TWI890103B - Method of synthesizing calcium molybdate crystals from molybdenum-contained wastewater by using fluidized-bed crystallization technology - Google Patents

Abstract

The present invention relates to a method of synthesizing calcium molybdate crystals from molybdenum-containing wastewater by fluidized bed crystallization technology. In addition to a high efficiency of molybdenum ions removal from water, the method also can reduce the usage of chemical agents. Furthermore, this method has no need to apply heterogeneous carriers in the fluidized-bed reactor. By varying the operation conditions of pH, the molar ratio of Ca to Mo, surface loading, hydraulic retention time and bed height, molybdenum ions can be removed from wastewaters in a fluidized-bed homogeneous crystallization (FBHC) system by recovering calcium molybdate crystals. This method has a very high application potential because of high treatment efficiency and crystal purity.

Description

Method for synthesizing calcium molybdate crystals from molybdenum-containing wastewater using fluidized bed crystallization technology

The present invention relates to a method for synthesizing and recovering calcium molybdate crystals from molybdenum-containing wastewater, and more particularly to a method for synthesizing and recovering low-water-content, high-purity calcium molybdate crystal particles from molybdenum-containing wastewater using fluidized bed crystallization technology.

Molybdenum metal has excellent electrical conductivity and a high melting point, making it widely used in the photovoltaic and steel industries as a conductor material and an additive to strengthen steel structures, respectively. Sources of molybdenum-containing wastewater include the photovoltaic panel industry, metallurgy, and petrochemicals, with concentrations ranging widely. Because molybdenum can be harmful to humans, such as slowed growth and neurological disorders, its removal is essential. The most common molybdenum removal technologies are ion exchange resins and chemical coagulation.

Taiwan Patent I731678 discloses a method for treating molybdenum-containing wastewater and recovering molybdenum-containing ferric oxide crystals, comprising: draining the molybdenum-containing wastewater into a first reactor, adding a trivalent iron ion reagent to the first reactor, and adding an acid to adjust the pH to an appropriate level; draining the molybdenum-containing wastewater from the first reactor into a second reactor, and adding an alkaline agent to the second reactor to cause the molybdenum ions in the molybdenum-containing wastewater to react with the iron ions to generate molybdenum-containing ferric oxide crystals, which are attached to the surface of the crystal substrate. The wastewater treated in the second reactor can overflow from the upper end of the second reactor and be recycled. The wastewater overflowing from the top of the second reactor is defined as treated water. This treated water is directed to subsequent treatment processes, with a portion of the treated water diverted to form circulating water. This circulating water is then injected into the second reactor, allowing any residual ferric molybdate crystalline components in the circulating water to adhere and accumulate on the surface of the crystalline substrate. When the ferric molybdate crystals adhering to the surface of the crystalline substrate in the second reactor reach a certain particle size, the crystalline substrate particles with ferric molybdate crystals adhering to their surfaces are removed from the second reactor, achieving the goal of treating molybdenum-containing wastewater and recovering molybdenum metal resources for reuse.

Taiwan Patent I725782 discloses a method and apparatus for treating molybdenum-containing wastewater. The method involves draining the molybdenum-containing wastewater to a first reaction unit, adding an iron salt reagent, and adjusting the pH value to 3-5. This allows the molybdenum in the wastewater to react with iron ions to form larger particles of molybdenum-ferric flocs. The wastewater is then drained to a first solid-liquid separation unit for solid-liquid separation, where the molybdenum-ferric flocs in the wastewater form molybdenum-ferric sludge. The clarified liquid from the first solid-liquid separation unit is then drained to a second reaction unit, where the pH value is adjusted to 6-8, allowing the iron ions in the wastewater to react to form larger particles of molybdenum-ferric flocs. The wastewater from the second reaction unit is then drained to the second solid-liquid separation unit for solid-liquid separation, where the iron ion flocs in the wastewater form iron hydroxide sludge. Since the molybdenum and iron have been removed from the clarified liquid in the second solid-liquid separation unit, it can be discharged. The iron hydroxide sludge in the second solid-liquid separation unit is then transferred to the sludge dissolution unit, where an acid is added to dissolve the iron hydroxide sludge to obtain a high-concentration iron ion solution. This high-concentration iron ion solution is then returned to the first reaction unit to recover the iron ions for reuse.

