Separation process and reactor

Abstract

The present disclosure relates to a separation process and a reactor. The present disclosure provides methods and systems for precipitating CaSO4 from a supersaturated solution of CaSO4. The precipitate may form gypsum particles having an average diameter of about 25 [mu] m. Precipitation may be controlled to reduce or avoid fouling. The present disclosure also provides methods and systems in which CaSO4 fouling can be removed.

Description

Technical Field

[0002] The present disclosure relates to methods and reactors for removing calcium sulfate from wastewater. Background Art

[0004] The following paragraphs do not admit that anything discussed therein is prior art or part of the knowledge of those skilled in the art.

[0005] Various industrial processes, such as desalination, coal mine drainage, flue gas desulfurization, and lime neutralization of acidic wastewater, produce aqueous effluents that include calcium sulfate. Calcium sulfate may undesirably precipitate to form scale on the surfaces of processing equipment, thereby interfering with operating efficiency. Summary of the Invention

[0007] The following introduction is intended to introduce the reader to this specification but does not define any invention. One or more inventions may exist in combinations or sub - combinations of the device elements or method steps described below or in other parts of this document. The inventors do not waive or disclaim their right to any one or more inventions disclosed in this specification by not describing such one or more other inventions in the claims.

[0008] 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, there is a desire to develop methods and processing equipment that reduce or avoid lime softening, the production of sodium sulfate, or both. Compared to conventional lime - softening methods, such methods and processing equipment can result in cost savings. It has been shown that it is 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 the equipment surfaces.

[0009] The present disclosure discloses various methods and devices that can be operated individually or combined into larger systems or devices. As described above, the larger systems or devices according to the present disclosure can be sub - combinations of the disclosed methods and devices.

[0010] In some embodiments, the present disclosure provides a separation method that includes: 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) into the multi - stage CSTR; flowing the solution through the multi - stage CSTR and operating the seeded precipitation and the multi - stage CSTR under shear conditions to produce fine gypsum seeds having an average diameter of about 20 μm to about 40 μm; and transferring the fine gypsum seeds as a mixture of fine gypsum seeds in an aqueous solution to a separator.

[0011] The authors of the present disclosure have determined that the efficacy of gypsum precipitation can be enhanced by using a multi-stage continuous stirred tank reactor (compared to a single large-stage reactor); and fine gypsum seed particles with an average diameter of about 20 μm to about 40 μm are produced by operating the multi-stage CSTR under shear conditions.

[0012] In a particular example, the present disclosure provides a separation method that includes: receiving an oversaturated 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; flowing the solution through the first CSTR and at least one additional CSTR to produce a mixture of fine gypsum seed particles in the aqueous solution; and transferring the fine gypsum seed particles as a mixture of fine gypsum seed particles in the aqueous solution to a separator. Each CSTR independently has a height (H) and a diameter (D), where the H:D ratio is from about 1:1 to about 2:1. Stirring in each CSTR is independently carried out by a pitched blade impeller at a speed of from about 50 rpm to about 200 rpm, where each impeller independently has a width (d), where the d:D ratio is from about 1:3 to about 1:2. The residence time in each CSTR is independently from about 2 to about 10 minutes. Operating such a multi-stage CSTR under these conditions results in gypsum seeds of the desired size and concentration. In the context of the present disclosure, those skilled in the art will understand that the reference to the width (d) of the impeller refers to the radius of the circle formed when the impeller is stirred.

[0013] The present disclosure also provides a precipitation reactor that includes a multi-stage continuous stirred tank reactor (CSTR). The precipitation reactor is in fluid communication with a source of an oversaturated aqueous solution of CaSO4. At least one pitched blade impeller is disposed in at least one stage of the multi-stage CSTR, where the size of the impeller and the size of the vessel in which it is disposed are selected to produce fine gypsum seed particles 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 fine gypsum seed particles in the aqueous solution to the separator. The separator can be, for example, the solid / liquid separator described herein.

[0014] In certain embodiments, the present disclosure provides a precipitator reactor that includes a plurality of continuously stirred tank reactors (CSTRs) in series, where each of the plurality of CSTRs independently has a height (H) and a diameter (D), and where the H:D ratio is from about 1:1 to about 2:1. The 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 is disposed in at least the first of the plurality of CSTRs, where each impeller independently has a width (d), and where the d:D ratio is from about 1:3 to about 1:2. A coagulant source is in fluid communication with (a) the feed stream to the first of the plurality of CSTRs or (b) the first of the plurality of CSTRs. The precipitation reactor is in fluid communication with a separator to provide a mixture of fine gypsum seed particles in the aqueous solution to the separator. The separator can be, for example, a solid / liquid separator as discussed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0017] Figure 1 is a process flow diagram of an exemplary solid / liquid separator according to the present disclosure.

