Separation method and reactor

Abstract

The present disclosure relates to separation methods and reactors. The present disclosure provides methods and systems for precipitating CaSO4 from a supersaturated solution of CaSO4. The precipitation can form gypsum particles with an average diameter of about 25 μm. The precipitation can be controlled to reduce or avoid scaling. The present disclosure also provides methods and systems in which CaSO4 scaling can be removed.

Description

Technical Field

[0001] The present disclosure relates to methods and reactors for removing calcium sulfate from wastewater. Background Art

[0002] The following paragraphs are not an admission that anything discussed therein is prior art or part of the general knowledge of a person skilled in the art.

[0003] Various industrial processes, such as desalination, coal mining drainage, flue gas desulfurization, and lime neutralization of acidic wastewater, produce aqueous effluents that include calcium sulfate. Calcium sulfate can undesirably precipitate to form scale on the surfaces of process equipment, interfering with operating efficiency. Summary of the invention

[0004] The following introduction is intended to introduce the specification to the reader, but does not limit any invention. One or more inventions may exist in the combination or sub-combination of the device elements or method steps described below or in other parts of this document. The inventor does not waive or deny his rights to any one or more inventions disclosed in this specification simply by not describing such one or more other inventions in the claims.

[0005] The formation of calcium sulfate scale can be mitigated by adding lime and sodium carbonate to the calcium sulfate-containing effluent, which can result in the formation of sodium sulfate. However, it is desirable to develop methods and process equipment that reduce or avoid lime softening, the generation of sodium sulfate, or both. Such methods and process equipment can result in cost savings compared to conventional lime softening methods. It has proven difficult to remove calcium sulfate from its supersaturated solution using conventional coagulation / flocculation methods and equipment because high concentrations of sulfate often result in scaling on equipment surfaces.

[0006] The present disclosure discloses various methods and apparatuses that can operate individually or be combined into larger systems or apparatuses. As described above, a larger system or apparatus according to the present disclosure can be a sub-combination of the disclosed methods and apparatuses.

[0007] In some embodiments, the present disclosure provides a separation method comprising: receiving a mixture of gypsum seed fines in an aqueous solution from a reactor; adding an anionic flocculant and optionally a coagulant to the aqueous solution; aggregating the gypsum seed fines and the flocculant into flocs; separating the mixture into a turbidity-reduced effluent and a flocculated gypsum slurry; and exposing a portion of the flocculated gypsum slurry to a shear stress sufficient to convert the flocculated gypsum seeds into unflocculated gypsum seed fines, and transferring the fines to a reactor.

[0008] The disclosed method controls the conversion of gypsum seeds between (I) a flocculated portion that can be separated into a turbidity-reduced effluent and a flocculated gypsum slurry and (II) a non-flocculated portion of gypsum seeds that can be returned to a reactor and used to precipitate additional calcium sulfate. The flocculated portion is converted to a non-flocculated portion by exposing the flocculated portion to shear stress, such as by pumping the flocculated gypsum seeds through a centrifugal pump with an open impeller.

[0009] In other embodiments, the present disclosure provides a separation method comprising: receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR); adding a coagulant to (a) the feed stream of the multi-stage CSTR, or (b) the multi-stage CSTR; flowing the solution through the multi-stage CSTR, and operating the seed-assisted precipitation and the multi-stage CSTR under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm; and transferring the gypsum seed fines to a separator as a mixture of gypsum seed fines in aqueous solution.

[0010] The authors of the present disclosure have determined that the efficiency of gypsum precipitation can be enhanced by using multiple continuous stirred tank reactors (as compared to a single large reactor); and by operating the multiple CSTRs under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm.

[0011] In a specific example, the present disclosure provides a separation method, which includes: receiving a supersaturated aqueous solution of CaSO4 into a first continuous stirred tank reactor (CSTR); adding a coagulant to (a) the feed stream of the first CSTR, or (b) the first CSTR; passing the solution through the first CSTR and at least one additional CSTR to produce a mixture of gypsum seed fines in aqueous solution; and transferring the gypsum seed fines as a mixture of gypsum seed fines in aqueous solution to a separator. Each CSTR independently has a height (H) and a diameter (D), wherein the H:D ratio is about 1:1 to about 2:1. Agitation in each CSTR is independently provided by a pitched blade paddle at a speed of about 50 rpm to about 200 rpm, wherein each paddle independently has a width (d), wherein the ratio of d:D is about 1:3 to about 1:2. The residence time in each CSTR is independently about 2 to about 10 minutes. Operating such a multi-stage CSTR under these conditions results in gypsum seeds of a desired size and concentration. In the context of the present disclosure, the skilled person will understand that reference to the width (d) of a paddle refers to the radius of the circle formed by the paddle when agitated.

[0012] In other embodiments, the present disclosure provides a precipitation method comprising receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are vertically stacked, wherein the internal outflow port from one stage substantially corresponds to the internal feed port of a subsequent downstream stage; flowing the solution vertically upward through the multi-stage CSTR to produce a gypsum seed fines mixture in aqueous solution; and transferring the gypsum seed fines mixture in aqueous solution to a separator.

[0013] Since the outflow port of one stage corresponds to the feed port of the subsequent stage, the surface area of ​​this vertical multi-stage CSTR is smaller than other comparable reactors, which include additional fluid conduits connecting the different stages of the reactor. Flowing the solution vertically upward through this vertical multi-stage CSTR results in less fouling than in other comparable reactors.

[0014] In other specific examples, a method is provided in which a supersaturated aqueous solution of CaSO4 is received into a multistage continuous stirred tank reactor (CSTR) for seed-assisted precipitation, wherein the stages of the reactor are stacked vertically, wherein the internal outflow port from one stage substantially corresponds to the internal feed port of the subsequent downstream stage. A coagulant is added to the feed stream of (a) the multistage CSTR, or (b) the multistage CSTR. The solution is caused to flow vertically upward through the multistage CSTR. The seed-assisted precipitation and the multistage CSTR are operated under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. The gypsum seed fines are transferred to a separator. An anionic flocculant is added to the feed stream of (a) the separator, or (b) the separator. The gypsum seed fines and the flocculant are aggregated into flocs. The flocculated mixture is separated into an effluent with reduced turbidity and a flocculated gypsum slurry. A portion of the flocculated gypsum slurry is exposed to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines. At least a portion of the fines are transferred to a multi-stage CSTR.

[0015] The present disclosure further provides a method comprising vibrating a portion of a CaSO4-precipitation reactor or a portion of a liquid conduit connected to a feed port of the reactor, wherein scale is present on at least a portion of the vibrated portion. The vibrated portion is at least partially made of a low friction material or at least partially coated with a low friction material. The vibration is sufficient to remove at least some of the scale from at least some of the low friction material.

[0016] The present disclosure also provides a solid / liquid separator, which includes: a settling tank; a fluid inlet in the settling tank for receiving a gypsum seed fine particle mixture in an aqueous solution from a precipitation reactor; and an anionic flocculant source in fluid communication with the settling tank. The reactor also includes a first fluid outlet in the settling tank for discharging an effluent with reduced turbidity; a second fluid outlet in the settling tank for discharging flocculated gypsum slurry; and a liquid conduit connecting the second fluid outlet to an inlet in the reactor. An applicator of shear stress is arranged in the liquid conduit connecting the second fluid outlet to the inlet in the reactor.

[0017] In other embodiments, the present disclosure provides a precipitation reactor comprising a multi-stage continuous stirred tank reactor (CSTR). The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4. At least one pitched blade paddle is disposed in at least one stage of the multi-stage CSTR, wherein the size of the paddle and the size of the container in which it is disposed are selected to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. There is a source of coagulant in fluid communication with the precipitation reactor, and the precipitation reactor is in fluid communication with a separator to provide a mixture of gypsum seed fines in aqueous solution to the separator. The separator can be, for example, a solid / liquid separator as described above.

