Each process test link of the nuclear fuel cycle will produce low-level radioactive wastewater (referred to as “low-level wastewater”), such as uranium-containing fluorine-containing low-level wastewater from uranium conversion and purification, boric acid-containing low-level wastewater from nuclear power plants, and various types of nuclear fuel reprocessing. Complex low-level wastewater, high-salt low-level wastewater generated from spent fuel reprocessing, etc.
To protect the environment and human health, these wastewaters must be disposed of in a safe, economical and efficient manner. At present, although this kind of wastewater has been effectively treated and disposed, there are still problems such as complex treatment process, high cost and low efficiency.
Membrane distillation has gradually attracted widespread attention as a new membrane separation technology. Compared with traditional low-level wastewater treatment methods, such as evaporation, chemical precipitation, filtration, ion exchange or a combination of these processes, membrane distillation The technology has many advantages such as simple process flow, low application cost and high efficiency, especially for the concentration and crystallization treatment of high-salinity wastewater, which has irreplaceable advantages.
In this study, the high-salt and low-level discharge wastewater produced by a nuclear facility was treated as the object, and the main chemical compositions Ca(NO3)2 and Al(NO3)3 were used as the simulated source items to prepare the feed liquid, and the air-gap membrane distillation module was studied. The effect of system operating temperature, flow rate and concentration on membrane flux and rejection rate was verified, and the purification effect of the system on real high-salt low-level waste liquid was verified.
Experimental part
1.1 Key performance evaluation parameters of membrane distillation
Membrane distillation is a new type of membrane separation process using a microporous hydrophobic membrane with the vapor pressure difference on both sides of the membrane as the driving force for mass transfer.
As shown in Figure 1, the aqueous solutions on the cold and hot sides are separated by a hydrophobic microporous membrane. Due to the hydrophobicity of the membrane, the aqueous solutions on both sides cannot pass through the membrane pores and enter the other side, while the water vapor at the interface between the aqueous solution on the hot side and the membrane cannot enter the other side. When the partial pressure is higher than the cold side, the water vapor will pass through the membrane pores from the hot side (high vapor pressure) and enter the cold side (low vapor pressure) to condense. This evaporation-mass transfer-condensation process is called membrane distillation.
Membrane flux refers to the volume of distillate passing through a unit membrane area per unit time. The calculation formula is:
In the formula, J is the membrane flux, V is the volume of distillate within a certain period of time, A is the effective evaporation membrane area, and t is the time required to collect V distillate.
The retention rate, which characterizes the decontamination effect of the membrane distillation process, is calculated as:
In the formula, R is the retention rate, and ρF and ρP are the mass concentrations of ions in the raw material liquid and the distillate, respectively.
The purification coefficient DF refers to the ratio of the radioactive activity AF before the waste liquid treatment to the activity AP of the purified waste liquid.
1.2 Materials and Equipment
Calcium nitrate (Ca(NO3)2·4H2O), aluminum nitrate (Al(NO3)3·9H2O), analytically pure; deionized water, low-level wastewater, a nuclear company.
Membrane module, tubular module (PTFE); magnetic circulation pump, DR-3; constant temperature circulator, DTY-8A; spectrum analyzer, 300X; electronic balance, WT-B50001; conductivity meter, DDSJ-308A.
1.3 Experimental principle
Tubular membrane modules were used to carry out experiments by air-gap membrane distillation. The experimental system of membrane distillation is shown in Figure 2.
When the hot-side feed liquid is heated to the specified temperature in the raw water tank, the magnetic circulation pump passes through the pre-filter to the hot side of the membrane module and then returns to the original water tank. The cold side returns to the cold water tank, so that the circulation of the hot side and the cold side is established, the hot side material volatilizes under the action of the steam pressure difference, and the water vapor is cooled to form purified water after passing through the membrane.
1.4 Experimental method and content
The simulated feed solution was prepared using Ca(NO3)2 and Al(NO3)3 as source terms to prepare mixed solutions of nitrates with different contents of Al3+ and Ca2+ molar ratio of 1:1. Thermal experiment analysis detected that the total α radioactive specific activity Σα of the wastewater was 1.26kBq/L, and the total β radioactive specific activity Σβ was 30.4kBq/L. At the same time, the waste liquids with different salt contents were adjusted and prepared to carry out thermal verification experiments.
In the experimental process, the hot-side flow rate, hot-side temperature, and hot-side salt content are used as variables, and the nitrate mixed solution is used as the raw material solution to obtain the optimized process operation control parameters. The influence of membrane distillation effect was verified by thermal experiments under optimized process operating conditions.
Data collection started after the experimental system was stable for 30 minutes. The raw material liquid and the condensed liquid were taken every 10 minutes, and the conductivity was measured after cooling to room temperature. The data were collected in parallel for 5 times under a single control condition.
The cold experimental results are calculated by the conductivity (conductivity and the mass concentration of the solute in the solution have a linear relationship within a certain range) to characterize the mass concentration of salt, and the membrane flux and rejection rate are calculated; the thermal experimental results are calculated by the radioactive specific activity of the waste liquid , calculate the purification coefficient.
Results and discussion
2.1 Influence of circulation flow on the hot side
The mass concentration of the hot side solution salt is 10g/L, the temperature is 65°C, the circulation volume flow rate of the cold side is qV1=2m3/h, and the temperature is 20°C. The influence of the hot side circulation flow rate qV2 on the membrane flux and rejection rate under the operating conditions is shown in the table. 1.
