Abdel Rehim, A. M.

Khalil, Sobhy O.

English

Sintering of zircon with calcium carbonate or oxide was reported to take place completely at temperature range 1400—1500 °C. This work represents a thermal investigation of sintering of Rosetta zircon with calcite in presence of graphite by using derivatograph. The products of sintering were identified by using a Siemens crystalloflex diffractometer. The process of sintering results in the formation of calcium zirconate CaZr03 and calcium silicate of composition 3 Ca0-Si02

Abdel Rehim A. M.

Khalil Sobhy O.

University of Szeged

Acta Mineralogica-Petrographica, Szeged, XXIIjl, 147—155, 1975 S I N T E R I N G O F R O S E T T A Z I R C O N W I T H C A L C 1 T E A . M . A B D E L R E H I M , S . O . K H A L I L ABSTRACT Sintering of zircon with calcium carbonate or oxide was reported to take place completely at temperature range 1400—1500 °C. This work represents a thermal investigation of sintering of Rosetta zircon with calcite in presence of graphite by using derivatograph. The products of sintering were identified by using a Siemens crystalloflex diffractometer. The process of sintering results in the formation of calcium zirconate CaZr0 3 and calcium silicate of composition 3 Ca0 -S i0 2 . INTRODUCTION Among the minerals of the black sand of Egypt is zircon which is found in an economic amount ( 7 . 2 9 % ) . These sands are found along the Mediterranean coast of Nile Delta. These sands are enriched by gravity concentration methods followed by magnetic and electrostatic separation [ A B D E L R E H I M , 1 9 7 4 and A N W A R et al., 1 9 7 0 ] . There are different methods of chemical processing of zircon to obtain zirconium oxide [ A N W A R et al., 1970; M I R S O N et al., 1965 and Z E L I K M A N et al., 1964]. These include sintering of zircon with soda, alkali or calcium oxide and hydrometallurgical leaching with alkali solutions. Sintering of zircon with calcium oxide takes place only at 1400—1500°C. Addition of some components such as CaCl2 and CaF2 leads to the decrease of temperature of the process [ G A L K I N E ? al, 1971; M I R S O N et al., 1965 and Z E L I K M A N et al., 1964]. Non metamict zircon did not give any thermal reaction. Several data from literatures can be found on the thermal investigations of zircon, calcite and other calcium silicates [ E Y S E L et al., 1 9 7 4 ; F R O N D E L , 1 9 5 3 ; G A L K I N et al., 1 9 7 1 ; G A R R E L S et al., 1 9 6 0 ; I V A N O V A , 1 9 6 1 ; K O N D R A S H O V A et al., 1 9 5 2 ; M A C K E N Z I E , 1 9 6 4 & 1 9 6 2 . ] The thermogram of calcite shows a strong endothermic peak at 880 °C which repre-sents its dissociation to calcium oxide and the liberation of carbon dioxide. Little is known about the thermal character of reaction of sintering of zircon with calcium carbonate. This work represents a thermal investigation of sintering of Rosetta zircon with calcite in presence of graphite at considerably lower temperature than the temperature range reported earlier by using a derivatograph. Also, it includes a study of the products of sintering by X-ray powder diffraction analysis. EXPERIMENTAL WORK This work was carried out with zircon, separated from Egyptian black sand. Its chemical, mineralogical and X-ray analysis are given in Tables 1, 2 and 3, respecti-vely. The X-ray powder data of calcite used in the mix are given in Table 4. 10* 147 Chemical composition of zircon concentrate TABLE 1 Chemical components Content, % Si02 32.40 Z r 0 2 64.21 Ti0 2 0.73 Fe 2 0 3 1.57 AI2O3 0.68 TABLE 2 Mineralogical analysis of zircon concentrate Mineral Content, % Zircon 96.70 Rutile 0.45 Ilmenite 0.68 Monazite 0.20 Epidote 0.53 Garnet 0.76 Amphiboles 0.10 TABLE 3 X-ray powder diffraction data of zircon d{k) d(A) / / hkl ASTM Observed ASTM Observed 4.434 4.438 45 65 101 3.302 3.304 100 100 200 2.650 2.653 7 12 211 2.518 2.520 45 65 112 2.336 2.338 10 25 220 2.217 2.223 8 12 202 2.066 2.068 20 45 301 1.908 1.911 14 12 103 1.751 1.752 11 20 321 1.712 1.712 40 60 312 1.651 1.652 14 40 400 1.547 1.548 4 4 411 1.495 1.498 3 2 004 1.477 1.478 8 20 420 As shown from Table 2, contaminants include some ilmenite, rutile, monazite, garnet, amphibole and others in fine grains and in small amounts. Non of the con-taminants was detected by X-ray analysis, therefore, no individual mineral may be present as a major constituent which is in consistence with the mineralogical data. The total Zr0 2 content in pure zircon should be 67.22%. 