稀有金属(英文版) 2020,39(04),448-454

A two-step zircon decomposition method to produce zirconium oxychloride:alkali fusion and water leaching

Jing Song Jing-Fei Fan Jing-Chong Liu Ran Liu Jing-Kui Qu Tao Qi

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Institute of Process Engineering,Chinese Academy Sciences

School of Chemistry and Chemical Engineering,Shandong University

作者简介：*Tao Qi e-mail:tqgreen@ipe.ac.cn;

收稿日期：18 April 2014

基金：financially supported by the National Science Foundation for Distinguished Young Scholars of China (No.51125018);

A two-step zircon decomposition method to produce zirconium oxychloride:alkali fusion and water leaching

Jing Song Jing-Fei Fan Jing-Chong Liu Ran Liu Jing-Kui Qu Tao Qi

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Institute of Process Engineering,Chinese Academy Sciences

School of Chemistry and Chemical Engineering,Shandong University

Abstract：

In this work,a two-step zircon method to produce zirconium oxychloride was introduced,and the alkali fusion technique with NaOH and water leaching process were investigated.The effects of the operating conditions on the decomposition of zircon were determined,and the optimal conditions are as follows:alkali/zircon mass ratio of 0.7 at the first step and 0.6 at the second step(0.7+0.6),fusion temperature of 700℃and fusion time of 0.5 h at the first step and 0.5 h at the second step(0.5+0.5 h).Under these conditions,the decomposition alloy of zircon sand can reach 97.25%.In the fusion process of zircon sand,the products of first step are mainly Na₂ZrO₃ and ZrSiO₄,the products of second step are Na₂ZrO₃ and Na₂SiO₃,and the diffraction peaks of Na₂ZrSiO₅ are not observed.The conditions of water leaching process were investigated as well,and the optimal conditions are as follows:liquid-solid ratio of 5:1,leaching time of 0.5 h,leaching temperature of 50℃ and leaching three times.Under these conditions,the contents of leaching products SiO₂ and Na₂O are 3.51%(40% ZrO₂) and 4.46%(40% ZrO₂),respectively.The crystal phase structures of Na₂ZrO₃ and Na₂SiO₃ are formed in water leaching process.

Keyword：

Zirconium oxychloride; Zircon; Two-step alkali fusion; Water leaching;

Received： 18 April 2014

1 Introduction

Zirconium oxychloride (ZOC) is an important basic zirconium salt,which can be used as various additives,such as media dyemordant,color-fixing agent,deodorant,flame retardant,stabilizer,etc.Further,another important application of zirconium oxychloride is its use as middle raw material of deep processing.Using zirconium oxychloride,a series of zirconium chemicals can be produced,such as industrial-grade and high zirconium oxide and nano-zirconia.When adding stabilizer,stable and semi-stable zirconium oxide can be prepared,and zirconium carbonate,zirconium sulfate,zirconium nitrate,and other zirconium products can be also obtained.Besides,zirconium oxychloride can be used in producing zirconium and hafnium metals by proper separating technique.These zirconium chemicals are widely used in ceramics,chemicals,electronics,military projects,and other industries.China is a country that produces zirconium oxychloride in a great quantity.According to the statistics conducted by China Nonferrous Metals Industry Association,zirconium oxychloride production in China had been over 2×10⁵ ton by2010,ranking the first in the world [ 1, 2, 3, 4, 5, 6] .

