WO2017174839A1

WO2017174839A1 - Method for obtaining magnesium hydroxide from saline effluent

Info

Publication number: WO2017174839A1
Application number: PCT/ES2017/070095
Authority: WO
Inventors: Hicham El Bakouri; Abel Riaza Frutos; Jose Morillo Aguado; Jose Usero Garcia
Original assignee: Abengoa Water, S.L.
Priority date: 2016-04-05
Filing date: 2017-02-22
Publication date: 2017-10-12
Also published as: ES2641269B1; ES2641269A1

Abstract

The invention relates to a method for obtaining magnesium hydroxide from saline effluent, comprising an acidification and desorption step for the elimination of carbon species, and alkalinisation and basification steps for the purpose of precipitating the magnesium hydroxide.

Description

PROCEDURE FOR OBTAINING MAGNESIUM HYDROXIDE FROM

OF EFFLUENT SALINOS

DESCRIPTION

The present invention relates to a process for obtaining magnesium hydroxide (Mg (OH) 2) from saline effluents.

The invention is part of the technical sector of water treatment, more specifically in the treatment of effluents of high salt content.

Background of the invention

Magnesia (MgO), magnesium oxide that can be obtained from the calcination of Mg (OH) 2, has an important international import and export market. Mg (OH) 2 is used primarily for water treatment in the chemical, pharmaceutical and medical industries. It is also used in the construction sector and in rubber processing.

Currently, the production of MgO is mainly carried out in two ways; the first from mining extraction, with higher daily costs that make it necessary to access magnesites whose MgO content exceeds 40%, and the second from high-salinity stream water, mainly seawater, which It has now become the most competitive alternative with the previous one. The process to obtain MgO from seawater was carried out for the first time in the United Kingdom in 1937 and, from 1942, its use was extended in the USA in order to meet the needs of refractory brick of MgO associated with the steel industry.

In general, patent documents related to this technique focus on equipment development or process optimization in order to obtain a better quality of magnesium salts. The most important patent documents in this context are described below. Patent document with publication number US4229423 describes a method of production of Mg (OH) 2. In this procedure, Mg (OH) 2 is obtained from an effluent by means of a first stage of CO2 removal by acidification and a second stage of precipitation of Mg (OH) 2 with excess of calcined dolomite or calcium oxide (CaO), to subsequently separate the Mg (OH) 2 from the brine; part of the Mg (OH) 2 is used as nucleation particles or "seed" of this second stage, and the rest of Mg (OH) 2 is reacted with more brine. In this way, Mg (OH) 2 is obtained with a low amount of boron and CaO. Finally, this process includes a stage of filtration and final washing in order to eliminate residual boron.

The patent document with publication number US1505202, referring to the recovery of magnesium compounds from brines, describes the precipitation of Mg (OH) 2 by the addition of calcium hydroxide (Ca (OH) 2). In this document the effluent is treated with Ca (OH) 2 after filtration and removal of

CO2 by acidification.

Patent document with publication number US2595314 describes a process for the production of Mg (OH) 2 from seawater precipitating it with lime or calcined dolomite. In the process, a portion of the thickener suspension that is in series with the reactor is recycled. Mg (OH) 2 precipitates in the presence of seed crystals that must be present in at least an amount of 300% of the amount of Mg (OH) 2 that precipitates in the reaction zone, or must be in a ratio of 3: 1 to 10: 1 if high purity is required in the Mg (OH) 2 crystals. The calcined dolomite is added with particle size between 20 and 200 meshes (75-850 μηι). Particles between 48 and 100 meshes (150-350 μηι) favor sedimentation of the precipitate. Therefore, in this document it is necessary to use dolomite with a specific grain size to obtain an adequate production of Mg (OH) 2.

