Detailed Description
Seed Slurry Technology (SST) can be used in wastewater concentration processes such as membrane filtration, electrodialysis, or thermal crystallization. When water is concentrated, SST reduces fouling by supersaturated components (e.g., caSO4). CaSO4 may precipitate and/or harden, blocking one or more water treatment systems (e.g., tanks, pipes, and pumps) during the water concentration process or the downstream filtration process.
In one aspect, the present disclosure provides a separation process comprising receiving a gypsum seed particle mixture in an aqueous solution from a reactor, adding an anionic flocculant and optionally a coagulant to the aqueous solution, agglomerating the gypsum seed particles and flocculant into a flocculate, separating the mixture into an effluent of reduced turbidity 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 to non-flocculated gypsum seed particles and transferring the particles to the reactor. The aqueous solution from the reactor may be directly received into the deposition tank.
In the context of the present disclosure, it should be understood that the phrases "accept from [ X ] and" accept from [ X ] A "refer to direct and indirect acceptance. For example, if reactor Z is disclosed as "receiving fluid A from reactor X," it is to be understood that reactor Z may be (i) coupled directly to reactor X such that fluid A is received directly from reactor Z, or (ii) coupled indirectly to reactor X such that process equipment Y receives fluid A from reactor X and reactor Z receives fluid A from equipment Y.
As described above, the separation process controls the transition of gypsum seeds between (i) a flocculated fraction that may be separated into a turbidity reduced effluent and a flocculated gypsum slurry and (ii) gypsum seeds that may be returned to the reactor and used to precipitate additional calcium sulfate (e.g., from a supersaturated solution of CaSO4) in the non-flocculated fraction.
The anionic flocculant may be a polyacrylamide flocculant. The anionic flocculant may be a low charge density, high molecular weight polymeric flocculant. Anionic flocculants may exhibit reduced flocculation activity when exposed to high shear or excessive agitation. One example of a suitable anionic flocculant is PolyFlocTM AP1100. The flocculant may be added before the aqueous solution is received in the separator, allowing the flocculant to be thoroughly mixed into the solution before the solution enters the separator.
The coagulant may be a trivalent metal salt based coagulant, such as an iron or aluminum based coagulant. Specific examples of such coagulants include FeCl3、Fe2(SO4)3, ferric polysulfate 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, prior to the addition of the anionic flocculant, or (d) any combination thereof. The coagulant may be added in an amount sufficient to precipitate at least a portion of any anti-fouling agent present in the aqueous solution received from the reactor. For example, feCl3 may be added in a sufficient amount to provide a concentration in the deposition tank of at least 10ppm. In some particular examples, the final concentration of FeCl3 is at least 30ppm.
The flocs may be stirred in a settling tank to prevent the gypsum slurry from hardening. For example, the flocs may be stirred with a paddle at 50rpm or less, wherein the diameter of the paddle is about 1/2 to about 3/4 of the diameter of the settling tank. The bed height of settled flocs may be about 1/5 to about 1/3 of the height of the settling tank. The local concentration of settled gypsum seed flocs at the bottom of the settling tank may be from about 8 to about 25 weight percent.
Slurries comprising flocs of gypsum seeds produced according to the method have 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 coagulants and flocculants used to flocculate gypsum seeds act as lubricants and wetting agents to inhibit hardening of the gypsum seeds.
The turbidity reduced effluent produced according to the process can be a clarified effluent having a turbidity of less than 3NTU (nephelometric turbidity units), or from about 3 to about 5NTU. When the turbidity of the clarified effluent is less than 3NTU, the method may further comprise nanofiltration of the clarified effluent without prior ultrafiltration. The effluent with reduced turbidity may be treated with a lamella clarifier (also known as an inclined plate settler), for example when turbidity is greater than 3 NTU.
