit is operated for 1 hour in a total recirculation mode (retentate and permeate are recirculated to the feed vessel). A 50 ml permeate sample is then collected and the Ca retention is determined by measuring the Ca concentrations of a permeate sample and a retentate sample after acidification and dilution with nitric acid using simultaneous inductively coupled plasma emission spectrometry (ICP-ES). This is a blind test. In a second experiment, 300 ppm of an additive is added to the aforementioned synthetic crude salt solution. If a precipitate is formed, the additive is not considered to be suitable for use as a positive retention improving component PREC in the process according to the invention. If the formation of a precipitate is not observed visually, the nanofiltration test just described is repeated. The additive is considered to be a positive retention-improving component PREC if an absolute increase of at least 5% is observed for the Ca retention compared to the blank sample.

It has surprisingly been found that when the above process is used, an absolute increase in the retention of calcium of at least 5%, preferably 7%, most preferably at least 10% is observed in step (iii) compared to a process in which the same aqueous solution without the positive retention-improving component PREC is subjected to such a nanofiltration step. Said at least 5% absolute increase in retention can be achieved for all indicated aqueous salt solutions, i.e. solutions with a sodium chloride concentration in the range from 100 g/l up to the concentrations which are saturated and even supersaturated in their sodium chloride concentration. It is noted that if in a process in which an aqueous solution not comprising a positive retention-enhancing component is subjected to a nanofiltration step (i.e. in a blank), the observed retention of calcium is already between 90% and 97%, but application of the method according to the present invention will still result in an increase in retention, but the absolute increase will be less than 5% although it will be at least one percent. It is noted that if the retention of calcium is already more than 97% for the blank, an increase in retention after the addition of a positive retention-enhancing component must still be expected, but this will no longer have practical application. The method according to the present invention is therefore preferably used for a salt solution in which the retention of calcium in the absence of a positive retention-improving component is between 2 and 97%, more preferably between 4 and 90%, most preferably between 5 and 75%.

It is further noted that an increase in the retention of other polyvalent cations present in the aqueous salt solution, such as magnesium, strontium, iron barium, and/or aluminium, is also observed. The observed absolute increase in retention usually also exceeds 5%.

The PREC used in the method according to the present invention is preferably selected from the group consisting of polycarboxylic acids, polyacrylates, polymaleic acids, and optionally also phosphino; sugars in the form of glucose, sucrose, sucrose or sodium gluconate.

Nitrogenous components are less preferred as they will cause difficulties in electrolysis operations due to the formation of NCl 3 . In particular, when NCl 3 accumulates, which is the case if chlorine gas is liquefied as usual in commercial electrolysis operations, its formation is highly undesirable because the resulting product is explosive.

In an even more preferred embodiment, the PREC is selected from the group consisting of ecologically healthier components: (poly)carboxylic acid, phosphinocarboxylic acid, polyacrylic acid, polymaleic acid, glucose, sucrose, sucrose and sodium gluconate.

Most preferably, a PREC is selected from the group of the following compounds with large molecules: phosphinocarboxylic acid, preferably used as the 40% aqueous solution "Bellsperse" 164 from Jianghai Chemical Co.; polymaleic acid, preferably used as the 50% aqueous solution "Drewsperse" 747A from Ashland Inc.

Typically, the total amount of PREC components that must be added in step (ii) of the method of the present invention to be effective (ie to effect an absolute 5% increase in retention for at least the polyvalent cation calcium compared to the blank ), at least 15 ppm. Preferably, the total amount of PREC components added in step (ii) of the method is at least 25 ppm, more preferably at least 35 ppm, and most preferably at least 50 ppm. Preferably, the total amount of PREC components added in step (ii) in the method according to the present invention is less than 5,000 ppm, more preferably less than 1,000, even more preferably less than 500 ppm, and most preferably less than 350 ppm. The PRC components can be added to the aqueous salt solution in pure form (solid or liquid) or as a solution in water.