Taiwan Patent I511938 discloses a method for recycling wastewater containing molybdenum in photoelectric etching. The method involves adsorbing the molybdenum ions in the wastewater containing molybdenum in an anion exchange resin and then performing separation and analysis. The obtained separation solution contains high concentrations of molybdenum metal ions and is recycled. The process technology includes purification, impurity removal, crystallization, and forging to produce high-purity and high-solubility catalyst-grade molybdenum oxide, high-purity refined molybdenum acid, high-purity sodium molybdate, and ammonium molybdate, among other molybdenum salt products. It also reduces the molybdenum ion concentration in molybdenum-containing etching wastewater from the optoelectronics industry from approximately 10 ppm to below 0.5 ppm.

Taiwan Patent I427155 discloses a method for recovering metals from spent molybdenum-containing catalysts, comprising: a leaching step in which a mixture of spent molybdenum-containing catalysts is immersed in a highly oxidizing acid solution, causing the sulfur in the mixture to react with the acid solution to produce sulfur-containing compounds and vapors. After the sulfur in the mixture reacts with the acid solution completely, an effluent containing oxidized metals and a residual slag are obtained; and A refining step involves dumping the metals in the slag to obtain a slag liquor, and extracting and precipitating the metals enriched in the molybdenum-containing waste catalyst from the dumped liquor and the slag liquor, respectively. The vaporized product generated in the leaching step is re-reacted to form the highly oxidizing acidic solution for reuse in the leaching step. The highly oxidizing acidic solution is nitric acid, hypochlorous acid, chloric acid, chlorous acid, perchloric acid, or nitrous acid.

Taiwan Patent I535664 discloses a method for treating molybdenum-containing wastewater, comprising: adding a molybdenum-selective adsorbent to the molybdenum-containing wastewater; mixing the wastewater and the molybdenum-selective adsorbent so that the molybdenum-selective adsorbent adsorbs molybdenum ions in the wastewater; separating the wastewater and the molybdenum-selective adsorbent to obtain a used molybdenum adsorbent and treated wastewater; and discharging the treated wastewater, retaining the used molybdenum adsorbent. A method for recovering molybdenum from molybdenum-containing wastewater treatment comprises: adding a molybdenum desorption agent to a used molybdenum adsorbent; mixing the used molybdenum adsorbent and the molybdenum desorption agent to remove molybdenum ions from the used molybdenum adsorbent to form a molybdenum ion solution and a regenerated molybdenum selective adsorbent; separating the molybdenum ion solution from the regenerated molybdenum selective adsorbent; and discharging and collecting the molybdenum ion solution, while retaining the regenerated molybdenum selective adsorbent.

While chemical coagulation sedimentation is simple to operate and effective in removing molybdenum, it requires the addition of large amounts of chemicals during the molybdenum removal process. The resulting solid waste has a high moisture content (i.e., high-molybdenum sludge), which impacts the environment. The low purity of the solids creates difficulties and hazards during subsequent solid-liquid separation and recovery.

To improve upon the challenges of chemical coagulation, a technology has been proposed for recovering high-concentration metals from wastewater using a fluidized bed crystallization process. The resulting high-concentration metal particles can be reused, reducing costs and environmental concerns compared to traditional chemical coagulation. However, the conventional fluidized bed crystallization process requires the addition of carriers such as silica sand and brick powder to the reactor for crystallization. This results in the inclusion of carriers in the metal crystals, resulting in low purity and negatively impacting their value for reuse. Furthermore, there is currently no technology for removing and recovering molybdenum from wastewater using fluidized bed crystallization.

Molybdenum is widely used in various industries and is in demand as a raw material for certain products. If molybdenum can be removed from wastewater and recovered as commercially valuable molybdenum salt, it can achieve the sustainable value of resource reuse.

Therefore, the primary objective of this invention is to provide a method for synthesizing calcium molybdate crystals from molybdenum-containing wastewater using fluidized bed crystallization technology. This method not only removes molybdenum but also recovers low-water-content, high-purity calcium molybdate crystal particles from the molybdenum-containing synthetic wastewater. These particles have potential for recycling and reuse.