It can be seen from Table 1 that with the increase of the circulating flow rate on the hot side, the membrane flux increases significantly, while the rejection rate remains above 99.8%. The reason is that the increase of the circulating flow rate on the hot side can change the liquid flow state at the membrane surface, increase the heat and mass transfer coefficient, and reduce the temperature difference and content difference between the membrane surface and the main body of the feed liquid, thereby reducing the temperature difference polarization and content polarization. effect, the membrane flux increases.
The highest membrane flux occurs when the circulating flow rate on the hot side is 2m3/h, and the flux exceeds this flux with a slight downward trend. A slight downward trend in volume.
The increase of the hot-side flow rate can increase the membrane flux, and the rejection rate is kept at a high level. According to the experimental data, the hot-side circulating flow rate should be kept at 2m3/h.
2.2 Influence of circulating temperature on the hot side
The salt mass concentration of the hot side solution is 10g/L, the circulation volume flow rate of the hot side and the cold side is 2m3/h, and the temperature of the cold side is 20℃. The influence of the hot-side solution temperature on the membrane flux and rejection rate under this operating condition is shown in Table 2.
It can be seen from Table 2 that with the increase of the hot-side solution temperature, the membrane flux increases, especially at 55°C to 65°C, and then tends to be stable, indicating that the hot-side temperature has a greater impact on the membrane flux; The retention rates were kept above 99.8%.
However, the retention rate decreased slightly when the hot side temperature exceeded 65 °C. The reason is that the increase in the temperature of the hot side increases the temperature difference on both sides of the membrane, thereby increasing the driving force of water vapor through the membrane wall, resulting in more vapor permeating through the membrane pores;
In addition, increasing the temperature can reduce the viscosity of the solution, weaken the effect of concentration polarization, and increase the diffusion coefficient of water vapor. Therefore, when the temperature increases, the flux increases more obviously, and with the increase of temperature, the driving force increases, resulting in a small number of ions permeating, so the interception rate decreases slightly.
The hot-side temperature has a positive effect on the increase of flux, and the retention rate is affected by temperature, but remains at a high level. According to the data obtained from the experiment, considering factors such as economic energy consumption, it is advisable to keep the temperature of the hot side at 65 °C.
2.3 Influence of the liquid salt content of the hot side feed
The circulation volume flow rate of the hot side and the cold side are both 2m3/h, and the temperature of the hot side and the cold side are 65 and 20 °C, respectively.
It can be seen from Table 3 that with the increase of the solution content on the hot side, the membrane flux decreases significantly, and it decreases by about 1/3 from the mass concentration of 10 g/L to 320 g/L. The reason is that the higher the salt content of the thermal solution, the lower the steam driving pressure on both sides of the membrane, thus reducing the membrane flux.
The interception first increased and then decreased, but remained at a relatively high level. The reason for the analysis was that when the salt content increased within a certain range (preliminarily determined that the mass concentration was lower than 150g/L), the soluble salt ions were attached to the membrane surface, reducing the membrane size. The cracks in the structure, that is, the pore size of the membrane is reduced, which is beneficial to the increase of the rejection rate, but when the range is exceeded, the number of salt ions increases and a few ions can pass through the membrane and enter the cold side, thereby reducing the rejection rate.
2.4 Effect of acidity of feed liquid
Considering that this technology may treat high-acid wastewater in the future, the influence experiment of different nitric acid content was carried out (the mass concentration of salt is 10g/L). After the liquid circulation of the cold and hot sides runs for a period of time, when the water production of the device reaches a stable level, sampling is performed every 10 minutes. The results are shown in Table 4.
It can be seen from Table 4 that the flux and retention rate did not change much under each nitric acid content, indicating that this type of membrane material can also operate well under acidic conditions.
2.5 Thermal Experiment Verification
Adjust the circulation volume flow rate of the hot side to 2m3/h and the temperature to 65°C, and adjust the circulation volume flow of the cold side to 2m3/h and the temperature to be 12°C. The experimental results of thermal experiments are shown in Table 5.
It can be seen from Table 5 that the total α radioactivity (Σα) and total β radioactivity (Σβ) in the distillate are basically stable, and are far lower than the GB8978-1996 emission standards (Σα≤1Bq/L, Σβ≤10Bq/L ), the purification coefficient is greater than 103.
Conclusion
The influence of the hot side circulation flow on the membrane distillation flux is more significant. Increasing the circulation flow on the hot side can increase the flux of the air-gap membrane distillation. The circulating volume flow rate on the hot side is selected as 2m3/h, which will not bring too much energy consumption and ensure the best membrane flux.
The hot-side temperature has a significant effect on the membrane distillation flux. For the experimental temperature, the increasing trend of flux slowed down after 65 °C, and the retention rate decreased slightly. Under the premise of ensuring the flux of membrane distillation, it is more appropriate to select 65 °C for the hot side temperature.
With the gradual increase of the salt content of the hot side solution, the membrane flux shows a downward trend, and the rejection rate also decreases slightly when the salt content reaches a certain level. A decrease of 0.08%, which verifies the feasibility of this type of module for the treatment of waste liquid with high salt content.
To sum up, when the temperature of the cold side and the hot side are maintained at 20 and 65°C, respectively, and the volume flow of the cold side and the hot side are both controlled at 2 m3/h, the operation effect of membrane distillation is better.
Thermal verification experiments show that this technology has a good purification effect on high-salt and low-level wastewater. Within the scope of the experiment, the purification coefficient of the distillate produced by the air-gap component is greater than 103, and the control index is far lower than the GB8978-1996 emission standard.
The research results show that membrane distillation technology is basically suitable for the treatment of multiple types of low-level wastewater, and can be used for purification and concentration of low-level wastewater from other nuclear-related units.