简介概要

Mixed rare earth concentrate leaching with HCl-AlCl₃ solution

来源期刊：Rare Metals2013年第3期

论文作者：Mei Li Xiao-Wei Zhang Zhao-Gang Liu Mi-Tang Wang Jia Liu Jian-Ping Yang

文章页码：312 - 317

摘要：A method of Baotou mixed rare earth concentrate leaching was investigated. According to the principle of complex leaching, the quadratic orthogonal regression experiment was used to investigate effect of six factors. An optimal condition of low energy and environment friendly was determined as follows: HCl acidity is 4 mol·L-1 , AlCl3 concentration is 2mol·L-1 , reaction temperature is 85℃, reaction time is 90 min, liquid to solid ratio is 35ml·g-1 , and stirring speed is 100 r·min-1 . According to this condition, the leaching ratio of mixed rare earth concentrate is 76.5 %. The analyses of X-ray diffraction shows that bastnasite disappears, and the main peaks of filter residue is monazite and cheralite after leaching of HCl-AlCl3 solution. The result indicates that REFCO3 of the mixed rare earth concentrate can be dissolved, but REPO4 remains in the leaching residue. This method provides a way to separate and extract rare earth.

Rare Metals 2013,32(03),312-317+2

Mixed rare earth concentrate leaching with HCl-AlCl₃ solution

College of Materials Science and Engineering, Beijing University of Chemical Technology

School of Material and Metallurgy, Inner Mongolia Science and Technology University, Key Laboratory of Inner Mongolia New Technologies of Modern Metallurgy and Application of Rare

摘要：

A method of Baotou mixed rare earth concentrate leaching was investigated. According to the principle of complex leaching, the quadratic orthogonal regression experiment was used to investigate effect of six factors. An optimal condition of low energy and environment friendly was determined as follows: HCl acidity is 4 mol·L-1 , AlCl3 concentration is 2mol·L-1 , reaction temperature is 85℃, reaction time is 90 min, liquid to solid ratio is 35ml·g-1 , and stirring speed is 100 r·min-1 . According to this condition, the leaching ratio of mixed rare earth concentrate is 76.5 %. The analyses of X-ray diffraction shows that bastnasite disappears, and the main peaks of filter residue is monazite and cheralite after leaching of HCl-AlCl3 solution. The result indicates that REFCO3 of the mixed rare earth concentrate can be dissolved, but REPO4 remains in the leaching residue. This method provides a way to separate and extract rare earth.

收稿日期：31 August 2012

基金：financially supported by the National Nature Science Foundation of China (No. 51174115);National Outstanding Youth Science Foundation of China (No. 51025416);Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1065);

Mixed rare earth concentrate leaching with HCl–AlCl₃ solution

Abstract：

A method of Baotou mixed rare earth concentrate leaching was investigated. According to the principle of complex leaching, the quadratic orthogonal regression experiment was used to investigate effect of six factors. An optimal condition of low energy and environment friendly was determined as follows: HCl acidity is 4 mol·L-1 , AlCl3 concentration is 2mol·L-1 , reaction temperature is 85℃, reaction time is 90 min, liquid to solid ratio is 35ml·g-1 , and stirring speed is 100 r·min-1 . According to this condition, the leaching ratio of mixed rare earth concentrate is 76.5 %. The analyses of X-ray diffraction shows that bastnasite disappears, and the main peaks of filter residue is monazite and cheralite after leaching of HCl–AlCl3 solution. The result indicates that REFCO3 of the mixed rare earth concentrate can be dissolved, but REPO4 remains in the leaching residue. This method provides a way to separate and extract rare earth.

Keyword：

HCl–AlCl3 solution; Mixed rare earth concentrate; Leaching;

Received： 31 August 2012

1 Introduction

Bastnasite (REFCO₃) is a magma-derived fluorocarbonate mineral containing 65 wt%–75 wt%rare earth oxides (REO) .The two major sources in the world for lanthanides are bastnasite deposits at Mountain Pass, California (USA) solely for REE production, and Bayunebo, Inner Mongolia (China) , mined primarily for iron ore and REE as a by-product.Monazite (REPO₄) is an REE phosphate containing 55 wt%–65 wt%REO, associated with granites and beach sands in Australia, Brazil, and India[1].The mixed rare earth ore is a paragenetic mineral that is composed of REFCO₃ (bastnasite) and REPO₄ (monazite and cheralite) ;it is an important rare earth resource, and it contains about40%important ore of rare earth industry minerals in the world[2].This mineral is distributed in the Baiyunebo of Baotou and is considered to be the mineral that is hardest to decompose due to its complex composition[3, 4].The proportion of REFCO₃and REPO₄in minerals is 9:1–1:1 in1990s[5], and according to the analysis of minerals, this number becomes 6.5:3.5 in recent years[6].Nowadays the sulfuric acid calcinations method is mainly used to decompose mixed rare earth;however, the problem of this method releases SO₂and HF gases, which are difficult to be recycled economically.Environmental pollution is serious[7].Some new green technologies of mixed rare earth concentrate have been investigated by many researchers such as the method of Roasting with Ca O and Na Cl[8], the carbochlorination–oxidation method[9], or the method of roasting with Ca O–Na Cl–Ca Cl₂[10], etc., but these methods have not been applied to industry production.