Gypsum seeds can be returned to the reactor and used to precipitate additional calcium sulfate. The flocculated gypsum seeds are not equally effective in precipitating calcium sulfate because anionic flocs inhibit such precipitation. The flocculated gypsum seed is converted to non-flocculated gypsum seed fines by exposing the flocculated material to shear stress.
The bottom of the settling tank may be fluidly connected to the reactor and a pump may be disposed therebetween to transfer concentrated gypsum seed floe from the settling tank into the reactor. The pump may be a centrifugal pump with an open impeller, for example operating at a rate of at least 500 rpm. Such pumps can provide sufficient shear force to convert flocculated gypsum seeds into non-flocculated fines. Such pumps are also more tolerant of concentrated slurry than closed impeller pumps, thereby reducing the likelihood of forming a slurry pile within the pump. The method may include flushing the pump with fresh water whenever the pump is stopped to reduce the likelihood of gypsum seeds depositing and hardening within the pump.
For example, if the pump does not provide shear force, the shear stress may be provided by a mechanical stirrer disposed between the deposition tank and the reactor. The mechanical stirrer may be, for example, at the inlet of the reactor, at the outlet of the deposition tank, or in close proximity to the pump.
A sufficient amount of gypsum seed may be returned to the reactor to maintain the concentration of gypsum seed fines in the reactor in the range of about 0.5 wt% to about 10 wt%, such as in the range of about 1 wt% to about 7 wt%.
The method may further comprise maintaining the aqueous solution in the reactor at a pH of about 4 to about 10, such as a pH of about 6 to about 8, such as a pH of about 6.5 to about 7.
The reactor may be operated under conditions that produce gypsum seed fines having 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 CaSO4 precipitation particularly effective for removal of CaSO4 from its supersaturated solution. The authors also found that when particles of this size were flocculated with an anionic polymeric flocculant, the desired moisture content was maintained even after a period of time of removal from the aqueous solution. The period of time may 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 CaSO4 from hardening so that the flocculated particles can disperse into water after a period of time.
The present disclosure also provides a solid/liquid separator. The separator includes a settling tank, a fluid inlet in the settling tank for receiving the gypsum seed particle mixture in the aqueous solution from the precipitation reactor, a first fluid outlet in the settling tank for discharging effluent of reduced turbidity, a second fluid outlet in the settling tank for discharging flocculated gypsum slurry, and an optional agitator. The second fluid outlet may be located in the bottom third of the settler. The settling tank may also be referred to as a settling tank or clarifier. The settling tank may directly receive fluid from the precipitation reactor.
In one example, the separator comprises a cylindrical settling tank having a height to diameter ratio of about 1:1 to about 8:1, preferably about 2:1 to 5:1, and a stirrer having blades having a diameter of about 3/4 to about 5/6 of the diameter of the tank, wherein the blades are located about 1 to about 10cm, preferably about 2 to about 5cm, above the bottom of the tank. The stirrer may be operated at a rate of about 10 to about 40 rpm. The separator constructed and operated under these conditions can maintain a stable suspension of the floc slurry at a concentration of about 15 wt% to about 35 wt% with a well-defined boundary between the suspension of the floc slurry and the supernatant.
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 the inlet in the reactor.
The 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.
The shear stress applicator may be a centrifugal pump with an open impeller or a mechanical stirrer. The mechanical stirrer may be, for example, at the inlet of the reactor, at the outlet of the deposition tank, or in close proximity to the pump.
The solid/liquid separator may 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 apparatus. For example, the effluent may be transferred directly to a sand filtration pretreatment unit of a nanofiltration unit.
The present disclosure also provides an apparatus comprising 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 the gypsum seed particle mixture in the aqueous solution, and the apparatus includes a liquid conduit connecting the fluid outlet of the reactor and the fluid inlet of the settling tank. A liquid conduit connecting the second fluid outlet to the reactor fluidly connects the second fluid outlet to a fluid inlet in the reactor.
The apparatus may additionally include one or more of a gypsum seed source 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.
