A method for treating molybdenum tail liquid in hydrometallurgy

Abstract

The present application relates to water treatment, and provides a treatment method of molybdenum tail liquid in hydrometallurgy, which comprises the following operation steps: (1) pre-filtering, filtering the tail liquid after adjusting the pH of the tail liquid, so that the SS after filtering is less than 15 mg / L; (2) ultrafiltration, performing ultrafiltration on the solution obtained in step (1), and obtaining ultrafiltration liquid and concentrated water after ultrafiltration; (3) electrodialysis concentration, performing electrodialysis concentration on the ultrafiltration liquid obtained in step (2), and obtaining electrodialysis concentrated water and electrodialysis fresh water after electrodialysis concentration; (4) performing adsorption and filtration operations on the electrodialysis concentrated water obtained in step (3); (5) nanofiltration, performing nanofiltration separation on the liquid obtained in step (4), and obtaining sodium chloride solution, sodium sulfate solution and mother liquor, and returning the mother liquor to step (1) for treatment. The treatment method of molybdenum tail liquid in hydrometallurgy can efficiently treat high-salt molybdenum tail liquid, and the treated tail liquid can be directly reused.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and more specifically, to a method for treating tailings from hydrometallurgical molybdenum smelting. Background Technology

[0002] In the process of molybdenum hydrometallurgy, a significant amount of molybdenum smelting tail liquor is generated, such as acidic molybdenum tail liquor from the classic water washing method and high-salt molybdenum tail liquor from ion exchange. Under the current concept of green environmental protection, the treatment of molybdenum smelting tail liquor has become a core process in the molybdenum hydrometallurgy industry. How to rationally treat and utilize molybdenum smelting wastewater has become an important issue for the molybdenum smelting industry. In particular, the high-salt molybdenum tail liquor generated by ion exchange has a complex composition. In addition to the main components of sodium chloride (10-30 g / L), sodium sulfate (2-5 g / L), and sodium carbonate (1-3 g / L), it also contains acid anion impurities of elements such as Mo and P (10-100 mg / L), as well as trace impurities such as Si, Al, and Fe (5-20 mg / L) that easily form colloids. Due to the oxygen-rich nature of water, a small amount of bacterial substances are also present.

[0003] The existing treatment methods mainly include: (1) ion exchange adsorption under acidic conditions, but quicklime is added, resulting in a large amount of additional by-products, which can easily cause secondary pollution. In addition, due to the presence of other salts, the water cannot be directly reused; (2) spiral wound membrane treatment, but it has high requirements for water quality because some water contains a lot of colloidal and bacterial substances, which makes nanofiltration membranes and reverse osmosis membranes very easy to be clogged, thus making it impossible to achieve large-scale industrialization; (3) direct use of two-stage reverse osmosis, but it is costly. Although it allows for large-scale water reuse, the by-product salts produced by MVR have complex compositions and cannot meet the national standards for industrial salt. Therefore, it is difficult to continue to use them.

[0004] Therefore, a more efficient and reasonable method for treating high-salt molybdenum tailings needs to be designed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for treating molybdenum tailings from hydrometallurgical processes, which can efficiently treat high-salt molybdenum tailings and allow the treated tailings to be directly reused.

[0006] The embodiments of the present invention are achieved through the following technical solution: The method for treating the tailings of hydrometallurgical molybdenum smelting of the present invention includes the following operation steps:

[0007] (1) Pre-filtration: Add hydrochloric acid to adjust the pH of the tail liquid to 6-7. After adjusting the pH of the tail liquid, filter the tail liquid (to filter particles >5um and mechanical impurities, such as obvious calcium silicate, iron hydroxide precipitate, etc.). Use a column filter. The bottom of the column filter is filled with 200-300mm 10-20 mesh quartz sand; the top of the column filter is filled with 1000-1500mm 20-40 mesh quartz sand. The pressure is 0.2-0.5MPa, the temperature is 10-40℃, the linear velocity is 5-15m / s, and the SS after filtration is <15mg / L. By adjusting the pH, trace amounts of Ca and Mg ions in the solution can be dissolved (in a slightly acidic environment, calcium sulfate, calcium carbonate, magnesium hydroxide, magnesium carbonate, etc. are dissolved into ionic state), so as to avoid Ca and Mg precipitation when the solution concentration increases later, thus affecting the water concentration process.