[0018] Figure 2 is a graph showing the concentration (volume %) of seed particles in a precipitation reactor and a deposition tank used in a solid-liquid separation method according to the present disclosure.

[0019] Figure 3 is a graph showing the turbidity of an effluent stream from a solid-liquid separation method according to the present disclosure.

[0020] Figure 4 is a process flow diagram of an exemplary precipitation reactor according to the present disclosure.

[0021] Figure 5 is a process flow diagram of an exemplary apparatus according to the present disclosure.

[0022] Figure 6 is a graph showing the size distribution of particles produced in a method according to the present disclosure.

[0023] Figure 7 is a graph showing the size distribution of particles produced in a comparative method.

[0024] Figure 8 is a diagram of an exemplary precipitation reactor according to the present disclosure.

[0025] Figure 9 is a diagram of another exemplary precipitation reactor according to the present disclosure.

[0026] Figure 10 is a process flow diagram of an exemplary apparatus according to the present disclosure. Detailed implementation mode

[0028] The seed slurry technology (SST) can be used in wastewater concentration methods, such as membrane filtration, electrodialysis or thermal crystallization. When water is concentrated, SST reduces the scaling of supersaturated components (such as CaSO4). However, CaSO4 may precipitate and / or harden, thus blocking one or more water treatment systems (such as tanks, pipes and pumps) in the water concentration method or downstream filtration method.

[0029] In one aspect, the present disclosure provides a separation method, which includes: receiving a mixture of fine gypsum seeds in an aqueous solution from a reactor; adding an anionic flocculant and an optional coagulant to the aqueous solution; aggregating the fine gypsum seeds and the flocculant into flocs; separating the mixture into an effluent with reduced turbidity and a flocculated gypsum slurry; and exposing a part of the flocculated gypsum slurry to a shear stress sufficient to convert the flocculated gypsum seeds into non-flocculated fine gypsum seeds, and transferring the fine seeds to the reactor. The aqueous solution from the reactor can be directly received into a sedimentation tank.

[0030] In the context of the present disclosure, it should be understood that the phrases "receive from [X]" and "receive [A] from [X]" refer to direct and indirect reception. For example, if reactor Z is disclosed as "receiving fluid A from reactor X", it should be understood that reactor Z can (i) be directly coupled to reactor X so that fluid A is directly received from reactor Z; or (ii) be indirectly coupled to reactor X so that process equipment Y receives fluid A from reactor X, and reactor Z receives fluid A from equipment Y.

[0031] As described above, the separation method controls the transformation of gypsum seeds between (i) a flocculated part that can be separated into an effluent with reduced turbidity and a flocculated gypsum slurry and (ii) a non-flocculated part that can be returned to the reactor and used to precipitate additional calcium sulfate (such as from a supersaturated solution of CaSO4).

[0032] The anionic flocculant can be a polyacrylamide flocculant. The anionic flocculant can be a polymer flocculant with a low charge density and a high molecular weight. When exposed to high shear or excessive stirring, the anionic flocculant may exhibit reduced flocculation activity. An example of a suitable anionic flocculant is PolyFloc TM AP1100. The flocculant can be added before the aqueous solution is received into the separator, thereby allowing the flocculant to be fully mixed into the solution before the solution enters the separator.

[0033] 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 adding 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 scale inhibitor present in the aqueous solution received from the reactor. For example, a sufficient amount of FeCl3 can be added to make the concentration in the sedimentation tank at least 10 ppm. In some specific examples, the final concentration of FeCl3 is at least 30 ppm.

[0034] The flocs can be stirred in the sedimentation tank to prevent the gypsum slurry from hardening. For example, the flocs can be stirred with a paddle at a speed of 50 rpm or lower, where the diameter of the paddle is about 1 / 2 to about 3 / 4 of the diameter of the sedimentation tank. The bed height of the settled flocs can be about 1 / 5 to about 1 / 3 of the height of the sedimentation tank. The local concentration of the settled gypsum seed flocs at the bottom of the sedimentation tank can be about 8 to about 25 wt%.

[0035] The slurry comprising the flocs 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.

[0036] The effluent with reduced turbidity produced according to the method can be a clarified effluent with a turbidity of less than 3 NTU (nephelometric turbidity unit), or about 3 to about 5 NTU. When the turbidity of the clarified effluent is less than 3 NTU, the method can further include nanofiltration of the clarified effluent without prior ultrafiltration. The effluent with reduced turbidity can be treated with a lamella clarifier (also known as a inclined plate settler), for example, when the turbidity is greater than 3 NTU.