One or more sources of one or more coagulants may each independently be in fluid communication with (a) a reactor, (b) a liquid conduit connecting a fluid outlet of the reactor and a fluid inlet of a settling tank, or (c) both. Each coagulant may independently be a trivalent metal salt based coagulant, such as an iron or aluminum based coagulant, e.g., feCl3、FeSO4, ferric polysulfate, or polyaluminum chloride. As discussed above, the coagulant may be added in an amount sufficient to precipitate at least a portion of any anti-fouling agent present in the aqueous solution received from the reactor.
Fig. 1 shows a process flow diagram of an exemplary solid/liquid separator (110) according to the present disclosure in combination with a precipitation reactor (112). A precipitation reactor (112) provides an aqueous mixture (114) of gypsum seed particles that is received into a precipitation tank (116). A coagulant (118) is added to the reactor (112). A pH adjuster (120) may be added to the reactor (112) to maintain 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 reactor (112) with the settling tank (116). The settling tank (116) produces a flocculated gypsum slurry (124) and a turbidity reduced effluent (126). 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 gypsum seed fines (130).
Example 1
A pilot plant using the process shown in fig. 1 was established to treat a concentrated waste stream from a coal-to-chemical production process. The concentrated reject stream is derived from the reverse osmosis treatment of coal chemical wastewater. Typical water quality is shown in table 1.
TABLE 1
The waste stream is first concentrated at least 3-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 was adjusted to 6.5-7 and again an amount of 30ppm FeCl3 based on the volume of the influent was added at the same location. Gypsum seeds are dispersed in the reactor and supersaturated CaSO4 precipitates out. The concentration of gypsum seeds is maintained between 2-5% (v/v) by recovering the seed slurry from the settling tank.
In the outlet tube of the precipitation reactor, an additional amount of 10ppm FeCl3 was added to adjust the surface properties of the seed crystals. The effluent is transferred to a downstream settling tank using a pump. Immediately prior to pumping, flocculant was added to the effluent in an amount of 0.5 ppm.
Flocs form and settle in the settling tank. The height of the concentrated slurry never increases to 1/4 of the height of the tank under slow agitation by a mechanical paddle. The supernatant from the top of the settling tank is sent to a downstream membrane processing unit. Some of the slurry from the bottom of the settling tank was pumped back to the settling reactor by an open impeller operating at about 800RPM, which provided sufficient shear stress to reduce the flocculation activity of the flocculant.
About 1-2% of the recycled slurry is discharged into the blowdown stream. The amount in the blowdown stream is set based on the mass balance of the overall system. The removal of gypsum seeds from the system in the blowdown stream avoids potential seed aging problems. But 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 influent waste stream. CaSO4 in the influent reject stream was calculated to be 113% to 140% of the saturation level and CaSO4 in the effluent of the settling tank was measured to be 100% to 110% of the saturation level (see table 2).
| 11 Months and 30 days | 12 Months and 4 days | 12 Months 7 days | 12 Months and 10 days |
| An inlet | 140.20% | 123.50% | 113.30% | 136.40% |
| An outlet | 96.20% | 103.90% | 103.50% | 100.80% |
TABLE 2
The concentration of gypsum seeds in the precipitation reactor and the precipitation tank was tracked. The results are shown in fig. 2. The test was run for 500 hours and the concentration of gypsum seeds in the settling tank was as high as 30% vol/vol. No pump or pipe is blocked by the gypsum slurry. Other comparable systems using closed impellers rather than open impellers do not run for the same length of time due to the pump body being plugged with concentrated gypsum slurry.
The flocculated gypsum seeds were found to disperse rapidly into water even after several weeks of drying. It was also found that flocculated gypsum seeds could be stored for months without hardening. Without wishing to be bound by theory, the authors of the present disclosure believe that the gypsum seed particles that retain moisture and are readily dispersible have a reduced tendency to form scale in the pipe, and that the polyacrylamide flocculant added to the inflow of the settling tank surrounds the gypsum seed and provides these desirable properties. The polyacrylamide flocculant may act as a wetting agent in the seed and accelerate the dispersion of the gypsum seed when added to water.