[0018] In a particular embodiment, the present disclosure provides a precipitator reactor comprising a plurality of continuous stirred tank reactors (CSTRs) connected in series, wherein each of the plurality of CSTRs independently has a height (H) and a diameter (D), wherein the H:D ratio is from about 1:1 to about 2:1. The first of the plurality of CSTRs is fluidly connected to a source of a supersaturated aqueous solution of CaSO4. At least one pitched blade paddle is disposed in at least the first of the plurality of CSTRs, wherein each paddle independently has a width (d), wherein the d:D ratio is from about 1:3 to about 1:2. A coagulant source is fluidly connected to (a) a feed stream to the first of the plurality of CSTRs or (b) the first of the plurality of CSTRs. The precipitation reactor is fluidly connected to a separator to provide a mixture of gypsum seed fines in aqueous solution to the separator. The separator can be, for example, a solid / liquid separator as described above.

[0019] In other embodiments, the present disclosure provides a precipitation reactor comprising a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are connected in series, wherein the internal flow outlet from one stage substantially corresponds to the internal feed inlet of the subsequent downstream stage. For example, the stages of the reactor can be stacked vertically, wherein the internal flow outlet from one stage substantially corresponds to the internal feed inlet of the subsequent downstream stage. The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4 and in fluid communication with a separator to provide a mixture of gypsum seeds and fine particles in aqueous solution to the separator.

[0020] The present disclosure also provides an apparatus comprising a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are connected in series, wherein the internal flow outlet from one stage substantially corresponds to the internal feed inlet of the subsequent downstream stage. The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4. At least one pitched blade paddle is arranged in at least one stage. The size of the paddle and the size of the container in which it is arranged are selected to produce gypsum seed fines with an average diameter of about 20 μm to about 40 μm. The apparatus further includes: a coagulant source in fluid communication with the multi-stage CSTR; a settling tank in fluid communication with the multi-stage CSTR for receiving a mixture of gypsum seed fines in aqueous solution from the multi-stage CSTR; an anionic flocculant source in fluid communication with the settling tank. A first fluid outlet is in the settling tank for discharging an effluent with reduced turbidity; a second fluid outlet is in the settling tank for discharging a flocculated gypsum slurry; a liquid conduit connects the second fluid outlet to the multi-stage CSTR. An applicator of shear stress is arranged in a liquid conduit connecting the second fluid outlet to the multi-stage reactor.

[0021] In other embodiments, the present disclosure provides a precipitation reactor or a liquid conduit connected to a feed port of the reactor, wherein the reactor or the liquid conduit is at least partially made of or at least partially coated with a low friction material. The low friction material is located at a location that will be exposed to a supersaturated solution of CaSO4. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0023] Figure 1 is a process flow diagram of an exemplary solid / liquid separator according to the present disclosure.

[0024] Figure 2 is a graph showing the concentration (volume %) of seed particles in a precipitation reactor and a sedimentation tank used in the solid-liquid separation method according to the present disclosure.

[0025] Figure 3 is a graph showing the turbidity of an effluent stream from a solid-liquid separation process according to the present disclosure.

[0026] Figure 4 is a process flow diagram of an exemplary precipitation reactor according to the present disclosure.

[0027] Figure 5 is a process flow diagram of an exemplary apparatus according to the present disclosure.

[0028] Figure 6 is a graph showing the size distribution of particles produced in a method according to the present disclosure.

[0029] Figure 7is a graph showing the size distribution of particles produced in a comparative process.

[0030] Figure 8 is a diagram of an exemplary precipitation reactor according to the present disclosure.

[0031] Fig. 9 is a diagram of another exemplary precipitation reactor according to the present disclosure.

[0032] Fig.10 is a process flow diagram of an exemplary apparatus according to the present disclosure. DETAILED DESCRIPTION

[0033] Seeded slurry technology (SST) can be used in wastewater concentration processes such as membrane filtration, electrodialysis or thermal crystallization. SST reduces scaling of supersaturated components (e.g., CaSO4) as water is concentrated. However, CaSO4 may precipitate and / or harden, thereby blocking one or more water treatment systems (e.g., tanks, pipes and pumps) in the water concentration process or downstream filtration process.

[0034] In one aspect, the present disclosure provides a separation method comprising: receiving a mixture of gypsum seed fines in an aqueous solution from a reactor; adding an anionic flocculant and optionally a coagulant to the aqueous solution; aggregating the gypsum seed fines and the flocculant into flocs; separating the mixture into a turbidity-reduced effluent and a flocculated gypsum slurry; and exposing a portion of the flocculated gypsum slurry to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines and transferring the fines to a reactor. The aqueous solution from the reactor can be received directly into a settling tank.

[0035] In the context of the present disclosure, it is understood that the phrases "receive from [X]" and "receive [A] from [X]" refer to both direct and indirect receipt. For example, if reactor Z is disclosed as "receiving fluid A from reactor X," it is understood that reactor Z can be (i) directly coupled to reactor X such that fluid A is received directly from reactor Z; or (ii) indirectly coupled to reactor X such that process equipment Y receives fluid A from reactor X and reactor Z receives fluid A from equipment Y.

[0036] As described above, the separation process controls the transition of gypsum seeds between (i) a flocculated portion that can be separated into a turbidity-reduced effluent and a flocculated gypsum slurry and (ii) a non-flocculated portion of gypsum seeds that can be returned to the reactor and used to precipitate additional calcium sulfate (e.g., from a supersaturated solution of CaSO4).

[0037] The anionic flocculant may be a polyacrylamide flocculant. The anionic flocculant may be a low charge density, high molecular weight polymer flocculant. When exposed to high shear or excessive agitation, the anionic flocculant may exhibit reduced flocculation activity. An example of a suitable anionic flocculant is PolyFloc TM AP 1100. The flocculant may be added before the aqueous solution is received into the separator, thereby allowing the flocculant to be thoroughly mixed into the solution before the solution enters the separator.

[0038] The coagulant can be a coagulant based on a trivalent metal salt, such as an iron or aluminum based coagulant. Specific examples of such coagulants include: FeCl3, Fe2(SO4)3, polyferric sulfate or polyaluminum chloride. The coagulant can be added: (a) to the feed stream of the reactor; (b) to the aqueous solution in the reactor; (c) to the aqueous solution received from the reactor, for example, before the addition of an anionic flocculant; or (d) any combination thereof. The coagulant can be added in an amount sufficient to precipitate at least a portion of any antiscalant present in the aqueous solution received from the reactor. For example, a sufficient amount of FeCl3 can be added to provide a concentration of at least 10 ppm in the sedimentation tank. In some specific examples, the final concentration of FeCl3 is at least 30 ppm.

[0039] The flocs may be stirred in the settling tank to prevent the gypsum slurry from hardening. For example, the flocs may be stirred at a speed of 50 rpm or less with a paddle having a diameter of about 1 / 2 to about 3 / 4 the diameter of the settling tank. The bed height of the settled flocs may be about 1 / 5 to about 1 / 3 the height of the settling tank. The local concentration of the settled gypsum seed flocs at the bottom of the settling tank may be about 8 to about 25 weight percent.

[0040] The slurry including floccules of gypsum seeds produced according to the method has a reduced tendency to harden and can be dispersed and transferred. Without wishing to be bound by theory, the authors of the present disclosure hypothesize that the coagulant and flocculant used to flocculate the gypsum seeds act as lubricants and wetting agents to inhibit the hardening of the gypsum seeds.

[0041] The effluent with reduced turbidity produced according to the method can be a clarified effluent having a turbidity of less than 3 NTU (nephelometric turbidity units), or from about 3 to about 5 NTU. When the turbidity of the clarified effluent is less than 3 NTU, the method can further include subjecting the clarified effluent to nanofiltration without prior ultrafiltration. The effluent with reduced turbidity can be treated with a thin plate clarifier (also known as an inclined plate settler), for example when the turbidity is greater than 3 NTU.

