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 having an average diameter of approximately 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 surfaces of process equipment, interfering with operational efficiency. Summary of the Invention

[0004] The following description is intended to introduce the reader to this specification and does not limit any invention. One or more inventions may reside in any combination or subcombination of the device elements or method steps described below or elsewhere in this specification. The inventors do not waive or disclaim their 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] Calcium sulfate scale formation 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 processing equipment that reduce or avoid lime softening, the generation of sodium sulfate, or both. Such methods and processing equipment can result in cost savings compared to conventional lime softening methods. Removing calcium sulfate from its supersaturated solution using conventional coagulation / flocculation methods and equipment has proven difficult because high sulfate concentrations 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 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 as a mixture of gypsum seed fines in aqueous solution to a separator.

[0008] 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.

[0009] In a specific example, the present disclosure provides a separation process comprising: receiving a supersaturated aqueous solution of CaSO4 into a first continuously 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 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 from about 1:1 to about 2:1. Agitation in each CSTR is independently provided by a pitched blade paddle at a speed of from about 50 rpm to about 200 rpm, wherein each paddle independently has a width (d), wherein the d:D ratio is from about 1:3 to about 1:2. The residence time in each CSTR independently ranges from about 2 to about 10 minutes. Operating the multi-stage CSTR under these conditions results in gypsum seed crystals of the desired size and concentration. In the context of this 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.

[0010] The present disclosure also 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. A coagulant source is in fluid communication with the precipitation reactor, and the precipitation reactor is in fluid communication with a separator to provide the gypsum seed fines mixture in aqueous solution to the separator. The separator can be, for example, a solid / liquid separator as described herein.

[0011] 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. 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 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 source of coagulant 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 gypsum seed fines in aqueous solution to the separator. The separator can be, for example, a solid / liquid separator as discussed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0013] Figure 1 is a process flow diagram of an exemplary solid / liquid separator according to the present disclosure.

[0014] 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.

[0015] Figure 3 is a graph showing the turbidity of an effluent stream from a solid-liquid separation process according to the present disclosure.

[0016] Figure 4 is a process flow diagram of an exemplary precipitation reactor according to the present disclosure.

[0017] Figure 5 is a process flow diagram of an exemplary apparatus according to the present disclosure.

[0018] Figure 6 is a graph showing the size distribution of particles produced in a method according to the present disclosure.

[0019] Figure 7 is a graph showing the size distribution of particles produced in a comparative process.

[0020] Figure 8 is a diagram of an exemplary precipitation reactor according to the present disclosure.

[0021] Figure 9 is a diagram of another exemplary precipitation reactor according to the present disclosure.

[0022] Figure 10 is a process flow diagram of an exemplary apparatus according to the present disclosure. DETAILED DESCRIPTION

[0023] Seeded slurry technology (SST) can be used in wastewater concentration processes such as membrane filtration, electrodialysis, or thermal crystallization. SST reduces scaling from supersaturated components (e.g., CaSO4) as the water is concentrated. However, CaSO4 can precipitate and / or harden, potentially blocking one or more water treatment systems (e.g., tanks, pipes, and pumps) in the water concentration process or downstream filtration process.

[0024] 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 seed crystals into non-flocculated gypsum seed fines, and transferring the fines to a reactor. The aqueous solution from the reactor can be directly received into a settling tank.

[0025] 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 it receives fluid A 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.

[0026] 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 the gypsum seeds that can be returned to the reactor and used to precipitate additional calcium sulfate (e.g., from a supersaturated solution of CaSO4).

[0027] 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 it enters the separator.

[0028] The coagulant may 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 may be 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, for example, before the addition of an anionic flocculant; or (d) any combination thereof. The coagulant may 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, sufficient FeCl3 may be added to provide a concentration in the sedimentation tank of at least 10 ppm. In some specific examples, the final concentration of FeCl3 is at least 30 ppm.

[0029] The flocs may be agitated in the settling tank to prevent the gypsum slurry from hardening. For example, the flocs may be agitated at 50 rpm or less using 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% by weight.

[0030] 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.

[0031] 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.

