Journal of Non-Crystalline Solids 140 (1992) 331-334 North-Holland

] O U R N A L O F

NON-CRYSTALLINE SOLI])S

ZrOC12 for fluoride glass preparation

Z a n Ling and Z h a n g C h e n g s h a n Infrared Material Laboratory, Wuhan Unic, ersity, Wuhan, People's Republic of China

D o n g Gaox ian and W a n g Kangkang Hubei Institute of Geology, Wuhan, People's Republic of China

A method is described for removing trace metals (Fe, Co, Ni, Cu) from ZrOC12 aqueous solution by s imultaneous solvent extraction. The system involving methyl isobutyl ketone (MIBK) as solvent, and ammon ium pyrrolidine dithiocarbonate or diethyldithiocarbonate as chelating agent has been researched. The effect of matrix concentration and extraction pH are discussed. The trace metals in a purified ZrOC1 a sample were determined by GFAAS and the results indicate that the Fe content is below 10 ppb, while Co, Ni and Cu contents are each below 5 ppb.

1. Introduction 2. Experimental

High purity ZrF 4 is needed for the preparation of fluoride fibers with ultra-low optical losses. To obtain high purity ZrF 4, many purification meth- ods are being developed [1]. In a previous paper [2], we reported work on purification of ZrF 4. In the present paper, a new method is described. The impurities (Fe, Co, Ni, Cu) are extracted from ZrOCI 2 aqueous solution with ammonium pyrrolidine dithiocarbonate or diethyl dithiocar- bonate (APDC or DDTC) as chelating agent and methyl isobutyl ketone (MIBK) as second phase. The A P D C / M ! B K system and D D T C / M I B K system are widely used in determining the trace metals in sea water and natural water [3,4]. T h e Fe content in the purified ZrOC12 sample is below 10 ppb, and Co, Ni, Cu content are each below 5 ppb. The high purity ZrF 4 is then ob- tained from the following reactions:

ZrOC12 + NH 3 • H 2 0 ~ ZrO 2 - n H 2 0 , (pure) (pure)

ZrO 2 " n H 2 0 + HF ~ Z r F 4. (pure)

Glassware for all operations was soaked in 10 vol.% nitric acid for one week before initial use and was subsequently stored under 3 N hydro- chloric acid. All experiments were carried out in a super clean room or clean glove box. A PHS-3 pH meter was used.

A working standard was prepared by appropri- ate dilutions of stock standards so that the final metal concentrations were 1 p~g/ml Fe, Co, Ni, Cu, respectively.

Super pure water:deionized water was filtered through MilLi-Q water filter. Analytical grade MIBK was distilled twice. A buffer solution was prepared by dissolving 500 g AR NH4Ac in 1 1 deionized water. The solution was purified by the addition of 25 ml of 5 vol.% APDC, followed by three successive extractions with 20 ml portions of MIBK.

An aqueous 5 vol.% solution of APDC was prepared by dissolving 5 g AR APDC in 100 ml deionized water each day. Since a fraction of the APDC is often water-insoluble, the solution was filtered. Portions (100 ml) of the solution were

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332 Zan Ling et aL / ZrOCI 2 for fluoride glass preparation

Table 1 Working conditions of the AAS

Element Light A Slit Sample volume source (nm) (nm) (~1)

Fe HCL 248.3 0.7 30 Cu HCL 324.7 0.7 30 Co HCL 240.7 0.7 50 Ni HCL 232.0 0.7 80

purified by a single extraction with 20 ml of MIBK. All four elements (Fe, Co, Ni, C u ) w e r e removed from the APDC. A 5 vol.% solution of DDTC was prepared by dissolving AR DDTC in water, filtered and purified in the same way as APDC.

A Perk in-Elmer 3030 AAS and H G A 500 graphite furnace (GF) for atomic absorption measurements were used. Table 1 shows the working conditions of the AAS and table 2 shows the heating program of the GF.

Sufficient buffer was added to adjust the pH of the standard solution of the required value in the range 1-6. After the addition of the chelate solu- tion, APDC or DDTC and MIBK, the separating funnels were shaken 300 times. After at least 5 min for APDC, 20 min for DDTC, the organic phase was collected in a dry tube and subse- quently analyzed by GFAAS. The aqueous phase was collected for pH measurement.

Table 2 Graphite furnace heating program

Program Element Temperature Ramp Hold ( ° C) (s) (s)

Drying 1 Drying 2

Ashing

Atomization

Cleaning

Fe Cu Co Ni

Fe Cu Co Ni

130 10 20 300 5 10

1400 5 15 1000 5 15 1300 5 15 1300 5 15

2500 1 6 2500 1 6 2600 1 6 2600 1 6

2700 1 3

In the raw ZrOC12 • 8 H 2 0 sample, the transi- tion element TE (Fe, Co, Ni, Cu) content is above several ppm. The sample was recrystallized first, the TE content being decreased below 1 ppm. The recrystallized ZrOCI 2 • 8 H 2 0 was then dissolved in superpure water. The pH of solution was adjusted carefully to 3 with NH4Ac buffer solution. The solution of 5% APDC of DDTC was added and shaken. MIBK was then added, and the separating funnel shaken 300 times (about 2 rain). After 5 rain for APDC, 20 min for DDTC, the aqueous phase was collected and extracted again in the same way in a new separating funnel. The last aqueous phase was collected and ana- lyzed. Three or four successive extractions were needed in order to obtain satisfactory results.