[0037] The gypsum seeds can be returned to the reactor and used to precipitate additional calcium sulfate. However, the flocculated gypsum seeds are not as effective in precipitating calcium sulfate because the anionic flocs inhibit this precipitation. The flocculated gypsum seeds are transformed into non-flocculated gypsum seed fines by exposing the flocculated material to shear stress.

[0038] The bottom of the sedimentation 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 sedimentation tank to the reactor. The pump can be a centrifugal pump with an open impeller, for example operating at a rate of at least 500 rpm. Such a pump can provide sufficient shear force to transform the flocculated gypsum seeds into non-flocculated fine particles. Compared with a closed impeller pump, this pump also has a higher tolerance to concentrated slurry, thereby reducing the possibility of forming a slurry pile 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 depositing and hardening in the pump.

[0039] For example, if the pump does not provide shear force, the shear stress can be provided by a mechanical agitator arranged between the sedimentation tank and the reactor. The mechanical agitator can be, for example, at the inlet of the reactor, at the outlet of the sedimentation tank, or adjacent to the pump.

[0040] An adequate amount of gypsum seeds can be returned to the reactor to maintain the concentration of gypsum seed fine particles in the reactor within the range of about 0.5 wt% to about 10 wt%, for example within the range of about 1 wt% to about 7 wt%.

[0041] The method can also include maintaining the aqueous solution in the reactor at a pH of about 4 to about 10, for example at a pH of about 6 to about 8, for example at a pH of about 6.5 to about 7.

[0042] The reactor can operate under conditions that produce gypsum seed fine particles 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 CaSO4 precipitation particularly effective for removing CaSO4 from its supersaturated solution. The authors also found that when particles of this size are flocculated with an anionic polymer flocculant, they can maintain an ideal moisture content even after being removed from the aqueous solution for a period of time. The period of time can be, for example, one, two, or three months. In the context of the present disclosure, the ideal moisture content will be understood as the moisture content that prevents CaSO4 from hardening so that the flocculated particles can be dispersed in water after a period of time.

[0043] The present disclosure also provides a solid / liquid separator. The separator includes a sedimentation tank, a fluid inlet in the sedimentation tank for receiving a mixture of gypsum seed fine particles in an aqueous solution from a precipitation reactor; a first fluid outlet in the sedimentation tank for discharging the effluent with reduced turbidity; a second fluid outlet in the sedimentation tank for discharging the flocculated gypsum slurry, and an optional agitator. The second fluid outlet can be located at the bottom third of the sedimentation tank. The sedimentation tank can also be referred to as a sedimentation tank or a clarifier. The sedimentation tank can directly receive the fluid from the precipitation reactor.

[0044] In one example, the separator includes a cylindrical settling tank having a height to diameter ratio of from about 1:1 to about 8:1, preferably from about 2:1 to 5:1; an agitator with blades having a diameter of about 3 / 4 to about 5 / 6 of the diameter of the tank, where 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 operate at a rate of about 10 to about 40 rpm. The separator configured in this way and operating under these conditions can maintain a stable suspension of the flocculant slurry at a concentration of about 15 wt% to about 35 wt%, with a well-defined boundary between the suspension of the flocculant slurry and the supernatant.

[0045] The separator further includes a source of anionic flocculant in fluid communication with the settling tank, a liquid conduit connecting the second fluid outlet to an inlet in the reactor, and an applicator of shear stress disposed in the liquid conduit connecting the second fluid outlet to an inlet in the reactor.

[0046] The source of anionic flocculant can be in fluid communication with the liquid conduit connecting the reactor to the fluid inlet in the settling tank. The anionic flocculant can be a polyacrylamide flocculant.

[0047] The applicator of shear stress can be a centrifugal pump or a mechanical agitator with an open impeller. The mechanical agitator can be, for example, at the inlet of the reactor, at the outlet of the sedimentation tank, or adjacent to the pump.

[0048] The solid / liquid separator can be configured to discharge the turbidity-reduced effluent from the first fluid outlet to a 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.

[0049] The present disclosure also provides an apparatus that includes the above-described solid / liquid separator and a precipitation reactor, such as the following precipitation reactor. The precipitation reactor includes a fluid outlet for discharging a mixture of fine gypsum seeds 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.

[0050] The apparatus can additionally 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, for example, a supersaturated aqueous solution of CaSO4 from a membrane separation unit.

[0051] 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 the fluid outlet of the reactor and the fluid inlet of the sedimentation tank, or (c) both. Each coagulant can independently be a coagulant based on a trivalent metal salt, 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 scale inhibitor present in the aqueous solution received from the reactor.

