Metal recovery from reverse osmosis concentrate

Metal recovery from reverse osmosis concentrate

Journal of Cleaner Production 17 (2009) 703–707 Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevi...

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Journal of Cleaner Production 17 (2009) 703–707

Contents lists available at ScienceDirect

Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Metal recovery from reverse osmosis concentrate T. Jeppesen, L. Shu, G. Keir, V. Jegatheesan* School of Engineering, James Cook University, Townsville, QLD 4811, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 18 December 2008

The use of reverse osmosis (RO) membranes is becoming increasingly common in desalination plants, though disposal of the highly concentrated brines poses significant environmental risks. The targeted extraction of some metals from the concentrate can have multiple environmental and economic benefits. This is particularly apparent with recent developments in the development of zero liquid discharge desalination systems. This study has shown that recovery of sodium chloride from RO concentrate can significantly lower the cost of potable water production if employed in conjunction with thermal processing systems. Additionally, the recovery of rubidium from seawater may be a potential source of revenue, however further work is needed to characterise the economics of the rubidium extraction process. Finally, removal of phosphorus from RO concentrate provides little economic benefit, but may become increasingly necessary as environmental restrictions increase in the future. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Desalination Metal recovery Nanofiltration Reverse osmosis membranes Salt Zero liquid discharge

1. Introduction

2.1. Disposal to surface water or sewer

Desalination of seawater and brackish water sources is a common method for providing fresh drinking water around the world. The use of reverse osmosis (RO) membranes is becoming an increasingly practical method of desalination, due to significant improvements in energy recovery systems and pre-treatment processes over the past couple of decades. However, disposal of the concentrated brine produced by the desalination process poses significant environmental issues, due to the high concentrations of metals and salts. Recovery of selected elements from RO concentrate would provide environmental benefits, both in reducing the magnitude and environmental impact of disposal, and economic benefits in production of valuable metals.

Disposal to surface water and the sewer are typically the simplest and lowest cost disposal options of the reverse osmosis (RO) concentrates. Combined, these options are used in over 80% of the desalination plants in the USA [2]. Surface water disposal requires concentrate to be transported via pipeline to the disposal site, where outfall structures (such as diffusers) are used to transfer the concentrate to the surface water body. Design of appropriate outfall structures is necessary to ensure the discharge is well mixed and will not cause damage to the receiving waters and environments. The design must consider factors such as ambient conditions (conditions of the receiving water including bathymetry, salinity, density, velocity, and temperature), and discharge conditions (conditions of the outfall structure including structure geometry, discharge rate and physical and chemical factors as listed above) [2]. For RO plants, the concentrate is highly saline and typically denser than the receiving water, so diffuser systems are typically employed to avoid spreading of the discharge over the sea floor and benthic communities, which are most affected by RO discharge [3]. Disposal to sewer typically involves conveyance of the concentrate to sewer, and negotiation of fees with local wastewater treatment authorities [2]. The concentrate is then discharged as a part of the wastewater treatment plant effluent.

2. Concentrate disposal Concentrate from desalination plants is conventionally disposed of by several means, the most common of which is disposal to surface waters (including the ocean) or sewers. Other methods include disposal via deep well injection, disposal in evaporation ponds, and land application of concentrate [1]. Recent developments include the use of ‘zero liquid discharge’ systems, which are designed to further process the concentrate to produce dry salts. A brief description of each disposal option is presented in the following paragraphs.

* Corresponding author. Tel.: þ61 7 4781 4871; fax: þ61 7 4781 6788. E-mail address: [email protected] (V. Jegatheesan). 0959-6526/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2008.11.013

2.2. Deep well injection In this method, concentrate is disposed of by injection into porous rock formations below the ground surface. This method is not as prevalent as disposal to surface water, as it is generally only

