Fang Zhou, Lisen Zhang, Zhiwei Wang, Yixin Zhang, Ruan Chi*

Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China

Keywords:Weathered crust elution-deposited rare earth ores Surfactant Kinetics Mass transfer

A B S T R A C T Surfactants were proposed to be added into magnesium sulfate solution to improve the leaching process of weathered crust elution-deposited rare earth ores(WREOs).Effects of surfactants and their concentration on the seepage of leaching solutions and the leaching efficiency of rare earth(RE)and aluminum(Al)were investigated,and the leaching kinetics,the mass transfer process,the adhesion work and the adhesion work reduction factor were analyzed to reveal its strengthening leaching mechanism.The results show that cetyltrimethylammonium bromide (CTAB) has a better strengthening effect on the leaching process than dodecyl trimethyl ammonium bromide(DTAB),sodium dodecyl sulfate(SDS),sodium oleate and oleic acid.In the presence of 0.04%CTAB in 0.2 mol/L solution,the permeability coefficient of WREOs increases from 0.945×10-5 to 1.640×10-5 cm∙s-1,and the leaching efficiency of RE increases from 80%to 90%, confirming the promotion of surfactants on the leaching process of WREOs.Kinetic analysis shows that the leaching process conforms to the inner diffusion control model, and the leaching kinetics equations of RE and Al related to CTAB content are obtained.Mass transfer discussion shows a smaller height equivalent to theoretical plate(HETP)of RE and Al at CTAB content of 0.04%,suggesting the higher mass transfer efficiency here.According to the interfacial properties of leaching solutions,the calculated adhesion work and the adhesion work reduction factor further demonstrate the strengthening leaching effect of CTAB on the leaching process of WREOs.

1.Introduction

Weathered crust elution-deposited rare earth ores (WREOs) is rich in medium and heavy rare earth elements,with complete rare earth distribution, low radioactivity and great comprehensive utilization value [1,2].Rare earths in such ores are mainly adsorbed on clay minerals as hydrated ions or hydroxyl hydrated ions.The in-situ leaching process is widely used in industry,and ammonium salts such as ammonium sulfate are applied as leaching agents to leach rare earths through ion-exchange reaction.However, after leaching by the ammonium salt, the residual ammonium salt in the ores can leak into the soil and water around the mining area,resulting in serious ammonia nitrogen pollution in the soil of the mining area and eutrophication of the water body [3].In order to control ammonia nitrogen pollution,in recent years,many experts have carried out a lot of researches on ammonium-free leaching agents such as magnesium salts [4,5].It is found that 0.2 mol/L of magnesium sulfate shows a good leaching efficiency[6].Magnesium salt and magnesium hydroxide can be used as leaching agent and precipitating agent to recover rare earth without ammonium salt.However,these researches mainly focused on the agent selection and the process optimization.Actually, it is also the key to strengthening the in-situ leaching process of WREOs such as accelerating the permeability velocity of leaching solution in the orebody and improving the mass transfer efficiency of ion exchange reaction between leaching solution and rare earth ions.There are few studies on the seepage, kinetics and mass transfer of leaching agents in the in-situ leaching process of WREOs [7,8].It was also found that a certain amount of leaching aids can effectively improve the seepage and mass transfer of the leaching solution during the in-situ leaching process of WREOs [9].

Due to the porous property of WREOs,the surface tension of the leaching solution is an important factor affecting the entry of the leaching solution into these pores [10].The molecular structure of surfactants is amphiphilic, and its hydrophilic groups can affect the surface tension of the leaching solution thereby effectively improving the wettability of the leaching solution on the mineral surface[11].Furthermore,surfactants can promote the adsorption of leaching agent on the surface of mineral particles,which is conducive to the entry of leaching agent into pores.The surfactants can promote the leaching of potassium in phosphate-potassium associated ore [12].The added surfactant adsorbed on the cracks in the ore body promotes a better penetration of the leaching solution into the ore body for chemical reactions, thereby improving leaching efficiency [13].The research showed that nature biosurfactant, saponin, can extract rare earth elements from fly ash[14–17].The interaction mechanism due to the adopted saponin with the fly ash surface phase during the leaching of these metals based on hydrophobicity and hydrophilicity was proposed.