A further advantage of the method according to the present invention is that sulfate ions which may be present in an aqueous salt solution will also be removed during the nanofiltration step due to the generally good to excellent sulfate retention properties of the nanofiltration membranes suitable for use in the present process. The low sulfate content of the purified aqueous salt solution (ie the permeate from step (iii) of the present process) has the following advantages. First, the electrolysis step will be less disturbed by the presence of hindering sulfate ions. Secondly, the flushing current in the final oxidation step, i.e. the electrolysis step, is significantly reduced. In e.g. chlorine production process, depleted brine is recycled to the salt dissolver to re-saturate the brine originating from the electrolysis cells back to preferably about 310 g/l. A recirculation flow of depleted salt solution in conventional processes typically has the disadvantage that contaminants in the process will accumulate, which requires the presence of a purification. Usually, the size of the purge flow is determined by the concentration of sulfate in the depleted brine to be recycled. With the nanofiltration process according to the invention, the amount of sulfate in the solution subjected to the electrolysis step can be reduced in comparison with the previous processes which do not include a nanofiltration step. Consequently, the cleaning flow is reduced and makes the process even more economically attractive.

In order to achieve the polyvalent cation removal stated above, no subsequent crystallization step or other treatment of the permeate is needed.

As a result of the preceding process, the purified aqueous salt solution resulting from step (iii) has such purity that it can be used directly for the subsequent step (IV) in which the purified aqueous salt solution is a raw material, i.e. the production of the chlorine-containing compounds in question .

The chlorine-containing compounds that can be produced using the method according to the invention are chlorine and sodium chlorate. Sodium chlorate, NaClO3, is a white, hygroscopic crystalline solid. The rapid growth in demand for sodium chlorate during the last decades is largely due to the introduction of chlorine-derived chlorine dioxide bleaching within the pulp and paper industry. Its other main use is as an intermediate in the production of chlorates of other metals (eg potassium chlorate used in matches and explosives, barium chlorate used in fireworks, and calcium chlorate used as a herbicide). Other uses are as an oxidizing agent in metallurgical operations and as an additive in agricultural products and dyes.

Sodium chlorate is produced according to this invention by subjecting the permeate obtained in step (iii) of the process according to the invention to a conventional electrolysis step, to produce chlorine, sodium hydroxide and hydrogen. The chlorine and sodium hydroxide are reacted to form sodium hypochlorite which is then converted to chlorate and chloride, preferably under controlled conditions of pH and temperature, as is generally known in the art. Preferably, the permeate is first made slightly acidic before being subjected to the electrolysis step to produce sodium chlorate. Furthermore, the solution preferably contains small amounts of oxidizing agents such as e.g. potassium dichromate to prevent the hydrogen released in the electrolysis from reducing the chlorate. Solid chlorate can be separated from the cell effluent by fractional crystallization, as is generally known in this field.

Most preferred is the production of chlorine. Chlorine is produced according to this invention by subjecting the permeate obtained in step (iii) of the process according to the invention to an electrolysis step, to produce chlorine, sodium hydroxide, and hydrogen, with the electrolysis cell in which the electrolysis step is carried out preferably by being constructed so that the reaction between chlorine and caustic soda is prevented, as is generally known in this area. The electrolysis step according to the invention includes electrolysis processes such as e.g. membrane electrolysis, diaphragm electrolysis, chlorate electrolysis, and mercury electrolysis. Most preferably, the step is a membrane electrolysis step.

An optional "polishing" step can be carried out in the method according to the invention. In a preferred embodiment, a further "polishing" step takes place between step (iii) and step (iv) of the process. The "polishing" step may include feeding the aqueous solution to an ion exchange process to remove the last traces of polyvalent ions from the system. Especially when step (iv) involves a membrane electrolysis step, a further ion exchange step between step (iii) and step (iv) is strongly preferred. It is noted that in the aqueous salt solution resulting from step (iii) in the method according to the invention, so many polyvalent ions have already been removed so that the possible ion exchange step can be carried out with very low chemical costs.

Preferably, the aqueous salt solution in the method according to the present invention comprises at least 150 g/l sodium chloride, more preferably at least 200 g/l, even more preferably at least 250 g/l, and even more preferably at least 300 g/l; most preferred is a saturated sodium chloride solution.