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Nomenclature Cc Co Cp k R Vc Vo Vp

concentration of an ion in the concentrate stream concentration of ion at the intake concentration of an ion in the permeate stream rejection coefficient recovery rate volume (or flow rate) of concentrate stream intake volume (or flow rate) volume (or flow rate) of permeate stream

economically viable for larger plants [1]. Design of deep well disposal systems is generally dictated by site selection and consideration of geologic and hydrologic factors, to prevent the movement of the concentrate into potable groundwater supplies [2]. 2.3. Evaporation pond disposal The use of evaporation ponds is a simple and effective means of concentrate disposal, and is most suited to warm, dry climates with high evaporation rates (which are by necessity generally areas where desalination is necessary) [2]. However, they are generally only economical for small volume plants, and can cause contamination of underground aquifers if improperly designed or constructed, especially in the case of unlined ponds [4]. 2.4. Land application Land application systems, including spray irrigation, have similar characteristics to evaporation pond disposal systems. They are typically suitable only for small desalination plants, and can pose serious risks to groundwater and surface water resources if improperly designed [2]. 2.5. Zero liquid discharge systems There are multiple reports in the literature of zero liquid discharge systems [1,2,5,6]. These entail further processing of the concentrate until dry salts are obtained, which can be disposed to landfill or be used in other ways. These systems have multiple benefits: avoiding discharge to surface or ground waters, flexibility in site selection, and efficient reuse of water [2]. From an environmental perspective, such a system is obviously desirable. However, further concentration of the brine must be achieved by thermal processes, which add significantly to the overall cost of desalination. The major technical barrier to achieving zero liquid discharge systems has historically been the problem of scale formation (precipitation of alkaline earth metals such as CaCO3, Mg(OH)2 and CaSO4) in highly concentrated brines [6]. The I.E.S. company of Germany has reported favourable results from a proprietary decalcination process by developing an adsorbent for these scale forming ions that is regenerated in a self-sustaining process [5]. Assuming this process or similar techniques can be adopted, this allows for production of highly concentrated brines to allow thermal treatment to a dry product and recovery of selected chemical resources from the brines. 3. Recovery of resources from seawater Seawater contains almost all elements in the periodic table from hydrogen to uranium [6]. Conventionally, four components are extracted from seawater by evaporation: principally table salt

(sodium chloride), and the by-products potassium chloride, magnesium salts and bromide salts. Extraction of other components may be feasible, provided the elements are sufficiently valuable or rare on land. Several researchers [6,7] have proposed extraction schemes for a range of elements. Le Dirach et al. [7] identified eight elements as being potentially economically and technically viable, which are listed in Table 1. The extraction process proposed by Le Dirach et al. is shown in Fig. 1. In this paper, we examine the potential for economic extraction of rubidium and phosphorus, as well as analysing the potential cost of potable water production for varying the levels of extraction of sodium chloride. For all calculations a typical, large RO plant with an average intake of 100,000 m3 day1, has been assumed. 3.1. Basis for economic calculations A typical RO desalination plant incorporates pre-treatment processes such as ultrafiltration (UF) and nanofiltration (NF). Each system progressively improves the quality of the saline solution, with particulate matter being removed in the UF process, and hardness ions being removed in the NF process. The cost of potable water production can, theoretically, be reduced by further treatment of the RO concentrate by Multi-Stage Flash (MSF) distillation, a thermal process capable of producing potable water from extremely saline solutions. A conceptual diagram of the desalination process is shown in Fig. 2. The concentration of dissolved ions in the outlet stream depends on the rejection coefficient k of the NF membrane, which lies between zero and one and is defined by Eq. (1). Typical rejection coefficients for common ions are given in Table 2.

k ¼ 1

Cp Cc

(1)

Each process consists of one intake stream and two outlet streams (permeate/distillate stream and concentrate stream). The recovery rate R is the ratio of the volume of solution Vp passed through the membrane to the initial intake volume Vo, as shown in Eq. (2). Recovery rates and unit costs for each process have been assumed as shown in Table 3.

R ¼

Vp Vc ¼ 1 Vo Vo

(2)

The concentration of ions in the permeate and concentrate streams is thus given by:

Cp ¼

i Co h 1  ð1  RÞ1k R

(3)

Cc ¼ Co ð1  RÞk

(4)

Using these values, the concentration of metals and salts in the permeate and concentrate streams can be determined, and are shown in Table 4 and Table 5 respectively.