According to one embodiment of the present invention, the method for synthesizing calcium molybdate crystals from molybdenum-containing wastewater using fluidized bed crystallization technology includes: providing a fluidized bed reactor, the reactor having a lower section and an upper section, the lower section having a solution inlet and a reagent inlet, the upper section having a water outlet, and a reflux line between the lower section and the upper section; respectively, introducing the molybdenum-containing solution and the reagent into the fluidized bed reactor from the solution inlet and the reagent inlet; The flow port is introduced into the fluidized bed reactor for mixing, wherein the reagent is a calcium precipitant containing calcium ions; the molybdenum-containing solution mixed with the reagent flows from the lower section of the reactor to the upper section of the reactor; and the molybdenum-containing solution mixed with the reagent is refluxed to the lower section through the reflux pipe for circulation, so that the molybdenum ions in the molybdenum-containing solution react with the reagent to produce calcium molybdate particles, wherein the pH value of the reactor is _R ) is controlled between 6 and 11, the feed molar concentration ratio of calcium ions in the reagent to molybdenum ions in the molybdenum-containing solution (Ca/Mo) is controlled between 2 and 4, and the cross-sectional load of the molybdenum-containing solution (L _Mo ) is controlled between 1.8 and 22 kg m ^-2 h ^-1 .

In a preferred embodiment, the agent is calcium chloride or calcium hydroxide.

In a preferred embodiment, a molybdenum-containing solution and reagents are first mixed in the reactor to produce molybdenum oxide crystal particles as a carrier. The particle bed height is controlled between 35 cm and 65 cm to achieve high molybdenum ion removal and crystallization rates.

In a preferred embodiment, the hydraulic retention time (HRT) of the reactor is controlled between 33.9 and 59.2 minutes to achieve high molybdenum ion removal and crystallization rates.

In a preferred embodiment, the feed molar concentration ratio of calcium ions in the reagent to molybdenum ions in the molybdenum-containing solution (Ca/Mo) is controlled between 3 and 4 to achieve a high molybdenum ion removal rate and crystallization rate.

In a preferred embodiment, the pH value of the reactor is controlled between 6 and 9 to obtain a high molybdenum ion removal rate and crystallization rate.

In a preferred embodiment, the cross-sectional loading of the molybdenum-containing solution is between 3.67 and 22 kg m ^-2 hr ^-1 to achieve high molybdenum ion removal efficiency and crystallization rate.

Other objects, advantages and features of the present invention will be understood from the following detailed description of the preferred embodiments with reference to the accompanying drawings.

10: Reactor

12: Next paragraph

14: Upper section

16: Solution inlet

18: Medication inlet

20: Water outlet

22: Return pipe

23: Water outlet pipe

24: pH detector

26: Sedimentation Tank

28: Pump

30: Molybdenum solution

32:Medication

Figure 1 is a schematic diagram of a fluidized bed reactor according to one embodiment of the present invention.

Figure 2 shows the relationship between the effect of different pH values (pH _R ) on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) in a fluidized bed granulation system for the synthesis of calcium molybdate.

Figure 3 shows the relationship between the effect of different feed molar ratios (Ca/Mo) on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) in a fluidized bed granulation system for the synthesis of calcium molybdate.

Figure 4 shows the relationship between the effect of different particle bed heights (H) on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) in a fluidized bed granulation system for the synthesis of calcium molybdate.

Figure 5 shows the relationship between the effect of different cross-sectional loadings (L _Mo ) of the molybdenum-containing solution on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) in a fluidized bed granulation system for the synthesis of calcium molybdate.

Figure 6 shows the relationship between the effect of different hydraulic retention times (HRT) on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) in a fluidized bed granulation system for the synthesis of calcium molybdate.

Figure 7 shows the relationship between the effect of different hydraulic retention times (HRT) on the molybdenum removal efficiency (TR%) and crystallization rate (CR%) under varying feed molybdenum concentrations in a fluidized bed granulation system for the synthesis of calcium molybdate.

This invention proposes a method for synthesizing calcium molybdate crystals from molybdenum-containing wastewater using fluidized bed crystallization technology. This method removes and recovers molybdenum from molybdenum-containing solutions (e.g., molybdenum-containing wastewater), reducing the use of chemical reagents. High-purity, low-water-content calcium molybdate crystal particles are synthesized in the system without the addition of foreign carriers, thereby recovering heavy metal elements from the wastewater and increasing the added value of the recovered molybdenum salt crystal particles.