In an alternative embodiment of the method according to the present invention, the concentration of sodium chloride can be regulated to be at least 100 g/l in a step after or simultaneously with the addition of at least one positive retention-improving component PREC to the aqueous solution and before the nanofiltration step (iii).

It is noted that the term "salt source" as used herein is intended to name all salts of which more than 25% by weight is NaCl. Such salt preferably contains more than 50% by weight of NaCl. More preferably, the salt contains more than 75% by weight NaCl, while a salt containing more than 90% by weight NaCl is most preferred. The salt can be a solar salt (salt obtained by evaporating water from salt solution using solar heat), rock salt, and/or underground salt beds. When said salt source is dissolved in water to give an aqueous salt solution comprising at least 100 g/l sodium chloride it will comprise a total amount of at least 0.01 ppm polyvalent cationic contaminants. The required amount of one or more positive retention-improving components PREC according to the present invention is added to the thus prepared aqueous solution. However, it is also possible to add the PREC components to the salt source before the dissolution step or to the water before the dissolution step. A combination of these procedures is also possible.

Preferably, the salt source is an underground salt deposit that is exploited using solution mining. If the salt source is rock salt or solar salt, it is preferably transported to a salt solution to which water is added to produce the aqueous salt solution according to the present invention. The water can be from any water source conventionally used for this purpose, such as e.g. well water or surface water.

At least part of the retentate is recycled to the dissolution step (i). Consequently, it is recycled to the dissolver or to the underground salt bed. Recycling the retentate to the dissolver or underground salt bed has the following advantages. The aqueous salt solution subjected to the nanofiltration step comprises at least one positive retention-enhancing component PREC. A recycling of the retentate to the solvent thus reduces the amount of PREC that must be added to said aqueous salt solution. Furthermore, because polyvalent ionic contaminants are significantly retained by the nanofiltration membrane, these will accumulate in the salt solution recycled to the dissolver or subsurface salt bed. They will eventually reach their solubility limits and thus be deposited, e.g. in the form of anhydrite or polyhalite, at the bottom of the dissolver, where they can be easily removed via the sludge, or at the bottom of the cavity for the underground salt bed.

In this embodiment, it is possible to add one or more conventional retarding agents such as e.g. described in EP 1 404 614 to the dissolver or underground salt bed to further reduce the amount of contaminants present in the salt source and which will dissolve in the aqueous salt solution.

In a further embodiment, the polyvalent cations include, in addition to calcium, the polyvalent cations magnesium, strontium, iron, barium, aluminum or a mixture of two or more of these cations. In a further embodiment, the contaminating amount of polyvalent cations in the aqueous solution to be subjected to a nanofiltration step is less than 20,000 ppm and at least 0.01 ppm, more preferably less than 10,000 ppm, even more preferably less than 4,000 ppm and most preferably less than 2,000 ppm. Preferably, the contaminant amount is at least 0.1 ppm, even more preferably at least 10 ppm, most preferably at least 100 ppm.

In a further embodiment of the method according to the invention, the amount of either calcium or magnesium in the aqueous salt solution to be subjected to a nanofiltration step is less than 2,000 ppm, preferably less than 1,800 ppm, more preferably less than 1,600 ppm, most preferably less than 1,400 ppm. More preferably, the combined amount of calcium and magnesium is less than 2,500 ppm, preferably less than 2,000 ppm.

In yet another embodiment of the method according to the invention, the amount of sulfate ions in the aqueous solution to be subjected to the nanofiltration step is less than 75,000 ppm, preferably less than 50,000 ppm, more preferably less than 25,000 ppm, most preferably less than 10,000 ppm, and most preferably less than 8,000 ppm.