Zircon sand is the main raw material of zirconium oxychloride production.The chemical inertness of zircon renders its processing a difficult task,and a large number of studies were carried out on this problem;several methods are used for industrial processing of zircon,including chlorination [ 7, 8, 9] ,sodium hydroxide fusion [ 10, 11, 12, 13, 14, 15, 16, 17] ,sodium carbonate fusion [ 11, 12, 17] ,mixed alkali fusion [ 18, 19, 20] ,liming fusion or other alkali metal oxides [ 21, 22, 23] ,sodium hydroxide solution leaching [ 24, 25] ,fluorosilicate fusion [ 26] ,plasma thermal dissociation in the presence of carbon [ 27, 28] .Alkali fusion with NaOH is the most commonly used method in China due to the advantages of technically simplicity and low reaction temperature.The general process of producing hydrated zirconium oxychloride includes alkali fusion,water leaching,acid leaching,evaporation and crystallization.Alkali fusion method has become a unique step to decompose zircon in order to get zirconium compounds.In conventional processes,the alkali/zircon mass ratio is about 1.3-1.5,the content of alkali in raw materials is 60 wt%,and this process is very difficult to realize continuous implementation in consideration of the viscosity of molten alkali,so the alkali fusion are mostly batch production,which results in inhomogeneous reaction,as well as pollution of alkali mist.

In the present work,a two-step zircon decomposition method was put forward.The alkali fusion technique with NaOH and water leaching process were investigated.The effects of the operating factors on the decomposition of zircon were determined,and the fused products were characterized and analyzed.The water leaching conditions were investigated and the leaching process was analyzed,aiming to provide a theoretical reference for the development of new process.

2 Experimental

2.1 Raw materials

Zircon sand used for the experiments was provided by Jiangxi Jing'an Hi-Technology Co.Ltd.The mean particle size of zircon sand was 140μm,and the chemical compositions were as follows:Zr(Hf)O₂≥65 wt%,SiO₂≥33 wt%,TiO₂ 0.1 wt%-0.3 wt%,Fe₂O₃ 0.10 wt%-0.25 wt%,P₂O₅ 0.2 wt%-0.3 wt%,A1₂O₃ 0.2 wt%-0.3 wt%,U(Th) 0.06 wt%-0.08 wt%.X-ray diffraction(XRD) pattern of the ground zircon sand is shown in Fig.1,which demonstrates that it mainly consists of ZrSiO₄.The other materials employed were chemically pure anhydrous NaOH and HCl.Water was deionized with Millipore filtration system.

Fig.1 XRD pattern of ground zircon sample

2.2 Procedure

10 g zircon sand was weighed,sodium hydroxide was weighed in certain mass ratio with zircon sand,and the raw materials were put in a nickel crucible and mixed thoroughly;then the nickel crucible was placed in an electric furnace.The mixtures were calcined for a required time period at certain temperatures and then cooled to room temperature in the air.Then the fused products were leached with water to remove soluble sodium silicate.The leaching experiments were conducted in a plastic cup in oil bath at certain temperatures.The fused products were mixed with water via mechanical stirring.Solid-liquid separation was subsequently carried out.The leaching conditions,such as liquid-solid ratio,were determined.

The decomposition rate was measured according to the following method:the fused products were weighed and leached with a certain amount of water,then liquid-solid separation was conducted,and the washing products were obtained.The washing products were leached with excessive hydrochloric acid of 5.5 mol·L^-1 at 90℃to dissolve the zirconium compound,un-decomposed zircon precipitated during this process,and then it was filtered from the solution,dried carefully,and weighed.The decomposition rate of zircon sand was calculated according to the following equation:

where R is the decomposition rate of zircon sand,m₀ is the weight of original zircon sand,and m₁ is the weight of undecomposed zircon sand.

The contents of Na₂O,SiO₂,and ZrO₂ of washing samples were measured by inductively coupled plasma atomic emission spectra (ICP-AES).

2.3 Analytical methods

The component contents of samples were analyzed using ICP-AES (IRIS/AP,Thermo Electron Corporation).Hydrogen and nitrogen in the xerogel were determined using PerkinElmer SeriesⅡCHNS/O Analyzer 2400 (USA).The microstructures of the solid particles were taken using scanning electron microscope (SEM,JSM-35CF,Japan Electron Optics Laboratory Co,Ltd).The physical structure was evaluated using X-ray diffractometer (XRD,D/max-RB,Rigaku,Japan) with the following conditions:40 kV,CuKαradiation with a graphite monochromator,and electric current of 40 mA.The patterns were obtained within 5°-90°at 2θangular interval of 0.05°and counting time of 1 s.

