System and method for low-energy crystallisation of minerals from brine

Abstract

The crystallisation of minerals 308 from brine 102 using a forward osmosis system 106. The forward osmosis system comprises a semipermeable membrane 302 comprising a material having a surface tension

Description

TECHNICAL FIELD

[0001] The present disclosure relates to low-energy crystallisation of minerals from brine, and more particularly, though not exclusively to low-temperature membrane crystallisation of seawater brine from a desalination plant. The present disclosure is also directed to a method and system for regenerating a draw solution for use in a mineral crystalliser, which enables the system to be used as part of a zero liquid discharge process. BACKGROUND

[0002] The seawater desalination industry mainly focuses on maximizing the potable water recovery from seawater using energy efficient methods. A seawater desalination plant usually produces two streams, namely potable water that has a low salinity (the permeate) and a brine that contains the salts and minerals from the seawater. Conventionally, the brine is discharged back into the sea which increases the salinity of seawater making it environmentally harmful to aquatic habitats. Recently, more environmentally friendly solutions are being developed to reduce the negative impact on the aquatic ecosystems and natural environments.

[0003] The brine is also known as a reject stream, as it cannot be used for consumption and is generally considered to have little to no commercial value as it contains mixtures of salts. A growing industry is the brine valorisation industry, which is focused on the recovery and purification of individual salts / minerals from brine for commercial use. Recovering a single salt / mineral from brine can be complicated and energy intensive, with conventional thermal evaporation-based concentrators and crystallisers using about 50 to 75 kWh of energy per cubic metre of brine.

[0004] The recovery and crystallisation of salts and minerals from brine or seawater requires large amounts of liquid to be processed in order to extract sufficient quantities of a mineral for commercial use. As such, this is a continuous process that consumes large amounts of energy, and more energy-efficient systems are being sought. Any reductions in the energy use of crystallisation processes offers a great advantage in decreasing the total amount of energy consumed by a crystallisation system, as well as a decrease in the costs of operation.

[0005] A lower-energy intensive method of salt and mineral recovery from brine is offered by membrane-based processes, which are based on the physical separation of the salts and water. Conventional membrane-based systems use standard polysulfone membranes which are easily fouled due to the hydrophobic nature of polysulfone. Membrane fouling decreases the quantity of crystals produced per unit of time for an area of the membrane and requires frequent membrane cleaning. This may shorten the lifespan of the membrane and leads to membranes having to be replaced frequently which increases the costs of operation.

[0006] Membrane crystallisation systems have also been combined with forward osmosis (FO) techniques. In such a system, the brine and a “draw solution” are applied on opposite sides of a membrane creating a transmembrane osmotic pressure gradient, which causes water from the brine to pass into the draw solution through the membrane, thereby diluting the draw solution. To maintain a sufficient transmembrane osmotic pressure gradient, the draw solution must be continuously replenished or regenerated (reconcentrated), which is also a highly energy-intensive process.

[0007] US11040904B2 describes methods and systems for treating wastewater via forward osmosis. In particular, using a forward osmosis membrane module to filter one or more precipitated salts from wastewater. The membrane module includes a membrane sheet spirally wound about a draw tube. Whilst the membrane module filters salts from wastewater, it recovers salts in an amorphous form and does not describe a process for the recovery of crystallised target salts. Additionally, US11040904B2 describes the reconcentration of the diluted draw solution using a reverse osmosis process using one or more standard reverse osmosis (RO) membrane modules, wherein the RO membrane modules simply extract water from the draw solution to help reconcentrate the draw solution.

[0008] US10835870B2 describes methods of manufacturing a multi-leaf membrane for filtering product fluid flow. These membranes may have a variety of applications, including wastewater treatment and desalination of seawater. However, a process or system for recovering crystallised target minerals from a product fluid flow, such as seawater or brine, is not described.

[0009] An objective of the current disclosure is therefore to address at least one of the limitations outlined above. SUMMARY OF THE PRESENT DISCLOSURE

[0010] According to one aspect of the present disclosure there is provided a system for crystallising a selected mineral from brine, the system comprising: a forward osmosis (FO) system including: a semipermeable membrane comprising a material having a surface tension below 50% of a surface tension of the selected mineral in crystallised form, the membrane being positioned substantially vertically providing fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution; and a crystal collector provided at the lowest edge of the substantially vertically positioned semipermeable membrane, the crystal collector being arranged to collect crystals of the selected mineral formed on the membrane which have fallen due to gravity.

[0011] This aspect of the disclosure enables the crystallisation to be carried out using significantly less energy than a thermal crystalliser. The scale of difference in energy use is that the crystallisation can be carried out at relatively low temperatures (for example room temperatures) rather than at 100 degrees Celsius as per a thermal crystalliser. This lower operating temperature provides a significant reduction in the operation and maintenance costs of a membrane crystalliser, according to the present disclosure, which is approximately 10 times cheaper than the operational and maintenance costs of a comparable thermal crystalliser. Furthermore, the capital cost of a membrane crystalliser, according to the present disclosure, is approximately 50% cheaper than the capital cost of a comparable thermal crystalliser.

[0012] These advantages are realised as the present aspect of the disclosure uses the natural forces rather than machine forces to affect the crystallisation. For example, osmotic potential for a forward osmosis system is used to increase the concentration of a selected mineral by drawing water and non-selected mineral in brine into a draw solution. As this concentrates the selected mineral in the brine forcing it to saturation, crystals form on the surface of the semipermeable membrane which eventually become big enough to have a weight pulling them down by the natural force of gravity to the crystal collector.