[0032] The gypsum seeds can be returned to the reactor and used to precipitate additional calcium sulfate. However, flocculated gypsum seeds do not precipitate calcium sulfate as effectively because anionic flocculants inhibit this precipitation. By exposing the flocculated material to shear stress, the flocculated gypsum seeds are converted into non-flocculated gypsum seed fines.

[0033] The bottom of the settling tank can be fluidically connected to the reactor, and a pump can be positioned therebetween 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, operating at a speed of at least 500 rpm. Such a pump can provide sufficient shear force to convert the flocculated gypsum seed crystals into non-flocculated fine particles. Compared to closed impeller pumps, such pumps are also more tolerant of concentrated slurries, thereby reducing the likelihood of slurry buildup within the pump. The method can also include flushing the pump with clean water whenever the pump is stopped to reduce the likelihood of gypsum seed crystals settling and hardening within the pump.

[0034] For example, if the pump does not provide shearing force, shearing 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.

[0035] 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 %.

[0036] 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.

[0037] 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-to-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 an anionic polymer flocculant, they retain a desired moisture content even after being removed from the aqueous solution for a period of time. This period of time can be, for example, one, two, or three months. In the context of the present disclosure, a desired moisture content will be understood as a moisture content that prevents the CaSO from hardening so that the flocculated particles can disperse into water after a period of time.

[0038] 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 crystals and fine particles in aqueous solution from a precipitation reactor; a first fluid outlet in the settling tank for discharging effluent with reduced turbidity; a second fluid outlet in the settling tank for discharging flocculated gypsum slurry; and an optional agitator. The second fluid outlet can be located in the bottom third of the settling tank. The settling tank can also be referred to as a sedimentation tank or clarifier. The settling tank can directly receive fluid from the precipitation reactor.

[0039] In one embodiment, 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; and an agitator having blades having a diameter of about ¾ to about 5 / 6 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 speed of about 10 to about 40 rpm. A separator constructed in this manner and operated under these conditions can maintain a stable suspension of floc slurry at a concentration of about 15% to about 35% by weight, with a well-defined boundary between the floc slurry suspension and the supernatant.

[0040] 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 a shear stress applicator disposed in the liquid conduit connecting the second fluid outlet to the inlet in the reactor.

[0041] 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.

[0042] The shear stress applicator can be a centrifugal pump with an open impeller or a mechanical stirrer. The mechanical stirrer can be, for example, at the inlet of the reactor, at the outlet of the sedimentation tank, or in the immediate vicinity of the pump.

[0043] The solid / liquid separator can be configured to discharge the turbidity-reduced effluent 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.

[0044] The present disclosure also provides an apparatus comprising the above-described 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 to a fluid inlet of a settling tank. The liquid conduit connecting the second fluid outlet to the reactor fluidly connects the second fluid outlet to a fluid inlet in the reactor.

[0045] The apparatus may further comprise 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.

[0046] 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 settling 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.

[0047] 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 settling tank (116) as non-flocculated gypsum seed fines (130).

[0048] Example 1

[0049] Established the use Figure 1 The process was pilot tested 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.

[0050]

[0051]

[0052] Table 1

[0053] The waste stream is first concentrated at least three-fold 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 of FeCl₃ is added at the same location based on the volume of the influent. Gypsum seeds are dispersed in the reactor, and the supersaturated CaSO₄ precipitates out. The gypsum seed concentration is maintained between 2-5% (vol / vol) by recovering the seed slurry from the settling tank.

[0054] In the outlet pipe of the precipitation reactor, an additional 10 ppm of FeCl3 was added to adjust the surface properties of the seed crystals. The effluent was transferred to the downstream sedimentation tank using a pump. Immediately before pumping, a flocculant was added to the effluent at a level of 0.5 ppm.

[0055] Flocs form and settle in the settling tank. Gently stirred by mechanical paddles, the concentrated slurry never grows higher than one-quarter of the tank's height. The supernatant from the top of the settling tank is sent to a downstream membrane treatment unit. Some of the slurry from the bottom of the settling tank is pumped back to the precipitation reactor via an open impeller operating at approximately 800 RPM, which provides sufficient shear stress to reduce the flocculant's flocculating activity.