As disclosed in the state of the art, many procedures have been developed for obtaining magnesium salts from saline effluents but today, procedures that improve the purity of Mg (OH) 2 obtained especially if the effluent are needed saline has a high ion concentration calcium (Ca ²⁺ ) and sulfates (S0 ₄ ^{2 "} ) -

Description of the invention To solve the problem described in the state of the art, which makes it necessary to improve the purity of Mg (OH) 2 obtained from saline effluents with high concentration of Ca ²⁺ and S0 ₄ ^{2 "} , it has been developed a process for obtaining Mg (OH) 2 from saline effluents, where the Mg (OH) 2 obtained has a high purity.

The process of the invention presents a stage for the removal of carbonates (CO3 ^{2 "} ) from the saline effluent using an acid and a desorption stage and subsequent stages of precipitation of Mg (OH) 2. Therefore a first aspect of the The present invention relates to a process for obtaining Mg (OH) 2 from saline effluents comprising the steps of: a) acidifying a saline effluent with an acidic stream at a pH between 3 and 5, obtaining an acidified stream;

b) perform a desorption of the carbon dioxide (CO2 from carbonates) present in the acidified stream obtained in step a), said acidified stream passing through a filler material;

c) passing the effluent obtained in step b) through a bed of calcined dolomite until reaching a pH between 9 and 10;

d) precipitate Mg (OH) 2 from the effluent obtained in step c) by adding a base until a pH of 1.01 to 11.5 is reached;

e) decant the precipitate of Mg (OH) 2 from the solution obtained in d).

In step b) of desorption, the acidified stream obtained in step a) is contacted with a filler material. The landfill must have a high surface area to improve CO2 transfer. Preferably in step b) of desorption, the acidified stream obtained in step a) is contacted in a countercurrent with a gaseous stream through a filler material. More preferably, the gas stream is an air stream. As mentioned, the effluent from stage b) is contacted with a bed of calcined dolomite until a pH range between 9 and 10 is reached.

Optionally after stage b) of desorption an absorption stage b1) would take place in which the gaseous stream charged with CO2 obtained in b) is brought into contact with an alkaline solution preferably NaOH, obtaining sodium carbonate (Na2COs) .

The precipitation of stage d) maximizes production and minimizes impurities.

In the present invention saline effluents are effluents with a high concentration of salts, in particular residual effluents from desalination plants and saline effluents with high calcium content, greater than 500 mg / l, and sulfates, greater than 1000 mg / l. Particularly effluents from water purification processes where reverse osmosis membranes are used.

In the event that the effluent has a high calcium content, the process optionally comprises a first stage a1), of carbonation prior to stage a), where calcium is removed so as to reduce the joint precipitation of the calcium carbonate (CaCOs) and calcium sulfate (CaS0 ₄ ) with Mg (OH) ₂ .

In the event that the saline effluent has a high sulfate content, the described method comprises an optional stage aO) prior to stage a1) of carbonation, for the elimination or reduction of the sulfate content.

Likewise, the described method may additionally comprise a final step e1) of purification of the precipitate.

Brief description of the drawings

For a better understanding of how much has been exposed, some drawings are attached in which, schematically and only by way of non-limiting example, a practical case of realization is represented. Figure 1 shows a schematic of the process of the invention.

Figure 2 shows a scheme of a preferred embodiment of the process of the invention.

Description of a preferred embodiment

As said, the first aspect of the invention is a process for obtaining Mg (OH) 2 from saline effluents comprising the steps of:

a) acidifying a saline effluent (1) with an acidic stream (2) at a pH between 3 and 5 obtaining an acidified stream (3);

b) perform a desorption of carbon dioxide (4) (CO2 from carbonates) present in the acidified stream obtained in step a), said acidified stream passing through a filler material;

c) passing the effluent (5) obtained in step b) through a bed of calcined dolomite until reaching a pH between 9 and 10;

d) precipitate the Mg (OH) 2 from the effluent (6) obtained in step c) by adding a base (7) until a pH of 1.01 to 11.5 is reached;

e) decanting the precipitate (9) of Mg (OH) 2 from the solution (8) obtained in d).