The chemical composition of the gypsum seed produced was analyzed using X-ray fluorescence. In addition to CaSO4 (95 wt% of the main component), srSO4 was co-precipitated from solution (2 wt%). This suggests that other sparingly soluble ions can be removed simultaneously in this process, further reducing the risk of fouling in downstream membrane filtration. The remaining components were 1 wt% Fe (OH)3 and 2 wt% Na2SO4.
The supernatant flow from the settling tank was pale yellow and contributed to turbidity readings even without any particles. Thus, turbidity during this pilot run was not tracked. But in another test area where the same method was used to treat mine drainage, the supernatant was colorless and the turbidity of the effluent was tracked. As shown in fig. 3, the turbidity measured throughout the cycle was 3NTU or less. This water quality may be suitable for downstream membranes. For example, where colloid in water is not a major issue, for example, water having <3NTU may be fed directly to the nanofiltration membrane process without first passing the water through the ultrafiltration unit.
In another aspect, the present disclosure provides a seed-assisted precipitation process, which may be performed at ambient temperature, e.g., about 18 to about 25 ℃. The method may exclude lime softening, for example by excluding the addition of calcium hydroxide. The process comprises receiving a supersaturated aqueous solution of CaSO4 into a multistage Continuous Stirred Tank Reactor (CSTR), and adding a coagulant to either (a) the feed stream to the multistage CSTR, or (b) the multistage CSTR. The method further includes flowing the solution through a multistage CSTR and operating the seed assisted precipitation and multistage CSTR under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. The gypsum seed particles are transferred to the separator as a gypsum seed particle mixture in an aqueous solution.
The present disclosure also provides a precipitation reactor. The precipitation reactor comprises a multistage Continuous Stirred Tank Reactor (CSTR). The precipitation reactor is in fluid communication with a source of supersaturated aqueous solution of CaSO4. The precipitation reactor comprises at least one inclined blade paddle arranged in at least one stage of the multistage CSTR. The size of the paddles and the size of the vessel in which they are disposed are selected to produce gypsum seed particles 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 may be in fluid communication with a source of gypsum seed fines. The precipitation reactor may be in fluid communication with the separator to provide the gypsum seed particulate mixture in the aqueous solution to the separator. As mentioned above, the separator may be a separator according to the invention.
The coagulant is used to neutralize a sufficient amount of the anti-scaling agent that is normally present in supersaturated solutions of CaSO4 to precipitate CaSO4 on gypsum seed particles. The coagulant may be a trivalent metal salt based coagulant, such as an iron or aluminum based coagulant. Specific examples of such coagulants include FeCl3、Fe2(SO4)3, ferric polysulfate or polyaluminum chloride. FeCl3 can be added at the inlet of the reactor to result in a concentration of 30 to 50 ppm.
Seed assisted precipitation and multistage CSTR are operated under shear conditions to produce the desired gypsum seed fines. In particular examples of such shear conditions, each CSTR may independently have a height (H) and a diameter (D), wherein the H to D ratio is from about 1:1 to about 2:1, such as from about 1:1 to about 1.5:1. Agitation in each CSTR can be independently performed with a pitched blade paddle at a speed of from about 50rpm to about 200rpm, such as from about 120rpm to about 150rpm, wherein each paddle independently has a width (D), wherein the ratio of D to D is from about 1:3 to about 1:2. The flow rates and dimensions of the CSTRs may be provided such 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% (supersaturation) to less than about 120%, for example, about 100% (saturation), thereby reducing the risk of scaling in downstream processes and equipment. A pitched blade with a larger blade may operate at a lower rpm than a pitched blade with a smaller blade.