In this paper, Baotou mixed rare earth concentrate was leached out by the method of hydrometallurgy.REFCO₃is a composite compounds, and the compositions are RE₂ (CO₃) ₃and REF₃.Rare earth carbonate is easily dissolved in hydrochloric acid, but the rare earth fluoride is hard to be dissolved in hydrochloric acid.Previous investigations showed that Al^3?can form complex with F^-strongly[11].The HCl–ACl₃solution was used to leach out rare earth oxide and Ca F₂, the Baotou mixed rare earth concentrate was decomposed by Ca O–Na Cl–Ca Cl₂.The experiment results show that the leaching ratio of RE₂O₃and Ca F₂is up to 70.8%and 55.8%, and the leaching ratio of RE₂O₃and Ca F₂is to99.62%and 98.56%, respectively, when quintic contraflow leaching at the same parameters[12].However, in this experiment, the unroasted Baotou mixed rare earth concentrate was leached by the HCl–ACl₃solution.Due to its high electronegativity and small ionic size, the fluoride ion is classified as a hard base, which has a strong affinity toward metal ions including Al^3?, Fe^3?, and La^3?.Al^3?has a good affinity with fluoride ions based on the soft and hard acid base concept, which is frequently used in defluoridation[13–16].Al^3?and F^-easily generates a stable complex ion[Al F₆]^3-with stability constant of 6.9 9 10¹⁹.The metals in bastnasite and fluorite can be leached out by forming complex ions with Al^3?.The important chemical reactions are given as follows:

The advantage of this method is that there is no HF and SO₂, and the fluorine element can be transformed to a useful product in the follow-up process[17].The leaching residue can be decomposed by Na OH.The whole process not only reduces energy consumption and environment pollution but also makes the resources more rationally utilized.

2 Experimental

2.1 Experimental materials

The mixed rare earth concentrate was supplied by Baogang Gaoke Rare Earth Co.Ltd.The particle size of the gray mineral by floating is below 40 lm.The chemical compositions are listed in Table 1.Other reagents in the experiment are AR grade.

Table 1 Chemical compositions of mixed rare earth concentrate (wt%) 下载原图

Table 1 Chemical compositions of mixed rare earth concentrate (wt%)

2.2 Experimental process and analysis

The leaching experiments were performed in a 500 ml three neck glass flask connecting with a condenser pipe.A mechanical stirrer and a thermostat were facilitated to keep the reaction medium at constant temperature[18].In the leaching process, HCl solution and Al Cl₃solution was put into the reactor and then distilled water was added to a certain volume according to the orthogonal regression experiment design.After the reaction temperature was reached, a specific amount of the mixed rare earth concentrate sample was added to the HCl–Al Cl₃solution and stirred.After a certain period of time, the solution was filtered.Leaching residue was dried in an oven with constant temperature at 120°C for 30 min.

The mixed rare earth concentrate leaching ratio (%) = (mass of the mixed rare earth concentrate-mass of leaching residue) /mass of the mixed rare earth concentrate 9 100%.The calculation of theory indicated that the maximum leaching ratio of mixed rare earth is less than77.98%.The reproducibility of the measurements was averaged in triplicates.Relative standard deviations were within±2.0%.The phase compositions were determined in a D/max 2400 diffraction instrument with the Cu Ka.