Referring to FIG. 1 , the method of the present invention first provides a fluidized bed reactor 10 having a tubular lower section 12 and a tubular upper section 14. The lower section 12 is provided with a solution inlet 16 and a reagent inlet 18, while the upper section 14 is provided with a water outlet 20. A recirculation line 22 is provided between the lower and upper sections 12, 14. In this embodiment, the bottom of the lower section 12 of the reactor 10 is conical, which facilitates uniform distribution of the recirculation flow force. An outlet pipe 23 is provided at the outlet 20. A pH detector 24 is mounted on the outlet pipe 23 to monitor the pH value at the outlet. The outlet pipe 23 is connected to a sedimentation tank 26. Next, a molybdenum-containing solution (e.g., molybdenum-containing wastewater) 30 and a reagent 32 are introduced into the lower section 12 of the reactor 10 through the solution inlet 16 and the reagent inlet 18 for mixing. The molybdenum-containing solution 30 mixed with the reagent 32 then flows from the lower section 12 to the upper section 14. The molybdenum-containing solution 30 mixed with the reagent 32 is then circulated back to the lower section 12 via the recirculation line 22. This allows the molybdenum ions in the molybdenum-containing solution 30 to react with the calcium ions in the reagent 32 to form calcium molybdate particles. Reagent 32 is a calcium precipitant containing calcium ions. In this embodiment, calcium chloride (CaCl ₂ ) is used as the precipitant. Calcium chloride has the advantages of high solubility and ease of solution preparation. In a feasible embodiment, calcium hydroxide (Ca(OH) ₂ ) is used as the precipitant. The hydroxide radicals in calcium hydroxide maintain the pH during the reaction, preventing a significant pH drop.

According to the method of the present invention, the reactor pH (pH _R ), the feed molar concentration ratio of calcium ions in the reagent 32 to molybdenum ions in the molybdenum-containing solution 30 (Ca/Mo), the bulk bed height within the reactor 10 (the height of the particles within the reactor, H), the cross-sectional load of the molybdenum-containing solution (L _Mo ), and the hydraulic retention time (HRT) of the reactor 10 treating the molybdenum-containing wastewater will respectively affect the molybdenum ion removal rate (molybdenum removal efficiency) in the molybdenum-containing solution 30 and the granulation rate (crystallization rate) after particle stabilization. Furthermore, with or without reflux, the molybdenum-containing solution 30 and the reagent 32 can be introduced into the reactor 10 and mixed to produce calcium molybdate particles as a carrier. This provides sufficient surface area for the growth of new crystals, facilitating the attachment of newly generated crystals to form new particles, thereby preventing the formation of a gel-like precipitate containing a large amount of water.

According to test results, in a system for synthesizing calcium molybdate crystals, the pH value (pH _R ) should be controlled between 6 and 11, the feed molar concentration ratio (Ca/Mo) of calcium ions in the reagent 32 relative to the molybdenum ions in the molybdenum-containing solution 30 should be controlled between 1 and 4, the bed height (H) should ideally be controlled between 35 cm and 65 cm, the cross-sectional load (L _Mo ) should be controlled between 1.8 and 22 kg m ^-2 hr ^-1 , and the hydraulic retention time should be controlled between 33.9 and 59.2 min.

Referring to Figure 2 , it shows the effects of changes in pH (pH R ) on crystallization rate ( _{CR%) and molybdenum removal efficiency (TR%) in a calcium molybdate synthesis system operating under the following conditions: an initial molybdenum concentration (C Mo,in} ) of 2000 mg/L in the molybdenum-containing solution, a hydraulic retention time (HRT) of 33.9 min, a feed molar ratio (Ca/Mo) of 3.0 of calcium ions in reagent 32 to molybdenum ions in the molybdenum-containing solution 30, a bed height (H) of 40 cm, a feed upflow velocity ( _U ) of 30.4 m/hr, a cross-sectional load (L _Mo ) of 3.67 kg m ^-2 hr ^-1 , and a reflux ratio (R) of 7.29. Experimental studies have shown that when pH _R < 6, the solubility of calcium molybdate increases with decreasing pH _R. Consequently, calcium molybdate precipitates are less likely to form in this environment, and both the total molybdenum removal rate and crystallization rate decrease significantly. The total molybdenum removal rate (TR _R ) in the reactor, the crystallization rate (CR _R ) in the reactor, and the total molybdenum removal rate (TR _P ) in the settling tank drop from 90%, 90%, and 94% to 28%, 28%, and 34%, respectively. When pH _R is between 6 and 9, the TR _R, CR _R, and TR _P are approximately 90%, 90%, and 94%, respectively. The solubility curve shows that the solubility of calcium molybdate is lowest when pH _R is between 6 and 9. When pH _R reaches 11, the total molybdenum removal rate and crystallization rate begin to decline, with TR _R , CR _R , and TR _P dropping to 85%, 84%, and 91%, respectively. An investigation into the factors affecting pH _R revealed that controlling pH _R between 6 and 9 yields the best TR _R , CR _R , and TR _P . However, to determine the optimal pH _R , a _pH of 6 is recommended. This is because the acidic range makes the pH less susceptible to the dissolution of carbonic acid in water, resulting in a more stable pH _R.