In this study, five surfactants such as CTAB, DTAB, SDS, sodium oleate and oleic acid were selected and compared to improve the leaching process of WREOs in the magnesium sulfate leaching system.The effects of surfactant concentration on the permeability of ore-body, and the leaching efficiency of RE and Al were investigated to find the optimum surfactant amount.The leaching kinetics,the mass transfer process,the adhesion work and the adhesion work reduction factor were analyzed to demonstrate the strengthening leaching effect of surfactant as leaching aids on the leaching process of WREOs with magnesium sulfate.

2.Experimental

2.1.Materials

Reagent grade magnesium sulfate, zinc oxide, ethylenediaminetetraacetic acid disodium, ascorbic acid, sulfosalicylic acid,hexamethylenetetramine, xylenol orange, hydrochloric acid,ammonia,and kaolinite were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai.CTAB, DTAB, SDS, sodium oleate, oleic acid and montmorillonite were purchased from McLin Biochemical Technology Co., Ltd., Shanghai.Illite were purchased from Wengjiang Chemical Reagent Co., Ltd., Guangdong.

WREOs samples are from Dingnan, Jiangxi Province, China.The particle size analysis of WREOs was screened by sieve.The results show that in the particle size range of 0.830, -0.830–0.250,-0.250–0.150,-0.150–0.105 and-0.105 mm,the mass proportion of rare earth ore(REO)is 34.28%,20.25%,10.31%,2.52%and 32.64%,respectively.

The chemical composition of WREOs samples was analyzed by the EDX-4500 X-ray fluorescence spectrometer (Jiangsu Tianrui Instrument Co., Ltd.).The analysis shows that the mass ratio of REO is 0.13%.The distribution of rare earth ions in the WREOs samples was determined by the 7900 inductively coupled plasma mass spectrometer(Agilent Technologies Co.,Ltd.).The results show that the mass ratio of medium and heavy rare earths in the ionic phase in the ore sample is 51.81%.

The X-ray diffraction measurement for WREOs samples was performed by the Empyrean XRD (D8-ADVANCE, Bruker, Karlsruhe).The results shown in Fig.1 were analyzed by X’pert Highscore software.The results show that the clay minerals in WREOs samples are mainly composed of quartz,kaolinite,illite,halloysite,feldspar and mica,and the clay mineral content is 32.2%,indicating a highly weathered WREOs samples.

Fig.1.X-ray diffraction pattern of WREOs samples.

2.2.Experimental method

2.2.1.Column leaching

To simulate the leaching process of WREOs, a column leaching is used in this study.For the penetration experiment is shown in Fig.2a, 35 g dried WREOs are loaded into a glass column with an inner diameter of 30 mm, and 1–2 layers of filter paper are laid on the surface of the ore layer.The leaching solution is sent to the top of ore layer to form a liquid column height of 8 cm, and the height of the liquid column remains unchanged during the leaching process to keep the water head constant.The leaching solution is collected at the bottom of the glass column, and the time and volume of the leaching solution are recorded at intervals.For the leaching experiment is shown in Fig.2b, 35 g of dried WREOs are weighed and slowly and evenly loaded into the glass column.1–2 layers of filter paper are laid on the surface of the ore layer, and 70 mL leaching solution with a liquid-solid ratio of 2:1 is added from the top of the glass column by a constant flow pump at 0.6 mL/min.When the leachate flows out from the lower end of the glass column,the timer begins.At intervals,the leachate is collected and the time and the volume recorded, and then the content of rare earth and aluminum in leachate are analyzed using ethylene diamine tetraacetic acid(EDTA)complexometric titration[18–20].

Fig.2.Schematic illustration of leaching process.

