ES2388252A1 - Method for converting brines into acids and bases, and products obtained - Google Patents

Abstract

The invention relates to a method for treating brines, which includes the steps of: subjecting a stream of brines (4, 13) to a pretreatment (5, 14) by means of membrane separation (15) designed such as to obtain a stream of brines that are substantially free of divalent ions (7, 16) and a residual stream which includes an electrolytic solution (6, 17) subjecting the stream of brines (7, 16) obtained in the preceding step to a treatment (8, 18) by means of electrodialysis using bipolar membranes designed such as to obtain a stream of hydrochloric acid (9, 28) and a stream of sodium hydroxide (10, 29), as well as a stream of desalinated water (30). The invention also relates to a hydrochloric acid solution obtained by means of the preceding method. The invention also relates to a sodium hydroxide solution obtained by means of the preceding method.

Description

PROCESS OF CONVERSION OF SALMUERAS IN ACIDS AND BASES AND PRODUCTS OBTAINED FIELD OF THE INVENTION

The present invention belongs to the field of water desalination processes and treatments and, in particular, the recovery of brackish residual currents for obtaining reusable acids and bases in the process and / or marketable.

BACKGROUND OF THE INVENTION

Water desalination is an emerging and promising alternative to obtain drinking water. An increasing number of desalination plants use membrane technologies, which together with the desalinated water stream, generate a concentrated water stream, rejection or brine, which is characterized by having a high concentration of salts, mainly sodium chloride (NaCl), which it gets to double the value of the starting water. When brines are discharged into the sea, as is the case in coastal desalination plants, they cause a series of adverse environmental effects on the receiving marine environment that are subject to recent studies (Water Research, 44 (18) (2010), Elsevier Science Publishers BV, Amsterdam, pp. 51175128, by DA Roberts, EL Johnston, NA Knott: "Impacts of desalination plant discharges on the marine environment: a critical review of published studies"; Journal of Industrial Ecology 14 (3) (2010), Wiley-Blackwell Publishers, Hoboken, NJ (USA), pp. 512-527, by M. Meneses, JC Pasqualino, R. Céspedes-Sánchez, F. Castells: "Alternatives for reducing the environmental impact of the main residue from a desalination plant ").

The situation is further aggravated in desalination plants located in the interior, given that, since there is no possibility of direct discharge to the sea, the management of concentrates is an even greater problem. Traditionally, the options used to minimize the effect of these concentrates are the generation of salts by evaporation in lagoons, which requires large areas of land, and injection into underground wells, which can lead to salinization of underground aquifers and soils.

Therefore, it is urgent to develop treatments that minimize these spills giving priority to those processes that enable the recovery of materials, as indicated in Water Research, 46 (2) (2012), Elsevier Science Publishers B.V., Amsterdam, p. 267-283, by A.Pérez-González, A.M. Urtiaga, R.lbañez. LOrtiz: "State of the art and review on treatment technologies of water reverse osmosis concentrates".

The treatment of brines from desalination plants, in the cases in which it is carried out, focuses on the recovery of the maximum amount of treated water possible, either by reverse osmosis desalination processes in series stages (with

or without intermediate treatments), or by the combination of treatments such as evaporation or precipitation to have a zero liquid discharge.

Due to the large amount of salts contained in the brines, the treatments focused on the recovery of substances are focused on the recovery of these salts, applying precipitation or selective crystallization processes.

Brines from desalination plants are particularly rich in sodium and chloride ions, so that the recovery of the corresponding acids and bases entails significant benefits not only from an environmental point of view, but also as a source of chemicals that can be reused therein. plant or marketed.