[0052] Figure 1 A process flow diagram of an exemplary solid / liquid separator (110) in accordance with the present disclosure in combination with a precipitation reactor (112) is shown. The precipitation reactor (112) provides an aqueous mixture (114) of fine gypsum seeds, which is received into a sedimentation tank (116). A coagulant (118) is added to the reactor (112). A pH adjuster (120) can be added to the reactor (112) to bring or maintain the pH at a value of about 4 to about 10. An anionic flocculant (122) is added to the mixture in the fluid conduit connecting the reactor (112) and the sedimentation tank (116). The sedimentation tank (116) produces a flocculated gypsum slurry (124) and a clarified 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 reactor (116) as non-flocculated fine gypsum seeds (130).

[0053] Example 1

[0054] A pilot plant using the Figure 1 method shown was established to treat a concentrated waste stream from a coal-to-chemicals production process. The concentrated waste stream is from the reverse osmosis treatment of coal-to-chemicals wastewater. Typical water quality is shown in Table 1. Table 1

[0055] First, the waste stream was concentrated at least 3-fold to provide a supersaturated solution of CaSO4. The resulting supersaturated solution was fed into a precipitation reactor. Upon entering the precipitation reactor, the pH of the feed stream was adjusted to 6.5 - 7, and an amount of 30 ppm FeCl3 was added at the same location based on the volume of the influent. Gypsum seeds were dispersed in the reactor, and the supersaturated CaSO4 precipitated out. The concentration of the gypsum seeds was maintained between 2 - 5% (volume / volume) by recycling the seed slurry from the sedimentation tank.

[0056] In the outlet pipe of the precipitation reactor, an additional amount of 10 ppm FeCl3 is added to adjust the surface properties of the seed crystals. A pump is used to transfer the effluent to a downstream sedimentation tank. Just before pumping, a flocculant is added to the effluent in an amount of 05 ppm.

[0057] Flocs form and settle in the sedimentation tank. Under slow agitation by a mechanical paddle, the height of the concentrated slurry never grows higher than 1 / 4 of the tank height. The supernatant from the top of the sedimentation tank is sent to a downstream membrane treatment unit. Some of the slurry from the bottom of the sedimentation tank is pumped back to the precipitation reactor by an open impeller operating at about 800 RPM, which provides sufficient shear stress to reduce the flocculation activity of the flocculant.

[0058] 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 performance of the recycled gypsum seeds in the precipitation reactor, the recycled seeds maintain the ability to reduce the supersaturation level in the incoming waste stream. The calculated CaSO4 in the incoming waste stream is 113% to 140% of the saturation level, and the measured CaSO4 in the effluent from the sedimentation tank is 100% to 110% of the saturation level (see Table 2). November 30th December 4th December 7th December 10th Entrance 140.20％ 123.50％ 113.30％ 136.40％ Exit 96.20％ 103.90％ 103.50％ 100.80％ Table 2

[0059] The concentrations of gypsum seeds in the precipitation reactor and the sedimentation tank are tracked. The results are shown in Figure 2 . The test was run for 500 hours and the concentration of gypsum seeds in the sedimentation tank was as high as 30% vol / vol. No pumps or pipes were blocked by the gypsum slurry. Other comparable systems using a closed impeller instead of an open impeller could not operate for the same length of time due to blockage of the pump body by the thick gypsum slurry.

[0060] It was found that the flocculated gypsum seeds disperse rapidly into water even after being dried for several weeks. It was also found that the flocculated gypsum seeds can be stored for months without hardening. Without wishing to be bound by theory, the authors of the present disclosure believe that gypsum seed particles that retain moisture and are easily dispersible have a reduced tendency to form scale in pipes, and the polyacrylamide flocculant added to the influent of the sedimentation tank surrounds the gypsum seeds and provides these desirable properties. The polyacrylamide flocculant can act as a wetting agent in the seeds and accelerate the dispersion of the gypsum seeds when added to water.

[0061] The chemical composition of the resulting gypsum seeds was analyzed using X-ray fluorescence. In addition to CaSO4 (the main component being 95 wt%), STSO4 was also co-precipitated from the solution (2 wt%). This indicates that other slightly soluble ions can be removed simultaneously in this method, thereby further reducing the scaling risk in downstream membrane filtration. The remaining components are 1 wt% Fe(OH)3 and 2 wt% Na2SO4.

[0062] The supernatant flow from the sedimentation tank was light yellow and contributed to the turbidity reading even without any particles. Therefore, the turbidity was not tracked during this pilot process. However, in another test area where coal mine drainage was treated using the same method, the supernatant was colorless, and the turbidity of the effluent was tracked. As Figure 3 shown, the turbidity measured throughout the cycle was 3 NTU or less. This water quality may be suitable for downstream membranes. For example, in cases where colloids in water are not a major problem, water with < 3 NTU can be directly fed into a nanofiltration membrane process without first passing the water through an ultrafiltration unit.