2.2.2.Contact angle measurement

Since the presence of surfactant can change the interfacial properties of the leaching solutions,and then affect its interaction with WREOs, the interfacial property of leaching solutions was mainly discussed in this study.The WREOs surface was replaced by a glass slide with smooth surface and negative electricity to test the contact angle of the leaching solutions on WREOs to investigate the effect of interfacial property of leaching solutions on the penetration and the leaching process.The contact angle was measured at ambient temperature by the JC-2000DM contact angle measuring instrument (Shanghai Zhongchen Digital Technology Equipment Co., Ltd.).The contact angle measurements were repeated three times for the same location and three sets of locations were used for a given experimental condition.The average values of nine measured average contact angle values of a given sample were reported.

2.2.3.Surface tension measurement

The surface tension of leaching solution was measured by the JK99D automatic tension measuring instrument (Shanghai Zhongchen Digital Technology Equipment Co.,Ltd.),and the test temperature was controlled at (20±0.5) ℃.

2.3.Analytical methods

2.3.1.Permeability coefficient

Permeability velocity and permeability coefficient were calculated by Darcy’s Law shown as follows [21]:

where Q is the seepage discharge, mL/s; ʋ the penetration velocity,cm/s;A the transverse area,cm2;κ the permeability coefficient,cm/s; △H the head loss, cm; L the ore height, cm; and J the hydraulic gradient.

2.3.2.Kinetic analysis

The leaching kinetics of WREOs by magnesium sulfate with addition of surfactant was analyzed by the following four models[22]:

(1) Chemical reaction controls:

(2) Outer diffusion controls:

(3) Inner diffusion controls:

(4) Mixed controls:

where α is the leaching efficiency; k the apparent rate constant,min-1; t the leaching time, min; C0the initial concentration of leaching solution, mol/L; M the quality of rare earth ore, g; r0the initials particle size of rare earth ore,mm;and ρ the molar concentration of rare earth ore.

2.3.3.Mass transfer analysis

Based on the chromatographic plate theory, Gauss fitting was carried out on the mass transfer process of WREOs to calculate the theoretical plate number(n)and the height equivalent to a theoretical plate (HETP) [23]:

where n is the theoretical plate number;VRthe retention volume of the outflow curve, mL; V1/2the half-height width of the outflow curve, mL; and L the ore height, mm.

2.3.4.Adhesion work and adhesion work reduction factor

The adhesion work(W,J/m2)between the leaching solution and the mineral particles is significantly affected by the surface tension(σ,mN/m)of leaching solution and the contact angle(θ)of leaching solution on the mineral, which can be calculated by the following equation [24]:

To compare the influence of surfactant on adhesion work, the adhesion work reduction factor (E) is determined as follows:

where the subscript 1 and 0 represent the leaching solution with and without surfactant, respectively.

3.Results and discussion

3.1.Effect of different surfactants on leaching process

The exploring experiment results show that all of the five surfactants can promote the leaching process.However, oleic acid shows a relatively low solubility among the five surfactants.In order to better compare the effects of the five surfactants,different surfactants with a mass concentration of 0.02% were added to 0.2 mol/L magnesium sulfate solution to discuss their effects on the permeability coefficient of WREOs, as shown in Fig.3, where A stands for the original leaching solution;B the CTAB;C the DTAB;D the SDS;E the sodium oleate;F the oleic acid.It can be seen from Fig.3 that the presence of surfactants can increase the permeability coefficient whatever the surfactant is.However, CTAB shows a better promoting permeability effect than other surfactants.When different surfactants are added to the magnesium sulfate leaching system, the permeability coefficient from large to small is CTAB,DTAB, SDS, sodium oleate and oleic acid.

Fig.3.Effects of different surfactants on permeability coefficient of WREOs in magnesium sulfate leaching system.