Hydrochloric acid (Hel) is one of the most commonly used acids both in industry and in other applications. Hydrochloric acid is obtained in the industry as a byproduct in the organic chlorination reactions of organic compounds with elemental chlorine or in the chlor-alkali industry, in which a common salt solution (NaCl) is hydrogenated, producing chlorine, hydroxide of sodium and hydrogen The chlorine gas thus obtained can be combined with the hydrogen gas, forming chemically pure Hel gas. The Hel that is in the market usually has a concentration of 38% or 25% by weight. Solutions of concentration somewhat higher than 40% are chemically possible, but the evaporation rate in them is so high that extra storage and handling measures have to be taken. In the market it is possible to acquire solutions for current use with a concentration between 10% and 12% by weight, mainly used in cleaning. In desalination plants it is used in much lower concentrations for pH regulation processes and equipment cleaning. Currently, sulfuric acid is used more because it is cheaper, but it favors the appearance of fouling phenomena such as fouling, reducing the effectiveness of cleaning processes. Sodium hydroxide (NaOH) is a basic hydroxide used primarily

in the manufacture of paper, fabrics and detergents. It is mostly obtained in chloro-alkali electrolysis.

The recovery process of Hel and NaOH from brines from water desalination simplifies the process of obtaining these compounds for applications in which the quality and purity requirements are not high, while at the same time taking advantage of these residual currents The brine treatment must be integrated into the global water desalination process in order to convert the desalination of water into a sustainable process.

SUMMARY OF THE INVENTION

The present invention seeks to solve the aforementioned drawbacks by a brine treatment process comprising the steps of: subjecting a brine stream to a pretreatment by membrane separation designed to obtain a brine stream substantially free of divalent ions and a stream residual comprising an electrolytic solution; subject the brine stream obtained in the previous stage to a treatment by electrodialysis with bipolar membranes designed to obtain a stream of hydrochloric acid and a stream of sodium hydroxide, in addition to a stream of desalinated water.

Preferably, the brine stream comes from a desalination plant and comprises the previous desalination stage from brackish water, by treatment with membranes, this prior stage producing said brine stream.

Preferably, the pretreatment step for obtaining a brine stream substantially free of divalent ions is performed by separation by nano filtration membranes.

In a particular embodiment, the step of obtaining a stream of hydrochloric acid and a stream of sodium hydroxide by electrodialysis with bipolar membranes is performed by at least one unit formed by a first feed compartment separated from a second acid compartment by a membrane anionic, and a third base compartment separated from said first feed compartment by means of a cationic membrane, wherein said unit is delimited by two bipolar membranes that in turn respectively separate the base compartment and the acid compartment, from a compartment by the circulating the electrolyte solution obtained in said pretreatment stage, the set of membranes being subjected to the influence of an electrical potential.

Preferably, the hydrochloric acid stream obtained has a concentration of hydrochloric acid ranging between 2% and 5% by weight.

Preferably, the sodium hydroxide stream obtained has a concentration

of sodium hydroxide that varies between 1.5% and 4% by weight.

In another aspect of the invention, there is provided a solution of hydrochloric acid with a concentration between 2% and 5% by weight and a solution of sodium hydroxide with a concentration between 1.5% and 4% by weight, obtained by the process described above.

The process described here is an important step forward, since the problem of brine management is solved at the same time that an improvement in the overall costs of the process is obtained.

Other advantages of the invention will become apparent in the following description.

BRIEF DESCRIPTION OF THE FIGURES

In order to help a better understanding of the characteristics of the invention, in accordance with a preferred example of practical realization thereof, and to complement this description, a set of drawings is attached as an integral part thereof, whose character is Illustrative and not limiting. In these drawings:

Figure 1 shows a global flow chart of the process, according to an embodiment of the invention.

Figure 2 shows a scheme of the process according to a possible embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In this text, the term "comprises" and its variants should not be understood in an exclusive sense, that is, these terms are not intended to exclude other technical characteristics, additives, components or steps.

In addition, the "approximately", "substantially", "around", "ones", etc. they should be understood as indicating values close to those mentioned by those tenors, since due to calculation or measurement errors, it is impossible to achieve those values with complete accuracy.

Brines from water desalination are residual streams from the process of obtaining desalinated water, with a concentration in salts that approximately doubles the concentration in salts of the starting water. In the coastal plants the brines are poured directly into the sea, while in the interior plants they condition their location, due to the need for storage of the brines.

