Extraction of commercially valuable mineral salt from reverse osmosis brine using a spray dry process

Garudachari Bhadrachari; Mansour Ahmad; Rajesha Kumar Alambi; Jibu P. Thomas

doi:10.4491/eer.2022.299

Environ Eng Res > Volume 28(4); 2022 > Article

Bhadrachari, Ahmad, Alambi, and Thomas: Extraction of commercially valuable mineral salt from reverse osmosis brine using a spray dry process

Research

Environmental Engineering Research 2023; 28(4): 220299.

Published online: September 22, 2022

DOI: https://doi.org/10.4491/eer.2022.299

Extraction of commercially valuable mineral salt from reverse osmosis brine using a spray dry process

Garudachari Bhadrachari^†

, Mansour Ahmad, Rajesha Kumar Alambi, Jibu P. Thomas

Water Research Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait

^†Corresponding author: E-mail: garuda.achar@gmail.com, Tel: +965 98724398, Fax: +965 24879238

Received June 27, 2022 Revised September 02, 2022 Accepted September 21, 2022

(open-access):

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

As the need for drinkable water increases, thermal and membrane desalination of seawater have emerged as the two most effective solutions. Both thermal and membrane desalination systems, however, release large quantities of concentrated brine back into the sea. The continuous discharge of enormous amount of brine into the sea will disrupt the aquatic ecosystem and increase the seawater salinity level. In this study, spray dry technology is utilized for isolation of minerals from reverse osmosis brine and there by achieving zero liquid discharge concept. The procedure involves spraying feed into a chamber, followed by the introduction of heated air. The flow rates of the spray dryer and hot air can be adjusted. Consequently, the amount and concentration of feed residue in the chamber can be altered. The residue from the first vaporization chamber was cooled to a specified temperature and centrifuged to separate the minerals in crystal form. The centrifuge reject was fed to a second vaporization chamber for further concentration and separation of pure mineral salts. This cycle was repeated between two and six times to isolate all significant minerals from the brine. The vaporized water from all chambers was condensed to generate pure water as a by-product.

Keywords: Desalination brine, Mineral recovery, Spray dry process, Zero-liquid discharge

Graphical Abstract

Keywords: Desalination brine, Mineral recovery, Spray dry process, Zero-liquid discharge

1. Introduction

The generation of fresh water by seawater desalination technology will be one of the primary means of meeting the rising demand for fresh water in the near future [1–4]. Desalinating seawater in a cost-effective manner is possible using a variety of processes available around the world [5, 6]. Thermal and membrane processes are the mainly used desalination technologies [5–8]. Direct heating sources are used in thermal desalination to evaporate saline water, which is subsequently collected and condensed to produce fresh water on the opposite side. The thermal desalination process has a number of flaws, including reactor and pipeline corrosion, low pure water recovery, and issues with feed and reject pre- and post-treatment [9]. Various membrane technologies with high water recovery, reduced pre-treatment, and post-treatment approaches have been developed in recent years [5,10–12]. However, the majority of the concentrated brine is returned to sea by both thermal and membrane processes. Continuously dumping massive amounts of brine into the sea will disrupt the aquatic ecosystem and raise salinity levels [13–17]. Researchers from all over the world have been working on brine management and mineral extraction in recent years [18–22]. One way to reduce the environmental impact of rejected brine is to use zero liquid discharge (ZLD) approach and recover minerals from brine using the chemical precipitation process [23, 28]. Other methods are membrane distillation, vacuum distillation, solar evaporation, etc [29–32]. The techno-economic assessment of minimal liquid discharge (MLD) and ZLD desalination using brine from desalination methods such as reverse osmosis (RO), multi-effect distillation (MED), and multi-stage flash distillation (MSF) yielded promising results [33–35]. However, the processes described in the literature are complex.