[0063] On the other hand, the present disclosure provides a seed-assisted precipitation method that can be carried out at ambient temperature, for example, from about 18 to about 25 °C. The method can exclude lime softening, for example, by excluding the addition of calcium hydroxide. The method includes receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR); and adding a coagulant to (a) the feed stream of the multi-stage CSTR, or (b) the multi-stage CSTR. The method further includes flowing the solution through the multi-stage CSTR and operating the seed-assisted precipitation and the multi-stage CSTR under shear conditions to produce fine gypsum seeds with an average diameter of about 20 μm to about 40 μm. The fine gypsum seeds are transferred as a mixture of fine gypsum seeds in an aqueous solution to a separator.

[0064] 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 impeller disposed in at least one stage of the multi-stage CSTR. The size of the impeller and the size of the vessel in which it is disposed are selected to produce fine gypsum seeds 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 can be in fluid communication with a source of fine gypsum seeds. The precipitation reactor can be in fluid communication with a separator to provide a mixture of fine gypsum seeds in an aqueous solution to the separator. As described above, the separator can be a separator according to the present invention.

[0065] A coagulant is used to neutralize a sufficient amount of scale inhibitor normally present in a supersaturated solution of CaSO4 so that CaSO4 precipitates on 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.

[0066] Seed-assisted precipitation and multi-stage CSTRs are operated under shear conditions to produce the desired fine gypsum seeds. In specific examples of such shear conditions, each CSTR can independently have a height (H) and a diameter (D), where the H:D ratio is from about 1:1 to about 2:1, such as from about 1:1 to about 1.5:1. Stirring in each CSTR can be independently carried out with pitched blade impellers at a speed of about 50 rpm to about 200 rpm, such as about 120 rpm to about 150 rpm; where each impeller independently has a width (d), where the d:D ratio is from about 1:3 to about 1:2. The flow rate and size of the CSTRs can be provided such that the residence time in each CSTR is independently from about 2 minutes to about 10 minutes, such as 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%, such as about 100% (saturated), thus reducing the risk of fouling in downstream processes and equipment. Pitched blade impellers with larger blades can operate at a lower rpm than those with smaller blades.

[0067] A smaller d:D ratio has an increased shear effect, while a larger d:D ratio has an increased mixing effect. Shear and mixing control the size of the gypsum seeds. Vigorous stirring associated with increased shear breaks up larger particles and produces smaller particles. Increasing mixing enhances the crystallization of the supersaturated CaSO4 solution and results in larger particles within the same crystallization time. A d:D ratio of from about 1:3 to about 1:2 provides an acceptable balance between shear and mixing.

[0068] The concentration of the gypsum seeds can be controlled by removing the gypsum seeds from the reactor and by optionally adding the gypsum seeds to the reactor. The gypsum seeds can be added by recycling the removed gypsum seeds back to the reactor. A higher concentration of gypsum seeds results in a faster crystallization rate, but also increases the operating load of any recycling unit. The higher concentration also increases the risk of fouling formation downstream of the reactor. The method 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%, such as about 1 wt% to about 7 wt%.

[0069] A multi-stage CSTR may include at least two, such as at least three, stages. The total residence time in the multi-stage CSTR may be from about 8 to about 40 minutes. The authors of the present disclosure have determined that the precipitation rate in the multi-stage CSTR is faster than that in a single-stage CSTR of the same total volume.

[0070] Figure 4 A process flow diagram of an exemplary precipitation reactor according to the present invention is shown. In the apparatus (210), a multi-stage CSTR (212) consists of three stages (212a, 212b, and 212c). The multi-stage 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 regulator (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 meet the above H:D ratio and d:D ratio. The final stage produces a mixture (220) of gypsum fines. The multi-stage CSTR (212) includes a feed (222) for gypsum seed particles.

[0071] The three stages (212a, 212b, and 212c) may be positioned to allow liquid to flow from one stage to the next by gravity. For example, the three stages may be positioned such that the first stage (212a) is higher than the second stage (212b) by, for example, about 10 cm, and the liquid flows from the first stage (212a) into the second stage (212b) by gravity; and the second stage (212b) is higher than the third stage (212c) by, for example, about 10 cm, and the liquid flows from the second stage (212b) into the third stage (212c) by gravity.

[0072] The gypsum seed fines produced by this method can be used in the above separation method. The precipitation reactor can be combined with the above separator for use in an apparatus.

[0073] Figure 5 A process flow diagram of an exemplary apparatus according to the present disclosure is shown. The apparatus (310) includes the above 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 impellers are not shown. A flocculant (122) is optional.