The acidification step a) is preferably carried out with an inorganic acid (2) and in particular with hydrochloric acid (HCI). In a preferred embodiment, stage b) of desorption is carried out in a tower where the acidified current (3) of stage a) is preferably introduced through the top of the tower through a arrester and distributed evenly. on the filling material, while the gas stream (10) is preferably introduced from the bottom. In a preferred embodiment, the gas stream (10) is an air stream. The effluent (5) obtained in step b), leaves the tower from the bottom; the stream of CO2-charged air (1 1) exits from the top. The filling material is preferably formed by plastic parts that have a high surface to volume ratio. Optionally after stage b) of desorption there is an absorption stage b1) in which the stream of CO2-laden air (1 1) obtained in b) is contacted in countercurrent with an alkaline solution (23), preferably NaOH, obtaining Na2CÜ3 (12).

Preferably, the particle size of the calcined dolomite used in step c) is larger than 20 mm. This particle size improves the hydration of MgO, which leads to an increase in the purity of the Mg (OH) 2 obtained. More preferably, the particle size of the calcined dolomite is in the range between 20 mm and 40 mm.

Preferably the basification of step d) is performed with calcined dolomite (7). Preferably step d) is carried out in a two-zone reactor (13), followed by a two-zone flocculation chamber (14). More preferably in the event that step d) is carried out in a two-zone reactor (13) followed by a two-zone flocculation chamber (14), the two-zone reactor (13) and the two-zone flocculation chamber (14) are purged and between 25% and 30% of each purge is recirculated, the purge that starts from the two-zone reactor (15) and the purge that starts from the flocculation chamber (16), is recirculated to the two-zone reactor (13). It has been found that recirculation rates greater than 30% increase the impurity content in the product. On the other hand, a recirculation rate of less than 25% results in a decrease in the amount of product as the reaction yield decreases.

In a preferred embodiment, once the Mg (OH) 2 is decanted, said precipitate (9) is subjected to a step e1) of purification of the precipitate in which it is washed with acidified fresh water (17), subsequently reprecipitated with a base and it filters. Preferably the base is soda, lime or calcined dolomite.

In another preferred embodiment, in the event that the saline effluent has a high calcium content, the method optionally comprises a first carbonation stage a1) prior to stage a) where calcium is removed so that it is possible to reduce the joint precipitation of the CaCC> 3 and CaSCU with the Mg (OH) ₂ .

This carbonation stage a1) is carried out in three alternative ways. - NaaCC is added to excess saline effluent in a reactor. The advantage associated with this stage is that it is possible to precipitate at least 90% of the calcium present in the effluent, without the need to raise the pH with a base and without losing magnesium in the effluent or;

- the pH is raised to a pH between 9 and 10.5 preferably 9.5 with NaOH and together Na2CÜ3 is added in a reactor. The advantage associated with this stage is to achieve the precipitation of at least 97% of the calcium present in the effluent or;

- the pH is raised with Ca (OH) 2 or with CaO at least between 9 and 10.5 and Na2CÜ3 is added in a reactor, in this way the precipitation of 97% of the calcium present in the effluent is also achieved. Preferably CaO is added so that the temperature of the solution is increased which favors the formation of insoluble carbonates.

This optional carbonation stage a1) is carried out in a reactor for a hydraulic retention time between 10 minutes and 30 minutes. Particularly during a hydraulic retention time of 15 minutes.

In the event that at this stage a1) of carbonation two NaOH and Na2C03 reagents are used; or Na2CÜ3 with Ca (OH) 2 or CaO, the stage is carried out in two series reaction equipment, in the first of rapid mixing where the Na2C03 is added, and in the second the NaOH is added, Ca (OH ) 2nd CaO.