3 Results and discussion

3.1 Decomposition of zircon sand

3.1.1 Effect of alkali/zircon mass ratio

Decomposition of zircon sand with NaOH at high temperature is a well-known procedure.Figure 2 shows that the decomposition rate of zircon and the fused products/zircon mass ratio vary with the overall alkali/zircon mass ratio under the fused temperature of 700℃and fused time of 1 h.It is evident that decomposition rate increases with the alkali/zircon mass ratio,and it is up to 97.4%when the alkali/zircon mass ratio is 1.3,and the increasing rate decreases when the alkali/zircon mass ratio surpasses 1.The fused products/zircon mass ratio increases linearly with alkali/zircon mass ratio.Here,the range of 1.1-1.3 of alkali/zircon mass ratio is chosen to carry out two-step fusion experiments.

Fig.2 Effect of alkali/zircon mass ratio on zircon decomposition rate by one-step method (temperature:700℃;time:0.5h)

Figure 3 illustrates the effect of alkali/zircon mass ratio on decomposition rate of zircon when using two-step fusion method.It can be seen that the decomposition rate of zircon increases in the alkali/zircon mass ratio range of1.1-1.3,and the alkali amount of first step also influences the decomposition noticeably.Taking alkali/zircon mass ratio of 1.3 as an example,the decomposition rate of zircon increases from 92.18%to 98.25%when the alkali/zircon mass ratio at the first step changes from 0.4 to 0.9,while the value by one-step method is 97.4%.The results for alkali/zircon mass ratios of 1.1 and 1.2 are similar,and the decomposition rates increase from 90.8%to 93.14%and from 94.7%to 95.98%,respectively,by the change of fusion mode.These results show that when the alkali amount of first step reaches a certain value,the decomposition rate of zircon sand using two-step method is higher than that using one-step method.When the alkali/zircon mass ratio is 0.7 at the first step and 0.6 at the second step (0.7+0.6),the decomposition rate reaches97.26%,so the appropriate alkali/zircon mass ratio is0.7+0.6.

3.1.2 Effect of temperature

The effect of fused temperature on decomposition of zircon sand was investigated.As shown in Fig.4,the decomposition rate of zircon rises with the increase in temperature,and when the alkali/zircon mass ratio is 0.7+0.6,the decomposition rate is 92.48%at 500℃,93.99%at600℃,97.25%at 700℃,and up to 97.83%at 800℃;the growth rates for 700 and 800℃are lower.During the experiments,it is observed that the fused product lumps easily at the bottom of the crucible when the fused temperature is 800℃,and consequently,700℃is the optimum temperature.

Fig.3 Effect of alkali/zircon (A/Z) mass ratio on zircon decomposition rate by a one-step method and b two-step method (temperature:700℃;time:0.5+0.5 h).A',B',C':A/Z mass ratio=1.1,1.2,1.3;A1-A5 (A/Z mass ratio=1.1):A/Z mass ratio at the first step=0.4-0.8;B1-B5(A/Z mass ratio=1.2):A/Z mass ratio at the first step=0.4-0.8;C1-C6 (A/Z mass ratio=1.3):A/Z mass ratio at the first step=0.4-0.9

3.1.3 Effect of fusion time

Figure 5 shows the effect of fusion time of two steps on zircon decomposition,supposing that the fusion time of two steps is the same.It is shown that the decomposition rate of zircon sand increases gradually and hardly changes after 0.5 h.Therefore,the optimum fusion time is 0.5 h at the first step and 0.5 h at the second step (0.5+0.5 h).