[0042] The gypsum seeds can be returned to the reactor and used to precipitate additional calcium sulfate. However, flocculated gypsum seeds are not as effective at precipitating calcium sulfate because anionic flocculants inhibit such precipitation. The flocculated gypsum seeds are converted to non-flocculated gypsum seed fines by exposing the flocculated material to shear stress.

[0043] The bottom of the settling tank can be fluidly connected to the reactor, and a pump can be arranged between them to transfer the concentrated gypsum seed flocs from the settling tank to the reactor. The pump can be a centrifugal pump with an open impeller, for example, running at a rate of at least 500 rpm. Such a pump can provide enough shear force to convert the flocculated gypsum seeds into non-flocculated fine particles. Compared with a closed impeller pump, this pump is also more tolerant to concentrated slurries, thereby reducing the possibility of slurry piles forming in the pump. The method can include flushing the pump with clean water whenever the pump is stopped to reduce the possibility of gypsum seeds being deposited and hardened in the pump.

[0044] For example, if the pump does not provide shear force, shear stress can be provided by a mechanical stirrer arranged between the settling tank and the reactor. The mechanical stirrer can be, for example, at the inlet of the reactor, at the outlet of the settling tank, or in close proximity to the pump.

[0045] A sufficient amount of gypsum seeds may be returned to the reactor to maintain the concentration of gypsum seed fines in the reactor in a range of about 0.5 wt % to about 10 wt %, such as in a range of about 1 wt % to about 7 wt %.

[0046] The method can further include maintaining the aqueous solution in the reactor at a pH of about 4 to about 10, such as at a pH of about 6 to about 8, such as at a pH of about 6.5 to about 7.

[0047] The reactor can be operated under conditions that produce gypsum seed fines with an average diameter of about 20 μm to about 40 μm, for example about 25 μm. Without wishing to be bound by theory, the authors of the present disclosure believe that particles of this size have a surface / volume ratio that makes surface-assisted CaSO precipitation particularly effective for removing CaSO from its supersaturated solution. The authors have also found that when particles of this size are flocculated with anionic polymer flocculants, they can maintain a desired moisture content even after being removed from the aqueous solution for a period of time. The time period can be, for example, one, two or three months. In the context of the present disclosure, the desired moisture content will be understood as the moisture content that prevents the CaSO from hardening so that the flocculated particles can be dispersed into water after a period of time.

[0048] The present disclosure also provides a solid / liquid separator. The separator includes a settling tank, a fluid inlet in the settling tank, for receiving a mixture of gypsum seed and fine particles in an aqueous solution from a precipitation reactor; a first fluid outlet in the settling tank, for discharging an effluent with reduced turbidity; a second fluid outlet in the settling tank, for discharging a flocculated gypsum slurry, and an optional agitator. The second fluid outlet can be located at the bottom third of the settling tank. The settling tank can also be called a sedimentation tank or a clarifier. The settling tank can directly receive fluid from the precipitation reactor.

[0049] In one example, the separator comprises a cylindrical settling tank having a height to diameter ratio of about 1:1 to about 8:1, preferably about 2:1 to 5:1; an agitator having blades having a diameter of about 3 / 4 to about 5 / 6 of the diameter of the tank, wherein the blades are located about 1 to about 10 cm, preferably about 2 to about 5 cm above the bottom of the tank. The agitator can be operated at a rate of about 10 to about 40 rpm. The separator of this configuration and operated under these conditions can maintain a stable suspension of floc slurry at a concentration of about 15% by weight to about 35% by weight, with a well-defined boundary between the suspension of floc slurry and the supernatant.

[0050] The separator also includes an anionic flocculant source in fluid communication with the settling tank, a liquid conduit connecting the second fluid outlet to the inlet in the reactor, and an applicator of shear stress disposed in the liquid conduit connecting the second fluid outlet to the inlet in the reactor.

[0051] A source of anionic flocculant may be in fluid communication with a liquid conduit connecting the reactor to a fluid inlet in the settling tank.The anionic flocculant may be a polyacrylamide flocculant.

[0052] The shear stress applicator may be a centrifugal pump with an open impeller or a mechanical stirrer. The mechanical stirrer may be, for example: at the inlet of the reactor, at the outlet of the sedimentation tank, or in the immediate vicinity of the pump.

[0053] The solid / liquid separator can be configured to discharge the effluent with reduced turbidity from the first fluid outlet to the nanofiltration unit without first passing the effluent through an ultrafiltration process device. For example, the effluent can be directly transferred to a sand filtration pretreatment unit of the nanofiltration unit.

[0054] The present disclosure also provides an apparatus, which includes the above-mentioned solid / liquid separator, and a precipitation reactor, such as the precipitation reactor described below. The precipitation reactor includes a fluid outlet for discharging a mixture of gypsum seeds and fine particles in an aqueous solution, and the apparatus includes a liquid conduit connecting the fluid outlet of the reactor and the fluid inlet of the settling tank. The liquid conduit connecting the second fluid outlet to the reactor fluidly connects the second fluid outlet to the fluid inlet in the reactor.

[0055] The apparatus may further include one or more of the following: a source of gypsum seeds in fluid communication with the reactor; one or more sources of one or more coagulants; a pH sensor for measuring the pH of the liquid in the reactor; or a fluid inlet for receiving a supersaturated aqueous solution of CaSO4, for example from a membrane separation unit.

[0056] One or more sources of one or more coagulants can each independently be in fluid communication with (a) the reactor, (b) a liquid conduit connecting a fluid outlet of the reactor and a fluid inlet of the sedimentation tank, or (c) both. Each coagulant can independently be a trivalent metal salt-based coagulant, such as an iron or aluminum-based coagulant, such as FeCl3, FeSO4, polyferric sulfate, or polyaluminum chloride. As discussed above, the coagulant can be added in an amount sufficient to precipitate at least a portion of any antiscalant present in the aqueous solution received from the reactor.

[0057] Figure 1 A process flow diagram of an exemplary solid / liquid separator (110) according to the present disclosure in combination with a precipitation reactor (112) is shown. The precipitation reactor (112) provides an aqueous mixture of gypsum seed fines (114), which is received into a settling tank (116). A coagulant (118) is added to the precipitation reactor (112). A pH adjuster (120) may be added to the precipitation reactor (112) to adjust or maintain the pH at a value of about 4 to about 10. An anionic flocculant (122) is added to the mixture in a fluid conduit connecting the precipitation reactor (112) to the settling tank (116). The settling tank (116) produces a flocculated gypsum slurry (124) and an effluent (126) with reduced turbidity. A portion of the flocculated gypsum slurry (124) is exposed to an applicator of shear stress, illustrated as a centrifugal pump (128), and recycled to the precipitation reactor (112) as non-flocculated gypsum seed fines (130).

[0058] Example 1

[0059] Established using Figure 1 Pilot test of the process shown to treat a concentrated waste stream from a coal-to-chemicals production process. The concentrated waste stream was derived from reverse osmosis treatment of coal-to-chemicals wastewater. Typical water qualities are shown in Table 1.

[0060]

[0061] Table 1

[0062] The waste stream is first concentrated at least 3 times to provide a supersaturated solution of CaSO4. The resulting supersaturated solution is fed to a precipitation reactor. Upon entering the precipitation reactor, the pH of the stream is adjusted to 6.5-7 and 30 ppm FeCl3 is added again at the same location based on the volume of the influent. Gypsum seeds are dispersed in the reactor and supersaturated CaSO4 is precipitated. The concentration of gypsum seeds is maintained between 2-5% (volume / volume) by recovering the seed slurry from the sedimentation tank.

[0063] In the outlet pipe of the precipitation reactor, an additional amount of 10 ppm FeCl3 was added to adjust the surface properties of the seed crystals. A pump was used to transfer the effluent to a downstream sedimentation tank. Just before pumping, a flocculant was added to the effluent in an amount of 0.5 ppm.