The effects of different surfactants on the leaching efficiency of RE and Al are shown in Fig.4.It is obvious from Fig.4 that the addition of surfactants improves the leaching efficiency of RE.The added surfactant adsorbs on the leaching solution-mineral particle interface, which significantly reduce the surface tension of the leaching solution and improve the wetting performance on the mineral particle surface.The leaching solution more easily enters into the pores of the ore-body so as to accelerate the wetting speed as well as increase the contact area between the leaching solution and the ore-body.Furthermore, Fig.3 shows that the addition of surfactant promotes the seepage process.A higher permeability coefficient suggests a higher flow velocity of the leaching solution in the ore-body, which can enlarge the concentration difference between the leaching agent cation and the rare earth ions thereby strengthening the mass transfer and accelerating the rare earth ion exchange reaction.The leaching efficiency of rare earth in the presence of five different surfactants from large to small is CTAB,DTAB,SDS, sodium oleate and oleic acid.

Fig.4.Effects of different surfactants on leaching efficiency of RE and Al in magnesium sulfate leaching system.

For the leaching process of Al, all the five surfactants show a certain aluminum inhibition.The reason why the surfactant can reduce the leaching efficiency of Al may be that the surfactant converts the ion-exchanged aluminum into the adsorbed hydroxyl aluminum and remains in the ore body.The leaching efficiency of Al from large to small is oleic acid,DTAB,SDS,CTAB and sodium oleate, and the leaching efficiency of CTAB and SDS is close.

It can be seen from Figs.3 and 4 that in the magnesium sulfate leaching system, five selected surfactants promote the leaching process of WREOs.The increasing leaching efficiency of RE and the reducing leaching efficiency of Al indicate that surfactants can effectively promote the leaching of RE and inhibit the leaching of Al.Furthermore, the cationic surfactant CTAB shows a better reinforcement on leaching process, and CTAB is thus selected as the leaching aids of magnesium sulfate in this study.

3.2.Effects of CTAB content on leaching process

In the in-situ leaching process of WREOs, the concentration of leaching agent, the leaching temperature, the pH of leaching solution,and the liquid-solid ratio will affect the leaching efficiency of rare earth.Among them,the concentration of leaching agent shows the greatest influence.The research showed that a higher leaching temperature can accelerate the thermal movement of ions and improve the leaching efficiency of rare earth [19,20].Although acidic conditions can slightly improve the leaching efficiency of rare earth, it increases the content of impurity ions such as Al3+.The initial pH of magnesium sulfate leaching solution is generally 5.5–6.0.In the actual mining process, WREOs is generally leached at room temperature and without adjusting the pH of the leaching solution.The study showed the effect of magnesium sulfate on the leaching process of WREOs, and found that 0.2 mol/L magnesium sulfate solution shows a good leaching efficiency of rare earth[4,5].Here, the purpose of this study is mainly to explore the surfactant influence on the leaching process of WREOs.Therefore,0.2 mol/L magnesium sulfate solutions with different additions of CTAB were discussed at room temperature and the liquid-solid ratio of 2:1.

3.2.1.Effects of CTAB content on permeability coefficient of clay minerals

Since the RE in the WREOs mainly adsorbs on the clay minerals,the effect of CTAB content on permeability coefficient of clay minerals such as kaolinite,montmorillonite and illite was studied and shown in Fig.5.It can be seen from Fig.5 that the permeability coefficient of clay minerals increases in the presence of CTAB with different concentrations,and all of them increase significantly with the CTAB content in the range of 0–0.04%.When the CTAB content is greater than 0.04%, the permeability coefficients of three clay minerals show no change.At the same CTAB content, the permeability coefficients of the three clay minerals are montmorillonite,illite and kaolinite from large to small.

Fig.5.Effects of CTAB content on permeability coefficient of clay minerals in magnesium sulfate leaching system.

When 0.04%CTAB was added into the magnesium sulfate solutions, the permeability coefficients of kaolinite, montmorillonite and illite increase from 0.605×10-5, 0.724×10-5and 0.678×10-5cm∙s-1to 1.121×10-5, 1.357×10-5and 1.261×10-5cm∙s-1,respectively,which indicates the strengthening permeability effect of 0.04% of CTAB on clay minerals as well as the enhancement on the permeability of WREOs.

3.2.2.Effect of CTAB content on permeability coefficient of WREOs

The effect of CTAB content on the permeability coefficient of WREOs is shown in Fig.6.As predicted,the permeability coefficient of WREOs increases by adding CTAB in the magnesium sulfate leaching solution.The permeability coefficient increases with the CTAB content and then achieves equilibrium when the CTAB content is 0.04%.