A smaller dD ratio has an increased shearing effect, while a larger dD ratio has an increased mixing effect. Shearing and mixing control the size of the gypsum seeds. Vigorous agitation associated with increased shear breaks up larger particles and produces smaller particles. Increasing the mixing enhances crystallization of the supersaturated CaSO4 solution 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 shear and mixing.
The concentration of 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 seed can be added by recycling the removed gypsum seed back to the reactor. Higher concentrations of gypsum seeds result in faster crystallization rates, but also increase the operating load of any recycle unit. Higher concentrations also increase the risk of scale formation downstream of the reactor. The process may be operated under conditions that result in a seed concentration in the reactor in the range of about 0.5 wt% to about 10wt%, such as about 1 wt% to about 7 wt%.
The multi-stage CSTR may comprise at least two, e.g., at least three, stages. The total residence time in the multi-stage CSTR may be about 8 to about 40 minutes. The authors of the present disclosure have determined that the precipitation rate in a multistage CSTR is faster than in a single stage CSTR of equivalent total volume.
Fig. 4 shows a process flow diagram of an exemplary precipitation reactor according to the present invention. In the apparatus (210), the multi-stage CSTR (212) is comprised of three stages (212 a, 212b and 212 c). The multi-stage CSTR (212) receives a supersaturated solution (214) of CaSO4 into the first stage (212 a). A coagulant (216) is added to the feed stream of the first stage (212 a). An optional pH adjuster (not shown) may be added. All three stages include pitched blade paddles (218 a, 218b, 218 c). All three stages and their respective paddles are sized to meet the H: D ratio and D: D ratio described above. The final stage produces a mixture of gypsum fines (220). The multi-stage CSTR (212) includes a feed (222) for gypsum seed particles.
The three stages (212 a,212b, and 212 c) may be positioned to flow liquid from one stage to the next by gravity. For example, the three stages may be positioned such that the first stage (212 a) is higher than the second stage (212 b), e.g., about 10cm, and liquid flows by gravity from the first stage (212 a) into the second stage (212 b), and the second stage (212 b) is higher than the third stage (212 c), e.g., about 10cm, and liquid flows by gravity from the second stage (212 b) into the third stage (212 c).
The gypsum seed particles produced by this method can be used in the separation process described above. The precipitation reactor may be used in combination with the separator described above in the apparatus.
Fig. 5 shows a process flow diagram of an exemplary device according to the present disclosure. The apparatus (310) comprises the above-described precipitation reactor (210) and a solid/liquid separator (110). The mixture (220) produced by the final stage (212 c) of the CSTR corresponds to the mixture (114) received by the solid/liquid separator (110). The oblique blade paddles are not shown. Flocculant (122) is optional.
Example 2
A pilot plant using the method shown in fig. 5 was established, but without any flocculant added to the settling tank to treat supersaturated solutions of CaSO4 produced by the nanofiltration process for treating coal mine drainage. The pilot process uses a multistage CSTR consisting of three precipitation reactors. The concentration of gypsum seeds in each reactor was about 3 to 5 wt.%. The reactor was stirred using a mechanical stirrer at a stirring speed of about 100 to about 140 rpm. The width of the metal paddles of the stirrer in the reactor is about half the diameter of the reactor. The height of the reactor was 1000mm, the diameter was 600mm, and the diameter of the circle formed by the paddles was 300mm. This corresponds to a H:D ratio of 5:3 and a d:D ratio of 1:2. The total residence time in the series of three reactors is from 30 to 40 minutes. The average diameter of the gypsum seeds in the reactor was about 25 μm.
The gypsum seed crystals were transferred to a solid-liquid separator having a height of 1200mm and a diameter of 600mm (H: D ratio of 2: 1). The agitator has blades with a diameter of about 500mm and operates at about 20RPM to about 40 RPM. The larger gypsum seed floc obtained from the bottom of the solid-liquid separator was sheared and recycled and dispersed into the first reaction tank with vigorous stirring by a transfer pump comprising an open impeller operating at about 800 RPM.