[0064] Floccules form and settle in the settling tank. With slow stirring by mechanical paddles, the height of the concentrated slurry never grows higher than 1 / 4 of the tank height. The supernatant from the top of the settling tank is sent to the downstream membrane treatment unit. Some of the slurry from the bottom of the settling tank is pumped back to the precipitation reactor by an open impeller running at about 800 RPM, which provides sufficient shear stress to reduce the flocculation activity of the flocculant.

[0065] Approximately 1-2% of the recycled slurry is discharged into the blowdown stream. This amount in the blowdown stream is set based on the mass balance of the entire system. Removing the gypsum seeds from the system in the blowdown stream avoids potential seed aging problems. However, based on the desaturation properties of the recycled gypsum seeds in the precipitation reactor, the recycled seeds retain the ability to reduce the supersaturation level of the incoming waste stream. The CaSO4 in the incoming waste stream was calculated to be 113% to 140% of the saturation level, and the CaSO4 in the effluent of the sedimentation tank was measured to be 100% to 110% of the saturation level (see Table 2).

[0066] November 30 December 4 December 7 December 10 Entrance 140.20％ 123.50％ 113.30％ 136.40％ exit 96.20％ 103.90％ 103.50％ 100.80％

[0067] Table 2

[0068] The concentration of gypsum seeds in the precipitation reactor and the settling tank was tracked. The results are shown in Figure 2 The test was run for 500 hours and the concentration of gypsum seeds in the settling tank was as high as 30% vol / vol. No pumps or pipes were blocked by the gypsum slurry. Other comparable systems using closed impellers instead of open impellers could not be run for the same length of time due to the pumps being blocked by the concentrated gypsum slurry.

[0069] The flocculated gypsum seed crystals were found to disperse rapidly into water even after being dried for weeks. It was also found that the flocculated gypsum seed crystals could be stored for months without hardening. Without wishing to be bound by theory, the authors of the present disclosure believe that the gypsum seed crystal particles, which retain moisture and are easily dispersed, have a reduced tendency to form scale in the pipe, and that the polyacrylamide flocculant added to the influent of the sedimentation tank surrounds the gypsum seed crystals and provides these desirable properties. The polyacrylamide flocculant may act as a wetting agent in the seed crystals and accelerate the dispersion of the gypsum seed crystals when added to water.

[0070] The chemical composition of the produced gypsum seeds was analyzed using X-ray fluorescence. In addition to CaSO4 (main component 95 wt%), SrSO4 was also co-precipitated from the solution (2 wt%). This shows that other sparingly soluble ions can be removed simultaneously in this process, thereby further reducing the scaling risk in downstream membrane filtration. The remaining components are 1 wt% Fe(OH)3 and 2 wt% Na2SO4.

[0071] The supernatant stream from the settling tank was pale yellow and contributed to the turbidity reading even without any particles. Therefore, the turbidity during this pilot was not tracked. However, in another pilot area where coal mining drainage was treated with the same method, the supernatant was colorless and the turbidity of the effluent was tracked. Figure 3 As shown in , the turbidity measured throughout the cycle is 3 NTU or less. This water quality may be suitable for downstream membranes. For example, water with <3 NTU can be fed directly to a nanofiltration membrane process without first passing the water through an ultrafiltration unit, such as where colloids in the water are not a big problem.

[0072] In another aspect, the present disclosure provides a seed-assisted precipitation process that can be performed at ambient temperature, e.g., about 18 to about 25° C. The process can exclude lime softening, e.g., by excluding the addition of calcium hydroxide. The process includes receiving a supersaturated aqueous solution of CaSO4 into a multistage continuous stirred tank reactor (CSTR); and adding a coagulant to (a) the feed stream of the multistage CSTR, or (b) the multistage CSTR. The process further includes flowing the solution through the multistage CSTR, and operating the seed-assisted precipitation and the multistage CSTR under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. The gypsum seed fines are transferred to a separator as a mixture of gypsum seed fines in aqueous solution.

[0073] The present disclosure also provides a precipitation reactor. The precipitation reactor includes a multi-stage continuous stirred tank reactor (CSTR). The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4. The precipitation reactor includes at least one pitched blade paddle arranged in at least one stage of the multi-stage CSTR. The size of the paddle and the size of the container in which it is arranged are selected to produce gypsum seed fines with an average diameter of about 20 μm to about 40 μm. A coagulant source is in fluid communication with the precipitation reactor. The precipitation reactor may be in fluid communication with a source of gypsum seed fines. The precipitation reactor may be in fluid communication with a separator to provide a mixture of gypsum seed fines in an aqueous solution to the separator. As described above, the separator may be a separator according to the present invention.

[0074] The coagulant is used to neutralize a sufficient amount of the antiscalant that is usually present in the supersaturated solution of CaSO4 to cause the CaSO4 to precipitate on the gypsum seed particles. The coagulant can be a coagulant based on a trivalent metal salt, such as an iron or aluminum based coagulant. Specific examples of such coagulants include: FeCl3, Fe2(SO4)3, polyferric sulfate or polyaluminum chloride. FeCl3 can be added at the inlet of the reactor to result in a concentration of 30 to 50 ppm.

[0075] Seed assisted precipitation and multi-stage CSTRs are operated under shear conditions to produce the desired gypsum seed fines. In a specific example of such shear conditions, each CSTR can independently have a height (H) and a diameter (D), wherein the H:D ratio is about 1:1 to about 2:1, for example, about 1:1 to about 1.5:1. Agitation in each CSTR can be independently carried out with a pitched blade paddle at a speed of about 50 rpm to about 200 rpm, for example, about 120 rpm to about 150 rpm; wherein each paddle independently has a width (d), wherein the ratio of d:D is about 1:3 to about 1:2. The flow rate and size of the CSTR can be provided so that the residence time in each CSTR is independently about 2 minutes to about 10 minutes, for example, about 2.5 to about 5 minutes. Operating under these conditions can reduce the saturation level of gypsum from about 200% (supersaturated) to less than about 120%, for example, about 100% (saturated), thereby reducing the risk of scaling in downstream processes and equipment. A pitched blade impeller with larger blades can be run at a lower rpm than a pitched blade impeller with smaller blades.

[0076] Smaller d:D ratios have an increased shearing effect, while larger d:D ratios have an increased mixing effect. Shearing and mixing control the size of the gypsum seed crystals. Vigorous agitation associated with increased shear breaks up larger particles and produces smaller particles. Increased mixing enhances crystallization of supersaturated CaSO4 solutions and results in larger particles within the same crystallization time. A d:D ratio of about 1:3 to about 1:2 provides an acceptable balance between shearing and mixing.

[0077] The concentration of gypsum seeds can be controlled by removing gypsum seeds from the reactor and by optionally adding gypsum seeds to the reactor. Gypsum seeds can be added by recycling the removed gypsum seeds back to the reactor. Higher concentrations of gypsum seeds result in faster crystallization rates, but also increase the operating load of any recycling unit. Higher concentrations also increase the risk of scaling downstream of the reactor. The process can be operated under conditions that result in a seed concentration in the reactor in the range of about 0.5 wt % to about 10 wt %, for example, about 1 wt % to about 7 wt %.

[0078] The multistage CSTR may include at least two, for example at least three stages. The total residence time in the multistage CSTR may be about 8 to about 40 minutes. The authors of the present disclosure have determined that the precipitation rate in the multistage CSTR is faster than the precipitation rate in a single-stage CSTR of equivalent total volume.

[0079] Figure 4 A process flow diagram of an exemplary precipitation reactor according to the present invention is shown. In the precipitation reactor (210), a multistage CSTR (212) consists of three stages (212a, 212b and 212c). The multistage CSTR (212) receives a supersaturated solution (214) of CaSO4 into the first stage (212a). A coagulant (216) is added to the feed stream of the first stage (212a). An optional pH adjuster (not shown) may be added. All three stages include pitched blade impellers (218a, 218b, 218c). The sizes of all three stages and their respective impellers satisfy the above-mentioned H:D ratio and d:D ratio. The final stage produces a mixture of gypsum fines (220). The multistage CSTR (212) includes a feed (222) for gypsum seed particles.