Fig.6.Effects of CTAB content on permeability coefficient of WREOs in magnesium sulfate leaching system.

The permeability coefficient of WREOs increases from 0.945×10-5to 1.640×10-5cm∙s-1in the presence of 0.04% CTAB,which is basically consistent with the influence of CTAB content on the permeability coefficient of clay minerals.Furthermore, the influence of CTAB content on the permeability coefficient of WREOs is greater than that of clay minerals.

3.2.3.Effects of CTAB content on leaching efficiency of RE and Al

To further demonstrate the enhancement of CTAB on the magnesium sulfate leaching process of WREOs,the effect of CTAB content on the leaching efficiency of RE and Al were discussed and shown in Fig.7.It can be seen from Fig.7 that the addition of CTAB with different concentrations improves the leaching efficiency of RE and reduces the leaching efficiency of Al.The leaching efficiency of RE increases with the CTAB content, and the leaching efficiency of Al decreases.When the CTAB content reaches 0.04 %, the leaching efficiency of RE increases from 80% to 90%, and the leaching efficiency of Al decreases from 85% to 67%.

Fig.7.Effects of CTAB content on leaching efficiency of RE and Al in magnesium sulfate leaching system.

As the surfactant,CTAB can help the leaching solution enter the mineral particle pores as well as ore-body through reducing the surface tension of leaching solution.The more the contact between the leaching solution and mineral particles occurs,the more effectively the ion exchange reaction between the leaching agent and rare earth ions can be strengthened.In addition, Fig.6 shows that the presence of CTAB with different concentrations increases the permeability coefficient of WREOs, indicating that CTAB accelerates the renewal of leaching solution in ore-body and promotes the leaching of RE.

According to Figs.6 and 7,0.04%of CTAB is a good leaching aid for WREOs which not only enhances the seepage of leaching solution and improves the leaching efficiency of RE but also inhibits the leaching of Al.

3.3.Effects of CTAB content on leaching kinetics

3.3.1.Effects of CTAB content on leaching kinetics of RE and Al

In order to better understand the effect of CTAB content on leaching efficiency and reveal the leaching enhancement mechanism of CTAB.The effects of different CTAB contents on the leaching kinetics of RE and Al were studied as shown in Fig.8.It can be seen from Fig.8 that the leaching efficiency of RE increases with the leaching time, and the leaching efficiency of Al decreases until the leaching equilibrium, and with the increase of CTAB content,the leaching equilibrium time of both is significantly shortened.When the CTAB content reaches 0.04%, the leaching equilibrium time of RE is the shortest, about 240 min, which is about 140 min shorter than that without CTAB.The leaching equilibrium time of Al is also the shortest,about 300 min.With the continuous increase of CTAB content, the leaching efficiency and the leaching equilibrium time of RE and Al does not change.The results show that 0.04% CTAB can not only accelerate the leaching process of RE and Al, but also promote the leaching of RE and inhibit the leaching of Al.

Fig.8.Effects of CTAB content on leaching process of RE and Al in magnesium sulfate leaching system.

3.3.2.Kinetic analysis

The leaching process of WREOs is a liquid-solid reaction system.This process is generally divided into slow and unstable initial stage, fast and stable intermediate stage and the final equilibrium stage.Obviously,the fast and stable intermediate stage of leaching process has a great influence on the leaching efficiency,which can be controlled by four model steps, chemical reaction controls,outer diffusion controls, inner diffusion controls and mixed controls.The data in Fig.8 are fitted by the above four kinetics control models.The experimental data and fitting results show that the leaching process of RE and Al is an inner diffusion control process.The results are shown in Fig.9, and the apparent rate constant (k)and the fitting coefficient (R2) are shown in Table 1.