It is noted that the "nanofiltration membrane" housed within a membrane nanofiltration unit, as referred to throughout this disclosure, is intended to denote any conventional nanofiltration membrane designed to selectively reject divalent and other polyvalent anions and having a molecular weight threshold of at least 100 Da, preferably at least 150 Da, and wherein the molecular weight threshold is at most 25,000 Da, preferably at most 10,000 Da, more preferably at most 2,500 Da, and most preferably at most 1,000 Da. The nanofiltration system preferably uses semipermeable membranes of the nanofiltration type, such as e.g. those sold as "Film Tec" NF270 (The Dow Chemical Company), "DESAL" 5DK, "DESAL" 5DL and "DESAL" 5HL (all from GE/Osmonics), "NTR" 7250 (Nitto Denko Industrial Membranes), and "AFC"-30 (from PCI Membrane Systems Ltd). These and similar membranes suitable for use in the process of the present invention are effective in rejecting a high percentage of all divalent anions and especially sulfate and carbonate, as indicated by an observed sulfate retention of greater than 80% and preferably greater than 90% during processing of a 1 g/l MgSO4 solution in demineralized water in full recycling operation, while allowing the passage through the membrane of a high percentage of all monovalent anions and especially chloride and bromide, as indicated by a chloride retention below 80% and preferably below 70% during the processing of a 1 g/l NaCl solution in demineralized water in full recirculation operation. Tests with these solutions should be carried out at ambient temperature, a membrane flux of between 20 l/m2�h and 30 l/m2.h, and a cross-flow rate to avoid strong concentration polarization. Sodium chloride and magnesium sulfate retentions in these tests can be determined using calibrated conductivity measurements. Although a nanofiltration type semipermeable membrane such as the membrane types mentioned earlier are preferred, other nanofiltration membranes with these high divalent ion rejection characteristics can be obtained commercially and can alternatively be used.

The method according to the present invention is further illustrated by means of the following examples.

Examples

In the examples, the following definition is used:

Retention = {1-(concentration of component in permeate/concentration of component in retentate)} x 100%

Example 1

An experiment was conducted using two membrane types, a 10 cm spiral wound "NF" 270 polyamide thin film "NF" membrane (from DOW Chemical Company "FilmTec") and a 10 cm spiral wound "Desal" 5DK polyamide "NF" membrane ( from GE/Osmonics) with 7.6 m2 and 8.4 m2 membrane surface area respectively. The membrane modules were tested in parallel in a pilot plant operated in continuous feed and draw operation mode with a cross flow rate of approximately 3 m3/h per membrane module. Mother liquor obtained from a sodium chloride crystallizer was fed to the unit. The pH of the mother liquor was reduced to pH 10.7 using a concentrated (35%) H2SO4 solution. Furthermore, 101 ppm of the positive retention improving component was added to the mother liquor by adding 202 ppm of "Drewsperse" 747A from Ashland Inc., which is a 50% aqueous solution of polymaleic acid. The resulting mother liquor sent to the membrane test unit contained, among other things, 280 g/l NaCl, 0.25 meq/l calcium, 0.06 meq/l strontium and 1,190 meq/l SO 2-4. The bulk of the retentate was circulated to the membrane feed line (cross-flow operation) while a portion of the retentate was flushed out with the permeate to achieve a concentration factor (the ratio of fresh feed flow to the flushed retentate flow) of approximately 1.3. During membrane filtration at 32 bar pressure and 40 ºC, calcium retentions of 99% were achieved for "Desal" 5DK and "NF" 270 and strontium retentions of 88%.

Comparative example 2

A further experiment was carried out using two membrane types, planar sheet "NF" 270 polyamide thin film "NF" membranes (from DOW Chemical Company "FilmTec") and planar sheet "Desal" 5DK polyamide "NF" membrane (from GE/Osmonics ). The membrane types were tested simultaneously in a DSS lab stack unit, which was operated in continuous feed and draw operation mode at a cross flow rate of 600 l/h. A total of 0.144 m2 membrane surface area was installed. Mother liquor obtained from a sodium chloride crystallizer was supplied to the unit. The pH of the mother liquor was reduced to pH 10.8 using a concentrated H 2 SO 4 solution. No positive retention enhancing component was added. The mother liquor sent to the DSS unit contained, among other things, 1,150 meq/l SO 2-4, 296 g/l NaCl, 1.3 mg/l Ca2+, and 655 mg/l Br-. Membrane filtration was carried out at 50 bar pressure and 32 ºC temperature. The main part of the retentate was recycled to the membrane feed line (cross-flow operation) while part of the retentate was pumped together with permeate to achieve a concentration factor of about 1.3. The membranes showed calcium retentions below 32%.