[0080] The three stages (212a, 212b, and 212c) can be positioned so that liquid flows from one stage to the next by gravity. For example, the three stages can be positioned so that the first stage (212a) is higher than the second stage (212b), for example, by about 10 cm, and liquid flows from the first stage (212a) to the second stage (212b) by gravity; and the second stage (212b) is higher than the third stage (212c), for example, by about 10 cm, and liquid flows from the second stage (212b) to the third stage (212c) by gravity.

[0081] The gypsum seed fines produced by this method can be used in the above separation method. The precipitation reactor can be used in combination with the above separator in the device.

[0082] Figure 5A process flow diagram of an exemplary apparatus according to the present disclosure is shown. The apparatus (310) includes the above-described precipitation reactor (210) and a solid / liquid separator (110). The mixture (220) produced by the final stage (212c) of the CSTR corresponds to the mixture (114) received by the solid / liquid separator (110). The pitched blade paddle is not shown. The flocculant (122) is optional.

[0083] Example 2

[0084] Established using Figure 5 A pilot test of the process shown, but without adding any flocculant to the sedimentation tank, was conducted to treat a supersaturated solution of CaSO4 produced by a nanofiltration process for treating coal mine drainage. The pilot test process used a multistage CSTR consisting of three precipitation reactors. The concentration of gypsum seeds in each reactor was about 3 to 5% by weight. The reactor was stirred with a mechanical stirrer at a stirring speed of about 100 to about 140 rpm. The width of the metal paddle of the stirrer in the reactor was about half the diameter of the reactor. The reactor had a height of 1000 mm, a diameter of 600 mm, and a diameter of 300 mm of the circle formed by the paddle. This corresponds to a H:D ratio of 5:3; a d:D ratio of 1:2. The total residence time in the series of three reactors was 30 to 40 minutes. The average diameter of the gypsum seeds in the reactor was about 25 μm.

[0085] The gypsum seeds are transferred to a solid-liquid separator having a height of 1200 mm and a diameter of 600 mm (H:D ratio of 2:1). The agitator has blades of about 500 mm in diameter and is operated at about 20 RPM to about 40 RPM. The flocs of larger gypsum seeds obtained from the bottom of the solid-liquid separator are sheared and recirculated and dispersed into the first reaction tank under strong stirring by a transfer pump including an open impeller operating at about 800 RPM.

[0086] Stabilizers, also known as antiscalants, are added to the supersaturated effluent produced by the nanofiltration process to reduce or avoid scaling. FeCl3 in an amount of 30 to 50 ppm is added as a coagulant to the influent of a multi-stage CSTR to accelerate the destabilization of supersaturated CaSO4.

[0087] The size distribution of the gypsum seeds produced in this test was measured. The size distribution of the gypsum seeds produced in the comparative process was also measured. The comparative process used only one of the stages of the CSTR settling tank described above and was operated with a residence time of 30 to 40 minutes, but was otherwise the same as the pilot described above. As described above, the exemplary process according to the present disclosure produced particles with an average size of about 25 μm. The comparative process produced particles with an average size of about 80 μm. The size distributions were respectively Figure 6 and 7 Shown in.

[0088] The water quality of the influent and effluent streams of the CSTR was analyzed and the saturation of CaSO4 was calculated using the following equation:

[0089]

[0090] The composition of the influent and effluent streams are shown in Table 3, and the supersaturation level of CaSO4 was calculated for four different time points. It was determined that the average supersaturation level decreased from about 200% at the influent stream to about 120% at the effluent stream.

[0091] Ca Fe K Mg Na Si Sr SO4 Cl Trough Inflow 1250 2.3 68.4 639 6490 10.8 12.7 18750 16.5 Trough outflow 435 2.4 63.2 630 6300 10.4 8.6 16180 49.4

[0092] Table 3

[0093] It was determined that the average supersaturation level of the influent and effluent streams of the comparative reactor decreased from about 230% at the influent stream to about 190% at the effluent stream. It was observed that the supersaturation level of about 190% resulted in fouling downstream of the comparative reactor.

[0094] In another aspect of the present disclosure, a precipitation method is provided. The method includes receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are vertically stacked, wherein the internal outflow port from one stage substantially corresponds to the internal feed port of the subsequent downstream stage. The method includes flowing the solution vertically upward through the multi-stage CSTR to produce a gypsum seed fines mixture in aqueous solution; and transferring the gypsum seed fines mixture in aqueous solution to a separator.

[0095] The present disclosure also provides a precipitation reactor comprising a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are connected in series, wherein the internal outflow port from one stage corresponds to the internal feed port of the subsequent downstream stage. The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4, and is in fluid communication with a separator to provide a mixture of gypsum seeds and fine particles in aqueous solution to the separator. The separator may be a separator as described above. The source of the supersaturated CaSO4 solution may be a membrane separation unit.

[0096] The stages of the reactor are preferably stacked vertically. The source of the supersaturated aqueous solution of CaSO4 can provide the solution at a static pressure sufficient to drive the solution vertically upward through the stages of the CSTR.

[0097] The outflow port and the feed port may be connected by a short fluid conduit, or may not be connected by a fluid conduit. When not connected by a fluid conduit, the outflow port and the inflow port may refer to the same hole between two adjacent stages. Because the outflow port of one stage substantially corresponds to the inflow port of the subsequent stage, the disclosed precipitation method reduces or avoids CaSO4 precipitation in the fluid conduits, such as pipes, connecting different stages of the reactor.

[0098] The multi-stage CSTR may be operated under conditions that produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. Exemplary conditions are discussed above.

[0099] The reactor may include three stages, wherein the second stage of the reactor is directly on top of the first stage of the reactor, the third stage of the reactor is directly on top of the second stage of the reactor, the outflow port of the first stage of the reactor corresponds to the feed inlet of the second stage of the reactor, and the outflow port of the second stage of the reactor corresponds to the feed inlet of the third stage of the reactor.

[0100] The size of the orifice can be designed to prevent or reduce backmixing, which occurs when fluid from one stage flows downward through the inlet into the lower stage. Preventing or reducing backmixing through the inlet can be achieved when the diameter of the inlet is about 5% to about 10% of the diameter of the reactor stage.

[0101] In this vertical multi-stage CSTR, each stage can be agitated using an agitator on the same single agitator shaft.

[0102] The reactor may include a source of gypsum seeds in fluid communication with the reactor, such as in fluid communication with a first stage of the reactor. The reactor may include a source of coagulant in fluid communication with the reactor, such as in fluid communication with a first stage of the reactor.

[0103] Figure 8 An exemplary precipitation reactor according to the present disclosure is shown. In the precipitation reactor (410), a vertical multi-stage CSTR (412) consists of three stages (412a, 412b, and 412c). The vertical multi-stage CSTR (412) receives a supersaturated solution (414) of CaSO4 into the bottom, in the first stage (412a). A coagulant (416) is added to the feed stream of the first stage (412a). An optional pH adjuster (not shown) may be added. All three stages include pitched blade paddles (418a, 418b, 418c) on the same agitator shaft. The dimensions of all three stages and their respective paddles satisfy the above-mentioned H:D ratio and d:D ratio. The final stage produces a gypsum fines mixture (420). The first stage (412a) is in fluid communication with a gypsum seed fines source (422). The outlet of the first stage corresponds to the inlet (424a) of the second stage. The outlet of the second stage corresponds to the inlet (424b) of the third stage.

