Removing aluminum from a low-concentration lixivium of weathered crust elution-deposited rare earth ore with neutralizing hydrolysis
来源期刊:Rare Metals2017年第8期
论文作者:Xian-Ping Luo Li-Ping Zou Pei-Long Ma Cai-Gui Luo Jing Xu Xue-Kun Tang
文章页码:685 - 690
摘 要:Aluminum,the main impurity in the lixivium of weathered crust elution deposited rare earth ore,not only results in an increasing consumption of precipitant in the rare earth precipitation process,but also lowers the purity of final rare earth product.Aluminum in the weathered crust elution-deposited rare earth ore lixivium should be removed.Neutralizing hydrolysis method was employed to remove aluminum from the lixivium.Hexamethylenetetramine was found to be the optimum pH regulator for the removal of aluminum in the low concentration.When used to adjust the pH value of the lixivium to 5.0,aluminum in the lixivium can be effectively removed in the form of aluminum hydroxide precipitation with removal rate of 97.60%.It shows that hexamethylenetetramine has a good effect on the removing of aluminum ions from the lowconcentration lixivium.Moreover,hexamethylenetetramine in removing aluminum from lixivium has little adverse effect on the RE precipitation process.
稀有金属(英文版) 2017,36(08),685-690
Xian-Ping Luo Li-Ping Zou Pei-Long Ma Cai-Gui Luo Jing Xu Xue-Kun Tang
Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology
Jiangxi Key Laboratory of Mining and Metallurgy Environmental Pollution Control
Key Laboratory of Southern Ionic-Type Rare Earth Resources Development and Application,Ministry of Education
Faculty of Engineering,University of Alberta
收稿日期:4 October 2014
基金:financially supported by the National Natural Science Foundation of China (No.51404112);theNational Key Technology R&D Program (No.2012BAC11B07);the Technology Program for Benefitting the People (No.2013GS360203);the Program for Talents Training of “Gan Po Excellence 555 Project”;the Program for New Century Excellent Talents in University (No.NCET-10-0183);the Program for Main Subject, Technology Leaders of Jiangxi Province (No.2010DD01200);
Xian-Ping Luo Li-Ping Zou Pei-Long Ma Cai-Gui Luo Jing Xu Xue-Kun Tang
Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology
Jiangxi Key Laboratory of Mining and Metallurgy Environmental Pollution Control
Key Laboratory of Southern Ionic-Type Rare Earth Resources Development and Application,Ministry of Education
Faculty of Engineering,University of Alberta
Abstract:
Aluminum,the main impurity in the lixivium of weathered crust elution deposited rare earth ore,not only results in an increasing consumption of precipitant in the rare earth precipitation process,but also lowers the purity of final rare earth product.Aluminum in the weathered crust elution-deposited rare earth ore lixivium should be removed.Neutralizing hydrolysis method was employed to remove aluminum from the lixivium.Hexamethylenetetramine was found to be the optimum pH regulator for the removal of aluminum in the low concentration.When used to adjust the pH value of the lixivium to 5.0,aluminum in the lixivium can be effectively removed in the form of aluminum hydroxide precipitation with removal rate of 97.60%.It shows that hexamethylenetetramine has a good effect on the removing of aluminum ions from the lowconcentration lixivium.Moreover,hexamethylenetetramine in removing aluminum from lixivium has little adverse effect on the RE precipitation process.
Keyword:
Weathered crust elution-deposited rare earth ore; Lixivium; Aluminum removing; Hexamethylenetetramine;
Author: Xian-Ping Luo,e-mail:lxp9491@163.com;
Received: 4 October 2014
1 Introduction
Weathered crust elution-deposited rare earth (RE) ore which mainly distributed in China is the main RE resource in the world
However,there are many aluminum ions whose properties are similar to those of RE ions in the ore
However,when using this method to treat the RE lixivium with low concentration (<2 g·L-1),the aluminum removal process is always associated with low removal rate(about 60%-80%) and high RE loss (about 15%-20%)
Thus,in this paper,researches on removing aluminum ions from low-concentration lixivium were carried out.The pH regulators (ammonia,ammonium bicarbonate,hexamethylenetetramine,sodium acetate and sodium sulfide,respectively) which may have good effect on removing aluminum ions were chosen
2 Experimental
2.1 Materials
A low-concentration lixivium of a lean weathered crust elution-deposited RE ore (grade of 0.065%,collected from Zudong Rare Earth Mine area located in Longnan County,Jiangxi,China) was used.The RE concentration of the lixivium determined by ethylene diamine tetraacetic acid (EDTA) volumetric titration method was1.61 g·L-1.The aluminum concentration of the lixivium determined by chromazurol S spectrophotometry method was 100.26 mg.L-1.