[0013] In some exemplary embodiments, the semipermeable membrane comprises a polycellulose acetate material. Polycellulose acetate is synthetically generated plastic material with the structure of natural cellulose and has a very smooth surface. This in turn provides a very low surface tension value of the membrane which facilitates requirement of the present embodiments. However, semipermeable membranes composed of other materials can also be used, so long as the surface tension is low enough to meet the requirements for crystal detachment from the membrane by gravity. This leads to a further advantage in that the membrane does not have to have the crystals manually removed from the membrane as the crystals automatically, as a result of the orientation of the membrane, fall to the crystal collector when they become large enough to overcome the surface tension of the semipermeable membrane on which they grow.

[0014] In some embodiments, the surface tension of the semipermeable membrane is in the range 30 N / m to 50 N / m. This provides a relative low surface tension for the semipermeable membrane which can be used to crystalise out different minerals from brine.

[0015] The FO system is, in some embodiments, configured with a selected draw solution to apply an osmotic draw pressure at least two times larger than that of the osmotic pressure of the brine. This provides enough required osmotic force to make the system work commercially. More preferably in other embodiments, the FO system is configured with a selected draw solution to apply an osmotic draw pressure at least three times larger than that of the osmotic pressure of the brine. The greater the osmotic draw pressure that can be applied the faster and more efficient the process. In this regard, in some embodiments the selected draw solution comprises MgCh. This has been found to be particularly useful for creating a large osmotic draw force for crystalising NaCI, the main constituent of brine derived from sea water for example. Similarly, the selected draw solution can be NaCI which has been found to generate a large osmotic pressure across the semipermeable membrane when the mineral to be crystalised from brine is CaSO4. It is noted that these two examples are particularly useful for seawater desalination plants where NaCI and CaSO4 make up the largest constituent minerals in seawater and hence seawater-derived brine.

[0016] In some embodiments, the concentration of the draw solution is selected to be in the range 100,000 mg / L to 330,000 mg / L. This tends to be several times higher than the concentration of brine thus leading to a relatively high osmotic potential across the semipermeable membrane. In some embodiments, the FO system operates at a relatively low temperature range of 15 to 35 degrees Celsius thereby reducing energy requirements of the system.

[0017] In an exemplary embodiment, the system further comprises an osmotically assisted reverse osmosis (OARO) system coupled to the FO system. This provides an effective feedback loop for the draw solution. More particularly, the OARO system is arranged to reconcentrate the draw solution by removing water from the draw solution and to recirculate the reconcentrated draw solution back to the FO system. This advantageously provides a simple way of regenerating the draw solution for use once again in the FO system which is constantly diluting the draw solution during crystallisation of the selected mineral. It is to be appreciated that the OARO system for concentrating the draw solution mentioned above, can provide the advantages described herein when used with any crystalliser system which can crystallise a selected mineral from brine which requires a concentrated draw solution to operate.

[0018] The OARO system, in some embodiments, applies a pressure of between 68 and 72 bars to the draw solution as part of a reverse osmosis process.

[0019] In some embodiments, the OARO system comprises a plurality of OARO units each reconcentrating the draw solution by a given amount, the plurality of OARO units being connected in series whereby a second OARO unit further concentrates the draw solution concentrated by a first OARO unit before recirculating the further concentrated draw solution back to the FO system. This enables the limitations of a OARO unit to be accommodated and also helps to reduce costs as cheaper OARO units can be used which have a given limited ability to concentrate the draw solution.

[0020] In some embodiments, the OARO system reconcentrates the draw solution comprising MgChor NaCI. As mentioned above, these are the most common draw solutions used to achieve the greatest osmotic potential across the semipermeable membrane for crystallisation of selected minerals.

[0021] The OARO system, in some embodiments, reconcentrates the draw solution to a concentration of approximately 200,000 mg / L to 330,000 mg / L before recirculating the draw solution to the FO system. This concentration is considered to enable the system to work optimally.

[0022] In some embodiments, the OARO system comprises a semipermeable membrane comprising molecular pore openings of 5 to 15 nanometres in size. More preferably the molecular pore openings have a size in the range 5 to 10 nm. This size permits water and small minerals to pass through the membrane as solute but is small enough to stop the draw solution molecules from traversing the semipermeable membrane. Because these molecular pores are slightly bigger than that of a conventional Sea Water Reverse Osmosis (SWRO) membrane, they will usually let more minerals pass through the OARO membranes on the permeate side of these membranes as compared to SWRO membranes which will mainly pass pure water and very little minerals. These extra minerals on the clean side of the membranes create a natural additional osmotic force draw of water of about 70 bars, which adds to the feed pressure of the OARO membranes, thereby advantageously allowing more water to pass through the OARO membranes and thereby concentrating the feed brine into the OARO to higher levels than the feed pressure will allow alone. This additional osmotic draw force also reduces the amount of energy required in reconcentration of the spent draw solution.

[0023] The OARO system of the embodiments can pass a permeate of the reverse osmosis to a further mineral extraction system, wherein the mineral extraction system uses low-energy crystallisation techniques using membranes with a selective affinity to a mineral to be extracted to further extract the mineral in the permeate. Such a system is described in our co-pending US patent application number US 63 / 607,049 (the contents of which are herein incorporated by reference and a copy of which is provided in Annex 1).