148 TABLE ( X-ray powder diffraction data of calcium silicate (3CaO • SiO«) d(k) / / hkt ASTM Observed ASTM Observed 3.86 3.850 12 12 102 3.035 3.035 100 100 104 2.845 2.840 3 3 006 2.495 2.498 14 16 110 2.285 2.284 18 22 113 2.095 2.095 18 18 202 1.927 1.926 5 6 204 1.913 1.913 17 17 108 1.875 1.875 17 22 116 1.626 1.626 4 4 211 1.604 1.604 8 10 212 1.587 1.586 2 2 1.0.10 1.525 1.524 5 6 214 1.518 1.521 4 4 208 1.510 1.509 3 2 119 1.473 1.472 2 2 215 1.440 1.438 5 6 300 1.422 1.434 3 3 0.0.12 TECHNIQUES OF WORK Starting materials Starting materials usually consisted of zircon mixes. Reagents used in mixes are calcite and graphite in particular amounts. Mixes were processed by repeated grind-ing in an automated agate mortar and sieving till all the powder pass through 200 mesh sieve and pestle for one hour to achieve homogeneity. Sample container and apparatus Sintering runs were carried out in ceramic crucibles, heated in an electrical furnace. The character of the reactions of sintering of zircon with calcite in presence of graphite was studied by thermal analysis using F. P A U L I K , J . P A U U K and L. E R D E Y derivatograph [ P A U L I K et ah, 1 9 6 6 ] . This apparatus records simultaneously four thermal curves: (T).the change of temperature o i the sample, (DTA) differential thermal analysis, (TG) thermogravimetric (quantitatively in mg'l and (DTG) derivative thermogravimetric, on a single sample under controlled conditions. DTA and temperature measuring thermocouples are Pt/Pt-Rh wires. Ceramic crucible and a ceramic sample holder were used. Alumina, calcined at 1000 °C, was used as a reference material. The parameters during tests were as follows: Weight of sample 1 gm, T—1200, DTA — 1/10, DTG — 1/10, TG — 500 mg and heating rate of 10 °C per minute. All determinations were carried out in air atmosphere under suction of carbon dioxide. Phase identification and characterization The end products of zircon sintering with calcite were studied both microscopi-cally and by X-ray analysis. 149-X-ray Procedure A Siemens Crystalloflex diffractometer was used with nickel filtered copper radiation. Exposure was one hour and scanning speed was 1° 20 per minute at 1 cm per minute chart speed. Intensities were collected to maximum20 = 65°. The sensi-tivity of the experiment was 1X104 impl./min., and the statistical error was 1.5%. Chemical analysis of the end product of sintering was carried out for the deter-mination of the eificiency of sintering. Before studying the thermal character of the reaction of sintering of zircon with calcite, a calculation of its thermodynamic constants was estimated. The standard free energy (F°) and the equilibrium constant (K) of the reaction were —20.60 Kcal/mol and 1.29 XlO15, respectively. RESULTS AND DISCUSSION The derivatogram of zircon mixture with calcite of amount 15 % of theoretical value and graphite in the suitable amount 15% of zircon charge is shown in. Fig. 1. The evaluation of the DTA curve of zircon mixture is based on its comparison with that of calcite and other components of the mix. The first small endothermic peak at Fig. 1. Derivatogram of zircon sintering with calcite in presence of graphite. Weight of sample 1000 mg. Heating rate 10 °C/min. 150 70 °C represents the dehydration of the mix. The sharp exothermic peak at 580 °C may be due to the burning of graphite and the combustion of volatiles. This is accompanied by some decrease in weight (TG). The large endothermic peak at 880 °C, represents the intensive dissociation of calcite and the reaction between zircon and the resultant