2.1 Chemicals

TRPO was obtained from CYTEC, USA. Its commercial name is Cyanex 923. It mainly comprises four trialkylphosphine oxides, namely, R₂R’PO (42%), R₃PO (14%), R’₃PO (8%), and RR’₂PO (31%), where R’ and R are n-hexyl and n-octyl groups, respectively [27]. TRPO was diluted with kerosene (Jinzhou Refinery Factory, China) to 30% TRPO-kerosene (v/v), and purified before use, as described in a previous study [12]. The viscosity, density, and surface tension of the 30% TRPO-kerosene are 2.82×10^-3 Pa•s, 787.8 kg/m³, and 32.35×10^-3 N/m, respectively [28]. HNO₃ and Nd(NO₃)₃ obtained from Beijing Chemical Works, China, were employed as analytical reagents.

2.2 Description of the industrial-scale ACE

The industrial-scale ACE includes four main parts, namely, the housing, the rotor, the drive unit, and the motor, as shown in Fig. 1. The rotor’s inner diameter is 260 mm. When the rotor speed is 1600 r/min, the relative centrifugal force generated by the spinning rotor is about 372 G. The explosion-proof motor (3-phase 380V AC, 5.5 kW) is adopted for fire prevention. A magnetic coupling and "hanging" rotor structure are applied in the drive unit of the industrial-scale ACE. The magnetic coupling provides torque to the rotor and enables the motor to be independently and easily replaced [16]. The magnetic coupling and the "hanging" rotor structure provide ease of remote maintenance and a radioactive seal. Table 1 lists the major geometric dimensions of the industrial-scale ACE.

Fig. 1Full-screen View

(Color online) Sketch (a) and photograph (b) of the industrial-scale ACE

As shown Fig. 1(a), immiscible feed liquids with different densities flow into the annular mixing zone through a separate inlet, and are then rapidly mixed by the spinning rotor to form a dispersion [29]. In general, the denser phase is the aqueous phase, while the less dense phase is the organic phase. Radial vanes attached to the housing stop rotation of the dispersion when it flows down to the bottom of the housing, and direct the dispersion through the rotor inlet into the rotor. Here, it is accelerated to the rotor speed towards the rotor wall by vertical baffles inside the rotor, and quickly separated into two phases by their density difference as it flows upwards under high centrifugal force [30]. The aqueous phase moves towards the peripheral wall of the rotor, while the organic phase is forced to move towards the center of the rotor. The separation rate of the dispersion is a function of the settling velocity under the centrifugal action (rω²), the drop size distribution, the surface tensions and viscosities of the two phases, as well as the density difference between the aqueous and organic phases [31]. Lastly, the separated aqueous and organic phases flow over their respective weirs in the rotor into their respective collectors and tangential outlets in the housing, and flow into adjacent stages or collection vessels by gravity.

2.3 Hydraulic tests

Hydraulic tests of the single-stage industrial-scale ACE were performed using the experimental setup shown in Fig. 2. Here, kerosene was used as the organic phase and deionized water was used as the aqueous phase. The hydraulic performance of the industrial-scale ACE was evaluated by measuring the maximum throughput (sum of the aqueous and organic flowrates) for which the maximum allowable cross-phase entrainment did not exceed 1% at a given rotor speed and flow ratio (aqueous/organic, A/O). Cross-phase entrainment is defined as aqueous phase carryover in the organic effluent stream, or organic phase carryover in the aqueous effluent stream. For most applications, phase separation is generally considered satisfactory if each effluent from the ACE has entrained <1% of the other phase. This condition results in high stage efficiency [32]. Each effluent stream was collected in a vessel to determine the amount of cross-phase entrainment according to a predetermined time after the industrial-scale ACE reached steady-state operation. After the collected fluid in each vessel was completely separated, the cross-phase entrainment in each vessel was measured by volume measurement using a graduated cylinder.