Stabilizers, also known as scale inhibitors, are added to the supersaturated effluent resulting from the nanofiltration process to reduce or avoid scaling. FeCl3 in an amount of 30 to 50ppm was added as a coagulant to the inflow of the multistage CSTR to accelerate destabilization of supersaturated CaSO4.
The size distribution of gypsum seeds produced in this test was measured. The size distribution of gypsum seeds produced in the comparative method was also measured. This comparative method uses only one of the stages of the CSTR deposition tank described above and operates with a 30 to 40 minute residence time, but otherwise is identical to the pilot plant described above. As described above, exemplary methods according to the present disclosure produce particles having an average size of about 25 μm. The comparative method produced particles with an average size of about 80 μm. The size distribution is shown in fig. 6 and 7, respectively.
The water quality of the inflow and outflow streams of the CSTR was analyzed and the saturation of CaSO4 was calculated using the following equation:
the composition of the influent and effluent streams is shown in table 3 and the supersaturation levels of CaSO4 were calculated for four different time points. It has been determined that the average supersaturation level decreases from about 200% at the inflow stream to about 120% at the outflow 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 effluent | 435 | 2.4 | 63.2 | 630 | 6300 | 10.4 | 8.6 | 16180 | 49.4 |
TABLE 3 Table 3
It has been determined that the average supersaturation level of the influent and effluent streams of the comparative reactor is reduced from about 230% at the influent stream to about 190% at the effluent stream. A supersaturation level of about 190% was observed to result in fouling downstream of the comparative reactor.
In another aspect of the present disclosure, a precipitation method is provided. The method comprises receiving a supersaturated aqueous solution of CaSO4 into a multistage Continuous Stirred Tank Reactor (CSTR), wherein the stages of the reactor are vertically stacked, wherein the internal outflow from one stage substantially corresponds to the internal feed inlet of a subsequent downstream stage. The method includes flowing the solution vertically upward through a multi-stage CSTR to produce a gypsum seed particulate mixture in an aqueous solution, and transferring the gypsum seed particulate mixture in the aqueous solution to a separator.
The present disclosure also provides a precipitation reactor comprising a multistage Continuous Stirred Tank Reactor (CSTR) wherein the stages of the reactor are in series, wherein the internal outflow from one stage corresponds to the internal feed inlet of a subsequent downstream stage. The precipitation reactor is in fluid communication with a source of supersaturated aqueous solution of CaSO4 and in fluid communication with the separator to provide a gypsum seed particulate mixture in the aqueous solution to the separator. The separator may be a separator as described above. The source of supersaturated CaSO4 solution may be a membrane separation unit.
The reactor stages are preferably vertically stacked. The source of supersaturated aqueous solution of CaSO4 may provide the solution at a static pressure sufficient to drive the solution vertically upward through the various stages of the CSTR.
The outflow and the feed opening may or may not be connected by a short fluid conduit. When not connected by a fluid conduit, the outflow and inflow may refer to the same orifice between two adjacent stages. Because the outflow of one stage substantially corresponds to the inflow of a subsequent stage, the disclosed precipitation method reduces or avoids precipitation of CaSO4 in fluid conduits, such as pipes, connecting the different stages of the reactor.
The multistage CSTR may be operated under conditions that produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. Exemplary conditions are discussed above.
The reactor may comprise 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 opening of the first stage of the reactor corresponds to the feed opening of the second stage of the reactor, and the outflow opening of the second stage of the reactor corresponds to the feed opening of the third stage of the reactor.
The orifices may be sized to prevent or reduce back mixing, which occurs when fluid from one stage flows down through the inlet into the lower stage. Preventing or reducing back mixing through the inlet may be achieved when the diameter of the inlet is about 5% to about 10% of the diameter of the reactor stage.
In this vertical multistage CSTR, agitators on the same single agitator shaft can be used to agitate each stage.