calcium oxide. This process is connected with a remarkable decrease in weight (TG) due to the liberation of carbon dioxide. According to the thermal curve of this mixture, the beginning of sintering and decomposition of «calcite follows directly the end of the exothermic reaction. The exothermic peak at 945 °C may be connected with the reaction between zircon and calcium oxide and probably transition of the resulted calcium silicate to another form. The transition temperature is previously recorded near to this value [ E Y S E et al., 1 9 7 4 ] . There is a small exothermic peak at 1340 °C which is under study and probably due to the congruent melting of resulted sphene, since the temperature of melting of sphene was recorded as 1382 °C. The X-ray powder diffraction pattern shows the first appearance of calcium zirconate and silicate above 700 °C as shown in figures 2 and 3. The sintering process results in the formation of calcium zirconate and calcium silicate of composition 3CaO • Si02 instead of CaO • SiOa and 2CaO • Si02 as recorded previously [ M I R S O N et al., 1 9 6 5 and Z E L I K M A N et al, 1 9 6 4 ] . Their X-ray powder diffraction data are given in Tables 5 & 6. The obtained calcium zirconate will be directed for dissolution process using hydrochloric or sulphuric acid for extraction of zirconium. For studying the effect of temperature on sintering of zircon, some runs were carried out using zircon mixes with calcite and graphite in amount 15% of zircon charge, at temperature ranging from 850 °C up to 1050 °C during 5 hours. From the obtained results (Table 7), it is shown that the sintering efficiency of zircon sharply increases with temperature. It is observed that nearly complete sin-3 10 QS 30 35 40 45 SO SS B0 2 8 ° ( C u k a ) • 18 ZC 65 Fig. 2. X-ray powder diffraction pattern of the products of zircon sintering at 700 °C 1 — Calcium zirconate 2 — Calcium silicate 3 — Zircon 151 i 11 30 is *0 45 To ~S5 60 28°(Cuka) »- 12 C Fig. 3. X-ray powder diffraction pattern of the products of zircon sintering with calcite in presence of graphite (A), (B), (C) and (D) at 850, 900, 950 and 1050 °C. 1 — Calcium zirconate 2 — Calcium silicate 3 — Zircon 152 TABLE ( X-ray powder diffraction data of calcium s i l i c a t e (3CaO • SiO«) ¿(A) d(k) / I hkl ASTM Observed ASTM Observed 4.01 4.006 80 90 200, 020 3.58 '3.587 40 20 120, 210 2.87 2.872 80 60 202 2.83 2.836 100 100 220,022 2.79 2.794 80 68 202 2.70 2.714 10 28 212 2.55 2.547 10 28 103, 301 2.51 2.513 40 25 103, 301 2.43 2.430 40 25 113,311 2.41 2.409 40 25 131 2.39 2.400 40 25 113,311 2.33 2.338 20 22 222 2.29 2.287 20 111 222 2.22 2.219 40 20 023, 321 2.13 2.128 10 9 132,231 2.00 2.002 100 90 400,040 1.957 1.945 20 22 401,104 1.939 1.932 40 36 014,041 1.845 1.839 10 14 133,331 1.828 1.829 10 13 133, 331 1.811 1.812 40 20 204,302 1.789 1.792 60 50 024,042 1,751 1.753 10 9 142,241 1.727 1.713 10 9 214,412 1.651 1.651 80 78 224, 422 1.642 1.643 6 6 242 1.625 1.628 60 40 242 1.617 1.624 80 50 224, 422 1.498 1.499 40 15 234, 432 tering of zircon (96.6%) takes place at 1050 °C. The products of sintering of these runs were identified microscopically and by X-ray diffractometer. By microscopic examination of thin sections of the products of sintering at 850, 900, 950 °C, it is observed that considerable numbers of zircon grains were detected in the runs at 850 and 900 °C. At 950 °C the amount of zircon grains decreases in the sintered product. This gives good idea about incomplete sintering. The product obtained in the run at 1050 °C consists mainly of calcium zirconate and silicate with few fine grains of zircon. The X-ray powder diffraction patterns of these products of sintering at 850,. 900, 950 and 1050 °C are shown in Fig. 3. Zircon is piesent in large amounts in the runs A, B and C and its peaks are completely disappeared in the product of run D at 1050 °C. It is observed that the X-ray peaks of zircon are less intense as the temperatures rises. The X-ray peaks of calcium zirconate and silicate are narrow and intense, suggesting good crystallinity. The microscopic study of the product phases of zircon sintering with calcite is in good agreement with the X-ray powder diffraction patterns. 