Preferably, the Na2CÜ3 used in carbonation stage a1) prior to stage a) where calcium is removed, is Na2CÜ3 obtained in absorption stage b1). The Na2CÜ3 obtained in stage b1) is recirculated to the reactor of stage a1) thus reducing CO2 emissions, obtaining a more environmentally friendly process.

In another preferred embodiment shown in FIG. 2, in the event that the saline effluent (1) has a high sulfate content, the described method comprises a step aO), of elimination or reduction of the sulfate content, initial prior to step a1) of carbonation for the elimination or reduction of sulfate content. In this case, the saline effluent (1) Ca (OH) 2 (18) is added, until a pH between 9.5 and 10.5 is reached, preferably 10 and then calcium chloride (CaC) (19) is added. CaC (19) is added in a concentration that allows the CaSCU solubility limit to be exceeded by at least 50%. This precipitation in the form of crystals is very slow, so to avoid subsequent supersaturation and precipitation, the reaction must be carried out in the presence of a large concentration of nucleation particles. Therefore it is necessary to maintain a high concentration of CaSCU. Subsequently, the sulfate-free saline effluent (20) is conducted to carbonation stage a1), in which the calcium described above is removed. Therefore, in a preferred embodiment, the process comprises a step aO), of elimination or reduction of the sulphate content, prior to step a1) where Ca (OH) 2 effluent is added until a pH between 9.5 and 10.5 and CaC and the effluent obtained is conducted to step a1). In FIG. 2 shows a preferred embodiment where the saline effluent without sulfates (20) but with a high calcium content is carbonated by raising the pH with Ca (OH) 2 (21) and adding Na2CÜ3 (12) obtained in step b1). The saline effluent without sulfates and without calcium (22) is acidified in step a) and the process continues.

EXAMPLES Example 1 The process carried out in a pilot plant with the effluent of a reverse osmosis seawater desalination plant with the composition indicated in Table 1 is described below.

Parameters Value

Conductivity (mS / cm) 73

PH 7.9

Dissolved solids (mg / L) 54.6

Calcium (mgCa ² 7L) 960 Magnesium (mgMg ² 7L) 2, 1

Sodium (mgNa7L) 15.3

Chlorides (mgCIVL) 28.7

Sulfates (mgS0 ₄ ² 7L) 4.6

Lithium (mgLi / L) 0.6

Potassium (mg / L) 720

Carbonates (mgCOsCa / L) n.d.

Alkalinity (mgC0 ₃ Ca / L) 445

Table 1. Composition of residual effluent from seawater desalination plant. The first stage of the process consisted in the removal of calcium from the effluent through a carbonation process. For this, a dose of Na2CÜ3 of 2 kg / m ³ of treated effluent was added, a reaction time of 15 minutes was maintained and at a later stage of decantation the solid phase was separated. The analysis of the treated effluent revealed the removal of 90% of calcium, without significantly affecting the magnesium content of the brine (see table 2). Tests were also carried out by adjusting the pH in the reaction stage with NaOH or Ca (OH) 2, obtaining good results in relation to the removal of calcium (see tables 3 and 4).

Table 2. Effluent treated with Na2CÜ3. Na ₂ C0 ₃ NaOH Ca Mg

PH% Ca% Mg

(kg / m ³ ) (kg / m) (kg / m ³ ) (kg / m ³ )

final deleted removed added added final final

2.5 80 9.0 30.9 96.8 1989 2.0

3.0 40 9.0 24.7 97.4 2022 0.4

Table 3. Treatment at pH 9 with MaOH.