Previous research studies have found that mineral extraction technologies from the desalination brine depends on several significant factors such as feed-water salinity and its physiochemical characteristics, land availability, climate change, location, and purity demand [36–38]. Previous research introduced evaporation ponds, wind-aided intensified evaporation (WAIV), evaporation and crystallization, membrane distillation (MD), two-stage RO, forward osmosis (FO), electrodialysis (ED), salt solidification and sequestration process (SAL-PROC), ion exchange (IX), and integration processes [36, 37, 39]. Mineral with low concentrations have not been recovered from seawater because their market values are much lower than the capital and operational costs of extraction. Seawater RO (SWRO) brine from the desalination plants in the Kingdom of Saudi Arabia contains high concentrations of the main elements such as sodium (Na), calcium (Ca), magnesium (Mg), and potassium (K), with an estimated market value of $US18 billion per year [40]. The literature identified several common products (e.g., gypsum, NaCl, Mg(OH)₂, CaCl₂, CaCO₃, and Na₂SO₄) that can potentially be recovered from SWRO desalination plant concentrates [41, 42]. Various configurations, methods, and systems for recovering these chemical compounds have been developed over the years, including the SAL-PROC and Reverse-Osmosis-SAL-PROC (ROSP) systems, evapo-cooling systems, and other integrated systems [43–46]. All of these innovative research studies were proposed with the goal of developing specific chemical compounds that can be commercialized. Unfortunately, none of these novel approaches have been commercialized.

Spray dry technologies have been popular in the dairy and pharmaceutical industries in recent years [47–49]. The feed solution is sprayed as droplets in a reactor to generate solid powder/fast evaporation/encapsulation in the spray dry process. Spray dry technology has shown to be efficient in the dairy and pharmaceutical industries. This paper covers the outcome of a study that evaluates the performance of spray dry process for SWRO brine concentration and mineral extraction. In the study, the spray dry approach was found to be more effective at purifying water and recovering mineral salts.

In this investigation, a custom-designed unit was built by combining many processes, including spray drying, cooling, and centrifugation. The key innovation in the study is the method of integrating several processes to improve the water recovery of the desalination plant and the extraction of pure mineral salt. Hot air is used as a carrier gas in the process to improve feed vaporization. The Gulf Cooperation Council (GCC) countries has very lengthy and hot summers, with air temperatures ranging from 36 °C to 47 °C [50]. The direct application of hot air to the system decreases energy usage in the process. The system, on the other hand, is made of anticorrosive materials such as glass and stainless steel and is free of vacuum and pressure in the reactor vessel. This will provide additional benefits in the process of extending the life of the system. The study's primary goals are to integrate and evaluate the process to improve water recovery and extract pure mineral salt.

2. Experimental Section

2.1. Materials

SWRO brine from the Desalination Research Plant (DRP) of Kuwait Institute for Scientific Research (KISR) was used as feed. The DRP SWRO desalination plant is located in Kuwait at the Doha East Power Generation and Water Desalination Plant site. The brine produced from the DRP SWRO plant has a TDS concentration of 54,000 mg/L on average. The elemental composition of the sea-water and SWRO brine is shown in supplementary Table S1. The water samples were analysed at ISO-certified laboratories of Water Research Center (WRC) of KISR. The instruments were calibrated and examined using international standards and techniques. The spray dry system was custom designed and built in the DRP laboratory using a glass reactor (5 L capacity, custom fabrication from India), a heat exchanger (AB Plate Heat Exchanger, 3"x8" 20 Plates, ALFA HEATING SUPPLY, USA), a chiller (Büchi® Recirculating Chiller F-100, Germany), a sprayer (Nozzle Stainless Steel 0.30 mm, AmFog, USA), and a centrifuge (Table Top Scale Centrifuge Machine, Ace Industries Pvt Ltd, India). Thermo Scientific ORION STAR A221 and ORION STAR A222 meters were used to measure pH and conductivity. The EDTA titration method was used to analyze calcium and magnesium, while the spectrophotometer LANGE DR2800 was used to analyze sulfate. Other elements contained in all streams were evaluated using the THERMO SCIENTIFIC, iCAP 6000.