[0104] Fig. 9 Another exemplary precipitation reactor according to the present disclosure is shown. Reactor (510) and Figure 8 The reactor shown in is similar to that shown in , except that the three stages do not share a common stirring shaft. The vertical reactor (512) still receives a supersaturated solution (514) of CaSO4 into the bottom, the first stage (512a). A coagulant (516) is added to the feed stream of the first stage. An optional pH adjuster (not shown) may be added. All three stages include pitched blade paddles (518a, 518b, 518c). The paddles may stir at the same or different rates. The dimensions of all three stages and their respective paddles satisfy the above-mentioned H:D ratio and d:D ratio. The final stage produces a gypsum fines mixture (520). The first stage (512a) is in fluid communication with a source of gypsum seed fines (522). The outlet of the first stage corresponds to the inlet (524a) of the second stage. The outlet of the second stage corresponds to the inlet (524b) of the third stage.

[0105] The gypsum seed fines produced by the precipitation method or in the precipitation reactor can be used in the above-mentioned separation method or reactor. For example, a method is provided, which includes receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR) for seed-assisted precipitation, wherein the stages of the reactor are vertically stacked, wherein the internal outflow port from one stage corresponds to the internal feed port of the subsequent downstream stage. A coagulant is added to the feed stream of (a) the multi-stage CSTR, or (b) the multi-stage CSTR. The solution flows vertically upward through the multi-stage CSTR. The seed-assisted precipitation and the multi-stage CSTR are operated under shear conditions to produce gypsum seed fines with an average diameter of about 20 μm to about 40 μm. The fines are transferred to a separator, and an anionic flocculant is added to the feed stream of (a) the separator, or (b) the separator. The gypsum seed fines and the flocculant are aggregated into flocs. The flocculated mixture is separated into an effluent with reduced turbidity and a flocculated gypsum slurry. A portion of the flocculated gypsum slurry is exposed to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines. At least a portion of the fines are transferred back to the multi-stage CSTR.

[0106] In an example of a combined precipitation reactor and solid / liquid separator, an apparatus includes a multistage continuous stirred tank reactor (CSTR) wherein the stages of the reactor are connected in series, wherein the internal flow outlet from one stage corresponds to the internal feed inlet of the subsequent downstream stage. The precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4. At least one pitched blade paddle is arranged in at least one stage, wherein the size of the paddle and the size of the container in which it is arranged are selected to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. The apparatus further includes a coagulant source in fluid communication with the multistage CSTR; and a settling tank in fluid communication with the multistage CSTR for receiving a mixture of gypsum seed fines in aqueous solution from the multistage CSTR. The settling tank includes a first fluid outlet for discharging an effluent having reduced turbidity; and a second fluid outlet for discharging a flocculated gypsum slurry. The apparatus includes an anionic flocculant source in fluid communication with the settling tank. There is a liquid conduit connecting the second fluid outlet to the multistage CSTR; and an applicator of shear stress is arranged in the liquid conduit.

[0107] As described above, the stages of a multi-stage CSTR may be stacked vertically.The features of the precipitation reactor and solid / liquid separator are discussed in more detail above.

[0108] Fig.10 A process flow diagram of an exemplary apparatus according to the present disclosure is shown. The apparatus (610) includes the precipitation reactor (410) and the solid / liquid separator (110) described above. The mixture (420) produced by the CSTR (412) corresponds to the mixture (114) received by the solid / liquid separator (110). The gypsum seed fines (130) produced by the open centrifugal pump (128) correspond to the gypsum seeds received by the CSTR (412). The flocculant (122) is optional.

[0109] Example 3

[0110] Established using Figure 8 A pilot test of the reactor shown was performed to treat a supersaturated solution of CaSO4 produced by a nanofiltration process for treating coal mine drainage.

[0111] The vertical CSTR includes three stages. The height and diameter of each stage have an H:D ratio of about 1:1. The width (d) of the paddle is such that the d:D ratio is about 1:3. In this pilot, a supersaturated solution of CaSO4 (saturation of 200%) is received at the bottom, in the first stage, at a flow rate of 500 L / h. Gypsum seed fines at a concentration of 20-35 wt% are added at a flow rate of 100 L / h. Sufficient amount of FeCl3 is added to the feed stream of the first stage to produce a concentration of 30-40 ppm. The pitch blade paddle is stirred at a rate of about 110 rpm. The final stage produces a mixture of gypsum fines. As shown in Table 4, the gypsum seed concentration of each of the three stages is measured to be about 3 to about 9 wt%. The precipitation is operated so that the total residence time is about 28 minutes.

[0112]

[0113] Table 4

[0114] The effluent from the nanofiltration process includes an antiscalant to reduce or avoid scaling. The supersaturation level of the nanofiltration effluent is about 140% of the saturation level, with calcium at about 1800 ppm (recorded as CaCO3). The effluent produced by the final stage of the vertical multi-stage CSTR reaches about 100% of the saturation level, with calcium at about 1200 ppm (recorded as CaCO3), as shown in FIG.

[0115] As shown in Table 5.

[0116]

[0117] Table 5

[0118] In another aspect, the present disclosure provides a method for removing scale from (a) process equipment, such as a CaSO4 precipitation reactor, a solid / liquid separator, or a fluid conduit, or (b) a portion of process equipment. The method includes vibrating or deforming the process equipment or a portion of the process equipment to remove at least some of the scale. The vibrated process equipment or portion thereof is at least partially made of or coated with a low friction and optionally hydrophobic material. The scale is present on at least some of the low friction material.

[0119] The present disclosure also provides process equipment or a portion of process equipment, wherein the process equipment or portion thereof is at least partially made of or at least partially coated with a low friction and preferably hydrophobic material. The low friction material is in a position to be exposed to a supersaturated solution of CaSO4. The low friction material can be in a position to be sufficiently vibrated or deformed to remove at least some of the scale present on the low friction material.

[0120] The parts of the process equipment that may be vibrated or deformed may be side walls, baffles, liquid conduits within the reactor or stirring blades.

[0121] The low friction material may be polyethylene (PE), polypropylene (PP) or polytetrafluoroethylene (PTFE).

[0122] The vibration may be at a frequency of about 0.1 to 10 Hz, and / or may include moving the low friction material at an amplitude of about 1 to about 5 mm.

[0123] Any of the process equipment discussed above may be made of or coated with a low friction material, and at least some of the scale may be removed by vibrating at least a portion of the equipment. When the equipment is a reactor (e.g., a precipitation reactor or a solid-liquid separation reactor), the reactor may include a slag discharge port, and the dislodged scale may be removed from the reactor through the slag discharge port.

[0124] In the foregoing description, for the purpose of explanation, many details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that these specific details are not necessary. Therefore, what has been described is only used to illustrate the application of the described embodiments, and according to the above teachings, many modifications and variations are possible. Since the above description provides embodiments, it will be understood that those skilled in the art may modify and vary the specific embodiments. Therefore, the scope of the claims should not be limited by the specific embodiments set forth herein, but should be interpreted in a manner generally consistent with the specification.

Claims

1. A separation method comprising: receiving a mixture of gypsum seed crystals and fine particles in aqueous solution from a reactor; adding an anionic flocculant to the aqueous solution; Aggregating gypsum seed crystal particles and flocculants into flocs; separating the mixture into a turbidity-reduced effluent and a flocculated gypsum slurry; and A portion of the flocculated gypsum slurry is exposed to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines, and the fines are transferred to a reactor.

2. The separation method according to claim 1, wherein the anionic flocculant is a polyacrylamide flocculant.

3. The separation method according to claim 1 or 2, wherein: Aggregating the gypsum seed crystals and the flocculant into flocs comprises: The floccules are agitated in the settling tank to prevent the gypsum slurry from hardening.

4. The separation method according to claim 3, wherein: Aggregating the gypsum seed crystals and the flocculant into flocs includes stirring the flocs in the settling tank using a paddle at the bottom of the settling tank.

5. The separation method according to claim 1 or 2, wherein: The reduced turbidity effluent is a clarified effluent having a turbidity of less than 3 NTU (nephelometric turbidity units) or a turbidity of 3 to 5 NTU.

6. The separation method according to claim 5, wherein: The clarified effluent has a turbidity of less than 3 NTU, and the method further comprises nanofiltration of the clarified effluent without prior ultrafiltration.