[0024] The present disclosure also extends to a low-energy multiple mineral crystallisation system for extracting a first and a second mineral from brine, the multiple mineral crystallisation system comprising: a first system for crystallising a first selected mineral from brine having an FO system and an OARO system as has been described above, and a second system for crystallising a second selected mineral from brine having an FO system and an OARO system as has been described above; wherein the OARO system of the first system passes a permeate of the reverse osmosis to an input of the FO system of the second system.

[0025] According to another aspect of the disclosure there is provided a method of crystallising a selected mineral from brine, the method comprising: providing a brine to a forward osmosis (FO) system; providing fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution in the FO system, by providing a substantially vertically positioned semipermeable membrane comprising a material having a surface tension below 50% of a surface tension of the selected mineral in crystallised form; and collecting crystals of the selected mineral formed on the membrane which have fallen due to gravity in a crystal collector positioned at the lowest edge of the substantially vertically positioned semipermeable membrane.

[0026] According to a further aspect of the present disclosure, there is provided a system for crystallising a selected mineral from brine, the system comprising a forward osmosis system including: a brine chamber for receiving a brine feed comprising the brine; a draw solution chamber for holding a draw solution; a semipermeable membrane positioned between the brine chamber and the draw solution chamber, the semipermeable membrane comprising a cellulose acetate material having a surface tension below 50% of a surface tension of the selected mineral in crystalised form and being positioned substantially vertically providing fluid communication between the brine chamber and the draw solution chamber; and a collector provided at the lowest edge of the substantially vertically positioned semipermeable membrane; wherein, in use, the draw solution creates an osmotic pressure gradient across the semipermeable membrane drawing non-selected minerals and water from the brine into the draw solution chamber, causing saturation of a selected mineral of the brine in the brine chamber and the formation of at least one crystal of the selected mineral at the semipermeable membrane surface; and wherein when the weight of the at least one crystal exceeds the selected surface tension, the at least one crystal falls under gravity to the collector.

[0027] According to a yet further aspect of the present disclosure, there is provided a system for crystallising a selected mineral from brine, the system comprising: a forward osmosis (FO) system including: a semipermeable membrane comprising a polycellulose acetate material having a surface tension below 50% of a surface tension of the selected mineral in crystallised form, the membrane being positioned substantially vertically providing fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution; and a collector provided at the lowest edge of the substantially vertically positioned semipermeable membrane; wherein, in use, the draw solution creates an osmotic pressure gradient across the semipermeable membrane drawing non-selected minerals and water from the brine into the draw solution chamber, causing saturation of a selected mineral of the brine in the brine chamber and forming at least one crystal of the selected mineral at the semipermeable membrane surface; and wherein when the weight of the at least one crystal exceeds the selected surface tension, the at least one crystal falls under gravity to the collector.

[0028] According to another yet further aspect of the present disclosure there is provided a method of crystallising a selected mineral from a brine using a forward osmosis system, the method comprising: providing the forward osmosis system with a semipermeable membrane comprising a material having a surface tension below 50% of a surface tension of the selected mineral in crystallised form, positioning the membrane substantially vertically with the forward osmosis system to provide fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution; positioning a collector at the lowest edge of the substantially vertically positioned semipermeable membrane; and operating the forward reverse osmosis system such that the draw solution creates an osmotic pressure gradient across the semipermeable membrane drawing nonselected minerals and water from the brine into the draw solution chamber, causing 7 saturation of a selected mineral of the brine in the brine chamber; forming at least one crystal of the selected mineral at the semipermeable membrane surface; and collecting the at least one crystal in the collector when the weight of the at least one crystal exceeds the selected surface tension and the at least one crystal falls under gravity to the collector. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and other features and advantages of the present disclosure will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:

[0030] Figure 1 is a block diagram of a crystallisation system in accordance with an embodiment of the present disclosure, showing a forward osmosis (FO) crystallisation system and an osmotically assisted reverse osmosis (OARO) unit, as well as inputs and outputs of the crystallisation system;

[0031] Figure 1A is a block diagram of a crystallisation system in accordance with another embodiment of the present disclosure, the crystallisation system comprising a FO crystallisation system and an OARO unit, which comprises two sequential stages of OARO processing;

[0032] Figure 2 is a block diagram of another embodiment of a multi-mineral crystallisation system in accordance with the present disclosure, where the multi-mineral crystallisation system is configured to recover at least two target minerals from brine, and the multi-mineral crystallisation system comprises the crystallisation system of Figure 1 and a further crystallisation system in order to recover crystals of the two target minerals;

[0033] Figure 3 is a series of schematic osmosis transport diagrams each showing different stages of operation of the FO crystallisation system of Figure 1 as a function of time;

[0034] Figure 4 is a series schematic osmosis transport diagrams showing what is happening in the OARO system of Figure 1 as a function of time;

[0035] Figure 5 is a flow diagram illustrating a method of operation of the crystallisation system of Figure 1; and

[0036] Figure 6 is a graph showing the osmotic pressures of different solutions at different concentrations, enabling selection of the appropriate draw solution for a given crystal recovery for any of the embodiments of the present disclosure. DETAILED DESCRIPTION

[0037] Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the disclosure or as a limitation on the scope of the disclosure. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described.

[0038] The figures are schematic and simplified for clarity, and they merely show details which aid understanding of the disclosure, while other details have been left out Throughout, the same reference numerals are used for identical or corresponding parts.