Table 1 List of potentially economic extracts from seawater [7]. Element

Concentration (mg L1)

Major use

Selling price ($ kg1)

Na Mg K Rb P In Cs Ge

10,500 1350 380 0.12 0.07 0.02 0.0005 0.00007

Fertilizers Alloys Fertilizers Lasers Fertilizers Metallic protection Aeronautics Electronics

0.13 2.80 0.15 79,700 0.02 300 63,000 1700

Raw Sea water Salinity = 35 g/L

Potable Water

Concentrated Brine Reject Salinity = 65 g/L

Pre-concentrate Salinity = 200 g/L

Alum (mixture of iron and aluminium sulphides)

Removal of Phosphorus by precipitation at pH 8 or 9

HCl and Calixerene C5 (0.1 mol/L) in TPH modified by monomide 2 (0.5 mol/L)

Removal of Cesium by a liquid-liquid extraction

Cesium in solution

Scrubbing and stripping operations using HNO3 D2EHPA EHPNA PIA226

Removal of Indium by a liquid-liquid extraction

Recovery of Rubidium using cation exchange resin - M2Ti2SiO4.nH2O

Indium and by-product Gallium - separate using counter-current processes Regenerate resin and separate from Potassium by-product with membranes

Total evaporation to form carnallite crystals KMgCl3.6H2O Gaseous HCl (90°C at 1 bar)

Recovery of Germanium

KCl - use electrolytic process to obtain pure metal

Hydrolysis to convert GeCl4 to GeO2, then roast under reducing atmosphere

Magnesium separated based on solubility

MgCl2 - use electrolytic process to obtain pure metal

Hot lixiviation to separate Sodium and Potassium

NaCl - use electrolytic process to obtain pure metal

Fig. 1. Extraction process proposed by Le Dirach et al. [7].

Ultrafiltration Intake

Nanofiltration

UF Permeate

UF Concentrate

Reverse Osmosis

NF Permeate

NF Concentrate

RO Concentrate

RO Permeate

Multi-Stage Flash

MSF Distillate

MSF Concentrate Fig. 2. Typical desalination process diagram.

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which is further substantially reduced by the subsequent recovery of sodium chloride.

Table 2 Typical rejection coefficients for NF processes. Ion

Rejection coefficient k

Naþ Mg2þ Ca2þ Cl SO2 4

0.10 0.87 0.83 0.10 0.93

3.3. Rubidium

Table 3 Recovery rates and unit costs of treatment processes. Treatment process

Recovery rate R

Unit cost ($ m3 of feed solution)

UF NF RO MSF

0.9 0.7 0.65 0.7

0.08 0.20 0.70 1.00

Table 4 Predicted metal and salt concentrations in permeate/distillate streams. Ion

Concentration (g L1) Seawater

UF permeate

NF permeate

RO permeate

MSF distillate

Naþ Mgþ Caþ Cl SO2 4

26.70 1.35 4.00 19.00 2.65

26.70 1.35 4.00 19.00 2.65

25.24 0.28 1.06 17.96 0.31

0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00

Table 5 Predicted metal and salt concentrations in concentrate streams. Ion

Concentration (g L1) Seawater UF concentrate NF concentrate RO concentrate MSF concentrate

Naþ 26.70 Mgþ 1.35 4.00 Caþ 19.00 Cl 2 2.65 SO4

26.70 1.35 4.00 19.00 2.65

30.12 3.85 10.87 21.43 8.12

72.10 0.80 3.02 51.31 0.87

240.34 2.66 10.07 171.03 2.91

3.2. Salt The cost of salt production is assumed to be $1.92 per cubic metre of concentrate sent to the evaporation ponds and the market price of salt is assumed to be $30 per ton. It is also assumed that 85% of the salt can be recovered from the reverse osmosis concentrate stream [8]. The resulting cost of potable water production for a UF– NF–RO system, both with and without sodium chloride recovery and MSF distillation, is shown in Table 6. The results indicate that for a conventional UF–NF–RO system, the return obtained from the sale of sodium chloride does not justify the additional extraction costs. However, the addition of MSF treatment slightly reduces the cost of potable water production,