Referring to Figure 3, it shows the effects of varying feed molar ratios (Ca/Mo) on crystallization rate (CR%) and molybdenum removal efficiency (TR%) in a calcium molybdate synthesis system, operating under the following conditions: an initial molybdenum concentration (C _Mo,in ) of 2000 mg/L, a pH value ( _{pH R} ) of 6, a hydraulic retention time (HRT) of 33.9 min, a feed upflow velocity (U) of 30.4 m/hr, a cross-sectional load (L Mo ) of 3.67 kg m ^-2 hr ^-1 , a reflux ratio (R) of 7.29, and a bed height (H) of 50 cm. Experiments have found that when [Ca]/[Mo] is less than 1, the amount of calcium precipitant is insufficient to react with molybdate ions in the solution to form calcium molybdate. As a result, TR _R, CR _R , and TR _p are poor, less than 74%, 72%, and 81%, respectively. When [Ca]/[Mo] is between 1 and 3, an increase in calcium precipitant is observed, which also increases the reaction driving force and accelerates the formation rate of calcium molybdate. As a result, TR _R, CR _R , and TR _p can be increased from 73%, 71%, and 80% to 91%, 90%, and 94%, respectively. The dissolved molybdenum ion concentration in the reactor (C _Mo,TRR ) is 87.6 mg/L, and the dissolved calcium ion concentration in the reactor (C _Ca,TRR ) is 916 mg/L; the dissolved molybdenum ion concentration in the settling tank (C _Mo,TRP ) is 56.9 mg/L, and the dissolved calcium ion concentration in the reactor (C _Ca,TRP ) is 794 mg/L. When [Ca]/[Mo] is set above 3, TRR _{, CRR} , and _TRP can reach 91%, 90%, and 94%, respectively. The dissolved molybdenum ion concentration in the reactor (CMo _,TRR ) is 87.6 mg/L, and the dissolved calcium ion concentration in the reactor (CCa _,TRR ) is 916 mg/L. The dissolved molybdenum ion concentration in the settling tank (CMo _,TRP ) is 56.9 mg/L, and the dissolved calcium ion concentration in the reactor (CCa _,TRP ) is 794 mg/L, reducing the dissolved molybdenum ion concentration in the water to below 60 ppm.

Referring to Figure 4 , it shows the effects of different bed heights (H) on crystallization rate (CR%) and molybdenum removal efficiency (TR%) in a calcium molybdate synthesis system, operating under the following conditions: an initial molybdenum concentration (C _Mo,in ) of 2000 mg/L in the molybdenum-containing solution, a pH value (pH _R ) of 6, a hydraulic retention time (HRT) of 33.9 min, a feed molar ratio (Ca/Mo) of 3.0 of calcium ions in reagent 32 to molybdenum ions in the molybdenum-containing solution 30, a feed upflow velocity (U) of 30.4 m/hr, a cross-sectional load (L _Mo ) of 3.67 kg m ^-2 hr ^-1 , and a reflux ratio (R) of 7.29. Experimental results show that when the bed height (H) is less than 30 cm, the system lacks sufficient effective surface area for crystal growth, resulting in excessive molybdenum ions remaining in the solution. This leads to excessive supersaturation and the generation of large amounts of sludge. When the bed height is greater than 40 cm, sufficient effective surface area for crystal growth and homogenized particles are provided, resulting in a better crystallization rate (CR) and accelerated reaction. The total molybdenum removal rate (TR _R ) in the reactor can reach over 91%, the crystallization rate (CR) over 90%, and the total molybdenum removal rate (TR _p ) in the settling tank over 95%.