[0008] (2) Ultrafiltration: The solution obtained from step (1) is subjected to ultrafiltration. The ultrafiltration membrane precision is less than 0.01 μm (removing particulate matter and mechanical impurities of 0.01–5 μm), and the pressure is 0.02–0.15 MPa. During the ultrafiltration process, 1–3 ppm of 10–20% isothiazolinone and 1–3 ppm of 1–5% dibromopropionamide are added to inhibit bacteria in the water (by breaking the protein bonds of bacteria), thus preventing bacterial fouling of the membrane. At the same time, fouling materials and colloidal substances are discharged from the concentrate side. After ultrafiltration, ultrafiltrate and concentrated water are obtained in a ratio of 1:10-15. The ultrafiltrate is sent to the next process. The resulting concentrated water contains certain colloidal substances such as Al, Si, and Fe. These colloidal substances are suspended in the solution, making the solution unusable. Direct use of flocculants has very limited effect. Al colloids are positively charged, Si colloids are negatively charged, and Fe colloids change according to pH value. Therefore, the concentrated water also needs to be treated. This includes heating the concentrated water to 60-80℃, stirring for 30-60 minutes, adding soluble Fe salts to neutralize the colloidal charge through the principle of charge neutralization. The pH is adjusted to 5-8, and 1% of the volume of concentrated water with a 1‰ concentration of polyacrylamide solution is added. The mixture is stirred for 30-60 minutes and then filtered using a filter press. The filtered filtrate is then sent back to step (1) for pre-filtration.

[0009] (3) Electrodialysis concentration (using homogeneous anion and cation exchange membranes, using a two-sided electrolysis method to enrich the salt in the concentration chamber. Due to the characteristics of the functional groups on the membrane, the functional groups of the anion exchange membrane are positively charged and repel cations, while the functional groups of the cation exchange membrane are negatively charged and repel anions. Thus, the brine is enriched by using a two-sided electrolysis method). The ultrafiltrate obtained after ultrafiltration mainly consists of sodium chloride 10-30 g / L, sodium sulfate 2-5 g / L, sodium carbonate 1-3 g / L, etc., and anion impurities of elements such as Mo and P 5-80 mg / L. Colloidal impurities of Al, Si, and Fe <1 mg / L. The ultrafiltrate obtained in step (2) is concentrated by electrodialysis, wherein the homogeneous ion exchange membrane group is 550*1100 mm, and the membrane area to self-circulation flow rate ratio is 20-30 m². 2 1m 3 / h, after electrodialysis concentration, electrodialysis concentrate and electrodialysis desalination are obtained. The conductivity of electrodialysis concentrate is 130-200mS / cm, and the conductivity of electrodialysis desalination is 8-30mS / cm. The electrodialysis concentrate is sent to the next process. It also includes reverse osmosis treatment of electrodialysis desalination (by passing through a reverse osmosis membrane, ionic substances are intercepted to obtain low conductivity water). After reverse osmosis treatment, reverse osmosis desalination with a conductivity of less than 500μS / cm and reverse osmosis concentrate with a conductivity of greater than 500μS / cm are obtained. The reverse osmosis concentrate is sent back to step (2) for ultrafiltration.

[0010] (4) At this time, the electrodialysis concentrate contains 100-170 g / L sodium chloride, 5-30 g / L sodium sulfate, 8-20 g / L sodium carbonate, and 40-500 mg / L of anion impurities such as Mo, P, and As. According to the colloidal adsorption mechanism, it is removed in stages. Therefore, the electrodialysis concentrate obtained in step (3) is subjected to adsorption and filtration operations; including a first adsorption, a first filtration, a second adsorption, and a second filtration in sequence; wherein the first adsorption is to add 1 / 100-1 / 140 of the volume of 2-8% iron salt solution to the electrodialysis concentrate, and add 5-20% hydrochloric acid to adjust the pH to 4-6 (at pH 4-6, MoO4 2- WO4 2- The Fe ions polymerize heteropolyacid ions. Simultaneously, under these conditions, Fe ions form positively charged ferric hydroxide colloids, which adsorb the polymerized heteropolyacid tungsten-molybdenum ions. Stir for 30-60 minutes. First filtration is performed at 0.3-0.5 MPa using a 3μm PP filter cloth. Secondary adsorption involves adding 1 / 100-1 / 80 (by volume) of a 2-8% iron salt solution to the filtrate after the first filtration, and adjusting the pH to 7-8 using 5-30% sodium hydroxide (at pH 7-8, the Fe salt colloids carry a higher positive charge, which is beneficial for PO42-4 ions). 3- AsO4 3- SiO3 2-It has strong adsorption properties, and at the same time, SiO3 2- It will also self-hydrolyze to form silica precipitate. Stir for 30-60 minutes, and the parameters for the second filtration are the same as those for the first filtration.