The reactor may include a gypsum seed source in fluid communication with the reactor, such as in fluid communication with the first stage of the reactor. The reactor may include a coagulant source in fluid communication with the reactor, for example in fluid communication with the first stage of the reactor.
Fig. 8 illustrates an exemplary precipitation reactor according to the present disclosure. In the precipitation reactor (410), a vertical multistage CSTR
(412) Consists of three stages (412 a, 412b and 412 c). A vertical multi-stage CSTR (412) receives supersaturated solution (414) of CaSO4 at the bottom, in a first stage (412 a). Coagulant (416) is added to the feed stream of the first stage (412 a). An optional pH adjuster (not shown) may be added. All three stages include pitched blade paddles (418 a, 418b, 418 c) on the same stirring shaft. All three stages and their respective paddles are sized to meet the H: D ratio and D: D ratio described above. The final stage produces a gypsum fines mixture (420). The first stage (412 a) is in fluid communication with a gypsum seed fine particle source (422). The outlet of the first stage corresponds to the inlet (424 a) of the second stage. The outlet of the second stage corresponds to the inlet (424 b) of the third stage.
Fig. 9 illustrates another exemplary precipitation reactor according to the present disclosure. The reactor (510) is similar to the reactor shown in fig. 8 except that the three stages do not share a common stirring shaft. The vertical reactor (512) would still receive a supersaturated solution (514) of CaSO4 in the bottom, first stage (512 a). A coagulant (516) is added to the feed stream to the first stage. An optional pH adjuster (not shown) may be added. All three stages include pitched blade paddles (518 a, 518b, 518 c). The paddles may agitate at the same or different rates. All three stages and their respective paddles are sized to meet the H: D ratio and D: D ratio described above. The final stage produces a gypsum fines mixture (520). The first stage (512 a) is in fluid communication with a gypsum seed fine particle source (522). The outlet of the first stage corresponds to the inlet (524 a) of the second stage. The outlet of the second stage corresponds to the inlet (524 b) of the third stage.
Gypsum seed fines produced by the precipitation process or in the precipitation reactor can be used in the separation process or reactor described above. For example, a method is provided that includes receiving a supersaturated aqueous solution of CaSO4 into a multistage Continuous Stirred Tank Reactor (CSTR) for seed-assisted precipitation, wherein the stages of the reactor are vertically stacked, wherein an internal outflow from one stage corresponds to an internal feed inlet of a subsequent downstream stage. The coagulant is added to (a) the feed stream of a multistage CSTR, or (b) the multistage CSTR. The solution flows vertically upward through the multiple stages of CSTRs. Seed assisted precipitation and multistage CSTR are operated under shear conditions to produce gypsum seed fines having an average diameter of about 20 μm to about 40 μm. Transferring the fines to a separator and adding an anionic flocculant to (a) the feed stream to the separator, or (b) the separator. The gypsum seed fines and flocculant are agglomerated into a floe. The flocculated mixture is separated into a turbidity reduced effluent 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 to non-flocculated gypsum seed fines. At least a portion of the fines is diverted back to the multi-stage CSTR.
In the example of a combined precipitation reactor and solid/liquid separator, an apparatus comprises a multistage Continuous Stirred Tank Reactor (CSTR) in which the stages of the reactor are connected in series, with the internal outflow from one stage corresponding to the internal feed inlet of the subsequent downstream stage. The precipitation reactor is in fluid communication with a source of supersaturated aqueous solution of CaSO4. At least one pitched blade is disposed in at least one stage, wherein the dimensions of the blade and the dimensions of the vessel in which it is disposed are selected to produce 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 the gypsum seed particulate mixture in the aqueous solution from the multi-stage CSTR. The settling tank includes a first fluid outlet for discharging effluent of reduced turbidity and a second fluid outlet for discharging flocculated gypsum slurry. The apparatus includes a source of an anionic flocculant in fluid communication with a 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.