Referring to Figure 5 , it shows the effects of different cross-sectional loads (L Mo ) on crystallization rate ( _CR %) and molybdenum removal efficiency (TR%) in a calcium molybdate synthesis system operating under the following conditions: a pH value (pH R ) of 6, a hydraulic retention time (HRT) of 33.9 min, a feed molar ratio (Ca/Mo) of 3.0 of calcium ions in reagent 32 to molybdenum ions in molybdenum-containing solution 30, a feed upflow velocity ( _U ) of 30.4 m/hr, a reflux ratio (R) of 7.29, and a particle bed height (H) of 50 cm. Experimental results show that when the cross-sectional load is between 0.18 and 3.67 kg m ^-2 hr ^-1 and the feed molybdenum concentration is between 100 and 2000 mg/L, TR _R , CR _R and TR _P increase significantly with the increase of cross-sectional load, from 44%, 42%, and 49% to 91%, 90%, and 94%, respectively. The concentrations of dissolved molybdenum ions in the reactor (C _{Mo, TRR} ) are 27.9 mg/L, 74.5 mg/L, 108.9 mg/L, and 87.6 mg/L, respectively, and the concentrations of dissolved calcium ions in the reactor (C _{Ca, TRR} ) were 58.1 mg/L, 261.3 mg/L, 486.3 mg/L, and 916 mg/L, respectively; the dissolved molybdenum ion concentrations in the sedimentation tank (C _Mo,TRP ) were 25.3 mg/L, 55.5 mg/L, 67 mg/L, and 56.9 mg/L, respectively; and the dissolved calcium ion concentrations in the reactor (C _Ca,TRP ) were 57.1 mg/L, 253.5 mg/L, 470.5 mg/L, and 794 mg/L, respectively. When the cross-sectional load is between 3.67 and 22 kg m ^-2 hr ^-1 and the feed molybdenum concentration is between 2000 and 12000 mg/L, TR _R , CR _R and TR _P still increase with the increase of cross-sectional load, but the increase tends to be gentle. When the cross-sectional load is 22 kg m ^-2 hr ^-1 and the feed molybdenum concentration is 12000 mg/L, TR _R , CR _R and TR _P can reach 99.2%, 98.9% and 99.6% respectively. The concentration of soluble molybdenum ions in the reactor (C _Mo,TRR ) is 53.2 mg/L, and the concentration of soluble calcium ions in the reactor (C _Ca,TRR ) was 5170 mg/L; the dissolved molybdenum ion concentration in the sedimentation tank (C _Mo,TRP ) was 27.9 mg/L; and the dissolved calcium ion concentration in the reactor (C _Ca,TRP ) was 5142 mg/L. After varying the cross-sectional load, it was determined that for the calcium molybdenum oxide FHBC system, when the cross-sectional load was greater than 3.67 kg m ^-2 hr ^-1 and the feed molybdenum concentration was greater than 2000 mg/L, the TR _R, CR, and TR _p values could reach over 91%, 90%, and 94%, respectively. The dissolved molybdenum ion concentration in the reactor (C _{Mo, TRR} ) was 87.6 mg/L, and the dissolved calcium ion concentration in the reactor (C _{Ca, TRR} ) was 916 mg/L. The dissolved molybdenum ion concentration in the settling tank (C _{Mo, TRP} ) was 56.9 mg/L, and the dissolved calcium ion concentration in the reactor (C _{Ca, TRP} ) was 794 mg/L.

Referring to Figure 6 , it shows the effects of different hydraulic retention times (HRT) on crystallization rate (CR _{%) and molybdenum removal efficiency (TR%) in a calcium molybdate synthesis system, operating under the following conditions: an initial molybdenum concentration (C Mo,in} ) of 2000 mg/L in the molybdenum-containing solution, a pH value (pH _R ) of 6, a feed molar ratio (Ca/Mo) of 3.0 of calcium ions in reagent 32 to molybdenum ions in molybdenum-containing solution 30, a reflux ratio (R) of 7.29, and a particle bed height (H) of 50 cm. Experiments have shown that when the HRT ranges from 16.9 to 23.7 minutes, the system cross-sectional loads are 7.34 kg m ^-2 hr ^-1 and 5.24 kg m ^-2 hr ^-1 , respectively. Because the reaction time is insufficient, the calcium molybdate does not fully react, resulting in the total molybdenum removal rate (TR _R ), crystallization rate (CR _R ) in the reactor, and total molybdenum removal rate (TR _P ) in the settling tank all being below 90%. When the HRT is between 33.9 and 59.2 min, the cross-sectional load of the system is 3.67 to 2.1 kg m ^-2 hr ^-1 . Due to sufficient time for reaction, the TR _R can reach above 91%, the CR _R can reach above 90%, and the total molybdenum removal rate (TR _p ) in the sedimentation tank can reach above 94%. The concentration of dissolved molybdenum ions in the reactor (C _{Mo, TRR} ) is 76 to 90 mg/L, and the concentration of dissolved calcium ions in the reactor (C _{Ca, TRR} ) is 870 to 916 mg/L; the concentration of dissolved molybdenum ions in the sedimentation tank (C _{Mo, TRP} ) is 32 to 57 mg/L, and the concentration of dissolved calcium ions in the reactor (C _{Ca, TRP} ) ranged from 794 to 846 mg/L. After analyzing the HRT, it was determined that optimal TR _R, CR _R , and TR _p were achieved when the HRT was greater than 33.9 minutes. Combined with the previously investigated cross-sectional load variables, due to the unique characteristics of the calcium molybdenum FHBC system, in addition to cross-sectional load, reaction time must also be considered. Combining these two factors, it was determined that when the feed molybdenum concentration was 2000 mg-Mo/L, an HRT of 33.9 minutes yielded the best TR _R, CR _R , and TR _p , reaching over 91%, 90%, and 94%, respectively.