2.3 Quadratic orthogonal regression experiment design

The initial HCl acidity of mixed solution, Al Cl₃concentration of mixed solution, reaction temperature, reaction time, liquid to solid ratio, and stirring speed were selected as the studying factors.Forty-eight experiments were designed in accordance with these six factors regression in the quadratic orthogonal regression experiment including four experiments of zero level.We used the half implementing method.Table 2 is the encode table of six factors in the quadratic orthogonal regression experiment c=1.896[19–21].The results of the mixed rare earth concentrates leaching ratio by the regression orthogonal experiment are listed in Table 3.The regression equation is obtained from the results shown in Table 3.

x_i (x₁, x₂, …, x₆) is the code of the orthogonal regressive table factors, which take linear transformation, x_i

Table 2 Encode of factors 下载原图

Table 2 Encode of factors

Where y is the leaching ration of mixed rare earth concentrate, a₁is the initial HCl concentrate (mol?L^-1) , a₂is Al Cl₃concentration (mol?L^-1) , a₃is temperature (°C) , a₄is reaction time (min) , a₅is liquid to solid ratio (ml?g¹) , and a₆is the stirring speed (r?min^-1) , D_jis change spacing of a_i.The F test of the regression equation is listed as follows:

S_total?12243:22;

Sregression?10973:82;

Sremain?Stotal?Sregression?12243:22;

F=6.4[F_(0.01)=2.81[20, 21].The equation is conspicuous at 0.01 level.It is shown that the effects of the experiment on the leaching method of mixed rare earth concentrate can be described by this equation.

3 Results and discussion

3.1 Effect of the initial concentration of HCl acid

When Al Cl₃concentration of mixed solution is1.5 mol?L^-1, liquid to solid ratio is 20 g?ml^-1, reaction time is 90 min, stirring speed is 300 r?min^-1with reaction temperatures at 45, 65, and 85°C, respectively.The mixed rare earth concentrate leaching ratio versus the concentration of HCl acid at different temperatures were calculated based on the regression equation (Fig.1) .

The leaching ratio increases obviously with increasing the reaction temperature at the specific initial concentration of HCl acid (Fig.1) .When the concentration of HCl increases from 1 to 3 mol?L^-1, the leaching ratio rapidly increases for all reaction temperatures in 45, 65, and 85°C The leaching ratio reaches maximum in 4 mol?L^-1HCl concentration in each curve.The leaching ratio increases19.8%, while the HCl concentration increases from 1 to4 mol?L^-1at 85°C.

3.2 Effect of Al Cl3concentration

The leaching ratio versus Al Cl₃concentration was calculated based on the regression equation (Fig.2) .The Al Cl₃concentration increased from 0.5 to 2.5 mol?L^-1, while the reaction temperature is 65°C, liquid to solid ratio is20 g?ml^-1, reaction time is 90 min, and the stirring speed is300 r?min^-1.Under this condition, the mixed rare earth concentrate leaching curves in 1, 2, and 3 mol?L^-1are obtained (Fig.2) .

The mixed rare earth concentrate leaching ratio increases with increasing the Al Cl₃concentration from 0.5 to2 mol?L^-1and reaches the maximum value at the Al Cl₃concentration of 2 mol?L^-1for the three curves of specific HCl concentration.The leaching ratio almost do not change when the Al Cl₃concentration increases from 2 to2.5 mol?L^-1for the curves of 3 and 4 mol?L^-1.This indicates that the Al Cl₃concentration of 1.5 mol?L^-1is optimal enough for the best leaching efficiency.The leaching ratio increases 7%when HCl concentration is 4 mol?L^-1, and the Al Cl₃concentration increases from 0.5 to 2 mol?L^-1.

Table 3 The results of the regression orthogonal experiment 下载原图

Table 3 The results of the regression orthogonal experiment

Fig.1 Mixed rare earth concentrate leaching ratio versus HCl concentration

Fig.2 Mixed rare earth concentrate leaching ratio versus Al Cl3concentration

3.3 Effect of reaction temperature

The mixed rare earth concentrate leaching ratios versus the reaction temperature were obtained under the following conditions:HCl concentration is 4 mol?L^-1, Al Cl₃concentration is 2 mol?L^-1, reaction time is 90 min, and liquid to solid ratio is 20 ml?g^-1with the stirring speed of 200, 300, and 400 r?min^-1 (Fig.3) .

The leaching ratio of rare earth concentrate rapidly increases with the temperature changing from 25 to100°C.However, leaching ratio effect of the stirring speed is not critical.When the stirring speed is 300 and400 r?min^-1, the leaching ratio increases by 57.19%and49.69%, with the reaction temperature increasing from 25to 100°C, respectively.Then, the reaction temperature of85°C is the optimal condition considering the volatility of the hydrochloric acid and energy saving.

Fig.3 Mixed rare earth concentrate leaching ratio versus reaction temperature

Fig.4 Mixed rare earth concentrate leaching ratio versus reaction time

3.4 Effect of the reaction time

The effect of the reaction time on the mixed rare earth concentrate leaching ratio was obtained with stirring speed of 100, 200, and 300 r?min^-1, while concentration of HCl is 4 mol?L^-1, Al Cl₃concentration is 2 mol?L^-1, the reaction temperature is 85°C, and the liquid to solid ratio is20 ml?g^-1 (Fig.4) .