[0011] (5) Nanofiltration: The liquid obtained in step (4) is subjected to nanofiltration separation (the nanofiltration membrane can only pass through monovalent salts, thus saving polyvalent salts and achieving the purpose of separation), wherein the molecular weight cutoff is 100-200 Daltons, the operating pressure is 1.5-4 MPa, and sodium chloride solution, sodium sulfate solution and mother liquor are obtained. The sodium chloride solution and sodium sulfate solution are evaporated, and the mother liquor is returned to step (1) for further processing.

[0012] The technical solution of the present invention has at least the following advantages and beneficial effects: The method for treating molybdenum tailings from hydrometallurgical processes of the present invention can treat high-salt molybdenum tailings efficiently and at low cost. It can not only effectively treat colloidal and microbial substances, but also the treated liquid can be directly reused, achieving zero discharge of wastewater. Detailed Implementation

[0013] Example 1

[0014] This embodiment provides a method for treating molybdenum tailings from hydrometallurgical processes, including the following steps:

[0015] (1) Pre-filtration: Add hydrochloric acid to adjust the pH of the tail liquid to 6. After adjusting the pH of the tail liquid, filter the tail liquid using a column filter. The bottom of the column filter is filled with 200mm 10-mesh quartz sand; the top of the column filter is filled with 1000mm 20-mesh quartz sand. The pressure is 0.2MPa, the temperature is 10℃, the linear velocity is 5m / s, and the SS after filtration is <15mg / L.

[0016] (2) Ultrafiltration: The solution obtained by filtration in step (1) is subjected to ultrafiltration. The ultrafiltration membrane precision is less than 0.01 μm and the pressure is 0.02 MPa. During the ultrafiltration process, 1 ppm of 10% isothiazolinone and 1 ppm of 1% dibromopropionamide are added. After ultrafiltration, ultrafiltrate and concentrated water are obtained. The ratio of ultrafiltrate to concentrated water is 1:10. The ultrafiltrate is sent to the next process. The concentrated water is also treated by heating the concentrated water to 60°C, stirring for 30 min, adding soluble FeCl2, adjusting the pH to 5-6, adding 1% of the volume of concentrated water with 1‰ concentration of polyacrylamide solution, stirring for 30 min, and using a filter press for filtration. The filtrate obtained by filtration is sent back to step (1) for prefiltration.

[0017] (3) Electrodialysis concentration: The ultrafiltrate obtained in step (2) is concentrated by electrodialysis, wherein the homogeneous ion-exchange membrane module is 550*1100mm, and the membrane area to self-circulation flow rate ratio is 20m². 2 1m3 / h, after electrodialysis concentration, electrodialysis concentrate and electrodialysis desalination are obtained. The conductivity of electrodialysis concentrate is 130-200mS / cm, and the conductivity of electrodialysis desalination is 8-30mS / cm. The electrodialysis concentrate is sent to the next process. It also includes reverse osmosis treatment of electrodialysis desalination. After reverse osmosis treatment, reverse osmosis desalination with a conductivity of less than 500μS / cm and reverse osmosis concentrate with a conductivity of greater than 500μS / cm are obtained. The reverse osmosis concentrate is sent back to step (2) for ultrafiltration.

[0018] (4) Perform adsorption and filtration operations on the electrodialysis concentrate obtained in step (3); including a first adsorption, a first filtration, a second adsorption, and a second filtration in sequence; wherein the first adsorption is to add 1 / 100 of the volume of 2% iron salt solution to the electrodialysis concentrate, add 5% hydrochloric acid to adjust the pH to 4-5, and stir for 30 min; the first filtration is to filter at 0.3 MPa using a 3 μm PP filter cloth; the second adsorption is to add 1 / 100 of the volume of 2% iron salt solution to the filtrate after the first filtration, use 5% sodium hydroxide to adjust the pH to 7-8, and stir for 30 min; the parameters for the second filtration are the same as those for the first filtration.