Referring to FIG. 7 , it shows the effect of varying hydraulic retention times (HRT) on crystallization rate ( _CR %) and molybdenum removal efficiency (TR%) when the feed molybdenum concentration is varied in a calcium molybdate synthesis system operating at a pH R of 6, a feed molar ratio (Ca/Mo) of calcium ions in reagent 32 to molybdenum ions in molybdenum-containing solution 30 of 3.0, a reflux ratio (R) of 7.29, and a particle bed height (H) of 50 cm. Experiments revealed that when the HRT ranged from 16.9 to 23.7 minutes, the cross-sectional loadings for the experimental group with a feed molybdenum concentration of 2000 mg/L were 7.34 kg m ^-2 hr ^-1 and 5.24 kg m ^-2 hr ^-1 , respectively. Due to insufficient reaction time, the total molybdenum removal rate (TR _R ), crystallization rate (CR _R ) in the reactor, and total molybdenum removal rate (TR _P ) in the settling tank were all below 90%, and the dissolved molybdenum ion concentration (C _{Mo,TR R} ) in the reactor ranged from 160 to 295 mg/L. When the HRT ranged from 16.9 to 23.7 minutes, the cross-sectional loadings for the experimental group with a feed molybdenum concentration of 8000 mg/L were 29.34 kg m ^-2 hr ^-1 and 20.96 kg m ^-2 hr ^-1 , respectively. Its TR _R , CR _R , and TR _P were all above 92%. C _{Mo, TRR} ranged from 150 to 225 mg/L, and C _{Ca, TRR} ranged from 3250 to 3300 mg/L. C _{Mo, TRP} ranged from 80 to 100 mg/L, and C _{Ca, TRP} ranged from 3080 to 3180 mg/L. When the HRT ranged from 33.9 to 59.2 minutes, the cross-sectional loading for the experimental group with a feed molybdenum concentration of 2000 mg/L was 3.67 to 2.1 kg m ^-2 hr ^-1 . Due to ample reaction time, TR _R , CR _R , and TR _P reached over 91%, 90%, and 94%, respectively. The dissolved molybdenum ion concentration in the reactor (C _{Mo, TRR} ) was 87.6 mg/L, and the dissolved calcium ion concentration (C _{Ca, TRR} ) was 916 mg/L. When the HRT ranged from 33.9 to 59.2 min, the experimental group with a feed molybdenum concentration of 8000 mg/L had a cross-sectional loading of 14.67 to 8.38 kg m ^-2 hr ^-1 , and TR _R , CR _R , and TR _P values exceeding 97%, 96.5%, and 98.5%, respectively. The dissolved molybdenum ion concentration (C _{Mo, TRR} ) in the reactor ranged from 60 to 120 mg/L, and the dissolved calcium ion concentration (C _{Ca, TRR} ) in the reactor ranged from 3096 to 3425 mg/L. At the same HRT, the experimental group with a feed molybdenum concentration of 8000 mg/L had better TR _R , CR _R and TR _P than the experimental group with a feed molybdenum concentration of 2000 mg/L. When the HRT was controlled at 33.9 min and the feed molybdenum concentration was 8000 mg/L, TR _R , CR _R and TR _P reached 98.4%, 98.0% and 99.2% respectively. The dissolved molybdenum ion concentration in the reactor (C _{Mo, TRR} ) was 60.9 mg/L, the dissolved calcium ion concentration in the reactor (C _{Ca, TRR} ) was 3425 mg/L; the dissolved molybdenum ion concentration in the settling tank (C _{Mo, TRP} ) was 33.1 mg/L, and the dissolved calcium ion concentration in the reactor (C _Ca,TRP ) was 3439 mg/L. Increasing molybdenum concentration increases the reaction driving force of calcium molybdate, leading to higher removal and crystallization rates. This demonstrates that increasing the feed molybdenum concentration can compensate for a short reaction time.