The leaching ratio increases with increasing the reaction time and the stirring speed.However, time and energy consumption need to be balanced.For time, 90 min is selected in this experiment with the leaching ratio 69.32%at 300 r?min^-1

3.5 Effect of the liquid to solid ratio

The liquid to solid ratio is an important factor in chemical reaction.The following conditions have been used4 mol?L^-1HCl acidity, 2 mol?L^-1Al Cl₃, 85°C as the reaction temperature, and 300 r?min^-1.The results for 1560, and 90 min have been compared (Fig.5) .

The mixed rare earth concentrate leaching ratio increases with liquid to solid ratio increase from 10 to 30 ml?g^-1.The maximum leaching ratio reaches to 76.73%when reaction time is 120 min with the liquid to solid ratio is30 ml?g^-1.Effect of the reaction time is not obvious when the liquid to solid ratio is low.However, at higher liquid to solid ratio, the effects are obvious, while the leaching ratio is 3.9%higher at 120 min than at 60 min reaction time.From the analysis above, the liquid to solid ratio of30 ml?g^-1is the optimal ratio (Fig.6) .

3.6 Effect of stirring speed

The optimal conditions of five factors have been determined by the results analyzed above based on the orthogonal regression equation 4 mol?L^-1HCl acid mixed with2 mol?L^-1Al Cl₃as the liquid to solid ratio is 30 ml?g^-1and react at 85°C for 90 min.

The leaching ratio slightly decreases with increasing the stirring speed under this condition.The stirring speed of100 r?min^-1is determined as the optimal condition with77.41%leaching ratio.

3.7 Experimental verification on the optimal conditions

The optimal conditions of six major factors were obtained via the analysis based on the orthogonal regression equation and the consideration of energy saving and HCl volatility as follows:4 mol?L^-1HCl acid mixed with2 mol?L^-1Al Cl₃, the liquid to solid ratio being 30 ml?g^-1, and reacting at 85°C with 100 r?min^-1for 60 min.A triplet experiment set was preformed to verify this optimal condition.The F mixed rare earth concentrate exhibits an average value of 76.5%, only 0.91%difference with the value from the analysis results of orthogonal regression experiment.Therefore, the results obtained from the orthogonal regression experiment is much more reliable.

Fig.5 Mixed rare earth concentrate leaching ratio versus liquid to solid ratio

Fig.6 Mixed rare earth concentrate leaching ratio versus stirring speed

Fig.7 XRD spectrum of mixed rare earth concentrate

Fig.8 XRD spectrum of leaching residue in optimal experiment condition

3.8 XRD analysis

The mixed rare earths concentrate and leaching residue from the leaching experiment at the optimal experiment condition were analyzed by X-ray diffractometer (Figs.7, 8) .

The XRD analysis shows that the mixed rare earths concentrate mainly consisted of REFCO₃, REPO₄, Fe O_nand Ca CO₃ (Fig.7) , and the leaching residue mainly consisted of REPO₄.Figure 8 shows that bastnasite dis appears, and monazite still remains.It can be explained that REFCO₃, Fe O_n, and Ca CO₃were almost dissolved in the leaching progress by HCl–Al Cl₃solution.Rare earth recovering from the leaching solution and complex anion of fluorine and aluminum retained in the solution tha transformed into cryolite will be studied in othe investigation.

4 Conclusion

The mixed rare earths concentrate was leached by HCl–Al Cl₃solution.Using the method of quadratic orthogona regression experiment, the effect of six factors on the mixed rare earth concentrate leaching ratio was investi gated, and the regression equation was obtained.The optimal condition is determined as follows:HCl concen trate is 4 mol?L^-1, Al Cl₃concentration is 2 mol?L^-1reaction temperature is 85°C, reaction time is 90 min, and stirring speed is 100 r?min^-1.Using this condition fo leaching experiment, the mixed rare earths concentrate leaching ratio reaches 76.5%, which only has 0.91%difference from the analysis results based on orthogona regression equation.The result obtained with the orthog onal regression experiment is much more reliable.

The result of XRD analysis reveals that the majo composition of the leaching residue is monazite, and cheralite, REFCO₃, Fe O_n, and Ca CO₃in the mixed rare earth concentrate are dissolved in the leaching solution The result indicates that the REPO₄ (monazite and chera lite) are not dissolved by HCl–Al Cl₃solution in this experiment condition.