Table 1 shows that the apparent rate constant (k) of RE increases with the CTAB content, and the apparent rate constant(k) of Al decreases.When the CTAB content reaches 0.04%, the apparent rate constant (k) of RE and Al almost no longer changes with the increase of CTAB content.According to the expression of inner diffusion kinetics Eq.(4), it can be transformed into the expression of inner diffusion control model related to CTAB content as follows:

Combined with Eq.(4), it can be found that:

Available through mathematical conversion:

where k and k0are the apparent rate constant of RE with CTAB and without CTAB, min-1; k′andthe apparent rate constants of Al with and without CTAB, min-1; nREand nAlthe empirical reaction orders of RE and Al; C0the content of CTAB, g/L; and kREand kAlthe apparent rate constants of RE and Al related to CTAB content,min-1.

The data in Table 1 are substituted into the Eqs.(14) and (15),and the relationship equation with RE and Al is obtained.The linear fitting is carried out, where the slope of the fitting line is nREand nAl, and the intercept is ln kREand ln kAl, respectively.

Table 1 Apparent rate constant (k) and fitting coefficient (R2) of RE and Al inner diffusion control model.

The fitting relationship equations are:

The nREis 0.91247, and the kRErelated to CTAB content can be calculated as 0.000931.The nAlis 0.6306, and the kAlrelated to CTAB content is 0.000195.Therefore, in magnesium sulfate leaching system, the leaching kinetics equations of RE and Al related to CTAB content are:

It can be seen from the apparent rate constant and the empirical kinetic equation that the leaching efficiency of RE and Al can be accelerated by appropriately increasing the content of CTAB, and the influence on RE is particularly obvious.

3.4.Effects of CTAB content on mass transfer process

The mass transfer process of WREOs includes ion exchange process and ion diffusion process.In order to better understand the effect of CTAB content on ion exchange reaction and ion diffusion migration between magnesium ions and rare earth ions, so as to reveal the reason why CTAB enhances the leaching efficiency of RE, the effect of CTAB content on the mass transfer process of RE and Al is further studied and shown in Fig.10.It can be seen that with the increase of the leaching solution volume, the concentrations of RE (CRE2O3) and Al () firstly increase to the peak and then sharply decrease to zero.The peak concentration of RE increases with the CTAB content.The peak concentration of Al decreases with the CTAB content, and the peak concentration of RE is much higher than that of Al, which indicates that the mass transfer efficiency of RE is better than that of Al.When the content of CTAB is greater than 0.04%, the peak concentration of RE and Al do not change significantly.

Fig.10.Leaching mass transfer curves of RE and Al in magnesium sulfate leaching system.

The change of RE concentration in mass transfer process may be due to the reduced surface tension of the leaching solution in presence of CTAB,which makes the solution easier to leach into the ore body, and promotes the ion exchange reaction between magnesium ions and rare earth ions, resulting in the increased RE peak concentration.Since the critical micelle content of CTAB is 0.04%,the surface tension of the leaching solution decreases with the CTAB content until reaching the critical micelle concentration[25].

The chromatographic plate theory is used to analyze the leaching curves of RE and Al with CTAB content, and the theoretical plate number and related parameters were calculated.The results are shown in Tables 2 and 3.

It can be seen from Tables 2 and 3 that in the magnesium sulfate leaching system, the retention volume of RE and Al increases with the CTAB content, and the HETP of RE and Al first decreases to the minimum and then slightly.When CTAB content is 0.04%,the HETP of RE and Al reaches the minimum.The results show that in the critical micelle concentration range of CTAB (0–0.04%), the mass transfer efficiency of RE and Al increases with the CTAB content.In the presence of CTAB, the increased permeability coefficient and the strengthened seepage process improve the contact between the leaching solution and ore body, which is conducive to the mass transfer process.

Compared with Tables 2 and 3, under the same conditions, the retention volume and HETP of RE are less than those of Al,indicating that the leaching of RE is prior to the leaching of Al, and the mass transfer efficiency of RE is greater than that of Al.

3.5.Strengthening leaching mechanism of CTAB

3.5.1.Effects of CTAB content on interfacial properties of leaching solution

In the magnesium sulfate leaching system, CTAB with different concentrations was added to the leaching solution.The effect of CTAB content on the interfacial properties of the leaching solution was further investigated by measuring the surface tension σ and the contact angle θ of the solution as shown in Fig.11.