[0039] Figure 1 shows the overview of an exemplary system 100 comprising a crystallisation system 118. The crystallisation system 118 comprises a forward osmosis (FO) crystallisation system 106 and an osmotically assisted reverse osmosis (OARO) unit 110. The FO crystallisation system 106 receives brine 102 (typically from a seawater desalination plant, namely the output of a SWRO system) and a draw solution 104 and outputs a crystallised mineral 112 and a diluted draw solution 108. The diluted draw solution 108 is subsequently fed to the OARO unit 110, which outputs a reconcentrated draw solution 116 that is recirculated to the FO crystallisation system 106 for reuse in the FO process occurring there. Whilst the embodiment illustrated in Figure 1 shows brine derived from seawater as the input to the FO crystallisation system 106, in other embodiments the input may include brine derived from other sources or brackish water.

[0040] In addition to the reconcentrated draw solution 116, the OARO unit 110 outputs a permeate (water) containing other salts / minerals 114 that may have passed through the FO crystallisation system 106 into the diluted draw solution 108 during the crystallisation process of the FO crystallisation system 106. These other salts / minerals are typically smaller in size than the water molecules and so pass through the FO membrane under osmotic pressure. This permeate 114 can be processed through a Selective Membrane System comprising chemically modified membranes that are semipermeable to water and have an affinity to a target mineral. One such Selective Membrane System is as described in our copending US patent application number US 63 / 607,049 (the contents of which are herein incorporated by reference and a copy of which is provided in Annex 1).

[0041] Figure 1A shows an alternative embodiment of the system 100 shown in Figure 1, comprising the crystallisation system 118. As in the embodiment of Figure 1, the FO crystallisation system 106 operates in the same manner as has been described in relation to Figure 1, namely that the crystallisation system 106 receives brine 102 and a draw solution 104 and outputs a crystallised mineral 112 and a diluted draw solution 108 which is then fed to the OARO unit 110 for reconcentration. However, in the embodiment of Figure 1A, the OARO unit 110 comprises two stages of OARO 110a, 110b. The diluted draw solution 108 is fed to the first stage of OARO 110 where it is partially reconcentrated. The partially reconcentrated draw solution is then subsequently passed to the second stage of OARO 110b where it is further reconcentrated. The second stage of OARO 110b outputs the fully reconcentrated draw solution 116 that is recirculated to the FO crystallisation system 106. The solute of each of the first and second stages 110a, 110b is combined and output as the permeate with remaining minerals 114.

[0042] Each stage of OARO 110a, 110b increases the concentration of the diluted draw solution 108 by a limited amount. In one non-limiting example, each OARO stage increases the concentration of the diluted draw solution 108 by 60,000 mg / Lto 70,000 mg / L of total dissolved solids (TDS). In this example, after two stages of OARO 110a, 110b, the concentration of the diluted draw solution 116 is increased by 120,000 mg / L to 140,000 mg / L of TDS. Although the current embodiment shows two stages of OARO 110a, 110b, other embodiments may comprise fewer or more stages of OARO. The number of stages required is dependent on the mineral which is being crystalised and in particular the saturation level of the mineral being crystallised. For example, for the crystallisation of NaCI, because of its high saturation point of 330,000 mg / L (at standard atmospheric pressure and at 25 degrees Celsius) of TDS, a two-stage OARO unit is considered optimal. Crystallisation of other minerals with a lower saturation point require less concentrated draw solution and so fewer OARO stages can be used.

[0043] Figure 2 shows an embodiment of a multi-mineral crystallisation system 200 comprising the crystallisation system 118 depicted in Figure 1 and a second crystallisation system 218 positioned consecutively to the first crystallisation system 118. Brine 102 containing target minerals A and B and other salts / minerals is fed into the first crystallisation system 118. The first crystallisation system 118 operates in the same manner as has been described in relation to Figure 1 to crystallise Mineral A 112. The second crystallisation system 218 also operates in a similar manner as the first crystallisation system 118 as described in relation to Figure 1, except for the fact that the draw solution 210 is different to the draw solution 104 of the first crystallisation system 118 in order to crystallise a different mineral (Mineral B212).

[0044] The first crystallisation system 118 outputs a crystallised mineral A 112 and a permeate (water) containing target mineral B and the other salts / minerals 114. The permeate containing target mineral B and the other salts / minerals 114 is fed into the second crystallisation system 218. The second crystallisation system 218 outputs a crystallised mineral B 212 and a permeate containing the other salts / minerals 214 (namely salts / minerals in the original brine 102 excluding Minerals A and B which have been crystalised out). Although the current embodiment illustrates two crystallisation systems 118, 218 that are used to produce crystallised minerals A and B 112, 212, other embodiments may include more crystallisation systems sequentially arranged with the current system 200. For example, a third crystallisation system (not shown) may receive the permeate with other salts / minerals 214 output by the second crystallisation system 218, wherein the permeate with other salts / minerals 214 comprises a third target mineral (not shown) that is crystallised in the third crystallisation system.

[0045] Figure 3 shows a series of schematic osmosis transport diagrams each showing different stages of operation of the forward osmosis (FO) crystallisation system 106 of Figures 1 and 2 as a function of time. The first block 106a depicts the first stage, namely the FO crystallisation system 106 at the start of the crystallisation process. The FO crystallisation system 106 comprises a single flat sheet semipermeable membrane 302 separating a brine chamber 324 that holds the brine 102 and a draw solution chamber 322 that holds the draw solution 104. In the present embodiment, the semi-permeable membrane 302 is positioned substantially vertically as shown in Figure 3. The brine 102 is essentially comprised of water molecules 312, the target salt / mineral molecules 308 and other salt / mineral molecules 310. The difference in the saturation levels of the draw solution 104 and the brine 102, create an osmotic pressure difference 304 which drives the water molecules 312 in a first direction 306 through the membrane 302 into the draw solution chamber 322. The osmotic pressure of the draw solution 104 is initially at least twice the osmotic pressure of the brine 102.