3.1.4 Analysis of two-step alkali fusion process

The fusion experiments of zircon sand were carried out in the conditions that alkali/zircon mass ratio of 0.7+0.6,time of 0.5+0.5 h,and temperatures of 500,600,700,and 800℃.XRD patterns of the final fused products are shown in Fig.6.The results indicate that the Na₂ZrO₃compound is predominant,and it is similar at different temperatures.It is worthwhile to note that the diffraction peaks of Na₂ZrSiO₅ are not observed,and Na₂ZrO₃ is present as a main product,which further indicates that silicon mainly exists in the form of soluble materials,such as sodium silicate (Na₂SiO₃).The fused products of two steps at alkali/zircon mass ratio of 0.7+0.6,time of0.5+0.5 h,and temperature of 700℃are also shown in Fig.7,which exposes that the phase compositions of firststep products are mainly Na₂ZrO₃ and ZrSiO₄.The unreacted ZrSiO₄ is continually reacted with NaOH in the second step.According to the above results,it is determined that during the decomposition process of zircon sand,the main reaction equations are as follows:

Fig.4 Effect of fused temperature on zircon decomposition rate(time:0.5+0.5 h)

Fig.5 Effect of fused time on zircon decomposition rate (A/Z mass ratio:0.7+0.6;temperature:700℃)

Fig.6 XRD patterns of fused products obtained at different temper-atures (A/Z ratio:0.7+0.6;time:0.5+0.5 h)

Fig.7 XRD patterns of two-step fused products (A/Z:0.7+0.6;temperature:700℃;time:0.5+0.5 h)

Fig.8 SEM images of zircon sand and fused products:a zircon sand,b fused product of first step,and c fused product of second step

Since the diffraction peak of Na₂ZrSiO₅ is not observed in the products,it is obvious that Reaction (4) does not happen.The results are consistent with Barawy's research [ 12] .

SEM images of fused products with alkali/zircon mass ratio of 0.7+0.6,time of 0.5+0.5 h,and temperature of700℃are shown in Fig.8.Figure 8a is the SEM image of zircon sand,and Fig.8b,c shows SEM images of fused products of two steps,respectively.It can be seen that the crystal phase is destroyed and decomposition of zircon sand happens in alkali fusion process.

3.2 Water leaching

The fused products obtained from the fusion of zircon by two-step method with alkali/zircon mass ratio of 0.7+0.6,time of 0.5+0.5 h,and temperature of 700℃were leached with water to remove soluble sodium silicate.For convenience,the contents of SiO₂ and Na₂O were calculated based on 40%ZrO₂.

Fig.9 Effect of L/S ratio on removal of SiO₂ and Na₂O (temper-ature:50℃,time:0.5 h;leaching once)

3.2.1 Effect of liquid-solid ratio

The effect of liquid-solid ratio (L/S ratio) ranging from 2to 7 on leaching rate of SiO₂ and Na₂O from fused products was investigated,and the results are shown in Fig.9.It is indicated that the contents of SiO₂ and Na₂O decrease with the increase in L/S ratio,and the content of SiO₂ can reduce to 3.5%(40%ZrO₂) when the L/S ratio is 7:1.Meanwhile,the leaching rates of SiO₂ and Na₂O gradually descend when the L/S ratio is more than 5:1,so the optimal L/S ratio is 5:1.

3.2.2 Effect of leaching time

Figure 10 shows the effect of water leaching time on removal of SiO₂ and Na₂O.It is seen that leaching time has small impact on removal of SiO₂ and Na₂O,and therefore,the optimum leaching time is chosen as 0.5 h.

3.2.3 Effect of leaching temperature

The effect of 30-90℃leaching temperatures on removal of SiO₂ and Na₂O was investigated.Figure 11 shows that the removal of SiO₂ and Na₂O is basically steady in the temperature range.It is noteworthy that at temperature of30℃,the actual temperature of slurry is more than 30℃,which can be attributed to the heat release behavior of NaOH dissolution.The measured temperature of slurry is50℃when the temperature of added water is 30℃.Considering that temperature has few effects on leaching,the optimal temperature is chosen as 50℃.