[0019] (5) Nanofiltration: The liquid obtained in step (4) is separated by nanofiltration, wherein the molecular weight cutoff is 100 Daltons and the operating pressure is 1.5 MPa, to obtain sodium chloride solution, sodium sulfate solution and mother liquor. The sodium chloride solution and sodium sulfate solution are evaporated and the mother liquor is returned to step (1) for further processing.

[0020] Example 2

[0021] This embodiment provides a method for treating molybdenum tailings from hydrometallurgical processes, including the following steps:

[0022] (1) Pre-filtration: Add hydrochloric acid to adjust the pH of the tail liquid to 7. After adjusting the pH of the tail liquid, filter the tail liquid using a column filter. The bottom of the column filter is filled with 300mm of 20-mesh quartz sand; the top of the column filter is filled with 1500mm of 40-mesh quartz sand. The pressure is 0.5MPa, the temperature is 40℃, the linear velocity is 15m / s, and the SS after filtration is <15mg / L.

[0023] (2) Ultrafiltration: The solution obtained by filtration in step (1) is subjected to ultrafiltration. The ultrafiltration membrane precision is less than 0.01 μm and the pressure is 0.15 MPa. During the ultrafiltration process, 3 ppm of 20% isothiazolinone and 3 ppm of 5% dibromopropionamide are added. After ultrafiltration, ultrafiltrate and concentrated water are obtained. The ratio of ultrafiltrate to concentrated water is 1:15. The ultrafiltrate is sent to the next process. The concentrated water is also treated by heating the concentrated water to 80°C, stirring for 60 min, adding soluble FeCl2, adjusting the pH to 8, adding 1% of the volume of concentrated water with 1‰ concentration of polyacrylamide solution, stirring for 60 min, and using a filter press for filtration. The filtrate obtained by filtration is sent back to step (1) for prefiltration.

[0024] (3) Electrodialysis concentration: The ultrafiltrate obtained in step (2) is concentrated by electrodialysis, wherein the homogeneous ion exchange membrane module is 550*1100mm, and the membrane area to self-circulation flow rate ratio is 30m². 2 1m 3 / h, after electrodialysis concentration, electrodialysis concentrate and electrodialysis desalination are obtained. The conductivity of electrodialysis concentrate is 130-200mS / cm, and the conductivity of electrodialysis desalination is 8-30mS / cm. The electrodialysis concentrate is sent to the next process. It also includes reverse osmosis treatment of electrodialysis desalination. After reverse osmosis treatment, reverse osmosis desalination with a conductivity of less than 500μS / cm and reverse osmosis concentrate with a conductivity of greater than 500μS / cm are obtained. The reverse osmosis concentrate is sent back to step (2) for ultrafiltration.

[0025] (4) Perform adsorption and filtration operations on the electrodialysis concentrate obtained in step (3); including a first adsorption, a first filtration, a second adsorption, and a second filtration in sequence; wherein the first adsorption is to add 1 / 140 of the volume of 2-8% iron salt solution to the electrodialysis concentrate, add 20% hydrochloric acid to adjust the pH to 5-6, and stir for 60 min; the first filtration is to filter at 0.5 MPa using a 3 μm PP filter cloth; the second adsorption is to add 1 / 80 of the volume of 8% iron salt solution to the filtrate after the first filtration, use 30% sodium hydroxide to adjust the pH to 7-8, and stir for 60 min; the parameters for the second filtration are the same as those for the first filtration.

[0026] (5) Nanofiltration: The liquid obtained in step (4) is separated by nanofiltration, wherein the molecular weight cutoff is 200 Daltons and the operating pressure is 4 MPa, to obtain sodium chloride solution, sodium sulfate solution and mother liquor. The sodium chloride solution and sodium sulfate solution are evaporated and the mother liquor is returned to step (1) for further processing.