Fig.11.Effects of CTAB content on interfacial properties of leaching solution.

It can be seen from Fig.11 that the surface tension of the leaching solution can be reduced by adding CTAB, and decreases with the CTAB content.When the content of CTAB is greater than 0.04%,no change can be observed because of the saturated adsorption of surfactant on the solution surface.When the concentration reaches a certain value,too many surfactants will self-aggregate to form micelles in a specific way, which is the critical micelle concentration.When the surfactant concentration is less than the critical micelle concentration, the surface tension of the solution decreases with the surfactant concentration.When the surfactant concentration is greater than the critical micelle concentration,the surface tension of the solution almost no longer decreases.The critical micelle content of CTAB, 0.04% suggests the optimum content of CTAB.

The change of wettability is basically the same.The contact angle increases with the CTAB content until 0.04%.The hydrophilic base band of cationic surfactant CTAB is positively charged,adsorbed on the surface of mineral, and the hydrophobic group is outward, resulting in the weakened wettability as well as the increased contact angle.Thus, the contact angle increases with the CTAB adsorption amount on the solution surface until the critical micelle concentration.

3.5.2.Effects of CTAB content on adhesion work and adhesion work reduction factor

The surface tensions and contact angles of the leaching solution as the factor of CTAB content suggests the interaction between CTAB and the leaching solution.The addition of CTAB can promote the leaching solution to enter the ore body and improve the adhesion of ore body to the leaching solution.Therefore, the effect of CTAB content on the adhesion work and the adhesion work reduction factor is further investigated shown in Fig.12.

Fig.12.Effects of CTAB content on adhesion work and adhesion work reduction factor.

It can be seen from Fig.12 that the adhesion work can be reduced by adding CTAB to the leaching solution,and the adhesion work reduction factor decreases with CTAB content until 0.04%.It can be seen from Fig.6 that the change of the permeability coefficient of WREOs is just opposite to that of the adhesion work and the adhesion work reduction factor, which may be that the presence of CTAB reduces the adhesion force of the ore body to the leaching solution, promotes the seepage velocity of the solution in the ore body and improves the permeability of the ore body.It is also proved that the adhesion work and the adhesion work reduction factor can be used to explain the seepage strengthening effect of surfactant.

Table 2 Theoretical plate number and related parameters of RE in magnesium sulfate leaching system.

4.Conclusions

(1) In the magnesium sulfate leaching system, the presence of surfactants, CTAB, DTAB, SDS, sodium oleate and oleic acid can improve the permeability coefficient,increase the leaching efficiency of RE and reduce the leaching efficiency of Al.Among them, CTAB shows a better strengthening effect on the leaching process.

(2) Addition of 0.04% CTAB into 0.2 mol/L magnesium sulfate solution accelerates the permeability coefficient of WREOs from 0.945×10-5to 1.640×10-5cm∙s-1,improves the leaching efficiency of RE from 80% to 90%, and inhibits the leaching efficiency of Al from 85% to 67%.

(3) Kinetic analysis shows that the leaching process of RE and Al conforms to the internal diffusion control model in the magnesium sulfate leaching system.And Eqs.(18) and (19) are,repectviely, the leaching kinetics equations of RE and Al related to CTAB content.

(4) Smaller retention volumes and HETP indicates the increased mass transfer efficiency in the presence of CTAB, which proves the enhanced effect of CTAB on the leaching process of WREOs.

(5) The reduced adhesion work and adhesion work reduction factor were calculated by the surface tension and the contact angle, indicating the enhanced seepage of magnesium sulfate leaching solution in the presence of CTAB so as to improve the leaching process of WREOs.

Acknowledgments

Financial supports for this work from National Natural Science Foundation of China(Nos.22078252 and 52274266)and the Graduate Education Innovation Foundation of Wuhan Institute of Technology (No.CX2021463) and the Young Top-notch Talent Cultivation Program of Hubei Province are greatly appreciated.