Fig.10 Effect of time on removal of SiO₂ and Na₂O (temperature:50℃,L/S ratio:5:1;leaching once)

3.2.4 Effect of leaching times

Considering that SiO₂ and Na₂O cannot be removed thoroughly by leaching only once,the effect of leaching times was investigated,and the results are shown in Fig.12.As can be seen,the leaching times have a significant effect on removal of Na₂O,but have a small effect on removal of SiO₂.Besides,the filtering speed would slow down after leaching four or five times.Thus,the optimal leaching time is three times.The contents of SiO₂ and Na₂O are 3.51%(40%ZrO₂) and 4.46%(40%ZrO₂),respectively.

Fig.11 Effect of leaching temperature on removal of SiO₂ and Na₂O(L/S ratio:5:1;time:0.5 h;leaching once)

Fig.12 Effect of leaching times on removal of SiO₂ and Na₂O(temperature:50℃;L/S ratio:5:1;time:0.5 h)

3.2.5 Analysis of water leaching process

From the above analyses,it is known that the components of fused products are Na₂ZrO₃,Na₂SiO₃,and un-reacted alkali.In water leaching process,most of SiO₂ and Na₂O are removed,which leads to the preliminary separation with ZrO₂.The component contents of leaching products with leaching temperature of 50℃,L/S ratio of 5:1,leaching time of 0.5 h,and leaching three times are:ZrO₂45.86%,SiO₂ 3.51%(40%ZrO₂),and Na₂O 4.46%(40%ZrO₂).The three leaching products were analyzed using XRD,as shown in Fig.13.

The mechanism of water leaching was studied in earlier work [ 29] .As the results indicated,Na₂ZrO₃ peaks disappeared,which could be attributed to the transformation of ZrO₂ during water leaching process.According to Ref. [ 3] ,Na₂ZrO₃ tends to hydrolyze when the alkalinity is low,and the chemical equation is as follows:

The particles of ZrO(OH)₂ are very small,and it is difficult to filter and precipitate them,which explains the fact that the leaching slurry filtration becomes more and more difficult with the leaching times increasing.

The content of SiO₂ is almost constant in the water leaching process,due to that most Si0₂ is removed in the first leaching.Na₂SiO₃ or Na₄SiO₄ is water soluble;the hydrolysis of sodium silicate can be expressed as follows:

So when the alkalinity is low,the hydrolysis reaction of Na₂SiO₃ will happen and H₂SiO₃ is generated,which is not water soluble.H₂SiO₃ is gel forming in filter cake.So increasing the leaching time or L/S ratio is not beneficial to the removal of SiO_2.

Fig.13 XRD patterns of leaching products obtained (temperature:50℃;L/S ratio:5:1;time:0.5 h)

Based on the above analysis,it can be concluded that though the removal efficiency of impurities increases when the leaching times increase,the water consumption and energy consumption also increase.

4 Conclusion

A two-step alkali fusion method of zircon was proposed,and the alkali fusion and water leaching processes were studied.In the process of alkali fusion,alkali/zircon mass ratio,temperature,and fusion time have effects on decomposition of zircon sand,and the optimal conditions are alkali/zircon mass ratio of0.7+0.6,temperature of 700℃,and fusion time of0.5+0.5 h.Under these conditions,the decomposition rate of zircon sand is 97.25%.The transformation of zircon sand was also studied.At the first step,products are mainly Na₂ZrO₃ and ZrSiO₄,and the products of second step are Na₂ZrO₃ and Na₂SiO₃,but the diffraction peaks of Na₂ZrSiO₅are not observed.For water leaching process,the optimal conditions are liquid-solid ratio of 5:1,leaching time of0.5 h,leaching temperature of 50℃,and leaching three times.Under these conditions,the contents of leaching products SiO₂ and Na₂O are 3.51%(40%ZrO₂) and4.46%(40%ZrO₂).The crystal phase structures of Na₂ZrO₃ and Na₂SiO₃ are transformed in water leaching process.

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