[0056] Approximately 1-2% of the recycled slurry is discharged into the blowdown stream. This amount in the blowdown stream is set based on a 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 from the sedimentation tank was measured to be 100% to 110% of the saturation level (see Table 2).

[0057] 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％

[0058] Table 2

[0059] The concentration of gypsum seeds in the precipitation reactor and sedimentation tank was tracked. The results are shown in Figure 2 The test ran for 500 hours with gypsum seed concentrations in the sedimentation tank reaching up to 30% vol / vol. No pumps or pipes became clogged by the gypsum slurry. Comparable systems using closed impellers, rather than open impellers, were unable to operate for the same length of time due to pump blockage by the concentrated gypsum slurry.

[0060] The flocculated gypsum seed crystals were found to disperse rapidly in water even after being dried for several weeks. The flocculated gypsum seed crystals were also found to be able to 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 disperse easily, have a reduced tendency to form scale in pipes, 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.

[0061] The chemical composition of the resulting gypsum seed crystals was analyzed using X-ray fluorescence. In addition to CaSO₄ (the main component, 95 wt%), SrSO₄ also coprecipitated from the solution (2 wt%). This demonstrates that this method can simultaneously remove other sparingly soluble ions, further reducing the risk of scaling in downstream membrane filtration. The remaining components were 1 wt% Fe(OH)₃ and 2 wt% Na₂SO₄.

[0062] The supernatant stream from the sedimentation tank was pale yellow and contributed to the turbidity reading even without any particles. Therefore, turbidity was not tracked during this pilot test. However, in another test 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 Figure 1, the turbidity measured throughout the entire cycle was 3 NTU or less. This water quality may be suitable for downstream membranes. For example, if colloids in the water are not a major issue, water with a turbidity of <3 NTU can be fed directly to the nanofiltration membrane process without first passing the water through an ultrafiltration unit.

[0063] In another aspect, the present disclosure provides a seed-assisted precipitation process that can be performed at ambient temperature, for example, about 18 to about 25° C. The process can eliminate lime softening, for example, by eliminating the addition of calcium hydroxide. The process 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 process 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 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.

[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 paddle disposed 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 disposed are selected to produce gypsum seed fines having 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 gypsum seed fines. The precipitation reactor can be in fluid communication with a separator to provide the gypsum seed fines mixture in aqueous solution to the separator. As described above, the separator can be a separator according to the present invention.

[0065] The coagulant is used to neutralize the amount of antiscalant typically present in the supersaturated solution of CaSO4, sufficient to cause the CaSO4 to precipitate on the gypsum seed particles. The coagulant can be 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 a concentration of 30 to 50 ppm.

[0066] 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 from about 1:1 to about 2:1, for example, from about 1:1 to about 1.5:1. Agitation in each CSTR can independently be performed with a pitched blade paddle at a speed of from about 50 rpm to about 200 rpm, for example, from about 120 rpm to about 150 rpm; wherein each paddle independently has a width (d), wherein the d:D ratio is from about 1:3 to about 1:2. The flow rate and size of the CSTRs can be provided so that the residence time in each CSTR is independently from about 2 minutes to about 10 minutes, for example, from 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 propeller with larger blades can be run at a lower rpm than a pitched blade propeller with smaller blades.

[0067] Smaller d:D ratios increase shearing, while larger d:D ratios increase mixing. Shearing and mixing control the size of the gypsum seed crystals. Vigorous agitation associated with increased shear breaks up larger particles and produces smaller ones. Increased mixing enhances crystallization from 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.

[0068] The concentration of gypsum seeds can be controlled by removing gypsum seeds from the reactor and optionally adding gypsum seeds to the reactor. Gypsum seeds can be added by recycling removed gypsum seeds back into the reactor. Higher concentrations of gypsum seeds result in faster crystallization rates, but also increase the operating load of any recirculation units. Higher concentrations also increase the risk of scale formation 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% to about 10% by weight, for example, about 1% to about 7% by weight.