[0074] Example 2

[0075] A setup was used Figure 5Pilot tests of the method shown were conducted without adding any flocculants to the sedimentation tank to treat the supersaturated solution of CaSO4 produced by the nanofiltration method for treating coal mine drainage. The pilot test method used a multi-stage CSTR consisting of three precipitation reactors. The concentration of gypsum seeds in each reactor was about 3 to 5 wt%. A mechanical stirrer was used to stir the reactor 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 of the reactor diameter. The height of the reactor was 1000 mm, the diameter was 600 mm, and the diameter of the circle formed by the paddle was 300 mm. This corresponded to an 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.

[0076] The gypsum seeds were transferred to a solid-liquid separator with a height of 1200 mm and a diameter of 600 mm (H:D ratio of 2:1). The stirrer had blades with a diameter of about 500 mm and operated at about 20 RPM to about 40 RPM. The larger gypsum seed flocs obtained from the bottom of the solid-liquid separator were sheared by a transfer pump and recycled and dispersed into the first reaction tank under strong stirring. The transfer pump included an open impeller operating at about 800 RPM.

[0077] A stabilizer, also known as a scale inhibitor, was added to the supersaturated effluent produced by the nanofiltration method to reduce or avoid scaling. An amount of 30 to 50 ppm of FeCl3 was added as a coagulant to the influent of the multi-stage CSTR to accelerate the destabilization of the supersaturated CaSO4.

[0078] The size distribution of the gypsum seeds produced in this test was measured. The size distribution of the gypsum seeds produced in the comparative method was also measured. The comparative method used only one stage of the above CSTR sedimentation tank and operated with a residence time of 30 to 40 minutes, but was otherwise the same as the above pilot test. As described above, the exemplary method according to the present disclosure produced particles with an average size of about 25 μm. The comparative method produced particles with an average size of about 80 μm. The size distributions are shown in Figure 6 and 7 respectively.

[0079] 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:

[0080] The components 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. Ca Fe K Mg Na Si Sr SO4 Cl Tank Inflow 1250 2.3 68.4 639 6490 10.8 12.7 18750 16.5 Tank Outflow 435 2.4 63.2 630 6300 10.4 8.6 16180 49.4 Table 3

[0081] It has been determined that the average supersaturation level of the influent and effluent streams of the comparison reactor is reduced from approximately 230% at the influent stream to approximately 190% at the effluent stream. A supersaturation level of approximately 190% was observed to cause fouling downstream of the comparison reactor.

[0082] 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, and wherein the internal flow outlet from one stage substantially corresponds to the internal feed inlet of the subsequent downstream stage. The method includes flowing the solution vertically upward through the multi-stage CSTR to produce a mixture of fine gypsum seed particles in the aqueous solution; and transferring the mixture of fine gypsum seed particles in the aqueous solution to a separator.

[0083] The present disclosure also provides a precipitation reactor that includes a multi-stage continuous stirred tank reactor (CSTR), wherein the stages of the reactor are in series, and 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 and is in fluid communication with a separator to provide the mixture of fine gypsum seed particles in the aqueous solution to the separator. The separator can be the separator as described above. The source of the supersaturated CaSO4 solution can be a membrane separation unit.

[0084] The stages of the reactor are preferably vertically stacked. 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 respective stages of the CSTR.

[0085] The flow outlet and the feed inlet can be connected by a short fluid conduit or can be not connected by a fluid conduit. When not connected by a fluid conduit, the flow outlet and the inlet may refer to the same orifice between two adjacent stages. Since the flow outlet of one stage substantially corresponds to the inlet 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.

[0086] The multi-stage CSTR can be operated under conditions that produce fine gypsum seed particles having an average diameter of from about 20 μm to about 40 μm. Exemplary conditions were discussed above.

[0087] The reactor can include three stages, where 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 outlet of the first stage of the reactor corresponds to the inlet of the second stage of the reactor, and the outlet of the second stage of the reactor corresponds to the inlet of the third stage of the reactor.

[0088] 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. When the diameter of the inlet is about 5% to about 10% of the diameter of the reactor stage, prevention or reduction of backmixing through the inlet can be achieved.

[0089] In this vertical multi-stage CSTR, agitators on the same single agitator shaft can be used to agitate each stage.

[0090] The reactor can include a source of gypsum seeds in fluid communication with the reactor, such as in fluid communication with the first stage of the reactor. The reactor can include a source of coagulant in fluid communication with the reactor, such as in fluid communication with the first stage of the reactor.