3. Results

Figure 1 and fig. 2 show the pH dependence of extraction with APDC or DDTC, as determined by the absorbance of the solution.

The impurities in purified ZrOC12 were deter- mined by GFAAS using the methods of standard addition and extraction. The results are shown in

Abs.

0.5

0.3

_ _ ° • G o ' . I I N [

0,1

/ I I I I I

2 3 4 5 6 pH

Fig. 1. pH dependence of extraction with APDC.

Zan Ling et al. / ZrOCl 2 for fluoride glass preparation 333

0.5

Cu 0,3 Co

Ni

Fe

0,1

I I I I I

Abs.

1 2 3 4 5 6 pH

Fig. 2. pH dependence of extraction with DDTC.

table 3 and table 4. Detec t ion limits were calcu-

lated by the formula

D L = 3 S b / S W = 2 S J W ,

Table 3 The impuritiy contents (ppb) in purified ZrOCl 2 samples

System Fe Cu Co Ni

APDC/MIBK < 10 < 5 < 5 < 5 DDTC/MIBK < 10 < 5 < 5 < 5

where S b iS the s tandard deviat ion of the b lank signal, S c is the s t andard devia t ion of the b lank concent ra t ion , S is the slope of the s tandard curve, and W is the sample weight phase as a funct ion of time. We have found all the metal

chelates in the A P D C system to be stable for 24 h, in the D D T C system the Co and Cu chelates to be stable for 24 h, and Fe and Ni chelates for

10 h. We p repared the ZrOC12 aqueous solut ion in

0.1 g / m l , 0.2 g / m l , 0.3 g / m l and 0.4 g / m l con-

centrat ions . Z r O C l 2 was easily hydrated. If the concen t ra t ion of the aqueous solut ion was greater than 0.3 g / m l , the solut ion became colloidal as the buffer solut ion was added. In the 0.1-0.3

g / m l range, the matrix concen t ra t ion effects on extractions were similar.

Table 4 Detection limits and RSD% of impurities in ZrOCl 2

Element Total blanks Average Standard Detection limit RSD Recovery data value deviation (ng/g) (%) (%)

Fe 0.038,0.079,0.041,0.047 0.058,0.044,0.058,0.057 0.068,0.035,0.057,0.053 0.049 0.013 6.3 7.5 95 0.034,0.057,0.048,0.040 0.023,0.056,0.036,0.064

Cu 0.126,0.078,0.076,0.0167 0.093,0.087,0.074,0.158 0.083,0.101,0.127,0.106 0.105 0.020 5 7.1 89 0.094,0.094,0.064,0.135 0.111,0.089,0.088,0.160

Co 0.005, - 0.001,0.011,0.002 0.005,0.005,0.010, - 0.004 0.005,0.004,0.008,0.005 0.0044 0.0041 1 4.0 97 0.002,

Ni 0.012,0.015,0.011,0.020 0.007,0.004,0.015,0.013 0.005,0.011,0.022,0.009 0.013 0.0071 3 11.3 91 0.029,

334 Zan Ling et al. / ZrOCl 2 for fluoride glass preparation

4. Discussion

From fig. 1 and fig. 2 we can see that ~ pH 3 is the best extraction pH value in the D D T C / M I B K system and p H 3 - 6 is a bet ter extraction pH range in the A P D C / M I B K system. In this way, the A P D C / M I B K system is superior. Under the same conditions, the extraction efficiency of the A P D C / M I B K system is higher with Fe but lower with Co, Ni, Cu than the D D T C / M I B K system.

Table 3 shows that the A P D C / M I B K and D D T C / M I B K systems have obtained similar re- sults in that the Fe content is below 10 ppb, .and Co, Ni, and Cu contents are each below 5 ppb.

The stability tests show that the Fe and Ni chelates in the A P D C system are more stable than in the D D T C system. All the metal chelates in A P D C or in D D T C are stable enough to separate from aqueous phase and be analyzed.

5. Conclusion

In this paper, we have reported a method for removing trace metals (Fe, Co, Ni, Cu)

from ZrOC12 aqueous solution by extraction. The D D T C / M I B K system and A P D C / M I B K system are available and can decrease the content of Fe, Co, Ni, Cu. The Fe content achieved was below 10 ppb and Co, Ni, and Cu contents below 5 ppb. The best pH value is ~ 3 and matrix concentra- tion 0.3 g/1. The A P D C is superior to D D T C in p H conditions, phase separation time, and stabil- ity of metal chelates.

The authors would like to thank Mr G.J. Dai and Mr J. Cheng for their helpful contribution to experimental investigation and Mr K.K. Wan, Mr C.H. Xing and Miss Z.Y. Cheng for determina- tion of TE content.

References

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