[0046] Although the present embodiment shown in Figure 3 illustrates a single flat sheet membrane, the optimum number of membranes will depend on the sustainable flux of the membrane. The sustainable flux of the membrane depends on the target salt / mineral. For example, considering a system wherein the draw solution 104 comprises a magnesium chloride (MgCI2) solution and the membrane 302 comprises a polycellulose acetate membrane with a low surface tension of 30 to 50 N / m, the required sustainable flux to produce sodium chloride (NaCI) crystals and calcium sulphate crystals (CaSO4) is different. Specifically, to produce sodium chloride crystals the required sustainable flux is 3 litres / m2 / hour, wherein the surface tension of sodium chloride crystals is 114 N / m. However, the production of calcium sulphate crystals requires a sustainable flux of 8 litres / m2 / hour, wherein the surface tension of calcium sulphate crystals is 156 N / m. The sustainable flux can also influence the required membrane surface area as well as the selection of the draw solution 104.

[0047] Although the present embodiment shows brine 102 as an input, seawater may also be fed into the FO crystallisation system 106. Seawater has a much lower concentration of salts / minerals than brine. For example, the concentration of sodium chloride in seawater can be up to a magnitude less than in brine. As such, to process seawater, a larger membrane surface area is required when the input comprises seawater compared to when the input comprises brine. A lower membrane surface area is required when the input comprises brine as the concentration of minerals is higher, and so processing brine is more cost efficient than processing seawater.

[0048] Block 106b shows a second stage of the crystallisation process. In this stage, due to the high osmotic pressure of the draw solution 104, the water molecules 312 are “drawn” from the brine chamber 324 through the membrane 302 into the draw solution chamber 322. In addition to water molecules 312, other salt / mineral molecules 310 are also drawn through the membrane 302. This causes the saturation level of the target mineral 308 in the brine solution 102 to increase overtime whilst the draw solution 104 becomes increasingly dilute. At the saturation point of the target mineral 308, crystals 320 naturally form as microscopic nuclei on the surface of the membrane 318 as a result of the draining effect of the osmotic draw force on the opposite side of the membrane 302. The naturally occurring draw force is generated by the difference in the osmotic pressures 304 of the draw solution 104 on one side of the membrane 302 and the brine 102 on the other side of the membrane 302. Crystallisation stops when the difference in osmotic pressures 304 between the brine 102 and the draw solution 104 is less than 10%.

[0049] Block 106c shows a third stage of the crystallisation process. In this stage once the crystals 320 of the target salt 308 have reached a critical weight, they roll (fall) off the surface of the membrane 318 due to gravity and the relatively low surface tension of the membrane and are collected in a collection basin 316. The critical weight depends on the surface tension of the crystals 320, which in turn depends on the size of crystals 320 formed and therefore varies for different target minerals. Depending on the surface tension difference between the target mineral crystals and the surface of the membrane 318, as well as the osmotic pressure difference 304 between the brine and the draw solution, crystals 320 can accumulate over a time period between 20 minutes and 2 hours. Advantageously, the FO membrane crystallisation system 106 described herein uses 7.85 kWh of energy per cubic metre of brine, which is approximately ten times less than the energy used by conventional thermal crystallisers.

[0050] The FO membrane crystallisation system 106 operates at the temperatures of crystallisation of the target salt, which are between 15 and 35 degrees Celsius. On the other hand, other methods of crystallisation such as thermal crystallisation require temperatures above 100 degrees Celsius. Therefore, the FO membrane crystallisation system 106 advantageously requires less energy than a conventional thermal crystallisation system.

[0051] Figure 4 shows two schematic osmosis transport diagrams showing what is happening in an OARO stage 110a / b comprised in any of the OARO units 110 of Figures 1, 1A and 2 as a function of time. An OARO stage 110a / b comprises reconcentration chamber 400, a semi-permeable membrane 402 and a solute chamber 404. In the first stage (upper diagram of Figure 4) the diluted draw solution 108 is fed into the reconcentration chamber 400 of the OARO stage on one side of the membrane 402. The diluted draw solution 108 comprises draw solution molecules 314, water molecules 312 and other salts / minerals 310 from the FO crystallisation system 106, as described above. In the second stage (as shown in the lower diagram of Figure 4), during the OARO process, the water 312 and other salts / minerals 310 are driven through the membrane 402 (as shown by arrow 406), thereby reconcentrating the draw solution 108. The reconcentrated draw solution 116 is then recirculated to the FO crystallisation system 106.