[0091] 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 at the bottom, in the first stage (412a). A coagulant (416) is added to the feed stream of the first stage (412a). An optional pH regulator (not shown) can be added. All three stages include pitched blade impellers (418a, 418b, 418c) on the same agitator shaft. The sizes of all three stages and their respective impellers satisfy the above 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 source of gypsum seed fines (422). The outlet of the first stage corresponds to the inlet of the second stage (424a). The outlet of the second stage corresponds to the inlet of the third stage (424b).

[0092] Figure 9 Another exemplary precipitation reactor according to the present disclosure is shown. The reactor (510) is in communication with Figure 8The reactor shown in is similar, except that the three stages do not share a common stirring shaft. The vertical reactor (512) still receives the supersaturated solution (514) of CaSO4 at the bottom, in the first stage (512a). The coagulant (516) is added to the feed stream of the first stage. An optional pH regulator (not shown) may be added. All three stages include pitched blade impellers (518a, 518b, 518c). The impellers may stir at the same or different rates. The dimensions of all three stages and their respective impellers meet the above H:D ratio and d:D ratio. The final stage produces a mixture of gypsum fines (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 of the second stage (524a). The outlet of the second stage corresponds to the inlet of the third stage (524b).

[0093] The gypsum seed fines produced by this precipitation method or in this precipitation reactor can be used in the separation method or reactor described above. For example, a method is provided that includes receiving a supersaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR) for seeded precipitation, where the stages of the reactor are vertically stacked, and the internal flow outlet from one stage corresponds to the internal feed inlet of the subsequent downstream stage. The coagulant is added to (a) the feed stream of the multi-stage CSTR, or (b) into the multi-stage CSTR. The solution flows vertically upward through the multi-stage CSTR. The seeded 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 (a) the feed stream of the separator, or (b) into 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 is transferred back to the multi-stage CSTR.

[0094] In an example of a combined precipitation reactor and solid / liquid separator, an apparatus includes a multi-stage continuous stirred tank reactor (CSTR) where the stages of the reactor are in series and where the internal effluent 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 fluid of a supersaturated aqueous solution of CaSO4. At least one pitched blade impeller is disposed in at least one stage, where the size of the impeller and the size of the vessel in which it is disposed are selected to produce fine gypsum seed particles having an average diameter of from about 20 μm to about 40 μm. The apparatus further includes a coagulant source in fluid communication with the multi-stage CSTR; and a settling tank in fluid communication with the multi-stage CSTR for receiving a mixture of fine gypsum seed particles in an aqueous solution from the multi-stage 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 multi-stage CSTR; and an applicator of shear stress is disposed in the liquid conduit.

[0095] As described above, the stages of the multi-stage CSTR can be vertically stacked. The features of the precipitation reactor and the solid / liquid separator are discussed in more detail above.

[0096] Figure 10 A process flow diagram of an exemplary apparatus according to the present disclosure is shown. 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 fine gypsum seed particles (130) produced by the open centrifugal pump (128) correspond to the gypsum seed (422) received by the CSTR (412). The flocculant (122) is optional.

[0097] Example 3

[0098] A pilot plant using Figure 8 the reactor shown was established to treat a supersaturated solution of CaSO4 produced by a nanofiltration method for treating coal mine drainage.

[0099] The vertical CSTR comprises three stages. The height-to-diameter H:D ratio of each stage is approximately 1:1. The width of the paddle (d) is such that the d:D ratio is approximately 1:3. In this pilot test, a supersaturated solution of CaSO4 (saturation degree of 200%) was fed to the bottom, into the first stage, at a flow rate of 500 L / h. Fine gypsum seeds with a concentration of 20 - 35 wt% were added at a flow rate of 100 L / h. A sufficient amount of FeCl3 was added to the feed stream of the first stage to produce a concentration of 30 - 40 ppm. The pitched blade paddle was agitated at a rate of approximately 110 rpm. The final stage produced a mixture of fine gypsum particles. As shown in Table 4, the gypsum seed concentration in each of the three stages was measured to be approximately 3 to approximately 9 wt%. The precipitation was operated to give a total residence time of approximately 28 minutes. Table 4

[0100] The effluent of the nanofiltration method includes a scale inhibitor to reduce or avoid scaling. The supersaturation level of the nanofiltration effluent is approximately 140% of the saturation level, where calcium is approximately 1800 ppm (recorded as CaCO3). The effluent produced by the final stage of the vertical multi-stage CSTR reaches approximately 100% of the saturation level, where calcium is approximately 1200 ppm (recorded as CaCO3), as shown in Table 5. Table 5

[0101] In another aspect, the present disclosure provides a method for removing scale from the following equipment: (a) process equipment, such as a CaSO4 precipitation reactor, a solid / liquid separator, or a fluid conduit, or (b) a part of the process equipment. The method includes vibrating or deforming the process equipment or a part of the process equipment to remove at least some of the scale. The process equipment or the part thereof that is vibrated is at least partially made of or coated with a low-friction and optionally hydrophobic material. Scale is present on at least some of the low-friction materials.