2.2 Reagents
Unless otherwise stated,all chemicals used in aluminum removing experiments were of analytical reagent grade (asreceive without further purification).Moreover,deionized water was used in preparation of all the solutions in aluminum removing experiments.Reagents used in analysis of RE contents in the lixivium were acetylacetone,α-benzoin oxime hexamethylenetetramine,ascorbic acid,sulfosalicylic acid,EDTA and xylenol orange (Chemical Reagent Factory of Tianjin Damao,China).Reagents used in analysis of Al3+content in the lixivium were chromazurol S,oxalic acid,perchloric acid,and ascorbic acid (Chemical Reagent Factory of Tianjin Damao,China).
2.3 Analytical methods
The morphology of the RE products were measured by a scanning electron microscope (SEM,EVO-Zeiss Jena,Germany).The RE content in lixivium was determined by EDTA volumetric titration methods
Al3+concentration in lixivium was determined by chromazurol S spectrophotometry
Fig.1 Absorption spectra of clathrate generated by different con-centrations of Al3+reacting with chromazurol S
Fig.2 Calibration curve of aluminum
3 Results and discussion
3.1 Mechanism of removing aluminum from lixivium with neutralizing hydrolysis
The existing forms of aluminum (Al) and RE under various pH solutions are shown in Fig.3
3.2 Effects of ammonia on removing aluminum
In this stage,ammonia is used as pH regulator to remove aluminum from lixivium.Ammonia with different dosages was added into lixivium at a uniform velocity to adjust pH value of lixivium to 4.4,4.6,4.8,5.0,5.2 and 5.4,respectively.The results are shown in Fig.4.
From Fig.4,it can be seen that aluminum removal rate increases along with pH value of lixivium.Meanwhile,the RE loss rate also increases.When pH value is 5.0,the aluminum removal rate reaches 94.4%and RE loss rate is10.4%.When pH value exceeds 5.0,the aluminum removal rate barely changes.Clearly,the aluminum removing process of low-concentration lixivium has a high RE loss rate when using ammonia as pH regulator.Thus,ammonia is not suitable for removing aluminum from lixivium.
Fig.3 Existential form of aluminum (Al) and RE under various pH solutions (where
Fig.4 Effects of ammonia dosage on Al removal rate and RE loss rate
3.3 Effects of ammonium bicarbonate on removing aluminum
When NH4HCO3 was added into the lixivium,Al3+would be precipitated.It can be described by the following reaction
However,there exists side reaction at the same time,which can be described by the following reaction
Thus,in the aluminum removing process,the dosage of ammonium bicarbonate should be controlled strictly to avoid the side reaction.The ammonium bicarbonate used in the experiment was dissolved into deionized water.The concentration of the prepared ammonium bicarbonate solution is 1.83 mol·L-1.Moreover,the pH value of ammonium bicarbonate solution is 8.2.In the experiment,ammonium bicarbonate solution with different dosages was added into the lixivium at a uniform velocity to adjust pH value of the lixivium to 4.4,4.6,4.8,5.0,5.2,and 5.4,respectively.The results are shown in Fig.5.
Figure 5 shows that the aluminum removal rate and the RE loss rate increase significantly with the increase of ammonium bicarbonate dosage.When the aluminum removal rate of is 91.3%,the RE loss rate reaches 20.1%.The aluminum in the lixivium would exhaust the ammonium bicarbonate,resulting in a change of pH value.Under this condition,side reaction becomes strong to make an increase of RE loss rate
3.4 Effects of sodium acetate on removing aluminum
Sodium acetate solution with concentration of 1.83 mol·L-1(pH value of 8.7) was used to remove aluminum from lixivium.Sodium acetate solution with different dosages was added into lixivium at a uniform velocity to adjust pH value of lixivium to 4.4,4.6,4.8,5.0,5.2 and 5.4,respectively.The results are shown in Fig.6.