Na ₂ C0 ₃ Ca (OH) ₂ Ca Mg

PH% Ca% Mg

(kg / m ³ ) (kg / m ³ ) (kg / m ³ ) (kg / m ³ )

final deleted removed added added final final

2.5 367 9.5 33.2 96.5 2004 1, 3

3.0 367 9.6 27.9 97, 1 1989 2.0

3.5 367 9.6 14.5 98.5 2003 1, 3

Table 4. Treatment at pH 9.5 with Ca (0 H) ₂ and Na ₂ C0 ₃

Next, the treated effluent was transferred to a reactor to acidify it to pH 3.5 with inorganic acids (0.36 kg of H ₂ S04 or 0.22 kg of HCI was needed to acidify each m ³ of effluent). Subsequently, desorption (stripping) was performed with air to recover the C0 _2, this gas stream is led to an absorption tower wherein contacted countercurrently with a stream of NaOH, obtaining a solution close to saturation Na ₂ C0 ₃ (200 g / L). In this way it is possible to recover the Na ₂ C0 ₃ that is recirculated to the reactor of stage a1) of carbonation, thus having an environmentally friendly process in relation to the emissions of C0 ₂ .

The acidified brine stream was transported to a downflow filter type bed formed of 20 mm grain size calcined dolomite particles. The process was developed by gravity with a speed of 50 L / min / m ² to achieve an optimal residence time and that the effluent pH was between 9.0 and 10.0.

The resulting effluent was transported to the bizone reactor in which calcined dolomite was added in the form of a slurry by adjusting the pH in a range between 1.1 and 11.5. The bizone reactor supernatant was transported to the bizone flocculator with a residence time of another 15 minutes. The bottom of the reactor and the flocculator are purged to prevent impurities from accumulating in both. 30% of the purge current of the bizone reactor and 30% of the bizone flocculator is returned to the reactor thus improving the reaction process and the reaction yield, by increasing the depletion of the reagent. The supernatant of the flocculator is taken to a decanter, the decanted stream is conducted to a thickener, and the thickened product is washed and filtered to obtain a solid composed mainly of Mg (OH) 2, with a purity of this compound of 87.2 % (acidification with H2SO4) and 90% (acidification with HCI). The precipitate was calcined at 1,100 ° C to obtain a product with the composition indicated in Table 5.

Example 2

The process carried out in a pilot plant with the effluent of a reverse osmosis seawater desalination plant with the composition indicated in Table 1 is described below.

The first stage of the process consisted of acidification to pH 3.5 with inorganic acid (0.36 kg of H2SO4 or 0.22 kg of HCI is needed to acidify each m ³ of effluent). Stripping was performed with air to remove CO2. In this process, a countercurrent flow of the brine was contacted acidified and air in a column through a filler material. The filling supplies a high surface area to improve the transfer of CO2 from the liquid phase to the gas phase. The acidified brine stream was introduced from the top of the tower by a diffuser and distributed evenly over the landfill, while the air stream is introduced through the bottom. The brine stream leaves the column from the bottom and the gas stream from the top. The filler material used in this case was formed by plastic parts (raschig rings). The acidified brine stream was transferred to a downflow filter type bed formed of 20 mm grain size calcined dolomite particles. The process was developed by gravity with a speed of 50 L / min / m ² to achieve an optimal residence time and increase the pH of the effluent between 9.0 and 10.0.

The resulting effluent was transported to the bizone reactor in which the reagent was added in the form of a slurry by adjusting the pH in a range between 1.0 and 11.5.

The bizone reactor supernatant was transported to the bizone flocculator with a residence time of another 15 minutes. The bottom of the reactor and the flocculator are purged to prevent impurities from accumulating in both. 30% of the purge current of the bizone reactor and 30% of the bizone flocculator is returned to the reactor thus improving the reaction process and the reaction yield, by increasing the depletion of the reagent. In the experiments carried out, it has been found that superior recirculation causes an increase in product impurities. On the other hand a lower recirculation, produces a decrease in the amount of product to decrease the yield of the reaction. The flocculator supernatant was taken to a decanter, the decanted stream was led to a thickener, and the thickened product was washed and filtered to obtain a solid composed mainly of Mg (OH) 2, with a purity of this compound of 84.9 % (acidification with H2SO4) and

88.2% (acidification with HCI). The precipitate was calcined at 1,100 ° C to obtain a product with the composition indicated in Table 6.