Fig. 2Full-screen View

The setup for both hydraulic performance and mass-transfer tests

The hydraulic tests were performed with continuous recycling of the two effluent streams to the feed tanks. For each operation, the industrial-scale ACE was first started at the predetermined rotor speed. Then, the aqueous phase was pumped into the industrial-scale ACE at a constant flowrate by a magnetic drive pump (10.8 m³/h, 40CQ-20, Shanghai Lian Quan Pump Manufacturing Co. Ltd., China). If the aqueous phase flowed out from the heavy phase outlet, the organic phase was pumped into the industrial-scale ACE at a constant flowrate by another magnetic drive pump. Otherwise, the organic phase flowed out through the heavy phase outlet. The flowrate of each phase liquid was adjusted by the respective rotameter. After the ACE reached steady-state, the aqueous and organic effluent streams were sampled and visually examined for cross-phase entrainment. Generally, the operation of the ACE should be close to steady-state after three residence times (total liquid hold-up volumes) of the fluid have passed though the ACE (or three total liquid hold-up volumes of the ACE) [32]. In the present hydraulic tests, steady-state operation of the industrial-scale ACE was reached after more than four total liquid hold-up volumes (4×0.035 m³) of two phases had passed through the ACE. If cross-phase entrainment was <1% in both collected samples, the flowrate of one phase was kept constant, while the flowrate of the other phase was increased incrementally. These processes were repeated until cross-phase entrainment in either effluent stream was ≥1%. Later, the flowrate of the phase whose initial flowrate was kept constant was increased to a designed flowrate, and the flowrate of the other phase was incrementally increased. Each time, samples from the effluent streams were collected to check for cross-phase entrainment after reaching steady state operation. Experiments were carried out for different rotor speeds and flow ratios (A/O). A frequency converter (7.5 kW, Micromaster 420, SIEMENS, Germany) was used to adjust the rotor speed of the ACE and ensure smooth start-up of the motor.

The minimum rotor speed of the ACE operation was determined according to the following equation [33]:

For the industrial-scale ACE, when ΔH is equal to the rotor length, ω is the minimum rotor speed, namely, 990 r/min. However, considering mass transfer efficiency, the normal rotor speed should be higher than the minimum rotor speed.

The liquid hold-up volume affects the interfacial area, mixing, and mass transfer efficiency of the ACE [29], which depends on rotor speed, total flowrate, rotor geometry, and housing geometry. The liquid hold-up volume in both the rotor and the housing of the industrial-scale ACE was determined by a liquid discharging method [34]. Firstly, when the industrial-scale ACE reached steady-state, the liquids in the housing were drained into a vessel by opening the discharge valve under the housing bottom after the pumps were stopped. Secondly, the motor was shut down to stop rotation of the rotor, and the liquid in the rotor was drained into another vessel. Lastly, volumes of the two phases in each vessel were determined after complete settling of the dispersion using a measuring cylinder. Tests for obtaining the liquid hold-up volume were performed for different rotor speeds, total flowrates, and flow ratios.

2.4 Mass transfer tests

In the TRPO process, 30% TRPO-kerosene can efficiently extract trivalent actinides and lanthanides from HLLW. Trivalent actinides and lanthanides have similar chemistry, so extraction of trivalent actinides (Am) can be simulated by extraction of Nd³⁺ from HNO₃ solution using 30% TRPO-kerosene in the experimental studies. The mass-transfer efficiency of the industrial-scale ACE was evaluated by extracting Nd³⁺ and HNO₃ from HNO₃ solution containing Nd³⁺ using 30% TRPO-kerosene. The setup for mass-transfer tests is the same as that for hydraulic tests (Fig. 2). For each test, the aqueous phase was first pumped by a magnetic drive pump at a constant flowrate into the industrial-scale ACE, and the organic phase was then pumped by another magnetic drive pump at a constant flowrate into the industrial-scale ACE. Each time, the flowrate of each effluent stream was measured to obtain the exact flowrate of each phase. Samples for Nd³⁺ and HNO₃ concentration analysis were taken from each effluent stream after the ACE reached steady-state operation. For calculating the extraction stage efficiency of the industrial-scale ACE, two phase feeds were equilibrated at a determined phase ratio by violently mixing them to obtain equilibrium concentrations of both the aqueous and organic phases. The Nd³⁺ concentration of the aqueous phase was obtained by ICP-AES (Inductive Coupled Plasma). The Nd³⁺ concentration of the organic phase was obtained by firstly stripping with 5.5 mol/L HNO₃ three times, and then analyzing the Nd³⁺ concentration of the stripping solution by ICP-AES [20]. The HNO₃ concentrations of the organic and aqueous phases were obtained by titration using a standard NaOH solution, and K₂C₂O₄ was added as the masking agent. Both the initial Nd³⁺ and HNO₃ concentrations of the organic phase were 0, while the initial Nd³⁺ and HNO₃ concentrations of the aqueous phase were 97.5 mg/L and 0.5 mol/L, respectively. The Murphree extraction stage efficiency relationship was used to express the mass transfer efficiency as follows [33]

Mass transfer tests were conducted for different flow ratios (A/O), rotor speeds, and total flowrates.