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

[0070] Figure 4 A process flow diagram of an exemplary precipitation reactor according to the present invention is shown. In the precipitation reactor (210), a multi-stage CSTR (212) consists of three stages (212a, 212b, and 212c). The multi-stage CSTR (212) receives a supersaturated solution of CaSO4 (214) 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 paddles (218a, 218b, 218c). The dimensions of all three stages and their respective paddles meet the above-mentioned H:D ratio and d:D ratio. The final stage produces a mixture of gypsum fines (220). The multi-stage CSTR (212) includes a feed (222) for gypsum seed particles.

[0071] 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.

[0072] 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 an apparatus.

[0073] Figure 5 A process flow diagram of an exemplary apparatus according to the present disclosure is shown. Apparatus (310) includes the precipitation reactor (210) described above 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). Pitched blade impellers are not shown. Anionic flocculant (122) is optional.

[0074] Example 2

[0075] Established the use Figure 5A pilot test of the process described, but without adding any flocculant to the sedimentation tank, was conducted to treat a supersaturated solution of CaSO₄ generated by a nanofiltration process for treating coal mine drainage. The pilot test used a multi-stage CSTR consisting of three precipitation reactors. The concentration of gypsum seeds in each reactor was approximately 3 to 5% by weight. The reactors were stirred using a mechanical stirrer at a stirring speed of approximately 100 to 140 rpm. The width of the metal paddles of the stirrers in the reactors was approximately half the diameter of the reactors. The reactors were 1000 mm high and 600 mm in diameter, and the diameter of the circle formed by the paddles was 300 mm. This corresponds to an H:D ratio of 5:3 and a d:D ratio of 1:2. The total residence time in the three reactors in series was 30 to 40 minutes. The average diameter of the gypsum seeds in the reactors was approximately 25 μm.

[0076] The gypsum seeds were 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 had blades with a diameter of approximately 500 mm and was operated at approximately 20 to 40 RPM. The flocs of larger gypsum seeds obtained from the bottom of the solid-liquid separator were sheared and recirculated and dispersed into the first reaction tank under strong agitation by a transfer pump having an open impeller operating at approximately 800 RPM.

[0077] Stabilizers, also known as antiscalants, are added to the supersaturated effluent produced by the nanofiltration process to reduce or avoid scaling. FeCl3 is added as a coagulant to the influent of a multi-stage CSTR in an amount of 30 to 50 ppm 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 a comparative process was also measured. This comparative process used only one of the stages of the CSTR settling tank described above and was run with a residence time of 30 to 40 minutes, but was otherwise identical to the pilot plant described above. As described above, the exemplary process according to the present disclosure produced particles with an average size of approximately 25 μm. The comparative process produced particles with an average size of approximately 80 μm. The size distributions were respectively Figure 6 and 7 Shown in.

[0079] The water quality of the CSTR influent and effluent streams was analyzed, and the saturation of CaSO4 was calculated using the following equation:

[0080]

[0081] 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.

[0082] 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 effluent 435 2.4 63.2 630 6300 10.4 8.6 16180 49.4

[0083] Table 3

[0084] 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.The supersaturation level of about 190% was observed to cause fouling downstream of the comparative reactor.

[0085] In another aspect of the present disclosure, a precipitation method is provided. The method includes introducing a supersaturated aqueous solution of CaSO into a multi-stage continuous stirred tank reactor (CSTR), wherein the reactor stages are vertically stacked, and wherein the internal outflow port from one stage substantially corresponds to the internal feed port of a subsequent downstream stage. The method includes flowing the solution vertically upward through the multi-stage CSTR to produce a mixture of gypsum seed crystals and fine particles in aqueous solution; and transferring the mixture of gypsum seed crystals and fine particles in aqueous solution to a separator.

[0086] The present disclosure also provides a precipitation reactor comprising a multi-stage continuous stirred tank reactor (CSTR), wherein the reactor stages are connected in series, wherein the internal outflow port from one stage corresponds to the internal feed port of a 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 the 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.

[0087] 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.

[0088] 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 fluid conduits, such as pipes, connecting different stages of the reactor.

[0089] The multi-stage CSTR can 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.

[0090] 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 port of the second stage of the reactor, and the outflow port of the second stage of the reactor corresponds to the feed port of the third stage of the reactor.