Table 6. Composition of the calcined magnesite obtained (%).

Claims

1. Procedure for obtaining Mg (OH) 2 from saline effluents comprising the steps of:

a) acidify a saline effluent with an acidic stream at a pH between 3 and 5 obtaining an acidified stream;

d) precipitate Mg (OH) 2 from the effluent obtained in step c) by adding a base until a pH of 11.0 to 11.5 is reached;

e) decant the precipitate of Mg (OH) 2 from the solution obtained in d).

2. Procedure for obtaining Mg (OH) 2 according to claim 1 wherein the acidification step a) is carried out with an inorganic acid.

3. Method for obtaining Mg (OH) 2 according to claims 1-2 wherein in step b) the acidified stream obtained in step a) is contacted with a gaseous stream.

4. Method for obtaining Mg (OH) 2 according to claim 3 wherein stage b) of desorption is carried out in a tower where the acidified current of stage a) is introduced by the top of the tower and is distributed in a manner even on a filling material, and the gaseous stream is introduced through the bottom of the tower, the acidified stream from step a) leaves the tower on the bottom and the gaseous stream loaded with CO2 from the top.

5. Method of obtaining Mg (OH) 2 according to claims 1-4 wherein the filler material is formed by plastic parts.

6. Method for obtaining Mg (OH) 2 according to claims 1-5, where after stage b) of desorption there is an absorption stage b1) in which it is placed in contact the gaseous stream charged with CO2 obtained in b) in countercurrent with an alkaline solution.

7. Method for obtaining Mg (OH) 2 according to claim 6 wherein the alkaline solution to be used is a NaOH solution, to obtain Na2CÜ3.

8. Method of obtaining Mg (OH) 2 according to claims 1-7 wherein the gas stream of step b) is air.

9. Method of obtaining Mg (OH) 2 according to claims 1-8 wherein the particle size of the calcined dolomite used in the bed of step c) is greater than 20 mm.

10. Method of obtaining Mg (OH) 2 according to claim 9 characterized in that the particle size is between 20 mm and 40 mm.

1 1. Procedure for obtaining Mg (OH) 2 according to claims 1-10 wherein step d) is carried out in a two-zone reactor followed by a two-zone flocculation chamber.

12. Method of obtaining Mg (OH) 2 according to claim 11 wherein the bizone reactor and the bizone flocculation chamber are purged and between 25% and 30% of each purge is recycled to the reaction chamber.

13. Method for obtaining Mg (OH) 2 according to claims 1-12, where prior to step a) there is a carbonation stage a1), where excess Na2CÜ3 saline effluent is added in a reactor.

14. Method for obtaining Mg (OH) 2 according to claims 1-12, where prior to stage a) there is a stage a1) of carbonation, where NaOH saline effluent is added in a reactor together with Na2CÜ3, up to a pH between 9 and 10.5.

15. Procedure for obtaining Mg (OH) 2 according to claims 1-12, where prior to stage a) there is a stage a1) of carbonation, where it is added in a reactor to the Ca (OH) 2 or CaO saline effluent, until a pH between 9 and 10.5 is obtained and Na2CÜ3 is added.

16. Method of obtaining Mg (OH) 2 according to claims 13-15 wherein the Na2CÜ3 used in step a1) comes from the Na2CÜ3 obtained in step b1) of absorption.

17. Method for obtaining Mg (OH) 2 according to claims 13-16 wherein the hydraulic retention time in the reactor is between 10 minutes and 30 minutes.

18. Method of obtaining Mg (OH) 2 according to claims 13-16 comprising a step aO), of elimination or reduction of the sulfate content, prior to step a1) where Ca (OH) 2 effluent is added up to reaching a pH between 9.5 and 10.5 and CaC and the effluent obtained is conducted to step a1).