[0102] The present disclosure also provides a process equipment or a part of the process equipment, wherein the process equipment or the part thereof is at least partially made of a low-friction and preferably hydrophobic material or at least partially coated with a low-friction and preferably hydrophobic material. The low-friction material is in a position that will be exposed to the supersaturated solution of CaSO4. The low-friction material can be in a position that can be sufficiently vibrated or deformed to remove at least some of the scale present on the low-friction material.

[0103] The part of the process equipment that can be vibrated or deformed can be a side wall, a baffle, a liquid conduit inside the reactor, or a stirring blade.

[0104] The low-friction material can be polyethylene (PE), polypropylene (PP), or polytetrafluoroethylene (PTFE).

[0105] Vibrations can be carried out at a frequency of about 0.1 to 10 Hz, and / or can include moving a low-friction material with an amplitude of about 1 to about 5 mm.

[0106] Any of the process equipment discussed above can be made of or coated with a low-friction material, and at least some of the fouling can be removed by vibrating at least a portion of the equipment. When the equipment is a reactor (such as a precipitation reactor or a solid-liquid separation reactor), the reactor can include a slag discharge port, and the removed fouling can be removed from the reactor through the slag discharge port.

[0107] In the foregoing description, for purposes of explanation, numerous details have been 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. Thus, what has been described is merely used to illustrate the application of the described embodiments, and many modifications and variations are possible in light of the above teachings.

[0108] Since the above description provides embodiments, it will be understood that those skilled in the art can make modifications and variations to the specific embodiments. Accordingly, the scope of the claims should not be limited by the specific embodiments set forth herein, but should be construed in a manner generally consistent with the specification.

Claims

1. Seed-assisted precipitation method, comprising: Receiving an oversaturated aqueous solution of CaSO4 into a multi-stage continuous stirred tank reactor (CSTR); Adding a coagulant (a) to the feed stream of the multi-stage CSTR, or (b) to 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 fine gypsum seeds with an average diameter of 20 μm to 40 μm; and Transferring the fine gypsum seeds as a mixture of fine gypsum seeds in an aqueous solution to a separator.

2. The seed-assisted precipitation method according to claim 1, wherein each CSTR independently has a height (H) and a diameter (D), and the H:D ratio is from 1:1 to 2:

1.

3. The precipitation method according to claim 2, wherein the method comprises flowing the mixture through at least three CSTRs.

4. The precipitation method according to claim 2 or 3, wherein the stirring in each CSTR is independently carried out with a pitched blade impeller at a speed of 50 rpm to 200 rpm, and wherein each impeller independently has a width (d), and the ratio of d:D is from 1:3 to 1:

2.

5. The precipitation method according to claim 4, 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 with an average diameter of 25 μm.

6. The precipitation method according to claim 2 or 3, wherein the H:D ratio is from 1:1 to 1.5:

1.

7. The precipitation method according to claim 2 or 3, wherein each stirring speed is independently 120 rpm to 150 rpm.

8. The precipitation method according to claim 2 or 3, wherein the residence time in each CSTR is independently 2 to 10 minutes.

9. The precipitation method according to claim 2 or 3, wherein the residence time in each CSTR is independently 2.5 to 5 minutes.

10. The precipitation method according to any one of claims 1 to 3, wherein, The seed-assisted precipitation includes adding gypsum seeds to the reactor.

11. The precipitation method according to claim 10, wherein, Adding gypsum seeds to the reactor by transferring the gypsum seeds from the separator back to the reactor.

12. The precipitation method according to claim 10, wherein the transferred gypsum seeds are non-flocculated gypsum seeds.

13. The precipitation method according to any one of claims 1 to 3, wherein the oversaturated aqueous solution of CaSO4 is received from a membrane separation unit.

14. The precipitation method according to any one of claims 1 to 3, wherein, The coagulant is a coagulant based on a trivalent metal salt.

15. The precipitation method according to claim 14, wherein, The coagulant based on a trivalent metal salt is an iron or aluminum-based coagulant.

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

17. The precipitation method according to any one of claims 1 to 3, wherein the coagulant is added in an amount sufficient to destabilize any scale inhibitor in the reactor.

18. The precipitation method according to any one of claims 1 to 3, wherein the method excludes adding calcium hydroxide.

19. The precipitation method according to any one of claims 1 to 3, wherein the method is carried out at a temperature of 18 °C to 25 °C.

20. The precipitation method according to any one of claims 1 to 3, wherein, The calcium concentration of the mixture transferred to the separator is 400 to 1300 ppm.

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