[0027] Example 3

[0028] This embodiment provides a method for treating molybdenum tailings from hydrometallurgical processes, including the following steps:

[0029] (1) Pre-filtration: Add hydrochloric acid to adjust the pH of the tail liquid to 6-7. After adjusting the pH of the tail liquid, filter the tail liquid using a column filter. The bottom of the column filter is filled with 250mm of 15-mesh quartz sand; the top of the column filter is filled with 1200mm of 300-mesh quartz sand. The pressure is 0.4MPa, the temperature is 25℃, the linear velocity is 10m / s, and the SS after filtration is <15mg / L.

[0030] (2) Ultrafiltration: The solution obtained by filtration in step (1) is subjected to ultrafiltration. The ultrafiltration membrane precision is less than 0.01 μm and the pressure is 0.1 MPa. During the ultrafiltration process, 2 ppm of 15% isothiazolinone and 2 ppm of 3% dibromopropionamide are added. After ultrafiltration, ultrafiltrate and concentrated water are obtained. The ratio of ultrafiltrate to concentrated water is 1:12. The ultrafiltrate is sent to the next process. The concentrated water is also treated by heating the concentrated water to 70°C, stirring for 45 min, adding soluble FeCl2, adjusting the pH to 6-7, adding 1% of the volume of concentrated water with 1‰ concentration of polyacrylamide solution, stirring for 450 min, and using a filter press for filtration. The filtrate obtained by filtration is sent back to step (1) for prefiltration.

[0031] (3) Electrodialysis concentration: The ultrafiltrate obtained in step (2) is concentrated by electrodialysis, wherein the homogeneous ion exchange membrane module is 550*1100mm, and the membrane area to self-circulation flow rate ratio is 25m². 2 1m 3 / h, after electrodialysis concentration, electrodialysis concentrate and electrodialysis desalination are obtained. The conductivity of the electrodialysis concentrate is 150mS / cm and the conductivity of the electrodialysis desalination is 20mS / cm. The electrodialysis concentrate is sent to the next process. It also includes reverse osmosis treatment of the electrodialysis desalination. After reverse osmosis treatment, reverse osmosis desalination with a conductivity of less than 500μS / cm and reverse osmosis concentrate with a conductivity of greater than 500μS / cm are obtained. The reverse osmosis concentrate is sent back to step (2) for ultrafiltration.

[0032] (4) Perform adsorption and filtration operations on the electrodialysis concentrate obtained in step (3); including a first adsorption, a first filtration, a second adsorption, and a second filtration in sequence; wherein the first adsorption is to add 1 / 120 of the volume of 5% iron salt solution to the electrodialysis concentrate, add 15% hydrochloric acid to adjust the pH to 4-6, and stir for 45 min; the first filtration is to filter at 0.4 MPa using a 3 μm PP filter cloth; the second adsorption is to add 1 / 90 of the volume of 5% iron salt solution to the filtrate after the first filtration, use 20% sodium hydroxide to adjust the pH to 7-8, and stir for 45 min; the parameters for the second filtration are the same as those for the first filtration.

[0033] (5) Nanofiltration: The liquid obtained in step (4) is separated by nanofiltration, wherein the molecular weight cutoff is 150 Daltons and the operating pressure is 3 MPa, to obtain sodium chloride solution, sodium sulfate solution and mother liquor. The sodium chloride solution and sodium sulfate solution are evaporated and the mother liquor is returned to step (1) for further processing.

[0034] Experimental Example

[0035] The elemental content of the solutions obtained in Examples 1, 2, and 3 was determined, and the results are shown in Table 1 below.

[0036] Table 1. Element content in the purified solutions of Examples 1-3

[0037] element Example 1 Example 2 Example 3 unit Cr trace amounts trace amounts trace amounts ppm Cd trace amounts trace amounts trace amounts ppm Ba 0.0001 0.0002 0.0001 ppm Cl 108.35 100.6 110.32 g / L Cu 0.4 0.38 0.41 mg / L Ca 5.3 4.9 4.5 mg / L SO4 0.92 0.9 1.1 g / L Ni trace amounts trace amounts trace amounts mg / L Co trace amounts trace amounts trace amounts mg / L Mn 0.006 0.005 0.005 mg / L Mg 4.1 4.32 4.18 mg / L Mo 0.4 0.4 0.5 mg / L

[0038] In Table 1, "trace amount" indicates that the amount is below the detection line. By comparing with GB 8978-1996 "Integrated Wastewater Discharge Standard", the content of all elements in the molybdenum tailings treated by this application is far below the standard. Furthermore, the treated water not only does not need to be discharged, but also meets the requirements for secondary reuse and can be reused, thus truly achieving zero wastewater discharge.