2.2. Process Description

The process includes seven steps: (1) high-pressure feed dosing, (2) spray droplet and flow rate control, (3) hot air blower with temperature control, (4) heat exchanger to condense and collect pure water, (5) scraping of solid particles deposited on the walls of the vaporization chamber, (6) cooling to crystallize minerals, and (7) centrifugation to separate solid particles.

The concentrated brine solution was fed to the sprayer at a specific pressure by a feed pump, and the sprayer flow rate and droplet size were controlled by the sprayer. The sprayed brine solution was evaporated using hot air from the evaporation chamber. Evaporated water was condensed in a heat exchanger, separated water was collected in a tank, and air was recirculated to the heater via a gas blow system. The solid that had accumulated on the vaporization chamber walls was scraped away using a mechanical stirring scraper and deposited at the bottom. The concentrated brine was transferred to the cooling unit for crystallization and then centrifuged to separate the first stage crystals (major salt is calcium sulfate). Then, using a sprayer and a controlled flow rate, the centrifuge reject was fed to the second evaporation chamber to extract the second fraction of concentrated brine, which contains sodium chloride (NaCl) as the primary salt. The concentration and separation of pure mineral salts was continued by repeating the extraction process for all main mineral pure salts. Figures 1, and 2 show a flow diagram and a process schematic diagram of integrated laboratory scale unit, respectively.

2.3. Experimental Procedure

The testing was carried out in an in-house constructed spray drying chamber (5L capacity) with an integrated cooler and centrifuge system. The feed solution was fed to the fluid nozzle at the top of the spray dryer using a high-pressure pump at flow rates of 25, 35, 45, and 55 Ml/min, and hot air at 60, 70, and 80°C was supplied in a 90-degree angle to the feed. Evaporated hot air was collected by condensation outside the evaporation chamber and recirculated. To separate the crystalized salts, the saturated solution was cooled to 10°C and transferred to a centrifuge machine. The filtrate from the centrifugation system was fed into second spray dryer chamber with integrated cooler and centrifuge to concentrate and isolate the crystallized salts. The above process was repeated for two, four, and six cycles. The mineral concentration was analysed using Inductively Coupled Argon Plasma Emission Spectrometer [ICAP]. The compositions of the separated minerals were examined using energy-dispersive X-ray spectroscopy (EDX). All studies were performed in triplicate, and mean data were given.

3. Results and Discussion

3.1. Effect of Flow Rate and Temperature

The feed flow rates and hot air temperatures were varied during experimentation. The results show that the increase of feed flow rate at constant hot air temperature decrease the residual brine concentration in the evaporation chamber. On the other hand, increase of hot air temperature at constant feed flow rate increased the residual brine concentration in the evaporation chamber. This is mainly due to increase of evaporation inside the reactor at higher temperature [51, 52]. Purity of the extracted salts increased when the feed flow rate was increased from 14 to 45 Ml/min, but further increase of feed flow rate to 55 Ml/min decreased extracted salt purity. The residual time of feed vapors in the evaporation chamber was good enough to obtain high residual brine concentration at feed flow rates 14 to 45 Ml/min. Further increase in the flow rate to 55 Ml/min may shorten the residence time feed droplet and reduce the residual brine concentration [53–56]. The preliminary experimental results show that 45 Ml/min and 80°C are the optimum condition to obtain better residual brine concentration to separate the pure mineral salts.

The optimal feed flow rate and temperature were used to control the amount of residual brine in the reactor in the remaining trials. The evaporation stages were 2, 4 and 6, respectively for trial 1, 2 and 3.

3.2. Extraction of Mineral Using Two Stages of Spray-Drying Process (Trial 1)

During the two-stage experiment, as shown in Figures 1 and 2, RO brine went through two full cycles of the spray-drying process. As shown in Table 1, the residual brine concentration in the reactor after the first cycle was about 162,260 mg/L, and it was cooled to 10°C to separate the divalent salt. A centrifuge was used to separate the salts that had settled out, and the reject was used in the second cycle of the process. The residual brine concentration after the second cycle was about 204,400 mg/L, and the solution was cooled and the crystallized salt was taken out.