Figure 6 shows that the aluminum removal rate is very low (<15%) even when the sodium acetate dosage of at a high level.Clearly,the effect of sodium acetate on removing aluminum from the low-concentration lixivium is very poor.Thus,it is not suitable to use as pH regulator.
3.5 Effects of sodium sulfide dosage on removing aluminum
Sodium sulfide solution (pH value of above 12) was used as pH regulator.The results are shown in Fig.7.When sodium sulfide solution was added into lixivium,S2-and Al3+in lixivium have double hydrolysis reaction
Fig.5 Effects of ammonium bicarbonate dosage on Al removal rate and RE loss rate
Fig.6 Effects of sodium acetate dosage on Al removal rate and RE loss rate
Fig.7 Effects of sodium sulfide dosage on A1 removal rate and RE loss rate
It can be seen from Fig.7 that the aluminum removal rate is very low when using sodium sulfide as pH regulator,due to that the H2S generated in the reaction would make the precipitated aluminum dissolve again
3.6 Effects of hexamethylenetetramine on removing aluminum
Hexamethylenetetramine solution is alkaline and has a certain buffer action.In this stage,hexamethylenetetramine solution (concentration of 1.43 mol·L-1 and pH value of above 12) was used to remove aluminum from low-concentration lixivium.Hexamethylenetetramine with different dosages was added into lixivium at a uniform velocity to adjust pH value of lixivium to 4.4,4.6,4.8,5.0,5.2,and5.4,respectively.The results are shown in Fig.8.
It can be seen from Fig.8 that the aluminum removal rate reaches 97.6%when the pH value of lixivium is 5.0.Under this condition,the RE loss rate is only 4.02%.Clearly,compared to other pH regulators,hexamethylenetetramine has good effect on removing aluminum from low-concentration lixivium.
The main reason that hexamethylenetetramine has good effect on removing aluminum mostly lies in two aspects.Firstly,hexamethylenetetramine solution has a certain buffer action.With the addition of hexamethylenetetramine,the pH value of lixivium keeps at a stable range.It makes a high aluminum removal rate and a low RE loss.Secondly,compared to other pH regulators,there are not side chemical reactions existing when using hexamethylenetetramine.It effectively reduces the RE loss rate in aluminum removing process.
Obviously,in the four pH regulators (ammonia,ammonium bicarbonate,hexamethylenetetramine,sodium acetate and sodium sulfide),hexamethylenetetramine is the optimum choice for the removing of aluminum from lowconcentration RE lixivium.
3.7 Effects of aluminum on RE precipitation process
The existence of aluminum in lixivium has adverse effects on precipitating RE from lixivium.These adverse effects are mainly caused by the co-precipitation of aluminum ions with RE ions.Firstly,the purity of RE product will decline.And secondly,the precipitated aluminum is colloid,which would wrap on the surface of RE precipitation
Fig.8 Effects of hexamethylenetetramine dosage on Al removal rate and RE loss rate
Fig.9 SEM images of RE product,a prepared from purified lixivium and b prepared from unpurified lixivium
Figure 9a shows SEM image of final RE products precipitated from unpurified lixivium (aluminum has not been removed)
image of final RE products precipitated from lixivium,which contains little aluminum (most of aluminum has been removed by hexamethylenetetramine).It shows that the RE product is crystal solid,which has large particle size and is easy to filter.It again proves that hexamethylenetetramine has little adverse effect on the precipitation process of RE.
4 Conclusion
The existence of aluminum ions in the lixivium has a significant adverse effect on rare earth precipitation process.The aluminum in the lixivium would be co-precipitated with the rare earth precipitation,resulting in the decrease of the purity of the final rare earth products.Beside,its existence in the lixivium makes the precipitation form flocculent rare earth carbonates (not beneficial for further processing) rather than crystalline precipitation.
The effect of sodium acetate and sodium sulfide on removing aluminum from the low-concentration rare earth lixivium is very bad.Large amount of rare earth is loss when they were used as pH regulator to remove aluminum.In contrast,hexamethylenetetramine shows a significantly better performance on removing aluminum.When using hexamethylenetetramine as pH regulator,the removal rate of aluminum and the loss rate of rare earth reach 97.6%and 4.02%,respectively.Hexamethylenetetramine is the optimum choice for removing aluminum from low-concentration rare earth lixivium.
参考文献