7. The separation method according to claim 4, wherein: The separation includes settling the floccules and maintaining the bed height of the gypsum slurry at 1 / 5 to 1 / 3 of the height of the settling tank.

8. The separation method according to claim 1 or 2, wherein: Exposing a portion of the flocculated gypsum seeds to a shear stress includes pumping the flocculated gypsum seeds through a centrifugal pump having an open impeller.

9. The separation method according to claim 8, wherein: The pumping includes operating an open impeller at a speed of at least 500 rpm.

10. The separation method according to claim 1 or 2, wherein: In the reactor, the method includes precipitating gypsum from a supersaturated solution of CaSO4 using the gypsum seed fines.

11. The separation method according to claim 1 or 2, wherein: The method further comprises: The concentration of gypsum seed fines in the reactor was maintained in the range of 0.5 wt % to 10 wt %.

12. The separation method according to claim 11, wherein: The concentration of gypsum seed fines in the reactor was maintained in the range of 1 wt % to 7 wt %.

13. The separation method according to claim 1 or 2, wherein: The method further comprises: The coagulant is added: (a) to the feed stream to the reactor; (b) to the aqueous solution in the reactor; (c) to the aqueous solution received from the reactor prior to the addition of the anionic flocculant; or (d) to any combination thereof.

14. The separation method according to claim 13, wherein: The coagulant is a trivalent metal salt-based coagulant.

15. The separation method according to claim 14, wherein: The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

16. The separation method according to claim 15, wherein: The iron or aluminum based coagulant is FeCl3, Fe2(SO4)3, polyferric sulfate or polyaluminum chloride.

17. The separation method according to claim 1 or 2, wherein: The method further comprises: The aqueous solution in the reactor was maintained at a pH of 4 to 10.

18. The separation method according to claim 17, wherein: The aqueous solution in the reactor was maintained at a pH of 6 to 8.

19. The separation method according to claim 18, wherein: The aqueous solution in the reactor was maintained at a pH of 6.5 to 7.

20. The separation method according to claim 1 or 2, wherein: The average diameter of the gypsum seed fine particles is 20 μm to 40 μm.

21. The separation method according to claim 20, wherein: The average diameter of the gypsum seed fine particles is 25 μm.

22. The separation method according to claim 1 or 2, wherein: At the bottom of the settling tank, the local concentration of settled gypsum seed flocs was 8 to 25% by weight.

23. A method comprising a seed-assisted precipitation method in combination with a separation method according to any one of claims 1 to 22, wherein: The separation process receives gypsum seed fines from the seed-assisted precipitation process and the seed-assisted precipitation process comprises: A supersaturated aqueous solution of CaSO4 is received into a multi-stage continuous stirred tank reactor (CSTR); adding a coagulant (a) to a feed stream to a multi-stage CSTR, or (b) to the multi-stage CSTR; Passing the solution through a multistage CSTR and operating the seed-assisted precipitation and the multistage CSTR under shear conditions to produce gypsum seed fines having an average diameter of 20 μm to 40 μm; and The gypsum seed fines are transferred to the separation process as a mixture of gypsum seed fines in aqueous solution.

24. A method comprising a seed-assisted precipitation method in combination with a separation method according to any one of claims 1 to 22, wherein: The separation process receives gypsum seed fines from the seed-assisted precipitation process and the seed-assisted precipitation process comprises: receiving a supersaturated aqueous solution of CaSO4 into a first continuous stirred tank reactor (CSTR); adding a coagulant (a) to a feed stream to the first CSTR, or (b) to the first CSTR; flowing the solution through a first CSTR and at least one additional CSTR to produce a mixture of gypsum seed fines in aqueous solution; transferring gypsum seed fines to the separation process as a mixture of gypsum seed fines in aqueous solution; wherein each CSTR independently has a height (H) and a diameter (D), wherein the ratio of H:D is 1:1 to 2:1; wherein agitation in each CSTR is independently performed with a pitched blade paddle at a speed of 50 rpm to 200 rpm, and wherein each paddle independently has a width (d), wherein the ratio of d:D is 1:3 to 1:2; and The residence time in each CSTR is independently between 2 and 10 minutes.

25. The method of claim 24, wherein the precipitation process comprises flowing the mixture through at least three CSTRs.

26. The method of claim 24 or 25, wherein the H:D ratio of the CSTR, the d:D ratio of the CSTR, the stirring speed of the CSTR and the residence time of the CSTR are selected to obtain gypsum seeds having an average diameter of 20 μm to 40 μm.

27. The method according to claim 26, wherein gypsum seeds having an average diameter of 25 μm are obtained.

28. The method of claim 24 or 25, wherein the H:D ratio is from 1:1 to 1.5:

1.

29. The method of claim 24 or 25, wherein each stirring speed is independently 120 rpm to 150 rpm.

30. The process of claim 24 or 25, wherein the residence time in each CSTR is independently from 2.5 to 5 minutes.

31. The method according to any one of claims 23 to 25, wherein: The seed-assisted precipitation includes adding gypsum seeds to the reactor.

32. The method according to claim 31, wherein: Gypsum seeds are added to the reactor by transferring gypsum seeds from the separator back to the reactor.

33. The method of claim 32, wherein the transferred gypsum seeds are non-flocculated gypsum seeds.

34. The method according to any one of claims 23 to 25, wherein the supersaturated aqueous solution of CaSO4 is received from a membrane separation unit.

35. The method according to any one of claims 23 to 25, wherein: The coagulant is a trivalent metal salt-based coagulant.

36. The method of claim 35, wherein: The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

37. The method of claim 36, wherein: The iron or aluminum based coagulant is FeCl3, Fe2(SO4)3, polyferric sulfate or polyaluminum chloride.

38. The process of any one of claims 23 to 25, wherein the coagulant is added in an amount sufficient to destabilize any antifouling agent in the reactor.

39. The method of any one of claims 23 to 25, wherein the precipitation process excludes the addition of calcium hydroxide.

40. The method of any one of claims 23 to 25, wherein the precipitation process is carried out at a temperature of 18°C ​​to 25°C.

41. The method according to any one of claims 23 to 25, wherein: The mixture transferred to the separator has a calcium concentration of 400 to 1300 ppm.

42. A method comprising: receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR) for seed-assisted precipitation; adding a coagulant (a) to a feed stream of the multi-stage CSTR, or (b) to the multi-stage CSTR; Passing the solution through a multistage CSTR and operating the seed-assisted precipitation and the multistage CSTR under shear conditions to produce gypsum seed fines having an average diameter of 20 μm to 40 μm; transferring the gypsum seed fines into a separator; adding an anionic flocculant (a) to a feed stream to the separator, or (b) to the separator; Aggregating the gypsum seed particles and the flocculant into flocs; separating the flocculated mixture into a turbidity-reduced effluent and a flocculated gypsum slurry; and A portion of the flocculated gypsum slurry is exposed to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines, and at least a portion of the fines is transferred to a multi-stage CSTR.

43. A method comprising: receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR) for seed-assisted precipitation, wherein the stages of the reactor are vertically stacked, wherein the internal outflow port from one stage corresponds to the internal feed port of a subsequent downstream stage; adding a coagulant (a) to a feed stream to the multi-stage CSTR, or (b) to the multi-stage CSTR; allowing the solution to flow vertically upward through the multi-stage CSTR and operating the seed-assisted precipitation and the multi-stage CSTR under shear conditions to produce gypsum seed fines having an average diameter of 20 μm to 40 μm; transferring the gypsum seed fines into a separator; adding an anionic flocculant (a) to a feed stream to the separator, or (b) to the separator; Aggregating the gypsum seed particles and the flocculant into flocs; separating the flocculated mixture into a turbidity-reduced effluent and a flocculated gypsum slurry; and A portion of the flocculated gypsum slurry is exposed to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated gypsum seed fines and transfer at least a portion of the fines to the multi-stage CSTR.