As shown in Table 2, the elemental analysis shows that the first stage of the process separated more than 94% of the calcium and sulfate. The calcium and sulfate separation may be due to its solubility limit in the saline water [55]. After two stages of extraction, the final residue is mostly made up of sodium, chloride, and magnesium due to higher solubility of the ions. The analysis of the salt that was taken out showed that the salt in the first fraction was CaSO₄ and that the salt in the second fraction was a mix of CaSO₄ and Na₂SO₄. Sodium, potassium, and chloride are the main impurities in the salt that was taken from the first fraction. Since these impurities dissolve easily in water, a water wash can improve the purity of the salt that was taken from the first fraction [56].

3.3 Extraction of Mineral Using Four Stages of Spray-Drying Process (Trial 2)

The second set of tests were done in four stages with an air flow rate of 45 Ml/min and at 80°C temperature. During the four-stage experiment, RO brine went through the spray-dry process four times. As shown in Table 3, the residual brine concentrations are 64,700 mg/L for the first fraction, 96,300 mg/L for the second fraction, 124,700 mg/L for the third fraction, and 216,800 mg/L for the fourth fraction. The elemental analysis tabulated in Table 4 shows that the first stage separated the calcium sulfate salt and the sodium chloride impurities, and these observations are in line with literatures [55, 56]. In second fraction more percentage of impurities along with calcium sulfate was observed. In the third stage, calcium sulfate impurities are found in high concentrations along with sodium chloride. In the fourth stage, calcium sulfate impurities were found in lower concentrations than in the third stage. When compared to other elements, there was more magnesium in the leftovers from a four-stage experiment. In four-step experiments, calcium sulfate and sodium chloride are the only pure salts that can be taken out. When compared to the two-stage process, the purity of calcium sulfate obtained from the four-stage process was better.

3.4. Extraction of Mineral Using Six Stages of Spray-Drying Process (Trial 3)

The third set of tests was done in six stages. During the six-stage experiment, RO brine went through the spray-dry process a total of six times. Table 5 shows that the concentration of reject went from 152,000 mg/L to 307,200 mg/L and then went down to 276,450 mg/L. The elemental analysis shows that the first three stages separated the calcium sulfate as the main salt and the sodium chloride as little as possible. In the fourth stage, sodium chloride and impurities made up of calcium sulfate were separated. Table 6 shows that the mineral salt extracted after fifth and sixth stages had more sodium chloride than the fourth stage. Table 7 shows that a six-step process can be used to obtain calcium chloride of 91 percent purity and sodium chloride of 94 percent purity.

4. Conclusions

The spray dry process is an effective method to extract minerals from desalination brines. Pure mineral salts were separated using the spray-dry procedure and by the integration of spray drying, cooling, and centrifuging processes. The experimental study shows that at hot air temperature and feed spraying flow rate has significant effect on the residual brine concentration. The four-stage spray dry method was comparatively better to separate calcium sulfate and sodium chloride, according to the findings of the studies. The extracted calcium sulfate was approximately 93% pure whereas the sodium chloride was around 90% pure. The experimental results demonstrated that the mineral salt solubility limitations in concentrated brine are as follows. CaSO₄<NaCl<MgCl₂<KCl [57]. The elemental analysis of extracted salt reveals that calcium sulfate precipitates first in concentrated brine, followed by sodium chloride, magnesium chloride, and potassium chloride.

The experimental investigation of designed system showed promising results to extract pure mineral salt from the SWRO brine. This will create new opportunities to diversify the land mining for mineral extraction and reduce the desalination brine management cost. Additionally, this process will enhance the overall water recovery in the desalination plant and reduce the environmental impact from the brine discharge. The spray dry process showed better performance in terms of purity of extracted mineral salt compared with available literature and is free from additive or chemicals. The existing desalination plants, wastewater treatment plants, etc. can be retrofitted with spray dry process to recover minerals and reduce the brine volume. The research team recommends more research and development to evaluate energy consumption and commercial viability of the process.