Rubidium is found in small amounts in seawater and river water, with average concentrations of 1.3 mmol L1 (110 mg L1) and 0.6 mmol L1 (50 mg L1), respectively. Rubidium is not as economically important as some of the lighter alkali metals, but is used in semiconductor technology, as a medicament and also as a catalyst [9]. There are two potential methods of rubidium extraction from seawater: extraction by ion-exchange resins, and liquid–liquid extraction techniques. The use of cation exchange resins is generally the most efficient and economical method for the extraction of rubidium [7]. The chemically stable and macroporous phenol–formaldehyde resins with weakly dissociating phenolic ion-exchange groups are very efficient at separating rare alkali metals, and sulfonated phenol– formaldehyde resins have been found to have comparatively high selectivity towards rubidium and cesium [10]. However, further separation is required to remove other alkali metals such as cesium and potassium. Rubidium and potassium are very difficult to separate due to their similar properties, though separation is possible through the use of potassium selective membranes [7]. Rubidium can also be extracted from solution using simple benzylphenols via a liquid–liquid extraction technique. The recovery ratio for rubidium using BAMBP [4-tert-butyl-2-(a-methylbenzyl) phenol or 4-sec-butyl-2-(a-methylbenzyl) phenol] is more than 80%. However the extraction of rubidium with BAMBP is decreased in the presence of other ions [11]. Little information is available on extraction costs for rubidium. However, if we conservatively assume a rejection coefficient of 0.10 for Rbþ (consistent with other monovalent ions in Table 2) and a recovery rate of 10%, the daily production of rubidium is approximately 0.71 kg. This corresponds to a daily return of $56,950 based on the selling price shown in Table 1. Evidently the extraction of rubidium from seawater warrants further investigation so that the cost of extraction can be fully quantified. 3.4. Phosphorus Research has indicated that conventional phosphorus resources could be exhausted within the next 100–250 years [12]. Recovery from seawater therefore, represents a potential source of phosphorus that may be sustained for a longer period. In addition, removal of phosphorus from concentrate disposal streams can minimise the environmental impacts associated with phosphorus in the marine environment, including algal blooms, eutrophication and damage to coral organisms [13]. Assuming the typical concentration of phosphorus in seawater is approximately 0.07 mg L1, simple calculations indicate revenue that could be obtained from phosphorus recovery to be in the order of $50 per year. Clearly at current prices, phosphorus recovery from seawater is not economically feasible. However, extraction provides environmental benefits in reducing discharge of phosphorus to the

Table 6 Comparison of potable water costs for different desalination systems. System

Daily production cost ($)

Daily volume of potable water produced (m3)

By-product recovery costs ($)

Revenue from by-product sale ($)

Total cost of potable water ($ m3)

UF–NF–RO UF–NF–RO–NaCl recovery UF–NF–RO–MSF UF–NF–RO–MSF–NaCl recovery

70,100 70,100 92,150 92,150

40,950 40,950 56,385 56,385

– 42,336 – 12,701

– 40,542 – 40,542

1.71 1.76 1.63 1.14

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aquatic environment, and may become more economically viable as phosphorus reserves diminish in the future. 4. Conclusion The following conclusions can be drawn from the analysis: 1. The adoption of zero liquid discharge desalination systems can reduce the environmental effects associated with desalination concentrate management. In addition, such systems are also well suited for the extraction and exploitation of mineral byproducts from the concentrate stream. 2. The economy of extraction of sodium chloride from the distillation concentrate is significantly improved by the use of MSF distillation processes to further treat the concentrate from UF, NF, and RO processes. 3. The extraction of rubidium can yield income in the same order of magnitude as the operating costs of the plant, but detailed extraction costs are presently unknown. Further investigation into the economics of rubidium extraction is well justified. 4. The extraction of phosphorus is not economically viable but has substantial environmental benefits. Extraction may become worthwhile in the future as discharge restrictions increase and other phosphorus sources decline. Acknowledgements A grant awarded to the corresponding author by the Australia– Korea Foundation in 2007–2008 to conduct research on this topic is greatly appreciated.

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