[0091] The orifice can be sized to prevent or reduce backmixing, which occurs when fluid from one stage flows downward through the inlet into a 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.

[0092] In this vertical multi-stage CSTR, each stage can be agitated using an agitator on the same single agitator shaft.

[0093] 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.

[0094] 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, 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 source of gypsum seed crystals (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.

[0095] Figure 9 Another exemplary precipitation reactor according to the present disclosure is shown. Reactor (510) and Figure 8The reactor shown in FIG is similar to that shown in FIG, except that the three stages do not share a common stirring shaft. The vertical reactor (512) still receives a supersaturated solution of CaSO4 (514) 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.

[0096] 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 accepting 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 stacked vertically, 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 having 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.

[0097] In an example of a combined precipitation reactor and solid / liquid separator, an apparatus includes a multi-stage continuously stirred tank reactor (CSTR) wherein the reactor stages are connected in series, wherein the internal outflow port from one stage corresponds to the internal feed port of a 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 disposed in at least one stage, 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. 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 the gypsum seed fines mixture in aqueous solution from the multi-stage CSTR. The settling tank includes a first fluid outlet for discharging a turbidity-reduced effluent; and a second fluid outlet for discharging a flocculated gypsum slurry. The apparatus includes a source of anionic flocculant in fluid communication with the settling tank. There is a liquid conduit connecting the second fluid outlet to the multi-stage CSTR; and a shear stress applicator is disposed in the liquid conduit.

[0098] As described above, the stages of a multi-stage CSTR can be stacked vertically.The features of the precipitation reactor and solid / liquid separator are discussed in more detail above.

[0099] 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 solid / liquid separator (110) described above. The gypsum fines 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 centrifugal pump (128) correspond to the gypsum seed crystals (422) received by the CSTR (412). Anionic flocculant (122) is optional.

[0100] Example 3

[0101] Established the use Figure 8 The reactor is shown pilot-scaled to treat a supersaturated solution of CaSO4 produced by a nanofiltration process for treating coal mine drainage.

[0102] The vertical CSTR comprises three stages. The height and diameter of each stage have an H:D ratio of approximately 1:1. The width (d) of the paddles is such that the d:D ratio is approximately 1:3. In this pilot plant, a supersaturated solution of CaSO4 (200% saturation) is received at a flow rate of 500 L / h into the bottom, first stage. Gypsum seed fines at a concentration of 20-35 wt% are added at a flow rate of 100 L / h. Sufficient FeCl3 is added to the feed stream of the first stage to produce a concentration of 30-40 ppm. The pitched blade paddles are stirred at a rate of approximately 110 rpm. The final stage produces a mixture of gypsum fines. 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 settling was operated so that the total residence time was approximately 28 minutes.

[0103]

[0104] Table 4

[0105] The effluent from the nanofiltration process includes an antiscalant to reduce or avoid scaling. The supersaturation level of the nanofiltration effluent is approximately 140% of the saturation level, with calcium at approximately 1800 ppm (recorded as CaCO3). The effluent generated by the final stage of the vertical multi-stage CSTR reaches approximately 100% of the saturation level, with calcium at approximately 1200 ppm (recorded as CaCO3), as shown in FIG.

[0106] As shown in Table 5.

[0107]

[0108] Table 5

[0109] In another aspect, the present disclosure provides a method for removing scale from: (a) process equipment, such as a CaSO precipitation reactor, a solid / liquid separator, or a fluid conduit, or (b) a portion of process equipment. The method comprises vibrating or deforming the process equipment or a portion of the process equipment to dislodge at least some of the scale. The vibrated process equipment or portion 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.

[0110] The present disclosure also provides process equipment or a portion of process equipment, wherein the process equipment or portion is at least partially made of or coated with a low-friction, preferably hydrophobic material. The low-friction material is positioned to be exposed to a supersaturated solution of CaSO4. The low-friction material can be positioned to be sufficiently vibrated or deformed to dislodge at least some scale present on the low-friction material.

[0111] Parts of the process equipment that may be vibrated or deformed may be side walls, baffles, liquid conduits within the reactor or stirring blades.

[0112] The low friction material may be polyethylene (PE), polypropylene (PP) or polytetrafluoroethylene (PTFE).