[0052] The membranes 402 used in an OARO stage 110a / b, have a slightly larger membrane thin film separation layer than conventional seawater reverse osmosis (SWRO) membranes. In addition, conventional SWRO membranes have typical pore sizes of less than 2 nm typically 1 to 2 nm, therefore only allowing water molecules to pass through them and very few minerals resulting in a salinity of approximately 100 mg / L to 200 mg / L of TDS. However, the chemical structure of the OARO membranes 402 used herein results in molecular pore openings of greater than 5 nm, preferably the range can be 5 nm to 15 nm and more preferably in some embodiments 5 nm to 10 nm. The larger pore openings of the OARO membranes 402, allow water molecules and small mineral molecules to pass through the membrane 402 into the solute chamber 404. The water and minerals that pass through the OARO membranes 402 comprise a permeate 114 with a salinity of approximately 35,000 mg / L. The minerals comprised in the permeate 114 create a natural osmotic draw force for water of about 70 bars. This additional natural draw pressure adds to the applied feed pressure of the OARO membranes 402, thereby driving more water through the membranes 402. The additional draw pressure enables the diluted draw solution 108 to be concentrated to higher levels than when using the feed pressure alone, thereby increasing the effectiveness of the OARO process.

[0053] In an embodiment with two or more stages of OARO, such as the embodiment shown in Figure 1A, each stage of OARO further concentrates the diluted draw solution 108 as described above. Additionally, each stage of OARO increases the concentration of minerals in the permeate 114. As the mineral concentration in the permeate 114 increases, the osmotic draw force also increases. This increases the effectiveness of the OARO unit 110. The required number of OARO stages will depend on the desired osmotic draw force and the osmotic pressure of the reconcentrated draw solution 116.

[0054] Figure 5 shows a method 500 used in the FO crystallisation system 106 and OARO unit 110 to crystallise a target mineral from brine 102 and reconcentrate the spent draw solution (or diluted draw solution 108). In an embodiment of the present system, brine 102 is input, at Step 502, into the crystallisation system 118 and a draw solution 104 is input, at Step 504, into the crystallisation system 118. The target salt / mineral is crystallised, at Step 506, using forward osmosis (FO) membrane crystallisation. The crystallised target salt / mineral 112 is collected, at Step 508, in the form of crystals 320 in a collection basin 316. The crystallisation process outputs, at Step 508, a spent draw solution that is processed, at Step 510, by a first stage of OARO comprised in the OARO unit 110 to reconcentrate the spent draw solution. As in the embodiment of Figure 1 A, two or more stages of OARO 110a / b might be required to reconcentrate the spent draw solution 108 to its initial concentration. Therefore, in the embodiment shown in Figure 1A, the spent draw solution is subsequently processed, at Step 512, by a second stage of OAROHOb. The reconcentrated draw solution 116 is then recirculated, at Step 514, to the FO crystallisation system 106. Therefore, the reconcentrated draw solution 116 can be used to continue the crystallisation process, at Step 506, of the target salt / mineral.

[0055] The present system 100 can be used to recover various target minerals from brine, including calcium sulphate, sodium chloride, potassium chloride and magnesium sulphate. The same type of polycellulose acetate membrane can be used, with the same surface tension, but with different draw solutions as different solutions have different osmotic pressure differences. The higher the osmotic pressure difference between the brine 102 and the draw solution 104, the faster the target mineral can be crystallised as the number of times that the diluted draw solution 108 has to be reconcentrated is reduced. As such, it is preferable for the osmotic pressure of the draw solution 104 to be at least two times higher than the osmotic pressure of the brine 102 and, more preferably in some embodiments, at least three times higher. In order to determine appropriate draw solutions 104 for specific target mineral solutions, a graph such as the one shown in Figure 6 can be used.

[0056] Figure 6 shows a graph 600 of the osmotic pressure of various solutions at a temperature of 25 degrees Celsius, as a function of their concentration. This experimental data can be used for the design of any of the embodiments of the present disclosure. For example, to crystallise sodium chloride crystals from a concentrated sodium chloride solution, such as brine derived from the desalination of seawater, magnesium chloride is the preferred draw solution as it has the highest osmotic pressure difference to the sodium chloride solution at all concentrations. However, the graph 600 indicates that another suitable draw solution candidate is a calcium chloride solution, though the osmotic draw force will be lower than when using magnesium chloride. Other suitable draw solution compositions and concentrations can be determined from this graph by comparison of the osmotic draw pressures, wherein the osmotic pressure of the draw solution has to be at least twice the osmotic pressure of the target mineral solution. More preferably, as can be seen from the graph of Figure 6, the draw solution can be selected to have an osmotic pressure of at least three times the osmotic pressure of the target mineral solution.

[0057] Having described the structure of the systems 100, 200 and generally described their operation to achieve crystallisation of a desired mineral from brine, some more detailed, but non-limiting, examples of use of the above systems are now described.

[0058] In one non-limiting example of the system 100, the brine 102 fed into the crystallisation system 118 is derived from the desalination of seawater. The target mineral is sodium chloride, which is the most abundant salt in brine derived from seawater. An advantage of processing brine in the present system 100, is that its salinity level is too high for the survival of organisms and therefore biofouling of the membranes used in this system 100 is avoided. The draw solution 104 comprises a magnesium chloride solution with an initial concentration of 300,000 mg / L of total dissolved solids (TDS). The semipermeable membrane 302 separating the brine 102 from the draw solution 104 in the FO crystallisation system 106 is substantially vertical to maximise the gravitational pull on the crystals as they are formed. The membrane comprises a single flat sheet polycellulose acetate membrane with a surface tension of 30 to 50 N / m. To decrease the environmental burden of using magnesium chloride as a draw solution, magnesium chloride can be recovered from seawater using a Selective Membrane System, such as the one mentioned above and described in our co-pending US patent application (application number US 63 / 607,049) set out in Annex 1.