153-TABLE ( X-ray powder diffraction data of calcium silicate (3CaO • SiO«) «/(A) d(k) / / hkl ASTM Observed ASTM Observed 3.07 3.090 90 40 201 3.01 3.020 30 15 202 2.84 2.838 70 80 • 009 2.787 2.794 100 100 204 2.739 2.744 6 10 116 2.649 2.639 100 70 205 2.362 2.365 20 12 207 2.331 2.338 6 7 211 2.303 2.310 4 4 212 2.224 2.220 50 30 119 2.198 2.189 16 22 214 2.006 2.004 12 40 303 1.971 1.986 18 50 210,010 1.858 1.861 8 14 301 1.830 1.829 2 4 1.1.12 1.789 1.792 55 60 220 1.669 1.678 4 10 309 1.659 1.662 16 30 314 1.575 1.576 10 10 2.1.12 1.537 1.539 4 7 402 1.513 1.516 25 16 229, 318 1.505 1.510 6 6 404 1.482 1.478 12 10 405 TABLE 7 Effect of temperature on sintering efficiency of zircon Temperature, °C Sintering efficiency, % 850 56,8 900 67.3 950 83.7 1050 96.6 C O N C L U S I O N S From the thermal investigation of sintering of Rosetta zircon with calcite of -amount 15% in presence of graphite has revealed that nearly its complete sintering -can be reached at 1050 °C. This temperature is distinctly lower than the temperature range reported earlier. The microscopic and X-ray diffraction study shows that the sintering of zircon Tesults in the formation of calcium metazirconate and calcium silicate of composition -CaZr03 and 3CaO • Si02 respectively. 154-ACKNOWLEDGEMENTS This work was carried out in the Institute of the Mineralogy, ELTE University, Budapest, to whom the writer is much indebted. Help received from P R O F . D R . J. Kiss is gratefully acknowledged. REFERENCES ABDEL REHIM, A. M. [1974]: Desilication of zircon with aluminium fluoride in presence of graphite. Read before the Fourth Internat. Conf. Therm. Anal., Budapest. ANWAR, Y. M., ABDEL REHIM, A. M. [1970]: High pressure hydrometallurgy of alkaline leaching of Egyptian zircon. — Bull. Fac. Sci. Alex. Univ., 10, 173—193. EYSEL, W . , WOERMANN, E . a n d HAHN, TH. [1974]: D T A of c a l c i u m s i l ica te (3 C a O . S i 0 2 ) so l id solutions. — Fourth Internat. Conf. Therm. Anal., Budapest. FRONDEL, C. [1953]: Hydroxyl substitution in thorite and zircon. — Amer. Min., 38, 1007. CALKIN, U . M . , TSHUKHLANTSEV, V . G . a n d LEVAKOV, E . V. [1971] : E f f ec t o f s o m e sa l t s o n ve loc i t y of decomposition of zircon with calcium carbonate. — Non Fer. Metals., 7, 54—56. GARRELS, R. M., THOMPSON, M. E. and SIEVER, R. [I960]: Stability of some carbonates. — Amer. Jour. Sci., 258, 402-418. IVANOVA, V. P. [1961]: Thermograms of minerals. — Zapisky Vsiesoiusnova mineralogisheskova obsh. 90, 50—90. Acad. Sci. USSR. KONDRASHOVA, P. S. et at. [1972]: Formation of dicalcium silicate in presence of calcium ferrite. — Metals., 1, 62—65. KULP, J . L . VOLCHOK, H . L . a n d HOLLAND, H . D . [1952]: A g e f r o m m e t a m i c t m i n e r a l s . — A m e r . M i n e r a l . , 37, 7 0 9 — 7 1 8 . MACKENZIE, R. C. [1964]: Scifax Differential Thermal Analysis Data Index, First Supplement. Macmillan, London. MACKENZIE, R. C. [1962]: Scifax DTA Data Index. Cleaver-Hume Press, London. MIRSON, G. A., ZELIKMAN, A. N. [1965]: Metallurgy of rare metals. — Metallurgizdat. PAULIK, F., PAULIK, J. and ERDEY, L. [1966]: Derivatography. A complex method in thermal analysis. — Talanta 13, 1405—1430. ROSEN, E., NUEN, A., [1965]: Stability of zircon in temperature range 1180 to 1360 °C. — Jour. Amer. Ceram. Soc., 48, 603—604. TAUBER, E. and JAMES, R. J. [1971]: Zircon and its uses. — Jour. Austral. Ceramic. Soc., 7, 27—33. ZELIKMAN, A. N., SAMSONOV, G. V. and KREIN, O. E. [1964]: Metallurgy of rare metals. — Izdatelstva metailurgia, M. Manuscript received, January 7, 1975 D R . A . M . ABDEL REHIM D R . S. O . KHALIL Dept. of Geology, Alexandria University, Alexandria Egypt 155 

Sintering of rosetta zircon with calcite

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Sintering of rosetta zircon with calcite

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