In turn, "brackish water" means water whose salt content exceeds a threshold threshold, above 0.05% by weight, as is the case with groundwater. Seawater, meanwhile, usually has a salt content of approximately 3% by weight. According to this composition of the starting water, the total salt content of the brines generated during the desalination process varies between 2% to 8% by weight.

The following preferred embodiments are provided by way of illustration, and are not intended to be limiting of the present invention. In addition, the present invention covers all possible combinations of particular and preferred embodiments indicated herein. For those skilled in the art, other objects, advantages and features of the invention will be derived partly from the description and partly from the practice of the invention.

The process of treating brines from desalination plants for the recovery of acids and bases is described below. Specifically, the invention relates to obtaining hydrochloric acid and sodium hydroxide from rejection water or brine from the desalination of brackish water.

As the general flow diagram of the process, illustrated in Figure 1, shows, the process uses rejection or brine water (4) that is generated in obtaining desalinated water (3) from brackish water (1) by means of a desalination stage (2). The brine (4) is subjected to a pre-treatment (5) in which compounds that can interfere with the quality of the final acids and bases, mainly calcium and magnesium salts and sulfates (6) are eliminated. This stream (6) will be used in the main stage of acid and base generation. Other compounds present in the brine (6) that may also interfere with the quality of the acids and final bases, such as strontium or iron, appear in a proportion lower than 0.1% by weight, so that their interference is minimal . The pre-treatment (5) comprises one or several stages of membrane separation, not illustrated in detail in Figure 1, after which the softened brine (7) is subjected to the main treatment (8) in which acid is produced hydrochloric, HCl, (9) and sodium hydroxide, NaOH, (10) from sodium chloride, NaCl, the main component of brines.

Figure 2 illustrates the process in detail. The process incorporates a series of stages of treatment properly interconnected by means of the relevant pipes and pumping systems. The brackish water stream (11) is subjected to a conventional membrane treatment stage (12), which constitutes the main desalination stage. One skilled in the art knows that, depending on the type of brackish water, this stage of treatment with membranes (12) can be by reverse osmosis, electrodialysis, or others. Hence two streams arise: the desalinated water stream (11 '), which depending on its quality can be used for various purposes, such as irrigation or drinking water, and the rejection or brine stream (13).

Said brine stream (13) is subjected to a pre-treatment process (14), since it also contains in addition to NaCl a high concentration in other salts, mainly calcium and magnesium, and sulfates, as indicated in relation to the Figure 1, from which it is desired to separate and recover the sodium chloride, NaCl, from which the final streams of hydrochloric acid, Hel, and sodium hydroxide, NaOH are subsequently generated. Divalent ions (such as calcium, magnesium and sulfates), in addition to decreasing the purity of the recovered products, can cause scaling salts and cause fouling phenomena in the stage of obtaining Hel and NaOH if they are not eliminated. Therefore, divalent ions such as calcium, magnesium and sulfates should be eliminated mainly.

The pretreatment stage (14) used to remove these salts is based on a membrane filtration (15). Commercial nanofiltration membranes are preferably used. The membranes (15) of this stage have as their main characteristic that they are capable of penetrating monovalent ions and rejecting divalent ions, because they have a pore size of the order of 1 nm and are electrically charged. In this way, calcium, magnesium and sulfate (divalent) ions are removed (17) from the brine (16). In particular, this pretreatment step (14) results in a brine stream substantially without encrusting salts (16), which is also known as a soft brine stream, and an electrolyte solution (17) rich in sulfates primarily. The pretreatment stage (14) may be connected by one or more modules based on these membranes (15). The specific arrangement of membrane modules (15) depends on several aspects, such as the feed rate, the composition of the brine to be treated and the quality of the brine that is to be obtained, mainly. The brine stream (16), whose major component is NaCl, passes to the next stage (18) in which the acid and basic streams are obtained.