[0059] In use, the magnesium chloride solution draws 306 the water and other minerals contained in the brine through the membrane 302, thereby increasing the concentration of the sodium chloride. At the saturation point of sodium chloride of 330,000 mg / L of TDS, sodium chloride crystals spontaneously form on the surface of the membrane 318, on the brine side of the membrane 302. When the crystals 320 reach a critical size between 1 and 4 nm with a surface tension of 114 N / m, gravity causes them to roll off (fall from) the surface of the membrane 318 into the collection basin 316. The sodium chloride crystals collected initially have a purity of 99.9%. As the crystallisation process continues, the purity of the sodium chloride crystals decreases. The crystallisation process is stopped when the purity of the sodium chloride crystals is 99.6%, which is the lower limit of commercially viable crystals. Advantageously, the FO crystallisation process described herein allows the purity of the produced crystals to be controlled and additionally produces commercially viable crystals.

[0060] There are two main advantages of engineering the membrane 302 to have a surface tension that is below 50% of the surface tension of the produced crystals 320. Firstly, the collection of the crystals 320 does not require mechanical intervention or other energy-intensive recovery methods. Recovery advantageously simply uses the natural force of gravity. Secondly, the smoothness of the membrane 302 prevents other crystals forming on the surface of the membrane 318 that would cause scaling or clogging up of the membrane 302 and thus the membrane can be used for longer without manual intervention for cleaning.

[0061] During the crystallisation of sodium chloride, water is drawn through the membrane 302 to the draw solution side of the membrane 302. This causes the draw solution 104 to become increasingly diluted over time. At a concentration of 100,000 mg / L of TDS the osmotic pressure of the magnesium chloride draw solution 104 becomes comparable to the osmotic pressure of the brine 102. Therefore, no more water can effectively be drawn from the brine side of the membrane 302 to the draw solution side of the membrane 302. In order to reuse the diluted draw solution 108, it is sent to an OARO unit 110 to be reconcentrated.

[0062] In the present example, the OARO unit 110 comprises two stages of OARO as shown in the embodiment of Figure 1A, wherein each stage concentrates the diluted draw solution by an additional 60,000 mg / L to 70,000 mg / L of TDS. The pressure used is a combination of feed pressure of 70 bar and additional osmotic draw pressure of 68 to 72 bar naturally created by the other minerals in the solute 114. The OARO unit 110 concentrates the diluted draw solution back to its initial concentration of 300,000 mg / L of TDS. In order to reconcentrate the diluted draw solution 108, each stage of OARO removes a permeate 114 from the magnesium chloride solution. The permeate 114 comprises water, other minerals contained in the brine that passed through the FO crystallisation membrane 302 and less than 10% of the magnesium chloride draw solution. The permeate is collected on the low salinity side of the OARO membrane 402 and can be fed into a second crystallisation system 218, such as the one depicted in the embodiment of the multi-crystallisation system 200 of Figure 2, for the crystallisation of other minerals.

[0063] In another non-limiting example, the target mineral comprised in the input brine 102 is calcium sulphate. In order to recover calcium sulphate crystals, the draw solution 104 comprises a sodium chloride solution with an initial concentration of 330,000 mg / L of total dissolved solids (TDS). As in the example above, the semi-permeable membrane separating the brine from the draw solution in the FO crystallisation system 106 is substantially vertical and comprises a single flat sheet polycellulose acetate membrane with a surface tension of 30 to 50 N / m.

[0064] The sodium chloride draw solution increases the concentration of calcium sulphate to its saturation point of 260,000 mg / L (at standard atmospheric pressure and at 25 degrees Celsius), causing crystals 320 to spontaneously form on the surface of the membrane 318, on the brine side of the membrane 302. When the calcium sulphate crystals reach a critical size between 1 and 4 nm with a surface tension of 156 N / m, gravity causes them to roll off the surface 318 into the collection basin 316. The calcium sulphate crystals collected initially have a purity of 99.9%. As described in the example of the crystallisation of sodium chloride, the purity of the calcium sulphate crystals decreases over time. The crystallisation process is stopped when the purity of calcium sulphate crystals is 99.6% and can therefore be used for commercial purposes.

[0065] At a concentration of 100,000 mg / L of TDS the osmotic pressure of the sodium chloride draw solution 104 becomes comparable to the osmotic pressure of the brine 102. Therefore, substantially no more water can be drawn from the brine side of the membrane 302 to the draw solution side of the membrane 302 and the diluted sodium chloride draw solution 108 is directed to the OARO unit 110 to be reconcentrated.

[0066] In the present example, the OARO unit 110 comprises two stages of OARO, as illustrated in the embodiment of Figure 1A. The pressure used is a combination of feed pressure with 70 bars and additional osmotic draw pressure of 68 to 72 bars. The OARO unit 110 concentrates the diluted sodium chloride draw solution back to its initial concentration of 330,000 mg / L. In order to reconcentrate the diluted draw solution, the OARO unit 110 removes a permeate from the sodium chloride solution. The permeate comprises water, other minerals contained in the brine that passed through the FO crystallisation membrane 302 and less than 10% of the sodium chloride draw solution. The permeate is collected on the low salinity side of the OARO membrane 402 and can be fed into a second crystallisation system 218, such as the one depicted in the embodiment of the multi-mineral crystallisation system 200 of Figure 2, for the crystallisation of other minerals.