Comparative example 3

A further experiment was carried out using two membrane types, flat sheet "NF" 270 polyamide thin film "NF" membrane (from DOW Chemical Company "FilmTec") and flat sheet "Desal" 5DK polyamide "NF" membrane (from GE/Osmonics) . The membrane types were tested simultaneously in a DSS lab stack unit, which was operated in a total recirculation mode (total retentate and permeate were recycled to the membrane feed vessel) with a crossflow rate of 600 l/h. A total of 0.36 m2 membrane surface area was installed. Raw salt solution obtained from a salt source was supplied to the device. No positive retention enhancing component was added. The raw salt solution sent to the DSS lab stack unit contained, among other things, 1.21 g/l SO 2-4, 273 g/l NaCl, 3.3 mg/l strontium, 10.3 mg/l magnesium, and 494 mg/l Ca2+. Membrane filtration was carried out at 21 bar pressure and 22 ºC temperature. "Desal" 5DK and "NF" 270 showed 36% and 24% calcium retention respectively, and strontium retentions below 59%. The magnesium retentions for "Desal" 5DK and "NF" 270 were found to be 68% and 66% respectively. The sulfate retentions for "Desal" 5DK and "NF" 270 were found to be 94.2% and 95.9% respectively.

Example 4

An experiment was conducted using two membrane types, a 10 cm spiral wound "NF" 270 polyamide thin film "NF" membrane (from DOW Chemical Company "FilmTec") and a 10 cm spiral wound "Desal" 5DK polyamide "NF" membrane (from GE/Osmonics) with respectively 7.6 m2 and 8.4 m2 membrane surface area. The membrane modules were tested in parallel in a pilot plant operated in a continuous feed and draw operation mode at a cross flow rate of 3.1 m3/h and 2.6 m3/h per membrane module for "NF" 270 and "Desal" 5DK respectively. Mother liquor obtained from a sodium chloride crystallizer was fed to the unit. The pH of the mother liquor was reduced to pH 10.6 using a concentrated (35%) H2SO4 solution. Furthermore, 96 ppm of a positive retention improving component was added to the mother liquor by the addition of 192 ppm "Drewsperse" 747A (see Example 1). The resulting mother liquor sent to the membrane test plant contained, among other things, 280 g/l NaCl, 0.046 meq/l completely dissolved calcium, and 1,125 meq/l SO 2-4. The bulk of the retentate was recycled to the membrane feed line (cross-flow operation) while part of the retentate was flushed out with the permeate to give a concentration factor (the ratio of fresh feed flow to the flushed retentate stream) of approximately 1.3 and 1.2 for "NF" 270 respectively "Desal" 5DK. During membrane filtration at 32 bar pressure and 34 ºC and 39 ºC respectively for "NF" 270 and "Desal" 5DK, calcium retentions of 96% and 97% were obtained for "Desal" 5DK and "NF" 270 respectively.

Example 5

A further experiment was carried out using two membrane types, flat sheet "NF" 270 polyamide thin film "NF" membrane (from DOW Chemical Company "FilmTec") and flat sheet "Desal" 5DK polyamide "NF" membrane (from GE/Osmonics) and a crude saline solution from the same source as indicated in Comparative Example 3. Furthermore, 300 ppm of a positive retention enhancing component was added to the crude saline solution by adding 600 ppm "Drewsperse" 747A (see Example 1). The membrane types were tested simultaneously in a DSS lab stack unit operated in total recirculation mode (total retentate and permeates were recycled to the membrane feed vessel) at a crossflow rate of 600 l/h. A total of 0.216 m2 of membrane surface area was installed. Raw salt solution obtained from a salt source was supplied to the device. The raw salt solution sent to the DSS lab stack unit contained, among other things, 1.11 g/l SO 2-4, 289 g/l NaCl, 3.0 mg/l strontium, 10.1 mg/l magnesium and 490 mg/l Ca2+. Membrane filtration was carried out at 31 bar pressure and 21 ºC temperature. "Desal" 5DK and "NF" 270 showed 79% and 50% calcium retention respectively, strontium retentions of 90% and 70% respectively and magnesium retentions of 93% and 79% respectively. Both membranes showed 0.6% chloride retention. "Desal" 5DK and "NF" 270 showed 96.8% and 98.5% sulfate retention, respectively.