Supplementary Information

eer-2022-299-suppl.pdf

Acknowledgments

Authors are thankful to the Kuwait Institute for Scientific Research (KISR) for funding and supporting the implementation of this research work.

Notes

Conflict-of-Interest

The authors declare that they have no conflict of interest.

Author Contributions

G.B (Associate Research Scientist) is the main author and performed the experimentation. M.A (Research Scientist), R.K.A (Research Scientist) and J.P.T (Research Associate) are the co-authors and were involved in experimentation.

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Fig. 1

Process Flow diagram.

Fig. 2

Schematic diagram of the spray dry process.

Table 1

Residual brine concentration at two stages of spray-drying process

Parameters	Seawater	RO brine	First fraction	Second fraction
TDS (mg/L)	45,377	54,900	162,260	204,400
Conductivity (ms/cm)	58.3	69.4	204.7	220.8
pH	7.3	7.13	7.54	7.21

TDS: total dissolved solids, mg/L: parts per million, ms/cm: millisiemens/centimetre

Table 2

Purity of isolated minerals (%) obtained from two stages of spray-drying process

Minerals	First fraction salt	Second fraction salt	Final residue
Mg²⁺ (%)	0	0	9.505
Ca²⁺ (%)	93.95	88.77	1.47
Na⁺ (%)	3.01	8.65	84.21
K⁺ (%)	3.03	3.3	3.62
Cl⁻ (%)	0	1.15	93.03
SO₄²⁻ (%)	100	98.84	6.67
Br⁻ (%)	0	0	0.29

%: percentage, Mg²⁺: magnesium ion, Ca²⁺: calcium ion, Na⁺: sodium ion, K⁺: potassium ion, Cl⁻: chloride ion, SO₄²⁻: sulfate ion, Br⁻: bromide ion.

Table 3

Residual brine concentration at four stages of spray-drying process

Parameters	First fraction	Second fraction	Third fraction	Fourth fraction
TDS (mg/L)	64,700	96,300	124,700	216,800
Conductivity (ms/cm)	97.0	162.0	208.0	230.0
pH	7.25	7.59	7.55	7.21

Table 4

Purity of isolated minerals (%) obtained from four stages of spray-drying process

Minerals	First fraction salt	Second fraction salt	Third fraction salt	Fourth fraction salt	Final residue
Mg²⁺ (%)	0	0	0	0	41.98
Ca²⁺ (%)	96.02	51.44	8.67	2.62	5.92
Na⁺ (%)	3.9	47.94	91.3	96.17	38.99
K⁺ (%)	0	0	0	1.20	13.09
Cl⁻ (%)	7.96	78.16	95.67	89.23	73.01
SO₄²⁻ (%)	92.03	21.83	4.32	2.382	29.48
Br⁻ (%)	0	0	0	0.38	1.33

Table 5

Residual brine concentration at six stages of spray-drying process

Parameters	First fraction	Second fraction	Third fraction	Fourth fraction	Fifth fraction	Sixth fraction
TDS (mg/L)	152,000	168,620	193,400	220,000	307,200	276,450
Conductivity (ms/cm)	191.4	199.0	215.0	221.0	292.8	402.5
pH	7.66	7.65	7.55	7.10	8.34	8.78

Table 6

Purity of isolated minerals (%) obtained from six stages of spray-drying process

Minerals	First fraction salt	Second fraction salt	Third fraction salt	Fourth fraction salt	Fifth fraction salt	Sixth fraction salt	Final residue
Mg²⁺ (%)	0	0	0	0	0	0	19.05
Ca²⁺ (%)	88.88	89.04	91.83	9.45	3.233	3.24	1.96
Na⁺ (%)	8.23	7.12	5.22	87.83	94.48	94.033	71.77
K⁺ (%)	2.88	3.82	2.94	2.71	2.28	1.95	7.21
Cl⁻ (%)	4.31	2.9	1.67	90.35	93.84	92.6	87.82
SO₄²⁻ (%)	95.68	97.09	98.32	9.32	5.93	3.18	11.52
Br⁻ (%)	0	0	0	0.31	0.22	0.21	0.64