As described above, the stages of a multi-stage CSTR can be vertically stacked. The characteristics of the precipitation reactor and the solid/liquid separator are discussed in more detail above.
Fig. 10 shows a process flow diagram of an exemplary device according to the present disclosure. The apparatus (610) includes the above-described precipitation reactor (410) and solid/liquid separator (110). The mixture (420) produced by the CSTR (412) corresponds to the mixture (114) received by the solid/liquid separator (110). The gypsum seed fines (130) produced by the open centrifugal pump (128) correspond to gypsum seed (422) accepted by the CSTR (412). Flocculant (122) is optional.
Example 3
A pilot plant using the reactor shown in fig. 8 was set up to treat supersaturated solutions of CaSO4 produced by the nanofiltration process for treating coal mine drainage.
The vertical CSTR comprises three stages. The H: D ratio of the height and diameter of each stage is about 1:1. The width (D) of the paddles is such that the ratio of D to D is about 1:3. In this pilot plant, a supersaturated solution of CaSO4 (200% saturation) was received at the bottom, in the first stage, at a flow rate of 500L/h. Gypsum seed particles were added at a concentration of 20 to 35% by weight at a flow rate of 100L/h. Sufficient FeCl3 is added to the feed stream to the first stage to produce a concentration of 30-40 ppm. The pitched blade paddles agitate at a rate of about 110 rpm. The final stage produces a mixture of gypsum fines. As shown in table 4, the gypsum seed concentration for each of the three stages was measured to be about 3 to about 9 wt.%. The precipitation was operated so that the total residence time was about 28 minutes.
TABLE 4 Table 4
The effluent of the nanofiltration process includes an anti-fouling agent to reduce or avoid fouling. The supersaturation level of the nanofiltration effluent was about 140% of the saturation level, with calcium being about 1800ppm (recorded as CaCO3). The effluent produced from the final stage of the vertical multistage CSTR reached a saturation level of about 100% with about 1200ppm calcium (recorded as CaCO3) as shown in Table 5.
TABLE 5
In another aspect, the present disclosure provides a method of removing scale from (a) a process plant, such as a CaSO4 precipitation reactor, a solid/liquid separator, or a fluid conduit, or (b) a portion of a process plant. The method includes vibrating or deforming the process equipment or a portion of the process equipment to remove at least some of the scale. The process equipment or parts thereof that are vibrated are 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 material.
The present disclosure also provides a process apparatus or a part of a process apparatus, wherein the process apparatus or part thereof is at least partly made of or at least partly coated with a low friction and preferably hydrophobic material. The low friction material is in a location that would be exposed to supersaturated solutions of CaSO4. The low friction material may be in a location that is sufficiently vibrated or deformed to dislodge at least some of the scale present on the low friction material.
The part of the process equipment that may be vibrated or deformed may be a side wall, a baffle, a liquid conduit within the reactor or a stirring blade.
The low friction material may be Polyethylene (PE), polypropylene (PP) or Polytetrafluoroethylene (PTFE).
The vibration may be at a frequency of about 0.1 to 10Hz and/or may include moving the low friction material at an amplitude of about 1 to about 5 mm.
Any of the process equipment discussed above may be made of or coated with a low friction material and at least some of the scale may be removed by vibrating at least a portion of the equipment. When the apparatus is a reactor (e.g., a precipitation reactor or a solid-liquid separation reactor), the reactor may include a slag discharge, and removed scale may be removed from the reactor through the slag discharge.
In the previous description, for purposes of explanation, numerous details were set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. Thus, what has been described is merely an application to illustrate the described embodiments and many modifications and variations are possible in light of the above teachings.
Since the above description provides embodiments, it will be understood that modifications and variations of the specific embodiments will occur to those skilled in the art. Therefore, the scope of the claims should not be limited by the specific embodiments set forth herein, but should be construed in a manner consistent with the specification as a whole.