Comparative example 6

An electrolysis trial was conducted using a laboratory membrane electrolysis cell with the permeate produced in Comparative Example 3 as a feed stream. The experiments were carried out with a constant current of 5A between the anode and the cathode. After about 3 hours, the electrolysis process was stopped because the potential difference across the membrane exceeded 4V.

Example 7

An electrolysis experiment was carried out with the same electrolysis equipment and under the same process conditions as described in comparative example 6, but now using the permeate produced in example 5. After 6 hours of operation, the laboratory setup was still in operation and the potential difference across the membrane was below 4V.

As illustrated by the previous examples, the retention of calcium and strontium ions using the method according to the invention is significantly improved compared to processes where no positive retention-enhancing component is added to the aqueous salt solution before it is subjected to a nanofiltration step, and therefore the resulting aqueous salt solution can be directly and advantageously used to convert the chloride anions into chlorine in an electrolysis process.

Claims (8)

Patent claims 1. Process for the production of chlorine or sodium chlorate using an aqueous salt solution containing at least 100 g/l sodium chloride and a contaminating amount of polyvalent cations, comprising the steps (i) preparing an aqueous salt solution containing at least 100 g/l sodium chloride and at least 0.01 ppm (parts per million) polyvalent cations such as calcium by dissolving a sodium chloride source in water, wherein the polyvalent cations include, in addition to calcium, the polyvalent cations the cations magnesium, strontium, iron, barium, aluminum or a mixture of two or more of these cations, (ii) adding an effective amount of at least one positive retention-enhancing component selected from the group consisting of (poly)carboxylic acids, phosphinocarboxylic acids, polyacrylic acids, polymaleic acids, glucose, sucrose, sucrose and sodium gluconate to the aqueous solution, wherein the total amount of positive retention-enhancing components that is added is at least 15 ppm, (iii) then subjecting the solution to a nanofiltration step using a nanofiltration membrane with a molecular weight threshold of between 100 Da and 10,000 Dalton so that the solution is separated into a retentate which is enriched in polyvalent cations, and a permeate which is the purified aqueous salt solution, ( iv) converting the chloride anions in the permeate to chlorine or sodium chlorate by means of an electrolysis step, and (v) recycling at least a portion of the retentate to the dissolution step (i). 2. Method according to claim 1, characterized in that the concentration of sodium chloride can be regulated to be at least 100 g/l in a step after or simultaneously with step (ii) with the addition of the positive retention-improving component to the aqueous solution and before the nanofiltration step (iii). 3. Method according to claim 1 or 2, characterized in that the aqueous solution comprises at least 150 g/l sodium chloride, preferably at least 200 g/l, and even more preferably at least 300 g/l, and is most preferably a saturated sodium chloride solution. 4. Method according to any one of the preceding claims 1 to 3, characterized in that the positive retention-improving component is added in step (ii) to the aqueous solution in an amount of at least 15 ppm, preferably at least 25 ppm, most preferably at least 50 ppm . 5. Method according to any one of the preceding claims 1 to 4, characterized in that the positive retention improving agent is added in step (ii) to the aqueous solution in an amount of less than 5,000 ppm, more preferably less than 1,000 ppm, and most preferably less than 350 ppm. 6. Method according to claim 5, characterized in that in step (i) any of calcium or magnesium is present in the aqueous solution in an amount less than 2,000 ppm, preferably less than 1,800 ppm, more preferably less than 1,600 ppm, and most preferably less than 1,400 ppm. 7. Method according to any one of the preceding claims 1 to 6, characterized in that in step (i) sulfate anions are present in the aqueous salt solution in an amount of less than 75,000 ppm, preferably less than 50,000 ppm, more preferably less than 25,000 ppm, most preferably less than 8,000 ppm. 8. Method according to any one of the preceding claims 1 to 7, characterized in that the sodium chloride source is a natural salt source containing more than 75% by weight NaCl and most preferably more than 90% by weight NaCl.