3.1 Hydraulic performance

3.1.1 Maximum throughput

The maximum throughput Q_m of the ACE was estimated by the following equation [33]

$Q_{\text{m}}=1.386\times {10}^{-4}\omega D_i^{2}L.$ (4)

For the industrial-scale ACE, ω is 1600 r/min, D_i is 26 cm, and L is 46 cm, so Q_m is approximately 6.9 m³/h. Effects of the flow ratio (A/O) on the throughput of the industrial-scale ACE at different rotor speeds are presented in Fig. 3. The single-stage hydraulic performance of the industrial-scale ACE is excellent. The maximum throughput can reach 10 m³/h for a kerosene-water system at a rotor speed of 1600 r/min, which is higher than the estimated value (approximately 6.9 m³/h). The maximum throughput varies with the flow ratio (A/O) at a defined rotor speed. The normal limit to the ACE throughput is determined by the thickness of the dispersion band in the rotor (Fig. 1). This band is actually a hollow dispersion cylinder. At a given rotor speed, the dispersion band thickness increases as total flowrate increases until, at the maximum throughput, the dispersion band reaches either the light phase weir (with subsequent appearance of the aqueous phase in the organic effluent stream) or the underflow (with subsequent appearance of the organic phase in the aqueous effluent stream) [13]. The flow ratio (A/O) has an important effect on the ACE throughput. As the flow ratio (A/O) decreases, the liquid height over the light phase weir increases, while the liquid height over the heavy phase weir decreases. This shifts the dispersion band outward from the light phase weir to the aqueous underflow. Thus, at high flow ratios (A/O), phase entrainment (when the maximum throughput is exceeded) will be due to the aqueous phase in the organic effluent stream. At low flow ratios (A/O), phase entrainment (when the maximum throughput is exceeded) will be due to the organic phase in the aqueous effluent stream. At intermediate flow ratios (A/O), the full volume of the separating zone will be used, and both effluent phases will experience other-phase entrainment at the same time if the maximum throughput is exceeded. At this flow ratio (A/O), the ACE capacity is at a maximum. It is also observed that, with an increase in rotor speed, the maximum throughput increases at the same flow ratio (A/O), because the increase in rotor speed enhances separation of the two phases due to the increased centrifugal force.

Fig. 3Full-screen View

(Color online) Effects of flow ratio on the throughput of the industrial-scale ACE at different rotor speeds

The maximum throughput differs for different extraction systems. A good engineering design practice is to keep the nominal throughput in an ACC to no more than 67% of the nominal maximum throughput [32, 35]; thus, the industrial-scale ACE can reach 7 m³/h for a 30% TRPO-kerosene-HNO₃ solution system, which is almost equal to the estimated value (about 6.9 m³/h).

3.2 Mass transfer efficiency

During mass transfer, the dispersed phase is converted into small droplets by inputting the mixing energy in the form of shear caused by the differential velocity between inner and outer layers of the annular mixing zone. As the bulk dispersed phase is converted into small drops, the surface area available for mass transfer increases several times. Generally, the higher the rotor speed, the higher the mass-transfer efficiency. Tables 3 and 4 present the extraction stage efficiency of the single-stage industrial-scale ACE for extracting Nd³⁺ and HNO₃, respectively. In all tests, the extraction stage efficiencies are more than 98%. It is clear that excellent phase mixing is achieved, even at relatively low rotor speed. The mass transfer efficiency of the ACE is also a complex function of various operating variables such as rotor speed, total flowrate, and interfacial tensions between the two phases [37]. The interfacial tension changes with the flow ratio (A/O). Generally, a lower total flowrate, lower flow ratio (A/O), or higher rotor speed results in an increased extraction efficiency [20].

The residence time in the mixing chamber varies from 18 s to 5 s. These test results demonstrate that equilibrium in Nd and HNO₃ extraction is rapidly attained. Moreover, no clear trends are observed according to rotor speed, total flowrate, or flow ratio (A/O). The data show good mass transfer efficiency in the industrial-scale ACE. There are two possible explanations for extraction stage efficiencies over 100%. One may be analytical errors, and the other may be the large flowrate of each phase making it difficult to precisely control each flowrate. Accordingly, the phase ratio used to obtain the equilibrium concentration is inconsistent with the actual flow ratio. Compared with the laboratory-scale ACE (20 mm rotor diameter) used for extracting Nd³⁺ from HNO₃ solution containing Nd³⁺ [36], and the pilot-scale ACE (70 mm rotor diameter) used for extracting HNO₃ from HNO₃ solution with 30% TRPO-kerosene [20], the extraction stage efficiency of the industrial-scale ACE for extracting Nd³⁺ and HNO₃ with 30% TRPO-kerosene from nitric acid solution containing Nd³⁺ is higher. The mixing intensity experienced by the liquid in the mixing zone is proportional to the power consumption per unit of annular volume. Generally, the larger the ACE, the greater power consumption per unit of annular volume [37]. Thus, the mixing intensity of the two phases in the industrial-scale ACE is higher than that in both the laboratory-scale and pilot-scale ACEs. In addition, the volume fluctuations in the larger ACE are much smaller relative to the total liquid volume than in the smaller ACE, particularly the laboratory-scale ACE (20 mm rotor diameter).