In the step (18) of obtaining Hel hydrochloric acid and NaOH sodium hydroxide, cationic (23) and anionic (24) monopolar membranes are preferably used which are placed together with membranes that combine the two types, called bipolar (22), in series alternatives working under the influence of an electric potential. To generate this electrical potential, the membrane assembly (22,23,24) is placed between two electrodes, cathode (25) and anode (26), connected to a power source (not illustrated in Figure 2). Membranes called bipolar (22), which combine an anionic membrane and a cationic membrane, are designed to generate hydroxyl ions and protons from water molecules by dissociating said molecules in the presence of an electrical potential. Figure 2 illustrates a possible embodiment, with a three compartment configuration: feed (20), acid (21) and base (19). In the embodiment of Figure 2 this three compartment configuration is implemented twice, constituting 2 repeating units. Each repeating unit is formed by the set of bipolar (22), cationic (23) and anionic (24) membranes, being delimited by bipolar membranes (22). The number of repeat units can be increased to improve separation efficiency. In accordance with the configuration shown in Figure 2, the brine stream substantially free of encrusting salts (16) is passed through the feed compartment (20) between the cationic and anionic membranes (23) and (24). Sodium cations cross the cationic membrane (23) into the base compartment, delimited by this membrane (23) and by the bipolar membrane (22) arranged with its cationic face towards the cathode (25) to generate hydroxyl ions that combine with sodium cations, forming sodium hydroxide (29). In turn, the chloride anions cross the anionic membrane (24) and pass into the acid compartment (21), delimited by this membrane (24) and by the bipolar membrane (22) arranged with its anionic face towards the anode (26) to generate protons that combine with chloride anions, forming hydrochloric acid (28). In this way the brine stream (16) becomes a stream of desalinated water (30) that can be directly discharged without adverse environmental effects.

The bipolar membranes (22) are located in front of the electrodes delimiting a compartment (27) through which the electrolytic solution (17) from the pretreatment stage (14) circulates. This solution (17) is characterized by containing ions that favor the generation of electrical potential, but by the arrangement of the membranes (22) these ions do not reach the base compartment

(19) or the acid compartment (21). Therefore, the residual current (17) generated in step (14) as a result of the brine pretreatment process, mainly sulfate rich, can be used as an electrolytic solution (17).

The system has a purge (31) to empty the electrolyte compartment when necessary. This purge stream (31) rich in sulfates can be directly discharged without adverse effects, and if necessary, by special sensitivity of the receiving medium, can be diluted by mixing with the stream (30).

In this way, the main outflows of the global process are the desalinated water stream (30), the hydrochloric acid stream (28) and the sodium hydroxide stream (29). Thanks to the described configuration, a stream of hydrochloric acid (28) is obtained with a concentration that varies between 2% and 5% by weight and a stream of sodium hydroxide (29) with a concentration that varies between 1.5 % and 4% by weight.

Preferably, the acidic HCl stream is used in the desalination process to improve chlorination and reduce the risk of bicarbonate precipitation. The basic NaOH stream is preferably used in chemical cleaning and for pH correction in product water. As mentioned in the state of the art, sulfuric acid is currently used more for being cheaper, but it favors the appearance of fouling phenomena such as fouling, reducing the efficiency of cleaning processes. For this reason, obtaining diluted hydrochloric acid as a byproduct of the process is an improvement both from a technical and economic point of view. In addition to the reuse of these currents in the desalination process itself, these products can be used in other water treatment processes or in the production of generic cleaning agents.

The process described in the present invention is applicable, among others, in coastal desalination plants, in which brines are usually dumped directly into the sea, and in interior desalination plants, whose brines condition their location, due to The need for brine storage. The process described here is therefore an advance both from an environmental and economic point of view, due to the possibility of using products.

In sum, the integrated process of obtaining acids and bases from the brines generated in desalination processes, of the invention, is an obvious novelty within its field of application since it manages to solve, for example, the environmental and logistical problem associated to the management of these brines by deep injection or evaporation, at the same time that it constitutes an advantage in the overall balance of process costs since the acids and bases obtained are reusable within the global process and / or marketable.