The experimental results shown above demonstrate that under optimized conditions (C _Mo,in = 2000 mg-Mo/L, [Ca]/[Mo] = 3, pH _R = 6±0.3, HRT (hydraulic retention time) = 33.9 min, U (upflow velocity) = 30.4 m/hr, H (bed height) = 50 cm), the total molybdenum removal rate (TR _R ) in the reactor can reach over 91%, the total molybdenum removal rate (TR _P ) in the settling tank can reach over 95%, and the crystallization rate (CR _R ) can reach over 90%, demonstrating the excellent ability of calcium molybdate to form crystal particles. Furthermore, solid state analysis using XRD revealed that the particles synthesized using FBHC are crystalline calcium molybdate (CaMoO ₄ ). Surface analysis using SEM revealed that the calcium molybdate particles synthesized by FBHC ranged in size from 60μm to 100μm and were spherical in appearance.

The method of the present invention can treat molybdenum-containing wastewater, efficiently removing molybdenum ions from the water and recovering calcium molybdate crystals for effective reuse. Furthermore, the method of the present invention utilizes homogeneous nucleation crystallization technology, eliminating the need for prior addition of heterogeneous carriers to the fluidized bed reactor. This results in crystals of high purity, making them suitable for subsequent treatment applications. Therefore, the method of the present invention not only replaces chemical coagulation to achieve excellent treatment results, but also avoids the drawbacks of traditional chemical or biological methods, achieving product resource utilization. It also boasts the advantages of high efficiency, low cost, and the absence of sludge.

In the foregoing description, the present invention is described with respect to specific embodiments only. However, various variations and modifications are possible based on the features of the present invention. Therefore, obvious substitutions and modifications that can be made by persons skilled in the art will still be included within the scope of the patent claimed by the present invention.

10: Reactor 12: Lower section 14: Upper section 16: Solution inlet 18: Chemical inlet 20: Water outlet 22: Circulation line 23: Water outlet pipe 24: pH detector 26: Sedimentation tank 28: Pump 30: Molybdenum solution 32: Chemical

Claims (9)

A method for synthesizing calcium molybdate crystals from molybdenum-containing wastewater using fluidized bed crystallization technology comprises: providing a fluidized bed reactor having a lower section and an upper section, the lower section having a solution inlet and a reagent inlet, the upper section having a water outlet, and a recirculation pipe body between the lower and upper sections; introducing a molybdenum-containing solution and a reagent into the fluidized bed reactor through the solution inlet and the reagent inlet, respectively, for mixing; The molybdenum-containing solution mixed with the reagent is flowed from the lower section of the reactor to the upper section of the reactor; and the molybdenum-containing solution mixed with the reagent is circulated back to the lower section through the reflux line so that molybdenum ions in the molybdenum-containing solution react with the reagent to produce calcium molybdate particles. The pH value of the reactor is controlled between 6 and 11, the feed molar concentration ratio of calcium ions in the reagent to molybdenum ions in the molybdenum-containing solution is controlled between 1 and 4, and the cross-sectional load of the molybdenum-containing solution is controlled between 1.8 and 22 kg m ^-2 h ^-1 . The method as claimed in claim 1, wherein the agent is calcium chloride. The method as described in claim 1, wherein the agent is calcium hydroxide. The method as described in any one of items 1 to 3 of the patent application, wherein a molybdenum-containing solution and a reagent are first mixed in the reactor to produce calcium molybdate crystal particles as a carrier. The method as described in any one of items 1 to 3 of the patent application, wherein the hydraulic retention time of the reactor is controlled between 33.9 and 59.2 minutes. The method of any one of claims 1 to 3, wherein the feed molar concentration ratio of calcium ions in the reagent to molybdenum ions in the molybdenum-containing solution (Ca/Mo) is controlled to be between 3 and 4. The method as described in any one of items 1 to 3 of the patent application, wherein the pH value of the reactor is controlled between 6 and 9. The method of any one of claims 1 to 3, wherein the cross-sectional loading of the molybdenum-containing solution is between 3.67 and 22 kg m ^-2 hr ^-1 . The method of any one of items 1 to 3 of the patent application, wherein the initial molybdenum concentration of the molybdenum-containing solution is controlled to be 2000 mg/L, the pH of the reactor is controlled to be 6 ± 0.3, the feed molar concentration ratio of calcium ions in the reagent to molybdenum ions in the molybdenum-containing solution is controlled to be 3.0, and the hydraulic retention time of the reactor is controlled to be between 33.9 and 59.2 minutes.