44. A solid / liquid separator, the separator comprising: Sedimentation tanks; A fluid inlet in the settling tank is used to receive the gypsum seed and fine particle mixture in aqueous solution from the precipitation reactor; a source of anionic flocculant in fluid communication with the settling tank; a first fluid outlet in the settling tank for discharging an effluent having reduced turbidity; A second fluid outlet in the settling tank is used to discharge the flocculated gypsum slurry; a liquid conduit connecting the second fluid outlet to an inlet in the reactor; and An applicator of shear stress is arranged in a liquid conduit connecting the second fluid outlet to the inlet in the reactor.

45. A separator according to claim 44, wherein The anionic flocculant source is in fluid communication with a liquid conduit connecting the reactor to a fluid inlet in the settling tank.

46. ​​A separator according to claim 44 or 45, wherein The shear stress applicator is a centrifugal pump with an open impeller.

47. The separator of claim 44 or 45, further comprising an agitator arranged in the settling tank.

48. A separator according to claim 44 or 45, wherein The second fluid outlet is located in the bottom third of the settling tank.

49. A separator according to claim 44 or 45, wherein The first fluid outlet discharges the reduced turbidity effluent directly to a nanofiltration unit.

50. A separator according to claim 44 or 45, wherein: The anionic flocculant is a polyacrylamide flocculant.

51. An apparatus comprising a separator according to any one of claims 44 to 50 and the precipitation reactor, wherein: The reactor comprises a fluid outlet for discharging a mixture of gypsum seeds and fine particles in aqueous solution, and the apparatus comprises a liquid conduit connecting the fluid outlet of the reactor and the fluid inlet of the settling tank, and A liquid conduit connecting the second fluid outlet to the reactor fluidly connects the second fluid outlet to a fluid inlet in the reactor.

52. The apparatus of claim 51, wherein: The apparatus further includes a source of gypsum seeds in fluid communication with the reactor.

53. The device according to claim 51 or 52, wherein: The apparatus further includes one or more sources of one or more coagulants, each independently (a) in fluid communication with the reactor, (b) in fluid communication with a liquid conduit connecting a fluid outlet of the reactor and a fluid inlet of the settling tank, or (c) both.

54. The apparatus of claim 53, wherein: The one or more coagulants are each independently a trivalent metal salt based coagulant.

55. The apparatus of claim 54, wherein: The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

56. The apparatus of claim 55, wherein: The iron or aluminum based coagulant is FeCl3, FeSO4, polyferric sulfate or polyaluminum chloride.

57. The apparatus of claim 51 or 52, wherein: The apparatus further includes a pH sensor for measuring the pH of the liquid in the reactor and in fluid communication with the reactor: (a) an acid source, b) a base source, or (c) both.

58. The apparatus of claim 51 or 52, wherein: The reactor comprises a fluid inlet for receiving a supersaturated aqueous solution of CaSO4 from a membrane separation unit.

59. An apparatus comprising a precipitation reactor and a separator according to any one of claims 44 to 50, wherein the precipitation reactor comprises: a multi-stage continuous stirred tank reactor (CSTR), wherein the precipitation reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4; at least one pitched blade paddle disposed in at least one stage of the multi-stage CSTR, wherein the size of the paddle and the size of the vessel in which it is disposed are selected to produce gypsum seed fines having an average diameter of 20 μm to 40 μm; and a coagulant source in fluid communication with the precipitation reactor; The precipitation reactor is in fluid communication with a separator to provide a mixture of gypsum seeds and fine particles in aqueous solution to the separator.

60. An apparatus comprising a precipitation reactor and a separator according to any one of claims 44 to 50, wherein the precipitation reactor comprises: a plurality of continuous stirred tank reactors (CSTRs) connected in series, wherein each of the plurality of CSTRs independently has a height (H) and a diameter (D), wherein the H:D ratio is between 1:1 and 2:1, wherein a first of the plurality of CSTRs is in fluid communication with a source of a supersaturated aqueous solution of CaSO4; at least one pitched blade impeller disposed in at least a first one of the plurality of CSTRs, wherein each impeller independently has a width (d), wherein the ratio of d:D is between 1:3 and 1:2; and a coagulant source in fluid communication with (a) a feed stream to a first one of the plurality of CSTRs or (b) a first one of the plurality of CSTRs; The precipitation reactor is in fluid communication with a separator to provide a mixture of gypsum seeds and fine particles in aqueous solution to the separator.

61. The apparatus of claim 60, wherein the H:D ratio in at least one of the plurality of CSTRs is from 1:1 to 1.5:

1.

62. The apparatus of claim 59, wherein each CSTR includes a paddle disposed therein.

63. The apparatus of any one of claims 60 to 62, wherein the reactor comprises at least three CSTRs.

64. The apparatus of any one of claims 59 to 62, further comprising a source of gypsum seeds in fluid communication with the reactor.

65. The device according to any one of claims 59 to 62, wherein: The reactor includes a fluid inlet in communication with the separator for receiving gypsum seeds from the separator.

66. The apparatus of claim 65, wherein the received gypsum seeds are non-flocculated gypsum seeds.

67. The device according to any one of claims 59 to 62, wherein: The coagulant is a trivalent metal salt-based coagulant.

68. The apparatus of claim 67, wherein: The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

69. The apparatus of claim 68, wherein: The iron or aluminum based coagulant is FeCl3, FeSO4, polyferric sulfate or polyaluminum chloride.

70. The apparatus of any one of claims 59 to 62, further comprising a pH sensor for measuring the pH of the liquid in the reactor and in fluid communication with the reactor: (a) an acid source, (b) an alkali source, or (c) both.

71. The device according to any one of claims 59 to 62, wherein: The reactor comprises a fluid inlet for receiving a supersaturated aqueous solution of CaSO4 from a membrane separation unit.

72. The device according to any one of claims 59 to 62, wherein The reactor comprises a fluid outlet for discharging the mixture of gypsum seeds and fine particles in aqueous solution, and the apparatus comprises a liquid conduit connecting the fluid outlet of the reactor and the fluid inlet of the settling tank; and A liquid conduit connecting a second fluid outlet in the settling tank to the reactor fluidly connects the second fluid outlet to a fluid inlet in the reactor.

73. An apparatus comprising: a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are connected in series, wherein the internal flow outlet from one stage corresponds to the internal feed inlet of the subsequent downstream stage, wherein the reactor is in fluid communication with a source of a supersaturated aqueous solution of CaSO4, at least one pitched blade paddle disposed in at least one of the stages, wherein the size of the paddle and the size of the container in which it is disposed are selected to produce gypsum seed fines having an average diameter of 20 μm to 40 μm; and a coagulant source in fluid communication with the multi-stage CSTR; a settling tank in fluid communication with the multi-stage CSTR for receiving the gypsum seed and fines mixture in aqueous solution from the multi-stage CSTR; a source of anionic flocculant in fluid communication with the settling tank; a first fluid outlet in the settling tank for discharging an effluent having reduced turbidity; a second fluid outlet in the settling tank for discharging the flocculated gypsum slurry; connecting the second fluid outlet to a liquid conduit of the multi-stage CSTR; and An applicator of shear stress is arranged in a liquid conduit connecting the second fluid outlet to the multi-stage reactor.

74. The apparatus of claim 73, wherein: The stages of the multi-stage CSTR are stacked vertically.

75. The apparatus of claim 74, wherein: The source of the supersaturated aqueous solution of CaSO4 provides the solution at a static pressure sufficient to drive the solution vertically upward through the stages of the CSTR.

76. The device according to any one of claims 73 to 75, wherein Each stage of the CSTR independently has a height (H) and a diameter (D), wherein the ratio of H:D is 1:1 to 2:1; and wherein each paddle independently has a width (d), wherein the ratio of d:D is 1:3 to 1:

2.

77. Apparatus according to any one of claims 73 to 75, wherein: The shear stress applicator is a centrifugal pump with an open impeller.

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