[0067] In another non-limiting example, multiple target salts / minerals can be crystallised as illustrated by the embodiment of a multi-mineral crystallisation system 200 shown in Figure 2. In this example, one of the target minerals A and B comprises sodium chloride and the other comprises calcium sulphate. The brine comprising target minerals A and B, as well as other salts / minerals, is fed into the first crystallisation system 118. The first crystallisation system outputs crystallised mineral A (sodium chloride or calcium sulphate) and the permeate with mineral B is subsequently fed into the second crystallisation system 218 for the crystallisation of mineral B 212. The two FO crystallisation systems 106 comprised in the two crystallisation systems 118, 218, comprise the same type of single flat sheet polycellulose membrane with a surface tension of 30 to 50 N / m. The difference between the two FO crystallisation systems 106 is the composition of the draw solution 104, as detailed in the examples above. Namely, to crystallise sodium chloride the draw solution 104, 210 comprises a magnesium chloride solution whereas to crystallise calcium sulphate the draw solution 104, 210 comprises a sodium chloride solution. Advantageously, the same type of membrane, with the same surface tension, can be used to extract more than one target mineral.

[0068] Although features have been shown and described, it will be understood that they are not intended to limit the claimed disclosure, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit 18 and scope of the claimed disclosure. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed disclosure is intended to cover all alternatives, modifications, and equivalents.

Claims

1. A system for crystallising a selected mineral from brine, the system including a forward osmosis (FO) system, the FO system comprising:a semipermeable membrane comprising a material having a surface tension less than 50% of a surface tension of the selected mineral in crystalised form, the membrane being positioned substantially vertically providing fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution; anda crystal collector provided at the lowest edge of the substantially vertically positioned semipermeable membrane, the crystal collector being arranged to collect crystals of the selected mineral formed on the membrane which have fallen due to gravity.

2. A system as claimed in Claim 1, wherein the semipermeable membrane comprises a polycellulose acetate material.

3. A system as claimed in Claim 1 or 2, wherein the surface tension of the semipermeable membrane is in the range 30 N / m to 50 N / m.

4. A system as claimed in any one of Claims 1 to 3, wherein the FO system is configured with a selected draw solution to apply an osmotic draw pressure at least two times larger than that of the osmotic pressure of the brine.

5. A system as claimed in any one of Claims 1 to 3, wherein the FO system is configured with a selected draw solution to apply an osmotic draw pressure at least three times larger than that of the osmotic pressure of the brine.

6. A system as claimed in any one of Claims 1 to 5, wherein the selected draw solution comprises MgCh7. A system as claimed in any one of Claims 1 to 6, wherein the concentration of the draw solution is in the range 100,000 mg / Lto 330,000 mg / L.

8. A system as claimed in any one of Claims 1 to 7, wherein the selected mineral comprises NaCI.

9. A system as claimed in any one of Claims 1 to 7, wherein the selected mineral comprises CaSCU.

10. A system as claimed in Claim 9, wherein the selected draw solution comprises NaCI.

11. A system as claimed in any one of Claims 1 to 10, wherein the FO system operates at a temperature range of 15 to 35 degrees Celsius.

12. A system according to any one of Claims 1 to 11, further comprising an osmotically assisted reverse osmosis (OARO) system coupled to the FO system, the OARO system being arranged to reconcentrate the draw solution by removing water from the draw solution and to recirculate the reconcentrated draw solution back to the FO system.

13. A system according to Claim 12, wherein the OARO system applies a pressure of between 68 and 72 Bars to the draw solution as part of a reverse osmosis process.

14. A system according to Claim 12 or 13, wherein the OARO system comprises a plurality of OARO units each reconcentrating the draw solution by a given amount, the plurality of OARO units being connected in series whereby a second OARO unit further concentrates the draw solution concentrated by a first OARO unit before recirculating the further concentrated draw solution back to the FO system.

15. A system according to any of Claims 12 to 14, wherein the OARO system reconcentrates the draw solution comprising MgChor NaCI.

16. A system according to any of Claims 12 to 15, wherein the OARO system reconcentrates the draw solution to a concentration of approximately 200,000 mg / Lto 330,000 mg / L before recirculating the draw solution to the FO system.

17. A system according to any of Claims 12 to 16, wherein a semipermeable membrane of the OARO system comprises molecular pore openings of 5 to 15 nanometres in size.

18. A system according to Claim 17, wherein a semipermeable membrane of the OARO system comprises molecular pore openings of 5 to 10 nanometres in size.

19. A system according to any of Claims 12 to 18, wherein the OARO system passes a permeate of the reverse osmosis to a further FO mineral extraction system, wherein the mineral extraction system uses low-energy crystallisation techniques using membranes with a selective affinity to a mineral to be extracted to further extract the mineral in the permeate.

20. A low-energy multiple mineral crystallisation system for extracting a first and a second mineral from brine, the multiple mineral crystallisation system comprising:a first system for crystallising a first selected mineral from brine according to any of Claims 12 to 19, anda second system for crystallising a second selected mineral from brine according to any of Claims 12 to 19;wherein the OARO system of the first system passes a permeate of the reverse osmosis to an input of the FO system of the second system.

21. A method of crystallising a selected mineral from brine, the method comprising:providing a brine to a forward osmosis (FO) system;providing fluid communication between a brine chamber holding the brine and a draw solution chamber holding a draw solution in the FO system, by providing a substantially vertically positioned semipermeable membrane comprising a material having a surface tension less than 50% of a surface tension of the selected mineral in crystalised form; andcollecting crystals of the selected mineral formed on the membrane which have fallen due to gravity in a crystal collector positioned at the lowest edge of the substantially vertically positioned semipermeable membrane.

Images