An experimental example of the process for obtaining hydrochloric acid and sodium hydroxide is described below, in accordance with the described embodiment:

Description of an embodiment

The operating conditions and the results obtained in a variant of interest of the invention are described by way of example below:

A brine from a desalination plant that uses reverse osmosis as a desalination technology is treated. This brine or rejection water has a total salt content of 6.5% by weight, of which 5.6% is sodium chloride, with its majority composition being: 34.9 giL chlorides, 5.3 giL sulfate, 2.5 giL magnesium, 1 giL calcium. This brine is subjected to a nanofiltration pretreatment stage, using a commercial polyamide membrane with a maximum operating temperature of 45 ° C and a maximum operating pressure of 41 bar. The pretreatment by nanofiltration is carried out at a pressure of 10 bars and at a constant temperature of 20 ° C. Under these conditions a sulfate rejection of 95% is achieved, rejection of

80% calcium and 53% magnesium rejection. Chloride rejection is from

Only 5%.

Therefore, after the pretreatment stage the softened brine has a content

Total in salts of 5.34% by weight, of which 5.3% is sodium chloride. With this softened brine, the step of obtaining acids and bases is carried out. For this, commercial cationic, anionic and bipolar membranes are used, arranged in two repeating units as detailed in Figure 2. The working mode of the system is in recirculation so that after a treatment period of between 4

At 6 hours, working at a current density of between 800 and 1000 Nm2, a current of 3.25% hydrochloric acid by weight and a sodium hydroxide stream of 1.9% by weight are obtained. The resulting desalinated water stream has a salt content of approximately 0.5% by weight, corresponding to the brackish water entering the global process.

Claims (7)

1. A brine treatment process characterized by the stages of:

-

subjecting a brine stream (4, 13) to a pretreatment (5, 14) by membrane separation (15) designed to obtain a brine stream substantially free of divalent ions (7, 16) and a residual stream comprising a solution electrolytic (6.17);

-

subject the brine stream (7, 16) obtained in the previous stage to a treatment (8, 18) by electrodialysis with bipolar membranes designed to obtain a hydrochloric acid stream (9, 28) and a stream of sodium hydroxide (10, 29), in addition to a stream of desalinated water (30).

2.

The process of claim 1, wherein said brine stream (4, 13) comes from a desalination plant and comprises the previous stage of obtaining desalinated water (3, 11 ') from brackish water (1, 11), by membrane treatment (2, 12), this previous stage producing said brine stream (4, 13).

3.

The process of any of the preceding claims, wherein said pretreatment step (5, 14) for obtaining a brine stream substantially free of divalent ions (7, 16) is carried out by separation by nanofiltration membranes (15).

Four.

The process of any of the preceding claims, wherein said step (8, 18) of obtaining a stream of hydrochloric acid (9, 28) and a stream of sodium hydroxide (10, 29) by electrodialysis with bipolar membranes is carried out by less a unit formed by a first feed compartment (20) separated from a second acid compartment (21) by an anionic membrane (24), and a third base compartment (19) separated from said first feed compartment (20) by a cationic membrane

(23), where said unit is bounded by two bipolar membranes (22) which in turn separate the base compartment (19) and the acid compartment (21), respectively, from a compartment (27) through which the solution circulates electrolytic (6, 17) obtained in said pretreatment stage (5, 14), the whole being 5 membranes subjected to the influence of an electrical potential (25, 26). 5. The process of any of the preceding claims, wherein said hydrochloric acid stream obtained (9, 28) has a concentration of hydrochloric acid ranging between 2% and 5% by weight. 6. The process of any of the preceding claims, wherein said stream of sodium hydroxide obtained (10, 29) has a concentration of sodium hydroxide ranging between 1.5% and 4% by weight. A solution of hydrochloric acid (9, 28) with a concentration between 2% and 5% by weight, obtained by the process of any one of claims 1 to 6. 8. A solution of sodium hydroxide (10, 29) with a concentration between 1.5% 20 and 4% by weight, obtained by the process of any of claims 1 to 6.