[0039] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for treating tailings from hydrometallurgical molybdenum smelting, characterized in that, The following steps are included: (1) Pre-filtration: After adjusting the pH of the tail liquid, filter the tail liquid to make the SS < 15 mg / L after filtration. (2) Ultrafiltration: The solution obtained by filtration in step (1) is subjected to ultrafiltration. After ultrafiltration, ultrafiltrate and concentrated water are obtained. The ultrafiltrate is sent to the next process. (3) Electrodialysis concentration: The ultrafiltrate obtained in step (2) is concentrated by electrodialysis. After electrodialysis concentration, electrodialysis concentrate and electrodialysis desalinated water are obtained. The electrodialysis concentrate is sent to the next process. (4) The electrodialysis concentrate obtained in step (3) is subjected to adsorption and filtration operations; step (4) includes a first adsorption, a first filtration, a second adsorption, and a second filtration performed in sequence; The first adsorption step involves adding 1 / 100-1 / 140 of the volume of a 2-8% iron salt solution to the electrodialysis concentrate, adjusting the pH to 4-6 with 5-20% hydrochloric acid, and stirring for 30-60 minutes. The first filtration step involves filtering the solution at 0.3-0.5 MPa using a 3μm PP filter cloth. The second adsorption step involves adding 1 / 100-1 / 80 of the volume of a 2-8% iron salt solution to the filtrate after the first filtration, adjusting the pH to 7-8 with 5-30% sodium hydroxide, and stirring for 30-60 minutes. The parameters for the second filtration step are the same as those for the first filtration step. (5) Nanofiltration: The liquid obtained in step (4) is separated by nanofiltration to obtain sodium chloride solution, sodium sulfate solution and mother liquor. The sodium chloride solution and sodium sulfate solution are evaporated and the mother liquor is returned to step (1) for further processing. In step (1), hydrochloric acid is added to adjust the pH of the tail liquid to 6-7.

2. The method for treating molybdenum tailings from hydrometallurgical processes according to claim 1, characterized in that, The pre-filtration in step (1) uses a column filter, with 200-300mm of 10-20 mesh quartz sand at the bottom and 1000-1500mm of 20-40 mesh quartz sand at the top.

3. The method for treating molybdenum tailings from hydrometallurgical processes according to claim 1, characterized in that, In step (2), the ratio of ultrafiltrate to concentrated water is 1:10-15, the ultrafiltration membrane precision is less than 0.01μm, and 1-3ppm of 10-20% isothiazolinone and 1-3ppm of 1-5% dibromopropylamide are added during the ultrafiltration process.

4. The method for treating molybdenum tailings from hydrometallurgical processes according to claim 1, characterized in that, Step (2) also includes treating the concentrate, which includes heating the concentrate to 60-80℃, stirring for 30-60 minutes, adding soluble Fe salt, adjusting the pH to 5-8, adding 1% of the concentrate volume of 1‰ polyacrylamide solution, stirring for 30-60 minutes, using a filter press for filtration, and sending the filtrate obtained by filtration back to step (1) for pre-filtration.

5. The method for treating molybdenum tailings from hydrometallurgical processes according to claim 1, characterized in that, In step (3), the homogeneous ion-exchange membrane module used in electrodialysis is 550*1100mm, and the membrane area to self-circulation flow rate ratio is 20-30m². 2 1m 3 / h, adjust the conductivity of the electrodialysis concentrate to 130-200mS / cm, and the conductivity of the electrodialysis desalinated water to 8-30mS / cm.

6. The method for treating molybdenum tailings from hydrometallurgical processes according to claim 1, characterized in that, Step (3) also includes reverse osmosis treatment of the electrodialysis desalinated water. After reverse osmosis treatment, reverse osmosis desalinated water with a conductivity of less than 500 μS / cm and reverse osmosis concentrate with a conductivity of greater than 500 μS / cm are obtained. The reverse osmosis concentrate is sent back to step (2) for ultrafiltration.