[0113] The vibrations may be at a frequency of approximately 0.1 to 10 Hz and / or may include moving the low friction material at an amplitude of approximately 1 to approximately 5 mm.

[0114] Any of the process equipment discussed above can be made of or coated with a low-friction material, and at least some scale can 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 can include a slag discharge port, and dislodged scale can be removed from the reactor through the slag discharge port.

[0115] 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 essential. Therefore, what has been described is merely illustrative of the application of the described embodiments, and many modifications and variations are possible in light of the above teachings.

[0116] Since the above description provides examples, it will be understood that those skilled in the art may make modifications and changes to 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 consistent with the description as a whole.

Claims

1. A seed-assisted precipitation method comprising: The 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 transferring the gypsum seed fines as a mixture of gypsum seed fines in aqueous solution to a separator, wherein each CSTR independently has a height (H) and a diameter (D), wherein the ratio of H:D is between 1:1 and 2:1; wherein agitation in each CSTR is independently provided by a pitched blade paddle at a speed between 50 rpm and 200 rpm, and wherein each paddle independently has a width (d), wherein the ratio of d:D is between 1:3 and 1:2; and wherein the residence time in each CSTR is independently from 2 to 10 minutes.

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

3. The precipitation process of claim 2, wherein the process comprises passing the mixture through at least three CSTRs.

4. The precipitation process according to claim 2 or 3, 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 20 μm to 40 μm.

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

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

10. The precipitation method according to claim 9, wherein Gypsum seeds are added to the reactor by transferring them from the separator back to the reactor.

11. The precipitation method of claim 9, wherein the transferred gypsum seeds are non-flocculated gypsum seeds.

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

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

14. The precipitation method according to claim 13, wherein The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

15. The precipitation method according to claim 14, wherein The iron or aluminum based coagulant is FeCl3, Fe2(SO4)3, polyferric sulfate or polyaluminum chloride.

16. The precipitation process of any one of claims 1 to 3, wherein the coagulant is added in an amount sufficient to destabilize any antifouling agent in the reactor.

17. The precipitation process according to any one of claims 1 to 3, wherein the process excludes the addition of calcium hydroxide.

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

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

20. A precipitation reactor comprising: 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 dimensions of the paddle and the dimensions 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.

21. The precipitation reactor of claim 20, wherein each CSTR includes a paddle disposed therein.

22. A precipitation 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 1:1 to 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 paddle disposed in at least a first one of the plurality of CSTRs, wherein each paddle independently has a width (d), wherein the ratio of d:D is from 1:3 to 1:2; and a coagulant source 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 gypsum seeds and fine particles in aqueous solution to the separator.

23. The precipitation reactor of claim 22, wherein the H:D ratio in at least one of the plurality of CSTRs is from 1:1 to 1.5:

1.

24. The precipitation reactor of any one of claims 21 to 23, wherein the reactor comprises at least three CSTRs.

25. The precipitation reactor of any one of claims 20 to 23, further comprising a source of gypsum seed crystals in fluid communication with the reactor.

26. The precipitation reactor according to any one of claims 20 to 23, wherein The reactor includes a fluid inlet in communication with the separator for receiving gypsum seeds from the separator.

27. The precipitation reactor of claim 26, wherein the received gypsum seeds are non-flocculated gypsum seeds.

28. The precipitation reactor according to any one of claims 20 to 23, wherein The coagulant is a trivalent metal salt-based coagulant.

29. The precipitation reactor according to claim 28, wherein The trivalent metal salt based coagulant is an iron or aluminum based coagulant.

30. The precipitation reactor according to claim 29, wherein The iron or aluminum based coagulant is FeCl3, FeSO4, polyferric sulfate or polyaluminum chloride.

31. The precipitation reactor of any one of claims 20 to 23, further comprising a pH sensor for measuring the pH of the liquid in the reactor and being in fluid communication with the reactor: (a) an acid source, (b) an alkali source, or (c) both.

32. The precipitation reactor according to any one of claims 20 to 23, wherein The reactor includes a fluid inlet for receiving a supersaturated aqueous solution of CaSO 4 from a membrane separation unit.

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